Spin labeling ESR investigation of the molecular environment of soil interacting with chemical organic contaminants

Abstract

This manuscript elucidates the influence of special functional groups of organic xenobiotic chemicals on partly simultaneous molecular binding mechanisms. Organic xenobiotics are released as contaminants into the environment and are partially bound to soil constituents, such as humic substances (HS) and the mineral phase. The interaction of chemical contaminants with HS of soil was investigated using a labeling technique of soil samples with stable nitroxide radicals as model contaminants. These nitroxide radicals only differed in one functionality, a hydroxy or an amino functional group. Electron spin resonance (ESR) analyses of contaminated soil samples showed that the interaction was a multistage process that comprised an initial increase in the concentration of free organic radicals of soil influenced by contaminants with hydroxy or amino functionality, a decrease in the polarity of the molecular environment of soil, and a change in mobility of contaminant molecules. Towards the end of interaction, the concentration of free organic radicals usually decreased. Binding of contaminants to soil constituents via specific functionality was revealed detecting a change in their ESR spectra. Basing on the ESR analyses, a two-way exchange of protons' electron pairs between contaminants and HS was hypothesized to interpret the beginning of interaction.

A new insight into a problem of mediating effects of humic acids on binding and biological effects of organic pollutants in natural soil

“…The EPR signals of chernozem and its HA, measured before the SL EPR experiment, are typical of natural soils and HA of different origin (Paul et al 2006;Skrzypczak-Bonduelle et al 2008;Aleksandrova 2013). Around a factor g=2, the paramagnetic signal comes from free organic radicals (OR) in the insoluble organic matter of soil or HA and originates from complex organic molecules with polyphenolic and quinoid units.…”

“…Modeled pollutant molecules with amines in the SL EPR experiment are observable within time limits influenced by OR of chernozem or its HA. OR cause a gradual reduction of nitroxide radicals (NR) in the soil environment via donating them H-atoms or electron-proton pairs (Berliner 1998;Foti et al 2008;Aleksandrova 2013). …”

“…(2 0 11 ) . I n e a r l i e r w o r k s (Aleksandrova et al 2011;Aleksandrova 2013), an effect of the specific functionality of contaminants was found using nitroxide spin labels with different substitutes, such as 4-Amino-TEMPO, 4-Hydroxy-TEMPO, and TEMPO, that were applied to luvisol and cambisol. Binding of organic chemicals to soil via the aromatic amine to luvisol and chernozem was considered in Aleksandrova et al (2013) and Aleksandrova (2014), using analyses of inhomogeneous broadening of EPR spectra of SL incubated with soil samples (Sankaram and Marsh 1998).…”

Using spin labels to study molecular processes in soils: Covalent binding of aromatic amines to humic acids of soils

“…In [29][30][31], the method of paramagnetic probing of soil was described, and it was shown; the redistribution of nitroxyl radicals 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO), and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (4-oxo-TEMPO) among the soil microareas of different polarities as well as changes in their mobilities and concentrations over time.…”

Can Carbamates Undergo Radical Oxidation in the Soil Environment? A Case Study on Carbaryl and Carbofuran

“…According to the literature a number of substituted aromatic pesticides undergo oxidation reactions that involve the generation of stable radicals in soil through removal of a proton or an electron from the molecule . Nonetheless, the role of radical species in the binding mechanism of contaminants to soil constituents is still under debate and further information about environmentally important radical reactions involving various organic compounds can be found in the literature. ,,− …”