Effect of Cu(II) ions on the enhancement of tetracycline adsorption by Fe₃O₄@SiO₂-Chitosan/graphene oxide nanocomposite

Highlights

• Fe₃O₄@SiO₂-Chitosan/GO adsorbent (MSCG) was successfully prepared.

• MSCG exerts enhanced interaction with the tetracycline (TC).

• Well explained the role of Cu(II) as the bridge between TC and MSCG.

• Magnetic separation technology was used to separate MSCG with TC from aqueous solution.

• The regenerated MSCG exhibited excellent reusability for the adsorption of TC with and without Cu(II).

Abstract

Fe₃O₄@SiO₂-Chitosan/GO (MSCG) nanocomposite was investigated by various techniques (SEM, TEM, XRD, VSM, FT-IR, XPS) for the removal of tetracycline (TC). Effects of pH, zeta potential and initial contaminant concentration were studied in detail. Four background cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺) with a concentration of 0.01 M showed little influence on the TC adsorption at the studied pH range while the divalent heavy metal cation Cu(II) could significantly enhance the adsorption. The results indicated that the highest adsorption capacity of TC were 183.47 mmol/kg and 67.57 mmol/kg on MSCG with and without Cu(II), respectively. According to mechanism investigation for the adsorption of TC by pH impact study and XPS analysis, besides electrostatic interaction and π–π interactions, the Cu(II) also acts as a bridge between TC and MSCG, which significantly improve the adsorption of TC. This study provided valuable guidance and effective method for the removal of TC from aquatic environments.

Introduction

Tetracycline (TC), the second most widely used antibiotics worldwide, has been extensively used as human medicines, veterinary drugs, and growth promoters in animal cultivation with little attention for a long time (Lian, Song, Liu, Zhu, & Xing, 2013). It is reported that only small amount of TC are absorbed during metabolism, and the majority are excreted via urine and feces as unchanged form. Residues of TC have been frequently detected in wastewater (Liu, Xu, Lee, Yu, & Lihong, 2016), surface water, and even in groundwater (Gottschall et al., 2012). Their presence within the environment may cause a risk to human health by promoting antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARG) (Sharma, Johnson, Cizmas, McDonald, & Kim, 2016). Therefore, to prevent its deleterious impact on the ecosystem and public health, it is of great importance to develop effective and low-cost technologies for its removal from surface and wastewater. To date, four most promising treatment methods are ion-exchange, membrane filtration, photocatalytic degradation, and adsorption. Compared to other techniques, adsorption is considered simple and economical, and remains one of the most attractive methods for antibiotic removal (Álvarez-Torrellas, Rodríguez, Ovejero, & García, 2016).

In environmental engineering, polymers have been used for different purposes, but the use of natural polymers has attracted considerable attention from the perspectives of cost, environmental and safety concern. Among these, chitosan is a highly deacetylated derivative of chitin, and known to be a biodegradable and non-toxic natural polymer in addition to its excellent capability of adsorption (Ge, Hua, & Chen, 2016). Previous studies have reported that chitosan and its derivatives as biosorbents have already been used in TC treatment (Jia et al., 2016). For example, Caroni et al. did a kinetic analysis of the sorption of TC on chitosan and reported that it is a promising sorbent for TC (Caroni, De Lima, Pereira, & Fonseca, 2009). Oladoja et al. explored a novel magnetic macro-reticulated cross-linked chitosan for the adsorption of TC from aquatic systems and high removal efficiencies were achieved (Oladoja, Adelagun, Ahmad, Unuabonah, & Bello, 2014). However, despite the numerous advantages and unique properties of chitosan, its use in a wider range of applications is limited because of its poor mechanical and electrical properties (Giannakas, Grigoriadi, Leontiou, Barkoula, & Ladavos, 2014). An effective method for improving the physical and mechanical properties of CS is to form organic–inorganic composites through incorporation of nanofillers, such as metal nanoparticles, clays, carbon nanotubes and graphene oxide (Huang et al., 2015).

Graphene, a monolayer of hexagonally arrayed sp²-bonded carbon atoms, due to its excellent physical and chemical properties, has been studied world-wide for several purposes, since its discovery in 2004 (Kyzas, Koltsakidou, Nanaki, Bikiaris, & Lambropoulou, 2015). Chemical structure of graphene oxide (GO) is reported as oxidized graphene, decorated with various oxygenated functionalities such as hydroxy, epoxy on the basal plane and carbonyl, carboxylic acid at the edges (Mukherjee, Bhunia, & De, 2016).These oxygen hydrophilic functionalities make GO dispersible in water as well as some organic solvents extensively, and easier to intercalate. From earlier reported literatures, it is known that the aromatic compound can be easily adsorbed on GO and graphene by π– π stacking (Lin, Xu, & Li, 2013). TC consists of four aromatic rings with various functional groups on each ring, which can be strongly deposited on the GO surface via π–π interaction and cation–π bonding. Therefore, intercalation of CS into GO could not only enhance the physical and chemical properties derived synergistically from both components but also the adsorption capacity of TC.

Since this composite material is preferably soluble in water, separation is difficult. To improve the efficiency of separation, magnetic separation technology has attracted much attention (Han, Cao, Ouyang, Sohi, & Chen, 2016; Tang et al., 2012), but fewer were used to separate TC from aqueous solution directly. Thus, introducing magnetic properties to the adsorbent can be a research hotspot, magnetic nanoparticles, especially Fe₃O₄, have attracted more and more attentions because of the outstanding properties such as easy separation and low toxicity (Shan, Yan, Yang, Hao, & Du, 2015). However, there are two major challenges. One is related to the reunion, poor dispersion of Fe₃O₄ in water. The other is the easy oxidation/dissolution of iron nanoparticles, especially at high concentrations of acid solution. To compensate for these shortages, surface modifications of magnetic nanoparticles based on covalent binding or physical coating have been widely explored (Fan, Li, Zhou, & Liu, 2016; Muliwa, Leswifi, Onyango, & Maity, 2016). For example, Magnetic Fe₃O₄@poly(m-phenylenediamine) particles (Fe₃O₄@ PmPDs) with well-defined core−shell structure were designed for high performance Cr(VI) removal by taking advantages of the easy separation property of magnetic nanoparticles (Wang et al., 2015). In comparison with these organic coating materials, SiO₂ can serve as a more ideal shell component because of its stability under acidic conditions, and abundance of surface hydroxyl groups are able to link special functional groups (Lai, Xie, Chi, Gu, & Wu, 2016). Thus, the silica shell is convenient for the material to be grafted onto chitosan, which can also make the core–shell structure more stable.

The objective of this paper focused on the TC removal ability by the graphene oxide composites, namely Fe₃O₄@SiO₂-Chitosan/GO nanocomposite (MSCG). The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared spectrum (FT-IR), vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS). Meanwhile, to fully understand the sorption behavior of TC on MSCG, and how pH and Cu(II) affect TC adsorption on the MSCG, a series of sorption experiments were conducted to determine the sorption properties. The adsorption mechanism was investigated by XPS analysis. Our results demonstrate that MSCG exhibits tremendous potential for effective removal of TC and fast separation performance in aqueous media simultaneously.