Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil

Unveiling a Technosol-based remediation approach for enhancing plant growth in an iron-rich acidic mine soil from the Rio Tinto Mars analog site

This paper explores the potential of Technosols made from non-hazardous industrial wastes as a sustainable solution for highly acidic iron-rich soils at the Rio Tinto mining site (Spain), a terrestrial Mars analog. These mine soils exhibit extreme acidity (pHH2O = 2.1-3.0), low nutrient availability (non-acid cation saturation < 20 %), and high levels of Pb (3420 mg kg-1), Cu (504 mg kg-1), Zn (415 mg kg-1), and As (319 mg kg-1), hindering plant growth and ecosystem restoration. To address these challenges, the study systematically analyzed selected waste materials, formulated them into Technosols, and conducted a four-month pot trial to evaluate the growth of Brassica juncea under greenhouse conditions. Technosols were tailored by adding varying weight percentages of waste amendments into the mine Technosol, specifically 10 %, 25 %, and 50 %. The waste amendments comprised a blend of organic waste (water clarification sludge, WCS) and inorganic wastes (white steel slag, WSS; and furnace iron slag, FIS). The formulations included: (T0) exclusively mine Technosol (control); (T1) 60 % WCS + 40 % WSS; (T2) 60 % WCS + 40 % FIS; and (T3) 50 % WCS + 16.66 % WSS + 33.33 % FIS. The analyses covered leachate quality, soil pore water chemistry, and plant response (germination and survival rates, plant height, and leaf number). Results revealed a significant reduction in leachable contaminant concentrations, with Pb (26.16 mg kg-1), Zn (4.94 mg kg-1), and Cu (2.29 mg kg-1) dropping to negligible levels and shifting towards less toxic species. These changes improved soil conditions, promoting seed germination and seedling growth. Among the formulations tested, Technosol T1 showed promise in overcoming mine soil limitations, enhancing plant adaptation, buffering against acidification, and stabilizing contaminants through precipitation and adsorption mechanisms. The paper stresses the importance of tailoring waste amendments to specific soil conditions, and highlights the broader implications of the Technosol approach, such as waste valorization, soil stabilization, and insights for Brassica juncea growth in extreme environments, including Martian soil simulants.

Desorption kinetic and sequential extraction of Pb and Zn in a contaminated soil amended with phosphate, lime, biochar, and biosolids

The mining and metallurgical industry sector activities often release potential toxic elements (PTE) surrounding exploitation area. We evaluated the addition of phosphate and lime using the dosage of 0.5:1, 1:1, and 2:1 molar ratio of PO43- and CO32- to the sum of PTE, respectively, and also, biochar and biosolids using the dosage of 2.5, 5, and 10% (m:m) to immobilize PTE in contaminated forest soil (Pb (270 mg kg-1) and Zn (858 mg kg-1)) near an abandoned mine site in Brazil. The desorption by stirred flow kinetics revealed that 15% of the total Zn and 12% Pb contents are mobile before any amendment application. Phosphate amendment decreased Pb desorption but increased Zn desorption. Biochar and biosolids immobilize high amounts of Zn and Pb because of their high cation exchange capacities and alkaline properties; however, 20% biosolid dose increased Pb desorption. X-ray absorption spectroscopy suggested Zn-kerolite as the major species in the contaminated soil, likely from mine dust. The change in Zn speciation after soil amendment addition indicated that biochar and lime kept a high proportion of Zn-Al species, whereas phosphate and biosolids led to more Zn-Fe species. Our results pointed out that lime might reduce both Pb and Zn mobilities; however, field trials are crucial to confirm the immobilization efficiency of lime and other amendments over long term.

Phosphogypsum/titanium gypsum coupling for enhanced biochar immobilization of lead: Mineralization reaction behavior and electron transfer effect

The hazards caused by Pb pollution have received worldwide attention. Phosphogypsum (PG) and titanium gypsum (TG) have the disadvantage of limited adsorption capacity and poor dispersion when used as heavy metal adsorbents on their own. The excellent pore and electron transfer capacity of biochar makes it possible to combine with PG and TG to solidify/stabilize Pb2+. In this study, the mechanism of Pb2+ adsorption/immobilization by rice husk biochar (BC) combined with PG/TG was investigated in terms of both mineral formation and electron transfer rate. The removal rate of Pb2+ by BC composite PG (BC/PG-Pb) or TG (BC/TG-Pb) was as high as 97%-98%, an increase of 120.9% and 122.5% over BC. Adsorption kinetics and mineral precipitation results indicate that the main removal of Pb2+ from BC/PG-Pb and BC/TG-Pb is achieved by PG/TG induced Pb-sulfate and Pb-phosphate formation. The addition of PG/TG significantly enhances the formation of stable Pb-minerals on the biochar surface, with the proportion of non-bioaccessible forms exceeding 50%. The four-step extraction results confirm that P and F in PG/TG are key in facilitating the conversion of Pb minerals to pyromorphite. The rich pore structure of biochar not only disperses the easily agglomerated PG/TG onto the biochar surface, but also attracts Pb2+ for uniformly dispersed precipitation. Furthermore, the excellent electrical conductivity and smooth electron transfer channels of biochar facilitate the reaction rate of Pb2+ mineralization. Overall, the use of biochar in combination with PG/TG is a promising technology for the combination of solid waste resourceisation and Pb remediation.

Biochar applications for treating potentially toxic elements (PTEs) contaminated soils and water: a review

Environmental pollution with potentially toxic elements (PTEs) has become one of the critical and pressing issues worldwide. Although these pollutants occur naturally in the environment, their concentrations are continuously increasing, probably as a consequence of anthropic activities. They are very toxic even at very low concentrations and hence cause undesirable ecological impacts. Thus, the cleanup of polluted soils and water has become an obligation to ensure the safe handling of the available natural resources. Several remediation technologies can be followed to attain successful remediation, i.e., chemical, physical, and biological procedures; yet many of these techniques are expensive and/or may have negative impacts on the surroundings. Recycling agricultural wastes still represents the most promising economical, safe, and successful approach to achieving a healthy and sustainable environment. Briefly, biochar acts as an efficient biosorbent for many PTEs in soils and waters. Furthermore, biochar can considerably reduce concentrations of herbicides in solutions. This review article explains the main reasons for the increasing levels of potentially toxic elements in the environment and their negative impacts on the ecosystem. Moreover, it briefly describes the advantages and disadvantages of using conventional methods for soil and water remediation then clarifies the reasons for using biochar in the clean-up practice of polluted soils and waters, either solely or in combination with other methods such as phytoremediation and soil washing technologies to attain more efficient remediation protocols for the removal of some PTEs, e.g., Cr and As from soils and water.

Influence of Fertilization on Growth and Lead Content of Pepper under Lead Stress

To investigate the effect of fertilization on Pb content in vegetables, pepper was planted in L1645 (the 5 influencing factors are fertilizers (N, P, K), organic fertilizers (sheep manure) and Pb2+; the 4 levels are blank, low, medium and high; a total of 16 treatments) pot orthogonal experiment. The effects of fertilizers on the growth and Pb content in various parts of pepper under Pb stress were analyzed. The results showed that: (1) The Pb content in pepper fruit ranged from 0.011 mg·kg-1 to 0.085 mg·kg-1, which did not exceed the limit value (0.1 mg·kg-1) in the National Standard for Food Safety-Limit of Contaminants in Food (GB2762-2017); (2) The effect order of fertilization on pepper fruit weight was P2O5 > sheep manure > N > K2O; The horizontal combination of factors that promoted the maximum fruit weight of pepper was N (0.15 g·kg-1), P2O5 (0.225 g·kg-1), K2O (0.15 g·kg-1) and sheep manure (9 g·kg-1); (3) The order of fertilizer effects on Pb content in pepper fruit was Pb2+ > K2O > N = sheep manure > P2O5; the factor level combination that resulted in the maximum Pb content in pepper fruits was N (0.15 g·kg-1), P2O5 (0 g·kg-1), K2O (0.45 g·kg-1), sheep manure (6 g·kg-1) and Pb2+ (350 mg·kg-1); (4) Based on the soil fertility characteristics of Urumqi, the recommended optimal fertilizer application rate was: high phosphorus fertilizer P2O5 (495 kg·hm-2), low-level potassium fertilizer K2O (330 kg·hm-2), medium-level nitrogen fertilizer N (660 kg·hm-2) (or low-level nitrogen fertilizer N (330 kg·hm-2) + high-level organic manure sheep manure (19,800 kg·hm-2), which can achieve high yield while ensuring that the Pb content in the fruits does not exceed the standard. Strengthening control of effective and reasonable fertilization methods in Urumqi agricultural land is helpful to reduce the Pb content in vegetables.

