Optimized Degradation And Resource Recovery Of Polyethylene Terephthalate In Tree-dimensional Electrochemical Reactor

Polyethylene terephthalate(PET)has been widely used for its high mechanical stability,good heat resistance and corrosion resistance.A large number of waste PET is difficult to be effectively degraded and recycled.So far,PET degradation and resource recovery methods mainly include mechanical recovery and chemical degradation recovery.Although waste plastic recycling technology is widely used,it has disadvantages of high recovery cost and low value of recycled products.Therefore,the development of cost-effective plastic recycling technology is an urgent problem in the field of waste plastic disposal.Three dimensional electrochemical reactor(Threedimensional electrochemical reactor,3DER)developed by our former research has achieved a good degradation effect on waste PET.In this paper,on the basis of the research done by our research,further research on the optimization of PET degradation by 3DER.Multiple response surface was used to optimize the operating parameters of 3DER degraded PET,and to regulate and control the degradation of PET and the accumulation of terephthalic acid(TA),so as to achieve efficient degradation of PET,accumulation and resource recovery of TA.Main conclusions are as follows:(1)Under degradation of the electric field,PET film on the particle electrode is broken to form intermediate degradation products,which enter the reactor in the form of dissolved organic matter and form DOC.Meanwhile,DOC in the liquid is degraded and mineralized simultaneously in 3DER.The degradation of PET by 3DER has the advantage of quick start.Under the condition of 12 V,after the reactor ran for3 min,the concentration of TC in the liquid was 5.4±0.2 mg/L,and after the reaction for 3 min,PET was degraded into dissolved intermediates and entered the liquid.Within 0-1 h,TC concentration increased rapidly.After 1 h,the concentration of TC reached 68.5±4.4 mg/L.(2)A single factor experiment was conducted on the four main parameters(voltage,electrolyte,degradation time and the amount of particle electrode PET film)that affected the PET degradation effect of 3DER.The results showed that the concentrations of dissolved total carbon(TC)and terephthalic acid(TA)in the liquid phase increased significantly at 6-12 V,and the TC concentration reached 141.5±3.0mg/L at 12 V for 3 h after degradation.When the electrolyte was sodium chloride,the concentrations of TC and TA in the solution were 77.4±2.5 mg/L and 10.4±1.3 mg/L after reaction for 1 h,which were higher than those in the solution with sodium sulfate and sodium nitrate.When the reaction time is 30-60 min,the dissolution kinetics constant of TA is 10.72,which is higher than that of other periods.The current value at 100% film load is 0.034 A after 1 h of electrification,which is lower than that at other film load.(3)Reaction time,electrolyte concentration and voltage were set as factors,TC concentration,energy consumption and TA concentration were set as response values,and response surface methodology(RSM)was used to optimize PET degradation effect of 3DER.The influence weight of voltage on TC concentration,energy consumption and TA concentration is the maximum of the three factors,and the influence weight of voltage on the three response values is 0.7941,0.6120 and 0.8300.The operating parameters corresponding to maximum TC concentration(488.1 mg/L),minimum energy consumption(15.8 k Wh/kg PET)and maximum TA concentration(65.5 mg/L)were calculated separately.The optimal operating parameters were determined as reaction time 40.7 min,electrolyte concentration 1.0 g/L and voltage11.5 V through multiple response surface optimization.Under these conditions,the TC concentration of 3DER degraded PET is 441.2 mg/L,the energy consumption is 44.8k Wh/kg,and the concentration of PET and TA is 61.7 mg/L.After verification experiment,the actual value of each response value is in good agreement with the predicted value,which proves the reliability of the optimal operation parameters.(4)PET degradation and TA accumulation process has high degradation efficiency in the initial stage.In order to efficiently degrade PET and recover TA,the degradation mode of short time and multiple rounds is adopted.After 8 rounds of degradation,the degradation rate of PET is 95.8±8.5%,and the energy consumption of PET degradation is 4.1 k Wh/kg PET,about 2.5 yuan /kg PET.Compared with the traditional technology of PET hydrolysis,the energy consumption is greatly reduced.The 8-round degradation in short time is adopted in this paper to discuss the mode of efficient degradation and energy consumption reduction.In practical engineering,several rounds of degradation and the degradation time of each round should be flexibly carried out according to specific product requirements and cost performance requirements.(5)The acid extraction method was used to recover TA.The shape of the recovered TA was regular prism,and the XRD and FT-IR spectra were highly similar to those of the standard TA.After 8 rounds of PET degradation,24.5 ± 2.1 mg TA crystal was extracted with the recovery of 18.1%.The wastewater produced in the process of PET degradation and TA recovery was treated by activated carbon bioreactor.The bioreactor effectively degraded the acid wastewater.The effluent COD value was 22.7-36.4 mg /L,the COD degradation rate was 81.3%,and the effluent p H was 6.23-6.56.In this paper,the work on TA crystallization is mainly to give the feasibility of TA recovery.Specific TA crystallization optimization process and subsequent purification process still need to rely on professionals to improve.(6)3DER degradation of PET is a nearly natural method,using natural tourmaline particles as particle electrode,water as reactor medium,sodium chloride as electrolyte,at room temperature and pressure,the degradation process only consumes electric energy.By regulating the degradation process,TA can be recycled and reused.The waste water produced in the process is simple in composition and is only the intermediate product of PET degradation.Using activated carbon biological method to treat wastewater can meet the discharge standard.(7)The 8-round degradation in a short time is adopted in this paper to discuss the mode of efficient degradation and energy consumption reduction.In practical engineering,several rounds of degradation and the degradation time of each round should be flexibly carried out according to specific product demands and cost performance requirements.In this paper,waste PET was fused with organic solvents such as trifluoroacetic acid and hung on the surface of tourmaline,which can significantly improve the binding area of PET and catalytic site.Moreover,the active oxidizing substances such as hydroxyl radicals produced in the electric field can also react with the substrate PET molecules in a fast and close distance,thus improving the degradation efficiency.In practical engineering,the method of in-situ membrane hanging and in-situ recovery and reuse of organic solvent can be adopted,which is simple and easy to operate.