Sources and toxicity of hexavalent chromium

Uptake of hexavalent chromium by Lactuca sativa and Triticum aestivum plants and mediated effects on their performance, linked with associated public health risks

Plants exposed to toxic Cr(VI) concentrations show reduced biomass production and yield. The present study examined (1) the impacts of increasing Cr(VI) exposure (0.05, 0.5, 1, 5 and 10 mg L-1 K2Cr2O7 in irrigation water) on the growth and development, yield, and stress physiology of two important crop species, such as wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.), and (2) the associated human health risks due to the consumption of these commodities. Plants in all Cr(VI) treatments preserved similar growth rates and phenotypes with control, untreated plants, with no remarkable modulations in biomass yield (fresh/dry weight, plant height, straw/grain weight). Grains harvested from plants exposed to all Cr(VI) treatments had increased size and 1000 grain weight, partially suggesting hormetic effect. Elevated K2Cr2O7 concentrations (5 and 10 mg L-1) resulted in increased photosynthetic pigments' levels in lettuce leaves. Lipid peroxidation and H2O2 content also revealed the absence of oxidative stress in lettuce plants. Non-carcinogenic (target hazard quotient) and carcinogenic risks from the consumption of these commodities grown under the conditions described in this study were found de minimis. However, more studies are needed in order to obtain more solid information on the safety of Cr(VI)-contaminated water irrigation.

Facile synthesis of economical feasible fly ash-based zeolite-supported nano zerovalent iron and nickel bimetallic composite for the potential removal of heavy metals from industrial effluents

Heavy metals contamination of water is one of the environmental issue globally. Thus prepared fly ash-based zeolite (FZA)-supported nano zerovalent iron and nickel (nZVI/Ni@FZA) bimetallic composite from low-cost fly ash waste for the potential treatment of anion (Cr(VI) and cation Cu(II)) heavy metals from industrial effluents at pH 3 and 5, respectively in this study. The systematic interaction between FZA and nZVI/Ni and the adsorptive removal mechanism was studied. The mean surface area of the nZVI/Ni@FZA (154.11 m²/g) was much greater than that of the FZA (46.6 m²/g) and nZVI (4.76 m²/g) independently, as determined by BET-N₂ measurements. The effect of influence factors on the removal of Cr(VI) and Cu(II) by nZVI/Ni@FZA, such as pH effect, initial concentration effect, time effect, temperature effect, coexisting metals, and ionic strength, and cumulative loading ability, were discussed. The maximum adsorption capacity of nZVI/Ni@FZA was 48.31 mg/g and 147.06 mg/g towards Cr(VI) and Cu(II), respectively. These were higher than those of nZVI@FZA and FZA. It demonstrated that Ni could play an important role in enhancing the reduction ability of nZVI. Furthermore, isothermal and kinetic results revealed that both heavy metal adsorption processes were rate limiting monolayer Langmuir adsorption on homogeneous surfaces. Thermodynamic results suggested that the adsorptive removal of metal ions was endothermic with spontaneity. The applicability of nZVI/Ni@FZA on real industrial wastewater treatment results demonstrate that the concentration of heavy metals were removed under the acceptable standard levels. Further the adsorption capacity of nZVI/Ni@FZA was higher than the nZVI@FZA and FZA. The overall results demonstrated that nZVI/Ni@FZA was a promising, efficient, and economically feasible sorbent for potential wastewater treatment. Moreover this is first report on the preparation nZVI/Ni@FZA bimetallic composite.

Occurrence and Distribution of Hexavalent Chromium in Ground and Surface Waters in Cyprus

The origin and distribution of hexavalent chromium, Cr (VI) over four seasonal cycles was investigated through a conceptual model that included three aquifer systems in Cyprus. An extensive water sampling grid covered two sea water intruded coastal aquifers, namely the Kokkinochoria (A1) and Kiti-Pervolia (A2) aquifers and the Troodos massif (A3). Analytical results give a first insight to the presence of Cr (VI) in Cyprus ground and surface waters. The highest Cr (VI) value of 26 μg L^-1 is observed in the Troodos area (A3) where Cr(VI) is detected in all sampled water systems (surface and ground/spring). Nonetheless, the highest median values of 4.6 and 4.5 μg L^-1 are observed in the Kokkinochoria (A1) and Kiti-Pervolia (A2) study areas respectively, where the local aquifers are under increased pressure from agriculture, urbanization and seawater intrusion. Stable isotopes were used to discern the effect of return irrigation and seawater intrusion. Stable isotope analyses from Troodos samples show two distinct groups of meteoric and near meteoric waters whereas isotopically enriched water is shown to correspond to the Kiti and Kokkinochoria area.

Effect of hexavalent chromium on the environment and removal techniques: A review

Despite the importance of chromium (Cr) in most anthropogenic activities, the subsequent environmental adulteration is now a source of major concern. Cr occurs in numerous oxidation states, with the furthermost stable and frequently occur states being Cr(0), Cr(III) and Cr(VI). Cr(0) and Cr(III) are vital trace elements while Cr(VI) is dispensable and noxious to living organisms. Predominantly in plants, Cr at low concentrations of about 0.05-1 mg/L assist to boost growth as well as increase productivity. However, accumulation of Cr could represent a potential threat to living organisms. Cr absorption, displacement and accretion depend on its speciation, which also determines its toxicity which is often diverse. Indications of its toxicity include; reduction of seed germination, retardation of growth, reduction of yield, inhibition of enzymatic activities, weakening of photosynthesis, nutrient, oxidative disparities and genetic mutation in plants as well as several injurious diseases in animals and humans. In this study, we have presented a comprehensive review as well as an informative account of the influence of Cr on the environment drawn from researches carried out over the years following an analytical approach. Uniquely, this work presents a review of the effects and remediation of Cr from soil and wastewater drawn from several evidence and meta-data-based articles and other publications. Accordingly, the write-up is intended to appeal to the consciousness of the general public that the significance of Cr notwithstanding, its environmental toxicity should not be taken for granted.

Descriptive Analysis of Heavy Metals Content of Beef From Eastern Uganda and Their Safety for Public Consumption

In this study, we initiated an effort to generate information about beef safety in Uganda. Our entry point was to assess by atomic absorption spectrophotometry the levels of essential elements copper (Cu), cobalt (Co), iron (Fe) and zinc (Zn), and non-essential elements lead (Pb), chromium (Cr), nickel (Ni), and cadmium (Cd) in 40 beef samples collected from within and around Soroti (Uganda). The information was used to evaluate the safety of consuming such beef against the World Health Organization (WHO) limits. The latter was accomplished by (i) estimating the daily intake (EDI) of each metal in the study area, (ii) modeling the non-cancer health risk using the target hazard quotient (THQ) and (iii) modeling the cancer risk using the incremental lifetime cancer risk (ILCR). The study finds that the mean concentrations (±95% CI) and EDI were in the order of Fe > Zn > Cr > Ni > Pb > Co > Cu > Cd. Cancer risk was found to be due to Ni > Cr > Cd > Pb and significantly higher in children than adults. The latter particularly demonstrates the importance of Ni poisoning in the study area. Overall, while essential elements in our beef samples were below WHO limits (hence no health risks), non-essential elements had high health and cancer risks due to higher levels of Cr and Ni.

