Buffering Mechanisms in Acidic Mining Lakes

1. Buffering Mechanisms in Acidic Mining Lakes Totsche, O., Pothig, R., Uhlmann, W., Buttcher, H., & Steinberg, C. E. (2004).      Buffering Mechanisms in Acidic Mining Lakes -- A Model-Based Analysis. Aquatic Geochemistry, 9, 343-359. by Alex Stalboerger NDSU Geol 628 Geochemistry 2010

2. Introduction • Extensive open cast lignite mining was done in eastern Germany for several decades before German reunification • After the mines were closed approximately 200 acid mine lakes were formed through natural inflow of groundwater, surface runoff, and man-controlled flooding

3. Introduction cont. • The weathering of sulfide minerals and low carbonate content of the soil resulted in extreme acidification of many of the lakes • These lakes are not suitable sources for drinking water, fishing or recreational purposes due to the high acidity • It is also possible that the highly acidic water in these lakes could potentially contaminate neutral groundwater

4. Buffering Mechanisms • The main problem with neutralizing these lakes is the extremely high acidity produced by very strong buffering systems • Hydrogen sulfate buffering • Iron buffering • Aluminum buffering • Buffer based on ion exchange and mineral transformation

5. Buffering Mechanisms • I will be focusing on neutralizing one of the major buffering mechanisms that is characterized by the formation of Goethite (FeOOH(s)), from Schwertmannite (Fe16O16(OH)16-2x(SO4)x(s)): • Fe16O16(OH)16-2x(SO4)x(s) + 2xH2O  16 FeOOH + xSO42- + 2xH+

6. Theoretical Situation • To neutralize the buffering mechanism I will propose a theoretical situation • I will assume that near the acid mine lakes there is a farming community • The farmers use traditional fertilizers that contain N and P. Also within the soil there are sulfate compounds • Water containing N, P, and Sulfate have spilled into acid mine lakes

7. Objective • Using data for standard farm runoff concentrations of N and P of 0.0074 mmol/L and 0.00084 mmol/L respectively (Mitsch & Gosselink 2007), I will attempt to find a concentration of Sulfate that will begin to neutralize the Schwertmannite/Goethite buffering mechanism

8. Input File

9. Solution 1 Output • Phase SI log IAP log KT • Goethite 4.28 3.28 -1.00 FeOOH • pH = 2.55 • Within normal acid mine lake conditions Goethite is supersaturated and present in mineral form

10. Solution 1 &2 Mixed Output • Phase SI log IAP log KT • Goethite -0.09 -1.09 -1.00 FeOOH • pH = 2.45 • At a Sulfate concentration of 0.84084 mmol/L, the SI of Goethite drops to -0.09 and is undersaturated and begins to dissolve in solution

11. SI Trends

12. Conclusion • When acid mine lake water is mixed with water containing N, P and Sulfate it can affect the buffering mechanisms within the water creating such highly acidic conditions. • When water containing a Sulfate concentration of 0.84084 mm0l/L, along with N and P concentrations of 0.0074 mmol/L and 0.00084 mmol/L the Schwetmannite/Goethite buffering mechanism is effectively reversed.

13. Conclusion cont. • Even though the buffering mechanism appears to be reversed the system still maintains highly acidic conditions. • In fact the pH move from 2.55 to 2.45 • Efforts are still being made to understand the mechanisms and strength of the buffer systems