Preparation and Properties of a Novel Semi-IPN Slow-Release

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Preparation and properties of a novel semi-IPNs slowrelease fertilizer with the function of water retention Yang Xiang, Xudong Ru, Jinguo Shi, Jiang Song, Haidong Zhao, Yaqing Liu, Dongdong Guo, and Xin Lu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03827 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Journal of Agricultural and Food Chemistry

1

Preparation and properties of a novel semi-IPNs slow-release

2

fertilizer with the function of water retention

3

Yang Xiang, Xudong Ru, Jinguo Shi, Jiang Song, Haidong Zhao, Yaqing Liu,*

4

Dongdong Guo, and Xin Lu

5

Research Center for Engineering Technology of Polymeric Composites of Shanxi

6

Province, School of Materials Science and Engineering, North University of China,

7

Taiyuan 030051, China

1

ACS Paragon Plus Environment


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Abstract

9

A new semi-interpenetrating polymer networks (semi-IPNs) slow-release fertilizer

10

(SISRF) with water absorbency, based on kaolin-g-poly (acrylic acid-co-acrylic amide)

11

(kaolin-g-P(AA-co-AM)) network and linear urea-formaldehyde oligomers (UF), was

12

prepared by solution polymerization. Nutrients phosphorus and potassium were

13

supplied by adding dipotassium hydrogen phosphate during the preparation process.

14

The structure and properties of SISRF were characterized by various characterization

15

methods. SISRF showed excellent water absorbency of 68 g/g in tap water. The

16

slow-release behavior of nutrients and water-retention capacity of SISRF were also

17

measured.

18

pseudo-second-order kinetic model. Results suggested the formation of SISRF with

19

simultaneously good slow-release and water-retention capacity, which was expected

20

to apply in modern agriculture and horticulture.

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Keywords: semi-IPNs, slow-release fertilizer, water retention, kaolin-g-poly (acrylic

22

acid-co-acrylic amide), urea-formaldehyde

Meanwhile,

the

swelling

kinetics

was

2


well

described

by

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INTRODUCTION

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It’s well-known that the crops need fertilizer and water to grow up. Taking perfect

25

utilization of fertilizer nutrients and water sources is of great significance to increase

26

food production. However, about 40-70% of nitrogen, 80-90% of phosphorous and

27

50-70% of potassium contained in common compound fertilizers cannot be absorbed

28

by plants. The main reason is that these fertilizers possess high water solubility.1 After

29

applying to the soils, a large percentage of nutrients cannot be utilized by plants and

30

then lose to the surrounding environment, resulting in serious waste and

31

environmental pollution.2

32

Use of slow-release fertilizers is an effective method, which can reduce nutrients

33

loss and environmental hazards.3 Slow-release fertilizers have various advantages

34

over the common types, including the controlled release rate of nutrients, prolonged

35

period of fertilizer, improved fertilizer using efficiency, reduced environment

36

problems caused by nutrients loss, improved economic efficiency of fertilization, and

37

guaranteed quality of agriculture products.4 Coated fertilizer, as a main kind of

38

slow-release fertilizer, slows down the dissolution rates of nutrients by coating various

39

materials on the surface of conventional fertilizers.5 Although coated fertilizer

40

possesses the capability of sustained-release, it also present several problems as

41

shown in following: (i) The coating process is a multistep preparation

42

procedures,leading to the increase of production cost; (ii) The nutrient content of

43

coated fertilizer is limited due to the presence of coating material; (iii) The coating

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material may pollute the soil when being left and unabsorbed by plants.6,7 Using

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urea-formaldehyde is an alternative way to solve these problems. The nutrient release

46

characteristics of urea-formaldehyde is controlled by the microbial degradation.8

47

Superabsorbent

is

a

moderate

crosslinked

hydrophilic

3


material

with


48

three-dimensional polymer network structures, which has strong ability of water

49

uptake and been widely using in agriculture and horticulture.9-13 However, there are

50

still some problems on the large-scale application of superabsorbent in agriculture.

51

The most important problem is that majority superabsorbents are made of pure

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poly(sodium acrylate), resulting in high cost and poor salt resistance in the

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saline-alkali soil.14 Clay-based superabsorbent composites show low cost and

54

enhanced performance of swelling, gel strength and salt tolerance.15 Kaolin with one

55

Si-O tetrahedral layer and one Al-O octahedral layer structure is a desirable material

56

of construction for the clay-based superabsorbent composite system. Furthermore,

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superfine kaolin powder can react with a super absorbent resin and then form

58

networks due to the abundant hydroxyl groups (-Si(Al)-OH) and active sites (the

59

exchangeable cation, permanent and changeable charge) on the surface of kaolin.16

60

Thus, graft copolymerization of vinyl monomers onto kaolin can obtain desired

61

properties and broaden the field of potential application of kaolin.

