Physicochemical properties, metal availability and bacterial community structure in heavy metal-polluted soil remediated by montmorillonite-based amendments

Reducing the migration of cadmium, lead and nickel in soil using rice straw-based humic acid modified bentonite

With the continuous advancement of industrial development, the contamination of soil by potentially toxic element ions has emerged as an escalating issue, posing significant threats to both ecological systems and human health. Clay minerals have long been considered as green and economic remediation agents for polluted soil. However, their current application is hampered by low and inconsistent fixation efficiency. Here, a new clay composite BHA@B was prepared by modifying bentonite with biomass humic acid (BHA) prepared from rice straw. The BHA@B was characterized by FTIR, SEM and XRD and then tested as a remediation agent for potentially toxic element ion-contaminated soil. The effect of BHA@B on the prevalence of different fractions of Cd, Pb and Ni in soil and its ability to stabilize these metals were investigated using the BCR sequential extraction method. Treatment with BHA@B reduced the weak acid-extractable and reducible fractions of cadmium, lead and nickel by 22.5 and 8.1%, 17 and 11.4%, and 19.8 and 14%, respectively, compared with untreated soil. BHA@B transforms potentially toxic elements into oxidizable, residual, and other stable fractions, which greatly reduces the ability of potentially toxic element ions in the soil to migrate into the groundwater. BHA@B is a new, green, low-cost and efficient soil remediation agent.

Development of multifunctional immobilized bacterial agents for multi-pesticides degradation and environment remediation

The proliferation of weeds, pests, and plant diseases in crop cultivation has driven the increased application of herbicide lactofen, insecticide acetamiprid, and fungicide carbendazim, contributing to environmental pollution. Microorganisms are requently employed to remove pesticide residues from the environment. However, Liquid bacterial agents encounter difficulties in transportation and preservation during application and the current immobilized bacterial agents have a single degradation function. This study developed immobilized bacterial agents containing the lactofen-degrading strain Bacillus sp. Za, the acetamiprid-degrading strain Pigmentiphaga sp. D-2, and the carbendazim-degrading strain Rhodococcus sp. djl-6. Preparation conditions, including activated carbon concentration, sodium alginate (SA), CaCl2, and immobilization time, were optimized using the response surface method (RSM). The degradation performance of the immobilized bacteria was evaluated, with degradation rates exceeding 70% for all three pesticides under conditions of 30 °C, pH 7.0, and 6% inoculation over 48 h. The immobilized bacterial agents were stored at pH 7.0 and 4 °C for 180 days, maintaining a preservation rate of 51.26% with a viable cell count of 1.04 × 108 CFU/g. These agents effectively remediated soil and water contaminated with multi-pesticides, achieving degradation rates of 92.50% and 98.50% for lactofen, 91.05% and 99.89% for acetamiprid, 88.43% and 98.99% for carbendazim within 21 in soil and 7 days in water, respectively. This study provides essential technical support for developing microbial agents capable of degrading multi-pesticides residues, with significant potential applications in agriculture and environmental protection.

Effect of sludge-based biochar on the stabilization of Cd in soil: experimental and theoretical studies

Soil heavy metal contamination and sludge disposal have become globally environmental issues problems of great concern. Utilizing sludge pyrolysis to produce biochar for remediating heavy metal-contaminated soil is an effective strategy to solve these two environmental problems. In this study, municipal sewage sludge and papermaking sludge were used as feedstock to prepare co-pyrolyzed biochar, which was then applied to reduce the toxicity of Cd in soil. The results indicated that the application of co-pyrolyzed biochar significantly increased soil pH, CEC, and enzyme activity, while decreasing the content of available Cd in the soil. Following the application of 3% co-pyrolyzed biochar, the proportion of acid-soluble Cd in the soil decreased to below 46%, as the biochar facilitated the conversion of leachable acid-soluble Cd to stable oxidizable and residual forms through precipitation and complexation. The DFT computational results indicate that the aromatics in co-pyrolyzed biochar can adsorb Cd ions through cation-π interactions, while carboxyl, hydroxyl, aldehyde, and amide groups can provide more electrons for the adsorption of Cd ions, resulting in stronger adsorption capacities. The study findings provide a feasible solution for the resourceful treatment of sludge and the remediation of heavy metal-contaminated soil.

A Review on Remediation Technology and the Remediation Evaluation of Heavy Metal-Contaminated Soils

With the rapid development of industry and agriculture, soil contamination has become a significant environmental issue, and the heavy metal contamination of soils is an important part of it. The main methods for the remediation of heavy metal-contaminated soils include physical methods, chemical methods, biological methods, and combined remediation methods have been proposed as research deepens. However, the standards and evaluation methods for the remediation of heavy metal-contaminated soils are still not well-established. This article discusses the sources and contamination status of heavy metals in soils, the advantages and disadvantages of remediation technology for heavy metal-contaminated soils, remediation standards, and post-remediation evaluation methods. It also proposes scientific issues to be addressed in future research and provides an outlook on future development, hoping to assist in subsequent remediation studies of heavy metal-contaminated soils.

Application of acid-activated attapulgite clay on effective immobilization of Cd in paddy soils

This study investigated the effects of different amounts of acid-activated attapulgite clay on soil environmental risks and ecotoxicity after incubation as a soil amendment. The experimental soil was alkaline clay soil with a pH of 7.54. The optimal addition and incubation times for the remediation of Cd-contaminated soil were determined. The results showed that the optimal period for Cd stabilization was 24 days, and the optimal addition amount was 8% (w/w). The addition of 8% attapulgite clay significantly reduced the bioavailable Cd concentration in soil. Water-Cd, DTPA-Cd, TCLP-Cd, and HCl-Cd reduced the bioavailable Cd concentration in the soil by 26.43%, 42.75%, 35.22%, and 25.89%, respectively. Meanwhile, the addition of 8% attapulgite clay converted the chemical speciation of exchangeable-Cd into residual-Cd, and the stabilizing component increased by 48% after 48 days. The ecological and environmental risk indices decreased significantly, reaching 10.89% and 4.22%, respectively. The pot experiment showed that attapulgite clay promoted corn growth while decreasing concentrations of Cd in aboveground parts and roots by 9.01-31.82% and 9.52-40.48%, respectively, and limited Cd transfer from roots to stems, minimizing its presence in edible portions as much as possible. The accumulation of heavy metals in the growth of corn plants was significantly and positively correlated with the bioavailability of Cd extracted from different extractants and exchangeable speciation, and significantly and negatively correlated with soil residual speciation. Attapulgite is a promising soil amendment that can effectively reduce ecological and environmental risks, reduce the ecological toxicity of Cd, and improve soil quality.

Ameliorating the detrimental effects of chromium in wheat by silicon nanoparticles and its enriched biochar

Increased anthropogenic activities over the last decades have led to a gradual increase in chromium (Cr) content in the soil, which, due to its high mobility in soil, makes Cr accumulation in plants a serious threat to the health of animals and humans. The present study investigated the ameliorative effect of foliar-applied Si nanoparticles (SiF) and soil-applied SiNPs enriched biochar (SiBc) on the growth of wheat in Cr-polluted soil (CPS). Two levels of CPS were prepared, including 12.5 % and 25 % by adding Cr-polluted wastewater in the soil as soil 1 (S1) and soil 2 (S2), respectively for the pot experiment with a duration of 40 days. Cr stress significantly reduced wheat growth, however, combined application of SiF and SiBc improved root and shoot biomass production under Cr stress by (i) reducing Cr accumulation, (ii) increasing activities of antioxidant enzymes (ascorbate peroxidase and catalase), and (iii) increasing protein and total phenolic contents in both root and shoot respectively. Nonetheless, separate applications of SiF and SiBc effectively reduced Cr toxicity in shoot and root respectively, indicating a tissue-specific regulation of wheat growth under Cr. Later, the Langmuir and Freundlich adsorption isotherm analysis showed a maximum soil Cr adsorption capacity ∼ Q(max) of 40.6 mg g-1 and 59 mg g-1 at S1 and S2 respectively, while the life cycle impact assessment showed scores of -1 mg kg-1 and -211 mg kg-1 for Cr in agricultural soil and - 0.184 and - 38.7 for human health at S1 and S2 respectively in response to combined SiF + SiBC application, thus indicating the environment implication of Si nanoparticles and its biochar in ameliorating Cr toxicity in different environmental perspectives.

