Magnetically Recoverable Fe–Co–N Functionalized Pomegranate Peel–Derived Pyrolyzed Hydrochar for High-Efficiency Malachite Green Remediation: Integrated Kinetic, Isotherm, and Mechanistic Insights

Abstract

In this study, waste biomass was strategically valorized to fabricate a magnetically recoverable Fe–Co–N functionalized pyrolyzed hydrochar (Fe–Co–N/HCpp) composite derived from pomegranate peel via a two-stage thermal treatment (300/700 °C), and its performance for Malachite Green (MG) removal from aqueous media was systematically investigated. FT-IR, XRD, SEM and EDX analyses comprehensively elucidated the structural, chemical, and morphological characteristics, confirming effective surface functionalization and successful heteroatom incorporation. The synthesized composite exhibited pronounced magnetic responsiveness, enabling rapid and straightforward adsorption without secondary contamination. Batch adsorption experiments were conducted to assess MG removal efficiency as a function of contact time, adsorbent dosage, initial dye concentration, and solution pH. Equilibrium and kinetic data were rigorously analyzed using established isotherm and rate models to clarify the governing adsorption mechanism. The results revealed that MG removal efficiency significantly increased with increasing pH, attaining a maximum at pH 8, while adsorption capacity increased proportionally with higher initial dye concentrations due to the enhanced mass-transfer driving force. Equilibrium data were best described by the Langmuir isotherm, yielding a maximum monolayer adsorption capacity of 333.40 mg g⁻¹, whereas kinetic analysis demonstrated superior conformity to the pseudo-second-order model. The adsorption system reached equilibrium within 45 min, during which most accessible active sites became saturated, as evidenced by the negligible additional uptake beyond this contact duration. The outstanding adsorption performance of the composite is attributed to synergistic contributions from electrostatic interactions, π–π electron donor–acceptor interactions between aromatic dye molecules and the carbonaceous matrix, surface complexation mechanisms, and coordination interactions involving Fe–Co catalytic centers. Overall, the findings indicate that fruit peel biomass can be effectively transformed into an environmentally sustainable, cost-efficient, and high-performance magnetic adsorbent, offering a promising alternative to commercial adsorbents for advanced wastewater treatment applications.