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