Removal of phosphate from solution by adsorption and precipitation of calcium phosphate onto monohydrocalcite.

The sorptionbehavior and mechanism of phosphateon monohydrocalcite(CaCO3⋅H2O: MHC) were examined using batch sorptionexperiments as a function of phosphateconcentrations, ionic strengths, temperatures, and reaction times. The mode of PO4 sorptionis divisible into three processes depending on the phosphateloading. At low phosphateconcentrations, phosphateis removed by coprecipitation of phosphateduring the transformation of MHC to calcite. The sorptionmode at the low-to-moderate phosphateconcentrations is most likely an adsorption process because the sorption isothermat the conditions can be fitted reasonably with the Langmuirequation. The rapid sorptionkinetics at the conditions is also consistent with the adsorption reaction. The adsorption of phosphateon MHC depends strongly on ionic strength, but slightly on temperature. The maximum adsorption capacities of MHC obtained from the regression of the experimental data to the Langmuirequation are higher than those reported for stable calcium carbonate ( calciteor aragonite) in any conditions. At high phosphateconcentrations, the amount of sorptiondeviates from the Langmuirisotherm, which can fit the low-to-moderate phosphateconcentrations. Speciation–saturation analyses of the reacted solutions at the conditions indicated that the solution compositions which deviate from the Langmuirequation are supersaturated with respect to a certain calcium phosphate. The obtained calcium phosphateis most likely amorphous calcium phosphate(Ca3(PO4)2⋅xH2O). The formation of the calcium phosphatedepends strongly on ionic strength, temperature, and reaction times. The solubility of MHC is higher than calciteand aragonitebecause of its metastability. Therefore, the higher solubility of MHC facilitates the formation of the calcium phosphatesmore than with calciteand aragonite.