Sodium sulfide nonahydrate is widely used in the textile industry as a key component in the process of sulfur dyeing, where it helps to dissolve sulfur dyes and improve their affinity for fabric fibers. It is also employed in the leather industry for dehairing hides during the tanning process, as it effectively breaks down keratin proteins. In water treatment, it acts as a reducing agent to remove heavy metals from wastewater by converting them into insoluble sulfides. Additionally, it finds application in the pulp and paper industry for the digestion of wood chips in the production of paper. Its reducing properties are also utilized in chemical synthesis and metallurgy for the extraction of certain metals.