Zinc speciation and desorption kinetics in a mining waste impacted tropical soil amended with phosphate

Mining is an important component of the Brazilian economy. However, it may also contribute to environmental problems such as the pollution of soils with zinc and other potentially toxic metals. Our objective was to evaluate changes in the chemical speciation and mobility of Zn in a soil amended with phosphate. Soil samples were collected from a deactivated mining area in the state of Minas Gerais, Brazil, and amended with NH₄H₂PO₄ saturated with deionized water to 70 % of maximum water retention and incubated at 25 ± 2 °C in open containers for 60 days. The soil was chemically and mineralogically characterized, and sequential extraction, desorption kinetics, and speciation were carried out using synchrotron bulk-sample and micro-X-ray Absorption Near-Edge Structure (XANES/μ-XANES) spectroscopy at the Zn K-edge, and X-ray fluorescence microprobe analysis (μ-XRF). The combination of μ-XRF and μ-XANES techniques made it possible to identify Zn hotspots in the main species formed after phosphate remediation. The best fit combination for bulk XANES and μ-XANES was observed in Zn-montmorillonite, Zn-kerolite, Zn-ferrihydrite, and gahnite. In the course of phosphate treatment, gahnite, Zn layered double hydroxides (Zn-LDH), Zn₃(PO₄), and ZnO were identified by bulk XANES, while Zn-ferrihydrite, Zn-montmorillonite, and scholzite were identified by μ-XANES. Zinc in the phosphate-amended soil had the strongest partial correlations (r' > 0.05) with Ni, Co, Fe, Cr, Mn, Si, P, Cd, Pb, and Cd, while the unamended soil showed the strongest correlation with Cu, Pb, Fe, and Si. The application of NH₄H₂PO₄ altered Zn speciation and favored an increase in Zn desorption. The most available Zn contents after phosphate amendment were correlated with the release of exchangeable Zn fractions, associated with carbonate and organic matter.

Effect of phosphate on cadmium immobilized by microbial-induced carbonate precipitation: Mobilization or immobilization?

Microbial-induced carbonate precipitation (MICP) is a promising technology to immobilize/remediate heavy metals (HMs) like cadmium (Cd). However, the long-term stability of MICP-immobilized HMs is unclear, especially in farmland where chemical fertilization is necessary. Therefore, we performed MICP treatment on soils contaminated with various Cd compounds (CdCO₃, CdS, and CdCl₂) and added diammonium phosphate (DAP) to explore the impact of phosphate on the MICP-immobilized Cd. The results showed that MICP treatment was practical to immobilize the exchangeable Cd but to mobilize the carbonate and Fe/Mn oxide-bound Cd. After applying DAP, soil pH declined due to ammonium nitrification. At high P/Ca molar ratios (1/2 and 1), partial previously immobilized Cd was released due to the carbonate dissolution. Contrarily, exchangeable Cd transformed to less mobilizable Fe/Mn oxide-bound at low P/Ca molar ratios (1/4 and 1/8). Meanwhile, other treatments were also helpful in avoiding the release of immobilized Cd, such as applying non-ammonium phosphate and adding lime material after soil acidification. Our investigation suggested that the long-term stability of HMs in remediated sites should be carefully evaluated, especially in agricultural areas with phosphate and nitrogen fertilizer input.

The era of low-permeability sites remediation and corresponding technologies: A review

Rational utilization of soil resources and remediation of contaminated soils are imperative due to the rapidly growing demand for clean soils. Currently, many in-situ remediation technologies are less suitable at low-permeability sites due to the limitations of soil permeability. This work defines a low-permeability site as a site with hydraulic conductivity less than 10^-4 cm/s, and summarizes the migration characteristics of representative contaminants at low-permeability sites, and discusses the principles and practical applications of different technologies suitable for the remediation of low-permeability sites, including electrokinetic remediation technology, polymer flushing technology, fracturing technology, and in-situ thermal remediation technology. Enhanced and combined remediation technologies are further described because one remediation technology cannot remediate all contaminants. The prospects for the application of remediation technologies to low-permeability sites are also proposed. This work highlights the necessity of low-permeability sites remediation and the urgent need for new remediation technologies, with the hope to inspire future research on low-permeability sites.

Whole-Process Risk Management of Soil Amendments for Remediation of Heavy Metals in Agricultural Soil-A Review

Reducing the mobility and bioavailability of heavy metals in soils by adding exogenous materials is a technology for remediating soils contaminated with heavy metals. Unlike industrial sites, the use of such techniques in agricultural soils requires consideration of not only reducing the mobility of heavy metals but also avoiding adverse effects on soil fertility and the growth of plants. Due to the uncertainty of the stability of amendments applied to agricultural soil, the application of amendments in farmland soil is controversial. This article reviewed the field studies in which amendments were used to immobilize heavy metals, and identified the potential environmental impacts of all aspects of soil amendment usage, including production and processing, transportation, storage, application to soil, long-term stability, and plant absorption. Results of the study indicated that after identifying the environmental risks of the whole process of the application of improvers in agricultural fields, it is necessary to classify the risks according to their characteristics, and design differentiated risk control measures for the safe application of this type of technology.

Effects of Pretreatment and Polarization Shielding on EK-PRB of Fe/Mn/C-LDH for Remediation of Arsenic Contaminated Soils

In this study, coupling electrokinetic (EK) with the permeable reactive barriers (PRB) of Fe/Mn/C-LDH composite was applied for the remediation of arsenic-contaminated soils. By using self-made Fe/Mn/C-LDH materials as PRB filler, the effects of pretreatment and polarization shielding on EK-PRB of Fe/Mn/C-LDH for remediation of arsenic contaminated soils were investigated. For the pretreatment, phosphoric acid, phosphoric acid and water washing, and phosphate were adopted to reduce the influence of iron in soil. The addition of phosphate could effectively reduce the soil leaching toxicity concentration. The removal rate of the soil pretreated with phosphoric acid or phosphoric acid and water washing was better than with phosphate pretreatment. For the polarization shielding, circulating electrolyte, electrolyte type, anion and cation membranes, and the exchange of cathode and anode were investigated. The electrolyte circulates from the cathode chamber to the anode chamber through the peristaltic pump to control the pH value of the electrolyte, and the highest arsenic toxicity removal rate in the soil reaches 97.36%. The variation of total arsenic residue in soil using anion and cation membranes is the most regular. The total arsenic residue gradually decreases from cathode to anode. Electrode exchange can neutralize H⁺ and OH^- produced by electrolyte, reduce the accumulation of soil cathode area, shield the reduction of repair efficiency caused by resistance polarization, enhance current, and improve the removal rate of arsenic in soil.

Novel Fuller Earth, Rock Phosphate, and Biochar for Phytomanagement of Toxic Metals in Polluted Soils

The present study was aimed to assess the efficacy of individual and combined effects of novel fuller earth, rock phosphate, and biochar (grapefruit peel) at 1% dosage on maize plant growth, soil chemical properties anduptake of toxic metals (TMs), such as Cu, Zn, Fe, and Cd, by maize plant sown in Korangi (district of Karachi, Pakistan) heavily polluted and Korangi less polluted (K-HP and K-LP) soils. The obtained results indicate that the dry biomass of maize crop increased by 14.13% with combined (FE1% + GBC1%) on K-HP soil and 18.24% with combined (FE 1% + GBC 1%) effects on K-LP soil. The maximum immobilization of Cu, Zn, Fe, and Cd was observed by 36% with GBC1%, 11.90% with FE1%, 98.97% with combined RP1% + GBC1%, 51.9% with FE1% + GBC1% for K-HP, 11.90% with FE1%, 28.6% with FE1%, 22.22% with RP1% + GBC1%, and 57.05% with FE 1% + GBC 1% for K-LP soil. After the addition of proposed substances, modification of soil OM, SOC, TOC, and pH level appeared this lead to the changes in the phyto-availability of Cu, Zn, Fe, and Cd in maize plant. It was concluded that the application of individual and combined effects of novel fuller earth, rock phosphate, and biochar (grapefruit peel) have potential to stabilize pollutants from multi-metal polluted soils for safe crop production.

