Fractionation of elements in sediment samples collected in rivers and harbors at Lake Balaton and its catchment area
Introduction
At present, it is evident that element speciation has become a major aspect in analytical and bioinorganic chemistry. In spite of a number of impressive achievements over the past few years, a huge amount of questions have not yet been answered. For example, a deeper insight into interaction phenomena between living organisms and elements is still urgently needed, therefore, element speciation is one of the main targets of research groups. In an IUPAC guideline for terms related to speciation of trace elements: ‘definitions, structural aspects and analytical methods’, definitions of terms related to speciation and fractionation are [1]:
- 1.
Speciation (in chemistry) of an individual element refers to its occurrence in or distribution among different species (chemical speciation).
- 2.
Speciation analysis is the analytical activity of identifying and quantitating one or more chemical species of an element present in a sample.
- 3.
Species (in chemistry) denotes an element in a specific and unique molecular, electronic, or nuclear structure (chemical species).
Fractionation is the process of classification of an analyte or a group of analytes into a certain matrix according to physical (size, solubility) or chemical (bonding, reactivity) properties.
Concerning natural waters (lakes and rivers), the mobility, transport and partitioning of trace elements in natural aquatic systems is the function of the chemical form of the elements. The process is controlled by the physico-chemical and biological characteristics of that system. Major variations of these characteristics are found in time and space due to the dissipation and flux of energy and materials involved in the biogeochemical processes which drive the speciation reaction. Solid components in sediments govern the dissolved levels of these elements via sorption–desorption and dissolution–precipitation reactions. For the assessment of the environmental impacts of pollutant some questions must be applied in a sediment–water system: (i) what is the reactivity of the metals introduced with solid materials from anthropogenic activities (hazardous waste, sewage sludge, atmospheric deposits, etc.) by comparison with the natural components; (ii) are the interactions of crucial metals between solution and solid phases comparable for natural and contaminated systems; (iii) what are the rules of solid–solution interactions on the weaker bonding of certain metal species; and (iv) are the processes of remobilization effective in contaminated as compared with the natural system [2].
A general goal of selective chemical leaching is the accurate determination of the partitioning of elements among different discrete phases of a sample. Mineralogical components of sediment, which are important in controlling metal concentrations in sediments, are hydrous oxides of iron and manganese, organic matter, and clays. Fractionation is usually performed by a sequence of selective chemical extraction techniques, which include the successive removal, or dissolution of these phases and their associated metals. The concept of chemical leaching is based on the idea that a particular chemical solvent is either specific to a particular phase or specific in its action. Although a differentiated analysis is advantageous over investigations of bulk chemistry of sediments, verification studies indicate that there are many problems associated with operational speciation by partial dissolution techniques. There is no general agreement on the solutions preferred for the various sediment components to be extracted, due to mostly the matrix effect involved in the heterogeneous chemical processes. The most appropriate extractants are determined by the aim of the study, by the type of the solid material and by the elements of interest.
Partial dissolution techniques should include reagents that are sensitive to only one of the various sediment components significant in trace metal binding. In sequential multiple extraction techniques, chemical extractants of various types are applied successively to the sample of sediment, each follow-up treatment being more drastic in chemical action or different in nature from the previous one. Selectivity for a specific phase or binding form cannot be expected for these procedures. Whatever the extraction procedure chosen, the validity of selective extraction results primarily depends on the sample collection and preservation prior to analysis. Care has to be taken to collect sediment samples from the same layers during monitoring.
In environmental science the development of a monitoring system is of importance. Monitoring is done in order to gain information on the present level of harmful or potentially harmful pollutants in discharges to the environment, within the environment itself, or in living creatures, including ourselves, that may be effected by these pollutants. Objectives of monitoring are, among others, to assess pollution effects on man and his environment; to identify any possible cause and effect relationship between pollutant concentrations and health effects or climate changes. Sediments are basic components of the lake, as they provide foodstuffs for living organisms and serve as sinks for deleterious chemical species. The composition of sediments reflects the degree of degradation and leaching introduced by weathering and the influence of external inputs such as contamination from anthropogenic sources. It is necessary to know the mechanisms of transport of trace elements and their complexes in lakes to understand their chemical cycles in nature.
Lake Balaton is the largest lake of Central Europe with a surface of 596 km2 and an average depth of 3.25 m. The shallow lake is one of the most valuable and important recreation zones. Monitoring of the lake with respect to the determination of toxic pollutants in sediments is fundamental to the solution of environmental problems.
In our recent work [3], [4] a four-step method was used for determination of partitioning of elements in bottom sediments collected in the Lake Balaton. Fifteen sampling points were set and samples were collected during three seasons, spring, summer and fall. The environmentally mobile fractions were compared to the sum of the four-step extraction. In this paper, results of fractionation of elements of sediment samples collected in small rivers of the catchment area around the lake and in harbors are reported. For comparison of data on element speciation and the validation of leaching method developed, sediment samples were air-dried and homogenized. Certified reference materials (BCR 601) for three-step leaching were used to estimate the systematic error characterized by accuracy and reproducibility.
Section snippets
Sampling
Sediment samples were collected from the top 10-cm layer of the bottom of harbors at Lake Balaton and rivers at catchment areas in three seasons. A fiberglass boat was driven to the selected sampling site and a sampling device was used to collect the sediments. The sampler is 60 cm long, 20×20 cm square in cross-section and made from plastic. The bottom is cut in a 45° angle and a cover operated by a spring is installed. There is a 1.5-m handling bar fastened to the side of the sampler. The
Results and discussion
First the mineralogical composition of sediments is discussed. Based upon the results the following conclusions can be drawn: the main constituent of samples are quartz, calcite, aragonite, dolomite, muscovite, chlorite, feldspar, smectite, kaolinite, pyrite and some amorphous material (Fig. 2). The feldspar, in most samples, was found in the form of plagioclase (albite), but in some sample, K-feldspar was also identified. They were largely found (together with quartz) in sandy sediments, where
Conclusions
A monitoring system was developed to characterize the environmental conditions of the bottom sediment of Lake Balaton and catchment area. Fractionation by the selective sequential leaching process has the advantage of simulating, to a certain extent, various natural environmental factors. In the study a modified leaching process was applied, and samples were taken in three seasons at eight sampling points from rivers and seven from harbors. Concentrations of elements were determined from the
Acknowledgements
The financial support of the Hungarian National Science Foundation (OTKA 029250), the FKFP 0084/1999, and Balaton Secretariat of the Prime Minister's Office is greatly acknowledged.
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