Modeling the sorption kinetics of divalent metal ions to hematite

The sorption kinetics of the divalent metals Zn, Co, Ni, and Cd to hematite were studied in single sorbate systems with high sorbate/sorbent ratios (from 1.67 to 3.33 mol sorbate/mol sorption sites) in 10 mM Na-piperazine N,N′-bis 2-ethane sulfonic acid (Na-PIPES) solution at pH 6.8. The experimental data showed a rapid initial sorption (half-time about 1 min) followed by slower sorption that continued for 1–5 days. The sequence of fast to slow sorption kinetics was modeled by slow inner-sphere (IS) complexation in equilibrium with outer-sphere (OS) complexes. Although the OS reaction was fast and considered to be in equilibrium, the extent of OS complexation changed over time due to increased surface potential from the IS complexes. For example, the model showed that the dimensionless OS complexation function, Kos, decreased from 0.014 initially to 0.0016 at steady state due to sorption of 4×10−5 M Zn(II) to 2 g L−1 hematite. Sorption rate constants, kads, for the various divalent metals ranged from 6.1 to 82.5 M−1 s−1. Desorption rate constants, kdes, ranged from 5.2×10−7 to 6.7×10−5 s−1. This study suggests that the conversion from OS to IS complex was the rate-determining step for the sorption of divalent metals on crystalline adsorbents.