Methodological Studies On The Degradation And Elimination Of Microcystins

Microcystins (MCs) are a group of monocyclic heptapeptide toxins produced mainly by some species of cyanobacteria, including Microcystis, Anabaena, Oscillatoria, Anabaenopsis, Hapalosipon, Nostoc and Planktothrix, which frequently occur in fresh water body, such as lake, river and reservior. MCs are intracellular toxins, and they release into water body after the disruption of algal cells. MCs usually show potent hepatotoxicity and tumor-promoting activity in animal and humam being. The World Health Organization (WHO) recommended the drinking water provisional guideline-value of 1μg/L of microcystin-LR for people daily exposure. Recently, many researchers have found that MCs can be detected in many water bodies and even the source of drinking water. Therefore it is vital to remove MCs from drinking water or the source of drinking water bodies due to the toxicity and threat of MCs to human beings. In recent years, there have been a lot of reports regarding MCs degradation, however, few studies on the MCs elimination by the water supply and purification plant in their actual water purification according to local conditions are available. In the present study, chlorination, coagulation and their combination using are introduced to remove MC from water based on the technological parameters of a local waterworks in Xinxiang, China to find the best way of MCs elimination. In addition, nano-titanium dioxide (TiO2) was also used to eliminate MCs in water by photocatalytic degradation in this test.Single species cultures of Microcystis aeruginosa were isolated from a reservoir located in Henan Province, China, and algal cell extracts were isolated according to the method of boiling-water bath extraction, and MCs used in the experiments were obtained by methanol extraction and ODS-cartridge (C18) purification. The contents of MCs in the solutions of MC degradation /elimination was determined by ELISA. The main results of this study are showed as following:(1) The results of MC- degradation by sodium hypochlorite show that pH plays an important role in the reaction, and lower pH of the reaction solution promotes the oxidation process. The reason for this result may be that lower pH enhances the activity of HClO.(2) We suggest that the best concentration of available chlorine in the reaction solution of chlorination should be 3.6 mg/L and the chlorination time of sodium hypochlorite be controlled within 30 minutes acoording to the result of this study. As we know, the higher the concentration of MC is while the lower the MC degradation rate will be, under certain concentration of available chlorine in the solution.(3) The pH value of the MC-elimation solution is an important factor controlling the eliminating rate of MC by coagulation, and the results of this study indicate that MC elimination rate is higher under lower pH value. Meanwhile we also find that the MC-eliminating ablilty of FeSO4·7H2O is limited under this condition.(4) The results of combination using of coagulation and chlorination in MC-elimination tests reveal that the concentration of available chlorine in the solution is a key factor of removing MC. With the increase of available chlorine dosage in the solution, MC-elimination rate goes up. In combination using of pre-chlorination + coagulation + post-chlorination to remove MC, we find that pre-chlorination plays an important role in MC eliminating when the total dosage of available chlorine in the treatment-solution is set up. When the pre-chlorination dosage of available chlorine is 1.8mg/L, MC-elimination rate reaches to 95.82%.(5) In the tests of MC-removing by nano-titanium dioxide (TiO₂), the results show that, TiO₂ can adsorbs more MC under the acidity condition (pH=6.24) than neutral and /or alkalinous condition of reaction solution.In conclusion, these results of the present studies indicate that chlorination and coagulation can effectively remove MC in water under lab conditions. But photocatalytic degradation of MC by TiO₂ decrease in value of MC-elimination in the water plant because the reaction is too dependent on the pH value of treatment solution.