Chromium speciation in a contaminated groundwater: redox processes and temporal variability

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.

What happens to gut microorganisms and potential repair mechanisms when meet heavy metal(loid)s

Heavy metal (loid) pollution is a significant threat to human health, as the intake of heavy metal (loid)s can cause disturbances in intestinal microbial ecology and metabolic disorders, leading to intestinal and systemic diseases. Therefore, it is important to understand the effects of heavy metal (loid)s on intestinal microorganisms and the necessary approaches to restore them after damage. This review provides a summary of the effects of common toxic elements, such as lead (Pb), cadmium (Cd), chromium (Cr), and metalloid arsenic (As), on the microbial community and structure, metabolic pathways and metabolites, and intestinal morphology and structure. The effects of heavy metal (loid)s on metabolism are focused on energy, nitrogen, and short-chain fatty acid metabolism. We also discussed the main solutions for recovery of intestinal microorganisms from the effects of heavy metal (loid)s, namely the supplementation of probiotics, recombinant bacteria with metal resistance, and the non-toxic transformation of heavy metal (loid) ions by their own intestinal flora. This article provides insight into the toxic effects of heavy metals and As on gut microorganisms and hosts and provides additional therapeutic options to mitigate the damage caused by these toxic elements.

Reductive Cr(VI) Removal under Different Reducing and Electron Donor Conditions—A Soil Microcosm Study

Increased groundwater and soil contamination by hexavalent chromium have led to the employment of a variety of detoxification methods. Biological remediation of Cr(VI) polluted aquifers is an eco-friendly method that can be performed in situ by stimulating the indigenous microbial population with organic and inorganic electron donors. In order to study the effect of different redox conditions on microbial remediated Cr(VI) reduction to Cr(III), microcosm experiments were conducted under anaerobic, anoxic, and sulfate-reducing conditions and at hexavalent chromium groundwater concentrations in the 0–3000 μg/L range, with groundwater and soil collected from an industrial area (Inofyta region). As electron donors, molasses, emulsified vegetable oil (EVO), and FeSO4 were employed. To quantitatively describe the degradation kinetics of Cr(VI), pseudo-first-order kinetics were adopted. The results indicate that an anaerobic system dosed with simple or complex external organic carbon sources can lead to practically complete Cr(VI) reduction to Cr(III), while the addition of Fe2+ can further increase Cr(VI) removal rate significantly. Furthermore, Cr(VI) microbial reduction is possible in the presence of NO3− at rates comparable to anaerobic Cr(VI) microbial reduction, while high sulfate concentrations have a negative effect on Cr(VI) bioreduction rates in comparison to lower sulfate concentrations.

Development of Solid Phase Extraction Method Based on Ion Imprinted Polymer for Determination of Cr(III) Ions by ETAAS in Waters

In this work, a new solid phase extraction method for the determination of chromium species in water samples by electrothermal atomic absorption spectrometry was developed. For selective separation of Cr(III) ions under dynamic conditions, two ion imprinted polymers containing Cr(III)-1,10-phenanthroline complex (Cr(III)-phen) were prepared with the use of one (styrene, ST) or two (styrene and 4-vinylpyridine, ST-4VP) functional monomers. The physicochemical properties of those solid sorbents towards Cr(III) ions were studied and compared. It was found that Cr(III) ions were retained on the Cr(III)-phen-ST and Cr(III)-phen-ST-4VP polymers with high efficiency and repeatability (91.6% and 92.9%, RSD < 2%) from solutions at pH 4.5. The quantitative recovery of the analyte (91.7% and 93.9%, RSD < 4%) was obtained with 0.1 mol/L EDTA solution. The introduction of 4VP, an additional functional monomer, improved selectivity of the Cr(III)-phen-ST-4VP polymer towards Cr(III) ions in the presence of Cu(II), Mn(II) and Fe(III) ions, and slightly decreased the sorption capacity and stability of that polymer. The accuracy of procedures based on both polymeric sorbents was proved by analyzing the standard reference material of surface water SRM 1643e. The method using the Cr(III)-phen-ST polymer was applied for determining of Cr(III) ions in tap water and infusion of a green tea.

