Chromium speciation in solid matrices and regulation: a review

Efficient selective adsorption of Cr(VI) by S-doped porous carbon prepared from industrial lignin: Waste increment and wastewater treatment

Industrial lignin is a waste product of the paper industry, which contains a large amount of oxygen group structure, and can be used to treat industrial wastewater containing Cr(VI). However, lignin has very low reactivity, so how to enhance its adsorption performance is a major challenge at present. In this study, a two-stage hydrothermal and activation strategy was used to activate the lignin activity and doping S element to prepare high-performance S-doped lignin-based polyporous carbon (S-LPC). The results show that the surface of S-LPC is rich in S and O groups and has a well-developed pore structure, which is very beneficial to Cr(VI) uptake -reduction and mass transfer on the material. In the wastewater, the utmost adsorption potential of Cr(VI) by S-LPC achieved 882.83 mg/g. After 7 cycles of regeneration, the adsorption of S-LPC decreased by only approximately 18 %. Ion competition experiments showed that S-LPC has excellent specificity for Cr(VI) adsorption. In factory wastewater, the adsorption performance of S-LPC for Cr(VI) remained above 95 %, which shows the excellent performance of S-LPC in practical applications. The results are of great significance for green chemical utilization of waste lignin, treatment of industrial wastewater and sustainable development.

A novel synthesis method of magnetic Janus particles for wastewater applications

HYPOTHESIS

Magnetic particles are widely used in many adsorption and removal processes. Among the many types of magnetic colloids, magnetic Janus particles offer significant possibilities for the effective removal of several components from aqueous solutions. Nevertheless, the synthesis of structures integrating different types of materials requires scalable fabrication processes to overcome the limitations of the available methodologies. Herein, we hypothesized a fabrication process for dual-surface functionalized magnetic Janus particles.

EXPERIMENTS

The primary silica particles with surface-attached amine groups are further asymmetrically modified by iron oxide nanoparticles, exploiting Pickering emulsion and electroless deposition techniques. The dual surface functionality of the particles is designed for its versatility and demonstrated in two wastewater-related applications.

FINDINGS

We show that our design can simultaneously remove chromium (VI) and phenol from aqueous solution. The fabricated magnetic-responsive Janus particles are also an effective adsorbent for genomic Deoxyribonucleic acid (DNA) and show superior performance to commercial magnetic beads. Thus, this study provides a novel platform for designing magnetic Janus particles with multifunctional surfaces for wastewater treatment applications.

Carcinogenic metal(loid)s in house dust compared to soil: Concentrations and gastric bioaccessibility

As a sink and a source of chemicals, house dust represents a relevant medium to assess indoor exposure to metal(loid)s via incidental ingestion or inhalation. However, nationally representative indoor data are scarce. Results from the Canadian House Dust Study (CHDS, 2007-2010; n = 1025) provide nationally representative mean, median and 95th percentile concentrations for 38 elements in typical urban house dust, along with their gastric bioaccessibility. Total concentrations (median/95th percentile) of carcinogenic metal(loid)s in Canadian house dust (μg g-1) are as follows: As (9.0/40), Be (0.4/0.9), Cd (3.5/17), Co (5.6/19), Cr (99/214), Ni (62/322) and Pb (100/760). Total As and Pb concentrations in house dust exceed residential soil guidelines for the protection of human health in about one-third of Canadian homes. Percent bioaccessibilities (median) are: Cd (65%) > Pb (63%) > Be ∼ Ni (36%) > Co (35%) > As (20%) > Cr (15%). Lead, Cd and Co concentrations are significantly greater in older houses (< 1976). Data from two pilot studies (n = 66 + 51) further demonstrate the distinct geochemistry of house dust compared to soils, notably enrichment of carcinogenic metal(loid)s and their increased bioaccessibility. These results provide essential baseline values to refine risk assessment and inform on health risk at contaminated sites.

Implementing magnetically-active Sn-based nanocomposites in hexavalent chromium removal from drinking water

A novel nanocomposite consisting of Fe3O4-loaded tin oxyhydroxy-chloride is demonstrated as an efficient adsorbent for the removal of hexavalent chromium in compliance to the new drinking water regulation. This study introduces a continuous-flow production of the nanocomposite through the separate synthesis of (i) 40 nm Fe3O4 nanoparticles and (ii) multilayered spherical arrangements of a tin hydroxy-chloride identified as abhurite, before the application of a wet-blending process. The homogeneous distribution of Fe3O4 nanoparticles on the abhurite's morphology, features nanocomposite with magnetic response whereas the 10 % loaded nanocomposite preserves a Cr(VI) uptake capacity of 7.2 mg/g for residual concentrations below 25 μg/L. Kinetic and thermodynamic examination of the uptake evolution indicates a relative rapid Cr(VI) capture dominated by interparticle diffusion and a spontaneous endothermic process mediated by reduction to Cr(III). The efficiency of the optimized nanocomposite was validated in a pilot unit operating in a sequence of a stirring reactor and a rotary magnetic separator showing an alternative and competitive application path than typical fixed-bed filtration, which is supported by the absence of any acute cellular toxicity according to human kidney cell viability tests.

Novel method of removing metals from estuarine water using whole microbial mats

This study addresses the limited understanding of chromium-microbial mat interactions in estuarine tidal flats. The aims were to evaluate (1) the efficiency of the microbial consortium in Cr(III) removal from seawater; (2) the elemental and mineralogical composition of the microbial mat as a natural system in the Cr removal, (3) the effects of metal on microphytobenthos, and (4) possible interactions of Cr with other metals present in the consortium. Microbial mats were exposed to Cr(III) solutions at different concentrations (2-30 mg Cr/L). Analysis such as metal concentration, organic matter content, chlorophyll a and phaeopigment concentrations, abundance of diatoms and cyanobacteria, SEM-EDS, and XRD were performed. Most of the Cr(III) was deposited, as chromium oxide/hydroxide, on the surface of all microbial mat components. The complete microbial mat, comprising sediments, detritus, EPS, and diverse microorganism communities, exhibited a remarkable capacity to accumulate Cr(III), retaining over 87% in the solution.

Atomic-scale Ru anchored on chromium-shavings as a precursor for a pH-universal hydrogen evolution reaction electrocatalyst

In the leather manufacturing industry, the management of substantial quantities of solid waste containing chrome shavings remains a formidable challenge. Concurrently, there is a pressing need for the development of pH-universal and economically viable electrocatalysts for the hydrogen evolution reaction (HER). In response to these intertwined challenges, this study proposes an innovative approach wherein the amino groups present on the surface of chrome shavings are utilized to immobilize single ruthenium atoms during pyrolysis, thereby facilitating the synthesis of hydrogen evolution electrocatalysts. The optimized sample, denoted as CN/Cr2O3/Ru-1, demonstrates exceptional electrocatalytic performance, exhibiting an ultra-low overpotential of -28 mV in 1.0 M KOH at a current density of 10 mA cm-2, and it also exhibits good performance in acidic and neutral electrolytes. Importantly, these overpotentials surpass those reported for many previous ruthenium-based catalysts. Density functional theory (DFT) calculations elucidate that both oxygen (O) and chromium (Cr) moieties within Cr2O3 can engage in favorable interactions with the coordination patterns of the ruthenium (Ru) atoms, thereby elucidating the synergistic enhancement conferred by the chromium element in CN/Cr2O3/Ru, which ultimately facilitates and promotes the catalytic activity of the ruthenium atoms serving as the catalytic center. This facile synthesis route not only presents a green solution for addressing waste chromium pollutants but also offers a promising avenue for the development of high-performance, cost-efficient electrocatalysts.

