Removal of nalidixic acid from aqueous solutions using a cathode containing three-dimensional graphene

Journal of Water Process Engineering 32 (2019) 100978

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Removal of nalidixic acid from aqueous solutions using a cathode containing three-dimensional graphene

T

⁎

Mahmoud Zareia, , Farzaneh Beheshti Nahanda, Alireza Khataeeb,c, Aliyeh Hasanzadehb a

Research Laboratory of Environmental Remediation, Department of Applied Chemistry, University of Tabriz, 51666-16471, Tabriz, Iran Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 5166616471, Tabriz, Iran c Department of Environmental Engineering, Gebze Technical University, 41400, Gebze, Turkey b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electro-Fenton Emerging pollutants Three-dimensional graphene Advanced oxidation processes Nalidixic acid

Herein three-dimensional graphene (3DG) was synthesized by solution combustion method. A nanostructured 3DG modified carbon paper as the cathode was employed in the homogeneous electro-Fenton method for degradation of nalidixic acid from aqueous solutions and as an anode electrode, graphite was used. The physicchemical properties of 3DG powder and modified carbon paper were studied by the SEM, XRD, AFM, FTIR, CV and BET analyses. The ability of the resultant 3DG modified carbon paper cathode for hydrogen peroxide production was investigated in solution fed with air. Results showed that the value of hydrogen peroxide that was electrogenerated by using 3DG electrode is 4 times more than bare carbon paper and graphene oxide modified electrode. The consequence of running parameters including current intensity (mA), initial solution pH, initial concentration of nalidixic acid (mg L−1) and process time were investigated comprehensively in a series of batch experiments which were optimized by response surface method (RSM) and the optimized amounts of each parameter were respectively 300 mA, 3.5, 15 mg L−1 and 300 min. The pharmaceutical removal efficiency was 90% under these optimum conditions. The total organic carbon (TOC) measurements showed 87.3% mineralization of 15 mg L−1 drug at 7 h. The gas chromatography (GC) linked to a mass spectrometry (MS) was used to recognize the generated intermediates through the nalidixic acid degradation.

1. Introduction The role of bioactive chemical pollutants such as pharmaceuticals in the environment has received increasing attention in the last two decades [1]. The accumulation of these pollutants in aquatic environments has been contemplated as an important public health challenge [2]. To keep away from the hazardous environmental accumulation of the pharmaceuticals, it is necessary to expand powerful treatment processes to remove such emerging organic pollutants. Different treatment methods have been recently studied and developed to remove organic contaminants from aqueous solutions [3–7]. However, advanced oxidation processes (AOPs) as strong, green and environmentally friendly methods have been noticed [8]. AOPs are based on an extremely reactive non-selective species such as hydroxyl radicals (%OH) that oxidize organic pollutants [9]. Electrochemical advanced oxidation processes (EAOPs) are very attractive among AOPs for wastewater decontamination because of their ability to remove persistent organic pollutants

(POPs) even in low concentrations [10]. In these methods, H2O2 is regularly provided to an acidic solution by the O2 gas reduction of normally at cathodes such as carbon-polytetrafluoroethylene (PTFE) O2-diffusion [11,12] and carbon-felt [13] by reaction (1) [14]: O2(g) + 2H+ + 2e− → H2O2

(1) 2+

Adding a small amount of Fe as catalyzer leads to extremely enhancement of the oxidizing power of H2O2 according to the Fenton reaction (2) [15]: Fe2+ + H2O2 → Fe3+ + %OH + OH−

(2)

The cathodes fabricated by carbon-based materials have been broadly utilized to the electrogeneration of H2O2 because of non-toxicity, great H2 evolution overpotential, low catalytic activity for H2O2 decomposition, high stability, conductivity and chemical resistance of them [8]. Several carbonaceous cathodes such as gas diffusion electrode (GDE) [16,17], graphite felt [18], carbon felt [19], graphene [20],

⁎

Corresponding author. E-mail addresses: [email protected] (M. Zarei), [email protected] (F. Beheshti Nahand), [email protected] (A. Khataee), [email protected] (A. Hasanzadeh). https://doi.org/10.1016/j.jwpe.2019.100978 Received 26 July 2019; Received in revised form 17 September 2019; Accepted 21 September 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 Characteristics of Naldixic acid. Structural

λmax(nm)

Formula

Mw (g.mol−1)

251

C12H12N2O3

235.232

Table 2 Coded levels and the natural values of the independent test variables. Variables

Ranges and levels

−1

[Nalidixic acid]0 (mg.L ) (X1) pH (X2) Applied current (mA) (X3) Time (min) (X4)

