Deposition/adsorption of colloids to surface-modified granules: Effect of surface interactions

Abstract

Granular media with positive surface charges were prepared by surface modification with polypyrrole (PPy) and were used in a fixed bed column for removing colloidal substances, such as kaolin particles and humic acid. The surface-modified granules were found to possess high positive ζ potentials at pH < 10.5, whereas kaolin particles and humic acid have negative ζ potentials at pH >2.5 and pH >1.8, respectively. Deposition and adsorption experiments showed that the granules with positive surface charges significantly enhanced the removal of the negatively charged kaolin particles and humic acid, indicating the importance of electrostatic surface interactions in filtration and adsorption. X-ray photoelectron spectroscopy (XPS) revealed that the positive surface charges of the surface-modified granules can be attributed to the N+ present in the PPy coating, and kaolin particle deposition or humic acid adsorption reduced the amount of N+ in the PPy coating, suggesting the existence of charge neutralization during the filtration or adsorption process. Scanning electron microscope (SEM) studies visually showed that a substantial amount of kaolin particles or humic acid was deposited or adsorbed to the surface-modified granules. The study concluded that surface interactions play an important role in the removal of colloidal substances in a granular media filtration or adsorption system.