Clay minerals inhibit the release of Cd(II) during the phase transformation of Cd(II)-ferrihydrite coprecipitates

Clay minerals and iron (hydr)oxides are important geosorbents in controlling the migration of heavy metal cations in the environment. Despite the widespread occurrence of clay minerals/iron (hydr)oxides composites, their complex mutual effects on the fate of heavy metal cations are not well recognized. In this work, we investigated the effect of clay minerals on the redistribution of Cd(II) during the phase transformation of ferrihydrite containing coprecipitated Cd(II) (Cd-Fh). Three systems were considered: i.e., Cd-Fh, Cd-Fh/kaolinite composite, and Cd-Fh/montmorillonite composite. Our results showed that the transformation of Fh into goethite and hematite caused the release of Cd(II), while the presence of kaolinite and montmorillonite inhibited the phase transformation of Fh and the release of Cd(II), with montmorillonite being more effective in these process. Multiple factors contributed to the reduced release of Cd(II), including the retarded transformation of Fh, the buffering of solution pH, and the re-adsorption of the released Cd(II). Our findings show that clay minerals have multiple effects in reducing the release of heavy metal cations from Fh during its transformation process, which sheds new light on understanding the critical roles of nanominerals in modulating the migration and bioavailability of heavy metal cations in the environment.