Repurposing drinking water treatment sludge for wastewater remediation: Performance and mechanistic study on antibiotic removal

The adsorption properties of drinking water treatment sludge (DWTS) rich in activated carbon were studied to remove antibiotics under batch and fixed-bed operating conditions using distilled water and wastewater. Kinetic and equilibrium adsorption tests were performed using sulfamethoxazole and trimethoprim as target molecules. The results showed that DWTS was more effective at removing trimethoprim compared to sulfamethoxazole across all tested matrices, achieving maximum adsorption capacities up to 70.9 mg/g for trimethoprim (TMP) and 69.1 mg/g for sulfamethoxazole (SMX) in single-solute systems. Even under competitive conditions in binary mixtures, TMP adsorption capacities remained high, up to 74.8 mg/g, whereas the maximum capacity for SMX significantly decreased to 17.5 mg/g. To understand this behavior, density functional theory calculations modelled the adsorbent using a carboxyl-functionalized graphene structure. The energy interactions showed that a molecular aggregate between sulfamethoxazole and trimethoprim can be formed if both antibiotics are present in the same solution, thus favoring the adsorption of trimethoprim in the presence of sulfamethoxazole in the tested matrices. The fixed-bed experiments further confirmed the material's selectivity, with TMP achieving a column removal efficiency of 57.6 % in distilled water, significantly outperforming SMX. The results support the use of this waste product as an adsorbent for removing both antibiotics. These results are useful to valorize this waste as a promising material to reduce the carbon footprint and operating cost of water treatment systems to remove contaminants of emerging concern.