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Novel Fabrication of Biodegradable Superabsorbent Microspheres with Diffusion Barrier through Thermo-Chemical Modification and Their Potential Agriculture Applications for Water Holding and Sustained Release of Fertilizer Diejing Feng, Bo Bai, Honglun Wang, and Yourui Suo J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b01849 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017
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Journal of Agricultural and Food Chemistry
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Novel Fabrication of Biodegradable Superabsorbent Microspheres with Diffusion
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Barrier through Thermo-Chemical Modification and Their Potential Agriculture
3
Applications for Water Holding and Sustained Release of Fertilizer
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Diejing Feng 1, 2, Bo Bai *, 1, 2, Honglun Wang 3, Yourui Suo 3
5
1
Land and Resources of China, Xi’an 710075, China
6 7
2
10
Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education, Chang’an University, Xi’an, 710054, P.R. China.
8 9
Key Laboratory of Degraded and Unused Land Consolidation Engineering, The Ministry of
3
State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining, 810016, P.R. China
11 12
Corresponding Author: Bo Bai
13
Tel: +86 298 233 0952; Fax: +86 298 233 9961; Email:
[email protected] 14
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ABSTRACTS: Synergistic utilization of water and fertilizer has vital contribution to the modern
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production of agriculture. This work reports on a simple and facile strategy to prepare biodegradable
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yeast/sodium alginate/poly(vinyl alcohol) superabsorbent microspheres with a diffusion barrier merit
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by thermo-chemical modification route. The integrated performances, including water absorbency,
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water retention, water evaporation ratio, leaching loss control, sustained-release behaviors and
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degradation in soil were systematically investigated. The results revealed that the modified
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microspheres were a triumphant water and fertilizer manager to effectively hold water and control
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unexpected leakage of fertilizer for sustained release. Therefore, this work provides a promising
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approach to ameliorate the utilization efficiency of water and fertilizer in potential agriculture
24
applications.
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Keywords: superabsorbent microspheres, diffusion barrier, denser cross-linked network, water
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holding, sustained release, biodegradation
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Journal of Agricultural and Food Chemistry
1. INTRODUCTION
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Water and fertilizer have an extremely vital contribution to the modern production of agriculture.1
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In this respect, the high-efficient utilization of water and fertilizer always was encouraged in the
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drought areas where insufficient water supply and fertilizer loss into groundwater by leaching,2
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which maybe lead to soil degradation, water eutrophication or impose greater risk to the ecosystem.3,
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4
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development of agriculture is becoming a critical job. To achieve this goal, superabsorbent
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composites are deemed to be a potential candidate employed in agriculture. The main reason is due
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to that the superabsorbent composites could act as an integrated water and fertilizer manager to
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absorb and retain water from tens to thousands times its weight, reduce irrigation water consumption,
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supply fertilizer nutrients sustainably, decrease fertilizer loss rate, lower application frequency, and
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minimize potential negative effects associated with overdosage.5-7 However, the application of
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traditional superabsorbent composites primarily based on synthetic polymers like acrylic acid or
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acrylamide have brought about environmental hazards as a result of their poor biodegradability.8 As a
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consequence, currently much attention has been attracted to exploit eco-friendly and biodegradable
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bio-based raw materials for superabsorbent composites, including chitosan,8 sodium alginate,9
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polydopamine,10 and starch.11
So, simultaneously improving the efficiency of water and fertilizer utilization in the sustainable
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Sodium alginate (SA) is a polyanionic linear copolymer of 1,4-linked-α-L-guluronic acid and
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β-D-mannuronic acid residues found in brown seaweeds. Sodium alginate has been regarded as an
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brilliant polysaccharide for water and fertilizer manager because of its unique hydrophilicity,
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biocompatibility, biodegradability and non-toxicity.12 In order to obtain the sodium alginate-based
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superabsorbent composites, the ionotropic gelation has been verified as a facile and effective
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cross-linking technique that enables the formation of Ca-SA spherical microspheres with reticulated
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structure, regular shape, uniform size and smooth surface, when sodium alginate molecules contacts
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with Ca2+ ions.13 Moreover, it has been ascertained that sodium alginate-based polymer 3 ACS Paragon Plus Environment
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encapsulating water and fertilizer is superior to non-encapsulated commercial formulation in
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extending agriculture applications. However, the release properties of pure Ca-SA microspheres
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regretfully often suffered from burst release and quick breakdown in the in vitro release process,14
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which would give rise to unexpected leaching loss and decreasing utilization efficiency. To address
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this dilemma, several efforts have been made to blend with other polymers or fillers to improve the
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comprehensive performance of Ca-SA microspheres. Successful attempt also involves furnishing the
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Ca-SA microspheres with a diffusion barrier.15 For example, the classical thermo-chemical
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modification approach, which was extensively used to improve water affinity and densify
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microstructure through cross-linking between citric acid and target matrix in past years,16-18 has been
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alternatively introduced to endow the surface of superabsorbent microspheres with a denser
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cross-linked diffusion barrier, and consequently the diffusion of water and other soluble molecules
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have been controlled precisely due to adjustable emigrating behavior from the inner matrix to the
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external
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thermo-chemical modification was rich in carboxyl groups, which increased the adsorption sites and
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improved pH-responsible swelling behavior of the cross-linked network.19
surface.
