PhreeqcUsers Discussion Forum

Solubility => Dissolution/precipitation => Topic started by: edward on July 11, 2018, 04:54:09 PM

Title: Distribution of species and SI
Post by: edward on July 11, 2018, 04:54:09 PM
Hello ,
I need that LiCl to appear in the species distribution section,  but since it is not in the database, I added it in the following way,in PHASES, with what only appears in the saturation index section. Also according to literature LiCl should precipitate a certain amount, but with the saturation index that Phreeqc calculates, precipitation does not occur (SI is very negative) .Thanks

DATABASE c:\phreeqc\database\wateq4f.dat

TITLE Evaporation Preconcentration

SOLUTION 1  Evaporaci˛n muestra 1
        units           mg/L
        pH              8   # estimated
        temp          50
   pe              8.451
        Li              150
        Ca             160
        Mg             15
        Na             231
        K               88
        B               4.45
        Sr              33
   Ni              0.0066
   Zn            0.005
        Cl              19400
   Mn             225
   Rb              0.12
        Alkalinity    115 as HCO3
        S(6)           2860
        N(-3)          0.208
        N(5)           0.237
   O(0)           1.0     O2(g) -0.7
   -water         1 #Kg
USE Solution 1
REACTION 1
        H2O     -1.0
        53 moles
PHASES
LiCl
 LiCl:H2O= Li+ + Cl- + H2O
        log_k     10.02
SAVE solution 1
END
Title: Re: Distribution of species and SI
Post by: dlparkhurst on July 11, 2018, 11:00:19 PM
You are confusing PHASES and SOLUTION_SPECIES. PHASES is used to define minerals and gases; saturation indices will be calculated for all the phases for which all of the components (elements) of the phase are present in the solution.

SOLUTION_SPECIES is used to define aqueous species. If you think that LiCl(aq) exists, then you would define it in SOLUTION_SPECIES. llnl.dat has an LiCl(aq) definition.

A phase with a log K of 10 is extremely soluble. Halite log K is 1.5, which results in a solubility of more than 6 molal.
Title: Re: Distribution of species and SI
Post by: edward on July 12, 2018, 07:26:33 PM
Thanks!, I used that database that you suggested to me


My other question is quite basic. How can I know in the output file how many compounds have precipitated? (For LiCl and NaCl mainly)
Title: Re: Distribution of species and SI
Post by: John Mahoney on July 12, 2018, 08:20:21 PM
Look at the saturation indices first and identify phases that have saturation indices greater than 0.0.

Then,

you need to select phases that may precipitate and include those phases in an EQUILIBRIUM_PHASES block, so typically you would set up something along the line of

EQUILIBRIUM_PHASES
Calcite 0.0  0


This says set the calcite saturation index to zero (first 0.0), and you do not have any precipitated calcite in the system initially (the second 0).    This is sometimes called a possible phase.   

This allows for calcite precipitation to occur if the solution is oversaturated with respect to calcite.  There are a lot of other options look at discussion for equilibrium_phases.

For a lot of reasons, some phases like the iron bearing ones are going appear strongly oversaturated, but will not likely form at ambient conditions over the short term.   So you should probably not include them in your list.  Also even if you have lots of oversaturated phases in the list of EQUILIBRIUM_PHASES block only a few will precipitate because of phase rule considerations.  As the most stable phases in the list of possible phases precipitate the other phases become undersaturated, and cannot precipitate. 

Look at the previous literature related to your problem to get a sense of possible phases.