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Being able to predict the behavior of trace elements in the environment is crucial for environmental risk assessment studies. For this reason, modeling, in addition to experimental methods, has become an indispensable tool to better understand the (bio)-geochemistry of trace elements and the processes involved in their availability, transport and ecotoxicity. In this chapter we briefly outline the development of geochemical modeling over time and its basic principles. A comprehensive description of the state-of-the-art ion-binding and surface complexation models presently available for dissolved and particulate organic matter, metal (hydr)oxides of aluminum, iron, manganese and silica and clay minerals is given. A significant part of this chapter is dedicated to the application of these models for studying surface waters and soils. The most common model platforms used for this purpose together with the available (thermodynamic) databases of model parameters are summarized. In two separate sections we highlight the application of an assemblage model (with submodels for the various adsorbents) to describe trace element solid-solution partitioning and speciation in surface waters and soils; here particular attention is given to the derivation of site-specific inputs concerning the geochemical reactive metal content and the contents of adsorbents metal (hydr)oxides, clay and organic matter). Consideration is therefore given to the most recent developments in bio-geochemical modeling to link metal speciation to bioavailability, biotic accumulation and toxicity. Finally, future prospects of geochemical modeling are discussed, giving an overview of the potential directions for development.

Arkhipova et al. caution that the limitations of structural models be taken into account when interpreting crystallographic data., Crystal structures provide visual models of biological macromolecules, which are widely used to interpret data from functional studies and generate new mechanistic hypotheses. Because the quality of the collected x-ray diffraction data directly affects the reliability of the structural model, it is essential that the limitations of the models are carefully taken into account when making interpretations. Here we use the available crystal structures of members of the glutamate transporter family to illustrate the importance of inspecting the data that underlie the structural models. Crystal structures of glutamate transporters in multiple different conformations have been solved, but most structures were determined at relatively low resolution, with deposited models based on crystallographic data of moderate quality. We use these examples to demonstrate the extent to which mechanistic interpretations can be made safely.

The anionic charge of humic substances (HS) plays a major role in the interaction of HS with other components. Therefore, the potential of the polyelectrolyte titration technique to obtain the charge density of HS in simple 1–1 electrolyte solutions has been investigated. Titrations are carried out with an automatic titrator combined with the “Mutek particle charge detector” which allows determination of the Mutek potential and the pH as a function of the added amount of titrant which is a solution of poly-diallyldimethylammonium chloride (polyDADMAC), a cationic strong polyelectrolyte. When the Mutek potential reverses its sign the iso-electric point (IEP) of the polyDADMAC–HS complex is reached. The polyDADMAC/HS mass ratio at the IEP gives information on the HS charge density and from the pH changes in solution an estimate of the charge regulation in the HS–polyDADMAC complex can be obtained. In general, for polyDADMAC–HS complexes an increase in the dissociation of the acid groups of HS is found (charge regulation). The charge regulation decreases with increasing concentration of 1–1 background electrolyte. Cation incorporation can be neglected at 1–1 electrolyte concentrations ⩽ 1 mmol L−1 and a 1–1 stoichiometry exists between the polyDADMAC and HS charge. However, at these low salt concentrations the charge regulation is substantial. A detailed analysis of purified Aldrich humic acid (PAHA) at pH 5 and a range of KCl concentrations reveals that the anionic charge of PAHA in the complex increases at 5 mmol L−1 KCl by 30% and at 150 mmol L−1 KCl by 12%. On the other hand, increasing amounts of K+ become incorporated in the complex: at 5 mmol L−1 KCl 5% and at 150 mmol L−1 KCl 24% of the PAHA charge is balanced by K+. By comparing at pH 5 the mass ratios polyDADMAC/PAHA in the complex at the IEP with the theoretical mass ratios of polyDADMAC/PAHA required to neutralize PAHA in the absence of charge regulation and K+ incorporation, it is found that at 50 mmol L−1 KCl the extra negative charge due to the interaction between polyDADMAC and PAHA is just compensated by K+ incorporation in the complex. Therefore, a pseudo 1–1 stoichiometry exists at about 50 mmol L−1 1–1 electrolyte concentration and only at this salt concentration polyDADMAC titrations and conventional proton titrations give identical results. Most likely this is also true for other HA samples and other pH values. For FA further study is required to reveal the conditions for which polyDADMAC and proton titrations give identical results.

The charge of poly(acrylic acid) (PAA) in dilute aqueous solutions depends on pH and ionic strength. We report new experimental data and test various models to describe the deprotonation of PAA in three different NaCl concentrations. A simple surface complexation approach is found to be very successful: the constant capacitance model requires one pKa value and one capacitance for excellent fits to the data, with both parameters depending on ionic strength. The use of a self-consistent set of diffuse double layer parameters with one pKa for flat, spherical, and cylindrical geometry does not result in a satisfactory description of the data, and a number of adjustments to that model were tested to improve the fit. The basic Stern model (BSM) was tested with both plate and cylinder geometry. The cylinder geometry along with strong electrolyte binding was found to be superior to a similar approach involving weak electrolyte binding both in terms of goodness of fit and self-consistency of the parameters. The third approach, the non-ideal competitive consistent adsorption-Donnan (NICCA-Donnan) model, involving one functional group, allows an excellent description of the experimental data. Finally, the polyacid chain was modeled using a mechanistically more realistic self-consistent field (SCF) approach, which allows for radially inhomogeneous distributions of the charges and radial variations in the polymer density and electrostatic potential, while the functional groups can be in protonated, deprotonated, or complexed states. One functional group was insufficient for a satisfactory description of the data. With two segments (one monoprotic, the other diprotic) a reasonable description of the data, including the ionic strength dependence, is achieved, and the tendency of the size of the macro-ion with pH and ionic strength is as expected. This model has the fewest adjustable parameters and is considered the most realistic and comprehensive among the models tested