–2

–1

0

+1

+2

10 2 100 60

20 4 200 120

30 6 300 180

40 8 400 240

50 10 500 300

activated carbon fiber [21], carbon sponge [22] and C60-carbon nanotubes composite [12] have been studied for H2O2 electrogeneration. Three-dimensional graphene (3DG) aerogel is a novel encouraging candidate as cathode martial because of its elevated particular region, elevated porosity and low density martial [23,24]. This research, therefore, included two primary components: for the first moment, 3DG-PTFE GDE was manufactured as a cathode electrode and its effectiveness was compared with bare carbon paper and graphene oxide (GO)-PTFE electrodes. The structure and electrochemical behavior of the so-prepared electrodes were studied by field emission scanning electron microscopy (FESEM), Atomic force microscopy (AFM) and cyclic voltammetry (CV). Nalidixic acid (1-ethyl-1,4-dihydro-7-methyl-4-oxo-1,8-naftiridine-3-carboxylic acid) is a pharmaceutical antibacterial agent used as an antibacterial agent to manage chicken infection and improve the productivity of fish farming by enhancing water quality and preventing infection [25–27]. The presence of nalidixic acid has been reported in environmental waters, wastewater treatment plants effluents, hospital effluents and soils [28–30]. So the concerns about its environmental and human health effects have increased. At low concentrations, it acts as bacteriostatic and inhibits bacterial growth, while it exhibits bacterial effects at high concentrations [31]. Extensive manufacture of pharmaceutical by plants, big quantities of elapsed drugs, unchanged pharmaceutical products excretion by animals, their metabolites and human cause to occurrence of drugs in the aquatic environment [32–34]. Thus, existence of these compounds has serious effects on human and animal health [35]. Literatures study shows that various methods for removing nalidixic acid from aqueous solutions were used, including an embedded MBR-ozonation scheme [36], powdered activated carbon [37], Adsorption onto anion-exchange and neutral polymers [38] and UV and UV/H2O2 processes [39]. But the previous studies have not still considered the toxicity evolvement of the aqueous solutions of nalidixic acid during the time of electrolysis. In fact, it is worth emphasizing that treating poisonous organic contaminants aquatic solutions through sophisticated oxidation procedures is not needfully followed through a reduction in toxicity as the oxidation reaction could create intermediates more dangerous than the original contaminant. Thus, nalidixic acid degradation by the electro-Fenton method via nanostructured 3DG modified electrode as the cathode was investigated at the second part of our work. Gas chromatography–mass spectrometry (GC–MS) analysis was used to investigation of degradation intermediates. Response surface methodology (RSM) has been also used to investigation the influence of operational parameters on the degradation efficiency of

Fig. 1. (a) SEM image of the prepared 3DG. (b) XRD patterns of GO and 3DG. (c) FTIR spectra of 3DG and GO.

nalidixic acid. 2. Experimental 2.1. Chemicals In this work, nalidixic acid (a synthetic antibiotic quinolone with 2

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Fig. 2. FESEM images of (a) carbon paper before bonding of GO-PTFE or 3DG-PTFE layer, (b) GO-PTFE electrode, (c) and (d) 3DG-PTFE electrode.

2.3. Preparation of the gas-diffusion electrode

purity > 98.5%) was selected as the model compound and its features were provided in Table 1. It was purchased from Sigma-Aldrich Co. and utilized without additional purifying. Analytical grade ethanol, potassium permanganate, graphite powder, glycine, anhydrous sodium sulfate, sodium hydroxide, n-butanol (≥99.5%), ammonium molybdate tetrahydrate, phosphoric acid (80%), potassium hydrogen phthalate, sulfuric acid (98%), hydrochloric acid (37%), potassium iodide, potassium chloride, iron (II) sulfate, N, O-bis(trimethylsilyl) acetamide and dichloromethane (≥99.9%) were purchased from Fluka and Merck. Carbon papers and PTFE were obtained from Pars Hydro Pasargad and Electro Chem, Iran, respectively.

Suitable quantities of prepared 3DG (0.1 g), distilled water (60 mL), n-butanol (3%) and PTFE (0.42 g) were mixed and ultrasonicated to produce a highly dispersed mixture for 10 min. The resulted blend was made warm at 80 °C until it looked like an appearance ointment. A carbon paper loaded with 50% of PTFE was bonded by this ointment and sintered under inert conditions (N2) at 350 °C for 30 min. Operational 3DG-PTFE cathodes (25 mm in diameter and about 0.6 mm thickness) were obtained by cutting of the resulting electrode. Prepared cathodes were put at the lowest point of a cylinder-shaped container of polypropylene that had an inner graphite ring as current collector. A copper cable as electrical connection was in contact with this graphite ring.

