High-efficiency and sustainable washing of waste oil-based mud using CO2-responsive amine-free hydrophobic deep eutectic solvent-based microemulsion

Efficiently cleaning waste oil-based mud using microemulsions while avoiding secondary pollution from surfactants is a huge challenge. Surfactant-free microemulsions (SFMEs) may provide a potential solution to this issue. In this work, a series of hydrophobic deep eutectic solvents (HDESs) were prepared by mixing valproic acid and 4-methoxyphenol. All obtained HDESs displayed reversible phase-switching behavior upon stimulation with NaOH and CO 2 . One of HDESs achieved the fastest phase transition response time of 2.15 min and a peak regeneration efficiency of 98.67%. Accordingly, this optimal HDES was chosen to prepare surfactant-free microemulsions (SFMEs) featuring three distinct structures: oil-in-water, bicontinuous, and water-in-oil (W/O). These SFMEs exhibited NaOH/CO 2 -responsive solubilization behavior toward n-heptane, a model oil, and were subsequently used to wash waste mud. The W/O microemulsion achieved the highest total petroleum hydrocarbon (TPH) removal rate of 96%, attributed to its minimal interfacial tension at the oil phase and the lowest contact angle on the solid surface. Response surface methodology was used to optimize W/O microemulsion washing parameters, affording a peak TPH removal efficiency of 97.37%. GC–MS analysis revealed that the composition of the recovered TPH closely resembled that of the original TPH, demonstrating its potential for reuse in formulating oil-based mud. Importantly, these microemulsions can be efficiently regenerated through CO 2 bubbling after washing and TPH release, enabling their sustainable utilization. This strategy offers a promising solution for the effective and sustainable washing of a broader range of oily solid wastes.