| 其他摘要 | Nature organic matter(NOM) in nature waters (e.g. ground water, lake, river and ocean) receives much attention because of its remarkable ecology and the environment function. As an important componet of the global carbon cycle, NOM is the main source of carbon and energy for hetero-microorganism. Furthermore, NOM influences many physical and chemical processes in lake ecosystem: the organic acids of NOM contribute significantly to the acidity of freshwaters and can buffer freshwaters against acid deposit; NOM can aslo affect light condition in nature waters by attenuating both visible and ultraviolet light. By attenuating light, DOM may decrease the growth of autotrophs but at the same time protects aquatic organisms against harmful UV radiation. More importantly, NOM can affect the fate of the contaminants in environment. For instance, the interactions between the contaminants and NOM can change their solubility, transport, toxicity and bioavailability, attenuating their harm for organisms.Polycyclic aromatic hydrocarbons (PAHs) are organic substances composed of at least two condensed aromatic ring structures. Most of the PAHs are toxic and carcinogenic; furthermore, they tend to bioaccumulate to aquatic organisms. Therefore, the determination or prediction of the intensity of PAHs-NOM interaction，which is commonly defined as the partitioning coefficient (Kdoc) normalized to the mass of dissolved organic carbon , is very important. A lot of Kdoc values of various PAHs binding to dissolved organic matter from a variety of different sources were determined by many investigators with various approaches. Generally, these studies showed that the magnitude of Kdoc largely depended on the hydrophobicity of PAHs , solution chemistry and properties of NOM including origin of NOM, its polarity, molecular weight, aromaticity and aliphaticity, etc. However, contradictory observations were obtained by many investigators when studying the impact of physicochemical properties on the magnitude of partitioning coefficient. Aquatic humic substances (hydrophobic fraction of aquatic NOM) derived from higher plants constitute approximately 40-60% of aquatic NOM, and their interaction with PAHs had been studied by many investigators. However, the role of the nonhumic materials (hydrophilic fraction) microbially derived organics from algae and bacteria in binding with PAHs was rarely investigated due to difficulty of isolating.
In this study, six humic and non-humic fractions were isolated using two-column array XAD-8 and XAD-4 resins based on Leenheer's fractionation protocol (Leenheer, 1981). The six NOM fractions were extensively characterized using many analytical techniques such as elemental analysis, high-performance size exclusion chromatography (HPSEC), total dissolved organic carbon analysis and specific UV absorbance. The fluorescence spectroscopy techniques were used to investigate the effect of solution chemistry on the conformational changes of humic substances and to determine partitioning coefficients and conditional stability constants of the NOM-PAHs interactions; furthermore, the influence of molecular weight, molar absorptivities at 280nm and atomic ratios of C/H, (O+N)/C on the magnitude of partitioning coefficients and different interaction mechanisms underlying binding of the different fractions to the PAHs were also discussed. The main outcomes from this study were summarized as following:
1. The effect of water chemistry on their fluorescence properties and molecular conformation:
The molecular comformation of humic substances have significant effect on the interaction between NOM and PAHs. However, the water chemstry can aslo affect the molecular conformation of NOM. In this study, 3DEEM and fluorescence polarization technique were used to investigate the effect of water chemisrty on the fluorescence properties and molecular conformation of humic substances. The fluorescence properties (e.g. intensity, shift of emission peak and fluorescence polarization) of the humic substances have changed to some extent with increasing concentration, pH and ionic strength, indicating the altering of the molecular conformation of the humic substances.
2. The interaction of the NOM fractions isolated from Lake Hongfeng with Perylene, Phenanthrene and Anthracene was investigated by fluorescence quenching. This study showed that binding capacity of aquatic NOM for PAHs was strongly related to aromatic structures in NOM macromolecules. Strong correlation of the logKdoc values with molecular weight and molar absorptivities at 280 nm were observed, while the atomic C/H ratio was not a better indicator for aromaticity (unsaturated carbon). Simultaneously, amorphous polymethylene carbon existed in paraffinic carbon region (0-50 ppm in 13C NMR) of NOM had a great impact on its PAHs binding ability. A converse correlation of (C+O)/C ratio with log Kdoc value was also obtained, indicating the significance of polarity in sorption PAHs. Furthermore, the effect of varying PAHs on NOM binding characteristics was quite different due to their different hydrophobicity.
3. Generally, more linear Stern-Volmer plots were observed when binding of perylene to the NOM fractions, indicating preNOMinant partitioning mechanism controlling the binding process, while sorption isotherms of phenanthrene and anthracene were less linear, suggesting superior sorption mechanism. As for the NOM fractions, the hydrophobic fractions showed higher binding capacity (Kdoc) relative to the hydrophilic fractions; furthermore, the Stern-Volmer plots obtained for the hydrophobic fractions binding with the PAHs were more linear. Although the HON fractions was not superior in aromaticity (i.e, ∈280 and molecular weight) relative to the HA fraction, it exhibited higher PAHs sorption capacity when binding to phenanthrene and anthracene. This may result from π-π electron donor-acceptor interaction besides hydrophobic interaction; furthermore, its enhanced sorption capacity would also be attributed to its low polarity.
4. The solution chemistry such as pH and ionic strength had some extent effect on the partitioning coefficients of NOM-PAHs interaction. In general, the partitioning coefficients decreased with increasing pH, while the effect of ionic strength on the partitioning coefficients was quite complex. Generally, NOM sorption capacity for PAHs increased with increasing ionic strength.
5. The interaction of the NOM fractions isolated from Lake Hongfeng with Anthracene was investigated by fluorescence polarization. This study showed that the fluorescence polarization of Anthracene decreased with increasing NOM concentration, indicating turning of Anthracene conformation to columned. Among the NOM samples, the conditional stability constant of Amherst HA-Anthracene interaction was the largest (logK=5.6 and 5.4 at pH 4 and 6, respectively). As for the NOM fractions, the logK of the hydrophobic fractions exhibited in the area of (4.5-5.2)，while the logK of the hydrophilic fractions lay in the range of (4.3-4.8). This indicated that the sorption capacity of the hydrophobic fractions was larger relative to that of hydrophilic fractions. |
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