A review of the distribution coefficients of trace elements in soils: influence of sorption system, element characteristics, and soil colloidal properties

Cadmium, zinc, and copper leaching rates determined in large monolith lysimeters

Soil mass balances are used to assess the risk of trace metals that are inadvertently applied with fertilizers into agroecosystems. The accuracy of such balances is limited by leaching rates, as they are difficult to measure. Here, we used monolith lysimeters to precisely determine Cd, Cu, and Zn leaching rates in 2021 and 2022. The large lysimeters (n = 12, 1 m diameter, 1.35 m depth) included one soil type (cambisol, weakly acidic) and distinct cropping systems with three experimental replicates. Stable isotope tracers were applied to determine the direct transfer of these trace metals from the soil surface into the seepage water. The annual leaching rates ranged from 0.04 to 0.30 for Cd, 2.65 to 11.7 for Cu, and 7.27 to 39.0 g (ha a)-1 for Zn. These leaching rates were up to four times higher in the year with several heavy rain periods compared to the dry year. Monthly resolved data revealed that distinct climatic conditions in combination with crop development have a strong impact on trace metal leaching rates. In contrast, fertilization strategy (e.g., conventional vs. organic) had a minor effect on leaching rates. Trace metal leaching rates were up to 10 times smaller than fertilizer inputs and had therefore a minor impact on soil mass balances. This was further confirmed with isotope source tracing that showed that only small fractions of Cd, Cu, and Zn were directly transferred from the soil surface to the leached seepage water within two years (< 0.07 %). A comparison with models that predict Cd leaching rates in the EU suggests that the models overestimate the Cd soil output with seepage water. Hence, monolith lysimeters can help to refine leaching models and thereby also soil mass balances that are used to assess the risk of trace metals inputs with fertilizers.

Mitigating sediment cadmium contamination through combining PGPR Enterobacter ludwigii with the submerged macrophyte Vallisneria natans

Sediment cadmium contamination poses risks to aquatic ecosystems. Phytoremediation is an environmentally sustainable method to mitigate cadmium contamination. Submerged macrophytes are affected by cadmium stress, but plant growth-promoting rhizobacteria (PGPR) can restore the health status of submerged macrophytes. Herein, we aimed to reduce sediment cadmium concentration and reveal the mechanism by which the combined application of the PGPR Enterobacter ludwigii and the submerged macrophyte Vallisneria natans mitigates cadmium contamination. Sediment cadmium concentration decreased by 21.59% after submerged macrophytes were planted with PGPR, probably because the PGPR colonized the rhizosphere and roots of the macrophytes. The PGPR induced a 5.09-fold increase in submerged macrophyte biomass and enhanced plant antioxidant response to cadmium stress, as demonstrated by decreases in oxidative product levels (reactive oxygen species and malondialdehyde), which corresponded to shift in rhizosphere metabolism, notably in antioxidant defence systems (i.e., the peroxidation of linoleic acid into 9-hydroperoxy-10E,12Z-octadecadienoic acid) and in some amino acid metabolism pathways (i.e., arginine and proline). Additionally, PGPR mineralized carbon in the sediment to promote submerged macrophyte growth. Overall, PGPR mitigated sediment cadmium accumulation via a synergistic plantmicrobe mechanism. This work revealed the mechanism by which PGPR and submerged macrophytes control cadmium concentration in contaminated sediment.

Enhancing remediation efficiency of hyperaccumulators through earthworm addition: Evidence from a pot study on cadmium-contaminated soil

Soil cadmium (Cd) contamination is an urgent environmental problem, which endangers human health through the food chain. Bioremediation attracted extensive attention around the world due to the high cost-efficiency. However, the remediation efficiency of different plant and earthworm species of soil Cd pollution is still unclear, it is thus of great significance to explore the combined effects of different remediation plants and earthworm species to improve the bioremediation capacity. In the present study, we consequently selected three species of Cd hyperaccumulator plants (vetiver, P. vittata and S. emarginatum) and three species of earthworms (E. fetida P1, E. fetida P2, and P. guillelmi) to compare the differences in Cd accumulation among various earthworm-plant combinations. Results indicated that the changes of soil pH and SOM in plant-animal combined application induced the higher soil Cd removal efficiency. The Cd removal efficiency showed highest in combination groups P. vittata-E. fetida P2 and P. vittata-P. guillelmi. Meanwhile, the improvements of biomass of plants and animals also were consistent with the increasing of Cd concentration in both plants and earthworms after combined application. It showed that the Cd concentrations in P. vittata were the highest while the TFs of Cd in S. emarginatum displays significantly more than that in others. In conclusion, the recommended combined system of earthworm-plant (P. vittata-E. fetida P2 and P. vittata-P. guillelmi) to provide reference for soil Cd bioremediation system in practice.

One-point calibration and matrix-matching concept for quantification of potentially toxic elements in wood by LA-ICP-MS

The aim of this work is to evaluate two quantitative methods, based on the external calibration applied in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis, known as (i) analytical curve and (ii) one-point calibration, using the concept of matrix matching to quantify three potentially toxic elements (PTEs) in wood samples. These can biologically register changes in the abiotic environment and be applied to monitoring climate change or environmental toxicity. In this case, standard sample preparation was evaluated to prepare the standard pellets using Pinus taeda species as a matrix-matching concept. Six pellets of P. taeda, with different Pb, Cd, and Ba concentrations, were prepared to build the analytical curve and one-point calibration strategies. The LA-ICP-MS parameters were optimised for 206Pb, 208Pb, 112Cd, 114Cd, 137Ba, and 138Ba isotope analysis in wood samples. The two calibration strategies provided 74-110% analytical recovery from certified reference materials and similar results to those obtained by ICP-MS through the acid digestion of environmental wood samples from São Paulo City (Brazil). This demonstrated the applicability of the one-point calibration strategy in quantifying PTEs in wood samples, which could be used with environmental analyses. Differences observed between the Ba isotope results obtained via LA-ICP-MS and ICP-MS quantification were related to sampling by LA-ICP-MS and the ICP-MS sample introduction, as well as to laser matrix and transport effects because of the difference between the wood species evaluated.

Comparison of water-soluble organic matter (WSOM)-containing and WSOM-free biochars for simultaneous sorption of lead and cadmium

The effects of individual biochar constituents and natural environmental media on the immobilization behaviors and chemical activities of toxic heavy metals are still poorly understood. In this work, the physicochemical properties of raw corn straw (CS) and CS-derived biochar materials as well as their sorption abilities and retention mechanisms for lead (Pb) and cadmium (Cd) were evaluated by combining batch experiments and spectral approaches. According to the spectral analysis results and single variable principle, the setting of biochars after soaking in solution as the control group was suggested when evaluating their retention mechanisms for Pb and Cd. The rising of ionic strength did not apparently affect the immobilization of Pb by biochar prepared at 500 °C (i.e., CB500) and Pb/Cd by water-soluble organic matter (WSOM)-free CB500 (i.e., DCB500), while slightly inhibited the sorption of Cd by CB500. Pb and Cd exhibited a mutual inhibition effect on their sorption trends with a higher sorption preference of Pb. The dominant fixation mechanism of Pb by CB500 and DCB500 was identified to be mineral precipitation. In contrast, the main sorption mechanism of Cd changed from mineral precipitation in the single-metal system to surface complexation in the binary-metal system. The sorption ratios of Pb and Cd on CB500 were comparable to those on DCB500 with the coexistence of mixed natural organic matters (NOM) and ferrihydrite. The current experimental findings suggested that DCB500 was a suitable remediation agent for regulating the migration behaviors of toxic Pb and Cd in acidic and NOM-rich soil and water systems.

Assessing the cadmium content of cacao crops in Arauca, Colombia

The district of Arauca is the second-largest producer of cacao in Colombia. However, despite its quality, it faces issues for export due to levels of cadmium (Cd) higher than the regulatory thresholds. A central question is how it may impact agricultural performance in the presence of Cd in cacao and chocolates. This study quantified Cd in cacao plantations from Arauca. Thus, 180 farms were assessed in the municipalities of Arauquita, Fortul, Saravena, and Tame. Five sample types (soil, irrigation channel sediment, soil litter, cacao seeds, and chocolates) were assessed for Cd. As a technological innovation, the new MXRF technology was used for Cd in chocolates. The sequence of Cd content was soil litter > chocolate > soils > cacao seeds > irrigation-channel sediment. A gradient north-south of Cd content in soil was observed, where highest content was found in farms near the Arauca River, and lower farther away. In irrigation channel sediment, Cd levels averaged 0.07 mg kg-1. The Cd content in cacao seeds was 0.78 mg kg-1 on average. Cd content in chocolates was above the threshold (1.10 mg kg-1 on average, including several cacao mass percentages). These artisanal chocolate bars produced by single farms were near the limit of Cd set by the European Union (up to 0.8 mg kg-1). Therefore, mixing beans from different farms could reduce their Cd content. The present study underscores the complexity of Cd distribution, emphasizing the importance of integrating soil, crop, and landscape features in managing and mitigating Cd levels in cacao.

