Direct electro-deposition of metallic chromium from K2CrO4 in the equimolar CaCl2-KCl molten salt and its reduction mechanism

Constructed wetlands as a sustainable technology for wastewater treatment with emphasis on chromium-rich tannery wastewater

Water scarcity is a major threat to agriculture and humans due to over abstraction of groundwater, rapid urbanization and improper use in industrial processes. Industrial consumption of water is lower than the abstraction rate, which ultimately produces large amounts of wastewater such as from tannery industry containing high concentration of chromium (Cr). Chromium-contaminated tannery industry wastewater is used for irrigation of food crops, resulting in food safety and public health issues globally. In contrast to conventional treatment technologies, constructed wetlands (CWs) are considered as an eco-friendly technique to treat various types of wastewaters, although their application and potential have not been discussed and elaborated for Cr treatment of tannery wastewater. This review briefly describes Cr occurrence, distribution and speciation in aquatic ecosystems. The significance of wetland plant species, microorganisms, various bedding media and adsorbents have been discussed with a particular emphasis on the removal and detoxification of Cr in CWs. Also, the efficiency of various types of CWs is elaborated for advancing our understanding on Cr removal efficiency and Cr partitioning in various compartments of the CWs. The review covers important aspects to use CWs for treatment of Cr-rich tannery wastewater that are key to meet UN's Sustainable Development Goals.

Study on the Behavior of Electrochemical Extraction of Cobalt from Spent Lithium Cobalt Oxide Cathode Materials

The molten salt electrochemical method was used to reduce the Co in spent LiCoO₂. The reduction mechanism of Co (III) in LiCoO₂ was analyzed by cyclic voltammetry, square wave voltammetry, and open circuit potential. The reduction process of Co (III) on Fe electrode was studied in NaCl-CaCl₂-LiCoO₂ molten salt system at 750 °C. The results show that the reduction process of Co (III) is a two-step reduction: Co (III) → Co (II) → Co (0) and they are all quasi-reversible processes controlled by diffusion. Phase analysis (XRD) shows that Li⁺ and Cl^2- in the molten salt form LiCl electrolysis experiments with different voltages were carried out, which proved the stepwise reduction of Co in LiCoO₂.

Chromium Determination in Leather and Other Matrices: A Review

Leather industry plays an essential role in the world's economy; however, it also has a negative environmental impact due to the generation of significant quantities of wastes, some of which are classified as hazardous chemicals. Chrome tanning, the most popular tanning process, employs chromium salts, acids, and some other chemicals. Some dyes can be also a source of chromium. As a result, hexavalent chromium, a known carcinogenic and mutagenic, can be found in leather products and cause allergic dermatitis or trigger other diseases. For this reason, it is important to quantify the total amount of chromium in final leather goods, as well as the oxidation state in which this element is found. This paper aims to summarize chromium contamination due to the leather production processes, and to review the analytical methods that have been used to determine chromium's most abundant species: Cr(III) and Cr(VI) in leather and other matrices (foodstuffs, cosmetic products, environmental, and pharmaceutical samples). The international and European regulations are presented as well as the last academic developments to extract and quantify chromium species. The future outlook of pretreatment and quantification techniques are also discussed in this work, with a special focus on chromium interconversions.

The electrochemical reduction mechanism of Fe3O4 in NaCl-CaCl2 melts

In order to study the process of Fe3O4 reduction by melt electro-deoxidation. Electrochemical method was used to analyze the reduction mechanism of Fe3O4 in NaCl-CaCl2 melts. The effects of cell voltage and time on the product were discussed through constant cell voltage electrolysis. The results showed: (1) The reduction of solid Fe3O4 to metallic Fe is a two-step process for obtaining electrons. (2) The transformation process (600 min, 0–1.0 V) of the electrolysis products with the increase of the cell voltage is as follows: Fe3O4 → FeO → FeO + Fe → Fe. (3) The intermediate product Ca2Fe2O5 was formed (2.0 V, 10–300 min), which inhibited the deoxygenation process in the early stage of the reaction. When the electrolysis time exceeds 60 min, the main reaction is the reduction of Ca2Fe2O5 to Fe.

Electrochemical Behavior of Fe3O4 in NaCl-CaCl2 Melts

The reduction and dissolution mechanism of Fe3O4 in NaCl-CaCl2 melts system are analyzed by thermodynamics. The electrochemical methods of Fe3O4 in NaCl-CaCl2 melts are studied by cyclic voltammetry, square wave voltammetry and open-circuit chronopotentiometry. The effects of temperature on reduction potential and peak current are investigated. The diffusion coefficients of Fe3+ and Fe2+ at different temperatures are calculated, and the activation energy E is obtained. Electrolytic products are obtained after 10 min of potentiostatic electrolysis in the melts of NaCl-CaCl2-Fe3O4 at 1073 K and 1.9 V. The results of XRD show that no other compounds are formed in the melts, indicating that Fe3O4 do not react chemically in the NaCl-CaCl2 melts system at 973 K, which is consistent with the theoretical analysis. Fe3O4 is reduced by two-step quasi-reversible reaction in the melts of NaCl-CaCl2 at 973 K to obtain iron: Fe3+ → Fe2+ → Fe. The two-step reaction is controlled by diffusion. The diffusion coefficients are 4.53 × 10–6 cm2·s–1 and 1.17 × 10–6 cm2·s–1, respectively. Fe3O4 is reduced in two steps. The potential shifts with temperature and decreases with the increase of the proportion of Fe3O4. The activation energies of the two-step reaction are 62.38 kJ·mol–1 and 77.52 kJ·mol–1. A deposited layer of uniform distribution, high coverage and dense iron is prepared.