Radiat. Phys. Chem. Vol. 37, No. 2. pp. 325-329, 1991 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain. All rights reserved

0146-5724/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press pie

THE ROLE OF THE ALUMINOSILICATE MATRIX IN THE GAMMA-IRRADIATION ENERGY TRANSFER TO THE

OXALATE MOLECULE

LASZLO HORVATH, VESNA NOTHIG-LASLO and HALKA BIL1NSKI

"Ruder BogkoviC' Institute, 41001 Zagreb, Croatia, Yugoslavia

(Received 14 November 1989; in revised form 20 March 1990)

Abstract--Three types of aluminosilicate matrices, prepared from oxalate solutions, were y-irradiated and investigated by ESR spectroscopy. In the aluminosilicates with AI:Si = I:1 that can be considered as the end member of allophane series upon 7-irradiation at room temperature in the presence of air, no stable paramagnetic species were formed, even if the aluminosilicate was precipitated in the presence of oxalate. On the other hand, in the evacuated sample upon ;,-irradiation at room temperature a single line ESR spectrum at g = 2.0036 was detected that was ascribed to oxalate radical

Oxalate radical was observed in the other two types of aluminosilicates even in the presence of air. Upon ~,-irradiation at 77 K, in addition to the described oxalate radical, primary irradiation sites were observed and identified as paramagnetic centres derived from chlorides and hydr(gen atoms. Radicals from chlorides were observed in all samples except in NaAI(OH)4(H4SiO4) 2 (hydrous feldspathoid), the same as were found recently in protoimogolite allophane. Hydrogen atoms were found in AI(OH) 3 • H4SiO4 and NaAI(OH)4(H4SiO,) 7. The results suggest that oxalate acts as a trap for v-irradiation energy.

INTRODUCTION

The present work is a continuation of the study of paramagnetic molecular centres in the model clay compounds, that have been prepared and character- ised recently (Bilinski et al., 1986, 1990; Horvath et al., 1988, 1990; N6thig-Laslo et al., 1990; Matkovi6- (~alogovi6 et al., 1987).

These model compounds contained oxalate ions either coordinated in aluminium hydroxo-oxalates, or present as an impurity, i.e. in the trace amount, as in the aluminosilicates.

Upon ) '-irradiation of aluminium hydroxo- oxalates stable paramagnetic species that were found were ascribed to the oxalate radical. We suggested that oxalate ion acts as a trap, i.e. the storage site for the ),-irradiation energy (Horvath et al., 1988; N6thig-Laslo et al., 1990).

Here, we ) '-irradiated several aluminosilicates of different composition. We were interested to find out whether oxalate ion impurity acts as a trap for the ) '-irradiation energy in different aluminosilicate matrices.

with ethanol in order to remove NaCI. Solids were dried in a desiccator over silica gel.

Chemicals used, AICI3"6H20, NaCI, NaOH, (COOH)2 • 2H20, LiCI (Merck, Darmstadt, Germany), Na2SiO3"9H20 (Baker, Deventer, Holland) were p.a. pure.

Freshly prepared samples were 7-irradiated at room temperature in the test tubes opened to the air. For the ) '-irradiation at 77 K the samples were evac- uated before irradiation, at the pressure of 0.0133 Pa, in the special tubes attached to the standard ESR tubes.

The Co-60 source (1.2" 10~SBq) was used at the dose rate 4 Gy/s and the dose per sample was 10 kGy.

The ESR spectra were taken at Varian E-12 cen- tury line spectrometer, equipped with the Varian variable temperature control unit. The temperature was measured by the thermocouple located above the ESR cavity. The temperature was varied from 87 to 300 K.

i.r. spectra were recorded on a Perkin-Elmer infra- red spectrometer model 580B, using KBr pellets.

