Preparation And Ammonia Separation Performance Of PVDF Mixed Matrix Membrane

Ammonia nitrogen is a major nutrient in biogas slurry and its improper disposal and utilization can render the environment pollution.Membrane distillation（MD）has been sought after for ammonia separation,in which the driving force of ammonia and water transfer across the membrane is the partial press vapor.During ammonia recovery,the extremely high-water content of biogas slurry results in low ammonia selection performance and poor permeation performance in the MD process without p H adjustment.Considering that a large amount of mass transfer of water vapor across membrane can decrease the ammonia concentration of the absorbent,thus the increase in energy consumption.From the perspective of water recovery,the poor ammonia rejection performance and water permeability still remain challenges for biogas slurry treatment by MD process.Electrospun nanofiber membranes have the advantages of high porosity,high specific surface area and easy to achieve superhydrophobicity,which have potential to enhancing the permeability of the MD process.However,most single polymer nanofiber membranes have low mechanical strength and poor separation performance of ammonia.Functionalized carbon nanotubes（CNTs）and zeolite molecular sieves（ZSM-5）have good mechanical strength and hydrophobicity,and they are capable to adsorb ammonia nitrogen.They were doped into polyvinylidene fluoride（PVDF）matrix to prepare mixed matrix membranes（MMMs）by using electrospinning technology and phase separation method in this paper.The MMMs’ammonia separation performance in the MD process was investigated,the main results are as follows:（1）The parameters of electrospinning membrane production were optimized.The results showed that changes in PVDF molecular weight,spinning fluid concentration,solvent system,voltage,spinning distance and feed flow rate all affect the size and distribution of membrane fiber diameter.Among them,PVDF molecular mass and spinning fluid concentration have the greatest influence on membrane fiber diameter.Aiming to produce smooth,bead-free nanofiber membranes with small fiber diameters,the optimal spinning parameters are:4×10⁵g/mol PVDF molecular mass,10%PVDF concentration,6/4 solvent ratio（DMF/acetone）,16 k V spinning voltage,15 cm spinning distance and 0.5 m L/h feed flow rate.（2）CNTs/PVDF mixed matrix nanofiber membranes were fabricated to enhance the ammonia separation performance in the MD process.The results showed that all the prepared mixed matrix nanofiber membranes had high hydrophobicity and porosity.Without p H adjustment for biogas slurry with TAN concentration of 1000 mg/L,the 0.05wt.%C-CNT/8%PVDF mixed matrix nanofiber membranes showed the best ammonia separation performance,in which the total ammonia mass transfer coefficient,ammonia removal efficiency,ammonia flux and ammonia separation coefficient of 34.1×10^-7m/s,50.40%,15.30 g/（m²h）and 2.47,respectively.This is an improvement of about 82%,10%,53%and 47%respectively compared with the commercial membranes.The ammonia separation performance of the carboxylated CNTs-modified group was better than that of the hydroxylated CNTs-modified group,mainly because the carboxyl groups in C-CNT could adsorb NH₄⁺and thus promote the transfer of ammonia.（3）ZSM-5/PVDF mixed matrix nanofiber membranes with enhanced permeability for the MD process were prepared.When the concentration of ZSM-5 molecular sieve was increased from 0 wt.%to 10 wt.%,the average fiber diameter of the membrane decreased from 368.90 nm to 346.21 nm and then increased to about 522.89 nm.When the ZSM-5 doping concentration was 15 wt.%,the nanofiber membrane formation was failed.Proper ZSM-5 doping could improve the hydrophobicity,porosity,liquid entry pressure（LEP）and mechanical strength of the mixed matrix nanofiber membranes.Furthermore,under the same MD conditions,7.5 wt.%ZSM-5/PVDF mixed matrix nanofiber membrane can achieve a water flux of 12.14 kg/（m²h）,which is approximately88%and 122%higher than commercial membranes and CNTs/8%PVDF series membranes,respectively.When the ZSM-5 doping was increased from 2.5 wt.%to 10wt.%,the ammonia flux of the corresponding membrane increased from 7.13 g/（m²h）to17.35 g/（m²h）,an increase of about 143.34%.This indicated that the presence of ZSM-5could improve the ammonia permeability of the membrane.In the isothermal MD test,when the concentration of ZSM-5 molecular sieve increased from 2.5 wt.%to 10 wt.%,the total ammonia nitrogen transfer coefficient increased from 28×10^-7m/s to 55.54×10^-7m/s,which is 1-3 times higher than that of commercial membranes.This is mainly due to the adsorption effect of ZSM-5 on NH₄⁺,therefore increased the NH₄⁺concentration near the membrane surface on the feed side and weakened the negative impact of concentration polarization.（4）ZSM-5/PVDF/PTFE Janus mixed matrix membranes with adsorption properties on NH₄⁺were prepared based on a non-solvent phase separation method.The results showed that ZSM-5 would reduce the porosity and pore size of the coated membrane,it also enhanced the mechanical strength and LEP value of the membrane.MD test indicated that the ZSM-5/PVDF/PTFE Janus membrane was suitable for the removal of ammonia nitrogen from low concentration ammonia nitrogen wastewater（Ammonia nitrogen concentration<20 mg N/L）.The values of ammonia nitrogen concentration on the permeate side was 0-0.2 mg N/L regardless of the ZSM-5 doping amount selected.This indicated that the ZSM-5 molecular sieve was adsorptive retention of ammonia nitrogen in wastewater with low ammonia nitrogen concentration.Meanwhile,the adsorption capacity of the membrane could be regenerated by washing and drying.This indicated the advantage of the MD-based zeolite mixed matrix membrane for ammonia nitrogen removal compared to the direct adsorption of zeolite powder and mixed matrix membranes.In summary,three types of membranes prepared in this paper can treat wastewater with different ammonia nitrogen concentrations and can be used in different stages of the MD treatment process of biogas slurry,which is conducive to obtaining ammonia fertilizer and pure water,thus realizing the reduction of biogas slurry treatment and resource utilization.