Toxic Cr Research Articles

Efficient reduction of Cr(VI) and recovery of Fe from chromite ore processing residue by waste biomass

The traditional chromite ore processing residue (COPR) detoxification process is carried out in strong acid or anoxic high-temperature condition, which not only has a high input cost but leading difficulties for resource utilization of treated COPR. This study first reported the efficient reduction of Cr(VI) and recovery of Fe from COPR by waste molasses-assisted hydrothermal treatment. Under optimal conditions (i.e., the Na2CO3 dosing of 0.5 mol/L, the molasses/COPR mass ratio of 10%, hydrothermal treatment at 140 °C for 150 min), the Cr(VI) reduction efficiency reached 99.96 ± 0.01%, and the leaching toxicity of total Cr met the USEPA regulatory limit of 5 mg/L. Moreover, Fe was recycled in the form of magnetic magnesioferrite with Fe2O3 grade of 52.58 ± 0.90%, which can be used as raw material in steel industry. The Cr(VI) reduction reaction conforms to a pseudo-first-order kinetic model, and the reaction rate constants increase with increasing temperature (120 ∼160 °C). As a potential mechanism of the reaction, the Mg-Al hydrotalcite (LDHs) structure collapses in the presence of Na2CO3 during the hydrothermal treatment, and the embedded Cr(VI) is released, which is then reduced to Cr(OH)3 by reducing substances such as fructose, sucrose, and glucose in waste molasses. From the standpoint of a circular economy, this study may provide a potential strategy for minimizing resource input and waste/emission production by harmless disposal and resource utilization of COPR and waste molasses.

Calorific Value of Zea mays Biomass Derived from Soil Contaminated with Chromium (VI) Disrupting the Soil’s Biochemical Properties

One of the major challenges faced by contemporary agriculture is how to achieve better yields of crops and, consequently, higher biomass, even in unfavorable environmental conditions. This challenge corresponds to the assumptions of sustainable development, wherein it is envisaged that plant biomass should be used on a large scale for heat generation or conversion of biofuels. Keeping pace with observed trends, the following study was conducted in order to determine the effect of Cr(VI) on the net calorific value of Zea mays, to assess the impact of this element on soil enzymatic activity, and to identify the effectiveness of compost and humic acids in alleviating possible negative effects of Cr(VI) toxicity. These aims were pursued by setting up a pot experiment, in which soil either uncontaminated or contaminated with increasing doses of Cr(VI) of 0, 15, 30, 45, and 60 mg Cr kg−1 d.m. was submitted to biostimulation with compost and the preparation HumiAgra, a source of humic acids, and cropped with Zea mays. The plant height, yield, and net calorific value of the aerial parts of maize, as well as its root yield, were determined. Additionally, the activity of seven soil enzymes and the values of the impact indices of compost and HumiAgra relative to the analyzed parameters were determined. It was found that Cr(VI) decreased the amount of energy obtained from the plants by decreasing maize biomass, and additionally by distorting the biochemical balance of the soil. Dehydrogenases, urease, and arylsulfatase proved to be particularly sensitive to this element. It was demonstrated that HumiAgra was more effective than compost in mollifying the adverse effects of Cr(VI) on the activity of soil enzymes and, consequently, on the biomass of Zea mays.

Sulfur supplementation enhances nitric oxide efficacy in reversal of chromium-inhibited Calvin cycle enzymes, photosynthetic activity, and carbohydrate metabolism in wheat

The present study demonstrated that exogenously-sourced nitric oxide (as SNP, sodium nitroprusside; NO donor) and sulfur (S) protected photosynthesis against chromium (Cr) stress in wheat (Triticumaestivum L. cv. HD 2851). Plants grown with 100 µM Cr exhibited higher reactive oxygen species (ROS) production, resulting in photosynthetic damage. The individual application of 50 µM NO increased carbohydrate metabolism as well as photosynthetic parameters, antioxidant system with higher transcriptional gene levels that encode the key enzymes for the Calvin cycle under Cr stress. These effects were more prominent when NO was applied with 1.0 mM SO42−. An increase in the reduced glutathione (GSH) content obtained with NO was further enhanced by S and resulted in higher protection against Cr stress. The protective effect of NO with S against Cr toxicity on photosynthesis was reversed when buthionine sulfoximine (BSO; GSH biosynthetic inhibitor) was used. Application of BSO reversed the impact of NO plus S on photosynthesis under Cr stress, verifying that the ameliorating effect of NO was through S-assimilation and via GSH production. Thus, the availability of S to NO application can help reduce Cr toxicity and protect photosynthetic activity and expression of the Calvin cycle enzymes in leaves through the GSH involvement.

