Water and fertilizer integrated hydrogel derived from the

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Agricultural and Environmental Chemistry

Water and fertilizer integrated hydrogel derived from the polymerization of acrylic acid and urea as a slow release N fertilizer and water retention in agriculture Dongdong Cheng, Yan Liu, Guiting Yang, and Aiping Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00872 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Journal of Agricultural and Food Chemistry

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Water and fertilizer integrated hydrogel derived from the polymerization of

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acrylic acid and urea as a slow release N fertilizer and water retention in

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agriculture

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Authors: Dongdong Cheng1,2,*, Yan Liu1, Guiting Yang1, Aiping Zhang3

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Affiliations:

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1

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Resources, National Engineering & Technology Research Center for Slow and

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Controlled Release Fertilizers, College of Resources and Environment, Shandong

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Agricultural University, Tai’an, Shandong 271018, China

National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer

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2

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Kingenta Ecological Engineering Co., Ltd, Linyi, Shandong 276700, China

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3

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of Agricultural Sciences, Beijing, 100081, China

State Key Laboratory of Nutrition Resources Integrated Utilization, Shandong

Institute of agricultural environment and sustainable development, Chinese Academy

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*Corresponding authors: Dongdong Cheng

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Telephone: +86-538-8241531

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E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT:

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To reduce the preparation cost of superabsorbent and improve the N release rate at the

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same time, a novel low cost superabsorbent (SA) with the function of N slow release

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was prepared by chemical synthesis with neutralized acrylic acid (AA), urea,

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potassium persulfate (KPS) and N,N'-methylenebisacrylamide (MBA). The influence

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order of influence factors on water absorbency property was determined by

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orthogonal L18(3)7 experiment. Based on the optimization results of orthogonal

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experiment, the effects of single factor on the water absorption were investigated, and

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the highest water absorbency (909 g/g) was achieved on the conditions: 1.0 mol/mol

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urea to AA mole ratio, 100% of AA neutralized, K+, 1.5% KPS to AA mass fraction,

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0.02% MBA to AA mass fraction, 45 oC reaction temperature and 4.0 h reaction time.

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The optimal sample was characterized by scanning electron microscopy (SEM) and

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Fourier transform infrared spectroscopy (FTIR). Swelling behaviors of the

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superabsorbent were investigated in distilled water, various soil and salt solution. The

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water release kinetics of SA in different negative pressure and soil were

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systematically investigated. Additionally, the maize seed germination in various types

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of soil with different amount of SA was proposed, and the N could release 3.71% after

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being incubated in distilled water for 40 days. After 192 h, the relative water content

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of SA-treated sandy soil, loam and paddy soil was 42%, 56%, and 45%, respectively.

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All the results in this work showed that SA had good water retention and N slow

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release properties, which are expected to have potential application in sustainable

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modern agriculture. 2


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KEY WORDS: Integrated water and fertilizer; Superabsorbent; Urea; Acrylic acid

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INTRODUCTION

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Water is one of the main factors that improve the crop production. However, due to

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rising water and irrigation costs, drought and water shortage have been puzzling the

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world agricultural development. According to estimates, 84% of cultivated area in the

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1 billion 440 million world’s arable land was dry cropland, the loss of agricultural

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production caused by drought and water shortage every year in the world amounts to

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more than the sum of the losses caused by other factors,1 hence it is important and

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necessary to use water resource efficiently. In the past few years, the study of

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superabsorbent (SA) as water management materials for agricultural and horticulture

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applications has received increasing attention.2-4 Practical applications are also

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indicated that superabsorbent polymer displays a promising future for the applications

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of increasing the survival rate and simultaneously alleviating the drought stress effects

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on the plants in arid and semi-arid areas.5-7

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At the same time, nutrient is another important factor which limits the growth

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and yield of crops.8 Nitrogen (N) is a key element in plant nutrition and greatly affects

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the crop yield. However, the utilization rate of N-based fertilizer is relatively low, and

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only about 30-50%.9 Inefficient nitrogen absorption increases farmers’ input costs and

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brings many environmental problems. So it is a major challenge to improve the use

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efficiency of nitrogen, to reduce farmers’ input costs, and to decrease the

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environmental impact of N losses while to maintain the crop yield. These 3



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shortcomings can be overcome by using the slow release fertilizer (SRFs), which

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release fertilizers to plants gradually at a certain rate to coincide with the plant’s

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nutritional requirement and reduce fertilizer loss simultaneously.10, 11 Water scarcity,

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environmental pollution caused by excessive application of fertilizers, and the high

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costs of irrigation and fertilizers, demand for greater increase than ever in grain yield

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of crop with less water and less fertilizers in China.12 Thus, there is an increasing need

