Chemical speciation and mobilization of copper and zinc in naturally contaminated mine soils with citric and tartaric acids

MGDA-assisted plant washing agent for improving the removal of Cd and Cu from farmland soils

Utilizing plant extracts for soil washing is a promising and cost-efficient strategy to permanently remove toxic metals from farmland soils. However, the efficiency of current plant extracts in heavy metals removal is constrained by the need of excessively high liquid-solid ratios (>10:1), which limits their practical application in the farmland soils. To address this challenge, we developed a novel binary washing agent consisting of Fructus mume residue extract (RPM) and methylglycinediacetate acid (MGDA). Through optimization experiments, we determined an optimal composition of 50 g L-1 RPM and 0.34 g L-1 MGDA (RPMG3), which exhibited a remarkable synergistic effect on the removal of Cd and Cu from two polluted farmland soils. Compared to the single use of RPM or MGDA, RPMG3 increased metal removal efficiency by 44.6%-303.8% in the alkaline dryland soil and 55.8%-141.9% in the slightly acidic paddy soil, at an optimum liquid-solid ratio of 2:1. The contents of soil Cd and Cu can be reduced to below the risk screening values via acid activation, metal ion exchange, and complexation of functional groups. Moreover, the potential ecological risks associated with Cd in the soils were significantly mitigated due to the decrease in exchangeable and reducible Cd fractions following RPMG3 washing. Additionally, RPM and RPMG3 washing led to enhancements in soil organic carbon and nutrient concentrations, as well as increased activities of soil enzymes including catalase, urease, and β-glucosidase. Notably, RPMG3 washing exhibited the most pronounced promotion effect on wheat seed germination and growth. Taken together, the binary complex of RPMG3 demonstrates potential as an environmentally friendly green washing agent capable of in reducing heavy metals from farmland soil with great efficiency, even at low liquid-solid ratios.

Changes in chemical fractionation of copper and zinc in soil as a function of incubation moisture content and organic matter amendments

Copper and zinc are essential micronutrients that are potentially toxic when present in excess in soils. Their bioavailability depends on their speciation in soil, but this may vary with environmental conditions. Aeration and hence redox conditions, and organic matter amendments are among the factors likely to cause variation on metal fractionation. We have monitored the chemical fractionation of both native and added copper and zinc in a clay loam top soil during a 5-month laboratory incubation. The effects of aeration (moist soil or flooded) and addition of two organic matter amendments, alfalfa straw or leaf compost, were studied. Metal spike was more labile than legacy metal, and was slowly redistributed over the incubation period. Organic matter caused short-lived flushes of metals, attributed to metal chelation with soluble organic matter. This effect was greater for straw than for more stable compost. There was no evidence that added organic matter increased the capacity of soil organic matter to immobilise metal. Flooding solubilized soil metal (hydr)oxides, releasing legacy Cu and Zn, but with less effect on the capacity to immobilise metal spike. Effects of flooding and organic matter addition were not additive. Both metals appear to be precipitated as sulphides under reducing conditions, and accounted for in the acid soluble phase. Monitoring the dynamics of metal distribution gives a more comprehensive understanding of underlying processes than would a single measurement, and is closer to in campo conditions than slurry microcosms.

Microwave-assisted recovery of lead from electrolytic manganese anode sludge using tartaric acid and NaOH

In this study, a new metallurgical system for treating electrolytic manganese anode sludge by microwave roasting and alkaline leaching system was developed, and the lead leaching behaviour was studied. The XRD results show that Pb in anode sludge is mainly in the form of PbSO₄ after microwave roasting at 850°C, as a result, the leaching rate of Pb is improved. The results show that the leaching rate of lead can reach 93.89% under the conditions of liquid-solid ratio of 7:1, leaching time of 30 min, leaching temperature of 40°C, and the concentration of sodium hydroxide of 8%. The addition of tartaric acid can further improve the lead leaching rate, FT-IR analysis showed that the coordination form of lead and tartaric acid. Lead and tartaric acid ions (L) form three coordination compounds, PbL, Pb₂L₂ and Pb₂L₃, which can only exist in alkaline solution.

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.

Alleviation of metal stress in rape seedlings (Brassica napus L.) using the antimony-resistant plant growth-promoting rhizobacteria Cupriavidus sp. S-8-2

This study investigated an effective strategy for remediating antimony (Sb)-contaminated soil using the bacterial strain screened from Sb-contaminated fern rhizospheres due to its superior growth-promoting, heavy-metal(loid) resistant, and antibiotic-tolerant characteristics. The strain that belongs to Cupriavidus sp. was determined by 16S rRNA sequencing and showed no morphological changes when grown with high concentrations of Sb (608.8 mg/L). The strain showed prominent indole acetic acid (IAA), phosphate-solubilizing abilities, and ACC deaminase activity under Sb stress. Moreover, IAA and soluble phosphate levels increased in the presence of 608.8 mg/L Sb. Inoculation of rape seedlings with Cupriavidus sp. S-8-2 enhanced several morphological and biochemical growth features compared to untreated seedlings grown under Sb stress. Inoculation of Cupriavidus sp. S-8-2 increased root weight by more than four-fold for fresh weight and over two-fold for dry weight, despite high environmental Sb. The strain also reduced Sb-mediated oxidative stress and malondialdehyde contents by reducing Sb absorption, thus alleviating Sb-induced toxicity. Environmental Scanning Electron Microscope (ESEM) imaging and dilution plating technique revealed Cupriavidus sp. S-8-2 is localized on the surface of roots. Identifying the Sb-resistant plant growth-promoting bacterium suggested its usefulness in the remediation of contaminated agricultural soil and for the promotion of crop growth. We highly recommend the strain for further implementation in field experiments.

