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Re: Aquatic Plants Digest V3 #204

     Roger Miller wrote:
     
     >Under favorable conditions, hydroxylapatite is essentially insoluble. 
     >I turned around a calculation (perhaps a little too quickly?) and 
     >found that at pH 8 with 4 degrees of general hardness (all as 
     >calcium) hydroxylapatite solubility would allow something like 0.6 
     >part per trillion (not a typo) of HPO4 in solution.  With 8 degrees 
     >of general hardess this drops to about 0.2 parts per trillion.
     
     <snip>
     
     >All that aside, I wasn't aware that hydroxylapatite played a role in 
     >water treatment.  Someone might set me straight on that.  <snip>
     
     Roger,
     
     I got it from:
     
     Manahan, S.  1979.  Environmental Chemistry.  Willard Grant Press, 
     Boston.
     
     Chapter 8 is entitled "Water treatment", and phosphorus removal is 
     covered in section 8.18.
     
     My tap water has virtually no phosphate, but ample calcium (18 ppm Ca 
     and 6 GH) and high pH (9.8).  I suspect our treatment plant is 
     removing phosphate (and lowering hardness), but I do not know how they 
     do it.
     
     Your calculations indicate that in the presence of Ca, there is very 
     little soluble PO4.  Many people report phosphate levels of >1 ppm in 
     their tanks.  Does this mean they have very little Ca?  Or is the 
     phosphorus in another form (organic?) that will not precipitate?
     
     Regards,
     
     Mark