The transition from anaerobic to aerobic rice (Oryza sativa L.) cultivation has been reported to decrease Zn bioavailability. To determine and understand the differences in plant Zn uptake between anaerobic and aerobic rice cultivation systems, a field plot experiment was conducted with direct-seeded rice grown on a low-Zn soil under both anaerobic and aerobic conditions. Results showed that the mass fraction of Zn in shoots and Zn uptake of plants in the aerobic field was significantly lower than in the anaerobic field. Visual Zn deficiency symptoms as whitish-brown necrotic patches on leaves were more severe on plants grown in aerobic soils than plants in anaerobic soils, indicating reduced Zn bioavailability. To determine whether the underlying causes could be of a geochemical nature, a controlled soil incubation experiment was done. Using a fully parameterized model, we found that the difference in plant Zn uptake between the two cultivation systems was small compared with that predicted by geochemical modeling, assuming Zn uptake is controlled by the soluble fraction in the soil. Whereas individual effects of soil factors (e.g., pH) were demonstrated to cause large variations in Zn concentration, only about a 15% difference in plant Zn uptake was observed. This large discrepancy reveals that the consideration of separate soil chemical processes, instead of their integrated assessment, may be inappropriate. We speculated that the discrepancy could also be associated with the chemical conditions in the rhizosphere, where the local acidity, dissolved organic C, and redox conditions may deviate significantly from the bulk soil on which the geochemical analysis was based

The performance of a multisurface sorption model approach, composed of well-accepted surface complexation models in combination with published "generic" parameter sets, is evaluated for its possible use in risk assessment. For that purpose, the leaching of a broad range of potential soil contaminants (Ni, Cu, Zn, Cd, Pb, Ba, Cr, Co, Mo, V, Sn, Sb, S, As, Se) from eight diffusely and industrially contaminated soils is predicted simultaneously without any parameter optimization. The model approach includes aqueous speciation in combination with sorption to organic matter (NICA-Donnan model), Fe/Al(hydr)oxides (Generalized Two-Layer Model), and clay (Donnan model). Independent data generated by pH-static leaching experiments, performed with individual subsamples over a wide pH range (pH 0.4-12), provide a sensitive evaluation of the model performance. Root-mean-squared error values between predicted and measured log concentrations over the entire pH range, RMSE(log), are < 0.5 for Cu, Ni, Cd, Co, S, and Se, and RMSE(log) < 1 for Zn, Ba, Cr, Pb, Sn, Mo, Sn and As. The approach without parameter optimization has led to recommendations for further research with particular emphasis on identification of leaching mechanisms for Pb, Mo, Sb, and V and further expansion of the data sets to reduce the uncertainty of the available generic sorption parameters for Sn, Sb, Ba, Cr, and V.

Interactions of purified Aldrich humic acid (PAHA) with the protein lysozyme (LSZ) are studied with dynamic light scattering and isothermal titration calorimetry by mixing LSZ and PAHA at various mass ratios. In solution LSZ is positive and PAHA is negative at the investigated pH values. Up to moderate KCl concentrations no aggregation occurs for LSZ and for PAHA aggregated particles with an average radius of 80 nm are present Complexation of PAHA with LSZ starts as soon as PAHA is added to LSZ and is followed by aggregation when the isoelectric-point (IEP) of the complexes is approached. Aggregation is gradual for 50 mM KCl and sudden for low KCl concentrations. The aggregate size is at its maximum at the IEP of the complexes. At mass ratios beyond the IEP the aggregates partially disaggregate. Positively charged complexes of PAHA and LSZ, formed in the absence of salt, strongly aggregate upon salt addition. Mixing of LSZ and PAHA is initially enthalpically driven. Near the IEP complexation and aggregation are due to hydrophobic forces (structural reorganization) and counterion release. The observations are relevant for other HA-protein systems when the protein is positively charged.

Four soils were collected from different sites of China in Lechang (LC, Guangdong province), Changsha (CS, Hunan province), Jiaxing UX, Zhejiang province) and Hangzhou (HZ, Zhejiang province), and were spiked with Cu (50 mg kg(-1)) and Cd (5 mg kg(-1)). The effects of rice straw addition (6%) on the chemical distribution of both metal ions were studied by measuring the soluble metal ion concentrations and the free metal ion concentrations (using the Donnan membrane technique) after 1-, 3- and 6-month incubation. Results show that the addition of rice straw increased soil pH by about 0.4 pH unit on average, and increased DOC significantly in soil LC, CS and JX, but not in soil HZ. With the addition of rice straw, total soluble Cu concentration increased from 0.82 mu mol l(-1) (0.26-2.4 mu mol l(-1)) to 1.22 mu mol l(-1) (0.70-3.60 mu mol l(-1)), whereas total soluble Cd concentration decreased from 20 nmol l(-1) (2-70 nmol l(-1)) to 15 nmol l(-1) (2-56 nmol l(-1)). When rice straw was added, both free Cu 2 and Cd 21 concentrations decreased, for free Cu2+ concentration from 217 nmol l(-1) (31 to 369 nmol l(-1)) to 124 nmol l(-1) (22 to 263 nmol l(-1)) and for free Cd2+ concentration from 16 nmol l(-1) (1-55 nmol l(-1)) to 12 nmol l(-1) (1-43 nmol l(-1)). With the increase of incubation time, free Cu2+ concentration tended to increase but free Cd2+ concentration decreased. Speciation model calculations show that compared to the binding capacity of soil organic matter, the capacity of rice straw is much less important. The decrease of free metal ion concentration upon rice straw addition can be attributed mainly to the increased pH. The higher DOC content in the rice straw treatment could be the reason for higher soluble Cu concentration when rice straw was added. Adsorption to DOC is much less important for Cd than for Cu. Calculation also shows that adsorption to clay minerals plays a more important role for Cd than for Cu, which may explain the stronger ageing effects on Cd distribution. (c) 2008 Elsevier B.V. All rights reserved.