EXPERIMENTAL

The concentrat ion conditions under which precipi- tates of aluminosilicates were prepared according to Bilinski et al. (1990), are presented in Table 1. The samples were filtered and the precipitate was washed

RESULTS AND DISCUSSION

i.r. spec tra l characteris t ics o f alurninosil icates

Five aluminosilicates (Table 1) were isolated from solution close to the precipitation boundary as was described recently (Bilinski et al., 1990). All the

325

326 LXSZLO HORVAra et al.

Table 1. Aluminosilicates precipitated from 0.6 M NaCI according to Bilinski et al. (1990), which have been "/-irradiated and studied by ESR

Concentration of solution (mmol - 3) Complex

predominant Sample Precipitated solids AICI 3 H:C204 NaOH Na2SiO J pH in solution* %C %Na %AI %Si

I AI(OH)3H4SiO, 10 0 10 10 5.41 Not known <0 0 17.12 15.64 2 AI(OH)~H4SiO 4 and 3 3 3.5 3 5.1 AI[CrO4]' 1.8 0 15.80 13.72

traces of AI3(OH)TC204 • 3H20 3 AI(OH)3(H4SiO4)7 10 13 9 10 4.29 A1[C204] z <0.20 0.58 3.30 25.26 4 NaAI(OH)4(H4SiO4)~ 10 50 89 10 6.78 AI[C~O4]~ 3 <0.20 9.08 3.75 19.22 5 NaAI(OH),,(H4SiO4)2 3 15 26.3 3 7.39 AIICzO4]~ ~ <0.20 6.64 12.13 23.34

'Complexes, calculated using equilibrium model in Bilinski et al. (1986).

precipitated solids obtained were X-ray amorphous, but had a characteristic i.r. spectra. Using i.r. spectra and chemical analysis, it was possible to distinguish pure phases from the mixture of the phases. The i.r. spectra of the samples 1, 2 and 5 have been published (Fig. I, spectra b, e and h respectively, in Bilinski et al. , 1990). The i.r. spectra of samples 3 and 4 are shown in this work in Fig. 1, spectra a and b.

i.r. spectra of all five samples in Table 1, tend to be simple with relatively few main vibrations, which are somewhat broad and lacking fine structure. In Table 2 the i.r. characteristic bands are compared to illus- trate the structural differencies, in the absence of X-ray data.

Namely, our attempt failed to crystallise these samples by ageing in contact with original solution for 5 years at room temperature. Samples 1 and 2 with Si:AI = 1:1 can be considered as one of the end-members of allophane series, according to Wada (1967). Sample 2 was precipitated from oxalate solu- tion and contained some oxalate impurity in the form of coprecipitated compound AI3(OH)TC204 • 3H20, which has been described in details (Bilinski et al., 1986; Horvath et al., 1988). From the microelement analysis the amount of carbon was 1.8%. Strong bands at 1730 and 1710cm -~ correspond to VasymCOO- and the medium ones at 1430 and 1310 cm- 1, to V,~mCOO- of AI3(OH)TC204" 3H20.

The main asymmetric stretch frequency of Si--O is at the same position (1030 cm -~) in samples 1 and 2. Samples 3 and 4 have been prepared at high silicate concentration (10mM) at which silica-rich phase precipitates. From the microelement analysis carbon in this compounds was less than 0.2%. Correspond-

i

t ~ 4000 5000 2000 1600 1200 800600400200

Wavenurnber/cm -~

Fig. 1. The i.r. spectra of AI(OH)3(H4SiO4)7 (sample 3)-- spectrum a, and NaAI(OH)4(H4SiO4)7 (sample 4)--

spectrum b.

ing carboxylate frequencies are observed only in the form of very weakly pronounced shoulders.

The main asymmetric stretch frequency of S i4 ) is at similar position (1070 and 1050cm t), also is the symmetric stretch (800 and 780 cm ~). Strong band observed at 460 and 450 cm- t, assigned to an interval tetrahedral mode is equally pronounced in samples 3 and 4. These samples could not be identified with any natural clay mineral. In i.r. spectrum of sample 5 no oxalate stretching was observed. The main asymmet- ric stretch frequency of Si-O is at 1020 cm-~, that is at the lowest position in all studied samples. Although the position of most bands is at a similar position in samples 1 and 5, the significant difference is observed in the stronger intensity of 4 4 0 c m ' frequency of the last sample, corresponding to in- ternal tetrahedral mode,

Although an internal tetrahedral mode is less sen- sitive to frame work structural variation, it shows some structural specificity.

Samples 5 was identified (Bilinski et al., 1990) with hydrous feldspathoids, according to Farmer et al.

