Preparation Of The Forward Osmosis Membrane With Modified Selective Layers And Its Application In Heavy Metal Ion Removal

In recent years,freshwater resources have been seriously polluted by heavy metals,which are harmful to people’s life and health.Therefore,it is urgent to develop energy-saving and effective technologies to remove heavy metal ions in water.Because of the advantages of high rejection rate,low energy consumption,low membrane pollution and high water recovery rate,forward osmosis（FO）technology has potential applications in wastewater treatment.However,the selectivity and permeability of FO membranes are limited by the density and roughness of the selective layers.Therefore,the modification of the selective layers was applied for the preparation of FO membranes with high performance,which are used to remove heavy metal ions in water,while keeping the high retention rate.Reducing the adverse effect of internal concentration polarization（ICP）and increasing the water flux is one of the research hotspots in the study of FO membrane performances.The selective layer modification is a simple and effective method to improve the selectivity and permeability of the FO membrane.In the first part of this paper,the FO membranes were prepared via the initial modification of dopamine（DA）and the subsequent layer-by-layer（LbL）self-assembly technique,which effectively improved the thickness,compactness and surface morphology of the selected layers,and enhanced the selectivity and water flux of the prepared FO membrane.In the second part,the selective layer of the FO membrane was synthesized by the interfacial polymerization of polyamide amine dendrimer（PAMAM）and m-phenylenediamine（MPD）,which effectively improved the free volume,surface morphology and hydrophilicity of the polyamide（PA）selective layer,and significantly increased the water flux,selectivity and chemical stability of the prepared FO membrane.The main results of this paper are as follows:（1）The prepared FO membrane with multi-layer polymer network was fabricated via LbL-assembling of multiple bilayers of polyethylenimine（PEI）and sodium alginate（SA）on a polydopamine（PDA）-functionalized polyvinylidene fluoride（PVDF）supporting membrane.The prepared FO membrane possessed high rejection of five heavy metal ions,i.e.,Cu²⁺,Ni²⁺,Pb²⁺,Zn²⁺and Cd²⁺.Results showed that the water flux was 42.0 LMH and reverse salt flux was 9.3 gMH in AL-DS（the selective layer facing the draw solution）mode for 3-（SA/PEI）membrane,when the draw solution concentration was 2.0 M MgCl₂ and the feed solution was DI water.When the draw solution concentration was 2.0 M and the feed solution contained2.0 g/L heavy metal ions,in the mode of AL-DS and AL-FS,the water flux was higher than35.5 and 19.6 LMH,and the rejection of the five heavy metal ions was higher than 95.3%and98.7%,respectively.The experimental results revealed that the number of bilayers of the polyelectrolyte pairs,the hydrated radii of heavy metal ions and the adsorption of heavy metal ion all contributed to the high rejection of the heavy metal ions.（2）The PAMAM was incorporated into the PA selective layer via the interfacial polymerization（IP）between the mixtures of PAMAM/MPD and trimesoyl chloride（TMC）on PVDF substrates to form the thin-film composite（TFC）-type FO membranes with highly permeable performances.We systematically investigated the effect of PAMAM addition on the properties of the TFC-type FO membranes.Results showed that proper addition of PAMAM can improve the separation performance of TFC-type FO membrane.The water flux of M-2 membrane（addition of 0.2%PAMAM during preparation）was 1.7 times higher than that of M-0 membrane（without the addition of PAMAM）.In addition,the water flux was38.5 LMH and reverse salt flux was 4.6 gMH in AL-DS mode for M-2 membrane,when 2.0M MgCl₂ was used as the draw solution and DI water was used as the feed solution.When2.0 M MgCl₂ was used as the draw solution and 2.0 g/L heavy metal ion solution was used as the feed solution,in the mode of AL-DS and AL-FS,the pure water flux of M-2 membrane was higher than 34.5 and 18.2 LMH,and the rejection of the five heavy metal ions was higher than 98.1%and 99.5%,respectively.This study proves that the PAMAM polymerized in the PA selective layer with intramolecular cavity may serve as a water molecule channel and the nearly spherical nanostructure may increase the relatively free volume and hydrophilia of the PA layer,which possibly resulted in the increase of the water flux.In general,the participation of PAMAM as a reactive monomer in the PA layer formation can promote the reaction between MPD and TMC,increase the free volume of the selective layer and adsorb the heavy metal ions via the free amine groups to form the electrostatic repulsive force against the other heavy metal ions in the solution.