Plasma-TiO2 Catalytic Method for High-Efficiency Remediation of p

ARTICLE pubs.acs.org/est

Plasma-TiO2 Catalytic Method for High-Efficiency Remediation of p-Nitrophenol Contaminated Soil in Pulsed Discharge Tie Cheng Wang,† Na Lu,†,‡ Jie Li,†,‡,* and Yan Wu†,‡ † ‡

Institute of Electrostatics and Special Power, Dalian University of Technology, Dalian 116024, PR China Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education of the People’s Republic of China, Dalian 116024, PR China

bS Supporting Information ABSTRACT: Nonthermal discharge plasma and TiO2 photocatalysis are two techniques capable of organic pollutants removal in soil. In the present study, a pulsed discharge plasma-TiO2 catalytic (PDPTC) technique by combining the two means, where catalysis of TiO2 is driven by the pulsed discharge plasma, is proposed to investigate the remediation of p-nitrophenol (PNP) contaminated soil. The experimental results showed that 88.8% of PNP was removed within 10 min of treatment in PDPTC system and enhancing pulse discharge voltage was favorable for PNP degradation. The mineralization of PNP and intermediates generated during PDPTC treatment was followed by UV-vis spectra, denitrification, total organic carbon (TOC), and COx selectivity analyses. Compared with plasma alone system, the enhancement effects on PNP degradation and mineralization were attributed to more amounts of chemically active species (e.g., O3 and H2O2) produced in the PDPTC system. The main intermediates were identified as hydroquinone, benzoquinone, catechol, phenol, benzo[d][1, 2, 3]trioxole, acetic acid, formic acid, NO2
, NO3
, and oxalic acid. The evolution of the main intermediates with treatment time suggested the enhancement effect of the PDPTC system. A possible pathway of PNP degradation in soil in such a system was proposed.

’ INTRODUCTION Nitrophenols are toxic and biorefractory organic compounds, used extensively as raw materials and intermediates in the production of explosives, pharmaceuticals, pesticides, pigments, dyes, wood preservatives, and rubber chemicals.1 Three nitrophenols (2-nitrophenol, 4-nitrophenol, and 2, 4-dinitrophenol) have been listed in the USEPA list of priority pollutants.2 Nitrophenols were released to soil as fugitive emissions during their production and use, causing serious health hazards. Taking China as an example, some contaminated lands, where nitrophenols extensively exist, have been left at the center of the city after some chemical factories migrated to the suburb in the industrial rearrangement. These lands are of high economic values because of their locations, and hence, need to be remedied rapidly. Several technologies such as physical method,3 chemical method,4 bioremediation,5 electrokinetic remediation,6 thermal technology,7 and photocatalysis8 have been employed to remediate organic pollutants contaminated soils. With the increasing strictness of industrial standard and the increment of economic values of lands, high-efficient and rapid soil remediation method is becoming a necessity. In this case, the conventional remediation technologies would not meet the requirement of high-efficient and rapid r 2011 American Chemical Society

remediation due to the drawbacks such as second pollution and time-consuming. In our previous study, pulsed discharge plasma technology, one of the advanced oxidation processes, has been employed to remediate pentachlorophenol contaminated soil, and great performance of soil remediation was obtained in a short remediation period.9 In pulsed discharge plasma process, discharge energy is released in forms of high energy electrons, strong electric field, and UV light radiation, etc., some of which can excite gases in plasma region to generate chemically active species. However, the utilization efficiency of the discharge energy is still needed to be enhanced in order to satisfy the requirement of practical application. Catalysis is suggested to be viable by introducing the active catalyst into the discharge plasma soil remediation system. Anatase TiO2, an economic and photosensitive semiconductor material with a band gap of about 3.2 eV, can be excited by strong electric field and UV light radiation to generate pairs of electrons and holes, resulting in more numerous chemically Received: April 26, 2011 Accepted: September 16, 2011 Revised: September 15, 2011 Published: September 16, 2011 9301

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Figure 1. Effect of pulsed discharge voltage on PNP degradation in PDPTC system.

