Speciation Bioavailability and effects of trace metals in the environment

Assessments of the fate and biological effects of trace metals in the environment based solely on total concentrations of the metals in the various media (water, sediment or soil) is no longer state-of-the-art or scientifically justified. Although water, sediment and soil quality criteria for metals in most countries are still based on the total concentrations, it is now becoming more and more evident also for regulatory authorities that it is the actual metal species that determines mobility, bioavailability and toxicity of a metal, but also that metal speciation depends on the site-specific seasonal and spatial variations existing in a particular water, sediment or soil system.

The scientific community has vigorously responded to this new understanding by swiftly developing a whole array of new methods to describe and quantify the distribution and dynamics of various metal species occurring under different physico-chemical conditions in the environment. New, detailed models and descriptions of the mechanisms of formation of metal species and their transformations have been developed. A broad review of the new investigative tools and how to use them for widening our understanding of the behaviour and effects of trace metals in the environment is given in Chapters 5-7 of this monograph.

0.3.1 In the water column - BLM as a tool for prediction of toxicity

It has been repeatedly demonstrated that the toxicity, e.g. the acute median lethality, LC50, of a trace metal to a single aquatic species, varies widely between different tests. Variations in toxicity of the same metal are directly related to variations in water hardness, pH, content of suspended solids and the concentration of organic ligands that can form complexes with the metal. All these water quality characteristics modify the speciation of a metal and, thereby, its bioavailability. The most available (and toxic) species of a divalent metal in aqueous solution usually is the free ion (e.g. Cu2+, Ni2+, Zn2+), but also a few inorganic complexes (e.g. CuOH+ and CuCO3) have the potential to contribute to the total toxicity of these metals to aquatic organisms. It has also been shown that various common cations (Ca2+, Mg2+, Na+ and H+) compete with the trace metals for binding sites at the organism-water interface and, thus, tend to reduce the overall toxic response.

The recently developed "Biotic Ligand Models" (BLMs) for copper, nickel, silver and zinc interacting with fish, daphnids and algae, are able to handle all the above mentioned sources of apparent variation in toxicity and are now, after refinement, powerful tools for making accurate predictions of the toxicity of a trace metal in natural waters with widely differing chemical composition. A basic assumption of the BLM is that metal toxicity occurs as a result of metal ions reacting with binding sites at the organism-water interface, e.g. the gills, thus forming a metal-biotic ligand (metal-BL) complex. The concentration of this metal-BL complex directly reflects the magnitude of the resulting toxic effect, independent of the physico-chemical characteristics of the surrounding water. Hence, the metal toxicity can be predicted when metal speciation, the activity of each cation in solution, and the stability constant for each cation binding to the BL of the actual organism are known.

It has been demonstrated that the concentrations of trace metals in gills of fish were constant predictors of the acute toxicity of the metals to the fish, although water hardness varied up to tenfold. In contrast, total metal concentrations (e.g., for copper, nickel or zinc) in the water or the free-ion activities of these metals could not be used as accurate toxicity predictors. Also with regard to the prediction of chronic toxicity, the BLM has been successfully used. Chronic toxicities of copper and zinc to rainbow trout, Daphnia magna and green microalgae could be predicted within a factor of two compared to the observed values, when performing toxicity tests both in laboratory waters and in waters collected from the field. Dietary uptake of copper in daphnids did not enhance the chronic toxicity of waterborne copper, an observation that does not exclude, however, that dietary uptake of trace metals may influence the toxic response, e.g., in typically particle-ingesting organisms.

0.3.2 In aquatic sediments - AVS as a tool for prediction

Bioavailability and toxicity of sediment-associated trace metals towards bottom-dwelling organisms are governed by site-specific factors like sediment properties, the redox potential, the chemistry of pore-water and overlying water, and by the physiology and feeding behaviour of the fauna. In oxidized sediment layers, iron and manganese oxy-hydroxides and particulate organic carbon mainly control the bioavailability of trace metals, while in anoxic sediments, the most important regulating factors are pH and sulphides. Undisturbed sediments rich in organic matter may have a very sharp gradient in redox potential, so that anoxic conditions appear at a depth of 2-5 mm, although the sediment surface is well oxidized.

A multitude of speciation methods is now available to determine typical species of trace metals in sediments, their relative distribution among various sediment fractions, or the kind and kinetics of transformations between different metal forms. Examples are wet chemical methods like sequential extractions, which successively extract metals from the sediment matrix by using increasingly powerful extractants. The outcome provides information about the strength of binding between a metal and the major sediment fractions, and hence on its potential mobility and bioavailability. Other, more sophisticated approaches include measurements of the sediment's capacity to buffer protons and electrons, thermodynamic and kinetic solubility calculations, new spectroscopic techniques (like X-ray absorption spectroscopy) and the determination of the acid-volatile sulphides (AVS).