Assessment of potential exposure to As, Cd, Pb and Zn in vegetable garden soils and vegetables in a mining region

Mining and smelting activities can contaminate soils and affect farming due to high emissions and input of potentially toxic elements (PTE) into the environment. Soils (sampled from two depths) and market vegetables from vegetable gardens located within the vicinity of unconfined slag deposits from decades of mining and smelting activities in Kutná Hora, Czechia were assessed to determine to what extent they pose a health hazard to communities that use these gardens. Pseudo-total As concentrations in the soils exceeded background levels (4.5 mg kg⁻¹) 1.9–93 times, with higher concentrations in the deeper layer. The pseudo-total concentrations of PTE in soils ranked in the order As > Zn > Cd > Pb. Phyto-available concentrations of PTE in soils were relatively low, compared to pseudo-total concentrations. Concentration of As, Cd, Pb and Zn in the vegetables exceeded guideline values, with the highest concentrations found in the fruits of cucumber, peppers, and zucchini. Despite low phyto-available PTE concentrations in soils, all the PTE concentrations in the vegetables surpassed the guidelines set by the Czech Ministry of Health and EU directive, indicating a health hazard to consumers.

Toxicity of historically metal(loid)-contaminated soils to Folsomia candida under the influence of climate change alterations

Global warming is drastically altering the climate conditions of our planet. Soils will be among the most affected components of terrestrial ecosystems, especially in contaminated areas. In this study we investigated if changes in climate conditions (air temperature and soil moisture) affect the toxicity of historically metal(loid)-contaminated soils to the invertebrate Folsomia candida, followed by an assessment of its recovery capacity. Ecotoxicity tests (assessing survival, reproduction) were performed in field soils affected by metal(loid)s under different climate scenarios, simulated by individually changing air temperature or soil moisture conditions. The scenarios tested were: standard conditions (20°C + 50% soil water holding capacity-WHC); increased air temperature (daily fluctuation of 20-30°C + 50% WHC); soil drought (20°C + 25% WHC); soil flood (20°C + 75% WHC). Recovery potential was assessed under standard conditions in clean soil. Increased temperature was the major climate condition negatively affecting collembolans performance (decreased survival and reproduction), regardless of metal(loid) contamination. Drought and flood conditions presented less pronounced effects. When it was possible to move to the recovery phase (enough juveniles in exposure phase), F. candida was apparently able to recover from the exposure to metal(loid) contamination and/or climate alterations. The present study showed that forecasted climate alterations in areas already affected by contamination should be considered to improve environmental risk assessment.

Effects of phosphorus-containing material application on soil cadmium bioavailability: a meta-analysis

Diverse phosphorus-containing materials (PCMs) were widely applied in remediation of cadmium-contaminated soils, and their effects on the change of soil cadmium availability (SCA) varied with their physicochemical characteristics and environmental conditions. Investigation on the effect of various PCMs on reducing SCA under different conditions favors the safe utilization of Cd-contaminated soil. Herein, a meta-analysis of literature published before August 2021 was carried out. A total of 342 independent observations were obtained from 42 published papers which included 9 factors that may affect the passivation effect of fertilizer content: phosphorus type, phosphorus application rate, soil pH, soil CEC, soil organic matter, experiment type, and time. Results of boosted regression tree analysis showed that the application rate is the most important factor contributing to the SCA, followed by soil pH and duration. Results of this meta-analysis showed that medium P input shows potential for reactivating the SCA. Under alkaline soil conditions and high soil CEC values, PCM input can better deactivate SCA. In addition, the difference from the previous understanding is that under the medium input of phosphorus-containing fertilizer (90-500 mg P∙kg^-1), it will significantly increase the content of available cadmium in soil. In addition, future recommendation for exploring novel PCMs and suitable strategies for controlling the SCA though PCM application were also proposed. Our works may promote the interpretation of the interference factors on the SCA changes and fill the research gaps on utilization of PCM in Cd-polluted soil remediation.

Biogeochemical Behavior of Lead and Nickel as Influenced by Phosphatic Fertilizer Applied to Rice (Oryza sativa L.) Cultivars Grown under City Effluent Irrigation

The hydrology of arid to semi-arid regions is substantially different from that in humid regions due to scarce hydrological data distributions with dry climates and strong evapo-transpirative conditions. In the perspective of global water shortage, food security for all of the living beings has become a matter of great concern. Efficient use of water resources both in urban and rural environments and application of non-conventional water resources for irrigation are becoming increasingly important. In order to sustain crop production, the re-use of treated wastewater for irrigation of crops could be a good option. A pot experiment was set up to evaluate effects of different doses of di-ammonium phosphate (DAP) fertilizer on lead (Pb) and nickel (Ni) phyto-availability by two cultivars of rice irrigated with city effluent. Experiment was conducted in a completely randomized design (CRD) each with three replications. The results showed effective immobilization of Pb with applied phosphatic fertilizer. Among all of the tested treatments, the most effective treatment to reduce phyto-availability of Pb was T4 (248 kg P ha−1) due to antagonistic interaction. While Ni showed inconsistent behavior with both synergistic and antagonistic interaction (biphasic) to applied phosphorus (P) rates. Data regarding various growth parameters such as plant height, number of tillers, shoot and root dry weights, straw and grain yields, and physiological attributes such as total chlorophyll contents, photosynthetic and transpiration rates showed significant (p ≤ 0.05) responses to P application. An increasing trend was revealed in determined parameters with increased P application rates, with the exception of decreased plant height. The conclusion of the present remedial approach indicated that addition of P can significantly reduce the risks of rice grown under city effluent irrigation containing high Pb and Ni contents.

Lead immobilization in simulated polluted soil by Douglas fir biochar-supported phosphate

This study compared the lead (Pb²⁺) immobilization efficacy of biochar-supported phosphate to conventional in-situ heavy metal immobilization methods (with lime, neat biochar and phosphate). The biochar-supported phosphate was obtained by treating Douglas fir biochar (BC) with anhydrous calcium chloride and potassium dihydrogen phosphate. The amount of Pb²⁺ immobilized was determined by comparing the concentration of ammonium nitrate extractable Pb²⁺ lead from lead-spiked soil (without amendment) to that of a 30 d incubation with (a) lead-spiked soil plus 5% (wt./wt.) biochar supported-phosphate, (b) lead-spiked soil plus 5% (wt./wt.) untreated Douglas fir biochar, (c) lead-spiked soil plus 5% (w/w) lime and (d) lead-spiked soil plus 5% (wt./wt.) potassium dihydrogen phosphate. The control (lead-spiked soil without amendment) produced the largest quantity (96.08 ± 9.22 mg L^-1) of NH₄NO₃-extractable Pb²⁺, while lead-spiked soil treated with 5% (wt./wt.) biochar-supported phosphate resulted in the lowest quantity of NH₄NO₃ extractable Pb²⁺ (0.3 ± 0.2 mg L^-1). The mechanism for immobilization of Pb²⁺ by BP occurs at pH < 7 through dissolution of hydroxyapatite embedded in BP during modification, followed by precipitation of insoluble Pb₁₀(PO₄)₆(OH)₂. The residual lead fraction in the lead-spiked soil increased by 20.9% following amendment with BP. These results indicate that biochar-supported phosphate is a candidate to reduce lead mobility (bioavailability) in polluted soil. This amendment may lower Pb²⁺ uptake into plants while minimizing the potential for water contamination due to Pb²⁺mobility.

Investigating triple superphosphate for lead removal from aqueous solutions

The aim of this research is to investigate monocalcium phosphate monohydrate [Ca(H₂PO₄)_2. H₂O] also called triple superphosphate (TSP) for the removal of lead (Pb) from aqueous solutions. In this study, TSP was selected amongst various phosphate-based materials and fertilizers to act as the source of orthophosphate (PO₄^3-) which is a powerful tool for metal fixation in soil and water. Thermodynamic equilibrium dissolution-precipitation relationships for the systems of Pb-H₂O and Pb-PO₄^3--H₂O were drawn with the aid of Eh-pH stability diagrams to determine the predominance areas of different species. The lead phosphate compounds, identified through the stability area diagrams, were verified with the batch precipitation tests performed with standard solutions of lead and TSP at different conditions. It was observed that, depending upon solution conditions, TSP can precipitate 99.9% of the lead from the solution. Lead precipitates, analyzed by x-ray diffraction, showed the formation of lead phosphate compounds. The mechanism of TSP for the removal of lead from aqueous solutions is discussed.