Speciation Analysis of Cr(VI) and Cr(III) in Water with Surface-Enhanced Raman Spectroscopy

Identifying and quantifying chromium in water are important for the protection of precious water resources from chromium pollution. Standard methods however are unable to easily distinguish toxic hexavalent chromium, Cr(VI), from innocuous trivalent chromium, Cr(III), are time-consuming, or require large sample quantity. We show in this report that Cr(VI) and Cr(III) in water can be differentiated based on their distinct spectral features of surface-enhanced Raman scattering (SERS). Their SERS signals exhibit different pH dependences: the SERS features of Cr(VI) and Cr(III) are most prominent at pH values of 10 and 5.5, respectively. The obtained limit of detection of Cr(VI) in water is below 0.1 mg/L. Both concentration curves of their SERS signals show Langmuir sorption isotherm behavior. A procedure was developed to quantify Cr(VI) concentration based on the direct retrieval or addition method with an error of 10%. Finally, the SERS detection of Cr(VI) is shown to be insensitive to co-present Cr(III). The developed SERS procedure offers potential to monitor toxic chromium in fields.

Synthesis and application of cationised cellulose for removal of Cr(VI) from acid mine-drainage contaminated water

Background: Acid mine drainage (AMD) leads to contamination of surface and ground water by high levels of toxic metals including chromium. In many cases, these waters are sources of drinking water for communities, and treatment is therefore required before consumption to prevent negative health effects. Methods: Cationised hemp cellulose was prepared by etherification with two quaternary ammonium salts: 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPTAC) and glycidyltrimethylammonium chloride (GTMAC) and examined for (i) the efficiency of Cr(VI) removal under acid mine-drainage (AMD) conditions, and (ii) antibacterial activity. Adsorbents were characterised by electron microscopy, Fourier transform infrared (FTIR), CP-MAS 13C nuclear magnetic resonance (NMR) spectroscopy, elemental composition and surface charge. Results: FTIR and solid state 13C NMR confirmed the introduction of quaternary ammonium moieties on cellulose. 13C NMR also showed that cationisation decreased the degree of crystallisation and lateral dimensions of cellulose fibrils. Nevertheless, 47 %  - 72 % of Cr(VI) ions were removed from solutions at pH 4, by 0.1 g of CHPTAC and GTMAC-cationised cellulose, respectively. Adsorption kinetics followed the pseudo-second order model and isotherms were best described by the Freundlich and Dubinin-Radushkevich models. When GTMAC-modified cellulose was applied to AMD contaminated water (pH 2.7); however, Cr(VI) removal decreased to 22% likely due to competition from Al and Fe ions. Cationised materials displayed considerable antibacterial effects, reducing the viability of Escherichia coli by up to 45 % after just 3 hours of exposure. Conclusions: Together, these results suggest that cationised cellulose can be applied in the treatment of Cr(VI)-contaminated mine water particularly if pre-treatments to reduce Fe and Al concentrations are applied.

The Competitive Adsorption of Chromate and Sulfate on Ni-Substituted Magnetite Surfaces: An ATR-FTIR Study

With similar chemical properties and geometrical configurations, sulfate and chromate display interesting competitive adsorption on mineral surfaces. Although such issues have been investigated on several Fe (hydr)oxide surfaces, e.g., ferrihydrite, goethite and hematite, the competitive adsorption on magnetite surfaces and the constraint mechanism have seldom been studied. This impedes the understanding of the transfer and fate of chromate and sulfate on magnetite surfaces, as magnetite is not only a useful adsorbent but also an efficient reductant to decrease the mobility and toxicity of chromium. In the present study, the geometries of the competitive adsorption of chromate and sulfate on Ni-substituted magnetite surfaces over a pH range of 4–9 were investigated using in situ attenuated total reflectance Fourier transform infrared spectroscopy and two-dimensional correlation analysis. In individual adsorption, nonprotonated monodentate mononuclear (NMM) complexes dominated chromate adsorption, accompanied by a few bidentate binuclear (BB) complexes. For sulfate, NMM complexes and outer-sphere (OS) species predominated under acidic and neutral–alkaline conditions, respectively. The above variation in adsorption configuration resulted in the different adsorption competitiveness between chromate and sulfate at different pH values. Specifically, the NMM complexes of chromate were substituted by NMM sulfate complexes under acidic conditions and vice versa. However, under neutral and alkaline conditions, the OS species of sulfate scarcely affected the adsorption of chromate. The adsorption affinity of chromate and sulfate on Ni-substituted magnetite increased in the following order: OS complex (sulfate) < NMM complexes (chromate) < NMM complexes (sulfate).

In vitro microcosm of co-cultured bacteria for the removal of hexavalent Cr and tannic acid: A mechanistic approach to study the impact of operational parameters

Industrial wastes, for instance, tannery wastes are rich soups of resistant and bioremediation-potent bacteria. In the present work, Chromium (Cr) and tannic acid (TA) resistance bacterial strains were isolated from tannery effluent and identified as Bacillus subtilis (MCC 3275) and Bacillus safensis (MCC 3283) based on its 16S Ribosomal RNA homology. Hexavalent Cr is highly toxic and mutagenic due to its high mobility and reactivity. Whereas, TA is known to inhibit enzyme activity, substrate deprivation, and interaction with membranes and matrix-metal ions. The developed In vitro co-cultured microcosm of B. subtilis and B. safensis was able to remove Cr(VI) up to 95% and TA up to 23%. The bacteria cultures separately were able to degrade Cr(VI) to 88% by B. subtilis and 91% by B. safensis and TA up to 27%. Plackett Burman design (PBD) followed by Response surface methodology (RSM) was applied for the optimization of physio-chemical parameters. The optimized conditions for co-culture development were recorded as K₂HPO₄ = 0.2 g/L, MgSO₄ = 0.2 g/L, NH₄Cl = 0.5 g/L, glucose - 0.2 g/L, TA - 5%, Cr = 200 ppm, incubation period of 96 h, agitation speed of 110 rpm, pH = 5.0, temperature= 30 °C and inoculum size = 3%. Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) revealed the thorough mechanism of cellular uptake followed by degradation of Cr(VI) and TA. The efficiency of co-culture for other heavy metals was observed as follows: Zn 65%, Pb 63%, Cd 65%, and Ni 65%. Bioremediation using bacteria is an economical and environmentally better alternative to conventional remediation methods. The isolated bacteria are useful in the effluent treatment of tannery or related industries and in metal recovery in mining processes.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Effect assessment of chelators on bioaccessible and total chromium extraction in soils polluted by chromite processing wastes

Solid wastes from chromium-processing facilities, when used as fill material, have generated chromium point source contamination of soils and groundwaters, that remains after decades in many urban and peri urban areas all over the world, causing environmental impacts that need to be addressed in a sustainable economic, social and environmental way. Representative surface soil samples, from a polluted area affected by chromite wastes burials, were used to carry on a feasibility study on Phytoextraction applicability, and chelating agents' selection to reduce environmental risks, considering chromium bioaccessible forms reduction as an objective. Tests for natural (only vegetation), and induced phytoextraction (plants + chelating agents) applicability were performed at greenhouse facilities under controlled conditions at two rounds. Natural phytoextraction tests, using first wheat and then corn plants, allowed to observe a low but significant 4,8% reduction in soil chromium. At induced phytoextraction trials, when comparing the action of the different chelating agents versus a blank on final soil chromium concentration, the results were not so conclusive globally (0,5% less soil chromium in trials in which the first crop was wheat followed by corn, and 2% in trials in which the first and the second crop was corn). In each group, the most "effective" chelating agent differed: EDDS for the first set, MGDA for the second one.The addition of chelating compounds in the soil could result in an increasing of the Cr oxidizable fraction mainly composed by Cr species associated to the organ mineral complexes.

