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Controlled release fertilizer prepared using a biodegradable aliphatic copolyester of poly(butylene succinate) and dimerized fatty acid Krzysztof Lubkowski, Aleksandra Smorowska, Barbara Grzmil, and Agnieszka Koz#owska J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b00518 • Publication Date (Web): 26 Feb 2015 Downloaded from http://pubs.acs.org on March 4, 2015
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Journal of Agricultural and Food Chemistry
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Controlled-Release Fertilizer Prepared Using a Biodegradable Aliphatic
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Copolyester of Poly(butylene succinate) and Dimerized Fatty Acid
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Krzysztof Lubkowski*, Aleksandra Smorowska, Barbara Grzmil, Agnieszka Kozłowska
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West Pomeranian University of Technology, Institute of Chemical and Environment
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Engineering, ul. Pułaskiego 10, 70-322 Szczecin, Poland
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*
Corresponding author (Tel: +48 91 449 4730; Fax: +48 91 449 4686; E-mail:
[email protected]) 1 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
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ABSTRACT
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The preparation and characterization of a controlled-release multicomponent (NPK)
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fertilizer with the coating layer consisting of a biodegradable copolymer of poly(butylene
5
succinate) and a butylene ester of dilinoleic acid (PBS/DLA) is reported. The morphology and
6
structure of the resulting polymer-coated materials and the thickness of the covering layers
7
were examined using X-ray diffraction and scanning electron microscopy coupled with
8
energy dispersive X-ray analysis. The mechanical properties of these materials were
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determined with a strength testing machine. Nutrient release was measured in water using
10
spectrophotometry, potentiometry and conductivity methods. The results of the nutrient
11
release experiments from these polymer-coated materials were compared with the
12
requirements for controlled-release fertilizers. A conceptual model is presented describing the
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mechanism of nutrient release from the materials prepared in this study. This model is based
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on the concentrations of mineral components inside the water-penetrated fertilizer granules,
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the diffusion properties of the nutrients in water and a diffusion coefficient through the
16
polymer layer. The experimental kinetic data on nutrient release were interpreted using the
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sigmoidal model equation developed in this study.
18 19
KEYWORDS: controlled-release fertilizers, biodegradable materials; aliphatic copolyesters
20 21 22 23 24 25
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Journal of Agricultural and Food Chemistry
INTRODUCTION
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Global consumption of mineral fertilizers has systematically increased in recent years
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from 135.4 Mt (80.8 Mt N, 32.4 Mt P2O5, 22.2 Mt K2O) in 2000/2001 to 177.2 Mt (107.9 Mt
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N, 41.4 Mt P2O5, 28.0 Mt K2O) in 2011/2012, with a slight decrease in consumption
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occurring in 2008/2009 due to the crisis in the banking system. Fertilizer consumption in the
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2012/2013 season was 178.6 Mt, while worldwide demand is forecast to reach 187.9 Mt and
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200.3 Mt in 2014/2015 and 2018/2019, respectively.1
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While fertilizer production and soil fertilization continue to increase, nutrient use
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efficiency (NUE) remains relatively low. Nutrient assimilation by crops is estimated to reach
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50-60% for nitrogen and potassium in the first year and 10-25% for phosphorous in the first
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year, with a further 1-2% per year reduction in following years.2-4 The low effectiveness of
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mineral component assimilation causes serious problems in view of environmental protection,
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and it also adversely affects the human and animal health.3,5-7 This low NUE has adverse
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economic effects as well, including material losses, energy expenditures and wasted labor,
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negatively affecting the overall economic balance of the agrochemical production process.
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Additionally, non-renewable energy sources such as natural gas are often used for the
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production of mineral fertilizers.8
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Improvements in nutrient use efficiency resulting from developments in fertilizer
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production and utilization have been thoroughly discussed and summarized elsewhere.9,10
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Increases in the effectiveness of nutrient assimilation can be achieved by the production of
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liquid fertilizer both in solution and suspension forms (elimination of drying and granulation
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stages of the process), stabilized fertilizers (with nitrification or urease inhibitors) and urea
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supergranules or urea supergranules containing P and K nutrients for deep placement. These
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increases can be also achieved by developing, producing and applying fertilizers with a
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delayed release of mineral nutrients. The so-called "intelligent fertilizers" release mineral
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components according to the nutrient requirements of the plants4: examples of such materials
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are slow release fertilizers (SRF) and controlled-release fertilizers (CRF).
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According to AAPFCO (Association of American Plant Food Control Officials),11 slow
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release fertilizers (SRF) are chemically or biologically degradable materials with a high
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molecular weight, complex structure and low solubility, whereas controlled-release fertilizers
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(CRF) are materials that release the mineral components through a polymer layer or a
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membrane. A fertilizer is commonly referred to as a slow or controlled-release fertilizer if the
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following requirements are fulfilled: no more than 15% of nutrients are released within 24 h,
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no more than 75% of nutrients are released within 28 days, and at least 75% of nutrients are
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released by the stated release time.12 The application of SRF/CRF that release their nutrients
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in a way that better matches plant requirements improves the effectiveness of fertilization by
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minimizing losses between application and absorption. At the same time, using SRF/CRF
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reduces the negative environmental influence of fertilizers by avoiding the deposition of an
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excessively large amount of highly soluble nitrogen compounds. As SRF/CRF release their
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nutrients slowly and gradually during all vegetation seasons, they only need to be applied
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once, greatly reducing both time and energy consumption. A better and more efficient use of
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nutrients can lead both to a reduction of waste material produced by the fertilizer industry and
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to a reduction in natural gas consumption.
