What reaction are you going to carry out in your reactor? Keep in mind, that Pourbaix diagram is based on thermodynamic properties and does not describe quantitatively the speciation in the system. 

It is intended to remove chloride and phosphates from the effluent. So, it is desired to know ideal voltage and pH by applying Pourbaix diagram.

The removal of chloride and phosphate are NOT chemical reaction. The reaction is A + B -> C + D.  The Pourbaix diagram in your system is likely to be 4-dimensional. 

"Treatment of textile wastewaters by

electrocoagulation using iron and

aluminum electrodes"

Mehmet Kobya, Orhan Taner Can, Mahmut Bayramoglu

Main reactions at the electrodes are:

Al → Al3+(aq) + 3e (anode) (1)

3H2O + 3e → 3 2H2 + 3OH− (cathode) (2)

Meanwhile, if anode potential is sufficiently high, secondary reactions may occur also, suchas direct oxidation of organic compounds and of Cl− ions present in wastewater:

2Cl− → Cl2 + 2e (3)

The chlorine produced is a strong oxidant that can oxidize same organic compounds and promote electrode reactions. In addition, the cathode, may be chemically attacked by OH− ions generated during H2 evolution at high pH values [25]:

2Al + 6H2O + 2OH− → 2Al(OH) 4− + 3H2 (4) Al3+(aq) and OH− ions generated by electrode reactions (1) and (2) react to form various mono-meric species such as Al(OH)2+, Al(OH) 2+, Al2(OH)2 4+, Al(OH) 4−, and polymeric species such as Al6(OH)15 3+, Al7(OH)17 4+, Al8(OH)20 4+, Al13 O4(OH)24 7+, Al13(OH)34 5+, which transform finally into Al(OH)3 according to complex precipitation kinetics.

On the other hand, electrogenerated ferric ions may form monomeric ions, ferric hy-

droxo complexes with hydroxide ions and polymeric species, depending on the pH range.

These are: FeOH2+, Fe(OH)2 +, Fe2(OH)2 4+, Fe(OH) 4−, Fe(H2O) 2+, Fe(H2O)5OH 2+,

Fe(H2O)4(OH) 2+, Fe(H2O)8(OH)2 4+, Fe2(H2O)6(OH)4 2+, which transform finally into

Fe(OH)3.