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Surface complexation mass balance issue.

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When performing surface complexation, we have a sorbing surface and basis surface species. The basis surface species are formed after some reactions that have been input in the data base. At time t=0 there is some concentration of basis surface species present in the aquifer which should not be possible given at t=0 no reaction has taken place.

In the attached screenshot for the file 3metals.x1t, in the initial phase there is 1e-12mmolal of Hg+. Given this how do we have FeOHg+ of the magnitude 2e-5 in the system. Where does the extra mercury ion come from to form the complex species if none was there at t=0.

image (1).png

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By default, the program assumes the provided concentration describes the mass of dissolved solutes in fluid. At the start of the calculation, X1t calculates an additional mass adsorbed to the surface of the mineral that is in equilibrium with the solution. This is what you observe in 3metals.x1t.

Alternatively, you can set the concentration in your Initial pane as the total dissolved solute in solution and sorbed on the mineral surface. If you would like to set this constraint, go to 'Config' menu -> 'Options...' -> and check 'sorbate include'.

See Reference Manual for more details on "sorbate include". To see an example of two-layer surface complexation, see chapter 7.7.1 Surface Complexation in the GWB Essentials Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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