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Re: laterite & phosphate

On Mon, 25 Jan 1999, Steve Pushak wrote:

> Does anybody know the equivalent of CEC (AEC?) for anions as a measure
> for laterite? Does it matter if the laterite is finely powdered or
> granular? Jim Kelly mentioned this phenomenon about phosphates and
> laterite but I haven't heard anything further of an analytical measure.
> One thing I do remember (vaguely) was somebody saying the fixing process
> for P in iron hydroxide crystals was very slow. (Roger?)

Hmm.  Don't know if that was me or not.

I don't think that the association of phosphate with laterite is an
exchange process analogous to cation exchange.  It is caused when
phosphate combines with hydrous iron oxyhydroxides in the laterite to form
a stable "surface" complex.  The stability constants for these complexes
are high.  The database for the USGS Phreeqc model contains constants for
these reactions.

Hfo_wOH + PO4-3 + 3H+ = Hfo_wH2PO4 + H2O               log_k=   31.29

Hfo_wOH + PO4-3 + 2H+ = Hfo_wHPO4- + H2O               log_k=   25.39

Hfo_wOH + PO4-3 + H+ = Hfo_wPO4-2 + H2O                log_k=   17.72

In these reactions, "Hfo_w" is a symbol for the weak binding site of
hydrous ferric oxyhydroxide.

With these reactions the amount of phosphates in the water depends on how
much laterite there is in contact with the water, how many available
binding sites (Hfo_wOH) there are in the laterite, how many of those
binding sites are already occupied by phosphate (Hfo_wPO4-2, etc) and by
the pH of the water.  Low pH favors formation of the surface complex.

I imagine that the number of potential binding sites in laterite would be
quite variable.  Only binding sites on the grain surface in direct contact
with the water are readily available for the reaction, so very fine grain
size (hence high surface/mass ratio) should make the effect stronger.

Roger Miller