Methylene blue adsorption on factory-rejected tea activated carbon prepared by conjunction of hydrothermal carbonization and sodium hydroxide activation processes

The hydrochar from factory-rejected tea (FRT) was prepared by hydrothermal carbonization and was used as precursor for activated carbon and later activated by NaOH to remove methylene blue (MB) as model dye from aqueous solution. The hydrochar of FRT introduced carbon-rich materials with different functional groups on the surface. This surface functionality was enhanced when chemical modification of NaOH was conducted. The batch adsorption study was performed to remove MB from the aqueous solution using the selected adsorbent. The influences of different adsorption parameters, such as solution pH (3-13), initial concentration (25 mg/L to 500 mg/L), and contact time (0.5-25 h), were investigated. Adsorption kinetics was better described by the pseudo-second-order model and adsorption isotherm was well defined by the Langmuir isotherm. The maximum monolayer adsorption capacity of the selected adsorbent for MB dye was 487.4 mg/g at 30 °C, which has great potential for the removal of organic dye from aqueous solution. This study suggests that the hydrothermal carbonization is a very promising way to produce quality adsorbents for wastewater treatment.