Remediation of multiple heavy metal-contaminated soil through the combination of soil washing and in situ immobilization

Zinc oxide/graphene oxide nanocomposites specifically remediated Cd-contaminated soil via reduction of bioavailability and ecotoxicity of Cd

From both environment and health perspectives, sustainable management of ever-growing soil contamination by heavy metal is posing a serious global concern. The potential ecotoxicity of cadmium (Cd) to soil and ecosystem seriously threatens human health. Developing efficient, specific, and long-term remediation technology for Cd-contaminated soil is impending to synchronously minimize the bioavailability and ecotoxicity of Cd. In the present study, zinc oxide/graphene oxide nanocomposite (ZnO/GO) was developed as a novel amendment for remediating Cd-contaminated soil. Our results showed that ZnO/GO effectively decreased the available soil Cd content, and increased pH and cation exchange capacity (CEC) in both Cd-spiked standard soil and Cd-contaminated mine field soil through the interaction between ZnO/GO and soil organic acids. Using Caenorhabditis elegans (C. elegans) as a model organism for soil safety evaluation, ZnO/GO was further proved to decrease the ecotoxicity of Cd-contaminated soil. Specifically, ZnO/GO promoted Cd excretion and declined Cd storage in C. elegans by increasing the expression of gene ttm-1 and decreasing the level of gene cdf-2, which were responsible for Cd transportation and Cd accumulation, respectively. Moreover, the efficacy of ZnO/GO in remediating the properties and ecotoxicity of Cd-contaminated soil increased gradually with the time gradient, and could maintain a long-term effect after reaching the optimal remediation efficiency. Our findings established a specific and long-term strategy to simultaneously improve soil properties and reduce ecotoxicity of Cd-contaminated soil, which might provide new insights into the potential application of ZnO/GO in soil remediation for both ecosystem and human health.

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

Reducing toxic element leaching in mine tailings with natural zeolite clinoptilolite

The use of natural zeolite clinoptilolite to reduce the leaching rate of potentially toxic elements such as Cd, Pb, and Mn in soil from mine tailings was studied. Soil from the surroundings of the mine El Bote in Zacatecas, Mexico, was analyzed, and the zeolite was characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, and nitrogen physisorption. An ammonium-exchange method for the zeolite was employed. Leaching experiments using packed columns with polluted soil and zeolite mixtures were carried out and the effect of the pH of the carrier solutions was studied. Incorporation of zeolite in the soil achieved a beneficial increase in pH, from 5.03 to 6.95. The concentration of Cd and Mn was reduced when zeolite was present in the column and the ammonium-modified zeolite with ammonia also enhanced the concentration reduction of metallic species in leachates in a range of 28 to 68%. The first-order model best fits the experimental data, suggesting that the leaching rate is controlled by concentration difference between the liquid and the soil matrix. These results demonstrate the potential for using natural zeolite clinoptilolite to reduce the leaching rate of potentially toxic elements in soil from mine tailings.

Review of soil environment quality in India near coal mining regions: current and future predictions

Production and utilization of coal are one of the primary routes of accumulation of Toxic Elements (TEs) in the soil. The exploration of trends in the accumulation of TEs is essential to establishing a soil pollution strategy, implementing cost-effective remediation, and early warnings of ecological risks. This study provides a comprehensive review of soil concentrations and future accumulation trends of various TEs (Cr, Ni, Pb, Co, Cu, Cd, Zn, Fe, Mn, and As) in Indian coal mines. The findings revealed that average concentrations of Cr, Mn, Ni, Cu, Zn, Pb, and Co surpass India's natural background soil levels by factors of 2, 4.05, 5.32, 1.77, 9.6, and 6.15, respectively. Geo-accumulation index values revealed that 27.3%, 14.3%, and 7.7% of coal mines are heavily polluted by Ni, Co, and Cu, respectively. Also, the Potential Ecological Risk Index indicates that Cd and Ni are primary contaminants in coal mines. Besides, the health risk assessment reveals oral ingestion as the main exposure route for soil TMs. Children exhibit a higher hazard index than adults, with Pb and Cr being major contributors to their non-carcinogenic risk. In addition, carcinogenic risks exist for females and children, with Cr and Cu as primary contributors. Multivariate statistical analysis revealed that TEs (except Cd) accumulated in the soil from anthropogenic sources. The assessment of future accumulation trends in soil TE concentrations reveals dynamic increases that significantly impact both the ecology and humans at elevated levels. This study signifies a substantial improvement in soil quality and risk management in mining regions.

Recent advances in phyto-combined remediation of heavy metal pollution in soil

The global industrialization and modernization have witnessed a rapid progress made in agricultural production, along with the issue of soil heavy metal (HM) pollution, which has posed severe threats to soil quality, crop yield, and human health. Phytoremediation, as an alternative to physical and chemical methods, offers a more cost-effective, eco-friendly, and aesthetically appealing means for in-situ remediation. Despite its advantages, traditional phytoremediation faces challenges, including variable soil physicochemical properties, the bioavailability of HMs, and the slow growth and limited biomass of plants used for remediation. This study presents a critical overview of the predominant plant-based HM remediation strategies. It expounds upon the mechanisms of plant absorption, translocation, accumulation, and detoxification of HMs. Moreover, the advancements and practical applications of phyto-combined remediation strategies, such as the addition of exogenous substances, genetic modification of plants, enhancement by rhizosphere microorganisms, and intensification of agricultural technologies, are synthesized. In addition, this paper also emphasizes the economic and practical feasibility of some strategies, proposing solutions to extant challenges in traditional phytoremediation. It advocates for the development of cost-effective, minimally polluting, and biocompatible exogenous substances, along with the careful selection and application of hyperaccumulating plants. We further delineate specific future research avenues, such as refining genetic engineering techniques to avoid adverse impacts on plant growth and the ecosystem, and tailoring phyto-combined strategies to diverse soil types and HM pollutants. These proposed directions aim to enhance the practical application of phytoremediation and its integration into a broader remediation framework, thereby addressing the urgent need for sustainable soil decontamination and protection of ecological and human health.

Statistical modelling, optimization, and mechanistic exploration of novel ureolytic Enterobacter hormaechei IITISM-SA3 in cadmium immobilization under microbial inclusive and cell-free conditions through microbially induced calcite precipitation

The study aimed to evaluate the potential of a novel isolated ureolytic Enterobacter hormaechei IITISM-SA3 in cadmium bioremoval through MICP. The optimization and modelling of the biotic and abiotic factors affecting the process of mineralization were also performed. In addition, the underlying mechanism of MICP-driven Cd mineralization under microbial-inclusive and cell-free conditions was revealed and supported through the characterization of the bio-precipitates obtained using various characterization techniques. The results indicated that the isolate could remove 97.18% Cd2+ of 11.4 ppm under optimized conditions of 36.86 h, pH 7.63, and biomass dose of 1.75 ml. Besides, the presence and absence of bacterial cells were found to influence both the morphologies and crystalline structures of precipitates. The precipitates obtained under microbial-inclusive conditions showed typical rhombohedral crystalline structures of the composition comprising CaCO3, CdCO3, and 0.67Ca0.33CdCO3. However, the crystalline nature of the precipitate reduced to a nano-sized granular structure in cell-free media. Unlike the cadmium mineralization process under microbial-inclusive media, where bacterial cells serve as nucleation sites for crystallization, the carbonate precipitation effectively captures Cd2+ through co-precipitation, chemisorption, or alternative mechanisms involving interactions between metal ions and CaCO3 under cell-free conditions. The findings presented suggest that using cell-free culture supernatant enriched with carbonate ions provides an avenue that could be harnessed for sustainable metal remediation.

