Murthy A. Vairavamurthy

Characterization of humic acid from leonardite coal: an integrated study of PY-GC-MS, XPS and XANES techniques

Abstract We studied the humic-acid fraction isolated from a Spanish leonardite coal (Torrelapaja, Cretaceous basin belonging to the Utrillas facies) using a suite of chromatographic and spectroscopic techniques to characterize the structure of the carbon skeleton and the nature of the sulfur-containing compounds. In particular, analytical pyrolysis coupled to gas chromatography-mass spectrometry was used to gain detailed molecular information on the organic structures. Pyrolysis in the presence of a methylating agent (tetramethyl-ammonium hydroxide, TMAH) was used to characterize polar moieties. We employed non-destructive techniques, XANES and XPS, to investigate the composition of the sulfur functionalities (such as sulfide, polysulfide, thiophene, sulfoxide, sulfonate and sulfate). The combination of these different approaches allows a more complete understanding of the organic sulfur structures in the leonardite coal. In agreement with previous studies, our results show that oxidized sulfur functionalities, such as sulfonate and sulfate, represent the major forms of sulfur in leonardite coal.

The interaction of hydrophilic thiols with cadmium: investigation with a simple model, 3-mercaptopropionic acid ☆

Abstract Environmental contamination by cadmium is widely recognized as a serious problem. All cadmium compounds are considered toxic and are regulated by the US Environmental Protection Agency. In organisms, thiols or organic sulfhydryl compounds are primarily involved in mobilizing and detoxifying cadmium through the formation of cadmium–thiol complexes inside the cell. This complexation is also of environmental importance because a variety of thiols are generated by biotic and abiotic processes in anaerobic environments, such as estuarine wetlands and coastal sediments, which have increasingly become the targets of metal pollution. We investigated the complexation of cadmium by 3-mercaptopropionic acid (MPA), using it as a simple model to better understand the potential pathways of cadmium transformations by hydrophilic thiols. Our results show that cadmium forms primarily 1:1 and 1:2 complexes with MPA in aqueous solutions at near-neutral pH values. While the dithio complex was highly soluble in water, the monothio complex rapidly precipitated from solution. Based on mass spectrometric and X-ray absorption spectroscopic data, we propose a cyclic, charge-neutralized structure for the monothio complex, and a linear ionic structure for the dithio complex. These results suggest that the type of complex formed is an important determinant of the mobility of cadmium.

Interactions of thiols with sedimentary particulate phase: studies of 3-mercaptopropionate in salt marsh sediments from Shelter Island, New York

Abstract 3-Mercaptopropionate (3-MPA) is a major hydrophilic thiol in salt marsh sediments from Shelter Island, New York. It is associated mainly with the sedimentary particulate phase, although the mechanism of its binding has not been clear. In this study, we show that humic substances coated on clay minerals are involved in binding 3-MPA through di- and poly-sulfide linkages. With increasing depth, the abundance of 3-MPA in the fulvic acid fraction decreases with a concomitant increase in the humic acid fraction, suggesting that, with depth, fulvic-bound 3-MPA is converted into humic-bound 3-MPA. We modeled the depth variation with a multi-G model which suggests that there are two types of reactive components in both fulvic acid-bound and humic acid-bound 3-MPA, one changing about 10 times more rapidly than the other. These two 3-MPA pools probably reflect different types of sulfur bridging, with the polysulfide-bridged pool changing more rapidly than the disulfide-bridged pool. As free thiol, 3-MPA is attacked relatively rapidly by anaerobic bacteria, its degradation following firstorder kinetics. However, in the sediment core we studied, humic-bound 3-MPA was present throughout the sediment column, down to 22 cm, corresponding to an age of about 90 years. These results suggest that thiols can escape bacterial degradation in sediments by binding to humic substances. The sedimentary distribution of 3-MPA is an important illustration of the interplay of biological and chemical processes in the fate of thiols in anoxic marine sediments.

Effect of magnesium ion in the analysis of carboxylated compounds by electrospray-mass spectrometry

The divalent magnesium ion generates various adducts with low-molecular-weight carboxylated compounds (e.g. propionic acid) in the analysis of aqueous solutions by electrospray ionization mass spectrometry (ESI-MS). The adducts detected correspond to singly charged cluster ions of magnesium with the general formula (MgnL2n+1)−, where L represents the different anions in solution. Mono-functionalized compounds, such as propionic acid, mainly formed dinuclear complexes, whereas poly-functionalized compounds, such as glutathione, generated significant amounts of mononuclear and other types of polynuclear complexes, in addition to dinuclear complexes. An equimolar mixture of five carboxylic acids (propionic-heptanoic) with magnesium chloride gave numerous peaks distributed in the 300–1000 m/z range in five Gaussian-shaped groups, each representing gas-phase ions with a particular molecular formula. We propose a statistical approach which explains that the Gaussian distribution of peaks arises from the various combinations of different fatty acids required for a specific molecular formula for a particular gas-phase cluster.