Easily recoverable bismuth-based catalytic materials for integrated capture and solar removal of polyphenols from water

Significance and Relevance The removal of polyphenols from wastewater remains a key challenge due to their dual nature as valuable bioactive compounds and persistent pollutants. This contribution presents a novel, eco-friendly strategy based on recoverable bismuth-alginate systems that integrate adsorption and solar photocatalysis within a single materials platform. Rather than focusing only on removal efficiencies, the coupled process is assessed in terms of complementary material functions. By combining Bi3+-modified alginate beads for high-capacity capture and BiOBr-based magnetic alginate composites for solar-driven degradation, the results demonstrate that adsorption and photocatalysis play distinct but synergistic roles, providing a more rigorous, process-oriented and sustainable framework for advanced water treatment. Introduction and Motivations Polyphenols are increasingly detected in agro-industrial wastewaters, particularly those generated by olive oil production, due to their high stability and poor removal by conventional treatment technologies. Despite their well-known antioxidant and bioactive properties, polyphenols become environmentally harmful at elevated concentrations, contributing to water toxicity and ecosystem imbalance. Moreover, their persistence and complex interaction with treatment processes make their effective management particularly challenging. Among advanced treatment approaches, adsorption and heterogeneous photocatalysis have attracted significant attention for polyphenol management. However, when applied individually, both strategies suffer from intrinsic limitations, including incomplete removal, poor selectivity, catalyst recovery issues, and limited process integration. Photocatalytic systems based on powdered materials often display promising degradation kinetics while neglecting recoverability and scalability aspects. The combination of adsorption and solar-driven photocatalysis within recoverable materials has recently emerged as a promising strategy. By exploiting bio-inspired supports and visible-light-active bismuth-based phases, it becomes possible to decouple capture and degradation steps within a single platform. In this framework, the present study moves beyond single-process evaluation and critically investigates integrated bismuth–alginate systems, focusing on how complementary catalytic functions can be rationally combined within a single materials platform. Materials and Methods Alginate spheres were prepared by ionotropic gelation of sodium alginate in an aqueous CaBr2 solution, while magnetic alginate spheres were obtained by dispersing magnetite nanoparticles in the alginate solution prior to gelation, enabling magnetic recovery. Bi3+-modified alginate spheres were synthesized following the same procedure, substituting Ca2+ with Bi3+ ions as crosslinking agents, resulting in alginate networks exclusively coordinated by Bi3+ centers. BiOBr/magnetic alginate spheres were prepared through a sequential immersion procedure, promoting the controlled growth of a thin BiOBr layer on the surface of magnetic alginate spheres. The materials were characterized by several physico-chemical techniques. Gallic acid and 3,4,5-trimethoxybenzoic acid were selected as model polyphenols. Adsorption tests were performed under dark conditions, while photocatalytic experiments were carried out under simulated solar irradiation. A sequential adsorption–photodegradation protocol was applied to evaluate the integrated process performance. Results and Discussion Adsorption and photocatalytic behavior of the investigated materials revealed a clear structure–function relationship. Bare and magnetic alginate spheres exhibited negligible adsorption toward both polyphenols, with removal efficiencies below 10%, confirming the inert nature of the investigated support. The inclusion of magnetite did not alter adsorption performance, but solely enabled efficient post-treatment recovery. BiOBr/magnetic alginate spheres displayed only a moderate affinity toward gallic acid and no detectable adsorption of 3,4,5-trimethoxybenzoic acid, highlighting the limited chelating ability of BiOBr species compared to exposed Bi3+ centers. In contrast, Bi3+-modified alginate spheres showed an enhanced adsorption capacity, particularly toward gallic acid, reaching approximately 80% removal within 180 min. Adsorption of 3,4,5-trimethoxybenzoic acid remained significantly lower, consistently reflecting the reduced interaction between methoxy-substituted aromatic rings and Bi3+ sites1. These results clearly demonstrate that adsorption efficiency is governed by both the accessibility of metal centers and the molecular structure of the target polyphenols, as schematically illustrated by the proposed chelation models. Photocatalytic experiments under simulated solar irradiation confirmed that BiOBr/magnetic alginate spheres retained photocatalytic activity toward both polyphenols. However, degradation efficiency progressively decreased at increasing initial concentrations, underlining the intrinsic limitations of photocatalysis when applied as a standalone process for highly loaded systems2. This evidence provided the rationale for integrating adsorption and photocatalysis within a sequential treatment strategy. The coupled adsorption–photodegradation approach proved highly effective: initial capture of polyphenols by Bi3+-modified alginate spheres followed by solar irradiation in the presence of BiOBr/magnetic alginate composites enabled almost complete abatement, achieving overall removal efficiencies close to 99%. Overall, these results demonstrate that decoupling capture and degradation functions within complementary, recoverable bismuth–alginate materials enables a rational and scalable catalytic strategy, representing a significant process-level advancement for sustainable polyphenol removal1.