Valorization of Spent Hops (Humulus lupulus L.) into Biochar and Activated Carbon: Characterization, Phenol Adsorption, and Cost Assessment

Spent hops generated during the dry-hopping stage of beer production were investigated as a precursor for the production of biochar and ZnCl2-activated carbon for phenol adsorption from aqueous solutions. The effects of production conditions on adsorption performance were evaluated through factorial experimental designs, and the resulting materials were characterized by SEM, FTIR, and N2 adsorption–desorption analyses. Chemical activation significantly enhanced the textural properties of the material, increasing the specific surface area from 113.6 to 755.9 m2 g−1 and the pore volume from 0.087 to 0.442 cm3 g−1 compared with biochar. FTIR analysis revealed the presence of oxygen-containing functional groups and aromatic structures that may contribute to phenol adsorption. The activated carbon produced under optimized conditions (700 °C, 30 min, and 130 μm particle size) exhibited an adsorption capacity of 57.32 mg g−1. Kinetic experiments showed rapid adsorption, with equilibrium reached within approximately 60 min, and the pseudo-second-order model provided the best fit to the experimental data (R2 = 0.999). Equilibrium studies demonstrated that adsorption capacity increased with temperature, reaching a Langmuir maximum adsorption capacity of 727.46 mg g−1 at 55 °C. Economic analysis showed that activated carbon presented a higher production cost than biochar (24.35 versus 13.40 US$ kg−1). However, its superior adsorption performance resulted in a lower performance-adjusted cost (0.516 versus 0.856 US$ g−1 of phenol adsorbed). These findings demonstrate that spent hops are a promising feedstock for the production of activated carbon, contributing to both wastewater treatment and the valorization of brewing-industry residues.