Arbuscular Mycorrhizal Fungi Are an Influential Factor in Improving the Phytoremediation of Arsenic, Cadmium, Lead, and Chromium

The increasing expansion of mines, factories, and agricultural lands has caused many changes and pollution in soils and water of several parts of the world. In recent years, metal(loid)s are one of the most dangerous environmental pollutants, which directly and indirectly enters the food cycle of humans and animals, resulting in irreparable damage to their health and even causing their death. One of the most important missions of ecologists and environmental scientists is to find suitable solutions to reduce metal(loid)s pollution and prevent their spread and penetration in soil and groundwater. In recent years, phytoremediation was considered a cheap and effective solution to reducing metal(loid)s pollution in soil and water. Additionally, the effect of soil microorganisms on increasing phytoremediation was given special attention; therefore, this study attempted to investigate the role of arbuscular mycorrhizal fungus in the phytoremediation system and in reducing contamination by some metal(loid)s in order to put a straightforward path in front of other researchers.

Passive treatment of acid mine drainage from the Sidi-Kamber mine wastes (Mediterranean coastline, Algeria) using neighbouring phosphate material from the Djebel Onk mine

Passive abiotic treatment of acid mine drainage (AMD) was investigated using phosphate mining residuals (raw low-grade phosphate ore, phosphatic limestone wastes, and phosphate mine tailings) from the Djebel Onk mine, Algeria. Laboratory batch tests were performed using the main expected lithologies of phosphate materials in contact with synthetic AMD, which had a low pH (3.08) and contained high concentrations of Fe (600 mg/L), Mn (40 mg/L), Mg (10 mg/L), Zn (20 mg/L), Cu (25 mg/L), As (50 mg/L), and sulfate (3700 mg/L). Phosphate materials were used as an oxic limestone drain to evaluate the increase in the pH of the AMD and metal removal by sorption and precipitation mechanisms. The results showed that all phosphatic lithologies were efficient in the passive treatment of AMD. The pH rapidly increased from 3.08 to 8.47 during water-rock interactions. The neutralization potential comparisons also showed that the phosphatic limestone wastes neutralized more acid than other lithologies. In addition, metals were efficiently removed (95.5% to 99.9%) by all materials. The results of batch sorption tests showed that the concentrations of metals in residual leachates did not exceed the Algerian criteria for industrial liquid effluents. Overall, these findings indicate that passive systems using phosphatic materials from the Djebel Onk mine can be effective for AMD treatment. The use of these mine wastes for passive treatment of AMD would allow the development of integrated management strategies for these residual materials in the context of sustainable development of phosphate mining.

Immobilization of lead by amendments in a mine-waste impacted soil: Assessing Pb retention with desorption kinetic, sequential extraction and XANES spectroscopy

Chemical stabilization is an in-situ remediation that uses amendments to reduce contaminant availability in polluted soils. Rates of phosphate, lime, biochar, and biosolids were evaluated as affecting Pb speciation and mobility in soil samples of a mining area located in Vazante, state of Minas Gerais, Brazil. Chemical and mineralogical characterization, desorption kinetics, sequential extraction, leaching evaluation in columns and speciation using X-ray absorption near edge structure were performed. Pb adsorbed on bentonite and on anglesite were the predominant species in the unamended soil. The treatments with phosphate and lime transformed part of the Pb species to pyromorphite. Conversely, part of Pb species was transformed to Pb adsorbed on citrate in the soil amended with biochar, while PbCl₂ was formed in soil samples amended with biosolids. Phosphate and lime increased the Pb extracted in the residual fraction, thus showing that more recalcitrant species, such as pyromorphite, were formed. Biosolids and biochar treatments decreased the Pb in the residual fraction, and the fraction associated to organic matter increased after the addition of biosolids. Phosphate and lime were effective to immobilize Pb and to decrease Pb desorption kinetics, but the organic amendments increased the desorption kinetics of Pb in all rates applied. The soil amended with phosphate decreased the Pb leached in the experiment with leaching columns.

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.

In situ removal of cadmium by short-distance migration under the action of a low-voltage electric field and granular activated carbon

Cd pollution in soil is a global environmental issue of great concern. The secondary release and low removal rate of Cd are obstacles to the use of adsorption techniques. To develop a sustainable and effective remediation technique, low-voltage direct current (DC) and granular activated carbon (GAC) were applied for in situ Cd removal. The results showed that a low-voltage gradient was more favourable than a high-voltage gradient for Cd removal. A voltage gradient of 0.2 V cm^-1 acted as a driving force for Cd migration while limiting the side effects caused by DC. As an auxiliary enhancement measure, polarity exchange was effective in maintaining uniform distributions of soil moisture and temperature as well as a stable pH while improving Cd removal by weakening inhibition caused by OH^- generated at the cathodes. The average removal rates of total and bioavailable Cd were 61.05% and 76.96%, respectively. The potential mobility of Cd in soil was assessed by the mobility factor (M_F). The M_F was lowered from 42.66% to 8.96%, indicating that the risks of Cd mobility were reduced to low levels. The energy consumption and utilization efficiency of the method were 5.65 KWh m^-3 and 11.25, respectively. The energy utilization efficiency was significantly higher than the efficiencies of other methods that use DC to improve Cd removal. The results suggested that the in situ removal of Cd by low-voltage DC and GAC was efficient and avoided the secondary release of Cd.

Assessing the Capability of Chemical Ameliorants to Reduce the Bioavailability of Heavy Metals in Bulk Fly Ash Contaminated Soil

In-situ rehabilitation of fly ash at dumping sites has rarely been addressed for crop production due to growth-related constraints, largely of heavy metal (HM) contamination in soils and crops. Current communication deals with a novel approach to identify a suitable management option for rejuvenating the contaminated soils. In this background, a 60-days incubation experiment was conducted with different fly ash-soil mixtures (50 + 50%, A1; 75 + 25%, A2; 100 + 0%, A3) along with four ameliorants, namely, lime (T1), sodium sulphide (T2), di-ammonium phosphate (T3), and humic acid (T4) at 30 ± 2 °C to assess the ability of different fly ash-soil-ameliorant mixtures in reducing bio-availability of HMs. Diethylenetriaminepentaacetic acid (DTPA)-extractable bio-available HM contents for lead (Pb), cadmium (Cd), nickel (Ni), and chromium (Cr) and their respective ratios to total HM contents under the influence of different treatments were estimated at 0, 15, 30, 45, and 60 days of incubation. Further, the eco-toxicological impact of different treatments on soil microbial properties was studied after 60 days of experimentation. A1T1 significantly recorded the lowest bio-availability of HMs (~49-233% lower) followed by A2T1 (~35-133%) among the treatments. The principal component analysis also confirmed the superiority of A1T1 and A2T1 in this regard. Further, A1T1 achieved low contamination factor and ecological risk with substantial microbial biomass carbon load and dehydrogenase activity. Thus, liming to fly ash-soil mixture at 50:50 may be considered as the best management option for ameliorating metal toxicity. This technology may guide thermal power plants to provide the necessary package of practices for the stakeholders to revive their contaminated lands for better environmental sustainability.

Phytostabilization of a contaminated military site using Miscanthus and soil amendments

Military activities can contaminate productive land with potentially toxic substances. The most common trace metal contaminant on military bases is lead (Pb). A field experiment was begun in 2016 at Fort Riley, KS, in an area with total soil Pb concentrations ranging from 900 to 1,500 mg kg^-1 and near-neutral pH. The main objectives were to test the potential of Miscanthus sp. for phytostabilization of the site and to evaluate the effects of soil amendments on Miscanthus growth, soil-plant Pb transfer, bioaccessibility of soil Pb, and soil health. The experimental design was a randomized complete block, with five treatments and four replications. Treatments were (a) existing vegetation; (b) Miscanthus planted in untilled soil, no amendments; (c) Miscanthus planted in tilled soil; (d) Miscanthus planted in tilled soil amended with inorganic P (triple superphosphate applied at 5:3 Pb:P); and (e) Miscanthus planted in tilled soil amended with organic P (Class B biosolids applied at 45 Mg ha^-1 ). Tilling and soil amendments increased dry matter yields only in the establishment year. Total Pb uptake, plant tissue Pb concentration, and soil Pb bioaccessibility were significantly less in the Miscanthus plots amended with biosolids than the Miscanthus plots with no added P across all 3 yr. Enzyme activities, organic carbon, and microbial biomass were also greater in biosolids-treated plots. Results show that planting-time addition of soil amendments to Pb-contaminated soil supported Miscanthus establishment, stabilized and reduced bioaccessibility of soil Pb, reduced concentration and uptake of Pb by Miscanthus, and enhanced soil health parameters.

