Chromite Ore Processing Residue Research Articles

Detoxification of chromium slag using corn stalk for the hierarchical preparation of ultrafine aluminum hydroxide and iron hydroxide

The harmless treatment and resource utilization of chromite ore processing residue (COPR), a consequential byproduct of chromate production, carry profound significance. In this study, corn stalk, an agricultural waste, was used to generate reducing substances through high-temperature and vacuum treatments and was successfully used to detoxify COPR. The acid leaching method combined with multi-step purification was employed to extract iron and aluminum, two crucial metallic elements, from COPR. The successful preparation of iron hydroxide and ultrafine aluminum hydroxide (ATH) was achieved. The prepared ATH has an average particle size of 8.124 μm, with a highly uniform distribution, enabling it to be a promising environmentally friendly inorganic flame retardant. This study not only effectively addresses the issue of pollution caused by COPR but also presents a novel approach to utilizing industrial and agricultural wastes, thereby promoting the maximization of resource utilization.

Research on the environmental stability performance of chromite ore processing residue solidified products.

Chromite ore processing residue (COPR) is a hazardous waste because of leachable chromium, especially Cr(vi). Therefore, ascorbic acid (AA) and blast furnace slag (BFS) have been used to detoxify and solidify COPR. On this basis, environmental stability experiments with high temperature and freeze-thaw cycles were carried out to explore the stability performance of a solidified body with 40% COPR. The environmental stability performance was analyzed through changes in edge length, mass loss, compressive strength development, and leaching concentration of Cr(vi). The result indicated that the high-temperature environment had much more effect on the solidified body than the freeze-thaw cycle environment in these four aspects: after being maintained at 900 °C for 2 h, the compressive strength of the solidified bodies reached its minimum value (35.76 MPa). However, in the freeze-thaw cycle experiments, the compressive strength of the solidified bodies consistently remained above 80 MPa, and the leaching of hexavalent chromium was below the limit (5 mg L-1). In addition, X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR) analysis verified that COPR was effectively solidified through physical and chemical means. Moreover, high temperature changes the molecular structure of the solidified body, thus reducing the compressive strength and curing ability of the solidified body, while the freeze-thaw cycle experiment has little effect on it.

LED-driven photocatalytic degradation of tetracycline by chromite ore processing residue (COPR): Mechanism, pathway, toxicity assessment

Soda-based chromite ore processing residue (COPR) is a hazardous waste produced from chromium salt plants. In this study, detoxified water-washed chromite ore processing residue (DW-COPR) was investigated as a catalyst to activate peroxymonosulfate (PMS) to degrade tetracycline (TC) in the presence of visible light. Performance, mechanism, pathways of TC degradation process in the DW-COPR/PMS/light catalytic system and the toxicity analysis of the intermediates were systematically analyzed by EPR, LC-MS, and Toxicity Estimation Software Tool (T.E.S.T). The results indicated that the DW-COPR had excellent properties on TC degradation and practical application, with optimal efficiency of 94.37% in the DW-COPR/PMS/light system, due to the combined participation of radical •OH, SO4•-, O2•-, 1O2, h+, and e-. Simultaneously, the study outlined four potential degradation pathways for TC: deamidation, hydroxylation, benzene ring opening, and carbonylation. It was revealed that the naturally formed spinel structure in DW-COPR played a dominant role in the reusability and recyclable performance. The present work provides a new perspective on the high-value-added resourcing of DW-COPR as catalyst.

Effective Cr(VI) reduction and immobilization in chromite ore processing residue (COPR) contaminated soils by ferrous sulfate and digestate: A comparative investigation with typical reducing agents

