Regular ArticleEnhanced adsorption of diclofenac sodium on the carbon nanotubes-polytetrafluorethylene electrode and subsequent degradation by electro-peroxone treatment
Graphical abstract
Introduction
Pharmaceuticals and personal care products (PPCPs) have received considerable attention in recent years due to their widely distribution in the aquatic environment [1]. Diclofenac sodium (DS) is a widely used non-steroidal anti-inflammatory drug which has potential threats to ecological environments and human health, and it is also one of the most frequently detected pharmaceuticals in surface waters and municipal wastewaters [2]. Adsorption and advanced oxidation processes (AOPs) are widely studied and used to remove different PPCPs from wastewater [3], [4], [5], [6], [7], [8], [9].
Carbon nanotubes (CNTs) have been used to adsorb various contaminants including inorganic metallic ions and organic compounds from water [10], [11], [12], [13]. CNTs can be used as electrodes to remove polar pollutants, and electrosorption can enhance their adsorption rate and capacity. For example, electrosorption significantly increased the adsorption capacity of perfluorinated compounds on the CNTs electrode [14]; 4-nonylphenol was effectively removed by multi-walled CNTs at a potential of 0.6 V [15].
On the other hand, CNTs are also applied to photocatalysis, electrolysis and Fenton reactions for the removal of organic pollutants due to the high electrical conductivity, good chemical stability and remarkable mechanical strength [16], [17], [18]. It is reported that the CNT networks were used as anode and could electrochemically degrade organic pollutions during the filtration process [19], [20], but the anodic oxidation might destroy the CNTs and form new polymers (such as polyphenol and polyoxyphenylene) on the electrode during the oxidative degradation of organic pollutants [20], which prevented the further degradation of organic compounds.
Ozone (O3) has been increasingly used as an oxidant to degrade pharmaceuticals in waters, but it is a highly selective oxidant, less effective than hydroxyl radical (OH) [21], [22], [23], [24]. We developed the novel electro-peroxone process combining conventional ozonation and electrochemical processes [25], [26], and it utilizes the residual O2 in the ozonation process to produce H2O2 on the cathode (). The in-situ generated H2O2 may react with sparging O3 to produce OH (), which can enhance the mineralization of PPCPs [26]. Carbon-based electrodes have been verified to efficiently convert O2 to H2O2 due to their high overpotential for H2 evolution and low catalytic activity for H2O2 decomposition [27], [28].
Due to the low concentrations of PPCPs in waters, it is attractive to concentrate them by adsorption, following by oxidative degradation to realize the adsorbent regeneration and pollutant degradation. Although the removal of some PPCPs from waters using adsorption or oxidation alone has been reported [11], [15], [24], the combination of electrosorption and oxidative degradation of PPCPs, namely, first adsorption of PPCPs on electrodes and then degradation of the adsorbed PPCPs in the regeneration process have not been reported. In this study, the carbon nanotubes-polytetrafluorethylene (CNTs-PTFE) electrode was prepared and used as an anode to adsorb DS from water, and then used as a cathode to in-situ degrade the adsorbed DS in the regeneration process using the novel electro-peroxone treatment. The electrode preparation and electrosorption behavior of DS on the CNTs-PTFE anode in the static and dynamic experiments were investigated. After the dynamic adsorption, the spent electrode was used as the cathode to in-situ degrade the adsorbed DS by the electro-peroxone process. The effects of ozone concentrations and electric current intensity on DS removal as well as DS mineralization were also evaluated, and the complete degradation conditions for DS were obtained. Finally, the good reusability was verified via five successive adsorption-degradation cycles.
Section snippets
Chemicals and materials
Multi-walled carbon nanotubes (MWCNTs) with the outer diameter of 10–20 nm and length of 10–30 μm were purchased from Chengdu Organic Chemicals Co. (China) and used as received. PTFE dispersion (60%) was purchased from DuPont China Holding Co. Ltd. Diclofenac sodium (DS, >98%) was purchased from Sigma-Aldrich. Potassium titanium (IV) oxalate was purchased from Sinopharm Chemical Reagent Co., China. Other chemicals (Na2SO4, NaOH and H2SO4) were purchased from Modern Eastern Fine Chemical Co.
DS adsorption and H2O2 production by CNTs-PTFE electrode
Different electrodes were prepared at different CNTs/PTFE mass ratios and pressures, and the adsorbed amounts of DS on these CNTs-PTFE electrodes were used to evaluate their adsorption capacity (Fig. 2). When the CNTs-PTFE electrodes were prepared at 20 MPa and the variable CNTs/PTFE mass ratios of 4:1, 5:1 and 9:1, the adsorbed amounts of DS were 2.3, 2.7 and 3.5 mg/g, respectively (Fig. 2a). The adsorbed amounts increased with the increase of CNTs/PTFE mass ratios, but the electrode cannot be
Conclusions
This study demonstrated the enhanced electrosorption of DS on the CNTs-PTFE anode and subsequent in-situ degradation by the electro-peroxone process in the self-made device. The adsorption of DS on the CNTs-PTFE anode was enhanced in the presence of the applied voltage due to the electrosorption between anionic DS and positive charges on the anode. The optimal CNTs-PTFE electrode had a strong reactivity for H2O2 and OH production in the electro-peroxone process, and the adsorbed DS on the
Acknowledgments
We thank the National Nature Science Foundation of China (Project No. 51378282), Tsinghua University Initiative Scientific Research Program (20141081174), and Collaborative Innovation Center for Regional Environmental Quality for financial support.
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2022, Journal of Molecular LiquidsCitation Excerpt :Moreover, the adsorbent can be in-situ regenerated by switching off the electric field or even changing the polarity of the electrodes, which prolongs the half-life and reduces the operating cost [11]. Similarly, the in-situ regeneration of the saturated carbon electrodes could be achieved by a subsequent degradation process using electro-Fenton or electro-peroxone treatment [12,18]. Although electrochemical-based techniques have been successfully used for the removal of pharmaceuticals [29], few works have analysed the electrosorption / electrodesorption capacity of carbonaceous materials for the removal of pharmaceuticals and personal care products [11,54,57,58].