Gerhard Furrer

The coordination chemistry of weathering: I. Dissolution kinetics of δ-Al2O3 and BeO

Abstract The dissolution kinetics of most slightly soluble oxides and silicates are controlled by chemical processes at the surface. The reaction controlling steps can be interpreted in terms of a surface coordination model. In dilute acid solutions, in the absence of complex-forming ligands, the dissolution kinetics are controlled by the surface bound protons. The rate of the proton-promoted reaction of δ - Al 2 O 3 is R H = k H ( C H 3 ) 3 where C h 3 is the proton concentration per unit area on the oxide surface. The mechanism can be described by the attachment of three protons to the reaction site prior to the detachment of an Al species into the solution. The dissolution rate of BeO is proportional to (C H 2 ) 2 . For δ - Al 2 O 3 at pH ⩽ 3.5 dissolution rate is independent of pH; at this pH maximum surface concentration of protons is reached. The organic ligand-promoted dissolution, R L , is of first order with respect to concentration of surface chelates: R L = k L {M L} where {M L} is the concentration of surface chelates per unit area. Detachable surface complexes result from surface coordination of metal ions of the hydrous oxides with bidentate ligands. Especially efficient are bidentate ligands that form mononuclear surface complexes. The sequence of rate constants shows that five- and six-membered chelate rings (oxalate, catechol, malonate and salicylate) enhance the dissolution reactions to a greater extent than seven-membered rings (phthalate, succinate). Monodentate ligands (benzoate ion), though readily adsorbed, do not enhance dissolution rates. However, they can inhibit dissolution by displacing ligands that catalyze this reaction.

The coordination chemistry of weathering: II. Dissolution of Fe(III) oxides

The dissolution of goethite and other iron oxides is kinetically controlled by the detachment of an Fe center from the surface. Surface protonation and surface complex formation with ligands are reaction steps preceding the detachment. The dissolution rate is enhanced markedly by oxalate which can form bidentate surface chelates with an Fe center in the oxide surface. An additional catalytic effect is observed under reducing conditions. The electron transfer from the reductant to the surface Fe(III) center precedes the detachment step. This may occur either by direct reduction, subsequent to the adsorption of reductants, or, indirectly by electron transfer from dissolved Fe(II) ions which are generated by the same reductants in solution.

Weathering-associated bacteria from the Damma glacier forefield: physiological capabilities and impact on granite dissolution.

ABSTRACT Several bacterial strains isolated from granitic rock material in front of the Damma glacier (Central Swiss Alps) were shown (i) to grow in the presence of granite powder and a glucose-NH4Cl minimal medium without additional macro- or micronutrients and (ii) to produce weathering-associated agents. In particular, four bacterial isolates (one isolate each of Arthrobacter sp., Janthinobacterium sp., Leifsonia sp., and Polaromonas sp.) were weathering associated. In comparison to what was observed in abiotic experiments, the presence of these strains caused a significant increase of granite dissolution (as measured by the release of Fe, Ca, K, Mg, and Mn). These most promising weathering-associated bacterial species exhibited four main features rendering them more efficient in mineral dissolution than the other investigated isolates: (i) a major part of their bacterial cells was attached to the granite surfaces and not suspended in solution, (ii) they secreted the largest amounts of oxalic acid, (iii) they lowered the pH of the solution, and (iv) they formed significant amounts of HCN. As far as we know, this is the first report showing that the combined action of oxalic acid and HCN appears to be associated with enhanced elemental release from granite, in particular of Fe. This suggests that extensive microbial colonization of the granite surfaces could play a crucial role in the initial soil formation in previously glaciated mountain areas.

