DRIFT Infrared Spectroscopy Studies of Organic Matter Interactions at Mineral Surfaces

Abstract

The distribution and reactivity of organic carbon in the environment are strongly affected by the interaction of organics with mineral surfaces. The fine particulate nature of soils and marine sediments lend itself to infrared spectroscopy studies using the DRIFT technique. This technique requires minimal preparation for samples which already exist as powders and can give information on surface species. Spectral information of adsorbed species can be obtained by collecting spectra from the sample with adsorbed species of interest and from a mineral "blank" and then computing the difference spectrum. Since clays and other minerals typically have adsorbed water molecules on their surface an investigation was carried out to determine how results using DRIFT differed from existing studies reported in the literature where the mineral with adsorbed organic was in situ in an aqueous environment. The DRIFT results obtained for salicylic acid adsorbed onto ã-alumina were closely comparable with those obtained by Biber and Stumm (1994), using ATR with the mineral in situ in the solution. A series of materials of geochemical significance were prepared using kaolinite and ã-alumina as the mineral phases. The organics studied included ringed and straight chained carboxylic acids and a selection of short chain amino acids with one or two carboxylic acid groups and one or two amino groups. Studies with lysine showed the amino acid present on the surface of ã-alumina and kaolinite after exposure to a 0.01M solution with a pH in the region of the iso-electric point of the lysine (previously reported as the most favorable condition for adsorption). The 1700 – 1300 cm-1 region provided a window to observe amino acid peaks. The peak positions observed for lysine were consistent those observed at the same pH by McQuillan et al.(1998) for lysine adsorption onto TiO2, using in situ ATR –IR spectroscopy. The adsorption of long chain organics, octacosane and myristic acid (both insoluble in water) was also investigated. Hexane was used as the solvent. Octacosane was detected on the surface of the minerals, even after washing with fresh hexane. Myristic acid interacted with both ã-alumina and kaolinite surfaces forming linkages which result in a carboxylate group. This implies the displacement of the H+ from the carboxyl group which could be accommodated by a negatively charged surface site. There was also evidence of carboxyl groups, particularly on the unwashed samples. This could be a contribution from subsequent multiple layers of myristic acid on the surface or from more than one type of adsorption of the molecule onto the mineral surface. The intensity of the myristic acid was significantly stronger on the ã-alumina surface compared with the kaolinite samples. The study is being extended to investigate organic molecules on the surface of marine and esturine sediments.