Dynamic metal speciation analysis in aquatic ecosystems is emerging as a powerful basis for development of predictions of bioavailability and reliable risk assessment strategies. A given speciation sensor is characterized by an effective time scale or kinetic window that defines the measurable metal species via their labilities. Here we review the current state of the art for the theory and application of dynamic speciation sensors. We show that a common dynamic interpretation framework, based on rigorous flux expressions incorporating the relevant diffusion and reaction steps, is applicable for a suite of sensors that span a range of time scales. Interpolation from a kinetic spectrum of speciation data is proposed as a practical strategy for addressing questions of bioavailability. Case studies illustrate the practical significance of knowledge on the dynamic features of metal complex species in relation to biouptake, and highlight the limitations of equilibrium-based models.

Republication or reproduction of this report or its storage and/or dissemination by electronic means isAbstract: This paper presents definitions of concepts related to speciation of elements, more particularly speciation analysis and chemical species. Fractionation is distinguished from speciation analysis, and a general outline of fractionation procedures is given. We propose a categorization of species according to isotopic composition of the element, its oxidation and electronic states, and its complex and molecular structure. Examples are given of methodological approaches used for speciation analysis. A synopsis of the methodology of dynamic speciation analysis is also presented.

The free-ion activity model for the biouptake of metals from complex media is limited to cases where mass transfer is not flux-determining. This paper considers the simultaneous effects of bioconversion kinetics and metal transport in the medium coupled with metal complex dissociation kinetics. For the two kinetically limiting situations of inert and fully labile complexes, the bioavailabilities of bioinactive metal complexes are analyzed under conditions where (i) the actual biouptake follows a Michaelis−Menten type of steady-state flux and (ii) the supply of free metal is governed by diffusion of free metal or coupled diffusion of the different labile metal species. The resulting steady-state fluxes are given in terms of two fundamental quantities, i.e., the relative bioaffinity parameter (a) and the ratio between the limiting uptake flux and the limiting transport flux (b). For labile complexes, these variables are differentiated by a complexation parameter defined by the ratio between the free metal ion activity and the total labile metal activity. Limits of the uptake flux for extreme values of the bioaffinity parameter a and the limiting flux ratio b are easily derived from the general flux expression. The analysis precisely shows under what conditions labile complex species contribute to the biouptake process or, equivalently, under what conditions the free-ion activity model is not obeyed.

Due to the complexity of the humic substances (HS), mathematical models have often been employed to understand their roles in the environment. Since no consensus exists with respect to the structure and conformation of the HS, models have alternatively given them properties corresponding to impermeable hard spheres or fully permeable polyelectrolytes. In this study, the hydrodynamic permeability of standard HS (Suwannee River fulvic, humic, and peat humic acids) are evaluated as a function of pH and ionic strength. A detailed theoretical model is used to determine the softness parameter (lambda0), which characterizes the degree of flow penetration into the HS on the basis of measured values of electrophoretic mobilities, diffusion coefficients, and electric charge densities. Their motion in an electric field is evaluated by a rigorous numerical evaluation of the governing electrokinetic equations for soft particles. The hydrodynamic impact of the polyelectrolyte chains is accounted for by a distribution of Stokes resistance centers and partial dissociation of the hydrodynamically immobile ionogenic groups distributed throughout the polyelectrolyte. The results demonstrate thatthe studied HS are small (radius ca. 1 nm), highly charged (500-650 C g(-1) when all sites are dissociated), and very permeable (typical flow penetration length of 25-50% of the radius, depending on pH). The HS also coagulate slightly when lowering the pH of the solution. Modeling of the HS as hard spheres with a charge and slip plane located at the surface is thus physically inappropriate, as are a number of analytical theories for soft particles that hold for low to moderate electrostatic potentials and large colloids. The shortcomings of these simpler approaches, when interpreting the electrophoretic mobilities of HS, are highlighted by comparison with rigorous theoretical predictions.