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Arsenic and chromium, common polyvalent heavy metals found in soil, are a growing environmental concern, yet research on soil threshold values for these metals is limited. This study presents a method for deriving soil guidelines based on published literature, considering two land types (parkland and industry), two exposure pathways (direct contact and food chain for mammalian and avian), and two toxicity levels (NOEC/LOAEL and EC50/LC50). Soil guidelines for inorganic arsenic (III & V) and chromium (III & VI) were derived separately for parkland and industry, factoring in direct contact and food chain exposures (applicable only to parkland) and both lower and upper toxicity levels from available data. The study discusses the similarities and differences between the proposed guideline values and international standards. Additionally, a new method is introduced for deriving soil guidelines for total arsenic and chromium, applicable to naturally occurring mixed valence states. Future research should refine these guidelines by addressing sample limitations, soil properties, and regional variations to enhance their accuracy and applicability in environmental management. Both methods can serve as references for managing other pollutants. These soil guideline values, particularly the total soil guidelines, offer valuable guidance for the Chinese government in managing arsenic and chromium levels in soil. [Display omitted] • A method was developed to derive soil guidelines by assessing land types, exposure scenarios, and toxicity levels. • Guidelines for arsenic (III & V) and chromium (III & VI) were established based on this assessment. • A novel approach for addressing arsenic and chromium in mixed valence states was proposed. • Guidelines for total arsenic and chromium were also formulated, considering land, exposure, and toxicity. [ABSTRACT FROM AUTHOR]

Frequent wildfires have accumulated the pyrogenic carbon (PyC) colloids in the environment, where they undergo environmental aging processes. The altered properties of aged PyC colloids may affect their ability to facilitate transformation and transport of contaminants in post-fire environments, posing unknown threats to ecological security. This study investigated the effect of chemical aging on the PyC colloid-facilitated transformation and transport of chromium (Cr) using batch experiments, column experiments, and transport model simulations. Results showed that aged PyC colloids exhibited weaker electron-donating capacity, and the reduction of Cr (VI) to Cr (III) decreased from 37.6 % to 13.5 % with the increasing aging time. Although PyC colloid transport increased with aging time, the PyC colloid-facilitated Cr (III) transport decreased because of the weakened reduction of Cr (VI). The transport of PyC colloid-facilitated Cr (III) was weaker at low pH. The reactive solute transport model well simulated the aged PyC colloid-facilitated transformation and transport of Cr (VI). Our findings highlight the significance of aging processes and environmentally relevant conditions in influencing the PyC colloid-facilitated transformation and transport of Cr, which is crucial for assessing risks of wildfire-driven Cr pollution and the potential of PyC for in-situ pollution control. [Display omitted] • Chemical aged PyC colloids showed more negative and weaker electron-donating capacity. • Chemical aging decreased the reduction of Cr (VI) to Cr (III) by PyC colloids. • More PyC colloid-adsorbed Cr (III) and weaker transport were observed at low pH. • Modeling well simulated PyC colloid-facilitated Cr transformation and transport. • PyC colloid-facilitated Cr transport should be considered in post-fire environments. [ABSTRACT FROM AUTHOR]

Chromium (Cr) soil contamination is a critical global environmental concern, with hexavalent chromium (Cr[VI]) being especially perilous due to its high mobility, bioavailability, and phytotoxicity. This poses a significant threat to the cultivation of crops, particularly rice, where the mechanisms of Cr(VI) absorption remain largely unexplored. This study uncovered a competitive interaction between Cr(VI) and essential nutrients—sulfate and phosphate during the uptake process. Notably, deficiencies in sulfate and phosphate were associated with a marked increase in Cr(VI) accumulation in rice, reaching up to 76.5 % and 77.7 %, respectively. Employing q-PCR, this study identified significant up-regulation of the sulfate transporter gene, OsSultr1;2 , and the phosphate transporter gene, OsPht1;1 , in response to Cr(VI) stress. Genetic knockout studies have confirmed the crucial role of OsSultr1;2 in Cr(VI) uptake, with its deletion leading to a 36.1 % to 69.6 % decrease in Cr uptake by rice roots. Similarly, the knockout of OsPht1;1 resulted in an 18.1 % to 25.7 % decrease in root Cr accumulation. These findings highlight the key role of the sulfate transporter OsSultr1;2 in Cr(VI) uptake, with phosphate transporters also contributing significantly to the process. These insights are valuable for developing rice varieties with reduced Cr(VI) accumulation, ensuring the safety of rice grain production. [Display omitted] • A notable competitive interaction between Cr(VI) and the uptake of sulfate and phosphate. • During Cr(VI) uptake in rice, the sulfate transporter OsSultr1;2 plays a key role. • Phosphate transporters OsPht1;1 are responsible for the absorption of Cr(VI) in rice. [ABSTRACT FROM AUTHOR]

3-(2-Aminoethylamino)propyltriethoxysilane and carboxyethylsilanetriol sodium salt were grafted on silica-coated Fe 3 O 4 nanoparticles via sol-gel process to prepare novel amine- and carboxyl-bifunctionalized magnetic nanocomposites (SMNPs-(NH 2 + COOH)). After well characterized, this doubly functionalized material was used as magnetic solid-phase extraction (MSPE) adsorbent to separate and enrich inorganic chromium species followed by inductively coupled plasma-mass spectrometry detection. The optimization of MSPE operation parameters including pH was conducted. It is reasonably elucidated that the adsorption mechanisms of zwitterionic SMNPs-(NH 2 + COOH) towards chromium species are electrostatic and/or coordination interactions. Cr(VI) and Cr(III) can be adsorbed around pH 3.0 and around 10.0 respectively with strong anti-interference ability not only from other co-existing ions but also from the two labile species each other, and eluted by dilute nitric acid solution. With a 15-fold enrichment factor, the limits of detection of Cr(VI) and Cr(III) were 0.008 and 0.009 μg L−1, respectively, profiting from the maximum adsorption capacities of 7.52 and 6.11 mg g−1. The just one magnetic extraction matrix based speciation scheme possesses excellent convenience and friendliness to Cr(VI) and Cr(III) without any oxidation or reduction prior to capture of these two species. This protocol has been successfully applied to the speciation analysis of inorganic chromium in real-world environmental water samples. [Display omitted] • A novel amine- and carboxyl-bifunctionalized magnetic nanocomposite was developed. • The zwitterionic material was used as MSPE adsorbent for chromium species enrichment. • No oxidation or reduction is needed for Cr(VI) and Cr(III) before separation by MSPE. • The stability of inorganic chromium species is kept during the adsorption procedure. • The facile MSPE method has been applied to environmental water analysis with ICP-MS. [ABSTRACT FROM AUTHOR]

Chromium (Cr) pollution may threaten food safety in China. In this study, the concentration, pollution level, distribution, and non-cancer risk of Cr in wheat grains grown in 186 areas across 28 provinces in China were investigated. Results indicated that mean concentration of Cr was 0.28 ± 2.5 mg/kg, dry mass (dm). Of the samples, 7.5 % were found to be polluted with Cr. The mean concentrations were in the following order: Northwest > Northeast > South > East > North > Southwest > Central China. Based on deterministic models, mean hazard quotient (HQ) values for adult males, adult females, and children were 0.11 ± 3.4, 0.11 ± 3.4, and 0.13 ± 3.5, respectively with < 6 % of HQ values ≥ 1. Eleven sites in northern China were identified as hotspots, whereas Gansu Province and Northwestern China were labeled as priority provinces and regions for risk control. The mean HQ values estimated by probabilistic risk assessment were two times greater than those estimated using deterministic models. The risk probabilities for adult males, adult females, and children were 4.81 %, 3.78 %, and 6.55 %, respectively. This study provides valuable information on Cr pollution in wheat grains and its risks at a national scale in China. [Display omitted] • First national-scale investigation of Cr in wheat grains and health risks. • Cr pollution and deterministic and probabilistic health risks were assessed. • Hotspots, priority provinces and regions for risk control were identified. • 7.5 % of wheat grains polluted; 3.78–6.55 % of inhabitants at non-cancer risks. • Children were not undergoing larger risk than adult males/females. [ABSTRACT FROM AUTHOR]

