Potential hazards of hexavalent chromate in our drinking water

Occurrence, fate, and potential impacts of wood preservatives in the environment: Challenges and environmentally friendly solutions

Wood preservation has gained global prevalence in recent years, primarily owing to the renewable nature of wood and its capacity to act as a carbon sink. Wood, in its natural form, lacks intrinsic resilience and is prone to decay if left untreated; hence, wood preservatives (WPs) are used to improve wood's longevity. The fate and potential hazards of wood preservatives to human health, ecosystems, and the environment are complex and depend on various aspects, including the type of the preservative compounds, their physicochemical properties, application methods, exposure pathways, environmental conditions, and safety measures and guidelines. The occurrence and distribution of WPs in environmental matrices such as soil and water can result in hazardous pollutants seeping into surface water, groundwater, and soil, posing health hazards, and polluting the environment. Bioremediation is crucial to safeguarding the environment and effectively removing contaminants through hydrolytic and/or photochemical reactions. Phytoremediation, vermicomposting, and sustainable adsorption have demonstrated significant efficacy in the remediation of WPs in the natural environment. Adsorbents derived from biomass waste have been acknowledged for their ability to effectively remove WPs, while also offering cost-efficiency and environmental sustainability. This paper aims to identify wood preservatives' sources and fate in the environment and present a comprehensive overview of the latest advancements in environmentally friendly methods relevant to the removal of the commonly observed contaminants associated with WPs in environmental matrices.

An integrated experimental and theoretical approach to probe Cr(vi) uptake using decorated halloysite nanotubes for efficient water treatment

Halloysite nanotubes (HNTs) were surface functionalized using four distinct chemical moieties (amidoxime, hydrazone, ethylenediamine (EDA), and diethylenetriamine (DETA)), producing modified HNTs (H1-H4) capable of binding with Cr(vi) ions. Advanced techniques like FTIR, XRD, SEM, and EDX provided evidence of the successful functionalization of these HNTs. Notably, the functionalization occurred on the surface of HNTs, rather than within the interlayer or lumen. These decorated HNTs were effective in capturing Cr(vi) ions at optimized sorption parameters, with adsorption rates ranging between 58-94%, as confirmed by atomic absorption spectroscopy (AAS). The mechanism of adsorption was further scrutinized through the Freundlich and Langmuir isotherms. Langmuir isotherms revealed the nearest fit to the data suggesting the monolayer adsorption of Cr(vi) ions onto the nanotubes, indicating a favorable adsorption process. It was hypothesized that Cr(vi) ions are primarily attracted to the amine groups on the modified nanotubes. Quantum chemical calculations further revealed that HNTs functionalized with hydrazone structures (H2) demonstrated a higher affinity (interaction energy -26.33 kcal mol-1) for the Cr(vi) ions. This can be explained by the formation of stronger hydrogen bonds with the NH moieties of the hydrazone moiety, than those established by the OH of oxime (H1) and longer amine chains (H3 and H4), respectively. Overall, the findings suggest that these decorated HNTs could serve as an effective and cost-efficient solution for treating water pollution.

NiFe Layered Double Hydroxides with Controlled Composition and Morphology for the Efficient Removal of Cr(VI) from Water

A layered double hydroxide (LDH) composed of Ni2+ and Fe3+ with a Fe3+/(Ni2+ + Fe3+) ratio of 0.05, which is not commonly available, was successfully prepared by coprecipitation from an aqueous solution of glycerol containing nickel nitrate and iron nitrate. Precipitation using NaOH as a precipitating agent at room temperature or 120 °C under hydrothermal conditions gave products with micrometer-sized aggregates of nanometer-sized unshaped particles, while that using urea yielded LDHs with a foam-like porous architecture composed of platy particles with a size of 100-300 nm. The products were examined to remove Cr(VI) from an acidic (pH = 3) aqueous solution of K2Cr2O7 by adsorption and photocatalytic reduction. The foam-like porous NiFe-LDH exhibited the highest adsorbed amount (122 mg g-1) and rate (0.017 g mg-1 min-1) in the dark and the highest rate (0.012 min-1) of photocatalytic Cr(VI) reduction among the NiFe-LDHs prepared in the present study, which can be explained as a positive effect of the foam-like porous architecture. These performances were superior to those of other reported LDHs, showing the importance of the composition and the particle morphology to boost the removal of Cr(VI).

Mechanism of Cr(VI) removal by efficient Cr(VI)-resistant Bacillus mobilis CR3

Cr(VI) is a hazardous environmental pollutant that poses significant risks to ecosystems and human health. We successfully isolated a novel strain of Bacillus mobilis, strain CR3, from Cr(VI)-contaminated soil. Strain CR3 showed 86.70% removal capacity at 200 mg/L Cr(VI), and a good Cr(VI) removal capacity at different pH, temperature, coexisting ions, and electron donor conditions. Different concentrations of Cr(VI) affected the activity of CR3 cells and the removal rate of Cr(VI), and approximately 3.46% of total Cr was immobilized at the end of the reaction. The combination of SEM-EDS and TEM-EDS analysis showed that Cr accumulated both on the cell surface and inside the cells after treatment with Cr(VI). XPS analysis showed that both Cr(III) and Cr(VI) were present on the cell surface, and FTIR results indicated that the presence of Cr on the cell surface was mainly related to functional groups, such as O-H, phosphate, and -COOH. The removal of Cr(VI) was mainly achieved through bioreduction, which primarily occurred outside the cell. Metabolomics analysis revealed the upregulation of five metabolites, including phenol and L-carnosine, was closely associated with Cr(VI) reduction, heavy metal chelation, and detoxification mechanisms. In addition, numerous metabolites were linked to cellular homeostasis exhibited differential expression. Cr(VI) exerted inhibitory effects on the division rate and influenced critical pathways, including energy metabolism, nucleotide metabolism, and amino acid synthesis and catabolism. These findings reveal the molecular mechanism of Cr(VI) removal by strain CR3 and provide valuable insights to guide the remediation of Cr(VI)-contaminated sites.

One step synthesis of highly photoluminescent red light-emitting carbon dots from O-phenylenediamine and 2,4-diaminophenol as fluorescent probes for the detection of pH and Cr(VI)

In this study, new red light-emitting carbon dots (R-CDs) that can selectively recognize Cr(VI) were prepared using a strategy that utilizes 2,4-diaminophenol to enhance the fluorescence of O-phenylenediamine based carbon dots. The results showed that 2,4-diaminophenol increased the quantum yield (QY) of the carbon quantum dots (CDs), and that the QY of the CDs increased from their original value of 8.7% to 20.1% (R-CDs). The R-CDs show sensitivity to acidic conditions and maintain good linearity between pH = 1.00-4.00, making them useful as pH probes. Furthermore, the prepared R-CDs possess good solubility in water and are responsive to changes in Cr(VI) concentrations in aqueous environments. The quenching of the R-CDs fluorescence was linearly correlated with the Cr(VI) concentration within a range of 0-20 μM, with a lower detection limit of 66 nM. The detection mechanism is attributed to the formation of hydrogen bonds between Cr(VI) and the R-CDs, resulting in the fluorescence quenching of the R-CDs. The R-CDs can be considered effective multifunctional fluorescent probes for both pH and Cr(VI) in aqueous environments. This study will provide new R-CD design strategies for probes that selectively identify specific target substances.

