Surface–active bismuth ferrite as superior peroxymonosulfate activator for aqueous sulfamethoxazole removal: Performance, mechanism and quantification of sulfate radical

A surface–active Bi₂Fe₄O₉ nanoplates (BF–nP) was prepared using a facile hydrothermal protocol for sulfamethoxazole (SMX) removal via peroxymonosulfate (PMS). The catalytic activity of BF–nP was superior to other catalysts with the following order of performance: BF–nP > Bi₂Fe₄O₉ (nanocubes) >> Co₃O₄ > Fe₂O₃ (low temperature co–precipitation method) > Fe₂O₃ (hydrothermal method) ∼ Bi₂O₃ ∼ Bi³⁺ ∼ Fe³⁺. The empirical relationship of the apparent rate constant (k_app), BF–nP loading and PMS dosage can be described as follows: k_app = 0.69[BF–nP]^0.6[PMS]^0.4 (R² = 0.98). The GC–MS study suggests that the SMX degradation proceed mainly through electron transfer reaction. The XPS study reveals that the interconversion of Fe³⁺/Fe²⁺ and Bi³⁺/Bi⁵⁺ couples are responsible for the enhanced PMS activation. The radical scavenging study indicates that SO₄[rad]⁻ is the dominant reactive radical (>92% of the total SMX degradation). A method to quantify SO₄[rad]⁻ in the heterogeneous Bi₂Fe₄O₉/PMS systems based on the quantitation of benzoquinone, which is the degradation byproduct of p–hydroxybenzoic acid and SO₄[rad]⁻, is proposed. It was found that at least 7.8 ± 0.1 μM of SO₄[rad]⁻ was generated from PMS during the BF–nP/PMS process (0.1 g L⁻¹, 0.40 mM PMS, natural pH). The Bi₂Fe₄O₉ nanoplates has a remarkable potential for use as a reusable, nontoxic, highly–efficient and stable PMS activator.