Phytoremediation, an eco-friendly approach for mitigating heavy metal contamination, is reliant on hyperaccumulators. This study focused on Leersia hexandra Swart, a known chromium (Cr) hyperaccumulator with demonstrated tolerance to multiple heavy metals. Our objective was to investigate its response to simultaneous Cr and nickel (Ni) stress over 12 days. Results from physiological experiments demonstrated a significant increase in the activities of antioxidant enzymes (APX, SOD, CAT) and glutathione (GSH) content under Cr and Ni stress, indicating enhanced antioxidant mechanisms. Transcriptome analysis revealed that stress resulted in the differential expression of 27 genes associated with antioxidant activity and metal binding, including APX, SOD, CAT, GSH, metallothionein (MT), and nicotinamide (NA). Among them, twenty differentially expressed genes (DEGs) related to GSH metabolic cycle were identified. Notably, GSTU6 , GND1 , and PGD were the top three related genes, showing upregulation with fold changes of 4.57, 6.07, and 3.76, respectively, indicating their crucial role in metal tolerance. The expression of selected DEGs was validated by quantitative real-time PCR, confirming the reliability of RNA-Seq data. Metabolomic analysis revealed changes in 1121 metabolites, with amino acids, flavonoids, and carbohydrates being the most affected. Furthermore, glucosinolate biosynthesis and amino acid biosynthesis pathways were represented in the KEGG pathway of differentially expressed metabolites (DEMs). This study provides insights into the tolerance mechanisms of L. hexandra under the co-stress of Cr and Ni, offering a new perspective for enhancing its remediation performance. [Display omitted] • Novel tolerance mechanisms in L. hexandra have been analyzed. • Co-stress with Cr and Ni increased amino acid and flavonoid content in L. hexandra. • Upregulation of ABCC10 , SODCC2 , GSTM1 , and NAS3 contributes to Cr and Ni tolerance. • GSH metabolic pathways and TCA cycle are vital for Cr and Ni detoxification. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • A sandbox experiment was used for Cr migration and diffusion patterns. • Low water-table conditions led to Cr vertical migration range expansion. • Approximately 75 % of the total Cr was immobilized in the soil. • The model achieved a good fit for Cr retention and morphological distribution. [ABSTRACT FROM AUTHOR]

Clay minerals (CMs) and pyrogenic carbons (PCs) often co-exist in the environment and participate in the redox cycling of pollutants. This study unveiled the dual role of PCs in CM-dominated chromium transformation in both aqueous and agar solidification media. The findings showed that CMs and PCs adsorbed minimal Cr(VI), while reduced CMs and PCs displayed a substantial difference by directly reducing Cr(VI) to solid/dissolved Cr(III) through reactive structural Fe(II) and functional groups, respectively. Moreover, dissolved PCs were found to mediate electron transfer from reduced CMs to Cr(VI) in aqueous and solid media. Interestingly, the effect of solid PCs on Cr(VI) reduction by reduced CMs was concentration-dependent. At lower concentrations, solid PCs dispersed reduced CMs, acting as electron mediators and facilitating both direct and indirect Cr(VI) reduction, resulting in solid Cr(III) rather than dissolved Cr(III). Conversely, at higher concentrations, solid PCs served as redox buffers, storing electrons transferred from reduced CMs to Cr(VI). In either case, the transformed chromium was primarily immobilized on the surface of CMs rather than PCs. These findings offer valuable insights into pollutant transformations associated with CMs and PCs, deepening our understanding of their geochemical processes. [Display omitted] • Dissolved pyrogenic carbon can mediate electron transfer from clay minerals to Cr(VI). • Solid pyrogenic carbon can store electrons and inhibit Cr(VI) transformation. • Aqueous and solid media were developed to verify electron mediation. • The formed Cr(III) is mainly immobilized on the clay mineral's surface. [ABSTRACT FROM AUTHOR]

Sulfidated nano-scale zerovalent iron (S-nZVI) has emerged as an advanced functional nanomaterial for efficiently remediating Cr(VI) contamination in aqueous environments. However, there is an insufficient understanding of its coherent process, removal pathway, and hydrochemical reactive mechanisms, presenting potential challenges for its future environmental applications. To address this gap, this study successfully synthesized S-nZVI through a chemical precipitation method and effectively applied it for the removal of Cr(VI). Additional characterization revealed that the removal of Cr(VI) followed a sequence of rapid chemisorption and intraparticle diffusion processes, concomitant with an increase in pH and a decrease in oxidation-reduction potential. The remediation mechanism encompassed a synergistic reduction of Cr(VI) to Cr(III) and simultaneous immobilization via Cr 2 FeO 4 coprecipitation. The highest Cr(VI) removal capacity of 75 mg/g was attained during dynamic removal experiments in the sand column packed with S-nZVI. Further computational analysis, employing density functional theory calculations based on the experimental data, revealed the involvement of multiple molecular orbitals of Cr(VI) in the removal process. It also elucidated a step-by-step reduction pathway for Cr(VI) characterized by decreasing free energy. These findings provide evidence-based insights into Cr(VI) remediation using S-nZVI and can serve as valuable technical support for future environmental management of heavy metals. [Display omitted] • S-nZVI exhibits a desirable removal capacity and remediation efficiency of Cr(VI). • Cr(VI) reduction pathway involves multi-step reaction with decreasing free energy. • Cr(VI) removed by S-nZVI encompasses adsorption, reduction and coprecipitation. • Cr(VI) removal leads to obviously concurrent pH increase and ORP decrease. • This study provides evidence-based insights into Cr(VI) remediation using S-nZVI. [ABSTRACT FROM AUTHOR]

Co-contaminants and complex subsurface conditions pose great challenges to site remediation. This study demonstrates the potential of electrokinetic bioremediation (EK-BIO) in treating co-contaminants of chlorinated solvents and heavy metals in low-permeability soils with elevated sulfate. EK-BIO columns were filled with field soils, and were fed by the electrolyte containing 20 mg/L trichloroethylene (TCE), 250 μM Cr(VI), 25 μM As(III), 10 mM lactate, and 10 mM sulfate. A dechlorinating consortium containing Dehalococcoides (Dhc) was injected several times during a 199-d treatment at ∼1 V/cm. Sulfate reduction, Cr/As immobilization, and complete TCE biodechlorination were observed sequentially. EK-BIO facilitated the delivery of lactate, Cr(VI)/As(III), and sulfate to the soils, creating favorable reductive conditions for contaminant removal. Supplementary batch experiments and metagenomic/transcriptomic analysis suggested that sulfate promoted the reductive immobilization of Cr(VI) by generating sulfide species, which subsequently enhanced TCE biodechlorination by alleviating Cr(VI) toxicity. The dechlorinating community displayed a high As(III) tolerance. Metagenomic binning analysis revealed the dechlorinating activity of Dhc and the potential synergistic effects from other bacteria in mitigating heavy metal toxicity. This study justified the feasibility of EK-BIO for co-contaminant treatment and provided mechanistic insights into EK-BIO treatment. [Display omitted] • Feasibility of EK-BIO in treating TCE-Cr/As-contaminated soils was justified. • EK-BIO constructed suitable conditions for removing aqueous Cr/As and TCE. • Sulfate enhanced Cr immobilization and subsequent TCE biodechlorination. • Aqueous Cr(VI) inhibited sulfate reduction and TCE biodechlorination. • Dhc was the essential OHRB with dechlorinating activity. [ABSTRACT FROM AUTHOR]

