Iso Christl

Relating Ion Binding by Fulvic and Humic Acids to Chemical Composition and Molecular Size. 2. Metal Binding

Binding of Cu(II) and Pb(II) to a soil fulvic acid, humic acid, and two different size fractions of the humic acid was investigated with metal titration experiments at pH 4, 6, and 8. Proton and free metal ion activities in solution were monitored after each titration step using pH and ion selective electrodes (ISE), respectively. The amounts of base required to maintain constant pH conditions were recorded and used to calculate stoichiometric proton-to-metal ion exchange ratios. Despite clear differences in chemical composition and protonation behavior, the fulvic acid and all humic acid fractions exhibited very similar metal binding behavior. Binding of Cu(II) and Pb(II) generally increased with increasing pH and total metal concentration. At low to moderate metal ion concentrations, Cu(II) was bound more strongly to the humic substances than Pb(II). Only at high free metal concentrations, the amounts of metal ions sorbed were higher for Pb(II) than for Cu(II). The molar proton-to-metal ion exchange ratios ranged from 1.0 to 1.8 for Cu(II) and from 0.6 to 1.2 for Pb(II), suggesting that Cu(II) was bound as monodentate and bidentate complexes, while Pb(II) was bound predominantly as monodentate complexes. The metal ion binding data were quantitatively described with the consistent NICA-Donnan model. The best description of an entire multicomponent data set consisting of proton titration, Cu(II), and Pb(II) binding data was achieved when the entire data set was fitted simultaneously. To reduce the number of fitting parameters, results from size exclusion chromatography and solid state 13C NMR spectroscopy were used to estimate two of the NICA-Donnan model parameters. The values of the remaining NICA-Donnan parameters for the humic substances are within a narrow range, suggesting that generalized model parameters may be useful in geochemical modeling involving humic substances.

COMPETITIVE SORPTION OF COPPER AND LEAD AT THE OXIDE-WATER INTERFACE : IMPLICATIONS FOR SURFACE SITE DENSITY

Abstract The competitive sorption of Cu(II) and Pb(II) to colloidal hematite was investigated as a function of pH and total metal concentration. Acid–base titrations of the hematite and single-metal sorption experiments for Cu and Pb at low to medium surface coverages were used to calibrate two surface complexation models, the triple layer model, and a 2-pK basic Stern model with ion-pair formation. The surface site density was systematically varied from 2 to 20 sites/nm2. Three different metal surface complexes were considered: (1) an inner-sphere metal complex; (2) an outer-sphere metal complex; and (3) an outer-sphere complex of singly hydrolyzed metal cations. Both models provided excellent fits to acid–base titration and single-metal sorption data, regardless of the surface site density used. With increasing site density, ΔpK of the stability constants for protonation reactions increased and metal surface complexes decreased steadily. The calibrated models based on different site densities were used to predict competitive sorption effects between Cu and Pb and single-metal sorption at higher total metal concentrations. Precipitation of oversaturated solid phases was included in the calculations. Best predictions of competitive sorption effects were obtained with surface site densities between 5 and 10 sites/nm2. The results demonstrate that surface site density is a key parameter if surface complexation models are exposed to more complex, multicomponent environments. We conclude that competitive metal sorption experiments can be used to obtain additional information about the relevant surface site density of oxide mineral surfaces.

Interaction of copper and fulvic acid at the hematite-water interface

The influence of surface-bound fulvic acid on the sorption of Cu(II) to colloidal hematite particles was studied experimentally and the results were compared with model calculations based on the linear additivity assumption. In the first step, proton and Cu binding to colloidal hematite particles and to purified fulvic acid was studied by batch equilibration and ion-selective electrode titration experiments, respectively. The sorption data for these binary systems were modeled with a basic Stern surface complexation model for hematite and the NICA-Donnan model for fulvic acid. In the second step, pH-dependent sorption of Cu and fulvic acid in ternary systems containing Cu, hematite, and fulvic acid in NaNO3 electrolyte solutions was investigated in batch sorption experiments. Sorption of fulvic acid to the hematite decreased with increasing pH (pH 3–10) and decreasing ionic strength (0.01–0.1 M NaNO3), while the presence of 22 μM Cu had a small effect on fulvic acid sorption, only detectable at low ionic strength (0.01 M). Sorption of Cu to the solid phase separated by centrifugation was strongly affected by the presence of fulvic acid. Below pH 6, sorption of Cu to the solid phase increased by up to 40% compared with the pure hematite. Above pH 6, the presence of fulvic acid resulted in a decrease in Cu sorption due to increasing concentrations of dissolved metal-organic complexes. At low ionic strength (0.01 M), the effects of fulvic acid on Cu sorption to the solid phase were more pronounced than at higher ionic strength (0.1 M). Comparison of the experimental data with model calculations shows that Cu sorption in ternary hematite-fulvic acid systems is systematically underestimated by up to 30% using the linear additivity assumption. Therefore, specific interactions between organic matter and trace metal cations at mineral surfaces must be taken into account when applying surface complexation models to soils or sediments which contain oxides and natural organic matter.