La-doped MIL-88B(Fe)-NH2: a mixed-metal-organic framework photocatalyst for highly efficient reduction of Cr(vi) in an aqueous solution

With the aim to resolve the problem of water pollution, we herein propose a new photocatalyst based on metal-organic frameworks (MOFs), called La-doped MIL-88B(Fe)-NH2 (MIL-88B((1 - x)Fe/xLa)-NH2), which was designed and employed for the photocatalytic reduction of Cr(vi) in aqueous solutions. MIL-88B((1-x)Fe/xLa)-NH2 materials with different x values were synthesized via a one-pot solvothermal method. Their characteristics were investigated using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) analysis, Fourier-transform infrared (FT-IR) spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). We found that compared to pristine MIL-88B(Fe)-NH2 with a photocatalytic efficiency of 67.08, MIL-88B((1 - x)Fe/xLa)-NH2 materials with x = 0.010, 0.025 and 0.050 exhibit excellent photocatalytic efficiencies reaching 88.21, 81.19 and 80.26%, respectively, after only 30 minutes of irradiation at a small catalyst dosage of 0.2 g L-1. These La-doped MIL-88B(Fe)-NH2 photocatalysts can work well under mild conditions (pH = 6). Furthermore, they are robust-can be recycled for at least four consecutive runs without any activity loss. This novel material is promising for the photocatalytic degradation of pollutants.

Effective chromium mitigation using phosphorous doped bio carbon electrode via capacitive deionisation

Capacitive deionisation (CDI) is an emerging eco-economic water reclamation technology that can remove inorganic salts and heavy metals. Biomass-derived carbon electrodes have attracted the scientific communities in recent years due to their economic feasibility and sustainability. However, electrochemical performance needs to be improved to achieve durability and reusability. Hence, the present study develops rice straw-derived phosphorous-doped (P-doped) carbon as an electrode for mitigating Cr(VI) ions. Phosphorus doping of biocarbon electrodes enhances their electrochemical properties, including increased electrical conductivity, improved charge storage capacity, and enhanced ion adsorption capabilities. Here, Phosphoric acid plays a dual role of activation and doping that enhances the physico-electrochemical properties. The synthesised material was found to be P-doped carbon with better pore distribution, which was confirmed through FESEM-EDX analysis. Further, the physicochemical properties of the electrode material are enriched with carbon and possess an enhanced surface area of 753 m2/g. The cyclic voltammetry shows the specific capacitance of 67 F/g for the Cr(VI) ions, which was found to be 15 times more than the non-doped carbon. CDI studies were performed with optimisation of operational parameters and found that mitigation of Cr(VI) ions was efficient at pH 2 for the applied voltage of 2V. The electrode's performance with real-time chrome wash effluent confirms its potentiality and has better scope upon optimisation. The experimental data fits well with pseudo first-order kinetics, which ensures the nature of electrosorption is physisorption.

Synthesis and characterization of iron oxide-commercial activated carbon nanocomposite for removal of hexavalent chromium (Cr6+) ions and Mordant Violet 40 (MV40) dye

Iron Oxide-commercial activated carbon nanocomposite (CAC-IO) was prepared from commercial activated carbon (CAC) by the co-precipitation method, and the resulting nanocomposite was used as an adsorbent to remove hexavalent chromium (Cr6+) ions and Mordant Violet 40 (MV40) dye from wastewater. The produced materials (CAC, CAC after oxidation, and CAC-IO) were comparatively characterized using FTIR, BET, SEM, EDX TEM, VSM, and XRD techniques. The adsorption mechanism of Cr6+ ions and MV40 dye on CAC-IO was examined using Langmuir and Freundlich isotherm models.. Different models were applied to know the adsorption mechanism and it was obtained that Pseudo-second order fits the experimental data better. This means that the adsorption of the adsorbate on the nanocomposite was chemisorption. The maximum removal percent of Cr6+ ions by CAC-IO nanocomposite was 98.6% determined as 2 g L-1 adsorbent concentration, 100 mg L-1 initial pollutant concentration, solution pH = 1.6, the contact time was 3 h and the temperature was room temperature. The maximum removal percentage of Mordant Violet 40 dye (C.I. 14,745) from its solutions by CAC-IO nanocomposite was 99.92% in 100 mg L-1 of initial dye concentrations, 1.0 g L-1 of adsorbent concentration, solution pH = 2.07, the contact time was 3 h. The MV40 dye adsorption on CAC-IO was the most fitted to the Freundlich isotherm model. The maximum adsorption capacity was calculated according to the Langmuir model as 833.3 mg g-1 at 2 g L-1 of adsorbent concentration and 400 mg L-1 of initial MV40 dye concentration. The Cr6+ ions adsorption on CAC-IO was more fitted to the Freundlich model with Qmax, equal to 312.50 mg g-1 at 1 g L-1 adsorbent concentration and 400 mg L-1 of Cr6+ ions initial concentrations.

Evidence for the accumulation of toxic metal(loid)s in agricultural soils impacted from long-term application of phosphate fertilizer

Phosphate fertilizers may contain elevated concentrations of toxic metals and metalloids and therefore, their excessive application can result in the accumulation of both phosphorus (P) and metal(loid)s in agricultural soils. This study aims to investigate the occurrence, distribution, and potential plant-availability of metal(loid)s originating from phosphate fertilizer in a long-term experimental field at the Tidewater Research Station in North Carolina, where topsoil (10-20 cm deep) and subsoil (up to 150 cm deep) samples were collected from five plots with consistent and individually different application rates of P-fertilizer since 1966. We conducted systematic analyses of P and metal(loid)s in bulk soils, in the plant available fraction, and in four sequentially extracted soil fractions (exchangeable, reducible, oxidizable, and residual). The results show that P content in topsoils were directly associated with the rate of P-fertilizer application (ρ = 1, p < 0.05). Furthermore, P concentrations were highly correlated with concentrations of Cd, U, Cr, V, and As in the bulk topsoil (ρ > 0.58, p < 0.05), as well as the potential plant-available fraction (ρ > 0.67, p < 0.01), indicating the accumulation of the fertilizer-derived toxic metal(loid)s in the topsoil. Significant correlations (p < 0.001) of metal(loid)s concentrations between the bulk soil and the potential plant-available fraction raises the possibility that P-fertilizer application could increase the accumulation of toxic metal(loid)s in plants, which could increase human exposure. Results from sequential leaching experiments revealed that large portions of the trace elements, in particular Cd, occur in the soluble (exchangeable and reducing) fractions of topsoil with higher P-fertilizer input, whereas the levels of redox-sensitive elements (As, V, U, Cr) were higher in the reducible and oxidizable fractions of the soils. Overall, the data presented in this study demonstrate the effect of long-term P-fertilizer application on the occurrence and accumulation of a wide range of toxic metal(loid)s in agricultural topsoil.

