Selective Separation And Anti-fouling Mechanism Of The Polypyrrole Conducting Membranes

Membrane separation is one of the main technologies of water treatment and desalination.However,anti-fouling and selective separation are the main challenges of membrane separation technology.In this study,the polypyrrole/dopant composite membrane was used as both membrane and electrode to take advantage of the unique electrochemical properties of polypyrrole.The pore size,the wettability and chargeability of the polypyrrole composite membrane could be dynamically adjusted by varying the electrode potential.Thus,the membrane fouling could be controlled,and relieved,and selective separation could be realized.This study integrated electrochemistry and membrane separation,designing novel membrane materials for electrochemically regulating membrane separation,which has theoretical significance and application value for promoting the development of membrane water treatment technology.The main results are as follows:(1)A polypyrrole-doped sodium dodecylbenzene sulfonate(PPy-DBS)ultrafiltration membrane was successfully synthesized.With the merit of the electrically responsive structure relaxation of polypyrrole,the pore size of the PPy-DBS membrane could be adjusted by applying redox potential to the membrane.The size of the membrane pores can undergo a telescopic transformation of about ± 25% by varying the redox potential control.The insertion/extraction of hydrated cations will cause the volume change of PPy-DBS,so the membrane pores of PPy-DBS can be adjusted.By combining the the membrane pore size tuning and physical cleaning,the membrane fouling can be effectively relieved.As a result,the specific flux of the PPy-DBS membrane was increased by 21.91% and showed great recycling performance.Meanwhile,the molecular weight distribution of solute after permeation could be dynamically adjusted by tuning the membrane pore size.The results showed that molecular weight distribution of humic acid after permeation was significantly decreased.This proved that the smaller pores of the reduced PPy-DBS membrane enhanced the screening effect,thus achieving the selective separation of organics with different sizes.(2)A polypyrrole-doped bis(2-ethylhexyl)sodium sulfosuccinate(PPy-AOT)microfiltration membrane was synthesized.Electrically responsive tunable wettability,separation of oil-water emulsion and mitigated membrane fouling were achieved with this PPy-AOT membrane.The PPy-AOT membrane in the oxidation state was superhydrophobic and super lipophilic underwater,and the separation efficiency of water-in-oil emulsion was above 80%.After applying the reduction potential to the membrane surface,the sulfonic acid group of AOT would reorient,thus the reduced PPy-AOT membrane exhibited super hydrophilicity and super oleophobic underwater.The results showed that reduced PPy-AOT membrane could separate the oil-in-water emulsion with efficiency over 90%.Moreover,reduced PPy-AOT membrane exhibited the properties of anti-fouling and self-cleaning for heavy oil underwater.Based on density functional theory calculations,strong electrostatic attraction existed between oxidized/reduced PPy and hydrophilic/hydrophobic groups of AOT.The reversible wettability switching of PPy-AOT membrane was due to the reorientation of AOT anions during the redox process,which exposes the hydrophilic sulfonic acid groups or hydrophobic alkyl chains on the outermost surface.(3)A reduced graphene oxide-polypyrrole nanotubes(RGO-PPyNTs)composite nanofiltration membrane was synthesized.When applying RGO-PPyNTs nanofiltration membrane as electrode,high water flux and high salt rejection were achieved simultaneously,breaking the balance between permeability and selectivity.The results showed that the incorporation of PPyNTs into the RGO formed three-dimensional "sandwich" structure,creating more water transport channels.Moreover,the structure relaxation of PPyNTs occurred in the redox state with the interlayer spacing of the composite membrane changing to a certain extent.Accordingly,the flux and desalination efficiency of the reduced RGO-PPy NTs membrane were increased by 11.93% and 4.22% respectively compared with those without potential application.This is due to the volume expansion of PPyNTs under reduction potential,which leads to the increase of pore size and hydrophilicity of membrane surface.Therefore,the membrane flux is increased.At the same time,the applied potential will increase the charge density of membrane surface,which would increase its electrostatic repulsion to ions,improving the salt rejection performance.After applying the oxidation potential to the surface of the RGO-PPyNTs membrane,the membrane flux was reduced due to the shrinkage of the membrane pores,which in turn increased the salt rejection.The desalination efficiency of the oxidized RGO-PPyNTs membrane was increased by 5% compared with those without potential application.