Elsevier

Geochimica et Cosmochimica Acta

Volume 66, Issue 18, 15 September 2002, Pages 3211-3224
Geochimica et Cosmochimica Acta

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

https://doi.org/10.1016/S0016-7037(02)00930-4Get rights and content

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.

Introduction

Aluminium and iron are abundant and reactive elements, with a variety of geochemical and environmental chemical roles. In the dissolved and particulate phases of freshwaters, sediments and soils, they undergo significant interactions with natural organic matter (chiefly humic substances). In the case of Al, it has been shown that interactions with organic matter are central to the chemistry of the element in organic-rich soils (e.g., Tipping et al 1995, Berggren and Mulder 1995, Skyllberg 1999, and consequently to its transfer to surface waters. They also play a crucial role in determining Al toxicity (e.g., Driscoll et al., 1980). Dissolved iron–organic complexes and iron oxide–humic colloids are important in the transport and fate of the element in rivers and estuaries Sholkovitz and Copland 1981, Ross and Sherrell 1999. Adsorption of humic matter alters the surface chemistry and colloid stability of iron oxides (Tipping, 1986). Photochemical processes involving Fe and natural organic matter lead to the decomposition of DOM, the production of reactive species (superoxide anion, hydrogen peroxide, hydroxyl radical), and the generation of dissolved, bioavailable, ferrous iron (e.g., Collienne 1983, Stumm and Morgan 1996, Voelker et al 1997. Iron and humic matter influence phosphorus speciation in lakes (Jones et al., 1993).

Whereas the presence in freshwaters of dissolved complexes of (monomeric) Al with natural organic matter is well established Driscoll 1984, LaZerte 1984, the situation with regard to Fe(III) is less clear. According to Perdue et al. (1976) and Koenings (1976), soluble iron–humic complexes are significant components of surface freshwaters. Others Shapiro 1966, Cameron and Liss 1984 attribute the high apparent solubility of Fe in freshwaters to the presence of small iron oxide particles stabilized by adsorbed humic matter. Estimates of the distribution of Fe(III) between the organically complexed and colloidal oxide forms have been made by kinetic analysis, i.e., by determining the rate of conversion of Fe(III) to a spectroscopically detectable complex, following the addition of an excess of a suitable ligand. The starting species are then identified and quantified by fitting the kinetic data to a model in which each species has a rate constant in a defined range. Tipping et al. (1982) found that ca. 30% of the Fe(III) in the supernatant of a centrifuged lake water sample was present as a fast-reacting component, possibly organically complexed Fe. Sojo and de Haan (1991) reported that ca. 60% of the Fe(III) in a filtered (0.2 μm) neutral lake water (DOC 9 mg L−1) was present as organic complexes. In both studies, Fe(III) not complexed by organic matter was considered to be present as colloidal oxides.

The interactions of Al and Fe with humic substances affect not only the two metals, but also the organic matter itself. Their binding may alter the tendency of humic substances to aggregate and to adsorb to surfaces Ong and Bisque 1968, Tipping et al 1988b, Theng and Scharpenseel 1975, and may affect the binding of other metals. Thus, Al has been shown to compete with Eu Susetyo et al 1990, Bidoglio et al 1991, Pb Mota et al 1996, Pinheiro et al 2000, and Cd (Pinheiro et al., 2000), and competition by Fe(III) for Am and Cu has been proposed (Peters et al., 2001). In view of the high concentrations of Al and Fe(III), compared to those of trace metals, such competition effects may be highly significant in natural systems. Clearly, therefore, a full understanding of metal chemistry in natural waters needs to take into account the competitive reactions of Al, Fe, and other metals with humic matter.

The aim of the present work was to bring together existing and new information about Al and Fe binding by humic matter in freshwaters, and to examine how that binding will affect interactions with trace metals. As a framework, we have used Humic Ion-Binding Model VI, a discrete-site, electrostatic, model of cation–humic interactions, which has been parametrized with a large number of data sets from experiments with isolated humic substances (Tipping, 1998). Model VI permits knowledge gained from laboratory studies to be applied to field situations. An important assumption of the model is that all binding sites are available to all metals.

In the following text, square brackets indicate concentrations. Fulvic acid, humic acid, and humic substances are abbreviated by FA, HA, and HS, respectively, dissolved organic carbon and dissolved organic matter by DOC and DOM. The variable v is moles of metal bound per gram of humic matter or DOM. For simplicity, oxide phases are represented by Al(OH)3 and Fe(OH)3. The term “filterable” is used to indicate components that pass through a filter (typically with a pore size in the range 0.1–1 μm), and which might be classed as “dissolved” for some practical purposes. Such components will often include colloidal species, notably Fe(OH)3 and also Al(OH)3 and aluminosilicates.

Section snippets

Humic ion-binding model VI

The model was described in detail by Tipping (1998). It uses a structured formulation of discrete, chemically plausible binding sites for protons, in order to allow the creation of regular arrays of bidentate and tridentate binding sites for metals. Metal aquo ions (Ca2+, Fe3+, Cu2+, etc.) and their first hydrolysis products (CaOH+, FeOH2+, CuOH+, etc.) compete with each other, and with protons, for binding. The same intrinsic equilibrium constant is assumed to apply to the aquo ion and its

Isolation of humic acid

The procedure followed that of Swift (1996). One kilogram of peat was collected from a site in the Pennines (N. England), sieved, mixed with 1 liter of 0.1 M HCl and stirred overnight. The resulting suspension was centrifuged for 30 min at 4400 g, and the supernatant discarded. The pellet was mixed with 2 liters of 0.15 M NaOH and stirred overnight under an atmosphere of N2 in the dark. The basic soil suspension was centrifuged, the supernatant removed and acidified to pH ∼ 1 with 6 M HCl. The

Laboratory data for Al(III)

The application of Model VI to some laboratory data for Al–humic interaction has been described already (Tipping, 1998). Additional data have subsequently been published by Kinniburgh et al. (1999) for “purified peat humic acid” (PPHA). Model VI was applied by fixing all the default values (Table 1), except that the value of KMA for Al was optimized by least-squares minimization (Fig. 1). The fitting is successful in that the model provides the correct pH dependence, and the optimized log KMA

Discussion

Aluminium and iron have complex chemistries, and their binding reactions with humic substances have been subjected to relatively little experimental investigation. Humic substances themselves are complex and variable with respect to source. Consequently, our ability to predict equilibrium humic binding of Al and Fe in natural waters is imprecise. Nonetheless, the modelling results presented here, together with earlier findings, indicate a reasonably coherent picture of the interactions of the

Acknowledgements

We thank E.J.Smith and P.A.Stevens for collecting the peat sample, and the staff of the CEH Windermere Analytical Chemistry Laboratory for performing chemical analyses. C.Rey-Castro was supported by an FPU grant from the Spanish Ministerio de Educación, Cultura y Deporte, S.E.Bryan by a grant from the Freshwater Biological Association. The constructive comments of W.H. van Riemsdijk led to improvements in the paper.

Associate editor: P. A. Maurice

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