Adsorption of the Elements on Inorganic Ion Exchangers from Nitrate Media WILLIAM J. MAECK, MAXINE
E.
KUSSY, and JAMES E. REIN
Atomic Energy Division, Phillips Pefroleum Co., ldaho Falls, ldaho
,The distribution coefficients of 60 metal ions have been measured for four inorganic ion exchangers from nitrate media over the pH range of 1 to 5. The exchangers are hydrous zirconium oxide, zirconium phosphate, zirconium tungstate, and zirconium molybdate. The data are presented as plots of log distribution coefficient vs. pH in a series of four periodic charts. Separations of analytical interest are presented.
T
HE use of inorganic compounds for ion exchange separation has been gaining popularity. The emphasis in research to date has been the synthesis of various inorganic compounds and experiments t o characterize the ion exchange mechanisms. The main use has been in the nuclear energy industry for the separation of selected nuclides from dissolved, spent, reactor fuel solutions. Recently, four inorganic compoundshydrous zirconium oxide, zirconium molybdate, zirconium phosphate, and zirconium tungstate-have become commercially available for the express use as ion exchangers. This paper presents adsorption data for 60 metal ions for these four exchangers from nitrate media over the pH range of 1 to 5. Potentially useful separations to the analyst and radiochemist are presented. Compared to the organic ion exchangers, the inorganic exchangers are more stable to high temperature, radiation damage, and oxidation; however, the solubility of the hydrous oxide exchangers becomes appreciable in acidic media and the acid salt types are appreciably soluble in basic media. Based on the excellent studies of Kraus et al. ( 5 ) , dmphlett ( I ) , Gal and Gal ( d ) , and others (2, S), the adsorption properties of the inorganic exchangers can be summarized as follows. Hydrous Zirconium Oxide. I n acid (at p H levels below the isoelectric point) , the positively charged polymer is electrically neutralized by exchange-
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ANALYTICAL CHEMISTRY
able anions. The anion exchange characteristics of the material are these. Adsorption of anions decreases with increasing pH; polyvalent anions are more strongly adsorbed than are monovalent, anions; negatively charged metal complexe. are adsorbed; complexing reactions with the exchanger can cause abnormally high adsorption -i.e., the D for fluoride on hydrous zirconium oxide approaches infinityand the capacity of the exchanger is on the order of 1 to 2 meq. per gram. (The commercial exchanger used in this study has a stated capacity of 1.05 meq. of Crz0;2 per gram a t pH 1.) At pH above the isoelectric point, the hydrous zirconium oxide polymer is negatively charged and is electrically neutralized with exchangeable cations. Its cation exchange characteristics are: Increased pH gives increased cation adsorption; the more strongly adsorbed polyvalent cations are adsorbed a t
Table I.
Periodic group IA IIA
Remarks Low adsorption Low adsorption
relatively low pH values; and the distribution coefficients for ammoniatransition element complexes are extraordinarily large. Zirconium Acid Salts. The three exchangers studied-zirconium phosphate, molybdate, and tungstateare sufficiently acidic to act only as cation exchangers with the following exchange characteristics: (1) With increased pH, adsorption of cations increases; ( 2 ) a t low pH, the selectivity for alkali metals is noteworthy, characterized by increasing D values with increasing number; (3) this high selectivity for the alkalies and also for the alkaline earths is lost a t higher pH (with the exchangers in the ammonium form, the alkali metals can be eluted as one group and the alkaline earths as another group); and (4)the capacities of the acid salt exchangers are on the order of 1meq. per gram. The capacities of the exchangers used in this study
Exchange Characteristics of Elements
Potential analytical usefulness
Be separated from IA and IIA elements at pH 2 5 Adsorption increases with increasing Ag separated from most cations IB, IIB at pH 1 pH, except Ag with high adsorption over pH range 1 to 5 IIIA, I V S Adsorption increases with increasing Sc separated from IIIA elements at pH 3 to 5 (rare earths) pH. Sc and Zr most strongly adsorbed. Zr added from fluoride s o h . may be adsorbed as fluoride complex or through heterogeneous exchange reaction with exchanger Maximum adsorption at pEI 3 to 5 ex- TI(1) separated from IIIB eleIIIB ments at pH 3 to 5 cept TI(1) which is low over pH range 1t o 5 IVB, VB, VIB Strong adsorption, as anionic species, Pb separated from IVB, VB, and VIB elements at pH 1 over pH range 1 to 5 except Pb(I1). D of Pb(I1) maximum a t pH 3 Nb and Ta added from fluoride media. VA Increasing D with increasing pH attributed t o increasing stability of fluoride complexes
n i m l
HYDROUS ZIRCONIUM OXIDE
E
ACTINIDES
~
Figure 1.
