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

BEATRIZ ELENA BONOLA; FABIOLA S. SOSA-RODRÍGUEZ; ULISES M. GARCÍA-PÉREZ; ISSIS ROMERO-IBARRA; EDUARDO RUBÉN HENQUÍN; JORGE VAZQUEZ-ARENAS

Journal of Hazardous Materials Advances

ELECTROCHEMICAL SOC INC

Año: 2021

2772-4166

The Cr(III) removal (3585 mg L-1) from real tanning discharges is herein conducted using electro-precipitation 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 determine the optimal conditions in the reactor, and compare against the real effluent. Cr removal, current efficiencies, energy consumptions and cell voltages are calculated to rationalize these variations in the experimental conditions. Thermodynamic calculations based on a Fraction Cr(III)-pH diagram reveal that Cr2O3(s) and FeCr2O4(s) precipitates are the most dominant species at pH > 3.5 in these sulfate solutions, in the absence and presence of Iron, respectively. 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 its pH needs to be modified to pH 5 or 6. The influence of the cathode material particularly occurs at current densities lower than 20 mA cm-2, where the Cr removal kinetic proceeds faster on the TiO2/RuO2 electrode than 316L SS. When the applied current density is increased to 30 mA cm-2, no significant differences in the Cr removal are detected between both cathodes. Residence times of 4800 (80 min) and 3600 s (60 min) are enough to remove all the Cr concentration at pH 5 and 6, respectively. The precipitates are mainly integrated by Cr2FeO4 (Chromite) regardless of the experimental condition used in the removal process, followed by minor traces of and . A reaction mechanism is proposed for the Cr(III) electro-precipitation relying on the thermodynamic diagrams, and characterizations of the precipitates carried out using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM)-EDAX. Keywords: chromium removal; Cr(III); tanning wastewater; electro-precipitation; Iron dissolution