A study of the mechanism of the adsorption on columns of salts The sorption of europium on cadmium oxalate*

Adsorption chromatography has been little stutlicd as a technique for tile

separation of metal cations since xqss. The cliscovcry of the ion-csclx~iigc properties

and tlic selectivity of inorganic materials, such as zirconiuni phosplintc or zirconium

aside, IILLS, however, c:~uscd n revival of interest in tllis type of stationnqV l)lln~ic

and 11as led scvcral authors to invcstigatc typical adsorption c:llrc,matogrnl,Ilic tccliniqucs. Most of tlic t-went stutlics of sin-iplc salts as column niatcri;lls 11nw Iwx ccmwrnctl with tlx tlcvclopiiicnt of sclectivc r;itliochcl~~ical separations.

In tliis conrcst tlie rescarcl~ cffcctccl by GII~AI<l)I1 slioulcl lx! mcnti~mccl. ‘I’liis autlior 11~12s csaniinctl s_ystcniatimlly tlic soq>tioii cliaractcristics of columns of sinlplc salts and has cornl~lctcd stutlics of, among others, cxlmiuni sulfide ant1 c’oppel

sulf idc, c-oppcr chloride ant1 lend fluoriclc ; w\*cral intcrcstilig sqxirntic.ms arc possilAc. Otljer esa~nplcs arc tlie sorption of Iinlidcs sucli a5 ioclitlc, on silver broniitlc! as stuclictl

by ‘Ec I< FLr\ I< 11’1’ ct d. 2, and tllc sclxm~tion of ,2iiiCric~iiilii(\‘l) frolii (*uriuni(III) on c;ilciuiii fluoride 3s csaniinccl hy I-Ior_c:oaI 1i3. ‘I’lrc sorption of scvc~al tiictal cations on lllctill sulfides Ilas been studied carefully l>>r I’ZfII_l,ll~S ASI) I<I<:\IJS.L, \\yllc> c3Xl~lUClCXl

tll;it tllc cscliangc rcac’tioiis Ix2twecii tlic! rilctril ions in tlic stxtion;u-j* I~llasc! zulcl mobilc~ pl~asc occur througl~ inctatlicscs sucli as:

M,S+M2 ---t M~S+M, (I)

wllcrc Ml ancl l&I1 represent cations ancl II bar rcprcscnts tllc stationmy ~>l~asc. In an investigation of tlic possibility of using osalxtcs i1S tile stationary ]>llilSC

for radiochcinical column separations, tllc mechanism of tlie sorption on suc11

materials, and especially tile sorption of curopium on cadmium osalnte were in-

vestigated.

Cadmium osalatc was prcparecl in 30-50-g batcllcs by precipitation of a :jr:,{,

osalic acid solution with a stoichiomctric cluantity of tlie metal ion (“stoicliio-

metric” osalate) or with a xon/0 escess (“non-stoichiomctric” oxalatc). The precipi-

tates wet-c washed twice by decantation with water, filtered off on porous glass

filters, washed with ca. 50 ml of water and dried overnight at 75-80”.

Ratch experiments were performed to determine sorption percentngcs or

distribution coefficients unclcr completely equilibrium conditions. Column esperi-

nients were performed in glass columns with an internal diameter of 8 mm. ‘kc

distribution of europium was followed with 152mEu, prepared by neutron activation

in the Thetis reactor of this Institute.

Inf.lzience of the quantity of e~iwopiabm. Figure 1 shows the percentage of europium

adsorbed on equilibration with IOO mg of cadmium osalate in 0.1 &_Z nitric acicl.

On “stoichiometric” cadmium osalate, a nearly complete sorption occurs for

quantities of europium between 0.75 mg and IO mg. While the increasing loss of

+ Communic~.tccl in part to the \‘th Symposium on Chrom~~togmphy ancl IZlcctropliorcsis. Brussels, xgG8.

sorption ~wwer at Jliglicr rJuantities is csJxxtctJ, the Josses at low loacling cannot be easily csJ>JainccJ on tllc basis of a metathesis reaction. ‘I’lle bchaviour of “non- stoicl~io~iictric~” cxclniiunl oxnlatc is still more cr~mplicatccl. It is identical wit11 “stoicl~ic~nictric” caclrniutn osnlate at medium and Jligller loading. At low loading, tlic sorJ,tion clccronscs witli clccrcasing lrz~ling until the range x0--20 /~g of curopium is rcacl~cxl. l;rom tlicrc on, tlic sorJ~tion J>ower increases again.

o.01 0.1 I IO IO0 Ino Eu

13x:. I. ‘I’llc iLtlSO~ptiC~ll of cur~~pillIll in 0.1 Al nitric ilCiCl 011 caclnliiinl rm;rl;Ltc ils a fclwtioll of the

;rnlount Of I31 carrier. (-----) stoichiolnctric product; ( -) non-stoichiomctric product.

Tlic beliaviour of “stoicl~iomctric” cadmiuin osalate can lx esplaincd if one Jx&.ulates that the sorption of curoJ$nn on caclmiunl osalate (and probably on many otlxr osalatcs since the same effects were notccl on Jcacl osalatc) does not occur tlirougli nictatlicsis, hut tiirougli Jx-cciJ>itatioti with tlic osalntc anion in solution. ‘I’liis is rcpresentccl by tlic following reaction scllemc :

MIS # M,-t5 (2)

M:!+S e M& (3) According to this scJ~2me, reaction (3) would take place only wlicn tlx solubility

Jxocluct for M& is escceclecl. The beliaviour of europium at very low loading on “non-stoichiometric”

cadmium osalate can bc e.splainecl by a secondary process. WJlen a salt is prccipi- tatecl in the presence of an excess of one of the constituent ions, this ion is adsorbed on the surface. This ion, a caclnliutn(I1) ion in tllis case. can probably be eschanged against the curol~iun~(III) cation wl~ich forms a less soluble compound with the osalatc. This is certainly tile case for caclnnu~:~ i~snlnte, precipitated wit11 a x07/0 excess of osalate anion, with whicll exactI>- tile same sorption results are obtained as wit11 the so-called “non-stoichiotnetric” cadmium osalate. As leas been shown by I<OL’rHOFFS, the negative charge on the surface is in such a case compensated by the adsorption of positive ions. According to the l’aneth-Fajans-Hahn rule, the ion wllich forms the lesser dissociated compound with the anion is preferentially adsorbed, which in this case will lead to the adsorption of europium(II1). Because it is a surface phenomenon, however, the capacity as determined by this reaction alone is very low.