The effect of different amendments on Cd availability and bacterial community after three-year consecutive application in Cd-contaminated paddy soils

In situ immobilization is a widely used measure for passivating Cd-contaminated soils. Amendments need to be continuously applied to achieve stable remediation effects. However, few studies have evaluated the impact of consecutive application of amendments on soil health and the microecological environment. A field experiment was conducted in a Cd-contaminated paddy (available Cd concentration 0.40 mg kg-1) on the Chengdu Plain to investigate the changes in soil Cd availability and response characteristics of soil bacterial communities after consecutive application of rice straw biochar (SW), fly ash (FM) and marble powder (YH) amendments from 2018 to 2020. Compared with control treatment without amendments (CK), soil pH increased by 0.6, 0.5 and 1.5 under SW, FM and YH amendments, respectively, and the soil available Cd concentration decreased by 10.71%, 21.42% and 25.00%, respectively. The Cd concentration in rice grain was less than 0.2 mg kg-1 under YH amendment, which was within the Chinese Contaminant Limit in Food of National Food Safety Standards (GB2762-2022) in the second and third years. The three amendments had different effects on the transformation of Cd fractions in soil, which may be relevant to the specific bacterial communities shaped under different treatments. The proportion of Fe-Mn oxide-bound fraction Cd (OX-Cd) increased by 11% under YH treatment, which may be due to the promotion of Fe(III) and Cd binding by some enriched iron-oxidizing bacteria, such as Lysobacter, uncultured_Pelobacter sp. and Sulfurifusis. Candidatus_Tenderia and Sideroxydans were enriched under SW and FM amendments, respectively, and were likely beneficial for reducing Cd availability in soil through Cd immobilization. These results revealed the significance of the bacterial community in soil Cd immobilization after consecutive application of amendments and highlighted the potential of applying YH amendment to ensure the safe production of rice in Cd-contaminated soil.

Nanoscale chestnut soil pore characteristics changes induced by non-growing season precipitation in a temperate grassland

Changes in precipitation patterns during the non-growing season can affect soil moisture storage in temperate grasslands. However, there is a lack of comprehensive understanding regarding how these changes influence microscale soil pore characteristics and nutrient cycling in the context of climate change. Therefore, we carried out a 3-year artificial precipitation experiment during the non-growing season, along with N2 adsorption experiments of soil pore distribution and surveys of soil nutrient content. The aim was to clarify the influence of non-growing season precipitation variations on nanoscale soil pore characteristics and explore the potential correlations of the soil physicochemical properties. The results showed that: (1) The precipitation sheltering treatment during the non-growing season led to a significant 9.80 % increase in soil porosity at the 0-15 cm depth compared to the control. (2) Compared to the control, alterations in non-growing season precipitation (both increase and sheltering treatments) led to a significant increase in soil specific surface area (SSA), with an average increase of 23.2 %. Additionally, soil micropores, mesopores, macropores, and total pore volume (PV) increased by an average of 24.2 %, 14.0 %, 30.1 %, and 23.1 %, respectively. (3) Significant correlations were observed between soil microscale pore characteristics and soil C, soil organic matter (SOM), C: N ratio, and available P (AP). Redundancy analysis showed that soil microscale pore characteristics effectively accounted for the variations in soil nutrients with an explanatory degree of 94.23 %. (4) Influence pathways analysis by structural equation modeling indicated that dramatic variability in non-growing season precipitation promoted increases in mesopore and macropore volume, as well as the transformation of mesopores into macropores, thereby facilitating soil carbon accumulation. Our study suggests that soil microscale pore characteristics, acquired through adsorption experiments, assist in elucidating these potential synergistic mechanisms among physicochemical properties under varying non-growing season precipitation patterns. Given the escalating impacts of climate change, our findings provide novel insights and evidence for the assessment of climate change impacts in temperate arid grassland ecosystems.

Redox-mediated changes in the release dynamics of lead (Pb) and bacterial community composition in a biochar amended soil contaminated with metal halide perovskite solar panel waste

This study explored the redox-mediated changes in a lead (Pb) contaminated soil (900 mg/kg) due to the addition of solar cell powder (SC) and investigated the impact of biochar derived from soft wood pellet (SWP) and oil seed rape straw (OSR) (5% w/w) on Pb immobilization using an automated biogeochemical microcosm system. The redox potential (Eh) of the untreated (control; SC) and biochar treated soils (SC + SWP and SC + OSR) ranged from -151 mV to +493 mV. In SC, the dissolved Pb concentrations were higher under oxic (up to 2.29 mg L-1) conditions than reducing (0.13 mg L-1) conditions. The addition of SWP and OSR to soil immobilized Pb, decreased dissolved concentration, which could be possibly due to the increase of pH, co-precipitation of Pb with FeMn (hydro)oxides and pyromorphite, and complexation with biochar surface functional groups. The ability and efficiency of OSR for Pb immobilization were higher than SWP, owing to the higher pH and density of surface functional groups of OSR than SWP. Biochar enhanced the relative abundance of Proteobacteria irrespective of Eh changes, while the relative abundance of Bacteroidota increased under oxidizing conditions. Overall, we found that both OSR and SWP immobilized Pb in solar panel waste contaminated soil under both oxidizing and reducing redox conditions which may mitigate the potential risk of Pb contamination.

CaAl-Layered Double Hydroxides-Modified Biochar Composites Mitigate the Toxic Effects of Cu and Pb in Soil on Pea Seedlings

Compound contamination of soil with heavy metals copper (Cu) and lead (Pb) triggered by mining development has become a serious problem. To solve this problem, in this paper, corncob kernel, which is widely available and inexpensive, was used as the raw material of biochar and modified by loading CaAl-layered double hydroxides to synthesize biochar-loaded CaAl-layered double hydroxide composites (CaAl-LDH/BC). After soil remediation experiments, either BC or CaAl-LDH/BC can increase soil pH, and the available phosphorus content and available potassium content in soil. Compared with BC, CaAl-LDH/BC significantly reduced the available content of Cu and Pb in the active state (diethylenetriaminepentaacetic acid extractable state) in the soil, and the passivation rate of Cu and Pb by a 2% dosage of CaAl-LDH/BC reached 47.85% and 37.9%, respectively. CaAl-LDH/BC can significantly enhance the relative abundance of beneficial microorganisms such as Actinobacteriota, Gemmatimonadota, and Luteimonas in the soil, which can help to enhance the tolerance and reduce the enrichment ability of plants to heavy metals. In addition, it was demonstrated by pea seedling (Pisum sativum L.) growing experiments that CaAl-LDH/BC increased plant fresh weight, root length, plant height, catalase (CAT) activity, and protein content, which promoted the growth of the plant. Compared with BC, CaAl-LDH/BC significantly reduced the Cu and Pb contents in pea seedlings, in which the Cu and Pb contents in pea seedlings were reduced from 31.97 mg/kg and 74.40 mg/kg to 2.92 mg/kg and 6.67 mg/kg, respectively, after a 2% dosage of CaAl-LDH/BC, which was a reduction of 90.84% and 91.03%, respectively. In conclusion, compared with BC, CaAl-LDH/BC improved soil fertility and thus the plant growth environment, and also more effectively reduced the mobility of heavy metals Cu and Pb in the soil to reduce the enrichment of Cu and Pb by plants.

