Water Research 84 (2015) 18e24
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Evaluation of the potential of p-nitrophenol degradation in dredged sediment by pulsed discharge plasma Tiecheng Wang a, b, *, Guangzhou Qu a, b, Qiuhong Sun c, Dongli Liang a, b, Shibin Hu a, b a
College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi 712100, PR China c Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2015 Received in revised form 26 June 2015 Accepted 13 July 2015 Available online 16 July 2015
Hazardous pollutants in dredged sediment pose great threats to ecological environment and human health. A novel approach, named pulsed discharge plasma (PDP), was employed for the degradation of pnitrophenol (PNP) in dredged sediment. Experimental results showed that 92.9% of PNP in sediment was smoothly removed in 60 min, and the degradation process fitted the first-order kinetic model. Roles of some active species in PNP degradation in sediment were studied by various gas plasmas, OH radical scavenger, hydrated electron scavenger and O$ 2 scavenger; and the results presented that O3, OH radical, $ $ e aq and O2 all played significant roles in PNP removal, and eaq and O2 mainly participated in other oxidising active species formation. FTIR analysis showed that PNP molecular structure was destroyed after PDP treatment. The main degradation intermediates were identified as hydroquinone, benzoqui none, phenol, acetic acid, NO 2 and NO3 . PNP degradation pathway in dredged sediment was proposed. It is expected to contribute to an alternative for sediment remediation by pulse discharge plasma. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pulse discharge plasma Dredged sediment Sediment remediation p-Nitrophenol
1. Introduction Water sediment is one of the most important constituent parts of aquatic ecosystem. With the quick development of industrial manufacture, large numbers of hazardous pollutants have been entering water body via various ways such as atmospheric deposition, wastewater discharge, and rain flushing (Borgnino et al., 2006; Payne et al., 2013). These hazardous contaminants will deposit into the water bottom and sequentially gather in the sediment, resulting in sediment pollution (Payne et al., 2013). The main pollutants in the sediment are heavy metals and persistent organic pollutants. Even worse, the sediment that contains various contaminants is not only a “reservoir” where hazardous pollutants are deposited but also an active compartment with a fundamental role in the redistribution of the hazardous pollutants into the overlying water. That is, sediment will become the secondary pollution source of the water body when external pollution sources of rivers are effectively controlled (Borgnino et al., 2006). Thus, the presence of the hazardous pollutants in the sediment is a serious threat to
* Corresponding author. College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.watres.2015.07.022 0043-1354/© 2015 Elsevier Ltd. All rights reserved.
the aquatic environment and human health (Zhang et al., 2013). Sediment remediation is a quite significant pathway to resolve river pollution problem. Conventional sediment remediation technologies available include in-situ remediation and ex-situ dredging. Typical in-situ remediation methods include capping, inactivation, chemical oxidation and reduction, and the reduction of bioavailability using amendments (Truex et al., 2011; Josefsson et al., 2012). However, many concerns are present when the in-situ remediation methods are implemented due to the possibility of river storage capacity decrease, riverbed shallows, residual contamination, and biointrusion etc. (Redell et al., 2011; Truex et al., 2011; Josefsson et al., 2012). At present, ex-situ dredging is widely used for sediment remediation, and then other techniques are employed to remove pollutants from dredged sediment, such as drip washing (Kumar et al., 2013), bioremediation (Seidel et al., 2004), pyrolysis (Hu et al., 2007), electrokinetic remediation (Li et al., 2009), and landfilling and incineration (USEPA, 1994). Some problems such as secondary pollution, and time-consuming still exist for posttreatment of dredged sediment. Therefore, it is highly recommended to explore high-efficient and rapid method to treat dredged sediment. Recently, chemical oxidation by advanced oxidation processes
T. Wang et al. / Water Research 84 (2015) 18e24
(AOPs) and ozone have been received great emphasis on sediment remediation because of their ability to rapidly oxidize refractory organic contaminants in sediment, such as Fenton reaction (Ferrarese et al., 2008), peroxy acids (Levitt et al., 2003) and ozonation (Hong et al., 2008). Pulsed discharge plasma, one of the AOPs, has aroused considerable interest in pollution control because of its high removal efficiency and environmental compatibility, such as wastewater treatment (Bian et al., 2007). During pulsed discharge plasma processes, the ensuring electronemolecule interactions generate highly reactive non-thermal plasma, which are strongly oxidizing environments due to the presence of large number of chemically active species, such as þ þ ozone, H2O2, and OH radicals, O atoms, and ions (O 2 , O2 , H3O ); simultaneously, physical effects concomitantly generated during discharge plasma process such as ultraviolet light can also improve pollutants removal. Our previous studies have also demonstrated that organic compounds in soil could be effectively removed by pulsed discharge plasma (Wang et al., 2011). Hence, it is believed that it may be powerful enough for pulsed discharge plasma to remove organic pollutants in dredged sediment, which usually contains 80%e95% water content. When pulse discharge plasma occurs, high-energetic electrons are formed and then various active species (such as OH, H2O2 and O3) are produced; on the one hand, organic pollutants in dredged sediment may be excited, ionized and dissociated by high-energetic electrons; on the other hand, various active species can react with the organic pollutants rapidly; furthermore, the presence of natural metallic minerals in sediment may enable the Fenton reaction to proceed due to the generation of H2O2 by pulsed discharge plasma. Because of the strong oxidation environment of pulse discharge plasma, organic pollutants in the dredged sediment may be removed effectively and rapidly. However, to the best of our knowledge, little has been reported about organic pollutants removal from sediment by pulsed discharge plasma. This paper describes research on p-nitrophenol (PNP) degradation in sediment using pulsed discharge plasma. PNP has been listed as the 129 priority toxic pollutants by U.S. Environmental Protection Agency (Larry and William, 1979). The present study was primarily focused on evaluating the efficacy of pulse discharge plasma for PNP degradation in sediment. The roles of various active species in PNP removal from dredged sediment were discussed. PNP removal processes in dredged sediment in such a system was also proposed. This study is expected as preliminary investigation for development of a new approach that can be used as a fastacting, and ex-situ remediation technique for dredged sediment. 2. Materials and methods 2.1. Materials PNP was used in the study, and its detailed introduction was presented in S1 of Supporting Information (SI). Sediment samples were collected using a grab sampler in Liaohe River, China.
2.3. Treatment of contaminated sediment The schematic diagram of the experimental apparatus was illustrated in Fig. 1. The reaction system consisted of a pulsed high voltage power supply and a reactor vessel, the detailed introduction on power supply and reactor were presented in S3. The typical pulse discharge voltage and current waveforms obtained in the experiment were shown in Fig. S1 in the SI. For each bench test, 10 g PNP contaminated sediment was transferred into discharge plasma reactor, and then a solid matrix suspension (sediment slurry) was prepared by adding 50 mL deionized water into the sediment. After stirring the suspension for a few minutes, so as to obtain homogeneous slurry, the pulse discharge plasma was triggered. In order to control reaction temperature, the reactor vessel was wrapped by a cooling system in the whole discharge plasma process. Air was used as carrier gas in this study, unless special illustration, which entered the reactor through the hypodermic needles. PNP concentration in the sediment was measured after desirable treatment time. 2.4. Extraction and analysis After discharge plasma treatment, PNP and its degradation intermediates in sediment were extracted immediately, and the extraction and analysis procedures were described in S4. FTIR analysis, PNP degradation efficiency and energy yield calculation were all described in S4. H2O2 concentration was determined by UVeVis spectrophotometry at wavelength of 400 nm (Sellers, 1980). All experiments were conducted in duplicates. 3. Results and discussion 3.1. PNP degradation in dredged sediment In order to evaluate the efficacy of PNP removal from sediment by pulse discharge plasma system, PNP degradation characteristics in sediment under different pulse peak discharge voltages were firstly investigated, and the results were shown in Fig. 2. Herein, the air flow rate was 0.8 L min1. Obviously, great PNP degradation performance was observed and its degradation efficiency increased with the pulse peak discharge voltage. At peak discharge voltage of 19.0 kV, about 92.9% of PNP was removed from sediment within 60 min's discharge plasma treatment, while approximately 84.1% and 68.8% of PNP were degraded at peak discharge voltages of 17.0 and 15.0 kV within the same time, respectively. PNP removal process in sediment by pulse discharge plasma could be described by the first-order kinetic model, and the kinetic curves under different peak discharge voltages were shown in Fig. 2 (the inset figure). As presented, with the pulse peak discharge voltage increased from 15.0 to 19.0 kV, the reaction rate constant enhanced from 0.019 to 0.055 min1. It is generally believed that
2.2. Sediment and slurry preparation The physical and chemical properties of sediment samples were presented in S2. The contents of main metal oxides of the sediment were analyzed by XRF (S4 EXPLORER, Germany), and the results were shown in Table S1 in the SI; the main metal oxides were SiO2, Al2O3, Fe2O3, CaO, MgO, K2O and Na2O, and their contents were 55.1%, 6.12%, 3.82%, 3.06%, 1.24%, 0.21% and 0.03%, respectively; in addition, loss on ignition took up 8.54%. The preparation processes of PNP spiked sediment were presented in S2, and finally its concentration in the dry sediment was 1000 mg kg1.
