Indi an Journal of Chemistry Vol. 44A, July 2005, pp. 1378- 1382

Application of proton induced X-ray emission (PIXE) in estiJnation of trace metals entrapped in silica matrix

P K Jal ,·b, M Sudarshanb, A Sahab

, Sabita Patel ' & B K Mishra'· * 'Centre of Studies in Surface Science and Techno logy, Department of Chemistry, Sam balpur Uni versity ,

Jyo ti Vihar 768 01 9, India

Email: bijaym @hotmail.com

blnter Uni versity Consortium for DAE Fac ilities, HIILB-8, Bidhan Nagar, Ko lkata 700 098, Ind ia

Received 27 November 2002; revised 9 May 2005

Proton induced X-ray e mi ss ion techni que is used fo r multielemental analys is o f metal ions adsorbed on nallosi lica sur­face. At pH 3.5, si lica traps uraniu m se lecti ve ly from a mi xture of soluti ons of 13 di fferent metal ions viz., 1-' (1 ), Ca(ll). Fe(IlI), Mn(lI), Co(II), Ni(Il), Cu(II ), Z n(II ), Sr(II ), Cd(II ). 8 a(11 ), Hg( lI ) and U0 2(V I).

IPC Code: Int. CI. 7 GO IN2 1/00

The search for a solid support to separate and precon­centrate organic compounds or inorganic metal ions in solutions converges mostly to sili ca. It remains the firs t choice as a model to study the sorption of Ura­nium(Vl) on silica surfaces 1.2. Recently, Vlasoya has reported adsorpti on of Cu2

+ ions onto silica surface from aqueous solution containing organi c substances3

.

Extensive works are reported on preconcentration of metal ions ~y using modi fied sili ca surface2

. On suit­able modi fication by chelating groups, silica can trap preferentially certain metal ions. For qualitati ve as wel l as quantitative analysis of metal ions from a solid si lica matri x, neutron acti vation analysis and X-ray fl uorescence spectroscopy have become the most im­portant tools.

Proton induced X-ray emi ss ion spectroscopy (PIXE) is a unique tool for mul tielementl analys is in trace amount. Minagawa et 0 1.4 reported the analysis of ura­nium fro m wastewater using PIXE. However, in gen­eral, sensiti vity of the techn ique red uces because of the low energy background fro m the water matri x, when ilTadi ated directl y. Therefore, extrac ti ve concentration and ion exchange separation of trace elements is essen­tial prior to instrumental analysi s. Among the methods of analysis of water using PIXE technique, a 1Il0dllS

operandi fo llows the sequence: (i) extractive concen­rr'ation of elements, (ii) non-selective preconcentration of trace elements from the water samples using suitable chelating agent, (i ii ) isolation or metal ions by su itable

reagent, followed by (iv) deposition of the metal ion on suitable surface fo r PIXE analysis5

.8

. Lochmuller et

'

9 10 db· d . h . o . . use a mem rane Impregnate Wit a cation ex-change resin to concentrate the trace elements in water and then irradiated the membrane fo r elemental analy­sis using PIXE.

For the first time we have reported the use of PIXE for multi elemental analys is of entrapped metal ions on a silica matri x J . By immobili zing carboxy hydrazide li gand on silica surface, attempt has been made to trap metal ions like K(I), CrOll ), Co(l!), Ni(Il ), Cu(IT), Zn(II), Hg(JI) and U(Vl) fro m a mixture of their aqueous solutions. ]n continu ati on to our earli er work l

the present study a: ms at recovery of metal ions by using nanosili ca fro m their mi xture of aqueous solu­tions at different p H and concentrat ions.

Proton induced X-ray emission (PIXE) has con­tributed towards the developments of several analyti­cal procedures. When ions pass through matter, the ions interact with the electrons in the iltoms and, oc­casionally, a vacancy is produced in an electronic shell. When thi ~ occu.s in an inner shell , the vacancy is fiiled by an electronic transition from an outer shell , and an X-ray of characte ri sti c energy is emitted. Thi s em:ssion is a characteristic signature of that particular element. The area under the X-ray peak determines the amollnt of the element present.

Continuolls and characteristic X-rays emiSSion along with auger electron emission takes place on

JAL el ai.: PIXE ANALYSIS OF TRACE METALS LN SILICA MATRIX 1379

irradiation of sample with energetic charge particles such as protons obtained from the accelerator. Ener­getic protons of 2-3MeY are reported to be the opti­mum choice for X-ray emission due to a relatively high ionization cross section of K shell electron II. The X-ray energy spectrum consists of characteristic X­ray lines of the atoms present in the sample. The X­rays are detected using a Si(Li) detector and the pul ses from the detector are amplified and finally registered in a pulse height analyzer. The number of pulses in each peak is a measure of concentration of the corresponding element.

