Manganese-based nanostructure-decorated hydrochar derived from pomegranate-peel wastes for highly efficient removal of methylene blue

Abstract This study advances circular-economy principles by converting pomegranate peels into effective sorbents for methylene blue (MB) removal. Accordingly, two spherical MnO x /hydrochar nanocomposites, HCMn0.1 and HCMn0.2, were engineered via one-pot hydrothermal treatment of pomegranate peels with 0.1 and 0.2 M KMnO 4 , respectively, and benchmarked against pristine pomegranate peel-derived hydrochar (PPHC). Comprehensive characterization using SEM–EDX, BET, XRD, XPS, and ZPC demonstrated that KMnO 4 concentration governs MnOx phase evolution and textural properties. The XRD analysis supports crystalline Mn-based nanostructured domains, with rhodochrosite-like and β-MnO 2 -type reflections alongside enhanced crystallinity observed in HCMn0.2 relative to HCMn0.1. MnOx loading also modulated surface area (S BET ), increasing from 6.46 m 2 g −1 for PPHC to 14.17 m 2 g −1 for HCMn0.1, but decreasing to 9.64 m 2 g −1 for higher loading (HCMn0.2). XPS/FTIR confirmed abundant oxygen-containing functionalities (> C=O, –COO⁻, C–OH) and Mn 3+ /Mn 4+ surface species. Optimal MB sorption occurred at pH ~7.0 to 9.0, with nanocomposites reaching equilibrium within ⁓60 min, approximately twice as fast as PPHC. Kinetic data followed pseudo-second-order model, while equilibrium data were best fitted by the Sips model, giving capacities of 323.78, 273.76, and 196.60 mg g −1 for HCMn0.1, HCMn0.2, and PPHC, respectively, at 298 K. Structure–activity analysis reveals that HCMn0.1 outperforms due to well-dispersed MnOx (2.25 mass%/(m 2 g −1 )), ensuring accessible active sites with minimal pore blockage, whereas excessive loading in HCMn0.2 (5.45 mass%/(m 2 g −1 )) promoted aggregation and pore occlusion, reducing S BET and sorption capacity. The enhanced removal reflects synergistic multi-mechanistic MnO x /hydrochar interactions and pore-assisted diffusion. Sorption was exothermic and reversible, with high MB recovery using 0.1 M NaCl and good performance retained over nine cycles. After nine cycles, capacity losses were only 9.5–13.0%, demonstrating good stability and promising reusability for wastewater treatment.