19
Fig. 1. Schematic diagram of the experimental setup.
20
T. Wang et al. / Water Research 84 (2015) 18e24
Fig. 2. Effect of pulse peak discharge voltage on PNP degradation in sediment. Fig. 3. Effect of gas types on PNP degradation in sediment.
more high-energetic electrons are produced at higher pulse peak discharge voltage, leading to the accelerated generation of active species such as O3, OH radical and H2O2 (Bian et al., 2007); these chemically active species have strong oxidation capacity, which can promote PNP removal. In our previous research, more ozone and H2O2 were also detected at higher pulse peak discharge voltage, which in turn enhanced PNP removal from soil particles (Wang et al., 2011). Because of lack of research reports on PNP removal from sediments, methods on PNP removal from soil slurry were collected and listed in Table 1 in order to be preliminarily compared with the present method. It could be seen that PNP removal efficacy obtained by Fenton reaction (Ye and Lemley, 2009) was comparable with that of the present research; almost all of PNP in montmorillonite clay slurry was removed within 30 min's treatment by Fenton reaction, while 92.9% of PNP was removed from sediment in the present research with higher initial PNP concentration and handling capacity. Biotreatment is a traditional method for pollutant degradation, and it can also be employed to highefficiently remove PNP from soil slurry (Tomei et al., 2013); however, the degradation processes were much slower than that in the present study, and it took 24 h to obtain about 75%e93% of PNP removal efficiency. Hence, it is believed that it is an alternative and efficient method for pulse discharge plasma to remove PNP in sediment. In addition, the energy yields for PNP degradation in sediment after 60 min's discharge treatment were 572, 469, and 407 g mg kWh1 at 15.0, 17.0, and 19.0 kV, respectively. Considering PNP degradation and energy efficiency comprehensively, later experiments were conducted at 17.0 kV. 3.2. Roles of active species in PNP degradation Roles of some active species in PNP degradation in sediment were studied by various gas plasmas, OH radical scavenger,
hydrated electron scavenger, and O$ 2 scavenger. 3.2.1. Effect of various gas plasmas The varieties and numbers of oxidative species are considerably different in various gas atmospheres (Sun et al., 1997). PNP degradation experiments were conducted separately under O2, N2, air, argon (Ar), and pure ozonation, respectively, and the results were shown in Fig. 3. Obviously, PNP degradation efficiency in O2 plasma was the highest among all gas atmospheres and the degradation efficiency of 95.9% was achieved within 60 min's treatment; while which were 84.1%, 58.1%, and 33.5% in air, Ar, and N2 plasmas within the same treatment time, respectively. In addition, only 44.1% of PNP was removed by pure ozonation, and herein O3 concentration was equal to that obtained by the discharge treatment under O2 atmosphere. These results indicated that there were other active species involved with PNP reactions in addition to O3 in the case of O2 plasma; in the case of Ar and N2 plasmas, PNP degradation was attributed mainly to OH radical. Furthermore, it was worth mentioning that the formation rate of OH radical was ranked in decrease as Ar > N2 > air > O2 in discharge plasma system containing water molecules (Bian et al., 2007), while the trends of PNP degradation efficiency in the present research were quite different with OH radical formation trends; these different trends also suggested that there were other active species involved in PNP degradation in addition to OH radical. Under O2 atmosphere, the input energy was mainly consumed to produce the O-reagents, whose concentrations were larger than those under air atmosphere, resulting in higher PNP degradation efficiency. For pure ozonation, only O3 participated in PNP degradation, and thus very low degradation efficiency was obtained. It is known that the bond dissociation energy of N2 (9.82 eV) is higher than that of O2 (5.12 eV), and thus it is much easier for O2 to dissociate than N2 when pulse discharge occurs under air atmosphere, and therefore O radical and O3 may be generated more
Table 1 The brief comparison of the present method with other techniques. System
Initial PNP concentration (mg L1)
Treatment time
Removal efficiency (%)
Moisture content (%)
Solid amount (g)
Reference
Fenton reaction Two-phase bioreactor Slurry batch bioreactor This study
100 2000 200 200
30 24 24 60
z100 93 75 92.9
z99 z83 z83 z83
1 200 200 10
Ye and Lemley, 2009 Tomei et al., 2013 Tomei et al., 2013 e
min h h min
T. Wang et al. / Water Research 84 (2015) 18e24 þ easily than N, Nþ 2 and N (Fancey, 1995). More importantly, the oxidation capacities of O radical and O3 are much stronger than þ those of atomic N, Nþ 2 and N , so PNP degradation efficiency in sediment was much higher in air plasma than that in N2 plasma, although more OH radical could be generated in N2 atmosphere that in air atmosphere. As compared with N2, the inert gas Ar is monoatomic and its atom has a closed-shell structure, and electron collisions are essentially elastic, and therefore electrons in Ar plasma have higher energy than in N2 plasma, which promoted OH radical formation (Bian et al., 2007) and benefited PNP degradation.
3.2.2. Effect of OH radical scavenger OH radical is very short-lived active species, and some scavengers such as n-butanol, salicylic acid and isopropanol were usually employed to indirectly characterize its roles in pollutants degradation. The effect of n-butanol on PNP degradation in sediment was evaluated and the result was shown in Fig. 4. The addition of n-butanol into sediment exhibited obvious inhibiting effects on PNP degradation. About 84.1% of PNP in sediment was degraded within 60 min's discharge plasma treatment without n-butanol addition, while there was a 25.3% decline in PNP degradation efficiency with 0.2 mmol L1 n-butanol addition, and PNP degradation efficiency further decreased to 44.6% with 0.8 mmol L1 n-butanol addition. These results suggested that OH radical played an important role in PNP degradation in sediment. Similar phenomenon was also found in our previous research, where the addition of n-butanol into polluted soil scavenged OH radical generated by multi-channel pulse discharge plasma, and then inhibited PNP degradation in soil (Wang et al., 2014). 3.2.3. Effect of hydrated electrons scavenger The formation and yield of active species are dependence on hydrated electrons (e aq) to a certain extent. Phosphate and nitrate can react rapidly with e aq, and they are usually chosen as eaq scavengers (Buxton et al., 1995; Yakabuskie et al., 2011; Zhang et al., 2012). 2 e aq þ NO3 /NO3
k ¼ 9:7 109 M1 s1
$ NO2 3 þ H2 O/ NO2 þ2OH
k ¼ 5:5 104 M1 s1
Fig. 4. Effect of n-butanol additive on PNP degradation in sediment.
