Biochemical Process Of Released Heavy Metals On The Sediment-water Interface Of Eutrophic Lakes

Heavy metals in lake water body are widely researched pollutants due to their higher toxicity, lower biological degradability, and higher bioaccumulation. The processes of heavy metals on Sediment-Water Interface are complex, involving adsorption/desorption, dissolution/absorption, complexation/dissociation and a series of physical and chemical effects and biological oxidation, reduction, methylation and so on, and they are a typical biochemical processes. In Eutrophic lake water, it is noteworthy that nitrogen, phosphorus and heavy metals always exist at the same time, and there are close relationships between eutrophication and pollution. Besides, eutrophic lakes are rich in a variety of organic matters and a great number of microbes which have strong surface activity. The degradation of organic matter, microbial metabolism will affect the shape and behavior of heavy metals. For these reason, the research on migration and transformation of heavy metals in sediment-water interface of eutrophic lakes has important theoretical and realistic significance to solving the issue of eutrophication and heavy metal pollution.This paper was funded by the National Basic Research Program: “Water environmental quality evolution and water quality criteria in lakes”(2008CB418200). In this paper, three different rich eutrophic lake sediments of Tai lake, Nansi lake, and Bai Yangdian were studied as the research objects. Using the combination of research methods of wild field sampling, laboratory analysis and simulation experiment, the characteristics of sediment in eutrophic lakes were analyzed and evaluated. On the basis of the analysis of heavy metals’ release potential with extraction procedure, the simulation experiments of static and dynamic working conditions was carried out to explore the release kinetics characteristics of heavy metals in different eutrophic lake sediments. Through the comparison between sterilization and non-sterilization, we explored the chemical and biochemical process of heavy metals’ release on eutrophic lake sediment-water interface, and clarified the role of microorganisms in the sediment release process of heavy metals. Furthermore, through the study of the influence factors such as the particle size, nutrient level, eternal enviornment temperature, and p H, the process and mechanism of the geochemical behavior of heavy metals on the interfaces are further revealed.The main research results are as followed :(1) In the three lakes, the concentrations of TN and TP in the overlying water were 0.31-7.74 mg/L and 0.014-0.482 mg /L separately, it showed that the lakes were in highly eutrophic status. In sediments, the contents of organic matters, total nitrogen and total phosphorus were 9.54-108.10 mg/g, 0.58-4.82 mg/g, and 0.448-2.148 mg/g respecively. The organic pollution in Tai Lake was classified to clean status, while Nansi and Bai yangdian lakes were in pollution status.(2) The distribution rules of heavy metal contents in sediments were in the sequence of Zn>Pb>Cu>Cd. In these metals, Cd pollution was the most serious but Cu and Zn pollution is slighter. From the perspective of pollution load indexs, the heavy metal pollution in Bai yangdian was the most serious, Nansi Lake was secondary polluted, and Tai Lake was the lightest. Analysis on the potential ecological risk of heavy metals in lake sediment was: Nansi lake>Bai Yanydian> Tai Lake.(3) Based on a single method of extracting agent, heavy metals release potential analysis indicated the largest release potential was Cd, and the minimum was Zn. Based on continuous extraction method, Pd in the sediments of Taihu Lake had the greatest potential to release, and Cu had the highest potential in the three lakes. In Nansi and Bai Yangdian lakes, Cd had the greatest release potential, however the Pb’s potential could not be ignored. The correlation analysis for the single extraction amount, pollution load index and potential ecological risk of heavy metals shows that, Cu extracted by HCl,Zn and Pb extracted by DTPA, and the active fractions had positive correlations(represent the potential release).(4) Under the simulated dynamic condition, the release of heavy metals Cu, Zn, Pb and Cd can be divided into two phases which is called quick release and release balance. In different sampling areas, the release quantity of Cu had insignificant difference, with release ratio from 1.5% to 2.5%. The highest release intensity was Cd, and the release ratio was from 10% to 37%. For Pb, the release rate was from 0.8% to 2.5%, and for Zn, the release was less than 1%. The release kinetics process could significantly fit Elovich equation, double constant rate equation and pseudo secondary dynamics. But the pseudo secondary dynamics equation showed the best fitting effect. From the perspective of fitting characteristics value in Nansi Lake, it had the biggest intensity of heavy metals release in domestic sewage area, but had the lowest intensity in aquatic vegetation area. Based on the correlation analysis among heavy metal release characteristics in dynamic condition, the single extraction and continuous extraction experiments showed that HCl extraction method could be uesd as a kind of quick and easy characterization of the release quantity of Cu and Zn.(5) At the simulated static condition, the release characteristics of heavy metals Cu and Zn can be divided into three phases which is called slow release, accelerated release, and release balance. Balance release quantity of Cu is from 0.2 mg/kg-0.3 mg/kg, and Zn is from 0.3 mg/kg-0.5 mg/kg. Under the static condition, the release rate of Pb whose the balance release quantity of 0.15 mg/kg-0.45 mg/kg is faster than Cu and Zn. Through the comparison of heavy metals’ release characteristics between dynamic and static working conditions, it could be found that the release rate and intensity of heavy metals under the static condition is less than that in dynamic condition(except Zn, whose release quantity is higher under static conditions than that under dynamic condition). Furthermore, the release process was much more complicated.(6) Among the influence factors of heavy metal release, the size of sediment grain can affect the release of heavy metals: the more content of small size sediment is, the bigger of specific surface area to adsorbent to heavy metals is, so heavy metals are not easy to release. The contents of phosphorus in eutrophic lakes can promote the adsorption to heavy metals. Microorganisms have inhibitory effects on the release of Cu, Cd and Pb in sediments, but have promoting effect on the release of Zn. The release quantity of heavy metals decreases with the increase of p H value. When temperature rising, the release quantity of Cu increases, which is not obvious. On the contrary, Zn releases the largest quantity at low temperature.(7) Fluorescence spectrum of dissolved organic matter in sediments show that, DOM contents in the sediments show the tendency of first decreasing and then increasing in the process of heavy metal release. This is mainly attributed to the decreasing or disappearing of protein-like and the regenerating of the fulvic acid-like or humic acid-like.