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Zeta Potential and Surface Area of Calcium Carbonate as Related to Phosphate Sorption¹

ABSTRACT

Orthophosphate (OP) sorption by eight reagent-grade CaCO₃ samples and a natural limestone, as influenced by the surface area and zeta potential (ZP), was investigated in the absence and presence of pyrophosphate (PP) as a crystal growth inhibitor. Reacting the nine CaCO₃ samples for 24 h with 1 mmol OP L^–1 indicated a rapid 0.08-h adsorption reaction and a 2-h induction period that preceded OP precipitation. The amount of adsorbed OP was proportional to the CaCO₃ surface area which ranged from 250 to 12 780 m² kg^–1 and accounted for 92.8% of the variation in OP adsorption. The surface area was generally proportional to the ZP, as five CaCO₃ samples were positively charged with ZP values ranging from 17.7 to 25.3 mV, three samples were noncharged, and only limestone was negatively charged with a ZP of –6 mV. The ZP decreased with OP adsorption and again increased after OP precipitation. Addition of PP did not interfere with OP adsorption, but inhibited OP precipitation during the 24-h reaction period by extending the induction period, and markedly reduced the ZP. Negativity of the CaCO₃ particles per se was not considered the main factor in retarding P precipitation, as polyacrylamide was very effective in lowering ZP but had no effect on OP sorption. The added PP was also subject to adsorption by the CaCO₃ and, therefore, the amount of PP needed for inhibiting growth of the calcium phosphate crystals depended on CaCO₃ reactivity as well as on OP concentration.

¹ Contribution from the Fertilizer Efficiency Project, Soil and Water Science Dep., Univ. of Alexandria, Egypt. The project is supported in part by the Int. Dev. Res. Center, Canada.

² Professor of Soils, Graduate Research Assistant, and Professor of Physical Chemistry, respectively, Univ. of Alexandria, Egypt.

Received for publication August 6, 1984. Accepted for publication May 10, 1985.

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