Transformation Of Selected Organic Pollutants In Humification Processes

Humification processes is a transformation pathway of organic compounds in environment,leading to their coupling with soil organic matter to form non-extractable bound residue.A thorough understanding of compounds transformation in environment is critical for scientifically assessing the environmental risks associated with its application and for devising effective management and remediation means to mitigate its negative environmental and human health impacts.In this study,we demonstrated that neucleophilic substitution could serve an important pathway causing ATZ sequestration.The carbon bonded to the chlorine in ATZ molecule is partially positively charged due to the strong electronegativity of chlorine and is susceptible to the attack of nucleophiles such as aniline.Since aromatic amines are relatively rare in natural soils,amino acids/peptides were hypothesized to act as the main nucleophiles in real environment.However,substantially ATZ transformation was only observed in the presence of those species containing thiol functionality.Thus,we speculated that it was the thiol group in amino acids/peptides acting as the nucleophile.Nitrogen in amino acids was in fact not an active nucleophile toward ATZ.In addition to the sulfur-containing amino acids,other thiol compounds,and sulfide were also proved to be reactive to ATZ.Thus,the sequestration potential of ATZ probably correlates to the availability of thiol compounds in soil.Transformation of halophenol phenols(XPs)in laccase-catalyzed oxidation processes were also explored in this study.We first examined the intrinsic reaction kinetics and found that the transformation of XPs appeared first order to the concentrations of both XPs and laccase.A numerical model was developed to describe the role of HA in this process.It was demonstrated that HA could reverse the oxidation of XPs by acting as the inner filtrator of XP radical intermediates formed upon reactions between the substrates and laccase.The extent of such reversion was proportional to HA concentration.Copper had a complicate influence on the reaction rate.At concentration less than 1 mmol·L-1,it enhanced the transformation appreciably.However,at higher concentration,the reaction was suppressed.MS analysis in combination with quantum chemistry computation suggested that coupling products were generated.XPs coupled to each via C-C or C-O pathways,generating hydroxylpolyhalogenated biphenyl ethers and hydroxyl polyhalogenated byphenyls,respectively.Many of these polyhalogenated products were demonstrated to be estrogenic and hazardous to the ecosystem and human health.We speculate laccase-catalyzed transformation of XPs might responsible for their occurrence in the environmental media.We also expolred that reaction mechanisms and products of XPs in laccase-catalyzed systems in the presence of 2 model humic constituents.The results showed that vary concentrations of DHBA inhibited the transformation of XPs.Resorcinol had a suppressed influence on the reaction rate.At concentration 0.1 mmol L-1,it inhibited the transformation appreciably.However,at higher concentration,the reaction rate was unchanged.MS analysis suggested that cross-coupling products were generated.XPs coupled to 2 model humic constituents,generating dioxin-containing functional compounds,respectively.