Binding of P AHs from dissolved and adsorbed humic material was accomplished within 3 min for all samples; some reactions with dissolved humic substances appeared to equilibrate within 20 seconds. The binding of a P AH compound to Suwannee River humic material appears to depend on the size of the solute molecule and its ability to fit into hydrophobic cavities in the humic structure. A major effect of the solution chemistry was to alter the adsorption mechanisms of the organic substances, which in turn dictated the types of surface complexes formed.
Chapter 6
INTRODUCTION
Gain a molecular-level understanding of the mechanisms by which PAH compounds partition between dissolved humic material and water. Investigate the binding of a PA AH compound by an organically coated water oxide surface and determine what effect the nature of the surface coating has on the ability of the PA AH probe to associate with the surface. Gain a better understanding of the mechanisms of adsorption of a P AH compound on a bare mineral surface.
THE EFFECTS OF AQUEOUS CHEMISTRY ON THE BINDING OF
POLYCYCLIC AROMATIC HYDROCARBONS BY DISSOLVED HUMIC MATERIALS
ABSTRACT
It has been shown that the distribution of non-ionic hydrophobic organic compounds (HOC) between water and surface soils or sediments mainly depends on the hydrophobicity of the compound and the fraction of organic carbon ((,c) in the sorbent (Karickhoff et al., 1979 Means et al., 1980; Chiou et al. Several excellent reviews have been published on the relevance of hydrophobic distribution in environmental systems (McDowell-Boyer et al., 1986; Weber et al., 1991). the effects of the "quality" of natural organic matter (e.g., Garbini and Lion, 1986; Chiou et al., 1986; Gauthier et al., 1987), well-characterized humic materials were chosen as models to represent DOM.
FLUORESCENCE QUENCHING AND ENERGY TRANSFER
The Forster theory predicts that the Coulombic interaction for radiationless energy transfer will be most preferred if (Turro, 1978): a) there is a large overlap between the fluorescence emission spectrum of the excited donor molecule and the absorption spectrum of the acceptor molecule. Concentrated stock solutions were prepared by dissolving weighed amounts of the humic material in deionized, distilled water (D2H2O). The fluorescence emission spectrum of a pure substance is independent of the excitation wavelength (Parker, 1968).
RESULTS AND DISCUSSION
Fluorescence quenching results from the kinetic study of anthracene binding by humic acid were analyzed using equation (2.1). Regardless of pH, an increase in ionic strength resulted in a small decrease in the binding of anthracene by fulvic acid in NaCl solutions. At pH 4 humic acid solutions with Ca2+ showed a decreasing ability to bind anthracene compared to NaCl solutions of the same ionic strength.
In the presence and absence of DOM, measurements of the fluorescence intensity of perylene decreased exponentially with time according to equation (2.3). As the pH increases from 4 to 10 for constant ionic strength, a decrease in perylene binding is observed for all samples (Figure 1la). The presence of Ca2+ (1 mM) had little effect on the partition coefficient compared to NaCl solutions of the same ionic strength.
The polarity of the micellar core was observed to be approximately the same as that of benzene. However, few studies have been conducted on the effects of water chemistry on the distribution of nonpolar organic compounds with DOM. The characterization of humic material as an organic phase was the basis of the thermodynamic treatment proposed by Chiou et al.
Their modification included orientational and inductive force components of van der Waals forces in the calculation of the Flory-Huggins interaction parameter. Due to the competing reactions for perylene (binding by DOM and adsorption to walls), the kinetics of perylene partitioning was observed indirectly. It can only be estimated that the quantum yield of anthracene bound by humic acid will be less than or equal to that of the anthracene-fulvic acid complex.
SUMMARY AND CONCLUSIONS
Based on concepts similar to the molecular-level representation of Freeman and Cheung (1981), the model uses Flory-Huggins theory to estimate dissolved activity coefficients in the humic phase. The Flory-Huggins model is based on the idea that humic material forms "microscopic organic environments," similar to micelles, in which hydrophobic solutes can dissolve; the relatively low molecular weight humic substances from the Suwannee River are apparently too small to form such an organic phase. Pinckney (1989), Molecular Size and Weight of Fulvic and Humic Acids from the Suwannee River, in Humic Substances in the Suwannee River, Georgia: Interactions, Properties, and Proposed Structures, R.C.
Maccarthy (1989) Acid-base titration and hydrolysis of fulvic acid from the Suwannee River, in humic substances in the Suwannee River, Georgia: interactions, properties and proposed structures, R.C. Suffet (1983), Interactions between dissolved humic and fulvic acids and pollutants in aquatic environments, in Fate of Chemicals in the Environment, R.L. 1989), Estimation of the effects of dispersed organic polymers on the uptake of contaminants by natural solids. 1985), Geochemistry of humic substances in lake sediments, in humic substances in soil, sediment and water: geochemistry, isolation and characterization, GR. 1985), Intennolecular and Surface Forces, Academic Press, London.
Shiu (1977), Solubility of polynuclear aromatic hydrocarbons in water, 1 Chem. 1980), Solubility Behavior of Polycyclic Aromatic Hydrocarbons in Aquatic Systems, in Petroleum in the Marine Environment, L. Banwart (1980), Sorption of Polynuclear Aromatic Hydrocarbons by Sediments and Soils, Environ. 1981), Standards in Fluorescence, Chapman Hall, New York. Gschwend (1987), The Role of Colloids in the Partitioning of Solutes in Natural Waters, in Aquatic Surface Chemistry, W. Stumm, ed., Wiley-Interscience, New York.
