Efficient recovery of phenol from coal tar processing wastewater with tributylphosphane/diethyl carbonate/cyclohexane: Extraction cycle and mechanism study

Abstract Sodium sulfate wastewater is a kind of coal tar processing effluent containing highly concentrated phenol. Owing to its high toxicity, phenol pose a serious threat to the natural environment and human health. In this paper, a novel ternary extractant, tributylphosphane (TBP)/diethyl carbonate (DEC)/cyclohexane, was utilized to extract phenol from the wastewater. The combination of 20% TBP, 20% DEC, and 60% cyclohexane yielded a high extraction efficiency of 99.79% under the suggested conditions (time of 6 min, organic–aqueous volume ratio of 1, pH of 5.05 and temperature of 298.15 K). Fourier-transform infrared spectroscopy analysis illustrates that the extraction of phenol was enabled by intermolecular hydrogen bonding with both TBP and DEC. The stripping of phenol from the phenol-loaded extractant was carried out using an alkali solution, realizing the recovery of phenol resource and the regeneration of extractant. The stripping efficiency reached 96.29% when using 1 mol/L NaOH solution under certain conditions (contact time of 4 min, aqueous–organic volume ratio of 1, and temperature of 313.15 K). Additionally, the pH response analysis shows that the stripping process could be divided into four stages and more than 8 min of contact time leaded to the hydrolysis of the DEC. The ultra-high phenol recovery, up to 99.78%, was achieved from the actual wastewater via a two-stage extraction and back-extraction.