Granular Slow-Release Fertilizer from Urea-formaldehyde, Ammonium

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Agricultural and Environmental Chemistry

Granular Slow-Release Fertilizer from Ureaformaldehyde, Ammonium Polyphosphate and Amorphous Silica Gel: A New Strategy Using Cold Extrusion Yang Xiang, Xudong Ru, Jinguo Shi, Jiang Song, Haidong Zhao, Yaqing Liu, and Guizhe Zhao J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02349 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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Journal of Agricultural and Food Chemistry

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Granular Slow-Release Fertilizer from Urea-formaldehyde, Ammonium

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Polyphosphate and Amorphous Silica Gel: A New Strategy Using Cold Extrusion

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

4

Guizhe Zhao*

5

Research Center for Engineering Technology of Polymeric Composites of Shanxi

6

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

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Taiyuan 030051, China

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ACS Paragon Plus Environment


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Abstract

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A new granular slow-release fertilizer prepared by a cold extrusion strategy

10

(GSRFEx) based on urea-formaldehyde (UF), ammonium polyphosphate (APP) and

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amorphous silica gel (ASG) was presented. Characterizations showed that there were

12

a strong hydrogen bond interaction and good compatibility among UF, APP and ASG

13

in GSRFEx. Mechanical properties as well as slow-release properties of GSRFEx

14

were greatly enhanced after adding APP and ASG into UF. Rape pot experiments

15

indicated that GSRFEx could improve N use efficiency dramatically and thereby

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facilitate the growth of rapes. Importantly, as an economical, effective and

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environment-friendly technology, the cold extrusion had a great potential to be

18

applied in horticulture and agriculture. We hope that our work could offer an

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alternative method for the design of slow-release fertilizer with desirable properties.

20 21

Keywords: granular slow-release fertilizer; urea-formaldehyde; ammonium polyphosphate; amorphous silica gel; cold extrusion

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Introduction

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Slow-release fertilizers have received increasing attention lately because the use of

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them could improve nutrients use efficiency and reduce the environmental hazards.1, 2

26

Coated fertilizers, prepared by physically coating conventional inorganic fertilizers

27

with a variety of materials to reduce their solubility in water, are widely applied as

28

slow-release fertilizers at present.3, 4 The release rates of core fertilizers are regulated

29

by controlling the thickness and structures of coated layer.5 Various coating materials

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have been developed by using synthetic polymers such as polyolefines, polyurethane,

31

polystyrene and polysulfonate.6-9 However, the accumulation of residual or

32

undegraded synthetic polymers in the soil may contaminate the environment.10

33

Therefore, biodegradable-based coating materials have been paid much attention

34

recently because of their biodegradability, non-toxicity and environmentally friendly

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properties.11 Biodegradable polymers including chitosan, starch, sodium alginate and

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cellulose could be directly used or further processed as the coating materials.12-15

37

Unfortunately, due to relatively high-cost and the complicated coating processes,

38

biodegradable-based coated fertilizers have still not satisfied the requirement for

39

large-scale application, especially in the crop field.16-18

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Using chemically controlled releasing fertilizers is one possible way to mitigate

41

those problems.19 So far, urea-formaldehyde (UF) is a main species in the world. UF

42

is a long-chain polymer derived from the reaction between urea and formaldehyde,

43

and could be biodegraded by the microorganisms, and then have the ability to release

44

nutrient element N slowly in the soil. But unfortunately, the releasing rate of nutrient 3



45

N is too slow to match the requirements for the normal growth of crops.20 The main

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reason for this is that the macromolecules and crystalline regions in UF could hardly

47

be decomposed by microbial action in a short time. Therefore, the share of UF in the

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global market for slow-release fertilizers has been falling significantly.

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The slow-release property of UF could be improved by decreasing its crystallinity,

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because water and microorganisms are easier to penetrate the amorphous regions in

51

UF and then attack the fragile ureido groups.21 Ammonium polyphosphate (APP) and

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amorphous silica gel (ASG) contain a large number of phosphorus-oxygen double

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bonds and hydroxyls respectively. These groups could form an extensive hydrogen

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bond network with the ureido of UF.22-24 Hence, when APP and ASG are introduced

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into UF, the regular molecular arrangement of pure UF will be destroyed and then the

56

UF with a reduced crystallinity will be formed, thus improving the slow-release

57

property. To our best knowledge, few papers have been reported on improving the

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slow-release property of UF by decreasing its crystallinity.

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Besides the unfavorable slow-release property, poor processability is another

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important factor which restricts the large-scale application of UF. Since the raw

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material formaldehyde generally contains 63% by weight of water, the large-scale

62

preparation of UF granules is practically limited.25 For now, some granulation

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methods for UF have been developed, such as disc granulation, extruding granulation

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and so on. However, there are many deficiencies in these granulation processes. For

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example, the UF product had to be dried and then crushed into powder before

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granulation. This process would inevitably lead to large in dust and excessive energy 4


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consumption, and then cause heavy environment pollution and significant costs of

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granulation.26, 27 To alleviate these problems, some new granulating processes have

69

been developed. Guo et al.28 reported that UF hydrogel beads were prepared by

70

mixing UF with sodium alginate. However, their preparation technology at large scale

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is typically a complex process. Extrusion is another way to prepare granulated UF

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recently.25 In order to shape UF, plenty of montmorillonites with super plasticity were

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added into UF in the extrusion process. However, the nutrient content of fertilizers

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produced by this method is limited due to the presence of a large number of

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montmorillonites. Moreover, in order to achieve a good slow-release capacity, UF has

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to be inserted into the interlayers of montmorillonites under the high shearing force of

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the extrusion equipment, which will inevitably lead to excessive energy consumption,

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and therefore lead to the significant costs of granulation.

