Influence of molecular weight on polyacrylic acid flocculation of sub-micron titanium dioxide

This paper reports the effect of the molecular weight of polyacrylic acid (PAA) on the flocculation of sub-micron titanium dioxide particles. The average molecular weight (MW) of the three studied PAAs were 140,000, 512,000 and 1,483,000 g/mol, respectively. The flocculation process was studied by several techniques including focused beam reflectance measurement (FBRM), photometric dispersion analyses (PDA), settling tests in graduated cylinders. The adsorption density of the PAAs on the titanium dioxide surface was measured by an elemental analyzer, and the floc structure was studied by scanning electron microscope and fractal dimension analysis using an optical microscope. The results showed that the flocs formed with the high MW PAA had larger sizes than the flocs generated with medium MW PAA. However, the medium MW PAA resulted in better flocculation performance, with faster settling rate and clearer supernatant. This observation was in contrast to the general perception in the literature that high MW polymer flocculants have better flocculation performance. The porosity of the flocs generated by the high MW PAA was larger than flocs by the medium MW PAA, leading to lower density flocs. This was found to be the main reason for the slow settling rate of the high MW PAA despite the larger floc size. Finally, despite the different settling rates, the PAAs with different MWs all showed their optimum flocculation performance at about 3.3 % of a monolayer coverage on the TiO2 surfaces.