Bioresource Technology 43 (1993) 27-33 F ~

NATURAL ZEOLITES A N D SEPIOLITE AS AMMONIUM A N D AMMONIA ADSORBENT MATERIALS

M. E Bernal & J. M. Lopez-Real

Department of Biochemistry and Biological Sciences, Wye College, University of London, Wye, Ashford, Kent TN25 5.411, UK

(Received l0 February 1992; revised version received 5 March 1992; accepted 7 March 1992)

Abstract Zeofite clinoptilolite is a natural mineral with ion- exchange and gas-adsorption properties. Ammonium- and ammonia-adsorption processes were studied in the zeolite cfinoptilolite and zeolite-like sepiolite minerals. Maximum adsorption capacities of both ammonium and ammonia were determined by fitting the experi- mental results of the adsorption isotherm to Langmuir and first-order models. Ammonium-adsorption capaci- ties of the zeolites were from 8"149 to 15"169 mg N g- i; up to 16~3 times higher than that of sepiolite. Ammo- nium-adsorption capacity increased with the surface charge density of the material, due to the readily avail- able exchange sites in the surface.

Ammonia-adsorption capacities of the zeolites were between 6"255 and 14"155 mg N g-i. Because of its large surface area, sepiolite had a capacity of ammonia adsorption three times higher than that of the ammo- nium ion. The complexity of the ammonia-adsorption process meant that no individual characteristics of the materials influenced directly the adsorption capacity. However, ammonia adsorption was enhanced on the ammonium adsorption in zeolites with relatively low surface charge density.

Key words: Adsorption isotherm, ammonia, ammo- nium, Langmuir equation, sepiolite, zeolite clinopti- lolite.

INTRODUCTION

The applications of natural zeolites include pollution control, energy conservation, agriculture and aqua- culture, mining and metallurgy (Mumpton, 1981). All the applications of natural zeolites make use of one or more of their physical and chemical properties: ion- exchange, adsorption and related molecular sieve properties, dehydration and rehydration, and siliceous composition. About 39 naturally occurring zeolite species have been subjected to study.

Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

27

The ion-exchange capacity is basically a function of the degree of substitution of aluminium for silicon in the framework structure. The greater the substitution, the greater the charge deficiency, and the greater the number of cations required for electrical neutrality. In practice, however, the ion-exchange depends on other factors, notably the nature of the cation species (size, charge, etc.), the temperature, the concentration of the cation species in solution and the structural charac- teristics of the zeolite. The cation-exchange equili- brium between a zeolite and a solution is usually depicted by an ion-exchange isotherm. Clinop- tilolite is one of the commenest natural zeolites. The small amount of aluminium substitution for silicon in the framework (Si:AI>4) results in a relatively low ion-exchange capacity. Its cation selectivity has been reported as: C s > R b > N H 4 > Ba > Sr > Na > Ca > Fe > AI > Mg > Li (Ames, 1960) and Pb > NH4 > Cu, Cd > Zn, Co > Ni > Hg (Blanchard et aL, 1984).

Its characteristics in cation-exchange capacity and cation selectivity have led to its frequent use in waste- water treatments, mostly for waters with high levels of ammonium (Hagiwara & Uchida, 1978; Murphy et al., 1978; Baumann et aL, 1981; Liberti et aL, 1981; Tara- sevich, 1988); in agriculture and aquaculture (Barba- rick & Pirela, 1984); and in radioactive-wastewater treatments (Mercer & Ames, 1978).

Crystalline zeolites are unique adsorbent materials due to their large central cavities and entry channels. Most of the surface area is found within the zeolite structure and represents the inner surface of dehy- drated channels and cavities. Molecules having dia- meters small enough to pass through the channels are readily adsorbed in the dehydrated channels and central cavities. The unique geometries contained in zeolitic channels and cavities create selective sorption properties. Zeolites are active drying agents (Flaningen & Mumpton, 1981; Dyer, 1988), they have been used for reducing the odour due to ammonia in agricultural wastes (Miner, 1984) and in livestock buildings (Koelli- ker et al., 1980). Due to their channel sizes, they also have molecular-sieve properties, and they can be used in the purification of gases (Hayhurst, 1978) and in catalysis (Chi etal., 1978).

