Removal of complexed iron by chemical oxidation and/or alum coagulation

Abstract

The fate of iron complexed by various organic compounds was investigated as a function of both chemical oxidative and coagulation removal methods. Dissolved organic carbon (DOC) utilized in the studies was obtained from a variety of sources and included humic and fulvic acids, tannic acid and oxalic acid. Oxidants evaluated were potassium permanganate, free chlorine, and chlorine dioxide. Both laboratory-scale and field monitoring studies were performed. The relative molecular weight distribution (MWD) of the DOC present was analyzed to evaluate how changes in this parameter affected the efficiency of soluble iron removal by oxidation. In addition, the MWD of selected coagulated samples was evaluated to determine how this parameter affected the fate of complexed iron during the coagulation of dissolved organic matter with alum.

A high degree of ferrous iron complexation occurred with the DOC dominated by higher molecular weight organics. This complexation rendered the iron stable against the addition of each of the oxidants evaluated. However, soluble Fe(II) complexed by low molecular weight organics was successfully removed by chemical oxidation. Potassium permanganate was found to be the most effective oxidant of the three oxidants utilized in the study.

The results indicated that soluble Fe(II) complexed by high molecular weight DOC can be efficiently removed by alum coagulation. The pH and alum dose utilized to produce effective DOC removal was also found to promote efficient complexed Fe(II) removal.