A Study On The Phototransformation Mechanism Of Alkylphenols In Aquatic Environment Under Simulated Sunlight

The typical endocrine disrupting chemicals, nonylphenol （NP） and octylphenol（OP）, have been widely used in various industrial processes and they are ubiquitous inthe aquatic environment. The strong biologically cumulative effect, stability andestrogenic character of NP and OP have caused public concern. It is therefore of greatsignificance to investigate their environmental fate. The primary route of exposure toNP and OP for human and wildlife is through water, and phototransformation is animportant abiotic process for the elimination of NP and OP in water. Therefore, wechose4-n-NP and4-t-OP as target compounds to investigate theirphototransformation pathways and to elucidate the effects of common waterconstituents on the phototransformation mechanisms of NP and OP. The majorconclusions of the study are as following:The influence of each common environmental factor on the phototransformationof NP and OP has been assessed through contrasting the residual rate of NP and OP.In pure water, dissolved oxygen （DO） played a key role in the phototransformation ofNP, as the degradation rate obviously decreased when the concentration of DO wasdecreased. Other factors such as NO₃-, HCO₃^-, Fe（Ⅲ）, H₂O₂and HA increased thephototransformation of NP. However, the influences of NO₃-, H₂O₂and HA on theNP phototransformation were more obvious. The photolysis rate of NP was morerapid in an alkaline solution than in an acidic medium. In the condition of higher lightintensity or higher temperature, the phototransformation rate of NP increased. Theeffects of common water constituents on the OP phototransformation in pure waterwere also assessed. And DO was also a significant influencing factor on the OPphototransformation and other factors （NO₃^-, H₂O₂, HA） accelerated thephototransformation rate of OP due to their photoactivity. Although the photolysisrates of NP and OP were different in the three water systems, the four factors （DO,NO₃^-, H₂O₂, HA） were significant on the phototransformation of NP and OP. The phototransformation pathways in different conditions have been proposedaccording to the intermediate products identified by GC-MS. In pure water, when NPreacted with DO,4-nonyl-catechol, nonanol, nonanal and nonoic acid have beenidentified as the major degradation products, while the accumulated amount of nonoicacid was the most. We therefore proposed that4-nonyl-catechol and ortho-quinonederivative were produced after the formation of4-nonylphenoxyl radical andsuperoxide radical anions （O₂^-）, then the intermediates underwent conjugate additionto produce nonanol, nonanal and nonoic acid. In the presence of H₂O₂,4-n-akylphenol(HOC₆H₄-CnH_2n+1, n=2⁸),4-nonyl-catechol, nonanol, nonanal and nonoic acid werethe major products, and nonanal was the most accumulated one. We proposed that thehydroxyl radicals （OH） generated by the photolysis of H₂O₂attacked the electronicgathered positions of NP molecules. In the presence of NO₃^-, the same products werealso detected as in the presence of H₂O₂in addition to2-nitryl-4-nonylphenol. Theirradiated NO₃-can produce OH and NO2, and NO₂attacked the ortho position ofphenolic hydroxyl to generate2-nitryl-4-nonylphenol. In the condition of coexistenceof Cl-and H₂O₂in an aqueous solution, two opposite effects of Cl-were observed: itcan make full use of OH with surplus amount of H₂O₂, resulting in an accelerateddegradation rate of NP, while Cl-compete with NP when the amount of H₂O₂is verylimited, leading to a retarded degradation rate of NP. In any case, the same productswere detected in the coexistence of Cl-and H₂O₂as in the presence of H₂O₂, inaddition to nonanoyl chloride. The fact that nonanal was the most accumulatedproduct let us believe that it was most probable that the chlorine radical （·Cl）（generated from the reaction of OH and Cl-） reacted with nonanal to producenonanoyl chloride.The degradation rate of NP in natural seawater was slower than in pure water.Only4-nonylcatechol was identified as the intermediate, no other products werefound in the irradiated samples. There are many photoactive species in naturalseawater. These species may rapidly react with intermediate products. This may bethe reason why it is difficult to detect those small molecules. When OP reacted with DO,4-octylcatechol was the main degradation product.The proposed mechanism was that4-octylcatechol derivative was produced after theformation of4-nonylphenoxyl radical and superoxide radical anions （O₂^-）. However,since other small molecules were not detected during the process of OP degradation, itis not possible to propose further transformation pathways of4-octylcatechol. In thepresence of H₂O₂, OH generated by the photolysis of H₂O₂is the active species toreact with OP.4-octylcatechol and4-amylphenol were found in this condition and theformer one was the most abundant intermediate. The proposed mechanism was that4-octylcatechol was formed after that· OH attacked the ortho position of phenolichydroxyl. Also, with the reaction of·OH，tertiary carbon turned to secondary carbon,then after the rearrangement of secondary carbon structure, the heptylphenol that hastwo tertiary carbons was formed. However, heptylphenol was not detected in thephotolysis samples due to its very active nature of OH. At last,4-amylphenol wasformed. In the presence of NO₃-,2-nitryl-4-octylphenol and4-octylcatechol werefound as the intermediate products and the former one was the most accumulatedproduct of OP phototransformation owing to the stability of2-nitryl-4-octylphenol.We proposed that OH and NO2（produced by irradiated NO₃-） attacked the orthoposition of phenolic hydroxyl. In natural seawater, the phototransformation of OP wasslower than in pure water. The existence of many species in natural seawater maycompete with OP, resulting in a retarded degradation of OP. The main intermediateproduct of OP phototransformation in natural seawater was2-nitryl-4-octylphenol,while other products were not detected. We therefore postulated that the existence ofNO₃-may play a key role in degradation of OP in natural seawater. This is obviouslydifferent from the case of NP phototransformation in natural seawater. The alkylchain of OP may be responsible for its different pathway of photolysis in naturalseawater and the stability of2-nitryl-4-octylphenol may account for the fact that itwas the only product detected during OP phototransformation.