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Leaching of nitrate in calcareous soils as influenced by its adsorption on calcium carbonate
Geoderma, 20 (1978) 271--279 271 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
L E A C H I N G O F N I T R A T E I N C A L C A R E O U S S O I L S AS I N F L U E N C E D BY ITS A D S O R P T I O N O N C A L C I U M C A R B O N A T E
BIJAY SINGH and G.S. SEKHON
Department of Soils, Punjab Agricultural University, Ludhiana 141004 (India) (Received January 11, 1977; accepted January 9, 1978)
ABSTRACT Singh, B. and Sekhon, G.S., 1978. Leaching of nitrate in calcareous soils as influenced by its adsorption on calcium carbonate. Geoderma, 20:271--279. Adsorption of the nitrate ion on calcium carbonate and its leaching in calcareous soils were examined by equilibrium and elution techniques. Nitrate ions adsorbed on the surface of CaCO 3 fitted the Langmuir model well at an equilibrium concentration of 40 ppm NO~. Sulphate ions reduced adsorption of nitrate. Data for elution of surface-applied nitrate from laboratory soil columns, when plotted in the form of elution curves and semi-log plots, indicated interactions of nitrate with the soils. The elution curves had long trailing portions due to desorption of nitrate. The length of the trailing portion of a curve was determined by the amount of nitrate adsorbed which in turn seems to depend upon the total surface area of CaCO 3. Sulphate ions when present in the displacing fluid seem to desorb nitrate ions from the surface of the CaCO 3 whereas chloride ions have little or no effect.
INTRODUCTION The n i t r a t e ion is regarded as a w a t e r p o l l u t a n t because o f its p o t e n t i a l as a health h a z a r d w h e n its c o n c e n t r a t i o n increases b e y o n d 45 p p m NO~- (WHO, 1963). Moreover, dissolved nitrate leaches u n i m p e d e d t h r o u g h the soil i n t o g r o u n d w a t e r (Eliassen a n d T c h o b a n o g l o u s , 1 9 6 9 ; J o h n s o n et al., 1 9 6 5 ) and b e c o m e s inaccessible t o plants. Its a d s o r p t i o n on s o m e soil c o m p o n e n t s , however, can r e d u c e such losses, m a k i n g it available t o g r o w i n g plants f o r a time. T h e a d s o r p t i o n o f n i t r a t e in soils has o n l y been s t u d i e d t o a limited extent. Singh and K a n e h i r o ( 1 9 6 9 ) r e p o r t e d on t h e a d s o r p t i o n o f nitrate b y t w o acidic soils f r o m Hawaii and used the F r e u n d l i c h i s o t h e r m to describe the ads o r p t i o n process. F o l l o w i n g a s t u d y b y S o m a s u n d a r a n a n d Agar ( 1 9 6 7 ) , w h i c h s h o w e d t h a t CaCO3 u n d e r certain c o n d i t i o n s develops a positively charged surface and suggested t h a t n i t r a t e i n t e r a c t i o n with CaCO3 is a d i s t i n c t possibility, J u r i n a k and Griffin ( 1 9 7 2 ) investigated t h e a d s o r p t i o n o f nitrate o n solid-phase CaCO3 b y equilibrium and c o l u m n t e c h n i q u e s . I n a s m u c h as CaCO3 is a p r e d o m i n a n t c a r b o n a t e mineral in alkaline soils o f semi-arid and arid regions, the a d s o r p t i o n o f NO~ o n the surface o f CaCO3 c o u l d be i m p o r t a n t
272
in determining the transport of NO3 in calcareous soils. The studies included in the present paper pertain to adsorption of nitrate ions on pure calcium carbonate and to leaching of nitrate through columns containing calcareous soil materials. An a t t e m p t was also made to investigate the roles of C1- and S O : 2 in desorbing the adsorbed NO~. MATERIAL AND METHODS
Equilibrium adsorption studies were conducted at 30 ° C_+2 ° C with the following adsorbents: (1) calcium carbonate (May and Baker Ltd., Dagenham, England; lot C 93678; (2) Ludhiana loamy sand; (3) Gurdaspur sandy loam; (4) Kanganwal sandy clay loam; and (5} Langrian loamy sand. Selected characteristics of the soil samples (0--20 cm layer) are given in Table I. 25-ml portions of standard NaNO3 solutions (5, 10, 20, 40 and 50 ppm NO~) were added to 250 ml Erlenmeyer flasks containing 4 g of CaCO3. The flasks were shaken for an hour to attain equilibrium. The solid phase was then separated by filtration and nitrate ion concentration in the equilibrium solution was determined by a phenoldisulphonic acid m e t h o d (Bremner, 1965) with a slight alteration (Singh and Sekhon, 1976). The a m o u n t of adsorbed nitrate was estimated from the difference between initial and final nitrate ion concentrations in solution. Equilibrium adsorption studies with soil samples were carried o u t in the same manner, b u t with the alteration