Soil amendments alter cadmium distribution and bacterial community structure in paddy soils

Soil amendments play a pivotal role in ensuring the safety of food production by inhibiting the transfer of heavy metal ions from soils to crops. Nevertheless, their impact on soil characteristics and the microbial community and their role in reducing cadmium (Cd) accumulation in rice remain unclear. In this study, pot experiments were conducted to investigate the effects of three soil amendments (mineral, organic, and microbial) on the distribution of Cd speciation, organic components, iron oxides, and microbial community structure. The application of soil amendments resulted in significant reductions in the soil available Cd content (16 %-51 %) and brown rice Cd content (16 %-78 %), facilitating the transformation of Cd from unstable forms (decreasing 10 %-20 %) to stable forms (increasing 77 %-150 %) in the soil. The mineral and organic amendments increased the soil cation exchange capacity (CEC) and plant-derived organic carbon (OC), respectively, leading to reduced Cd accumulation in brown rice, while the microbial amendment enhanced OC complexity and the abundances of Firmicutes and Bacteroidota, contributing to the decreased rice Cd uptake. The synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy indicated that soil amendments regulated soil Cd species by promoting iron oxides and OC coupling. Moreover, both organic and microbial amendments significantly reduced the diversity and richness of the bacterial communities and altered their compositions and structures, by increasing the relative abundances of Bacteroidota and Firmicutes and decreasing those of Acidobacteria, Actinobacteria, and Myxococcota. Soil microbiome analysis revealed that the increase of Firmicutes and Bacteroidota associated with Cd adsorption and sequestration contributed to the suppression of soil Cd reactivity. These findings offer valuable insights into the potential mechanisms by which soil amendments regulate the speciation and bioavailability of Cd, and improve the bacterial communities, thereby providing guidance for agricultural management practices.

Formation of cation bridges and its promoting mechanism for sorption of sulfamethoxazole by montmorillonite

The coexistence of metal cations is often accompanied by organic pollution and could affect the environmental fate of organics by mediating the formation of cation bridges. However, the environmental fate and risk of organics in cation co-existing environments are poorly understood due to the lack of accurate identification of cation bridge formation and stability. In this study, the sorption of sulfamethoxazole (SMX) on montmorillonite (MT) with the coexistence of three different valence metal cations (Na+, Ca2+, and Cr3+) was investigated. Ca2+ and Cr3+ can significantly promote the sorption of SMX on MT for about 5∼10 times promotion, respectively, while Na+ bridges displayed little effect on the sorption of SMX. The sorption binding energy of SMX with MT-Ca (-44.01 kcal/mol) and MT-Cr (-64.57 kcal/mol) bridges was significantly lower than that with MT-Na (-38.45 kcal/mol) and MT (-39.39 kcal/mol), indicating that the sorption affinity of SMX on Cr and Ca bridges was much stronger. The higher valence of the cations also resulted in a more stable adsorbed SMX with less desorption fluctuation. In addition, the relatively higher initial concentration of SMX and the valence of cations increased the bonding density of the cation bridges, thus promoting the apparent sorption of SMX on MT to a certain extent. This work reveals the formation and function of cation bridges in the sorption of SMX on MT. It lays a theoretical foundation for further understanding the environmental fate and risk of organics.

Heavy metal migration dynamics and solid-liquid distribution strategy in abandoned tailing soils

The tailings soil originating from an abandoned sulfur-iron mine in Sichuan Province, China, exhibits elevated concentrations of heavy metals (HMs) and possesses limited soil conservation capacity. Variability soil particle size fractions (PSFs) contributes to an increased risk of HMs ion migration. Existing research on HMs behavior has focused on the bulk soil scale, resulting in a dearth of comprehensive information concerning different particle sizes and colloid scales. We collected soil samples from upstream source (XWA), migration path (XWB), and downstream farmland (XWC) of an abandoned tailing and categorized into sand, silt, clay, colloid and dissolved, respectively. The investigation primarily aimed to elucidate the solid-liquid distribution trade-off strategies of soil HMs along migration pathway. Results show that PSFs composition predominantly influences HMs solid-liquid distribution. In the mining area, large particles serve as the principal component for HMs enrichment. However, along the migration pathway, the proportion of highly mobile fine particles increases, shifting HMs from solid to liquid phase. Furthermore, inorganic elements such as Mg, Al, and Fe influence on HMs distribution within PSFs through various reactions, whereas organic matter and glomalin-related soil protein (GRSP) also exert regulatory roles. Increasing the proportion of large particles can reduce the risk of HMs migration.

Preparation and characterization of pectin-alginate-based microbeads reinforced by nano montmorillonite filler for probiotics encapsulation: Improving viability and colonic colonization

Hydrogel microbeads can be used to enhance the stability of probiotics during gastrointestinal delivery and storage. In this study, the pectin-alginate hydrogel was enhanced by adding montmorillonite filler to produce microbeads for encapsulating Lactobacillus kefiranofaciens (LK). Results showed that the viscosity of biopolymer solutions with 1 % (PAMT1) and 3 % (PAMT3) montmorillonite addition was suitable for producing regular-shaped microbeads. A layered cross-linked network was formed on the surface of PAMT3 microbeads through electrostatic interaction between pectin-alginate and montmorillonite filler, and the surrounding LK with adsorbed montmorillonite was encapsulated inside the microbeads. PAMT3 microbeads reduced the loss of viability of LK when passing through the gastric acid environment, and facilitated the slow release of LK in the intestine and colonic colonization. The maximum decrease in viability among all filler groups was 1.21 log CFU/g after two weeks of storage, while PAMT3 freeze-drying microbeads only decreased by 0.46 log CFU/g, indicating that the gel layer synergized with the adsorbed layer to provide dual protection for probiotics. Therefore, filler-reinforced microbeads are a promising bulk encapsulation carrier with great potential for the protection and delivery of probiotics and can be developed as food additives for dairy products.

The Effects of Heavy Metal Pollution on Soil Nitrogen Transformation and Rice Volatile Organic Compounds under Different Water Management Practices

One of the most concerning global environmental issues is the pollution of agricultural soils by heavy metals (HMs), especially cadmium, which not only affects human health through Cd-containing foods but also impacts the quality of rice. The soil's nitrification and denitrification processes, coupled with the release of volatile organic compounds by plants, raise substantial concerns. In this review, we summarize the recent literature related to the deleterious effects of Cd on both soil processes related to the N cycle and rice quality, particularly aroma, in different water management practices. Under both continuous flooding (CF) and alternate wetting and drying (AWD) conditions, cadmium has been observed to reduce both the nitrification and denitrification processes. The adverse effects are more pronounced in alternate wetting and drying (AWD) as compared to continuous flooding (CF). Similarly, the alteration in rice aroma is more significant in AWD than in CF. The precise modulation of volatile organic compounds (VOCs) by Cd remains unclear based on the available literature. Nevertheless, HM accumulation is higher in AWD conditions compared to CF, leading to a detrimental impact on volatile organic compounds (VOCs). The literature concludes that AWD practices should be avoided in Cd-contaminated fields to decrease accumulation and maintain the quality of the rice. In the future, rhizospheric engineering and plant biotechnology can be used to decrease the transport of HMs from the soil to the plant's edible parts.

