Removal of heavy metals from polluted soil using the citric acid fermentation broth: a promising washing agent

Preparation of landscape gardening soil using undersized fraction from aged MSW by EDTA or citric acid coupled with humic acid: Effect assessment, properties, and optimization

Undersized fraction from aged municipal solid waste (UFAMSW), as a kind of soil-like material, has been proved effective in providing a large amount of organic matter and nutrients for soil and plants. The characteristics and effectiveness of heavy metal pollution removal in UFAMSW attracted tremendous research interest from scientists recently. In this study, the heavy metal removal efficiencies and bioavailability of washing on contaminated UFAMSW were evaluated with three washing reagents including ethylene diamine tetra acetic acid (EDTA), citric acid (CA), and humic acid (HA). The effects of chelating agent concentration, pH, and washing time on metal removal were investigated and response surface methodology (RSM) was employed to optimize the washing conditions. The results indicated that the removal efficiencies of Cu, Zn, and Mn could be 53.68%, 52.12%, and 30.63% by EDTA/HA washing and 42.36%, 39.67% and 28.49% by CA/HA washing, respectively. The European Community Bureau of Reference (BCR) sequential extraction was applied to analyze the fraction change of heavy metals in UFAMSW before and after washing, and it was found that chelating agent combined with HA could contribute to the removal of the exchangeable fraction. Physical and chemical properties of UFAMSW were improved to some extent after washing with mixed HA and chelating agent and could achieve the quality standard of landscape gardening soil. Accordingly, the mixture of HA and other chelating agents could be a promising washing process for preparation of landscape gardening soil using UFAMSW.Implications: Our manuscript studies the removal of heavy metals from the contaminated undersized fraction from aged municipal solid waste (UFAMSW). UFAMSW, as a kind of soil-like material, has been proved effective in providing a large amount of organic matter and nutrients for soil and plants however often limited by heavy metal pollution. The UFAMSW used in this experiment was collected after the excavation and screening-sorting of aged refuse from Changshankou Domestic Waste Sanitary Landfill in Wuhan City, Hubei Province, Southern China. This study investigated the effects of EDTA, CA, HA, mixed EDTA/HA, and mixed CA/HA washing on heavy metal removal (Cu, Zn, and Mn), bioavailability of residual heavy metal and properties. The effects of chelating agent concentration, pH, and washing time on metal removal were investigated and then response surface methodology was employed to optimize the washing conditions. The results showed that washing by CA/HA and EDTA/HA, had a higher removal efficiency of heavy metals (Cu, Zn, and Mn) in UFAMSW compared to single HA. Meanwhile, HA has a higher removal for exchangeable fraction of heavy metals, the exchangeable concentration of Cu, Zn, and Mn in CA/HA and EDTA/HA washed UFAMSW were lower compared with UFAMSW washed by single CA and EDTA. Thus, mixing HA with EDTA or CA makes a less risk to environmental and the removal efficiency is acceptable. Additionally, CA/HA and EDTA/HA washing tend to improve soil physicochemical properties and soil fertility. Thus, mixing HA with different washing agent are potential methods for preparation of landscape gardening soil using UFAMSW.

Nanoparticles modulate heavy-metal and arsenic stress in food crops: Hormesis for food security/safety and public health

Heavy metal and arsenic (HM-As) contamination at the soil-food crop interface is a threat to food security/safety and public health worldwide. The potential ecotoxicological effects of HM-As on food crops can perturb normal physiological, biochemical, and molecular processes. To protect food safety and human health, nanoparticles (NPs) can be applied to seed priming and soil amendment, as 'manifestation of hormesis' to modulate HM-As-induced oxidative stress in edible crops. This review provides a comprehensive overview of NPs-mediated alleviation of HM-As stress in food crops and resulting hormetic effects. The underlying biochemical and molecular mechanisms in the amelioration of HM-As-induced oxidative stress is delineated by covering the various aspects of the interaction of NPs (e.g., magnetic particles, silicon, metal oxides, selenium, and carbon nanotubes) with plant microbes, phytohormone, signaling molecules, and plant-growth bioregulators (e.g., salicylic acid and melatonin). With biotechnical advances (such as clustered regularly interspaced short palindromic repeats (CRISPR) gene editing and omics), the efficacy of NPs and associated hormesis has been augmented to produce "pollution-safe designer cultivars" in HM-As-stressed agriculture systems. Future research into nanoscale technological innovations should thus be directed toward achieving food security, sustainable development goals, and human well-being, with the aid of HM-As stress resilient food crops.

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.