The rising heavy metal contamination of soils imposes toxic impacts on plants as well as other life forms. One such highly toxic and carcinogenic heavy metal is hexavalent chromium [Cr(VI)] that has been reported to prominently retard the plant growth. The present study investigated the potential of silicon (Si, 10 µM) to alleviate the toxicity of Cr(VI) (25 µM) on roots of wheat (Triticum aestivum L.) seedlings. Application of Si to Cr(VI)-stressed wheat seedlings improved their overall growth parameters. This study also reveals the involvement of two phytohormones, namely auxin and cytokinin and their crosstalk in Si-mediated mitigation of the toxic impacts of Cr(VI) in wheat seedlings. The application of cytokinin alone to wheat seedlings under Cr(VI) stress reduced the intensity of toxic effects of Cr(VI). In combination with Si, cytokinin application to Cr(VI)-stressed wheat seedlings significantly minimized the decrease induced by Cr(VI) in different parameters such as root-shoot length (10.8% and 13%, respectively), root-shoot fresh mass (11.3% and 10.1%, respectively), and total chlorophyll and carotenoids content (13.4% and 6.8%, respectively) with respect to the control. This treatment also maintained the regulation of proline metabolism (proline content, and P5CS and PDH activities), ascorbate-glutathione (AsA-GSH) cycle and nutrient homeostasis. The protective effect of Si and cytokinin against Cr(VI) stress was minimized upon supplementation of an inhibitor of polar auxin transport- 2,3,5-triiodobenzoic acid (TIBA) which suggested a potential involvement of auxin in Si and cytokinin-mediated mitigation of Cr(VI) toxicity. The exogenous addition of a natural auxin – indole-3-acetic acid (IAA) confirmed auxin is an active member of a signaling cascade along with cytokinin that aids in Si-mediated Cr(VI) toxicity alleviation as IAA application reversed the negative impacts of TIBA on wheat roots treated with Cr(VI), cytokinin and Si. The results of this research are also confirmed by the gene expression analysis conducted for nutrient transporters (Lsi1 , CCaMK , MHX , SULT1 and ZIP1) and enzymes involved in the AsA-GSH cycle (APX , GR , DHAR and MDHAR). The overall results of this research indicate towards possible induction of a crosstalk between cytokinin and IAA upon Si supplementation which in turn stimulates physiological, biochemical and molecular changes to exhibit protective effects against Cr(VI) stress. Further, the information obtained suggests probable employment of Si, cytokinin and IAA alone or combined in agriculture to maintain plant productivity under Cr(VI) stress and data regarding expression of key genes can be used to develop new crop varieties with enhanced resistance against Cr(VI) stress together with its reduced load in seedlings. [Display omitted] • Silicon regulates the cytokinin and IAA crosstalk and improve the growth. • Silicon reduces the CrVI accumulation through cytokinin and IAA crosstalk. • Auxin-cytokinin crosstalk modulates Si-mediated regulation of photosynthetic attributes of Cr(VI)-stressed wheat seedlings. • Auxin and cytokinin are crucial companions for Si-mediated nutrient homeostasis against Cr(VI)-stress. • Si balances the ascorbate-glutathione cycle through auxin-cytokinin crosstalk under Cr(VI) stress. [ABSTRACT FROM AUTHOR]

Rania-Khan Chandpur site, (Kanpur Dehat, Uttar Pradesh, India), one of the highly Chromium (Cr) contaminated sites in India due to Chromite Ore Processing Residue (COPR), has been investigated at the field-scale. We found that the area around the COPR dumps was hazardously contaminated with the Cr where its concentrations in the surface water and groundwater were > 40 mgL-1, its maximum contents in the COPRs and in the soils of the adjoining lands were 9.6 wt% and 3.83 wt%, respectively. By exploring the vegetation and microbial distribution across the site, we advocate the appropriateness of Cynodon dactylon , Chrysopogon zizanioides , Cyperus sp., and Typha angustifolia as the most suitable phytoremediation agent because their association with Cr remediating bacterial species (Pseudomonas sp. , Clostridium sp. and Bacillus sp.) was strong. Using this remarkable information for the bioremediation projects, this site can be re-vegetated and bioaugmented to remediate Cr in soils, waterlogged ditches, surface water, and in groundwater systems. [Display omitted] • A globally known COPR contaminated site was investigated at the field-scale. • Soil-water systems impacted by the hazardous chromium have been studied. • Only a few plant species can grow in Cr contaminated zone or near COPR sites. • Cr content (up to 9.6 wt%) in soils was one of the key drivers of microbiome. • Selected native plants and microbes can improve soil-water quality at COPR sites. [ABSTRACT FROM AUTHOR]

In this study, municipal waste pyrolytic char (PEWC) was prepared by pyrolysis from municipal solid waste extracted in landfills, and Fe-based modified pyrolytic char (Fe-PEWC) was prepared by modification. Focusing on the evaluation of the stabilization capacity of Fe-PEWC for vanadium (V) and chromium (Cr) in soils, the effects of PEWC addition on soil properties, bioavailability and morphological distribution of V and Cr, ryegrass growth, and V and Cr accumulation were thoroughly investigated. The results of pot experiment showed that the application of PEWC and Fe-PEWC significantly (P < 0.05) improved soil properties (such as pH, EC, total nitrogen, available phosphorus, available potassium, and organic matter). After 42 days of cultivation, Fe-PEWC has a better fixation effect on heavy metals, and the bioavailable V and Cr of 3% Fe-PEWC decreased by 14.96% and 19.48%, respectively. The exchangeable state and reducible state decreased, while the oxidizable state and residual state increased to varying degrees. The Fe-PEWC can effectively reduce the accumulation of V and Cr in ryegrass by 71.25% and 76.43%, respectively, thereby reducing their toxicity to plants. In summary, modified pyrolytic char can effectively solidify heavy metals in soil, improve soil ecology and reduce the toxicity to plants. The use of excavated waste as a raw material for the preparation of soil heavy metal curing agent has the significance of resource recycling, low price, and practical application., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

• The bacterial strains Klebsiella sp. strain CPSB4 and Enterobacter sp. strain CPSB49 showed plant growth promoting ability. • The isolates enhanced the nutrient uptake and growth of the test plants in pot scale studies. • They enhanced the chromium uptake in roots and shoots of Helianthus annuus (L.). Prevalence of metal pollutants exerts negative effects on human health and environment, thus propounding an urgent need for a safer substitute. This study was conducted to compare the chromium bioremediation and plant growth promotion ability of two bacterial strains, Klebsiella sp. strain CPSB4 (MH266218) and Enterobacter sp. strain CPSB49 (MH532567), isolated from the rhizospheric soils. A pot scale experiment was setup with Helianthus annuus (L.) as a test plant to compare the efficiency of both isolates in enhancement of plant growth, nutrients uptake, anti-oxidative enzymes production, lipid peroxidation, and chromium bioremediation. Inoculation of strains, CPSB4 and CPSB49 enhanced plant biomass, plant growth, nutrient uptake, anti-oxidative enzymes, and chromium bioremediation, while reduction in lipid peroxidation was observed compared to uninoculated control under chromium stress. The maximum increase in plant growth and nutrient uptake was found in treatments inoculated with CPSB49, while maximum chromium uptake by sunflower was observed in treatments inoculated with CPSB4. Moreover, an increase in anti-oxidative enzyme production and decrease in lipid peroxidation was observed on inoculation of the selected strains. Thus, the strains Klebsiella sp. and Enterobacter sp. can be effectively used in chromium bioremediation and plant growth promotion under chromium stress conditions. [ABSTRACT FROM AUTHOR]

• A novel technological route for selective recovery of chromium from ferronickel slag was developed. • The Cr 2 O 3 in the ferronickel slag was converted to NaCrO 2 by alkaline roasting with addition of Na 2 O 2. • The loose structure of NaCrO 2 was beneficial to subsequent water dissolution. • The formation of highly toxic Cr (VI) was restrained in the process. Chromium was selectively recovered from ferronickel slag by roasting the slag with addition of Na 2 O 2 , followed by water leaching. The thermodynamic analysis revealed that in the presence of Na 2 O 2 at appropriate temperatures, the Cr 2 O 3 in the ferronickel slag can be converted to NaCrO 2 , instead of Na 2 CrO 4 , which prevents the formation of highly toxic Cr (VI). The experimental results confirmed that under the optimal alkaline roasting and water leaching conditions of the mass ratio of ferronickel slag to Na 2 O 2 of 1, roasting temperature of 600 °C, roasting time of 1 h, leaching temperature of 50 °C, leaching time of 1 h, and liquid-to-solid ratio of 10 mL/g, 92.33% of Cr was leached with 64.28% of Na and 11.16% of Si and only 0.06 wt % Cr was left in the leaching residue. The high leaching percentage of Cr was a result of the transformation of Cr 2 O 3 in the ferronickel slag to NaCrO 2 with a loose structure during alkaline roasting that was beneficial to water dissolution. Compared to the traditional alkaline roasting process, the proposed more environmentally friendly method did not produce toxic Cr (VI) during recovery of chromium and the resulting residue has potential to be used as a good construction material. [ABSTRACT FROM AUTHOR]

