Chromium contamination accident in China: viewing environment policy of China

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Optimization and Production of Exopolysaccharides (EPS) and Indole-3-Acetic Acid (IAA) Under Chromium by Halophilic Bacteria Oceanobacillus oncorhynchi W4

The current study assessed the levels of exopolysaccharides (EPS) and indole-3-acetic acid (IAA) and the impacts of halophilic bacteria Oceanobacillus oncorhynchi W4 under Cr (VI). The effects of W4 were tested for its ability to remove Cr (VI) at several concentrations, the removal rate was reached after 48 h at 58.4%, 53.3%, 49.2%, and 43.1%). After 12-48 h, the maximum removal rate of 29-58% was found at an initial concentration of 50 mg/L (Cr (VI)). The Box-Behnken design based on response surface methodology was utilized to optimize the EPS, including pH, sucrose concentration, and incubation period. The highest EPS yield (314.5 mg/L) was obtained under 96 h at pH 7.0, with 5% sucrose concentration. The strain Oceanobacillus oncorhynchi W4 was tested for its ability to create EPS at various concentrations of Cr (VI). After 96 h, it generated the maximum amount of EPS (216.3 mg/L) at a concentration of 50 mg/L. By using FT-IR spectrum measurements, it was confirmed that hexavalent chromium and EPS had surface chemical interactions. At various Cr (VI) concentrations, the isolate W4 was tested for its ability to secrete Indole-3 acetic acid. IAA secretion (control) without Cr (VI) achieved a maximum of 1.45 mg/ml at 120 h. At 200 mg/L Cr (VI) concentration, 1.65 mg/ml of IAA was also produced after 48 h. According to the findings, Oceanobacillus oncorhynchi W4 was a promising isolate in a stressful environment.

Characteristics and numerical simulation of chromium transportation, migration and transformation in soil-groundwater system

Understanding chromium (Cr) migration and dispersion patterns in the soil-groundwater system is critical for the control and remediation of subsurface Cr contamination. In this study, a typical Cr-contaminated site from the Pearl River Delta (PRD) in China was simulated with a three-dimensional (3D) sandbox experiment to investigate the migration and transformation behavior of Cr. Results revealed that under the combined influence of rainfall and groundwater flow, a complex flow field favorable for 3D migration and solute dispersion was formed. The flow field characteristics were influenced by water-table depth, which in turn affected Cr behavior in the system. Moreover, downward flow field expansion under low water-table conditions led to Cr vertical migration range expansion, causing greater contamination in the deep soil. The migration process was accompanied with Cr(VI) reduction, during which approximately 75 % of the total Cr was immobilized in soils. The reactive transport model achieved a good fit for Cr retention and morphological distribution in the solid phase. The model indicates that Cr is more readily transported and dispersed with groundwater, and Cr migrated and spread downstream by 15 m during the eighth year. Therefore, managing water-table depth could be a strategy to minimize the Cr vertical migration and contamination.

Isolation of chromium resistant bacteria from tannery waste and assessment of their chromium reducing capabilities - A Bioremediation Approach

Every year different industries generate numerous toxic environmental polluting agents throughout the world. Among the polluting agents, chromium (Cr) toxicity is a great concern nowadays. It is continuously released in soil and water, causing environmental and health problems thereby raising several public health issues in developing countries like Bangladesh. The primary goal of this study was to provide a bioremediation option to reduce toxic hexavalent chromium to a less toxic trivalent form by isolating chromium resistant bacteria from Cr contaminated environments. Bacterial isolates were obtained from seven tannery waste samples collected from Hazaribag and Hemayetpur, Savar, Dhaka. Twenty morphologically distinct colonies were screened, of which six showed the highest resistance. These were designated as A1, A2, B1, F1, K1, and P1. Their maximum tolerance to Cr (VI) was determined through growth assays in varying chromium concentrations up to 8000 mg/L on LB agar media. Strains A2 and B1 exhibited the highest resistances to chromium at 7700 mg/L and 7200 mg/L respectively. Bacterial strains A2 and B1 were identified through several biochemical tests and after PCR analysis finally identified as Bacillus sp. and Micrococcus sp. respectively. Their Cr (VI) reduction capabilities were assessed quantitatively using the diphenylcarbazide colorimetric assay. Both strains exhibit approximately 100% reduction of chromium from 100 mg/L concentration to non-toxic form within 48 h using accurate analytical methods. This study demonstrates the isolation of highly chromium-resistant bacteria from tannery waste that can efficiently bioremediate Cr (VI) pollution, thus providing an eco-friendly and cost-effective bioremediation approach.

Heavy metals pollution from smelting activities: A threat to soil and groundwater

Throughout the literature, the word "heavy metal" (HM) has been utilized to describe soil contamination; in this context, we characterize it as those elements with a density greater than 5 g per cubic centimeter. Contamination is one of the major global health concerns, especially in China. China's rapid urbanization over the past decades has caused widespread urban water, air, and soil degradation. This study provides a complete assessment of the soil contamination caused by heavy metals in China's mining and smelting regions. The study of heavy metals (HMs) includes an examination of their potential adverse impacts, their origins, and strategies for the remediation of soil contaminated by heavy metals. The presence of heavy metals in soil can be linked to both natural and anthropogenic processes. Studies have demonstrated that soils contaminated with heavy metals present potential health risks to individuals. Children are more vulnerable to the effects of heavy metal pollution than adults. The results highlight the significance of heavy metal pollution caused by mining and smelting operations in China. Soil contaminated with heavy metals poses significant health concerns, both carcinogenic and non-carcinogenic, particularly to children and individuals living in heavily polluted mining and smelting areas. Implementing physical, chemical, and biological remediation techniques is the most productive approach for addressing heavy metal-contaminated soil. Among these methods, phytoremediation has emerged as a particularly advantageous option due to its cost-effectiveness and environmentally favorable characteristics. Monitoring heavy metals in soils is of utmost importance to facilitate the implementation of improved management and remediation techniques for contaminated soils.

Toxic effect of chromium on nonspecific immune, bioaccumulation, and tissue structure of Urechis unicinctus

The contamination of toxic heavy metals in aquatic environments has garnered significant global attention due to its detrimental effects on marine organisms and human health. Hexavalent chromium is a typical environmental and occupational heavy metal pollutant, identified as carcinogenic heavy metal. This study aimed to assess the impact of different Cr (VI) concentrations (0.05-2.5 mg/L) on Urechis unicinctus (U. unicinctus) by investigating bioaccumulation, antioxidant defense system, expression of resistance-related genes, and histological issues. A clear concentration-effect relationship was observed in the bioaccumulation of Cr (VI) in muscle tissues of U. unicinctus. Moreover, exposure to Cr (VI) can alter the activities of lysozyme (LSZ), catalase (CAT), and superoxide dismutase (SOD) to enhance cellular defense mechanisms in U. unicinctus. Likewise, maintained the normal protein structure and functional stability by regulating protein folding. The heat shock cognitive protein (HSC70) gene showed an upward and then downward trend after Cr (VI) exposure. At 12 h, the HSC70 gene expression reached the maximum values of 4.75 and 4.61-fold in the 0.1 and 1.5 mg/L groups, respectively. The organism produced a large number of free radicals, and elevated level of metallothionein (MT) was used to scavenge free radicals and alleviate oxidative stress. Additionally, histopathological examination revealed disorganization in the midgut, atrophic changes in intestinal connective tissue, uneven distribution in respiratory tissues, and irregular shape with a significant reduction in epithelial cells within the gastric cavity. These findings can serve as a valuable reference for elucidating the toxicity mechanisms of heavy metals towards marine benthic organisms and enhancing water environment monitoring strategies.

Effects of acute and chronic chromium stress on the expression of heat shock protein genes and activities of antioxidant enzymes in larvae of Orthetrum albistylum

The dragonfly species Orthetrum albistylum, can accumulate heavy metals from its aquatic environment and thus serves as a biological indicator for monitoring and evaluating water quality. Heat shock proteins (HSPs) play important biological roles in resistance to various types of environmental stress. The full-length cDNA sequences of the heat shock cognate (hsc) 70 and heat shock protein (hsp) 70 genes were cloned from O. albistylum larvae. Relative levels of expression of hsc70 and hsp70 in the head, epidermis, midgut, and adipose tissue were measured by qRT-PCR after chronic and acute contamination of 5-8 instar larvae with chromium (Cr) solution, and under control conditions. Activities of superoxide dismutase (SOD) and catalase (CAT) in chronically contaminated larvae were also measured. Phylogenetic analysis revealed that the cloned hsc70 and hsp70 genes were highly homologous to known HSP70 family members reported in other insects. The mRNA levels of hsc70 and hsp70 did not differ significantly in various larval tissues. Under chronic chromium stress, hsc70 and hsp70 expression were upregulated to a maximum and then downregulated; hsp70 mRNA levels were higher than those of hsc70 at all concentrations of chromium. Under acute chromium stress, hsc70 expression was inhibited at low chromium concentrations and upregulated at chromium concentrations higher than 125 mg/L. However, hsp70 expression was higher than that in the control group and markedly higher than that of hsc70. Changes in SOD and CAT activities displayed consistent trends for different chronic chromium concentrations, first increasing and then decreasing over time. Collectively, these findings demonstrated the response of the HSP family of genes and antioxidant enzymes following exposure to heavy metal stress, as well as their potential applicability as biomarkers for monitoring environmental pollutants.

