## 7 Conclusions and Future Perspectives  
In this review, the recent advances in the application of carbon-based (CNTs, GNPs, CQDs, CNFs, CNs) and metal-oxide nanoparticles (ZnO, CuO, Ag\({}_{2}\)O, TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\)) in wastewater treatment have been evaluated. Based on reviewing the literature, the following conclusion can be drawn:  
* CNTs, GNPs, and CQDs exhibit unique properties in water treatment processes. These materials, with their high surface area, adsorption capabilities, and catalytic properties, contribute to the removal of diverse contaminants, including heavy metals and organic compounds. CNTs, with their extensive specific surface area, show proficiency in adsorbing contaminants and enhancing filtration processes. Graphene-based materials, such as GO and rGO, contribute to water purification through their unique structural and catalytic properties. CQDs showed their excellence in removing heavy metals from wastewater. * Although the studies for CNFs and CNs are rare, they demonstrated high potential for water treatment due to their large surface area and unique properties. CNFs can be functionalized to enhance adsorption capacity in electrochemical processes, while CNs offer an alternative to activated carbon with enhanced contaminant trapping capacity. * The study of the process and mechanism of CNT functionalization is very important. Generally, several factors, including surface chemistry, surface charge, functional groups, the surfactants' pore properties, and the altered chemical structure, control the adsorption of pollutants by CNTs. * The experimental parameters, including temperature, pH, and adsorbent/adsorbate concentration, have an impact on the effectiveness of the pollutant removal rate. Based on the findings, functionalization has consistently increased the efficiency of CNTs in comparison with unfunctionalized CNTs. In the pH range of 4-6, most of the pollutants exhibited a greater affinity for the carbonaceous adsorbents and functionalized CNTs. From the findings in Table 2, it can be concluded that the removal rate of pollutants using CNPs was in the range of 75 to 98% removal within 2 h.  
The diverse modifications and applications of MONPs highlighted their potential in water treatment processes. Each type of MONP is capable of the adsorption of different pollutants based on their properties as listed below:  
* ZnO NPs exhibit superior photocatalytic properties, making them effective in different types of dye reduction. The range of dye removal using different types of ZnO nanomaterials was from a minimum of 60% for methyl orange in 120 min to a maximum of \(\sim\)97% for Rhodamine blue dye in 75 min. * CuO NPs exhibit remarkable adsorption capacities for various contaminants, including heavy metals, with a minimum removal of 64.4% for Cd\({}^{2+}\) and a maximum removal of 91.4% for Cr\({}^{4+}\). * Ag\({}_{2}\)O, as a photocatalytic agent, exhibits excellent capacity for breaking down organic pollutants and potential antimicrobial properties. The synthesized Ag\({}_{2}\)O nanofibers from cauliflower waste had a 97.57% removal of MB dye in 150 min, demonstrating cost-effective and environmentally friendly materials for efficient pollutant removal. * TiO\({}_{2}\) NPs have extensively shown their photocatalytic activity, chemical stability, and low cost. They exhibit impressive capabilities in the breakdown of pollutants, including pesticides and organic compounds, through the release of reactive oxygen species. Ag-coated TiO\({}_{2}\) NPs exhibited a notable 90.9% photocatalytic degradation of MB dye in 120 min, showing enhanced stability and reusability compared to TiO\({}_{2}\). * Fe\({}_{2}\)O\({}_{3}\) nanoparticles with different crystalline forms, such as hematite and magnetite, stand out for their unique magnetism, making them effective adsorbents for heavy metal ions and excellent candidates for photocatalytic degradation.