Polymer Adsorption and Conformation in Dispersion-Flocculation of Concentrated Suspensions

Abstract

Performance of polymers as stabilizers or floccultants for controlling the properties of concentrated suspensions depends upon both the extent of the polymer adsorption as well as conformation at solid-liquid interfaces. Solids loading, polymer molecular weight and fractionation, and dissolved solid species, among others, are important factors that control the polymer adsorption and conformation and the system behavior. In this study, polymer molecular weight fractionation and adsorption changes were monitored using stepwise adsorption tests with techniques such as TOC (total organic carbon) and GPC/LS (gel-permeation chromatography/light scattering). Fluorescence and ESR (electron spin resonance) spectroscopy were used to monitor the conformation/orientation of the polymers adsorbed on the solid particles. Dissolved aluminum species in the residual solutions was monitored using ICP (inductively coupled plasma) atomic emission. Zeta potential measurements of the suspensions were made using a Zeta-Meter Model D to investigate effects of the dissolved aluminum species on the system behavior. The adsorption of PAA on alumina did not change significantly with increase in solids loading from 2 vol. % to 15 vol. % as a whole. However, there is large scattering of data at low solids loading (2 vol. %) and fixed initial high PAA concentration as found in our previous study which showed a "greater" adsorption density at low solids loading with a marked decrease in adsorption with increase in solids loading in the same range. Interestingly, the polymer adsorption behavior at high solids loading is different from that at low solids loading when the polymer sample contains impurities such as dioxane. Under the test conditions, smaller polymer molecules were found to preferentially adsorb first at the interfaces. Polymer molecular weight fractionation due to such preferential adsorption is more evident at low solids loading than at high solids loading, suggesting an increasing effect of particle-particle interaction on the polymer diffusion. Concentration of the dissolved aluminum species in the residual solutions increased significantly as the system becomes denser. Dissolved aluminum species was found to affect the conformation of PAA in the solution: PAA molecules become more coiled in the presence of the species at pH<7 even though the concentration of the species is low. Zeta potential of PAA-alumina suspensions with additions of dissolved aluminum species was markedly higher than that of PAA-alumina systems at pH 4 to pH 10, suggesting that the alumina particle surface becomes more positive in this pH range due to increasing complexation of PAA with the dissolved alumina species. Our earlier fluorescence and ESR results had suggested that the adsorbed PAA molecules tend to stretch out or dangle more into the solution as the system becomes denser. It is clear that conformation of the adsorbed polymer is an important parameter for controlling flocculation/dispersion behavior of concentrated suspensions.