(1) (2)
21
· e aq þ NO2 /NO2
k ¼ 1:0 1010 M1 s1
2 e aq þ H2 PO4 /H2 PO4
k ¼ 1:9 107 M1 s1
(3) (4)
H2PO 4
The effect of on PNP degradation in sediment was presented in Fig. 5. Obviously, the addition of H2PO 4 into sediment exhibited inhibiting effects on PNP degradation. PNP degradation efficiency reached 84.1% within 60 min's discharge plasma treatment without H2PO 4 addition, while it decreased to 48.1% with 0.2 mmol L1 H2PO 4 addition, and it further decreased to 34.1% with 0.8 mmol L1 H2PO 4 addition. These results suggested that eaq played a significant role in PNP degradation in sediment. When a certain concentration of H2PO 4 was added into polluted sediment, the hydrated electrons would be scavenged by H2PO-4, and then the formation and yields of chemically active species would be reduced because the primary chemical effects of non-thermal discharge plasma is assumed to generate active species from the dissociation and/or ionization of water molecules by electrons collisions, which was not beneficial for PNP degradation. Similar result was also reported by Leitner et al. (2005), who found that atrazine removal efficiency decreased significantly in pulsed arc electro-hydraulic discharge system with H2PO-4 addition. In addition, microcystinLR degradation efficiency also decreased obviously in water by glow discharge plasma oxidation when a certain amount of HPO 4 was added into water solutions (Zhang et al., 2012). 3.2.4. Effect of O$ 2 scavenger During discharge plasma process, H is produced from electrons impact dissociation of water molecules, and it will react with O2 rapidly to form HO2 that has a pH dependent equilibrium with O$ 2 (Sahni and Locke, 2006), HO2 and O$ 2 are both important intermediates which promote H2O2 formation (see reactions 5e9). Tetranitromethane, nitroblue tetrazolium chloride and 1,4benzoquinone are usually used to capture O$ (Zhang et al., 2 2006). The effect of 1,4-benzoquinone on PNP degradation in sediment was evaluated and the result was shown in Fig. 6. The presence of 1,4-benzoquinone exhibited obviously inhibiting effect on PNP degradation, and PNP degradation efficiency decreased to 68.8% and 52.0% with 0.2 mmol L1 and 0.8 mmol L1 1,4benzoquinone added into sediment within 60 min's discharge plasma treatment, respectively. In the presence of 1,4benzoquinone, lots of O$ 2 would be captured, and then the yield
Fig. 5. Effect of H2PO 4 additive on PNP degradation in sediment.
22
T. Wang et al. / Water Research 84 (2015) 18e24
Fig. 8. Total ion chromatogram of intermediates of PNP degradation after 45 min's discharge treatment.
Fig. 6. Effect of 1,4-benzoquinone additive on PNP degradation in sediment.
of H2O2 would decrease, which could be seen in Table S2. These results suggested that O$ and H2O2 both participated in PNP 2 removal process. Zhang et al. (2006) employed 1,4-benzoquinone as O 2 scavenger to evaluate its contribution to acid orange 7 degradation in water, and also found that about 93.8% of acid orange 7 was degraded by O 2 reaction.
e þ H2 O/· OH þ · H þ e
(5)
HþO2 /HO$2
(6)
·
pKa¼4:8
þ HO$2 ! O$ 2 þH
(7)
þ HO$2 þ O$ 2 þ H /H2 O2 þ O2
(8)
HO$2 þ e þ Hþ /H2 O2
(9)
3.3. PNP degradation process in sediment PNP degradation process in pulse discharge plasma system was studied by FTIR analysis and degradation intermediates detection.
3.3.1. FTIR analysis The FTIR spectra of dry sediment samples before and after treatment were depicted in Fig. 7. The wide band at 3100e3700 cm1 was OeH functional group, and the narrow band at 1033 cm1 was usually assigned to CeO stretches in lactonic, ether and phenol groups (Wang et al., 2010). As shown in Fig. 7, the intensity of OeH and CeO functional groups both decreased obviously after pulse discharge plasma treatment, especially at 1033 cm1. This result suggested that PNP molecular structure in sediment was disrupted after discharge plasma treatment. 3.3.2. Degradation intermediates PNP degradation intermediates in sediment were preliminarily analyzed using ion chromatography (IC) and HPLC. Hydroquinone, benzoquinone, phenol, acetic acid, NO 2 and NO3 were detected by HPLC and IC, and the total chromatograms of degradation intermediates after 45 min's discharge plasma treatment were shown in Fig. 8 and Fig. 9. Herein, PNP degradation experiment in sediment by pulse discharge plasma was conducted in O2 atmosphere, in order to eliminate the influence of N2 on NO 2 and NO3 formation. In PNP molecular structure, the phenolic hydroxyl group is electron-donating, and the eNO2 group is electron-withdrawing. The electron-donating substituent increases electron density at the -ortho and -para positions, while the electron-withdrawing substituent is strongly deactivating and -meta directing. Therefore, the attacks of chemically active species such as OH radical will occur preferentially in -ortho and -para positions with respect to the eOH group (Lukes and Locke, 2005). The nitro group in nitroaromatics is a leaving group which can be easily eliminated (Wang et al., 2011). Therefore, the 1,4-benzosemiquinone intermediate could be generated by the attacks of active species to PNP molecule, accompanied by HNO2 elimination; and subsequently, 1,4benzosemiquinone was further disproportionated into hydroquinone and benzoquinone (Liu et al., 2010), as shown in reaction 10.
(10) Fig. 7. FTIR spectra of PNP contaminated sediment before and after treatment.
The attacks of active species to -para position with respect to the
T. Wang et al. / Water Research 84 (2015) 18e24
eOH group could also generate hydroquinone (Di Paola et al., 2003), accompanied by eNO2 elimination; and subsequently, hydroquinone would be further oxidized into benzoquinone, as shown in reaction 11.
23
the formation of ring-opening byproducts such as organic acid, accompanied by NO x releasing. This study was a fundamental research effort, trying to offer a possible way for the application of pulse discharge plasma in sediment remediation. Acknowledgements
(11) The direct breakage of CeN bond could result in the formation of phenol during PNP oxidation process (Dai et al., 2008), and then phenol was further oxidized by discharge plasma into hydroquinone and benzoquinone, as presented in reaction 12.
The authors thank the Project funded by China Postdoctoral Science Foundation (2014M562460), and the Initiative Funding Programs for Doctoral Research of Northwest A&F University (2013BSJJ121) for the financial supports to this research. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.watres.2015.07.022. References
(12) The aromatic rings of quinones are opened easily by cycloaddition reactions of O3, and followed by the formation of small organic acids. Therefore, further attacks of O3 to benzoquinone in the present research would lead to the formation of acetic acid.
4. Conclusions The application of pulsed discharge plasma in removing PNP in dredged sediment was studied. Great performance for PNP degradation in sediment was obtained, and the degradation process fitted the first-order kinetic model. PNP degradation experiments under various gas atmospheres and different species scavengers $ showed that O3, OH radical, e aq and O2 played important roles in PNP degradation in sediment. PNP molecular structure was destroyed significantly after discharge plasma treatment. The intermediate products were monitored by HPLC and IC. Identified products were hydroquinone, benzoquinone, phenol, acetic acid, NO 2 and NO3 . The obtained intermediates suggested that the attacks of active species at CeN position in PNP molecular structure triggered its degradation to generate hydroxylated intermediates, and further attacks on these hydroxylated intermediates resulted in
Fig. 9. Total liquid chromatogram of intermediates of PNP degradation after 45 min's discharge treatment.