Impacts of atmospheric deposition on the heavy metal mobilization and bioavailability in soils amended by lime

Atmospheric deposition is an important source of heavy metal in agricultural soils, but there is limited research on the mobility of these metals in soil and their impact on soil amendment. Here, we performed a dust incubation experiment in soils in the laboratory and a factorial transplant experiment at three field sites with a gradient of atmospheric deposition to examine the impacts of atmospherically deposited heavy metals (Cu, Cd, and Pb) on the mobility and bioavailability in soils with and without lime applications. Results showed that the atmospherically deposited heavy metals showed high mobility and were primarily presented in the soluble ionic fractions in the wet part and acid-exchangeable and reducible fractions in the dry part of atmospheric deposition. Atmospheric dust addition caused the 2p3/2 and 2p1/2 electrons of Cu atoms in uncontaminated soils to transition the 3d vacant states, resulting in similar copper absorption peaks as atmospheric particles by the observation of X-ray absorption near-edge spectroscopy (XANES). In the field, atmospheric deposition can only increase the mobile fractions in the surface soils, but not in the deeper layers. However, the deposition can increase the soluble and diffusive gradients in thin films (DGT)-measured bioavailable fractions in profile along with the soil depth. Lime applications cannot significantly reduce the mobile fractions of heavy metals in the surface soils exposed to atmospheric deposition, but significantly reduce the heavy metal concentrations in soil solutions and the DGT-measured bioavailable concentrations, particularly in the deeper layer (6-10 cm). The major implication is that atmospherically deposited heavy metals can significantly increase their bioavailable concentrations in the plough horizon of soil and constrain the effects of soil amendments on heavy metal immobilization, thereby increasing the risks of crop uptake.

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

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

Current technologies for heavy metal removal from food and environmental resources

Exposure to heavy metals in water and food poses a significant threat to human well-being, necessitating the efficient removal of these contaminants. The process of urban development exacerbates heavy metal pollution, thereby increasing risks to both human health and ecosystems. Heavy metals have the capacity to enter the food chain, undergo bioaccumulation and magnify, ultimately resulting in adverse effects on human health. Therefore, implementing effective pollution control measures and adopting sustainable practices are crucial for mitigating exposure and associated health risks. Various innovative approaches, including adsorption, ion exchange, and electrochemical technology, are currently being actively investigated to cope with the issue of heavy metal contamination. These innovative methods offer benefits such as efficient recycling, cost-effectiveness and environmental friendliness. In this review, we summarize recent advances for removing heavy metals from water, soil and food, providing valuable guidance for environmental engineers and researchers seeking to address contamination challenges.

Research progress and hotspots on microbial remediation of heavy metal-contaminated soil: a systematic review and future perspectives

Microbial remediation technology has received much attention as a green, ecological, and inexpensive technology, and there is great potential for the application of microbial remediation technology for heavy metals (HMs) contaminated soil alone and in conjunction with other technologies in environmental remediation. To gain an in-depth understanding of the latest research progress, research hotspots, and development trends on microbial remediation of HMs-contaminated soil, and to objectively reflect the scientific contributions and impacts of relevant countries/regions, institutions, and individuals of this field, in this manuscript, ISI Web of Knowledge's Web of Science™ core collection database, data visualization, and analysis software Bibliometrix, VOSviewer, and HistCite Pro were used to collect and analyze the relevant literature from 2000 to 2022, and 1409 publications were subjected to scientometric analyses. It involved 327 journals, 5150 authors, 75 countries/regions, and 2740 keywords. The current progress and hotspots on microbial remediation of HMs-contaminated soil since the twenty-first century were analyzed in terms of the top 10 most productive countries (regions), high-yielding authors, source journals, important research institutions, and hotspots of research directions. Over the past 22 years, China, India, and the USA have been the countries with the most articles. The institution and author with the most publications are the Chinese Acad Sci and Zhu YG, respectively. Journal of Hazardous Materials is the most productive journal. The keywords showed 6 co-occurrence clusters. These findings revealed the research hotspots, knowledge gaps, and future exploration trends related to microbial remediation of HMs-contaminated soil.

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.

Management of arsenic-contaminated excavated soils: A review

Ongoing global sustainable development and underground space utilization projects have inadvertently exposed many excavated soils naturally contaminated with geogenic arsenic (As). Recent investigations have revealed that As in certain excavated soils, especially those originating from deep construction projects, has exceeded regulatory limits, threatening the environment and human health. While numerous remediation techniques exist for treating As-contaminated soil, the unique characteristics of geogenic As contamination in excavated soil require specific measures when leachable As content surpasses established regulatory limits. Consequently, several standard leaching tests have been developed globally to assess As leaching from contaminated soil. However, a comprehensive comparative analysis of these methods and their implementation in contaminated excavated soils remains lacking. Furthermore, the suitability and efficacy of most conventional and advanced techniques for remediating As-contaminated excavated soils remained unexplored. Therefore, this study critically reviews relevant literature and summarize recent research findings concerning the management and mitigation of geogenic As in naturally contaminated excavated soil. The objective of this study was to outline present status of excavated soil globally, the extent and mode of As enrichment, management and mitigation approaches for As-contaminated soil, global excavated soil recycling strategies, and relevant soil contamination countermeasure laws. Additionally, the study provides a concise overview and comparison of standard As leaching tests developed across different countries. Furthermore, this review assessed the suitability of prominent and widely accepted As remediation techniques based on their applicability, acceptability, cost-effectiveness, duration, and overall treatment efficiency. This comprehensive review contributes to a more profound comprehension of the challenges linked to geogenic As contamination in excavated soils.

Mercapto-palygorskite efficiently immobilizes cadmium in alkaline soil and reduces its accumulation in wheat plants: A field study

Cadmium (Cd) contamination in wheat fields has become a major environmental issue in many regions of the world. Mercapto-palygorskite (MPAL) is a high-performance amendment that can effectively immobilize Cd in alkaline wheat soil. However, MAPL as an in-situ Cd immobilization strategy for alkaline wheat soil remains to be evaluated on a field-scale and the underlying mechanisms requires further evaluation. Here, MPAL were used as soil amendment to evaluate their immobilization efficiency on Cd-contaminated alkaline soil in the field experiments. The field experiments showed that MPAL application significantly reduced wheat grain Cd concentration from 0.183 mg/kg to 0.056 mg/kg, with Cd concentration in wheat grain treated with MPAL all falling below the limit value of 0.1 mg/kg as defined in China's food safety standard (GB 2762-2022). The maximal immobilization efficiency of MPAL on soil Cd figured out by diethylenetriaminepentaacetic acid (DTPA) extraction was 61.5%. The mechanisms involved in Cd immobilization by MPAL were mainly related to the enhanced sorption of Cd onto Fe oxides, and the removal of amorphous or free Fe oxides from soil had a substantial impact on Cd immobilization efficiency by MPAL. Furthermore, the antagonistic effect between Mn and Cd uptake may also contribute to the reduction of wheat Cd accumulation after MPAL application. The current research can provide theoretical and technical support for the large-scale application of MPAL in Cd-contaminated wheat fields.

Evaluation of newly designed flushing techniques for on-site remediation of arsenic-contaminated excavated debris

Excavated debris (soil and rock) contaminated with geogenic arsenic (As) is an increasing concern for regulatory organizations and construction stakeholders. Chelator-assisted soil flushing is a promising method for practical on-site remediation of As-contaminated soil, offering technical, economic, and environmental benefits. Ethylenediaminetetraacetic acid (EDTA) is the most prevalent chelator used for remediating As-contaminated soil. However, the extensive environmental persistence and potential toxicity of EDTA necessitate the exploration of eco-compliant alternatives. In this study, the feasibility of the conventional flushing method pump-and-treat and two newly designed immersion and sprinkling techniques were evaluated at the laboratory scale (small-scale laboratory experiments) for the on-site treatment of As-contaminated excavated debris. Two biodegradable chelators, L-glutamic acid-N,N'-diacetic acid (GLDA) and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS), were examined as eco-friendly substitutes for EDTA. Additionally, this study highlights a useful post-treatment measure to ensure minimal mobility of residual As in the chelator-treated debris residues. The pump-and-treat method displayed rapid As-remediation (t, 3 h), but it required a substantial volume of washing solution (100 mL g-1). Conversely, the immersion technique demonstrated an excellent As-extraction rate using a relatively smaller washing solution (0.33 mL g-1) and shorter immersion time (t, 3 h). In contrast, the sprinkling technique showed an increased As-extraction rate over an extended period (t, 48 h). Among the chelators employed, the biodegradable chelator HIDS (10 mmol L-1; pH, 3) exhibited the highest As-extraction efficiency. Furthermore, the post-treatment of chelator-treated debris with FeCl3 and CaO successfully reduced the leachable As content below the permissible limit.

