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SiO2, HCO3 absorb with Fe(OH)3


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Dear GWB support forum team,

 

My name is Yongtae kim, in GIST, South Korea.

 

I'm doing experiment with Electrocoagulation technology.

when electrocoagulation process started. Fe2+, OH- ions generated and make Fe(OH)3.

That precipitate absorb arsenic in solution.

 

In real experiment, PO4, SiO2, HCO3 are in competition with arsenic removal by absorption.

So, arsenic removal efficiency is reduced by that anions.

However, in GWB reaction modeling, only PO4 act negative roles.

SiO2 is absorbed with Fe(OH)3 but that fraction is too small in GWB software.

HCO3 is not absorbed in GWB.

 

I was wondering if the mechanism was not included in the background of the GWB program or I was wrong.

 

A file is attached and I'd appreciate it if you could answer it.

 

Thank you.

Yongtae Kim

EC_As removal efficiency_Yongtae.pptx

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Hi Yongtae Kim,

The Dzombak and Morel two-layer model is a very thorough compilation of cation and anion sorption reactions involving hydrous ferric oxide. It is in no way complete, however. The FeOH.sdat dataset included with the GWB contains reactions and log Ks that Dzombak and Morel derived from actual experiments, while the FeOH+.sdat includes some estimates for additional sorbing species. I believe there is a reaction for SiO2 sorption in the FeOH+ dataset, but not in the FeOH dataset, for example, because they didn’t have any experimental data available. You should keep in mind that the Dzombak and Morel datasets are examples. The mineral they studied (and its surface properties) might be close to the iron mineral in your experiments, but they are not necessarily the same. 

In your models, the SiO2(aq) and HCO3- components do not significantly decrease the efficiency of arsenic removal. As I mentioned above, the FeOH.sdat dataset contains no sorption reactions for SiO2, while neither FeOH.sdat nor FeOH+.sdat include any reactions for HCO3- species. They do, however, have reactions for As(III) species and PO4---, which is why you observe both of those components to sorb. You can view the datasets yourself using the TEdit app. TEdit can also be used to add reactions that you need. You’ll need to supply a reaction for each surface complex along with an equilibrium constant, expressed as a log K. There should be information for sorption of HCO3- to iron oxyhydroxides in the literature, and there may be additional  information for SiO2 as well. Or, you can perform sorption experiments and derive log Ks yourself. For more information on TEdit, please see chapter 9 Using TEdit in the GWB Essentials Guide.

If you have a redox equilibrium model enabled (the default), the arsenic will be present as both As(V) and As(III), but mostly As(V), reflecting the high DO concentration you’ve specified. If you believe that the arsenic should be present in only one valence, you should decouple the redox reactions involving arsenic and add only the appropriate basis entry. For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide.

By the way, if your reactor remains in contact with the atmosphere throughout the experiment, you should fix the fugacity of O2(g) in your model (and CO2(g) as well, when you include the HCO3- component). If it is closed to the atmosphere, then you most likely should not fix the fugacity of these gases. Please see section 3.5 Buffered paths in the GWB Reaction Modeling Guide for more information. 

Hope this helps,

Brian Farrell
Aqueous Solutions LLC
 

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