Surface complex HFo and Sorption

[rfembilejr]

Hi,

I just want to be be sure about my understanding of the HFO for SURFACE. As HFo and other surfaces are generally written using the following format:

HFo_w A B C

where A is the number of sites or site density, B is the surface area, and C is the mass of the sorbent.

From Dzombak and Morel (1990), the range for site density for the weak sites is 0.1 to 0.3.
Question: Should I be confined on this range only? Or can I also use values like 10 or even 100?

For the surface area, Dzombak and Morel (1990) also pointed out a range of 200-840 m2/g. Is this why 600 m2/g is generally used, as an average?

For the mass of the sorbent, Dzombak and Morel (1990) assumed HFO to be Fe2O3 H2O ie. 89 g HFO/mol Fe.
How can we calculate the mass of the sorbent, in this case?

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For EXCHANGE, I have also read that this is user-specified to define amount and composition of an "exchanger" phase.
If I just want to use X to represent exchange, the moles of X can be calculated from CEC (cation exchange capacity) where X is:

X = CEC/((100/Sw)(theta/(1-theta))), where Sw is dry wt of soil, theta is porosity

If Sw = 2.65 and theta is 0.3, X is CEC/16.2.
In this case, X basically depends on the CEC of the soil?



Thanks and I appreciate any guidance on this.

Rodrigo

[dlparkhurst]

>  Question: Should I be confined on this range only? Or can I also use values like 10 or even 100?

You can use whatever you want, but I don't think 10 or 100 would be physically reasonable. The effective surface area is difficult to estimate, and in fitting experimental data, surface area and log K are usually highly correlated parameters. Note that if you increase the site density, you will be increasing the importance of the electrical potential factor, whereas a large surface area and smaller number of sites will decrease the importance of the potential factor.

> Is this why 600 m2/g is generally used, as an average?

Yes, I think it is the value they suggest, although it might be from a chapter by Davis and Kent.

> How can we calculate the mass of the sorbent, in this case?

You can start from an analysis of iron in the sediment, or better, an analysis of the easily soluble iron.

> In this case, X basically depends on the CEC of the soil?

Absolutely.

[rfembilejr]

Thank you, as always.