Evaporation Phenomena during Thermal Decontamination of Soils

Abstract

Effects of thermal treatment conditions on removal of a low volatility aromatic contaminant from soil are quantified. Shallow beds (10 mg) of a clay soil pretreated with 8 ( 0.4 wt % pyrene were heated in a thermogravimetric analysis at 5, 25, or 50 °C/min or isothermally. Effects of temperature, time, initial soil mass, initial contaminant mass, heating rate, and flow rate of carrier gas (helium at standard conditions) were studied, as were weight loss of uncontaminated soil and evaporation rates of pure pyrene. Extensive pyrene removal from foil (85-90%) was found between 200 (at 5 °C/ min) and 300 °C (at 50 °C/min). Soil decontamination was modeled as pyrene transport without chemical reaction, i.e., (1) evaporation from a liquid sheath encasing each soil particle; (2) diffusion through the soil pile; and (3) diffusion to the ambient through a concentration boundary layer between the soil pile and the top of the thermobalance crucible. Evaporation is important in removing most, i.e., 70% or more, of the pyrene. However, rates of pyrene release from the soil bed are about 5-fold lower than those for evaporation of pure pyrene. Pyrene evaporation is confined largely to thin zones that recede downward through the soil pile as decontamination proceeds.

Thermal removal of pyrene contamination from soil: basic studies and environmental health implications.

“…Assuming the difference in weight loss method is valid, Figure 13 suggests that there is little effect of heating rate on pyrene removal at a peak temperature of 550°C, when heating rate was reduced from 5000 to 1000°C/sec or further from 1000 to 100°C/sec ( Figure 13). However, at somewhat lower temperatures and considerably lower heating rates, Gilot et al (27) found that a 10-fold reduction in heating rate, i.e., from 50 to 5°C/min, lowered the temperature to achieve 85% pyrene removal from a clay soil, from 275 to 200°C. Further mathematical modeling of heating rate and temperature effects on soil thermal decontamination, for broad ranges of these two variables, would shed further light on these observations.…”

“…Differences in soil type, heating apparatus, and soil bed configuration may also be important, i.e., Gilot et al. (27) heated a shallow bed of soil in a thermogravimetric analyzer.…”

“…[PAH] precursor in high temperature reactions, see below) and liquids of medium to low volatility (tars). Gilot et al (27) found extensive removal (85-90%) of pyrene contamination from a clay soil by heating from room temperature to 200°C (5°C/min heating rate) or 300°C (500C/min). They concluded that while evaporation could account for most (70% or more) of the pyrene release, other processes, e.g., desorption and possibly chemical reactions, are important in removing the last 5 to 10% (27).…”

Chemical Characterization and Bioactivity of Polycyclic Aromatic Hydrocarbons from Non-Oxidative Thermal Treatment of Pyrene-Contaminated Soil at 250-1,000 degrees C

“…Giot et al (10) used a thermogravimetric analyzer (TGA; Cahn System 113, Cahn Instruments, Madison, WI) to study pyrene removal (inferred from weight loss data) during much slower heating (5-50°CImin) of shallow beds of clay particles contaminated with approximately 8 wt% (by weight) pyrene. They found that rates of pyrene release from the soil bed were approximately 5-fold lower than those from heating pure pyrene under similar conditions.…”

“…Rates [8][9][10] hr in a specially treated, inverted wide-mouth jar (8). The resulting residue, that is, the contaminated soil, was then redried in the desiccator before use.…”