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Precipitation and hydrolysis of uranium (VI) in aqueous solutions-VI
J . inorg . nucl . Chem ., 1969, Vol . 31, pp . 1771 to 1782 . Pergamon Press .
Printed in Great Britain
PRECIPITATION AND HYDROLYSIS OF URANIUM (VI) IN AQUEOUS SOLUTIONS-VI INVESTIGATION ON THE PRECIPITATION OF AMMONIUM URANATES B . TOMAZIC, M . SAMARZIJA and M . BRANICA Department of Physical Chemistry, "Ruder Boskovic" Institute, Zagreb, Croatia, Yugoslavia (First received 29 July 1968 ; in revisedform 23 October 1968)
Abstract-The precipitation boundary has been studied in the aqueous system : U02 (NO3)2 - N H,OH -NH,NO 3. Assuming that the first precipitation product is ammonium heptauranate, the solubility product constant has been estimated to be log K,, .25 = - 14 . 3 . A mechanism of the formation of ammonium uranates from alkaline solutions was proposed assuming the presence of anionic uranyl hydroxo complexes . The morphology of precipitates isolated from alkaline solutions is described . INTRODUCTION THE PROBLEMS of precipitation and hydrolysis of uranium (V I) in different aque-
ous systems were extensively studied in this laboratory [ 1-4] . In spite of the great variety of observed phenomena, which can be attributed to the fairly complex behavior of the uranyl ion, there are some common phenomena which can be ascribed to the unique complexation reactions of the uranyl ion . The idea of complexation of the uranyl ion (Werner type complexes) was developed by Chernyaev et al. [5, 6] . The hydrolysis reaction is presented as a substitution of hydroxyl ions for water molecules in the complex sphere of the hydrated uranyl ion resulting in the formation of a genetic series of uranyl hydroxo complexes . Consequently, one uranyl ion can bind six hydroxyl ions . The experimental verification for only the first few members of this series formed in the system : UO22+-MOH (M = NH4 +, K+, Na+) was found[6] . The maximum ratio OH - : U02 2+ obtained was 3, and the formed complex Me[UO 2 (OH)3 (H 2O),t] corresponds to the well known metal diuranate . Chernyaev neglected the concept of polynuclear complex formation, which had been introduced by the Swedish school and developed by the mathematical treatment of potentiometric data [7] . The "core +link" theory introduced by Sillen [8] has shown that the process of polymerization results in the formation of an unlimited series of complexes with a general formula [UO 2(UO2(OH)2)n] 2 + Unfortunately, very few attempts have been made so far to present physical evidence of the polymerization of the 1 . B . Tomazlc, M . Branica and B . Teiak, Croat. Chem. Acta 34, 41 (1962). 2 . H . Fiiredi and B. Teiak, Croat. Chem. Acta 36, 119 (1964) . 3 . H . Fdredi . Croat. Chem . Acta 195 36, (1964). 4. B . Tomazic and M . Branica, Croat. Chem . Acta 38, 249 (1966) . 5 . 1 . 1. Chernyaev, V . A . Golovnya and G . V . Ellert, Zh. neorg . Khim . 5, 1481 (1960) . 6. I . 1 . Chernyaev, Kompleksnye soedineniya urana . Nauka, Moscow (1964) . 7 . A . Arhland, S . Hietanen and L . G . Sillen,Acta chem . scand. 8, 1907 (1954) . 8 . L. G . Sillen and S . Hietanen, Acta chem . scand . 8, 1607 (1954) . 1771
1772
B . TOMA2IC, M . SAMAR2IJA and M . BRANICA
uranyl ion . From cryoscopic measurements Sutton has shown that polymerization exists [9] . Ultracentrifugation measurements have shown that polymerization takes place up to three nuclear species[ 10] . The hydrolysis of the uranyl ion results in the formation of positively charged polynuclear complexes . The formation constants for these complexes have been evaluated by many authors [7, 9, 10] . From spectrophotometric measurements, Sutton has proposed the formation of negatively charged complexes of the form U308(OH),,-(n-2' and estimated the corresponding formation constants [9]. The existence of such species in alkaline solutions of the uranyl ion has also been confirmed by conductometric [9] and polarographic [ 11 ] measurements . It has been shown previously that the progressive alkalization in the system : U02(NO3)2-KOH leads to the formation of negatively charged colloid particles, which could be "coagulated" by means of various "neutral electrolytes" [1, 4] . In the present paper an attempt has been made to show the dependence of ammonium uranate composition on the starting concentration of reacting components . Special attention has been paid to the first appearance of the precipitate (precipitation boundary) in the broad concentration range. The morphology of some precipitates was studied by use of light microscopy, electron microscopy and X-ray powder pattern, and a comparison with the published data [ 12] is given . The mechanisms of hydrolyzation and of precipitation of uranium (VI) with ammonia are discussed . EXPERIMENTAL Materials . All solutions were prepared from reagent grade chemicals, dissolved in twice distilled water. The concentration of uranium was determined gravimetrically as U 308 [13] . The concentration of ammonium hydroxide was determined by volumetric titration . The concentration of ammonium nitrate was determined by the ion exchange method [ 14] . Systems . All systems for the determination of precipitation boundary were prepared by mixing equal volumes of the precipitating components . The solution of ammonium hydroxide was added to the mixture of uranyl nitrate and ammonium nitrate, the mixing was repeated 7 times in 20 sec . The final volumes were 10 ml . After the mixing the tubes were closed with rubber stoppers and placed in the water bath at 25°C for a given period . For the determination of the composition of ammonium uranates systems of 50 ml of 500 ml were prepared . A solution of ammonium hydroxide was added to the mixture of uranyl nitrate and ammonium nitrate while the solution was vigorously mixed with a vibration mixer . These systems were kept one month in the water bath at 25°C in order to attain equilibrium . Analysis method . The separation of precipitates from mother liquors was performed by repeated centrifugation at 6000 RPM . In order to prevent pronounced hydrolysis and peptization the precipitates were not washed . An exception was made with precipitates obtained from 10 -3 M uranium (V I) solutions, which were washed with 10 -3 M ammonium nitrate solution adjusted to the pH of the mother liquor solution . Uranium was analyzed polarographically[15] and gravimetrically as U 308, [ 13] . The ammonia content was determined by the Kjeldahl method . Apparatus . The turbidity measurements were performed with a Zeiss tyndallometer joined to a Pulfrich photometer supplied with a green filter (wave length 530 nm) . The pH measurements were 9 . 1 . J . Sutton, J . chem. Soc . S. 1 . No. 2, S 275 (1949) . 10 . R. M. Rush, J . S. Johnson and K . A. Kraus, lnorg. Chem. 1, 378 (1962) . 11 . EURAEC-876, Quart. Rep. No . 12, July 1-September 30 (1963) . 12 . P. C. Debets and B . 0 . Loopstra, J. inorg . nucl. Chem. 25, 945 (1963) . 13 . C. J. Rodden, Analytical Chemistry of the Manhattan Project . McGraw-Hill, New York (1950) . 14 . 0. Samuelson, Ion Exchangers in Analytical Chemistry . Wiley, New York (1950) . 15 . V. Vouk, M . Branica and 0 . Weber, Ark . Kemi. 25, 225 (1953) .