Highly efficient Cr(VI) removal by cationic covalent organic framework

<p>Developing an efficient sorbent for removing the highly toxic Cr(VI) is very important, which tackles the threat of Cr(VI) pollution to ecological environment and human health. Herein, a cationic covalent organic framework bearing cationic guanidine (named TATGCl) was synthesized from terephthalaldehyde (TA) and triaminoguanidinium chloride (TGCl), and exploited to discard Cr(VI) from water. The Cr(VI) removal ability of TATGCl strongly relies on the aqueous pH, and is strong at low pH. The removing ability is obviously also affected by ionic strength. The Cr(VI) sorption process on TATGCl obeys pseudo-second-order and Langmuir models. Batch adsorption experiments and characterization show that the guanidine group on the skeleton of TATGCl is pivotal in capturing Cr(VI) through hydrogen bonding and anion exchange/electrostatic interaction. A reduction mechanism also exists in the Cr(VI) disposal by a small amount of unreacted aldehyde group of TATGCl. The large removal ability makes TATGCl a prospective candidate for discarding Cr(VI) from water.</p>

Efficient hexavalent chromium removal by nano-cerium oxide functionalized biochar: Insight into the role of reduction

The strong toxicity of hexavalent chromium (Cr(VI)) have spurred great interests in developing an effective technology to remove it from water. In this study, a cost-effective and recyclable composite nano-cerium oxide functionalized peanut shell biochar (BC-Ce) was synthetized via impregnation-precipitation-pyrolysis process and employed for simultaneous adsorption and reduction of Cr(VI) in water. The BC-Ce possessed better Cr(VI) adsorption potential, and its maximum Cr(VI) adsorption capacity calculated by Langmuir achieved 47.83 mg g−1. Simultaneously, the as-prepared composite displayed obvious reduction behavior for Cr(VI) and partially adsorbed Cr(VI) was reduced to less toxic Cr(III). The effects of composite dosage (0.2–4 g L−1), initial Cr(VI) concentration (20–200 mg L−1) and pH (3−9) on Cr(VI) removal were also investigated. Characterization results indicated that the Cr(VI) reduction induced by BC-Ce was mainly attributed to the comprehensive effect of oxygen-containing functional group (OFGs), carbon-centered persistent free radicals (PFRs), oxygen vacancies and graphitic structure in BC-Ce. The CeO2 supported on the surface of BC facilitated the generation of more OFGs (phenolic-OH and C-O), PFRs and oxygen vacancies, thus provided abundant sites for chemical reduction of Cr(VI). Moreover, the graphitic structure in BC-Ce promoted the transfer of electrons from graphite carbon to Cr(VI). This study provided a new insight into the removal mechanisms of Cr(VI) in water by BC-Ce. Except for the adsorption, the reduction behavior during Cr(VI) removal process by BC-based materials should not be overlooked.

Functionalized Titanium-Based MOF for Cr(VI) Removal from Wastewater.

The removal of toxic Cr(VI) is a hot topic in the environmental remediation field. In this work, a Ti-based metal-organic framework (MOF) (MIL-125(Ti)-NH2) was successfully functionalized by introducing amidoxime groups for the first time. The functionalized material (MIL-125(Ti)-AO) exhibited excellent Cr(VI) adsorption performance with a maximum adsorption capacity of 271 mg·g-1 according to Langmuir fitting. More importantly, during the adsorption process, Cr(VI) could be simultaneously reduced to less toxic Cr(III) species, and the residual concentration of chromium in the treated water was below the drinking water limit (0.05 mg·L-1) recommended by WHO. The effects of initial pH, contact time, and the initial concentration of Cr(VI) and the presence of competitive ions on the Cr(VI) adsorption performance of MIL-125(Ti)-AO were systematically investigated. The excellent Cr(VI) removal performance of MIL-125(Ti)-AO was attributed to the synergistic effects of simultaneous adsorption of Cr(VI) and Cr(III) by a [Ti-O] bond when Cr(VI) was reduced to Cr(III) by amidoxime groups.

Titanium dioxide nanoparticles potentially regulate the mechanism(s) for photosynthetic attributes, genotoxicity, antioxidants defense machinery, and phytochelatins synthesis in relation to hexavalent chromium toxicity in Helianthus annuus L.

