Green-Synthesized Manganese Oxide Nanoparticles for Efficient Dye Degradation: A Sustainable Approach Toward Achieving SDG 6

Abstract

Introduction: The development of eco-friendly and sustainable nanomaterials has gained significant attention for environmental remediation applications. Green synthesis using plant extracts offers an environmentally benign alternative to conventional chemical routes by acting as both reducing and capping agents. Manganese oxide nanoparticles (MnO NPs) are particularly attractive photocatalysts due to their narrow band gap, chemical stability, low toxicity, and strong visible-light absorption. Textile dyes such as Congo red are persistent organic pollutants that pose serious threats to aquatic ecosystems and human health. Therefore, the fabrication of efficient photocatalysts for dye degradation under natural sunlight is crucial for sustainable wastewater treatment and for achieving Sustainable Development Goal 6 (Clean Water and Sanitation).

Objectives: The primary objective of this study was to synthesize manganese oxide nanoparticles via a green and eco-friendly approach using Crocus sativus L. plant extract and to evaluate their structural, optical, and morphological properties. Additionally, the study aimed to investigate the photocatalytic efficiency of the synthesized MnO nanoparticles toward the degradation of Congo red dye under UV and solar light irradiation.

Methods: Manganese oxide nanoparticles were synthesized using Crocus sativus L. plant extract as a natural reducing and stabilizing agent. The synthesized MnO nanoparticles were systematically characterized using UV–Visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Optical properties were examined using UV–Visible absorption spectra, and the optical band gap was calculated using a Tauc plot. Photocatalytic activity was evaluated by monitoring the degradation of Congo red dye under UV and sunlight irradiation. Reaction kinetics were analyzed using a pseudo-first-order kinetic model.

Results: UV–Visible spectroscopy revealed a strong absorption band in the 280–320 nm range. The Tauc plot confirmed a direct optical band gap energy of 2.94 eV, indicating effective visible-light harvesting capability. XRD analysis confirmed the crystalline nature of MnO nanoparticles with an average crystallite size of approximately 28.99 nm. SEM images showed agglomerated, porous, and uneven surface morphologies, which are favorable for photocatalytic reactions. FTIR spectra confirmed the formation of Mn–O bonds, while EDX analysis verified manganese and oxygen as the major elemental constituents. Photocatalytic studies demonstrated significant degradation of Congo red dye, with higher degradation efficiency under sunlight irradiation compared to UV light. Kinetic analysis showed that the degradation followed pseudo-first-order kinetics, with a higher reaction rate under sunlight due to enhanced visible-light absorption and reduced electron–hole recombination.

Conclusions: This study successfully demonstrates the green synthesis of manganese oxide nanoparticles using Crocus sativus L. plant extract. The synthesized MnO nanoparticles exhibited excellent structural and optical properties along with superior photocatalytic performance toward Congo red dye degradation, particularly under solar irradiation. The enhanced activity is attributed to the narrow band gap, high surface area, and efficient generation of reactive oxygen species. These findings highlight the potential of green-synthesized MnO nanoparticles as promising photocatalysts for sustainable wastewater treatment and their contribution toward achieving Sustainable Development Goal 6 (Clean Water and Sanitation).