The Binding Of Organic Contaminants By Dissolved Humic Acid And The Impact On Organic Contaminant Degradation

Modern industrialized societies have developed thousands of synthetic organic compounds in recent decades.Organic contaminants often end up in the environment as pollutants during and after their application.The contaminants accumulate and transformation in environment,thereby render hazardous impacts to life and ecological systems.Dissolved organic matter(DOM)widely exists in natural environment and plays a critical role in the transport,partitioning,mobility and bioavailability of organic compounds because of their strong interactions.Through various interactions(such as Vander Waals forces,hydrophobic interactions,and electron donor-acceptor),DOM binds organic contaminants and affects their environmental behavior.It is considered to be the most important environmental components controlling the fate and risks of organic contaminants in terrestrial and aquatic systems,and thus the role of DOM played in regulating the environmental behavior of organic contaminants has become a research hotspot.To investigate the interactions between DOM and organic contaminants,the very first step is to quantify DOM-contaminant interactions.So far,a great deal of researches focused on the measurement of binding coefficient(Koc)of organic contaminants in different aqueous chemistry environments,and then the environmental fate and risk are evaluated according to Koc values.Many methods have been used to evaluate the Koc values between organic contaminants and DOM.However,Koc differ greatly between which obtained from different methods(higher binding coefficients using fluorescence quenching in comparison to other methods),which make it difficult to accurately quantify organic contaminants Koc.Furthermore,only quantifying the interaction is not enough to provide more comprehensive and reliable information for us to control the risk of organic contaminants.Although most of researchers evaluated the environmental fate and risk of organic contaminants according with the obtained Koc,the environmental behavior of organic contaminants after binding to DOM in the complicated natural environment is still not clear.Up to date,there is rare information about the environmental behavior of freely dissolved and bound organic contaminants.To gain a more complete understanding of the influence of DOM on the environmental behavior of organic contaminants,the accurate quantification of the organic contaminants/DOM interaction,and the role of DOM played in the degradation of organic contaminants are investigated in this study.The key technique for quantification the interactions between DOM and organic contaminants is to separate the freely dissolved and DOM-bound contaminants.Fluorescence quenching technique is recommended as a sensitive and fast method thus it has been widely applied.Fluorescence quenching could be identified as static(binding)and dynamic quenching(molecular collision).Dynamic fluorescence quenching is not real binding,thus it complicates the binding data interpretation and misleads the analysis of conclusion.However,the contribution of dynamic quenching is rarely quantified and how a particular structure in DOM contributes to the static and dynamic fluorescence quenching of a fluorescer is still unknown.The quantification of static and dynamic quenching and the analysis of particular structural contribution are beneficial to accurately quantify organic contaminants/DOM interaction.We thus specifically designed a work to quantify dynamic quenching as affected by DOM structural characteristics,sequentially extracted and chemically modified dissolved humic acids(DHA)were used as model DOM in binding experiments for ofloxacin(OFL),phenanthrene(PHE),9-phenanthrol(PTR)and naphthalene(NAP)in this study.We observed that the binding of PHE to DHA increased with DHA hydrophobicity,while OFL-DHA interaction showed the opposite.The very significant contribution of dynamic quenching(up to 82%)suggested that this process could not be overlooked in fluorescence quenching experiments.The dynamic quenching of PHE decreased from 50%?82%to 11%?58%and OFL dynamic quenching increased from 2%?27%to 31%?61%with DHA hydrophobicity.Considering DHA characteristics,we speculated carboxyl groups may make the main contribution to PHE dynamic quenching,while interact with OFL through both static and dynamic quenching.Identification of DHA structures indicated that aromatic components and carbohydrates in DHAs primarily showed static quenching(bound)rather than dynamic quenching(collide)with PTR.Aromatic components showed static quenching to PHE,NAP,and PTR,while carboxyl groups primarily showed dynamic quenching.Carboxyl groups showed interactions with PTR through dynamic quenching only when carboxyl groups were on the benzene ring.Although Stern-Volmer curves are apparently linear,dynamic quenching makes a great contribution to the interaction between DHA and organic contaminants.Ignoring the dynamic quenching will lead to a completely erroneous conclusion.We took a close look at the impact of binding on the environmental behavior and fate of organic contaminants.Binding of organic contaminants to DOM can occur when chemicals come in to the natural environment,which decreases the concentration of freely dissolved contaminants,and consequently impact on their bioavailability and risk.Triclosan(TCS)was taken as the model organic contaminants in this work.The results showed that TCS can be anaerobically degraded by a metal-reducing bacterium,Shewanella putrefaciens CN32.As expected,TCS degradation was inhibited by DHA at 15-100 mg C/L,which was resulted from the significant interactions between TCS and DHA(binding).This observation is consistent with the earlier statement that binding with DHA decrease the bioavailability of organic contaminants.However,TCS degradation was substantially facilitated by low-concentration DHA(0-15 mg C/L).We thus hypothesized that in addition to binding,DHA may act as an electron shuttle to mediate the degradation of organic contaminants.The electron accepting capacity was also measured for DHA in this study.DHA can accelerate the degradation through facilitating extracellular electron transport,and decrease the degradation by inhibiting the bioavailability of TCS.DHA acted as both an electron shuttle and a sorbent in regulating the degradation of TCS.The dual role of ubiquitous DHA in the reaction of TCS governs the environmental degradation and persistence of TCS.The important complex role of DOM in regulating the persistence of organic contaminants needs to be considered for evaluating the ecological risk and effects of organic contaminants occurring in natural environments.This study is a positive step toward accurately quantifying the interaction between organic contaminants and DOM.The established relationship between DHA properties and fluorescence quenching contributions will greatly facilitate the development of fluorescence quenching method in studying DOM-organic contaminant interactions.This study highlighted that DOM may control the environmental behavior of emerging trace organic contaminants in natural environmental through binding and electron transfer mediating,with implications in their engineering application and environmental risk regulation.