• Electrochemical reduction was employed to improve Cr(VI) removal by magnetite. • Cr(VI) was reduced to Cr(III) and subsequently precipitated on magnetite in KNO 3. • The highest Cr removal capacity of magnetite reached as high as 514.7 mg g−1. • Relatively low pH and applied potential facilitated Cr(VI) removal. • Cr(VI) was completely reduced in K 2 SO 4 electrolyte without Cr(OH) 3 formation. Aqueous hexavalent chromium (Cr(VI)) poses serious threats to ecological environments. Magnetite is a potential adsorbent for Cr(VI). However, its adsorption capacity is limited due to the formation of Fe(III) oxide coating on magnetite surface. Herein, constant potential reduction was conducted to improve the Cr(VI) removal capacity of magnetite, and the influence of pH, potential, and supporting electrolytes including KNO 3 , KCl, and K 2 SO 4 on the adsorption capacity was also investigated. The results showed that the highest Cr(VI) reduction percentage reached 93.7% with a total Cr removal capacity of 514.7 mg g−1 at optimized pH 2 and −0.2 V (vs. SCE) in supporting electrolyte of KNO 3. Cr(VI) was reduced to Cr(III) on the surface of magnetite due to the direct electrochemical reduction at low potentials and reduction by Fe2+ aq electrochemically generated from magnetite. The Cr(III) was subsequently removed and easily separated due to the formation of Cr(OH) 3 precipitate on magnetite surface when KNO 3 and KCl were used as supporting electrolyte; however, when K 2 SO 4 was used instead, Cr(OH) 3 precipitate was not observed. The decrease in pH and electrical potential was found to facilitate the reduction and removal of Cr(VI). This work proposes a facile method to enhance Cr(VI) removal by iron oxides. [ABSTRACT FROM AUTHOR]

• Waste water treatment with no sludge formation and easy handling, scale up and reusable process developed. • Aqueous sol-gel route was employed in the synthesis of reusable Rebar Multiwalled CNT/Alumina Composite (RMAC) nodules. • Effective removal of Methylene Blue dye and Cr (VI) ion was achieved using RMAC nodules. • The mesoporous RMAC nodules were fully characterized using BET, XRD, SEM, TEM, FTIR and Raman techniques. • The kinetic and thermodynamic study of the adsorption process was carried out to examine the efficiency of RMAC nodules. The distinctive and tuneable physical, chemical and configurational properties of carbon nanotubes (CNTs), has prompted their combination with metal oxides to contrive carbon composites showing entrancing adsorption property with incredible potential in water treatment. MWCNT/Alumina (RMAC) nodules with effective adsorption capacity were synthesized following aqueous sol-gel route. Batch sorption experiments examined the efficiency of removal of dyes and heavy metal ions from an aqueous solution on RMAC nodules. The factors affecting adsorption were studied for adsorption of methylene blue dye (MB) and hexavalent chromium by altering the MWCNT concentration from 1 wt.% to 5 wt.%. The adsorption experiment demonstrated an adsorption capacity of 187.5 and 597 mg g−1 at 25 °C for MB and Cr (VI) respectively. Various characterization techniques such as XRD, BET, TEM, Raman, FTIR, TPD and CHN were employed to study the initial development of the material. Multiple adsorption interaction mechanisms (electrostatic interactions, hydrogen bonding, π−π electron-donor-acceptor interactions) may be credited for the remarkable adsorption capacity of these nodules. Results of this work are of great significance for environmental applications of Alumina/MWCNT composite as a promising adsorbent nanomaterial for organic pollutants from aqueous solutions. Apart from high sorption ability, these nodules offer ease of separation with splendid regeneration ability. [ABSTRACT FROM AUTHOR]

• Diffusion coefficients estimation of Cr-OABL (Organic acid-based ligands) complexes. • The maximum Cr removal efficiency of OABL was slightly less than inorganic acids. • Soil ecotoxicity improved significantly with nutrient enrichment before treatment. We assessed whether soil with high Cr contamination could be reclaimed by alkali, mineral, and organic acid-based ligands (OABLs) washing. We tested HNO 3 , H 2 SO 4 , HCl, NaOH, H 2 O 2 , lactic acid (LA), malic acid (MA), oxalic acid (OA), and citric acid (CA), together with EDTA, obtaining the highest efficiencies in presence of 1 M sulfuric acid (98%). Nonetheless we noted that using OABLs, we obtained a Cr(III) removal efficiency similar to the one obtained using mineral acids. Indeed 1 M of LA and MA and 0.8 M of OA allowed obtaining, respectively, 88%, 75%, and 67% removal percentage. The extraction process with OABLs was strongly dependent on intraparticle diffusion of Cr-LA, Cr-MA, and Cr-OA complexes. We also determined the apparent diffusion coefficients. Residual toxicity of treated soils was tested with the nematode, Caenorhabditis elegans. The OABL washing generally allowed getting a soil without Cr and with reduced toxicity. However, the washing process also removed other cations that acted as nutrients. Consequently, we conducted toxicity tests on enriched soil and found that the mortality index improved. In some cases (LA and MA), mortality was comparable to that observed with uncontaminated control samples. In contrast, when contaminated soils were washed with sulfuric acid, in all conditions, we observed significant ecotoxicity. Therefore, we concluded that only the OABL treatment provided a non-toxic soil that could be reused for anthropic activities. [ABSTRACT FROM AUTHOR]

• Cr(VI) enhanced accumulation of Cr, lignin and ROS, decreased NO content and arrested cell cycle at G 2 /M checkpoint. • This resulted into decreased photosynthetic performance and hence reduced growth. • Ca and S prevent arrest of the cell cycle and accumulation of Cr, lignin and ROS, and enhance antioxidant potential. • This resulted into improved photosynthetic performance and hence better growth. • In Ca and S-mediated management/amelioration of Cr(VI) toxicity, NO plays a positive role. To reduce pressure of toxic metals on crop plants, several strategies are being employed of which nutrient management is gaining much importance. Moreover, whether nitric oxide (NO), has any role in nutrients-mediated management/amelioration of metal toxicity is still not known. Therefore, the role of Ca and S in managing Cr(VI) toxicity was investigated in tomato and brinjal with an emphasis on possible involvement of NO. Cr(VI) reduced growth in both vegetables which was accompanied by increased accumulation of Cr(VI), lignin and reactive oxygen species (ROS), and altered cell cycle dynamics and photochemistry of photosynthesis. However, external addition of either Ca or S reversed these effects and hence improved growth noticed in both vegetables. Cr(VI) toxicity was further increased by NG-nitro- l -arginine methyl ester even with additional Ca and S while sodium nitroprusside either restored growth up to the control level or increased it in both vegetables, even in the presence of L-NAME, suggesting that NO might have a positive role in nutrients-mediated management/amelioration of Cr(VI) toxicity. In this study, role of Ca, S and NO with reference to Cr(VI) and NO accumulation, components of phenylpropanoid pathway, cell cycle dynamics, photosynthesis, ROS and antioxidant potential in managing Cr(VI) toxicity is discussed. [ABSTRACT FROM AUTHOR]

• Chromium doped mesoporous CeO 2 catalysts were synthesized by a nanocasting route. • The Cr doped CeO 2 catalysts have a high surface area and uniform pore structure. • Induced Cr3+ into CeO 2 can greatly extend the spectral response property. • Enhanced surface chemisorbed oxygen species were observed in Cr doped CeO 2. • The Cr doped CeO 2 catalysts exhibited an enhanced activity for CO 2 photoreduction. Chromium doped mesoporous CeO 2 catalysts were synthesized via a simple nanocasting route by using silica SBA-15 as the template and metal nitrates as precursors. The effect of Cr doping concentration (5%, 10%, 15% and 20% of the initial Cr/(Cr + Ce) molar percentage) on the structures of these catalysts and their photocatalytic performances in reduction of CO 2 with H 2 O were investigated. The results indicated that the introduction of Cr species could effectively extend the spectral response range from UV to visible light region (400–700 nm) and improve the electronic conductivity for the mesoporous CeO 2 catalysts which exhibited an enhanced photocatalytic activity in the reduction of CO 2 with H 2 O when compared with the non-doped counterpart. The highest CO and CH 4 yield of 16.2 μmol/g-cat. and 10.1 μmol/g-cat., respectively, were acquired on the optimal chromium doped CeO 2 catalyst with the initial Cr(Cr + Ce) molar percentage of 15% under 8 h visible-light irradiation, which were more than twice as high as that of bare CeO 2. The remarkably increased photocatalytic performance should be attributed to the advantageous structural and compositional features of the chromium doped mesoporous CeO 2. [ABSTRACT FROM AUTHOR]

