Adsorption of sulfamethoxazole on polypyrrole decorated volcanics over a wide pH range: Mechanisms and site energy distribution consideration

As a widely used antibiotic, sulfamethoxazole (SMX) has attracted increasing research attention. However, acidic condition was normally required for efficient SMX removal, and the energy variation of adsorption system is not fully elucidated. In this study, based on a typical mineral adsorbent of volcanic rock (VR), anion exchange was introduced by Polypyrrole (PPy) modification to overcome the electrostatic repulsion in alkaline environment, and site energy distribution (SED) theory was applied to explore adsorption behavior and mechanism on SMX from an energy perspective. The batch adsorption experiments and a series of complementary characterization were combined, confirming that pore filling and hydrogen bonding are the dominant adsorption mechanisms for SMX removal by VR. While the adsorption distinction between PPy supported VR (PPy-VR) and VR could be explained by electrostatic attraction, π-π electron donor-acceptor interaction (π-π EDA) and ion exchange, which further explained the wide pH application potential of modified sorbent. Moreover, Dubinin-Ashtakhov (DA) model was used to calculate the energy distribution, and PPy-VR exhibited a stronger adsorption affinity (Em = 8.55–11.57KJ/mol) and energy heterogeneity (δe*=2.15–2.57) for SMX. The high-energy adsorption centers increased significantly (92.63%) after modification, which mainly attributed to π-π EDA and ion exchange introduced by PPy loading. The surface modification method provides a feasible idea for the improvement of application environment of adsorbents, and the analysis of SED is expected to promote the understanding of the connection between site energy and adsorption mechanism.