Impact of Al and Mn doping on the catalytic activity of magnetite spinel for sulfamethoxazole degradation: kinetics and toxicity assessment†

This study examined the impact of redox-active manganese (Mn) and redox-inactive aluminium (Al) substituted magnetite, both encapsulated in a carbon matrix, on the catalytic wet peroxide oxidation of (10 ppm) antibiotic sulfamethoxazole (SMX). The Lewis acid character of Al in Fe(FeAl)₂O₄@C and the high electronegativity of Mn in Fe(FeMn)₂O₄@C effectively polarized the neighbouring Fe^3+(δ+). This interaction was evidenced by a shift in the Fe³⁺ peak to a higher binding energy of about 1.1 eV in the XPS analysis of both catalysts. However, under the optimized conditions, Fe(FeAl)₂O₄@C decomposed H₂O₂ with a three times higher k_obs value (0.11 min⁻¹) compared to Fe(FeMn)₂O₄@C (0.037 min⁻¹), though both redox-active Fe and Mn are capable of generating HO· from H₂O₂ in the Fe(FeMn)₂O₄@C catalyst. This difference in the kinetics can be attributed to the partial neutralization of the polarization effect of Mn on Fe³⁺ due to competition between H₂O₂ and neighbouring Fe^3+(δ+) for the oxidation of Mn²⁺ in the spinel structure. Consequently, the Fe(FeAl)₂O₄@C catalyst exhibited superior catalytic performance for the degradation of SMX with 60% TOC removal, compared to 50% and 18% attained from Fe(FeMn)₂O₄@C and Fe₃O₄@C, respectively. Furthermore, at higher pH levels, Fe(FeAl)₂O₄@C selectively decomposed H₂O₂ while the Fe(FeMn)₂O₄@C catalyst produced O₂ and H₂O by non-selective decomposition of H₂O₂. The effects of various inorganic anions, organic acids, and water matrices on SMX degradation were investigated over all the catalysts. The Fe(FeAl)₂O₄@C catalyst effectively detoxifies the effluent within 30 minutes. Conversely, effluent from Fe(FeMn)₂O₄@C remains more toxic, showing a 60% mortality rate in acute toxicity assessment after the same reaction time.