Competition of Salts with Sulfamethoxazole in an Anionic Ion Exchange Process

Abstract

Sulfamethoxazole is an emergent contaminant in waters. It is an antibiotic for humans and animals that is difficult to biodegrade in drinking water and sewage treatment plants. As the most frequent sulfonamide, its treatment and removal is difficult. In its anionic form, it can be retained by ion exchange, although competition with other ions, especially salts present in natural and treated water, needs to be taken into consideration because of competition between the anions for the active sites of the resin. This work investigated the removal of sulfamethoxazole (SMX) from low saline waters. An anionic ion exchange resin, Lewatit MP5, was used to remove SMX in synthetic solutions in the presence of chloride, sulfate, and nitrate salts. Multicomponent system solutions containing a fixed concentration of salts and different concentrations of SMX were tested in batch experiments. The presence of Cl−, SO42−, and NO3− suppressed the adsorption of SMX, especially the presence of SO42−, which showed the highest adsorption capacity onto the resin. Adsorption equilibrium constants were determined using the Langmuir isotherm and the extended Langmuir isotherm for multicomponent systems. The maximum adsorption capacity for the SMX and salts system was 15 mg/mL wet resin and the equilibrium constants 1.4 L/g SMX, 6 L/g NO3−, 9 L/g SO42−. Kinetics was analyzed using the pore diffusion model, getting a diffusivity value of 1.3×1 cm2/s for SMX. Finally, two operational load and elution cycles were carried out in a fixed-bed column using mixture synthetic solutions of SMX and salts to obtain the corresponding breakthrough curves. Results show that the resin is able to retain SMX despite the high competition of the other anions and can be regenerated completely after each cycle.