Bian, W.J., Zhou, M.H., Lei, L.C., 2007. Formations of active species and by-products in water by pulsed high-voltage discharge. Plasma Chem. Plasma Process 27 (3), 337e348. Borgnino, L., Avena, M., De Pauli, C., 2006. Surface properties of sediments from two Argentinean reservoirs and the rate of phosphate release. Water Res. 40 (14), 2659e2666. Buxton, G.V., Mulazzani, Q.G., Ross, A.B., 1995. Critical-review of rate constants for reactions of transients from metal-ions and metal-complexes in aqueous-solution. J. Phys. Chem. Ref. Data 24 (3), 1055e1349. Dai, Q.Z., Lei, L.C., Zhang, X.W., 2008. Enhanced degradation of organic wastewater containing p-nitrophenol by a novel wet electrocatalytic oxidation process: parameter optimization and degradation mechanism. Sep. Purif. Technol. 61 (2), 123e129. Di Paola, A., Augugliaro, V., Palmisano, L., Pantaleo, G., Savinov, E., 2003. Heterogeneous photocatalytic degradation of nitrophenols. J. Photochem. Photobiol. A 155 (1e3), 207e214. Fancey, K.S., 1995. An investigation into dissociative mechanisms in nitrogenous glow-discharges by optical-emission spectroscopy. Vacuum 46 (7), 695e700. Ferrarese, E., Andreottola, G., Oprea, I.A., 2008. Remediation of PAH-contaminated sediments by chemical oxidation. J. Hazard. Mater. 152 (1), 128e139. Hong, P.K.A., Nakra, S., Kao, C.M.J., Hayes, D.F., 2008. Pressure-assisted ozonation of PCB and PAH contaminated sediments. Chemosphere 72 (11), 1757e1764. Hu, Z.B., Navarro, R., Nomura, N., Kong, H.N., Wijesekara, S., Matsumura, M., 2007. Changes in chlorinated organic pollutants and heavy metal content of sediments during pyrolysis. Environ. Sci. Pollut. Res. 14 (1), 12e18. Josefsson, S., Schaanning, M., Samuelsson, G.S., Gunnarsson, J.S., Olofsson, I., Eek, E., Wiberg, K., 2012. Capping efficiency of various carbonaceous and mineral materials for in situ remediation of polychlorinated dibenzo-p-dioxin and dibenzofuran contaminated marine sediments: sediment-to-water fluxes and bioaccumulation in boxcosm tests. Environ. Sci. Technol. 46 (6), 3343e3351. Kumar, M., Furumai, H., Kurisu, F., Kasuga, I., 2013. Potential mobility of heavy metals through coupled application of sequential extraction and isotopic exchange: comparison of leaching tests applied to soil and soakaway sediment. Chemosphere 90 (2), 796e804. Larry, K., William, T., 1979. ES&T special report: priority pollutants: I-a perspective view. Environ. Sci. Technol. 13, 416e423. Leitner, N.K.V., Syoen, G., Romat, H., Urashima, K., Chang, J.S., 2005. Generation of active entities by the pulsed arc electrohydraulic discharge system and application to removal of atrazine. Water Res. 39 (19), 4705e4714. Levitt, J.S., N'Guessan, A.L., Rapp, K.L., Nyman, M.C., 2003. Remediation of alphamethylnaphthalene-contaminated sediments using peroxy acid. Water Res. 37 (12), 3016e3022. Li, T.P., Yuan, S.H., Wan, J.Z., Lin, L., Long, H.Y., Wu, X.F., Lu, X.H., 2009. Pilot-scale electrokinetic movement of HCB and Zn in real contaminated sediments enhanced with hydroxypropyl-beta-cyclodextrin. Chemosphere 76 (9), 1226e1232. Liu, Y.J., Wang, D.G., Sun, B., Zhu, X.M., 2010. Aqueous 4-nitrophenol decomposition and hydrogen peroxide formation induced by contact glow discharge electrolysis. J. Hazard. Mater. 181 (1e3), 1010e1015. Lukes, P., Locke, B.R., 2005. Degradation of substituted phenols in a hybrid gasliquid electrical discharge reactor. Ind. Eng. Chem. Res. 44 (9), 2921e2930. Payne, R.B., Fagervold, S.K., May, H.D., Sowers, K.R., 2013. Remediation of polychlorinated biphenyl impacted sediment by concurrent bioaugmentation with anaerobic halorespiring and aerobic degrading bacteria. Environ. Sci. Technol. 47 (8), 3807e3815. Redell, C.J., Elmore, A.C., Burken, J.G., Stringer, R.D., 2011. Waterjet injection of powdered activated carbon for sediment remediation. J. Soil. Sediment. 11 (6), 1115e1124. Sahni, M., Locke, B.R., 2006. Quantification of reductive species produced by high
24
T. Wang et al. / Water Research 84 (2015) 18e24
voltage electrical discharges in water. Plasma Process. Polym. 3 (4e5), 342e354. Seidel, H., Loser, C., Zehnsdorf, A., Hoffmann, P., Schmerold, R., 2004. Bioremediation process for sediments contaminated by heavy metals: feasibility study on a pilot scale. Environ. Sci. Technol. 38 (5), 1582e1588. Sellers, R.M., 1980. Spectrophotometric determination of hydrogen-peroxide using potassium titanium (IV) oxalate. Analyst 105, 950e954. Sun, B., Sato, M., Clements, J.S., 1997. Optical study of active species produced by a pulsed streamer corona discharge in water. J. Electrostat. 39 (3), 189e202. Tomei, M.C., Angelucci, D.M., Annesini, M.C., Daugulis, A.J., 2013. Ex situ remediation of polluted soils by absorptive polymers, and a comparison of slurry and twophase partitioning bioreactors for ultimate contaminant degradation. J. Hazard. Mater. 262, 31e37. Truex, M.J., Macbeth, T.W., Vermeul, V.R., Fritz, B.G., Mendoza, D.P., Mackley, R.D., Wietsma, T.W., Sandberg, G., Powell, T., Powers, J., Pitre, E., Michalsen, M., Ballock-Dixon, S.J., Zhong, L., Oostrom, M., 2011. Demonstration of combined zero-valent iron and electrical resistance heating for in situ trichloroethene remediation. Environ. Sci. Technol. 45 (12), 5346e5351. USEPA, 1994. Remediation Guidance Document, EPA-905-B94-003, Assessment and Remediation of Contaminated Sediments Program. Great Lakes National Program Office, Chicago, Illinois. Wang, T.C., Lu, N., Li, J., Wu, Y., 2010. Evaluation of the potential of pentachlorophenol degradation in soil by pulsed corona discharge plasma from soil
characteristics. Environ. Sci. Technol. 44 (8), 3105e3110. Wang, T.C., Lu, N., Li, J., Wu, Y., 2011. Plasma-TiO2 catalytic method for highefficiency remediation of p-nitrophenol contaminated soil in pulsed discharge. Environ. Sci. Technol. 45 (21), 9301e9307. Wang, T.C., Qu, G.Z., Li, J., Liang, D.L., 2014. Evaluation of the potential of soil remediation by direct multi-channel pulsed corona discharge in soil. J. Hazard. Mater. 264, 169e175. Yakabuskie, P.A., Joseph, J.M., Stuart, C.R., Wren, J.C., 2011. Long-term gammaradiolysis kinetics of NO-3 and NO-2 solutions. J. Phys. Chem. A 115 (17), 4270e4278. Ye, P., Lemley, A.T., 2009. Adsorption effect on the degradation of 4,6-o-dinitrocresol and p-nitrophenol in a montmorillonite clay slurry by AFT. Water Res. 43 (5), 1303e1312. Zhang, H., Huang, Q., Ke, Z.G., Yang, L.F., Wang, X.Q., Yu, Z.L., 2012. Degradation of microcystin-LR in water by glow discharge plasma oxidation at the gas-solution interface and its safety evaluation. Water Res. 46 (19), 6554e6562. Zhang, X.W., Li, G.T., Wang, Y.Z., Qu, J.H., 2006. Microwave electrodeless lamp photolytic degradation of acid orange 7. J. Photochem. Photobiol. A 184 (1e2), 26e33. Zhang, X.Y., Guo, Q., Hu, Y.Y., Lin, H., 2013. Effects of monorhamnolipid and dirhamnolipid on sorption and desorption of triclosan in sediment-water system. Chemosphere 90 (2), 581e587.
