| 其他摘要 | Dissolved organic matter (DOM) is a mixture of various organic substances including organic macromolecules (humic substances) derived from the remains of plants and animals through complicated physical, chemical and biological processes naturally occurring in water, soils and sediments. DOM is extremely complicated molecular structure, with its molecular weight varying from several hundreds to several hundred thousands. The elemental compositions and its chemical structure are varied with time, space as well as with source. It has been reported that humic substances are the major components of DOM, and accounted for approximately 50 to 80% of DOM, in terms of dissolved organic carbon (DOC), in the aquatic environments. In accordance with its varying solubility under different acidity and alkalinity conditions, Humic substances can be separated into fulvic acid (soluble at all pHs), humic acid (insoluble at pH 1), and humin (insoluble at all pHs).
Previous studies show that DOM can affect some biogeochemical processes. For example, DOM acts as a large organic carbon reservoir, which can influence on microbial food chains and global carbon cycle. DOM is emerging as a critical variable akin to nutrient loading and food web structure in significance as a determinant of the general properties and dynamics of aquatic ecosystems. It can absorb the sunlight, thus has a strong effect on the penetration of ultraviolet and photo-synthetically active radiation. DOM mainly comprises organic acidic and alkali fuctional groups, which affects pH of natural aquatic systems. It can also greatly influence the speciation, toxicity, transformation and bioavailability of trace metallic ions and organic contaminants by complexation and adsorption reactions. Moreover, DOM can also act as a precursor in the formation of the cancer-causing substances thihalomethanes and other by-products during the disinfections of municipal waters.
Tracer metal ions (such as: Cu2+ and Hg2+) in natural waters are present predominantly in the form of complex compounds with DOM. DOM mainly controlled copper speciation in oceanic waters, and more than 90% of copper in freshwater. Previous researches show that DOM can inhibit the assimilation of Hg by plankton, while an increase the cumulation of Hg by fish because of the complextion of Hg with DOM. Up to now, little is known about the mechanisms of transport and conversion of metal ions in environments, especially about the circulation of toxic heavy metal ions in the case of the involvement of DOM. The conditional stability constant is a fundamental parameter to characterize the capacities of complexation of metal ions with DOM. Binding properties of DOM with metal ions have been studied with a variety of analytical methods e. g., ion selective electrode potentiometry, anodic stripping voltammetry, ultrafiltration and fluorescence quenching titration etc. Ultraviolet (UV) absorbance titration technique was an alternative for investigating the binding constants between pure compounds and metal ions. However, a little is known about whether it is applicable to the naturally occurring DOM, which was a complicated mixture of organic macromolecules and no significant peaks in the UV absorbance spectrum. In this paper, UV absorbance titration, for the first time, was used to investigate the interactions between DOM and tracer metal ions. The UV absorbance titration method has the advantages of rapid determination, and simple performance so that it may become a powerful approach to study the interactions between DOM and trace metallic ions.
Pharmaceuticals and personal care products (PPCPs) occurred and have been detected in natural environments. It has been evidences that some of those compounds may influence organisms in ecosystems, e.g., changes in sex ratios, resulting in subtle modification in plant growth, failure of larvae to hatch, and varying degrees of anatomical deformities. Carbamazepine (CBZ) is an established drug used for the control of grand mal and psychomotor epilepsy, and is widely used in the treatment of trigeminal neuralgia and bipolar depression. Unfortunately, toxicological researches demonstrated that CBZ in natural environments had significantly damaged the health of aquatic animals such as rainbow trout liver and immune system of mussels. CBZ was used as a tracer for anthropogenic activity effects and evaluation of the efficiency of PPCPs removal in sewage treatment plant because of the general large consumption, high concentrations in aquatic environments, and the persistence even in normal wastewater treatment and groundwater infiltration. But a little is known about its biogeochemical cycling and influencing factors. The interactions between DOM and organic contaminants can influence on their circulations of biogeochemistry. To the best of our knowledge, there is no reports available about the interactions between CBZ and DOM. The major objectives of our study were to inestigate: (i) the binding behavior of DOM with CBZ using fluorescence spectroscopy, (ii) the functional groups of DOM responsible for the binding with that compounds, (iii) influencing factors on the binding behavior between DOM and CBZ, and (iv) the possible role of DOM in affecting its behaviors in natural waters. This may help the understanding of biogeochemical cycling of other PPCPs, and the prediction of their transport, conversion, fate, and potential risk.
In this paper, UV absorb titration and fluorescence-quenching titration were applied to study the interactions of trace metal ions (Cu2+ and Hg2+) and DOM from different sources. The binding of CBZ and DOM was also investigated using three-dimensional excitation emission matrix fluorescence spectroscopy and synchronous scan fluorescence spectroscopy. The followings results can be summarized:
1. UV absorbance titration was used, for the first time, to investigate the interactions between DOM and tracer metal ions. And this technique has the advantages of simple performance, rapid determination and high popularization rate of equipment. Even if the concentrations of DOM are very low (about 10-5 ~ 10-7 mol/L), ultraviolet absorbance titration is also powerful enough to determine directly the conditional stability constants without conducting the complicated pre-enrichment of DOM. Therefore, it may become a new powerful approach to study the interactions between DOM and trace metallic ions.
2. Metal ions (Hg2+and Cu2+) binding with DOM have some related to carboxyl and phenolic hydroxyl group. Those complexations increased dislocation of electron, resulting an increase in absorbance. While intrinsic fluorescence of DOM quenched by metal ions e.g., Hg2+and Cu2+, could be attributed to changes the electronic polarization and effect of paramagnetic cupric ion. There was a significant and negative correlation between UV absorbance and fluorescence intensity during DOM titrations with Cu2+or Hg2+ (R2=0.99, p<0.001). Those indicated that the fluorescence intensity and ultraviolet absorbance reflect the complexation of metal ions with DOM.
3. The conditional stability constants (log K) detected with UV absorbance titration method agree well with those detected with fluorescence quenching titration simultaneously, raging from 3.5 ~ 5.5. The strong complexations of DOM with metal ions lead to change of biogeochemical circulations of heavy metal ions. The results also showed that sources of DOM and pH both can affect their binding behavior.
4. CBZ and DOM show a strongly bound. The binding constants (log K) range from 3.41 to 5.04. The binding of CBZ with DOM can significantly reduce the concentration of carbamazbepine, and increase its solubility, mobility and transformation in natural environments. These results may help to understand the biogeochemical cycle of CBZ and other PPCPs.
5. The intrinsic fluorescence of DOM was significantly quenched upon the addition of CBZ, and the static quenching was the primary mechanism detected with three-dimensional excitation emission matrix fluorescence spectroscopy and synchronous scan fluorescence spectroscopy. Hydrophobic adsorption/partitioning could be the major force that drives CBZ binding to DOM. The influence of pH on DOM-CBZ complex is minimal and the Cu2+ partly competes with CBZ for fluorescence binding sites of DOM. |
修改评论