Geochemistry of manganese oxides: metal adsorption in freshwater and marine environments

Abstract

Abstract The effect of pH and metal ion concentration on the adsorption of Cu, Co, Cd, Mn, Ni and Zn onto manganese oxides was investigated. The oxides used in the experiments were deposited around individual sand grains in water treatment filter beds in NE Scotland and show reproducible adsorption profiles regardless of location or age. The oxides remained amorphous throughout the four-year study period, and show an inferred pH ZPC of 1.5. Metal adsorption increases towards more alkaline pH. The adsorption series Co > Cu > Cd > Zn > Mn > Ni is shown at pH 3 and Cu > Co > Cd > Mn > Zn > Ni at pH 6. Changing the concentration of the metal ions in solution leads to an increase in the mass of metal adsorbed, though the relationship is not linear as the proportion of metal taken up falls as concentration rises. Adsorption from an artificial seawater solution shows a significant reduction in the amount of metal taken up in all cases except Cu which shows an adsorption profile comparable with that in deionized water. The linear Freundlich isotherm accurately and reproducibly models the adsorption profile of all metals at each pH and concentration examined. The data do not, however, replicate the enrichment sequence observed in manganese nodules. The study demonstrates that for the enrichments observed in manganese nodules to be attained, the active oxide surface must be replenished by regular precipitation of fresh oxide material to enable continued adsorption to take place.