Exploring metal(loid)s dynamics and bacterial community shifts in contaminated paddy soil: Impact of MgO-laden biochar under different water conditions

In this study, a soil incubation experiment was conducted to explore the influence MgO-treated corn straw biochar (MCB) on the bioavailability and chemical forms of cadmium (Cd), lead (Pb), and arsenic (As), alongside the impact on the bacterial community within paddy soil subjected to both flooded and non-flooded conditions. Raw corn straw biochar (CB) served as the unmodified biochar control, aiding in the understanding of the biochar's role within the composite. The results showed that even at a minimal concentration of 0.5 %, MCB exhibited higher effectiveness in reducing the bioavailability of Pb and Cd compared to 1 % CB. In non-flooded conditions, 0.5 % MCB reduced the bioavailable Pb and Cd by 99.7 % and 87.4 %, respectively, while NaH2PO4-extracted As displayed a 14.5 % increase. With increasing MCB concentrations (from 0.5 % to 1.5 %), soil pH, DOC, EC, available phosphorus, and bioavailable As increased, while bioavailable Pb and Cd exhibited declining tendencies. Flooding did not notably alter MCB's role in reducing Pb and Cd bioavailability, yet it systematically amplified As release. Heavy metal fractions extracted by acetic acid increased in the MCB groups under flooding conditions, especially for As. The inclusion of 0.5 % MCB did not noticeably affect bacterial diversity, whereas higher doses led to reduced diversity and substantial changes in community composition. Specifically, the groups with MCB showed an increase in the Bacteroidetes and Proteobacteria phyla, accompanied by a decrease in Acidobacteria. These alterations were primarily attributed to the increased pH and EC resulting from MgO hydrolysis. Consequently, for Pb/Cd stabilization and soil bacterial diversity, a low dosage of MgO-treated biochar is recommended. However, caution is advised when employing MgO-treated biochar in soils with elevated arsenic levels, particularly under flooded conditions.

Effect of montmorillonite modified straw biochar on transfer behavior of lead and copper in the historical mining areas of dry-hot valleys

Due to the rapid development of human beings, heavy metals are occurred in the Yunnan-Guizhou Plateau and Panxi Plateau, the special dry and hot climate areas. Pb and Cu can be quickly transferred through water-plant-animal, further harm to human health by food chain. Therefore, the study of heavy metal treatment is imminent. In this study, Biochar-montmorillonite composites were prepared by co-pyrolysis and characterized, and their ability to remove lead and copper from water-soil process were tracked. And their effectiveness in remediating soil contaminated by lead and copper was documented. The composite material has the rich pore structure, large specific surface area (81.5 m2/g) and a variety of surface functional groups such as C-C, CO, ester-metal and metal-oxygen bonds. Pb and Cu can be effectively adsorbed and fixed to the level of no harm to human health. The adsorption reaction of lead and copper on the Biochar-montmorillonite composites is more suitable to be described by Langmuir adsorption and pseudo-second-order kinetics models. The saturation adsorption capacity of the composite for Pb was measured as 212.5 mg/g. For Cu, it was 136.5 mg/g. The data were fitted by a two-compartment first-order kinetic model. ffast for Pb and Cu is estimated to be 0.81 and 0.78, respective. Fast adsorption is dominant and belongs to typical chemical adsorption, which is consistent with the second-order kinetic results. With 5 % of the composite, approximately 80 % of exchangeable heavy metals in those soils collected from the Yunnan-Guizhou Plateau and Panxi Plateau were reduced. The biochar-montmorillonite composites made Pb and Cu change to stable residual state, up to 35 %. Besides, it effectively restored the activity of urease and sucrase in soils. Results indicated that biochar-montmorillonite composites can be effectively used as an environment-friendly adsorbent or passivator to purify heavy metals in soils.

Safe Production Strategies for Soil-Covered Cultivation of Morel in Heavy Metal-Contaminated Soils

Morel is a popular edible mushroom with considerable medicinal and economic value which has garnered global popularity. However, the increasing heavy metal (HM) pollution in the soil presents a significant challenge to morels cultivation. Given the susceptibility of morels to HM accumulation, the quality and output of morels are at risk, posing a serious food safety concern that hinders the development of the morel industry. Nonetheless, research on the mechanism of HM enrichment and mitigation strategies in morel remains scarce. The morel, being cultivated in soil, shows a positive correlation between HM content in its fruiting body and the HM content in the soil. Therefore, soil remediation emerges as the most practical and effective approach to tackle HM pollution. Compared to physical and chemical remediation, bioremediation is a low-cost and eco-friendly approach that poses minimal threats to soil composition and structure. HMs easily enriched during morels cultivation were examined, including Cd, Cu, Hg, and Pb, and we assessed soil passivation technology, microbial remediation, strain screening and cultivation, and agronomic measures as potential approaches for HM pollution prevention. The current review underscores the importance of establishing a comprehensive system for preventing HM pollution in morels.

Effect of cornstalk biochar on phytoremediation of Pb-contaminated soil by females and males of Populus deltoides (Salicaceae)

Soil pollution with lead (Pb) has become a serious global concern, adversely affecting the forest ecosystem. This study was conducted to investigate the effects of corn straw on the remediation efficiency of Pb-contaminated soil using Populus deltoides. Female and male P. deltoides cuttings were subjected to soil spiked with 900 mg kg-1 Pb and amended with 5% (v/v) corn straw biochar for 90 days. Under Pb stress, the addition of biochar significantly increased the total biomass accumulation by 29% in females and 26% in males. However, without the addition of biochar, the biomass accumulation was significantly reduced by 11% in females and 3% in males under Pb stress. Females showed a higher uptake and accumulation of Pb in roots and leaves, while males accumulated more Pb in roots and stems and exhibited an increased anti-oxidative capacity. Biochar addition alleviated Pb toxicity in both male and female P. deltoides by immobilizing Pb ion in the soil, reducing Pb uptake and translocation, promoting nutrient uptake, and improving the diversity and stability of the soil bacteria community. Under Pb stress, the relative abundances of metal-resistance bacteria significantly increased, such as the abundance of Bacteroidetes in females and the abundances of Actinobacteria, Firmicutes, and Planctomycetes in males. In brief, the males under biochar addition exhibited promising potential as candidates for phytoremediation of Pb-contaminated soil. This study provides new insights into mechanisms underlying sexually differential responses to Pb stress in the presence of biochar amendment.

Comparative analysis of temporal variation of heavy metal accumulation by two sea urchin species from a harbour region, including pre and post COVID 19 lock down period

Sea urchins are marine invertebrates belonging to phylum Echinodermata, recognized as relevant biological tool for assessing environmental pollution. In the present study, we assessed the bioaccumulation potential of different heavy metals by two sea urchin species, Stomopneustes variolaris Lamarck, 1816 and Echinothrix diadema Linnaeus, 1758, collected from a harbour region, along the south west coast of India, during four different sampling periods for 2 years, from the same sea urchin bed. Heavy metals like Pb, Cr, As, Cd, Co, Se, Cu, Zn, Mn and Ni were analysed from water, sediment and different body parts of sea urchins, such as shell, spine, tooth, gut and gonad. The sampling periods also included the pre and post COVID 19 lockdown period during which the harbour activities were closed. The bio-water accumulation factor (BWAF), bio-sediment accumulation factor (BSAF) and the metal content/test weight index (MTWI) were calculated, in order to compare the bioaccumulation of metals by both the species. The results showed that S. variolaris had higher bioaccumulation potential than E. diadema, for metals like Pb, As, Cr, Co and Cd especially in the soft body parts like gut and gonad. The hard parts of S. variolaris like shell, spine, and tooth also accumulated more Pb, Cu, Ni and Mn than E. diadema. Following the lockdown period, there was a decline in the concentration of all heavy metals in water, whereas in sediment, Pb, Cr, and Cu levels were reduced. The gut and gonad tissues of both the urchins showed a decrease in the concentration of most of the heavy metals following the lockdown phase and no significant reduction was observed in the hard parts. This study reveals the use of S. variolaris as an excellent bioindicator of heavy metal contamination in the marine environment which can be employed for coastal monitoring programs.

