Use of commercial adsorbents for the removal of antibiotics from agricultural soil. Process efficacy and interaction mechanisms

Abstract Soil contamination by antibiotic residues poses an increasing risk to terrestrial and water ecosystems and human health, particularly due to its contribution to the development and spread of antimicrobial resistance. This study evaluates the effectiveness of a commercial organoclay-based adsorbent (PM199), selected from an initial pool of eight candidate materials, for the immobilization of antibiotics frequently detected in soils. Batch experiments were conducted to evaluate removal efficiency, adsorption kinetics, and equilibrium behaviour under controlled soil-solution contact conditions. Eight antibiotics were analyzed simultaneously, including amoxicillin, trimethoprim, sulfamethoxazole, clindamycin, ofloxacin, tetracycline, ciprofloxacin, and azithromycin. The incorporation of PM199 into the soil (80:20, w/w) significantly improved the retention of amoxicillin, trimethoprim, sulfamethoxazole, and clindamycin, with retention efficiencies ranging from 57.5% to 84.6%, and these compounds were subsequently selected for detailed kinetic and isothermal analyses. Adsorption kinetics were described using a biphasic PFO + PSO model for the four selected antibiotics and a simple PFO model for the remaining compounds, reflecting differences in surface accessibility and rate-limiting mechanisms. The equilibrium data were best described by the Freundlich isotherm, which is consistent with adsorption on an energetically heterogeneous surface. The multicomponent model indicated that competitive interactions among the four antibiotics were limited, suggesting preferential interaction with distinct adsorption sites. Overall, these results support the potential of soils enriched with organoclays as an effective strategy for the simultaneous retention of antibiotic mixtures in contaminated soils, to prevent these highly hazardous pollutants from entering natural water bodies.