CdEDTA²- adsorption to weathered shale-limestone saprolite: modeling the adsorption mechanism and effect of Al-oxide co-dissolution on Fe-oxide dissolution dynamics

Partially weathered bedrocks that retain the fabric and structure of the parent rocks, commonly known as saprolite, exist extensively in the subsurface. A large quantity of Fe-, Al- and Mn-oxides is often present in saprolite soils. These surface oxides are very important in promoting and regulating geochemical reactions in the subsurface. The objective of this research is to identify and characterize the mechanisms of CdEDTA surface complexation in a saprolite derived from a shale-limestone bedrock on the Oak Ridge Reservation of the US Department of Energy. CdEDTA is a compound of two widely used industrial chemicals. As a result of the complexation of Cd and EDTA, the mobility of Cd is largely increased. This has had grave implications to the environment and the industrial facilities that use EDTA as a chelating agent for various purposes. The existence of Fe- and Al-oxides in the saprolite with highly pH-buffered soil solutions may result in the co-dissolution of Fe and Al oxides under the influences of pH and EDTA. In this research, a series of laboratory batch experiments was conducted with varying initial concentrations of CdEDTA in the presence of the saprolite. Three surface complexation mechanisms were evaluated in terms of their likelihood of describing the dynamics of CdEDTA adsorption and oxide dissolution. The modeling results verified a previous hypothesis that the formation rate of AlEDTA was faster than that of FeEDTA, largely because of the much larger initial concentration of Al ions in the soil solution. The subsequent CdEDTA dissociation and acid-promoted Al-oxide dissolution was thus identified as mechanisms that may interfere or inhibit the transformation of a surface CdEDTA ternary complex to FeEDTA and explain the slower formation rate of FeEDTA. It was concluded that acid-promoted dissolution of amorphous Al-oxide may exert certain degree of control on the dissolution dynamics of amorphous Fe-oxide.