Research On The Photocatalytic Degradation Of Nodularin By Using UV And Visible Light

Nodularin（NOD） is the cyclic pentapeptide cyanobacterial hepatotoxin produced by Nodularia spumigena. The main target organ of NOD is liver, meanwhile, it has strong genotoxicity, embryotoxicity and genetoxic for human and animals. NOD can be enriched and transformed in vivo and environment, prolonged contact with it may increase the risk of cancer. NOD has stable chemical properties; traditional sewage treatment methods can’t remove it effectively, so it is urgent to find a safe and efficient NOD removal method. As a new sewage treatment technology, photodegradation has played an important role in the removal of environmental pollutants, the advantages of low cost, high treatment efficiency, easy construction of equipment, no pollution, etc. that the method have overcomed the defects of existing methods for the removal of algae toxins. Generally speaking, photodegradation technology has an excellent application prospect.In order to determine the effect of photodegradation, it it critical to determine the concentration of NOD in water. A method using solid phase extraction（SPE） combined with high performance liquid chromatography（HPLC） to determinate NOD in water. Pre washed C18 column with 5 m L methanol, then activated C18 column with 5 m L water, samples without adding catalyst direct enrichment while samples that added catalyst centrifuged 10 min under 4 ℃, 8000r/min. The supernatants were filtrated with 0.45 μm filter and then enriched with C18 column. Then, using 5 m L 10% methanol leaching C18 column twice, made the column liquid completely drained. Finally, eluted C18 column with 5 m L 90% methanol（containing 0.1% TFA） and collected the eluent. Evaporated the eluent nearly to dryness at 40℃ with nitrogen, then the residues dissolved with methanol to 0.5 m L and filtrated with 0.45 μm filter. HPLC was used to determine the concentration of NOD, the conditions were as follows: sample size 20 μL, mobile phases were methanol and water（containing 0.1% TFA）（58:42 V/V）, flow rate 1.0m L/min, detection temperature 30℃, UV detection wavelength 238 nm. The results showed that this method can effectively determine NOD content in water, NOD detection limit was 0.013mg/L. The concentration of NOD showed good linearity with the peak area（R2=0.9994） in the concentration range of 0.05-5 mg/L in water. The average recoveries of different concentrations of NOD were above 90% and the precision were high（1.30%-4.7%）.Purified NOD and self-made light reaction device was used to study the removal efficiency of nodularin under different light condition including dark, visible light, dark with TiO₂ as catalyst, visible light with TiO₂ and ultraviolet light（UVA, UVB,and UVC）. In the subsequent experiments, UVC was chosen as the best light source to test the influence of NOD initial concentration, temperature, pH and light intensity on the removal efficiency of NOD and the reaction kinetics of photolysis was developed. The results showed that the dark reaction, visible light, visible light with TiO₂,UVA and UVB had no obvious removal effect of NOD and the highest removal rate was about 20%. UVC could remove NOD rapidly and the process could be well described by the second order kinetics. In the UVC treatment experiments, pH had no significant effect on NOD photolysis, finally removal rate were all above 90%, the removal rate of NOD increased slowly with the increase of temperature, but the difference between groups was no significant. Meanwhile, the removal rate decreased with the increase of NOD initial concentration, and increased rapidly with the increase of light intensity but remained at a stable condition when reached a certain threshold. The effect sequences of three main factors on NOD removing were as follows: light intensity > time > temperature. After treated for 4h, 99.60% of NOD with initial concentration of 0.1 μg/m L could be removed and the remaining content complied with the algal toxin limit of WHO and China’s rules when treated with light intensity 318μW/cm2, pH 7 and temperature 30 ℃.Nitrogen-droped TiO₂（N-TiO₂） with visible light activity were prepared by sol-gel method and characterized by UV-VIS spectrophotometric analyzer, X-ray diffraction（XRD） and X-ray photoelectron energy spectrometer（XPS）, then the removal effect of NOD under visible light irradiation was verified. The effects of nitrogen doping content, NOD initial concentration, catalyst dosage, pH and light intensity on the photocatalytic degradation of NOD were discussed and the optimum conditions were determined. The results showed that nanometer N-TiO₂ prepared is anatase, the average particle size was 14.8nm and N element replaced the part oxygen atom of TiO₂ lattice formed O-Ti-N key that made it has obvious absorption in visible light range and it did not reduce the absorption for ultraviolet region at the same time. Compared the removal results of NOD under different light source, TiO₂ and N-TiO₂ had certain adsorption to NOD, the adsorption rate of TiO₂ was 15% while N-TiO₂ was above 20% after 18 h dark reaction. N-TiO₂ with visible photocatalysis can effectively remove NOD in water with increasing removal rate according to the increase of nitrogen doping content while the growth slowed of nitrogen content（[N]/[Ti]） is 4 after 12 h. The optimum dosage of catalyst was 0.75g/L under the same reaction time.Meanwhile, a higher NOD initial concentration resulted in a lower removal rate of NOD and the removal process was in accordance with the first order reaction kinetics. pH and light intensity have a significant impact on the removal rate of NOD. The higher the pH was, the slower the reaction rate get. The degradation level was positively correlated with the light intensity but remained at a stable condition when reached a certain threshold. The effect sequences of four main factors on NOD removing were as follows: time >light intensity > catalyst dosage > nitrogen doping content. After treated for 16 h, 99.80% of NOD with initial concentration of 0.1 μg/m L could be removed and the remaining content complied with the algal toxin limit of WHO and China’s rules when treated with light intensity5064 lux, catalyst dosage 0.75g/L and nitrogen doping content（[N]/[Ti]=3）.Ultraviolet and visible light photocatalytic can effectively remove NOD in water. The two methods have their own advantages and disadvantages, UV is highly effective but requires human intervention, visible light is cheap and easy to get but the reaction efficiency is relatively low. The future research focus shoule on improving the efficiency of photodegration, reducing the energy consumption, developing large reaction equipment and exploring the effect in practical application.