ARTICLE

pubs.acs.org/IECR

j-Carrageenan
Sodium Alginate Beads and Superabsorbent Coated

Nitrogen Fertilizer with Slow-Release, Water-Retention, and
Anticompaction Properties
Yanfang Wang, Mingzhu Liu,* Boli Ni, and Lihua Xie
State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province, and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China.

ABSTRACT: A multifunctional slow-release nitrogen fertilizer (SRNF) has been developed to improve fertilizer use efficiency and
reduce environmental pollution. k-Carrageenan
sodium alginate (kC
SA) and cross-linked kC-g-poly(acrylic acid)/Celite
superabsorbent were used as inner and outer coating materials, which were coated consecutively on the granule core urea in a pan
granulator. Elemental analysis result showed that the product contained 22.6% nitrogen. The water evaporation as well as the
nitrogen release behavior of SRNF in soil was explored. In addition, the coated urea had a higher average crushing strength in
comparison with uncoated urea. Furthermore, the incorporation of SRNF in soil could effectively prevent the compaction of soil.
These studies showed that the product prepared by a simple route with good slow-release and water-retention properties may be
expected to have wide potential applications in modern agriculture and horticulture.

1. INTRODUCTION widespread use in agriculture because the science and technology

Modern agriculture now feeds 6000 million people,1 and of polymers has undergone explosive growth during recent years
fertilizer is a vital input material for the sustainable development and they can be designed to release nutrients in a more controlled
of crop production and food security.2 Worldwide experiences in manner.8
agricultural development have proved that rational fertilization is The current trend of using environmentally friendly products
one of the most efficient and important methods to increase crop has resulted in an extensive exploitation of new materials obtained
yields. Fertilizer consumption in China is also challenging the from renewable resources. Preparation of polysaccharide-based
acceptance limit of resource and environment. From 1977 to composites, blends, or mixtures represents a new category of
2005, annual synthetic nitrogen (N) fertilizer application in- environmentally safe materials for newer applications, due to
creased from 7.07 to 26.21 million tons (a 271% increase) in their water solubility, renewability, nontoxicity, and biodegrad-
China.3 However, compared with 45% in developed countries, ability.9,10 Sodium alginate (SA) is an anionic natural macro-
the degree of utilization of N fertilizer in China is only in the molecule, which is composed of poly-β-1,4-D-mannuronic acid
range of 30
35%. Meanwhile, nitrate leaching and groundwater and α-1,4-L -guluronic acid in varying proportions by 1
4
contamination have been shown to be related to N fertilizer use linkages. It can be extracted from marine algae or produced by
in agriculture. Additionally, some recent studies have shown that bacteria.11 k-Carrageenan (kC) is a negatively charged poly-
overuse of N fertilizer has already induced serious environmental saccharide extracted from red seaweed that has a linear sulfated
hazards, including soil acidification, heavy metal contamination, backbone of alternating α-1,4- and β-1,3-linked galactose
and greenhouse gas emission.4 Among the N fertilizers, the most residues.12 SA together with kC was used in the drug delivery
widely used one is urea, because of its high nitrogen content systems and special membranes.13
15 Their joint use as coating
(46%) and comparatively low cost of production. After being material in agriculture has not been reported. Most recently, we
applied to soil, it can be rapidly hydrolyzed to NH3 and CO2 by prepared kC
SA complex beads by dropping an aqueous
soil urease, followed by NO3
formation through nitrification. solution of kC and SA into calcium chloride (CaCl2) and
Therefore, ammonia loss and nitrate leaching are environmental potassium chloride (KCl) mixed solution to form ionically
concerns in regions where urea is applied.5 When concerns about cross-linked hydrogel beads, which were used as a fully natural
the sustainability of input-intensive agriculture and the economic, coating material. Mixing of kC with SA could improve its
ecological, and environmental effects of N fertilizer overuse mechanical properties and reduce brittleness of the hydrogel.16
are taken into consideration, it is clear that use of slow-release In addition, there is a need to explore the utilization of abun-
fertilizers (SRFs) is one attempt to enhance the efficiency of dantly available seaweed polysaccharides.
fertilizer to mitigate these problems.6 SRFs allow the active Besides fertilizer, water is another important factor that limits
component to slowly diffuse toward the soil, thus making it the production of agriculture. China is one of the world’s most
available in the field for a longer period of time.7 In case of urea,
slow-release products are typically made by reacting urea with Received: September 8, 2011
various aldehydes to reduce the solubility of the material. Accepted: December 21, 2011
Another method of regulating N fertilizer release is the applica- Revised: December 15, 2011
tion of a coating. Polymer-coated SRFs look promising for Published: December 21, 2011

r 2011 American Chemical Society 1413 dx.doi.org/10.1021/ie2020526 | Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

