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Re: phosphate
>
> 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.
>
Thanks. Sounds interesting.
> 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.
Probably they're using lime softening. That involves adding lime to push
the pH very high. That converts all CO2 to carbonate and precipitates a
lot of the hardness (Ca and Mg) as carbonate minerals. This also strips a
lot of trace elements and may help pull phosphate down, as well.
Low phosphates in tap water shouldn't be real surprising. I think that
phosphate (normally sited as P) are typically in the ppb range in most tap
water.
> 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?
>
It's always necessary for concentrations to go higher than this kind of
calculation indicates before deposits form. Also, minerals will sometimes
just not form (the crystals will not nucleate) or form very slowly even
when chemistry says they should, and I think the chance of that happening
is pretty good at low concentrations.
Many of us (not me) have pH below 7.2, where this reaction may just not
happen at all. Or, we may have phosphate (and pH etc) high enough but
the apatite forms so slowly that it doesn't really balance the rate that
phosphate is added to the tank with food and fertilizer.
It might be interesting to circulate some tank water with elevated
phosphate and a pH of 7.4 or higher through a bone meal filter and see
what happens. Bone meal contains hydroxylapatite. If the problem is just
that hydroxylapatite crystals won't form on their own, then maybe it would
precipitate on the bone meal. This would have to be run with a control to
make sure that the phosphate wouldn't just drop out on its own over the
same period of time.
Even if that does work in a test there would be some danger in applying
that sort of treatment in a tank. The pH on the surface of the bone meal
might drop (probably due to biological activity) and the bone meal would
then release phosphate back into the water.
Roger Miller