Number of strong and weak adsorption centers

[Dejavu]

Hello everyone,
I want to evaluate the sorption of cations and anions when HFO as Fe (OH)3 is added to water at a concentration of  0.88 g / L or 0.008 Mol.
According to Dzombak and Morel (1990), the surface area of HFO is assumed to be 600 m2/g, and the density of strong and weak centers is 0.005 mol centers/mol HFO and 0.2 mol centers / mol HFO, respectively.
My question is whether I should recalculate the number of strong and weak adsorption centers in proportion to the number of moles of Fe (OH) 3 (as shown in example 2) below? Please, advise which option is correct:

1) SURFACE 1
    Hfo_sOH   0.005     600   0.88
    Hfo_wOH   0.2
or
2) SURFACE 1
    Hfo_sOH   0.00004   600   0.88
    Hfo_wOH   0.00165
Thanks

[dlparkhurst]

Option 2 would be correct.

There is also an option to define the site density--sites per nanometer squared--by using the -site_units density option. If you use this option, then the total number of sites are calculated from the area, which makes it easier to ensure consistency between the area and the total number of sites.

For your definitions with the gram formula weight of Fe(OH)3, produces the number of sites per nanometer squared, as calculated by the script below. If you use -site_units density, you would substitute these values for 0.00165 and 0.000004 respectively.

Code: [Select]

Weak sites per nm^2:      1.8777e+00
Strong sites per nm^2:    4.6941e-02

Dzombak and Morel use a gram formula weight for HFO of 89, which gives the following values for sites per nanometer squared:

Code: [Select]

Weak sites per nm^2:      2.2547e+00
Strong sites per nm^2:    5.6367e-02

I'm not saying one is better than the other, only that your values may differ from straight D&M calculations because of the gram formula weight. HFO only persists for hours or days, so there is plenty of uncertainty as to the structure and number of sites any given environment.

Code: [Select]

USER_PRINT
10 gwt = GFW("Fe(OH)3")
#11 gwt = 89
12 PRINT "GFW Fe(OH)3:           ", gwt
15 FeOH3_mass = 0.88
20 Fe_mol = FeOH3_mass / gwt
25 PRINT "Fe, mol:               ", Fe_mol
30 weak_mol = Fe_mol * 0.2
35 PRINT "Weak, mol:             ", weak_mol
40 strong_mol = Fe_mol * 0.005
45 PRINT "Strong, mol:           ", strong_mol
50 area = 600 * FeOH3_mass
55 PRINT "Area, m^2:             ", area
60 weak_sites_nm2 = weak_mol / area * 6.02e23 / (1e9)^2
65 PRINT "Weak sites per nm^2:   ", weak_sites_nm2
70 strong_sites_nm2 = strong_mol / area * 6.02e23 / (1e9)^2
75 PRINT "Strong sites per nm^2: ", strong_sites_nm2

SOLUTION
END


[Dejavu]

Thank you very much