Ag Nanoparticles Modified Pectin/Agar Hydrogel Beads for Catalytic Reduction of Nitroaromatic Pollutants and Organic Dyes in Water: Performance, Reusability, and Ecotoxicity Assessment

The pollution and management of water/wastewater caused by organic pollutants (nitroaromatic or synthetic organic dyes) originating from industries has become a highly significant issue worldwide. These highly recalcitrant pollutants have a notorious reputation for being difficult to effectively eliminate using conventional methods. Catalytic reduction has emerged as a promising method for the removal of these pollutants due to its advantages such as operation under mild conditions, high reaction efficiency, and a more environmentally friendly approach. In this study, we developed hybrid system of Ag nanoparticles doped pectin–agar hydrogel beads as heterogeneous catalyst (Ag@Pct–AG) and then characterized it using various techniques, including TEM, XRD, EDS, FT-IR, and FE-SEM, to confirm the formation of well-dispersed Ag nanoparticles. The Pct–AG hydrogel beads, serving as a carrier, successfully stabilized well-dispersed Ag NPs with an average particle size of approximately 10 nm. The catalytic role of Ag@Pct–AG hydrogel beads was evaluated in the reduction of several nitroaromatics (4-nitro-o-phenylenediamine (4-PDA), 4-nitroaniline (4-NA), 2-nitroaniline (2-NA), and -4-nitrophenol (4-NP) and organic dyes (methylene blue (MB), methyl orange (MO) and rhodamine B (RhB)) applying NaBH4 as reducing agent. Ag@Pct–AG hydrogel beads demonstrated its resourcefulness as a catalyst by reducing nitroaromatics within 15–60 s with rate constants ranging from 0.013 to 0.11 s⁻1, and organic dyes within 0–65 s with rate constants between 0.030 and 0.032 s⁻1. Furthermore, Ag@Pct–AG hydrogel beads demonstrated their operational stability by being repeatedly reused over six consecutive cycles. Possible toxicity of Ag@Pct–AG hydrogel beads in aquatic ecosystems were assessed by a model plant Lemna minor. Correspondingly, our findings indicate that the application of 0.15 g/7.6 cm2 Ag@Pct-AG did not induce any observable toxicity symptoms in aquatic ecosystems. However, when aquatic organisms were exposed to concentrations exceeding 0.2 g/7.6 cm2, toxicity can observe.