Restauração de água eutrofizadas do semiárido brasileiro através da técnica Flock & Lock

Abstract

The Flock & Lock technique was indicated as one of the best alternatives for the recovery of eutrophicated lakes and reservoirs. This is a series of applications of flocculants (Flock) and clays (Lock) to flocculate / sediment and inactive, respectively, the phosphorus of the water column. The present study evaluated the efficiency of aluminum sulphate (SUL), alkali polychloride (PAC) and chitosan (QUI) flocculants, isolated and combined with natutal (BEN) and lanthanum modified bentonite (BML) eutrophic dust from the Brazilian semiarid, as well as the beneficial effects on cyanobacteria density and the intra and extracellular forms of cyanotoxins. Bench scale tests using water samples from the Argemiro Figueiredo reservoir, located in the municipality of Itatuba, Paraíba, Brazil. These tests were conducted in glass beakers containing 200 mL of water from the reservoir, samples being collected at the top and bottom of the beakers. Cyanobacteria density was determined by inverted microscope and the intra and extracellular concentrations of the cyanotoxins microcystin, saxitoxin and cylindrospermopsin by the Enzyme-Linked Immuno Sorbent Assay (ELISA) method. Analysis of one-way variance (ANOVA one-way) was performed to verify significant differences between treatments. It was observed that the SUL and PAC flocculants efficiently reduced the PT, PSR, turbidity, chlorophyll-a, cyanobacteria density and intracellular microcystin efficiency, whereas the chitosan showed low removal power of the evaluated parameters. The BML showed to be more efficient than the BEN, however, the application of these clays combined with the flocculants was not strictly necessary in the success of the Flock & Lock technique, since the SUL and PAC alone proved to be sufficient in the significant removal of the parameters studied. The efficiency of flocculants in the removal of cyanobacteria cells varied between species. Small-sized species such as A. delicatissima, M. glauca and M. tenuissima were effectively removed regardless of treatment, including those with QUI and BEN, while filamentous cyanobacteria C. raciborskii, G. amphibium, P. agardhii and P. Catenata were removed only with the application of PAC, SUL and BML isolated and combined. The intracellular concentrations of saxitoxin and cylindrospermopsin and the extracellular fraction of these cyanotoxins and microcystin were not influenced by the application of flocculants and clays. These results show that cyanotoxin types differ in the response to these compounds and that the successful application of the Flock & Lock technique in the recovery of eutrophic lakes and reservoirs can be compromised in bodies of water with high concentration of dissolved toxins.