Mechanisms for potential Pb immobilization by hydroxyapatite in a soil-rice system

This study aimed to investigate the mechanism of lead (Pb) immobilization by hydroxyapatite (HAP) in a soil-rice system, a pot experiment was conducted using Pb-contaminated soil amended with various rates of HAP and planted with rice (Oryza sativa L.). The Pb species in the soil and rice roots were determined using Pb L3-edge X-ray absorption near edge structure (XANES). Application of HAP increased soil pH and induced the dissolution of phosphate, subsequently promoting the formation of chloropyromorphite, an insoluble Pb species, in the soil. Therefore, the acid soluble and DTPA-extractable Pb concentrations decreased significantly with increasing levels of applied HAP. HAP reduced the retention of Pb in the iron plaque on the root surface at maturity, thereby alleviating Pb uptake by rice roots. The amount of phosphate in roots were increased with increasing rate of application of HAP, but was negatively correlated with Pb in rice stems and leaves. Application of 32 g kg^-1 of HAP triggered the precipitation of Pb₅PO₄Cl in roots, limiting Pb translocation from roots to shoots. In addition, HAP may induce the redistribution of Pb in rice nodes, lowering the transfer factor of Pb from the stem (or leaf) to rice grains. When the rate of application of HAP exceeds 4 g kg^-1, the Pb concentration in brown rice could be reduced to less than the Chinese National Standard of 0.2 mg kg^-1 (GB2762-2017).

Improved heavy metal immobilization of compacted clay by cement treatment

This study examines the use of cement treatment in improving the lead immobilization properties of a compacted clay liner for sanitary landfill applications. The compaction and strength characteristics of the cement treated clay at cement contents of 0%, 3%, 6%, 9%, 12%, 15% and 18% by weight of dry soil are studied via standard compaction and unconfined compressive strength tests. The lead immobilization characteristics of the cement treated clay are also investigated using atomic absorption spectroscopy. The cement contents of 6% and 9% significantly affect the permeability coefficient and lead absorption of the clay. The permeability coefficient of the cement treated clay meets the requirement for a waterproof material in landfill, i.e., <1.49 × 10^-11 m/s. Lead immobilization is shown to increase with increasing cement content. When the lead nitrate solution in the form of Pb²⁺ ions seeps through the cement treated clay, the hydrolysis reaction results in the formation of Ca²⁺ and OH^- ions. The solution with high alkalinity from this reaction dissolves SiO₂ and Al₂O₃ in the clay. The Pb²⁺ ions are therefore absorbed by SiO₂ and Al₂O₃ and Pb₃SiO₅ is formed. As a result, the lead content in the effluent from the cement treated clay is significantly lower than that from untreated clay. The results from this research can be translated into a regulatory framework for managing the contamination dissipation of industrial waste from landfill.

About plant species potentially promising for phytoextraction of large amounts of toxic trace elements

There is no information yet about plant species capable of accumulating many different metals/metalloids. The plants feasible for phytoremediation aims must grow fast, have high biomass, deep roots, and should accumulate and tolerate a range of toxicants in their aerial parts. In our research, greenhouse and field experiments have been performed to investigate accumulation and tolerance of not well-studied trace elements such as Br, Eu, Sc, Th (and also U) in couch grass and wheat. We compared bioaccumulation abilities of the plants with those of some other plant species grown under the same conditions. Additionally, we tested the effects of inoculation of seeds with Cellulomonas bacteria on phytoextraction of the trace elements from contaminated soils. For determination of elements, we used neutron activation analysis and ICP-MS. It was found that couch grass and wheat can grow in heavily contaminated soils and accumulate different toxic trace elements to levels that exceed physiological requirements typical for most plant species. Infection of seeds with bacteria resulted in a significant increase in the uptake of various trace elements and their translocation to upper plant parts. The use of couch grass and/or wheat, either alone or in combination with microorganisms, is a promising way to phytoextract metals/metalloids from contaminated soils.

Phytoremediation of Heavy Metals in Tropical Soils an Overview

The geomorphological characteristics of the materials inherent in tropical soils, in addition to the excessive use of fertilizers and pesticides, industrial waste and residues, and novel pollutants derived from emerging new technologies such as nanomaterials, affect the functionality and resilience of the soil-microorganism-plant ecosystem; impacting phytoremediation processes and increasing the risk of heavy metal transfer into the food chain. The aim of this review is to provide a general overview of phytoremediation in tropical soils, placing special emphasis on the factors that affect this process, such as nanoagrochemicals, and highlighting the value of biodiversity among plant species that have the potential to grow and develop in soils impacted by heavy metals, as a useful resource upon which to base further research.

Study on safe usage of agricultural land in karst and non-karst areas based on soil Cd and prediction of Cd in rice: A case study of Heng County, Guangxi

It is well-known that Cd concentration in the soil, Cd bioavailability, and Cd bioaccumulation in rice (Oryza sativa L.) grains vary greatly in different soil parent materials. Therefore, the classification of agricultural land environmental quality and agricultural land safe usage based on the Cd content in the soil has a wide deviation. By the application of systematic sampling and analysis of soil and rice samples, and in comparison with the different soil parent materials in Heng County, it was found that soil derived from carbonate rocks has higher total Cd and lower mobile Cd proportion than soil from the non-karst areas. This result indicated that soil carbonates raised soil pH and the adsorption of Fe/Mn oxide/hydroxide on Cd significantly reduced the bioavailability of Cd in karst areas. In contrast, acidic soils with relatively lower CaO, TFe₂O₃, Mn and total Cd contents, the grown rice plants accumulated higher Cd in their grains. Further research confirmed significant differences in Cd bioaccumulation abilities in the soil between karst and non-karst areas. On this basis, the bioaccumulation factor prediction models of Cd in rice grains were developed in karst and non-karst areas. According to the total concentration of Cd in topsoil samples obtained from the area survey and the predicted Cd content in rice grains, an agricultural land safe usage scheme was put forward. The results showed that the agricultural land classification method based on Cd concentration in the soil and rice grains was more accurate and scientific than that based on the Cd contents in the soil alone.

Metal immobilization and nitrate reduction in a contaminated soil amended with zero-valent iron (Fe0)

Technologies based on zero-valent iron (Fe⁰) are increasingly being used to immobilize metals in soils and remove metals and nitrate from waters. However, the impact of nitrate reduction on metal immobilization in metal contaminated soils has been poorly investigated so far. Here, different concentrations of Fe⁰ filings (1%, 2% and 5%; wt%) were applied to a metal contaminated soil. The resulting nitrate reduction and metal (Cd and Zn) immobilization was investigated using a column leaching experiment for 12 weeks. Corrosion of Fe⁰ filings and precipitation of Fe oxyhydroxydes (FeOOH) on the surfaces of the filings were observed using SEM-EDS and EMPA-WDS at the end of the experiment. Compared to the untreated soil, total nitrate amounts released were lowered by 47%, 59% and 87% in the presence of 1%, 2% and 5% of Fe⁰, respectively. Concomitantly with nitrate reduction, Cd and Zn concentrations in leachates were strongly alleviated in the presence of Fe⁰, which was partly attributed to the rise of soil pH subsequent to nitrate reduction. More importantly, biotests with Lupinus albus L. revealed that the mechanisms involved in metal immobilization are stable to root-induced acidification. However, Fe⁰ was not efficient to reduce Cd concentration in Lolium multiflorum Lam., indicating that root processes other than acidification may re-mobilize metals.