that 5 g portions of each sample were equilibrated with 25 ml portions of 5, 10, 20, 40 and 50 ppm NO~ solutions. Adsorption studies with samples of CaCO3, Ludhiana loamy sand (l.s.) and Kanganwal sandy clay loam (s.c.1.) were also conducted with standard NO~ solutions containing 5 ppm C1- (as NaC1) + 25 ppm SO42 (as Na2 SO4) or 25 ppm C1- + 5 ppm SO~-2. Column elution studies were conducted in 5.2 cm ID glass tubes. The columns were made up of a b o t t o m layer of glass wool and a disc of Whatman No. 42 filter paper overlaid by the adsorbent which was covered at the surface by another disc of filter paper. The adsorbents used for column studies were samples of Ludhiana 1.s., Gurdaspur s.1. (sandy loam) and Kanganwal s.c.1. Total air-dried weight of the adsorbent in each column was 500 g. The adsorbent columns were initially eluted with distilled water till there was no nitrate ion in the leachate. Then, with the water surface flush with top of the adsorbent column, 1 mg NO~ (as NaNO3) was added in a 1 ml aliquot to each. The nitrate was then eluted by continued addition of distilled water at a specific rate, applied by a constant-head device. The leachate fractions were collected at a fixed volume interval (depending upon the specific rate of addition) and analysed for NO~. Samples of Ludhiana 1.s. and Kanganwal s.c.1, were also eluted with solutions containing 5 ppm Cl- + 25 ppm SO~-2 and 25 ppm C1- + 5 ppm SO~-2. The elution data were plotted in the form of elution curves and according to the m e t h o d of Thomas (1963) who found that the rate of removal of a nonreactive anion in a saturated soil column is given by:
I
55.6 55.2 87.3 82.6
G u r d a s p u r s.l., H a p l u s t a l f K a n g a n w a l s.c.l., U s t o c h r e p t L a n g r i a n l.s., C a l c i o r t h i d L u d h i a n a 1.s., U s t i p s a m m e n t
10.1 24.6 5.5 8.9
Clay*~ (%)
0.61 0.75 0.33 0.34
O r g a n i c .2 carbon (%)
0.90 13.10 1.05 0.40
C a l c i u m *~ carbonate (%)
6.96 12.18 3.83 4.35
CEC mequiv./ 100 g
7.5 9.1 8.9 8.6
p H .4
Specific surface area o f C a C O 3 ~l (m2/g) 44.7 10.9 79.3 71.5
Electrical c o n d u c t i v i t y *~ (mmhos/cm)
0.16 0.33 0.14 0.11
percentage of clay + sand. • 2 Walkley a n d Black, 1 9 3 4 . • 3 Pllri, 1 9 4 9 . • 4 A t soil w a t e r r a t i o o f 1 : 2. • s Holford and Mattingly, 1975.
• 1 D i a m e t e r s o f clay a n d s a n d p a r t i c l e s are < 0 . 0 0 2 m m a n d 2 . 0 - - 0 . 0 2 m m , r e s p e c t i v e l y ; p e r c e n t a g e o f silt ( 0 . 0 2 - - 0 . 0 0 2 r a m ) is given b y 1 0 0 m i n u s
Sand*l (%)
Soils: location, type and group
S e l e c t e d c h a r a c t e r i s t i c s o f t h e soil s a m p l e s
TABLE
274
-dAt/dt = kAt
Ill
where k is a constant and At is the amount of anion in the column at time t during elution. When t = 0, the amount of anion in the column is Ai. Integrating eq. 1 and substituting In A i for the constant of integration gives: log A t = log Ai - k t / 2 . 3 0 2 6
(2)
When nitrate reacted with the adsorbent in the column a distinct change occurred in the slope of the straight line plot defined by eq. 2. Because the flow rate of elution was kept constant for a column, the volume of effluent expressed as fraction of the total pore volume has been substituted for the time variable in eq. 2. RESULTS AND DISCUSSION
A d s o r p t i o n studies
Nitrate adsorption as a function of the amount of CaCO3 in the system is shown in Fig. 1. The linear relation between total nitrate adsorption and the a m o u n t of CaCO3 shows that nitrate adsorption is a function of the interfacial area and that no apparent secondary reaction occurs. Data pertaining to nitrate adsorption by CaCO3 and by samples of Ludhiana 1.s., Kanganwal s.c.l., Gurdaspur s.1. and Langrian 1.s. are plotted in Fig.2. according to the Langmuir adsorption equation which, in its linear form, is:
(3)
Ce/S = 1/gaSm + Ce/Sm
where Ce is the final solution concentration of NO~ in ppm, S represents the amount of NO~ adsorbed in pg/g of adsorbent, Sm gives the adsorption maxi-
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o
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Equilibrium nitrate concentration, Ce (ppm)
Fig. 1. A d s o r p t i o n of nitrate f r o m solution by calcium carbonate as a f u n c t i o n o f the a m o u n t o f CaCO3. Y = 10.2 + 3 5 . 7 X ; r = 0 . 9 9 4 " * . Fig. 2. L a n g m u i r i s o t h e r m s f o r n i t r a t e a d s o r p t i o n b y soil samples and c a l c i u m c a r b o n a t e . - CaCO3; - L u d h i a n a l.s.; . . . . . . . L a n g r i a n 1.s.; . . . . . . = G u r d a s p u r s.1. ; . . . . . K a n g a n wal s.c.1.