62

In order to further increase the gel strength of superabsorbent after absorbing water,

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semi-IPNs technology has been using to prepare the superabsorbent with water

64

retention and slow-release capacity.17,18 However, in essence, nutrients and

65

water-absorbing components are completely isolated two phases in the semi-IPNs

66

structure reported so far. In these literatures, nutrients are only embedded in the

67

semi-IPNs structure as the small molecules, thus the slow-release properties aren’t

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obviously improved. Whereas, UF, as a long chain molecule, can entangle

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superabsorbent’s network to form the semi-IPNs structure, which not only improves

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the gel strength of water absorbent resin, but also contributes to excellent slow-release

71

property.

72

A novel semi-IPNs slow-release fertilizer (SISRF), in which all components were 4


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integrated together, was presented. The new preparation process based on graft

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copolymerization and semi-IPNs technology was developed, in which the

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methylolurea and phosphate were added directly into the mixture during the

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preparation of the SISRF. SISRF did not only show better slow-release and

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water-retention properties, but also showed lower production costs and better

78

biodegradability comparing to the conventional superabsorbent composite.

79

MATERIALS AND METHODS

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Materials. Acrylic acid (AA), acrylamide (AM), potassium hydroxide (KOH),

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dipotassium hydrogen phosphate (K2HPO4), formaldehyde, urea, ammonium

82

persulfate (APS), and kaolin were all applied by Damao factory, Tianjin, China. All

83

chemicals were of analytical grade and used directly without further purification. A

84

homemade meshed board (Hole diameter: 6mm) was used as the granulating tool of

85

SISRF. Distilled water was utilized in the preparation of SISRF.

86

Preparation of methylolurea. 4.0g of formaldehyde, 2.5g of H2O and 6.0g of urea

87

was added to a 100mL round bottom flask and stirred constantly. After dissolution of

88

the urea, the solution pH was adjusted carefully to 8.0 with 5% KOH solution in a

89

water bath at 40 °C for 2 h.

90

Preparation of SISRF. 5.0 g AA and 2.0 g AM were firstly dissolved in 20 mL

91

distilled water and then neutralized with KOH (neutralization value, 80%) in a beaker.

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0.7 g kaolin powder was suspended in the above solution. 0.021 g APS, 0.5g K2HPO4

93

and 6 g methylolurea solution were then added into the beaker in sequence and the

94

mixture was stirred in an ice water bath for 20 min. The mixture solution was moved

95

to a single neck flask equipped with a magnetic stirrer and a nitrogen line, and then

96

the flask was placed in water bath kept at 60 °C. Under nitrogen atmosphere, the

97

viscous product with white color was obtained after 4 h reaction. 5



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The white viscous product was pressed into a meshed board. Afterwards the

99

meshed board filled with the resulting product was dried in an oven at 60 °C, and

100

finally the cylindrical white particles SISRF was obtained.

101

For the sake of contrastive analysis, UF was synthesized by polycondensation

102

between methylolurea and urea in acid medium. Instead of removing the raw material

103

methylolurea, the preparation process of kaolin-g-P(AA-co-AM) was similar to the

104

SISRF.

105

Characterization.

Fourier

transform

infrared

(FTIR)

spectra

of

UF,

106

kaolin-g-P(AA-co-AM) and SISRF obtained under the optimum conditions were

107

characterized at room temperature by Nicolet IS50 FTIR spectrometer with an ATR

108

attachment. A diamond crystal plate was employed as a reflector. The absorbance

109

measurements were carried out in the range of 500-4000 cm-1.

110

The Tg values of the samples were measured with a differential scanning

111

calorimeter (TA DSC Q200). The dried samples were heated from 30 to 250 °C at a

112

rate of 5 °C /min under a nitrogen atmosphere. The onset of the abrupt decrease in the

113

heat flow was taken as Tg.

114

A scanning electron microscope (SEM) (Hitachi U8010, Japan) was used to analyze

115

the morphology of the samples. The samples were swollen to equilibrium in tap water

116

at room temperature for 24 h, then frozen in liquid nitrogen and snapped immediately,

117

and freeze-dried. Before the test, the fracture surface of samples was sputter-coated

118

using an Au-Pd target (MSP1S, SHINKKU VD).