Cation adsorption on oxides and clays: The aluminum case

The sorption mechanisms for trace metal ions on montmorillonite have been investigated. Complexation with surface hydroxyl groups located on the brocken edges of platelet particles is found to occur over a pH range similar to that observed on silica and other oxides, at comparable metal/site ratios. A second mechanism involving cation exchange on the negatively charge basal plane, which does not involve proton exchange in our experimental conditions, has been invoked to explain the low pH behavior. Consistent with this cation exchange mechanism, adsorption at low pH is strongly ionic strength dependant. A quantitative model which involves both mechanisms is presented and tested against both cation and proton adsorption data.

The Effects of Complex-Forming Ligands on the Dissolution of Oxides and Aluminosilicates

The composition of natural waters is controlled to a significant extent by processes at the solid (particle)/water interface, above all by the dissolution and precipitation of minerals.

Microbial reactions, chemical speciation, and multicomponent diffusion in porewaters of a eutrophic lake

Abstract A mathematical model for the study of early diagenesis in lake sediments is presented. The computer program STEADYQL has been extended into a multibox model that allows the calculation of porewater profiles at steady state. The model includes microbial and geochemical reactions, inorganic chemical speciation of the dissolved components, and diffusion of the individual species between the boxes. The application of the approach to a set of porewater data from the eutrophic Lake Sempach (Switzerland) yields the following results. The assumption of solubility equilibrium with respect to calcite produces a pH profile that is in close agreement with the measurements. The model predicts calcite precipitation in the zone of Fe reduction. The contributions of methanogenesis, sulfate reduction, denitrification, and Fe reduction to anaerobic mineralization are calculated as 47, 27, 17, and 9%, respectively. Based on the assumptions of Redfield ratios, the model predicts the profiles of ammonium and phosphate remarkably well. The analysis of the diffusion pathways shows that 35% of the Fe(II) flux across the sediment-water interface occurs in the form of inorganic complexes. However, effects of complex formation on diffusive fluxes are smaller than the experimental error of the measured chemical gradients.

Siderophores, NTA, and citrate: potential soil amendments to enhance heavy metal mobility in phytoremediation.

ABSTRACT Phytoremediation of heavy metal-contaminated sites is often limited by the low bioavailability of the contaminants. Complexing agents can help to improve this technique by enhancing heavy metal solubility. We investigated the effect of three organic chelating agents, that is, the siderophore desferrioxamine B (DFOB), nitrilotriacetate (NTA), and citrate on binding of Cu, Zn, and Cd by either Namontmorillonite, kaolinite, or goethite. The different effects of the complexing agents on metal sorption can mainly be explained by the differences in stability constants and surface charge characteristics. In the presence of clay minerals, NTA was the most efficient ligand with respect to mobilization of heavy metals. In goethite suspensions, the effect of DFOB was more pronounced. In all systems, Cu proved to be the most affected element by the presence of the ligands. In batch experiments with heavy metal-contaminated soils from field sites, NTA was the most efficient metal mobilizer.

On the chemistry of the keggin Al13 polymer: kinetics of proton-promoted decomposition