275
mum and K a is a constant related to the bonding energy of NO~- to the adsorbent (ppm -1). The statistical significance of the correlation coefficient of Ce/S versus Ce was the criterion by which the fit of the data to the Langmuir isotherm was tested. Adsorption data for all soil samples fitted the Langmuir adsorption equation well. The adsorption of nitrate by CaCO3 is attributed to the steric compatibility of nitrate ion with the calcite crystal lattice. Both NO~- and COl 2 anions have planar configurations with similar dimensions. These anions are plane triangles with oxygen atoms at the corners. Indeed NaNO3 and CaCO3 are isomorphous (Partington, 1961). However, the energy of nitrate interaction is not expected to be large, as verified from values of AH of adsorption by Jurinak and Griffin (1972). The pH values of the CaCO3--NaNO3 suspensions were 8.3, which is close to the theoretical iso-electric point for calcite as calculated by Somasundaran and Agar (1967). Hence, the positive charge density and adsorption capacity of the CaCO3 for NO~- are expected to be low. The Langmuir coefficients (Sm and Ka) calculated from the equations for the best-fitting straight lines are given in Table II. It seems that the native CaCO3 of soils plays an important role in determining nitrate adsorption. This is evident from the Sm value for the Kanganwal sample, which is almost double the Sm values for the other three soil samples. The latter samples do not contain more than 1% CaCO3. Thus, magnitude of'nitrate adsorption is not proportional to the amount of CaCO3 in a calcareous soil. Possibly differences in specific surface areas of CaCO3 in different soils (Table I) are a factor in governing nitrate adsorption by CaCO3. Values of K a given in Table II further indicate that the bonding energy between nitrate ions and CaCO3 is least in the Kanganwal sample which adsorbs the greatest amount of nitrate per gram. The calcium carbonate in that sample may be less active than the carbonates in the Ludhiana, Langrian and Gurdaspur soil samples. Langmuir plots for nitrate adsorption by pure CaCO3 and by Kanganwal samples from equilibrium solutions at two levels of each of chloride and sulphate ions are shown in Fig.3. Note that data for the Kanganwal samples fit T A B L E II L a n gmuir coefficients for nitrate a d s o r p t i o n by calcium carbonate and soil samples in presence and absence o f C l - and SO~"2
Adsorbent
Calcium carbonate L u d h i a n a l.s. K a n g a n w a l s.c.l. G u r d a s p u r s.l. L a n g r i a n l.s.
N o C I - o r SO4 -=
25 p p m C1- + 5 p p m SO~-=
5 p p m C1- + 25 p p m SO~-2
Sm ug/g
Ka "10-2 ml/ug
Sm ug/g
Ka '10-2 ml/ug
Sm ug/g
Ka "10-2 ml/-g
80.7 17.6 45.5
4.4 3.5 1.6
71.5 17.3 41.6
3.8 3.8 2.7
66.1 12.3 23.5
2.9 4.1 3.2
18.0 24.2
3.3 2.4
--
---
--
--
276
the Langmuir equation and that the intercept of the Y-axis by the plot for the greater amount of sulphate ions is itself greater. The data for the Ludhiana sample (not plotted in Fig.3) also followed a similar trend. The effects of sulphate and chloride ions on nitrate adsorption are shown more clearly, however, by the Sm and K a values given in Table II. Maximum adsorption (Sm ) on CaCO3 is reduced from 80.64 pg/g to 71.48 pg/g in the presence of 25 ppm C1- + 5 ppm SO~-2 and to 66.09 pg/g in the presence of 5 ppm C1- + 25 ppm SO~ 2. A similar trend is observed for the soil samples. These data thus indicate that SO~ 2 ions c o m p e t e with adsorption of nitrate and reduce the amount of adsorbed nitrate appreciably. The decrease in Sm values in the presence of 25 ppm C1- + 5 ppm SO4-2 is always small and is perhaps due to sulphate ions. The data indicate a negligible effect of chloride ions on nitrate adsorption, an observation consistent with the earlier findings of Jurinak and Griffin (1972). 0
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Equilibrium nitrate concentration, Ce (ppm) Fig. 3. Langmuir isotherms for nitrate adsorption by calcium eaxbonate and Kanffanwa! samples in the presence o f C ] - and SO 42 ions in the e q u i l i b r i u m solution.- . . . . CaCO3, 25 p p m C ] - + 5 p p m SO~'2; . . . . . . CaCO 3, 5 p p m C ] - + 25 p p m SO~'~; = Kanganwa] s.c.]., 25 p p m C ] - + 5 p p m S0~'2; - Kanganwa] s.e.L, 5 p p m C ] - + 25 p p m SO~ 2.
Elution studies Data on the elution of nitrate ions through columns of Ludhiana, Kanganwal and Gurdaspur soil samples are plotted in the form of elution curves and also according to eq. 2 in Fig.4. In all the elution curves, the peaks are at a void volume of one. The area under each peak represents the dissolved nitrate which moves through the column, spreading as it goes, and the data form the expected normal curve. This fast-removal portion of the elution curve can also be approximately defined by a first order removal process as shown in the initial portions of the semi-log straight line plots. Curves in Fig.4 are not symmetrical. The lack of symmetry indicates nitrate adsorption in the soil columns. The long descending portion of each curve arises from desorption of nitrate from adsorption sites, with the length dependent on the a m o u n t of nitrate adsorbed. In fact as the peak in the curve diminishes, the trailing portion of the elution curve became more pronounced because of increased nitrate adsorption. Distinct changes in the slopes of the semi-log plots, which define nitrate elution as a first order process, are ascribed
277
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0.9
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1.5
1'7
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Equilibrium nitrate concentration, Ce (ppm)
Fig.4. N i t r a t e e l u t i o n c u r v e s a n d s e m i - l o g f i r s t o r d e r n i t r a t e r e m o v a l p l o t s o f e l u t i o n d a t a f o r t h r e e soil s a m p l e s . • - K a n g a n w a l s.c.l., f l o w r a t e 0 . 8 9 m l / m i n ; o= Ludh i a n a 1.s., f l o w r a t e 1 . 1 2 m l / m i n ; ~ . . . . . . = G u r d a s p u r s.l., f l o w r a t e 0 . 9 8 m l / m i n .