The Metallotolerance and Biosorption of As(V) and Cr(VI) by Black Fungi

A collection of 34 melanized fungi isolated previously from anthropogenic contaminated sites were assessed for their tolerance to toxic concentrations of As(V) and Cr(VI) anions. Three strains of the species Cyphellophora olivacea, Rhinocladiella similis, and Exophiala mesophila (Chaetothyriales) were identified as hyper-metallotolerant, with estimated IC50 values that ranged from 11.2 to 16.9 g L-1 for As(V) and from 2.0 to 3.4 g L-1 for Cr(VI). E. mesophila and R. similis were selected for subsequent assays on their biosorption capacity and kinetics under different pH values (4.0 and 6.5) and types of biomass (active and dead cells and melanin extracts). The fungal biosorption of As(V) was relatively ineffective, but significant removal of Cr(VI) was observed from liquid cultures. The Langmuir model with second-order kinetics showed maximum sorption capacities of 39.81 mg Cr6+ g-1 for R. similis and 95.26 mg Cr6+ g-1 for E. mesophila on a dry matter basis, respectively, while the kinetic constant for these two fungi was 1.32 × 10-6 and 1.39 × 10-7 g (mg Cr6+ min)-1. Similar experiments with melanin extracts of E. mesophila showed maximum sorption capacities of 544.84 mg Cr6+ g-1 and a kinetic constant of 1.67 × 10-6 g (mg Cr6+ min)-1. These results were compared to bibliographic data, suggesting that metallotolerance in black fungi might be the result of an outer cell-wall barrier to reduce the diffusion of toxic metals into the cytoplasm, as well as the inner cell wall biosorption of leaked metals by melanin.

Evaluation of the lead and chromium removal capabilities of Bacillus subtilis-induced food waste compost-based biomedia

Food waste compost (FWC) is a sustainable recycling approach employed in soil media, offering extensive advantages to urban areas by promoting resource circulation and effectively managing water pollution. To improve value, Bacillus subtilis (B. subtilis)-induced FWC-based biomedia (BIBMFWCs) was produced via a secondary treatment involving selective meso-thermophilic stages. During the production of BIBMFWCs, physicochemical properties were found to have favorable characteristics for the efficient removal of metal ions. The produced organic-carbonate complex structure demonstrated the synergistic effect involving simultaneous sorption/precipitation mechanisms for the removal of Pb(II) and Cr(III). Also, the dose of B. subtilis has an impact on the pseudo-second-order (PSO) and intra-particle diffusion (IPD) reaction, leading to distinct removal capacities for Pb(II) and Cr(III) [24.26-24.74 mg g-1 in Pb(II) and 12.7-23.93 mg g-1 in Cr(III)]. Furthermore, B. subtilis, an inducing mediator for microbial metabolites, exhibits the potential to facilitate the removal of Pb(II) and Cr(III) through biological modification of raw materials, which are transformed, facilitating the presence of hydroxyl groups, immobilizing metal ions, and enabling ion exchange via biogenic carbonate formation processes. Finally, the developed BIBMFWCs could be used as a nature-based solution (NBS) material without in-situ pH control.

Efficient Removal of Hexavalent Chromium (Cr(VI)) from Wastewater Using Amide-Modified Biochar

The utilization of biochar, derived from agricultural waste, has garnered attention as a valuable material for enhancing soil properties and serving as a substitute adsorbent for the elimination of hazardous heavy metals and organic contaminants from wastewater. In the present investigation, amide-modified biochar was synthesized via low-temperature pyrolysis of rice husk and was harnessed for the removal of Cr(VI) from wastewater. The resultant biochar was treated with 1-[3-(trimethoxysilyl) propyl] urea to incorporate an amide group. The amide-modified biochar was characterized by employing Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. During batch experiments, the effect of various parameters, such as adsorbent dosage, metal concentration, time duration, and pH, on Cr(VI) removal was investigated. The optimal conditions for achieving maximum adsorption of Cr(VI) were observed at a pH 2, an adsorbent time of 60 min, an adsorbent dosage of 2 g/L, and a metal concentration of 100 mg/L. The percent removal efficiency of 97% was recorded for the removal of Cr(VI) under optimal conditions using amide-modified biochar. Freundlich, Langmuir, and Temkin isotherm models were utilized to calculate the adsorption data and determine the optimal fitting model. It was found that the adsorption data fitted well with the Langmuir isotherm model. A kinetics study revealed that the Cr(VI) adsorption onto ABC followed a pseudo-second-order kinetic model. The findings of this study indicate that amide-functionalized biochar has the potential to serve as an economically viable substitute adsorbent for the efficient removal of Cr(VI) from wastewater.

Structured LDH/Bentonite Composites for Chromium Removal and Recovery from Aqueous Solutions

This study focuses on chromium removal through adsorption and ion exchange using structured calcined layered double hydroxide (LDH) (MgAl)-bentonite composites. Firstly, the powders were structured into granulates to study the effect on Cr sorption kinetics to circumvent the limitations of working with powders in real-life applications. Secondly, the regeneration of the structured composites was optimized to enable multi-cycling operation, which is the key for their applicability beyond laboratory scale. Firstly, the LDH/bentonite ratio was optimized to obtain the best performance for the removal of Cr3+ and Cr6+ species. In powder form, the calcined adsorbent containing 80 wt% LDH and 20 wt% bentonite performed best with an adsorption capacity of 48 and 40 mg/g for Cr3+ and Cr6+, respectively. The desorption was optimized by studying the effect of the NaCl concentration and pH, with a 2 M NaCl solution without pH modification being optimal. The kinetic data of the adsorption and desorption steps were modelled, revealing a pseudo-second order model for both. This was also demonstrated using XRD and Raman measurements after the Cr3+ and Cr6+ adsorption tests, indicating successful uptake and revealing the adsorption mechanism. Finally, five consecutive adsorption-desorption cycles were performed, each showing nearly 100% adsorption and desorption.

