Published: December 21, 2011

r 2011 American Chemical Society 1413 dx.doi.org/10.1021/ie2020526 | Ind. Eng. Chem. Res. 2012, 51, 1413–1422

ARTICLE

pubs.acs.org/IECR

j-Carrageenan�Sodium Alginate Beads and Superabsorbent CoatedNitrogen Fertilizer with Slow-Release, Water-Retention, andAnticompaction PropertiesYanfang Wang, Mingzhu Liu,* Boli Ni, and Lihua Xie

State Key Laboratory of AppliedOrganic Chemistry, Key Laboratory of NonferrousMetal Chemistry and Resources Utilization of GansuProvince, and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China.

ABSTRACT: Amultifunctional slow-release nitrogen fertilizer (SRNF) has been developed to improve fertilizer use efficiency andreduce environmental pollution. k-Carrageenan�sodium alginate (kC�SA) and cross-linked kC-g-poly(acrylic acid)/Celitesuperabsorbent were used as inner and outer coating materials, which were coated consecutively on the granule core urea in a pangranulator. Elemental analysis result showed that the product contained 22.6% nitrogen. The water evaporation as well as thenitrogen release behavior of SRNF in soil was explored. In addition, the coated urea had a higher average crushing strength incomparison 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 beexpected to have wide potential applications in modern agriculture and horticulture.

1. INTRODUCTION

Modern agriculture now feeds 6000 million people,1 andfertilizer is a vital input material for the sustainable developmentof crop production and food security.2 Worldwide experiences inagricultural development have proved that rational fertilization isone of the most efficient and important methods to increase cropyields. Fertilizer consumption in China is also challenging theacceptance limit of resource and environment. From 1977 to2005, annual synthetic nitrogen (N) fertilizer application in-creased from 7.07 to 26.21 million tons (a 271% increase) inChina.3 However, compared with 45% in developed countries,the degree of utilization of N fertilizer in China is only in therange of 30�35%. Meanwhile, nitrate leaching and groundwatercontamination have been shown to be related to N fertilizer usein agriculture. Additionally, some recent studies have shown thatoveruse of N fertilizer has already induced serious environmentalhazards, including soil acidification, heavy metal contamination,and greenhouse gas emission.4 Among the N fertilizers, the mostwidely used one is urea, because of its high nitrogen content(46%) and comparatively low cost of production. After beingapplied to soil, it can be rapidly hydrolyzed to NH3 and CO2 bysoil urease, followed by NO3

� formation through nitrification.Therefore, ammonia loss and nitrate leaching are environmentalconcerns in regions where urea is applied.5When concerns aboutthe sustainability of input-intensive agriculture and the economic,ecological, and environmental effects of N fertilizer overuseare taken into consideration, it is clear that use of slow-releasefertilizers (SRFs) is one attempt to enhance the efficiency offertilizer to mitigate these problems.6 SRFs allow the activecomponent to slowly diffuse toward the soil, thus making itavailable in the field for a longer period of time.7 In case of urea,slow-release products are typically made by reacting urea withvarious aldehydes to reduce the solubility of the material.Another method of regulating N fertilizer release is the applica-tion of a coating. Polymer-coated SRFs look promising for

widespread use in agriculture because the science and technologyof polymers has undergone explosive growth during recent yearsand they can be designed to release nutrients in amore controlledmanner.8

The current trend of using environmentally friendly productshas resulted in an extensive exploitation of new materials obtainedfrom renewable resources. Preparation of polysaccharide-basedcomposites, blends, or mixtures represents a new category ofenvironmentally safe materials for newer applications, due totheir water solubility, renewability, nontoxicity, and biodegrad-ability.9,10 Sodium alginate (SA) is an anionic natural macro-molecule, which is composed of poly-β-1,4-D-mannuronic acidand α-1,4-L-guluronic acid in varying proportions by 1�4linkages. It can be extracted from marine algae or produced bybacteria.11 k-Carrageenan (kC) is a negatively charged poly-saccharide extracted from red seaweed that has a linear sulfatedbackbone of alternating α-1,4- and β-1,3-linked galactoseresidues.12 SA together with kC was used in the drug deliverysystems and special membranes.13�15 Their joint use as coatingmaterial in agriculture has not been reported. Most recently, weprepared kC�SA complex beads by dropping an aqueoussolution of kC and SA into calcium chloride (CaCl2) andpotassium chloride (KCl) mixed solution to form ionicallycross-linked hydrogel beads, which were used as a fully naturalcoating material. Mixing of kC with SA could improve itsmechanical properties and reduce brittleness of the hydrogel.16

In addition, there is a need to explore the utilization of abun-dantly available seaweed polysaccharides.

Besides fertilizer, water is another important factor that limitsthe production of agriculture. China is one of the world’s most

Received: September 8, 2011Accepted: December 21, 2011Revised: December 15, 2011

1414 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 amajor threat to long-term food security. Therefore, efficientmanagement of soil moisture is important for agriculturalproduction in response to scarce water resources. Superabsor-bents are slightly cross-linked hydrophilic polymers that canabsorb huge volumes of water without dissolving or losing theirthree-dimensional structures and can retain the absorbed watereven under certain pressure.17 Because of their excellent char-acteristics of water retention and absorbency, superabsorbentshad been used in agriculture and horticulture since their advent in1969.18,19 Recently, several studies20�22 have confirmed theimportance of superabsorbents in improving physical propertiesof the soil such as porosity, structure, and water-holding capacity.Johnson23 reported 171�402% increases in water retentioncapacity when superabsorbents were incorporated in coarse sand.Islam et al.4 reported that the application of superabsorbentscould conserve soil�water, making it available to plants forincreased biomass accumulation and reduced oxidative stress,especially under severe water stress. Similarly, plant height, leafarea, and number of grains as well as protein, soluble sugar, andstarch contents in the grain also increased with superabsorbenttreatment. Mikkelsen et al.24 found that addition of superabsor-bent to the fertilizer solutions reducedN leaching losses from soilcolumns by as much as 45% during the first 4 weeks in heavilyleached conditions compared with N fertilizer alone. Moreover,the use of superabsorbent materials as carrier and regulator ofnutrient release was helpful in reducing undesired fertilizer losseswhile sustaining vigorous plant growth.25