Adsorption of randomly branched polyelectrolytes, "hairy" particles and internally structured macromolecules, collectively denoted as heterogeneously charged nanoparticles, on charged surfaces is important in many technological and natural processes. In this paper, we will focus on (1) the charge regulation of both the nanoparticle and the surface and (2) the surface complexation between the particle functional groups and the surface sites and will theoretically study the adsorption using the extended surface complexation approach. The model explicitly considers the electrochemical potential of a nanoparticle with an average (smeared-out) structure and charge both in bulk solution and on the surface to obtain the equilibrium adsorption. The chemical heterogeneity of the particle is described by a distribution of the protonation constant. Detailed analysis of the chemical potential of the adsorbed nanoparticle reveals that the pH and salt dependence of the adsorption can be largely explained by the balance between an energy gain resulting from the particle and surface charge regulation and the surface complexation and an energy loss from the unfavorable interparticle electrostatic repulsion close to the surface. This conclusion is also supported by the strong impacts that the chemical heterogeneity of the particle functional groups, the magnitude of the surface complexation, the number of the functional groups, and the size of the particle have on the adsorption.

The development of speciation and surface complexation models to calculate partitioning of metals between the soil solid phase and soil solution has made a large progress. We evaluated the use and practical applicability of an assemblage of complexation models to predict sorption of Cd, Cu, Ni, Pb and Zn. We used 353 samples which cover a wide range of soil properties and metal contents. For sorption of cations to solid and dissolved organic matter the NICA-Donnan model was used, to clay minerals a non-specific Donnan type exchange model, and to metal(hydr)oxides a two-site DDL model. Calculated solution concentrations agree well with measurements, except for Pb where calculations overestimate concentrations by one order of magnitude on average. Some highly Zn contaminated samples show relatively large deviations between measured and calculated concentrations. This is most likely caused by Zn containing minerals which are included in the total amount of reactive metals in the soil as determined by acid extraction. Hence, such models can be used for practical applications if one is aware of the possible complications. When the “total reactive metal content” includes a substantial amount of precipitated metals, which may occur for heavily contaminated soils, the model tends to overestimate concentrations in solution.

The adsorption of humic acids (HA) to goethite (at pH 3-11) and the proton co-adsorption (at pH 4.0, 5.5, and 7.0) were measured, and the results were compared to those of fulvic acids (FA). Compared to FA, the adsorption of HA is stronger and more ionic strength dependent. The adsorption of both HA and FA decreases with increasing pH. The relative change of the adsorption with pH is bigger for HA than for FA at relatively low pH. At relatively high pH, it is the opposite. Protons are released at pH 4.0 and co-adsorbed at pH 5.5 and 7.0 upon the adsorption of both HA and FA. The observed pH dependency of HA and FA adsorption is in agreement with the proton co-adsorption data. Model calculations show that the adsorbed FA particles are on average located in the Stern layer, whereas the adsorbed HA particles protrude beyond the Stern layer. The closer location to the surface of the adsorbed FA leads to stronger electrostatic interactions between the FA particles and the surface, which explains the larger amount of protons released at low pH and co-adsorbed at high pH with each mass unit of FA adsorbed than that with HA adsorbed. The model also reveals that for FA a mean-field (smeared-out) approximation is reasonable, but for HA a patchwise approach is more appropriate at relatively low loading.

In this paper, we characterize the leaching of heavy metals (Ni, Cu, Zn, Cd, and Pb) from eight contaminated soils over a wide range of pH (pH 0.4-12) using an original approach based on batch pH-static leaching experiments in combination with selective chemical extractions and geochemical modeling. The leached concentrations of the heavy metals are generally much lower than the total concentrations and show a strong pH dependency, resulting in "V-shaped" leaching curves with orders of magnitude changes in solution concentrations. The "multisurface" model used incorporates adsorption to dissolved and solid organic matter (NICA-Donnan), iron/aluminum (hydr)oxide (generalized two-layer model) and clay (Donnan model). These models were applied without modifications, and only the standard set of binding constants and parameters was used (i.e., without any fitting). The model predictions of heavy metal leaching are generally adequate and sometimes excellent. Results from speciation calculations are consistent with the well-recognized importance of organic matter as the dominant reactive solid phase in soils. The observed differences between soils with respect to element speciation in the solid phase correspond to the relative amounts of the reactive surfaces present in the soils. In the solution phase, complexes with dissolved organic matter (DOM) are predominant over most of the pH range. Free metal ions (Me2+) are generally the dominant species below pH 4. The combination of the experimental and modeling approach as used in this study is shown to be promising because it leads to a more fundamental understanding of the pH-dependent leaching processes in soils. The "multisurface" modeling approach, with the selected sorption models, is shown to be able to adequately predict the leaching of heavy metals from contaminated soils over a wide range of conditions, without any fitting of parameters.