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Evaluation of the potential of p-nitrophenol degradation in dredged sediment by pulsed discharge plasma Tiecheng Wang a, b, *, Guangzhou Qu a, b, Qiuhong Sun c, Dongli Liang a, b, Shibin Hu a, b a
College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi 712100, PR China c Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2015 Received in revised form 26 June 2015 Accepted 13 July 2015 Available online 16 July 2015
Hazardous pollutants in dredged sediment pose great threats to ecological environment and human health. A novel approach, named pulsed discharge plasma (PDP), was employed for the degradation of pnitrophenol (PNP) in dredged sediment. Experimental results showed that 92.9% of PNP in sediment was smoothly removed in 60 min, and the degradation process fitted the first-order kinetic model. Roles of some active species in PNP degradation in sediment were studied by various gas plasmas, OH radical scavenger, hydrated electron scavenger and O$ 2 scavenger; and the results presented that O3, OH radical, $ $ e aq and O2 all played significant roles in PNP removal, and eaq and O2 mainly participated in other oxidising active species formation. FTIR analysis showed that PNP molecular structure was destroyed after PDP treatment. The main degradation intermediates were identified as hydroquinone, benzoqui none, phenol, acetic acid, NO 2 and NO3 . PNP degradation pathway in dredged sediment was proposed. It is expected to contribute to an alternative for sediment remediation by pulse discharge plasma. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pulse discharge plasma Dredged sediment Sediment remediation p-Nitrophenol
1. Introduction Water sediment is one of the most important constituent parts of aquatic ecosystem. With the quick development of industrial manufacture, large numbers of hazardous pollutants have been entering water body via various ways such as atmospheric deposition, wastewater discharge, and rain flushing (Borgnino et al., 2006; Payne et al., 2013). These hazardous contaminants will deposit into the water bottom and sequentially gather in the sediment, resulting in sediment pollution (Payne et al., 2013). The main pollutants in the sediment are heavy metals and persistent organic pollutants. Even worse, the sediment that contains various contaminants is not only a “reservoir” where hazardous pollutants are deposited but also an active compartment with a fundamental role in the redistribution of the hazardous pollutants into the overlying water. That is, sediment will become the secondary pollution source of the water body when external pollution sources of rivers are effectively controlled (Borgnino et al., 2006). Thus, the presence of the hazardous pollutants in the sediment is a serious threat to
* Corresponding author. College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.watres.2015.07.022 0043-1354/© 2015 Elsevier Ltd. All rights reserved.
the aquatic environment and human health (Zhang et al., 2013). Sediment remediation is a quite significant pathway to resolve river pollution problem. Conventional sediment remediation technologies available include in-situ remediation and ex-situ dredging. Typical in-situ remediation methods include capping, inactivation, chemical oxidation and reduction, and the reduction of bioavailability using amendments (Truex et al., 2011; Josefsson et al., 2012). However, many concerns are present when the in-situ remediation methods are implemented due to the possibility of river storage capacity decrease, riverbed shallows, residual contamination, and biointrusion etc. (Redell et al., 2011; Truex et al., 2011; Josefsson et al., 2012). At present, ex-situ dredging is widely used for sediment remediation, and then other techniques are employed to remove pollutants from dredged sediment, such as drip washing (Kumar et al., 2013), bioremediation (Seidel et al., 2004), pyrolysis (Hu et al., 2007), electrokinetic remediation (Li et al., 2009), and landfilling and incineration (USEPA, 1994). Some problems such as secondary pollution, and time-consuming still exist for posttreatment of dredged sediment. Therefore, it is highly recommended to explore high-efficient and rapid method to treat dredged sediment. Recently, chemical oxidation by advanced oxidation processes
T. Wang et al. / Water Research 84 (2015) 18e24
(AOPs) and ozone have been received great emphasis on sediment remediation because of their ability to rapidly oxidize refractory organic contaminants in sediment, such as Fenton reaction (Ferrarese et al., 2008), peroxy acids (Levitt et al., 2003) and ozonation (Hong et al., 2008). Pulsed discharge plasma, one of the AOPs, has aroused considerable interest in pollution control because of its high removal efficiency and environmental compatibility, such as wastewater treatment (Bian et al., 2007). During pulsed discharge plasma processes, the ensuring electronemolecule interactions generate highly reactive non-thermal plasma, which are strongly oxidizing environments due to the presence of large number of chemically active species, such as þ þ ozone, H2O2, and OH radicals, O atoms, and ions (O 2 , O2 , H3O ); simultaneously, physical effects concomitantly generated during discharge plasma process such as ultraviolet light can also improve pollutants removal. Our previous studies have also demonstrated that organic compounds in soil could be effectively removed by pulsed discharge plasma (Wang et al., 2011). Hence, it is believed that it may be powerful enough for pulsed discharge plasma to remove organic pollutants in dredged sediment, which usually contains 80%e95% water content. When pulse discharge plasma occurs, high-energetic electrons are formed and then various active species (such as OH, H2O2 and O3) are produced; on the one hand, organic pollutants in dredged sediment may be excited, ionized and dissociated by high-energetic electrons; on the other hand, various active species can react with the organic pollutants rapidly; furthermore, the presence of natural metallic minerals in sediment may enable the Fenton reaction to proceed due to the generation of H2O2 by pulsed discharge plasma. Because of the strong oxidation environment of pulse discharge plasma, organic pollutants in the dredged sediment may be removed effectively and rapidly. However, to the best of our knowledge, little has been reported about organic pollutants removal from sediment by pulsed discharge plasma. This paper describes research on p-nitrophenol (PNP) degradation in sediment using pulsed discharge plasma. PNP has been listed as the 129 priority toxic pollutants by U.S. Environmental Protection Agency (Larry and William, 1979). The present study was primarily focused on evaluating the efficacy of pulse discharge plasma for PNP degradation in sediment. The roles of various active species in PNP removal from dredged sediment were discussed. PNP removal processes in dredged sediment in such a system was also proposed. This study is expected as preliminary investigation for development of a new approach that can be used as a fastacting, and ex-situ remediation technique for dredged sediment. 2. Materials and methods 2.1. Materials PNP was used in the study, and its detailed introduction was presented in S1 of Supporting Information (SI). Sediment samples were collected using a grab sampler in Liaohe River, China.
2.3. Treatment of contaminated sediment The schematic diagram of the experimental apparatus was illustrated in Fig. 1. The reaction system consisted of a pulsed high voltage power supply and a reactor vessel, the detailed introduction on power supply and reactor were presented in S3. The typical pulse discharge voltage and current waveforms obtained in the experiment were shown in Fig. S1 in the SI. For each bench test, 10 g PNP contaminated sediment was transferred into discharge plasma reactor, and then a solid matrix suspension (sediment slurry) was prepared by adding 50 mL deionized water into the sediment. After stirring the suspension for a few minutes, so as to obtain homogeneous slurry, the pulse discharge plasma was triggered. In order to control reaction temperature, the reactor vessel was wrapped by a cooling system in the whole discharge plasma process. Air was used as carrier gas in this study, unless special illustration, which entered the reactor through the hypodermic needles. PNP concentration in the sediment was measured after desirable treatment time. 2.4. Extraction and analysis After discharge plasma treatment, PNP and its degradation intermediates in sediment were extracted immediately, and the extraction and analysis procedures were described in S4. FTIR analysis, PNP degradation efficiency and energy yield calculation were all described in S4. H2O2 concentration was determined by UVeVis spectrophotometry at wavelength of 400 nm (Sellers, 1980). All experiments were conducted in duplicates. 3. Results and discussion 3.1. PNP degradation in dredged sediment In order to evaluate the efficacy of PNP removal from sediment by pulse discharge plasma system, PNP degradation characteristics in sediment under different pulse peak discharge voltages were firstly investigated, and the results were shown in Fig. 2. Herein, the air flow rate was 0.8 L min1. Obviously, great PNP degradation performance was observed and its degradation efficiency increased with the pulse peak discharge voltage. At peak discharge voltage of 19.0 kV, about 92.9% of PNP was removed from sediment within 60 min's discharge plasma treatment, while approximately 84.1% and 68.8% of PNP were degraded at peak discharge voltages of 17.0 and 15.0 kV within the same time, respectively. PNP removal process in sediment by pulse discharge plasma could be described by the first-order kinetic model, and the kinetic curves under different peak discharge voltages were shown in Fig. 2 (the inset figure). As presented, with the pulse peak discharge voltage increased from 15.0 to 19.0 kV, the reaction rate constant enhanced from 0.019 to 0.055 min1. It is generally believed that
2.2. Sediment and slurry preparation The physical and chemical properties of sediment samples were presented in S2. The contents of main metal oxides of the sediment were analyzed by XRF (S4 EXPLORER, Germany), and the results were shown in Table S1 in the SI; the main metal oxides were SiO2, Al2O3, Fe2O3, CaO, MgO, K2O and Na2O, and their contents were 55.1%, 6.12%, 3.82%, 3.06%, 1.24%, 0.21% and 0.03%, respectively; in addition, loss on ignition took up 8.54%. The preparation processes of PNP spiked sediment were presented in S2, and finally its concentration in the dry sediment was 1000 mg kg1.
