Recently, the nanofiber is attracting attention in widely diversified fields and being developed for various applications. One of the typical applications of the nanofiber material is a nanofiber membrane, and it is considered to be promising as an alternative to conventional membrane. The nanofiber membrane with a nanosized effect and a high porosity is expected to serve as a new filter for water treatment since both performances of water permeation and particle retention are very high. Furthermore, if such functions as adsorption or ion exchange are added to the surface of nanofibers, the functionalized membrane developed by extremely high specific surface area can be produced for application to the water purification process.
In this study, two kinds of functionalized nanofiber membranes were developed to purify contaminated water. One is a nanofiber membrane supported by carbon black with solute adsorption capacity, and another is a nanofiber membrane supported by iron ferrocyanide with cesium ion adsorption capacity.
1. Development of Carbon Black–Supported Nanofiber Membrane for Adsorption of Solute
The nanofiber membrane supplied by Japan Vilene Co. was used as a base material. This membrane is made of polyacrylonitrile and has mean fiber diameter of 400 nm. Carbon black (CB) with original particle size of 20 nm was used as an adsorbent and dispersed in pure water at the concentration of 10 g/L. The nanofiber membrane was immersed in the CB dispersion and shaken for 2 hours. After taking out and rinsing it lightly, CB was immobilized on the surface of nanofibers by heating for 1 hour at 150°C.
The CB particles were subjected to any of the following pretreatments (1) - (3):
(1) In the preparation of the CB dispersion, pH was adjusted to a given value by the addition of HCl or NaOH.
(2) The CB particles were activated by heating for 1 hour at 800°C under N2 gas atmosphere after mixing KOH and CB.
(3) The CB particles were oxidized by mixing CB with 70% nitric acid at a rate of 1 g to 1 ml and heating for 2 hours with agitation.
The adsorption test of prepared CB-supported nanofiber membranes was conducted by soaking them into the aqueous solutions of methylene blue used as an adsorbate which were prepared at various concentrations. As a result, the saturated amount of adsorption was 1.9 mg/g for original nanofiber membrane, 5.0 mg/g for untreated CB-supported nanofiber membrane, 6.1 mg/g for CB-supported nanofiber membrane prepared under the acid environment, 18.3 mg/g for activated CB-supported nanofiber membrane, and 96.2 mg/g for oxidized CB-supported nanofiber membrane.
(1) When CB-supported nanofiber membrane was prepared under the acid environment, the amount of supported CB was slightly increased due to the reduction of electrostatic repulsion between CB particles and polyacrylonitrile fibers.
(2) The adsorption amount was increased by the activation of CB because of the increase in specific surface area of CB from 105 m2/g to 774 m2/g. However, the activation induced the flocculation of CB and consequently reduced the rate of increase in the amount of supported CB, resulting in not so much adsorption amount as expected.
(3) The adsorption amount was significantly increased by the oxidization of CB because the surface of CB particles became hydrophilic and as a result the size of CB particles became original one.
In conclusion, the oxidization of CB is an essential pretreatment to attain high adsorption performance.
2. Development of Iron Ferrocyanide–Supported Nanofiber Membrane for Removal of Cesium
With the aim of the establishment of an effective method for purifying water contaminated with radioactive cesium, the functionalized membrane with large cesium adsorption capacity per unit mass was prepared by combining nanofibers having a large specific surface area with iron ferrocyanide having a high selectivity for cesium adsorption. Iron ferrocyanide has a unique 3D framework structure, which can capture radioactive cesium ion selectively.
As in the study of the previous section, the polyacrylonitrile nanofiber membrane with mean fiber diameter of 400 nm supplied by Japan Vilene Co. was used. After being cut into 3 cm square and preweighed, the nanofiber membranes were immersed in the dispersions of iron ferrocyanide with various concentrations CIF and pH values and shaken under 80 rpm for 2 hours. After taking them out, they were dried for 2 hours at various temperatures T and then non-immobilized iron ferrocyanide was completely removed through water washing and subsequent ultrasonic cleaning for 2 hours. Finally, the functionalized nanofiber membrane was completed after drying for 2 hours at 50°C.
Then, the weight of prepared iron ferrocyanide-supported nanofiber membrane was measured to estimate the amount qIF of iron ferrocyanide immobilized by the nanofiber membrane from the difference from the initial weight of nanofiber membrane. Moreover, prepared iron ferrocyanide-supported nanofiber membranes were soaked in the non-radioactive cesium chloride solution for 2 hours with shaking under 80 rpm, and the adsorption amount qCs of cesium onto the functionalized nanofiber membrane was obtained by measuring the cesium ion concentrations before and after the adsorption test.
In a series of operations described above, the conditions that can immobilize more iron ferrocyanide on the surface of nanofibers were explored based on measurement of qIF and qCs, resulting in the following conclusions (1) – (3).
(1) Both qIF and qCs increased significantly with the increase in the concentration CIF of iron ferrocyanide dispersion and subsequently showed a tendency to decrease rapidly with CIF, resulting in both maximums of qIF and qCs at CIF of 3 wt%. This is because the increase in the viscosity of iron ferrocyanide dispersion and the aggregation of iron ferrocyanide occured under conditions of high concentration, and consequently iron ferrocyanide became impenetrable to the inside of the nanofiber membrane.
(2) Both qIF and qCs increased as pH became lower. This is because the negative charge on the surface of PAN nanofibers, which interfered with the approach of iron ferrocyanide, was neutralized by hydrogen ions in an acidic atmosphere. On the other hand, in an alkaline atmosphere iron ferrocyanide is at risk of releasing toxic and hazardous cyanide due to decomposition. Thus, it is more effective to lower the dispersion pH.
(3) Both qIF and qCs increased as the temperature rose. However, when the nanofiber membrane after soaking was dried at a temperature above the glass transition temperature 104°C of polyacrylonitrile, the breakage of the nanofiber membrane occurred in subsequent washing process. Therefore, it is optimal to dry it at as high temperature as possible without exceeding this limit temperature.