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One of the drawbacks of CRFs, particularly polymer-coated CRFs, is that after nutrient
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consumption, a considerable amount of non-functional polymer remains in the soil,
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amounting to approximately 50 kg/ha per year.4 It should be kept in mind in that context that
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the average consumption of nutrients (NPK) in European Union is approximately 100 kg/ha
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per year. One possible solution that has not yet been implemented on a large scale, is to
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produce CRF using biodegradable materials,13 either natural materials or biosynthetic
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materials manufactured from renewable raw materials. Among the various biodegradable
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materials used for this purpose, starch and its derivatives seem to be the most extensively
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investigated,14,15 although cellulose and its derivatives,16-18 chitosan,19,20 and polylactic acid21
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have also recently been examined for this application as described in the literature.
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All the products with prolonged nutrient release, available on the market, including
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commercially successful ones like OsmocoteTM, MulticoteTM, MeisterTM, NutricoteTM,
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BasacoteTM or PolyonTM, are manufactured with the use of polymers that are not
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biodegradable. The objective of this research was to obtain materials with controlled-release
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properties by coating granules of a mineral fertilizer with a layer of biodegradable aliphatic
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copolymer of poly(butylene succinate) and a butylene ester of dilinoleic acid (PBS/DLA)
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using an immersion method, the characterization of the prepared materials and evaluation of
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the nutrient release kinetics. As far as we know this kind of copolyester (PBS/DLA) has not
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been used in the preparation of controlled-release fertilizer, however copolyesters of succinic
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acid with starch22 and maleic anhydride23 have been evaluated as biodegradable matrix
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materials for the controlled release of bacterial fertilizers.
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MATERIALS AND METHODS
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Materials
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The source of active, mineral, macronutrient components (nitrogen, phosphorous,
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potassium) for this experiment was the commercial, granulated, multicomponent fertilizer
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NPK(S) 6-20-30-(7). This fertilizer contains at least 92% of particles within the range of 2-5
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mm. Its bulk density is approximately 0.9-1.0 kg/dm3. For the experiment, the granule fraction
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with a diameter of 3-4 mm was selected.
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The fertilizer was commercially produced according to the Dorr-Oliver method.24 The
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mixture of phosphoric and sulphuric acid was neutralized in the saturator with ammonia to
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obtain monoammonium phosphate and ammonium sulphate slurry, which was then granulated
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in the blunger with potassium chloride and small amounts of magnesium carbonate as a filler.
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Fertilizer granules were dried, crushed, sieved and conditioned to obtain the final product.
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The contents of the three main components of the fertilizer, namely nitrogen (5.9 wt% N-
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NH4+), phosphorous (19.8 wt% total P2O5, 18.2 wt% water-soluble P2O5), and potassium
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(29.5 wt% K2O) were determined by potentiometry with an Orion 9512 ion selective
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electrode (Thermo Fisher Scientific, Beverly, MA), spectrophotometry with a Spekol 11
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instrument (Carl Zeiss, Oberkochen, Germany) and emission flame photometry with a BWB-
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1 instrument (BWB Technologies, Newbury, U.K.), respectively. These measured contents of
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the mineral components are consistent with the data declared by the fertilizer producer.
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The aliphatic copolymer of poly(butylene succinate) and a butylene ester of dilinoleic
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acid (PBS/DLA) was used as a coating material. The aliphatic copolyester was synthesized
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through direct melt polycondensation in the presence of 1,4-butanediol.25 This copolyester is a
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multi-block thermoplastic elastomer composed of soft sequences containing flexible chains of
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butylene ester of dilinoleic acid (DLA) and poly(butylene succinate) (PBS) as hard segments.
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The obtained copolymer contains 60 wt% PBS and 40 wt% DLA. This material was shown to
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be biodegradable in biodegradation tests carried out on thin polymer films (60×60×0.5 mm)
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under controlled conditions at 50 °C for 3 months in a mixture of sand and compost,
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according to the previously reported procedure.26 The weight losses of the PBS/DLA
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copolyester were approximately 5 wt% after one month, 8 wt% after 2 months and 10 wt%
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within 3 months of incubation,25 however there are reports on complete biodegradation of
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PBS and PBS blends with various organic materials.22,23,27
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Despite the fact that the use of PBS/DLA in the preparation of controlled-release
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fertilizers may bring many benefits, especially in the context of environment protection, it
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should be kept in mind that poly(butylene succinate) is not a cost-effective material. The price
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of commercially available PBS is approximately 5.4-5.6 euro/kg and it is comparable with
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other synthetic biodegradable polyesters28 but slightly higher than the price of bio-based,
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biodegradable polymers from natural resources and several times higher than the price of
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conventional, not biodegradable synthetic polymers. The high price of PBS, being the main
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obstacle of its possible acceptance as a fertilizer component, can be lowered by the
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preparation of PBS blends with various organic materials from natural resources, for example
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with dilinoleic acid.
