(247c) Photochemical Treatment of Herbicide/Pathogen Contaminated Agricultural Water in the Rio Grande Basin

The paper describes the photochemical treatment of chemical (herbicide) and biological (pathogens) contaminants in agricultural water. We investigated the photolysis of dissolved herbicide atrazine and photocatalytic sterilization of pathogen E-coli, common in the Rio Grande valley agricultural run-off water and rivers. Atrazine (C₈H₁₄ClN₅), a

widely-used herbicide used in the cane and corn fields, can cause prostate cancer, cardiovascular damage, muscle and adrenal degeneration, and congestion of heart, lungs, and kidneys. It also causes sexual deformities in endangered species (e.g., Leopard frogs) [1-3]. Without treatment, the atrazine-contaminated water is hazardous and detrimental to water sustainability in the Rio Grande Basin.

  Fig. 1 Photolysis of Atrazine in Water monitored with Agilent 1100 HPLC

Generally speaking, the photochemical treatment includes photolysis using UVC and photocatalysis using UVA [4-9].  The former directly breaks down chemical bonds or kills pathogens while the latter can use lower energy photon such as UVA or visible light over a photocatalyst to generate electron-hole pairs and free radicals.  Photocatalysis not only has the potential to use sunlight but also leads to a more complete mineralization. In our lab, atrazine rapidly decomposes from 40 ppb to below 3 ppb in 23 minutes under UVC (3.0 W) irradiation. The photolysis results in two major and two minor byproducts that can further decompose to CO₂ and water, Fig. 1.  The two known major byproducts (butyric acid and hydroxyatrazine) and a known minor byproduct (2-ethyl-2-hexenal) are relatively harmless compared to atrazine. The kinetic data of the photochemical decomposition of atrazine can be fitted very well with a first-order rate equation: dC/dt = - kC                                                 (1)

(Fig. 2, R²= 0.996). The first-order rate equation states that under a constant UVC irradiation, the atrazine decomposition rate is proportional to its concentration. At the same concentration, the atrazine decomposition rate is proportional to the UVC output squared (Fig. 3): k = 0.0129 P²                                                (2)

where P is the UVC output

Photochemical sterilization of E-coli in our lab showed one order of magnitude decrease (89% reduction) in Colony Forming Units (CFU) when a 4W UVA lamp (1.5 in away) was shined upon a vessel containing Degussa P-25 TiO₂ photocatalyst in 40 minutes.  The blank tests showed TiO₂ alone can reduce E. Coli CFU by 80% and UVA alone by 36% in 40 minutes, Figure 4. More experiments on the light intensity effect are needed.  

   Fig.2  Atrazine photolysis follows a first-order rate law       Fig.3  The first-order rate constant is proportional to the second power of the UVC light output

Fig. 4 Photochemical sterilization of E. Coli (UVA + TiO2) vs. blank tests     References

1. US EPA, Ambient Aquatic Life Water Quality Criteria for Atrazine-Revised Draft, EPA-822-R-03-03, 2003.  
2. V. Hequet, C. Gonzalez, P. L. Cloirec, Water Research (2001) 18, 4254-4260
3. Hayes, T., Haston, K., Tsui, M., Hoang, A., Haeffele C., and Vonk, A., Environmental Health Perspectives, V. 111 (4), Apr. 2003.
4. National Renewable Energy Laboratory (2001, October). Photochemical treatment of pollutants (CDS-SS25-B001). CO. www.nrel.gov/research/industrial_tech/pollution.html
5. Fujishima, A., Hashimoto, K. & Wuatanabe, T. (1999). TiO₂ photocatalysis fundamentals and applications (p. 126 ? 156). Tokyo: Bkc, Inc.
6. Ye, X., Chen, D.H., and Li, K. "Oxidation of PCE with a UV LED Photoreactor", Chemical Engineering & Technology, 28, No. 1, 95-97 (2005).
7. Devahasdin, S., C. F., K. Li, and D. H. Chen ?TiO₂ Photocatalytic Oxidation Of Nitric Oxide:Transient Behavior And Reaction Kinetics?, Journal of Photochemistry and Photobiology A; Chemistry, 156, 161-170 (2003).
8. Chen, D. H., C. Huang, and K. Li, "Photocatalytic Oxidation of Butyraldehyde over Titania in Air: Byproduct Identification and Reaction Pathways,? Chem. Eng. Comm., 190, 373-392 (2003).
9. Chen, D. H., K. Li, S.Y.C. Liu, C. Huang, and S. Esariyaumpai "TiO2 Photocatalytic Oxidation of Butyraldehyde and PCE in the Air through Concentric Reactors," J. Adv. Oxid. Technol, 5, 227-232 (2002).