Superfund Research Program
Fundamental Studies of Thermal Decontamination of Soils
Project Leader: Jack B. Howard
Grant Number: P42ES004675
Funding Period: 1995 - 2000
Recent results on production of PAH during O2-free heating of pyrene (PY)-contaminated soil have been further analyzed. The data were generated by heating batch samples of a Superfund-related synthetic soil matrix pretreated with 5.08 wt% PY in a tubular furnace under a constant flow of helium gas, at 250, 500, 750, and 1000 oC. Dichloromethane (DCM) extracts of cooled residues of heated soil, and of volatiles condensed on a cold finger (c. 20oC) after 1 second at furnace temperature, were assayed gravimetrically and analyzed for PAH by HPLC, HPLC-MS, and GC-MS, drawing on resources in the Analytical Chemistry Core Laboratory. All four temperatures volatilized PY and generated PY derivatives. Heating at 750 and 1000oC gave rise to additional PAH in the cold finger extracts including bioactive compounds: cyclopenta[cd]pyrene (CPEP) at 750 and 1000oC, and benzo[a]pyrene (BaP) at 1000oC. PY and DCM extracts were undetected in the soil residue only at 750oC. Control experiments with uncontaminated soil, PY, and Ottawa sand +4.89 wt % PY revealed no CPEP or BaP from the soil, but imply that PY interactions with soil decomposition products, soil-bound silica, or other soil ingredients stimulate or augment CPEP and BaP production. Candidate PY-to-PAH conversion pathways are: (1) reaction with light gas species (e.g., soil- or PY-derived acetylene); (2) loss of C2 units followed by reaction with a PAH; (3) dimerization with further molecular weight growth via cyclodehydrogenation.
Results of earlier computational studies were prepared for journal publication. Those studies utilized molecular mechanics methods [MM3(92)] to assess, for a limiting case of thermodynamic equilibrium, the importance of molecular weight growth by acetylene addition and intramolecular rearrangement (isomerization) in modifying the structure and bioactivity of polycyclic aromatic hydrocarbons (PAH). Temperatures (300-1100oC) and the chemical environment (C2H2/H2 molar ratios) were selected for relevancy to thermal treatment of PAH-contaminated soils under O2-free conditions. Thirty PAH with empirical formulae C14H10, C16H10, C18H10, C18H12, C20H10, and C20H12 were studied: 11 PAH containing only six-membered rings, and 19 PAH containing five- and six-membered rings. The results revealed that: (i) the most stable PAH isomer in a given formula group undergoes significant isomerization, with compositional diversity increasing with increasing temperature; (ii) that isomerization partially transforms non-mutagens to mutagens (e.g., PY and benzo[e]pyrene to flouranthene and BaP respectively), and partially converts CPEP and chrysene, both human cell mutagens, to one and three additional human cell mutagens; (iii) acetylene addition transforms the non-mutagens phenanthrene and PY to the mutagens triphenylene and CPEP, respectively.