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Author Topic: Laboratory vs PHREEQC Redox States  (Read 1590 times)

awillou4

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  • Posts: 16
Laboratory vs PHREEQC Redox States
« on: June 25, 2017, 12:16:58 AM »
I'm dealing with a situation where lab redox & pH imply a particular redox state; however PHREEQC concentration outputs only match lab data when using a different redox state.  In my case my lab solution chemistry implies As(5); however PHREEQC isotherms only fit when As(3) is dominant (As(3) has a much smaller affinity for HFO).  I've included a basic version of my input with complete water chemistry below.

I'm wondering if its possible that the dominant species in my lab isotherms is actually As(3), even though redox and pH suggest otherwise.  If not, perhaps the native soil I used could have introduced competing ions which allowed As(5) to sorb far less during my batch runs?

SURFACE 1
    Hfo_wOH    6.71E-06    600       0.00504
SOLUTION 1
    temp      25
    pH        6.27
    pe        4
    redox     pe
    units     ppm
    density   1
    As(3)     0.403
    B         0.397
    Ca        20
    Cd        0.446
    Cr        0.516
    Mg        5
    Mo        0.389
    S(6)      25
    Se        0.636
    Tl        0.624
    V         0.245
    Na        10
    C(4)      10
    -water    0.2 # kg

USE solution 1
USE surface 1
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dlparkhurst

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Re: Laboratory vs PHREEQC Redox States
« Reply #1 on: June 25, 2017, 12:59:48 AM »
If you measured A(5) then I would tend to believe it is A(5), in which case, you could consider whether the characteristics and log Ks of Hfo are adequate, or whether the sorption characteristics of your soil may be different.

If you measured redox with a platinum electrode, I would not put too much faith in it. What the electrode responds to is in question, but it probably is not responding to As. So, I think you would need other evidence to try to estimate the state of the As in your system.
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