119

Measurement of water absorbency of SISRF. Water absorbency of SISRF was

120

determined according to ref 19. 1 g of SISRF was immersed in 500mL of tap water at

121

room temperature until equilibrium swelling was reached, nearly 6 h. Then the

122

80-mesh sieve was used to separate the swollen SISRF from tap water, and the surface 6


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of swollen SISRF was dried with absorbent cotton. Finally, the SISRF was weighed,

124

and the water absorbency Qeq (g/g) was calculated according to following Eq. (1):

125

126 127

Qeq =

M − M0 M0

(1)

Here M and M0 denote the weights of the water-swollen sample and the dry sample, respectively.

128

The swelling kinetics of SISRF in tap water was studied according to previous

129

study.18 SISRF was ground into powder (80 mesh) from which 1 g was put in a nylon

130

net bag (300 meshes), and then immersed in tap water. At planned intervals (0.5, 1,

131

1.5, 2, 2.5, 3, 4, 9, 14, 19, 24, 34, 44, 54, 74, 94, and 114 min), the bag was quickly

132

taken out of water and then weighed. The Qt was defined as the water absorbency of

133

SISRF at time t, which was calculated by the mentioned equation.

134

Measurement of slow-release behavior of SISRF in soil. The testing method of

135

slow-release behavior of SISRF in this study was similar to previous work reported by

136

Liu et al.20 1 g dried samples was thoroughly mixed with 200g of dried soil (below 40

137

mesh) and placed in a 250 mL plastic bottle. Then the mixture was incubated at room

138

temperature, and the soil moisture in the bottle was maintained at 30wt% throughout

139

the experiment. After 1, 3, 5, 7, 10, 15, 20, 25 and 30 day’s incubation periods, the

140

remaining granulated SISRF in the bottles were retrieved, washed carefully with

141

distilled water and dried at 80 °C to a constant weight. The remaining contents of N, P

142

and K were analyzed using the Kjeldahl method of distillation, ultraviolet

143

spectrophotometer and flame spectrophotometry, respectively.21-23

144

Measurements of largest water-holding ratio and water-retention of SISRF in

145

soil. The largest water-holding ratio and water-retention of SISRF was used to study

146

the effect of SISRF on the water-holding capacity of soil following the procedures of 7



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previous studies.19,24 To prepare a mixture of dry soil (below 40 mesh) and SISRF,

148

different application rates (0%, 1%, 2%, 3%) of SISRF were examined.

149

The above-mentioned mixtures were placed in a 4.5cm diameter, 15cm long PVC

150

tube. The bottom of the tube was sealed using nylon fabric (300 mesh) and weighed

151

(marked M1). Then the tube was hung on the iron support stand vertically. The

152

mixtures were soaked slowly by tap water until water seeped out from the bottom.

153

The tube was weighed again (marked M2) when there was no water seeping from the

154

bottom. The largest water-holding ratio (WH%) of soil was calculated using following

155

Eq. (2):

156

WH% =

M 2 - M1 × 100 100

(2)

157

The mixtures were kept in a glass beaker and weighed (marked M0). Then the

158

mixtures were carefully drenched with tap water until saturation (the amount of water

159

was calculated from previous calculation results), and the beaker was weighed again

160

(marked M1). The beakers were kept at room temperature and weighed every 2 days

161

(marked Mi). All measurements were done within 30 days. The water-retention ratio

162

(WR %) of soil was calculated using following Eq (3):

163

164

WR% =

Mi - M0 × 100 M1 - M 0

(3)

RESULTS AND DISCUSSION

165

Reaction mechanism for the synthesis of SISRF. The proposed mechanism for

166

synthesis of SISRF is outlined in Scheme 1. First, the MU and DMU were prepared

167

by the chemical reaction between urea and formaldehyde under pH of 8.0 and

168

temperature of 40 °C for 2 h. Subsequently, trace amounts of formaldehyde reacted

169

with AM to form MBA in the acidic condition. Then, graft copolymerization reaction

170

took place among kaolin, AA and AM. Meanwhile, UF was synthesized by MU or 8


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DMU with urea in acid medium. Finally, the end vinyl groups of MBA reacted with

172

P(AA-co-AM) to form cross-linked structure during chain propagation.25 With this

173

method, a novel SISRF was formed gradually. Scheme 2 shows a schematic

174

illustration of SISRF.

