Humic Acids Restrict the Transformation and the Incorporation of Cd to Iron (Hydr)Oxides

Abstract

Iron (hydr)oxides and their association with organic matter significantly affect the mobility of heavy metals in natural soils and sediments. However, the behavior of Cd during crystalline iron (hydr)oxide formation in the presence of humic acid (HA) is still unknown. In this study, the speciation of Cd in iron (hydr)oxide-HA coprecipitates were studied by extraction, surface complexation model (SCM) calculation and characterization of the composites during the aging. The results showed that aging promoted the incorporation of ~30-50% of the added Cd ions into minerals in the binary iron (hydr)oxide systems. The reduction of Cd occurred earlier than hematite formation, indicating that the aggregation of amorphous iron (hydr)oxide led to the initial incorporation of Cd. The presence of HA restricted the crystallization of iron (hydr)oxide by the formation of tight mineral nanoparticle-HA aggregates, while there were negligible changes in the speciation of Cd and Fe during aging at high HA concentrations. Therefore, HA promoted the adsorption of Cd onto amorphous iron (hydr)oxide but limited the incorporation of Cd in mineral phases. The knowledge about the role of HA in iron (hydr)oxide transformation and Cd speciation is of great significance for the prediction of heavy metal behavior in soils and sediments.

Keywords: Ferrihydrite, Heavy metal pollution, Composites, Aging, Incorporation