Surface areas of calcium carbonate in soils

Geoderma, 13 (1975) 247--255 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

S U R F A C E A R E A S O F C A L C I U M C A R B O N A T E IN S O I L S

I.C.R. HOLFORD' and G.E.G. MATTINGLY Rothamsted Experimental Station, Harpenden (Great Britain) (Received February 27, 1974; accepted for publication October 4, 1974)

ABSTRACT Holford, I.C.R. and Mattingly, G.E.G., 1975. Surface areas of calcium carbonate in soils. Geoderma, 13: 247--255. In 24 calcareous soils derived from Jurassic limestone in southwest England (Sherborne series), the specific surface areas of the carbonate component were an inverse hyperbolic function of the percentage of CaCO3 they contained, and ranged from 16 to about 500 m2/g. The surface areas of three samples of Jurassic oolitic limestone varied from 1.0 to 1.5 m2/g, so weathering processes had greatly increased the specific surface areas of calcium carbonate in these soils. The total surface areas of calcium carbonate were a slightly inverse and linear function of the percentage of CaCO 3, and ranged from 4.0 to 8.5 m:/g soil.

INTRODUCTION Calcium c a r b o n a t e is o n e o f the m o s t surface-active c o n s t i t u e n t s o f calcareous soils. Its effects, as a soft a m e n d m e n t , o n t h e p r o p e r t i e s o f acid soils have b e e n w i d e l y studied b u t less is k n o w n a b o u t t h e r e a c t i v i t y o f native soil carbonates. It is s o m e t i m e s assumed t h a t t h e p e r c e n t a g e o f CaCO3 in t h e soil, d e t e r m i n e d chemically, provides a satisfactory i n d e x o f its t o t a l area. However, this a s s u m p t i o n implies t h a t all CaCO3 consists o f a p p r o x i m a t e l y equalsized particles - - i.e., t h a t t h e specific surface area o f CaCO3 does n o t vary f r o m o n e soil t o a n o t h e r . T h e limitations o f t h e a b o v e a s s u m p t i o n were first d e m o n s t r a t e d b y B o i s c h o t et al. ( 1 9 5 0 ) , w h o s h o w e d t h e i m p o r t a n c e o f t h e surface areas o f CaCO3 in p h o s p h a t e f i x a t i o n b y calcareous soils. T h e i r estimates o f CaCO3 surface areas, calculated f r o m t h e m e a n d i a m e t e r o f t h e soil particles, ranged f r o m 13.5 cm2/g in soils c o n t a i n i n g 90% CaCO3 and u p t o 1 2 0 cm2/g in soils c o n t a i n i n g 47% CaCO3 - - i.e., t h e t o t a l surface area o f CaCO3 in t h e least calcareous soil was nine times larger t h a n in t h e m o s t calcareous soil. T a l i b u d e e n and A r a m b a r r i ( 1 9 6 4 ) used a 4SCa i s o t o p e - e x c h a n g e p r o c e d u r e t o m e a s u r e CaCO3 specific surface areas and f o u n d t h a t these ranged f r o m 20 t o 51 m2/g 1Present address: Agricultural Research Centre, RMB 944, Tamworth, N.S.W. (Australia).

248 in soils containing from 7 to 81% CaCO3. They related these surface areas, with some success, to the P buffer capacities of the soils but did not measure the extent of other P-adsorbing surfaces in their soils. Because CaCO3 is an important adsorbent of phosphate in calcareous soils (Cole et al., 1953), P adsorption capacity should be positively correlated with % CaCO3 in the soil. However, recent work shows that both the P adsorption capacities of lake sediments (Shukla et al., 1971) and the P buffer capacities of a group of unrelated soils (Webber and Mattingly, 1970) were negatively correlated with their CaCO3 contents. These unexpected relationships could arise from large variations in the surface reactivity of CaCO3 present in the sediments or soils. The purpose of the following work was to measure the relationships between the percentage of CaCO3 and the surface reactivity of the CaCO3 in a group of 24 soils all formed on Jurassic oolitic limestone. The surface areas of the parent limestones were also measured to estimate the changes which occur during weathering. MATERIALS AND METHODS