Chromium removal from aqueous solution using bimetallic Bi0/Cu0-based nanocomposite biochar

Chromium (Cr), due to its greater contamination in aquifers and distinct eco-toxic impacts, is of greater environmental concern. This study aimed to synthesize nanocomposites of almond shells biochar (BC) with zerovalent bismuth and/or copper (Bi0/BC, Cu0/BC, and Bi0-Cu0/BC) for the removal of Cr from aqueous solution. The synthesized nanocomposites were investigated using various characterization techniques such as XRD, FTIR spectroscopy, SEM, and EDX. The Cr removal potential by the nanocomposites was explored under different Cr concentrations (25-100 mg/L), adsorbent doses (0.5-2.0 g/L), solution pH (2-8), and contact time (10-160 min). The above-mentioned advanced techniques verified successful formation of Bi0/Cu0 and their composite with BC. The synthesized nanocomposites were highly effective in the removal of Cr. The Bi0-Cu0/BC nano-biocomposites showed higher Cr removal efficiency (92%) compared to Cu0/BC (85%), Bi0/BC (76%), and BC (67%). The prepared nanocomposites led to effective Cr removal at lower Cr concentrations (25 mg/L) and acidic pH (4.0). The Cr solubility changes with pH, resulting in different degrees of Cr removal by Bi0-Cu0/BC, with Cr(VI) being more soluble and easier to adsorb at low pH levels and Cr(III) being less soluble and more difficult to adsorb at high pH levels. The experimental Cr adsorption well fitted with the Freundlich adsorption isotherm model (R2 > 0.99) and pseudo-second-order kinetic model. Among the prepared nanocomposites, the Bi0-Cu0/BC showed greater stability and reusability. It was established that the as-synthesized Bi0-Cu0/BC nano-biocomposite showed excellent adsorption potential for practical Cr removal from contaminated water.

Sources, impacts, factors affecting Cr uptake in plants, and mechanisms behind phytoremediation of Cr-contaminated soils

Chromium (Cr) is released into the environment through anthropogenic activities and has gained significant attention in the recent decade as environmental pollution. Its contamination has adverse effects on human health and the environment e.g. decreases soil fertility, alters microbial activity, and reduces plant growth. It can occur in different oxidation states, with Cr(VI) being the most toxic form. Cr contamination is a significant environmental and health issue, and phytoremediation offers a promising technology for remediating Cr-contaminated soils. Globally, over 400 hyperaccumulator plant species from 45 families have been identified which have the potential to remediate Cr-contaminated soils through phytoremediation. Phytoremediation can be achieved through various mechanisms, such as phytoextraction, phytovolatilization, phytodegradation, phytostabilization, phytostimulation, and rhizofiltration. Understanding the sources and impacts of Cr contamination, as well as the factors affecting Cr uptake in plants and remediation techniques such as phytoremediation and mechanisms behind it, is crucial for the development of effective phytoremediation strategies. Overall, phytoremediation offers a cost-effective and sustainable solution to the problem of Cr pollution. Further research is needed to identify plant species that are more efficient at accumulating Cr and to optimize phytoremediation methods for specific environmental conditions. With continued research and development, phytoremediation has the potential to become a widely adopted technique for the remediation of heavy metal-contaminated soils.

Effective removal of Cr(VI) in water by bulk-size polyaniline/polyvinyl alcohol/amyloid fibril composite beads

With the rapid expansion of industrial activities, chromium ions are discharged into the environment and cause water and soil pollution of various extents, which seriously endangers the natural ecological environment and human health. In this study, polyaniline/polyvinyl alcohol/amyloid fibril (PANI/PVA/AFL) composite gel beads (PPA) were prepared from polyaniline and amyloid fibrils with HCl as doping acid and PVA as a cross-linking agent. The results showed that PPA was an irregular composite bead with a diameter of 6 mm. The adsorption of Cr(VI) on the PPA gel beads followed the pseudo-second-order kinetics model, suggesting that chemical reactions were the controlling step in the Cr(VI) adsorption process. Though the Redlich-Peterson isotherm model had the best fit for the adsorption data, the isothermal adsorption process can be simplified using the Langmuir model. The maximum adsorption capacity for Cr(VI) in water was 51.5 mg g-1, comparable to or even higher than some PANI-based nanomaterials. Thermodynamic parameters showed that the adsorption process was a spontaneous, endothermic, and entropy-increasing process. Microscopic analysis revealed that the capture of Cr(VI) on PPA was mainly governed by electrostatic attraction, reduction, and complexation reactions. PPA can be used as a kind of effective remediation agent to remove Cr(VI) in water.

Exogenous glutathione can alleviate chromium toxicity in kenaf by activating antioxidant system and regulating DNA methylation

Glutathione (GSH) participates in plant response to heavy metals (HMs) stress, however, the epigenetic regulating mechanisms of GSH in HMs detoxification remains unclear. In this study, to reveal the potential epigenetic regulating mechanisms, kenaf seedlings were treated with/without GSH under chromium (Cr) stress. A comprehensive physiological, genome-wide DNA methylation and gene functional analysis were performed. Results showed that external GSH obviously recovered Cr-induced growth inhibition, significantly decreased H₂O₂, O₂^.- and MDA accumulation, increased the activities of antioxidant enzymes (SOD, CAT, GR and APX) in kenaf exposed to Cr. In addition, the expression level of the main DNA methyltransferase (MET1, CMT3 and DRM1) and demethylase (ROS1, DEM, DML2, DML3 and DDM1) genes were investigated by qRT-PCR. The result indicated that Cr stress decreased DNA methyltransferase genes expression while increased demethylase genes expression; however, apply exogenous GSH led to the recovery trend. These indicating exogenous GSH alleviation Cr stress on kenaf seedlings by increasing DNA methylation level. At the same time, the MethylRAD-seq genome-wide DNA methylation analysis showed the DNA methylation level was significantly increased after GSH treatment compared with Cr treatment alone. The differentially methylated genes (DMGs) were uniquely enriched in DNA repair, flavin adenine dinucleotide binding and oxidoreductase activity. Furthermore, a ROS homeostasis-associated DMG, HcTrx, was selected for further functional analysis. Results showed that the knock-down of HcTrx kenaf seedlings displayed yellow-green phenotype and impaired antioxidant enzyme activity; in contrast, the overexpression lines of HcTrx improved chlorophyll levels and enhanced Cr tolerance in Arabidopsis. Taken together, our results illustrate the novel role of GSH-mediated Cr detoxification in kenaf by modulating the DNA methylation, and thus further affect the activation of antioxidant defense systems. The present characterized Cr tolerant gene resource could be further used for kenaf Cr tolerant breeding via genetic improvement.

A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Asymmetric Alternative Current Electrochemical Method Coupled with Amidoxime-Functionalized Carbon Felt Electrode for Fast and Efficient Removal of Hexavalent Chromium from Wastewater

A large amount of Cr (VI)-polluted wastewater produced in electroplating, dyeing and tanning industries seriously threatens water ecological security and human health. Due to the lack of high-performance electrodes and the coulomb repulsion between hexavalent chromium anion and cathode, the traditional DC-mediated electrochemical remediation technology possesses low Cr (VI) removal efficiency. Herein, by modifying commercial carbon felt (O-CF) with amidoxime groups, amidoxime-functionalized carbon felt electrodes (Ami-CF) with high adsorption affinity for Cr (VI) were prepared. Based on Ami-CF, an electrochemical flow-through system powered by asymmetric AC was constructed. The mechanism and influencing factors of efficient removal of Cr (VI) contaminated wastewater by an asymmetric AC electrochemical method coupling Ami-CF were studied. Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) characterization results showed that Ami-CF was successfully and uniformly loaded with amidoxime functional groups, and the adsorption capacity of Cr (VI) was more than 100 times higher than that of O-CF. In particular, the Coulomb repulsion effect and the side reaction of electrolytic water splitting were inhibited by the high-frequency anode and cathode switching (asymmetric AC), the mass transfer rate of Cr (VI) from electrode solution was increased, the reduction efficiency of Cr (VI) to Cr (III) was significantly promoted and a highly efficient removal of Cr (VI) was achieved. Under optimal operating conditions (positive bias 1 V, negative bias 2.5 V, duty ratio 20%, frequency 400 Hz, solution pH = 2), the asymmetric AC electrochemistry based on Ami-CF can achieve fast (30 s) and efficient removal (>99.11%) for 0.5-100 mg·L^-1 Cr (VI) with a high flux of 300 L h^-1 m^-2. At the same time, the durability test verified the sustainability of the AC electrochemical method. For Cr (VI)-polluted wastewater with an initial concentration of 50 mg·L^-1, the effluent concentration could still reach drinking water grade (<0.05 mg·L^-1) after 10 cycling experiments. This study provides an innovative approach for the rapid, green and efficient removal of Cr (VI) containing wastewater at low and medium concentrations.

