Sorption Dynamics and Energetics of Cinnamic Acid and Its Derivatives at the Ferrihydrite–Water Interface Determined by Flow-Adsorption Microcalorimetry

Abstract

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.