Subscriber access provided by NEW YORK UNIV
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 40
Journal of Agricultural and Food Chemistry
1
Novel Fabrication of Biodegradable Superabsorbent Microspheres with Diffusion
2
Barrier through Thermo-Chemical Modification and Their Potential Agriculture
3
Applications for Water Holding and Sustained Release of Fertilizer
4
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
1 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
15
ABSTRACTS: Synergistic utilization of water and fertilizer has vital contribution to the modern
16
production of agriculture. This work reports on a simple and facile strategy to prepare biodegradable
17
yeast/sodium alginate/poly(vinyl alcohol) superabsorbent microspheres with a diffusion barrier merit
18
by thermo-chemical modification route. The integrated performances, including water absorbency,
19
water retention, water evaporation ratio, leaching loss control, sustained-release behaviors and
20
degradation in soil were systematically investigated. The results revealed that the modified
21
microspheres were a triumphant water and fertilizer manager to effectively hold water and control
22
unexpected leakage of fertilizer for sustained release. Therefore, this work provides a promising
23
approach to ameliorate the utilization efficiency of water and fertilizer in potential agriculture
24
applications.
25
Keywords: superabsorbent microspheres, diffusion barrier, denser cross-linked network, water
26
holding, sustained release, biodegradation
27
2 ACS Paragon Plus Environment
Page 2 of 40
Page 3 of 40
28
Journal of Agricultural and Food Chemistry
1. INTRODUCTION
29
Water and fertilizer have an extremely vital contribution to the modern production of agriculture.1
30
In this respect, the high-efficient utilization of water and fertilizer always was encouraged in the
31
drought areas where insufficient water supply and fertilizer loss into groundwater by leaching,2
32
which maybe lead to soil degradation, water eutrophication or impose greater risk to the ecosystem.3,
33
4
34
development of agriculture is becoming a critical job. To achieve this goal, superabsorbent
35
composites are deemed to be a potential candidate employed in agriculture. The main reason is due
36
to that the superabsorbent composites could act as an integrated water and fertilizer manager to
37
absorb and retain water from tens to thousands times its weight, reduce irrigation water consumption,
38
supply fertilizer nutrients sustainably, decrease fertilizer loss rate, lower application frequency, and
39
minimize potential negative effects associated with overdosage.5-7 However, the application of
40
traditional superabsorbent composites primarily based on synthetic polymers like acrylic acid or
41
acrylamide have brought about environmental hazards as a result of their poor biodegradability.8 As a
42
consequence, currently much attention has been attracted to exploit eco-friendly and biodegradable
43
bio-based raw materials for superabsorbent composites, including chitosan,8 sodium alginate,9
44
polydopamine,10 and starch.11
So, simultaneously improving the efficiency of water and fertilizer utilization in the sustainable
45
Sodium alginate (SA) is a polyanionic linear copolymer of 1,4-linked-α-L-guluronic acid and
46
β-D-mannuronic acid residues found in brown seaweeds. Sodium alginate has been regarded as an
47
brilliant polysaccharide for water and fertilizer manager because of its unique hydrophilicity,
48
biocompatibility, biodegradability and non-toxicity.12 In order to obtain the sodium alginate-based
49
superabsorbent composites, the ionotropic gelation has been verified as a facile and effective
50
cross-linking technique that enables the formation of Ca-SA spherical microspheres with reticulated
51
structure, regular shape, uniform size and smooth surface, when sodium alginate molecules contacts
52
with Ca2+ ions.13 Moreover, it has been ascertained that sodium alginate-based polymer 3 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Page 4 of 40
53
encapsulating water and fertilizer is superior to non-encapsulated commercial formulation in
54
extending agriculture applications. However, the release properties of pure Ca-SA microspheres
55
regretfully often suffered from burst release and quick breakdown in the in vitro release process,14
56
which would give rise to unexpected leaching loss and decreasing utilization efficiency. To address
57
this dilemma, several efforts have been made to blend with other polymers or fillers to improve the
58
comprehensive performance of Ca-SA microspheres. Successful attempt also involves furnishing the
59
Ca-SA microspheres with a diffusion barrier.15 For example, the classical thermo-chemical
60
modification approach, which was extensively used to improve water affinity and densify
61
microstructure through cross-linking between citric acid and target matrix in past years,16-18 has been
62
alternatively introduced to endow the surface of superabsorbent microspheres with a denser
63
cross-linked diffusion barrier, and consequently the diffusion of water and other soluble molecules
64
have been controlled precisely due to adjustable emigrating behavior from the inner matrix to the
65
external
66
thermo-chemical modification was rich in carboxyl groups, which increased the adsorption sites and
67
improved pH-responsible swelling behavior of the cross-linked network.19
surface.
