Supercritical CO₂ Removes Residual Ammonium Perfluorooctanoate In Polyvinylidene Fluoride

While the rapid development of chemical industry has brought tremendous material wealth to human society,it has also had a profound negative impact on the environment.It has become the consensus of researchers to explore green chemical processes.In recent years,supercritical fluids(SCF)have attracted more and more attention due to its environmental advantages.As an environment-friendly solvent,it can replace many traditional organic solvents and be applied to many fields,such as extraction,catalysis,etc.SCF has both the diffusion coefficient similar to the gas and the density similar to the liquid.As well as its physical properties,such as the density and dielectric constant,are very sensitive to the change of temperature and pressure near the critical state.Therefore,it is possible to adjust the properties of SCF by changing the temperature and pressure of the system without changing the composition of the system.As the most typical representative of SCF,supercritical carbon dioxide(scCO2)has its own unique advantages in addition to the characteristics of the general SCF such as nontoxic,nonflammable,easily recyclable,and environmentally benign.These advantages have important theoretical significance and practical value for its application in the field of extraction.Therefore,the research and application of supercritical carbon dioxide extraction(SFE-CO2)technology has attracted extensive attention of researchers.Based on the intermolecular interaction hypothesis proposed by previous studies,perfluorooctanoic acid(PFOA)is selected as a research model to investigate the intermolecular interaction between PFOA and scCO2 and develop a method about improving extraction efficiency of ammonium perfluorooctanoate(APFO).This work mainly includes three parts:(1)Monitoring the solvation process of PFOA+scCO2 systemWith the support of the hypothesis of intermolecular-interaction dynamically induced solvation effect,the solvation and dispersion processes of the PFOA are in-situ monitored at 333,343,354,and 373 K by using the high-pressure in-situ ATR-FTIR monitoring system and studied on the solvation and the intermolecular interaction mechanism of dispersoid in scCO2 from the microscopic point of view.According to the spectral vibration absorption evolution curves of PFOA+scCO2 binary system under different temperatures and pressures,it is known that for the determined binary system,the transition pressure(PT)of the system is determined and unique under the isothermal condition,and the P-T increases with the increase of temperature.From 333 to 373 K,P-T of the PFOA+CO2 system increases from 12.0 to 20.2 MPa,in which merely 40 K rise in temperature.This is attributed to the dynamic change of σ(A-B)and σ(B-B)between the dispersate molecule and CO2 as the pressure changes.(2)Extraction and detection of residual APFOIn light of the deficiencies and problems in the current methods,the experimental methods were re-improved,and optimized the APFO dissolution process parameters in polymer samples.The optimal conditions are as follows:methanol as solvent;ultrasonic power=500 W;ultrasonic times=twice;ultrasonic time=15 min;standing time=30 min;centrifugal speed=10000 rpm;centrifugation time=5 min.Meanwhile,the conditions for the quantitative analysis of APFO and the uncertainty of the experiment are optimized,and the initial concentration of APFO in the polymer sample is determined to be 19.96 mg/L.(3)Exploration of factors affecting the efficiency extraction of APFOIn order to find out the effects of flow rate,time,temperature and pressure on the extraction efficiency,the individual effects of each factor are systematically studied.The extraction efficiency is improved by changing the flow rate of CO2 under the conditions of 333 K,10.5 MPa,60 min of extraction time.When the flow rate is 0.3 m3/h,the extraction efficiency reaches a maximum and is over 80%.The optimal extraction time is 60 min and maximum extraction efficiency is higher than 90%.The residue of APFO is the lowest in the sample,which is 1.53 mg/L,and the extraction efficiency of APFO is 92.3%.The PT is considered as the best extraction pressure by analyzing of the results of experiment in the extraction process of APFO under isothermal condition.