Chromium (VI) is one of the hazardous heavy metal, heavily discharged into the soil and severely hampers the plants yield. The TiO2 NPs was selected due to its potential to alleviate the heavy metals toxicity. This manuscript unravels the mechanisms for Cr(VI) induced toxicity and how foliar application of TiO2 NPS potentially ameliorate the toxicity by regulating the photosynthetic attributes, DNA damage, antioxidants defense machinery, and phytochelatins synthesis in Helianthus annuus L. Plants were exposed to Cr(VI) concentrations [0, 15, 30, and 60 mg Cr(VI) kg−1 of soil], and TiO2 NPS (15 mg L−1, 25 nm size) were foliar sprayed thrice to the plants at three days interval. The maximum accumulation of total chromium was recorded in root (12.53 µg g−1 DW) followed by shoot (5.67 µg g−1 DW) at 60 mg Cr(VI) treatment. The presence and localization of TiO2 NPs inside the plant leaf cells were confirmed by TEM-EDS analysis. The results revealed that Cr(VI) exposure had a dose-dependent inhibitory effects on photosynthetic attributes, structure of guard and epidermal cells, photosynthetic pigments; inducing impacts on H2O2 and MDA productions, DNA damage, AsA-GSH cycle, and most importantly on PC2, and PC3 synthesis which is rarely reported. However, TiO2 NPs exposure minimized Cr(VI) induced toxicity through reduction of total chromium accumulation, H2O2 and MDA productions, thereby reducing DNA damage reported first time under combined treatment of Cr(VI)+ TiO2 NPs as evidenced through comet assay. It also positively regulate the photosynthetic pigments, AsA-GSH cycle, and modulates PC2 and PC3 synthesis which have crucial impacts on ROS quenching and Cr(VI) detoxification, respectively, and in turn, minimizes Cr(VI) toxicity in H. annuus L. Besides, this study strengthens the less acknowledged report that Cr(VI) is an inducer of PCs synthesis and also confirms that TiO2 NPs potentially counteract Cr(VI) toxicity.

Preferential, synergistic sorption and reduction of Cr(VI) from chromium–rhodamine B mixed wastewater by magnetic porous biochar derived from wasted Myriophyllum aquaticum biomass

Eradication of heavy metals and dyes simultaneously from wastewater is urgently needed to safeguard public and environmental health. In this study, magnetic porous biochar derived from wasted Myriophyllum aquaticum (MPMaB) was synthesized by KOH-activation and co–precipitation method to treat chromate and rhodamine B (RhB)-bearing wastewater. The KOH activation significantly improved the pore structure of biochar with a high specific surface area of 937.1 m2 g−1. The sorption performance of MPMaB for Cr(VI) and RhB in single and co-solutes conditions was evaluated. In single system, a pH-dependent sorption pattern for Cr(VI) by MPMaB was revealed and the estimated sorption capability reached 175.4 mg g−1, whereas the Langmuir-based sorption capacity of RhB was 175.4 mg g−1 pH-independently. MPMaB partially transformed Cr(VI) to less toxic Cr(III) (approximately 59.3%). Synergistic sorption of Cr(VI) with the coexistence of RhB was observed, where synergistic effect ranged from 119% to 527% depending on pH. For example, the sorption capacity of Cr(VI) on MPMaB, at pH 2, augmented from 175.4 mg g−1 (single system) to 208.3 mg g−1 (binary system). Preferential sorption of Cr(VI) was found and was further confirmed by the post-sorption of Cr(VI) (or RhB) by MPMaB pre-sorbed with RhB (or chromate). Chromate sorption mechanisms mainly include electrostatic interactions and complexation, while the sorption of RhB is ascribed to π–π interactions, pore filling and hydrogen bonding. Additionally, MPMaB showed excellent reusability and maintained high removal efficiency after 5 cycles. In short, MPMaB can efficiently treat chromium and dyes–containing wastewater as sustainable and environmentally friendly adsorbent.

