Multifunctional Environmental Smart Fertilizer Based on l-Aspartic Acid

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Multifunctional Environmental Smart Fertilizer Based on L-Aspartic Acid for Sustained Nutrient Release Shaoyu Lü, Chen Feng, Chunmei Gao, Xinggang Wang, Xiubin Xu, Xiao Bai, Nannan Gao, and Mingzhu Liu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b01133 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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Journal of Agricultural and Food Chemistry

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Multifunctional Environmental Smart Fertilizer

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Based on L-Aspartic Acid for Sustained Nutrient

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Release

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Shaoyu Lü *, 1, Chen Feng 1, Chunmei Gao 1, Xinggang Wang 2, Xiubin Xu 1, Xiao Bai 1, Nannan

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Gao 1, Mingzhu Liu *, 1

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Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou

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University, Lanzhou 730000, People’s Republic of China

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2

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State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal

Research Institute of Lanzhou Petrochemical Corporation, Petrochina Lanzhou Petrochemical

Company, Lanzhou 730060, People’s Republic of China

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*S. Lü. Tel.: +86-931-8912387. Fax: +86-931-8912582. E-mail: [email protected]

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*M. Liu. Tel.: +86-931-8912387. Fax: +86-931-8912582. E-mail:[email protected]

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1 Environment ACS Paragon Plus


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ABSTRACT

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Fertilizer is one of the most important element of modern agriculture. However, conventional

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fertilizer, when applied to crops, is vulnerable to losses through volatilization, leaching,

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nitrification, or other means. Such a loss limits crop yields and pollutes the environment. In an

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effort to enhance nutrient use efficiency and reduce environmental pollution, an environmental

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smart fertilizer was reported in the current study. Poly(aspartic acid) and a degradable

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macrocross-linker based on L-aspartic acid were synthesized and introduced into the fertilizer as

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a superabsorbent to improve the fertilizer degradability and soil moisture-retention capacity.

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Sustained release behavior of the fertilizer was achieved in soil. Cumulative release of nitrogen

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and phosphorus was 79.8% and 64.4% after 30 days, respectively. The water-holding and water-

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retention capacity of soil with the superabsorbent are obviously higher than the control soil

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without superabsorbent. For the sample of 200 g soil with 1.5 g superabsorbent, the water-

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holding capacity is 81.8%, and the water-retention capacity remains 22.6% after 23 days. All of

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the current results in this study indicated that the as-prepared fertilizer has a promising

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application in sustainable modern agriculture.

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Keywords ： Poly(aspartic acid); Sustainable agriculture; Sustained release; Multifunctional

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fertilizer; Water holding; Water retention


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Introduction

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Spectacular changes in agriculture have been observed to satisfy the expansion of the global

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population since the food demand is increasing rapidly in the last 40 years. For this purpose,

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large amounts of nutrient inputs, pesticides and water were utilized which result in a great

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increase in cereal production on a worldwide basis.1-3 However, high nutrient loss via

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volatilization and leaching and the low nutrient utilization efficiency contribute to a serious

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waste of resource and severe environmental pollution, such as eutrophication, global greenhouse

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gas, and acidic rain.4, 5 To alleviate these problems, controlled/slow-release fertilizers (CSRFs)

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have been widely produced to enhance nutrient utilization efficiency and the effective nutrients

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uptake in the modern agriculture.6 These fertilizers are also referred to as “environmental smart

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fertilizer” due to their capacity to reduce environmental pollution from nutrient loss. 7

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During the last several decades, many researchers devote a great deal of effort to develop

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varies of CSRFs. In fact, many CSFRs have already been used to supply nutrients in the modern

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agriculture, such as urea-formaldehyde (UF), polyphosphates, sulfur coated urea (SCU), and

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polymer coated fertilizers (PCFs). Among these fertilizers, PCFs have a promising application

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because of the explosive growth of the science and technology of polymers. Various petroleum-

