EFFECT OF MAGNESIUM ON PHOSPHATE ADSORPTION BY CALCIUM CARBONATE1

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Abstract
The effect of Mg2+ as MgSO4 on P adsorption by CaCO3 was determined at 0.5-100 ppm P and 0-40 ppm Mg concentrations. At low P (0.5-10.0 ppm P), adding 1-2 ppm Mg significantly reduced P adsorption by CaCO3. With increasing solution P concentration, more Mg is required to inhibit the adsorption of P by CaCO3. The molar ratio of P/Mg at which adsorption of P by CaCO3 is affected most is .apprx. 2-1.5. Nearly 1/2-1/3 of the Ca in the dicalcium and octacalcium phosphates and hydroxyapatite apparently is displaced by Mg, which hinders nucleation and crystal growth of these phosphate precipitates. Soluble MgSO4 is much better than slightly soluble MgCO3 in unpairing the adsorption of P by CaCO3. The possible formation of octamagnesium phosphate or trimagnesium phosphate on the surface of MgCO3 will increase P adsorption when more MgCO3 is mixed with CaCO3.

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