28 M. P. Bernal, J. M. Lopez-Real

Adsorption on crystalline zeolites is usually charac- terized by Langmuir-type isotherms (Flannigen & Mumpton, 1981), assuming that the surface upon which the sorbed molecule sits is of uniform energy (Dyer, 1988). When the aim is to obtain the maximum adsorption to compare with other adsorbents, Lang- muir, Freundlich and first-order kinetics equations are generally used (Weber et al., 1983; Hunt & Adamsen, 1985; Witter & Kirchmann, 1988).

In this work the ammonium- and ammonia-adsorp- tion process in zeolite clinoptilolite and in sepiolite minerals was studied, with the aim of determining their maximum adsorption capacity. The influence of the physical and chemical properties of the materials upon the ammonium- and ammonia-adsorption is also dis- cussed.

METHODS

extract. Water-holding capacity was determined at 1/3 atmosphere in a pressure cell apparatus (Klute, 1986). A simplified ethylene glycol monoethyl ether proce- dure was used for determining the specific surface area (Cihacek & Bremner, 1979). Porosity was calculated from bulk and particle density values, previously deter- mined by core method (Blake & Hartge, 1986a) and pycnometer method (Blake & Hartge, 1986b) respect- ively. Total nitrogen was determined by a Kjeldahl method (Bremner, 1965). Ammonium nitrogen was extracted with 2N KCI, and determined colorimetri- cally by a salicylate method (Krom, 1980). The ammo- nium acetate saturation procedure was used for determining the cation-exchange capacity (Chapman, 1965). Exchangeable cations were analysed from the ammonium acetate extract; Na and K by flame photo- metry and Ca and Mg by means of atomic absorption spectrometry.

Four clinoptilolite zeolites and one sepiolite were used. Zeolite 1 (ZI) was from S.P. Health & Hygiene Pro- ducts, and the other three zeolites (Z2, Z3 and Z4) were provided by Teage Mineral Products. Their main differences were in cation-exchange capacity, surface area, and density. The sepiolite sample was from Orera in Zaragoza (Spain), provided by MYTA. Sepiolite is a magnesium silicate with low aluminium content. Its structure includes internal channels which provide zeolitic properties (Zelazny & Calhoun, 1977). Although its cation-exchange capacity is lower than those in zeolites, it has a great surface area which generates a high adsorption capacity.

Analytical methods All samples were ground to < 2 mm. Their chemical analyses are listed in Table 1. The pH values were determined in 1:5 water and 1N KCI suspensions. Electrical conductivity was measured in 1:5 water

Ammonium-adsorption isotherm Samples of zeolite and sepiolite were extracted with different solutions of NH4CI + NaCI (total 0-2~) in a ratio 1:10 g:ml. Isonormal solutions were used in order to maintain a constant ionic strength in the sample-solution mixtures. The initial NH4CI concen- tration in the extracting solution was from 0"001 to 0"160M. Amounts of ammonium adsorbed by the samples were calculated from the reduction of ammo- nium in the solution after shaking for 2 h, and centrifu- gation. A separate study of the ammonium adsorption by the zeolite against time showed that the ammonium adsorption was practically complete after 1 h of shak- ing (Table 2). Significant differences were observed after 5 h of shaking.

