Effects of sorbed and dissolved organic carbon on molybdenum retention by iron oxides

Molybdenum occurs in soil solution mostly as MoO42- and HmoO4. At low pH it is sorbed very strongly to iron oxides. Studies conducted with acid forest soils imply that dissolved organic carbon (DOC) and organic substances sorbed to iron oxides increase the mobility of Mo. The aims of the experiments presented here were to elucidate the role of dissolved and sorbed organic C in the retention of Mo by iron oxides. In our study we used pure goethite and goethite covered with organic carbon., The goethite samples were analyzed by XRD and BET measurement. Afterward, we incubated C free and C coated iron oxide samples with Mo for 12, 24, and 48h at pH 4. Molybdenum desorption kinetics from the incubated iron oxides were analyzed by anion exchange resin. In addition, XPS analyses of incubated iron oxides were conducted. DOC treatment did not affect the crystallinity of the iron oxides; in contrast, pore volume of the goethite samples clearly decreased after C covering. The desorption kinetics of Mo were biphasic. A fast desorption was followed by only slow Mo mobilization at longer desorption times. Desorbability of Mo from C-coated iron oxides, as well as mobilization rate, were higher than from pure oxide samples. We suggest the fast mobilization represents Mo desorption from the outer surfaces of iron oxides, while slow mobilization is due to diffusion out of the micropores and interdomains of iron oxides. XPS analyses show that, in the presence of C, the percentage of Mo sorbed at outer surfaces is increased. Carbon coverage seems to reduce the accessibility of the pores and may thus decrease the diffusion of Mo into the iron oxides. To prove whether there are chemical interactions between organic substances and Mo, dialysis experiments with DOC and p-jump experiments with pure and coated iron oxides were performed. The results imply that Mo is complexed by DOG. Molybdenum/DOC co-sorption seems to be possible. From our results we conclude that Mo fixation to iron oxides is mainly due to the diffusion of Mo into the pores of iron oxides. Organic substances seem to clog the pores and may thus act as diffusion barriers for oxyanions. In addition, Mo-DOG co-sorption and competition for sorption sites may be relevant for the Mo mobility in soils of high DOC concentration.