The present disclosure provides methods for producing carboxylated nanocelluloses. Compared with conventional methods, the methods of the present disclosure are simple and cost-effective in the production of carboxylated (or carboxy) nanocelluloses, in embodiments nanofibers and/or nanowhiskers, directly from raw biomass, including lignocellulose wood, non-wood sources, non-lignocellulose wood, lignocellulose or pure cellulose. The carboxy groups on the surface of nanocellulose thus produced can then be easily modified into functional derivatives such as amide, acetate, ether, ester, etc. The resulting nanocelluloses may be used to form purifying agents and/or filters to remove impurities from wastewater.