Mineralogical and sorption characterization of lateritic soils from Southwestern Nigeria for use as landfill liners

Lateritic soils are prevalent in the tropical regions, and they are used for various construction purposes including landfill liner applications. However, their contaminant attenuation potentials through sorption and the influence of parent rocks on this property are poorly understood. This study investigates lateritic soils from southwestern Nigeria as barrier to leachate migration in engineered landfills and related waste containment facilities. The lateritic soils were investigated through X-ray diffraction (XRD), geochemical analysis and batch equilibrium sorption test to evaluate the competitive sorption of Mn, Cd, Pb, Cu and Cr which are common in landfill leachates. The XRD analysis shows that the kaolinite and dickite are the dominant clay minerals present in the lateritic soils, implying low desiccation cracking and low shrink-swell behaviour. The geochemical analysis indicate that the lateritic soils are silico-alumino-feruginuous with average major oxide composition of SiO2, Al2O3and Fe2O3 of 50.86 wt%, 29.83 wt% and 14.29 wt%), respectively. Additional oxides with lower abundance include TiO2 (1.55 wt%), Na2O (0.01 wt%), MgO (0.36 wt%), CaO (0.15 wt%) and K2O (1.52 wt%). The lateritic soils contain trace amount of heavy metals with average concentrations of Cd (0.039 ppm), Pb (0.548 ppm), Cr (0.189 ppm), Cu (0.964 ppm), Mn (0.145 ppm). Furthermore, the low abundance of sodium oxide in the lateritic soils indicates that the soil particles are not susceptible to dispersion while the presence of considerable amount of iron and manganese oxides signify its good heavy metal retention. The batch equilibrium sorption analysis shows that the lateritic soils derived from granite-gneiss and charnockite exhibit better sorption potential than those derived from schist and quartzite. This high sorption capacity is intricately related to the presence of goethite in the soils. The sorption of these trace metals onto the lateritic soils follows Langmuir type isotherm and these isotherms deviate from the corresponding desorption isotherms to different degrees indicating various extents of hysteresis. The sorption hysteresis indices for these trace metals range from 0.63 to 0.99 and imply that the trace metals may re-leached to the surrounding soils and groundwater. Thus, it is recommended that landfill liners utilizing these lateritic soils are design as a composite containment facility by integrating compacted soil liners, leachate collection systems and monitoring networks to ensure effective environmental protection.

Effects of selenium on biogeochemical cycles of cadmium in rice from flooded paddy soil systems in the alluvial Indus Valley of Pakistan

This study delves into the pollution status, assesses the effects of Se on Cd biogeochemical pathways, and explores their interactions in nutrient-rich paddy soil-rice ecosystems through 500 soil-rice samples in Pakistan. The results showed that 99.6 % and 12.8 % of soil samples exceeded the World Health Organization (WHO) allowable Se and Cd levels (7 and 0.35 mg/kg). In comparison, 23 % and 6 % of the grain samples exceeded WHO's allowable Se and Cd levels (0.3 and 0.2 mg/kg), respectively. Geographically Weighted Regression (GWR) model results further revealed spatial nonstationarity, confirming diverse associations between dependent variables (Se and Cd in rice grain) and independent variables from paddy soil and plant tissues (root and shoot), such as Soil Organic Matter (SOM), pH, Se, and Cd concentrations. High Se:Cd molar ratios (>1) and a negative correlation (r = -0.16, p < 0.01) between the Cd translocation factor (Cd in rice grain/Cd in root) and Se in roots suggest that increased root Se levels inhibit the transfer of Cd from roots to grains. The inverse correlation between Se and Cd in paddy grains was further characterized as Se deficiency, no risk, high Cd risk, Se risk, Cd risk, and Se-Cd co-exposure risk. There was no apparent risk for human co-consumption in 42.6 % of grain samples with moderate Se and low Cd. The remaining categories indicate differing degrees of risk. In the study area, 31 % and 20 % of grain samples with low Se and Cd indicate Se deficiency and risk, respectively. High Se and low Cd levels in rice samples suggest a potential hazard for severe Se exposure due to frequent rice consumption. This study not only systematically evaluates the pollution status of paddy-soil systems in Pakistan but also provides a reference to thoroughly contemplate the development of a scientific approach for evaluating human risks and the potential dangers associated with paddy soils and rice, specifically in regions characterized by low Se and low Cd concentrations, as well as those with moderate Se and high Cd concentrations. SYNOPSIS: This study is significant for understanding the effects of Se on Cd geochemical cycles and their interactions in paddy soil systems in Pakistan.

Multi-metal contaminant mobilizations by natural colloids and nanoparticles in paddy soils during reduction and reoxidation

Naturally-occurring colloids and nanoparticles are crucial in transporting heavy metal contaminants in soil-water systems. However, information on particle-bound metals' size distribution and elemental composition in paddy soils under redox-fluctuation is scarce. Here, we investigated the mobilization of Cu, Cd, and Pb-containing nanoparticles and colloids in four contaminated soils with distinctive geochemical properties during reduction and subsequent re-oxidation. Using AF4-UV-ICP-MS and STEM-EDS, we observed that particle-bound metals were primarily associated with two sizes ranges: 0.3-40 kDa (F1) and 130 kDa-450 nm (F2), which mainly consisted of organic matter (OM), iron hydroxide and clay minerals. Cu and Pb were more likely bound to colloid than Cd. Colloidal Cu, Pb and Cd accounted for averages of 83.2%, 72.4% and 19.8% of their total concentration in solution (<0.45 µm) during soil reduction, and decreased during soil re-oxidation. This proportion was also positively correlated with aqueous pH and DOC but negatively correlated with Eh. Further quantitative analysis demonstrated that Cu/Cd positively correlated with OM at nanometric scale (F1). This study provides quantitative insights into the size, composition and abundance of polymetallic pollutant-carrying particles in paddy soils during redox fluctuation, and highlights the importance of nanometric interactions between OM and toxic cationic metals for their release.

Study on the remediation of heavy metal contaminated soils by citric acid and polyepoxysuccinic acid complex leaching

Soil leaching remediation has attracted extensive attention because of its good removal effect, short operation period and stable removal effect of heavy metals. The key to reduce the harm of heavy metal contaminated soil to the environment and human health is to use appropriate leachate to repair heavy metal contaminated soil. In this study, citric acid (CA), iron nitrate (Fe(NO3)3) and polyepoxysuccinic acid (PESA) with different concentrations were used as research reagents to explore the best combination of leaching effects of heavy metals (Cu, Zn, Pb, Cd) in contaminated soil. The effects of concentration of eluent, liquid to solid ratio and leaching time on leaching efficiency of heavy metals and the changes of soil physical and chemical properties before and after leaching were studied. The results showed that 0.5 mol/L CA and 0.05 mol/L PESA were combined according to the volume ratio of 7:3, and the leaching effect was the best when the liquid-solid ratio was 15 and the leaching time was 240 min. Under the optimal leaching condition, the four heavy metals in the soil had significant removal effects, and the removal rates were, respectively, 86.06% Cu, Zn 74.55%, Pb 67.88% and Cd 91.63%. The X-ray spectrum and Fourier infrared spectrum analysis of soil before and after leaching showed that CA-PESA combined leaching had little effect on soil structure change. This study provided theoretical support for the development and application of suitable leaching agents for the remediation of heavy metal-contaminated soil.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

Exploring the mechanism of Cd uptake and translocation in rice: Future perspectives of rice safety

Cadmium (Cd) contamination in rice fields has been recognized as a severe global agro-environmental issue. To reach the goal of controlling Cd risk, we must pay more attention and obtain an in-depth understanding of the environmental behavior, uptake and translocation of Cd in soil-rice systems. However, to date, these aspects still lack sufficient exploration and summary. Here, we critically reviewed (i) the processes and transfer proteins of Cd uptake/transport in the soil-rice system, (ii) a series of soil and other environmental factors affecting the bioavailability of Cd in paddies, and (iii) the latest advances in regard to remediation strategies while producing rice. We propose that the correlation between the bioavailability of Cd and environmental factors must be further explored to develop low Cd accumulation and efficient remediation strategies in the future. Second, the mechanism of Cd uptake in rice mediated by elevated CO2 also needs to be given more attention. Meanwhile, more scientific planting methods (direct seeding and intercropping) and suitable rice with low Cd accumulation are important measures to ensure the safety of rice consumption. In addition, the relevant Cd efflux transporters in rice have yet to be revealed, which will promote molecular breeding techniques to address the current Cd-contaminated soil-rice system. The potential for efficient, durable, and low-cost soil remediation technologies and foliar amendments to limit Cd uptake by rice needs to be examined in the future. Conventional breeding procedures combined with molecular marker techniques for screening rice varieties with low Cd accumulation could be a more practical approach to select for desirable agronomic traits with low risk.