Nitric oxide and hydrogen peroxide mediated regulation of chromium (VI) toxicity in wheat seedlings involves alterations in antioxidants and high affinity sulfate transporter

Chromium contamination of the soil is a major scientific concern with reference to crop productivity and human health. In recent years, several approaches are being employed in managing metal toxicity in crop plants. Here, we have investigated about potential and probable crosstalk of nitric oxide (NO) and hydrogen peroxide (H2O2) in mitigating hexavalent chromium [Cr(VI)] toxicity in wheat seedlings. Cr(VI) toxicity reduced the fresh mass and overall growth due to accumulation of reactive oxygen species (ROS) and decreased efficiency of AsA-GSH cycle and downregulation of high affinity sulfate transporter. However, exogenous treatment of NO and H2O2 significantly alleviated Cr toxicity. Application of NO and ROS scavengers reversed stress mitigating effects of NO and H2O2, respectively suggesting that endogenous NO and H2O2 are necessary for rendering Cr toxicity tolerance. Furthermore, NO rescued negative effect of diphenylene iodonium (DPI, NADPH oxidase inhibitor) and H2O2 reversed the negative effect of c-PTIO suggesting that they exhibit independent signalling in mitigating Cr stress. Altogether, data indicated that NO and H2O2 rendered mitigation of Cr stress by up-regulating enzymes (activity and relative gene expression) and metabolites of AsA-GSH cycle, high affinity sulfate transporter (relative gene expression) and glutathione biosynthesis which collectively controlled occurrence of oxidative stress.

Adsorption‐Induced In‐Situ Construction of TiO2 Hollow Sphere/UiO‐66‐NH2 Heterostructures with Boosted Photocatalytic Activity toward Cr(VI) Reduction

AbstractWater contamination caused by highly toxic Cr(VI) should be resolved imminently with a more efficient photocatalytic approach. The interaction between photocatalyst heterostructure is critical for charge separation which largely affects the photocatalytic efficiency. Herein, TiO2 hollow sphere/UiO‐66‐NH2 (TiO2/UiO‐66‐NH2) heterostructures were successfully synthesized by an adsorption‐induced in‐situ solvothermal growth strategy. The UiO‐66‐NH2 particles were well dispersed on TiO2 hollow spheres by solvothermal treatment of Zr4+ adsorbed TiO2 hollow spheres with NH2‐BDC ligand. The photocatalytic activities of TiO2/UiO‐66‐NH2 heterostructures were investigated by reduction of Cr(VI) with visible light irradiation. With the introduction of UiO‐66‐NH2, the reduction efficiency toward Cr(VI) by TiO2/UiO‐66‐NH2 heterostructures reached 89% after 180 min of irradiation, which is much higher than those of TiO2 hollow spheres (10%) and UiO‐66‐NH2 crystals (29%). Moreover, the TiO2/UiO‐66‐NH2 heterostructures display good regeneration ability and can still maintain 70% of Cr(VI) removal ability after four cycles. The integration of TiO2 and UiO‐66‐NH2 could construct a type‐II heterojunction to broaden the absorption range of light and suppress the recombination of photoproduced electrons and holes. As a result, such unique composite structure significantly enhance the photocatalytic performance towards Cr(VI) reduction. This work not only inspires a new approach for in‐situ solvothermal synthesis of semiconductor@MOFs composites, but provides a novel photocatalyst for highly efficient removal of Cr(VI) from wastewater.

Chromium Toxicity in Plants: Signaling, Mitigation, and Future Perspectives

Plants are very often confronted by different heavy metal (HM) stressors that adversely impair their growth and productivity. Among HMs, chromium (Cr) is one of the most prevalent toxic trace metals found in agricultural soils because of anthropogenic activities, lack of efficient treatment, and unregulated disposal. It has a huge detrimental impact on the physiological, biochemical, and molecular traits of crops, in addition to being carcinogenic to humans. In soil, Cr exists in different forms, including Cr (III) "trivalent" and Cr (VI) "hexavalent", but the most pervasive and severely hazardous form to the biota is Cr (VI). Despite extensive research on the effects of Cr stress, the exact molecular mechanisms of Cr sensing, uptake, translocation, phytotoxicity, transcript processing, translation, post-translational protein modifications, as well as plant defensive responses are still largely unknown. Even though plants lack a Cr transporter system, it is efficiently accumulated and transported by other essential ion transporters, hence posing a serious challenge to the development of Cr-tolerant cultivars. In this review, we discuss Cr toxicity in plants, signaling perception, and transduction. Further, we highlight various mitigation processes for Cr toxicity in plants, such as microbial, chemical, and nano-based priming. We also discuss the biotechnological advancements in mitigating Cr toxicity in plants using plant and microbiome engineering approaches. Additionally, we also highlight the role of molecular breeding in mitigating Cr toxicity in sustainable agriculture. Finally, some conclusions are drawn along with potential directions for future research in order to better comprehend Cr signaling pathways and its mitigation in sustainable agriculture.

