Optical Properties of Humic Substances and CDOM: Relation to Structure

Abstract

The spectral dependencies of absorption and fluorescence emission (emission maxima (λ_max), quantum yields (ϕ), and mean lifetimes (τ_m)) 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 λ_max, ϕ, and τ_m was very similar for all samples, suggesting a common photophysical and thus structural basis. A strong decrease of ϕ and τ_m 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. They further 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.