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Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: Radical versus non-radical mechanism
Accepted Manuscript Title: Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism Author: Jiabin Chen Liming Zhang Tianyin Huang Wenwei Li Ying Wang Zhongming Wang PII: DOI: Reference:
S0304-3894(16)30670-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.07.038 HAZMAT 17894
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-3-2016 29-6-2016 17-7-2016
Please cite this article as: Jiabin Chen, Liming Zhang, Tianyin Huang, Wenwei Li, Ying Wang, Zhongming Wang, Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.07.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism
Jiabin Chen a, Liming Zhang a, Tianyin Huang a,*, Wenwei Li b, Ying Wang a, Zhongming Wang a a
School of Environmental Science and Engineering, Suzhou University of Science and
Technology, Suzhou, 215001, P. R. China b
School of Chemistry and Materials Science, University of Science and Technology of China,
Hefei, 230026, P. R. China
* Corresponding Authors. Phone: +86 0512 68096895; Fax: +86 0512 68096895. Email: [email protected] (Tianyin Huang).
Manuscript submitted to Journal of Hazardous Materials
June 28, 2016 (Manuscript word count: 4989)
1
Highlights
Carbon nanotube effectively activated peroxymonosulfate
Radical-induced decolorization took place on the surface of carbon nanotube
Cl- exhibited a dual effect on orange acid 7 decolorization
High dosage of Cl- induced rapid decolorization without generation of radicals
Different products and mineralization appeared in the presence/absence of Cl-
Abstract Carbon nanotube (CNT) has been shown to effectively activate peroxymonosulfate (PMS) to remove contaminants, whereas controversial activation mechanisms (radical vs non-radical mechanism) were previously proposed. Here we report that radical-induced decolorization of acid orange 7 (AO7) dominated in the CNT activated PMS system, but non-radical mechanism was also involved at high Cl- concentration. CNT exhibited high activity in activating PMS to decolorize AO7. The decolorization rate of AO7 increased with increasing PMS dosages and CNT loadings, rising temperature and higher pH. Radical quenching and photoluminescence techniques confirmed the decolorization of AO7 in the CNT/PMS system was caused by the radical oxidation, which dominantly took place on the surface of CNT, rather than the bulk solution. The presence of Cl- exhibited a dual effect on AO7 decolorization. Low concentration of Cl- slightly inhibited AO7 decolorization, but further raising the concentration to above 0.1 M significantly accelerated its decolorizaition. Cl- was confirmed to react with PMS to generate HClO, which effectively bleached AO7 through non-radical process rather than radical process. The decolorization of AO7 induced from the non-radical process exhibited different degradation products and less mineralization in comparison to that derived from radical process.
Keywords: peroxymonosulfate, carbon nanotube, azo dye, chloride, mechanism.
1. Introduction Azo dyes, characterized by azo bonds (-N=N-), are the most widely used dyes in modern industries, such as textile manufacture, paper printing, and leather processing [1, 2]. Their 2
accumulation in the environment pose potential risks on the eco-environment and human health due to their toxicity, carcinogenic and mutagenic nature [3]. Most azo dyes are bio-refractory in aerobic treatment and are usually reduced under anaerobic condition to more toxic intermediates. Hence they are generally not amenable for conventional wastewater treatment process [4, 5]. In recent years, some advanced oxidation processes (AOPs), such as Fenton and Fentonlike reactions, photo-catalysis and electrochemical treatment have emerged as promising alternatives to treat persistent contaminants, e.g., azo dyes. Among the various AOPs for degradation of the recalcitrant pollutants, sulfate radical (SO4·-)-based AOPs (SR-AOPs) have attracted considerable attentions [6]. Generally, SO4·- possessing a high standard redox potential (2.5-3.1 V), could be generated by activating peroxymonosulfate (PMS) or persulfate (PS) with base, heat, UV, transition metal [7]. However, the wide application of heating and UV irradiation method is limited due to high cost and energy input [8]. Although metal ion is one of the most frequently used activators for PMS owing to their low cost and natural abundance, the challenges of its reuse and toxicity also restrict its widespread application in the homogeneous activation process. Employment of supported metal as heterogeneous activator can overcome the drawbacks of the homogeneous activation to some extent, but metal leaching can not be completely avoided [9]. Metal-free carbonaceous materials can be used as a promising alternative due to the prevention of metal leaching and secondary contamination to water environment [10]. Activated carbon (AC) has been suggested as a catalyst of electron-transfer mediator owing to the oxygen functional groups on the surface [11, 12]. AC has been used to activate PMS (or PS) for AO7 degradation at ambient temperature, and remarkable synergistic effect exists in the AC/PMS combined system [13, 14]. Nanostructured carbons, such as carbon nanotube (CNT), have also demonstrated to be effective in various catalytic processes due to their high surface area, favorable thermal conductivity, and sp2-hybridized carbon configuration [15, 16]. Recently, CNT has been reported to be effective to activate PMS to degrade phenolic compound. Moreover, PMS activation was related to the type and surface property of CNT [10, 17]. However, controversial activation mechanisms (radical versus non-radical mechanism) were previously proposed. For example, Sun et al. proposed that the sp2 carbons and oxygen3
containing functional groups on the surface could effectively activate PMS to generate SO 4·for oxidation of phenols [10]. While Lee et al. proposed that oxidation of phenols was induced by the reactive complex of CNT and PMS without generation of SO4·- [17]. The controversial mechanism of CNT activated PMS needs to be further explored. On the other hand, CNT activated PMS has not been applied to treat azo dye wastewater before. Noting the recalcitrant nature of dye wastewater derived from high salinity, high temperature and variable pHs, it was imperative to evaluate the impact of different operating conditions (e.g., NaCl, pH and temperature) on azo dye degradation. Herein, we investigated the performance of CNT activated PMS on the decolorization of a model azo dye, acid orange 7 (AO7). The impact of different operating condition parameters e.g., NaCl, pH, and temperature on AO7 decolorization were evaluated. In particular, the role of Cl- in AO7 decolorization was explored in the CNT/PMS system. Radical quenching experiment and photoluminescence technique were applied to investigate whether radicals were generated in the CNT/PMS system. Afterwards, the UV-vis spectrum and the TOC were monitored to assess the product characteristic and mineralization of AO7, respectively. Finally, the activation mechanism of PMS by CNT was proposed in the presence/absence of Cl-.
2. Materials and Methods 2.1. Chemicals Sources of chemicals and materials are provided in the Supporting Information (SI) Text S1. 2.2. Characterization of CNT X-Ray diffraction (XRD) patterns of CNT were analyzed on Bruker D8 Advance Power Xray diffractometer (Germany) with a Co Kα at 40 kV and 40 mA. The morphology of CNT was analyzed on a scanning electron microscope (SEM, FEI Quanta250, USA). Raman spectra were recorded on a DXR Raman microscope (Thermo Fisher Scientific, USA). 2.3. Experimental procedure The batch experiments were conducted in 250-ml glass flasks at room temperature (23 ± 2 C). The solution in the flasks were constantly mixed by magnetic stirring. Reactions were initiated by adding CNT to the flasks containing AO7 and PMS. The sample aliquots taken at 4
the predetermined time intervals were quenched with excess NaNO2 immediately. Afterwards, the quenched sample was filtered through 0.45 μm membrane, and then analyzed on a UV-vis spectrophotometer. The initial pH of the solution was adjusted by H2SO4 and NaOH to the designated value. All the experiments were conducted in duplicate or more. 2.4. Analytical methods The concentration of AO7 was monitored by measuring the maximum absorbance at λ = 484 nm on a UV-vis spectrophotometer (Mapada UV-1600 PC). The wavelength scan (200 – 650 nm) of AO7 during decolorization was also performed on this spectrophotometer. TOC was measured by Shimadzu TOC-LCPH analyzer. Metal impurities in CNT were determined on an ICP-MS (Agilent 7500). The concentration of PMS was measured by iodometric titration [18, 19]. A photoluminescence technique with terephthalic acid as probe compound was used to detect the generated HO· [20], and the photoluminescence intensity was measured on a Hitachi F-4500 fluorescence spectrophotometer with 315 nm and 425 nm for excitation and emission wavelength, respectively. Degradation products of AO7 were analyzed by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GCMS) (Text S2).