Insight into the oxidation mechanisms of vanadium slag and its application in the separation of V and Cr

Vanadium slag contains valuable element vanadium and heavy metal element chromium. V and Cr in the traditional oxidation roasting process were simultaneously oxidized by oxidant O2 in the air, which made the separation of V and Cr difficult, and produced highly toxic Cr6+ that polluted seriously the environment. The valence states of Fe, Cr and V in vanadium slag increased during oxidation roasting. In order to reveal the oxidation mechanisms, the oxidative roasting of Na2CO3–Cr2O3, Na2CO3–FeCr2O4 and Na2CO3–V2O3 was investigated. The results showed that Cr2O3 could be oxidized by O2 and the presence of Fe2+ reduced the oxidation ratio of Cr3+. Moreover, it was found for the first time that the V3+ could be oxidized by CO2. The calculation of FactSage 8.0 showed that V2O3 could be oxidized by Na2CO3. However, V2O3 could not be directly oxidized to VO2 or V2O5 by CO2. Due to the lack of NaVO2 data in the thermodynamic database, the reaction of NaVO2 and CO2 was firstly simulated by Vienna ab initio simulation package (VASP). The results indicated that NaVO2 could be oxidized by CO2. On the basis of revealing the oxidation mechanisms of V, Cr, and Fe, an innovative method for selective oxidation of vanadium by oxidant CO2 was proposed. The extraction ratio of vanadium could reach 90.5% at 800 °C, in which the mass ratio of V/Cr was 32,400. Mass ratio of V/Cr in this work was 1448 to 20,250 times larger than that in the literatures. Based on the high vanadium extraction ratio, the problems of separation of V and Cr and the presence of Cr6+ were solved from the source. This novel discovery may provide an inspiration for the vanadium metallurgical revolutionary.

The mobility of Cr(VI) on the ferrihydrite-Cr(VI) co-precipitates: The effect of co-existing tartaric acid and Cu(II)

Ferrihydrite is a crucial scavenger for hexavalent chromium (Cr(VI)) via coprecipitation processes. Foreign impurities widely exist in soils and sediments, which influence mineral recrystallization and transformation processes. Under the circumstances, the speciation and migration behavior of Cr(VI) will change accordingly. In this research, we explored the impact of tartaric acid and Cu(II) on ferrihydrite transformation, as well as the sequestration of Cr(VI) on iron minerals. Our results manifested that tartaric acid affected the valence of Cr on iron oxides. The adsorbed Cr(VI) was reduced to Cr(III) by tartaric acid on the surface of iron oxides and not released into the aqueous phase, which greatly reduced its toxicity. It was conducive to the conversion of ferrihydrite into hematite when tartaric acid concentration was 1.0 mM, because tartaric acid could facilitate the hematite nucleation to induce the generation of hematite from ferrihydrite. Moreover, Cu(II) promoted the formation of hematite and played a synergistic role in Cr(VI) fixation. Therefore, tartaric acid and Cu(II) are important factors decreasing the toxicity of Cr(VI) when considering Cr(VI) retention on iron minerals in complicated environments. This work has a vital implication for understanding the impact of the co-existent metals and organic substances on Cr(VI) retention in complex environments.

Multifunctional CdS Nanoparticle-Decorated CeO2 as Efficient Visible Light Photocatalysts and Toxic Cr(VI) Sensors.

Several one-dimensional and three-dimensional CdS@CeO2 nanocomposites were synthesized by a solvothermal route. A nanoflower-shaped CdS@CeO2 nanocomposite (CdS-NF@CeO2) was selected as the model catalyst after various characterizations. It was, then, employed directly as a luminescent sensor for Cr(VI) detection in an aqueous medium. A good linear quenching was observed in the range of 0-0.5 μM with a detection limit of 0.04 μM. The quantum yield of the catalyst was found to be 73%. Moreover, our catalyst is highly selective toward Cr(VI) and can be applied as an efficient sensor for real water analysis. The efficiency of the catalyst was also tested in controlling the photocatalytic activity for oxidation of benzylamine to N-benzylidenebenzylamine under a domestic LED bulb with molecular O2 as a sole, green oxidant. Conversion (>99.9%) and selectivity as high as 100% were observed for the CdS-NF@CeO2 photocatalyst. These results show the potential applications of CdS-NF@CeO2 nanocomposites as an efficient photocatalyst for organic transformation and environmental remediation.

