Inhibition of calcite precipitation by orthophosphate: Speciation and thermodynamic considerations

The inhibition of heterogeneous calcite precipitation by orthophosphate was investigated under four different solution compositions using a pH-stat system. The system composition was designed to maintain a constant degree of supersaturation with respect to calcite, but with different carbonate/calcium ratios and pH values during precipitation. Inhibition in the presence of orthophosphate was found to be more effective at lower carbonate/calcium ratios and lower pH values. With the assumption that the calcite precipitation rate is proportional to the surface concentration of active crystal-growth sites, the reduction in the rate of calcite precipitation by phosphate can be explained by a Langmuir adsorption model using a conditional equilibrium constant and total phosphate concentration. Through a detailed analysis of chemical speciation in the solution phase and calcite surface speciation using chemical equilibrium computer modeling, the “conditional” equilibrium constants obtained at different solution compositions were found to converge to a single “non-conditional” value if only CaHPO4(aq)0 was considered in the adsorption reaction. This suggests that CaHPO4(aq)0 is the responsible species for inhibition of calcite precipitation because it adsorbs to the surface and blocks the active crystal-growth sites. The standard enthalpy change (ΔH0) and standard entropy change (TΔS0) of the adsorption reaction, determined by experiments performed from 15 to 45 °C, were 58.5 and 98.3 kJ/mol, respectively. The high positive values of the standard enthalpy change and the standard entropy change suggest that the adsorption reaction is an endothermic reaction, chemisorptive in nature, and driven by the entropy change, most likely resulting from the dehydration process that accompanies the adsorption of CaHPO4(aq)0 onto the calcite surface.