4.1. Streaming Potential Data
In this study, the streaming potential measurements were conducted between pH values of 3 to 10. As can be seen in
Figure 2, the isoelectric point (IEP) of the membrane in a KCl solution was found to be at a pH of 4.7 below which the surface becomes positively charged.
Specific adsorption SO
42− anions could be expected to shift the IEP to a lower pH value. In any case, at pH 7, for which all standard rejection experiments using magnesium sulfate were performed, Membrane A is negatively charged. At this pH value, rejection of sulfate is greater than the chloride due to stronger electrostatic repulsion (higher valency of SO
42− vs. Cl
−), thus resulting in the Donnan effect, namely preferential rejection of divalent to monovalent ions of the same charge [
23]. Passage of Cl
− ions is more prominent at higher salt concentrations due to the screening effect brought about by increasing Na
+ concentration. The presence of Na
+ shields the negative charge on the membrane surface causing a decreased Cl
− rejection. Due to the higher valency and larger size of sulfate, sulfate rejection remained high for all tested concentrations (see
Section 4.2).
4.2. 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.
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 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. Al Rashdi 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. This shows that Membrane A produces more consistent and stable rejection results compared to NF270, which might be also associated with its denser polymeric structure and, therefore, the stronger contribution of steric (size-exclusion) compared to charge-related effects.
Sulfate rejection was determined in two different single salt solutions: 4000 mg/L magnesium sulfate and 100 mg/L copper (II) sulfate. It might be expected that in these solutions, due to complex co-adsorption of both the cations and the anions on the membrane surface, the IEP remains the same, but the overall surface charge becomes lower. As seen in
Figure 3d, the rejection of sulfate in both solutions was relatively constant above the membrane IEP. This shows that the rejection of salt was governed mainly by the electrostatic repulsion of sulfate (SO
42−) and the negatively charged membrane.
Below the IEP, a sharp drop in sulfate rejection was observed, most remarkably at pH 2, which is close to the pKa value of sulfate (pKa = 1.92). This could be attributed to the increase in bisulfate (HSO
4−) concentration in the feed solution due to chemical speciation of sulfate at lower pH values, which could trigger the Donnan effect [
32]; at pH 2, sulfate ions are present as SO
42− and HSO
4− with a ratio of ca. 10% and 50%, respectively [
33].
The co-ion Mg2+ was rejected at a greater extent than H+ due to its higher valence charge. Therefore, SO42− anion remained with Mg2+ cation to maintain the overall electroneutrality on the feed side. At the same time, the monovalent HSO4− ions coupled with H+ ions present in the solution, passing through the membrane to the permeate side, thus resulting in a lower permeate pH. The latter is also favored by the significantly higher concentration of very mobile protons (a “conductivity jump”) at a feed pH lower than 3, which can lead to protons permeation (together with Cl−) across the membrane. The overall membrane surface charge is also more shielded under such conditions.
As a result of the permeation of HSO
4−, the overall sulfate rejection significantly decreased at pH values below the membrane IEP. This phenomenon was also observed by Visser et al. [
34] and Mullett et al. [
14], where sulfate rejection in a mixed system decreased drastically at pH 2 and 3. The fall in sulfate rejection at low pH (36.3% at pH = 2) was more drastic for the CuSO
4 experiment compared to the MgSO
4 experiment. The explanation for this phenomenon is that the higher permeation of SO
42− ions in the CuSO
4 solution can be attributed to the reduced effect of membrane charge shielding due to the lower salt concentration (100 mg/L of CuSO
4, in comparison to 4000 mg/L of MgSO
4).
4.3. 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 (∑ Charge of Cations = ∑ 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
42− counter ions to pass through the membrane as HCl and H
2SO
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.
For single salt sodium silicate solution at pH 7, uncharged H
2SiO
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
42− 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).
4.4. Simulated AMD Solutions
In the simulated AMD solution, rejection minima were observed at pH 4 (
Figure 8). This value is close to the IEP of Membrane A for which the net charge of the membrane approaches zero. At the IEP, size exclusion is the remaining rejection mechanism, therefore, an increased passage of metal ions is registered. The observed rejection minimum (at pH = 4.0) was lower than the IEP obtained from the zeta potential measurement at membrane IEP of pH 4.5–4.7. The observed shift of minimal rejection can be attributed to the fact that the IEP measurements are dependent on the solution chemistry that the membrane is exposed to. Thus, the observed difference in the IEP for the model AMD solution may be due to the adsorption of divalent sulfate anions present in the solution onto the membrane.
With the exception of copper, metal ions rejection increased with decreasing pH due to stronger electrostatic repulsion brought about by a more positively charged membrane. An opposite trend was observed for copper, for which rejection increases with increasing pH. This is possibly due to the appearance of a negatively charged copper complexes, such as Cu(OH)
3−, which become increasingly rejected by the more negatively charged membrane with increasing pH. The permeate flux (shown in
Figure 9) remains relatively constant throughout the tested pH range. The permeate pH values were lower than the feed pH ones, especially for feed pH lower than 4 as H
+ couples with anions and passes through the membrane.
Sodium rejection was lower than that for the larger size divalent cations. In general, sodium rejection increases with increasing solution pH (a more negatively charged membrane). As seen can be in
Figure 10, at higher pH, sodium rejection displays a higher rejection value as accompanying divalent anions (SO
42−) are highly rejected. However, sodium rejection was lower than expected. This may possibly be due to the permeation of sodium cations with monovalent anions (nitrate and chloride). The anions rejection increases with increasing pH due to a stronger electrostatic repulsion from the negative membrane. Due to their monovalent nature and smaller size, nitrate and chloride expectedly showed lower rejection values than those for sulfate.
While not substituting the necessity of performing validation tests using real acid mine drainage water in a defined case, the results obtained with the simulated AMD solutions can be used as a fast and preliminary evaluation tool of expected performance of the membrane in nanofiltration of real water with a known ionic composition and pH.