Study On Chlorination Characteristics And Chlorine-Resistant Modification Of Polypiperazine Amide Composite Nanofiltration Membrane

Nanofiltration membrane technology has broad application prospects in drinking water treatment and industrial separation due to its unique separation performance,among which polypiperazine amide thin layer composite membrane is the most widely used nanofiltration membrane type at present.However,in the practical application process,the deterioration of membrane separation performance caused by biological pollution and chlorination degradation hinders its further popularization,and also brings a series of economic and environmental problems.In view of the lack of systematic studies on the chlorination characteristics and mechanism of polypiperazine amide composite nanofiltration membrane,this paper firstly combined with static and dynamic accelerated chlorination experiments to explore the chlorination characteristics and chlorination mechanism of polypiperazine amide composite nanofiltration membrane under different chlorination conditions.Based on this,the chlorination resistance of polypiperazine amide composite nanofiltration membrane modified by surface coating was explored.The pollution characteristics and the feasibility of using active chlorine to clean and remove pollutants were explored.The main research results are as follows:（1）The chlorination characteristics and mechanism of piperazine amide thin layer composite films（PIP films）were investigated under different p H and active chlorine intensity.Optimized experimental conditions for homemade PIP membranes under the condition of 4 bar,4.1g·L^-1 Mg SO₄ feeding solution,the water flux is 21.4±1.7L·m^-2·h^-1,and the Mg SO₄ interception rate is 98.1±1.2%.The static accelerated chlorination experiments were carried out at p H 4.0,7.0 and 10.0.The results showed that the deterioration of PIP film performance became more and more serious with the decrease of p H value or the increase of active chlorine intensity.When p H=4.0,after 16000mg·L^-1·h chlorination,Mg SO₄ salt retention rate is only 30.7±0.9%.When p H=7.0,it was68.6±1.1%.When p H=10.0,it fluctuates between 97.1%and 97.8%.When p H=7.0 and active chlorine concentration of 10 mg·L^-1 were used for dynamic chlorination attack experiment,Mg SO₄ interception rate was 73.1%after 337h operation,and the deterioration degree was more serious than static chlorination under the same total chlorine intensity.Based on the chemical structure and morphology changes of PIP films under different chlorination conditions,the chlorination mechanism of PIP films is mainly oxidation degradation mechanism,pathway I（p H=4.0）and pathway II（p H=7.0,10.0）.The mechanism of oxidative degradation pathway I includes:chlorination of amide bond,oxidation to form unstable bivalent piperazine cation,C-C bond breaking to form imine intermediate,and finally oxidation to carboxylic acid group.Oxidative degradation pathway II includes amide bond chlorination and conversion of amino group to hydroxylamine and imide.（2）The evolution of chlorine resistance of PIP film modified by polyvinyl acetate was investigated.The optimal modified membrane（0.05%modified membrane）was provided with 4bar,4.1g·L^-1 Mg SO₄ feed solution,the brine flux was 19.9±0.3 L·m^-2·h^-1,and the salt retention rate was 98.0±0.3%.The results of static accelerated chlorination experiment show that the retention rate of Mg SO₄ of 0.05%modified film is maintained at 91.1±0.1%after 16000 mg·L^-1·h total chlorine intensity treatment at p H 4.0.at98.7±0.3%when p H=7.0;When p H=10.0,there was no significant change.The experimental results of dynamic chlorination attack showed that the salt retention rate of0.05%modified film was 95.1±0.2%after 337h operation.Combined with the chemical structure and morphology changes of the modified film under different chlorination conditions,it was proved that the chlorination resistance of the modified film can be effectively improved by using polyethylene-acetate surface coating.（3）The contamination resistance of PIP membrane before and after modification and the feasibility of using active chlorine to clean the surface of PIP membrane were investigated.With HA and SA as typical pollutants,the dynamic operation pollution experiment was carried out on PIP film and 0.05%modified film.The results showed that for PIP film and 0.05%modified film,the attenuation amplitude of specific flux caused by SA was greater than that of HA,and the attenuation amplitude of specific flux of PIP film was greater than that of 0.05%modified film.After 8h contamination of PIP membranes and 0.05%modified membranes with HA and SA as single pollutants and HA,SA and BSA as mixed pollutants,the surface of the membranes was dynamically cleaned with active chlorine solution.The specific flux of PIP membranes increased rapidly after recovering to 1,and the retention rates decreased to less than 50%,indicating that the separation layer of PIP membranes had been irreversibly damaged.The specific flux of the 0.05%modified film recovered to 1 and tended to be stable,and the retention rate remained above 90%,indicating that the chlorine resistance and pollution resistance of the modified film were improved simultaneously after the polyethylene acetate surface modification.