Removal Of Bacteria From Water By Modified Starch-based N-free Flocculants

Inorganic suspended solid particles,organic colloid and microorganism are common components of drinking water.Turbidity and color removal,inhibition/sterilization are the main technical units of drinking water production process.Drinking water production process is based on coagulation,precipitation,filtration and disinfection.Among them,flocculating technology is usually used for the removal of solid particles and organic colloids,while disinfection is mainly used for sterilization,so as to obtain clean and safe water.For example,quaternary ammonium salt cationic polymer is a kind of polymer material widely used in drinking water production process to flocculate to remove negatively charged particles and dissolved organic matter?DOM?.But,their use in disinfection units following the production process may produce disinfection byproducts with carcinogenic risk.If a multi-functional flocculant with both turbidity and color removal and inhibition/sterilization performance can be developed,it can not only effectively reduce the dosage of flocculant and disinfectant,reduce the cost,but also reduce the production of disinfection byproducts.Therefore,it is necessary to develop a new multi-functional N-free flocculant.In this paper,two new N-free starch-based flocculants with bactericidal function were synthesized,respectively:?1?Cationic etherified starch flocculant?3-bromopropyl triphenyl brominated phosphine modified starch,S-BTP?,?2?cationic esterified starch flocculant??4-carboxybutyl?triphenyl brominated phosphine modified starch,TPS?.Firstly,the structure of the products was characterized by FTIR,¹H NMR,XRD and Zeta potential measurement.The flocculation and sterilization performance of the new flocculants in the treatment of bacteria-containing water was studied by using both jar test and a bench-scale continuous flow flocculation-sedimentation-ultrafiltration process.The influence of cation substitution degree?DS?,flocculant dosage,water p H value and model pollutant on flocculation performance was investigated.In the continuous flow flocculation-sedimentation-ultrafiltration test,the flocculation and antibacterial properties of the new flocculant were studied.The main conclusions are as follows:?1?Cationic etherified starch flocculant S-BTP showed good flocculation performance for single-,binary-,and trinary-contaminant simulated water samples,and its performance was similar to that of quaternary ammonium flocculant and conventional alum;For single-contaminant simulated water,the surface charge properties of S-BTP flocculant with kaolin particles,humic acid and bacteria determine the ultimate flocculation effect.For binary-contaminant simulated water,humic acid in the water plays a decisive role,and kaolin has a certain synergistic effect on the removal of other pollutants.In the treatment of the trinary-contaminant simulated water,under the optimal dosage of 40 mg/L,S-BTP kills 99%of E.coli in water,the turbidity removal rate is close to 100%,and the removal rate of UV₂₅₄ is up to 90%through charge attraction,polymer bridging and antibacterial action.Moreover,the bactericidal performance of S-BTP is obviously better than that of alum?killing bacteria 41.4%?.SEM and 3DEEM visualized that S-BTP flocculant significantly damaged the cell wall of E.coli,and could control the release of intracellular substances by regulating the amount of flocculant,thus controlling the generation of secondary pollutants.S-BTP flocculant showed better performance in water quality monitoring and membrane pollution control compared with commercial alum flocculant.By scanning electron microscopy?SEM?and laser confocal microscopy?CLSM?analysis of ultrafiltration membrane surface,the thickness of filter cake layer and the influence of polysaccharides,proteins and bacteria on membrane fouling were visualized.It was observed that compared with S-BTP,the filter cake layer on the ultrafiltration membrane of alum system was thicker and contains more polysaccharides and bacteria,resulting in more hydraulic resistance and irreversible membrane contamination.?2?Through comparative study,it was found that compared with S-BTP flocculant,cationic esterified starch flocculant TPS was more mild in the synthesis condition,and could react at room temperature,and the amount of quaternary phosphine monomer was lower when the degree of substitution was close,which reduced the cost.In addition,it was found that cationic esterified starch flocculant TPS not only had good killing performance against gram-negative E.coli,but also had good removal effect against gram-positive S.aureus.For single-contaminant simulated water,The removal of kaolin and bacteria by TPS is similar to that of S-BTP,but in the removal of UV₂₅₄,the optimal dosage of TPS?25-30 mg/L?is lower than that of S-BTP?40-50 mg/L?,which may be due to the structural changes of flocculants caused by different modification methods;For binary-contaminant simulated water,humic acid in the water plays a decisive role,and kaolin has a certain synergistic effect on the removal of other pollutants;In the treatment of the trinary-contaminant simulated water,,the turbidity removal rate of TPS for the water containing two different species of bacteria can reach100%,and the removal rate of UV₂₅₄ is about 94%.The removal rate was 99.94%for E.coli and 98.16%for S.aureus.Through the observation of 3DEEM and SEM after flocculation,Water containing S.aureus to release the aromatic protein and soluble microbial product is less,and And the shape of the bacteria in the floc is only partially destroyed.Compared with alum,TPS shows an advantage in membrane pollution control for both kinds of water,but water containing S.aureus caused more irreversible pollution to ultrafiltration membrane.Through the comparative study of the two systems,it is shown that the N-free cationic starch flocculant has obvious advantages as a substitute for traditional chemical substances in water treatment.The two modified starch flocculants had not only good flocculation performance,but also inhibition/sterilization effect.Therefore,the above two modified starch flocculants not only had good flocculation performance,but also have inhibition/sterilization effect.This dual function is mainly attributed to the introduction of cationic phosphine group on the starch chain,so that the flocculant molecules could not only flocculate and settle kaolin particles and bacterial cells through bond bridging and electrical neutralization,but also effectively destroy the bacterial cell wall and kill bacteria,control membrane contamination and reduce the risk of disinfection byproducts.