Adsorption Investigation of Polyacrylic Acid on Sub-micron Titanium Dioxide Surface and Associated Effects on Flocculation

Abstract

This work investigated the flocculation of sub-micron titanium dioxide (TiO2) by polyacrylic acids (PAAs), with a focus on studying the correlation between PAAs adsorption conformation and flocculation efficiency/flocs property. The adsorption analysis of PAAs on the TiO2 surface was studied by attenuated total reflection Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), CHNS elemental analyzer coupled with quartz crystal microbalance with dissipation monitoring (QCM-D). Adsorption densities were fitted using Langmuir and Freundlich isotherm models. The flocculation process of TiO2 by PAAs of different molecular weights (MWs) was monitored in real-time by photometric dispersion analyzer (PDA), and the floc structure was studied by scanning electron microscope (SEM). Results indicated that PAAs adsorption followed the Langmuir isotherm, forming monolayer adsorption on the TiO2 surface regardless of MWs through hydrogen and bidentate chelating chemical bonds. The adsorption layer thickness increased with increasing MW, which could promote flocs size by providing molecular “bridges”; but impeded the flocculation rate. For sub-micron particles, polyacrylic acid with a medium MW (512 kDa) was the optimum polymer owing to obtain the fastest flocculation rate and relatively dense flocs in the present research. PAA with high MW resulted in larger flocs with more porositise, while the low MW PAA can only generate smaller flocs with slower flocculation rate.