Simultaneous photocatalytic removal of organic dye and heavy metal from textile wastewater over N-doped TiO2 on reduced graphene oxide

Methylene blue (MB) and hexavalent chromium Cr(VI) are hazardous pollutants in textile waste and cannot be completely removed using conventional methods. So far, there have been no specific studies examining the synthesis and activity of N-TiO₂/rGO as a photocatalyst for removing MB and Cr(VI) from textile wastewater. This work especially highlights the synthesis of N-TiO₂/rGO as a photocatalyst which exhibits a wider range of light absorption and is highly effective for simultaneous removal of MB-Cr(VI) under visible light. Titanium tetrachloride (TiCl₄) was used as the precursor for N-TiO₂ synthesis using the sol-gel method. Graphite was oxidized using Hummer's method and reduced with hydrazine to produce rGO. N-TiO₂/rGO was synthesized using a hydrothermal process and then analyzed using several characterization instruments. The X-ray diffraction pattern (XRD) showed that the anatase N-TiO₂/rGO phase was detected at the diffraction peak of 2θ = 25.60°. Scanning electron microscopy and transmission electron microscopy (SEM-EDS and TEM) dispersive X-ray spectrometry images show that N-TiO₂ particles adhere to the surface of rGO with uniform size and N and Ti elements are present in the N-TiO₂/rGO combined investigated. Gas absorption analysis data (GSA) shows that N-TiO₂/rGO had a surface area of 77.449 m²/g, a pore volume of 0.335 cc/g, and a pore size of 8.655 nm. The thermogravimetric differential thermal analysis (TG-DTA) curve showed the anatase phase at 500-780 °C with a weight loss of 0.85%. The N-TiO₂/rGO composite showed a good photocatalyst application. The photocatalytic activity of N-TiO₂/rGO for textile wastewater treatment under visible light showed higher effectiveness than ultraviolet light, with 97.92% for MB and 97.48% for Cr(VI). Combining N-TiO₂ with rGO is proven to increase the light coverage in the visible light region. Removal of MB and Cr(VI) can be carried out simultaneously and results in a removal efficiency of 95.96%.

Photoreduction of Cr(VI) by TiO2 adsorbed gold nanoparticles and perylene as a novel organic-inorganic hybrid photocatalyst

The photoreduction of hexavalent chromium (Cr(VI)) using TiO₂ adsorbed gold nanoparticles and perylene (Au/Pe/TiO₂) as a novel organic-inorganic hybrid photocatalyst has been studied. The irradiation by a Xe lamp of a Cr (VI) aqueous solution (0.1 mM) with the Au/Pe/TiO₂ powder resulted in the reduction of the Cr(VI). The rate of Cr(VI) reduction reached 98.3% by the irradiation for 60 min. The reaction rate constant using Au/Pe/TiO₂ (0.0545 min^-1) was higher than that of TiO₂ (0.0218 min^-1), Pe/TiO₂ (0.0303 min^-1), or Au/TiO₂ (0.0393 min^-1). Gold nanoparticles and perylene synergistically accelerated the TiO₂ photocatalytic reaction. This result is due to the Z-scheme electron transfer between Pe and TiO₂ and the suppression of charge recombination by the gold nanoparticles. The irradiation of sunlight also led to the photocatalytic reduction of the Cr(VI) by Au/Pe/TiO₂. In addition, successive reduction of the Cr(VI) was achieved by using a column packed with the Au/Pe/TiO₂ powder immobilized by calcium alginate gel.

A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Biosorption of toxic metal ions (Cr+6, Cd2+) and nutrients (PO43-) from aqueous solution by diatom biomass

This paper evaluates diatom biomass as a biosorbent for removing Cr⁺⁶, Cd²⁺, and PO₄^3- ions from water. The diatom was characterized by X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), and Scanning Electron Microscopy (SEM-EDS) for its crystallinity, functional groups, and morphology. A batch sorption study was conducted to evaluate the parameters influencing Cr⁺⁶, Cd²⁺, and PO₄^3- ions adsorption, and the mechanisms were explored. The FTIR spectra revealed Si-O, O-H, N-H, and C-O as the main functional groups present on the surface of the adsorbent. The SEM showed a rough and irregular-shaped morphology, while the EDS indicated that the diatom biomass is an aluminosilicate material. The rate-limiting steps for Cr⁺⁶ and Cd²⁺ were pseudo-first order, and pseudo-second order sorption favored PO₄^3- based on their R² values. Moreover, the dominant adsorption model that best described the equilibrium data was the Freundlich isotherm. The maximum adsorption capacities obtained for Cr⁺⁶ was 5.66 (mg/g), and Cd²⁺ was 5.27 (mg/g) at 313 K while PO₄^3- was 19.13 (mg/g) at 298 K. The thermodynamic data revealed that the reaction was endothermic for Cd²⁺ and exothermic for Cr⁺⁶ and PO₄^3-, respectively. Diatom biomass was observed to be a promising bio-sorbent for removing Cr⁶⁺, Cd²⁺ and PO₄^2- from wastewater.

Online column preconcentration for speciation and selective determination of Cr(III) in natural water samples using flow injection with chemiluminescence detection

A simple, rapid, sensitive and inexpensive approach is described in this work based on a combination of solid-phase extraction of 8-hydroxyquinoline (8HQ), for speciation and preconcentration of Cr(III) and Cr(VI) in river water, and the direct determination of these species using a flow injection system with chemiluminescence detection (FI-CL) and a 4-diethylamino phenyl hydrazine (DEAPH)-hydrogen peroxide system. At different pH, the two forms of chromium [Cr(III) and Cr(VI)] have different exchange capacities for 8HQ, therefore two columns were constructed; the pH of column 1 was adjusted to pH 3 for retaining Cr(III) and column 2 was adjusted to pH 1 for retaining of Cr(VI). The sorbed Cr(III) and Cr(VI) species were eluted from columns using 3.0 ml of 0.1 N of HCl and 3.0 ml of 0.1 N of NaOH, respectively. The flow injection-chemiluminescence (FI-CL) method is based on light emitted due to the oxidation of DEAPH by the H₂ O₂ in the presence of Cr(III), which catalyzes the reaction. The flow cell is a transparent coiled tube made from glass (2.0 × 4.0, inner and outer diameter) and located close to the photodetector. The flow parameters: flow rate, sample volume, flow cell length, and distance to the CL detector were studied and optimized. Under optimum flow conditions, the Cr(III) concentration can be determined over the range 5-350 μg L^-1 with a limit of detection of 1.2 μg L^-1 , as the Cr(III) concentration is proportional to the intensity of the CL signal. The relative standard deviations (%) for 10 and 50 μg L^-1 Cr(III) were 1.2% and 3.2%, respectively. The effects of Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Co(II), Cu(II), Ni(II), Mn(II), Ca(II), and Fe(III) were investigated. The proposed method is highly selective and sensitive, enabling a rapid determination of the Cr(III) amount in the presence of other interfering metals. Finally, the FI-CL method was examined in five river water samples with excellent recoveries.