Precipitation and hydrolysis of uranium (V I) in aqueous solutions- V I
1773
performed with a standard glass/calomel electrode pair connected to a PYE Master pH meter . Polarographic measurements were performed with a Cambridge Instrument Pen Recording polarograph . The precipitates were examined by X-ray diffraction using a Philips diffractometer PW 1050/PW 1051 with a recorder . Microscopic equipment from E . Reichert was used for dark field microscopic observations of precipitates . Electron micrographs were performed using an electrostatic electron microscope, Model KM4 from Trub Tauber and Co . Spectra were performed with a Perkin-Elmer Mod . 137 UV spectrophotometer . RESULTS AND DISCUSSION
The system
UO 2 (NO 3 ) 2
-NH 4 OH
The precipitation of uranium with ammonium hydroxide starts at a certain ratio Z = NH 40H/UO2 (NO 3)2 which is dependent on the initial concentration of uranyl nitrate . The first precipitation point was detected by turbidity measurements . The first precipitation as a rule was accompanied by a sudden drop in the pH value . This effect was reported previously in similar systems [4] and also shown by other authors [16, 17] . Several reactions can account for this effect, but most probably this is due to the slow formation of uranates at the given conditions . Uranyl hydroxide is probably formed as a metastable species which then reacts with a cation . This is shown schematically as : UO 2 (OH)2 ₂m H .,O+n NH 4 +
UO2 (OH)2 _ n (ONH 4 )„+n H+ .
(1)
The pH d op can be impl a ocia ed o he e en of he p ecipi a ion of ani m. The elea ed p o on e en iall change he ol ion compo i ion e l ing in a e lo a ainmen of he p ecipi a ion eq ilib i m . In Fig . I pH al e of he fi p ecipi a ion em a a f nc ion of he ini ial concen aion of an l ni a e a e p e en ed. C e 4 define he end of Z a io a hich he fi p ecipi a ion a ob e ed. Thi c e emain nchanged af e aging fo ei he 24 h o 30 da . On he o he hand, c e 3 ho ing he end of pH again an l ni a e concen a ion in p ecipi a ed em fo eq ilib a ion pe iod of 24 h ignifican l diffe f om c e 1 gi en fo em 30 da af e mi ing. Thi ho ha he he e ogeneo eq ilib i m i eached a a imecon ming p oce . In Table I he e pe imen al da a de c ibing c e I and 2 a e gi en . The e pe imen al poin of pH . log . concen a ion of an l ni a e a eq ilib i m can be fi ed b a linea eq a ion of he gene al fo m : pH = - ]In log UO2(OH~
n)
+
I /n
log KI(2 _n) -log K,,,
(2)
he e n de e mine he a e age n mbe of OH - ion pe one ani m a om eq i ed fo he fo ma ion of olid pha e of ani m (VI) f om aq eo ol ion . The egmen C2 _ n on he pH a i con aining he al e of he ol bili p od c i defined [181 a : C2_„ -
I
In
log K,(2-,,)-Iog K . .
16. V . Ba an and M . T mpl,J. ino g . n cl . Chem . 28,89 (1966) . 17 . A . Dep la, N kleonika No 4, 265 (1962) . 18 . H . Bilin ki, H . Fa edi, M . B anica and B . Te ak, C oa . Chem . Ac a 35, 19 (1963) .
(3)
1 774
B. TOMA2IC, M. SAMARZIJA and M . BRANICA
I 1,2 30 da 3,6 _26 25
ho
af e
mi ing
af e
mi ing
,
C
PRECIPITATE
pH
4
-h
, 3
-3
i'
CLEAR SOLUTION
X
.
Old I PRECI
ITATE
,a[Il"1
1
-2
-3
-4
LOG. CONCN. U02(N03)2 [M] Fig. 1 . The p ecipi a ion bo nda
of he
em: UO2 (NO 3 ) 2 -NH,OH-NH 4 NO3 .
U ing he lea q a e me hod, he lope a de e mined a -0 . 8 hich lead o n = 1 . 25, and ha of C2-,,= 2 . 55. Th , one can concl de ha onl 1 . 25 OH- pe one ani m i nece a o fo m he p ecipi a e . The fo ma ion of an l h d o ide f om a ol ion of nh d ol ed an l ion o ld eq i e o h d o l ion pe one an l ion . Ho e e , he fo ma ion of ammoni m hepa ana e heo e icall e l in he con mp ion of 2 . 28 h d o l ion fo one an l ion, a ho n b a implified eac ion cheme : 7 UO2 2 ++ 16 OH - + 2 NH4 + = (NH4)2U7022+8 H2O The al e fo n = 1 . 25 can be in e p e ed in e m of o con ec i e eac ion aking place in he ol ion of p eh d ol ed an l ion leading o he fo ma ion of ani m (VI) p ecipi a e : UO2(OH)+"' +n' OH - ± UO2(OH)2 UO2(OH)2+n"NH4 OH -1 UO2 (OH)2
_
A
ONH4)„~+n"/2 H 2O .
(5) (6)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion - V I
1775
Table I E peimen al da a of he p ecipi a ion bo nda in he em : U02 (N0 3)2 -NH4OH-0 .2 M NH4NO3 . Eq ilib a ed fo one mon h a 25 C
U02 (NO3 )2 M 0-1 0-05
0-02 0.01 0-008 0-005 0-003 0.002 0-001 0.0008 0.0005 0-0003 0-0002
pH in ab ence of NH4 NO3
pH in p e ence of 0-2 M NH 4NO3
3-40 3-65 3. 95 4-12 4-28 4-35 4-46 4-60 4-90
3-50 3-70 3 . 95 4 . 20 4-22 4-40 4-60 4-70 4-90 5 . 00 5-07 5-30
5-00
5-20 5-30 5-60
5-50
The p ecipi a ion p od c i mo e o le pol me i ed, b he p e en me hod can no di ing i h be een monome and pol me . The al e n i , he efo e, ell he a mp ion ha he p ehe m of o al e n' and n", hich fi dominan comple in he ol ion of h d ol ed an l in hi em eq i e one h d o l ion pe one ani m o fo m he an l h d o ide (n' = 1) and 0-28 h d o l ion (e peimen all ob ained n" = 1 . 25- I = 0-25) o fo m "ammoni m hep a ana e" . The li e a e da a[19] and o anal i ha e ho n ha he fi p ecipi a ion p od c a e clo el ela ed o he compo nd ammoni m hepa ana e. The efo e, i i po ible o e al a e he al e of he ol bili p od c con an : log K ,_ = - 14-3 of ammoni m hep a ana e, ing Eq a ion (3). In Fig. 2, he h ee-dimen ional diag am : log concn . NH,OH-log concn. U0 2 (N0 3 ) 2 -pH (T bidi ) i p e en ed. The lope pH - .log.concn . UO., (NO 3 ) 2 a he eq imola a io of eac ing componen a calc la ed f om e pe imen al poin o he abo -1 (-0-956) . Thi de e mine he a e age 2 + OH - / UO 2 a io of he comple in he homogeneo comple ol ion, ppo ing p e io a emen ha ch h d ol ed ani m (VI) comple i in eq ilib i m i h he p ecipi a e. The fi p ecipi a ion i a mode a el fa p oce and ake place nea he po i ion of he fi inflec ion poin of he pH c e, hich af e anding p od ce a d op in he pH . Wi h an inc ea ing Z a io (NH 4 OH/UO 2 (NO 3 ) 2 ) he p ecipi a ion i enhanced. A he po i ion of he econd inflec ion (a Z --- 2-6) of he pH c e, he em i h a concen a ion of ani m lo e han 5 10 -3 M ho he appea ance of pe manen bidi . The bidi appea abo 1 h af e mi ing and i inc ea ed i h ime . One da af e mi ing i i con an , and a able ol-like pen ion occ . S ch a em i ligh l p ecipi a ed (af e I mon h) . In all he bid em he p e ence of nega i el cha ged pa icle a de ec ed b mean of an elec o19 . E . H . P. Co df nke, J. ino g. n cl . Chem . 24,303 (1962) .