Effect of citric acid on phytoextraction potential of Cucurbita pepo, Lagenaria siceraria, and Raphanus sativus plants exposed to multi-metal stress

Phytoextraction is a novel technique that involves using plants to remove heavy metals from contaminated soils. An outdoor pot experiment was designed to evaluate the phytoextraction potential of three plant species Cucurbita pepo, Lagenaria siceraria, and Raphanus sativus in soil contaminated with multiple metals (Cd, Co, Cr, Cu, Ni, Pb, and Zn) under the application of citric acid. The results showed that Raphanus sativus, out of all the studied plants, had the highest root and shoot dry weight and the capacity to accumulate all heavy metals at higher concentrations except for Cu. The application of citric acid into the polluted soil significantly increased plant growth, biomass, and heavy metal uptake. High bioconcentration values indicate that Raphanus sativus is a promising plant for absorbing and accumulating Cd and Ni from the soil. The maximum values of bioconcentration were also observed by the application of citric acid. The values of metal translocation from the root to the shoot were varied by plant species and the citric acid application. Regarding the biomass, metal content, as well as removal metal percentage values, it became apparent that the Raphanus sativus plant was the most effective crop in removing heavy metals from multi-metal contaminated Soil. Generally, these findings emphasize that the application of citric acid could be a useful approach to assist Cd and Ni phytoextraction by Raphanus sativus plants. When these plants are growing as vegetable crops, more attention should be given to evaluating the heavy metal content in them, especially when adding citric acid to their soil through fertigation systems to avoid food chain contamination.

Investigating the use of Aspergillus niger fermentation broth as a washing treatment for arsenic and antimony co-contaminated soil

High concentrations of arsenic and antimony contamination in soil are a potential risk to the ecological environment and human health. Soil washing can effectively and permanently reduce the soil contamination. This study used Aspergillus niger fermentation broth as a washing agent to remove As and Sb from contaminated soil. Characterization of organic acids in the fermentation broth by high-performance liquid chromatographic (HPLC) and chemically simulated leaching experiments revealed that oxalic acid played a significant role in removing As and Sb from the soil. The effect of washing conditions on the metal removal rate of Aspergillus niger fermentation broth was investigated by batch experiments, and the optimal conditions were determined: no dilution, pH 1, L/S ratio 15:1, and leaching at 25 °C for 3 h. The soils were washed three times under optimal conditions, with 73.78%, 80.84%, and 85.83% removal of arsenic and 65.11%, 76.39%, and 82.06% removal of antimony, respectively. The results of metal speciation distribution in the soil showed that the fermentation broth could effectively remove As and Sb on amorphous Fe/Al hydrous oxides in soil. The analysis of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) of soils before and after washing showed that the washing of Aspergillus niger fermentation broth had a minor effect on the structural changes of soils. After washing, soil organic matter and soil enzyme activity were increased. Thus, Aspergillus niger fermentation broth shows excellent potential as a washing agent for removing As and Sb from soils.

Green remediation of Ni, Zn, and Cu in an electroplating contaminated site by wood vinegar with optimization and risk assessment

Wood vinegar (WV) is a renewable organic compound, possessing characteristics such as high oxygenated compound content and low negative impact on soil. Based on its weak acid properties and complexing ability to potentially toxic elements (PTEs), WV was used to leach Ni, Zn, and Cu contaminated soil in electroplating sites. In addition, the response surface methodology (RSM) based on the Box-Behnken design (BBD) was established to clarify the interaction between each single factor, and finally completed the risk assessment of the soil. The amounts of PTEs leached from the soil climbed with the increase of WV concentration, liquid-solid ratio, and leaching time, while they surged with the decrease of pH. Under optimal leaching circumstances (the concentration of WV= 100 %; washing time= 919 min; pH= 1.00), the removal rates of Ni, Zn, and Cu could reach 91.7 %, 57.8 %, and 65.0 %, respectively, and the WV-extracted PTEs were mainly from the Fe-Mn oxides fraction. After leaching, the Nemerow integrated pollution index (NIPI) decreased from an initial value of 7.08 (indicating severe pollution) to 0.450 (indicating no pollution). The potential ecological risk index (RI) dropped from 274 (medium level) to 39.1 (low level). Additionally, the potential carcinogenic risk (CR) values reduced by 93.9 % for both adults and children. The results revealed that the washing process significantly reduced the pollution level, potential ecological risk, and health risk. Coupled with FTIR and SEM-EDS analysis, the mechanism of WV removal of PTEs could be explained from three aspects: acid activation, H⁺ ion exchange, and functional group complexation. In summary, WV is an eco-friendly and high-efficiency leaching material for the remediation of PTEs polluted sites, which will maintain soil function and protect human health.

