## 3 Results
### Membrane Performance  
* UF Experiments  
The UF performance and the anti-fouling properties of the developed membranes were investigated using HA as foulant model revealing a rejection superior to 95%. In particular, the measured HA rejections were 95.7%, 96.03%, and 99.9% for M0, M8, and M15, respectively. Hence, the increasing of the PAA amount in the casting solution positively affected the UF performance of CA membranes. Figure 4: Contact angle of the prepared membrane. Figure 5: Water flux of the CA and CA/PAA blended membranes. Figure 6 shows the 2D fluorescence spectra of the studied HA solutions, before (a) and after ultrafiltration membrane process (b, c, and d), not corrected for Rayleigh scattering peaks that appeared in the form of diagonal bands. The presence of HA in the feed caused two regions of fluorescent activity located at around Ex/Em = 265 nm/525 nm and Ex/Em = 360 nm/520 nm due to the presence of large quantities of aromatic structures and unsaturated fatty chains in humic substances [47], as observed in Figure 6a. Obviously, the decreasing of the HA in the permeates provoked a diminishing of the fluorescent activities, thus Ex/Em bands were less evident by increasing the rejection. In fact, the results showed that fluorescent activities decreased by increasing the concentration of PAA in the membranes and the emission signal raising from HA disappeared in the permeate collected after the UF processes carried out with the membrane M15 (Figure 6d). It is well known that the main factors contributing to the separation in the UF process dominated by size-excluding mechanism are the membrane pore size and the steric hindrance of the target contaminants. Thus, the reduction of the pore size from 12.9 nm to 9.6 nm by increasing the PAA concentration from 0 wt. % to 15 wt. % with respect to CA positively affected the coefficient of rejection. Nevertheless, the chemical nature of the membranes played a key role on the separation performance: PAA lead to the incorporation of carboxylic groups, improving rejection via electrostatic repulsions with humic acid components that are negatively charged, as demonstrated by the negative potential zeta (\(-\)27.1 mV) reported in Table 2.  
* Anti-fouling Properties  
Fouling is a critical issue in UF negatively affecting the long-term stability, productivity, and cost of the process. Results summarized in Figure 7 show the benefits of blending CA with PAA in terms of flux and anti-fouling resistance. Figure 6: 2D fluorescence spectra of feed (**a**) and permeates of UF experiments performed with M0 (b), M8 (c), and M15 (**d**), respectively. In fact, water flux (J\({}_{\text{W}}\)), HA solution flux (J\({}_{\text{HA}}\)), and water flux after cleaning (J\({}_{\text{W2}}\)) increased by enhancing the PAA concentration in the blend with CA. Nevertheless, the effect of the presence of HA in the feed solution was a consistent drop in the flux with a reduction in the range of 7-13%. Subsequently, the flux dramatically decreased during the time due to the permeation of water and the consequent improvement of HA concentration in the feed that intensified the concentration polarization and fouling. The fouling consists of extra resistance to the water permeation as clarified by the Darcy's law, which is useful for quantitatively describing the hydraulic resistance of the membrane (R) from the driving force (\(\Delta\)P), the water viscosity (\(\upmu\)), and the measured permeate flux (J), as follows:  
\[\text{R}=\Delta\text{P}/\text{(J}\times\upmu) \tag{10}\]  
The ultimate result of 2 h of UF practices with a feed solution containing HA (1 st Cycle) was an improvement of ca. 26% compared to the hydraulic resistance of the membrane of M0 caused by the fouling, whereas the impact on the resistance of M15 membrane was of ca. 20%. Moreover, primary fouling serves active sites for the subsequent deposition of macromolecules on the membrane surface, accelerating the decline of membrane performance over the time [48].