Leaching Remediation of Dredged Marine Sediments Contaminated with Heavy Metals

There are more than 150 ports in China and a considerable proportion of dredged sediments in ports and waterways are contaminated with heavy metals as the typical contaminants. It is mandatory to remediate the contaminated dredged sediments prior to further resource utilization. The over-arching objective of this study was to use natural organic acids (oxalic acid, citric acid, tartaric acid, and malic acid) as leaching agents to remove heavy metals (Cu, Cd, and Pb) from contaminated dredged sediments. Batch experiments were conducted to investigate the factors governing the removal rate of heavy metals and leaching kinetics. Citric acid had the best leaching effect on heavy metals Cu, Cd, and Pb with an optimal leaching concentration of 20 mmol/L and a solid-to-liquid ratio of 1:20. The average removal rates of Cu, Cd, and Pb were 85%, 73%, 56%, and 35% for citric acid, malic acid, tartaric acid, and oxalic acid, respectively. The leaching kinetics showed that the removal of heavy metals increased rapidly with time and then gradually reached the maximum value which was best described by the Elovich equation model. The outcomes of this study suggest that citric acid is an effective and environmentally friendly leaching agent for removing heavy metals from marine dredged sediments.

Organic acid leaching was an efficient approach for detoxification of metal-containing plant incineration ash

Metal-containing plant incineration ash (MPIA), which was the by-product for metal extraction from soil by phytoextraction process, contains various kinds of heavy metal that have post potential risk to the environment. This study investigated the leaching efficiency and metal redistribution of MPIA using organic acid as leaching agents. The MPIA before and after leaching was characterized using the toxicity characteristic leaching procedure (TCPL) test, X-ray diffraction, scanning electronic microscopy, and Fourier transform infrared spectroscopy. The results showed that all tested organic acids resulted in the dissolution of metals, especially 1 mol L-1 citric acid leaching achieved for the dissolution efficiency of 84% Mn, 87.01% Cd, 66.97% Zn, and 55.83% Pb. During leaching progress, the synergetic of chelation and acid soluble action accelerated the metal release and redistribution, and the dissolution of Mn, Zn, Pb, and Cd fit best to the shrinking core model of chemical control. Meanwhile, the leaching residue reached the regulatory standard. Thus, organic acid leaching may be a feasible strategy for detoxification of MPIA.

Effects of exogenous citric acid on the concentration and spatial distribution of Ni, Zn, Co, Cr, Mn and Fe in leaves of Noccaea caerulescens grown on a serpentine soil

The aim of this study was to show the potential of citric acid in increasing the concentration of Ni, Zn, Co, Cr, Mn and Fe in leaves of the hyperaccumulator Noccaea caerulescens. Synchrotron x-ray fluorescence (μ-XRF) images were collected to assess the distribution of metals in leaves. Applying citric acid (20 mmol kg^-1) to soil increased in 14-, 10-, 7-, 2- and 1.4- fold the concentration of Mn, Fe, Co, Ni, and Cr, respectively, compared to the control. The μ-XRF imaging revealed that Ni and Zn were not spatially correlated across the leaf. We observed a clear partitioning of Zn between veins and surrounding leaf cells while Ni was more evenly distributed between veins and leaf blade. The accumulation of metals in citric acid treated plants did not change the Ni and Zn distribution pattern in leaves but altered the Mn distribution. It seems that Mn reached toxic concentrations in leaves and we hypothesize that a mechanism driven by transpiration through the xylem was used to excrete the metal. Our results show that citric acid can enhance metal accumulation by N. caerulescens and have impact for soil remediation by either decreasing the time for clean up or increasing the access to non-labile pools of metals in soil.

Removal of Cd, Pb, Zn, Cu in smelter soil by citric acid leaching

Soil washing has been verified as a feasible technology for source reduction for contaminated soil with heavy metals. We conducted batch and column leaching experiments to investigate the removal of Cd, Pb, Cu and Zn from smelter soil by citric acid. The removal efficiency of heavy metals by batch leaching reached a maximum (89.1% Cd, 26.8% Pb, 41.7% Zn, 14.2% Cu) at a concentration of 0.1 M and a pH of 5. Citric acid also removed 91.3%, 11.1%, 39.2% and 11.1% of Cd, Pb, Zn, and Cu respectively after column leaching. Citric acid mainly removed exchangeable, carbonate bound and oxide bound Cd, Pb, Cu and Zn fractions. Vertical distributions of Cd, Cu and Zn similarly increased with increasing soil depth. Chestnut shells were applied to the recovery of citric acid from the waste eluent, which removed 84.4%, 97.5%, 74.6%, 70.3% of Cd, Pb, Zn, and Cu, respectively, due to chestnut shell chelation. Fresh and regenerated citric acid was used in batch leaching of heavy metal-contaminated soil, and they showed similar ability to extract heavy metals.