to interactions of nitrate ions with the soil materials. The areas under the elution curves represent the amounts of nitrate eluted from the columns. The smallest a m o u n t of nitrate is eluted from the Kanganwal soil sample, which contains 13.1% CaCO3. From these adsorption studies, it seems that the a m o u n t of nitrate which interacts with different soils is greatly dependent on the specific surface of CaCO3. The effect of replacing water with solutions containing 25 ppm C1- + 5 ppm SO~ 2 or 5 ppm C1- + 25 ppm S O ~ as a displacing fluid for eluting nitrates in the columns containing Ludhiana and Kanganwal soil samples is shown by curves and semi-log first order nitrate removal plots in Fig. 5. With an increase in the SO~ 2 content of the displacing fluid, the height of the peak in the elution curve increases and trailing portion becomes smaller. This indicates that with an increasing a m o u n t of SO~-2 in the displacing solution, the a m o u n t of nitrate which reacts with the soil decreases. This p h e n o m e n o n is also depicted by increased slopes of both parts of the semi-log plots. Sulphate ions seem to desorb nitrate ions from the surface of CaCO3, whereas chloride ions in the displacing fluid have little or no effect. These findings are in accord with those reported earlier by Jurinak and Griffin (1972), who attributed the lack of effectiveness of chloride ions to their steric incompatibility with the crystal lattice of calcite. Inferences drawn from the elution experiments are thus consistent with the results of the equilibrium adsorption studies.
278
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48
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40
~ 32 ~ 36
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0.04
0-02
.,~ 09
I H
1,3
1"5
1'7
1'9
Pore volume of effluent
Fig. 5. Nitrate elution curves and semi-log first-order nitrate removal plots of elution data from the Ludhiana and Kanganwal samples when eluted with solutions containing different amounts of C1- and SO~ 2. o = Ludhiana 1.s., 25 ppm C1- ÷ 5 ppm SO~"~, flow rate 1.09 ml/min; • = Ludhiana l.s., 5 ppm CI- + 25 p p m SO~ 2, flow rate 1.09 ml/min; ~ Kanganwal s.c.l., 25 p p m CI- + 5 ppm SO~2, flow rate 0~96 rrd/min; A - Kanganwal s,c.L, 5 ppm C1- + 25 p p m SO~"~, flow rate 0.96 ml/min.
ACKNOWLEDGEMENT
The first author is grateful to Indian Council of Agricultural Research for the award of a Senior Research Fellowship during the course of this investigation.
REFERENCES Bremner, J.M., 1965. Inorganic forms of nitrogen. In: C.A. Black (Editor), Method of Soil Analysis. American Society of Agronomy, Madison, Wisc., Part 2, pp. 1179--1237. Eiiassen, R. and Tchobanoglous, G., 1969. Removal of nitrogen and phosphorus from waste water. Environ. Sci. Technol., 3: 536--541. Holford, I.C.R. and Mattingly, G.E.G., 1 9 7 5 . Surface area of calcium carbonate in soils. Geoderma, 13: 247--255. Johnson, W.R., Ittihadieh, F. and Pillsbury, A.R., 1965. Nitrogen and phosphorus in tile drainage effluent. Soil Sci. Soc. Am. Proc., 29: 287--289. Jurinak, J.J. and Griffin, R.A., 1972. Nitrate ion adsorption by calcium carbonate. Soil Sci., 113: 1 3 0 - - - 1 3 5 . Partington, J.R., 1961. A Text-Book of Inorganic Chemistry. MacMillan, New York, N.Y. 996 pp.
279
Puri, A.N., 1949. Soils, their Physics and Chemistry. Reinhold, New York, N.Y. Singh, B.R. and Kanehiro, Y., 1969. Adsorption of nitrate in amorphous and kaolinitic Hawaiian soils. Soil Sci. Soc. Am. Proc., 33: 681--683. Singh, B. and Sekhon, G.S., 1976. Some measures of reducing leaching loss of nitrates beyond potential rooting zone, I. Proper coordination of nitrogen splitting with water management. Plant Soil, 44: 193--200. Somasundaran, P. and Agar, G.E., 1967. The zero point charge of calcite. J. Colloid Interfacial Sci., 24: 433--440. Thomas, G.W., 1963. Kinetics of chloride desorption from soils. J. Agric. Food Chem., 11: 201--203. Walkley, A. and Black, I.A., 1934. An examination of the Degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci., 37: 29--38. WHO, 1963. International Standards for Drinking Water. World Health Organisation, Geneva.
