Light regime, riboflavin, and pH effects on 2,4-D photodegradation in water

A laboratory study was conducted to determine the effects of light regime, riboflavin, and pH on photodegradation of 2,4-D in aqueous solution. In con trolled-environment chamber experiments, riboflavin sensitized 2,4-D photol ysis in a concentration-dependent manner under both attenuated UV (-UV) and enhanced UV (+UV) light regimes. The photolysis half-life of 2,4-D in solu tions containing 10 mg L-1 riboflavin was 9.7 and 12.5 h when exposed to +U V and -UV, respectively, compared to no photolysis in the absence of ribofl avin. In contrast, the extrapolated half-life of 2,4-D in solutions contain ing 2.5 mg L-1 riboflavin was 46 h under +UV and 72 h under -UV. The rate o f 2,4-D photolysis in the presence of riboflavin increased under both light regimes as initial pH of the solution was decreased from 7.5 to 4.5. The h alf-life of 2,4-D in the presence of 10 mg L-1 riboflavin at pH 4.5 and exp osed to +UV was 1.6 h. Lumichrome, a principal photoproduct of riboflavin, did not photosensitize 2,4-D. Concentrations of 2,4-dichlorophenol formed a s a result of riboflavin-sensitized 2,4-D photolysis were higher under the -UV than the +UV regime. These results indicate that riboflavin concentrati on, solution pH, and light regime are interacting factors that may be manip ulated to enhance rates of aqueous 2,4-D photolysis.