Measurement of specific surface areas of soils by p-Nitrophenol adsorption

Applied Clay Science, 4 (1989) 521-532 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

521

Measurement of Specific Surface Areas of Soils by p-Nitrophenol Adsorption G.G. RISTORI, E. SPARVOLI, L. LANDI and C. MARTELLONI Centro Studio CoUoidi Suolo, Piazzale deUe Cascine, 28, 50144 - - Firenze (Italy) (Received February 1, 1988; accepted after revision February 20, 1989 )

ABSTRACT Ristori, G.G., Sparvoli, E., Landi, L. and Martelloni, C., 1989. Measurement of specific surface areas of soils by p-Nitrophenol adsorption. Appl. Clay Sci., 4: 521-532. The adsorption of p-Nitrophenol (pNP) was used for measuring the specific external and total surface area of Na +- and Ca 2+-saturated soil samples. Adsorption isotherms at 20°C from a pNP solution in xylene were obtained after heating the samples at ( 1 ) 160 ° C or (2) 90 ° C. With soils not containing swelling clay minerals L-type isotherms were obtained independently of the pretreatments. The final plateau was assumed to indicate the completion ofpNP monolayer coverage of the surface of the samples. The derived specific surface area (SSA) was in good agreement with the specific area obtained by N2 sorption. In the presence of swelling phyllosilicates, only the Na + -saturated samples preheated at 160 ~C showed L-type isotherms quite similar to those typical of the first group of soils. Under the other adsorption conditions, isotherms having a first branch of L-type and a second branch of C (constant partition ) -type were obtained. The C-branch was found to reveal the penetration of pNP in the interlayer spaces of swelling clay minerals. Only the curves relative to Ca 2+saturated samples heated at 90 ° C terminated in a plateau, assumed to be the result of the complete monolayer coverage of both external and internal surfaces. The onset of a linear segment (point X) was considered to indicate the pNP complete coverage of the external surface. The SSA calculated at this point agrees well with values obtained by N~ sorption. The total (internal + external) area measured at the plateau is lower than the EGME (ethylene glycol monoethyl ether ) area, but it was considered more realistic. The influence of the pretreatments on the behaviour of swelling soils was ascribed mainly to the different polarizing power of the monovalent and divalent exchange cations.

INTRODUCTION

The specific surface area (SSA) plays an important role in determining soil physical and chemical-physical properties and therefore has a great influence on processes related to soil conservation, fertility and pollution. 0169-1317/89/$03.50

© 1989 Elsevier Science Publishers B.V.

522 Several adsorption methods have been employed for measuring surface areas. The adsorption of N2 at 77 K and the application to the isotherms of the BET equation is a widely recognized method for calculating the "external" surface area of degassed soil particles. Although "total area" of soils containing swelling clay minerals cannot be determined, the method remains a useful means of comparing two different soils. Adsorption of polar molecules, like water or ethylene glycol (EG) have also been proposed and widely employed but, because of some undeterminable multilayer absorption, such methods often overestimate surface areas especially of soils with low CEC (Newman, 1983). The adsorption from solutions was suggested by Giles et al. (1960, 1962, 1974) for measuring surface areas of many finely divided solids, and specifically for soils by Greenland and Quirk (1964). These authors used cetyl pyridinium bromide in aqueous solution and found that the method works well for some soils, despite some important limitations, as noted also by Slade et al. (1978). p-Nitrophenol (pNP) in xylene solutions was tested in a preliminary study (Ristori et al., 1985) for the determination of the external and total surface area of pure smectite minerals. Values of external surface, in very good agreement with those obtained by N2 sorption, were calculated from the plateau of adsorption isotherms (20 ° C) of samples which were saturated with monovalent cations, heated at 160 °C and kept dehydrated to avoid the penetration of the organic molecule in the interlayer spacings. Values of total surface areas very close to those calculated from the structural formulas were obtained from the adsorption isotherms (20 ° C) of samples which were saturated with divalent cations and equilibrated at low values (about 10% ) of relative humidity (RH). The purpose of this work was: ( 1 ) to test the possibility of measuring SSA of soils by adsorption ofp-Nitrophenol (pNP) from solution; (2) to determine by the same method the total and external surface area: (3) to evaluate the influence of the nature of the interlayer cation (Na +, Ca 2+ ) on the adsorption of a polar organic molecule in the presence of swelling phyllosilicates. MATERIALSAND METHODS

