Insight and mechanisms of tetracycline adsorption on sodium alginate/montmorillonite composite beads

The presence of antibiotics in soil and water raises great health concerns because they may increase the development of harmful antibiotic-resistant bacteria. In this study, composite beads prepared from 2:1 or 1:1 ratios of montmorillonite (Mt) and sodium alginate (SA) were used to adsorb tetracycline hydrochloride (TC), an antibiotic commonly found in aqueous systems. The equilibrium time for TC adsorption onto the Mt/SA was 8 h, and the kinetic data were consistent with the pseudo-second-order model. The adsorption isotherm was described with the Langmuir equation. The maximum amounts of TC adsorbed were 745, 689, and 445 mg g−1 for the 2:1- and 1:1-Mt/SA and the original Mt, respectively. The Mt/SA composite beads exhibited porous structures; however, this did not play a key role in TC removal, as previously reported. Cation exchange was the major adsorption mechanism, and electrostatic attraction and hydrogen bonding between the SA and TC also contributed to TC adsorption on the Mt/SA composite beads. In addition, the migration of a small amount of TC into the inner spaces of the beads led to the intercalation of the TC in the Mt interlayers and enhanced TC adsorption by the Mt/SA composite beads.