175

Morphology and composition of SISRF. The characteristics of SISRF, i.e., main

176

composition, diameters of dried samples and swollen samples and percentage of the

177

elements were presented in Table 1. Fig. 1 showed the morphologies of the SISRF

178

samples. As shown in Fig. 1 (a), the samples were cylindrical white particles. From

179

Fig. 1 (b), it was obvious that the SISRF granules were capable of taking up plenty of

180

water. Meanwhile, it could be seen that the kaolin-g-P(AA-co-AM) superabsorbent

181

composite were miscible with the UF chains after absorbing water.

182

FTIR analysis. The FTIR spectra of kaolin, kaolin-g-P(AA-co-AM), urea, UF, and

183

SISRF were shown in Figure 2. The characteristic peaks at 3365 cm−1 and 3196 cm−1

184

could be assigned to –NH stretching of -CONH2 groups of kaolin-g-P(AA-co-AM) as

185

shown in Fig.2 (b). At the same time, the characteristic absorption peaks at 1650 cm−1

186

and 1550 cm-1 were ascribed to C=O stretching of -CONH2 groups and -COO-

187

asymmetric stretching vibration, respectively. And other two weak absorption peaks at

188

767cm-1 and 700 cm-1 were assigned to Si-O-Al bonds. The results confirmed the

189

formation of kaolin-g-P(AA-co-AM). In addition to this, in a comparison with the

190

spectrum of pure urea (Fig. 2(c)) and pure UF (Fig. 2(d)), strong absorption peaks at

191

3327cm−1 and 1550 cm−1 were observed in the IR spectrum of UF, indicating the

192

formation of secondary amide by the reaction of urea with formaldehyde. Meanwhile,

193

the peaks at 3030 cm-1 and 2965 cm-1 assigned to the C-H stretching vibration of UF

194

were also observed. Fig.2(e) shows the IR spectrum of SISRF. It clearly showed that

195

all the characteristic peaks of the components appeared. Accordingly, SISRF 9



196

contained both kaolin-g-P(AA-co-AM) and UF molecular simultaneously.

197

SEM analysis. The micro-structure of kaolin-g-P(AA-co-AM) and SISRF were

198

presented in Fig.3. In Fig. 3 (a) and 3 (b), there were lots of tiny cracks on the surface

199

of samples after being dried in an oven, whereas irregular folds and inhomogeneous

200

bulges were clearly observed on the surface of SISRF (Fig. 3 (b)). The reason was that

201

the addition of methylolurea could intensify the denser crosslinked diffusion barrier

202

on the surface of SISRF and the addition of K2HPO4 might form small crystallites to

203

adhere to the surface of SISRF. Meanwhile, the irregular surface morphology might

204

affect the releasing behaviors as well as the water-holding and water-retention

205

capacity of SISRF. The interior morphologies of swollen kaolin-g-P(AA-co-AM) and

206

SISRF were showed in Fig. 3 (c) and Fig. 3 (d), respectively. From Fig. 3 (c), the

207

cross-sections of kaolin-g-P(AA-co-AM) showed well-defined, interconnected,

208

three-dimensional porous network structures. In general, the average pore size of

209

kaolin-g-P(AA-co-AM) hydrogel was about 10µm to 50 µm. Interestingly, the average

210

pore size of SISRF (Fig. 3 (d)) was less than 2 µm, which was much smaller than that

211

of the kaolin-g-P(AA-co-AM) hydrogel. Furthermore, SISRF exhibited more irregular

212

pore structure comparing to kaolin-g-P(AA-co-AM) hydrogels. It looked more like

213

that the pores of SISRF were formed by lamellar materials. These changes about pore

214

size could result in the increase of crosslink point. Some UF chains might act as a

215

cross-linking agent in the kaolin-g-P(AA-co-AM) network hydrogel system. These

216

physical crosslinks caused the formation of additional free volume in the polymer

217

composite. These physical crosslinks would restrict the mobility of polymer chains,

218

thus limiting the swelling capacity of SISRF. The remaining UF chains, which did not

219

develop entanglements with kaolin-g-P(AA-co-AM), formed irregular aggregates on

220

the surface of the hydrogel network. The differences between kaolin-g-P(AA-co-AM) 10


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221

and SISRF could prove the formation of SISRF with semi-interpenetrating networks

222

structure.

223

DSC analysis. Although SISRF was a crosslinked polymer, the high water

224

absorption of SISRF was defined in this study (the result was discussed in detail

225

below), suggesting the favorable flexibility and elasticity of the polymer segment. So

226

the dried SISRF should show obviously Tg. The thermal behavior of UF, SISRF-X (X

227

refers to the amount of methylolurea solution) and kaolin-g-P(AA-co-AM) were

228

investigated by means of DSC analyses. It clearly showed Tg for SISRF. As shown in

229

the Fig.4, Tg values of the SISRF could be affected by different amounts of

230

methylolurea solution. From Fig. 4a, the Tg of kaolin-g-P(AA-co-AM) was 83.6 °C.

