Green Nanomaterials Rapidly Destroy Industrial Pollutants Threatening Human Health

Industrial wastewater contaminated with para-nitrophenol (p-NP) and methylene blue (MB) poses serious risks to aquatic ecosystems and human health, causing respiratory disorders, skin diseases, and neurological damage even at low concentrations. Conventional remediation methods — including adsorption, coagulation, and biological treatment — suffer from low efficiency, high costs, slow kinetics, and generation of secondary toxic by-products, motivating the search for superior alternatives.

This comprehensive review examines green-synthesized nanomaterials as nanocatalysts for the reductive degradation of p-NP into p-aminophenol (p-AP) and MB into colorless leucomethylene blue. Green synthesis leverages natural precursors — plant extracts rich in polyphenols, flavonoids, and alkaloids, as well as agricultural waste, e-waste, and household biowaste — to replace toxic chemical reducing and stabilizing agents. These biomolecules act simultaneously as reducing, capping, and stabilizing agents during nanoparticle formation, yielding materials with controlled particle size, high surface area, and strong catalytic activity.

The review covers multiple nanomaterial classes: metal nanoparticles (Ag, Au, Cu, Pd), metal oxide nanoparticles (Fe3O4, TiO2, ZnO), carbon-based nanostructures (carbon dots, graphene, graphene quantum dots), and polymer-based NMs. For p-NP reduction, the accepted Langmuir-Hinshelwood mechanism involves borohydride ion adsorption onto the catalyst surface, dissociation to generate reactive electron-proton pairs, adsorption of p-NP via the nitro group, and stepwise hydrogenation through nitroso and hydroxylamine intermediates to yield p-aminophenol. For MB reduction, the mechanism involves surface adsorption of MB, light-induced charge carrier generation, reactive oxygen species (ROS) formation, and oxidative/reductive cleavage of the chromophoric conjugated structure.

Notable examples include a CuNP/hydrochar nanocomposite from organic peel waste achieving 100% degradation of both p-NP and MB within ~11 minutes, stable across eight reuse cycles. Iron oxide NPs from Tinospora cordifolia leaf extract removed 88% of MB within 60 minutes with five-cycle reusability. Gold NPs from Korean red ginseng extract demonstrated efficient dual reduction of p-NP and MB. Ag2S NPs from lemon citrus extract achieved 85% MB removal at 10 mg dosage, with particle size and surface defects identified as key determinants of electron-hole separation efficiency.

The review highlights alignment with the UN Sustainable Development Goals (SDGs) and circular economy principles, emphasizing low-cost, non-toxic, and scalable synthesis routes. However, the authors note that most studies remain at laboratory scale, and real-world translation — involving mixed industrial effluents, variable pH, competing ions, and large-volume processing — requires further investigation before clinical or industrial deployment.