Metabolic pathway of Cr(VI) reduction by bacteria: A review

Heavy metal wastes, particularly hexavalent chromium [Cr(VI)], are generated from anthropogenic activities, and their increasing abundance has been a research concern due to their toxicity, genotoxicity, carcinogenicity and mutagenicity. Exposure to these dangerous pollutants could lead to chronic infections and even mortality in humans and animals. Bioremediation using microorganisms, particularly bacteria, has gained considerable interest because it can remove contaminants naturally and is safe to the surrounding environment. Bacteria, such as Pseudomonas putida and Bacillus subtilis, can reduce the toxic Cr(VI) to the less toxic trivalent chromium Cr(III) through mechanisms including biotransformation, biosorption and bioaccumulation. These mechanisms are mostly linked to chromium reductase and nitroreductase enzymes, which are involved in the Cr(VI) reduction pathway. However, relevant data on the nitroreductase route remain insufficient. Thus, this work proposes an alternative metabolic pathway of nitroreductase, wherein nitrate activates the reaction and indirectly reduces toxic chromium. This nitroreductase pathway occurs concurrently with the chromium reduction pathway.

The interaction of ZnO nanoparticles, Cr(VI), and microorganisms triggers a novel ROS scavenging strategy to inhibit microbial Cr(VI) reduction

Cr(VI) contaminated water usually contains other contaminants like engineered nanomaterials (ENMs). During the process of microbial treatment, the inevitable interaction of Cr(VI), ENMs, and microorganisms probably determines the efficiency of Cr(VI) biotransformation, however, the corresponding information remains elusive. This study investigated the interaction of ZnO nanoparticles (NPs), Cr(VI), and Pannonibacter phragmitetus BB (hereafter BB), which changed the process of microbial Cr(VI) reduction. ZnO NPs inhibited Cr(VI) reduction, but had no effect on bacterial viability. In particular, Cr(VI) induced BB to produce organic acids and to drive Zn²⁺ dissolution from ZnO NPs inside and outside of cells. The dissolved Zn²⁺ not only promoted Cr(VI) reduction to Cr(V)/Cr(IV) by strengthening sugar metabolism and inducing increase in NAD(P)H production, but also hindered Cr(V)/Cr(IV) transformation to Cr(III) through down-regulating Cr(VI) reductase genes. A novel bacterial driven ROS scavenging mechanism leading to the inhibition of Cr(VI) reduction was elucidated. Specifically, the accumulated Cr(VI) and Cr(V)/Cr(IV) formed a redox dynamic equilibrium, which triggered the disproportionation of superoxide radicals mimicking superoxide dismutase through the flip-flop of Cr(VI) and Cr(V)/Cr(IV) in bacterial cells. This study provided a realistic insight into design the applicability of biological remediation technology for Cr(VI) contaminant and evaluating environmental risks of ENMs.

Sodium Nitroprusside Improves Bamboo Resistance under Mn and Cr Toxicity with Stimulation of Antioxidants Activity, Relative Water Content, and Metal Translocation and Accumulation

Sodium nitroprusside (SNP), as a single minuscule signaling molecule, has been employed to alleviate plant stress in recent years. This approach has a beneficial effect on the biological and physiological processes of plants. As a result, an in vitro tissue culture experiment was carried out to investigate the effect of high and low levels of SNP on the amelioration of manganese (Mn) and chromium (Cr) toxicity in a one-year-old bamboo plant, namely Pleioblastus pygmaea L. Five different concentrations of SNP were utilized as a nitric oxide (NO) donor (0, 50, 80, 150, 250, and 400 µM) in four replications of 150 µM Mn and 150 µM Cr. The results revealed that while 150 µM Mn and 150 µM Cr induced an over-generation of reactive oxygen species (ROS) compounds, enhancing plant membrane injury, electrolyte leakage (EL), and oxidation in bamboo species, the varying levels of SNP significantly increased antioxidant and non-antioxidant activities, proline (Pro), glutathione (GSH), and glycine betaine (GB) content, photosynthesis, and plant growth parameters, while also reducing heavy metal accumulation and translocation in the shoot and stem. This resulted in an increase in the plant's tolerance to Mn and Cr toxicity. Hence, it is inferred that NO-induced mechanisms boosted plant resistance to toxicity by increasing antioxidant capacity, inhibiting heavy metal accumulation in the aerial part of the plant, restricting heavy metal translocation from root to leaves, and enhancing the relative water content of leaves.

Molecular mechanisms of Chromium(III) sorption by organo-ferrihydrite coprecipitates induced by crop straws

Agricultural organo-ferrihydrite (Fh) coprecipitates (OFCs), resulting from the coprecipitation of Fe(III) and dissolved organic carbon (DOC) from returned straws, significantly affect the bioavailability of heavy metals in farmland. However, the molecular sorption mechanisms of Cr(III) by the OFCs remain unclear. Here, we explored the sorption behaviors of Cr(III) by the OFCs formed with wheat or maize straws derived-DOC (wheat-DOC or maize-DOC) under various environmental conditions, and further underlying molecular sorption mechanisms using Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy. Results showed that high C loadings reduced the specific surface areas (SSAs) and Cr(III) sorption capacities of the OFCs, implying the blockage of binding sites by C loading. Additionally, although the wheat-DOC induced OFC had a smaller SSA than the maize-DOC induced OFC, their Cr(III) sorption were comparable, which was likely to be compensated by the more carboxyl in the wheat-DOC. Moreover, at a higher ionic strength, the increased or slightly decreased Cr(III) sorption indicated that the inner-sphere sorption was dominant regardless of high or low C loadings, which was also supported by the extremely low Cr(III) extraction percentage. The Cr K-edge XANES spectroscopy suggested that Cr(III) could be immobilized by both the Fh and organic fractions, with the Fh fractions playing a significant role. These findings contribute to a molecular-level mechanistic understanding of Cr(III) sorption by the OFC, which will aid in the prevention and control of Cr-contaminated agricultural soils.

Reduction of hexavalent chromium using bacterial isolates and a microbial community enriched from tannery effluent

We investigated microbial growth in increasing concentrations of hexavalent chromium (Cr(VI)) and its reduction by a microbial community enriched from tannery effluent and by the bacterial strains isolated from the enriched community. The bacterial growth was monitored by measuring the optical cell density (OD₆₅₀), while the Cr(VI) concentration in the samples was determined using spectrophotometry and liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). At a Cr(VI) concentration of 100 mg/L, the isolates affiliated with Pseudomonas aeruginosa (P. aeruginosa) reached higher optical cell densities, but were in general less effective for Cr(VI) reduction than the isolates affiliated with Mammaliicoccus sciuri (M. sciuri). All three M. sciuri isolates and only one of the seven P. aeruginosa isolates were able to reduce 50% of the Cr(VI) with an initial concentration of 100 mg/L within 24 h (pH 7.1), while the six isolates affiliated with P. aeruginosa were less effective. Compared to the isolated, individual bacterial strains, the enriched microbial community was better adapted to the elevated Cr(VI) concentrations, but needed a longer time (48 h) to reduce the Cr(VI) with the same efficacy as the most efficient individual isolates. The ability of the enriched microbial community and the isolated bacterial strains to reduce the Cr(VI) highlights their potential for use in the rapid bioremediation of wastewaters contaminated with Cr(VI).

Pollution and Health Risk Assessments of Potentially Toxic Elements in the Fine-Grained Particles (10−63 µm and <10 µm) in Road Dust from Apia City, Samoa

Fine road dust is a major source of potentially toxic elements (PTEs) pollution in urban environments, which adversely affects the atmospheric environment and public health. Two different sizes (10−63 and <10 μm) were separated from road dust collected from Apia City, Samoa, and 10 PTEs were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Fine road dust (<10 μm) had 1.2−2.3 times higher levels of copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), antimony (Sb), lead (Pb), and mercury (Hg) than 10−63 μm particles. The enrichment factor (EF) value of Sb was the highest among PTEs, and reflected significant contamination. Cu, Zn, and Pb in road dust were also present at moderate to significant levels. Chromium (Cr), cobalt (Co), and nickel (Ni) in road dust were mainly of natural origins, while Cu, Zn, Sb, and Pb were due to traffic activity. The levels of PTEs in road dust in Samoa are lower than in highly urbanized cities, and the exposure of residents in Samoa to PTEs in road dust does not pose a noncarcinogenic health risk. Further studies of the effects of PTEs contamination in road dust on the atmosphere and living organisms are needed.