Decreasing dissolved oxygen enhances in situ curtailment of intermediate Cr(VI) during photo-oxidative decomplexation of Cr(III)-EDTA

Cr(III)-organic complexes are stably presented in tanning, electroplating, and other industrial wastewaters, and their safe and efficient removal remains a current challenge. Available oxidation processes can remove Cr(III) complexes but readily result in highly toxic Cr(VI) accumulation. Herein, negligible Cr(VI) accumulation was achieved during photo-oxidation of Cr(III) complexes using a simple strategy of decreasing dissolved oxygen (DO). At the DO concentration of 5.0 mg·L-1 or less, the in-process formation of intermediate Cr(VI) was totally abated by in situ formed reductive species, and total Cr was reduced from 9.0-11.0 mg·L-1 to below 1.0 mg·L-1. A complete curtailment of Cr(VI) was observed after 30-60 min at pH 6.0-9.0. Increasing Cr(III)-EDTA concentration and decreasing pH value facilitated the in situ reduction of intermediate Cr(VI). Based on the identification of intermediates and additional Cr(II) and quenching experiments, the possible key species involved in intermediate Cr(VI) reduction were the photogenerated Cr(II) and some C-centered radicals from Cr(III)-EDTA decomplexation, and the possible mechanisms of Cr(III)-EDTA decomplexation and intermediate Cr(VI) reduction were thus proposed. The process also showed efficient treatment on other Cr(III) complexes (citrate, oxalate, and tartrate) and realistic Cr(III) complexed wastewater. This study would provide an insignificant Cr(VI)-accumulated alternative for efficient and safe removal of Cr(III) complexes from contaminated water.

Green desorption combined with peptization technology for disposing of Cr(VI)-V(V)-containing hazardous aluminum waste to prepare high-valued pseudo-boehmite

Cr(VI)-containing aluminum residues are inevitable industrial hazardous waste in the chromate industry. To reduce and utilize these residues, a combined desorption and peptization technology for disposing of Cr(VI)-V(V)-containing boehmite hazardous waste (γ-AlOOH-Cr-V) with NaOH solutions was proposed, which realized the removal of V(V) and Cr(VI) effectively and the improvement of peptization index (PI) of obtained pseudo-boehmite (PB) synergistically. The results showed that PB with high PI (98.1%) and low content of V(V) (0.0656%) and Cr(VI) (0.0018%) could be achieved by desorbing the γ-AlOOH-Cr-V, while the surface area and pore volume would be decreased during desorption with the increase of NaOH concentration. Besides the leaching toxicity of Cr(VI) of obtained PB of 0.09 mg/L was extremely lower than that of the standard of hazardous waste. It was noted that the evolution of structural hydroxyl groups governed the improvement of peptization performance. DFT results showed that proper interlayer water content was beneficial to the adsorption of hydroxyl ions on the structural hydroxyl groups of PB. This work not only reduced the emission of Cr(VI)-V(V)containing hazardous waste, but also paved the way for the high-valued utilization of co-associated aluminum and vanadium resources in chromite.

Application of Metallic Iron and Ferrates in Water and Wastewater Treatment for Cr(VI) and Organic Contaminants Removal

Iron species can act as electron donors, electron acceptors or serve as a sorbent to co-precipitate contaminants. These properties, along with its relatively low cost as a material, make iron an ideal compound for environmental applications in the removal of pollutants from water and wastewater. This study assesses the use of metallic iron as a reductant for the removal of toxic Cr(VI) from aqueous solutions, as well as the use of hexavalent iron (ferrates) for the removal of organic compounds, turbidity and biological contaminants from water and wastewater. Laboratory-scale experiments show that the Cr(VI) removal efficiency of metallic iron filling materials, such as scrap iron fillings, via reduction to Cr(III) and the subsequent precipitation/filtration of aggregates can reach values over 99.0%. Moreover, the efficiency of ferrates, in situ synthesized via a low-cost Fe0/Fe0 electrochemical cell, in the removal of organic compounds, turbidity and biological contaminants from high-strength industrial wastewater, biologically treated wastewater and natural water can also reach values over 99.0%. The results showed that iron species can be applied in low-cost and environmentally friendly technologies for natural water remediation and wastewater treatment. Furthermore, the study showed that the challenge of an iron material’s surface passivation, as well as of ferrates’ procurement cost and stability, can be resolved via the application of ultrasounds and via in situ ferrate electrosynthesis.