1980), The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed., Wiley, New York. 1989), Nuclear Magnetic Resonance Spectrometry Investigations of Fulvic and Humic Acids from the Suwannee River, in Humic Substances of the Suwannee River, Georgia: Interactions, Properties, and Proposed Structures, R.C. 1985), Organic Geochemistry of Natural Waters, Martinus Nijhoff/.
THE ADSORPTION OF AQUATIC
HUMIC SUBSTANCES ON COLLOIDAL-SIZED ALUMINUM OXIDE PARTICLES
A STUDY OF THE
The presence of Ca2+ increased humic acid adsorption but had little effect on fulvic acid adsorption. In the environment, NOM adsorbs to particle surfaces and can dominate the properties of the solid-liquid interface. The samples were not stirred during the adsorption period to prevent particle coagulation.
For example, for the adsorption of humic substances on minerals, rm is assumed to represent the monolayer coverage of the solid by the humic material. Another factor in the effect of pH on oxyanion adsorption is the p-value of the conjugated Bnmsted acid. For example, Stone (1989) studied the hydrolysis of the anionic carboxylic acid ester monophenyl terephthalate (MPT) in the.
Instead, a hydrophobic binding mechanism appears to be at work, accounting for at least a fraction of the adsorption. Despite the increase in humic acid adsorption, Langmuir-type isotherms were observed in the presence of Ca2+ in all studies, contrary to the observations of Tipping (1981). At low NaCl concentrations, the number of carboxyl groups present in the adsorbed layer for humic acid is ~3.3 COO"/nm2. The majority of these carboxyl groups are most likely bound directly to the alumina surface via complexes in the inner sphere, because the concentration of NaCl is not expected to promote adsorption mechanisms other than ligand exchange.
Qualitative interpretations of the adsorption processes were made using the isothermal Langmuir ~ds and rm· parameters. Other differences in humic and fulvic acid adsorption were: (1) fulvic acid adsorption showed more pH dependence than humic acid; (2) the affinity of aluminum for humic acid was much greater than for fulvic under identical solution conditions; and (3) the presence of Ca2+ at pH 4 increased humic acid adsorption but had little effect on fulvic acid adsorption. From the adsorption data, it appears that a maximum of 3.3 carboxyl groups per nm2 bind directly as inner-sphere complexes from the surface of the alumina particles used here.
BINDING OF A FLUORESCENT HYDROPHOBIC ORGANIC PROBE BY DISSOLVED
HUMIC SUBSTANCES AND ORGANICALLY- COATED ALUMINUM OXIDE SURFACES
The presence of Ca2+ had little effect on perylene binding by the dissolved humic substances. The ability of weakly adsorbed humic acid to bind perylene, and possibly many of its other properties, approaches that of the dissolved species. The importance of the organic carbon content of sediments and soils suggests that several components of natural organic matter (NOM) bind nonionic HOC in solution and on mineral surfaces.
The combined results of these studies indicate that the binding of hydrophobic organic pollutants to DOM depends on the chemical and structural characteristics (i.e., "quality") of the DOM and may depend on the chemistry of the water in the system. In the environment, organic material adsorbs to particle surfaces and can dominate the properties of the solid-liquid interface. Analysis and characterization of the organic material in solution versus that on the particle surface was not done, nor was the fractional binding of perylene between dissolved and adsorbed organic material determined.
An important objective of this study was to distinguish between binding of the hydrophobic probe to humic substances in solution versus humic material adsorbed on the alumina surface. There were concerns that the separation of humic substances adsorbed on the aluminum surface from that remaining in solution would upset the equilibria and lead to variable estimates of binding constants, similar to that observed in the partitioning of equilibrated contaminants between two phases (Gauthier et al., 1986). . To elucidate the effects of water chemistry, in addition to the effects of natural organic material quality (e.g., Gauthier et al., 1987; Garbini and Lion, 1986; Chiou et al., 1986), well-characterized humic materials were selected. as models to represent NOM.
Aquatic humic substances make up 40 to 60 percent of the dissolved organic carbon, and make up the largest fraction of NOM, in water (Thurman, 1985).
CJI12
Chemical properties of humic acid and fulvic acid are shown in Table II, and some physical properties are listed in Table III. Because the received humic materials are very hygroscopic, they were stored in a desiccator until use. The solutions were mixed and allowed to stand in the dark for at least 24 hours.
The humic stock solutions were filtered through pre-washed polycarbonate Nuclepore filters with 0.2 µm pores to remove potential particulate contamination and then stored in amber borosilicate glass bottles, similar to the perylene solution. Absorption spectra and measurements at the wavelength of 250 nm were taken from the humic acid and fulvic acid stock solutions before and after filtering to check for possible loss and/or fractionation of material. Alumina C is produced by flame hydrolysis of anhydrous aluminum chloride and De bye-Scherrer X-ray diffraction patterns show that it has primarily a gamma structure with a slight delta structure.
The chemical purity is > 99.6% and the density is 2.9 g/cm3• Pretreatment and preparation cf. parent particle suspensions are reported in chapter 3.