1 77 6
B . TOMAZIC, M . SAMARZIJA and M . BRANICA
pH
1
-2 -3 -4 LOG. CONCN . NH 4 OH [M]
Fig. 2 . The h ee-dimen ional p e en a ion of p ecipi a ion em : U02(NO3)2 NH,OH . T bidi and pH mea emen e e pe fo med one mon h af e mi ing ( =25 C) . F ll line in he NH,OH-U02 (N0 3 )2 plane de e mine he bo nda of pe manen bidi ( haded a ea). B oken c e ho he dden i e in bidi ha ing a hei ba e he haded egmen ho ing he e i ence of p ecipi a e.
pho e ic l amic o cope . The p e ence of nega i el cha ged pa icle e plain he colloidal abili of he bid em . B a f he inc ea e in he Z a io he p ecipi a ion of ani m (VI) i dec ea ed, a i can be een f om Fig. 3 . P obabl i i d e o he fo ma ion of ol ble and able anionic an l h d o o comple e of he gene al fo m U3011 (OH),,-In-2), a i ha al ead been p opo ed [9] ., The alkaline ol ion (pH = 9. 5) of 3 10-3 M an l ni a e emained clea p o one ho , ho ing a pec m of h d ol ic anionic comple e [9] . U an l ca bona e comple e e e e cl ded ince i a ho n ha he pec m a no changed af e he addi ion of ca bona e p o a a io of 1 .5 C032-/U022 + . The fo ma ion of ani m (VI) p ecipi a e i dec ea ed b a f he inc ea e of he pH of he ol ion. Thi effec i p ono nced fo a concen a ion of ani m lo e han 5 10-3 M. All he ani m emain in ol ion e en one mon h af e mi ing in he ca e of 1 X 10-4 M an l ni a e ol ion alkal ed o pH = 11 . The follo ing cheme of p ecipi a ion f om dil e ol ion of ani m (V I) b addi ion of ammoni m h d o ide i p opo ed : pH > 7 -n [(U02 (OH)2)3(OH)n] U02+
pH < 7 [U02 (OH)(2_, .)]n+n(2- )
e lo p oce nega i el (U 0 2(OH)2) P cha ged pa icle fa p oce (U02 (OH)2)P
NH4- ana e ( edimen )
(7)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion -V I
p H (1 m o
h )
1777
9
pH
.
0 001 M U0 2 (N0 3 ) 2 25` C
100 a ~, U (24h) M W
6
0 X
4
Z D 2 U (lmon h)
0 -3
Fig. 3 . The p ecipi a ion of
-2
-1
LOG. CONCN . NH 4 0H
[M]
ani m (VI) in he NH } OH . 25 C .
em : 0-001 M UO,(NO, j ),- a .
The chemical compo i ion of i ola ed ammoni m ana e depend on he pH al e of he pe na an ol ion . In Fig . 4 and 5, he compo i ion of ammoni m ana e fo med in 0 . 1 and 0 .01 M an l ni a e ol ion a e ho n. The a io of U/N H 3 a g ad all changed i h an inc ea e in he pH al e. A he po i ion of he econd inflec ion he pH c e ("po en iome ic") of he em anal ed, he compo i ion apidl changed app oaching he al e U/NH 3 = 1, indica ing he fo ma ion of ammoni m di ana e . Thi i no in ag eemen i h ome li e a e da a [19] . I a no po ible o i ola e ana e f om 10-3 M an l ni a e ol ion d e o he lo and incomple e p ecipi a ion . S d ing he po ibili of efficien i ola ion of ammoni m ana e , A . Dep la[20] fo nd ha "an ea il fil e able p ecipi a e of ani m concen a e i ob ained, hen addi ion of ani m concen a e i ob ained, hen addi ion of ammonia i in e p ed fo ca . 100 min a pH 5 . 8-6 . 0, and he mi e i b eq en l alkal ed n il pH 9. 0-9 . 5 i eached" . A lo e pH al e , he po i i e and ne al comple e a e he p edominan pecie [9], and he eadil nde go p ecipi a ion i h he b eq en addi ion of ba e . If he o al amo n of he ba e a q ickl added p o pH = 9, he edimen a ion a a e lo p oce , ha i in good ag eemen i h o da a, and hi can be e plained b he e i ence of nega i el cha ged pa icle . We belie e ha nega i el cha ged colloid pa icle a e fo med a a e l of agg ega ion of pa icle hich bind on hei face he nega i el cha ged comple hich i in a q a i-eq ilib i m a e i h he olid pha e . 20. A. Dep la, N kleonika No 5, 341 (1962) .
1 778
B . TOMA2IC, M . SAMAR2IJA and M . BRANICA
0.1 M U02(N03)2 25 NH3
%U in
ol
ion
U
NH3/U
180
1. 0
10
PH
C
8
0
0
08
pH
06
6
0 .4
0
0.2 -20
4
0/0 U ~G ~Q
0.0
3
Q 4
5
Q 6
7
3
8
10
[NH40H] / [U02(NO3)2
Fig . 4. The p ecipi a ion of ani m (VI) and compo i ion of ana e i ola ed fom he em : 0 . 1 M UO,(NO,), - a . NH,OH, 24 h af e mi ing, 25 C.
10
0 .01 M U 0 2(NO3)2 25 C NH3 U
1.0
U in
ol
pH
I ion
_1DOQ-
NH3 / U 8
08
pH
06 -60
6
04 -40 1
a2 -20
`
Q~ ,Q 0.0
0
%U Q
I
1
2
3
4
5
6
7
8
9
Q 10
INH40H] /[U02(NO3) 2]
Fig. 5 . The p ecipi a ion of ani m (VI) and compo i ion of ana e i ola ed f om he em: 0. 01 M U02(NO3)2- a . NH4OH, 24 h af e mi ing, 25 C .
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol
ion - V I
1779
The em U0 2 (NO3)2 -NH,OH-NH,NO3 p ecipi a ion of The p e ence of ne al elec ol e accele a ed he fi em : 10-3 M UO2 (N03)1- KOH [ 1, 4] . ani m (VI) a al ead ho n in he The p e ence of 0. 2 M NH,NO3 doe no change he condi ion fo he fi p ecipi a ion of ani m (VI) i h ammoni m h d o ide in he b oad concen ap ecipi a ion commence a ion ange of an l ni a e (10 - ' - 10- M) . The fi e 2 in Fig. 1 gi e he lope pH - log. concn. e en iall he ame Z a io . The c U0,(NO3)2 hich a calc la ed b lea q a e me hod in he concen a ion ange f om I 10-' - 2 10 -4 M . Fo he lo e concen a ion of an l ni a e e e em i a no po ible o ob ain con i en ep od cible da a. The eq ilib a ed a 25 C fo I mon h. The e pe imen al da a a e o lined in Table 1 . The al e n a e ima ed o be 1 . 35, he egmen on pH a i being 2 . 75 . The e em i ho he al e a e p ac icall iden ical i h ho e ob ained fo p e ence of ammoni m ni a e . One can concl de ha he p oce of he fi em i h and i ho amp ecipi a ion p oceed in he ame a in he moni m ni a e, e l ing in he ame p od c . Thi e l diffe f om he e l ob ained i h po a i m h d o ide he e he p e ence of ne al elec ol e ca ed he fi p ecipi a ion o commence a a ignifican l lo e KOH/ UO2 (NO3 )2 a io. The ac ion of ca ion a p e io l a c ibed o coag la ion effec [ 11, b la e i a ho n ha he fo med ana e con ained he ca ion of a ne al elec ol e [4] . The efo e, i a concl ded ha ne al elec ol e di pla ed combined coag la ion and con i ion effec . F om he e e l i can be concl ded ha he ammoni m ion f om he ammoni m h d o ide p e en mo n he c i ical p ecipi a ion al e (CPV) in hi em . Ho e e , he infl ence of ne al elec ol e i empha i ed in he alkaline em a i ho n in Fig . 6 . The p ecipi a ion i a lo p oce hich i ma kedl accele a ed b he p e ence of ammoni m ni a e . The p e ence of ammoni m ni a e e l in he q an i a i e p ecipi a ion of ani m, follo ed b he d op in he pH al e d e o i b ffe ing ac ion. The ac ion of ammoni m and ba i m ni a e on he p ecipi aion of ani m f om 10-3 M ol ion i p e en ed in Fig . 7 hich i o be compa ed i h he p ecipi a ion in he em i ho ne al elec ol e. A i co ld be an icipa ed, ba i m ni a e ha he mo e p ono nced effec , fo ming ba i m ana e . Gene all peaking, he ne al elec ol e accele a e he p ecipi a ion p oce e and comple el dimini h he colloidal abili of he died em . gge he follo ing cheme fo p ecipi a ion a ing f om The efo e, e alkaline ol ion : U0 2 2+
[(U02(OH)2)3(OH)"]-" ~ (U02(OH)2)( N ) NH4NO3
nega i el cha ged . NH 4- ana e ( edimen ) . colloidal pa icle
(8)
The chemical compo i ion of ana e i ola ed f om 10-3M an l ni a e and 0. 2 M ammoni m ni a e ol ion i gi en in Fig. 8 . The pH al e did no e ceed 8 i h he addi ion of ammoni m ni a e . Con eq en l , he ammoni m di ana e i no fo med in hi em. The e i ence of o pha e ha ing he mola a io U/NH3 0. 3 and 0. 5 i empha i ed . The X- a pa e n e emble he compo nd
B. TOMAZIC, M . SAMAR21JA and M . BRANICA
1780
3M
3 -10
I U0 2 (N03 ) 2 -4 10 3 M NH 4 OH-
o
P
3
- a . NH,N0 -25 * C
4
h
M~+ " i_ ∎~
10 9 pH
∎
~~24h)
8
014
24 h j 03 J
J J Q Z a1
2h H
0'
o-0 -2
-4
-3
LOG . CONCN . NH 4 N0 3 [M]
Fig. 6. The infl ence of a io concen a ion of NH 4NO3 on he p ecipi a ion of ani m (V I) f om he em: 10-'M U02(NO3)2 -4 10-3 M NH 4OH, 25 C.
