The influence of cathode material, current density and pH on the rapid Cr(III) removal from concentrated tanning effluents via electro-precipitation

The Cr(III) removal (3585 mg L−1) from real tanning discharges is herein conducted using an electro-precipitation process in a rotating cylinder electrode (RCE) reactor. A 1018-type carbon steel anode and two different cathodes (TiO2/RuO2 or 316L stainless steel, 316L SS) are tested within this process, using three current densities (10, 20 and 30 mA cm−2), and two initial pH values for the real solution. A synthetic solution is initially evaluated to analyze more ideal conditions in the reactor. The Cr(III) electroprecipitation is not favorable in the original pH of the real tannery wastewater (∼3.55) unlike the synthetic solution (pH 2.8), presumably since the Fe-Cr interaction is hindered by impurities in the solution, whereby the pH of the real discharge needs to be modified to pH 5 or 6. Residence times of 4800 (80 min) and 3600 s (60 min) using 30 mA cm−2 are enough to remove all the Cr concentration at pH 5 and 6, respectively. Energy consumptions of 10 and 7.5 kWh m−3 are calculated for these treatments, respectively. The precipitates are mainly comprised of Cr2FeO4(s) (Chromite) regardless of the experimental condition used in the removal process, followed by minor traces of Cr(OH)3(s) and CrO(OH)(s). A reaction mechanism is proposed for the Cr(III) electro-precipitation relying on the thermodynamic diagrams, and characterizations of the precipitates.