Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

Asymmetric Alternative Current Electrochemical Method Coupled with Amidoxime-Functionalized Carbon Felt Electrode for Fast and Efficient Removal of Hexavalent Chromium from Wastewater

A large amount of Cr (VI)-polluted wastewater produced in electroplating, dyeing and tanning industries seriously threatens water ecological security and human health. Due to the lack of high-performance electrodes and the coulomb repulsion between hexavalent chromium anion and cathode, the traditional DC-mediated electrochemical remediation technology possesses low Cr (VI) removal efficiency. Herein, by modifying commercial carbon felt (O-CF) with amidoxime groups, amidoxime-functionalized carbon felt electrodes (Ami-CF) with high adsorption affinity for Cr (VI) were prepared. Based on Ami-CF, an electrochemical flow-through system powered by asymmetric AC was constructed. The mechanism and influencing factors of efficient removal of Cr (VI) contaminated wastewater by an asymmetric AC electrochemical method coupling Ami-CF were studied. Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) characterization results showed that Ami-CF was successfully and uniformly loaded with amidoxime functional groups, and the adsorption capacity of Cr (VI) was more than 100 times higher than that of O-CF. In particular, the Coulomb repulsion effect and the side reaction of electrolytic water splitting were inhibited by the high-frequency anode and cathode switching (asymmetric AC), the mass transfer rate of Cr (VI) from electrode solution was increased, the reduction efficiency of Cr (VI) to Cr (III) was significantly promoted and a highly efficient removal of Cr (VI) was achieved. Under optimal operating conditions (positive bias 1 V, negative bias 2.5 V, duty ratio 20%, frequency 400 Hz, solution pH = 2), the asymmetric AC electrochemistry based on Ami-CF can achieve fast (30 s) and efficient removal (>99.11%) for 0.5-100 mg·L^-1 Cr (VI) with a high flux of 300 L h^-1 m^-2. At the same time, the durability test verified the sustainability of the AC electrochemical method. For Cr (VI)-polluted wastewater with an initial concentration of 50 mg·L^-1, the effluent concentration could still reach drinking water grade (<0.05 mg·L^-1) after 10 cycling experiments. This study provides an innovative approach for the rapid, green and efficient removal of Cr (VI) containing wastewater at low and medium concentrations.

Selective electrochemical extraction of copper from multi-metal e-waste leaching solution and its enhanced recovery mechanism

Recycling activity for waste electrical and electronic equipment is always accompanied with leaching solution containing copper. Its selective extraction is of environmental and economic significance, and is beneficial for subsequent resource purification procedure. Compared with techniques such as chemical precipitation and solvent extraction, potentiostatic electrodeposition is outstanding with the advantage of high selectivity, electron as clean reagent, and minimal chemical usage. However, key factors affecting copper electrodeposition behavior as well as its kinetic process remain unclear, which blocks its further application. In this study, selective copper electrochemical extraction from multi-metal leaching solution of waste liquid crystal display panels is explored. Copper electrodeposition is analyzed from electrochemical and mass transport point of view, and the main results are summarized: (i) copper can be first electrodeposited due to its higher reduction potential compared with indium; (ii) applied potential and agitation are the most influential factors towards space-time yield and current efficiency; (iii) a semi-empirical kinetic model could quantitatively describe the influence of agitation and the time-current-concentration relationship. The model-predicted extraction rate agreed well with experimental data throughout electrodeposition; (iv) electrodeposition experiments show over 95% of copper can be selectively extracted as ultrafine copper powder (~150 nm) at 0.05 V (vs. SHE).

Improving photoelectrochemical reduction of Cr(VI) ions by building α-Fe2O3/TiO2 electrode

Photoelectrochemical process is an environmentally friendly technology and has a wide application in the control of environmental pollutants. Efficient nanophotocatalysts responsive to visible light are still highly attractive. In this work, α-Fe₂O₃/TiO₂ were grown on fluorine doped tin oxide (FTO) substrates by hydrothermal method for photoelectrochemical reduction of Cr(VI). Compared with the separate α-Fe₂O₃ and TiO₂ electrodes, the composite α-Fe₂O₃/TiO₂ electrodes show higher photocurrent density. Under visible light irradiation, 100% removal efficiency of Cr(VI) was obtained after 40 min treatment. The composite α-Fe₂O₃/TiO₂ electrodes showed an enhanced absorbance in visible light region and had good stability to photoelectrochemical reduction of Cr(VI). The role of hole scavengers (citric acid and oxalic acid) and pH values was systematically investigated. This novel intensification approach provides new insight on the application of photoelectrochemical reduction in environmental remediation.

Alkaline electrochemical advanced oxidation process for chromium oxidation at graphitized multi-walled carbon nanotubes

Alkaline electrochemical advanced oxidation processes for chromium oxidation and Cr-contaminated waste disposal were reported in this study. The highly graphitized multi-walled carbon nanotubes g-MWCNTs modified electrode was prepared for the in-situ electrochemical generation of HO₂^-. RRDE test results illustrated that g-MWCNTs exhibited much higher two-electron oxygen reduction activity than other nanocarbon materials with peak current density of 1.24 mA cm^-2, %HO₂^- of 77.0% and onset potential of -0.15 V (vs. Hg/HgO). It was originated from the highly graphitized structure and good electrical conductivity as illustrated from the Raman, XRD and EIS characterizations, respectively. Large amount of reactive oxygen species (HO₂^- and ·OH) were in-situ electro-generated from the two-electron oxygen reduction and chromium-induced alkaline electro-Fenton-like reaction. The oxidation of Cr(III) was efficiently achieved within 90 min and the conversion ratio maintained more than 95% of the original value after stability test, offering an efficient and green approach for the utilization of Cr-containing wastes.

Tuning α-Fe2O3 nanotube arrays for the oxygen reduction reaction in alkaline media

α-Fe₂O₃ nanotube arrays were fabricated and employed as low cost non-noble electrocatalysts for the oxygen reduction reaction (ORR). As-prepared α-Fe₂O₃ nanotube arrays exhibit excellent ORR catalytic activity and durability in alkaline media.

Electrolytic recovery of bismuth and copper as a powder from acidic sulfate effluents using an emew® cell

Effective removal of bismuth is a primary concern during copper electrorefining.

Recent advances in electrochemical detection of toxic Cr(vi)

The recent achievements and future directions in electrochemical quantification of toxic hexavalent chromium were outlined.

Integrated lignin-mediated adsorption-release process and electrochemical reduction for the removal of trace Cr(vi)

Hexavalent chromium Cr(vi) is extremely toxic and is classified as a human carcinogen, even at trace concentrations.