Rohit Misraab,
Nageswara Nao Neti*a,
Dionysios D. Dionysiouc,
Mahendra Tandekara and
Gajanan S. Kanaded
aWastewater Technology Division, CSIR-National Environmental Engineering Research Institute, Nagpur, 440020, India. E-mail: nn_rao@neeri.res.in; Tel: +91 712 2249885
bInstitute of Water & Wastewater Technology, Durban University of Technology, Durban, South Africa
cDepartment of Biomedical, Chemical and Environmental Engineering (DBCEE), University of Cincinnati, USA
dAnalytical Instrumentation Division, CSIR-National Environmental Engineering Research Institute, Nagpur, 440020, India
First published on 24th December 2014
Three-dimensional carbon bed electrochemical reactors have been recently applied for the degradation of several organic pollutants. However, the carbon particles in such reactors slowly undergo attrition. We fabricated a novel flow-through three-dimensional anode using granular activated carbon (GAC) particles and polyvinylidene fluoride (PVDF) binder that potentially avoids such attrition. Optimization of the composition of GAC and PVDF with respect to mechanical integrity and electrical conductivity is reported. The anodes were tested in the electro oxidation of the reactive dyes: Reactive Orange-16 (RO-16), Reactive Red-2 (RR-2), and Reactive Blue-4 (RB-4). A tentative mechanism of dye degradation was proposed based on the observed role of the supporting electrolyte and the cyclic voltammetric, UV-vis, FT-IR and GC-MS data. The decolorization efficiencies were 75 ± 3, 81 ± 5 and 88 ± 4% for RB-4, RO-16 and RR-2, respectively. The integrated 3-D anodes are advantageous because of the absence of carbon attrition, which is otherwise found when a bed of GAC is used in the electrochemical reactors.
The advanced oxidation processes (AOPs), which are based on the generation of highly oxidizing species such as hydroxyl radicals (HO˙), have demonstrated the capability to mineralize recalcitrant dye compounds.6–8 The treatment of synthetic dye-colored wastewater using AOPs is particularly interesting.3,6–12 One noteworthy approach is photocatalysis using titanium dioxide and sunlight. However, as can be drawn from the previously reported work this method suffers from many setbacks: (i) low oxidation efficiency from real wastewater due to severe matrix ion interference (ii) engineering difficulties in recovering the spent catalyst (iii) low photon-to-OH radical conversion efficiencies (iv) higher capital costs even in pilot scale applications when artificial UV lamps are proposed to be used and (v) large land area requirements for the TiO2/sunlight reactors. In the past decade, the electrochemical oxidation of organic pollutants has attracted wide attention due to the effectiveness and scale up possibilities of the process.13–19 Electrochemical oxidation is also attractive due to its amenability to selectively tune the electrode potential/current to attain higher reaction rates. More importantly, it can also make use of the chloride ions present in many types of common industrial effluents to cause indirect oxidation via the Cl2/OCl-redox couple [8]. Electrochemical oxidation is conveniently carried out in two-dimensional (2-D) parallel plate electrode reactors or three-dimensional carbon bed electrode reactors (TDR).20 TDR have particularly attracted attention due to their high surface-area-to-volume ratios, which result in lower electrical energy consumption, high treatment efficiency, improved mass transfer of pollutants, generation of oxidants in high concentration, and adsorption of pollutants on carbon [8]. Thus, TDR have been successfully used for the treatment of various refractory pollutants such as acid-orange-II,21 formic acid,22 reactive blue 4,8 phenol,23 reactive brilliant red X-3B24 and acid orange-7.25 TDR have also been used to treat actual wastewater, including landfill leachate,26,27 chemical industry wastewater28 and heavy oil refinery wastewater.29 However, the granular activated carbon (GAC) particles forming the carbon bed in TDR are found to undergo slow attrition, which leads to operational problems.27,30
In this study, a novel flow-through three-dimensional (3-D) anode was fabricated using GAC particles and PVDF binder with the aim of avoiding carbon attrition. While the choice of GAC was based on its high surface area and attractive adsorption properties, the choice of PVDF was directed by its tenacious binding properties and thermal stability. PVDF is soluble in N-methylpyrrolidone, which can be washed away with hot water without removing PVDF. The 3-D anode incorporated in a cylindrical stainless steel reactor was applied for the electro oxidation of three reactive dyes; viz., Reactive Orange-16 (RO-16), Reactive Red-2 (RR-2), and Reactive Blue-4 (RB-4). Our results demonstrate that GAC particles can be shaped into an integrated 3D-anode using the PVDF binder and electro oxidation in a three-dimensional flow-through carbon anode reactor (TDFCR), which delivers clean effluent free from the carbon dust that otherwise, arises from carbon attrition. It is pertinent to mention that although PVDF has been frequently used as a binder in the fabrication of carbon based thin super capacitors31 and carbon nanotube–PVDF filter electrodes,32 it has not been employed previously for the preparation of bulk three dimensional integrated particle electrodes similar to the one reported in this study.
