Hexavalent chromium reduction through redox electrolytic cell with urea and cow urine as anolyte

The present study demonstrates the potential utilization of urea/cow urine as anolyte for Cr(VI) reduction via a simple three-chambered electrolytic cell. The inherent chemical energy in the dual-waste stream (Cr(VI)-urea/urine) is employed for its self-oxidation-reduction without the need for any external energy supply. Ni foam as electroactive anode and catalyst-free carbon felt as cathode, along with the appropriate positioning of ion-selective separators, indirectly improved the cell performance by impeding electrolyte crossover. A fundamental study involving five different membrane configurations was conducted herein to improve Cr(VI) reduction efficiency. The Cr(VI) reduction efficiencies were 11.84 ± 0.27%, 10.55 ± 0.17%, 77.24 ± 0.38% at 24 h, 13.57 ± 0.25% at 72 h with glass frit, cation exchange membrane (CEM), sandwiched membrane, and anion exchange membrane (AEM) as separators in a dual-chambered H-cell, respectively, with an initial Cr(VI) concentration of 100 mg/L. The fifth configuration, consisting of a middle chamber between the anode and cathode with the CEM close to the anode and the AEM close to the cathode resulted in a reduction efficiency of 79.98 ± 2.24% within 45 min for an initial Cr(VI) concentration of 400 mg/L. The first order rate constants were determined to be 0.024, 0.018, and 0.013 min^-1 for Cr(VI) concentrations of 100, 200, and 400 mg/L, respectively. Moreover, when urea was replaced with cow urine as anolyte, a reduction efficiency of 98.94 ± 1.28% was achieved at pH 2 in 45 min with 400 mg/L as initial Cr(VI) concentration. Furthermore, the XPS spectra of reduced Cr corresponding to binding energies of 579.4 eV and 589.3 eV, respectively, confirmed the presence of low-toxic Cr(III). The effect of applied load, initial Cr(VI) and urea concentration, Cr(VI) reduction under different initial H₂SO₄ concentrations were succinctly investigated to evaluate the performance of the electrolytic cell. The redox electrolytic cell can thus be an alternative to the conventional chemical or energy intensive processes for the reduction of hexavalent chromium.