The AVS approach has attracted a lot of interest, since it turned out to be a rapid, simple and cheap method for predicting at least the absence of trace metal-related toxicity of aquatic sediments. It is well known that sulphide, derived from sulphate-reducing bacteria in the pore-water of anoxic sediment layers, forms relatively insoluble complexes with various trace metals, thereby rendering them non-bioavailable and non-toxic. These metal sulphides (AVS) can be liberated from sediments by treatment with 1 N HCl. In fact, sediments containing an excess of AVS over "simultaneously extracted metals" (SEM) are characterized by very low pore-water metal concentrations, as well as low metal bioavailability and toxicity. Both the U.S.EPA and the Environmental Directorate of the European Commission have now proposed the ZSEM / AVS ratio, or the ZSEM - AVS difference, as a reliable measure of the bioavailability of sediment-associated metals, e.g. cadmium, lead, copper, nickel and zinc.

In general, the model has been found to be a good predictor of the non-toxicity of trace metals in sediments, especially after normalisation to the sediment organic matter content, but may not be fully reliable in predicting that a certain sediment is toxic to the benthic fauna. In other words, the SEM/AVS approach tends to overestimate the toxicity of a sediment, i.e. a greater number of sediments are predicted to be toxic than what is really the case.

Although the SEM/AVS approach has been quite successful, so far, in predicting the bioavailability and toxicity of trace metals in sediments, recent studies have pointed to some weaknesses in the model that leave room for future improvements. Moreover, there is a need to apply more fine-scale sediment sampling techniques, such as the "diffusive gradients in thin films" (DGT) technique, which allows the documentation of metal releases from solid phases in discrete locations within a sediment core, thus providing a better understanding of the exact interactions between burrowing organisms and sediment-associated trace metals.

0.3.3 In soils - laboratory versus field tests

In spite of the many differences between sediment and soil systems, their geochemical similarity has justified the development and use of similar methods for metal speciation in both media. A first approximation for the speciation of soil-associated metals may be achieved by experimentally determining solid-liquid partitioning coefficients (Ad). Such coefficients help to differentiate between metals dissolved in the soil solution and those bound to particulate phases. But they tell us very little about the dynamic relationships existing between free, labile and non-labile metal species. Measured kd values depend above all on the dissolved organic matter (DOM) content and on the total dissolved metal concentration in the soil solution. Thus, it is evident that every factor influencing soil DOM will also affect metal partitioning. Therefore, competitive adsorption models rather than simple solid-solution ratios (as expressed by kd) are increasingly used to predict concentrations of dissolved metals.

Dissolved metal concentrations in soils vary widely with seasonal changes in redox potential, which may be triggered by light and temperature variations and, hence, by the activity of micro-organisms. Effects of soil aging on trace metal fluxes usually become manifest as a stronger adsorption and reduced extractability of metals. Depletion of trace metals in the vicinity of plant roots, due to uptake, promotes metal transfer from solid phases into solution. Plants can actively mobilise essential metals such as iron, copper and zinc from solid phases under deficiency conditions. Due to these site-specific phenomena, it is increasingly important to describe the kinetic, rather than only the thermodynamic equilibria for metals in soils.

In order to predict trace metal bioavailability to plants, the new "effective concentration" (CE) concept has attracted considerable interest, because it fulfils the requirement of including kinetic aspects into the assessment. It relies upon using the "diffusive gradients in thin films" (DGT) technique, by which a metal chelating resin is introduced into the soil to mimic metal uptake by plants. Like plants, the DGT responds to the labile metal pool, re-supplied from both the soil solution and the solid phases. Measured DGT fluxes can be quantitatively related to CE, and thus, provide a measure of the potential hazard of metals in contaminated soils.

In general, soil quality guidelines for trace metals are still not expressed in terms of bioavailable metals. A problem is, of course, that the bioavailable fraction of a trace metal in soil is typically variable and organism-specific, i.e. it may not be the same for a higher plant, a soil invertebrate or micro-organism species. Some soil scientists have stressed that it is necessary to consider the regional, natural background concentration of the metal in question and to formulate permissible levels in terms of "critical enhancement", i.e. the number of times that a metal is allowed to increase in the soil relative to the background level. However, metals freshly introduced into the soil matrix have a different behaviour (mobility, bioavailability and toxicity) than metals gradually introduced over a long period of time, so the question on metal aging in the soil has to be taken into account. This was demonstrated in some recent studies of copper, lead and zinc. In one major study, zinc toxicity was shown to be consistently lower in field contaminated soils than in corresponding zinc-spiked soils. The importance of soil properties for the degree of toxicity, that a specific zinc addition will cause, became evident. We may even deepen our understanding of the observed metal-related toxicity variations in soils by normalizing the toxicity data to the particular soil type, thereby deriving specific "soil sensitivity factors".

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