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Constructing slow-release formulations of metribuzin based on degradable poly(3-hydroxybutyrate) Anatoly Nikolayevich Boyandin, Natalia Olegovna Zhila, Evgeniy Gennadievich Kiselev, and Tatiana Volova J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b05896 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016
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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.
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Journal of Agricultural and Food Chemistry
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Title: CONSTRUCTING SLOW-RELEASE FORMULATIONS OF METRIBUZIN
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BASED ON DEGRADABLE POLY(3-HYDROXYBUTYRATE)
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Authorship: Anatoly Nikolayevich Boyandin*, Natalia Olegovna Zhila, Evgeniy
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Gennadievich Kiselev, Tatiana Grigorievna Volova
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Institute of Biophysics of Siberian Branch of Russian Academy of Sciences, 50/50
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Akademgorodok, Krasnoyarsk 660036, Russia
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*Corresponding author. E-mail:
[email protected] 8
Keywords: metribuzin, degradable poly(3-hydroxybutyrate), controlled release, release
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kinetics, herbicide
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ACS Paragon Plus Environment
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ABSTRACT
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Experimental formulations of herbicide metribuzin embedded in matrices of
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degradable natural polymer poly(3-hydroxybutyrate) (P3HB) and its composites with
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polyethylene glycol (PEG), polycaprolactone (PCL), and wood powder have been prepared
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in the form of pressed pellets containing 75% of polymeric basis (pure P3HB or its
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composite with a second component at a ratio of 7:3) and 25% of metribuzin. Incubation of
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formulations in soil laboratory systems led to the degradation of the matrix and herbicide
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release. The most active release of metribuzin (about 60% of the embedded herbicide over
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35 days) was detected for the P3HB/PEG carrier as compared with P3HB, P3HB/wood and
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P3HB/PCL forms (30-40%). Thus, the study shows that herbicide release can be controlled
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by the matrix formulation. Metribuzin formulations exerted a significant herbicidal effect
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on the plant Agrostis stolonifera, used as a weed plant model. Application of these long-
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term formulations will make it possible to reduce environmental release of chemicals,
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which will restrict the rate of their accumulation in trophic chains of ecosystems and abate
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their adverse effects on the biosphere.
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Journal of Agricultural and Food Chemistry
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INTRODUCTION
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Intensive agricultural technologies require implementation of a wide variety of
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chemicals for pest and weed control. However, only a minor part of pesticides applied and
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released into the environment reaches their target. The major part of those chemicals
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accumulates in the environment and biota, and pollutes soil and water.1 An actual area of
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focus is development of ecologically safe new-generation preparations with targeted and
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controlled agent release using special coating and/or matrices (carriers) from biodegradable
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materials. The key aspect of production of this type of preparations is availability of a
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suitable material possessing special properties including environmental safety, chemical
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compatibility with agricultural preparations, long-term storage quality and controlled
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degradability without forming toxic products.
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There were synthetic and natural biodegradable materials among materials studied
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for construction of new-generation agricultural preparations. Aside from individual
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polymers, composites and mixtures including polymers with different filling materials,
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among which are carboxymethylcellulose and sodium alginate with bentonite and
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anthracite,2-4 carboxylmethylcellulose with clay,5 alginate with bentonite and activated
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carbon,6 composite of bentonite or nanobentonite with polymers of acrylic acid and
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acrylamide7, ethylcellulose and lignin,8 and others have been studied for embedding.
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Studying composite matrices for preparation embedding is performed for the agent release
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kinetics regulation, and also for involvement of more available materials.
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Metribuzin is one of the modern herbicides which is intensively studied and used at
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the present time. This systemic-action wide-range herbicide eliminates broad-leaved and
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poaceous weeds by repressing photosynthesis; it is effective against different pests, such as
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ragweed, foxtail, shepherd's purse, crabgrass, wild oats, barnyardgrass, amaranth,
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cornflower, pigweed, daisy, wild mustard, sow thistle, veronica, cocklebur, etc. Metribuzin
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is easily absorbed by roots and sprouts. Degradation of the preparation can last from 1 to 3
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months depending on soil structure and climatic conditions.9 Decrease of metribuzin
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activity is usually caused by its leaching to lower horizons of soil profile.7 Because of these
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reasons, metribuzin is appropriate herbicide for using in controlled delivery systems. By
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now, it was described developing slow release metribuzin formulations with different
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natural and synthetic materials,2,7,10-18 which should enable slow release of an active
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component, be fully biodegrade in the environment and have reasonable price.
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Polymers of microbiological origin – polyhydroxyalkanoates (PHA) – are of
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particular importance among biodegradable materials. These polymers are potentially
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productive for application as a degradable biopacking material and also in specific areas
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including medicine and pharmacology, agriculture.19,20 Long-term biodegradability of PHA
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and possibility of obtaining polymeric products from them in variable physical states
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(powders, solutions, melts) open a prospect of constructing long-term preparations on their
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basis suitable for soil and pre-emergence application. Works, focused on creation of such
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matrices on the basis of PHA are on the conceptual stage. By this moment, PHA were used
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for embedding of fungicides Sumilex and Ronilan,21 insecticide malathion;22 herbicides
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atrazine and ametryn.23,24 Works of our team have studied PHA have as platforms for
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embedding insecticides α-hexochlorocyclohexane and lindane,25,26 herbicide Zellek Super
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(haloxyfop-P-methyl).27
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It is impossible not to note that increase of implementation of PHA polymers at the
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present time is still lowered by their high price. Searching for means of cost reduction of
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PHA based products is actual for PHA technical implementation.19,20 Not only pure PHA
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but also mixture of these polymers with more obtainable materials can be a way of
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embedding agricultural preparations. Filling a polymer matrix with variable materials also
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Journal of Agricultural and Food Chemistry
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gives access to regulation of the polymeric carrier degradation and release of the agent into
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the environment.
