Bio-templated green synthesis of ZnO nanostructures using herbal seed Mucilages: A sustainable route for dye adsorption

Abstract

Conventional methods for removing toxic dyes from aqueous media, often impose high costs or result in the release of secondary pollutions. So as an alternative solution, utilization and assessment of sustainable adsorbents obtained from green chemistry principles are interesting and progressive. This study investigates the synthesis, characterization, and adsorption performance of herbal-mediated zinc oxide (ZnO) nanoparticles to remove Congo red (CR) dye from aqueous solutions. The novelty of this study lies in the bio-templated green hydrothermal synthesis of ZnO nanoparticles using mucilages extracted from four herbal seeds, such as Plantago ovata, Alyssum homalocarpum, Plantago major, and Cydonia oblonga as biogenic directing agents. This approach marks the first report of such a morphologically controlled synthesis using the plant-based precursors, yielding a nanopowder with high crystallinity and mesoporous architecture (average pore size of 24.49 nm and surface area of 10.36 m²/g). Adsorption studies revealed a maximum capacity of 120.48 mg/g, following pseudo-second-order kinetics (R² = 0.9929) and the Langmuir isotherm model (R²=0.997). Thermodynamic analysis confirmed the exothermic nature of the process, with spontaneous and entropy-driven characteristics. The adsorbent exhibited pH-dependent performance, with optimal removal efficiency (98 %) at acidic conditions, attributed to electrostatic interactions between protonated ZnO surfaces and anionic CR species. Regeneration studies showed a 25 % capacity loss over five cycles, linked to mesopore occlusion and surface hydroxyl depletion. This work demonstrates the potential of plant-mediated ZnO as a sustainable adsorbent for textile wastewater treatment while providing mechanistic insights into its structure-performance relationship.