# Nanofiltration of Simulated Acid Mine Drainage: Effect of pH and Membrane Charge
## 4 Results and Discussion
### NF Performance in Mixed-Salt Solutions  
Due to the complexity of real acid mine drainage solutions, assessing the membrane performance during AMD filtration is precarious. The complexities arise from the interaction between the membrane surface and the ions in the bulk solution. In addition, the solution chemistry of the bulk solution may change the membrane properties, resulting in different performance as previously tested in single salt solutions. In order to better understand how the membrane behaves in a more complex solution, three different mixed salt solutions were prepared as shown in Table 2.  
Chloride rejection (Figure 4) was obtained by performing a charge balance (\(\sum\) Charge of Cations = \(\sum\) Charge of Anions) on both the feed and permeate side while taking into account the speciation of sulfate at pH lower than 3. At pH 2, it can be expected that the co-ions, H+, coupled with either Cl\({}^{-}\) or SO\({}_{4}\)\({}^{2-}\) counter ions to pass through the membrane as HCl and H\({}_{2}\)SO\({}_{4}\), while Na\({}^{+}\) remains in the feed side. At relatively low concentrations of H+, within the range of pH 5 to 7, Na\({}^{+}\) rejection (Figure 4) remained high and almost constant.  
A slight decrease in permeate pH was observed for a feed range of pH 5 to 7 (Figure 5). This may be due to the increased rejection of the trace amounts of OH\({}^{-}\) brought about by the more negatively charged membrane at higher pH.  
Figure 4: Apparent sulfur rejection (dashed line) sodium rejection (full line) and chloride rejection (dotted line) as a function of pH for Na\({}_{2}\)SO\({}_{4}\) and NaCl mixed salt solution. Experimental conditions: feed pressure = 10 bar, T = 25 \({}^{\circ}\)C. Concentrations of Na\({}_{2}\)SO\({}_{4}\) and NaCl = 2000 mg/L and 700 mg/L, respectively.  
Figure 5: Permeate pH as a function of feed pH for Na\({}_{2}\)SO\({}_{4}\) and NaCl mixed salt solution.  
For single salt sodium silicate solution at pH 7, uncharged H\({}_{2}\)SiO\({}_{3}\) was the dominant silicate species in the solution and HSiO\({}_{3}\)\({}^{-}\) was present in trace amounts. Size exclusion was the main rejection mechanism for silicate, thus accounting for the low rejection value of 52.42% (Figure 6). When magnesium sulfate was introduced into the single salt sodium silicate solution, silicate rejection fell further to 38.56%. This was due to the effect of charge shielding by Mg\({}^{2+}\) on the negatively charged membrane. HSiO\({}_{3}\)\({}^{-}\) was rejected to a lesser extent. Similarly, magnesium sulfate rejection falls in a mixed salt solution due to the shielding effect of the negatively charged membrane by Na\({}^{+}\) ions.  
At pH 7, Membrane A is negatively charged. At this pH value, rejection of sulfate is greater than the chloride due to stronger electrostatic repulsion (higher valency in SO\({}_{4}\)\({}^{2-}\) vs. Cl\({}^{-}\)), thus resulting in a system behavior influenced by the Donnan effect related phenomenon, occurring due to preferential rejection of divalent compared to monovalent ions. Passage of Cl\({}^{-}\) is more prominent at higher salt concentrations due to the screening effect brought about by increasing Na\({}^{+}\) concentrations. The presence of Na\({}^{+}\) shields the negative charge on the membrane surface, thus causing a decreased Cl\({}^{-}\) rejection. Due to its higher valency and larger size, the rejection of sulfate was high for all tested concentrations and was less dependent on the membrane charge (see Figure 7).  
Figure 6: Magnesium sulfate (grey) and sodium nitrate apparent rejection (white) in the single (S) and mixed (M) solutions. Experimental conditions: feed pressure = 10 bar, 25 \({}^{\circ}\)C.