Characteristics of Nitrogen Release From Synthetic Zeolite Na-P1 Occluding NH4NO3

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Characteristics of nitrogen release from synthetic zeolite

Na-P1 occluding NH4 NO3

Man Park a,*, Jong Su Kima , Choong Lyeal Choia , Jang-Eok Kima , Nam Ho Heo b,Sridhar Komarnenic, Jyung Choia

a

Department of Agricultural Chemistry, Kyungpook National University, Teagu, 701-702, Korea b Department of Industrial Chemistry, Kyungpook National University, Teagu, 701-702, Koreac205 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA

Received 23 March 2004; accepted 21 February 2005

Available online 15 June 2005

Abstract

Zeolites can accommodate a considerable amount of occluded salt such as NH 4 NO3, which can serve as a good source of

slow-release plant nutrient. This study evaluates the kinetics of ion release from NH4 NO3-occluded Na-P1 (N-NaP) using a

simulated soil solution and deionized water as leaching solutions. The patterns of ion releases were examined as a function of

leaching time under both static and continuous-flow conditions for more than one month. Releases of both NH4

+

and NO3

from N-NaP were found to be slow and steady under both the above conditions. The soil solution affected the release of NH4

+ and

NO3 differently, while deionized water released nearly the same equivalents of these ions. This clearly indicates that ion release

from salt-occluded zeolite involves two different reactions, cation exchange and dissolution. The kinetics of ion release from

occluded NH4 NO3 under static condition was best described by the standard Elovich model while the power function model

best expressed these under continuous-flow condition. The initial ion release patterns under both conditions exhibited

considerable deviation from the simulated models, probably as a result of the presence of hydrated occluded NH4 NO3.

Flow condition and the presence of electrolytes in leaching solution affected the release kinetics significantly. Release of

occluded NH4 NO3 was delayed by the presence of the NH4 NO3 coated on zeolite crystals. These results indicate that the ion

release property of occluded salt could be predicted and controlled. This study clearly shows that NH 4 NO3-occluded zeolites

could be developed as slow release fertilizers.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Salt occlusion; Zeolite; Slow release; Fertilizer

1. Introduction

Aluminosilicate zeolites (generally zeolites) are

well known to exert beneficial effects on soil envi-

ronment because of their excellent cation exchange

0168-3659/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jconrel.2005.02.029

* Corresponding author. Tel.: +82 53 950 5717; fax: +82 53 953

7233.

E-mail address: [email protected] (M. Park).

Journal of Controlled Release 106 (2005) 44–50

www.elsevier.com/locate/jconrel



capacity (CEC), remarkable cation selectivity and

discrete microporosity [1,2]. Their potential has been

exploited for a wide range of applications such as

selective adsorbents [3–6], fertilizer matrices [7–13],artificial soils [7,14] and soil amendments [15,16]. In

particular, many attempts have been made to develop

zeolite-based fertilizers with slow release property.

However, zeolites as fertilizer matrices suffer from

low nutrient holding capacity in spite of their high

CEC. Although natural zeolites have CECs of about

200 cmol/kg, they can only hold about 3% N content.

Recently, it was found that N holding capacity of

zeolites could be increased by salt occlusion i.e., the

formation of stable salt complexes in zeolite pores

[11,12,17]. Furthermore, salt-occluded zeolites exhib-ited slow-release properties of nutrients, which were

different from those of cation-saturated zeolites [12].

Unfortunately, there are no systematic studies on the

release properties of the occluded salt complexes that

are essential to the application of salt-occluded zeo-

lites as slow-release fertilizers.

Salt occlusion is one of the unique phenomena in

microporous zeolites [17–19]. When zeolites are trea-

ted with molten salts, the salt either in solid or in molten

state could be introduced into zeolite pores by replace-

ment of adsorbed water. Depending on the proper size

fit between the pore window of zeolite and each ion

pair of salt, the introduced salt could be stabilized

(occluded) by geometric fit (especially in synthetic

aluminophosphate molecular sieves) and/or electro-

static interaction with zeolite framework. Likewise,

nearly all kinds of salts could be occluded, including

plant nutrient components like NH4 NO3, KNO3, and

KH2PO4, as well as chemically or biologically active

components like KClO3 and AgNO3. In addition, the

occluded salt is somewhat stable to water washing.

