Werner Stumm

The interaction of anions and weak acids with the hydrous goethite (α-FeOOH) surface

Adsorption of F−,SO2−4, acetate, H2SiO2−4, PO3−4 and their conjugate acids on the surface of α-FeOOH (goethite) has been investigated with acid—base titrations and adsorption experiments. A surface complexation model is used to describe the dependence of the extent of adsorption on pH and other solution variables. A set of equilibrium constants has been determined which permits calculation of the amount bound to the surface and of the surface charge as a function of pH and total concentrations. The equilibrium constants for the adsorption of a series of ligands on the α-FeOOH surface can be correlated with the complex formation constants of the same ligands with Fe3+ in solution.

Interaction of Pb2+ with hydrous γ-Al2O3☆

Abstract Significant adsorption of Pb(II) on to hydrous γ-Al2O3 from dilute solutions is observed even at pH values far below the zero point of charge. The specific binding of Pb2+ on hydrous γ-Al2O3 in dilute electrolyte is interpreted as surface complex formation. The amphoteric properties of the coordinating surface are characterized in 0.1 M NaC1O4 in terms of a two-protic acid-base system (pHZPC = 8.3). The extent of Pb2+ interaction with the Al2O3 surface was determined both from measurements of residual [Pb2+] with a Pb2+ electrode and from the displacement of alkalimetric titration curves by the presence of Pb(II). The complex forming properties of the surface coordination reaction can be quantified by the equilibria: ≡ AlOH + Pb2+ ⇄ ≡ AlOPb+ + H+; 2 ≡ AlOH +Pb2+ ⇄ (≡ AlO)2Pb + 2H+. Hydrolysis need not be invoked to describe the interaction.

The cycling of iron in natural environments: Considerations based on laboratory studies of heterogeneous redox processes

The various pathways for the oxygenation of ferrous iron and for the dissolution of Fe(III) (hydr) oxides, especially by reducing ligands with oxygen donor atoms in thermal and photochemical processes, are assessed on the basis of laboratory experiments for application to natural systems. The typically large specific surface area of Fe-bearing solids in natural systems and the ability of these surfaces to interact chemically (surface complexation, ligand exchange) with reductants and oxidants facilitates electron transfer as well as dissolution and precipitation. Adsorption of Fe(II) to particle surfaces (complexation with surface hydroxyl groups) enhances the oxygenation rate of Fe(II) in a similar way as hydrolysis in solution (complexation with OH− ions). Surface processes (and not transport processes) control the dissolution kinetics. The rate of dissolution is proportional to the surface complexes formed on the surface of Fe(III) (hydr)oxides. Thus, a reductant, such as ascorbate, exchanges electrons with a surface Fe(III) ion subsequent to its inner-sphere coordination to the oxide surface. The Fe(II) thereby formed becomes more easily detached from the surface. Complex formation reactions of Fe(III) and Fe(II) with organic and inorganic ligands to form solute and solid complexes makes it possible that electron cycling of Fe(III)-Fe(II) transformations can occur over the entire EH range within the stability of water (EH from −0.5 V to +1.1 V). Solid and solute Fe(II) complexes with silicates, with hydrous oxides (e.g., Fe3O4), and with sulfides are very efficient reductants from a thermodynamic as well as from a kinetic point of view. Photosynthetic processes occurring on some inorganic Fe-bearing surfaces (semiconductors) and with iron species may be looked at as “primitive” alternatives or precursors to biological photosynthesis. In light-induced reductive dissolution of Fe(III) (hydr)oxides, dissolved Fe(II) (example: reduction of solid Fe(III) phases with an organic ligand such as oxalate) is formed. In the heterogeneous photoredox reaction, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficiency of the electron transfer.

The surface complexation of organic acids on hydrous γ-Al2O3

The specific adsorption of the aromatic acids, catechol, salicylic acid, benzoic acid, and phthalic acid, on γ-Al2O3 is interpreted in terms of a ligand exchange model where anions of the organic acids replace the surface hydroxo groups of the Al2O3 surface. Surface complex formation equilibrium constants and the type of surface species formed have been evaluated from measurements on the extent of adsorption and alkalimetric/acidimetric titration curves of the Al2O3 dispersion in presence and absence of the organic acids. Thus, the specific adsorption depends on the acid-base properties of the surface hydroxo groups, of the specifically adsorbable ligand, and of the affinity of the surface metal ion for the ligand. The tendency of the organic ligands to form surface complexes with Al2O3 is similar to that of organic ligands to form complexes with Al3+ in solution. It is possible to estimate from stability constants in solutions the tendency to form surface complexes with hydrous oxides and to predict with the help of surface equilibrium constants the extent of specific adsorption of ligands and its dependence upon pH and other solution variables.

Metal ion binding by biological surfaces: Voltammetric assessment in the presence of bacteria☆

Voltammetric techniques (differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV)) were evaluated for their capability to distinguish, without prior separation of the solid phase (e.g. filtration, centrifugation), between dissolved and particulate concentrations of Zn(II), Pb(II) and Cu(II), and to measure the extent of binding of these metals to the surface of a bacterium (Klebsiella pneumonia, formaldehyde treated). From titration curves of bacterial cell suspensions with metals the specific adsorption of metals was determined and quantified in terms of average surface complex formation constants and differential equilibrium functions. The following stability sequence for surface complexes was found: Cu2+ greater than Pb2+ greater than Zn2+ much greater than Ca2+. Simultaneous analytical determination permitted the measurement of both the binding of Cu(II) to the cell surface, and the binding to the solute exudate ligands. The affinity of the metal ions for the functional groups of the cell surface is strongly pH-dependent, and, at a given pH, decreases with increasing metal loading of the bacterial surfaces. This indicates that metal ions bind first to the highest affinity surface ligands and subsequently to those of lesser activity. Copper(II) appears to form stronger surface complexes with the high affinity ligands of the bacterial surface than with the functional groups of hydrous oxides.