Removal of Cd from contaminated farmland soil by washing with residues of traditional Chinese herbal medicine extracts

Soil washing is one of the effective methods for permanent removal of heavy metals from farmland soil, and selection of washing agents determines heavy metal removal efficiency. However, there is still a lack of cost-efficient and eco-friendly washing agents. In this study, three residues of traditional Chinese herbal medicine (RTCHM) extracts: residues of Prunus mume (Sieb.) Sieb. et Zucc. (RPM), residues of Schisandra chinensis (Turcz.) Baill. (RSC), and residues of Crataegus pinnatifida Bunge (RCP), were tested for their potential of Cd removal. The variations in amounts and compositions of dissolved organic carbon (DOC) and citric acid were responsible for the difference in Cd removal efficiencies of RTCHM extracts. Fourier-transform infrared spectrophotometer (FTIR) analysis showed that hydroxyl, carboxyl, and amine were the main functional groups of RTCHM extracts to chelate with heavy metals. The optimum conditions for RTCHM extracts were 100 g L^-1 concentration, solid-liquid ratio 1:10, pH 2.50, and contact time of 1 h, and the highest Cd removal efficiencies of RPM, RSC, and RCP extracts reached 35%, 11%, and 15%, respectively. The ecological risk of Cd decreased significantly due to the decrease of exchangeable and reducible Cd fractions. RTCHM extracts washing alleviated soil alkalinity and had little effect on soil cation exchange capacity. Meanwhile, the concentrations of soil organic matter and nitrogen were enhanced significantly by RPM extracts and the activities of soil catalase and urease were also improved. Overall, among the tested extracts, RPM extracts was a much more feasible and environment-friendly washing agent for the remediation of Cd-contaminated farmland soil.

Leaching and characterization studies of heavy metals in contaminated soil using sequenced reagents of oxalic acid, citric acid, and a copolymer of maleic and acrylic acid instead of ethylenediaminetetraacetic acid

In this work, the removal performance of three environmentally friendly reagents, oxalic acid (OA), citric acid (CA), and a copolymer of maleic and acrylic acid (PMAA), on heavy metals in polluted soil was studied at the optimum conditions and compared their sequenced performance. The results showed that the consecutive washing with the individual acids significantly improved the removal percentage of heavy metals in the soil compared to that of EDTA (10.2%, 71.3%, 29.8%, 61.6%, and 52.4% removal for As, Cd, Cu, Pb, and Zn, respectively). The removal of As, Cd, Cu, Pb, and Zn in the sequence of CA-OA was 65.6%, 79%, 59.1%, 64.6%, and 63.5%, respectively. In addition, the organic acids had little influence on the soil physicochemical properties after washing with slight reductions of acidity (pH) and soil organic matter (SOM), which are the major determinants of the usability of washed soils for plant growth. The germination rate of Sorghum bicolor in CA-OA-washed soils reached over 70% on the 7th day. CA-OA-washed soils collectively stand out in using washed soils for plant growth with the following advantages: simultaneous removal of cationic and anionic metals, less harmful impact on soil properties, and successful support for the germination of crops. Based on the findings, we recommend the CA-OA sequence as the best alternative to EDTA with higher metal removal efficiency and germination success.

Tea saponin enhanced bioleaching of Fusarium solani to remove hexavalent chromium from soil

Hexavalent chromium pollution is one of the most serious types of site pollution. In this study, a microorganism was screened to remove most hexavalent chromium from soil by leaching in 24 h. After ITS sequencing, the microorganism was identified as belonging to the genus Fusarium solani. The optimization experiment of leaching conditions determined that the removal rate reached the maximum 80% when the rotation speed was 200 rpm, the liquid-soil ratio was 15:1, the temperature was 35℃, and the pH was 7. The study has also shown that tea saponin can effectively strengthen the leaching of Fusarium solani to remove hexavalent chromium from the soil. Compared with tea saponin, the strengthening effect of glucose and rhamnolipid was relatively small. The removal rate of hexavalent chromium reached 85% when the added amount of tea saponin was 0.02 g/mL. The leaching solution destroyed part of the iron-manganese nodule structure of the soil, and its hydroxyl, carboxyl, and other groups complexed metal ions into the solution to achieve the purpose of removing hexavalent chromium. However, since the main crystal of the soil was SiO₂, there was no obvious change in the XRD of the soil. Toxicity test showed that after leaching, the content of hexavalent chromium leached was 0.28 mg/L (< 1.5 mg/L), which meet the entry standard of the landfill site.