Chromium Induces Toxicity at Different Phenotypic, Physiological, Biochemical, and Ultrastructural Levels in Sweet Potato (Ipomoea batatas L.) Plants

Crop productivity is enormously exposed to different environmental stresses, among which chromium (Cr) stress raises considerable concerns and causes a serious threat to plant growth. This study explored the toxic effect of Cr on sweet potato plants. Plants were hydroponically grown, and treatments of 0, 25, 50, 100, and 200 µM Cr were applied for seven days. This study exhibited that a low level of Cr treatment (25 µM) enhanced the growth, biomass, photosynthesis, osmolytes, antioxidants, and enzyme activities. However, significant deleterious effects in growth, biomass, photosynthetic attributes, antioxidants, and enzymes were observed at higher levels of Cr treatment. The remarkable reduction in plant growth traits was associated with the over-accumulation of H2O2 and MDA contents (410% and 577%, respectively) under the highest rate of Cr (200 µM). Under 200 µM Cr, the uptake in the roots were 27.4 mg kg-1 DW, while in shoots were 11 mg kg-1 DW with the highest translocation rate from root to shoot was 0.40. The results showed that the higher accumulation of Cr negatively correlated with the phenotypic and physiological parameters. It may be proposed that Cr toxicity causes oxidative damage as sustained by augmented lipid peroxidation, reactive oxygen species, and reduced photosynthetic rate, chlorophyll, and stomatal traits. The chloroplastic ultrastructure was damaged, and more apparent damage and size reduction were observed at higher Cr levels. Furthermore, aggregated Cr concentration positively correlates with the increase of osmolytes and superoxide dismutase (SOD) activity in the leaves of sweet potato. Moreover, improved osmolytes and SOD do not help protect sweet potato against high Cr stress. Overall, these findings will improve the understanding of the defense mechanisms of sweet potato to Cr stress.

Asymmetrical alternating current electrochemically-mediated washing method for sustainable remediation of Cr(VI)-contaminated soil

The demands for genuine remediation of heavy metal contaminated soil have triggered extensive studies in the soil washing method. However, numerous soil washing methods show poor sustainability for target soil, due to the tremendous cost, hidden secondary pollution and severe soil deterioration. Here, an asymmetrical alternating current electrochemically-mediated remediation platform (ACRP) is developed by fabricating an amidoxime-functionalized electrode (Ami-electrode). The real soil contaminated with 1200 mg/kg Cr(VI) is remediated efficiently to less than safety level (30 mg/kg), meanwhile no exorbitant soil nutrient loss is observed and no secondary pollution occurs. Furthermore, the consumption of washing effluents for the ACRP method is 24 times lower than the traditional washing method. Ami-electrode with asymmetrical alternating current promote the electrocatalytic efficiency by inhibiting the Coulomb repulsion between Cr(VI) species and cathode. With the aid of Ami-electrode and positive bias, Cr(VI) species in effluents are adsorbed on chelating site. By subsequent negative bias, Cr element is reduced and recycled in the less hazardous form of amorphous Cr(III) hydroxide, and effluents are regenerate concurrently in a cyclic system. Durability experiment and cost calculation verify the exceptional sustainability and feasibility for remediation practices. This work provides a sustainable remediation method for Cr(VI)-contaminated soil, and then paves the way to develop electrochemically soil remediation platform for practical applications.

New insight for the diffusion-resupply kinetics of Cr(VI) in contaminated soil using DGT/DIFS

Chromium (Cr) is a widespread pollutant with high toxicity and mobility. However, the diffusion-resupply kinetics of Cr(VI) between the solid phase and solution in the soils remain unclear. Here, we quantified the contributions of the soil solution and solid phase to the diffusion-resupply process of Cr(VI) in the contaminated soils using the diffusive gradients in thin-films (DGT) and DGT-induced fluxes in soils model. Based on the solution extraction result, Cr(VI) was the main available Cr species in the contaminated soils. Comparing the two diffusion-resupply stages of the kinetic process, the potential hazards due to the resupply from the solid phase can reach 10.71-50.66 %, although the soil solution accounted for the largest proportion of the effective concentration of Cr(VI) (49.34-89.29 %), which was ignored in the traditional equilibrium method. The kinetic parameters can be used to interpret the dynamic process. The resupply ability of the solid phase was closely related to the response time (T_c). The longer T_c was consistent with the low desorption constant, indicating a kinetic limitation. The magnitude of the resupply from the solid phase was related to labile pool size of Cr(VI) and soil organic carbon content. This study established a new quantification method for assessing diffusion-resupply kinetics of Cr(VI) in the soil， indicating the underestimation of Cr(VI) risk based on the use of traditional equilibrium methods. Our data provided a scientific basis for ecological risk assessment, pollution prevention, surface- and groundwater control, and environmental governance in areas with Cr contaminated soil.

Raspberry stalks-derived biochar, magnetic biochar and urea modified magnetic biochar - Synthesis, characterization and application for As(V) and Cr(VI) removal from river water

Raspberry stalks-derived biochar (BC), magnetic biochar-iron oxide composite (BC-Fe) and its derivative modified with urea (BC-Fe-U) were synthesized, characterized and tested as(V) and Cr(VI) ion sorbents. The surface area of BC, BC-Fe and BC-Fe-U was 259, 163 and 117 m² g^-1, respectively. The structure of BC was dominated by micropores, while in BC-Fe and BC-Fe-U mesopores predominated. Based on the XRD results, it was found that the magnetic properties of the biochar-iron oxide composites are due to the presence of ferrimagnetic magnetite (Fe₃O₄) and maghemite (Fe₂O₃). The optimal pH of As(V) and Cr(VI) adsorption onto the studied sorbents is in the range of 2.3-5.7. Pristine biochar (BC) does not adsorb As(V) ions; however, it enables rapid adsorption of Cr(VI) with the static adsorption capacity of 19.2 mg g^-1. The maximum static adsorption capacities of As(V) and Cr(VI) ions onto BC-Fe and BC-Fe-U are within the range of 13.5-16.3 mg g^-1. For most adsorption systems tested, adsorption equilibrium is reached within 4 h, though even a few minutes is enough to reach half of the adsorption static value. Phosphates over 0.005 mol L^-1 hinder adsorption of As(V) and Cr(VI) ions. Application of at least 5 mol L^-1 nitric acid allows about 95% of Cr(VI) and As(V) to be desorbed from adsorbate-loaded BC-Fe material. For other materials, the desorption efficiencies are significantly lower. BC-Fe and BC-Fe-U materials were successfully used for simultaneous Cr(VI) and As(V) removal from river water.

Rapid ultrasensitive detection of hexavalent chromium in soil and groundwater by a microProbing imaging platform

Rapid detection methods are needed to investigate the environmental quality risk of soil and groundwater in contaminated lands. Currently there is lack of rapid detection methods to sensitively and accurately analyze contaminations of hexavalent chromium in soil due to the challenge of complex sample pretreatment or expensive instrumentation. Here we report a rapid accurate detection platform for quantifying hexavalent chromium in soil and groundwater with ultrasensitivity. The platform consists of a novel sensor of microProbing beads and a portable microscope. Each microProbing bead was a nanoliter reactor to selectively sequestrate Cr (VI) with the enrichment factor up to 150 ×. The microProbing beads presented the signal uniformity of ~97% for the statistical colorimetric imaging analysis. Combined with a miniaturized microscope, the microProbing beads allowed for detecting aqueous Cr (VI) and soluble Cr (VI) in soil within 45 min. The platform achieved high sensitivity with the detection limits of 0.003 ppb for aqueous Cr (VI) and 0.07 ppm for soil Cr (VI). It accurately detected soil and groundwater samples from a chromium contaminated land in Yangtze River Basin of China. The consistency to the laboratory standard methods was achieved with the low cost of ~0.20 US dollar per test. The microProbing imaging platform with the operational simplicity and device portability is highly promising for the field analysis of Cr (VI) in contaminated lands.

