Fenton Like Reactions Proceed Via a Variety of Mechanisms

Dan Meyerstein, Chemistry, Ariel University/Ben-Gurion University, Beer-Sheva, Israel
Ariela Burg, Chemical Engineering, Shamoon College of Engineering, Beer-Sheva, Israel
Haya Kornweitz, Biological Chemistry, Ariel University, Ariel, Israel

The Fenton like reactions are of key importance in advanced oxidation processes, AOPs, and in biology. Their mechanism was in debate for many years: the question being whether the active intermediates formed are hydroxyl radicals or L_m-1Mⁿ⁺²=O complexes. It is now accepted that usually the mechanism involves two steps:

                                 -L, -H⁺

(1)   MⁿL_m + H₂O₂  <===> L_m-1Mⁿ(O₂H)/L_m-1Mⁿ(H₂O₂)

                                   L          

                                  ---> Mⁿ⁺¹L_m  + ^.OH  +  OH^-

 
(2)    L_m-1Mⁿ(H₂O₂)  ---> L_m-1Mⁿ⁺²=O  + H₂O

                                  RH

                                  ---> Mⁿ⁺¹L_m + R^. + H₂O   + OH^‑

It was also shown that at least for M = Fe the following reaction is of major importance:

                                                           2H⁺

(3)  (H₂O)₅Fe(OOH)²⁺ + Fe(H₂O)₆²⁺  --->  ((H₂O)₅Fe^IV=O + Fe(H₂O)₆³⁺)/(2Fe(H₂O)₆³⁺ + ^.OH)

Recent results point out that the reactions:

(4)  (H₂O)_6-l-1Mⁿ(OOH)_l(H₂O₂)  ---> (H₂O)_6-l-1Mⁿ(^.OOH)(OOH)_l-1(OH) +  ^.OH

(5)  (H₂O)_6-l-1Mⁿ(OOH)_l(H₂O₂)  ---> (H₂O)_6-l-1Mⁿ⁺¹(^.OOH)(OOH)_l-1(OH)   + OH^-

are the source of the ROS when reaction (2) is endothermic. Furthermore in the presence of HCO₃^-/CO₃^2-, which is present in AOPs and in biology, the reactions:

(6) LMⁿ(CO₃)(H₂O)_n + H₂O₂ <==> LMⁿ(CO₃)(H₂O)_n-1(OOH)

(7)  LMⁿ(CO₃)(H₂O)_n-1(OOH) ---> LMⁿ(CO₄)(H₂O)_n-1

(8) LMⁿ(CO₄)(H₂O)_n-1 ---> LMⁿ⁺¹(OH)(H₂O)_n + CO₃^.-

(9) LMⁿ(CO₄)(H₂O)_n-1 + H₂O₂ <==> LMⁿ(CO₄)(H₂O)_n-2(OOH) + H⁺

(10)     LMⁿ(CO₄)(H₂O)_n-2(OOH)  ---> LMⁿ(OH^-)₂(^.OOH)(H₂O)_n-2 + CO₃^.-

have to be considered. This means that in these systems carbonate anion radicals are the major reactive species.