Desorption Kinetics of Cadmium On Al Hydroxides Formed Under the Influence of Tannic Acid.

Abstract:

Tannic acid is one of the common high-molecular-weight organic acids (HMWOAs) in the biosphere of the Earth¡¦s Critical Zone. The structure of tannic acid has both phenolic hydroxyl and carboxyl groups, which are metal-bonding ligands and the presence of tannate in the hydrolytic reaction of Al ion can substantially affect the structural and surface properties of the resultant Al-hydroxide precipitates. Because Al hydroxides are one of the most important adsorbents in determining the bioavailability of heavy metals, understanding the interactions between heavy metals and Al hydroxides formed under the influence of tannate will lead to better understanding of heavy metal availability in the rhizosphere. In this study, the desorption kinetics of Cd adsorbed on Al hydroxides which were formed in the presence of tannate was investigated. The perturbation of the formation of Al hydroxides increased with an increasing tannate/Al molar ratio. Consequently, Cd adsorption increased due to the increases in the specific surface area, reactive Al-OH and Al-OH₂ sites, and tannate ligands co-precipitated in the adsorbents. Cd was desorbed from Al hydroxides by citrate, malonate and acetate, which are common low-molecular-weight organic acids (LMWOAs) present in root exudates and microbial metabolites. The effects of LMWOAs on the Cd desorption followed the order of citrate>malonate>acetate. The Cd desorption decreased with an increasing tannate/Al molar ratios of Al precipitates. The Cd desorption by LMWOAs was attributed to the formation of Cd-LMWOAS complexes and the enhanced dissolution of Al precipitates by LMWOAS. The results of this work illustrated the impact of structural perturbation of Al hydroxides by HMWOAs such as tannic acid on the Cd desorption of short-range ordered Al precipitates by LMWOAs such as citric, malonic, and acetic acids. Therefore, the significant roles of these biomolecules in influencing the mobility and toxicity of Cd at the soil-root interface merits attention.