Resource utilization of hazardous Cr/Fe-rich sludge: synthesis of erdite flocculant to treat real electroplating wastewater

Cr/Fe-bearing sludge is a hazardous solid waste, produced at mass production in smelting, plating and surface finishing industries. Such waste is commonly treated by chemical detoxification and safety landfill, whereas only a few Cr-rich sludge is recycled as a tanning reagent. In this study, a novel route was developed to recycle Cr/Fe-bearing sludge as erdite-bearing flocculant for wastewater treatment. Results showed that two sludges were irregular aggregates, one of which contained 1.6 wt.% Cr (short for LS) and the other contained 4.2 wt.% Cr (HS). After hydrothermal treatment, stable Cr(III)/S-bearing product was formed from the Cr(VI) reduction in the sludges. Conversely, erdite was generated in nanorod form with diameter and length of 200 nm and 0.5-1 μm from LS, respectively, whereas grew radially to 1.5-2.5 μm for HS. The two erdite-bearing products were spontaneously hydrolysed to Fe/S-bearing flocs and showed similar performance in the treatment of real electroplating effluent with 91.55, 1.94 and 0.25 mg/L of Zn, Ni and Cr, respectively. For instance, by adding 1 g/L product of LS, the release of Cr from the products did not occur, and the residual Zn, Ni and Cr in the effluent was 0.25, 0.65 and 0.17 mg/L, respectively, which met the discharge standard of the electroplating industry. With the two converted products, the residual Zn/Ni/Cr concentrations were apparently lower than those of the raw sludges and other common reagents (e.g. polymeric ferric sulphate, activated carbon and diatomite). Thus, such erdite-bearing products could serve as a flocculant and then be applied in electroplating wastewater treatment.

Removal of hexavalent chromium from water by Z-scheme photocatalysis using TiO2 (rutile) nanorods loaded with Au core-Cu2O shell particles

All-solid-state Z-scheme photocatalysts, containing Cu₂O, TiO₂ (rutile), and Au as the electron mediator, were prepared and applied to the reduction of Cr(VI) in aqueous solutions. The Cu₂O-Au-TiO₂ composites were prepared by loading Au core-Cu₂O shell hemisphere particles on TiO₂ (rutile) nanorods using a two-step photocatalytic deposition process. Under ultraviolet-visible (UV-vis) light illumination, the Cu₂O-Au-TiO₂ composites exhibited higher photocatalytic Cr(VI) reduction activities than those exhibited by single TiO₂ (rutile) and Cu₂O. In this reaction, a precipitate containing Cr, which was considered to be Cr(OH)₃, was deposited site-selectively on the Au core-Cu₂O shell particles of the composites, indicating that the reduction site of the composite was Cu₂O, and the reaction proceeded according to the Z-scheme. The Cu₂O-Au-TiO₂ composites also exhibited photocatalytic activity under visible light illumination. The oxidation state of Cu in the Cu₂O-Au-TiO₂ composite gradually changed from Cu(I) to Cu(II) during the photocatalytic Cr(VI) reduction. However the composite maintained its high photocatalytic performance even after oxidation. The role of Au in the Cu₂O-Au-TiO₂ composite was examined by comparing the properties of the Cu₂O-Au-TiO₂ composite with those of the Cu₂O-TiO₂ composite prepared via direct Cu₂O deposition on TiO₂.

Lung function assessment and its association with blood chromium in a chromate exposed population

Hexavalent chromium [Cr(VI)] and its compounds have been associated with various respiratory diseases, while few studies have attempted to determine its adverse effect on lung function. To explore the potential early indicators of health surveillance for respiratory diseases induced by chromate exposure, a longitudinal cohort study including 515 workers with 918 measurements across 2010-2017 was conducted to investigate the impact of individual internal exposure on lung function. Inductively coupled plasma mass spectrometry (ICP-MS) and spirometry were used to measure whole blood chromium (blood Cr) and lung function respectively. In the linear mixed-effects analysis, each 1- unit increase in Ln- transformed blood Cr was significantly associated with estimated effect percentage decreases of 1.80 (0.35, 3.15) % in FEV1, 0.77 (0.10, 1.43) % in FEV1/FVC, 2.78 (0.55, 4.98) % in PEF, and 2.73 (0.59, 4.71) % in FEF25-75% after adjusting for related covariates. Exposure- response curve depicted the reduction of lung function with blood Cr increase, and the reference value of blood Cr was proposed as 6 μg/L considering the lung function as health outcome. Based on the repeated-measure analysis, compared with the low frequency group, subjects with high frequency of high exposure across 2010-2017 had an additional reduction of 5.65 (0, 11.3) % in FVC. Subjects with medium frequency showed more obvious declines of 9.48 (4.16, 14.87) % in FVC, 8.63 (3.49, 13.97) % in FEV1, 12.94 (3.34, 22.53) % in PEF and 10.97 (3.63, 18.30) % in MVV. These findings suggested that short- term high exposure to Cr associated with obstructive ventilatory impairment, and long- term exposure further led to restrictive ventilatory impairment.

Effect of nano bentonite on direct yellow 50 dye removal; Adsorption isotherm, kinetic analysis, and thermodynamic behavior

Developing countries suffering from the toxicity of different industrial effluents especially dyes. This study successfully prepared and characterized nano-bentonite for anionic dye removal (DY 50). The prepared nanoparticles were characterized by X-Ray Diffraction (XRD), X-ray Fluorescence (XRF), Scanning Electron Microscope (SEM), EDAX analysis, FT-IR, and TGA and the obtained results indicated the formation of nanoparticles with an average size of 15 nm. The effect of different operating conditions was studied using different pH, dose, contact time, temperature, and initial DY 50 concentrations. The obtained results indicated that nano bentonite was able to adsorb about 78.3 and 100% for initial concentrations of 100±8.1 and 20 ±1.62 mg/L, respectively. The optimum removal conditions were observed at acidic media (pH 3) using sorbent material dosage 1 g/L for 45 min and 30°C. The adsorption isotherm, kinetic analysis, and thermodynamic behavior were studied by using linear equation form, and the adjusted R2 was compared to detect the preferred models. The adsorption isotherm indicated that heterogeneous, as well as multilayer adsorption, plays an important role in the removal of dye. Kinetic studies indicated the chemisorption interaction between sorbed and adsorbed molecules. Thermodynamic behavior indicated the reaction is exothermic with ∆H equal to −5.24 KJ/mol and ∆S equal −74.2 J/K.mol. Finally, this study strongly recommended using nano bentonite for DY 50 removal from an aqueous solution. The RSM relations show significant relations in all removal models with p-value <0.001. The ANN results indicated that the most effective operating conditions are the effect of nano bentonite dose followed by the pH effect.

Recovery of trivalent and hexavalent chromium from chromium slag using a bipolar membrane system combined with oxidation

Chromium slag (CS) with large quantities of multivalent Cr species (III and VI) generated during chromium salt production is hazardous to nature and living organisms. Furthermore, CS discharge leads to considerable resource wastage. Herein, a bipolar membrane electrodialysis (BMED) system was employed along with hydrogen peroxide (H₂O₂) oxidation for simultaneously recovering Cr(III) and Cr(VI) from CS in the form of Na₂CrO₄. A bipolar membrane was used to produce OH^- under a direct electric field, providing an alkaline environment for the oxidative conversion of Cr(III) to Cr(VI) in the presence of H₂O₂, followed by the recovery of Cr(III) and Cr(VI) as Na₂CrO₄. The effect of H₂O₂content on Cr(III) oxidation and that of the current density on chromium recovery, current efficiency and specific energy consumption were investigated. Moreover, the morphology of chromium in CS before and after the BMED treatment was analysed. The H₂O₂ content affected the Cr oxidation rate from Cr(III) to Cr(VI). The current density affected chromium removal, current efficiency and specific energy consumption. At a current density of 2 mA/cm², the total chromium recovery exceeded 67% and the remaining chromium was mainly in the residual state (RES). When the number of CS compartments increased, the current efficiency was enhanced and the specific energy consumption decreased. Binding state analysis show that Cr(III) and different species of Cr(VI) could be transformed into exchangeable Cr(VI) after H₂O₂ oxidation and BMED treatment. After the treatment, 92% of the remaining chromium in CS was in the RES. Thus, the employed method can effectively recover chromium from CS and other chromium-contaminated solid waste.

Efficient immobilization and utilization of chromite ore processing residue via hydrothermally constructing spinel phase Fe2+(Cr3+X, Fe3+2-x)O4 and its magnetic separation

Chromite ore processing residue (COPR) has been a severe environmental contaminant which is worthy of attention. In this study, we developed an eco-friendly and practical technology for effectively stabilizing and recovering Cr(VI) in COPR via combining FeSO₄ reducing agent and the hydrothermal treatment. A stable spinel phase product was formed during detoxification. In addition, the ferrochrome resources in the treated COPR can be obtained by magnetic separation. As we studied, the hydrothermal environment promoted the release of unstable Na₂CrO₄ from COPR into the solution, and the released CrO₄^2- was reduced to Cr(III) by FeSO₄. Subsequently, Cr(III), Fe(II) and Fe(III) were hydrothermally mineralized to form the magnetic spinel phase Fe²⁺(Cr³⁺_X, Fe³⁺_2-x)O₄ (FeCr spinel substance), which was conducive to the magnetic separation of ferrochrome resources. Under the optimal hydrothermal conditions (0.15 g FeSO₄/2 g COPR, treatment at 180 °C for 8 h), the total Cr leaching concentration of treated COPR (COPR-HT) was decreased from 120.51 mg L^-1 to 0.23 mg L^-1, well below the regulatory limit of 1.5 mg L^-1 (HJ/T 301-2007, China EPA). After 300 days aging under atmospheric conditions, the total Cr leaching concentration of COPR-HT was still below 1.5 mg L^-1. Besides, the COPR-HT after magnetic separation contained 11.52 wt% Cr₂O₃ and 53.44 wt% Fe₂O₃, which can be used as the raw material for steel industry. The underlying mechanism of COPR stabilization was explained by XRD, XPS and SEM-EDS analysis. This work converted the toxic and unstable Cr(VI) in COPR into the long-term stable FeCr spinel substance that is easy to magnetically separate. It has important reference for the harmless disposal and resource utilization of other chromium-containing hazardous wastes including chromium slag and electroplating sludge.