To satisfy these requirements, we prepared a double-coatedslow-release and water-retention nitrogen fertilizer. Its core isurea fertilizer granule, the first coating layer is kC�SA bead, andthe second coating layer is kC-g-poly(acrylic acid)/Celite super-absorbent. The choice of the coatings is largely dictated by theproblems to be addressed above. By doing so, first, superabsor-bents were used as the outer coating instead of blending orpolymerizing with fertilizers; this process reduced the loss offertilizer without altering the properties of water retention andabsorbency. Second, the kC�SA layer was incorporated into thefertilizer production to make the products cheaper and easier tobiodegrade.26�29 Meanwhile, hydrophilic groups of kC�SAmatrix were cross-linked completely by potassium and calciumions. This approach turned hydrophilic kC and SA into thehydrophobic kC�SA bead, which contributed to the slow-release behavior of fertilizer. In addition, the coated fertilizer isexpected to retard nitrogen release, improve soil moisture,reduce the use of water, and alleviate environmental hazardscaused by excessive fertilization. Therefore, the main purpose ofthis study was to determine the release characteristics of thecoated fertilizer and its effect on improving the water-holdingcapacity of soil. Research has been ongoing to prepare a multi-functional fertilizer that can be used in agro-industries.

2. EXPERIMENTAL SECTION

2.1. Materials. The source of nitrogen used was commercialpelleted urea, which was previously sieved to be between 2.0 and2.5 mm in diameter. Sodium alginate (SA; the viscosity of a 2%solution is 3200 mPa 3 s at 25 �C) was obtained from QingdaoHaiyang Chemical Co. (Qingdao, China). k-Carrageenan (kC)was purchased from the Golden Phoenix of k-Carrageenan Co.Ltd. (Tengzhou, China) and used without further purification.Acrylic acid (AA, chemical grade; Beijing Eastern Chemical

Works, Beijing, China) was distilled at reduced pressure beforeuse. N,N0-Methylenebis(acrylamide) (NNMBA) was recrystal-lized from 95% ethanol prior to use. Ammonium persulfate(APS) was recrystallized from distilled water before use. Celite,supplied by Shanghai Chemical Reagent Factory (Shanghai,China), was calcined at 450 �C, and particle sizes <10 mm wereabout 90 wt %. It contains 14�16 wt % calcium sulfate and0.02 wt % chloride, and the content of iron is <0.05 wt %. The pHof the solution (10 wt % diatomite) is 6.5�7.5. All other reagentsused were of analytical grade, and all solutions were preparedwith distilled water.2.2. Synthesis of jC-g-PAA/Celite Superabsorbent Com-

posites. A series of superabsorbent composites with differentcontents of kC, Celite, and cross-linker were prepared accordingto the following procedure. A certain amount of kC was firstdissolved in 10 mL of distilled water, which was under vigorousstirring in a four-necked flask equipped with a stirrer, a thermo-meter, a condenser, and a nitrogen line. The flask was immersedin a thermostated water bath preset to the desired tempera-ture (80 �C). After complete dissolution of kC to form ahomogeneous solution, certain amounts of preneutralized AA,NNMBA, and Celite powder were simultaneously added to thereaction mixture. Afterward, the solution was stirred and purgedwith nitrogen for 15 min to remove the dissolved oxygen. Then, adefinite amount of APS solution was added dropwise to thereaction flask under continuous stirring to generate free radicals.The water bath was heated slowly to 80 �C andmaintained at thistemperature for 3 h to complete polymerization. Finally, theresulting product was dried, milled, screened, and stored forfurther use. All samples used had a particle size in the range of40�90 mesh.2.3. Preparation of jC�SA Complex Beads. kC�SA com-

plex beads were prepared according to the following procedure.kC solution (1%, w/v) and SA solution (2%, w/v) were preparedby dissolving 4 g of kC and 8 g of SA into 400 mL of distilledwater in a 1000 mL three-necked flask equipped with a mecha-nical stirrer, a reflux condenser, and a thermometer. The flask wasthen placed in a water bath, which was heated slowly to 75 �C andmaintained at this temperature for 30 min to dissolve kC and SAcompletely. The polysaccharide solution was then dropped into400 mL of a stirred salt solution mixture of CaCl2 (3%, w/v) andKCl (3%, w/v). To complete gelation, the beads weremaintainedin the salt solution for 30 min and filtered, followed by washingwith distilled water, and then were allowed to dry at 35 �C.2.4. Preparation of Slow-Release Nitrogen Fertilizer. First,

an amount of urea granules (2�2.5 mm in diameter) was placedon a rotating pan in batches. Subsequently, kC�SA complexpowder (below 200 mesh) as the inner coating was adhered tothe fertilizer cores under water atomization. Then, the granulescoated with a thin layer of kC�SA powder were removed anddried at 30 �C. Multiple kC�SA coatings were prepared byrepeatedly atomizing the previously coated granules for adheringkC�SA powder. Thus, kC�SA powder-coated urea granuleswith different coating thicknesses were obtained. Finally, kC-g-PAA/Celite superabsorbent powder (below 200 mesh) as theouter coating was coated on the surface of the granules underrotating. The process was completed until a compact andhomogeneous coating formed on the fertilizer granules. Thecoated granules were dried in an oven at 30 �C to obtain the finalproducts.2.5. Characterizations. Fourier transform infrared (FTIR)

spectra of samples were recorded on a Nicolet Nexus 670 FTIR

1415 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 KBrpellets, with samples extracted in distilled water for 72 h at roomtemperature. The average diameter of coated fertilizers wasdetermined on a micrometer for 20 granules. The nitrogencontent of SRNF was determined by an elemental analysisinstrument (Germany Elemental Vario EL Corp., model 1106).Micrographs of samples were examined with scanning electronmicroscopy (SEM) (JSM-5600LV, JEOL, Ltd.). Before theSEM observation, all samples were fixed on aluminum stubsand coated with gold.2.6. Measurements of Equilibrium Water Absorbency of

the Superabsorbents. An accurately weighed quantity of thesuperabsorbent composite (0.1 g, 40�90 mesh) was immersedinto a certain amount of tap water at room temperature for60 min to reach the swelling equilibrium. The swollen sampleswere filtered and weighed. Water absorbency (WA) was calcu-lated from eq 1:

WA ¼ m2 �m1

m1ð1Þ

where m2 and m1 refer to the weights of swollen and driedsuperabsorbents, respectively. WA was calculated as grams ofwater per gram of sample. In all cases, three parallel samples wereused and the averages are reported in this paper.2.7. Determination of Soluble Fraction of Superabsor-

bent. The soluble fraction (sol) is the sum of all water-solublespecies including non-cross-linked oligomers and nonreactedstarting materials such as residual monomers. The sol content issimply determined by extraction of superabsorbent sample indistilled water. A certain amount of crude sample particles waspoured into an excess amount of distilled water and dispersedwith mild magnetic stirring at room temperature for 24 h. Thewater was refreshed every 8 h in order to remove the solublefraction. Afterward, the swollen sample was filtered and dewa-tered in excess anhydrous ethanol for 12 h. Finally, the samplewas oven-dried at 50 �C to a constant weight. The sol contentwas calculated as the weight loss of the initial crude sample.2.8. Saline Solution Absorbency of Superabsorbent under

Load. A hollow macroporous plexiglas cylinder with an internaldiameter of 2.9 cm was placed in a 100 mL beaker. An accuratelyweighed portion (0.1 g) of superabsorbent composite was spreaduniformly on the surface of the polyester gauze located on theplexiglas cylinder. A plastic cylinder (106 g) that could slip freelyin a glass cylinder was used to apply the load to the dry super-absorbent samples (P≈ 2060 Pa). Then 60 mL of 0.9 wt %NaClaqueous solution was added slowly into the beaker. The entiresetup was covered to prevent surface evaporation and prob-able change in the saline concentration. The superabsorbentswere then separated from unabsorbed saline solution andweighed at set intervals. This process was repeated until theweight of the superabsorbents remained constant. The salineabsorbency under load (AUL) at different time intervals wascalculated according to eq 2:

saline AUL ¼ m2 �m1 �m0

m0ð2Þ

where m0 and m1 denote the weights of dry superabsorbentsample and glass cylinder, and m2 is the weight of glass cylinderwith swollen superabsorbent sample.2.9. Determination of Average Crushing Strength for

SRNF. Crushing strength is a measure of the resistance ofgranules to deformation or fracture under pressure. High

crushing strength is better for avoiding breakage and pellet strainduring handing and bag storage.30 In this study, the crushingstrength was measured by applying pressure to individualgranules of a diameter range 2.7�3 mm. The magnitude ofpressure exerted on the fertilizer granule was continuouslyincreased until destruction of the granule was observed.This maximum load value (average from 20 measurements)was considered to be the crushing strength.31,32 For uncoatedurea granules, the average crushing strength was 1121g/granule, and for double-coated SRNF, the value was 1234g/granule, which was about 10% higher than the uncoatedurea. Therefore, this process would facilitate the transporta-tion and use practice and improve the durability and integrityof the SRNF.2.10. Slow-Release Behavior of Nitrogen from SRNF in

Soil. To study the slow-release behavior of nitrogen from SRNFin soil, 1 g of SRNF was buried in sealed nonwoven bags approx-imately 6 cm beneath the surface of the soil (below 26 mesh)in a glass beaker at ambient temperature. Throughout theexperiment, the soil moisture was kept at 20%. After 0.5, 1, 2,3, 5, 10, 15, and 25 days, the bags were retrieved and air-dried.Then the fertilizer granules were removed from the bags and thecontent of nitrogen was estimated.2.11. Measurement of Water-Holding Capacity of Soil

with SRNF. The study of the effect of SRNF on water-holdingcapacity of soil was carried out. Different amounts of SRNF werewell blended with 200 g of dry soil (below 26 mesh) and placedinto a poly(vinyl chloride) (PVC) tube of 4.5 cm in diameter.The bottom of the tube was sealed with nylon fabric (with anaperture of 0.076 mm) and weighed (marked m1). The soilsample was slowly drenched by tap water from the top of the tubeuntil the water seeped out from the bottom. The tube wasweighed (markedm2) again when there was no water seeping outat the bottom. A control experiment without SRNF was alsocarried out. Three fertilizer application rates (1, 2, and 3 wt %)were examined. The water-holding capacity (WH, %) of the soilwas calculated from eq 3:

WH ¼ ðm2 �m1Þ � 100200

ð3Þ

2.12.Measurement of theWater Retention of SRNF in Soil.Different amounts of SRNF were well mixed with 100 g of drysoil (below 26mesh) and placed in a glass beaker. An appropriateamount of tap water was added into the beaker to make the soilsaturated, and then the beakers were kept under ambienttemperature. The initial masses of the mixture of dry soil withdifferent amounts of SRNF in the beakers were measured(marked m0). The weights of the mixture of soil saturated withtap water were also recorded daily (marked mi) to compare thewater retention of SRNF. Meanwhile, a control experimentwithout SRNF was carried out. Three fertilizer application rates(1, 2, and 3 wt %) were examined. The water retention capacity(WR, %) of the soil was calculated from eq 4:

WR ¼ mi �m0

m0� 100 ð4Þ

3. RESULTS AND DISCUSSION

3.1. Fourier Transform Infrared Analysis. The FTIR spectraof kC, kC-g-PAA, kC-g-PAA/Celite, and Celite are shown in

1416 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

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 FTIRspectrum of kC-g-PAAwas compared with that of kC, new bandsat 1727 cm�1 (CdO stretching of�COOHgroups), 1561 cm�1

(asymmetrical stretching vibration of �COO� groups), and1409 cm�1 (symmetrical stretching vibration of�COO� groups)were observed in the spectrum of kC-g-PAA (Figure 1b).Besides, the characteristic absorption band of kC at 1252 cm�1

was observed in the spectrum of kC-g-PAA. In addition, thecharacteristic absorption bands of kC at 1070, 1123, and1159 cm�1 (stretching vibration of C�OH groups) wereobviously weakened after reaction and shifted to 1041, 1110,and 1171 cm�1 for kC-g-PAA. This result indicated that AAmonomers were grafted onto the kC backbone. The �COO�

band of kC-g-PAA at 1561 cm�1 shifted to 1567 cm�1 afterformation of kC-g-PAA/Celite composite (Figure 1c), whichimplied that the incorporation of Celite decreased the hydrogen-bonding interaction among polymer chains, in contrast to kC-g-PAA. As depicted in Figure 1d, the bands at 3435 and 793 cm�1

were due to the vibration of free silanol groups (SiO-H), and theband at 1629 cm�1 represented H�O�H bending vibration ofwater, which bonded to the surface hydroxyl groups of Celite viaH-bonds.34,35 When the FTIR spectrum of Celite was comparedwith that of kC-g-PAA/Celite, the band at 1629 cm�1 shifted to1636 cm�1 and the intensity was obviously weakened. Mean-while, the characteristic absorption band of Celite at 793 cm�1

disappeared after reaction. This information gave direct evidencethat Celite participated in the graft polymerization reactionthrough its active silanol groups.According to the information from FTIR spectra, it can be

concluded that a graft-copolymerization reaction takes placeamong kC, Celite, and AA and a network structure is formed.Scheme 1 shows the possible synthetic route of kC-g-PAA/Celite superabsorbent. The sulfate anion radicals produced fromthe thermal decomposition of APS abstract hydrogen fromthe hydroxyl groups of kC12 or Celite36,37 to form correspond-ing radicals on the substrates. The radicals in active centers onthe substrates initiate the polymerization reaction and give