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based synthetic materials, like polyurethane, polyvinyl chloride, polysulfone, and polystyrene

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have been extensively used as coating materials to physically encapsulate water-soluble

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fertilizers.8-10 Although the coating materials can slow down the release rate of nutrients, a new

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kind of environmental pollution emerges as a result of non-degradability and non-renewability of

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the coating materials, which could lead to the decrease of soil fertility and the loss of agricultural

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land.11 Therefore, coating materials based on natural polysaccharides are drawing significant

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attention owing to their biodegradability, recyclability, low-cost, and renewability in recent



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years.12, 13 Konjac glucomannan (KGM) is a high-molecular weight natural polysaccharide with

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good film-forming property. Nevertheless, the sensitivity to moisture of KGM film which owes

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to the large amounts of hydroxyl groups on the repeating units of KGM limits its applications in

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agriculture. Recently, many researches demonstrated that the water resistance of deacetylated

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KGM film is better than that of natural KGM film, and deacetylated KGM film can be easily

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biodegraded in soil.14, 15 In terms of outstanding properties of this film, it will be a potential

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coating material to encapsulate water-soluble fertilizer for controlled release of nutrients.

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It is well recognized that both nutrient and water are indispensable to crops. However, drought

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and nutrient deficiency, so far, are still two main constraints for global agriculture.16 To

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overcome these problems, a technology that combining fertilizer with superabsorbent has been

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put forward in recent years, and studies indicated that several CSRFs can both control fertilizer

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loss and retain large amounts of water after fertilization.17-19 Although these fertilizers have

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many advantages, such as enhancing nutrients use efficiency, improving the retention of water in

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soil, alleviating the environmental pollution and reducing the irrigation water consumption,

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practical applications of them are limited due to most of superabsorbent polymers are made from

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non-degradable petroleum-based monomers and cross-linkers.20 In order to expand the scope of

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application of these fertilizer, the development of biodegradable superabsorbent is necessary and

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urgent.

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Polypeptides and their derivatives have both biodegradation and biocompatibility properties

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because of their protein-like chain structure. These lead to the use of them for many applications,

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such as drug delivery,21 regenerative medicine,22 and gene therapy.23 Among various

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polypeptides, poly(aspartic acid) (PAsp) has aroused tremendous attention owing to its water-

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solubility and facile synthesis. With a large number of carboxylic groups and amide groups on its


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molecular chain, PAsp is extensively used as an environment-friendly scale inhibitor or a sand

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fixing agent.24 Meanwhile, PAsp can also be served as a controlled-release agent in agriculture to

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enhance plant nutrient uptake and reduce nitrogen loss by inhibiting the nitrification and

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ammoniation of urea in soil.25, 26 As the previous studies reported, thermal condensation of L-

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aspartic acid to produce polysuccinimide (PSI) and afterwards hydrolysis in a basic medium is

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the most common method to synthesize PAsp.27, 28 In addition, the precursor polymer, PSI, has a

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high reactivity towards primary amines to produce its derivatives, for example,

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poly(hydroxyethylaspartimide) (PHEA).29, 30 Despite PAsp and PSI having so many advantages,

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only a few studies have been carried out with them in agriculture.31, 32 In the study, we design a

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semi-interpenetrating polymer network (IPN) hydrogel by employing PAsp and a biodegradable

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cross-linker based on PHEA with the purpose of promoting nutrient absorption by plants and

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enhancing the degradability of the fertilizers.

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Herein, we report on the synthesis of an environmental smart fertilizer on the basis of urea as

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fertilizer core, deacetylated KGM film as an inner coating material and degradable

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superabsorbent as an outer coating material. The release behavior was systematically examined

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in soil. Meanwhile, degradation behavior of the superabsorbent, the water-holding and water-

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retention capacity of soil were also investigated in detail.

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MATERIALS AND METHODS

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Materials.

