Influence of key cations and anions on phosphate removal by Fe (0) electrocoagulation

The electrocoagulation technology is efficient for the removal of phosphate. The removal efficiency of phosphate in the secondary effluent is higher than that of in the synthetic wastewater. In order to identify the key factors for promoting the removal of phosphate in Fe(0) electrocoagulation (EC), this study investigated typical coexisted substances in the secondary effluent, including Ca2+, Mg2+, NH4+, HCO3-, SO42-. The results indicated that Ca2+ and Mg2+ were the key factors promoting the removal of phosphate by Fe(0) EC both under normal (5-6 mg/L) and low dissolved oxygen (DO) (<= 1 mg/L) conditions. Under low and normal DO conditions, the phosphate removal efficiency increased from 28 % to 98 % +/- 2 % (in 5 min) at 10 mA/cm(2) with the addition of Ca2+ (0.2-20 mM) or Mg2+ (0.2-2 mM) but decreased to approximately 70 % with the addition of a Mg2+ of 20 mM. Under low and normal DO conditions, the aggregation time of iron oxide flocs decreased from 10 min to 5 min when the concentration of Ca2+ increased from 0 to 20 mM and Mg2+ increased from 0 to 2 mM. The aggregation stability of flocs was not sufficient when the Mg2+ concentration was 20 mM at 5 mA/cm(2). Our results confirmed that Fe(0) EC was suitable for the removal of phosphate in the secondary effluent, especially containing moderate amounts of Ca2+ and Mg2+ regardless of the concentration of DO.