Mass of water question

[Olasz Tamas]

Dear Community!

I am confused by how PHREEQC determines mass of water. Below I will put my working model for a red mud sample for which I am trying to determine its equilibrium composition. However, if I put in 0.2 kg for the mass of water for the initial solution, and I equilibrate it with minerals present in red mud, after the reaction the mass of water becomes 0.77. Where does the water come from in this case? Or does PHREEQC recalculate for 1kg of water somehow? I am using the llnl database for this case.

My code:

SOLUTION 1
    temp      25
    pH        7 charge
    units     mol/kgw
    density   1

    -water    0.2 # kg


PHASES
NdPO4
   NdPO4 = PO4-3 + Nd+3
   log_k      -26.2

Nd2(CO3)3
   Nd2(CO3)3 = 3 CO3-2 + 2 Nd+3
   log_k      -33

Nd(OH)3
   Nd(OH)3 + 3 H+ = 3 H2O + Nd+3
   log_k      18.09


NdCl3
   NdCl3 = Nd+3 + 3 Cl-
   log_k    -4.16


Nd2(SO4)3
   Nd2(SO4)3 = 2 Nd+3 + 3 SO4-2
   log_k    -12.515

SOLUTION_SPECIES


   Nd(CO3)2- = 2 CO3-2 + Nd+3
   log_k      -12.73


   Nd(SO4)2- = Nd+3 + 2 SO4-2
   log_k    -5.1


   Nd2(OH)2+4 + 2 H+ = 2 H2O + 2 Nd+3
   log_k      13.894


   NdCO3+ = CO3-2 + Nd+3
   log_k      -7.53


   NdOH+2 + H+ = H2O + Nd+3
   log_k      7.997


   NdSO4+ = Nd+3 + SO4-2
   log_k      -3.66

EQUILIBRIUM_PHASES 1
Corundum 0 2.88
Quartz 0 3.84
Hematite 0 4.64
Anatase 0 0.88
Shcherbinaite 0 0.02
Lime 0 6.47
Periclase 0 0.43
Zincite 0 0.01
Pyrolusite 0 0.09
Afwillite 0 0
Akermanite 0 0
Albite 0 0
Albite_high 0 0
Albite_low 0 0
Amesite-14A 0 0
Analcime 0 0
Andalusite 0 0
Andradite 0 0
Anorthite 0 0
Anthophyllite 0 0
Antigorite 0 0
Aragonite 0 0
Beidellite-Ca 0 0
Beidellite-H 0 0
Beidellite-Mg 0 0
Boehmite 0 0
Brucite 0 0
Burkeite 0 0
Ca-Al_Pyroxene 0 0
Ca2Al2O5:8H2O 0 0
Ca2V2O7 0 0
Ca3V2O8 0 0
Ca4Al2O7:13H2O 0 0
Ca4Al2O7:19H2O 0 0
CaAl2O4 0 0
CaAl2O4:10H2O 0 0
CaAl4O7 0 0
Calcite 0 1.14
CaV2O6 0 0
Chalcedony 0 0
Chrysotile 0 0
Clinochlore-14A 0 0
Clinochlore-7A 0 0
Clinoptilolite-Ca 0 0
Clinoptilolite-dehy-Ca 0 0
Clinoptilolite-hy-Ca 0 0
Clinoptilolite-hy-Na 0 0
Clinoptilolite-Na
Clinozoisite 0 0
Coesite 0 0
Cordierite_anhyd 0 0
Cordierite_hydr 0 0
Cristobalite(alpha) 0 0
Cristobalite(beta) 0 0
Dawsonite 0 0
Diaspore 0 0
Dicalcium_silicate 0 0
Diopside 0 0
Dolomite 0 0
Dolomite-ord 0 0
Enstatite 0 0
Epidote 0 0
Epidote-ord 0 0
Ferrite-Ca 0 0
Ferrite-Dicalcium 0 0
Ferrite-Mg 0 0
Ferrite-Zn 0 0
Forsterite 0 0
Foshagite 0 0
Gaylussite 0 0
Gehlenite 0 0
Gibbsite 0 0
Gismondine 0 0
Goethite 0 0
Grossular 0 0
Gyrolite 0 0
Hatrurite 0 0
Hillebrandite 0 0
Jadeite 0 0
Kaolinite 0 0
Katoite 0 0
Kyanite 0 0
Larnite 0 0
Laumontite 0 0
Lawsonite 0 0
Margarite 0 0
Merwinite 0 0
Mesolite 0 0
Mg2V2O7 0 0
MgV2O6 0 0
Mirabilite 0 0
Monticellite 0 0
Montmor-Ca 0 0
Montmor-Mg 0 0
Montmor-Na
Na2CO3 0 0
Na2CO3:7H2O 0 0
Nahcolite 0 0
Natrolite 0 0
Natron 0 0
Nepheline 0 0
Nontronite-Ca 0 0
Nontronite-H 0 0
Nontronite-Mg 0 0
Nontronite-Na 0 0
Okenite 0 0
Paragonite 0 0
Pirssonite 0 0
Portlandite 0 0
Prehnite 0 0
Pseudowollastonite 0 0
Pyrophyllite 0 0
SiO2(am) 0 0
Rankinite 0 0
Ripidolite-14A 0 0
Ripidolite-7A 0 0
Rutile 0 0
Saponite-Ca 0 0
Saponite-H 0 0
Saponite-Mg 0 0
Saponite-Na 0 0
Scolecite 0 0
Smithsonite 0 0
Sepiolite 0 0
Sillimanite 0 0
Spinel 0 0
Talc 0 0
Thenardite 0 0
Tobermorite-11A 0 0
Tobermorite-14A 0 0
Tobermorite-9A 0 0
Tremolite 0 0
Tridymite 0 0
Wairakite 0 0
Wollastonite 0 0
Xonotlite 0 0
Zincite 0 0
Zn2SiO4 0 0
Zn2TiO4 0 0
ZnCO3:H2O 0 0
Zoisite 0 0
Na2O 0 2.92
Gypsum 0 0.29
Whitlockite 0 0.11
O2(g) -0.679 10
CO2(g) -3.5 10
NdPO4 0 0
Nd2(CO3)3 0 0
Nd(OH)3 0 0.002784
NdCl3 0 0
Nd2(SO4)3 0 0

end

[dlparkhurst]

I am a little skeptical of the set of minerals that you are including.

However, the mass of water is calculated as part of the equilibration. A simple example would be dissolution of gypsum, where the mass of water will increase as gypsum dissolves because of the formula CaSO4:2H2O. Other reactions also produce water Fe(OH)3 + 3H+ = Fe+3 + 3H2O, for example. The number of moles of H and O is conserved between the initial solution and the moles of minerals initially present.

[Olasz Tamas]

Thank you for your answer!

The minerals included are those that had a SI close to 0 or higher in the initial output file (without allowing any minerals to precipitate). Later I also mix the sample with different acids (HCl, HNO3, H3PO4, H2SO4) so all minerals are included which can form. But only 10-15 of these actually precipitate or dissolve along the process.

Yes I was thinking about the minerals "releasing" water as well, but the minerals that have inital amounts are all oxides, and in smaller amounts calcite, gypsum and whitlockite. So there is no H in any of them, how is it still possible that the amount of water increases?

[dlparkhurst]

I think by default you introduced 10 moles of Montmor-Na and Clinoptilolite-Na. Setting these to zero, you get a decrease in water mass by about 75% due to the dissolution and precipitation reactions.

[Olasz Tamas]

Oh well, now I feel stupid I didn't check thoroughly. But I guess thats why you define only the relevant minerals so you dont get lost. Thank you so much for your help.