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react caco3 with NH4Cl solution

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I've been away from GWB for a while, so forgive me if this is obvious. I want to react calcite with a 0.1 molar solution of NH4CL and can't seem to get this to go. I'm forced to swap NO3 for NH4 and then specify the soluble oxygen content. I do not retain my NH4 at any reasonable oxygen content. I must be missing something.

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Hi,

 

Assuming you're using a thermodynamic dataset like thermo.dat, NO3- is the default basis species for N. If you take a look at the entry for NH4+ in thermo.dat you'll see that in order to form NH4+, you'll need NO3-, H+, H2O, and O2(aq) in your system. Alternatively, just go to program Rxn and type in "react NH4+" and it will spit out the reaction: NH4+ + 2 O2(aq) = NO3- + 2 H+ + H2O.

 

Since you don't want to consider oxygen in your system, or redox reactions between NH4+ and NO3-, you need to decouple the redox pair NH4+/NO3-. This will allow you to add NH4+ to your basis without swapping it for NO3- and adding O2(aq). As long as you don't additionally add NO3- to your basis, no N species in that redox state will be considered in your calculations.

 

The thermo.com.v8.r6+ dataset, on the other hand, uses NH3(aq) to represent N species. In this case, there is no need to decouple the reaction between NH3 and NO3- for your particular simulation.

 

You should look into section 2.4 Redox couples of the GWB Essentials Guide or chapter 7 in the Geochemical and Biogeochemical Reaction Modeling text for more on redox disequilibrium.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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Hi,

 

I have made some progess and am now using thermo.com.v8.r6+ dataset. I am now up against a question of how to best constrain the system. This is a closed reactor, about 1/2 water rest is air. There is excess calcite. I have tried two constraints 1) swap HCO3- for CO2 gas and set LogfCO2=-3.5, charge balance on H+ or 2) inital pH=7, charge balance on HCO3-. The results are quite different. Is there a better way to choose the basis?

 

hubert king

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Hi Hubert,

 

What exactly is known about your system? Swapping CO2(g) in for HCO3- is often a good idea if the reactor is open to the atmosphere, but may or may not be in a closed reactor, since the atmosphere is a much larger reservoir than the headspace of a reactor and its CO2(g) partial pressure is more likely to remain unchanged.

 

Do you have a chemical analysis of the reactor fluid (or a recipe to make it)? A pH measurement?

 

Since you have Calcite in your system, you might swap that in for either Ca++ or HCO3-.

 

Another question, how would you best describe your system? Are you dropping Calcite grains into a 0.1 M NH4Cl solution, or adding some volume of 0.1 M NH4Cl into a solution saturated with Calcite?

 

Regards,

Brian

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Hi Brian,

 

We have meaured pH before and after- little change from neutral. Although we have analyzed for various elements, we have not done that for this simple system

 

We have excess calcite. The starting point is a reactor with 1 gm of calcite and 23.5 gm of deionized water and 0.1 molar NH4Cl. We heat that up to 70C.

 

I would like to track the calcite solubility, pH, Ca++ content, alkalinity, etc. Once I have that under control, I will add other minerals and begin to introduce kinetics.

 

Hope that helps define better what we are trying to do. I've run similar simulations without the NH4CL and I am also confused about the correct formulation there.

 

hubert

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Okay, it looks like everything should be set up on the Basis pane. I would probably swap 1 free g Calcite for HCO3- and balance on Ca++. Then just set pH to the measured initial value, and .1 mol/l each of NH3 and Cl-. Then create a polythermal path from the starting value to 70.

 

Hope this helps,

Brian

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Using above basis, everything looks very sensible. Thanks

 

One question about GPLOT, why can't I plot pressure? For example, as I raise the temperature, pressure goes up and the calculations provide the values. I can plot fugacity, which is close to the pressure, but not the acutal pressure that you provide on the calculation panel.

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Glad that helped. The pressure you see in the output file is not really a calculated result. Rather, it is simply the pressure specified in the thermodynamic dataset which corresponds to the current temperature. Typically, thermodynamic data is collected at 1 atm (1.013 bars) up to 100 C. Above this the pressure refers to the vapor pressure of water.

 

Brian

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I am analyzing the results of my calculation. Cannot reconcile some of the numbers in react_output file. I decided to run under fixed log(PCO2)= -3.5, for now only water and calcite in basis, balance on H+.

 

at 25 C, I get pH= 8.275 and carbonater alkalanity = 49.11 mg/kg soln

 

the H+ concentration is given as -1.71e-7 moles in fluid - why is that negative?

 

The carbonate alkalanity I calculate is HCO3 + 2CO3 = 0.97014 mM/kg = 48.507 mg/kg-- not equal to the GWB value why?

 

Thanks

 

hubert

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Hi Hubert,

 

Can you attach a script (or just type in the commands, since there won't be many) because I'm getting a slightly different result.

 

The concentration of the H+ component is negative, but not the free concentration of the H+ species (which is small but still positive). Type "print species = long" to see the concentration of all species (under "Aqueous species"), not just those present at high concentrations. I think you're looking at the section "Original basis total moles" which refers to total component concentrations, not actual species.

 

If you're unsure how a component can have negative mass, consider a simple system with only the components H2O and H+. The species that can be formed from this basis set include H2O, H+, and OH- (OH- = H2O - H+). Since the pH is higher than 7, there will be more OH- than H+, and the overall solution is described by a positive amount of water component and negative amount of H+ component. Chapter 3 of the Geochemical and Biogeochemical Reaction Modeling text will be helpful here.

 

Cheers,

Brian

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# React script, saved Fri Aug 24 2012 by LB391

data = "C:\Program Files\Gwb\Gtdata\thermo.com.V8.R6+.dat" verify

temperature initial = 25 C, final = 180 C

H2O = .0235 kg

balance on H+

swap Calcite for Ca++

Calcite = 1 free g

swap CO2(g) for HCO3-

CO2(g) = -3.5 log fugacity

fix fugacity of CO2(g)

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