(1979). This phase shows tetrahedral coordination of aluminium (Horvath et al., 1990). Gels of similar composition were found in natural lake sediments (Eugster and Jones, 1968).

Table 2. Infrared spectra in the 4000-200cm L'range

Sample no. I 2 3 4 5

3450 vs,b 3500 vs,b 3460 vs,b 3460 vs,b 3460 vs,b 1730s 1730 sh 1730sh 1710s 1710sh

1640 m 1650 sh 1650 m 1650 s 1650 m 1430 m 1430 w 1430 w

1345 sh 1310m 1325w

l l50sh 1170 sh 1190 sh 1200 sh 1150sh

1070 vs,b 1050 vs.b 1030 v~i,b 1030 vs,b 1020 ~s,b 880 sh 940 sh 915 sh

880 sh

690 sh

580 m,b 580 s,b 580 m.b

440 m,b

810w 800 w

780 v,

560 sh 560 sh 500 sh

460 s 450 s 430 sh

700 m,b

440s

v = Very; s=st rong; m = medium; w = weak; sh = shoulder: b = broad.

ESR, y-irradiated aluminosilicate 327

ESR spectral characteristics o f aluminosilicates y- irradiated at room temperature

When samples 1 and 2 (Table 1) were y-irradiated at room temperature in the tubes opened to the air, no ESR spectra were detected, i.e. no stable paramag- netic species were formed. Similarly, the composites of aluminosilicates, i.e. AI(OH)3 and H4 SiO4 gave no ESR spectra when the compounds were y-irradiated at room temperature. Also the compound AI3(OH)TC204 3HzO which is present in sample 2 as an impurity gave no ESR spectra when y-irradiated under same conditions (Horvath et al., 1988).

The other three aluminosilicates (samples 3, 4 and 5, in Table 1) showed the single line ESR spectrum at g -- 2.0036 of 0.8 mT linewidth. This suggests the unpaired electron to be located at the carbon atom. Such spectrum was recently observed (Horvath et al., 1988) in the y-irradiated aluminium hydroxo-oxalate Na4AI2(OH)z(C204)4" 6H20, in which oxalate mol- ecule was coordinated as a bridging ligand between aluminium and sodium (Matkovi6-(~alogovi6 et al., 1987).

We have ascribed this spectrum to

radical. On the other hand, when sample 2 was y-irradiated

at room temperature, in the evacuated tube the one line ESR spectrum was observed at g = 2.0036.

In the solution of samples 3-5, the oxalate was determined in Table 1 as a negatively charged alu- minium oxalate complex according to Bilinski et al. (1990) and was most probably incorporated into the precipitate in this form. We suppose that in these compounds (samples 3-5) it is an intrinsic part of the aluminosilicate matrix. The same radical was ob- served also in sample 2 when y-irradiated at room temperature in the absence of oxygen, but the radical disappeared soon after opening the tube to the air, while in all the other samples the oxalate radical was stable in the presence of air for several weeks.

Aluminosilicates "/-irradiated at 77K

In order to determine the primary irradiation sites in the aluminosilicates, the samples were y-irradiated at 77 K, in the liquid nitrogen.

In Fig. 2 the ESR spectra taken at 87 K of the y-irradiated samples 1 and 2 are presented, spectra a and b, respectively. The hyperfine doublet of 54.0 mT splitting (spectrum a) is usually ascribed to the hydro- gen atoms (Weeks and Abraham, 1965). These lines disappeared upoia warming the sample to 139 K. As all the compounds were prepared from 0.6 M NaCI solution, both chloride and sodium ions remained in the precipitate as an impurity (2-46 ppm). In order to assign ESR lines to paramagnetic species that may derive from chloride ions we y-irradiated LiCI pow-

n

Fig. 2. ESR spectra of ~,-irradiated at 77 K aluminosilicate AI(OHhH~SiO4' 3H20 prepared in the absence of oxalate (spectrum a), and in the presence of oxalate (spectrum b). Spectrum c--LiCl powder ~,-irradiated at 77 K. The aster- isks denote paramagnetic centres due to chloride ions. ESR

spectra were taken at 87 K.

der at 77 K (spectrum c, in Figure 2). The hyperfine doublet of 54 mT splitting is obviously due to hydro- gen atoms formed from the water molecules in LiCI powder. The lines denoted by stars in the spectra in Fig. 2 suggest the presence of (~lf and ¢~1 radicals, as we deduced by the comparison with the literature data (Griscom, 1974, 1980). These lines disaplmared at about 209 K. At room temperature no ESR spec- trum was detected.