Highlights • Dichromate anions in concentration ranges of 50–1000 μg/L were removed by means of aqueous dispersions of TiO 2 doped with tungsten. • The best results for Cr (VI) removal were obtained with pH values from 2 to 4. • The CO 2 contained in the ambiental air that had been injected into the photocatalytic reactor for the chromium removal experiments was captured by the photocatalyst. Abstract Removal of hexavalent chromium was accomplished by using photocatalyst materials of TiO 2 doped with tungsten oxide, environmental air as oxygen supply and white light as irradiation source. Dichromate anions in concentration ranges of 50 to 1000 μg/L were removed by means of aqueous dispersions of TiO 2 doped with tungsten. The aqueous chromium analyses were performed by Differential Pulse Voltammetry technique. Additionally, mineralization of CO 2 gas was promoted by the photocatalysis process, as was clearly shown by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) analyses obtained from the TiO 2 samples recovered after photocatalytic experiments. Results of sample analyses by Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) are presented and discussed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • ZVI coupled with H 2 O 2 could remove aniline, Sb and Cr simultaneously. • Aniline was degraded by OH but Sb, Cr removal did not depend on Fenton. • Fe(0) corrosion promoted by H 2 O 2 and in-situ generated nano iron(hydr)oxides. • Nano iron(hydr)oxides had abundant adsorption and co-precipitation sites. • Sb and Cr was mainly removed as the states of Sb(III) and Cr(III). Abstract Aniline, antimony (Sb) and chromium (Cr) are typical and regulated co-contaminants in textile wastewater, but their removal was often investigated individually. In this work, simultaneous removal of aniline, Sb and Cr by ZVI coupled with H 2 O 2 was studied. With the dosage of 0.5 g L−1 ZVI and 2 mM H 2 O 2 , aniline, Sb and Cr can be removed completely at pH 3. Experiment with iso -propanol as the radical scavenger confirmed that OH derived from Fenton reaction accounts for aniline degradation, but not for Sb and Cr removal. H 2 O 2 accelerated Fe(0) corrosion and generated the nanoscale iron(hydro)oxides. Aniline was degraded by OH first and then the degradation products were removed by iron(hydro)oxides via adsorption and co-precipitation. Both Fe(0) and iron(hydro)oxides were responsible for Sb and Cr removal, yet iron(hydro)oxides were identified as the major contributor. X-ray photoelectron spectroscopy analysis demonstrated that Sb and Cr were removed mainly as the states of Sb(III) and Cr(III). The real textile wastewater investigation confirmed that ZVI coupled with H 2 O 2 can eliminate aniline, Sb and Cr effectively, which has important implications for the advanced treatment of textile wastewater. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Anion exchange method was proposed to separate and extract hazardous Cr(VI) & V(V). • Cr(VI) was in-situ reductively desorbed from resin column selectively by ethanol. • Desorbed form of Cr(VI) was Cr3+ that can be directly precipitated for recovery. • 95.59% of Cr(VI) and 94.54% of V(V) in AVP wastewater were recovered separately. Abstract In China, the wastewater produced after vanadate precipitation (AVP wastewater) from industrial vanadium extraction contains toxic V(V) and carcinogenic Cr(VI). When considering environmental protection and wastewater use, V(V) and Cr(VI) must be extracted and separated from the hazardous AVP wastewater. However, separating V(V) and Cr(VI) is difficult because of their highly similar physicochemical properties. Herein, we propose a novel anion exchange method based on the in situ selectively reductive desorption of Cr(VI) to separate and extract V(V) and Cr(VI) using a weak organic reductant (ethanol) to selectively reduce Cr(VI) anions and transform them into Cr3+ cations, while maintaining V(V) in a H 2 V 10 O 28 4− anion form. We indicate that the efficient separation of Cr(VI) from V(V) can be attributed to selective Cr(VI) anion reduction via ethanol. We applied this anion exchange method to separate and recover Cr(VI) and V(V) in AVP wastewater with a Cr(VI) recovery of 95.59% and a V(V) recovery of 94.54%. The final Cr 2 O 3 and V 2 O 5 products had a purity of 98.03% and 96.82%, respectively. This study provides novel insights into the simultaneous separation and extraction of analog transition metals and a comprehensive method to use hazardous wastewater. [ABSTRACT FROM AUTHOR]

Highlights • Cr(VI) concentrations had distinct effects on the microbial communities and system performance. • Cr(VI) inhibited metabolic pathway and functional genes of the microbial communities in sludge. • Cr(VI) especially inhibited gene abundance those involving in denitrification pathway. • Microbe comprising functional genes had very high diversity and were adversely affected by Cr(VI). Abstract This is the first study exploring the effects of persistent Cr(VI) treatment on microbial communities and function as well as the process efficiency of an A 2 O system. The inhibitory effect was clearly higher at a high Cr(VI) concentration than a low Cr(VI) concentration, and different Cr(VI) concentrations had distinct effects on the microbial communities as well as on the performance efficiency of the system. Functional annotation analysis indicated that Cr(VI) stress inhibited most of the metabolic pathway and functional genes of the microbial communities, especially those involved in the denitrification pathway. Network analysis was used to investigate the co-occurrence patterns between denitrification genes and microbial taxa; the results indicated that microorganisms with functional genes had high diversity and were adversely affected by Cr(VI) exposure. This study is the first to establish a relationship between Cr(VI) stress and microbial communities and function as well as to determine the underlying mechanisms and roles of Cr(VI) in A 2 O sludge. [ABSTRACT FROM AUTHOR]

Highlights • The Cr(Ⅵ) removal by Oceanobacillus oncorhynchi W4, a novel Cr(Ⅵ) reducing bacterium, were firstly investigated. • Biological reduction was the major way for Cr(Ⅵ) removal by Oceanobacillus oncorhynchi W4. • The cell envelop was the main location for Cr(Ⅵ) reduction. • C O, C OH and C O C groups played major roles in correlation with the interaction of cell and chromium species. Abstract The Cr(Ⅵ) removal way and Cr(Ⅵ) reducing site of Oceanobacillus oncorhynchi W4, a novel Cr(Ⅵ) reducing bacterium, were investigated in this study. Results showed that about 74.2% of Cr(Ⅵ) was removed from solution by growing cells within 72 h. Moreover, heating-killed resting cells had little Cr(Ⅵ) removal capacity, which was significantly lower than that of resting cells, which reached nearly 80% removal rate, suggesting that the way of Cr(Ⅵ) removal mainly relied on biological reduction rather than biosorption. And the Cr(Ⅵ) reduction was found to be significantly enhanced by some electron donors, especially glycerin, which further verified enzyme-mediated biological reduction as the way for Cr(Ⅵ) removal. Experiments of Cr(Ⅵ) removal by permeable cells indicated that there was no significant difference in chromium reduction between the impermeable cells and the permeable cells. The cell envelop fraction had a Cr(Ⅵ) removal rate of 82.9%, apparently higher than cytoplasmic fraction (11.1%), indicating that the cell envelop was the main location for Cr(Ⅵ) reduction, which were further demonstrated by Scanning Electron Microscope and Transmission electron microscopy plus EDS analysis. Furthermore, analysis of X-ray photoelectron spectroscopy manifested that C O, C–OH and C–O C groups on the surfaces played major roles in correlation with chromium species. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Schwertmannite formed typical ball-with-whiskers morphology by a slow H 2 O 2 supply. • The specific surface area of schwertmannite increased by 35% with a slow H 2 O 2 supply. • The pore volume of schwertmannite increased by 50% with slow H 2 O 2 supply. • A slow H 2 O 2 supply increased by 6–11% of Cr(VI) absorption of schwertmannite. • The Cr(VI) maximum sorption capacity by schwertmannite was 1.89 mmol/g at pH = 6.0. Abstract Schwertmannite has attracted increasing interest for its excellent sorption for pollutants such as arsenite [As(III)] and arsenate [As(V)]. Limited studies were conducted with hexavalent chromium [Cr(VI)], especially for schwertmannite synthesized through Fe2+ oxidation. The effect of the hydrogen-peroxide (H 2 O 2) supply rate on the structural characteristics and Cr(VI) adsorption capacity of schwertmannite is unclear. The morphology, crystallinity, specific surface area (SSA), pore volume and Cr(VI) adsorption of schwertmannite through Fe2+ oxidation at different H 2 O 2 supply rates were analyzed. A slow H 2 O 2 supply could improve the schwertmannite SSA and pore volume. Schwertmannite changed from nanoparticle aggregates (169.31 m2/g SSA, 0.20 cm3/g pore volume) to ball-with-whisker-shaped particles (228.75 m2/g SSA, 0.30 cm3/g pore volume) with a lower H 2 O 2 supply rate. The Cr(VI) adsorption capacity increased by 6.25%–11.70% at different given Cr(VI) concentrations. The maximum Cr(VI) adsorption capacity of schwertmannite was 1.89 mmol/g at pH 6.0. Cr(VI) adsorption onto schwertmannite may be attributed mainly to ion exchange with the structural SO 4 2–. More than 91% of the original Cr(VI) adsorption capacity was maintained after four recycles. This study provides novel insights into the effects of H 2 O 2 supply rate on the microstructure of schwertmannite and its adsorption capacity for Cr(VI) in aqueous medium. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A Cr(VI) reducing Bacillus strain TCL isolated from derelict open-cast coalmine of Jharia, India. • TCL show multifarious morphological and biochemical stress response in ≥ Cr 500. • Its Cr(VI) reduction activity is constitutive and facilitated by diverse mechanisms. • The strain has high bioremediation potential. Abstract Microbial reduction of Cr(VI) to Cr(III) can mitigate environmental chromium toxicity. A chromium, cadmium and nickel tolerating strain TCL with 97% 16S rRNA gene sequence homology to Bacillus cereus was isolated from a derelict open-cast, T asra C oalmine L ake of Jharia, India. It could tolerate up to Cr 2000 [2,000 mg L−1 Cr(VI)] and completely reduce Cr 200 within 16 h under heterotrophic condition. TCL grown in ≥ Cr 500 exhibited multifarious stress responses particularly in its prolonged lag-phase, like cell aggregation, up to two-fold elongation, increased exopolysaccharide production, and stress enzyme activities. These were relieved by increasing inoculum size or nutrient content. Chromium reduction was constitutive, with maximum activities detected in loosely-bound exopolysaccharides and membrane fractions, followed by cytoplasm and spent media. Cr(VI) was efficiently reduced to Cr(III) and >90% was released in spent media. Cells also expressed Cr-induced active efflux pumps. Growing cells or its crude enzyme extracts could efficiently reduce Cr(VI) in diverse temperatures (15–45 °C), pH (5–9); and in presence of other metals (Cd, Cu, Mo, Ni, Pb), oxyanions (SO 4 −2, NO 2 −), and metabolic inhibitors (phenol, NaN 3 , EDTA). Growth and reduction were also detected in nutrient-limited minimal salt media, and contaminated leather industry effluent thereby making TCL a potential candidate for bioremediation. [ABSTRACT FROM AUTHOR]

