Fate of calcium, magnesium, and phosphate during Fe electrocoagulation: An investigation for mitigating cathodic scaling in phosphate removal process

Abstract

Fe electrocoagulation (Fe-EC) is efficient for the removal of phosphate (PO43−-P). The fate of Ca2+, Mg2+, and PO43−-P during the Fe-EC process is unclear, resulting in the difficulty for mitigating the cathodic scaling. After Fe-EC treatment for 10 min at 10 mA/cm2, PO43−-P was mainly distributed in the sediment (>90 %). Ca and Mg was mainly distributed in the solution (54 % − 97 %), followed by in the sediment (2 % − 44 %) and on the cathode (0 % − 6 %). Although only a small amount of Ca and Mg was precipitated on the cathode, with a long term operation, cathodic scaling would still occur. The precipitation of Ca on the cathode was mainly caused by the presence of HCO3− with other precipitates which directly precipitated with OH− as crystal seeds, because the precipitation of CaCO3 did not depend on the local pH near the cathode. The precipitation of Mg on the cathode was mainly caused by a lower applied current, because Mg(OH)2 which formed on the cathode would be prevented by H2 evolution with a higher current. Accordingly, the strategies for preventing cathode scaling are proposed. The precipitation of Ca can be prevented by increasing the current both with HCO3− and crystal seeds, and by increasing the current only with PO43−-P. It is hard to prevent the precipitation of Ca by controlling the current only with HCO3−. The precipitation of Mg can be prevented by increasing the current. With the initial concentrations of PO43−-P, HCO3−, Ca2+, and Mg2+ in the real wastewater 1.0 mg/L, 48 mg/L, 150 mg/L, and 30 mg/L, respectively, the amounts of Ca and Mg deposited on the cathode were much less at 10 mA/cm2 (5.8 ± 2.9 mg/L, 1.1 ± 0.3 mg/L) than those at 1 mA/cm2 (28.6 ± 4.6 mg/L, 5.0 ± 0.4 mg/L). With the above controlling strategies, the cathode scaling can be alleviated under different water quality conditions.