Electrochemical oxidation of diethyl phthalate at two dimensional graphite sheet electrodes: optimization and analysis of degradation in water with HRMS

Concentration of phthalates in the environment has been steadily increasing due to their high utilization rate and the inability of conventional wastewater and water treatment to remove them. Electrochemical oxidation at Boron doped diamond (BDD) was effective for phthalate degradation but costly and unaffordable in many situations. Hence, we carried out the electrochemical oxidation of Diethyl Phthalate (DEP) at two dimensional graphite electrodes as a low cost alternative to oxidation at BDD electrodes. Optimization studies found that high concentrations of DEP (56–112 mg L−1) can be effectively removed from water with acidic pH (pH 3), 60 mA cm−2 current density, 81 cm2 anode surface area and 10 mmol sodium sulfate with 60 min treatment time. At 56 mg L−1 DEP concentration, chemical oxygen demand (COD) and total organic carbon (TOC) removal were 92.5 and 70%, respectively. Fourier transform infrared spectroscopy (FTIR) studies were conducted to find out whether any adsorption or electrosorption had taken place at the electrode surface. High performance liquid chromatography-photo diode array (HPLC–PDA) analysis found that 97.3% of 84 mg L−1 DEP got oxidized or degraded. High resolution mass spectrometry (HRMS) studies utilizing Ultra performance liquid chromatography quadrupole time of flight mass spectrometry (UPLC-Q-ToF–MS) were conducted for finding the degradation byproducts and possible degradation pathway was proposed with the degradation mechanism. The major byproducts were dimethyl phthalate, phthalic acid, phthalic anhydride and phenol.


Introduction
Phthalic acid esters (PAEs) or phthalates, more commonly known as plasticizers, have been a worrying presence in the environment due to three important reasons: their ubiquitous, toxic and persistent nature to conventional wastewater treatment [1,2]. The increasing concentration of PAEs in the environment and their presence in the human body in various studies [3,4] have caused the scientific community to look for alternate treatment technologies such as advanced oxidation processes (AOPs) for their degradation in water and wastewater. PAEs have been classified as endocrine disrupting compounds causing hormonal imbalances, reproductive disorders and developmental abnormalities in animals and humans. Some of them are teratogenic causing fetal abnormalities and carcinogenic [3,4].
Among PAEs, diethyl phthalate (DEP) have been given less attention for treatment and removal due to comparatively less toxicity compared to other phthalates [4]. However, recent studies underscore the need for DEP degradation for their annihilation from water and wastewater due to their rising concentration in the environment that results from high usage of pharmaceuticals, adhesives, personal care products and plastic products [2,4]. Table 1 shows the concentration of DEP found in various aquatic environments around the world. DEP's presence in various artificial products ranges from 80 to 36,500 µg g −1 while in industrial wastewater their concentration exceeded 50 mg L −1 reaching 100 mg L −1 [2,5,6,[8][9][10][11][12]. 60.3 µg L −1 of monoethyl phthalate-a metabolite of DEP, was found in the urine samples collected from diapers of infants and toddlers [13]. DEP has been found to cause developmental and reproductive disorders in humans and animals [3,4].
Various AOPs such as photo catalytic, sono fenton and catalytic ozonation had been employed over the years for the elimination of PAEs from water. In photocatalysis, light penetration was a major factor hence, opaque water sources such as wastewater hinders its applicability. In sono fenton, the production of sludge and its disposal required additional treatment techniques. Catalytic ozonation was comparatively costlier than other AOPs and struggled to maintain optimal degradation efficiency when the concentrations of PAEs were high (> 50 mg L −1 ) [42,[47][48][49].
Among AOPs, electrochemical advanced oxidation process (EAOPs) have been found to effectively oxidize and degrade emerging contaminants such as PAEs. Table 2 describes the level of DEP degradation achieved with different oxidation and reduction techniques including electrochemical and others. Various active and non-active electrodes such as mixed metal oxide based (Pb/PbO 2 , Ce-Ti/ PbO 2 , etc.) and carbon-based anodes (BDD) have been studied for their oxidation abilities for the degradation of emerging contaminants such as phthalates [14][15][16][17][18][19]46]. BDD anode, had been studied well for their oxidation abilities. It is also one of the most powerful electrodes for oxidizing emerging contaminants in water and wastewater [42,[20][21][22][23]. However, the high cost associated with BDD electrode hampers its wide scale application and affordability especially in the developing and under developed economies. MMO electrodes are less costly than BDD but the inability to produce optimal quantity of hydroxyl radicals and their dependence upon electrolytes for the production of oxidation species are some of the drawbacks of various MMO electrodes available in the market [15,16,24,25].
Studies on the effectiveness of affordable active carbonbased electrodes such as graphite for the degradation of emerging contaminants such as pharmaceuticals, personal care products and plasticizers are limited [16,26]. We couldn't find scientific papers that explored the application of graphite electrodes for the oxidation of plasticizers such as DEP.
The main objectives of the present study were to explore the efficiency of graphite as an electrode material for electrochemical oxidation of high concentrations of DEP by optimization studies using chemical oxygen demand (COD) and total organic carbon (TOC) analysis. Fourier transform infrared spectroscopy (FTIR) studies were undertaken to confirm whether adsorption or electrosorption occurred at the electrode and also to assess the structure of graphite before and after treatment. High performance liquid chromatography (HPLC) and high resolution mass spectrometry (HRMS) were employed to deduce the extent of DEP degradation and its mechanism, respectively.

