Why In Struvite formation HPO42- is favored over PO43-?

[demander]

Cheers community, I have been studying the Struvite formation from respective ions. Being the general formula for Struvite MgNH₄PO₄.6H₂O I intuitively tought the reaction would be
Mg²⁺ + NH₄⁺ + PO₄^3- + 6H₂O↔ MgNH₄PO₄.6H₂O.

However from literature and from practice, not only the optimal pH seems to be around the range from 7 to 10/11 where HPO₄^2- is predominant as when Struvite forms tends to slightly decrease the pH of solutuon, which gives an hint for the release of a proton(H⁺), what suggests the real formula for struvite is
Mg²⁺ + NH₄⁺ + HPO₄^2- + 6H₂O↔ MgNH₄PO₄.6H₂O + H⁺.

My question is wouldn't it be more favorable for the system to generate Struvite directly from PO₄^3- instead of HPO₄^2-? doesn't it create a further energy consuming step for the deprotonation of the Hydrogen Phosphate?
From what I checked being pKa = 12.346 for reaction HPO₄^2- = H⁺ + PO₄^3-, the Free energy Gibbs Variation is ≅30,6 kJ mol^-1, implying from what I know that this deprotonation step is not a spontaneous reaction.

Might there be a reason I'm negleting or forgetting that might explain why this reaction is driven by HPO₄^2- instead of PO₄^3-?

Thanks in advance for any insight you might have.

 

[DrDu]

I think at higher pH values, rather than NH4+, NH3 would be the predominant species in solution which is not even ionic and therefore might be more difficult to embed into the crystal lattice.

 

[DrDu]

I just tried to calculate the pH where the solubility of Struvite is minimal. If I got my maths right, the optimal pH is $(\mathrm{pK_a(HPO_4^{2-})+pK_a(NH_4^{+}))/2 }$, which is about 10.9.

 

[demander]

Thank you DrDu, I was so focused on Phosphorus I didn't thought to remember the NH₄⁺ turning to NH₃ an evolving from solution.

I just tried to calculate the pH where the solubility of Struvite is minimal. If I got my maths right, the optimal pH is $(\mathrm{pK_a(HPO_4^{2-})+pK_a(NH_4^{+}))/2 }$, which is about 10.9.

That is an inteteresting value. What expression(s) did you use to get there to this sum of Pkas I don't remember this experession(maybe because I was too many years without working with solutions)?

 

[DrDu]

I used the equations
$K_{sp}=[Mg^{2}][NH_4^+][PO_4^{3-}]$
$K_a(NH_4^{+})= [NH_3][H^+]/[NH_4^+]$
$K_a(HPO_4^{2-})=[PO_4^{3-}][H^+]/[HPO_4^{2-}]$
$[Mg^{2+}]=[NH_3]+[NH_4^+]=[HPO_4^{2-}]+[PO_4^{3-}]$
And looked for a minimum of $[Mg^{2+}]$ as a function of $[H^+]$.