Elsevier

Toxicon

Volume 45, Issue 6, May 2005, Pages 745-752
Toxicon

Degradation of microcystin-RR by UV radiation in the presence of hydrogen peroxide

https://doi.org/10.1016/j.toxicon.2005.01.012Get rights and content

Abstract

Experiments were conducted to investigate the degradation of microcystin-RR in order to assess the effectiveness and feasibility of the combined UV/H2O2 catalytic system for purification of water polluted by microcystins. The operating parameters such as hydrogen peroxide dosage, pH value, UV light intensity, initial concentration of microcystin-RR and reaction time were evaluated, respectively. The degradation efficiency increased nonlinearly with increasing UV light intensity and hydrogen peroxide dosage, respectively. There existed an optimal hydrogen peroxide dosage, beyond which the reagent exhibited an inhibitory effect, for degrading microcystin-RR. The degradation process could be fitted by both of the pseudo-first-order and second-order kinetics well and primarily followed a mechanism of both direct photolysis and hydroxyl radical oxidation. Compared with the treatment using UV radiation and hydrogen peroxide individually, the combined UV/H2O2 system could significantly enhance the degradation efficiency due to the synergetic effect between UV radiation and hydrogen peroxide oxidation. The observed rate constants decreased and the corresponding half-lives prolonged as the concentrations of microcystin-RR increased. The combined UV/H2O2 process provides an effective technology for the removal of microcystins from drinking water supplies.

Introduction

The increase of population and the consequent intensification of agricultural and industrial activities have led to the enhancement of eutrophication in superficial freshwater bodies, and then it has led to cyanobacteria blooms more frequent worldwide. Microcystins are a family of hepatotoxic peptides mainly produced by freshwater cyanobacteria such as Microcystis, Oscillatoria, Nostoc, Aphanizomenon and Anabaena (Carmichael, 1992, Dawson, 1998). Their occurrence in drinking water is of concern since chronic exposure to these toxins causes tumor promotion (Helasmo and Meriluoto, 1998, Ito et al., 2002). There have many reports of toxication of plant, fish and other animals and of even humans by microcystins (Jochimsen et al., 1998, Azevedo et al., 2002, Magalhães et al., 2003, Jacquet et al., 2004). Over 70 different analogues of microcystins, one of which is microcystin-RR (MC-RR, C49H75N13O12, mw 1038.2), have been isolated from natural blooms or laboratory cultures of cyanobacteria (Codd, 2000, Haider et al., 2003). Many of the water bodies where cyanobacterial blooms form are used for drinking water supplies. However, microcystins are known to be chemically stable compounds, possibly as a result of their cyclic structure (Lahti et al., 1997). It is, therefore, not surprising that conventional drinking water treatments have only limited efficacy in removing dissolved microcystins (Himberg et al., 1989, Takenaka and Tanaka, 1995, Vasconcelos and Pereira, 2001, Svrcek and Smith, 2004). Microcystins can be degraded by natural sunlight when pigments present as photosensitizer (Tsuji et al., 1994, Welker and Steinberg, 1999). Chlorination and ozonation are effective for the destruction of dissolved microcystins (Nicholson et al., 1994, Tsuji et al., 1997, Rositano et al., 1998, Newcombe and Nicholson, 2004), but cost may prove to be prohibitive particularly since contamination with microcystins is typically seasonal and unpredictable (Lawton and Robertson, 1999). Photocatalytic systems such as UV/TiO2 and UV/H2O2/TiO2 do appear to be extremely effective for the removal of microcystins from drinking water (Robertson et al., 1997, Robertson et al., 1998, Shephard et al., 1998, Lawton et al., 1999, Feitz et al., 1999, Cornish et al., 2000, Shephard et al., 2002, Lawton et al., 2003).

In comparison with other AOPs, such as Fenton, ozone, UV/O3, UV/TiO2, etc., the combined UV/H2O2 process can be more practical due to involving single-step dissociation of hydrogen peroxide to form two hydroxyl radicals (OH) as follows:H2O2hv2O·H

The hydroxyl radicals generated can nonselectively oxidize a wide variety of refractory organic pollutants in water. During the last decades, some investigators have reported the successful applications of combined UV/H2O2 system for dyes and pharmaceutical intermediates or toxic chemical treatment (Glaze, 1995, Galindo and Kalt, 1998, Kurbus et al., 2003, Lopez et al., 2003, Shu et al., 2004). Under the appropriate conditions, the organic pollutants can be completely mineralized into H2O, CO2 and other inorganics without introducing any secondary pollution. This system shows some advantages such as no phase transfer problems, no sludge formation, simplicity of operation and lower investment costs, and then it is often adapted for purification of micro-polluted water (Colonna et al., 1999).

The type of microcystins produced by cyanobacteria mainly depends on the temperature (Rapala and Sivonen, 1998). MC-RR is far more common in water supplies in Asia due to the higher temperature (Yan et al., 2004) and it would make more sense to use MC-RR for the present study. Consequently, the degradation of selected MC-RR was investigated in detail using UV/H2O2 oxidation system.

Section snippets

Materials

HPLC-grade MC-RR standard was purchased from ALEXIS Biochemicals (San Diego, USA) and methanol from Concord (Tianjin, China). H2O2 (30%, w/w), Na2SO3, NaOH and H2SO4, purchased from Shanghai Chemical Reagent Co. (Shanghai, China), were of analytical grade and used without further purification. Microcystis aeruginosa (FACHB-7820) was purchased from Institute of Hydrobiology, Chinese Academy of Sciences (Wuhan, China). Deionized-distilled water was obtained using Milli-Q-Water system (Millipore,

Effect of reaction time

The degradation of MC-RR at different reaction times was shown in Fig. 2. It could be observed that the reaction was fast at the initial stage and 84% MC-RR could be removed within 30 min and 94.83% MC-RR removal occurred within 60 min. Extended reaction time had no notable effect on the further degradation of MC-RR. This could be attributed to the relative high concentrations of MC-RR and oxidants resulted in rapid reaction rate in the batch reactor at the beginning. As the oxidation reactions

Conclusions

The following conclusions can be drawn from the results of the experiments.

MC-RR degradation efficiency could reach 94.83% under the optimal conditions of MC-RR 0.72 mg/L, H2O2 dosage 1.0 mmol/L, reaction temperature 25.5±1 °C, pH 6.8, UV light intensity 3.66 mW/cm2 and reaction time 60 min. Increasing H2O2 dosage did not always lead to an improvement of the degradation efficiency of MC-RR by combined UV/H2O2 system. The initial MC-RR concentration had an important effect on the performance of

Acknowledgements

This study was supported by China Education Ministry project titled ‘Study for advanced treatment processes of wastewater and micro-polluted water’ and by the National Key Project for Basic Research on the Process of Lake Eutrophication and the Mechanism of Cyanobacterial Blooming (No. 2002CB412301).

References (43)

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