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The purpose of this study was constructing and research of sustained release
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formulations of herbicide metribuzin with application of mixtures of poly(3-
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hydroxybutyrate), the best-known polymer of PHA class, with synthetic and natural
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materials.
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MATERIALS AND METHODS
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Materials. Poly(3-hydroxybutyrate) (P3HB) (the weight average molecular weight
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Mw 920 kDa; polydispersity 2.52) was synthesized according to previously described
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technology.28 Poly-ε-caprolactone (PCL) with the number average molecular weight Mn 80
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kDa in the form of granules and also poly(ethylene glycol) with Mw 300 kDa in the form of
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powder were produced by Aldrich (USA). Wood powder was obtained by birch wood
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disintegrating with the MD 250-85 wood-carving working bench (Stanko-Premier, Russia),
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followed by drying at 60°C for 120 h until it reached its constant weight and particle
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fraction selecting by means of screen sizing with 0.5 mm mesh. Metribuzin was purchased
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from “Ecolan” (Moscow, Russia). Empirical formula: C8H14N4OS. Molecular mass: 214.3.
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IUPAC name: 4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one. CAS
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name:
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substance: colorless needles. Melting point of the active material: 126.2ºС.
4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one.
Pure
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Pure metribuzin was used as positive control 1. A commercial metribuzin
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formulation, Sencor Ultra, containing 600 g/kg of the active ingredient was purchased from
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Bayer CropScience AG (Monheim am Rhein, Germany), and used as positive control 2.
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Preparation of P3HB/filling material mixtures. For obtaining P3HB/filling
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material mixtures, polymers (P3HB and PCL) were ground using a ZM 200 mill (Retsch,
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Germany), and particle fractions under size of 1 mm were selected. Particle size distribution
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of the obtained powder was determined with an analytical sieving machine AS 200 control
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(Retsch, Germany): particle fraction of size under 0.50 mm totaled 60%; of size from 0.50
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to 1.00 mm – 40%. Pellets, 13 mm in diameter, containing 150 mg of pure P3HB or its
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mixtures with birch sawdust, powder of PCL or PEG in the ratio of 7:3 (105 mg of P3HB
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and 45 mg of the second component) were obtained by cold pressing powdered components
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using a Carver Auto Pellet 3887 press (Carver, U.S.); pressing force was 14 000 F.
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Metribuzin embedding in the P3HB/filling material mixture and obtaining the
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formulations. Pure P3HB powder and its mixtures with birch sawdust, PCL and PEG
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powder at a source ratio of 7:3 (i.e. 70% of P3HB and 30% of the second component) were
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used for construction of pellets loaded with 25% metribuzin. Specimens obtained by
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mixing of powders (150 mg of pure P3HB or 105 mg of P3HB and 45 mg of the second
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component) with 50 mg of metribuzin were cold-pressed by the same way as it was
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described in the previous section.
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Metribuzin analysis. Metribuzin was detected by gas chromatography using the
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gas chromatograph 7890/5975C (Agilent Technologies, U.S.) equipped with a mass
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spectrometer as it was described earlier.29
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Kinetics of metribuzin release from the polymeric samples was studied in laboratory
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systems. The specimens were sterilized using the Sterrad NX system (Johnson & Johnson,
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U.S.). In water experiments, the specimens were put into sterile conical flasks of 500 ml
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volume filled with 100 ml of sterile distilled water and incubated in an Innova 44 (New
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Brunswick, USA) shaker at 30°C and 150 rpm. After specified intervals, a part of samples
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was ejected; water samples from flasks were chosen and analyzed for metribuzin.
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Metribuzin was extracted with chloroform three times to determine its concentration. The
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chloroform extracts were passed through sodium sulfate. Chloroform was removed in a
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rotary vacuum evaporator.
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Journal of Agricultural and Food Chemistry
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For metribuzin concentration analysis in soil, 50 g of dry soil were overflown by
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200 ml of chloroform and left for 24 h. Then the soil was glass-filtered using a vacuum
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pump, rinsing it with chloroform, repeating the operation 3 times. United obtained extracts
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were concentrated on a rotary evaporator, then quantitatively transferred into smaller flasks
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using chloroform. Chloroform was removed in the rotary vacuum evaporator.
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The amount of metribuzin released was determined as percentage of the metribuzin encapsulated in the polymer matrix. Mathematical analysis of release kinetics of metribuzin was performed using the Korsmeyer – Peppas model:29-31
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Mt / M∞= Ktn,
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where Mt and M∞ are the amounts of the metribuzin which were released at time t
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and over a very long time, respectively, K is a kinetic constant and n is the diffusional
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exponent. M∞ generally corresponds to the initial loading. For our case (cylindrical pellets)
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n ≥ 0.45 diffusion mechanism conforms to Fick’s law, 0.45 < n 0.89 to Super Case II transport
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mechanisms. For calculating the exponent n only the part of the release curve with
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Mt/M∞
