Ion-binding Model Research Articles

Dynamic retention of thallium(I) on humic acid: Novel insights into the heterogeneous complexation ability and responsiveness

Widely distributed soil humic acid (HA) would significantly affect the environmental migration behavior of Tl(I), but a quantitative and mechanistic understanding of the dynamic Tl(I) retention process on HA is limited. A unified kinetic model was established by coupling the humic ion-binding model with a stirred-flow kinetic model, which quantified the complexation constants and responsiveness coefficients during dynamic Tl(I)-HA complexation. Furthermore, the heterogeneous complexation mechanism of HA and Tl(I) was revealed by batch adsorption experiments, stirred-flow migration experiments, and 2D-FTIR-COS analysis. An increase in pH significantly improved the responsiveness of HA organic binding sites, promoting Tl(I) dynamic retention. Monodentate carboxyl groups induced rapid Tl(I) complexation (kd = 1.9 min−1) in strongly acidic environments. Under weakly acidic conditions, Tl(I) retention on HA was mainly attributed to the synergistic complexation effect of carboxyl and amide groups. Among the groups, multidentate carboxyl-phenolic hydroxyl sites could achieve sustained Tl(I) retention due to their stable complexing properties (logK = 4.48∼7.46) and slow response (kd = 1.1 × 10−3 min−1). These findings are crucial for a comprehensive understanding of the environmental interactions of Tl(I) with humic substances in swamp environments.

Rare earth elements binding humic acids: NICA–Donnan modelling

Environmental context Rare earth elements (REEs) are technologically critical elements released into the environment by various anthropogenic activities, and whose ecotoxicological impacts are still largely unknown. REE binding to natural organic matter (NOM) is key to understand their fate and bioavailability in the environment. With this work, it is now possible to predict REE binding to NOM in various environments using various speciation software (ECOSAT, ORCHESTRA, Visual MINTEQ). Rationale Understanding rare earth element (REE) speciation in different natural environments is important to evaluate their environmental risks because different chemical species of an element may have different bioavailability and toxicity. REEs have a great affinity for particulate and dissolved organic matter, particularly fulvic and humic acids (HAs). Thus, the use of humic ion binding models may help to understand and predict the behaviour and speciation of these species in surface waters, groundwaters and soils. Methodology In this work, we used previously published experimental datasets to parameterise the NICA–Donnan model for REEs binding with HAs, using the model optimisation tool PEST-ORCHESTRA. We propose using linear free energy relationships (LFERs) to constrain the number of parameters to optimise. Results We determined a coherent NICA–Donnan parameter set for the whole REEs series being compatible with available generic NICA–Donnan parameters for other metals. The impact of pH, ionic strength and REE/HA ratio as well as the presence of competitors (Fe3+, Al3+ and Cu2+) on model results is analysed. Discussion We consolidate confidence in our derived NICA–Donnan parameters for REEs by comparing them with the Irving–Rossotti LFER. We also show the general applicability of this relationship to predict and constrain metal-binding parameters for the NICA–Donnan model. We discuss observed shortcomings and provide suggestions for potential improvement of NICA–Donnan modelling.

Study on silver ions binding to β-lactoglobulin

The drug-resistant pathogen phenomenon, resulting in infections and deaths that are increasingly difficult to treat, requires research into searching new potential antimicrobial agents. The presented study is focused on the investigation of impact of silver ions (Ag+ ions) to β-lactoglobulin (βLG) structure and mechanism formation of silver-β-lactoglobulin nanocomposites, that could find potential applications in medicine. To determine the physicochemical characteristics of silver ion binding, kinetics and isothermal models were used. The presence of functional groups involved in the binding process was investigated by spectroscopic methods (FTIR-ATR, Raman spectroscopy). The binding ability and nanocomplexes formation was determined by instrumental analyses (SEM, TEM, EDX). Based on the obtained results, the binding of Ag+ ions to βLG were heterogeneous in nature consisting of three main steps: rapid sorption of Ag+ ions on the βLG surface, intramolecular diffusion of Ag+ ions, and chemical equilibrium. Microscopic studies showed a change in the surface morphology of βLG and the appearance of silver nanoparticles. Spectroscopic studies indicated that acidic (Glu−, Asp−) and Lys, Tyr, Met amino groups play a key role in the formation of the AgβLG nanocomplex. Finally, molecular dynamics (MD) and density functional theory (DFT) calculated studies as a comparative and complementary method have proven contribution of respective amino acids in the binding process.