Metal-Organic Framework-Based Fiber Optic Sensor for Chromium(VI) Detection

This study demonstrates a novel fiber optic sensing strategy for selective adsorption and rapid detection of Cr(VI) ions by exploiting a suitable metal-organic framework matrix and the characteristic spectral absorption of Cr(VI) at 395 nm wavelength, respectively. U-bent fiber optic sensor (U-FOS) probes that exhibit remarkably high evanescent wave-based absorbance sensitivity were employed to efficiently detect the Cr(VI) ions that are adsorbed to a stable zeolitic imidazolate framework (ZIF-67) matrix immobilized on the probe surface. A facile technique was developed for the fabrication of ZIF-67-coated U-FOS probes (FOS/ZIF-67) involving an in situ deposition process followed by heat treatment. Selectivity of the FOS/ZIF-67 probes to Cr(VI) was confirmed by optical absorption spectral investigations with 14 other heavy metals and interfering ions. The sensor performance was evaluated with a compact light-emitting diode-photodetector-based setup. FOS/ZIF-67 probes demonstrate an ability to detect Cr(VI) ions with a limit of detection of 1 ppb and a wide linear dynamic range from 0.005 to 100 ppm within a short response time of 5 to 10 min. These sensors show good recovery rates with real water samples and a shelf-life of at least 4 weeks under ambient conditions, thereby demonstrating their viability for real-world application.

Simple solid-phase colorimetry for trace Cr(VI) by combination of complexation with diphenylcarbazide and ion-pair solid-phase extraction with sedimentable dispersed particulates

A highly sensitive and simple solid-phase colorimetry for Cr(VI) was proposed. It was based on the ion-pair solid-phase extraction of Cr-diphenylcarbazide (DPC) complex with sedimentable dispersed particulates. The concentration of Cr(VI) was measured from the color tones obtained by image analysis of the photo of sediment. Various conditions, e.g., material and amounts of adsorbent particulates, chemical properties and concentration of counter ions, and pH, were optimized for the formation and quantitative extraction of the complex. In the recommended procedure, 1 mL of sample was put into a 1.5 mL microtube where powder form adsorbent and reagents, i.e., XAD-7HP particles, DPC, sodium dodecyl sulfate, amido sulfuric acid, and sodium chloride had been packed. The analytical operation was completed within 5 min by gently shaking the microtube and allowing it to stand until enough amounts of particulates were deposited to take a picture. Chromium (VI) up to 2.0 ppm was determined, and the detection limit was 0.0034 ppm. The sensitivity was enough to determine Cr(VI) at lower concentrations than the water quality of standard (0.02 ppm). This method was successfully applied to the analysis of simulated industrial wastewater samples. The stoichiometry of the extracted chemical species was also investigated by applying the same equilibrium model as the ion-pair solvent extraction.

Occupational exposure to nickel and hexavalent chromium and the risk of lung cancer in a pooled analysis of case-control studies (SYNERGY)

There is limited evidence regarding the exposure-effect relationship between lung-cancer risk and hexavalent chromium (Cr(VI)) or nickel. We estimated lung-cancer risks in relation to quantitative indices of occupational exposure to Cr(VI) and nickel and their interaction with smoking habits. We pooled 14 case-control studies from Europe and Canada, including 16 901 lung-cancer cases and 20 965 control subjects. A measurement-based job-exposure-matrix estimated job-year-region specific exposure levels to Cr(VI) and nickel, which were linked to the subjects' occupational histories. Odds ratios (OR) and associated 95% confidence intervals (CI) were calculated by unconditional logistic regression, adjusting for study, age group, smoking habits and exposure to other occupational lung carcinogens. Due to their high correlation, we refrained from mutually adjusting for Cr(VI) and nickel independently. In men, ORs for the highest quartile of cumulative exposure to CR(VI) were 1.32 (95% CI 1.19-1.47) and 1.29 (95% CI 1.15-1.45) in relation to nickel. Analogous results among women were: 1.04 (95% CI 0.48-2.24) and 1.29 (95% CI 0.60-2.86), respectively. In men, excess lung-cancer risks due to occupational Cr(VI) and nickel exposure were also observed in each stratum of never, former and current smokers. Joint effects of Cr(VI) and nickel with smoking were in general greater than additive, but not different from multiplicative. In summary, relatively low cumulative levels of occupational exposure to Cr(VI) and nickel were associated with increased ORs for lung cancer, particularly in men. However, we cannot rule out a combined classical measurement and Berkson-type of error structure, which may cause differential bias of risk estimates.

Removal of Pb2+, CrT, and Hg2+ Ions from Aqueous Solutions Using Amino-Functionalized Magnetic Nanoparticles

In this paper, a circular economy approach with the adsorption and desorption of heavy metal (HM) ions—i.e., lead (Pb2+), chromium (CrT), and mercury (Hg2+)—from aqueous solutions was studied. Specific and selective binding of HM ions was performed on stabilized and amino-functionalized iron oxide magnetic nanoparticles (γ-Fe2O3@NH2 NPs) from an aqueous solution at pH 4 and 7. For this purpose, γ-Fe2O3@NH2 NPs were characterized by thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), specific surface area (BET), transmission electron microscopy (TEM), EDXS, and zeta potential measurements (ζ). The effects of different adsorbent amounts (mads = 20/45/90 mg) and the type of anions (NO3−, Cl−, SO42−) on adsorption efficiency were also tested. The desorption was performed with 0.1 M HNO3. The results showed improvement of adsorption efficiency for CrT, Pb2+, and Hg2+ ions at pH 7 by 45 mg of g-Fe2O3@NH2 NPs, and the sequence was as follows: CrT > Hg2+ > Pb2+, with adsorption capacities of 90.4 mg/g, 85.6 mg/g, and 83.6 mg/g, respectively. The desorption results showed the possibility for the reuse of γ-Fe2O3@NH2 NPs with HNO3, as the desorption efficiency was 100% for Hg2+ ions, 96.7% for CrT, and 91.3% for Pb2+.

Chromium in plant-soil nexus: Speciation, uptake, transport and sustainable remediation techniques

Heavy metal (HM) pollution has become a serious global problem due to the non-biodegradable nature of the HMs and their persistence in the environment. Agricultural soil is a non-renewable resource that requires careful management so that it can fulfill the increasing demand for agricultural food production. However, different anthropogenic activities have resulted in a large-scale accumulation of HMs in soil which is detrimental to soil and plant health. Due to their ubiquity, increased bioavailability, toxicity, and non-biodegradable nature, HM contamination has formed a roadblock in the way of achieving food security, safety, and sustainability in the future. Chromium (Cr), specifically Cr(VI) is a highly bioavailable HM with no proven role in the physiology of plants. Chromium has been found to be highly toxic to plants, with its toxicity also influenced by chemical speciation, which is in turn controlled by different factors, such as soil pH, redox potential, organic matter, and microbial population. In this review, the different factors that influence Cr speciation were analyzed and the relationship between biogeochemical transformations of Cr and its bioavailability which may be beneficial for devising different Cr remediation strategies has been discussed. Also, the uptake and transport mechanism of Cr in plants, with particular reference to sulfate and phosphate transporters has been presented. The biological solutions for the remediation of Cr contaminated sites which offer safe and viable alternatives to old-style physical and chemical remediation strategies have been discussed in detail. This review provides theoretical guidance in developing suitable approaches for the better management of these remediation strategies.