Effect of organic acids and fluoride on the dissolution kinetics of hydrous alumina. A model study using the rotating disc electrode

Abstract The dissolution of aluminium oxide was studied with an oxide film covered rotating disc aluminium electrode. This allows us to make measurements under conditions of well defined mass transport under conditions representative of those found in natural waters (conc. of Al, organic acids and fluoride), and permits us to distinguish between surface-controlled and transport-controlled rates. Under steady-state conditions, the dissolution current is a direct measure of the flux of dissolving Al ions at the aqueous interface of the amorphous hydrous oxide film. At pH 3–6 and in presence of organic ligands, dissolution is controlled by a surface process, i.e. the rate of detachment of surface complexes. Fluoride ions in concentrations ≥ 10−6 M increase dramatically the dissolution rate: at pH = 4 the process is controlled by convertive diffusion of F− from the solution to the surface (kF- = (3.6 ± 0.5) × 10−2cms−1). Competitive and reversible adsorption of organic ligands (10−6 − 10−2M) displacing fluoride slows down the rate of detachment of the surface complex which becomes the rate-limiting step. The affinity of ligands for the Al2O3 surface sites increases in the sequence: formate ~ chloride ~ carbonate The results are compared with simulated weathering experiments and interpreted in terms of the surface complexation model.

Effects of root exudates and humic substances on weathering kinetics

The effect of complex natural organic ligands on the weathering kinetics of aluminum oxide was investigated in laboratory experiments. A peat-derived humic substance and root exudates obtained from ectomycorrhizal (Picea abies — Hebeloma crustuliniforme) and non-mycorrhizal Norway Spruce trees; and γ-Al2O3 were used as a model system. The experimental weathering rates are in accordance with a surface-controlled dissolution mechanism. The effect of the humic material on dissolution rates appears to depend on the degree of protonation of the humic (macro)molecules: we observed dissolution-enhancement or -inhibition at pH 3 and 4, respectively. Ectomycorrhizal exudates proved to be effective weathering agents at pH 4, as opposed to humic material and non-mycorrhizal exudates. Our results suggest that (i) the role of humic materials in mineral weathering and podzolization is different from what is commonly thought, and (ii) mineral weathering rates in the rhizosphere may be higher than in the bulk soil.

Coordinative interactions between soil solids and water — An aquatic chemist's point of view

Abstract Almost all the problems associated with understanding the rate processes that control the composition of our environment concern interfaces. Oxides, especially those of Si, Al, Fe and Mn are abundant components of the earths crust. The oxygen donor atoms present on the hydrous oxide surfaces tend to undergo protolysis and to form complexes with metal ions, and to become exchanged for other ligands (anions or weak acids). Many of these surface complexes are of an inner-sphere nature. The rates of processes occurring at the hydrous oxide surface, such as precipitation (heterogeneous nucleation on oxide surfaces) of minerals and dissolution of mineral phases — of importance in the weathering of rocks, in the formation of soils and sediments, in the corrosion of metals and their inhibition — are critically dependent on the coordinative interactions taking place on these surfaces. Rate laws showing dependence on the concentration of surface ligand complexes and on surface protonation are derived.

Interpretation and measurement of redox intensity in natural waters

Frevert deserves credit for proposing—for equilibrium systems—a distinction between a conceptually defined redox intensity, pε, and an operationally defined redox condition under stationary states, pe, as given by the response of a sensor electrode, and for pointing out that pε need not relate to pe.We would like to re-emphasize (1, 2, 3) that in defining a redox intensity, pε=−log{e}, we have treated the electron conceptually as a basic redox component which, as a species in aqueous solution, does not have an existence of its own. Morel (4) has elaborated on the use of the electron as a (phase rule) component in redox reactions. As he shows, it obviously can be treated equivalent to O2, i.e. O2=(H+)−4(e−)−4(H2O)2. We define (3) pε as “the hypothetical electron activity at equilibrium which measures the relative tendency of a solution to accept or transfer electrons”. This free energy change ΔG can be expressed as a redox potential (electrode potential) in volts (i.e., as a free energy change per mole of electrons associated with a given reduction). Electron activities may be defined in any equilibrium systems where the free activities of reductants {Red}, and oxidants {Ox}, are defined. Thus, pε (like pH) is a derivative form of free energy.Using electrons in redox reactions and as components does not at all imply that such electrons exist as species in waters. In the compilation of “Stability Constants” of the Chemical Society (London), Sillén and Martell (1964) treat the electron as an inorganic ligand and establish an electron activity scale that corresponds to the definition given.

Partikelgrössenverteilung und Natürliche Koagulation im Zürichsee

AbstractThe size distribution of suspended particles in Lake Zürich water shows always the same shape, irrespective of the total concentration of particles, depth or season. The particle size distribution can be described by a function of the form