3. Results and Discussion 3.1. Characterization of CNT The SEM image of CNT was shown in Fig. 1A. CNT was stacked onto each other, and each CNT was several microns long. The XRD patterns of CNT before and after reaction were shown in Fig. S1. The diffraction peaks at 26.5 and 42.4 could be attributed to the hexagonal graphite structures (002) and (100) [21, 22]. The XRD of CNT after reaction was almost identical to that before reaction, indicating CNT remained graphitic structure. The Raman spectra of CNT were shown in Fig. 1B. Both spectra exhibited two distinct bands in the spectral region between 1000 and 2000 cm-1 (D and G band). The D band at around 1340 cm-1 reflects the disorder and defects in the carbon lattice or the presence of amorphous carbon [23, 24]. While the G band at 1500-1600 cm-1 signify the in-plane tangential stretching of the carbon-carbon bonds in the graphene structure [25]. The ratio between the intensities of these two bands (ID/IG) has been used to assess the purity, and extent of functionalization of 5
CNT. After the modification of CNT, this ratio always increases [24, 26]. Indeed, the ratios of ID/IG slightly increased after the reaction in this work (from 0.8 to 1.05). The CNT surface might get oxidized as oxygenated species after the reaction, which could increase the extent of functionalization of CNT surface, thus induced the increase of ID/IG. The G’ band is a Raman characteristic region which is sensitive to the charge transfer effect on the surface of CNT [24, 27]. No significant shift of G’ band was observed after reaction, thus indicating no evidences of covalent bonding between CNT and organic chemicals. 3.2. Decolorization of AO7 by PMS activated by CNT Decolorization of AO7 was investigated in the presence of CNT and PMS (Fig. 2A). CNT or PMS alone showed slight decolorization efficiency for AO7, whereas combination of CNT and PMS exhibited remarkable decolorization. To be specific, about 10% and 20% AO7 were decolorized within 50 min in the presence of PMS and CNT, respectively. Complete decolorization, however, was achieved within 50 min in the CNT/PMS system. Rapid decolorization of AO7 indicated a synergistic effect of PMS and CNT during the reaction. Such synergistic effect was possibly due to the activation of PMS by CNT. AC activated PMS has been well documented before [13]. Only slight decolorization of AO7 was observed in the presence of AC (0.1 g/L) and PMS. When AC loading increased to 1 g/L, AO7 decolorization was still much lower than that in the CNT/PMS system (Fig. 2A). Hence, CNT exhibited a much better performance on activating PMS to decolorize AO7 (Text S3, Fig. S2). The decomposition of PMS was also observed in the CNT/PMS system (Fig. 2B). In the absence of AO7, however, the decomposition of PMS was still observed, and the decomposition efficiency was faster than that in the presence of AO7. This result indicated that CNT was effective to activate PMS. Moreover, the reduction in the decomposition efficiency of PMS in the presence of AO7 might be attributed to the loading of AO7 or its intermediates, which could reduce the active surface sites available for PMS activation [14]. We further investigated the potential effect of metal residues in CNT. CNT was treated in nitric acid, and the dissolved metal ions were analyzed by ICP-MS. The results indicated that metal impurities, such as Fe, Co were present in CNT (Table S1). The acid-pretreated CNT exhibited similar activity to pristine CNT for PMS activation to decolorize AO7 (Fig. S3). Moreover, the acid solution from the treatment slightly decolorize AO7 in the presence of PMS. 6
Therefore, the contribution of metal impurities to AO7 decolorization in the CNT/PMS system could be excluded. Effect of CNT loadings and PMS dosages on AO7 decolorization in the CNT/PMS system were explored. The decolorization of AO7 well conformed to the pseudo first-order kinetics (Fig. 3A). The rate constants of AO7 increased with increasing CNT loadings or PMS dosages. The increment amount of rate constants was faster at higher CNT loadings, which might be explained by the larger amount of AO7 adsorbed on CNT when higher loadings of CNT were present (Fig. S4). Nonetheless, plots of rate constants and PMS dosage approximated to the linear relationship, which might be attributed to the relatively low dosages of PMS used in this work. CNT rendered enough reactive sites for PMS activation. The proportion of rate constants with the oxidant dosages was also observed in degradation of MTBE [28] and carbamazepine [29] in SR-AOPs. 3.3. Impact of pH, temperature and Cl- on AO7 decolorization Dye wastewater typically has broad pH variation, large temperature fluctuations, and high salt concentration. Thus we then investigated the impacts of such parameters on the decolorization of AO7. 3.3.1. Impact of pH The pH impact on AO7 decolorization in the CNT/PMS system was shown in Fig. 3B. The rate constants increased from pH 2 to pH 6, and then levelled off before it reached pH 8. Thereafter, there was a remarkable increase when pH further increased to 10. It was noted that significant decolorization of AO7 was observed at pH 10 in the presence of PMS alone owing to the base activated PMS. If the impact of base activated PMS was subtracted, the rate constants levelled off from pH 6 to pH 10. The pHpzc of CNT was determined to be 4.98, thus the surface of CNT was positively charged at pH 2 and 4, but negatively charged at pH 6 or higher. Acidic pHs (e.g., pH 2 and 4) were generally favorable for the adsorption of AO7, a anionic dye, on CNT (Fig. S5), but unfavorable for CNT activated PMS because some active sites of CNT were occupied by AO7 [14]. At pH 2, the formation of H-band between H+ and – O-O- group of PMS was assumed to be more significant, attaching positive charge to PMS, thus inhibiting its interaction with the positive CNT surface [30]. Therefore, the rate constant of AO7 at pH 2 was much lower than those at other pHs. 7
3.3.2. Impact of temperatures Effect of temperature on AO7 decolorization in the CNT/PMS system was shown in Fig. 4A. AO7 decolorization increased with elevated temperatures. Moreover, the rate constants well conformed to Arrhenius behavior, generating the activation energy (Ea) of 45.64 kJ/mol. Ea of AO7 in this work was slight lower than the values in PMS activated by AC/Co (59.7 kJ/mol) [31] or AC/ RuO2 (61.4 kJ/mol) [32], indicating the reaction in this work might take place easily. 3.3.3. Impact of ClThe impact of salts, e.g., NaCl, on AO7 decolorization was shown in Fig. 4B. NaCl exhibited promoting effect on AO7 adsorption, and such promoting effect gradually increased with increasing dosage of NaCl. The addition of salts could increase the dye dimerization in solutions, and the aggregation of dye molecules forced by the action of salt ions would further increase the extent of dye adsorbed onto CNT [33]. In the CNT/PMS system, however, NaCl with different concentrations showed a dual effect on AO7 decolorization. For example, a slight decrease of AO7 decolorization was observed upon addition of NaCl (0.01-0.05 M), but further addition of NaCl (0.1–0.3 M) clearly accelerated its decolorization (Fig. 4B). Interestingly, decolorization of AO7 was also observed in the presence of PMS and NaCl, and complete decolorization of AO7 could be achieved after 30 min when 0.3 M of NaCl was added into PMS (Fig. 4B). Therefore, Cl- might participate in AO7 decolorization in the CNT/PMS system, which would be addressed in the following sections. 3.4. Activation mechanism of PMS in the absence of ClGenerally, the mechanism of CNT activated PMS was controversial, with two alternative mechanisms previously proposed, i.e. radical process [10] versus non radical process [17]. The non-radical process proceeded via the formation of outer-sphere complexes between CNT and PMS, increasing its reactivity towards target compound. It was reported that the increase of ionic strength significantly influenced the out-sphere interactions between solid surface and solute [34]. The increasing ionic strength (i.e. NaClO4) did not influence AO7 decolorization in the CNT/PMS system (Fig. S6), indicating activation of PMS by CNT was not resulted from the outer-sphere interactions (Text S4). In order to check whether radicals were generated or not, the radical quenching experiment 8
was frequently used in SR-AOP. In a previous study about CNT activated PMS, the addition of excess radical scavengers (MeOH and dimethyl sulfoxide (DMSO)) exhibited negligible effect on phenol degradation, thus non-radical mechanism was considered as the main reaction mechanism [17]. Indeed, MeOH and TBA slightly depressed the decolorization of AO7 in the CNT/PMS system (Fig. 5A). However, it should be noted that CNT activated PMS belonged to the heterogeneous activation. Both MeOH and TBA are hydrophilic compounds, thus are relatively difficult to approach to the surface of the heterogeneous catalyst [35]. In contrast, phenol (a strong scavenger for both SO4·- and HO·) is relatively hydrophobic, thus is easier to approach to the surface [36, 37]. As shown in Fig. 5A, phenol exhibited a remarkable inhibitory effect on AO7 decolorization even when the phenol/AO7 ratio was 100 in the CNT/PMS system. When the ratio was further raised to 2000, AO7 decolorization was almost identical to its adsorption on CNT (Fig. 5A vs Fig. 2A). It was thus suggested that the radical-induced decolorization of AO7 mainly took place on the surface of CNT, consistent with the results in the activation of PMS by AC [13]. To further verify that the radicals bounded on the CNT surface were responsible for AO7 decolorization, KI was used as a radical quencher in the CNT/PMS system, because KI can react with surface-bound free radicals [38, 39]. Indeed, the addition of KI almost completely retarded the oxidative degradation of AO7 (Fig. 5B). In addition, PMS could be still detected after the complete decolorization of AO7 (data not shown), indicating that the depression of AO7 degradation was not resulted from the consumption of PMS by KI. Therefore, the surfacebound reactive radicals (i.e., SO4·- and HO·) played a dominant role in AO7 decolorization. The formation of HO· on the surface of CNT was further detected by a photoluminescence technique with terephthalic acid as a probe compound. The HO· can quantitatively convert nonfluorescent terephthalic acid to highly fluorescent product, 2-hydroxyterephthalic acid with a maximum emission at 425 nm [20, 40]. The intensity of the fluorescent peak at 425 nm is proportional to the amount of HO· in the system. As shown in Fig. 5C, an intensive fluorescent peak was observed in PMS solution after the addition of CNT, and the intensity at the maximum emission gradually increased as the reaction proceeded. This evidence strongly suggested the formation of HO· in the CNT/PMS system. Therefore, the activation mechanism of PMS by CNT was radical mechanism rather than non-radical mechanism. In the previous studies, both 9
sp2-hybridized carbon and oxygen-containing groups were assumed to contribute to the catalytic activity of nanocarbons, such as reduced graphene oxide (rGO) [41] and CNT [10]. Also, the catalytic activity of rGO was proved to be related to the content of oxygen-containing groups [42]. Therefore, sp2 carbon and oxygen-containing groups on CNT could activate PMS to generate SO4·- and HO·, and then decolorize AO7. 3.5. Activation mechanism of PMS in the presence of ClThe influence of Cl- on contaminant degradation in SR-AOPs was dependent on the target pollutant [43]. For instance, addition of Cl- accelerated 2,4-DCP degradation during SO4·oxidation [44], but slightly inhibited carbamazepine degradation [45], and even significantly reduced iopromide degradation [30]. In the activated PMS process, Cl- could be oxidized by SO4·- to form Cl·, and subsequent transformed to Cl2·- and other chlorine radical species (equations 1-6). Also, Cl- could participate in PMS decomposition via non-radical mechanism, generating HClO and Cl2 (equations 7-8) [46, 47]. Therefore, three types of reactive species might contribute to the overall contaminant degradation, i.e. 1) HO· and SO4·-, 2) HClO and Cl2, and 3) Cl·, Cl2·-, and other chlorine radical species. Cl· and other chlorine radical species are significantly less reactive to AO7 than SO4·-, whereas HClO could rapidly decolorize the azo dyes [46, 48]. Therefore, the adverse effect of Cl- at low concentration could be explained by the consumption of PMS and SO4·- by Cl-, and formation of less reactive chlorine radical species. On the contrary, the accelerating effect of Cl- at higher concentration might be resulted from the generation of highly reactive HClO. SO4·- + Cl- → SO42- + Cl·
