WHAM humic acid model with sodium?

[peterwadeuk]

Esteemed colleagues,

A question for the chemists.

I am modelling an agricultural soil that contains a high concentration of organic matter.

The humic acid component of the organic matter may act as one of the principal ion exchangers in the system.

Organic matter as an exchanger is modelled as a SURFACE using the database http://www.hydrochemistry.eu/a&p/7/T_H.DAT which emulates the WHAM (Windermere humic acid model) by Tipping and Hurley.

I am simulating weathering of various silicate minerals into the pore waters of the agricultural soil.

I discover that one of the important processes in the system is substitution of calcium on the organic phase (Surf_H) by magnesium.

The release of sodium into solution by some of the silicate minerals is also an important process,  but there are no surface complexation constants defined in the T_H.DAT database for sodium.

Is this a problem?

Does sodium bind so weakly to the carboxyl and phenolic sites of the humic acids by comparison with binding of divalent metals with humic acids that the contribution of sodium binding to the humic acids may be discounted?

All the best,
Peter