Growing food in polluted soils: A review of risks and opportunities associated with combined phytoremediation and food production (CPFP)

Innumerable private households and small-scale producers currently operate on polluted soils. Phytoremediation is one of the most cost-effective remediation options but as a stand-alone technology, it is often not lucrative enough to make it appealing for farmers, especially in economically vulnerable regions. Economic incentives are crucial for remediation projects to materialise and synergies can be obtained by integrating phytoremediation with other profitable activities including food production. This review aims to synthesise state-of-the-art scientific data to provide a general understanding of opportunities and risks for sustainable remediation of agricultural soil by the use of combined phytoremediation and food production (CPFP). The results show that strategies based on CPFP may be appropriate options for most pollutants in virtually all climatic or socioeconomic contexts but a number of challenges need to be surpassed. The challenges include remediation-technological issues such as undeveloped post-harvest technology and inadequate soil governance. The need for remediation solutions for polluted fields is increasingly urgent since many farmers currently operate on polluted land and the scarcity of soil resources as the human population continuously increases will inevitably force more farmers to cultivate in contaminated areas. We conclude that, although large scale CPFP has not yet reached technological maturity, appropriate combinations of soil types, plant species/cultivars, and agronomic practices together with thorough monitoring of the pollutants' pathways can potentially allow for safe food production on polluted soil that restricts the transfer of a number of pollutants to the food chain while the soil pool of pollutants is gradually reduced.

Immobilization of Lead and Zinc Leached from Mining Residual Materials in Kabwe, Zambia: Possibility of Chemical Immobilization by Dolomite, Calcined Dolomite, and Magnesium Oxide

Massive amount of highly contaminated mining residual materials (MRM) has been left unattended and has leached heavy metals, particularly lead (Pb) and zinc (Zn) to the surrounding environments. Thus, the performance of three immobilizers, raw dolomite (RD), calcined dolomite (CD), and magnesium oxide (MO), was evaluated using batch experiments to determine their ability to immobilize Pb and Zn, leached from MRM. The addition of immobilizers increased the leachate pH and decreased the amounts of dissolved Pb and Zn to different extents. The performance of immobilizers to immobilize Pb and Zn followed the following trend: MO > CD > RD. pH played an important role in immobilizing Pb and Zn. Dolomite in RD could slightly raise the pH of the MRM leachate. Therefore, the addition of RD immobilized Pb and Zn via adsorption and co-precipitation, and up to 10% of RD addition did not reduce the concentrations of Pb and Zn to be lower than the effluent standards in Zambia. In contrast, the presence of magnesia in CD and MO significantly contributed to the rise of leachate pH to the value where it was sufficient to precipitate hydroxides of Pb and Zn and decrease their leaching concentrations below the regulated values. Even though MO outperformed CD, by considering the local availability of RD to produce CD, CD could be a potential immobilizer to be implemented in Zambia.

Cadmium, copper, lead and zinc accumulation in wild plant species near a lead smelter

Smelting activities have been shown to increase the likelihood of environmental heavy metal accumulation and bioaccumulation potential within relative proximity to smelter sites. This investigation focused specifically on cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) accumulation in 43 wild plant species and corresponding soils near a Pb smelting contaminated area. Soils in the study area had elevated Cd, Cu, Pb and Zn concentrations, with pollution indices ranked as Cd > Pb > Cu > Zn. Aboveground plant heavy metal concentrations ranked Pb > Zn > Cd > Cu, with plants having greater bioconcentration factor (BCF) values for Cd than for Pb, Cu and Zn. Plant Cd BCF averaged 1.42, while Pb, Cu and Zn averaged 0.128, 0.256 and 0.560, respectively. The greatest Cd BCF value was 5.40 for Dendranthema indicum; Cd accumulation for this species has not been reported previously. The greatest shoot Cd concentration (66.5 mg kg^-1) was found in Viola verecunda. Significant correlations existed between plant shoot Cd and Pb concentrations and soil total and DTPA-extractable Cd and Pb. Plant species with greater heavy metal accumulation could potentially be used for phytoextraction in the study area, while those having less accumulation potentially being used to reduce heavy metal flow into the food chain, reducing the health risks associated with elevated heavy metal soil contamination.

Evaluation of Different Types and Amounts of Amendments on Soil Cd Immobilization and its Uptake to Wheat

Using amendments is a cost-effective method to soil cadmium (Cd) remediation, whereas knowledge about how different amendments and rates affect remediation efficiency remains limited. This study aimed to evaluate the impacts of different types and amounts of amendments on soil Cd immobilization and its uptake by plants. Biochar (BC), zeolite (ZE), humic acid (HA), superphosphate (SP), lime (L), and sodium sulfide (SS) were applied at three rates (low, medium, and high) ranging from 0.5 to 5%. The concentration of CaCl₂-extractable Cd was considerably affected by the amendments, except HA, and the high doses achieved better immobilization effects than the low doses did. The addition of amendments decreased weak acid soluble Cd by 4.1-44.0% but slightly increased the fractions of oxidizable and residual Cd. These amendments (except BC and HA dose of 1%) decreased Cd accumulation in grains by 1.3-68.8% and (except SP) in roots by 16.3-65.5% compared with the control. The SP efficiently immobilized Cd but posed a potential soil acidification risk. Moreover, SS treatment increased the soil electrical conductivity (EC) value and restricted the growth of wheat, possibly due to high-salt stress. BC, ZE, and L exerted significant effects on the reduction in available Cd as the application rate increased. These amendments enhanced Cd immobilization mainly by changing Cd availability in soil and influencing its redistribution in different fractions in soil and root uptake by plants. This study concluded that BC-5%, ZE-1%, and L-0.5% can be used for Cd immobilization in acidic or neutral soils.

Simultaneous removal of arsenic, cadmium, and lead from soil by iron-modified magnetic biochar

Effective and economically viable method to remove elevated metal(loid)s from farm and industrial lands remains a major challenge. In this study, magnetic biochar-based adsorbents with Fe₃O₄ particles embedded in a porous biochar matrix was synthesized via iron (Fe) treated biochar or thermal pyrolysis of Fe treated cedar sawdust. Application and separation of the adsorbent to a multi-contaminated soil slurry simultaneously removed 20-30% of arsenic, cadmium and lead within 24 h. Fast removal of multi-metal(loid)s result from the decrease in all operationally defined fractions of metal(loid)s, not limited to the exchangeable fraction. The direct removal of arsenic-enriched soil particles was observed via micro X-ray fluorescence maps. Furthermore, through comparison of biochars with different production methods, it has been found that magnetization after pyrolysis treatment leads to stronger metals/metalloids adsorption with a higher q_e (bound sorbate) than other treatments but pyrolysis after magnetization stabilized Fe oxides on the biochar surface, indicating a higher biochar recovery rate (∼65%), and thus a higher metal(loid)s removal efficiency. The stability of Fe oxides on the surface of biochar is the determining factor for the removal efficiency of metal(loid)s from soil.

Transformation of Pb, Cd, and Zn Minerals Using Phosphates

Heavy metal contamination in soils has become one of the most critical environmental issues. The most efficient in-situ remediation technique is chemical immobilization that uses cost-effective soil amendments such as phosphate compounds to decrease Pb, Cd and Zn accessibility in the contaminated soils. The present study examined the effectiveness of KH2PO4 in immobilizing Pb, Cd and Zn in three samples of contaminated soils collected from ZGH “Bolesław” (Mining and Smelting Plant “Bolesław”). Effectiveness was evaluated using the following methods: a toxicity characteristic leaching procedure (TCLP)-based experiment, sequential extraction, X-ray diffraction analyses (XRD), and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). The most efficient percentage reduction of total leachable metal concentration assessed by TCLP was observed for lead (50%–80%), and the least reduction was observed for zinc (1%–17%). The most effective immobilization of stable compounds assessed by sequential extraction was noted for lead, while the weakest immobilization was noted for cadmium. New insoluble mineral phases were identified by SEM-EDS analysis. Cd, Zn, and Pb formed new stable mineral substances with phosphates. The predominant crystal forms were dripstones and groups of needles, which were easily formed by dissolved carbon rock surfaces containing zinc ions. The alkaline nature of the soil and a large number of carbonates mainly influenced the formation of new structures.