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to develop water-saving and N-fertilizer efficient technologies for economical and

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environment-friendly production of crop. The water-holding capacity and nutrient

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retention of sandy soils can be improved via the combination of superabsorbent and

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SRFs. The aeration and microbial activity of soil can be increased, the influence of

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water-soluble fertilizers on the environmental will be mitigated, and the frequency of

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irrigation can be lowered at the same time.13 Currently, the multifunctional slow

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release fertilizer with water conservation and nutrient release is prepared by coating a

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water absorbing resin in the out of the coated fertilizer. 14-16 However, the coating

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process is limited by the surface characteristics of fertilizer granules, coating material

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and production process, which is bound to result in complex processes, rising costs

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and limiting their use in most high-value crops.17 Therefore, it is necessary to prepare

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materials with retention and fertilizer release function from other ways. Chemical

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reaction grafting is a simple material multifunctional method, if the graft reaction

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between fertilizer and water absorbent will be carried out, it will solve the cost

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problem caused by the expensive process and material.

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Nowadays, most of super water absorbents (SA) are obtained by solution or 4


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inverse-suspension polymerization techniques from acrylic acid, its salts, and

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acrylamide (AM).18 The above raw materials are not only expensive, but the

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acrylamide is harmful to human health.19 It is well known that superabsorbent can

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absorb hundred times of its own weight because of the chemical or physical

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crosslinkings of individual polymer chain.20-22 The highly water-absorptive polymer

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possesses linear structure, which contains many strong water absorption groups

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including -COO-, -OH, -NH2.21-23 Therefore, increasing the number of water

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absorbent groups is an important way to increase the water absorption of a material.

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Urea is a low-cost material with amide structure. It has excellent hydrophilicity and is

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also a major source of crop nitrogen. The price of urea is just 1/10 to 1/15 that of

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acrylic amide, showing potential application in water retention materials with nitrogen

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sustained release function. However, urea is a hydrophilic substance with small

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molecule that can be quickly dissolved in water. Therefore, it should be modified to

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be employed as a high water absorbent resin. Herein, a low cost super absorbent

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hydrogel (SA) was prepared by crosslinking reaction of acrylicacid (AA) and urea

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with potassium sulfate and N,N’-methylenebis acrylamide (MBA) as initiator and

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cross-linker. The synthetic conditions and properties of SA were studied to find out

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the optimal synthesis parameters. This work not only innovated the theory and

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method of “water and fertilizer integration”, but also achieved simultaneous slow

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release of water and fertilizer, so as to achieve the goal of water and fertilizer

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cooperation in farmland.

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MATERIALS AND METHODS 5



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Materials. Acrylic acid (AA, chemical purity, Sinopharm Chemical Reagent Co., Ltd,

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Shanghai, China) was used after vacuum distillation. Potassium persulfate (KPS) and

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N,N'-methylenebisacrylamide (MBA, chemical purity, Tianjin Kermel Chemical

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Reagent Co., Ltd., Tianjin, China) were used as received. NaOH, KOH, and urea

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(analytical grade, Tianjin yongda Chemical Co., Ltd., China) were used as received.

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Other agents were all in analytical grade, and the solutions were prepared in distilled

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water.

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Preparation of superabsorbent. Figure 1 shows the schematic illusion of

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preparation of the low cost superabsorbent. A series of superabsorbent with different

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water absorption capacity were prepared as follows: Initially, 100 mL of AA was

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neutralized with 20 wt% KOH or 20 wt% NaOH solution in a 1 L glass beaker under

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ice bath condition and mixed thoroughly. Secondly, an appropriate amount of urea and

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N,N'-methylenebisacrylamide (MBA) were added in the beaker with stirring until

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dissolved. After that, all the mixed solution was transferred into a 250 mL three-neck

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flask equipped with a mechanical stirrer, nitrogen line and reflux condenser. The

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reactor was immersed in an oil bath. Before adding the initiator, the oxygen free

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nitrogen gas was bubbled into the solution for 30 min. Then, an aqueous solution with

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a certain amount of KPS was added. The oil bath was kept at a target temperature to

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complete the polymerization process with a prescriptive reaction time. After reaction,

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the product was dried to constant weight at 85 oC. All samples used for tests were

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grind into particles with diameters between 40 and 70 mm.

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Fig. 1…………………………………………………………. 6


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Determination of water absorbency. A series of weighed dried samples (0.1±

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0.0001 g) were immersed in over distilled water to achieve swelling equilibrium at

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room temperature (about 30 min). Then, swollen samples (M2) were filtered through a

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100-mesh nylon net and suspended for 10 min until there was no free water on the

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surface. The equilibrium water absorbency (Qeq, g/g) was calculated according to the

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following equation:

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Qeq 

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M 2  M1 M1

(1)

Where Qeq (g/g) is the water absorbency per gram of dried sample; M1 (g) and

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M2 (g) are the weights of the dry and the swollen sample, respectively.

