Determining Fe(III) left in solution

[jwcohen]

Hi,

I'm new to PHREEQC and I'm trying to determine how much total Fe(3) regardless of species is left in solution at low O2 in a seawater medium I've prepared. I used the following code:

SOLUTION 1
    temp      20
    pH        6.5
    pe        14.14
    redox     O(-2)/O(0)
    units     mmol/kgw
    density   1
    Alkalinity 10 as HCO3
    Ca        9.5232
    Cl        630.101
    Fe(3)     10 uMol/kgw
    K         10.2319
    Mg        84.0151
    N(-3)     18.6947
    Na        480.568
    O(0)      10 uMol/kgw
    P         287.026 uMol/kgw   as HPO4
    S(6)      27.5084
    -water    1 # kg
USE solution 1
EQUILIBRIUM_PHASES 1
    Fe(OH)3(a) 0 0
    Goethite  0 0
END

Am I on the right track here? I noticed that there are Fe(2) species in solution when I run this, which I didn't expect since I don't think there would be anything reducing the Fe(3). Same for some S- species showing up.

Also is there a way to graph total Fe(3) over a range of pH?

Thanks in advance.

[dlparkhurst]

You have define 18 mmol/kgw of ammonia. That will reduce your Fe(3) to Fe(2) and S(6) to S(-2) to some extent. The reaction solution has some N(0), which is the result of oxidation of N(-3).

If you remove N(-3) from the SOLUTION definition, you will precipitate virtually all of the Fe as goethite, which is 5 orders of magnitude more insoluble than Fe(OH)3(a).

The Basic function TOT("Fe(3)") will return the concentration of Fe(3) in solution, which is the sum of all ferric ion pairs and complexes. As for adjusting pH, you can add varying amounts of NaOH or HCl with REACTION. Also look at example 8; it is a bit complicated by the automatic generation of code, but basically you can use a phase (Fix_H+) to add a reaction to a given pH. Also look at the footnote in example 8 in case you are unsure whether you need to add acid or base.