Influence of pH on the Sorption of Co2+, Zn2+ and Ca2+ by a Hydrous Manganese Oxide1
- P. Loganathan2,
- R. G. Burau3 and
- D. W. Fuerstenau4
The uptake of Co2+, Zn2+, and Ca2+ by δ-MnO2 at 24.0 ± 0.5°C as a function of pH and salt concentration was investigated. At pH values < 5, δ-MnO2 released structural Mn to solution phase during the sorption of Co2+ and to a lesser extent during Zn2+ sorption. Calcium sorption did not release any Mn to solution phase. In addition Co2+, Zn2+, and Ca2+ were shown to adsorb specifically at the δ-MnO2 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 Co2+ and Zn2+ in narrow pH ranges were observed whereas no such marked increase of Ca2+ 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-MnO2 in the presence of Co2+, Zn2+ and Na+ suggested that Co(OH)2 or Zn(OH)2 may be nucleated at the δ-MnO2 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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