Treatment of Cd2+ and Cu2+ Ions Using Modified Apatite Ore

Apatite ore from Lao Cai (Vietnam) has large reserves and low prices. Its main component is fluorapatite. The purification and modification of apatite ore can produce a material that can be used as an absorbent for heavy metals with high efficiency. The molecular structure, phase component, specific surface area, element component, and morphology of modified apatite ore from Lao Cai province, Vietnam, were characterized by IR, XRD, BET, EDX, and SEM methods. The IR and XRD results show that the modified process transformed apatite ore from fluorapatite to nanohydroxyapatite. The specific surface area of modified apatite ore (100.79 m2/g) is much higher than the original ore (3.97 m2/g). The modified apatite ore was used to adsorb Cd2+ and Cu2+ ions in water. The effect of adsorbent mass, pH, contact time, and initial concentration of Cd2+ and Cu2+ on the adsorption efficiency and capacity was investigated. Besides, the isotherm adsorption model was determined using Freundlich and Langmuir theories.

Amino-modified Graphene Oxide/Fe3O4 for Dispersive Solid-Phase Extraction of Cadmium Ions in Rice, Lentil, and Water Samples

In this study, amino groups were directly coated on reduced graphene oxide sheets and applied for the extraction of cadmium(II) ions from well water, aqueduct (water coming from mountain), lentils and rice prior to measurements by flame atomic absorption spectrometry. The properties of the adsorbent were investigated by field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray, Fourier-transform infrared spectroscopy and a vibrating sample magnetometer. Some parameters related to the adsorption and desorption stages were optimized. After preconcentration, the linear determination range of cadmium(II) was 0.5 - 40 μg L^-1. The limit of quantification, relative standard deviation and preconcentration factor were obtained as 0.5 μg L^-1, 0.39 - 2.18% and 100, respectively.

Impact of spent mushroom substrate on Cd immobilization and soil property

This study aims to evaluate the potential of fresh spent mushroom substrate (SMS) in Cd immobilization and soil improvement, compared with spent mushroom substrate biochar (SMSB) and spent mushroom substrate compost (SMSC). A simulating remediation experiment was conducted with soil at Cd concentration of 0.6, 1.2, 1.8, and 2.4 mg kg^-1 and amendment addition ratio of 0.5%, 1%, 2%, and 4% for 90 days. At the end of incubation, it was found that 4%SMS addition showed the best effect both on Cd immobilization and soil improvement. It decreased Cd exchangeable fraction ratio by 52.77% (16.30% higher than 4%SMSC) and increased residual fraction ratio by 65.28% (36.34% and 49.64% higher than 4%SMSB and 4%SMSC, respectively); increased soil pH, EC, and CEC by 10.43% (3.83% higher than 4%SMSC), 11.54%, and 29.72%; and increased urease activity, sucrase activity, and catalase activity by 125.61% (43.90% and 8.54% higher than 4%SMSB and 4%SMSC, respectively), 79.46% (35.35% and 14.02% higher than 4%SMSB and 4%SMSC, respectively), and 75.68% (29.73% higher than 4%SMSB), compared with control treatment (CK) respectively. The results demonstrate that 4%SMS can be used as amendments for cadmium-contaminated soils.

Phytoremediation Potential of Macrophytes of Urban Waterbodies in Central India

BACKGROUND

India's rapidly increasing population and growing urbanization pose a great challenge for wastewater treatment efforts, leading to increased pollution of surrounding waterbodies.

OBJECTIVES

A field sampling-based study was conducted to analyze water quality, heavy metals and bioconcentration and bioaccumulation in the roots and shoots of naturally growing vegetation in an urban lake, Laxmi Taal. The lake receives domestic sewage from Jhansi city in Central India.

METHODS

Temperature, pH, electrical conductivity, turbidity, and water-soluble ions were measured with appropriate instruments. Plant accumulation of metals was measured with the bioconcentration factor (BCF), the ratio of metal concentration in the root to wastewater. The translocation factor (TF) was estimated as the ratio of metal concentration in the shoot to the root.

RESULTS

Water quality and heavy metal concentrations were found to be within the prescribed limit as per Indian standards IS-2296 "D". In the present study, BCF was assessed to be >1 and the plants Typha angustifolia and Echhornia crassipus were determined to be accumulator plants. The TF study revealed that translocation of all the metals studied were significant, except for manganese (Mn), where concentration was found to be below detection limit.

CONCLUSIONS

The present study validated that Typha angustifolia and Echhornia crassipus could be used for bioremediation purposes in cases of urban waterbodies receiving varying amounts of domestic wastewaters which have relatively limited concentrations of toxic metals.

COMPETING INTERESTS

The authors declare no competing financial interests.

Evaluation of several phosphate amendments on rare earth element concentrations in rice plant and soil solution by X-ray diffraction

The exploitation and smelting of rare earth resources lead to serious pollution of rare earth elements (REEs) in farmland around mining area. The influence of four kinds of phosphate amendments-phosphate rock (PR), superphosphate (SSP), bone char (BC), and calcium magnesium phosphate (CMP)-on the bioavailability of REEs and the uptake and accumulation of 15 types of REE in rice were conducted in this study. Soil solutions were collected at tillering stage, heading stage and maturing stage, and rice was harvested at maturing stage. The mechanism of phosphate amendments reducing the bioavailability of REEs was studied by X-Ray diffraction and ICP-MS. PR treatment inhibited rice growth, but SSP, BC and CMP treatments all promoted rice growth, improved biomass of roots, shoots and grains, and promoted the uptake of phosphorous in rice. When compared with the CK, SSP, BC and CMP reduced the total REE concentrations in rice roots by 82.2%, 67.9% and 89.6%, shoots by 75.4%, 40.1% and 65.5%, grains by 23.8%, 29.0% and 29.3%, respectively. PR, SSP, BC and CMP significantly reduced the concentrations of REEs in the soil solution at three stages of rice growth. Analytic results of X-ray diffraction shows that adding PR, SSP, BC and CMP can lead to the formation of rare earth phosphate in the soil, thus reduce the activity of the REEs in the soil. Because SSP releases H+ during its dissolution, which has the risk of activating REEs in soil, CMP and BC are potential materials for remediation of REE-contaminated soil.

Effects of an Integrated Carbide Slag-Mushroom Dreg-Calcium Superphosphate Amendment on the Stabilization Process of Pb, Cu, Zn and Cd in Contaminated Soils

In the present study, the integrated use of organic and inorganic amendments (CS–MD–CSP) including carbide slag (CS), calcium superphosphate (CSP) and mushroom dreg (MD) are employed for the stabilization of heavy metals in contaminated soils. A response surface methodology combined with immobilization efficiency was employed to evaluate and optimize the proportion of the integration amendments on the stabilization process. The results predicted by a Box–Behnken design indicated that the maximum immobilization efficiencies of soil Pb, Cu, Zn and Cd could be achieve up to 99.88, 96.11, 99.78 and 87.88%, respectively, when the dosage of CS, CSP and MD were set as 4, 1 and 6%, respectively. European Community Bureau of Reference (BCR) speciation analysis indicated that the acid-soluble proportion of Pb, Cu, Zn and Cd in the soils decreased by 72.68, 37.03, 9.36 and 12.94%, respectively. Thus, this CS–MD–CSP integration amendment could potentially be used for the remediation of Pb, Cu, Zn and Cd in contaminated soils.

Bio-organic stabilizing agent shows promising prospect for the stabilization of cadmium in contaminated farmland soil

In situ immobilization of cadmium (Cd) has been considered as a cost-effective and non-disruptive remediation technique for Cd-contaminated soils. In this study, several immobilization approaches were compared in a Cd-contaminated agricultural farmland. The soil was treated with different combinations of the immobilizing agents such as biochar (C), rice straw (RS), lime (L), and engineered bacteria P. putida X4/pIME (B). The plant yield and Cd uptake of lettuce as well as soil Cd fractionations were measured. The Cd content in lettuce leaves and roots decreased by 46.8~67.2% and 36.8~60.2%, respectively. Among the five treatments, combined rice straw, lime, and engineered bacteria treatment showed the lowest Cd concentration in lettuce leaves (0.14 mg/kg) and the highest plant yield (21.5 t/ha). The alleviating effects are assigned to the significant transformation of water soluble and exchangeable Cd to humic substance bound, strong organic bound and residual Cd in the soils. This study suggests that this bio-organic stabilizing agent is more cost-effective than some other immobilization agents reported previously, and shows a great application prospect in improving agriculture production of heavy metal-polluted agricultural soils.