2.2. Production of 3D graphene Graphite powder was oxidized to synthesize GO using H2SO4, H3PO4 and KMnO4 as claimed by the improved Hummers’ method. In this method, a 9:1 mixture of concentrated H2SO4/H3PO4 (120:13.33 mL) was added to a mixture of graphite powder (1 g) and KmnO4 (6 g). The reaction was then heated to 50 °C and stirred for 12 h. The reaction was cooled and poured onto Ice with H2O2 (10 mL) [40]. For the fabrication of 3D structured graphene by way of a solution combustion method, aqueous suspensions of GO and glycine as fuel were prepared. The combined solution was heated after ultrasonic and the resulting colloidal solution was then heated in air. The resulted powder was washed with ethanol and deionized water then dried. The product was designated as 3DG [41].

2.4. Instruments All of the experiments were taken place in an undivided cell using a DC power supply. The pH of the solutions was measured with an AZ 86,502 pH-meter, Taiwan. The elimination of nalidixic acid was monitored by using a Specord 250, Analytik Jena UV–Vis spectrophotometer, Germany. Cyclic voltammetry tests were performed by a standard three-electrode cell in combination with a multichannel potentiostat (Autolab/PGSTAT 100, Netherland) regulated by a computer with a scan rate of 10 mV/s. A Skala-Formics total organic carbon (TOC) analyzer (the Netherlands) was used to determine the TOC values by catalytic oxidation. Nanosurf Mobile S (Switzerland) AFM 3

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record of FTIR spectra. For the Brunauer-Emmett-Teller (BET) analysis, nitrogen adsorption/desorption at 77 K with a Belsorp mini II device (Bel, Japan) was executed. 2.5. Electrolytic system A 300 mL undivided cylinder-shaped glass cell fitted with double electrodes and conducted at a steady current was utilized in all of the experiments at ambient temperature (25 °C). The above glass cell was equipped with a 1 cm2 Pt sheet as anode and the electrode 3DG-PTFE as cathode. Solutions have been magnetically stirred in all studies. Air at a flow rate of 140 mL.min−1 was injected to the diffusion cathode for the generation of H2O2 by reaction (1). In the aqueous medium comprising 0.05 mol.L−1 Na2SO4 as electrolyte, nalidixic acid samples (250 mL) at initial pH 3.0 with drug concentration between 10 and 50 mg.L−1 were removed relatively. The pH 3.0 value was chosen as the optimal value for Fenton’s reaction (2) [42]. At the end of the reaction, the Residual Concentration of the nalidixic acid was determined spectrophotometrically at λmax = 251 nm. The degradation efficiency (DE%) is calculated by the following equation:

A⎞ DE% = ⎛1 − × 100 A o⎠ ⎝ ⎜

⎟

(3)

Ao and A in this equation are the nalidixic acid absorbance at time 0 and t, respectively. 2.6. Method of analysis At well-ordered time intervals, samples were taken out from the reactor and the elimination of nalidixic acid was assessed by measuring the solution absorbance at λmax equals 251 nm. The iodide method was used to determine hydrogen peroxide concentration spectrophotometrically (detection limit of approximately 10−6 mol.L−1) as follows [43]. Iodide reagent (0.06 mol.L−1 NaOH, ≈10-4 mol.L−1 ammonium molybdate, 0.4 mol.L-1 potassium iodide) (3 mL) and 0.1 mol.L−1 potassium biphthalate (3 mL) were added to aliquots (2–3 mL) of samples and were diluted up until 10 mL. After that the treated solution absorbance was measured at λmax = 351 nm with a UV–vis spectrophotometer. For identification of the intermediates of the drug, 250 mL aqueous solution contains 30 mol.L−1 of nalidixic acid was degraded for 5 h at 300 mA and at ambient temperature (25 °C). The resulted solution was acidified to pH 1.0 with H2SO4 and saturated with Na2SO4, before three times extracting of organic parts with 30 mL diethyl ether [44]. After evaporating of the gathered organic solution, the remaining solid was dissolved in 100 μL of N,O-bis-(trimethylsilyl)acetamide at 60 °C under heating and stirring for 10 min. After that the resulting silylated compounds were analyzed by GC–MS with a follows temperature program of 50 °C for 4 min, 8 °C min−1 up to 300 °C and hold time 4 min. The temperature of inlet, transfer line and detector were 250, 250 and 300 °C, respectively [16].

