Adsorption of sulfonamides on polyamide microplastics in an aqueous solution: behavior, structural effects, and its mechanism

Although microplastics (MPs) and sulfonamides (SAs) are emerging aquatic pollutants, the structural effects of SAs following their adsorption on MPs and underlying mechanisms of the same remain unelucidated. In this study, the characteristics of the adsorption of 10 SAs on polyamide (PA) were analyzed, and significant structural effects of the former were observed at different pH values (P < 0.01). The highest adsorption distribution coefficient (Kd) was ∼7.83–43.57 times the lowest one. The key processes and micromechanisms of SA adsorption on PA were revealed using the quantitative structure–activity relationship model and partial least squares structural equation model (PLS-SEM). The results suggest that both the octanol–water partition coefficients (Kow) of SAs and the valid adsorption energies (Ead-v) between PA and SAs are vital in the observed structural effects of SAs. Furthermore, the density functional theory results demonstrated that during the adsorption of SAs on PA, the different adsorption energies were engendered by equilibrium configurations and interactions, such as hydrophobic, hydrogen bond (H-bond), van der Waals, and electrostatic interactions. Moreover, the developed PLS-SEM models revealed that during the adsorption of cationic (PA–SA+) and anionic (PA–SA−) SAs, the structural effects were partially driven by the electrostatic interactions. Conversely, the structural effects of neutral SA (SA0) adsorption on PA were partially due to H-bond interactions. Overall, we pioneered a theoretical basis to understand the structural effects of SAs following their adsorption on MPs.