Hydrochar from Spent Coffee Grounds: Characterization and Application for Phenol Removal from Aqueous Solutions Under Static and Dynamic Conditions

In this work, spent coffee grounds (SCGs) waste is converted into an activated hydrochar (AHC) through hydrothermal carbonization (HTC) at 180 °C followed by steam activation at 850 °C for 1 h. The AHC was firstly characterized and then tested for phenol removal from synthetic aqueous solutions in both batch and column modes. Characterization results show that the AHC is an amorphous material rich in numerous functional groups and has interesting textural properties, with a surface area and a total pore volume of 219.2 m2 g−1 and 0.156 cm3 g−1, respectively. These promising properties permit the AHC to successfully remove phenol from aqueous solutions under wide-batch experimental conditions with an adsorption capacity of 226.4 mg g−1. Moreover, this process was found to be spontaneous, endothermic, and favorable, involving physical and chemical mechanisms. Furthermore, even in column mode, the AHC retains a strong ability to remove phenol, with an adsorbed amount of 42.3 mg g−1 observed for the lowest flow rate (14 mL min−1). The phenol breakthrough curves (BTCs) from the column were best fitted with a Thomas model, indicating that limitations of external and intraparticle mass-transfer of phenol are relatively small. Finally, the phenol-loaded AHC can be efficiently regenerated with hot water for at least two cycles. Indeed, the average adsorption efficiencies in the second cycle were evaluated to be 97.7% and 72.7% of those measured in the first cycle, in batch and column modes, respectively. These promising technical results should be consolidated with a detailed life-cycle assessment study in the future.