Al(III) and Fe(III) binding by humic substances in freshwaters, 5 and implications for trace metal speciation

Available experimental data describing Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by 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 5 competition data involving Al(III) and 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. By adjusting 10 the model parameter characterising binding affinity within acceptable limits, and assuming equilibrium with a relatively soluble form of Fe(OH)3, the model can be made to explain data from 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 15 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 Al 3+ , AlOH 2+ being 20 dominant at pH >6; the principal bound form of Fe(III) is FeOH 2+ 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 25 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 “purified” humic matter. 21/01/08 E.Tipping et al. Al(III) and Fe(III) binding by humic substances in freshwaters / REVISION 3