# Nanofiber-Based Materials for Persistent Organic Pollutants in Water Remediation by Adsorption
## 6 Recommendations and Conclusions  
Nanofiber-based materials showed their fast adsorption kinetics and required low adsorbent dose compared to benchmark adsorbents in this study. For instance, polyacrylamide nanofibers attained equilibrium after 0.5 h in the adsorption of Atrazine, for an adsorbent dose of 1 g/L (Table 4, \(N5\)) [41]. Regarding the adsorption performance, engineered nanofibers yielded over 95% removal efficiency with high adsorption capacities comparable to granular activated carbons and acid-treated biomass. For example, activated carbon nanofibers against Phenol showed a capacity of 256.1 mg/g (Table 7, \(N20\)), and cross-linked cellulose acetate-polybenzoxazine nanofibers demonstrated against Phenantrene 98.5% removal (Table 20, \(N1\)) [61; 113]. Such a nanofiber web presented adequate structural and chemical surface characteristics for the adsorption of organic contaminants. Essential structural characteristics for a nanofiber adsorbent were found to be surface pores and density of functional sites. Indeed, the size of pores across the fiber surface was shown to have a major impact on the saturation of the adsorbent, with pore channels engineered to take into account the size or cross section of the contaminant showing higher performance. Activated carbon nanofibers for phenol adsorption present a dual pore repartition of 0.4 nm and 0.8 nm, adapted to the phenol molecule size, with a smallest cross section of 0.43 nm [61]. Molecular orientation of the contaminant is also considered to prevent size-exclusion effects when one molecule is already adsorbed along a pore wall, as highlighted by the performance comparison of Dieldrin and stereoisomer Endrin adsorption [73]. Adsorbent contact surface area ruled by the nanofiber average diameter impacted on the contaminant capture, with diameters below 150 nm yielding higher performance in the capture of Atrazine [39]. Higher surface specific areas led in general to higher adsorption capacities, provided there was contaminant diffusion along the surface pore channels. Activated carbon nanofibers demonstrated a high BET of 2921 m\({}^{2}\)/g [61]. Regarding adsorbent surface chemistry, a total hydrophobic surface is preferable to show affinity for adsorption--in the case of surface grafting of hydrophobic groups, adsorption may be impeded along the pore walls [28]. Compared to hydrophobic interactions, electrostatic interactions andforemost electron interactions were shown to be more efficient adsorption mechanisms [41; 113]. To this end, adsorbent isoelectric point should be systematically determined to improve the contribution of electrostatic interactions in the contaminant capture. Electron interaction in this study happens via \(\pi\)-\(\pi\) stacking of electrons between adsorbate and adsorbent unsaturated bonds, justifying the high efficiency of carbon-based adsorbents [61]. Other than pore and surface area characteristics, carbon adsorbent crystallinity and carbon content are features of interest that could be investigated in order to improve adsorption by electron interaction. The tuning of the carbon adsorbent surface chemistry to shift its isoelectric point could also be investigated to improve the contaminant adsorption by electrostatic interaction contribution. Other structures such as cellulose acetate crosslinked with polybenzoxazine lead to a highly hydrophobic surface presenting unsaturated bonds, resulting in high performance in the capture of Phenantrene [113]. Material functionalisation to reach a surface chemistry allowing the three adsorption mechanism defined above could be the focus of future research in order to yield complete contaminant removal. Testing conditions in terms of initial contaminant concentration were found to vary between 0.5 \(\upmu\)g/L and 700 mg/L in this study [16; 81].