Comparison of the heterogeneous GO-FePO4/electro-Fenton against the homogeneous Fe(II) ion and Fe(III)-oxalate complex/electro-Fenton for the degradation of metronidazole
Graphical abstract
Introduction
Nowadays, water pollution caused by organic pollutants has reached serious levels and has become an important problem for all living things on earth. Antibiotics are one of the most important of these organic pollutants. In particular, the widespread and broad-spectrum use of antibiotics makes them the most important group of contaminants in environmental waters. Metronidazole (MTZ) is one of the commonly used antibiotics, is used to treat a wide variety of infections caused by some bacteria and parasites. MTZ is identified in several wastewater effluents like urban, hospital, fish farms, and meat production facilities which produce large amounts of wastewaters [1]. In the United States, MTZ was 119th most commonly prescribed medication in 2018 [2]. It is also on the World Health Organization's List of Essential Medicines [3]. On the other hand, as a result of the studies, metronidazole was reported to be carcinogenic [4]. Metronidazole has high mobility in the soil because of its KOC (soil adsorption coefficient) value [5] and it has been reported to be low biodegradable in the environment [[6], [7], [8]]. Due to the lack of effective treatment in effluent treatment plants, high concentrations of such pollutants are released into the environment. For instance, MTZ levels as high as 34 mg L−1 have been determined in the sample of the Waste Water Treatment Plant of a pharmaceutical manufacturing company in Nigeria [9,10]. Therefore, effective studies to be carried out on the degradation of MTZ in wastewater will help to solve an important problem.
Electrochemical advanced oxidation techniques (EAOPs) have attracted great attention in consequence of their environmental-friendliness, moderate application condition, compatibility, versatility, high efficiency, limited operative costs, and suitability for automation [[11], [12], [13], [14]]. EAOPs can be used to effectively degrade persistent organic pollutants [15]. Electro-Fenton (EF) method is also a prominent member of EAOPs. In the Fenton method, the ferrous ion and H2O2, which are named Fenton reagents, react to produce hydroxyl radical in the bulk solution (Eq. (1)).
However, in the EF method, it is possible to be electro-generated H2O2 at a suitable cathode fed with O2/air (Eq. (2)) or produce Fe2+ ions using a sacrificial iron anode. It can also be applied by adding catalytic amounts of iron ions (Fe2+ or Fe3+) into the reaction medium. In EF process, regeneration of Fe3+/Fe2+ performs by direct reduction on the cathode (Eq. (3)) [16].
Also, coupling electro-Fenton processes, which are also environmentally friendly, with biological treatment techniques, allows the use of environmentally friendly high-efficiency new methods by improving existing biological treatment processes. Aboudalle et al. performed the mineralization of metronidazole using combined the Fenton process with biological treatment processes [17,18]. They showed that when electro-Fenton was applied before biological treatment, the rate of metronidazole removal doubled compared to using two different bacterial strains alone for 120 h. As a result, by combining the two processes, they achieved a mineralization efficiency of 97% with a significant reduction in 16-day processing time compared to previous traditional biological treatment methods [17]. Some chemicals, such as pesticides, accumulate in the soil and may get mixed with groundwater after irrigation or rains. Contreras et al. proposed an EF pre-treatment to improve the biodegradability of polluted soil washing effluent with clopyralid, also showed that the groundwater can be used as a natural electrolyte instead of adding synthetic electrolytes [19].
Traditionally, homogeneous Fenton applications have some limitations, such as the application of acidic pH (~pH = 3) [20] and the inability to recover Fe2+ ions. Also, reproducing the Fe2+ ion, which is depleted as a result of the Fenton reaction (Eq. (1)), limits the Fenton process. It has been reported that adding low molecular weight organic acids to the solution in a homogeneous photo-Fenton process increases the amount of degradation. For this purpose, oxalic acid, acetic acid, citric acid, malic acid, and tartaric acid were tested [[21], [22], [23], [24]]. These acids form a soluble strong complex with the Fe(III) ion [25]. The optimum molar ratio was reported as 1:3 for the Fe(III)-oxalate complex and it was determined as 1:1 for the others, too [[26], [27], [28]]. The reasons for increasing the degradation efficiency of the Fe(III)-oxalate complex; i) facilitating the Fe(III)/Fe(II) cycle in the photo-Fenton process [[29], [30], [31], [32]], ii) reacting faster with hydrogen peroxide than the free Fe3+ ion (Fe(III)-aqua complexes), and iii) providing degradation in a wide pH range [21].