Stripping chronopotentiometry at scanned deposition potential (SSCP) allows chemical heterogeneity in metal speciation to be unambiguously identified. In the labile regime, use of the Freundlich binding isotherm allows straightforward determination of parameters to describe the apparent stability and heterogeneity of metal complexes with humic substances. The extent of heterogeneity of metal binding by several humic substances follows the order Cu(ii) >> Pb(ii) > Cd(ii). The lability of metal complexes decreases from the foot to the top of the wave, and the greater the degree of heterogeneity, the more readily lability is lost. In the kinetic current regime, the Koutecký–Koryta approximation allows an expression to be obtained for the SSCP wave that provides a good estimate of the experimental data for metal complexes with moderate degrees of heterogeneity.

The primary purpose of this study was to determine how flooding and draining cycles affect the redox chemistry of metal (hydr)oxides and organic matter in paddy soils and how the pH influences these processes. Our secondary purpose was to determine to what extent a geochemical thermodynamic equilibrium model can be used to predict the solubility of Mn and Fe during flooding and draining cycles in paddy soils. We performed a carefully designed column experiment with two paddy soils with similar soil properties but contrasting pH.We monitored the redox potential (Eh) continuously and took soil solution samples regularly at four depths along the soil profile during two successive flooding and drainage cycles. To determine dominant mineral phases of Mn and Fe under equilibrium conditions, stability diagrams of Mn and Fe were constructed as a function of Eh and pH. Geochemical equilibrium model calculations were performed to identify Mn and Fe solubility-controlling minerals and to compare predicted total dissolved concentrations with their measured values. Flooding led to strong Eh gradients in the columns of both soils. In the acidic soil, pH increased with decreasing Eh and vice versa, whereas pH in the alkaline soil was buffered by CaCO3. In the acidic soil, Mn and Fe solubility increased during flooding due to reductive dissolution of their (hydr)oxides and decreased during drainage because of re-oxidation. In the alkaline soil, Mn and Fe solubility did not increase during flooding due to Mn(II) and Fe(II) precipitation as MnCO3, FeCO3, and FeS. The predicted levels of soluble Mn and Fe in the acidic soil were much higher than their measured values, but predictions and measurements were rather similar in the alkaline soil. This difference is likely due to kinetically limited reductive dissolution of Mn and Fe (hydr)oxides in the acidic soil. During flooding, the solubility of dissolved organic matter increased in both soils, probably because of reductive dissolution of Fe (hydr)oxides and the observed increase in pH. Conclusions Under alternating flooding and draining conditions, the pH greatly affected Mn and Fe solubility via influencing either reductive dissolution or carbonate formation. Comparison between measurements and geochemical equilibrium model predictions revealed that reductive dissolution of Mn and Fe (hydr)oxides was kinetically limited in the acidic soil. Therefore, when applying such models to systems with changing redox conditions, such rate-limiting reactions should be parameterized and implemented to enable more accurate predictions of Mn and Fe solubility.

The lanthanum (La) modified bentonite Phoslock ® has been proposed as dephosphatisation technique aiming at removing Filterable Reactive Phosphorus (FRP) from the water and blocking the release of FRP from the sediment. In the modified clay La is expected the active ingredient. We conducted controlled laboratory experiments to measure the FRP removal by Phoslock ® in the presence and absence of humic substances, as La complexation with humic substances might lower the effectiveness of La (Phoslock ® ) to bind FRP. The results of our study support the hypothesis that the presence of humic substances can interfere with the FRP removal by the La-modified bentonite. Both a short-term (1 d) and long-term (42 d) experiment were in agreement with predictions derived from chemical equilibrium modelling and showed lower FRP removal in presence of humic substances. This implies that in DOC-rich inland waters the applicability of exclusively Phoslock ® as FRP binder should be met critically. In addition, we observed a strong increase of filterable La in presence of humic substances reaching in a week more than 270 μg La l −1 that would infer a violation of the Dutch La standard for surface water, which is 10.1 μg La l −1 . Hence, humic substances are an important factor that should be given attention when considering chemical FRP inactivation as they might play a substantial role in lowering the efficacy of metal-based FRP-sorbents, which makes measurements of humic substances (DOC) as well as controlled experiments vital.

The soil bacterium Streptomyces reticuli secretes the octameric protein HbpS that acts as a sensory component of the redox-signalling pathway HbpS-SenS-SenR. This system modulates a genetic response on iron-and haem-mediated oxidative stress. Moreover, HbpS alone provides this bacterium with a defence mechanism to the presence of high concentrations of iron ions and haem. While the protection against haem has been related to its haem-binding and haem-degrading activity, the interaction with iron has not been studied in detail. In this work, we biochemically analyzed the iron-binding activity of a set of generated HbpS mutant proteins and present evidence showing the involvement of one internal and two exposed D/EXXE motifs in binding of high quantities of ferrous iron, with the internal E78XXE81 displaying the tightest binding. We additionally show that HbpS is able to oxidize ferrous to ferric iron ions. Based on the crystal structure of both the wild-type and the mutant HbpS-D78XXD81, we conclude that the local arrangement of the side chains from the glutamates in E78XXE81 within the octameric assembly is a pre-requisite for interaction with iron. The data obtained led us to propose that the exposed and the internal motif build a highly specific route that is involved in the transport of high quantities of iron ions into the core of the HbpS octamer. Furthermore, physiological studies using Streptomyces transformants secreting either wild-type or HbpS mutant proteins and different redox-cycling compounds led us to conclude that the iron-sequestering activity of HbpS protects these soil bacteria from the hazardous side effects of peroxide-and iron-based oxidative stress.

