Are my Sorption Sites filled?

[bkazamel]

Hello,

Long time reader, first time poster here. We have a reactive transport model in which Ni and Co are sourced from the dissolution of pyrite, and are subsequently adsorbed by Fe(OH)3 precipitated from the dissolution of pyrite (FeS2) and siderite (FeCO3). The conceptual model is that with time, the Fe(OH)3 content of the system increases and Ni and Co concentrations in the porewater are limited by adsorption. Ni is roughly 10x more abundant than Co in the pyrite.

Currently, our model output shows that very little Co is being adsorbed, while most of the Ni is adsorbed (See the attached images). However the system we are trying to model has measured Co concentrations that much less than our model, and Ni concentrations that are higher. At first, we thought that Co might not be adsorbing because Ni was taking all of the adsorption sites. We checked the mol fractions for the adsorption sites, and discovered that most of the adsorption sites are still unfilled (see attached image). I think that most of the adsorption sites are unfilled based on 60% of the strong sites being occupied by "sOH", which is a master species for the surface. Is this the correct way to interpret the mol fractions?

When we run the model with only allowing Co to adsorb, the Co concentrations are no different than when Ni and Co adsorb together. We removed all other sorbing species and still got the same result. So to me it doesn't look like all of the sorption sites are filled, and that it has to do directly with the log_k values for Ni and Co adsorption.

Any help with this issue would be greatly appreciated! Attached the Input file here (takes ~5 min to run).

Cheers,
Brent

[dlparkhurst]

No, your sorption sites are not all filled with Co+2 and Ni+2.

I'm not sure I am using the same model as you because Hfo_sOHCa+2 is a major species in llnl.dat for Hfo_s and Hfo_wOMg+ for Hfo_w in my version. Hfo_s appears to be the dominant sorbed species for Ni+2 and Co+2, but Hfo_w species are within an order of magnitude lower.

You have a lot of moving parts here, but here is what I think is going on. Fortunately a steady condition applies after one pore volume of water passes through the column. Even though the number of surface sites continues to increase with increased precipitation of Fe(OH)3, the proportions of surface species become constant in each cell. Likewise, the solution compositions become constant in each cell. I used 50 cells and 75 steps to produce a steady condition.

Looking at just Co+2 sorption on Hfo_s, the equilibrium condition that obtains in every cell is

Code: [Select]

K = f(Hfo_sOCo+)/f(Hfo_sOH) * [H+]/[Co+2] *  exp(F*psi/RT),         

 where f is mole fraction of Hfo_s sites and [] indicates activity.

The attached graphs show the steady state conditions with distance in the column after 75 shifts. Sites of Hfo_s (red squares) continue to increase, but other curves are constant with time. The product of the blue, green, and orange values for each cell is equal to the equilibrium constant for the Hfo_sOCo+ reaction.

Conditions change with distance in the column, but are steady at each cell. The change with distance is due to continuous dissolution of FeS2 and dolomite as each parcel of water transits the column. The second graph shows Co, Ni, and pH with distance. The main controls on Ni and Co are the concentration released in the kinetic dissolution reaction, and the log Ks for the sorption reactions, given the kinetic reactions. Results would certainly differ if dolomite and siderite dissolve slower, so that pH were lower, in which case sorption would probably be less important.

[Sen]

Dear Mr.Parkhust

The uploaded files can no longer be downloaded. Could you re-post these files?

warm regards

[dlparkhurst]

Sorry, the former attached files are lost to time.