Increased Adsorption of Ciprofloxacin by Systematic Variation of the Composition of Synthetic Montmorilloinites

Abstract

A series of layered silicates with a montmorillonite structure Na_2x(Al_2(1–x),Mg_2x)Si₄O₁₀(OH)₂·nH₂O, where (0 < x ≤ 1), have been synthesized under hydrothermal conditions. The studied samples have a layered morphology, with individual layers with a thickness of about 20–30 nm forming the secondary porous structure of the samples. The ability of synthetic clays to remove the antibiotic drug ciprofloxacin from aqueous solutions was studied. The samples were studied using X-ray phase analysis, scanning electron microscopy and energy-dispersive X-ray (EDX) analysis, low-temperature nitrogen adsorption method, differential thermal analysis, method of adsorption of acid–base indicators, Fourier transform infrared (FTIR)-spectroscopy, and electrokinetic (ζ) potential determination. Isotherms and kinetic curves of ciprofloxacin adsorption were plotted. Adsorption kinetic curves were processed using pseudo-first and pseudo-second-order models. Langmuir, Freundlich, and Temkin models were used to process the adsorption isotherms. A comparison of the sorption capacity of synthetic clays with raw bentonite clay and activated carbon was made. It has been shown that as the degree of isomorphic substitution increases, the sorption capacity of synthetic clays increases and can reach 298 mg/g, which is 3 times more than the sorption capacity of activated carbon. The increase in sorption capacity may be associated with an increase in the negative ζ-potential of the surface as the degree of isomorphic substitution increases. It has been established that the extraction of ciprofloxacin by synthetic clays, in addition to electrostatic interactions, is also determined by their cation exchange capacity, porous-textural characteristics, and by the presence of Brønsted acid sites on the surface. The possibility of regenerating sorbents based on synthetic clays has been demonstrated. Recommendations are made on the selection of optimal compositions of synthetic clays for their use as effective universal sorbents for the extraction of antibiotics from wastewater.