(1)
Cl· + Cl- → Cl2·-
(2)
Cl2·- + Cl2·- → Cl2 + 2Cl-
(3)
H2O + Cl2·- →-ClOH·- + H+ + Cl-
(4)
ClOH·- + H+ → Cl· + H2O
(5)
ClOH·- →Cl- + HO·
(6)
HSO5- + Cl- →SO42- + HClO
(7)
HSO5- + 2Cl- + H+ →SO42- + Cl2 + H2O
(8)
To verify the involvement of HClO in AO7 decolorization in the CNT/PMS system in the presence of high concentration of NaCl, we subsequently carried out the following experiments. 10
1) decolorization of AO7 in the presence of PMS and NaCl. According to equations 7-8, HClO (Cl2) could be directly generated from redox reaction between PMS and Cl- without the involvement of radical species. Thus we hypothesized that AO7 was susceptible to decolorization in the solution containing both PMS and NaCl. Indeed, AO7 was stable in the presence of PMS alone, but was rapidly decolorized after addition of NaCl into PMS (Fig. 4B ). Moreover, the decolorization rate of AO7 increased with increasing dosages of Cl-. This experiment provided a direct evidence for the involvement of HClO in AO7 decolorization. 2) Simultaneous addition of radical quencher (i.e., phenol) and NaCl in the CNT/PMS system. Addition of excess phenol into CNT/PMS system could completely scavenge the radicals of SO4·- and HO·. However, further addition of NaCl could promote AO7 decolorization, and the accelerating effect was apparent as high dosage of NaCl was added (Fig. 6A). It was thus suggested that AO7 decolorization was not derived from the radical-induced reaction, but most likely from other reactive oxidants, e.g., HClO. 3) Addition of NH4+ in the CNT/PMS/NaCl system. Significant decrease of AO7 decolorization was observed after addition of NH4+ from 0.01 to 0.1 M (Fig. 6B). NH4+ could react with HClO to generate chloramines, a weak oxidants in comparison to HClO [49, 50]. All these evidences strongly suggested that HClO participated in the decolorization of AO7. We further monitored the UV-vis spectra of AO7 solution during the reaction in CNT/PMS (Fig. 7A), NaCl/PMS (Fig. S7) and CNT/NaCl/PMS systems (Fig. S8). Four apparent peaks were observed in the spectrum of AO7. The peaks at 230 and 310 nm represented the benzene and naphthalene rings on AO7, respectively [51], while the other peaks of 430 and 484 nm reflected the hydrazone form and azo form [52]. In all the three systems, the four peaks gradually decreased as the reaction proceeded, and a new peak appeared at about 250 nm (Text S5). To better investigate the spectra characteristics of AO7 solution, we then compared the UV-vis spectra in different systems when about 80% AO7 was decolorized (Fig. 7B). An apparent band shift was observed from 280 to 380 nm as PMS and 0.3 M NaCl were present. Moreover, the new peak was apparently shifted to 245 nm in the NaCl/PMS system in comparison to 254 nm in the CNT/PMS system. These evidences clearly indicated that the degradation products induced by HClO were significantly different from that induced by reactive radical species. In the CNT/PMS/0.3 M NaCl system, the new peak at 250 nm could 11
be attributed to the product mixture generated from HClO oxidation and radical oxidation. To better clarify the degradation products from radical process (i.e. CNT/PMS) and nonradical process (i.e. NaCl/PMS), we further analyzed the degradation products of AO7 by HPLC. As shown in Fig. 8A, the parent compound AO7 was observed at the retention time (RT) of about 13.50 min. In the CNT/PMS and NaCl/PMS system, several new peaks also appeared when about 80% AO7 was decolorized, indicating formation of degradation products. Besides the common products at about 2.5 3.5 and 7.3 min in both systems, other new products were also observed in the NaCl/PMS system. This result indicated that the degradation products in the NaCl/PMS system were much more complicated. The degradation products of AO7 in these systems were further identified by GC-MS, and the proposed structures were shown in Fig. 8B. Six products were identified in the CNT/PMS system. They could be classified into three groups: three naphthalene-type compounds (1-nitro-2-naphthalenol, coumarin, and 1,2naphthoquinone), one fused heterocyclic compound (phthalic anhydride), and two aromatic compounds (phthalic acid, benzoic acid). All these degradation products have been previously reported [46, 53-55]. Naphthalene-type products were supposed to be primary intermediates after the cleavage of azo band in AO7. Afterwards, they were subsequently oxidized to generate fused heterocyclic compounds, then other smaller molecule products, and finally mineralized to CO2 and H2O [46]. In the NaCl/PMS system, more products were identified, consistent with the results in HPLC. Besides the products observed in CNT/PMS, several chlorinated organic products were also observed, such as 7-chloro-2-naphthalenol, 2,4-dichloro-1-naphthalenol, 3chloroisocoumarin, 5-chloroisobenzofuran-1,3-dione, which were previously reported in the UV/TiO2/NaCl [56] and Co/PMS/NaCl system [46]. To make clear whether AO7 was mineralized or not, TOC variation was monitored during AO7 decolorization (Fig. 9). Negligible TOC removal was observed after the complete decolorization of AO7 (180 min) in the NaCl/PMS system, suggesting that HClO was highly efficient to decolorize the azo band of AO7, but ineffective to further mineralize it to CO2 and H2O. In the CNT/PMS and CNT/NaCl/PMS systems, TOC removal was fast in the initial 5 min, and then slowed down. About 20% TOC removal was observed at 60 min when the parent AO7 was completely decolorized. Hence, the CNT/PMS (CNT/PMS/NaCl) system could effectively destruct AO7 into small molecular products, which might be difficult to further mineralize. 12
Such low mineralization capacity was also observed in the AC activated PMS system [13]. To improve the mineralization in CNT/PMS system, CNT could be impregnated with transition metals [31] or combined with other technologies [57]. 3.6. Overall reaction scheme Generally, both radical process (I) and non-radical process (II) were involved in the activation of PMS (Fig. 10). (I) The electrons were transferred from sp2-hybridized carbon and functional groups on CNT to PMS, and thus SO4·- and HO· were generated on the surface of CNT. The radicals would decolorize the AO7 accumulated on the surface of CNT rather than AO7 in the bulk solution. (II) The Cl- in the bulk solution could be directly oxidized by PMS through two electron transfer pathway, with reactive HClO generated in the solution. HClO was effective to bleach AO7 in the bulk solution. Although the activated PMS (via radical process or non-radical process) were effective to decolorize AO7, the decolorization of AO7 was likely to proceed through different ways. The radicals generated from the activated PMS possessed high oxidation capacity, thus effectively destruct AO7, and finally mineralize to CO2 and H2O. On the other hand, AO7 could be rapidly bleached by HClO generated from the non-radical process, but is possibly transformed to other oxidized products, rather than mineralized to CO2 and H2O.
4. Conclusions CNT exhibited excellent performance in heterogeneously activating PMS to decolorize the azo dye, i.e. AO7. Decolorization efficiency of AO7 increased with increasing pH, raising PMS dosage and CNT loadings. The decolorization of AO7 conformed to pseudo first-order kinetics at different temperatures, and the activation energy was determined to be 45.64 kJ/mol. The decolorization of AO7 dominantly took place on the surface of CNT, rather than the bulk solution. Such decolorization was induced by the surface-bound radicals generated from PMS activated by CNT. Interestingly, NaCl, a common component in the dye wastewater, exerted dual effect in the decolorization of AO7, i.e. inhibitory effect on AO7 decolorization at low concentration, but accelerating effect at high concentration. Cl- was directly oxidized by PMS to generate HClO, which was effective to decolorize AO7 through non-radical reaction. In comparison to the radical-induced decolorization of AO7, different product profiles and less 13
mineralization were derived from that through non-radical process.
Acknowledgements We sincerely thank the National Natural Science Foundation of China (51478283, 51509175) for financially supporting this work.
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References [1] H. Lin, H. Zhang, L. Hou, Degradation of C. I. Acid Orange 7 in aqueous solution by a novel electro/Fe3O4/PDS process, J. Hazard. Mater. 276 (2014) 182-191. [2] I. Grčić, S. Papić, N. Koprivanac, I. Kovačić, Kinetic modeling and synergy quantification in sono and photooxidative treatment of simulated dyehouse effluent, Water Res. 46 (2012) 5683-5695. [3] C. Cai, H. Zhang, X. Zhong, L. Hou, Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a bimetallic Fe–Co/SBA-15 catalyst for the degradation of Orange II in water, J. Hazard. Mater. 283 (2015) 70-79. [4] M. Wang, X. Liu, B. Pan, S. Zhang, Photodegradation of Acid Orange 7 in a UV/acetylacetone process, Chemosphere 93 (2013) 2877-2882. [5] D. Han, J. Wan, Y. Ma, Y. Wang, M. Huang, Y. Chen, D. Li, Z. Guan, Y. Li, Enhanced decolorization of Orange G in a Fe(II)-EDDS activated persulfate process by accelerating the regeneration of ferrous iron with hydroxylamine, Chem. Eng. J. 256 (2014) 316-323. [6] Y. Qian, X. Zhou, Y. Zhang, P. Sun, W. Zhang, J. Chen, X. Guo, X. Zhang, Performance of alpha-methylnaphthalene degradation by dual oxidant of persulfate/calcium peroxide: Implication for ISCO, Chem. Eng. J. 279 (2015) 538-546. [7] B.T. Zhang, Y. Zhang, Y.H. Teng, M.H. Fan, Sulfate Radical and Its Application in Decontamination Technologies, Crit. Rev. Env. Sci. Tec. 45 (2015) 1756-1800. [8] H. Liu, T.A. Bruton, F.M. Doyle, D.L. Sedlak, In Situ Chemical Oxidation of Contaminated Groundwater by Persulfate: Decomposition by Fe(III)- and Mn(IV)-Containing Oxides and Aquifer Materials, Environ. Sci. Technol. 48 (2014) 10330-10336. [9] H. Sun, H. Liang, G. Zhou, S. Wang, Supported cobalt catalysts by one-pot aqueous combustion synthesis for catalytic phenol degradation, J. Colloid Interf. Sci. 394 (2013) 394-400. [10] H. Sun, C. Kwan, A. Suvorova, H.M. Ang, M.O. Tadé, S. Wang, Catalytic oxidation of organic pollutants on pristine and surface nitrogen-modified Carbon nanotubes with sulfate radicals, Appl. Catal. B-Environ. 154–155 (2014) 134-141. [11] M. Kimura, I. Miyamoto, Discovery of the Activated-Carbon Radical AC+ and the Novel Oxidation-Reactions Comprising the AC/AC+ Cycle as a Catalyst in an Aqueous Solution, 15
B. Chem. Soc. Jpn. 67 (1994) 2357-2360. [12] C. Liang, Y.T. Lin, W.H. Shih, Treatment of Trichloroethylene by Adsorption and Persulfate Oxidation in Batch Studies, Ind. Eng. Chem. Res. 48 (2009) 8373-8380. [13] J. Zhang, X.T. Shao, C. Shi, S.Y. Yang, Decolorization of Acid Orange 7 with peroxymonosulfate oxidation catalyzed by granular activated Carbon, Chem. Eng. J. 232 (2013) 259-265. [14] S. Yang, X. Yang, X. Shao, R. Niu, L. Wang, Activated Carbon catalyzed persulfate oxidation of Azo dye acid orange 7 at ambient temperature, J. Hazard. Mater. 186 (2011) 659-666. [15] J. Zhang, D. Su, A. Zhang, D. Wang, R. Schlögl, C. Hébert, NanoCarbon as robust catalyst: Mechanistic insight into Carbon-mediated catalysis, Angew. Chem. Int. Edit. 46 (2007) 7319-7323. [16] J. Luo, F. Peng, H. Yu, H. Wang, W. Zheng, Aerobic Liquid-Phase Oxidation of Ethylbenzene to Acetophenone Catalyzed by Carbon Nanotubes, ChemCatChem 5 (2013) 1578-1586. [17] H. Lee, H.J. Lee, J. Jeong, J. Lee, N.B. Park, C. Lee, Activation of persulfates by Carbon nanotubes: Oxidation of organic compounds by nonradical mechanism, Chem. Eng. J. 266 (2015) 28-33. [18] R.E. Ball, J.O. Edwards, M.L. Haggett, P. Jones, A kinetic and isotopic study of the decomposition of monoperoxyphthalic acid, J. Am. Chem. Soc. 89 (1967) 2331-2333. [19] Y. Güpta, Iodometric determination of persulphate in sulphuric acid solution, Anal. Chem. 180 (1961) 260-265. [20] J. Yu, W. Wang, B. Cheng, B.L. Su, Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment, J. Phys. Chem. C 113 (2009) 6743-6750. [21] W. Li, C. Liang, W. Zhou, J. Qiu, Zhou, G. Sun, Q. Xin, Preparation and Characterization of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells, J. Phys. Chem. B 107 (2003) 6292-6299. [22] M.S. Liu, M. Ching Cheng Lin, I.T. Huang, C.C. Wang, Enhancement of thermal conductivity with Carbon nanotube for nanofluids, Int. Commun. Heat Mass 32 (2005) 16
