Preparation Of Lignin-based Cationic Polyacrylamide Flocculant And Its Application For Water Flocculation

In recent years,the water pollution problem in China has become increasingly serious,and the sewage treatment industry has received unprecedented attention.As the key to sewage treatment,flocculation sedimentation is a commonly used method to improve the water treatment efficiency in the world.Linear ultra-high molecular weight cationic polyacrylamide flocculant is one of the high-end flocculants commonly used in industry at present,but it has some disadvantages such as high viscosity and long dissolution time when it was used,the ionic monomers of it are randomly distributed on the polymer chain,and the effective charge utilization rate is really low.Besides,there are limitations such as high production costs,technical difficulties,difficulty in producing independently by China,etc.Therefore,developing a new type of cationic polyacrylamide flocculant is one of the keys to improving sewage treatment technology,and it also meets the requirements of high performance of traditional products.Lignin is the second largest biomass resource after cellulose in plants,which has a wide source and low cost,possessing a three-dimensional network structure.Lignin contains many reactive functional groups,which can adsorb polar organic substances dissolved in water.The net capture colloidal particles and suspended particles can be used as a natural flocculant material.This work innovatively employed the"grafting to"synthetic route to prepare lignin grafted cationic polyacrylamide flocculant,and researched its flocculation performance in sewage treatment.Firstly,acrylamide?AM?was used as the monomer,methacryloyloxyethyl trimethylammonium chloride?DMC?was used as the cationic monomer,4-chloromethylstyrene?CS?was used as the chain transfer agent,and ammonium persulfate?APS?was used as the initiator,linear cationic polyacrylamide prepolymer?CPA?was prepared by free radical polymerization method,which introduced a chlorine atom at the end of the polyacrylamide chain through a chain transfer agent;and then utilized the numerous phenolic hydroxyl groups in the lignin structure to react with the chlorine on the polyacrylamide chain under alkaline conditions atomic reaction,grafting polyacrylamide prepolymers with different cation degrees to lignin.Thereby,lignin-based cationic polyacrylamide flocculant?L-CPA?was obtained.L-CPA was used for the flocculation settling of kaolin suspension.The results showed that:when the graft ratio of CPA to lignin in the flocculant was 3/1?5/1,the addition amount of flocc ulant was 4 mg/L,the p H of the solution is 5?9 and the concentration of kaolin is 500 mg/L,the light transmittance of the solution can reach more than 85%after 20 minutes of flocculation settling.It was found that the flocculant L-CPA prepared by this method has similar characteristics to amphiphilic polymers through characterization of SEM,TEM and particle size testing.It can self-assemble to form an octopus-like hydrophilic micelle structure in water,the CPA fragments are dissolved in water,the hydrophobic lignin skeleton is wrapped in the core.The flocculation mechanism of L-CPA is electrical neutralization and bridging effect.In order to further develop the multifunctional lignin-based polyacrylamide flocculant,carboxymethylated lignin-based cationic polyacrylamide flocculant?CL-CPA?was also prepared by the grafting method of"grafting to",which can be applied to the treatment of carboxymethylation to prepare carboxymethylated lignin?CL?;then the carboxymethylated lignin was grafted with CPA to prepare carboxymethylated lignin-based cationic polyacrylamide flocculant?CL-CPA?under alkaline conditions.Then,CL-CPA was used to treat the wastewater suspension containing Cu²⁺.The results showed that:when the p H of the solution was 3?5,the graft ratio of CPA to CL in the flocculant was 2/1 and 3/1,the addition amount of flocculant was 4-8 mg/L and the concentration of Cu²⁺was 50 mg/L,the removal rate of Cu²⁺by the flocculant could reach more than 80%.The removal efficiency of Cu²⁺per unit mass of flocculant reached 5.46×10³ mg/g,which was much higher than the adsorption efficiency of various biomass adsorbents reported in the current literature.In addition,the flocculant can also be used to treat the mixed suspension containing kaolin and Cu²⁺.When the concentration of kaolin was 300 mg/L and the concentration of Cu²⁺was 50 mg/L,the light transmittance of the supernatant after CEL₂-CPA-4 treatment could reach more than 90%within 2 h,and the removal rate of Cu²⁺could reach more than 80%.In this work,the"grafting to"method was used to prepare the lignin-based cationic polyacrylamide flocculant,which can effectively overcome the low grafting ef ficiency and uncontrollable lignin-based flocculant chain structure of traditional"grafting from"free radical graft polymerization resulting from the free radical scavenging function of lignin.The prepared lignin grafted cationic polyacrylamide floccula nt performed similar feature to amphiphilic polymers,which could self-assemble in water to form an octopus-like hydrophilic micelle structure.The charge utilization efficiency of the cationic monomer was improved,the viscosity of L-CPA was low,and it could dissolve easily when used.The synthesis process was simple and efficient,which effectively overcome the difficulties in the preparation and use of the traditional linear ultra-high molecular weight cationic polyacrylamide flocculant.