A Kinetic Approach to the Inactivation of Zinc by Various Soil Constituents

The adsorption reaction of zinc onto prochlorite, pyroxene, and biotite minerals, from dilute solution (water) was studied under isothermal conditions by agitated nonflow experiment method. The equilibrium concentration of zinc in the solution was determined both by radioactive tracer technique and adsorption spectrophotometer methods. The data were collected at three temperatures. The data were treated by the equation: ∂θ/∂t = k1(1-θ)C - k-1θ where θ is the fraction of total surface coverage and C is the equilibrium concentration of zinc in solution, k1 and k-1 are the rate constants for the adsorption and reverse process and t is the time of reaction. The specific rate constant for the adsorption reaction (k1) depends on the nature of the mineral and mineral particle fraction. The k1 increases with rise in temperature. The values for the desorption rate constant (k-1) are small, indicating that zinc ions are preferentially adsorbed over other exchangeable ions in the system. The activation energy for the sorption process lay between 3 to 6 kcals/mole, eliminating chemical adsorption. The activation energy indicates the physical process of diffusion as the mechanism controlling step in the present study. The positive standard entropy of activation (ΔSŦ) is related to the change in the volume of activation complex (θŦ) during the course of reaction. The monolayer capacities for the three minerals and mineral fractions were evaluated. The percent of the surface area occupied by the adsorbed zinc ion was about 50 percent, 65 percent and 66 percent for prochlorite, pyroxene and biotite, respectively. This suggests that zinc is adsorbed on certain specific sites. The monolayer capacities increase as the temperature increases. The change in the standard free energy of adsorption, -Δ°G , with the increased fraction of surface coverage allows speculation that: The sites are energetically heterogenous. There is mutual repulsion of adsorbed zinc ions at higher surface coverage. The data show that zinc adsorption onto the minerals used in the present study is exchange adsorption. The initial adsorption plateau in the case of prochlorite and pyroxene is described as a result of exchange of solution zinc for weakly adsorbed sodium. The change in the nature of the curve beyond this region is speculated to the progressive exchange of other cations. The data for biotite are not conclusive and need further study. It appears that exchange adsorption is a step wise process and more than one plateau may be obtained if the system is allowed to reach its ultimate equilibrium where all the exchange sites are completely saturated by the adsorbate. The small differential heat of adsorption (Δ°H) confirms the contention that the process is simply an exchange adsorption in the systems studied