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Konjac glucomannan (KGM, viscosity of 11000~12000 mPa.s in 1% (w/v) aqueous solution at

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25 oC) was obtained from Wealthy Chemical Industry Co., Ltd. (Suzhou, China). L-aspartic acid

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(98%, Aladdin), ethanolamine (99%, Aladdin), and acr]yloyl chloride (AC, 96%, Aladdin) were

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used as received, without further purification. Acrylic acid (AA, Beijing Oriental Chemical



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Factory, Beijing, China) was distilled under reduced pressure to remove the polymerization

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inhibitor prior to use. Acrylamide (AM, Shanghai Chemical Factory, Shanghai, China) was used

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directly without further purification. Triethylamine (TEA) was dried over CaH2 overnight,

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distilled under reduced pressure and stored with activated molecular sieves. N,N-

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dimethylformamide (DMF) was distilled over CaH2 under reduced pressure prior to use. All

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other reagents, ammonium persulfate (APS), ammonium polyphosphate (APP), phosphoric acid,

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sodium hydroxide, and mesitylene were analytical reagent grade and used as received. Water

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used throughout the experiment was deionized.

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Synthesis of polysuccinimide (PSI) and poly(aspartic acid) (PAsp).

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The preparing of PSI was carried out according to a previous literature.28 Briefly, L-aspartic

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acid (10 g) and phosphoric acid (85%, 3.0 mL) were placed in a mortar, and fully mixed by

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grinding afterwards. The mixture was added to a three-neck round-bottom flask, followed by the

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addition of mesitylene (25 mL). Then, the reaction was kept at 180~200 oC for 5 hours. A Dean-

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Stark trap with a reflux condenser was employed to remove by-product water at the same time.

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Finally, the reaction was cooled to room temperature, and DMF was added to flask in order to

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dissolve the crude product. After filtration of the mixture, the resulting solution was precipitated

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in deionized water under vigorous stirring for 10 min. Then the white precipitate was filtered,

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washed several times with water and dried under vacuum for 2 days.

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Sodium polyaspartate (PAspNa) was obtained by the simple hydrolysis of PSI in alkaline

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solution as the reported method.33 Firstly, hydrolysis was conducted by dispersion of PSI in a

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basic medium. Subsequently, the reaction mixture was precipitated in ethanol to receive

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PAspNa, washed, and dried at room temperature. Finally, PAsp was achieved through an ion

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exchange column.


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Synthesis of poly(hydroxyethylaspartimide) (PHEA) and biodegradable macrocross-linker

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(PHEA-AC).

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Typical processes for the synthesis of PHEA are as follows. PSI and DMF were mixed in a

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breaker with magnetic stirring for an hour to make sure PSI was completely dissolved.

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Subsequently, the aminolysis of PSI was achieved by the addition of ethanolamine (2.0

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equivalents of PSI), and the reaction was carried out for 48 h. The reaction mixture was dialyzed

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against deionized water for 5 days to remove excessive reactant and fresh deionized water was

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replaced every four hours. Finally, PHEA was obtained by lyophilized afterwards.

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PHEA-AC was synthesized according to the following method reported by Cao et al.34 PHEA

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(4.0 g) was dissolved in 50 mL dry DMF containing 3.0 mL TEA in a round bottom flask with

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vigorous stirring. After the complete dissolution of PHEA, a mixture of AC (2.5 ml) in 20 ml dry

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DMF was added dropwise into the flask over 1 h, and the whole system was immersed in a

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water/ice bath for 24 h. Then the resulting solution was filtrated to remove triethylamine

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hydrochloride salts, the filtrate was dialyzed against deionized water for 3 days with periodic

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fresh water changes every four hours. The final product was obtained by lyophilized for 24 h.

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Synthesis of KGM-g-P(AA-co-AM)/PAsp semi-IPN superabsorbent hydrogel (KAP).