Removal of heavy metals from industrial sludge with new plant-based washing agents

Washing is one of the techniques for permanent removal of heavy metals from industrial sludge, for which washing agents are a key influence factor. However, high-efficiency, eco-friendly, and inexpensive agents are still lacking. In this study, the solutions derived from the three plant materials including Fatsia japonica, Hovenia acerba and Pterocarya stenoptera were employed to remove Cd, Cu, Pb, and Ni from industrial sludge. The effects of washing solution concentration, pH, washing time and temperature on metal removal were investigated. The metal removal efficiencies were found to increase with increasing solution concentrations or washing temperatures, decline with increasing pH, and presented various trends with increasing washing time. Among the three agents that derived from H. acerba showed relatively high removal for Cu (75.81%), Pb (63.42%), Ni (27.52%), and Cd (56.99%). After washing, environmental risks of residual metals were markedly diminished in sludge, attributable to decrease in their exchangeable forms. Furthermore, the applications of the plant washing agents increased sludge organic carbon, alkali-hydrolysable nitrogen, available phosphorus, and available potassium. Fourier transform infrared spectroscopy analysis suggested that the hydroxyl, carboxyl, ether, and amide may be the main functional groups in the three plant materials binding the heavy metals. Overall, the agent derived from H. acerba appears to be a feasible washing material for heavy metals removal from sludge.

Citric acid-assisted accumulation of Ni and other metals by Odontarrhena muralis: Implications for phytoextraction and metal foliar distribution assessed by μ-SXRF

Odontarrhena muralis is one of the most promissing plant species for Ni phytomining, and soil amendments can further increase its Ni phytoextraction ability. Here we investigated whether Ni phytomining/phytoremediation using this Ni hyperaccumulator can benefit from applying citric acid to a serpentine soil that is naturally enriched in Ni (>1000 mg kg^-1). Synchrotron micro X-ray fluorescence (μ-SXRF) was used to image Ni and other metal distributions in whole fresh leaves of O. muralis. Leaf Ni accumulation in plants grown on citric acid-amended soil increased up to 55% while Co, Cr, Fe, Mn, and Zn concentrations were 4-, 14-, 6-, 7- and 1.3-fold higher than the control treatment. O. muralis presented high bioconcentration factors (leaf to soil concentration ratio) to Ni and Zn whereas Cr was seemingly excluded from uptake. The μ-SXRF images showed a uniform distribution of Ni, preferential localization of Co in the leaf tip, and clear concentration of Mn in the base of trichomes. The citric acid treatments strongly increased the Co fluoerescence intensity in the leaf tip and altered the spatial distribution of Mn across the leaf, but there was no difference in Ni fluorescence counts between the trichome-base region and the bulk leaf. Our data from a serpentine soil suggests that citrate treatment enhances Ni uptake, but Co is excreted from leaves even in low leaf concentrations, which can make Co phytoming using O. muralis unfeasible in natural serpentine soils.

Tracing source and mobility of arsenic and trace elements in a hydrosystem impacted by past mining activities (Morelos state, Mexico)

The Sierra Huautla (Morelos State, Mexico) is a biological reserve with historical mines of Ag and Pb. In this area, waters used by inhabitants are contaminated by arsenic (As). An integrated environmental survey was realized both in waters and sediments to better constrain the source and the mobility of As and other trace elements. Two areas of interest were selected: (1) the Nexpa River ecosystem to determine the local geochemical background, and (2) the Huautla area, affected by past mining activities. This study allowed the definition of the local geochemical baseline in sediments or in waters, demonstrated uncontaminated by TE in the Nexpa area, except for As in the dissolved phase or for Cd in Suspended Particulate Matters (SPM). In the Huautla area, TE contents in water were higher than the World Health Organization (WHO) limits for Al, As and Mn in unfiltered waters, and only for As in the dissolved phase. Speciation analyses revealed arsenic to be present only as the toxic-inorganic arsenate species, As(+V). In SPM, Ag, As, Cd and Zn concentrations were higher than Sediment Quality Guidelines (SQG). The different geochemical indice (EF: 5, PLI: 3, EF: Igeo: 5-3) demonstrated that SPM were significantly contaminated and consistute an health risk for Huautla inhabitants exposed to As-contaminated waters and TE-rich SPM. The combination of mineralogy, chemistry, C and S stable isotopes with thermodynamic modeling indicate dissolutions of minerals from local geological formations, sorption-desorption phenomena from clays and oxy-hydroxides, and the weathering responsible for the transport of the TE-rich SPM (1.8 wt% for 17% of the total TE concentration). Moreover, the past mining activity would be a source of the contamination only for As in waters from flooded mines.

