Phase equilibrium and mechanism analysis of trioxane in wastewater extraction by hydrophobic deep eutectic solvent

Trioxane (Tox) is an important source of anhydrous formaldehyde, which is used in large quantities in industrial production resulting in large quantities of low concentration Tox wastewater. Liquid-liquid extraction is one of the commonly used methods for wastewater resource recycling. This study presents a method on the separation of Tox from wastewater using hydrophobic deep eutectic solvents (HDES). Firstly, Thy-BA and Thy-DA were screened from 12 HDES by extraction performance and solubility in water. The effects of different molar ratios of HDES, extraction temperature and addition of acetic acid as impurity on the extraction performance were further investigated. For the measured ternary phase data of HDES-Tox-water, the reliability of the data was verified using the Othmer-Tobias and Hand equations, and the data were fitted using the NRTL model. Quantum chemistry was used to calculate the interaction energy of HDES with Tox and to establish a correlation with the extraction efficiency. Hydrogen bonding was found to be the main force for separating Tox by electrostatic potential and interaction region indicator analysis. Molecular dynamics computing found that Thy was a substance that plays a major role in Thy-BA and Thy-DA. Ultimately, it was discovered that the chosen HDES has great recycling and thermal stability characteristics. This work explores three aspects of phase equilibrium, thermodynamics and microscopic separation mechanism revelation to support the systematic recovery of Tox wastewater.