H O e HO HO 2 2 +→ + − − i (5)
Hydroxyl radicals can be also produced from the reactions:
Dye or RH HO Dye or RH HO RH H O + +− ( ) + → + →+ ( ) ii i i 2 22 (6)
Dye H O Dye HO H + + +→++ i i 2 (7)
The produced protons can be introduced in reaction (3) or
react with hydroxide ion in solution:
H HO H O + − + → 2 (8)
Thus, the production of water will lead to a decrease in the
hydroxide ion in the solution. A significant observation is that
the decolorization of the dye was accompanied with a depression
in pH by 2–3 units.
Experimental results revealed that higher degradation of the
dye occurred in basic medium than in acidic medium.The rate of
photo-decolorization increased with increase in pH and a possible
reason for this behavior might be due to the formation of more
hydroxyl radicals (Ochoa et al., 2001). A similar behavior was also
observed in earlier studies on Acid brown dye (Daneshvar et al.,
2003; Sakthivel et al., 2003) and methyl orange (Mehra and
Sharma, 2012). In acidic medium, the concentration of hydroxide
ions is quite low to form hydroxyl radicals enhancing the
degradation process as shown for extract solutions at pH 1.
•
RH (equation 6) can react with oxygen to form:
i
RH O Products CO +→ + 2 2 (9)
and RH RH HR RH i i +→− (10)
or RH OH R H O i i + →+ 2 (11)