Holliday Junctions are critical DNA intermediates central to double strand break repair and homologous recombination. The junctions can adopt two general forms: open and stacked-X, which are induced by protein or ion binding. In this work, fluorescence spectroscopy, metal ion luminescence and thermodynamic measurements are used to elucidate the ion binding site and the mechanism of junction conformational change. Förster resonance energy transfer measurements of end-labeled junctions monitored junction conformation and ion binding affinity, and reported higher affinities for multi-valent ions. Thermodynamic measurements provided evidence for two classes of binding sites. The higher affinity ion-binding interaction is an enthalpy driven process with an apparent stoichiometry of 2.1 ± 0.2. As revealed by Eu(3+) luminescence, this binding class is homogeneous, and results in slight dehydration of the ion with one direct coordination site to the junction. Luminescence resonance energy transfer experiments confirmed the presence of two ions and indicated they are 6-7 Å apart. These findings are in good agreement with previous molecular dynamics simulations, which identified two symmetrical regions of high ion density in the center of stacked junctions. These results support a model in which site-specific binding of two ions in close proximity is required for folding of DNA Holliday junctions into the stacked-X conformation.

The entire substrate translocation pathway in the human GABA transporter (GAT-1) was explored for the endogenous substrate GABA and the anti-convulsive drug tiagabine. Following a steered molecular dynamics (SMD) approach, in which a harmonic restraining potential is applied to the ligand, dissociation and re-association of ligands were simulated revealing events leading to substrate (GABA) translocation and inhibitor (tiagabine) mechanism of action. We succeeded in turning the transporter from the outward facing occluded to the open-to-out conformation, and also to reorient the transporter to the open-to-in conformation. The simulations are validated by literature data and provide a substrate pathway fingerprint in terms of which, how, and in which sequence specific residues are interacted with. They reveal the essential functional roles of specific residues, e.g. the role of charged residues in the extracellular vestibule including two lysines (K76 (TM1) and K448 (TM10)) and a TM6-triad (D281, E283, and D287) in attracting and relocating substrates towards the secondary/interim substrate-binding site (S2). Likewise, E101 is highlighted as essential for the relocation of the substrate from the primary substrate-binding site (S1) towards the cytoplasm.

The chemodynamics of Cu(II) complexation by humic acid is interpreted in terms of recently developed theory for permeable charged nanoparticles. Two opposing electric effects are operational with respect to the overall rate of association, namely, (i) the conductive enhancement of the diffusion of Cu2+, expressed by a coefficient f(e nu), which accounts for the accelerating effect of the negative electrostatic field of the humic particle on the diffusive transport of metal ions toward it, and (ii) the ionic Boltzmann equilibration with the bulk solution, expressed by a factor f(B), which quantifies the extent to which Cu2+ ions accumulate in the negatively charged particle body. These effects are combined in the probability of outer-sphere metal site complex formation and the covalent binding of the metal ion by the complexing site (inner-sphere complex formation) as in the classical Eigen mechanism. Overall "experimental" rate constants for CuHA complex formation, k(a), are derived from measurements of the thermodynamic stability constant, K*, and the dissociation rate constant, k(d)(*), as a function of the degree of metal ion complexation, theta. The resulting k(a) values are found to be practically independent of theta. They are also compared to theoretical values; at an ionic strength of 0.1 mol dm(-3), the rate of diffusive supply of metal ions toward the particles is comparable to the rate of inner-sphere complex formation, indicating that both processes are significant for the observed overall rate. As the ionic strength decreases, the rate of diffusive supply becomes the predominant rate-limiting process, in contrast with the general assumption made for complexes with small ligands that inner-sphere dehydration is the rate-limiting step. The results presented herein also resolve the discrepancy between experimentally observed and predicted dissociation rate constants based on the above assumption.

Various dynamic soil chemistry models have been developed to gain insight into impacts of atmospheric deposition of sulphur, nitrogen and other elements on soil and soil solution chemistry. Sorption parameters for anions and cations are generally calibrated for each site, which hampers extrapolation in space and time. On the other hand, recently developed surface complexation models (SCMs) have been successful in predicting ion sorption for static systems using generic parameter sets. This study reports the inclusion of an assemblage of these SCMs in the dynamic soil chemistry model SMARTml and applies this model to a spruce forest site in Solling Germany. Parameters for SCMs were taken from generic datasets and not calibrated. Nevertheless, modelling results for major elements matched observations well. Further, trace metals were included in the model, also using the existing framework of SCMs. The model predicted sorption for most trace elements well.

Ion binding models such as the nonideal competitive adsorption-Donnan model (NICA-Donnan) and model VI successfully describe laboratory data of proton and metal binding to purified humic substances (HS). In this study model performance was tested in more complex natural systems. The speciation predicted with the NICA-Donnan model and the associated uncertainty were compared with independent measurements in soil solution extracts, including the free metal ion activity and fulvic (FA) and humic acid (HA) fractions of dissolved organic matter (DOM). Potentially important sources of uncertainty are the DOM composition and the variation in binding properties of HS. HS fractions of DOM in soil solution extracts varied between 14 and 63% and consisted mainly of FA. Moreover, binding parameters optimized for individual FA samples show substantial variation. Monte Carlo simulations show that uncertainties in predicted metal speciation, for metals with a high affinity for FA (Cu, Pb), are largely due to the natural variation in binding properties (i.e., the affinity) of FA. Predictions for metals with a lower affinity (Cd) are more prone to uncertainties in the fraction FA in DOM and the maximum site density (i.e., the capacity) of the FA. Based on these findings, suggestions are provided to reduce uncertainties in model predictions.