Exploring the mechanism of enhanced Cr(VI) removal by Lysinibacillus cavernae microcapsules loaded with synthetic nano-hydroxyapatite

In this study, nano-scale hydroxyapatite (HAP) powder was successfully synthesized from waste eggshells and combined with Lysinibacillus cavernae CR-2 to form bio-microcapsules, which facilitated the enhanced removal of Cr(VI) from wastewater. The effects of various parameters, such as bio-microcapsule dosage, HAP dosage, and initial Cr(VI) concentration on Cr(VI) removal, were investigated. Under different treatment conditions, the Cr(VI) removal followed the order of LC@HAP (90.95%) > LC (78.15%) > Free-LC (75.61%) > HAP (6.56%) > NM (0.23%) at the Cr(VI) initial concentration of 50 mg L-1. Relative to other reaction systems, the LC@HAP treatment exhibited a considerable decrease in total Cr content in the solution, with removal rates surpassing 70%. Additionally, the bio-microcapsules maintained significant biological activity after reacting with Cr(VI). Further characterization using SEM, FTIR, XPS, and XRD revealed that the Cr(VI) removal mechanisms by bio-microcapsules primarily involved biological reduction and HAP adsorption. The adsorption of Cr(III) by HAP predominantly occurred through electrostatic interactions and surface complexation, accompanied by an ion exchange process between Cr(III) and Ca(II). Hence, bio-microcapsules, created by combining L. cavernae with HAP, represent a promising emerging material for the enhanced removal of Cr(VI) pollutants from wastewater.

Tin Oxide Nanoparticles via Solar Vapor Deposition for Hexavalent Chromium Remediation

Tin oxide nanoparticles optimized to capture low concentrations of hexavalent chromium from water were developed through a facile, scalable, and low-cost one-step solar vapor deposition methodology. Considering the preservation of high electron donation capacity as the key to support the reduction of mobile Cr(VI) into insoluble forms, the growth of SnO nanoparticles was favored by the co-evaporation of SnO2 with Fe powders at various mass ratios. Characterization techniques indicated that the percentage and the stability of SnO is proportional to the Fe content in the target with a requirement of at least 50% wt to inhibit the formation of a passive SnO2 surface layer. The produced particles were evaluated regarding their efficiency to capture Cr(VI) under conditions similar to water treatment for drinking purposes (pH 7). It was revealed that passivation-free SnO nanoparticles deliver significant improvement in the adsorption capacity corresponding to the residual concentration of 25 μg/L, reaching a value of 1.74 mg/g for the sample prepared with 50% wt Fe in the target. The increase of water acidity was found responsible for the activation of more reduction sites on the particle surface, as reflected through the elevation of efficiency by more than 20% at pH 6.

Colorimetric sensor for Cr (VI) by oxidative etching of gold nanotetrapods at room temperature

Hexavalent chromium (Cr(VI)) is highly carcinogenic and mutagenic, which is seriously harmful to human health. Hence, it is important to create a probe that can detect Cr(VI) effectively. In this work, gold nanotetrapods (Au NTPs) were applied in colorimetric detection for the first time. Based on the oxidative etching principle of Cr(VI) on Au NTPs, a sensitive and multicolor response detection method for Cr(VI) was established. The oxidative etching of Au NTPs by Cr(VI) resulted in the blue shift of plasmon resonance absorption peak of Au NTPs with the change of morphology. As the etching progress processed, Au NTPs solution exhibited obvious color changes from gray-green to blue-violet and then to pink. This multicolor response design is very convenient for naked-eye detection. The limit of detection (LOD) of Cr(VI) is 3 nM for the naked eyes and 0.5 nM for UV-vis spectrum, both of which are lower than the toxicity level of Cr(VI) (0.2 μM) set by United States Environmental Protection Agency. This sensing method exhibits good linearity between the wavelength shift and Cr(VI) concentration in the range of 0.5 nM to 8 nM. The detection results of Cr(VI) in actual environmental samples demonstrate that the Au NTPs colorimetric probe (Au-N-Probe) is expected to be applied to the detection of Cr(VI) in water environmental samples such as lake water and industrial wastewater.

Sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms in the aquatic environment

Microorganisms can oxidize Mn(II) to biogenic Mn oxides (BioMnOx), through enzyme-mediated processes and non-enzyme-mediated processes, which are generally considered as the source and sink of heavy metals due to highly reactive to sequestrate and oxidize heavy metals. Hence, the summary of interactions between Mn(II) oxidizing microorganisms (MnOM) and heavy metals is benefit for further work on microbial-mediated self-purification of water bodies. This review comprehensively summarizes the interactions between MnOM and heavy metals. The processes of BioMnOx production by MnOM has been firstly discussed. Moreover, the interactions between BioMnOx and various heavy metals are critically discussed. On the one hand, modes for heavy metals adsorbed on BioMnOx are summarized, such as electrostatic attraction, oxidative precipitation, ion exchange, surface complexation, and autocatalytic oxidation. On the other hand, adsorption and oxidation of representative heavy metals based on BioMnOx/Mn(II) are also discussed. Thirdly, the interactions between MnOM and heavy metals are also focused on. Finally, several perspectives which will contribute to future research are proposed. This review provides insight into the sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms. It might be helpful to understand the geochemical fate of heavy metals in the aquatic environment and the process of microbial-mediated water self-purification.

Systemic impact of heavy metals and their role in cancer development: a review

Heavy metals are well-recognised as environmental hazards due to their toxicity, environmental persistence, and bioaccumulation in living organisms. Human health is a crucial concern related to terrestrial and aquatic ecosystems poisoned by harmful heavy metals. Most heavy metals pollute the air, water, and soil, which can be fatal to humans. Humans and other species can be exposed to heavy metals through the food chain if the metals oxidise or combine with other environmental elements (such as water, soil, or air). Their entry into the food chain assures interactions with biological macromolecules in living systems, including humans, resulting in undesirable outcomes. Human poisonings have typically been caused by mercury, lead, chromium, cadmium, and arsenic. The build-up of these metals in living organisms causes various harmful consequences on different organs and tissues. The gravitas of heavy metal toxicity regarding molecular impact and carcinogenesis needs in-depth understanding despite the plethora of available data. Hence, additionally, we attempt to elaborate on the multi-level impact of five heavy metals and emphasise their role in cancer development. The rationale of this essay is thus to understand the role of five heavy metals, viz., lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg), in carcinogenesis. Heavy metals interfere with various biological functions, including proliferation, differentiation, repair of damage, and apoptosis. By comparing their modes of action, we see that these metals share common mechanisms for inducing toxicity, such as reactive oxygen species (ROS) production, antioxidant defence weakening, enzyme inactivation, and oxidative stress.