Soils The 24 calcareous surface softs (Ap horizon, sampled to depths of 15--20 cm) were taken from 8 different sites on the Sherborne series in southwest England and their analyses were given by Russell (1963). In the soil classification used by Avery (1973), this soft series includes brown rendzinas, with limestone brash at 30 cm or less, and brown calcareous earths. These softs were described as Lithic Udorthents and Lithic Eutrochrepts respectively by Ragg and Clayden (1973). Three limestone samples were taken from fresh exposures at quarries within the areas occupied by the Sherborne soil series; their analyses and properties are given elsewhere (Holford and Mattingly, 1975). Analytical methods CaC03 contents in soils. CaCO3 concentrations were determined by a manometric method described by Williams (1948). All soils with less than 6% carbonate were also analysed by the titrimetric method of Tinsley et al. (1951). Surface area o f soil carbonates. The specific surface areas of soil carbonates were determined by the method described by Talibudeen and Arambarri (1964), which is based on the principle that the difference between the isotopically exchangeable Ca (Cae) in an intact soil and in the same soil after decalcification will be directly related to the surface area of the CaCO3 it contains. The surface area is calculated from the percentage of CaCO3 and

249

the surface area (20 A 2) occupied by each Ca atom; l#g Ca e is equivalent to an area of 3.03 • 10 -3 m 2. Soils were suspended in M CaCl2 and decalcified with N HCI added, from a burette, at a rate which maintained the p H of the suspension above 4. Even with slightheating (to about 40 o C) and continuous agitation,the most calcareous soilstook up to 14 days to decalcify because the coarsest carbonate fraction was very resistant.Decalcification was considered complete when the p H of the suspension was 4.5 • 0.2 after shaking overnight. The p H of the 1 :I0 soil:water suspension increased by 1.0 to 2.0 units during washing, indicating that Ca saturation of the exchange complex had not been significantly decreased during decalcification. The decalcified soil (10.00 g) and weights of whole soil containing an equivalent amount of carbonate-free residue were equilibrated with 100 ml 0.01M CaCl2 for 16 h on a reciprocating shaker (200 oscillationsper min) at 23 _+ 2 o C before labellingwith 2 pCi 4SCa. Labelled soil suspensions were centrifuged at 2 000 rpm for 10 rain at 24 + I o C before sampling, and aliquots (0.50 ml) were mixed with 5 ml scintillatorfor radioactive assay using a Beckman LS system. All soilswere analysed in duplicate and counting errors were 1 % and rates were 5 000--9 000 c p m for soil suspensions and 15 000 to 20 000 c p m for the controls. The 4SCa exchange was usually complete within 30 rain in the decalcified soil and within 60 rain in the whole soil;there was no further exchange for at least 24 h. The mean fractional activity (f) was 0.386 on the decalcified soils and 0.355 on the whole soils,where f = the ratio of the amount of 4SCa in solution after time, t, to the amount of 4SCa present initially(t = 0). Exchangeable Ca for each soil was calculated from: Ca conc. in solution after exchange (Ce) - Ca conc. before exchange (Co)

f where Co, the concentration before exchange (in 0.01M CaCl2), is 400 ~g Ca/ml. Table I gives fractional activities,Ca concentrations in solution after exchange and the surface-exchangeable Ca (Cae) in the soils,calculated as ~g Ca/g CaCO3 from: Ca e = (Celf-Co)lw where w = weight of CaCO3/ml of extracting solution.