Salt-occluded zeolites possess two different kinds

of nutrient sources, exchangeable cations and occlud-ed salts. Zeolites accommodate much more amount

of nutrient by salt occlusion than by cation exchange

reaction. However, release properties of occluded

salts remain unexplored at present. To date, only

simple release patterns of ions from NH4 NO3-oc-

cluded natural zeolites were reported [12]. Therefore,

systematic studies on the release properties of oc-

cluded salts are highly needed to explore the poten-

tialities of salt-occluded zeolites as slow-release

fertilizers and soil conditioners. This study deals

with the release kinetics of NH4+ and NO3

from

NH4 NO3-occluded Na-P1 zeolite under both static

and continuous-flow conditions to evaluate the re-

lease property of occluded salt.

2. Materials and methods

A synthetic zeolite Na-P1 was used for this study

because it has the similar pore structure to the natural

zeolite phillipsite which could occlude a high amount

of NH4 NO3. It was hydrothermally synthesized by

microwave irradiation to have a Si/Al molar ratio of

1.12. The synthesis conditions and chemical compo-

sition were reported previously [17]. NH4 NO3-oc-cluded Na-P1 (hereafter referred as N-NaP) was

prepared in an electronic furnace as follows;

NH4 NO3 (4 g) and air-dried Na-P1 (1 g) were well

mixed in an Al2O3 crucible and thermally treated at

185 (F2) 8C for 8 h. After the crucible was cooled to

room temperature, the treated mixture was transferred

to a centrifuge tube with deionized water and rapidly

washed with 30 ml of distilled water by centrifuging

at 3000 rpm for 5 min. This washing step was re-

peated 7 times to ensure the complete removal of free

salt. The resulting N-NaP was dried at 105 8C and

stored in tightly capped glass bottles for further char-

acterization. Elemental analysis indicated that N-NaP

contained 13.1 wt.% of occluded NH4 NO3 and 1.82

wt.% of exchangeable NH4+. Additionally, a mixture

of NH4 NO3 (1 g) and Na-P1 (1 g) was thermally

treated as above and ground without water-washing

for the preparation of the unwashed N-NaP.

Two different solutions, deionized water and a

simulated soil solution, were used as leaching solu-

tions to examine the property of ion release from N-

NaP. The simulated soil solution consisted of 5 mM

CaCl2, 1 mM MgCl2, and 0.25 mM KCl [12]. The ionrelease was examined under two different conditions,

static and continuous-flow conditions. In static condi-

tion, 100 mg of N-NaP was immersed in 50 ml of the

leaching solution. The ion concentrations released into

the solution were monitored with immersion time. In

continuous-flow condition, 100 mg of N-NaP was

deposited on a 0.22-Am pore size membrane filter

which was placed inside a 1.5 cm-diameter glass

column with glass septum. The top end of the column

was connected to a reservoir of the leaching solution,

M. Park et al. / Journal of Controlled Release 106 (2005) 44–50 45



which was attached to a peristaltic pump. The leach-

ing solution was continuously eluted through the col-

umn at a flow rate of 2.0 (F0.2) ml/h. Effluent was

collected in 0.5 ml aliquots at selected intervals for determination of concentration of released NH4

+ and

NO3 (initially 0.1 ml aliquots were collected up to a

time of 24 h). Ions released into the solutions were

analyzed by ion chromatography [17]. All experi-

ments were conducted in triplicate. The leached N-

NaP was air-dried and pressed into a disc together

with KBr for infra-red spectroscopic analysis.

3. Results and discussion

3.1. Ion release property of NH 4 NO3-occluded Na-P1

The N-NaP releases both NH4+ and NO3

ions.

Figs. 1 and 2 show the release patterns of these ions

in the presence of soil solution and deionized water,

respectively.

Releases of both NH4+ and NO3

from N-NaP were

found to be slow and steady in static condition.

Releases of both ions continued even after one

month. NH4+ release was greatly affected by the pres-

ence of counter cations in solution. Both the initial

release rate and the cumulative released amount were

much higher in the soil solution than in deionized

water. On the other hand, NO3 release was very

similar in both soil solution and deionized water.

Only the cumulative amount of released NO3 was

slightly higher in deionized water than in the soil

solution. In addition, NH4+ and NO3

releases by

deionized water were found to be nearly the same in

both the pattern and the cumulative molar amount,

although the molar amount of released NO3 was

slightly higher than that of released NH4+.