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Measurement of water retention in different soil types. The three types of soils are

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sandy loam, loam, paddy soil (Table1). 0.5 g of SA and 50 g of dry soil (below10

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mesh) were fully mixed and placed in a plastic cup, and then 417 mL of distilled

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water was slowly dripped into the beaker and weighed (W1). The controlled

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experiment was also carried out (W2) without SA. The beaker was kept at 25 oC and

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weighed (Wt) at selected time points (hours 0, 24, 48, 72, 96, 120, 168, 192). All the

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water holding tests were conducted three times, and the average value was used for

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plotting. The water retention (WR%) of soil was calculated from Equation (2):

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WR% 

Wt  W2  100% W1  W2

(2)

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Where WR% is the soil water retention; W1 and Wt are the total mass of 0.5 g SA,

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soil, plastic cup and water content of the different treatment before and after soil

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culture, respectively, and W2 is the total mass of the control (no SA). 7



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Table 1 ………………………………………………………….

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Measurement of water retention in different negative pressure. 0.1 g of dried SA

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was maintained in 250 mL of distilled water for swelling equilibrium at room

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temperature. The swollen sample was weighed (W) and then placed in a pressure

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membrane instrument and weighed at various negative pressures. The water retention

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ratio at different negative pressures was calculated by Equation (3):

WR%  1 -

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W  Wp W

100%

(3)

Where WR% is the water retention percentage and Wp is the mass of the sample

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at negative pressure p.

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Water absorbency in different salt solutions. The water absorbency in different salt

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solution (KH2PO4, K2SO4, NH4Cl) with a concentration of 0.2%, 0.6%, and 1.0%

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were tested according to the same procedures described in Section 2.3.

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Characterization. FTIR spectra were recorded on a NEXUS-470 series FTIR

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spectrometer (Thermo Nicolet, NEXUS). The samples were initially dried under

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vacuum until they reached a constant weight. Later on, the dried samples were ground

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into powder, mixed with KBr powder, and then pressed to make a pellet for FTIR

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characterization. SEM images were taken on a JSM-5600LV equipment after the

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sample was coated with gold film for good conductivity.

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Growth experiments. Sandy loam, loam, and paddy soil (＜0.20 mm) were used as

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the growth medium. Corn seeds with similar sizes germinated in a growth chamber

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(GXZ-SMART, Ningbo Jiangnan Instrument Co., China) at 25 oC and grown in soil

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for 10 days. In each soil, four different concentrations of SA (0%, 0.2%, 0.5%, and 8


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1.0%) were applied to the germination test, and only distilled water was added to soil.

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For each test, 100 g of soil and the designed SA were well mixed and packed in a 100

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mL plastic cup with one corn seed placed 1 cm away from top layer. Another 50 mL

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of distilled water was then added to the container to saturate the soil. The plants were

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grown for 10 days. The roots and seedling were washed carefully with distilled water

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and dried with bibulous paper, and then the length was measured. Each treatment was

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repeated three times.

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Urea release in water. The kinetic of urea release in water at 25oC was examined

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according to the previous reports.24 Briefly, 1.0 g of SA was embedded into a tea bag

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and then immersed in 1 L distilled water. After each incubated period (days 1, 3, 5, 7,

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10, 20, 30, 40), 10 mL solution was sampled for N release determination, and the

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other 10 mL distilled water was added into the release medium to maintain a constant

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volume of the solvent. The release experiments were conducted in triplicate and

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average value was taken as the result.

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RESULTS AND DISCUSSION

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Optimization of SA absorption synthesis condition. Superabsorbent (SA)

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synthesized from urea and acrylic acid under different conditions exhibited different

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water absorbencies properties due to their different structures. To seek the suitable

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conditions of SA synthesis, orthogonal experiments with seven factors and three

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levels were set up. “A” indicates the molar ratio of urea to AA, and there are three

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levels of 0.4, 1.0, 1.6 mol/mol. “B”, “C”, “D”, “E”, “F” and “G” stand for the

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percentage of AA neutralized by 20 wt% alkaline solution, cationic species, KPS to 9



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AA mass fraction, MBA to AA mass fraction and reaction temperature, respectively.

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According to the orthogonal table of L18(3)7 shown in Table 2, the following

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experiments were carried out (Table 3) and the range analysis was exhibited in Table 4.