Increasing occurrence of heavy metals (HMs) in sewage sludge threatens its widespread land utilization in China due to its potential impact on nutrient cycling in soil, requiring a better understanding of HM-induced impacts on nitrification. Herein, lab-scale experiments were conducted over 185-day, evaluating the effect of sludge-derived chromium (Cr3+), nickel (Ni2+), and lead (Pb2+) on soil nitrification at different concentrations. Quantitative polymerase chain reaction and linear regression results revealed an inhibitory sequence of gene abundance by HMs' labile fraction: ammonia-oxidizing bacteria (AOB)-ammonia monooxygenase (amoA)> nitrite oxidoreductase subunit alpha (nxrA)> nitrite oxidoreductase subunit beta (nxrB). The toxicity of HMs' incremental labile fraction decreased in the order of Ni2+>Cr3+>Pb2+, with respective threshold values of 5.01, 24.03 and 38.42 mg·kg−1. Furthermore, extending incubation time reduced HMs inhibition on ammonia oxidation, mainly related to their fraction bound to carbonate minerals. Random Forest analysis, variation partitioning analysis, and Mantel test indicated that soil physicochemical properties primarily affected nitrification genes, especially in the test of Cr3+ on AOB- amoA , nxrA , nxrB , Ni2+ for complete ammonia-oxidizing bacteria- amoA , and Pb2+ for nxrA and nxrB. These findings underline the importance of labile HMs fractions and soil physicochemical properties to nitrification, guiding the establishment of HM control standards for sludge utilization. [Display omitted] • Bioavailable heavy metals inhibit nitrifying gene abundance: AOB-amoA>nxrA> nxrB. • Soil's labile Ni, Cr, Pb incremental thresholds (mg·kg-1) are 5.01, 24.03, 38.42. • Bound to carbonate mineral of Cr, Ni, Pb affect ammonia oxidation capacity in soil. • Cumulative importance of soil properties for nitrifying gene abundance exceed 90%. [ABSTRACT FROM AUTHOR]

The presence of cadmium (Cd) and chromium (Cr) contamination in soil poses an environmental risk to food safety. Microorganisms are crucial for biotransforming heavy metals, but their limited survival in contaminated soils hinders their application in bioremediation. Here, we isolated Lysinibacillus sp. OR-15, which effectively removed Cd(II) and reduced Cr(VI) from the culture. Proteomic analyses and heterologous expression assays showed that QueF, an NADPH-dependent reductase, enhanced bacterial resistance to Cd(II) and Cr(VI), enhancing metal removal capacity. The dry bacterial powder, produced through deep liquid fermentation and spray drying, contained 2.3 × 1010 viable cells per gram. It effectively reduced Cd and Cr levels in rice grains and maintained a concentration of 105 per gram in soils even after 60 days of cultivation following one application. Scanning and microscopy analysis revealed that Lysinibacillus sp. OR-15 facilitated the formation of iron plaque on rice roots. The formation of iron plaque on the root surface serves as a protective barrier against the upward translocation of heavy metals. Our findings suggest that Lysinibacillus sp. OR-15 exhibits stable and effective properties as a factor for Cd and Cr adsorb on rice iron plaque, thus mitigating the levels of Cd and Cr in rice grains. [Display omitted] • Lysinibacillus sp. OR-15 exhibits the ability to remove Cd(II) and reduce Cr(VI). • The reductase QueF contributes to the resistance of OR-15 against Cd and Cr. • The fermented powder exhibits stable colonization in both soils and rice roots. • The OR-15 strain enhances the formation of Fe plaque on the surface of rice roots. • The metal content in grain decreases due to the adsorption of metals by Fe plaque. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is a redox-sensitive element in contaminated coastal urban soils. Sea level rise (SLR) with subsequent soil inundation may facilitate Cr transformation and mobilization through alterations in local redox conditions and porewater ion composition. We investigated the impact of water salinity and redox conditions on Cr chemistry in these environments. Synchrotron-based X-ray spectroscopy and wet chemical analyses revealed that the soils contained very high levels of Cr (up to 4320 mg kg-1) and that chromite (∼52%) and Fe-Cr hydroxide coprecipitates (∼44%) were the predominant Cr species. The abundance of these two components resulted in low Cr mobility under non-flooded conditions. Chromium(II) was identified in the soils, potentially derived from the waste parent material. Seawater and anoxic conditions resulted in lower Cr release compared to freshwater and aerobic conditions. Up to three to eight times more Cr was released under aerobic conditions versus anaerobic conditions in the freshwater versus saltwater, respectively, with total dissolved Cr values remaining below 0.02 mg L-1. The decrease in Cr release was likely due to Cr reduction by Fe(II) and sulfide. This work provides important information on how salinity and redox fluctuations impact Cr cycling which is likely to occur during SLR. [Display omitted] • Chromite and Fe-Cr hydroxide coprecipitates were the two predominant Cr species. • Cr(II) is uncommon in environmental samples but was observed in these soils. • Redox and water salinity conditions greatly alter Cr release. • Soil exposure to seawater and anoxic conditions decreased Cr release. • Soil exposure to freshwater and oxic conditions increased Cr release. [ABSTRACT FROM AUTHOR]