Ammonia-adsorption isotherm Each dry material (5 g) was placed on a watch glass in a closed container of known volume (between 10 and 11

Table 1. Characteristics of the materials

Z 1 Z2 Z3 Z4 S

pH (H20) 8-15 8.05 7.78 6'79 7"68 pH (KCI) 6'80 6"90 6"65 5"75 7"45 WHC (%) 133"53 132"01 128-89 168"74 170"56 Surface area (m 2 g- 1) 175 172 193 214 297 Bulk density (g cm-3) 0.982 0.796 0.858 0.561 0.684 Particle density (g cm- 3) 2,378 2.395 2.353 2.287 2.555 Porosity (%) 58"7 66.77 63.54 75.46 73.22 CEC (me kg- t) 930.29 1016.36 1396.16 1501-30 175.40 SCD" (/~e m- 2) 5.32 5.91 7.23 7.02 0.59 NH4-N (mg kg- l) 107 5 5 72 17

Exchangeable cations Na (me kg- t) 161.7 161.8 198.5 285-8 1"00 K (me kg- l) 101-4 93.2 539.9 258.0 4.10 Ca (me kg- t) 654.8 630"5 306.6 687.0 69"8 Mg (me kg- ~) 41.6 47.7 10.7 100.7 127.0

aSurface charge density. Z -- zeolite. S = sepiolite.

Ammonium and ammonia in zeofites 29

Table 2. Influence of the extraction time in the ammonium-adsorption isotherm

Extraction time (h)

1 2 3 5 7

0.761 b 0.769b 0.781 b 0.840a 0.854a

Analysis of variance

NH 4 adsorbed (mg N/g zeolite)

Source DF Sum of square Mean square F

Treatments 4 0.014 83 0.003 71 Residual 5 1.8 x 10 -4 3"62 x 10 -5 Total 9 0-015 01

102"407"**

*** Results are significant at the probability level of 0.001. Numbers followed by the same letter are not statistically different at the probability level of 0"01 according to the DMRT test.

litres) over 48 h with a constant atmosphere of ammo- 7"-" 4.0 ~t0

nia. Ammonia concentration in the container was 3.5 Z

0"17-15"70 mg N litre -1. The constant atmosphere ~ 3.o was obtained by several additions of 100 ml of 5N v 2.5

NaOH to a predetermined amount of NH4CI during 2.o the incubation time. Ammonia adsorbed by the mate- ~ 1.s rials was determined by KCI extractions, acidified with

1.0 HCi (pH 4) to prevent any ammonia loss during the

-* 0 . 5 analysis. The Kjeldahl-N method was compared to KCI extracts in one zeolite; the results indicated that all 0.0 ammonia adsorbed in the zeolite was KCl-extractable (data not shown).

Statistical analyses Results from both ammonia- and ammonium-adsorp- tion isotherms were fitted to Langmuir and first-order kinetic models by non-linear least squares, using SAS procedures (SAS Institute, 1985). Analysis of variance and Duncan's multiple range test (DMRT) were used to compare data obtained from different treatments.

RESULTS AND DISCUSSION

Influence of the zeolite particle size in the ammonium adsorption Two different particle sizes of zeolite 1 were tested in the adsorption of ammonium. One of them was less than 0.5 mm, and another between 2"0 and 0"5 mm. Six sequential extractions with a 0"01M NH4CI+0"09M NaCI solution were carried out (extraction ratio 1:10 g ml-~). After each individual equilibrium was reached no statistical differences in the amount of ammonium adsorbed were observed between the two sizes tested (Fig. 1 ). The particle sizes used did not have any effect on the amount of NH4-N adsorbed in the zeolite. This is logical since, according to Flaningen and Mumpton ( 1981 ), the external surface of the particle accounts for only about 1% of the total surface area of the zeolite. For this reason, zeolite and sepiolite samples used in the following experiments were ground to < 2 mm only.

I I i I I I I

0 1 2 3 4 5 6 7 E x t r a c t i o n t i m e s

Fig. I. |nfluence of the panicle size of the zeolite in the ammonium adsorption by sequential extractions with solution NH4CI 0-01N+NaC! 0"09N (O, size <0"5 mm; A,

0"5 < size < 2 mm).

Ammonium-adsorption isotherm Figure 2 represents the experimental results in the adsorption isotherm of ammonium, and the curve fitt- ing for the four zeolites and the sepiolite.

The basic Langmuir equation is usually written in the form:

n = MbC/( 1 + bC)

where n is the amount adsorbed per unit of adsorbent, M is the maximum adsorption, b is a constant, and C is the equilibrium solution concentration.

