Precipitation–Dissolution Switchable Surfactant-Based Microemulsions: Fabrication and Potential Applications in Remediating <i>p</i> -Nitrochlorobenzene-Contaminated Soil

In surfactant-enhanced soil washing (SESW) using microemulsions as extractants, the efficient and rapid postprocessing of well-emulsified SESW eluents, such as effectively releasing the solubilizate and recycling surfactants, poses significant challenges that need to be addressed. Therefore, a precipitation-dissolution switchable surfactant, sodium dodecyl seleninate (DSeNa), was used to prepare DSeNa-based microemulsions via the phase diagram approach. When acidified with HCl, water-soluble DSeNa was converted into dodecyl seleninic acid (DSeH), an insoluble solid, and by this means, the monophasic DSeNa-based microemulsions were rapidly and efficiently demulsified. Irrespective of the types of microemulsions, a distinct three-phase system consisting of DSeH, oil, and aqueous phases was obtained. This facilitates the recycling of DSeH through filtration with a recycling rate of over 96.5%. The recycled DSeH, with a purity approaching 100%, can be easily and almost quantitatively regenerated into DSeNa by reacting with NaOH. The oil-in-water type microemulsions show potential as extractants for the SESW of <i>para</i>-nitrochlorobenzene (<i>p</i>NCB) from soil, attaining a <i>p</i>NCB removal rate of up to 99.9%. Subsequently, approximately 96.9% of DSeH was recycled after the HCl-induced demulsification of the SESW eluent. Additionally, 99.9% of the total <i>p</i>NCB in the SESW eluent was separated into the oil phase. After being adsorbed by activated carbon and anion-exchange resin, the chemical oxygen demand value of the aqueous phase was reduced to around 7.8 mg O₂ L^-1.