water-deficient economies, and the scarcity of water is viewed as a Works, Beijing, China) was distilled at reduced pressure before
major threat to long-term food security. Therefore, efficient use. N,N0 -Methylenebis(acrylamide) (NNMBA) was recrystal-
management of soil moisture is important for agricultural lized from 95% ethanol prior to use. Ammonium persulfate
production in response to scarce water resources. Superabsor- (APS) was recrystallized from distilled water before use. Celite,
bents are slightly cross-linked hydrophilic polymers that can supplied by Shanghai Chemical Reagent Factory (Shanghai,
absorb huge volumes of water without dissolving or losing their China), was calcined at 450 °C, and particle sizes <10 mm were
three-dimensional structures and can retain the absorbed water about 90 wt %. It contains 14
16 wt % calcium sulfate and
even under certain pressure.17 Because of their excellent char- 0.02 wt % chloride, and the content of iron is <0.05 wt %. The pH
acteristics of water retention and absorbency, superabsorbents of the solution (10 wt % diatomite) is 6.5
7.5. All other reagents
had been used in agriculture and horticulture since their advent in used were of analytical grade, and all solutions were prepared
1969.18,19 Recently, several studies20
22 have confirmed the with distilled water.
importance of superabsorbents in improving physical properties 2.2. Synthesis of jC-g-PAA/Celite Superabsorbent Com-
of the soil such as porosity, structure, and water-holding capacity. posites. A series of superabsorbent composites with different
Johnson23 reported 171
402% increases in water retention contents of kC, Celite, and cross-linker were prepared according
capacity when superabsorbents were incorporated in coarse sand. to the following procedure. A certain amount of kC was first
Islam et al.4 reported that the application of superabsorbents dissolved in 10 mL of distilled water, which was under vigorous
could conserve soil
water, making it available to plants for stirring in a four-necked flask equipped with a stirrer, a thermo-
increased biomass accumulation and reduced oxidative stress, meter, a condenser, and a nitrogen line. The flask was immersed
especially under severe water stress. Similarly, plant height, leaf in a thermostated water bath preset to the desired tempera-
area, and number of grains as well as protein, soluble sugar, and ture (80 °C). After complete dissolution of kC to form a
starch contents in the grain also increased with superabsorbent homogeneous solution, certain amounts of preneutralized AA,
treatment. Mikkelsen et al.24 found that addition of superabsor- NNMBA, and Celite powder were simultaneously added to the
bent to the fertilizer solutions reduced N leaching losses from soil reaction mixture. Afterward, the solution was stirred and purged
columns by as much as 45% during the first 4 weeks in heavily with nitrogen for 15 min to remove the dissolved oxygen. Then, a
leached conditions compared with N fertilizer alone. Moreover, definite amount of APS solution was added dropwise to the
the use of superabsorbent materials as carrier and regulator of reaction flask under continuous stirring to generate free radicals.
nutrient release was helpful in reducing undesired fertilizer losses The water bath was heated slowly to 80 °C and maintained at this
while sustaining vigorous plant growth.25 temperature for 3 h to complete polymerization. Finally, the
To satisfy these requirements, we prepared a double-coated resulting product was dried, milled, screened, and stored for
slow-release and water-retention nitrogen fertilizer. Its core is further use. All samples used had a particle size in the range of
urea fertilizer granule, the first coating layer is kC
SA bead, and 40
90 mesh.
the second coating layer is kC-g-poly(acrylic acid)/Celite super- 2.3. Preparation of jC
SA Complex Beads. kC
SA com-
absorbent. The choice of the coatings is largely dictated by the plex beads were prepared according to the following procedure.
problems to be addressed above. By doing so, first, superabsor- kC solution (1%, w/v) and SA solution (2%, w/v) were prepared
bents were used as the outer coating instead of blending or by dissolving 4 g of kC and 8 g of SA into 400 mL of distilled
polymerizing with fertilizers; this process reduced the loss of water in a 1000 mL three-necked flask equipped with a mecha-
fertilizer without altering the properties of water retention and nical stirrer, a reflux condenser, and a thermometer. The flask was
absorbency. Second, the kC
SA layer was incorporated into the then placed in a water bath, which was heated slowly to 75 °C and
fertilizer production to make the products cheaper and easier to maintained at this temperature for 30 min to dissolve kC and SA
biodegrade.26
29 Meanwhile, hydrophilic groups of kC
SA completely. The polysaccharide solution was then dropped into
matrix were cross-linked completely by potassium and calcium 400 mL of a stirred salt solution mixture of CaCl2 (3%, w/v) and
ions. This approach turned hydrophilic kC and SA into the KCl (3%, w/v). To complete gelation, the beads were maintained
hydrophobic kC
SA bead, which contributed to the slow- in the salt solution for 30 min and filtered, followed by washing
release behavior of fertilizer. In addition, the coated fertilizer is with distilled water, and then were allowed to dry at 35 °C.
expected to retard nitrogen release, improve soil moisture, 2.4. Preparation of Slow-Release Nitrogen Fertilizer. First,
reduce the use of water, and alleviate environmental hazards an amount of urea granules (2
2.5 mm in diameter) was placed
caused by excessive fertilization. Therefore, the main purpose of on a rotating pan in batches. Subsequently, kC
SA complex
this study was to determine the release characteristics of the powder (below 200 mesh) as the inner coating was adhered to
coated fertilizer and its effect on improving the water-holding the fertilizer cores under water atomization. Then, the granules
capacity of soil. Research has been ongoing to prepare a multi- coated with a thin layer of kC
SA powder were removed and
functional fertilizer that can be used in agro-industries. dried at 30 °C. Multiple kC
SA coatings were prepared by
repeatedly atomizing the previously coated granules for adhering
2. EXPERIMENTAL SECTION kC
SA powder. Thus, kC
SA powder-coated urea granules
with different coating thicknesses were obtained. Finally, kC-g-
2.1. Materials. The source of nitrogen used was commercial PAA/Celite superabsorbent powder (below 200 mesh) as the
pelleted urea, which was previously sieved to be between 2.0 and outer coating was coated on the surface of the granules under
2.5 mm in diameter. Sodium alginate (SA; the viscosity of a 2% rotating. The process was completed until a compact and
solution is 3200 mPa 3 s at 25 °C) was obtained from Qingdao homogeneous coating formed on the fertilizer granules. The
Haiyang Chemical Co. (Qingdao, China). k-Carrageenan (kC) coated granules were dried in an oven at 30 °C to obtain the final
was purchased from the Golden Phoenix of k-Carrageenan Co. products.
Ltd. (Tengzhou, China) and used without further purification. 2.5. Characterizations. Fourier transform infrared (FTIR)
Acrylic acid (AA, chemical grade; Beijing Eastern Chemical spectra of samples were recorded on a Nicolet Nexus 670 FTIR
1414 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