The first-order function is also an asymptotic growth curve, and it can be written as:

n= M ( 1 - e -k,)

where n, M and C represent the same factors as in the Langmuir model, and k is the first-order model con- stant.

Linearity of the Langmuir model (i.e. C/n plotted versus C) was not used, because plottingJ~ C) versus C decreases data variability and always provides a statis- tically significant correlation coefficient (Harter, 1984).

As Table 3 shows, the Langmuir model fitted the experimental results better than the first-order model.

30 M. P. Bernal, J. M. Lopez-Real

T

z

,= 3

,~ 2 e~

Z 1 I

Z 0

f

200 400 600 800 1000

E q u i l i b r i u r t i c o n e . ( l i i g N 1-1)

I

1200

- " " 8 I

Z

0~ 4

2 Z

I

z 0

(b)

I I I I I I I

0 2 0 0 4 0 0 600 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0

E q u i l i b r i u m COILC. ( I n g N 1-1)

Fig. 2.

,,--, I

z

o.

z I

i z

10

8

6

4

2

0

(o)

I I I I I I I

200 400 600 800 1000 1200 1400 - l

E q u i l i b r i u t n uol l t : . ( l l l~ N l )

- ' " 10 I

z 8

E 6

4

Z 2 I

Z 0

(4)

I I I I I I I

200 400 600 800 1000 1200 1400

E q u i l i b r i u m c o n e . ( n t g N I - l )

1.4 I

Z. 1.2

~ 1.o

'~ 0 . 8

~ 0 . 6

~ 0 . 4

L 0 . 2

Z 0.0

(e)

f

I I I I I

0 200 400 600 800 I000

E q u i l i b r i u m c o n c . ( m g N 1-1)

Experimental results (o) and Langmuir curve fitting ( - - ) of the ammonium adsorption isotherm in zeolite 1, 2, 3, 4 and sepiolite (a, b, c, d and e, respectively).

Although, according to the values of R 2 all the equa- tions were significant at the 0.001 probability level in both models, the lowest values of the standard error of estimate were obtained with the Langmuir equation. In the first-order model the maximum adsorption was underestimated by 19"27- 31%.

The maximum ammonium adsorptions calculated by the Langmuir model were 62"57, 74"61, 77"61, 61-12 and 59"86% of the cation-exchange capacity in zeolites 1, 2, 3, 4 and sepiolite, respectively, with an average value of 67" 15%. Therefore, only a fraction of the exchange sites were accessible to the ammonium ion. In a previous paper Witter and Kirchmann (1989) calculated ammonium-adsorption capactiy by Lang- muir and first-order models, but the values obtained by the Langmuir model were only 10% greater than those predicted by the first-order model. In that zeolite 79"5% of the exchange sites were available for the ammonium adsorption.

The maximum adsorption of NH4-N was between 8.149 and 15.169 mg N g-l for zeolite, and 1.470 mg N g- l for sepiolite. Although sepiolite had the greatest surface area its capacity to adsorb ammonium was very low, relating to its cation-exchange capacity. Zeolite was up to 10.3 times more effective in NH4-N adsorp- tion than sepiolite. The different adsorption capacity of these minerals was related to both their cation- exchange capacity and surface area. The surface- charge density (CEC/surface area) was correlated with the ammonium adsorption capacity, and the regression equation was significant at the 0.01 probability level:

N,d~, = 1"892 SCD - 0"212; r-- 0"971

where N,a s is the ammonia adsorption capacity (mg N g-1) and SCD is the surface-charge density (/~e m-Z). Ammonium-adsorption capacity in- creased with the surface-charge density of the zeolite,

Ammonium and ammonia in zeolites 31

Table 3. Parameter values, coefficients of determination and standard error of estimate for ammonium-adsorption isotherm