Best management practices for minimizing undesired effects of thermal remediation and soil washing on soil properties. A review

The use of remediated soils as end-of-life materials raises some challenges including policy and regulation, permits and specifications, technological limitations, knowledge and information, costs, as well as quality and performance associated with using them. Therefore, a set of procedures must be followed to preserve the quality and fundamental properties of soil during a remediation process. This study presented a comprehensive review regarding the fundamental impacts of thermal desorption (TD) and soil washing (SW) on soil characteristics. The effects of main operating parameters of TD and SW on the physical, chemical, and biological properties of soil were systematically reviewed. In TD, temperature has a more remarkable effect on physic-chemical and biological characteristics of soil than heating time. Therefore, decrease in temperature within a suitable range prevents unreversible changes on soil properties. In SW, more attention should be paid to extraction process of contaminants from soil particles. Using the right dosage and type of chelating agents, surfactants, solvents, and other additives can help to avoid problems with recovery or treatment using conventional methods. In addition, this review introduced a framework for implementing sustainable remediation approaches based on a holistic approach to best management practices (BMPs), which, besides reducing the risks associated with different pollutants, might provide new horizons for decreasing the unfavourable impacts of TD and SW on soil.

Effect of ultrasound-assisted EDTA and citric acid washing on heavy metal removal, residual heavy metal mobility, and sewage sludge quality

We investigated the effects of ultrasound-assisted ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) washing on heavy metal (HM) removal, residual HM mobility, and sewage sludge quality. EDTA and CA washing of sewage sludge successfully reduced the total concentration of HMs after one round of washing, but the mobility of residual HMs increased significantly. The eluate had a high concentration of HMs and nutrients (nitrogen, phosphorus, potassium, and total organic carbon), although the nutritional content of the sludge remained high. The three-phase ratio of the sludge after six rounds of washing by CA was closest to the ideal three-phase ratio, and the degree of influence on the physical structure of the soil after a land application was reduced, according to the fluctuation of generalized soil structure index (GSSI) and soil three-phase structure distance (STPSD) values. The results indicate that CA as an environmental-friendly washing agent can be the superior choice for sludge HM washing; single washing of sewage sludge may increase the mobility of residual HMs, so multiple washings should be considered for land application of sludge.

Immobilization of cadmium in paddy soil using a novel active silicon-potassium amendment: a field experimental study

The rapid development of industrialization and agriculture has led to extensive environmental issues worldwide such as cadmium (Cd) pollution of paddy soils, posing a potential threat to environmental safety and food health. Therefore, there is an urgent need to reduce the Cd contents in paddy soils. In this study, a newly active silicon-potassium amendment was first prepared from potassium hydroxide-assisted potassium feldspar at a low temperature, and then was used to remediate a contaminated paddy soil by Cd over a long period. The obtained results demonstrated the effectiveness of the applied active silicon-potassium in promoting rice growth in the experimental field. In addition, soil pH values increased to 6.89-7.03, thus decreasing the bioavailability of Cd bioavailability by 8.61-13.7%. The soil enzyme activities and available nutrients (Si, Ca, Mg, N, and P) were also significantly increased. In particular, the Cd contents in the rice grains decreased from 0.279 to 0.179-0.194 mg/kg following the application of the active silicon-potassium amendment, reaching the food crop standard level of China (< 0.2 mg/kg). The detailed remediation mechanisms of the Cd-contaminated paddy soil involved several processes, including ion exchange, ligand complexation, electrostatic attraction, and precipitation. Overall, the active silicon-potassium material is a promising amendment for achieving effective control of Cd-contaminated paddy 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.

Analog soil organo-ferrihydrite composites as suitable amendments for cadmium and arsenic stabilization in co-contaminated soils

Remediation of CdAs co-contaminated soils has long been considered a difficult problem to solve, as Cd and As have distinctly different metallic characters. Amending contaminated soils with traditional single passivation materials may not always work well in the stabilization of both Cd and As. Here, we reported that analog soil organo-ferrihydrite composites made with either living or non-living organics (bacterial cells or humic acid) could achieve stabilization of both Cd and As in contaminated soils. BCR and Wenzel sequential extractions showed that organo-ferrihydrite, particularly at 1 wt% loading, shifted liable Cd and As to more stable phases. Organo-ferrihydrite amendments significantly (p < 0.05) increased soil urease, alkaline phosphatase and catalase enzyme activities. With organo-ferrihydrite amendments, the bioavailable fraction of Cd decreased to 35.3 % compared with the control (65.1 %), while the bioavailable As declined from 29.4 % to 12.4%. Soil pH, microbial community abundance and diversity were almost unaffected by organo-ferrihydrite. Ferrihydrite and organo fractions both contributed to direct Cd-binding, while the organo fraction probably maintained the Fe-bound As via lowering ferrihydrite phase transformation. Compared to pure ferrihydrite, organo-ferrihydrite composites performed better not only in reducing liable Cd and As, but also in maintaining soil quality and ecosystem functions. This study demonstrates the applications of organo-ferrihydrite composites in eco-friendly remediation of CdAs contaminated soils, and provides a new direction in selecting appropriate soil amendments.

Cooperative effect of slow-release ferrous and phosphate for simultaneous stabilization of As, Cd and Pb in soil

The different chemical behavior of anionic As and cationic Cd and Pb makes the simultaneous stabilization of soils contaminated with arsenic (As), cadmium (Cd), and lead (Pb) challenging. The use of soluble, insoluble phosphate materials and iron compounds cannot simultaneously stabilize As, Cd, and Pb in soil effectively due to the easy re-activation of heavy metals and poor migration. Herein, we propose a new strategy of "cooperatively stabilizing Cd, Pb, and As with slow-release ferrous and phosphate". To very this theory, we developed ferrous and phosphate slow-release materials to simultaneously stabilize As, Cd, and Pb in soil. The stabilization efficiency of water-soluble As, Cd and Pb reached 99% within 7d, and the stabilization efficiencies of NaHCO₃-extractable As, DTPA-extractable Cd and Pb reached 92.60%, 57.79% and 62.81%, respectively. The chemical speciation analysis revealed that soil As, Cd and Pb were transformed into more stable states with the reaction time. The proportion of residual fraction of As, Cd, and Pb increased from 58.01% to 93.82%, 25.69 to 47.86%, 5.58 to 48.54% after 56 d, respectively. Using ferrihydrite as a representative soil component, the beneficial interactions of phosphate and slow-release ferrous material in stabilizing Pb, Cd, and As were demonstrated. The slow-release ferrous and phosphate material reacted with As and Cd/Pb to form stable ferrous arsenic and Cd/Pb phosphate. Furthermore, the slow-release phosphate converted the adsorbed As into dissolved As, then the dissolved As reacted with released ferrous to form a more stable form. Concurrently, As, Cd and Pb were structurally incorporated into the crystalline iron oxides during the ferrous ions-catalyzed transformation of amorphous iron (hydrogen) oxides. The results demonstrates that the use of slow-release ferrous and phosphate materials can aid in the simultaneous stabilization of As, Cd, and Pb in soil.