Figure 2. Evolution of UV-vis absorption spectra of PNP with treatment time.

active species generation.10 Recently, the combination of nonthermal discharge plasma with TiO2 photocatalysis for pollutant removal has drawn great attention.11
13 Previous studies presented that the introduction of TiO2 into discharge plasma system could enhance pollutants removal and promote energy efficiency.11 However, relevant research was mostly focused on wastewater treatment and little has been reported on soil remediation. In this work, a pulsed discharge plasma-TiO2 catalytic (PDPTC) technique is proposed to enhance the remediation of p-nitrophenol (PNP) contaminated soil. The study was focused on exploring the enhancement of PNP degradation in this PDPTC system, and the variances of the main intermediates between PDPTC system and plasma alone system were compared to evaluate the enhanced behavior. Possible mechanisms of such enhancement were discussed by analyzing the variances in the amounts of chemically active species. A possible pathway of PNP degradation in such a system was proposed.

Extraction and Analysis. After discharge treatment, PNP in soil was extracted immediately, and the extraction procedure was described in SI S4. The extractions produced average recoveries of 90.1
95.3%. PNP concentration, total organic carbon (TOC), intermediates, O3 and H2O2 concentrations, and CO2 and CO were analyzed and the details were shown in SI S5. The COx selectivity, CO2 conversion, and denitrification efficiency were defined in SI S6. All experiments were conducted in duplicates.

’ MATERIALS AND METHODS Materials. PNP was used in the study, and its detailed introduction was presented in S1 of the Supporting Information (SI). Soil samples were collected from a suburb of Dalian, China. The details were presented in SI S2. The original PNP concentration in the soil was 800 mg kg
1. TiO2 (Degussa, P25) (BET area =50 m2 g
1) was used as the catalyst. Treatment of Contaminated Soil. The schematic diagram of the experimental apparatus was illustrated in Figure S1 in the SI, which was similar with our previous work.9 The details of the reactor were showed in SI S3. The pulse frequency and pulseforming capacitance Cp were 100 Hz and 200 pF, respectively, and the input energies per pulse were 0.016, 0.020, and 0.023 J at pulse voltage of 16, 18, and 20 kV, respectively. The experiments were all conducted at 20 kV unless special illustration. In each experiment, a certain amount of TiO2 was added into PNP contaminated soil and then homogenized. TiO2 amount in the soil was 2 wt %. The soil sample (approximately 2.0 g) was spread on the ground electrode with a thickness of about 1.3 mm. Prior to discharge treatment, the moisture content of soil was adjusted to 10% with deionized water. Air was injected for one side of the reactor and out from the other side with the flow rate of 0.5 L min
1.

’ RESULTS AND DISCUSSION PNP Degradation in PDPTC System. Figure 1 showed the effect of pulse discharge voltage on PNP degradation in soil in PDPTC system. On the one hand, the introduction of TiO2 enhanced PNP removal in soil. At pulsed discharge voltage of 20 kV, PNP degradation efficiency reached 88.8% after 10 min of discharge treatment in the PDPTC system, which was 78.1% in plasma alone system. In the case of TiO2 catalyst, conduction band electrons and holes (h+) would be generated when TiO2 was irradiated with high energy input, and the photogenerated electrons and holes could react with PNP directly or indirectly, increasing PNP degradation. Hoffmann14 has reported that the conduction band electrons, holes (h+) and the reactive oxygen species such as •OH radicals and superoxide radicals generated on the illuminated catalyst could promote pollutant removal. On the other hand, increase in pulsed discharge voltage greatly enhanced PNP degradation efficiency in the PDPTC system. For example, at discharge voltage of 16 kV, only 65.2% of PNP was removed after 10 min of discharge treatment, while it increased to 88.8% at 20 kV. More energy is injected into the reactor when the discharge voltage increases, and then more plasma channels with strong energy are very effective to generate more amounts of chemically active species, and therefore PNP degradation efficiency is enhanced; meanwhile, the effects including high energy electrons, strong electric field and UV light radiation etc would also become stronger at higher discharge voltage. In this case, more conduction band electrons and holes (h+) would be generated, and then the formation of chemically active species was accelerated, which promoted PNP removal. In addition, the energy efficiencies for PNP removal in the PDPTC system after 10 min of discharge treatment were 3.90, 3.84, and 3.70 g kWh
1 at pulse voltages of 16, 18, and 20 kV, respectively, as presented in SI Table S1. Considering PNP degradation and energy efficiency comprehensively, 20 kV was used in the following experiments. 9302

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Figure 3. Evolution of NO2
and NO3
with treatment time.

