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Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction
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JECE 664 1–9 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction
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3 Q1 4 5 6 7 8
Wenchao Wang a , Gang Liu a , Jun Shen a, * , Honghong Chang a , Ruifeng Li a , Jiankui Du b , Zhifeng Yang c , Qingbai Xu d a
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Shanxi Lulujia Science & Technology Co., Ltd., Taiyuan 030002, China Research Institute of Highway Ministry of Transport, Beijing 100088, China d Fushun Research Institute of Petroleum and Petrochemicals, China Petroleum & Chemical Corporation, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 April 2015 Accepted 26 May 2015
This study was conducted to assess the feasibility of using potassium permanganate (KMnO4) as an oxidant to reduce 7 carcinogenic polycyclic aromatic hydrocarbons (PAHs) content in two coal tar pitches (CTPs), and to research the oxidation mechanism of KMnO4. It had been observed reduction of major PAHs in CTP after KMnO4 oxidation. The best benzo[a]pyrene equivalency (BaPeq) reduction rate of 82% in n-hexane soluble in a high temperature CTP (HCTP) was achieved with KMnO4 concentration of 0.2 M treated for 5 h. Specifically, a higher phenanthrene reduction rate (90%) was obtained by using the Soxhlet extraction with n-hexane as solvent on the KMnO4 oxidized HCTP. The single PAH removal rate roughly increases with increasing rings number of PAH, following the order of phenanthrene < fluoranthene < pyrene < benzo[a]pyrene < benzo(b)fluoranthene < indeno [1,2,3-ed]pyrene < benzo[g,h,i]perylene under the same conditions. By using gel penetration chromatography (GPC) and gas chromatography–mass spectrometry (GC–MS), electrophilic substitution reaction was speculated as the oxidation mechanism of CTP by KMnO4. ã2015 Published by Elsevier Ltd.
Keywords: Coal tar pitch Potassium permanganate Oxidation Polycyclic aromatic hydrocarbons (PAHs)
9
Introduction
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Coal tar pitch (CTP), the remnant of distillation process from coal tar, contains about 200 polycyclic aromatic hydrocarbons (PAHs) [1], PAHs are aromatic compounds containing from two to six benzene rings, many of them are highly toxic, carcinogenic, teratogenic and thermally stable [2–4]. 16 PAHs (Table 1) in CTP have been identified as priority carcinogenic compounds by the US Environmental Protection Agency (US EPA) [5,6]. CTP is commonly found in many industrial areas like former manufactured gas plants, steel making plants, mesocarbon/microbeads electrodes for the metallurgy and material of pavement industry [7–10]. Coal tar have accumulated over decades in soils of the related fields, it can penetrate the entire 4–5 m thickness of soil, refractory to biodegradation, and persistent in soil and water [11–14], therefore, in recent years, coal tar or the related product is considered the fundamental PAHs source among all the known sources [15,16], especially in places like coal-tar-based sealcoat, it has been
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Q2
* Corresponding author. Tel.: +86 351 6018554; fax: +86 351 6018554. E-mail addresses: [email protected], [email protected] (J. Shen).
identified that CTP contributes to a major source of PAHs, PAHs concentration in dust is about 1000 times higher than asphalt-based sealcoat pavement [17]. PAHs in particles can be easily transported by wind and rainwater to nearby soil sediments, water and air, lead to increased cancer risk for human beings [18–20]. Therefore, CTP is thought as a potential pollutant for its toxic PAHs components [21], and considerable restriction of the Q3 application areas for CTP is currently observed. For those reasons, to prepare less toxic CTP is a matter of extreme importance to environment and human health. Several researches have reported removing carcinogenic PAHs from CTP [22–27]. Boyd et al., Dix and Marnett explained the transformation mechanism of benzo[a]pyrene (BaP) which is the most common and most dangerous PAHs carcinogens to human beings and investigated the carcinogenic mechanism. They pointed out that BaP would be converted into harmless as some functional groups substitute the active position of BaP. This mechanism can be summarized according to the reaction sequence in Fig. 1 [28,29]. Some researchers used polymers at high temperature for a long time to reduce the carcinogenic PAHs content in CTP, such as unsaturated polyester resin (UPR), polyethylene glycol (PEG), 1,4-benzenedimethanol (PXG), divinylbenzene (DVB) and so on
http://dx.doi.org/10.1016/j.jece.2015.05.024 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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Table 1 Relative potency factor (RPF) of each PAH and property according to the US EPA OPPTS. Chemical formula
Number of aromatic rings
Molecular weight (g/mol)
Boiling point ( C)
RPF
Abbreviation
Naphthalene
C10H8
2
128.16
217.9
0.000
NAP
Acenaphthylene
C12H8
3
152.2
275
–
ACY
Acenaphthene
C12H10
3
154.21
279
–
ACP
Fluorene
C13H8
3
166.22
298
0.000
FLR
Phenanthrene
C14H10
3
178.23
340
0.000
PHE
Anthracene
C14H10
3
178.23
345
0.000
ANT
Fluoranthene
C16H10
4
202.26
367
0.034
FLT
Pyrene
C16H10
4
202.26
393.5
0.000
PYR
Benzo(a)anthracene
C18H12
4
228.29
438
0.033
BaA
Benzo(k)fluoranthene
C20H12
5
252.3
481
0.010
BbF
Benzo(b)fluoranthene
C20H12
5
252.3
481
0.100
BbF
Benzo[a]pyrene
C20H12
5
252.3
500
1.000
BaP
Indeno[1,2,3-ed]pyrene
C22H12
6
276
–
0.100
IcP
Dibenzo[a,h]anthracene
C22H14
5
278.35
–
1.400
DhA
PAH species
Chemical structure
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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Table 1 (Continued) PAH species
Chemical structure
Benzo[g,h,i]perylene
Chemical formula
Number of aromatic rings
Molecular weight (g/mol)
Boiling point ( C)
RPF
Abbreviation
C22H12
6
276.6
542
–
BgP
“–” means the data were not found.
12
1
11 10
Nu-R
O
2 3
9 4
8 7
6
5
HO
HO
HO HO
(I)
HO
(II)
HO
(III)
(IV)
Fig. 1. Reaction of BaP (benzo[a]pyrene) causing conversion of carcinogenic component (III) to substituted non-carcinogenic component (IV).
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[23,25,30,31]. Zielinski et al. [30] studied the effect of various polymers, 46% BaP content was reduced by using a compound containing PVC, anthracene oil and butadiene/styrene rubber latex when heated for 1 h at 120–150 C. Zhang et al. [31] used coumarone–indene resin and reduction rate of BaP was 46.78% at 270 C. However, there are many shortcomings in those modification methods, for example, the modification processes are often very complicated, and always need huge doses and/or high temperature (120–270 C); some modifiers such as paraformaldehyde can give off poisonous methanol in the operation process, which lead to generate secondary pollution. To solve the problem, the aim of this study is to reduce PAHs content in CTP as low as possible by a simple and efficient method. Due to potassium permanganate (KMnO4) is a common oxidant for remediation of contaminated soil, it can be easily monitored, and has high redox potential [32–35]; therefore, it may be regarded as an alternative to decrease the toxic substances in CTP with lower environmental impact. Here, the objectives of the paper were to (1) evaluate the potential of KMnO4 as a modifier to lessen the carcinogenic PAHs contents supervised by US EPA in CTP and investigate the process parameters including reaction time, doses of KMnO4 and kinds of CTP, (2) find the rules of single PAH removal in the experiment, and (3) identify the oxidation mechanism.
