Formic Acid and Alkalinity

[sreuter]

I have RO permeate that contains formic acid that needs to be pH adjusted (i.e., titrated) with NaOH before discharge. I do not have the alkalinity of the water because pH<4.3 and the lab does not provide negative alkalinity results. Below I have attempted to add the solution, add formic acid to the database, equilibrate with atmospheric CO2, and then titrate with NaOH. The results show that the alkalinity of the solution is negative until pH >6.3. I did not think you could have negative alkalinity above pH = 4.3, so trying to understand what I have done wrong. Any help would be appreciated.

TITLE Titrating NCL Wastewater with 50% caustic
SOLUTION 1 NCL
    temp      15
    pH        2.85
    pe        4
    redox     pe
    units     mg/l
    density   1
    Ca        0.1
    Cl        0.5
    F         0
    Fe        0.3
    Fr        2470
    K         0.59
    Mg        0
    Na        0.59
    S(6)      0.38
    -water    1 # kg


SOLUTION_MASTER_SPECIES
   Fr   Fr-   0.0      46.03      46.03
   
SOLUTION_SPECIES
Fr- = Fr-
   log_k   0.0
Fr- + H+ = HFr
   log_k   3.75

EQUILIBRIUM_PHASES 1
   CO2(g)   -3.5   

SAVE solution 1
END   

[dlparkhurst]

Alkalinity can be more complicated when you are not dealing with a carbonate system. In PHREEQC, you have to choose a reference state for each redox state of each element. It probably makes the most sense to assign 1.0 to Formate-, so that its titration to HFormate would get counted as alkalinity.

However, a pure Formate- solution would have an alkalinity end point of about pH 1.75, instead of about 4.3 for HCO3-. If you use this definition, alkalinity will be calculated to be positive for all pH above 1.75.

Code: [Select]

SOLUTION_MASTER_SPECIES
    Fr            Fr-              1     46.03           46.03

For your calculation, alkalinity is not really important. You want the amount of NaOH to reach a given pH. The amount of NaOH is nearly equal to the Fr concentration (at pH 2.85 about 87 percent is HFr, which needs to be titrated). The amount added from CO2(aq) is negligible.

BTW, Fr is the only significant ion in your SOLUTION. Is that right?

Code: [Select]

SOLUTION_MASTER_SPECIES
    Fr            Fr-              1     46.03           46.03
SOLUTION_SPECIES
Fr- = Fr-
   log_k   0.0
Fr- + H+ = HFr
   log_k   3.75
END
PRINT
-alk
TITLE Titrating NCL Wastewater with 50% caustic
SOLUTION 1 NCL
    temp      15
    pH        2.85
    pe        4
    redox     pe
    units     mg/l
    density   1
    Ca        0.1
    Cl        0.5
    F         0
    Fe        0.3
    Fr        2470
    K         0.59
    Mg        0
    Na        0.59
    S(6)      0.38
    -water    1 # kg
EQUILIBRIUM_PHASES 1
   CO2(g)   -3.4   
SAVE solution 1
END 
USE solution 1
REACTION
NaOH 1
100 mmol  in 20 steps
USER_GRAPH 1
    -headings               rxn pH Alkalinity
    -axis_titles            "NaOH added" "pH" "Alkalinity, eq/kgw"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y -LA("H+")
30 GRAPH_SY ALK
  -end
END