# Nanofiber-Based Materials for Persistent Organic Pollutants in Water Remediation by Adsorption
## 3 Phenolic Compounds
### Nanofibers for Disinfectant Adsorption
#### 3.2.3 Impact of Adsorbent Surface Charge and Solution pH  
Adsorbent isoelectric point (IEP) corresponds to the pH at which the adsorbent carries no net electrical charge and is used to determine adsorbent global surface charge in an aqueous medium in function of the solution pH. As phenol and phenolic compounds are polar molecules, the adsorbent surface charge can significantly enhance the phenolic compound adsorptive performance with the attraction of two opposite charges between adsorbent and adsorbate. Phenol adsorption up to pH 7 has been experimented on with carbon nanofibers, resulting in a 31% removal capacity [89]. Nevertheless, the adsorbent IEP was not reported. Considering that phenol is a weak acid with a pKa of 9.95 (Table 6), an iso-electric point between pH 4 and pH 6.5 would be ideal to get the phenol molecule to bind to a carbon adsorbent surface for a reaction pH slightly below the adsorbent theoretical IEP and the phenol pKa. Adsorption studies at a reaction pH superior to the adsorbate pKa have been carried out for meta-aramid/polyacrylic acid nanofibers on another phenol compound, namely 2,4-dichlorophenol [90]. It was reported that the phenol compound dissociated into chlorophenolate anions, thus impeding the adsorption performance, given the electrostatic repulsion between identical charged target molecules and fiber active sites [90]. Solution pH therefore plays a determinant role in the efficiency of adsorption by electrostatic interaction. Ionic strength also has an impact on the adsorption process. Indeed, electrical double layers can form on the adsorbent surface [41]. In a case of a neutral pH, when falling below the adsorbent isoelectric point the adsorbent surface would carry a positive charge, and would therefore attract the anions present in the solution to the interface to form what is called a stern layer. The protonated contaminant would also carry a positive surface charge and would be found in the diffuse layer, attracted by anions in the stern layer. The thickness of the electrical double layers can prevent the adsorption reaction. However, when the ionic strength increases, the electrolyte will compress the electrical double layer and thus influence electrostatic interactions between adsorbate and adsorbent [41].  
Figure 7 shows the variation in adsorption performance in function of the solution pH. It can be seen from all graphs that, starting from a solution pH of 6, the performance decreases with increasing pH. A solution pH between 4.5 and 6 seems to be the optimal window for metal oxide- and carbon-based adsorbents, confirming the theory explained above. The microfiber/nanofiber carbon composite adsorbent was surface-functionalized with ammonia [57]. It was reported that the inclusion of nitrogen groups on the adsorbent surface increased the adsorbent basicity, thus facilitating phenol adsorption at a close to neutral pH [57].  
Figure 7: _Cont._Horseradish peroxidase immobilized on polymer nanofibers was tested in the adsorption of Pentachlorophenol in several solution pH conditions (Table 9, \(N\)1). Sorption capacities were reported of 15.27 mg/L, 14.33 mg/L and 11.85 mg/L for an increasing pH of 2, 3, and 4, with no sorption occurring at pH 5 [78]. This direct proportion to solution acidity can be related to the protonation degree of Pentachlorophenol [78]. Since the hydrophobicity of Pentachlorophenol was found to influence the sorption rate, hydrophobic interactions can be identified as a significant capture mechanism. Other mechanisms reported \(\pi\)-\(\pi\) electron interactions between the Pentachlorophenol benzene ring and \(\pi\) electrons of the ketone group from the polymer molecular skeleton, as well as hydrogen bonding between the polymer ketone groups and Pentachlorophenol hydroxyl group [78].