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Materials and methods
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Materials
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Two CTPs were used in the experiment. One was medium temperature CTP (MCTP) obtained from Datuhe (Shanxi) Coking & Chemical Co., Ltd. The other was high temperature CTP (HCTP)
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produced by China Pingmei Shenma Group in Henan province. Two CTPs were crushed separately and sieved to <0.074 mm, air-dried at 25 C for 2 days, then kept in brown bottles with lid. Two CTPs composition are shown in Table 2. The standard PAH compounds with purity (>99%), phenanthrene, fluoranthene, pyrene, benzo[a]pyrene, benzo(b)fluoranthene, indeno[1,2,3-ed]pyrene, benzo[g,h,i]perylene, were purchased from Aldrich Chemical Co. KMnO4 (>99%), n-hexane (AR), dichloromethane (AR) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.
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The experimental process
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The oxidation experiment Two CTPs were crushed separately and sieved to <0.074 mm, air-dried at 25 C for 2 days, then 20 g CTP (dry mass) was weighted and placed in 250 mL flasks; and 100 mL distilled water was added in. Homogeneous slurry was obtained by stirring at 150 rpm for 1 h. Then the required doses of solid KMnO4 was slowly added in, with concentration range from 0.05 and 0.6 M, the reaction time was from 0.5 to 24 h, the slurry was shaken at a speed of 250 rpm, the oxidation process proceeded at room temperature and at natural pH (7.2). After the oxidation, the CTP suspension was transferred to a Buchner funnel, filtered by suction, and the filtered cake was dried in a vacuum oven at 60 C for 4 h then the modified CTP was crushed and passed through an 80-mesh sieve for next extraction experiment.
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The extraction experiment To further remove the light PAHs in the oxidized CTPs, 3 g oxidized CTP was extracted with 150 mL n-hexane by the Soxhlet extraction for 12 h until the solvent in siphon was colorless.
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Table 2 The original properties of untreated CTP samples. Samples
HCTP MCTP
SP ( C)
101.50 87.50
Elemental analysis (wt%) C
H
O
S
N
92.91 92.53
4.39 4.46
0.96 1.23
0.89 0.97
0.86 0.81
PHE
FLT
PYR
BbF
BaP
IcP
BgP
8.97 7.96
14.49 11.76
11.56 8.41
13.64 10.77
9.78 7.76
7.45 4.74
10.58 5.59
Determination method: SP (softening point) (GB/T 2293-60; ring and ball method). The unit of the above PAHs is g PAHs/kg CTP (dry mass).
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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The controlled experimental In order to evaluate the impact of agitation time on PAHs removal, 20 g HCTP sample (dry mass, 0.074 mm) was weighted and placed in 250 mL flasks, 100 mL distilled water was subsequently added in, the solution was stirred for 12 h at 250 rpm (25 C), then filtered by suction, air-dried. Another 20 g HCTP sample without any treated were crushed and passed through an 80-mesh for extraction experiment, the conditions of extraction experiment was consistent with the previous.
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Analytical methods
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The quantitative analysis of PAHs The PAHs concentrations in two CTP samples were determined by Linghua 9890a GC (Shanghai). External standards were used for GC calibration. The CTP samples were first extracted with n-hexane for 12 h. Then the extract was condensed to approximately 0.5 mL by a rotary evaporation (RE-52AA, Shanghai) and the residue was diluted in 10 mL CH2Cl2 for GC analysis. A 1-mL sample solution was injected into the GC equipped with a 30 0.25 mm Rxi-1 capillary column with a 0.25 mm film thickness, initial oven temperature was 100 C for 0.5 min; ramp to 300 C at 5 C/min and held for 10 min. The flame ionization detector was maintained at 320 C, retained for 50 min until complete elution of all species.
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The qualitative analysis of PAHs GC–MS (Shimadz, QP2010ultra) and GPC (Waters) were used to analyze the constituents in CTP and molecular weight, respectively. GC–MS analyses were performed on a GC using the same column and chromatographic conditions as given above for the PAH quantitative analysis. A 2-mL sample solution was injected in the GC–MS, the Shimadz mass spectrometer was run in the electron impact mode, and m/z was 50–550. An automated GPC system consisting of Waters 600E pump, two gel columns connected in series packed with Waters styrage HT2 and HT3 (7.8 300 mm); stationary phase: polystyren-divinyl benzene; column temperature: 35 C; mobile phase: tetrahydrofuran (HPLC grade), flow rate: 1 mL/min 20 mL sample of solvent was injected in the GPC. All the preparation step of CTP samples for GC–MS and GPC were consistent with the given above for GC analysis. All the experiments were run in triplicate.
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The calculation methods
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Many studies have showed different PAHs have different effects on human health, the calculation methods of PAHs toxicity are always based on BaP equivalent value. In the experiment, the carcinogenicity of different PAHs in CTP was calculated in terms of not the sum of 7 PAHs but BaP equivalents (BaPeq) [36], the equation of BaPeq is shown below: BaPeq ¼ S ½PAHi RPFi (1)
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Results and discussion
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The original properties of the two CTPs
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The properties of two CTPs are shown in Table 2. The seven PAHs contents in two CTPs were measured by GC. The total PAHs content in HCTP is about 76.50 g/kg CTP (dry mass). Light PAHs (FHE, FLT) are 23.46 g/kg; heavy PAHs (PAHs with 4–6 rings except FLT) are 53.04 g/kg; the total PAHs content in MCTP is about 56.99 g/kg, light PAHs are 19.72 g/kg; heavy PAHs are 37.27 g/kg [39]. The HCTP has higher PAHs content than MCTP, the difference is supposed to have influences on the KMnO4 oxidation results.
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The PAHs in CTP oxidation with KMnO4
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The BaPeq removal of HCTP with different KMnO4 concentrations Fig. 2 shows the relationship between BaPeq removal of PAHs in HCTP and concentration of KMnO4 (reaction time = 3 h). As seen from Fig. 2, different KMnO4 concentrations resulted in different removal rates, the sum of seven PAHs removal are always lower than BaPeq removal, because heavy PAHs have higher reactivity and toxicity than light PAHs, BaPeq removal is more accurate than the sum of PAHs removal in reflecting the CTP toxicity, so we mainly discuss BaPeq removal. The BaPeq removal rate in HCTP varied from about 28.96% to 62.50%. Obviously, it was a non-linear correlation between BaPeq removal rate and KMnO4 concentrations, BaPeq removal rate in HCTP increases significantly with KMnO4 concentration till to the highest at 0.2 M, the maximum BaPeq reduction was 62.50% with the consumption of KMnO4 dosage was 9.5% of HCTP. Higher oxidant concentrations could have a negative influence on the PAHs removal, likely because too powerful and quick reactions prevent further contract between KMnO4 and PAHs. This phenomenon may be explained with prior studies relating to remediation of PAH-contaminated sediments by chemical oxidation, the study pointed out that excess oxidant concentration could lead to generate strong foam and decrease the oxidation efficiency, the optimal oxidant dosage should be strictly determined during reducing PAHs content [39]. Meanwhile, because MnO2 was formed from KMnO4, the oxidation product showed brownish, however, the presence of MnO2 does not cause an environment problem [40].
