Kinetic Reactions Rate EQUATIONS

[kennedyantwi1]

Dear Phreeqc Users,
I'm new to Phreeqc and trying to model the geological storage of hydrogen using phreeqc.
I've been reading a fellow researcher's published work on the same topic and came across the reaction RATE EQUATION as shown below:

20 rate = 2.3e-09*(TOT("C(4)")/1e-3 + TOT("C(4)")) + 9.26e-8*(TOT("S(6)")/(1.e-4 + TOT("S(6)")))

Please, would anyone be kind enough to throw some light on the following?
1. what RATE EQUATION has been used
2. how the numbers 2.3e-09; 1e-3; 9.26e-8; 1.e-4 were obtained

[dlparkhurst]

I'm not sure you transcribed the equation exactly. I think the C(4) term should be analogous to the S(6) term. If you examine this term, it is constant at 9.26e-8, probably with units mol/sec, until the S(6) concentration decreases to the range of about 1e-4. At lower concentrations, the rate tends to zero.

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9.26e-8*(TOT("S(6)")/(1.e-4 + TOT("S(6)")))

You don't give the entire rate script or the KINETICS definition, but maybe it is a rate expression for consumption of H2? I don't know where the rate constants come from, perhaps from fitting empirical data or the literature. The numbers 1e-3 and 1e-4 are somewhat arbitrary to designate where the rates start to diminish.

[nickliam]

You cite the Hemme et al. paper from 2018. Go to Eqn. (4) of that paper (Monod equation).
It has the general form y = ymax (X/(a+X)); right under that equation you fill find the input values for carbon=C(4) and sulphur=S(6), respectively:
Ymax = 2.3e-9 and 9.26e-8 for C and S
a = 1e-3 and 1e-4 for C and S
X = C or S; both terms are added up for overall growth
I am sure by looking closer at the equation and the few written lines underneath it, it will all become clear to you