# Nanofiber-Based Materials for Persistent Organic Pollutants in Water Remediation by Adsorption
## 3 Phenolic Compounds
### Nanofibers for Disinfectant Adsorption
#### 3.2.2 Impact of Adsorbent Pore Size  
The impact of adsorbent pore size on the adsorption capacity against phenol is illustrated on Figure 6. There is a noticeable trend, characterised by an exponential-type decrease in the adsorption capacity with the increase of adsorbent pore size.  
For phenol capture, adsorbent pore size was reported from 0.43 nm (activated carbon nanofibers, Table 7, N20) to 15 nm (diatomite nanocomposites, Table 7, N23), corresponding to an adsorption capacity of 256.1 and 2 mg/g, respectively, being the maximal and minimal capacities here reported. Maximum adsorption capacities were observed for a pore size in a range between 0.43 and 2.2 nm (Table 7, N20, N4). The optimum pore size range of the adsorbent can be linked to the kinetics of pore blocking (i.e., phenol diffusion rate), adsorbent pore geometry and the single orientation of the phenol molecule (Table 6). Considering that the smallest cross section of the phenol molecule is 0.43 nm, mesopores (from 2 to 50 nm) may insufficiently promote adsorbent/adsorbate contact, thus resulting in a potentially lower performance depending on the adsorption mechanisms at stake (Table 7, N7, \(N32\)). On the other hand, activated carbon nanofibers were shown to possess an open pore structure and a dual pore size repartition of 0.43 nm and 0.8 nm, allowing the penetration of one or two phenol molecules, respectively (Table 7, N20). Dual pore size distribution on adsorbent surfaces can be further advocated by the fitting of the adsorption reaction to a Freundlich isotherm.  
Furthermore, average adsorbent pore size has been calculated from a Barrett-Joyner-Halenda (BJH) method, and may not accurately reflect the adsorbent pore size distribution, depending on the actual pore tortuosity. However, a pore size calculated by BJH method and falling in the optimal range defined above may suggest an increased volume of micropores enhancing phenol adsorption. Using a BET/BJH method, the presence of micropores leads to an isotherm of type I, with increased nitrogen adsorption at low pressures [53]. Similarly to a Langmuir isotherm, the height of the plateau on the isotherm would reflect the volume of micropores, also called micropore capacity at the adsorbent surface. The extension of the isotherm hysteresis to relatively low pressures would give an indication of the micropores' diameter [53].  
Beside the adsorbent morphology, voids between the nanofibers in the case of fibrous mats could also impact the diffusion of the phenol molecules; however, over 95% of the studies reported a nanofiber web fabrication by a solution blowing or electrospinning process, hence ensuring webs of high porosity and of suitably large pore sizes to enable phenol diffusion. This last aspect is critical as the hydrodynamics of the solution diffusion, and therefore the impact of the webs' wettability, is critical to the design of nanofibrous adsorbents.  
Figure 6: Phenol adsorption capacity related to the pore size of different adsorbent structures (**A**) and compositions (**B**).