Influence of pH and Ionic Strength on the Adsorption of Copper and Zinc in Bentonite Clay

Abstract

This study concerned with the possibility of adsorption of some metal cations such as Cu 2+ and Zn 2+ on bentonite ore which were used as an inorganic fertilizer. Maximum adsorption was obtained at pH 5-5.5. The relationship between the amounts of metal cations adsorbed was found to follow the Freundlich adsorption isotherm. Preferential adsorption of both ions on adsorption sites was found to take place; Cu 2+ is being adsorbed at greater extent than Zn 2+ . Bentonite samples which allowed adsorbing Cu 2+ and Zn 2+ were then subjected to desorption experiments using EDTA/OAc. Sequential leaching of the adsorbed ions revealed that the amounts of desorbed ion are linearly proportional to the amount previously adsorbed. The phytotoxic threat of Zn and the risk of its contaminating surface or groundwater depend on its concentration in the soil solution, which is related to its adsorption and desorption from soil colloids. Owing to their adsorption capacity, aluminosilicate and mineral oxides are capable of removing many metals over a wide pH range representing an efficient way for the remediation of contaminated soils. Extensive research has assessed the metal adsorption characteristics of natural or modified clays and oxides and has demonstrated that pH variation influences zinc adsorption. Lacin et al . has found that the pH has positive effects on zinc adsorption by bentonite clay 2 . Usually, raising the pH increases the capacity for zinc adsorption 3-4 . The increase in adsorption is probably due to cation hydrolysis, since hydroxyl complexes are adsorbed preferentially over uncomplexed cations or to the increase in the negative surface charge of the adsorbent related to the increase in the concentration of hydroxyl ions, leading to an increase in attraction for cation in solution