# Nanofiltration of Simulated Acid Mine Drainage: Effect of pH and Membrane Charge
## 4 Results and Discussion
### NF Performance in Single Salt Solutions  
To assess the membrane performance with minimal interferences, single salt experiments with selected compounds were performed, namely magnesium sulfate, copper sulfate, and manganese nitrate. In addition, sulfate rejection was determined separately. Figure 3a shows the magnesium rejection and permeate flux as a function of feed pH. Figure 2: Graph of zeta potential of Membrane A as a function of pH. Figure 3: Cont. As can be seen, the rejection of Mg\({}^{2+}\) ions remained relatively constant (above 99.5%) throughout the investigated pH range, except for a slight dip at pH 5. This may be due to the proximity of this pH value to the membrane IEP, where a minimum ion rejection is to be expected [24; 25]. The permeate flux, on the other hand, was higher as the pH value increased. This is contributed by the fact that the membrane became more negatively charged with increasing pH value as can be seen in Figure 2, resulting in a higher hydrophilicity [26; 27; 28; 29]. As reported in the literature, dissociating groups in the membrane are more negatively charged with increasing pH value. This is due to the fact that the membrane is more negatively charged with increasing pH value. Figure 3: (**a**) Effect of feed pH on magnesium rejection (open symbol) and permeate flux (closed symbol). (Experimental conditions: MgSO\({}_{4}\) = 4000 mg/L, feed pressure = 10 bar, T = 25 \({}^{\circ}\)C). (**b**) Effect of feed pH on copper rejection (open symbol) and permeate flux (closed symbol) (Experimental conditions: CuSO\({}_{4}\) = 100 mg/L, feed pressure = 10 bar, T = 25 \({}^{\circ}\)C). (**c**) Effect of feed pH on apparent manganese rejection (open symbol) and permeate flux (closed symbol). (Experimental conditions: Mn(NO\({}_{3}\))\({}_{2}\) = 165 mg/L, feed pressure = 10 bar, T = 25 \({}^{\circ}\)C). (**d**) Effect of feed pH on apparent sulfate rejection. Feed solution containing 4000 mg/L MgSO\({}_{4}\) (full line) and 100 mg/L CuSO\({}_{4}\) (dotted line). (Experimental conditions: feed pressure = 10 bar, T = 25 \({}^{\circ}\)C). polymer skin layer can make the surface more hydrophilic and looser, when charges of the polymer chains start to repel each other at elevated pH [17]. As copper is one of the most commonly found elements in AMD, copper rejection was determined in a single salt rejection experiment, and the data obtained are presented in Figure 3b. It was found that Cu rejection increased slightly with increasing pH, with the lowest rejection of 98.6% at pH = 2 and the highest rejection of 99.1% at pH = 5.4. However, these rejection values can be interpreted as relatively constant after accounting for the standard deviation. For polyamide-based TFC membranes such as Desal DK and NF 270, a trend where the rejection of Cu ions increased with decreasing pH was reported [30; 31]. This was due to electrostatic repulsion from the increasing positive charge. Consequently, the rejection of Cu in Membrane A was less dependent on the membrane charge as compared to commercial polyamide-based membranes for Cu rejection (see Figure 3b). In terms of manganese rejection, Membrane A maintained relatively high values (above 97%) for the investigated pH range of 2 to 5 (Figure 3c). This can be explained by the increasing retention of positively charged manganese cations with respect to decreasing pH. Specific adsorption of Mn\({}^{2+}\) cations could be expected to shift the IEP to a higher pH value due to the shielding of the negative membrane charge. As the pH decreased, a stronger electrostatic repulsion between Mn\({}^{2+}\) and positive membrane charges occurred. [PERSON] et al. [31] observed similar rejection values using NF270 where rejection was at a maximum of 90% at pH 1.5. However, the rejection of NF270 dropped rapidly to 20% near the membrane's IEP value.