PAA branches on the kC or Celite backbones and lead to agraft copolymer. Since a cross-linking agent (NNMBA) isintroduced into the system, the copolymer has a cross-linkedstructure. The Celite existing in the polymer matrix can beclassified as two kinds: chemically bonding and physicallyfilling.38

To obtain additional evidence of grafting, a similar polymer-ization was conducted in the absence of cross-linker.39 Theresulting product was precipated by pouring the reaction mixtureinto 100 mL of ethanol. Then 0.1 g of the dried product waspoured into 50 mL of dimethylformamide solution (a suitablesolvent for homopolymer).40 The mixture was stirred gently atroom temperature for 24 h. After complete removal of thehomopolymer, the kC-g-PAA/Celite superabsorbent compositewas filtered, washed with ethanol, and dried in an oven at 50 �Cto reach a constant weight, an appreciable amount of syntheticpolymer percentage of the graft polymer (89%) was concluded.FTIR spectra of the graft polymer were obtained; the result wasvery similar to that shown in Figure 1c and is not presented in thispaper. Also according to preliminary measurements, the solcontent of the superabsorbent was 18%. This fact practicallyproved that most of the monomers were involved in the polymernetwork.3.2. Effect of Cross-Linker Content on Water Absorbency.

One of the important properties of SRCF is the water absorbencydue to the coating of kC-g-PAA/Celite superabsorbent poly-mers. To improve the water absorbency capacity of the products,the reaction parameters were optimized. Water absorbency as afunction of NNMBA content was investigated for cross-linkedkC-g-PAA/Celite superabsorbents. As we can see from Figure 2,the water absorbency decreased with increasing cross-linkercontent from 0.2 to 0.5 wt %. This was attributed to the factthat when the cross-linker content was larger than 0.2 wt %, a

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

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

1417 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

further increase in the cross-linker content resulted in a highercross-linking density and decreased the space between polymerchains, and consequently, the resulting highly cross-linked rigidstructure cannot be expanded and hold a large quantity of water.However, when the cross-linker content was lower than 0.2 wt %,the three-dimensional network that was necessary for super-absorbent cannot be perfectly formed, leading to the presence ofnon-cross-linked PAA chains, which did not contribute to waterabsorption. These results were in accordance with Flory’s net-work theory.41

3.3. Effect of Initiator Content on Water Absorbency. Asthe mechanism of synthesis process of the superabsorbent wasfree-radical polymerization, the initiator content had remarkableinfluence on the whole reaction course and the ultimate waterabsorbency of the resultant superabsorbent. Figure 3 depicts thewater absorbency dependency on initiator content in tap water.Maximum water absorbency (261 g/g) was obtained at 0.3 wt %initiator content. Initiator content more or less than 0.3 wt %gave superabsorbent with decreased swelling capacity. Lowerinitiator content resulted in a low quantity of free radicals andhence an imperfectly formed network. But an excess of initiators

may cause a short average kinetic chain length and many morepolymer chain ends. Both cases lead to deterioration of absor-bency properties of the product. Also, large quantities of freeradicals may cause a transfer reaction to polymer chains, whichwould result in an increase in the cross-linked density and adecrease in the water absorbency consequently. Furthermore,the free radical degradation of kC backbones by sulfate radicalanions was an additional reason for swelling loss at higherinitiator content. The proposed mechanism for this possibilitywas reported in previous work.42 A similar oxidative degradationof chitosan chains by potassium persulfate was reported byHsu et al.43

3.4. Effect of Neutralization Degree of AA on WaterAbsorbency. Neutralization degree is defined as the molarpercentage of carboxyl groups in AA neutralized by sodiumhydroxide. Neutralization degree not only affects the polymer-ization rate but also determines the charge density of the three-dimensional network; therefore, it has a remarkable influence onwater absorbency. Figure 4 demonstrates that there is also amaximum of water absorbency dependent on the neutralizationdegree of AA. Water absorbency increased with increasingneutralization degree from 10% to 30% and decreased withfurther increase in the neutralization degree of AA. This behaviormay be explained as follows: within a certain range of neutraliza-tion degree, the carboxyl acid groups of AA turned into carbox-ylate groups, so the charge density of the network would increase,which resulted in the destruction of hydrogel bonds as well as anelectrostatic repulsion that tended to expand the network, andthen an increase in water absorbency. However, when theneutralization degree of AA was larger than 30%, the screeningeffect of sodium ions on the polyanion chains would reduce theelectrostatic repulsion of the network, which resulted in thedecrease of water absorbency of coating superabsorbent. Mean-while, the activity of acrylic acid was higher than that of acrylate,so the larger the neutralization degree was, the lower thepolymerization rate was.44 The low polymerization rate wouldresult in an increase in the content of the oligomers, andconsequently the water absorbency decreased.3.5. Effect of jC Content on Water Absorbency. The water

absorbency dependency on kC content is shown in Figure 5. Ascan be seen, the water absorbency increased with increasing kCcontent until it reached amaximumof 343 g/g atkCcontent 4wt%

Figure 3. Effect of initiator content on water absorbency of the super-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.

Figure 4. Effect of neutralization degree of AA on water absorbency ofthe superabsorbent in tapwater: AA, 4 g;NNMBA, 0.2wt%;APS, 0.3wt%;kC, 0.2 g; Celite, 0.1 g; reaction time, 3 h at 80 �C.

