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.