•The optimum molar ratios of H2O2 and KMnO4 to Fe(II) were determined.•The advantage of Fe(III) formed in situ was more obvious at lower Fe/As ratio.•Feb species were crucial for As(V) removal bys Fe(III) formed in situ.•Smaller floc sizes and growth rates facilitated the uptake of As(V).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 H2O2–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 Feb 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.Graphical abstract