Precipitation: Fixating pH on measured pH & Acidification with HCl

[cdelvalleve]

Hello,


I am currently constructing a code to model mineral precipitation on a wastewater sludge sample. To do so, I have measured concentrations of certain ions that can interact between them, such as PO4, NH4, Ca, Fe, Mg, K, Al, Na and Cl. I have also measured alkalinity of this wastewater (as carbonates) as well as short-chain fatty acids (as acetate).


That said, I am using the feature EQUILIBRIUM_PHASES that include a few minerals that are more likely to precipitate. My goal is to enter the total concentration of the ions mentioned above, as well as other chemical parameters, like pH, all of this in order to have an idea what minerals are mostlikely to be present in this wastewater.


However, after assameblage and precipitation, the pH changes. I do not want change on pH since this is the p

Code: [Select]

SOLUTION 1 Wastewater
    temp      25
    pH        8.23
    pe        -3
    redox     pe
    units     mmol/l
    density   1
    Acetate   66.18
    Al        5.26
    C(4)      285.69 charge # as carbonates
    Ca        37.95
    Cl        40.91
    Fe        2.16
    K         66.68
    Mg        22.13
    N(-3)     261.29
    Na        26.12
    P         26.2
    S         0.1
    -water    1 # kg


EQUILIBRIUM_PHASES 1
    Al(OH)3(a) 0 0
    Al2O3     0 0
   AlPO4     0 0
    Aragonite 0 0
    Ca3(PO4)2(am1) 0 0
    Ca3(PO4)2(am2) 0 0
    Ca3(PO4)2(beta) 0 0
    Ca4H(PO4)3:3H2O 0 0
    CaHPO4    0 0
    CaHPO4:2H2O 0 0
    Calcite   0 0
    Diaspore  0 0
    Dolomite  0 0
    Dolomite(dis) 0 0
    Gibbsite  0 0
    Hercynite 0 0
    Huntite   0 0
    Hydromagnesite 0 0
    Hydroxyapatite 0 0
    Hydroxylapatite 0 0
    Kstruvite 0 0
    Magnesite 0 0
    MgHPO4:3H2O 0 0
    Pyrite    0 0
    Siderite  0 0
    Spinel    0 0
    Struvite  0 0
    Vivianite 0 0
END

I know I am missing a lot of ions that might add some information regarding the charge balance (is on the carbonates), and therefore, the pH, but I would love to know if there is a way I can fix the pH to not change it after the EQUILIBRIUM_PHASES?


Also, I am planning to model the cidification of this digestate, but I am aware that probably this is going to have the same problem after the first reaction with HCl.


Till now, the only solution is to modify my input pH in order to reach the real pH value (increasing it) after the precipitation and charge balance.


Thank you very much,


C

[dlparkhurst]

You cannot magically set the pH with PHREEQC (except in SOLUTION). If minerals dissolve or precipitate, then there will be pH effects. PHREEQC requires you to add acid or base to adjust the pH.

Here is a script based on Example 8 in the manual, that will add acid (HBr) or base (LiOH) to obtain a specified pH. I picked Li and Br arbitrarily to demonstrate the way the calculation works. Two fictitious phases in PHASES and EQUILIBRIUM_PHASES are used to arrange the additions. The amount of base added is the mole transfer for Fix_H+, while the amount of acid added is the mole transfer of LiBr.

Code: [Select]

PHASES
Fix_H+
H+ = H+
log_k 0

LiBr
LiBr = Li+ + Br-
log_k -15
END

SOLUTION 1 Wastewater
    temp      25
    pH        8.23
    pe        -3
    redox     pe
    units     mmol/l
    density   1
    Acetate   66.18
    Al        5.26
    C(4)      285.69 #charge # as carbonates
    Ca        37.95
    Cl        40.91
    Fe        2.16
    K         66.68
    Mg        22.13
    N(-3)     261.29
    Na        26.12
    P         26.2
    S         0.1
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    Fix_H+ -8.23  LiOH
    LiBr    0  10
    Calcite 0 0
    Gibbsite
    Siderite
    Vivianite 0 0
END