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 °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 adsorption mechanism.