Characteristics and mechanism of phosphate removal by lanthanum modified bentonite in the presence of dissolved organic matter

Lanthanum modified bentonite (LMB) is a widely used adsorbent for removing inorganic phosphorus from polluted water to prevent eutrophication. However, its efficiency can be affected by various environmental factors, including dissolved organic matter (DOM), which is still unclear. In this study, we systematically explored the influence of model DOMs, including HA, bovine serum albumin (BSA), and sodium alginate (SA), on phosphate adsorption by LMB, as well as to elucidate the underlying adsorption mechanisms. Our results showed that only HA had a significant effect on phosphate adsorption by LMB, causing inhibition. When three DOMs were mixed with phosphate in different proportions and DOM was mainly HA, the performance of phosphate adsorption on LMB became worse, while BSA can slightly offset this impact. The kinetics of HA and phosphate adsorption on LMB followed the pseudo-second-order kinetic model, and isotherms fitted the Langmuir model, with a maximum adsorption capacity of 5.7 mg g-1 for P and 12 mg g-1 for HA. However, when HA and phosphate were mixed based on their Qm, a C/P molar ratio of 5.35, LMB preferentially adsorbed phosphate. HA invasion was also disadvantageous for phosphate removal by LMB, in which P adsorption was less efficient at low-concentrations. However, during co-adsorption the adsorption capacity for HA was higher. With a secondary addition of higher levels of P, both pollutants were adsorbed more effectively. In the natural water experiment, phosphate concentration decreased with increasing shaking time, while the UV254 value showed a downward trend, indicating that LMB also adsorbed HA. Characterization results showed that La and phosphate formed LaPO4 precipitation, forming La-O-P inner-sphere complexes as the main mechanism of phosphate removal by LMB. La and HA formed La-HA complexes, with O-CO bonds from HA competing for lanthanum with phosphate. Despite HA obstructing pores from adsorbent, LMB still maintained a good binding ability with phosphate. It may form La-P-HA ternary complexes during adsorption to keep HA adsorption amount.

Evaluation of the Phytoremediation Potential of the Sinapis alba Plant Using Extractable Metal Concentrations

Testing the feasibility of soil phytoremediation requires the development of models applicable on a large scale. Phytoremediation mechanisms include advanced rhizosphere biodegradation, phytoaccumulation, phytodegradation, and phytostabilization. The aim of this study was to evaluate the phytoremediation potential of the Sinapis alba. Identification of the factors influencing the extraction process of metals from contaminated soils in a laboratory system suitable for evaluating the phytoavailability of these metals in three solutions (M1-CaCl2, M2-DTPA, and M3-EDTA) included the following: distribution of metals in solution (Kd), soil properties and mobile fractions (SOC, CEC, pH), response surface methodology (RSM), and principal component analysis (PCA). The evaluation of the phytoremediation potential of the Sinapis alba plant was assessed using bioaccumulation coefficients (BACs). The accumulation of heavy metals in plants corresponds to the concentrations and soluble fractions of metals in the soil. Understanding the extractable metal fractions and the availability of metals in the soil is important for soil management. Extractable soluble fractions may be more advantageous in total metal content as a predictor of bioconcentrations of metals in plants. In this study, the amount of metal available in the most suitable extractors was used to predict the absorption of metals in the Sinapis alba plant. Multiple regression prediction models have been developed for estimating the amounts of As and Cd in plant organs. The performance of the predictive models generated based on the experimental data was evaluated by the adjusted coefficient of determination (aR2), model efficiency (RMSE), Durbin-Watson (DW) test, and Shapiro-Wilk (SW) test. The accumulation of the analyzed metals followed the pattern Root > Pods > Leaves > Seeds, stems > Flowers for As and Leaves > Root > Stem > Pods > Seeds > Flowers for Cd in soil contaminated with different metal concentrations. The obtained results showed a phytoremediation potential of the Sinapis alba plant.

XRF analysis searching for fingerprint elemental profile in south-eastern Sicily tomatoes

The implementation of analytical techniques able to certify food quality and origin in a fast and non-destructive way is becoming a widespread need in the agri-food sector. Among the physical non-destructive techniques, X-ray fluorescence (XRF) spectrometry is often used to analyze the elemental composition of biological samples. In this study, X-ray fluorescence (XRF) elemental profiles were measured on tomato samples belonging to different geographical areas in Sicily (Italy). The purpose of this investigation was aiming to establish a protocol for in-situ measurement and analysis able to provide quality assessment and traceability of PGI agri-food products, specifically sustaining health safety and self qualifying bio-chemical signature. In detail, sampling was performed in one of the most tomato productive area of south-eastern Sicily (Pachino district), characterised by a relative higher amount of Organic Carbon and Cation Exchange Capacity, and compared with samples from other growing areas of Sicily, falling in Ragusa province and Mt. Etna region. Experimental data were analyzed in the framework of multivariate analysis by using principal component analysis and further validated by discriminant analysis. The results show the presence of specific elemental signatures associated to several characterizing elements. This methodology establishes the possibility to disentangle a clear fingerprint pattern associated to the geographical origin of an agri-food product.

Soil contamination by trace metals and assessment of the risks associated: the dumping site of Safi city (Northwest Morocco)

The main objective of this work was to determine the soil contamination with trace metals within and around the dumpsite of Safi city (Morocco) and to evaluate the potential environmental risk associated. The results showed that the average soil concentrations of trace metals had the following order: Fe > Zn > Cu > Cr > Cd and exceeded the world and the upper continental background concentrations except for Fe. In addition, the concentrations of Zn, Cu, and Cd remained beyond the limit standards given by the WHO/FAO. Geoaccumulation index, enrichment factor, and pollution load index (PLI) indicated that the dumpsite soil is highly contaminated and deteriorated, presenting evidence of high ecological risk proved by the values of the potential ecological risk index (PERI). Correlation analyses revealed a strong relationship between the organic matter & [Fe, Zn, Cr, Cd], calcium carbonates & [Zn, Cr], and Cr & Cu inside the dumpsite soil. Principal component analysis confirmed the temporal and spatial classification of Zone A as the oldest and Zone C as the youngest and indicated that the regrouped trace metals could have the same behavior and or the same origin. The interpolation of trace metals concentrations and PERI revealed a plausible extension outside the landfill, confirmed by PLI values.

Microorganisms and Biochar Improve the Remediation Efficiency of Paspalum vaginatum and Pennisetum alopecuroides on Cadmium-Contaminated Soil

Phytoremediation can help remediate potential toxic elements (PTE) in soil. Microorganisms and soil amendments are effective means to improve the efficiency of phytoremediation. This study selected three microorganisms that may promote phytoremediation, including bacteria (Ceratobasidium), fungi (Pseudomonas mendocina), and arbuscular-mycorrhizal fungi (AMF, Funneliformis caledonium). The effects of single or mixed inoculation of three microorganisms on the phytoremediation efficiency of Paspalum vaginatum and Pennisetum alopecuroides were tested under three different degrees of cadmium-contaminated soil (low 10 mg/kg, medium 50 mg/kg, and high 100 mg/kg). The results showed that single inoculation of AMF or Pseudomonas mendocina could significantly increase the biomass of two plants under three different degrees of cadmium-contaminated soil, and the growth-promoting effect of AMF was better than Pseudomonas mendocina. However, simultaneous inoculation of these two microorganisms did not show a better effect than the inoculation of one. Inoculation of Ceratobasidium reduced the biomass of the two plants under high concentrations of cadmium-contaminated soil. Among all treatments, the remediation ability of the two plants was the strongest when inoculated with AMF alone. On this basis, this study explored the effect of AMF combined with corn-straw-biochar on the phytoremediation efficiency of Paspalum vaginatum and Pennisetum alopecuroides. The results showed that biochar could affect plant biomass and Cd concentration in plants by reducing Cd concentration in soil. The combined use of biochar and AMF increased the biomass of Paspalum vaginatum by 8.9-48.6% and the biomass of Pennisetum alopecuroides by 8.04-32.92%. Compared with the single use of AMF or biochar, the combination of the two is better, which greatly improves the efficiency of phytoremediation.

Spirulina platensis extract improves the production and defenses of the common bean grown in a heavy metals-contaminated saline soil

Plants have to cope with several abiotic stresses, including salinity and heavy metals (HMs). Under these stresses, several extracts have been used as effective natural biostimulants, however, the use of Spirulina platensis (SP) extract (SPE) remains elusive. The effects of SPE were evaluated as soil addition (SA) and/or foliar spraying (FS) on antioxidant defenses and HMs content of common bean grown in saline soil contaminated with HMs. Individual (40 or 80 mg SPE/hill added as SA or 20 or 40 mg SPE/plant added as FS) or integrative (SA+FS) applications of SPE showed significant improvements in the following order: SA-80+FS-40 > SA-80+FS-20 > SA-40+FS-40 > SA-40+FS-20 > SA-80 > SA-40 > FS-40 > FS-20 > control. Therefore, the integrative SA+FS with 40 mg SP/plant was the most effective treatment in increasing plant growth and production, overcoming stress effects and minimizing contamination of the edible part. It significantly increased plant growth (74%-185%) and yield (107%-227%) by enhancing net photosynthetic rate (78.5%), stomatal conductance (104%), transpiration rate (124%), and contents of carotenoids (60.0%), chlorophylls (49%-51%), and NPK (271%-366%). These results were concurrent with the marked reductions in malondialdehyde (61.6%), hydrogen peroxide (42.2%), nickel (91%-94%), lead (80%-9%), and cadmium (74%-91%) contents due to the improved contents of glutathione (87.1%), ascorbate (37.0%), and α-tocopherol (77.2%), and the activities of catalase (18.1%), ascorbate peroxidase (18.3%), superoxide dismutase (192%), and glutathione reductase (52.2%) as reinforcing mechanisms. Therefore, this most effective treatment is recommended to mitigate the stress effects of salinity and HMs on common bean production while minimizing HMs in the edible part.

