Fractionation Of Dissolved Organic Matter And The Impact Of The Fractions On The Bioavailability And Sorption-desorption Of Pyrene

Dissolved organic matter (DOM) is widely present in the aquatic environment,which is often defined operationally as a continuum of organic molecules ofdifferent sizes and structures that passes through a filter of0.45μm pore size, withthe molecular weight from several hundred to several ten thousand Da, which resultsin its heterogeneous nature. It contains carboxyl, phenol, ethanol, hydroxyl, carbonyl,amine and thiol functional groups and has been shown to be one of the key factorsaffecting the fate, transport, bioavailability, and chemical and biological reactivity ofHOCs as the result of combining with them.Polycyclic aromatic hadrocarbons (PAHs), the representative persistent organicpollutants (POPs), have drawn the world-wide concern. A proportion of PAHs endup in lakes or oceans, bind to DOM, and exert hazardous effects on aquaticorganism via atmospheric sedimentation, accumulation on land surface, and rainwashing. The transport and transformation behaviours of PAHs and its influence onthe aquatic organism are affected by DOM, which mechanism is quite complicated.In this study, DOM fractions with different molecular weight and polarity wereselected to study the binding coefficient with pyrene and its influence on thebioavailability and sorption/desorption behaviours.The main conclusions drawn in this dissertation are as following:(1)The DOMbulkwas extracted by water from the sediment of the BeitangEstuary，Tianjin，China. Then it was separated into three different molecular weight(MW) fractions by dialysis technique,<1000Da (DOM<1000),1000-14000Da(DOM1000-14000) and>14000Da (DOM>14000). Additionally, six hydrophobic andhydrophilic fractions, i.e. hydrophobic acid (HOA), hydrophobic base (HOB),hydrophobic neutral (HON), hydrophilic acid (HIA), hydrophilic base (HIB),hydrophilic neutral (HIN), were isolated using resins. All the fractions have good relationships with its optical properties, elemental ratio and the content of functionalgroups.(2) KDOMvalues of pyrene were measured using fluorescence quenchingmethod, and possible binding mechanisms were discussed. Different cationic ionshad varied impacts on KDOMof pyrene, which has relation with the charge-density.With increasing concentration of cationic ions, KDOMof pyrene showed a complexpattern of increasing initially, then decreasing, and finally stabilizing at a plateau,which followed four-stage variation model．It is showed that the ion strength has ancomplex effect on the pyrene binding to DOM, which is explained by the salt-outeffect and DOM conformation. The tested anionic ions did not show significantinfluence since the DOM itself are negatively charged.(3) DOM fraction with larger MW (14000Da) showed a greater bindingcapacity (KDOM=2.02×105) as compared to those of smaller MW, with KDOMbeing1.16×105for the fraction with MW of100014000Da,1.13×105for the fractionwith MW of1000Da. Positive correlation was acquired between KDOMand ε280and negative for KDOMand (O+C)/N, indicating the positive effect of DOMaromacity and negative effect of DOM polarity. Infrared spectroscopy revealedstronger interaction between pyrene and DOM fraction with larger MW. KDOMvaried in a complex pattern with increasing pH and cation concentration, and thiscould be ascribed to the change in DOM conformation. Aggregation leads to areduction in binding sites. however when aggregate acts as an hydrophobic moiety,its binding capacity increases; further increase in aggregation makes interior sitesless accessed and binding capacity does not increase further. The changing of KDOMof different DOM MW fractions is of similar pattern, but with different rates.(4) The KDOMof pyrene binding to the six DOM fractions with various polaritywere determined by fluorescence quenching. The KDOMvalues binding to thehydrophobic fractions were larger than those to the hydrophilic fractions, with thelargest value for HON. Besides hydrophobic interaction, there is π-π electron donoracceptor interactions. A significant correlation of KDOMwith ε280was observed,while atomic ratio of C/H was found to be a poor indicator for aromaticity. Otherstructural properties, such as polarity influenced the DOM-fraction ability to bind pyrene. If the hydrophobic domains were less covered by the surface polarcomponents, the PAHs may have accessed to more alkyl and aromatic domains inthe DOM-fractions, resulting in a high sorption capacity.(5) The effect of different MW DOM on the bioaccumulation of pyrene inBrocarded Carp and semi-permeable membrane (SPM) was studied. DOM reducedthe accumulation of pyrene in SPM and the muscles of fish and enhanced those inthe gills and guts of fish, and as a whole enhanced pyrene accumulation in fish.DOM with larger MW exhibited stronger influence. Additionally, the bound pyreneby DOM was partly available for accumulation by fish but the free dissolved form isthe primarily source. The accumulation of pyrene in SPM-ME (Semipermeablemembrane-microextraction) accumulation of pyrene compared with the traditionalSPM and it can be preferably applied to simulate the fish accumulation of freeHOCs in aqueous environment. The release rate of pyrene in fish is very fast, andmost of the accumulated pyrene was released in24h. DOM can make the stepquicker.(6) DOM significantly reduce the sorption capacity of peat for pyrene andaccelerated desorption of pyrene from peat. However, the sorption and desorption ofpyrene on kaolinte was accelerated by DOM. The larger MW DOM, the strongereffect, which is affected by many factors, such as aromaticity, polarity, molecularconformation.