Optical Properties of Humic Substances and CDOM: Relation to Structure

Abstract

The spectral dependencies of absorption and fluorescence emission (emission maxima (lamdamax), quantum yields (phi), and mean lifetimes (taum)) were acquired for a commercial lignin, Suwannee River humic (SRHA) and fulvic (SRFA) acids, and a series solid phase extracts (C18) from the Middle Atlantic Bight (MAB extracts). These parameters were compared with the relative average size and total lignin phenol content (TLP). TLP was strongly correlated with absorption at 280 and 355 nm for the MAB extracts, SRHA, and SRFA. The spectral dependence of lamdamax, phi), and taum was very similar for all samples, suggesting a common photophysical and thus structural basis. A strong decrease of phi and taum with increasing average size indicates that intramolecular interactions must be important. When combined with previous work, the results lead us to conclude that the optical properties commonly associated with terrestrial humic substances and chromophoric dissolved organic matter arise primarily from an ensemble of partially oxidized lignins derived from vascular plant sources. Theyfurther provide additional support for an electronic interaction model in which intramolecular energy transfer, excited-state electron transfer, as well as charge transfer likely play important roles in producing the observed optical and photochemical properties of these materials.

Predicting Dissolved Lignin Phenol Concentrations in the Coastal Ocean from Chromophoric Dissolved Organic Matter (CDOM) Absorption Coefficients

“…Intramolecular charge-transfer interactions among chromophores in aromatic molecules such as lignin have been proposed as a mechanism to explain the exponential shape of the CDOM absorption spectrum and its absorption at wavelengths >350 nm (Del Vecchio and Blough, 2004;Boyle et al, 2009;Sharpless and Blough, 2014). In this model, charge-transfer interactions among electron donors (e.g., phenols and/or methoxylated phenols in lignin) and electron acceptors (e.g., quinones or aromatic ketones/aldehydes in lignin) are thought to produce lower-energy optical transitions and enhance CDOM absorption at wavelengths >350 nm.…”

“…Lignin has the chemical moieties necessary to make it an important chromophore (Boyle et al, 2009;Sharpless and Blough, 2014). It is important to recognize, however, that a variety of other chromophoric components in terrigenous DOM likely co-vary strongly with lignin, and can contribute significantly to the CDOM absorption despite the strong linkage observed between CDOM and lignin.…”

“…These opposing trends are possibly linked to differences in the molecular structures of dissolved black carbon and dissolved lignin. Black carbon contains condensed aromatic compounds that typically have very high molar absorptivities in the UV (Chin et al, 1994;Weishaar and Aiken, 2001;Russo et al, 2012), whereas partially oxidized lignin contains the moieties responsible for intramolecular charge-transfer interactions, which are particularly effective at absorbing at wavelengths >350 nm (Del Vecchio and Blough, 2004;Boyle et al, 2009;Sharpless and Blough, 2014). Soils are a major source of black carbon and lignin, suggesting these two components likely correlate strongly in river-influenced coastal areas.…”

“…Lignin is an aromatic macromolecule, and is therefore an efficient absorber of ultraviolet radiation in natural waters (Chin et al, 1994;Weishaar and Aiken, 2001;Schmidt, 2010). The complex macromolecular structure of lignin is also thought to promote a continuum of intramolecular chargetransfer interactions that extend the absorption properties of lignin well into the visible region of the solar spectrum (Furman and Lonsky, 1988a,b;Del Vecchio and Blough, 2004;Boyle et al, 2009). As a result, strong linear relationships between dissolved lignin concentrations and absorption and fluorescence properties of CDOM have been observed in various estuarine and coastal environments (Hernes and Benner, 2003;Spencer et al, 2008;Hernes et al, 2009;Walker et al, 2009;Osburn and Stedmon, 2011;Fichot and Benner, 2012;Fichot et al, 2013;Yamashita et al, 2015).…”

Fluorescence quenching as an indicator of the conformational change of humic<br>acids

“…HA spectra does not arise from a superposition of many independent chromophores (Del Vecchio and Blough, 2004), but from a continuum of coupled states formed through charge transfer interactions of a few distinct chromophores. Emission maxima shift continuously to red with increasing excitation wavelength, while fluorescence quantum yields monotonically decrease (Boyle et al, 2009). …”