Microbial extracellular polymeric substances alleviate cadmium toxicity in rice (Oryza sativa L.) by regulating cadmium uptake, subcellular distribution and triggering the expression of stress-related genes

Cadmium (Cd) accumulation in crops causes potential risks to human health. Microbial extracellular polymeric substances (EPS) are a complex mixture of biopolymers that can bind various heavy metals. The present work examined the alleviating effects of EPS on Cd toxicity in rice and its detoxification mechanism. The 100 μM Cd stress hampered the overall plant growth and development, damaged the ultrastructures of both leaf and root cells, and caused severe lipid peroxidation in rice plants. However, applying EPS at a concentration of 100 mg/L during Cd stress resulted in increased biomass, reduced Cd accumulation and transport, and minimized the oxidative damage. EPS application also enhanced Cd retention in the shoot cell walls and root vacuoles, and actively altered the expression of genes involved in cell wall formation, antioxidant defense systems, transcription factors, and hormone metabolism. These findings provide new insights into EPS-mediated mitigation of Cd stress in plants and help us to develop strategies to improve crop yield in Cd-contaminated soils in the future.

Impacts of Nanobubbles in Pore Water on Heavy Metal Pollutant Release from Contaminated Soil Columns

This study investigated the release of heavy metals from polluted soil under the pore water flow containing nanobubbles (NBs) to simulate natural ebullition. Three types of NBs (CH₄, H₂, and CO₂) were generated in water and characterized, including bubble size, zeta potential, liquid density, and tension. The flow rate used in column tests was optimized to achieve proper soil fluidization and metal desorption or release. The leachate chemistries were monitored to assess the effect of NBs on conductivity, pH, oxidation-reduction potential (ORP), and dissolved oxygen (DO). The results showed that NBs in the pore water flow were significantly more effective in releasing Pb compared to DI water, with CO₂ NB water being the most effective and H₂ NB water being the least effective. CO₂ NB water was also used to rinse column soil contaminated with four different metals (Pb, Cu, Zn, and Cr), which exhibited different leaching kinetics. Moreover, a convective-dispersion-deposition equation (CDDE) model accurately simulated the leaching kinetics and explained the effects of NBs on the key parameters, such as the deposition rate coefficient (K_d), that affect the released metal transport. The findings could provide new insights into soil pollutant release under ebullition and soil remediation using water wash containing NBs.

Development of a potassium-based soil washing solution using response surface methodology for efficient removal of cesium contamination in soil

The overuse of chelating soil washing agents for removal of heavy metal can release soil nutrients and negatively affect organisms. Therefore, developing novel washing agents that can overcome these shortcomings is necessary. In this study, we tested potassium as a main solute of novel washing agent for cesium-contaminated field soil, owing to the physicochemical similarities between potassium and cesium. Response surface methodology was combined with a four-factor, three-level Box-Behnken design to determine the superlative washing conditions of the potassium-based solution for the removal of cesium from the soil. The parameters that were considered were the following: potassium concentration, liquid-to-soil ratio, washing time, and pH. Twenty-seven sets of experiments were conducted using the Box-Behnken design, and a second-order polynomial regression equation model was obtained from the results. Analysis of variance proved the significance and goodness of fit of the derived model. Three-dimensional response surface plots displayed the results of each parameter and their reciprocal interactions. The washing conditions that achieved the highest cesium removal efficiency (81.3%) in field soil contaminated at 1.47 mg/kg were determined to be the following: a potassium concentration of 1 M, a liquid-to-soil ratio of 20, washing time of 2 h, and a pH of 2.

Heavy metal uptake by plant parts of Populus species: a meta-analysis

Populus species are well documented for being potentially suitable for phytoremediation purposes regarding their accumulation characteristics. However, published results are contradictory. Based on the data gathered during an extensive literature search, we aimed to assess and revise the metal accumulation potential in the root, stem, and leaf of Populus species growing in contaminated soils, with meta-analysis. We evaluated the influences of pollution level, soil pH, and exposure time on the metal uptake patterns. We found accumulations of Cd, Cr, Cu, Pb, and Zn to be significant in each plant part, while that was only moderate for Ni, and limited for Mn. By calculating the soil pollution index (PI), we observed significantly intensive, PI-independent accumulation for Cd, Cr, Cu, Ni, Pb, and Zn. A decrease in soil pH significantly increased the uptake of Mn and significantly decreased the accumulation of Pb in the stem. Metal uptake was significantly influenced by exposure time as well; Cd concentration was significantly decreased in the stem, while concentrations of Cr in the stem and leaf, and Mn in the stem were significantly increased with time. These aforementioned findings support a well-founded metal-and-growth condition-specific application of poplars in phytoremediation processes, also triggering further in-depth assessments to enhance the efficiency of relevant poplar-based technologies.

A review on the clean-up technologies for heavy metal ions contaminated soil samples

The soil contamination with heavy metal ions is one of the grave intricacies faced worldwide over the last few decades by the virtue of rapid industrialization, human negligence and greed. Heavy metal ions are quite toxic even at low concentration a swell as non-biodegradable in nature. Their bioaccumulation in the human body leads to several chronic and persistent diseases such as lung cancer, nervous system break down, respiratory problems and renal damage etc. In addition to this, the increased concentration of these metal ions in soil, beyond the permissible limits, makes the soil unfit for further agricultural use. Hence it is our necessity, to monitor the concentration of these metal ions in the soil and water bodies and adopt some better technologies to eradicate them fully. From the literature survey, it was observed that three main types of techniques viz. physical, chemical, and biological were employed to harness the heavy metal ions from metal-polluted soil samples. The main goal of these techniques was the complete removal of the metal ions or the transformation of them into less hazardous and toxic forms. Further the selection of the remediation technology depends upon different factors such as process feasibility/mechanism of the process applied, nature and type of contaminants, type and content of the soil, etc. In this review article, we have studied in detail all the three technologies viz. physical, chemical and biological with their sub-parts, mechanism, pictures, advantages and disadvantages.

Bioremediation system consisted with Leclercia adecarboxylata and nZVI@Carbon/Phosphate for lead immobilization: The passivation mechanisms of chemical reaction and biological metabolism in soil

Bioremediation is one of the most promising strategies for heavy metal immobilization. A new remediation system was demonstrated in this research, which combined phosphate solubilizing bacteria (PSB) with nZVI@Carbon/Phosphate (nZVI@C/P) composite to remediate lead contaminated soil. Experimental results indicated that the new system (nZVI@C/P + PSB) could effectively convert the labile Pb into the stable fraction after 30 days of incubation, which increased the maximum residual fraction percentage of Pb by 70.58%. The characterization results showed that lead may exist in the forms of Pb₅(PO₄)₃Cl, PbSO₄ and 3PbCO₃·2Pb(OH)₂·H₂O in the soil treated with nZVI@C/P + PSB. Meanwhile, soil enzyme activities and Leclercia abundance were enhanced in the treated soil compared with CK during the incubation time. In addition, the specialized functions (e.g. ABC transporters, siderophore metabolism, sulfur metabolism and phosphorus metabolism) in PSB and nZVI@C/P + PSB group were also enhanced. These phenomena proved that the key soil metabolic functions may be maintained and enhanced through the synergistic effect of incubated PSB and nZVI@C/P. The study demonstrated that this new bioremediation system provided feasible way to improve the efficacy for lead contaminated soil remediation.

Effect of combined aging treatment on biochar adsorption and speciation distribution for Cd(II)

As a passivation material for heavy metals in-situ remediation, biochar (BC) has often been expected to maintain long-term adsorption performance for target pollutants. There is still lack of consensus about the impact of aging processes on biochar properties, particularly with respect to its long-term sorption performance. In this study, the changes to immobilization mechanisms as well as the speciation distribution of Cd(II) triggered by combined aging simulation (dry-wet, freeze-thaw cycle and oxidation treatment) on BC prepared under three levels of pyrolysis temperatures (300, 500 and 700 °C) were investigated. The results showed significant inhibition of aging on adsorption performance with the adsorptive capacity of BC300, BC500 and BC700 for Cd(II) decreased by 31.12 %, 50.63 % and 14.94 %, respectively. However, sequential extraction results indicated little influence of the aging process on the relative fractionation of Cd(II) speciation. The distribution of readily bioavailable, potentially bioavailable and non-bioavailable fractions of Cd(II) on BC showed only minimal changes post-aging. Overall, there was less Cd(II) sorption following aging, but the fractional availability (in relative terms) remained the same. Compared with 300 and 700 °C, the biochar prepared under 500 °C accounted the highest fraction of non-bioavailable Cd(II) (67.23 % of BC500, 59.17 % of Aged-500), and thus showed most promising for Cd(II) immobilization. This study has important practical significance for the long-term application of biochar in real environment.