231

As shown in Fig. 4b and Fig. 4c, the endothermic peak of SISRF shifted to a lower

232

temperature with the increase of the amount of methylolurea solution, and Tg for

233

SISRF-1.5 and SISRF-3 were 81.1 °C and 72.5 °C, respectively. When a very small

234

amount of methylolurea solution was added into the kaolin-g-P(AA-co-AM), the

235

well-known plasticizing effect of oligomer of methylolurea and unreacted urea would

236

lead to a decrease of Tg value. However, there was a slight increase of Tg with the

237

continuous increase of the methylolurea solution. The reason for the trend may be

238

molecular chain entanglement of UF and kaolin-g-P(AA-co-AM) becoming the

239

dominant factor. When the motion of the polymer segment was blocked, the value of

240

Tg increased. Therefore, the Tg of SISRF-6 from Fig. 4d was raised up to 76.6 °C. Fig.

241

4e showed the apparent endothermic peaks of urea-formaldehyde decomposition at

242

180 °C for SISRF-9 comparing to Fig. 4f. It showed that when the amount of

243

methylolurea solution was too much, part of methylolurea molecules would be

244

separated from kaolin-g-P(AA-co-AM) network to form the aggregates of

245

urea-formaldehyde. According to the above analysis, the kaolin-g-P(AA-co-AM) 11



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246

network was indeed inserted into some UF chains to form the semi-interpenetrating

247

networks structure. Since it was not obvious that phase separation of

248

kaolin-g-P(AA-co-AM) network and UF chains was existed in SISRF-6, it was

249

chosen for following experiments.

250

Eff ffect of the Monomer Ratio on Water Absorbency. The amount of hydrophilic

251

groups is a critical factor to affect the water absorbency of superabsorbents.26 The

252

amount of hydrophilic groups could be controlled by variation of the weight ratio of

253

AM/AA in kaolin-g-P(AA-co-AM) system.20 The changes of water absorbency with

254

the weight ratio of AM/AA were shown in Fig. 5. When the weight ratio of AM/AA

255

was about 0.5, there existed a maximum water absorbency. It’s well-known that AA

256

and AM are anion and nonionic monomer, respectively. The common-ion effect and

257

salt effect were weakened by the synergy of anion group (-COO-) and nonionic group

258

(-CONH2)

259

Nˊ-methylenebisacrylamide(MBA) could be synthesized by using AM and

260

formaldehyde as raw materials in the acidic condition.28 MBA could be used as a

261

cross-linker

262

three-dimensional network could not be effectively formed with less cross-linker.

263

Therefore, when the weight ratio of AM/AA was less than 0.5, the water absorbency

264

increased with the increase of the proportion of AM. On the other hand, because of the

265

fact that -COO- group is more hydrophilic than -CONH2, further increasing of AM

266

content would reduce the water absorbency.30 Furthermore, the crosslinking degree in

267

kaolin-g-P(AA-co-AM) polymeric system was greater at the higher AM content. So,

268

the polymer network becomes more compact and the mobility of polymer chains was

269

reduced.20 All of which led to the polymer network could not be effectively stretched

270

after water absorption, thus reducing the water absorbency of SISRF.

during

of

the

water

P(AA-co-AM).

absorbing

Based

on

process.27

Flory’s

12


Moreover,

network

the

theory,29

N,

the

Page 13 of 42


271

Eff ffect of kaolin content on Water Absorbency. The effect of kaolin content on

272

the water absorbency was shown in Fig.6. The water absorbency firstly increased and

273

then decreased with the increasing of the content of kaolin. When the content of

274

kaolin was about 10%, the largest water absorbency was obtained. The result was

275

attributed to the following two reasons. Firstly, the hydroxyl groups on the surface of

276

kaolin

277

kaolin-g-P(AA-co-AM) was prepared by graft polymerization of the hydroxyl radicals

278

and vinyl monomer (AA or AM).19 Consequently, kaolin which acted as a cross-linker

279

to a certain degree made an effect on crosslinking density of superabsorbent, and in

280

turn affected the water absorbency of superabsorbent. Therefore, when the content of

281

kaolin was lower than 10 wt%, there was few crosslinking points in SISRF, which

282

lead to dissolution of part molecular chain of SISRF in tap water, thus reducing its

283

water absorbency. Secondly, excessive amounts of kaolin would fill in the polymer