Competitive sorption of carbonate and arsenic to hematite: Combined ATR-FTIR and batch experiments

The competitive sorption of carbonate and arsenic to hematite was investigated in closed-system batch experiments. The experimental conditions covered a pH range of 3-7, arsenate concentrations of 3-300 μM, and arsenite concentrations of 3-200 μM. Dissolved carbonate concentrations were varied by fixing the CO(2) partial pressure at 0.39 (atmospheric), 10, or 100 hPa. Sorption data were modeled with a one-site three plane model considering carbonate and arsenate surface complexes derived from ATR-FTIR spectroscopy analyses. Macroscopic sorption data revealed that in the pH range 3-7, carbonate was a weak competitor for both arsenite and arsenate. The competitive effect of carbonate increased with increasing CO(2) partial pressure and decreasing arsenic concentrations. For arsenate, sorption was reduced by carbonate only at slightly acidic to neutral pH values, whereas arsenite sorption was decreased across the entire pH range. ATR-FTIR spectra indicated the predominant formation of bidentate binuclear inner-sphere surface complexes for both sorbed arsenate and sorbed carbonate. Surface complexation modeling based on the dominant arsenate and carbonate surface complexes indicated by ATR-FTIR and assuming inner-sphere complexation of arsenite successfully described the macroscopic sorption data. Our results imply that in natural arsenic-contaminated systems where iron oxide minerals are important sorbents, dissolved carbonate may increase aqueous arsenite concentrations, but will affect dissolved arsenate concentrations only at neutral to alkaline pH and at very high CO(2) partial pressures.

Reduction and reoxidation of humic acid: influence on spectroscopic properties and proton binding.

Previous studies on proton and metal binding to humic substances have not considered a potential influence of reduction and oxidation of functional groups. Therefore, we investigated how proton binding of a purified soil humic acid was affected by reduction. Reduction of the humic acid was carried out using an electrochemical cell that allowed us to measure the amounts of electrons and protons involved in reduction reactions. We further applied spectroscopic methods (UV-vis, fluorescence, FT-IR, C-1s NEXAFS) to detect possible chemical changes in the humic acid induced by reduction and reoxidation. The effect of reduction on proton binding was determined with acid-base titrations in the pH range 4-10 under controlled redox conditions. During reduction, 0.54 mol kg(-1) protons and 0.55 mol kg(-1) electrons were transferred to humic acid. NICA-Donnan modeling revealed an equivalent increase in proton-reactive sites (0.52 mol kg(-1)) in the alkaline pH-range. Our results indicate that reduction of humic acid increased the amount of proton-reactive sites by 15% compared to the untreated state. Spectroscopic differences between the untreated and reduced humic acid were minor, apart from a lower UV-vis absorption of the reduced humic acid between 400 and 700 nm.

Reduction and Reoxidation of Humic Acid: Influence on Speciation of Cadmium and Silver

Naturally occurring variations of redox conditions are considered to affect the interactions between trace metals and humic substances in a 2-fold manner. First, additional proton binding sites of humic substances formed under reducing conditions may also act as binding sites for trace metals. Second, reduced humic substances may transfer electrons to redox-active trace metals. In this study, we investigated the influence of electrochemical reduction of a purified soil humic acid on the binding of two chalcophile metal cations of environmental concern, Cd(2+) and Ag(+), with metal titrations conducted under monitored redox conditions. The binding of cadmium to reduced humic acid was slightly enhanced compared to humic acid reoxidized by O(2) and quantitatively in excellent agreement with the increase in binding sites formed upon reduction. Competitive experiments with calcium indicated that sulfur-containing sites played a minor role in cadmium binding, although sulfur K-edge XANES revealed that 36% of humic sulfur was in a reduced oxidation state. In all experiments with silver, the formation of Ag(0) was detected with transmission electron microscopy. Free Ag(+) activities under reducing conditions were controlled by Ag(0) formation rather than by binding to humic acid.

Physical and Chemical Characterization of Therapeutic Iron Containing Materials: A Study of Several Superparamagnetic Drug Formulations with the β-FeOOH or Ferrihydrite Structure

The effectiveness of therapeutically used iron compounds is related to their physical and chemical properties. Four different iron compounds used in oral, intravenous, and intramuscular therapy have been examined by X-ray powder diffraction, iron-57 Mössbauer spectroscopy, transmission electron microscopy, BET surface area measurement, potentiometric titration and studied through dissolution kinetics determinations using acid, reducing and chelating agents. All compounds are nanosized with particle diameters, as determined by X-ray diffraction, ranging from 1 to 4.1 nm. The superparamagnetic blocking temperatures, as determined by Mössbauer spectroscopy, indicate that the relative diameters of the aggregates range from 2.5 to 4.1 nm. Three of the iron compounds have an akaganeite-like structure, whereas one has a ferrihydrite-like structure. As powders the particles form large and dense aggregates which have a very low surface area on the order of 1 m2 g−1. There is evidence, however, that in a colloidal solution the surface area is increased by two to three orders of magnitude, presumably as a result of the break up of the aggregates. Iron release kinetics by acid, chelating and reducing agents reflect the high surface area, the size and crystallinity of the particles, and the presence of the protective carbohydrate layer coating the iron compound. Within a physiologically relevant time period, the iron release produced by acid or large chelating ligands is small. In contrast, iron is rapidly mobilized by small organic chelating agents, such as oxalate, or by chelate-forming reductants, such as thioglycolate.