More
importantly,
the
surface
of
superabsorbent
microspheres after
68
Yeast is a cost-effective, easily available, and safe industry microorganism. In terms of its
69
structure, yeast is a classic and ubiquitous aquatic unicellular eukaryotic microorganism, which is
70
constituted of cell wall, cell membrane, cytoplasm, nucleus, vacuoles and mitochondria.20 The cell
71
wall of yeast is composed of approximately ~90% polysaccharides, mainly polymers of mannose,
72
glucose, N-acetylglucosamine, and a small portion of proteins and lipids,21-23 which possess ample
73
functional groups including hydroxyl, carboxylate, amine, phosphate, and acylamino groups. The
74
cell wall has considerable natural tensile strength to protect yeast cell against the outside destructive
75
intrusion and also to prevent the hollow shape from serious shrinkage. Also, the cell wall has an
76
inherent advantage of semi-permeability, which permits the passage of small molecules, especially
77
water, with size exclusion estimated to be 30−60 kDa.24 These above-mentioned unique physical and 4 ACS Paragon Plus Environment
Page 5 of 40
Journal of Agricultural and Food Chemistry
78
chemical characteristics have made yeasts as excellent bio-reinforcing fillers for the preparation of
79
superabsorbent composites.25 In addition, the hydrophilic groups, which inherited from yeast cell
80
wall, have also provided prerequisite loading sites for fertilizer molecules through strong hydrogen
81
bonds.
82
Inspired by these backgrounds, we herein put forward a simple strategy for fabrication of novel
83
biodegradable
yeast/sodium
alginate/poly(vinyl
alcohol)
(yeast/SA/PVA)
superabsorbent
84
microspheres with a diffusion barrier through thermo-chemical modification route. The performance
85
of enhanced water-holding capacity and synchronously sustained release of fertilizer have been
86
realized. The systematical evaluations of usage, including water absorbency, water retention, water
87
evaporation in soil, leaching loss control, sustained-release behavior and degradation in soil, were
88
conducted to estimate their feasibility and practical value in ecological agriculture. The experimental
89
work has provided a promising approach to ameliorate the utilization efficiency of water and
90
fertilizer in the future agriculture applications.
91
2. MATERIALS AND METHODS
92
2.1 Materials. Yeast powder was purchased from Angel Yeast Corp. (Wuhan, China) and was
93
washed by water and ethanol beforehand. PVA with a degree polymerization of 1750±50 (86~90%
94
hydrolyzed and MW=72600~81400) was supplied from Tianjin Yongcheng Fine Chemical Co., Ltd.
95
Sodium alginate, sodium hydroxide, potassium hydroxide and ethanol was provided by Tianjin
96
Chemical Reagent Factory (Tianjin, China). Citric acid, sodium chloride, calcium chloride,
97
aluminium chloride, hydrochloride acid, phenolphthalein and were furnished by Xi’an Chemical
98
Reagent Factory (Shaanxi, China). Indole-3-butyric acid (IBA) was afforded by Zhengzhou Xinlian
99
Chemical Technology Co., Ltd (Zhengzhou, China). All agents were of analytical grade and used
100
without further purification.
101
2.2 Synthesis of Yeast/SA/PVA Microspheres. The preparation of yeast/SA/PVA microspheres was
102
divided into dual cross-linking processes. In the ionotropic gelation process, 5% (w/v) SA solution 5 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
103
and 5% (w/v) PVA solution were mixed by stirring at 25 ºC for 2 h in order to acquire a homogenous
104
solution of the two polymers. A certain amount of yeast powders was added into the mixed polymer
105
solution and dispersed uniformly. Then, the mixed solution was dripped into a 5% (w/v) CaCl2
106
solution by a glass syringe without the needle under continuous magnetic stirring (100 rpm),
107
allowing the coacervation of Ca-SA microspheres to occur and to entangle with PVA and embed
108
yeast cells inside. Then, the gelled microspheres were filtered and washed with distilled water to
109
remove the residual CaCl2 solution on the surface. In the thermo-chemical modification process, the
110
above washed microspheres were soaked in a citric acid solution, semi-dried in a hot air oven at
111
50 °C, and then cured at 130 °C for 20 min, resulting in a denser cross-linked network on the surface
112
of the microspheres. Finally, the resultant yeast/SA/PVA microspheres were washed with distilled
113
water and acetone in order to remove the untreated entities.
114
2.3 Characterization Techniques. The morphologies of the yeast/SA/PVA microspheres were
115
investigated by scanning electron microscope (SEM) using a Hitachi S-4800, Japan. All the samples
116
were placed on round brass stubs and sputter coated with gold to make them conductive, and then
117
scanned at an accelerating voltage of 20 kV. The Fourier transform infrared (FTIR) spectra of the
118
samples were analyzed in KBr discs using a Nicolet FTIR spectrometer in the range of 4000–500
119
cm-1. The samples were prepared through mixing and grounding with potassium bromide and the
120
pressed into a pellet.
