Factors affecting the catalytic oxidative removal of soluble manganese in natural water by superfine powdered activated carbon and free chlorine

Abstract

Catalytic oxidation of dissolved divalent Mn ion (Mn2+) by free chlorine and superfine powdered activated carbon (SPAC) before microfiltration has been emerged as a new technology to remove Mn from natural water for drinking water production. However, Mn was not sufficiently removed in pilot-plant experiments yet. The present study clarified underlaying removal mechanisms. In natural water, soluble Mn exists as colloidal Mn with a molecular weight ≥ 10,000 Da in the oxidized form in addition to Mn2+. Mn2+ was removed by SPAC‑chlorine, but Ca2+, Mg2+, and NOM decreased the Mn2+ removal rate as a result of competitive adsorption. The colloidal Mn was not removed by SPAC‑chlorine, but could be removed by coagulation and microfiltration or ultrafiltration alone. With the addition of coagulant (poly‑aluminum chloride), the soluble Mn concentration increased because of redissolution of precipitated-oxidized Mn due an increase in local acidity, in particular at low temperature, and coagulation reduced soluble Mn removal rate due to hindrance of mass transfer. These negative impacts may be partially attenuated by using high-intensity mixing or waiting to add the coagulant after the oxidation of Mn2+ has progressed sufficiently. Mn removal rate changed with temperature and mixing intensity in the same trend as predicted by the mass-transfer model, but it was smaller than the prediction. Together with the effects of Ca2+, Mg2+, and NOM, it appears that not only mass transfer but also the process of adsorption is a factor affecting the overall Mn removal rate.