Modeling the sorption of Ni(II) and Zn(II) by Mn oxide-coated sand: Equilibrium and kinetic approaches.

The behavior of metal cations in oxide-dominated systems is controlled by sorption reactions, which in turn depend on pH. Descriptions of such reactions are of interest for contaminant monitoring or remediation efforts; however, widely used isotherms such as Freundlich or Langmuir neglect the effect of pH and are therefore limited in their applicability. Two pH-dependent isotherms and their kinetic analogs were developed and evaluated regarding their ability to describe equilibrium and time-dependent sorption of Ni and Zn by Mn oxide-coated sand (MOCS). The sorption of Ni and Zn by MOCS at pH 4.0, 5.5, and 7.0 was investigated using batch equilibration and stirred-flow techniques. The affinity of MOCS for either metal cation was highly pH dependent, with greater affinity at higher pH. Both isotherms described the batch data well. Flow interruption during stirred-flow experiments indicated that chemical nonequilibrium existed between solution and sorbed phases of both Ni and Zn and that such nonequilibrium was greater with increasing pH. Both kinetic models provided good descriptions of the solution data from stirred-flow experiments and correctly captured the effect of pH on chemical nonequilibrium. These models offer simple alternatives to surface complexation approaches and are expected to be easily applied to describe equilibrium and time-dependent sorption of a wide range of metal cations by variably charged minerals or oxide-coated media.