spectrometer in the 4000
400 cm
1 region by use of KBr crushing strength is better for avoiding breakage and pellet strain
pellets, with samples extracted in distilled water for 72 h at room during handing and bag storage.30 In this study, the crushing
temperature. The average diameter of coated fertilizers was strength was measured by applying pressure to individual
determined on a micrometer for 20 granules. The nitrogen granules of a diameter range 2.7
3 mm. The magnitude of
content of SRNF was determined by an elemental analysis pressure exerted on the fertilizer granule was continuously
instrument (Germany Elemental Vario EL Corp., model 1106). increased until destruction of the granule was observed.
Micrographs of samples were examined with scanning electron This maximum load value (average from 20 measurements)
microscopy (SEM) (JSM-5600LV, JEOL, Ltd.). Before the was considered to be the crushing strength.31,32 For uncoated
SEM observation, all samples were fixed on aluminum stubs urea granules, the average crushing strength was 1121
and coated with gold. g/granule, and for double-coated SRNF, the value was 1234
2.6. Measurements of Equilibrium Water Absorbency of g/granule, which was about 10% higher than the uncoated
the Superabsorbents. An accurately weighed quantity of the urea. Therefore, this process would facilitate the transporta-
superabsorbent composite (0.1 g, 40
90 mesh) was immersed tion and use practice and improve the durability and integrity
into a certain amount of tap water at room temperature for of the SRNF.
60 min to reach the swelling equilibrium. The swollen samples 2.10. Slow-Release Behavior of Nitrogen from SRNF in
were filtered and weighed. Water absorbency (WA) was calcu- Soil. To study the slow-release behavior of nitrogen from SRNF
lated from eq 1: in soil, 1 g of SRNF was buried in sealed nonwoven bags approx-
m2
m1 imately 6 cm beneath the surface of the soil (below 26 mesh)
WA ¼ ð1Þ in a glass beaker at ambient temperature. Throughout the
m1 experiment, the soil moisture was kept at 20%. After 0.5, 1, 2,
where m2 and m1 refer to the weights of swollen and dried 3, 5, 10, 15, and 25 days, the bags were retrieved and air-dried.
superabsorbents, respectively. WA was calculated as grams of Then the fertilizer granules were removed from the bags and the
water per gram of sample. In all cases, three parallel samples were content of nitrogen was estimated.
used and the averages are reported in this paper. 2.11. Measurement of Water-Holding Capacity of Soil
2.7. Determination of Soluble Fraction of Superabsor- with SRNF. The study of the effect of SRNF on water-holding
bent. The soluble fraction (sol) is the sum of all water-soluble capacity of soil was carried out. Different amounts of SRNF were
species including non-cross-linked oligomers and nonreacted well blended with 200 g of dry soil (below 26 mesh) and placed
starting materials such as residual monomers. The sol content is into a poly(vinyl chloride) (PVC) tube of 4.5 cm in diameter.
simply determined by extraction of superabsorbent sample in The bottom of the tube was sealed with nylon fabric (with an
distilled water. A certain amount of crude sample particles was aperture of 0.076 mm) and weighed (marked m1). The soil
poured into an excess amount of distilled water and dispersed sample was slowly drenched by tap water from the top of the tube
with mild magnetic stirring at room temperature for 24 h. The until the water seeped out from the bottom. The tube was
water was refreshed every 8 h in order to remove the soluble weighed (marked m2) again when there was no water seeping out
fraction. Afterward, the swollen sample was filtered and dewa- at the bottom. A control experiment without SRNF was also
tered in excess anhydrous ethanol for 12 h. Finally, the sample carried out. Three fertilizer application rates (1, 2, and 3 wt %)
was oven-dried at 50 °C to a constant weight. The sol content were examined. The water-holding capacity (WH, %) of the soil
was calculated as the weight loss of the initial crude sample. was calculated from eq 3:
2.8. Saline Solution Absorbency of Superabsorbent under
Load. A hollow macroporous plexiglas cylinder with an internal ðm2
m1 Þ  100
diameter of 2.9 cm was placed in a 100 mL beaker. An accurately WH ¼ ð3Þ
200
weighed portion (0.1 g) of superabsorbent composite was spread
uniformly on the surface of the polyester gauze located on the 2.12. Measurement of the Water Retention of SRNF in Soil.
plexiglas cylinder. A plastic cylinder (106 g) that could slip freely Different amounts of SRNF were well mixed with 100 g of dry
in a glass cylinder was used to apply the load to the dry super- soil (below 26 mesh) and placed in a glass beaker. An appropriate
absorbent samples (P ≈ 2060 Pa). Then 60 mL of 0.9 wt % NaCl amount of tap water was added into the beaker to make the soil
aqueous solution was added slowly into the beaker. The entire saturated, and then the beakers were kept under ambient
setup was covered to prevent surface evaporation and prob- temperature. The initial masses of the mixture of dry soil with
able change in the saline concentration. The superabsorbents different amounts of SRNF in the beakers were measured
were then separated from unabsorbed saline solution and (marked m0). The weights of the mixture of soil saturated with
weighed at set intervals. This process was repeated until the tap water were also recorded daily (marked mi) to compare the
weight of the superabsorbents remained constant. The saline water retention of SRNF. Meanwhile, a control experiment
absorbency under load (AUL) at different time intervals was without SRNF was carried out. Three fertilizer application rates
calculated according to eq 2: (1, 2, and 3 wt %) were examined. The water retention capacity
m2
m1
m0 (WR, %) of the soil was calculated from eq 4:
saline AUL ¼ ð2Þ
m0 mi
m0
WR ¼  100 ð4Þ
where m0 and m1 denote the weights of dry superabsorbent m0
sample and glass cylinder, and m2 is the weight of glass cylinder
with swollen superabsorbent sample. 3. RESULTS AND DISCUSSION
2.9. Determination of Average Crushing Strength for
SRNF. Crushing strength is a measure of the resistance of 3.1. Fourier Transform Infrared Analysis. The FTIR spectra
granules to deformation or fracture under pressure. High of kC, kC-g-PAA, kC-g-PAA/Celite, and Celite are shown in
1415 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

