Removal of arsenate from water by coagulation with in situ formed versus pre-formed Fe(III)

In this study the performance of Fe(III) formed in situ, generated by oxidizing Fe(II) with H₂O₂ or KMnO₄ (denoted as H₂O₂-Fe(II) or KMnO₄-Fe(II) process), for As(V) removal was compared with FeCl₃ coagulation and the mechanisms were explored. The optimum oxidant/Fe(II) molar ratios for the H ₂O₂-Fe(II) and KMnO₄-Fe(II) processes were 1:2 and 1:3, respectively. The advantage of Fe(III) formed in situ over pre-formed Fe(III) for As(V) removal was strongly dependent on the Fe/As ratio and the advantage became less significant at high Fe/As ratio. The hydrolysis of Fe(III) formed in situ generated much more Fe_b species than the pre-formed Fe(III) did. Moreover, Fe(III) formed in situ hydrolyzed more slowly and generated flocs with smaller size compared to the pre-formed Fe(III) did over the pH range of 6.0-8.0. An introduction of high shear force for 1 min in the flocculation stage to break the aggregates into small particles enhanced the uptake of As(V) by pre-formed Fe(III) to a larger extent than that by Fe(III) formed in situ. The good performance of Fe(III) formed in situ for As(V) removal should be ascribed primarily to slow floc aggregation rate and small floc size and secondly to the formation of large amount of polymeric hydrolysis species.