Leaching behavior of metals from iron tailings under varying pH and low-molecular-weight organic acids

The migration of metals (e.g., Fe, Cd, Co, Cr, Cu, Mn, Ni, and Zn) in both of iron tailings under different pH leachates was studied by laboratory static leaching experiments. The results indicated that Fe showed the highest leaching concentration at an initial pH of 2, reaching 16.19 and 51.72 mg L^-1 in the Qian'anling (Q0) and Majuanzi (M0) iron tailings, respectively. Metal ions manifested a strong pH dependence. In addition, the leaching behavior of Cd, Cr, Fe, and Cu for the two tailings was also evaluated under leaching by three low-molecular-weight organic acids (LMWOAs). The results indicated the leaching of Cd and Fe followed the order of citric acid > malic acid > oxalic acid and that the leaching order for Cr and Cu was citric acid > oxalic acid > malic acid. The concentration of Fe was low in 5 mM oxalic acid leaching for 20 days because of the hydrolysis precipitation of iron ions and the complexation with organic ligand. The crystal lattice on the tailings was significantly damaged after leaching. The CO₃^2- peak appeared in M0 with different treatments, and the proportion of COO- fitting peak areas increased markedly after leaching with LMWOAs.

Finding a relationship between physicochemical characteristics and ionic composition of River Nworie, Imo State, Nigeria

Water has been described as a universal solvent, and this is perhaps the strength behind its many uses. Despite this unique property, anthropogenic activities along its course and natural factors often determine the composition of water. In the current research, the portion of River Nworie having past Owerri town was sampled in the dry season 2017 to determine its ionic composition at predestinated points and to relate such properties to its physicochemical characteristics. Studies relating physicochemical properties and dissolved toxic ions in water could develop a body of knowledge that could enable detection and quantification of potential risk of ions such as heavy metals from natural water to aquatic ecosystem, animal and human health without actually involving aquatic organism, animal and human. Clean sterile plastic bottles were used for collecting surface water. A total of 30 sub-samples from five points at 300 m apart were sampled in the morning. Physicochemical properties were determined using standard methods and ionic composition of water was determined according methods of APHA. Results revealed that Ca2+ had a mean 23.60 ± 0.67 mg/l and was the highest while K+ with a mean 0.72 ± 0.30 was the least amongst major cations. Amongst the major anions Cl− had mean of 31.58 ± 4.47 mg/l while mean of PO43− was 1.42 ± 0.13 mg/l. The ionic balance calculate as % balance error showed high values for all sampling sites ranging from 30 to 39.42% indicating that there is massive input from anthropogenic activities. The computed relationships for selected heavy metals, cations and anions revealed that R2 values were ranging between  ± 0.012 to 1 indicating some form of relationship existing. The water pH weakly correlated with dissolved cations and anions while moderate with pH only, due to the pH level (5.2–6.2). The cations and anions were more influenced by the water temperature than the heavy metals. Therefore, high temperature ranges of 31–32.4 °C will favour more dissolution of cations and anions in natural water. Cations showed stronger relationship with EC while only heavy metals showed no relationship with DO (Dissolved oxygen). Dissolved oxygen relationship with cations and anions was in the order; K+ > Mg2+ > Ca2+ > Na+ while anions was SO42− > NO3− > Cl− > PO43−, respectively. Information here could be used to predict the effects of using this water for various purposes including water for agricultural purposes, in the management of ion polluted waters, and also to inform on the mitigation process to be taken.

Finding a relationship between mobility factors of selected heavy metals and soil particle size in soils from children's playgrounds

In the 3-year study, heavy metals mobility factors for soils of public playgrounds within Owerri metropolis were correlated with soil particle size. Such studies could develop a body of knowledge that could enable detection and quantification of potential risk of metals to children from playground soil without actually involving children. Calculated values of mobility factors were subjected statistical treatment and to obtain descriptive statistics. Regression analysis of mobility factors and particle size were determined. Copper and zinc were most mobile elements, while cobalt was least mobile. Result showed that clay particle size had strong influence on mobility factors. The significance of this study is that recent estimation of mobility factors of metals are used to expand its possible understanding and application to fundamental and practical problems of ecotoxicology of metals which could reflect metals potentials to cause harm thereby permitting the evaluation of metals toxicity without direct involvement of children.

Effects of pH Conditions and Application Rates of Commercial Humic Substances on Cu and Zn Mobility in Anthropogenic Mine Soils

We studied the effects of commercial humic substances derived from leonardite at different rates (0, 0.25, 2, 10 g kg−1) and pH (4.5, 6.0, 8.0) on Cu and Zn mobility, to evaluate their use for remediation of metal contaminated mine soils and to optimize their application conditions. We conducted a single-step extraction experiment and analyzed extracts for metal concentrations, soluble organic carbon and their E4/E6 ratio (ratio of absorption at 465 to 665 nm). Metal speciation in a soil solution was simulated by the non-ideal competitive adsorption-Donnan (NICA-Donnan) model. Increasing the amount of humic substances and the pH caused higher release rates of soluble organic carbon with a lower humic/fulvic acids ratio. This led to a higher mobility of metals (up to 110 times Cu concentration in control and 12 times for Zn) due to the formation of soluble metal-humic complexes. Speciation modeling predicted that increasing rates of humic substances would result in a higher proportion of Cu and Zn associated with fulvic acids, more mobile than the humic acids fraction. Application of commercial leonardite humic substances at 2–10 g kg−1 and with pH levels similar to or below natural soil could be useful for assisted-phytoextraction of contaminated anthropogenic soils. High rates of humic substances in more alkaline conditions could entail a considerable risk of metal leaching to groundwater, toxicity and transfer to the trophic chain.