Scheme 1. Synthetic Route of jC-g-PAA/Celite

Superabsorbent

Figure 1. FTIR spectra of (a) kC, (b) kC-g-PAA, (c) kC-g-PAA/Celite,

and (d) Celite.

Figure 1. The characteristic absorption bands observed at 845,

930, 1260, and 3431 cm
1 can be assigned to D-galactose-4-
sulfate, 3,6-anhydro-D-galactose, ester sulfate, and
OH stretch-
ing vibrations of kC, respectively33 (Figure 1a). When the FTIR
spectrum of kC-g-PAA was compared with that of kC, new bands
at 1727 cm
1 (CdO stretching of
COOH groups), 1561 cm
1
(asymmetrical stretching vibration of
COO
groups), and
1409 cm
1 (symmetrical stretching vibration of
COO
groups) PAA branches on the kC or Celite backbones and lead to a
were observed in the spectrum of kC-g-PAA (Figure 1b). graft copolymer. Since a cross-linking agent (NNMBA) is
Besides, the characteristic absorption band of kC at 1252 cm
1 introduced into the system, the copolymer has a cross-linked
was observed in the spectrum of kC-g-PAA. In addition, the structure. The Celite existing in the polymer matrix can be
characteristic absorption bands of kC at 1070, 1123, and classified as two kinds: chemically bonding and physically
1159 cm
1 (stretching vibration of C
OH groups) were filling.38
obviously weakened after reaction and shifted to 1041, 1110, To obtain additional evidence of grafting, a similar polymer-
and 1171 cm
1 for kC-g-PAA. This result indicated that AA ization was conducted in the absence of cross-linker.39 The
monomers were grafted onto the kC backbone. The
COO
resulting product was precipated by pouring the reaction mixture
band of kC-g-PAA at 1561 cm
1 shifted to 1567 cm
1 after into 100 mL of ethanol. Then 0.1 g of the dried product was
formation of kC-g-PAA/Celite composite (Figure 1c), which poured into 50 mL of dimethylformamide solution (a suitable
implied that the incorporation of Celite decreased the hydrogen- solvent for homopolymer).40 The mixture was stirred gently at
bonding interaction among polymer chains, in contrast to kC-g- room temperature for 24 h. After complete removal of the
PAA. As depicted in Figure 1d, the bands at 3435 and 793 cm
1 homopolymer, the kC-g-PAA/Celite superabsorbent composite
were due to the vibration of free silanol groups (SiO-H), and the was filtered, washed with ethanol, and dried in an oven at 50 °C
band at 1629 cm
1 represented H
O
H bending vibration of to reach a constant weight, an appreciable amount of synthetic
water, which bonded to the surface hydroxyl groups of Celite via polymer percentage of the graft polymer (89%) was concluded.
H-bonds.34,35 When the FTIR spectrum of Celite was compared FTIR spectra of the graft polymer were obtained; the result was
with that of kC-g-PAA/Celite, the band at 1629 cm
1 shifted to very similar to that shown in Figure 1c and is not presented in this
1636 cm
1 and the intensity was obviously weakened. Mean- paper. Also according to preliminary measurements, the sol
while, the characteristic absorption band of Celite at 793 cm
1 content of the superabsorbent was 18%. This fact practically
disappeared after reaction. This information gave direct evidence proved that most of the monomers were involved in the polymer
that Celite participated in the graft polymerization reaction network.
through its active silanol groups. 3.2. Effect of Cross-Linker Content on Water Absorbency.
According to the information from FTIR spectra, it can be One of the important properties of SRCF is the water absorbency
concluded that a graft-copolymerization reaction takes place due to the coating of kC-g-PAA/Celite superabsorbent poly-
among kC, Celite, and AA and a network structure is formed. mers. To improve the water absorbency capacity of the products,
Scheme 1 shows the possible synthetic route of kC-g-PAA/ the reaction parameters were optimized. Water absorbency as a
Celite superabsorbent. The sulfate anion radicals produced from function of NNMBA content was investigated for cross-linked
the thermal decomposition of APS abstract hydrogen from kC-g-PAA/Celite superabsorbents. As we can see from Figure 2,
the hydroxyl groups of kC12 or Celite36,37 to form correspond- the water absorbency decreased with increasing cross-linker
ing radicals on the substrates. The radicals in active centers on content from 0.2 to 0.5 wt %. This was attributed to the fact
the substrates initiate the polymerization reaction and give that when the cross-linker content was larger than 0.2 wt %, a
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Industrial & Engineering Chemistry Research ARTICLE

Figure 2. Effect of cross-linker content on water absorbency of the Figure 4. Effect of neutralization degree of AA on water absorbency of
superabsorbent in tap water: AA, 4 g; APS, 0.5 wt %; kC, 0.2 g; Celite, the superabsorbent in tap water: AA, 4 g; NNMBA, 0.2 wt %; APS, 0.3 wt %;
0.1 g; neutralization degree of AA, 50%; reaction time, 3 h at 80 °C. kC, 0.2 g; Celite, 0.1 g; reaction time, 3 h at 80 °C.