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Preparation
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The granules of the multicomponent fertilizer in the 3-4 mm fraction were weighed and
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coated with a layer of biodegradable copolyester of PBS/DLA by the immersion method. For
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this purpose, appropriate amounts of solid polymer were dissolved, gradually and with the use
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of a magnetic stirrer, in appropriate amounts of chloroform (preferably 10 g ± 0.1 g of
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PBS/DLA and 90 g ± 0.1 g of chloroform) to obtain 10 wt% solution. Fertilizer granules were
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placed separately as a monolayer on the stainless steel wire cloth (opening size 1.6 mm x 1.6
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mm) and dipped into the solution, with a contact time of 2-3 s, at a constant temperature of 25
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°C, and then dried at 60 °C to evaporate the solvent residue. Repeating this dipping/drying
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procedure controlled and determined the amount of polymer layer coating the granules. After
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preparation, the coated fertilizer granules were weighed again. This method was used to
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prepare materials with various polymer to fertilizer (P/F) mass ratios.
147 148
Methods
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The morphology of the polymer-coated granules and the thickness of the coating layers
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were examined using an SU8020 scanning electron microscope with cold field emission and
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resolving power 1.3 nm (Hitachi High-Technologies Corporation, Tokyo, Japan) coupled with
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an energy dispersive NSS 312 X-ray analyzer (Thermo Fisher Scientific, Madison, WI).
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Granules of the prepared fertilizer materials were mechanically polished to achieve a flat
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cross section and then placed on double-sided adhesive tape kept on an aluminum stub.
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Granules were sprayed with a thin conductive film of chromium in a Q150T ES turbo-
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pumped sputter coater (Quorum Technologies, Laughton, U.K.). Coated fertilizers were
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observed at an accelerating voltage of 138 eV. Cross sections through the granules of the
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prepared fertilizer materials were recorded at a magnification of 20X. The thickness of the
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polymer layers was measured at a magnification of 100X. EDX mapping was also performed.
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XRD experiments were performed on an X′Pert PRO powder diffractometer
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(PANalytical, Almelo, Netherlands) with a proportional detector, computer-aided data
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acquisition and a copper lamp as an X-ray radiation source. The XRD patterns were recorded
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over the 2θ range from 10-60o.
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The mechanical strength of the prepared material granules was measured using a
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commercial table-top Zwick/Roell Z2.5 strength testing machine (Zwick GmbH, Ulm,
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Germany) with the TestExpert II ver. 2.2 software. The granules were individually
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compressed in the radial direction between two rigid plates with a maximum testing force
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Fmax of 2.5 kN at a constant crosshead test speed of 10 mm/s. The load at which the granule
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fractured was recorded as the crushing strength. For each sample, a total of 25 parallel
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measurements were carried out. Crushing strength was normalized to the granule diameter
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and compared with the mechanical properties of a commercial fertilizer.
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Nutrient release from the prepared fertilizer materials was measured over time in water.
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A sample of the prepared material (preferably five granules, weighted with the accuracy of
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0.0001 g) was placed in a plastic cap beaker with 50 mL ± 0.5 mL of distilled water. The
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sample was incubated at a constant temperature of 25 oC ± 0.5 oC. After 1 h of incubation the
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solution was decanted and the beaker was again filled with 50 mL ± 0.5 mL of distilled water.
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This procedure was repeated after 3, 6, and 24 h and after 3, 5, 7, 14, 21 and 28 days.
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Spectrophotometric and potentiometric methods were used to determine the amounts of
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released phosphates and ammonium, respectively.
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The overall release of water-soluble mineral components of the fertilizer was also
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monitored and analyzed with the use of CPC 505 pH/conductivity meter with temperature
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compensation (Elmetron, Zabrze, Poland). Polymer-coated granules with a given polymer to
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fertilizer ratio (P/F) were placed into distilled water at ambient temperature, and the
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conductivity was measured automatically every hour for six days (144 h). At the end of the
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experiment, the granules were mechanically crushed to determine the final conductivity value.
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Data analysis software system STATISTICA version 10.0 was used to evaluate
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descriptive statistics, i.e. standard deviation and standard error. To describe experimental data
188
related to nutrient release experiments with model lines OriginPro 8 software was used.
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RESULTS AND DISCUSSION
191 192
Morphology and structure
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Granules of the NPK fertilizer are initially of regular, spherical shape and uniform light-
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grey to dark-grey color. No permanent caking of the fertilizer granules are observed: they are
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hard, although when subjected to adequate crushing force they can be easily crumbled into
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small pieces that can then be milled into a powder. XRD measurements (Figure 1)
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demonstrate that the NPK fertilizer includes nitrogen, phosphorus and potassium mainly in
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the form of dihydrogen ammonium phosphate, ammonium chloride and potassium chloride.
