[sweetsedated]

Can you please spare me your precious time and share your expertise?
Thanks a lot in advance.


1. Manual calculation of the saturation state for fluorite (CaF2)


Groundwater analyses from Rajasthan in India and Maarum in Denmark are listed below. Concentrations are given as ppm (mg/kg).


Rajasthan              Maarum

pH                   7.3                   7.8

Temp. °C          25.                   8.0

Ca2+                7.816               42.08

Mg2+                19.08               28.92

Na+                 1100                439

K+                   5.865               14.0

HCO3-              901                  1086

SO42-               500                  0.0

Cl-                   615                  201

NO3-                484                  1.98

F-                     6.76                 1.69


(a) Calculate for the two water analyses, the Ion Activity Product (IAP) and the Saturation Index (SI =log(IAP/K)) of this water for fluorite (CaF2, logK = -10.57 at 25 °C and logK = -10.83 at 8 °C) without corrections for ionic strength (I) and complexes. Plot the Ca2+ and F- concentrations in the diagram (p. 3).

(b) Calculate the ionic strength (I) for both samples.

(c) Calculate activity coefficients for Ca2+ (ãCa) and F-( ãF) for both samples. Use Davis equation with A = 1.17 and b = 0.3. Ignore the temperature dependence of A for the Maarum sample.

(d) Calculate the SI for fluorite including activity coefficients and compare with the result from a). Plot the Ca2+ and F- activities in the diagram (p. 3).

(e) Answer the following 4 sub questions: i) What are the effects of the activity corrections? ii) Why is the effect larger for the Rajasthan sample than for the Maarum sample? iii) What effect did the inclusion of activity coefficients have on the saturation state for the Rajasthan waters? iv) What would be the effect of also including aqueous complexes in the calculations?


2. Calculation of the common ion effect

The fluoride content of the Rajasthan groundwater is well above the WHO limit of 1.7 mg/l. The possibility of decreasing the fluoride content of the Rajasthan water by addition of gypsum (CaSO4, log K = -4.58) will be considered.
 
(a) To what level could the fluoride concentration in Rajasthan groundwater be reduced by gypsum addition if you neglect the effects of ionic strength and complexes? Assume that equilibrium for both fluorite and gypsum is attained. You can do the calculations stepwise like for A, B and C in the diagram. Plot the resulting Ca2+ and F- concentrations in the diagram. If you are using the Tableau method to solve the problem then remember that we are looking at several compositions (A, B and C) with changes in-between them and remember that CaF2(s) is precipitating. 

(b) Verify all the assumptions that you made.

(c) What is the resulting sulfate concentration? (i.e. how much gypsum would be dissolved in the process?). Is that a problem from a water quality perspective?

(d) Evaluate, without doing calculations (use the diagram), what the effect of gypsum addition would be on the fluoride concentration of the Maarum water.

 
(e) Assuming a water composition as in point C in the diagram and simultaneous equilibrium with CaF2(s) and CaSO4(s). How would the oxidation of the mineral pyrite (FeS2(s)) with 0.3 mM of O2 affect the system in regards to the concentration of F- ? Write down the reaction equation for pyrite oxidation with O2.

[AWK]

There are special programs for such calculation. Some of them are even a freware

[eugenedakin]

Hello sweetsedated,

I would like to help, but I require that attempt the calculations yourself first (see the forum rules).

AWK is quite right, there are many programs which can help you out.

Sincerely,

Eugene