Recently, the increasing use of pharmaceuticals and personal care products has led to widespread discharge of such products into water environment. Since these products pose a potential threat to the water environment and human health, this issue has attracted attention. CIP, NOR and OFL are fluoroquinolone antibiotics widely used as veterinary and human drugs. It offers high antibacterial activity against Gram negative and Gram positive bacteria by inhibiting DNA gyrase [Fang et al. 2020]. CIP, NOR, OFL antibiotics can be found in animal husbandry, hospital wastewater and sewage treatment plants. Therefore, it is imperative to develop an effective method to remove antibiotics in wastewater. Commonly used methods for removing antibiotics include adsorption [Chen et al. 2015, Fang et al. 2020], membrane filtration [Azhar et al. 2017, Koyuncu et al. 2008], catalysis [Chen et al. 2019] and photocatalytic degradation [Hu et al. 2019]. However, membrane filtration is high cost and prone to cause membrane fouling [Azhar et al. 2017]. Photocatalysis and catalytic degradation may decompose antibiotics into small molecules. In addition, the cost of catalysts is a constraint for large-scale application, and it is difficult to determine the harmfulness of by-products of catalysts [Mehrjouei et al. 2014]. Compared with other treatment methods, adsorption has been considered to be a more effective method to remove contaminants in aqueous systems [Yan et al. 2017]. Referring to the recent studies, various materials are used as adsorbents to remove fluoroquinolones from wastewater, such as natural mineral materials [Jiang et al. 2013] and carbon-based materials [Carabineiro et al. 2012, Wang et al. 2011] for CIP removal; barley straw [Yan et al. 2017], porous resin and carbon nanotubes [Yang et al. 2012] have also been utilized for NOR adsorption; similarly, mesoporous alumina, cork-bio-mass and silica have been investigated [Crespo-Alonso et al. 2013] for OFL adsorption. However, the removal efficiency of these traditional adsorbents for antibiotics remains to be improved. Therefore, the development of renewable adsorbents with high adsorption capacity has been widely studied.
Amino acid can be used to remove contaminants in water environment because of its good chelation [Koilraj et al. 2019]. In order to achieve high efficiency and stability, amino acids are considered to be functionalized on solid surfaces with excellent properties. There are some reports on the contaminants removal by amino acid functionalized adsorbents in the literature. Glycine functionalized europium hydroxide [Alemtsehay et al. 2018] and arginine modified hydroxyapatite [Yang et al. 2017] were used to remove radioactive elements in wastewater. Similarly, FeSO4 grafted lysine modified polymer [Jing et al. 2018] and arginine and lysine functionalized Fe3O4 [Singh et al. 2016, Zhang et al. 2014] have been developed to remove organic matter, Ni (I) and arsenate [Jing et al. 2018, Singh et al. 2016, Zhang et al. 2014]. These reports showed that amino acid functionalized materials are multifunctional adsorbent, and the treatment effect of wastewater containing pollutants is considerable.
Layered double hydroxides ( LDHs ) are synthetic clays, whose molecular formula is [M(II)1−xM(III)(OH)2]x+ (An−)x/m·mH2O, where M (II), M (III) and An− are divalent and trivalent metal ions and interlayer charge equilibrium anions, respectively. Because of their high surface area, exchangeable anions, low cost and non-toxic properties, LDHs is condemned as a promising adsorbent to remove heavy metals and organic compounds (including dyes, pharmaceuticals and personal care products) in water treatment. Therefore, it is considered to functionalize amino acids on LDHs to achieve better adsorption performance.
Amino acid functionalized LDHs are used to remove different types of pollutants. A-alanine functionalized MgFe-LDH [Hong et al. 2014], glycine functionalized MgAl-LDH [Asiabi et al. 2017] and histidine functionalized MgAl-LDH [Tran et al. 2018] have been reported to be used to remove oxygen anions, heavy metal cations and organic compounds. Compared with the LDHs reported in other literature, the prepared amino acid functionalized LDHs have higher removal capacity. Based on these observations, we studied the effect of lysine-functionalized LDHs for the antibiotics’ efficient removal in aqueous systems.
As far as we know, there is no report about amino acid functionalized LDHs adsorption for antibiotics. In this study: (1) the controllable preparation of Ly @ FeZn was optimized by RSM. It means an efficient RSM model was established with the least number of experiments. Then, high adsorption capacity adsorbent was prepared at low cost. By the response surface optimization analysis, the qe for CIP could reach the highest (202 mg/g) at temperature of 60°C, Fe / Zn molar ratio of 0.5 and the lysine dosage of 5.8 mmol. (2) the optimized Ly @ FeZn was characterized by SEM, XRD, FT-IR and XPS. Batch technique (including adsorbent dosage, pH, contact time, antibiotic concentration and temperature) were performed. The results showed that the removal rate of CIP and NOR by Ly @ FeZn was above 95%. Moreover, different removal mechanism of Ly @ FeZn for CIP, NOR and OFL was investigated.