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Fig. 1 (a) SEM images of GAC, (b) nitrogen adsorption–desorption isotherms of typical porous GAC material and (c) pore size distribution plot. |
Composition (%PVDF) | Compressive strength (psi) | Electrical resistance (R = V/I) | Attributes |
---|---|---|---|
a NA = not adequate; NM = not measurable. | |||
5 | NA | NM | All GAC particles were not held together by the binder and a self-supporting anode could not be obtained as it readily collapsed during removal from the mold |
8 | 5.1 | NM | A self-supporting TDCA was obtained, but the TDCA disintegrated with the application of slight pressure |
10 | 15.8 | V = 12 V, I = 1 A (R = 12 Ω) | Good conduction was seen and the GAC particles were held together by the binder, resulting in greater mechanical strength |
15 | 35.1 | V = 15 V, I = 0.3 A (R = 50 Ω) | Poor conduction was seen but all GAC particles were held together by the binder, resulting in greater mechanical strength. The GAC particles were visibly coated with a whitish PVDF polymer layer, which prevents particle–particle contact and an increase in the electrical resistance. The drying time was also greater |
RR-2 showed four characteristic bands in its UV-vis spectra (Fig. 2a), one broad band in the visible region (450–600 nm), which corresponds to the conjugated structure connected with the azo group; the other three peaks appear at 331, 285 and 235 nm in the ultraviolet region, resulting from the unsaturated structures of the naphthalene, triazine and benzene rings, respectively. The intensity of all four absorption peaks decreased after electro oxidation, indicating that the azo bond of the dye molecule was ruptured and that the naphthalene, triazine and benzene rings were also transformed during the electro oxidation process in the TDFCR.
The UV-vis spectra of RO-16 recorded for the test samples, before and after electrolysis, (Fig. 2b) show that the electro oxidation process leads to very significant structural modifications. The initial absorption spectrum of RO-16 mainly shows four peaks; 250 and 300 nm in the UV region, and at 390 and 500 nm in the visible region. The absorbance band at ∼500 nm may be due to the n–π* transition of the chromophore azo group (–NN–), and the band at ∼390 nm is due to the π → π* transitions related to the aromatic rings bonded to the azo group. The intensity of the peaks at 500, 390, 300 and 254 nm confirms a rapid decrease in the color concentration due to transformation of the azo group as well as a loss of conjugation in the aromatic groups.
The initial UV-visible spectra obtained for the RB-4 dye displayed absorption bands at 599, 370 and 296 nm, which is a characteristic of the anthraquinone group, and another band at 256 nm that can be attributed to the aromatic and chlorotriazine groups (Fig. 2c). The final solution after electro oxidation treatment did not show absorption at the wavelengths of maximum absorption, i.e., 599, 370–296, and 256 nm. Both the chromophore (–NN–) and the intermediate compounds appeared to have degraded in the reactor.
