Microcystins (MCs) are toxic secondary metabolites produced by cyanobacteria, often present in eutrophic waters. Since the need of drinking exploitation of eutrophic lakes and reservoirs is increasing worldwide, effective and economic methods to remove cyanobacteria and MCs in the potable treatment of eutrophic water are required. Since traditional treatments present significant drawbacks in this process, the membrane pressure-driven filtration is emerging as a possible option; furthermore, being its cost mainly related to pore size, the use of low density membranes, and specifically Ultrafiltration (UF), seems the most appropriate solution. UF can effectively remove cyanobacteria biomass by size exclusion, but a certain capacity to remove significant amounts of microcystins, despite the fact that MC molecules are smaller than UF membrane pores, was also observed, This is due to the mechanisms governing the ratios of MC presence in intracellular and dissolved forms, or to possible adsorption of MCs by inorganic colloids or natural organic matter contained in the water, or to adsorption on the membrane itself. The ratio between intracellular and dissolved IvICs is mainly considered related to the release of MCs by cyanobacterial cells under stress conditions, or during their senescent stages. Nevertheless, on-field statistical studies assume the possibility of further unknown mechanisms, since the release mechanisms alone seem not clearly related to any ecological or climatic conditions. While the highest cyanobacteria proliferation happens in highly eutrophic water bodies, the highest concentration of dissolved MCs seems to happen mainly in oligotrophic or mesotrophic waters. In this study, we present the results of several laboratory and pilot plant experiments, aimed to deepen the knowledge of the mechanisms governing the migration of Microcystin-LR (MC- LR) from dissolved to particulate-bound phase and vice versa, and their effect over the UF removal of microcystins. Firstly, we analysed the presence of MC-LR, in filtered water after 3 months of pure culture of Microcystfs Aeruginosa and different cycles of freezing, showing an increase of dissolved MC- LR due to cell break deriving from the refrigeration cycles. Secondly, we performed tests of filtration in a pilot ultrafiltration plant equipped with a polysulfone membrane, using samples of demineralised water and raw water from an eutrophic lake (Massaciuccoli lake, Tuscany, Italy), both spiked with MC-LR. The lake water was characterized for presence of inorganic and organic compounds, as well as colonies of microorganisms, including total cyanobacteria. The UF filtration tests resulted in significantly different residuals of MC-LR. Specifically, the raw lake water spiked with MC-LR resulted in a higher MC-LR removal efficiency. The results of the experiments allowed to discuss the role of the absorption-release process over the efficiency of the ultrafiltration process, and the role of MC-LR release by cyanobacteria cells under stress conditions so as to allow the exploration of non-conventional operation rules aimed to maximize the removal of Iv1Cs by UF.
Effects of intracellular/dissolved ratios of microcystin-LR onto its ultrafiltration removal
IANNELLI, RENATO;CARDUCCI, ANNALAURA;CECCARINI, ALESSIO;FUOCO, ROGER
2009-01-01
Abstract
Microcystins (MCs) are toxic secondary metabolites produced by cyanobacteria, often present in eutrophic waters. Since the need of drinking exploitation of eutrophic lakes and reservoirs is increasing worldwide, effective and economic methods to remove cyanobacteria and MCs in the potable treatment of eutrophic water are required. Since traditional treatments present significant drawbacks in this process, the membrane pressure-driven filtration is emerging as a possible option; furthermore, being its cost mainly related to pore size, the use of low density membranes, and specifically Ultrafiltration (UF), seems the most appropriate solution. UF can effectively remove cyanobacteria biomass by size exclusion, but a certain capacity to remove significant amounts of microcystins, despite the fact that MC molecules are smaller than UF membrane pores, was also observed, This is due to the mechanisms governing the ratios of MC presence in intracellular and dissolved forms, or to possible adsorption of MCs by inorganic colloids or natural organic matter contained in the water, or to adsorption on the membrane itself. The ratio between intracellular and dissolved IvICs is mainly considered related to the release of MCs by cyanobacterial cells under stress conditions, or during their senescent stages. Nevertheless, on-field statistical studies assume the possibility of further unknown mechanisms, since the release mechanisms alone seem not clearly related to any ecological or climatic conditions. While the highest cyanobacteria proliferation happens in highly eutrophic water bodies, the highest concentration of dissolved MCs seems to happen mainly in oligotrophic or mesotrophic waters. In this study, we present the results of several laboratory and pilot plant experiments, aimed to deepen the knowledge of the mechanisms governing the migration of Microcystin-LR (MC- LR) from dissolved to particulate-bound phase and vice versa, and their effect over the UF removal of microcystins. Firstly, we analysed the presence of MC-LR, in filtered water after 3 months of pure culture of Microcystfs Aeruginosa and different cycles of freezing, showing an increase of dissolved MC- LR due to cell break deriving from the refrigeration cycles. Secondly, we performed tests of filtration in a pilot ultrafiltration plant equipped with a polysulfone membrane, using samples of demineralised water and raw water from an eutrophic lake (Massaciuccoli lake, Tuscany, Italy), both spiked with MC-LR. The lake water was characterized for presence of inorganic and organic compounds, as well as colonies of microorganisms, including total cyanobacteria. The UF filtration tests resulted in significantly different residuals of MC-LR. Specifically, the raw lake water spiked with MC-LR resulted in a higher MC-LR removal efficiency. The results of the experiments allowed to discuss the role of the absorption-release process over the efficiency of the ultrafiltration process, and the role of MC-LR release by cyanobacteria cells under stress conditions so as to allow the exploration of non-conventional operation rules aimed to maximize the removal of Iv1Cs by UF.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