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where [PAH] is each PAH concentration in the CTP; RPF represents relative potency factor of each PAH toxicity relative to BaP. Several relative potency factors of PAHs are shown in Table 1, which were estimated by the US EPA OPPTS [37]. The BaPeq removal was calculated as: ðBaPeqÞfinal (2) 100 The BaPeq removalð%Þ ¼ 1 ðBaPeqÞinitial where (BaPeq)final and (BaPeq)initial were the value of BaPeq respectively after and before the oxidation reaction [38]. In the experiment, it must be pointed out that all the PAHs value in CTP means the content of PAHs in n-hexane soluble of CTP.
Fig. 2. BaPeq (benzo[a]pyrene equivalents) removal of HCTP (high temperature coal tar pitch) with different KMnO4 (potassium permanganate) concentrations. Reaction time = 3 h.
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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Fig. 4. BaPeq removal concentration = 0.2 M.
of
HCTP
5
with
different
reaction
times.
KMnO4
Fig. 3. Single PAH removal for HCTP by KMnO4 oxidation. The oxidation conditions: time = 3 h; KMnO4 concentration = 0.2 M.
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The single PAH removal in HCTP Single PAH removal with different aromatic rings number in HCTP is plotted in Fig. 3 (the oxidation conditions: time = 3 h; KMnO4 concentration = 0.2 M). It can be seen that KMnO4 is effective for removing carcinogenic PAHs from CTP, and more effective for heavy PAHs than light PAHs. The PAH removal effect ranks increasingly PHE < FLT < PYR < PHE < pyrene < BaP < BbF < IcP < BgP under the same conditions. These results are consistent with that obtained by Forsey and Julien Lemaire on the oxidation of PAHs, which showed the degradation rate increased with the number of cycles due to the decrease of the bonding energy of cycles [38,41]. It can also be explained by Clard theories [42], PAHs contain three different kinds of keys (Table 3), most PAHs have true double bond characteristics, thus make them readily oxidized by KMnO4, meanwhile, each key attributes to different reactive ability of PAH, and the heavy species are more available to reactants than light PAHs [43]. Forsey et al. also proved that the rate of oxidation– reduction reactions increase with the number of PAH; however, we
must clearly understand that the reaction rate toward KMnO4 is a generalization, because there are other factors such as steric interactions, oxidation condition and the combination mode of the aromatic rings, those differences can also affect the reaction rate under certain conditions [41]. Therefore, we found the BaP removal slightly lower than the BbF in the experiment.
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The BaPeq removal of HCTP with different reaction times Fig. 4 shows the time (from 0.5 to 24 h) influence on BaPeq removal in HCTP while KMnO4 concentration was fixed at 0.2 M. From Fig. 4, the time influence on the PAHs removal rate roughly shows the same trend as shown in Fig. 2, the optimal reaction time in the experiment is 5 h. With the increase of time, BaPeq removal in HCTP first increases and then decreases, varying from 36.34% to 78.05%. The BaPeq removal grows slowly in the first hour during the oxidation of PAHs in CTP, which may be due to a strong adsorption characteristics between PAHs and CTP. They form a very stable interacting mode in a complex molecular network structure in CTP
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Table 3 Clar structure showing number of true double bonds, stabilizing aromatic sextets, and shared double bonds for several PAHs. PAHs
True double bonds
Stabilizing sextets
Shared double bonds
BaP
Clar structure
1
2
1
PYR
1
2
1
PHE
1
0
0
FLT
0
2
3
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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W. Wang et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
Fig. 5. Extraction rate with different agitation times. HCTP: 20 g (dry mass, 0.074 mm); distilled water: 100 mL; stirring rate: 250 rpm.
Fig. 6. BaPeq removal of MCTP (medium temperature coal tar pitch) with different KMnO4 concentrations. Reaction time = 3 h.
which may prevent KMnO4 to attack PAHs. With the extension of time and the influence of agitation, the network structure of CTP becomes loose, PAHs and KMnO4 begin to have a better contact during the second hour, the BaPeq removal begin to rise to 66.94%, then achieves the peak value of 78.05% at 6 h. The removal rate decreases if the time increases above 6 h. The phenomenon has some differences from other related findings during remediation of PAH-contaminated sediments by KMnO4 [39], which may be due to a great difference in composition among soil, waste water and CTP, PAHs is naturally present in CTP, while major PAHs in soil can be seen as foreign, and the content of PAHs in CTP are far higher than the PAH-contaminated sediments or water, especially, with the increase of agitation time, CTP structure becomes more and more loose, and easily extracted by n-hexane in the subsequent experiments. To clarify this point, a controlled trial was run, as expected, the result showed that the extraction rate with 12 h agitation time is 27.49% while the untreated CTP is only 14.05%, as shown in Fig. 5, it means PAHs would be more easily extracted as the agitation time goes on. Extraction rate rises higher above 6 h, it is consistent with previous research that the removal rate decreases if the time increases above 6 h as a result, the PAHs removal seemingly change lower at a long reaction time; therefore, although KMnO4 is a very persistent oxidant, the factor of agitation time must also be taken into account, or it could affect the end result.
PAHs, as noted above, show higher reactivity than light PAHs, which contributes to improve the PAHs removal. From above, we know that not all PAHs species in CTPs have a good oxidation result when KMnO4 used as a modifier. It indicates that the modifier kinds should be changed according to CTP species in the next study.
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The extraction with n-hexane as solvent on the oxidized HCTP From the previous study, we know that the KMnO4 is an effective oxidant for HCTP to destroy PAHs, but it has relatively low performances for the light PAHs during the oxidation modification, therefore, the extraction technology by using n-hexane as solvent was adopted in order to achieve the further reduction of light PAHs. The result is shown in Fig. 7. As expected, the use of solvent extraction to enhance the light PAHs removal is very effective. Much light PAHs have been extracted, especially; the PHE total removal rate is beyond 90%.