The continuous background at the base of the char­acteri stic X-ray peak determines the limit of detection of an element. The background arises due to mainly proton bremsstrahlung, electron bremss trahlung and nuclear reaction y-rays. The bremsstrahlung produced in PIXE is mainly due to secondary effect, i.e., elec­tron bremsstrahlung and not the primary background contribution as in electron microprobes and X-ray fluorescence analysis. Perhaps the main reason for high sensitivity of trace element detection in PIXE is the reduced bremsstrahlung background compared to the electron microprobe. The increased signal-to­noise ratio allows trace impurity detection down to I ppm or less; up to two orders of magnitude better than electron microprobes.

Use of proton beam as an excitation source with the ab ility to transport to a longer distance without any loss in intensity offers several advantages, e.g., (a) higher rate of data accumulati on across the ent ire spectrum, which allows faster analysis, and (b) better overa ll sensiti vities, especia ll y for the lower atomic number elements. This is due to a lower brem­ss trahlung background resulting from the de­accelerati on of ejected electrons, as compared to electron excitation; and the lack of a background con­tinuum. Further, this technique has the ability to measure spatial distribution of elements us ing proton microprobe and it provides non-destructive methods of analys is. Another interes ting feat ure about PIX E is its constant sensitivity over a wide range of elements 20<Z<40 and 75<Z<92 of the periodic table. Its ab i 1-ity to co mbine with auxi liary nuclear techniques like PIG E for y- ray measurement and RBS for information of matrix compos ition is an added advantage l 2

. In ad­dition , the advent of powerfu l software for data analy­sis, GUPfX u makes this method an easy choice to · anaiyze elements in a comp lex matrix with high acc u­racy.

Materials and Methods Nanosilica was prepared by precipitation from al­

kaline silicate solution l4. Salt solutions of differen t

metal ions, viz., K(I), Ca(U), Fe(III) , Mn(II ), Co(II), Ni(II), Cu(II), Zn(JI) , Sr(II), Cd(ll) . Ba(II) , Hg(lI ) and U02(Y£) were prepared by dissolving weighed amount of solid salts of their chloride or nitrate in tri­ply distilled water. Graphite powder was obtained from E. Merck.

Adsorption of metal ions on silica In order to investigate the binding capaci ty and se­

lectivity of silica surface towards the metal ions at different pHs (2.6, 3.5, 5 .0 and 6.1), a mixture of so­lutions of metal ions (1 x 10.3 and 1 x 104 M of each metal ion) was prepared. The adsorption of metal ions was carried out by shaking 10 ml of the mix ture of the solution in 100 mg of nanosilica at 298 ± I K on a shaking machine for 6 h. The metal bound silica was then filtered and washed with triply di stilled water and finally dried in an oven at 313 ± I K for 24 h.

Target preparation After the adsorpti on of metal ions, silica was

crushed with an agate mortar pestle and then thor­oughly mixed with graphite powder in I : 1 ratio by weight (75 mg each). Thick targets were prepared by press ing these mixtures into pellets, each hav ing 10 mm diameter and I mm thickness using hydrau lic press. Similar pellets were prepared for coal fly ash (standard reference material from NIST, USA) using the same procedure for standardizi ng the PIXE set-up.

Measurement of PIXE A PIXE set-up consists of a van de Graff or low

energy pelletron accelerator for proton acce leration, an analyzing and switching magnet, quad ruple magnet for focusing , a set of co llimeters, a target ho lder in­side the scattering chamber and a X-ray detecting and pulse analyzing system in volving a Si (Li) detector, preamplifier, amplifi er and a multichannel ana lyzer. The protons coming out of the accelerator are inj ected into the beam line through an analyzing magnet and then directed horizontally to the scattering chamber using quadruple magnets . The targets are mounted on a multiple target ladder o ri en ted at 45° to the proton beam axis within the scattering chamber maintained at a pressure of 10'6 torr. The targets were positioned in the path of the beam maintaining the required projec­tile - target - detector geometry. Finally, the beam fa ll s on the target after passing through a series of collimeters in the scattering chamber. Irradiation for

1380 INDlAN J CHEM, SEC A, JULY 2005

5-10 minutes with a beam current of 5- IOnA is good enough for a good stati stics. The characteri stic X-rays produced from the targets are collected usi ng a Si(Li) detector with resoluti on of 150-180eV at 5.9KeV Mn X-ray. The detector is placed as close as possibl e to the target. The characteristic X-ray spectra are re­corded on a multiparameter analyzer. For the identifi­cation of the X-ray, the multiparameter analyser is calibrated using 24 1 Am X-ray source with a wide range of X-ray peaks.

