Molecular Features of Humic Acids and Fulvic Acids from Contrasting Environments

Abstract

Insight in the molecular structure of humic acid (HA) and fulvic acid (FA) can contribute to identify relationships between their molecular properties, and further our quantitative abilities to model important organic matter functions such as metal complexation and association with mineral surfaces. Pyrolysis gas chromatography/mass spectrometry (Py-GC-MS) is used to compare the molecular composition of HA and FA. A systematic comparison was obtained by using samples from different environmental sources, including solid and aqueous samples from both natural and waste sources. The chemical signature of the pyrolysates was highly variable and no significant difference between HA and FA was found for major chemical groups, that is, carbohydrates, phenols, benzenes, and lignin phenols, together accounting for 62-96% of all quantified pyrolysis products. However, factor analysis showed that within each sample, FAs consistently differed from corresponding HAs in a larger contribution from mono- and polyaromatic hydrocarbons and heterocyclic hydrocarbons, together accounting for 3.9-44.5% of the quantified pyrolysis products. This consistent difference between FAs and corresponding HAs, suggests that their binding properties may, in addition to the carboxyl and phenolic groups, be influenced by the molecular architecture. Py-GC-MS may thus contribute to identify relationships between HA and FA binding- and molecular-properties.

Exploring the Potential Role of Terrestrially Derived Humic Substances in the Marine Biogeochemistry of Iron

“…Since the latter are more abundant in HA than in FA, it is also consistent with Laglera and van den Berg's (2009) measurements of the iron binding capacity of the Suwannee River humic acid (32 nmol Fe per mg HA) being higher than fulvic acid (17 nmol Fe per mg FA). Finally, it supports the notion (Aiken et al, 2011;Nuzzo et al, 2013;Fujii et al, 2014;Schellekens et al, 2017) that the iron binding properties of humic substances are driven by recognition chemistry (highly selective binding sites) and structural constraints (molecular architecture) rather than by the abundance of carboxylic and phenolic groups or some other bulk property.…”

“…Because of this continuum, it seems reasonable to use the term humic, to refer collectively to humic and fulvic acids. Additional support for this terminology comes from recent findings that all humic and fulvic substances appear to contain the same functional groups, i.e., carboxylic, phenolic, and carbonyl, only their abundances differ (Blazevic et al, 2016;Schellekens et al, 2017). On the basis of such findings, Orlowska et al (2017b) synthesized a range of model compounds in an attempt to represent the main components of humic substances, namely lignin decomposition products.…”

Vascular plants affect properties and decomposition of moss-dominated peat, particularly at elevated temperatures

“…First, the n-alkenes and n-alkanes were pooled to a single variable since these products were large in number (10 and 9, respectively) but in sum only contributed less than 1 % to total abundance. 175 Accordingly, it was avoided that this large set of correlated variables with small abundance was allocated to the first principal component (Schellekens et al, 2017). Second, key molecular parameters (Sect.…”

Characterization of coal-based fulvic acid and the construction of a fulvic acid molecular model

“…While the composition of FA has been reported, research on the construction of the FA molecular structure is lacking. Schellekens et al 11 used pyrolysis gas chromatography/mass spectrometry to compare the molecular compositions of humic acid (HA) and FA and found no signicant differences among their pyrolysis products. The main chemical groups included carbohydrates, phenols, benzene, and lignin phenols.…”