Simultaneous dissolution of mineral and CO2

[Pakan]

Hello,
My name is Noble. I am new to phreeqc and pardon me for probably naive question. I am bubbling CO2 gas  in a solution of a determined pH, eg, pH 8 and at the same time I am dissolving a mineral in the solution. I want to track the carbonate speciation in the solution and also the mineral speciation along with pH changes. My system is open to the atmosphere. I am a confused how to write the rate equations by taking account the dissolution of the CO2 gas and the mineral at the same time. Do I need two rate equations here? I assume at some point the CO2 will be in equilibrium but I want the system to determine that by itself. Can anyone help with this?
Thanks in advance

BR
Noble

[dlparkhurst]

For starters, you should fix the P(CO2) at the partial pressure of the gas that you are bubbling. You would use EQUILIBRIUM_PHASES and the log10 partial pressure of CO2 for the modeling.

There is a thread about kinetically dissolving CO2 (https://phreeqcusers.org/index.php/topic,2260.msg8413.html#msg8413), but I really don't think that is what you want to do.

[Pakan]

Hi Dr,

Thank you for the swift reply. Yes I want to do it much like the thread you showed. I want the CO2 to bubble and reach equilibrium with the solution and atmosphere since the system is open. One more question, when I make the dissolution of CO2 kinetic, how do I call the kinetics of the mineral and CO2 together in the code.?


BR
Noble

[Pakan]

DATABASE c:\phreeqc\database\llnl.dat
SOLUTION 1
water 0.1
pH 12
temp 25
EQUILIBRIUM_PHASES 1
CO2(g) -3.4
SAVE solution 1
END
USE solution 1
INCREMENTAL_REACTIONS true
kinetics 2
Wollastonite
 m 860
 m0 860
 parms 200 1
 steps 0 5 10 15 20 25 hour
 INCREMENTAL_REACTIONS true
RATES 2
##############
#wollastonite
##############
wollastonite
# from Palandri and Kharaka 2004
# experimental condition range T=25-65C, pH=1.6-12.4

-start
1 rem unit should be mol,kgw-1 and second-1
2 rem parm(1) is surface area in the unit of m2/kgw
3 rem calculation of surface area can be found in the note
4 rem M is current moles of minerals. M0 is the initial moles of minerals
5 rem parm(2) is a correction factor
10 rem acid solution parameters
11 a1=1.60E+04      
12 E1=54651
13 n1=0.400
20 rem neutral solution parameters
21 a2=5   
22 E2=54651
30 rem base solution parameters
31 a3=0
32 E3=0
33 n2=0
36 rem rate=0 if no minerals and undersaturated
40 SR_mineral=SR("wollastonite")
41 if (M<0) then goto 200
42 if (M=0 and SR_mineral<1) then goto 200
43 if (M0<=0) then SA=PARM(1) else SA=PARM(1)*(M/M0)^0.67
50 if (SA<=0) then SA=1
60 R=8.31451
75 Rate1=a1*EXP(-E1/R/TK)*ACT("H+")^n1  #acid rate expression
80 Rate2=a2*EXP(-E2/R/TK)               #neutral rate expression
85 Rate3=a3*EXP(-E3/R/TK)*ACT("H+")^n2    #base rate expression
90 Rate=(Rate1+Rate2+Rate3)*(1-Sr_mineral)*SA*parm(2)
100 moles= rate*Time
200 save moles
-end
END

[dlparkhurst]

You can have multiple kinetic reactants defined in KINETICS.

I think you are over-thinking your modeling of CO2. At least measure the pH and alkalinity to calculate a PCO2 at points in your experiment.

[Pakan]

Hi Dr,

Thanks for your kind reply. I tracked the pH over 24 hours but I didn’t measure the alkalinity hoping that I can infer from the modelling. But thanks for your answer. I am interested in the minerals that precipitate and how the pH evolves over time.