Average electrostatic potentials of purified Aldrich humic acid (PAHA) are measured, based on quenching of the PAHA fluorescence by neutral and cationic quencher molecules with similar structures. The obtained negative potentials increased with increasing pH at a given salt concentration and weakly decreased with increasing salt concentration at given pH. The trends and the magnitudes of the potentials correspond with the few experimental results available in the literature. Comparison with potentials calculated by the electrostatic models from the charge of PAHA revealed that the average potentials obtained from the ion permeable sphere model and Donnan-EDL model agreed with the measured average potentials. With these two models it is assumed that a part of the PAHA charge is neutralized by the electrical double layer (EDL) around the PAHA particle in line with the view that humic acid is a nano-size particle partly permeable to small ions and solvent molecules. The potentials calculated by these two models are averages of the potential “inside” the particle (which governs the proton binding) and that in the EDL. The Donnan models, with both the fixed charges and the counter charges distributed over a volume that is greater than the particle volume itself, result in more negative potentials than the measured average potentials of PAHA. These observations suggest that the quenchers are located close to the functional groups but diffusely bound.

Interactions of purified Aldrich humic acid (PAHA) with lysozyme (LSZ) are investigated. In solution LSZ is moderately positively and PAHA negatively charged at the investigated pH values. The proton binding of PAHA and of LSZ is determined by potentiometric proton titrations at various KCl concentrations. It is also measured for two mixtures of PAHA-LSZ and compared with theoretically calculated proton binding assuming no mutual interaction. The charge adaptation due to PAHA-LSZ interaction is relatively small and only significant at low and high pH. Next to the proton binding, the mass ratio PAHA/LSZ at the iso-electric point (IEP) of the complex at given solution conditions is measured together with the pH using the Mutek particle charge detector. From the pH changes the charge adaptation due to the interaction can be found. Also these measurements show that the net charge adaptation is weak for PAHA-LSZ complexes at their IEP. PAHA/LSZ mass ratios in the complexes at the IEP are measured at pH 5 and 7. At pH 5 and 50 mmol/L KCl the charge of the complex is compensated for 30-40% by K(+); at pH 7, where LSZ has a rather low positive charge, this is 45-55%. At pH 5 and 5 mmol/L KCl the PAHA/LSZ mass ratio at the IEP of the complex depends on the order of addition. When LSZ is added to PAHA about 25% K(+) is included in the complex, but no K(+) is incorporated when PAHA is added to LSZ. The flocculation behavior of the complexes is also different. After LSZ addition to PAHA slow precipitation occurs (6-24 h) in the IEP, but after addition of PAHA to LSZ no precipitation can be seen after 12 h. Clearly, PAHA/LSZ complexation and the colloidal stability of PAHA-LSZ aggregates depend on the order of addition. Some implications of the observed behavior are discussed. (c) 2008 Elsevier Ltd. All rights reserved.

For a period of 2 a, pore water composition in a heavily contaminated river floodplain soil was monitored in situ. Pore water samples were collected 12 times over all seasons in a profile ranging from aerobic to sulphidic redox conditions, and As, Cd, Cr, Cu, Pb, Zn, Mn, Fe, Ca, Cl, SO4. DOC,IOC and pH were determined. The variability of pH, IOC, DOC and Ca was found to be rather small during the year and within the profile (rsd

It is well established that dissolved organic matter (DOM) mobilizes copper (Cu) in soils but it is unknown to what extent variable DOM quality affects this. During a 5 month period, 250 leachates of an uncontaminated agricultural soil were sampled at 45 cm depth using passive capillary wick samplers. The dissolved Cu and organic carbon (DOC) concentrations varied sevenfold and were weakly correlated (r = 0.56). The [Cu] : [DOC] ratio varied fivefold and exhibited a significant positive correlation (r = 0.77) with the specific UV-absorbance of DOM at 254 nm (SUVA), indicating that the more aromatic DOM had higher Cu affinity. The dissolved Cu concentrations were predicted by an assemblage model in WHAM6 using the composition of the solid phase above the wick samplers and that of the solution, including DOC. The predicted [Cu] : [DOC] ratio was almost constant when assuming default DOM properties with 65% of all DOM active as fulvic acid (%AFA). The %AFA was subsequently varied proportionally to the SUVA of DOM and using the SUVA of pure FA (SUVA(FA)) as a fitting parameter. In that case, the variation in the predicted [Cu] : [DOC] ratio was much larger and the predicted Cu concentrations were within a factor of 1.4 of the measured values for 90% of the samples. The fitted SUVA(FA) was 38 l g(-1) cm(-1), in excellent agreement with that of Suwannee River FA (SUVA(FA) = 37 l g(-1) cm(-1)). It is concluded that the DOM quality, e.g. the aromaticity, should be taken into account when estimating Cu mobility in soils. ispartof: European journal of soil science vol:59 issue:6 pages:1087-1095 status: published

