Hydroxynaphthoic acid isomer sorption onto goethite.

Understanding how organic acids interact with mineral surfaces is essential for predicting their mobility and reactivity in natural systems. While prior research has established that carboxylic and phenolic groups influence sorption behavior, the specific mechanisms remain unclear for complex isomers like hydroxynaphthoic acid (HNA). Existing studies have focused on simpler compounds or single functional groups, leaving a knowledge gap regarding the role of intramolecular hydrogen bonds (IHB) in surface complexation. This uncertainty limits the ability to model sorption in heterogeneous environments such as soils. The presence of IHB in HNA isomers complicates the interpretation of vibrational spectroscopy data, which is commonly used to infer surface structures. No prior work has resolved how IHB affects sorption modes on iron oxides like goethite. This gap motivated the need for a combined experimental and computational approach to clarify the interfacial behavior of HNA isomers.

The aim of this study was to investigate the sorption mechanisms of hydroxynaphthoic acid (HNA) isomers on goethite surfaces using a combination of experimental and computational methods. The specific problem addressed was the unclear role of intramolecular hydrogen bonds (IHB) in shaping sorption structures at mineral-water interfaces. The researchers sought to determine whether IHB influences the dominant sorption mode and whether vibrational spectroscopy could distinguish between different surface species. By using both batch and ATR-FTIR flow-through techniques, the study aimed to overcome limitations in interpreting coupled vibrational modes. The motivation for this work stemmed from the need to improve predictive models of organic ligand behavior in natural systems. The study also aimed to evaluate whether sorption structures on pure minerals like goethite are analogous to those observed in iron oxide-rich soils.

The study employed batch experiments and attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) flow-through techniques to assess HNA isomer sorption onto goethite. Computational chemistry was used to model vibrational modes and interfacial structures. Two HNA isomers, 1-hydroxy-2-naphthoic acid and 2-hydroxy-3-naphthoic acid, were selected for their intramolecular hydrogen bonds (IHB) between carboxyl and hydroxyl groups. Batch experiments measured sorption isotherms and solution pH effects, while ATR-FTIR provided in situ spectroscopic data on surface species. Computational methods helped interpret the coupled vibrational modes observed in the spectra. The study also compared HNA sorption on goethite with that on the clay fraction of an iron oxide-rich soil. The combination of experimental and theoretical approaches allowed the researchers to propose surface complexation structures despite the limitations of overlapping vibrational modes.

The strongest finding was that surface complexation via a bidentate structure involving both carboxylate and phenolate groups was the dominant mode of HNA isomer sorption onto goethite. A secondary surface species was observed at pH 4, where sorption appeared to involve only the carboxylate group. The phenolic group in this species was either involved in intramolecular hydrogen bonding (IHB) or hydrogen-bonded to the solvent or surface hydroxyl groups. Spectral data from ATR-FTIR and batch experiments were consistent with the proposed bidentate structure. The study also found that the lack of unique vibrational modes for key functional groups did not prevent the identification of interfacial structures. Computational modeling supported the interpretation of coupled vibrational modes. The spectral similarity between HNA sorbed onto goethite and onto the clay fraction of an iron oxide-rich soil suggested analogous sorption mechanisms. These results highlight the importance of both functional groups and pH in determining HNA isomer behavior at mineral-water interfaces.

The authors concluded that the dominant mode of HNA isomer sorption onto goethite involves a bidentate structure with both carboxylate and phenolate groups. A secondary surface species was observed at pH 4, where sorption appeared to involve only the carboxylate group. The phenolic group in this species was either involved in intramolecular hydrogen bonding (IHB) or hydrogen-bonded to the solvent or surface hydroxyl groups. The study demonstrated that despite the lack of unique vibrational modes for key functional groups, the experimental approach successfully proposed interfacial structures. The researchers acknowledged the limitations in differentiating between mono- and binuclear complexes using the available methods. The spectral similarity between HNA sorbed onto goethite and onto the clay fraction of an iron oxide-rich soil suggested analogous solute interactions in pure phase minerals and soils. These findings provide insights into how intramolecular hydrogen bonds and pH influence sorption structures at mineral-water interfaces. The study supports the use of combined experimental and computational approaches to overcome spectroscopic limitations in surface complexation studies.