In this work, direct As(III) removal without pre-oxidation by the in-situ generated Fe(III) hydroxide derived from ferrous sulfate on the surface of calcium carbonate was investigated. The coagulation sediments were characterized by XPS, Zeta potential, AFM and SEM, respectively and the mechanism of the high As(III) removal efficiency was discussed and compared with the traditional neutralization of Fe2(SO4)3 at the same pH around 6. In the FeSO4 + CaCO3 system, Fe(II) ions dispersedly absorb on the surface of the CaCO3 particles. Accompanied with the slowly hydrolyzed OH− from CaCO3 and the O2 in air, Fe(III) hydroxide is in-situ generated, which maintains nanosized particles without evident agglomeration into larger ones as usually observed with the alkali neutralization and therefore presents high activity to As(III). Although Fe arsenite has higher solubility than the arsenate, the in situ formed Fe(III) hydroxide demonstrates high reactivity to form basic Fe arsenite with Fe/As molar ratio at 4.5, resulting in almost complete arsenic precipitation. By this pathway, it is possible to remove directly arsenic at the unchanged trivalent state, without the usual requirement to oxidize As(III) to As(V), verified by the measurements of arsenic valence and hydroxyl radicals (OH) formation during the conversion of Fe(II) into Fe(III). Display omitted •In-situ generated Fe(III) hydroxide achieves great As(III) removal effect.•Fe(II) ions dispersedly absorb on the surface of the CaCO3 particles.•Fe(III) hydroxide maintains a nanosized state without evident agglomeration.•As(III) directly removed as basic iron arsenite without usual oxidization to As(V).