Isothermal calorimetry measurements of ion binding and DNA condensation as a function of DNA length

Despite decades of theoretical and experimental research, the mechanism of ion-driven DNA condensation remains elusive. Numerous models successfully describe this condensation, including those invoking dynamic counterion fluctuations, salt bridges, static counterion lattices, and models that invoke the underlying DNA geometry, all while presenting different models for ion binding, DNA self-attraction, and the underlying energy landscape. We present research on isothermal calorimetry (ITC) of DNA condensation to extract the thermodynamic parameters that drive this process.

Estimation of WHAM7 constants for GaIII, InIII, SbIII and BiIII from linear free energy relationships, and speciation calculations for natural waters

Environmental contextNatural organic matter exerts a powerful control on chemical conditions in waters and soils, affecting pH and influencing the biological availability, transport and retention of metals. Modelling can help to predict these effects, but for many metals, model parameters are missing. We report parameters for four technology-critical elements in a chemical speciation model, and consider the chemistries of the elements in natural waters. AbstractWe compiled the equilibrium constants for the interactions of the technology-critical elements (TCEs) GaIII, InIII, SbIII and BiIII with ammonia, fluoride, hydroxyl and ligands with oxygen atoms. We then combined them with predictive equations to estimate parameters for Humic Ion-Binding Model VII, which permits the calculation of metal binding by natural organic matter (fulvic acid, FA, and humic acid, HA). Derived values of the Model VII parameter quantifying the interaction of metal ions with carboxyl-type groups (log KMA) were among the highest estimated so far, as were the values for the parameter (ΔLK2) that quantifies the tendency of the metal ion to interact with softer ligand atoms (N and S). The Windermere Humic Aqueous Model, version 7 (WHAM7), which incorporates Model VII, was then used to estimate the chemical speciation of each TCE element.

Modeling anaerobic soil organic carbon decomposition in Arctic polygon tundra: insights into soil geochemical influences on carbon mineralization

Abstract. Rapid warming of Arctic ecosystems exposes soil organic matter (SOM) to accelerated microbial decomposition, potentially leading to increased emissions of carbon dioxide (CO2) and methane (CH4) that have a positive feedback on global warming. Current estimates of the magnitude and form of carbon emissions from Earth system models include significant uncertainties, partially due to the oversimplified representation of geochemical constraints on microbial decomposition. Here, we coupled modeling principles developed in different disciplines, including a thermodynamically based microbial growth model for methanogenesis and iron reduction, a pool-based model to represent upstream carbon transformations, and a humic ion-binding model for dynamic pH simulation to build a more versatile carbon decomposition model framework that can be applied to soils under varying redox conditions. This new model framework was parameterized and validated using synthesized anaerobic incubation data from permafrost-affected soils along a gradient of fine-scale thermal and hydrological variabilities across Arctic polygonal tundra. The model accurately simulated anaerobic CO2 production and its temperature sensitivity using data on labile carbon pools and fermentation rates as model constraints. CH4 production is strongly influenced by water content, pH, methanogen biomass, and presence of competing electron acceptors, resulting in high variability in its temperature sensitivity. This work provides new insights into the interactions of SOM pools, temperature increase, soil geochemical feedbacks, and resulting CO2 and CH4 production. The proposed anaerobic carbon decomposition framework presented here builds a mechanistic link between soil geochemistry and carbon mineralization, making it applicable over a wide range of soils under different environmental settings.

Vibrational Relaxation in EDTA Is Ion-Dependent.

Ion binding by carboxylate groups is common in biomolecules such as metalloproteins, but dynamical aspects of ion binding are not fully understood. We present ultrafast spectroscopic measurements of vibrational relaxation in the ion-coordinating carboxylate groups of EDTA, which we use as a model of carboxylate-mediated ion binding, as EDTA binds a series of divalent and trivalent metal ions with high affinity. The measurements are interpreted using a Redfield-based anharmonic model of vibrational relaxation that rationalizes trends in vibrational lifetimes in terms of vibrational energy transfer between EDTA's asymmetric carboxylate stretching vibrational modes and lower-lying modes. Results show ion-dependent changes in complex structure and dynamics well outside the temporal and spatial resolution of common structural methods and demonstrate how vibrational relaxation measurements may contribute to exploration of ion-binding dynamics on ultrashort length and time scales.