Effect of goethite-loaded montmorillonite on immobilization of metal(loid)s and the micro-ecological soil response in non-ferrous metal smelting areas

In this work, the immobilization stabilization and mechanism of heavy metal(loid)s by goethite loaded montmorillonite (GMt) were investigated, and the soil microbial response was explored. The simulated acid rain leaching experiment showed that GMt had a higher acid tolerance and the more stable heavy metal(loid)s fixation ability. The soil incubation demonstrated that GMt significantly decreased the available Cd, Zn, Pb and As concentration. Interestingly, higher immobilization of heavy metals was observed by GMt in highly acid leached and acidic soils. The richness and diversity of bacterial communities improved after the addition of GMt. GMt induced the enrichment of the excellent functional bacteria of the phylum Proteobacteria as well as the genus Massilia and Sphingomonas. The main immobilization mechanisms of heavy metal(loid)s by GMt include electrostatic interaction, complexation, precipitation and oxidation. The addition of the GMt also optimizes the soil bacterial community structure, which further facilitates the immobilization of heavy metal(loid)s. Our results confirm that the novel GMt has a promising application in the immobilization and stabilization of heavy metal(loid)s contaminated soils in non-ferrous metal smelting areas.

Performance and mechanisms for Cd(II) and As(III) simultaneous adsorption by goethite-loaded montmorillonite in aqueous solution and soil

A series of goethite-modified montmorillonite (GMt) materials was synthesized for the amelioration of cationic cadmium (Cd) and anionic arsenic (As) complex contaminants in soil and water bodies. The results showed that goethite (Gt) was successfully loaded onto the surface of montmorillonite (Mt), which possessed more functional groups (such as Fe-O, and Fe-OH) and a larger specific surface area. GMt-0.5 (Mt loaded with Gt at a ratio of 0.5:1) showed the highest adsorption capacity for Cd(II) and As(III) with the maximum of 50.61 mg/g and 57.58 mg/g, respectively. The removal rate of Cd(II) was highly pH dependent, while the removal rate of As(III) showed little dependence on pH. The goethite on montmorillonite might contribute to the formation of surface complexes with As(III) and oxidation of As(III) to As(V). In the binary system, both, synergistic and competitive adsorption existed simultaneously. Importantly, in the binary system, the removal of As(III) was more favorable because of the electrostatic interaction, formation of a ternary complex, and co-precipitation. In addition, the amendment of GMt-0.5 significantly reduced the availability of Cd and As in the soil. This study suggests that GMt-0.5 is a promising candidate for the simultaneous immobilization of metal (loid)s in both, aqueous solution and mine soil.

Nonthermal plasma-irradiated polyvalent ferromanganese binary hydro(oxide) for the removal of uranyl ions from wastewater

Nonthermal plasma (NTP) irradiation was employed to adjust the morphological structures and valence distribution of ferromanganese (Fe-Mn)-based binary hydro (oxide) to enhance the heterogeneous adsorption of uranyl ions. The output voltage and the liquid-plate distance played a more vital role among the NTP factors in the irradiation system in influencing the polyvalent Fe-Mn binary hydro (oxide) (poly-Fe-Mn). The formation of plates, flakes, and nanoscale nodules was specifically observed, which caused more pores and fractures in the poly-Fe-Mn binary hydro (oxide). The poly-Fe-Mn performed explicitly better in the adsorption of uranium ions in comparison with the counterpart of the Fe-Mn, which was appropriately fitted by the pseudofirst-order kinetic and Elovich models. Maximum equilibrium adsorption capacities of 663.92 and 923.45 mg/g were obtained for the Fe-Mn and poly-Fe-Mn binary hydro (oxides) toward U ions in the orthogonal design, respectively. The maximum monolayer adsorption capacity achieved by the fitting of the Langmuir model was 1091.10 mg/g. Both physisorption and chemisorption contributed to the heterogeneous process of the poly-Fe-Mn toward uranium ions. The employment of NTP irradiation changed the monolayer adsorption of the traditional Fe-Mn materials and diversified the reaction mechanisms between the interface of the Fe-Mn materials and uranium ions. The elements, including O, N, and U exhibited higher compatibility and overlapped in the samples. The highly effective capture of uranium ions from the solution by the poly-Fe-Mn binary hydro (oxide) was mainly related to the chemical deposition of O and N radicals.

Long-term immobilization of cadmium and lead with biochar in frozen-thawed soils of farmland in China

The problem of potentially toxic elements (PTEs) in farmland is a key issue in global pollution prevention and control and has an important impact on environmental safety, human health, and sustainable agricultural development. Based on the climate background of high-latitude cold regions, this study simulated freeze-thaw cycles through indoor tests. Different initial conditions, such as biochar application rates (0%, 1%, 2%) and different initial soil moisture contents (15%, 20%, 25%), were set to explore the morphological changes in cadmium (Cd) and lead (Pb) in soil and the response relationship to the changes in soil physicochemical properties. The results indicate that soil pH decreases during freeze-thaw cycles, and soil alkalinity increases with increasing biochar content. Freeze-thaw cycles caused the total amount of PTEs to have a U-shaped distribution, and the amount of PTEs in the soluble (SOL) and reducible (RED) fraction increased by 0.28-56.19%. Biochar reduced the amount of Cd and Pb migration in the soil, and an increase in soil moisture content reduced the availability of Cd and Pb in the soil. Freezing and thawing damaged the soil structure, and biochar reduced the fractionation of small particle aggregates by enhancing the stability of soil aggregates, thereby reducing the soil's ability to adsorb Cd and Pb. In summary, for farmland soil remediation and pollution control, the application of biochar has a certain ability to optimize soil properties. Considering the distribution of PTEs in the soil and the physicochemical properties of the soil, the application of 1% biochar to soil with a 20% moisture content is optimal for regulating seasonally frozen soil remediation.

One stone two birds: Bone char as a cost-effective material for stabilizing multiple heavy metals in soil and promoting crop growth

Remediation of farmland soils contaminated with high levels of multiple heavy metals near PbZn smeltery is still a great challenge. It is of great significance to find cost-effective green remediation technologies for stabilization of multiple heavy metals in soil and reduce metal accumulation in crops with ensured yield. In this study, we demonstrated that bone char (BC) is an effective heavy metal stabilizer which can substantially increase residual fractions of heavy metals and reduce metal accumulation in pea (Pisum sativum) with its enhanced growth. We chose the soils contaminated with high levels of Pb, Zn, Cu and Cd near the Baiyin PbZn smeltery as the tested soil. After 2 months of BC application, the relative mobile fractions (non-residual fractions) of Cu, Zn, Pb and Cd in the contaminated soil decreased while the residual fraction increased significantly. The leachability of Cu, Zn, Pb and Cd decreased by 91.2%, 38.6%, 67.6% and 54.3%, respectively compared with the control. BC application remarkably promoted pea growth and reduced accumulation of heavy metals in shoots. The mechanisms for stabilization of multiple heavy metals BC include ion exchange, surface complexation and subsequent mineralization, accompanied with release of Ca and phosphate. The immobilization of heavy metals led to their reduced toxicity to plant, and thus increased pea growth. The results show that BC is a cost-effective and sustainable heavy metal stabilizer with phosphate fertilization function. It can simultaneously immobilize multiple heavy metals in soil and facilitate crop production.

