The textile industry's rapid growth has led to the discharge of large quantities of hardly degradable organic dyes into water bodies, posing a significant threat to human health and the environment. Reactive dyes, commonly used in textile dyeing, are particularly challenging to degrade due to their complex structures. Traditional wastewater treatment methods often fail to completely remove these pollutants, necessitating the development of efficient, economical, and environmentally friendly alternatives. Advanced oxidation processes (AOPs), which utilize strong reactive oxygen species (ROS) to degrade organic pollutants, have emerged as effective treatment methods. Among these, sulfate radical-based AOPs, particularly those employing peroxymonosulfate (PMS) activation, have gained considerable attention. Transition metals, such as cobalt, can efficiently activate PMS to generate sulfate radicals (SO₄•⁻). However, homogeneous cobalt catalysts suffer from drawbacks including low recyclability, metal leakage, and sludge disposal issues. To overcome these limitations, this study explores the use of a heterogeneous catalyst by loading cobalt onto a two-dimensional transition metal carbide/nitride (MXene) material. MXenes possess numerous surface functional groups that facilitate metal cation adsorption and anchoring, improving catalytic activity, stability, and recyclability. Furthermore, the low toxicity of MXene makes it suitable for environmental remediation applications. This research investigates the efficacy of a Co@MXene composite catalyst in degrading C.I. Reactive Red 195 (RR195), a model reactive dye, and evaluates the potential for recycling the treated wastewater for subsequent dyeing processes.

Existing literature extensively covers the challenges posed by reactive dye wastewater and the application of AOPs for their treatment. Studies have highlighted the effectiveness of sulfate radical-based AOPs using PMS activation, with transition metals like cobalt showing promising results. However, the limitations of homogeneous cobalt catalysts, including low recyclability and metal leakage, have been widely discussed. The use of supporting materials to enhance the catalytic performance and reusability of cobalt catalysts has also been explored. Recent research has focused on the application of MXenes as supporting materials due to their unique properties, including high surface area and abundant functional groups. However, the use of cobalt-loaded MXene catalysts for reactive dye degradation and the subsequent recycling of treated wastewater remain relatively unexplored areas.

The Co@MXene catalyst was synthesized using a molten salt method. Ti₂AIN MAX phase powder and CoCl₂ were mixed at a molar ratio of 1:2 and heated to 1000 °C for 7 hours. The resulting material was treated with 3 M HCl to remove excess Co particles and subsequently washed and dried. The degradation experiments involved using 100 mL of 0.06 g/L RR195 dye solution. The effects of various parameters, including Co@MXene concentration (0.1-2.0 g/L), PMS concentration (1-5 g/L), initial pH (3, 7, 10), temperature (25-60 °C), and NaCl concentration (0-25 g/L), were investigated. The degradation percentage was calculated based on UV-Vis absorbance measurements at the maximum absorption wavelength of RR195 (542 nm). Kinetic studies were performed to determine the degradation rate constants. Free radical scavenging experiments using methanol, tert-butanol, and furfuryl alcohol were conducted to identify the dominant reactive oxygen species involved in the degradation process. The reusability of the Co@MXene catalyst was assessed through ten consecutive cycles of dye degradation. Finally, the treated wastewater was used to dye cotton fabrics with RR195, and the dyeing performance was compared with conventional dyeing using deionized water. Characterization techniques such as SEM, EDS, XRD, FTIR, and XPS were employed to analyze the morphology, elemental composition, crystalline structure, functional groups, and oxidation states of the catalyst.

The Co@MXene catalyst demonstrated exceptional performance in degrading RR195 dye solution. Under optimized conditions (1.0 g/L Co@MXene, 3 g/L PMS, 25 °C, pH 7), a degradation percentage of 97.1% was achieved within 36 minutes. The addition of NaCl significantly increased the degradation rate, reaching 0.4984 min⁻¹ at 20 g/L NaCl, a 5.57-fold increase compared to the rate without NaCl (0.0894 min⁻¹). The enhanced degradation was attributed to the synergistic action of hydroxyl radicals (OH•), sulfate radicals (SO₄•⁻), and singlet oxygen (¹O₂), with singlet oxygen contributing most significantly (66.71%). The Co@MXene catalyst showed high reusability, maintaining 93.5% degradation efficiency after ten cycles. XPS analysis revealed a clear Co³⁺/Co²⁺ redox cycle, facilitated by the MXene support, which enhanced PMS activation. Reusing the treated wastewater for dyeing cotton fabrics resulted in faster dyeing rates compared to conventional methods, with the dyed fabrics exhibiting slightly darker colors and a more reddish hue. The color fastness properties remained comparable to conventionally dyed fabrics.

The findings demonstrate the effectiveness of the Co@MXene catalyst in activating PMS for the degradation of reactive dyes, significantly outperforming the performance of PMS alone or the adsorption capacity of the Co@MXene. The enhanced degradation rate in the presence of NaCl is attributed to the formation of additional reactive chlorine species, contributing to the overall oxidation process. The high reusability of the catalyst is a significant advantage, reducing operational costs and environmental impact. The successful recycling of the treated wastewater for dyeing further emphasizes the sustainability of this approach. The results provide valuable insights into the design and development of efficient and reusable catalysts for the treatment of reactive dye wastewater and contribute to the advancement of sustainable textile dyeing technologies. The observed changes in color parameters of the dyed fabrics using treated wastewater, while subtle, could be further optimized through adjustments in the dyeing process parameters.

This study successfully demonstrated the effectiveness of a novel Co@MXene catalyst in rapidly degrading reactive dye wastewater, achieving high degradation efficiency and reusability. The treated wastewater was successfully recycled for dyeing cotton fabrics, showcasing the potential of this approach for sustainable textile production. Future research could focus on exploring different MXene types and cobalt loading strategies to further optimize catalyst performance and investigate the scalability and applicability of this technology in industrial settings. Exploring other types of reactive dyes and investigating the long-term stability and environmental impact of the catalyst are also crucial areas for future research.

The study focused on a single model reactive dye (C.I. Reactive Red 195), and the generalizability of the findings to other types of reactive dyes requires further investigation. The long-term stability of the catalyst under continuous industrial-scale operation needs to be assessed. While the treated wastewater was successfully reused for dyeing, a comprehensive life-cycle assessment is needed to fully evaluate the environmental impact of the entire process. The study primarily focused on laboratory-scale experiments; therefore, further research is needed to scale up the process for industrial applications.