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Flours can be plastically formed when mixed with water because proteins in flours

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can form more hydrogen bonds with water when mechanical force stretches the

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dough.29 The sticky mixture of UF and water has very similar structure to the dough.

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So UF can also be plastically formed just like dough.

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We describe herein the preparation of granular slow-release fertilizer from UF, APP

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and ASG by a cold extrusion strategy, which is environmental friendly, easy to

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operate, and suitable for industrial production. The introduction of APP and ASG

86

could enhance the mechanical properties of GSRFEx effectively. In addition, there are

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strong hydrogen bond interactions among UF, APP and ASG, which could improve

88

the slow-release property of UF in GSRFEx greatly. Hence, GSRFEx, as a new 5



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member of UF-based fertilizers, has a great potential to be applied in horticulture and

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agriculture.

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Materials and Methods

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Materials. Formaldehyde, urea, calcium hydrogen phosphate (CaHPO4•2H2O),

93

potassium chloride (KCl), potassium hydroxide (KOH) and ammonium dihydrogen

94

phosphate (NH4H2PO4) were provided by Damao factory, Tianjin, China. Potassium

95

silicate (K2O•2.72SiO2) was purchased from Guangzhou Suixin Chemical Reagent

96

Co., Ltd. All reagents were of analytical grade and used directly. A homemade

97

extrusion device (Hole diameter 3mm) was used to granulate GSRFEx (Figure S1).

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Distilled water was utilized in the preparating process of GSRFEx.

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Preparation of GSRFEx. The synthesis process of GSRFEx was shown in Figure

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1. Here, 24 g of urea and 20.3 g of 37% (w/w) formaldehyde were added into a

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250mL round bottom flask and then stirred constantly. Then pH of the solution was

102

adjusted to 9.0 with 5% KOH solution. The reaction was charged at temperature

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50 °C for 2 h. Next, 9 g of NH4H2PO4 and 7 g of potassium silicate were added

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rapidly and then the reaction was charged at 60 °C for 1 h.

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Fertilizer granules were obtained by a plunger extrusion molding process. For this

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purpose, above reaction product was first solidified at room temperature for 30 min to

107

achieve the desired viscosity for extrusion molding, and then extruded into cylindrical

108

strips using a homemade extrusion device. Afterwards, the cylindrical strips were

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placed into the oven and dried at 160 °C for 30 min, and then cooled down to 80 °C

110

and dried to a constant weight. Finally, the dried stripped products were cut-up, then 6


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the cylindrical fertilizer granules were obtained.

112

For comparison, ASG was prepared by mixing NH4H2PO4 and potassium silicate at

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160 °C. Ammonium polyphosphate (APP) was prepared by melt polycondensation of

114

NH4H2PO4 and urea at 160 °C. UF was synthesized by solution polycondensation

115

with methylolurea and urea in acid medium. The preparation process of SRNP was

116

similar to that of GSRFEx except that no raw material potassium silicate was added.

117

Table 1 lists the composition and characteristics of above slow-release fertilizers,

118

including the nutrient contents, average dimeter and length of samples.

119

Characterizations of ASG, APP, UF, SRNP and GSRFEx. The surface

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functionalities of samples obtained under the optimum conditions were analyzed by a

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Fourier transform infrared spectroscopy (FTIR, Nicolet IS50, America) with an ATR

122

attachment at the wave number ranged from 500 to 4000 cm-1. The surface elemental

123

compositions and distribution were determined by an X-ray photoelectron

124

spectrometer (XPS, EscaLab 250Xi, America). The crystal structures of samples were

125

recorded by X-ray diffractometer (XRD, HAOYUAN DX-2700B, China) in the 2θ

126

range of 5-70°. The thermal stabilities of samples were evaluated in nitrogen

127

atmosphere by thermogravimetric analysis (TGA, TA Q50, America). The surface

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morphologies of samples were observed by scanning electron microscopy (SEM,

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Hitachi U8010, Japan). The surface elemental compositions and distribution were

130

measured by an energy dispersive X-ray spectroscopy (EDX) detector attached to the

131

SEM.

132

Measurement

of

processing

properties

of

GSRFEx

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with

different


133

urea/formaldehyde (U/F) mole ratio. The dynamical rheological properties of

134

samples were carried out by using a laboratory scale torque rheometer (KECHUANG

135

XSS-300, China). The torque curves were recorded with a rotation speed of 20 rpm at

136

room temperature. The weight was kept constantly at 60g for all samples. To

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minimize the test error caused by the evaporation of sample moisture, all samples

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were tested for the dynamical rheological properties within 30 minutes after being

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made.