Heavy metal removal using an advanced removal method to obtain recyclable paper incineration ash

Various agents, including ethylenediaminetetraacetic acid, oxalic acid, citric acid, and HCl, were applied to remove heavy metals from raw paper incineration ash and render the ash recyclable. Among these prepared agent solutions, ethylenediaminetetraacetic acid showed the highest efficiency for Pb removal, while oxalic acid showed the highest efficiencies for Cu, Cd, and As removal. Additionally, three modes of an advanced removal method, which involved the use of both ethylenediaminetetraacetic acid and oxalic acid, were considered for use at the end of the rendering process. Among these three modes of the advanced removal method, that which involved the simultaneous use of ethylenediaminetetraacetic acid and oxalic acid, i.e., a mixture of both solutions, showed the best heavy metal removal efficiencies. In detail, 11.9% of Cd, 10% of Hg, 28.42% of As, 31.29% of Cu, and 49.19% of Pb were removed when this method was used. Furthermore, the application of these three modes of the advanced removal method resulted in a decrease in the amounts of heavy metals eluted and brought about an increase in the CaO content of the treated incineration ash, while decreasing its Cl content. These combined results enhanced the solidification effect of the treated incineration ash. Thus, it was confirmed that the advanced removal method is a promising strategy by which recyclable paper incineration ash can be obtained.

Efficient Inorganic/Organic Acid Leaching for the Remediation of Protogenetic Lead-Contaminated Soil

In this study, inorganic acid and organic acid were used to leach and remediate superheavy, lead-contaminated protogenetic soil with a lead pollution level of 8043 mg∙kg−1. Among the compounds studied, HCl and citric acid (CA) presented the best effects, respectively. Under the optimal experimental conditions, the remediation efficiency of 0.05 mol∙L−1 CA reached 53.6%, while that of 0.2 mol∙L−1 HCl was 70.3%. According to the lead morphology analysis, CA and HCl have certain removal ability to different fractions of lead. Among them, the removal rates of acid-soluble lead in soil by HCl and CA are 93% and 83%, and the soil mobility factor (MF) value decreased from 34.4% to 7.74 % and 12.3%, respectively, indicating that the harm of lead in soil was greatly reduced. Meanwhile, the leaching mechanisms of CA and HCl were studied. The pH values of the soil after leaching with HCl and CA were 3.88 and 6.97, respectively, showing that HCl leaching has caused serious acidification of the soil, while the process of CA leaching is more mild. CA has a relatively high remediation efficiency at such a low concentration, especially for the highly active acid-soluble fraction lead when maintaining the neutrality of the leached soil. Hence, CA is more suitable for the remediation of lead-contaminated soil.

Newly-synthesized iron-oxide nanoparticles showed synergetic effect with citric acid for alleviating arsenic phytotoxicity in soybean

In the current investigation, we presented the success of the modified hydrothermal method for synthesizing the iron-oxide nanoparticles (Fe2O3-NPs) efficiently. These NPs were further characterized by using different techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) micrographs, energy-dispersive X-ray spectroscopy (EDAX)/Mapping pattern, Raman Spectroscopy Pattern, ultra violet (UV) and Photoluminescence (PL). All these analyses revealed highly pure nature of Fe2O3-NPs with no internal defects, and suggested its application for plant growth improvement. Therefore, we further investigated the separate as well as combined effects of the Fe2O3-NPs and citric acid (CA) in the alleviation of arsenic (As) toxicity in the soybean (Glycine max L.), by evaluating the different plant growth and metabolic attributes. Results of our study revealed that As-induced growth inhibition, reduction of photosynthesis, water use efficiency (WUE), and reactive oxygen species (ROS) accumulation whereas application of the Fe2O3-NPs and CA significantly reversed all these adverse effects in soybean plants. Moreover, the As-stress induced malondialdehyde (MDA) and hydrogen peroxide (H2O2) production were partially reversed by the Fe2O3-NPs and CA in the As-stressed plants by 16% and 10% (MDA) and 29% and 12% (H2O2). This might have resulted due to the Fe2O3-NPs and CA induced activities of the antioxidant defense in plants. Overall, the Fe2O3-NPs and CA supplementation separately and in combination positively regulated the As tolerance in soybean; however, the effect of the combined application on the As tolerance was more profound relative to the individual application. These results suggested the synergetic effect of the Fe2O3-NPs and CA on the As-tolerance in soybean. However, in-depth mechanism underlying the defense crosstalk between the Fe2O3-NPs and CA needs to be further explored.