Electrochemical experiments
The pH measurement for the experiments was carried out using pH and ORP Tester (HI98121) supplied by HANNA instruments. The electrolysis was carried out utilizing AC-DC converter (30 V/5A) supplied by Saba Electronics, Chennai at room temperature. A scheme of the experimental procedure is given in Fig. 1.
The batch mode glass reactor had 1000 mL capacity which was placed on top of a magnetic stirrer (RMS-114) supplied by Rotek Instruments. The reactor had a working volume of 750 mL. Graphite sheet electrodes procured from M/s Carbone Lorraine was used as both the anode and cathode that had 1 mm thickness. The inter electrode distance was fixed at 2 cm.
Samples for analysis were procured at 15 min time intervals starting from 0 and ending at 60 min. The electrochemically treated samples were filtered by Macherey Nagel Filter paper (110 nm pore size) to remove the graphite particles that eroded away from the electrode surface before further analysis. The graphite sheets were counter polarized and washed with distilled water after each treatment for reuse of the electrodes.

Electrode characterization
The FTIR spectra of the powder and liquid samples were analyzed in the mid-IR region by attenuated total reflection (ATR) mode using Jasco FTIR spectrometer. The spectra were assessed from wavenumber 4000-400 cm −1 . The ATR mode allowed the assessment of both powder and liquid samples.

Analytical procedures
Chemical oxygen demand (COD) measurement was carried out using COD open reflux method utilizing 12 inch Friedrich's condenser and Soxhelt extraction mantle for 2 h digestion at 150 °C.
The TOC analysis was conducted utilizing the Analyt-icJena multi N/C 3100 TOC analyzer. A sample aliquot of 500 µL was injected each time to the combustion tube. Pure oxygen was the carrier gas. Quantification was performed with non-dispersive infrared spectrometry. The degradation byproducts and degradation pathway were established by High resolution mass spectrometry (HRMS) studies utilizing Ultra performance liquid chromatography quadrupole time of flight mass spectrometry (UPLC-Q-ToF-MS). The Acquity UPLC system (Waters) involves TUV detector (J12TUV750A), a column chamber (J12 CHA730G), a quaternary solvent manager (H12 QSM632A), and a sample manager FTN (K12 SDI069G). A reversed-phase BEH C18 column (dimensions: 50 mm × 2.1 mm × 1.7 µm) of flow rate 0.3 mL min −1 was utilized for chromatographic separation (Waters). The mobile phase was a combination of water and acetonitrile with 0.1% formic acid in gradient mode. The UHPLC system was connected to the quadrupole time of flight mass spectrometer (Waters Xevo G2 Q TOF) with electronspray ionization (ESI) interface. The injection volume was 10 µL. All samples were analyzed in ESI positive ionization mode, with scan range of m/z between 50 and 1000. The desolvation gas flow was 900 L h −1 and the temperature was 350 °C. The mass spectra were procured from collision energy range 5-30 eV. The control of instrument and acquisition of data about the molecular weight and chemical composition were carried out by MassLynx software (v 4.1).

