Geochemist's Workbench Support Forum

# Apus

Members

2

1. ## Rate constant for the kinetic oxidation of Fe(II) by oxygen

Hi, As part of my lecture on reaction kinetics at University of Bayreuth, I have discussed the rate law of the abiotic oxidation of Fe(II) by molecular oxygen (Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters Wiley-Interscience, New York ) with my students. The reaction is Fe++ + .25*O2(aq) + 2.5*H2O -> Fe(OH)3(ppd) + 2*H+ and the rate law is given by: rate_con * activity("OH-")^2 * pO2 * molality("Fe++") with a rate constant of 8 * 10^13 M-2 atm-1 min-1 (at 20°C). I am now trying to simulate the kinetic oxidation of Fe(II) according to this rate law using GWB with my students. For initial rates, the rate law above should equal the example presented at the GWB academy which uses the buit-in rate equation rate_con * activity("OH-")^2 * molality("O2(aq)") * molality("Fe++") * (1-Q/K). The rate constant provided by GWB for this reaction is 7*10^2 kg mol-1 s-1. I understand that the latter rate constant refers to reaction with dissolved O2 on a basis of seconds at (probably?) 25°C, while the constant given above refers to a rate law expression with at O2 in air, on a basis of minutes at 20°C. I was wondering how these tow values compare, i.e. how I can calculate the rate constant from Stumm and Morgan in the units given by GWB. I tried to consider Henry’s constant (again, at 25 °C?) and the transformation from min to seconds, but this gives a completely different number. Also, I was wondering if the rate constant given by GWB refers to a temperature of 25°C (which is then adjusted by Arrhenius’ law, I assume?). If both rate constants cannot be readily compared, do you have a reference for the value used in GWB? The script I want to use with my students is given below, and it pretty much aligns with the file provided on the GWB homepage. Thanks a lot Kerstin Script FeII oxidation_built_in_RxRate.rea
2. ## Arc2: Sb speciation over pH as a function of sulfide

I am trying to plot Sb(III) speciation over pH as a function of sulfide, which itself speciates over pH (using Arc2). The x axis is pH and the y axis should be the sum of HS- and H2S concentrations. I have added the thermodynamc data for Sb sulfide complexes to the database, but sulfide does not speciate as HS-/H2S. I managed to plot Sb speciation with either HS- or H2S as the y axis and I have attached these files. Could you please advise me if (and how) it is possible that sulfide speciates over pH (no redox reactions required) to merge the two diagrams into one? Thanks a lot KH
×

• #### Activity

×
• Create New...