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Changes in the nonlinearity of pyrene sorption isotherms on humic substance (HS)-coated minerals (kaolinite and hematite) due to HS adsorptive fractionation processes were examined in model environmental systems at low mass fraction organic carbon (f oc ) levels (0.0001–0.0011) using purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). At a constant pH of 7, higher molecular weight (MW) fractions of PAHA were preferentially adsorbed on kaolinite whereas no adsorptive fractionation of PAHA occurred on hematite. At a constant f oc level of 0.0005, preferential adsorption of higher MW PAHA fractions on kaolinite was enhanced with increasing pH. Nonlinear pyrene sorption isotherms were observed with the bulk PAHA-coated mineral systems, whereas more linear pyrene sorption isotherms were observed for the PAHA-mineral systems undergoing adsorptive fractionation. Although the degree of isotherm linearity may be affected by pH and/or structural rearrangement of the adsorbed HS fractions on minerals, this study suggests that HS adsorptive fractionation is more important than are changes in pH and f oc levels with regard to the resulting pyrene sorption isotherms. Similar effects were not observed with SRFA, suggesting that the impacts of HS adsorptive fractionation on pyrene sorption isotherm nonlinearity are also influenced by the source and other biogeochemical characteristics of HS. [ABSTRACT FROM AUTHOR]

Changes in pyrene binding by dissolved and mineral-associated humic substances (HS) due to I-IS adsorptive fractionation processes were examined in model environmental systems using purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). For PAHA, carbon- normalized pyrene binding coefficients for nonadsorbed, residual fractions (Koc(res)) were different from the original dissolved PAHA Koc value (Koc(orig)) prior to contact with the mineral suspensions. A strong positive correlation between pyrene log Koc(res) and log weight-average molecular weight (MWw) for residual PAHA fractions was observed, which was relatively independent of the specific mineral adsorbent used and hypothesized fractionation processes. A strong positive correlation between log Koc(ads) and log MWw was also found for PAHA fractions adsorbed to kaolinite at low mass fraction organic carbon levels, although the relationship was statistically different from the one found with residual PAHA fractions. The same trends and correlations found for PAHA were not observed with SRFA, suggesting that the impacts of HS adsorptive fractionation on changes in hydrophobic organic contaminants binding are also influenced by the source and other biogeochemical characteristics of HS. [ABSTRACT FROM AUTHOR]

Molecular weight (MW) fractionation of Suwannee River fulvic acid (SRFA) and purified Aldrich humic acid (PAHA) by adsorption onto kaolinite and hematite was investigated in equilibrium and rate experiments with a size-exclusion chromatography system using ultraviolet (UV) light detection. The extent of adsorptive fractionation based on UV detection was positively correlated with the percent carbon adsorption for both humic substances (HS), although the specific fractionation pattern observed depended on the particular HS and mineral used. Higher MW fractions of SRFA, an aquatic HS, were preferentially adsorbed to both kaolinite and hematite whereas the fractionation trends for PAHA, a terrestrial peat HS, differed for the two minerals. The contrasting fractionation patterns for SRFA versus PAHA can be explained reasonably well by the different structural trends that occur in their respective MW fractions and the underlying adsorption processes. Rate studies of adsorptive fractionation revealed an initial rapid uptake of smaller HS molecules by the mineral surfaces, followed by their replacement at the surface by a much slower uptake of the larger HS molecules present in aqueous solution. [Copyright &y& Elsevier]

Abstract: Changes in the characteristics of a leaf litter-derived humic substance (LLHS) that resulted from its adsorption onto kaolinite or exposure to simulated solar irradiation were tracked using selected spectroscopic descriptors, apparent weight-average molecular weight (MWw) and pyrene binding. Heterogeneity within the original bulk LLHS was confirmed by a range of different characteristics obtained from ultrafiltration-based size fractions. In general, trends of some changing LLHS characteristics were similar for the adsorption and irradiation processes when tracked against percent carbon removal. For example, the overall values of specific ultraviolet absorbance (SUVA), MWw, and humification index (HIX) all decreased with increasing irradiation time and with increasing concentration of mineral adsorbent in the respective experiments, indicating that both processes resulted in less aromatic and smaller-sized LLHS components remaining in solution. In addition, both the adsorption and irradiation experiments resulted in enrichment of the relative distribution of protein-like fluorescence (PLF), implying the PLF-related components had low affinities for phototransformation and mineral surface adsorption. Despite these apparently similar overall trends in LLHS characteristics caused by the adsorption and irradiation processes, closer examination revealed considerable differences in how the two processes altered the original material. Net production of intermediate-sized constituents was observed only with the irradiation experiments. In addition, residual LLHS resulting from the adsorptive fractionation experiments exhibited consistently higher pyrene binding versus the irradiated LLHS despite having comparable MWw values. Changes in LLHS characteristics due to adsorption by kaolinite were likely caused by physical mechanisms (primarily hydrophobic interactions between LLHS components and the kaolinite surface) whereas the irradiation-induced changes appear to have been governed by the combined effects of several alteration mechanisms, including the transformation of more condensed aromatic structures to less aromatic constituents, conformational changes resulting from selective photooxidation, and the photochemical disruption of intramolecular charge-transfer interactions. [Copyright &y& Elsevier]

Abstract: Real-time or near real-time in-situ monitoring of dissolved organic matter (DOM) composition in natural waters and engineered treatment systems provides critical information to water quality scientists and engineers, particularly when the monitoring techniques can provide some information about the chemical nature of DOM. The efficacy of various indices derived from rapid, low-cost spectroscopic and chromatographic techniques to discriminate DOM composition was tested for samples prepared from well-defined mixtures of purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). Sensitivities of the discrimination indices were examined by comparing (1) the differences between measured values and those predicted based from mass balance and the end member characteristics, and (2) the linear correlations between index values and mass ratios of the DOM mixtures. Size exclusion chromatography (SEC) results revealed that the weight-average molecular weight (MWw) may be a useful approach for tracking DOM mixing processes, although the number-average molecular weight (MWn) may be better for distinguishing different DOM compositions. Specific ultraviolet absorbance measured at 254nm (SUVA254) performed better as a discrimination index than did two previously recommended absorbance ratios, both in terms of making better predictions of intermediate compositions and in exhibiting a more linear correlation with PAHA mass ratio. Several well-defined peaks in the derivative absorption spectra (301 and 314nm for the first derivative, 217nm for the third derivative, and 211 and 224nm for the fourth derivative) also were found to be promising potential DOM discrimination indices. Finally, a fluorescence ratio based on humic- versus fulvic-like fluorescence proved to be a superior DOM discrimination index for the two DOM end members studied here. In general, this study illustrates the evaluation process that should be followed to develop rapid, low-cost discrimination indices to monitor DOM compositions based on end member mixing analyses. [Copyright &y& Elsevier]