Precipitation-Dissolution switchable surfactants with the potential of simultaneous retrieving of surfactants and hydrophobic organic contaminants from emulsified and micellar eluents

Simultaneous recovery of surfactants and retrieval of hydrophobic organic contaminants (HOC) from the eluent in surfactant-enhanced washing is essential for saving useful resources and avoiding secondary contamination. A pH-triggered precipitation-dissolution switchable surfactant, sodium 3-laurylaminopropanesulphonic acid (LMPS), for both emulsified and micellar eluents was reported. After tuning the pH from 12.5 to 7.5, the water-soluble LMPS is converted to its oil–water insoluble inner salt LMP; then LMP is converted back to LMPS by varying pH to 12.5, by which the LMPS-based micelles and emulsions are of reversibly switchable under pH stimuli. The removal ratio of tetradecane (15 g/30 g sand) and phenanthrene (Phe, 3.5 mg/5g sand) is no less than 99 % by washing with 150 g LMPS solution (1.0 × 10−2 mol/L) and 100 g LMPS solution (0.5 wt%), respectively. The retrieval ratio of tetradecane from the emulsified eluents was around 94 % and without obvious adverse effects over 5 cycles. The retrieval ratio of Phe from the micellar eluents via a combination of co-precipitation and Soxhelt extraction was around 98 %, which can be further simplified and increased to ∼100 % by spiking the micellar eluents with a small amount of tetradecane (1 g /100 g eluent). The recovery ratio of LMPS from micellar eluents was approximately 96 %, and in the case of emulsified eluents it remained around 99 % after 5 cycles. After a simple and conventional treatment with anion-exchange resin and active carbon (0.25 g/100 g waste water), the chemical oxygen demand (COD) of the waste water was remarkably reduced from 500 to 1700 mg O2 L−1 to less than 30 mg O2 L−1.