100 R..∎
∎∎
0 -J
2 I-
∎∎
∎.
B
4
eo
60
C
,o
%U
pH e
.
C3
40
JPA
11
-
A
20
JAM 0
2 .0
A
M~e
Wed -----
I'T
10
1.5
3M
3
IW-M
6 a a
ho
ol
NH 4 0H e:
i
B.
IIII
,I -U
7
WW. PIP A
al A
Nam 2
L
U0 2 ( N03 )2 + a
+ ne
1-0
0-5
9
T
~I .
0-02M NBa(~N0 3 ) 2 24 h af e mi ing ; 25 . .... . . . . . .
C~ ~
4 3 4 2 2 CONCENTRATION OF NH 4 0H [mM]
Fig. 7 . The infl ence of NH4NO„ and Ba(NO 3 )2 on he p ecipi a ion of f om he em : 10-3 M UO 2(NO3)2 - a . NH4OH, 25 C .
3 ani m (VI)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion - V I
1781
D-001 M U0 2 (N0 3 ) 2 02 M NH 4 N0 3 25 C
NH 3
U
% U in
ol
ion
1009
pH
1 .0
08
80-
0. 6
60-
pH
-
6 0
04 -+40-
-
NH3/U 0
i/ ..
0.2 --20
-
0-0
1
2
3
4
5
6
7
10
[NH 4 0H] / [U0 2 (N0 3 ) 2 } Fig. 8. The p ecipi a ion of ani m (V I) and compo i ion of ana e i ola ed fom he em : 10 -' M U0 2 (N0,1 )2 - a . NH 4 0H-0 . 2 M NH,N0 3 , 24 h af e mi ing, 25 C .
11 (3U0 3 N H 3 . 5H2 0) and III (2U0 3 N H 3 . 3H2 0) de c ibed b Co df nke[19] and Debe and Loop a[12], hich a e in good ag eemen i h o da a. em 10-3 M an l ni a e -1 . 5 X 10' M Mo pholog of p ecipi a e . The ammoni m h d o ide a allo ed o eq ilib a e o e o mon h a oom em a fi nifo ml bid, b af e anding fo app o iempe a e. The ma el one mon h he e appea ed h ee ha pl defined la e . A he bo om an oil-like p ecipi a e a fo med. The o ppe la e e e bid pen ion e e anal ed b con aining nega i el cha ged pa icle . The e h ee la e mean of a io mic o copic echniq e and X- a diff ac ion . In he ligh mic o cope (da k field) i a ob e ed ha he oil-like p ecipi a e a compo ed of in pa icle in B o nian mo emen . A fe la ge pa icle , p obabl c al , e e al o ob e ed . In he elec on mic o cope hi ma e ial a ho n o be compo ed of hin i odiame ic pla e , 0 . 1-0 . 5 µm dia., ho ing a di inc diff acion. La ge c al ( e e al µm dia.) e e al o p e en (Fig . 9a) . The p ecipi a e a cen if ga ed and e en da la e i a een in he ligh mic o cope (da k field) ha he n mbe of la ge c al inc ea ed hile he concen a ion of in pa icle in B o nian mo emen a m ch lo e . The clo e con ac of in pa icle af e cen if ga ion eem o ca e he p ono nced c alli a ion o agglome a ion . The ppe and he lo e bid pha e e e compo ed onl of in i odiame ic pla e , 0. 1-0. 5µm in diame e (Fig. 9b and 9c) ho ing he p ono nced diff ac ion. So fa he ea on fo he epa a ion of he o liq id la e i no clea . The oil-like p ecipi a e, he lo e and he ppe pha e e e cen if ga ed, and an X- a anal i a pe fo med. The e l e e in ag eemen i h
1 782
B. TOMAZIC, M . SAMARZIJA and M . BRANICA
he mic o copic da a a all he h ee ample in e iga ed e e c alline . The X- a pa en e e iden ical fo all he h ee ample and clo el ela ed o he de cibed pa e n of he compo nd IV (2NH3. 3UO 3. 4H2 O) [ l2] . CONCLUSION
The fo ma ion of he ani m (VI) p ecipi a e f om he ol ion con aining ammoni m h d o ide i e m ch infl enced b he a ing Z a io of p ecipi aing componen . Thi i pa ic la l empha i ed fo he alkaline ol ion con aining anionic an l h d o o comple e . Fo he alkaline ol ion of ani m (VI) a concen a ion lo e han 5 X10-3 M, he appea ance of able " ol-like" pen ion con aining nega i el cha ged pa icle i de ec ed. The p e ence of ammoni m ni a e a ne al elec ol e make i po ible o p ecipia e all he ani m (VI) f om dil e ol ion e en f om highl ammoniacal ol ion . In he p e ence of ammoni m ni a e he p ecipi a e a e i ola ed a lo e pH al e , hich e l in lo e ammonia con en . The ma imal NH , /U i h NH,NO3 p e en did no e ceed a io in p ecipi a e i ola ed f om em 0 . 5 . The ol-like pen ion con ain he in pa icle 0 . 1-0. 5µm in diame e , alini of ho ing he p ono nced diff ac ion. X- a anal i confi med he c he e pa icle , he pa e n e embling he ana e i ola ed a high pH al e . Ackno ledgemen -The elec on mic og aph
cop , "R de Bo ko ic" In i
e.
e e p epa ed in Labo a o
fo Elec on Mic o-
Fig. 9. The elec on mic og aph of pa icle fo med in he em : 10 -3 M U02(N03)2 --2 . 5 10 I M NH4 OH, 2 mon h af e mi ing . Room empe a e : (a) oil-like p ecipi a e: (b) pen ion, lo e la e ; (c) pen ion, ppe la e , paladi m hado ed ample .
