The effects of pH changes on the absorbance spectra of fulvic acids, which constitute a major part of natural organic matter (NOM) were examined based primarily on the example of standard Suwannee River fulvic acid (SRFA). Differential absorbance spectra of SRFA generated at varying pH values were determined to be rich in features that are absent in the conventional spectra. Analysis of the differential absorbance revealed the existence of two predominant types of pH-active chromophores which produced features that emerged in the pH ranges associated with carboxylic and phenolic functional groups of fulvic acids. Comparison of the differential absorbance spectra of SRFA and those of the fraction of hydrophobic acids of South Platte River fulvic acid show the existence of differences indicative of fulvic acids's site-specificity. The evolution of the pH-differential spectra of fulvic acids was in accord with predictions made based on the NICA-Donnan theory. These results show that pH-differential absorbance spectroscopy can be used to quantify in situ the protonation and site-specificity of fulvic acids.
Experiments with immobilized lead dioxide showed that this solid was reduced by natural organic matter (NOM) isolated from Potomac River water. Kinetically, the process was slow and occurred throughout many weeks of exposure. The amount of mobilized lead was affected by the concentration of NOM and exposure time but not significantly influenced by the type of NOM used in the experiments. The interactions of NOM with PbO2 were quantified using differential absorbance spectroscopy. It showed that the oxidation of chromophoric groups in NOM was strongly correlated with lead release. Because lead release yields were higher thatthose predicted based on the depletion of the aromatic groups, it is hypothesized that NOM moieties otherthan aromatic functionalities are engaged in the reduction of PbO2 by NOM and/or lead mobilization involves the formation of mixed Pb(II)/Pb(IV) soluble and colloidal species.
The deprotonation-protonation properties of dissolved organic matter (DOM) in drinking water produced at critical treatment points were quantified using absorbance spectra in combination with DOM fractionation data. Analysis of differential spectra of DOM present in inlet, settled and filtered waters from two large treatment plants and their fractions were obtained. The data demonstrated the presence of six Gaussian bands largely associated with carboxylic and phenolic DOM functionalities. Properties of the protonation-active groups of DOM in raw and treated waters were further examined based on data of potentiometric titrations at pH from 2.5 to 10. Interpretation of the differential log-transformed absorbance at wavelength 350 nm (DlnA350) based on the NICA-Donnan model showed that the normalized concentrations of low- and high-affinity protonation-active groups in residual DOMs increases as a result of water treatment. This was consistent with the results of DOM fractionation. This study demonstrates that changes of the composition and reactivity of DOM found in drinking water treatment sequences can be quantified based on the examination of their optical properties.
This study examined the effects of changes in pH and copper concentration on the absorbance spectra of natural organic matter (NOM) from a reservoir in Western Australia. Differential absorbance spectra generated for this NOM under changing pH and copper concentration conditions revealed features that could be correlated to the activity of distinct types of chromophores. A comparison of results with those generated for experiments with standard Suwannee River fulvic acid highlighted important differences in chemistry between the two samples.
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