pH and electrolyte concentration
pH was found to be a significant factor for phthalate degradation [18,19,27]. At graphite electrodes, acidic pH was favorable [26,28,29]. Figure 2a shows the COD removal of 112 mg L −1 DEP sample at various pH. pH 3 was found optimal for DEP oxidation. The COD removal decreased as pH increased from 3. The negative COD removal or increase in COD concentration at pH 9 is attributed to the solubilization of DEP. DEP shows higher solubility at acidic pH such as pH 3 with decrease in water solubility as pH increases. In case of pH 9, the entire 112 mg L −1 DEP was not solubilized initially before treatment. However, the acidification of the sample during electrochemical oxidation prompted the remaining DEP to solubilize hence the increase in COD at pH 9. The second reason is the inadequate production of hydroxyl radicals and the formation of hydroxide ion at higher pH 7 [7]. The induction of acidic condition through the addition of H 2 SO 4 might have also resulted in the production of peroxy disulfate radicals (S 2 O 8°− ) that have oxidation potential higher than sulfate radicals. Peroxy disulfate species might have played a role in DEP oxidation resulting in the favoring of acidic pH by the degradation mechanism. The production of S 2 O 8 radicals and sTable ions are given from Eqs. 1-4.
(1) Concentration of electrolyte was found to be positively influential for electrochemical oxidation in some studies [26,31] while others found that there is a negative influence for the electrolyte concentration at some electrodes with some electrolytes [30][31][32]. Commonly used electrolytes for electrochemical oxidation are sodium sulfate, sodium chloride and sodium nitrate. Sodium chloride electrolyte can produce powerful chlorine oxidation species that can degrade organic compounds but, risks the formation of toxic organo chlorine byproducts. Sodium nitrate is an inert electrolyte that induces conductivity but doesn't aid in the production of oxidation radicals [17,32,43,45]. Hence sodium sulfate was utilized as the electrolyte for inducing conductivity in water, preventing the risk of toxic byproducts and for the production of sulfate oxidation species. Figure 2b shows the effect of sodium sulfate electrolyte concentration on the COD removal at 60 min treatment time, pH 3, 81 cm 2 electrode surface area, 112 mg L −1 DEP concentration and 60 mA cm −2 current density. At graphite sheet electrodes, concentration of sodium sulfate beyond 10 mmol neither inhibited nor enhanced the degradation. However, for inducing a current density of 60 mA cm −2 at 81 cm 2 electrode surface area, 10 mmol sodium sulfate was imperative. Experiments conducted with 6 mmol and 8 mmol sodium sulfate failed to induce 60 mA cm −2 current density due to lack of conductivity of the DEP sample At carbon-based electrodes such as graphite and BDD, the major mechanism of oxidation was through the direct electron transfer and hydroxyl radical production [17,22,33,34]. The mechanism for °OH production at graphite anode is given from Eq. (5) The mechanism of how hydrogen peroxide and ozone is produced is given from Eqs. 6-9 The lack of effect of sodium sulfate concentration beyond 10 mmol can be ascribed to the low oxidation potential of sulfate radicals and the role that sulfate ions play in the transformation of hydroxyl radicals to hydroxide ions [21,26,35,36]. Mechanism for the production of sulfate radicals and the resultant production of hydroxide ions from hydroxyl radicals is given in Eq. (10)

Electrode surface area and current density
In electrochemical oxidation, surface area of electrode was found to be a crucial factor as it was directly proportional to the amount of hydroxyl radical production and direct electron transfer at the anode [30,[37][38][39]. Figure 3a shows the effect of electrode surface area on COD removal at 112 mg L −1 of DEP concentration, 22 mA cm −2 current density and pH 3 in 60 min treatment time. 81 cm 2 was found optimal for maximum removal of COD for the electrochemical degradation of DEP. Figure 3b shows the effect of current density on COD removal at 112 mg L −1 DEP concentration, pH 3, 81 cm 2 surface area and 60 min treatment time. 60 mA cm −2 was found to be better than other current densities for COD removal tested for the study. Higher current density was . 3 a Effect of current density on COD removal at 112 mg L −1 DEP concentration, pH 3, 81 cm 2 , 60 min treatment time and 10 mM sodium sulfate concentration. b Effect of electrode surface area on COD removal at 60 min treatment time, 22 mA cm −2 , pH 3 and 10 mM sodium sulfate with 112 mg L −1 DEP concentration correlated with higher hydroxyl radical production [38,39] which also meant lesser current efficiency and higher energy consumption [36,39,40].

