Lars Lövgren

Acid/base reactions and Al(III) complexation at the surface of goethite

Acid/base reactions and Al(III) complexation at the goethite-solution interface have been investigated at 298.2 K in NaNO3 solutions at a constant ionic strength of 0.1 M. Equilibrium measurements were performed as potentiometric titrations using a glass electrode. The experimental data were evaluated on the basis of the constant capacitance model. The acid/base properties are described by the equilibria and the intrinsic equilibrium constants: FeOH + H+ ⇆ FeOH+2; log βs1,1,0(int) = 7.47 ±0.02, and FeOH ⇆ FeO− + H+; log βs−1,1,0(int) = −9.51 ± 0.04. The specific capacitance was determined to be 1.28 F m−2. Aluminium (III) complexation within the range 3.0 < −log [H +] < 8.5 results in the formation of the monodentate species Fe-O-Al-OH+ (log βs−2,1,1(int) = −1.49 ± 0.04) and Fe-O-Al-(OH)20 (log βs−3,1,1(int) = −9.10 ± 0.08). Whereas the acid base reactions were fully reversible, the aluminium desorption was found to be extremely slow and showed poor reversibility.

Competitive surface complexation of o-phthalate and phosphate on goethite (alpha-FeOOH) particles

Complexation of o-phthalate (1,2-benzenedicarboxylate) and competitive complexation of phosphate and phthalate at the goethite-water interface have been studied in 0.1 M Na(NO3) media at 298.2 K within the range 3.0 < -log [H+] < 8.5. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate and phthalate analyses. The binary and ternary chemical subsystems H+-goethite and H+-goethite-H2PO4- have been investigated earlier and described according to the constant capacitance model. The adsorption of phthalate showed a strong ionic strength dependence which indicated that phthalate is adsorbed as outer-sphere complexes. The experimental data in the subsystem H+-goethite-phthalate were evaluated on the basis of an extended constant capacitance model with the aid of the computer program FITEQL, version 2.0. One plane for inner sphere complexation and one plane for outer-sphere complexation, each with an associated constant capacitance, were included in the extended constant capacitance model. Surface complexation of phthalate is described by two outer-sphere complexes, =FeOH(2)(+)L(2-) and =FeOH(2)(+)HL(-). In the experiments with simultaneous complexation of phosphate and phthalate, the complexation of phosphate was not influenced by the presence of phthalate. On the other hand, the complexation of phthalate was decreased even at low phosphate concentrations. The equilibrium models determined for the subsystems were used to predict the adsorption of phosphate and phthalate in the quaternary system. It was found that these predictions were in good agreement with experimental titration and phosphate/phthalate adsorption data. Diffuse reflectance IR-spectra were recorded to obtain structural information of the phthalate complexes. The spectroscopic data did not contradict the outer-sphere model. However, because of the complexity of the phthalate molecule conclusive structural assignment could not be made. (Less)

Potentiometric and spectroscopic studies of sulfate complexation at the goethite-water interface

The complexation of sulfate ions at the goethite-water interface has been studied by equilibrium measurements at 298.2 +/- 0.5 K and by infrared spectroscopy. A unified set of data from potentiometric titrations and batch adsorption experiments were evaluated according to an extended Constant Capacitance Model. The obtained equilibrium model consists of two outer-sphere complexes: = FeOH2+, SO42- (log beta(1,1,1(intr)(out)) = 8.3 +/- 0.03) and drop FeOH2+, HSO4- (log beta(2,1,1(intr)(out)) = 13.5 +/- 0.07) and an inner capacitance, C-1, of 4.5 and an outer capacitance, C-2, of 1.8. The outer-sphere model was confirmed by the infrared data and adsorption experiments at different ionic strengths. Furthermore, the infrared measurements provided direct evidence for protonation of the sulfate ions at the interface. (Less)

Complexation of Pb(II) at the goethite (α-FeOOH)/water interface: The influence of chloride

