Study On Reduction Of Toxic Polycyclic Aromatic Hydrocarbons In Coal Tar Pitch By Chemical Oxidation

Along with the road construction inceasing development in our country,petroleum asphalt is not enough to supply. Compared with petroleum asphalt,coal tar pitch(CTP) has the advantages of low cost and good road performance.If CTP could be applied to road paving, it will not only clear out the warehouses of CTP, but also can largely change the serious shortage of asphalt materials supply in China and offer a new economic growth point. However, the environmental protection is the biggest problem during CTP application,because CTP is thought as a potential pollutant for its toxic polycyclic aromatic hydrocarbon(PAHs) components. Serious health risks may occur through the release of toxic pollutants into the environment through treatment or disposal.Therefore, the focus of this topic is how to reduce the toxic PAHs content in CTP.The optimum process for reducing content of toxic PAHs by KMnO4 was studied; Using 16 kinds of PAHs as model compounds to evaluate the toxicity of byproducts in the oxidation process; Differences of toxic PAHs content in CTP fume emissions and water-soluble before and after modification were analyzedand compared. The mechanism and the kinetic between PAH and KMnO4 was discussed by using fluoranthene as the model compound. The effects and mechanism between fluoranthene and persulfate was also discussed. The variations of oxidation-reduction potential(ORP) was observed to reveale the oxidizing conditions of system. Main research results in this paper are obtained as follows.1. The reduction of toxic PAHs by selected modifiers ranked in the following order: KMnO4 > Cinnamic aldehyde > Paraformaldehyde >Terephthalaldehyde > Waste rubber powder > phenolic resin. The reaction mechanism of modification CTP by Cinnamic aldehyde and Paraformaldehyde was discussed. The Ba P chemical conversion mechanism probably is C-alkylation. The modification of coal tar structure tends to be the resin of COPNA B. KMnO4, as a new modifier, was applied to modify CTP for the first time. Compared to the traditional modifiers, CTP modification by KMnO4 can achieve the highest removal rate of Ba Peq content. Therefore, KMnO4 displayed good potential for future applications.2. PAHs in CTP decreased significantly after treated with KMnO4 oxidation and n-hexane extraction. 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, 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 increaseswith 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; The results of GC-MS for reaction between KMnO4 and 16 PAHs show that the acenaphthene, fluorene and anthracene produce some by-products after KMnO4 incomplete oxidation, such as 1-acenaphthenone, 9-fluorenone,9,10-anthraquinone. However, there are no oxidation byproducts of large PAHs were detected, such as fluoranthene, benzo[a]anthracene and Ba P, it may be due to the large PAHs have been thoroughly degraded to small molecule compounds that difficult to detect by GC-MS. Meanwhile, oxy-PAHs are found have higher boiling point and less toxicity than their corresponding parent PAHs.3. The content of phenanthrene, anthracene, fluoranthene, pyrene in CTP fumes decreases after KMnO4 modification. Especially, the fluorene content reduced up to 65%. In the water-soluble substances in CTP, the content of fluorene, acenaphthene, phenanthrene, and anthracene were also decreased, the fluorene removal rate as high as 48.6%. Therefore, CTP by KMnO4 modification can reduce the toxic PAHs contents in CTP fumes and water-soluble substances and decrease their hazard to the environment.4. The fluoranthene removal rate improves as KMnO4 concentration increases. When the potassium permanganate dosage increases to 0.1025 g/L,fluoranthene removal rate reaches to 94% at 390 min. The batch experiments show that the kinetics of the KMnO4/fluoranthene reaction follows asecond-order rate law with an apparent rate constant of 0.00832 L·min-1·g-1.From the point of view of ORP, it indicates that increasing the concentration of KMnO4 can promote solution ORP, thus improve fluoranthene degradation rate.Under the condition of strong acid or strong alkaline, fluoranthene degradation rate may be suppressed, namely, fluoranthene by KMnO4 oxidation in acidic and alkaline does not improve the reaction rate. Conversely, under the condition of neutral, fluoranthene degradation rate can reach the maximum.5. Fluoranthene degradation rate was exponentially dependent on reaction time in the oxidation process. The reaction rate increases with the increasing of temperature. The activation energy of the KMnO4/fluoranthene reaction was calculated to be 50.22 kJ·mol-1. In general, the activation energy of chemical reaction is 60~250 kJ mol-1, and the reaction activation energy is slightly lower than the general value. It means the reaction is likely to happen. The initial concentration of fluoranthene has almost no effect on the oxidation reaction rate.6. Fe(II)-persulfate oxidation can produce hydroxyl radical(·OH) and sulfate radical(SO4-·) with high ORP. Fluoranthene can be completely oxidized under certain condition(T=40℃. [persulfate]0=0.084 mol/L. [CA]0=0.104 mol/L.[Fe(II)]0=0.036 mol/L). Raising the temperature could effectively improve the degradation rate of fluoranthene. Acidic conditions and right amount of citric acid(CA) could promote the degradation of fluoranthene rate, but excess CA can inhibit the degradation of fluoranthene.7. SO4-· and ·OH has been verified by using molecular experiment inFe(II)-persulfate system and SO4-· was identified as the predominant radical species responsible for fluoranthene degradation, while ·OH only occupies a very small proportion, meanwhile, ·OH is more prevalent in cidic and alkaline.There were no intermediate products detected by GC-MS and High Performance Liquid Chromatography(HPLC), it can be concluded that fluoranthene could be completely degraded in Fe(II)-persulfate system.8. From the point of view of ORP, increasing the concentration of sodium persulfate can effectively improve the system of ORP value, so it can promote the fluoranthene removal rate. In particular, ORP variations of oxidation system can explain the effect of ratio between ferrous sulfate and persulfate on fluoranthene removal rate. An excessively high ferrous sulfate concentration could reduce ORP of system. In general, an observation of ORP variations can reveal the oxidizing conditions of system quite well.