Control experiments with O3 and H2O2 addition in the absence of plasma were conducted, and the results were presented in SI Table S2. Herein, method of ozonation experiment was the same as our previous study.9 The results in SI Table S2 suggested that O3 played a decisive role for PNP degradation, and H2O2 also played a certain role. Mineralization of PNP in PDPTC System. Mineralization of PNP in the PDPTC system was studied by changes of UV
vis spectra, NO3
and NO2
formation, TOC removal, and CO2 and CO generation. UV-vis Absorption Spectra. The change of the UV
vis absorption spectra of PNP during degradation process in the PDPTC system was shown in Figure 2. The absorption peak at 400 nm disappeared quickly with the increase of treatment time, indicating that PNP was removed gradually. Formation of NO2
and NO3
. Since there is a nitro-group in PNP molecule, tracing changes of nitrogen forms is an approach to evaluate the degree of PNP degradation. To our knowledge, the nitro-group can be converted into NO3
and NO2
ions.15,16 Therefore, NO3
and NO2
ions were both monitored during PNP degradation in soil, and as control experiments, the formation of NO2
and NO3
were also analyzed in clean soil (uncontaminated soil) during pulsed discharge process. The concentrations of NO3
and NO2
released from PNP were calculated by subtracting the concentrations in clean soil from those in contaminated soil, respectively. The evolution of NO3
and NO2
concentrations released from PNP with treatment time was shown in Figure 3, and their concentrations generated in clean soil during pulsed discharge plasma were presented in SI Figure S2. The NO2
concentration in the PDPTC system declined after an initial increase in Figure 3, probably due to its further oxidation to other nitrogen forms, whereas the NO3
concentration increased gradually with treatment time. Similar trends were also presented in plasma alone system. Moreover, the NO3
concentration increased slowly in the initial 10 min, and then the rate increased gradually. These results indicated that the NO2
mainly resulted from PNP degradation, and NO2
was formed first when the nitrogen-tocarbon single bond (—N—C—) of the PNP was broken down, and then it was oxidized into NO3
. Active species reacted rapidly with nitrophenol to produce NO2
, and the NO2
concentration quickly reached a maximum and then decreased rapidly, and during the process it was oxidized into NO3
.15,16 More importantly, as shown in Figure 3, more amounts of NO3
and NO2
were formed in the PDPTC system than in plasma alone system in the initial 10 min, whereas higher NO3
and lower NO2
concentrations occurred in the PDPTC system after

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Figure 4. Evolution of TOC removal with treatment time.

Figure 5. Changes of COx selectivity with treatment time in PDPTC system.

30 min of treatment, compared with those in plasma alone system. These results suggested that more amounts of NO2
were oxidized into NO3
in the PDPTC system, which was attributed to the intense oxidation environment caused by TiO2 catalyst. The enhanced oxidation environment in the PDPTC system could also be further confirmed by denitrification efficiency. The evolution of denitrification efficiency of PNP with treatment time was presented in SI Figure S3. It was found that 81.3% of denitrification efficiency was achieved after 30 min of treatment in the PDPTC system, and there was a 20% rise as compared with that in plasma alone system. TOC Removal. The TOC values have been related to the total concentration of organic compounds, and the decrease of TOC with treatment time can reflect the degree of mineralization. Therefore, the TOC removal during PNP degradation process in the PDPTC system was shown in Figure 4. TOC removal efficiency achieved 55.1% in the PDPTC system after 30 min of treatment, compared with that of 42.9% in plasma alone system. These results demonstrated that more PNP and intermediates were mineralized to smaller organic molecules or to CO2 in the PDPTC system. Generation of CO2 and CO in Offgas. The changes of UV
vis absorption spectra, the formation of NO3
and NO2
, and the TOC removal only reflect indirectly the mineralization extent of PNP, whereas the generation of CO2 and CO can reflect directly its mineralization. Therefore the generation of CO2 and CO during PNP degradation was measured. Figure 5 presented the evolution of COx selectivity with treatment time during PNP degradation. With the treatment time continued, CO2 selectivity increased and CO selectivity decreased in the 9303

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Table 1. Comparison of O3 and H2O2 Concentrations and Their Energy Costs with/Without TiO2 in Plasma Process uncontaminated soil

contaminated soil

CO3 (mgL-1)

CH2O2 (mmol L
1)

O3 energy costs (mgkJ-1)

H2O2 energy costs (mgkJ-1)

CO3 (mgL-1)