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

1418 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

and then decreased with higher content of kC. It is well-knownthat the hydroxyl groups of kC may react with the initiator andliberate free radicals, on which the polymerization will take placeand give PAA branches on the kC backbone.45 This reaction willmake the kC bond to the polymer chains and become the cross-linking point. Since the water absorbency of the superabsorbentswould display a maximum at a certain content of cross-linker, itwas understandable that the maximum of water absorbencyappeared at kC content 4 wt %. In addition, when the kCcontent was low, the AA monomers turned out to be a homo-polymer, which in turn resulted in an increase in soluble materialsat fixed cross-linking density. When the content of kC was largerthan 4 wt%, the viscosity of the reactionmixture increased, whichhindered the movement of the reactants. Thus, the grafting ratioand the molecular weight of the grafted PAA chains decreased,resulting in a decrease in water absorbency.3.6. Effect of Celite Content on Water Absorbency. As

shown in Figure 6, the water absorbency of the superabsorbentcomposite first increased with increasing Celite content andreached a maximum at 2.5 wt %. Celite is rigid and contains largeamounts of active Si�OH groups on its surface, which can take

part in the polymerization reaction as well as construction of the3D network. As a result, the intertwining of polymeric chains wasprevented and the H-bonding interactions between hydrophilicgroups (such as �COOH, �COO�, �OH, and �SO4

�) wereweakened. Thus the degree of physical cross-linking decreasedand the network voids for holding water regularly formed, andthe water absorbency of the superabsorbent was improved.However, the excessive addition of Celite induced a decreaseof water absorbency. According to a previous report, the func-tional group of the filler particle surface involved an esterificationreaction with the carboxylic groups of the acrylic chains in thesuperabsorbent composite.46 It implied that the filler particlestook action like a multifunctional cross-linker or cross-linkpoints, inherently driving the network toward a higher densityof cross-linking. In addition, the excessive Celite powder mayphysically stack in the network, which may plug up the networkvoids for holding water. On the other hand, high content ofCelite powder in superabsorbents relatively reduced the ratio ofhydrophilic groups per unit volume, leading to a loss of waterabsorbency. It should be mentioned that the water absorbency ofthe superabsorbent composite was still higher than the super-absorbent without Celite even when the content of Celitereached 5 wt %, which was extremely favorable to reduce theproduction cost.3.7. Saline Absorbency under Load Analysis. Saline AUL

can be taken as a simple measurable simulation of the realcircumstances of the applications of superabsorbents. In nearlyall academic literature, the swelling capacity values of super-absorbents are reported as load-free swelling data, measuredusually in distilled water. It is obvious that swelling conditions(i.e., distilled water and lack of pressure), and hence the resultingdata, are not real, because in all applications of superabsorbents(hygienic, agriculture, etc), the swelling particles must absorbaqueous solutions while they are under pressure.47,48 Meanwhile,during many years of working on superabsorbent materials, weused to empirically realize a linear relationship between theAUL values and the strength of the swollen superabsorbents.So, AUL can be considered as a measure of the strength of thesuperabsorbents.49,50 To determine the saline AUL of the super-absorbents, we used optimized final products that absorbedsaline solution under load (P≈ 2060 Pa). As shown in Figure 7,the minimum time needed for the highest saline AUL of thekC-g-PAA/Celite superabsorbent was estimated to be 240 min.

Figure 6. Effect of Celite content on water absorbency of the super-absorbent in tap water: AA, 4 g; NNMBA, 0.2 wt %; APS, 0.3 wt %;neutralization degree of AA, 30%; kC, 0.2 g; reaction time, 3 h at 80 �C.

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-PAA.Figure 5. Effect of kC content on water absorbency of the super-

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 hat 80 �C.

1419 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

After this time, the saline AUL values were almost unchanged. Inaddition, a control experiment without Celite was also carried outwith other conditions keep constant. At the applied pressure,maximum swelling was found to be 39 and 32 g/g for thesuperabsorbents with and without Celite, which showed thatthe introduction of Celite could obviously improve the salineAUL, meaning improved strength of the superabsorbents.3.8. Nitrogen Release Behavior of Coated SRNF in Soil.

One of themost important characteristics of coated SRNFwas itsslow-release property. Plots of the released percentage of nitro-gen against time (days) are shown in Figure 8 for coated SRNFfertilizer in soil. As can be seen, the nitrogen in SRNF released39%, 72%, and 94% on the second, fifth, and 25th days,respectively. The rapid release rate in the early stage (in the first5 days) could be mainly ascribed to the dissolution of urea in thefertilizer cores. The coated superabsorbent polymers wouldabsorb the water and swell slowly after being added into soil,which would contribute to an increase in the pore size of thethree-dimensional network and benefit the diffusion of thefertilizer solution into the hydrogel network. There existed adynamic exchange between the free water in the hydrogel andthat in soil, and then the urea would diffuse out of the kC�SAlayer and enter into the kC-g-PAA/Celite layer, and then releaseinto the soil through the grids with dynamic exchange. Therelease rate after 5 days became slower. This was mainly becausethe concentration of urea in the cores decreased with time, whichcaused the decline of osmotic pressure inside and outside thefertilizer granules. Meanwhile, more ions and soil particles wereabsorbed by the coating superabsorbents. These ions and soilparticles played a role of physical barrier, together with the innerand outer coating layers. Additionally, due to the existenceof many kinds of ions in soil solution, the swelling degree ofthe coating superabsorbents was less in soil than in tap water, sothe diffusion of soluble urea in it would be difficult, whichalso contributed to the slow-release of nitrogen from SRNF.Thus, the urea had a slower release rate, compared with theuntreated urea granules, from which 98.5 wt % of N was releasedwithin 12 h.51

3.9.Water-Holding Capacity of Soil with SRNF.The coatingsuperabsorbent polymer can absorb a lot of water during rainfalland irrigation, which would be released slowly to the soil in drytimes. This is of great importance in drought-prone areas wherethe availability of water is insufficient. Taking into account the

interesting particular characteristic of the water absorptioncapacity of SRNF, we studied its effect on water-holding capacityof soil. As shown in Figure 9, the water-holding capacity of soilwas 39%, 46.5%, 53.5%, and 59% for SRCF application rates of 0,1, 2, and 3 wt %, respectively. It was noted that, with the increaseof SRCF samples, the superabsorbent contents increased andthus the water-holding capacity of soil increased. In soil, eachSRCF granule was surrounded by soil particles and subjected to aconfining pressure by these particles. Therefore, the swellingdegree of the superabsorbent in soil was limited compared withthat in tap water. However, compared with the control (soilwithout SRNF), the SRNF effectively improved the water-holding capacity of soil, even though at a low application rate.Moreover, it was also observed that the water flow rate throughthe soil was slowed down when SRNF was added to the soil.Consequently, the use of SRNF in the agricultural field couldreduce water losses by infiltration.3.10. Water Retention Behavior of SRNF. For application in

soil, not only the water-holding capacity but also the waterretention of the superabsorbent materials is of vital importance.Furthermore, the retention of water after absorption has to be ashigh as possible. As Figure 10 presents, the dry soil mixed withSRNF absorbed initially more water than the soil without SRNF.The water retention capacity of the soil was 46%, 67%, 86%, and

Figure 8. Slow-release behavior of nitrogen from SRNF in soil.Figure 9. Water-holding capacity of the soil mixed with 0, 1, 2, and3 wt % SRNF.