Synchronous stabilization of As, Cd, and Pb in soil by sustained-release of iron-phosphate

Anionic arsenic (As) exhibits geochemical behavior opposite to those of cationic cadmium (Cd), and lead (Pb), which makes the synchronous remediation of As, Cd, and Pb challenging. The synchronous stabilization of As, Cd, and Pb to form Cd/Pb-phosphate and iron‑arsenic precipitates is a promising strategy. However, the effectiveness of soluble phosphate or iron-based materials is limited by the activation of Cd, Pb, or As, while low mobility hinders insoluble particles. In this study, we developed an amorphous structure that releases iron and phosphate at a sustained rate. Thus, the stabilization efficiencies of NaHCO₃-extractable As, DTPA-extractable Cd and Pb reached 44.6 %, 40.8 %, and 48.1 %, respectively. The proportion of residual fraction of As, Cd, and Pb increased by 12.1 %, 14.5 %, and 36.4 %, respectively, after 28 d. Ferrihydrite was chosen as the soil component to monitor the chemical behavior and speciation transformation of As, Cd, and Pb in the reaction. During the process, the released iron directly reacted with dissolved As to form iron‑arsenic precipitation and phosphate directly reacted with Cd/Pb to form Cd/Pb-phosphate precipitation. Simultaneously, phosphate replaced the adsorbed As and transformed into a dissolved state, which could be re-precipitated with the released iron ions. Thus, this study provides a reliable strategy for the remediation of As, Cd, and Pb combined pollution in soil.

Remediation of Cu and As contaminated water and soil utilizing biochar supported layered double hydroxide: Mechanisms and soil environment altering

Preparing materials for simultaneous remediation of anionic and cationic heavy metals contamination has always been the focus of research. Herein a biochar supported FeMnMg layered double hydroxide (LDH) composites (LB) for simultaneous remediation of copper and arsenic contamination in water and soil has been assembled by a facile co-precipitation approach. Both adsorption isotherm and kinetics studies of heavy metals removal by LB were applied to look into the adsorption performance of adsorbents in water. Moreover, the adsorption mechanisms of Cu and As by LB were investigated, showing that Cu in aqueous solution was removed by the isomorphic substitution, precipitation and electrostatic adsorption while As was removed by complexation. In addition, the availability of Cu and As in the soil incubation experiments was reduced by 35.54%-63.00% and 8.39%-29.04%, respectively by using LB. Meanwhile, the addition of LB increased the activities of urease and sucrase by 93.78%-374.35% and 84.35%-520.04%, respectively, of which 1% of the dosage was the best. A phenomenon was found that the richness and structure of microbial community became vigorous within 1% dosage of LB, which indirectly enhanced the passivation and stabilization of heavy metals. These results indicated that the soil environment was significantly improved by LB. This research demonstrates that LB would be an imaginably forceful material for the remediation of anionic and cationic heavy metals in contaminated water and soil.

Enhanced removal of arsenic and cadmium from contaminated soils using a soluble humic substance coupled with chemical reductant

Soil washing is an efficient, economical, and green remediation technology for removing several heavy metal (loid)s from contaminated industrial sites. The extraction of green and efficient washing agents from low-cost feedback is crucially important. In this study, a soluble humic substance (HS) extracted from leonardite was first tested to wash soils (red soil, fluvo-aquic soil, and black soil) heavily contaminated with arsenic (As) and cadmium (Cd). A D-optimal mixture design was investigated to optimize the washing parameters. The optimum removal efficiencies of As and Cd by single HS washing were found to be 52.58%-60.20% and 58.52%-86.69%, respectively. Furthermore, a two-step sequential washing with chemical reductant NH₂OH•HCl coupled with HS (NH₂OH•HCl + HS) was performed to improve the removal efficiency of As and Cd. The two-step sequential washing significantly enhanced the removal of As and Cd to 75.25%-81.53% and 64.53%-97.64%, which makes the residual As and Cd in soil below the risk control standards for construction land. The two-step sequential washing also effectively controlled the mobility and bioavailability of residual As and Cd. However, the activities of soil catalase and urease significantly decreased after the NH₂OH•HCl + HS washing. Follow-up measures such as soil neutralization could be applied to relieve and restore the soil enzyme activity. In general, the two-step sequential soil washing with NH₂OH•HCl + HS is a fast and efficient method for simultaneously removing high content of As and Cd from contaminated soils.

Green remediation of cadmium-contaminated soil by cellulose nanocrystals

Cellulose nanocrystals (CNC) were used as a novel, green eluent to remediate Cd-contaminated soil in this study. The influence of washing conditions on the removal of Cd, including CNC concentration, pH value, liquid/solid (L/S) ratio, contact time and temperature were investigated. The effect of CNC remediation of Cd-contaminated soil on soil health and the possible remediation mechanism were also explored. The results showed that CNC concentration, pH value and contact time had a significant effect on the removal efficiency of Cd. CNC rapidly removed heavy metals in soil within 30 min. When the pH value of the eluent was 9.0, the removal efficiency of Cd could reach 86.3 %. The eluent mainly removed exchangeable and reducible fractions of Cd, which could effectively reduce the bioavailability of heavy metals. CNC washing had no negative effects on seed growth, species abundance and Shannon index. C-O, -COO^- groups on CNC played an important role in the reaction between CNC and soil Cd, and other oxygen-containing functional groups on CNC could also assist in adsorption, ion exchange and chemical complexation processes. Therefore, cellulose nanocrystals had the potential to remediate heavy metal-contaminated soils in a green and efficient manner.

Variation in the Optical Properties of PEO-Based Composites via a Green Metal Complex: Macroscopic Measurements to Explain Microscopic Quantum Transport from the Valence Band to the Conduction Band

In this study, a green chemistry method was used to synthesize polymer composites based on polyethylene oxide (PEO). The method of the remediation of metal complexes used in this study is an environmentally friendly procedure with a low cost. Zinc metal ion (Zn²⁺)-polyphenol (PPHNL) complexes were synthesized for two minutes via the combination of a black tea leaf (BTL) extract solution with dissolved Zn-acetate. Then, UV-Vis and FTIR were carried out for the Zn-PPHNL complexes in a liquid and solid. The FTIR spectra show that BTLs contain sufficient functional groups (O-H, C-H, C=O, C=C, C-O, C-N, and N-H), PPHNL, and conjugated double bonds to produce metal complexes by capturing the cations of Zn-acetate salt. Moreover, FTIR of the BTL and Zn-PPHNL complexes approves the formation of the Zn-PPHNL complex over the wide variation in the intensity of bands. The UV absorption spectra of BTL and Zn-PPHNL indicate complex formation among tea PPHNL and Zn cations, which enhances the absorption spectra of the Zn-PPHNL to 0.1 compared to the figure of 0.01 associated with the extracted tea solution. According to an XRD analysis, an amorphous Zn-PPHNL complex was created when Zn²⁺ ions and PPHNL interacted. Additionally, XRD shows that the structure of the PEO composite becomes a more amorphous structure as the concentration of Zn-PPHNL increases. Furthermore, morphological study via an optical microscope (OM) shows that by increasing the concentration of Zn-PPHNL in a PEO polymer composite the size of the spherulites ascribed to the crystalline phase dramatically decreases. The optical properties of PEO: Zn-PPHNL films, via UV-Vis spectroscopy, were rigorously studied. The E_g is calculated by examining the dielectric loss, which is reduced from 5.5 eV to 0.6 eV by increasing the concentration of Zn-PPHNL in the PEO samples. In addition, Tauc's form was used to specify the category of electronic transitions in the PEO: Zn-PPHNL films. The impact of crystalline structure and morphology on electronic transition types was discussed. Macroscopic measurable parameters, such as the refractive index and extinction coefficient, were used to determine optical dielectric loss. Fundamental optical dielectric functions were used to determine some key parameters. From the viewpoint of quantum transport, electron transitions were discussed. The merit of this work is that microscopic processes related to electron transition from the VB to the CB can be interpreted interms of measurable macroscopic quantities.

