Our research on arsenate removal by iron electrocoagulation (EC) produced highly variable results, which appeared to be due to Fe2+ generation without subsequent oxidation to Fe3+. Because the environmental technology literature is contradictory with regard to the generation of ferric or ferrous ions during EC, the objective of this research was to establish the iron species generated during EC with iron anodes. Experimental results demonstrated that Fe2+, not Fe3+, was produced at the iron anode. Theoretical current efficiency was attained based on Fe2+ production with a clean iron rod, regardless of current, dissolved-oxygen (DO) level, or pH (6.5-8.5). The Fe2+ remaining after generation and mixing decreased with increasing pH and DO concentration due to rapid oxidation to Fe3+. At pH 8.5, Fe2+ was completely oxidized, which resulted in the desired Fe(OH)3(s)/ FeOOH(s), whereas, at pH 6.5 and 7.5, incomplete oxidation was observed, resulting in a mixture of soluble Fe2+ and insoluble Fe(OH)3(s)/FeOOH(s). When compared with Fe2+ chemical coagulation, a transient pH increase during EC led to faster Fe2+ oxidation. In summary, for EC in the pH 6.5-7.5 range and at low DO conditions, there is a likelihood of soluble Fe2+ species passing through a subsequentfiltration process resulting in secondary contamination and inefficient contaminant removals.

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Arsenite [As(III)] and arsenate [As(V)] removal using zirconium and titanium coagulants was compared with removal using ferric chloride (FeCl3) and alum in a standard challenge water. As(III) adsorption was significantly lower than As(V) adsorption at all pH levels with all coagulants. The highest As(V) loadings both on mass and molar bases were observed with FeCl3. As(V) removal increased with decreasing pH with all coagulants. The small amount of As(III) removal observed was generally independent of pH. Alum did not remove any As(III). The highest As(III) loadings were observed with titanium(III) [Ti(III)], which appeared to be oxidized to As(V) by peroxide resulting from Ti(III) hydrolysis. The types of sludge produced by all coagulants passed the toxicity characteristic leaching procedure and the waste extraction test. When chemical costs were compared, FeCl3 was the most cost‐effective coagulant for As(III) and As(V) at all three pH values, and As(III) chemical coagulation costs were five to 20 times higher than those for As(V). Alum was four to eight times the cost of FeCl3 for As(V) removal.

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