Removal of Heavy Metal Ions from Water by Used Green Tea Leaves

Abstract

The effective utilization of used green tea leaves was investigated for removing heavy metal ions in an aqueous solution. The removal of the heavy metal ions by the used green tea leaves increased with increasing extraction time of the tea leaves. The optimum conditions for removing the heavy metal ions were achieved when 1 g of the used green tea leaves extracted five times with hot water was used as an adsorbent in an aqueous solution of 50 ml, using a stirring time of 30 min at 25℃ and pH4 for Cr⁶⁺, Fe³⁺ and Pb²⁺, and at pH6 for Cd²⁺, Co²⁺, Cu²⁺, Mn²⁺, Ni²⁺ and Zn²⁺. The catechin content of the extracted aqueous solution decreased with increasing amount of green tea leaves, there being a negative correlation between this catechin content and the percentage removal of the heavy metal ions. The removal of metal ions by used green tea leaves is suggested to involve physical adsorption by the tissue porosity and complexation between the metal ions and polymerized catechins in the leaves. The used green tea leaves can be used repeatedly as a heavy metal adsorbent after washing out the adsorbed ions with an acid solution.