19
Fig. 1. Schematic diagram of the experimental setup.
20
T. Wang et al. / Water Research 84 (2015) 18e24
Fig. 2. Effect of pulse peak discharge voltage on PNP degradation in sediment. Fig. 3. Effect of gas types on PNP degradation in sediment.
more high-energetic electrons are produced at higher pulse peak discharge voltage, leading to the accelerated generation of active species such as O3, OH radical and H2O2 (Bian et al., 2007); these chemically active species have strong oxidation capacity, which can promote PNP removal. In our previous research, more ozone and H2O2 were also detected at higher pulse peak discharge voltage, which in turn enhanced PNP removal from soil particles (Wang et al., 2011). Because of lack of research reports on PNP removal from sediments, methods on PNP removal from soil slurry were collected and listed in Table 1 in order to be preliminarily compared with the present method. It could be seen that PNP removal efficacy obtained by Fenton reaction (Ye and Lemley, 2009) was comparable with that of the present research; almost all of PNP in montmorillonite clay slurry was removed within 30 min's treatment by Fenton reaction, while 92.9% of PNP was removed from sediment in the present research with higher initial PNP concentration and handling capacity. Biotreatment is a traditional method for pollutant degradation, and it can also be employed to highefficiently remove PNP from soil slurry (Tomei et al., 2013); however, the degradation processes were much slower than that in the present study, and it took 24 h to obtain about 75%e93% of PNP removal efficiency. Hence, it is believed that it is an alternative and efficient method for pulse discharge plasma to remove PNP in sediment. In addition, the energy yields for PNP degradation in sediment after 60 min's discharge treatment were 572, 469, and 407 g mg kWh1 at 15.0, 17.0, and 19.0 kV, respectively. Considering PNP degradation and energy efficiency comprehensively, later experiments were conducted at 17.0 kV. 3.2. Roles of active species in PNP degradation Roles of some active species in PNP degradation in sediment were studied by various gas plasmas, OH radical scavenger,
hydrated electron scavenger, and O$ 2 scavenger. 3.2.1. Effect of various gas plasmas The varieties and numbers of oxidative species are considerably different in various gas atmospheres (Sun et al., 1997). PNP degradation experiments were conducted separately under O2, N2, air, argon (Ar), and pure ozonation, respectively, and the results were shown in Fig. 3. Obviously, PNP degradation efficiency in O2 plasma was the highest among all gas atmospheres and the degradation efficiency of 95.9% was achieved within 60 min's treatment; while which were 84.1%, 58.1%, and 33.5% in air, Ar, and N2 plasmas within the same treatment time, respectively. In addition, only 44.1% of PNP was removed by pure ozonation, and herein O3 concentration was equal to that obtained by the discharge treatment under O2 atmosphere. These results indicated that there were other active species involved with PNP reactions in addition to O3 in the case of O2 plasma; in the case of Ar and N2 plasmas, PNP degradation was attributed mainly to OH radical. Furthermore, it was worth mentioning that the formation rate of OH radical was ranked in decrease as Ar > N2 > air > O2 in discharge plasma system containing water molecules (Bian et al., 2007), while the trends of PNP degradation efficiency in the present research were quite different with OH radical formation trends; these different trends also suggested that there were other active species involved in PNP degradation in addition to OH radical. Under O2 atmosphere, the input energy was mainly consumed to produce the O-reagents, whose concentrations were larger than those under air atmosphere, resulting in higher PNP degradation efficiency. For pure ozonation, only O3 participated in PNP degradation, and thus very low degradation efficiency was obtained. It is known that the bond dissociation energy of N2 (9.82 eV) is higher than that of O2 (5.12 eV), and thus it is much easier for O2 to dissociate than N2 when pulse discharge occurs under air atmosphere, and therefore O radical and O3 may be generated more
Table 1 The brief comparison of the present method with other techniques. System
Initial PNP concentration (mg L1)
Treatment time
Removal efficiency (%)
Moisture content (%)
Solid amount (g)
Reference
Fenton reaction Two-phase bioreactor Slurry batch bioreactor This study
100 2000 200 200
30 24 24 60
z100 93 75 92.9
z99 z83 z83 z83
1 200 200 10
Ye and Lemley, 2009 Tomei et al., 2013 Tomei et al., 2013 e
min h h min
T. Wang et al. / Water Research 84 (2015) 18e24 þ easily than N, Nþ 2 and N (Fancey, 1995). More importantly, the oxidation capacities of O radical and O3 are much stronger than þ those of atomic N, Nþ 2 and N , so PNP degradation efficiency in sediment was much higher in air plasma than that in N2 plasma, although more OH radical could be generated in N2 atmosphere that in air atmosphere. As compared with N2, the inert gas Ar is monoatomic and its atom has a closed-shell structure, and electron collisions are essentially elastic, and therefore electrons in Ar plasma have higher energy than in N2 plasma, which promoted OH radical formation (Bian et al., 2007) and benefited PNP degradation.
3.2.2. Effect of OH radical scavenger OH radical is very short-lived active species, and some scavengers such as n-butanol, salicylic acid and isopropanol were usually employed to indirectly characterize its roles in pollutants degradation. The effect of n-butanol on PNP degradation in sediment was evaluated and the result was shown in Fig. 4. The addition of n-butanol into sediment exhibited obvious inhibiting effects on PNP degradation. About 84.1% of PNP in sediment was degraded within 60 min's discharge plasma treatment without n-butanol addition, while there was a 25.3% decline in PNP degradation efficiency with 0.2 mmol L1 n-butanol addition, and PNP degradation efficiency further decreased to 44.6% with 0.8 mmol L1 n-butanol addition. These results suggested that OH radical played an important role in PNP degradation in sediment. Similar phenomenon was also found in our previous research, where the addition of n-butanol into polluted soil scavenged OH radical generated by multi-channel pulse discharge plasma, and then inhibited PNP degradation in soil (Wang et al., 2014). 3.2.3. Effect of hydrated electrons scavenger The formation and yield of active species are dependence on hydrated electrons (e aq) to a certain extent. Phosphate and nitrate can react rapidly with e aq, and they are usually chosen as eaq scavengers (Buxton et al., 1995; Yakabuskie et al., 2011; Zhang et al., 2012). 2 e aq þ NO3 /NO3
k ¼ 9:7 109 M1 s1
$ NO2 3 þ H2 O/ NO2 þ2OH
k ¼ 5:5 104 M1 s1
Fig. 4. Effect of n-butanol additive on PNP degradation in sediment.