140

Measurement of mechanical strength of UF, SRNP and GSRFEx with different

141

amount of potassium silicate. Compression tests of samples were carried out by

142

using a universal testing machine equipped with a 20000 kgf load cell (MTS

143

CMT5105, America). To achieve uniform-sized samples, the sticky reaction product

144

was pressed into a homemade meshed board (hole diameter 6 mm) (Figure S2). After

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that, the meshed board filled with the resulting product was dried at 160 °C for 30 min,

146

and then cooled down to 80 °C and dried to a constant weight. Finally, the cylindrical

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samples were obtained. The cylindrical particles were individually compressed in the

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longitudinal direction between two rigid plates at a constant rate of 5 mm min-1 until a

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maximum deformation of 40%. All treatments were replicated 15 times, and statistical

150

analysis of significant differences between treatments was determined by Duncan's

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multiple range test.

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Measurement of slow-release behavior of UF, SRNP and GSRFEx in soil. To

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simulate the real application scenarios, the slow-release experiment was carried out by

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mixing fertilizer granular and soil directly. The procedure for testing the slow-release 8


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behavior in this study was similar to the method used by wu et al.30 First, 1g of

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fertilizer granular was mixed with 200g of dried soil (below 40 mesh) homogeneously,

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then the mixture was kept in a plastic bottle and incubated at room temperature. In

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order to ensure the aerobic respiration of the microorganisms in soil, the bottle was

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not sealed. The soil moisture was maintained at about 30 wt % by weighing and

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adding tap water regularly throughout the experiment. At each incubation period (1, 3,

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5, 7, 10, 14, 21, 28, 42, and 56 day), the remaining granulated fertilizers in the bottle

162

were retrieved, washed carefully with distilled water, and then dried at 80 °C to a

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constant weight. The remaining contents of N were estimated by the Kjeldahl method

164

of distillation.31 The degradation rate was calculated by the following equation:

M 0 -Mi × 100 M0

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degradation (%) =

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Where M0 and Mi are the weights of the fertilizers before incubation and at each

167

incubation period, respectively.

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N use efficiency. A pot trial was carried out to investigate the effect of GSRFEx on

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N use efficiency and N loss. Plastic pots (diameter 10.5 cm, height 9.5 cm) were used

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to plant the rape (Brassica campestris L). The soil used in this study was collected

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from Taiyuan city, Shanxi province, China. The physicochemical properties of soil

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were listed in Table S1. Four types of treatments with five replicates were prepared as

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follows: (1) CK: soil without fertilizer; (2) UF; (3) SRNP; (4) GSRFEx. As shown in

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Table S2, for all types of treatments, the same amount of N (150 mg of N), P (54 mg

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of P2O5) and K (9 mg of K2O) were mixed with 450 g of soil in per pot. Rape seeds 9



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were germinated in seedling bed soil. Each of uniform pregerminated rape seedlings

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with two leaves was transplanted to one pot. The pots were incubated indoor during

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the whole period. Soil water content was maintained at about 60% of maximum water

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capacity by daily watering with deionized water. After 45 days’ culture, the rapes were

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carefully removed from the soil. The root length, fresh weight, dry weight and

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chlorophyll content of rape leaves were measured. The N content of the dried rape

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samples was measured by the Kjeldahl method of distillation.31 The N uptake and N

183

use efficiency were then calculated.

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Statistical Analysis. The statistical analyses were conducted using SPSS Statistics

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20.0. It showed significant differences among means when P < 0.05, which

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determined by Duncan's multiple range test.

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Results and Discussion

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Synthesis Mechanism of GSRFEx. The synthesis mechanism of each component

189

of GSRFEx is shown in scheme 1. First, methylolurea (MU) and dimethylolurea

190

(DMU) were obtained by the reaction between formaldehyde and urea under alkaline

191

condition. Then UF was prepared by polycondensation of MU or DMU with urea in

192

the acid environment obtained by ionization of NH4H2PO4. (NH4)2HPO4, K2HPO4 and

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H4SiO4 were formed by mixing potassium silicate with NH4H2PO4. When the

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resulting products achieved desired viscosity, the strip products were produced by

195

cold extrusion. During the cold extrusion process, UF could be plastically formed

196

with the aid of the water in the viscous products. As scheme 2A shows, the structural

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change of viscous products is similar to that of dough during the extrusion process.29 10


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Eventually, at high temperature, APP was prepared by polycondensation of

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NH4H2PO4 or (NH4)2HPO4 with urea, and ASG was made by chemical dehydration of

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H4SiO4. Scheme 2B exhibits a schematic illustration of the final product GSRFEx. As

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shown in Scheme 2B, there are strong hydrogen bond interactions among UF, APP

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and ASG, which might affect the slow-release behaviors of GSRFEx.

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Morphology and Composition of GSRFEx. SEM and EDX were used to analysis

204

the structure of GSRFEx. For the SEM images of GSRFEx (Figure 2 A and B), some

205

irregularly shaped micropores were found on the surface of GSRFEx, which would be

206

resulted from the air holes produced by water evaporation or urea decomposition and

207

volatilization at high temperature in the process of preparing GSRFEx. Meanwhile,

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some small particles could be observed, as indicated by the red arrows in Figure 2B.

