Speciation studies aquatic distribution

In soil research, the term speciation is often applied to operationally defined fractionation of heavy metals into five or more components.25 Typically, water soluble, exchangeable, organically bound (which includes what is in biomass), amorphous oxide bound, crystalline oxide bound, and residual fractions are measured.26 Sometimes residual fractions are further subdivided according to particle size distributions to give amounts in sand, silt, and clay fractions. Similar fractionation procedures are often applied to aquatic sediments.27 In arid regions, often the calcium carbonate bound fractions of heavy metals are also measured.28 Because of the constraints of detection limits, generally only cadmium, copper, iron, manganese, and zinc are usually monitored by flame spectrometry in such heavy metal speciation studies.28... 

When TBTO is released into ambient water, a considerable proportion becomes adsorbed to sediments, as might be expected from its lipophilicity. Studies have shown that between 10 and 95% of TBTO added to surface waters becomes bound to sediment. In the water column it exists in several different forms, principally the hydroxide, the chloride, and the carbonate (Figure 8.5). Once TBT has been adsorbed, loss is almost entirely due to slow degradation, leading to desorption of diphenyl-tin (DPT). The distribution and state of speciation of TBT can vary considerably between aquatic systems, depending on pH, temperature, salinity, and other factors. 

Though several studies (5-9) have described the concentrations of trace elements in surface waters, most of them did not differentiate between species of a particular element. Some (10) have considered the distribution between dissolved and particulate forms. However, few attempts (11) have been made to evaluate the distribution of metals between various solid phase components of the suspended material. The present study provides quantitative estimates of dissolved (dissolved is defined as those aquatic components that could not be removed by centrifugation from liquid phase) and various solid phase associated metal fractions in southeastern United States streams. Between November 83 and August 85, 46 bi-weekly samples were taken from six SRP associated watersheds to determine dissolved (filterable) and total element concentrations. As one of several goals of the study was to assess the impact of natural and production related activities on trace element behavior in these aquatic systems, knowledge of speciation within solid and dissolved phases was essential for data interpretation. The research described herein used sequential extraction and a thermodynamics approach to define solid and dissolved phase species of Cu, Cd, Fe, Mn, Ni and Zn. The study also evaluated the effects of natural and production related processes on the distribution of metals in aquatic systems at SRP. 

Classical chemistry literature provides comprehensive lists of rate data that can also be applied for predicting abiotic reaction rates of environmental importance when the pathways are well identified. For some reactions, such as hydrolysis or OH radical reactions, enough data are available to determine siructure-reactivity correlations that enable one to interpolate within series of chemically related compounds of known reactivity [for review, see Brezonik (Chapter 4, this volume) and Lyman et al. (1982)]. The aquatic chemist is, however, more often confronted with the fact that many classical studies have been performed in organic solvents, but that the speciation of many dissolved chemicals in aquatic systems may vary with pH (caboxylic acids, phenols, etc.) and with ligand concentrations (e.g., all dissolved heavy metal species) or that the chemical species may be adsorbed on surfaces or absorbed by colloidal organic materials. In all these cases, reaction-rate constants must be determined for each individual aqueous species contributing to the overall kinetics. The equilibrium distribution of these species must also be accounted for. Reductions of the... 

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