Pilot-scale subcritical water remediation of polycyclic aromatic hydrocarbon- and pesticide-contaminated soil

Subcritical water (hot water under enough pressure to maintain the liquid state) was used to remove polycyclic aromatic hydrocarbons (PAHs) and pesticides from highly contaminated soils. Laboratory-scale (8 g of soil) experiments were used to determine conditions for the pilot-scale (8 kg of soil) extractions. Pilot-scale remediations of a PAH-contaminated manufactured gas plant soil (2200 ppm total PAHs ranging from naphthalene to benzo[ghi]perylene) with 275 degrees C water reduced all low and high molecular weight PAHs to below detectable levels (<0.5 ppm) in as little as 35 min. in contrast, removals of higher molecular weight (mutagenic) PAHs were much poorer with either bioremediation for 1 year or supercritical carbon dioxide extraction. Subcritical water extraction at 250 degrees C of 8 kg of soil contaminated with 70-400 mg/kg levels each of trifluralin, atrazine, cyanazine, pendimethalin, alachlor, and metolachlor also removed all pesticides to below detection limits. While neither the PAH- or pesticide-contaminated soils could support plant growth before extraction, both soils were fertile without additional treatment after extraction (based on germination of lettuce, radishes, and corn); and earthworm toxicity was reduced from 100% to 0%.