surface. There are also no more exchangeable anions on the outer surface of solid phase while due to the possible M(OH)2 formation or hydroxo complexes of Cr3+, Cu2+, Zn2+ and Ni2+ ions at high pH
recoveries will be reduced. At pH lower than optimum value due to high protons abundance, the electrostatic interactions between charged cations and positively charged adsorption sites was occur that lead to a decrease in metal ion adsorption