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These particles were mainly made up of APP and K2HPO4, which could be

210

demonstrated by the elemental distribution maps of P and K (Figure 2D). As shown in

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these maps, most of P and K were uniformly distributed on the surface of GSRFEx,

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indicating that most of the APP and K2HPO4 were well dispersed in UF matrix. Figure

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2C showed that GSRFEx contained elements C, O, N, P, K and Si. As shown in

214

Figure 2D, only elements C, O and N were evenly distributed on the surface of

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GSRFEx. Distribution of element Si on the surface of GSRFEx was uniform but

216

sparse because of the low amount of ASG. Obviously, GSRFEx was significantly

217

different from the coated fertilizers in terms of its structure and compositions.5

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FTIR Analysis. FTIR spectra was measured to investigate the interactions in

219

GSRFEx system. The typical FTIR spectra of ASG, APP, UF, SRNP and GSRFEx are 11



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shown in Figure 3. Characteristic absorption peaks at 3023 cm-1 could be assigned to

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N-H stretching vibration of APP (Figure 3Ab).32 SRNP and GSRFEx possessed the

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characteristic absorption peaks at 3334, 3026 and 2962 cm-1 (Figure 3Ac, 3Ad and

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3Ae), which were ascribed to N-H and C-H stretching vibration of UF, indicating that

224

both of them have been loaded with UF.33 Moreover, compared with APP, the P-O

225

symmetric stretching vibration in SRNP blueshifted from 1062 to 1096 cm-1, and the

226

P-O asymmetric stretching vibration blueshifted from 875 to 898 cm-1 (Figure 3Bb

227

and 3Bd), indicating that hydrogen bonds existed between APP and UF (Scheme

228

2B).34 In addition, when ASG was combined with SRNP to form GSRFEx, the main

229

peaks of ASG (1060 cm-1 for the asymmetric stretching vibration of Si-O-Si, 795 cm-1

230

for the vibration of Si-O) shifted to 1115 and 926 cm-1 (Figure 3Ba and 3Be).35 These

231

results may be due to the formation of hydrogen bonds among ASG, APP and UF,

232

which result in the formation of the ASG/APP/UF network (Scheme 2B). Besides, no

233

new peaks or peak shifts were observed in the spectra of GSRFEx compared with

234

those of ASG, APP and UF, which illustrated that no obvious chemical reaction

235

occurred among ASG, APP and UF in GSRFEx.36

236

XPS Analysis. To further verify the hydrogen bond interactions in GSRFEx system,

237

XPS spectrum was conducted. As shown in Figure 4a, the peak of C1s appeared at

238

291.47 eV represented the contribution of ureido carbon of pure UF. There was no

239

difference in the C1s spectra between SRNP and UF, which suggested that the

240

hydrogen bond interactions between UF(-NH-) and APP (P=O) had no effect on the

241

chemical environment of ureido carbon of UF in SRNP (Figure 4b). Comparing the 12


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C1s spectra for UF and SRNP, the peak position of C1s was moved toward a higher

243

binding energy region in GSRFEx (Figure 4c), indicating that the electron density

244

around ureido carbon atoms in GSRFEx had been influenced by the introduction of

245

ASG into the system, possibly by forming hydrogen bonds between ASG and UF.37

246

XRD Analysis. XRD patterns were conducted to study the crystal structures of

247

samples. As shown in Figure 5b, the characteristic peaks at 2θ = 22.5°, 25.0° and 31.3°

248

confirmed the existence of crystalline regions in cured UF, which is probably due to

249

the formation of strong hydrogen bonding between the molecular chains of UF (C=O

250

and -NH-) and could contribute to ordered arrangement of the UF chains at a certain

251

scale.

252

and 31.3° and APP at 2θ = 15.8°, 16.9° and 23.9° (Figure 5b, 5d and 5e), suggesting

253

the successful binding of UF and APP.40 Meanwhile, the characteristic peaks of APP

254

in SRNP had shifted compared with pure APP. These shifts provided a vital evidence

255

for the interactions of APP and UF in SRNP. It was noteworthy that ASG appeared

256

one weak broad peak at about 23° (Figure 5c), suggesting the amorphous nature of it.

257

Moreover, the intensities of some main characteristic peaks of GSRFEx at 2θ = 16.9°,

258

22.5°, 23.9°, 29.4°, and 45.3° (Figure 5f) decreased after the combination of SRNP

259

and ASG, implying a uniform dispersion and interpenetration of ASG in GSRFEx.