In situ growth of Fe3O4 on montmorillonite surface and its removal of anionic pollutants

An environmentally functional material for the efficient removal of anionic pollutants in water was prepared for our study. Montmorillonite (M) was modified by hydrochloric acid (HCl) and cetyltrimethylammonium bromide (CTAB). Fe₃O₄ was grown in situ to prepare modified Fe₃O₄/montmorillonite (AC Fe₃O₄–Mt) composites. The number of hydroxyl sites on the surface of Mt and the surface tension were increased by HCl and CTAB modification, respectively, which enabled higher Fe₃O₄ surface growth, promoting the multi-directional crystallisation of Fe₃O₄ and improving reactivity. The XRD results show that Fe₃O₄ grows on the surface of Mt and has good crystallinity and high purity, meaning that AC Fe₃O₄–Mt is more reactive than Fe₃O₄. When AC Fe₃O₄–Mt is used to remove anionic pollutants in water, the removal effect under the synergistic action of adsorption-redox is significantly improved, and the maximum removable quantities of Cr(vi) and 2-4-dichlorophenol can reach 41.57 mg g⁻¹ and 239.33 mg g⁻¹, respectively. AC Fe₃O₄–Mt is a high efficiency and environmentally functional material with green environmental protection, which can be used in the field of sewage treatment.

An environmentally functional material for the efficient removal of anionic pollutants in water, was prepared for our study.

Source apportionment, contamination levels, and spatial prediction of potentially toxic elements in selected soils of the Czech Republic

The sustenance of humans and livestock depends on the protection of the soil. Consequently, the pollution of the soil with potentially toxic elements (PTEs) is of great concern to humanity. The objective of this study is to investigate the source apportionment, concentration levels and spatial distribution of PTEs in selected soils in Frýdek-Místek District of the Czech Republic. The total number of soil samples was 70 (topsoil 49 and 21 subsoils) and was analysed using a portable XRF machine. Contamination factor and the pollution index load were used for the assessment and interpreting the pollution and distribution of PTEs in the soils. The inverse distance weighting was used for the spatial evaluation of the PTEs. The results of the analysis showed that the area is composed of low-to-high pollution site. PTEs displayed spatial variation patterns. The average PTE concentration decreases in this Fe > Ti > Ba > Zr > Rb > Sr > Cr > Y>Cu > Ni > Th order for the topsoil and also decreases in this Fe > Ti > Zr > Ba > Rb > Sr > Cr > Y > Cu > Ni > and Th order for the subsoil. These PTEs Cr, Ni, Cu, Rb, Y, Zr, Ba, Th, and Fe were far above the baseline European average value and the World average value level, respectively. The source apportionment showed the dominance of Cr, Ni, Rb, Ti, Th, Zr, Cu, Fe in the topsoil, while the subsoil was dominated by all the PTEs (factor 1 to 6) except Ba. The study concludes that indiscriminate human activities have an enormous effect on soil pollution.

Study on the mechanism of tunable ferromagnetic composites with different rare earth ions

Size-controlled Fe₃O₄ nanoparticles doped with rare earth (RE) ions (La³⁺, Ce³⁺, and Dy³⁺) varying from 15 nm to 30 nm were successful synthesized by a hydrothermal method for potential applications in the fields of biomedicine, environmental protection and magnetic memory devices. They possessed good dispersibility, adjustable particle size and nearly spherical shape. The particle grain size was uniformly distributed and showed a low degree of agglomeration in comparison with undoped Fe₃O₄ nanoparticles. The FTIR results showed that the RE elements partially replaced Fe²⁺, occupied the octahedral position, and enhanced the vibration of the Fe–O bond. The XPS study further revealed that the valence states of La, Ce, and Dy are both positive trivalent. The XPS Fe 2p valence band spectra observed a shift in the peak position toward higher binding energy after RE doping, confirming the existence of RE ions in the octahedral position. This paper explains the mechanism of rare earth doping with Fe₃O₄, and clarifies the influence of the doping of different RE ions on its magnetic properties. The detailed analysis of RE-doped ferrite materials can open a new perspective in designing biomedical and spintronics materials with tailored properties by choosing suitable cation substitution.

After doping RE elements, the corresponding M_s decreases, and the magnetization decreases with the increase of doping concentration. The RE ions were more inclined to replace some Fe²⁺ ions in the octahedral position from the VSM results.

Bio-sorption for effective removal of chromium(VI) from wastewater using Moringa stenopetala seed powder (MSSP) and banana peel powder (BPP)

Chromium is an extremely toxic metal in the form of Cr(VI) that causes severe environmental and health problems. Therefore, the aim of this study was to remove chromium ions from wastewater by using cost effective and environmentally friendly bio-sorbents; Moringa stenopetala seed powder (MSSP), and banana peel powder (BPP) and to evaluate its adsorption capacities as bio-sorbents. FT-IR characterization of the adsorbents showed that there was a change in the functional groups of the structure of both adsorbents before and after the adsorption that might be due to the adsorption processes taken place on the surface of adsorbent. Adsorption experiments were carried out as batch studies with different contact times, pH, adsorbent dose, initial metal ion concentration, and temperature. Results showed maximum removal efficiency for Cr(VI) at 120 min contact time, adsorbent dose of 20 g/L and pH 2 by MSSP and pH 4 by BPP. The percentage removal of Cr(VI) increased with increasing adsorbent dose (from 5 to 20 g/L) and contact time (from 60 to 120 min). Freundlich isotherm model showed a better fit to the equilibrium data than the Langmuir model. The kinetics of adsorption for chromium was well represented by pseudo-second order kinetic model and the calculated equilibrium sorption capacity of the model showed good agreement with the sorption capacity obtained from experimental results.

Detoxification of wood-combustion ashes containing Cr and Cd by thermal treatment

This study assesses the potential of thermal processing for detoxification of wood-combustion ashes that contain high levels of Cr and Cd. Thermal treatment (1000 °C) of bottom ash and fly ash in an oxidising gas (air) atmosphere resulted in: low volatilisation of Cd and most other heavy metals, oxidation of Cr in the ashes to Cr (VI), and, in the case of the fly ash, significantly increased leaching of Cr and Mo. Thermal treatment in a nitrogen atmosphere resulted in local reducing conditions due to oxidation of ash-derived carbon to CO (g). Thermal treatments in this atmosphere and in a reducing atmosphere consisting of 10 % H₂ and the balance N₂ detoxified the ashes in at least two ways: (i) by substantially removing Cd, Pb, Bi, Tl, and, in the case of the fly ash, Zn from the ashes by volatilisation; and (ii) by thermal reduction of Cr (VI) in the ashes. There was at least a 100-fold reduction in the leaching of total Cr from both the bottom ash and the fly ash following the thermal treatments in reducing conditions. Chromium only leached from the detoxified bottom ash to a significant extent in acidic conditions (pH < 4).