Material" Langmuir model First-order model

Nm~ b R 2 SE Nm~ k R 2 SE

ZI 8"149 0"024 0"998*** 0"1745 6"132 0"0027 0"997*** 0"2125 Z2 10"616 0"0014 0"998*** 0"2121 7"682 0"001 7 0"998*** 0"228 7 Z3 15"169 0"001 1 0"997*** 0"3135 10"467 0"0014 0"996*** 0"3376 Z4 12"847 0"0017 0"997*** 0"4770 9"581 0"0019 0"991"** 0"5290 S 1"470 0"0039 0"992*** 0"0903 1"187 0"0039 0"989*** 0"1019

Nm. , = Maximum adsorption of ammonium in the material (mg N g- '). b-- Langmuir model constant (litres mg- i N). k = First-order model constant (litres mg- ~ N). R -~ -- Coefficient of determination. SE-- Standard error of estimate. *** Results are significant at the probability level of 0.001.

10

Z 8

6

Z 0

- "" 1 2 ( a ) / • ' =D

~ + + . . . . . z I 0 + + + ~ 4

2 I '" I I I I I I I Z 0

2 4 6 8 10 1 2 1 4 1 6

E q u i l i b r i u l n c o n e . ( rag N l - t )

I I I 1 I I I

2 4 6 8 10 12 14

E q u i l i b r i u m c o n e . ( rag N l - t )

I

16

7 I

Z 6

5

4

3

2 Z

i 1 e~

Z 0

. '-" 10 (c) ,

~ 6

~ 4

Z 2 I

i i i , , i i Z 0

2 4 6 8 10 12 1 4

E q u i l i b r i u m c.ozlc. ( rag N l -L)

(d)

f . I I I I I

2 4 6 8 10

E q u i l i b r i u m c o n e . ( rag N l -L)

I

12

Fig. 3.

5 7

Z 4

3

2

Z 0

(e)

I I i I I

0 1 2 3 4 5

E q u i l i b r i u m c o n e . ( r a g N l - t )

Experimental results (0), Langmuir ( ) and first-order ( - - - - ) curve fitting of the ammonia-adsorption isotherm in zeolite 1, 2, 3, 4 and sepiolite (a, b, c, d and e, respectively).

32 M. P. Bernal, J. M. Lopez-Real

due to the readily available exchange sites for this ion on the surface,

A m m o n i a - a d s o r p t i o n i s o t h e r m

Experimental results of the ammonia-adsorption iso- therm were also fitted to Langmuir and first-order models. Figure 3 shows the experimental results and the curve-fitting.

The zeolite samples were not wetted before expo- sure to the ammonia atmosphere, in order to avoid any possible blocking of the internal channels by water adsorption. According to Hayhurst (1978), in the zeo- lite phillipsite, when both ammonia and water were present, the adsorption rate and capacity were both reduced from that found for pure ammonia adsorption. This effect would be expected in clinoptilolite, because of its higher kinetic pore diameter compared to phillip- site (Hayhurst, 1978).

The experimental results obtained fitted to both Langmuir and first-order models. As with the ammo- nium-adsorption isotherm, coefficients of determina- tion in the Langmuir model were also higher than those of the first-order model (Table 4), and the standard error of estimate was always lower in the Langmuir model. For these reasons, the Langrnuir model more accurately described the adsorption process. Maximum adsorptions of ammonia calculated by the first-order model were 9.74-19.7% lower than the values found with the Langmuir equation.

The maximum adsorption of ammonia in zeolite was between 6.255 and 14.155 mg N g -k Sepiolite also adsorbed less NH3-N than the zeolites (4"55 mg N g-~). Nevertheless, its capacity for ammonia adsorption was three times higher than that for the ammonium ion. The large surface area of the sepiolite resulted in a great increase in the nitrogen adsorption as ammonia gas, with respect to the ammonium-ion form. Increases in ammonia adsorption in contrast to ammonium were also observed in zeolites 1 and 2 (24 and 33.3%, respectively), while decreases occurred in zeolites 3 and 4 (58.8 and 34.9%, respectively).

