Nanoflower-structured BiOX (X = Cl, Br) for advanced photocatalytic degradation of the antidepressant venlafaxine

Pharmaceuticals are stubborn pollutants that resist degradation, lingering in wastewater systems as conventional activated sludge processes in treatment plants fail to degrade them effectively. As a result, these compounds accumulate in waterways or sludge, posing significant risks to ecosystems and public health. Growing concerns over these pollutants have fuelled the search for more efficient degradation processes. Semiconductor photocatalysis has emerged as a promising approach due to its capacity to degrade micropollutants and detoxify water. In this study, BiOCl and BiOBr nanostructures were synthesised by a microwave-assisted method using cetyltrimethylammonium chloride (CTAC) and Cetyltrimethylammonium Bromide (CTAB) precursors at different concentrations (5–15 mM). The resulting nanoflowers exhibited high porosity, stability in water, and tuneable photocatalytic performance. Under UV irradiation, BiOCl@15 mM CTAC and BiOBr@5 mM CTAB achieved nearly complete venlafaxine (VNX) removal within 8 h, with rate constants of 0.406 and 0.454 h −1 , respectively, and full detoxification confirmed by Vibrio fischeri assays. In larger-scale tests, BiOBr@5 mM CTAB reached almost complete VNX degradation in only 2 h (k = 1.526 h −1 ). The same material also showed visible-light activity, removing ~95 % of malachite green (MG) after 9 h (k = 0.273 h −1 ). Cycling experiments demonstrated high stability and reusability, with BiOCl@15 mM CTAC maintaining >97 % removal over four cycles and BiOBr@5 mM CTAB showing >94 % removal in the first two cycles. Identified transformation products revealed hydroxylation and demethylation as dominant degradation pathways. These findings demonstrate the potential of BiOCl and BiOBr nanocatalysts as efficient and reusable photocatalysts for pharmaceutical wastewater treatment under both UV and visible light.