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The BaPeq removal with different kinds of CTPs Based on different softening points, CTP is divided into high temperature (95–120 C), medium temperature (75–95 C) and soft CTP (35–75 C), to investigate the adaptability of KMnO4 to different CTPs, the MCTP was treated with different concentrations of KMnO4, and the result is shown in Fig. 6. At the same concentration of KMnO4, the BaPeq removal in MCTP is always much lower than HCTP. The best BaPeq removal in MCTP is only 41.65%, about 20% lower than that in HCTP. The low degradation may be due to the different production technology of CTPs, resulting in different combining degree between PAHs and CTPs, and the different original PAHs content may contribute to this phenomenon. For example, some studies have shown that distribution of PAH content between light component and heavy component can also influence the degradation, the heavy PAHs fraction of 69.94% in HCTP is higher than MCTP (55.60%), the heavy
Fig. 7. Content of PAHs (polycyclic aromatic hydrocarbons) under the conditions of oxidation/extraction. The oxidation conditions: time = 6 h; KMnO4 concentration = 0.2 M; the extraction conditions: time = 12 h; the extraction solvent is n-hexane.
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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Fig. 8. GC–MS (gas chromatography–mass spectrometry) for oxidized HCTP. The oxidized HCTP (high temperature coal tar pitch) treated with 0.2 M KMnO4 for 5 h.
Fig. 9. Reaction process between KMnO4 and fluorene in HCTP.
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The oxidation mechanism of CTP by KMnO4 According to Clar’s research on PAHs, which suggested that PAHs are self contained a true carbon–carbon double bonds, stabilizing aromatic sextets and shared carbon–carbon double bonds. An increase in these bonds number results in an increase in reactivity except the stabilizing aromatic sextets, these factors make PAHs have a certain activity degree [43–46], which brings up the possibility of KMnO4 to attack the aromatic rings. The interaction between KMnO4 and CTP is mainly electrophilic substitution reaction. Specifically, the p-electron cloud of the aromatic rings attacked by the OMnO3 generates a new p-complex, and then there is a donation of two electrons from the p-complex to form a s-complex, the carbon atom linked to OMnO3 changes from sp2 hybrid orbital to sp3 hybrid orbital, and the change can lead to the closed conjugate system destroyed, and no more p-orbital here. As we all know, the more energy one chemical structure has, the more unstable. s-complex as an instable system would lose a proton from the carbon atom of sp3 hybrid orbital to reform a closed system of conjugate, which reduces the system energy to generate a relatively stable substituted oxide [41,43,47]. The above mechanism indicates that some oxidation PAH products should be generated. To verify it, the GS–MS analysis for the oxidized HCTP by KMnO4 is shown in Fig. 8. As expected, a new compound of 9-fluorenone (retention time = 24.107 min), which may be the product of fluorene oxidation, is detected in the oxidized HCTP, and the speculated reaction process may follow that shown in Fig. 9, which shows electrophilic substitution reaction happened between KMnO4 and HCTP [41]. Although lots of toxic PAHs content was reduced, their
by-product was not detected; these findings were in agreement with many researchers’, the reason why some oxidation products were not found in the CTP remains puzzling [49–51]. From Fig. 10, the GPC spectrum of oxidized HCTP has a tendency to migrate to the left, compared to the original HCTP. And the spectrum band widens after oxidation, it represents the heavy
Fig. 10. GPC (gel penetration chromatography) spectrum of the untreated HCTP and the oxidized HCTP. The oxidized HCTP treated with 0.2 M KMnO4 for 5 h.
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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PAHs ascends after oxidation. The GPC analysis is mainly according to the molecular volume rather than molecular weight, the calibration curve method cannot be used to determine the molecular weight of CTP. However, the relative molecular mass changing trend before and after the oxidation process can be determined. The HCTP with KMnO4 oxidation changes the small molecular compounds distribution in CTP. According to the study of Yao [52], manganese acid ion attacking to aromatic molecules tend to form the ester ring intermediate, and then it would form aldehyde, ketone or carboxylic acid after the intermediate burst; therefore, the small molecules compound had a tendency to increase on account of the oxygen atoms introduced. The findings are consistent with the previous experimental results. Due to the composition of CTP complexity, the new derivatives commonly found in CTP are in very low concentrations after chemical oxidation, such as 5,12-naphthacenedione, 1-phenyl-pyrene, 7,8-dihydro-benzo(a)pyrene, dibutyl-phthalate, etc. The toxic degree between those PAHs oxidized derivatives and their parent compounds are hard to determine one by one. However, some oxygenated aromatic compounds (oxy-PAHs) like 9-fluorenone has been identified that has lower toxicity than its parent compound, and those oxy-PAHs could be easier degraded for further handling [51]. Furthermore, those oxy-PAHs have different physical and chemical properties than their parent PAHs. Oxy-PAHs have a higher aqueous solubility, meaning they can be more easily to leach by organisms and more availability for natural degradation, thus have a meaningful impact on environmental risks [39].
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Conclusions
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This article describes a novel method for producing new CTPs of low toxicity, involving oxidation and extraction treatment to remove PAHs from CTPs. KMnO4 is a favorable oxidant with excellent BaPeq removal rate (above 82%) at 0.2 M for 5 h. Moreover, a further light PAHs removal can be achieved for oxidized HCTP by n-hexane extraction, especially, the total PHE removal is beyond 90%. The oxidation rate increases with increasing rings number of PAH and the PAH-removal rate ranks PHE < FLT < PYR < PHE < pyrene < BaP < BbF < IcP < BgP under the same conditions. By GPC and GC–MS analyses, electrophilic substitution reaction is speculated as the oxidation mechanism of CTP by KMnO4. KMnO4 oxidation and n-hexane extraction can be an efficient, simple and saving energy treatment for the production of low toxicity CTP, however, other modifiers and the toxicity of the possible by-products should be researched in the future study. The other thing need noticed is, during the oxidation experiment, too high doses of KMnO4 or processing of a long time must be avoided, which can decrease the modification efficiency. Therefore, the optimal dose must be carefully determined under a certain condition. This work also found that KMnO4 might not be well suited for all the kinds of CTPs. As to the cost, the KMnO4 reagent relatively inexpensive, just cost 1.3 US $ kg1, the consumption of KMnO4 dosage was no more than 9.5 g per 100 g CTP, the cost of n-hexane has not been considered, because it can be reused, furthermore, KMnO4 is easily handled, readily available to use in riskless conditions than traditional modification methods on reducing PAHs content in CTP. Taken the cost, safety and effectiveness together, thus, this result is very positive for further application.
