(308h) Semipermeable Barriers for Subsurface Oil-Water Separation in Polluted Sites

Petroleum hydrocarbons are common groundwater pollutants because of their widespread industrial use. Sponges and filters for separation of oil from surface waters exist, whereas research on oil-water separation in the subsurface is scant. Also, no previous study investigated the effect of surfactant type on oil-water separation in the subsurface. This unexplored aspect is relevant when using surfactants to enhance hydrocarbon mobility, to promote their recovery from polluted groundwater (surfactant flushing).

Our study fills the current gaps, and uses dilute solutions of chitosan, hydroxyethylcellulose (HEC) and quaternized hydroxyethylcellulose (HEC+) to produce injectable oil-water separation filters. Polymers injected into sandy media adsorb onto the sand, and do not desorb from it upon flushing with deionized water (as demonstrated using a quartz-crystal microbalance with dissipation monitoring). Water could flow through our injectable filters, whereas non-polar organic solvents (e.g. hexane and toluene) were retained in the absence of emulsifiers. The concentrations of hexane and toluene in water eluted from the filter were in the low ppm range, as demonstrated using Gas Chromatography Mass Spectroscopy and fluorescence spectroscopy. Our injectable filters could not exclude organic, polar solvents (e.g. tetrahydrofuran, THF, and tetrachloroethylene, TCE).

Polymer coated surfaces were oleophobic upon immersion in water, where contact angles formed with non-polar solvents (hexane and toluene) were >90⁰. While hexane and toluene adhered onto chitosan and HEC coated surfaces, they did not adhere onto HEC+ (because of its higher hydrophilicity).

TCE could also not wet polymer-coated surfaces (the contact angles were >90⁰). However, TCE likely had greater affinity for the polymer-coated surfaces compared to hexane and toluene, because the contact area of TCE droplets sitting on polymer-coated surfaces was greater.

Emulsifiers affected oil-water separation efficiency to different degrees, depending on their characteristics. Classical surfactants such as Tween 20 and oleic acid completely impeded oil-water separation. Classical surfactants expose their hydrophilic heads to the water size when forming interfacial films, hence favoring oil water droplet passage through the filter. The corn protein zein also prevented separation when it was dissolved in water at pH=13. Instead, emulsions were partially separated when they were stabilized with humic acids (>97% of toluene and hexane were separated) and zein particles. Zein particles were produced by using KCl to salt out zein dissolved at pH=13 (approximately 60% of toluene and hexane were separated). Zein particles easily flowed through the filters, and oil-water separation was therefore not due to filter clogging. Instead, it is speculated that partial oil-water separation with zein particles was due to the increased hydrophobicity of the water-oil droplet interface with zein particles (relative to dissolved zein). It is also speculated that the water-oil droplet interface was only partially hydrophilic with humic acids, explaining why polymer coated sand filters could partially separate emulsions stabilized by humic acids.

In summary, our study indicates that oil-water separation is correlated with the characteristics of the emulsifiers used. Separation is not effective with classical surfactants, because the surface of interfacial films exposed to the water side is hydrophilic. Separation efficiency increases by increasing the hydrophobicity of the side of interfacial films exposed to water, which interacts with the surface of the filters.

These results are valuable when using surfactant flushing in conjunction with injectable oil-water separation barriers, which were first developed in our study. We propose using suitable emulsifiers to mobilize solvents in the subsurface, to promote their migration towards the injectable filters. Here, solvents are separated from groundwater and can be easily pumped to the surface.