Advanced oxidation degradation kinetics as a function of ultraviolet LED duty cycle

Abstract

Ultraviolet (UV) light emitting diodes (LEDs) may be a viable option as a UV light source for advanced oxidation processes (AOPs) utilizing photocatalysts or oxidizing agents such as hydrogen peroxide. The effect of UV-LED duty cycle, expressed as the percentage of time the LED is powered, was investigated in an AOP with hydrogen peroxide, using methylene blue (MB) to assess contaminant degradation. The UV-LED AOP degraded the MB at all duty cycles. However, adsorption of MB onto the LED emitting surface caused a linear decline in reactor performance over time. With regard to the effect of duty cycle, the observed rate constant of MB degradation, after being adjusted to account for the duty cycle, was greater for 5 and 10% duty cycles than higher duty cycles, providing a value approximately 160% higher at 5% duty cycle than continuous operation. This increase in adjusted rate constant at low duty cycles, as well as contaminant fouling of the LED surface, may impact design and operational considerations for pulsed UV-LED AOP systems.

Advanced Oxidation of Tartrazine and Brilliant Blue with Pulsed Ultraviolet Light Emitting Diodes

“…For example, the 30% duty cycle resulted in 61% BB removal, the 20% duty cycle resulted in 43% BB removal, and the lowest duty cycles removed only small fractions of the BB. These results are consistent with the findings of Duckworth et al (2015) for the degradation of MB in a UV LED AOP of the same design. The first-order degradation model fit the data generated at 100% and 10%, and 5% duty cycles well with R 2 > 0.95 for 100% and 10%, R 2 = 0.87 for 5%.…”

“…This adsorption may have implications on LED reactor design based on characteristics of the dissolved target substances and background matrixes. Interestingly, the effects of duty cycle on the observed and normalized reaction rate constants for BB and TAR were in good agreement with the results of Duckworth et al (2015) for MB, in spite of MB adsorption onto LED lenses. Thus, even though dye adsorption reduced the LED output in the Duckworth et al study, their overall conclusions were upheld by the results of the current study, including their conclusions about the implications of pulsing on UV LED reactor design.…”

“…LED lens staining was observed in Duckworth et al (2015) with a similar PUV reactor for the degradation of MB. In that study, the power output of the LEDs was gradually reduced because MB adsorbed to the surface of the quartz lenses.…”

“…A UV LED reactor was constructed as described by Duckworth et al, (2015). The reactor includes drive electronics to control the duty cycle of the LEDs, a flow through water reactor containing 240 nm LEDs, and a pump to provide a continuous flow of solution (see Supplemental Figure S1).…”

“…Liang et al (2003) used PUV to remove 98% of influent N-Nitrosodimethylamine (NDMA) from groundwater and found that rate constants increased with more frequent pulsing. Duckworth et al (2015) investigated the effect of duty cycle on the degradation of methylene blue and found that the normalized rate constants were greater for 5% and 10% duty cycles than for larger duty cycles and approximately 1.6 times higher than CUV operation. Unfortunately, methylene blue is a cationic dye that adsorbed to the surface of the LEDs' quartz lenses, thereby reducing LED power output and complicating data interpretation.…”