| Coal tar pitch(CTP) is the remnant of coal tardistillation process, after light oil, phenol oil, naphthalene oil, wash oil, anthracene oil are distilled. It is mainly composed of polycyclic aromatic hydrocarbons(PAHs) and heterocyclic compounds containing O, N, S and a few small carbon granule. In China, CTPhas huge output with low prices.CTP has good wetting, adhesion and resistance to erosion, and high road surface friction coefficient, therefore, many scholars devoted to research CTP performance in pavement. CTP contains 16 kinds of toxic PAHs supervised by EPA. As road materials, the 16 PAHs can cause greater harm to human and environment.Therefore, this article focuses on the detecting methods of 16 toxic PAHs in CTP, and their concentration during immerseing in water and in asphalt fumeduring heating.5 kinds of CTPs were selected and extracted with toluene using Soxhlet extraction. The extracts were analyzed with GC-MS. Gas chromatography(GC) method and column chromatography-high performance liquid chromatography(HPLC) method for analysing the 16 toxic PAHswere established and comparedrespectively. The influences of immerseing time, immerseingtemperature, liquid-solid ratio, p H, and CTP types on the rule of immerseing CTP in water were studied by GC-MS and GC. In addition, the influencesof heating rate, terminal temperature, CTP type on the contents of the 16 toxic PAHs in CTP fume were studied by a self-building fume generation and enrichment device. The main conclusions are as follows:(1)Using toluene as solvent for CTP extraction can get good extraction effect at 10 h. The extractsare mainly PAHs with 4, 5 ringsand N, O, S heterocyclic compounds. The 16 toxic PAHs contents measured by GCare between 91.88g/kg~116.96 g/kg in 5 CTPs. The 16 toxic PAHs contents in medium temperature CTP are higher than that of high temperature CTP, but BaP equivalent toxicity in medium temperature CTPis less than the high temperature CTP.(2) 16 kinds of toxic PAHs in toluene extractsareseparatedby column chromatography,elution solvents followed by petroleum ether, petroleum ether and toluene mixture(v: v = 1:1), toluene.Column chromatography-HPLC method can analyze the 16 kinds of toxic PAHs in CTP with good accuracy, especially for the isomers of benzene[b]fluoranthene and benzo[k]fluoranthene.(3) Compared two kinds of analysis methods, GC is more convenient and quick, but can’t separatebenzene[b]fluoranthene and benzo[k]fluoranthene, while column chromatography-HPLC method can do that. However, the pretreatment process is complex, the operation is time-consuming, and the cost is high.(4)According to the results of GC-MS, only 6 toxic PAHs were detected in the CTP immersed in water,namely acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene. Heterocyclic compounds were also detected; the relative content of N compounds accounted for 65.93%, O compounds was 23.79%. The leaching quantity of PAHs increases with the increase of time, and nearly approaching equilibrium after 2h. The higher the temperature, the greater the leaching amount. The concentration of PAHs in 30℃water is 122.92 ug/L, but 382.35 ug/L at 60℃, and acidic conditions can increase the leaching amount of PAHs. The leaching amount of PAHs in medium temperature CTP is higher than that of high temperature CTP, and so is BaP equivalent toxicity.(5)The pyrolysis process of CTP containsthree stages: dry dehydration, light component volatilization and heavy components decomposition, thermal polycondensation. The second stage is the main stage of weightloss. By determining PAHs in 2 CTPs’ fume at 120℃, 140℃, 160℃, 180℃, we conclude that medium temperature CTP produces more PAHs under the same temperature than high temperature CTP, and the higher the temperature, the more content of PAHs. Only the temperature is higher than 160℃, can 5 rings PAHs be released in fume. |
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