Water Quality Assessment of Dye Wastewater after Photocatalytic Treatment with Reduced Graphene Oxide–Metal Oxide Nanocomposites

Abstract

Textile dyeing effluents remain among the most persistent categories of industrial wastewater owing to their intense color, complex aromatic structures, and resistance to conventional biological treatment. This study reports the post-treatment physicochemical assessment of two structurally distinct textile dyes, the cationic triarylmethane dye Methyl Violet (MV) and the anionic xanthene dye Rose Bengal (RB), following batch photocatalytic treatment with four reduced graphene oxide (rGO)–metal oxide nanocomposites: rGO–CeO₂, rGO–CuO, rGO–MnO₂, and rGO–ZnO. Untreated (baseline) and catalyst-treated dye solutions were independently analyzed by an accredited third-party testing laboratory for color, pH, electrical conductivity (EC), and the dye-associated counter-ion species chloride, nitrogen, iodine, and potassium, and the results were benchmarked against the Bureau of Indian Standards drinking-water specification IS 10500:2012. Relative to the untreated baseline, chloride and nitrogen loads in the MV system fell by 18–74% and 36–81%, respectively, depending on the catalyst, while in the RB system, chloride, iodine, and potassium loads fell by 30–71%, 51–87%, and 23–75%, respectively. Electrical conductivity rose in every treated sample, consistent with the release of smaller, more mobile ionic fragments during oxidative breakdown of the parent dye chromophores. A composite ionic-load reduction index (CILRI), defined here as the mean percentage reduction across the measured dye-associated ionic species, ranked rGO–MnO₂ (55.1%) marginally ahead of rGO–CeO₂ (53.8%) and rGO–CuO (51.6%) for MV, and rGO–CuO (70.9%) clearly ahead of rGO–CeO₂ (58.2%), rGO–ZnO (60.0%), and rGO–MnO₂ (46.8%) for RB. Most treated parameters fell within the desirable limits of IS 10500:2012, indicating that rGO–metal oxide photocatalysis can move textile dye wastewater substantially toward potable-quality benchmarks for these specific parameters. However, color readings at or below the instrument quantification limit in both untreated and treated samples, and the absence of direct spectrophotometric dye concentration or catalyst characterization data in this data set, are noted as limitations that warrant follow-up study.