More
importantly,
the
surface
of
superabsorbent
microspheres after
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Yeast is a cost-effective, easily available, and safe industry microorganism. In terms of its
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structure, yeast is a classic and ubiquitous aquatic unicellular eukaryotic microorganism, which is
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constituted of cell wall, cell membrane, cytoplasm, nucleus, vacuoles and mitochondria.20 The cell
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wall of yeast is composed of approximately ~90% polysaccharides, mainly polymers of mannose,
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glucose, N-acetylglucosamine, and a small portion of proteins and lipids,21-23 which possess ample
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functional groups including hydroxyl, carboxylate, amine, phosphate, and acylamino groups. The
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cell wall has considerable natural tensile strength to protect yeast cell against the outside destructive
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intrusion and also to prevent the hollow shape from serious shrinkage. Also, the cell wall has an
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inherent advantage of semi-permeability, which permits the passage of small molecules, especially
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water, with size exclusion estimated to be 30−60 kDa.24 These above-mentioned unique physical and 4 ACS Paragon Plus Environment
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chemical characteristics have made yeasts as excellent bio-reinforcing fillers for the preparation of
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superabsorbent composites.25 In addition, the hydrophilic groups, which inherited from yeast cell
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wall, have also provided prerequisite loading sites for fertilizer molecules through strong hydrogen
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bonds.
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Inspired by these backgrounds, we herein put forward a simple strategy for fabrication of novel
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biodegradable
yeast/sodium
alginate/poly(vinyl
alcohol)
(yeast/SA/PVA)
superabsorbent
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microspheres with a diffusion barrier through thermo-chemical modification route. The performance
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of enhanced water-holding capacity and synchronously sustained release of fertilizer have been
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realized. The systematical evaluations of usage, including water absorbency, water retention, water
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evaporation in soil, leaching loss control, sustained-release behavior and degradation in soil, were
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conducted to estimate their feasibility and practical value in ecological agriculture. The experimental
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work has provided a promising approach to ameliorate the utilization efficiency of water and
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fertilizer in the future agriculture applications.
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2. MATERIALS AND METHODS
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2.1 Materials. Yeast powder was purchased from Angel Yeast Corp. (Wuhan, China) and was
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washed by water and ethanol beforehand. PVA with a degree polymerization of 1750±50 (86~90%
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hydrolyzed and MW=72600~81400) was supplied from Tianjin Yongcheng Fine Chemical Co., Ltd.
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Sodium alginate, sodium hydroxide, potassium hydroxide and ethanol was provided by Tianjin
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Chemical Reagent Factory (Tianjin, China). Citric acid, sodium chloride, calcium chloride,
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aluminium chloride, hydrochloride acid, phenolphthalein and were furnished by Xi’an Chemical
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Reagent Factory (Shaanxi, China). Indole-3-butyric acid (IBA) was afforded by Zhengzhou Xinlian
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Chemical Technology Co., Ltd (Zhengzhou, China). All agents were of analytical grade and used
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without further purification.
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2.2 Synthesis of Yeast/SA/PVA Microspheres. The preparation of yeast/SA/PVA microspheres was
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divided into dual cross-linking processes. In the ionotropic gelation process, 5% (w/v) SA solution 5 ACS Paragon Plus Environment
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and 5% (w/v) PVA solution were mixed by stirring at 25 ºC for 2 h in order to acquire a homogenous
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solution of the two polymers. A certain amount of yeast powders was added into the mixed polymer
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solution and dispersed uniformly. Then, the mixed solution was dripped into a 5% (w/v) CaCl2
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solution by a glass syringe without the needle under continuous magnetic stirring (100 rpm),
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allowing the coacervation of Ca-SA microspheres to occur and to entangle with PVA and embed
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yeast cells inside. Then, the gelled microspheres were filtered and washed with distilled water to
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remove the residual CaCl2 solution on the surface. In the thermo-chemical modification process, the
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above washed microspheres were soaked in a citric acid solution, semi-dried in a hot air oven at
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50 °C, and then cured at 130 °C for 20 min, resulting in a denser cross-linked network on the surface
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of the microspheres. Finally, the resultant yeast/SA/PVA microspheres were washed with distilled
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water and acetone in order to remove the untreated entities.
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2.3 Characterization Techniques. The morphologies of the yeast/SA/PVA microspheres were
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investigated by scanning electron microscope (SEM) using a Hitachi S-4800, Japan. All the samples
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were placed on round brass stubs and sputter coated with gold to make them conductive, and then
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scanned at an accelerating voltage of 20 kV. The Fourier transform infrared (FTIR) spectra of the
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samples were analyzed in KBr discs using a Nicolet FTIR spectrometer in the range of 4000–500
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cm-1. The samples were prepared through mixing and grounding with potassium bromide and the
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pressed into a pellet.
