Formatted Title

Synthetic and Natural Surfactants for NAPL Mobilization: From Batch Experiments to Laboratory-Scale Soil Flushing Simulation

Background/Objectives

Pump-and-treat (P&T), which is the most traditional technology for non-aqueous phase liquids (NAPLs) remediation, is well known to be affected by strong limitations, i.e., low efficiencies, low removal rate of contaminants from secondary sources, long operative times, and high treatment costs.

In recent years, surfactant enhanced aquifer remediation (SEAR) technology, based on in situ soil flushing or ex situ soil washing processes, has been proved to be an efficient method for NAPLs remediation. Surfactants can reduce the interfacial tension between immiscible phases and, when their concentration is equal to or greater than the critical micellar concentration (CMC) they aggregate into micelles, increasing the apparent solubility of poorly soluble compounds and favoring the mobilization and the mass transfer of NAPLs in aqueous phase. Thanks to these properties, SEAR technology can offer several advantages over P&T: greater contaminants removal efficiency, shorter operative time, and lower costs.

In this context, this work aims to provide guidelines for selecting the best surfactants and conditions for the mobilization of strongly adsorbed NAPLs through an experimental lab-scale process study in sequential steps. Five sugar-based biodegradable surfactants, both synthetic and natural, belonging to the families of alkyl-polyglycolides (APG 1 and APG 2), sophorolipids (SL), and rhamnolipids (RL 1 and RL 2) were investigated.

Approach/Activities

The study involved a preliminary chemical-physical characterization of the surfactants for the determination of their CMCs and surface behavior. Then, selecting toluene and perchloroethylene as reference pollutants (Light and Dense NAPLs, respectively), a mobilization study of adsorbed contaminants was carried out through two experimental phases.

In the first phase, a batch thermodynamic study (adsorption isotherms) was performed as an indirect approach to study surfactants’ solubilization ability, based on the evaluation of their effects on NAPLs adsorption on a reference sorbent material. Three different surfactant concentrations were tested in this step: half of the CMC (C = 0.5x CMC), five times the CMC (C = 5x CMC), and 5% V/V in water (C >> CMC).

Instead, the second phase involved a continuous configuration column experiment with which a soil flushing process enhanced with the use of surfactants at a concentration of 5x CMC was simulated on a laboratory scale, allowing a direct assessment of surfactants’ solubilization ability of strongly adsorbed NAPLs, in a representative context of a polluted aquifer under laboratory-controlled conditions.

Results/Lessons Learned

The results showed good interfacial properties and globally low CMC values of all investigated surfactants. Moreover, a significant reduction of NAPLs adsorption on porous media was observed at a surfactant concentration higher than the CMC. In particular, the synthetic surfactant APG 2 and the biosurfactant RL 2 showed interesting characteristics, such as low CMC and the highest ability to reduce the adsorption of NAPLs. Hence, these surfactants at 5x CMC concentration were used in the simulation of the soil flushing process, leading to the removal of 69% and 82% of toluene and 92% and 97% of PCE, respectively.

In conclusion, RL 2 biosurfactant showed the best characteristics such as the greater mobilization capacity of NAPLs and its biological origin, which makes this product completely biodegradable and eco-compatible, minimizing the environmental impact associated with its possible in situ application.