Ammonia level influences the assembly of dissimilatory nitrate reduction to ammonia bacterial community in soils under different heavy metal remediation treatments

Heavy metal remediation treatments might influence functional microbial community assembly. Dissimilatory nitrate reduction to ammonia (DNRA) contributes to the nitrogen retention processes in soil ecosystems. We assumed that remediation might reduce heavy metal toxicity and increase some available nutrients for the DNRA microbes, thus balancing the deterministic and stochastic process for DNRA community assembly. Here, we investigated the process of DNRA bacterial community assembly under different heavy metal remediation treatments (including control, biochar, limestone, rice straw, rice straw + limestone, and biochar + limestone) in an Alfisol soil. The abundance of DNRA bacteria diverged across treatments. The α-diversity of the DNRA bacterial community was correlated with pH, available phosphorus (AP), ammonium (NH₄⁺), and extractable Fe (EFe). Metal Cd and Fe significantly affected the abundance of the nrfA gene. The β-diversity was associated with pH, NH₄⁺, and EFe. Deterministic processes dominantly drove the assembly processes of the DNRA bacterial community. NH₄⁺ level played an essential role in the assembly processes than the other soil physicochemical properties and metal availability. High, moderate, and low levels of NH₄⁺ could advocate stochastic process plus selection, heterogeneous selection to stochastic process, and heterogeneous selection, respectively. Network analysis highlighted a predominant role of NH₄⁺ in regulating DNRA bacterial community assembly. However, the relative abundance of modules and some keystone species also were influenced by pH and EFe, respectively. Therefore, the DNRA bacterial community assembly under different heavy metal remediation treatments in this study was dominantly driven by nitrogen availability. pH, phosphorus, and metal availability were auxiliary regulators on DNRA bacterial community.

Geochemical partitioning and spatial distribution of heavy metals in soils contaminated by lead smelting

Heavy metals (HMs) pollution is a universal and complex problem at lead smelting sites. Further understanding on the distribution, coexistence relationship and occurrence form of multi-metals in soils should be taken prior to restoration on the contaminated sites. In this study, 222 soil samples in a typical abandoned lead smelting site were investigated to understand the spatial distribution and geochemical partitioning of HMs. The results showed that soil quality was seriously threatened by As, Pb and Cd, which expressed high spatial heterogeneity. Integration of sequential extraction, X-ray photoelectron spectroscopy and mineral liberation analysers were employed to qualify the geochemical partitioning of HMs. The data showed that Pb and As were mainly partitioned in the reducible phase and residue phase, where the maximum of As were 18% and 79%, and the maximum of Pb were 31% and 64%, respectively, whilst Cd was mainly partitioned with residue phase (about 25%) and weakly acid soluble phase (about 18%). Paulmooreite was the major important mineral host for Pb and As, whereas Cd predominantly existed in willemite. These minerals containing HMs could usually with Fe reside in the octahedral layer of clay minerals such as montmorillonite, and may also reside in the interlayer. Quartz, montmorillonite and goethite were closely associated with HMs minerals in contaminated soils, which limited vertical migration of HMs and potential risks to groundwater. The results enhanced the understanding of spatial distribution and occurrence behavior of HMs, whilst providing potential benefits to heavy metal stabilization and risks control at abandoned non-ferrous metal smelting sites.

Evaluating the remediation potential of MgFe2O4-montmorillonite and its co-application with biochar on heavy metal-contaminated soils

In this work, a novel and efficient magnesium ferrite-modified montmorillonite (MgFe2O4-MMT) compound was prepared. MgFe2O4-MMT and biochar were mixed at 0:10, 1:9, 3:7, 4:6, and 10:0 w/w combinations and were used for heavy metal immobilization in soil polluted with multiple heavy metals. MgFe2O4-MMT can significantly increase soil alkalinity, and it exhibited the most optimal effect in immobilization of heavy metals in soil. The amounts of Cd, Pb, Cu, and Zn that were extracted by the toxicity characteristic leaching procedure (TCLP) decreased by 58.4%, 50.3%, 42.9%, and 24.7%, respectively. MgFe2O4-MMT can immobilize heavy metals through electrostatic interactions and cation exchange processes. Although, the immobilization of potentially toxic elements by MgFe2O4-MMT and biochar was inferior to that by MgFe2O4-MMT. The combined application of MgFe2O4-MMT and biochar dramatically increased the diversity and richness of the soil bacterial community. The Chao1 index for M3B7 treatment group was 1.7 and 1.2 times higher than that for the control and MgFe2O4-MMT treatment groups, respectively. The combination of biochar and MgFe2O4-MMT might be a cost-effective and ecological remediation approach for mild Pb and Cd contamination.

Rhizospheric mechanisms of Bacillus subtilis bioaugmentation-assisted phytostabilization of cadmium-contaminated soil

Plant growth promoting (PGP) traits of inoculation in bioaugmentation assisted phytostabilization of heavy metal-contaminated soil have been well documented. The property of inoculation to immobilize heavy metals is another major contributor to phytostabilization efficiency. This study investigated the effects of inoculation with different concentrations of rhizobacteria Bacillus subtilis on the cadmium (Cd) bioavailability and distribution, enzyme activities, and bacterial community structure in soil planted with ryegrass (Lolium multiflorum L.). Addition of a high dosage of Bacillus subtilis decreased plant malondialdehyde (MDA) amount, increased plant antioxidant enzyme and soil nutrient cycling-involved enzyme activities, and subsequently enhanced biomass by 20.9%. In particular, the inoculation reduced the Cd bioavailability in soil, bioaccumulation coefficient (BCF), translocation factors (TF), and accumulation in ryegrass by 39.1%, 36.5%, 24.2%, and 27.9%, respectively. Furthermore, 16S rRNA gene sequencing analysis of rhizosphere soil revealed microbial community structure alterations (e.g., enrichment of Proteobacteria), eight phenotype regulations, and seventeen Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway transformations accounted for the stress mitigation and Cd immobilization in the presence of inocula. Besides, intracellular accumulation and biofilm sequestration were proposed as primary immobilization mechanisms induced by bioaugmentation.

Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components

Heavy metal (HM) pollution of soils is a globally important ecological and environmental problem. Previous studies have focused on i) tracking pollution sources in HM-contaminated soils, ii) exploring the adsorption capacity and distribution of HMs, and iii) assessing phyto-uptake of HMs and their ecotoxicity. However, few reviews have systematically summarized HM pollution in soil-plant systems over the past decade. Understanding the mechanisms of interaction between HMs and solid soil components is consequently key to effectively controlling and remediating HM pollution. However, the compositions of solid soil phases are diverse, their structures are complex, and their spatial arrangements are heterogeneous, all leading to the formation of soil micro-domains that exhibit different particle sizes and surface properties. The various soil components and their interactions ultimately control the speciation, transformation, and bioavailability of HMs in soils. Over the past few decades, the extensive application of advanced instrumental techniques and methods has greatly expanded our understanding of the behavior of HMs in organic mineral assemblages. In this review, studies investigating the immobilization of HMs by minerals, organic compounds, microorganisms, and their associated complexes are summarized, with a particular emphasis on the interfacial adsorption and immobilization of HMs. In addition, methods for analyzing the speciation and distribution of HMs in aggregates of natural soils with different particle sizes are also discussed. Moreover, we also review the methods for speciating HMs at mineral-organic micro-scale interfaces. Lastly, developmental prospects for HM research at inorganic-organic interfaces are outlined. In future research, the most advanced methods should be used to characterize the interfaces and in situ characteristics of metals and metal complexes. In particular, the roles and contributions of microorganisms in the immobilization of HMs at complex mineral-organic interfaces require significant further investigation.