1202-1210. [23] A. Gupta, S.R. Vidyarthi, N. Sankararamakrishnan, Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled Carbon nanotubes, J. Hazard. Mater. 274 (2014) 132-144. [24] A.P.M. Tavares, C.G. Silva, G. Dražić, A.M.T. Silva, J.M. Loureiro, J.L. Faria, Laccase immobilization over multi-walled Carbon nanotubes: Kinetic, thermodynamic and stability studies, J. Colloid Interf. Sci. 454 (2015) 52-60. [25] R.H. Gangupomu, M.L. Sattler, D. Ramirez, Comparative study of Carbon nanotubes and granular activated Carbon: Physicochemical properties and adsorption capacities, J. Hazard. Mater. 302 (2016) 362-374. [26] Y. Lu, Z. Zhu, Z. Liu, Catalytic growth of Carbon nanotubes through CHNO explosive detonation, Carbon 42 (2004) 361-370. [27] W. Yan, L. Yan, J. Duan, C. Jing, Sorption of organophosphate esters by Carbon nanotubes, J. Hazard. Mater. 273 (2014) 53-60. [28] K.C. Huang, R.A. Couttenye, G.E. Hoag, Kinetics of heat-assisted persulfate oxidation of methyl tert-butyl ether (MTBE), Chemosphere 49 (2002) 413-420. [29] J. Deng, Y. Shao, N. Gao, Y. Deng, S. Zhou, X. Hu, Thermally activated persulfate (TAP) oxidation of antiepileptic drug carbamazepine in water, Chem. Eng. J. 228 (2013) 765-771. [30] T. Zhang, H. Zhu, J.P. Croué, Production of Sulfate Radical from Peroxymonosulfate Induced by a Magnetically Separable CuFe2O4 Spinel in Water: Efficiency, Stability, and Mechanism, Environ. Sci. Technol. 47 (2013) 2784-2791. [31] P.R. Shukla, S.B. Wang, H.Q. Sun, H.M. Ang, M. Tade, Activated Carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution, Appl. Catal. B-Environ. 100 (2010) 529-534. [32] S. Muhammad, P.R. Shukla, M.O. Tadé, S. Wang, Heterogeneous activation of peroxymonosulphate by supported ruthenium catalysts for phenol degradation in water, J. Hazard. Mater. 215–216 (2012) 183-190. [33] C. Y. Kuo, C. H. Wu, J. Y. Wu, Adsorption of direct dyes from aqueous solutions by Carbon nanotubes: Determination of equilibrium, kinetics and thermodynamics parameters, J. Colloid Interf. Sci. 327 (2008) 308-315. 17
[34] T. Zhang, Y. Chen, Y.R. Wang, J. Le Roux, Y. Yang, J.P. Croue, Efficient Peroxydisulfate Activation Process Not Relying on Sulfate Radical Generation for Water Pollutant Degradation, Environ. Sci. Technol. 48 (2014) 5868-5875. [35] H.Y. Liang, Y.Q. Zhang, S.B. Huang, I. Hussain, Oxidative degradation of p-chloroaniline by copper oxidate activated persulfate, Chem. Eng. J. 218 (2013) 384-391. [36] M.E. Lindsey, M.A. Tarr, Inhibition of Hydroxyl Radical Reaction with Aromatics by Dissolved Natural Organic Matter, Environ. Sci. Technol. 34 (2000) 444-449. [37] J. Ziajka, W. Pasiuk-Bronikowska, Rate constants for atmospheric trace organics scavenging SO4−. in the Fe-catalysed autoxidation of S(IV), Atmos. Environ. 39 (2005) 1431-1438. [38] Z. Huang, H. Bao, Y. Yao, W. Lu, W. Chen, Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst, Appl. Catal. B-Environ. 154-155 (2014) 36-43. [39] L. Xu, J. Wang, Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient FentonLike Heterogeneous Catalyst for Degradation of 4-Chlorophenol, Environ. Sci. Technol. 46 (2012) 10145-10153. [40] Q. Xiao, Z. Si, J. Zhang, C. Xiao, X. Tan, Photoinduced hydroxyl radical and photocatalytic activity of samarium-doped TiO 2 nanocrystalline, J. Hazard. Mater. 150 (2008) 62-67. [41] H. Sun, S. Liu, G. Zhou, H.M. Ang, M.O. Tadé, S. Wang, Reduced Graphene Oxide for Catalytic Oxidation of Aqueous Organic Pollutants, ACS. Appl. Mater. Inter. 4 (2012) 5466-5471. [42] W. Peng, S. Liu, H. Sun, Y. Yao, L. Zhi, S. Wang, Synthesis of porous reduced graphene oxide as metal-free Carbon for adsorption and catalytic oxidation of organics in water, J. Mater. Chem. A. 1 (2013) 5854-5859. [43] Y. Yang, J.J. Pignatello, J. Ma, W.A. Mitch, Comparison of Halide Impacts on the Efficiency of Contaminant Degradation by Sulfate and Hydroxyl Radical-Based Advanced Oxidation Processes (AOPs), Environ. Sci. Technol. 48 (2014) 2344-2351. [44] G.P. Anipsitakis, D.D. Dionysiou, M.A. Gonzalez, Cobalt-Mediated Activation of Peroxymonosulfate and Sulfate Radical Attack on Phenolic Compounds. Implications of 18
Chloride Ions, Environ. Sci. Technol. 40 (2006) 1000-1007. [45] Q. Zhang, J. Chen, C. Dai, Y. Zhang, X. Zhou, Degradation of carbamazepine and toxicity evaluation using the UV/persulfate process in aqueous solution, J. Chem. Technol. Biotechnol. 90 (2015) 701-708. [46] R. Yuan, S.N. Ramjaun, Z. Wang, J. Liu, Effects of chloride ion on degradation of Acid Orange 7 by sulfate radical-based advanced oxidation process: Implications for formation of chlorinated aromatic compounds, J. Hazard. Mater. 196 (2011) 173-179. [47] K.H. Chan, W. Chu, Degradation of atrazine by cobalt-mediated activation of peroxymonosulfate: Different cobalt counteranions in homogenous process and cobalt oxide catalysts in photolytic heterogeneous process, Water Res. 43 (2009) 2513-2521. [48] Z. Wang, R. Yuan, Y. Guo, L. Xu, J. Liu, Effects of chloride ions on bleaching of azo dyes by Co2+/oxone regent: Kinetic analysis, J. Hazard. Mater. 190 (2011) 1083-1087. [49] X.Y. Lou, Y.G. Guo, D.X. Xiao, Z.H. Wang, S.Y. Lu, J.S. Liu, Rapid dye degradation with reactive oxidants generated by chloride-induced peroxymonosulfate activation, Environ Sci Pollut Res. 20 (2013) 6317-6323. [50] M. Deborde, U. von Gunten, Reactions of chlorine with inorganic and organic compounds during water treatment - Kinetics and mechanisms: A critical review, Water Res. 42 (2008) 13-51. [51] W. Feng, D. Nansheng, H. Helin, Degradation mechanism of azo dye C. I. reactive red 2 by iron powder reduction and photooxidation in aqueous solutions, Chemosphere 41 (2000) 1233-1238. [52] C. Bauer, P. Jacques, A. Kalt, Photooxidation of an azo dye induced by visible light incident on the surface of TiO2, J. Photochem. Photobiol. A-Chem. 140 (2001) 87-92. [53] H.Z. Zhao, Y. Sun, L.N. Xu, J. R. Ni, Removal of Acid Orange 7 in simulated wastewater using a three-dimensional electrode reactor: Removal mechanisms and dye degradation pathway, Chemosphere 78 (2010) 46-51. [54] X. Chen, J. Chen, X. Qiao, D. Wang, X. Cai, Performance of nanoCo3O4/peroxymonosulfate system: kinetics and mechanism study using Acid Orange 7 as a model compound, Appl. Catal. B-Environ. 80 (2008) 116-121. [55] J. Wu, H. Zhang, J. Qiu, Degradation of Acid Orange 7 in aqueous solution by a novel 19
electro/Fe2+/peroxydisulfate process, J. Hazard. Mater. 215 (2012) 138-145. [56] R. Yuan, S.N. Ramjaun, Z. Wang, J. Liu, Photocatalytic degradation and chlorination of azo dye in saline wastewater: Kinetics and AOX formation, Chem. Eng. J. 192 (2012) 171178. [57] C. Cai, L. Wang, H. Gao, L. Hou, H. Zhang, Ultrasound enhanced heterogeneous activation of peroxydisulfate by bimetallic Fe-Co/GAC catalyst for the degradation of Acid Orange 7 in water, J. Environ. Sci. 26 (2014) 1267-1273.
20
Figure captions Fig. 1 (A) SEM image, and (B) Raman spectra of CNT before (a) and after reaction (b). Fig. 2 Decolorization of AO7 (A) and decomposition of PMS (B) under different conditions. [AO7]0 = 0.057 mM, [PMS]0/[AO7]0 = 20, [CNT] = 0.1 g/L, pH0 = 7. Note: a 0.1 g/L AC, b 1 g/L AC. Fig. 3 Effect of PMS dosages, CNT loadings (A) and initial pH (B) on the rate constants (kobs) of AO7 in the CNT/PMS system. (A) [AO7]0 = 0.057 mM, pH0 = 7, (a) [CNT] = 0.1 g/L, (b) [PMS]0/[AO7]0 = 20 : 1. (B) [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1. Fig. 4 Effect of temperature (A) and NaCl (B) on AO7 decolorization in the CNT/PMS system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7. (B) Solid lines and open symbol: adsorption of AO7 in the presence of CNT and NaCl; solid lines and solid symbols: decolorization of AO7 in the presence of CNT, NaCl and PMS; dot lines and open symbols: decolorization of AO7 in the presence of PMS and NaCl. Fig. 5 Effect of alcohols, phenol (A) and KI (B) on AO7 decolorization in the CNT/PMS system; and (C) photoluminescence spectra of hydroxyl radical generated in the CNT/PMS system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, [KI] = 10 mM, pH0 = 7. Fig. 6 (A) Effect of NaCl on AO7 decolorization in the CNT/PMS/phenol system, (B) Effect of NH4+ on AO7 decolorization in the CNT/PMS/NaCl system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7, (A) [phenol] = 114 mM, (B) [NaCl] = 0.1 M. Control: AO7 decolorization in the CNT/PMS system. Fig. 7 (A) UV-vis spectra of AO7 in the CNT/PMS system; (B) UV-vis spectra variation in different activated PMS system (after about 80% AO7 removal). [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7. Control: spectrum of AO7 solution in the absence of CNT and NaCl. Fig. 8 Degradation product analysis in the CNT/PMS and NaCl/PMS systems by HPLC (A) and GC/MS (B). (A) the NCl/PMS system (a), the CNT/PMS system (b), and the standard chemical of AO7 (c). Fig. 9 TOC variation during the reaction in different systems. Fig. 10 Overall reaction scheme. 21
(A)
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(A) (a) 80.1% decolorization of AO7
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S0304-3894(16)30670-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.07.038 HAZMAT 17894
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-3-2016 29-6-2016 17-7-2016
Please cite this article as: Jiabin Chen, Liming Zhang, Tianyin Huang, Wenwei Li, Ying Wang, Zhongming Wang, Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.07.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: radical versus non-radical mechanism
Jiabin Chen a, Liming Zhang a, Tianyin Huang a,*, Wenwei Li b, Ying Wang a, Zhongming Wang a a
School of Environmental Science and Engineering, Suzhou University of Science and
Technology, Suzhou, 215001, P. R. China b
School of Chemistry and Materials Science, University of Science and Technology of China,
Hefei, 230026, P. R. China
* Corresponding Authors. Phone: +86 0512 68096895; Fax: +86 0512 68096895. Email: [email protected] (Tianyin Huang).
Manuscript submitted to Journal of Hazardous Materials
June 28, 2016 (Manuscript word count: 4989)
1
Highlights
Carbon nanotube effectively activated peroxymonosulfate
Radical-induced decolorization took place on the surface of carbon nanotube
Cl- exhibited a dual effect on orange acid 7 decolorization
High dosage of Cl- induced rapid decolorization without generation of radicals
Different products and mineralization appeared in the presence/absence of Cl-
Abstract Carbon nanotube (CNT) has been shown to effectively activate peroxymonosulfate (PMS) to remove contaminants, whereas controversial activation mechanisms (radical vs non-radical mechanism) were previously proposed. Here we report that radical-induced decolorization of acid orange 7 (AO7) dominated in the CNT activated PMS system, but non-radical mechanism was also involved at high Cl- concentration. CNT exhibited high activity in activating PMS to decolorize AO7. The decolorization rate of AO7 increased with increasing PMS dosages and CNT loadings, rising temperature and higher pH. Radical quenching and photoluminescence techniques confirmed the decolorization of AO7 in the CNT/PMS system was caused by the radical oxidation, which dominantly took place on the surface of CNT, rather than the bulk solution. The presence of Cl- exhibited a dual effect on AO7 decolorization. Low concentration of Cl- slightly inhibited AO7 decolorization, but further raising the concentration to above 0.1 M significantly accelerated its decolorizaition. Cl- was confirmed to react with PMS to generate HClO, which effectively bleached AO7 through non-radical process rather than radical process. The decolorization of AO7 induced from the non-radical process exhibited different degradation products and less mineralization in comparison to that derived from radical process.