1782
Printed in Great Britain
PRECIPITATION AND HYDROLYSIS OF URANIUM (VI) IN AQUEOUS SOLUTIONS-VI INVESTIGATION ON THE PRECIPITATION OF AMMONIUM URANATES B . TOMAZIC, M . SAMARZIJA and M . BRANICA Department of Physical Chemistry, "Ruder Boskovic" Institute, Zagreb, Croatia, Yugoslavia (First received 29 July 1968 ; in revisedform 23 October 1968)
Abstract-The precipitation boundary has been studied in the aqueous system : U02 (NO3)2 - N H,OH -NH,NO 3. Assuming that the first precipitation product is ammonium heptauranate, the solubility product constant has been estimated to be log K,, .25 = - 14 . 3 . A mechanism of the formation of ammonium uranates from alkaline solutions was proposed assuming the presence of anionic uranyl hydroxo complexes . The morphology of precipitates isolated from alkaline solutions is described . INTRODUCTION THE PROBLEMS of precipitation and hydrolysis of uranium (V I) in different aque-
ous systems were extensively studied in this laboratory [ 1-4] . In spite of the great variety of observed phenomena, which can be attributed to the fairly complex behavior of the uranyl ion, there are some common phenomena which can be ascribed to the unique complexation reactions of the uranyl ion . The idea of complexation of the uranyl ion (Werner type complexes) was developed by Chernyaev et al. [5, 6] . The hydrolysis reaction is presented as a substitution of hydroxyl ions for water molecules in the complex sphere of the hydrated uranyl ion resulting in the formation of a genetic series of uranyl hydroxo complexes . Consequently, one uranyl ion can bind six hydroxyl ions . The experimental verification for only the first few members of this series formed in the system : UO22+-MOH (M = NH4 +, K+, Na+) was found[6] . The maximum ratio OH - : U02 2+ obtained was 3, and the formed complex Me[UO 2 (OH)3 (H 2O),t] corresponds to the well known metal diuranate . Chernyaev neglected the concept of polynuclear complex formation, which had been introduced by the Swedish school and developed by the mathematical treatment of potentiometric data [7] . The "core +link" theory introduced by Sillen [8] has shown that the process of polymerization results in the formation of an unlimited series of complexes with a general formula [UO 2(UO2(OH)2)n] 2 + Unfortunately, very few attempts have been made so far to present physical evidence of the polymerization of the 1 . B . Tomazlc, M . Branica and B . Teiak, Croat. Chem. Acta 34, 41 (1962). 2 . H . Fiiredi and B. Teiak, Croat. Chem. Acta 36, 119 (1964) . 3 . H . Fdredi . Croat. Chem . Acta 195 36, (1964). 4. B . Tomazic and M . Branica, Croat. Chem . Acta 38, 249 (1966) . 5 . 1 . 1. Chernyaev, V . A . Golovnya and G . V . Ellert, Zh. neorg . Khim . 5, 1481 (1960) . 6. I . 1 . Chernyaev, Kompleksnye soedineniya urana . Nauka, Moscow (1964) . 7 . A . Arhland, S . Hietanen and L . G . Sillen,Acta chem . scand. 8, 1907 (1954) . 8 . L. G . Sillen and S . Hietanen, Acta chem . scand . 8, 1607 (1954) . 1771
1772
B . TOMA2IC, M . SAMAR2IJA and M . BRANICA
uranyl ion . From cryoscopic measurements Sutton has shown that polymerization exists [9] . Ultracentrifugation measurements have shown that polymerization takes place up to three nuclear species[ 10] . The hydrolysis of the uranyl ion results in the formation of positively charged polynuclear complexes . The formation constants for these complexes have been evaluated by many authors [7, 9, 10] . From spectrophotometric measurements, Sutton has proposed the formation of negatively charged complexes of the form U308(OH),,-(n-2' and estimated the corresponding formation constants [9]. The existence of such species in alkaline solutions of the uranyl ion has also been confirmed by conductometric [9] and polarographic [ 11 ] measurements . It has been shown previously that the progressive alkalization in the system : U02(NO3)2-KOH leads to the formation of negatively charged colloid particles, which could be "coagulated" by means of various "neutral electrolytes" [1, 4] . In the present paper an attempt has been made to show the dependence of ammonium uranate composition on the starting concentration of reacting components . Special attention has been paid to the first appearance of the precipitate (precipitation boundary) in the broad concentration range. The morphology of some precipitates was studied by use of light microscopy, electron microscopy and X-ray powder pattern, and a comparison with the published data [ 12] is given . The mechanisms of hydrolyzation and of precipitation of uranium (VI) with ammonia are discussed . EXPERIMENTAL Materials . All solutions were prepared from reagent grade chemicals, dissolved in twice distilled water. The concentration of uranium was determined gravimetrically as U 308 [13] . The concentration of ammonium hydroxide was determined by volumetric titration . The concentration of ammonium nitrate was determined by the ion exchange method [ 14] . Systems . All systems for the determination of precipitation boundary were prepared by mixing equal volumes of the precipitating components . The solution of ammonium hydroxide was added to the mixture of uranyl nitrate and ammonium nitrate, the mixing was repeated 7 times in 20 sec . The final volumes were 10 ml . After the mixing the tubes were closed with rubber stoppers and placed in the water bath at 25°C for a given period . For the determination of the composition of ammonium uranates systems of 50 ml of 500 ml were prepared . A solution of ammonium hydroxide was added to the mixture of uranyl nitrate and ammonium nitrate while the solution was vigorously mixed with a vibration mixer . These systems were kept one month in the water bath at 25°C in order to attain equilibrium . Analysis method . The separation of precipitates from mother liquors was performed by repeated centrifugation at 6000 RPM . In order to prevent pronounced hydrolysis and peptization the precipitates were not washed . An exception was made with precipitates obtained from 10 -3 M uranium (V I) solutions, which were washed with 10 -3 M ammonium nitrate solution adjusted to the pH of the mother liquor solution . Uranium was analyzed polarographically[15] and gravimetrically as U 308, [ 13] . The ammonia content was determined by the Kjeldahl method . Apparatus . The turbidity measurements were performed with a Zeiss tyndallometer joined to a Pulfrich photometer supplied with a green filter (wave length 530 nm) . The pH measurements were 9 . 1 . J . Sutton, J . chem. Soc . S. 1 . No. 2, S 275 (1949) . 10 . R. M. Rush, J . S. Johnson and K . A. Kraus, lnorg. Chem. 1, 378 (1962) . 11 . EURAEC-876, Quart. Rep. No . 12, July 1-September 30 (1963) . 12 . P. C. Debets and B . 0 . Loopstra, J. inorg . nucl. Chem. 25, 945 (1963) . 13 . C. J. Rodden, Analytical Chemistry of the Manhattan Project . McGraw-Hill, New York (1950) . 14 . 0. Samuelson, Ion Exchangers in Analytical Chemistry . Wiley, New York (1950) . 15 . V. Vouk, M . Branica and 0 . Weber, Ark . Kemi. 25, 225 (1953) .
