Removal of residual organic matter from secondary effluent by iron oxides adsorption

Residual organic matter (ROM) removal is an important issue in wastewater reclamation and reuse processes. Use of iron oxide particles (IOPs) as adsorbents for further treatment of the secondary effluent was investigated to mainly remove non-biodegradable residual organic matter under various operating conditions. ROM removal by IOP adsorption was evaluated in terms of the changes of UV absorbance, COD, and DOC concentrations in feed (secondary effluent) and treated water. The respective relationship of UV removal with COD and DOC removal efficiencies exhibited somewhat different profiles, possibly due to the influence of ROM properties on adsorptive removal mechanisms by IOP, such as hydrophobicity and coordinative capability. The types of IOPs such as ferrihydrite, goethite, and hematite were compared, and it was found that amorphous ferrihydrite was most effective in ROM removal. Maximum ROM removal was occurring at a solution pH of approximately 6.0, which might be ascribed to the ability of ligand exchange related to the association and dissociation of ROM molecules and IOPs at differing pH levels. Substantial reduction in the oxidation state (OS) of the treated water at high ROM removal indicated the coordination of highly oxidized groups in ROM at the IOP surface, such as carboxylic groups.