Removal of atorvastatin in water mediated by CuFe2O4 activated peroxymonosulfate
Graphical abstract
Introduction
Statins are widely used around the world for the treatment of hyperlipidemia by inhibiting the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which is the key enzyme to the production of cholesterol [1], [2]. The currently marketed statins include atorvastatin (ATV), simvastatin, fluvastatin, lovastatin, pitavastatin, rosuvastatin and pravastatin [3], in which ATV was the most prescribed one with annual sales of 10 billion dollars worldwide [4]. In 2008, the total retail sales of ATV was 5.88 billion dollars in the United States [5]. Increasing amount of ATV were released into wastewater treatment plant (WWTP) and aquatic environment because of its extensive usage and human excretion [6]. The average concentration of ATV in influent and effluent samples collected from 11 wastewater treatment plants located in Ontario, Canada were 166 and 77 ppt, respectively [7]. The influent concentration of ATV was reported as 1.56 ppb in a medium scale sewage treatment plant located in southeastern USA [8]. Furthermore, the concentration of ATV found in Tennessee River water was 101.3 ppt [9]. Therefore, attention needs to be paid to the potential risk of ATV and its degradation products to aquatic organisms.
Some reports have shown that ATV has certain threat to aquatic organisms. Slightly thicker yolk extension and pericardial edema were observed in zebrafish embryos exposed to 10 μ mol dm−3 ATV for 48 h [10]. The numbers of hemorrhagic zebrafish caused by vessel rupture in the head region were increased with increasing ATV concentration when the 6 h post-fertilization zebrafish embryo were exposed to different ATV concentrations (0.05, 0.15, 0.3, 0.5 and 1 μ mol dm−3) for 48 h [11]. In plants, ATV also inhibited the synthesis of sterols. When L. gibba was exposed to ATV, the concentration of sterols, which regulated the water permeability of phospholipid bilayer and affected the function of membrane-bound proteins were decreased with the increasing of ATV concentration, and the EC50 value was 64 ppb [12]. Although the acute toxicity of ATV to aquatic organisms was negligible at environmentally relevant concentrations, it may have a synergistic effect with other stains in the actual environment. In this sense, it is imperative to develop an effective treatment technique for the removal of ATV and its transformation intermediates in wastewater.
Previous studies have shown that ATV can be removed by indirect photolysis in aqueous solution with long half lifetime (4.89 × 103 s) [13]. It is difficult to completely remove ATV by biodegradation, resulting in the frequent detection of ATV in sludge [5], [14]. Advanced oxidation processes (AOPs), such as ozone oxidation, photocatalytic oxidation, Fenton oxidation, has been widely explored in wastewater treatment [15], [16], [17], [18]. Sulfate radical (SO4−)-based AOPs have been recognized as effective alternative method to degrade organic pollutants in wastewater because SO4− is more selective and possesses higher mineralization ability towards organic pollutants than hydroxyl radical (OH) [19], [20], [21]. SO4−· can be generated by activation of peroxodisulfate (S2O82−, PS) or peroxymonosulfate (HSO5−, PMS) via heat, UV, transition metals, mixed metal and alkaline [22], [23], [24], [25], [26], [27]. Mixed metal catalysts have attracted great interests in activation of PS and PMS because of their polyfunctionality, stability and better catalytic activity [28]. Copper ferrite (CuFe2O4) has been reported as an effective heterogeneous catalyst in activating persulfates. Zhang et al. have found that CuFe2O4 showed higher activity and lower Cu2+ leaching than CuO at the same dosage [29]. Guan et al. and Jaafarzadeh et al. have reported that atrazine and 2,4-dichlorophenoxyacetic acid could be degraded quickly by using CuFe2O4 to activate PMS [30], [31]. In addition, Ding et al. have found that 10 ppm TBBPA could be completely removed in 30 min by using 100 ppm CuFe2O4 and 0.2 m mol dm−3 PMS [32]. Therefore, CuFe2O4 was used to activate PMS for the degradation of ATV in this study.
The main purpose of this study is to systematically investigate the oxidative degradation process of ATV by CuFe2O4/PMS. Firstly, the key factors influencing the transformation of ATV were evaluated including solution pH, PMS concentration and CuFe2O4 dosage. Secondly, major oxidative species were identified by radical quenching experiment. Thirdly, the identification of transformation intermediates was studied using TOF-LC-MS. Fourthly, the degradation pathways of ATV were proposed, which was verified by the Gaussian theoretical calculations. Then, the distribution of transformation intermediates was analyzed. Finally, the removal efficiency of ATV in actual wastewater was examined.
Section snippets
Chemicals and reagents
Atorvastatin (ATV, 98.0%) was purchased from J&K Scientific Co. Ltd. Peroxymonosulfate (Oxone, KHSO5·0.5KHSO4·0.5K2SO4, KHSO5 ≥ 47%), copper iron oxide (CuFe2O4, 98.5%) and 2,2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) ammonium salt (ABTS, 98%) were obtained from Sigma-Aldrich (Shanghai, China). Standards of ATV lactone and 2-hydroxy ATV calcium salt were purchased from Toronto Research Chemicals (Toronto, Canada). HPLC grade methanol was obtained from Tedia Company (Fairfield, USA).
Degradation of ATV at different reaction conditions
The effect of initial pH on the degradation of ATV was illustrated in Fig. 1. The removal rate of ATV increased with the increase of pH from acid to neutral and decreased under alkaline condition (Fig. 1a). The removal rate of ATV in 30 min varied from 9.8% to 73.9% at pH range from 3.0 to 10.5 with the highest rate at pH 7 (73.9%). The phenomenon can be explained in the following aspects: CuFe2O4 nanoparticles are easily dissolved under acidic conditions to release metal ions (shown in Fig. S5a
Conclusion
Effective removal of ATV from water was achieved by CuFe2O4/PMS. The results revealed that increasing the PMS concentration and CuFe2O4 dosage enhanced the degradation efficiency. Optimized removal of ATV was achieved under neutral condition, which was determined by pKa of ATV, species of PMS, and pHpzc of CuFe2O4. Four degradation pathways were proposed by the structural analysis of intermediates and the studies of frontier electron densities. The distribution of ATV intermediates at different
Acknowledgments
The authors are very grateful for the financial support from the Commonwealth and Environmental protection project for the MEP grant (201509053) and the National Natural Science Foundation of China (No. 21577059).
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