Remediation of cadmium, lead and zinc in contaminated soil with CETSA and MA/AA

Soil washing, which is used to remove heavy metals from soil, is dependent on suitable washing agents. However, there is still a lack of economical, environmentally friendly washing agents with high removal efficiency. In this study, three washing agents, carboxyalkylthiosuccinic acid (CETSA), copolymer of maleic and acrylic acid (MA/AA) and ethylenediamine tetra acetic acid (EDTA), were used to remove heavy metals from contaminated soil. The influence of washing solution concentration, pH and washing time on heavy metals removal was also investigated. The cadmium (Cd), lead (Pb), and zinc (Zn) removal efficiencies increased as washing solution concentrations increased from 0 to 60 g L^-1, while they declined as pH increased from 3 to 8. Despite fluctuations between 90 and 120 min, heavy metal removal efficiencies increased continuously from 10 to 90 min. The three agents also effectively reduced the potential risks of Cd, Pb, and Zn in contaminated soil, but only CETSA and MA/AA produced no significant changes in chemical properties. Fourier transform infrared spectroscopy revealed that the hydroxyl, carboxyl, carbonyl, methoxyl, and sulfur groups were related to the heavy metal ions from the soil colloids. Thus, CETSA and MA/AA were suitable washing agents for remediation of heavy metals contaminated soil.

Complexation of Iron and Copper in Ambient Particulate Matter and Its Effect on the Oxidative Potential Measured in a Surrogate Lung Fluid

We investigated the complexation state of atmospheric iron and copper and its impact on the oxidative potential (OP) of ambient PM_2.5 (PM, particulate matter). A novel fractionation scheme was developed to segregate three different fractions of Fe and Cu present in ambient PM_2.5: (i) complexed with hydrophobic organic compounds, (ii) complexed with hydrophilic organic compounds, and (iii) free or inorganic metal fraction. A solid phase extraction (C-18) column was used to separate these fractions. The fractionation scheme applied to the ambient PM_2.5 samples collected from an urban site showed that up to 70-90% of water-soluble Fe and Cu were complexed with the organic compounds. The capability of the complexes of Fe(II) and Cu(II) with Suwanee river fulvic acid (SRFA), a proxy for the atmospheric organic compounds, to generate reactive oxygen species (ROS) (·OH and H₂O₂) in a surrogate lung fluid (SLF) was measured. The complex of Fe-SRFA showed a strong synergistic effect in the generation of ·OH in SLF, while that of Cu-SRFA showed an additive effect. Overall, our results indicate that organic complexation of the metals in ambient PM could significantly alter their OP and needs to be considered while assessing their health impacts.

Fractionation and release of Cd, Cu, Pb, Mn, and Zn from historically contaminated river sediment in Southern China: Effect of time and pH

The release of in situ heavy metals (cadmium [Cd], copper [Cu], lead [Pb], manganese [Mn], and zinc [Zn]) from contaminated river sediment near a Cu-iron mine in South China was experimentally studied. The effects of pH values and the kinetics of heavy metal release were investigated. In addition, species of the 5 metals in the sediment were determined as a function of pH and time. Rates and extents of metal desorption were studied over 30 d, and a 3-parameter, 2-compartment model was used to analyze the desorption kinetics. The rate constants (k_s ) for the slowly desorbing fraction of heavy metals were found to be 5 to 6 orders of magnitude lower than the corresponding rate constants (k_r ) for the rapidly desorbing fraction, suggesting that slow desorption is the rate-limiting step. The partition coefficients (K_d ) varied significantly among metals, on the order Pb > Cu >Zn > Cd > Mn, indicating that the sediment had a much higher retention capability for strongly hydrolyzed metals than for weakly hydrolyzed ones. The amount of metals released from the sediment decreased dramatically at the final pH of 2 to 4 and leveled off in the pH range of 5 to 8. Release time and pH exhibited a varied influence on the fractionation of metals, and had a more remarkable influence on more mobile fractions and hardly any effect on the residual fraction during the desorption process. The fractionation of strongly adsorbing metals like Pb was not as dependent on release time and pH as it was for the weakly bonded metals like Mn and Cd. The results indicate that in situ heavy metals may exhibit a sustainable potential for release and may increase health risks when the pH of the river is lowered. Environ Toxicol Chem 2019;38:464-473. © 2018 SETAC.

