EFFECT OF SOLUTION PROPERTIES ON TETRACYCLINE REMOVAL BY IRON-BASED DRINKING WATER TREATMENT RESIDUALS

Veterinary antibiotics (VAs) are contaminants of growing concern because of their potential to pollute both aquatic and soil environments. In the current batch sorption study, we examined the effect of solution chemistry (pH , ionic strength, sorbate: sorbent ratio (SSR)), and reaction kinetics (0.16-96 h) on tetracycline (TTC) sorption by a waste byproduct of the drinking-water treatment process, namely, Fe-based drinking-water treatment residuals (Fe-WTR) as a function of initial TTC concentration under aerobic conditions. The effect of a competing ligands (phosphate and sulfate) and complexing metal (calcium) on TTC sorption envelopes was also evaluated. Empirical modeling, surface complexation modeling, and ATR-FTIR studies were carried out to better understand possible sorption mechanism. Sorption of TTC to Fe-WTR increased with increasing pH up to pH 7, above which it decreased at higher pH values (8-11). A concentration of 40 g L⁻¹ was deemed as optimum SSR for Fe-WTR where more than 90% of the TTC was sorbed and equilibrium was reached in 5 h regardless of the initial TTC concentration. The reaction kinetics data best fit second order reaction kinetics (r²=0.99). The data best fit the linearized form of the Freundlich isotherm (R²=0.98). No significant effect (p<0.05) of ionic strength (IS) on sorption of TTC was observed between 0.05 and 0.5 mM. However, at higher initial concentrations (>1 mM), IS dependence on TTC sorption was observed. Sorption of TTC by surfaces of Fe-WTR decreased in the presence of P(V), exhibiting strong pH dependence. However, presence of sulfate had a minimal/negligible effect on the sorption of TTC by Fe-WTR. Significant negative effect (p<0.001) on the adsorption of TTC by Fe-WTR was observed in the pH range below 5 and at higher TTC: Ca ratios, probably due to TTC-Ca complex formation. Surface complexation modeling and FTIR analysis indicated the possibility of TTC forming mononuclear monodentate surface complex through strong inner-sphere type bonds on Fe-WTR. Results from the batch sorption study indicate high affinity of Fe-WTR for TTC in the pH range 4-8 (the normally encountered pH in the environment) in the presence of competing ligands and complexing metal. Our research is expected to help develop a cost-effective sorbent for TTC removal from aqueous media.