I have analysed a water samples taken from a water-rock interaction experiments for its major anions and cations. These major ions were further analysed with Visual Minteq for saturation indices. Expect species of Fe, other species are under saturated. For Fe species, over saturated and some values are given.
The SI of ferryhydrite of one sample is 2,52 while another has as 2.66. What does different values mean? Positive values mean oversaturation and what is the threshold for precipitation according to these values?
SI should have been calculated against the temperature of the experiment. In principle, a mineral is expected to precipitate if the SI is higher than zero and to dissolve if this value is below zero.
SI is dependent on the temperature and pH of the sample. do they have the same pH and temperature. If these parameters are higher in the sample #2, this would be the reason you got a higher number for SI. Since both SI numbers are positive, it denotes that both samples are over-saturated with the ferrihydrite and will cause precipitation.
I hope this answer helps. When are you going back to Sri Lanka. I might be back probably next year.
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