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Nonetheless, as the multicomponent NPK fertilizer is an example of a complex fertilizer,
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some small amounts of additional compounds resulting from reactions between initial
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substrates may also be observed and identified.
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Fertilizer granules were coated with PBS/DLA copolyester by the immersion method to
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create over forty samples of materials with different polymer to fertilizer mass ratios and with
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various layer thicknesses. The mass ratio of polymer to fertilizer P/F was determined based on
205
the mass balance of the process. Fertilizer materials have been obtained with mass ratios P/F
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ranging from 0.18 up to 0.26. The fertilizer granules are completely covered with the
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polymer. The prepared materials consist of a fertilizer core surrounded by an inert polymer
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layer. The surface of the prepared materials is uniform, dense and compact. There are no
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visible cracks or fractures on the material surfaces. The degree of coverage of granules with
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the polymer, i.e., the polymer to fertilizer mass ratio, increased with the number of immersion
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cycles and the contact time.
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Cross sectional SEM images of the NPK fertilizer granules covered with PBS/DLA at
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two different polymer to fertilizer mass ratio extremes of 0.18 and 0.26 are presented in
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Figure 2. As the thickness of the layer varies around the granule, twenty measurements were
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made to ensure accurate evaluation, with five measurements for every quarter of the granule
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cross-section. Based on the performed measurements, the mean value of the polymer coating
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thickness was determined to be 190.4 µm (with a standard deviation of 9.21 and a standard
218
error of 2.06) in the case of the material with P/F = 0.18 (Figure 2A) and 231.2 µm (with a
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standard deviation of 8.81 and a standard error of 1.97) in the case of the material with the
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mass ratio P/F = 0.26 (Figure 2B).
221
The distribution of elements in the prepared materials was characterized using a
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scanning electron microscope coupled with the energy dispersive X-ray analyzer. The EDX
223
mapping of the cross-section of the material with P/F = 0.18 is presented in Figure 3. The
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EDX analysis demonstrates that only carbon and oxygen are present in the polymer coating
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layer. This observation is consistent with the chemical composition of the polymer used in the
226
experiment. The structure of the formed layer is rather homogeneous, with carbon and oxygen
227
atoms spread quite uniformly throughout the layer. The EDX analysis demonstrates that the
228
components of the polymer coating do not penetrate into the granule core, which appears to
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be quite heterogeneous. The EDX mapping exhibits areas with higher concentrations of
230
potassium (red dots) and chlorine (bright pink dots). The increased incidence of these
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elements results from the presence of larger particles of potassium chloride in those places. It
232
is expected that the concentrations of oxygen (green dots) and phosphorus (dark blue dots), as
233
the components of ammonium phosphate salts, are significantly lower in these areas. The
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EDX mapping also indicates that small amounts of sulphur (light blue dots) are spread evenly
235
throughout the granule. Setting aside the location and arrangement of the elements in the
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granule core, the EDX analysis demonstrates that fertilizer components from the granule core
237
do not pass into the polymer layer. To confirm that observation, a granule covered with a
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polymer layer was cut in half and the mineral components of the fertilizer were rinsed off
239
with water. A meticulous and thorough inspection of the polymer layer revealed no
240
differences in appearance between the inside and outside surfaces of the layer.
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To investigate whether any interactions occur between the fertilizer components and the
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polymer material used to cover the granules, XRD analysis was also performed. The XRD
243
spectra of the prepared materials (not presented here) were compared with the spectrum of the
244
initial NPK fertilizer. These spectra are highly similar, with no differences observed in the
245
position and intensity of the peaks between the spectra of the prepared materials and the initial
246
NPK fertilizer. All recorded peaks can be assigned only to the fertilizer components. Based on
247
the fact that no new peaks are present in the spectra of the prepared materials, no new phases
248
are formed during the coating process. Therefore, the interactions between the fertilizer
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components of the granule and the polymer layer components are physical but not chemical in
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nature.
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Mechanical properties
253
Crushing strength was measured for samples of prepared materials with the lowest and
254
highest polymer to fertilizer mass ratios of 0.18 and 0.26 respectively, and for the initial NPK
255
fertilizer. For each sample, 25 parallel measurements were performed and the mean value of
256
the crushing strength was then calculated. The dependence of the crushing strength on
257
deformation for the polymer coated materials and for the initial NPK fertilizer are presented
258
in Figures 4.
259
Fertilizers can be considered typical materials with a brittle failure mode. Fertilizer
260
granules are porous, containing various mineral compounds with many flaws, dislocations and
261
crystal edges.29,30 Any discontinuity in the fertilizer granule bulk should be treated as a defect
262
and the reason for the stress concentration. The mechanical damage to the fertilizer granule
263
results from brittle failure due to an abrupt disastrous increase of a decisive defect under
264
tensile stress induced in the material bulk. In the case of the initial NPK fertilizer, fracture or
265
rather cracking of the granule occurred at the crushing strength of 21 N, with the mean
266
deformation of 17% (Figure 4). The obtained crushing strength (21 N) corresponds to
267
pressure of 2.1 kg/granule and is comparable with the literature data for various granulated
268
fertilizer compounds: urea (1.5-3.5 kg/granule), triple superphosphate (TSP) (1.5-3
269
kg/granule), monoammonium phosphate (MAP) (2-3 kg/granule) or ammonium sulphate (1.5-
270
2.5 kg/granule).31 Compared with the starting NPK fertilizer, polymer-coated granules
271
exhibited significantly higher resistance towards the crushing force (Figure 4). For both
272
prepared and examined materials, the crushing force continuously increased with deformation
273
up to 50%, and no fracture or cracking of the samples were observed. The crushing force
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reached the value of almost 170 N in the case of the material with the polymer to fertilizer
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mass ratio of 0.26, and 120 N for the material with the polymer to fertilizer mass ratio of 0.18.