Soil samples Soils of different origin and characteristics were selected. Group I. Soils without swelling phyllosilicates: three mediterranean Terra Rossa soils; two Regosols on Pliocenic clays. Group H. Soils containing swelling phyllosilicates: two Vertisols; one Regosol on Pliocenic clays; one Cambisol on basic rocks (gabbro and diabase). Their main characteristics are reported in Table I. After sieving at 2 mm the soil samples were Na +- or Ca2+-saturated by

523 TABLE I Characteristics of soils used for studies of p-Nitrophenol adsorption

Group I: Mediterranean Siena Terra rossa Fasano Gravina Regosols Monteuliveto Montespertoli Group II: Vertisols Regosols Cambisols

Canicatti Sardara Rumianca S.S. Marie

Sand Silt (%) (%)

Clay Org. (%) mat

CaC03 CEC (%) (meq/100g)

Mineral clay fract, comp.

34 50 57 47 38

31 28 23 23 32

35 22 20 30 30

3.6 4.0 4.2 2.6 1.5

3 15 20

24 34 29 20 11

Illite, kaolinite Illite, kaolinite Illite, kaolinite Illite chlorite, kaolinite Illite chlorite, kaolinite

20 30 47 70

23 17 28 17

57 53 25 13

3.5 3.6 2.7 2.2

4 2 24 -

60 51 13 32

Smectite, kaolinite Smectite Illite, chlorite, smectite Vermiculite, smectite

(%)

washing with 1 M chloride salt solution of the cations; the excess salt was then removed with distilled H20 by repeated centrifugations, and the samples were air dried.

pNP solutions A stock (50 raM/l) of reagent grade p N P in xylene (mixture of isomers) was prepared and diluted as needed to obtain a wide range of concentrations (1-50 mM/1), with the pure solvent. The solutions and the solvent were stored over anhydrous Na2SO4.

Adsorption isotherms 10-ml portions of p N P solutions of different concentrations were added to 100 mg of two series of Na +- and Ca2+-saturated soil samples in glass tubes and heated for 3 h: ( 1 ) at 160 ° C or (2) at 90 oC, and not allowed to rehydrate. The stoppered tubes were shaken overnight at 20 ° C. The heating at 90 °C was found more suitable than the equilibration at 10% RH, used in the previous paper (Ristori et al., 1985 ) for dehydrating samples in order to reduce water competition with p N P for adsorption sites. The treatment at 90°C was not so drastic as to produce the complete collapse (to 0.96 nm basal spacing) of swelling clay minerals, obtained by heating at 160°C. This facilitates the penetration of the organic molecules into the interlayer spaces and hence the covering of internal surfaces. The amount of p N P adsorbed was calculated by difference. An appropriate aliquot of the equilibrium solution of p N P in xylene was transferred to a vol-

524 umetric flask containing water adjusted to about pH 12: since the pNP is completely partitioned into the aqueous phase, the residual small amount of supernatant xylene can be taken off. The absorbance of the developed yellow solution was measured photometrically at 400 nm. The amount of pNP adsorbed (micromoles/g of air-dry soil) was plotted against the concentration of the equilibrium solution of pNP in xylene (mM/1). All measurements were carried out in duplicate.

Calculation of specific surface area The SSA for a given pNP adsorption value, corresponding to the completion of the monolayer coverage, has been calculated as follows: SSA (m2/g) =XmNA" 10 -24

(1)

where: Xm = pNP adsorbed (micromoles/g); N = Avogadro number (6.02.1022); A =cross-sectional area of pNP molecule, assumed adsorbed flat (0.525 nm2; Giles and Nakhwa, 1962). The A value must be doubled (1.05 nm e) when a pNP monolayer is adsorbed into the interlayer region of swelling clay minerals (see below).

