Geochemical Modeling of Iron and Aluminum Precipitation during Mixing and Neutralization of Acid Mine Drainage

Abstract

Geochemical modeling of precipitation reactions in the complex matrix of acid mine drainage is fundamental to understanding natural attenuation, lime treatment, and treatment procedures that separate constituents for potential reuse or recycling. The three main dissolved constituents in acid mine drainage are iron, aluminum, and sulfate. During the neutralization of acid mine drainage (AMD) by mixing with clean tributaries or by titration with a base such as sodium hydroxide or slaked lime, Ca(OH)2, iron precipitates at pH values of 2–3 if oxidized and aluminum precipitates at pH values of 4–5 and both processes buffer the pH during precipitation. Mixing processes were simulated using the ion-association model in the PHREEQC code. The results are sensitive to the solubility product constant (Ksp) used for the precipitating phases. A field example with data on discharge and water composition of AMD before and after mixing along with massive precipitation of an aluminum phase is simulated and shows that there is an optimal Ksp to give the best fit to the measured data. Best fit is defined when the predicted water composition after mixing and precipitation matches most closely the measured water chemistry. Slight adjustment to the proportion of stream discharges does not give a better fit.

Buffering Copper Tailings Acid Mine Drainage: Modeling and Testing at Fushë Arrëz Flotation Plant, Albania

“…The observations are consistent with the results of previous work [3,38], in which both the presented geochemical model and experimental results show precipitation of elements such as Al, Cu, Fe, Mn and Zn in the same pH ranges.…”

“…In particular, some elements, such as Al, As, Cu and Fe, undergo appreciable concentration decreases along the flat pH stretch at pH = 4-4.5. These results are consistent with the data in the literature [3,12,[37][38][39][40][41][42][43], showing that there is a correlation between chemical species in solution (and their oxidation state) and their precipitation in specific compounds as a pH function.…”

Chemistry, Growth, and Fate of the Unique, Short‐Lived (2019–2020) Water Lake at the Summit of Kīlauea Volcano, Hawaii

“…Results from PHREEQC simulations (Figure 11) suggest that hematite, jarosite, and goethite are highly supersaturated and ferric oxyhydroxide is slightly above saturation at 70 and 80°C and with 50% and 100% Fe 3+ . However, thermodynamic data on iron minerals at temperatures of 70-80°C are sparse, and studies have shown that there are significant discrepancies between calculated saturation indices and observations in natural systems for the various iron phases (e.g., Nordstrom, 2011;Nordstrom, 2020). In particular, there are no thermodynamic data in the different mineral databases used by PHREEQC for schwertmannite, which is one of the most common minerals to form via direct precipitation of iron in waters at the pH of the Kīlauea lake waters (Schoepfer & Burton, 2021).…”

Settling of Iron and Aluminum Particles in Acid Solutions for Acid Drainage Remediation

“…We used the wateq4f.dat database, as it is the most appropriate for systems associated with acid drainage [44]. Additional reactions for schwertmannite [45] and hydrobasaluminite [7] were added to the code, as they do not form part of the waterq4f.dat database.…”

“…Schwertmannite, Fe(OH) 3 (a), hydrobasaluminite, and Al(OH) 3 were the iron and aluminum phases chosen to precipitate in the simulations. These meta-stable phases were selected, as they are the most probable minerals to rapidly form in systems affected by acid mine drainage, at the pH range studied (pH 4.5-6.5) [7,34,45,46]. Hydrobasaluminite was preferred to basaluminite because the latter is formed by the dehydration of hydrobasaluminite [34], a process that is not expected in our suspensions.…”