Effects of microwave and plastic content on the sulfur migration during co-pyrolysis of biomass and plastic

In order to reduce the risks of sulfur-containing contaminants present in biofuels, the effects of microwave and content of hydrogen donor on the cracking of C-S bonds and the migration of sulfur were studied by co-pyrolysis of biomass and plastic. The synergistic mechanism of microwave and hydrogen donor was explored from the perspective of deducing the evolution of sulfur-containing compounds based on microwave thermogravimetric analysis. By combining temperature-weight curves, it was found that microwaves and hydrogen radicals promoted the cracking of sulfur-containing compounds and increased the mass loss of biomass during pyrolysis. The mixing ratio of hydrogen donor (plastic) was the key parameter resulting in the removal of sulfur from oil. By adjusting the mixing ratio, the yield of co-pyrolyzed oil was three times higher than that of cow dung pyrolysis alone and the relative removal rate of sulfur reached 73.67%. The relative content of sulfur in the oil was reduced by 73.77% due to the escape of sulfur-containing gases (H₂S, COS and C₂H₅SH) and the formation of sulfate crystals in the char. Microwave selectively heated sulfur-containing organics and hydrogen radicals stimulated the breaking of C-S bonds, which improved the cracking efficiency of the oil. This breaking will provide a theoretical and technological reference for the environmentally friendly treatment of biomass and biofuels.

Decreased in vitro bioaccessibility of Cd and Pb in an acidic Ultisol through incorporation of crop straw-derived biochar

Studies analyzing the in vitro bioaccessibility (BAc) of heavy metals in biochar-amended soils are currently lacking. The present study aimed to assess the metal BAc in Cd- and Pb-spiked acidic Ultisol samples treated individually with 2% (w/w) maize, rice, wheat, soybean, and pea straw-derived biochar. The results indicate that the Cd-BAc simulated in gastric phase (GP) decreased from 78.4% to 66.5-72.3% and the Pb-BAC decreased from 74.3% to 67.2-69.2%; however, the Cd-BAc in the intestinal phase (IP) decreased from 35.6% to 27.9-33.5% and the Pb-BAc decreased from 34.7% to 29.7-32.9% after 120 d of incubation with biochar application compared to the un-amended Ultisol. The Cd- and Pb-BAc in both GP and IP were significantly negatively correlated with soil pH, CEC, and organic carbon (P < 0.05), which increased after biochar application. The soybean straw-derived biochar amendment has the greatest potential to decrease the BAc of Cd and Pb in the GP and IP, owing to the highest level of CEC, SOC, TC and TN among all soil samples. Moreover, the BAc was positively correlated with the exchangeable, and exchangeable + carbonate-bound Cd and Pb fractions (P < 0.05), indicating these fractions had a dominant influence on the BAc of cationic heavy metals. Therefore, crop straw-derived biochar amendment can decrease the BAc of Cd and Pb in acidic Ultisol, and thus mitigate the health risks posed by these metals from incidental ingestion.

Efficient Removal of Heavy Metals from Contaminated Sunflower Straw by an Acid-Assisted Hydrothermal Process

The removal of heavy metals is crucial to the utilization of contaminated biomass resources. In this study, we report an efficient process of hydrothermal conversion (HTC) of sunflower straw (Helianthus annuus L.) to remove heavy metals. The effect of different HTC temperatures and concentrations of HCl additives on heavy metal removal efficiency was investigated. The results revealed that increasing the temperature or concentration of HCl promoted the transfer of heavy metals from hydrochar to liquid products during HTC. The heavy metals removed to the liquid products included up to 99% of Zn and Cd, 94% of Cu, and 87% of Pb after hydrothermal conversion with a temperature of 200 °C and HCl 2%. The species of heavy metals in hydrochars converted from unstable to stable with an increase in temperature from 160 °C to 280 °C. The stable fractions of heavy metals in the acidic condition decreased as the acid concentration increased. This aligns well with the high transfer efficiency of heavy metals from the solid phase to the liquid phase under acidic conditions. The FTIR indicated that the carboxy and hydroxy groups decreased significantly as the temperature increased and the concentration of HCl increased, which promoted the degradation of sunflower straw. A scan electron microscope showed that the deepening of the destruction of the initial microstructure promotes the transfer of heavy metals from hydrochars to liquid phase products. This acid-assisted hydrothermal process is an efficient method to treat biomass containing heavy metals.

Effect of multiple washing operations on the removal of potentially toxic metals from an alkaline farmland soil and the strategy for agricultural reuse

Few studies have carried out soil washing experiments using pot experiments to simulate in situ soil washing operations, particularly for alkaline soils. This study explored the effects of multiple washing operations using pot experiments on the removal efficiencies of potentially toxic metals (PTM) from alkaline farmland soil and the reuse strategy of washed soil for safe agricultural production. The results showed that the removal efficiencies of Cd, Pb, Cu, and Zn after seven washings with a mixed chelator (EDTA, GLDA, and citric acid) were 41.1%, 47.1%, 14.7%, and 26.5%, respectively, which was close to the results of the EDTA treatment. For the alkaline soil studied, the second washing with the mixed chelators most effectively removed PTM owing to the activation of them after the first washing operation. The mixed chelator more effectively increased the proportion of stable fraction of PTM and maintained soil nutrients (e.g., nitrogen content) than EDTA, indicating little disturbance of alkaline soil quality after washing with the mixed chelator. After the amendment of the washed soil, there was no visible difference in the biomass weight of crops from the soils washed with different agents, indicating that the inhibitory effect of both washing agents on plant growth was effectively alleviated. The Cd and Pb contents in Z. mays were below the threshold of Hygienical Standard for Feeds of China (GB 13078-2017) (1 and 30 mg·kg^-1). Moreover, after three cropping operations, the available concentrations of PTM in the soil washed with the mixed chelator were lower than those in the soil washed with EDTA, indicating the value and potential of agricultural reuse of alkaline farmland soil washed with the mixed chelator.

Reduction of cadmium bioavailability in paddy soil and its accumulation in brown rice by FeCl3 washing combined with biochar: A field study

Cadmium (Cd) removal from paddy soil to reduce Cd accumulation in brown rice is essential for agroecology, food safety, and human health. In this study, we demonstrate that ferric chloride (FeCl₃) washing combined with biochar treatment efficiently remediates Cd-contaminated paddy soil in field trials. Our results showed that 30.9 % of total Cd and 41.6 % of bioavailable Cd were removed by the addition of 0.03 M FeCl₃ at a liquid/soil ratio of 1.5:1. The subsequent addition of 1 % biochar further reduced bioavailable Cd by 36.5 and 41.5 %, compared with FeCl₃ washing or biochar treatment alone. The principal component regression analysis showed that the Cd content in brown rice was primarily affected by the bioavailable Cd in soil. The combined remediation contributed to the decreased Cd contents in brown rice by 45.5-62.5 %, as well as a 2.7-11.8 % increase in rice yield. The Cd contents in brown rice decreased to 0.12 and 0.04 mg kg^-1 in two cultivars of rice (Zhuliangyou189 and Zhuliangyou929), lower than the national food safety standard limit value of China (0.2 mg kg^-1). Meanwhile, the combined remediation promoted the restoration of soil pH and organic matter as well as the improvement of available nutrients. This finding suggests that the combination of FeCl₃ washing and biochar is an effective remediation strategy to minimize Cd bioavailability in paddy soil, and improves soil quality, thus contributing to food safety.

