ArticleAl(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation
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
References (66)
- et al.
The role of organic matter in controlling aluminium solubility in acidic mineral soil horizons
Geochim. Cosmochim. Acta
(1995) - et al.
Complexation of Eu and Tb with fulvic acids as studied by time-resolved laser-induced fluorescence
Talanta
(1991) Observations on the use of iron(II) complexing agents to fractionate the total filterable iron in natural water samples
Water Res
(1984)- et al.
Copper binding by dissolved organic matterII. Variation in type and source of organic matter
Geochim. Cosmochim. Acta
(1988) - et al.
The stabilization of “dissolved” iron in freshwaters
Water Res
(1984) - et al.
The simultaneous speciation of aluminium and iron in a flow-injection system
Anal. Chim. Acta
(1995) - et al.
Constantes de formation des complexes hydroxydes de l’aluminium en solution aqueuse de 20 à 70°C
Geochim. Cosmochim. Acta
(1984) A practical guide to pH measurement in freshwaters
Trends Anal. Chem
(1990)- et al.
Testing a humic speciation model by titration of copper-amended natural waters
Environ. Int
(1998) - et al.
‘Acid rain’, dissolved aluminium and chemical weathering at the Hubbard Brook Experimental Forest, New Hampshire
Geochim. Cosmochim. Acta
(1981)
Ion binding to natural organic mattercompetition, heterogeneity, stoichiometry and thermodynamic consistency
Colloid Surf. A
Europium binding by fulvic acids
Anal. Chim. Acta
The solubility of iron hydroxide in sodium chloride solutions
Geochim. Cosmochim. Acta
An assemblage model for cation-binding by natural particulate matter
Geochim. Cosmochim. Acta
Complexation study of humic acids with cadmium(II) and lead(II)
Anal. Chim. Acta
Colorimetric flow-injection analysis of dissolved iron in high DOC waters
Water. Res
The coagulation, solubility, and adsorption properties of Fe, Mn, Cu, Ni, Cd, Co and humic acids in a river water
Geochim. Cosmochim. Acta
Aluminium binding to humic substances determined by high performance cation exchange chromatography
Geochim. Cosmochim. Acta
Some aspects of the interactions between particulate oxides and aquatic humic substances
Mar. Chem
WHAM—A chemical equilibrium model and computer code for waters, sediments and soils incorporating a discrete site/electrostatic model of ion-binding by humic substances
Comput. Geosci
Conditions required for the precipitation of aluminium in acidic natural waters
Water Res
The complexation of protons, aluminium and calcium by aquatic humic substancesa model incorporating binding-site heterogeneity and macroionic effects
Water Res
Humic substances in acid surface watersmodelling aluminium binding, contribution to ionic charge-balance, and control of pH
Water Res
Calibration of copper ion selective electrode response to pCu 19
Anal. Chem
Hydrolysis of Cations
Metal-ion binding to humic substances—application of the nonideal competitive adsorption model
Environ. Sci. Technol
Interactive influences of bioactive trace metals on biological production in oceanic waters
Limnol. Oceanogr
Synchronous fluorescence spectra of metal-fulvic complexes
Environ. Sci. Technol
Photoreduction of iron in the epilimnion of acidic lakes
Limnol Oceanogr
The chemistry of a typical Dutch reservoir, the Tjeukemeer, in relation to its water management between 1970 and 1986
Freshwater Biol
Trace metal chemistry of a Dutch reservoir, the Tjeukemeer
Freshwater Biol
A procedure for the fractionation of aqueous aluminium in dilute acidic waters
Int. J. Environ. Anal. Chem
Effect of aluminium speciation on fish in dilute acidified waters
Nature
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