ABSTRACT

The coprecipitation of phosphate ions with calcium carbonate precipitated from calcium bicarbonate solution and the adsorption of phosphate ions on suspended calcium carbonate have been studied experimentally. (1) There are some differences in the mode of coprecipitation of phosphate ions with calcium carbonate formed from calcium bicarbonate solution between stirring and non-stirring (keeping still) the solution as follows: The amount of phosphate ions coprecipitated with calcium carbonate is larger when stirred than when kept still. In the keeping still system the amount of phosphate ions coprecipitated with calcite is approximately equal to that with aragonite, whereas in the stirring system the amount increases with increasing proportion of aragonite in calcium carbonate formed. (2) Phosphate ions in a parent solution favor the formation of calcite. (3) Phosphate ions are more easily adsorbed on suspended aragonite than on suspended calcite. The adsorption of phosphate ions on suspended calcium carbonate occurs in a solution containing fluoride ions by the following three steps: (a) The first remarkable adsorption occurs within several hours independent of the chemical composition of the solution. (b) The adsorption apparently reaches saturation. (c) Then the great adsorption occurs again after about 80 hours. This final adsorption is observed very clearly in a solution containing a large amount of fluoride ions. The adsorption of fluoride ions occurs with the final adsorption of phosphate ions. The marked adsorption of phosphate and fluoride ions on suspended calcium carbonate is related to the formation of apatite.