The role of transition metal ions on oxyhalogen redox reactions

The r e a c t i o n between c h l o r i t e i o n and c h l o r i n e and/or hypochlorous a c i d t o fo rm t h e h i g h l y r e a c t i v e C1202 in te rmed ia te i s v e r y r a p i d i n a c i d s o l u t i o n f o l l o w e d by t h e p r imary p roduc t i on o f c h l o r i n e d i o x i d e and under somewhat more d i l u t e c o n d i t i o n s reasonable concen t ra t i ons o f c h l o r a t e i o n . I n l e s s a c i d i c s o l u t i o n s , t h e p r i n c i p a l pathway i n v o l v e s t h e format i o n o f c h l o r a t e i o n by means o f d i r e c t e l e c t r o n t r a n s f e r between t r a n s i t i o n metal spec ies and t h e c h l o r i n e c o n t a i n i n g i n te rmed ia tes . I n t h e absence o f t r a n s i t i o n metal i ons i n t h e near n e u t r a l pH range, t h e reac t i o n proceeds n e g l i g i b l y slow. Emphasis w i l l be on t h e r o l e o f t r a n s i t i o n metal i o n s i n oneand twoe l e c t r o n t r a n s f e r r e a c t i o n s i n redox processes i n v o l v i n g h i g h l y r e a c t i v e non-metal species. INTRODUCTION The decomposi t ion o f hydrogen pe rox ide i n t h e presence o f t r a n s i t i o n metal i o n s demons t r a t e s t h e r o l e and importance o f s p e c i f i c f r e e r a d i c a l species. Fenton's reagent i s ob ta ined by m i x i n g aqueous i r o n ( I 1 ) and hydrogen pe rox ide which i s a s t r o n g e r o x i d i z i n g agent than hydrogen pe rox ide a lone ( r e f . 1) . It can o x i d i z e dihydrogen, i n i t i a t e po lymer i z a t i o n and o x i d i z e o rgan ic substances t h a t a re d i f f i c u l t t o o x i d i z e . subs t ra te , t h e o v e r a l l r e a c t i o n I n t h e absence o f a 2FeZ+ t H,O, t 2H+ -L. 2FeS+ t 2H,O i s accompanied by c a t a l y t i c decomposi t ion o f hydrogen perox ide . e x p l a i n t h e enhanced o x i d i z i n g power. r e a c t i o n has been analyzed i n t o s imp le r r e a c t i o n s : Th is , however, does n o t The exp lana t ion i s c l e a r o n l y a f t e r t h e o v e r a l l Fez+ t H,O, + FeOH2+ t OH OH t H,O, + H,O t HO, HO, t H,O, + H,O t 0, t OH Fe3+ t H,O, + Fez+ t HO, t H+ Fe3+ t HO, -, Fez+ t 0, t H+ Fez+ t OH + FeOH2+ FeOH2+ + Fe3+ t OHOH t S + Products where S denotes t h e s u b s t r a t e t o be o x i d i z e d (e.g. , H2) and Products denotes t h e produc ts o f t h e r e a c t i o n . 0,which a re d e r i v e d by p r o t o n l o s s f rom H,O, and HO,. Depending on t h e pH, f u r t h e r r e a c t i o n s can be w r i t t e n i n v o l v i n g H0,and Even these r e a c t i o n s may be