L E A C H I N G O F N I T R A T E I N C A L C A R E O U S S O I L S AS I N F L U E N C E D BY ITS A D S O R P T I O N O N C A L C I U M C A R B O N A T E
BIJAY SINGH and G.S. SEKHON
Department of Soils, Punjab Agricultural University, Ludhiana 141004 (India) (Received January 11, 1977; accepted January 9, 1978)
ABSTRACT Singh, B. and Sekhon, G.S., 1978. Leaching of nitrate in calcareous soils as influenced by its adsorption on calcium carbonate. Geoderma, 20:271--279. Adsorption of the nitrate ion on calcium carbonate and its leaching in calcareous soils were examined by equilibrium and elution techniques. Nitrate ions adsorbed on the surface of CaCO 3 fitted the Langmuir model well at an equilibrium concentration of 40 ppm NO~. Sulphate ions reduced adsorption of nitrate. Data for elution of surface-applied nitrate from laboratory soil columns, when plotted in the form of elution curves and semi-log plots, indicated interactions of nitrate with the soils. The elution curves had long trailing portions due to desorption of nitrate. The length of the trailing portion of a curve was determined by the amount of nitrate adsorbed which in turn seems to depend upon the total surface area of CaCO 3. Sulphate ions when present in the displacing fluid seem to desorb nitrate ions from the surface of the CaCO 3 whereas chloride ions have little or no effect.
INTRODUCTION The n i t r a t e ion is regarded as a w a t e r p o l l u t a n t because o f its p o t e n t i a l as a health h a z a r d w h e n its c o n c e n t r a t i o n increases b e y o n d 45 p p m NO~- (WHO, 1963). Moreover, dissolved nitrate leaches u n i m p e d e d t h r o u g h the soil i n t o g r o u n d w a t e r (Eliassen a n d T c h o b a n o g l o u s , 1 9 6 9 ; J o h n s o n et al., 1 9 6 5 ) and b e c o m e s inaccessible t o plants. Its a d s o r p t i o n on s o m e soil c o m p o n e n t s , however, can r e d u c e such losses, m a k i n g it available t o g r o w i n g plants f o r a time. T h e a d s o r p t i o n o f n i t r a t e in soils has o n l y been s t u d i e d t o a limited extent. Singh and K a n e h i r o ( 1 9 6 9 ) r e p o r t e d on t h e a d s o r p t i o n o f nitrate b y t w o acidic soils f r o m Hawaii and used the F r e u n d l i c h i s o t h e r m to describe the ads o r p t i o n process. F o l l o w i n g a s t u d y b y S o m a s u n d a r a n a n d Agar ( 1 9 6 7 ) , w h i c h s h o w e d t h a t CaCO3 u n d e r certain c o n d i t i o n s develops a positively charged surface and suggested t h a t n i t r a t e i n t e r a c t i o n with CaCO3 is a d i s t i n c t possibility, J u r i n a k and Griffin ( 1 9 7 2 ) investigated t h e a d s o r p t i o n o f nitrate o n solid-phase CaCO3 b y equilibrium and c o l u m n t e c h n i q u e s . I n a s m u c h as CaCO3 is a p r e d o m i n a n t c a r b o n a t e mineral in alkaline soils o f semi-arid and arid regions, the a d s o r p t i o n o f NO~ o n the surface o f CaCO3 c o u l d be i m p o r t a n t
272
in determining the transport of NO3 in calcareous soils. The studies included in the present paper pertain to adsorption of nitrate ions on pure calcium carbonate and to leaching of nitrate through columns containing calcareous soil materials. An a t t e m p t was also made to investigate the roles of C1- and S O : 2 in desorbing the adsorbed NO~. MATERIAL AND METHODS
Equilibrium adsorption studies were conducted at 30 ° C_+2 ° C with the following adsorbents: (1) calcium carbonate (May and Baker Ltd., Dagenham, England; lot C 93678; (2) Ludhiana loamy sand; (3) Gurdaspur sandy loam; (4) Kanganwal sandy clay loam; and (5} Langrian loamy sand. Selected characteristics of the soil samples (0--20 cm layer) are given in Table I. 25-ml portions of standard NaNO3 solutions (5, 10, 20, 40 and 50 ppm NO~) were added to 250 ml Erlenmeyer flasks containing 4 g of CaCO3. The flasks were shaken for an hour to attain equilibrium. The solid phase was then separated by filtration and nitrate ion concentration in the equilibrium solution was determined by a phenoldisulphonic acid m e t h o d (Bremner, 1965) with a slight alteration (Singh and Sekhon, 1976). The a m o u n t of adsorbed nitrate was estimated from the difference between initial and final nitrate ion concentrations in solution. Equilibrium adsorption studies with soil samples were carried o u t in the same manner, b u t with the alteration that 5 g portions of each sample were equilibrated with 25 ml portions of 5, 10, 20, 40 and 