Migration and fate of characteristic pollutants migration from an abandoned tannery in soil and groundwater by experiment and numerical simulation

The tannery industry is an integral part of economic development in many developing countries, and the environmental pollution caused by the tannery industry cannot be ignored. In this study, soil and groundwater samples at different depths were collected from an abandoned tannery to investigate the temporal and spatial distribution of characteristic pollutants produced by tanning. The concentrations of Cr, Cl, F and NH₄⁺-N in the soil from the sludge temporary storage area were higher than those from the liming and unhairing workshop, chrome tanning workshop, wastewater outlet, and around wastewater pond. The concentration of Cr(VI) in all sampling sites was below the detection limit. The main species of Cr in the groundwater were Cr(NH₃)₆Cl²⁺ and CrO₄^2- based on the simulation of Visual MINTEQ. The saturation index was negative and changed with time indicating that Cr existed in the dissolved phase. The proportion of Cr(VI) to total Cr was negatively correlated with the saturation index in village 1 and village 3. The simulation results from Visual MODFLOW and MT3DMS showed that the migration of Cr, NH₄⁺-N, Cl^- and F^- mainly occurred in the Quaternary system. The coverage of the pollution plume of pollutants in villages 1 and 3 was as follows: Cr > NH₄⁺-N > Cl^- > F^-. Two decay rate calculation methods of pollutants with migration time and distance were put forward to provide a basis for the actual investigation of the pollution migration scope and time determination.

Long-term hexavalent chromium exposure facilitates colorectal cancer in mice associated with changes in gut microbiota composition

Colorectal cancer (CRC) is among the leading causes of cancer-related mortality worldwide. Hexavalent chromium [Cr(VI)] is often present in groundwater. Chronic Cr(VI) exposure is suggested to be one of the main factors inducing cancer. However, the correlation between Cr(VI) and CRC remains unclear. In this study, we investigated the role of Cr(VI) in CRC by establishing a mouse CRC model induced by 1, 2-dimethylhydrazine (DMH). The results showed that Cr(VI) increased weight loss in DMH-induced mice and promoted the formation of tumors. Cr(VI) also increased DMH-induced proliferating cell nuclear antigen (PCNA) levels. Investigation of the underlying mechanisms found that Cr(VI) significantly decreased DMH-induced SOD, GSH and CAT levels, while, the MDA level increased. Metagenomic analyses found that the abundance of Firmicutes and Bacteroidetes in the DMH + Cr group was down-regulated. Interestingly, the combination of Cr(VI) and DMH significantly increased the abundance of Verrucomicrobia. At the family and genus levels, families Akkermansiaceae and Saccharimonadaceae and genus Akkermansia were more abundant in the DMH + Cr group, whereas the abundance of short-chain fatty acid (SCFA)-producing bacteria (family Muribaculaceae, family Lachnosipiraceae, genus Lachnospiraceae_NK4A136_group, and genus Roseburia) decreased. These results indicate that Cr(VI) might aggravate CRC by altering the composition of the gut microflora.

Cr(VI) remediation from aqueous environment through modified-TiO2-mediated photocatalytic reduction

Cr(VI) exhibits cytotoxic, mutagenic and carcinogenic properties; hence, effluents containing Cr(VI) from various industrial processes pose threat to aquatic life and downstream users. Various treatment techniques, such as chemical reduction, ion exchange, bacterial degradation, adsorption and photocatalysis, have been exploited for remediation of Cr(VI) from wastewater. Among these, photocatalysis has recently gained considerable attention. The applications of photocatalysis, such as water splitting, CO2 reduction, pollutant degradation, organic transformation reactions, N2 fixation, etc., towards solving the energy crisis and environmental issues are briefly discussed in the Introduction of this review. The advantages of TiO2 as a photocatalyst and the importance of its modification for photocatalytic reduction of Cr(VI) has also been addressed. In this review, the photocatalytic activity of TiO2 after modification with carbon-based advanced materials, metal oxides, metal sulfides and noble metals towards reduction of Cr(VI) was evaluated and compared with that of bare TiO2. The photoactivity of dye-sensitized TiO2 for reduction of Cr(VI) was also discussed. The mechanism for enhanced photocatalytic activity was highlighted and attributed to the resultant properties, namely, effective separation of photoinduced charge carriers, extension of the light absorption range and intensity, increase of the surface active sites, and higher photostability. Advantages and limitations for photoreduction of Cr(VI) over modified TiO2 are depicted in the Conclusion. The various challenges that restrict the technology from practical applications in remediation of Cr(VI) from wastewater were addressed in the Conclusion section as well. The future perspectives of the field presented in this review are focused on the development of whole-solar-spectrum responsive, TiO2-coupled photocatalysts which provide efficient photocatalytic reduction of Cr(VI) along with their good recoverability and recyclability.

Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: Evidence for natural attenuation of Cr(VI)

Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H₂O₂). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

Caffeic acid phenethyl ester protects the brain against hexavalent chromium toxicity by enhancing endogenous antioxidants and modulating the JAK/STAT signaling pathway

Hexavalent chromium [Cr(VI)] is commonly used in industry, and is a proven toxin and carcinogen. However, the information regarding its neurotoxic mechanism is not completely understood. The present study was designed to scrutinize the possible protective effects of caffeic acid phenethyl ester (CAPE), a bioactive phenolic of propolis extract, on Cr(VI)-induced brain injury in rats, with an emphasis on the JAK/STAT signaling pathway. Rats received 2mg/kgK₂CrO₄ and concurrently treated with 20mg/kg CAPE for 30 days. Cr(VI)-induced rats showed a significant increase in cerebral lipid peroxidation, nitric oxide and pro-inflammatory cytokines, with concomitantly declined antioxidants and acetylcholinesterase. CAPE attenuated oxidative stress and inflammation and enhanced antioxidant defenses in the cerebrum of rats. Cr(VI) significantly up-regulated JAK2, STAT3 and SOCS3, an effect that was reversed by CAPE. In conclusion, CAPE protects the brain against Cr(VI) toxicity through abrogation of oxidative stress, inflammation and down-regulation of JAK2/STAT3 signaling in a SOCS3-independent mechanism.

Gallic acid attenuates chromium-induced thyroid dysfunction by modulating antioxidant status and inflammatory cytokines

Hexavalent chromium-mediated oxidative stress causes severe organ damage. The present study was designed to investigate the possible thyroprotective effect and underlying mechanisms of gallic acid using rat model of potassium dichromate-induced thyroid dysfunction. Forty adult male albino rats were divided into 4 groups: control, gallic acid (20mg GA/kg b. wt), potassium dichromate (2mg PD/kg b. wt) and the fourth group was co-treated with PD and GA. PD-injection resulted in decreased serum free triiodothyonine (FT3), free thyroxine (FT4) with concomitant significant increase in thyroid stimulating hormone (TSH) levels. Superoxide dismutase (SOD), glutathione-S-transferase (GST) activities and their respective mRNA expression and reduced glutathione (GSH) content were significantly decreased. Thyroid nitrosative stress marker (NO level and iNOS mRNA and protein expression) and pro-inflammatory cytokines (serum TNF-α, IL-6 and thyroid TNF-α, IL-6 and COX-2 gene and protein expression levels) were disturbed. Histopathological changes revealed distended, collapsed and degenerated follicles with vacuolated cytoplasm. GA co-treatment attenuated pro-inflammatory cytokines, the thyroid expression of iNOS, TNF-α, IL-6 and COX-2, decreased the elevated lipid peroxidation biomarkers and NO level and up- regulated SOD and GST mRNA expression levels. In conclusion, GA has shown strong modulatory potential against PD-induced inflammation and oxidative stress in albino rats.