Precipitation and hydrolysis of uranium (V I) in aqueous solutions- V I
1773
performed with a standard glass/calomel electrode pair connected to a PYE Master pH meter . Polarographic measurements were performed with a Cambridge Instrument Pen Recording polarograph . The precipitates were examined by X-ray diffraction using a Philips diffractometer PW 1050/PW 1051 with a recorder . Microscopic equipment from E . Reichert was used for dark field microscopic observations of precipitates . Electron micrographs were performed using an electrostatic electron microscope, Model KM4 from Trub Tauber and Co . Spectra were performed with a Perkin-Elmer Mod . 137 UV spectrophotometer . RESULTS AND DISCUSSION
The system
UO 2 (NO 3 ) 2
-NH 4 OH
The precipitation of uranium with ammonium hydroxide starts at a certain ratio Z = NH 40H/UO2 (NO 3)2 which is dependent on the initial concentration of uranyl nitrate . The first precipitation point was detected by turbidity measurements . The first precipitation as a rule was accompanied by a sudden drop in the pH value . This effect was reported previously in similar systems [4] and also shown by other authors [16, 17] . Several reactions can account for this effect, but most probably this is due to the slow formation of uranates at the given conditions . Uranyl hydroxide is probably formed as a metastable species which then reacts with a cation . This is shown schematically as : UO 2 (OH)2 ₂m H .,O+n NH 4 +
UO2 (OH)2 _ n (ONH 4 )„+n H+ .
(1)
The pH d op can be impl a ocia ed o he e en of he p ecipi a ion of ani m. The elea ed p o on e en iall change he ol ion compo i ion e l ing in a e lo a ainmen of he p ecipi a ion eq ilib i m . In Fig . I pH al e of he fi p ecipi a ion em a a f nc ion of he ini ial concen aion of an l ni a e a e p e en ed. C e 4 define he end of Z a io a hich he fi p ecipi a ion a ob e ed. Thi c e emain nchanged af e aging fo ei he 24 h o 30 da . On he o he hand, c e 3 ho ing he end of pH again an l ni a e concen a ion in p ecipi a ed em fo eq ilib a ion pe iod of 24 h ignifican l diffe f om c e 1 gi en fo em 30 da af e mi ing. Thi ho ha he he e ogeneo eq ilib i m i eached a a imecon ming p oce . In Table I he e pe imen al da a de c ibing c e I and 2 a e gi en . The e pe imen al poin of pH . log . concen a ion of an l ni a e a eq ilib i m can be fi ed b a linea eq a ion of he gene al fo m : pH = - ]In log UO2(OH~
n)
+
I /n
log KI(2 _n) -log K,,,
(2)
he e n de e mine he a e age n mbe of OH - ion pe one ani m a om eq i ed fo he fo ma ion of olid pha e of ani m (VI) f om aq eo ol ion . The egmen C2 _ n on he pH a i con aining he al e of he ol bili p od c i defined [181 a : C2_„ -
I
In
log K,(2-,,)-Iog K . .
16. V . Ba an and M . T mpl,J. ino g . n cl . Chem . 28,89 (1966) . 17 . A . Dep la, N kleonika No 4, 265 (1962) . 18 . H . Bilin ki, H . Fa edi, M . B anica and B . Te ak, C oa . Chem . Ac a 35, 19 (1963) .
(3)
1 774
B. TOMA2IC, M. SAMARZIJA and M . BRANICA
I 1,2 30 da 3,6 _26 25
ho
af e
mi ing
af e
mi ing
,
C
PRECIPITATE
pH
4
-h
, 3
-3
i'
CLEAR SOLUTION
X
.
Old I PRECI
ITATE
,a[Il"1
1
-2
-3
-4
LOG. CONCN. U02(N03)2 [M] Fig. 1 . The p ecipi a ion bo nda
of he
em: UO2 (NO 3 ) 2 -NH,OH-NH 4 NO3 .
U ing he lea q a e me hod, he lope a de e mined a -0 . 8 hich lead o n = 1 . 25, and ha of C2-,,= 2 . 55. Th , one can concl de ha onl 1 . 25 OH- pe one ani m i nece a o fo m he p ecipi a e . The fo ma ion of an l h d o ide f om a ol ion of nh d ol ed an l ion o ld eq i e o h d o l ion pe one an l ion . Ho e e , he fo ma ion of ammoni m hepa ana e heo e icall e l in he con mp ion of 2 . 28 h d o l ion fo one an l ion, a ho n b a implified eac ion cheme : 7 UO2 2 ++ 16 OH - + 2 NH4 + = (NH4)2U7022+8 H2O The al e fo n = 1 . 25 can be in e p e ed in e m of o con ec i e eac ion aking place in he ol ion of p eh d ol ed an l ion leading o he fo ma ion of ani m (VI) p ecipi a e : UO2(OH)+"' +n' OH - ± UO2(OH)2 UO2(OH)2+n"NH4 OH -1 UO2 (OH)2
_
A
ONH4)„~+n"/2 H 2O .
(5) (6)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion - V I
1775
Table I E peimen al da a of he p ecipi a ion bo nda in he em : U02 (N0 3)2 -NH4OH-0 .2 M NH4NO3 . Eq ilib a ed fo one mon h a 25 C
U02 (NO3 )2 M 0-1 0-05
0-02 0.01 0-008 0-005 0-003 0.002 0-001 0.0008 0.0005 0-0003 0-0002
pH in ab ence of NH4 NO3
pH in p e ence of 0-2 M NH 4NO3
3-40 3-65 3. 95 4-12 4-28 4-35 4-46 4-60 4-90
3-50 3-70 3 . 95 4 . 20 4-22 4-40 4-60 4-70 4-90 5 . 00 5-07 5-30
5-00
5-20 5-30 5-60
5-50
The p ecipi a ion p od c i mo e o le pol me i ed, b he p e en me hod can no di ing i h be een monome and pol me . The al e n i , he efo e, ell he a mp ion ha he p ehe m of o al e n' and n", hich fi dominan comple in he ol ion of h d ol ed an l in hi em eq i e one h d o l ion pe one ani m o fo m he an l h d o ide (n' = 1) and 0-28 h d o l ion (e peimen all ob ained n" = 1 . 25- I = 0-25) o fo m "ammoni m hep a ana e" . The li e a e da a[19] and o anal i ha e ho n ha he fi p ecipi a ion p od c a e clo el ela ed o he compo nd ammoni m hepa ana e. The efo e, i i po ible o e al a e he al e of he ol bili p od c con an : log K ,_ = - 14-3 of ammoni m hep a ana e, ing Eq a ion (3). In Fig. 2, he h ee-dimen ional diag am : log concn . NH,OH-log concn. U0 2 (N0 3 ) 2 -pH (T bidi ) i p e en ed. The lope pH - .log.concn . UO., (NO 3 ) 2 a he eq imola a io of eac ing componen a calc la ed f om e pe imen al poin o he abo -1 (-0-956) . Thi de e mine he a e age 2 + OH - / UO 2 a io of he comple in he homogeneo comple ol ion, ppo ing p e io a emen ha ch h d ol ed ani m (VI) comple i in eq ilib i m i h he p ecipi a e. The fi p ecipi a ion i a mode a el fa p oce and ake place nea he po i ion of he fi inflec ion poin of he pH c e, hich af e anding p od ce a d op in he pH . Wi h an inc ea ing Z a io (NH 4 OH/UO 2 (NO 3 ) 2 ) he p ecipi a ion i enhanced. A he po i ion of he econd inflec ion (a Z --- 2-6) of he pH c e, he em i h a concen a ion of ani m lo e han 5 10 -3 M ho he appea ance of pe manen bidi . The bidi appea abo 1 h af e mi ing and i inc ea ed i h ime . One da af e mi ing i i con an , and a able ol-like pen ion occ . S ch a em i ligh l p ecipi a ed (af e I mon h) . In all he bid em he p e ence of nega i el cha ged pa icle a de ec ed b mean of an elec o19 . E . H . P. Co df nke, J. ino g. n cl . Chem . 24,303 (1962) .
