arsenic speciation

[ruth]

Hi Tom,

 

I am trying to model the theoretical chemical speciation of arsenic using the bulk chemical composition of the water.

My question is if the program can distinguish that I am using the total concentration of arsenic as input for the program of the GWB?

This is because the basis species for the arsenic is As(OH)4-

 

Thank you very much

[Tom Meuzelaar]

Hi Ruth:

 

This should work fine- just be sure to:

 

• constrain the oxygen concentration in the system
  
• specify a pH
  
• convert your total As concentration to the arsenic basis species
  

 

GWB will speciate all of your arsenic among all dissolved arsenic species in the thermo database according to their equilibrium constants.

 

Let me know how I can be of further help.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

[twq]

Hi Tom,

 

How to convert the total As concentration to the arsenic basis species? Since, usually ,the data we get from labaoratory is the total concentration.

 

 

Thanks

 

Tianwei

[twq]

Hi Brian,

 

I am a little confused about definition on 'component' and 'basis species' in GWB. What's the differences between them?

 

 

Regards

 

Tianwei

[Brian Farrell]

Hi Tianwei,

 

The basis species are simply what we choose to be our thermodynamic components. In aqueous geochemical modeling, it's generally accepted that using actual species is easier than using elements as components. What are listed in the thermo datasets as basis species are simply defaults. By performing basis swaps in the GWB programs, you can choose alternate basis species to represent the thermodynamic components.

 

Try taking a look at the beginning of chapter 3 in the Geochemical and Biogeochemical Reaction Modeling text for more information.

 

Regards,

Brian

[twq]

Hi Brian,

 

Then, How do we input the concentration in Gss ? Now we have the total concentration of Se, but the species can be selected in the basis species were SeO3--, SeO4-- and Se-- .

 

 

Regards

 

Tianwei

[Brian Farrell]

Hi Tianwei,

 

The options available in the spreadsheet are the basis species SeO4-- and the redox species (different oxidation states) SeO3-- and Se--. Unless you know the specific oxidation state, you should probably choose the basis species SeO4--. By choosing the basis species, you are specifying that the various redox species remain coupled to the basis species. So if you specify the Dissolved Oxygen concentration or Eh, you'll be able to figure out the distribution of Selenium between the various species in each oxidation state.

 

As for how to enter values, it may be helpful to use "elemental equivalents". For example, if the lab analysis reports results "as selenium," then you should right-click on the unit cell for your selenium species, then choose "as Se." Section 3.1 in the GWB Essentials Guide should be helpful here.

 

Regards,

Brian