The interaction of water-soluble polymers with solid substrates. Implications on the kinetics of crystal growth

Abstract

The precipitation of calcium carbonate was investigated at pH 8.5 and at temperatures of 25 and 80 °C in stable, supersaturated, synthetic high-salinity solutions (0.62 M NaCl) seeded with calcite, in the presence of fulvic acid (FA), humic acid (HA) and poly(acrylic acid) (PAA), which acted as inhibitors. The rates of crystal growth and the effect of the inhibitors on the mineral formation kinetics were evaluated under conditions of sustained supersaturation. Concentrations of the inhibitors tested in the range 0.02-0.1 ppm for solutions with a supersaturation value (SR) of 8.39 with respect to calcite suppressed the rates of crystal growth by up to 95%. For 0.1 ppm PAA as well as for 0.1 ppm HA the rates of crystal growth showed a linear increase with SR_calcite of the working solution (SR_calcite = 7.31-13.9) at 80 °C. The second-order dependence of the rate on the solution supersaturation suggested surface diffusion control. The inhibitory activity of the compounds tested was ascribed to their interaction with the solid surface. Kinetics analysis of the results obtained for the formation of calcium carbonate in the presence of the compounds tested, based on the assumption of Langmuir-type adsorption on the calcite seed crystals, fitted satisfactorily the data for PAA and FA, while for HA the interaction did not follow this model. In the latter case aggregation phenomena may occur during the growth of crystals, resulting in rate reduction.