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

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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.
Funding Information
  • Int. Dev. Res. Center, Canada