Assessment of environmentally persistent free radicals in soils and sediments from three Superfund sites

Abstract

We previously reported the presence of environmentally persistent free radicals (EPFRs) in pentachlorophenol (PCP) contaminated soils at a closed wood treatment facility site in Georgia. The reported EPFRs were pentachlorophenoxyl radicals formed on soils under ambient conditions via electron transfer from PCP to electron acceptors in the soil. In this study, we present results for soil and sediment samples from additional Superfund sites in Montana and Washington. Paramagnetic centers associated with different chemical environments were characterized by distinct g-factors and line widths (ΔHp-p). EPFR concentrations in contaminated samples were ~30x, ~12x, and ~2x higher than background samples at the Georgia, Montana, and Washington sites, respectively. EPR signals in the Montana contaminated soils were very similar to those previously observed for pentachlorophenol contaminated soils at the Georgia site, i.e., g = 2.00300 and ΔHp-p = 6.0 G, whereas signals in the Washington sediment samples were similar to those previously observed for other PAH contaminated soils, i.e., g = 2.00270 and ΔHp-p = 9.0G. Total carbon content measurements exhibited direct correlation with EPFR concentration. The presence of radicals in sites contaminated a decade to a century ago suggests continuous formation of EPFRs from molecular contaminants in the soil and sediment.

Formation of environmentally persistent free radical (EPFR) in iron(iii) cation-exchanged smectite clay

“…37,43 The complex nature of the soil matrix, which can consist of polymeric aromatic components of soil organic matter (SOM) and assorted transition metals (Fe(III), Cu(II), Mn(III)), 13-14,36,43 may account for the longer lifetimes encountered in the contaminated soil system due to local effects, such as hydrophobic associations and π-stacking, which are common with the SOM systems. 13-14,43 Additionally, the long EPFR lifetimes for real PM 2.5 samples have been previously explained by the presence of trapped EPFRs, also known as ‘internal radicals’. This trapping, or internalization, protects radicals from molecular oxygen in the environment, hindering the oxidation, and hence, slowing decay processes of these ‘internal’ EPFRs.…”

“…13 The complex tri-component biological/mineral/SOM soil system is an analytical challenge, especially for mechanistic studies. This study is an initial step in gaining a mechanistic understanding of how EPFR form in contaminated soils by simplifying the soil system to its mineral components.…”

“…7 EPFRs have recently been found at elevated concentrations in numerous soil/sediment samples from a range of pentachlorophenol-polluted Superfund sites. 13,14 A detailed top-down study found that the EPFRs were almost solely associated with the clay/humic soil fraction. 14 …”

“…14 Iron was chosen as the redox centre of interest as it was found to be the most abundant transition metal in the contaminated Superfund sites. 13 Phenol was utilized as a simplified model of organic contaminants as it is a well known soil contaminant itself and can be easily produced by microbial reductive dechlorination of chlorophenol and polychlorophenols in contaminated soils, 17 and is considered as priority pollutant by U.S. EPA. 18 The following analytical techniques were combined in this study: 1) Fourier transformed infrared (FTIR) spectroscopy and X-ray diffraction (XRD) to demonstrate the sorption of the model pollutant, phenol, 2) X-ray photoelectron spectroscopy (XPS) to study the changes in chemical environment of Fe species, the redox centre of interest in this study, and other elements in the clay, 3) electron paramagnetic resonance (EPR) spectroscopy for radical detection and quantification, and 4) X-ray absorption near edge structure (XANES) spectroscopy to investigate the redox change of the Fe centres.…”

Remediation of hydrocarbon polluted soils using 2.45GHz frequency-heating: Influence of operating power and soil texture on soil temperature profiles and contaminant removal kinetics

“…However, these treatments may be ineffective, too expensive or lengthy (Chien, 2012). Ex-situ conventional thermal desorption was also successfully applied to remedy contaminated soil (dela Cruz et al, 2014;Falciglia et al, 2011b;Li et al, 2014;Thuan and Chang, 2012), however it may be expensive due to the energy costs (Falciglia et al, 2013).…”