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With the purpose to investigate water absorbency of the superabsorbent, a series of samples

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with different amounts of AA, AM, KGM, PAsp and PHEA-AC were synthesized according to

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the following procedure. Firstly, KGM (0.1 g) and deionized water (20 mL) were added into a

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three-necked round bottom flask under vigorous stirring. Meanwhile, the solution was deaerated

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by purging with nitrogen for 30 min. After degassed, a certain amount of monomer (AA was

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partially neutralized with sodium hydroxide solution), cross-linker, APS, and PAsp were

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immediately added into the flask. Subsequently, the whole system was heated to 75 oC in an oil



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bath and kept for 4 h. In the end, the product was washed with a great deal of water to eliminate

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unreacted monomer and dried at 45 oC for 2 days.

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Synthesis of multifunctional sustained release fertilizer (MSRF).

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Three steps are adopted to prepare MSRF, and detailed procedures are as follows. KGM and

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APP (7:3, w/w) were grinded to powder and homogeneously mixed. Then the mixture was added

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to a rotating pan with urea granules (about 1.0~1.3 mm in diameter) in batches. During this step,

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the fertilizer core (about 1.4~1.6 mm in average diameter) was obtained by atomization of water

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and dried at 40 oC. In addition, KGM (1.0 g), sodium carbonate (0.3 g), and PEG-400 (150 µL)

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were dissolved in 100 mL deionized water with magnetic stirring for 30 min. In order to form

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inner polymer-coating, subsequent step was employed by spraying the resulting solution onto the

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surface of the rotating fertilizer core, and the same treatment was carried out several times until a

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certain thickness of the polymer-coating was achieved, and the average diameter of the single-

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layer coated fertilizer was 1.6~1.8 mm. Finally, KAP powder (below 200 mesh) used as the outer

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absorbent material was adhered to the surface of the granules under water atomization. Then the

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products were dried at 35 oC prior to use. The double coated products were about 2.0~3.0 mm in

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average diameter.

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Characterization.

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Fourier transforms infrared (FTIR) spectra of PSI, PHEA, PHEA-AC, PASP, KGM,

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and KGM-g-P(AA-co-AM)/PASP obtained under the optimum conditions were

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characterized at room temperature by Nicolet NEXUS 670 FTIR spectrometer (USA)

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with a KBr pellet at a scanning range from 4000 to 500 cm-1. The 1H NMR spectra of PSI,

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PHEA, and PHEA-AC were determined by a Bruker 400 MHz NMR spectrometer. The

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surface morphologies of fertilizer samples were examined using scanning electron


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microscopy (SEM, JSM-5600LV, Japan). To investigate the morphology of KGM-g-

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P(AA-co-AM)/PAsp, the superabsorbent hydrogel was allowed to swell in distilled water

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for 24 h and subsequently lyophilized for 12 h before the SEM observation. The

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molecular weight and polydispersity index (PDI) of PSI were evaluated by gel permeation

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chromatography (GPC) with a flow rate of 1.0 mL/min using DMF with 0.05 M LiBr as

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the eluent. Throughout the experiment, an elemental analysis instrument (Germany

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Elemental Vario EL Corp., Model 1106) and an IRIS Advantage ER/S inductively couple

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plasma emission spectrometer (TJA, USA) were used respectively to characterize the

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content of nitrogen and phosphorus of MSRF.

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Sustained release behavior of MSRF in soil.

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Soil used in the release study is a representative sample of semiarid region, which comes from

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Lanzhou in China. The soil texture is silt loam, containing 9.27% clay, 61.89% silt and 28.84%

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sand. The organic matter content is 1.76%. Before the release study, soil sample was passed

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through a 30-mesh sieve and dried at room temperature to eliminate moisture for about a month.