Chemical reduction combined with microbial stabilization is a green and efficient method for the remediation of hexavalent chromium (Cr(VI)) contaminated soil. In this study, the combination of ferrous sulfate with kitchen waste digestate was applied to reduce and immobilize Cr(VI) in chromite ore processing residue (COPR) contaminated soils, and systematically evaluated the remediation performance of Cr(VI) compared with several typical reducing agents (i.e., ferrous sulfate, zero valent iron, sodium thiosulfate, ferrous sulfide, and calcium polysulfide). The results showed that the combination of ferrous sulfate and digestate had superior advantages of a lower dosage of reducing agent and a long-term remediation effect compared to other single chemical reductants. Under an Fe(II):Cr(VI) molar ratio of 3:1% and 4% digestate (wt), the content of Cr(VI) in the soil decreased to 5.07 mg/kg after 60 days of remediation. Meanwhile, the leaching concentrations of Cr(VI) were below detection limit, which can meet the hazardous waste toxicity leaching standard. The risk level of Cr pollution was decreased from very high risk to low risk. The X-ray photoelectron spectroscopy (XPS) results further demonstrated that the combined treatments were beneficial to Cr(VI) reduction and stabilization. The abundance of bacteria with Cr(VI) reducing ability was higher than other treatments. Moreover, the high abundance of carbon and nitrogen metabolism in the combined treatments demonstrated that the addition of digestate was beneficial to the recovery and flourishing of Cr(VI)-reducing related microorganisms in COPR contaminated soils. This work provided an alternative way on Cr(VI) remediation in COPR contaminated soils.

Mycosynthesized magnetic iron-oxide nanoparticles for the remediation of heavy metals – An insight into the mechanism of adsorption, process optimization using algorithmic approach and its application for the treatment of groundwater

In the present study, mycosynthesized magnetic iron-oxide (γ-Fe2O3) nanoparticles (IO-NPs) were synthesized using manglicolous fungi isolated from the sediment samples of Indian Sundarbans. Out of the 15 strains isolated from the sediment samples, cell-free filtrate of Corynespora cassiicola – CNMP1 reduced the Fe(NO3)3 solution leading to the formation of IO-NPs (100 nm). It was observed that the fungal extracellular biomaterial embedding the IO-NPs provided stability to the IO-NPs in the aqueous suspension. The mycosynthesized IO-NPs were employed to adsorb heavy metals, viz. Pb, Ni, Cu, Zn, Cd, and Cr from water. Studies on adsorption isotherms delineated the energetically heterogeneous nature of IO-NPs. Batch adsorption studies showed that 1 g/L of IO-NPs at 313 K temperature and pH 6 is able to remove more than 90% of Pb, Ni, Cu, and Zn ions from the 10 mg/L solution. However, more than 90 % removal efficacy was obtained for Cr with a pH of 2, keeping the other adsorption parameters the same. Thermodynamic studies laid bare the spontaneity and endothermic nature of the adsorption process for each studied heavy metal. A multi-metric approach involving response surface methodology (RSM), and artificial neural network (ANN) analysis was used to validate the adsorption of heavy metals by IO-NPs, and it was observed that 1 g/L of IO-NPs at 313 K temperature and pH 6.1 can adsorb 92.86% of Pb, 94.93% of Ni, 95.92% of Cu, 98.11% of Zn, 98.91% of Cd and 82.12 % of Cr from water with an initial metallic concentration of 11.96 mg/L. Further, with the optimized IO-NP dose and temperature, the IO-NPs were able to remove 94 % of Cr from Cr-rich groundwater (12.68 mg/L) collected from a region around chromite ore processing residues (COPR) dumping. The magnetic nature of the IO-NPs eased their separation from the treated water.

Vertical migration in the soil of Cr(VI) and chromite ore processing residue: Field sampling and benchtop simulation

Chromite ore processing residue (COPR) keeps releasing Cr(VI) over time, and the mixing of residual COPR into soil makes the remediation of COPR-contaminated sites challenging. In this study, a sample of COPR and two soil profiles were collected from a typical historical COPR-contaminated site, and the vertical migration of Cr(VI) and COPR particles in contaminated soil was simulated in the laboratory. Cr(VI) was detected in the upper layer of the field samples at thousands of milligrams per kilogram even after decades of aging, and it can be leached out and migrate vertically deep into the surrounding soil and groundwater. In the COPR-containing soil, more diverse hydrated minerals of brownmillerite were produced than the COPR in the open air on the site. Minerals with high Cr content in COPR-containing soils have a relatively high proportion of particles smaller than 10 µm. COPR particles smaller than 5 µm were found to have migrated downward into the deep soil. During simulated one-year of precipitation, 578.9 mg Cr(VI)/kg was leached from COPR, while 35.5% of the COPR particles smaller than 5 µm had the potential to migrate vertically. The management of COPR particles should be emphasized during risk management or remediation of COPR-contaminated sites.