A review on the bioleaching of toxic metal(loid)s from contaminated soil: Insight into the mechanism of action and the role of influencing factors

The anthropogenic activities in agriculture, industrialization, mining, and metallurgy combined with the natural weathering of rocks, have led to severe contamination of soils by toxic metal(loid)s. In an attempt to remediate these polluted sites, a plethora of conventional approaches such as Solidification/Stabilization (S/S), soil washing, electrokinetic remediation, and chemical oxidation/reduction have been used for the immobilization and removal of toxic metal(loid)s in the soil. However, these conventional methods are associated with certain limitations. These limitations include high operational costs, high energy demands, post-waste disposal difficulties, and secondary pollution. Bioleaching has proven to be a promising alternative to these conventional approaches in removing toxic metal(loid)s from contaminated soil as it is cost-effective, environmentally friendly, and esthetically pleasing. The bioleaching process is influenced by factors including pH, temperature, oxygen, and carbon dioxide supply, as well as nutrients in the medium. It is crucial to monitor these parameters before and throughout the reaction since a change in any, for instance, pH during the reaction, can alter the microbial activity and, therefore, the rate of metal leaching. However, research on these influencing factors and recent innovations has brought significant progress in bioleaching over the years. This critical review, therefore, presents the current approaches to bioleaching and the mechanisms involved in removing toxic metal(loid)s from contaminated soil. We further examined and discussed the fundamental principles of various influencing factors that necessitate optimization in the bioleaching process. Additionally, the future perspectives on adding omics for bioleaching as an emerging technology are discussed.

Effects of biochar application on the loss characteristics of Cd from acidic soil under simulated rainfall conditions

Biochar is widely used for immobilizing heavy metals in soil as a kind of high-effective passivator. This research conducted incubation and simulated rainfall experiments to study the effects of biochar application on the loss characteristics of runoff and sediment, as well as the transportation of the Cd during the water erosion process. Two rainfall intensities (60 and 120 mm h^-1) and five biochar application rates (0%, 1%, 3%, 5%, and 7%) were considered in the experiment. The result showed that slaking had a greater effect than mechanical stirring in aggregate breakdown of the soil, and the addition of biochar generally increased the sensitivity of the soil to wet stirring, while had no obvious influence on the resistance to slaking. The H₂O and CaCl₂ extractable Cd in soil significantly decreased with the increase of biochar application rate. The runoff yields decreased with the increase of biochar application rate at both the two rainfall intensities, while the eroded sediment generally decreased at the 120 mm h^-1 rainfall intensity. The addition of biochar tended to increase the loss of the middle-sized (1-0.05 mm) aggregates at the 60 mm h^-1 rainfall intensity, whereas reduced their loss at the 120 mm h^-1 rainfall intensity. Biochar application could significantly reduce the concentration of Cd in the runoff and decreased the total loss amount of Cd (sediment+runoff) in most of the cases. Excessively high level (7%) of biochar application may aggravate soil erosion and result in more Cd loss.

A review for recent advances on soil washing remediation technologies

Contaminated soils have caused serious harm to human health and the ecological environment due to the high toxicity of organic and inorganic pollutants, which has attracted extensive attention in recent years. Because of its low cost, simple operation and high efficiency, soil washing technology is widely used to permanently remove various pollutants in contaminated soils and is considered to be the most promising remediation technology. This review summarized the recent developments in the field of soil washing technology and discusses the application of conventional washing agents, advanced emerging washing agents, the recycling of washing effluents and the combination of soil washing and other remediation technologies. Overall, the findings provide a comprehensive understanding of soil washing technology and suggest some potential improvements from a scientific and practical point of view.

Whole cell evaluation and the enzymatic kinetic study of urease from ureolytic bacteria affected by potentially toxic elements

Microbially induced carbonate precipitation (MICP) is a biomineralization process that has various applications in environmental pollution remediation and restoration of a range of building materials. In this study, a ureolytic bacterium, Lysinibacillus sp. GY3, isolated from an E-waste site, was found as a promising catalyst for remediation of heavy metals via the MICP process. This bacterial isolate produced significant amounts of urease and showed a great persistence in immobilization of potentially toxic elements. A reference ureolytic strain, Bacillus megaterium VS1, was selected in order to compare the efficiency of Lysinibacillus sp. GY3. Study on urease localization indicated 80 % more urease activity secreted extracellularly as for Lysinibacillus sp. GY3 compared to B. megaterium VS1. From the investigation on effects of metals on both intra- and extra-cellular urease, it was clear that Lysinibacillus sp. GY3 produced the most stable urease under conditions of metal pressure, especially retaining more than 70 % activity in the presence of 1 g/L Pb²⁺ and Zn²⁺. These results suggest that this isolated microorganism could be promisingly introduced in the MICP process to stabilize complex heavy metal pollutions, with reference to the regulating ability under harsh conditions to stabilize urease activity. This species is so important both for its biological features and environmental impacts. In addition, the present study will bring new insight in the field of metal remediation coupled with enzyme engineered biotechnology.

Mobilization of contaminants: Potential for soil remediation and unintended consequences

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

Recent advances in soil remediation technology for heavy metal contaminated sites: A critical review

With the increasing development of industry and urbanization, heavy metal contaminated sites have become progressively conspicuous, particularly by unreasonable emissions from electroplating, nonferrous metals smelting, mine tailing, etc. In recent years, soil remediation technologies for heavy metal contaminated sites have developed rapidly. New and effective remediation technologies have emerged successively, and more successful practical applications have appeared. Therefore, systematical summarization of the current progress is essential. As a result, in this paper, some mainstream soil remediation technologies for heavy metal contaminated sites, including physical remediation (soil thermal desorption and soil replacement), bioremediation (phytoremediation and microbial remediation), chemical remediation (chemical leaching, chemical stabilization, electrokinetic remediation-permeable reactive barrier, and chemical oxidation/reduction), as well as various combined remediation are comprehensively reviewed. The influencing factors, advantages, disadvantages, remediation mechanism, and practical applications are also deeply discussed. Besides, the corresponding remediation strategies are put forward for the remediation of heavily polluted sites such as the chemical industry, smelting, and tailing areas. Overall, this review will be beneficial for the in-depth understanding and provide references for the reasonable selection and development of soil remediation technology for heavy metal contaminated sites.

Co-high-efficiency washing agents for simultaneous removal of Cd, Pb and As from smelting soil with risk assessment

Soil washing is considered a highly efficient technology due to its higher removal rate of multiple heavy metals from contaminated soil. However, previous studies on Cd, Pb and As washing agents for soils with complex contaminations did not consider the differences in As and Cd/Pb properties, resulting in the lack of effective washing compounds and washing conditions for soils with complex contaminations. Moreover, most traditional washing agents can cause secondary pollution. In this study, HEDTA and lactic acid (LA) treatments resulted in a higher Cd and Pb removal, while 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) was more effective in As removal. Most importantly, a new washing strategy was proposed with a new combined high-efficiency washing agents consisting of HEDP + LA + FeCl₃ with a ratio of 6:3:1. Considering washing efficiency and consumption under optimal washing conditions, i.e. the soil/liquid (S/L) ratio of 1:20 and washing time of 48 h, the rates of Cd, Pb and As removal were 79.93%, 69.84% and 61.55%, respectively. In addition, washing process could influence the speciation of heavy metals, especially oxidizable and residual Cd and Pb fractions, as well as reducible As fraction. The washing process using the new washing agent can significantly reduce the pollution level and health risk of Cd, Pb and As contamination. The results of this study can provide an efficient washing agent for the remediation of heavy metal-contaminated soils at smelting sites, which will help protect human health.

