Magnesium-iron modified sludge biochar for methyl orange removal via adsorption and periodate-based advanced oxidation

Abstract

To enhance dye removal efficiency, this study employed a bimetallic modification technique combining magnesium and iron to composite the adsorbent with a catalyst. This resulted in the preparation of a biochar composite material (SBC-a) rich in iron oxide and MgFe2O4, enabling both dye enrichment and in situ degradation. The successful incorporation of magnesium and iron into the biochar matrix was confirmed via comprehensive characterization techniques, including SEM, XRD, FTIR, and XPS. The experimental results show that the adsorption kinetics of SBC-a follows the pseudo-second-order reaction model. Additionally, the Freundlich adsorption isotherm fits well, indicating the existence of heterogeneous surface adsorption. The adsorption process occurs in three stages and is endothermic in nature. In subsequent adsorption-coupled advanced oxidation experiments, under optimized conditions (pH = 3.0, SBC-a dosage of 0.15 g L-1, and sodium periodate concentration of 0.8 mM), the removal efficiency of methyl orange (MO), the target pollutant, reached 93.3%, demonstrating high effectiveness in pollutant degradation under acidic conditions. The observed reaction rate constant during the oxidation stage (kobs = 0.0179 min-1) was 3.7 times higher than that of the original biochar, demonstrating enhanced catalytic performance due to the synergistic effect of magnesium and iron bimetallic components. Free radical quenching experiments and mechanistic analysis reveal that the activation by oxygen-containing functional groups and the Fe2+/Fe3+ redox cycle activate sodium iodate, thereby generating multiple reactive species, including 1O2, Fe(IV), IO3•, and O2•−. In addition, liquid chromatography-mass spectrometry (LC-MS) was used to identify four possible decomposition pathways and intermediate products during the MO degradation process. The biochar composite material developed in this study provides a promising strategy for the value-added utilization of municipal sludge and the effective treatment of dye-contaminated wastewater.