Staffan Sjöberg

Silica in aqueous environments

Abstract The aqueous chemistry of silica (SiO2) is regulated by a number of coupled processes such as dissolution/precipitation, complexation to cations and anions in the aqueous phase and complexation to cations and anions at the particle-water interface. Further, uptake by biota is significant in parts of the hydrosphere. The dissolution of silica is strongly pH-dependent in that silicic acid (Si(OH)4) is formed at pH values ≤ 9. In more alkaline solutions, monosilicates (SiO(OH)3−), SiO2(OH)22−) and polysilicates are formed. Composition and thermodynamic stability of these polysilicates are still not fully understood. The surface chemistry of silica has been extensively studied throughout the years. Based on recent surface complexation models, thermodynamic data describing acid/base properties, as well as cation (metal ion) and anion adsorption/desorption reactions have been presented. The acidic properties of a silica surface have been described by an acidity constant for surface SiOH groups with pKa(intr.) = 6.8. Further, surface complexation constants for several metal ions have been published showing a strong correlation between hydrolytic properties of the metal ion in aqueous solution and at the SiO2-water interface.

Modeling proton binding at the goethite (α-FeOOH)–water interface

The basic charging behaviours of goethite particles with different surfaces area (23, 37 and 85 m2 g-1) in 0.003-2.0 M NaNO3 were interpreted using surface complexation theory with the basic Stern ...

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.

Equilibrium and structural studies of silicon(iv) and aluminium(iii) in aqueous solution. V. Acidity constants of silicic acid and the ionic product of water in the medium range 0.05–2.0 M Na(CI) at 25°C

Abstract The apparent ionization constants for silicic acid, k 1 and k 2 , and the ionic product of water, k w , have been determined in 0.05, 0.1, 0.2, 0.4 and 2.0 M Na(CI) media at 25°C. The medium dependence of these constants was found to fit equations of the form log k i = log K i +a i I 1 2 (1+I 1 2 )+b i I where K 1 is the ionization constant in pure water, α i and b i are parameters of which b i has been adjusted to present data. The following results were obtained (α i , b i ): p K 1 = 9.84, (1.022, −0.11); p K 2 = 13.43, (2.044, −0.20); and p K w = 14.01 (1.022, −0.22). k i values are collected in Tables I and II. Attempts have been made to explain the medium dependence of k 1 and k 2 with weak sodium silicate complexing according to the equilibria Na + + SiO(OH) − 3 ⇌ NaSiO(OH) 3 ;k 11 Na + + SiO 2 (OH) 2 2 ⇌ NaSiO 2 (HO) − 2 ; k 21 giving k 11 = 0.37M −1 and k 21 = 3.0M −1 . However, these weak interactions cannot be interpreted unambiguously from potentiometric data at different 1 -levels. Probably the medium dependence could equally well be expressed by variations in the activity coefficients. The measurements were performed as potentiometric titrations using a hydrogen electrode. The average number of OH - reacted per Si(OH) 4 , Z, has been varied within the limits 0 ⩽ Z ⩽ 1.1 and B 1 , the total concentration of Si(OH) 4 , between 0.001 M and 0.008 M. k 1 was evaluated from experimental data with B ⩽ 0.003 M, and k 2 with B ⩽ 0.008 M and Z ≳ 0.95.

Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+– Cl–, OH–, CO32–, SO42–, and PO43– aqueous systems (IUPAC Technical Report)

This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic < 3.0 mol dm–3. Recommended values of log10βp,q,r° and the associated reaction enthalpies, ∆rHm°, valid at Im = 0 mol kg–1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10βp,q,r° values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO32– and PO43–. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

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)

Benzenecarboxylate surface complexation at the goethite (alpha-FeOOH)/water interface: II. Linking IR spectroscopic observations to mechanistic surface complexation models for phthalate, trimellitate, and pyromellitate

A study combining information from infrared spectroscopy and adsorption experiments was carried out to investigate phthalate, trimellitate, and pyromellitate complexes at the goethite (-FeOOH)/wate ...

Chemical speciation of environmentally significant metals with inorganic ligands Part 2: The Cu2+-OH-, Cl-, CO32-, SO42-, and PO43- systems (IUPAC Technical Report)

The numerical modeling of CdII speciation amongst the environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10βp,q,r° valid at Im = 0 mol kg–1 and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ε , required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆rH, are reported where available. Unfortunately, with the exception of the CdII-chlorido system and (at low ionic strengths) the CdII-sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log10 {[H+]/c°} < 6), in the absence of organic ligands (e.g., humic substances), CdII speciation is dominated by Cd2+(aq), with CdSO4(aq) as a minor species. In this respect, CdII is similar to CuII [2007PBa] and PbII [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 < –log10 {[H+]/c°} < 8.6, the speciation of CdII is still dominated by Cd2+(aq), whereas for CuII [2007PBa] and PbII [2009PBa] the carbonato- species MCO3(aq) dominates. In weakly acidic saline systems (–log10 {[H+]/cϒ} < 6; –log10 {[Cl–]/c°} < 2.0) the speciation is dominated by CdCln(2–n)+ complexes, (n = 1–3), with Cd2+(aq) as a minor species. This is qualitatively similar to the situation for CuII and PbII. However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of CdII because of the relatively low stability of CdCO3(aq). In contrast, the speciation of CuII [2007PBa] and PbII [2009PBa] in seawater is dominated by the respective species MCO3(aq). There is scope for additional high-quality measurements in the Cd2+ + H+ + CO32– system as the large uncertainties in the stability constants for the Cd2+-carbonato complexes significantly affect the speciation calculations.

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.

Antibacterial Effect of Zinc Oxide in Vitro

Antibacterial activity, zinc concentrations and pH were measured in Müller-Hinton broth containing different amounts of zinc oxide and inoculated with Staphylococcus aureus (10(6) colony forming units/ml). The minimum inhibitory concentrations (MIC) of zinc oxide to different clinical isolates were determined using the Müller-Hinton agar dilution tests. Gram-positive bacteria were most susceptible. Gram-negative aerobic bacteria and streptococci were usually not inhibited even at the highest concentrations used (1024 micrograms/ml), but staphylococci--particularly some isolates of Staphylococcus epidermidis--were sensitive enough to allow determination of their MIC.