# Nanosponges for Water Treatment: Progress and Challenges
## 2 Nanosponges for Water Treatment
Notably, the efficiency of adsorption does not noticeably alter at various temperatures; however, adsorption proficiency can be affected by the concentration and type of cross-linking agents. Consequently, the maximum adsorption energy and capacity for these nanosponges, endowed with high porosity and rigidity, are reported to be 1.837 L mg\({}^{-1}\) and -1.0 mg g\({}^{-1}\), respectively [45]. In addition, surface-functionalized cis diol comprising mesoporous nanosponges were evaluated for rapid elimination of organic micropollutants and boric acid from water (Figure 2) [46]. These nanosponges exhibit an efficient capacity for boron adsorption, like the commercial resins, in addition to faster adsorption (up to 60 times). Furthermore, bisphenol A can be up-taken by these nanosponges with the equilibrium adsorption achieved in less than two min. The deployment of immobilized polyols can induce synergistic effects because of secondary bonding, simultaneously reducing the time of adsorption and enhancing the capacity of adsorption. Polyol-functionalized  
Figure 1: (**A–C**) The processes of adsorption/desorption by applying magnetic nanoparticle immobilized \(\beta\)-cyclodextrinrin-based nanosponges with highly efficient enrichment potentials. Adapted with permission from Ref. [44]. Copyright 2021, Springer Nature, CC BY 4.0.  
mesoporous nanosponges with high sensitivity / selectivity can be considered promising candidates for water treatment appliances [46]. Nanosponges constructed from \(\beta\)-cyclodextrintrim polyurethane with insolubility, recyclability, and great surface area (352.5 m\({}^{2}\) g\({}^{-1}\)) features are additionally altered with phosphorylated MWCNTs and adorned with Ag nanoparticles and TiO\({}_{2}\) via the amidation reaction, cross polymerization deploying diisocyanate (as a linker), and the sol-gel process. The designed nanosponges can adsorb Congo red dye and trichloroethylene from wastewater with maximum capacities of 146.96 mg g\({}^{-1}\) and 27,507 mg g\({}^{-1}\), respectively [29]. [PERSON] et al. [47] synthesized hyper-branched polymers functionalized with long aliphatic chains for encapsulating/capturing lipophilic polycyclic aromatic pollutants such as pyrene and fluoranthene from water with inclusion formation constants of 2.0 \(\times\) 10\({}^{8}\)-6.3 \(\times\) 10\({}^{6}\) M\({}^{-1}\) and 3.8 \(\times\) 10\({}^{6}\)-4 \(\times\) 10\({}^{5}\) M\({}^{-1}\), respectively. Notably, the chemical structure of the parent hyper-branched polymers and the type of polycyclic aromatic compounds can affect the loading capacity of nanosponges for fluoranthene (6-31 mg g\({}^{-1}\)), phenanthrene (15-54 mg g\({}^{-1}\)), and pyrene (6-35 mg g\({}^{-1}\)) [47]. Cyclodextrin nanospheres with reusability are designed via cross linking of 1,2,3,4-butanetetracarboxylic acid with \(\beta\)-cyclodextrinrin in the company of poly(vinyl alcohol) for the adsorptive elimination of cationic contaminants from water [48]. The maximum adsorption was reported for the removal of paraquat (-120.5, 97.0%), safranin (-92.6, 96.7%), and malachite green (-64.9 mg/g, 98.3%) [48]. In addition, biodegradable cyclodextrin-based nanosponges with high biosafety features are constructed using a one-step solvothermal technique using \(\beta\)-cyclodextrinrin and diphenyl carbonate for removing dyestuffs from the waste stream. The highest adsorption capacity for Basic red 46 and Rhodamine B was -101.43 mg g\({}^{-1}\) and 52.33 mg g\({}^{-1}\), respectively. The amount of adsorbent, the molar ratio of \(\beta\)-cyclodextrinrin and diphenyl carbonate, pH solution, initial concentration, and time of contact can affect the efficiency of nanosponges for pollutant removal [49]. [PERSON] et al. [50] utilized cyclodextrin bead polymers for the removal of micropollutants with \(>\)80% eff  
Figure 2: The preparative process for mesoporous cyclodextrin polymers using a copper-free synthesis technique and functionalization of polyols immobilized on their surfaces. Adapted with permission from Ref.