To improve the long-term environmental risk assessment of waste applications, a predictive “multi-surface” modelling approach has been developed to simultaneously predict the leaching and reactive transport of a broad range of major and trace elements (i.e., pH, Na, Al, Fe, Ca, SO4, Mg, Si, PO4, CO3, Cl, Ni, Cu, Zn, Cd, Pb, Mo) and fulvic acids from MSWI bottom ash. The geochemical part of the model approach incorporates surface complexation/precipitation on Fe/Al (hydr)oxides, complexation with humic and fulvic acids (HA and FA, respectively) and mineral dissolution/precipitation. In addition, a novel approach is used to describe the dynamic leaching of FA, based on the surface complexation of FA on Fe/Al (hydr)oxides. To enable reactive transport calculations, the geochemical part of the model is combined with advective/dispersive transport of water and first-order mass transfer between mobile and stagnant zones. Using a single, independently determined set of input parameters, adequate model predictions are obtained for the leaching of a broad range of elements under widely different conditions, as verified with data from the European standardised pH-static and percolation leaching tests (TS 14997 and TS 14405, respectively). The percolation tests were operated at different flow velocities and with flow interruptions to enable verification of the local equilibrium assumption. Although the combination of experimental and modelling results indicates that the leaching of major solubility-controlled elements occurs largely under local equilibrium conditions, this study has led to the identification of physical non-equilibrium processes for non-reactive soluble salts, as well as possible sorption-related non-equilibrium processes for the leaching of Mo, FA and associated trace metals. Further improvement of the reactive transport model can be achieved by a more mechanistic description of the (dynamic) leaching behaviour of humic substances. As the modelling approach outlined in this study is based on the fundamental processes that underlie leaching, the approach is expected to be also applicable to other granular contaminated materials application scenarios and conditions. Therefore, the combination of standardized leaching test methods, selective chemical extractions and mechanistic modelling, constitutes a promising generic approach to assess the long-term environmental impact of the application of granular contaminated materials in the environment.

The use of the [S,S]-isomer of EDDS to enhance phytoextraction has been proposed for the remediation of heavy metal contaminated soils. Speciation of metals in soil solution in the presence of EDDS and dissolved organic matter (DOM) received, however, almost no attention, whereas metal speciation plays an important role in relation to uptake of metals by plants. We investigated the influence of EDDS on speciation of dissolved metals in batch extraction experiments using four field-contaminated soils with pH varying between 4.7 and 7.2. Free metal concentrations were determined with the Donnan membrane technique, and compared with results obtained with the chemical speciation program ECOSAT and the NICA-Donnan model using a multicomponent approach. Addition of EDDS increased total metal concentrations in our soil extracts by a factor between 1.1 and 32 (Al), 2.1?48 (Cu), 1.1?109 (Fe), 1.1?5.5 (Ni), and 1.3?17 (Zn). In general, Al, Cu, Fe, and Zn had the largest total concentrations in the EDDS-treated extracts, but the contribution of these metals to the sum of total metal concentrations varied significantly between our soils. Free metal concentrations varied between 7.0 and 8.9 (pCd2+), 3.9?9.9 (pCu2+), 6.3?10.2 (pNi2+), and 5.2?7.0 (pZn2+). Addition of EDDS decreased free metal concentrations by a factor between 1.4 and 1.9 (Cd), 3.4?216 (Cu), 1.3?186 (Ni), and 1.3?3.3 (Zn). Model predictions of free metal concentrations were very good, and predicted values were mostly within 1 order of magnitude difference from the measured concentrations. A multicomponent approach had to be used in our model calculations, because competition between Fe and other metals for binding with EDDS was important. This was done by including the solubility of metal oxides in our model calculations. Multicomponent models can be used in chelant-assisted phytoextraction experiments to predict the speciation of dissolved metals and to increase the understanding of metal uptake by plants.

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTED incorporating NICA-Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available for the flux to GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data

Commercial surfactants are introduced into the environment either through waste products or site-specific contamination. The amphiphilic nature of both surfactants and humic substances (HS) leads to their mutual attraction especially when surfactant and HS are oppositely charged. Binding of the cationic surfactants dodecyl-pyridinium chloride (DPC) and cetyl- or hexadecyl-pyridinium chloride (CPC) to purified Aldrich humic acid (PAHA), Dando humic acid (DHA), Inogashira humic acid (IHA), Laurentian fulvic acid (LFA) and Strichen Bs fulvic acid (SFA) is studied at pH 4.5–5 at 0.005 M NaCl. For PAHA CPC binding is also studied at pH 5 and 0.1 M NaCl. Measurements with the Mutek Particle Charge Detector (PCD) and poly-DADMAC, a strong cationic polyelectrolyte, are used to determine the charge of the HS samples. PCD measurements with the surfactants reveal that the surfactant–HS complexes reach their iso-electric-point (IEP) before the critical micelle concentration (CMC) is reached. At the IEP the adsorption values (mol/g) of CP + and DP + to PAHA are the same, i.e. at the IEP the charge associated with the HS is neutralized by bound surfactant ions. For the other humic acids (HAs) CPC binding at the CMC corresponds with the charge obtained with poly-DADMAC, but for the fulvic acid (FA) samples CP(C) adsorption at the IEP is larger than the FA charge. The surfactant–HS complexes flocculate around the IEP. Binding isotherms are obtained using surfactant electrodes. The results for CPC and DPC to the HA samples show a pseudo-plateau near the IEP, which is missing in the isotherms to the FA samples. The CP + –PAHA isotherms at 0.005 M and 0.1 M intersect at the IEP. The affinity of CP + binding to PAHA is larger than that of DP + due to the longer aliphatic tail of CPC. The bound amount of DP(C) decreases in the order PAHA ≫ IHA ≈ DHA ≫ LFA ≈ SFA. The results demonstrate that cationic surfactant binding to HS is due both to electrostatic and hydrophobic attraction and that the fate of HS in aqueous environmental systems can be strongly affected by cationic surfactants.