Keywords: peroxymonosulfate, carbon nanotube, azo dye, chloride, mechanism.
1. Introduction Azo dyes, characterized by azo bonds (-N=N-), are the most widely used dyes in modern industries, such as textile manufacture, paper printing, and leather processing [1, 2]. Their 2
accumulation in the environment pose potential risks on the eco-environment and human health due to their toxicity, carcinogenic and mutagenic nature [3]. Most azo dyes are bio-refractory in aerobic treatment and are usually reduced under anaerobic condition to more toxic intermediates. Hence they are generally not amenable for conventional wastewater treatment process [4, 5]. In recent years, some advanced oxidation processes (AOPs), such as Fenton and Fentonlike reactions, photo-catalysis and electrochemical treatment have emerged as promising alternatives to treat persistent contaminants, e.g., azo dyes. Among the various AOPs for degradation of the recalcitrant pollutants, sulfate radical (SO4·-)-based AOPs (SR-AOPs) have attracted considerable attentions [6]. Generally, SO4·- possessing a high standard redox potential (2.5-3.1 V), could be generated by activating peroxymonosulfate (PMS) or persulfate (PS) with base, heat, UV, transition metal [7]. However, the wide application of heating and UV irradiation method is limited due to high cost and energy input [8]. Although metal ion is one of the most frequently used activators for PMS owing to their low cost and natural abundance, the challenges of its reuse and toxicity also restrict its widespread application in the homogeneous activation process. Employment of supported metal as heterogeneous activator can overcome the drawbacks of the homogeneous activation to some extent, but metal leaching can not be completely avoided [9]. Metal-free carbonaceous materials can be used as a promising alternative due to the prevention of metal leaching and secondary contamination to water environment [10]. Activated carbon (AC) has been suggested as a catalyst of electron-transfer mediator owing to the oxygen functional groups on the surface [11, 12]. AC has been used to activate PMS (or PS) for AO7 degradation at ambient temperature, and remarkable synergistic effect exists in the AC/PMS combined system [13, 14]. Nanostructured carbons, such as carbon nanotube (CNT), have also demonstrated to be effective in various catalytic processes due to their high surface area, favorable thermal conductivity, and sp2-hybridized carbon configuration [15, 16]. Recently, CNT has been reported to be effective to activate PMS to degrade phenolic compound. Moreover, PMS activation was related to the type and surface property of CNT [10, 17]. However, controversial activation mechanisms (radical versus non-radical mechanism) were previously proposed. For example, Sun et al. proposed that the sp2 carbons and oxygen3
containing functional groups on the surface could effectively activate PMS to generate SO 4·for oxidation of phenols [10]. While Lee et al. proposed that oxidation of phenols was induced by the reactive complex of CNT and PMS without generation of SO4·- [17]. The controversial mechanism of CNT activated PMS needs to be further explored. On the other hand, CNT activated PMS has not been applied to treat azo dye wastewater before. Noting the recalcitrant nature of dye wastewater derived from high salinity, high temperature and variable pHs, it was imperative to evaluate the impact of different operating conditions (e.g., NaCl, pH and temperature) on azo dye degradation. Herein, we investigated the performance of CNT activated PMS on the decolorization of a model azo dye, acid orange 7 (AO7). The impact of different operating condition parameters e.g., NaCl, pH, and temperature on AO7 decolorization were evaluated. In particular, the role of Cl- in AO7 decolorization was explored in the CNT/PMS system. Radical quenching experiment and photoluminescence technique were applied to investigate whether radicals were generated in the CNT/PMS system. Afterwards, the UV-vis spectrum and the TOC were monitored to assess the product characteristic and mineralization of AO7, respectively. Finally, the activation mechanism of PMS by CNT was proposed in the presence/absence of Cl-.
2. Materials and Methods 2.1. Chemicals Sources of chemicals and materials are provided in the Supporting Information (SI) Text S1. 2.2. Characterization of CNT X-Ray diffraction (XRD) patterns of CNT were analyzed on Bruker D8 Advance Power Xray diffractometer (Germany) with a Co Kα at 40 kV and 40 mA. The morphology of CNT was analyzed on a scanning electron microscope (SEM, FEI Quanta250, USA). Raman spectra were recorded on a DXR Raman microscope (Thermo Fisher Scientific, USA). 2.3. Experimental procedure The batch experiments were conducted in 250-ml glass flasks at room temperature (23 ± 2 C). The solution in the flasks were constantly mixed by magnetic stirring. Reactions were initiated by adding CNT to the flasks containing AO7 and PMS. The sample aliquots taken at 4
the predetermined time intervals were quenched with excess NaNO2 immediately. Afterwards, the quenched sample was filtered through 0.45 μm membrane, and then analyzed on a UV-vis spectrophotometer. The initial pH of the solution was adjusted by H2SO4 and NaOH to the designated value. All the experiments were conducted in duplicate or more. 2.4. Analytical methods The concentration of AO7 was monitored by measuring the maximum absorbance at λ = 484 nm on a UV-vis spectrophotometer (Mapada UV-1600 PC). The wavelength scan (200 – 650 nm) of AO7 during decolorization was also performed on this spectrophotometer. TOC was measured by Shimadzu TOC-LCPH analyzer. Metal impurities in CNT were determined on an ICP-MS (Agilent 7500). The concentration of PMS was measured by iodometric titration [18, 19]. A photoluminescence technique with terephthalic acid as probe compound was used to detect the generated HO· [20], and the photoluminescence intensity was measured on a Hitachi F-4500 fluorescence spectrophotometer with 315 nm and 425 nm for excitation and emission wavelength, respectively. Degradation products of AO7 were analyzed by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GCMS) (Text S2).
3. Results and Discussion 3.1. Characterization of CNT The SEM image of CNT was shown in Fig. 1A. CNT was stacked onto each other, and each CNT was several microns long. The XRD patterns of CNT before and after reaction were shown in Fig. S1. The diffraction peaks at 26.5 and 42.4 could be attributed to the hexagonal graphite structures (002) and (100) [21, 22]. The XRD of CNT after reaction was almost identical to that before reaction, indicating CNT remained graphitic structure. The Raman spectra of CNT were shown in Fig. 1B. Both spectra exhibited two distinct bands in the spectral region between 1000 and 2000 cm-1 (D and G band). The D band at around 1340 cm-1 reflects the disorder and defects in the carbon lattice or the presence of amorphous carbon [23, 24]. While the G band at 1500-1600 cm-1 signify the in-plane tangential stretching of the carbon-carbon bonds in the graphene structure [25]. The ratio between the intensities of these two bands (ID/IG) has been used to assess the purity, and extent of functionalization of 5
CNT. After the modification of CNT, this ratio always increases [24, 26]. Indeed, the ratios of ID/IG slightly increased after the reaction in this work (from 0.8 to 1.05). The CNT surface might get oxidized as oxygenated species after the reaction, which could increase the extent of functionalization of CNT surface, thus induced the increase of ID/IG. The G’ band is a Raman characteristic region which is sensitive to the charge transfer effect on the surface of CNT [24, 27]. No significant shift of G’ band was observed after reaction, thus indicating no evidences of covalent bonding between CNT and organic chemicals. 3.2. Decolorization of AO7 by PMS activated by CNT Decolorization of AO7 was investigated in the presence of CNT and PMS (Fig. 2A). CNT or PMS alone showed slight decolorization efficiency for AO7, whereas combination of CNT and PMS exhibited remarkable decolorization. To be specific, about 10% and 20% AO7 were decolorized within 50 min in the presence of PMS and CNT, respectively. Complete decolorization, however, was achieved within 50 min in the CNT/PMS system. Rapid decolorization of AO7 indicated a synergistic effect of PMS and CNT during the reaction. Such synergistic effect was possibly due to the activation of PMS by CNT. AC activated PMS has been well documented before [13]. Only slight decolorization of AO7 was observed in the presence of AC (0.1 g/L) and PMS. When AC loading increased to 1 g/L, AO7 decolorization was still much lower than that in the CNT/PMS system (Fig. 2A). Hence, CNT exhibited a much better performance on activating PMS to decolorize AO7 (Text S3, Fig. S2). The decomposition of PMS was also observed in the CNT/PMS system (Fig. 2B). In the absence of AO7, however, the decomposition of PMS was still observed, and the decomposition efficiency was faster than that in the presence of AO7. This result indicated that CNT was effective to activate PMS. Moreover, the reduction in the decomposition efficiency of PMS in the presence of AO7 might be attributed to the loading of AO7 or its intermediates, which could reduce the active surface sites available for PMS activation [14]. We further investigated the potential effect of metal residues in CNT. CNT was treated in nitric acid, and the dissolved metal ions were analyzed by ICP-MS. The results indicated that metal impurities, such as Fe, Co were present in CNT (Table S1). The acid-pretreated CNT exhibited similar activity to pristine CNT for PMS activation to decolorize AO7 (Fig. S3). Moreover, the acid solution from the treatment slightly decolorize AO7 in the presence of PMS. 6
Therefore, the contribution of metal impurities to AO7 decolorization in the CNT/PMS system could be excluded. Effect of CNT loadings and PMS dosages on AO7 decolorization in the CNT/PMS system were explored. The decolorization of AO7 well conformed to the pseudo first-order kinetics (Fig. 3A). The rate constants of AO7 increased with increasing CNT loadings or PMS dosages. The increment amount of rate constants was faster at higher CNT loadings, which might be explained by the larger amount of AO7 adsorbed on CNT when higher loadings of CNT were present (Fig. S4). Nonetheless, plots of rate constants and PMS dosage approximated to the linear relationship, which might be attributed to the relatively low dosages of PMS used in this work. CNT rendered enough reactive sites for PMS activation. The proportion of rate constants with the oxidant dosages was also observed in degradation of MTBE [28] and carbamazepine [29] in SR-AOPs. 3.3. Impact of pH, temperature and Cl- on AO7 decolorization Dye wastewater typically has broad pH variation, large temperature fluctuations, and high salt concentration. Thus we then investigated the impacts of such parameters on the decolorization of AO7. 3.3.1. Impact of pH The pH impact on AO7 decolorization in the CNT/PMS system was shown in Fig. 3B. The rate constants increased from pH 2 to pH 6, and then levelled off before it reached pH 8. Thereafter, there was a remarkable increase when pH further increased to 10. It was noted that significant decolorization of AO7 was observed at pH 10 in the presence of PMS alone owing to the base activated PMS. If the impact of base activated PMS was subtracted, the rate constants levelled off from pH 6 to pH 10. The pHpzc of CNT was determined to be 4.98, thus the surface of CNT was positively charged at pH 2 and 4, but negatively charged at pH 6 or higher. Acidic pHs (e.g., pH 2 and 4) were generally favorable for the adsorption of AO7, a anionic dye, on CNT (Fig. S5), but unfavorable for CNT activated PMS because some active sites of CNT were occupied by AO7 [14]. At pH 2, the formation of H-band between H+ and – O-O- group of PMS was assumed to be more significant, attaching positive charge to PMS, thus inhibiting its interaction with the positive CNT surface [30]. Therefore, the rate constant of AO7 at pH 2 was much lower than those at other pHs. 7
