Adsorption behavior of humic acid onto polypyrrole-coated nylon 6,6 granules

Abstract

One of the challenges in water supply is the removal of humic substances as the unit treatment processes in a conventional water treatment plant are usually not effective for this purpose. In this study, nylon 6,6 granules were surface-modified by chemical polymerization of pyrrole and were used as a granular adsorbent for removing humic acid from water. Zeta potential study showed that the coated granules have positive surface charges over a wide range of pH values. X-Ray photoelectron spectroscopy (XPS) indicated that the positive surface charges of the coated granules are attributed to the protonated nitrogen atoms (i.e. N⁺) in the polypyrrole (PPy) coating. Batch adsorption experiments were conducted under various conditions to examine the removal efficiency and mechanisms. PPy-coated nylon 6,6 granules were found to have much better removal of humic acid, as compared to the uncoated nylon 6,6 granules, suggesting the importance of electrostatic interactions between the positively charged granules and the negatively charged humic acid in the process. Evidence showed that although most adsorption may take place through non-specific electrostatic attraction, some adsorption occurred at specific sites and resulted in charge neutralization. It was found that low pH and high ionic concentration favored the adsorption performance and the adsorption kinetics involved both diffusion-controlled and attachment-controlled mechanisms.