1 77 6
B . TOMAZIC, M . SAMARZIJA and M . BRANICA
pH
1
-2 -3 -4 LOG. CONCN . NH 4 OH [M]
Fig. 2 . The h ee-dimen ional p e en a ion of p ecipi a ion em : U02(NO3)2 NH,OH . T bidi and pH mea emen e e pe fo med one mon h af e mi ing ( =25 C) . F ll line in he NH,OH-U02 (N0 3 )2 plane de e mine he bo nda of pe manen bidi ( haded a ea). B oken c e ho he dden i e in bidi ha ing a hei ba e he haded egmen ho ing he e i ence of p ecipi a e.
pho e ic l amic o cope . The p e ence of nega i el cha ged pa icle e plain he colloidal abili of he bid em . B a f he inc ea e in he Z a io he p ecipi a ion of ani m (VI) i dec ea ed, a i can be een f om Fig. 3 . P obabl i i d e o he fo ma ion of ol ble and able anionic an l h d o o comple e of he gene al fo m U3011 (OH),,-In-2), a i ha al ead been p opo ed [9] ., The alkaline ol ion (pH = 9. 5) of 3 10-3 M an l ni a e emained clea p o one ho , ho ing a pec m of h d ol ic anionic comple e [9] . U an l ca bona e comple e e e e cl ded ince i a ho n ha he pec m a no changed af e he addi ion of ca bona e p o a a io of 1 .5 C032-/U022 + . The fo ma ion of ani m (VI) p ecipi a e i dec ea ed b a f he inc ea e of he pH of he ol ion. Thi effec i p ono nced fo a concen a ion of ani m lo e han 5 10-3 M. All he ani m emain in ol ion e en one mon h af e mi ing in he ca e of 1 X 10-4 M an l ni a e ol ion alkal ed o pH = 11 . The follo ing cheme of p ecipi a ion f om dil e ol ion of ani m (V I) b addi ion of ammoni m h d o ide i p opo ed : pH > 7 -n [(U02 (OH)2)3(OH)n] U02+
pH < 7 [U02 (OH)(2_, .)]n+n(2- )
e lo p oce nega i el (U 0 2(OH)2) P cha ged pa icle fa p oce (U02 (OH)2)P
NH4- ana e ( edimen )
(7)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion -V I
p H (1 m o
h )
1777
9
pH
.
0 001 M U0 2 (N0 3 ) 2 25` C
100 a ~, U (24h) M W
6
0 X
4
Z D 2 U (lmon h)
0 -3
Fig. 3 . The p ecipi a ion of
-2
-1
LOG. CONCN . NH 4 0H
[M]
ani m (VI) in he NH } OH . 25 C .
em : 0-001 M UO,(NO, j ),- a .
The chemical compo i ion of i ola ed ammoni m ana e depend on he pH al e of he pe na an ol ion . In Fig . 4 and 5, he compo i ion of ammoni m ana e fo med in 0 . 1 and 0 .01 M an l ni a e ol ion a e ho n. The a io of U/N H 3 a g ad all changed i h an inc ea e in he pH al e. A he po i ion of he econd inflec ion he pH c e ("po en iome ic") of he em anal ed, he compo i ion apidl changed app oaching he al e U/NH 3 = 1, indica ing he fo ma ion of ammoni m di ana e . Thi i no in ag eemen i h ome li e a e da a [19] . I a no po ible o i ola e ana e f om 10-3 M an l ni a e ol ion d e o he lo and incomple e p ecipi a ion . S d ing he po ibili of efficien i ola ion of ammoni m ana e , A . Dep la[20] fo nd ha "an ea il fil e able p ecipi a e of ani m concen a e i ob ained, hen addi ion of ani m concen a e i ob ained, hen addi ion of ammonia i in e p ed fo ca . 100 min a pH 5 . 8-6 . 0, and he mi e i b eq en l alkal ed n il pH 9. 0-9 . 5 i eached" . A lo e pH al e , he po i i e and ne al comple e a e he p edominan pecie [9], and he eadil nde go p ecipi a ion i h he b eq en addi ion of ba e . If he o al amo n of he ba e a q ickl added p o pH = 9, he edimen a ion a a e lo p oce , ha i in good ag eemen i h o da a, and hi can be e plained b he e i ence of nega i el cha ged pa icle . We belie e ha nega i el cha ged colloid pa icle a e fo med a a e l of agg ega ion of pa icle hich bind on hei face he nega i el cha ged comple hich i in a q a i-eq ilib i m a e i h he olid pha e . 20. A. Dep la, N kleonika No 5, 341 (1962) .
1 778
B . TOMA2IC, M . SAMAR2IJA and M . BRANICA
0.1 M U02(N03)2 25 NH3
%U in
ol
ion
U
NH3/U
180
1. 0
10
PH
C
8
0
0
08
pH
06
6
0 .4
0
0.2 -20
4
0/0 U ~G ~Q
0.0
3
Q 4
5
Q 6
7
3
8
10
[NH40H] / [U02(NO3)2
Fig . 4. The p ecipi a ion of ani m (VI) and compo i ion of ana e i ola ed fom he em : 0 . 1 M UO,(NO,), - a . NH,OH, 24 h af e mi ing, 25 C.
10
0 .01 M U 0 2(NO3)2 25 C NH3 U
1.0
U in
ol
pH
I ion
_1DOQ-
NH3 / U 8
08
pH
06 -60
6
04 -40 1
a2 -20
`
Q~ ,Q 0.0
0
%U Q
I
1
2
3
4
5
6
7
8
9
Q 10
INH40H] /[U02(NO3) 2]
Fig. 5 . The p ecipi a ion of ani m (VI) and compo i ion of ana e i ola ed f om he em: 0. 01 M U02(NO3)2- a . NH4OH, 24 h af e mi ing, 25 C .
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol
ion - V I
1779
The em U0 2 (NO3)2 -NH,OH-NH,NO3 p ecipi a ion of The p e ence of ne al elec ol e accele a ed he fi em : 10-3 M UO2 (N03)1- KOH [ 1, 4] . ani m (VI) a al ead ho n in he The p e ence of 0. 2 M NH,NO3 doe no change he condi ion fo he fi p ecipi a ion of ani m (VI) i h ammoni m h d o ide in he b oad concen ap ecipi a ion commence a ion ange of an l ni a e (10 - ' - 10- M) . The fi e 2 in Fig. 1 gi e he lope pH - log. concn. e en iall he ame Z a io . The c U0,(NO3)2 hich a calc la ed b lea q a e me hod in he concen a ion ange f om I 10-' - 2 10 -4 M . Fo he lo e concen a ion of an l ni a e e e em i a no po ible o ob ain con i en ep od cible da a. The eq ilib a ed a 25 C fo I mon h. The e pe imen al da a a e o lined in Table 1 . The al e n a e ima ed o be 1 . 35, he egmen on pH a i being 2 . 75 . The e em i ho he al e a e p ac icall iden ical i h ho e ob ained fo p e ence of ammoni m ni a e . One can concl de ha he p oce of he fi em i h and i ho amp ecipi a ion p oceed in he ame a in he moni m ni a e, e l ing in he ame p od c . Thi e l diffe f om he e l ob ained i h po a i m h d o ide he e he p e ence of ne al elec ol e ca ed he fi p ecipi a ion o commence a a ignifican l lo e KOH/ UO2 (NO3 )2 a io. The ac ion of ca ion a p e io l a c ibed o coag la ion effec [ 11, b la e i a ho n ha he fo med ana e con ained he ca ion of a ne al elec ol e [4] . The efo e, i a concl ded ha ne al elec ol e di pla ed combined coag la ion and con i ion effec . F om he e e l i can be concl ded ha he ammoni m ion f om he ammoni m h d o ide p e en mo n he c i ical p ecipi a ion al e (CPV) in hi em . Ho e e , he infl ence of ne al elec ol e i empha i ed in he alkaline em a i ho n in Fig . 6 . The p ecipi a ion i a lo p oce hich i ma kedl accele a ed b he p e ence of ammoni m ni a e . The p e ence of ammoni m ni a e e l in he q an i a i e p ecipi a ion of ani m, follo ed b he d op in he pH al e d e o i b ffe ing ac ion. The ac ion of ammoni m and ba i m ni a e on he p ecipi aion of ani m f om 10-3 M ol ion i p e en ed in Fig . 7 hich i o be compa ed i h he p ecipi a ion in he em i ho ne al elec ol e. A i co ld be an icipa ed, ba i m ni a e ha he mo e p ono nced effec , fo ming ba i m ana e . Gene all peaking, he ne al elec ol e accele a e he p ecipi a ion p oce e and comple el dimini h he colloidal abili of he died em . gge he follo ing cheme fo p ecipi a ion a ing f om The efo e, e alkaline ol ion : U0 2 2+
[(U02(OH)2)3(OH)"]-" ~ (U02(OH)2)( N ) NH4NO3
nega i el cha ged . NH 4- ana e ( edimen ) . colloidal pa icle
(8)
The chemical compo i ion of ana e i ola ed f om 10-3M an l ni a e and 0. 2 M ammoni m ni a e ol ion i gi en in Fig. 8 . The pH al e did no e ceed 8 i h he addi ion of ammoni m ni a e . Con eq en l , he ammoni m di ana e i no fo med in hi em. The e i ence of o pha e ha ing he mola a io U/NH3 0. 3 and 0. 5 i empha i ed . The X- a pa e n e emble he compo nd
B. TOMAZIC, M . SAMAR21JA and M . BRANICA
1780
3M
3 -10
I U0 2 (N03 ) 2 -4 10 3 M NH 4 OH-
o
P
3
- a . NH,N0 -25 * C
4
h
M~+ " i_ ∎~
10 9 pH
∎
~~24h)
8
014
24 h j 03 J
J J Q Z a1
2h H
0'
o-0 -2
-4
-3
LOG . CONCN . NH 4 N0 3 [M]
Fig. 6. The infl ence of a io concen a ion of NH 4NO3 on he p ecipi a ion of ani m (V I) f om he em: 10-'M U02(NO3)2 -4 10-3 M NH 4OH, 25 C.