The Effect of Flooding and Drainage Duration on the Release of Trace Elements from Floodplain Soils

Floodplains downstream of urban catchments are sinks for potentially toxic trace elements. An intensification of the hydrological cycle and changing land use will result in floodplains becoming inundated for longer durations in the future. We collected intact soil cores from a floodplain meadow downstream of an urban catchment and subjected them to an inundation/drainage cycle in the laboratory to investigate the effect of flood duration on trace element concentrations in the soil porewater. The porewater concentrations of Ni, Cr, and Zn increased, whereas Cu and Pb decreased with flood duration. All the Cr present in porewaters was identified as Cr(III). Copper concentrations increased after drainage but Pb mobility remained suppressed. Both pH and dissolved organic carbon (DOC) increased with flood duration but were lower in treatments that were drained for the longest duration (which were also the treatments flooded for the shortest duration). The porewater concentrations of Cr and Ni decreased after drainage to levels below those observed before inundation, mirroring the DOC concentrations. We concluded that the duration of floodplain inundation does have an influence on the environmental fate of trace elements but that flooding does not influence all trace elements in the same way. The implications of an intensification of the hydrological cycle over the coming decades are that floodplains may become a source of some trace elements to aquatic and terrestrial ecosystems. Environ Toxicol Chem 2020;39:2124-2135. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Reinforcement of Quaternary Ammonium Modified Silica (QAMS) with Magnetite and its Application by Solid Phase Adsorption (SPA) to Adsorb Chromate Ions

Chromium (VI) in the form of chromate anions that have toxic properties needs to be overcome. This study aims to reinforce cationic sorbent quaternary amine-modified silica with magnetite (QAMS-Fe3O4) to adsorb chromate ions. QAMS prepared by reflux methylation ammine modified silica (AMS) obtained from destruction silicate from rice husk ash followed by the addition of 3-APTMS. Characterization QAMS-Fe3O4 by FT-IR showed successfully of methylation process indicated by disappearing absorbance at 1388 cm-1, and emerging absorbance at 2939 cm-1 in QAMS and QAMS-Fe3O4 indicated a transformation of N-H from -NH2 group to [-N+(CH3)3]. XRD analysis denotes 2θ = 30.15°, 35.53°, 43.12°, 57.22°, and 62.90° (JCPDS No. 00-033-0664) fathomed as a characteristic peak of magnetite. SEM-EDX reveals the homogenous topological spherical form with an average particle size 0.006 µm that is dominated by Si element (52.81%) with magnetic moment value = 34.1 emu/g. The stability test shows that this material stable in an acid condition. The adsorption of chromate ions was conducted by the SPA method. Optimal pH obtained by pH range 4-7 with more than 90% adsorbed chromate ions. Variation of increasing series flow rate from 0.05 to 1.5 mL min-1 resulted in decreased adsorbed chromate ions. The use of SPA methods offered simpler and easier handling than the batch method without overriding the adsorption process effectiveness.

Hexavalent Chromium Release in Drinking Water Distribution Systems: New Insights into Zerovalent Chromium in Iron Corrosion Scales

Upon cast iron corrosion in contact with residual disinfectants, drinking water distribution systems have become potential geogenic sources for hexavalent chromium Cr(VI) release. This study investigated mechanisms of Cr(VI) release from cast iron corrosion scales. The oxidation of the corrosion scales by residual disinfectant chlorine released Cr(VI) and exhibited a three-phase kinetics behavior: an initial 2 h fast reaction phase, a subsequent 2-to-12 h transitional phase, and a final 7-day slow reaction phase approximately 2 orders of magnitude slower than the initial phase. X-ray absorption spectroscopy analysis discovered that zerovalent Cr(0) coexisted with trivalent Cr(III) solids in the corrosion scales. Electrochemical corrosion analyses strongly suggested that Cr(0) in the corrosion scales originated from Cr(0) in the cast iron alloy. Cr(0) exhibited a much higher reactivity than Cr(III) in the formation of Cr(VI) by chlorine. The presence of bromide in drinking water significantly accelerated Cr(VI) release because of its catalytic effect. Meanwhile, chlorine consumption was mainly attributed to the oxidation of organic matter and ferrous iron. Findings from this study point to a previously unknown but important pathway of Cr(VI) formation in drinking water, that is, direct oxidation of Cr(0) by chlorine, and suggest new strategies to control Cr(VI) in drinking water by inhibiting Cr(0) reactivity.

Comprehensive review of the basic chemical behaviours, sources, processes, and endpoints of trace element contamination in paddy soil-rice systems in rice-growing countries

Rice is the leading staple food for more than half of the world's population, and approximately 160 million hectares of agricultural area worldwide are under rice cultivation. Therefore, it is essential to fulfil the global demand for rice while maintaining food safety. Rice acts as a sink for potentially toxic metals such as arsenic (As), selenium (Se), cadmium (Cd), lead (Pb), zinc (Zn), manganese (Mn), nickel (Ni), and chromium (Cr) in paddy soil-rice systems due to the natural and anthropogenic sources of these metals that have developed in the last few decades. This review summarizes the sources and basic chemical behaviours of these trace elements in the soil system and their contamination status, uptake, translocation, and accumulation mechanisms in paddy soil-rice systems in major rice-growing countries. Several human health threats are significantly associated with these toxic and potentially toxic metals not only due to their presence in the environment (i.e., the soil, water, and air) but also due to the uptake and translocation of these metals via different transporters. Elevated concentrations of these metals are toxic to plants, animals, and even humans that consume them regularly, and the uniform deposition of metals causes a severe risk of bioaccumulation. Furthermore, the contamination of rice in the global rice trade makes this a critical problem of worldwide concern. Therefore, the global consumption of contaminated rice causes severe human health effects that require rapid action. Finally, this review also summarizes the available management/remediation measures and future research directions for addressing this critical issue.

The Common Ice Plant (Mesembryanthemum crystallinum L.)-Phytoremediation Potential for Cadmium and Chromate-Contaminated Soils

The common ice plant (Mesembryanthemum crystallinum L.) is a widely studied model due to its tolerance to numerous biotic and abiotic stresses. In this study, carried out in model pots, the plants were treated with variant doses of Cd(II) and Cr(VI) and proved resistant to extreme levels of these heavy metals. Initial toxicity symptoms were observed upon final concentrations of 818 mg Cd kg-1 soil d.w., and 1699 mg Cr kg-1 applied as potassium chromate. Biometric analyses revealed that none of the Cr(VI) doses affected dry weight of the plant organs thus maintaining the shoot-to-root ratio. The Cd and Cr hypertolerance strategies were divergent and resulted in different accumulation patterns. For the case of Cd(II), an excluder-like mechanism was developed to prevent the plant from toxicity. For chromate, high accumulation potential together with Cr(VI) root-to-shoot translocation at sublethal concentrations was revealed (up to 6152 mg Cr kg-1 shoot at 4248 mg Cr kg-1 soil). It is concluded that M. crystallinum reveals considerable phytoremediation capabilities due to unique growth potential in contaminated substrates and is suitable for bioreclamation of degraded soils. The plant is especially applicable for efficient phytoextraction of chromate-contamination, whereas for Cd-affected areas it may have a phytostabilizing effect.

Contamination of heavy metals in poultry eggs: a study presenting relation between heavy metals in feed intake and eggs

This study aims to quantify heavy metals (Ni, Pb, Zn, Mn, Cr, Cu and Se) in eggs obtained from poultry farms and backyard raised hens by comparing the concentration of metals in their feed intake. Overall, 90 samples of egg and 12 samples of poultry feed (6 each with food and water) were collected from 3 different poultry farms and backyards located in peri-urban areas of Lahore. A di-acid digestion method was adopted for digestion, after which digested samples were analyzed under atomic absorption spectrophotometer. Results showed that the concentration of Pb, Cr and Se in egg white (Pb = 0.6578, Cr = 0.18 and Se = 0.2161), egg yolk (Pb = 0.7011, Cr = 0.2617 and Se = 0.2656), feed (Pb = 2.585, Cr = 1.3039 and Se = 0.9411) and water (Pb = 0.5483, Cr = 0.1006 and Se = 0.3461) were above permissible limits in both poultry farms (study group 1) and backyards (study group 2). The concentration of metals such as Pb, Mn, Cr, Cu and Se in poultry farms eggs were higher than backyard hen eggs, which may be due to the intake of contaminated feed. So, the current study concluded that the higher concentration of metals in eggs has a positive correlation with the intake of feed contaminated with heavy metals.

