Do the characteristics of crude oil in contaminated soils affect its removal by washing?

Contamination of soils by petroleum is a very common problem in areas of hydrocarbon exploration and production; refineries; storage stations; and other petroleum industry facilities. However, soils can also be contaminated by high concentrations of crude oil and oil fractions, due to problems during the transport of crude oil. There are several remediation techniques that can be used, but for soils recently contaminated with high concentrations of crude oil and requiring a quick response, surfactant-enhanced soil washing is probably the best method. In a previous work, Torres et al. (2004) described soils highly contaminated with three types of Mexican crude oil, i.e. El Batab (25.53°API), Paredon (34.93°API), and El Carrizo (14.13°API). (°API is the American Petroleum Association definition for oil density, and it is related to the water density at certain temperature and pressure conditions.) Surfactant-enhanced soil-washing processes were applied to these soils, using single surfactants, surfactant mixtures, and surfactant-plus-salt mixtures. The salts used were NaCl and Na-metasilicates. Freshwater and seawater soil-washing experiments were run for every contaminated soil. TPH removals were used as an indication of the efficiency of the processes. The experiments were performed as follows: 6 g of soils and 20 mL of washing solution were gently agitated for 23 hours at 20 ± 1 °C, followed by 1 hour of sedimentation. Maximum TPH removal for the soil contaminated with El Batab crude was as high as 24.2% (for the SDBS product, very close to the SDS + Na-Si assessment result). In the case of the soil contaminated with Paredon crude, the maximum TPH removal was only 15.8%, and for the soil contaminated with the El Carrizo crude, it was 76.4%. In the last two cases, the surfactant mixture employed was SIDS + Na-metasilicate (7%). The question is: do the type and other characteristics of crude oil in contaminated soils affect its rate of removal by washing? In this work we analysed whether the density of crude (°API); the initial total petroleum hydrocarbons (TPH) concentration; the benzene, toluene, ethylbenzene and xylenes (BTEX); polycyclic aromatic hydrocarbons (PAH) content; TPH_diesel; TPH_gasoline fractions; total metals content; or the Ca + Mg contents, are responsible for the different TPH removal values. The properties of contaminated soil that correlated well with the removal of TPH were TPH_gasoline = crude-oil density (°API) = BTEX > TPH_diesel = Ca + Mg > TPH. Total metals and PAH were never correlated with the amount of contamination removed by washing.