50 ppm NO~ solutions. Adsorption studies with samples of CaCO3, Ludhiana loamy sand (l.s.) and Kanganwal sandy clay loam (s.c.1.) were also conducted with standard NO~ solutions containing 5 ppm C1- (as NaC1) + 25 ppm SO42 (as Na2 SO4) or 25 ppm C1- + 5 ppm SO~-2. Column elution studies were conducted in 5.2 cm ID glass tubes. The columns were made up of a b o t t o m layer of glass wool and a disc of Whatman No. 42 filter paper overlaid by the adsorbent which was covered at the surface by another disc of filter paper. The adsorbents used for column studies were samples of Ludhiana 1.s., Gurdaspur s.1. (sandy loam) and Kanganwal s.c.1. Total air-dried weight of the adsorbent in each column was 500 g. The adsorbent columns were initially eluted with distilled water till there was no nitrate ion in the leachate. Then, with the water surface flush with top of the adsorbent column, 1 mg NO~ (as NaNO3) was added in a 1 ml aliquot to each. The nitrate was then eluted by continued addition of distilled water at a specific rate, applied by a constant-head device. The leachate fractions were collected at a fixed volume interval (depending upon the specific rate of addition) and analysed for NO~. Samples of Ludhiana 1.s. and Kanganwal s.c.1, were also eluted with solutions containing 5 ppm Cl- + 25 ppm SO~-2 and 25 ppm C1- + 5 ppm SO~-2. The elution data were plotted in the form of elution curves and according to the m e t h o d of Thomas (1963) who found that the rate of removal of a nonreactive anion in a saturated soil column is given by:
I
55.6 55.2 87.3 82.6
G u r d a s p u r s.l., H a p l u s t a l f K a n g a n w a l s.c.l., U s t o c h r e p t L a n g r i a n l.s., C a l c i o r t h i d L u d h i a n a 1.s., U s t i p s a m m e n t
10.1 24.6 5.5 8.9
Clay*~ (%)
0.61 0.75 0.33 0.34
O r g a n i c .2 carbon (%)
0.90 13.10 1.05 0.40
C a l c i u m *~ carbonate (%)
6.96 12.18 3.83 4.35
CEC mequiv./ 100 g
7.5 9.1 8.9 8.6
p H .4
Specific surface area o f C a C O 3 ~l (m2/g) 44.7 10.9 79.3 71.5
Electrical c o n d u c t i v i t y *~ (mmhos/cm)
0.16 0.33 0.14 0.11
percentage of clay + sand. • 2 Walkley a n d Black, 1 9 3 4 . • 3 Pllri, 1 9 4 9 . • 4 A t soil w a t e r r a t i o o f 1 : 2. • s Holford and Mattingly, 1975.
• 1 D i a m e t e r s o f clay a n d s a n d p a r t i c l e s are < 0 . 0 0 2 m m a n d 2 . 0 - - 0 . 0 2 m m , r e s p e c t i v e l y ; p e r c e n t a g e o f silt ( 0 . 0 2 - - 0 . 0 0 2 r a m ) is given b y 1 0 0 m i n u s
Sand*l (%)
Soils: location, type and group
S e l e c t e d c h a r a c t e r i s t i c s o f t h e soil s a m p l e s
TABLE
274
-dAt/dt = kAt
Ill
where k is a constant and At is the amount of anion in the column at time t during elution. When t = 0, the amount of anion in the column is Ai. Integrating eq. 1 and substituting In A i for the constant of integration gives: log A t = log Ai - k t / 2 . 3 0 2 6
(2)
When nitrate reacted with the adsorbent in the column a distinct change occurred in the slope of the straight line plot defined by eq. 2. Because the flow rate of elution was kept constant for a column, the volume of effluent expressed as fraction of the total pore volume has been substituted for the time variable in eq. 2. RESULTS AND DISCUSSION
A d s o r p t i o n studies
Nitrate adsorption as a function of the amount of CaCO3 in the system is shown in Fig. 1. The linear relation between total nitrate adsorption and the a m o u n t of CaCO3 shows that nitrate adsorption is a function of the interfacial area and that no apparent secondary reaction occurs. Data pertaining to nitrate adsorption by CaCO3 and by samples of Ludhiana 1.s., Kanganwal s.c.l., Gurdaspur s.1. and Langrian 1.s. are plotted in Fig.2. according to the Langmuir adsorption equation which, in its linear form, is:
(3)
Ce/S = 1/gaSm + Ce/Sm
where Ce is the final solution concentration of NO~ in ppm, S represents the amount of NO~ adsorbed in pg/g of adsorbent, Sm gives the adsorption maxi-
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Equilibrium nitrate concentration, Ce (ppm)
Fig. 1. A d s o r p t i o n of nitrate f r o m solution by calcium carbonate as a f u n c t i o n o f the a m o u n t o f CaCO3. Y = 10.2 + 3 5 . 7 X ; r = 0 . 9 9 4 " * . Fig. 2. L a n g m u i r i s o t h e r m s f o r n i t r a t e a d s o r p t i o n b y soil samples and c a l c i u m c a r b o n a t e . - CaCO3; - L u d h i a n a l.s.; . . . . . . . L a n g r i a n 1.s.; . . . . . . = G u r d a s p u r s.1. ; . . . . . K a n g a n wal s.c.1.