Recently studies conducted with the heterogeneous EF method showed that pH values can be changed from acidic to neutral and the iron source can be reused. Generally, Fe2+ (or Fe3+, Fe(III)-Ligand complexes) is used in homogeneous Fenton applications but in recent studies, the surface of heterogeneous structures containing iron such as goethite (α-FeOOH) [33,34], wustite (FeO) [33], magnetite (Fe3O4) [33,35,36], hematite (α-Fe2O3) [33], iron molybdophosphate (FeMoPO) [37], iron-loaded alginate beads [38], zero-valent iron (ZVI) [36], pyrite (FeS2) [[39], [40], [41]], laterite [42], ZnFe2O4 nanoparticles [43], and Fe-Cu allophane [44] acts as the active site for heterogeneous electro-Fenton applications.Moreover, it is possible to use heterogeneous catalysts obtained by loading iron sources on various support groups such as sepiolite [45], zeolite [46], carbon [47], organic polymers [48], and resin [49]. One of these support materials is graphene oxide (GO). GO has oxygenated functional groups in its two-dimensional lamellar structure which allows the obtaining of composite materials with various compositions [50,51]. Graphene, graphene oxide, and reduced graphene oxide composite materials such as GO-Fe3O4, GO-FeIIFeIII, FeIIFeII-Carbon felt, graphene-FeO dispersed onto Ni foam, graphene/carbon nanotube−based carbon aerogel/Fe3O4, iron-graphene-based particle, γ-FeOOH graphene polyacrylamide carbonized aerogel, can be used as a cathode material or catalyst in the electro-Fenton process [[52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62]]. In these structures, the p orbitals of graphene-based support materials overlap with the d orbitals of the transition metal atoms to form composite structures with appropriate geometry. Composite materials significantly increase catalytic activity and stability through synergistic interaction [62]. In addition, electrostatic interactions may occur between oxygenated functional groups in the GO structure and metal oxide structures [[63], [64], [65]]. Consequently, we can summarize some of the most important advantages of these synergetic interactions as follows; i) metal oxides are dispersed in the high surface area of the GO active areas, ii) agglomeration is prevented [65], iii) leakage of metal ions is reduced and finally catalyst stability is increased [66]. In Fenton reactions, one of the iron sources used in graphene-based composite materials is FePO4 [67,68]. Guo et al. [67] investigated the effect of GO-FePO4 composite on the degradation of rhodamine B by the photo-Fenton process and achieved 100% removal with the use of 100 mM H2O2 and 1 g L−1 catalyst. Zhou et al. (2018), also performed photo-Fenton degradation of methylene blue under simulated sunlight irradiation using GO-FePO4 composite (GO 10% w/w) and showed that this catalyst would be used 3 times without losing its photocatalytic activity and stability [68].
This work aimed to synthesize and characterize GO-FePO4 and to investigate the removal of metronidazole from aqueous solutions by the heterogeneous electro-Fenton process. The effects of catalyst amount, pH, and electrolysis time on degradation and mineralization were investigated. In addition, the reusability of GO-FePO4 catalyst, and Fe ion release into solution were investigated. To compare the performance of GO-FePO4 in the heterogeneous Fenton reactions, Fe(II) ions, and Fe(III)-oxalate compounds were selected as homogeneous catalysts. Finally, degradation products of MTZ were identified.
Section snippets
Chemicals
Graphite powder (325 mesh, 99.99% purity), and metronidazole (99%), were obtained from Alfa Aesar. Phosphorus pentoxide (P2O5), potassium persulfate (K2S2O8), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), iron (II) sulfate heptahydrate (FeSO4·7H2O), sodium sulfate (Na2SO4), sodium hydroxide (NaOH), nitric acid (HNO3), ammonium dihydrogen phosphate (NH4H2PO4), iron (III) nitrate Fe(NO3)3, sodium fluoride (NaF), ferric chloride (FeCl3), salicylic acid (C7H6
Characterization of GO-FePO4
As seen in Fig. 1a, the GO-FePO4 catalyst showed a uniform particle size distribution with a maximum of 141.3 nm. While C, O, Fe, and N elements were seen in the EDS spectrum belonging to the GO-FePO4 structure, no result for phosphorus was obtained. However, the reason for the absence of phosphorus may be its low amount (the FePO4 structure constitutes only 10% of the composite by mass) or overlapping with the spectra of the elements (Pt, and Pd) used in the coating (Fig. 1b).
In Fig. 2, the
Conclusions
The degradation of MTZ antibiotic solution was compared using heterogeneous and homogeneous EF processes. In the work, GO-FePO4 catalyst was used as a heterogeneous catalyst, Fe2+ ion and Fe(III)-oxalate complex were used as homogeneous catalysts. The characterization of the GO-FePO4 structure was performed by XRD, FE SEM-EDS, and FT-IR analysis. It was determined that the GO-FePO4 composite was more effective at pH 3 in the heterogeneous EF process, and the maximum mineralization percentages
Declaration of competing interest
Authors declare no conflict of interest.
Acknowledgments
We would like to thank Mersin University Advanced Technology Education, Research, and Application Center (MEITAM) where the analyses were carried out.
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