Figure 10. Water retention behavior of the soil mixed with 0, 1, 2, and3 wt % SRNF.

1420 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

113.5% for SRNF application rates of 0, 1, 2, and 3 wt %,respectively. It can also be observed that the rate of waterloss appeared to be identical for all samples investigated. After10 days, the soil without SRNF had nearly given off all water, butthe soil with 1, 2, and 3 wt % SRNF still retained 22%, 40.5%, and62% water. It was also noted that with the increase of SRNFsamples, the water retention capacity of the soil increased.Compared with the control (soil without SRNF), the SRNFeffectively improved the water retention capacity, even though ata low application rate. Therefore, if the SRNF was applied infarmland, it would be like a subminiature reservoir to retainand supply moisture to crops over time as the soil underwentalternate wet and dry periods.3.11. Anticompaction Property of SRNF in Soil.The criteria

used to identify compaction in the field include increase in soilstrength, changes to soil structure, distribution of soil moisture,etc.52 As Figure 11a shows, the soil used in the experimenthardened and cracked after 5 days at room temperature. It is asign of soil compaction, which was related to a reduction in thenumber and size of pores, a decrease in structural aggregation andintegrity, and an increase in strength and bulk density.53 However,the soils mixed with SRNF (Figure 11b�d) still kept their moistand continuous configuration. At the same time, lots of granularstructures were formed after the addition of SRNF to the soil.54 Ithad been reported in the literature55 that these granules wouldhelp to form soil aggregates, improve soil aeration and perme-ability, and prevent soil from becoming harder. Seeds requireoxygen and water to germinate, and pore space between theaggregates allows for storage and movement of air and water insoil. Thus, a good environment for crops to growwould be created.3.12. Morphology of SRNF. SEM images of the surface and

cross-section of SRNF are presented in Figure 12. It can be seenfrom Figure 12a that the surface of SRNF is rugged and there aremany apertures on it, so water can be absorbed quickly by thefertilizer granules because of the high specific surface area, inaddition to the hydrophilic groups on the polymer chains.Therefore, when SRNF is dipped in water, it can form a swollenhydrogel quickly. The freeze-dried swollen kC-g-PAA/Celitesuperabsorbent (Figure 12b) displays large apertures and thinwalls, which are responsible for the high water absorbency and

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 stickssolidly to the core urea, serves as a barrier for mass transfer,thereby reduced the rate of water diffusion into the granules andthemigration of the urea outside the granules. This layer providesthe SRNF with good slow-release properties. The kC-g-PAA/Celite superabsorbent is encapsulated onto the kC�SA coatedgranule as the outer layer. It is also in close contact with the firstlayer, although with a much rougher surface. This outer layer cannot only absorb lots of water and preserve the soil moisture butalso regulate the slow-release behavior of the SRNF.56

4. CONCLUSIONS

Adouble-coated, slow-release andwater-retention urea fertilizerwas prepared and characterized, which possessed a three-layerstructure: the core was pure granular urea, the inner coating layerwas kC�SA bead, and the outer coating layer was cross-linkedkC-g-PAA/Celite superabsorbent. The reaction variables thataffected the swelling capacity of the superabsorbents were studiedand optimized. The optimum conditions, under which the max-imum water absorbency (343 g/g) was achieved, were found asfollows: cross-linker content was 0.2 wt %, initiator content was0.3 wt %, neutralization degree of AA was 30%, kC content was4 wt %, and Celite content was 2.5 wt %. Elemental analysis showedthat the nitrogen content of the product was 22.6%. The additionof SRNF into soil could significantly improve the water-holdingcapacity and water-retention properties of soil. Meanwhile, it canprevent soil from becoming harder. Moreover, the product hadgood slow-release properties: nutrient N had a release value of94.2% after being incubated in the soil for 25 days. In addition, it isworth noting that the kC�SA hydrogel material is biodegradableand is expected to be applied in agriculture as a kind of coatingmaterial to alleviate the environmental pollution caused by con-ventional nondegradable superabsorbent polymer materials.

’AUTHOR INFORMATION

Corresponding Author*Telephone: +86 931 8912387. Fax: +86 931 8912582. E-mail:mzliu@lzu.edu.cn.

’ACKNOWLEDGMENT

We gratefully acknowledge the financial support of the SpecialDoctorial Program Fund of the Ministry of Education of China

Figure 11. Surface images of soil mixed with (a) 0, (b) 1, (c) 2, and(d) 3 wt % SRNF after 5 days at room temperature.

Figure 12. Scanning electron micrographs of the surface of (a) SRNF,(b) freeze-dried kC-g-PAA/Celite superabsorbent, and (c) cross-sectionof SRNF.

1421 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 ofScience and Technologies (Grant 0804WCGA130).

’REFERENCES

(1) Tilman, D.; Cassman, K. G.; Matson, P. A.; Naylor, R.; Polasky, S.Agricultural sustainability and intensive production practices. Nature2002, 418, 671.(2) Yan, X.; Jin, J. Y.; He, P.; Liang, M. Z. Recent advances on the

technologies to increase fertilizer use efficiency. Agric. Sci. Chin. 2008,7, 469.(3) Ni, B. L.; Liu, M. Z.; L€u, S. Y.; Xie, L. H.; Wang, Y. F. Environ-

mentally friendly slow-release nitrogen fertilizer. J. Agric. Food. Chem.2011, 59, 10169.(4) Islam, M. R.; Mao, S.; Xue, X.; Eneji, A. E.; Zhao, X.; Hu, Y.