276
Based on the experiments and dependencies presented here, it is not possible to determine the
277
maximum crushing strength for the prepared polymer-coated materials because of their
278
ductile properties. Instead of a brittle failure, only a plastic deformation of these samples can
279
be observed, evidently within the deformation region from 0-50%.
280 281
Nutrient release
282
All prepared polymer-coated materials were tested in water release experiments. This
283
kind of approach (nutrient release in water) is fully acceptable: several papers using that
284
approach have been published recently.16,18,32-35 Nutrient release in water and nutrient release
285
in soil are quite different aspects of the topic. Undoubtedly, field experiments are necessary in
286
the further and final assessment of the product, but these experiments will be addressed in
287
another paper. Besides, as it was clearly indicated in the relevant references,16,36 nutrient
288
release in soil can be easily predicted from release experiments performed in water, although
289
the nutrient release rate depends on the medium used, with the fastest release rate observed in
290
water, followed by water-saturated sand, and finally by sand.36
291
Figure 5 presents the results of the investigation of phosphate release from the prepared
292
polymer-coated materials over 24 h. The data presented here show that for the group of
293
materials with polymer to fertilizer mass ratios in the range from approximately 0.23-0.26,
294
less than 15% of phosphates is released within 24 h. Additionally, long-term release
295
experiments have demonstrated that the release of phosphate from this group of materials is
296
less than 75% within 28 days. On the other hand, for the group of materials with polymer to
297
fertilizer mass ratios less than 0.23, more than 15% of phosphates are released within 24 h.
298
Based solely on these phosphate release results, the materials prepared here with polymer to
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fertilizer mass ratios ranging from approximately 0.23-0.26 meet the first and second criteria
300
of controlled-release fertilizers, while those with mass ratios less than 0.23 do not.
301
A series of papers36-39 reported that the release rate of phosphates is lower than those of
302
K, N-NH4+ and N-NO3-. There is no consensus as to the margin of that discrepancy. In some
303
experiments, the release rate of phosphates was 45-70% slower than that of nitrates,36 but only
304
a 10% difference was observed in another study.38 The presence of strong interactions among
305
nutrients in the fertilizer granule and differences in nutrient solubility could lead to this
306
observed difference in release rate.36 Generally, the phosphate release rate can be treated as a
307
“bottleneck” for overall nutrient release. The kinetics of chemical processes are known to be
308
controlled by their slowest steps. The release of phosphates from controlled-release fertilizers
309
is the slowest step, therefore their release rate determines the overall nutrient release from that
310
type of material. To define controlled-release fertilizers, however, rather than knowing which
311
fertilizer component is released the slowest, it is important to know which is released the
312
fastest or to determine the overall nutrient release rate.
313
Additional experiments have been conducted to more accurately assess the release
314
properties of the materials prepared here. The release of ammonium and phosphate ions for
315
the sample of prepared material with the highest mass ratio of polymer to fertilizer (P/F =
316
0.26) is compared in Figure 6A. Throughout the whole experiment, the release of phosphates
317
is slower than the release of ammonium ions. However, the most significant difference in
318
release is observed during the first 24 h. After that time, the cumulative release for phosphate
319
and ammonium ions is 13% and 37%, respectively, whereas after 7 days it is 58% and 80%.
320
The average difference between the release of phosphate and ammonium ions is 19%.
321
Figure 6B presents the results of nutrient release measured by conductivity for the
322
sample of prepared material with the highest mass ratio of polymer to fertilizer (P/F = 0.26).
323
The release of the mineral components of the fertilizer R(%) was calculated according to the
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following equation, where σt represents the measured conductivity at time t and σF represents
325
the final conductivity value:
σ t ⋅ 100 σF
326
R=
327
Several studies have shown36,40,41 that nutrient release from polymer-coated CRF can be
328
described in three stages: lag period, linear stage and decay period. In the experiment
329
presented here (Figure 6B), the lag period is quite long. Placing the tangent line onto the
330
experimental curve at the inflection point (31st hour) enables a rough estimate of the lag time,
331
which is between 5 and 14 h. The linear stage of the release profile cannot be identified
332
because the release rate constantly increases until the maximum release rate is reached at the
333
thirty first hour of the process. From that point, the release rate slowly decays, almost
334
reaching zero after 140 h. Two further observations are worth mentioning. First, between
335
hours 8 and 14, the release rate substantially increases, which might be related to the earlier
336
differences in the release rates of fertilizer components. Secondly, after 24 h the release of all
337
water-soluble fertilizer components is approximately 11%, significantly below the limit of the
338
first requirement of slow/controlled-release fertilizers.