Surface areas of Jurassic limestones. Surface areas of three Jurassic limestones (LM 52, LM 67 and LM 70), ground <~0.5 ram, were measured directly b y 4SCa exchange on 10.00 g limestone equilibrated for 16 h in 100 ml O.O1M CaC12 before labelling. Methods of sampling and counting were the same as for the soil carbonates. Contrary to the results with soil carbonates, 4SCa exchange continued for

250

TABLE I ~Ca fractional activities, solution Ca concentrations, and isotopically exchangeable Ca after equilibrating soil suspensions in O.O1M CaCI~ Soil No. 1

CaCO~ (%)

Fractional activity

Ca conc. (ug/ml)

decalc, soil

whole soil

decalc, soil

whole soil

Exchangeable Ca (~g Cae/g CaCO~ )

A 11049 50 51

24.2 5.0 20.0

0.392 0.417 0.398

0.364 0.389 0.366

416 398 404

450 427 439

5 403 ± 890 26 893 + 1 056 7 422 ± 297

52 53 54

16.9 21.8 16.8

0.375 0.324 0.339

0.345 0.308 0.313

400 405 404

443 447 438

10 501 ± 925 7 140 ± 1 554 10 420 ± 2 110

55 56 57

8.9 9.6 12.8

0.408 0.415 0.438

0.380 0.381 0.403

405 401 403

438 443 438

16 009 + 2 278 17 789 + 2 442 10 936 ± 1 416

58 59 60

17.0 13.4 15.7

0.387 0.343 0.355

0.332 0.319 0.330

402 413 414

456 450 448

16 456 ± 462 13 215 + 1 088 10 238 + 991

61 62 63

7.1 4.9 5.6

0.390 0.411 0.425

0.358 0.372 0.374

397 400 400

440 435 440

27 402 ± 5 837 37 381 ± 3 926 39 879 ± 2 509

64 65 66

0.8 1.5 1.0

0.426 0.391 0.353

0.400 0.350 0.346

398 399 405

434 428 453

168 783 ± 6 099 135 294 ± 4 950 159 864 + 9 064

67 68 69

8.7 10.8 10.6

0.360 0.366 0.368

0.340 0.343 0.344

406 403 400

450 431 444

20 104 ± 989 13 589 ± 33 17 406 ± 1 385

70 71 72

3.0 2.5 3.1

0.426 0.379 0.392

0,371 0.338 0.357

398 400 400

428 430 431

70 649 ± 3 169 95 467 + 13 874 57 669 • 1 186

F o r additional analyses on these soils, see Russell (1963).

many hours. The initial rapid decrease in the fractional activity (f) ceased, however, after about 6 h, and thereafter f decreased at a uniform rate (Fig. 1). The linear sections of the fractional activity-time curves are usually attributed to self-diffusion of 4SCa into the calcite crystal (Lahav and Bolt, 1964), and their absence in soil carbonates may be due to coatings, such as silicates, on the carbonate surface (Lahav and Bolt, 1963). The values obtained by plotting fractional activity against time and extrapolating the linear sections of the curve, obtained after 6 h, to zero time (Fig. 1), were used to measure true surface exchange. This method, which is similar to that described by Inks and Hahn (1967) differs, however, in the use of 0.01M CaC12 as an ambient electrolyte and it

251 1 .0

0.9

LM C~7 -

o

0.8

.o_ P h

0.7

0

I 5

I I I 10 15 20 Time a f t e r labelling (hours)

I 25

Fig. 1. Effects of time of shaking on 4SCa fractional activity on limestones.

avoids changes in the solid:solution ratio during successive measurements of the fractional activity. The use of CaC12 instead of water also minimises the retention of solid CaCO3 in suspension after centrifugation, which could produce errors in the concentration of Ca. The removal of large aliquots (2 ml from 25 ml suspensions by Inks and Hahn) for 4SCa counting can cause significant losses of Ca from the exchange system and consequent errors in the measurement of Cae. Shaking in water, rather than in 0.01M CaC12, would also increase the possibility of recrystallisation. RESULTS AND DISCUSSION

Specific surface areas in relation to % CaC03 in soils

There was an inverse linear relationship between specific surface area (S.S.A.) and % CaCOs for soils with more than 12% CaCOs. However, where the CaCOs was less than 12% the inverse relationship became hyperbolic and the specific surface areas increased from about 50 m2/g for soils with about 10% CaCOs to about 500 m2/g for soils with about 1% CaCO3 (Fig. 2).