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.

Biogenic carbon encapsulated iron oxides mediated oxalic acid for Cr(VI) reduction in aqueous: Efficient performance, electron transfer and radical mechanisms

Carbonaceous materials have a potential to mediated oxalic acid (OA) for Cr(VI) reduction, but the rational modification is needed for boosting the mediation of electron transfer. Herein, we utilized polyvinyl alcohol to envelop schwertmannite synthesized by Acidithiobacillus ferrooxidans biomineralization, and pyrolyzed them to obtain the carbon encapsulated iron oxides (C-2.0-Sch-PVA). SEM and TEM results demonstrated that a moderate calcination temperature would yield a neural network-like carbon encapsulated structure. C-2.0-Sch-PVA efficiently mediated OA to reduce Cr(VI), 98.4% of Cr(VI) (40 mg L^-1) was reduced with 0.75 g L^-1 C-2.0-Sch-PVA and 4 mM OA in 60 min. It still performed excellent results in a wide pH range, multiple anions and different water matrixes. The carbon encapsulated structure as electron shuttle mediated the electron transfer, and the O-moieties on its surface were a premise for initiating the Cr(VI) reduction process. The electron transfer from the inner iron oxides to the conjugated structure of the outer carbon shells facilitated Cr(VI) reduction as well. Moreover, OA raised the persistent free radicals' level in C-2.0-Sch-PVA as another important pathway for Cr(VI) reduction. Overall, C-2.0-Sch-PVA provides an excellent demonstration in the carbonaceous materials modification for mediating OA to reduce Cr(VI) in aqueous.

Groundwater Quality, Health Risk Assessment, and Source Distribution of Heavy Metals Contamination around Chromite Mines: Application of GIS, Sustainable Groundwater Management, Geostatistics, PCAMLR, and PMF Receptor Model

Groundwater contamination by heavy metals (HMs) released by weathering and mineral dissolution of granite, gneisses, ultramafic, and basaltic rock composition causes human health concerns worldwide. This paper evaluated the heavy metals (HMs) concentrations and physicochemical variables of groundwater around enriched chromite mines of Malakand, Pakistan, with particular emphasis on water quality, hydro-geochemistry, spatial distribution, geochemical speciation, and human health impacts. To better understand the groundwater hydrogeochemical profile and HMs enrichment, groundwater samples were collected from the mining region (n = 35), non-mining region (n = 20), and chromite mines water (n = 5) and then analyzed using ICPMS (Agilent 7500 ICPMS). The ranges of concentrations in the mining, non-mining, and chromite mines water were 0.02-4.5, 0.02-2.3, and 5.8-6.0 mg/L for CR, 0.4-3.8, 0.05-3.6, and 3.2-5.8 mg/L for Ni, and 0.05-0.8, 0.05-0.8, and 0.6-1.2 mg/L for Mn. Geochemical speciation of groundwater variables such as OH^-, H⁺, Cr⁺², Cr⁺³, Cr⁺⁶, Ni⁺², Mn⁺², and Mn⁺³ was assessed by atomic fluorescence spectrometry (AFS). Geochemical speciation determined the mobilization, reactivity, and toxicity of HMs in complex groundwater systems. Groundwater facies showed 45% CaHCO₃, 30% NaHCO₃, 23.4% NaCl, and 1.6% Ca-Mg-Cl water types. The noncarcinogenic and carcinogenic risk of HMs outlined via hazard quotient (HQ) and total hazard indices (THI) showed the following order: Ni > Cr > Mn. Thus, the HHRA model suggested that children are more vulnerable to HMs toxicity than adults. Hierarchical agglomerative cluster analysis (HACA) showed three distinct clusters, namely the least, moderately, and severely polluted clusters, which determined the severity of HMs contamination to be 66.67% overall. The PCAMLR and PMF receptor model suggested geogenic (minerals prospects), anthropogenic (industrial waste and chromite mining practices), and mixed (geogenic and anthropogenic) sources for groundwater contamination. The mineral phases of groundwater suggested saturation and undersaturation. Nemerow's pollution index (NPI) values determined the unsuitability of groundwater for domestic purposes. The EC, turbidity, PO₄^-3, Na⁺, Mg⁺², Ca⁺², Cr, Ni, and Mn exceeded the guidelines suggested by the World Health Organization (WHO). The HMs contamination and carcinogenic and non-carcinogenic health impacts of HMs showed that the groundwater is extremely unfit for drinking, agriculture, and domestic demands. Therefore, groundwater wells around the mining region need remedial measures. Thus, to overcome the enrichment of HMs in groundwater sources, sustainable management plans are needed to reduce health risks and ensure health safety.

Advances in Sorptive Removal of Hexavalent Chromium (Cr(VI)) in Aqueous Solutions Using Polymeric Materials

Sorptive removal of hexavalent chromium (Cr(VI)) bears the advantages of simple operation and easy construction. Customized polymeric materials are the attracting adsorbents due to their selectivity, chemical and mechanical stabilities. The mostly investigated polymeric materials for removing Cr(VI) were reviewed in this work. Assembling of robust functional groups, reduction of self-aggregation, and enhancement of stability and mechanical strength, were the general strategies to improve the performance of polymeric adsorbents. The maximum adsorption capacities of these polymers toward Cr(VI) fitted by Langmuir isotherm model ranged from 3.2 to 1185 mg/g. Mechanisms of complexation, chelation, reduction, electrostatic attraction, anion exchange, and hydrogen bonding were involved in the Cr(VI) removal. Influence factors on Cr(VI) removal were itemized. Polymeric adsorbents performed much better in the strong acidic pH range (e.g., pH 2.0) and at higher initial Cr(VI) concentrations. The adsorption of Cr(VI) was an endothermic reaction, and higher reaction temperature favored more robust adsorption. Anions inhibited the removal of Cr(VI) through competitive adsorption, while that was barely affected by cations. Factors that affected the regeneration of these adsorbents were summarized. To realize the goal of industrial application and environmental protection, removal of the Cr(VI) accompanied by its detoxication through reduction is highly encouraged. Moreover, development of adsorbents with strong regeneration ability and low cost, which are robust for removing Cr(VI) at trace levels and a wider pH range, should also be an eternally immutable subject in the future. Work done will be helpful for developing more robust polymeric adsorbents and for promoting the treatment of Cr(VI)-containing wastewater.