The complexation of Pb(II) on a hydrous goethite surface has been investigated. Equilibrium measurements have been performed as potentiometric titrations at 298.2 K, within the range 2.7 < −log [H+] < 8.5. Three different ionic media were used: 0.1 mol dm−3 NaNO3, 0.1 mol dm−3 NaCl, and a mixture (1:1) of both these media. The evaluation of experimental data was made, using the constant capacitance model, within which a previously determined model for the acid-base reactions was included. The formation of the following complexes within the system H+ — FeOH − Pb2+, was found to describe the surface complexation: FeOHPb2+ (logβ0,1,1,0s(int) = 8.20 ± 0.06); FeOPb+(logβ−1,1,1,0s(int) = 0.17 ± 0.04); FeOPbOH(logβ−2,1,1,0s(int) = −8.85 ± 0.08). Within the four component system H+-FeOH — Pb2+ — Cl−, the following complexes were added to the model: FeOHPbCl+ (logβ0,1,1,1s(int) = 7.50 ± 0.08); FeOPbCl(logβ−1,1,1,1s(int) = −0.35 ± 0.16); FeOPbOHCl−(logβ−2,1,1,1s(int) = −8.00 ± 0.12). In the presence of an excess of Pb(II) ions to surface binding sites, the formation of a polynuclear surface Pb(II) species is postulated. The proposed model has been verified by independent determination of total soluble Pb (II) concentrations. Model calculations, using concentrations typical for natural waters, demonstrate a strong adsorption of Pb(II) even at low concentrations of surface binding sites. The experimental data show fully reversible reactions.

Equilibrium approaches to natural water systems—7. Complexation reactions of copper(II), cadmium(II) and mercury(II) with dissolved organic matter in a concentrated bog-water

Trace metal complexation by the dissolved organic matter in a concentrated bog-water has been studied using a potentiometric method (glass electrode). Bog-waters, sampled on five different times, were concentrated with a freezing-out technique and the complexation reactions involving Cu2+, Cd2+ and Hg2+ were investigated in 0.1 M ionic media. In the mercury(II) system, chloride was used as a competing ligand. Formation of binary CuL, CdL and HgL complexes was found. In the mercury(II) system a ternary HgH−1L complex was formed as well. The following values were obtained: log kCuL = 3.63 (Oct. 1983), 4.10 (Feb. 1984) and 3.99 (Sept. 1984); log kCdL = 2.84 (Jan. 1986); log kHgL = 10.37 and pKaHgL = 4.40 (Aug. 1985). Experimental data were evaluated using the least squares computer program LET-AGROPVRID. Distribution diagrams describing the speciation in experimental and natural conditions were constructed using the computer program SOLGASWATER.

Impact of water saturation level on arsenic and metal mobility in the Fe-amended soil

The impact of water saturation level (oxidizing-reducing environment) on As and metal solubility in chromium, copper, arsenic (CCA)-contaminated soil amended with Fe-containing materials was studied. The soil was mixed with 0.1 and 1 wt% of iron grit (Fe(0)) and 1, 7 and 15 wt% of oxygen scarfing granulate (OSG, a by-product of steel processing). Solubility of As and metals was evaluated by a batch leaching test and analysis of soil pore water. Soil saturation with water greatly increased As solubility in the untreated as well as in the Fe-amended soil. This was related to the reductive dissolution of Fe oxides and increased concentration of As(III) species. Fe amendments showed As reducing capacity under both oxic and anoxic conditions. The cytotoxicity of the soil pore water correlated with the concentration of As(III). The Fe-treatments as well as water saturation of soil were less significant for the solubility of Cu, Cr and Zn than for As. The batch leaching test used for waste characterization substantially underestimated As solubility that could occur under water-saturated (anaerobic) conditions. In the case of soil landfilling, other techniques than Fe-stabilization of As containing soil should be considered.

Limitations of the potentiometric titration technique in determining the proton active site density of goethite surfaces

Density of proton active surface sites at mineral surfaces is a property of fundamental importance in equilibrium modeling of surface complexation reactions. In this article, methods for an experim ...

