Adsorption of tetracycline using chitosan-alginate-bentonite composites

Nanocomposites composed of biopolymers and clay minerals have applications in several fields, particularly in environmental remediation. In this study, a series of new adsorbents for tetracycline was prepared by the microwave-assisted reaction of two biopolymers, chitosan (CS) and sodium alginate (SA), with sodium bentonite (Na-Bent), a clay mineral. The nanocomposites were prepared by first mixing CS with Na-Bent at 100, 200, and 300% of Na-Bent's cation-exchange capacity (CEC) at 50 degrees C for 30 min. Subsequently, the resulting mixture was reacted with SA at a 1:1 mass ratio under microwave-assisted heating. X-ray diffraction analyses indicated the formation of intercalated nanocomposites (with basal spacings of 1.96-2.26 nm) with increasing amount of CS, as quantified by CHN elemental analysis (the samples obtained using CS at 300% CEC of Na-Bent had higher carbon contents of 5.82% or 6.95 mmol/ g). Maintenance of the flake-like morphology of the clay was confirmed by scanning electron microscopy. The tetracycline-adsorption studies, viz., kinetics and equilibria at pH 5.5, using the bionanocomposites were conducted using 10-550 mg L-1 tetracycline solutions. Compared with that of Na-Bent (69% removal at 75 mg dosage), the nanocomposites exhibited a higher adsorptive capacity (>75%) for the antibiotic at 10 mg L-1, even when a lower dosage of the nanocomposites (25 mg) was used in comparison with that of Na-Bent. A better performance was obtained using the bionanocomposite prepared with CS at 100% CEC of Na-Bent, and the tetracycline removal reached 97.7% in 30 min. The addition of SA did not significantly change the adsorption capacity of the nanocomposites, and the tetracycline removal reached 81.8-82.7% at 45-90 min (adsorbent concentration: 10 mg L-1). Long-and short-range interaction characteristics and zeta potential measurements indicated that electrostatic interactions and ion-exchange processes drove the adsorp-tion mechanism.