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The result of the range analysis showed that the order of influencing factors on water

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absorbency was E＞B＞F＞A＞C＞D＞G. MBA to AA mass fraction is the most

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influential factor, the percentage of AA neutralization degree followed, and then

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reaction temperature, urea to AA mole ratio, cationic species, KPS to AA mass

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fraction and reaction time are the last. According to the orthogonal experiment results,

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the best combination of synthesis condition is A2B3C2D2E1F1G2, and the

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corresponding conditions are as follows: 1.0 mol/mol urea to AA mole ratio, 100% of

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AA neutralized, K+, 2.5% KPS to AA mass fraction, 0.02% MBA to AA mass fraction,

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45 oC reaction temperature and 4.0 h reaction time.

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Table 2………………………………………………………….

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Table 3………………………………………………………….

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Table 4………………………………………………………….

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Effect of the quality ratio of MBA to AA on water absorbency. In order to get the

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product with highest water absorption, the effect of single factor on water absorbent

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performance of SA was studied by changing the single factor based on the

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optimization results of orthogonal tests. As described by the Flory’s network theory,

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the degree of cross-linking is an important factor in controlling swelling.25 The

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properties of superabsorbent will face great change when a relatively small changes

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exist in cross-linking density. In view of the water absorbency of SA, the relationship 10


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between water absorbency and mass ratio of MBA to AA was studied. As the mass

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ratio of MBA and AA increased from 0.005% to 0.08%, the water absorption

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decreased significantly (Figure 2B). It is generally known that the cross-linking

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density is determined by the amount of cross-linker. Obviously, higher MBA levels

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produce more cross-linking, which also causes additional network formation. These

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networks will further reduce the available free volume inside the superabsorbent

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polymer and reduce the water absorption rate even more difficult. When the weight

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ratio of MBA to AA was less than 0.005 wt%, the absorbency of superabsorbent was

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very low. The less cross-linking point not only decreased the cross-linking density, but

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also leaded to the increase of soluble material. Similar observations were also

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previously reported by Riedel et al.26 and Li et al.27

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Fig. 2………………………………………………………….

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Effect of neutralization degree of AA on water absorbency. The influence of

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neutralization degree of acrylic acid (AA) on the water absorbency in distilled water

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is illustrated in Figure 2C. The results showed that the water absorbency of SA

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increased with the neutralization degree of AA increasing from 75% to 85%, and then

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decreased with the neutralization degree further increasing to 100%. The optimal

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water absorption of SA was 504 g/g when the degree of AA neutralization reached

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85%. The above phenomena can be explained as follows. At the low degree of

239

neutralization, the number of strong hydrophilic groups -COOK increased with the

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increase of neutralization degree. When such -COOK groups dissociate, the carboxyl

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group with a negative charge is suspended on the polymer chain and the electrostatic 11



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repulsion is established, leading to expand of the network. At the same time, the

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concentration of ion in polymers network structure increased and the osmotic pressure

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rose, which was conducive to the infiltration of water molecules into the network

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structure to improve the water absorbency. However, when the neutralization degree

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exceeded 85%, there were abundant and strong hydrogen bonds between water

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molecules and ionic polymers. The water molecules had specific spatial orientation,

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and adjacent hydrogen bonds interfered with each other due to the directional

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hydrogen bonds. Additionally, the electrostatic repulsion of contiguous -COO- groups

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limited the free movement of bonds, and weakened the storage capacity of polymers

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microspore structure. The water absorbency of hydrogels was reduced due to the

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resulted dense network structure.

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Effect of reaction temperature on water absorbency. With the other parameters

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unchanged, the influence of reaction temperature from 25 to 75 oC was investigated.

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Figure 2D shows that the water absorbency increased significantly when temperature

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increased from 25 to 45 oC, and then decreased relatively with the temperature further

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increasing. When the reaction begins, the chain formed by the polymerization is short

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chain, which is bad for water absorption. On the other hand, a long time reaction will

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lead to an increase in the degree of polymer cross-linking. Multiple branched chains

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were formed in the network structure, and they tangled with each other to produce

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small holes in three-dimensional networks, obstructing the expansion of polymer.

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Hence, too long or too short reaction time resulted in a low water absorbency.28

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Effect of the ratio of urea to AA on water absorbency. Different components and 12


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monomer ratio have obvious influence on the properties of superabsorbent resin. To

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increase the absorption performance and reduce the manufacturing costs of the

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superabsorbent, urea was introduced as a non-ionic water absorbent monomer. At the

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same time, as another monomer, AA was used to improve the water absorbency ability

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(Qeq). It can be seen in Figure 2E that the maximum value of Qeq was obtained when

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the mole ratio of urea/AA was 1.6. An initial increase of the Qeq was apparently

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observed when the mole ratio was less than 1.6. On the contrary, the Qeq decreased

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when the mole ratio was greater than 1.6. Such phenomenon was caused by the

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cooperation effect of the two monomers. Urea can improve the water ability of

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superabsorbent, which result in the increase of Qeq in water at the initial stage.

