Silicon precipitation

[Anya]

Hello, I?m using PHREEQC and have faced some difficulties, so could you please help with these questions below.
For the input:
1) If I have not just water for the analysis but a mixture with water and gas, can I indicate that an element is in a gas state? (Like in the output we can have O2(g) which stands for oxygen in a gas state, does PHREEQC have something like this for the input?)
2) I?ve read about ?pe? in the instructions. It says that stands for charge or phase, but I don?t understand how it can be connected. What I should enter there?
3) Why don?t we differ CO3 and HCO3, they are all named as C(+4) in the input?

For the output:
4) Can I somehow find the volume of each element that I get? As I understand it, SI shows, in simple terms, the ?probability? of precipitation, but is there a way to calculate exactly the amount of precipitation?

[dlparkhurst]

1) If I have not just water for the analysis but a mixture with water and gas, can I indicate that an element is in a gas state? (Like in the output we can have O2(g) which stands for oxygen in a gas state, does PHREEQC have something like this for the input?)

The SOLUTION data block is used to define dissolved concentrations. You can only enter concentrations for the aqueous phase.

You can handle gases in two ways. (1) You can fix the partial pressure of a gas by using EQUILIBRIUM_PHASES. The following would dissolve or exsolve CO2 to reach equilibrium with atmospheric P(CO2):

Code: [Select]

EQUILIBRIUM_PHASES
CO2(g) -3.4 10

or (2) You can define a GAS_PHASE. Gas phases are gas bubbles that have either a fixed pressure or a fixed volume. You have to list the initial composition of the gas phase in the GAS_PHASE data block. When a fixed-pressure gas phase reacts, the composition and volume of the gas phase changes. When a fixed-volume gas phas reacts, the composition and pressure of the gas phase changes.

Note that N2(g) will react with O2(g) and other dissolved redox species. If you don't want N2(g) to participate in redox reactions, use Ntg(g) as defined in phreeqc.dat and Amm.dat.

2) I?ve read about ?pe? in the instructions. It says that stands for charge or phase, but I don?t understand how it can be connected. What I should enter there?

Look in the forum for discussions of pe. I don't feel like repeating the discussions here. Simply put, the pe determines the distribution of redox elements among their redox states.

3) Why don?t we differ CO3 and HCO3, they are all named as C(+4) in the input?

PHREEQC only allows entry of total dissolved inorganic carbon--C(4)--or Alkalinity. Alkalinity includes all aqueous species that would participate in an alkalinity titration, including OH-, CO3-2, BO3-3, H3SiO4-, and others.

For the output:
4) Can I somehow find the volume of each element that I get? As I understand it, SI shows, in simple terms, the ?probability? of precipitation, but is there a way to calculate exactly the amount of precipitation?

A phase included in EQUILIBRIUM_PHASES will dissolve or precipitate to reach equilibrium.

[Anya]

Thank you very much!
There are SiO2 (am-gel) and SiO2(am-ppt) in my output, could not find them in the guide. Are they amorphous silica gel and amorphous precipitation or something else?

[dlparkhurst]

You could probably find the sources for those phases in the Wateq4f manual (https://wwwbrr.cr.usgs.gov/projects/GWC_chemtherm/pubs/wq4fdoc.pdf). They are less crystalline forms of SiO2 that might form in a rapidly precipitating environment, but would be expected to recrystallize into more stable forms with time.