Biosorption of Cr(VI) by Halomonas sp. DK4, a halotolerant bacterium isolated from chrome electroplating sludge

This study evaluated Cr(VI) biosorption by a halotolerant gram-negative bacterium Halomonas sp. DK4 isolated from chrome electroplating sludge. The bacterium could withstand high concentrations of Cr(VI) exhibiting a minimal inhibitory concentration (MIC) of 250 mg/L. Plackett-Burman design confirmed glucose, KH₂PO₄, NaCl, inoculum size, and initial Cr(VI) concentration as significant variables influencing the Cr(VI) removal ability of the bacterium. The suspended culture of Halomonas sp. DK4 was able to remove 81% (100 mg/L) of Cr(VI) in optimized MSM medium from aqueous solutions within 48 h. The bacterium also removed 59% Cr(VI) in the presence of 15% NaCl concentration within 72 h. The main mechanism involved in Cr(VI) removal by Halomonas sp. DK4 was determined to be biosorption which was best explained using the Langmuir isotherm model, wherein the maximum adsorption of 150.7 mg/g was observed under equilibrium conditions. Kinetic studies reveal that chemisorption of Cr(VI) by Halomonas sp. DK4 was a rate-limiting process which followed pseudo-second-order kinetics (R² = 0.99). Bacterial biomass exhibited maximum adsorption of 70.3% Cr(VI) at an initial concentration of 100 mg/L under optimal conditions. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of hydroxyl, carboxyl, amide, and phosphate groups on the bacterial surface which may be involved in Cr(VI) adsorption. Scanning electron microscopy coupled energy dispersive X-ray (SEM-EDX) analysis revealed morphological changes in the bacterial cell and accumulation of Cr(VI) on the cell surface. These results suggest the potential application of Halomonas sp. DK4 in the removal of Cr(VI) from saline chromium-containing industrial wastewaters.

Recovery of chromium, copper and vanadium combined with electricity generation in two-chambered microbial fuel cells

Microbial fuel cells (MFCs) offer a promising solution towards recovery and treatment of heavy metal pollutants. In this study, two-chambered MFCs were employed for recovery of chromium, copper and vanadium (Cr (VI), Cu (II) and V (V)). One g/L concentrations of K2Cr2O7, CuCl2 and NaVO3 served as catholytes, while a mixed culture was used as anolyte. Cr (VI), Cu (II) and V (V) were reduced biologically into less toxic forms of Cr (III), Cu and V (IV) respectively. Power density and cathodic efficiency were calculated for each of the catholytes. Cr (VI) gave the maximum power density and cathodic efficiency due to its high redox potential. Current produced depended on the concentration of the catholyte. Over a period of time, biological reduction of catholytes lead to decrease in the metal concentrations, which demonstrated the application of MFC technology towards heavy metal treatment and recovery in a reasonably cost-effective manner.

Chromium Pollution in European Water, Sources, Health Risk, and Remediation Strategies: An Overview

Chromium is a potentially toxic metal occurring in water and groundwater as a result of natural and anthropogenic sources. Microbial interaction with mafic and ultramafic rocks together with geogenic processes release Cr (VI) in natural environment by chromite oxidation. Moreover, Cr (VI) pollution is largely related to several Cr (VI) industrial applications in the field of energy production, manufacturing of metals and chemicals, and subsequent waste and wastewater management. Chromium discharge in European Union (EU) waters is subjected to nationwide recommendations, which vary depending on the type of industry and receiving water body. Once in water, chromium mainly occurs in two oxidation states Cr (III) and Cr (VI) and related ion forms depending on pH values, redox potential, and presence of natural reducing agents. Public concerns with chromium are primarily related to hexavalent compounds owing to their toxic effects on humans, animals, plants, and microorganisms. Risks for human health range from skin irritation to DNA damages and cancer development, depending on dose, exposure level, and duration. Remediation strategies commonly used for Cr (VI) removal include physico-chemical and biological methods. This work critically presents their advantages and disadvantages, suggesting a site-specific and accurate evaluation for choosing the best available recovering technology.

Chromium resistant microbes and melatonin reduced Cr uptake and toxicity, improved physio-biochemical traits and yield of wheat in contaminated soil

Melatonin and metal resistant microbes can enhance plant defense responses against various abiotic stresses, but little is known about the combined effects of melatonin and chromium (Cr) resistant microbes on reducing Cr toxicity in wheat (Triticum aestivum L.). In current study, we examined the effects of combined application of melatonin (0, 1, 2 mM) and Bacillus subtilis (with and without inoculation) on wheat physio-biochemical responses and Cr uptake under different levels of Cr (0, 25, 50 and 100 mg Cr kg^-1 DM soil). Chromium stress decreased the wheat growth, biomass, chlorophyll and relative water contents by causing oxidative damage in the form of overproduction of electrolyte leakage, hydrogen peroxide and malondialdehyde. However, foliar application of melatonin enhanced the plant growth, biomass and photosynthesis by alleviating the oxidative damage and Cr accumulation by plants. Melatonin significantly increased the enzymatic and non-enzymatic antioxidant activities as compared with respective control. Inoculation with microbes further enhanced the positive impacts of melatonin on wheat growth and reduced the Cr uptake by plants. Compared with non-inoculation and melatonin treatment, the inoculation with B. subtilis increased cholorophyll a by 27%, cholorophyll b by 49%, ascorbic acid in leaves by 50% and soluble proteins by 72% in wheat grwon with 50 mg Cr kg^-1 DM soil. The application of B. subtilis reduced oxidative stress and Cr toxicity by transforming the Cr⁶⁺ to Cr³⁺ in shoots and roots of wheat. Furthermore, B. subtilis reduced the Cr⁶⁺ uptake by wheat plants. The result of the present study revealed that the combined application of melatonin and B. subtilis might be a feasible approach aiming to reduce the Cr toxicity and its accumulation by wheat and probably in other plants.

Consumption of Raw Herbal Medicines Is Associated with Major Public Health Risks amongst Ugandans

Background

Community consumption of herbal plants in developing countries is a common practice, however, scarcity of information on their physiochemical composition is a major public health concern. In Uganda, Vernonia amygdalina is of interest in rural communities due to its therapeutical action on both bacterial and protozoal parasites, however no studies have been conducted to assess the heavy metal concentrations in traditional plants used in alternative medicine. The aim of the study was to establish concentrations of heavy metals in Vernonia amygdalina, model the estimated daily intake (EDI), and assess both the non-cancer-related health risk using the target hazard quotient (THQ), and the risk related to cancer through the incremental lifetime cancer risk (ILCR) for the Ugandan population.

Methods

Leaves of Vernonia amygdalina were collected from 20 georeferenced villages and processed into powder in the laboratory using standard methods. These were then analyzed in the laboratory using an atomic absorption spectrometer for lead (Pb), chromium (Cr), copper (Cu), zinc (Zn), cobalt (Co), iron (Fe), cadmium (Cd), and nickel (Ni). Concentrations were compared against the World Health Organization (WHO) limits. The EDI, THQ, and ILCR were modelled and significance was measured at 95% confidence.

