Temperature Dependence of Zinc Sorption by Goethite
Abstract
Sorption of Zn by synthetic goethite [surface area 95.2m2 g-', pH(ZPC) 7.9] was studied in equilibrium conditions as functions of temperature and pH. Two sets of isotherms of Zn sorption by goethite were conducted at 25°C and 44°C, each at pH 5.4, 6.2, 7.0 and .5. At each pH level, the initial Zn concentration ranged from 0.025 to 7.0 mg Zn (i.e., from 3.82 x 101 to 1-.07 x 104 mol Zn Zinc sorption by goethite increased with an increase in both temperature and pH. The maximum Zn sorption experimentally achieved at 25°C at pH 5.4, 82, 7.0 and 8.5 were, respectively, 14, 54, 185 and 370 umol Zn g'. Sorption of Zn was markedly endothermic at all pH levels, and the temperature-induced increase in sorption was generally most pronounced at low pH and high:syst_Am loadings. Mean i&G"values ranged from -47 kJ mot' at pH 8.5 to -38 kJ mot' at pH5.4;from27 kJ riot' at pH 8.5 to 61 kJ mot' at pH 5.4; and A..S" from 242 J mot' deg -' at pH 8.5 to 322 J mot' deg, at pH 5.4. lsosteric heats of Zn sorption, Q,, ranged from 4 to 113 kJ mot', and were generally higher at low pH than high pH.
