Fabrication of a Recyclable Mn–Fe@biochar Composite from Marine Manganese Nodules and Algae Biochar: Experimental and Theoretical Studies of Phenanthrene Adsorption

A magnetic nanocomposite, derived from naturally occurring marine manganese nodules (Mn–Fe) supported on Enteromorpha biochar (designated as HTTM), was synthesized for efficient phenanthrene (PHE) removal from aqueous solutions. Characterization results confirmed its highly ordered 3D layered architecture with intrinsic magnetic properties, attributable to the formation of magnetic oxides (e.g., Mn3O4, γ-Fe2O3, Fe3O4) and abundant surface polar functional groups derived from the oceanic precursor. The composite achieved 97% PHE removal within 2 h under optimized conditions, which is attributed to the synergistic effect of its large surface area and favorable physicochemical properties. The equilibrium and kinetic data were well fitted by the Langmuir isotherm and pseudo-second-order kinetic models, respectively. Furthermore, Mn–Fe@HTTM exhibited considerable regeneration capability and stability, maintaining high PHE removal efficiency over four consecutive adsorption–desorption cycles, wherein its magnetic behavior facilitated convenient separation and recycling. Frontier molecular orbital (FMO) analysis, including the LUMO–HOMO energy gap and electron density distribution, further elucidated the microscopic adsorption mechanism. Adsorption energy calculations revealed that Mn3O4 and Fe3O4 enhanced the composite’s affinity for PHE by strengthening π–π and electron donor–acceptor interactions. This study demonstrates a sustainable route for synthesizing magnetic composites from natural marine manganese nodules (utilizing their intrinsic magnetism) and Enteromorpha biomass, thereby contributing to the green design of functional adsorbents for environmental remediation.