Additionally, the TDFCR was tested repeatedly at different RB-4 concentrations (50, 100 and 150 mg L−1) by passing the dye solution in single pass mode under applied potential (10.0 V) and current (1.0 A). The UV-visible spectral features are similar for the treated dye solutions obtained over 5 cycles in the continuous mode, with each cycle contributing to one void volume of treated dye solution. The corresponding data given in Fig. 3 suggest that it was possible to maintain a very low residual RB-4 concentration compared to the initial concentration, and the treated water had very faint color even with a higher initial dye concentration. The performance of the TDFCR was tested repeatedly, and a treated volume equivalent to five void volumes (5 × 80 mL) was collected. It can be predicted that the reactor performance could be extended to even more void volumes. Furthermore, RB-4 removal efficiency was 74% in the batch mode compared to 90% under continuous mode. It may be inferred that as the adsorbed dye is electro oxidized at the TDCA, more dye is adsorbed, which further undergoes electro oxidation.
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Fig. 3 Percent dye removal versus void volume of the reactor, demonstrating consistent performance with variation in RB-4 concentration. |
Name of compound | Structure | MW | RT (min) | m/z | Compound present (✓)/absent (×) in (treated) effluent | ||||
---|---|---|---|---|---|---|---|---|---|
RB 4 | RO16 | RR2 | |||||||
4 h | 20 h | 4 h | |||||||
a Post treatment with RO16 (16 h), the reactor was washed with MeOH, dried and reconstituted with 2 mL acetonitrile. | |||||||||
1 | 4-Methyl benzamide (toulamide) | ![]() |
135 | 28.52 | 135 (M+); 119 (M+ − 15), 91 (M+ − 44), 78 (M+ − 59), 65 (M+ − 74) | ✓ | × | × | × |
2 | 3-Hydroxy butyric acid | ![]() |
104 | 5.14 | 104 (M+), 75 (M+ − 29), 59 (M+ − 45), 47 (M+ − 57), 43 (M+ − 60) | ✓ | — | ✓ | ✓ |
3 | 4-(Methylamino) butyric acid | ![]() |
117 | 9.45–10.05 | 117 (M+); 100 (M+ − 17); 72 (M+ − 44); 44 (M+ − 73) | ✓ | ✓ | ✓ | ✓ |
4 | N-Nitroso-1-phenylmethanamine | ![]() |
136 | 24.96/29.90 | 136 (M+); 105 (M+ − 31), 91 (M+ − 45) | × | — | ✓ | × |
5 | Di-butyl phthalate | ![]() |
278 | 33.0 | 278 (M+), 147 (M+ − 131) | ✓ | ✓ | ✓ | ✓ |
6 | 1-Nitro-2-propanol | ![]() |
105 | 3.35 | 105 (M+); 62, 61 (M+ − 46) | ✓ | × | × | × |
7 | 2-Pyrrolidinone, 1-methyl | ![]() |
99 | 9.48 | 99 (M+); 84 (M+ − 15); 70 (M+ − 29); 55 (M+ − 44); 44 (M+ − 55) | × | ✓ | ✓ | ✓ |
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|||||||||
Reactor washingsa | |||||||||
8 | Diazene, bis(1,1-dimethylethyl)- | ![]() |
142 | 12.14 | 142 (M+); 71 (M+ − 71); 57, 41, 29 | ✓ | |||
9 | 1-Benzenesulfonylpiperidine-4-carboxylic acid allylamide | ![]() |
308 | 30.93 | 308 (M+); 252 (M+ − 56); 223 (M+ − 85); 167 (M+ − 141); 141 (M+ − 167); 119; 77 (M+ − 172), 55 | ✓ | |||
10 | 4-Amino-N-methyl phthalimide | ![]() |
176 | 37.76 | 176 (M+); 132 (M+ − 44), 119 (M+ − 57); 91 (M+ − 85), 77 (M+ − 95) | ✓ | |||
11 | 2-Hydroxy-3-methyl-1,4-naphthoquinone | ![]() |
188 | 38.78 | 188 (M+); 160 (M+ − 28); 132 (M+ − 56); 105 (M+ − 83); 77 (M+ − 111); 55 (M+ − 133) | ✓ | |||
12 | N,N′-Bis(allyl)-1,4-benzene dicarboxamide | ![]() |
244 | 38.56 | 244 (M+); 188 (M+ − 56), 160 (M+ − 84), 132 (M+ − 110); 104 (M+ − 140), 75 (M+ − 168), 56, 41 | ✓ | |||
13 | 1,2,4,5-Tetrazin-3-amine | ![]() |
97 | 4.09 | 97 (M+); 69 (M+ − 28); 55 (M+ − 42); 42 (M+ − 55) | ✓ |
Furthermore, after reaction with RO16, the TDFCR was thoroughly washed with water and dried. The compounds that may have remained adsorbed on the TDFCA were extracted by equilibrating the reactor with 120 mL MeOH for 1 h. The MeOH extract was evaporated to dryness under a mild N2 flow. The remaining solid residue was reconstituted in 2 mL acetonitrile and analyzed. The extract contained several degradation products (Sr. no. 8–13 in Table 2), including 4-amino-N-pthalimide, substituted napthadione, and substituted benzamide, which are highly relevant to the structures of the dye compounds used in this study.
