Simultaneous removal of arsenic, fluoride, and hydrated silica from deep well water by electrocoagulation using hybrid Al-Fe electrodes

Abstract

This paper deals with the simultaneous removal of arsenic, fluoride, and hydrated silica (HSi) from natural deep well water (26 µg L−1 As, 1.65 mg L−1F−, 160 mg L−1 HSi, 0.3 mg L−1PO43−, pH 7.5, and conductivity 450 µS cm−1) by electrocoagulation (EC). An up-flow EC reactor made up of an eight-cell stack using aluminum and iron plates as sacrificial electrodes in horizontal mode was used. The influence of the mean linear flow velocity (1.2 <u < 4.8 cm s−1) and current density (3 < j < 5 mA cm−2) applied to the EC reactor on the removal efficiency of As, F−and HSi was systematically examined. The best electrocoagulation trial was obtained at j = 5 mA cm−2 and u = 1.2 cm s−1, producing an aluminum and iron coagulant dose of 33.73 and 68.4 mg L−1, respectively, attaining residual concentrations of arsenic (CAs = 0.02 μg L−1) and fluoride (CF− = 0.88 mg L−1) below the World Health Organization (WHO) guideline (As < 10 μg L−1 and F− < 1.5 mg L−1). The residual HSi concentration was CHSi = 33 mg L−1, with electrolytic energy consumption and total operating cost of EC of 2.07 KWh m−3 and 0.29 USD m−3, respectively. The SEM-EDS, FXRD, XRD, FTIR, and Raman analyses of the Al-Fe flocs evidence the formation of iron silicates and aluminosilicates formed by the reaction between iron and aluminum with silica, respectively, and the existence of iron oxides and iron oxyhydroxides. The arsenic was removed by adsorption on aluminum and iron aggregates, while fluoride substitutes a hydroxyl from aluminum flocs.