# Nanosponges for Water Treatment: Progress and Challenges
## 2 Nanosponges for Water Treatment  
Cyclodextrin-based nanosponges exhibit excellent potentials as sorbents for removing heavy metal ions from aqueous solutions, providing magnificent opportunities for eliminating organic/inorganic contaminants from water (Table 2) [30]. For instance, cyclodextrin-based nanosponges (0.7-1.2 nm) are synthesized with a strong affinity for absorbing hazardous organic contaminants in wastewater because of the presence of cyclodextrin cavities that provide a hydrophobic environment [31]. Cyclodextrin-based nanosponges can also be applied for fast removal of pollutants from water (\(\sim\)90%), with an utmost adsorption capability of 2 mg g\({}^{-1}\). These nanostructures exhibit great potentials for removing some pharmaceutical contaminants from water, namely carbendazim, diclofenac, sulfamethoxazole, and furosemide [32]. Additionally, polymeric nanobiocomposites with multifunctionality are constructed via a cross-linking oligomerization of cyclodextrin deploying phosphorylated multi-walled carbon nanotubes (MWCNTs) trailed by a sol-gel step to incorporate silver (Ag) and titanium dioxide (TiO\({}_{2}\)) nanoparticles. These composites can be deployed as potential filter nanosponges for eliminating pollutants (e.g., organic/inorganic materials and pathogenic microorganisms) from water and wastewater. Fourier-transform infrared (FTIR) analysis illustrates that oxygen-containing groups are subsisted on these composites, and carbamate bearing characteristic peaks (at 1645 cm\({}^{-1}\)) can also be recognized because of the polymerization reactions [33]. Polymer nanosponges constructed from \(\beta\)-cyclodextrintrin covalently-cross-linked tannic acid through a condensation reaction can be applied for capturing Pb\({}^{2+}\) from wastewater with high selectivity/sensitivity, and phenolic hydroxyl groups of tannic acid with the capability of effective binding to Pb\({}^{2+}\) generate stable structures [40]. A significant adsorption saturation competence of \(\sim\)136.8 mg g\({}^{-1}\) for Pb\({}^{2+}\) was reported at an original strength of 200 mg L\({}^{-1}\), with the highest capability of \(\sim\)116.5 mg g\({}^{-1}\). The efficiency of elimination was \(\sim\)81% Pb\({}^{2+}\) within 3 min, and the adsorption was completed within \(\sim\)50 min because of plentiful adsorption locations in tannic acid and proficient mass transfer rates maintained by cyclodextrin [40]. Cross-linking polymerization, sol-gel, and amidation reaction techniques are deployed together for producing phosphorylated multi-walled carbon nanotube-cyclodextrintrin/Ag-doped TiO\({}_{2}\) nanosponges for removing Pb\({}^{2+}\) and Co\({}^{2+}\) metal ions from wastewater [41]. One of the crucial factors with significant effects on the adsorption capacities of nanosponges is the pH solution, which can affect speciation, the ionization degree of pollutants, and the surface charge of adsorbents. By increasing the pH of a solution, the efficacy of Pb\({}^{2+}\) and Co\({}^{2+}\) removal is enhanced. On the other hand, by temperature increment, adsorption capability is increased with an enhancement in time of contact. The mechanism of adsorption and removal of these metal ions comprises