Influence of chemical compositions and molecular weights of humic acids on Cr(VI) photo-reduction

► Low molecular weights (Mw) of HA bear more polar and aromatic C in its structure. ► The polar sites of HA dominate the photo-reduction of Cr(VI). ► Low Mw of HA exhibits greater photochemical efficiency for Cr(VI) reduction. ► Cr(VI) adsorption on HA is indiscernible, particularly on the small Mw of HA. ► Upon Cr(VI) reduction by HA, most of Cr(III) are released into the solution.