Efficient Adsorption of Pollutants from Aqueous Solutions by Hydrochar-Based Hierarchical Porous Carbons

Three-dimensional hierarchical porous carbons (HPCs) created through hydrothermal carbonization and the subsequent chemical activation of miscanthus were tested as adsorbents of Pb²⁺ and methylene blue from the aqueous solution. The HPC pore structure was customized using various hydrochar precursors obtained through a longer reaction time and by adding acetic acid. HPC obtained from hydrochar derived from acetic acid’s addition exhibited the highest specific surface area due to a larger micropore volume. This adsorbent proved to be the most efficient in removing lead from aqueous solutions. The Langmuir isotherm best described the lead adsorption process onto HPC with q_m = 155.6 mg g⁻¹ and the pseudo-second-order kinetic model. HPC obtained from hydrochar produced with a longer reaction time exhibited improved methylene blue adsorption properties. The pseudo-second-order kinetic model and the Freundlich isotherm best described the experimental data. The theoretical maximum adsorption capacity for methylene blue was 316.0 mg g⁻¹. The type of hydrochar significantly impacted the yield and physical structure of HPCs, while having a lesser effect on the composition of surface functional groups. The results revealed the binding mechanism of each pollutant, highlighting the importance of biomass pretreatment on the structure of the resulting HPC and its effectiveness in water purification.