To be able to understand this it is necessary to take into account the different processes associated with the

adsorption of ammonia gas in these materials. The adsorption processes include the following individual reactions:

- - ammonia diffusion into the atmosphere; - - intraparticular diffusion of ammonia in the gase-

ous phase; -- adsorption of ammonia gas on the zeolite surface

(internal channels); - - equilibrium between the ammonia gas and ammo-

nia in solution (due to the partial pressure of H20 in the atmosphere, and the moisture content of the solid phase);

- - intraparticular diffusion of ammonia in solution; - - chemical equilibrium between ammonia and the

ammonium ion (NH 3 + H20 ~ NH4 ÷ + O H - ); and - - adsorption of the ammonium equilibrium form to

the exchange sites by cation exchange.

The complexity of the process means that no indi- vidual characteristic of the materials has a direct influ- ence on the maximum adsorption of ammonia. However, a greater ratio of NHa/NH 3 adsorbed occurred in zeolites with higher surface-charge density. The ion-exchange process prevailed in materials with high surface-charge density, diminishing the ammonia adsorption. To increase the NH3-N adsorption in these materials the protonation equilibrium, NH3 + H20 ~NH4 ÷ + O H - , should be directed to the ammonium ion formation. In this case the acid pre- treatment of the zeolite could stimulate the process, owing to the presence of hydrogen ions (protons) on the exchange sites. Prior to the actual ion-exchange process, a protonation of ammonia occurs on the acid exchange-sites. On the other hand, in materials with relatively low surface-charge density, the adsorption process is enhanced by gas adsorption in the internal channels of the zeolitic structure. Thus, pH and base saturation could also play an important role in the predominance of ion-exchange or gas adsorption processes.

Witter and Kirchmann (1989) found the ammo- nium-adsorption capacity of zeolite clinoptilolite to be more than 10 times greater than the ammonia adsorp-

T a b l e 4 . P a r a m e t e r v a l u e s , e o e f f i c i e n t s o f d e t e r m i n a t i o n a n d s t a n d a r d e r r o r o f e s t i m a t e for a m m o n i a - a d s o r p t i o n i s o t h e r m

Material ~ Langmuir model First-order model

Nm~ b R 2 SE Nm~ k R 2 SE

Z1 10.103 0-5529 0"993*** 0.6323 8"950 0.4197 0"984*** 0"9532 Z2 14"155 0.266 1 0-994*** 0.7878 11-368 0.2586 0"990*** 0"9966 Z3 6"255 1"3718 0.993*** 0.4663 5"646 0-9643 0-983*** 0.7497 Z4 8"365 1"519 7 0.993*** 0.653 3 7.413 1"1891 0.984*** 0"965 5 S 4"552 1-7068 0"996*** 0.2729 3"913 1.3925 0-989*** 0"4327

Nm~ = Maximum adsorption of ammonia in the material (mg N g- l). b = Langmuir model constant (litres mg- l N). k= First-order model constant (litres mg- t N). R 2 ~ Coefficient of determination. SE-- Standard error of estimate. ***Results are significant at the probability level of 0.001.

A m m o n i u m and ammonia in zeolites 33

tion. They concluded that the high pH avoided the transformation of ammonia into ammonium, this being the limiting factor, resulting in low ammonia adsorp- tion for the high CEC value. However, the strong. adsorption preference of ammonia gas over other gas compounds by zeolites has been shown (Hayhurst, 1978). The reason for the low ammonia-adsorption capacity found by Witter and Kirchmann (1989) could be due to the procedure, in which zeolite was wetted before exposure to the ammonia atmosphere. The water molecules present in the zeolite could have blocked the internal channels for the ammonia adsorp- tion in the zeolite.

ACKNOWLEDGEMENTS

The authors wish to thank the Ministry of Education and Science of Spain for the financial support awarded to Dr M. P. Bernal; Dr R. Manly, Kingston Polytechnic, for supplying some zeolite samples; Dr R. Ortiz, Uni- versity of Murcia (Spain), for the identification of the zeolite types by X-ray diffraction; also D. Haymes for his comments on the manuscript.

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