Polyaniline coated Pt/CNT as highly stable and active catalyst for catalytic hydrogenation reduction of Cr(VI)

Liquid phase catalytic hydrogenation reduction is a feasible method to eliminate Cr(VI) in water, while supported noble metal catalysts are liable to deactivation. In this study, carbon nanotube supported Pt catalyst (Pt/CNT) coated by polyaniline (Pt/CNT@PANI) was prepared and applied in the liquid phase catalytic hydrogenation of Cr(VI). Characterization results disclose that after coating Pt/CNT is completely wrapped by PANI layers and active Pt particles are no longer accessible. Despite complete embedment of Pt particles by PANI layers, Pt/CNT@PANI remains highly active for Cr(VI) reduction in liquid phase catalytic hydrogenation. The catalytic Cr(VI) reduction on Pt/CNT@PANI can be described by a PANI oxidation-reduction mechanism, by which PANI is first oxidized by Cr(VI) to form Cr(III), and oxidized PANI is reduced by catalytic hydrogenation. The Cr(VI) reduction on Pt/CNT@PANI complies with the Langmuir-Hinshelwood model, reflecting the pivotal role of Cr(VI) adsorption. Furthermore, the catalytic activity of Pt/CNT@PANI differs with PANI layer thickness and Cr(VI) reduction is positively correlated with reaction temperature. Catalyst recycling results show that after 4 cycles Pt/CNT loses 92.4% of catalytic activity, while the initial activity of Pt/CNT@PANI slightly decreases by 11.6%, demonstrating its high catalyst stability.

Biosorption of Cr (VI) by acid-modified based-waste fungal biomass from Calocybe indica fruiting bodies production

The world is going through a colossal drinking water scarcity. Unchecked discharge (even at trace levels) of Cr (VI) from industries into water bodies is a serious environmental concern. Here, we report waste fungal biomass (WFB) for the detoxification and removal of chromium ions. Biomass understudy was collected from Calocybe indica fruiting bodies. WFB was used after drying and pretreatment with two distinctive chemical methods, which improved the remediation effectiveness of Cr (VI). Light microscope and Field emission Scanning microscope (FESEM) were employed to elucidate the surface morphology of waste fungal biomass. While Fourier-Transform Infrared-Spectroscopy (FTIR) and Energy Dispersive X-Ray analysis (EDAX) were deployed to explore the mechanism of interaction between Cr (VI) anion and waste fungal biomass. X-ray Photoelectron Spectroscopy (XPS) analyses demonstrated considerable conversion of Cr (VI) into nontoxic Cr (III) species. The most favorable condition for optimum Cr (VI) remediation of 99.66% by treated waste fungal biomass (TWFB) occurred at pH 3, contact time 10 min, adsorbent dosage 3 gL^-1, Cr (VI) concentration 4 mgL^-1, stirring speed 140 rpm, and temperature 320 K, where for untreated waste fungal biomass (UWFB) the optimum of 85% remediation occurred at a contact time 15 min, and adsorbent dosage 2 gL^-1 whereas other experimental conditions remained identical as TWFB biosorbent. Pseudo-second-order kinetics (R² > 0.99) model matched the adsorption rate. And, the Freundlich isotherm model (R² > 0.99) is shown to be a better match for the experimental data. The optimum amount of Cr (VI) adsorbed by the TWFB and UWFB were 205.8 ± 10.1 and 72.85 ± 2.36 mgg^-1, respectively. Thermodynamic parameters revealed that the adsorption was spontaneous (ΔG ˂ 0), endothermic (ΔH > 0), and entropy-driven (ΔS > 0). The generated WFB adsorbent also has significant recycling potential. After five cycles of regeneration and adsorption. It can still keep up good remediation effectiveness of Cr (VI) ions to 85.5.

Quercetin ameliorates chromium toxicity through improvement in photosynthetic activity, antioxidative defense system; and suppressed oxidative stress in Trigonella corniculata L

Environmental stresses, including heavy metals accumulation, have posed an immense threat to the agricultural ecosystem, leading to a reduction in the yield of crucial crops. In this study, we evaluated the role of quercetin (Qu) in the alleviation of chromium (Cr) stress in Fenugreek (Trigonella corniculata L.). Different levels of Qu were prepared during the experiment, i.e., 15, 25, and 40 μM. For Cr toxification in potted soil, potassium chromate (K₂Cr₂O₇) was used. Cr toxification reduced growth of T. corniculata seedlings. Cr stress also reduced fiber, ash, moisture, carbohydrate, protein, fats, and flavonoid contents. However, seed priming with Qu improved growth and physiochemical characteristics of T. corniculata seedlings grown in normal and Cr-contaminated soil. Seed priming with Qu escalated intercellular CO₂ concentration, stomatal conductance, transpiration rate, and photosynthetic rate in T. corniculata seedlings. Application of Qu also increased the activity of antioxidative enzymes, i.e., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) in T. corniculata seedlings exposed to normal and Cr-contaminated soil. Application of Qu incremented the activity of SOD, POD, CAT, and APX, which were increased by 28, 22, 29, and 33%, respectively, in T. corniculata grown in Cr-toxic soil as compared to control treatment. Chromium stress alleviation was credited to the enhanced activity of the antioxidative defensive system in T. corniculata seedlings. It is proposed that Qu supplementation can be used to mitigate other abiotic stresses in plants.

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.

Enhanced sorption and reduction of Cr(VI) by the flowerlike nanocomposites combined with molybdenum disulphide and polypyrrole

Developing high-performance adsorbent for hexavalent chromium (Cr(VI)) elimination presents an enticing prospect in environmental remediation. Herein, three-dimensional flowerlike nanospheres composed of molybdenum disulphide and polypyrrole (MoS₂@PPy) were successfully prepared via a one-pot hydrothermal and subsequent carbothermal reduction process for the removal of Cr(VI). The effects of pH, adsorbent dosage, co-existing ions, initial Cr(VI) concentration and temperature were investigated systematically by batch experiments. Benefiting from the incorporation of MoS₂, the obtained MoS₂@PPy composites showed a dramatic increase of specific surface area (149.82 m²·g^-1) and adsorption capacity (230.97 mg·g^-1) when compared with the pure PPy nanoparticles. Based on the thermodynamics study and X-ray photoelectron spectroscopy analyses, the removal process of Cr(VI) was proved to be exothermic and spontaneous, and accessible under-coordinated Mo(IV) and pyrrolic N groups coupled with redox reactions were conducive to the efficient removal of Cr(VI). Attractively, the MoS₂@PPy acted as the electron donor could also activate peroxymonosulphate for the efficient degradation of organic contaminants. These results suggested that the MoS₂@PPy was promising in Cr(VI) elimination and other kinds of organic pollutants removal in wastewater.

Simultaneous reduction and sequestration of hexavalent chromium by magnetic β-Cyclodextrin stabilized Fe3S4

In this study, β-Cyclodextrin (CD) modified Fe₃S₄ nanomaterials were synthesized by a one-step facile strategy and investigated for the removal of Cr(VI). The resulted CD-Fe₃S₄ exhibited enhanced removal efficiency toward Cr(VI) than bared Fe₃S₄ with a maximum capacity of 220.26 mg·g^-1 as the molar ratio of CD-to-Fe₃S₄ at 0.2. The effective performance of CD-Fe₃S₄ toward Cr(VI) could well maintain under oxic conditions and a wide pH range of aqueous solution. A high selectivity for Cr(VI) was achieved in the presence of coexisting cations and anions. More significantly, a single treatment step of CD-Fe₃S₄ effectively removed chromium from actual electroplating wastewater to the detection limit of 0.004 mg·L^-1 that far below the WHO limitation of Cr (VI) (<0.05 mg·L^-1) combing with the rapid magnetic separation without adjusting the pH value of wastewater at 7. The effective removal of Cr (VI) by CD-Fe₃S₄ involved a complex process of surface adsorption/reduction, and solution homogenous reduction and subsequent sequestration of Cr(III) achieving the effective removal of aqueous total Cr. The superior Cr (VI) removal capability and facial separation of CD-Fe₃S₄ attained its prominent potential application as an effective material for the Cr(VI) removal.

