> The question is, exactly what is happening during the reaction simulation? Are small quantities of Fe(OH)3 and CH2O being allowed to react to equillibrium at each time step, with the quantity of reactants dictated by the rate expressions in the RATES block? Basically, yes, although the quantities or may not be small, depending on the rate expression. The reaction increases the concentrations of Fe, O, H, and C in the dissolved state, in accordance with the stoichiometry of -formula and the moles calculated by the rate expression. > I assume that this must be the case, since the KINETICS block does not contain information on the amount of HCO3- liberation or H+ consumption associated with the overall reaction: Fe(OH)3 + 0.25 CH2O + 1.75H+ => Fe2+ + HCO3- + 2.5H2O > Although this approach is of course quite acceptable, I'm wondering whether it is possible to use Phreeqc in a more generic way, so that the consumption of reactants and production of end-products associated with the reductive dissolution reaction are accounted for explicitly, without having to assume global redox equilibrium. It is possible, but I don't think advantageous. In the above equation, how confident are you that the products are not FeOH+, CO2, CO3-2, FeHCO3-, FeCO3(aq), etc? How do you know that the reaction does not switch to methanogenesis? If you want to do things explicitly, then you start having to write kinetic reactions for a lot of things. I am still a little concerned with the definition that adds both Fe(OH)3 and CH2O, because it ignores the point where methanogenesis would start, so your kinetics would have to consider all these possibilities. So you can define [Fe3] and [Fe2] as separate elements, you can use "" to allow numbers in the element name. You would have to write all the aqueous species for Fe3 and Fe2, as well as all the phases. Then you could define kinetic reactions that remove Fe3 and replace it with Fe2. To do everything explicitly, you would have to do this for every redox state of every redox element and then determine appropriate reaction kinetics for all the transformations. It is possible, but you should probably use some other model to do it that is designed with some of these kinetics already established. PHREEQC might have a hard time if the resulting kinetic expressions formed a stiff set of equations. > The reason I ask is that I do not commonly think about biogeochemical reaction systems in terms of global equilibrium; rather, instinct leads me to consider everything as a kinetically-controlled reaction - with the exception of aqueous phase speciation and surface complexation reactions. Hence, I would like to learn to use Phreeqc in this more general way, which I assume is possible if the input file is constructed properly. Thanks very much in advance for your input. Note that only part of the system is in equilibrium. In this example, the organic substrate is not in equilibrium with the rest of the system. I think the key is to isolate the real nonequilibrium part and treat it kinetically, for the rest, equilibrium is often an adequate approximation. In general, you do not see O2 and Fe(+2) in the same water; if you look at sediment pore waters, the redox couples are not exactly in equilibrium, but they are not far off. And it makes sense because the bacteria that drive the reactions can only do what is thermodynamically feasible. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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