Ion release from salt-occluded zeolite involves two

different reactions, cation exchange and dissolution.

Exchangeable cations of zeolite framework are readily

exchanged by counter cations in solution. As is already

well established, the cation exchange rate is signifi-

cantly affected by type of zeolite as well as concen-

tration and species of cations in solution [5,7,20]. On

the other hand, occluded salt is released mainly by

dissolution because it exists as neutral complex inside

the zeolite pores [17]. Release by dissolution occurs

through hydration and diffusion inside zeolite pores.

The simulated soil solution leads to both cation ex-

change and dissolution due to the presence of counter

cations, whereas deionized water results only in dis-

solution of occluded salt. Thus, the soil solution

resulted in faster and more release of NH4+ than that

of NO3

, while deionized water led to nearly equimolar release of NH4

+ and NO3. Slightly higher amount of

released NO3 in deionized water seemed to result

from adsorption of NH4+ by Na-P1. A slight difference

in the cumulative amount of released nitrate between

the soil solution and deionized water seemed to result

from the presence of electrolytes that could inhibit

hydration of occluded salt.

Figs. 3 and 4 show the release patterns of NH4+ and

NO3, respectively, under the continuous-flow condi-

tion. These release patterns were comparable to those in

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 200 400 600 800

Immersion time (h)

R e l e a s e d a m m o n i u m

i n s o l u t i o n ( m M )

DW

SS

Fig. 1. Ammonium ion release from NH4 NO3-occluded Na-P1

under static condition over the time immersed in either deionized

water (DW) or simulated soil solution (SS) with relative variation

shown by the size of the symbol marking each measurement.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 200 400 600 800

Immersion time (h)

R e l e a s e d n i t r a t e

i n s o l u t i o n

( m M )

DW

SS

Fig. 2. Nitrate ion release from NH4 NO3-occluded Na-P1 under

static condition over the time immersed in either deionized water

(DW) or simulated soil solution (SS) with relative variation shown

by the size of the symbol marking each measurement.

M. Park et al. / Journal of Controlled Release 106 (2005) 44–5046



the static solutions. The soil solution resulted in much

higher initial release rate and amount of NH4+ than those

ofNO3, whereas nearly equimolar releases of NH4

+ and

NO3 were observed in deionized water. In the soil

solution, released ammonium and nitrate approached

an equimolar ratio as elution proceeded. Nitrate release

was similar in both the soil solution and deionized

water. Slight differences were found in the released

concentration between NH4+ and NO3

in deionized

water as well as in the released NO3 concentration

between the soil solution and deionized water.

Rapid decrease in the release rate during the initial

period appears to result from the various hydration

states and locations of occluded salts. It was impos-

sible to completely avoid hydration of occluded salt

during preparation and storage procedures because the

preparation process of salt-occluded zeolite requires

washing with water. During the preparation process,some of the occluded NH4 NO3 would be in the var-

ious hydration states such as fully hydrated one (pre-

sumably out of zeolite pore), partially hydrated one

(presumably near pore entrance), and anhydrous one.

The former two could be easily released to result in a

high initial release rate. On the other hand, the loca-

tion of occluded salt in zeolite pores could also affect

dissolution rate. Due to diffusion through narrow pore

passage within small zeolite single crystals, the salt

occluded at the edge of pore near the surface of the

crystal could be more easily released compared to thesalt inside the pore. Deeply occluded salt could ex-

hibit a high resistance against dissolution even in

continuous-flow condition.

Slow and steady release of occluded salt was clear-

ly indicated by the persistence of occluded nitrates in

the N-NaP that had been eluted in soil solution (or

deionized water) for a long period. As reported pre-

viously [17], occluded nitrate salt exhibits an IR

absorption band at about 1400/cm which is distin-

guished from that of free nitrate salt at about 1385/

cm. Fig. 5 clearly indicates the presence of 1400/cm

band, although broad, even after 1 month of elution.

This result confirms high resistance of occluded salt

against dissolution even in continuous-flow condition.

