Ulrich Förstner

Binding and mobilization of heavy metals in contaminated sediments affected by pH and redox potential

In a contaminated sediment from Hamburg harbour, acid production, mobilization of Cu, Zn, Pb, and Cd at different redox- and pH-conditions, and the transfer and change of those particle bound metals are studied. During oxidation the pH value in the sediment suspension decreases from about 7 to 3.4 because of the low acid neutralization capacity (ANC). This leads to a mobilization of heavy metals whereby Cd and Zn as distinguished from Cu and Pb are strongly released. Also a change in particulate phase specific heavy metal binding forms is found. Generally the easily and moderately reducible fractions increase during oxidation while the sulfidic fraction decreases. Also great changes were found for the dissolved Zn-and Cd-concentrations which increased continuously. The results show that at identical pH values obviously more metals are mobilized in the oxidized sediment than in the reduced state. But although the pH value is the key factor - in particular if it sinks below 4.5 - the redox conditions may also control the mobilization of certain metals. Consequently, also in well buffered sediments a mobilization of metals during oxidation cannot be excluded.

Trace metal analysis on polluted sediments

Abstract Sediment analyses are used to pin‐point major sources of metal pollution and to estimate the toxicity potential of dredged materials on agricultural land. For source assessments (Part I of the present review) standardization is needed with respect to grain size effects, commonly achieved by analyzing the sieve fraction <63μm. Further aspects include sampling methods, evaluation of background data and extent of anthropogenic metal enrichment.

Chemical extraction of heavy metals in polluted river sediments in central Europe

Abstract A five-step sequential extraction technique was used to determine the chemical association of heavy metals (Zn, Cd, Pb, Cu) with major sedimentary phases (exchangeable cations, easily and moderately-reducible compounds, organic/sulfidic phases, residual components) in samples from polluted rivers in Central Europe (Middle Rhine River, Lower Rhine/Rotterdam Harbor, Weser Estuary, Neckar River). Data gained suggest that the surplus of metal contaminants introduced into the aquatic system from anthropogenic sources usually exists in relatively unstable chemical forms. The more important accumulative phases for trace metals are found in the easily-reducible fractions. Extraction with acidified hydroxylamine solution seems to yield the metal fractions which may predominantly participate in short-term geochemical and biochemical processes. Rates of mobilization were significantly higher for zinc and cadmium than for copper and lead.

Metal Speciation - General Concepts and Applications

Abstract Several aspects of metal speciation in sediments are reviewed: (1) Due to the instability of polluted solid materials, sample handling and storage prior to analysis is problematic. In particular, changes from reducing to oxidizing conditions, which involve transformations of sulfides and a shift to more acid conditions, increase the mobility of critical metals. (2) Simple “standard” leaching tests can be used for easily soluble components such as halides or sulfates, but in most cases are not adequate for assessing mobility of trace metals. With sequential extraction procedures rearrangements of specific solid “phases” can be evaluated prior to their actual remobilisation. (3) Estimation of long-term release of metals from solids can be made using a test procedure, which combines data from column experiments applying controlled intensivation of parameters such as pH-value, redox potential and temperature with results from sequential extraction of the contaminated solids before and after exposure....

Concept of subaqueous capping of contaminated sediments with active barrier systems (ABS) using natural and modified zeolites

Abstract In this study the concept of subaqueous capping of contaminated sediments of lakes, rivers and coastal waters with active barrier systems (ABS) in order to minimise the contaminant release into the surface water is developed. This concept is supposed to provide a low-cost alternative to conventional methods in water protection. Active barrier systems, i.e. reactive geochemical barriers on the basis of low-cost materials, shall actively inhibit contaminant release from the sediment into the surface water, without the hydraulic contact between sediment and surface water being disturbed. Theoretical considerations and first experimental results regarding the retention of Pb2+ by four different zeolitic rocks suggest that natural zeolite as a reactive material in sediment capping meets all the economical and technical requirements posed by the active barrier concept. Natural zeolites are capable of demobilising large amounts of cationic pollutants by sorption, as shown for Pb2+, and, furthermore, they are capable of demobilising non-polar organics and anionic contaminants, when the zeolite surface is pre-treated with cationic surfactants. Active barrier systems on the basis of natural zeolites thus can be applied to nearly any type of contaminated sediment. Additionally, the physical characteristics of zeolitic rocks, as grain size and density, facilitate their use in subaqueous applications.