Fig. 3. Three-dimensional (3D) AFM images of (a) GO-PTFE, (b) 3DG-PTFE. Table 3 Surface roughness parameters of as prepared GO-PTFE and 3DG-PTFE electrodes. Samples

Area

Roughness parameters Sa (nm)

Sq (nm)

Sy (nm)

26.336 70.073

31.819 86.28

310.65 483.33

2.7. Experimental design GO-PTFE electrode 3DG-PTFE electrode

In this research, as the most common one in RSM, the central composite design (CCD) with five levels was used to the optimization of the electro-Fenton method. So as to investigate the effect of the operational parameters on the degradation efficiency of nalidixic acid, four main factors were chosen: initial nalidixic acid concentration (X1), pH (X2), applied current (X3) and electrolysis time (X4). In this research, a total of 31 studies were used, including 24 = 16 cube points, 7 central point replications and 8 axial points. In order to statistical computations the variables Xi were coded as xi in accordance with the next connection:

assessed the surface morphology of produced electrodes. The surface of the samples was monitored by the SEM (Tescan Mira3 FEG-SEM, Czech) after coating with gold. For the GC–MS analysis, an Agilent 6890 GC system with a HP-5MS capillary column (30 m–0.25 mm) combined with a HP 5989A mass spectrometer. The system was working in EI mode at 70 eV. Philips PW1730 (Netherlands) took patterns of XRD. Bruker Tensor 27 FTIR spectrophotometer (Germany) was used to 4

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(5)):

Y= b0 +

∑ bi xi + ∑ bii x2i + ∑ bijxixj+ε

(5)

In this equation, Y is the computed response, b0 is the constant, bi, bii and bij are the linear, quadratic and interaction coefficients, respectively, xi and xj are factors (i = 1, 2 and 3; j = 1, 2 and 3) and ε is the residual part. For regression analysis of the experimental data, a statistical software package (Minitab® Release 15) was utilized. Analysis of variance (ANOVA) was used to test the goodness of fit of the obtained regression model and the significance of each term in the equation. Counter plots (two-dimensional) and curves of the response surface (three-dimensional) were evolved too. The numerical optimization procedure was applied to achieve the best degradation of nalidixic acid.

3. Results and discussion

Fig. 4. Cyclic voltammograms of (a) bare GDE, (b) GO-PTFE and (c) 3DG-PTFE in aqueous solution at pH 3.5 with O2 atmosphere. Sweep rate = 10 mV.s−1, [Na2SO4] =0.05 mol.L−1, room temperature (25 °C).

3.1. Characterization of the synthesized 3DG Fig. 1a shows FESEM image of synthesized 3DG. As can be seen in this figure, an interconnected porous network has been created in the 3DG structure. The creation of this network can be related to ignition and reaction of the glycine with oxidants such as oxygen or carboxyl, hydroxyl groups on the GO surface after the combustion. This lead to generation of a lot of bubbles composed of N2, CO2 and H2O in the GO suspension and between GO sheets. So the formed pores have random sizes and the product has a foam-like structure with many pores and expanded volume in comparison to the GO [41]. The XRD patterns of graphene oxide and the synthesized 3D structured graphene have been shown in Fig. 1b. As indicated in this figure, a sharp peak at 2θ near 10° was observed for the GO [41,46]. Furthermore, disappearance of GO diffraction peak (2θ = 10°) and appearance a new broad diffraction peak at 2θ = 25˚ can be related to the exfoliation of the multilayer GO after the combustion and the conversion to graphene 2D sheets with removal of surface functional groups [41,47]. Fig. 1c presents the asprepared 3D graphene FTIR spectrum. This figure shows characteristic eOH at ∼3400 cm−1 that is related to the hydroxyl groups on the surface of the synthesized 3DG. The characteristic peaks due to C]C (∼1620 cm−1), and carbonyl (∼1730 cm−1, 1400 cm−1 and 1060 cm−1) also appeared in the FTIR spectrum of the 3DG which indicates continues range of original graphene structure in some ranges. To assess the variation in the specific surface area of the synthesized GO and 3DG the BET analysis was carried out. 3DG structure displayed a higher surface area (132 m2. g−1) than that of GO (23.5 m2. g−1). Therefore, the prepared 3DG with a large specific surface area is an advantage for creating electrode materials in the electrochemical systems.