3.2. Release kinetics of occluded salt

Occluded salts exist in the neutral form inside the

zeolite pores. Typically, they are released into solution

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 200 400 600 800

Elution time (h)

R e l e a s e d a m m o n i u m

i n s o l u t i o n ( m

M )

Water (Initial conc. = 3.01 mN)

Soil solution (Initial conc. = 7.96 mN)

Fig. 3. Ammonium ion release from NH4 NO3-occluded Na-P1

under continuous-flow condition over the time immersed in either

deionized water (DW) or simulated soil solution (SS) with rela-

tive variation shown by the size of the symbol marking each

measurement.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 200 400 600 800

Elution time (h)

R e l e a s

e d n i t r a t e


DW (Initial conc. = 3.44 mN)

SS (Initial conc. = 3.71 mN)

Fig. 4. Nitrate ion release from NH4 NO3-occluded Na-P1 under con-

tinuous-flow condition over the time immersed in either deionized

water (DW) or simulated soil solution (SS) with relative variation

shown by the size of the symbol marking each measurement.

1385(free)

1400(occluded)

500 700 900 1100 1300 1500 1700

Wavenumber / cm

After 10 days

After 35 days

Fig. 5. Infra-red spectra of NH4 NO3-occluded Na-P1 leached under

continuous-flow condition.




as cations and anions. Although released cations result

from both exchangeable cations and occluded salts,

anions exclusively originate from occluded salts. It is

suggested that anion release patterns could represent the release behavior of occluded salt. In this study, the

NO3 release patterns (Figs. 1 and 3) were chosen to

evaluate the release kinetics of occluded NH4 NO3

from N-NaP.

Release kinetics of occluded salt was found to be

greatly affected by the flow of the leaching solution.

Static condition resulted in a logarithmic relationship

of cumulative amount to immersion time, whereas an

exponential relationship of released amount to elution

time was observed in continuous-flow condition. The

presence of electrolytes in the leaching solutionexerted little effect on the type of the release kinetics.

The logarithmic relationship in static condition could

be best expressed in the form of the Elovich equation,

C t = a + b ln(t ). On the other hand, the exponential

relationship in continuous-flow condition could be

best fitted into the power-function equation, C t = at b

or ln C t =ln(a) + b ln(t ). In these equations, C t is the

amount of ion released at time t , while a and b are

constants. Although any clear definitions were not

available to interpret the significance of these con-

stants, it seems reasonable that the constants a and b

represent the initial release rate and the release rate,

respectively, because t Y0 as C t Y0 [21,22].

The best-fitting plots for the release patterns in

static condition are shown in Fig. 6. The overall

patterns could not be fitted into the Elovich model,

which exhibited considerable discrepancy in initial

period (up to 10 h). The best linear plot of the Elovich

model was obtained when t 0 was adjusted to the value

of 48 h. This indicates that the release kinetics of

occluded salt in static condition was satisfactorily

described by the standard Elovich model, C t = a + b

ln(t + t 0) rather than the Elovich model. On the other

hand, the release patterns in continuous-flow condi-

tion were best described by the power function model

(Fig. 7), although they also exhibited a significant

deviation from the simulated patterns especially dur-

ing the initial period (up to 6 h). When the observed

data of the initial period were excluded, the plot of the

power function model was greatly enhanced (see the

insert in Fig. 7). The calculated constants and regres-

sion coefficients are given in Table 1. These results

clearly show that different reactions are involved in

the initial release of ions from N-NaP under both

1.0

1.2

1.4

1.6

1.8

2.0

2.22.4

2.6

2.8

3.0

3.0 4.0 5.0 6.0 7.0

Immersion time (Ln (to+h))


i n s o l u t i o n ( m

M )

DW

SS

Fig. 6. The standard Elovich model of NO3 release from NH4 NO3-

occluded Na-P1 under static condition. SS: Simulated soil solution;

DW: Deionized water.

0.01

0.10

1.00

1 10 100 1000

Elution time (h)



DW (Initial conc. = 3.44 mN)

SS (Initial conc. = 3.71 mN)

0.01

0.1

1

10 100 1000

Fig. 7. The power function model of NO3

release from NH4 NO3-occluded Na-P1 under continuous-flow condition. SS: Simulated

soil solution; DW: Deionized water.