Exchange characteristics of elements on hydrous zirconium oxide
(milliequivalentr of Cs per gram a t p H 4) were :
on Hydrous Zirconium Oxide from Nitrate Media
Periodic group VIA
VIIA
VI11
Actinides
Remarks
D of W (it8 W O I - ~and ) Mo (as
Potential analytical usefulness Cr(II1) separated from Fe, Co and Ni
high and independent of pH. Cr (111) :hdsorption pattern similar to IIIA elements Lower D for Tc(VI1) and Re(VI1) than other anions attributed to singly charged species. Adsorption of Mn (11) siinilar to Group IIA D of Co(I1) and Ni(I1) similar to IIA elements. D of Fe(II1) and Ru (111) similar to IIIA elements. Decreabing D with increasing pH for Pd(I1) may be due to formation of nonadriorbing hydrolysis product. Ir(IV), added from chloride media, strongly adsorbed aa chloride complex Decreasiig D of Th(1V) with increas- U eeparated from T h ing pII attributed to formation of nonadriorbing hydrolysis product. D valiies of U, Np, Pu present in +6 oxidation state approach useful high values a t pH 5
Zirconium phosphate, 1.05 Zirconium molybdate, 1 . 1 2 Zirconium tungstate, 0.77 EXPERIMENTAL
Reagents. The four exchangers (Bio-Rad Laboratories, Richmond, Calif.) of 100 t o 200 mesh were used without treatment. Radiotracers either were prepared by neutron irradiation in the Materials Testing Reactor or were purchased from the Isotopes Division of Oak Ridge National Laboratory. Procedure. T o a 50-ml. polyethylene centrifuge tube were added 0.50 gram of one of the four inorganic exchangers and 10 ml. of a 0.005M solution of a metal ion and its radiotracer in equilibrium. Oxidation-reduction cycling was used to effect chemical identity for multivalent ions. The metalion solution had been preadjusted with nitric acid or ammonium hydroxide t o p H 1, 3, or 5. A polyethylene stopper was VOL. 35, NO. 13, DECEMBER 1 9 6 3
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Figure 2.
Exchange characteristics of elements on zirconium phosphate
inserted and equilibration was made for 1 hour on a 33-r.p.m. wheel. After centrifugation, 1-ml. aliquots were analyzed. The beta-gamma tracers were counted in a well crystal; the alpha tracers were plated on stainless steel planchets and counted in a zinc sulfide alpha chamber. Suitable radiotracers were not available for several of the elements. Aluminum, beryllium, lead, magnesium, titanium, and vanadium were determined by rotating disk emission spectrography, lithium by flame photometry, and uranium by sodium fluoride pellet fluorophotometry. The distribution coefficient, D, amount of metal ion adsorbed per gram of exchanger/amount of metal ion per milliliter of contacting solution, for the above conditions, is:
D
=
20
(2-
1)
in which Cb = counts of tracer or amount of metal per milliter, before equilibration, and Ca = Cb, after equilibration.
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ANALYTICAL CHEMSTRY
Table
II.