Quantitative analysis

Manually estimating the total number of counts under a peak and subtracting the background, counts can be con·elated with the actual concentration. How­ever, this is a tedious process with spectra containing

Table I-H and Z (atomic number) values for concerned elements

Element Z H x 105

CI 17 130

K 19 130

Ca 20 145

Ti 22 145

Cr 24 155

Mn 25 155

Fe 26 155

Ni 28 160

CII 29 160

Zn 30 160

As 33 160

Br 35 160

Sr 38 160

Zn 40 160

Pb 82 180

U 92 190

many peaks. A computer code, which fits a Gaussian with exponential tail to each peak and a polynomial to the background serve the purpose best. GUPIX 13

, a popular PIXE spectral data analyzing software, that models a theoretical spectrum of a particular element usi ng the database of K, L and M X-ray energies, relative X-ray intensities and flu orescence and Koster-Cronig probabilities, is rhen fitted with the experimental spectrum with respect to the known ma­trix. The software package provides non-linear Ieast­square fittings of the spectrum, together with subse­quent conversion of X-ray pea:< intensities to ele­mental concentration via a defined standardizati on technique involving X-ray energy dependent H values relative charge and user-defined instrument constants. The software takes care of matrix effect, proton stop­ping power, X-iay mass attenuation coefficient and secondary fluorescence contribution for spectral fit­ting. The concentration of an element (Cz) is given as l3 ,

where, Y m and Yt are the measured and. theoretical X­ray yields respectively, Q is the real charge accumu­lated, c is the intrinsic effici ency of the detector and T is the transmi ssion through absorber between target and detector. H value is the product of detector solid angle and a charge correction factor l s

. Dutta et al. 16

have shown that H values are lower for low energy X­rays and become almost constant for Z=28 onwards.

The H value (Table I) and charge were normalized for coal fly ash reference material by closely match­ing the analyzed concentration with reported concen­tration of the e lements present (Table 2). The PIXE spectra of the metal ions bound on silica surface at three different pH are given in Figures 1-4 and the

Table 2- PIXE analysis results of coal fl y ash (NIST SRM) [ 1633b]

Trace Cert. values Found (mglkg) elements . (mg/kg) Coal fly ash .l Coal fly ash 2 Coal fl y ::Ish 3

Chromium 198.2±4.7 205.4± 12.5 208.6±11.2

Manganese 131.8±1.7 137 .6:t22. 7

Nickel 120.6± 1.8 112.5± 13.4 124.9± 17.2 11 3.G±8.8

Copper 11 2.8±2.6 108.4±9.7 145.8±13.3 115 .7iI 7.8

Zinc 2 10 222.0± 13.7 2 11.5± 14.3 2 11.9±7.8

Arsenic 136.2±2.6 129± 16 124.9±17.2 140.0±9 . t

Rubidium 140 148.0±23.8 142±20 144.6i I4.3

Strontium 104 1± 14 1087.0±41.3 1039.3±39.5 1 060.5±27.6

Uranium 8.79±0.36 8.3±0.43 8.3+0.25

JAL ef 01.: PIXE ANALYSIS OF TRACE METALS IN SILICA MATRIX 138 1

amounts of metal ion present on the silica surface as analysed from the results of PIXE are given in Tables 3 and 4.

Results and Discussion The sensitivity of PIXE method depends on several

factors like energy of proton beam, nature of target and background. Silica being a nonconducting target, the background is due to the positive charge built-up around the target, which increases the bremsstrahlung background considerably . This increase in back­ground not only reduces the detection sensitivity but also masks the weaker X-ray peaks completely. To reduce the positive charge built-up around the silica base target, graphite was mixed with the sample dur­ing target preparation.

Figures 1-4 represent histograms of X-ray counts versus X-ray energy, known as the PIXE spectrum, generated by bombardment of 3MeY proton beam on the metal ion embedded nanosilica surface. Each

1000

~

" 100 ~

0

U

10

Energy in KeV

10 1> '0 25 30

100 200 300 400 500 600 700 800 90010001100 12001 30014 00 1500

Chennel Number

Fig. I- PIXE spectrum of nanosilica embedded with metal ions recovered from IO·3M mi xture of aqueous solut ion of pH 2 .6.