Table 1

The calculated constants and regression coefficients for the standard

Elovich (static experiments) and power function (continuous-flow

experiments) equations of the nitrate releases

Condition Static Continuous-flow

Overall Modifieda

Constant DW SS DW SS DW SS

a 0.2666 0.2809 0.3502 0.3857 0.2525 0.2853

b 0.3868 0.3679 0.3295 0.3511 0.2678 0.2943

R2 0.9877 0.9879 0.9333 0.9442 0.9786 0.9740

DW: Deionized water for extraction. SS: Soil solution for extraction.a The constants and coefficients were calculated after t 0 was

adjusted to the value of 48 h, as shown in the inserted box in Fig. 7.




static and dynamic conditions. The major reaction

involved is the release of hydrated occluded salt that

takes place in the initial period.

Both the Elovich and power function models have been employed to describe the reactions in soils like

desorption, release and dissolution of phosphorous

and potassium [8,9,21–25]. Aharoni and Sparks [23]

suggested that these two models could be used to

describe the kinetics of slow soil reactions in which

rate-limiting processes are the activated diffusions at

solid surface, in micropore and in bulk solution. The

standard Elovich model has been adopted to describe

the kinetics including bulk and surface diffusion

[8,26]. On the other hand, the power function model

could best describe the slow reactions controlled by acombination of the activated transport processes [8].

As mentioned before, the release of occluded salt

takes place through hydration and diffusion. In static

condition, the release rate of occluded salt seems to be

greatly affected by the diffusion of fully hydrated salt

from the surface of zeolite particle into bulk solution.

On the other hand, continuous-flow condition results

in the so-called dilution or washing-out effect [8] that

greatly increases the rate of bulk and surface diffu-

sion. Likewise, hydration and diffusion of occluded

salt in confined space, i.e., zeolite pore, could play a

crucial role in the release of occluded salt under

continuous-flow condition. Therefore, static condition

led to the standard Elovich equation, whereas the

power function equation resulted from the continu-

ous-flow condition. Fast release of hydrated occluded

salt was clearly reflected by the observed discrepancy

initially. Between deionized water and the soil solu-

tion, the soil solution resulted in higher value of the

constant a, while deionized water led to a higher

value of constant b. The former suggests that the

initial release rate is enhanced by cation exchange

reaction due to counter cations in solution, whereasthe latter indicates that the overall release rate is

significantly affected by the hydration ability of

leaching solution. These results strongly suggest that

release of occluded salt proceeds mainly by the acti-

vated diffusion mechanism.

Thermal treatment for preparation of salt-occluded

zeolite inevitably leads to the mixture in which exter-

nal surface of salt-occluded zeolite is coated with

molten salt. Because the coated salt inhibits occluded

salt from being hydrated, they could delay release of

occluded salt. Fig. 8 shows the NO3 release pattern

of the thermally treated mixture of Na-P1 and

NH4 NO3 in continuous-flow condition. The mixture

contained both coated and occluded salts. The NO3

concentration in the initially eluted solution was ex-

tremely high because the coated salt dissolved. As

coated salt was washed off, the concentration of

released NO3 seemed to decrease exponentially.

None of the above models described the overall re-

lease pattern properly. However, when the overall

pattern was separated into two parts, before 12 h and

after 48 h, each pattern seemed to conform to power

function model. This result indicated that two differ-

ent forms of the nitrate, i.e., coated and occluded

ones, were sequentially released from the mixture,as can be expected. Between the elution time of 12

h and 48 h, releases of both coated and occluded salt

occurred. In this experiment, it was hard to find a

difference in the concentration of released NO3 be-

tween the soil solution and the deionized water. In

addition, this result confirms that the power function

model could be employed to describe the release

kinetics of occluded salt under continuous-flow con-

dition. Therefore, the release of occluded salt could be

delayed by the presence of the coated salt on external

surface of zeolite crystals, which could offer another

advantage. Not only this coated salt could be utilized

as fast-release fertilizer but also control the release of

occluded salt. Thus, these results suggest that it is

feasible to control the release property of occluded

salts in zeolites.

0.01

0.1

1

10

100

1 10 100 1000

Elution time (h)



DW

SS

Fig. 8. Linear plot of NO3 release from the thermally treated

mixture of Na-P1 and NH4 NO3 (the unwashed N-NaP) under

continuous-flow condition. SS: Simulated soil solution; DW: Deio-

nized water.




Acknowledgments

This research was supported both ARPC and

KEMOR of Korea. One of the authors (S.K.)acknowledges the support of College of Agricultural

Sciences under Station Research Project.

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Characteristics of Nitrogen Release From Synthetic Zeolite Na-P1 Occluding NH4NO3

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