3.3.2. Impact of temperatures Effect of temperature on AO7 decolorization in the CNT/PMS system was shown in Fig. 4A. AO7 decolorization increased with elevated temperatures. Moreover, the rate constants well conformed to Arrhenius behavior, generating the activation energy (Ea) of 45.64 kJ/mol. Ea of AO7 in this work was slight lower than the values in PMS activated by AC/Co (59.7 kJ/mol) [31] or AC/ RuO2 (61.4 kJ/mol) [32], indicating the reaction in this work might take place easily. 3.3.3. Impact of ClThe impact of salts, e.g., NaCl, on AO7 decolorization was shown in Fig. 4B. NaCl exhibited promoting effect on AO7 adsorption, and such promoting effect gradually increased with increasing dosage of NaCl. The addition of salts could increase the dye dimerization in solutions, and the aggregation of dye molecules forced by the action of salt ions would further increase the extent of dye adsorbed onto CNT [33]. In the CNT/PMS system, however, NaCl with different concentrations showed a dual effect on AO7 decolorization. For example, a slight decrease of AO7 decolorization was observed upon addition of NaCl (0.01-0.05 M), but further addition of NaCl (0.1–0.3 M) clearly accelerated its decolorization (Fig. 4B). Interestingly, decolorization of AO7 was also observed in the presence of PMS and NaCl, and complete decolorization of AO7 could be achieved after 30 min when 0.3 M of NaCl was added into PMS (Fig. 4B). Therefore, Cl- might participate in AO7 decolorization in the CNT/PMS system, which would be addressed in the following sections. 3.4. Activation mechanism of PMS in the absence of ClGenerally, the mechanism of CNT activated PMS was controversial, with two alternative mechanisms previously proposed, i.e. radical process [10] versus non radical process [17]. The non-radical process proceeded via the formation of outer-sphere complexes between CNT and PMS, increasing its reactivity towards target compound. It was reported that the increase of ionic strength significantly influenced the out-sphere interactions between solid surface and solute [34]. The increasing ionic strength (i.e. NaClO4) did not influence AO7 decolorization in the CNT/PMS system (Fig. S6), indicating activation of PMS by CNT was not resulted from the outer-sphere interactions (Text S4). In order to check whether radicals were generated or not, the radical quenching experiment 8
was frequently used in SR-AOP. In a previous study about CNT activated PMS, the addition of excess radical scavengers (MeOH and dimethyl sulfoxide (DMSO)) exhibited negligible effect on phenol degradation, thus non-radical mechanism was considered as the main reaction mechanism [17]. Indeed, MeOH and TBA slightly depressed the decolorization of AO7 in the CNT/PMS system (Fig. 5A). However, it should be noted that CNT activated PMS belonged to the heterogeneous activation. Both MeOH and TBA are hydrophilic compounds, thus are relatively difficult to approach to the surface of the heterogeneous catalyst [35]. In contrast, phenol (a strong scavenger for both SO4·- and HO·) is relatively hydrophobic, thus is easier to approach to the surface [36, 37]. As shown in Fig. 5A, phenol exhibited a remarkable inhibitory effect on AO7 decolorization even when the phenol/AO7 ratio was 100 in the CNT/PMS system. When the ratio was further raised to 2000, AO7 decolorization was almost identical to its adsorption on CNT (Fig. 5A vs Fig. 2A). It was thus suggested that the radical-induced decolorization of AO7 mainly took place on the surface of CNT, consistent with the results in the activation of PMS by AC [13]. To further verify that the radicals bounded on the CNT surface were responsible for AO7 decolorization, KI was used as a radical quencher in the CNT/PMS system, because KI can react with surface-bound free radicals [38, 39]. Indeed, the addition of KI almost completely retarded the oxidative degradation of AO7 (Fig. 5B). In addition, PMS could be still detected after the complete decolorization of AO7 (data not shown), indicating that the depression of AO7 degradation was not resulted from the consumption of PMS by KI. Therefore, the surfacebound reactive radicals (i.e., SO4·- and HO·) played a dominant role in AO7 decolorization. The formation of HO· on the surface of CNT was further detected by a photoluminescence technique with terephthalic acid as a probe compound. The HO· can quantitatively convert nonfluorescent terephthalic acid to highly fluorescent product, 2-hydroxyterephthalic acid with a maximum emission at 425 nm [20, 40]. The intensity of the fluorescent peak at 425 nm is proportional to the amount of HO· in the system. As shown in Fig. 5C, an intensive fluorescent peak was observed in PMS solution after the addition of CNT, and the intensity at the maximum emission gradually increased as the reaction proceeded. This evidence strongly suggested the formation of HO· in the CNT/PMS system. Therefore, the activation mechanism of PMS by CNT was radical mechanism rather than non-radical mechanism. In the previous studies, both 9
sp2-hybridized carbon and oxygen-containing groups were assumed to contribute to the catalytic activity of nanocarbons, such as reduced graphene oxide (rGO) [41] and CNT [10]. Also, the catalytic activity of rGO was proved to be related to the content of oxygen-containing groups [42]. Therefore, sp2 carbon and oxygen-containing groups on CNT could activate PMS to generate SO4·- and HO·, and then decolorize AO7. 3.5. Activation mechanism of PMS in the presence of ClThe influence of Cl- on contaminant degradation in SR-AOPs was dependent on the target pollutant [43]. For instance, addition of Cl- accelerated 2,4-DCP degradation during SO4·oxidation [44], but slightly inhibited carbamazepine degradation [45], and even significantly reduced iopromide degradation [30]. In the activated PMS process, Cl- could be oxidized by SO4·- to form Cl·, and subsequent transformed to Cl2·- and other chlorine radical species (equations 1-6). Also, Cl- could participate in PMS decomposition via non-radical mechanism, generating HClO and Cl2 (equations 7-8) [46, 47]. Therefore, three types of reactive species might contribute to the overall contaminant degradation, i.e. 1) HO· and SO4·-, 2) HClO and Cl2, and 3) Cl·, Cl2·-, and other chlorine radical species. Cl· and other chlorine radical species are significantly less reactive to AO7 than SO4·-, whereas HClO could rapidly decolorize the azo dyes [46, 48]. Therefore, the adverse effect of Cl- at low concentration could be explained by the consumption of PMS and SO4·- by Cl-, and formation of less reactive chlorine radical species. On the contrary, the accelerating effect of Cl- at higher concentration might be resulted from the generation of highly reactive HClO. SO4·- + Cl- → SO42- + Cl·
(1)
Cl· + Cl- → Cl2·-
(2)
Cl2·- + Cl2·- → Cl2 + 2Cl-
(3)
H2O + Cl2·- →-ClOH·- + H+ + Cl-
(4)
ClOH·- + H+ → Cl· + H2O
(5)
ClOH·- →Cl- + HO·
(6)
HSO5- + Cl- →SO42- + HClO
(7)
HSO5- + 2Cl- + H+ →SO42- + Cl2 + H2O
(8)
To verify the involvement of HClO in AO7 decolorization in the CNT/PMS system in the presence of high concentration of NaCl, we subsequently carried out the following experiments. 10
1) decolorization of AO7 in the presence of PMS and NaCl. According to equations 7-8, HClO (Cl2) could be directly generated from redox reaction between PMS and Cl- without the involvement of radical species. Thus we hypothesized that AO7 was susceptible to decolorization in the solution containing both PMS and NaCl. Indeed, AO7 was stable in the presence of PMS alone, but was rapidly decolorized after addition of NaCl into PMS (Fig. 4B ). Moreover, the decolorization rate of AO7 increased with increasing dosages of Cl-. This experiment provided a direct evidence for the involvement of HClO in AO7 decolorization. 2) Simultaneous addition of radical quencher (i.e., phenol) and NaCl in the CNT/PMS system. Addition of excess phenol into CNT/PMS system could completely scavenge the radicals of SO4·- and HO·. However, further addition of NaCl could promote AO7 decolorization, and the accelerating effect was apparent as high dosage of NaCl was added (Fig. 6A). It was thus suggested that AO7 decolorization was not derived from the radical-induced reaction, but most likely from other reactive oxidants, e.g., HClO. 3) Addition of NH4+ in the CNT/PMS/NaCl system. Significant decrease of AO7 decolorization was observed after addition of NH4+ from 0.01 to 0.1 M (Fig. 6B). NH4+ could react with HClO to generate chloramines, a weak oxidants in comparison to HClO [49, 50]. All these evidences strongly suggested that HClO participated in the decolorization of AO7. We further monitored the UV-vis spectra of AO7 solution during the reaction in CNT/PMS (Fig. 7A), NaCl/PMS (Fig. S7) and CNT/NaCl/PMS systems (Fig. S8). Four apparent peaks were observed in the spectrum of AO7. The peaks at 230 and 310 nm represented the benzene and naphthalene rings on AO7, respectively [51], while the other peaks of 430 and 484 nm reflected the hydrazone form and azo form [52]. In all the three systems, the four peaks gradually decreased as the reaction proceeded, and a new peak appeared at about 250 nm (Text S5). To better investigate the spectra characteristics of AO7 solution, we then compared the UV-vis spectra in different systems when about 80% AO7 was decolorized (Fig. 7B). An apparent band shift was observed from 280 to 380 nm as PMS and 0.3 M NaCl were present. Moreover, the new peak was apparently shifted to 245 nm in the NaCl/PMS system in comparison to 254 nm in the CNT/PMS system. These evidences clearly indicated that the degradation products induced by HClO were significantly different from that induced by reactive radical species. In the CNT/PMS/0.3 M NaCl system, the new peak at 250 nm could 11
be attributed to the product mixture generated from HClO oxidation and radical oxidation. To better clarify the degradation products from radical process (i.e. CNT/PMS) and nonradical process (i.e. NaCl/PMS), we further analyzed the degradation products of AO7 by HPLC. As shown in Fig. 8A, the parent compound AO7 was observed at the retention time (RT) of about 13.50 min. In the CNT/PMS and NaCl/PMS system, several new peaks also appeared when about 80% AO7 was decolorized, indicating formation of degradation products. Besides the common products at about 2.5 3.5 and 7.3 min in both systems, other new products were also observed in the NaCl/PMS system. This result indicated that the degradation products in the NaCl/PMS system were much more complicated. The degradation products of AO7 in these systems were further identified by GC-MS, and the proposed structures were shown in Fig. 8B. Six products were identified in the CNT/PMS system. They could be classified into three groups: three naphthalene-type compounds (1-nitro-2-naphthalenol, coumarin, and 1,2naphthoquinone), one fused heterocyclic compound (phthalic anhydride), and two aromatic compounds (phthalic acid, benzoic acid). All these degradation products have been previously reported [46, 53-55]. Naphthalene-type products were supposed to be primary intermediates after the cleavage of azo band in AO7. Afterwards, they were subsequently oxidized to generate fused heterocyclic compounds, then other smaller molecule products, and finally mineralized to CO2 and H2O [46]. In the NaCl/PMS system, more products were identified, consistent with the results in HPLC. Besides the products observed in CNT/PMS, several chlorinated organic products were also observed, such as 7-chloro-2-naphthalenol, 2,4-dichloro-1-naphthalenol, 3chloroisocoumarin, 5-chloroisobenzofuran-1,3-dione, which were previously reported in the UV/TiO2/NaCl [56] and Co/PMS/NaCl system [46]. To make clear whether AO7 was mineralized or not, TOC variation was monitored during AO7 decolorization (Fig. 9). Negligible TOC removal was observed after the complete decolorization of AO7 (180 min) in the NaCl/PMS system, suggesting that HClO was highly efficient to decolorize the azo band of AO7, but ineffective to further mineralize it to CO2 and H2O. In the CNT/PMS and CNT/NaCl/PMS systems, TOC removal was fast in the initial 5 min, and then slowed down. About 20% TOC removal was observed at 60 min when the parent AO7 was completely decolorized. Hence, the CNT/PMS (CNT/PMS/NaCl) system could effectively destruct AO7 into small molecular products, which might be difficult to further mineralize. 12
Such low mineralization capacity was also observed in the AC activated PMS system [13]. To improve the mineralization in CNT/PMS system, CNT could be impregnated with transition metals [31] or combined with other technologies [57]. 3.6. Overall reaction scheme Generally, both radical process (I) and non-radical process (II) were involved in the activation of PMS (Fig. 10). (I) The electrons were transferred from sp2-hybridized carbon and functional groups on CNT to PMS, and thus SO4·- and HO· were generated on the surface of CNT. The radicals would decolorize the AO7 accumulated on the surface of CNT rather than AO7 in the bulk solution. (II) The Cl- in the bulk solution could be directly oxidized by PMS through two electron transfer pathway, with reactive HClO generated in the solution. HClO was effective to bleach AO7 in the bulk solution. Although the activated PMS (via radical process or non-radical process) were effective to decolorize AO7, the decolorization of AO7 was likely to proceed through different ways. The radicals generated from the activated PMS possessed high oxidation capacity, thus effectively destruct AO7, and finally mineralize to CO2 and H2O. On the other hand, AO7 could be rapidly bleached by HClO generated from the non-radical process, but is possibly transformed to other oxidized products, rather than mineralized to CO2 and H2O.