Incorporation of zero valent iron nanoparticles in the matrix of cationic resin beads for the remediation of Cr(VI) contaminated waters

The objective of present study was to obtain the fixation of nano zero valent iron (nZVI) particles on a permeable matrix and evaluate the performance of this composite material for the removal of Cr(VI) from contaminated waters. The experiments were carried out using the cationic resin Dowex 50WX2 as porous support of the iron nanoparticles. The work was carried out in two phases. The first phase involved the fixation of nZVI on the resin matrix. The resin granules were initially mixed with a FeCl3 solution to obtain the adsorption of Fe(III). Then the Fe(III) loaded resin (RFe) was treated with polyphenol solutions to obtain the reduction of Fe(III) to the elemental state. Two polyphenol solutions were tested as reductants, i.e. green tea extract and gallic acid. Green tea was found to be inefficient, probably due to the relatively big size of the contained polyphenol molecules, but gallic acid molecules were able to reach adsorbed Fe(III) and reduce the cations to the elemental state. The second phase was focused on the investigation of Cr(VI) reduction kinetics using the nanoiron loaded resins (R-nFe). It was found that the reduction follows a kinetic law of first order with respect to Cr(VI) and to the embedded nanoiron. Compared to other similar products, this composite material was found to have comparable performance regarding reaction rates and higher degree of iron utilization. Namely the rate constant for the reduction of Cr(VI), in the presence of 1 mM nZVI, was equivalent to 1.4 h of half-life time at pH 3.2 and increased to 24 h at pH 8.5. The degree of iron utilization was as high as 0.8 mol of reduced Cr(VI) per mole of iron. It was also found that this composite material can be easily regenerated and reused for Cr(VI) reduction without significant loss of efficiency.

Cr (VI) induced oxidative stress and toxicity in cultured cerebellar granule neurons at different stages of development and protective effect of Rosmarinic acid

Chromium (Cr) is a widespread metal ion in the workplace, industrial effluent, and water. The toxicity of chromium (VI) on various organs including the liver, kidneys, and lung were studied, but little is known about neurotoxicity. In this study, neurotoxic effects of Cr (VI) have been investigated by cultured cerebellar granule neurons (CGNs). Immature and mature neurons were exposed to different concentrations of potassium dichromate for 24 h and cytotoxicity was measured by MTT assay. In addition, immature neurons were exposed for 5 days as regards cytotoxic effect in development stages. The reactive oxygen species (ROS), mitochondrial membrane potential (MMP) and the protective effect of Rosmarinic acid on mature and immature neurons exposed to potassium dichromate, were measured. Furthermore, lipid peroxidation, glutathione peroxidase (GPx), and acetylcholinesterase activity in mature neurons were assessed following exposure to potassium dichromate. The results indicate that toxicity of Cr (VI) dependent on maturation steps. Cr (VI) was less toxic for immature neurons. Also, Cr (VI) induced MMP reduction and ROS production in both immature and mature neurons. In Cr (VI) treated neurons, increased lipid peroxidation and GPx activity but not acetylcholinesterase activity was observed. Interestingly, Rosmarinic acid, as a natural antioxidant, could protect mature but not immature neurons against Cr (VI) induced toxicity. Our findings revealed vulnerability of mature neurons to Cr (VI) induced toxicity and oxidative stress.

Ultrastructure and subcellular distribution of Cr in Iris pseudacorus L. using TEM and X-ray microanalysis

Chromium pollution of freshwater is hazardous for humans and other organisms, and places a limitation on the use of polluted water sources. Phytoremediation, the use of plants to remove pollutants from the environment, is a cost-effective, environmentally friendly approach for water decontamination. To improve the efficiency of the process, it is essential to increase the current knowledge about Cr accumulation in macrophytes. Plants of Iris pseudacorus L. were treated with Cr(III) at 0.75 mM for 5 weeks to investigate Cr localization by means of transmission electron microscopy and energy dispersive X-ray analysis. Chromium induced severe ultrastructural alterations in the rhizodermis (cell wall disorganisation, thickening, plasmolysis, and electron-dense inclusions) and rhizome parenchyma (reduced cell size, cell wall detachment, vacuolation, and opaque granules). The highest Cr contents were found in the cell walls of the cortex in the roots and in the cytoplasm and intercellular spaces of the rhizome. The Cr concentration in root tissues was in the order cortex>rhizodermis>stele, whereas in the rhizome, Cr was evenly distributed. It is proposed that root and rhizome have distinct functions in the response of I. pseudacorus to Cr. The rhizodermis limits Cr uptake by means of Si deposition and cell wall thickening. The rhizome cortex generates vacuoles and granules where Cr co-occurs with S, indicating Cr sequestration by metal-binding proteins.