Strategic Brazilian Minerals Applied in the Photocatalytic Ozonation of Rhodamine B Using a Green Lithium Niobate Nanocatalyst Supported on Silica: Kinetic, Thermodynamic, Mechanism, Machine Learning, and Ecotoxicity Study

This study presents the synthesis and use of a novel heterogeneous lithium niobate nanocatalyst supported on silica (SiO2/LiNbO3) obtained from rice husk for the degradation of the polluting dye Rhodamine B (RhB) through photocatalytic ozonation. The main objective was to create an eco-friendly nanocatalyst from agro-industrial waste (such as rice husk and lemon) and strategic Brazilian minerals (Nb and Li), fostering a circular economy. Rice husk was used for SiO2 extraction, while LiNbO3 nanoparticles were biosynthesized by using the hydrothermal method, employing lemon peel extract (Citrus latifolia). Characterization confirmed the porous morphology and the creation of nanoparticles (34 nm) with a high surface area, ideal for dye diffusion. Using central composite rotatable design (CCRD), the SiO2/LiNbO3 system showed high photodegradation efficiency (>97%) in less than 60 min, adhering to the Langmuir–Hinshelwood kinetic model. Superoxide radicals (O2•–) and valence holes (h+) were the main agents responsible for the degradation process. The ecotoxicity of the final material was low. Furthermore, the use of machine learning (ML) to anticipate the formation of degradation intermediates highlighted the predictive ability of the K-nearest-neighbor (KNN) model.