Abstract The decomposition of the polynuclear Keggin Al13 species (Al13O4(OH)24(H2O)127+ or Al13) has been examined as a function of pH (2.06 ≤ pH ≤ 3.50), ionic strength (I = 0.1 and 1.0) and temperature (10°C ≤ T ≤ 65°C) using batch and flow-through reactors. The overall decomposition rates were found to be positively correlated with the activity of H+, ionic strength and temperature with half-lives ranging from 350 to 43,000 s. The decomposition rate was interpreted as a function of two parallel reactions, one first-order and one second-order with respect to [H+] − d[A1 13 ] dt =R 1 +R 2 (1) where R1 = k1 [H+] [Al13] and R2 = k2 [H+]2 [Al13]. For 25°C and I = 0.1, the rate constants k1 and k2 were determined as 0.0333 ± 0.0016 M−1 s−1 and 2.59 ± 0.62 M−2 s−1, respectively. The activation energies and the Arrhenius factors for an ionic strength of 1.0 were found to be Ea1 = 13.3 ± 1.9 kJ mol−1, Ea2 = 44.9 ± 4.9 kJ mol−1, A1(I=1.0) = 25.2 ± 19.2 M−1 s−1, A2(I=1.0) = 1.62 109 ± 3.12 109 M−2 s−1. While Ea1 and Ea2 do not depend on the ionic strength, the Arrhenius factors for I = 0.1 were obtained as A1(I=0.1) = 7.23 ± 0.19 M−1 s−1 and A2(I=0.1) = 1.86 108 ± 1.9 107 M−2 s−1. The apparent charge of the Al13 complex in the encounter reaction with a hydronium ion was calculated from the Arrhenius factors at various ionic strengths as 0.78. We postulate Al13 with a protonated bridging OH group as the precursor for the decomposition reaction, first-order in [H+]. The low value of the activation energy for the one-proton pathway is explained by an exothermic formation of the precursor HAl138+. The larger value of Ea2 indicates that the formation of a precursor with two adjacent protonated sites involves a positive reaction enthalpy. The half-life of several hundred hours at pH 5 indicates that Al13 may exist in natural waters, however, it may also be subject to continuous decomposition and reformation.

Pioneering fungi from the Damma glacier forefield in the Swiss Alps can promote granite weathering.

Fungi were isolated from fine granitic sediments, which were collected at 15 sampling points within a 20 m × 40 m area in front of the Damma glacier in the central Swiss Alps. From the 45 fungal isolates grown on nutrient-rich agar media at 4 °C, 24 isolates were selected for partial sequencing and identification based on the small subunit ribosomal DNA. Sequencing data revealed that the isolated fungi represented three fungal phyla and 15 species. The weathering potential of 10 of the 15 fungal species was tested with dissolution experiments using powdered granite material (<63 μm). The results showed that the zygomyceteous species Mucor hiemalis, Umbelopsis isabellina and Mortierella alpina dissolved the granite powder most efficiently due to the release of a variety of organic acids, mainly citrate, malate and oxalate. In particular, the high concentrations of Ca, Fe, Mg and Mn in the solutions clustered well with the high amounts of exuded citrate. This is the first report on fungi that were isolated from a non-vegetated glacier forefield in which the fungis capabilities to dissolve granite minerals were examined.

Comparison of NTA and Elemental Sulfur as Potential Soil Amendments in Phytoremediation

The effect of NTA (nitrilotriacetic acid) and elemental sulfur (S), two soil amendments suggested for the enhancement of metal phytoavailability in phytoextraction, on heavy metal uptake by Nicotiana tabacum (tobacco) and Zea mays (maize) were studied and compared in two Zn-, Cu-, Cd -, and Pb-contaminated soils from northern Switzerland. Experiments were performed in the greenhouse with topsoil (0 to 20 cm) material from two locations, Dornach and Rafz. The Dornach soil was calcareous and had been contaminated by dust emissions from a nearby brass metal smelter. The Rafz soil, free of carbonates, had been polluted by former sewage sludge application. Soil amendments with S increased the solubility (NaNO3 extraction) of Zn and Cd about 10-fold in Dornach soil and up to 30-fold in Rafz soil after 55 days. Zn and Cd removal by N. tabacum and Z. mays, however, increased only about 5.5- and 2.5-fold in these treatments in Rafz soil, respectively, while in the Dornach soil only a slight increase for Cd was found. Repeated NTA application increased soluble Zn, Cu, and Cd about 100-, 20-, and 19-fold in the Dornach soil and 13-, 4-, and 2-fold in the Rafz soil shortly after application. Soluble Pb was increased by NTA up to 50-fold in Rafz soil. After 90 days soluble heavy metal concentrations were only slightly elevated in both soils. Again, however, Zn, Cd, and Cu removal by N. tabacum and Z. mays increased only about 1.5- to 2.5-fold in the two soils, whereas Pb removal by N. tabacum increased about fivefold in the Rafz soil as a result of NTA application