Comparison methods For comparison the SSA has been determined: (1) for all soil samples by N2 sorption at 77 K after degassing the samples overnight at 80 oC, using a C. Erba Sorptomatic equipment and the classical BET equation for calculations: (2) for Ca samples only by the ethylene glycol monoethyl ether (EGME) adsorption method (Cihacek and Bremner, 1979). The variation of basal spacings after pNP adsorption of the swelling phyllosilicates present in one group of soils, was determined by X-ray powder diffraction (20=5 ° to 10 °, CuK~ radiation) in a controlled dry atmosphere, maintained, after heating the samples at 120°C for 20 min, by covering the sample holder with Mylar film. RESULTS AND DISCUSSION

Soils without swelling clay minerals The adsorption isotherms of all soil samples of this group exhibit the same shape, (Langmuir, L-type isotherms) independent of the soil characteristics and the pretreatments undergone (Ca e+- or Na + -saturation, heating at 160 ° C or 90 °C ). For this reason only the curves for two soils are reported in Figs. 1 and 2. The maximum amount of pNP adsorbed as indicated by the plateau of

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~

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i

2D

0

O

I

• -

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75.

n

-

• "

E ~ 5~

z

n

2s

pNP inXylene mMfl

Fig. L p N P adsorption isotherms (20 ° C ) of Na + and Ca 2+ Montespertoli soft (group I, Regosol), dried at 160 ° or 90°C. 25~ SIENA 225 2g~

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0

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pNP in Xylene mM/I F i g . 2. p N P a d s o r p t i o n i s o t h e r m s ( 2 0 ° C ) Terra Rossa), dried at 160 ° or 90°C.

o f N a + a n d C a 2÷ S i e n a s o i l ( g r o u p I, M e d i t e r r a n e a n

526 TABLE lI Specific surface area (m 2 g - ' ) air dry soil Na ÷ soils

Ca 2+ soils

N2

pNP EXT

N2

pNP EXT

pNP TOT

EGME

48 23 44

46" 24*' 48*'

63 52 62

63" 52*' 64*'

63"1 52*' 64*'

154 122 151

26 23

29*' 24 .I

33 32

32*' 32 .1

32*' 32*'

105 84

88 78

90 *2 76 *2

83 81

79 *3 85 *3

217 *4 246 *4

381 367

19

19 *2

38

36 *3

52 *4

125

18

16 *2

14

16 *3

86 *4

98

Group I Med. Terra Rossa: Fasano Gravina Siena Regosols: Monteuliveto Montespertoli

Group II: Vertisols: Canicatti Sardara Regosol: Rumianca Cambisoh S.S.Marie

* 1From plateau of samples conditioned at 90 ° or 160 ° C. *2From plateau or point X (change of isotherm slope ) of samples conditioned at 160 ° or at 90 ° C, respectively. *3From point X of samples conditioned at 90 ° or 160 ° C. *4From plateau of samples conditioned at 90 ° C.

the isotherms is related to the type of soil and to the exchange cations (Na + or Ca 2+ ), but not to the degree of dehydration of the samples. The adsorption value at the plateau is assumed to be the result of the completion of a monolayer coverage by p N P and is introduced in eq. 1 to calculate the surface area. The good agreement between the N2 and p N P SSA (Table II) supports this assumption. By contrast, the much higher values of the EGME surface areas suggest that also this method overestimates soil surface area as observed by Newman (1983) with EG.

Soils containing swelling clay minerals For soils of this group the pretreatments are of a great importance and therefore it is convenient to describe their different behaviours separately:

527

Na samples The p N P adsorption isotherms, after heating at 160 ° C, are quite similar to those obtained for the first group of soils, The L-type curves (Figs. 3, 4) terminate in a plateau from which a SSA in very good agreement with N2 area can be calculated (Table II). The same soils, after heating at 90 ° C, show composite isotherms: their first branch is still of the L-type, but a second linear branch is obtained at higher p N P concentrations. According to Giles et al. 1960), this second type of curve can be classified as a C (constant partition ) -isotherm and its significance is that new adsorption sites become available on the adsorbent as the concentration of the adsorbate increases. This is consistent with the hypothesis of a progressive penetration of p N P into the interlayer spaces of swelling clay minerals, as revealed by X-ray analyses of soils heated at 120°C for 20 min and not allowed to rehydrate. In Fig. 5 the Canicatt~ Na ÷ soil diffractograms are reported. The sample relative to the l-branch of the isotherm (Fig. 3) collapses like the untreated sample, while that relative to the C-branch remains partially swollen. A plateau, indicating the completion of a monolayer adsorption of the organic molecule, is not attained in these soils, probably because of the limit imposed by the solubility of p N P in xylene. Values of "external" surface area, almost identical to those calculated for samples heated at 160 ° C, are obtained by putting into eq. 1 the amount of p N P adsorbed at the onset of linear segments of the curves. This point X (Figs. 3, 4) is assumed to reveal both the completion of coverage of the external surface, and the starting of p N P penetration in the interlayer spaces.

