SIVO32- + 2 (FeIII)3+ + H2O 2 (FeII)2+ + 2 H+ + SVIO42-

This is an oxidation-reduction (redox) reaction:

SIV - 2 e- SVI (oxidation)

2 FeIII + 2 e- 2 FeII (reduction)

Change the oxidation states