Phosphate removal from water using freshly formed Fe–Mn binary oxide: Adsorption behaviors and mechanisms

Freshly formed metal oxide suspensions usually have high reactivity and are easy to achieve in-situ preparation and dosing in water treatment. In this study, a freshly formed Fe–Mn binary oxide (FMBO) suspension was prepared for phosphate removal using simultaneous oxidation and coprecipitation method. FMBO was characterized by SEM, TEM, XRD and zeta potential and compared with the mixture of ferric oxide and manganese oxides (FMMO), hydrous ferric oxide (HFO) and hydrous manganese oxide (HMO). Results indicated that ferric oxide was relatively enriched on the surface of nanostructured binary oxide, resulting in its pHZPC and XRD pattern close to HFO. At pH 3–10, phosphate removals followed trend: FMBO > FMMO > HFO > HMO. The pseudo-second-order kinetics model provided a better fit for phosphate adsorption onto FMBO, indicating that chemisorption may play a dominant role. The maximum adsorption capacity estimated from the Langmuir equation was 0.223 mol-P/mol-Fe at pH 5.0. The competition effects of coexisting substances such as sulfate, bicarbonate, silicate and DOC on phosphate adsorption using FMBO were not significant. Phosphate adsorption onto FMBO could be attributed to electrostatic interaction and replacement of surface hydroxyl groups via formation of monodentate and bidentate complexes.