Influence of pH on the Sorption of Co ²⁺ , Zn ²⁺ and Ca ²⁺ by a Hydrous Manganese Oxide

Abstract

Abstract The uptake of Co 2+ , Zn 2+ , and Ca 2+ by δ‐MnO 2 at 24.0 ± 0.5°C as a function of pH and salt concentration was investigated. At pH values < 5, δ‐MnO 2 released structural Mn to solution phase during the sorption of Co 2+ and to a lesser extent during Zn 2+ sorption. Calcium sorption did not release any Mn to solution phase. In addition Co 2+ , Zn 2+ , and Ca 2+ were shown to adsorb specifically at the δ‐MnO 2 surface by exchange with bound H. The pH values had very little influence on the sorption of these cations. At pH values above 6, abrupt increases in the sorption of Co 2+ and Zn 2+ in narrow pH ranges were observed whereas no such marked increase of Ca 2+ sorption was found up to pH 9.2. This type of sharp increase in sorption is due to specific adsorption and is related to hydroxylation of the metal ions. Measurements of microelectrophoretic mobility of δ‐MnO 2 in the presence of Co 2+ , Zn 2+ and Na + suggested that Co(OH) 2 or Zn(OH) 2 may be nucleated at the δ‐MnO 2 surface at pH values lower than those thermodynamically calculated (no supersaturation) which would result in precipitation of the pure hydroxides in bulk solution. At pH values between 5 and 6 any of the above mechanisms may be operative.