CH2O2 (mmol L
1)

without TiO2

42

0.038

0.76

0.014

23

0.022

with TiO2

57

0.062

1.03

0.022

30

0.041

•OH þ •OH f H2 O2

ð4Þ

O2 þ ecb
f O2 •


ð5Þ

O2 •
þ 2H2 O f H2 O2 þ 2OH
þ O2

ð6Þ

On the other hand, O3 concentration in the PDPTC system was always higher than that in plasma alone system, as shown in Table 1. O2 can be cleaved into single atomic oxygen radical anion (O•) by gas phase electrical pulses, and it can also be converted to superoxide radical anion (O2•
) by the effect of highly energized electrons (e
) and ecb
on the surface of TiO2. Subsequently, O•
and O2•
are transformed into more chemically active species (O and O2) by hvb+ on the surface of TiO2, resulting in more amounts of O3 production through the following possible reaction pathways:10,20,21 Figure 6. Evolution of main intermediates with treatment time.

O2 þ e
f O2 •


ð7Þ

PDPTC system. 96.8% of CO2 selectivity was obtained in the PDPTC system after 20 min of treatment, with 3.2% of CO selectivity. These results suggested that CO formed in the reaction was further oxidized into CO2. More importantly, higher CO2 selectivity and lower CO selectivity occurred in the PDPTC system than in plasma alone system. The intense oxidative environment in the PDPTC system was the reason for the enhanced CO2 selectivity and suppressed CO selectivity. Formation of Active Species. To explore the enhancement mechanisms of PNP degradation in the PDPTC system, the variances of chemically active species such as O3 and H2O2 were analyzed in clean and contaminated soils, respectively, and the results were presented in Table 1. Herein the discharge time was 20 min. As shown in Table 1, on the one hand, more amounts of H2O2 were generated in the PDPTC system both in clean soil and contaminated one, compared with those in plasma alone system. When an electron on the valence band (VB) of TiO2 absorbs some energy higher than the band gap between the VB and the conduction band (CB), it will be promoted to the CB and thus an electron
hole pair (ecb

hvb+) is formed. The holes will oxidize either H2O molecule or OH
anions to form •OH. The electrons on the CB react with O2 to generate superoxide radical anion (O2
). Then, the O2
will react with H2O to produce •OH. •OH can react with each other to form H2O2. Therefore, more amounts of H2O2 were generated through the following possible reaction pathways:17
19

O2 •
þ hvb þ f O2

ð8Þ

O•
þ hvb þ f O

ð9Þ

TiO2 þ hv sf ecb
þ hvb þ

energy

ð1Þ

hvb þ þ H2 O f Hþ þ •OH

ð2Þ

hvb þ þ OH
f •OH

ð3Þ

O þ O2 f O3

ð10Þ

As mentioned above, it is believed that the PDPTC system is effective to generate more chemically active species (such as O3 and H2O2), leading to the enhancement of PNP degradation in soil. Possible Degradation Pathways. The intermediates of PNP degradation in soil in the PDPTC system were analyzed using HPLC, HPLC/MS and IC. They mainly included hydroquinone, benzoquinone, catechol, phenol, benzo[d][1, 2, 3]trioxole, acetic acid, formic acid, NO2
, NO3
, and oxalic acid. Similar results were also reported by Oturan,22 where hydroquinone and benzoquinone were two main intermediates during PNP degradation by Fenton method. Hydroquinone, benzoquinone, phenol, formic acid, and oxalic acid were also detected as intermediates during PNP degradation by ozonation, and
NO2 group could be easily removed from the aromatic ring in the process.23 The evolution of some aromatic intermediates with treatment time in the PDPTC system and plasma alone system were analyzed, as depicted in Figure 6. Maximum concentrations of hydroquinone, benzoquinone and catechol in the PDPTC system were all lower than those in the plasma alone system. Hydroquinone reached the maximum concentration earlier in the PDPTC system, whereas benzoquinone and catechol reached the maximum concentrations almost at the same time in the two discharge systems. Phenol was generated earlier in the PDPTC system. Besides that, these intermediates in each reaction system all encountered further degradation with the 9304

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Figure 7. Possible degradation pathways of PNP in soil in PDPTC system.

