Transformation and removal of arsenic in groundwater by sequential anodic oxidation and electrocoagulation

摘要

Oxidation of As(Ⅲ) to As(Ⅴ) is generally essential for the efficient remediation of As(Ⅲ)-contaminated groundwater. The performance and mechanisms of As(Ⅲ) oxidation by an as-synthesized active anode, SnO_2 loaded onto Ti-based TiO_2 nanotubes (Ti/TiO_2NTs/Sb-SnO_2), were investigated. The subsequent removal of total arsenic by electrocoagulation (EC) was further tested. The Ti/TiO_2NTs/Sb-SnO_2 anode showed a high and lasting electrochemical activity for As(Ⅲ) oxidation. 6.67 μM As(Ⅲ) in synthetic groundwater was completely oxidized to As(Ⅴ) within 60 min at 50 mA. Direct electron transfer was mainly responsible at the current below 30 mA, while hydroxyl radicals contributed increasingly with the increase in the current above 30 mA. As(Ⅲ) oxidation was moderately inhibited by the presence of bicarbonate (20 mM), while was dramatically increased with increasing the concentration of chloride (0-10 mM). After the complete oxidation of As(Ⅲ) to As(Ⅴ), total arsenic was efficiently removed by EC in the same reactor by reversing electrode polarity. The removal efficiency increased with increasing the current but decreased by the presence of phosphate and silica. Anodic oxidation represents an effective pretreatment approach to increasing EC removal of As(Ⅲ) in groundwater under O_2-limited conditions.