Nanomaterials have been raised as a possible solution for sustainable water treatment due to their high efficiency and effectiveness even at low concentration. In this study, nanoparticles, nanofibers and organic polymers with nanoporosity have been synthesized to be utilized for arsenic removal and heavy metal adsorption. Based upon the different structures and physico-chemical properties of those nanomaterials, it has driven methods and alternatives to bring nanomaterials to field applications in sustainable way. In chapter 3, barium hexaferrite (BaFe12O19) nanoparticles have been prepared via solvothermal synthesis with subsequent annealing process. Annealed nanoparticles with 200 - 500nm size show enhanced arsenic removal efficiency than that of iron oxide nanoparticles of similar sizes. In following chapter 4, electrospun barium hexaferrite (BaFe12O19) nanofibers have been produced and utilized for magnetic separation of arsenic stained nanoparticles. It is found that <99.9% of nanoparticles which absorbed arsenic ions were separated from the original solutions with high level of arsenic removal efficiency (~97.6%). In the last chapter, covalent organic polymers (COPs) have been synthesized and applied for heavy metal adsorption. Due to the organized pore structure and the functionalities of COPs, crystalline oxide nanoparticles with uniform structure have been produced via metal adsorption / templation onto COPs. The morphologies of oxide nanoparticles are highly correlated with inner structure of amorphous COPs, therefore the synthetic process in this study could offer the way to look into the pore structure of amorphous polymers. Moreover, the composites with COPs and metal oxide nanoparticles show superior lithium ion battery performance with good cyclic stability and reversibility.