may cause a short average kinetic chain length and many more
polymer chain ends. Both cases lead to deterioration of absor-
bency properties of the product. Also, large quantities of free
radicals may cause a transfer reaction to polymer chains, which
would result in an increase in the cross-linked density and a
decrease in the water absorbency consequently. Furthermore,
the free radical degradation of kC backbones by sulfate radical
anions was an additional reason for swelling loss at higher
initiator content. The proposed mechanism for this possibility
was reported in previous work.42 A similar oxidative degradation
of chitosan chains by potassium persulfate was reported by
Hsu et al.43
3.4. Effect of Neutralization Degree of AA on Water
Absorbency. Neutralization degree is defined as the molar
percentage of carboxyl groups in AA neutralized by sodium
hydroxide. Neutralization degree not only affects the polymer-
ization rate but also determines the charge density of the three-
Figure 3. Effect of initiator content on water absorbency of the super-
dimensional network; therefore, it has a remarkable influence on
absorbent in tap water: AA, 4 g; NNMBA, 0.2 wt %; kC, 0.2 g; Celite,
0.1 g; neutralization degree of AA, 50%; reaction time, 3 h at 80 °C. water absorbency. Figure 4 demonstrates that there is also a
maximum of water absorbency dependent on the neutralization
degree of AA. Water absorbency increased with increasing
further increase in the cross-linker content resulted in a higher neutralization degree from 10% to 30% and decreased with
cross-linking density and decreased the space between polymer further increase in the neutralization degree of AA. This behavior
chains, and consequently, the resulting highly cross-linked rigid may be explained as follows: within a certain range of neutraliza-
structure cannot be expanded and hold a large quantity of water. tion degree, the carboxyl acid groups of AA turned into carbox-
However, when the cross-linker content was lower than 0.2 wt %, ylate groups, so the charge density of the network would increase,
the three-dimensional network that was necessary for super- which resulted in the destruction of hydrogel bonds as well as an
absorbent cannot be perfectly formed, leading to the presence of electrostatic repulsion that tended to expand the network, and
non-cross-linked PAA chains, which did not contribute to water then an increase in water absorbency. However, when the
absorption. These results were in accordance with Flory’s net- neutralization degree of AA was larger than 30%, the screening
work theory.41 effect of sodium ions on the polyanion chains would reduce the
3.3. Effect of Initiator Content on Water Absorbency. As electrostatic repulsion of the network, which resulted in the
the mechanism of synthesis process of the superabsorbent was decrease of water absorbency of coating superabsorbent. Mean-
free-radical polymerization, the initiator content had remarkable while, the activity of acrylic acid was higher than that of acrylate,
influence on the whole reaction course and the ultimate water so the larger the neutralization degree was, the lower the
absorbency of the resultant superabsorbent. Figure 3 depicts the polymerization rate was.44 The low polymerization rate would
water absorbency dependency on initiator content in tap water. result in an increase in the content of the oligomers, and
Maximum water absorbency (261 g/g) was obtained at 0.3 wt % consequently the water absorbency decreased.
initiator content. Initiator content more or less than 0.3 wt % 3.5. Effect of jC Content on Water Absorbency. The water
gave superabsorbent with decreased swelling capacity. Lower absorbency dependency on kC content is shown in Figure 5. As
initiator content resulted in a low quantity of free radicals and can be seen, the water absorbency increased with increasing kC
hence an imperfectly formed network. But an excess of initiators content until it reached a maximum of 343 g/g at kC content 4 wt %
1417 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

Figure 7. Swelling kinetic behavior of the superabsorbents in 0.9 wt %

NaCl aqueous solution under load: (a) kC-g-PAA/Celite; (b) kC-g-
Figure 5. Effect of kC content on water absorbency of the super-
PAA.
