Adsorption of copper and zinc ions on goethite : simulation and potentiometry

Abstract

The acid/base and complexation reactions of Cu2+and Zn2+ on goethite surface has been investigated by potentiometric titrations at 25.0±0.1℃ within the pH range 3.5 ≤ pH ≤ 9.5 in 0.001-0.500 M NaNO3. The computational evaluation of the potentiometric data revealed the complex species with the intrinsic equilibrium constants for each system as follows: the acid/base of Goethite system: =FeO- (log βint1.-1 = -9.9to -9.0) and =FeOH2+(log p βint1.-1 = 5.8 to 6.8),the Goethite-Cu2+ system: =FeOHCu2t (log βint1.0.1 = 4.5 to 6.0), =FeOCu+ (log βint1.-1 .1=0.8 to 1.3) and =FeOCuOH(log βint1.-2.1 = -5.4 to -8.0), the Goethite-Zn2+system: (=FeOHh Zn2+(log βint2.0.1 = 8.1 to 9.2), =FeOZn+ (leg βint1.-1.1 = -1.9 to -3.0) and =FeOZn(OH)-2(log βint1.-3.1 = -16.4 to -18.2), the Goethite-Cu2+-SO12system: =FeOHCuSO4(log βint1.0.1.1 = ll.0to 13.7), =FeOCuSO4 (log βint1.-1.1.1 = 6.0 to 9.0) and =Fe0CuOHSO42- ( log βint1.-2.1.1 = 1.7 to 2.5), the Goethite-Zn2+-SO42-system: (=FeOH)2ZnS04(log βint1.0.1.1 = 18.0 to 20.3),=FeOZnSO4(log βint1.-1.1.1 = 11.7 to 13.1) and =FeOZn(OH)2S O43- (log βint1.-3.1.1= -2.5). The distribution diagrams of the surface complex species on goethite from the adsorption simulation using the Constant Capacitance Model (CCM) showed that the adsorption of Cu2+ and Zn2+ increased with increasing pH and the adsorption edges were in the pH ranges of 4-7 and of 5-8 for Cu2+ and Zn2+, respectively. In the presence of the sulfate ion, the adsorption of Cu2+ and Zn2+ was enhanced at low pH due to the formation of the Cu2+-SO4 2- and Zn 2+-SO4 2- ternary surface complexes. Pmly little effect on the stability constants and the amount of Cu2+ and Zn 2- adsorbed on goethite surface was observed for 500-fold increase in the ionic strength of solution.