Treatment time and initial concentration
Studies show that treatment time is an important variable for electrochemical oxidation. Higher treatment time for EAOP results in higher TOC and COD removal rates but with higher energy consumption [39,41]. Figure 4a shows the COD and TOC removal of 112 mg L −1 DEP sample at optimal conditions. 60 min was found optimal for maximum TOC and COD removal of 63 and 89%, respectively. Approximately 26% difference was noticed between TOC and COD removal at different treatment times. The disparity would arise because of the difference between the inorganic and organic bond rupture and oxidation. While, COD is the total sum of all chemically oxidizable forms in a sample, TOC is the sum of only the organic carbon (C-H) remaining in a given sample. The difference is in line with other works cited in literature [39,41]. For assessing the effect of initial concentration of electrochemical oxidation of DEP, The treatment was executed at 56 mg L −1 , 84 mg L −1 and 112 mg L −1 of DEP at optimal conditions. Figure 4b demonstrates the influence of initial concentration of target compound on COD and TOC removal. An increase in the initial concentration of DEP decreased both the TOC and COD removal rates indicating inversely proportional relationship between target compound's initial concentration and degradation efficiency. 92.5% COD and 70% TOC was removed at 56 mg L −1 of DEP concentration with 60 mA cm −2 current density and 81 cm 2 electrode surface area.. There was considerable difference of approximately 25% between the TOC and COD removal rates with TOC removal being lower than COD removal in all the cases. Also an increase in initial concentration of compound results in higher molecule count to degrade resulting in lesser COD or TOC removal rates [35,36].
From optimization studies, pH 3, 10 mmol sodium sulfate concentration, 60 mA cm −2 current density, 81 cm 2 anode surface area, 56 mg L −1 of DEP sample and 60 min treatment time was found to give the best results for electrochemical oxidation at graphite electrodes.

FTIR studies
In order to find out whether adsorption or electrosorption has aided in DEP elimination, FTIR studies were utilized to find out the functional groups attached to the surface of the graphite sheets before and after treatment. Figure 5a shows the FTIR spectra of graphite sheet anode, graphite sheet cathode and DEP. There were no peaks of the functional groups of diethyl phthalate at the graphite sheet used as anode, cathode or the graphite settle during the treatment indicating that there was no adsorption or electrosorption of DEP at the graphite surface. Hence, the only mode of DEP elimination was direct and indirect oxidation brought on by direct electron transfer and °OH radicals, respectively [17,36,43,44], produced at graphite anode during electrochemical treatment. Figure 5b shows the FTIR spectra of spent graphite (graphite used for electrochemical oxidation) compared with virgin graphite (unused graphite). An additional stretching was noticed in the spectra of spent graphite at 1235 cm −1 that can be attributed to the intercalation of organic acids formed during DEP oxidation to exfoliated graphite structure [45].  Figure 6a and b shows the initial (84 mg L −1 ) and final DEP concentration after 60 min treatment time at optimal conditions. The peak at 5.528 retention time is ascribed to DEP. The other two peaks in Fig. 5b indicates the formation of degradation byproducts. Table 3 shows the initial and final peak area of DEP with the peak areas of degradation byproducts as well. There was 97.3% decrease in the DEP peak after 60 min of treatment