A lysimeter study of nitrate leaching, optimum fertilisation rate andgrowth responses of corn (Zea mays L.) following soil amendment withwater-saving super-absorbent polymer. J. Sci. Food Agric. 2011, 91, 1990.(5) Zhao, G. Z.; Liu, Y. Q.; Tian, Y.; Sun, Y. Y.; Cao, Y. Preparation

and properties of macromelecular slow-release fertilizer containingnitrogen, phosphorus and potassium. J. Polym. Res. 2010, 17, 119.(6) Wu, L.; Liu, M. Z. Preparation and properties of chitosan-coated

NPK compound fertilizer with controlled-release and water-retention.Carbohydr. Polym. 2008, 72, 240.(7) Chen, L.; Xie, Z. G.; Zhuang, X. L.; Chen, X. S.; Jing, X. B.

Controlled release of urea encapsulated by starch-g-poly(L-lactide).Carbohydr. Polym. 2008, 72, 342.(8) Mulder, W. J.; Gosselink, R. J. A.; Vingerhoeds, M. H.; Harmsen,

P. F. H.; Eastham, D. Lignin based controlled release coatings. Ind. CropsProd. 2011, 34, 915.(9) Meena, R.; Lehnen, R.; Schmitt, U.; Saake, B. Effect of oat spelt

and beech xylan on the gelling properties of kappa-carrageenan hydro-gels. Carbohydr. Polym. 2011, 85, 529.(10) Kuang, J.; Yuk, K. Y.; Huh, K. M. Polysaccharide-based super-

porous hydrogels with fast swelling and superabsorbent properties.Carbohydr. Polym. 2011, 83, 284.(11) Xu, J. B.; Bartley, J. P.; Johnson, R. A. Preparation and

characterization of alginate-carrageenan hydrogel films crosslinked usinga water-soluble carbodiimide (WSC). J. Membr. Sci. 2003, 218, 131.(12) Pourjavadi, A.; Hosseinzadeh, H.; Mazidi, R. Modified carra-

geenan. 4. Synthesis and swelling behavior of crosslinked kC-g-AMPSsuperabsorbent hydrogel with antisalt and pH-responsiveness proper-ties. J. Appl. Polym. Sci. 2005, 98, 255.(13) Chi, M.; Lyu, R.; Lin, L.; Huang, H. Characterization of bacillus

kaustophilus leucine aminopeptidase immobilized in Ca-alginate/k-carrageenan beads. Biochem. Eng. J. 2008, 39, 376.(14) Mohamadnia, Z.; Jamshidi, A.; Mobedi, H.; Ahmadi, E.;

Mohammad, J. Z. M. Full natural hydrogel beads for controlled releaseof acetate and disodium phosphate derivatives of betamethasone. Iran.Polym. J. 2007, 16, 711.(15) Mammarella, E. J.; Rubiolo, A. C. Study of the deactivation of

[beta]-galactosidase entrapped in alginate-carrageenan gels. J. Mol.Catal. B: Enzym. 2005, 34, 7.(16) Mohamadnia, Z.; Zohuriaan-Mehr, M. J.; Kabiri, K.; Jamshidi,

A.; Mobedi, H. Ionically cross-linked carrageenan-alginate hydrogelbeads. J. Biomater. Sci., Polym. Ed. 2008, 19, 47.(17) Kabiri, K.; Omidian, H.; Zohuriaan-Mehr, M. J.; Doroudiani, S.

Superabsorbent hydrogel composites and nanocomposites: A review.Polym. Compos. 2011, 32, 277.(18) Chan, A. W.; Whitney, R. A.; Neufeld, R. J. Semisynthesis of a

controlled stimuli-responsive alginate hydrogel. Biomacromolecules2009, 10, 609.(19) Lu, S. J.; Duan, M. L.; Lin, S. B. Synthesis of superabsorbent

starch-graft-poly(potassium acrylate-co-acrylamide) and its properties.J. Appl. Polym. Sci. 2003, 88, 1536.(20) Burke, D. R.; Akay, G.; Bilsborrow, P. E. Development of novel

polymeric materials for agroprocess intensification. J. Appl. Polym. Sci.2010, 118, 3292.

(21) Snyder, C. S.; Bruulsema, T. W.; Jensen, T. L.; Fixen, P. E.Review of greenhouse gas emissions from crop production systems andfertilizer management effects. Agric. Ecosys. Environ. 2009, 133, 247.

(22) Andry, H.; Yamamoto, T.; Irie, T.; Moritani, S.; Inoue, M.;Fujiyama, H. Water retention, hydraulic conductivity of hydrophilicpolymers in sandy soil as affected by temperature and water quality.J. Hydrol. 2009, 373, 177.

(23) Johnson, M. S. The effect of gel-forming polyacrylamides onmoisture storage in sandy soils. J. Sci. Food Agric. 1984, 35, 1196.

(24) Mikkelsen, R. L.; Behel, A. D.; Williams, H. M. Addition of gel-forming hydrophilic polymers to nitrogen fertilizer solutions. FertilizerRes. 1993, 36, 55.

(25) Islam, M. R.; Xue, X.; Mao, S.; Ren, C.; Eneji, A. E.; Hu, Y.Effects of water-saving superabsorbent polymer on antioxidant enzymeactivities and lipid peroxidation in oat (Avena sativa L.) under droughtstress. J. Sci. Food Agric. 2011, 91, 680.

(26) Cassidy, M. B.; Leung, K. T.; Lee, H.; Trevors, J. T. Survival oflac-lux marked Pseudomonas aeruginosa UG2Lr cells encapsulated ink-carrageenan and alginate. J. Microbiol. Methods. 1995, 23, 281.

(27) Autio, K.; Vesterinen, E.; Stolt, M. Rheological properties ofmixed starch-k-Carrageenan gels in relation to enzymatic digestibility.Food hydrocolloids. 2002, 16, 169.

(28) Pourjavadi, A.; Zeidabadi, F.; Barzegar, Sh. Alginate-basedbiodegradable superabsorbents as candidates for diclofenac sodiumdelivery systems. J. Appl. Polym. Sci. 2010, 118, 2015.

(29) Pourjavadi, A.; Barzegar, Sh.; Zeidabadi, F. Synthesis andproperties of biodegradable hydrogels of k-carrageenan grafted acrylicacid-co-2-acrylamido-2-methylpropanesulfonic acid as candidates fordrug delivery systems. React. Funct. Polym. 2007, 67, 644.

(30) Salman, O. A. Polyethylene-coated urea. 1. Improved storageand handling properties. Ind. Eng. Chem. Res. 1989, 28, 630.

(31) Tomaszewska, M.; Jarosiewicz, A. Use of polysulfone in con-trolled-release NPK fertilizer formulations. J. Agric. Food. Chem. 2002,50, 4634.