Coordination to lanthanide ions distorts binding site conformation in calmodulin

The Ca2+-sensing protein calmodulin (CaM) is a popular model of biological ion binding since it is both experimentally tractable and essential to survival in all eukaryotic cells. CaM modulates hundreds of target proteins and is sensitive to complex patterns of Ca2+ exposure, indicating that it functions as a sophisticated dynamic transducer rather than a simple on/off switch. Many details of this transduction function are not well understood. Fourier transform infrared (FTIR) spectroscopy, ultrafast 2D infrared (2D IR) spectroscopy, and electronic structure calculations were used to probe interactions between bound metal ions (Ca2+ and several trivalent lanthanide ions) and the carboxylate groups in CaM's EF-hand ion-coordinating sites. Since Tb3+ is commonly used as a luminescent Ca2+ analog in studies of protein-ion binding, it is important to characterize distinctions between the coordination of Ca2+ and the lanthanides in CaM. Although functional assays indicate that Tb3+ fully activates many Ca2+-dependent proteins, our FTIR spectra indicate that Tb3+, La3+, and Lu3+ disrupt the bidentate coordination geometry characteristic of the CaM binding sites' strongly conserved position 12 glutamate residue. The 2D IR spectra indicate that, relative to the Ca2+-bound form, lanthanide-bound CaM exhibits greater conformational flexibility and larger structural fluctuations within its binding sites. Time-dependent 2D IR lineshapes indicate that binding sites in Ca2+-CaM occupy well-defined configurations, whereas binding sites in lanthanide-bound-CaM are more disordered. Overall, the results show that binding to lanthanide ions significantly alters the conformation and dynamics of CaM's binding sites.

Cation binding properties of an anionic gemini surfactant monolayer

A new class of anionic, gemini surfactants based on N,N’-dialkyl-N-N’-diacetate ethylenediamine has recently been reported in the literature, with the compounds exhibiting controllable and, in some cases reversible, aggregation in response to solution pH and the presence of inorganic cations. In this work, the response of one member of this surfactant family (12 carbon long tail) to subphase Na+ and Ca2+ at basic pH was investigated at the air-water interface using Langmuir monolayer approaches. It was found that the anionic monolayer exhibited marked expansion in response to subphase cation concentration, with Ca2+ leading to significantly greater film expansion than Na+. The film’s response to inorganic ions in the subphase was modeled using a simple ion-binding model, with association constants that were generally smaller than those reported for comparator systems such as phospholipids, and a 1:1 and 2:1 surfactant to cation binding ratio, respectively. However, Brewster angle microscope measurements revealed that monolayer film expansion was accompanied by the formation of multimolecular aggregates, tentatively assigned to the formation of poorly-dispersed deposits of cation chelated with the surfactant. The impact of this effect on the apparent magnitude of film expansion, ion-binding and controlling aggregation in these systems, and the potential for this system in terms of controlling surfactant aggregation is discussed.

PEST-ORCHESTRA, a tool for optimising advanced ion-binding model parameters: derivation of NICA-Donnan model parameters for humic substances reactivity

Environmental contextThe environmental behaviour of trace metals in soils and waters largely depends on the chemical form (speciation) of the metals. Speciation software programs combining models for the binding of metals to soil and sediment constituents are powerful tools in environmental risk assessment. This paper describes a new combination of speciation software with a fitting program to optimise geochemical model parameters that describes proton and metal binding to humic substances. AbstractHere we describe the coupling of the chemical speciation software ORCHESTRA with the parameter estimation software PEST. This combination enables the computation of optimised model parameters from experimental data for the ion binding models implemented in ORCHESTRA. For testing this flexible tool, the NICA-Donnan model parameters for proton-, Cd- and Zn-binding to Laurentian fulvic acid were optimised. The extensive description of the method implementation and the examples provided facilitate the use of this tool by students and researchers. Three procedures were compared which derive the proton binding parameters, differing in the way they constrain the model parameters and in the implementation of the electrostatic Donnan model. Although the different procedures resulted in significantly different sets of model parameters, the experimental data fit obtained was of similar quality. The choice of the relation between the Donnan volume and the ionic strength appears to have a strong influence on the derived set of optimal model parameters, especially on the values of the protonation constants, as well as on the Donnan potential and Donnan volume. Optimised results are discussed in terms of their physico-chemical plausibility. Coherent sets of NICA-Donnan parameters were derived for Cd and Zn binding to Laurentian fulvic acid.

