Adsorption of Cadmium on Aluminum Precipitates in the Absence or Presence of Catechins

Abstract

The products of Al hydrolysis play significant roles in terrestrial and aquatic environments, particularly through their binding of nutrients and pollutants. However, the effect of catechins on the crystallization of Al precipitates and the resultant surface alteration of Al transformation products on their kinetics and mechanisms of adsorption of Cd still remain to be uncovered. The objective of this paper was to investigate the kinetics of Cd adsorption on the Al precipitates formed under the influence of catechin. The experiments were conducted with initial catechin/Al molar ratios, interaction time, concentration of Cd ions, and temperature as the variables. Six kinetic models were employed to investigate adsorption mechanisms, and data resembled more closely a second-order process. The structural perturbation of Al precipitates by catechin, resulting in the development of their microporosity and alteration of surface and charge properties, substantially enhanced the rate constants of both fast and slow reaction processes of Cd adsorption. The role of catechins, which vary in the structure and functionality and are common in natural environments in exerting the structural perturbation of Al transformation products and the impact on the dynamics and mechanisms of Cd transformation and transport in soils and waters, deserves increasing attention.

Keywords:

• aluminum (AL)
• cadmium (Cd)
• catechin
• kinetic study

ACKNOWLEDGEMENTS

The authors thank the National Science Council, Taiwan, The Republic of China, for financial support by NSC 98-2621-B-002-001, 99-2811-B-002-006, and 99-2811-B-002-072.

Notes

SE: Standard error.

LSD_0.05: The least significant difference (LSD) values were calculated based on SE and t values at appropriate degrees of freedom at 95% confidence levels.

^a Where C_AO are concentrations of Cd solution at time 0, C_A are concentrations of Cd remaining in solution at time t, and k_A is rate coefficients.

^b r²is the correlation coefficient; p is the probability level of significant difference; SE is the standard error.

^c Fast raction; 0.083–0.75 h.

^d Slow reaction; 0.75–4 h.

^a Where C_AO are concentrations of Cd solution at time 0, C_A are concentrations of Cd remaining in solution at time t, and k_A is rate coefficients.

^b r²is the correlation coefficient; p is the probability level of significant difference; SE is the standard error.

^c Fast raction; 0.083–0.75 h.

^d Slow reaction; 0.75–4 h.

^a LSD_0.05 = 5.5 for the activation energy.

^b LSD_0.05 = 6.2 × 10¹⁰ for the pre-exponential factor.