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Acknowledgments The authors would like to thank the opening funding supported by the Key Laboratory of Road Structure & Material Ministry of Transport, Beijing (grant no. KF201401), and thank the support from China Petroleum & Chemical Corporation, Shanxi Scientific and Technological Plan and Shanxi Lulujia Science & Technology Co., Ltd. References [1] Toxicological profile for wood creosote, coal tar creosote, coal tar, coal tar pitch, and coal tar pitch volatiles, U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA, September 2002 (available on the internet at http://www.atsdr.cdc.gov/ToxProfiles/tp85.pdf). [2] S. Gan, E.V. Lau, H.K. Ng, Remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs), J. Hazard. Mater. 172 (2–3) (2009) 532–549, doi:http://dx.doi.org/10.1016/j.jhazmat.2009.07.118. 19700241. [3] C.H. Yen, K.F. Chen, C.M. Kao, S.H. Liang, T.Y. 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Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction
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Wenchao Wang a , Gang Liu a , Jun Shen a, * , Honghong Chang a , Ruifeng Li a , Jiankui Du b , Zhifeng Yang c , Qingbai Xu d a
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Shanxi Lulujia Science & Technology Co., Ltd., Taiyuan 030002, China Research Institute of Highway Ministry of Transport, Beijing 100088, China d Fushun Research Institute of Petroleum and Petrochemicals, China Petroleum & Chemical Corporation, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 April 2015 Accepted 26 May 2015
This study was conducted to assess the feasibility of using potassium permanganate (KMnO4) as an oxidant to reduce 7 carcinogenic polycyclic aromatic hydrocarbons (PAHs) content in two coal tar pitches (CTPs), and to research the oxidation mechanism of KMnO4. It had been observed reduction of major PAHs in CTP after KMnO4 oxidation. The best benzo[a]pyrene equivalency (BaPeq) reduction rate of 82% in n-hexane soluble in a high temperature CTP (HCTP) was achieved with KMnO4 concentration of 0.2 M treated for 5 h. Specifically, a higher phenanthrene reduction rate (90%) was obtained by using the Soxhlet extraction with n-hexane as solvent on the KMnO4 oxidized HCTP. The single PAH removal rate roughly increases with increasing rings number of PAH, following the order of phenanthrene < fluoranthene < pyrene < benzo[a]pyrene < benzo(b)fluoranthene < indeno [1,2,3-ed]pyrene < benzo[g,h,i]perylene under the same conditions. By using gel penetration chromatography (GPC) and gas chromatography–mass spectrometry (GC–MS), electrophilic substitution reaction was speculated as the oxidation mechanism of CTP by KMnO4. ã2015 Published by Elsevier Ltd.
Keywords: Coal tar pitch Potassium permanganate Oxidation Polycyclic aromatic hydrocarbons (PAHs)
9
Introduction
10
Coal tar pitch (CTP), the remnant of distillation process from coal tar, contains about 200 polycyclic aromatic hydrocarbons (PAHs) [1], PAHs are aromatic compounds containing from two to six benzene rings, many of them are highly toxic, carcinogenic, teratogenic and thermally stable [2–4]. 16 PAHs (Table 1) in CTP have been identified as priority carcinogenic compounds by the US Environmental Protection Agency (US EPA) [5,6]. CTP is commonly found in many industrial areas like former manufactured gas plants, steel making plants, mesocarbon/microbeads electrodes for the metallurgy and material of pavement industry [7–10]. Coal tar have accumulated over decades in soils of the related fields, it can penetrate the entire 4–5 m thickness of soil, refractory to biodegradation, and persistent in soil and water [11–14], therefore, in recent years, coal tar or the related product is considered the fundamental PAHs source among all the known sources [15,16], especially in places like coal-tar-based sealcoat, it has been
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
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* Corresponding author. Tel.: +86 351 6018554; fax: +86 351 6018554. E-mail addresses: [email protected], [email protected] (J. Shen).
identified that CTP contributes to a major source of PAHs, PAHs concentration in dust is about 1000 times higher than asphalt-based sealcoat pavement [17]. PAHs in particles can be easily transported by wind and rainwater to nearby soil sediments, water and air, lead to increased cancer risk for human beings [18–20]. Therefore, CTP is thought as a potential pollutant for its toxic PAHs components [21], and considerable restriction of the Q3 application areas for CTP is currently observed. For those reasons, to prepare less toxic CTP is a matter of extreme importance to environment and human health. Several researches have reported removing carcinogenic PAHs from CTP [22–27]. Boyd et al., Dix and Marnett explained the transformation mechanism of benzo[a]pyrene (BaP) which is the most common and most dangerous PAHs carcinogens to human beings and investigated the carcinogenic mechanism. They pointed out that BaP would be converted into harmless as some functional groups substitute the active position of BaP. This mechanism can be summarized according to the reaction sequence in Fig. 1 [28,29]. Some researchers used polymers at high temperature for a long time to reduce the carcinogenic PAHs content in CTP, such as unsaturated polyester resin (UPR), polyethylene glycol (PEG), 1,4-benzenedimethanol (PXG), divinylbenzene (DVB) and so on
http://dx.doi.org/10.1016/j.jece.2015.05.024 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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Table 1 Relative potency factor (RPF) of each PAH and property according to the US EPA OPPTS. Chemical formula
Number of aromatic rings
Molecular weight (g/mol)
Boiling point ( C)
RPF
Abbreviation
Naphthalene
C10H8
2
128.16
217.9
0.000
NAP
Acenaphthylene
C12H8
3
152.2
275
–
ACY
Acenaphthene
C12H10
3
154.21
279
–
ACP
Fluorene
C13H8
3
166.22
298
0.000
FLR
Phenanthrene
C14H10
3
178.23
340
0.000
PHE
Anthracene
C14H10
3
178.23
345
0.000
ANT
Fluoranthene
C16H10
4
202.26
367
0.034
FLT
Pyrene
C16H10
4
202.26
393.5
0.000
PYR
Benzo(a)anthracene
C18H12
4
228.29
438
0.033
BaA
Benzo(k)fluoranthene
C20H12
5
252.3
481
0.010
BbF
Benzo(b)fluoranthene
C20H12
5
252.3
481
0.100
BbF
Benzo[a]pyrene
C20H12
5
252.3
500
1.000
BaP
Indeno[1,2,3-ed]pyrene
C22H12
6
276
–
0.100
IcP
Dibenzo[a,h]anthracene
C22H14
5
278.35
–
1.400
DhA
PAH species
Chemical structure
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Table 1 (Continued) PAH species
Chemical structure
Benzo[g,h,i]perylene
Chemical formula
Number of aromatic rings
Molecular weight (g/mol)
Boiling point ( C)
RPF
Abbreviation
C22H12
6
276.6
542
–
BgP
“–” means the data were not found.
12
1
11 10
Nu-R
O
2 3
9 4
8 7
6
5
HO
HO
HO HO
(I)
HO
(II)
HO
(III)
(IV)
Fig. 1. Reaction of BaP (benzo[a]pyrene) causing conversion of carcinogenic component (III) to substituted non-carcinogenic component (IV).
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[23,25,30,31]. Zielinski et al. [30] studied the effect of various polymers, 46% BaP content was reduced by using a compound containing PVC, anthracene oil and butadiene/styrene rubber latex when heated for 1 h at 120–150 C. Zhang et al. [31] used coumarone–indene resin and reduction rate of BaP was 46.78% at 270 C. However, there are many shortcomings in those modification methods, for example, the modification processes are often very complicated, and always need huge doses and/or high temperature (120–270 C); some modifiers such as paraformaldehyde can give off poisonous methanol in the operation process, which lead to generate secondary pollution. To solve the problem, the aim of this study is to reduce PAHs content in CTP as low as possible by a simple and efficient method. Due to potassium permanganate (KMnO4) is a common oxidant for remediation of contaminated soil, it can be easily monitored, and has high redox potential [32–35]; therefore, it may be regarded as an alternative to decrease the toxic substances in CTP with lower environmental impact. Here, the objectives of the paper were to (1) evaluate the potential of KMnO4 as a modifier to lessen the carcinogenic PAHs contents supervised by US EPA in CTP and investigate the process parameters including reaction time, doses of KMnO4 and kinds of CTP, (2) find the rules of single PAH removal in the experiment, and (3) identify the oxidation mechanism.