Fig. 5. The amount of generated H2O2 as a function of time at room temperature (25 °C), [Na2SO4] = 0.05 mol.L−1, I = 300 mA, pH = 3.0.

xi =

Xi − X o δX

(4)

In this equation, Xo refers to represent the Xi value at the central point and δX exhibits the step-change [45]. These factors coded levels and their natural values for nalidixic acid degradation are demonstrated in Table 2. 2.8. Analysis of data In order to evaluate the relationship between the response and independent variables a second-order polynomial equation was used (Eq. Table 4 Comparison of H2O2 generation with literatures. Cathode *

CNT /PTFE CNT/Graphite Modified graphite felt Nitrogen functionalized CNT Graphite CNT/PTFE CB**/PTFE GDE (CNT/PTFE) 3DG-PTFE GDE

Experimental conditions −1

150 m L, 0.05 mol.L Na2SO4, 1.2 V 1000 mL, 0.05 mol.L−1 Na2SO4, pH = 3, 100 mA −1 130 mL, 0.05 mol.L Na2SO4, 0.75 V/SCE 250 mL, 0.05 mol.L−1 Na2SO4, 0.85 V/SCE 50 mL, 0.035 mol.L−1 Na2SO4, pH = 3, 20 A.m-2 250 mL, 0.05 mol.L−1 Na2SO4, pH = 3, 100 mA 150 m L, 0.05 mol.L−1 Na2SO4, pH = 7, 100 mA 150 m L, 0.05 mol.L−1 Na2SO4, pH = 3, 100 mA 250 mL, 0.05 mol.L−1 Na2SO4, pH = 3, 300 mA

* Carbon nanotube. ** Carbon black. 5

H2O2 generation rate (mg. h−1. cm-2)

Refs.

0.004 0.04 1.61 2.04 2.04 6.04 15.25 17.60 28.19

[55] [56] [57] [58] [59] [16] [60] [61] This work

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PTFE and 3DG-PTFE electrodes were verified by AFM analysis. It can be seen in the images that the roughness over the prepared electrode with 3DG compared to GO was induced. For better comparisons, Table 3 shows surface roughness parameters including root mean square (Sq), roughness average (Sa), and peak-valley height (Sy). It obviously display that 3DG-PTFE has more active sites for generation of H2O2 than GO-PTFE and modifying with 3DG led to an increase in the average roughness. These results are coherent with the results of the FESEM images (Fig. 2b–d).

Table 5 The 4–factor central composite design matrix and the value of response function (DE%). Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

[Nalidixic acid]0 (mg.l−1)

pH

0 0 −2 0 −1 0 +1 −1 0 0 +1 0 0 0 −1 0 −1 −1 +1 +1 +1 +2 0 −1 0 +1 −1 +1 −1 +1 0

0 0 0 0 +1 0 +1 +1 0 +2 +1 0 0 0 +1 0 −1 −1 −1 +1 −1 0 0 0 0 −1 −1 −1 +1 +1 −2

Applied current (mA)

Time (min)

0 0 0 0 +1 0 −1 −1 −2 0 −1 0 0 0 −1 0 +1 −1 +1 +1 −1 0 +2 0 0 −1 −1 +1 +1 +1 0

0 −2 0 0 −1 +2 +1 −1 0 0 −1 0 0 0 +1 0 +1 −1 +1 +1 +1 0 0 0 0 −1 +1 −1 +1 −1 0

Degradation efficiency (DE%) Experimental 75.70 74.25 60.30 75.95 45.24 75.65 45.24 41.27 54.76 41.51 49.63 74.98 75.33 74.16 45.26 75.65 89.03 51.33 86.14 76.97 80.27 81.94 81.08 82.74 75.15 81.67 76.62 80.16 70.26 65.11 37.03

Predicted 76.43 75.24 60.69 75.59 49.10 76.63 45.80 42.12 55.72 42.15 50.76 75.84 76.51 75.69 46.24 76.10 90.69 52.69 87.41 77.44 81.69 82.59 82.53 83.68 76.15 82.29 77.69 82.69 74.43 68.68 40.25