Detoxification and resource utilization of soda ash chromite ore processing residue by copper slag

Chromite ore processing residue (COPR) and copper slag (CS) are industrial waste slags that cause environmental pollution and require effective treatment. This study proposes a new strategy for the co-treatment and resource utilization of COPR and CS. Under the optimal conditions (liquid-solid ratio of 9.99; H2SO4 concentration of 0.85 M; the CS/COPR mass ratio of 9.98%; temperature of 88.20 ℃; and duration of 60 min), the reduction efficiency of Cr(VI) reached 99.48%, and the total Cr leaching concentration of residue reached 1.35 mg/L, which was below the USEPA regulatory limit of 5 mg/L. In addition, the detoxicated slag containing 61.44 wt% Fe2O3 can be used as a raw material in the steel industry. Through kinetic, characterization, and DFT analyses, the effective remediation of Cr(VI) in COPR by CS is a combination of adsorption and reduction. The unreacted shrinkage nuclear reaction model under the control of the surface chemical reaction is the most suitable model to describe the process, and when the apparent activation energy is 63.90 kJ/mol, the apparent rate equation is:1-(1-x)1/3= 1·371×107[CS/COPR]2·35004 [H2SO4]2·3945 [L/S]1·95338 exp(–63·90/RT)This is an important study for waste-waste co-treatment and resource utilization of industrial waste.

Remediation of Cr(VI)-contaminated soil mixed with chromite ore processing residue by ferrous sulfate and enzyme residue

In this study, Cr(VI)-contaminated soil mixed with COPR by using ferrous sulfate (FeSO4), enzyme residue (ER), and their combination under aerobic or anaerobic condition were investigated. The concentration of Cr(VI) decreased from 1498.05 to 104.63 mg kg−1 after the simultaneous addition of FeSO4 (30 %, w/w as FeSO4·7H2O) and ER (30 %, w/w) at 45 d under the anaerobic condition with a reduction efficiency of 93.02 %, which is higher than that by single FeSO4 (72.39 %) or ER (75.47 %) under the anaerobic condition. XRD, XPS, FTIR, and fluorescence spectroscopy were conducted to characterize soil and ER composition. Metagenomic analysis was performed to reveal the reduction mechanisms of FeSO4 and ER. The anaerobic condition with lower Eh was beneficial for Cr(VI) reduction than aerobic condition, and Eh was the main driver for the evolution of Cr(VI) reduction-related microorganisms. Moreover, the addition of ER enriched the organic matter and microbials in the soil. During the decomposition of organic matter under the anaerobic condition, organic acids were generated, leading to a decrease in pH and promoting the release of Cr(VI) from minerals. They also served as electron donors in Cr(VI) reduction. Additionally, the addition of excess FeSO4 stimulated the growth of iron-reducing bacteria and sulfate-reducing bacteria, facilitating to Cr(VI) reduction. Metagenomic analysis showed that Acinetobacter, related to the nemA and nfsA genes, was the dominant Cr(VI) reduction genus. Thus, the combination of FeSO4 and ER is a promising method for the remediation of Cr(VI)-contaminated soils mixed with COPR.

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

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

Understanding and eliminating the reductant interference on Chromium VI measurement with USEPA method 3060A

Excessive reductants are used in engineering to ensure a reliable remediation effect of chromite ore processing residue (COPR), however, re-yellowing phenomenon of remediated COPR occurs after some time though the Cr(VI) content meets regulatory requirements after curing period. This problem is due to a negative bias on Cr(VI) determination using USEPA method 3060A. To address this issue, this study tried to reveal the interference mechanisms and proposed two methods to amend the bias. Results of ion concentrations, UV–Vis spectrum, XRD, and XPS together showed that Cr(VI) was reduced by ions (Fe2+, S52−) in the digestion stage of USEPA method 3060A, and as a result, method 7196A would not reflect the true Cr(VI) concentration. The interference on Cr(VI) determination generated by excess reductants mainly occurs during the curing period of remediated COPR, but it decreases over time as reductants being oxidized gradually by the air. Compared with the thermal oxidation, the chemical oxidation with K2S2O8 prior to alkaline digestion performs better to eliminate the masking effect brought by excess reductants. This study provides an approach on how to accurately determine the Cr(VI) concentration in the remediated COPR. It might be helpful to reduce the occurrence possibility of re-yellowing phenomenon.