Feasibility of four wastes to remove heavy metals from contaminated soils

Soil washing is one of the permanent techniques to remove heavy metals, and washing agent is a key influence factor for this technique, but there is still lack of high-efficiency, eco-friendly, and inexpensive agents. In this study, four wastes including pineapple peel (PP), soybean straw (SS), broad bean straw (BBS) and tea residue (TR) were employed to remove cadmium (Cd), lead (Pb) and zinc (Zn) in contaminated soils. The Fourier transform infrared spectroscopy (FTIR) analysis indicated that hydroxyl, carboxyl, amine, carbonyl and amide groups were involved in the interaction with metal ions by complexation or ion exchange. We then investigated the influences of various conditions including washing solution concentration, pH, and washing time. The metal removal efficiencies with these agents increased as the concentration augmented from 5 to 80 g L^-1, decreased or presented an asymmetric V-shaped curve with increasing pH from 2.5 to 7.5, and fit intraparticle diffusion or Elovich model with washing time increasing. PP has the highest removals for Cd (90.1%), Pb (18.6%), and Zn (15.2%) in soil A, and 85.8, 24.8, and 69.4% in soil B, respectively. The relatively high metal removal was mainly attributed to effective removal of the exchangeable and acid soluble fractions. Moreover, single washing not only lowered the potential ecological risk of the heavy metals, but moderated the effects on soil chemical properties. Therefore, PP was a feasible washing agent to remediate soils contaminated by heavy metals.

Extraction of rare earth elements from a contaminated cropland soil using nitric acid, citric acid, and EDTA

Rare earth elements (REEs) contamination to the surrounding soil has increased the concerns of health risk to the local residents. Soil washing was first attempted in our study to remediate REEs-contaminated cropland soil using nitric acid, citric acid, and ethylene diamine tetraacetic acid (EDTA) for soil decontamination and possible recovery of REEs. The extraction time, washing agent concentration, and pH value of the washing solution were optimized. The sequential extraction analysis proposed by Tessier was adopted to study the speciation changes of the REEs before and after soil washing. The extract containing citric acid was dried to obtain solid for the X-ray fluorescence (XRF) analysis. The results revealed that the optimal extraction time was 72 h, and the REEs extraction efficiency increased as the agent concentration increased from 0.01 to 0.1 mol/L. EDTA was efficient to extract REEs over a wide range of pH values, while citric acid was around pH 6.0. Under optimized conditions, the average extraction efficiencies of the major REEs in the contaminated soil were 70.96%, 64.38%, and 62.12% by EDTA, nitric acid, and citric acid, respectively. The sequential extraction analyses revealed that most soil-bounded REEs were mobilized or extracted except for those in the residual fraction. Under a comprehensive consideration of the extraction efficiency and the environmental impact, citric acid was recommended as the most suitable agent for extraction of the REEs from the contaminated cropland soils. The XRF analysis revealed that Mn, Al, Si, Pb, Fe, and REEs were the major elements in the extract indicating a possibile recovery of the REEs.

Combined effects of low-molecular-weight organic acids on mobilization of arsenic and lead from multi-contaminated soils

A batch experiment was conducted to examine the combined effects of three common low-molecular-weight organic acids (LMWOAs) on the mobilization of arsenic and lead in different types of multi-contaminated soils. The capacity of individual LMWOAs (at a same molar concentration) to mobilize soil-borne As and Pb varied significantly. The combination of the organic acids did not make a marked "additive" effect on the mobilization of the investigated three elements. An "antagonistic" effect on element mobilization was clear in the treatments involving oxalic acid for some soils. The acid strength of a LMWOA did not play an important role in controlling the mobilization of elements. While the mobilization of As and Pb was closely associated with the dissolution of soil-borne Fe, soil properties such as original soil pH, organic matter contents and the total amount of the element relative to the total Fe markedly complicated the mobility of that element. Aging led to continual consumption of proton introduced from addition of LMWOAs and consequently caused dramatic changes in solution-borne Fe, which in turn resulted in change in As and Pb in the soil solution though different elements behaved differently.

Selective dissolution followed by EDDS washing of an e-waste contaminated soil: Extraction efficiency, fate of residual metals, and impact on soil environment

To enhance extraction of strongly bound metals from oxide minerals and organic matter, this study examined the sequential use of reductants, oxidants, alkaline solvents and organic acids followed by a biodegradable chelating agent (EDDS, [S,S]-ethylene-diamine-disuccinic-acid) in a two-stage soil washing. The soil was contaminated by Cu, Zn, and Pb at an e-waste recycling site in Qingyuan city, China. In addition to extraction efficiency, this study also examined the fate of residual metals (e.g., leachability, bioaccessibility, and distribution) and the soil quality parameters (i.e., cytotoxicity, enzyme activities, and available nutrients). The reductants (dithionite-citrate-bicarbonate and hydroxylamine hydrochloride) effectively extracted metals by mineral dissolution, but elevated the leachability and bioaccessibility of metals due to the transformation from Fe/Mn oxides to labile fractions. Subsequent EDDS washing was found necessary to mitigate the residual risks. In comparison, prior washing by oxidants (persulphate, hypochlorite, and hydrogen peroxide) was marginally useful because of limited amount of soil organic matter. Prior washing by alkaline solvents (sodium hydroxide and sodium bicarbonate) was also ineffective due to metal precipitation. In contrast, prior washing by low-molecular-weight organic acids (citrate and oxalate) improved the extraction efficiency. Compared to hydroxylamine hydrochloride, citrate and oxalate induced lower cytotoxicity (Microtox) and allowed higher enzyme activities (dehydrogenase, acid phosphatase, and urease) and soil nutrients (available nitrogen and phosphorus), which would facilitate reuse of the treated soil. Therefore, while sequential washing proved to enhance extraction efficacy, the selection of chemical agents besides EDDS should also include the consideration of effects on metal leachability/bioaccessibility and soil quality.