Heavy metal accumulation in river sediments: A response to environmental pollution

Abstract As evidenced by catastrophic cadmium and mercury poisonings in japan, heavy metals belong to the most toxic environmental pollutants. Through the investigation of sediments, the extent, distribution and provenance of heavy metal contamination in rivers and lakes can be determined and traced. Eight heavy metals from the clay fraction of sediments from major rivers within the Federal Republic of Germany were determined by means of atomic adsorption spectrometry. Heavy metals especially known for their high toxicity are enriched most: mercury, lead and zinc by a factor of 10; cadmium by a factor of 50, as compared with the natural background of these elements. A mobilisation of heavy metals from the suspended load and from the sediments, as to be observed in rivers approaching the marine enviromment, could endanger marine organisms, thus negatively influencing the acquatic food chain. With a further increase of heavy metal pollution, a threat to the drinking water supplied by rivers and lakes cannot be excluded.

Heavy metals in sediments of the rhine and elbe estuaries: Mobilization or mixing effect?

On their way from the Rhine estuary into the North Sea and Dutch Wadden Sea, Rhine sediments “lose” large portions of their original heavy metal concentrations. Until now these losses were explained by a mobilization process, solubilization — the decomposition products of organic matter form soluble organometallic complexes with the metals of the sediment.Our investigations of the sediments of the Elbe clearly indicate that a mixing process, whereby highly polluted Elbe sediments mix with relatively non-polluted North Sea sediments, rather than solubilization, is the cause of the dilution of heavy metals in the sediments of the Elbe estuary. Because of the similarity of the Elbe data with those from the Rhine River, we propose that a mixing process is also effective in the Rhine estuary and adjacent North Sea areas.The mechanism by which heavy metals are “diluted” is important to the marine ecosystem. In the mixing process proposed in this paper, the heavy metals fixed to the suspected material are trapped in bottom sediments of the marine environment, whereas solubilization would increase the concentration of heavy metals in the sea water and thus they would be more available for uptake by aquatic organisms.

Traceability of sediment analysis

Chemical analysis of sediments provides an efficient tool for water-quality management. A basic sequence of measurements comprises three steps: sampling and sample preparation; grain size as a characteristic sediment feature; and, analytical procedures based on standardized extraction schemes and reference materials. These can be considered as an unbroken chain of comparisons. Further steps are split with regard to specific purposes: sediment-quality assessment, including biological effects; coupling of sediment-quality data with erosion-risk evaluation; chemical changes following resuspension of anoxic sediments; and, modeling of chemical sediment data. In the light of the economic value of these further steps for developing and executing far-reaching management plans, coordinated efforts should be made to improve their traceability (e.g., by organized dissemination of results from on-going research (ageing effects), official documentation of techniques and instruments in a relative new field (erosion effects), extension of standardized extraction schemes (anoxic sediments, capacity-controlling properties), and development of new reference materials (pore water)).

Metal Transfer Between Solid and Aqueous Phases

The greater part of the dissolved heavy metals transported by natural water systems is, under normal physicochemical conditions, rapidly adsorbed onto particulate material. However, heavy metals immobilized in bottom sediments do not necessarily stay in that condition, but may be released as a result of chemical changes in the aquatic milieu. The following sections will pay special attention to the interaction between water and sediment, and the possibilities of immobilization and remobilization of metals in unpolluted as well as in polluted systems.