Co-treatment of chromite ore processing residue and pyrolusite for simultaneous detoxification and resource utilization

Efficient and low-cost recovery of metals from pyrolusite and Chromite Ore Processing Residue (COPR) is of great significance for environmental protection and sustainable development of metal resources. This study proposes a new method for the simultaneous recovery of Cr and Mn from COPR and pyrolusite, to realize COPR and pyrolusite resource utilization and detoxification. Under the optimal conditions (i.e., the temperature of 150 ℃, the holding time of 1.5 h, the H2SO4 concentration of 3 M, the mass ratio of COPR to pyrolusite of 1:1, and the solid-liquid ratio of 1:20), the leaching efficiency of Cr and Mn reached 99.56% and 85.34% respectively. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration of the detoxified slag was 0.26 mg/L meeting the EPA standard (＜5 mg/L). The reaction mechanism was elucidated based on thermodynamics, kinetics, and mineral characterization. In addition, a high-quality utilization strategy was proposed, leaching solution to prepare manganese carbonate and sodium chromate, and the detoxified slag can be used for steel smelting. In brief, this method has good economic benefits and provides a new reference for the resource utilization of hazardous waste.

Collaborative resource utilization of hazardous chromium ore processing residue (COPR) and C-bearing dust during limonitic laterite sintering process

Much chromite ore processing residue (COPR), which is defined as hazardous solid waste due to virulent Cr (Ⅵ), is produced from chromium salt industry. C-bearing dust from steel industry is a kind of typically solid waste with poor surface activity. The iron ore sintering process has the potential on large-scale harmless utilization of COPR and C-bearing dust. This study proposes a new approach of co-utilizing COPR and C-bearing dust during limonitic laterite sintering process. The negative influence of COPR and C-bearing dust on permeability was offset by the fuel attribute of extra C from C-bearing dust. The results indicate that, while 6% COPR and 8% C-bearing dust were added, the tumble index and yield of the sinters were basically equal to those of the blank samples. The toxic Cr (Ⅵ) was reduced to Cr (Ⅲ) which was concentrated in high alumina spinel phases during the sintering process. The obtained sinters were identified as nontoxic, and no secondary waste was generated throughout the clean utilization process.

Visible Light Accelerates Cr(III) Release and Oxidation in Cr-Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence.

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)-Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photo-oxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO2 under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)-Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.

Solidification/stabilization of chromite ore processing residue via co-sintering with hazardous waste incineration residue.

In order to realize the harmless and resource disposal of hazardous waste incineration residue (HWIR) and chromite ore processing residue (COPR), this paper prepares glass-ceramics by HWIR. The COPR was co-sintered with the base glass of HWIR to realize the solidification and stabilization of COPR. The results shown that the single-stage sintering method has a simple process and low energy consumption, while the two-stage sintering method has better mechanical properties. Chromium in COPR may be solidified/stabilized by physical encapsulation and chemical fixation. When the content of COPR reaches 50%, the leaching concentration of Cr and Cr(VI) in the solidified body of HWIR solidified COPR (IRSC) is less than 5mg/L, which satisfies the US EPA and CN GB5085.3 standard limits. This study achieves waste control by waste and prepares solidified bodies (IRSC) with good mechanical properties, chemical corrosion resistance, and low leaching concentration of heavy metals, which provides feasibility for its engineering application.

