Abstract
Solid waste (SW) generated from extracting of essential oils from medicinal and aromatic plants (MAPs) is an abundant and renewable resource, but proper recycling is necessary to prevent negative environmental impacts from improper disposal. This study focused on converting SW from essential oil (EO) extraction industry by citric acid-catalyzed hydrothermal carbonization (c-HTC) coupled with chemical activation into a carbonaceous material to remove organic pollutants. For this purpose, an I-optimal design coupled with response surface methodology (RSM) was developed to investigate the relationship between the severity factor (SF) of the c-HTC process and citric acid dose (HTCcat) on carbon retention rate (CRR) and hydrochar mass yield (MY). Under optimal conditions (5.32 and 2 g for SF and HTCcat, respectively), an optimized hydrochar (HCop) was characterized by a CRR and MY of 71.02% and 56.14%, respectively. HCop was chemically activated by KOH solution (AHCop) and characterized by a specific surface area of 989.81 m2.g−1, a pore volume of 0.583 cm3.g−1, a higher heating value (HHV) of 37.3 MJ/Kg, oxygenated surface functions such as –OH, –COOH, C-O and a methylene blue (MB) removal rate of 90.71%. In addition, modelling of the adsorption isotherms found that the Freundlich isotherm better describes the experimental data, and the second-order model regulates the adsorption kinetics well. Furthermore, The maximum adsorption capacity of AHCop was 588.24 mg.g−1, with a separation factor of 0.625 under the operating conditions (t = 6 h, T = 25 °C, m(AHCop) = 0.2 g, [MB]0 = 300 mg.L-1, and pH = 7), indicating its potential for effectively removing organic pollutants. Together, these results provide crucial information on using c-HTC to convert waste biomass into functional carbon materials to remove organic pollutants efficiently. Therefore, several studies must be conducted out to discover other application fields of this material.