|Other Abstract||As an important componet of the global carbon cycle, dissolved organic matter (DOM) played a significant role in physical, chemical and biological processes in aquatic ecosystems: serving as the energy source for hetero-microorganism; affecting pH and acting as a buffer in some acidic waters; influencing the transport, toxicity and bioavailability of trace metals and organic pollutants; being the major precursors of chlorination byproducts that can cause certain cancers in drinking water treatment. DOM was a complicated compound and consisted of humic and nonhumic substances which had different environmental geochemical characteristics and behavior. So the study on the structural characteristics and environmental effects of DOM fractions will help us to understand their complex structure, their cycle and behavior nature, as well as to reveal their effective mechanism and contribution of their transport, toxicity and bioavailability, and to apply some fundamental data for the water evaluation, forcast and controls.
In this paper, DOM in Hongfeng Lake was concentrated and isolated with XAD resin isolation method into humic acid, fulvic acid and hydrophobic neutrals, hydrophilic acids, hydrophilic bases and hydrophilic neutrals. Analytical methods such as elemental analysis, organic carbon analyzer, Fourier transform infrared (FT-IR), UV-Vis absorbance, high performance size exclusion chromatography (HPSEC), three-dimensional excitation emission matrix fluorescence spectroscopy (3DEEM) and stable isotope mass spectrograph were selected to investigate the physical and chemical properties and origin of DOM fractions. The complexation between DOM fractions and copper were studied by using 3DEEM and fluorescence quenching titration. The chlorination activity and the main precursors of DOM fractions in chlorination were also compared by Gas chromatography. Some conclusions were abtained:
1. Modified the isolation procedure. Based on the XAD-8 resin separation method, another XAD-4 resin was used to separate DOM from Hongfeng Lake into humic acid, fulvic acid and hydrophobic neutrals, hydrophilic acids, hydrophilic bases and hydrophilic neutrals. The recovery was 82%. The composition of DOM in Hongfeng Lake was similar to that of other natural waters. Among these fractions, fulvic acid was dominant and accounted for 51% of the total DOM
2. Characterization of these six fractions. The elemental composition of hydrophobic fractions had higher carbon and sulfur than hydrophilic fractions; Higher O/C in hydrophilic fractions exhibited a largest oxygen and acidic functional group contents. FT-IR suggested there was a strong band at about 3424 to 3434 cm-1 attributed to O-H stretching and N-H stretching. Humic acid exhibited nitrogen-containing and polysaccharide structures, while fulvic acid and hydrophilic acid were characterized by a strong carboxyl peak.The molecular weight of these fractions were ranging from1338-2355. HPSEC showed the molecular weight of these six fractions were small. The Mw of hydrophobic fractions were larger than hydrophilic. A weak relationship was found between E2/E3 and Mw. 3DEEM showed these six fractions exhibited four different fluorescence peaks: two obvious humic-like fluorescence peaks (peak A and peak C) in all fractions, and two protein-like fluorescence peaks (peak B and peak D) only observed in HON. A strong relationship was observed between the fluorescence intensities of humic-like peaks and UV absorbance coefficient at 254 nm. This study indicated that there were subtle consistency and inter-relationship between molecular weight, fluorescence and absorbance characteristics, and also between hydrophilic and hydrophobic properties and sources among the six isolated DOM fractions. This study would help in the further understanding of the complexity and similarity of DOM, and its characteristics and environmental effects in aquatic environments.
3. The sources of these six fractions. 13C and C/N ratio together with fluorescenc spectroscope were applied to indicate the sources of DOM. DOM fractions in Hongfengh Lake had different sources. Fulvic acid was mainly derived from the organic material of terrestrial soils containing C3 plants. Other five fractions were derived from the both origin including allochthnonous and autochthonous origin in which humic acid was near the algae source and hydrophobic neutral and hydrophilic neutral were near the terrestrial source. Protein peaks in hydrophobic neutral showed in 3DEEM might indicate hydrophobic neutral partly derived from human activities. The results have significance for further understanding the diverse sources and the complex effect factors of DOM.
4. The complexation between Cu (ІІ) and DOM fractions in Hongfeng lake. Fluorescence quenching titration was performed to characterize the binding properties between Cu (ІІ) and DOM fractions. The fluorescence intensity was marked decreases aftering addition of Cu (ІІ). The wavelength of Em was blue shifted, while Em almostly constant. These phenomena indicated a marked modification of the electronic structure of the six fractions when interacting with Cu (ІІ), such as: the decrease of π electron density, conjugated and aromatic degree, and linear molecular converted into nonlinear molecular; The titration data were fitted to the modified Stern-Volmer equation. The values of stability constants (log K) were ranged from 4.73 to 5.16 for peak C and 4.65 to 5.24 for peak A. A strong positive correlation were observed between log K and total acidity and phenolic group contents, while a weak relationship between log K and carboxyl contents. These results suggest the Cu (ІІ) binding to DOM more generally preferred the strong (i.e. phenolic group) to weak (i.e. carboxylic-type group) binding sites. This study will help in the understanding of the difference and influencing factors of the complexation between DOM fractions in Hongfeng lake and metal binding and offer significant fundamental data for the transport and toxity of metal pollutant in aquatic system.
5. Chlorination activity of DOM. The reactions between DOM fraction in Hongfeng Lake and chlorine were studies with headspace technique with gas chromatography. The resulted showed that chloroform and dibromochloromethane were the primary chlorination byproducts. Fulvic acid produced more trihalomethanes and had higher reactivity with chlorine than other fractions. These results indicated that fulvic acid was the main precursor in chlorination which was derived from the terrestrial organic matter. So this study had a certain practical meaning in water disinfection.|