Sustainable enhancement of Cr(VI) bioreduction by the isolated Cr(VI)-resistant bacteria

Bioreduction of mobile Cr(VI) to sparingly soluble Cr(III) is an effective strategy for in situ remediations of Cr contaminated sites. The key of this technology is to screen Cr(VI)-resistant bacteria and further explore the sustainable enhancement approaches towards their Cr(VI) reduction performance. In this study, a total of ten Cr(VI)-resistant bacteria were isolated from a Cr(VI) contaminated site. All of them could reduce Cr(VI), and the greatest extent of Cr(VI) reduction (98%) was obtained by the isolated CRB6 strain. The isolated CRB6 was able to reduce structural Fe(III) in Nontronite NAu-2 to structural Fe(II). Compared with the slow bioreduction process, the produced structural Fe(II) can rapidly enhance Cr(VI) reduction. The resist dissolution characteristics of NAu-2 in the redox cycling may provide sustainable enhancement of Cr(VI) reduction. However, no enhancement on Cr(VI) bioreduction by the isolated CRB6 was observed in the presence of NAu-2, which was attributed to the inhibition of Cr(VI) on the electron transfer between the isolated CRB6 and NAu-2. AQDS can accelerate the electron transfer between the isolated CRB6 and NAu-2 as an electron shuttle in the presence of Cr(VI). Therefore, the combination of NAu-2 and AQDS generated a synergistic enhancement on Cr(VI) bioreduction compared with the enhancement obtained by NAu-2 and AQDS individually. Our results highlight that structural Fe(III) and electron shuttle can provide a sustainable enhancement of Cr(VI) reduction by Cr(VI)-reducing bacteria, which has great potential for the effective Cr(VI) in-situ remediation.

Preparation of sludge biochar rich in carboxyl/hydroxyl groups by quenching process and its excellent adsorption performance for Cr(VI)

A facile pyrolysis-quenching-reroasting process was developed to prepare a sludge-based biochar adsorbent, and its adsorption performance for Cr(VI) ions was investigated. The unquenched biochar (U-BC) and quenched biochar (Q-BC) were systematically compared and characterized. Fourier transform infrared spectroscopy (FTIR) results showed that more carbon and oxygen functional groups such as -COOH and -OH were formed on the surface of Q-BC. These functional groups could be used as active sites during the adsorption process and help to improve the adsorption performance of the material. The results of Brunauer-Emmett-Teller (BET) analysis showed that the specific surface area of U-BC biochar was 523.36 m²/g, while the specific surface area of Q-BC biochar after quenching treatment increased to 785.3 m²/g. The adsorption performance of Q-BC biochar was studied, and the effects of pH, contact time and temperature on the adsorption performance of the material were explored. The pseudo-second-order model and Langmuir isotherm model indicated that the removal of Cr(VI) by Q-BC biochar material was a chemical adsorption-based adsorption process. At a temperature of 298 K and a pH of 1, the maximum Cr(VI) adsorption capacity of the quenched Q-BC biochar is as high as 291.54 mg/g, which was much higher than the maximum adsorption capacity of U-BC biochar (91.46 mg/g). This pyrolysis-quenching-reroasting process to prepare modified biochar provides a new, economical and effective way for the preparation of high-performance adsorption materials from municipal sludge.

Salicylic acid alleviates chromium (VI) toxicity by restricting its uptake, improving photosynthesis and augmenting antioxidant defense in Solanum lycopersicum L

Contamination of agricultural soil by chromium (Cr) is a serious menace to environmental safety and global food security. Although potential of salicylic acid (SA) in mitigating heavy metal (HM) toxicity in plants is well recognized, detailed physiological mechanisms behind such beneficial effects under Cr-stress in tomato (Solanum lycopersicum L.) plant are far from being completely unravelled. The present study evaluated the efficacy of exogenously applied SA, in alleviating Cr-mediated alterations on photosynthesis and antioxidant defense in tomato exposed to three different concentrations of Cr(VI) [0, 50, and 100 mg Cr(VI) kg^-1 soil]. Exposure of tomato plants to Cr resulted in increased Cr-accumulation and oxidative damage, as signposted by high Cr concentration in root as well as shoot, augmented malondialdehyde (MDA) and superoxides levels, and inhibition in enzymes of ascorbate-glutathione (AsA-GSH) cycle. Furthermore, a significant (P ≤ 0.05) reduction in photosynthetic pigments and gas exchange parameters was also evident in Cr-stressed tomato plants. Findings of the present study showed that exogenous application of 0.5 mM SA not only promoted plant growth subjected to Cr, but also restored Cr-mediated disturbances in plant physiology. A significant (P ≤ 0.05) decrease in Cr acquisition and translocation as evidenced by improved growth and photosynthesis in SA-treated plants was observed. Additionally, exogenous SA application by virtue of its positive effect on efficient antioxidant system ameliorated the Cr-mediated oxidative stress in tomato plants as signposted by lower MDA and superoxide levels and improved AsA-GSH cycle. Overall, current study advocates the potential of exogenous SA application in amelioration of Cr-mediated physiological disturbances in tomato plant.

Exploring different mechanisms of biochars in removing hexavalent chromium: Sorption, reduction and electron shuttle

The role of different biochars in Cr(VI) removal process is not clear. Two raw materials with distinct components were prepared into biochars at various pyrolysis temperatures. The biochar derived from cotton stark (with less lignin than walnut shell) always had more surface functional groups. The pyrolysis temperature had a significant effect on the biochar. In the sorption experiments, the fitting results of two compartment models showed that slow compartment dominated by surface functional groups plays a leading role in the Cr(VI) removal by low-temperature char, while the contribution of the rapid or slow compartment to the process of high-temperature char was equal. In the experiment of biochar mediated the lactate to reduce Cr(VI), the biochar derived from walnut shell (richer lignin) had better conductivity due to its highly aromatic structure, which could directly transfer electrons. This study indicated that various biochars may play different roles in the Cr(VI) removal process.

Inhibition of Metamorphosis, Thyroid Gland, and Skeletal Ossification Induced by Hexavalent Chromium in Bufo gargarizans Larvae

Hexavalent chromium (Cr [VI]) is one of the major detrimental heavy metal pollutants. In the present study, Bufo gargarizans were exposed to 0, 52, 104, 208, and 416 μg/L Cr (VI) from Gosner stage 2 until metamorphosis; and growth, development, and histological characteristics of the thyroid gland and skeletal ossification were examined. The results demonstrated that the survival rate of larvae exposed to Cr (VI) was not different from that measured in animals from the control group. However, high levels of Cr (VI) (104, 208, and 416 μg/L) were associated with significantly delayed growth and development. The suppression of skeletal ossification was observed at high Cr (VI) levels. Besides, histological alterations of the thyroid gland, such as follicular cell hyperplasia, colloid depletion, and peripheral colloid vacuolation, were found in 52 to 416 μg/L Cr (VI) treatments. The results of the present study highlight reductions in growth and development as well as percent metamorphosis and skeletal ossification due to histological alteration of the thyroid gland during exposure to Cr (VI) in B. gargarizans larvae. The present investigation could provide a basis for understanding the detrimental effects of Cr (VI) in amphibian larvae. Environ Toxicol Chem 2021;40:2474-2483. © 2021 SETAC.

Simultaneous separation and immobilization of Cr(VI) from layered double hydroxide via reconstruction of the key phases

Layered double hydroxide (LDH) is one of the key host phases of Cr(VI) in the natural environment and chromite ore processing residue (COPR), causing serious pollution by Cr(VI). Therefore, efficient extraction or immobilization of the incorporated Cr(VI) in LDH is urgently needed. In this work, simultaneous separation and immobilization of Cr(VI) in LDH by using MgCl₂·6H₂O under thermal treatment is innovatively proposed. Cr was volatilized as CrCl₃ and was immobilized as MgCr₂O₄ accounted for 62.2% and 37.8%, respectively, under the optimal condition (the mole ratio of Cl/Cr is 9, 700 °C and 120 min). The underlying reaction mechanisms are as follows: (i) HCl produced by MgCl₂·6H₂O accelerates the destruction of Cr(VI)-LDH layer structure, completely exposing the incorporated Cr(VI), (ii) Cr(VI) is reduced to Cr(III) by Cl^-, part of which is directly immobilized as MgCr₂O₄, and the other part generates CrCl₃, which is volatilized or further combined with Mg²⁺ to form MgCr₂O₄. The total Cr leaching concentration of the practical COPR sample treated by this method dramatically decreases from 421 to 0.7 mg/L, well below the landfill standard limit (4.5 mg/L). This work provides an attainable strategy for thorough remediation of COPR and inspires the treatment of heavy metal-containing LDH.

