The effect of carbonate on the precipitation of calcium phosphate

In order to facilitate the recovery of phosphorus from wastewater, the effect of carbonate on the precipitation of calcium phosphate has been studied. The effect of carbonate concentrations up to 5.00 millimoles per liter on the precipitation of 20 milligrammes phosphorus per liter phosphate was studied. At pH 8.00 the precipitation rate of phosphate was greatly retarded by carbonate and the corresponding precipitation efficiency also decreased, but at pH values > or = 9.00 the effect of carbonate on the precipitation of phosphate was very small. This indicates that carbonate may decrease the precipitation rate and efficiency of calcium phosphate, but the solution pH value is still a key factor influencing the precipitation process. The effect of carbonate on the precipitation of phosphate was attributed to the formation of ion pairs between carbonate and calcium and the decrease of free calcium ions. This resulted in the decrease of the thermodynamic driving force for the precipitation of calcium phosphate, so the effect of carbonate on the precipitation of phosphate was in fact a competing one, although this competing effect was not so obvious at pH > or = 9.00. Carbonate may be coprecipitated with phosphate from solution, especially at pH 9.00-11.00, and this will decrease the relative phosphorus content of the precipitate. The increases in solution pH value and initial Ca/P ratio are two approaches to overcome the influence of carbonate on the precipitation of phosphate, but the solution pH values should be lower than 10.00 and the initial Ca/P ratios should not exceed 5.00.