Degradation Mechanism Of Anilines In The Atmosphere And Aqueous Solution

Anilines (ANs) are the simplest kind of aromatic amines. They are used as raw material and synthetic intermediates in many industries and discharged to the environment with the industrial waste water or slag. ANs are always hard to be biodegradable by the microorganism in water. They have the characteristics of environmental persistence and bioaccumulation. The strong toxicity of many ANs made considerable threat to the health of the aquatic organisms and human being. ANs can enter the bodies through inhalation, ingestion or skin exposure and make bodies poisoning. ANs could damage the blood system or nervous system and bring serious hemolytic jaundice, renal damage and toxic hepatitis. They could cause mutagenesis, teratogenesis or carcinogenesis which damage the human health significantly. So to investigate the degradation mechanism of the ANs and find the effective reaction pathway may have the significant environmental and social significance.In this paper, p-nitroaniline (p-NA), p-nitrophenol (p-NP) and p-chloroaniline (p-CA) are selected as the model compounds to investigate the microcosmic transformation mechanism of ANs in the environment. Based on quantum chemistry and reaction kinetics calculation methods, the degradation mechanism and the reaction rate constants of the compounds in gas phase and aqueous solution are available. It is expectd to make a contribution to find the effective method to remove the ANs in the environment.(1) The OH-initiated oxidation ofp-NA can take place in both gas phase and aqueous solution. The calculation results show that the OH-addition reactions between p-NA and OH radical are more feasible than the H-abstraction reactions in gas phase. The ortho-C of the amino group is the most favorable addition site and the H atom of the amino group is the easiest one to be abstracted. The major products are p-nitrophenol,2-amino-5-nitrophenol,5-amino-2-nitrophenol and p-aminophenol. The intermediates could react with O2 to generate oxygen-rich compounds including five-membered heterocycles or six-membered heterocycles and open the benzene ring finally. The OH-adducts of ortho-and meta-C atoms of amino group could make step reactions to form 2-amino-5-nitrophenol and 5-amino-2-nitrophenol, respectively. The calculated results show that the addition reactions of p-NA and OH radical are dominant in aqueous solution. The total rate constant of OH radical with p-NA at 298 K,760 Torr is determined to be 6.97×10-11 cm3 molecule-11 s-1. The life time of p-NA in atmosphere is estimated to be 4.1 hours.(2) As the calculation results, the OH-addition reaction and H-abstraction reaction of p-NP are competitive in gas phase and the major intermediates of the two reaction types are 1,2-dihydroxy-4-nitrocylohexadienyl radicals and 4-nitrophenoxy radicals,respectively. All the OH-addition reactions are barrierless and highly feasible in water. Comparing the primary reactions initiated by OH radical, HO2 radical and H with p-NP, OH-initiated degradation is the dominant one. OH radical, HO2 radical and O2 play important roles in further completed reactions of primary intermediates which can enhance the degradation extent of p-NP. Phenol, hydroquinone, benzene-1,2,4-triol, benzoquinone,4-nitrobenzene-1,2-diol,5-nitrobenzene-1,2,3-triol and 4-nitrocyclohexa-3,5-diene-1,2-dione are the products formed easily. On the whole said, the reactions in water are easier than those in gas phase. At 298 K and 760 Torr, the total rate constant of the initial reactions with OH radical is 1.4×10-13 cm3 molecule-1 s-1. The branching ratio of the OH-addition reaction and H-abstraction reaction are 0.49 and 0.51, respectively.(3) The main reaction between p-CA and OH radical is OH-addition process reacted at the para-C atom of the amino group. The OH-adducts could form the stable products p-chlorophenol,2-amino-5-chlorophenol,5-amino-2-chlorophenol and p-aminophenol through molecule decomposition reaction or H-abstraction reaction. O2 can attack the OH-adducts reacted at ortho-and meta-C of amino group to form 2-amino-5-chlorophenol and 5-amino-2-chlorophenol, respectively. 2-amino-5-chlorophenol can react with OH radical to form 4-chlorobenzene-1,2-diol, 5-chlorobenzene-1,2,3-triol and 3-amino-6-chlorobenzene-1,2-diol. The mineralization of p-CA can realize after the opening of benzene ring by several reaction steps. O2 could oxidize the H-abstracion intermediate and form oxygen-rich compounds with five or six-membered heterocycles. The amino group in p-CA can be transformed to nitro group. At 298 K and 760 Torr, the total rate constant of the initial reactions of p-CA and OH radical is 8.28×10-11 cm3 molecule-1 s-1, which is consistent with the experiment. Additionally, a negative correlation is confirmed between the initial rate constant and temperature under 200-400 K.