Translocation pattern of heavy metals in soil-rice systems at different growth stages: A case study in the Taihu region, Eastern China

Rice production is crucial for human nutrition and food safety globally. However, it has been a significant sink for potentially harmful metals because of intensive anthropogenic activities. The study was conducted to characterize heavy metal translocation from soil to rice at the filling, doughing and maturing stages, and influencing factors of their accumulation in rice. The distribution and accumulation patterns varied for metal species and growth stages. Cd and Pb accumulation mainly occurred in roots, Cu and Zn were readily transported to stems. Cd, Cu, and Zn accumulation in grains had a descending order of filling > doughing > maturing. Soil heavy metals, TN, EC, and pH exerted important impacts on heavy metals uptake by roots during the period from filling stage to maturing stage. Concentrations of heavy metals in grains were positively correlated with the translocation factors TF_stem-grain (from stem to grain) and TF_leaf-grain (from leaf to grain). Grain Cd exhibited significant correlations with total Cd and DTPA-Cd in the soil at each of the three growth stages. Moreover, Cd in maturing grain could be effectively predicted by soil pH and DTPA-Cd at the filling stage.

Evaluation of the effectiveness of amendments derived from vermicompost combined with modified shell powder on Cd immobilization in Cd-contaminated soil by multiscale experiments

The widespread heavy metal contamination of agricultural soils poses an enormous challenge to food safety. To evaluate the Cd immobilization potential of vermicompost combined with modified shell powder (VMSP) on Cd-contaminated soil, batch adsorption tests and field experiments were conducted. First, the Cd²⁺ removal characteristics and adsorption mechanisms of vermicompost (V), vermicompost combined with shell powder (VSP), and VMSP in an aqueous solution were investigated by batch tests. Then, 3 kg·m² V, VSP, and VMSP doses were applied to Cd-contaminated farmland soils as soil amendments to plant green garlic (Allium sativum L.) and investigate their Cd immobilization effects in Cd-contaminated soils. Batch adsorption tests showed that VMSP was most effective for Cd²⁺ removal, with adsorption rates as high as 85.7-99.79% and desorption rates of approximately 1.25-1.34%. Combining further characterization analysis of VMSP, it was demonstrated that the adsorption mechanism of Cd²⁺ was monolayer chemisorption, mainly involving the complexation reaction of Cd²⁺ with organic functional groups and the precipitation reaction of Cd²⁺ with mineral elements. The field experiment showed that adding V, VSP, and VMSP effectively inhibited the enrichment of Cd in green garlic, and the Cd content was reduced by 42.18%, 46.88%, and 68.75%, respectively. However, only the Cd content of green garlic treated with VMSP was lower than the national standard for food safety in China (Cd≤ 0.2 mg·kg^-1). V, VSP, and VMSP additions improved soil fertility and reduced Cd bioavailability in the soil by 15.5%, 18.9%, and 36.3%, respectively. In addition, V, VSP, and VMSP addition increased bacterial diversity and improved bacterial communities and functions in the soil by improving basic soil properties and reducing Cd-related toxicity. The results indicated that VMSP is a promising amendment for Cd immobilization in Cd-contaminated farmland soils.

Adsorption and desorption characteristics of heavy metals onto conventional and biodegradable plastics

Biodegradable plastics have been widely used to replace conventional plastics to minimize environmental impacts of plastic packaging. However, before biodegradable plastics decompose in the environment, they could pose a threat to terrestrial and aquatic creatures by acting as vectors of contaminants in the food chain. In this study, conventional plastic bags (CPBs) made of polyethylene and biodegradable plastic bags (BPBs) made of polylactic acid were examined for their heavy metal adsorption. Effects of solution pHs and temperatures on adsorption reactions were investigated. Because of a larger BET surface area, presence of oxygen-containing function groups, and smaller crystallinity, the heavy metal adsorption capacities of BPBs are significantly larger than those of CPBs. Among Cu (up to 791.48 mg⋅kg^-1), Ni (up to 60.88 mg⋅kg^-1), Pb (up to 1414.58 mg⋅kg^-1), and Zn (up to 295.17 mg⋅kg^-1), Pb and Ni show the largest and the lowest extents of adsorption onto the plastic bags, respectively. In the different waterbodies in nature, Pb adsorption on the CPBs and the BPBs were 318.08-379.91 and 528.41-764.22 mg⋅kg^-1, respectively. Consequently, Pb was selected as the target contaminant in the desorption experiments. After Pb was adsorbed onto the CPBs and the BPBs, Pb could be completely desorbed and released into simulated digestive systems in 10 h. In conclusion, BPBs could be potential vectors of heavy metals, and their suitability as a substitute for CPBs must be thoroughly investigated and confirmed.

Competitive adsorption of lead and cadmium on soil aggregate at micro-interfaces: Multi-surface modeling and spectroscopic studies

Aggregates are the basic structural units of soils and play a crucial role in metal migration and transformation. Combined contamination of lead (Pb) and cadmium (Cd) is common in site soils, and the two metals may compete for the same adsorption sites and affect their environmental behavior. Herein, the adsorption behavior of Pb and Cd on aggregates of two soils and contributions of soil components in single and competitive systems were studied by combining cultivation experiments, batch adsorption, multi-surface models (MSMs), and spectroscopic techniques. The results demonstrated that < 2 µm size aggregate was the dominant sink for Pb and Cd competitive adsorption in both soils. Compared with Pb, the adsorption capacity and behavior of Cd were affected greatly under competition. MSMs prediction revealed that soil organic matter (SOM) contributed the most to Cd and Pb adsorption on aggregates (> 68.4%), but the dominant competitive effect occurred on different sites for Cd adsorption (primarily on SOM) and Pb adsorption (primarily on clay minerals). Further, 2 mM Pb coexistence caused 5.9 - 9.8% of soil Cd conversion to unstable species (Cd(OH)₂). Thus, the competitive effect of Pb on Cd adsorption cannot be ignored in soils with high content of SOM and fine aggregates.

Metal distribution behavior based on soil aggregate size in a post-restoration coastal mining area

Soil aggregate size plays an important role in controlling the distribution and transport of metals. Metals immobilized in soil particles will pose potential risks through production/sink flow and infiltration. This study explored the distribution behavior of metals based on soil aggregate size in a restored coastal mining area by establishing Structural Equation Model (SEM) and column experiments. The results showed that hydrological factors and a high degree of weathering accelerated the dissolution of metals from the mine, the desorption of Wa-NH₄⁺-N, the release of F^-, and the leaching of NO₃^-. Driven by soil properties, natural factors, and anthropogenic activities, the total metal content (Total_metal) of Cr, Ni, Zn, Mn, and As showed significant spatial heterogeneity compared to Cd, Co, Cu, and Pb. The geochemical fraction of metals (Geo_metal) indicated that Cd, Co, Pb, Zn, As, and Cu are mainly present in iron‑manganese oxidation bound, organically bound, and residual fractions. The results of SEM showed that the physicochemical properties, Wa-NH₄⁺-N, nitrate nitrogen, and inorganic anions of the soil could explain 69.1 %, 76.4 %, 97.1 %, and 80.0 % of the variation in K_d-Mn, K_d-Pb, K_d-Ni, and K_d-Zn, respectively. While K_d-Cd, K_d-Cu, and K_d-Cr could be predicted by the Total_metal, but the Geo_metal seemed to have little influence on metal K_d. The results of column experiments showed that macroaggregates (>0.25 mm) significantly affected the distribution of Co, Cr, Cu, Mn, Ni, Pb, and Zn in the topsoil. The severe disruption of soil aggregate structure resulted in small fluctuations of anthropogenic Cu, Mn, Pb, Zn, and As in different layers of deep soil. In addition, mineral composition in >0.15 mm particle size was more likely to change. Overall, the hydrological cycle of coastal mines increases the uncertainty of their response to risk. Our study provides a basis for future strategies for priority control and risk prevention.

A state-of-the-art review on cadmium uptake, toxicity, and tolerance in rice: From physiological response to remediation process

Cadmium (Cd), a major contaminant of concern, has been extensively reviewed and debated for its anthropogenic global shifts. Cadmium levels in rice grains raise wide food safety concerns. The aim of this review is therefore to capture the dynamics of Cd in paddy soil, translocation pathways of Cd from soil to consumption rice, and assess its bio-accessibility in human consumption. In crop plants, Cd reduces absorption of nutrients and water, triggers oxidative stress, and inhibits plant metabolism. Understanding the mechanisms and behaviour of Cd in paddy soil and rice allows to explain, predict and intervene in Cd transferability from soil to grains and human exposure. Factors affecting Cd movement in soil, and further to rice grain, are elucidated. Recently, physiological and molecular understanding of Cd transport in rice plants have been advanced. Morphological-biochemical characteristics and Cd transporters of plants in such a movement were also highlighted. Ecologically viable remediation approaches, including low input cost agronomic methods, phytoremediation and microbial bioremediation methods, are emerging.