Ca samples Independent of the temperature at which the samples were preheated, all the adsorption isotherms for this group show the composite shape described above. The only significant distinctive feature is a maximum of the adsorption value (plateau) always seen, after the C-branch, on isotherms of samples preheated at 90 ° C (Figs 6, 7 ). This relatively moderate pretreatment allows p N P to more easily penetrate in the interlayer spaces, as demonstrated by the steep slope of the C-branch of these curves. By contrast, due to the greater difficulty for p N P to penetrate between the layers, the curves for fully dehydrated samples are flatter and the plateau is seldom attained. From samples preheated at 90 ° C the total SSA of the soils can be calculated, assuming the plateau to indicate the completion of p N P monolayer coverage including also the internal surface of swelling clay minerals. An accurate allocation of the amount of p N P adsorbed onto the external surface (indicated by point X) is very important because each interlayer region has a top and

528

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4~0~

•~

I

L>. 358 "0 -~C_ 389

98 "C dri

/

b 288

g

5~

pNP in Xylene remit

Fig. 3. p N P adsorption isotherms of Na + Canicatti soil (group II, Vertisol), dried at 90 ° and 1 60°C. 4~

368]

]

SARDAI~Na $AIIJRA'I'ED

J

II 99 *C dried

[68 "¢ deled

328

_

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L

~

zRe.

o

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1611.

~

128

zo.°- 881

pNP in Xylene mM/I Fig. 4. p N P adsorption isotherms (20°C) of Na + Sardara soil (group II, Vertisol), dried at 90 ° and 160°C.

529

I 5

I 6

I 7

DEGREES

1 8

I 9

10

26

Fig. 5. Na + Canicatti soil (group II, Vertisol). Smoothed X-ray diffraction tracings of samples dried at 90°C and equilibrated at different pNP concentrations in xylene: a, untreated; b, 11.3 raM/l; c, 45.7 mM/1. b o t t o m surface both covered by the same p N P monolayer. Therefore as noted before, the effective cross-sectional area to be considered in eq. 1 for calculating the internal fraction of the SSA is: 2 × 0.525 -- 1.05 nm. Point X is generally easy to identify on the isotherms for samples heated at 90 ° C, b u t sometimes, as found with pure smectites (Ristori et al., 1985 ), difficulties are encountered if the affinity of p N P for soil is high and the C-branch very steep. Such problems can be overcome by increasing the number of points on the curve in the field of interest and also by enlarging the scale. Otherwise, the isotherms obtained after heating at 160 ° C can be used: the "knee" on the plotting is more pronounced and, as a consequence, point X is more easily identified. In Table II the values of external surface area, obtained at point X, are compared with N2 surfaces, and the total p N P surface areas with those measured by the E G M E method. From these data, the selection of point X for calculating the external area of a soil containing swelling minerals seems confirmed. Total area (external + internal), calculated at the isotherm's plateau, is less than the E G M E

53O 600/ 550I

CAHICglII Ca SAIURAIEP

f d

5001 ! 458.

[I IGB "C dried

400.

~

30~ 25e

b

~_ zoo

~ 1110 50 8__

pNP in Xylene mM/I

Fig. 6. p N P adsorption isotherms (20 ° C ) of Ca 2+ Canicatti soil (group II, Vertisol), dried at 90 ~ and 160°C. |

5581

SARDfiR~Ca SiITUR~TED

458

FI 168 "C dried II 98 "C d~ied

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c_ .~ 258. fi

~ 158.