100 0

'" " 100 ~

0 U

10

Energy in KeV

1 0 15 2 0 25 30

200 3 00 400 500 60 0 700 800 9 00 100 0 1100 12 00 13 00 1400 150 0

C hennel Number

Fig. 2- PIX E spectrum o f nanosili ca embedded with metal ions recovered from 10" M mi xture o f aqueous so lu tion of pH 5.0.

spectrum is a collection of several peaks. The major energy lines correspond to ka/k(:l or la/l (:l X-ray lines . Presence of any metal ions in the silica matlix is pre­dicted from its characteristic X-ray peak posi tion. Presence of uranium is conftrmed from existence of lall (:l X-ray peaks at 13 .6 and 17.2 KeY. Similarly , the existence of other metal ions is confirmed from their corresponding kalk(:l lines in PIXE spectrum. The ka and k(:l values of the metal ions are K(I): 3.3 and 3.6; Ca(II) : 3.7 and 4.0; Fe(IJI): 6.4 and 7.0; Ni(II): 7.5 and 8.2, Cu(Il): 8.0 and 8.9; Zn(II): 8.6 and 9.6; Sr(II): 14.2 and 15 .9 KeY respectively . However, the concentration calculation is purely based on area un­der the principal X-ray peak after subtraction of back­ground at the base of the peak.

Silica surface has zero charge at pH around 2.0 17.18

•

Within the pH range 2.6 - 6.1 a major portion of si lica surface is ionized and electrostatic interaction plays a vital role for the adsorption of metal ions on silica surface. While investigating the effect of pH on metal

Ene rgy in KeV

10 1> 20 25 30

Fe 10 00

~

" ~ 100 0

U

10

1 +-'"""T-~-,-,,--,-..II..L, laO 2 00 300 400 5 00 BOO 7 00 800 9 00 l Q001 1 0Q 1200 13 00 1.H015 00

Chennel Number

Fig. 3- PIXE spectrum of nanosilica embedded with metal ions recovered from 104 M mi xture of aqueous solution o f pH 3. 5.

1000

'" c 100 ~

o U

10

Energy in KeV 10 15 20 25 30

100 2 00 300 4 00 5 00 600 7 0 0 800 900 10001 1001200 13001 4 0 0 1 500

C hennel Number

Fi g. 4- PIXE spectrum o f nanosilica embedded with mew I ion recovered from IO·4M mi xture of aqueous solution o f p H 6. I.

1382 INDIAN J CHEM. SEC A, JULY 2005

Table 3-Recovered metal ions from the nanosilica surface from 10·4M mixture

Nickel(lI)

Copper(II)

Zinc(IJ)

Strontium(lI)

Uranium(VI)

Metal content

(/lgll 0 ml)

58 .7

63.5

65.3

87.6

238.0

[Surface metal] atpH =3.5

13 .8±1.4

519.1±19.7

Recovery (% )

2.11

28.10

[Surface metal] Recovery atpH =6.1 (% )

9.2±0.7 1.56

18.5±0.9 2.9

32±1 4.9

1O.3±1.5 1.17

306.9±9.2 12.89

Table 4-Recovered metal ions from the nanosilica surface from 10·3M mixture

Metal ions Metal content [Surface metal]

(/lgIlO ml) atpH=2.6

Potassium(l) 391

Manganese(IJ) 549

Nickel(II) 587 42.8±1.7

Copper(ll) 635

Zinc(lI) 653 12.7± 1.3

Strontium(H) 1374

Uranium(VI) 2380 165 .6±11.1

ion extraction by modified silica, Mahmoud has observed that the extraction of metal ion has an opti­mum pH value for each metal ion when considered separately 19. Further, metal ions like copper (II) and zinc(ll) adsorb on silica surface with a sigmoidal ad­sorption isotherm with a negligible adsorption at low concentration2o. The results presented in Tables 3 and 4 reveal that at high concentration of metal ions (10.3 M) and at a pH of 5.0 almost all metal ions can be entrapped by silica except Fe(III) , Co(II), Cd(II), Ba(ll) and Hg(JI) . At low concentration of metal ions and at a pH of 6.1, silica can extract only Ni(II), Cu(ll), Zn(Il) and UOiYl). However, at a low pH of 3.5 only U02(YI) is found to be extracted. Among all the metal ions, uranium has the maximum affinity for the si lica surface.

Acknowledgement BKM is indebted to Inter University Consortium

for DAE Facilities, Calcutta Centre and UGC (DRS programme) for financial support.

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3 Vlasoya N N, Call Sill! 163 (2000) 125.

Recovery [Surface metal] Hecovery % at pH =5.0 %

lI07.2±16.6 28.3

25.1±2.2 0.45

0.73 18.7±1.3 0.32

22.7±1.4 0 .35

0.19 20.2±1.4 0.19

29 .1±4.2 0.21

0 .69 6298.8±56.7 26.46

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