Soil and plant contamination by potentially toxic and emerging elements and the associated human health risk in some Egyptian environments

The aim of this work was to assess the origins, mobility, bioavailability and potential health risks of V, Cr, Co, As, Se, Mo, Cd, Sn and Sb, which are not sufficiently studied in the terrestrial environment of Egypt. This has been carried out by employing a combination of chemical fractionation, plants uptake, mathematical modeling and risk assessment approaches on a wide range of soils and plants sampled from industrial, urban and agricultural locations across Egypt. The contents of As, Cd, Sn and Sb were elevated in the soils of some urban and industrial locations within Cairo, although their soil geo-accumulation (I_geo) indices remained ≤ 2, indicating only moderate contamination. Selenium showed moderate to heavy contamination levels (I_geo up to 4.7) in all sampling locations, and Sb was highly elevated (I_geo = 7.1; extreme contamination) in one industrial location. Therefore, Se was the most important contributor to the pollution load followed by Sb and Cd. Both principle component analysis (of total content) and geochemical fractionation (by sequential extraction) suggested that V, Cr and Co are mostly of geogenic origin, while Se and Sb contents appear to be highly influenced by anthropogenic inputs. The most mobile and bioavailable element was Cd with a large non-residual fraction in all soils (76% of total Cd). The bio-concentration factors of Cd in leafy and fruiting plants were 50 times larger than other elements (except Mo) indicating preferential systematic plant uptake of Cd. Risk assessment models showed an overall low noncarcinogenic and carcinogenic risks to the population of Egypt due to the studied elements with only a few anomalies.

Contribution of components in natural soil to Cd and Pb competitive adsorption: Semi-quantitative to quantitative analysis

Cadmium (Cd) and lead (Pb) are two of the most common elements found in contaminated sites. The behavior of specific metals in the soil may be affected by other metals because of the competition for adsorption sites. In this study, adsorption experiments after chemical extraction, multi-surface models, and advanced spectroscopy technology were jointly used to explain the adsorption mechanism of Cd and Pb and to determine the contribution of each component in the competitive system. The results show that pH is the key factor in determining the contribution of soil components to metal adsorption. Soil organic matter (SOM) is the dominant adsorbent for both Cd and Pb. Clay minerals play an adsorption role at low pH, whereas Fe/Al oxides adsorb metals primarily in the high pH range. Further, the competitive effect of Pb on Cd occurred primarily on SOM rather than on clay minerals. When the Pb concentration increased from 0 to 500 mg/L, the adsorption of Cd on SOM decreased by 132.0 mg/kg, whereas it decreased only by 1.9 mg/kg on clay minerals. Therefore, the competitive effect of Pb on Cd cannot be ignored in soils with high organic matter content.

Modeling the competitive sorption and transport of Ni(II) and Zn(II) in soils: Comparing two multicomponent approaches

The mobility of contaminants in soil is controlled by sorption reactions which can be affected by the presence of other solutes that compete for sorption sites. The ability to model such effects is necessary for evaluating the environmental risk of a given contaminant. In this study, the competitive sorption and transport of nickel (Ni) and zinc (Zn) in Olivier and Windsor soils was investigated using batch equilibration and miscible displacement experiments. During batch experiments, the sorption of Ni and Zn was mutually reduced in multicomponent systems, indicating that the metal cations compete for sorption sites. When applied concurrently, the retardation of both ions decreased and peak effluent concentrations increased relative to single ion experiments, demonstrating that competition increased the mobility of both ions during miscible displacement experiments. A novel Freundlich-type multicomponent isotherm (CDI) and its kinetic analog (CDIT) were developed and compared to the commonly used SRS isotherm and SRS-based kinetic approach (SRST) in describing the experimental data. The CDI provided a superior description of the competitive batch data, especially at low surface coverage, and may therefore be more applicable to multicomponent sorption than the SRS. The Olivier and Windsor transport data were best described by the CDIT and SRST, respectively, however, both models generally described the data well. Since both approaches gave comparable descriptions of the transport data while the CDI outperformed the SRS in describing the batch data, the CDI/CDIT may be more generally applicable to multicomponent systems and warrants further study.

Pros and Cons of Biochar to Soil Potentially Toxic Element Mobilization and Phytoavailability: Environmental Implications

While the potential of biochar (BC) to immobilize potentially toxic elements (PTEs) in contaminated soils has been studied and reviewed, no review has focused on the potential use of BC for enhancing the phytoremediation efficacy of PTE-contaminated soils. Consequently, the overarching purpose in this study is to critically review the effects of BC on the mobilization, phytoextraction, phytostabilization, and bioremediation of PTEs in contaminated soils. Potential mechanisms of the interactions between BC and PTEs in soils are also reviewed in detail. We discuss the promises and challenges of various approaches, including potential environmental implications, of BC application to PTE-contaminated soils. The properties of BC (e.g., surface functional groups, mineral content, ionic content, and π-electrons) govern its impact on the (im)mobilization of PTEs, which is complex and highly element-specific. This review demonstrates the contrary effects of BC on PTE mobilization and highlights possible opportunities for using BC as a mobilizing agent for enhancing phytoremediation of PTEs-contaminated soils.

Short-Term Effect of Fly Ash from Biomass Combustion on Spring Rape Plants Growth, Nutrient, and Trace Elements Accumulation, and Soil Properties

The short-term impact of biomass combustion fly ashes (BAs) fertilization on the spring rape growth, essential and trace elements accumulation in seeds, and selected soil and soil solution properties were analyzed in a pot experiment study. The pot experiment was carried out in the growing season (April-August) during the year 2018. The effect of BAs on the dry matter content in spring rape plants and the relative content of chlorophyll in leaves (SPAD) was analyzed. In addition, the effect of BAs on the accumulation of essential and trace elements in the seeds of this plant was analyzed. The impact of BAs on the basic physicochemical properties of soils was also assessed. Additionally, the solubility of compounds contained in BAs was monitored on the basis of the analysis of the changes in the physicochemical properties of soil solution during the experiment period. The present study demonstrated a positive effect of BAs fertilization on plant growth and development and improvement of soil physicochemical properties. A change has been achieved in the soil reaction class from a slightly acidic (control, NPK) to neutral (D1-D6), with the highest increase in pH induced by the highest ash dose of 3 mg ha^-1 (D6). It was shown that BAs contributed to a significant increase in the content of macroelements than trace elements in the analyzed soil. In turn, the accumulation of these elements in plant seeds exhibited an inverse relationship, which was mainly influenced by the soil pH and the content of N, Ca, Mg, K, and Na in the soil, as indicated by the correlation coefficients. The highest contents of Fe, Mn, Zn, Cu, Cr, and Ni were detected in the seeds of plants fertilized with BAs at a dose of 2.0 Mg ha^-1 (D4), and their respective values were 263, 363, 107, 51, 1835, and 137% higher than in the control. The Ca, Mg, S, and Na compounds introduced with BAs exhibited high solubility, as evidenced by the higher concentration of Ca²⁺, Mg²⁺, Na⁺, and SO₄^2- ions in the soil solutions and the dynamic changes in pH and EC observed during the experiment. The lowest solubility after the application of BAs was exhibited by N and P. The conducted PCA analysis to a large extent explained the variability between the applied fertilization and the factors analyzed in the experiment. Despite the positive impact of ashes, attention should be paid to the potential risks associated with their use. The use of higher doses of BAs may result in excessive alkalization and salinity of soils and may enhance the accumulation of trace elements in plants. These aspects should therefore be closely monitored, especially in the case of a long-term application of these wastes, in order to avoid serious environmental problems.

Sensitivity of Zea mays and Soil Microorganisms to the Toxic Effect of Chromium (VI)

Chromium is used in many settings, and hence, it can easily enter the natural environment. It exists in several oxidation states. In soil, depending on its oxidation-reduction potential, it can occur in bivalent, trivalent or hexavalent forms. Hexavalent chromium compounds are cancerogenic to humans. The aim of this study was to determine the effect of Cr(VI) on the structure of bacteria and fungi in soil, to find out how this effect is modified by humic acids and to determine the response of Zea mays to this form of chromium. A pot experiment was conducted to answer the above questions. Zea mays was sown in natural soil and soil polluted with Cr(VI) in an amount of 60 mg kg^-1 d.m. Both soils were treated with humic acids in the form of HumiAgra preparation. The ecophysiological and genetic diversity of bacteria and fungi was assayed in soil under maize (not sown with Zea mays). In addition, the following were determined: yield of maize, greenness index, index of tolerance to chromium, translocation index and accumulation of chromium in the plant. It has been determined that Cr(VI) significantly distorts the growth and development of Zea mays, while humic acids completely neutralize its toxic effect on the plant. This element had an adverse effect on the development of bacteria of the genera Cellulosimicrobium, Kaistobacter, Rhodanobacter, Rhodoplanes and Nocardioides and fungi of the genera Chaetomium and Humicola. Soil contamination with Cr(VI) significantly diminished the genetic diversity and richness of bacteria and the ecophysiological diversity of fungi. The negative impact of Cr(VI) on the diversity of bacteria and fungi was mollified by Zea mays and the application of humic acids.

Hazardous toxic metal(loid)s in top- and deep-soils during the transformation of aquaculture ponds restored to farmland

The pollution risks due to the soil migration of toxic metal(loid)s (TMs) are a greatly hazard to ecological environment as well as animal and human health. Previous studies have primarily focused on surface contamination while deep soil layers often contain dangerous levels of TMs. We used restored wheat and rice farmlands from aquaculture ponds as a case study to examine the ecological risk and distribution of TMs in soil profiles. The elements Cu, Zn, Cr, Cd, Hg and As were markedly enriched in the 60-180 cm soil layers of restored farmland, and their concentrations decreased in the several depths as follows: 120-180 cm > 60-120 cm > 0-60 cm. Concentrations of TMs were 9.5-128 % greater in the restored farmlands relative to farmlands not exposed to aquaculture practices. Cadmium and mercury were the most serious contaminants and increased the overall ecological risk. The subsoil of wheat farming system had the highest pollution risk versus the restored rice farmland at 60-120 cm due to elevated levels of Cu, Zn and Pb. Toxic metal(loid)s might be derived from natural sources in deep soil of conventional farmland whereas aquaculture practices were found to constitute the major contribution in the subsoil of restored farmland. Our results indicated that the TMs that were buried in deep soil layers migrated upward and were a significant pollution risk. Urgent actions should be taken to identify and alleviate the contamination sources of these deep soils in addition to the conventional leaching and migration processes of surface contaminants.

