MAGNESIUM AND PH EFFECT ON ZINC SORPTION BY GOETHITE

Zinc sorption by synthetic goethite (surface area 95.2 m2 g-1, pH(PZC) 7.9) was studied in equilibrium conditions as a function of Mg concentration and pH at 25-degrees-C and 1 x 10-3 M NaNO3 indifferent electrolyte. Two sets of isotherms of Zn sorption by goethite were conducted, in the absence of Mg and in the presence of 25 mg Mg L-1, each at pH 5.4, 6.2, 7.0, 7.8, and 8.5. At each pH level, the initial Zn concentration ranged from 0.025 to 7.0 mg Zn L-1 (i.e., from 3.82 x 10(-7) to 1.07 x 10(-4) mol Zn L-1). Magnesium effect on Zn sorption was influenced by pH and Zn levels. Magnesium reduced Zn sorption at high Zn loadings, but not at low Zn coverages, at pH 5.4, 6.2, and 7.0, while it had no effect on Zn sorption at pH 7.8 and 8.5 at any Zn coverage. Zinc sorption by goethite increased with an increase in pH; for example, Zn sorption achieved at the initial solution Zn concentration of 3.06 x 10(-5) mol Zn L-1 at pH 5.4, 5.2, 7.0, 7.8, and 8.5 in the absence of Mg was, respectively, 16, 56, 107, 117, and 121 mumol Zn g-1; in the presence of 25 mg Mg L-1 it was, respectively, 11, 40, 98, 117, and 121 mumol Zn g-1. The highest Zn sorption attained in this study, 390 mumol Zn g-1 at pH 8.5, represented 16.6% coverage of the goethite surface. Zinc sorption was specific; at and below pH 7.0 it was suggested as due mainly to a mechanism of Zn diffusion into goethite lattice with subsequent fixation through possibly isomorphous substitution of lattice Fe3+ by Zn2+. Zinc sorption at pH 7.8 and 8.5 was postulated to have occurred mainly through the processes of hydrolysis and sorption of hydrolyzed Zn species plus nucleation mediated by the goethite surface.