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Besides, the characteristic peaks of urea at 2θ = 29.4° and 45.3° (Figure 5a) were

261

observed in SRNP and GSRFEx, demonstrating that the unreacted substrate urea was

262

embedded in SRNP and GSRFEx in its initial form.37

263

38,39

SRNP possessed the characteristic reflections of UF at 2θ = 22.5°, 25.0°

Thermal Properties. The TG/DTG curves of UF, APP, ASG, SRNP, and GSRFEx 13



264

are shown in Figure 6. The TG curve of APP had two stages, 253-450 °C and

265

450-680 °C, with weight losses of 15.26% and 64.40% respectively, indicating that

266

APP possessed high thermal stability.40 However, SRNP containing APP and UF had

267

lower thermal stability than UF alone. The reason is that there were lots of hydrogen

268

bonds between carbonyl groups and secondary amide groups in pure UF. As a result,

269

the arrangement of UF molecular chains was relatively regular. But when added to UF,

270

APP caused breakage of the inter- and intra-molecular hydrogen bonds of UF and

271

then formed new hydrogen bond interactions with UF, which caused a substantial

272

increase in disordered structure in SRNP compared with pure UF.41 Consequently,

273

SRNP could be easily decomposed at lower temperature. In addition, ASG possessed

274

superior thermal stability, and the TG curve showed maximum 7.97% weight loss in

275

the temperature range from 110 to 459 °C, which was attributed to the decomposition

276

of the residual NH4H2PO4 and (NH4)2HPO4 on the surface or in the pores of ASG.

277

Compared the TG curves of GSRFEx with that of SRNP, it was found that GSRFEx

278

showed a bit higher thermal stability than SRNP. The main reason is that there were

279

lots of hydroxyl groups on the surface of ASG particles, thus strong

280

hydrogen-bonding networks were easily formed among ASG (-OH), UF (C=O) and

281

APP (P=O) (Scheme 2B). Hence, UF was firmly adsorbed on the surface of ASG,

282

which strongly restricted the movement of the UF chain, and further prohibited its

283

decomposition during heating process. As a result, GSRFEx had better thermal

284

stability than SRNP.

285

Processing Properties. Rheological property is the fundamental manufacturing 14


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characteristic of polymers. GSRFEx undergoes a plasticizing process during

287

extrusionwhich can be studied by torque variation versus time.42 Effects of the

288

different urea/formaldehyde (U/F) mole ratio on rheological behavior of GSRFEx are

289

shown in Figure 7. As seen in Figure 7A, all torque curves of GSRFEx showed a

290

similar trend: first, in the feeding stage, the torque increased quickly to the highest

291

value and then gradually decreased due to the breaking of the inter- and

292

intra-molecular hydrogen bonds of UF chains in GSRFEx and subsequent forming

293

new hydrogen bonds between water and UF chains. At last, the torque tended to a

294

certain value during blend homogenization.

295

To facilitate further discussion, GSRFEx is designated as GSRFEx-m-n, where m

296

and n represent the urea/formaldehyde (U/F) mole ratio and the amount of potassium

297

silicate, respectively. As shown in Figure 7A, the steady torque of GSRFEx-1.2-7 was

298

0.9 N·m. Meanwhile, the extrudates of GSRFEx-1.2-7 tended to stick together and

299

had poor strength. The steady torque of GSRFEx shifted to a higher value with the

300

increase of the U/F mole ratio, as shown in Figure 7A, the steady torque for

301

GSRFEx-1.4-7 and GSRFEx-1.6-7 were 1.9 and 4.8 N·m respectively, suggesting the

302

water content had a significant effect on the rheological and processing properties of

303

GSRFEx. The water content of GSRFEx gradually decreased with the increase of U/F

304

mole ratio, which led to the higher viscosity, thereby explaining the increase of torque.

305

Therefore, as seen in Figure 7B, lower water content was more conducive to reduce

306

adhesion among the extrudates and improve the strength of extrudates.

307

However, there was a decrease of steady torque with the continuous increase of U/F 15



308

mole ratio. The reason for this might be the UF chain length becoming the dominant

309

factor. The shorter UF chain length caused an increase in the molecular chain mobility

310

of UF, thus resulting in the decreasing of the viscosity of GSRFEx. Therefore, the

311

steady torque for GSRFEx-1.8-7 and GSRFEx-2.0-7 were reduced to 2.9 and 2.2 N·m,

312

respectively. Because of this, the extrudates of GSRFEx-1.8-7 and GSRFEx-2.0-7

313

exhibited more rough surface and lower strength (Figure 7B). In summary, the optimal

314

U/F mole ratio of GSRFEx was 1.6.

315

Mechanical Properties. Compression strength is an important indicator of the

316

fertilizer mechanical stability. As seen in Figure 8A, compression strengths of all

317

samples were lower in case of strain less than 10%, probably because there were lots

318

of residual micropores after urea decomposition and volatilization or water

319

evaporation inside these samples at high temperature. These micropores would

320

gradually be filled up with the further increase of the strain, so the compression

321

strength rapidly increased. After that, the compression strength and toughness of UF

322

were dramatically lower than those of SRNP and GSRFEx. These results indicated

323

that APP possessed perfect dispersity in UF matrix and there existed compatibility to a

324

certain extent between APP and UF, thus APP could bear and disperse the stress

325

applied on the UF matrix. On the other hand, the UF chains could be free to slide past

326

one another because of hydrogen bonding interaction between APP and UF, so that the

327

SRNP or GSRFEx could absorb more compression work when compressions were

328

applied, which resulted in the stronger compression toughness of SRNP and GSRFEx

329

than that of UF. In order to obtain the variation rule of mechanical property of 16


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GSRFEx with different amount of potassium silicate, average stress at 25% of strain

331

of these samples is shown in Figure 8B. The compression strengths of samples were

332

0.25, 0.28, 0.27, and 0.26 MPa for the potassium silicate amount of 0, 3, 5, and 7 g,

