EPFRs were initially reported to form from combustion-generated particulate matter (PM) via reactions of aromatic organic molecular precursors with redox-active transition metals at 150-500oC thermal reactions in a few seconds reaction times. Our previous study has shown that EPFRs could also be formed at ambient temperatures in soils and sediments contaminated with hazardous materials, such as the Superfund sites.            Probing into the mechanistic formation of EPFRs in soil, we investigate to which component of the soil is responsible and capable of stabilizing these EPFRs.  In this study, a smectite clay mineral cation-exchanged with redox-active transition metal (i.e. Fe(III) & Cu(II)-Calcium Montmorillonite) gas phase dosed at room temperature (20oC) with aromatic organic compound (i.e. Phenol) was investigated. Using EPR spectroscopy, EPFR concentrations and structural assignments were determined. Similarity of EPR signals in the Fe(III) and Cu(II) clays were observed (g = 2.00390 and ?Hp-p = 7.2 G) confirming the EPFRs detected were derived from Phenol. Several confirmational analyses (HPLC and FTIR analyses) strongly suggest that EPFRs was a phenoxyl type of radical. However, EPFRs concentrations differ significantly between the two clays, Fe(III) clay  was determined to be ~6x (6.0x1017 vs 1.0x1017 spins/gram of clay) higher than Cu(II) suggesting Fe(III) clay as a more efficient system in forming EPFRs. Additionally, kinetic studies resulted in a 1/e half-life of the EPFRs detected to about ~3-7 days at room temperature in ambient air. Chemisorption and electron transfer from Phenol to redox-active transition metals saturated clay are proposed to be responsible for EPFRs formation.The relatively long lifetimes indicated that EPFRs might persist in the environment long enough for human exposure. Thus, the existence of potentially toxic EPFRs questions the long held belief that sorption of an organic pollutant to a soil matrix is a method of mitigating its environmental impact.

Formation and characterization of environmentally persistent free radicals (EPFRs) in cation-exchanged smectite clays

245th National Meeting (2013)

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