Silicon fertilizers, humic acid and their impact on physicochemical properties, availability and distribution of heavy metals in soil and soil aggregates

It has been confirmed that silicon (Si) fertilizer and humic acid (HA) could effectively decrease the heavy metals in soil. Nonetheless, the impact of these additives on soil aggregate characteristics was ignored. Therefore, the effects of Si fertilizer, HA, and their combinations on the physicochemical characteristics, availability of heavy metals (Cu, Cd, Pb, Zn), and fraction changes in soils and soil aggregates were investigated in this research. The results showed that Si fertilizer and HA significantly modified soil properties such as soil pH, electrical conductivity total organic carbon, water-soluble organic carbon, and nitrate‑nitrogen. HA and Si-HA (SHA) supplementation significantly decreased the availability of Cu, Cd, Pd, and Zn. Besides, there was no significant difference in physicochemical properties between soil and soil aggregates. The availability of Cu, Cd, Pd, and Zn in soil aggregates could be significantly inhibited by the addition of HA and SHA, and the content in microaggregates was greater than that in macroaggregates. After the addition of the three additives, the main fractions of heavy metals in different particle sizes were changed and eventually transformed to the residue state. These results indicated that Si fertilizer, HA, and SHA were influential in physicochemical properties and metal availability in soil aggregates. Therefore, it is of great scientific significance to study the impact of heavy metal pollution on the ecological environment in different aggregates, which will provide reference data for future sustainable management of heavy-metal polluted soils.

Adsorption behaviors and mechanisms of Fe/Mg layered double hydroxide loaded on bentonite on Cd (II) and Pb (II) removal

In this study, FeMg-LDH loaded with bentonite (FeMg-LDH@bentonite) was prepared using the facile co-precipitating method in situ to remove heavy metals from water and then characterized using XRD, SEM, TEM, FTIR, BET, TGA, and XPS. Pb (II) and Cd (II) were selected as the representative heavy metals to evaluate the adsorption capability of the FeMg-LDH@bentonite. The batch adsorption method was adopted to test the effects of the contact time, pH, initial concentration, different cations, and temperatures. The kinetic study indicated that the adsorption of heavy metals onto FeMg-LDH@bentonite was well fitted by the pseudo-second-order method. Isotherms were effectively simulated based on the Langmuir model. The maximal adsorption capability of Cd (II) and Pb (II) can reach 510.2 mg/g and 1397.62 mg/g, exceeding those of conventional adsorbents. The adsorption mechanisms of FeMg-LDH@bentonite demonstrating that there may exist surface complexation, ion exchange, and chemical deposition between FeMg-LDH@bentonite and heavy metals. Moreover, FeMg-LDH@bentonite was found to have a promising application for practically treating wastewater with heavy metals and can be used for various environmental water pollution treatments. The material may be used for heavy metal contaminated soil in the future.

Soil microbiomes divergently respond to heavy metals and polycyclic aromatic hydrocarbons in contaminated industrial sites

Contaminated sites from electronic waste (e-waste) dismantling and coking plants feature high concentrations of heavy metals (HMs) and/or polycyclic aromatic hydrocarbons (PAHs) in soil. Mixed contamination (HMs + PAHs) hinders land reclamation and affects the microbial diversity and function of soil microbiomes. In this study, we analyzed HM and PAH contamination from an e-waste dismantling plant and a coking plant and evaluated the influences of HM and PAH contamination on soil microbiomes. It was noticed that HMs and PAHs were found in all sites, although the major contaminants of the e-waste dismantling plant site were HMs (such as Cu at 5,947.58 ± 433.44 mg kg-1, Zn at 4,961.38 ± 436.51 mg kg-1, and Mn at 2,379.07 ± 227.46 mg kg-1), and the major contaminants of the coking plant site were PAHs (such as fluorene at 11,740.06 ± 620.1 mg kg-1, acenaphthylene at 211.69 ± 7.04 mg kg-1, and pyrene at 183.14 ± 18.89 mg kg-1). The microbiomes (diversity and abundance) of all sites were determined via high-throughput sequencing of 16S rRNA genes, and redundancy analysis was conducted to investigate the relations between soil microbiomes and contaminants. The results showed that the microbiomes of the contaminated sites divergently responded to HMs and PAHs. The abundances of the bacterial genera Sulfuritalea, Pseudomonas, and Sphingobium were positively related to PAHs, while the abundances of the bacterial genera Bryobacter, Nitrospira, and Steroidobacter were positively related to HMs. This study promotes an understanding of how soil microbiomes respond to single and mixed contamination with HMs and PAHs.

The Effects of the Long-Term Freeze–Thaw Cycles on the Forms of Heavy Metals in Solidified/Stabilized Lead–Zinc–Cadmium Composite Heavy Metals Contaminated Soil

Heavy metals (HMs) exist in nature in different forms, and the more unstable the form of an HM, the higher its toxicity and bioavailability. The content of HMs in stable fractions can increase significantly through the stabilization/solidification (S/S) technology. Still, external environments such as freeze–thaw (F–T) cycles will affect the stability of HMs directly. Therefore, a long-term F–T study of S/S Pb–Zn–Cd composite HM-contaminated soil was conducted under six conditions (0, 3, 7, 14, 30, and 90 cycles) with each F–T cycle process up to 24 h. The improved Tessier method was employed, and the results show that the S/S technology makes HMs transform to a more stable fraction. Still, the transformation efficiency is different for each HM. More than 98% of lead and zinc were converted to stable forms, while for cadmium, there are only 75.1%. Meanwhile, the S/S HMs were rapidly transformed into unstable forms at 0–14 cycles, but after 14 cycles, the transformation speed was significantly reduced. Among stable forms, it is mainly that the carbonate-bound fraction of HMs changes to unstable forms, and the characteristic peaks of carbonate stretching vibration were found at 874 cm−1, and 1420 cm−1 by Fourier infrared spectroscopy proves the presence of carbonate-bound substances. As a result of this study, the change trend of contaminated soil with S/S HMs under the effect of long-term F–T cycle was revealed, and the crisis point of pollution prevention and control was found, which provides some theoretical basis for the safety of soil remediation project.

Mixed bacteria-loaded biochar for the immobilization of arsenic, lead, and cadmium in a polluted soil system: Effects and mechanisms

The present study explored the immobilization of mixed bacteria-loaded biochar on As, Pb, and Cd was explored. Physisorption and sodium alginate encapsulation were used to synthesize two kinds of mixed bacteria-loaded biochars, referred to as BCM and BCB. The observations of Scanning electron microscope, X-ray diffraction, and Fourier transform infrared spectroscopy distinctly demonstrated the colonization of mixed bacteria on biochar. Besides, the addition of BCM and BCB could increase soil pH with increasing incubation time. The residual fraction of heavy metals and soil dehydrogenase activities were also enhanced after 28 days of incubation. Pb was mainly immobilized by co-precipitation, which meant that Pb could be converted into a consistent crystalline form such as Pb₅(PO₄)₃OH. The X-ray photoelectron spectroscopy and X-ray diffraction analyses of materials identified the formation of Ca₂As₂O₇ and the presence of oxidation from trivalent arsenic to pentavalent arsenic. Cd was adsorbed by forming precipitations (CdCO₃) and exchanging ions with the BCM and BCB. Synergistic reactions between anions and cations also contributed to the immobilization of heavy metals, such as the formation of PbAs₂O₆ and Cd₃(AsO₄)₂. These results confirmed that mixed bacteria-loaded biochar was a feasible technology for the remediation of heavy metals contamination in site soils.