absorbent in tap water: AA, 4 g; NNMBA, 0.2 wt %; APS, 0.3 wt %;
neutralization degree of AA, 30%; Celite, 0.1 g; reaction time, 3 h
at 80 °C. part in the polymerization reaction as well as construction of the
3D network. As a result, the intertwining of polymeric chains was
prevented and the H-bonding interactions between hydrophilic
groups (such as
COOH,
COO
,
OH, and
SO4
) were
weakened. Thus the degree of physical cross-linking decreased
and the network voids for holding water regularly formed, and
the water absorbency of the superabsorbent was improved.
However, the excessive addition of Celite induced a decrease
of water absorbency. According to a previous report, the func-
tional group of the filler particle surface involved an esterification
reaction with the carboxylic groups of the acrylic chains in the
superabsorbent composite.46 It implied that the filler particles
took action like a multifunctional cross-linker or cross-link
points, inherently driving the network toward a higher density
of cross-linking. In addition, the excessive Celite powder may
physically stack in the network, which may plug up the network
voids for holding water. On the other hand, high content of
Celite powder in superabsorbents relatively reduced the ratio of
Figure 6. Effect of Celite content on water absorbency of the super- hydrophilic groups per unit volume, leading to a loss of water
absorbent in tap water: AA, 4 g; NNMBA, 0.2 wt %; APS, 0.3 wt %; absorbency. It should be mentioned that the water absorbency of
neutralization degree of AA, 30%; kC, 0.2 g; reaction time, 3 h at 80 °C. the superabsorbent composite was still higher than the super-
absorbent without Celite even when the content of Celite
and then decreased with higher content of kC. It is well-known reached 5 wt %, which was extremely favorable to reduce the
that the hydroxyl groups of kC may react with the initiator and production cost.
liberate free radicals, on which the polymerization will take place 3.7. Saline Absorbency under Load Analysis. Saline AUL
and give PAA branches on the kC backbone.45 This reaction will can be taken as a simple measurable simulation of the real
make the kC bond to the polymer chains and become the cross- circumstances of the applications of superabsorbents. In nearly
linking point. Since the water absorbency of the superabsorbents all academic literature, the swelling capacity values of super-
would display a maximum at a certain content of cross-linker, it absorbents are reported as load-free swelling data, measured
was understandable that the maximum of water absorbency usually in distilled water. It is obvious that swelling conditions
appeared at kC content 4 wt %. In addition, when the kC (i.e., distilled water and lack of pressure), and hence the resulting
content was low, the AA monomers turned out to be a homo- data, are not real, because in all applications of superabsorbents
polymer, which in turn resulted in an increase in soluble materials (hygienic, agriculture, etc), the swelling particles must absorb
at fixed cross-linking density. When the content of kC was larger aqueous solutions while they are under pressure.47,48 Meanwhile,
than 4 wt %, the viscosity of the reaction mixture increased, which during many years of working on superabsorbent materials, we
hindered the movement of the reactants. Thus, the grafting ratio used to empirically realize a linear relationship between the
and the molecular weight of the grafted PAA chains decreased, AUL values and the strength of the swollen superabsorbents.
resulting in a decrease in water absorbency. So, AUL can be considered as a measure of the strength of the
3.6. Effect of Celite Content on Water Absorbency. As superabsorbents.49,50 To determine the saline AUL of the super-
shown in Figure 6, the water absorbency of the superabsorbent absorbents, we used optimized final products that absorbed
composite first increased with increasing Celite content and saline solution under load (P ≈ 2060 Pa). As shown in Figure 7,
reached a maximum at 2.5 wt %. Celite is rigid and contains large the minimum time needed for the highest saline AUL of the
amounts of active Si
OH groups on its surface, which can take kC-g-PAA/Celite superabsorbent was estimated to be 240 min.
1418 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