In..hrem~e of the quwltit_~~ of cntiliiitrril o.ciilutr. \Vlien “stoicl~ion~ctric” cadmium oxalate is equilibrated in o.x N nitric acid with 1 mg or x pg of curopium, it was found that over tile range of 100 n1g to 2 g of stationary phase the quantity of stationary pl~nsa lias no influence on tlie percentage sorption. ‘l’llc quantity sorbed appe‘ars thus to be determined only by tlie quantity of free osalate in solution. ‘I’llc same result wits obtained for tile sorption of 1 mg of curopium on “noli-.s;toicllio- metric” cadmium oxalatc but not for tlie sorption of I-PC: quantities.

Table I suliimnrizes tllc results c)btainecl for tlic sorption of x and 10 /cc: of curopium in 0.x N nitric acicl on I00 n)K or 500 111~: of cadmium osalntc csprcssccl as o/o Eu in solution.

l’lle fact tliat tlic sorption of microgram cluantitics of europium on “non-

stoicliiomctric” cadmium osalntc is clcpcnclent on tlie quantity of stationary l)liase can be explainecl by consiclcring that, if tlie sorption of small quantities of caclmiuiii osalate is caused by an escliangc witli adsorhcl ions, the estcnt of the sorption depcncls on the total cluantity of cscl langeable cations present.

Group A represents a set of cspcriments in wliicli tlie inobile pliasc was acldecl one week before tlie equilibration ancl group I3 another set in whicll tlie equilibration with europium was carried out immediately after adclition of tile niobile phase. It is clear that the sorption is better for group 13. This is ascribed to a recrystallization process which leads to a decrease in surface area (Ostwalcl ripening).

C/tronzato~ru~hy. ‘I’lu2 foregoing results \vere all obtained by batch cquilibra- tion. Since cliromatography is a process of repeated equilibration. the precipitation plienomena discussed do not appear so clearly in column operation. However, it was found that “stoichion1etric” cadmium oxalate is a11 irrcproducible column material for the sorption of microgram quantities of curopium. The following sorption per- centages were obtained for 2 pug of europium on I Q of cadmium osalate in 0.1 f\’

nitric acid: g5.2%, SS.OO/~, 77.5%, Gz.~~/~, 17.8O/~. “Non-stoichiometric” caclmiunl oxalate yielded much better results since in a series of IO experiments no sorption percentage under 99.5 y0 was obtained.

It was also found that the preparation of “non-stoichiometric” columns a long time before use leads to a decrease in efficiency for the sorption of micro~raiii quantities of europium (IO to 20% for a preparation one week before use).

Cadmium oxalatc presents two favourable cllaracteristics as a column material. The capacity, for example, for the rare earths is very high. On a column of 500 1116 of cadmium osalate, x2g 1116 of samarium were sorbed (i.e. 50°jo of the column was converted to the san1ariun1 form) before break-throu&ll occurred. Furthermore, the attainment of equilibrium is fast so that flow rates of I 111l/cm” sn1in were possible.

‘I’lie dcmonstrntion of Inany phenomena cl~aractcristic of precipitation,

indicatc~s tllat tllc cxcl~angc mcchnnisni is rcprcsentecl by reactions (2) and (3). ‘fhis mecllanisni csists probably in many cases wllcrc rcplnccnicnt reactions on salts occur, but is not always easy to demonstrate. In tile cast of the sulfides, for csample, it is clifficult to obtain stoichiomctric salts ancl furthermore since they arc niucli less

solul~lc tlmi tlw osalatc discussctl hcrc, tlicrc wc~~ltl be no measurahlc downward trcntl in sorI>tion power at low loading.

On analytical curves and limits of detection in phosphorimetry

‘I’lie shapes of analytical curves and limits of cletcction in plxqAmrcscence

analysis can be prcclictccl from equations derived by S-r. Josh’, MCCARTHY AND \VINEFC~IZIINE~C who used a signal-to-noise approach*-2. Theory predicts a linear

relation lxtwcen phosphorescence intensity and concentration, i.e., analytical curves should have a slope of enc. Negative curvatures arc expected at high concentrations where complete absorption occurs and molecular aggregation ancl quenching may be significant. Positive curvatures near the limit of detection are a consequence of luminescent impurities in the solvent. Experimentally, it is difficult to apply signal- to-noise theory in determining phosphorescence limits of detection because of the variable background encountcrecl under most esperinlental conditions. WINEFORDNER,

MCCARTHS ANI) ST. JOHN have recommended an estrapolation procedure to estimate limits of clctection:‘. In this case, the limit of detection is defined as the concentration corresponding to the intersection of a line drawn through several background points with the estension of the linear portion of the analytical curve-(log-log plot). A simplification of this procedure which might give a rapid estimation of the limit of detection would be determination of a single analytical point and extrapolation of a line of slope one through this point to background. This procedure of course depends on the accuracy of the measured point and the validity of the assumption of a slope