(1) (2)
21
· e aq þ NO2 /NO2
k ¼ 1:0 1010 M1 s1
2 e aq þ H2 PO4 /H2 PO4
k ¼ 1:9 107 M1 s1
(3) (4)
H2PO 4
The effect of on PNP degradation in sediment was presented in Fig. 5. Obviously, the addition of H2PO 4 into sediment exhibited inhibiting effects on PNP degradation. PNP degradation efficiency reached 84.1% within 60 min's discharge plasma treatment without H2PO 4 addition, while it decreased to 48.1% with 0.2 mmol L1 H2PO 4 addition, and it further decreased to 34.1% with 0.8 mmol L1 H2PO 4 addition. These results suggested that eaq played a significant role in PNP degradation in sediment. When a certain concentration of H2PO 4 was added into polluted sediment, the hydrated electrons would be scavenged by H2PO-4, and then the formation and yields of chemically active species would be reduced because the primary chemical effects of non-thermal discharge plasma is assumed to generate active species from the dissociation and/or ionization of water molecules by electrons collisions, which was not beneficial for PNP degradation. Similar result was also reported by Leitner et al. (2005), who found that atrazine removal efficiency decreased significantly in pulsed arc electro-hydraulic discharge system with H2PO-4 addition. In addition, microcystinLR degradation efficiency also decreased obviously in water by glow discharge plasma oxidation when a certain amount of HPO 4 was added into water solutions (Zhang et al., 2012). 3.2.4. Effect of O$ 2 scavenger During discharge plasma process, H is produced from electrons impact dissociation of water molecules, and it will react with O2 rapidly to form HO2 that has a pH dependent equilibrium with O$ 2 (Sahni and Locke, 2006), HO2 and O$ 2 are both important intermediates which promote H2O2 formation (see reactions 5e9). Tetranitromethane, nitroblue tetrazolium chloride and 1,4benzoquinone are usually used to capture O$ (Zhang et al., 2 2006). The effect of 1,4-benzoquinone on PNP degradation in sediment was evaluated and the result was shown in Fig. 6. The presence of 1,4-benzoquinone exhibited obviously inhibiting effect on PNP degradation, and PNP degradation efficiency decreased to 68.8% and 52.0% with 0.2 mmol L1 and 0.8 mmol L1 1,4benzoquinone added into sediment within 60 min's discharge plasma treatment, respectively. In the presence of 1,4benzoquinone, lots of O$ 2 would be captured, and then the yield
Fig. 5. Effect of H2PO 4 additive on PNP degradation in sediment.
22
T. Wang et al. / Water Research 84 (2015) 18e24
Fig. 8. Total ion chromatogram of intermediates of PNP degradation after 45 min's discharge treatment.
Fig. 6. Effect of 1,4-benzoquinone additive on PNP degradation in sediment.
of H2O2 would decrease, which could be seen in Table S2. These results suggested that O$ and H2O2 both participated in PNP 2 removal process. Zhang et al. (2006) employed 1,4-benzoquinone as O 2 scavenger to evaluate its contribution to acid orange 7 degradation in water, and also found that about 93.8% of acid orange 7 was degraded by O 2 reaction.
e þ H2 O/· OH þ · H þ e
(5)
HþO2 /HO$2
(6)
·
pKa¼4:8
þ HO$2 ! O$ 2 þH
(7)
þ HO$2 þ O$ 2 þ H /H2 O2 þ O2
(8)
HO$2 þ e þ Hþ /H2 O2
(9)
3.3. PNP degradation process in sediment PNP degradation process in pulse discharge plasma system was studied by FTIR analysis and degradation intermediates detection.
3.3.1. FTIR analysis The FTIR spectra of dry sediment samples before and after treatment were depicted in Fig. 7. The wide band at 3100e3700 cm1 was OeH functional group, and the narrow band at 1033 cm1 was usually assigned to CeO stretches in lactonic, ether and phenol groups (Wang et al., 2010). As shown in Fig. 7, the intensity of OeH and CeO functional groups both decreased obviously after pulse discharge plasma treatment, especially at 1033 cm1. This result suggested that PNP molecular structure in sediment was disrupted after discharge plasma treatment. 3.3.2. Degradation intermediates PNP degradation intermediates in sediment were preliminarily analyzed using ion chromatography (IC) and HPLC. Hydroquinone, benzoquinone, phenol, acetic acid, NO 2 and NO3 were detected by HPLC and IC, and the total chromatograms of degradation intermediates after 45 min's discharge plasma treatment were shown in Fig. 8 and Fig. 9. Herein, PNP degradation experiment in sediment by pulse discharge plasma was conducted in O2 atmosphere, in order to eliminate the influence of N2 on NO 2 and NO3 formation. In PNP molecular structure, the phenolic hydroxyl group is electron-donating, and the eNO2 group is electron-withdrawing. The electron-donating substituent increases electron density at the -ortho and -para positions, while the electron-withdrawing substituent is strongly deactivating and -meta directing. Therefore, the attacks of chemically active species such as OH radical will occur preferentially in -ortho and -para positions with respect to the eOH group (Lukes and Locke, 2005). The nitro group in nitroaromatics is a leaving group which can be easily eliminated (Wang et al., 2011). Therefore, the 1,4-benzosemiquinone intermediate could be generated by the attacks of active species to PNP molecule, accompanied by HNO2 elimination; and subsequently, 1,4benzosemiquinone was further disproportionated into hydroquinone and benzoquinone (Liu et al., 2010), as shown in reaction 10.
(10) Fig. 7. FTIR spectra of PNP contaminated sediment before and after treatment.
The attacks of active species to -para position with respect to the
T. Wang et al. / Water Research 84 (2015) 18e24
eOH group could also generate hydroquinone (Di Paola et al., 2003), accompanied by eNO2 elimination; and subsequently, hydroquinone would be further oxidized into benzoquinone, as shown in reaction 11.
23
the formation of ring-opening byproducts such as organic acid, accompanied by NO x releasing. This study was a fundamental research effort, trying to offer a possible way for the application of pulse discharge plasma in sediment remediation. Acknowledgements
(11) The direct breakage of CeN bond could result in the formation of phenol during PNP oxidation process (Dai et al., 2008), and then phenol was further oxidized by discharge plasma into hydroquinone and benzoquinone, as presented in reaction 12.
The authors thank the Project funded by China Postdoctoral Science Foundation (2014M562460), and the Initiative Funding Programs for Doctoral Research of Northwest A&F University (2013BSJJ121) for the financial supports to this research. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.watres.2015.07.022. References
(12) The aromatic rings of quinones are opened easily by cycloaddition reactions of O3, and followed by the formation of small organic acids. Therefore, further attacks of O3 to benzoquinone in the present research would lead to the formation of acetic acid.
4. Conclusions The application of pulsed discharge plasma in removing PNP in dredged sediment was studied. Great performance for PNP degradation in sediment was obtained, and the degradation process fitted the first-order kinetic model. PNP degradation experiments under various gas atmospheres and different species scavengers $ showed that O3, OH radical, e aq and O2 played important roles in PNP degradation in sediment. PNP molecular structure was destroyed significantly after discharge plasma treatment. The intermediate products were monitored by HPLC and IC. Identified products were hydroquinone, benzoquinone, phenol, acetic acid, NO 2 and NO3 . The obtained intermediates suggested that the attacks of active species at CeN position in PNP molecular structure triggered its degradation to generate hydroxylated intermediates, and further attacks on these hydroxylated intermediates resulted in
Fig. 9. Total liquid chromatogram of intermediates of PNP degradation after 45 min's discharge treatment.
