Question about competition

[HFlower]

With the code below, if Ca adsorbs to the surface in an initial step, can Sr replace it in a subsequent step, even though no direct reaction is given for the exchange? I'm assuming such a reaction would be redundant, but I wanted to be sure. Thanks.

Hfo_sOH + Ca+2 = Hfo_sOHCa+2     -log_k   4.97

Hfo_sOH + Sr+2 = Hfo_sOHSr+2     -log_k   5.01

[dlparkhurst]

The first species on the right-hand-side of the equation is the one defined by the reaction. If you have two reactions defining the same species, only the last will be used.  So, no, you do not need to define an exchange reaction. If you did (and put it last), it would replace the original definition.

[HFlower]

Thank you for your clarification, Mr. Parkhurst.

[John Mahoney]

I think the question was not about the order of the reactions in a database, but rather Sr+2 replacing Ca+2 on the surface sites.  I suggest you answer your own question and try doing it, as a learning exercise.  Your question actually included its own answer, so you had the right idea.  Load up a surface with Ca+2, SAVE the SURFACE and END the first simulation, then in the next step USE  the  saved SURFACE and contact it with a different solution that has a high (always relative) concentration of Sr.  Some of the  Ca should leave the surface and go into solution and the Sr-surface complex should be present on the HFO. 

You could also do it in a single step. For example, use the REACTION keyword and add say SrCl2 in various amount.  look at the distribution of Ca and Sr surface complexes.