Study On Preparation Of Modified Polylactic Acid And Its Application As A Sustained-release Carbon Source For Effective Nitrate Removal

Nitrate pollution in water is widespread worldwide.It not only causes damage to the ecological environment,but also is a great threat to human health.Solid-phase denitrification(SPD)process is widely applied to the nitrate removal in water due to its excellent effect and simple manipulation.Bio-based plastic polylactic acid(PLA)is becoming an important substitute for traditional petroleum-based plastics,waste PLA has potential environmental problems.The disposal and re-utilization of waste PLA have gradually attracted people's attention.Studies have shown that polylactic acid with molecular weight less than 10,000 g/mol could be used as a solid-phase sustainedrelease carbon source in SPD.However,wasted PLA has a higher molecular weight and thus could not be directly used.As to solve this problem,this paper prepared modified PLA blends with different lactide content as sustained-release carbon sources to study the denitrification effects.It provided a new idea for the application of waste PLA in SPD technology:First,this paper analyzed the intrinsic characteristics,carbon release trait,and nitrate adsorption of modified PLA blends with different lactide content.It was indicated that the incorporation of lactide increased the surface roughness and organic carbon release capacity of polylactic acid.This provided a better place for attachment and proliferation of microorganisms,and these blends had a small adsorption effect to nitrate,so its effect on nitrate removal could be ignored.The response surface methodologywas used to analyze and optimize the influencing factors of denitrification system,in which the modified PLA blends working as sustained-release carbon source.Results showed that the lactide ratio,temperature,and p H in the system have put a comprehensive effect on denitrification effect,the optimal parameter combination of the denitrification system for efficient operation is: lactide ratio of 5.02%-6.69%,temperature at 25.58-33.86°C,and p H between 7.5-7.8.According to the results above,a polylactic acid with a lactide ratio of 0%(LPLA0)and a polylactic acid blend with a lactide ratio of 10%(LPLA10)were chosen as the negative control group and the positive control group respectively,polylactic acid with a lactide ratio 5 %(LPLA5)was chosen as test group,were used as solid-phase carbon sources for nitrate removal in SPD reactors.After 3 months of long-term experiments,the average removal rate of nitrate in LPLA5 reactor went above 98%,with the average concentration of nitrite less than 0.09 mg/L,and the average concentration of ammonia nitrogen less than 0.37 mg/L,the average concentration of DOC less than 25 mg/L;the average nitrate removal rate in LPLA10 reactor was 88%,the average concentration of nitrite was greater than 1.3 mg/L,the average concentration of ammonia nitrogen was greater than 2.1 mg/L,the average concentration of DOC was greater than 400 mg/L,and the effluent quality was seriously deteriorated;while the average nitrate removal rate in LPLA0 reactor was only 34%.Microbial community analysis showed that the microbial diversity and abundance in LPLA5 reactor were the highest.The denitrifying microorganisms prevailed in the LPLA0 and LPLA5 reactors,while denitrifying microorganisms in LPLA10 reactor were not dominant.This study showed that the modified polylactic acid blending with 5% lactide ratio as a solid-phase sustained-release carbon source could remove nitrates from water efficiently.