Binding of Cd(II) by Amorphous Aluminum Hydroxide-Organophosphorus Coprecipitates: From Macroscopic to Microscopic Investigation

The mobility of Cd(II) in soils, sediments, and aquatic systems is strongly dependent on adsorption behaviors occurring at the mineral-water interface, and this process may be influenced by the presence of organic phosphorus (OP). In this study, we investigate Cd(II) adsorption onto amorphous aluminum hydroxide (AAH), both in the presence and absence of OP, represented by the widely abundant myo-inositol hexakisphosphate (IHP). Isothermal adsorption experiment coupled with attenuated total reflection Fourier transform infrared (ATR-FTIR) and 1H solid-state NMR spectra were employed. Physiochemical characterization shows that IHP can increase the surface negative charge and the number of surface sites. Isothermal results show that high IHP loading enhances Cd(II) adsorption while no obvious increase is observed at low IHP loading. The overall effect of IHP on Cd(II) sorption depends on the extent of two positive processes, i.e., (1) IHP can form ternary complexes with adsorbed Cd(II) on AAH and (2) IHP can increase the negative surface charge of AAH, and a negative process, i.e., AAH competes with Cd(II) for AAH surface sites. ATR-FTIR results confirm the possible formation of three structurally distinct ternary complexes, i.e., the AAH-IHP-Cd, AAH-Cd-IHP, and AAH-Cd-IHP-Cd. The analysis of 1H solid-state NMR demonstrates that IHP only increases the number of surface OH groups rather than changes their chemical environment and speciation. Cd does not bind to the AAH surface but mainly binds with the OH groups of IHP. All findings of this work suggest that the presence of high dose of OP promotes the retention of Cd(II) in soils, thereby decreasing their bioavailability to biota.