Polypyrrole-coated granules for humic acid removal

Granules with positive surface charges were prepared by coating glass beads with polypyrrole (PPy). The coated glass beads were found to possess high positive zeta potentials over a wide range of pH values. Batch and fixed bed humic acid adsorption experiments using the coated glass beads as adsorbents were conducted. Scanning electron microscope (SEM) was used to examine the surface morphology of the coated glass beads before and after humic acid adsorption. X-ray photoelectron spectroscopy (XPS) was applied to assess the protonation and charge transfer of the PPy coating with and without adsorbed humic acid. SEM images showed that the PPy coating considerably increased the surface roughness of the granules and a significant amount of humic acid was adsorbed on the PPy coating. From XPS analysis, it was found that 28% of the nitrogen atoms in the PPy coating were protonated, leading to a highly positively charged surface at pH < 10.5. The results also showed that the amount of protonated nitrogen atoms decreased by up to 25% due to humic acid adsorption, suggesting that humic acid uptake by the PPy-coated glass beads was affected at least partly by charge neutralization. Humic acid adsorption also resulted in a reverse of the positive zeta potential of the PPy coating, indicating the importance of macromolecular adsorption in the process. The adsorption equilibrium data can be fitted with either the Langmuir or the Freundlich expression. Both pH and ionic concentration were found to affect the extent of humic acid adsorption by the PPy-coated granules. (C) 2001 Academic Press.