Ion Binding and Electrostatics in Condensed DNA Arrays Probed by Ion Counting Techniques

Despite decades of theoretical and experimental research, the mechanism of DNA condensation remains elusive. Numerous models successfully describe this condendsation, including those invoking dynamic counterion fluctuations, salt bridges, static counterion lattices, and models that invoke the underlying DNA geometry, all while presenting vastly different models for ion binding. Here we present new experimental data that quantifies the ions bound to condensed DNA arrays. By measuring the competition between condensing ions and various non-condensing ions we quantify the electrostatic interactions between the DNA and ions. Furthermore, by varying the inter-axial spacing of condensed DNA utilizing crowding agents, we report on the interplay between the packing force of DNA and competitive ion binding. Finally, we present a simple new Ion Binding Model (IBM) that captures much of the observed binding. These data should prove vital in determining the underlying electrostatic mechanisms that drive DNA condensation.

Recent Structures and Molecular Dynamics Simulations Offer New Perspective on Na+/H+ Antiporters

Na þ /H þ antiporters are vital to cells for maintaining homeostasis, especially in high-salt environments. New crystal structures for two such antiporters, cytoplasmic-open Escherichia coli NhaA and periplasmic-open Thermus Thermophilus NapA, show high structural similarity despite low sequence identity. Among their common features are a set of highly conserved charged residues (three aspartates and one lysine) near the putative ion binding site. Using molecular dynamics simulations, we observe that interaction with sodium is dependent on the charge states of the conserved aspartates. The lysine and the nearest asparate also form a previously unidentified salt bridge. Under simulated physiological pH the presence of a sodium ion disrupts and breaks the salt bridge in NhaA. Given the presence of this salt bridge in both protein structures, its behavior under simulation, and the known importance of the conserved lysine, we hypothesize that this salt bridge is directly involved in ion binding and transport. To address the question of sodium binding, we performed an ensemble of all-atom equilibrium molecular dyanamics simulations (over 10 ms in total), varying the protonation states of the conserved residues and quantifying the resulting sodium interaction. To address the question of proton binding, we performed heuristic pKa calculations on this ensemble of simulations, which support the hypothesis that the lysine binds protons in a sodium-dependent manner. In order to elucidate the structural basis for alternating access in NhaA, we created an outward-facing model of NhaA, based on the periplasmic-open NapA structure, and simulated conformational transitions of NhaA from its cytoplasmic- to periplasmic-open state using dynamic importance sampling MD. Taken together, the combination of recent structural and dynamic simulation data suggests a new model of ion binding and transport for the CPA2 class of antiporters. 987-Plat

The use of assemblage models to describe trace element partitioning, speciation, and fate: a review.

The fate of trace elements in soils, sediments, and surface waters is largely determined by their binding to reactive components, of which organic matter, metal oxides, and clays are considered most important. Assemblage models, combining separate mechanistic complexation models for each of the reactive components, can be used to predict the solid-solution partitioning and speciation of trace elements in natural environments. In the present review, the authors provide a short overview of advanced ion-binding models for organic matter and oxides and of their application to artificial and natural assemblages. Modeling of artificial assemblages of mineral components and organic matter indicates that the interactions between organic and mineral components are important for trace element binding, particularly for oxyanions. The modeling of solid-solution partitioning in natural systems is generally adequate for metal cations but less so for oxyanions, probably because of the neglect of organic matter-oxide interactions in most assemblage models. The characterization of natural assemblages in terms of their components (active organic matter, reactive oxide surface) is key to successful model applications. Improved methods for characterization of reactive components in situ will enhance the applicability of assemblage models. Collection of compositional data for soil and water archetypes, or the development of relationships to estimate compositions from geospatially available data, will further facilitate assemblage model use for predictive purposes.

Estimation of stability constants for metal–ligand complexes containing neutral nitrogen donor atoms with applications to natural organic matter

Linear free energy relationships (LFERs) were developed for estimating 1:1 metal–ligand stability constants (logKML) for small organic molecules containing neutral nitrogen donor atoms. A data set of 44 monodentate and 112 bidentate ligands for six metal ions: Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+ was employed to parameterize the LFER equations. Monodentate and bidentate logKML values were adequately described using Irving–Rossotti LFERs previously developed for ligands containing negatively-charged oxygen functional groups. Modifications to the LFER equations were necessary to account for steric hindrances to metal complexation by primary, secondary, and tertiary amines. The resulting LFER equations can be used to estimate logKML values for monodentate and bidentate ligands with neutral nitrogen donor groups where such values do not currently exist in the literature. Comparison of these results to our previous work with negatively-charged oxygen donor atoms reveals that neutral nitrogen functional groups are weaker than their oxygen counterparts for metal ions classified as “hard” on the basis of Hard–Soft Acid–Base (HSAB) theory. For “soft” metals, the opposite is true. These LFERs can be used to incorporate nitrogen functional groups in models for metal ion binding to natural organic matter (NOM).

