Microbiological and physicochemical changes occurring in a contaminated soil after surfactant-enhanced soil washing

A soil contaminated with petroleum hydrocarbons, arising from an old Mexican refinery, was previously characterized in terms of total petroleum hydrocarbons (TPHs), diesel and gasoline fractions, benzene-toluene-ethylbenzene-xylene (BTEX), polycyclic aromatic hydrocarbons (PAHs), and some metals. A health risk analysis determined that some PAHs should be reduced, and the Mexican standard indicates that high TPHs concentrations must be reduced to 2000 mg kg1. This soil was submitted to a surfactant-enhanced washing using sodium dodecyl sulfate (SDS) with and without NaCl, polyethoxylated sorbitan monoleate (TW80), and polyethoxylated nonylphenol (E600) (an anionic and two nonionic surfactants) at different concentrations. Based on these experiments, it was decided to wash soils with given doses of each surfactant in a continuous system, where 9 kg of soil was washed alternatively with surfactant solutions and plain water during 42 days. After this process, soils were drained, dried, and milled. Parameters, such as microbial count, total N, organic carbon and organic matter, available P, pH, dry and bulk densities, electrical conductivity (as a measure of salts content), Na, K, Ca, and Mg, were measured before and after the washing process. In general, all parameters were affected by the soil washing, but the most interesting changes were the increases in organic matter, electrical conductivity (for the experiments in which SDS was used), phosphorus, and total nitrogen (except for the experiment with TW80). Log total heterotrophs count was reduced to one half in most experiments, and Na and Mg were, in general, diminished, whereas K and Ca were augmented because of the surfactant soil washing. However, statistical analysis (analysis of variance [ANOVA]) indicated that only electrical conductivity and phosphorus were significantly affected (p 0.05). Changes are discussed and compared with changes occurring in soils caused by different nonanthropogenic events such as acid rain, organic fertilization, reseeding with native species, amending of soils with manure, etc.