On the Determination of Cr(VI) in Cr(III)-Rich Particulates: From the Failure of Official Methods to the Development of an Alternative Protocol

The goals of this work are the evaluation of the performances of official methods in the challenging determination of Cr(VI) in Cr(III)-rich particulate matter, and the development of a novel and robust analytical protocol for this issue. A liquid chromatography inductively coupled plasma mass spectrometry apparatus (LC-ICP-MS), together with an isotope-enriched spike addition technique, was used to allow the study of Cr(III)/Cr(VI) interconversions during the extraction step. An original separation strategy based on Cr(OH)₃ head-column stacking was developed to tolerate high concentrations of Cr(III) (up to 10 mg/kg, with a Cr(VI) limit of detection of 0.51 µg/kg) without the need of any sample pretreatment. After observing, the official extraction protocols always yield false positive values in the challenging situation of particulate matter of leather industries (where huge amounts of Cr(III) are present), a new extraction strategy was developed. The novel procedure involves a 48-h extraction at room temperature using a pH-8 phosphate buffer, which demonstrated that no Cr(III)/Cr(VI) interconversions occur during this phase. To get rid of any possible interference caused by co-extracted substances, the measurement of the redox potential, together with the addition of a Fe(II)/Fe(III) redox buffer was performed to fix chromium speciation during the overall analytical protocol.

Preparation of new MOF-808/chitosan composite for Cr(VI) adsorption from aqueous solution: Experimental and DFT study

In this study, a series of Zirconium-based MOF and chitosan composites (MOF-808/chitosan) were synthesized as efficient adsorbent for Cr(VI) ions elimination from aqueous solution. MOF-808/chitosan structure and morphology was characterized by FE-SEM, EDX, XRD, BET, zeta potential analysis, FT-IR, XPS techniques. The kinetic studies ascertained that Cr(VI) adsorption over MOF-808/chitosan followed pseudo-second-order kinetic model. The adsorption isotherms fitted the Langmuir isotherm model, implying on homogeneously adsorption of Cr(VI) on the surface of MOF-808/chitosan. According to the Langmuir model, the maximum capacity was obtained to be 320.0 mg/g at pH 5. Thermodynamic investigation proposed spontaneous (ΔG° < 0), disordered (ΔS° > 0) and endothermic (ΔH° > 0) for adsorption process. Besides, MOF-808/chitosan displayed an appropriate reusability for the elimination of Cr(VI) ions from their aqueous solutions for six successive cycles. DFT study of the adsorption process displayed and confirmed the role of hydrogen bonding and electrostatic attraction simultaneously.

Genesis of high hexavalent chromium groundwater in deep aquifers from loess plateau of Northern Shaanxi, China

Hexavalent chromium (Cr(VI)) groundwater usually exists in shallow aquifers related to ultramafic and serpentine formations, but knowledge of the genesis of dissolved Cr(VI) in deep sandstone aquifers is limited. Both groundwater and aquifer sediments were taken from the Jingbian County in the Loess Plateau of Northwestern Shaanxi to investigate distribution and genesis mechanism of high Cr(VI) groundwater in deep sandstone aquifers. Results showed that the Cr concentrations (median 142 μg/L) in groundwater from deep aquifers (>100 m) were relatively high, while the Cr concentrations in shallow groundwater were low (median 33.8 μg/L). Dissolved Cr mainly existed in the species of Cr(VI) (average, 90%). Deep groundwater with higher Cr(VI) concentrations generally had higher pH, Eh, and DO than shallow groundwater, indicating that the high Cr(VI) groundwater existed in relatively oxic environment. Cretaceous sandstones in deep aquifers had anomalously high contents of total Cr (average 115 mg/kg), where Cr was mainly present in silicates-bound form, and secondly in strongly adsorbed form. There were positive correlations between Mn and Cr in the unweathered silicate-bound form and adsorbed form, which were conducive to Cr(III) oxidation into Cr(VI) in an alkaline-oxic environment. The different ionic ratios (i.e. (Ca²⁺ + Mg²⁺)/(HCO₃^- + SO₄^2-)) also supported silicate weathering as the dominant rock-water interactions in the deep groundwater, which enhanced the release of the unweathered silicate-bound Cr. Relatively high pH and ionic strength mobilized the adsorbed Cr(VI) into groundwater. This investigation emphasizes the geological origin of high Cr(VI) groundwater in deep sandstone aquifers containing Mn oxides, which deserves more concerns for the purpose of drinking water supply.

Effects and bioaccumulation of Cr(III), Cr(VI) and their mixture in the freshwater mussel Corbicula fluminea

Chromium has two main oxidation states, Cr(III) and Cr(VI), that can occur simultaneously in natural waters. Current consensus holds that Cr(VI) is of high ecotoxicological concern, but regards Cr(III) as poorly bioavailable and relatively non-toxic. In this work, the effects and bioaccumulation of Cr(III), Cr(VI) and their mixture were studied using the freshwater clam Corbicula fluminea as a model organism. Mixture exposures were carried out using solutions isotopically enriched in ⁵⁰Cr(III) or ⁵³Cr(VI), allowing to quantify the contribution of each redox form to total Cr accumulation in the clams. Following exposure to individual redox forms, Cr(III) accumulated preferentially in the digestive glands and Cr(VI) in the gills of C. fluminea. In mixture exposures, both redox forms accumulated mainly in the gills; the concentration of Cr(III) in the digestive glands being much lowered compared with individual exposures. Both oxidation states affected the expression of biomarkers related to energy reserves, cellular damage and mitochondrial functioning, as well as the expression of mRNA for detoxification genes. The observed effects differed between gills and digestive glands. The present study suggests that Cr(III) is a bioavailable and biologically active elemental species deserving more consideration by the ecotoxicological community.

Gridded distribution of total suspended particulate matter (TSP) and their chemical characterization over Delhi during winter

In the present study, total suspended particulate matter (TSP) samples were collected at 47 different sites (47 grids of 5 × 5 km² area) of Delhi during winter (January-February 2019) in campaign mode. To understand the spatial variation of sources, TSP samples were analyzed for chemical compositions including carbonaceous species [organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC)], water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), polycyclic aromatic hydrocarbons (16 PAHs), water-soluble inorganic species (WSIS) (F^-, Cl^-, SO₄^2-, NO₂^-, NO₃^-, PO₄^3-, NH₄⁺, Ca²⁺, Mg²⁺, Na⁺, and K⁺), and major and minor trace elements (B, Na, Mg, Al, P, S, Cl, K, Ca, Ti, Fe, Zn, Cr, Mn, Cu, As, Pd, F, and Ag). During the campaign, the maximum concentration of several components of TSP (996 μg/m³) was recorded at the Rana Pratap Bagh area, representing a pollution hotspot of Delhi. The maximum concentrations of PAHs were recorded at Udhyog Nagar, a region close to heavily loaded diesel vehicles, small rubber factories, and waste burning areas. Higher content of Cl^- and Cl^-/Na⁺ ratio (>1.7) suggests the presence of nonmarine anthropogenic sources of Cl^- over Delhi. Minimum concentrations of OC, EC, WSOC, PAHs, and WSIS in TSP were observed at Kalkaji, representing the least polluted area in Delhi. Enrichment factor <5.0 at several locations and a significant correlation of Al with Mg, Fe, Ti, and Ca and C/N ratio indicated the abundance of mineral/crustal dust in TSP over Delhi. Principal component analysis (PCA) was also performed for the source apportionment of TSP, and extracted soil dust was found to be the major contributor to TSP, followed by biomass burning, open waste burning, secondary aerosol, and vehicular emissions.