Phosphate amendments for chemical immobilization of uranium in contaminated soil

Uranium (U) contamination is a major environmental problem associated with the mining and processing of nuclear materials for both weapons and power production. When possible, in situ soil remediation techniques are preferable for reducing the risk associated with diffuse low-level U contamination. Uranium is known to form sparingly soluble phosphate compounds that persist in the environment. Therefore, batch experiments were performed to evaluate the efficacy of three phosphate amendments, hydroxyapatite (HA), sodium phytate (IP6) and sodium tripolyphosphate (TPP), to immobilize U in contaminated sediments. The amendments were added at equivalent phosphorus (P) concentrations and then equilibrated under a range of test conditions, with changes in soluble U and P_total monitored at pre-set time intervals. Only HA was effective at reducing the soluble U soil fraction when compared to the control, with IP6 and TPP increasing the soluble U soil fraction. After equilibration, changes in contaminant partitioning in the amended sediments were evaluated using operational extraction methods. Sequential extraction results for HA generally indicated a transfer of U from labile to more recalcitrant phases, while the results for IP6 and TPP were more ambiguous.

Phosphate treatment alleviated acute phytotoxicity of heavy metals in sulfidic Pb-Zn mine tailings

Phytostabilization of sulfidic PbZn tailing landscapes may be one of interim options of tailings management, but which is limited by acute phytotoxicity of heavy metals in the tailings. The present study aimed to investigate the effectiveness of soluble phosphate (i.e., K₂HPO₄) in immobilizing soluble Pb, Cd and Zn and lowering their acute phytotoxicity. The addition of soluble phosphate improved the growth of native plants Acacia chisholmii and survival rate of A. ligulata, where the latter exhibited 100% survival rate. This was in contrast to effects of conventional organic amendment in the tailings on metal solubility (e.g., elevated metal levels in porewater) and plant survival (e.g., only 42%). Organic amendment with mulch did not lower the levels of water-soluble Cd, Pb and Zn and their concentrations in plant tissues after 56 days of plant growth in the treatment. In contrast, the tailings amended with K₂HPO₄ significantly decreased metal concentrations in the porewater and plant tissues by about 80-92% and 56-88%, respectively. The metal immobilization by phosphate was due to the formation of insoluble or sparingly soluble metal (Pb, Cd and Zn)-phosphate minerals in the tailings with circumneutral pH conditions, as revealed by using X-ray diffraction and scanning electron microanalyses. The reduced metal concentrations in roots and shoots of Acacia species after direct root contact with the K₂HPO₄ amended tailings suggested that metals (i.e., Pb, Cd and Zn) were effectively immobilized by the phosphate treatment of the tailings. These findings indicate that addition of high dosage of soluble phosphate may provide a low cost option to treat sulfidic PbZn tailings for rapid phytostabilization of the tailings surface, as an interim option to manage environmental risks of sulfidic PbZn tailings.

Phytoremediation of Heavy Metal-Contaminated Soil by Switchgrass: A Comparative Study Utilizing Different Composts and Coir Fiber on Pollution Remediation, Plant Productivity, and Nutrient Leaching

We investigated the effects of organic amendments (thermophilic compost, vermicompost, and coconut coir) on the bioavailability of trace heavy metals of Zn, Cd, Pb, Co, and Ni from heavy metal-spiked soils under laboratory conditions. To test switchgrass (Panicum virgatum) as a potential crop for phytoremediation of heavy metal from soil, we investigated whether the addition of organic amendments promoted switchgrass growth, and consequently, uptake of metals. Compost is a valuable soil amendment that supplies nutrients for plant establishment and growth, which is beneficial for phytoremediation. However, excess application of compost can result in nutrient leaching, which has adverse effects on water quality. We tested the nutrient leaching potential of the different organic amendments to identify trade-offs between phytoremediation and water quality. Results showed that the amendments decreased the amount of bioavailable metals in the soils. Organic amendments increased soil pH, electrical conductivity (EC), and soil nutrient status. Switchgrass shoot and root biomass was significantly greater in the amended soils compared to the non-amended control. Amended treatments showed detectable levels of heavy metal uptake in switchgrass shoots, while the control treatment did not produce enough switchgrass biomass to measure uptake. Switchgrass uptake of certain heavy metals, and concentrations of some leachate nutrients significantly differed among the amended treatments. By improving soil properties and plant productivity and reducing heavy metal solubility that can otherwise hamper plant survival, organic amendments can greatly enhance phytoremediation in heavy metal-contaminated soils.

The influence of global climate change on the environmental fate of anthropogenic pollution released from the permafrost: Part I. Case study of Antarctica

This article presents a review of information related to the influence of potential permafrost degradation on the environmental fate of chemical species which are released and stored, classified as potential influence in future Antarctic environment. Considering all data regarding climate change prediction, this topic may prove important issue for the future state of the Antarctic environment. A detailed survey on soil and permafrost data permitted the assumption that this medium may constitute a sink for organic and inorganic pollution (especially for persistent organic pollution, POPs, and heavy metals). The analysis of the environmental fate and potential consequences of the presence of pollutants for the existence of the Antarctic fauna leads to a conclusion that they may cause numerous negative effects (e.g. Endocrine disruptions, DNA damage, cancerogenicity). In the case of temperature increase and enhanced remobilisation processes, this effect may be even stronger, and may disturb natural balance in the environment. Therefore, regular research on the environmental fate of pollution is required, especially in terms of processes of remobilisation from the permafrost reserves.

Remediating Potentially Toxic Metal and Organic Co-Contamination of Soil by Combining In Situ Solidification/Stabilization and Chemical Oxidation: Efficacy, Mechanism, and Evaluation

Most soil remediation studies investigated single contaminants or multiple contaminants of the same type. However, in field conditions, soils are often contaminated with potentially both toxic metals and organic pollutants, posing a serious technical challenge. Here, batch experiments were conducted to evaluate the performance of combining in situ solidification/stabilization (ISS) and in situ chemical oxidation (ISCO) for the simultaneous removal of aniline (1000 mg/kg) and Cd (10 mg/kg). All four tested ISS amendments, especially quick lime and Portland cement, promoted in situ chemical oxidation with activated persulfate in contaminated soil. Combined ISS/ISCO remediation effectively removed aniline and reduced the bioavailable Cd content at optimal initial persulfate and ISS amendment concentrations of 1.08 mol/kg and 30 wt% with a seven-day curing time, and significantly reduced leaching. Persulfate inhibited the reduction of the bioavailable Cd content, and ISS amendment with persulfate did not synergistically remediate Cd in co-contaminated soil. Strong alkalinity and high temperature were the main mechanisms driving rapid pollutant removal and immobilization. The reaction of CaO with water released heat, and Ca(OH)₂ formation increased the pH. The relative contributions of heat vs. alkaline activation, as well as the contaminant removal efficiency, increased with ISS amendment CaO content. Combined treatment altered the soil physicochemical properties, and significantly increased Ca and S contents. Activated persulfate-related reactions did not negatively impact unconfined compressive strength and hydraulic conductivity. This work improves the selection of persulfate activation methods for the treatment of soils co-contaminated with both potentially toxic metals and organic pollutants.

Effects of red mud based passivator on the transformation of Cd fraction in acidic Cd-polluted paddy soil and Cd absorption in rice

Highly effective, economical, and replicable ways of Cd-polluted paddy field remediation (in situ) are urgently needed. In this work, a yearlong field experiment (both early and late rice) was conducted to investigate the effects of red mud based passivator [red mud, diatomite, and lime (5:3:2)] on remediation of an acidic Cd-polluted paddy field in Hunan Province. Compared with the control, the addition of red mud based passivator in the early and late rice reduced Cd concentration in each part of the rice plant (with the most significant decrease rate of 59.18% and 72.11% for brown rice in the early rice and late rice seasons respectively). The effect of Cd reduction in the rice plant was persistent in the next growing season. The addition of red mud based passivator also reduced the exchangeable fraction of Cd in the soil and converted the exchangeable fraction into other unavailable fractions. This study demonstrated that the pH in acidic soil increased after the application of red mud based passivator. Furthermore, red mud based passivator had no effect on the concentrations of Olsen-K, Alkaline-N, Olsen-P in the soil, but increased rice grain yield. Overall, the results of this study indicated that the red mud based passivator at 0.6 kg m^-2 could be a recommendation for Cd-polluted acidic paddy soil stabilization, and it would be a suitable method for remediation of Cd-polluted acidic paddy soil.