Adsorption of Dicarboxylic Acids by Clay Minerals as Examined by in Situ ATR-FTIR and ex Situ DRIFT

Abstract

The adsorption of dicarboxylic acids by kaolinite and montmorillonite at different pH conditions was investigated using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) and ex situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The sorption capacity of montmorillonite was greater than that of kaolinite. Adsorption of dicarboxylic acids (succinic acid, glutaric acid, adipic acid, and azelaic acid) was the highest at pH 4 as compared with those at pH 7 and 9. These results indicate that sorption is highly pH-dependent and related to the surface characteristics of minerals. The aliphatic chain length of the dicarboxylic acids highly influenced the sorption amount at acidic pH, regardless of the clay mineral species: succinic acid [HOOC(CH2)2COOH] < glutaric acid [HOOC(CH2)3COOH] < adipic acid [HOOC(CH2)4COOH] < azelaic acid [HOOC(CH2)7COOH]. With in situ ATR-FTIR analysis, most samples tend to have outer-sphere adsorption with the mineral surfaces at all tested pHs. However, inner-sphere coordination between the carboxyl groups and mineral surfaces at pH 4 was dominant from DRIFT analysis with freeze-dried complex samples. The complexation types, inner- or outer-sphere, depended on dicarboxylic acid species, pH, mineral surfaces, and solvent conditions. From the experimental data, we suggest that organic acids in an aqueous environment prefer to adsorb onto the test minerals by outer-sphere complexation, but inner-sphere complexation is favored under dry conditions. Thus, organic acid binding onto clay minerals under dry conditions is stronger than that under wet conditions, and we expect different conformations and aggregations of sorbed organic acids as influenced by complexation types. In the environment, natural organic material (NOM) may adsorb predominantly on positively charged mineral surfaces at the aqueous interface, which can convert into inner-sphere coordination during dehydration. The stable NOM/mineral complexes formed by frequent wetting-drying cycles in nature may resist chemical/microbial degradation of the NOM, which will affect carbon storage in the environment and influence the sorption of organic contaminants.

An Unexpected Fluctuating Reactivity for Odd and Even Carbon Numbers in the TiO2‐Based Photocatalytic Decarboxylation of C2‐C6 Dicarboxylic Acids

“…However, in the ATR-FTIR spectrum of pentanedioic acid (absorbed on the coated (Scheme 2 b ). [19,20,24] As shown in Figure 3 ) . This difference signified that both of the carboxylate groups coordinated to TiO 2 through the same bidentate chelating mode (Scheme 2 c).…”

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…The inducement of inner-sphere coordination on a mineral surface is due not only to dehydration but also to significant decreases in pH upon drying. This can be discerned from the observation of Kang and Xing (2007) that inner-sphere complexation was favored at lower pH, and the work of Dowding et al (2005) and Clarke et al (2011) illustrating drastic pH decreases (hydrogen ion activity increased 1000-fold) with increased drying. Although the spectra of powders or pressed pellets is applicable for understanding interactions under extremely dry conditions, it may provide false information regarding the types of surface complexes formed under well hydrated conditions.…”

“…Although the interference from water in a spectrum is typically diminished in transmission FTIR and DRIFTS analyses due to dehydration of the sample, these methods may provide inaccurate representation of interactions on a hydrated mineral surface. Kang and Xing (2007) and Kang et al (2008) noted that the carboxylic acid function groups of organic acids tend to form outer-sphere complexes with metal centers on minerals when the spectra of pastes were collected using in situ ATR-FTIR. However, inner-sphere complexes were more predominant in the DRIFTS spectra of freeze-dried samples as noted by shifts in the asymmetric COO − stretch to higher frequencies.…”

“…Formation of inner-sphere complexes between adsorbates with a carboxylic acid group and mineral surfaces is often indicated by substantial shifts (25-150 cm −1 ) in the frequency of asymmetric and symmetric COO − stretching relative to a spectrum of the aqueous adsorptive (e.g. Chorover and Amistadi, 2001;Duckworth and Martin, 2001;Gu et al, 1995;Gu and Karthikeyan, 2005b;Kang and Xing, 2007). With respect to the types of structural complexes formed, there are three common coordination modes developed between 1800 1600 1400 1200 1000…”

Wet–dry cycles impact DOM retention in subsurface soils

“…7) suggests that preferential adsorption of the carboxylic component was facilitated by the pre-existing soil-DOM phases of the dried soil. Prior work has shown that soil drying may promote conformational changes in pre-adsorbed DOM that promotes preferential desorption of O-alkyl relative to further inner-sphere coordination of carboxyl components (Kang et al, 2008;Kang and Xing, 2007). Additional support for the formation of inner-sphere carboxyl complexes is from the higher preferential adsorption of carboxyl over amide as observed in FTIR spectra of wet-dry compared to continuously wet treatments (Fig.…”