Sustainable Strategies for the Agricultural Development of Shaanxi Province Based on the Risk Assessment of Heavy Metal Pollution

Heavy metal elements in farmland soil can be absorbed by crops and endanger food security. To assess the risk of heavy metal elements in farmland soil to crops in Shaanxi Province, we collected 693 soil samples and analyzed the concentrations of nine heavy metals (As, Hg, Pb, Cd, Cr, Mn, Cu, Zn, and Ni). According to the National Standard (GB 15619-2018) of the People’s Republic of China, the proportions of soil sample points in which the concentration of heavy metals was higher than the risk screening value were 2.02% (Cd), 0.29% (Cr), 0.29% (Zn), 2.31% (Cu), 1.15% (Ni), and 0.14% (Pb). The proportions of areas in which the concentration of heavy metal was higher than the background value were as follows, from largest to smallest: Zn (53.20%) > Mn (49.86%) > Cd (29.51%) > Hg (26.77%) > As (26.58%) > Ni (14.95%) > Cu (13.90%) > Pb (6.49%) > Cr (1.40%). The assessment of the risk of heavy metal exposure (geo-accumulation index (Igeo), pollution load index (PLI), and potential ecological risk index (RI)) determined that Hg was the most concerning heavy metal in the farmland soil of Shaanxi Province. Moreover, 11.56% of these areas had Hg contamination, and they were mainly distributed in the western Guanzhong region. The farmland soil in the Guanzhong region was the most contaminated, followed by the southern Shaanxi region and then the northern Shaanxi region. The main sources of heavy metal contamination causing large-scale farmland soil pollution are agricultural production activities, transportation, and air pollution caused by coal combustion in Shaanxi Province. Therefore, sustainable strategies for the prevention and control of heavy metal pollution and agricultural development must be applied in different regions. Heavy metal pollution should be managed, and relevant policies should be created and enforced, such as the standardization of the use of qualified pesticides and fertilizers, improved treatment of livestock and poultry manure, development of the clean energy industry structure, and promotion of renewable energy vehicles. In terms of the high-quality development of agriculture, developing modern and local agriculture in different regions should be based on local geographical, climatic, and economic conditions.

Six fruit and vegetable peel beads for the simultaneous removal of heavy metals by biosorption

In this study, a comparison between the biosorption performance of six fruit and vegetable peels, namely kiwifruit (KP), apple, banana, cucumber, orange and potato immobilized on sodium alginate beads has been made. Inductively coupled plasma coupled with mass spectroscopy was used for measuring the concentration of metal ions in solution before and after biosorption. A range of kinetic models were also applied to the biosorption batch data. The results showed that biosorption percentage of the ions were different on the various beads. For example, the decreasing order of biosorption by one KP bead at equilibrium was Cd > Cu > Hg > Ni > Pb > Cr > As, with approximately 92%, 84%, 80%, 75%, 67%, 34%, and 17% simultaneous removal of ions, respectively. The fastest biosorption was seen with Cd and Pb, as both reached equilibrium by 24 h. Equilibrium time of all other ions occurred by 48 h. While all beads in their unmodified form were suitable for the removal of divalent cations, KP bead showed significantly higher removal of the anion hexavalent Cr. Biosorption of Cd, Hg and Ni was limited by both pseudo-first order and pseudo-second order reaction rates. For Cr and Cu, the reaction was controlled by film diffusion and pseudo-first order rates. At a higher solution concentration, the preference of ions biosorbed as well as their percentage removed changed. Overall, the results indicated that KP beads show promise as a cost-effective method for removing toxic ions by biosorption, especially hexavalent chromium from drinking water.

Tuning the Fe(II)/hydroxide Ratio during Synthesis of Magnetite Nanoparticles to Maximize Cr(VI) Uptake Capacity

The impact of hydroxyl excess as defined by the Fe(II)/hydroxide ratio during the synthesis of Fe3O4 nanoparticles by oxidative precipitation of FeSO4 was examined as a critical parameter determining the potential for Cr(VI) uptake from polluted water. Various samples were prepared by varying the OH− excess in the range of −0.10 up to +0.03 M and characterized according to their composition, morphology, and surface configuration. Their efficiency for Cr(VI) removal was evaluated by batch adsorption tests, carried out under similar conditions with drinking water purification in the concentration range below 10 mg/L. Results indicate that near the zero-excess point for hydroxyl balance, the uptake capacity for residual Cr(VI) concentration equal to 25 μg/L remains at very low levels (<0.5 mg/g). However, a small increase above +0.02 M features synthesized nanoparticles with an uptake capacity of 2.5 mg/g owed to the decrease in particles size (28 nm) and enhancement of the reducing potential (Fe2+/Fe3+ = 0.42). In addition, utilizing negative excess values below −0.05 M triggers a similar efficiency rise, although the morphology of the obtained aggregates is rather different. Such finding is attributed to a possible exchange mechanism between adsorbed sulfates and chromate anions that assist approach of Cr(VI) to the material’s surface. Overall, proper tuning of hydroxyl excess offers multiple options for the implementation of monodisperse magnetically responsive nanoparticles or larger aggregates with optimized purification efficiency in water technology.

Aging shapes Cr(VI) speciation in five different soils

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca²⁺) and Fe oxides.

Interfacial engineering of CuFeS2 quantum dots via platinum decoration with enhanced Cr(VI) reduction dynamics under UV-Vis-NIR radiation

Configuring reactive and stable catalytic interfaces is crucial to design efficient photocatalysts for Cr(VI) reduction. Herein, via the platinum decoration approach based on interfacial engineering, we developed an effective catalytic interface within novel semiconducting chalcopyrite quantum dots (Pt/CuFeS₂ QDs). Benefiting from the catalytic merits of the Pt modulated interfacial structure and electronic structure, Pt/CuFeS₂ QDs show a broader light absorption capability extending to near-infrared radiation (NIR) range with superior carriers separation performance and faster charge transfer efficiency, which delivers a three-folder faster photocatalytic Cr(VI) reduction efficiency comparing to the original CuFeS₂ QDs. Density functional theory (DFT) calculations unravel that Pt atoms prefer to be anchored with the surface S atoms to form a stable interfacial structure with faster electron transfer and Cr(VI) reduction dynamics. This work demonstrates that platinum decoration based on interfacial engineering is an effective strategy to simultaneously modulate the band structure and accelerate the interfacial reaction dynamics for semiconductor photocatalysts, which paves the way for designing highly efficient photocatalysts for light-driven environmental and energy engineering applications.