275
mum and K a is a constant related to the bonding energy of NO~- to the adsorbent (ppm -1). The statistical significance of the correlation coefficient of Ce/S versus Ce was the criterion by which the fit of the data to the Langmuir isotherm was tested. Adsorption data for all soil samples fitted the Langmuir adsorption equation well. The adsorption of nitrate by CaCO3 is attributed to the steric compatibility of nitrate ion with the calcite crystal lattice. Both NO~- and COl 2 anions have planar configurations with similar dimensions. These anions are plane triangles with oxygen atoms at the corners. Indeed NaNO3 and CaCO3 are isomorphous (Partington, 1961). However, the energy of nitrate interaction is not expected to be large, as verified from values of AH of adsorption by Jurinak and Griffin (1972). The pH values of the CaCO3--NaNO3 suspensions were 8.3, which is close to the theoretical iso-electric point for calcite as calculated by Somasundaran and Agar (1967). Hence, the positive charge density and adsorption capacity of the CaCO3 for NO~- are expected to be low. The Langmuir coefficients (Sm and Ka) calculated from the equations for the best-fitting straight lines are given in Table II. It seems that the native CaCO3 of soils plays an important role in determining nitrate adsorption. This is evident from the Sm value for the Kanganwal sample, which is almost double the Sm values for the other three soil samples. The latter samples do not contain more than 1% CaCO3. Thus, magnitude of'nitrate adsorption is not proportional to the amount of CaCO3 in a calcareous soil. Possibly differences in specific surface areas of CaCO3 in different soils (Table I) are a factor in governing nitrate adsorption by CaCO3. Values of K a given in Table II further indicate that the bonding energy between nitrate ions and CaCO3 is least in the Kanganwal sample which adsorbs the greatest amount of nitrate per gram. The calcium carbonate in that sample may be less active than the carbonates in the Ludhiana, Langrian and Gurdaspur soil samples. Langmuir plots for nitrate adsorption by pure CaCO3 and by Kanganwal samples from equilibrium solutions at two levels of each of chloride and sulphate ions are shown in Fig.3. Note that data for the Kanganwal samples fit T A B L E II L a n gmuir coefficients for nitrate a d s o r p t i o n by calcium carbonate and soil samples in presence and absence o f C l - and SO~"2
Adsorbent
Calcium carbonate L u d h i a n a l.s. K a n g a n w a l s.c.l. G u r d a s p u r s.l. L a n g r i a n l.s.
N o C I - o r SO4 -=
25 p p m C1- + 5 p p m SO~-=
5 p p m C1- + 25 p p m SO~-2
Sm ug/g
Ka "10-2 ml/ug
Sm ug/g
Ka '10-2 ml/ug
Sm ug/g
Ka "10-2 ml/-g
80.7 17.6 45.5
4.4 3.5 1.6
71.5 17.3 41.6
3.8 3.8 2.7
66.1 12.3 23.5
2.9 4.1 3.2
18.0 24.2
3.3 2.4
--
---
--
--
276
the Langmuir equation and that the intercept of the Y-axis by the plot for the greater amount of sulphate ions is itself greater. The data for the Ludhiana sample (not plotted in Fig.3) also followed a similar trend. The effects of sulphate and chloride ions on nitrate adsorption are shown more clearly, however, by the Sm and K a values given in Table II. Maximum adsorption (Sm ) on CaCO3 is reduced from 80.64 pg/g to 71.48 pg/g in the presence of 25 ppm C1- + 5 ppm SO~-2 and to 66.09 pg/g in the presence of 5 ppm C1- + 25 ppm SO~ 2. A similar trend is observed for the soil samples. These data thus indicate that SO~ 2 ions c o m p e t e with adsorption of nitrate and reduce the amount of adsorbed nitrate appreciably. The decrease in Sm values in the presence of 25 ppm C1- + 5 ppm SO4-2 is always small and is perhaps due to sulphate ions. The data indicate a negligible effect of chloride ions on nitrate adsorption, an observation consistent with the earlier findings of Jurinak and Griffin (1972). 0
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Equilibrium nitrate concentration, Ce (ppm) Fig. 3. Langmuir isotherms for nitrate adsorption by calcium eaxbonate and Kanffanwa! samples in the presence o f C ] - and SO 42 ions in the e q u i l i b r i u m solution.- . . . . CaCO3, 25 p p m C ] - + 5 p p m SO~'2; . . . . . . CaCO 3, 5 p p m C ] - + 25 p p m SO~'~; = Kanganwa] s.c.]., 25 p p m C ] - + 5 p p m S0~'2; - Kanganwa] s.e.L, 5 p p m C ] - + 25 p p m SO~ 2.
Elution studies Data on the elution of nitrate ions through columns of Ludhiana, Kanganwal and Gurdaspur soil samples are plotted in the form of elution curves and also according to eq. 2 in Fig.4. In all the elution curves, the peaks are at a void volume of one. The area under each peak represents the dissolved nitrate which moves through the column, spreading as it goes, and the data form the expected normal curve. This fast-removal portion of the elution curve can also be approximately defined by a first order removal process as shown in the initial portions of the semi-log straight line plots. Curves in Fig.4 are not symmetrical. The lack of symmetry indicates nitrate adsorption in the soil columns. The long descending portion of each curve arises from desorption of nitrate from adsorption sites, with the length dependent on the a m o u n t of nitrate adsorbed. In fact as the peak in the curve diminishes, the trailing portion of the elution curve became more pronounced because of increased nitrate adsorption. Distinct changes in the slopes of the semi-log plots, which define nitrate elution as a first order process, are ascribed
277
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•
0,8 0-6 (j
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0.4
.