Equilibrium partitioning behaviors and remobilization of trace metals in the sediment profiles in the tributaries of the Three Gorges Reservoir, China

Geochemical behaviors of trace metals in the sediment profiles are crucial for predicting the associated environmental risks in aquatic ecosystems. However, the comprehensive transport of trace metals under both equilibrium and dynamic conditions is still unclear under the changing hydrological regime. Here, the equilibrium partitioning behaviors and remobilization of five trace metals (Ni, Cu, Zn, As, and Pb) in sedimentary profiles within the tributaries of the Three Gorges Reservoir were explored by the partitioning coefficient (K_d), diffusive gradients in thin films (DGT), and DGT induced flux in sediments (DIFS) model. According to the K_d values, As posed the highest migration ability among the trace metals in the sediment profiles under equilibrium circumstances. Similarly, the dynamic processes of trace metals simulated by the DIFS model also suggested that As displayed the highest desorption rate despite having the lowest labile pool size. Moreover, all trace metals were classified as the "partially sustained" case, while the supply abilities of As and other trace metals were limited by the diffusion and the desorption kinetics, respectively. In addition, DGT-labile trace metals showed a diffusion trend from the sediment to the water column (except for Zn) at the sediment-water interface, indicating potential risks to water quality. Specifically, the equilibrium partitioning behaviors revealed the potential labile pool of trace metals in the solid phase, and the dynamic resupply process between the solid phase and porewater remained undetermined. In comparison, although DGT simulated the kinetic process of trace metals in the sediments, the labile pool of the trace metals could not be obtained. This study provided a holistic insight into the complementary trace metal behaviors under both equilibrium and dynamic conditions in the sediment and was beneficial to the water quality protection and internal pollution remediation in the aquatic environment.

Soil heterogeneity influence on the distribution of heavy metals in soil during acid rain infiltration: Experimental and numerical modeling

Acid rain is a global environmental problem that mobilizes heavy metals in soils, while the distribution and geochemical fraction of heavy metals during acid rain infiltration in heterogeneous soils are still unclear. In this study, we performed column experiments to investigate the distribution and geochemical fraction of Cu, Pb, Ni and Cd in heterogeneously layered soils during continuous acid rain infiltration. Chloride ion used as a conservative tracer was found to be uniformly distributed during acid rain infiltration, showing insignificant preferential flow effects in the column. In contrast, however, the distribution of heavy metals was highly non-uniform, especially in the silty soil at the lower part of the column, indicating a heterogeneous distribution of adsorption capacity. In addition, in the control experiments with neutral rain infiltration, uniform distribution of heavy metals was observed, indicating that the heterogeneous distribution of adsorption coefficient during acid rain infiltration was mainly caused by different pH buffering capacities. A numerical model considering water flow and solute transport was developed, where the average water-solid distribution coefficient (K_d) in Layer 2 was only 1.5-12.5% of that in Layer 1 during acid rain infiltration. The model could predict the variation of heavy metal concentrations in soil with the majority of error less than 35%, confirming that different K_d induced the heterogeneous distribution of heavy metals. In addition, the geochemical fraction of heavy metals in the upper coarse sand layer remained stable, while the acid-extractable fractions in the lower loam and silt loam layer gradually increased. Our findings suggest that soil heterogeneity, especially chemical heterogeneity affected by rainfall acidity, has an important influence on the infiltration, migration and geochemical fraction of heavy metals in soils. This study could help guide the risk assessment of heavy metal-contaminated sites that were polluted by acid rain or landfill leachate.

Influence of soil properties on the development of bacterial community tolerance to Cu, Ni, Pb and Zn

Pollution-Induced Community Tolerance (PICT) is a helpful and sensitive methodology to evaluate the effect of metal pollution in soils using microorganisms as indicators. PICT was used to determine the increase of bacterial community tolerance to Cu, Ni, Pb and Zn (Δlog IC₅₀), and to assess the influence of soil properties on the development of bacterial community tolerance to Cu, Ni, Pb, and Zn. Soil samples showed a wide range of properties, such as pH (3.96-7.47), texture (13.8-31.7% clay) or organic matter (9.7-30.7%). Bacterial growth measured by the [³H]-leucine incorporation method was used as the PICT endpoint. Bacterial communities generally developed tolerance in response to Cu, Ni and Zn additions to soils. However, bacterial communities showed no tolerance to Pb, probably due to high Pb sorption in studied soils. Soil properties influenced the development of bacterial community tolerance to Cu, Ni and Zn. Effective cation exchange and a soil sorption parameter (Freundlich's linearity index) were the selected variables to estimate Δlog IC₅₀ to Cu (R² = 0.65). Clay content and Ni-soluble are the main factors to estimate Δlog IC₅₀ to Ni (R² = 0.63). Organic matter content and a sorption parameter (maximum sorption capacity of the soil from Langmuir equation) are the soil properties to estimate Δlog IC₅₀ to Zn (R² = 0.45). Most of the variables exerted their effect in soil, i.e. PICT selection phase. However, clay content affected bacterial community tolerance determination (PICT detection phase), leading to overestimated measurements of bacterial community tolerance.

Mechanisms and influencing factors of yttrium sorption on paddy soil: Experiments and modeling

High-technology rare earth elements (REEs) as emerging contaminants have potentially hazardous risks for human health and the environment. Investigating the sorption of REEs on soils is crucial for understanding their migration and transformation. This study evaluated the sorption mechanisms and influencing factors of the rare earth element yttrium (Y) on paddy soil via integrated batch sorption experiments and theoretical modeling analysis. Site energy distribution theory (SEDT) combined with kinetics, thermodynamics, and isotherm sorption models were applied to illustrate the sorption mechanism. In addition, the effects of phosphorus (P), solution pH, particle size of soil microaggregates, and initial Y content on the sorption processes were evaluated by self-organizing map (SOM) and Boruta algorithm. The sorption kinetic behavior of Y on paddy soil was more consistent with the pseudo-second-order model. Thermodynamic results showed that the Y sorption was a spontaneous endothermic reaction. The generalized Langmuir model well described the isotherm data of Y sorption on heterogeneous paddy soil and soil microaggregates surface. The maximum sorption capacity of Y decreased with increasing soil particle size, which may be related to the number of sorption sites for Y on paddy soil and soil microaggregates, as confirmed by SEDT. The heterogeneity of sorption site energy for Y was the highest in the original paddy soil compared with the separated soil microaggregates. The SOM technique and Boruta algorithm highlighted that the initial concentration of Y and coexisting phosphorus played essential roles in the sorption process of Y, indicating that the addition of phosphate fertilizer may be an effective way to reduce the Y bioavailability in paddy soil in practice. These results can provide a scientific basis for the sustainable management of soil REEs and a theoretical foundation for the remediation of REEs-contaminated soils.

Mobilization of contaminants: Potential for soil remediation and unintended consequences

Land treatment has become an essential waste management practice. Therefore, soil becomes a major source of contaminants including organic chemicals and potentially toxic elements (PTEs) which enter the food chain, primarily through leaching to potable water sources, plant uptake, and animal transfer. A range of soil amendments are used to manage the mobility of contaminants and subsequently their bioavailability. Various soil amendments, like desorbing agents, surfactants, and chelating agents, have been applied to increase contaminant mobility and bioavailability. These mobilizing agents are applied to increase the contaminant removal though phytoremediation, bioremediation, and soil washing. However, possible leaching of the mobilized pollutants during soil washing is a major limitation, particularly when there is no active plant uptake. This leads to groundwater contamination and toxicity to plants and soil biota. In this context, the present review provides an overview on various soil amendments used to enhance the bioavailability and mobility of organic and inorganic contaminants, thereby facilitating increased risk when soil is remediated in polluted areas. The unintended consequences of the mobilization methods, when used to remediate polluted sites, are discussed in relation to the leaching of mobilized contaminants when active plant growth is absent. The toxicity of targeted and non-targeted contaminants to microbial communities and higher plants is also discussed. Finally, this review work summarizes the existing research gaps in various contaminant mobilization approaches, and prospects for future research.

Removal of lead (Pb+2) from contaminated water using a novel MoO3-biochar composite: Performance and mechanism

Removal of toxic chemicals from the environment using novel adsorbents is of great concern. In this study, a novel composite of molybdenum trioxide (MoO₃)-engineered biochar (MoO₃-BC) was derived from corn straw and synthesized for the removal of Pb(II) from water. The pyrolysis temperature of 600 °C was suitable for the thermal self-assembly of MoO₃-BC. Although MoO₃-BC had lower S_BET (59.3 m²/g) than the pristine BC (157.8 m²/g), it had a stronger adsorption affinity to Pb(II). The Pb(II) removal capacity of MoO₃-BC was 229.87 mg/g at pH 4.0, and the adsorptive removal of Pb(II) was fit using a pseudo-second-order model and the Langmuir model. High temperature favored the removal of Pb(II) by MoO₃-BC; However, the removal of Pb(II) was inhibited with increasing the ion strength. The MoO₃-BC revealed an acceptable stability and reusability, since the removal efficiency of Pb(II) remained above 80.7%, even after 8 cycles. The MoO₃-BC effectively reduced ≥99.9% of Pb(II) in the polluted irrigation water. The Pb(II) removal mechanisms involved surface electrostatic attraction, ion exchange and surface complexation. These findings conclude that the MoO₃-BC is a novel composite that can be used for the removal of Pb from contaminated water. More studies are needed to investigate the potentiality of MoO₃-biochar composite for the removal of other metals from water in a mono and competitive sorption system.