Self-Assembled Silver Nanoparticles Decorated on Exfoliated Graphitic Carbon Nitride/Carbon Sphere Nanocomposites as a Novel Catalyst for Catalytic Reduction of Cr(VI) to Cr(III) from Wastewater and Reuse for Photocatalytic Applications

Silver nanoparticles decorated on an exfoliated graphitic carbon nitride/carbon sphere (AgNP/Eg-C₃N₄/CS) nanocomposites were synthesized by an adsorption method with a self-assembled process. These nanoparticles were characterized by different techniques like UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Raman spectroscopy, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), electrochemical impedance spectroscopy (EIS), and ζ potential. AgNP/Eg-C₃N₄/CS nanocomposites showed a higher catalytic reduction activity for the conversion of Cr(VI) into Cr(III) with formic acid (FA) at 45 °C when compared to bulk graphitic carbon nitride (Bg-C₃N₄, Eg-C₃N₄, CS, and Eg-C₃N₄/CS). The kinetic rate constants were determined as a function of catalyst dosage, concentration of Cr(VI), pH, and temperature for the AgNP/Eg-C₃N₄/CS nanocomposite. This material showed higher reduction efficiency (98.5%, k = 0.0621 min^-1) with turnover frequency (0.0158 min^-1) for the reduction of Cr(VI) to Cr(III). It also showed great selectivity and high stability after six repeated cycles (98.5%). Further, the reusability of the Cr(III)-AgNP/Eg-C₃N₄/CS nanocomposite was also investigated for the photocatalytic degradation of methylene blue (MB) under visible light irradiation with various time intervals and it showed good degradation efficiency (α = 97.95%). From these results, the AgNP/Eg-C₃N₄/CS nanocomposite demonstrated higher catalytic activity, improved environmental friendliness, lower cost for the conversion of toxic Cr(VI) to Cr(III) in solutions, and also good reusability.

Investigations of Cr(VI) removal by millet bran biochar modified with inorganic compounds: Momentous role of additional lactate

In this study, millet bran biochars modified with inorganic compounds (H₃PO₄: P-BC, NaOH: Na-BC and K₂CO₃: K-BC) were prepared and applied for Cr(VI) removal to evaluate the effects of modification on biochars' physicochemical properties. The results showed that Cr(VI) reduction capacity complied with the order of Na-BC > BC > P-BC > K-BC, and reductive groups such as -OH and -NH₂ played considerable roles in electrons donating. Based on this, lactate was added for further investigation of electrons transferring. The results displayed that Cr(VI) removal of all biochars was enhanced tremendously and modified biochars exhibited better Cr(VI) reduction. This may be due to the bridging effect of lactate, which could not only chelate with Cr(VI) via -COOH (or -OH) but also form hydrogen bonds with oxygen or nitrogen containing groups on biochars through the other groups, thus facilitating electrons transferring between biochars and Cr(VI). This work provided an insight into evaluation of the influence of inorganic compounds modification on both electrons donating capability of biochars and electrons transferring potential of biochars combined with lactate in Cr(VI) removal.

Mechanistic study of Cr (VI) removal by modified alginate/GO composite via synergistic adsorption and photocatalytic reduction

A novel composite material was prepared by blending graphene oxide into polyethyleneimine grafted sodium alginate. The synthesized material was investigated as adsorbent and photocatalyst for the removal of hexavalent chromium (Cr (VI)) from aqueous solutions. The composite material has shown remarkable removal efficiency for Cr (VI) in high initial concentration solutions as the removal rate reached 86.16% and 99.92% for adsorption and photoreduction, respectively. We discovered experimentally that the adsorption was dominated via electrostatic interaction while the blending of GO could contribute in stimulating electrons for the photoreduction process. Moreover, the photoreduction can alter the surface charge of chromium species, thus electrostatic repulsion could regenerating the active sites of composite spontaneously. The conduction band energy was calculated as -2.04 eV, which proved that blending GO can narrow the bandgap of the composite material, thus enhance the light response and the photoreduction ability towards Cr (VI).

Sustainable Chromium Recovery From Wastewater Using Mango and Jackfruit Seed Kernel Bio-Adsorbents

Wastewater is a rich source of valuable chemicals of industrial importance. However, their economic recovery is crucial for sustainability. The objective of the present work is to recover hexavalent chromium (Cr VI) as a value-added transition metal from wastewater cost-effectively; the biosorbent derived from seed kernels of mango (M) and jackfruit (JF) were applied for removing the metal from simulated wastewater. The functional groups of the biomass were analysed with the help of Fourier transform infrared (FTIR) spectroscopy, micrographs were generated using a scanning electron microscope, and crystallinity was determined by an x-ray diffractometer (XRD). The concentration of Cr VI in wastewater was analysed by an inductively coupled plasma optical emission spectrometer (ICP-OES). Process parameters (pH, dose, contact time, temperature, and initial concentration) were optimized for efficient Cr VI adsorption using a response surface methodology-based Box-Behnken design (BBD) employing Design-software 6.0.8. The batch experiment at room temperature at pH 4.8 and Cr VI removal ∼94% (M) and ∼92% (JF) was achieved by using a 60-mg dose and an initial Cr (VI) concentration of 2 ppm in 120 min. The equilibrium Cr binding on the biosorbent was well explained using Freundlich isotherm (R ² = 0.97), which indicated the indirect interactions between Cr (VI) and the biosorbent. Biosorption of Cr (VI) followed the pseudo-order and intra-particle diffusion models. The maximum adsorption capacity of the M and JF bio-adsorbent is 517.24 and 207.6 g/mg, respectively. These efficient, cost-effective, and eco-friendly biosorbents could be potentially applied for removing toxic Cr (VI) from polluted water.

Study on leaching kinetics of hexavalent chromium from aged calcium-free chromium slag

To provide a scientific basis for the resource utilization of chromium slag, this article studies the release law of hexavalent chromium in the aged calcium-free chromium slag. XRD (X-ray diffractometer) and MLA (Mineral Liberation Analyzer) were used to analyze the composition of the chromium slag; using sulfuric acid-nitric acid as the leaching solution, the release law of hexavalent chromium in chromium slag and the leaching kinetics were studied. The results show that main components of the chromium slag are magnesioferrite, chromite, hematite, hydrargillite, and spinel; chromium is mainly present in chromite and magnesioferrite; the leaching rate of hexavalent chromium increases with the increase of temperature or the decrease of pH. The analysis of leaching kinetics shows the leaching rate is controlled by the internal diffusion reaction, and the apparent activation energy is 11.93 kJ·mol–1. The chromium slag is aged in high temperature seasons, which is conducive to the precipitation of hexavalent chromium in the chromium slag, can increase the yield of chromate in the roasting kiln, and is conducive to resource utilization; chromium slag should be stored in order to prevent acid rain erosion which leads to environmental pollution risk (e.g. drinking water).

