Study On The Reactivity And Mechanism Of Chlorine Dioxide With Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) represent an important group of persistent organic pollutants, which are known to be poor water-soluble, recalcitrant, microbial resistent and carcinogenic. PAHs have received considerable attention and been strictly controlled for its potential threat to environment and people's health. Nowadays, removal technologies for PAHs mainly include bioremediation, photolysis, adsorption, oxidation and sonolysis et al., however these techniques could not be applied to practice because researches in this field were just underway. As a sort of excellent disinfectant and oxidant, chlorine dioxide (ClO2) has promising future for its excellent ability in removing both organic and inorganic contaminants without forming halo-substituted organics such as trihalogenmethane (THMs). The reactivity and mechanism of PAHs with ClO2 and the feasibility of ClO2 in oxidizing PAHs were studied systematically in this dissertation, which established the theoretical basis for the application of ClO2 in removing PAHs in practical wastewater.The effects of chlorine dioxide (ClO2) on the degradation of nine typical PAHs including naphthalene (NPH), acenaphthene (ACE), fluorene (FLR), phenanthrene (PHN), anthracene (ANTH), fluoranthene (FLN), pyrene (PYR), benzo[a]anthracene (BaA) and benzo[a]pyrene (BaP) in aqueous solution were investigated systematically and the results showed that ClO2 has different removal ratio to these nine PAHs under identical conditions, with the best value above 90% for ACE, ANTH, BaA and BaP, and 30% to 70% for FLU, PHN and PYR, and less than 20% for NPH and FLN within 120min. Several factors included ClO2 dosage, reaction time and pH of solution influencing the removal ratio of ACE, ANTH, PYR, BaA and BaP have been studied by batch experiments. The results showed that the removal ratios of these PAHs were affected by the ClO2 dosage and reaction time instead of pH. The removal ratios of ACE, ANTH, PYR, BaA and BaP could reach to their maximum as approximately 96.0%,99.0%,62.8%,90.0% and 99.5% respectively under the conditions as follows: the ClO2 dosage 30, 5, 40, 35 and 5 mg/L, reaction time 90, 30, 120, 120 and 30 min, and pH 7.2. Kinetics about ACE, ANTH, PYR, BaA and BaP in simulated aqueous solution with ClO₂ were studied and the results showed that the reaction was pseudo-first order with respect to both PAHs and ClO₂, and the general reaction could be described as second order. Under the condition with pH and temperature of solution was 6.8 and 20℃, the reaction rate constants (k) was 0.141, 0.605, 0.722, 6.780 and 7.795L/(mol·s) for PYR, BaA, ACE, ANTH and BaP respectively, which has the same sequence with their removal ratios. The value of pH and concentration of ClO₂ have slight effects on the value of k, which however increased with the enhancement of temperature.ANTH was selected as the representative to study the reaction mechanism with ClO₂. The reaction activation energy was calculated as 44.235 kJ/mol, which revealed that the ANTH-ClO₂ reaction is an endothermic process and can occur under conventional water treatment conditions. The oxidation products formed in the ANTH-ClO₂ reaction were tentatively identified by gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared spectrum (FTIR) and the results showed that the main product was 9,10-anthraquinone. Through analyzing the reaction properties of ClO₂ and ANTH, the mechanism of ANTH-ClO₂ reaction was discussed and the possible pathway was proposed based on the theory of single electron transfer (SET).AM1 (Austin model 1) and PM3 (parametrisation 3 of modified neglect of diatomic overlap) methods in MOPAC computation application were used to calculate the molecular orbital energy and net charges on each atom of these nine PAHs. Their different reactivity with ClO₂ was expatiated by comparing the HOMO (highest occupied molecular orbit) energy and structural proterties of each PAHs. Respective quinonoid compounds of ACE, ANTH, BaA and BaP oxidized by ClO₂ were identified and the relationship between the oxidation positions and atom net charge of PAHs were examined, the results revealed that the most reactive positions were in accord with carbons with highest neglect charges. Parameters of quantum chemistry were induced to evaluate the reactivity and predicting the oxidation positions of PAHs in this study, which is a new tempt for parameters of quantum chemistry in describing properties of PAHs.Orthogonal experiment was used to discuss detailedly the influence factors as ClO₂ dosage, reaction time, temperature and pH in removing the residual PAHs and COD in secondary effluent from petroleum refinery by ClO₂ in this chapter. Results showed that after treated by ClO₂, the PAHs and biphenyl in secondary effluent were removed effectively and COD value was reduced further. Sequence of the influence factors investigated in the experiment is: ClO₂ dosage > reaction time > temperature > pH. The optimal parameters were as follows: ClO₂ dosage 10mg/L, reaction time 60min, pH 6.9 and temperature 20℃, under which the removal ratios of PAHs and COD were 64.55% and 75.62%, respectively. Work of this dissertation has great theoretical and practical value as it provided enough scientific basis for the application of ClO₂ in removing PAHs from aqueous solution.