333

respectively, but the differences were not significant. On the one hand, ASG, as a rigid

334

particle synthesized by the reaction of potassium silicate and ammonium dihydrogen

335

phosphate, could enhance the compression strength of UF matrix like APP. But on the

336

other hand, the decrease of compression strength of UF matrix resulted from the

337

decrease of condensation degree of UF matrix, which was because the condensation

338

processes of UF were catalyzed by acids, but the acids were neutralized by adding the

339

highly

340

GSRFEx-1.6-9 was only 0.14 MPa, significantly less than those of the other

341

treatments except for UF treatment. In addition, the reaction product was difficult to

342

solidify at room temperature, which could lead to the bad extrudates (Figure S3). This

343

was because that excessively low condensation degree of UF became the major factor

344

that influenced the compression strength of GSRFEx with the further increase of the

345

potassium silicate content. Considering from the compression strength and the

346

potassium nutrition contents, GSRFEx-1.6-7 was chosen as the optimal treatment, and

347

was used to study the slow-release behavior of GSRFEx.

alkaline

potassium

silicate.

Notably,

the

compression

strength

of

348

Slow-Release Behavior of UF, SRNP and GSRFEx in Soil. Figure 9A illustrates

349

the N release characteristics of UF, SRNP and GSRFEx in soil at room temperature. It

350

could be seen that more than 30% of the N content released within 1 day and about 58%

351

of the N content was “locked” in UF on the 56th day. Thus, for now, UF is only 17



352

applied to perennial plants, such as forests, lawns and so on just because of the long

353

release cycle of nutrient N.43-45 The N release rate from SRNP was fast in 10 days, and

354

then gradually slowed down. Additionally, the N cumulative release of SRNP only

355

reached 52.8% after 56 days. In contrast, for GSRFEx, the nutrient N was still

356

smoothly released into the soil after 10 days, and the N cumulative release was 72.1%

357

within 56 days. Therefore, most of the nutrient N in GSRFEx could be released in two

358

months, and could be widely used in various crop fields. In addition, the degradation

359

rate profiles of these fertilizers (Figure 9B) were similar to their N release

360

characteristics. After incubation in soil for 56 days, the degradation rates of UF, SRNP

361

and GSRFEx were 48.2%, 58.1% and 79.9% respectively. This result further indicated

362

that GSRFEx, with the highest degree of degradation, exhibited the optimal

363

slow-release behavior.

364

As shown in Figure 10, the slow-release mechanisms of UF, SRNP and GSRFEx

365

were explained by comparing the morphology changes before and after degradation.

366

The surface of UF before degradation was relatively smooth, and had many evenly

367

distributed rod-shaped stripes (Figure 10A), which were the evidences for the

368

existence of crystalline regions in UF. Similar morphology had already been found in

369

polycaprolactone (PCL).46 After 56 days of soil burial conditions, the overall structure

370

of UF was not destroyed, there were only a few small cracks appeared on the surface

371

(Figure 10D), which resulted from the quick dissolution of a small amount of

372

unreacted urea and oligomer of methylolurea with excellent water-solubility in UF.

373

These illustrated that the macromolecules and crystalline regions in UF could hardly 18


Page 18 of 51

Page 19 of 51


374

be hydrolyzed and biodegraded in a short time. Unlike UF, parts of needle-shaped

375

phosphate crystals and large APP crystals were deposited on the surface of SRNP or

376

GSRFEx, and APP in GSRFEx has a smaller grain size than that in SRNP (Figure 10B

377

and 10C). In addition, during the exothermic reaction of urea and ammonium

378

phosphate, a large amount of gaseous resultants such as water vapor, ammonia and

379

carbon dioxide spilt from the surface of SRNP and GSRFEx, consequently, lots of

380

void holes were formed both inside and outside of SRNP and GSRFEx. After 56 day’s

381

biodegradation, more large cavities and cracks were found on the SRNP surfaces

382

compared with UF (Figure 10E), resulting from the quick dissolution of water-soluble

383

phosphate and APP, which greatly increased the contact area between SRNP and

384

microorganisms, thus the degradation rate of SRNP was faster than that of UF. The

385

UF matrix in GSRFEx was more seriously eroded compared with SRNP, and many

386

irregular blocks were appeared at the surface of GSRFEx (Figure 10F), which were

387

comprised of the complex of ASG and hardly degradable long-chain UF, suggesting a

388

severe microbial attack. This illustrated that the degradation rate of GSRFEx was

389

higher than that of SRNP, and further confirmed the UF chains were more disorder

390

and easier to be hydrolyzed or degraded by microorganism, because of the relatively

391

short average chain length of UF and the formation of a strong inter-molecular

392

hydrogen-bonding system among ASG, APP and UF. Based on above analysis, the

393

order of erosion degree is as follows: UF < SRNP < GSRFEx, which is consistent

394

with the slow-release behavior shown in Figure 9.

395

N Use Efficiency. For the further evaluate the effects of GSRFEx on the N use 19



396

efficiency, pot experiments were also conducted. As shown in Figure 11, the growth

397

state of the rapes showed an order of GSRFEx > SRNP > UF > CK, suggesting that

398

GSRFEx could effectively provide nutrients for the growth of rapes. Detailed data

399

were summarized in Table 2. GSRFEx significantly increased fresh weight, dry

400

weight, root length, and chlorophyll content of the rape plants by 132.76%, 74.15%,

401

44.23% and 22.46%, respectively, as compared with UF, although were not

402

significantly greater than that of SRNP. This obviously confirmed that the applications

403

of GSRFEx benefited plant growth more than the applications of UF. Moreover, the

404

total N use efficiency of GSRFEx treatment was 84.14% greater than that of UF

405

treatment, confirming that the introducing APP and ASG into UF could greatly

406

improve the N use efficiency. GSRFEx shows great application prospect in modern

407

agriculture, especially in short-cycle crop field.

408

Evaluation of the Cost of GSRFEx. The synthesis of GSRFEx mainly requires

409

formaldehyde, urea, ammonium dihydrogen phosphate and potassium silicate, and

410

their cost are about $300/ton, $280/ton, 750/ton and $600/ton, respectively. According

411

to the cost of raw material and electrical energy consumption, the price of GSRFEx is

412

about $480/ton. The main slow-release fertilizers on the market are coated fertilizers

413

nowadays, for example, the price of a commercially available polymer-coated urea

414

(PCU) product produced by Shandong Kingenta Ecological Engineering Co., Ltd.,

415

China is about $2500,47 which is five times that of GSRFEx. Moreover, GSRFEx

416

formation is driven simply by conventional polycondensation and cold extrusion and

417

thus allows for facile linear scaling of the formulation from 250 mL to over 20 L 20


Page 20 of 51

Page 21 of 51


418

(Figure S4). Therefore, GSRFEx possess excellent industrial applications in the areas

419

of agriculture.

420

ASSOCIATED CONTENT

421

Supporting Information

422

Details of pot culture experiments; The internal structures of the homemade extrusion

423

device; The mesh board for granulating fertilizer in measuring its mechanical strength;

424

The cured products and extrudates of GSRFEx-1.6-9; Large-batch preparation of

425

GSRFEx.

426

AUTHOR INFORMATION

427

Corresponding Author

428

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

429

[email protected] (G.Z. Zhao)

430

Notes

431

The authors declare no competing financial interest.

432

ABBREVIATIONS USED

433

GSRFEx, granular slow-release fertilizer prepared by extrusion; SRNP, slow-release

434

fertilizer containing N and P prepared by the interaction of urea-formaldehyde and

435

ammonium polyphosphate; UF, urea-formaldehyde; APP, ammonium polyphosphate;

436

ASG, amorphous silica gel; FTIR, Fourier transform infrared; XPS, X-ray

437

photoelectron spectrometer; XRD, X-ray diffractometer; TGA, thermogravimetric

438

analyzer; SEM, scanning electron microscope; EDX, energy dispersive X-ray

439

spectroscopy 21



440

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J. Agric. Food. Chem. 1967, 15, 967-971.

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574

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28


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Figure captions Figure 1. Synthesis process of GSRFEx. Figure 2. SEM images (A and B), EDX spectra (C) and Distribution maps (D) of GSRFEx. Figure 3. FTIR spectra of ASG (a), APP (b), UF (c), SRNP (d) and GSRFEx (e) in the region of 600-4000 cm-1 (A) and 600-1200 cm-1 (B). Figure 4. XPS spectra of UF (a), SRNP (b) and GSRFEx (c). Figure 5. XRD patterns of urea (a), UF (b), ASG (c), APP (d), SRNP (e), and GSRFEx (f). Figure 6. TG curves (A) and DTG curves (B) of ASG, APP, UF, SRNP, and GSRFEx. Figure 7. Torque curves (A), Corresponding digital photographs (B) and water content (C) of GSRFEx-m-n (m and n denote the U/F mole ratio and the amount of potassium silicate, respectively) after being extruded at room temperature. Figure 8. Stress-strain curves (A) and average stress (B) at 25% of strain of UF, SRNP and GSRFEx-m-n. Different letters mean significant difference between treatments (the Duncan's multiple range tests; P < 0.05). Figure 9. Nitrogen cumulative release rate (A) and degradation rate (B) of UF, SRNP and GSRFEx in soil. Figure 10. SEM images of UF (A), SRNP (B) and GSRFEx (C) before degradation (0 day) and UF (D), SRNP (E) and GSRFEx (F) after 56 days of degradation in soil. Figure 11. Digital photographs of the rapes treated with the different fertilizers.

29



Page 30 of 51

Table 1 Characteristics of the Slow-Release Fertilizers characteristics

UF

nitrogen content (%)

40.83

SRNP

phosphorus (P2O5) content (%)

GSRFEx

36.02

34.76

13.19

11.01

potassium (K2O) content (%)

2.12

diameter of sample (mm)

3

3

3

length of sample (mm)

3-10

3-10

3-10

Table 2 Fresh Weight, Dry Weight, Root Length, Chlorophyll Content, N Uptake, and Total N Use Efficiency of rapes under Different Fertilizer Treatmentsa fertilizer

fresh

dry

root

chlorophyll

N uptake

total N

treatment

weight (g

weight (g

length

content

(mg

Use

plant-1)

plant-1)

(cm

(mg g-1

plant-1)

efficiency

plant-1)

plant-1)

(%)

CK

2.89 c

0.15 c

6.64 c

1.05 c

6.00 d

UF

22.80 b

1.47 b

19.08 b

1.38 b

57.55 c

34.37 c

SRNP

46.41 a

2.23 a

25.44 a

1.67 a

90.48 b

56.32 b

GSRFEx

53.07 a

2.56 a

27.52 a

1.69 a

100.94 a

63.29 a

a

Means within the same column followed by different letters are significantly different

(the Duncan's multiple range tests; P < 0.05).

