KINETICS OF CADMIUM ADSORPTION ON ALUMINUM PRECIPITATION PRODUCTS FORMED UNDER THE INFLUENCE OF TANNATE

Abstract

The impeding effects of organic substances on the crystallization of Al precipitation products have been studied for more than three decades. However, the impacts of organics-induced structural perturbation and the resultant surface alteration of Al transformation products on their kinetics and mechanisms of the adsorption of trace metals still remain to be uncovered. This paper describes the kinetics of Cd adsorption on the short-range-ordered (SRO) Al precipitation products formed under the influence of tannate. The kinetics of Cd adsorption on the SRO Al precipitation products formed in the presence of tannate at initial tannate/Al molar ratios (MRs) of 0, 0.001, 0.01 and 0.1 was studied at an initial Cd concentration of 0.89 ?M, pH 5.5, background electrolyte of 10-2 M NaNO3, and solid phase concentration of 0.08 g L-1 at 278, 288, 298 and 313 K using the conventional batch method. The results show that, among the six empirical kinetic models tested, the second-order rate equation best described the kinetic data. Cadmium adsorption on SRO Al precipitation products was a multi-step process involving an initial fast reaction (0.083-0.75 h) followed by a slow reaction (0.75-4 h). The structural perturbation of Al precipitation products by tannate and the resultant development of their microporosity and alteration of surface and charge properties substantially enhanced the rate constants of both the fast and slow reaction processes of Cd adsorption. The heat of activation values for the fast reaction ranged from 27 to 41 kJ mol-1, indicating that diffusion is the rate-limiting step in Cd adsorption. Except for the Al precipitation products formed at a tannate/Al MR of 0 and 0.001, the heat of activation was >47 kJ mol-1 for the slow reaction, indicating that the rate-limiting step is a chemical process, which apparently involves bond breaking and formation on the surfaces of the SRO Al precipitation products. The pre-exponential factor, an index of the frequency of Cd collision with the surface of Al precipitation products, significantly increased with the increase of the initial tannate/Al MR, which is attributed to the development of microporosity and the increase in the specific surface area of the Al precipitation products formed under the influence of tannate ligands. The findings merit attention to the role of natural organics, which vary in the structure and functionality, in developing the microporous structure of Al precipitation products and the impact on the dynamics and mechanisms of Cd transformation and transport in natural environments.