Concurrent arsenic, fluoride, and hydrated silica removal from deep well water by electrocoagulation: Comparison of sacrificial anodes (Al, Fe, and Al–Fe)

Abstract

Some studies have reported the removal of As (As) and fluoride (F−) using different sacrificial anodes; however, they have been tested with a synthetic solution in a batch system without hydrated silica (SiO2) interaction. Due to the above, concurrent removal of As, F−, and SiO2 from natural deep well water was evaluated (initial concentration: 35.5 μg L−1 As, 1.1 mg L−1F−, 147 mg L−1 SiO2, pH 8.6, and conductivity 1024 μS cm−1), by electrocoagulation (EC) process in continuous mode comparing three different configurations of sacrificial anodes (Al, Fe, and Al–Fe). EC was performed in a new reactor equipped with a small flow distributor and turbulence promoter at the entrance of the first channel to homogenize the flow. The best removal was found at j = 5 mA cm−2 and u = 1.3 cm s−1, obtaining arsenic residual concentrations (CAs) of 1.33, 0.45, and 0.77 μg L−1, fluoride residual concentration (CF−) of 0.221, 0.495, and 0.622 mg L−1, and hydrated silica residual concentration (CSiO2) of 21, 34, and 56 mg L−1, with costs of approximately 0.304, 0.198, and 0.228 USD m−3 for the Al, Fe and Al–Fe anodes, respectively. Al anode outperforms Fe and Al–Fe anodes in concurrently removing As, F− and SiO2. The residual concentrations of As and F− complied with the recommendations of the World Health Organization (WHO) (As < 10 μg L−1 and F− < 1 mg L−1). The spectroscopic analyses of the Al, Fe, and Al–Fe aggregates showed the formation of aluminosilicates, iron oxyhydroxides and oxides, and calcium and sodium silicates involved in removing As, F−, and SiO2. It is concluded that Al would serve as the most suitable sacrificial anode.