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Materials and methods
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
72
Materials
73
Two CTPs were used in the experiment. One was medium temperature CTP (MCTP) obtained from Datuhe (Shanxi) Coking & Chemical Co., Ltd. The other was high temperature CTP (HCTP)
74 75
produced by China Pingmei Shenma Group in Henan province. Two CTPs were crushed separately and sieved to <0.074 mm, air-dried at 25 C for 2 days, then kept in brown bottles with lid. Two CTPs composition are shown in Table 2. The standard PAH compounds with purity (>99%), phenanthrene, fluoranthene, pyrene, benzo[a]pyrene, benzo(b)fluoranthene, indeno[1,2,3-ed]pyrene, benzo[g,h,i]perylene, were purchased from Aldrich Chemical Co. KMnO4 (>99%), n-hexane (AR), dichloromethane (AR) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.
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The experimental process
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The oxidation experiment Two CTPs were crushed separately and sieved to <0.074 mm, air-dried at 25 C for 2 days, then 20 g CTP (dry mass) was weighted and placed in 250 mL flasks; and 100 mL distilled water was added in. Homogeneous slurry was obtained by stirring at 150 rpm for 1 h. Then the required doses of solid KMnO4 was slowly added in, with concentration range from 0.05 and 0.6 M, the reaction time was from 0.5 to 24 h, the slurry was shaken at a speed of 250 rpm, the oxidation process proceeded at room temperature and at natural pH (7.2). After the oxidation, the CTP suspension was transferred to a Buchner funnel, filtered by suction, and the filtered cake was dried in a vacuum oven at 60 C for 4 h then the modified CTP was crushed and passed through an 80-mesh sieve for next extraction experiment.
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The extraction experiment To further remove the light PAHs in the oxidized CTPs, 3 g oxidized CTP was extracted with 150 mL n-hexane by the Soxhlet extraction for 12 h until the solvent in siphon was colorless.
100
Table 2 The original properties of untreated CTP samples. Samples
HCTP MCTP
SP ( C)
101.50 87.50
Elemental analysis (wt%) C
H
O
S
N
92.91 92.53
4.39 4.46
0.96 1.23
0.89 0.97
0.86 0.81
PHE
FLT
PYR
BbF
BaP
IcP
BgP
8.97 7.96
14.49 11.76
11.56 8.41
13.64 10.77
9.78 7.76
7.45 4.74
10.58 5.59
Determination method: SP (softening point) (GB/T 2293-60; ring and ball method). The unit of the above PAHs is g PAHs/kg CTP (dry mass).
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The controlled experimental In order to evaluate the impact of agitation time on PAHs removal, 20 g HCTP sample (dry mass, 0.074 mm) was weighted and placed in 250 mL flasks, 100 mL distilled water was subsequently added in, the solution was stirred for 12 h at 250 rpm (25 C), then filtered by suction, air-dried. Another 20 g HCTP sample without any treated were crushed and passed through an 80-mesh for extraction experiment, the conditions of extraction experiment was consistent with the previous.
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Analytical methods
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The quantitative analysis of PAHs The PAHs concentrations in two CTP samples were determined by Linghua 9890a GC (Shanghai). External standards were used for GC calibration. The CTP samples were first extracted with n-hexane for 12 h. Then the extract was condensed to approximately 0.5 mL by a rotary evaporation (RE-52AA, Shanghai) and the residue was diluted in 10 mL CH2Cl2 for GC analysis. A 1-mL sample solution was injected into the GC equipped with a 30 0.25 mm Rxi-1 capillary column with a 0.25 mm film thickness, initial oven temperature was 100 C for 0.5 min; ramp to 300 C at 5 C/min and held for 10 min. The flame ionization detector was maintained at 320 C, retained for 50 min until complete elution of all species.
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The qualitative analysis of PAHs GC–MS (Shimadz, QP2010ultra) and GPC (Waters) were used to analyze the constituents in CTP and molecular weight, respectively. GC–MS analyses were performed on a GC using the same column and chromatographic conditions as given above for the PAH quantitative analysis. A 2-mL sample solution was injected in the GC–MS, the Shimadz mass spectrometer was run in the electron impact mode, and m/z was 50–550. An automated GPC system consisting of Waters 600E pump, two gel columns connected in series packed with Waters styrage HT2 and HT3 (7.8 300 mm); stationary phase: polystyren-divinyl benzene; column temperature: 35 C; mobile phase: tetrahydrofuran (HPLC grade), flow rate: 1 mL/min 20 mL sample of solvent was injected in the GPC. All the preparation step of CTP samples for GC–MS and GPC were consistent with the given above for GC analysis. All the experiments were run in triplicate.
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The calculation methods
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Many studies have showed different PAHs have different effects on human health, the calculation methods of PAHs toxicity are always based on BaP equivalent value. In the experiment, the carcinogenicity of different PAHs in CTP was calculated in terms of not the sum of 7 PAHs but BaP equivalents (BaPeq) [36], the equation of BaPeq is shown below: BaPeq ¼ S ½PAHi RPFi (1)
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Results and discussion
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The original properties of the two CTPs
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The properties of two CTPs are shown in Table 2. The seven PAHs contents in two CTPs were measured by GC. The total PAHs content in HCTP is about 76.50 g/kg CTP (dry mass). Light PAHs (FHE, FLT) are 23.46 g/kg; heavy PAHs (PAHs with 4–6 rings except FLT) are 53.04 g/kg; the total PAHs content in MCTP is about 56.99 g/kg, light PAHs are 19.72 g/kg; heavy PAHs are 37.27 g/kg [39]. The HCTP has higher PAHs content than MCTP, the difference is supposed to have influences on the KMnO4 oxidation results.
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The PAHs in CTP oxidation with KMnO4
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The BaPeq removal of HCTP with different KMnO4 concentrations Fig. 2 shows the relationship between BaPeq removal of PAHs in HCTP and concentration of KMnO4 (reaction time = 3 h). As seen from Fig. 2, different KMnO4 concentrations resulted in different removal rates, the sum of seven PAHs removal are always lower than BaPeq removal, because heavy PAHs have higher reactivity and toxicity than light PAHs, BaPeq removal is more accurate than the sum of PAHs removal in reflecting the CTP toxicity, so we mainly discuss BaPeq removal. The BaPeq removal rate in HCTP varied from about 28.96% to 62.50%. Obviously, it was a non-linear correlation between BaPeq removal rate and KMnO4 concentrations, BaPeq removal rate in HCTP increases significantly with KMnO4 concentration till to the highest at 0.2 M, the maximum BaPeq reduction was 62.50% with the consumption of KMnO4 dosage was 9.5% of HCTP. Higher oxidant concentrations could have a negative influence on the PAHs removal, likely because too powerful and quick reactions prevent further contract between KMnO4 and PAHs. This phenomenon may be explained with prior studies relating to remediation of PAH-contaminated sediments by chemical oxidation, the study pointed out that excess oxidant concentration could lead to generate strong foam and decrease the oxidation efficiency, the optimal oxidant dosage should be strictly determined during reducing PAHs content [39]. Meanwhile, because MnO2 was formed from KMnO4, the oxidation product showed brownish, however, the presence of MnO2 does not cause an environment problem [40].