ESR spectrum b, in Fig. 2 (sample 2), shows no hyperfine doublet of 54.0 mT splitting, i.e. in this compound hydrogen atoms could not be detected. In addition to the chloride paramagnetic centres, the predominant feature of the spectrum is the line at g = 2.0036. Upon warming of the sample the position of the line remained unchanged, with no significant changes in the intensity. If the sample was opened to the air, the line decreased and disappeared within about 10 min. This is in accordance with the result obtained upon y-irradiation at room temperature in the evacuated tube.

In Fig. 3 the ESR spectra are shown (taken at 77 K) of samples 3 and 4, spectrum a and b, respect- ively. The ESR lines characteristic for hydrogen atoms are present only in sample 4 at very high gain, but are absent in spectrum a of sample 3. They disappeared at the similar temperature as in sample 1, i.e. at 159 K. The lines characteristic for chloride

328 LAszI~ HORV^TI! et al.

o

- 'V--'"

Y + 8.0 mT

| i

Fig. 3. ESR spectra of),-irradiated at 77 K aluminosilicates: sample 3 (spectrum a), sample 4 (spectrum b) and sample 5 (spectrum c). The asterisks denote paramagnetic centres due

to chloride ions. The spectra were taken at 87 K.

paramagnetic centres are present in both samples and they disappear upon warming at 209 K.

The predominant feature in the ESR spectra of both samples is the line at g = 2.0036. This line remained unchanged upon warming the sample up to room temperature. For sample 5 (Fig. 3, spectrum c) only the line at g = 2.0036 was observed even at 87 K. Thus the chloride paramagnetic centres or hydrogen atoms could not be trapped by this matrix even at 87 K. The intensity of the line at g = 2.0036 increased upon warming the sample to room temperature and remained stable at that temperature. Sample 5 was identified with the hydrous feidspathoids (Farmer et al., 1979), and has a tetrahedral coordination of AI atoms as was determined by 27A1 MAS NMR (Hor- vath et al., 1990). It seems that in such a structure the primary irradiation sites cannot be trapped even at low temperature.

CONCLUSION

i.r. results suggested three different types of alumi- nosilicate matrices examined in this work. In all the three types ~-irradiation produced only one stable paramagnetic centre, the oxalate radical, i.e.

represented by one line at g = 2.0036, in the ESR spectrum. The stability of the oxalate radical de- pended on the type of aluminosilicate matrix. In the

presence of air in the first type of alurninosilicates no stable paramagnetic species were observed, while in the other two types, under these conditions the oxalate radical was stable for days.

Our previous results on ),-irradiated alumino- oxalates (Horvath et al., 1988) suggested that radicals formed depended on the type of alumino--oxalate matrix. Two types of ESR spectra were observed. A line at g = 2.000 was ascribed to t~Of and was the only stable paramagnetic centre in AI3(OH)TC204' 3H20. In NaAI(OH)2CaO 4. 3H20 in addition to this, a line at g = 2.0036 was observed that was ascribed to

radical. In the aluminosilicate, AI2 (OH)6 • H, SiO,, that was

identified as protoimogolite allophane (Bilinski et al., 1990; Farmer et al., 1980) and contained oxalate as an impurity, we have found only one ESR line at g =2.000 and ascribed it to (~O2 radical (N6thig-Laslo et al., 1990).

These facts suggest that the structural properties of aluminosilicate matrix may determine the type of the oxalate radical that will be formed by ),-irradiation, and the stability of the radical formed in the matrix. The mechanism of the 7-irradiation energy transfer deserves further investigation.

Acknowledgements--The work was supported by the Self- Management Community of Interest for Scientific research of SR Croatia (SIZ) and by U.S. Geological Survey (U.S.A.- Yu. joint project PN 823). We acknowledge the technical assistance of L. Rei~ in the preparation of the manuscript.

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