Vanadium removal by electrocoagulation with anodes of zinc

Abstract

This work demonstrates that floccules of hydroxides and zinc oxides (Zn(OH)2 and ZnO) can be produced with an optimal surface electrical charge to adsorb vanadium complexes, when using zinc anodes and selecting appropriate electrocoagulation process conditions. It was found that the electrical charge of the floccules was mainly controlled by chemical and complex species that are a function of the pH and ionic strength in the suspension. At a pH of 7.0 and a concentration of 0.01 M NaCl, the floccules got a positive electric charge (+24.75 mV) and readily adsorbed negatively charged vanadium complexes. The highest adsorption capacity of floccules produced with zinc anodes was 112.08 mg/g (for V(III)) and 158.76 mg/g (for V(V)) with an equilibrium time of 20 min. The floccules, before and after the adsorption of vanadium complexes, were characterized to identify the formation of oxyhydroxides (Zn3(OH)2(V2O7)* 2 H2O) and zinc oxides (Zn3(VO4)2 * 3 H2O). The optimal vanadium removal was obtained under the following experimental conditions: pH = 7.0, 50 mg/L of vanadium, 0.10 g/L of floccules, 25 °C, and 40 min of contact time between the floccules and the contaminated water. The percentage of vanadium removal were 97.4%, 89.76% and 66.08% for V(III) and 99.9%, 90.6% and 76.54% for V(V) while the concentration in the polluted water was 10, 30 and 50 mg/L, respectively. A pseudo–second–order model describes the vanadium adsorption kinetics. At 25 °C, the experimental data were well represented by the Freundlich isotherm, and the maximal vanadium adsorption capacity was 158.76 mg/g.