Results

The study showed that mean ± SEM concentrations of heavy metals were highest in the order of Cr, 121.8 ± 4.291 ppm > Ni, 84.09 ± 2.725 ppm > Zn, 53.87 ± 2.277 ppm > Pb, 40.61 ± 3.891 ppm > Cu, 28.75 ± 2.202 ppm > Fe, 14.15 ± 0.7271 ppm > Co, 7.923 ± 0.7674 ppm > Cd, 0.1163 ± 0.005714 ppm. Concentrations of Pb, Cr, Zn, Co, and Ni were significantly higher than the WHO limits. The EDI was significantly higher in children than in adults, demonstrating an increased risk of toxicity in children. The THQ and ILCR were over 1000 times higher in all Ugandans, demonstrating the undesirable health risks following oral consumption of Vernonia amygdalina due to very high Cr and Ni toxicities, respectively.

Conclusion

Consumption of raw Vernonia amygdalina was associated with a high carcinogenic risk, demonstrating a need to enact policies to promote physiochemical screening of herbal medicines used in developing countries against toxic compounds.

Characteristics and applications of biochar for remediating Cr(VI)-contaminated soils and wastewater

Chromium (Cr) is a common environmental contaminant due to industrial processes and anthropogenic activities such as mining of chrome ore, electroplating, timber treatment, leather tanning, fertilizer and pesticide, etc. Cr exists mainly in both hexavalent [Cr(VI)] and trivalent [Cr(III)] form, being Cr(VI) with non-degradability and potential to be hidden, thereby affecting surrounding environment and being toxic to human health. Therefore, researches on remediation of Cr pollution in the environment have received much attention. Biochar is a low-cost adsorbent, which has been identified as a suitable material for Cr(VI) immobilization and removal from soil and wastewater. This review incorporates existing literature to provide a detailed examination into the (1) Cr chemistry, the source and current status of Cr pollution, and Cr toxicity and health; (2) feedstock and characterization of biochar; (3) processes and mechanisms of immobilization and removal of Cr by biochar, including oxidation-reduction, electrostatic interactions, complexation, ion exchange, and precipitation; (4) applications of biochar for Cr(VI) remediation and the modification of biochar to improve its performance; (5) factors affecting removal efficiency of Cr(VI) with respect to its physico-chemical conditions, including pH, temperature, initial concentration, reaction time, biochar characteristics, and coexisting contaminants. Finally, we identify current issues, challenges, and put forward recommendations as well as proposed directions for future research. This review provides a thorough understanding of using biochar as an emerging biomaterial adsorbent in Cr(VI)-contaminated soils and wastewater.

Chromium Removal from Tannery Wastewater by Electrocoagulation: Optimization and Sludge Characterization

The treatment of tannery effluent is of great interest as it contains a complex mixture of pollutants, primarily chromium. The disposal of this wastewater can have adverse effects on the environment and aquatic life, which is an emerging problem for the environment. In this work, electrocoagulation is used to remove chromium from real tannery wastewater, focusing on performance optimization and sludge characterization. Electrocoagulation experiments were conducted using an electrochemical cell with iron electrodes immersed in a specific volume of tannery wastewater. Operating parameters, such as the initial chromium concentration, pH and current density as well as power consumption were evaluated to determine optimum chromium removal. The optimization was performed using Response Surface Methodology combined with central composite design. Analysis of variance (ANOVA) was used to determine the response, residual, probability, 3D surface and contour plots. The maximum chromium removal was 100% at the optimum values of 13 mA/cm2, 7 and 750 ppm for current density, pH and concentration, respectively.

Cyclic Sequential Removal of Alizarin Red S Dye and Cr(VI) Ions Using Wool as a Low-Cost Adsorbent

Alizarin red S (ARS) removal from wastewater using sheep wool as adsorbent was investigated. The influence of contact time, pH, adsorbent dosage, initial ARS concentration and temperature was studied. Optimum values were: pH = 2.0, contact time = 90 min, adsorbent dosage = 8.0 g/L. Removal of ARS under these conditions was 93.2%. Adsorption data at 25.0 °C and 90 min contact time were fitted to the Freundlich and Langmuir isotherms. R2 values were 0.9943 and 0.9662, respectively. Raising the temperature to 50.0 °C had no effect on ARS removal. Free wool and wool loaded with ARS were characterized by Fourier Transform Infrared Spectroscopy (FTIR). ARS loaded wool was used as adsorbent for removal of Cr(VI) from industrial wastewater. ARS adsorbed on wool underwent oxidation, accompanied by a simultaneous reduction of Cr(VI) to Cr(III). The results hold promise for wool as adsorbent of organic pollutants from wastewater, in addition to substantial self-regeneration through reduction of toxic Cr(VI) to Cr(III). Sequential batch reactor studies involving three cycles showed no significant decline in removal efficiencies of both chromium and ARS.

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

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

Interaction mechanism of dissolved Cr(VI) and manganite in the presence of goethite coating

Hexavalent chromium has aroused a series of environmental concerns due to its high mobility and toxicity. Iron and manganese oxides usually coexist in the environments and influence the speciation and geochemical cycling of chromium. However, the interaction mechanism of iron-manganese oxides with dissolved Cr(VI) remains largely unknown. In this work, the interaction processes of dissolved Cr(VI) and manganite in the presence of goethite coating were investigated, and the effects of pH (2.0-9.0) and iron oxide content were also studied. Manganite-goethite composites were formed with uniform micromorphologies in the system of manganite and Fe(II). In the reaction system of single manganite and Cr(VI), manganite could only adsorb but not reduce Cr(VI), with the adsorption amount decreasing at higher pHs. In the reaction system of manganite-goethite composites and Cr(VI), adsorbed Cr(VI) was reduced to Cr(III) by Fe(II) on composites surface. The generated Cr(III) was then retained as Cr(OH)₃ on the mineral surface. Goethite coating suppressed the re-oxidation of newly formed Cr(III) by manganite. The amounts of adsorbed Cr(VI) and generated Cr(III) increased with increasing iron oxide content, and increased first and then decreased with increasing pH. The Cr(III) formation and Cr(VI) adsorption amount reached the maximum at pH 5.0-6.0. The present work highlights the transformation and retention of Cr(VI) by iron-manganese oxides and provides potential implications for the use of such oxides in the remediation of Cr(VI) polluted waters and soils.

Enhanced biogeogenic controls on dichromate speciation in subsoil containment

In general, lab-based Cr (VI) reduction studies do not often corroborate the prevailing biogeochemical controls for on-site pollution abatement. To promulgate its importance, herein, we investigate the existing biogeogenic parameters of a contaminated site to attenuate the underground Cr (VI) toxicity. This study significantly assesses the speciation of dichromate by biogenic agents that are inherent and self-sustaining to treat the contaminated soil. Herein, a group of bacteria exposed to high concentrations of chromium (≥3500 mg/L) plays a vital role as an enhanced biogeogenic control for the detoxification of toxic Cr (VI). All identified bacteria were screened based on their ability to differentiate from extracellular speciation and harvested in a Cr (VI)-enriched molasses to achieve dichromate concentrations as low as 0.05 mg/L in 168 h. Under low O₂ condition, the bacterial growth rate and doubling time were monitored to establish the half-life period of Cr (VI) for adequate containment treatment. Furthermore, to understand the soil decontamination, Cr (VI) reactive transport was demonstrated to facilitate the contaminant reduction under both saturated and unsaturated groundwater conditions. Herein, Cr (VI) speciation to Cr (III) by the influence of abiogenic factors are unlikely or less probable as studied in existing geogenic conditions. Moreover, the evidence of biogenic reduction of Cr (VI) in microcosm suggests its effectiveness in enhanced detoxification of Cr (VI) up to ≤ 0.1 mg/L, within the reaction period of 144 h and 192 h, for saturated and unsaturated flow conditions, respectively.