HRMS
Usually in electrochemical oxidation, the degradation is initiated by the attack of the hydrocarbon bonds by hydroxyl radicals [17,36,41]. Figure 7a shows the HPLC spectra of electrochemically oxidized DEP sample (84 mg L −1 ) after 60 min of treatment at optimal conditions. The peak at 4.5 min indicates DEP. Figure 7b-d portrays the MS spectra of major byproducts. Table 4 shows the major degradation byproducts formed during the degradation of DEP with their molecular weight and retention time (RT). The most prominent peaks common among the spectra were phthalic anhydride (m/z 149; RT-2.4 min), phthalic acid (m/z-165; RT-2.8 min) and 2-hydroxy benzoic acid or salicylic acid (m/z-139; RT-2.7 min). There was a small peak of phenol (m/z-95, RT-2.4) common in all the spectra.
From the UPLC-Q-ToF-MS studies, the degradation pathway was proposed. Figure 8 shows the proposed pathway of DEP's electrochemical oxidation. The degradation was initiated by hydroxyl radical attack of aliphatic bonds. The major degradation mechanisms were hydroxylation, demethylation, dehydroxylation and decarboxylation. The final end products were carboxylic acid groups such as phthalic anhydride and phenol before getting oxidized into CO 2 and H 2 O. Contrary to electrochemical oxidation at BDD [17,41], graphite °OH was more keen on attacking the aliphatic bonds of the phthalate molecule. Although graphite °OH attacked the aromatic ring, the attack seemed unsuccessful in the initial stages. Degradation at graphite anode was more similar to oxidation at platinum anode than the carbon-based inactive BDD anode [17]. Similar to phthalate oxidation at other electrodes, formation of carboxylic acids was noticed in graphite as well [17,36,41]. However, formation of m/z-213 and formation of phenol before being oxidized to CO 2 and H 2 O was not common at other electrode materials during the electrochemical oxidation of phthalates [14-18, 23, 45]. Irrespective of the initial pH applied the final pH of the treated sample after 60 min at optimal conditions ranged between 3.5 and 4.5. The pH of DEP sample was found to be 4.4. In case of pH 3, the formation of phenolic groups and resident pH of DEP might have played their roles in making the final pH 3.5 after 60 min. In case of other pH ranges used for the study (5, 7, 8 and 9) the acidification of the sample to pH 3.5-4 was a result of the formation of carboxylic acid groups that can be observed from Fig. 8 and Table 4.
Each set of graphite sheet electrodes were reused up to eight times after counter polarization and Milli-Q water wash without considerable decrease in degradation efficiency levels. The counter polarization was executed by Fig. 5 a and b shows the FTIR-ATR spectra of graphite sheet anode, graphite sheet cathode, Diethyl phthalate virgin graphite and spent graphite reversing the potential of the used graphite electrodes (working surface area-81 cm 2 ) in the 1000 mL reactor. The reactor had 750 mL volume of deionized water with 50 mM sodium sulfate. The counter polarization was carried out for 10 min. Although, the electrode surface eroded away during treatment it seemed to have negligible effect on degradation efficiency of the electrode. Graphite exfoliation caused slight adsorption of organic acids at the electrode surface (Fig. 5b) that disappeared after counter polarization.
Overall, electrochemical oxidation at graphite electrodes can be applied as a post treatment step to conventional wastewater treatment and as pre-treatment to adsorption and filtration for water treatment for the elimination of emerging contaminants such as diethyl phthalate.  1 3

Conclusion
Present study showcased the electrochemical oxidation of high concentrations of diethyl phthalate in water at two dimensional graphite electrodes. Optimization studies found that high concentrations of phthalate can be oxidized to CO 2 and H 2 O by increasing the surface area of the electrode and current density at acidic pH. The optimal conditions found for maximum removal of COD (92.5%) and TOC (70%) of 56 mg L −1 of DEP sample were pH 3, 81 cm 2 electrode surface area, 60 mA cm −2 current density, 10 mM sodium sulfate and 60 min treatment time. Basic pH inhibited the degradation of DEP while electrolyte concentration above 10 mM had no effect on  COD and TOC removal. FTIR studies found no adsorption or electrosorption happening at the electrode surface. HPLC analysis found 97.3% degradation for 84 mg L −1 of DEP using the optimal conditions. From HRMS studies, the degradation byproducts formed were identified and possible degradation pathway was proposed. The major mechanisms involved for degradation were hydroxylation, demethylation, dehydroxylation and decarboxylation. Two dimensional graphite sheet electrodes can be considered as a less costly alternative to BDD electrodes for the electrochemical oxidation of phthalates in water. Pilot scale studies with cost assessment on a continuous scale in actual wastewater and natural water conditions are required for wide scale applications.