Equilibrium approaches to natural water systems—6. Acid-base properties of a concentrated bog-water and its complexation reactions with aluminium(III)

Abstract A filtered bog-water, concentrated by means of a freezing technique, has been studied with respect to acid-base properties and aluminium(III) complexation reactions. Sampling was performed during autumn and winter periods with a resulting acidity or alkalinity due to oxic (autumn) or anoxic (winter) conditions. The measurements were performed as potentiometric titrations in constant ionic media (0.02, 0.1 and 0.6 M NaCl) with the use of a glass electrode. The samples show buffer ranges at 3 ⪅ pH ⪅ 5 and pH ⪆ 7.5 . The first is ascribed to the presence of carboxylate groups and is characterized by fast equilibria. The second is due to phenolic OH− groups and precipitation reactions with resulting sluggish equilibria. For one sampling period comprehensive measurements were undertaken to study the possible polyelectrolytic character of the organic acids. Due to the small increase in apparent carboxylate pKa-values with the degree of dissociation at low (0.02 M) as well as at high (0.6 M) ionic strength, the possible polyelectrolytic feature of the acids was neglected. Instead, a good fit to data was obtained by introducing a di-protic acid (H2L) as a model compound. Furthermore the medium dependence of the two acidity constants could satisfactorily be fitted to the expression: log k = log K + aI 1 2 /( 1 + I 1 2 ) + bI , where K is the constant at infinite dilution, a and b parameters of which b has been adjusted to present data. The following K values were obtained: pK1 = 3.65 and pK2 = 4.30. The complexation with Al(III) could be described by the formation of AlL+, AlL2− and the ternary species AlLH−1. The stability constants (log k1 = 4.4 (winter), 4.2 (autumn); log k2 = 4.2, 4.7; pKa (AlL+) = 4.2, 4.2) show no significant trend with sampling period but indicate a stability of the complexes greater than for phtalic acid but lower than for oxalic acid. Finally, the theoretical solubility of the clay mineral kaolinite in the presence of bog-water was computer modelled. The calculations show up on a 10-fold increase with respect to soluble aluminium at pH = 5.

POTENTIOMETRIC TITRATION OF UNBLEACHED KRAFT CELLULOSE FIBRE SURFACES

Abstract The surface properties of unbleached kraft cellulose fibres in the pH range 2–8 have been investigated using a high precision potentiometric titration method. The pulps were digested in various ways, varying both digestion time (level of residual lignin) and the amount of excess alkali. Titrations were made at constant ionic strength (0.1 M NaCl). The results were evaluated by assuming the existence of different types of acidic groups in the surface and adjusting their intrinsic stability constants and the capacitance of the electrical double layer to obtain the best possible fit to experimental data. Fitting was done using the error square sum minimising program fiteql , version 2.0. Distribution diagrams were calculated with the program solgaswater . It was found that the amount of acidic groups decreases with increasing digestion time, while the amount of excess alkali is not important. The pulps contain two types of acidic groups that dissociate in the pH range 2.5–7.5 (intrinsic stability constants pK≈3.4 and pK≈5.5). The pK value of the stronger acid agrees with what could be expected for uronic acids in hemicellulose (xylan), while the amount of the weaker acid correlates directly with the amount of lignin in the pulps.

Phosphate complexation at the surface of goethite

AbstractPhosphate complexation at the goethite-water interface has been studied in 0.1 M Na(NO3) medium at 298.2 K in the range 3.0 <—log[H+] < 9.0. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate analyses. The experimental data were evaluated on the basis of the constant capacitance model and with the aid of the computer program FITEQL, version 2.0.The acid/base properties of the goethite-water interface have been investigated earlier and are described by two intrinsic equilibrium constants logβs 1,1,0(int) = 7.47 and logβs-1,1,0(int) = −9.51 and with a specific capacitance of 1.28 F m−2. The binding between phosphate and goethite was found to be strong within the investigated -log[H+] range, and the amount of adsorbed phosphate is slowly decreasing with increasing—log[H+] value. The model describing the phosphate complexation is represented by the following equilibria and intrinsic constants: Further support for the proposed model is gi...