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 Geochemistry2010

IntroductionExtensive 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

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

Buffering MechanismsThe main problem with neutralizing these

lakes is the extremely high acidity produced by very strong buffering systemsHydrogen sulfate bufferingIron bufferingAluminum bufferingBuffer based on ion exchange and mineral

transformation

Buffering MechanismsI 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+

Theoretical SituationTo neutralize the buffering mechanism I will

propose a theoretical situationI will assume that near the acid mine lakes

there is a farming communityThe 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

ObjectiveUsing 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

Input FileSOLUTION 1

pH 2.55temp 25pe units mmol/LAl 1.05Ca 4.64Cl 0.25Fe(2) 0.001Fe(3) 2.68Mg 1.14Mn 0.05K 0.08Na 0.31S 13.63 as SO4-2

SAVE solution 1 END

 

TITLE UntitledSOLUTION 2

pH 7.0 chargetemp 25pe units mmol/LN 0.0074P 0.00084S 0.84084 as SO4-2

SAVE solution 2 ENDTITLE MIX 1

1 0.702 0.30

END

Solution 1 OutputPhase SI log IAP log KTGoethite 4.28 3.28 -1.00 FeOOH

pH = 2.55

Within normal acid mine lake conditions Goethite is supersaturated and present in mineral form

Solution 1 &2 Mixed OutputPhase SI log IAP log KTGoethite -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

SI Trends

ConclusionWhen 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.

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 2.45Efforts are still being made to understand the

mechanisms and strength of the buffer systems