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However, AA has better hydrophilicity than urea, the Qeq decreased with the urea

275

further increasing.

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Effect of the initiator amount on water absorbency. The water absorption initially

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increased and then decreased with the increase of initiator dosage (Figure 2F). The

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maximum absorbency in distilled water could reach 909 g/g. It was found that the

279

amount of initiator had an obvious effect on water absorbency of hydrogels, which

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was consistent with the relationship between the initiator concentration and average

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chain length during the polymerization process. AA monomers could be polymerized

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because the KPS initiator could generate many free-radical reactive sites. When the

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dosage of KPS initiator was low, the SA superabsorbent with a three-dimensional

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structure was formed and the water absorbency was improved. However, excess KPS

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initiator accelerated AA self-polymerization, but it also reduced the amount of 13



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reaction between AA and urea. In the meantime, with the increase of the amount of

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KPS initiator, the three-dimensional structure of SA became denser, resulting in the

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decrease of water absorbency. From above single factor experiments, it can be

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concluded that the optimal conditions with maximum water absorbency (909 g/g)

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were found as follows: 1.0 mol/mol urea to AA mole ratio, 100% of AA neutralized,

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K+, 1.5% KPS to AA mass fraction, 0.02% MBA to AA mass fraction, 45 oC reaction

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temperature and 4.0 h reaction time.

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FT-IR measurements. Figure 2A shows the FTIR spectra of urea, SA, and MBA.

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The characteristic peaks in the urea spectra are ascribed as following: 3440 and 1687

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cm-1 could be assigned to the -NH2 and C=O stretching of -CONH2. For MBA, the

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peak of 3300 cm-1 corresponded to C=C stretching vibration. For SA, the

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characteristic peaks of urea was observed (1669 cm-1 for the -C=O stretching of

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-CONH2, 1160 cm-1 for the bending of the amide bands), and the active hydrogen of

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-NH2 in 3455 cm-1 was obviously weakened. The appearance of these bands confirms

300

the formation of SA material.

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Morphological analyses. In order to identify the surface and internal structure of SA,

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the scanning electron microscope (SEM) technique has been applied. The SEM image

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of SA (Figure 3(a)) showed that a large number of pore structures with sizes lower

304

than 0.1 mm were uniformly distributed in them. These porous structures were very

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favorable for the absorption and retention of water. Besides, it could be found that the

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interior was filled with three-dimensional polymeric networks in SA (Figure3(b)). The

307

highly porous structures with interlinked channels formed by urea, AA, and MBA are 14


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beneficial for more water molecules diffusing into the network of SA, leading to a

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higher swelling ratio.

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Fig. 3………………………………………………………….

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Water retention at various negative pressures. The water absorbed and transported

312

by plant was powered by root pressure and negative pressure of transpiration.29

313

Therefore, only the water absorbed in a certain negative pressure range is effective for

314

plants, not all water can be absorbed by plants. Figure 4 displayed the relationship

315

between negative pressure and water retention of SA. With the increase of negative

316

pressure, the moisture absorbed by SA gradually released. The water retention of A, B,

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C was 42.6 wt%, 29.9 wt% and 62.8 wt% at 0.1 MPa, respectively. When the negative

318

pressure was more than 0.3 MPa, the downward trend of the water retention of A, B,

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C became slow with increasing of negative pressure. According to the previous

320

literature,30 the optimum soil water potential is less 0.8 MPa for plant growing. It can

321

be found that the effective water content, that is, the water that can be used by plants

322

for A, B, C, were 94.3 wt%, 95.6 wt%, 72.4 wt%, respectively. Considering above,

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the product B has excellent water retention properties and important application valve,

324

and has broad application prospects in agriculture.

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Fig. 4………………………………………………………….

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Water absorbency in different salt solutions. The effect of various ions on water

327

absorbency can be concluded from Figure 5. The water absorption rate significantly

328

decreased when the salt concentration of the solutions increased from 0.2% to 1.0%,

329

and gently decreased in the concentration higher than 0.6%. For ionic hydrogels, the 15



330

additional cations caused an anion-anion electrostatic repulsion, which lead to a

331

decrease in osmotic pressure between the external solution and the polymer network.

