Efficient degradation of diclofenac by LaFeO3-Catalyzed peroxymonosulfate oxidation---kinetics and toxicity assessment
Introduction
In the past decade, pharmaceuticals, as emerging organic pollutants, have received ever-increasing concerns due to their ubiquity in water environment and documented harmful behavior (Corcoran et al., 2010; Gavrilescu et al., 2015). In 2013, pharmaceuticals were legally deemed as a potential hazard for the aquatic environment in an amendment of the EU water framework directive. This directive contains a watch list of compounds which demand recording, tracking and evaluating of their environmental risks to support their classification. Diclofenac (DCF) was included in the first watch list (EU, 2013).
Advanced oxidation processes (AOPs) are considered as desirable technologies for the degradation and mineralization of persistent organic pollutants (POPs) through radical oxidation. DCF has been subjected to many studies on its treatability by various AOPs, including photocatalysis (Chen et al., 2018), ozonation (Sein et al., 2008), sonication (Al-Hamadani et al., 2018) and Fenton oxidation (Zhou et al., 2018). In the past decade, there is an increasing interest in sulfate radical anion based oxidation for POPs removal. Sulfate radical with a redox potential of 2.5–3.1 V exhibited relatively high selectivity towards aromatic/unsaturated chemical structures (Neta et al., 1988) with a much longer half-life than hydroxyl radical (Janzen et al., 1992; Olmez-Hanci and Arslan-Alaton, 2013). The generation of sulfate radical can be achieved via the activation of peroxydisulfate (PDS) or peroxymonosulfate (PMS) by thermal (Yang et al., 2010), UV light (Lau et al., 2007; Guan et al., 2011), ultrasound (Hao et al., 2014) and transition metal ions (Rao et al., 2014). Among these methods, activation of PMS or PDS by transition metal ions is considered more cost-effective since the other processes demand continuous energy input. Although Cobalt (II) was an effective activator for PMS to generate sulfate radical among the transition metal ions, it is not the optimal choice due to its potential adverse health effects. Even for Co3O4 or supported cobalt oxide, concerns about leaching of cobalt from the solid phase still rein in their practical application in water treatment. On the other side, iron, the second most abundant element in the earth, is not considered to pose a potential risk on human health and ecological system. It has been reported both Fe2O3 and Fe3O4 can activate PMS to generate sulfate radicals in previous studies (Ji et al., 2013; Tan et al., 2014; Jaafarzadeh et al., 2017). Su et al. (2017) observed the perovskite oxide could promote an easier valence-state change of the B-site cation (cobalt ions) to accelerate a redox process. In recent years, using perovskite-type oxides (PTOs) to activate PMS has attracted increasing attention (Pang et al., 2016; Ben Hammouda et al., 2017; Solis et al., 2017; Su et al., 2017; Duan et al., 2018; Miao et al., 2018). Cobalt-based PTOs exhibited much higher activity toward PMS than Co3O4 did. However, the investigation on the use of Fe-based PTOs as an activator of PMS is limited. In our previous study, LFO also showed much better performance than Fe2O3 did in term of activating PMS for DCF degradation. The Fe (III) in LFO perovskite oxide can be reduced more easily than Fe (III) in Fe2O3. The generation of sulfate radicals which made a major contribution to DCF degradation is ascribed to the formation of inner-sphere complexation between Fe (III) and HSO5− and Fe (III)Fe (II)Fe (III) redox cycle (Rao et al., 2018).
The burgeoning applications of nanomaterials have invited environment, health and safety concerns in recent years. Servin et al. (2013) verified the accumulation of TiO2 nanoparticles in cucumber. CuO nanoparticles were reported to exert a toxic effect on Prokaryotic alga Microcystis aeruginosa and the presence of fulvic acid increased its toxicity (Wang et al., 2011). Furthermore, 2.5 mM PMS was observed to inhibit the growth of freshwater microalgae Pseudokirchneriella subcapitata significantly (Olmez-Hanci et al., 2014). However, the toxicity of PMS at low concentration to phytoplankton remains unknown. It was reported that the intermediates generated during the degradation of pharmaceuticals by some AOPs might be more toxic than parent compounds. In our previous study, the EC50–96h (Half maximal effective concentration for 96 h) of both solutions after 30-min and 60-min sonication is lower than that of initial carbamazepine solution (Rao et al., 2016). Chen et al. (2016) reported that the toxicity of intermediates in both ozonation and catalytic ozonation processes had increased, in comparison with that of DCF in the first 15 min of DCF degradation. Thus, the ecotoxicity evolution of the treated wastewater containing DCF should also be properly evaluated.
In this study, we focused on the optimization of LFO/PMS process conditions through examining the influence of various parameters such as pH value, LFO dosage, PMS concentration and DCF concentration on DCF degradation. The effect of calcination temperature on the activity of as-prepared catalyst LaFeO3 was evaluated. The potential toxicity of LFO and PMS was evaluated towards phytoplankton. The evolution of the acute toxicity was also examined during DCF degradation.
Section snippets
Chemicals
Iron nitrate nonahydrate (Fe(NO3)3·9H2O), citric acid monohydrate (C6H8O7·H2O), 2, 6-dichloroaniline and sodium tetraborate were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Diclofenac (2-[2,6-dichlorophenyl]-amino]-benzene acetic acid sodium salt), Oxone and Lanthanum nitrate hexahydrate (La(NO3)3·6H2O) were obtained from Sigma-Aldrich. All solvents were HPLC grade. All chemicals are in analytic purity and used without purification. All aqueous solutions were prepared in
XRD characterizations and pore structure of as-prepared LFO
Fig. S1 shows the XRD patterns of as-prepared LFO calcined at different temperature. The characteristic diffraction peaks located at 22.6°, 32.2°, 39.7°, 46.2°, 57.5°, 67.5° indicate the successful synthesis of phase-pure LFO. The molar ratio of La and Fe in LFO was determined to be 1:1.04 based on the ICP analysis of La and Fe content after the acid-digestion of LFO, further confirming the synthesized samples are LFO. As also illustrated in Fig. S1, the increasing intensity of those
Conclusions
In this study, LFO was synthesized. LFO was found to be an effective activator of PMS for the elimination of DCF in aqueous phase. DCF degradation data fit pseudo first-order kinetics well (R2 > 0.96). The catalytic activity of LFO depends on the calcination temperature of LFO. Experimental results also show that the performance of LFO/PMS was influenced by pH levels, LFO dose and PMS concentration. The optimal pH is around 7.0. The optimal concentration of PMS is 0.5 mM with the LFO dosage
Acknowledgements
This work was financially supported by National Science Foundation of China (No.41877480), Shaanxi Natural Science Foundation, China (No. 2017JM5074) and Open project of Jiangsu Key Laboratory for Bioresources of Saline Solis. The authors are also grateful to all anonymous reviewers who contribute to improving this work.
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