284

network physically.31 Meanwhile, kaolin acted as additional network points in the

285

network when its content was more than 10wt%. With the further increase of content

286

of kaolin, the superabsorbent with high crosslink density was achieved, which would

287

impede the stretch of molecular chain. This would decrease the water absorbency of

288

SISRF.

might

react with

initiator to

form

hydroxyl

radicals,

and

then

289

Slow-Release Behavior of SISRF in Soil. The semi-interpenetrating networks

290

made great effects on the slow-release property of fertilizers. Wen et al.17 reported that

291

60.8% of nitrogen was released from the semi-IPNs fertilizer prepared by them within

292

30 days. The investigation reported by Li et al.18 also showed that the release rate of

293

phosphorus (ca.85.10%) had reached equilibrium in the third hour, and the release rate

294

of nitrogen was about 75% until the sixth day. In contrast, in our study, SISRF showed

295

the better slow-release properties. More specifically, the release rate of nutrients 13



296

phosphorus (ca.58.8wt%) and potassium (ca.81.3wt%) was relatively faster than that

297

of nitrogen (ca.39.2wt%) within 30 days, respectively, as shown in Fig.7. The reason

298

was that K2HPO4 embedded in the swollen gel as the small molecules would be

299

dissolved once the gel was formed by mixing SISRF and soil solution. Then the

300

hydrogen phosphate ion and potassium ion diffused into the soil due to the presence of

301

concentration gradient between SISRF and the external soil.18 The releasing of

302

phosphorus and potassium would reach steady state in about 5 and 15 days,

303

respectively, mainly because of physical barrier of superabsorbent network and

304

molecular chain of UF. In the meantime, kaolin also contributed to slow-release of

305

nutrients phosphorus and potassium, due to the fact that kaolin had well-developed

306

pore channel and therefore had a large internal surface area.32 When the nutrients

307

phosphorus and potassium were dissolved in water, part of the nutrients could be

308

adsorbed by kaolin internal surfaces, thus slowing the release rate of nutrients.33

309

Compared with the relatively simple existing formation of phosphorus and potassium,

310

nitrogen consisted in the molecular chain of UF was difficult to dissolve in soil

311

solution. The slow-release process of nitrogen was attributed to the hydrolysis and

312

microbial degradation of UF in soil solution. Therefore, the nutrients nitrogen had a

313

slower release rate than phosphorus and potassium.

314

Water-Holding and Water-Retention Capacity of Soil with SISRF. The most

315

important influence factor of plant growth was that it required the supply of water. As

316

seen in Fig.8, the largest water-holding ratios of soil were 42.3±2.3%, 51.8±1.9%,

317

61.9±2.4%, and 73.9±2.1% for SISRF application rates of 0, 1%, 2% and 3%,

318

respectively. It can be seen obviously that the swelling ratio of SISRF in soil is much

319

less than that of SISRF in tap water. The reason might be that each SISRF granule was

320

surrounded by soil particles, thus the swelling of SISRF was hindered by the 14


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321

compression of these particles. Furthermore, highly-charged metal ions (such as Ca2+,

322

Mg2+, Al3+ and Fe3+) in soil solution had very high complexing capacities with the

323

hydrophilic groups of SISRF.34 However, the water content of the soil mixed with

324

SISRF was effectively improved comparing to the soil without SISRF. And the water

325

content gradually increased with increasing SISRF dosage in the soils. Consequently,

326

the water holding capacity of the soils would be greatly improved once SISRF were

327

applied to the soils. These would largely reduce the consumption of agricultural

328

irrigated water.

329

When it referred to the application of superabsorbent materials in soils, the

330

water-retention capacity was also extremely important. Besides, the water-retention

331

capacity was as high as possible. As shown in Fig.9, after 16 days’ test, the

332

water-retention capacities of the soil samples were about 18.1%, 31.7%, 41.9% and

333

49.2% for SISRF application rates of 0, 1wt%, 2wt%, and 3 wt%, respectively. The

334

soil without SISRF had nearly lost all of its absorbed water after 26 days, whereas the

335

soil samples with 1wt%, 2wt%, and 3 wt% SISRF still retained 3.8%, 14.1%, and

336

25.4% water. These results suggested that the water loss rate of soil without adding

337

SISRF was significantly higher than that of soil added with SISRF. At the same time,

338

with the increasing of the amount of SISRF, the water-retention capacity of soil

339

gradually increased. This was mainly due to the decrease of the free enthalpy of the

340

whole system after the water entered SISRF.35 If the water escaped from SISRF, the