Exchange Characteristics of Elements on
Periodic group IA
Remark8 Order of D ia ZP>ZT>ZM." Separation factors differ little for 3 exchangers, best at low pH IIA At comparable pH levels, D values are approximately 10 timee greater than for IA elementa. However, separation factors between members not as high 88 for Group IA. D of Ba(I1) decreases at high pH especially on ZP and ZT exchangera IB, IIB D values generally imcreaae with incresslng H exce t for Ag on ZT. Au, addeafrom chporide solution, not adaorbed IIIA All members strongly adsorbed. With (rare eartha) ZT, D valuee decrease with increwing pH differing from other two exIIIB IVA
IVB
Potential analytical usefulnew Separation of individual alkali metala Ba separated from Sr on ZP at pH 5
Ag separated from other membersatpH 1 onZM Sc separated from other members at pH 1 on ZP
ChsngeFs
D values of all membere increase with A1 aeparatad from other members on ZM at pH 5 increasing pH, except Al on ZM
Zr added aa fluoride complex. Decreasing D with inoreasin. pH attributed to increasing- stab& of fluoride complex Increasing D value an atomic number Separation of members a t pH 3 increwen as emected for increaainp to 6 on ZT metal charactei
-
Figure 3.
Exchange characteristics of elements on zirconium molybdate
DISCUSSION
Zirconium Acid Salt Exchangers from Nitrate Media
Periodic group T'A
T'B, VIB
17.4 VII.1 1-111
Actinides
a
ZP ZT
Zhl
= = =
Potential analytical usefulness Remarks Nb and 7'a added from fluoride media Decreasing D values with increating pH attributed to increasing stability cf fluoride complex Increasing D values as atomic number Separation of As from Sb on ZP increasss as expected for increasing a t pH 5 . Separate P o from Ri on Zhl at pH < 1 metal character. Decreasing D value for As with increasing pH attributed t o increasing- stabilitv of anionic form Cr(II1'I adsorption pattern similar to Separation of Cr(II1) from FefIII) on Z M at IIH 4. Group IIIA. D values of anionic Mo a n i W are low Sebaration of Cr(I11) fiom TT and hIo on ZT or ZP a t pH 4 Positively charged A h ( 11)most strongly adsorb1.d High D of Fe on ZP attributed to selec- Fe separation from Co, Si,Cu tive phosphate complexing on ZP a t pH 5 . Xi separation from Pd on ZT at DH < 1 Decreasing D of Pu(T'1) with increasing Th separated from U, T p ; Pu on pH attributed to hydrolysis. High Z M at pH < 1 D of T h and U on ZP attributed to selective phosphate complexing
Zirconium phosphate. Zirconium tungs1;ate. Zirconium molybdate.
The distribution coefficients, as defined under Procedure, for 60 metal ions in nitrate media from pH 1 t o 5 are presented in Figures 1 through 4, with one figure for each of the four exchangers. These data represent a total of 720 equilibrations; 4(eschangers) X 3(pH levels of 1, 3 2nd 5 ) X 60(metal ions). The pH range of 1 to 5 was selected because helotJ- pH 1 the solubility of hydrous zirconium oxide becomes appreciable and above pH 5 hydrolysis reactions of many of the metal ions become competitive. A nitrate medium was chosen for its minimal complexing action and because nitrate media commonly are encountered in the nuclear energy industry. Summaries of the exchange characteristics of the elements for the four exchanger materials are presented in Tables I and 11. Separations of potential analytical usefulness also are presented in these tables. VOL. 35, NO. 13, DECEMBER 1963
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ZIR C 0 NIUM T UN G S TAT E
ACTINIDES
1
1 Figure 4.
ACKNOWLEDGMENT
The authors thank K. R. Arnold for spectrochemical analyses. LITERATURE CITED
(I) Amphlett, C. B., Second Intern. Coni.
on Peaceful Uses of Atomic Energy,
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ANALYTICAL CHEMISTRY
Exchange characteristics of elements on zirconium tungstate
Geneva, Vol. 28, Paper 271, p. 17, United Nations. New York. 1058. (2) Baetsle, L., Huys, D., J . Inorg. Nucl. Chem. 21, 133 (1961). (3) Baetsle, L., Pebmaekers, J., Zbid., 21, 124 (1961). ( 4 ) Gal, I. J., Gal, 0. S.,Second Intern. C o d . on Peaceful Uses of Atomic Energy, Geneva, Vol. 28, Paper 468, p. 24, United Nations, New York, 1958.
( 5 ) Kraus, I
