One-Step Hydrothermal Carbonization of Ceratophyllum demersum under Acidic Conditions for Preparation of High-Performance Adsorbents

Large amounts of biomass waste were harvested from constructed wetlands and had to be treated and disposed of. In this study, a one-step hydrothermal carbonization (HTC) process was utilized to dispose of such biomass waste and prepare high-performance adsorption adsorbents with the assistance of sulfuric acid. The experimental parameters were optimized through response surface methodology (RSM) to achieve excellent adsorption performance for methylene blue and mass yield. Based on the RSM results, an HTC product (HTC70–2) obtained under the optimal conditions of 70 wt % sulfuric acid concentration and 2 h of reaction time had shown maximum adsorption capacity (Q_max = 384.08 mg/g) and a higher mass yield (42.90 ± 3.29%). The optimized products were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analysis. The results showed that HTC70–2 had rough surfaces formed by nanoparticles, developed pore structures, and abundant functional groups. Moreover, the adsorption capacity of the sample decreased by only 12.8% after five adsorption–desorption cycle experiments, showing an outstanding renewable performance. FTIR, XPS, and zeta potential analysis were used to investigate the adsorption mechanisms, revealing that electrostatic attraction and surface complexation played a main role in the adsorption process.