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2.4 Determination of Carboxyl Content and Degree of Esterification. Carboxyl content in the
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yeast/SA/PVA microspheres was estimated by using an acid-base titration according to the method of
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Salam.17 1.0 g of synthesized products was firstly dissolved in excess 0.1 M NaOH solution (pH 12.5)
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and the mixture was allowed to react for 1 h. The remaining excess amount of NaOH was determined
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by titration with 0.1 M HCl solution with phenolphthalein as an indicator. The carboxyl content (CC)
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in milliequivalents of acidity per 100 g can be calculated as: CC meq/100g =
− × × 100 1 6
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Where Va and Vb (mL) are the volume of HCl used to titrate in the presence and in the absence of
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sample, respectively. N is normality of used HCl, W (g) is the weight of dry samples.
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Degree of esterification was determined by titration involved complete basic hydrolysis of the
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ester linkages and potentiometric titration of the excess alkaline. 1 g of samples was added to 50 mL
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of 75% ethanol solution, and kept in the water bath (50 ºC) for 30 min with continuous agitation. As
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the slurry cooled down, 30 mL of 0.5 M KOH solution was added to saponify the ester linkages for
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72 h with stirring at room temperature. The excess alkali was back titrated with 0.5 M HCl using
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phenolphthalein as an indicator. Reference sample and duplicate sample were treated in a similar way.
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The degree of esterification (DE) followed the equation below.26 DE % =
− × × 158 × 10 × 100 2
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Where V0 and Vn (mL) are the volume of HCl used to titrate blank and sample, respectively. N is
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normality of used HCl. W (g) is the weight of dry sample. 158 is the molecular weight of citric acid
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acyl group.
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2.5 Determination of Swelling Ratio. The swelling ratio of the yeast/SA/PVA microspheres was
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determined by a conventional gravimetric method. Pre-weight dried samples were immersed in a
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certain amount of tap water and allowed to soak at 20 °C until the swelling behavior reached
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equilibrium stage. The swollen samples were filtered and weighted. The swelling ratio (St) of the
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microspheres was calculated using the following formula: g⁄g =
− 3
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Where W0 (g) and Wt (g) are the mass of the microspheres at dried state and at time t, respectively.
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The effects of different saline solutions (NaCl, CaCl2 and AlCl3) on swelling ratio were tested with
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the same methods.
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2.6 Determination of Water Retention. The yeast/SA/PVA microspheres were firstly soaked in tap
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water to achieve saturation. Then, the swollen microspheres were (I) placed at different temperatures 7 ACS Paragon Plus Environment
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(5~30 °C) for 10 h and (II) centrifuged with different rotation speeds (500~4000 rpm) for 30 min.
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The water retention capacity (R) was detected by the following equation: " % =
# − $ × 100 4 # −
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Where We (g) and WR (g) are the weight of the microspheres at swollen state and after treated by
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various temperatures and centrifugal forces, respectively.
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2.7 Measurement of Water Evaporation in Soil. The sandy soil used in this study is representative
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of the area of Xi’an, China. The soil was firstly air dried at room temperature to constant weight and
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sieved with a 20-mesh screen. A certain amount of the yeast/SA/PVA microspheres were mixed with
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200 g of dry soil and placed in a glass beaker. Then, the beaker was infiltrated by 200 g of tap water
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until the water exudation from the soil gaps appeared, and the whole container was weighed (W1).
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The control experiment without the yeast/SA/PVA microspheres was also carried out. The beaker
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was placed at 25 °C and weighed every 2 days (marked W2). The water evaporation ratio of soil was
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determined bythe following equation.27 Water evaporation ratio % =
/ − 0 × 100 5 200
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2.8 Investigation of Leaching Loss Control. The yeast/SA/PVA microspheres were firstly
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immersed into IBA solution until equilibrium adsorption. Then, 30 g of dry sand (150−200 mesh)
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was put into a 50 mL centrifuge tube with a hole about 2 mm in diameter at the bottom, and 5 mL of
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tap water was added to keep the sand humid. The IBA-loaded microspheres were buried in the sand
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(humidity of 30%) at 25 °C, and covered with another 10 g of dry sand. 50 mL of tap water was
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sprayed over the top of the sand layer to collect the leachate, in which the concentration of IBA was
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measured spectrophotometrically. The leaching loss control ratio was calculated by the formula:28 Leaching loss control ratio % =
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678 − 6799 × 100 6 678
Where Wloal (g) and Wloss (g) are the amount of IBA loading on and losing out from the microspheres, 8 ACS Paragon Plus Environment
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respectively.
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2.9 Investigation of Sustained Release Behaviors. The IBA-loaded yeast/SA/PVA microspheres
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were placed in 50 mL distilled water and the suspension was mildly shaken with maintained speed
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for predetermined time period. At fixed time intervals, the microspheres were withdrawn from the
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solution
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spectrophotometrically. The cumulative release was calculated according to the following equation:
and
the
residual
concentration
Cumulative release % =
of
IBA
in
the
supernatant
was
detected
678 −