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36
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i ;
--~ I~ o Z
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0-05
2o
12
:
0.9
16
o
o
I'I
l'3
1.5
1'7
I'9
Equilibrium nitrate concentration, Ce (ppm)
Fig.4. N i t r a t e e l u t i o n c u r v e s a n d s e m i - l o g f i r s t o r d e r n i t r a t e r e m o v a l p l o t s o f e l u t i o n d a t a f o r t h r e e soil s a m p l e s . • - K a n g a n w a l s.c.l., f l o w r a t e 0 . 8 9 m l / m i n ; o= Ludh i a n a 1.s., f l o w r a t e 1 . 1 2 m l / m i n ; ~ . . . . . . = G u r d a s p u r s.l., f l o w r a t e 0 . 9 8 m l / m i n .
to interactions of nitrate ions with the soil materials. The areas under the elution curves represent the amounts of nitrate eluted from the columns. The smallest a m o u n t of nitrate is eluted from the Kanganwal soil sample, which contains 13.1% CaCO3. From these adsorption studies, it seems that the a m o u n t of nitrate which interacts with different soils is greatly dependent on the specific surface of CaCO3. The effect of replacing water with solutions containing 25 ppm C1- + 5 ppm SO~ 2 or 5 ppm C1- + 25 ppm S O ~ as a displacing fluid for eluting nitrates in the columns containing Ludhiana and Kanganwal soil samples is shown by curves and semi-log first order nitrate removal plots in Fig. 5. With an increase in the SO~ 2 content of the displacing fluid, the height of the peak in the elution curve increases and trailing portion becomes smaller. This indicates that with an increasing a m o u n t of SO~-2 in the displacing solution, the a m o u n t of nitrate which reacts with the soil decreases. This p h e n o m e n o n is also depicted by increased slopes of both parts of the semi-log plots. Sulphate ions seem to desorb nitrate ions from the surface of CaCO3, whereas chloride ions in the displacing fluid have little or no effect. These findings are in accord with those reported earlier by Jurinak and Griffin (1972), who attributed the lack of effectiveness of chloride ions to their steric incompatibility with the crystal lattice of calcite. Inferences drawn from the elution experiments are thus consistent with the results of the equilibrium adsorption studies.
278
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E
0.6
48
0.4
44
40
~ 32 ~ 36
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.=_
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0.06
16
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0.04
0-02
.,~ 09
I H
1,3
1"5
1'7
1'9
Pore volume of effluent
Fig. 5. Nitrate elution curves and semi-log first-order nitrate removal plots of elution data from the Ludhiana and Kanganwal samples when eluted with solutions containing different amounts of C1- and SO~ 2. o = Ludhiana 1.s., 25 ppm C1- ÷ 5 ppm SO~"~, flow rate 1.09 ml/min; • = Ludhiana l.s., 5 ppm CI- + 25 p p m SO~ 2, flow rate 1.09 ml/min; ~ Kanganwal s.c.l., 25 p p m CI- + 5 ppm SO~2, flow rate 0~96 rrd/min; A - Kanganwal s,c.L, 5 ppm C1- + 25 p p m SO~"~, flow rate 0.96 ml/min.
ACKNOWLEDGEMENT
The first author is grateful to Indian Council of Agricultural Research for the award of a Senior Research Fellowship during the course of this investigation.
REFERENCES Bremner, J.M., 1965. Inorganic forms of nitrogen. In: C.A. Black (Editor), Method of Soil Analysis. American Society of Agronomy, Madison, Wisc., Part 2, pp. 1179--1237. Eiiassen, R. and Tchobanoglous, G., 1969. Removal of nitrogen and phosphorus from waste water. Environ. Sci. Technol., 3: 536--541. Holford, I.C.R. and Mattingly, G.E.G., 1 9 7 5 . Surface area of calcium carbonate in soils. Geoderma, 13: 247--255. Johnson, W.R., Ittihadieh, F. and Pillsbury, A.R., 1965. Nitrogen and phosphorus in tile drainage effluent. Soil Sci. Soc. Am. Proc., 29: 287--289. Jurinak, J.J. and Griffin, R.A., 1972. Nitrate ion adsorption by calcium carbonate. Soil Sci., 113: 1 3 0 - - - 1 3 5 . Partington, J.R., 1961. A Text-Book of Inorganic Chemistry. MacMillan, New York, N.Y. 996 pp.
279
Puri, A.N., 1949. Soils, their Physics and Chemistry. Reinhold, New York, N.Y. Singh, B.R. and Kanehiro, Y., 1969. Adsorption of nitrate in amorphous and kaolinitic Hawaiian soils. Soil Sci. Soc. Am. Proc., 33: 681--683. Singh, B. and Sekhon, G.S., 1976. Some measures of reducing leaching loss of nitrates beyond potential rooting zone, I. Proper coordination of nitrogen splitting with water management. Plant Soil, 44: 193--200. Somasundaran, P. and Agar, G.E., 1967. The zero point charge of calcite. J. Colloid Interfacial Sci., 24: 433--440. Thomas, G.W., 1963. Kinetics of chloride desorption from soils. J. Agric. Food Chem., 11: 201--203. Walkley, A. and Black, I.A., 1934. An examination of the Degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci., 37: 29--38. WHO, 1963. International Standards for Drinking Water. World Health Organisation, Geneva.