Study On The Content Of Microcystins In The Water Source Of Shanghai And The Optimal Removal Methods

In recent years, the occurrence of heavy cyanobacteria blooms in eutrophic fresh water has been a worldwide problem. One of the major problems caused by cyanobacteria is the producing and releasing of microcystins (MCs), in which microcystin-LR and microcystin-RR are the two most common species. Pre-oxidation. powdered activated carbon (PAC) adsorption, coagulation and enhanced coagulation and disinfection process were studied in this paper to explore the removal characteristics and mechanisms of the two microcystins. in addition, removal effects and influential factors of solar catalyzed oxidation process to microcystins and algae were studied in the meantime.The quantity of algae and the content of microcystin in the Shanghai water sources are relatively low: the quantity of algae are between 60,000 and 10,000,000 per liter, the concentration of total MC-LR and MC-RR are 0-250ng/L and 0-650ng/L, respectively, in Huangpu river; as to Yangtz River source, the value are even lower than that.Both pre-chlorination and potassium permanganate pre-oxidation can improve the removal effect to algae by coagulation to some extent. However, they have some discrimination in the extra-cellar microcystins' removal. Pre-chlorination can't reduce the concentration of extra-cellar microcystins but increase it to some certain extent. Potassium permanganate can't increase the concentration of extra-cellar microcystins, for it can oxidize microcystins prior.Powdered activated carbon can effectively remove microcystins from natural waters, the removal rate can reach 45% and 55% for MC-LR and MC-RR, respectively, at PAC dosage of 10mg/L. The removal percentage of microcystins at any contact time and at any PAC dose were observed to be independent of the initial microcystins concentration when the initial concentrations were below some given concentration. Equation derived from pore surface diffusion model (PSDM), ideal adsorbed solution theory (IAST) and equivalent background compounds (EBC) validated this result; Chlorine can enhance the removal effect of PAC to microcystins(about 20%), which was contrary to the former results in literatures; the reason may be lied in that the OH radicals produced through the way of reaction between PAC and chlorine.To high-algae raw water, adjusting the pH to 6.0 and adding 10mg/L PAC can remove algae effectively and keep the cellar intact, it can also resolve the problem of "algae-odour" in high algae water. Enhanced coagulation can improve the removal effect of extra-cellar microcystins obviously, of which removal rate was 60～70% and the removal mechanism mainly lies in that the carboxyl in microcystin molecular structure can form some compounds with metal hydrate at the condition of enhanced coagulation and these compounds can be easily removed in the process of coagulation. Enhanced coagulation can't enhance the removal of some trace SOCs such as atrazine and BPA, which doesn't has carboxyl group.Free chlorine can remove two kinds of microcystins to some extent, and the degradation process in aqueous solution followed second-order kinetics, which related closely with reaction time and free chlorine concentration. CT value was at least 101.88 mg L^-1 min at pH 7.25 to reduce 10μg/L microcystins to the national standard of 1μg/L, which is higher than normal CT value for disinfection; Acidic pH condition favored the degradation of the two kinds of microcystins, for the reaction betw(?) HOCl mocecular and microcystins are the main pathways of all reactions. Enhanced coagulation combined with chlorine and ammonia disinfection significantly decreased the production of DBPs, and ensured a good disinfection effect. The process could effectively remove alge and microcystins in water.Sunlight-indued catalytic oxidation has a good removal effect both on MC-RR and MC-LR. The degradation process of two substances followed pseudo-first order kinetics well. Humic acids had both improving and retardant effects on the degradation process. Microcystins removal was improved in sunlight/HA system, whereas it was retarded in sunlight/HA/TiO₂ system. Because HA adsorbed partial sunlight, which decrease the light intensity used by TiO₂ and OH radicals thereafter. And HA could react with OH·also. Sunlight-induced catalytic oxidation could inactivate algae in water efficiently, and control the production of algae in water resources. Furthermore, it could oxidize EDCs substances such as ametryn, BPA and atrazine. The removal efficiency is unrelated with the structure of organic substance, which suggest the OH·reaction is the main degradation pathway.