Spatial Characteristics Of Dissolved Organic Matter And Its Complexation Behavior With Copper In The Yuqiao Reservoir In China

Dissolved organic matter(DOM)is ubiquitous in various aquatic and soil environments,which is defined as the water soluble organic material that can pass through a 0.1-0.7 ?m filter,and consists of highly complex organic mixture of relatively low molecular weight compounds including humic-like substances,protein,other macromolecular substances and small molecular amino acids,carbohydrates,sugars and other organic substances.It plays a crucial role in the natural environment system.It is an essential source of energy and nutrition for heterotrophic microorganisms,as well as an important complex agent or adsorbent for environmental pollutants.Dissolved organic matter can form strong complexions with heavy metal ions,which affects the solubility and bioavailability of heavy metals,thus consequently affecting the transformation and ultimate fate of heavy metals.However,the variations on the composition characteristics of dissolved organic matter with different sources,components and molecular weight have significant influences on its heavy metal binding affinity and environmental behavior of other organic contaminants.In recent years,human activities have greatly changed the sources,structure and compositional nature of dissolved organic matter in aquatic systems.Among them,dam construction is the most dramatically and the most extensive anthropogenic disturbance event.With the increasing need of economic development,the construction of various reservoirs has become an important way to alleviate the contradiction between the supply and urgent demand of fresh water resources.Dam interception has significantly changed natural properties and internal biogeochemical process in natural rivers.Followed by which are the alterations of hydrological conditions including water depth,hydrodynamic conditions,water retention time,which strongly influence the contents,chemical structure,compositions of DOM and subsequently change the bioavailability of the DOM and its combination ability with pollutants.Additionally,this not only plays a very important role in the biogeochemical cycling of carbon,nitrogen and other biogenic elements in the aquatic ecosystem,but also has an influence on the environmental behavior such as the biological toxicity and migration of heavy metals and organic pollutants.Yuqiao reservoir is the major and crucial drinking water production for the population of Tianjin,China.In this study,the spatial variations on the chemical composition and structure characteristics of DOM and the effects of its heavy metal binding properties were investigated in Yuqiao Reservoir river-reservoir system by measuring elemental analyzer,UV-Visible absorption spectroscopy,Excitationemission matrix fluorescence spectroscopy combined with parallel factor analysis(EEM-PARAFAC)and Fourier transform infrared spectrum(FT-IR).Moreover,two dimensional correlation spectroscopy(2DCOS)integrated with UV-Visible absorption spectroscopy and synchronous fluorescence were adopted to depict the binding characteristics of copper to DOM.The main results are as follows:(1)The pH is alkaline in the water body of Yuqiao Reservoir catchment.The range of pH value was from 7.47 to 8.44 in this area.Significant spatial variations in the physical and chemical properties of water were observed,including the higher DIC concentration and the lower pH value and much lower DOC content as well as the alteration on major ions in the outgoing water compared to lake water.This suggests that river damming interception caused the increase of the water level in the reservoir area,changed the in-lake biogeochemical effect,thus affecting of the physiochemical properties of river water in the downstream reaches of the riverine-reservoir system,significantly changed destroyed the natural continuity of river.The distribution of organic carbon in the Yuqiao Reservoir catchment had obvious spatial heterogeneity.The DOC content was significantly higher in the reservoir water than that in the upstream and downstream water bodies.The mean DOC concentration of the incoming river was 4.68mg/L,9.53mg/L in the reservoir water,and 5.05 mg/L in the downstream river water.This suggests that river damming interception caused the increase of the water level in the reservoir area,which can significantly alter some physical degradation processes and in-lake DOM processes,such as production and microbial degradation,thereby can subsequently impact on the spatial distribution and export of DOM.(2)Dam interception of rivers resulted in significant spatial differences in DOM composition of water bodies in the study area.The results of EEM-PARAFAC modelling showed that two fulvic acid-like substance components(C1 and C2)and one protein-like components(C3)in the upstream water,a tryptophan-like component(C4),two fulvic acid-like components(C5 and C6),a tyrosine-like substance(C7)in the reservoir lake water and a tryptosphan-like component(C8),two fulvic acid-like components(C9 and C10)in the released river water.These results showed that three components(a C-type fulvic-acid like substance,a M-type fulvic-acid like substance and a protein-like substance or a typtophan-like substance)were commonly detected in all samples.But one more tyrosine-like component was detected in the lake DOM.These studies indicates that the aquatic ecosystem in the river-reservoir system has a series of complex physical,chemical and biological processes,and river damming significantly changes the composition of DOM in aquatic system.(3)No obvious differences in the fluorescence intensity of humic-like substances detected between the upstream and main reservoir areas,while the contents of proteinlike components in upstream sites were significantly lower than that of the main reservoir area due to changes of hydrological conditions in the reservoir area resulted from the presence of dam as well as the disturbances of human activities.Significantly decrease in fluorescence intensities of both humic-like and proteinlike substances with different decrease rates were observed in the released water DOM compared the reservoir water DOM.Among which,the tyrosine-like components were completely degraded,and the fluorescence intensity of tryptophan,C-like fulvic acidlike substances and M-like fulvic-acid substances decreased by 65%,74% and 47% respectively.These results suggests different interception efficiency resulted by dam barrier effect.These results also suggests that in winter season,humic-like substances with larger molecular weight and higher aromaticity were conservative in the transportation from upstream to the reservoir area before the dam by rivers,while dam interception caused several changes in biological,physical and chemical processes,thus altered the refractory nature of humic-like substances.(4)Our results revealed that DOC concentration,terrestrial humic-like substances amounts,protein-like substances contents,DOM absorption a280,a355,spectral slope SR,E2/E3,and SUVA254 value decreased significantly when water flow through dam from the reservoir to the downstream outgoing river water.This indicates the outgoing water had lower DOC contents with a relatively higher average molecular weight and a lower degree of aromaticity.The results of FT-IR indicate the degradation of functional groups(O-H bending,C=C vibrations of aromatic structures,and C=O stretching of quinone and/or conjugated ketone and amide groups,C-O alcohols,ethers and carbohydrates)in water DOM from upstream to downstream sites,which could be linked with some microbial and physical dam barrier degradation processes.(6)All fluorescent components were quenched after addition of Cu(II)by decreasing fluorescent intensity at their respective peak positions.Lake fluorescent components showed very similar decreasing trend of fluorescence intensity after addition of Cu(II)compared to those of outgoing waters.Complexation binding affinity(lgKCu-DOM,termed as conditional stability constant)for both FA-like substances(C type and M type)were similar to each other between incoming rivers and lake waters whilst lgK values for tryptophan-like substances were,on average,relatively higher in lake waters than in incoming river waters.LgKCu-DOM values for outgoing waters were relatively higher compared to those of lake and incoming rivers,suggesting that dam barrier interception could transform both FA-like substances that may responsible for relatively high binding affinity of functional groups in dam affected DOM to form complex with trace metals.Overall,these results revealed that dam interception had a strong influence on the amount and quality of DOM including the distribution of the CDOM component,the functional groups as well as the spatial dynamics of DOM in riverine-lake system,which may consequently alter the DOM-metal binding properties.Meanwhile,Complexation binding affinity between organic matter and copper showed different selection for different organic matter from various water sources in reservoir system.The studied results highlighted changes of components and structure of organic matter impacting by key biogeochemical processes in riverine-reservoir system.Of course,there exits further working to explore the coupling and link between dissolved organic matter and heavy metal,such as other heavy metal except copper.