The point of zero charge and sorption of cadmium (II) and strontium (II) ions on synthetic hydroxyapatite

Surface properties of synthetic well-crystallized hydroxyapatite were investigated. Points of zero charge (pH(PZC)) for different solid to solution ratios were determined by the batch equilibration technique, using KNO3 as a background electrolyte. It has been found that the decrease in solid to solution ratio from 1:100 to 1:500 leads to a decrease in pH(PZC) from 6.1 to 4.1, respectively. Sorption of Cd2+ ions is not influenced by the initial pH value in the investigated range (5-7), while sorption of Sr2+ -ions, determined in the wider pH range (4.5-11.5), depends only on the initial pH higher than 10, i.e. final pH values higher than 4.5. Negative surface charge established when pH of the solution is higher than pH,, leads to a more effective cation sorption. (C) 2000 Elsevier Science B.V. All rights reserved.