Natural additives contribute to hydrocarbon and heavy metal co-contaminated soil remediation

A biological treatment method was tested in laboratory conditions for the removal of hydrocarbons contained in a waste disposal soil sample consisting of excavated sandy soil from a former fueling station. Two fractions of hydrocarbons were quantified by GC-FID: diesel (C₁₀-C₂₁) and lubricant oil (C₂₂-C₄₀). Meat and bone meal (MBM, 1% w/w) was used as a bio-stimulant agent for soil organisms. Cyclodextrin, an oligosaccharide produced from starch by enzymatic conversion, was also used to assess its ability to improve the bioavailability/biodegradability of hydrocarbons in the soil. Parameters such as temperature, pH, water content and aeration (O₂ availability) were monitored and optimized to favor degradation processes. Two different experimental tests were prepared: one to measure the degradation of hydrocarbons; the other to monitor the mobility of some elements in the soil and in the leachate produced by watering with tap water. Soil samples treated with MBM and cyclodextrin showed, over time, a greater removal of the more persistent hydrocarbon fraction (lubricant oil). MBM-treated soils underwent a faster hydrocarbon removal kinetic, especially in the first treatment period. However, the final hydrocarbon concentrations are comparable in all treatments, including control. Over time, the effect of cyclodextrin on hydrocarbon degradation seemed to be relevant. MBM-treated soils sequestered lead in the very first weeks. These results highlight the intrinsic capacity of soil, and its indigenous microbial communities, to degrade petroleum hydrocarbons and suggest that MBM-induced bioremediation is a promising, environmentally friendly technology which should be considered when dealing with hydrocarbon/heavy metal co-contaminated soils.

Oyster shell amendment reduces cadmium and lead availability and uptake by rice in contaminated paddy soil

Contamination with cadmium (Cd) and lead (Pb) in rice and paddy soil threatens food safety and human health. This study determined the effects of oyster shell amendment (0, 6, and 12 g kg^-1) on the mobility and uptake of Cd and Pb by two rice plants (ZY18, japonica and DL5, indica) in contaminated paddy soil. Oyster shell amendment significantly increased the pH of soil and pore water, and decreased the DTPA-extractable Cd concentration in soil, but not the Cd concentration in pore water. Furthermore, the DTPA-extractable Pb concentration in rhizosphere soil was not significantly influenced by oyster shell addition. Application of oyster shell reduced the Pb concentration in pore water and Pb uptake by both cultivars, but excessive application (12 g kg^-1) resulted in Cd accumulation in tissues for ZY18. Furthermore, oyster shell addition significantly increased the dissolved organic carbon, calcium, and magnesium concentrations in soil pore water, and decreased glutathione and phytochelatin levels in roots, all of which alleviated heavy metal toxicity and improved rice growth. These results demonstrate that amendment with the appropriate amount of oyster shell, combined with rice cultivar type, could simultaneously reduce Cd and Pb accumulation in rice grown in heavy metal-contaminated soil.

Influence of soil properties, topography, and land cover on soil organic carbon and total nitrogen concentration: A case study in Qinghai-Tibet plateau based on random forest regression and structural equation modeling

The effects of topography, land cover type, and soil physicochemical properties on the distribution of soil organic carbon (SOC) and total nitrogen (TN) have drawn growing attention recently, but little is known about how these features-associated interactions impact SOC and TN. To elucidate how these interactions affect the preservation of carbon and nitrogen in soils, we used data-driven models (random forest regression and structural equation modeling) to identify the dominant environmental factors affecting the distribution of SOC and TN in two different soil layers (0-20 and 20-40 cm) of the Qinghai-Tibet plateau. In addition, an algorithm based on random forest ("Boruta") was chosen to identify the relevant influencing factors and partial dependence was used to depict the two most important factors. We found that rather than land cover type, environmental properties, such as soil physicochemical characteristics and altitude had the most significant effects on the distribution of SOC and TN. Our findings indicate that elevation and TN are the two most important factors influencing SOC in the surface and subsurface soil layers. Moreover, total potassium (TK) impacts TN content in the surface soil layer, but only in a specific range of concentrations, which could be attributed to anthropogenic activities such as applying nitrogen and potassium fertilizers to increase the yields of local food crop, Tibetan hulless barley. These findings provide a scientific perspective on soil nutrient preservation.

Geochemical assessment of trace element concentrations in the Farmington River, Connecticut, Northeastern, USA

The Farmington River, located in the northeastern USA, originates from Massachusetts and flows through several towns until it joins the Connecticut River in Windsor, Connecticut. Trace element concentrations within the drainage systems of the Farmington River are a growing concern, largely due to the presence of both historical and present land use activities along the river that include mining, farming, urbanization, industrialization, landfills, and shooting gun ranges. This study examined the impact of land use activities and geology on the chemical signature of major and trace elements within the Farmington River. A total of seventy-eight (78) stream sediment and corresponding water samples were collected and analyzed for twenty-one chemical elements (As, Be, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Mo, Ni, Pb, Sb, Se, Sr, Ti, Tl, V, and Zn) and basic river sediment parameters such as particle size distribution and organic matter contents. Results showed spatial variability in the concentration of elements along the river flow path, with the highest concentrations of As, Cd, Ni, Zn, and Pb determined at location FR31 due to various human activities within the surrounding environment. However, most sampling locations do not pose a major environmental concern. Multivariate statistical analysis showed inter-relationship among most elements, with a weak but significant positive relationship with fine particle sizes in sediment. Results of principal component analysis (PCA) suggested a combination of both geogenic and anthropogenic sources of trace elements to the Farmington River system.

Migration and Transformation of Multiple Heavy Metals in the Soil–Plant System of E-Waste Dismantling Site

E-waste generation has become a major environmental issue worldwide. Heavy metals (HMs) in e-waste can be released during inappropriate recycling processes. While their pollution characteristics have been studied, the migration and transformation of different multi-metal fractions in soil–plant system of e-waste dismantling sites is still unclear. In this study, pot experiments were conducted to investigate the migration and transformation of different multi-metal fractions (Cu, Pb, Zn and Al) in the soil–plant system using two Chinese cabbage cultivars (heavy metals low-accumulated variety of Z1 and non-low-accumulated Z2) treated with or without biochar. The result showed that the acid-soluble fraction of Cu, Pb, Zn and Al in soil decreased by 5.5%, 55.7%, 7.8% and 21.3%, but the residual fraction (ResF) of them increased by 48.5%, 1.8%, 30.9% and 43.1%, respectively, when treated with biochar and plants, compared to that of the blank soil (CK). In addition, Pb mainly existed as a reducible fraction, whereas Cu existed as an oxidisable fraction. Biochar combined with plants significantly increased the ResF of multi-metals, which reduced the migration ability of Pb among all other metals. The relative amount of labelled ¹³C in the soil of Z1 was higher than that of Z2 (25.4 fold); among them, the Gram-negative bacteria (18-1ω9c, 18-1ω7c) and fungi (18-2ω6c) were significantly labelled in the Z1-treated soil, and have high correlation with HM migration and transformation. In addition, Gemmatimonadete were significantly positive in the acid-soluble fraction of HMs, whereas Ascomycota mostly contributed to the immobilisation of HMs. Therefore, the distribution of fractions rather than the heavy metal type plays an important role in the HM migration in the soil–plant system of e-waste dismantling sites.

Technologies for the cobalt-contaminated soil remediation: A review

The cobalt-contaminated soil has exposed potential toxicity to humans, plants, and animals. Industrial activities like ore smelting, alloy manufacture, and electric and electronic devices production have induced the increased cobalt content in soil resulting in higher ecosystem risk in diverse environmental media. However, knowledge gaps in cobalt transfer in soil and the limited understanding of remediation techniques make it challenging to estimate their potential application scenarios. Thus, keeping in view the above facts, this paper summarizes the natural and anthropogenic sources arousing the increase of cobalt in soil and reviews the cobalt species in soil and factors that influence the mobilization of cobalt. Moreover, the status of the remediation technologies is critically evaluated, including phytoremediation, immobilization, and separation technologies (soil washing and electroremediation) with a focus on the application and mechanism of phytoremediation and immobilization. Based on the actual application, further improvements and prospects of all techniques are proposed. This comprehensive review might serve to guide technique selection and inspire more scientific exploration on the remediation of cobalt-contaminated soil.