Ion Competition in Condensed DNA Arrays in the Attractive Regime

Physical origin of DNA condensation by multivalent cations remains unsettled. Here, we report quantitative studies of how one DNA-condensing ion (Cobalt3+ Hexammine, or Co3+Hex) and one nonDNA-condensing ion (Mg2+) compete within the interstitial space in spontaneously condensed DNA arrays. As the ion concentrations in the bath solution are systematically varied, the ion contents and DNA-DNA spacings of the DNA arrays are determined by atomic emission spectroscopy and x-ray diffraction, respectively. To gain quantitative insights, we first compare the experimentally determined ion contents with predictions from exact numerical calculations based on nonlinear Poisson-Boltzmann equations. Such calculations are shown to significantly underestimate the number of Co3+Hex ions, consistent with the deficiencies of nonlinear Poisson-Boltzmann approaches in describing multivalent cations. Upon increasing the concentration of Mg2+, the Co3+Hex-condensed DNA array expands and eventually redissolves as a result of ion competition weakening DNA-DNA attraction. Although the DNA-DNA spacing depends on both Mg2+ and Co3+Hex concentrations in the bath solution, it is observed that the spacing is largely determined by a single parameter of the DNA array, the fraction of DNA charges neutralized by Co3+Hex. It is also observed that only ∼20% DNA charge neutralization by Co3+Hex is necessary for spontaneous DNA condensation. We then show that the bath ion conditions can be reduced to one variable with a simplistic ion binding model, which is able to describe the variations of both ion contents and DNA-DNA spacings reasonably well. Finally, we discuss the implications on the nature of interstitial ions and cation-mediated DNA-DNA interactions.

Investigating humic substances interactions with Th4+, [formula omitted], and [formula omitted] at high pH: Relevance to cementitious disposal of radioactive wastes

A number of geodisposal concepts for intermediate level radioactive waste involve geological emplacement within cementitious repositories. Such facilities, once rehydrated with groundwater, will create high pH environments due to aqueous phase reaction of the cements. This work focuses on the interactions of several important long-lived radionuclide cations with dissolved organic matter (DOM) constituents (humic and fulvic acids) under high pH conditions. We also sought to test the comprehensive speciation model WHAM/Humic Ion Binding Model VII for these specific conditions. Results for Th demonstrate high fractions present as organic complexes at all pH values. Binding of neptunyl to DOM shows a maximum over the pH range expected within an evolving repository. Uranyl exhibits decreasing binding with pH, however, the majority of metal in solution is present as organic complexes under the lower pH conditions investigated (10–10.5). We have updated the WHAM/Model VII binding values for UO22+, and have for the first time added NpO2+ values to the database. These updates now allow application of the model for more complex mixtures across the entire repository pH range. Calculations for three simulated cement interstitial waters (representing different degradation phases) suggest U(VI) and Np(V) are not likely to be significantly bound to DOM under these conditions.

Root-induced changes in pH and dissolved organic matter binding capacity affect copper dynamic speciation in the rhizosphere

Due to only few experimental evidences, the importance of root-induced alteration of metal dynamic speciation in the rhizosphere in the determination of metal bioavailability to plants is still a matter for debate. The present study thus investigated how root-induced changes in pH and dissolved organic matters (DOM) altered copper (Cu) dynamic speciation in the rhizosphere of durum wheat (Triticum turgidum durum L.). Plants were exposed to a Cu-contaminated soil previously alkalised by liming to cover soil pH values ranging from 4.8 to 7.5. A range of analytical techniques was deployed on soil exposed (i.e. in the rhizosphere) or not (i.e. in the bulk soil) to plant roots, including the measurement and the modelling (using the Humic Ion-Binding Model VI) of Cu2+ activity, the measurement of labile Cu concentration and Cu lability by Differential Pulse Anodic Stripping Voltammetry (DPASV) and Diffusive Gradients in Thin films (DGT). Due to root-induced alkalisation, pH reached about 7.3 in the rhizosphere whatever the initial bulk soil pH. Compared to the most acidic bulk soil (pH≈4.8), Cu2+ activity decreased by three orders of magnitude in the rhizosphere while DPASV-Cu concentration decreased by 6-fold. DOM became the key driver of Cu dynamic speciation in the rhizosphere, where roots induced up to an order of magnitude increase in DOM concentration compared to bulk soils. This resulted in an increase in labile-Cu (both DPASV and DGT) concentrations, in spite of a decrease in Cu2+ activity. Model VI calculations supported a decrease in DOM binding capacity towards Cu in the rhizosphere. DPASV measurements unequivocally demonstrated that the increase in Cu lability in the rhizosphere solution can be attributed to a greater lability of organically-bound Cu. Collectively, our data introduce a consistent picture of root-induced changes of Cu dynamic speciation in the rhizosphere that were notably related to substantial alterations of DOM binding capacity.