Capture of Toxic Oxoanions from Water Using Metal-Organic Frameworks

The effective capture of common water contaminants using metal-organic frameworks (MOFs) presents a remedy for current environmental concerns arising from the pollution of water sources. The crystalline porous nature of MOFs, their high internal surface area, and exceptional tunability make them suitable candidates for sequestration and removal of pollutants. However, the efficiency of capture depends largely on the nature of the interactions between the anions and the MOF. In this work, to elucidate the host-guest interactions involved in the capture of such pollutants, we explore three characteristically different MOFs: ZIF-8, iMOF-2c, and MOF-74. We demonstrate by ab initio electronic structure calculations the importance of exploiting qualitatively different binding modes for strong host-guest interactions available in the selected MOFs. Our simulations reveal the relative performance of neutral and cationic adsorbents while underscoring the importance of employing MOFs containing open metal sites for the efficient uptake of anions.

Enabling simultaneous redox transformation of toxic chromium(VI) and arsenic(III) in aqueous media-A review

Simultaneous conversion of most harmful As(III) and Cr(VI) to their less toxic counterparts is environmentally desirable and cost-effective. It has been confirmed that simultaneous oxidation of As(III) to As(V) and reduction of Cr(VI) to Cr(III) can occur via free radical or mediated electron transfer processes. While Cr(VI) is reduced by reacting with H^•, e_aq^-, photoelectron directly or undergoing ligand exchange with H₂O₂ and SO₃^2-, As(III) is oxidized by HO^•, SO₄^•-, O₂^•-, and holes (h⁺) in free radical process. The ability to concentrate Cr and As species on heterogeneous interface and conductivity determining the co-conversion efficiency in mediated electron transfer process. Acidity has positive effect on these co-conversion, while mediated electron transfer process is not much affected by dissolved oxygen (O₂). Organic compounds (e.g., oxalate, citrate and phenol) commonly favor Cr(VI) reduction and inhibit As(III) oxidation. To better understand the trends in the existing data and to identify the knowledge gaps, this review elaborates the complicated mechanisms for co-conversion of As(III) and Cr(VI) by various methods. Some challenges and prospects in this active field are also briefly discussed.

Modeling transmission of hexavalent chromium concentration and its health cost with a water quality analysis simulation program

In this study, the Water Quality Analysis Simulation Program (WASP7) was used to evaluate the transmission of hexavalent chromium (Cr(VI)) contamination in a water-sediment system and its flux into cultivated soils. The agricultural areas adjacent to the Wu River in Taiwan were taken as the study area, as these soils were heavily polluted with Cr(VI) concentrations of 2173-3271 μg/kg. The rates of accumulation of Cr(VI) are affected by the distance from the source of contamination and the size and type of cultivated areas. The highest concentrations of Cr(VI) (4.27 mg/kg) were detected in soil from Changhua city and correlated with the greater risk of gastric cancer in residents. Specifically, the risk of gastric cancer due to Cr(VI) contamination of agricultural soil was 3 × 10 - 7 - 15.2 × 10 - 6 in Taichung city (upstream) and 1.3 × 10 - 6 - 76.3 × 10 - 6 in Changhua county (downstream). The values of statistical life-years (VSLYs) were US$6.2-10 million for rice, US$42-60 million for corn, and US$360-580 million for other vegetables, respectively, each year. It is critical that techniques other than source reduction are used to reduce human exposure to Cr(VI), such as chemical oxidation or ion-exchange treatment to remove Cr(VI) from factory wastewaters, prior to their discharge into rivers. PRACTITIONER POINTS: This study evaluated the transmission of hexavalent chromium (Cr(VI)) contamination in a water-sediment-soil system. Maximum concentrations of Cr(VI) most rapidly accumulated in the smallest cultivated areas. The highest concentrations of Cr(VI) (3.3 mg/kg) were correlated with the greater risk of gastric cancer. Young children had a threefold greater risk of gastric cancer than adults. Techniques other than source reduction are prior to their discharge into rivers.

Polysaccharide-based (nano)materials for Cr(VI) removal

Chromium is a potentially poisonous and carcinogenic species, which originates from human activities and various industries such as leather, steel, iron, and electroplating industries. Chromium is present in various oxidation states, among which hexavalent chromium (Cr(VI)) is highly toxic as a natural contaminant. Therefore, chromium, particularly Cr(VI), must be eliminated from the environment, soil, and water to overcome significant problems due to its accumulation in the environment. There are different approaches such as adsorption, ion exchange, photocatalytic reduction, etc. for removing Cr(VI) from the environment. By converting Cr(VI) to Cr(III), its toxicity is reduced. Cr(III) is essential for the human diet, even in small amounts. Today, biopolymers such as alginate, cellulose, gum, pectin, starch, chitin, and chitosan have received much attention for the removal of environmental pollutants. Biopolymers, particularly polysaccharides, are very useful compounds due to their OH and NH₂ functional groups and some advantages such as biodegradability, biocompatibility, and accessibility. Therefore, they can be widely applied in catalytic applications and as efficient adsorbents for the removal of toxic compounds from the environment. This review briefly investigates the application of polysaccharide-based (nano)materials for efficient Cr(VI) removal from the environment using adsorption/reduction, photocatalytic, and chemical reduction mechanisms.

Development of a novel pyrite/biochar composite (BM-FeS2@BC) by ball milling for aqueous Cr(VI) removal and its mechanisms

As a natural reduction mineral, pyrite (FeS₂), was combined with biochar by simply ball milling technology to synthesize FeS₂@biochar composite (BM-FeS₂@BC) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. SEM, XRD, FTIR, and XPS characterization results showed that the FeS₂ and biochar were successfully combined and biochar suppressed the agglomeration of FeS₂. Batch sorption experiments showed that the BM-FeS₂@BC700 composite (mass ratio of FeS₂-to-biochar = 3:1) had enhanced Cr(VI) removal capacity of 134 mg·g^-1, which were 3-25 times higher than those of the corresponding pristine and ball-milled biochar and FeS₂. The removal of Cr(VI) by BM-FeS₂@BC700 was dosage and pH dependent. The addition of oxalic acid (OA) exhibited a promotion effect on the removal of Cr(VI) by increasing the removal rate of Cr(VI) from 56% to 100%. Reduction, adsorption, and surface complexation were the dominate mechanisms for Cr(VI) removal by BM-FeS₂@BC700. At the equilibrium Cr(VI) concentration of 15.7 mg·L^-1, 92.25% of Cr(VI) was removed through reduction/precipitation and 8.75% was removed by adsorption/surface complexation. The fitting results of the Langmuir model proved that the removal of Cr(VI) by BM-FeS₂@BC700 composite was chemical surface monolayer adsorption. This work demonstrates the potential of ball milling for the preparation of FeS₂@BC composite to remove Cr(VI) from water and wastewater.