(1)
339
Based on the experimental results presented here, the following conceptual model of
340
nutrient release from the prepared fertilizer material can be proposed. At the very beginning
341
of the process, after immersion of the polymer-coated fertilizer in water, water starts to
342
diffuse through the polymer layer to penetrate the inner part of the granule. The fertilizer
343
material is mainly composed of readily water-soluble compounds with similar water
344
solubilities (KCl – 342 g/dm3, NH4Cl – 370 g/dm3, NH4H2PO4 – 370 g/dm3). The fertilizer
345
components begin to dissolve in water. Based on the initial fertilizer composition and XRD
346
measurement results (Figure 1), it is obvious that the same amounts of ammonium and
347
phosphate ions result from the dissolution of dihydrogen ammonium phosphate. However, an
348
additional amount of ammonium ions is present within the granule due to ammonium chloride 15 ACS Paragon Plus Environment
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349
dissolution. Therefore, it seems reasonable to conclude that the concentration of ammonium
350
ions in the water-penetrated fertilizer granule is greater than the concentration of phosphate
351
ions. Moreover, the concentration of phosphate ions is lower because slight amounts of
352
phosphorous (approximately 8%) are bound up in the form of water-insoluble compounds.
353
Differences in these ion concentrations may be one of the factors affecting nutrient release
354
from polymer-coated fertilizer. Water-dissolved components of the fertilizer diffuse through
355
the polymer layer outside the granule. As the driving force for diffusion depends on the
356
component concentration gradient, the release of ammonium ions surpasses the release of
357
phosphate ions. In discussing the release of ions (nutrients) from polymer-coated fertilizer,
358
two other parameters should be considered, diffusion coefficients in water and ionic radii of
359
non-hydrated and hydrated ions (Supporting Information Table SI-A).42-46 As the ionic
360
potential |Zi|/ri increases, where |Zi| is the absolute value of the charge of ion i and ri is the
361
crystal ionic radius of ion i, the ionic diffusion in water decreases. The reason for that
362
dependence is that the hydration layer of the water molecules around the ion becomes thicker,
363
thus hindering movement of the ion. The diffusion of phosphate ions in water is around three
364
times less than the diffusion of ammonium ions in water, thus contributing to the slower
365
release of phosphate ions from the polymer-coated granule.
366
Summarizing the previous considerations, both the observed discrepancy between the
367
release of phosphate and ammonium ions (Figure 6A) and the substantial increase in the
368
release rate between hours 8 and 14 of the process (Figure 6B) can be explained by the
369
influence of nutrient concentration and diffusion properties of those nutrients in water.
370
However, based on the available experimental data, it is not possible to assess the mutual
371
interaction of these factors in this phenomenon.
372
The release of phosphates and nutrients in general from the prepared polymer-coated
373
materials depends on the polymer to fertilizer mass ratio (Figure 5). The phosphate release
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374
decreases as the mass ratio of polymer to fertilizer increases and as the layer thickness
375
increases. The diffusion mechanism of the release can be noted as a reason for that
376
correlation.36 As the layer thickness increases, the value of the diffusion coefficient D
377
decreases and the release process becomes increasingly hindered. The value of the diffusion
378
coefficient D through the polymer layer can be calculated according to the equation: h2 6 ⋅ t lag
379
D=
380
Taking the value of the polymer layer thickness (h = 231 µm) (Figure 2) and the value
381
of the lag period (tlag = 5-14 h) (Figure 6B) for the material with the highest polymer to
382
fertilizer mass ratio (P/F = 0.26), the estimated value of the diffusion coefficient D may be in
383
the range of 0.176-0.494⋅10-8 cm2/s. The diffusion coefficient through the polymer layer D is
384
around four orders of magnitude smaller than the diffusion coefficients in water Do presented
385
earlier. The obtained D value refers to the diffusion of all water-soluble components of the
386
fertilizer. We cannot provide detailed values for particular ions based on this experimental
387
data, but this will be addressed in future research.
(2)
388
Note that all release experiments presented here were conducted in water. As mentioned
389
earlier, nutrient release from polymer-coated controlled-release fertilizers is mainly controlled
390
by diffusion. When materials with biodegradable polymer coatings are investigated in those
391
conditions (water release experiments), the diffusion through the polymer layer has a critical
392
impact on the release pattern. If the release experiments from materials coated with these
393
biodegradable polymers were performed in soil, then some other factors and parameters
394
would be important, especially microorganism-induced biodegradation of the polymer layer.
395
While microorganism-related biodegradation of the polymer layer and its influence on
396
nutrient release is not addressed in the current work, that issue should be the topic of future
397
research.
