Investigation of adsorption properties of oxytetracycline hydrochloride on magnetic zeolite/Fe₃O₄ particles

Highlights

• OTC-HCL adsorption on zeolite/Fe₃O₄ best fitted to the Langmuir isotherm.

• Experimental data were fitted with pseudo second order kinetic model.

• Characterization results show that adsorption of OTC-HCL on zeolite/Fe₃O₄ successfully achieved.

• After adsorption, molecular clusters that form in bulk on the surface resulted from the formation of [Fe(TC)]_n³⁺ complexes.

• Adsorption process showed endothermic and spontaneously behavior.

Abstract

Iron oxides and ferrite-supported zeolites have been successfully used in the cheap and environmentally friendly removal of organic pollutants such as antibiotics. In this study, the adsorption of Oxytetracycline hydrochloride (OTC-HCL) on zeolite/Fe₃O₄ particles synthesized from natural Manisa-Gördes clinoptilolite by co-precipitation method was investigated in the batch system at 298–323 K. Various parameters such as pH, initial antibiotic concentration, adsorbent dosage, contact time, stirring speed and temperature were examined and optimum parameters were determined according to experimental data. Adsorption isotherms were investigated with Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Temkin models. Kinetic constants were determined according to pseudo first order, pseudo second order, intraparticle diffusion and Elovich models. OTC-HCL adsorption on zeolite/Fe₃O₄ best fitted to the Langmuir isotherm and pseudo second order kinetic models. The maximum adsorption capacity of zeolite/Fe₃O₄ was determined as 83.33 mg/g at 323 K. Adsorption of OTC-HCL on zeolite/Fe₃O₄ occurred spontaneously and endothermic. Physicochemical characterization of zeolite/Fe₃O₄ was performed before and after adsorption, by N₂ adsorption-desorption, XRD, FTIR, SEM-EDX, XPS, TEM analyses. Magnetic properties of zeolite/Fe₃O₄ were determined by VSM analysis. The BET specific surface area of zeolite/Fe₃O₄ decreased after adsorption of OTC-HCL. VSM and SEM-EDX results showed that zeolite/Fe₃O₄ had superparamagnetic property and OTC-HCL adsorbed on zeolite/Fe₃O₄ successfully.

Introduction

Antibiotics are pharmaceutical compounds widely used in human medicine and veterinary [1]. They enter the water cycle as main compounds and metabolites due to partially metabolized [2]. In recent studies, antibiotic residues were determined in soil [3], groundwater [4], wastewater [5], tap water [6], surface water [7]. Wastewaters that emerge from industry, hospitals, aquaculture, and households are the most important sources of antibiotic pollution in aquatic environments [8]. The presence of antibiotic pollution in the environment causes potential toxic effects on microorganisms, plants, animals, and humans [9]. Tetracyclines (TCs) are one of the most widely used antibiotics in aquafarming and veterinary medicine [10]. Most studies have shown that TCs do not fully remove from wastewater treatment plants [11], [12], [13]. For the removal of TCs from wastewater, adsorption [14], membrane filtration [15], advanced osmosis [16], electrochemical method [17] and photocatalysis [18] are used. Among these methods, adsorption is the most efficient and widely used treatment method [19]. Natural zeolites are hydrated aluminosilicate minerals with large specific surface area and high cation exchange capacity (CEC) due to extended isomorphic substitution of Si⁴⁺ by Al³⁺ in tetrahedra [20]. Recently, zeolites have been used frequently in the adsorption process due to their ease of accessibility, specific properties, and low cost [21]. The adsorption potential of the zeolite surface can be significantly improved by using various modification methods [22]. Among these modifications, the adsorbent can be easily separated from the solution by applying an external magnetic field, with particles having a relatively high surface area, a smaller particle size, and a porous structure by magnetic modification. In particular, the excellent magnetic property of the powder allows it to be easily recovered by magnetic separation technology after adsorption or regeneration [23], [24]. Since magnetite (Fe₃O₄) shows high dispersion stability and magnetic susceptibility, it is widely used as a magnetic material [25]. The methods used in the preparation of magnetic composites are sol–gel [26], co-precipitation [27], thermal degradation [28], microwave method [29], hydrothermal [30], solvothermal synthesis [31] and mechanochemical process [32]. The co-precipitation method is frequently preferred for the removal of impurities in water due to its advantages such as synthesizing particles of similar size, ease of application and less chemical use. In the chemical co-precipitation method, magnetic particles are generally obtained by adding a base to the aqueous solution of Fe(II) and Fe(III) salts [33]. The chemical reaction of Fe₃O₄ precipitation is given by the following equation [34].

Fe³⁺ + Fe²⁺ + 8OH⁻ ↔ 2Fe(OH)₃ (s) + Fe(OH)₂ (s) → Fe₃O₄ (s) + 4H₂O

Iron oxides have been used for zeolite modification in wastewater treatment. Iron compounds show high affinity for TC compounds due to surface complexation. The iron ion can increase the adsorption capacity by establishing a bond between the zeolite and TC molecule [35]. Removal of antibiotics such as cephalexin [36], ceftazidime [37], diclofenac, naproxen, gemfibrozil, ibuprofen [38] from aqueous solutions with magnetic zeolites has been successfully studied. Abadi et al. (2019) investigated TC adsorption on zeolite 13X and Fe³⁺ modified zeolite 13X. Adsorption data were best described with Langmuir isotherms. The maximum adsorption capacity of the modified zeolite for TC adsorption was calculated as 204 mg/g, which is approximately four times higher than the unmodified zeolite [35]. Liu et al. (2015) used modified zeolite X with zerovalent iron particles (NZVI) to remove TC from water. The adsorption capacity of the TC reached 476.2 mg/g by magnetic modification. Adsorption isotherms were well described by Langmuir isotherm model and adsorption kinetics followed both pseudo second order and intraparticle diffusion models [39]. In the present study, the adsorption of OTC-HCL on zeolite/Fe₃O₄ was investigated. It has been observed that zeolite/Fe₃O₄ can be used effectively in the removal of antibiotics from aqueous solutions.

The amount of OTC-HCL adsorbed per gram of zeolite/Fe₃O₄ and the adsorption efficiency were determined [40]. Adsorption of OTC-HCL on zeolite/Fe₃O₄ examined with Langmuir [41], Freundlich [35], Dubinin Radushkevich (D-R) [41] and Temkin [42] isotherm models. Adsorption of OTC-HCL on zeolite/Fe₃O₄ particles was investigated with pseudo first order, pseudo second order kinetic, intraparticle diffusion and Elovich models. Thermodynamic parameters were calculated between 298 and 323 K [43]. Experiments were performed using optimum values in Table 2.