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 Lm-1Mn+2=O complexes. It is now accepted that usually the mechanism involves two steps:


                                 -L, -H+


(1)   MnLm + H2O2  <===> Lm-1Mn(O2H)/Lm-1Mn(H2O2)


                                   L          

                                  ---> Mn+1Lm  + .OH  +  OH-

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


                                  RH

                                  ---> Mn+1Lm + 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)5FeIV=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-/CO32-, 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.


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