It may be inferred that as the adsorbed dye is electro oxidized at the TDCA, more dye is adsorbed, which undergoes further electro oxidation. Generally, the electrochemical oxidation of organic compounds proceeds through the generation of OH radicals, H2O2, Cl2/OCl−. In the absence of sufficient Cl− concentration, the reaction mainly involves OH radicals produced through water oxidation at the anode, while the complementary reaction at the cathode is usually hydrogen generation. Chen33 reported various aspects of the degradation mechanism during electrochemical oxidation and Rao et al.26 described the role of carbon particles in TDR. In this study, the reaction may be assumed to involve oxidation by the in situ generated OH radicals and chlorine based oxidants.
Based on the results, the various reactions that contribute to the overall electro oxidation of dyes in TDFCR can be written as follows. Reactions (1)–(3) represent water and chloride oxidation at the carbon anode, whereas (4) and (5) illustrate the pH dependent disproportion of electrochemically generated molecular chlorine. The electro oxidation of dye molecules involving hydroxyl radicals and hypochlorite is represented by eqn (6).
H2O → ˙OH + H+ + e− | (1) |
Cl− → Cl˙ + e− | (2) |
Cl˙ + Cl˙ → Cl2 | (3) |
Cl2 + H2O → HOCl + H+ + Cl− | (4) |
HOCl → H+ + OCl− | (5) |
Dye + ˙OH + (Cl2/H2O) → CO2 + intermediates + inorganic anions + water | (6) |
Though the results suggest that the dye structures are transformed and partially degraded, it was found that some intermediates were formed during electro oxidation in the TDFCR. If these intermediates are present in the treated solutions they can be expected to give rise to absorption peaks particularly in the UV range. However, in the UV-visible spectra of the treated RB-4 solutions it was observed that most spectral features, including those in the UV range, were absent, which implies that all these intermediates either remain adsorbed on the carbon or undergo degradation in the TDFCR. Since carbon is a good adsorbent for many organic compounds, it may be possible that these intermediates remain adsorbed on the carbon. To verify this, we conducted some additional electro oxidation experiments with RB-4 using GAC particulate bed in TDR (6 h electro oxidation reaction of 1000 mg L−1 RB4 solution), and the recovered GAC was processed for recording FT-IR spectrum in ATR mode. The IR spectrum of the recovered carbon was compared with that of carbon deliberately adsorbed with RB-4. The adsorbed RB-4 on carbon showed peaks at 2980–3050, 1743, 1580–1440, 1246, and 979 cm−1. These are attributable to the stretching frequencies of the aromatic –C–H, quinone CO, aromatic –C
C–&–C
N–, –SO3H group and C–Cl bonds, respectively. On the other hand, the recovered carbon prominently displayed stretching vibrations at 2980–3050 due to the aromatic –C–H bond, 1494 cm−1 due to the –C
N– bond of chlorotriazine, and 979 cm−1 due to the C–Cl groups of chlorotriazine. The other RB-4 peaks observed at 1743 cm−1 and 1246 cm−1 were clearly absent, indicating that the –SO3H groups were detached and that the anthraquinone mainframe structure was also destroyed during electro oxidation. The main degradation product appears to be chlorotriazine groups as their presence is identified by the peaks at 1494 cm−1 and 979 cm−1. The chlorotriazine groups were also reported as intermediates in similar studies by other researchers.34 Di Giulio et al.34 reported the chromatographic analysis of RB 4 electro oxidation products and observed that the molecule was broken into three fragments i.e., amino anthraquinone sulfonic acid, amino benzene sulfonic acid and the triazinic ring. The triazinic group was reported to resist electrooxidative treatment. On the basis of the GC-MS data of the MTBE extracts of the reacted dye solutions and that of the reactor washings, we may conclude that in this study, intermediates were also formed; moreover, the aqueous phase mainly appears to contain smaller molecular fragments while the larger intermediates are partitioned onto the carbon anode.