Removal of heavy metals in municipal solid waste incineration fly ash using lactic acid fermentation broth

Municipal solid waste incineration fly ash (MSWIFA) is considered as a hazardous solid waste because of the high mobility of heavy metals. In this study, the removal of heavy metals in MSWIFA using lactic acid fermentation broth (LAFB) under various leaching protocols (i.e. LAFB addition amount and timing) was investigated. Results revealed that compared with that in pure lactic acid solution, the synergistic effect of various substances in LAFB was more favourable to the dissolution of heavy metals. Although the content of acid-soluble heavy metals in MSWIFA decreased after leaching with LAFB, the leaching toxicity measured by acetic acid buffer solution method increased to varying degrees (except that of Cr). Moreover, the maximum leaching concentration of Pb was 14.1 mg/L (standard limit, 0.25 mg/L), which was not conducive to the landfill treatment of MSWIFA. However, if the LAFB-treated MSWIFA was used in cement kiln for co-disposal, the amount of MSWIFA entering the kiln was 6.0 percentage points higher than that in pure water leaching. Therefore, LAFB leaching instead of water leaching is expected to be an effective pre-treatment method for the utilisation of MSWIFA.

Managing soils of environmental significance: A critical review

Globally, environmentally significant soils (ESSs) mainly include acid sulfate, heavy metal(loid)-contaminated, petroleum hydrocarbon-contaminated, pesticide-contaminated, and radionuclide-contaminated soils. These soils are interrelated and have many common characteristics from an environmental management perspective. In this review, we critically evaluate the available literature on individual ESSs, aiming to identify common problems related to environmental quality/risk assessment, remediation approaches, and environmental regulation for these soils. Based on these findings, we highlight the challenges to, and possible solutions for sustainable ESS management. Contaminated land has been rapidly expanding since the first industrial revolution from the industrialized Western countries to the emerging industrialized Asia and other parts of the world. Clean-up of contaminated lands and slowdown of their expansion require concerted international efforts to develop advanced cleaner production and cost-effective soil remediation technologies in addition to improvement of environmental legislation, regulatory enforcement, financial instruments, and stakeholder involvement to create enabling environments. Two particular areas require further action and research efforts: developing a universal system for assessing ESS quality and improving the cost-effectiveness of remediation technologies. We propose an integrated framework for deriving ESS quality indicators and make suggestions for future research directions to improve the performance of soil remediation technologies.

Potential environmental pollution from copper metallurgy and methods of management

This paper presents the latest overview of the environmental impact of wastes from the non-ferrous metallurgical industry. Ashes, slags and dusts - by-products from mining and metal processing - are sources of toxic metals, such as Pb, Cd, Hg, As, Al, as well as particulate matter. Physical, chemical and biological processes transform industrial wastes and cause water, soil and air pollution. Improperly protected heaps are subject to wind erosion and rain water leaching. Heavy metals and particulate matter are transported over long distances, contaminating the soil, living areas, watercourses, while in combination with mist they create smog. Water erosion releases heavy metals, which are leached into groundwater or surface runoff. This paper focuses on the range of pollution emissions from non-ferrous metallurgy wastes, hazards, mechanisms of their formation and fallouts, on the current state of technology and technological risk reduction solutions. The impact of pollution on human health and the biosphere, and methods of waste reduction in this industry sector are also presented. A sustainable and modern mining industry is the first step to cleaner production.

Removal of Cu and Pb from contaminated agricultural soil using mixed chelators of fulvic acid potassium and citric acid

This study investigated the application of a specific soil washing method to remove Cu and Pb from contaminated agricultural soil. To develop an efficient leaching agent of heavy metal compounds for use in farmland soil, a mixed chelator (MC) was prepared using potassium fulvic acid (PFA, 3.2%) and citric acid (CIT, 0.16 M) in a volume ratio of 4:1 (PFA:CIT = 4:1); the optimal solid-liquid ratio (S/L = 1:20), initial pH value (4.51) and contact time (360 min) were also explored. Under optimal conditions, the removal efficiencies of MC for Cu and Pb were 42.92% and 50.46%, respectively, both of which performed better than PFA (27.86% of Cu and 17.91% of Pb) and CIT (42.04% of Cu and 41.46% of Pb). The effective states, bioavailability and relative mobilities of Cu and Pb in soil were also efficiently reduced by MC, which also increased the stability of these elements, thereby lowering the risk to soil health. More importantly, MC not only had little effect on the soil physicochemical properties (e.g., pH, organic matter (OM), cation exchange capacity (CEC), ammonium nitrogen (AN), available phosphorus (AP) and rapidly available potassium (AK)), but also improved the restored soil. Furthermore, soil structure, surface elements and the enzyme activity did not exhibit significantly loss. Therefore, MC has great potential for remediating agricultural soil.

