Tuning bismuth oxyhalides for efficient amoxicillin removal: insights from crystal facets, toxicity and sustainability

In recent decades, the increasing presence of antibiotics in wastewater has become a critical environmental issue, posing significant risks to both ecosystems and human health [1]. Among the available advanced oxidation processes, heterogeneous photocatalysis has emerged as an effective and versatile treatment technology. Within this framework, bismuth oxyhalides (BiOX, where X = Cl, Br, or I) have attracted substantial interest owing to their unique structural and electronic features [2]. These materials crystallize in a distinctive tetragonal structure in which positively charged [Bi₂O₂] layers alternate with double layers of halide anions, generating an internal static electric field perpendicular to the slabs [3]. This built-in electric field promotes the separation and migration of photoinduced electrons and holes, ultimately enhancing photocatalytic efficiency. The photocatalytic performance of BiOX can be further tuned by controlling their exposed crystal facets, which strongly influence surface reactivity, charge dynamics, and adsorption properties. Facet engineering can be achieved through rational design of synthesis conditions—modulating reaction temperature and time, selecting suitable solvents, introducing capping agents, and choosing appropriate halogen precursors [3]. In this study, BiOBr and BiOCl photocatalysts were synthesized using potassium (K) and calcium (Ca) salts as halogen sources, and their bulk and surface properties were comprehensively characterized using a wide set of physico-chemical techniques (FESEM, XRD, N₂ physisorption, Raman, photoluminescence, XPS, UV-Vis DRS, point of zero charge, EIS, Mott–Schottky analysis, and VB-XPS). XRD patterns highlighted the influence of the halogen precursor on crystal growth, with K-derived samples showing an increased exposure of the (110) facet [4]. This structural feature correlates well with the photocatalytic results obtained for amoxicillin (AMX) degradation ([AMX] = 10 mg L⁻¹; catalyst dosage = 0.25 g L⁻¹; irradiance = 35 W m⁻²): K-based BiOBr achieved complete AMX removal within 30 minutes, while the Ca-derived analogue reached ~70% only after 180 minutes. A similar trend was observed for BiOCl. The intrinsic toxicity of the materials was assessed using zebrafish embryo tests, confirming the biocompatibility of all samples. Furthermore, a Life Cycle Assessment (LCA) was performed to compare the environmental impacts of BiOX materials synthesized from different halide precursors, providing insights into greener and more sustainable production routes. In conclusion, the combination of facet engineering, toxicity assessment, and environmental impact analysis represents a robust strategy for optimizing BiOX photocatalysts. These results support the development of efficient, safe, and sustainable materials for the removal of antibiotics from wastewater.