Coagulation-flocculation of Microcystis aeruginosa by polymer-clay based composites

Abstract

Coagulation-flocculation is a critical treatment to reduce operational problems at Drinking Water Treatment Plants (DWTPs), created by episodes of cyanobacterial blooms in surface waters. There is a need for the search of good coaguflocculants that avoid release of intracellular toxins for a safer sustainable production. Clay-polymer interactions were examined for the development of efficient composites for their use as coaguflocculants for Microcystis aeruginosa (M. aeruginosa) suspensions. Polymers derived from poly-4-vinylpyridine (PVP) were quaternized on the pyridinic N by introducing methyl and hydroxyethyl moieties. These polymers were toxic per se; however, composites prepared either from their sorption or grafting on the clay surface of a montmorillonite exhibited null toxicity, The combination of infrared and X-ray diffraction data with thermal analysis showed a train conformation of the sorbed polymers on the clay surface whereas a brush conformation was elucidated for the grafted polymers. Only the composite prepared from grafting and subsequent quaternization with methyls was an efficient coaguflocculant, due to its high positive surface potential (+39 mV) which allowed a close contact between the quaternary moieties of the brushes and the negative cell wall. A patch flocculation mechanism was involved, with the riks of resuspension of the coagulated cells if the composite added in excess. The optimum ratio between the amount of coagulated cyanobacteria and this composite expressed as equivalent to polymer content was 7.2 × 107 cells/mg polymer. This ratio was determined in axenic cultures of M. aeruginosa, but was reduced 5-fold in natural surface waters due to natural organic matter (NOM) content and heterogeneity. This study has demonstrated the relevance of the type of modification of the clay mineral surface with polymers to obtain good coaguflocculants of cyanobacterial cells, that can be extended to other microorganisms.