Biopolymer-stabilized metal/metal oxide nanocomposites for efficient and sustainable removal of methylene blue dye and 4-nitrophenol from aqueous systems through the catalytic reduction process: A comprehensive review

Water contamination by hazardous organic dyes and nitrophenols, particularly methylene blue (MB) and 4-nitrophenol (4-NP), is one of the critical environmental challenges in recent times. Conventional remediation methods often face limitations, including inefficiency, high cost, and the generation of secondary pollution. Catalytic reduction using metal/metal oxide (M/MO) nanoparticles (NPs) is a promising alternative for converting these pollutants into less harmful substances. However, the practical application of bare M/MO NPs is hindered by issues of aggregation, instability, and leaching. This paper comprehensively discusses the innovative strategy for stabilizing M/MO NPs with eco-friendly and cost-effective biopolymers. The paper explores strategies for extraction of biopolymers, synthesis and characterization of biopolymer-stabilized M/MO nanocomposites (NCs), their effectiveness in reducing 4-NP and MB, their stability and reusability, as well as the mechanisms of the reduction process. Furthermore, the critical operational parameters influencing catalytic efficiency are discussed, and performance is evaluated through key catalytic performance metrics such as the conversion rate, rate constant, and turnover frequency. The finding of the review revealed that biopolymer incorporation into M/MO NPs significantly enhanced MB and 4-NP conversion efficiency while improving nanoparticle dispersion, stability, and reusability. Despite their considerable advantages, challenges such as potential nanoparticle leaching, biopolymer degradation, and the need for reducing agent optimization remain. The review finally presents future research directions, which include the development of greener alternatives to NaBH 4 , computational mechanistic studies, and scaling up the technology for real-world wastewater treatment applications. • The review examined the roles of biopolymers as stabilizers for catalytic reduction of 4-NP and MB. • Biopolymer-stabilized M/MO NCs demonstrated exceptional catalytic performance. • The NCs can achieve conversion rates of 90–100 % for both 4-NP and MB under optimized conditions. • The NCs demonstrated excellent reusability and stability, with 5–10 catalytic cycles. • The catalytic innovation provides eco-friendly wastewater treatment that aligns with UN SDG 6.