100 R..∎
∎∎
0 -J
2 I-
∎∎
∎.
B
4
eo
60
C
,o
%U
pH e
.
C3
40
JPA
11
-
A
20
JAM 0
2 .0
A
M~e
Wed -----
I'T
10
1.5
3M
3
IW-M
6 a a
ho
ol
NH 4 0H e:
i
B.
IIII
,I -U
7
WW. PIP A
al A
Nam 2
L
U0 2 ( N03 )2 + a
+ ne
1-0
0-5
9
T
~I .
0-02M NBa(~N0 3 ) 2 24 h af e mi ing ; 25 . .... . . . . . .
C~ ~
4 3 4 2 2 CONCENTRATION OF NH 4 0H [mM]
Fig. 7 . The infl ence of NH4NO„ and Ba(NO 3 )2 on he p ecipi a ion of f om he em : 10-3 M UO 2(NO3)2 - a . NH4OH, 25 C .
3 ani m (VI)
P ecipi a ion and h d ol
i of
ani m (V I) in aq eo
ol ion - V I
1781
D-001 M U0 2 (N0 3 ) 2 02 M NH 4 N0 3 25 C
NH 3
U
% U in
ol
ion
1009
pH
1 .0
08
80-
0. 6
60-
pH
-
6 0
04 -+40-
-
NH3/U 0
i/ ..
0.2 --20
-
0-0
1
2
3
4
5
6
7
10
[NH 4 0H] / [U0 2 (N0 3 ) 2 } Fig. 8. The p ecipi a ion of ani m (V I) and compo i ion of ana e i ola ed fom he em : 10 -' M U0 2 (N0,1 )2 - a . NH 4 0H-0 . 2 M NH,N0 3 , 24 h af e mi ing, 25 C .
11 (3U0 3 N H 3 . 5H2 0) and III (2U0 3 N H 3 . 3H2 0) de c ibed b Co df nke[19] and Debe and Loop a[12], hich a e in good ag eemen i h o da a. em 10-3 M an l ni a e -1 . 5 X 10' M Mo pholog of p ecipi a e . The ammoni m h d o ide a allo ed o eq ilib a e o e o mon h a oom em a fi nifo ml bid, b af e anding fo app o iempe a e. The ma el one mon h he e appea ed h ee ha pl defined la e . A he bo om an oil-like p ecipi a e a fo med. The o ppe la e e e bid pen ion e e anal ed b con aining nega i el cha ged pa icle . The e h ee la e mean of a io mic o copic echniq e and X- a diff ac ion . In he ligh mic o cope (da k field) i a ob e ed ha he oil-like p ecipi a e a compo ed of in pa icle in B o nian mo emen . A fe la ge pa icle , p obabl c al , e e al o ob e ed . In he elec on mic o cope hi ma e ial a ho n o be compo ed of hin i odiame ic pla e , 0 . 1-0 . 5 µm dia., ho ing a di inc diff acion. La ge c al ( e e al µm dia.) e e al o p e en (Fig . 9a) . The p ecipi a e a cen if ga ed and e en da la e i a een in he ligh mic o cope (da k field) ha he n mbe of la ge c al inc ea ed hile he concen a ion of in pa icle in B o nian mo emen a m ch lo e . The clo e con ac of in pa icle af e cen if ga ion eem o ca e he p ono nced c alli a ion o agglome a ion . The ppe and he lo e bid pha e e e compo ed onl of in i odiame ic pla e , 0. 1-0. 5µm in diame e (Fig. 9b and 9c) ho ing he p ono nced diff ac ion. So fa he ea on fo he epa a ion of he o liq id la e i no clea . The oil-like p ecipi a e, he lo e and he ppe pha e e e cen if ga ed, and an X- a anal i a pe fo med. The e l e e in ag eemen i h
1 782
B. TOMAZIC, M . SAMARZIJA and M . BRANICA
he mic o copic da a a all he h ee ample in e iga ed e e c alline . The X- a pa en e e iden ical fo all he h ee ample and clo el ela ed o he de cibed pa e n of he compo nd IV (2NH3. 3UO 3. 4H2 O) [ l2] . CONCLUSION
The fo ma ion of he ani m (VI) p ecipi a e f om he ol ion con aining ammoni m h d o ide i e m ch infl enced b he a ing Z a io of p ecipi aing componen . Thi i pa ic la l empha i ed fo he alkaline ol ion con aining anionic an l h d o o comple e . Fo he alkaline ol ion of ani m (VI) a concen a ion lo e han 5 X10-3 M, he appea ance of able " ol-like" pen ion con aining nega i el cha ged pa icle i de ec ed. The p e ence of ammoni m ni a e a ne al elec ol e make i po ible o p ecipia e all he ani m (VI) f om dil e ol ion e en f om highl ammoniacal ol ion . In he p e ence of ammoni m ni a e he p ecipi a e a e i ola ed a lo e pH al e , hich e l in lo e ammonia con en . The ma imal NH , /U i h NH,NO3 p e en did no e ceed a io in p ecipi a e i ola ed f om em 0 . 5 . The ol-like pen ion con ain he in pa icle 0 . 1-0. 5µm in diame e , alini of ho ing he p ono nced diff ac ion. X- a anal i confi med he c he e pa icle , he pa e n e embling he ana e i ola ed a high pH al e . Ackno ledgemen -The elec on mic og aph
cop , "R de Bo ko ic" In i
e.
e e p epa ed in Labo a o
fo Elec on Mic o-
Fig. 9. The elec on mic og aph of pa icle fo med in he em : 10 -3 M U02(N03)2 --2 . 5 10 I M NH4 OH, 2 mon h af e mi ing . Room empe a e : (a) oil-like p ecipi a e: (b) pen ion, lo e la e ; (c) pen ion, ppe la e , paladi m hado ed ample .
1782