Efficient adsorption of cationic and anionic dyes using hydrochar nanoparticles prepared from orange peel

Hydrochar nanoparticles biosorbent (HCNPs), derived from orange peel was prepared via an in-situ hydrothermal carbonization method. The HCNPs were tested as an effective biosorbent for removing cationic MB (methylene blue) and anionic MO (methyl orange) dyes. The prepared HCNPs was characterized using FTIR, CHN/O, SEM-EDX, UV–visible, XRD, PZC-titration, BET, and XPS. The biosorbent exhibits a good specific surface-to-volume ratio (28.23 m²/g and 0.069 cc/g, respectively) and mesopores size (2.41 nm). UV–Vis data confirm the graphitic nature of the carbon structures. XPS analysis reveals a high enrichment of multiple oxygen functionalities (including C-OH, C = O, and –COO^- groups) on the surface. The optimal initial pHs are 3.0 and 5.0–8.0 for removing MO and MB, respectively. Electrostatic attraction plays a substantial role, with other hydrophobic forces in dye removal and binding. Equilibrium is attained during 60/90 min, with half-biosorption-time (t_HST) for MB and MO are 4.72 and 6.71 min, respectively. The highest biosorption capacities are 203.4 mg/g for MB and 113.3 mg/g for MO, closely correlated with physiochemical characteristics. Experimental data follows Langmuir isotherm and PSO kinetic models, indicating monolayer chemisorption on a homogeneous surface. The biosorption process for both dyes is exothermic and spontaneous. HCNPs show high durability over multiple biosorption/desorption cycles using HCl and NaOH (0.1 M) for MB and MO, respectively. These findings highlight the promising potential of HCNPs as efficient biosorbents for removing both anionic and cationic dyes.