341

free enthalpy of the system would rise, which was unfavorable to the stability of the

342

system. Therefore, the water in SISRF could only be released gradually in the external

343

environment with water shortage. Therefore, when SISRF was applied in cropland,

344

the evaporation rate of soil water would decrease. It had great application prospects in

345

arid areas in the agriculture field. 15



Page 16 of 42

346

Swelling kinetics. The swelling kinetics was a crucial characteristic to describe the

347

water absorption. The swelling property of SISRF was shown in Fig.10. From the

348

swelling rate curve of the SISRF in tap water, it could be found that the swelling rate

349

of SISRF was fast at the first 10 min, and then the rate gradually slowed down. Finally,

350

the curve almost took about 75 min to reach the equilibrium state. In this section,

351

pseudo-second-order swelling kinetics model was adopted to help us understand the

352

swelling behavior of SISRF clearly, and the model could be expressed by the

353

following equation:36

354

t 1 t = + 2 Qt KQe Qe

355

Here, Qt (g g-1) was the swelling capacity at contact time t (min), Qe (g g-1) was the

356

theoretical equilibrium water absorbency, and K (g g-1 min-1) was a rate constant.

357

Based on the swelling rate data in Fig. 10, the plot of t/Qt versus t could give nice

358

straight line showed in Fig. 10, and the linear correlation coefficient (R2=0.9989) was

359

very close to one, thus suggesting that the pseudo-second-order swelling kinetics

360

model had made considerable fitting results for the swelling behaviors of SISRF.

361

Moreover, K and Qe parameters could be calculated from the intercept and slope of

362

the fitted straight line precisely. The K and Qe was 0.008 g g-1 min-1 and 68.97 g g-1,

363

respectively. Meanwhile, the Qe value obtained from the pseudo-second-order

364

swelling kinetics model was very close to the observed value in experiment. So, the

365

information indicated that the swelling process in tap water followed the

366

pseudo-second-order swelling kinetics model perfectly. It also demonstrated that the

367

semi-IPNs structure of SISRF did not change the water absorption mechanism of the

368

acrylic superabsorbent hydrogel.25

369

AUTHOR INFORMATION 16


(4)

Page 17 of 42


370

Corresponding Author

371

*Tel. & Fax: +86-351-3559669. E-mail address: [email protected] (Y.Q. Liu)

372

Notes

373

The authors declare no competing financial interest.

374

ABBREVIATIONS USED

375

AA, Acrylic acid; AM, acrylamide; KOH, potassium hydroxide; K2HPO4,

376

dipotassium hydrogen phosphate; APS, ammonium persulfate; UF, urea-formaldehyde;

377

kaolin-g-P(AA-co-AM), kaolin-g-poly (acrylic acid-co-acrylic amide); semi-IPNs,

378

semi-interpenetrating polymer networks; SISRF, semi-IPNs slow-release fertilizer;

379

FTIR, Fourier transform infrared; DSC, differential scanning calorimeter; SEM,

380

scanning electron microscope; Qeq, water absorbency; Qt, swelling capacity at contact

381

time t, Qe, the theoretical equilibrium water absorbency; WH, water-holding ratio;

382

WR, water-retention ratio

383

REFERENCES

384

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slow-release properties of a poly (acrylic acid-co-acrylamide)/sodium humate

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cellulose-based semi-IPNs superabsorbent with integration of water-retaining and

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of coated controlled-release fertilizer with the function of water retention. J. Agric.

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(20) Liu, M.; Wu, L. Slow-release potassium silicate fertilizer with the function of

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superabsorbent and water retention. Ind. Eng. Chem. Res. 2007, 46, 6494-6500.

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absorption spectrometric methods for determination of phosphorus and silicon by

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21



Figure captions Figure 1. Photographs of dry (a) and swollen (b) SISRF granules. Figure 2. FTIR spectra of kaolin (a), kaolin-g-P(AA-co-AM) (b), urea (c), UF (d), and SISRF (e). Figure 3. SEM images of dry kaolin-g-P(AA-co-AM) (a) and SISRF(b), swollen kaolin-g-P(AA-co-AM) (c) and SISRF(d). Figure 4. DSC thermograms of kaolin-g-P(AA-co-AM) (a), SISRF-1.5 (b), SISRF-3 (c), SISRF-6 (d), SISRF-9 (e) and UF (f). Figure 5. Effect of the monomer ratio on water absorbency of SISRF. Figure 6. Effect of kaolin content on water absorbency of SISRF. Figure 7. Slow-release profiles of nitrogen (a), phosphorus (b) and potassium (c) from SISRF in soil, respectively. Figure 8. The largest water-holding ratio of soil with different SISRF application rates. Figure 9. Water retention behavior of the soil mixed with different SISRF application rates. Figure 10. Swelling rate curve and swelling kinetic curve of SISRF in tap water.

