Biomedical subjects

Willem H van Riemsdijk

Publications and source records attributed to Willem H van Riemsdijk.

13 recordsLinked to original sources

PubMed2006

Surface speciation of As(III) and As(V) in relation to charge distribution.

The adsorption of As(III) and As(V) on goethite has been studied as a function of pH and loading. The data can be successfully described with the charge distribution (CD) model (extended Stern layer option) using realistic species observed by EXAFS. The CD values have been derived theoretically. Therefore, the Brown bond valence approach has been applied to MO/DFT optimized geometries of a series of hydrated complexes of As(III) and As(V) with Fe(III) (hydr)oxide. The calculated ionic CD values have been corrected for the effect of dipole orientation of interfacial water, resulting in overall interfacial CD coefficients that can be used to describe the surface speciation as a function of pH and loading. For As(III), the main surface species is a bidentate complex and a minor contribution of a monodentate species is found, which is in agreement with EXAFS. The CD values have also been fitted. Such an analysis of the adsorption data resulted in the same surface species. The fitted CD values for the bidentate complex points to the presence of strong AsO bonds with the surface and a weaker AsOH bond with the free OH ligand. This agrees quantitatively with the MO/DFT optimized geometry. Interpretation of free fitted CD values for As(V) binding suggests that the main surface species is a non-protonated bidentate complex (B) with a contribution of a singly protonated surface complex (MH) at sub-neutral pH and high loading. In addition, a protonated bidentate surface complex (BH) may be present. The same species are found if the theoretical CD values are used in the data analysis. The pH dependency of surface speciation is strongly influenced by the charge attribution of adsorbed species to the electrostatic surface plane while the effect of loading is primarily controlled by the amount of charge attributed to the 1-plane, illustrating the different action of the CD value. The MO/DFT geometry optimizations furthermore suggest that for As(V) the B, MH and BH surface complexes may have very similar AsFe distances which may complicate the interpretation of EXAFS data.

Adsorption↗

PubMed2006

Competitive sorption and diffusion of chromate and sulphate in a flow system with goethite in gel beads.

Column experiments and model simulations were employed to evaluate the processes involved in multicomponent solute transport in a system with heterogeneous flow. Column experiments were performed with goethite embedded in polyacrylamide gel beads. The gel forms an immobile water region that can be accessed by diffusion. A two-region transport model with diffusion into spheres was combined with a surface complexation model to predict reactive transport in the goethite-gel bead system. Chromate and sulphate breakthrough curves were measured in a set of transport experiments, along with corresponding changes in the pH of the effluent. Sorption and transport of sulphate and chromate in separate columns were predicted from independently measured sorption parameters. The model overestimated the pH changes in the effluent, possibly because of proton buffering by the polyacrylamide gel. The effect of competitive sorption on transport was examined in experiments with both anions present. The model predicted the effect of competition very well in a system initially equilibrated with sulphate, followed by infiltration with chromate. However, when sulphate was infiltrated after equilibration with chromate, chromate desorption and sulphate adsorption were clearly overestimated by the transport model. The exchange between the more strongly bound chromate and the sulphate added subsequently may be too slow to cause a substantial chromate peak in the effluent. This suggests that the local equilibrium assumption was not applicable in this case.

Adsorption↗

PubMed2006

Inner- and outer-sphere complexation of ions at the goethite-solution interface.

Formation of inner- and outer-sphere complexes of environmentally important divalent ions on the goethite surface was examined by applying the charge distribution CD model for inner- and outer-sphere complexation. The model assumes spatial charge distribution between the surface (0-plane) and the next electrostatic plane (1-plane) for innersphere complexation and between the 1-plane and the head end of the diffuse double layer (2-plane) for the outersphere complexation. The latter approach has been used because the distance of closest approach to a charged surface may differ for different ions. The surface structural approach implies the use of a Three-Plane model for the compact part (Stern layer) of solid-solution interface, which is divided into two layers. The thickness of each layer depends on the capacitance and the local dielectric constant. The new approach has been applied to describe the adsorption of magnesium, calcium, strontium, and sulfate ions. It is shown that the concept can successfully describe the development of surface charge in the presence of Ca(+2), Mg(+2), Sr(+2), and SO4(-2) as a function of loading, pH, and salt level, and also the shift in the isoelectric point (IEP) of goethite. The CD modeling revealed that, for the conditions studied, magnesium is mainly adsorbed as a bidentate innersphere complex, calcium can be a combination of bidentate innersphere and a monodentate inner- or outer-sphere complexes, and strontium is probably adsorbed as an outersphere complex. Sulfate is present as a mixture of inner- and outer-sphere monodentate complexes. Outersphere complexation is less pH dependent than innersphere complexation. The CD model predicts that the outersphere complexation of divalent cations and anions is relatively favorable at respectively low and high pH. Increase of ion loading favors the formation of innersphere complexes.

Journal Article↗

PubMed2006

Effects of humic acid and competing cations on metal uptake by Lolium perenne.

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.

Adsorption↗

PubMed2005

Dynamic speciation analysis and bioavailability of metals in aquatic systems.