Achieving strong Pb-Cr complexation in Mg/Al LDHs for ultrafast chromate ions separation and chrome recovery from complex water matrices

Layered double hydroxides with high anion exchange capacity and positive surface charge were being explored to remove metal oxo-anions. Environmental complexities such as the presence of other major interfering anions, organic matter, etc., drastically hampers their performance and limit their applicability. Based on the fundamental understanding of strong Pb-Cr complexation, in this work, Pb2+ incorporated LDH (LDH-Pb) was designed for selective chemisorption of chromate ions overcoming existing environmental limitations of LDH for chromium removal. LDH-Pb was designed by substituting Mg2+ ions of Mg/Al LDH with Pb2+ during crystallization process. Pb2+ have shown efficient incorporation at MII crystal sites of Mg/Al LDH and also sorbed in form of oxy-hydroxide on the surface. Pertaining to faster Pb-Cr complexation, strong binding, and formation of insoluble PbCrO4, LDH-Pb has shown efficient and higher removal of chromate ions than LDHs. Despite environmental complexities such as varying pH, competitive ions, organic matter, and natural matrices, efficiency of LDH-Pb remained less altered. Direct application (proof-of-concept) has been shown for extraction of chromate ions and chrome recovery from collected chromite ore processing residue leachate contaminated groundwater samples. Higher-order of removal kinetics and sorption capacity of LDH-Pb (n = 9.2, and 188.7 mg/g) further justified its candidature. Strong Pb-Cr complexation in LDH-Pb further resulted in faster and efficient continuous chromate filtration (47.3 mg/g) with adsorbent loaded syringe filters. Chrome recovery was achieved through easy acid dissolution of LDH from LDH-Pb-Cr waste (pH <3) resulting in chrome particles (monocrystalline micro rods) that have several potential industrial applications in various sectors.

One-step natural pyrite-assisted mechanochemical detoxification of Cr(VI) in soda ash Chromite Ore Processing Residue: Effectiveness, mechanisms, and life cycle analysis

Traditional wet and dry Chromite Ore Processing Residue (COPR) detoxification methods consume acid for pH adjustment and produce greenhouse gasses (CO2), yielding potential secondary pollution. It is imperative to develop an environmentally-friendly, efficient, and effective method for COPR treatment. This study reported the one-step natural pyrite-assisted mechanochemical detoxification of Cr(VI) in COPR. Under optimal conditions (i.e., the pyrite/COPR mass ratio of 5% and mechanochemical treatment at 600 rev./min for 2.5 h), Cr(VI) in COPR was efficiently reduced and immobilized, and the leaching concentration of total Cr met the regulatory limit of 5 mg/L. The X-ray absorption near-edge structure (XANES) analysis further verified that the Cr(VI) in treated COPR was 100% reduced to Cr(III). Potential mechanism could be that the enclosed non-exchangeable Cr(VI) in the chromite matrix was exposed to the COPR particle surface under the mechanical action; then, the redox reaction between pyrite and Cr(VI) were induced and enhanced through accumulated crystal defects and generated new surfaces with dangling bonds under mechanical forces. Life cycle assessment (LCA) results highlight that the high energy input is the major contributor eliciting environmental impacts for this current technology. More efficient and cleaner energy supplies should be sought alternatively to overcome the overall sustainability.

Treatment of methylene blue wastewater with nano-PbCrO4 photocatalyst prepared from chromite ore processing residue

Chromite ore processing residue (COPR) is an environmental threat due to its content of Cr (Ⅵ) which is a highly toxic substance. However, some metallic elements in COPR can create the intrinsic values of COPR. In this study, the nano-lead chromate (PbCrO4) was prepared by the chemical precipitation method for the water-soluble Cr (VI) leachate obtained by the simple water washing of COPR and the use of methylene blue (MB) degradation. Results demonstrated that the prepared lead chromate had good photocatalytic performance and stability. The degradation rate of MB was 76.78% with the conditions of the initial pH was 6, the initial concentration was 10 mg/L, the reaction time was 80 min, and the photocatalyst dosage was 1.0 g/L. Moreover, recycling results displayed that the degradation rate remained at 62.91% after five cycles which means it can be recycled. Meanwhile, the concentration of Cr (VI) in COPR decreased from 34.7 to 1.4 mg/L which is below the national emission standard of China (GB 5085.3–2007), that is 5 mg/L, achieving harmless treatment of COPR. Consequently, this study provides a new tactic for the resource utilization of COPR, and implements the governance concept of “waste control by turning waste into treasure”.

