Competitive adsorption and molecular interaction study of sulfamethoxazole and trimethoprim on biomass-based activated carbon

Abstract

This study investigated the adsorption of sulfamethoxazole and trimethoprim onto activated carbon from reed canary grass in single- and binary-solute systems. Trimethoprim showed higher adsorption capacity than sulfamethoxazole. Single solute adsorption isotherms were modeled using Nitta model to assess equilibrium and site energy. The adsorption of trimethoprim and sulfamethoxazole followed a monolayer pattern, with trimethoprim exhibiting a higher maximum adsorption capacity (0.69 mmol/g) compared to sulfamethoxazole (0.55 mmol/g). Trimethoprim also demonstrated a significantly greater adsorption affinity (521.89 L/mmol) than sulfamethoxazole (70.92 L/mmol). The ratio of adsorption sites occupied to adsorbed molecules was 0.81. Weighted mean of site energy was 11.5 kJ/mol for sulfamethoxazole and 16.7 kJ/mol for trimethoprim, suggesting stronger trimethoprim interactions. Competitive adsorption experiments showed that trimethoprim significantly reduced sulfamethoxazole adsorption due to its stronger interaction with the adsorbent and its larger molecular size, which blocked small pores and shielded the adsorbent surface, restricting sulfamethoxazole access to adsorption sites. In the binary-solute system, the maximum adsorption capacity increased to 0.93 mmol/g, surpassing the individual maximum capacities of sulfamethoxazole and trimethoprim in their single-solute systems. However, the adsorption affinities of sulfamethoxazole and trimethoprim during competitive adsorption, at 9.29 and 267.69 L/mmol respectively, were lower than their corresponding values in single-solute solutions. Scanning transmission X-ray microscopy combined with X-ray absorption near-edge structure spectroscopy revealed adsorption mechanisms of sulfamethoxazole on activated carbon. The aromatic CC peak shifted from 285.7 to 285.2 eV, indicating π–π electron donor-acceptor interaction. The carboxylic carbon peak at 288.1 eV increased in intensity, suggesting n–π interaction.