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where [PAH] is each PAH concentration in the CTP; RPF represents relative potency factor of each PAH toxicity relative to BaP. Several relative potency factors of PAHs are shown in Table 1, which were estimated by the US EPA OPPTS [37]. The BaPeq removal was calculated as: ðBaPeqÞfinal (2) 100 The BaPeq removalð%Þ ¼ 1 ðBaPeqÞinitial where (BaPeq)final and (BaPeq)initial were the value of BaPeq respectively after and before the oxidation reaction [38]. In the experiment, it must be pointed out that all the PAHs value in CTP means the content of PAHs in n-hexane soluble of CTP.
Fig. 2. BaPeq (benzo[a]pyrene equivalents) removal of HCTP (high temperature coal tar pitch) with different KMnO4 (potassium permanganate) concentrations. Reaction time = 3 h.
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Fig. 4. BaPeq removal concentration = 0.2 M.
of
HCTP
5
with
different
reaction
times.
KMnO4
Fig. 3. Single PAH removal for HCTP by KMnO4 oxidation. The oxidation conditions: time = 3 h; KMnO4 concentration = 0.2 M.
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The single PAH removal in HCTP Single PAH removal with different aromatic rings number in HCTP is plotted in Fig. 3 (the oxidation conditions: time = 3 h; KMnO4 concentration = 0.2 M). It can be seen that KMnO4 is effective for removing carcinogenic PAHs from CTP, and more effective for heavy PAHs than light PAHs. The PAH removal effect ranks increasingly PHE < FLT < PYR < PHE < pyrene < BaP < BbF < IcP < BgP under the same conditions. These results are consistent with that obtained by Forsey and Julien Lemaire on the oxidation of PAHs, which showed the degradation rate increased with the number of cycles due to the decrease of the bonding energy of cycles [38,41]. It can also be explained by Clard theories [42], PAHs contain three different kinds of keys (Table 3), most PAHs have true double bond characteristics, thus make them readily oxidized by KMnO4, meanwhile, each key attributes to different reactive ability of PAH, and the heavy species are more available to reactants than light PAHs [43]. Forsey et al. also proved that the rate of oxidation– reduction reactions increase with the number of PAH; however, we
must clearly understand that the reaction rate toward KMnO4 is a generalization, because there are other factors such as steric interactions, oxidation condition and the combination mode of the aromatic rings, those differences can also affect the reaction rate under certain conditions [41]. Therefore, we found the BaP removal slightly lower than the BbF in the experiment.
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The BaPeq removal of HCTP with different reaction times Fig. 4 shows the time (from 0.5 to 24 h) influence on BaPeq removal in HCTP while KMnO4 concentration was fixed at 0.2 M. From Fig. 4, the time influence on the PAHs removal rate roughly shows the same trend as shown in Fig. 2, the optimal reaction time in the experiment is 5 h. With the increase of time, BaPeq removal in HCTP first increases and then decreases, varying from 36.34% to 78.05%. The BaPeq removal grows slowly in the first hour during the oxidation of PAHs in CTP, which may be due to a strong adsorption characteristics between PAHs and CTP. They form a very stable interacting mode in a complex molecular network structure in CTP
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Table 3 Clar structure showing number of true double bonds, stabilizing aromatic sextets, and shared double bonds for several PAHs. PAHs
True double bonds
Stabilizing sextets
Shared double bonds
BaP
Clar structure
1
2
1
PYR
1
2
1
PHE
1
0
0
FLT
0
2
3
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Fig. 5. Extraction rate with different agitation times. HCTP: 20 g (dry mass, 0.074 mm); distilled water: 100 mL; stirring rate: 250 rpm.
Fig. 6. BaPeq removal of MCTP (medium temperature coal tar pitch) with different KMnO4 concentrations. Reaction time = 3 h.
which may prevent KMnO4 to attack PAHs. With the extension of time and the influence of agitation, the network structure of CTP becomes loose, PAHs and KMnO4 begin to have a better contact during the second hour, the BaPeq removal begin to rise to 66.94%, then achieves the peak value of 78.05% at 6 h. The removal rate decreases if the time increases above 6 h. The phenomenon has some differences from other related findings during remediation of PAH-contaminated sediments by KMnO4 [39], which may be due to a great difference in composition among soil, waste water and CTP, PAHs is naturally present in CTP, while major PAHs in soil can be seen as foreign, and the content of PAHs in CTP are far higher than the PAH-contaminated sediments or water, especially, with the increase of agitation time, CTP structure becomes more and more loose, and easily extracted by n-hexane in the subsequent experiments. To clarify this point, a controlled trial was run, as expected, the result showed that the extraction rate with 12 h agitation time is 27.49% while the untreated CTP is only 14.05%, as shown in Fig. 5, it means PAHs would be more easily extracted as the agitation time goes on. Extraction rate rises higher above 6 h, it is consistent with previous research that the removal rate decreases if the time increases above 6 h as a result, the PAHs removal seemingly change lower at a long reaction time; therefore, although KMnO4 is a very persistent oxidant, the factor of agitation time must also be taken into account, or it could affect the end result.
PAHs, as noted above, show higher reactivity than light PAHs, which contributes to improve the PAHs removal. From above, we know that not all PAHs species in CTPs have a good oxidation result when KMnO4 used as a modifier. It indicates that the modifier kinds should be changed according to CTP species in the next study.
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The extraction with n-hexane as solvent on the oxidized HCTP From the previous study, we know that the KMnO4 is an effective oxidant for HCTP to destroy PAHs, but it has relatively low performances for the light PAHs during the oxidation modification, therefore, the extraction technology by using n-hexane as solvent was adopted in order to achieve the further reduction of light PAHs. The result is shown in Fig. 7. As expected, the use of solvent extraction to enhance the light PAHs removal is very effective. Much light PAHs have been extracted, especially; the PHE total removal rate is beyond 90%.
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The BaPeq removal with different kinds of CTPs Based on different softening points, CTP is divided into high temperature (95–120 C), medium temperature (75–95 C) and soft CTP (35–75 C), to investigate the adaptability of KMnO4 to different CTPs, the MCTP was treated with different concentrations of KMnO4, and the result is shown in Fig. 6. At the same concentration of KMnO4, the BaPeq removal in MCTP is always much lower than HCTP. The best BaPeq removal in MCTP is only 41.65%, about 20% lower than that in HCTP. The low degradation may be due to the different production technology of CTPs, resulting in different combining degree between PAHs and CTPs, and the different original PAHs content may contribute to this phenomenon. For example, some studies have shown that distribution of PAH content between light component and heavy component can also influence the degradation, the heavy PAHs fraction of 69.94% in HCTP is higher than MCTP (55.60%), the heavy
Fig. 7. Content of PAHs (polycyclic aromatic hydrocarbons) under the conditions of oxidation/extraction. The oxidation conditions: time = 6 h; KMnO4 concentration = 0.2 M; the extraction conditions: time = 12 h; the extraction solvent is n-hexane.