Remediation of heavy metal contaminated soil by biodegradable chelator-induced washing: Efficiencies and mechanisms

Biodegradable chelators (BCs) are promising substitutes for conventional washing agents in the remediation of heavy metal contaminated soil with strong complexing ability and less cost. However, great challenges for the applications of BC-assisted washing still exist, such as the assessment of the factor affecting the efficiency of metal removal and the unclear of the metal removal mechanism. Batch washing was therefore explored to evaluate the potential for four BCs for removing Cd, Pb, and Zn from polluted soils. The soil spectroscopic characteristics before and after washing were also investigated. The results demonstrated that iminodisuccinic acid (ISA) and glutamate-N, N-diacetic acid (GLDA) were an appealing alternative to commonly used non-biodegradable ethylenediaminetetraacetic acid, but glucomonocarbonic acid (GCA) and polyaspartic acid (PASP) were less efficient. Optimal parameters of BCs were determined to be a concentration of 50 mmol L^-1, a pH of 5.0, a contact time of 120 min, and a solid/liquid ratio of 1:5, considering metal removal efficiencies and the suitable cost. A single removal washing could be up to 52.39% of Cd, 71.79% of Pb, and 34.13% of Zn from mine soil, and 98.28% of Cd, 91.10% of Pb, and 90.91% of Zn from polluted farmland soil. After washing, the intensity of heavy metal binding to soil colloids increased while the metal mobility reduced because of weakly bound fractions removed by BCs. The BCs-induced soil washing revealed that the possible mechanisms of metal removal included the acid dissolution, ion exchange, and surface complexation. Our findings highlight the potential application of especially ISA and GLDA as efficient washing agents to remove potentially toxic elements from contaminated soils.

Electro-enhanced leaching method for the mobilization of Cr(VI) in contaminated groundwater aquifer

Removal of hexavalent chromium [Cr(VI)] from soils and water has been widely studied for its high toxicity. Although leaching method is viewed as an effective approach to eliminate Cr(VI) and some studies attempted to enhance leaching performance via the external electric field, there is little knowledge about the influential factor in electro-leaching system on Cr(VI) removal performance. In this study, an electro-leaching technology was developed for removing Cr(VI) from groundwater aquifer to comprehensively discuss the correlation between the operational parameters and Cr(VI) removal efficiency. When the applied voltage was 20 V and the initial Cr(VI) concentration was 40 mg/kg, Cr(VI) removal efficiency achieved 99.9% in 120 min in the electro-leaching system, 15% higher than the system without the electric field. Cr(VI) removal efficiencies increased with the voltage demonstrating the significant enhancement of the electro-leaching method in removing Cr(VI). When Cr(VI) concentration climbed to 120 mg/kg, Cr(VI) removal efficiency remained above 85%. The effects of different voltages, Cr(VI) concentrations, pollutant distribution and salt content of leaching solution on the leaching effect were also investigated. Meanwhile, the relationship between the current intensity change and the amount of removed Cr(VI) during the electro-leaching process was first investigated, and the relevant model was fitted. There is a quadratic linear correlation between the amount of current change and the amount of removed Cr(VI). This novel electro-enhanced leaching method can effectively remove Cr(VI) from contaminated groundwater aquifer by enhancing the migration of charged contaminant ions during the leaching process, and it is worthy of further study of heavy metal remediation.

Simultaneous removal of multiple heavy metals from soil by washing with citric acid and ferric chloride

Citric acid and ferric chloride exhibited synergistic effect on the removal of multiple heavy metals from soil.

A Soluble Humic Substance for the Simultaneous Removal of Cadmium and Arsenic from Contaminated Soils

With abundant oxygen-containing functional groups, a humic substance (HS) has a high potential to remediate soils contaminated by heavy metals. Here, HS was first extracted from a leonardite and analyzed for its chemical compositions and spectroscopic characteristics. Then it was assessed for its ability as a washing agent to remove Cd and As from three types of soils (red soil, black soil, and fluvo-aquic soil) that were spiked with those contaminants (Cd: 40.5–49.1 mg/kg; As: 451–584 mg/kg). The operational washing conditions, including the pH and concentration of the HS, washing time and cycles, and liquid–soil ratio, were assessed for Cd and As removal efficiency. At pH 7, with an HS concentration (3672 mg C/L) higher than its critical micelle concentration and a liquid–soil ratio of 30, a single washing for 6–12 h removed 41.9 mg Cd/kg and 199.3 mg As/kg from red soil, 33.5 mg Cd/kg and 291.5 mg As/kg from black soil, and 30.4 mg Cd/kg and 325.5 mg As/kg from fluvo-aquic soil. The removal of Cd and As from the contaminated soils involved the complexation of Cd and As with the carboxyl and phenolic groups of HS. Outcomes from this research could be used to develop a tailor-made HS washing agent for the remediation of Cd- and As-contaminated soils with different properties.

