SARGASSUM TENERRIMUM SEAWEED-MEDIATED GREEN SYNTHESIS OF TERNARY SW/AG–ZNO NANOCOMPOSITES FOR ENHANCED PHOTOCATALYTIC DEGRADATION OF METHYLENE BLUE AND REACTIVE BLUE 198 DYES

Abstract

The accumulation of recalcitrant synthetic dyestuffs in aquatic ecosystems poses a severe threat to environmental sustainability and human health. This study reports the eco-friendly green synthesis of novel ternary nanocomposites comprising Sargassum tenerrimum seaweed biomatrix integrated with silver and zinc oxide nanoparticles (SW/Ag–ZnO NCs). Biosynthesis was driven by bioactive phytochemicals extracted from Sargassum tenerrimum, which functioned as natural reducing, capping, and stabilizing agents. Comparative evaluations were conducted across single, binary, and ternary compositions, including seaweed-supported silver nanoparticles (SW–Ag), seaweed-supported zinc oxide nanoparticles (SW–ZnO), silver–zinc oxide nanocomposites (Ag–ZnO), and the fully integrated ternary composite (SW/Ag–ZnO). Optical absorption spectroscopy revealed surface plasmon resonance (SPR) characteristic of Ag nanoparticles alongside shifted excitonic transitions in the ternary matrix. Optical band gap analysis using Tauc plots confirmed a reduction in energy band gap down to 2.5eV–3.0 eV for the heterostructure, facilitating efficient visible-light absorption. Fourier-transform infrared spectroscopy (FT-IR) confirmed functional group participating moieties (hydroxyl, carbonyl, carboxyl, and amine groups) originating from algal polysaccharides and proteins involved in metal ion coordination and lattice stabilization. X-ray diffraction (XRD) patterns verified the co-existence of face-centered cubic Ag and hexagonal wurtzite ZnO crystalline phases within the composite matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) confirmed the formation of highly dispersed, spherical to quasi-spherical nanoparticles anchored onto the biopolymer support. Thermogravimetric analysis (TGA) demonstrated superior thermal stability of the ternary composite, displaying distinct weight-loss stages corresponding to moisture release, biomatrix decomposition, and structural phase stabilization. Brunauer–Emmett–Teller (BET) analysis revealed a Type IV isotherm with H3 type hysteresis, indicating a porous mesoporous framework with enhanced specific surface area (>35 m2g) and pore volume. Photodegradation kinetics were evaluated under irradiation against target organic pollutants: cationic Methylene Blue (MB) and anionic Reactive Blue 198 (RB 198) dyes. The ternary SW/Ag–ZnO nanocomposite exhibited exceptional photocatalytic efficiency, achieving over 97.2 % degradation for MB and 99.1 % for RB 198 within150min, vastly outperforming individual components (SW–Ag, SW–ZnO) and binary systems (Ag–ZnO). Kinetic modeling verified pseudo-first-order degradation behaviour, where rate constants (k) were drastically enhanced due to synergistic charge transfer, plasmonic hot electron injection from Ag SPR, and reduced electron-hole recombination mediated by the integrated Sargassum tenerrimum biopolymer scaffold.