10 pages, 10 figures, 3 tables.-- Printed version published Sep 2007., Activated carbons were prepared by phosphoric acid activation of fruit stones in air at temperatures 400–1000°C. The surface chemistry was investigated by elemental analysis, cation exchange capacity, infrared spectroscopy and potentiometric titration. The porous structure was analyzed from adsorption isotherms (N2 at 77 K and CO2 at 273 K). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (2.2 mmol g−1) being attained at 700°C, which coincides with the maximum contents of phosphorus and oxygen. The use of air instead of argon during thermal treatment increased the amount of cation exchangeable surface groups for carbons obtained at 400–700°C. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK 1.8–3.1 (carboxylic and polyphosphates), 4.8–6.3 (second dissociation of carboxylic, weak acid in polyphosphates and enol structures) and 8.1–9.7 (phenols and enol structures). Carbons obtained in air at 400–600°C show enhanced copper adsorption from 0.001 mol L−1 Cu(NO3)2 in acidic solutions as compared to carbons obtained in argon. Carbons obtained in air show well-developed porous structure that is equivalent or higher as compared with carbons obtained in argon; the difference being progressively increased with increasing treatment temperature., This research was made possible in part by the NATO Collaborative Linkage Grant EST.CLG.979588.

The effect of accelerated aging of Municipal Solid Waste Incinerator (MSWI) bottom ash on the leaching of Cu and Mo was studied using a "multisurface" modeling approach, based on surface complexation to iron/aluminum (hydr) oxides, mineral dissolution/precipitation, and metal complexation by humic substances. A novel experimental method allowed us to identify that the solid/liquid partitioning of fulvic acids (FA) quantitatively explains the observed beneficial effect of accelerated aging on the leaching of Cu. Our results suggest that iron/aluminum (hydr)oxides are the major reactive surfaces that retain fulvic acid in the bottom ash matrix, of which the aluminum (hydr)oxides were found to increase after aging. A new modeling approach, based on the surface complexation of FA on iron/aluminum (hydr) oxides is developed to describe the pH-dependent leaching of FA from MSWI bottom ash. Accelerated aging results in enhanced adsorption of FA to (neoformed) iron/aluminum (hydr) oxides, leading to a significant decrease in the leaching of FA and associated Cu. Accelerated aging was also found to reduce the leaching of Mo, which is also attributed to enhanced adsorption to (neoformed) iron/aluminum (hydr) oxides. These findings provide important new insights that may help to improve accelerated aging technology.

Numerous models exist for predicting metal interactions in the environment, but few have been validated with field observations. Modeling the binding of metal ions and actinides to natural particles of various origins remains a challenging task. The use of models to understand the speciation and distribution of metals in the environment is essential for predicting metal bioavailability. In this article, we discuss the primary metal–particle interaction models and their limits, with an emphasis on natural organic matter (NOM). These models have varying levels of complexity, but we discuss only those based on parameters that are independent of environmental conditions. We also discuss the application of such models to field predictions.

Within the biotic ligand model, which describes relationships between chemical speciation and metal binding at an organism's surface, multicomponent (long-term) metal uptake by plants has seldom been studied. In the present work, we exposed perennial ryegrass to nutrient solutions with two levels of Cd, Cu, Ni, Pb, and Zn (1 and 0.1 microM) and with or without 30 mg/L of humic acid. Iron and Mn concentrations were constant over all treatments. The hypothesis tested was that humic acid lowers the free and labile metal concentration and, therefore, reduces the metal uptake and, finally, the metal content of the plant. The free metal ion concentrations in the nutrient solutions were measured by the Donnan membrane technique and labile metal concentrations by diffusive gradients in thin-films. The metal content of the shoots depends on the metal content of the roots. The metal content of the roots is a function of the adsorption of metals on the root surface. In a multicomponent system at metal concentrations of 1 microM, humic acid decreased Cu, Pb, and Fe adsorption at the root surface, but it increased Cd, Zn, and Mn adsorption at the root surface. Complexation of cations such as Cu, Pb, and Fe with high affinity for (dissolved) organic matter may lead to increased uptake of cations with low affinity for organic matter (Ni, Zn, and Cd) because of competition between cations at the root surface. The results suggest that competition between metal ions can play a major role in multicomponent metal uptake, which has to be taken into account during risk assessments of metal-polluted soils.

The aim of this study is to predict the solid-solution partitioning of heavy metals in river flood plain soils. We compared mechanistic geochemical modeling with a statistical approach. To characterize the heavy metal contamination of embanked river flood plain soils in The Netherlands, we collected 194 soil samples at 133 sites distributed in the Dutch part of the Rhine and Meuse river systems. We measured the total amounts of As, Cd, Cr, Cu, Ni, Pb, and Zn in the soil samples and the metal fraction extractable by 2.5 mM CaCl2. We found a strong correlation between heavy metal contamination and organic matter content, which was almost identical for both river systems. Speciation calculations by a fully parametrized model showed the strengths and weaknesses of the mechanistic approach. Cu and Cd concentrations were predicted within one log scale, whereas modeling of Zn and Pb needs adjustment of some model parameters. The statistical fitting approach produced better results but is limited with regard to the understanding it provides. The log RMSE for this approach varied between 0.2 and 0.32 for the different metals. The careful modeling of speciation and adsorption processes is a useful tool for the investigation and understanding of metal availability in river flood plain soils.