4. Conclusions CNT exhibited excellent performance in heterogeneously activating PMS to decolorize the azo dye, i.e. AO7. Decolorization efficiency of AO7 increased with increasing pH, raising PMS dosage and CNT loadings. The decolorization of AO7 conformed to pseudo first-order kinetics at different temperatures, and the activation energy was determined to be 45.64 kJ/mol. The decolorization of AO7 dominantly took place on the surface of CNT, rather than the bulk solution. Such decolorization was induced by the surface-bound radicals generated from PMS activated by CNT. Interestingly, NaCl, a common component in the dye wastewater, exerted dual effect in the decolorization of AO7, i.e. inhibitory effect on AO7 decolorization at low concentration, but accelerating effect at high concentration. Cl- was directly oxidized by PMS to generate HClO, which was effective to decolorize AO7 through non-radical reaction. In comparison to the radical-induced decolorization of AO7, different product profiles and less 13
mineralization were derived from that through non-radical process.
Acknowledgements We sincerely thank the National Natural Science Foundation of China (51478283, 51509175) for financially supporting this work.
14
References [1] H. Lin, H. Zhang, L. Hou, Degradation of C. I. Acid Orange 7 in aqueous solution by a novel electro/Fe3O4/PDS process, J. Hazard. Mater. 276 (2014) 182-191. [2] I. Grčić, S. Papić, N. Koprivanac, I. Kovačić, Kinetic modeling and synergy quantification in sono and photooxidative treatment of simulated dyehouse effluent, Water Res. 46 (2012) 5683-5695. [3] C. Cai, H. Zhang, X. Zhong, L. Hou, Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a bimetallic Fe–Co/SBA-15 catalyst for the degradation of Orange II in water, J. Hazard. Mater. 283 (2015) 70-79. [4] M. Wang, X. Liu, B. Pan, S. Zhang, Photodegradation of Acid Orange 7 in a UV/acetylacetone process, Chemosphere 93 (2013) 2877-2882. [5] D. Han, J. Wan, Y. Ma, Y. Wang, M. Huang, Y. Chen, D. Li, Z. Guan, Y. Li, Enhanced decolorization of Orange G in a Fe(II)-EDDS activated persulfate process by accelerating the regeneration of ferrous iron with hydroxylamine, Chem. Eng. J. 256 (2014) 316-323. [6] Y. Qian, X. Zhou, Y. Zhang, P. Sun, W. Zhang, J. Chen, X. Guo, X. Zhang, Performance of alpha-methylnaphthalene degradation by dual oxidant of persulfate/calcium peroxide: Implication for ISCO, Chem. Eng. J. 279 (2015) 538-546. [7] B.T. Zhang, Y. Zhang, Y.H. Teng, M.H. Fan, Sulfate Radical and Its Application in Decontamination Technologies, Crit. Rev. Env. Sci. Tec. 45 (2015) 1756-1800. [8] H. Liu, T.A. Bruton, F.M. Doyle, D.L. Sedlak, In Situ Chemical Oxidation of Contaminated Groundwater by Persulfate: Decomposition by Fe(III)- and Mn(IV)-Containing Oxides and Aquifer Materials, Environ. Sci. Technol. 48 (2014) 10330-10336. [9] H. Sun, H. Liang, G. Zhou, S. Wang, Supported cobalt catalysts by one-pot aqueous combustion synthesis for catalytic phenol degradation, J. Colloid Interf. Sci. 394 (2013) 394-400. [10] H. Sun, C. Kwan, A. Suvorova, H.M. Ang, M.O. Tadé, S. Wang, Catalytic oxidation of organic pollutants on pristine and surface nitrogen-modified Carbon nanotubes with sulfate radicals, Appl. Catal. B-Environ. 154–155 (2014) 134-141. [11] M. Kimura, I. Miyamoto, Discovery of the Activated-Carbon Radical AC+ and the Novel Oxidation-Reactions Comprising the AC/AC+ Cycle as a Catalyst in an Aqueous Solution, 15
B. Chem. Soc. Jpn. 67 (1994) 2357-2360. [12] C. Liang, Y.T. Lin, W.H. Shih, Treatment of Trichloroethylene by Adsorption and Persulfate Oxidation in Batch Studies, Ind. Eng. Chem. Res. 48 (2009) 8373-8380. [13] J. Zhang, X.T. Shao, C. Shi, S.Y. Yang, Decolorization of Acid Orange 7 with peroxymonosulfate oxidation catalyzed by granular activated Carbon, Chem. Eng. J. 232 (2013) 259-265. [14] S. Yang, X. Yang, X. Shao, R. Niu, L. Wang, Activated Carbon catalyzed persulfate oxidation of Azo dye acid orange 7 at ambient temperature, J. Hazard. Mater. 186 (2011) 659-666. [15] J. Zhang, D. Su, A. Zhang, D. Wang, R. Schlögl, C. Hébert, NanoCarbon as robust catalyst: Mechanistic insight into Carbon-mediated catalysis, Angew. Chem. Int. Edit. 46 (2007) 7319-7323. [16] J. Luo, F. Peng, H. Yu, H. Wang, W. Zheng, Aerobic Liquid-Phase Oxidation of Ethylbenzene to Acetophenone Catalyzed by Carbon Nanotubes, ChemCatChem 5 (2013) 1578-1586. [17] H. Lee, H.J. Lee, J. Jeong, J. Lee, N.B. Park, C. Lee, Activation of persulfates by Carbon nanotubes: Oxidation of organic compounds by nonradical mechanism, Chem. Eng. J. 266 (2015) 28-33. [18] R.E. Ball, J.O. Edwards, M.L. Haggett, P. Jones, A kinetic and isotopic study of the decomposition of monoperoxyphthalic acid, J. Am. Chem. Soc. 89 (1967) 2331-2333. [19] Y. Güpta, Iodometric determination of persulphate in sulphuric acid solution, Anal. Chem. 180 (1961) 260-265. [20] J. Yu, W. Wang, B. Cheng, B.L. Su, Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment, J. Phys. Chem. C 113 (2009) 6743-6750. [21] W. Li, C. Liang, W. Zhou, J. Qiu, Zhou, G. Sun, Q. Xin, Preparation and Characterization of Multiwalled Carbon Nanotube-Supported Platinum for Cathode Catalysts of Direct Methanol Fuel Cells, J. Phys. Chem. B 107 (2003) 6292-6299. [22] M.S. Liu, M. Ching Cheng Lin, I.T. Huang, C.C. Wang, Enhancement of thermal conductivity with Carbon nanotube for nanofluids, Int. Commun. Heat Mass 32 (2005) 16
1202-1210. [23] A. Gupta, S.R. Vidyarthi, N. Sankararamakrishnan, Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled Carbon nanotubes, J. Hazard. Mater. 274 (2014) 132-144. [24] A.P.M. Tavares, C.G. Silva, G. Dražić, A.M.T. Silva, J.M. Loureiro, J.L. Faria, Laccase immobilization over multi-walled Carbon nanotubes: Kinetic, thermodynamic and stability studies, J. Colloid Interf. Sci. 454 (2015) 52-60. [25] R.H. Gangupomu, M.L. Sattler, D. Ramirez, Comparative study of Carbon nanotubes and granular activated Carbon: Physicochemical properties and adsorption capacities, J. Hazard. Mater. 302 (2016) 362-374. [26] Y. Lu, Z. Zhu, Z. Liu, Catalytic growth of Carbon nanotubes through CHNO explosive detonation, Carbon 42 (2004) 361-370. [27] W. Yan, L. Yan, J. Duan, C. Jing, Sorption of organophosphate esters by Carbon nanotubes, J. Hazard. Mater. 273 (2014) 53-60. [28] K.C. Huang, R.A. Couttenye, G.E. Hoag, Kinetics of heat-assisted persulfate oxidation of methyl tert-butyl ether (MTBE), Chemosphere 49 (2002) 413-420. [29] J. Deng, Y. Shao, N. Gao, Y. Deng, S. Zhou, X. Hu, Thermally activated persulfate (TAP) oxidation of antiepileptic drug carbamazepine in water, Chem. Eng. J. 228 (2013) 765-771. [30] T. Zhang, H. Zhu, J.P. Croué, Production of Sulfate Radical from Peroxymonosulfate Induced by a Magnetically Separable CuFe2O4 Spinel in Water: Efficiency, Stability, and Mechanism, Environ. Sci. Technol. 47 (2013) 2784-2791. [31] P.R. Shukla, S.B. Wang, H.Q. Sun, H.M. Ang, M. Tade, Activated Carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution, Appl. Catal. B-Environ. 100 (2010) 529-534. [32] S. Muhammad, P.R. Shukla, M.O. Tadé, S. Wang, Heterogeneous activation of peroxymonosulphate by supported ruthenium catalysts for phenol degradation in water, J. Hazard. Mater. 215–216 (2012) 183-190. [33] C. Y. Kuo, C. H. Wu, J. Y. Wu, Adsorption of direct dyes from aqueous solutions by Carbon nanotubes: Determination of equilibrium, kinetics and thermodynamics parameters, J. Colloid Interf. Sci. 327 (2008) 308-315. 17
