Sorption of antibiotics onto montmorillonite and kaolinite: competition modelling

Antibiotic contaminants, which are generally present in bi-solute systems, can be competitively adsorbed onto clays. Single- and bi-solute sorptions of sulfadiazine (SDZ) and ciprofloxacin (CIP) onto montmorillonite and kaolinite were investigated at pH values of 5 and 8. Freundlich and Langmuir models were used and fit the experimental data well for single-solute sorption. The sorption isotherms were nonlinear (N-F = 0.265-0.730), and the maximum sorption capacities (q(mL)) of the SDZ and CIP onto montmorillonite were higher than those onto kaolinite. The octanol-water distribution ratio (D-ow), cation exchange capacity (CEC), Brunauer-Emmett-Teller (BET) surface area (A(BET)), pore size, point of zero charge (pH(PZC)), and basal spacing predominantly affected the Freundlich constant (K-F) and q(mL) of SDZ(0) and CIP+ at pH 5 more than SDZ(-) and CIP +/- at pH 8. For bi-solute sorption, the presence of CIP inhibited the SDZ sorption onto montmorillonite and kaolinite. Competitive sorption models such as Sheindorf-Rebhun-Sheintuch (SRS), Murali-Aylmore (M-A) and the modified extended Langmuir model (MELM) were used; of these, the MELM provided the best prediction with SDZ sorption onto montmorillonite at pH 8 and CIP onto kaolinite at pH 5 and 8 in SDZ/CIP system occurring synergistically, whereas others occurred antagonistically. The distribution coefficient (K-d) of the bi-solute sorption decreased with increasing pH in the order cationic > neutral > anionic for SDZ and cationic > zwitterionic > anionic for CIP, which resembled the K-d of single-solute sorption. Fourier transform infrared spectroscopy (FT-IR) spectra indicated that amine in SDZ and keto oxygen in CIP were responsible for the interactions with the montmorillonite and kaolinite.