Efficiency of several leaching reagents on removal of Cu, Pb, Cd, and Zn from highly contaminated paddy soil

The efficiency of five different single leaching reagents (tartaric acid (TA), citric acid (CA), CaCl₂, FeCl₃, EDTA) and two different composite leaching reagents (CA + FeCl₃, CA + EDTA) on removing Cu, Pb, Zn, and Cd from contaminated paddy soil in Hunan Province (in China) was studied. The results indicated that the efficiencies of CA, FeCl₃, and EDTA on extracting Cu, Pb, Cd, and Zn from soil were greater than that of TA and CaCl₂, and their extraction efficiencies were EDTA ≥ FeCl₃ > CA. The efficiencies of CA + FeCl₃ on extracting Cu, Pb, Cd, and Zn were higher than that of single CA or FeCl₃. The 25 mmol L^-1 CA + 20 mmol L^-1 FeCl₃ was a promising composite leaching reagent for paddy soil, and it could remove Cu (57.6 %), Pb (59.3 %), Cd (84.8 %), and Zn (28.0 %), respectively. With the same amount of leaching reagent, the efficiency of continuous leaching by several times was higher than that by once. In addition, the easily reducible and oxidizable fractions of heavy metals showed significant decrease during the process of leaching.

Biochemical and Molecular Mechanisms of Plant-Microbe-Metal Interactions: Relevance for Phytoremediation

Plants and microbes coexist or compete for survival and their cohesive interactions play a vital role in adapting to metalliferous environments, and can thus be explored to improve microbe-assisted phytoremediation. Plant root exudates are useful nutrient and energy sources for soil microorganisms, with whom they establish intricate communication systems. Some beneficial bacteria and fungi, acting as plant growth promoting microorganisms (PGPMs), may alleviate metal phytotoxicity and stimulate plant growth indirectly via the induction of defense mechanisms against phytopathogens, and/or directly through the solubilization of mineral nutrients (nitrogen, phosphate, potassium, iron, etc.), production of plant growth promoting substances (e.g., phytohormones), and secretion of specific enzymes (e.g., 1-aminocyclopropane-1-carboxylate deaminase). PGPM can also change metal bioavailability in soil through various mechanisms such as acidification, precipitation, chelation, complexation, and redox reactions. This review presents the recent advances and applications made hitherto in understanding the biochemical and molecular mechanisms of plant-microbe interactions and their role in the major processes involved in phytoremediation, such as heavy metal detoxification, mobilization, immobilization, transformation, transport, and distribution.

Effects of long term raw pig slurry inputs on nutrient and metal contamination of tropical volcanogenic soils, Uvéa Island (South Pacific)

In small Polynesian islands, family pig breeding is usually conducted without recovery of pig slurry. Raw pig slurry is spread onto the soil without any treatment. So far, most of the studies were carried out in temperate climate and for industrial digested pig slurry applications on agricultural lands. In the present case study, conducted in Uvéa Island, the aim is to determine if long term application of raw pig slurry on tropical soils, naturally rich in heavy metals has a significant influence on elements concentrations and mobility. Two types of tropical soils and two pig breeding systems, pig enclosure on small concrete pens or pig enclosure in large land pens, were investigated. Here we demonstrate that raw pig slurry inputs on soils can lead to an increase of total nitrogen and phosphorus content with high Contamination Factors. The Pollution Load Index values (1.3; 5.3; 2.5; 2.3) were indicative of multi-heavy metals pollution (Fe, Mn, Al, Cu, Zn, Cr and Ni) in mixed calcareous soils of the coastal area and they are exchangeable while they are immobilized or less mobile in inland pure ferralitic soils. For mixed calcareous soils of the coastal area, family pig breeding represents a drainage risk of soluble species (phosphorus, inorganic nitrogen, Fe, Mn, Al, Cu, Zn, Cr and Ni). For inland ferralitic soils, family pig breeding is more compatible with a sustainable management of the environment in Uvéa Island and by extension in volcanic tropical islands with respect to the investigated chemical elements.

