An old automotive industrial site
located at Mexico City with many years of operation and contaminated with heavy
oil hydrocarbons, particularly spent oils, was assessed for restoration using
the surfactant enhanced soil washing (SESW) process. The main goal of this study
was to characterize the contaminated soil in terms of TPHs, BTEX, PAHs, and
metals contents as well as microbiologically (total heterotrophs and specific
degrading microorganisms). We also aimed to determine the surfactant type and
concentration to be used in the SESW process for the automotive waste oil contaminated
soil. At the end, sixteen kg of contaminated soil were washed and the produced
wastewater (approximately 40 L) was characterized in terms of COD, BOD; solids, and other
physico-chemical parameters. The soil contained about 14,000 mg of TPH/kg soil
(heavy fraction), 0.13 mg/kg of benzo (k) fluoranthene and
0.07 mg/kg of benzo (a) pyrene as well as traces of some
metals. Metals concentrations were always under the maximum concentration
levels suggested by Mexican regulations. 15 different surfactants were used to
identify the one with the capability to achieve the highest TPH removal.
Surfactants included 5 anionics, 2 zwitterionic, 5 nonionics and 3 natural
gums. Sulfopon 30 at a concentration of 0.5% offered the best surfactant
performance. The TPH removals employing the different surfactants were in the
range from 38% to 68%, in comparison to the soil washing with water (10% of TPH
removal). Once the surfactant was selected, 70 kg of soil were washed and the
resulting water contained approximately 1300 mg/L of COD, 385 mg/L of BOD
(BOD/COD = 0.29), 122 mg/L of MBAS, and 212 mg/L of oil and greases, among
other contaminants.