To reduce heavy metals (HMs) contamination from tannery sludge, this study investigated the migration behaviour of arsenic (As) and chromium (Cr) at 700–900 °C using CO 2 gasification. The HMs enrichment results showed that As contents of ash decreased (6.42→1.87 mg/kg) while Cr contents increased (41.40→78.24 mg/kg) over 700–900 °C. More Si-O bonds and fewer Ca-O bonds with increasing temperature in ash primarily determined this migration behaviour of HMs. Meanwhile, the proportions of toxic As(III) and Cr(VI) declined from 96.02% and 64.26–76.96% and 21.24%, forming As(0) and Cr(III) with less toxicity. This reduction was conducted via two pathways: (i) carbon reduced As(III)/Cr(VI) and (ii) carbon reduced Fe(II)/Fe(III) to Fe(0), then Fe(0) reduced As(III)/Cr(VI) assisted with carbon via Fe(0)→Fe(II)→Fe(III). However, free calcium ions oxidized As(0)/Cr(III) to As(III)/Cr(VI) at 700 ○C. At higher temperatures, silicate glass conversion of ash immobilized free calcium ions and barely oxidized HMs. Furthermore, this study identified the positive effect of increasing temperature on enhancing the stability of HMs in ash by transforming bioavailable HMs into non-bioavailable HMs, which decreased the leaching toxicity and environmental risk. Regarding HMs emissions control and cold gas efficiency, CO 2 gasification treatment of tannery sludge is most effective at 800 °C. [Display omitted] • Silicate glass transformation of ash occurred above 800 °C under CO 2 gasification. • Temperatures exceed 800 °C greatly reduced As(III)/Cr(VI) to As(0)/Cr(III). • Over 88.16% and 87.63% of As and Cr in ash were non-bioavailable above 800 °C. • Environmental risk of ash was at low risk at 700–900 ○C under CO 2 gasification. • 800 °C was more suitable for tannery sludge disposal under CO 2 gasification. [ABSTRACT FROM AUTHOR]

Mushrooms are consumed worldwide as they constitute a part of traditional cuisine culture in many countries. However, chromium (Cr) accumulation in mushrooms may constitute a potential pathway for its chronical exposure to humans. In this work, the Cr contents, distribution and bioaccessibility in 140 cultivated mushrooms from 14 species in 10 top-producing provinces in China were examined. Total Cr contents were 0.09–4.71 mg·kg−1 dw (mean 0.74 mg kg−1), with 59% exceeding the 0.5 mg kg−1standard. Additionally, less Cr was accumulated in the caps than stipes, with Cr ratio in caps/stipes being 0.28–2.6, averaging 0.91. Based on the Solubility Bioaccessibility Research Consortium (SBRC) assay, the mean Cr bioaccessibility in the mushrooms was 24.8% and 50.1% in the gastric phase (GP) and intestinal phase (IP). However, samples from Guizhou show the lowest Cr bioaccessibility at 12.5% in GP and 24.8% in IP. Further, a negative correlation between total Cr contents and Cr bioaccessibility suggests that Cr bioaccessibility is critical for accurate assessment of Cr exposure. In addition, drying mushrooms increased their bioaccessibility in the gastric phase. This study shows a high Cr exceeding rate of cultivated mushrooms, which may indicate a potential exposure risk, with Cr contents and bioaccessibility showing species and regional variation. [Display omitted] • 140 mushrooms were from 14 species in 10 top-producing provinces in China. • Total Cr contents were 0.09–4.71 mg·kg−1, with 59% exceeding 0.5 mg kg−1 standard. • More Cr was concentrated in the stipes than caps (1.11 vs. 0.92 mg·kg−1). • Cr bioaccessibility negatively correlated with Cr contents via SBRC assay. • Greater Cr bioaccessibility in the gastric phase in dried mushrooms than in fresh samples. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is a hazardous heavy metal that negatively affects animals and plants. The micronutrients selenium (Se) and molybdenum (Mo) have been widely shown to alleviate heavy metal toxicity in plants. However, the molecular mechanism of Cr chelation on the cell wall by combined treatment with Se and Mo has not been reported. Therefore, this study aimed to explore the effects of Se-Mo interactions on the subcellular distribution of Cr (50 µM) and on cell wall composition, structure, functional groups and Cr content, in addition to performing a comprehensive analysis of the transcriptome. Our results showed that the cell walls of shoots and roots accumulated 51.0% and 65.0% of the Cr, respectively. Furthermore, pectin in the cell wall bound 69.5%/90.2% of the Cr in the shoots/roots. Se-Mo interactions upregulated the expression levels of related genes encoding galacturonosyltransferase (GAUT), UTP-glucose-1-phosphate uridylyltransferase (UGP), and UDP-glucose-4-epimerase (GALE), involved in polysaccharide biosynthesis, thereby increasing pectin and cellulose levels. Moreover, combined treatment with Se and Mo increased the lignin content and cell wall thickness by upregulating the expression levels of genes encoding cinnamyl alcohol dehydrogenase (CAD), peroxidase (POX) and phenylalanine amino-lyase (PAL), involved in lignin biosynthesis. Fourier-transform infrared (FTIR) spectroscopy results showed that Se + Mo treatment (in combination) increased the number of carboxylic acid groups (-COOH) groups, thereby enhancing the Cr chelation ability. The results not only elucidate the molecular mechanism of action of Se-Mo interactions in mitigating Cr toxicity but also provide new insights for phytoremediation and food safety. [Display omitted] • Se-Mo interaction promoted Cr chelation on the cell wall to reduce Cr toxicity. • Upregulated GAUT, GALE, UGP and PAL gene expressions in cell wall biosynthesis. • Conjunctive use of Se and Mo increased the cell wall thickness and lignin content. • Se-Mo interaction improved cell wall crystallinity and cell wall network structure. • Se-Mo interaction increased the number of carboxylic acid groups (-COOH). [ABSTRACT FROM AUTHOR]

