# Nanofiltration of Simulated Acid Mine Drainage: Effect of pH and Membrane Charge
## 1 Introduction  
Acid mine drainage (AMD) is an acidic, sulfur-rich, metallic wastewater, originating from mine discharge of both operational and abandoned mines [1]. During mining operations, sulfide minerals such as pyrite (FeS\({}_{2}\)), machinawite (FeNiS), and chalcogenite (Cu\({}_{2}\)S) are being exposed. These exposed minerals are oxidized by the combined effect of water and oxygen resulting in the release of metal and sulfates. For example, AMD can be formed from pyrite through a series of oxidative reactions (Equations (1)-(4)) where ferric iron acts as the primary oxidant [2, 3]:  
\[2\mathrm{FeS}_{2\,(8)}+7\mathrm{O}_{2\,(g)}+2\mathrm{H}_{2}\mathrm{O}_{\,(l)} \to 2\mathrm{Fe}^{2+}\ _{(aq)}+4\mathrm{SO}_{4}^{2-}\ _{(aq)}+4\mathrm{H}^{+}\ _{(aq)}, \tag{1}\]  
\[4\mathrm{Fe}^{2+}\ _{(aq)}+\mathrm{O}_{2\,(g)}+4\mathrm{H}^{+}\ _{(aq)}\to 4 \mathrm{Fe}^{3+}\ _{(aq)}+2\mathrm{H}_{2}\mathrm{O}_{\,(l)}, \tag{2}\]  
\[\mathrm{Fe}^{3+}\ _{(aq)}+3\mathrm{H}_{2}\mathrm{O}_{\,(l)}\to \mathrm{Fe}(\mathrm{OH})_{3\,(g)}+2\mathrm{H}^{+}\ _{(aq)}, \tag{3}\]\[\text{FeS}_{2\text{ (s)}}+14\text{Fe}^{3+}\text{ (aq)}+8\text{H}_{2}\text{O (I)}\to 15\text{Fe}^{2+}\text{ (aq)}+2\text{SO}_{4}{}^{2-}\text{ (aq)}+16\text{H}^{+}\text{ (aq)}. \tag{4}\]  
Typically, AMD is characterized by high acidity (pH 2 to 4), high sulfate concentration (1000 to 20,000 mg/L) and elevated concentration of metal mixtures which may include iron, manganese, copper, calcium, lead, magnesium, sodium, and nickel [4; 5]. When left untreated, AMD can contaminate soils and water sources (ground and surface) by introducing heavy metals and lowering pH, thus pollution by AMD endangers the well-being of humans, animals, plants, and the ecology as a whole. AMD composition can vary in pH and ionic composition, depending on the mine location and mineral rock types. As a result of these diverse conditions, the choice of treatment is dictated by both economic and environmental factors. For the selection of AMD treatment, [PERSON] and [PERSON] [3] suggested three guidelines. Firstly, decision-makers have to be wary of the hidden economic and environmental costs associated with each treatment. For example, the cost of fossil fuels for transportation of materials and the price of waste disposal are usually overlooked. Secondly, it is important to ensure the sustainability of the chosen remediation method. Ideally, the end products of mine water treatment should be viewed as a potential resource rather than waste and valorized, accordingly. Sales of recovered and recycled resources could be used to fund the cost of treatment. Lastly, legislation and regulation from the relevant environmental authorities is the overriding factor that determines the choice of the treatment system. The disposal and wastewater discharge criteria usually demand near complete removal of sulfate, metals, and acidity from the AMD. So far, lime neutralization and biological treatment have been used as conventional treatment approaches to AMD. However, alternative methods such as ion exchange and membrane separation have the advantage to recover metal and acid in addition to AMD mitigation, as an effort to achieve zero liquid discharge [6]. Lime (Ca(OH)2) or limestone (CaCO3) are added to AMD to precipitate sulfate as gypsum and metals as hydroxides. The gypsum sludge is then removed by gravity separation, which raises the pH [7]. Massive amounts of gypsum sludge with no economic value are generated and have to be disposed of in large land areas. Moreover, this process is expensive, labor-intensive, and unable to meet the discharge criteria in some instances [8]. Biological treatment systems can be classified based on either active systems (bioreactors) or passive systems (aerobic and anaerobic wetlands) [3]. In general, biological systems rely on the ability of sulfate-reducing bacteria found in carbon sources (e.g., manure, wood chips) to generate alkalinity in AMD and remove metals. Sulfate is reduced to sulfide to form metal precipitates.