[34] T. Zhang, Y. Chen, Y.R. Wang, J. Le Roux, Y. Yang, J.P. Croue, Efficient Peroxydisulfate Activation Process Not Relying on Sulfate Radical Generation for Water Pollutant Degradation, Environ. Sci. Technol. 48 (2014) 5868-5875. [35] H.Y. Liang, Y.Q. Zhang, S.B. Huang, I. Hussain, Oxidative degradation of p-chloroaniline by copper oxidate activated persulfate, Chem. Eng. J. 218 (2013) 384-391. [36] M.E. Lindsey, M.A. Tarr, Inhibition of Hydroxyl Radical Reaction with Aromatics by Dissolved Natural Organic Matter, Environ. Sci. Technol. 34 (2000) 444-449. [37] J. Ziajka, W. Pasiuk-Bronikowska, Rate constants for atmospheric trace organics scavenging SO4−. in the Fe-catalysed autoxidation of S(IV), Atmos. Environ. 39 (2005) 1431-1438. [38] Z. Huang, H. Bao, Y. Yao, W. Lu, W. Chen, Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst, Appl. Catal. B-Environ. 154-155 (2014) 36-43. [39] L. Xu, J. Wang, Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient FentonLike Heterogeneous Catalyst for Degradation of 4-Chlorophenol, Environ. Sci. Technol. 46 (2012) 10145-10153. [40] Q. Xiao, Z. Si, J. Zhang, C. Xiao, X. Tan, Photoinduced hydroxyl radical and photocatalytic activity of samarium-doped TiO 2 nanocrystalline, J. Hazard. Mater. 150 (2008) 62-67. [41] H. Sun, S. Liu, G. Zhou, H.M. Ang, M.O. Tadé, S. Wang, Reduced Graphene Oxide for Catalytic Oxidation of Aqueous Organic Pollutants, ACS. Appl. Mater. Inter. 4 (2012) 5466-5471. [42] W. Peng, S. Liu, H. Sun, Y. Yao, L. Zhi, S. Wang, Synthesis of porous reduced graphene oxide as metal-free Carbon for adsorption and catalytic oxidation of organics in water, J. Mater. Chem. A. 1 (2013) 5854-5859. [43] Y. Yang, J.J. Pignatello, J. Ma, W.A. Mitch, Comparison of Halide Impacts on the Efficiency of Contaminant Degradation by Sulfate and Hydroxyl Radical-Based Advanced Oxidation Processes (AOPs), Environ. Sci. Technol. 48 (2014) 2344-2351. [44] G.P. Anipsitakis, D.D. Dionysiou, M.A. Gonzalez, Cobalt-Mediated Activation of Peroxymonosulfate and Sulfate Radical Attack on Phenolic Compounds. Implications of 18
Chloride Ions, Environ. Sci. Technol. 40 (2006) 1000-1007. [45] Q. Zhang, J. Chen, C. Dai, Y. Zhang, X. Zhou, Degradation of carbamazepine and toxicity evaluation using the UV/persulfate process in aqueous solution, J. Chem. Technol. Biotechnol. 90 (2015) 701-708. [46] R. Yuan, S.N. Ramjaun, Z. Wang, J. Liu, Effects of chloride ion on degradation of Acid Orange 7 by sulfate radical-based advanced oxidation process: Implications for formation of chlorinated aromatic compounds, J. Hazard. Mater. 196 (2011) 173-179. [47] K.H. Chan, W. Chu, Degradation of atrazine by cobalt-mediated activation of peroxymonosulfate: Different cobalt counteranions in homogenous process and cobalt oxide catalysts in photolytic heterogeneous process, Water Res. 43 (2009) 2513-2521. [48] Z. Wang, R. Yuan, Y. Guo, L. Xu, J. Liu, Effects of chloride ions on bleaching of azo dyes by Co2+/oxone regent: Kinetic analysis, J. Hazard. Mater. 190 (2011) 1083-1087. [49] X.Y. Lou, Y.G. Guo, D.X. Xiao, Z.H. Wang, S.Y. Lu, J.S. Liu, Rapid dye degradation with reactive oxidants generated by chloride-induced peroxymonosulfate activation, Environ Sci Pollut Res. 20 (2013) 6317-6323. [50] M. Deborde, U. von Gunten, Reactions of chlorine with inorganic and organic compounds during water treatment - Kinetics and mechanisms: A critical review, Water Res. 42 (2008) 13-51. [51] W. Feng, D. Nansheng, H. Helin, Degradation mechanism of azo dye C. I. reactive red 2 by iron powder reduction and photooxidation in aqueous solutions, Chemosphere 41 (2000) 1233-1238. [52] C. Bauer, P. Jacques, A. Kalt, Photooxidation of an azo dye induced by visible light incident on the surface of TiO2, J. Photochem. Photobiol. A-Chem. 140 (2001) 87-92. [53] H.Z. Zhao, Y. Sun, L.N. Xu, J. R. Ni, Removal of Acid Orange 7 in simulated wastewater using a three-dimensional electrode reactor: Removal mechanisms and dye degradation pathway, Chemosphere 78 (2010) 46-51. [54] X. Chen, J. Chen, X. Qiao, D. Wang, X. Cai, Performance of nanoCo3O4/peroxymonosulfate system: kinetics and mechanism study using Acid Orange 7 as a model compound, Appl. Catal. B-Environ. 80 (2008) 116-121. [55] J. Wu, H. Zhang, J. Qiu, Degradation of Acid Orange 7 in aqueous solution by a novel 19
electro/Fe2+/peroxydisulfate process, J. Hazard. Mater. 215 (2012) 138-145. [56] R. Yuan, S.N. Ramjaun, Z. Wang, J. Liu, Photocatalytic degradation and chlorination of azo dye in saline wastewater: Kinetics and AOX formation, Chem. Eng. J. 192 (2012) 171178. [57] C. Cai, L. Wang, H. Gao, L. Hou, H. Zhang, Ultrasound enhanced heterogeneous activation of peroxydisulfate by bimetallic Fe-Co/GAC catalyst for the degradation of Acid Orange 7 in water, J. Environ. Sci. 26 (2014) 1267-1273.
20
Figure captions Fig. 1 (A) SEM image, and (B) Raman spectra of CNT before (a) and after reaction (b). Fig. 2 Decolorization of AO7 (A) and decomposition of PMS (B) under different conditions. [AO7]0 = 0.057 mM, [PMS]0/[AO7]0 = 20, [CNT] = 0.1 g/L, pH0 = 7. Note: a 0.1 g/L AC, b 1 g/L AC. Fig. 3 Effect of PMS dosages, CNT loadings (A) and initial pH (B) on the rate constants (kobs) of AO7 in the CNT/PMS system. (A) [AO7]0 = 0.057 mM, pH0 = 7, (a) [CNT] = 0.1 g/L, (b) [PMS]0/[AO7]0 = 20 : 1. (B) [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1. Fig. 4 Effect of temperature (A) and NaCl (B) on AO7 decolorization in the CNT/PMS system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7. (B) Solid lines and open symbol: adsorption of AO7 in the presence of CNT and NaCl; solid lines and solid symbols: decolorization of AO7 in the presence of CNT, NaCl and PMS; dot lines and open symbols: decolorization of AO7 in the presence of PMS and NaCl. Fig. 5 Effect of alcohols, phenol (A) and KI (B) on AO7 decolorization in the CNT/PMS system; and (C) photoluminescence spectra of hydroxyl radical generated in the CNT/PMS system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, [KI] = 10 mM, pH0 = 7. Fig. 6 (A) Effect of NaCl on AO7 decolorization in the CNT/PMS/phenol system, (B) Effect of NH4+ on AO7 decolorization in the CNT/PMS/NaCl system. [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7, (A) [phenol] = 114 mM, (B) [NaCl] = 0.1 M. Control: AO7 decolorization in the CNT/PMS system. Fig. 7 (A) UV-vis spectra of AO7 in the CNT/PMS system; (B) UV-vis spectra variation in different activated PMS system (after about 80% AO7 removal). [AO7]0 = 0.057 mM, [CNT] = 0.1 g/L, [PMS]0/[AO7]0 = 20 : 1, pH0 = 7. Control: spectrum of AO7 solution in the absence of CNT and NaCl. Fig. 8 Degradation product analysis in the CNT/PMS and NaCl/PMS systems by HPLC (A) and GC/MS (B). (A) the NCl/PMS system (a), the CNT/PMS system (b), and the standard chemical of AO7 (c). Fig. 9 TOC variation during the reaction in different systems. Fig. 10 Overall reaction scheme. 21
(A)
G
(B) D
Intensity (a.u.)
G'
(a) D
G
G'
(b)
500
1000
1500
2000
2500 -1
Raman shift (cm )
Fig. 1
22
3000
(A)
0.9
CNT PMS CNT + PMS a AC a AC + PMS b AC b AC + PMS
C/C0
0.6
0.3
0.0 0
15
30
45
t (min)
(B)
C/C0
0.9
PMS + AO7 CNT + PMS + AO7 CNT + PMS
0.6
0.3
0.0 0
15
30
45
t (min)
Fig. 2
23
0
PMS / AO7
50
100
0.9
150
200
(A)
0.6
y = 0.00323 x + 0.00474, 2 R = 0.983
kobs
(a) Effect of PMS dosage
0.3
(b) Effect of CNT loading
0.0 0.0
0.1
0.2
0.3
CNT (g/L)
(B)
0.3 CNT + PMS PMS CNT + PMS, subtract the contribution of base activation
k obs
0.2
0.1
0.0 2
4
6
8
pH
Fig. 3
24
10
(A) 60 C, only PMS 30 C 40 C -0.4 50 C 60 C
0.9
C/C0
lg k
0.6
2
y = -2384.2 * x + 6.893, R = 0.9837 Ea = 45.64 kJ/mol
-0.6 -0.8
0.3 -1.0 0.0030
0.0031
0.0032 -1
0.0033
1/T (K )
0.0 0
15
30
45
t (min)
(B)
0.9
C/C0
0.6 0.01 M 0.3 M
0M 0.01 M 0.1 M 0.3 M
0.3
0M 0.01 M 0.05 M 0.1 M 0.3 M
0.0 0
15
30
45
t (min)
Fig. 4
25
(A)
0.9
0.6
C/C0
without quenchers phenol : AO7 = 100 : 1 phenol : AO7 = 500 : 1 phenol : AO7 = 1000 : 1 TBA : AO7 = 1000 : 1 MeOH : AO7 = 1000 : 1
0.3
0.0 0
10
20
30
40
50
60
t (min)
(B)
0.9
C/C0
0.6 CNT + PMS CNT + PMS + KI CNT + KI
0.3
0.0 0
10
20
30
40
t (min)
26
50
1000 (C) 0 min 5 min 10 min 20 min 30 min 40 min 50 min
Intensity
800 600 400 200 0 400
450
500
Wavelength (nm)
Fig. 5
27
550
(A)
0.9
C/C0
0.6 control 0 M NaCl 0.01 M NaCl 0.1 M NaCl 0.3 M NaCl
0.3
0.0 0
15
t (min)
30
45
(B)
0.9
C/C0
0.6 control + 0 M NH4
0.3
0.01 M NH4
+
0.05 M NH4
+
0.1 M NH4
0.0 0
15
t (min)
30
45
Fig. 6
28
+
2.0
(A) with CNT alone 0 min 2.5 min 5 min 10 min 20 min 30 min 40 min 60 min
Abs
1.5
1.0
0.5
0.0 200
300
400
500
600
wavelength (nm)
2.0
CNT + 0.1 M NaCl CNT + 0.3 M NaCl
(B)
control CNT 0.3 M NaCl
245 nm
Abs
1.5
1.0
250 nm 200
220
254 nm
240
260
280
wavelength (nm)
0.5
0.0 200
300
400
500
wavelength (nm)
Fig. 7
29
(A) (a) 80.1% decolorization of AO7
(b) 79.5% decolorization of AO7
(c)
(B)
Fig. 8 30
CNT + PMS CNT + PMS +NaCl PMS + NaCl
TOC/TOC0
1.0
0.8
0.6
0
1
5
20
60
t (min)
Fig. 9
31
180
Fig. 10
32