The adsorption behavior and interaction mechanisms of extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 towards chromium (Cr), lead (Pb), and cadmium (Cd) were investigated. EPS-covered (EPS-C) cells exhibited significantly higher (p < 0.0001; two-way ANOVA) removal of Cr (85.58 ± 0.39%), Pb (81.98 ± 1.02%), and Cd (73.88 ± 1%) than EPS-removed (EPS-R) cells. Interactions between EPS-heavy metals were spontaneous (ΔG<0). EPS-Cr(VI) and EPS-Pb(II) binding were exothermic (ΔH<0), while EPS-Cd(II) binding was endothermic (ΔH>0) process. EPS bonded to Pb(II) via inner-sphere complexation by displacement of surrounding water molecules, while EPS-Cr(VI) and EPS-Cd(II) binding occurred through outer-sphere complexation via electrostatic interactions. Increased zeta potential of Cr (29.75%), Pb (41.46%), and Cd (46.83%) treated EPS and unchanged crystallinity (CI XRD =0.13), inferred EPS-metal binding via both electrostatic interactions and complexation mechanism. EPS-metal interaction was predominantly promoted through hydroxyl, amide, carboxyl, and phosphate groups. Metal adsorption deviated EPS protein secondary structures. Strong static quenching mechanism between tryptophan protein-like substances in EPS and heavy metals was evidenced. EPS sequestered heavy metals via complexation with C-O, C-OH, CO/O-C-O, and NH/NH 2 groups and ion exchange with -COOH group. This study unveils the fate of Cr, Pb, and Cd on EPS surface and provides insight into the interactions among EPS and metal ions for metal sequestration., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Surajit Das reports financial support was provided by Department of Science and Technology, Government of India, India., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Microbial fuel cell (MFC) technology has been developed for wastewater treatment in the anodic chamber, and heavy metal reduction in the cathodic chamber. However, the limited extracellular electron transfer (EET) rate of exoelectrogens remained a constraint for practical applications of MFCs. Here, a MFC system that used the electricity derived from anodic wastewater treatment to drive cathodic Cr 6+ reduction was developed, which enabled an energy self-sustained approach to efficiently address Cr 6+ contamination. This MFC system was achieved by screening exoelectrogens with a superior EET rate, promoting the exoelectrogenic EET rate, and constructing a conductive bio-anode. Firstly, Shewanella algae-L3 was screened from brewing wastewater acclimatized sludge, which generated power density of 566.83 mW m -2 . Secondly, to facilitate EET rate, flavin synthesis gene operon ribADEHC was overexpressed in engineered S. algae-L3F to increase flavins biosynthesis, which promoted the power density to 1233.21 mW m -2 . Thirdly, to facilitate interface electron transfer, carbon nanotube (CNT) was employed to construct a S. algae-L3F-CNT bio-anode, which further enhanced power density to 3112.98 mW m -2 . Lastly, S. algae-L3F-CNT bio-anode was used to harvest electrical energy from brewing wastewater to drive cathodic Cr 6+ reduction in MFC, realizing 71.43% anodic COD removal and 98.14% cathodic Cr 6+ reduction. This study demonstrated that enhanced exoelectrogenic EET could facilitate cathodic Cr 6+ reduction in MFC., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Poor plant growth and low pollutant bioavailability in contaminated soils limit phytoremediation efficiency. Pot experiments were conducted to investigate the effects and mechanisms of Piriformospora indica inoculation on the phytoremediation of Cd-Cr co-contaminated soils from farmland using king grass. P. indica colonization increased plant biomass by 20.4-24.6% and enhanced Cd/Cr accumulation in root, stem and leave tissues. Root vascular cylinder and cortex were the major structures for Cd/Cr transportation in plants. The amounts of Cd and Cr extracted by king grass considerably increased in the presence of P. indica (by 31.5-88.9% and 22.4-38.4%, respectively), as did the removal efficiency of both metals from soils (by 13.2-32.2% and 23.2-33.5%, respectively). Cd/Cr phytoextraction was closely related to the contents of alkanes, lipids and acids in root exudates. Following inoculation, the respiration of microbial sulfur compounds was promoted in soils at low and medium pollution levels, whereas nitrogen fixation and nitrification were reduced at high pollution level. This study demonstrates that P. indica inoculation enhances the phytoremediation efficiency of king grass for Cd-Cr co-contaminated soils through multiple regulation of plant growth, rhizosphere environment, root exudation and soil microbial function., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Graphical abstract Highlights • Synthesis of stable and porous thermally expended reduced-graphene oxide (TErGO/Ni 3 C-Ni) nanocomposite. • High performance reduction of Cr(Ⅵ) via TErGO/Ni 3 C-Ni nanocatalyst. • Fast reduction of dyes with the help of TErGO/Ni 3 C-Ni nanocatalyst. Abstract A high performance thermally expanded reduced graphene oxide (TErGO) nanocomposite decorated with Ni 3 C-Ni nanoparticles (TErGO/Ni 3 C-Ni) has been successfully synthesized by using a facile and eco-friendly approach. The morphology, textural features, surface composition and stability of TErGO/Ni 3 C-Ni nanocomposite are investigated by various physicochemical characterizations which revealed the uniform dispersion of crystalline metal nanoparticles inside TErGO matrix. The composite has been exhibited a large surface area and pore volume of 121 m2 g−1 and 0.791 cm3 g−1, respectively. The TErGO/Ni 3 C-Ni exhibited remarkable catalytic performance surpassing most metal-based catalysts with various kind of support matrices reported in recent literature. The reduction of Cr(Ⅵ) to Cr(Ⅲ) was achieved within 1 min with an excellent rate constant of 2.74 min−1 and phenomenally higher specific removal rate (SRR) of 0.29 mg Cr(VI) min−1. mg−1 of TErGO/Ni 3 C-Ni. While it also proved an excellent reducing catalyst for organic dyes via NaBH 4 with full reduction achieved within 30 s. Moreover, as prepared nanocatalyst possesses excellent stability and recyclability with easy magnetic separation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ag/Bi 4 O 7 /g-C 3 N 4 ternary metal/non-metal Z-scheme heterojunction was prepared. • A photodeposition experiment confirmed Z-scheme heterojunction in Ag/Bi 4 O 7 /g-C 3 N 4. • Ag adhered to g-C 3 N 4 nanosheets, constructing plasmonic metal/semiconductor system. • The visible-light photoreduction of aqueous Cr(VI) is enhanced. • The ternary Ag/Bi 4 O 7 /g-C 3 N 4 nanosheets have favorable cycling stability. Abstract In this paper, we successfully constructed a ternary metal/non-metal nanomaterial which can synergize Z-scheme heterojunction and plasmonic metal/semiconductor for enhancing visible light photoreduction of aqueous Cr(VI). The as-formed ternary metal/non-metal nanomaterial was composed of g-C 3 N 4 nanosheets mutual modified by Ag nanoparticles and Bi 4 O 7. In the ternary nanomaterial, g-C 3 N 4 nanosheets and Bi 4 O 7 fabricated Z-scheme heterojunction, g-C 3 N 4 nanosheets and Ag nanoparticles formed plasmonic metal/semiconductor system, synchronously. The photodeposition experiments were designed to indirect demonstrate the Z-scheme heterojunction via photocatalytic oxidation and reduction deposition reactions, AgNO 3 was used as the electron acceptor and Pb(NO 3) 2 as the electron donor, photocatalytic reduction product and oxidation product were selectively deposited on g-C 3 N 4 nanosheets and Bi 4 O 7 , respectively, revealing the Z-scheme heterojunction was definite formed. In addition, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), UV–vis absorption, and Fourier-transform infrared spectroscopy (FTIR) were studied to indicate that the plasmonic metal/semiconductor system was existent in the ternary nanomaterial. The visible light photoreduction of ternary Ag/Bi 4 O 7 /g-C 3 N 4 nanosheets was evaluated by photoreducing aqueous Cr(VI). The photoreduction results show that the ternary Ag/Bi 4 O 7 /g-C 3 N 4 nanosheets is much higher than the pristine bulk g-C 3 N 4 and g-C 3 N 4 nanosheets. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nanoadsorbents merge several magnetic materials and properties in a single object. • Nanoadsorbents present high adsorption capacity and selectivity for Cr(VI). • The mean size of nanoadsorbents strongly influences their adsorption efficiency. • Nanoadsorbents are magnetic retrievable and reusable for Cr(VI) removal. • Removal process can be extended to other heavy metal ions. Abstract Novel nanoadsorbents based on core-shell bimagnetic nanoparticles (CoFe 2 O 4 @ɣ-Fe 2 O 3) with two different mean sizes were elaborated, characterized and applied as potential sorbents for Cr(VI) removal from aqueous solutions through magnetically assisted chemical separation. The nanoadsorbents were characterized by XRD, TEM, FTIR, XPS, potentiometric-conductometric titrations, BET and vibrating sample magnetometry. The influence of contact time, shaking rate, pH, pollutant concentration, temperature and competing ions on Cr(VI) adsorption were evaluated. The results were analyzed in the framework of Langmuir and Freundlich models to evaluate the maximum adsorption capacity and the extent of affinity. The nanoadsorbents showed a good selectivity for Cr(VI) adsorption and were more effective at pH = 2.5, with a shaking rate of 400 RPM. The adsorption process was spontaneous, endothermic and presented an increased randomness. The contact time required to reach the equilibrium was relatively short and the kinetic date followed the pseudo-second-order model. The maximum adsorption capacity was nearly 40% higher for the nanoadsorbent of smaller mean size due to its higher surface area. Regeneration studies revealed that the nanoadsorbents can be recovered for reuse. These results indicate that prepared nanoadsorbents can be used as a powerful tool for Cr(VI) removal from contaminated water. [ABSTRACT FROM AUTHOR]

Highlights • A water-based silsesquioxane (POSS)-containing polymer, POSS-PAA, was synthesized by interfacial polymerization. • The pollution of leather tanning wastewater can be reduced by this star-shaped polymer. • The hydrothermal stability and fullness properties of leather can be improved by POSS-PAA. Abstract A water-based silsesquioxane (POSS)-containing polymer, POSS-PAA, was synthesized by using octavinyl-POSS (V-POSS) and acrylic acid (AA) via interfacial polymerization. The TEM of POSS-PAA showed that the polymer formed a core-shell structure in aqueous solution and was well-dispersed. The star-shaped POSS-PAA and linear PAA were both tanned with 3.5% chromium tanning agent, and leather hide was tanned with 3.5% chromium tanning agent as a control. The results showed that the shrinkage temperature of wet-blue leather treated by POSS-PAA was increased by 3.5 °C than that of the control. The thickening rate of the POSS-PAA treated wet-blue leather samples was increased by 21% and 96% than the linear PAA-treated leather and the control leather, respectively. The EDS results suggested that the POSS-PAA pre-treated leather had a higher chromium content than the others, and the chromium distribution from the leather flesh side to the grain side was uniform. Moreover, the Cr 2 O 3 content in both the POSS-PAA and the PAA pre-treated tanning waste water was reduced by about 50%, compared to the control waste. The COD and BOD of the POSS-PAA pre-tanning waste were decreased compared to the others. Therefore, POSS-PAA appeared to be promising for promoting the development of cleaner leather production. [ABSTRACT FROM AUTHOR]