Kinetic Study for Adsorption of Heavy Metals on Zeolite

Heavy metals-polluted water has negative impact on the ecosystem. In Malaysia, minimum discharge limit for Cu2+ and Zn2+ are 1 mg/L and 2 mg/L, respectively. Zeolite is a highly porous adsorbent and its performance is affected by various factors, including contact time and pH. Thus, the objective of this study is to identify an ideal operating condition to treat Cu2+ and Zn2+ solutions up to the allowable discharge limit, by considering the pH and contact time factors. Six kinetic models were studied to identify the adsorption mechanism of the heavy metal removal process. Single solute batch adsorption experiment was conducted within pH 3-11 from 30 to 150 min. Results showed that hydration enthalpy (ΔHhyd ) governed the selectivity of heavy metals, where a maximum of 90. 87％ Zn2+(ΔHhyd = -1955 kJ/mol ) and 82. 15％ Cu2+(ΔHhyd = -2010 kJ/mol) removals were found at pH≥7. Without pH adjustment, selectivity towards Cu2+ was higher compared with Zn2+ due to the size of hydration radii, where Cu2+ is 0. 11 ? smaller than Zn2+. By considering both pH and contact time factors, this study shows that by adjusting the pH of Zn2+ solutions to a minimum pH value of 7, the contact time required to achieve maximum Zn2+ removal rate was 90％, which can be achieved within 60 min. Meanwhile, zeolite performed better in Cu2+ removal without any pH adjustment where a maximum of 94％ was achieved at 120 min. Final concentration of 0.523 mg/L Cu2+ and 0. 981 mg/L Zn2+ were obtained in this study. Kinetic study showed that Ritchie ' s equation predicted Cu2+ adsorption the best, while Zn2+ adsorption could be represented by Elovich model. This suggested that the adsorption on the activate site governed Cu2+and Zn2+removal process. Hence, future work should focus on modifying zeolite surface to improve the adsorptive performance.