252 el

500

F'

i

i

400

t

E

U

300

100 S S A = 5.8 , ~-z-~:-~-_~ ,~,(" 3 -~)

"6 o

Io

u 200

l%

\

\\ • \o

100

%

I

0

\ ~.,o,,.°

I 5

I I 10 15 COC03 c o n t e n t (°Io)

I 20

I 25

Fig. 2. Relationship between specific surface area (S.S.A.) of CaCO, and CaCO~ content (%) of soils derived from Jurassic limestone (Sherborne series). The overall relationship was well described by the following equation: (100 S.S.A. = 5.8

% CaCO3

) -1

No record can be found in the literature of CaCO3 surface areas exceeding about 50 m2/g. The specific surface areas of CaCO3 measured by Talibudeen and Arambarri (1964) were 51 m2/g for a soil formed on lake marl, but less than 40 m2/g CaCO3 for six other soils. The probable explanation of this inverse relationship is that the specific

253

surface area (or particle size) is related to the degree of weathering. The more weathered the soft, the lower the % CaCO3, the smaller the particle size, and the larger the specific surface area. This explanation is supported by the high correlation (r=0.83) between specific surface area and % of Na-dithionitesoluble Fe in the 24 soils which ranged from 2.3 to 8.9% Fe. Total surface areas in relation to % CaC03 in soils

The most important consequence of the above results is the inverse relationship it gives between total surface areas (which ranged from 4.0 to 8.5 m 2 CaCO3 per g soil) and the % CaCO3 in the softs (Fig. 3). This shows that the higher the CaCO3 content of the softs the lower is its total surface area. This relationship obviously holds only within the range of CaCO3 contents present in these softs and the regression line in Fig. 3 cannot be extrapolated to zero % CaCO3. The specific surface areas of CaCO3 in soils containing less than about 4% CaCOs (Fig. 2), which range from 100 to 500 m2/g, suggest that the crystallites have equivalent spherical diameters of 250--50 A, which are comparable with the crystallite size of apatite in bone (Neuman and Neuman, 1953). The line fitted in Fig. 3 was calculated on the assumption that the total surface area was a function only of (100-% CaCO3 ) -- i.e. that it reached zero at 100% CaCO3. This oversimplification gave a better fit to the data than would have been obtained if a positive intercept at 100% CaCO3 had been included in the regression. The anomalously high CaCO3 surface area for one soil (A 11058) was omitted in calculating the lines fitted in Figs. 2 and 3. The very large total surface area in this soil may have been caused by deep ploughing bringing coarser CaCO3 into the sampled zone and thereby increasing the CaCO3 content. The method used to measure specific surface areas was subject to considerable experimental errors because: (1) it was based on a CaCO3 analysis made on another sub-sample of soil; (2) it was difficult to remove all the CaCO3 without reducing the Ca saturation of the cation-exchange complex; and (3) it was based on the difference in Cae between one whole soft sample and another decalcified sample. For example, the difference in Cae on a soil containing 1% CaCO3 is based on isotopic exchange on only 100 mg CaCO3 (from 10 g soil) whereas it is based on 2.50 g CaCO3 for a soil containing 20% CaCO3 (because the amount of whole soil, which contains 10.0 g of carbonate-free residue, is 12.5 g). Analytical errors therefore tend to be inversely related to the % CaCO~, a trend which is illustrated by the scatter of points in Fig. 3. Because of this variability, the data for CaCO3 surface areas were weighted according to the % CaCO3 in order to establish the slope of the regression line in Fig. 3.

254

10

S.A. = 0 . 0 5 8

(lO0-e/,CaC03)

0

8

~eil~

"a

~

•

oo

•

# e

~4

e

2L 0

0

I 10

I 20

I 30

CoCO 3 cor~en I ("/e)

Fig. 3. Relationship between total surface area o f CaCO 3 and CaCO 3 content (%) of soils derived from Jurassic limestone (Sherborne series).