Figure 9. Water-holding capacity of the soil mixed with 0, 1, 2, and

Figure 8. Slow-release behavior of nitrogen from SRNF in soil. 3 wt % SRNF.

After this time, the saline AUL values were almost unchanged. In
addition, a control experiment without Celite was also carried out
with other conditions keep constant. At the applied pressure,
maximum swelling was found to be 39 and 32 g/g for the
superabsorbents with and without Celite, which showed that
the introduction of Celite could obviously improve the saline
AUL, meaning improved strength of the superabsorbents.
3.8. Nitrogen Release Behavior of Coated SRNF in Soil.
One of the most important characteristics of coated SRNF was its
slow-release property. Plots of the released percentage of nitro-
gen against time (days) are shown in Figure 8 for coated SRNF
fertilizer in soil. As can be seen, the nitrogen in SRNF released
39%, 72%, and 94% on the second, fifth, and 25th days,
respectively. The rapid release rate in the early stage (in the first
5 days) could be mainly ascribed to the dissolution of urea in the
fertilizer cores. The coated superabsorbent polymers would
absorb the water and swell slowly after being added into soil, Figure 10. Water retention behavior of the soil mixed with 0, 1, 2, and
which would contribute to an increase in the pore size of the 3 wt % SRNF.
three-dimensional network and benefit the diffusion of the
fertilizer solution into the hydrogel network. There existed a interesting particular characteristic of the water absorption
dynamic exchange between the free water in the hydrogel and capacity of SRNF, we studied its effect on water-holding capacity
that in soil, and then the urea would diffuse out of the kC
SA of soil. As shown in Figure 9, the water-holding capacity of soil
layer and enter into the kC-g-PAA/Celite layer, and then release was 39%, 46.5%, 53.5%, and 59% for SRCF application rates of 0,
into the soil through the grids with dynamic exchange. The 1, 2, and 3 wt %, respectively. It was noted that, with the increase
release rate after 5 days became slower. This was mainly because of SRCF samples, the superabsorbent contents increased and
the concentration of urea in the cores decreased with time, which thus the water-holding capacity of soil increased. In soil, each
caused the decline of osmotic pressure inside and outside the SRCF granule was surrounded by soil particles and subjected to a
fertilizer granules. Meanwhile, more ions and soil particles were confining pressure by these particles. Therefore, the swelling
absorbed by the coating superabsorbents. These ions and soil degree of the superabsorbent in soil was limited compared with
particles played a role of physical barrier, together with the inner that in tap water. However, compared with the control (soil
and outer coating layers. Additionally, due to the existence without SRNF), the SRNF effectively improved the water-
of many kinds of ions in soil solution, the swelling degree of holding capacity of soil, even though at a low application rate.
the coating superabsorbents was less in soil than in tap water, so Moreover, it was also observed that the water flow rate through
the diffusion of soluble urea in it would be difficult, which the soil was slowed down when SRNF was added to the soil.
also contributed to the slow-release of nitrogen from SRNF. Consequently, the use of SRNF in the agricultural field could
Thus, the urea had a slower release rate, compared with the reduce water losses by infiltration.
untreated urea granules, from which 98.5 wt % of N was released 3.10. Water Retention Behavior of SRNF. For application in
within 12 h.51 soil, not only the water-holding capacity but also the water
3.9. Water-Holding Capacity of Soil with SRNF. The coating retention of the superabsorbent materials is of vital importance.
superabsorbent polymer can absorb a lot of water during rainfall Furthermore, the retention of water after absorption has to be as
and irrigation, which would be released slowly to the soil in dry high as possible. As Figure 10 presents, the dry soil mixed with
times. This is of great importance in drought-prone areas where SRNF absorbed initially more water than the soil without SRNF.
the availability of water is insufficient. Taking into account the The water retention capacity of the soil was 46%, 67%, 86%, and
1419 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