121
2.4 Determination of Carboxyl Content and Degree of Esterification. Carboxyl content in the
122
yeast/SA/PVA microspheres was estimated by using an acid-base titration according to the method of
123
Salam.17 1.0 g of synthesized products was firstly dissolved in excess 0.1 M NaOH solution (pH 12.5)
124
and the mixture was allowed to react for 1 h. The remaining excess amount of NaOH was determined
125
by titration with 0.1 M HCl solution with phenolphthalein as an indicator. The carboxyl content (CC)
126
in milliequivalents of acidity per 100 g can be calculated as: CC meq/100g =
− × × 100 1 6
ACS Paragon Plus Environment
Page 6 of 40
Page 7 of 40
Journal of Agricultural and Food Chemistry
127
Where Va and Vb (mL) are the volume of HCl used to titrate in the presence and in the absence of
128
sample, respectively. N is normality of used HCl, W (g) is the weight of dry samples.
129
Degree of esterification was determined by titration involved complete basic hydrolysis of the
130
ester linkages and potentiometric titration of the excess alkaline. 1 g of samples was added to 50 mL
131
of 75% ethanol solution, and kept in the water bath (50 ºC) for 30 min with continuous agitation. As
132
the slurry cooled down, 30 mL of 0.5 M KOH solution was added to saponify the ester linkages for
133
72 h with stirring at room temperature. The excess alkali was back titrated with 0.5 M HCl using
134
phenolphthalein as an indicator. Reference sample and duplicate sample were treated in a similar way.
135
The degree of esterification (DE) followed the equation below.26 DE % =
− × × 158 × 10 × 100 2
136
Where V0 and Vn (mL) are the volume of HCl used to titrate blank and sample, respectively. N is
137
normality of used HCl. W (g) is the weight of dry sample. 158 is the molecular weight of citric acid
138
acyl group.
139
2.5 Determination of Swelling Ratio. The swelling ratio of the yeast/SA/PVA microspheres was
140
determined by a conventional gravimetric method. Pre-weight dried samples were immersed in a
141
certain amount of tap water and allowed to soak at 20 °C until the swelling behavior reached
142
equilibrium stage. The swollen samples were filtered and weighted. The swelling ratio (St) of the
143
microspheres was calculated using the following formula: g⁄g =
− 3
144
Where W0 (g) and Wt (g) are the mass of the microspheres at dried state and at time t, respectively.
145
The effects of different saline solutions (NaCl, CaCl2 and AlCl3) on swelling ratio were tested with
146
the same methods.
147
2.6 Determination of Water Retention. The yeast/SA/PVA microspheres were firstly soaked in tap
148
water to achieve saturation. Then, the swollen microspheres were (I) placed at different temperatures 7 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
149
(5~30 °C) for 10 h and (II) centrifuged with different rotation speeds (500~4000 rpm) for 30 min.
150
The water retention capacity (R) was detected by the following equation: " % =
# − $ × 100 4 # −
151
Where We (g) and WR (g) are the weight of the microspheres at swollen state and after treated by
152
various temperatures and centrifugal forces, respectively.
153
2.7 Measurement of Water Evaporation in Soil. The sandy soil used in this study is representative
154
of the area of Xi’an, China. The soil was firstly air dried at room temperature to constant weight and
155
sieved with a 20-mesh screen. A certain amount of the yeast/SA/PVA microspheres were mixed with
156
200 g of dry soil and placed in a glass beaker. Then, the beaker was infiltrated by 200 g of tap water
157
until the water exudation from the soil gaps appeared, and the whole container was weighed (W1).
158
The control experiment without the yeast/SA/PVA microspheres was also carried out. The beaker
159
was placed at 25 °C and weighed every 2 days (marked W2). The water evaporation ratio of soil was
160
determined bythe following equation.27 Water evaporation ratio % =
/ − 0 × 100 5 200
161
2.8 Investigation of Leaching Loss Control. The yeast/SA/PVA microspheres were firstly
162
immersed into IBA solution until equilibrium adsorption. Then, 30 g of dry sand (150−200 mesh)
163
was put into a 50 mL centrifuge tube with a hole about 2 mm in diameter at the bottom, and 5 mL of
164
tap water was added to keep the sand humid. The IBA-loaded microspheres were buried in the sand
165
(humidity of 30%) at 25 °C, and covered with another 10 g of dry sand. 50 mL of tap water was
166
sprayed over the top of the sand layer to collect the leachate, in which the concentration of IBA was
167
measured spectrophotometrically. The leaching loss control ratio was calculated by the formula:28 Leaching loss control ratio % =
168
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
Page 8 of 40
Page 9 of 40
Journal of Agricultural and Food Chemistry
169
respectively.
170
2.9 Investigation of Sustained Release Behaviors. The IBA-loaded yeast/SA/PVA microspheres
171
were placed in 50 mL distilled water and the suspension was mildly shaken with maintained speed
172
for predetermined time period. At fixed time intervals, the microspheres were withdrawn from the
173
solution
174
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 −