How to recycle surfactant and retrieve hydrophobic organic contaminants (HOCs) in the remediation of HOCs-polluted soil by means of surfactant-enhanced soil washing (SESW) is still a significant challenge. Here, a proof-of-concept strategy for solubilizing a model HOC of phenanthrene (Phe) in artificial Phe-polluted sand and co-pricipitating surfactant-Phe from the SESW effluent by reversible tuning Krafft temperature (K T ) of surfactant via CO 2 /N 2 -bubbling was reported for the first time. In the presence of N,N,N’,N’ -tetramethyl-1,2-ethylenediamine (TEMDA), the K T of sodium alkyl sulphonates (C n AS, n = 12, 14 and 16) was reduced after CO 2 -bubbling because of the formation of TEMDA carbonates; in contrast, the TEMDA carbonates could be deprotonated after the removal of CO 2 by N 2 -bubbling, which enabled the K T to increase, allowing subsequent precipitation of C n AS. By alternating CO 2 - and N 2 -bubbling, the K T of C n AS-TEMDA could be reversibly switched over 10 cycles without causing obvious adverse distortions. The potential usage of this novel K T -tuning strategy was demonstrated in a conceptual C 12 AS-TEMDA-enhanced Phe-polluted sand washing. About 99.9% of Phe was solubilized by C 12 AS at room temperature, about 92% of C 12 AS and 99% of Phe could be co-precipitated, allowing both the total organic carbon and chemical oxygen demand of the residue wastewater were remarkably reduced to less than 25 mg L −1 after a simple and conventional treatment with active carbon and ion-exchange resin. The precipitated C 12 AS could be separated from Phe by a simple Soxhlet extraction.
The incorporation of hydroxy groups into long-chain quaternary ammonium surfactants can enhance their solubility and confer other desirable properties; however, the systematic investigation on the solution properties and the influence of molecular structures was less documented. In this work, two saturated C 18 -tailed quaternary ammonium surfactants with dual hydroxyethyl moieties, namely N , N -bis(2-hydroxyethyl)- N -methyloctadecan-1-aminium iodide (HEOAI) and N , N -bis(2-hydroxypropyl)- N -methyloctadecan-1-aminium iodide (HPOAI), were designed and synthesized. We investigated and compared their Krafft temperature ( T k ), surface activities, self-assembly behaviors, and rheological properties. Our findings demonstrate that the introduction of hydroxyethyl groups dramatically improves the solubility and surface activities. The T k of HEOAI and HPOAI is approximately 50 °C lower than that of the corresponding surfactant octadecyl trimethylammonium iodide. Both surfactants exhibit lower critical micelle concentration and equilibrium surface tension compared to those of corresponding conventional quaternary ammonium surfactant. This should be ascribed to the formation of hydrogen bonds between hydroxyl groups and water molecules. Moreover, HPOAI shows even better solubility and surface activities than HEOAI does. Additionally, the presence of two more methyl groups for HPOAI enhances its ability to form worm-like micelles in pure water. However, HEOAI shows better salt and temperature resistance for maintaining stability of worm-like micelles. A solution containing 50 mM HEOAI with 10 % NaCl exhibits temperature tolerance up to around 80 °C, indicating potential application as fracturing fluid for shallow and medium oil/gas reservoirs. These findings contribute to designing surfactants with improved solubility and diverse properties.
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