Figure 12. Scanning electron micrographs of the surface of (a) SRNF,

(b) freeze-dried kC-g-PAA/Celite superabsorbent, and (c) cross-section
of SRNF.

water retention capacity of SRNF. Figure 12c shows a cross-

section view of the double-coated structure of SRNF. In our case,
the inner coated kC
SA layer, which is homogeneous and sticks
Figure 11. Surface images of soil mixed with (a) 0, (b) 1, (c) 2, and solidly to the core urea, serves as a barrier for mass transfer,
(d) 3 wt % SRNF after 5 days at room temperature. thereby reduced the rate of water diffusion into the granules and
the migration of the urea outside the granules. This layer provides
113.5% for SRNF application rates of 0, 1, 2, and 3 wt %, the SRNF with good slow-release properties. The kC-g-PAA/
respectively. It can also be observed that the rate of water Celite superabsorbent is encapsulated onto the kC
SA coated
loss appeared to be identical for all samples investigated. After granule as the outer layer. It is also in close contact with the first
10 days, the soil without SRNF had nearly given off all water, but layer, although with a much rougher surface. This outer layer can
the soil with 1, 2, and 3 wt % SRNF still retained 22%, 40.5%, and not only absorb lots of water and preserve the soil moisture but
62% water. It was also noted that with the increase of SRNF also regulate the slow-release behavior of the SRNF.56
samples, the water retention capacity of the soil increased.
Compared with the control (soil without SRNF), the SRNF 4. CONCLUSIONS
effectively improved the water retention capacity, even though at A double-coated, slow-release and water-retention urea fertilizer
a low application rate. Therefore, if the SRNF was applied in was prepared and characterized, which possessed a three-layer
farmland, it would be like a subminiature reservoir to retain structure: the core was pure granular urea, the inner coating layer
and supply moisture to crops over time as the soil underwent was kC
SA bead, and the outer coating layer was cross-linked
alternate wet and dry periods. kC-g-PAA/Celite superabsorbent. The reaction variables that
3.11. Anticompaction Property of SRNF in Soil. The criteria affected the swelling capacity of the superabsorbents were studied
used to identify compaction in the field include increase in soil and optimized. The optimum conditions, under which the max-
strength, changes to soil structure, distribution of soil moisture, imum water absorbency (343 g/g) was achieved, were found as
etc.52 As Figure 11a shows, the soil used in the experiment follows: cross-linker content was 0.2 wt %, initiator content was
hardened and cracked after 5 days at room temperature. It is a 0.3 wt %, neutralization degree of AA was 30%, kC content was
sign of soil compaction, which was related to a reduction in the 4 wt %, and Celite content was 2.5 wt %. Elemental analysis showed
number and size of pores, a decrease in structural aggregation and that the nitrogen content of the product was 22.6%. The addition
integrity, and an increase in strength and bulk density.53 However, of SRNF into soil could significantly improve the water-holding
the soils mixed with SRNF (Figure 11b
d) still kept their moist capacity and water-retention properties of soil. Meanwhile, it can
and continuous configuration. At the same time, lots of granular prevent soil from becoming harder. Moreover, the product had
structures were formed after the addition of SRNF to the soil.54 It good slow-release properties: nutrient N had a release value of
had been reported in the literature55 that these granules would 94.2% after being incubated in the soil for 25 days. In addition, it is
help to form soil aggregates, improve soil aeration and perme- worth noting that the kC
SA hydrogel material is biodegradable
ability, and prevent soil from becoming harder. Seeds require and is expected to be applied in agriculture as a kind of coating
oxygen and water to germinate, and pore space between the material to alleviate the environmental pollution caused by con-
aggregates allows for storage and movement of air and water in ventional nondegradable superabsorbent polymer materials.
soil. Thus, a good environment for crops to grow would be created.
3.12. Morphology of SRNF. SEM images of the surface and ’ AUTHOR INFORMATION
cross-section of SRNF are presented in Figure 12. It can be seen
from Figure 12a that the surface of SRNF is rugged and there are Corresponding Author
many apertures on it, so water can be absorbed quickly by the *Telephone: +86 931 8912387. Fax: +86 931 8912582. E-mail:
fertilizer granules because of the high specific surface area, in mzliu@lzu.edu.cn.
addition to the hydrophilic groups on the polymer chains.
Therefore, when SRNF is dipped in water, it can form a swollen
hydrogel quickly. The freeze-dried swollen kC-g-PAA/Celite ’ ACKNOWLEDGMENT
superabsorbent (Figure 12b) displays large apertures and thin We gratefully acknowledge the financial support of the Special
walls, which are responsible for the high water absorbency and Doctorial Program Fund of the Ministry of Education of China
1420 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research ARTICLE

(Grant 20090211110004) and Gansu Province Project of (21) Snyder, C. S.; Bruulsema, T. W.; Jensen, T. L.; Fixen, P. E.
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