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Subsequently, soil sample (10 g) was transferred to a bottle with deionized water (50 mL) and

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stirred for 2 hours. The suspension was used to determine the eletroconductivity and the pH of

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soil sample using a conductivity meter (DDS-307, Shanghai Leici, China) and a pH meter (PHS-

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3B, Shanghai Leici, China), respectively. The eletroconductivity and the pH of soil are 21.50

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µS/cm and 8.13, respectively. To investigate the sustained release behavior of nitrogen (N) and

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phosphorus (P2O5) from MSRF, several experimental steps were carried out as follows: MSRF

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(1.0 g) was firstly placed in a non-woven plastic mesh bag and buried in a beaker filled with 200

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g of dry soil afterwards. Meanwhile, fertilizer samples were kept at approximately 5~6 cm below

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the surface of soil and the water content of the whole system was maintained at 20% (w/w)



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during the experiment. The mesh bags were taken out at selected time points (days 1, 3, 5, 10,

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15, 20, 25, 30) and dried at room temperature. Subsequently, nutrients in fertilizer samples which

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were collected from the mesh bags was estimated. The release behavior was evaluated by the

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differences between the total content of nitrogen (N) and phosphorus (P2O5) and the remaining

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content of nitrogen (N) and phosphorus (P2O5). Meanwhile, the release behaviors of the fertilizer

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core and the fertilizer core coated with deacetylated KGM film were also investigated in the

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same way.

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Measurement of water absorbency of KAP.

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A certain amount of superabsorbent (0.1 g, 40~90 mesh) with different compositions was

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soaked into tap water (100 mL) for one hour at ambient temperature (20±3 oC). Then the swollen

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superabsorbent was picked out by filtering and weighed. The water absorbency (Wa) was

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measured by eqn 1:

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=

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where Wd refers to the initial weight of the dry superabsorbent, and Ws is the weight of swollen

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superabsorbent.

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Measurement of water-holding and water-retention of soil with KAP.

(1)

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The well mixture of three different amounts of KAP (0.5 g, 1.0 g and 1.5 g) and 200 g soil

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(below 30 mesh) was carefully placed into PVC tubes, respectively. The bottom of PVC tube

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was sealed with two layers of nylon fabric (200 mesh) and weighed (defined as W0). Then tap

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water was slowly added into the mixture from the top of tube until water seeped out from the

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bottom. When there was no water seeped from the bottom, the PVC tubes were once more

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weighed (defined as W1). At the same time, the control treatment which has no superabsorbent

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was carried out as well. In this study, water-holding capacity and water-retention capacity of soil


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with or without KAP were measured at ambient temperature (20±3 oC). On the basis of W0 and

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W1, the value of water-holding in soil (Wh, refers to a saturated moisture of soil, which is the ratio

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of the total amount of moisture in the soil and the weight of soil when excess water is discharged

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by the effect of gravity) was calculated according to eqn 2:

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% =

× 100

(2)

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The above procedures were immediately followed by the study of water-retention capacity of

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soil with KAP. Throughout the experiment, the four treatments were maintained at ambient

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temperature (20±3 oC) and weighed every days in 23 days (defined as Wi). The value of water-

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retention (Wr) was calculated according to eqn 3:

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% = × 100

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(3)

Degradation of KAP outer materials.

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To examine the degradation behavior of the superabsorbent, the dry weight loss of KAP was

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determined. KAP with maximum absorbency (1.0~1.5 cm in thickness and 9~10 mm in

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diameter) was chosen to incubate in soil solution for 35 days, then the sample was picked up,

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dried, weighed at selected point during the experiment (defined as Mi). Meanwhile, the samples

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were weighed (defined as M0) before the incubation. The temperature was maintained at 25 oC

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during degradation experiment. Soil solution was prepared as described before, and its

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eletroconductivity and the pH are 21.50 µS/cm and 8.13, respectively. The degree of degradation

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(De) was calculated by eqn 4:

239 240

% =

× 100 (4)

Statistical method.



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All data were analyzed through a three-way analysis of variance (ANOVA). Difference at

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P

Multifunctional Environmental Smart Fertilizer Based on l-Aspartic Acid

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