# Nanofiber-Based Materials for Persistent Organic Pollutants in Water Remediation by Adsorption
## 3 Phenolic Compounds
### Nanofibers for Disinfectant Adsorption  
#### 3.2.1 Impact of Adsorbent Specific and Contact Surface Area  
Figure 5 shows the adsorption performance of phenol and chlorophenols related to the adsorbent specific surface area. A near-linear trend of increase in the adsorption capacity for increasing adsorbent \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Name**} & \multicolumn{2}{c}{**Molecular**} & \multirow{2}{*}{**Log K\({}_{\text{aw}}\)**} & \multirow{2}{*}{**Solubility in Water** (mg/L, 25 \({}^{\circ}\)C)**} & \multirow{2}{*}{**2D Structure**} \\ \cline{2-2} \cline{4-4}  & **Weight (g/mol)** & & & \\ \hline Phenol & 94.113 & C\({}_{\text{e}}\)H\({}_{\text{e}}\)O & 1.46 & 67 \\ \hline
2-Chlorophenol & 128.555 & C\({}_{\text{e}}\)H\({}_{\text{e}}\)CIOH & 2.15 & 28 \\ \hline Pentachlorophenol & 266.323 & C\({}_{\text{e}}\)HCl\({}_{\text{s}}\)OH & 5.12 & 14 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Characteristics of Phenol, 2-Chlorophenol, and Pentachlorophenol [26]. specific surface area can be observed in the case of phenol adsorption, as shown in Figure 5A. This near-linearity can be bound to a near-homogeneous surface saturation process, with a higher proportion in adsorptive sites, i.e., BET proportionally corresponding to a higher adsorption capacity. In over 80% of the reported kinetic and isotherm studies for phenol adsorption, adsorbents were found to follow pseudo-second order and Langmuir models, translating into a surface monolayer adsorption reaction that is limited by chemisorption, hence further suggesting homogeneous surface saturation (Table 7). Plain carbon nanofibers and iron-oxide-decorated carbon nanofibers demonstrated a capacity of 0.842 and 1.684 mg/g for a specific surface area of 40.7 and 72.4 m\({}^{2}\)/g, respectively (Table 7, \(N17\), \(N18\)). However, activated carbon nanofibers showed a higher performance of 256.1 mg/g for a higher BET of 2921 m\({}^{2}\)/g (Table 7, \(N20\)). Figure 5B details the adsorbent composition used for phenol capture, showing a majority of carbon or carbon-based adsorbents following the near-linear adsorption trend. An explanation of this adsorption performance trend formed by carbonaceous adsorbents relies on the donor-acceptor electron complex adsorption mechanism. Actors in the electron interaction for adsorption are the carbonyl groups on the carbon-adsorbent surface (electron donor), the proportion of which influences the specific surface area, and the phenol compound aromatic ring (electron acceptor) [4]. Figure 5: _Cont._  
The observed preference for the Langmuir isotherm model can further be linked to the role of \(\pi\)-\(\pi\) dispersion forces between the aromatic rings of phenol and activated carbon. Indeed, a process of localized monolayer adsorption could signify that each site is accommodating a single molecule with repulsive interactions between adsorbed molecules [4]. Ceramic and clay adsorbents, although less reported, also showed to follow the adsorption trend. Therefore, adsorbent structure rather than composition is affecting the performance for a given BET. Styrene divinylbenzene particles, here reported under polymer adsorbents and not following the trend, have a particle size in the macro-range, and offered a capacity of 8.83 mg/g for a BET of 1000 m\({}^{2}\)/g (Table 7, \(N32\)). In the case of carbon nanotubes, which do not fit the global performance trend, their capacity of 166.58 mg/g for a BET of 357 m\({}^{2}\)/g (Table 7, \(N25\)) may be explained by their hollow structure. Variations in test conditions may also significantly affect the adsorbent performance for a given specific surface area. For instance, temperature was shown to act as a catalyst for phenol adsorption on doped cryptomelane manganese nanofibers, with faster metal oxide surface hydroxylation reported for temperatures from 140 \({}^{\circ}\)C (Table 7, \(N19\)).