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ActII and Spec8 - Cadmium Eh-Ph


lzeng
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Hi Brian:

 

Per our discussion, I am submitting my questions through the forum.

 

I am trying to use the geobench to direct an in-situ remediation for cadmium.  Calcium polysulfide (CaS5) will be injected to the ground.  The injection solution will have 54 gram per liter of concentration.  At the injection point, the concentration of CaS5 will have the concentration of the injection solution.  At the injection radius of influence edge (i.e. 10 ft from the injection point), the concentration of CaS5 levels will be reduced to several orders of magnitudes lowers. 

 

My goal is to figure out:

  • The site-specific Eh-Ph diagram under the different CaS5 concentrations.

  • The sulfide concentration at which sulfide can not effectively precipitate cadmium.

 

First step:

I read the user guidance and forum and conduct the modeling follow the step

Input two sets of parameters in Spec8 as show in the attached file “S1-full” and “S7-full”:

Note that:

  • S1 is the simulation of the condition near the injection location and CaS5 concentration is the injection solution level. 

  • S7 is the simulation of the condition away from the injection location and CaS5 concentration is only 1/1000 of the injection solution.

  • The input requires pH and O2 level in the aquifer, the level of the reagent (CaS5) really determine the pH and O2 levels and I can not assume the constant pH and O2 levels.  I used the pH and O2 level measured in the field. 

Second step:

I used the output of the log activities from SpecE8 files and directly applied the log activities to the Act2 input activities for different constituents.  See the attached in-put Act 2 files: S1-input and S7-input. 

 

Please help me answer:

  • Is it Ok to use the Spec8 log activities output file directly?  If not, please direct me how to generate the correct activities?

  • I selected a few constituents that show highest log activities to the Act 2.  Meanwhile, when Ca++ and CaSO4 both showed high log activities, I used the constituent with high log activities – CaSO4 as the input for calcium.  Please let me know whether this is an appropriate procedure?

  • Because CaS5 really change pH, ORP, and DO levels, I changed pH and O2 level in the Spec8.  Is there way to actually estimate the CaS5 concentrations and corresponding pH and oxygen level?  or I should just use the site background pH and DO levels?

  • Lastly, which program can directly estimate sulfide concentration at which sulfide can not effectively precipitate cadmium anymore – meaning the aqueous concentration will be more than 0.003 ug/L.

 

Thank you!

     s7-full.sp8

s1-full.sp8

S1-input.ac2

S7-input.ac2

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Hi,

I think you’re trying to distill too many complicated processes into a single two-dimensional diagram. They can be quite useful to help you understand certain aspects of a system’s chemistry, but you need to set up some reaction path and reactive transport models to answer most of the questions you’re asking.

To see the effects of CaS5 addition on pH and oxidation state, as well as the fate of Cd and other constituents of your fluid, you’ll probably want to use React to titrate Ca++ and S5-- into your contaminated fluid. For more information, please see 3.1 Titration paths in the GWB Reaction Modeling Guide. You could do something similar to vary sulfide concentrations to see when CdS precipitates or remains undersaturated. You could probably use a titration path or a sliding activity path to vary the amount of sulfide in the system, and you might want to buffer certain aspects of the chemistry in this case to keep things simple. A simple log activity of sulfide vs. pH diagram for Cd++ could also help here. 

To see the spatial zone of influence of the CaS5 amendment, you’ll probably want to set up a reactive transport model. A 1D radial model, for example, with flow diverging from the injection point would help you understand how far the amendment travels, how much it is attenuated by dilution or reaction at any point, and how much Cd remains in solution at any point. 

As for your redox-pH diagram, Act2 simply reproduces the algorithm you would learn in a geochemistry or aquatic chemistry class to make diagrams by hand. The diagrams work in terms of species activity; they assume species on opposite sides of a reaction line have equal activity along those lines. Species with different charges, then, would have different activity coefficients, which implies that concentration changes along the diagram. A large number of other simplifications go into those types of diagrams, and I don’t think they handle complexing species the way you expect. For a simpler problem it might be ok to take values from SpecE8, as you’ve done, but most of your complexing species in Act2 have little to no effect on the Cd++, and they certainly don’t interact with each other. 

Computing power and software have improved quite a bit over the years, so more realistic calculations can be used now. Phase2 draws diagrams that look somewhat similar to Act2 in many cases, but the calculation is a complete solution to the equations describing the distribution of mass, just like in SpecE8 and React. You constrain a fluid in terms of concentration, rather than activity. Then you set up simple paths, like in React, to adjust the chemistry. In the staging path, you might slide Eh while holding pH constant to define the left edge of the plot. Then, you trace a series of scanning paths originating from intermediate points along the left edge. For the scanning paths, you might slide pH while holding Eh constant at each of its original values. In this way, mass balance is conserved throughout the diagram. All components in the fluid can interact with each other. And since the calculation is for a fluid as a whole, there’s no “main species” – you can display the predominant form of any basis entry or element. A predominant species is the one that accounts for the most mass of an element or basis species. When constructing diagrams of this sort, though, you should typically work in more restricted Eh and pH conditions. It’s not particularly useful to know conditions far outside the stability limits of water or at extreme pH values.  

A few other notes:

Elemental equivalent units: The mg/l Pb++ as Pb doesn’t hurt anything, but it’s not necessary since the Pb++ ion and the element Pb have the same mole weight. The option is useful for polyatomic species, like the sulfate oxyanion, when the instrument measures the mass of only part of the  molecule. If NO3-concentration is determined by actually measuring the amount of nitrogen, for example, you’d use mg/l NO3- as N. For more information, please see 7.1 Example calculation in the GWB Essentials Guide.

Free vs. bulk constraints: When setting the concentration of the O2(aq) component with a DO measurement, you almost always want to use the “free” constraint option. For more information, please see 7.2 Equilibrium models in the GWB Essentials Guide.

How do you conceptualize redox chemistry in your diagram? Should all redox coupling reactions remain in equilibrium? For example, should ferric iron react to form ferrous iron under reducing conditions, or should it remain stable as ferric iron throughout the calculation, while ferrous iron is ignored? For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide.

If you continue to use Act2, you might want to look into making a mosaic diagram. That way you can account, in a limited way, for the speciation of complexing ligands over the redox and/or pH conditions of your diagram. For more information, please see 5.3 Mosaic diagrams in the GWB Essentials Guide.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC
 

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