A laboratory pilot study of thermal decontamination of soils polluted by PCBs. Comparison with thermogravimetric analysis.

Real soils contaminated with a mixture of polychlorinated biphenyls were thermally decontaminated in a laboratory scale thermal desorption apparatus, under different operating conditions including two operating pressures (P= 0.01 and 0.1 MPa), different sample masses (80-320 g), carrier gas flow rates (0-30-280 Nl h(-1)) and two initial contamination levels. A standard European soil artificially contaminated with 4-chlorobiphenyl was decontaminated using a thermogravimetric analyser (TGA). The same parametric study as cited earlier was performed. With both techniques, the extent of decontamination was studied as a function of temperature during the heating programs (2-3 degrees C min(-1) for the thermal desorption apparatus; 5 degrees C min(-1) for TGA). Only a few differences were noticed between the two techniques. The decontamination starts when the melting temperature of the contaminant is reached (30 degrees C) and is complete before 350 degrees C. Thermogravimetric analysis which does not necessitate any chemical analysis appears to be a very attractive technique to investigate the feasibility of a decontamination and to quickly determine the best operating conditions.

Chemical characterization and bioactivity of polycyclic aromatic hydrocarbons from non-oxidative thermal treatment of pyrene-contaminated soil at 250-1,000 degrees C.

In this paper we report yields, identities, and mutagenicities of products from heating a polycyclic aromatic hydrocarbon (PAH)-contaminated, Superfund-related synthetic soil matrix without exogenous oxygen. We heated batch samples of soil pretreated with 5.08 wt% (by weight) pyrene in a tubular furnace under a constant flow of helium gas at 250, 500, 750, and 1,000 +/- 20 degrees C. Dichloromethane (DCM) extracts of cooled residues of heated soil and of volatiles condensed on a cold finger after 1 sec residence time at furnace temperature were assayed gravimetrically and analyzed for PAH by HPLC, HPLC coupled to mass spectrometry, and gas chromatography coupled to mass spectrometry. All four temperatures volatilized pyrene and generated other PAHs, including alkylated pyrenes. We detected bioactive PAHs in the product volatiles: cyclopenta[cd]pyrene (CPP) at 750 and 1,000 degrees C and benzo[a]pyrene (BaP) at 1,000 degrees C. We found a clean soil residue, i.e., no pyrene or other DCM extracts, only at 750 degrees C. Control experiments with uncontaminated soil, pyrene, and Ottawa sand plus 4.89 wt% pyrene revealed no CPP or BaP production from soil itself, but these experiments imply that pyrene interactions with soil, e.g., soil-bound silica, stimulate CPP and BaP production. We detected mutagenicity to human diploid lymphoblasts (in vitro) in volatiles from 1,000 degrees C heating of soil plus pyrene and sand plus pyrene, and in the residue from 500 degrees C heating of soil plus pyrene. Three plausible pathways for pyrene conversion to other PAHs are a) a reaction with light gas species, e.g., soil- or pyrene-derived acetylene; b) loss of C(2)-units followed by reaction with a PAH; and c) dimerization with further molecular weight growth via cyclodehydrogenation. This study shows that thermal treatment of PAH-polluted soil may generate toxic by-products that require further cleanup by oxidation or other measures.

Thermogravimetric study of thermal decontamination of soils polluted by hexachlorobenzene, 4-chlorobiphenyl, naphthalene, or n-decane.

To determine decontamination behavior as affected by temperature, shallow beds of a clay-rich, a calcerous, and a sedimentary soil, artificially polluted with hexachlorobenzene, 4-chlorobiphenyl, naphthalene, or n-decane, were separately heated at 5 degrees C min-1 in a thermogravimetric analyzer. Temperatures for deep cleaning of the calcerous and the sedimentary soil increased with increasing boiling point (bp) of the aromatic contaminants, but removal efficiencies still approached 100% well below the bp. Decontamination rates were therefore modelled according to a pollutant evaporation-diffusion transport model. For the calcerous and sedimentary soils, this model reasonably correlated removal of roughly the first 2/3 of the naphthalene, but gave only fair predictions for hexachlorobenzene and 4-chlorobiphenyl. It was necessary to heat the clay soil above the aromatics bp to achieve high decontamination efficiencies. Weight loss data imply that for temperatures from near ambient to as much as 150 degrees C, interactions of each aromatic with the clay soil, or its decomposition products, result in lower net volatilization of the contaminated vs. neat clay. A similar effect was observed in heating calcerous soil polluted with hexachlorobenzene from near ambient to about 140 degrees C. Decontamination mechanisms remain to be established, although the higher temperatures needed to remove aromatics from the clay may reflect a more prominent role for surface desorption than evaporation. This would be consistent with our estimates that the clay can accommodate all of the initial pollutant loadings within a single surface monolayer, whereas the calcerous and sedimentary soils cannot.