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.

Bacteria↗

PubMed2005

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions.

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.

Journal Article↗

PubMed2004

Surface complexation of carbonate on goethite: IR spectroscopy, structure and charge distribution.

The adsorption of carbonate on goethite has been evaluated, focussing on the relation between the structure of the surface complex and corresponding adsorption characteristics, like pH dependency and proton co-adsorption. The surface structure of adsorbed CO3(-2) has been assessed with (1) a reinterpretation of IR spectroscopy data, (2) determination of the charge distribution within the carbonate complex using surface complexation modeling, and (3) evaluation of the proton co-adsorption of various oxyanions, including carbonate, in relation with structural differences. Carbonate adsorption leads to a degeneration of the nu3 IR vibration. Currently, the magnitude of the Deltanu3 band splitting is used as a criterion for metal coordination. However, the interpretation is not unambiguous, since the magnitude of Deltanu3 is influenced by polarization and additional field effects, due to, e.g., H bonding. Our evaluation shows that for goethite the magnitude of band splitting Deltanu3 falls within the range of values that is representative for bidentate complex formation, despite contrarily assignments made in literature. Determination of the charge distribution (CD), derived by modeling available carbonate adsorption data, shows that a very large part (2/3) of the carbonate charge resides in the surface. Interpretation of this result with a bond valence and a ligand charge analysis strongly favors the bidentate surface complexation option for adsorbed carbonate. This option is also supported by the proton co-adsorption of carbonate. The H co-adsorption is very high, which corresponds closely to an oxyanion surface complex in which 2/3 of the ligands are common with the surface. The high H co-adsorption is in conflict with the monodentate option for adsorbed CO3(-2). The study shows that the H co-adsorption of CO3(-2) is almost equal to the experimental H co-adsorption obtained for SeO3(-2) adsorption, which can be rationalized supposing for both XO3(-2) complexes the same ligand distribution in the interface, i.e., bidentate complex formation.

Journal Article↗

PubMed2004

Adsorption of humic acid on goethite: isotherms, charge adjustments, and potential profiles.

The adsorption of natural organic matter (NOM) on mineral (hydr)oxide plays an important role in the evaluation of the speciation of toxic metal ions in the environment. Because both NOM and mineral oxide have variable charges that adjust upon adsorption, a good understanding of proton binding is required before the binding of metal ions can be understood. In this study, the adsorption of purified Aldrich humic acid (PAHA) on goethite was examined as a function of the environmental conditions (pH, salt concentration, and free concentration of PAHA) together with the proton adsorption to PAHA, goethite, and their mixtures. The induced charges on both components were separated on the basis of the difference between the charge/pH curves of the mixture and those of the single components. The electrostatic potential profile across the adsorbed layer was obtained as a numerical solution of the Poisson-Boltzmann equation using the charge density of the adsorbed PAHA and the goethite surface. From the quantitative evaluation of the induced charge on both components, it is revealed that the degree of the charge adjustment is related to the electrostatic affinity between the PAHA segments and the goethite surface, the electrostatic repulsion between the PAHA segments, and the electrostatic shielding by salt ions. Considering the charge distribution of the adsorbed PAHA at the goethite surface, it is concluded that the change of the charge adjustment is sensitive to that of the conformation of the adsorbed PAHA. From the detailed inspection of the assumptions made and the comparison with the reported theoretical calculations, the obtained potential profiles are considered to broadly reflect the true potential profiles. Because a charge adjustment is not frequently considered in detail in relation to the NOM adsorption on metal (hydr)oxides, the obtained results can form the basis for the further development of modeling of the adsorption of NOM on (hydr)oxide surfaces.

Journal Article↗

PubMed2004

Understanding the effects of soil characteristics on phytotoxicity and bioavailability of nickel using speciation models.

Acidity (pH) has been realized to be the most important soil characteristic that modulates bioavailability of heavy metals by affecting both the chemical speciation of metals in soil and the metal binding to the active sites on biota. In this work, we show that besides soil pH, metal bioavailability also depends to a certain extent on the type of soil. A better understanding of the role of soil type in regulating metal availability can be achieved with the analysis of soil composition and with calculations using chemical speciation models. Results of pot experiments, in which three different soils were spiked with nickel, show that the EC50 of total nickel in decreasing the biomass production of oats varies widely (0.7-22.5 mmol kg(-1) soil, more than 30 times). pH (4.7-7.0) is the most important factor, explaining up to a factor of 14 difference of nickel bioavailability in the soils. The remaining variation is caused by other differences in soil composition (soil type). The bioavailability and toxicity of nickel in the organic matter-rich soil studied is less than half of that in the sandy and clay soil studied at a similar pH. The chemical calculations using a multi-surface speciation model show that soil organic matter binds Ni much stronger than clay silicates and iron (hydr)oxides within the acidic pH range, which supports the experimental findings. In all three soils, the EC50 of Ni expressed in terms of Ni in 0.01 M CaCl2 soil extraction is rather stable (24-58 microM), suggesting the possibility to use this extraction as an estimation of metal availability in soil.