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Fig. 5 Schematics and experimental set-up of the flow through three-dimensional reactor (TDFCR) in (a) batch mode, and (b) continuous mode. |
The electrochemical station was equipped with General Purpose Electrochemical System software (Eco Chemie, the Netherlands). The electrode potential was varied from −0.75 to +1.25 V for RO-16, and from −0.75 to +0.75 V for RR-2 and RB-4 at a scan rate of 0.01 V s−1. Sodium sulphate was used as the supporting electrolyte and pure nitrogen gas was used for de-aeration of the test samples for CV studies. A Model JEOL JXA-840A Scanning Electron Microscope (SEM) was used for examining the surface morphology of the carbon. Infrared spectra (ATR) were recorded on a Perkin Elmer Auto Image FTIR spectrometer (Spectrum 100) to ascertain the presence of RB4 and intermediate products on the GAC particles. The electrical resistance of the dry TDCA was measured using an EXCEL DT 9205A digital multimeter. The compressive strength of the TDCA having different weight proportions of PVDF was determined using a hydraulic press (PCI, Mumbai, 0–4000 psi). Nitrogen adsorption and desorption isotherms were measured at −196 °C using a Micromeritics Tristar 3000 gas adsorption analyzer. Before the measurement, the samples were degassed under vacuum (10−4 mmHg at 200 °C) for at least 4 h. The pore size distributions were derived from the adsorption isotherm using the Barrett–Joyner–Halenda (BJH) model.
The samples for GC-MS analysis of the degradation products were processed as follows. Approximately 250 mL of reacted dye solution was placed in a conical flask, and the pH was adjusted to greater than 10 using sodium hydroxide. Then sodium chloride was added until the saturation point was reached. The sample then was extracted twice with 25 mL of methyl-tert-butyl ether (MTBE). The ether fractions were combined; and, moisture was removed over anhydrous sodium sulfate and the fractions were evaporated to dryness. Finally, the extract was reconstituted in 2 mL of methanol for GC/MS analysis. Alternatively, post reaction with RO16 for 16 h, the TDFCR was thoroughly washed with water and dried. The compounds that had possibly remained adsorbed on the TDFCA were extracted by equilibrating the reactor with 120 mL MeOH for 1 h. The MeOH extract was evaporated to dryness under nitrogen gas. The remaining solid residue was reconstituted in 2 mL acetonitrile. The samples were analyzed using GC-MS (Perkin Elmer Clarus 680 GC coupled with 600 C MS, single quadruple MSD) and separation was achieved on an RTX-5 capillary column (30 m length, 0.25 mm ID, 0.25 μm film thickness). The following temperature program was applied: oven temperature, 60 °C (5 min hold time), 60–280 °C@5 °C min−1, final temperature 280 °C (hold for 6 min.); split-less capillary injector temperature, 280 °C; carrier gas: helium@1 mL min−1. The intermediates were identified by comparing with the standard azo dye mix 1 (10 μg mL−1 in acetonitrile) as well as the NIST Library.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13550d |
This journal is © The Royal Society of Chemistry 2015 |