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531

area, but this was expected because of the overestimation of the external surface obtained with this method as suggested before. It has to be pointed out that the high incidence of the external surface (about 30-40% ) accounts for such overestimation also for our Vertisol samples. On the other hand, the values of total p N P SSA, calculated for pure smectites, are very close to the theoretical ones, as deduced from structural formulas (Ristori et al., 1985) and therefore appear to confirm the correctness of the data for soil samples. From the knowledge of the internal SSA it is also possible to calculate the approximate concentration of swelling phyllosilicates in soils, considering that generally the internal area is about 80% of total area (700-750 m2/g) in such minerals. CONCLUSIONS

The method for calculating the external and the total surface area of soils by p N P adsorption from xylene solution provides realistic and reproduceable results. The procedure is simple and rapid, particularly for soils without swelling minerals. In these cases, owing to the coincidence of the external and total surfaces, a few determinations at high p N P concentrations (40-50 mM/1) are sufficient to give the maximum adsorption values to be introduced in eq. 1, and complete plotting of the isotherms is not necessary. In the presence of swelling clay minerals, only the external SSA can be measured for Na + -saturated soils by using the adsorption values at the plateau, or at point X, of the isotherms depending on the preheating temperature. With Ca z+ soils, often corresponding to the saturation in the natural environment, it seems possible to calculate both the total and the external surface areas from a single complete isotherm if the samples are preheated to 90 ° C. As expected, the influence of water content and of the nature of exchange cation is considerable only in the second group of soils. In order to obtain the maximum adsorption value, the best compromise has to be found between the necessity of dehydrating the soil enough to reduce the competition of water for adsorption sites, and at the same time avoiding the complete collapse of the swelling minerals. A moderate preheating (90°C) facilitates the penetration of pNP, allowing the coverage of both external and internal surface to be completed in Ca 2+ samples. In the Na + samples, due to the lower affinity of the organic molecule for this cation, the coverage is not complete and the plateau in the isotherm is not attained. These differences of affinity appear to be related to the polarizing power of the exchange cations. After heating at 160°C, the p N P molecule is able to overcome the forces holding the layers together so that, in the fully dehydrated Ca 2+ samples, the adsorbate partially penetrates the interlayers. For Na +saturated samples, which are irreversibly collapsed by such heating, the coverage by p N P is limited to the external surface. The lower polarizing power of

532 t h i s m o n o v a l e n t c a t i o n is a p p a r e n t l y i n s u f f i c i e n t to e n a b l e p N P to p e n e t r a t e b e t w e e n t h e layers. ACKNOWLEDGEMENTS T h e a u t h o r s are g r a t e f u l to Mr. A. D o d e r o for c o n s c i e n t i o u s t e c h n i c a l h e l p in p r e p a r i n g t h e X - r a y d i f f r a c t o g r a m s . R e s e a r c h w o r k s u p p o r t e d b y C.N.R., Italy. Special g r a n t I.P.R.A. S u b - p r o j ect 1 - - P a p e r No. 1828.

REFERENCES Cihacek, L.J. and Bremner, J.M., 1979. A simplified ethylene glycol monoethyl ether procedure for assessment of soil surface area. Soil Sci. Soc. Am. J., 43: 821-822. Giles, C.H., McEwan, Z.H., Nakhwa, S.N. and Smith, D., 1960. Studies in adsorption, Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J. Chem. Soc., pp. 39733993. Giles, C.H. and Nakhwa, S.N., 1962. Studies in adsorption. XVI. The measurement of specific surface areas of finely divided solids by solution adsorption. J. Appl. Chem., 12,266-272. Giles, C.H., D'Silva, A.P. and Easton, I.A., 1974. A general treatment and classification of the solute adsorption isotherms, Part II. Experimental interpretation. J. Colloid Interface Sci., 47: 766-777. Greenland, D.J. and Quirk, J.P., 1964. Determination of the total specific surface area of soils by adsorption of cetyl pyridinium bromide. J. Soil Sci., 15: 178-191. Newman, A.C.D., 1983. The specific surface of soil determined by water sorption. J. Soil Sci., 34: 23-32. Ristori, G.G., Sparvoli, E., Fusi, P., Quirk, J.P. and Martelloni, C., 1985. Measurements of total and external surface area of homoionic smectites by p-Nitrophenol adsorption. Miner. Petrogr. Acta, 29-A: 137-143. Slade, P.G., Raupach, M. and Emerson, W.W., 1978. The ordering of cetylpyridinium bromide on vermiculite. Clays Clay Miner., 26: 125-134.