exponent for the term (A/V) in the reaction kinetics

[Tom]

From Nicolaus van Zweel from South-Africa

I have a question about a problem I am tinkering with. I modeled a humidity cell and the model seems to describe the leach tests data well, but a fitting parameter I used, I do not agree with and it is wrong in my opinion. It is the n value for the exponent for the term (A/V) in the reaction kinetics as described by Appelo & Postma, p162. I had to make this value much higher than values reported in literature. I think the shrinking core model will describe the data better an I just want to know if any one has attempted to use the shrinking core model in a kinetics block in PHREEQC.

[dlparkhurst]

The KINETICS data block has identifiers -M and -M0, which correspond to the current number of moles of the kinetic reactant and the initial moles of kinetic reactant. Usually, these are equal at the start of the run, and if you define only one, the other will be set equal to it. In a RATES equation, the Basic functions M and M0 refer to these values, where M0 is fixed by the KINETICS definition and M varies as the reactant is consumed (positive values of SAVE in RATES) or produced (negative values of SAVE in RATES). I think this provides the elements of the shrinking core model, where the surface area is assumed to be proportional to (M/M0)^n.

[nickvz]

Hello David, Tom

Tom thanks for posting my question. Hmm yes, I also see it that way David, but I want to bring in the shrinking core radius to calculate for the increasing resistance for diffusion of the formation of a 'ash' layer- or inerts 'shielding' pyrite reactive surface. I want to compensate for a mass transfer controlled reaction rate as the n value I used in the reaction kinetic term is really high (40) and the maximum reported by you guys on page p.162 is in the vicinity of 4 I think. So either my conceptual model is wrong (I modeled the cell as a semi-batch using a transport block and a stagnant cell) or I do need to find a better way to describe the solid/liquid system in terms of kinetics.

I would very much like it if you can comment on my approach. I am not a geochemist, I  would very much like to be one- but in South- Africa the there is not a lot of people working in this field that can assist in modelling in PHREEQC.

I have attached a picture of the data set I have at the moment fitted by my model. I will post my code shortly,

Thanks guys