Aluminum Silicates↗

PubMed2004

Effect of synthetic iron colloids on the microbiological NH(4)(+) removal process during groundwater purification.

Subsurface aeration is used to oxidise Fe in situ in groundwater that is used to make drinking water potable. In a groundwater system with pH>7 subsurface aeration results in non-mobile Fe precipitate and mobile Fe colloids. Since originally the goal of subsurface aeration is to remove iron in situ, the formation of non-mobile iron precipitate, which facilitates the metal's removal, is the desired result. In addition to this intended effect, subsurface aeration may also strongly enhance the microbiological removal of ammonium (NH(4)(+)) in the purification station. Mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH(4)(+) removal process. Therefore, the objective of this study was to assess whether synthetic iron colloids could improve the NH(4)(+) removal process. The effect of synthetic iron colloids on the NH(4)(+) removal process was studied using an artificial purification set-up on a laboratory scale. Columns that purified groundwater with or without added synthetic iron colloids were set up in duplicate. The results showed that the NH(4)(+) removal was significantly ( alpha = 0.05 ) increased in columns treated with the synthetic iron colloids. Cumulative after 4 months about 10% more NH(4)(+) was nitrified in the columns that was treated with the groundwater containing synthetic iron colloids. The results support the hypothesis that mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH(4)(+) removal process.

Chemical Precipitation↗

PubMed2003

Generic NICA-Donnan model parameters for metal-ion binding by humic substances.

A total of 171 datasets of literature and experimental data for metal-ion binding by fulvic and humic acids have been digitized and re-analyzed using the NICA-Donnan model. Generic parameter values have been derived that can be used for modeling in the absence of specific metalion binding measurements. These values complement the previously derived generic descriptions of proton binding. For ions where the ranges of pH, concentration, and ionic strength conditions are well covered by the available data,the generic parameters successfully describe the metalion binding behavior across a very wide range of conditions and for different humic and fulvic acids. Where published data for other metal ions are too sparse to constrain the model well, generic parameters have been estimated by interpolating trends observable in the parameter values of the well-defined data. Recommended generic NICA-Donnan model parameters are provided for 23 metal ions (Al, Am, Ba, Ca, Cd, Cm, Co, CrIII, Cu, Dy, Eu, FeII, FeIII, Hg, Mg, Mn, Ni, Pb, Sr, Thv, UVIO2, VIIIO, and Zn) for both fulvic and humic acids. These parameters probably represent the best NICA-Donnan description of metal-ion binding that can be achieved using existing data.

Benzopyrans↗

PubMed2003

Phytotoxicity and bioavailability of nickel: chemical speciation and bioaccumulation.

The effect of pH on the bioaccumulation of nickel (Ni) by plants is opposite when using a nutrient solution or a soil as a growing medium. This paradox can be understood if the pH effect on the bioaccumulation, on the chemical speciation in the soil solution, and on the binding to the soil of Ni are all taken into account. Using simple equations to describe the individual relationships, it is possible to quantify these effects once the relationships have been established. Increased Ni uptake leads to reduced plant dry weight production for a certain growing period. The median effective concentration (EC50) decreased from 23 to 1.7 microM Ni in the nutrient solution for pH 4.0 to 7.0, whereas the EC50 of added Ni in a sandy soil increased from 0.72 to 9.95 mmol Ni/kg soil for pH 4.7 to 6.8. Bioaccumulation, binding to the soil solid phase, and binding to the dissolved organic matter all increase with increasing pH. However, the magnitude of the effect is the least for bioaccumulation as a function of pH, causing the apparent paradox.

Biological Availability↗

PubMed2002

Transport of humic and fulvic acids in relation to metal mobility in a copper-contaminated acid sandy soil.

The transport of inorganic and organic pollutants in water and soil can be strongly influenced by the mobility of natural dissolved organic matter (DOM). In this paper, the transport of a humic acid (HA) and a fulvic acid (FA) in a copper-contaminated acid sandy soil was studied. The data showed that the transport behavior of HA differed from that of FA. The breakthrough curves (BTCs) of HA were characterized by a rapid relatively sharp front followed by a plateau at a lower HA concentration than in the influent solution. The increase of the Ca concentration decreased the HA concentration further. Compared to HA, the BTCs of FA were retarded and showed an extended tailing, approaching complete breakthrough. The increase of the Ca concentration decreased the FA concentration only temporarily. On the basis of our model calculation, the characterization of HA transport could be explained by the coagulation of HA largely upon the binding of Al. The increase of the Ca concentration resulted in further coagulation of HA because of the increased Ca adsorption, which occurred mainly in the Donnan phase. For FA, the adsorption to the soil matrix was more likely the process that controls its solubility and mobility. The mobility of Al and Cu in the soil column was closely related to the solubility and transport of the DOM in soil solution. The concentration of Ca in the effluent was lower than in the influent because Ca was retained in the soil due to the retardation of HA and FA and due to the compensation of the other cations released from the soil to the solution.

Adsorption↗