Dynamic experiment on the treatment of acidic chromium-containing wastewater by lignite loaded nano FeS

In terms of the problem of severe pollution to the ecological environment caused by the acidic chrome-containing wastewater produced in the tanning, electroplating, metallurgy, printing and dyeing and other industries, based on the good adsorbability, reducibility and other properties of heavy metals such as Cr(vi) by lignite and nano FeS, the lignite-loaded nano FeS adsorbing material (nFeS-lignite) was prepared by ultrasonic precipitation method. NFeS-lignite and lignite were used as fillers to construct 1# and 2# dynamic columns to carry out the dynamic treatment experiment of acidic chrome-containing wastewater. And nFeS-lignite and lignite were characterized by XRD, SEM and EDS to explore the regularity, long-acting properties and removal mechanism of acidic chrome-containing wastewater treated by NFeS-lignite and lignite. The results indicate that: ① during 25 days of operation, the average removal percentages of Cr(vi) in the 1# and 2# dynamic columns are 71.6% and 53.1%. The average removal percentages of total chromium in 1# and 2# dynamic columns are 54.4% and 28.8%, and the average effluent pH of 1# and 2# dynamic columns is 5.3 and 7.3. ② According to XRD, SEM, EDS and FTIR analysis, the reducing groups in the structure of nFeS-lignite, such as –CH₃, –CH₂, C–O and Ar-OH, participate in the reaction and are oxidized to C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C, CO and other groups. A large number of sediment crystals appeared on the particle surface, and new diffraction peaks such as FeOOH, Cr(OH)₃ and Cr₂S₃ appeared at the same time, indicating that after Cr(vi) was reduced to Cr(iii), it would be fixed on the surface of nFeS-lignite in the form of precipitation such as hydroxide and sulfide.

Based on the good adsorption and reducibility of Cr(vi) of lignite and nano-FeS, the lignite supported nano-FeS adsorption material (nFeS-lignite) was prepared by ultrasonic precipitation method to treat acidic chromium-containing wastewater.

Adsorption and Reduction of Aqueous Cr by FeS-Modified Fe-Al Layered Double Hydroxide

To remedy the widespread chromium (Cr) pollution in the environment, this study mainly used the ultrasonic-assisted co-precipitation and precipitation methods to prepare FeS-modified Fe-Al-layered double hydroxide (FeS/LDH) composite material. The experimental results showed that FeS/LDH has higher removal efficiency of Cr in aqueous solution and stronger anti-interference ability than unmodified LDH. Under the same reaction conditions, the removal efficiency of total Cr(Cr(T)) using LDH was 34.85%, and the removal efficiency of Cr(VI) was 46.76%. For FeS/LDH, the removal efficiency of Cr(T) and Cr(VI) reached 99.57% and 100%, respectively. The restoration of Cr(T) and Cr(VI) by FeS/LDH satisfied the Langmuir adsorption isotherm. The maximum adsorption capacity of Cr(T) and Cr(VI) achieved 102.9 mg/g and 147.7 mg/g. The efficient removal of Cr by FeS/LDH was mainly based on the triple synergistic effect of anion exchange between Cr(VI) and interlayer anions, redox of Cr(VI) with Fe2+ and S2−, and co-precipitation of Fe3+ and Cr3+.

Quantitative extraction of chromium VI and III from tanned leather: a comparative study of pretreatment methods

In this study, seven pretreatment methods for chromium speciation in tanned leather were evaluated: acidic mineralization, ethylenediaminetetraacetic acid (EDTA) extraction, diethylenetriaminepentaacetic acid (DTPA) extraction, alkaline extraction (NH4OH), ammonium nitrate extraction (NH4NO3), water extraction, and phosphate buffer extraction. Acidic mineralization permitted the decomposition of the organic matter and ensured the complete digestion of leathers, giving access to the total content of chromium in each sample using inductively coupled plasma-atomic emission spectrometry (ICP-AES). From all the extractant media tested, EDTA proved to be the most efficient, allowing the extraction of Cr(VI) and Cr(III) as a Cr(III)-EDTA complex, quantitatively. Method validation is presented for EDTA extraction and direct mineralization. For the EDTA extraction, method detection limit (MDL) and method quantification limit (MQL) for total Cr in leather were 3.4 ppb and 11.2 ppb (µg of total Cr per L of extraction solution), respectively. Due to the lack of leather certified reference materials (CRMs) for Cr(VI), accuracy was evaluated by spiking leather samples with a Cr(VI) solution. The spike recovery of EDTA microwave assisted extraction ranged from 91.0 to 108.6%. Interday precision was also evaluated and all variation coefficients were below 5%, for both mineralization and EDTA extraction. This article provides an efficient procedure to extract quantitatively chromium from leather, while maintaining the speciation, which can be further followed by ion chromatography-inductively coupled plasma-mass spectrometry (IC-ICP-MS).

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

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

Systematic review of biomonitoring data on occupational exposure to hexavalent chromium

Occupational exposure to hexavalent chromium (Cr(VI)) can cause serious adverse health effects such as lung cancer and irritation of the skin and airways. Although assessment of chromium (Cr) in urine is not specific for Cr(VI) exposure, the total amount of Cr in urine is the most used marker of exposure for biomonitoring of Cr(VI). The purpose of this systematic review was fourfold: (1) to assess current and recent biomonitoring levels in subjects occupationally exposed to Cr(VI), with a focus on urinary Cr levels at the end of a working week, (2) to identify variables influencing these biomonitoring levels, (3) to identify how urinary Cr levels correlate with other Cr(VI) exposure markers and (4) to identify gaps in the current research. To address these purposes, unpublished and published biomonitoring data were consulted: (i) unpublished biomonitoring data comprised urinary Cr levels (n = 3799) of workers from different industries in Belgium collected during 1998-2018, in combination with expert scores indicating jobs with Cr exposure and (ii) published biomonitoring data was extracted by conducting a systematic literature review. A linear mixed effect model was applied on the unpublished biomonitoring data, showing a decreasing time trend of 30% in urinary Cr levels. Considering the observed decreasing time trend, only articles published between January 1, 2010 and September 30, 2020 were included in the systematic literature search to assess current and recent biomonitoring levels. Twenty-five studies focusing on human biomonitoring of exposure to Cr(VI) in occupational settings were included. Overall, the results showed a decreasing time trend in urinary Cr levels and the need for more specific Cr(VI) biomarkers. Furthermore, this review indicated the importance of improved working conditions, efficient use of personal protective equipment, better exposure control and increased risk awareness to reduce Cr levels in biological matrices. Further investigation of the contribution of the different exposure routes is needed, so that better guidance on the use of control measures can be provided. In addition, this review support the call for more harmonization of human biomonitoring.

