Biogeochemical functioning of oxides and pyrogenic organic matter ( pyOM) are greatly influenced by surface and deprotonation characteristics. We present an energetics-based, logistic modeling approach for quantifying surface homogeneity (ϕ) and surface acidity ( pK ) for Brønsted-type surfaces. The ϕ, pK and associated deprotonation behavior of pyOM were quantified across feedstock (honey mesquite, HM; pine, PI; cord grass, CG) and heat-treatment-temperatures (HTT; 200-650 °C). At HTT200, lower ϕ [HM (0.86) > PI (0.61) > CG (0.42)] and higher pK [CG (4.4)> PI (4.2) > HM (4.1)] for CG indicated higher heterogeneity and lower acidity for Brønsted-type surface moieties on grass versus wood pyOM. Surface acidity of CG increased at HTT550/650 °C with no effect on ϕ; while the surface heterogeneity of both wood pyOMs increased, the acidity of HM increased and that of PI decreased. Despite different HTT-induced ϕ and pK trajectories, the deprotonation range for all pyOM was pH = [Formula: see text]. Therefore, higher heterogeneity pyOMs deprotonate more readily at lower pH, over a wider range and (for similar pK and cation exchange capacity) are better cation/metal binding surfaces at pH< pK . The approach also facilitates the evaluation of surface and deprotonation characteristics for mixtures and more complex surfaces.

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Photobleaching experiments with water-extracted dissolved organic matter (wDOM) from uncharred or charred grass (SG25 and SG400), softwood (EC25, AJ25, EC400, and AJ400), and hardwood (HM25 and HM400) biomass revealed that photodegradation proceeds along an energy-dependent, component-specific trajectory dictated by the nature of the aliphatic attachments associated with single-ring aromatic domains. Trends in wDOM photobleaching for both uncharred [EC25 = AJ25 (94%) > SG25 (46%) ≥ HM25 (43%)] and charred [EC400 = AJ400 (76%) > HM400 (55%) > SG400 (44%)] biomass pointed to biomass type and chemistry and its response to processing (here, charring) as key factors controlling photodegradation. Specifically, photobleaching was lower in wDOM where aliphatic attachments to the aromatic backbone had shorter alkyl chain lengths, was involved in heterocyclic ring formation, or was more oxidized. Photobleaching behavior, across the wDOMs, was captured by a three-component, UVA photoenergy (E UVA-V )based model comprising a fast, wDOM f , component requiring a mean E UVA-V flux of 42−204 kJ m −2 for photobleaching; a slow, wDOM s , component requiring a 3−24-fold higher E UVA-V flux; and a photobleaching resistant, wDOM res , component. Combining long-term daily E UVA-V across the State of Texas, the statewide distribution of vegetation types used in this study, and our photobleaching model suggested that an increase in the number of vegetation fires is likely to decrease photodegradative contributions to wDOM cycling in the conifer-dominated east, have no effect in the grass-dominated central regions, and increase contributions in the hardwood shrub-dominated west.

Lignin-derived aromatic structures are stabilized in soils and clay-rich fractions by iron oxyhydroxides. However, the dynamics and energetics of sorption have not been determined and are challenging to study directly in soil matrices due to the complex mineralogy and organic molecular diversity of soils. Flow adsorption microcalorimetry experiments conducted using cinnamate and its hydroxylated derivatives (coumarate, ferulate, and sinapate) were used to assess the impact of the position and type of R-group substituents (OH and OCH3) on the sorption behavior at the ferrihydrite–water interface. The molar heat of sorption of (hydroxy)­cinnamates was 1.17 kJ mol–1 and was consistent with an outer-sphere mechanism that comprised both electrostatic and physisorption interactions. Cinnamate sorption was endothermic and entropy-driven, whereas sorption of the hydroxylated derivatives was exothermic. While the OH substituent shifted the enthalpic response to exothermic, it had a minimal effect on the duration and total energy of the reaction. With each OCH3 substituent added, both the energy and duration of reaction were greater. Compared to OH, the OCH3 substituent contributed significantly less to the molar heat of sorption, suggesting that OCH3 facilitated the formation of intermolecular bonds between sorbate molecules. The OH and OCH3 substituents increased the energy of sorption by 54% but had a minimal effect on the proportion of sorbate that was desorbed by nitrate: 54% for cinnamate vs 51% for ferulate. These findings suggest that a significant fraction of (hydroxy)­cinnamate interactions with iron oxyhydroxides are weak electrostatic forces and, unless protected within the mineral framework, are highly susceptible to shifts in environmental conditions.