Oxytetracycline adsorption by clay-carbon hybrids: effects of pH, commercial formulation additives, and PET microplastics.

The removal of antibiotics from water under environmentally complex laboratory systems remains insufficiently understood, particularly in the presence of coexisting contaminants such as formulation additives and microplastics. This study evaluates the performance of clay-carbon hybrid materials for the removal of oxytetracycline from aqueous solutions under both controlled and complex conditions. Natural montmorillonite and two montmorillonite-carbon hybrids were investigated through batch adsorption experiments, including kinetic and equilibrium analyses at different pH values. One of these (MDac3-210-500) exhibited a high adsorption capacity at neutral pH, reaching a maximum value of ~ 0.6 mmol/g. Mechanistic analysis suggests that chemisorption plays a key role in montmorillonite, whereas intraparticle diffusion and surface heterogeneity appear to dominate in the hybrids. To approximate realistic scenarios, adsorption experiments were conducted using a veterinary commercial oxytetracycline formulation and in the presence of polyethylene terephthalate (PET) microplastics (particles < 112 μm at 0.5 g/L). The commercial formulation led to a marked decrease in adsorption capacity for hybrids (0.05-0.08 mmol/g), highlighting matrix effects and competition for active sites, whereas, under the tested experimental conditions, PET microplastics did not substantially modify oxytetracycline adsorption by the hybrid materials. These findings highlight the sensitivity of oxytetracycline adsorption to coexisting compounds and provide a basis for understanding the performance of clay-carbon hybrids under more complex water treatment conditions. In particular, the contrasting effects of formulation additives and PET microplastics emphasize the importance of considering matrix composition when evaluating adsorbent performance.