Abstract
We report a fully biogenic route to ZnO, Fe(3)O(4), and their hydrothermally coupled ZnO/Fe(3)O(4) heterostructure and establish a synthesis-structure-function link. Phase-pure, quasi-spherical wurtzite ZnO and finer inverse-spinel Fe(3)O(4) nanoparticles assemble into a biphasic interface without forming a solid solution; optical analysis yields E(g) = 2.36 eV (ZnO), 1.46 eV (Fe(3)O(4)), and 1.45 eV (ZnO/Fe(3)O(4)), while PL shows near-band-edge quenching and green-yellow defect reweighting at 490-560 nm, consistent with interfacial band bending. Magnetically, ZnO/Fe(3)O(4) is soft-ferrimagnetic with M(S)/M(R)/H(C) = 226 emu g(-1)/17 emu g(-1)/0.010 T (at 300 K), enabling rapid magnetic recovery. Under natural sunlight (572.6 ± 32 W m(-2)), adsorption-corrected methylene blue removal (10 mg L(-1); 10 mg in 50 mL) gives real degradation rates RDR = 90% (ZnO), 65% (ZnO/Fe(3)O(4)), and 30% (Fe(3)O(4)) at 180 min, with pseudo-first-order constants k = 1.9 × 10(-2), 0.7 × 10(-2), and 0.4 × 10(-2) min(-1), respectively; dark adsorption baselines are 10%, 14%, and 49%. Reusability over four cycles preserves pseudo-first-order kinetics (ZnO/Fe(3)O(4): 65% → 50%). Scavenger tests implicate OH• as the dominant oxidant in ZnO and ZnO/Fe(3)O(4), and O2-• in Fe(3)O(4). Taken together, the band alignment, photoluminescence quenching, radical-scavenger profiles, and kinetic synergy are consistent with a defect-rich S/Z-scheme-like ZnO/Fe(3)O(4) heterojunction, delivering a green, sunlight-operable, and recyclable platform for affordable wastewater remediation.