Fabrication of Redox Active Mn3O4 on Douglas fir Biochar for Aqueous Arsenic(III) Oxidation and Remediation

Aqueous As(III) is more toxic than As(V). Its remediation from wastewater is challenging due to its neutral charge at neutral water pH. Hausmannite (Mn 3 O 4 ) is the most stable Mn-oxide in geological settings. However, Mn 3 O 4 supported on biochar or any carbon matrix remains unexplored for As(III) redox remediation. In this study, Mn 3 O 4 -modified Douglas fir biochar (Mn 3 O 4 @DFBC), designed to adsorb and oxidize As(III) to As(V), was prepared by NaOH-induced precipitation, followed by passive air oxidation. Mn 3 O 4 @DFBC was characterized by XRD, FTIR, XPS, SEM, SEM-EDX and elemental analysis. Batch adsorption studies at As(III) concentrations of 100 µg/L to 250 mg/L revealed maximum uptake at pH 5-7. The adsorption kinetics best fit the pseudo-second-order kinetic model, while the equilibrium data fitted the Langmuir isotherm best, with a maximum monolayer adsorption capacity of 14.1, 13.0, and 13.1 mg/g at 5, 25, and 40 o C, respectively. Only negligible As uptake occurred on pristine DFBC. The effects of Mn 3 O 4 @DFBC dose, ionic strength, and lake water matrices on As(III) removal were studied. Ionic strength did not affect As(III) uptake while the co-existing ions in the lake water slightly decreased As(III) uptake at 1 g/L Mn 3 O 4 @DFBC dose. Regeneration experiment restored ∼76% of the As(III) removal capacity after three cycles. XPS analysis revealed 65.5% of the adsorbed As(III) was oxidized to As(V), accompanied by Mn(III) reduction to Mn(II). A new As-O XPS peak confirmed the formation of chemisorbed Mn-O-As complexes. In the near-neutral water pH range, As(V) could form monodentate-mononuclear ( 1 V), bidentate-binuclear ( 2 C) and bidentate-mononuclear ( 2 E) Mn-O-As chemisorbed complexes.