Retrieval and Reuse of Surfactants From Microemulsions Enabled by a pH-Triggered Precipitation-Dissolution Strategy.

How to retrieve and reuse surfactants efficiently from surfactant-based microemulsions (MEs) has long been a problem, which is full of challenges and needs to be solved urgently. To this end, a pH-triggered precipitation-dissolution (PTPD) strategy is developed. The surfactant sodium 3-(laurylamino)propane-1-sulfonate (LMPS) transforms into an insoluble precipitate (the inner salt of LMPS, LMP) after reaction with HCl, by which the monophasic LMPS-based MEs demulsified entirely, giving a separable mixture of oil, water and LMP. LMP can be retrieved efficiently (~95.3 %) regardless of the ME type, and can then be conveniently restored to LMPS via reactions with NaOH. Conceptually, the retrieval of LMPS (~96.6 %), toxic benzo[a]pyrene (BaP, ~99.5 %) and a mixture of co-surfactant n-butanol and the oil phase n-heptane (~97.1 %) from the sufficiently emulsified soil eluents is achievable by respectively using the PTPD strategy and distillation, wherein the soil eluents were generated from the remediation of BaP-contaminated soil using an oil-in-water LMPS-based ME as washing agent. It reveals a promising future for the PTPD strategy in the post-processing of soil eluents containing toxic hydrophobic organic contaminants and excessive surfactants.