Heterogeneity of humic/fulvic acids derived from composts explains the differences in accelerating soil Cd-hyperaccumulation by Sedum alfredii

The hyperaccumulating mechanism concerning heavy metal activation or passivation and plant response triggered by fulvic acid (FA) and humic acid (HA) recruitments are investigated herein. We carefully examine the Cd activation effect by various FA and HA, tracing from pig, goat, and duck manure composts to straw compost and commercial materials (i.e., PC, GC, DC, SC, and CM), as well as their roles in plant growth promotion and Cd uptake. Our results indicate that due to the decrease of soil pH and their multiple functional groups, the contents of available Cd (AE-Cd) increased by 4.3-4.8% and 3.6-6.3% when all FA and HA sources were applied for 30 days. A 13.1-19.9% increase in AE-Cd was observed when CFA, DFA, and PFA were applied for five days, and a 9.5% increment was found when PHA was applied for 10 days. In the pot experiment, the Cd accumulation in plants increased by 2.78 and 2.17 folds with PFA and PHA applications, respectively, compared to the blank control group. This result can be attributed to the stimulative effects of the simultaneous Sedum alfredii growth and Cd phytoavailability. Notably, the Cd accumulation increased by 2.26 times with the SFA amendment due to the predominant stimulation effect to the phytoavailable Cd rather than plant growth. However, slight inhibitory effects were observed upon plant growth or Cd uptake, which led to the reduction of the Cd accumulation with DHA, SHA, and CHA employments. Consistently, the corresponding soil Cd removal efficiencies were 43.5% and 34.6% with PFA and PHA, respectively, which hold abundant O- and N-containing groups. Our research aims to gain insights into the ternary interaction in the presence of heavy metal, humic substances, and S. alfredii to simultaneously accelerate Cd activation and hyperaccumulation.

Evaluation of three common alkaline agents for immobilization of multi-metals in a field-contaminated acidic soil

We investigated three common alkaline agents (NaOH, CaO, and Mg(OH)₂) for immobilization of four heavy metals (Pb, Zn, Cu, and Cd) in a field-contaminated soil and elucidated the underpinning principles. NaOH caused the highest pH spike in the soil, while CaO and Mg(OH)₂ served as a longer-lasting source of OH^-. Amending the soil with CaO or Mg(OH)₂ at ≥0.1 mol as OH^- (kg·soil)^-1 for 24 h was able to immobilize all four metals, while NaOH failed. NaOH leached up to 3 times more organic carbon than CaO and Mg(OH)₂, resulting in elevated leachability of the metals. Column elution tests showed that amendments by CaO and Mg(OH)₂ lowered the leachable Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ by 52-54%, 71-75%, 69-73%, and 68%, respectively, after 1440 pore volumes of elution. Sequential extraction revealed that the soil amendments converted the exchangeable fraction of the metals to the much less available forms. XRD and FTIR analyses indicated that formation of metal oxide precipitates and complexation with soil organic matter were responsible for the metals immobilization. Taken together the chemical cost, technical effectiveness, and environmental impact, CaO is the most suitable alkaline agent for remediation of soil contaminated with heavy metals.

New insights into the underlying influence of bentonite on Pb immobilization by undissolvable and dissolvable fractions of biochar

Biochar as a green amendment has been used to immobilize heavy metals in contaminated soil. Apart from the importance of the amendment itself, the interaction with soil components like clay minerals might also influence the immobilization behavior of biochar. Here, we examined the impact of a typical soil mineral, bentonite, on the immobilization of Pb by barley grass-derived biochar, and elucidated the underlying mechanisms by dividing biochar into dissolvable and undissolvable fractions. Results showed that biochar and bentonite could immobilize Pb through mechanism of electrostatic sorption, complexation, and precipitation. Compared to sole undissolvable biochar, coexistence of bentonite rapidly raised pH of the mixture over 7.0, leading the free Pb²⁺ transformed into more stable Pb₂CO₃(OH)₂ (K_sp = 1.3 × 10^-18) instead of PbCO₃ (K_sp = 1.5 × 10^-13), finally increased Pb²⁺ removal rate by 1.47 times. As for the dissolvable biochar, the generation of dissolvable biochar-bentonite-Pb²⁺ ternary complex raised the Pb²⁺ removal rate by 59.6% with the presence of bentonite. Small angel XRD analysis showed that the free Pb²⁺ and dissolvable biochar-associated Pb²⁺ could enter the interlayer space of bentonite and thus expanded the d-spacing from 1.28 nm to 1.36-1.50 nm, which might favor the formation of ternary complex. Findings of this study not only provided a new insight into the immobilization of heavy metals by biochar in soil, but also emphasized the importance of interaction between biochar and soil minerals.

DFT Simulation of the Water Molecule Interaction with the (00l) Surface of Montmorillonite

Montmorillonite is one of the principal mineralogical phases in clay minerals, where its interaction with water and other molecules represents one of the most important aspects and properties for basic science and specific applications. In fact, montmorillonite has many uses in various scientific and technological fields, ranging from environmental remediation to ceramics, food science, and construction/building materials. Several efforts have characterized its structure and physico-chemical properties, especially at the Tetrahedral-Octahedral-Tetrahedral TOT surface. For this purpose, in this work, the authors investigated the structural and electrostatic potential features of the (00l) surface of montmorillonite and the water adsorption process by first principle methods (density functional theory, DFT), considering both static and molecular dynamics approaches. The provided data further extend the knowledge of the modulation of the water molecule adsorption with this important clay mineral.

Simultaneous immobilization of multi-metals in a field contaminated acidic soil using carboxymethyl-cellulose-bridged nano-chlorapatite and calcium oxide

We prepared and tested carboxymethyl-cellulose-bridged nano-chlorapatite (CMC-CAP) for simultaneous immobilization of Pb, Zn, Cu, and Cd in a field-contaminated acidic soil. Amending the field-contaminated soil using 0.5 wt.% CMC-CAP and 0.1 wt.% CaO was most effective in immobilizing the four metals, which decreased the leachabilities by 98.2%, 98.3%, 96.3%, and 96.2% for Pb, Zn, Cu, and Cd, respectively, after 1 day of treatment. The acid-leached metals fluctuated in the first 60 days, and then approached to steady state after 180 days, where the acid-leachable concentrations all met the regulation levels, and the immobilization was further consolidated when further aged for 365 days. Column elution tests showed that the soil amendment lowered the peak metal concentrations by > 92.5%, and the total eluted masses by >71.9%. Sequential extraction revealed that the soil amendment converted the exchangeable fractions to the much less available Fe-Mn oxides bound and residual forms, and thus, lowered the risk levels to "low risk" for all the metals. The immobilization of the metals was facilitated through formation of stable metal (chloro)phosphates, surface complexation, and/or ion exchange reactions. Combined CMC-CAP and CaO may serve as an effective formulation for simultaneous and long-term immobilization of multiple heavy metals in acidic soil.