Removal, redistribution, and potential risks of soil Cd, Pb, and Zn after washing with various extractants

The effectiveness of four different washing extractants--HCl, FeCl3, citric acid, and EDTA--in removing Cd, Pb, and Zn from polluted soil was studied. The removal of these metals, their redistribution between fractions, and the potential risks posed by them, in soils washed with the tested extractants, were examined. Although all the rounds of washing removed Cd, Pb, and Zn from soil, the first round removed more metals than subsequent rounds. Each of the four extractants had different effects on the removal of the metals. At the end of the first round of washing, HCl, EDTA, and FeCl3 were the most effective in removing Zn, Pb, and Cd, respectively. Both the single round and five successive rounds of washing with various extractants resulted in significant increases in Pb in the exchangeable/acid extractable fraction. Washing with HCl, EDTA, and FeCl3 significantly reduced potential risks (calculated as the Potential Risk Index, PRI) posed by Cd in washed soil. The first round of washing, using all extractants, increased the risks posed by Pb and Zn. However, five successive rounds of washing with FeCl3 and EDTA reduced the risk posed by Pb, and washing with citric acid and FeCl3 increased the risks posed by Zn. EDTA and HCl were better for reducing Zn risks, and successive washing with EDTA and FeCl3 were more effective in reducing Pb risks than the other extractants. Finally, five successive rounds of washing, with all the extractants, effectively reduced the potential risks posed by Cd. Among the four reagents, EDTA was advised to be the alternative of the washing reagent by significantly reducing the PRI values of Cd, Pb, and Zn.

Enhanced efficiency of cadmium removal by Boehmeria nivea (L.) Gaud. in the presence of exogenous citric and oxalic acids

Boehmeria nivea (L.) Gaud. is a potential candidate for the remediation of Cd contaminated sites. The present investigation aims to explore Cd tolerance threshold and to quickly identify the role of exogenous organic acids in Cd uptake and abiotic metal stress damage. Elevated Cd levels (0-10mg/L) resulted in an obvious rise in Cd accumulation, ranging from 268.0 to 374.4 in root and 25.2 to 41.2mg/kg dry weight in shoot, respectively. Citric acid at 1.5 mmol/L significantly facilitated Cd uptake by 26.7% in root and by 1-fold in shoot, respectively. Cd translocation efficiency from root to shoot was improved by a maximum of 66.4% under 3 mmol/L of oxalic acid. Citric acid exhibited more prominent mitigating effect than oxalic acid due to its stronger ligand affinity for chelating with metal and avoiding the toxicity injury of free Cd ions more efficiently. The present work provides a potential strategy for efficient Cd remediation with B. nivea.

Effect of different zinc levels on activity of superoxide dismutases & acid phosphatases and organic acid exudation on wheat genotypes

A field study was carried out to investigate the effect of three Zn levels 0, 20 kg ZnSO4 ha(-1) and 20 kg ZnSO4 ha(-1)+ foliar spray of 0.5 % ZnSO4 on superoxide dismutase activity, acid phosphatase activity and grain yield and a pot experiment to study the effect of zinc deficient and sufficient conditions on organic acid exudation. Increasing Zn levels was established as beneficial in improving the enzyme activities of genotypes. Combined foliar and soil application of Zn proved to be superior of all the treatments. Zinc application resulted in a maximum increment limit of 96.8 % in superoxide dismutase activity, 75.76 % in acid phosphatase activity, and a decrement limit of 88.57 % in oxalic acid exudation irrespective of stages and year of study. The increased enzyme activities had a positive impact on grain yield. As an average of all genotypes an improvement of 19.88 % in 2009 and 21.29 % in 2010 due to soil application while of 16.45 % in 2009 and 13.01 % in 2010 due to combined application was calculated for grain yield. There existed a variation among genotypes in showing responses towards zinc application and the genotypes UP 2584 and PBW 550 were found to be more responsive.

Metal extractability patterns to evaluate (potentially) mobile fractions in periurban calcareous agricultural soils in the Mediterranean area-analytical and mineralogical approaches

A set of periurban calcareous agricultural Mediterranean soils was spiked with a mixture of Cd, Cu, Pb and Zn at two levels within the limit values proposed by current European legislation, incubated for up to 12 months, and subjected to various one-step extraction procedures to estimate mobile (neutral salts) and potentially mobile metal fractions (complexing and acidic extraction methods). The results obtained were used to study metal extractability patterns according to the soil characteristics. The analytical data were coupled with mineralogical investigations and speciation modelling using the Visual Minteq model. The formation of soluble metal-complexes in the complexing extracts (predicted by the Visual Minteq calculations) led to the highest extraction efficiency with complexing extractants. Metal extractability patterns were related to both content and composition of carbonate, organic matter, Fe oxide and clay fractions. Potentially mobile metal fractions were mainly affected by the finest soil fractions (recalcitrant organic matter, active lime and clay minerals). In the case of Pb, scarce correlations between extractable Pb and soil constituents were obtained which was attributed to high Pb retention due to the formation of 4PbCO3·3PbO (corroborated by X-ray diffraction). In summary, the high metal proportion extracted with complexing agents highlighted the high but finite capacity to store potentially mobilizable metals and the possible vulnerability of these soils against environmental impact from metal accumulation.