Hi  Jixing Cui,

I would try to explain it in the following lines

Rate Expression

-(dM/dt) = I0 phi f (1-exp(-At))

phi = quantum yield

I0 = Incident radiation flux

f = Ratio of light absorbed by the M (organic compound) and other components 

At = Total absorbance of the solution time a factor 2.3.

Which further reduces to:

rM = - ln(10) phi epsilon [M] I0            (Leifer, 1988)

Where

epsilon = Molar extinction co-efficient  (M-1 cm-1)

I would recommend you to read following research articles + books as much as you can

References

[1] Leifer A. The kinetics of environmental aquatic photochemistry: Theory and practice. ACS professional reference book (USA). 1988.

[2] Bolton J, Cater S. Homogeneous photodegradation of pollutants in contaminated water: an introduction. Aquatic and surface photochemistry. 1994:467-90.

[3] Oppenländer T. Photochemical Purification of Water and Air: Advanced Oxidation Processes (AOPs)-Principles, Reaction Mechanisms, Reactor Concepts: John Wiley & Sons; 2003.

[4] Alpert SM, Knappe DR, Ducoste JJ. Modeling the UV/hydrogen peroxide advanced oxidation process using computational fluid dynamics. Water Res. 2010;44:1797-808.

[5] Buxton GV, Greenstock CL, Helman WP, Ross AB. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅ OH/⋅ O− in aqueous solution. J Phys Chem Ref Data. 1988;17:513-886.

[6] Crittenden JC, Hu S, Hand DW, Green SA. A kinetic model for H2O2/UV process in a completely mixed batch reactor. Water Res. 1999;33:2315-28.

[7] Santoro D, Raisee M, Moghaddami M, Ducoste J, Sasges M, Liberti L, et al. Modeling hydroxyl radical distribution and trialkyl phosphates oxidation in UV− H2O2 photoreactors using computational fluid dynamics. Environ Sci Technol. 2010;44:6233-41.

Regards

Tipu Sultan