Green clay engineering via dimethyl sulfoxide, dimethylformamide, and urea-assisted exfoliation of serpentinite for efficient azithromycin adsorption as emerging pharmaceutical pollutant

Azithromycin (AZT), a persistent macrolide antibiotic, is an emerging environmental contaminant due to its high aqueous stability and role in antimicrobial resistance. In this study, natural serpentinite was chemically exfoliated using dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and urea (U) to enhance surface reactivity and adsorption performance toward AZT. Textural characterization revealed a marked increase in surface accessibility, with BET area rising from 6.1 m²/g (raw) to 23.7 m²/g (DMSO/SP), 18.8 m²/g (DMF/SP), and 15.3 m²/g (U/SP), while pore diameters expanded from 11.1 nm to ~ 16 nm, exposing more reactive hydroxyl groups. Batch adsorption experiments showed pH-dependent uptake, with maximum removal at pH 9. Adsorption kinetics followed the pseudo-first-order model (R² > 0.90), and intra-particle diffusion confirmed a multi-stage adsorption pathway. Statistical physics modeling revealed saturation capacities (Q_sat) of 329.7 mg/g for DMSO/SP, 292.3 mg/g for DMF/SP, and 279.9 mg/g for U/SP, confirming the superior adsorption of DMSO-assisted exfoliation. Active site density (N_m) reached 109.5 mg/g for DMSO/SP, compared to 77.5 mg/g for DMF/SP and 84.3 mg/g for U/SP. The number of AZT molecules per site (n) exceeded unity, indicating multi-molecular stacking: DMSO/SP: 3.01-3.44, DMF/SP: 3.77-4.41, U/SP: 3.32-4.40 molecules/site. The mean adsorption energy (ΔE ≈ 5 kJ/mol) confirmed a reversible, exothermic physisorption process dominated by electrostatic attraction. Expanded interlayer spacing facilitated multilayer stacking, while hydroxyl groups promoted hydrogen bonding, creating a synergistic mechanism for enhanced retention. These findings show solvent polarity and molecular geometry governs serpentinite delamination, site density, and adsorption, with DMSO producing the most open, high-capacity adsorbent.