22


Page 22 of 42

Page 23 of 42


Table 1 Characteristics of SISRF

Characteristics

Value

Carbon content

17.01%

Nitrogen content

34.81%

P2O5 content

1.31%

K2O content

11.32%

Diameter of dry sample

6mm

Diameter of swollen sample

30mm

O O H C H + H2 N C NH 2

OH-

NH 2

O C NH CH2OH MU

O O + NH H C H 2 C NH CH2OH

OH-

O HOCH 2 NH C NH CH2OH DMU

MU O

MU or

+ H 2 N C NH2

H+

NH 2

O O C NH CH 2 NH C NH H n

DMU

UF

O O 2 CH 2 CH C NH 2 + H C H

H+

O O CH 2 CH C NH CH2 NH C CH MBA

AM

Scheme 1. Synthesis Mechanism of SISRF

23


CH2


Scheme 2. Schematic Illustration of SISRF

Figure 1. Photographs of dry (a) and swollen (b) SISRF granules

24


Page 24 of 42

Page 25 of 42


Figure 2. FTIR spectra of kaolin (a), kaolin-g-P(AA-co-AM) (b), urea (c), UF (d), and SISRF (e).

Figure 3. SEM images of dry kaolin-g-P(AA-co-AM) (a) and SISRF(b), swollen kaolin-g-P(AA-co-AM) (c) and SISRF(d).

25



Figure 4. DSC thermograms of kaolin-g-P(AA-co-AM) (a), SISRF-1.5 (b), SISRF-3 (c), SISRF-6 (d), SISRF-9 (e) and UF (f).

Figure 5. Effect of the monomer ratio on water absorbency of SISRF.

Figure 6. Effect of kaolin content on water absorbency of SISRF.

26


Page 26 of 42

Page 27 of 42


Figure 7. Slow-release profiles of nitrogen (a), phosphorus (b) and potassium (c) from SISRF in soil, respectively.

Figure 8. The largest water-holding ratio of soil with different SISRF application rates.

Figure 9．Water retention behavior of the soil mixed with different SISRF application rates.

27



Figure 10. Swelling rate curve and swelling kinetic curve of SISRF in tap water.

28


Page 28 of 42

Page 29 of 42


Graphic for table of contents

29



Graphic for table of contents 82x44mm (300 x 300 DPI)


Page 30 of 42

Page 31 of 42


Photographs of dry (a) and swollen (b) SISRF granules. 84x40mm (300 x 300 DPI)



FTIR spectra of kaolin (a), kaolin-g-P(AA-co-AM) (b), urea (c), UF (d), and SISRF (e). 84x61mm (300 x 300 DPI)


Page 32 of 42

Page 33 of 42


SEM images of dry kaolin-g-P(AA-co-AM) (a) and SISRF(b), swollen kaolin-g-P(AA-co-AM) (c) and SISRF(d). 64x48mm (300 x 300 DPI)



DSC thermograms of kaolin-g-P(AA-co-AM) (a), SISRF-1.5 (b), SISRF-3 (c), SISRF-6 (d), SISRF-9 (e) and UF (f). 84x64mm (300 x 300 DPI)


Page 34 of 42

Page 35 of 42


Effect of the monomer ratio on water absorbency of SISRF. 65x51mm (600 x 600 DPI)



Effect of kaolin content on water absorbency of SISRF. 63x47mm (600 x 600 DPI)


Page 36 of 42

Page 37 of 42


Slow-release profiles of nitrogen (a), phosphorus (b) and potassium (c) from SISRF in soil, respectively. 63x48mm (600 x 600 DPI)



The largest water-holding ratio of soil with different SISRF application rates. 62x46mm (600 x 600 DPI)


Page 38 of 42

Page 39 of 42


Water retention behavior of the soil mixed with different SISRF application rates. 65x49mm (600 x 600 DPI)



Swelling rate curve and swelling kinetic curve of SISRF in tap water. 59x42mm (600 x 600 DPI)


Page 40 of 42

Page 41 of 42


Synthesis Mechanism of SISRF 71x56mm (300 x 300 DPI)



Schematic Illustration of SISRF 84x110mm (300 x 300 DPI)


Page 42 of 42

Preparation and Properties of a Novel Semi-IPN Slow-Release

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