The adsorbent used was highly-dispersed iron ferrocyanide suspension supplied by Kanto Chemical Co. It is viscous and blue aqueous suspension containing 10% iron ferrocyanide particle with a diameter of 10 to 20 nm. Since it is a nanoparticle, it is appropriate for immobilization on the surface of nanofibers.

On the other hand, the adsorbate used for evaluation of the adsorption performance of iron ferrocyanide or prepared membrane was cesium chloride supplied by Kanto Chemical Co. Non-radioactive cesium chloride was used for experiment because the use of radioactive cesium was extremely hazardous. The cesium ion solution was prepared by adding cesium chloride to ultrapure deionized water with agitation.

Polyaluminum chloride (PAC) supplied by Taki Chemical Co. was used as a flocculant for iron ferrocyanide in the adsorption test. This is one of the most popular flocculants to be applied to such general water treatment as surface water treatment, effluent treatment and sludge dewatering because it has very high flocculation per- formance for various sorts of particles.

The nanofiber membrane supplied by Japan Vilene Co. was used as a base material for immobilizing iron ferro- cyanide. This membrane was made of polyacrylonitrile (PAN) and produced by an electrospinning method. Two kinds of nanofiber membranes with different fiber diameters of 220 and 411 nm and the same deposition weight per unit base area of 17 g/m² were used. The specific surface area calculated from the fiber diameter and the density of PAN (1.17 g/cm³) was 15.5 m²/g for 220 nm fiber, and 8.32 m²/g for 411 nm fiber, respectively. An example of their SEM photographs is shown in Figure 2.

In order to evaluate the cesium adsorption performance of iron ferrocyanide, the static adsorption test was con- ducted by mixing various concentrations of cesium chloride and iron ferrocyanide. The iron ferrocyanide dispersion and the cesium chloride solution with twice the concentration of desired one were prepared by diluting them with ultrapure deionized water, respectively. By mixing the same amount of these two fluids with agitation of 300 rpm by a magnetic stirrer, the cesium adsorption test of iron ferrocyanide was started. After 30 minutes when the adsorption amount of cesium absolutely reached an equilibrium, dispersed iron ferrocyanide was floc- culated by the addition of PAC to separate iron ferrocyanide from unadsorbed cesium solution. This mixture was centrifuged under 3300 rpm for 5 minutes and then the supernatant was sampled immediately. The concentration C_Cs^* of cesium ion in the supernatant was measured by an atomic absorption spectrophotometer. The adsorption amount q_Cs of cesium to iron ferrocyanide was calculated from the difference between the cesium concentrations of solutions before and after the adsorption test.

The nanofiber membrane was cut into 3 cm square and preweighed. Nanofiber membranes were immersed in the dispersions of iron ferrocyanide with various concentrations C_IF and pH values in the petri dishes with the lid and shaked 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, and then its weight was measured to estimate the amount q_IF of iron ferrocyanide immobilized by the nanofiber membrane from the difference from the previous weight. In this series of operations, the condi- tions that can immobilize more iron ferrocyanide on the surface of nanofibers were searched.

Prepared iron ferrocyanide-supported nanofiber membranes were soaked in the cesium chloride solution and shaked under 80 rpm for 2 hours. The adsorption amount q_Cs of cesium onto the functionalized nanofiber mem- brane was obtained by measuring the cesium ion concentrations before and after the adsorption test, which also led to the evaluation of the amount of iron ferrocyanide immobilized by the nanofiber membrane.

The adsorption amount q_Cs of cesium per unit mass of iron ferrocyanide measured by the static adsorption test is plotted against the equilibrium cesium concentration C_Cs^* in Figure 3. This figure clearly demonstrates that the adsorption behavior can be described by the following Langmuir adsorption isotherm equation:

which indicates the saturated adsorption amount q_Cs,max of cesium to iron ferrocyanide is estimated to be 84 mg-Cs/g-IF.

The saturated cesium adsorption amount of generally-used zeolite has been reported to be approximately 200 mg-Cs/g-ZL, which is larger than that of iron ferrocyanide [1] . It is known, however, that the cesium selectivity of zeolaite decreases in such saline environment as seawater. Meanwhile, iron ferrocyanide keeps a stable ce- sium selectivity even in a solution containing a variety of ionic species [10] .

For investigation of the effect of the concentration of iron ferrocyanide dispersion on the immobilization of iron ferrocyanide, the functionalized nanofiber membranes were prepared under various conditions of iron ferrocya- nide concentration. In Figure 4, the photographs of prepared affinity membranes are shown. The membrane prepared at lower or higher iron ferrocyanide concentration turns a comparably pale blue color, while that at in- termediate concentration is stained bright blue. This indicates the existence of the optimal iron ferrocyanide concentration.

In Figure 5(a), the supported amount q_IF of iron ferrocyanide per unit mass of the nanofiber membrane is shown against the initial concentration C_IF of iron ferrocyanide dispersion used for preparation of the affinity membrane. In Figure 5(b), the adsorption amount q_Cs of cesium onto the affinity membrane is also shown against C_IF. The initial concentration C_Cs,0 of cesium ion in the adsorption test was adjusted to 24 ppm. This concentration was determined in consideration of the measurement accuracy of the atomic absorption spectro- photometer used in this study even though it is much higher than the cesium concentration of real contaminated water. Both q_IF and q_Cs increase significantly with the increase in C_IF and subsequently show a tendency to de- crease rapidly for any fiber diameter, resulting in both maximums of q_IF and q_Cs at C_IF of 3 wt%. This is because the increase in the viscosity of iron ferrocyanide dispersion and the aggregation of iron ferrocyanide occur under conditions of high concentration, and consequently iron ferrocyanide becomes impenetrable to the inside of the nanofiber membrane.

The nanofiber membrane with fiber diameter of 220 nm has about twice specific surface area of the nanofiber membrane with fiber diameter of 411 nm, and therefore the values of q_IF of the former membrane were also about twice those of the latter membrane. However, the increase of q_Cs with a twofold increase in specific sur- face area was considerably less than twice. This may be attributed to the deactivation of iron ferrocyanide ac- companying close contact between iron ferrocyanide particles. Thus, using thinner fibers is effective for attain- ing a higher adsorption performance of cesium, but further improvement of the supporting method of iron fer- rocyanide onto nanofibers is required to make the most of their performance.

The effect of pH of the iron ferrocyanide dispersion on the preparation of the affinity membrane was investi-

gated. In Figure 6(a), the supported amount q_IF of iron ferrocyanide onto the nanofiber membrane is plotted against pH of the iron ferrocyanide dispersion used for preparation of the affinity membrane. In Figure 6(b), the adsorption amount q_Cs of cesium onto the affinity membrane is also plotted against pH. As indicated in these figures, both q_IF and q_Cs increase as pH becomes lower. This is because the negative charge on the surface of PAN nanofibers, which interferes with the approach of iron ferrocyanide, is 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.

The affinity membranes were prepared at various drying temperatures (T = 50˚C to 150˚C) in the drying process after immersing a nanofiber membrane in the iron ferrocyanide dispersion. In Figure 7(a), the supported amount q_IF of iron ferrocyanide onto the nanofiber membrane is shown against the drying temperature T. In Figure 7(b), the adsorption amount q_Cs of cesium onto the affinity membrane is also shown against the drying temperature T. It is clearly demonstrated that both q_IF and q_Cs increase as the temperature rises. However, when the nanofiber membrane after soaking was dried at a temperature above the glass transition temperature 104˚C of PAN, 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.