Sorption and degradation

The HP1 code incorporates modules simulating
(1) transient water flow in variably-saturated media,
(2) transport of multiple components, and
(3) mixed equilibrium/kinetic geochemical reactions.
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docfrank
Posts: 3
Joined: Wed Oct 27, 2010 3:37 pm

Sorption and degradation

Post by docfrank » Thu Oct 28, 2010 8:54 am

Hi all,

I would like to use the HP-1 to simulate a compound exhibiting monod-kinetics for its degradation, which is fine. However, I'm not quite sure how I have to address the sorption of the compound. Do I have to include it in the rates and kinetics definition or do I have to define specific surface species?

Thanks in advance
regards
frank

diederik
Posts: 51
Joined: Fri Mar 10, 2006 7:57 am
Location: Belgium

Post by diederik » Fri Oct 29, 2010 1:40 am

That depends.

If you want to describe kinetic sorption, you do not have to defined a surface master species, a surface species, surface, but a rate and a kinetic defintion. If you want to consider equilibrium sorption, you define a surface master species, a surface species and a surface.

For the kinetic cases: you need to know the equilibrium sorption. If you have competitive sorption, you might need to define also a surface (with surface species) with a very small amount of sorption sites. As such, PHREEQC/HP1 will calculate the equilibrium concentration which then can be used in the rate equation (see website of T. Appelo for an example with kinetic cation exchange)

diederik

docfrank
Posts: 3
Joined: Wed Oct 27, 2010 3:37 pm

Post by docfrank » Fri Oct 29, 2010 8:41 am

Hi,

Thanks for your response. My intention is to simulate HOC sorption, degradation and transport in a soil profile similar to what was done in Environ. Sci. Technol., 2005, 39 (11), pp 4189–4197 by van Breukelen et. al, however, on a vertical axis, and including water transport (which is not the problem).

At first I'd like to have an equilibrium sorption implemented with organic matter as the sorption domain and Freundlich sorption.

All the examples I find do have either the HOC degradation or the sorption of a metal species illustrated.

Cheers
Frank

diederik
Posts: 51
Joined: Fri Mar 10, 2006 7:57 am
Location: Belgium

Post by diederik » Fri Oct 29, 2010 10:18 am

I will try to check this paper.

Thus, you have to define a surface, a surface reaction (for Freundlich sorption, thus you have to have -no_check and -molebalance) and a surface. If you do not know how to implement a Freundlich isotherm in PHREEQC/HP1, let me know, and I can send you an example. Note also that when you have a charged species in this adsorption reaction, you need to transport the component "Charge".

What do you mean with "All the examples I find do have either the HOC degradation or the sorption of a metal species illustrated.". Do you want to have degradation of the sorbed DOC species also, or only degradation in the aqueous phase, but combined with sorption of it.

An example of combined degradation in the aqueous phase and sorption, you can have a look at example 15 in the PHREEQC manual.

diederik

docfrank
Posts: 3
Joined: Wed Oct 27, 2010 3:37 pm

Post by docfrank » Fri Oct 29, 2010 11:33 am

Hi,

Thanks, for the response! O.K. you were right, the example 15 does have the HOC sorption included, but for a kinetic sorption type. For implementation of the monod-kinetics I consulted Appelo's website.

It'd be great if you could help me out with the Freundlch implementation.

Cheers
frank

diederik
Posts: 51
Joined: Fri Mar 10, 2006 7:57 am
Location: Belgium

Post by diederik » Sun Oct 31, 2010 10:42 pm

This is how to define the Freundlich isotherm in PHREEQC:


The Freundlich isotherm:

S = Kf C ^n

where
S sorbed concentration [Mc Mb]
C aqueous concentration [Mc L3]
Kf Freundlich sorption constant
n Freundlich exponent.

In the input, you need the following (for a hypothetical component Pola)

#Thermodynamic database information
SOLUTION_MASTER_SPECIES
Pola Pola 0.0 Pola 1.0

SOLUTION_SPECIES
Pola = Pola; log_k 0.0

SURFACE_MASTER_SPECIES
Surf Surf
SURFACE_SPECIES
Surf = Surf ; log_k 0

Sor + n Pola = SorPola
-log_k logK # where logK = log Kf + log bulk density - log TotSurf (see below)
-no_check #reaction equation is not balanced w.r.t. Pola, with no_check, PHREEQC does not check the mole balance
-mole_balance SorPola # to indicate which species to use for the reaction

#geochemical model

SURFACE 1
Surf 1e+100 # TotSurf, also used to calculate K
-no_edl
-equilibrate with solution 1


diederik

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