Amoxicillin electro-catalytic oxidation using Ti/RuO2 anode: Mechanism, oxidation products and degradation pathway
Introduction
Water pollution is one of the major concerns of mankind in the present era. Urbanization, industrialization and population growth has led to consumption of enormous gallons of water, which in turn leads to generation of hefty volumes of wastewater. A number of treatment technologies have been developed and implemented at production sites to eliminate detrimental pollutants from wastewater, in order to make it suitable to be discharged in to aquatic environments. Since the introduction of electrochemical advanced oxidation processes (EAOPs) in the late 1970s, they have fetched much of the attention of scientific groups worldwide for treatment of bio-refractory and toxic organic pollutants from water [[1], [2], [3], [4]]. EAOPs are based on in situ generation of hydroxyl radical (OH), a powerful oxidizing agent which unselectively destroys the organic content in wastewaters. Among different EAOPs, electro-oxidation (EO) method of water treatment has become quite popular for its application in industrial wastewater treatment and target pollutant destruction [5,6]. Major advantages of EO are: 1) no sludge generation, hence environmentally compatible; 2) minimum or no addition of chemicals required; 3) versatile; 4) amenable to automation; 5) operates at mild conditions and simple equipment, hence safe; 6) robust as reaction can be easily terminated by switching off the power [7]. EO technique destroys organic pollutants by two different pathways, i.e., “direct oxidation” at the anode surface and “indirect oxidation” in the bulk solution by oxidizing agents generated on the anode surface [5,8].
Performance and efficiency of EO technique have been investigated for degradation of persistent organic pollutants, such as pharmaceutical compounds, from wastewater [[9], [10], [11], [12], [13], [14], [15]]. Much focus has been given on removal of antibiotics by EO [[16], [17], [18], [19], [20]], since the studies in different parts of globe have reported presence of antibiotics in surface water, ground water and effluent from wastewater treatment plants (WWTPs) in the range of few ng L−1 to 100 mg L−1 [[21], [22], [23], [24], [25], [26], [27]]. Conventional physico-chemical and biological treatment methods used in WWTPs are unable to remove antibiotics from wastewater because of their biorefractory nature, thus resulting in their accumulation in surface and ground water [21,28]. It is a problematic issue as research has revealed continuous rise in antibiotic-resistant bacteria, which is dangerous for both humans and animals, because at present, there are quite limited alternatives to available antibiotics [[29], [30], [31]]. Amoxicillin trihydrate (AMT) is a semi-synthetic broad spectrum β-lactam antibiotic. It is one of the most prescribed antibiotics for children, adults and animals, for a number of bacterial infections, such as pneumonia, middle ear infections, strep throat, skin infections and urinary tract infections. It is commonly detected in sewage treatment plant effluents and surface waters [22,32], as only 20–30% of AMT is metabolized in human and livestock body systems and rest is excreted [33].
There were very few studies available in literature dealing with AMT removal using electrochemical oxidation technique [[34], [35], [36], [37]]. In an interesting study, Sopaj et al., compared the capability of different anode materials such as BDD (Boron doped diamond), Ti/RuO2-IrO2 (DSA), Pt, PbO2, carbon-fiber, carbon-graphite and carbon-felt for AMT removal. Complete mineralization was attained using BDD in 6 h after applying 41.66 mA cm−2 of current density. Nonetheless, for same reaction conditions, less than 25% mineralization was reported for DSA. Similarly, Ganiyu et al., compared the performance of BDD, Pt, DSA and Ti4O7 as anode materials, and carbon-felt as cathode material for EO treatment of AMT. Carbon-felt cathode assisted in formation of H2O2 from compressed air circulated in to the EO cell. Ti4O7 exhibited better results than Pt and DSA, but was found to be less effective than BDD. Frontistis et al., performed AMT oxidation for current density values of 30 and 50 mA cm−2 by means of BDD anode, and proposed possible pathways of degradation of AMT by BDD. Another study on BDD application for AMT removal was reported by Panizza et al. [60] wherein complete mineralization by electro-Fenton process was reported. However, the requirement of highly acidic pH conditions (pH ≈ 3.0) for Fenton's reaction is a major drawback of this process. Likewise, BDD anodes have proven their competence by being dimensionally stable and giving supreme performance for destruction of other varieties of antibiotics as well [[38], [39], [40], [41], [42]]. However, these are comparatively much more expensive, which restricts their usage and questions their application on large-scale [5]. In another remarkable study, Santos et al., prepared a dimensionally stable anode (DSA), Ti/Pt/SnO2-Sb2O4 and reported successful removal of AMT from wastewater. Nevertheless, ∼50% TOC abatement was achieved after 24 h of EO at 30 mA cm−2of current density.