Changes in surface activity of CaC03 during weathering The surface areas of the three samples o f Jurassic oolitic limestone (98% CaCO3 ), calculated from the data in Fig. 1, were 1.0 m2/g for LM 52 and LM 67 and 1.5 m2/g for LM 70. The most calcareous soil contained 24% CaCOs (S.S.A. = 16 m2/g) and the least calcareous soil contained 0.8% CaCO3 (S.S.A. = 512 m2/g). Weathering processes have increased the specific surface area of CaCOs from about 1.0 m2/g in the parent limestone to about 500 m2/g in the least calcareous soils. The inverse relationship established here between CaCO3 content and total surface area shows that the reactivity of the CaCOs is negatively related to the CaCO3 content of these soils and may explain the negative correlations between the P sorption characteristics of lake sediments and soils and their CaCOs contents previously described by Webber and Mattingly (1970) and Shukla et al. (1971).

255

ACKNOWLEDGEMENTS

The senior author was supported financially by the New South Wales Department of Agriculture and the Australian Wool Corporation. We thank Margaret Chater for CaC03 analyses of some soils, V. Cosimini for Ca analyses of soil solutions, R.W.M. Wedderburn for statistical analyses, and O. Talibudeen for advice and discussions.

REFERENCES

Avery, B.W., 1973. Soil classification in the soil survey of England and Wales. J. Soil Sci., 24:324--338 Boischot, P., Coppenet, M. and Hebert, J., 1950. Fixation de l'acide phosphorique sur le calcaire des sols. Plant Soil, 2:311--322 Cole, C.V., Oisen, S.R. and Scott, C.O., 1953. The nature of phosphate sorption by calcium carbonate. Soil Sci. Soc. Am. Proc., 17:352--356 Holford, I.C.R. and Mattingly, G.E.G., 1975. Phosphate sorption by Jurassic oolitic limestones. Geoderma 13:257--264 Inks, C.G. and Hahn, R.B., 1967. Determination of surface area of calcium carbonate by isotopic exchange. Anal. Chem., 39:625---628 Lahav, N. and Bolt, G.H., 1963. Interaction between calcium carbonates and bentonite suspensions. Nature (Lond.), 200:1343--1344 Lahav, N. and Bolt, G.H., 1964. Self-diffusion of Ca 45 into certain carbonates. Soil Sci., 97:293--299 Neuman, W.F. and Neuman, W.M., 1953. The nature of the mineral phase of bone. Chem. Rev., 53:1--45 Ragg, J.M. and Clayden, B., 1973. The classification of some British soils according to the comprehensive system of the United States. Soil Surv. Tech. Monogr., No. 3. Harpenden, 227 pp. Russell, R.D., 1963. Experiments on cumulative dressings of fertilisers on calcareous soils in south-west England. I. Description of field experiments and soil analysis for phosphorus residues. J. Sci. Food Agric., 14:622--628 Shukla, S.S., Syers, J.K., Williams, J.D.H., Armstrong, D. and Harris, R.F., 1971. Sorption of inorganic phosphate by lake sediments. Soil Sci. Soc. Am. Proc., 35:244--249 Talibudeen, O. and Arambarri, P., 1964. The influence of the amount and the origin of calcium carbonates on the isotopically-exchangeablephosphate in calcareous soils. J. Agric. Sci., Camb., 62:93---97 Tinsley, J., Taylor, T.G. and Moore, J.H., 1951. The determination of carbon dioxide derived from carbonates in agricultural and biological materials. Analyst. (Lond.), 76: 300--310 Webber, M.D. and Mattingly, G.E.G., 1970. Inorganic soil phosphorus. I. Changes in monocalcium phosphate potentials on cropping. J. Soil Sci., 21:111--120 WilliRmR~D.E., 1948. A rapid manometric method for the determination of carbonate in soils. Soil Sci. Soc. Am. Proc., 13:127--129