Thermochemical Treatment of Lignocellulosic Biomass <i>Ceratophyllum demersum</i> at Temperatures below 100 °C to Prepare the Carbon Adsorbent

Hydrothermal carbonization (HTC) is a thermal conversion process that has been widely studied in the field of waste biomass management. However, conventional HTC reactors require operation under high pressure, which leads to economic and safety issues during both manufacturing and operation processes. To address this challenge, a thermochemical treatment process with the assistance of sulfuric acid was adopted to promote carbonization of the typical lignocellulosic biomass Ceratophyllum demersum into carbon adsorbents at temperatures below 100 °C. After optimization by response surface method, the sample S80-8-70 obtained under a reaction temperature of 80 °C, 8 h of reaction time, and 70 wt % of sulfuric acid concentration showed excellent adsorption capacity (203.80 ± 17.88 mg/g), along with a higher mass yield (40.15 ± 0.69%). To explore the mechanism of low-temperature processes of biomass conversion, the characterizations of lignocellulosic components (cellulose, hemicellulose, and lignin) and their corresponding biochars under optimal conditions were also conducted. Scanning electron microscopy analysis showed that the surface morphology of cellulose and hemicellulose underwent significant changes during the thermochemical treatment process, while the lignin remained unchanged. The Brunauer–Emmett–Teller results indicated that sample S80-8-70 had well-developed mesoporous structures and a higher specific surface area compared to that of the biochars from lignocellulosic components. Fourier transform infrared spectroscopy showed that the functional groups of the S80-8-70 sample were similar to those of the biochars derived from cellulose and hemicellulose while retaining some characteristics of lignin. The ζ-potential analysis also indicated that the surface of the sample carried a negative charge, consistent with the biochars from cellulose and hemicellulose. These results demonstrate that lignocellulosic biomass was successfully converted into carbon adsorbent materials at temperatures below 100 °C. In the thermochemical process, cellulose and hemicellulose underwent carbonization, while the residual lignin had little impact on the carbonization and properties of the carbon materials. Moreover, by using a step-by-step washing method, high-concentration washing wastewater could be recovered and used for the next reaction.