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Activated carbon was produced by chemical activation of hydrothermally carbonized (HTC) banana empty fruit bunch (BEFB), using phosphoric acid (H3PO4) as the activating agent. The activation process was optimized using a Box-Behnken factorial design (BBD), with an outcome of 17 different experiments under the predefined conditions. Three different parameters (activation temperature (x1), activation time (x2), and the concentration of activating acid (x3)) were analyzed with respect to their influence on maximum adsorption percentage for divalent cations, Pb(II) (Y1) and Zn(II) (Y2), and carbon yield (Y3). All process parameters had strong positive effects on adsorption capacity up to a certain limit. The specific surface area of the hydrochar (HTC) was enhanced substantially after the activation process. Scanning electron microscopy (SEM) revealed that the morphology of the BEFB-based char changed noticeably after the acid impregnation and activation process. The Langmuir maximum monolayer adsorption capacity for Pb (II) and Zn (II) cations was 74.62 mg/g and 77.51 mg/g, respectively. Equilibrium isotherm data were in agreement with the Langmuir model. Thermodynamic characterization revealed that the equilibrium system was endothermic and spontaneous.

Phenol and p-nitrophenol (PNP) are priority pollutants widely present in wastewater. Developing superior or low-cost sorbents for their removal would be of great benefit. Here, corncob residues (CCR) were converted to hydrochars via hydrothermal carbonization (HTC) and further upgraded to carbon materials by thermal activation in an N2 atmosphere. The influence of HTC conditions including the temperature, residence time, and CCR/water weight ratio on the material properties and their performance for removing phenol and PNP from water were investigated and compared with those that were obtained from pyrochar (directly pyrolyzed CCR). Hydrochars showed lower adsorption capacities for phenols than pyrochar. The initial hydrothermal treatment at 220 °C and 2 h resulted in an improved porosity and 4- to 5-fold higher adsorption capacities for phenol and PNP compared with the pyrochar. However, hydrochars prepared at 250 °C or with a prolonged residence time (4 and 6 h) could not be upgraded to high performance carbon materials by thermal activation. The adsorption isotherms of both phenols on the best performance material were well correlated by the Sips model.