Chromium (VI) Inhibition of Low pH Bioleaching of Limonitic Nickel-Cobalt Ore

Limonitic layers of the regolith, which are often stockpiled as waste materials at laterite mines, commonly contain significant concentrations of valuable base metals, such as nickel, cobalt, and manganese. There is currently considerable demand for these transition metals, and this is projected to continue to increase (alongside their commodity values) during the next few decades, due in the most part to their use in battery and renewable technologies. Limonite bioprocessing is an emerging technology that often uses acidophilic prokaryotes to catalyse the oxidation of zero-valent sulphur coupled to the reduction of Fe (III) and Mn (IV) minerals, resulting in the release of target metals. Chromium-bearing minerals, such as chromite, where the metal is present as Cr (III), are widespread in laterite deposits. However, there are also reports that the more oxidised and more biotoxic form of this metal [Cr (VI)] may be present in some limonites, formed by the oxidation of Cr (III) by manganese (IV) oxides. Bioleaching experiments carried out in laboratory-scale reactors using limonites from a laterite mine in New Caledonia found that solid densities of ∼10% w/v resulted in complete inhibition of iron reduction by acidophiles, which is a critical reaction in the reductive dissolution process. Further investigations found this to be due to the release of Cr (VI) in the acidic liquors. X-ray absorption near edge structure (XANES) spectroscopy analysis of the limonites used found that between 3.1 and 8.0% of the total chromium in the three limonite samples used in experiments was present in the raw materials as Cr (VI). Microbial inhibition due to Cr (VI) could be eliminated either by adding limonite incrementally or by the addition of ferrous iron, which reduces Cr (VI) to less toxic Cr (III), resulting in rates of extraction of cobalt (the main target metal in the experiments) of >90%.

Long-Term Releasing Kinetics of Chromium from Leather

The release of chromium from leather inevitably results in potential risks and this study is conducted to investigate the long-term releasing behavior. The leaching tests proceed using water at solid to liquid ratio of 1:20 and rotational speed 60 r/min for 240 hours to simulate the release of chrome leather under natural conditions. The experimental data successfully fit with the Pseudo-second-order equation, Elovich equation, and Weber-Morris model, indicating the long-term leaching behavior of chromium in heterogeneous leather is controlled by liquid-solid film, while the interparticle and intraparticle diffusion also play important roles. The leachable chromium accounts for 2.8-4.5% total chromium in leather and increases with temperature. The Three-compartment model depicts the releasing process as rapid, slow, and very slow stages, and temperature mainly affected the very slow stage. The amount of released chromium in rapid and slow stages slightly increases with temperature, which could be used to assess the hazard of chrome leather.

Long-term Metal Fume Exposure Assessment of Workers in a Shipbuilding Factory

This study aims to assess the metal fume exposure of welders and to determine exposure rates for similar exposure groups in a shipyard through the use of Near-field/Far-field (NF/FF) mathematical models and Bayesian decision analysis (BDA) technique. Emission rates of various metal fumes (i.e., total chromium (Cr), iron (Fe), lead (Pb), manganese (Mn), and nickel (Ni)) were experimentally determined for the gas metal arc welding and flux cored arc welding processes, which are commonly used in shipyards. Then the NF/FF field model which used the emission rates were further validated by welding simulation experiment, and together with long-term operation condition data obtained from the investigated shipyard, the predicted long-term exposure concentrations of workers was established and used as the prior distribution in the BDA. Along with the field monitoring metal fume concentrations which served as the likelihood distribution, the posterior decision distributions in the BDA were determined and used to assess workers’ long-term metal exposures. Results show that welders’ Fe, Mn and Pb exposures were found to exceed their corresponding action levels with a high probability, indicating preventive measures should be taken immediately. The proposed approach provides a universal solution for conducting exposure assessment with usual limited number of personal exposure data.

Identification of a New Microalgal Strain From Chromite Mine Wastes for Detoxification of Hexavalent Chromium for Sustainable Crop Growth

Sukinda chromium mine is well known for its chromium (Cr) reserve in India. It accounts for 97% of Cr production in the country. The open cast mining results in the seepage and accumulation of chromium in the nearby paddy fields through soil runoff. Deposition of high concentrations of toxic Cr6+ adversely affected the growth and productivity of rice plants. It was studied that Cr6+ toxicity can be counteracted by the microbes especially algae. Hence, an attempt has been made for the exploration of an indigenous micro-algal strain for the detoxification of Cr6+ in the rice fields. Three different micro-algal strains were isolated from the waterlogged regions of the mine waste area and tested against Cr6+. The average concentration of Cr6+ in the soils of rice fields and its surrounding regions was estimated around 40ppm. In vitro study was conducted to determine the optimal growth parameters for the growth of the algal strains. The concentration of total chromium availability was determined by using ICP-OES (Inductively coupled plasma atomic emission spectroscopy. It showed that all the algal-stains were able to detoxify Cr6+, but the best result (89.63%) was observed in one strain ‘SM3’. SEM-EDX study also showed that there was no Cr adsorbed on the surface of the algal strain. Raman Spectroscopy study confirmed the reduction of Cr6+ to Cr3+ in algal strain. The strain was identified as Fischerella sp. (Accession no. MK422171) through morphological and molecular characterization. This algal strain can be used for the bioremediation of chromium contaminated crop fields.

