Predicting heavy metals' adsorption edges and adsorption isotherms on MnO₂ with the parameters determined from Langmuir kinetics

Although surface complexation models have been widely used to describe the adsorption of heavy metals, few studies have verified the feasibility of modeling the adsorption kinetics, edge, and isotherm data with one pH-independent parameter. A close inspection of the derivation process of Langmuir isotherm revealed that the equilibrium constant derived from the Langmuir kinetic model, K_S-kinetic, is theoretically equivalent to the adsorption constant in Langmuir isotherm, K_S-Langmuir. The modified Langmuir kinetic model (MLK model) and modified Langmuir isotherm model (MLI model) incorporating pH factor were developed. The MLK model was employed to simulate the adsorption kinetics of Cu(II), Co(II), Cd(II), Zn(II) and Ni(II) on MnO₂ at pH3.2 or 3.3 to get the values of K_S-kinetic. The adsorption edges of heavy metals could be modeled with the modified metal partitioning model (MMP model), and the values of K_S-Langmuir were obtained. The values of K_S-kinetic and K_S-Langmuir are very close to each other, validating that the constants obtained by these two methods are basically the same. The MMP model with K_S-kinetic constants could predict the adsorption edges of heavy metals on MnO₂ very well at different adsorbent/adsorbate concentrations. Moreover, the adsorption isotherms of heavy metals on MnO₂ at various pH levels could be predicted reasonably well by the MLI model with the K_S-kinetic constants.