Multi-step column leaching using low-molecular-weight organic acids for remediating vanadium- and chromium-contaminated soil

In soil, vanadium (V) contamination is commonly concomitant with chromium (Cr) contamination, which poses potential risks to humans, animals, and plants due to the transfer of toxic metals and the increase in their concentrations via the food chain or through direct exposure. This study applied a multi-step column leaching process using low-molecular-weight organic acids (LMWOAs) to treat V-contaminated soil from a smelter site that contains 2015.1 mg V kg^-1 and 1060.3 mg Cr kg^-1. After leaching three times with an equivalent solution/soil ratio of 0.3 mL/g using 1.0 M oxalic acid solution, the total removal rates reached 77.2% and 7.2% for V and Cr, respectively, while the removal rates of the extractable fractions reached 118.6% and 99.2% due to the reduction in residual fraction (F₄) of toxic metals. Simultaneously, the distribution and redistribution of geochemical fractions of V and Cr were determined with a sequential extraction technique, and the greater proportion of potential mobile fractions of V (65.1%) may increase its leaching from soil relative to Cr (7.1%). In addition, a lower pH of the leaching agent increased the efficiency of the leaching process to an extent. Compared with batch extraction with a typical solution to soil ratio of 10 mL/g, multi-step column leaching used less agent and hence produced less wastewater. This strategy could reduce the mobilization and bioavailability of toxic metals, and potentially enhance in situ soil flushing for the remediation of V- and Cr- contaminated soil.

Mixotrophic acidophiles increase cadmium soluble fraction and phytoextraction efficiency from cadmium contaminated soils

A profound concern in developing microbially-assisted phytoextraction is that introduced microbes not only remove heavy metals from contaminated soils but also enhance metal uptake into plant tissues from the treated soils. Cadmium (Cd) removal efficiencies were compared after leaching with deionized water (CK), acidified basal salts medium (acid control), cell-free spent medium (spent bioleaching) and mixotrophic acidophiles (two-step bioleaching). Two-step bioleaching using the mixotrophic acidophiles removed 34% of total Cd and 87% of available Cd, significantly more than CK (3% and 4%), acid control (12% and 51%) and spent bioleaching (26% and 75%). Pot experiments of water spinach growing in four treated soils were conducted to evaluate the Cd uptake performance in plants. Notably, the mixotrophic acidophiles increased Cd concentration in plant tissues by 78% compared to the CK group. More interestingly, the mixotrophic acidophiles were not colonized in soils but caused the compositional increase of indigenous microbes such as the genera of Alicyclobacillus, Clostridium sensu strict and Streptacidiphilus. Meanwhile, two-step bioleaching had little effects on soil structure and physicochemical properties determined by the spectroscopy characteristics analysis. These results implied that the mixotrophic acidophiles facilitated the development of microbially-assisted phytoextraction technology.

Investigating the use of synthetic humic-like acid as a soil washing treatment for metal contaminated soil

Humic acid can effectively bind several metals and is regarded as a promising soil washing agent. Previous studies indicate that carboxylic groups dominate metal binding to humic acid. In this study, a synthetic humic-like acid (SHLA) with high COOH content (5.03 mmol/g) was used as a washing agent to remove metals (Cu, Zn, Ni, Pb, As) from two contaminated agricultural soils (Soil 1 (pH: 6.17 ± 0.11; organic carbon: 5.91 ± 0.40%; Cu: 302.86 ± 3.97 mg/kg; Zn: 700.45 ± 14.30 mg/kg; Pb 323.56 ± 4.84 mg/kg; Ni: 140.16 ± 1.59 mg/kg) and Soil 2 (pH: 9.83 ± 0.01; organic carbon: 2.52% ± 0.25%; Cu: 242.81 ± 10.66 mg/kg; Zn: 841.00 ± 22.31 mg/kg, Pb 451.21 ± 1,92 mg/kg, As: 242.23 ± 5.24 mg/kg)). The effects of solution pH (4 to 11), liquid/solid ratio (L/S ratio, 5:1 (mL:g) to 80:1 (mL:g)), SHLA concentration (100 mg/L to 2000 mg/L), and contact time (0 to 1440 min) on % metal removal were investigated and optimum conditions identified: pH of 9, L/S ratio of 1:80, SHLA concentration of 1500 mg/L at 25 °C for 4 h. Under optimum conditions, a single washing removed 45.2% of Cu, 34.6% of Zn. 42.2% of Ni and 15.6% of Pb from Soil 1, and 30.6% of Cu, 28.1% of Zn. 14.6% of As and 18.1% of Pb from Soil 2. A modified BCR extraction of the two soils before and after washing indicated that the SHLA mainly removed metals in the exchangeable and acid soluble fraction and reducible fraction, which could effectively reduce bioavailability and environmental risk of metals. On a molar basis, SHLA was a more effective washing agent than commercial humic acid, Na₂EDTA, citric acid and tartaric acid. Overall, SHLA shows great potential for use as a soil washing agent.