Effect of different washing solutions on soil enzyme activity and microbial community in agricultural soil severely contaminated with cadmium

Soil enzyme activities and microbial communities have a good response to the remediation effect of heavy metal-contaminated soils. To evaluate the effect of three commonly used washing agents, ferric chloride (FC), ethylenediaminetetraacetic acid (EDTA) and ethylenediamine-tetra-methylenephosphonic acid (EDTMP) on soil enzyme activities and microbial community in cadmium (Cd)-contaminated agricultural soil were collected from farmland near a non-ferrous metal smelter. The soil enzyme activities, microbial community, chemical forms of Cd and some physicochemical properties of the soil washed with different washing solutions were determined. The results showed that the three washing solutions had moderate removal efficiencies for Cd in the tested soil and the breakdown product of EDTMP has a certain stabilizing effect on Cd. The geometric mean and the integrated total enzyme activity index showed that soil washing with FC and EDTA was more beneficial to the restoration of biochemical functions than that with EDTMP. After soil washing, the Chao1 index of bacteria increased, and the microbial community structure changed. Pearson correlation analysis and redundancy analysis (RDA) indicated that the three washing solutions affected soil enzyme activities and microbial community by altering soil nutrient, total Cd concentration and Cd fractions in soils.

Source analysis and ecological risk assessment of heavy metals in farmland soils around heavy metal industry in Anxin County

Studying the pollution status, spatial distribution characteristics, and sources of heavy metals in farmland soil in Anxin County will provide a method basis for the next step of soil remediation. This study investigates the contents of Zn, Cu, Pb, Cd, and Ni in wheat grains and soil samples. Moreover, different methods are used to evaluate soil heavy metal pollution. The results show that the soil in the study area is weakly alkaline. Cu, Zn, and Ni contents in the ground are lower than the risk screening values for soil contamination of agricultural land. In comparison, Cd and Pb contents are higher than the screening value of soil pollution risk of agricultural land, and the proportion of points lower than the control value of soil pollution risk of agricultural land are 64.58% and 16.67%, respectively. The farmland with high Cd and Pb content is mainly distributed near roads and factories and concentrated primarily on 0-20 cm topsoil. The Cd content in wheat grains meets the standard, but 4.17% of the samples are close to 0.1 mg kg⁻¹ (more than 0.09 mg kg⁻¹). The Pb content of 50% of the wheat grain samples exceeds the lead limit in the standard. The evaluation results of the single factor pollution index and geoaccumulation index show that the pollution degree of heavy metals in the soil is Cd > Pb > Cu > Zn > Ni. The potential ecological risk index in the study area is 288.83, and the soil heavy metal pollution is at a moderate-considerable ecological risk level. The average value of Cd's single-factor environmental risk index is 233.51, which belongs to the high environmental risk and is the main influencing factor. Cd and Pb in soil are significantly disturbed by the production activities of heavy metal processing enterprises around the farmland. It is speculated that there are two primary sources of soil heavy metal pollution in the study area. Cd, Pb, Zn, and Cu are mainly industrial and mobile sources, and Ni is primarily agricultural and natural sources.

Effect of the Co-Application of Eucalyptus Wood Biochar and Chemical Fertilizer for the Remediation of Multimetal (Cr, Zn, Ni, and Co) Contaminated Soil

Contamination of soil with heavy metals is a worldwide problem, which causes heavy metals to release into the environment. Remediation of such contaminated soil is essential to protect the environment. The aims of this study are: first, to compare the effect of biochar and the joint application of biochar with fertilizer for the phytoremediation of heavy metals-contaminated soil using Acacia auriculiformis; second, to study the effect of the application rate of biochar in improving the physicochemical properties of the soil. The soil samples were collected from an active coal mine dump and assessed for their physicochemical properties and heavy metals toxicity. Initial results indicated that the soil has poor physicochemical properties and was contaminated with the presence of heavy metals such as Zn, Ni, Cu, Cr, and Co. Later, the heavy metals-contaminated soil was mixed with the 400 and 600 °C biochar, as well as the respective biochar–fertilizer combination in varying mixing ratios from 0.5 to 5% (w/w) and subjected to a pot-culture study. The results showed that the application of both varieties of biochar in combination with fertilizer substantially improved the physicochemical properties and reduced the heavy metals toxicity in the soil. The biochar and fertilizer joint application also substantially improved the soil physiochemical properties by increasing the application rate of both varieties of biochar from 0.5 to 5%. The soil fertility index (SFI) of the biochar and biochar–fertilizer amended soil increased by 49.46 and 52.22%, respectively. The plant’s physiological analysis results indicated a substantial increase in the plant’s shoot and root biomass through the application of biochar and biochar–fertilizer compared to the control. On the other hand, it significantly reduced the heavy metals accumulation and, hence, the secretion of proline and glutathione hormones in the plant cells. Therefore, it can be concluded that the joint application of biochar with the application rate varying between 2.5 to 5% (w/w) with the fertilizer significantly improved the physicochemical properties of the soil and reduced the heavy metals toxicity compared to the controlled study.

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

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

Experimental and numerical study on Cu and Cd migration in different functional-area soils under simulated rainfall conditions

Natural rainfall exerts a significant influence on the migration of heavy metals in soil. However, the knowledge of migration characteristics and release kinetics of heavy metals in contaminated soils under different rainfall intensities still remains unclear. In this study, the simulated rainfall of different intensities was designed to experimentally and numerically investigate Cu and Cd movements in different functional-area (agriculture, industrial, urban) soils. A HYDRUS-2D model was optimized to simulate the migration process of Cu and Cd in soil under different rainfall conditions. The hydraulic properties and solute transport parameters used in the model were estimated based on isothermal adsorption and chloride ion penetration experimental measurements and related model fitting. Furthermore, Cu and Cd BTCs (Breakthrough Curves) were fitted using the HYDRUS-2D inverse solution function with two-site model. The results showed that the order of the migration capacity of Cu and Cd in different functional-area soils was agriculture soil > industrial soil > urban soil, and Cd had a greater risk of groundwater pollution than Cu. With the increase of rainfall intensity, the high proportion of the exchangeable state of Cu and Cd in contaminated soil is easy to be released. Furthermore, the model was proved to describe the distribution of Cu and Cd in the soil profile very well. The present results can improve understanding of the environmental behavior of Cu and Cd in different functional-areas soils and can be used as a basis for risk assessment of Cu and Cd polluting groundwater.

Valency distributions and geochemical fractions of arsenic and antimony in non-ferrous smelting soils with varying particle sizes

Arsenic and antimony are common toxic metalloids found in associated minerals. These metalloids generally cause high-concentration pollution in non-ferrous metal smelting soils; however, few studies have investigated the pollution characteristics of these two metalloids at non-ferrous smelting sites using varying soil particle sizes. In this study, the valency distributions and geochemical fractions were investigated with varying soil particle sizes (≤ 0.05, 0.05-0.25, 0.25-1, and 1-2 mm). Soils were mainly concentrated in ≤ 0.05 and 0.05-0.25 mm with mass percentages of 32.97% and 29.02%, respectively. The highest total As and Sb concentrations in ≤ 0.05 mm were found to be 20,350 and 3655 mg/kg, respectively. In addition, As(Ⅲ), As(Ⅴ), Sb(Ⅲ), and Sb(Ⅴ) concentrations in this soil particle size were found to be 224, 19,813, 1036, and 24 mg/kg, respectively. The geochemical fractions of As and Sb in varying soil particle sizes were mainly residual, accounting for 50% and 90% in the ≤ 0.05 mm. Soil may bind ≤ 0.25 mm due to the disparity found in the geochemical compositions and valency distributions of arsenic and antimony. X-ray diffraction and scanning electron microscopy/energy dispersive system analysis confirmed that arsenolite accumulated in particle sizes of ≤ 0.05 and 0.05-0.25 mm. The results of this study may provide a scientific reference for risk assessment and restoration strategies for non-ferrous metal smelting soils.