Systematic review of biomonitoring data on occupational exposure to hexavalent chromium

Occupational exposure to hexavalent chromium (Cr(VI)) can cause serious adverse health effects such as lung cancer and irritation of the skin and airways. Although assessment of chromium (Cr) in urine is not specific for Cr(VI) exposure, the total amount of Cr in urine is the most used marker of exposure for biomonitoring of Cr(VI). The purpose of this systematic review was fourfold: (1) to assess current and recent biomonitoring levels in subjects occupationally exposed to Cr(VI), with a focus on urinary Cr levels at the end of a working week, (2) to identify variables influencing these biomonitoring levels, (3) to identify how urinary Cr levels correlate with other Cr(VI) exposure markers and (4) to identify gaps in the current research. To address these purposes, unpublished and published biomonitoring data were consulted: (i) unpublished biomonitoring data comprised urinary Cr levels (n = 3799) of workers from different industries in Belgium collected during 1998-2018, in combination with expert scores indicating jobs with Cr exposure and (ii) published biomonitoring data was extracted by conducting a systematic literature review. A linear mixed effect model was applied on the unpublished biomonitoring data, showing a decreasing time trend of 30% in urinary Cr levels. Considering the observed decreasing time trend, only articles published between January 1, 2010 and September 30, 2020 were included in the systematic literature search to assess current and recent biomonitoring levels. Twenty-five studies focusing on human biomonitoring of exposure to Cr(VI) in occupational settings were included. Overall, the results showed a decreasing time trend in urinary Cr levels and the need for more specific Cr(VI) biomarkers. Furthermore, this review indicated the importance of improved working conditions, efficient use of personal protective equipment, better exposure control and increased risk awareness to reduce Cr levels in biological matrices. Further investigation of the contribution of the different exposure routes is needed, so that better guidance on the use of control measures can be provided. In addition, this review support the call for more harmonization of human biomonitoring.

Combined effects of perchlorate and hexavalent chromium on the survival, growth and reproduction of Daphnia carinata

Perchlorate and hexavalent chromium (Cr(VI)) are common cocontaminants in aquatic environments due to their high water solubility, stability, mobility, and some coapplications. However, few studies have investigated their combined toxicity to organisms. In this work, we studied the acute and chronic toxicities of perchlorate and Cr(VI), alone and in combination, with survival, growth, and reproduction as endpoints using Daphnia carinata as a model organism. For a single contaminant, Cr(VI) was found to be more toxic than perchlorate to D. carinata not only in terms of survival but also in terms of growth and reproduction. In regard to the combined pattern, the interactive effects on survival, growth, and reproduction were mainly additivity, antagonism, and synergism, respectively, suggesting that the interactive response of perchlorate and Cr(VI) is endpoint-specific. Due to significant synergism, over 21 days of observation, the inhibition of 0.1 mg/L perchlorate and 0.2 mg/L Cr(VI) on cumulative offspring per female in the first seven broods reached 63.9 ± 3.6%, suggesting that long-term exposure to perchlorate and Cr(VI) at environmentally relevant concentrations may affect D. carinata reproduction in the natural environment. Our results will be significant for understanding the complicated combined toxicity of perchlorate and Cr to aquatic organisms.

Enrichment and characterization of an effective hexavalent chromium-reducing microbial community YEM001

Chromium (Cr) is one of the most widely used heavy metals in industrial processes, resulting in water and soil pollution that seriously threaten environmental safety. In this paper, we have directionally enriched a Cr(VI)-reducing bacterial community YEM001 from no-Cr(VI) polluted pond sedimental sludge by selectively growing it in Cr(VI)-containing media. This community could effectively reduce Cr(VI) in laboratory rich media containing different concentrations of Cr(VI), such as 61% reduction at 435 mg/L Cr(VI), 85% reduction at 355 mg/L Cr(VI), and complete reduction at 269 mg/L Cr(VI) in 93.5 h. It was also able to completely reduce 100 mg/L and 300 mg/L Cr(VI) in landfill leachate and natural sludge in 48 h, respectively. Optimal pH for Cr(VI) reduction of the YEM001 is between 7 and 8 and the best efficiency for Cr(VI) reduction occurs at 30 °C. Metagenomic data demonstrated that the YEM001 community was composed of multiple bacteria, including well-known Cr(VI)-reducing bacteria and non-Cr(VI)-reducing bacteria. Delftia, Comamonas, Alicycliphilus, Acidovorax, Bacillus, and Clostridioides account for 83% of total community abundance. The stability of the composition of the YEM001 community and its Cr(VI)-reducing activity allows for its application in bioremediation of environmental Cr(VI) pollution.

Spectrofluorimetric determination of Cr(VI) and Cr(III) by quenching effect of Cr(III) based on the Cu-CDs with peroxidase-mimicking activity

The study presents a spectrofluorimetric method for the determination of Cr(III) based on 2,3-diaminophenazine (DAP) as fluorescent nano sensor. With the peroxidase-mimicking activity of copper-doped carbon dots (Cu-CDs), colorless o-phenylenediamine (OPD) was oxidized to fluorescent DAP in the presence of H₂O₂ via generation of hydroxyl radicals. The Cr(III) was found to decrease the fluorescent intensity of the Cu-CDs-mediated OPD oxidation system. Cr(VI) species were reduced to Cr(III) by employing 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) as the reductant, and a novel spectrofluorimetric method for the highly sensitive detection of speciation Cr(VI) and Cr(III) was developed. The Cu-CDs nanozyme was prepared from ethylenediaminetetraacetic acid copper disodium salt (Na₂[Cu (EDTA)]) by one-step pyrolysis method. Under optimal conditions, the fluorescence intensity of DAP is linearly proportional to the Cr(III) ion concentration in the range of 5 × 10^-6 to 1.5 × 10^-4 mol L^-1 and the detection limit is 1.2 × 10^-7 mol L^-1. The method has a good anti-interference performance against some metal ions, such as, Hg²⁺, Co²⁺, Cu²⁺, Mg²⁺, Ba²⁺, K⁺, Mn²⁺, Ni²⁺, Al³⁺, and Fe³⁺. The proposed method was successfully applied in the determination of Cr(VI) and Cr(III) in water samples.

Partitioning, leachability, and speciation of chromium in the size-fractions of soil contaminated by chromate production

Soil particle size significantly affects the distribution and migration of chromium (Cr) in soil. Limited studies have investigated the impact of soil particle size on Cr partitioning at chromate contaminated sites. In this study, the physicochemical properties of coarse sand, medium sand, fine sand, and silt-clay were analyzed. And the particle size effects on partitioning, leachability, and bioaccessibility of total Cr and Cr(VI) were determined. The results showed the distribution factor (DF) of Cr(VI) in the coarse sand, medium sand, fine sand, and silt-clay fractions were 0.70, 0.79, 1.35, and 1.60, respectively. The total Cr DF values also had the similar result. The leached concentrations of total Cr and Cr(VI) in silt-clay (562.89 mg/L and 551.71 mg/L) was higher than in coarse sand (238.55 mg/L and 228.68 mg/L) fraction. The bioaccessibility of total Cr and Cr(VI) in silt-clay (77.72% and 88.58%) was higher than in fine sand (60.72% and 79.55%) fraction. The total Cr proportion of the exchangeable fraction (45.92%-73.67%) was relatively high in the four soil particle size fractions and gradually increased as soil particle size decreased. These implied that finer soil particles are more capable of enriching, mobilizing, and bioaccessibility of Cr and Cr(VI) than the coarse particles, which was related to the higher organic matter, cation exchange capacity, specific surface area, and clay components in smaller particles. The results suggested that higher environmental risk occurred in the finer fraction than in the coarser fraction for the chromate production contaminated soil.