332

So the swelling decreased.31 Secondly, the order of water absorbency of the hydrogel

333

in the salt solutions followed KH2PO4 ＞ K2SO4 ＞ NH4Cl when the solution

334

concentrations were lower than 1.0 wt%. This was due to the water absorbency in

335

polyatomic monovalent cationic (NH4+) solution decreased more than that in single

336

atom monovalent cationic (K+) solution, and the sensitivity of superabsorbent to

337

various cations was PO43-< SO42-< Cl-.28

338

Fig. 5………………………………………………………….

339

Water retention in various soils. The water retention property of superabsorbent

340

used in agriculture and horticulture is a key property for soil water conservation. It

341

has positive effects on improving soil quality, raising the survival rate of seedlings

342

and promoting the growth of plants. In order to invest the water retention effect of SA

343

in different kind of soil, the experiments of water retaining agent were carried out in

344

three types of soil (sandy loam, loam, paddy soil). Figure 6 presents the water

345

absorbency of the SA in sandy loam, loam and paddy soil, standing for the typical soil

346

in different regions. It can be found that the addition of SA into sandy loam, loam and

347

paddy soil could significantly improve the water holding capacity and water-retention

348

properties of soil. After 192 h, the relative water content of sandy soil, loam and

349

paddy soil treated with SA was 42%, 56% and 45%, respectively. However, the

350

relative water content of soils (sandy loam, loam, paddy soil) without SA were only

351

2%, 18% and 4%. 16


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Fig. 6………………………………………………………….

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Effect of SA on maize germination. SA has a significant influence on the

354

development of maize seeds. Measurements of the seeding lengths showed the SA

355

treatment with less 0.2% content promoted root growth (Figure 7B). The average

356

seedling lengths of maize seeds treated by the SA were significantly larger than that of

357

the control groups, not only the sandy loam group but also the loam and paddy soil.

358

Among all the groups, the 0.2% treatments showed the longest seedling height (Figure

359

7A), which was almost twice of that of the 0% and 0.5%. Besides, it can be found that

360

when soil SA content exceeded 1%, maize seeds germinated only in loam soil (Figure

361

7). SA is a new type of water-retention slow release fertilizer containing a large

362

amount of nitrogen. SA releases nitrogen slowly after it is applied to the soil.

363

Therefore, more nitrogen would release into soil solution when the amount of SA

364

increased, and the high nitrogen concentration of the soil inhibited the growth of crop

365

roots. These results indicated that low concentration of SA (﹤0.2%) promote the

366

embryo development of the corn seeds.

367

Fig. 7………………………………………………………….

368

Slow release urea behavior of SA. One of the important properties of SA prepared

369

was its N sustained release performance. Figure 8 represented the N slow release

370

behavior of SA in water. It can be found that the N release curve approximates the

371

straight line, and N in SA released 0.18, 0.37, 0.58, 0.81, 1.06, 1.59, 2.67, 3.71 wt%

372

with in 1, 3, 5, 7, 10, 20, 30, 40 days, respectively. A release value of 3.71% for

373

nutrient N could be reached after being incubated in distilled water for 40 days. This 17



374

indicated that the N in SA released very slow, could be applicable for the crops with a

375

long duration of fertilizer.

376

Currently, the most of slow release fertilizer was prepared by coating common

377

granule fertilizer to isolate the moisture outside the membrane and reduce water

378

dissolution. SA was prepared through chemical synthesis to slow release dissolution.

379

The mechanism of SA was totally different from that of coated fertilizer. The nutrient

380

release mechanism of SA could be illustrated as follows: (1) under the action of water

381

absorbing group, the SA adsorbed water molecules and slowly swollen after it was

382

added into distilled water. Thereafter, the three-dimensional network structure of SA

383

became larger and the pores were filled with free water until the maximum water

384

absorption rate was reached. (2) The urea molecule was gradually hydrolyzed from

385

the molecular chain and dissolved in the three-dimensional network pores. The

386

dissolved urea would be slowly diffused out the network pores through the dynamic

387

exchange of free water.

388

Fig. 8………………………………………………………….

389 390 391 392 393 394 395 18


Page 18 of 39

Page 19 of 39


396 397

AUTHOR INFORMATION

398

Corresponding author

399

*Tel/Fax: +86-538-8241531. Email: [email protected]

400

ORCID

401

Dongdong Cheng: https://orcid.org/0000-0003-2939-1415

402

Notes

403

The authors declare no competing financial interest.

404

ACKNOWLEDGEMENTS

405

This study was supported by The National Key Research and Development Program

406

of China (2016YFD0201105, 2017YFD0200702, SQ2017ZY060105-06), the Natural

407

Foundation of Shandong Provice (ZR2017BC091), Shandong Quan Lin Jia Fertilizer

408

Co., Ltd., STANLEY fund (010-380261; QL2016-4).