Synergistic effect of PGPR and PSB for alleviation of chromium toxicity in Vigna radiata (L.) R. Wilczek seedlings

The current investigation depicts the individual and synergistic effects of two important plant growth promoting microbial groups viz. Plant Growth Promoting Rhizobacteria (PGPR) and Phosphate Solubilizing Bacteria (PSB) in alleviating the phytotoxic impacts of chromium in Vigna radiata (L) R. Wilczek (green gram) seedlings. Cr6+ (100 ppm) treatment caused a stiff decline of about 44%, 72%, 68%, and 49% reduction in root and shoot length as well as leaf number and leaf area respectively as compared to control after 90 d of exposure. However, combined amendment with PGPR and PSB causes a significant amelioration of Cr toxicity though doubling the shoot length and leaf area with a 4 times increase in root length and leaf number after 90 d of growth. Total chlorophyll synthesis showed a 68% reduction in Cr6+ (100 ppm) which was ameliorated by combined treatments of PGPR and PSB. It showed a 123% increased total chlorophyll content than Cr6+ (100 ppm) whereas individual application of PGPR and PSB showed a 46% and 27% increase respectively. Combined application of PGPR and PSB with a toxic dose of Cr showed significant boosting alleviation ability and indicates its ameliorative role for abatement of Cr-induced toxicity.

Reducing Chromium Toxicity in Chinese Cabbage through Synergistic Effects of Silicon and Selenium: A Study of Plant Growth, Chromium Content, and Biochemical Parameters

Silicon (Si) and selenium (Se) have been found to reduce chromium (Cr) toxicity in plants, which is important for crop production and human health. However, there is limited understanding of the interaction between Si and Se in mitigating Cr toxicity and its mechanisms. This study investigated the impact of Si and Se on Cr-treated Chinese cabbage growth, Cr content, and biochemical parameters in a hydroponic experiment. The results showed that both Si and Se effectively alleviate Cr toxicity and have a strong synergistic effect. They reduced Cr content in cabbage by 73.99% and increased photosynthetic pigments by 62.50% and 47.51%, respectively. Antioxidant enzyme activity increased by 28.20 and 21.37%, while non-enzyme antioxidants such as proline and GSH decreased by 27.44 and 28.51%. It was observed that the addition of Si and Se to Chinese cabbage under Cr stress resulted in a 29.58 and 134.37% increase in soluble protein and soluble sugar, respectively, as well as improved nutrient contents (N, P, K, Ca, and Mg). This suggests that Si and Se can improve the physiological ecology of Chinese cabbage, reducing the effects of Cr stress and contributing to the global control of heavy metal pollution in food crops.

Providencia alcalifaciens—Assisted Bioremediation of Chromium-Contaminated Groundwater: A Computational Study

In Saudi Arabia, seawater desalination is the primary source of acquiring freshwater, and groundwater contains a high concentration of toxic heavy metals. Chromium (Cr) is one of the heavy metals that is widely distributed in the environment, particularly in the groundwater of Madinah. Diverse techniques are employed to eliminate the toxicity of heavy metals from the environment, but, lately, the focus has shifted to biological remediation systems, due to their higher removal efficiencies, lower costs, and more ecologically benign characteristics than the conventional methods. Providencia bacteria engage in a variety of adsorption processes to interact with heavy metals. In this study, we aim to investigate the role of potential active site residues in the bioengineering of chromate reductase (ChrR) from Providencia alcalifaciens to reduce the Cr to a lesser toxic form by employing robust computational approaches. This study highlights Cr bioremediation by providing high-quality homology-modeled structures of wild type and mutants and key residues of ChrR for bioengineering to reduce the Cr toxicity in the environment. Glu79 is found to be a key residue for Cr binding. The mutant models of Arg82Cys, Gln126Trp, and Glu144Trp are observed to establish more metallic interactions within the binding pocket of ChrR. In addition, the wild type ChrR (P. alcalifaciens) has been found to be unstable. However, the mutations stabilized the structure by preserving the metallic contacts between the critical amino acid residues of the identified motifs and the Cr(VI). Therefore, the mutants discovered in the study can be taken into account for protein engineering to create reliable and effective enzymes to convert Cr(VI) into a lesser toxic form.