17 ACS Paragon Plus Environment
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398
Figure 7 presents the results of 28 day release experiments for the polymer-coated
399
materials with polymer to fertilizer mass ratios of 0.18, 0.20, 0.24 and 0.26 and the NPK
400
fertilizer. During these experiments, the amounts of released phosphates were determined as a
401
function of time at a constant temperature of 25 °C. The release of phosphates from the NPK
402
fertilizer was very fast, with all phosphates released within 55-60 min. Based on the results
403
presented here, all coated materials exhibit slower release of phosphates in comparison with
404
the starting fertilizer.
405
Nutrient uptake by plants exhibits a sigmoidal relationship with the point in the plant’s
406
vegetation cycle.3,4,46,47 Therefore, all experimental data presented related to nutrient release
407
from the prepared polymer-coated materials were described and interpreted with a kinetic
408
model. To interpret the kinetic data, a sigmoidal model equation was applied:
409
R=
410
where Mt/M∞ is the fraction of nutrients released at time t and a, b, c, d are the parameters of
411
the sigmoidal equation.
Mt = M∞
a−b +b t −d 1 + exp c
(3)
412
The experimental data of nutrient release from the prepared polymer-coated materials
413
and the predicted values calculated using the sigmoidal equation are compared in Figures 6-7.
414
The sigmoidal model satisfactorily fits the experimental data, with the best result obtained for
415
the experiment performed using the conductivity method (Figure 6B). Based on the applied
416
sigmoidal model, the constants and correlation coefficients of the equations were determined
417
and are given in Supporting Information Table SI-B.
418
The sigmoidal equation produces model lines that fit the experimental data very well as
419
expressed by correlation coefficients. Keeping in mind the fact that the nutrient uptake by
420
plants follows a sigmoidal curve and that the sigmoidal model has already been applied to
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Journal of Agricultural and Food Chemistry
421
describe controlled release,48-50 the sigmoidal equation used in the present work satisfactorily
422
predicts the release of phosphates from the prepared polymer-coated materials.
423 424
In conclusion, the new, innovative, biodegradable copolymer of poly(butylene
425
succinate) and the butylene ester of saturated dilinoleic acid was successfully applied in the
426
preparation of fertilizer materials with controlled and prolonged nutrient release.
427
Multicomponent fertilizer granules can be coated with a polymer layer using the immersion
428
method, which creates an incentive to use the process on a larger scale, for example by
429
spraying in a drum granulator or in the Wurster bed.
430
The obtained polymer-coated materials differ from each other in the layer thickness and
431
the polymer to fertilizer mass ratio. The mass ratio of polymer to fertilizer ranged from 0.18-
432
0.26, and the layer thickness ranged from 190-230 µm. The interactions between the fertilizer
433
components of the granule and the polymer layer components were found to be physical
434
rather than chemical in nature. The mechanical properties of the fertilizer granules coated
435
with the copolyester layer are significantly better than those of the NPK fertilizer.
436
Release experiments have shown that polymer-coated materials with polymer to
437
fertilizer mass ratios of 0.23-0.26 meet the requirements of controlled-release fertilizers. The
438
overall nutrient release was less than 15% within 24 h and approximately 75% after 28 days,
439
although the ammonium ion release during first 24 h of the experiment was significantly
440
higher than the release of phosphate ions. The concentrations of the mineral components
441
inside the water-penetrated fertilizer granules and the diffusion properties of the nutrients in
442
water have been indicated as the reasons for that discrepancy. Nutrient release from the
443
polymer-coated materials decreased with the increasing layer thickness. The diffusion
444
mechanism of release has been noted as a reason for that correlation. The diffusion coefficient
445
through the polymer layer was estimated as D = 0.176-0.494⋅10-8 cm2/s. The experimental
19 ACS Paragon Plus Environment
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446
kinetic data of nutrient release from the prepared polymer-coated materials have been
447
accurately described using a sigmoidal model equation.
448 449
SUPPORTING INFORMATION
450
Tables SI-A and SI-B. This material is available free of charge via the internet at
451
http://pubs.acs.org.
452 453
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454 455
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458
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8152ed74f306&=404%3bhttp%3a%2f%2fwww.fertilizer.org%3a80%2fen%2fimages%2
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fLibrary_Downloads%2f2014_ifa_sydney_summary.pdf (15 January 2015).
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Trenkel, M. E. Slow- and controlled-release and stabilized fertilizers: an option for
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Finck, A. Fertilizers and their efficient use. In World Fertilizer Use Manual; Halliday, D.
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18. Costa, M. M. E.; Cabral-Albuquerque, E. C. M.; Alves, T. L. M.; Pinto, J. C.; Fialho, R.
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20. Corradini, E.; de Moura, M. R.; Mattoso, L. H. C. A preliminary study of the
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incorporation of NPK fertilizer into chitosan nanoparticles. eXPRESS Polym. Lett. 2010,
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22. Wu, C.-S. Controlled release evaluation of bacterial fertilizer using polymer composites as matrix. J. Control. Release 2008, 132, 42-48. 23. Wu, C.-S. Promoting fertilizer use via controlled release of a bacteria-encapsulated film bag. J. Agric. Food Chem. 2010, 58, 6300-6305.
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24. Lutz, W. A. Production of ammonium phosphate. Patent US3310371, United States Patent Office, 1967.
