Photocatalyzed oxidation pathways of 2,4-dichlorophenol by CdS in basic and acidic aqueous solutions

Photocatalytic oxidation of 2,4-dichlorophenol (2,4-DCP) by CdS in the presence and absence of thioacetamide as a function of pH has been studied. Based upon the trends in the appearance of different intermediates such as benzene and hydroxylated biphenyl, it is proposed that 2,4-DCP oxidation by positive holes (h+) is favored at pH less than 6, while oxidation by hydroxyl radicals (•OH) becomes dominant at pH greater than 6. This is evidenced by the intermediates detected at different pH values. In the absence of thioacetamide (TA), benzene was detected as the only intermediate at pH ranging from 4 to 5. When pH changed from 5 to 9, no benzene was detected. Highly hydroxylated intermediates, such as hydroxylated biphenyl, were the major intermediates. When TA was present, it may act as a “bridge” between, 2,4-DCP and CdS through its thiol functional group. Therefore, a shift of the optimal pH for 2,4-DCP oxidation from 5 to 9 was observed and no hydroxylated biphenyl were detected.