(32) Timmons, R. J.; Ohio,M. U.S. Patent 4,636,242, January 13, 1987.(33) Pourjavadi, A.; Sadeghi, M.; Hosseinzadeh, H. Modified carra-

geenan. 5. Preparation, swelling behavior, salt- and pH-sensitivity ofpartially hydrolyzed crosslinked carrageenan-graft-polymethacrylamidesuperabsorbent hydrogel. Polym. Adv. Technol. 2004, 15, 645.

(34) Qi, X. H.; Liu, M. Z.; Chen, Z. B.; Liang, R. Preparation andproperties of diatomite composite superabsorbent. Polym. Adv. Technol.2007, 18, 184.

(35) Khraisheh, M. A. M.; Al-Ghouti, M. A.; Allen, S. J.; Ahmad,M. N. Effect of OH and silanol groups in the removal of dyes fromaqueous solution using diatomite. Water Res. 2005, 39, 922.

(36) Pourjavadi, A.; Soleyman, R.; Ghasemzadeh, H.; Salimi, H.CMC/Celite superabsorbent composites: effect of reaction variables onsaline-absorbency under load. Iran. Polym. J. 2010, 19, 571.

(37) Xie, L. H.; Liu, M. Z.; Ni, B. L.; Zhang, X.; Wang, Y. F. Slow-releasenitrogen and boron fertilizer from a functional superabsorbent formulationbased on wheat straw and attapulgite. Chem. Eng. J. 2011, 167, 342.

(38) Wang, W. B.; Xu, J. X.; Wang, A. Q. A pH-, salt- and solvent-responsive carboxymethylcellulose-g-poly(sodium acrylate)/medicalstone superabsorbent composite with enhanced swelling and responsiveproperties. Express Polym. Lett 2011, 5, 385.

(39) Pourjavadi, A.; Harzandi, A. M.; Hosseinzadeh, H. Modifiedcarrageenan 3. Synthesis of a novel polysaccharide-based superabsor-bent hydrogel via graft copolymerization of acrylic acid onto kappa-carrageenan in air. Eur. Polym. J. 2004, 40, 1363.

(40) Pourjavadi, A.; Harzandi, A. M.; Hosseinzadeh, H. Modifiedcarrageenan. 6. Crosslinked graft copolymer of methacrylic acid andkappa-carrageenan as a novel superabsorbent hydrogel with low salt- andhigh pH-sensitivity. Macromol. Res. 2005, 13, 483.

(41) Flory, P. J. Principles of polymer chemistry; Cornell UniversityPress: Ithaca, NY, 1953.

(42) Hosseinzadeh, H.; Pourjavadi, A.; Mahdavinia, G. R.; Zohuriaan-Mehr, M. J. Modified carrageenan. 1. H-carragPAM, a novel biopolymer-based superabsorbent hydrogel. J. Bioact. Compat. Polym. 2005, 20, 475.

1422 dx.doi.org/10.1021/ie2020526 |Ind. Eng. Chem. Res. 2012, 51, 1413–1422

Industrial & Engineering Chemistry Research ARTICLE

(43) Hsu, S.; Don, T.; Chiu, W. Free radical degradation of chitosanwith potassium persulfate. Polym. Degrad. Stabil. 2002, 75, 73.(44) Jin, S. P.; Yue, G. R.; Feng, L.; Han, Y. Q.; Yu, X. H.;

Zhang, Z. H. Preparation and properties of a coated slow-release andwater-retention biuret phosphoramide fertilizer with superabsorbent.J. Agric. Food. Chem. 2010, 59, 322.(45) Pourjavadi, A.; Amini-Fazl, M. S. Optimized synthesis of

carrageenan-graft-poly(sodium acrylate) superabsorbent hydrogel usingthe Taguchi method and investigation of its metal ion absorption. Polym.Int. 2007, 56, 283.(46) Pourjavadi, A.; Ayyari, M.; Amini-Fazl, M. S. Taguchi optimized

synthesis of collagen-g-poly(acrylic acid)/kaolin composite superabsor-bent hydrogel. Eur. Polym. J. 2008, 44, 1209.(47) Qi, X. H.; Liu, M. Z.; Zhang, F.; Chen, Z. B. Synthesis and

properties of poly(sodium acrylate-co-2-acryloylamino-2-methyl-1-pro-panesulfonic acid)/attapulgite as a salt-resistant superabsorbent com-posite. Polym. Eng. Sci. 2009, 49, 182.(48) Ramazani-Harandi,M. J.; Zohuriaan-Mehr,M. J.; Yousefi, A. A.;

Ershad-Langroudi, A.; Kabiri, K. Rheological determination of theswollen gel strength of superabsorbent polymer hydrogels. Polym. Test.2006, 25, 470.(49) Ramazani-Harandi,M. J.; Zohuriaan-Mehr,M. J.; Yousefi, A. A.;

Ershad-Langroudi, A.; Kabiri, K. Effects of structural variables on AULand rheological behavior of SAP gels. J. Appl. Polym. Sci. 2009, 113, 3676.(50) Pourjavadi, A.; Ghasemzadeh, H.; Mojahedi, F. Swelling prop-

erties of CMC-g-poly (AAm-co-AMPS) superabsorbent hydrogel.J. Appl. Polym. Sci. 2009, 113, 3442.(51) Liang, R.; Liu, M. Z. Preparation and properties of a double-

coated slow-release and water-retention urea fertilizer. J. Agric. Food.Chem. 2006, 54, 1392.(52) Batey, T.; Mckenzie, D. C. Soil compaction: identification

directly in the field. Soil Use Manage. 2006, 22, 123.(53) Batey, T. Soil compaction and soil management: a review. Soil

Use Manage. 2009, 25, 335.(54) Chen, P.; Zhang, W. A.; Luo, W.; Fang, Y. E. Synthesis of

superabsorbent polymers by irradiation and their applications in agri-culture. J. Appl. Polym. Sci. 2004, 93, 1748.(55) Zhan, F. L.; Liu, M. Z.; Guo, M. Y.; Wu, L. Preparation of

superabsorbent polymer with slow-release phosphate fertilizer. J. Appl.Polym. Sci. 2004, 92, 3417.(56) Tao, S. M.; Liu, J.; Jin, K. M.; Qiu, X. Y.; Zhang, Y.; Ren, X. Q.;

Hu, S. W. Preparation and characterization of triple polymer-coatedcontrolled-release urea with water-retention property and enhanceddurability. J. Appl. Polym. Sci. 2011, 120, 2103.