More recently, titanium coated with ruthenium dioxide (Ti/RuO2) has emerged as a promising anode material for degradation of organic pollutants because of its low cost, dimensional stability, high efficiency in strong acidic conditions, high electro-catalytic activity for generation of chloro-oxidant species (Cl2, OCl−, HOCl) and other strong oxidizing agents (OH, H2O2, O3) [[43], [44], [45], [46]]. Such strong mechanical and chemical properties along with high oxygen evolution overpotential (≈1.9 V) and low cost, make Ti/RuO2 a better alternative to rest of the anode materials. The degradation of organic pollutant (R) by EO treatment technique using Ti/RuO2 electrodes can follow the route of direct oxidation at the anode surface and/or indirect oxidation in the bulk solution. Direct anodic oxidation takes place via direct electron transfer from anode to organic molecule (R) as shown in following reaction:Ti/RuO2 + R → Ti/RuO2.(R)ad → nCO2 + oxidation products + yH+ + ze−
Electrolysis of water produces active oxygen in the form of hydroxyl radical at the anode surface which can either get physisorbed on to anode surface (Eq. (2)) or can get chemisorbed into the oxide matrix of the Ti/RuO2 anode, thus resulting in formation of Ti/RuO3 (Eq. (3)). When the pollutant species (R) come in contact of the physisorbed active oxygen, it results in to complete “combustion” as shown in Eq. (4). However, when R reacts with chemisorbed OH, it gets oxidised to form RO, which is also called electrochemical “conversion” (Eq. (5)) [47]. Further oxidation of RO would lead to formation of CO2, H2O and other degradation products and ions.Ti/RuO2 + H2O → Ti/RuO2.(OH)ad + H+ + e− (Physisorption)Ti/RuO2.(OH)ad → Ti/RuO3 + H+ + e− (Chemisorption)Ti/RuO2.(OH)ad + R → Ti/RuO2 + nCO2 + oxidation products + yH+ + ze−Ti/RuO3 + R → Ti/RuO2 + RO
However, the inevitable but undesirable reaction of oxygen evolution occurs simultaneously as shown in following reactions:Ti/RuO2.(OH)ad → Ti/RuO2 + 1/2O2 + H+ + e−Ti/RuO2+1 → Ti/RuO2 + 1/2O2 + H+ + e−
Despite the advantages, studies on application of Ti/RuO2 for removal of antibiotics from wastewater are very few [45,49]. The novelty of present study lies in application of Ti/RuO2 electrodes for degradation and mineralization of AMT by EO treatment methodology and generation of plausible degradation pathway using Ti/RuO2 anodes. Effect of important operating parameters, such as current density (i), initial pH of synthetic wastewater, supporting electrolyte concentration (S0) and initial AMT concentration (C0) on AMT removal efficiency (%ARE) and TOC removal efficiency (%TRE) of Ti/RuO2 anode was studied in detail. AMT decay kinetics was studied by varying i (mA cm−2) and C0 (mg L−1). Besides, mineralization current efficiency (%MCE) and specific energy consumption (SEC) were evaluated. Furthermore, the economic feasibility of the EO treatment technique using Ti/RuO2 anode was checked by calculating the operating cost. Major transformation products were identified using UPLC-Q-TOF-MS and a plausible degradation pathway of AMT was proposed.
Section snippets
Materials
Fresh synthetic wastewater was prepared at room temperature for each electro-oxidation experiment. Predefined quantity of AMT antibiotic (DSM Sinochem Pharmaceuticals, Punjab, India) and NaCl (Loba Chemie Pvt. Ltd., Mumbai, India) were dissolved in 1.5 L of double distilled water by placing the mixture over a magnetic stirrer for 1 h. NaCl was added as supporting electrolyte to enhance the conductivity of the synthetic wastewater. Initial pH of wastewater was adjusted using 0.1 M HCl or 0.1 M
Effect of initial pH on degradation and mineralization of AMT
Efficiency of electrooxidation process for abatement of organic pollutants is highly pH dependent. The nature of various oxidation species generated on the anode surface and in the bulk solution strongly depends on the pH of wastewater under treatment. Effect of initial pH (2–9) of the solution on %ARE and %TRE with time was studied at current density, i = 5.88 mA cm−2, S0 = 2 g L−1 and C0 = 50 mg L−1, and is shown in Fig. 1a and b, respectively. As shown in Fig. 1a, %ARE increased with an
Conclusions
Maximum removal efficiencies is terms of AMT and TOC were achieved at neutral pH of wastewater. Mineralization current efficiency decreased from 11.77% to 7.67% with increase in current density from 1.47 to 5.88 mA cm−2. This could be attributable to certain parasitic reactions occurring at higher current value. Thus, only a small portion of current intensity was used in degradation/mineralization of AMT in wastewater. Increase in NaCl concentration increased the amount of active chloro-oxidant
Acknowledgements
Authors are grateful to University Grants Commission, India for providing research fellowship (MANF-2014-15-SIK-PUN-43596) to the first author.
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