Re: Column model

["David L Parkhurst" <dlpark@xxxxxxxx>]

> I am modelling  F- removal with calcite in a column packed with granular
calcite (12 moles or ~1.2kg). The influent concentration of F- is 2300 mg/L
(as NaF) using ADVECTION with 40 cells & 600 shifts (15 pore volumes).
On analysis of the results I find that the entire amount of calcite is used
up within 6 pore volumes!!! and therefore 455.4 GRAMS of fluoride are
precipitated which would require about 200 litres of my solution.

I think the problem is the distinction between the column and the cell. At
each shift, a liter of water is added to the column. Six pore volumes is
6*40 = 240 liters of water. 240*2.3 ~ 500 g of fluoride, as you calculate.
So at a first read, it seems like the calculation is correct.

> I believe this is a due to the fact that the area of the column is not
specified (or controlled) and therefore the actual volume (and hence F
mass) passing through the column is much greater than it should be.

> Is there a way of limiting the volume of the influent solution passing
through the column or programming PHREEQ so that the surface area can be
taken into account??

I think the volume of water is probably the key. Here's the way I'm
guessing you should set the problem up. First, when you define
EQUILIBRIUM_PHASES for the column, you should define the amount of calcite
per cell, or 12/40 moles, when you use 40 cells. Second, you need to
determine the water-filled pore volume of a cell and define the infilling
solution to have that volume. There is no way to account automatically for
changes in mineral volumes in the column. You can define the water volume
in a solution in liters (really kilograms of water in the solution) in the
SOLUTION data block. Here is an abbreviated data block that would model
introduction of 0.25 L of water per shift.

SOLUTION 0
      -unit mg/L
      F  2300
      -water 0.25


> I am using PHREEQC Interactive Alpha version 2.4.2

PhreeqcI is finally officially released. The current version is 2.6.1,
available at the website listed below.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled