Znd. Eng. C h e m . Res. 1987,26, 1970-1977
1970
Adsorption of Uranium on Cross-Linked Amidoxime Polymer from Seawater Takahiro Hirotsu,* Shunsaku Katoh, and Kazuhiko Sugasaka Government Industrial Research Institute, Shikoku, Takamatsu 761, Japan
Nobuharu Takai and Manabu Sen6 Institute of Industrial Science, T h e University of Tokyo, Minato-ku, Tokyo 106, J a p a n
Takaharu Itagaki Central Research Laboratory, Mitsubishi Chemical Industry, Ltd., Midori-ku, Yokohama 227, Japan
Adsorptive properties of uranium on cross-linked polymers bearing amidoxime groups from seawater were examined especially on the dependence upon hydrophilicity and porosity of the polymer. The hydrophilicity of the polymer was evaluated from heat of immersion in water. Proton-binding behavior of the polymer examined by the method of potentiometric titration revealed the existence of amidoxime groups as neutral species in seawater that seems to be responsible for a high adsorptivity of the amidoxime polymer toward uranium and principally determines the hydrophilic property. T h e adsorption rate of uranium on the polymers was significantly dependent on the product of the content of amidoxime groups and the surface area; furthermore, the polymers cross-linked with tetraethylene glycol dimethacrylate (4EGDM) exhibited a much higher adsorption rate than those with divinylbenzene. These results suggest intensively the primary determination of the adsorption rate by the diffusion process of uranium into the gel phase. Thus, the marked increase of adsorption rate by using 4EGDM was reasonably interpreted on the basis of the hydrophilic property of the polymer.
Part 1: Divinylbenzene We have already demonstrated that a polymer bearing amidoxime groups as ligands shows high adsorptivity to uranium in seawater, principally owing to proton-binding and metal-complexing abilities of the ligand (Hirotsu et al., 1986a-c). I t is, however, not fully clarified what characters of the amidoxime polymer affect significantly the adsorption rate of uranium from seawater. The concentration of uranium, which is present as an assumed species of stable and bulky tricarbonatouranate(V1) ion, [U0,(C0,)3]4- in seawater (Ogata et al., 1971), is as low as ca. 3 pg dm-,. These imply that the diffusion of [UOz(CO,),lp into the amidoxime polymer may be one of the processes governing predominantly the adsorption rate of uranium from seawater. As reported previously, an adsorption rate of uranium on a dihydroxamic acid polymer from seawater depends considerably on the morphology of the polymer (Hirotsu et al., 1986d). Thus, the adsorption rate onto the porous polymer is much higher than that onto the gel-type one; furthermore, on the gel-type polymer the adsorption rate decreases with an increase of the degree of cross-linking with divinylbenzene, DVB, implying the significant dependence of the diffusion of uranium into the polymer and, therefore, the adsorption rate on the porous properties. On the other hand, a hydrophilic character of polymer would be assumed to enhance the mobility of [UOz(CO,),]" into the polymer domain. Hydrophilic properties of amidoxime polymer are supposed to be determined principally by the proton-binding nature of the ligand groups. Accordingly, in the first part, adsorptive properties of uranium from seawater on porous amidoxime polymers cross-linked with DVB are examined, and in particular the adsorption rate is discussed from proton-binding and porous properties of the polymer. Experimental Section Materials. A solution containing acrylonitrile, AN, and DVB in toluene was dispersed with vigorous stirring into
Table I. Preoaration of Polv(acrvlonitri1e) 56 % linear DVB, wt toluene,n polystyrene,b % AN. wt % wt % wt% Dolvmer 1 48 52 62 0 2 48 52 100 0 3 53 47 100 0 4 53 47 100 10 ~
~~
~
~
"The values are weight ratios per total weight of monomers. *The values are weight ratios per total weight of monomers. M , 22 000.
an aqueous solution at 80 "C for 8 h by the usual method of suspension polymerization to give particles of poly(acrylonitrile) (Hodge and Sherrington, 1980; Sederel et al., 1973). The experimental conditions for preparation of poly(acrylonitri1e)are listed in Table I. These polymer particles were treated with hydroxylamine in methanol at 60 "C for 8 h to afford a polymer bearing amidoxime groups. After washing well with methanol and then being dried in vacuo, the polymer was stored in a desiccator. The polymer was sieved, and a fraction of 32-60 mesh was employed in the adsorption experiments of uranium from seawater and a fraction of 60-100 mesh in the other experiments. The content of amidoxime groups was determined from the H+consumption (Colella et al., 1980). Potentiometric Titrations. Potentiometric titrations were carried out with a Horiba Model M-8s pH meter using a combination electrode, which was calibrated with the standard buffer solutions. All experiments were performed at 25 f 0.1 "C, and the ionic strengths were maintained at 0.1 mol dm-3with KNO, (Merck, Suprapur). The conversion of pH meter reading, pHM,to -(log [H+]), where [H+]refers to the hydrogen ion concentration, was made through the same method as described in a previous report (Hirotsu et al., 1986a): -(log [H+])= pHM - 0.065. The hydroxide ion concentration, [OH-], was also obtained by using the apparent ionic product of water, pKwt,determined by the same method as described in the literature (Hirotsu et al., 1986a): pKw. = pHM - log [OH-] = 13.95.
0888-5885/87/2626-1970$01.50/0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 1971
A potentiometric titration of amidoxime polymer 1was carried out at pH 3-11 by a batch titration method. Amidoxime polymer (0.1 g) immersed in a solution containing an equivalent molar quantity of HN03 (Merck, Suprapur) to that of the ligand, and a prescribed quantity of 0.05 mol dm-3 KOH (Merck, GR) under an atmosphere of nitrogen was sealed and maintained a t 25 OC. Here, the degree of neutralization of the solution was set up in a range from -1 to 1. The degree of neutralization is defined as ([OH], - [H],)/[L],, where [HI, and [OH], are concentrations of acid and base added, respectively, and [L], is a concentration of the ligand in 20 cm3of the solution phase. After equilibrium was attained, pH of the solution was measured under an atmosphere of nitrogen. Adsorptive Capacity for Cu(I1). An adsorptive capacity of amidoxime polymer for Cu(I1) was determined by the following batch method. Amidoxime polymer (0.1 g) immersed in 25 cm3 of a 0.05 mol dmT3copper(I1) dichloride solution, which was maintained a t pH 4.5 with an acetic acid-sodium acetate buffer solution, was sealed and maintained at 25 "C for 48 h with shaking. After filtration, the concentration of Cu(I1) in the filtrate was determined with a Perkin-Elmer Model 403 atomic absorption spectrophotometer. The adsorptive capacity was evaluated from the difference between the initial and the final concentrations of the solution. Adsorption of Uranium from Seawater. Amidoxime polymer (5 g) was charged in a glass column (inner diameter, 2.5 cm; height, 20 cm). Seawater a t 25 f 2 OC was passed upward through the column a t a flow rate of 100 f 20 cm3 min-* which was high enough to assure the maximal adsorption rate of uranium on the polymer. After a prescribed time, ca. 50 mg of the polymer was collected and then washed well with distilled water. Uranium adsorbed was eluted with an 1mol dm-3 HC1 solution with refluxing. The concentration of uranium in the filtrate was determined to obtain the amount of uranium adsorbed. On the other hand, the following method was employed in order to examine the effect of temperature on the adsorption rate of uranium on the amidoxime polymer 1from seawater. The polymer (0.07 g) immersed into 2 dm3 of filtered seawater in a glass vessel equipped with an agitator was stirred at 25 OC. Seawater was renewed by 2 dm3every 24 h. After a prescribed time, the polymer was collected and then washed well with distilled water. The amount of uranium adsorbed was determined by the same method as described above. Measurements. IR spectra of the polymer were obtained by using a Nicolet Model 7199A FT-IR spectrophotometer. The pore volume, surface area, and pore spectrum were determined by the mercury intrusion method using a Carlo Erba Model Series 200 porosimeter. Chemical elements adsorbed on the amidoxime polymer from seawater were qualitatively detected by using a Rigaku Denki X-ray fluorescence spectrometer. The concentration of uranium was determined by fluorimetry with an Aloka Model FMT-3B fluorimeter, and those of the other elements were determined by atomic absorption spectroscopy with a Perkin-Elmer Model 403 atomic absorption spectrophotometer. Results and Discussion An infrared spectrum of poly(acrylonitri1e)prepared in this research is shown in Figure la. The characteristic stretching band of nitrile groups appears at 2240 cm-'. On the other hand, the polymer obtained by treatment of poly(acrylonitri1e) with hydroxylamine exhibits some new bands accompanied by the reduced absorption band due to unreacted nitrile groups, as shown in Figure lb. The
4000
I500
2000
3300
1000
500
Wavenumber / cm-'
Figure 1. Infrared spectra of poly(acrylonitri1e) (a) and amidoxime polymer (b).
bands at 3460 and 3390 cm-I are assigned to an antisymmetric and a symmetric stretching mode of NH2 groups, respectively. The bands at 1660 and 915 cm-I are assigned to a C=N stretching mode and a N-0 stretching mode of oxime groups, respectively. These results show clearly the extensive conversion of the original nitrile groups to amidoxime groups through the treatment with hydroxylamine. Proton-Binding Behavior. As reported previously, an acid dissociation equilibrium of amidoxime polymer, LH, is represented by LHz+ LH + H+ pKal (1) LH
L-
+ H+
pKa2
(2)
where pKal and pKa2are relevant acid dissociation constants (Hirotsu et al., 1986~).In the present study, a titration curve for amidoxime polymer 1 was found to deviate significantly from the curve in the absence of the ligand at values of -(log [H+])lower than 7 and coincides well with the latter at the higher values. This result implies intensively that LH behaves as a very weak acid and only the proton dissociation process expressed by eq 1 occurs predominantly a t -(log [H+])of 3-7. Therefore, in the range of -(log [H+])lower than 7, only the equilibrium represented by eq 1 is responsible for the titration behavior; under these conditions, mass-balance equations can be represented by [LI, = [LH2+1+ [LHI [LH2+]+ [H']
+ [OH],, = [OH-] + [HI,
(3) (4)
From these equations, [LH2+]and [LH] could be determined experimentally. The equilibrium constant, Kal,and the degree of dissociation, a, which are defined as Ka, = [LHI [H+I/ [LH2+1
(5)
a = [LHl/[Ll, (6) respectively, are also determined. A plot of pKa, against a is shown in Figure 2. With an increase of a , the value of pKal increases and becomes close to ca. 5.9, which is the pKa, value of acetamidoxime (Hirotsu et al., 1986a), revealing the behavior of LH2+as a stronger acid with decreasing values of a. The Henderson-Hasselbach relation -(log [H+]) = pKalapP + n log ( a / l - a ) (7)
is shown in Figure 3, where n is a constant and pKalaPPis a value of pKa, at a = 0.5. Plots of -(log [H+])against log (a/1 - a ) meet satisfactorily to form a straight line, with the values of n and pKalaPP being 1.87 and 4.55, respectively. This suggests that the electrostatic interaction is responsible for the decrease of pK,, and therefore the behavior of LH2+as a stronger acid with a decrease in a
1972 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
i
d
t
P
I
1 0
IO
0.5
IO2
10'
Pore Radius / n m
1
M.
Figure 2. Plot of pK,, against n for amidoxime polymer 1 at 25 "C and ionic strength of 0.1 mol dm-3 (KNOB).
Figure 4. Pore spectra of amidoxime polymers obtained by the mercury intrusion method. The numbers represent those of polymer samples. Table 111. Porous Properties and Water Contents of Amidoxime Polymers water content,a VP, SP, polymer cm3 g-l m2 g-' wt 9i 1 0.20 99 44.8 2 0.38 150 57.1 3 0.46 146 55.0 4 1.14 32 59.0
q
i -
' 5
The values were obtained from ( W , - W,) / W,, where W , and W , are the weights of wetting and drying polymers, respectively.
Table IV. Adsorptive Abilities of Amidoxime Polymer for Metal Ions in Seawater -I
0 log ( c ( / 1
CP
I
-co
Figure 3. Plot of -(log [H+]) against log ( n / l - n ) for amidoxime polymer 1. Table 11. Contents of Amidoxime Groups and Adsorptive Cauacities for Cu(I1) of Amidoxime Polymers adsorptive capacity for content of amidoxime groups, Cu(I1): polymer mmol g-' mmol g-' 1 1.14 0.95 2 0.76 0.67 3 1.17 1.17 4 0.79 0.57 The values were obtained at p H 4.5, and their standard deviations were within 10.02 mmol g-l. (I
value. Therefore, the amidoxime ligands of polymers is presumed to exist as a neutral species, LH, in a seawater (PH 8.1-8.3). Adsorptive Capacities for Cu(I1). Adsorptive capacities of the amidoxime polymers for Cu(I1) were examined under the conditions of pH 4.5 and [L], 3 (Hirotsu et al., 1986~).The results are listed in Table I1 together with the contents of amidoxime groups. In all cases, the values of capacities are close to the values of amidoxime contents. This result means the formation of 1:l complexes with Cu(I1) and the effective participation of the ligands in chelation with Cu(I1). Porous Properties. The distributions of pore radii of the amidoxime polymers were obtained for pore radii larger than 7 nm by the mercury intrusion method, as shown in Figure 4. Polymer 4 possesses predominantly pores of radii around ca. lo2 nm, while polymers 1, 2, and 3 have
element Mg Ca Fe Ni
cu Zn U
pug g-l
455 362 139 54.1 33.6 93.6 68.9
mmol g-l 1.87 X 9.03 X 2.5 X 9.2 X 5.4 X 1.4 X 2.9 X
C., pg dm-3 CC ,,'; dm3 g-l 1.29 X lo6 3.53 X 4.12 X lo5 8.79 X lo-* 2 69.5 1.7 31.8 0.5 67.2 4.9 19.1 3.2 21.5
many pores in a range of radii smaller than 20 nm. The specific pore volumes, V,, and the specific surface areas, S,, are listed in Table 111. Interestingly, polymers 1-3, which have many pores of smaller radii, exhibit larger values of S, than polymer 4. The water contents of the amidoxime polymers are also listed in Table 111. The present polymer samples were found to scarcely swell when the dried samples are immersed into distilled water or seawater. Therefore, the similar pore structures would be kept in distilled water and also in seawater. Adsorptive Properties for Metal Ions in Seawater. Adsorptions of Mg, Ca, Fe, Ni, Cu, Zn, and U on amidoxime polymer 1, which was treated with seawater for 10 days by a column method under the same conditions as mentioned in the Experimental Section, were determined by X-ray fluorimetry. The amounts of these elements adsorbed, C,, are listed in Table IV together with the concentrations, C,, in seawater (Riley and Skirrow, 1975). Furthermore, the concentration factor, which is defined as C,Ccl and referred to the adsorptivity for the element (Wada et al., 1979), was calculated and listed for each element in Table IV. On the basis of these results, the amidoxime polymer shows metal-ion-adsorptive affinities of the following order: Mg < Ca
