The recovery of gallic acid from wastewater by extraction with tributyl phosphate/4-methyl-2-pentanone/-hexane, tributyl phosphate/-octanol/-hexane and -hexanol

The recovery of gallic acid (GA) from GA processing wastewater was studied via solvent extraction. Tributyl phosphate (TBP)/4-methyl-2-pentanone (MIBK)/ n -hexane, TBP/ n -octanol/ n -hexane and n -hexanol were used as the extractants. The effects of the component proportion in the mixed extraction solvent, stirring speed, extraction time, pH, phase ratio (extraction solvent/GA solution), temperature and salt on extraction rate were investigated. After univariate crossover experiments, the optimum conditions were obtained as a component proportion in the mixed extraction solvent of 1 : 2 : 1 at pH 1.5 with a phase ratio of 1 : 1, stirring speed of 150 rpm, extraction time of 60 s and temperature of 30 °C. The maximum extraction rates through the one-stage process were 88.2%, 82.5% and 76.7% for TBP/MIBK/ n -hexane, TBP/ n -octanol/ n -hexane and n -hexanol, respectively. FT-IR spectroscopy was used to characterize the extracted organic phases. Afterwards, Box-Behnken design was used to optimize the three most influential parameters including pH, temperature and phase ratio whose values were optimized to 0.5, 30 °C and 1 : 1, respectively and subsequently, the extraction rate through the one-stage process using TBP/MIBK/ n -hexane, TBP/ n -octanol/ n -hexane and n -hexanol further increased to 95.5%, 86.8% and 78.1%, respectively. TBP/MIBK/ n -hexane was proved to be the most effective extractant in this study and more than 94.4% of GA was recovered through a four-stage stripping process. Finally, TBP/MIBK/ n -hexane was used in actual GA processing wastewater with 92.5% of GA being extracted and more than 88.7% was recovered after the four-stage extraction and stripping process. The results can be referred to for the selection and design of processes to efficiently recover GA from GA processing wastewater. Gallic acid recovery was investigated using two extraction solvent systems. The three most influential parameters obtained through univariate experiments were further optimized by BBD resulting in an enhancement of the extraction rate.