Binding of Cd by ferrihydrite organo-mineral composites: Implications for Cd mobility and fate in natural and contaminated environments.

Adsorption and coprecipitation of organic matter with iron (hydr)oxides can alter iron (hydr)oxide surface properties and their reactivity towards nutrient elements and heavy metals. Organo−mineral composites were synthesized using humic acid (HA) and iron oxide, during coprecipitation with ferrihydrite (Fh) and adsorption to pre-formed Fh with two C loadings. The Fh−HA coprecipitated composites have a higher C content and smaller surface area compared to the equivalent adsorbed composites. NanoSIMS shows there is a high degree of spatial correlation between Fe and C for both composites, but C distribution is more uniform in the coprecipitated composites. The C 1s NEXAFS reveals a similar C composition between the Fh−HA coprecipitated and adsorbed composites. However composites at high carbon loading are more enriched in aromatic C, likely due to preferential binding of carboxyl functional groups on aromatic rings in the HA. The amount of Cd sorbed is independent of the composite type, either coprecipitated or adsorbed, but is a function of the C loading. Composites with low C loading show Cd sorption that is almost identical to pure Fh, while composites with high C loading show Cd sorption that is intermediate between pure Fh and pure HA, with sorption significantly enhanced over pure Fh at pH < 6.5. A bidentate edge-sharing binding was identified for Cd on pure Fh and Cd-carboxyl binding on pure HA. These findings have significant implications not only for the sequestration of Cd in contaminated environments but also the coupled biogeochemical cycling of Cd, Fe and C in the critical zone.