The Preparation Of A Cellulose Nanofiber Microfiltration Membrane Characterized By Absorption Of Metal Ions

With the rapid population growth and accelerating process of global industrialization, the concerns about global water shortage and contamination bring great challenges for water treatment industry. Among various kinds of water pollution, heavy metal pollution has become one of the most serious environmental problems, because heavy metals are not biodegradable and can be absorbed by living organisms, unlike organic contaminants, many heavy metal ions are known to be toxic. Once they enter the food chain, large concentrations of heavy metals tend to accumulate in the human body, which definitely will pose a serious threat to the health of humans and all life forms. Currently, various methods can be used for removal metal ions from wastewater, membrane filtration as a common method for wastewater treatment has some drawbacks: high cost, environmental concerns, poor fouling resistance, hydrophobic property. The development of new and more efficient membrane characterized by eco-friendly and feasible raw materials, low operation cost and high fouling resistance for water treatment is becoming an important task.The introduction of Nanotechnology into cellulose have extremely expanded its application fields and bring many astounding properties. It is well known that chemically modified cellulose materials exhibit higher adsorption capacities than unmodified forms. In this present study, we report a novel MF membrane characterized by high flux, low cost, good fouling resistance and metal ions removal produced by papermaking process. The modified cellulose nanofibers act as adsorbent and PET as support layer in this composite membrane. The main research contents are as follows:Softwood as raw materials, high consistency refining and screening were employed to produce Cellulose Nanofibers and SEM was used to observe micromorphology. TEMPO（2,2,6,6-tetramethylpiperidine-1-oxylradical） was employed to oxidate C6 hydroxyl groups on the cellulose fiber surface into carboxyl groups, the Carboxylated CNFs（C-CNFs） with different carboxylate content were fabricated by changing added Na Cl O amounts. The reaction between amination reagents（DETA and EA） and C-CNFs with EDC/NHS as the activating reagent for carboxyls was employed to fabricate Amino CNFs（A-CNFs）. Many methods were used for performance characterization, such as SEM, FTIR, XRD, TG and Element analyzer. The absorption mechanism was also studied. Two wet-laid nonwoven microfiltration membrane（C-CNFs-M and A-CNFs-M）were fabricated by papermaking process. Water flux, rejection rate, fouling resistance and metal ions removal were studied by dead-end filtration method.The main research results are as follows:（1） The amount of carboxylate groups formed from the hydroxyl groups of cellulose increased with the amount of Na Cl O added. Even though the increase in added Na Cl O from 9 to 12 mmol per gram of cellulose, the increased carboxylate content was only 0.05 mmol/g, the oxidation time remarkably increased from 3h to 6h. From FTIR and Element analysis, amino groups were successfully grafted on the carboxylated CNFs, and the content of grafted amino groups on CNFs could be controlled by changing amino agents. The decomposition temperature decreased after modification while the crystallinity increased, CNFs（62%）,C-CNFs（78%） and A-CNFs（79%）, which means that modified reaction mainly occurred on the surface and non-crystalline region.（2） At pH=5.5, C-CNFs reached the biggest absorption capacity for Pb²⁺（395mg/g）, at pH=4.5, A-CNFs reached the biggest absorption capacity for Cr⁶⁺（103mg/g）. As absorbents, their adsorption processes fitted for Langmuir adsorption isotherm and the Pseudo-Second-Order Model, which means that metal ions adsorption is monolayer and chemical process, besides, the adsorption is driven by ion exchange force.（3） The properties of wet-laid nonwoven MF membrane（3 g/m2 modified CNFs）are as follows: average pore size is 0.16 um, maximum pore size is 0.28 um, pure water flux is 1200L/（m2·h）（0.1MPa）, rejection rate for colibacillus is 100%（both A-CNFs-M and C-CNFs-M）, rejection rate for BSA（A-CNFs-M） is 98% and its recovery rate is 99%, rejection rate for BSA（C-CNFs-M） is 0.（4） A-CNFs-M has absorption capability of 85mg/g Cr（VI） at pH=4.5 and C-CNFs-M had absorption capability of 380mg/g Pb（II） at pH=5.5, while maintaining high water flux（12200L/（m2·h·MPa））, and the regeneration efficiency is very high（95% for C-CNFs membrane and for 90% A-CNFs membrane）. At relative lower concentrate range(Pb²⁺, 10mg/L-30mg/L), the absorption efficiency of C-CNFs-M is more than 90%; At relative lower concentrate range(Cr⁶⁺, 5mg/L-10mg/L), the absorption efficiency of A-CNFs-M is more than 95%. The desorption results gave the composite membrane a potential to be recycled.