Historical Contaminated Sediments and Soils at the River Basin Scale

Background, Aims, and ScopeData from the Elbe River and its tributaries indicate, despite extensive improvement in water quality during the last 15 years, that the respective sediment situation of many priority pollutants has not reached an acceptable level. For the coming decades, risks for downstream sites and stakeholders will persist, mainly due to secondary sources originating from historical pollution of soils and sediments in the catchment area. In practice, a catchment-wide assessment of historical contaminated soil and sediment should apply a three-step approach: (i) Identification of substances of concern (s.o.c.) and their classification into ’hazard classes of compounds’; (ii) identification of areas of concern (a.o.c.) and their classification into ‘hazard classes of sites’; (iii) identification of areas of risk (a.o.r.) and their assessment relative to each other with regard to the probability of polluting the sediments in the downstream reaches. The conversion of this concept has to consider the underlying philosophy of the EU Water Framework Directive, particularly with respect to the analysis and monitoring of priority substances in solid matrices. However, major deficiencies are still in the assessment and prognosis of resuspension processes, and potential approaches to fill this gap are described both in theory and from examples of the Elbe River.MethodsThe sediment stability testing facilities consist of a unique triple set developed by innovative experimental laboratory and field research. The instrumental facilities consisting of a tube corer and a pressurized channel allow one to measure not only the onset of erosion (critical bed shear stress), but also the erosion rate for different sediment layers. Undisturbed sediment samples were taken from contaminated sites, e.g. in near-bank groyne fields and floodplains, using (i) core sampler (diameter 14 cm, length 150 cm) for sediment erodibility depth profiling and (ii) box sampler (30*70 cm2 top view area, 28 cm depth) for comparing and upscaling the results from the laboratory to the field. Sediment properties such as grain size spectrum (laser beam attenuation), water and gas content were analyzed by a non-intrusive, high frequency, capacity measurement method and bulk density by γ-ray.Results and DiscussionSediment core samples from flooded areas in the Middle Elbe indicate, that, except from the uppermost 5 cm and at a depth of from 47 to 48 cm, where the critical shear stress is very low (0.5 Pa), the critical bottom shear stress is between 1.2 Pa and 3.4 Pa, i.e. at a moderate level. Major reasons for the distinct heterogeneity of the erosion stability are differences in consolidation processes, grain size distribution and in the composition of stabilizing exudates in the individual sediment layers. Similar to the erosion stability depth profile, the metal data exhibit short-range heterogeneities; the variations in the individual layers can be explained by different proportions of fine grained components and by an improvement of suspended matter quality in the course of time. A comparison of the metal contents of embanked alluvial soils and unembanked alluvial areas suggests the following causal chain: Recent floodplain areas at low mean water levels exhibiting high concentrations of organic carbon represent the most highly contaminated sites. On the other hand, insignificant pollution has occurred on alluvial areas, which were embanked already at pre-industrial times. In the case of flood events, due to the combination of flooding probability and flow conditions, the most favorable conditions for the deposition of nutrient- and contaminant-rich suspended particulate matter are found in the low level depressions with low current. Within a typical river section of 1 km length in the lower middle Elbe, the groyne fields are recognized as dominant, slack-water zones containing the following nutrient and trace metal loads (reference year 2001, anthropogenic proportions): 287 t organic carbon, 17.6 t phosphorous, 17.4 t nitrogen, and 16.7 t sulfur; 8.6 t zinc, 1.1 t copper, 0.9 t lead, 0.4 t chromium, and 0.2 t nickel, respectively. The estimated nutrient and pollutant loads, deposited on the floodplains and in the river course, clearly demonstrate the specific sink function of both sites. At the same time, however, the results suggest, in contrast to the deposits in the floodplains, that sediments within the river course may partly be remobilized. This means that the longterm sink function can at least temporarily become a significant source character, involving the hazard of a substantial deterioration of the downstream sections of the river basin.Conclusions and OutlookIn view of the findings of relative low erosion stabilities of groyne field sediments and, in particular, after the extreme Elbe flood from August 2002, a prime question relates to the remobilization risks of these sediments typically enriched in contaminants and nutrients. The combined view on substances, areas and processes of concern in the Elbe catchment - with special emphasis on historical contamination of floodplain soils and sediments, as well as on groyne field sediments, as significant secondary sources of pollution - is a typical example for the holistic river basin approach of the European Water Framework Directive (WFD), both with respect to assessment of ecological risks and the development of remediation measures. In the latter respect, recent developments in ‘soft’ (geochemical and biological) techniques on contaminated soils and sediments, both with respect to policy aspects as well as to technical developments have led to a stimulation of in-situ remediation options, such as sub-aqueous depots, active capping, and application of natural attenuation processes. Limited financial resources require a direction of investments to those sites with the highest efficiencies in risk reduction. Establishing a rough sediment dynamic model, building on tributary/Elbe dilution factors, sedimentation data, suspended particulate matter monitoring data as well as calculations of long-term costs and benefits, based on risk management, could be essential steps in a basin wide river management.