In Situ Examination of the Protonation Behavior of Fulvic Acids Using Differential Absorbance Spectroscopy

Abstract

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.

Structure and composition of Fe–OM co-precipitates that form in soil-derived solutions

“…Comparing measured spectra with reference spectra is the general idea of differential spectra and discloses features that are unrevealed in whole spectra (Dryer et al, 2008). In our study, component 2 from PMF can be understood as average ATR-FTIR spectrum of all Fe-OM coprecipitates from either Ah-derived or Ap-derived effluent.…”

Study of the pH influence on the optical properties of dissolved organic matter using fluorescence excitation–emission matrix and parallel factor analysis

“…A fluorescence analysis of model compounds has demonstrated that electron-donating functional groups (e.g., amine, hydroxyl) can also significantly increase fluorescence intensities [2]. It has been well recognized that carboxylic chromophores overwhelmingly contributed to the changes in fluorescence at pH values between 3.0 and 5.0, whereas phenolic chromophores are predominant at pH values between 8.0 and 10.0 [38].…”

Probing the pH Dependent Optical Properties of Aquatic, Terrestrial and Microbial Humic Substances by Sodium Borohydride Reduction

“…Optical titrations of lower concentration samples have been used to successfully model proton binding in Suwannee River Fulvic Acid (SRFA) by the use of pH dependent differential absorbance (A) (Dryer et al, 2008). The NICA-Donnan model describes two distributions of titratable groups centered at pH 4 and pH 7-8 respectively (Koopal et al 2005;De Wit et al 1993a;De Wit et al 1993b).…”

“…A direct comparison of divergent sources of fulvic and humic acid model compounds including an aquatic terrestrially derived fulvic acid, Suwannee River fulvic Acid (SRFA) and a microbial source of fulvic acid, Pony Lake fulvic Acid (PLFA) (Brown et al, 2004;McKnight et al, 1994), the soil derived humic acids Elliott humic acid (EHA) and Leonardite humic acid (LHA) and a terrestrially derived aquatic humic acid Suwannee River humic acid (SRHA) ( Table 1). Analysis and comparison of untreated and borohydride reduced model humic compounds particularly at short wavelength (< 350) using potentiometric and optical titrations, spectral slope values (S), difference plots (A) (Dryer et al, 2008), fluorescence emission spectra, (Jørgensen et al, 2011), and fluorescence difference spectra (F) (Del Vecchio & Blough, 2004) provide insight into how electronic interactions work universally in the environment as well as how differences that exist between sources humic materials can potentially augment existing knowledge about the fate and transport of humic substances in the environment (Hernes & Ronald Benner, 2002).…”