Fabrication And Characterization Of Novel Composite Nanofiltration Membranes Based On Zwitterionic O-carboxymethyl Chitosan

Nanofiltration membrane, a kind of energy-efficient separation membraneintermediate between reverse osmosis （RO） membrane and ultrafiltration （UF）membrane, has attracted more and more research attentions during the past few years.Generally speaking, its molecular weight cut-off ranges from200to1000Da, caneffectively intercept divalent/multivalent ions, micro-molecular organics, bacteriumand viruses. The unique separation performance of NF membranes mentioned aboveis the combinational result of "sieve effect" and "charged effect" of membrane surface.At present, most of the commercial NF membranes are composite membrane, whichconsists of active layer and permeable supporting layer. Zwitterionic polymers,containing both anionic and cationic groups in the same monomer unit, are importantas a new type of anti-fouling materials. Most often, common foulants areheterogeneous mixtures and involve a variety of functional groups, so zwitterionicmembranes may have advanced permeation performance in some complex systemsdue to their zwitterionic feature.Chitin, a kind of renewably marine biomass, is just less than cellulose in nature.Chitosan derived from the alkaline deacetylation of chitin is an important positivelycharged material because of its good biocompatibility, low toxicity, antimicrobialeffect and film forming ability. In this research, a series of novel compositenanofiltration membranes are prepared from O-carboxymethyl chitosan （O-CMC） andepoxy chloropropane （ECH） through coating and chemical cross-linking reaction onthe polyacrylonitrile （PAN） supporting layer. O-CMC is synthesized from chitosanand monochloroacetic acid. In experimental process, effect of different preparingconditions on permeation performance is characterized, and the optimal preparingconditions of NF membranes are confirmed. Besides, effect of different operating conditions on permeation performance of the optimal NF membrane is characterized.In addition, morphological structures and performance are characterized by infraredspectroscopy, atomic force microscopy, field emission scanning electron microscopy,contact angle detection and contact angle detection. At last, anti-fouling ability andstability of the optimal NF membrane were characterized to evaluate their practical.In this research, O-CMC is synthesized from chitosan and monochloroacetic acidthrough grafting modification method, and its substitution degree ofcarboxymethylation is about80%determined through potentiometric titration method.In the range examined, the optimum preparing conditions of NF membrane are3wt.%O-CMC solution,2.0wt.%ECH and12h cross-linking time at50oC. Separationperformance order of the optimal membrane to different inorganic salts is as follows:Na₂SO₄（494.73%）＞NaC（l42.78%）＞MgSO₄（423.69%）＞MgCl₂（29.04%）. Chemicalstructures of NF membranes before and after cross-linking reaction are characterizedthrough ATR-IR, and all characteristic peaks mentioned suggest that the cross-linkingreaction has taken place. Effect of casting solution concentration on the surfaceroughness of NF membranes is evaluated, and RMS value of the optimal NFmembrane is9.65nm. SEM images of membrane surface and cross-sectionmorphologies of the optimal NF membrane are obtained in different magnification.Change order of contact angles of the optimal NF membrane first rises and then dropwith the increase of casting solution concentration, and contact angle of the optimalNF membrane is44.21o. MWCO of the optimal NF membrane is tested as about689Da. Integral charge performance of the optimal NF membrane is negativelycharged in0.1mol·L^-1potassium chloride solution. In addition, the optimal NFmembrane expresses excellent anti-fouling ability and stability.