Calcium precipitation in a saline pond

[partrg]

Hi there Randy,

After reading your very interesting article in ReefKeeping on the effects of calcium concentration, alkalinity and pH on the saturation values of calcium carbonate, I was very interested to see if I could use this information to explain a large precipitation event we have recently experienced in a saline aquaculture pond. In fact there were two ponds, both with 15 ppt water, both with 180 ppm Ca and 160 ppm Mg and similar pH values of about 8.2 One had higher alkalinity than the other (320 V 280 ppm) and the one with the high ALK had a big precipitation problem, coating all the structures in the pond with what we assume was CaCO3. The low ALK pond had no precip. I am assuming to calculate whether these conditions resulted in supersaturation of CaCO3, I would need the Ksp values for calcium carbonate at 15 ppt and the proportion of ALK as CO3 at pH 8.2? Could you please advise if I'm on the right track in working out what happened in our ponds and if so suggest any literature where I could get such values? I realise this is an unusual posting for a reef forum, but any advice I can get would be much appreciated!

Cheers

Gavin

[Randy Holmes-Farley]

[welcome]

Calculating the Ksp will be tricky. Even if we could find it for 15 ppt seawater, the magnesium level that you have is more reflective of 4 ppt seawater. Since magnesium has an impact on the apparent Ksp (by ion pairing to the free carbonate ions in solution), I think that such calculations would be of limited value.

That said, I do think almost certainly true that the conditions that you state are well above the Ksp and could have caused a significant precipitation event. Even if the Ksp were the same as in normal seawater (it is not, it will be such that precipitation is more likely in lower salinity), you are above saturation. While the calcium is 0.43 times natural levels, the alkalinity is 2.5 times natural levels, and the pH is the same. Since normal seawater is well above saturation and only held in check by all the magnesium around, your situation is also supersaturated, with far less magnesium, making precipitation more likely.