Interaction Of DOM With PBDEs And Effects Of DOM On Photochemical Behavior Of PBDEs

In recent years, plybrominated diphenyl ethers (PBDEs) have been detected in diverse biota samples and environmental media. There has been a growing concern about the environmental behavior and eco-toxicological effects of PBDEs. Dissolved organic matter (DOM) is ubiquitous composition in aquatic environment. It can interact with hydrophobic organic pollutants (HOCs) through various mechanisms, thereby affects the environmental partition and tranformation of HOCs. PBDEs, with low solubility, are typical HOCs. DOM can interact with PBDEs and significantly increase the solubility of PBDEs in water. However, the interaction mechanisms between DOM and PBDEs are poorly explored. In surface waters, photodegradation is one of important transformation pathways for PBDEs. DOM is one of important factors to affect the photochemical transformation of pollutants in water.The effects of DOM on the photochemical behavior of PBDEs worth studying. In this study, theoretical computation methods were applied to study the interaction mechanisms of DOM with PBDEs, and photochemical experiments were conducted to explore the effects of DOM on the photochemical behavior of PBDEs.Quantum chemistry and molecular docking calculations were employed to reveal the interaction mechanisms of PBDEs/PCBs with DOM from the view of charge distribution, surface electrostatic potential distribution and molecular conformation. Effects of halogen substituent position and numbers on the interaction between PBDEs/PCBs and DOM were explored. It is shown that non-ortho-substituted isomers have larger stronger hydrophobicity than those with ortho-substituted halogen atoms. By the calculations of atomic charge and surface electrostatic potential distribution of target chemicals and DOM model molecules, electrostatic interaction (such as π-π) between the aromatic rings was proposed. By comparing the dihedral angle change of the aromatic rings and energy barrier, it is indicated that non-ortho-substituted PBDEs and PCBs have larger dihedral angle adjustment space and lower orientation vdw energy. Thus on-ortho-substituted isomers interact more effectively with DOM through twist and folding, and show larger binding constants than ortho-substituted isomers. With the increasing of substituted Br atoms, the steric hindrance between DOM and PBDEs are increased and larger orientation vdw energies are required for π-π interaction. Meanwhile, the difference of cumulative electron density between benzene rings of DOM and PBDEs are decreased, which means that π-π interactions are weakened. Higher bromine substituted PBDEs generally demonstrate stronger hydrophobility. It is proposed that with the increase of bromine substituted number, the contribution of π-π interaction between DOM and PBDEs will be decreased and the contribution of hydrophobic partition will be increased.To further study the influence of DOM on the photodegradation of PBDEs, BDE-47was selected as a model compound. Photodegradation experiments and computational methods based on density functional theory (DFT) were employed to investigate the effects of Leonardite humic acid (LHA) and Suwannee fulvic acid (SRFA) on the photodegradation of BDE-47. The react activities of BDE-47with reactive oxygen species (ROS)1O2and-OH was determinded. Experimental results show that SRFA and LHA can inhibit the photodegradation of BDE-47by light attenuation. From the ratio of the theory photolysis rate constant of free BDE-47to the photolysis rate constant of BDE-47in the presence of DOM, we presumed that DOM can quench excited states of BDE-47through energy or electron transfer from BDE-47*to DOM, thus may inhibiting the photodegradation of BDE-47. Furthermore, DFT computaional results indicated that SRFA and LHA can quench the excited states of BDE-47, thus inhibiting the photodegradation of BDE-47. Though BDE-47can react with1O2and-OH, the contribution of reaction of BDE-47with ROS to photodegradation is smaller than light shielding effects and quench effects of DOM.