Simultaneous removal of Cr(III) and V(V) and enhanced synthesis of high-grade rutile TiO2 based on sodium carbonate decomposition

Rutile TiO₂ is widely applied as the raw material to produce titanium dioxide and titanium sponge, whereas the Cr (III) and V (V) impurities in rutile TiO₂ significantly affect the performance of related products. In the present work, the sodium carbonate decomposition treatment on Panzhihua titanium slag was attempted, to improve the preparation process of rutile TiO₂ with high crystallinity and simultaneously reduce the chromium (Cr) and vanadium (V) content as hazardous elements. Effects of sodium carbonate decomposition treatment on the crystal composition, microstructure of rutile TiO₂ were determined using XRD, SEM and Raman characterization. The recovery of Cr(III) and V(V) was achieved through leaching the roasted titanium slag by dilute sulfuric acid, with the chromium and vanadium content in the residue decreasing up to 0.03 % and 0.04 %, respectively, followed by the final product rutile TiO₂ was produced by the leaching residue calcined at 1323.15 K with a duration time of 120 min, with 85.56 % of TiO₂ grade. The work highlights the feasibility of synchronously preparing rutile TiO₂ and removing hazardous Cr (III) and V (V) impurities from titanium slag using sodium carbonate decomposition.

Polyethylenimine-stabilized silver nanoclusters act as an oxidoreductase mimic for colorimetric determination of chromium(VI)

A new and efficient assay is proposed for the photometric determination of Cr⁶⁺ by employing polyethylenimine-stabilized Ag nanoclusters (PEI-AgNCs) as an oxidoreductase mimic. Cr⁶⁺ with certain oxidicability is able to specifically react with 3,3',5,5'-tetramethylbenzidine (TMB), giving a color change from colorless to blue indicating the presence of Cr⁶⁺. However, the redox kinetics is so slow that the sensitivity obtained for Cr⁶⁺ determination is very poor. It is interestingly found that PEI-AgNCs can act as an oxidoreductase-like nanozyme to significantly promote the sluggish reaction, making it possible to rapidly detect toxic Cr⁶⁺ with remarkably enhanced performance. With the use of PEI-AgNCs, fast and convenient determination of Cr⁶⁺ was realized, with a limit of detection as low as 1.1 μM. Additionally, the proposed assay exhibited excellent selectivity; other ions, including Cr³⁺, hardly affected the determination of Cr⁶⁺. Graphical abstract Polyethylenimine-stabilized silver nanoclusters (PEI-AgNCs) act as an oxidoreductase mimic to catalyze the redox reaction of Cr⁶⁺ and 3,3',5,5'-tetramethylbenzidine (TMB), enabling the high-performance colorimetric determination of toxic Cr⁶⁺.

Chromium (III) coordination capacity of chitosan

Polycationic chitosan has a strong coordination to heavy metal ions due to its multifunctional hydroxyl and amino groups. However, due to the fast and facile dissolution of chitosan in acidic medium, it is difficult to measure the exact adsorption amount or coordination capacity accurately. In this work, a simple method of lyophilization plus ethanol-washing was employed to separate and purify chitosan/Cr(III) complex for further determining the coordination capacity. Meanwhile, the coordination structure of Bridge-chitosan-N(OH)₃(H₂O) and morphology of regenerated fibrillar sponge of CS/Cr(III) were further certified. The coordination capacity of Cr(III) on chitosan increased with the rising concentration of Cr(III) ions till the maximum coordination capacity was reached up to 355.03 mg/g. The mechanisms and characteristic parameters of the adsorption process were fit using two-parameter isotherm models which revealed the following order (based on the coefficient of determination) of Langmuir > Halsey > Freundlich > Temkin > Dubinin-Radushkevich. A proposed coordination formula of CS/Cr (III) might be a good certificate for the homogeneous chemical combination nature of Cr(III) on the monolayer surface of chitosan in a molecular scale.

Occurrence and distribution of hexavalent chromium in groundwater from North Carolina, USA

Hexavalent chromium (Cr(VI)) is a groundwater contaminant that is potentially harmful to human health. Understanding the occurrence of Cr(VI) in groundwater resources is critical for evaluating its risks to human health. Here we report a large dataset (n = 1362) of Cr(VI) and total chromium (CrT) concentrations in public, private, and monitoring wells from different aquifers across North Carolina. These water quality data come from new and previous measurements conducted at Duke University, as well as data reported by the U.S. Environmental Protection Agency, the N.C. Department of Environmental Quality, and the U.S. Geological Survey. The data confirm that Cr(VI) is the predominant species of dissolved Cr and that groundwater from aquifers in the Piedmont region contain significantly higher concentrations than groundwater from the coastal plain. Though there is only one exceedance of the U.S. EPA Maximum Contaminant Level (100 µg/L for CrT) in the dataset, over half of all wells measured for Cr(VI) (470 out of 865) in the dataset exceeded the N.C. Health Advisory Level of 0.07 µg/L. Using information from this dataset, we explore three different approaches to predicting Cr(VI) in groundwater: (1) CrT concentrations as a proxy for Cr(VI); (2) Exceedance probabilities of health goals for groundwater from aquifers located in specific geologic areas; and (3) Censored linear regression using commonly measured field parameters (pH, electrical conductivity, dissolved oxygen) with relationships to Cr(VI) as regressors. Combining these approaches, we have identified several areas in the Piedmont region where Cr(VI) in drinking water wells is expected to be higher than the advisory level, which coincide with large population groundwater reliant populations. While this study focuses on N.C., the wide-spread occurrence of Cr(VI) in groundwater at concentrations above health guidelines in aquifers of the Piedmont region could pose high human health risks to large populations in the eastern U.S.

A review on the use of chelating agents as an alternative to promote photo-Fenton at neutral pH: Current trends, knowledge gap and future studies

In this review, we have critically examined the alternatives to conventional photo-Fenton process such as the strategies to perform it in circumneutral pH in the so-called photo-Fenton like process. They include iron chelation, iron replacement with another metal and use of iron immobilized on surfaces of solid materials, use of iron oxides, among others. The use of such strategies can be employed to overcome the challenges identified in conventional photo-Fenton, moreover, advantages and drawback of each technique must be clarified and the recent achievements should be shared with the scientific community. The use of a chelating agent to make iron soluble at circumneutral pH presents many advantages when compared to other current techniques. However, the correct understanding of the chelating process, complex activity and the complex resistance along with the mechanism of radical production should be taken into account to prepare an effective photo-Fenton with complexed iron. The review also identifies the current trends in chelate assisted photo-Fenton process and the unexplored areas in this field of study. A discussion about the environmental and safety issues in the application of these methods, with emphasis to the Fe chelation strategy, was also considered with detailed review over the past ten years.