Bioremediation of Chromium by Microorganisms and Its Mechanisms Related to Functional Groups

Heavy metals generated mainly through many anthropogenic processes, and some natural processes have been a great environmental challenge and continued to be the concern of many researchers and environmental scientists. This is mainly due to their highest toxicity even at a minimum concentration as they are nonbiodegradable and can persist in the aquatic and terrestrial environments for long periods. Chromium ions, especially hexavalent ions (Cr(VI)) generated through the different industrial process such as tanneries, metallurgical, petroleum, refractory, oil well drilling, electroplating, mining, textile, pulp and paper industries, are among toxic heavy metal ions, which pose toxic effects to human, plants, microorganisms, and aquatic lives. This review work is aimed at biosorption of hexavalent chromium (Cr(VI)) through microbial biomass, mainly bacteria, fungi, and microalgae, factors influencing the biosorption of chromium by microorganisms and the mechanism involved in the remediation process and the functional groups participated in the uptake of toxic Cr(VI) from contaminated environments by biosorbents. The biosorption process is relatively more advantageous over conventional remediation technique as it is rapid, economical, requires minimal preparatory steps, efficient, needs no toxic chemicals, and allows regeneration of biosorbent at the end of the process. Also, the presence of multiple functional groups in microbial cell surfaces and more active binding sites allow easy uptake and binding of a greater number of toxic heavy metal ions from polluted samples. This could be useful in creating new insights into the development and advancement of future technologies for future research on the bioremediation of toxic heavy metals at the industrial scale.

Double Z-scheme FeVO4/Bi4O5Br2/BiOBr ternary heterojunction photocatalyst for simultaneous photocatalytic removal of hexavalent chromium and rhodamine B

HYPOTHESIS

Fabrication of the heterojunction photocatalyst with appropriate band potentials as a promising method of inhibiting electron-hole pair recombination leading to enhanced photocatalytic properties.

EXPERIMENTS

Herein, BiOBr, Bi₄O₅Br₂, and binary BiOBr/Bi₄O₅Br₂ composite were selectively synthesized by employing a one-step microwave irradiation method. Then, double Z-scheme FeVO₄/Bi₄O₅Br₂/BiOBr ternary composites with different weight percentages (%wt) of FeVO₄ were fabricated and their photocatalytic applications were studied. The photodegradation of organic compounds (rhodamine B (RhB), methylene blue (MB) and salicylic acid (SA)), along with the photoreduction of hexavalent chromium (Cr(VI)) were investigated.

FINDINGS

Comparing with the single and binary photocatalysts, and a commercial TiO₂, the 1 %wt-FeVO₄/Bi₄O₅Br₂/BiOBr photocatalyst demonstrated superior visible-light-driven photocatalytic performance. In a Cr(VI)/RhB combined system, Cr(VI) photoreduction was further improved and coexisting RhB molecules were simultaneously degraded. Removal of Cr(VI) and RhB were maximized by adjusting both pH values and catalyst dosages. Based on UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, electrochemical investigations, active-species trapping, nitrotetrazolium blue transformation, and silver photo-deposition experiments, a double Z-scheme charge transfer mechanism with an RhB-sensitized effect was proposed. This special mechanism has led to significant enhancement in charge segregation and migration, along with higher redox properties of the ternary composite, which were responsible for the excellent photocatalytic activity.

Reductive materials for remediation of hexavalent chromium contaminated soil - A review

Chemical reduction of Cr(VI) to Cr(III) by reductive materials is the most widely used technology for the remediation of Cr(VI)-contaminated soil due to its high efficiency, adaptability and low cost. This paper reviews chromium chemistry and the materials that can effectively reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soil, namely iron-bearing reductants, sulfur-based compounds and organic amendments. Moreover, we discuss the corresponding mechanisms involved in the process of immobilization of Cr(VI) in polluted soil, and emphasize the relationship between the materials remediation performance and soil environmental conditions. Besides, perspectives on the potential future researches of novel materials design and technological development in the remediation of Cr(VI) contaminated soil are also put forward.

Preparation of environmental samples for chemical speciation of metal/metalloids: A review of extraction techniques

Chemical speciation is a relevant topic in environmental chemistry since the (eco)toxicity, bio (geo)chemical cycles, and mobility of a given element depend on its chemical forms (oxidation state, organic ligands, etc.). Maintaining the chemical stability of the species and avoiding equilibrium disruptions during the sample treatment is one of the biggest challenges in chemical speciation, especially in environmental matrices where the level of concomitants/interferents is normally high. To achieve this task, strategies based on chemical properties of the species can be carried out and pre-concentration techniques are often needed due to the low concentration ranges of many species (μg L^-1 - ng L^-1). Due to the significance of the topic and the lack of reviews dealing with sample preparation of metal (loid)s (usually, sample preparation reviews focus on the total metal content), this work is presented. This review gives an up-to-date overview of the most common sample preparation techniques for environmental samples (water, soil, and sediments), with a focus on speciation of metal/metalloids and determination by spectrometric techniques. Description of the methods is given, and the most recent applications (last 10 years) are presented.

Ammonia and chromate interaction explains unresolved Hyalella azteca mortality in Flanders' sediment bioassays

Agricultural, industrial and household chemicals are emitted in large rivers along populated areas, transported by water and deposited in sediments, posing (eco)toxicological risks. Sediments have received less attention than surface waters, likely because of the intrinsic complexity of interactions between sediment constituents complicating correct framing of exposures. Sadly, thorough assessment of the in situ behavior of sediment constituents in bioassays is often not practical. Alternatively, we related physicochemical properties of sediments from field testing to results from bioassays. The case study covers Flemish sediment (incl. Scheldt and Meuse) and mortality of Hyalella azteca, a sensitive bio-indicator. Though variable across Flanders' main water bodies, heavy metals and ammoniacal nitrogen dominate the observed toxicity according to toxic unit (TU) assessments. Depending on the water body we explain between 50 and 90% of the variance in the observed H. azteca mortality, substantially more than previous ecotoxicity studies. We attribute the remaining variance to potential incoherently documented biophysicochemical sediment properties and concentrations of non-target biocides, testing conditions/set-ups and/or species variabilities. We discuss the relative influence of heavy metals/metaloxides, nitrogen (e.g. fertilizer), polycyclic aromatics and organochlorides. We highlight both direct and indirect mortality mechanisms. We note potential synergetic mixture effects between ammoniacal nitrogen and chromium. Such synergy may be phenomenological of 'standard' aerobic bioassays, and prove a complementary method alongside the 'acid-volatile sulfide test' to more effectively link concentration to toxicity. Future study ought to include variation in biophysicochemical properties between sampling locations and batch bioassays. Our approach enables water managers to interpret their monitoring data by converting sediment concentrations to H. azteca mortality and prioritize substances that contribute most.

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.