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27. Reddy, M. M.; Mohanty, A. K.; Misra, M. Optimization of tensile properties
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thermoplastic blends from soy and biodegradable polyesters: Taguchi design of
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28. Changsha Green Mountain Chemical Co., Organic Products, Polybutylene succinate,
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http://greenmountchem.en.alibaba.com/product/526875229-
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213189775/Polybutylene_succinate.html (15 January 2015).
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29. Kelbaliyev, G. I.; Samedli, V. M.; Samedov, M. M.; Kasimova, R. K. Experimental study
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and calculation of the effect of intensifying additives on the strength of superphosphate
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30. Nie, X.; Besant, R. W.; Evitts, R. W. An experimental study of moisture uptake and transport in a bed of urea particles. Granul. Matter 2008, 10, 301-308. 31. Clayton, W. E. Physical properties of fertilizers. In Modern techniques in fertilizer distribution and handling, Muscle Shoals, Alabama, 1995.
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32. Bortolin, A.; Aouada, F. A.; de Moura, M. R.; Ribeiro, C.; Longo, E.; Mattoso, L. H. C.
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Application od polysacharide hydrogels in adsorption and controlled-extended release of
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fertilizers processes. J. Appl. Polym. Sci. 2012, 123, 2291-2298.
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33. Calabria, L.; Vieceli, N.; Bianchi, O.; de Oliveira, R. V. B.; Filho, I. N.; Schmidt, V. Soy
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protein isolate/poly(lactic acid) injection-molded biodegradable blends for slow release of
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34. Pereira, E. I.; Minussi, F. B.; da Cruz, C. C. T.; Bernardi, A. C. C.; Ribeiro, C. Urea-
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36. Du, C.; Zhou, J.; Shaviv, A. Release characteristics of nutrients from polymer-coated compound controlled release fertilizers. J. Polym. Environ. 2006, 14, 223-230. 37. Shoji, S.; Gandeza, A. T. Controlled release fertilizers with polyolefin resin coating, Kanno Printing Co. Ltd. Sendai, Japan, 1992.
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38. Huett, D. O.; Gogel, B. J. Longevities and nitrogen, phosphorus and potassium release
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patterns of polymer-coated controlled-release fertilizers at 30°C and 40°C. Commun. Soil
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39. Shaviv, A. Advances in controlled release of fertilizers. Adv. Agron. 2000, 71, 1-49.
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40. Du, C.; Zhou, J.; Shaviv, A.; Wang, H. Mathematical model for potassium release from
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polymer-coated fertilizer. Biosystems Eng. 2004, 88, 395-400.
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42. Li, J.-H.; Gregory, S. Diffusion of ions in sea water and in deep-sea sediments. Geochim. Cosmochim. Acta 1974, 38, 703-714.
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43. Gourary, B. S.; Adrian, F. J. Wave functions for electron-excess color centers in alkali halide crystals. Solid State Phys. 1960, 10, 127-247.
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45. Conway, B. E. Ionic Hydration in Chemistry and Biophysics (Studies in Physical &
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46. Shaviv, A. Controlled Release Fertilizers, IFA International Workshop on Enhanced-
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47. Shoji, S.; Takahashi, C. Innovative fertilization methods. In Meister controlled release
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fertilizer – properties and utilization, Konno Printing Company Ltd. Sendai, Japan, 1999.
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50. Shaviv, A.; Raban, S.; Zaidel, E. Modelling controlled nutrient release from a population
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582 583 584 585 586 587 588
25 ACS Paragon Plus Environment
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589
FIGURE CAPTIONS
590
Figure 1. XRD image of the commercial, multicomponent fertilizer NPK(S) 6-20-30-(7).
591
Figure 2. SEM images of the fertilizer granules coated with PBS/DLA with extreme polymer
592
to fertilizer mass ratios (A – P/F = 0.18; B – P/F = 0.26).
593
Figure 3. EDX mapping of the elements for the material with polymer to fertilizer mass ratio
594
P/F = 0.18. Elements correspond to the color code indicated at the top of each panel.
595
Figure 4. Crushing strength as a function of deformation for materials with different polymer
596
to fertilizer mass ratios and for the uncoated, starting NPK fertilizer.
597
Figure 5. Release of phosphates vs. the polymer to fertilizer mass ratio P/F after 24 h.
598
Figure 6. The comparison between the release of ammonium and phosphate ions (A) and the
599
nutrient release measured by conductivity (B) for the sample of prepared material with the
600
highest polymer to fertilizer mass ratio (P/F = 0.26).
601
Figure 7. Release of phosphates within 28 days from: A – polymer-coated materials with
602
polymer to fertilizer mass ratios of 0.18, 0.20, 0.24, and 0.26; B – uncoated NPK fertilizer.
26 ACS Paragon Plus Environment
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Journal of Agricultural and Food Chemistry
FIGURE 1
intensity, arb. u.
KCl NH4H2PO4 NH4Cl
10
20
30
40
50
60
70
2θ, deg
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FIGURE 2
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FIGURE 3
29 ACS Paragon Plus Environment
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FIGURE 4
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FIGURE 5
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FIGURE 6
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FIGURE 7
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