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Fig. 8. GC–MS (gas chromatography–mass spectrometry) for oxidized HCTP. The oxidized HCTP (high temperature coal tar pitch) treated with 0.2 M KMnO4 for 5 h.
Fig. 9. Reaction process between KMnO4 and fluorene in HCTP.
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The oxidation mechanism of CTP by KMnO4 According to Clar’s research on PAHs, which suggested that PAHs are self contained a true carbon–carbon double bonds, stabilizing aromatic sextets and shared carbon–carbon double bonds. An increase in these bonds number results in an increase in reactivity except the stabilizing aromatic sextets, these factors make PAHs have a certain activity degree [43–46], which brings up the possibility of KMnO4 to attack the aromatic rings. The interaction between KMnO4 and CTP is mainly electrophilic substitution reaction. Specifically, the p-electron cloud of the aromatic rings attacked by the OMnO3 generates a new p-complex, and then there is a donation of two electrons from the p-complex to form a s-complex, the carbon atom linked to OMnO3 changes from sp2 hybrid orbital to sp3 hybrid orbital, and the change can lead to the closed conjugate system destroyed, and no more p-orbital here. As we all know, the more energy one chemical structure has, the more unstable. s-complex as an instable system would lose a proton from the carbon atom of sp3 hybrid orbital to reform a closed system of conjugate, which reduces the system energy to generate a relatively stable substituted oxide [41,43,47]. The above mechanism indicates that some oxidation PAH products should be generated. To verify it, the GS–MS analysis for the oxidized HCTP by KMnO4 is shown in Fig. 8. As expected, a new compound of 9-fluorenone (retention time = 24.107 min), which may be the product of fluorene oxidation, is detected in the oxidized HCTP, and the speculated reaction process may follow that shown in Fig. 9, which shows electrophilic substitution reaction happened between KMnO4 and HCTP [41]. Although lots of toxic PAHs content was reduced, their
by-product was not detected; these findings were in agreement with many researchers’, the reason why some oxidation products were not found in the CTP remains puzzling [49–51]. From Fig. 10, the GPC spectrum of oxidized HCTP has a tendency to migrate to the left, compared to the original HCTP. And the spectrum band widens after oxidation, it represents the heavy
Fig. 10. GPC (gel penetration chromatography) spectrum of the untreated HCTP and the oxidized HCTP. The oxidized HCTP treated with 0.2 M KMnO4 for 5 h.
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PAHs ascends after oxidation. The GPC analysis is mainly according to the molecular volume rather than molecular weight, the calibration curve method cannot be used to determine the molecular weight of CTP. However, the relative molecular mass changing trend before and after the oxidation process can be determined. The HCTP with KMnO4 oxidation changes the small molecular compounds distribution in CTP. According to the study of Yao [52], manganese acid ion attacking to aromatic molecules tend to form the ester ring intermediate, and then it would form aldehyde, ketone or carboxylic acid after the intermediate burst; therefore, the small molecules compound had a tendency to increase on account of the oxygen atoms introduced. The findings are consistent with the previous experimental results. Due to the composition of CTP complexity, the new derivatives commonly found in CTP are in very low concentrations after chemical oxidation, such as 5,12-naphthacenedione, 1-phenyl-pyrene, 7,8-dihydro-benzo(a)pyrene, dibutyl-phthalate, etc. The toxic degree between those PAHs oxidized derivatives and their parent compounds are hard to determine one by one. However, some oxygenated aromatic compounds (oxy-PAHs) like 9-fluorenone has been identified that has lower toxicity than its parent compound, and those oxy-PAHs could be easier degraded for further handling [51]. Furthermore, those oxy-PAHs have different physical and chemical properties than their parent PAHs. Oxy-PAHs have a higher aqueous solubility, meaning they can be more easily to leach by organisms and more availability for natural degradation, thus have a meaningful impact on environmental risks [39].
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This article describes a novel method for producing new CTPs of low toxicity, involving oxidation and extraction treatment to remove PAHs from CTPs. KMnO4 is a favorable oxidant with excellent BaPeq removal rate (above 82%) at 0.2 M for 5 h. Moreover, a further light PAHs removal can be achieved for oxidized HCTP by n-hexane extraction, especially, the total PHE removal is beyond 90%. The oxidation rate increases with increasing rings number of PAH and the PAH-removal rate ranks PHE < FLT < PYR < PHE < pyrene < BaP < BbF < IcP < BgP under the same conditions. By GPC and GC–MS analyses, electrophilic substitution reaction is speculated as the oxidation mechanism of CTP by KMnO4. KMnO4 oxidation and n-hexane extraction can be an efficient, simple and saving energy treatment for the production of low toxicity CTP, however, other modifiers and the toxicity of the possible by-products should be researched in the future study. The other thing need noticed is, during the oxidation experiment, too high doses of KMnO4 or processing of a long time must be avoided, which can decrease the modification efficiency. Therefore, the optimal dose must be carefully determined under a certain condition. This work also found that KMnO4 might not be well suited for all the kinds of CTPs. As to the cost, the KMnO4 reagent relatively inexpensive, just cost 1.3 US $ kg1, the consumption of KMnO4 dosage was no more than 9.5 g per 100 g CTP, the cost of n-hexane has not been considered, because it can be reused, furthermore, KMnO4 is easily handled, readily available to use in riskless conditions than traditional modification methods on reducing PAHs content in CTP. Taken the cost, safety and effectiveness together, thus, this result is very positive for further application.
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[48].
Acknowledgments The authors would like to thank the opening funding supported by the Key Laboratory of Road Structure & Material Ministry of Transport, Beijing (grant no. KF201401), and thank the support from China Petroleum & Chemical Corporation, Shanxi Scientific and Technological Plan and Shanxi Lulujia Science & Technology Co., Ltd. References [1] Toxicological profile for wood creosote, coal tar creosote, coal tar, coal tar pitch, and coal tar pitch volatiles, U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, GA, September 2002 (available on the internet at http://www.atsdr.cdc.gov/ToxProfiles/tp85.pdf). [2] S. Gan, E.V. Lau, H.K. Ng, Remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs), J. Hazard. Mater. 172 (2–3) (2009) 532–549, doi:http://dx.doi.org/10.1016/j.jhazmat.2009.07.118. 19700241. [3] C.H. Yen, K.F. Chen, C.M. Kao, S.H. Liang, T.Y. 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Please cite this article in press as: W. Wang, et al., Reducing polycyclic aromatic hydrocarbons content in coal tar pitch by potassium permanganate oxidation and solvent extraction, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.024
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