Promising Biocompatible, Biodegradable, and Inert Polymers for Purification of Wastewater by Simultaneous Removal of Carcinogenic Cr(VI) and Present Toxic Heavy Metal Cations: Reduction of Chromium(VI) by Poly(ethylene glycol) in Aqueous Perchlorate Solutions

A spectrophotometric technique has been applied for studying the reduction of chromium(VI) by poly(ethylene glycol) (PEG) as water-soluble and nontoxic synthetic polymer at a constant ionic strength of 4.0 mol dm^-3 in the absence and presence of the ruthenium(III) catalyst. In the absence of the catalyst, the reaction orders in [Cr(VI)] and [PEG] were found to be unity and fractional first orders, respectively. The oxidation process was found to be acid-catalyzed with fractional second order in [H⁺]. The addition of Ru(III) was found to catalyze the oxidation rates with observation of zero-order reaction in [CrO₄ ^2-] and fractional orders in both [PEG] and [Ru(III)], respectively. The PEG reduces the soluble toxic hexavalent Cr(VI) as a model pollutant to the insoluble nontoxic Cr(III) complex, which is known to be eco-friendly and more safer from the environmental points of view. The acid derivative of PEG was found to possess high affinity for the removal of poisonous heavy metal ions from contaminant matters by chelation. Formation of the 1:1 intermediate complex has been kinetically revealed. A consistent reaction mechanism of oxidation was postulated and discussed.

Characterization of heavy metal pollution in an anthropogenically and geologically influenced semi-arid region of east India and assessment of ecological and human health risks

The present study evaluated the heavy metal contamination in groundwater and associated ecological and human health risks of a geologically and anthropogenically diverse semi-arid region of Birbhum district, India. For a reliable evaluation, concentrations of nine heavy metals in 680 groundwater samples (N = 680) which were collected during premonsoon and postmonsoon seasons of consecutive two years from 170 wells were measured. The human health risk assessment using the USEPA model which is based on single value for each parameter may inherit certain inaccuracy and uncertainties in the evaluation. Unlike earlier studies, a higher degree of accuracy in carcinogenic and noncarcinogenic health risk assessments was achieved through Monte Carlo simulations, sensitivity analysis and uncertainty analysis. The study revealed the occurrence of the target heavy metals in groundwater with mean dominance order of Fe > Zn > Sr > Mn > Cr > Pb > Ni > Cu > Cd where mean concentrations of the carcinogens, Pb and Fe exceeded their maximum permissible limits. The water quality status evaluated using the modified heavy metal pollution index, Nemerow index and Heavy metal evaluation index methods resulted in medium to high heavy metal contamination in groundwater within a large portion of the study area which indicated its unsuitability for drinking purpose. The study suspects a moderate to very high risk for the groundwater dependent ecosystems in major part of the study area. The study further revealed cancer risks, ranging from high to very high within the residents due to accumulative exposure of the carcinogenic heavy metals in groundwater through ingestion and dermal contact. Minor populations of the study area were found to be more vulnerable to the carcinogenic and noncarcinogenic diseases than teenagers and adults, mainly through oral exposure. The study recommends the residents to consume treated groundwater since the primary route of heavy metal exposure was identified to be the ingestion route.

Effects of metal cation substitution on hexavalent chromium reduction by green rust

Chromium contamination is a serious environmental issue in areas affected by leather tanning and metal plating, and green rust sulfate has been tested extensively as a potential material for in situ chemical reduction of hexavalent chromium in groundwater. Reported products and mechanisms for the reaction have varied, most likely because of green rust's layered structure, as reduction at outer and interlayer surfaces might produce different reaction products with variable stabilities. Based on studies of Cr(III) oxidation by biogenic Mn (IV) oxides, Cr mobility in oxic soils is controlled by the solubility of the Cr(III)-bearing phase. Therefore, careful engineering of green rust properties, i.e., crystal/particle size, morphology, structure, and electron availability, is essential for its optimization as a remediation reagent. In the present study, pure green rust sulfate and green rust sulfate with Al, Mg and Zn substitutions were synthesized and reacted with identical chromate (CrO₄^2-) solutions. The reaction products were characterized by X-ray diffraction, pair distribution function analysis, X-ray absorption spectroscopy and transmission electron microscopy and treated with synthetic δ-MnO₂ to assess how easily Cr(III) in the products could be oxidized. It was found that Mg substitution had the most beneficial effect on Cr lability in the product. Less than 2.5% of the Cr(III) present in the reacted Mg-GR was reoxidized by δ-MnO₂ within 14 days, and the particle structure and Cr speciation observed during X-ray scattering and absorption analyses of this product suggested that Cr(VI) was reduced in its interlayer. Reduction in the interlayer lead to the linkage of newly-formed Cr(III) to hydroxyl groups in the adjacent octahedral layers, which resulted in increased structural coherency between these layers, distinctive rim domains, sequestration of Cr(III) in insoluble Fe oxide bonding environments resistant to reoxidation and partial transformation to Cr(III)-substituted feroxyhyte. Based on the results of this study of hexavalent chromium reduction by green rust sulfate and other studies, further improvements can also be made to this remediation technique by reacting chromate with a large excess of green rust sulfate, which provides excess Fe(II) that can catalyze transformation to more crystalline iron oxides, and synthesis of the reactant under alkaline conditions, which has been shown to favor chromium reduction in the interlayer of Fe(II)-bearing phyllosilicates.

A review of chromite mining in Sukinda Valley of India: impact and potential remediation measures

Sukinda Valley, one of the highly polluted areas of the world is generating tons of mining waste and causing serious health and environmental issues in its surroundings. Several reports are available reporting the severity of hexavalent chromium, yet little efforts have been made to address the pollution and its remediation due to a lack of proper remedial measures. The review highlights the pros and cons of various physical, chemical and biological techniques used worldwide for the treatment of chromium waste and also suggests better and reliable bioremediation measures. Microbes such as Acidophilium and Acidithiobacillus caldus (Bioleaching), Pseudomonas, Micrococcus and Bacillus (Bioreduction), Aereobacterium and Saccharomyces (Biosorption), are widely used for bioremediation of hexavalent chromium owing to their unique metabolic activities, ionic movement through an extracellular membrane, and other cellular adsorptions and reduction properties. The use of native and hybrid combinations of microbes supported by organic supplements is projected as a fast and efficient technique that not only reduces chromium quantity but also maintains the integrity of the microbial sources. Innovation and emphasis on nano-based products like nanocomposite, nano adsorbent, nanoscale zerovalent iron (nZVI) particles and multifunctional plant-growth-promoting bacteria (PGPB) will serve as the next generation environmental remediation technologies in the near future.

Amido-amine derivative of alginic acid (AmAA) for enhanced adsorption of Pb(II) from aqueous solution

The present work reports the alternate synthesis of amido-amine derivative of alginic acid (AmAA) with high degree of functionalization. The AmAA have been characterized for percentage functionalization, functional group change, surface morphology and thermal decomposition behavior. The results indicate that the amido-amine derivatisation of alginic acid (AA) with >95% functionalization, significantly improves its Pb(II) adsorption efficiency (395.72 mg/g to 535.87 mg/g) over the AA. The equilibrium and kinetic studies showed that Langmuir and Freundlich adsorption isotherm models fitted well to the experimental data, and these followed pseudo-second order kinetic model. The FTIR (Fourier transform infrared spectroscopy) and ¹³C CP-MAS NMR (Cross-polarization magic angle spinning carbon-13 solid state nuclear magnetic resonance spectroscopy) analysis revealed that Pb(II) binds to the carboxyl group in case of AA and to the carbonyl & amine group in case of AmAA, which leads to increase in its adsorption efficiency. The study concludes that the functionalization of amido-amine on AA improves its adsorptive efficiency for Pb(II) from aqueous medium.