RO permeate post treatment

[Juicemode]

Hello Community,

Having only a limited working knowledge of Phreeqc I though id reach out for help with a question I have.  I am looking to develop a post treatment strategy for reverse osmosis permeate.  I am looking to increase the pH to 7.2 and add alkalinity and if feasible hardness.  My first inclination is to try a combination of sodium hydroxide and sodium bicarbonate.  Is it possible to set up a titration simulation where solutions of both are introduced into the source water?  If not would it be advised to fist run a simulation with NaOH to near the target pH and then do a separate titration with NaHCO3?  The reaction will not be open to the atmosphere.

Find below my solution: I am looking for guidance in optimizing the amounts of solution to add and the final speciation of my treated water.  Thanks in advance!

SOLUTION 1
    temp      12
    pH        5.5
    pe        4
    redox     pe
    units     mg/l
    density   1
    Br        0.024
    K         0.241
    Ca        0.753
    Mg        0.36
    Na        4.76
    Sr        0.008
    Ba        0.001
    Cl        1.695
    F         0.065
    B         0.183
    S(6)      1.494
    Alkalinity 12.071 as HCO3
    -water    1 # kg

[Juicemode]

I made a few little tweaks but my first attempt turns up a '0 moles added' in my user print. I am using llnl.dat

Did I input an error somewhere??
 
SOLUTION 1
    temp      12
    pH        5.49
    pe        4
    redox     pe
    units     mg/l
    density   1
    Br        0.024
    K         0.241
    Ca        0.753
    Mg        0.36
    Na        4.76
    Sr        0.008
    Ba        0.001
    Cl        1.695
    F         0.065
    B         0.183
    S(6)      1.494 as SO4
    C(4)      83.771
    -water    1 # kg
End
PHASES
Fix_pH
    H+ = H+
    log_k     0
END
USER_PRINT
10 PRINT "Moles of NaOH: ", TOT("NaOH")
USE solution 1
EQUILIBRIUM_PHASES 1
    Fix_pH    -7.2 NaOH 10
END

[dlparkhurst]

The function TOT takes an element name as an argument, like "Na" or "Ca". One way to show the amount of NaOH reacted to maintain a pH is to use the EQUI function as below.

The file below also adds NaHCO3 as a reaction, so the amount of NaOH added adjusts as NaHCO3 is added. Note that NaOH may be removed if necessary to produce a fixed pH.

Code: [Select]

SOLUTION 1
    temp      12
    pH        5.49
    pe        4
    redox     pe
    units     mg/l
    density   1
    Br        0.024
    K         0.241
    Ca        0.753
    Mg        0.36
    Na        4.76
    Sr        0.008
    Ba        0.001
    Cl        1.695
    F         0.065
    B         0.183
    S(6)      1.494 as SO4
    C(4)      83.771
    -water    1 # kg
End
PHASES
Fix_pH
    H+ = H+
    log_k     0
END
REACTION
NaHCO3 1
1 mmol in 2 steps
USE solution 1
EQUILIBRIUM_PHASES 1
    Fix_pH    -7.2 NaOH 10
USER_PRINT
10 PRINT "Moles of NaOH: ", 10 - EQUI("Fix_pH")
END


[Juicemode]

^Thanks for your reply. 

Does my code correctly handle carbon for a reaction not open to the atmosphere?
The concentration of NaOH used here is 10 molar correct?

I did not think it all the way through and I now see that by adding NaOH to my solution I will not only increase the pH but also drive bicarbonate formation, from dissolved CO2 I suppose.  I had previously incorrectly assumed I needed NaHCO3 to add alkalinity.

I would like to compare the results of this code to a similar simulation using Na2C03 and CaOH2.  Would your advice be to determine the molarity of a solution of these compounds based on there solubility (2.05M and 0.022M respectively)?

Thanks again to the community for the time and guidance.

[dlparkhurst]

Your system is closed to CO2, the only carbon in the system is in the initial solution, and, if used, the addition of NaHCO3. To be open, you would need to include CO2(g) in equilibrium phases or add it as a reaction or kinetic reaction.

There are 10 moles of NaOH available for reaction, but only enough dissolves (or is removed) to bring the pH to 7.2.

Addition of NaOH, Ca(OH)2, NaHCO3, or Na2CO3 will increase the alkalinity of the solution. Alkalinity will be the sum of the aqueous species OH-, HCO3-, and 2*CO3-2.

The solubility of Na2CO3 and Ca(OH)2 are irrelevant if you are considering near neutral pH. The 10 moles of NaOH is simply a large number sufficient to have enough NaOH to produce the desired pH. You can replace "NaOH" with "Na2CO3" or "Ca(OH)2" in the fix_pH definition, or you can include a REACTION adding sequential amounts of any of these reactants to produce a titration curve of pH vs amount added.

[Juicemode]

Oh I see.  I was thinking the fix_pH function was using a aqueous solution of NaOH similar to what I am actually trying to simulate.  When using the REACTION function and creating the said titration curves can I specify an aqueous solution of the compounds in place of the pure solids?

[dlparkhurst]

If the titrant is concentrated, to a first approximation you can just add the solutes in REACTION, like NaOH or Ca(OH)2, ignoring the relatively small amount of water that would be added.

If you want to do a more accurate titration, you can make the two solutions and MIX them together, but that gets a little tedious because you need a MIX for every step. Further, if you want to consider volume, you need to do some preliminary runs to determine the volumes of each solution.

You can also add water in the reaction in addition to the solute. This reaction would add about 1 mL of approximately 1 N base. It would be close enough for most purposes.

REACTION
NaOH 1
H2O   55.5
0.001 mole