Sustainable and fast elimination of high Cr(III) concentrations from real tannery wastewater using an electrochemical-chemical process forming Cr₂FeO₄

Here, an electroprecipitation process is proposed to eliminate high concentrations of Cr(III) (5424.3 mg L^-1), and organic impurities contained in a real tanning discharge. The electrochemical reactor is comprised of TiO₂/RuO₂ as cathode, and carbon steel (1018-type) as anode, evaluated at 10 and 20 mA cm⁻² (applied current density), and pH values equal 5 and 6. The electroprecipitation of Cr(III) cannot be conducted at the original wastewater pH (4.53), due to impurities hampering the occurrence of this reaction, and the need of hydroxyl ions to enhance the metal removal, whereby the original pH was varied to pH 5 or 6. Using 20 mA cm⁻² (current density), treatment times of 1 and 0.5 h are required to eliminate the entire Cr concentration at pH values of 5 and 6, respectively. 6 and 3.5 kWh m⁻³ of energy consumptions are estimated for these respective experimental conditions. This consumption considerably decays at 10 mA cm⁻². It is found that Cr₂FeO_4(s) is the main solid precipitate recovered from the treatment method, subsequently low contents of CrO(OH)_(s) and Cr(OH)_3(s) are collected, regardless of the applied current density and solution pH. Visual inspection and Scanning Electron Microscopy (SEM) analysis reveal that organic impurities are also removed. X-ray photoelectron spectroscopy (XPS) confirms this information along with the formation of other Cr and Fe species on the surface of the precipitates. The electroprecipitation mechanism for Cr(III) removal is discussed based on Pourbaix-type diagrams, and X-ray diffraction (XRD) analysis of the recovered solids.