Treatment of soda ash chromite ore processing residue by Waste-Molasses-Based Ball Milling: A new strategy for disposal of waste with waste

The current technology of soda ash Chromium Ore Processing Residue (COPR) detoxification consumes a large amount of acids (as in the wet reduction process) and requires high temperature, along with the generation of carbon dioxide (as in the dry reduction process), and thus, is not cost-effective and environmentally-friendly. Waste molasses (WM) is waste biomass consisting of various reducing sugars, a cost-effective reducing agent. According to the idea of treating waste with waste, this study put forward a ball milling + WM method to detoxify leached-soda ash COPR (e.g., dried COPR containing 1622 ± 28 Cr(VI)/kg). Through response surface experiments, it was determined that under the best conditions (i.e., the milling speed = 600 rpm, the milling time = 2 h, WM dosage = 7.5% of the dry mass of COPR, and the ball-to-powder weight ratio, BPR = 12), the reduction efficiency of Cr(VI) reached 98.35%. The Toxicity Characteristic Leaching Procedure (TCLP) total Cr concentration reached 1.82 mg/L, meeting the standard (total Cr < 5 mg/L) of the US Environmental Protection Agency (USEPA). The associated reaction mechanism was determined as: under the action of mechanical energy, the structure of Cr(VI)-embedded matrixes was destroyed, and the internal, otherwise non-exchangeable, Cr(VI) was exposed and underwent redox reactions with WM under alkaline conditions. In brief, WM-based ball milling to detoxify COPR is cost-effective and provides a new reference for the disposal of waste with waste.

Factors Affecting the Detection of Hexavalent Chromium in Cr-Contaminated Soil

The alkali digestion pretreatment method in the United States Environmental Protection Agency (USEPA) Method 3060A could underestimate the content of Cr(VI) in Cr-contaminated soils, especially for soils mixed with chromite ore processing residue (COPR), which leads to a misjudgment of the Cr(VI) level in soils after remediation, causing secondary pollution to the environment. In this study, a new pretreatment method to analyze Cr(VI) concentration in contaminated soils was established. The impacts of soil quality, particle size, alkali digestion time and the rounds of alkali digestion on Cr(VI) detection in contaminated soils was explored and the alkali digestion method was optimized. Compared with USEPA Method 3060A, the alkaline digestion time was prolonged to 6 h and multiple alkali digestion was employed until the amount of Cr(VI) in the last extraction was less than 10% of the total amount of Cr(VI). Because Cr(VI) in COPR is usually embedded in the mineral phase structure, the hydration products were dissolved and Cr(VI) was released gradually during the alkaline digestion process. The amount of Cr(VI) detected showed high correlation coefficients with the percentage of F1 (mild acid-soluble fraction), F2 (reducible fraction) and F4 (residual fraction). The Cr(VI) contents detected by the new alkaline digestion method and USEPA Method 3060A showed significant differences for soil samples mixed with COPR due to their high percentage of residual fraction. This new pretreatment method could quantify more than 90% of Cr(VI) in Cr-contaminated soils, especially those mixed with COPR, which proved to be a promising method for Cr(VI) analysis in soils, before and after remediation.

Ecological and human health risk assessment of chromite ore processing residue (COPR) dumpsites in Northern India: A multi–pathways based probabilistic risk approach

In this study, ecological and human health risk assessment (HHRA) of the subsurface environment was performed due to the presence of six different contaminants, i.e., Cr(VI), U, F-, As, NO3-, and Mo, in the proximity of chromite ore processing residue (COPR) dumping sites of Northern India. The analysis covered a probabilistic risk assessment approach comprised of 10,000 Monte Carlo simulations (MCS) for multi-exposure scenarios following soil and water pathways. The results of ecological risk indexes indicated considerable (contamination factor (CF) = 5.6) to very high (maximum CF ~ 11.0) contamination of soil near the Rania contaminant site. The maximum and 95th percentile of CF obtained due to Cr(VI) were 4.547 and 11.129, respectively, during the monsoon season. Cancer risk (CR) value for dermal contact exposure via soil pathway for the Rania site was > 1–2 orders higher than the safe limit, posing a serious threat to human health. Further, oral ingestion and skin dermal exposure to groundwater in the proximity of Chhiwali dumping site were posing the highest risk with CR = 18.5 and 54.9 (>10−6), respectively to the adult population. Overall, the study recommends an urgent need to take intensive remediation action for these COPR dumpsites to ensure human health safety.