Electrochemical Reduction for Metal Recovery in Water Treatment

Abstract

Groundwater is an essential source of drinking water, and it often contains contaminants in the form of dissolved metal ions that pose health risks and affect its suitability for consumption. Removal of these contaminants by conventional treatment methods, such as oxidation and filtration, results in additional treatment steps for managing sludge. New techniques are needed to prevent the formation of low-value sludge and provide better control over the drinking water treatment process. This study focused on understanding the mechanistics of electrochemical reduction for its utility as a groundwater treatment method. It was done by passing artificial groundwater containing dissolved metal ions Fe2+, Mn2+, and Al3+ through a stainless steel cathode in an electrochemical cell. It resulted in the removal of these ions through electrochemical reduction and the recovery of metals as deposits. The experiments were performed with very high metal ion concentrations (0.72 mmol/L) to obtain clear and noticeable results from their electrochemical reduction.
It was observed that while Fe2+ and Mn2+ were removed from the water and deposited on the cathode, Al3+ did not get electrochemically reduced. It was due to the system settings adopted for the study being unsuitable for Al3+ removal. It highlights the potential of electrochemical reduction as a selective treatment process that offers control by manipulating the system settings. Upto 51.4% removal was observed in Fe experiments, while for Mn experiments, up to 22.22% removal was observed. As the deposits grew with the volume of water treated during an experiment, the electrochemical reduction declined. The removal of metal ions from water became negligible when the volume of water treated reached 6.3 L. The decrease in the effective surface area of the cathode because of deposits and the changing water composition near the cathode, as the volume of the water treated was increasing, was detrimental to the transfer of electrons from the cathode to the dissolved metal ions. It was also observed that the voltage rises continuously as the water is treated due to increasing cell resistance.
Another observation was that the pH of the water matrix is an essential factor in the electrochemical reduction of the species, with cathodic depositions increasing as the pH increases. Fe depositions increased 5.8 times from 0.109 μm at pH 4 to 0.630 μm at pH 7, while Mn at pH 4 had negligible deposits, which rose to 0.213 μm at pH 7. As the pH decreases, the entropic barrier of H+ ions decreases, leading to H2 production and a decline in FE.
Further, it was observed that the electrochemical reduction performs better when a water matrix has only one type of metal ion (individual) instead of a water matrix with all three types of metal ions simultaneously (combined). The FE of Fe2+ ions in the individual case is 35.05% while it is 11.5% in combined, at pH 7. It is 14.90% for Mn2+ in individual and 1.75% in combined. This could be from the decreased availability of the free metal ions in the combined case - due to the formation of bonds between the ionic species and changes in the thermodynamic feasibility of electrochemical reduction resulting from changes in the water matrix composition.
Further investigations are required to check performance with natural groundwater samples, optimize the system settings and find cathode material that best fits the desired contaminant removal.