Arsenic sorption to nanoparticulate mackinawite (FeS): An examination of phosphate competition

•A competitive effect of PO43− was examined on sorption of As(V) and As(III) to nanoparticulate FeS.•PO43− did not affect sorption of either As(V) or As(III) to nanoparticulate FeS at pH 6, 7 and 9.•XANES spectroscopy indicated the involvement of several As redox transformations and formation of sorbed species.•Nanoparticulate FeS immobilized 99% As in sulfidic reduced environments, in the presence of PO43−.