Study On The Removal Techniques Of Microcystins From Micro-polluted Water

The increase of population and the consequent intensification of agricultural andindustrial activities have led to the enhancement of eutrophication in superficialfreshwater bodies, and then it has led to cyanobacteria blooms more frequentworldwide. Microcystins are mainly produced by freshwater cyanobacteria such asMicrocystis, Oscillatoria, Nostoc, Aphanizomenon and Anabaena. Microcystinsdisplay hepatotoxic behaviour and cause tumor promotion. Furthermore, theirchemical characteristic is so stable that the conventional water treatment processeshave only limited efficiency in removing soluble microcystins. The advancedoxidation processes (AOPs), which involve the production of reactive oxidativespecies, especially the hydroxyl radicals (·OH), are capable of mineralizing organiccontaminants into H2O, CO2 and other inorganics. AOPs provide promising treatmentoptions for microcystins-containing water.To meet the need of further research, microcystins were extracted firstly from theMicrocystis aeruginosa cultured in laboratory. The accumulation, separation andpurification procedures of microcystins were optimized, and then the solid phaseextraction and high performance liquid chromatography (SPE-HPLC) analyticalmethod was developed for analysis of microcystins. Experiments were carried out toinvestigate the degradation behaviours of microcystins using some AOPs includingthe homogeneous and heterogeneous oxidative techniques and other practicaltechniques. The main conclusions are as follows:The UV/H2O2 photooxidation was effective in removing microcystins in water,and UV photolysis and ·OH were responsible for the degradation of microcystins. Themajor destruction pathway of microcystin included the isomerization of microcystin,electrophilic addition of ·OH on the conjugated diene structure of ADDA moiety andpeptide bond, cleavage of dihyroxylated ADDA and the peptide bond, and furtheroxidation of the intermediates. The degradation process accorded approximativelywith the pseudo-first-order kinetics. Considering the contributions of individual UVphotolysis, H2O2 and ·OH oxidation in the combined homogeneous system, asimplified kinetic model for the degradation of microcystins was developed using thepseudo-first-order equation and steady-state approximation. This kinetic modelprovided better understanding for the effects of H₂O₂,HO₂ ,CO₃ and HCO₃ on thedegradation of microcystins. The control experiments showed that H2O2 was difficultto oxidize microcystins while UV direct photolysis could decompose microcystinspartially. However, the UV/H2O2 system could significantly enhance the degradationefficiency due to the synergetic effect between UV radiation and H2O2 oxidation.The ·OH generated from the decomposition of H2O2 under the UV irradiation wasresponsible for the synergetic effects. The variation of pH in the process indicated theformation of organic acids, which showed that microcystins were not mineralized toH2O, CO2 and other inorganics directly but converted to some intermediates firstly,then partial mineralization was achieved. The degradation efficiencies of microcystinsdepended on the following factors: A lower initial microcystin concentration led to afaster and more efficient degradation; The neutral and weak alkaline solutions weremore appropriate to carry out degradation reaction; In a lower H2O2 concentrationrange, the degradation of microcystins improved significantly with increasing H2O2concentration. Under the experimental conditions, the relationships between theobserved rate constants and H2O2 concentrations could be described as follows:kobs = 0.0378CH2O20.2115(MC-LR), kobs = 0.0512CH2O20.1467 (MC-RR). On the other hand, thedegradation of microcystins retarded obviously while H2O2 increased to a largerconcentration. It was attributed to the ions and radicals could scavenge ·OH in thesolution; Increasing UV light intensity could enhance but was not directlyproportional to the degradation rate of microcystins. Under the optimal conditions, thedegradation efficiencies of MC-LR and MC-RR could reach 80.8% and 94.8%,respectively. Fenton reagent oxidation was also effective in removing microcystins. In all cases,most of microcystins degradation occurred during the initial 10 min of reaction.Microcystins transformation was characterized by pseudo-first order kinetics duringthis brief initial phase, while the subsequent phase exhibited a sharp drop indegradation rate. This behavior suggested a mechanism where Fenton reagentproduced an initial surge of ·OH that resembling a "pulse"injection as opposed to the"continuous"injection observed in the UV/H2O2 system. There existed an optimalFe2+ dosage, and a higher addition resulted in brown turbidity that hindered theabsorption of the UV light required for photolysis and caused the recombinationof ·OH. In this case, Fe2+ reacted with ·OH as a scavenger. After reacting for 30 min,the degradation efficiencies of MC-LR and MC-RR could up to 92.4% and 95.8%under the experimental conditions, respectively. In the presence of UV radiation, theoxidation potential of Fenton and Fenton-like reagent could be enhanced vigorouslyand followed the sequence of UV/Fe2+/H2O2 ≈UV/Fe3+/H2O2 > Fe2+/H2O2 >Fe3+/H2O2. The effect of UV light attributed to the direct ·OH formation andregeneration of Fe2+ from the photolysis of the Fe(III) complex in solution. The effectof ferric chloride flocculation on soluble microcystins was negligible. However, itcould remove most organic materials that would be favourable for the degradation ofmicrocystins by Fenton and Fenton-like reagent oxidation. The experiments performed in the heterogeneous photocatalytic processesexhibited best degradation results of microcystins and followed the order:UV/TiO2-Fe3+/H2O2>UV/TiO2-Fe3+>UV/TiO2. The doping of Fe3+ greatly improvedthe photocatalytic reactivity of TiO2, and H2O2 appeared to significantly enhance thedegradation of microcystins due to the higher generation rate of ·OH. The experimentsresults showed that the degradation of microcystins on TiO2-Fe3+ was in accordancewith Langmiur-Hinshelwood (L-H) kinetic model well. Under the experimentalconditions, the apparent reaction rate constants of MC-LR and MC-RR were 0.2435and 0.4102 mg·L?1·min?1, and the corresponding Langmiur adsorption equilibriumconstants KA were 2.6301 and 0.7606 mg?1·L, respectively. The degradationefficiencies of microcystins mainly depended on the TiO2-Fe3+ dosage and UV lightintensity. Increasing the addition amount of TiO2-Fe3+ would increase both the surfacearea of catalyst available for adsorption and the Fe(III) complex. However, there...