The remediation potential and kinetics of cadmium in the green alga Cladophora rupestris

This study determined the subcellular distribution, chemical forms, and effects of metal homeostasis of excess Cd in Cladophora rupestris. Biosorption data were analyzed with Langmuir and Freundlich adsorption models and kinetic equations. Results showed that C. rupestris can accumulate Cd. Cd mainly localized in the cell wall and debris (42.8-68.2%) of C. rupestris, followed by the soluble fraction (22.1-38.4%) observed in C. rupestris. A large quantity of Cd ions existed as insoluble CdHPO₄ complexed with organic acids, Cd(H₂PO₄)₂, Cd-phosphate complexes (F_HAC) (43.2-56.0%), and pectate and protein-integrated Cd (F_NaCl) (30.8-43.2%). The adsorption data were well fitted by the Freundlich model (R² = 0.933) and could be described by the pseudo-second-order reaction rate (R² = 0.997) and Elovich (R² = 0.972) equations. Related parameters indicated that Cd adsorption by C. rupestris is a heterogeneous diffusion. Cd promoted Ca and Zn uptake by C. rupestris. Cu, Fe, Mn, and Mg adsorption was promoted by low Cd concentrations and inhibited by high Cd concentrations. Results suggested that cell wall sequestration, vacuolar compartmentalization, and chemical morphological transformation are important mechanisms of Cd stress tolerance by C. rupestris. This study suggests that C. rupestris has bioremediation potential of Cd.

Effect of soil washing with biodegradable chelators on the toxicity of residual metals and soil biological properties

Soil washing with chelators is a promising and efficient method of remediating metals-contaminated soils. However, the toxicity of residual metals and the effects on soil microbial properties have remained largely unknown after washing. In this study, we employed four biodegradable chelators for removal of metals from contaminated soils: iminodisuccinic acid (ISA), glutamate-N,N-diacetic acid (GLDA), glucomonocarbonic acid (GCA), and polyaspartic acid (PASP). The maximum removal efficiencies for Cd, Pb, and Zn of 85, 55, and 64% and 45, 53, and 32% were achieved from farmland soil and mine soil using biodegradable chelators, respectively. It was found that the capacity of ISA and GLDA to reduce the labile fraction of Cd, Pb, and Zn was similar to that of the conventional non-biodegradable chelator ethylenediaminetetraacetic acid (EDTA). The leachability, mobility, and bioaccessibility of residual metals after washing decreased notably in comparison to the original soils, thus mitigating the estimated environmental and human health risks. Soil β-glucosidase activity, urease activity, acid phosphatase activity, microbial biomass nitrogen, and microbial biomass phosphorus decreased in the treated soils. However, compared with EDTA treatment, soil enzyme activities distinctly increased by 5-94% and overall microbial biomass slightly improved in the remediated soils, which would facilitate reuse of the washed soils. Based on soil toxicity tests that employed wheat seed germination as the endpoint of assessment, the washed soils exhibited only slight effects especially after ISA and GLDA treatments, following high-efficiency metal removal. Hence, ISA and GLDA appear to possess the greatest potential to rehabilitate polluted soils with limited toxicity remaining.

Compound washing remediation and response surface analysis of lead-contaminated soil in mining area by fermentation broth and saponin

The development of eluent is the key to soil washing remediation, and a compound eluent was constructed using the prepared citric acid fermentation broth and saponin in this study. It displayed a good washing performance for Pb, Cu, Cr, and Cd in red soil, and the removal rates, especially Pb, gained an improvement compared with a single eluent. Based on this, the compound eluent was applied to remediation of Pb-contaminated soil in mining area; the desorption of Pb is a heterogeneous diffusion process, and Pb in large particle size soil is relatively easy to remove. An available response surface analysis model was established; its P < 0.0001 is very significant, and the P of the missing item is 0.1152. The degree of influence of three significant factors on removal of Pb is liquid-to-solid ratio > washing time > saponin concentration, and liquid-to-solid ratio and washing time show interaction. Moreover, the Pb removal rate can reach 56.20% under the optimized conditions: 0.25% saponin concentration, 20 mL/g liquid-to-solid ratio, and 320-min washing time, which is close to the predicted value of 56.20% with a difference of 1.41%. In addition, most of the active Pb was removed and environmental risks were lowered after washing.