treatment process went on, and lower concentrations occurred in the PDPTC system finally. The results presented that greater degradation performance occurred in the PDPTC system, which was attributed to the enhanced generation of chemically active species by the effect of TiO2 catalyst. On the other hand, from the sequence of accumulation of aromatic intermediates, phenol appeared later than hydroquinone, benzoquinone and catechol. Therefore, it could be concluded that hydroquinone, benzoquinone and catechol were generated more easily than phenol during PNP degradation. The differences in evolution of intermediates were due to the intensive oxidation environment in the PDPTC system. On the one hand, more amounts of H2O2 and O3 generated in PDPTC system could increase the formation of •OH radicals through trapping of photogenerated electrons, and they also interfered in recombination of electrons and positive holes due to the electrophilic properties of H2O2 and O3.24 On the other hand, active species generated in discharge plasma could act directly on the active sites of TiO2, and then accelerate TiO2 to trigger reactions, and meanwhile, the strong electric field in discharge plasma could inhibit the recombination of electrons and holes on TiO2 surface.25,26 Based on the intermediates and their evolution with treatment time, and the roles of O3, H2O2, and •OH radicals played in the present study, possible degradation pathways of PNP in soil were proposed in Figure 7. The patterns of intermediates indicated

that hydroxylation was the main oxidation pathway. Hydroxylated intermediates could result from electrophilic attack on PNP by O3 and •OH radicals. Aromatic ring of PNP contains two substituents,
OH and
NO2. The
OH is electron-donating and an ortho- and para-director, while
NO2 is electron-withdrawing and meta-director. •OH radicals preferentially attack the ortho- or para-position with respect to the
OH group due to the electrophilic nature. The •OH radicals may eliminate nitrous acid from PNP to yield 1,4-benzosemiquinone as an intermediate, which subsequently disproportionates into hydroquinone and benzoquinone. Similar results were reported by Liu et al.27 and Suarez et al.28 The possibility of a direct attack of •OH radicals at the position carrying the
NO2 group also exists, with resultant hydroquinone formation.29 In addition, the •OH radicals may attack the
NO2 group due to the relatively long length of C—N bond in PNP molecule, which is the longest bond and would be potential to be attacked to form phenol,30 and then hydroquinone, benzoquinone and catechol would be generated by further oxidation of phenol. Further reactions of these intermediates with •OH radicals lead to ring cleavage and formation of aliphatic compounds. O3 reacts with organic pollutants through nucleophilic, electrophilic and cyclo-addition reactions.31,32 The nucleophilic and electrophilic attacks of O3 on PNP proceeds preferentially on the ortho- and para-positions with respect to the
OH group to yield 9305

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Environmental Science & Technology the same hydroxylated intermediates as in the case of •OH radicals attack.23 Furthermore, the cyclo-addition reactions of O3 may cause the addition of O3 to PNP molecule structure, and finally form cyclo-addition intermediates,33 such as benzo[d][1, 2, 3]trioxole in the present study. Further attacks to these intermediates by O3 would lead to ring-cleavage of the aromatic ring to form acetic acid, formic acid, and oxalic acid. The formation of NO2
is the result of the denitrification of PNP. In the process, the NO2
concentration quickly reaches a maximum and then decreases rapidly to be oxidized into NO3
. This study opens a possible way to improve soil remediation in pulsed discharge plasma through TiO2 catalyst.

’ ASSOCIATED CONTENT

bS Supporting Information. Text S1
S6 include introduction of PNP and other reagents, details of the soil sample, reactor introduction, extraction procedure, analysis methods, COx selectivity, and denitrification efficiency. Figures S1
S3 include the reactor system, nitrite and nitrate concentrations in clean soil, and denitrification efficiency. Table S1 presents the energy efficiency within 10 min of discharge treatment at different discharge voltages. Table S2 presents the comparison of PNP degradation in soil by pulsed discharge plasma, ozonation, and H2O2 oxidation. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-411-84708576; fax: +86-411-84709869; e-mail: [email protected].

’ ACKNOWLEDGMENT We thank the National Natural Science Foundation, P.R. China (Project No. 40901150), the Ministry of Science and Technology, P.R. China (Project No. 2008AA06Z308), and Program for Liaoning Excellent Talents in University, China (Project No. 2009R09) for their financial support to this research. ’ REFERENCES (1) Uberoi, V.; Bhattacharya, S. K. Toxicity and degradability of nitrophenols in anaerobic systems. Water Environ. Res. 1997, 69 (2), 146
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