Preparation Of Flocculation / Antibacterial Bifunctional Starch Modified Flocculant And Its Water Treatment Performance

Inorganic suspended colloidal particles and pathogenic microorganisms are frequently the main contaminants in raw water. Thus, the major tasks in drinking water treatment are efficient turbidity removal and effective sterilization. Traditional technological processes for the production of potable water include coagulation/flocculation, sedimentation, sand filtration, and disinfection. Among them, flocculation is usually the first step in water purification, and the final flocculation efficiency clearly depends on the selected flocculant. Accordingly, the development of novel flocculants with the characteristics of high efficiency, low cost, and environmental friendliness is an important field of research. Moreover, it is a significant work to prepare multi-function flocculants, i.e., the efficient removal of turbidity as well as the effective inhibition of bacterial growth, in water, which could result in considerable cost reduction and notable diminution in the secondary-pollution risk of disinfection byproducts. However, traditional flocculants, such as those that are inorganic-metal-based as well as synthetic organic polymeric flocculants, have no evident sterilization activities. Moreover, they themselves carry many health risks, due to residual metal ions or the release of noxious polymeric monomers into the target water. Recently, natural-polymer-based flocculants have been the focus of intense interest, due to their widespread availability, environmental friendliness, and biodegradability. Among them, starch is an abundant natural resource and contains large numbers of hydroxyl groups on its saccharide ring, which can be easily modified chemically to satisfy various applications by the introduction of different functional groups onto its backbone. Over the years, many kinds of starch-based flocculants have been developed and applied in water treatment. They are inexpensive, biodegradable, environmentally friendly, shear stable, and highly efficient at low doses. However, the antibacterial properties of starch and its derivatives have hardly been studied.In this work, three kinds of novel starch-based flocculant were successfully prepared by introducing various functional groups,(1) 3-Chloro-2-hydroxypropyl trimethyl ammonium chloride modified starch, devoted as St-CTA, (2) starch-graft-poly[(2-methacryloyloxyethyl) trimethyl ammonium chloride, devoted as St-g-PDMC), and (3) carboxymethyl-starch-graft-aminomethylated-polyacrylamide, devoted as CMS-g-APAM. Fourier transform infrared spectroscopy (FTIR),’H nuclear magnetic resonance (NMR), and zeta-potential measurements were used to characterize the structure and solution properties of them. Their flocculation and antibacterial properties were systematically tested at different doses and under different pH conditions using a kaolin suspension and a suspension of Escherichia coli as well as a mixture thereof as synthetic effluents. Besides, the effects of structural factors such as substitution degree of CTA in St-CTA and grafting ratio of St-g-PDMC on their flocculation performance have been also tried. The experimental results have been drawn:(1) All the three kinds of starch-based flocculants exhibited effective turbidity removal properties via charge attraction. The charge properties of the flocculants vs. those of the kaolin and the E. coli suspensions mainly accounted for the final flocculation performance at the tested pH values. CMS-g-APAM achieved efficient removal of turbidity under acidic as well as neutral conditions while St-CTA and St-g-PDMC were effective at all measured pH conditions. The flocculation of the mixtures of kaolin and E. coli suspensions mainly depends on the organic components which is quite similar to but higher than that of solo E. coli suspensions due to the synergistic effect. Insights into the flocculation mechanisms were obtained by measurement of the zeta potential and floc property analyses for the flocculation of three kinds of simulated water samples. In general, simple charge neutralization plays an important role at acidic conditions, but patching becomes more dominant with the increase of the pH value.(2) All the three kinds of starch-based flocculants exhibited effective sterilization properties. From the direct SEM observation, the cell wall of bacteria turned anamorphic and damaged after treated with the starch-based flocculant. As for their 3D EEM spectra, the fluorescence signals of intracellular aromatic proteins became stronger with the addition of increasing amounts of flocculant, indicating the disruption of E. coli cells.Thus, taken together the above-described flocculation studies, the 3D EEM spectra and SEM analysis, all the three kinds of starch-based flocculants exhibits both efficient flocculation and partial bacterial sterilization. The dual effect can be explained by the introduction of the cationic functional groups onto the starch backbone, i.e.,the quaternary ammonium salt groups of St-CTA and St-g-PDMC as well as the tertiary amine group of CMS-g-APAM, which could not only attract kaolin particles and E. coli cells via electrostatic interactions, but also effectively break the cell wall of the bacterium. It is expectable that the well-prepared dual-function starch-based flocculants have a wide range of potential applications in water treatment.