Optical Properties of Humic Substances and CDOM: Effects of Borohydride Reduction

Abstract

Treatment of Suwanee River humic (SRHA) and fulvic (SRFA) acids, a commercial lignin (LAC), and a series of solid phase extracts (C18) from the Middle Atlantic Bight (MAB extracts) with sodium borohydride (NaBH(4)), a selective reductant of carbonyl-containing compounds including quinones and aromatic ketones, produces a preferential loss of visible absorption (> or = 50% for SRFA) and substantially enhanced, blue-shifted fluorescence emission (2- to 3-fold increase). Comparison of the results with those obtained from a series of model quinones and hydroquinones demonstrates that these spectral changes cannot be assigned directly to the absorption and emission of visible light by quinones/hydroquinones. Instead, these results are consistent with a charge transfer model in which the visible absorption is due primarily to charge transfer transitions arising among hydroxy- (methoxy-) aromatic donors and carbonyl-containing acceptors. Unlike most of the model hydroquinones, the changes in optical properties of the natural samples following NaBH(4) reduction were largely irreversible in the presence of air and following addition of a Cu(2+) catalyst, providing tentative evidence that aromatic ketones (or other similar carbonyl-containing structures) may play a more important role than quinones in the optical properties of these materials.

Linking Heterotrophic Microbial Activities with Particle Characteristics in Waters of the Mississippi River Delta in the Aftermath of Hurricane Isaac

“…These peaks are more representative of fluorophore molecules than the continuous longwave emission of humic substances (Ma et al, 2010). Characteristic of this pattern is the protein-like fluorescence (excitation max 275-280 nm, emission max 340-344 nm) similar to the amino acid tryptophan (T peak) which is linked to primary production (Coble, 1996).…”

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

“…The spectral slope is historically parameterized from 290 nm to 820 nm (Green & Blough, 1994). The wavelength maximum of many quinones is at shorter wavelength than 290 nm (Ma et al, 2010). Further many heterocyclic secondary amines have pka values that are close to or above pH 11.00 (Yamauchi & Odani, 1985).…”

“…The first model presented is the electronic interaction model which seeks to explain the unique optical properties associated with all CDOM/HS (Ma, 2010). The second model is a combination of the non-ideal competitive absorption model (NICA) and the Donnan gel model (Koopal et al 2005;De Wit et al 1993a;De Wit et al 1993b).…”

“…All show a pH dependent absorbance, exhibit increasing absorbance as wavelength decreases and a loss of absorbance upon borohydride reduction (Ma et al, 2010). A prominent theory linking humic model compounds is the electronic interaction model which highlights their ability to form electronic interactions and charge transfer complexes that extend long wavelength absorbance from source material that does not have absorbance spectra at long wavelengths (Ma et al, 2010).…”

“…Borohydride reduction targets carbonyl functional groups such as aromatic ketones and uinines (Tinnacher & Honeyman, 2007). By removing a moiety suspected of participating in electronic interaction, long wavelength absorbance is partially or entirely lost and fluorescence emission increases and blue shifts (Ma et al, 2010). 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).…”