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

 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

 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

 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

 I will be focusing on neutralizing one of the major buffering mechanisms that is characterized by the formation of Goethite (FeOOH
(s)
), from
Schwertmannite (Fe
16
O
16
(OH)
16-2x
(SO
4
) x(s)
):
 Fe
16
O
16
(OH)
16-2x
(SO
4
) x(s)
+ 2xH
2
O  16 FeOOH + xSO
4
2+ 2xH +

 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

 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

SOLUTION 1 pH 2.55
temp 25 pe units mmol/L
Al 1.05
Ca 4.64
Cl 0.25
Fe(2) 0.001
Fe(3) 2.68
Mg 1.14
Mn 0.05
K 0.08
Na 0.31
S 13.63 as SO4-2
SAVE solution 1
END
TITLE Untitled
SOLUTION 2 pH 7.0 charge temp 25 pe units mmol/L
N 0.0074
P 0.00084
S 0.84084 as SO4-2
SAVE solution 2
END
TITLE
MIX 1
1
2
0.70
0.30
END

 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

 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

 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.

 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