Bian, W.J., Zhou, M.H., Lei, L.C., 2007. Formations of active species and by-products in water by pulsed high-voltage discharge. Plasma Chem. Plasma Process 27 (3), 337e348. Borgnino, L., Avena, M., De Pauli, C., 2006. Surface properties of sediments from two Argentinean reservoirs and the rate of phosphate release. Water Res. 40 (14), 2659e2666. Buxton, G.V., Mulazzani, Q.G., Ross, A.B., 1995. Critical-review of rate constants for reactions of transients from metal-ions and metal-complexes in aqueous-solution. J. Phys. Chem. Ref. Data 24 (3), 1055e1349. Dai, Q.Z., Lei, L.C., Zhang, X.W., 2008. Enhanced degradation of organic wastewater containing p-nitrophenol by a novel wet electrocatalytic oxidation process: parameter optimization and degradation mechanism. Sep. Purif. Technol. 61 (2), 123e129. Di Paola, A., Augugliaro, V., Palmisano, L., Pantaleo, G., Savinov, E., 2003. Heterogeneous photocatalytic degradation of nitrophenols. J. Photochem. Photobiol. A 155 (1e3), 207e214. Fancey, K.S., 1995. An investigation into dissociative mechanisms in nitrogenous glow-discharges by optical-emission spectroscopy. Vacuum 46 (7), 695e700. Ferrarese, E., Andreottola, G., Oprea, I.A., 2008. Remediation of PAH-contaminated sediments by chemical oxidation. J. Hazard. Mater. 152 (1), 128e139. Hong, P.K.A., Nakra, S., Kao, C.M.J., Hayes, D.F., 2008. Pressure-assisted ozonation of PCB and PAH contaminated sediments. Chemosphere 72 (11), 1757e1764. Hu, Z.B., Navarro, R., Nomura, N., Kong, H.N., Wijesekara, S., Matsumura, M., 2007. Changes in chlorinated organic pollutants and heavy metal content of sediments during pyrolysis. Environ. Sci. Pollut. Res. 14 (1), 12e18. Josefsson, S., Schaanning, M., Samuelsson, G.S., Gunnarsson, J.S., Olofsson, I., Eek, E., Wiberg, K., 2012. Capping efficiency of various carbonaceous and mineral materials for in situ remediation of polychlorinated dibenzo-p-dioxin and dibenzofuran contaminated marine sediments: sediment-to-water fluxes and bioaccumulation in boxcosm tests. Environ. Sci. Technol. 46 (6), 3343e3351. Kumar, M., Furumai, H., Kurisu, F., Kasuga, I., 2013. Potential mobility of heavy metals through coupled application of sequential extraction and isotopic exchange: comparison of leaching tests applied to soil and soakaway sediment. Chemosphere 90 (2), 796e804. Larry, K., William, T., 1979. ES&T special report: priority pollutants: I-a perspective view. Environ. Sci. Technol. 13, 416e423. Leitner, N.K.V., Syoen, G., Romat, H., Urashima, K., Chang, J.S., 2005. Generation of active entities by the pulsed arc electrohydraulic discharge system and application to removal of atrazine. Water Res. 39 (19), 4705e4714. Levitt, J.S., N'Guessan, A.L., Rapp, K.L., Nyman, M.C., 2003. Remediation of alphamethylnaphthalene-contaminated sediments using peroxy acid. Water Res. 37 (12), 3016e3022. Li, T.P., Yuan, S.H., Wan, J.Z., Lin, L., Long, H.Y., Wu, X.F., Lu, X.H., 2009. Pilot-scale electrokinetic movement of HCB and Zn in real contaminated sediments enhanced with hydroxypropyl-beta-cyclodextrin. Chemosphere 76 (9), 1226e1232. Liu, Y.J., Wang, D.G., Sun, B., Zhu, X.M., 2010. Aqueous 4-nitrophenol decomposition and hydrogen peroxide formation induced by contact glow discharge electrolysis. J. Hazard. Mater. 181 (1e3), 1010e1015. Lukes, P., Locke, B.R., 2005. Degradation of substituted phenols in a hybrid gasliquid electrical discharge reactor. Ind. Eng. Chem. Res. 44 (9), 2921e2930. Payne, R.B., Fagervold, S.K., May, H.D., Sowers, K.R., 2013. Remediation of polychlorinated biphenyl impacted sediment by concurrent bioaugmentation with anaerobic halorespiring and aerobic degrading bacteria. Environ. Sci. Technol. 47 (8), 3807e3815. Redell, C.J., Elmore, A.C., Burken, J.G., Stringer, R.D., 2011. Waterjet injection of powdered activated carbon for sediment remediation. J. Soil. Sediment. 11 (6), 1115e1124. Sahni, M., Locke, B.R., 2006. Quantification of reductive species produced by high
24
T. Wang et al. / Water Research 84 (2015) 18e24
voltage electrical discharges in water. Plasma Process. Polym. 3 (4e5), 342e354. Seidel, H., Loser, C., Zehnsdorf, A., Hoffmann, P., Schmerold, R., 2004. Bioremediation process for sediments contaminated by heavy metals: feasibility study on a pilot scale. Environ. Sci. Technol. 38 (5), 1582e1588. Sellers, R.M., 1980. Spectrophotometric determination of hydrogen-peroxide using potassium titanium (IV) oxalate. Analyst 105, 950e954. Sun, B., Sato, M., Clements, J.S., 1997. Optical study of active species produced by a pulsed streamer corona discharge in water. J. Electrostat. 39 (3), 189e202. Tomei, M.C., Angelucci, D.M., Annesini, M.C., Daugulis, A.J., 2013. Ex situ remediation of polluted soils by absorptive polymers, and a comparison of slurry and twophase partitioning bioreactors for ultimate contaminant degradation. J. Hazard. Mater. 262, 31e37. Truex, M.J., Macbeth, T.W., Vermeul, V.R., Fritz, B.G., Mendoza, D.P., Mackley, R.D., Wietsma, T.W., Sandberg, G., Powell, T., Powers, J., Pitre, E., Michalsen, M., Ballock-Dixon, S.J., Zhong, L., Oostrom, M., 2011. Demonstration of combined zero-valent iron and electrical resistance heating for in situ trichloroethene remediation. Environ. Sci. Technol. 45 (12), 5346e5351. USEPA, 1994. Remediation Guidance Document, EPA-905-B94-003, Assessment and Remediation of Contaminated Sediments Program. Great Lakes National Program Office, Chicago, Illinois. Wang, T.C., Lu, N., Li, J., Wu, Y., 2010. Evaluation of the potential of pentachlorophenol degradation in soil by pulsed corona discharge plasma from soil
characteristics. Environ. Sci. Technol. 44 (8), 3105e3110. Wang, T.C., Lu, N., Li, J., Wu, Y., 2011. Plasma-TiO2 catalytic method for highefficiency remediation of p-nitrophenol contaminated soil in pulsed discharge. Environ. Sci. Technol. 45 (21), 9301e9307. Wang, T.C., Qu, G.Z., Li, J., Liang, D.L., 2014. Evaluation of the potential of soil remediation by direct multi-channel pulsed corona discharge in soil. J. Hazard. Mater. 264, 169e175. Yakabuskie, P.A., Joseph, J.M., Stuart, C.R., Wren, J.C., 2011. Long-term gammaradiolysis kinetics of NO-3 and NO-2 solutions. J. Phys. Chem. A 115 (17), 4270e4278. Ye, P., Lemley, A.T., 2009. Adsorption effect on the degradation of 4,6-o-dinitrocresol and p-nitrophenol in a montmorillonite clay slurry by AFT. Water Res. 43 (5), 1303e1312. Zhang, H., Huang, Q., Ke, Z.G., Yang, L.F., Wang, X.Q., Yu, Z.L., 2012. Degradation of microcystin-LR in water by glow discharge plasma oxidation at the gas-solution interface and its safety evaluation. Water Res. 46 (19), 6554e6562. Zhang, X.W., Li, G.T., Wang, Y.Z., Qu, J.H., 2006. Microwave electrodeless lamp photolytic degradation of acid orange 7. J. Photochem. Photobiol. A 184 (1e2), 26e33. Zhang, X.Y., Guo, Q., Hu, Y.Y., Lin, H., 2013. Effects of monorhamnolipid and dirhamnolipid on sorption and desorption of triclosan in sediment-water system. Chemosphere 90 (2), 581e587.
