Adsorption of Cd2+ on hydrous manganese dioxide from aqueous solutions

The adsorption of Cd2+ onto amorphous hydrous manganese dioxide (δ-MnO2) has been studied in (a) 0.5 M NaCl solution and (b) major ion seawater. The adsorption experiments were carried out in two methods: isotherms at constant pH in the presence of a buffer solution over a wide range of initial Cd2+ concentrations (0.0089 to 0.5357 mM) and varied pH (2-7.5) in the absence of a buffer solution at 0.0911 mM Cd2+ concentration. In order to describe the isotherm mathematically, the experimental data of the adsorption equilibrium were correlated by either the Langmuirian or Freundlich equations. Results indicated that the Langmuirian-type model gave a better fit to the experimental data than the Freundlich type in both electrolyte media. The proton stoichiometry derived from isotherms at two close pH values varies between 0.22 to 0.42 for the 0.5 M NaCl solution. The variation of fractional adsorption with pH can be modeled either with the formation of a SOCd+ complex or with a combination of SOCd+ and SOCdOH complex, depending upon the electrolyte medium. The equilibrium constants obtained from Kurbatov-type plots are found to be the most suitable for these model calculations.