Derivation of SURFACE parameters for organic carbon in an agricultural soil

[peterwadeuk]

Greetings, good people.

I am working on a model of agricultural soil that is 20% v/v pore water, and 55% v/v solid material. The air-filled porosity is 25% v/v. I denote the "solid material" as dry-soil here.

I am neglecting the volume contribution of the organic matter here, for simplicity.

Following Example 19b, I believe that for 1g dry-soil, with 1 Litre pore water, the following specification holds for one monoprotic carboxylic acid humic acid site:

SURFACE 1
# 1 g soil = 0.7% Organic Matter ~ 3.5 mg Organic Carbon = 3.50e-03 g OC.
# 7.1 meq charge per g OC
# For Psi vs I (= ionic strength) dependence, adapt specific surface area in PHRC:
# SS = 159300 - 220800/(I)^0.09 + 91260/(I)^0.18
# Example: SS = 46514 m2/g for I = 0.003 mol/l
# 3.5 mg OC, 0.025 meq total charge, distributed over the sites:
# charge on 4 nHA sites: -2.84 / 4 * 3.5e-3 / 1e3 (eq) = 2.48e-06
  H_a  2.48e-06 46.5e3 3.50e-03

The concentration of surface area is 46.5e3*3.50e-03 = 1.7e2 m^2. This expression holds for 1g_dry-soil, and 1L_porewater (denoted 1L_pw).

The agricultural soil has the following properties. I express the equation numbers in brackets.

SOIL BULK PROPERTIES      
Water filled porosity of soil         0.20   L_pw/L_soil       (1)
Air filled porosity of soil         0.25   L_air/L_soil       (2)
Soil minerals vol proportion in soil   0.55   L_dry-soil/L_soil    (3)
Soil minerals vol proportion in soil   2.75   L_dry-soil/L_pw    (4) = (3)/(1)
Density of soil minerals         2.65   kg_dry-soil/L_dry-soil (5)
Soil minerals concentration         7.29   kg_dry-soil/L_pw    (6) = (4)*(5)
Soil minerals concentration         7.29e3   g_dry-soil/L_pw (7) = (6)

Here is where I have a problem. I have 7.29e3 g_dry-soil/L_pw, not 1 g_dry-soil/L_pw.

If I consider scaling up the expression for H_a  by 7.29e3 I get a truly huge estimate for reactive surface area, (46.5e3 * 3.50e-03 * 7.29e3) = 1.19E+06 m^2/L_pw.

This figure results in a very large volume in the EDL.

I would be very grateful for directions to errors in my logic.

All the best,
Peter

[dlparkhurst]

Just looking at the numbers, 46000 m^2/g seems too high. For comparison, freshly precipitated HFO is 600 m^2/g, but in real systems that is usually decreased because of aging and shielding or lack of contact with the pore water.

25 g OC/L pw seems plausible, but here too, you need to consider the extent of contact between pore water and OC.

So, I don't have a good answer for you except to try smaller values of surface area.