3.2.2. Electrochemical behavior of modified electrodes The CV measurements were registered on the bare carbon paper before modification, GO-PTFE and 3DG-PTFE electrodes in the aqueous solution of 0.05 mol.L−1 Na2SO4 at pH 3.5 saturated by O2 with scan rate of 10 mV.s-1 (Fig. 4). A Pt sheet was used as the anode and the saturated calomel electrode (SCE) was performed as the reference electrode. The cathodic peaks corresponding to the reduction of oxygen and generation of H2O2 in the potential ranges of −0.3 to −0.5 V vs. SCE well-defined in the both of the two electrodes (GO and 3DG) [20]. As can be seen in this figure, the bare GDE (bare carbon paper) displayed very poor oxygen reduction activity. On the other hand, the onset potential for GO (graphene/carbon paper) and 3DG (3DG/carbon paper) were observed around −0.30 V vs. SCE. It could be seen that under the same condition, greater peak current density is seen in 3DG modified electrode’s CV curve. For the purpose of determining the amount of H2O2 produced by the bare GDE and 3DG-PTFE electrodes, electrolysis was executed at a steady current of 300 mA with 250 mL solution. A Pt sheet (area of 1.0 cm2) was used as anode in this electrolysis. Fig. 5 shows the results on the bare GDE and 3DG/GDE for the generation of H2O2. As it can be viewed in this figure, the concentration of H2O2 obtained via bare GDE and 3DG-PTFE electrodes was 10 and 40 mmol.L−1, respectively at the end of 180 min electrolysis. The amount of H2O2 obtained with 3DGPTFE electrode was almost four times greater than that of bare GDE electrode. The 3DG-PTFE cathode used in this work showed higher H2O2 generation rate (28.19 mg.h−1. cm-2) compared with the other literatures (Table 4). It can be concluded that 3DG could be a useful cathode material for H2O2 electrogeneration, according to these outcomes.

3.2. Modified GDE cathode characterization 3.2.1. Physical properties characterization Fig. 2a–d represents the carbon paper’s FESEM images before (Fig. 2a) and after bonding of GO-PTFE (Fig. 2b) and 3DG-PTFE (Fig. 2c and d) layer on it. As can be seen in FESEM images, the surface of the electrode modified with 3DG (Fig. 2c and d) was much uneven than that of the GO (Fig. 2b). Such a porous 3DG-PTFE electrode surface could enable rapid O2 diffusion to achieve high mass transfer rate and highly efficient H2O2 production. Fig. 3a and b represent the morphology results (3D graphs) of GO-

3.3. Central composite design model Response surface modeling of the process was done by a five-level CCD with sixteen factorial point, eight axial point and seven

Fig. 6. Coefficient of determination (R2) for the quadratic model and model terms. 6

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acceptable for comparing models with dissimilar numbers of independent variables. It is also an evaluation of goodness of a fit. Adj–R2 improves R2–value by using the degrees of liberty on its computations for the sample size and amount of phrases in the model. It can be smaller than R2 if there are many phrases in a model and the sample size is not very large [48,49]. Here, Adj–R2 value (0.804) and the corresponding R2 value are too close (Table 6). An appropriate fit of the model has to be acquired to guarantee the acceptability of the model used in optimizing a response surface. From a statistical point of view, the evaluation of the acceptability of the model requires three tests: the Fisher test, the Student’s t-test and Rsquare test. The variance test of Fisher known as the F-test used to test the reliability of the forecasted model. If the experimental outcomes well predicted by the model, F-value for a certain amount of degrees of liberty in the model at a meaning level of α has to be higher than the critical value of F-distribution. Table 6 shows the outcomes of the anticipated quadratic response surface model for the electro-Fenton method in the form of analysis of variance (ANOVA) of the regression parameters. The resulted F-value (84.94) at the confidence level of 95% is clearly higher than the F critical value (2.352 at the significance level of 95%). It means that the quadratic model forecast was statistically significant, and the regression equation can explain most of the variations in the response. As the ANOVA results present, the predicted model showed a great value of determination coefficient (R2 = 0.896 and Adj–R2 = 0.804). Every time you add a variable to a model, the R2 value increases. But the Adj-R2 increases only if the new term improves the model. So the Adj-R2 that takes the number of variables into account is commonly selected [50–52]. The relatively high R-squared value (0.896) displays good accordance between the calculated and experimental results. The model's proportionality also is assessed by the residuals (the difference between expected and experimental amounts). The observed residuals plotted against expected values are shown in Fig. 7. All points on the normal probability plot have to be approximately on a straight line if the residuals have a normal distribution, therefore it could be concluded that the estimated effects are the real and differ from noise [50]. Fig. 7 shows a normal distribution and randomly scattering of the residuals.

Table 6 Analysis of variance (ANOVA) for fit of degradation efficiency from central composite design. Source variations

Sum of squares

Degree of freedom

Adjusted of Mean square

F–value

Regression Residuals Total

4268.46 57.43 4325.90

14 16 30

304.89 3.59

84.94

R2 = 0.896, Adj–R2 = 0.804.

replications resulting in a total of 31 experiments. Table 5 presents the resulting CCD matrix with 4-factor and experimental outcomes. The polynomial response equation of the second-order (Eq. (5)) was used to correlate the dependent and autonomous variables. The graph for the coefficient of determination (R2) of the quadratic model and model terms is shown in Fig. 6. Quadratic model and model terms applied current, pH, initial nalidixic acid concentration and pHpH are significant (Fig. 6). Based on these outcomes, the following second-order polynomial equation developed between the response and independent variables subjected to -2 ≤ xi ≤ +2:

Y = 75.1178 + 4.3227X1 –7.3774X2 + 7.5009X3 + 3.0178X 4 –0.1657X1 X2 –1.5128X1 X3 –2.8461X1 X 4 + 1.9379X2 X3 –0.1657X2 X 4 + 1.4163X3 X 4 –0.4072X12 − 8.3696X22 − 1.2072X32 + 0.5515X42

(6)

The forecasted degradation efficiencies (DE%) by Eq. (6) have been shown in Table 5. These findings suggested excellent degradation effectiveness contracts between the experimental and forecasted amounts. The correlation between the experimental information and the expected reactions was assessed quantitatively by the correlation coefficient (R2). A comparison between the experimental results and the forecasted values obtained from the model (Eq. (6)) discovered that the expected values corresponded fairly well with R2 = 0.896 to the experimental values. This implies that the independent variables explain 89.6 percent of the differences for color removal and does not describe just about 10.4 percent of the variation. Adjusted R2 (Adj–R2) is more

Fig. 7. Residual plots for degradation efficiency of nalidixic acid. 7

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Fig. 8. Pareto graphic analysis.

Fig. 9. The response surface plot and contour plot of the degradation efficiency (DE. %) as the function of applied current (mA) and electrolysis time.

Fig. 10. The response surface plot and contour plot of the degradation efficiency (DE. %) as the function of initial nalidixic acid concentration (mol.L−1) and electrolysis time.

The Pareto analysis provides further meaningful data for interpreting the outcomes. In fact, according to the following relationship, the percentage impact of each factor was calculated by this assessment [53,54]:

It can be seen in the Pareto graphic analysis (Fig. 8) among the varying factors, pH-pH (b22, 30.90%), applied current (b3, 24.82%), pH (b2, 24.01%) and initial nalidixic acid concentration (b1, 8.24%) have the major impact on degradation efficiency.

b2 Pi ⎛⎜ i 2 ⎞⎟ × 100 (i≠0) ⎝ ∑ bi ⎠

3.4. Variables effects as response plots (7) Geometric depictions significantly enhance the interpretation of the 8

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Fig. 11. The response surface plot and contour plot of the degradation efficiency (DE. %) as the function of initial nalidixic acid concentration (mol.L−1) and initial pH. Table 7 Identified by-products during electrolysis of nalidixic acid. No.

Compound name

1

Structure

Retention time (min)

Main fragments

Benzoic acid

27.73

163, 189, 245, 273, 339

2

Alanine

2.15

52, 73, 94, 117, 147

3

phenethylamine

24.37

43, 186, 227, 270

4

2-Furanone

17.32

73, 110, 141, 172, 201

scheme with various inputs and outputs and, in particular, by using the contour and surface plots. Contour plots averse to pairs of significant explanatory variables demonstrate in what way these two factors are related to the reaction variable at a moment. 3D plots of the response surface also display the side response surface. Fig. 9 created the surface and contour plots on the basis of applied current and electrolysis time whilst the initial pH and concentration of the nalidixic acid were held constant at 6 and 30 mol.L−1, respectively. The applied current is one of the effective variables in electrochemical process. The rate of electrochemical reactions and the performance of electrodes is highly dependent on applied current [8]. As could be viewed in Fig. 9, increasing the applied current until about 500 mA causes to increase degradation efficiency. This degradation efficiency enhancing can be related to a greater generation of H2O2 with reaction (1). It should be noted that a further rise in the applied current led to a slight improvement in degradation efficiency. The utilization of higher currents not only increases energy loss but also enhances H2O electrolysis and produces oxygen instead of hydroxyl radical [9]. Hence, the electro-generation of oxidizing agents such as hydroxyl radical increases when of course, current rises by the optimum amount. Fig. 10 demonstrates the degradation efficiency’s response surface and contour plots as a function of reaction time and initial concentration of nalidixic acid. It can be viewed in Fig. 10, drug degradation efficiency reduced with the increasing initial concentration of the drug. This conduct is one of the features of advanced oxidation methods. While the pharmaceutical concentration is increased, certain amount of hydroxyl radicals are generated in the constant status of the electrolysis

Fig. 12. (a) Degradation efficiency of 3DG-PTFE electrode during repeatedbatch operations, b) FESEM images of 3DG-PTFE electrode after repeated runs.

9

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4. Conclusions

system (e.g. pH, constant current, etc.). Thus, this certain amount of hydroxyl radicals is not adequate to degrade the high concentrations of the pharmaceutical and degradation efficiency declines [1]. Fig. 11 displays the degradation efficiency’s response surface and contour plots based on the initial pH and nalidixic acid concentration of the 300 mA applied current and 180 min electrolysis time. Fig. 11 showed the highest degradation efficiencies occurred when initial pH was kept at acidic pHs (3.0–6.0) under all electrolysis time conditions.

A 3DG-PTFE gas diffusion cathode electrode was supplied and compared with bare carbon paper and GO-PTFE electrodes in this work. SEM, XRD, AFM, FTIR, CV and BET analyses were used to examine the resulting electrodes. The results displayed that the concentration of H2O2 obtained via bare carbon paper and 3DG-PTFE electrodes was 10 and 40 mmol.L−1, respectively at the end of the 180 min electrolysis. The value of obtained H2O2 with 3DG-PTFE electrode was almost four times greater than that of bare carbon paper electrode. In addition, the 3DG-PTFE electrode electro-Fenton enables 81% mineralization after 6 h of electrolysis. These results allowed concluding that 3DG-PTFE electrode can be used to electrogeneration of H2O2 and degradation of nalidixic acid as a good material. The comparison between SEM and AFM images of fabricated electrodes showed that 3DG-PTFE electrode has large surface area than other electrodes causing high efficient production of H2O2. On the Basis of experimental outcomes, the secondorder polynomial equation achieved and expressed an empirical connection between the response and autonomous variables. The response surface and contour plots established the effect of trial parameters on the removal efficiency of nalidixic acid. ANOVA test demonstrated high R2 and Adj-R2 values (0.896 and 0.804, respectively). GC–MS was used to analyze degraded samples and four compounds were effectively detected in order to identify nalidixic acid degradation by-products.

3.5. Optimization of nalidixic acid degradation Determining the optimum values of variables in the electro-Fenton process is the major purpose of the optimization. The required objective was described as "maximizing" in terms of degradation effectiveness to obtain the greatest performance of therapy. The results of optimization displayed that pharmaceutical removal efficiency presented the maximal result (90%) in the optimal condition of initial nalidixic acid concentration 15 mg.L−1, initial pH 3.5, applied current 350 mA and electrolysis time 300 min. Another experiment was done at the forecasted optimum conditions for verifying the optimization results. This test result (88.98%) was in close agreement with the model's predicted value and validated the results of the optimization of the response surface. The efficiency of the process using fabricated 3DG modified carbon paper cathode to mineralize nalidixic acid solutions was evaluated by The TOC decay. The experiment was done with initial nalidixic acid concentration 15 mg.L−1, initial pH 3.5, applied current 350 mA and electrolysis time 7 h. The result showed 87.3% mineralization of the drug after 7 h of process.

Declaration of Competing Interest None. Acknowledgement

3.6. Determination of nalidixic acid degradation products

The authors thank the University of Tabriz, Iran for all the supports.

Samples were evaluated using GC–MS to define reaction by-products that accompany nalidixic acid degradation. The examined constituents were recognized by combining their spectra with those registered in the MS library (Wiley 7n). Four compounds were satisfactorily identified, namely benzoic acid (1), alanine (2), phenethylamine (3) and 2-furanone (4) (Table 7). It should be noted that several other chromatographic peaks were also discovered other than the compounds effectively detected, but it was not possible to recognize favorably (i.e. the mass spectrum’s match factor was lower than 90%). It is evident that a broad variety of cleavage compounds are anticipated as quickly as the aromatic ring opens; unfortunately, such by-products have not been identified due to the employed analytical technique limitations or they were not considerably accumulated in the medium. On the other side, some of the intermediates were not recognized, likely because they are rapidly oxidized to their derivatives in the event of their generation. The findings of the product distribution indicate that the method of degradation is initialized to form intermediates 1–4 by oxidation of nalidixic acid. These intermediates can be converted to the short-chain carboxylic acids by further oxidation and then are converted directly into CO2 and H2O.

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3.7. Stability of the fabricated electrode In order to examine the stability of the prepared 3DG-PTFE gas diffusion cathode electrode, repeated-batch operations were performed. As can be seen in Fig. 12a, during seven repeated runs, the as-prepared 3DG-PTFE electrode showed almost the same degradation efficiency (DE%) that obtained from the first run. The results indicated that the 3DG-PTFE electrode possessed reasonable reusability in repetitive degradation operations and had an acceptable stability. Fig. 12b also represents FESEM image of the used 3DG-PTFE electrode after repeated runs. As can be seen in this image, the surface of the electrode has saved the same porous surface. 10

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