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Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation.

Tipping, Edward and Rey-Castro, Carlos and Bryan, Stephen E. and Hamilton-Taylor, John (2002) Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation. Geochimica and Cosmochimica Acta, 66 (18). pp. 3211-3224. ISSN 0016-7037

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    Abstract

    Published experimental data for Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by the Humic Ion-Binding Model VI. The model is based on conventional equilibrium reactions involving protons, metal aquo ions and their first hydrolysis products, and binding sites ranging from abundant ones of low affinity, to rare ones of high affinity, common to all metals. The model can also account for laboratory competition data involving Al(III), Fe(III) and trace elements, supporting the assumption of common binding sites. Field speciation data (116 examples) for Al in acid-to-neutral waters can be accounted for, assuming that 60–70 % (depending upon competition by iron, and the chosen fulvic acid : humic acid ratio) of the dissolved organic carbon (DOC) is due to humic substances, the rest being considered inert with respect to ion binding. After adjustment of the model parameter characterizing binding affinity within acceptable limits, and with the assumption of equilibrium with a relatively soluble form of Fe(OH)3, the model can simulate the results of studies of two freshwater samples, in which concentrations of organically complexed Fe were estimated by kinetic analysis. The model was used to examine the pH dependence of Al and Fe binding by dissolved organic matter (DOM) in freshwaters, by simulating the titration with Ca(OH)2 of an initially acid solution, in equilibrium with solid-phase Al(OH)3 and Fe(OH)3. For the conditions considered, Al, which is present at higher free concentrations than Fe(III), competes significantly for the binding of Fe(III), whereas Fe(III) has little effect on Al binding. The principal form of Al simulated to be bound at low pH is Al3+, AlOH2+ being dominant at pH >6; the principal bound form of Fe(III) is FeOH2+ at all pH values in the range 4–9. Simulations suggest that, in freshwaters, both Al and Fe(III) compete significantly with trace metals (Cu, Zn) for binding by natural organic matter over a wide pH range (4–9). The competition effects are especially strong for a high-affinity trace metal such as Cu, present at low total concentrations (1 nM). As a result of these competition effects, high-affinity sites in humic matter may be less important for trace metal binding in the field than they are in laboratory systems involving humic matter that has been treated to remove associated metals.

    Item Type: Article
    Journal or Publication Title: Geochimica and Cosmochimica Acta
    Additional Information: The final, definitive version of this article has been published in the Journal, Geochimica Cosmochimica Acta, 66 (18), 2002, © ELSEVIER.
    Subjects: G Geography. Anthropology. Recreation > GE Environmental Sciences
    Departments: Faculty of Science and Technology > Lancaster Environment Centre
    ID Code: 487
    Deposited By: Dr John Hamilton-Taylor
    Deposited On: 28 Mar 2007
    Refereed?: Yes
    Published?: Published
    Last Modified: 26 Jul 2012 18:12
    Identification Number:
    URI: http://eprints.lancs.ac.uk/id/eprint/487

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