The Comprehensive Toxicological Assessment of Total Chromium Impurities in Traditional Herbal Medicinal Product with Thymi herba (Thymus vulgaris L. and Thymus zygis L.) Available in Pharmacies in Poland: Short Communication

Scientific reports about elemental impurities in final pharmaceutical products are essential from a regulatory point of view; unfortunately, there is a lack of studies about this important toxicological topic. The aim of our short communication was determination of total Cr impurities in traditional herbal medicinal products (THMP) with Thymi herba (Thymus vulgaris L. and Thymus zygis L.) available in Polish pharmacies (n = 6, because only six manufacturers produce this kind of pharmaceutical products in Poland). The total content of Cr impurities was determined by atomic absorption spectrometry using electrothermal atomization (ETAAS). Applied comprehensive toxicological risk assessment was based on three main tiers: Tier 1, the comparison of raw results as total Cr impurities profile (metal per L of THMP) with ICH Q3D guideline standards; Tier 2, the estimation of total Cr exposure with a single dose; and Tier 3, the estimation of total Cr daily exposure. We confirmed that total Cr impurities were present in all analyzed THMP with thymi herba (the observed level was below 6.0 µg/L). Total Cr concentration in a single dose can be deceptively high in comparison to the raw results but is not a threat to patients (20.15–63.45 ng/single dose). Moreover, the estimation of total Cr daily exposure shows that all analyzed THMP are characterized by daily dose (40.30–181.41 ng/day) below PDE value (10,700 mg/day); hence, all products meet the standards of ICH Q3D elemental impurities guideline.

Synthesis, Optical Characterization, and Adsorption of Novel Hexavalent Chromium and Total Chromium Sorbent: A Fabrication of Mulberry Stem Biochar/Mn-Fe Binary Oxide Composite via Response Surface Methodology

In this study, a new generation chromium sorbent, mulberry stem biochar/Mn-Fe binary oxide composite (MBC-MFC), was fabricated by chemical precipitation on carbonized mulberry stem according to response surface methodology (RSM) results. RSM was more convenient to figure out the optimized preparation condition of MBC-MFC theoretically for achieving a maximum removal efficiency of Cr(VI) and total chromium (TCr), compared to labor-intensive orthogonal experiments. The RSM results showed that Fe/Mn concentration (CFe; CMn), MBC activation temperature after soaking in KOH solution (T), and pH during precipitation of Fe-Mn oxide were three main factors to significantly affect the efficiency of MBC-MFC (p < 0.05) in Cr(VI) and TCr removal. With the selected condition (CFe = 0.28 mol/L; CMn = 0.14 mol/L; T = 790°C; pH = 9.0), MBC-MFC was synthesized with a large surface area (318.53 m2/g), and the point of zero charge values of MBC-MFC was 5.64. The fabricated MBC-MFC showed excellent adsorption performance of Cr(VI) and TCr in an aqueous solution. The maximum Cr(VI) and TCr removal capacity of MBC-MFC was 56.18 and 54.97 mg/g (T = 25°C, pH = 3.0, t = 48 h, and dosage = 0.10 g/50 ml), respectively, and the maximum Cr(VI) adsorption of MBC-MFC was 4.16 times that of bare MBC, suggesting the synergistic effects of Fe/Mn oxides and MB on the performance of MBC-MFC in Cr(VI) and TCr removal. The adsorption mechanism of MBC-MFC on chromium was mainly contributed by surface complexation and electrostatic attraction. Our study offers valuable outlooks to develop high-performance biochar-based sorbents for heavy metal removal and sustainable environmental remediation.

Nanoscale zero-valent iron loaded vermiform expanded graphite for the removal of Cr (VI) from aqueous solution

Cr (VI) is indispensable in industrial manufacturing, and its extensive use leads to severe heavy-metal pollution in the water environment around people, posing a great danger to physical health and living environment of multitudinous organisms. Expanded graphite (EG) is considered as a typical material for adsorption, while nanoscale zero-valent iron (nZVI) can be applied to degrade and sedimentate various organic or inorganic pollutants. In this study, a simultaneous collaboration of EG and nZVI is carried out, with the investigation on the influence of different test conditions for adsorption performances. These findings demonstrate that nZVI@EG manifests favourable adsorptive performance on the removal of hexavalent chromium efficiently. nZVI, acting as an electron donor, is supposed to reduce Cr (VI) to Cr (III), turning itself into iron oxide or hydroxide. The whole process is an exothermic reaction, accompanying chemical reduction and physical adsorption. And Cr (III) is fastened on the appearance by deposition of chromium hydroxide or ferrochromium complex precipitation, which greatly reduces the total chromium content in the aqueous solution. Herein, as a new composite adsorbent, nZVI@EG shows promising prospects of practical applications in water contamination and environmental remediation.

Co-Occurrence of Metal Contaminants in United States Public Water Systems in 2013–2015

The United States Environmental Protection Agency monitors contaminants in drinking water and consolidates these results in the National Contaminant Occurrence Database. Our objective was to assess the co-occurrence of metal contaminants (total chromium, hexavalent chromium, molybdenum, vanadium, cobalt, and strontium) over the years 2013–2015. We used multilevel Tobit regression models with state and water system-level random intercepts to predict the geometric mean of each contaminant occurring in each public water system, and estimated the pairwise correlations of predicted water system-specific geometric means across contaminants. We found that the geometric means of vanadium and total chromium were positively correlated both in large public water systems (r = 0.45, p < 0.01) and in small public water systems (r = 0.47, p < 0.01). Further research may address the cumulative human health impacts of ingesting more than one contaminant in drinking water.