30


Page 31 of 51


Scheme 1. Synthesis mechanisms of UF, APP and ASG.

Scheme 2. Schematic illustration of the structure change of product in the cold extrusion (A) and the structure of final product GSRFEx (B).

31



Figure 1. Synthesis process of GSRFEx.

Figure 2. SEM images (A and B), EDX spectra (C) and Distribution maps (D) of GSRFEx.

32


Page 32 of 51

Page 33 of 51


Figure 3. FTIR spectra of ASG (a), APP (b), UF (c), SRNP (d) and GSRFEx (e) in the region of 600-4000 cm-1 (A) and 600-1200 cm-1 (B).

Figure 4. XPS spectra of UF (a), SRNP (b) and GSRFEx (c).

33



Figure 5. XRD patterns of urea (a), UF (b), ASG (c), APP (d), SRNP (e), and GSRFEx (f).

Figure 6. TG curves (A) and DTG curves (B) of ASG, APP, UF, SRNP, and GSRFEx.

Figure 7. Torque curves (A), Corresponding digital photographs (B) and water content (C) of GSRFEx-m-n (m and n denote the U/F mole ratio and the amount of potassium silicate, respectively) after being extruded at room temperature.

Figure 8. Stress-strain curves (A) and average stress (B) at 25% of strain of UF, SRNP 34


Page 34 of 51

Page 35 of 51


and GSRFEx-m-n. Different letters mean significant difference between treatments (the Duncan's multiple range tests; P < 0.05).

Figure 9. Nitrogen cumulative release rate (A) and degradation rate (B) of UF, SRNP and GSRFEx in soil.

Figure 10. SEM images of UF (A), SRNP (B) and GSRFEx (C) before degradation (0 day) and UF (D), SRNP (E) and GSRFEx (F) after 56 days of degradation in soil.

35



Figure 11. Digital photographs of the rapes treated with the different fertilizers.

36


Page 36 of 51

Page 37 of 51


Graphic for table of contents

37



Scheme 1. Synthesis mechanisms of UF, APP and ASG. 81x78mm (600 x 600 DPI)


Page 38 of 51

Page 39 of 51


Scheme 2. Schematic illustration of the structure change of product in the cold extrusion (A) and the structure of final product GSRFEx (B). 177x63mm (300 x 300 DPI)



Figure 1. Synthesis process of GSRFEx. 84x71mm (300 x 300 DPI)


Page 40 of 51

Page 41 of 51


Figure 2. SEM images (A and B), EDX spectra (C) and Distribution maps (D) of GSRFEx. 137x103mm (300 x 300 DPI)



Figure 3. FTIR spectra of ASG (a), APP (b), UF (c), SRNP (d) and GSRFEx (e) in the region of 600-4000 cm1 (A) and 600-1200 cm-1 (B). 176x69mm (300 x 300 DPI)


Page 42 of 51

Page 43 of 51


Figure 4. XPS spectra of UF (a), SRNP (b) and GSRFEx (c). 84x66mm (300 x 300 DPI)



Figure 5. XRD patterns of urea (a), UF (b), ASG (c), APP (d), SRNP (e), and GSRFEx (f). 84x66mm (300 x 300 DPI)


Page 44 of 51

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Figure 6. TG curves (A) and DTG curves (B) of ASG, APP, UF, SRNP, and GSRFEx. 176x68mm (300 x 300 DPI)



Figure 7. Torque curves (A), Corresponding digital photographs (B) and water content (C) of GSRFEx-m-n (m and n denote the U/F mole ratio and the amount of potassium silicate, respectively) after being extruded at room temperature. 176x54mm (300 x 300 DPI)


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Figure 8. Stress-strain curves (A) and average stress (B) at 25% of strain of UF, SRNP and GSRFEx-m-n. Different letters mean significant difference between treatments (the Duncan's multiple range tests; P < 0.05). 171x63mm (300 x 300 DPI)



Figure 9. Nitrogen cumulative release rate (A) and degradation rate (B) of UF, SRNP and GSRFEx in soil. 176x66mm (300 x 300 DPI)


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Figure 10. SEM images of UF (A), SRNP (B) and GSRFEx (C) before degradation (0 day) and UF (D), SRNP (E) and GSRFEx (F) after 56 days of degradation in soil. 173x86mm (300 x 300 DPI)



Figure 11. Digital photographs of the rapes treated with the different fertilizers. 71x91mm (300 x 300 DPI)


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Graphic for table of contents 82x45mm (300 x 300 DPI)


Granular Slow-Release Fertilizer from Urea-formaldehyde, Ammonium

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