Negative Alkalinity Modelling

[Idoboro]

Hi,

I'm modelling solutions of SWRO desalination brine that went through electrolysis.
As a result of the electrolysis, some of the brines ended with a negative alkalinity measured, and a very low DIC.
When trying to model these brines, PHREEQC sets their pH to be 7 (although it should be lower) and calculates small positive alkalinity. Most of this alkalinity is still HCO3-.
Why is that? Can't PHREEQC model solutions with negative alkalinity?

P.S. I'm using Pitzer database.

Thanks in advance

[dlparkhurst]

You cannot set a negative Alkalinity. However, PHREEQC can calculate negative alkalinities, for example a pH 3 HCl solution.

PHREEQC adjusts the C(4) concentration to attain the specified alkalinity and cannot calculate a negative C(4) concentration. So, you need a different way to define the solution composition with pH, C(4) concentration, equilibrium with a phase (CO2(g), for example, or other means.