409 410 411 412 413 414 415 416 417 19



418

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sustainable intensification improved nitrogen use efficiency and maintained high crop

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yield during 1980-2014 in Northern China. Sci. Total. Environ. 2017, 596, 61-68.

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boron fertilizer from a functional superabsorbent formulation based on wheat straw

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composites derived from corn stover and feather meals as double-coating materials

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Arias-Marin, E. Synthesis and swelling characteristics of semi-interpenetrating

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properties of a coated slow-release and water-retention biuret phosphoramide fertilizer

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with superabsorbent. J. Agr. Food. Chem. 2010, 59, 322-327.

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(21) Bansiwal, A.K.; Rayalu, S.S.; Labhasetwar, N.K.; Juwarkar, A.A.; Devotta, S.

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Surfactant-modified zeolite as a slow release fertilizer for phosphorus. J. Agr. Food.

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Chem. 2006, 54, 4773-4779.

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(22) Teodorescu, M. Lungu, A. Stanescu, P.O. Neamtu, C. Preparation and properties

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monomer prepared by a Baylis-Hillman reaction. Eur. Polym. J. 2013, 49, 1662-1672.

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(24) Tomaszewska, M. Jarosiewicz, A. Use of polysulfone in controlled-release NPK

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fertilizer formulations. J. Agric. Food. Chem. 2002, 50, 4634-4639.

490

(25) Flory, P.J. Principles of polymer chemistry; Cornell University Press: Ithaca, NY,

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1953.

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(26) Riedel, M. Relationship between water absorbency and reaction conditions in

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aqueous solution polymerization of polyacrylate superabsorbents. J. Appl. Polym. Sci.

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2000, 75, 808-814.

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superabsorbent composite. I. Synthesis and characterization. J. Appl. Polym. Sci. 2004,

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498

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characterization, and swelling behaviors of salt-sensitive maize bran-poly(acrylic acid)

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superabsorbent hydrogel. J. Agric. Food. Chem. 2014, 62, 8867-8874.

501

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roots at the micro- and macro-scale. J. Exp. Bot. 2015, 66, 2145-2154.

503

(30) Campbell, G.S. Soil water potential measurement: An overview, Irrigation. Sci.

504

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505

(31) Zhao, Y.; Su, H.; Fang, L.; Tan, T. Superabsorbent hydrogels from poly(aspartic 23



506

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507

5368-5376.

508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 24


Page 24 of 39

Page 25 of 39


528

FIGURE CAPTIONS

529

Figure 1. Synthesis schematic of SA.

530

Figure 2. (A) FI-IR Spectra of SA, urea and MBA; Influence of different factors on water

531

absorption: (B) the quality ratio of MBA:AA(g/g), (C) neutralization degree of AA (%), (D)

532

temperature, (E) urea:AA (mol), (F) initiator content.

533

Figure 3. SEM micrograph of (a) surface and (b) internal structure.

534

Figure 4. Water retention at various negative pressures.

535

(The water absorbency in the distilled water：A-909 g/g; B-885 g/g; C-835 g/g)

536

Figure 5. Water absorbency in different salt solutions.

537

Figure 6. Daily water retention property of sandy loam, loam and paddy soil amended with SA.

538

Figure 7. Effect of SA content in different tyepes of soil on seedling height (A) and root length (B)

539

in maize.

540

Figure 8. Slow release urea behavior of SA.

541 542 543 544 545 546 547 548 549 25



Page 26 of 39

TABLES 550

Table 1. Soil texture Soil types

Field capacity (%)

Clay (g/kg)

Sand (g/kg)

Silt (g/kg)

Loam

17.72

185

191

624

Sandy loam

6.27

61

793

146

Paddy soil

28.83

341

321

338

551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 26


Page 27 of 39


567

Table 2

568

Factors and levels of the orthogonal experiment (A: Urea to AA mole ratio; B: the percentage of

569

AA neutralized by 20% KOH solution; C: cationic species; D: KPS to AA mass fraction; E: MBA

570

to AA mass fraction; F: reaction temperature; G: reaction time). Factor level

A (mol/mol)

B (%)

C

D (%)

E (%)

F (℃)

G (h)

1

0.4

70

Na+

0.5

0.02

45

3.0

2

1.0

85

K+

2.5

0.10

55

4.0

3

1.6

100

Ca2+

5.0

0.20

65

5.0

571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 27



Page 28 of 39

586

Table 3

587

The orthogonal L18(3)7 experiment of Urea, the percentage of AA neutralized, cationic species,

588

KPS, MBA, reaction temperature and reaction time (A: Urea to AA mole ratio; B: the percentage

589

of AA neutralized by 20% KOH solution; C: cationic species; D: KPS to AA mass fraction; E:

590

MBA to AA mass fraction; F: reaction temperature; G: reaction time; Qd: the water absorbency in

591

the distilled water). Sample no.

