Intensifying heterogeneous TiO2 photocatalysis for bromate reduction using the NETmix photoreactor

Highlights: • For the first time, a mili-photoreactor was used for BrO₃⁻ photocatalytic reduction. • TiO₂ films showed good activity and stability for at least 13 successive reactions. • Front-side irradiation led to a 2.9-fold higher efficiency than back-side irradiation. • [BrO₃⁻] < 10 μg L⁻¹ was obtained after 2-hour reaction at near neutral pH. • Presence of natural organic and inorganic matter impaired the BrO₃⁻ reduction. -- Abstract: This work focuses on the intensification of BrO₃⁻ (200 μg L⁻¹) reduction by TiO₂-assisted heterogeneous photocatalysis, using the NETmix mili-photoreactor illuminated by UVA light-emitting diodes (UVA-LEDs). The mili-photoreactor was assembled in two configurations: i) catalyst deposition on the channels and chambers of a back stainless steel slab (SSS) and ii) catalyst deposition on the front borosilicate glass slab (BGS), allowing the study of front-side (FSI) and back-side (BSI) illumination mechanisms, respectively. The BrO₃⁻ reduction rate in aqueous solution was assessed as a function of: i) pH; ii) dissolved oxygen (DO); iii) addition of formic acid (CH₂O₂) as a sacrificial agent (SA); iv) photocatalyst film thickness; v) illumination mechanism; vi) irradiation intensity; vii) temperature; and viii) water matrix. Higher BrO₃⁻ reduction rates were observed using the FSI mechanism and lower pH values. Nitrogen injection (to eliminate DO) did not significantly improve the reaction rate and the addition of CH₂O₂ had a negative effect at pH 6.5. Neither temperature nor irradiance increase showed a considerable improvement on the reduction rate. Moreover, TiO₂ film remains stable for at least 13 consecutive reactions without significant catalyst leaching. The chemically pre-treated fresh water (FW) matrix negatively affected the reaction rate when compared with the synthetic water (SW), under the best operational conditions (SSS: pH = 5.5, 287 mg of TiO₂, 25 °C, SA absence, [DO] = 232–263 μM). This was associated with the presence of both inorganic and organic matter at much higher concentrations than BrO₃⁻. Notwithstanding, heterogeneous TiO₂ photocatalysis, using the NETmix mili-photoreactor, was successfully applied to fresh water, achieving [BrO₃⁻] < 10 μg L⁻¹ (guideline value) after 2-hour reaction.