Processes > Inverse modelling
Query about calculation of Saturation Indices
dlparkhurst:
First, consider that the Fe(3) is not usually measured, so the SI for Fe(3) minerals relies on the total Fe measurement and your choice of pe.
It sounds like you have an increase in Fe concentration, which means you need a mineral that dissolves. It is probably not hematite or any of the other oxyhydroxides like goethite or Fe(OH)3.
If the increase in Fe is significant, you should consider other possible sources--pyrite, aluminosilicates, others.
sally.xiaosa.0212:
Thanks for your help, Dr. Parkhurst. I?m now carrying out forward reaction path simulation, and some new problems have emerged.
I originally planned to use the amount of mineral transfer calculated in the reverse simulation as the input for the forward simulation, but found that the units did not agree.
The unit of mineral transfer for inverse modeling is mol/kgw, but the unit of mineral transfer input in forward modeling is mol. As it is impossible to acquire the water quantity in a natural groundwater system, how to solve the unit problem and continue the forward simulation?
dlparkhurst:
The units of mole transfers from inverse modeling are moles. The amount would change depending on the mass or volume of solution. In most cases you are using SOLUTION definitions with the default of 1 kg water, which will close to 1 L. So, effectively the mole transfers are numerically equal to mol/L.
Your water analyses are concentration, usually mg/L or ppm, which would apply regardless of the representative volume that you choose. If you use a representative volume of 1 L (~ 1 kgw) for your groundwater system, the inverse modeling results would be directly applicable. You would be tracking the evolution of 1 L of water as it moves through the groundwater system.
sally.xiaosa.0212:
Thanks for your reply, Dr. Parkhurst. I am now carrying out forward simulation for Fe and trying to find mainly contributors for excess Fe in solution. Only goethite and siderite were reported in XRD results.
Ferrihydrite (Fe5HO8?nH2O) is a kind of weakly crystalline iron hydroxide, which is often a precursor to more stable crystalline forms such as goethite and hematite. Although thermodynamically unstable, amorphous or poorly crystallized ferrihydrite is the main product of abiotic and biocatalytic iron oxidation. These minerals have significant differences in reactivity, with Ferrihydrite being the most active. Although ferrihydrite is not detected in XRD, it may make an important contribution to the accumulation of groundwater Fe. There are a few questions:
1. How to consider its mineral reaction in PHREEQC when ferrihydrite does not be included in mineral phases input? Is there any other software that contains ferrihydrite and do some simulation?
2. Is it the same chemical formula as goethite （FeOOH）, that is, goethite also stands for component of ferrihydrite?
3. What does Fe(OH)3 (a) represent in PHREEQC phases? Iron hydroxide precipitates?
dlparkhurst:
As you note, there are many ferric oxyhydroxide minerals that have a varying solubilities ranging from hematite to goethite to Fe(OH)3(a) (amorphous) and ferrihydrite (which is in the minteq.v4.dat database). For inverse modeling, it does not matter which ferric oxyhydroxide mineral you chose because you are only concerned with mole transfers, not quantitative saturation.
For other modeling, ferrric oxyhdroxides in any form are relatively insoluble, so if you include any one of them in EQUILIBRIUM_PHASES, the concentration of Fe(3) will be small except at low pH. At non-acid pH (say >5), I generally assume the iron is ferrous (Fe(2)), and the question is whether oxygen will enter the system to oxidize ferrous to ferric, which will precipitate.
For acid drainage, Fe(3) concentrations can be large, but Fe(2) could be present as well. Here you will genereally see red precipitates, so some ferric phase is precipitating. The environment is dynamic with Fe(2) oxidation, ferric precipitation (oxides and sulfates), and gradual conversion to more stable phases.
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