Copper redox transformation and complexation by reduced and oxidized soil humic acid. 1. X-ray absorption spectroscopy study.

Natural organic matter (NOM) exerts strong influence on copper speciation and bioavailability in soils and aquatic systems. In redox-dynamic environments, electron transfer reactions between copper and redox-active moieties of NOM may trigger Cu(I) and Cu(0) formation. To date, little is known about Cu-NOM redox interactions and Cu(I) binding to NOM. Here, we present X-ray absorption spectroscopy results on copper redox transformations upon addition of Cu(II) or Cu(I) to untreated and electrochemically reduced soil humic acid (HA) under oxic and anoxic conditions. Both untreated and reduced HA mediated copper redox transformations. Under anoxic conditions, Cu(II) and Cu(I) added to reduced HA were primarily complexed and thereby stabilized as Cu(I)-HA at low loadings, whereas high copper loadings resulted in the additional formation of Cu(0) nanoparticles (16-64% of total copper). Cu(I) bound to HA was predominantly 2-fold coordinated and to a lower extent 3- to 4-fold coordinated, with a contribution of at least one nitrogen and/or sulfur ligand group. Under oxic conditions, Cu(II)-HA complexes prevailed, but smaller fractions of copper were also stabilized as Cu(I)-HA in a 3- to 4-fold coordination. Our results show that Cu-HA redox interactions are strongly affected by binding of Cu(II) and Cu(I) to HA and that HA contributes to the stabilization of Cu(I) against disproportionation.

Ionic strength- and pH-dependence of calcium binding by terrestrial humic acids

Environmental context In terrestrial environments, humic substances act as major sorbents for calcium, which is an essential nutrient for organisms. This study shows that calcium binding by terrestrial humic acids is strongly dependent on pH and ionic strength. The results indicate that calcium binding by humic acids is primarily controlled by electrostatic forces and specific binding to carboxylic groups. Abstract Calcium binding by two terrestrial humic acids was investigated at 25 °C as a function of pH, ionic strength and Ca2+ activity with calcium titration experiments. A Ca2+-selective electrode was used for Ca2+ measurements to cover a wide range of Ca2+ activities (10–8.5–10–2.5). Experimental data were quantitatively described with the NICA–Donnan model accounting for electrostatic and specific calcium binding. The results showed that calcium binding as a function of Ca2+ activity was strongly affected by variations of pH and ionic strength indicating that electrostatic binding is an important mechanism for calcium binding by humic acids. Data modelling providing a good description of experimental data for both humic acids suggested that electrostatic binding was the dominant calcium binding mechanisms at high Ca2+ activities often observed in terrestrial environments. Specific calcium binding being quantitatively predominant only at low Ca2+ activities was exclusively attributed to binding sites exhibiting a weak affinity for protons considered to represent mainly carboxylic groups. Since the negative charge of the humic acids being prerequisite for electrostatic calcium binding was found to be mainly due to deprotonation of carboxylic groups except under alkaline conditions, carboxylic groups were identified to primarily control calcium binding of humic acids.

Polymerization of silicate on hematite surfaces and its influence on arsenic sorption.

Iron oxides and oxyhydroxides are important sorbents for arsenic in soils, sediments, and water treatment systems, but their long-term potential for arsenic retention may be diminished by the formation of polymeric silicate on their surfaces. To study these interactions, we first investigated the sorption of silicate to colloidal hematite (α-Fe(2)O(3)) in short-term (48 h) and long-term (210 days) batch experiments. The polymerization of silicate on the hematite surface was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The pH dependence of silicate sorption exhibited a maximum between pH 9.0 and 9.5. The condensation of silicate on hematite surfaces adsorbed from monomeric silicate solutions steadily continued over the 210 day period, whereby surface polymerization was slower at pH 3 than at pH 6. The effect of silicate surface polymerization on arsenate and arsenite sorption was studied by use of hematite pre-equilibrated with silicate for different time periods of up to 210 days. The competitive effect of silicate on arsenate and arsenite sorption increased with increasing silicate pre-equilibration time. Only under strongly acidic conditions (pH 3), where silicate sorption was weakest and surface polymerization was slowest, was arsenate and arsenite sorption not affected by the presence of silicate. We conclude that the long-term exposure to dissolved silicate can decrease the potential of natural iron (oxyhydr)oxides for adsorbing inorganic arsenic.