Study On The Anti-Fouling Capacity And Ammonia Nitrogen Rejection Performance Of Amine-Functionalized Forward Osmosis Membrane

In the context of the current shortage of freshwater and the continuous consumption of energy and nutrients such as nitrogen and phosphorus,the alternative treatment of the domestic wastewater for simultaneously recovering the freshwater,energy and nutrients is of great significance for the sustainable development of society.Recently,the emerging anaerobic osmotic membrane bioreactor?AnOMBR?has shown strong advantages in the recovery of the resources embedded in domestic wastewater.However,during the operation process,the severe membrane fouling and the poor ammonia nitrogen rejection capacity of the forward osmosis?FO?membrane greatly limited the development of AnOMBR.In order to overcome this defect,this study investigated the effects of the substrate structures and the active layer surface characteristics of the thin film composite FO membrane?TFC-FO?on the antifouling ability and the ammonia nitrogen rejection capacity of the FO membrane.Subsequently,a surface amine functionalization strategy was proposed to simultaneously improve the antifouling ability and the ammonia nitrogen rejection capacity of the TFC-FO membrane,besides,the function of the primary and the tertiary amines was systematically investigated.The PAMAM dendrimer grafted membrane prepared in this study showed superior antifouling ability and the ammonia nitrogen rejection capacity in concentrating domestic wastewater,and could effectively inhibit the formation of biofouling.This study first explored the effect of substrate structures on the antifouling ability and the ammonia nitrogen rejection capacity of the TFC-FO membrane.The results showed that the substrates with sponge-like structure contributed to the formation of a thinner polyamide?PA?layer with a slightly lower roughness,a higher crosslinking degree and a lower carboxylic density on the surface,and the corresponding TFC-FO membrane had a slightly stronger antifouling ability.Specially,the FO-PEG,which processed a fully sponge-like substrate,exhibited the best performance in the domestic wastewater concentration process.The substrate with macroporous structure was otherwise.The antifouling ability of the TFC-FO membrane was mainly determined by the surface properties of the PA layer,and its improvement by changing the substrate structure to optimize the PA layer was not obvious.The ammonia nitrogen rejection capacity of the TFC-FO membrane was barely related to the substrate structure and the PA layer density.The results showed that the selectivity of the FO membrane towards ammonia nitrogen should be concentrated on enhancing the repulsive force from membrane surface to the external ammonia nitrogen.In order to simultaneously improve the antifouling ability and the ammonia nitrogen rejection capacity of the TFC-FO membrane,the feasibility of amine functionalization of PA active layer surface by hydrophilic amine-rich molecules was explored.First,in this study,the linear-structure polyethyleneimine?PEI?was used to modify the surface of the pristine TFC-FO membrane?i.e.FO-PEG?.The results showed that the surface hydrophilicity,surface potential and the primary amine density of the TFC-FO membrane were greatly improved after the surface of the PA layer was grafted with PEI,and the surface roughness was also reduced.The selectivity of the PEI-grafted membrane towards ammonia nitrogen was significantly enhanced as the PEI loading on the surface of the PA layer increases,among them,the rejection rate of FO-P₂ for ammonium chloride solution with 50 mg/L NH₄⁺-N was 95.88%,which was much higher than the pristine membrane?70.36%?.During concentrating domestic wastewater,the proper amount of PEI load could slightly improve the antifouling ability of TFC-FO membrane,however,excess PEI load aggravated the membrane fouling.Meanwhile,the rejection capacity of the PEI-grafted membrane for ammonia nitrogen in domestic wastewater increased first and then decreased with the increase of PEI load.The rejection rate of FO-P_0.5,which suffered from the slightest membrane fouling,for ammonia nitrogen was peaked at86.11%.It can be concluded that the antifouling ability and the ammonia nitrogen rejection capacity of the amine-functionalized TFC-FO membrane were mainly determined by the primary amine density on the grafted membrane surface,and the extent of its synchronous improvement was mainly determined by the tertiary amine density on the grafted membrane surface.Therefore,it is crucial to reduce the proportion of tertiary amines in polyamine molecules or to hide tertiary amines in polyamine molecules for simultaneously improving the antifouling ability and the ammonia nitrogen rejection capacity of the amine-functionalized TFC-FO membrane.In view of this,the study subsequently used the spheroidal polyamidoamine?PAMAM?dendrimer as a grafting agent to modify the pristine membrane.The PAMAM dendrimer has a very high primary amine group density,and the terminal primary amine group at the periphery of the molecule can effectively encapsulate the tertiary amine group within the molecule.Compared with the PEI-grafted membrane,the PAMAM dendrimer-grafted membrane not only had significantly elevated primary amine density,but also achieved a significant increase in surface hydrophilicity and ammonia nitrogen selectivity over a lower surface potential increase?isoelectric point pH<5.4?.The results showed that FO-G2,which grafted with PAMAM dendrimer of generation 2,exhibited the strongest antifouling ability and the ammonia nitrogen rejection capacity?rejection rate up to 93.88%?in concentrating domestic wastewater.With the increase of the G2 PAMAM dendrimer on the grafted membrane surface,the antifouling ability and the ammonia nitrogen rejection capacity of the grafted membrane were significantly improved,indicating that it is feasible to use PAMAM dendrimer as graft agent in amine-functionalization to prepare the TFC-FO membrane with the superior antifouling ability and ammonia nitrogen rejection capacity.In addition,compared with the pristine membrane?ammonia nitrogen rejection:48.84⁵7.58%?,PAMAM dendrimer-grafted membrane exhibited stronger antibiofouling capacity and ammonia nitrogen rejection capacity?ammonia nitrogen rejection:90.08⁹3.66%?during long-term concentration of domestic wastewater,especially in the early stage.Considering that FO-P2 can effectively inhibite the formation of biofouling at the initial stage during long-term concentration of domestic wastewater,it is conceivable that if the membrane surface is cleaned in time,the PAMAM dendrimer-grafted membrane will have a more durable membrane surface to simultaneously resist ammonia nitrogen transmembrane diffusion and membrane fouling.