Melatonin Attenuates Chromium (VI)-Induced Spermatogonial Stem Cell/Progenitor Mitophagy by Restoration of METTL3-Mediated RNA N6-Methyladenosine Modification

Spermatogonial stem cells (SSCs) are the basis of spermatogenesis, and any damage to SSCs may result in spermatogenic disorder and male infertility. Chromium (Cr) (VI) is a proven toxin, mutagen, and carcinogen, perpetually detrimental to environmental organisms due to its intricate and enduring detoxification process in vivo. Despite this, the deleterious effects of Cr (VI) on SSCs and the underlying mechanisms remain poorly understood. In this study, we identified that Cr (VI) impaired male reproductive system in mouse testes and induced mitochondrial dynamic imbalance and mitophagy in SSCs/progenitors. Cr (VI) also downregulated the RNA N⁶-methyladenosine (m⁶A) modification levels in mitochondrial dynamic balance and mitophagy genes in SSCs/progenitors. Inspiringly, the toxic effects of Cr (VI) could be relieved by melatonin pretreatment. Melatonin alleviated Cr (VI)-induced damage to male reproductive system and autophagy in mouse testes. Melatonin also attenuated Cr (VI)-induced cell viability loss and reactive oxygen species (ROS) generation, as well as mitochondrial dynamic disorders and mitophagy in SSCs/progenitors. The protective roles of melatonin against Cr (VI)-induced mitophagy were exerted by restoration of METTL3-mediated RNA m⁶A modification and activation of mitochondrial fusion proteins MFN2 and OPA1, as well as inhibition of the mitophagy BNIP3/NIX receptor pathway. Thus, our study provides novel insights into the molecular mechanisms for RNA m⁶A modification underlying the gene regulatory network responsible for mitochondrial dynamic balance, and also lays new experimental groundwork for treatment of Cr (VI)-induced damage to male fertility.

Genetic Basis of Chromate Adaptation and the Role of the Pre-existing Genetic Divergence during an Experimental Evolution Study with Desulfovibrio vulgaris Populations

Hexavalent chromium [Cr(VI)] is a common environmental pollutant. However, little is known about the genetic basis of microbial evolution under Cr(VI) stress and the influence of the prior evolution histories on the subsequent evolution under Cr(VI) stress. In this study, Desulfovibrio vulgaris Hildenborough (DvH), a model sulfate-reducing bacterium, was experimentally evolved for 600 generations. By evolving the replicate populations of three genetically diverse DvH clones, including ancestor (AN, without prior experimental evolution history), non-stress-evolved EC3-10, and salt stress-evolved ES9-11, the contributions of adaptation, chance, and pre-existing genetic divergence to the evolution under Cr(VI) stress were able to be dissected. Significantly decreased lag phases under Cr(VI) stress were observed in most evolved populations, while increased Cr(VI) reduction rates were primarily observed in populations evolved from EC3-10 and ES9-11. The pre-existing genetic divergence in the starting clones showed strong influences on the changes in lag phases, growth rates, and Cr(VI) reduction rates. Additionally, the genomic mutation spectra in populations evolved from different starting clones were significantly different. A total of 14 newly mutated genes obtained mutations in at least two evolved populations, suggesting their importance in Cr(VI) adaptation. An in-frame deletion mutation of one of these genes, the chromate transporter gene DVU0426, demonstrated that it played an important role in Cr(VI) tolerance. Overall, our study identified potential key functional genes for Cr(VI) tolerance and demonstrated the important role of pre-existing genetic divergence in evolution under Cr(VI) stress conditions.

IMPORTANCE Chromium is one of the most common heavy metal pollutants of soil and groundwater. The potential of Desulfovibrio vulgaris Hildenborough in heavy metal bioremediation such as Cr(VI) reduction was reported previously; however, experimental evidence of key functional genes involved in Cr(VI) resistance are largely unknown. Given the genetic divergence of microbial populations in nature, knowledge on how this divergence affects the microbial adaptation to a new environment such as Cr(VI) stress is very limited. Taking advantage of our previous study, three groups of genetically diverse D. vulgaris Hildenborough populations with or without prior experimental evolution histories were propagated under Cr(VI) stress for 600 generations. Whole-population genome resequencing of the evolved populations revealed the genomic changes underlying the improved Cr(VI) tolerance. The strong influence of the pre-existing genetic divergence in the starting clones on evolution under Cr(VI) stress conditions was demonstrated at both phenotypic and genetic levels.

Synergistic/antagonistic effects and mechanisms of Cr(VI) adsorption and reduction by Fe(III)-HA coprecipitates

Widespread Fe(III)-humic acid (HA) coprecipitates (FHCs) have substantial impacts on the adsorption and reduction of Cr(VI) in soils and sediments, but whether this process is equal to the sum of their individual components remains unknown. In this study, ferrihydrite (Fh)- and HA-like FHCs (C/Fe<3 and C/Fe>3, respectively) were synthesized by controlling the initial C/Fe ratios (0.5-18) to explore the potential synergistic/antagonistic effects during the adsorption and reduction of Cr(VI). According to the results, antagonistic effects on Cr(VI) adsorption (5%-80%) were observed on Fh- and HA-like FHCs, where the antagonistic intensity increased with increasing HA proportions, respectively caused by the more serious occupation of adsorption sites and the stronger electrostatic repulsion to Cr(VI). Notably, significant synergistic reduction effects (5%-650%) occurred on Fh-like FHCs were found to be achieved by the activation of low-molecular HA (0.1-0.3 kDa) with primary/secondary hydroxylic groups, which might be induced by the inductive effect of Fh on complexed HA molecules according to density-functional theory (DFT) calculation. While slight antagonistic reduction effects (2%-45%) by HA-like FHCs were attributed to the decreasing accessibility of Cr(VI) to reductive phenolic groups, which might be blocked within FHC particles or complexed with Fe(III) ions through cation bridges.

Carbon-nitride-based micromotor driven by chromate-hydrogen peroxide redox system: Application for removal of sulfamethaxazole

In this study, a Janus Fe/C₃N₄ micromotor driven by a chromate-hydrogen peroxide (Cr(VI)/H₂O₂) redox system was developed and its movement was analyzed. The motion of the micromotor was tracked via nanoparticle tracking analysis (NTA) and the corresponding diffusion coefficients (D) were determined. The NTA results revealed that D = 0 in water in the absence of additives (Cr(VI) or H₂O₂). The addition of H₂O₂ resulted in an increase in D from 0 to 12 × 10⁶ nm² s^-1, which further increased to 20 × 10⁶, 26.5 × 10⁶, 29 × 10⁶, and 44 × 10⁶ nm² s^-1 with the addition of 0.5, 1, 2, and 5 ppm of Cr(VI), respectively. Cr(VI) alone did not efficiently propel the Fe/C₃N₄-based micromotor. Therefore, it was proposed that the Cr(VI)/H₂O₂ redox system generates O₂, which plays a major role in the movement of the C₃N₄-based micromotor. In addition, the formation of reactive species, such as OH and ¹O₂, was confirmed through electron spin resonance experiments. The reactive species efficiently degraded sulfamethaxazole (SMX), an organic pollutant, as demonstrated through degradation studies and product analyses. The effects of various parameters, such as H₂O₂ concentration, Cr(VI) concentration, and initial pH on the movement of micromotor and degradation of SMX were also documented.