Environmentally friendly chromium electrodeposition: effect of pre-electrolysis on a Cr(III) bath in an anion-exchange membrane reactor

Abstract

In order to tackle the problem of high energy consumption and toxic substances produced in Cr(III) electrodeposition, a pre-electrolysis method was proposed, and its effects on the current efficiency and stability of the cathodic electrolyte were studied. The results show that pre-electrolysis had a remarkable influence on the pH of the Cr(III) bath. Pre-electrolysis can accelerate the hydrolysis of [Cr(H₂O)₆]³⁺, and this can be considered to provide sufficient electrochemically active Cr(III) complexes for electrowinning. Different pretreatment methods influence the cathodic polarization. Without pre-electrolysis, the cathodic polarization curve has one plateau, corresponding to the evolution of hydrogen. After pre-electrolysis, three plateaus in the polarization curve were found, corresponding to the evolution of hydrogen, the reduction of [Cr(H₂O)_nL_m]^3−m to [Cr(H₂O)_nL_m]^2−m, and the electrodeposition of chromium from [Cr(H₂O)_nL_m]^2−m. Moreover, the cathodic potential shifts towards a negative value with the increase of pH. The secondary adjustment of pH after pre-electrolysis has a significant influence on the current efficiency of Cr(III) electrolysis. When the pH is less than 1.3, no chromium is produced; when the pH is 1.6, the current efficiency reaches the highest value of 75.4%. With the extension of electrolysis time, the average current efficiency decreases. However, the average current efficiency is still able to reach more than 50% in an 8 h test, which should be attributed to the effects of pre-electrolysis. For the separation characteristics of an ion-exchange membrane reactor, the problems of toxicant produced on both of the electrodes were solved.