Cr(III) removal from synthetic and real tanning effluents using an electro-precipitation method

An electro-precipitation method is proposed for the treatment of synthetic (2654.4 mg L⁻¹ total Cr, pH 3.9) and real tanning effluents (2775 mg L⁻¹ total Cr, pH 4.0) containing elevated concentrations of Cr(III). Aluminum and copper anodes are thermodynamically (Pourbaix-type diagrams) and electrochemically evaluated in the synthetic electrolyte, as potential candidates to generate the chromium precipitation. Copper is selected due to better dissolution properties and avoiding passivation. The choice of copper anode is effective in both solutions, generating Cu⁺² species which displace the chromium species from Cr(III), and obtaining chromium recoveries >99% for these cases. The total Cr concentration is decreased down to 0.12 mg L⁻¹ in the real wastewater. Visual inspection of the treated synthetic solution and FTIR analysis reveal that a chromium hydroxide is formed as insoluble compound after the treatment, while cupric sulfate remains in the supernatant. A similar result is obtained for the real effluent, but other impurities and compounds different than chromium or copper are present in the precipitate. Although these species are not identified due to their complexity, they could be related to the nature of the tanning process, being derived from hide, surfactants, fixers and dyes. X-ray diffraction conducted for the sludge electro-generated from the treatment of the real tanning effluent support the FTIR and electrochemical evaluations, and this technique enables to identify that a very hydrated chromium hydroxide is formed after the treatment. The drying of this insoluble compound produces a distinctive hexagonal crystal of Cr(OH)₃·3H₂O, and dehydrated phases of chromium oxides (e.g. Cr₂O₃). These findings suggest that the electro-precipitation method herein proposed is efficacious to decrease the total chromium concentration below the maximum permissible limits (0.5 mg L⁻¹).