Assembly of root-associated bacterial community in cadmium contaminated soil following five-year consecutive application of soil amendments: Evidences for improved soil health

Soil amendments have been extensively used to remediate heavy metal contaminated soils by immobilizing or altering edaphic properties to reduce the bioavailability of heavy metals. However, the potential influences of long-term soil amendments applications on microbial communities and polluted soil health are still in its infancy despite that have been applied for decades. We used amplicon sequencing and q-PCR array to characterize the root-associated microbial community compositions and rhizosphere functional genes in a five-year field experiment with consecutive application of four amendments (lime, biochar, pig manure, and a commercial Mg-Ca-Si conditioner). Compared with the control, soil amendments reduced the available Cd (CaCl₂ extractable Cd) in soils and strongly affected bacterial community compositions in four root-associated niches. Five rare keystone bacterial species were found belonging to the family Gallionellaceae (1), Haliangiaceae (1), Anaerolineaceae (2), and Xanthobacteraceae (1), which significantly correlated with soil pH and the functional genes nifH and phoD. Random forest analysis showed that rhizosphere soil pH and microbial functions, and root-associated keystone bacterial community compositions mainly influenced the Cd concentrations in rice grains. Altogether, our field data revealed five-year consecutive application of soil amendments regulated root-associated microbial community assembly and enhanced microbial functions, thereby improved rhizosphere health of Cd-contaminated soils.

Boron, Manganese, and Zinc Sorption and Leaf Uptake on Citrus Cultivated on a Sandy Soil

Solute fate in soil-plant continuum could either be soil or leaf uptake or leaching beyond the rooting zone. An adsorption coefficient (K_D) is an important chemical property to describe the interaction between the solute and soil, affecting the solute movement in soils from one point to another. Boron (B), manganese (Mn), and zinc (Zn) uptake are evident in the leaves as a constituent of photosynthesis and other plant body-building mechanisms for growth and development. This study investigates the availability of micronutrients (B, Mn, and Zn) to citrus trees through modified application methods and rates. Leaf samples were collected from experimental plots arranged in a randomized complete block design, with 4 micronutrient treatments: control, foliar ×1, foliar ×2, and soil ×1. Boron, Mn, and Zn rates were 1.12, 10.08, and 5.60 kg ha^-1, respectively. Composite soil samples were randomly collected at 5 points, and a 1-point adsorption study was conducted in 4 soil depths at an increment of 15 cm from the soil surface. Adsorption coefficient (K_D) for Mn and B was 22 and 3 times higher at 0-15 cm than at soil depth of 15-60 cm. The adsorption coefficient (K_D) for Zn was 2.5 times greater at 0-15 cm than 15-30 cm soil depth, while there was little or no sorption at 30-60 cm. Leaf Mn and Zn concentrations showed that foliar spray was 2 times higher than the soil application method, while B showed that the soil application method was 2 times higher than foliar application method for 2 seasons sampling events. Thus, the behavior of B, Mn, and Zn in the soil via adsorption coefficient (K_D) reflects the availability of B, Mn, and Zn in the citrus leaves.

Soil gallium speciation and resulting gallium uptake by rice plants

Gallium (Ga) is widely used in high-tech industries and is an emerging contaminant in the environment. This study aimed to determine Ga speciation in soils and Ga accumulation in rice plants (Oryza sativa L.) grown in three Ga-contaminated soils. The results showed that, among the soils, the acidic soil with a coarse texture had the highest soil Ga availability, which enhanced Ga uptake by rice roots. The Ga K-edge X-ray absorption near edge structure and sequential extraction results of the soils showed that the predominant species of Ga associated with iron hydroxides transformed to Ga(OH)₃ precipitates, and the residue fraction increased with rice-growing time, resulting in lower Ga uptake by rice roots in the second half period of rice cultivation. A large fraction of Ga was accumulated in the rice roots, with only a small portion of Ga was transferred to the shoots and then to the rice grains. This study revealed that Ga speciation in soil-rice plant systems varied during rice cultivation and determined soil Ga availability to rice plants. Gallium accumulated in rice grains is distributed homogenously in the endosperm of the grains, suggesting a potential risk to public health via the intake of rice grains harvested from Ga-contaminated paddy fields.

Monitoring of ion release, bioavailability and ecotoxicity of thallium in contaminated paddy soils under rice cultivation conditions

Paddy soils contaminated by thallium (Tl) have been frequently reported; however, their ecotoxicological impact in the paddy field is less known. We used a novel soil-fish exposure system with larvae of rice fish medaka (Oryzias latipes) to assess the bioavailability of Tl from soils to fish and causal toxicity under simulated conditions of rice cultivation. Two acidic soils [Pingzhen (Pc) and Sankengtzu (Sk)] spiked with monovalent Tl [Tl(I), 75-250 mg/kg] released higher Tl⁺ into pore or overlying waters than neutral soils [Sangkang (Su)], which resulted in higher mortality to exposed fish. The addition of K fertilizers into the system did not significantly reduce Tl release and fish mortality, but a drainage/re-flooding treatment worked effectively. The acidic Pc soil contaminated with low Tl(I) (2.5 and 15 mg/kg) caused higher sublethal toxicity in medaka than the neutral Su soil, including altered growth and swimming behavior with increased Tl body burden. These Tl-induced effects by low-Tl soils were significantly alleviated by K addition. The Tl/K ratios in aqueous phases were correlated with the mortality or Tl body burden in exposed fish. This study provides useful bio-analytical evidence that can help assess the ecological risks of Tl pollution in paddy field-related ecosystems.

Changeable effects of coexisting heavy metals on transfer of cadmium from soils to wheat grains

Cadmium (Cd) and other heavy metals usually coexist in soils. Effects of coexisting heavy metals on the accumulation and transfer of Cd in field soils by wheat remain poorly understood. Here we revealed changeable effects of coexisting Pb, Zn and Cu on the Cd transfer from soils to wheat grains. Soil burdens of Cd were found to exhibit positive correlations (r = 0.459-0.946) with those of coexisting Pb, Zn and Cu (particularly Pb). Effects of three coexisting metals on to the uptake of Cd by wheat varied in the directions and/or extents with types of metals and transfer processes of Cd. Coexisting Zn inhibited the uptake of Cd by wheat grains to higher extent than Pb and Cu. Soil Zn, along with soil Cd, soil pH and soil Ca, was used to construct the predictive model of grain Cd (R² = 0.868). External verifications of the model on 572 datasets of large representation performed well. The predictive accuracy was about 54%, 73% and 89% for a factor of 1, 2 and 5 above and below the ideal fit, respectively. This finding has practical interest in risk assessments and remediation measures of Cd-contaminated soil sites in regional scales.

Combined amendment reduces soil Cd availability and rice Cd accumulation in three consecutive rice planting seasons

The scientific application of stabilized materials has been considered an effective method for the in situ remediation of Cd-contaminated soil. This study aimed to investigate the persistence of the effect of a combined amendment of limestone and sepiolite (LS) on soil Cd availability and accumulation in rice grown in a mildly Cd-contaminated paddy field (0.45 mg/kg of Cd) over three consecutive rice seasons. 1125-4500 kg/ha of LS was applied to the soil before the first rice planting season and 562.5-2250 kg/ha of LS was supplemented before the third rice planting season. The application of LS (1125-4500 kg/ha) increased the soil pH by 0.44-1.09, 0.18-0.53, and 0.42-0.68 in the first, second, and third season, respectively, and decreased the soil acid-extractable Cd content by 18.2-36.4%, 17.7-33.5%, and 9.6-17.6%. LS application significantly decreased the Cd contents in the rice tissues. The application of 4500 kg/ha of LS decreased the Cd content in brown rice to below the National Food Limit Standard of 0.2 mg/kg (GB 2762-2017) in the three consecutive rice seasons. However, the effect of LS on the soil-rice system was significantly weakened in the third season. The supplementary application of 562.5-2250 kg/ha of LS further decreased the Cd content in brown rice by 26.1-56.5% and decreased the health risk index by 23.7-43.8%. Therefore, it was recommended to apply 4500 kg/ha of LS in the first season and to supplement 2250 kg/ha of LS in the third season to effectively guarantee the clean production of rice in three consecutive rice seasons.

Earthworms as candidates for remediation of potentially toxic elements contaminated soils and mitigating the environmental and human health risks: A review

Global concerns towards potentially toxic elements (PTEs) are steadily increasing due to the significant threats that PTEs pose to human health and environmental quality. This calls for immediate, effective and efficient remediation solutions. Earthworms, the 'ecosystem engineers', can modify and improve soil health and enhance plant productivity. Recently, considerable attention has been paid to the potential of earthworms, alone or combined with other soil organisms and/or soil amendments, to remediate PTEs contaminated soils. However, the use of earthworms in the remediation of PTEs contaminated soil (i.e., vermiremediation) has not been thoroughly reviewed to date. Therefore, this review discusses and provides comprehensive insights into the suitability of earthworms as potential candidates for bioremediation of PTEs contaminated soils and mitigating environmental and human health risks. Specifically, we reviewed and discussed: i) the occurrence and abundance of earthworms in PTEs contaminated soils; ii) the influence of PTEs on earthworm communities in contaminated soils; iii) factors affecting earthworm PTEs accumulation and elimination, and iv) the dynamics and fate of PTEs in earthworm amended soils. The technical feasibility, knowledge gaps, and practical challenges have been worked out and critically discussed. Therefore, this review could provide a reference and guidance for bio-restoration of PTEs contaminated soils and shall also help developing innovative and applicable solutions for controlling PTEs bioavailability for the remediation of contaminated soils and the mitigation of the environment and human risks.