Nitrogen-rich, potato peel-derived hydrochar as an effective dye adsorbent

Abstract

This study explores the potential of hydrochar derived from potato peels as a cost-effective option for efficient adsorption. Potato peel hydrochar (PPH) was produced via hydrothermal carbonization (HTC) with KOH activation at 200 °C for 2 h. The resulting hydrochar was then divided into three portions, two of which underwent mixing with urea followed by pyrolysis under N2 gas at 600 °C for 2 and 3 h, respectively, resulting in the formation of N-doped pyrochars (N-PPH.2 hrs and N-PPH.3 hrs). Characterization using Brunauer-Emmett-Teller (BET), Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) revealed distinctive properties. The results showed that nitrogen atoms were effectively incorporated into the pyrochar, increasing significantly from 3.7 to 8.3% and then to 45.6% as new nitrogen compounds. This modification impacted the acidity and basicity of the chemical surfaces of the pyrochar. The N-doped pyrochar, with a particle sizve of N-doped compounds at 296.11 nm, demonstrated the most efficient adsorption of Congo red (CR). This efficiency was achieved through various mechanisms, including pore filling, Lewis acid-base interaction, hydrogen bonding, and π–π interaction. The presence of highly available functional groups, primarily pyridinic-N, pyrrolic-N, and graphitic-N, further enhanced the adsorption capacity. The highest adsorption capacities were observed at 310 mg g−1 for N-PPH-2Hr and 400 mg g−1 for N-PPH-3Hr, respectively. Increased temperature and CR concentration correlated positively with enhanced adsorption, following Freundlich isotherm and pseudo second-order kinetics. Further, the results indicate that the adsorption of Congo red by the prepared chars is endothermic, increases disorder, and is a spontaneous process. This study highlights the cost-effectiveness of agro-industrial waste-derived chars for Congo red removal, highlighting economic and environmental benefits.