The effects of humic substances on the transport of radionuclides: Recent improvements in the prediction of behaviour and the understanding of mechanisms

Some recent developments made during the European Union 6th Framework Integrated Project FUNMIG in the understanding and prediction of behaviour in ternary systems of radionuclides, humic substances and mineral surfaces are described. These developments are placed in the context of the existing literature. The aim is to describe the current understanding of humic substance mediated radionuclide transport as it may be applied to calculations in support of Radiological Performance Assessment. Some improvements in experimental techniques that provide the raw data to calibrate metal ion binding models are explained. The various metal ion binding models that are available are described and contrasted, before the recent development of ternary system models, in particular the Ligand Charge Distribution model that can predict metal ion and humic substance behaviour in ternary systems. The kinetic effects in ternary systems are described along with the models that are used to describe them. Finally, the remaining challenges in making predictions of radionuclide transport for the Radiological Performance Assessment of radioactive waste repositories are discussed.

An improved description of the interactions between rare earth elements and humic acids by modeling: PHREEQC-Model VI coupling

The Humic Ion Binding Model VI (Model VI) – previously used to model the equilibrium binding of rare earth elements (REE) by humic acid (HA) – was modified to account for differences in the REE constant patterns of the HA carboxylic and phenolic groups, and introduced into PHREEQC to calculate the REE speciation on the HA binding sites. The modifications were shown to greatly improve the modeling. They allow for the first time to both satisfactorily and simultaneously model a large set of multi-REE experimental data with the same set of equations and parameters. The use of PHREEQC shows that the light rare earth elements (LREE) and heavy rare earth elements (HREE) do not bind to HA by the same functional groups. The LREE are preferentially bound to carboxylic groups, whereas the HREE are preferentially bound to carboxy–phenolic and phenolic groups. This binding differentiation might lead to a fractionation of REE–HA patterns when competition between REE and other metals occur during complexation. A survey of the available data shows that competition with Al 3+ could lead to the development of HREE-depleted HA patterns. This new model should improve the hydrochemical modeling of the REE since PHREEQC takes into account chemical reactions such as mineral dissolution/precipitation equilibrium and redox reactions, but also models kinetically controlled reactions and one-dimensional transport.

Aluminium speciation in streams and lakes of the UK Acid Waters Monitoring Network, modelled with WHAM

The Windermere Humic Aqueous Model (WHAM) incorporating Humic Ion-Binding Model VI was applied to analytical data from the United Kingdom Acid Waters Monitoring Network, collected for 22 streams and lakes over the period 1988–2007, to calculate the chemical speciation of monomeric aluminium (Almon) in 3087 water samples. Model outputs were compared with analytical measurements of labile and non-labile Almon concentrations, the former being equated with inorganic forms of Almon and the latter with organically-complexed metal. Raw analytical data were used, and also data produced by applying a correction for the possible dissociation of organically-complexed Almon, and therefore its underestimation, during passage through the analytical cation-exchange column. Model calibration was performed by finding the conversion factor, FFADOC, between the concentration of isolated fulvic acid, with default ion-binding properties, required by the model, and the measured concentration of dissolved organic carbon, [DOC]. For both uncorrected and corrected data, the value of FFADOC for streams was greater than for lakes, indicating greater binding activity towards aluminium. Model fits were better using uncorrected analytical data, but the values of FFADOC obtained from corrected data agreed more closely with previous estimates. The model provided reasonably good explanations of differences in aluminium speciation between sampling sites, and of temporal variations at individual sites. With total monomeric concentration as input, WHAM calculations might substitute for analytical speciation measurements, or aid analytical quality control. Calculated Al3+ activities, aAl3+, showed a pH-dependence similar to that previously found for other surface waters, and the modelling exercise identified differences between waters of up to two orders of magnitude in the value of aAl3+ at a given pH. The model gives the net charge of dissolved organic matter, which is calculated to have risen significantly at 15 of the AWMN sites, due to increases in pH and decreases in aluminium concentration.