Environ. Sci. Technol. 2005, 39, 6624-6631
Infrared Spectroscopic Evidence Supporting Heterogeneous Site Binding Models for Humic Substances DAVID G. LUMSDON* AND ANTHONY R. FRASER Catchment Management, Macaulay Land Use Research Institute, Aberdeen, AB15 8QH, United Kingdom
Infrared spectroscopy was used to corroborate predictions made by newly developed heterogeneous site binding models for humic substances. Experimental conditions to acquire the spectra of soil humic substances (humic and fulvic acid and a polysaccharide fraction) in an aqueous state using horizontal attenuated total reflectance Fourier transform infrared spectroscopy (HATR-FTIR) were established. Elimination of the water spectrum from that of the sample was achieved by spectral subtraction of the water peak at 2020 cm-1. A KSCN internal standard with an absorption band at 2067 cm-1 was used to verify the efficacy of the subtraction procedure. Spectral artifacts produced by the water spectrum subtraction and from contaminants within the humic materials have been identified. Three fulvic and one humic acid solution were examined in solutions of varying pH. Results show that the observed proportion of ionized carboxylate in relation to pH is consistent with models that assume electrostatic effects and a continuous distribution of proton association constants (log KH). The spectroscopic data were in accordance with calculations made using the generic humic and fulvic acid NICA-Donnan model parameters.
Introduction In soil and water, humic substances are important for influencing metal speciation and pollutant binding as well as buffering acidity and when present in the dissolved phase can facilitate the transport of pollutants and nutrients. Many of the chemical properties of humic substances in relation to cation binding and charge development depend on the type and quantity of acid organic functional groups associated with the humic molecules (1). Accordingly, most attempts to represent humic substances within geochemical speciation models involve descriptions of proton dissociation reactions and metal complexation reactions with organic functional groups (2-6). For most modeling, two types of approaches have been proposed. In the simplest one, the dissociation reactions of the organic acid functional groups are assumed to be discrete, and the protonation steps are modeled using a diprotic acid (2-3). A more sophisticated approach includes functional group heterogeneity in combination with electrostatics, and these models either describe the protonation reactions using a stepped series of several acid association * Corresponding author phone: +44-1224-498200; fax: +44-1224311556; e-mail:
[email protected]. 6624
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 17, 2005
constants (log KH) (e.g., WHAM (4)) or a continuous distribution function in which a median log KH value is defined together with a distribution parameter that determines the frequency distribution of the log KH values. A recent example of the continuous approach would be the NICA-Donnan model (5, 7). Both WHAM and the NICA-Donnan model have been parametrized using potentiometric titration data and have been used to aid understanding of pollutant behavior in soils (8, 9). To model proton binding by soil humic substances, the models assume a bimodal distribution of sites. The two binding sites of this distribution represent weak proton affinity sites that are believed to be carboxyl and those sites with stronger proton affinity that may be phenolic, although this assignment of functionality is unproven. However, it is not possible to determine via titration alone the functionality of these binding sites; for this, direct spectroscopic evidence is needed. A recent trend in the geochemical modeling literature has been to relate physical evidence from spectroscopic methods to the underlying structural assumptions of the models. This approach has been particularly successful for the surface complexation models that describe ion adsorption by mineral surfaces (10-12) but has not been attempted for humic models. Fourier transform infrared spectroscopy (FTIR) provides a possible tool to provide information on the protonation reactions of organic functional groups associated with humic molecules; however, to relate this information to aqueous speciation modeling calculations, it is essential to obtain spectroscopic data for humic substances in aqueous media. The aims of this study are to (i) report a method to obtain FTIR spectra of soil humic substances in aqueous media that are free from spectral artifacts, (ii) determine spectroscopically the relation between ionized carboxyl and pH, and (iii) use the spectroscopic data to test the hypothesis that models invoking binding site heterogeneity and electrostatics (e.g., NICA-Donnan) provide a physically more realistic model than those assuming a discrete acid approach.
Experimental Procedures Soil Organic Matter Fractions. Soil humic (HA) and fulvic acid (FA) samples were extracted from soils using the methods of the International Humic Substances Society (13). The soils were an organic surface (SO) horizon, a Bs horizon (SB) from a peaty podzol (Strichen Association), and a cultivated surface horizon (CA) from an iron humus podzol (Countesswells Association). The extraction of humic and fulvic acid and the purification of fulvic acid are described in detail elsewhere (14). Purification of the extracted humic acid consisted of dialysis (MWCO > 14 000) with high purity water to remove salt until the conductivity of the water remained constant. To lower the residual mineral content of the humic acid, it was treated with 0.3 M HF. The reduction of the mineral content was followed by monitoring the IR (infrared) spectra of aluminosilicate bands in freeze-dried humic acid subsamples examined in KBr disks. Finally, the humic acid preparations were freeze-dried. Ash contents were determined following combustion at 650 °C for 16 h. Elemental content of C and N was carried out using a Carlo Erba 1106 CHN analyzer (Table 1). The cultivated surface horizon from the Countesswells soil was fractionated to produce a polysaccharide using the procedure outlined by Cheshire et al. (15). Infrared Spectroscopy. For samples examined in KBr disks, samples of humic material, 0.8 mg was mixed with 170 10.1021/es050180i CCC: $30.25
2005 American Chemical Society Published on Web 07/23/2005
TABLE 1. Chemical Properties of Fulvic and Humic Acids
factor is described in eq 1
A ) (A + B) - Fsub(B)
elemental composition humic sample
%C
%N
ash Content %
SOFA SBFA CAFA SOHA
49.3 50.6 48.6 56.4
2.00 0.99 2.73 3.08