A (mol/mol)

B (%)

C

D (%)

E (%)

F (℃)

G (h)

Qd (g/g)

1

1(0.4)

1(70)

1(Na+)

1(0.5)

1(0.02)

1(45)

1(3.0)

157.7

2

1(0.4)

2(85)

2(K+)

2(2.5)

2(0.10)

2(55)

2(4.0)

171.5

3

1(0.4)

3(100)

3(Ca2+)

3(5.0)

3(0.20)

3(65)

3(5.0)

28.6

4

2(1.0)

1(70)

1(Na+)

2(2.5)

2(0.10)

3(65)

3(5.0)

51.8

5

2(1.0)

2(85)

2(K+)

3(5.0)

3(0.20)

1(45)

1(3.0)

142.3

6

2(1.0)

3(100)

3(Ca2+)

1(0.5)

1(0.02)

2(55)

2(4.0)

699.1

7

3(1.6)

1(70)

2(K+)

1(0.5)

3(0.20)

2(55)

3(5.0)

64.1

8

3(1.6)

2(85)

3(Ca2+)

2(2.5)

1(0.02)

3(65)

1(3.0)

317.0

9

3(1.6)

3(100)

1(Na+)

3(5.0)

2(0.10)

1(45)

2(4.0)

189.1

10

1(0.4)

1(70)

3(Ca2+)

3(5.0)

2(0.10)

2(55)

1(3.0)

40.2

11

1(0.4)

2(85)

1(Na+)

1(0.5)

3(0.20)

3(65)

2(4.0)

41.2

12

1(0.4)

3(100)

2(K+)

2(2.5)

1(0.02)

1(45)

3(5.0)

665.2

13

2(1.0)

1(70)

2(K+)

3(5.0)

1(0.02)

3(65)

2(4.0)

246.5

14

2(1.0)

2(85)

3(Ca2+)

1(0.5)

2(0.10)

1(45)

3(5.0)

226.6

15

2(1.0)

3(100)

1(Na+)

2(2.5)

3(0.20)

2(55)

1(3.0)

164.5

28


Page 29 of 39


16

3(1.6)

1(70)

3(Ca2+)

2(2.5)

3(0.20)

1(45)

2(4.0)

52.7

17

3(1.6)

2(85)

1(Na+)

3(5.0)

1(0.02)

2(55)

3(5.0)

231.2

18

3(1.6)

3(100)

2(K+)

1(0.5)

2(0.10)

3(65)

1(3.0)

94.7

592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 29



Page 30 of 39

611

Table 4

612

Analysis of the orthogonal L18(3)7 experiment (A: Urea to AA mole ratio; B: the percentage of

613

AA neutralized by 20% KOH solution; C: cationic species; D: KPS to AA mass fraction; E: MBA

614

to AA mass fraction; F: reaction temperature; G: reaction time). A

B

C

D

E

F

G

K1a

184.1

102.2

139.3

213.9

382.8

238.9

152.7

K2

255.1

188.3

230.7

237.1

129.0

228.4

233.4

K3

158.1

306.9

227.4

146.4

82.2

129.0

211.6

Rb

97.0

204.7

91.4

90.7

300.6

109.9

80.7

Influencing order Optimal

E＞B＞F＞A＞C＞D＞G A2

B3

C2

combination 615

a

616

b

D2

E1

A2B3C2D2E1F1G2

K1=(∑the water absorbency in the distilled water of single-factor)/4 R=max K1-min K1.

617 618 619 620 621 622 623 624 30


F1

G2

Page 31 of 39

625


FIGURES

626 627

Figure 1

628 629 630 631 632 633 634 635 636 637 31



638 639

Figure 2

640 641 642 643 644

32


Page 32 of 39

Page 33 of 39


645 646

Figure 3

647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 33



664 665

Figure 4

666 667 668 669 670 671 672 673 674 675 676 34


Page 34 of 39

Page 35 of 39


677 678

Figure 5

679 680 681 682 683 684 685 686 687 688 689 35



690 691

Figure 6

692 693 694 695 696 697 698 699 700 701 702 36


Page 36 of 39

Page 37 of 39


703 704

Figure 7

705 706 707 708 709 710 711 712 713 714 37



715 716

Figure 8

717 718 719 720 721 722 723 724 725 726 727 38


Page 38 of 39

Page 39 of 39


728 729

TOC Graphic

39


Water and fertilizer integrated hydrogel derived from the

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