Floating BiOCl photocatalytic platforms for solar removal of trace antibiotics from surface water

Introduction Heterogeneous photocatalysis has been widely investigated as a sustainable strategy for the removal of persistent organic contaminants from water, owing to its ability to generate highly reactive oxidative species under light irradiation. Despite the extensive research in this field, the practical implementation of photocatalytic processes remains challenging. Most studies rely on photocatalysts in powder form, which, although offering high surface area and catalytic efficiency, are difficult to recover after treatment and may require additional separation steps. These limitations represent a significant barrier for large-scale or environmental applications [1]. To address these issues, increasing attention has been directed toward immobilized photocatalytic materials. In particular, floating photocatalytic platforms have emerged as a promising approach for solar-driven water remediation. By remaining at the air–water interface, floating systems can maximize light exposure while enabling straightforward catalyst recovery and reuse, making them particularly attractive for treatment processes in natural water bodies [2]. At the same time, the presence of antibiotics in surface waters has become a growing environmental concern. Compounds such as Sulfamethoxazole (SMX) and Trimethoprim (TMP) are frequently detected in freshwater environments and can persist even at trace concentrations, raising concerns about their potential ecological effects and contribution to antimicrobial resistance. In this framework, the present work explores floating concrete platforms coated with Bismuth Oxychloride (BiOCl) as immobilized photocatalytic materials for the solar-driven degradation of SMX and TMP at environmentally relevant concentrations. The photocatalytic performance of the system was evaluated using both simulated solar irradiation and natural sunlight in ultrapure water and in real river water collected from the Pilões River (Brazil), with the aim of assessing the potential of floating photocatalytic materials for practical environmental applications. Material and Methods BiOCl was synthesized by co-precipitation following the procedure reported by Falletta and Bernasconi et al. [3]. The obtained powder was used to coat floating concrete pieces (1 cm3), previously washed with deionized water to remove surface impurities, following a procedure similar to that described by Davari et al. [4]. The resulting materials were characterized using complementary physicochemical techniques to investigate their structural and morphological properties. Photocatalytic experiments were carried out in a reflective batch reactor (V = 400 mL) containing 200 mL of antibiotic solution and irradiated with a simulated solar lamp (Osram Ultra-Vitalux 300 W) with an average power density of 35 Wm-2. The SMX and TMP photodegradation was monitored by LC–MS analysis. Results and Discussion The floating behavior of the concrete supports was first verified to ensure their suitability as floating photocatalytic platforms. The concrete pieces (1 cm3) remained stable at the air–water interface throughout the experiments, confirming their ability to float while maintaining structural integrity during irradiation and mixing conditions. This configuration allows the photocatalyst to remain at the water surface, maximizing light exposure and facilitating catalyst recovery after treatment. The BiOCl successful immobilization onto the floating concrete supports was confirmed by XRD analysis, which revealed its characteristic diffraction peaks. The homogeneity of the coating was further verified by SEM micrographs, showing the typical BiOCl morphology composed of nanosheets arranged in a flower-like structure. UV–Vis DRS investigation revealed a slightly lower band gap (3.02 eV) compared with BiOCl powder (3.32 eV). This behavior can be attributed to the composite nature of the BiOCl–concrete system, which may modify the electronic structure and light absorption properties. The photocatalytic performance of the floating materials was initially evaluated by investigating different catalyst loadings (0.3, 0.4, and 0.5 g of BiOCl). The results indicated that increasing the catalyst loading enhanced the photocatalytic degradation of SMX and TMP, with the highest loading providing the best performance under simulated solar irradiation. To simulate more realistic environmental conditions, experiments were performed using a mixture of SMX and TMP (4:1), a ratio typically found in surface waters, at environmentally relevant concentrations (100 µgL-1). Complete removal of both antibiotics was observed under irradiation, while negligible removal occurred under dark conditions, confirming that the degradation process was mainly driven by photocatalytic reactions. Additional tests were carried out in river water collected from the Pilões River (Santa Catarina, Brazil) spiked with the SMX:TMP mixture. Complete degradation of the investigated antibiotics was achieved also in this matrix, although at a slower rate compared with ultrapure water due to the presence of inorganic ions acting as natural scavengers of reactive species. The stability of the floating BiOCl-coated materials was evaluated over five consecutive reuse cycles (Figure 1). The photocatalyst maintained stable activity throughout the experiments, indicating good adhesion of the BiOCl layer and satisfactory durability of the floating platform. Finally, the photocatalytic activity was evaluated under natural sunlight conditions in Florianópolis (Santa Catarina, Brazil) (27°35'49" S, 48°32'56" W) during December 3RD 2025, under an average solar power density of 178 Wm-2 (see Graphical abstract), the floating photocatalytic system showed faster degradation rates compared with laboratory irradiation, confirming its potential for solar-driven remediation of contaminated surface waters. Figure 1. Reusability tests of the floating Bismuth Oxychloride-coated concrete platform for the photocatalytic degradation of SMX (full marks) and TMP (empty marks) mixture (4:1) at 100 µgL-1 under simulated solar irradiation (35 Wm-2). Conclusions Floating concrete platforms coated with BiOCl were successfully developed as immobilized photocatalytic materials for solar-driven water treatment. The floating configuration enabled efficient light exposure while avoiding the catalyst recovery issues typically associated with suspended photocatalysts. The system showed effective degradation of SMX and TMP at environmentally relevant concentrations, both in ultrapure and real river water matrices. The materials also exhibited good stability over repeated reuse cycles and maintained activity under natural sunlight irradiation. Overall, these results demonstrate the potential of floating photocatalytic platforms as a simple and scalable strategy for the solar-driven remediation of antibiotic-contaminated surface waters.