Sorption-desorption mechanisms and environmental friendliness of different surfactants in enhancing remediation of soil contaminated with polycyclic aromatic hydrocarbons

Abstract

The ionic and nonionic surfactants have different adsorption-desorption models for polycyclic aromatic hydrocarbons (PAHs) in soil-water system, due to the difference in the composition and charge of the hydrophilic groups. Surfactant eluents retained in the soil may also have a secondary effect on the soil environment. Thus, the aim of the study was to investigate sorption-desorption mechanisms and environmental toxicity of different surfactants in enhancing remediation of soil contaminated with PAHs. The distribution of PAHs between different surfactants and soil is influenced by the surfactant bilayer formation. The average molecular density, Sips isotherm, critical micelle concentration (CMC), and critical washing concentration (CWC) models were used to explore the mechanisms involved in the adsorption-desorption of cationic (cetyltrialkylammonium bromide, CTAB), anionic (sodium dodecyl benzene sulfonate, SDBS), and nonionic (TritonX-100, TX100) surfactants. These models were associated with the surfactant-enhanced remediation (SER) of naphthalene-contaminated soil. The mean concentrations and activity of indigenous bacteria were used to detect the toxic effect of the above surfactants on soil environment. The results of SER experiments showed that there are critical washing points (SDBS = 2.4 CMC, TX100 = 4.6 CMC, CTAB = 3.3 CMC) for different surfactants washing naphthalene (Nap). The values of CWC corresponding to critical washing points were key variables driving the need for added surfactants to remove or immobilize Nap. The CWC of different surfactants could be conveniently predicted by a model based on the surfactant-derived organic carbon. In addition, the mean concentration of viable bacteria in biological culture experiment was highest for TX100, followed by CTAB and SDBS. This study revealed the effects of average molecular density and surfactant-derived organic carbon on the distribution of Nap, and suggested a model for calculating CWC of different surfactants, to optimize SER technology. Biological culture experiment indicated that the high concentration and ionic surfactants had a significantly toxic effect on indigenous bacteria.