Simultaneous Removal of Trivalent Chromium and Hexavalent Chromium from Soil Using a Modified Bipolar Membrane Electrodialysis System

In this study, a modified bipolar membrane electrodialysis system equipped with a "back-to-back" soil compartment was fabricated for simultaneous removal of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) from contaminated soils. The results showed that the soil solution pH had a significant effect on the Cr(III) and Cr(VI) desorption, and the desorption data fit well with the Elovich kinetic model. Current density had an obvious effect on Cr(III) and Cr(VI) removal, cell voltage, soil pH, current efficiency, and specific energy consumption, and the optimal current density was 2.0 mA/cm². The removal efficiencies of Cr(III) and Cr(VI) were both 99.8%, while Cr(III) and Cr(VI) recoveries were somewhat lower at 87 and 90%, respectively, because some Cr(III) and Cr(VI) were adsorbed by the membranes. An energy consumption analysis indicates that the back-to-back soil compartment equipped system increased the current efficiency and decreased the specific energy consumption. When a system equipped with two back-to-back soil compartments was used to remove chromium from soil, the current efficiency increased to 28.8% and the specific energy consumption decreased to 0.048 kWh/g. The experimental results indicate that the proposed process has the potential to be an effective technique for the treatment of soil contaminated with heavy metals.

Characteristics and long-term effects of stabilized nanoscale ferrous sulfide immobilized hexavalent chromium in soil

Based on the phenomenon of soil polluted by Hexavalent chromium (Cr(VI)), this study systematically examined the efficiency, stability and feasibility of using sodium carboxymethyl cellulose-stabilized nanoscale ferrous sulfide (CMC-nFeS) to immobilize Cr(VI) in contaminated soil. The experiments described herein showed CMC-nFeS exhibited superior dispersity and a higher antioxidative effect than nFeS alone. Batch tests indicated the nanoparticles could effectively immobilize Cr(VI) in soil. At Cr(VI) concentrations of 56.01-502.21 mg/kg, the reducing capacity of CMC-nFeS was 54.68-198.74 mg Cr(VI)/g FeS. Following treatment with CMC-nFeS, the leachabilities of Cr(VI) and Cr_total determined by the Toxicity Characteristic Leaching Procedure (TCLP), Synthetic Precipitation Leaching Procedure (SPLP) and Physiologically Based Extraction Test (PBET) decreased significantly after 24 h and remained stable for 90 days. Column tests with water and simulated acid rain showed the injection of CMC-nFeS significantly increased the fixed Cr concentration and the procedure was environmentally friendly. Furthermore, analysis of the reaction mechanism demonstrated the best removal obtained in a neutral environment and Cr(VI) was reduced and immobilized in the form of Cr(OH)₃ and Fe_0.75Cr_0.25OOH confirmed by SEM-EDS and XPS analysis.

The acute toxic effects of hexavalent chromium on the liver of marine medaka (Oryzias melastigma)

Chromium is toxic to marine animals and can cause damage to many of their organs, including the liver. To test the toxicity of chromium on marine organisms, we exposed the liver of the marine medaka (Oryzias melastigma) with hexavalent chromium [Cr(VI)]. Our results show that Cr enrichment in the liver demonstrates a positive correlation to the exposure concentration. With the increase of Cr(VI) concentration, pathological changes including nuclear migration, cell vacuolization, blurred intercellular gap, nuclear condensation, become noticeable. To further study changes in gene expression in the liver after Cr(VI) exposure, we used RNA-seq to compare expression profiles before and after Cr(VI) exposure. After acute Cr(VI) exposure (2.61 mg/l) for 96 h, 5862 transcripts significantly changed. It is the first time that the PPAR pathway was found to respond sensitively to Cr(VI) exposure in fish. Finally, combined with other published study, we found that there may be some difference between Cr(VI) toxicity in seawater fish and freshwater fish, due to degree of oxidative stress, distribution patterns and detailed Cr(VI) toxicological mechanisms. Not only does our study explore the mechanisms of Cr(VI) toxicity on the livers of marine medaka, it also points out different Cr(VI) toxicity levels and potential mechanisms between seawater fish and freshwater fish.

Contents and Health Risk Assessment of Elements in Three Edible Ectomycorrhizal Fungi (Boletaceae) from Polymetallic Soils in Yunnan Province, SW China

Ectomycorrhizal fungi (EcMF) can mobilize mineral elements directly from insoluble mineral sources and accumulate various metallic elements and metalloids from soils to their fruiting bodies. Mushrooms from genus Boletus and its related genus are one of the most important EcMF which are consumed worldwide as wild edible mushrooms. Yunnan province (China) is a high biodiversity of genus Boletus mushrooms but is also an area with potential elevated contents of toxic elements in soil. Total contents of As, Ag, Ba, Cd, Co, Cr, Cs, Cu, Li, Mn, Ni, Pb, Rb, Sb, Sr, Tl, U, V, and Zn in three edible EcMF species collected from five sites of Yunnan were analyzed by inductively coupled plasma mass spectrometer. The highest contents for As, Cd, and Pb were 7.8 mg kg^-1 dry weight (dw) in the caps of Butyriboletus roseoflavus, 3.4 mg kg^-1 dw in the caps of B. roseoflavus, and 6.4 mg kg^-1 dw in the stipes of Hemileccinum impolitum. Health risk assessment of As, Cd, and Pb indicated that the estimated exposure due to intakes of some mushroom samples from the sites were above the limits recommended by the Joint FAO/WHO Expert Committee on Food Additives. Since EcMF were considered as bioexclusors of Cr, higher Cr contents in the mushroom samples, compared with previous studies, indicated high geochemical background value of Cr in the sampling sites. Relatively higher V contents in mushrooms from family Boletaceae could also associate with the high V contents in Yunnan soil. Further work is needed to identify the places in Yunnan with geochemical anomalies resulting in high levels of toxic elements in EcMF.

Enhanced Cr(VI) stabilization in soil by carboxymethyl cellulose-stabilized nanosized Fe0 (CMC-nFe0) and mixed anaerobic microorganisms

A collaborative system of carboxymethyl cellulose stabilized nanosized zero-valent iron (CMC-nFe⁰) and microorganisms was set up to enhance the stabilization of Cr(VI) in soil. In comparison with an aqueous-bound Cr(VI) removal of 18.9% in the nFe⁰ system, a higher Cr(VI) removal of 68.9% was achieved in the nFe⁰ and microorganisms system after 14 d remediation because the microorganisms on the nFe⁰ surface promoted nFe⁰ corrosion and enhanced abiotic and biotic Cr(VI) stabilization by generating highly active minerals such as magnetite, lepidocrocite and green rust on the nFe⁰ surface. As a stabilizing agent for nFe⁰ and an organic substrate for microorganisms, CMC on the nFe⁰ surface not only enhanced the dispersion of nFe⁰, but also boosted the activity of microorganisms, resulting in a promotion of 0.9 and 0.5 times higher aqueous-bound Cr(VI) removal via the improvement of nFe⁰ and microorganisms respectively, thus a total 4 times higher aqueous-bound Cr(VI) removal of 95.3% was achieved in the CMC-nFe⁰ and microorganisms system as compared to the nFe⁰ system. After 14 d remediation, easily available species of Cr(VI) and Cr_total, such as water soluble (WS), exchangeable (EX) and bounded to carbonates (CB), were mainly transformed to less available Fe-Mn oxides-bounded (OX) and residual (RS) species because of the production of ferrochrome precipitates (Cr_xFe_1-xOOH or Cr_xFe_1-x(OH)₃). Besides, the stabilization of Cr(VI) in the CMC-nFe⁰ and microorganisms system was pH-dependent and it increased with CMC-nFe⁰ dosage. Due to excellent Cr(VI) stabilization and Cr immobilization, coupled CMC-nFe⁰ and anaerobic microorganisms process is of great potential in remediating Cr(VI)-containing soil.

Ca2+ and SO42- accelerate the reduction of Cr(VI) by Penicillium oxalicum SL2

Some ions in soils may affect the growth and metabolism of microorganisms and subsequently alter the remediation efficiency of Cr(VI). Here, the effects of different Ca²⁺ and SO₄^2- levels on the reduction of Cr(VI) by Penicillium oxalicum SL2 were investigated. The results showed that Cr(VI) reduction by P. oxalicum SL2 in potato dextrose liquid (PDL) medium was accelerated by the presence of exogenous Ca²⁺ and SO₄^2-. The Cr(VI) reduction rates were increased by 52.5% (200 mg L^-1 Ca²⁺ treated) and 55.9% (2000 mg L^-1 SO₄^2- treated), respectively. High concentration of Ca²⁺ in medium resulted in the production of calcium oxalate crystals, which was contributed to the adsorption of chromium. In addition, X-ray absorption near-edge spectroscopy (XANES) analysis showed that P. oxalicum SL2 could reduce the toxicity of Cr(VI) by synthesizing cysteine (Cys) and reduced glutathione (GSH). The decrease of thiol compounds (Cys and GSH) in P. oxalicum SL2 mycelia treated with SO₄^2- proved the alleviation of oxidative stress. In conclusion, exogenous Ca²⁺ could reduce the damage of Cr(VI) to P. oxalicum SL2 by maintaining the integrity of cell wall, and the addition of SO₄^2- alleviated the Cr(VI) toxicity to P. oxalicum SL2, thus accelerating the reduction of Cr(VI).