Thermal desorption of a PAH-contaminated soil: a case study

Thermal desorption was tested to remediate a soil contaminated by PAHs (Polycyclic Aromatic Hydrocarbons) from a former Manufacturing Gas Plant site in northern Italy. Lab-scale experiments were carried out to assess the feasibility of this technology; in particular, the minimum desorption temperature required to reach the Italian soil standards was sought, as well as interferences of the natural organic matter on the on-line process control, performed by a Flame Ionization Detector (FID). The matrix was a sandy gravel; experiments were carried out on the fraction having particle size lower than 2 mm, whose PAH contamination was nearly 2700 mg/kg (total benzo-PAHs: 1100 mg/kg), and the fraction of organic carbon f(oc) 2.4% w/w. The apparatus was equipped with an indirectly-heated quartz reactor placed into a cylindrical furnace. In a typical experiment, a measured amount of soil was loaded into the reactor. Soil temperature was increased according to a linear schedule under an air flow, followed by a hold (60 min) at the final temperature T(max), from 250 to 650 degreesC, depending on the test being performed. Gases exiting from the reactor were analyzed by an on-line FID. Soil analysis was also performed following each test. The obtained results showed that remediation of soil could be accomplished with a maximum temperature between 300 degreesC and 350 degreesC, depending on the remediation limits to be reached. Removal efficiencies above 99.7% were obtained for all PAHs, when T(max) above 300 degreesC was applied. In the case of T(max) = 250 degreesC, the abatements decreased to values between 87.0% and 99.9%, depending on the PAH being measured. The on-line FID monitoring of the process fit the purpose, exhibiting results comparable with those obtained by chemical analyses carried out on the treated soil.