Removal of nonsteroidal anti-inflammatory drugs (NSAIDs) by electrocoagulation–flotation with a cationic surfactant
Graphical abstract
Introduction
Pharmaceuticals are emerging contaminants of concern [1], [2]. Due to the large variety of pharmaceuticals being used for medical purposes, elevated concentrations of these compounds can be found in hospital wastewaters [3], [4], [5]. Conventional wastewater treatment processes are not effective in removing these substances due to their complicated properties [6], [7], [8]. Among the pharmaceuticals, nonsteroidal anti-inflammatory drugs (NSAIDs) are most frequently found due to their widespread use [9], [10]. Although there is no evidence to suggest that NSAIDs are harmful to adults, they may be toxic to aquatic organisms and harmful to embryos, infants, children, and adults with weak constitutions and sensitivity to pharmaceuticals [11], [12], [13], [14], [15], [16].
The electrocoagulation–flotation (ECF) process has been applied as an economical physicochemical method for treatment of various types of wastewater such as textile wastewater [17], [18], [19], papermaking wastewater [20], chemical mechanical polishing (CMP) wastewater [21], [22], laundry wastewater [23], and fluoride-containing wastewater [24], [25], [26]. This method has also been successfully applied to treat wastewater containing such pharmaceuticals as salicylic acid [27], berberine hydrochloride [28], [29], and some antibiotics [30]. Chou et al. indicated that over 80% of salicylic acid compounds were removed with aluminum electrodes [27]. Ren et al. found that the removal efficiencies of berberine hydrochloride (a broad-spectrum antibiotic) with iron electrodes reached over 80% [28]. According to Martins et al. study, the reduction of the concentration of sulfamethoxazole by electrocoagulation-flotation process with aluminum electrodes was over 80% [30]. The main removal mechanisms for these pharmaceuticals by EC should be adsorption [27] and charge neutralization [29]. The target compounds of this study are NSAIDs, which are acidic in nature. NSAIDs dissociate under neutral and alkaline conditions, and may also be removed by electrocoagulation through adsorption and charge neutralization.
In electrocoagulation–flotation, metal ions and hydrogen gas are produced stoichiometrically and simultaneously when an electric current is passed through a sacrificial electrode (Eqs. (1), (2)):
Meanwhile, the metal ions are then hydrolyzed to various metal hydroxide complexes and metal hydroxide precipitates by the following reactions:
The soluble and colloidal pollutants can be removed by adsorption, ionic complexation, or ion exchange on active surface sites of the metal hydroxide flocs [31], [32]. The flocs are removed either by sedimentation of the ions or by flotation with hydrogen gas released from the cathode at the end of the reaction.
The ECF process emphasized the flotation performance of electrocoagulation [24], [25], [33], [34]. The duration of flotation is much shorter than that of sedimentation; moreover, less space is needed for the treatment plant. However, the flocs can rarely be completely removed by flotation in a conventional ECF process. The hydrogen bubbles generated by the cathodes is hydrophobic, and some of the bubbles would combine together, thus the flotation efficiency decreases with increasing bubble size as the total surface area and retention time of large bubbles are less than those of small bubbles. To make gas bubbles smaller and more stable, surfactants are employed as a frother to reduce the surface tension of solutions in ECF processes [24], [25]. They are also employed as a collector to increase the collection efficiency of pollutants by bubbles [24], [25]. A cationic surfactant may neutralize anionic NSAIDs and improve their removal in the ECF process.
To develop a promising way to remove NSAIDs from hospital wastewater, this study applied ECF to remove three NSAIDs: diclofenac, ibuprofen, and ketoprofen. A cationic surfactant, cethyltrimethyl-ammonium bromide (CTAB) was added during the ECF process as a collector and frother to improve NSAIDs adsorption and the removal efficiency during flotation. Furthermore, the effects of the surfactant dose ([CTAB]0), charge loading (Qe), current density and operating time (t) on the NSAIDs removal were investigated. Finally, a real hospital wastewater with spiked NSAIDs was treated under optimal ECF operating conditions.
Section snippets
Chemicals and materials
Diclofenac (⩾98%), ibuprofen (⩾98%), ketoprofen (⩾98%) and CTAB (⩾98%) were purchased from Sigma–Aldrich. NaCl (99.5%) was purchased from Wako. KH2PO4 (99%) was purchased from SHOWA. HPLC-grade methanol (99.99%) was purchased from Scharlau. HPLC-grade acetonitrile (99.9%) was purchased from J. T. Baker. Individual stock standard solutions were prepared on a weight basis using DI water and were stored in amber glass bottles at 4 °C for a maximum of 14 days. The physicochemical properties of the
Effects of pharmaceutical characteristics
The removals of the three NSAIDs as a function of reaction time by the ECF process are shown in Fig. 2. Removals were in the 10–45% range in the single-NSAID systems; these results are similar to those of Suarez et al. [35]. Due to its high lipophilicity, ibuprofen exhibited the highest removal (44%). Removals of diclofenac and ketoprofen were 14% and 10%, respectively. In the multiple-NSAIDs system, NSAID removals significantly decreased (Fig. 2(b)). This may be due to competitive adsorption
Conclusions
The ECF process with the addition of CTAB could effectively remove NSAIDs which have lipophilic characteristics and low surface tension in solution. Without addition of the cationic surfactant, the removal of NSAIDs in single-NSAID systems was in the range of 10–45%, which decreased to approximately 10% in multiple-NSAIDs systems. After adding a stoichiometric amount of CTAB, removal significantly improved in single-NSAID systems. The concentration of CTAB required to achieve an ideal
Acknowledgements
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research (contract no. NSC101-2221-E-038-011-MY2).
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