my dear 

the value of O(0). SOLUTION_SPECIES must contain H+, H2, H2O, O2, e-, and OH

Typically, if you dont give a value, it will use a default. The main input from the atmosphere is often the Pco2 which can affects the pH of a sample by several units. I have modelled and replicated much experimental data with Phreeqc without considering pe. Look into the manual to see which applications it matters for.

Thank you so much for the answers sir.

In the manual, I too found that no input is given for pe while the water is in contact with amtosphere. Default pe = 4 is used as such. 

However, while doing some search I found that pe can be equal to 12 if we consider the presence of  oxygen of the atmosphere. Will that be ok sir? 

Hi Raj,

Yes, pe is typically calculated from the available redox couples (in your case oxygen). If you are under oxic conditions, a value of 12 or higher would make sense I would say.

TITLE Adjusting pe to equilibrium with atmospheric O2. From Appelo and Postma (2005)

SOLUTION 1

pe 10.0 O2(g) -0.68 # SI of O2 = -0.68 (SI=log(0.21); 21%O2 in the atnmosphere)

END

# pe = 13.6

Thank you Sir...