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Controlled release formulations of alachlor based on calcium alginate
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controlled release ELSEVIER
Journal of Controlled Release 34 (1995) 17-23
Controlled release formulations of alachlor based on calcium alginate 1 Armand B. Pepperman *, Jui-Chang W. Kuan Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, LA70179, USA
Received 31 March 1994; accepted 7 October 1994
Abstract
Granular controlled release formulations of alachlor were prepared from sodium alginate-kaolin-linseed oil-herbicide slurries dropped onto calcium chloride to form spherical beads which were air-dried and evaluated for release rates in a static water extraction. These formulations gave considerably slower release of alachlor than a commercial formulation and/or the alginate formulation without oil. Factors affecting the rate of release included aging of the dried formulation and the ratio of oil/herbicide in the formulation. The size of the beads had no appreciable effect on the rate of release and several oils, including canola, corn, olive, safflower, peanut, soybean, and sunflower gave similar release rates to linseed oil formulations for alachlor. The use of lignin in combination with alginate also produced a granular formulation with reduced release rates. Keywords: Alachlor; Alginate; Kaolin; Linseed oil; Vegetable oil; Controlled release; Herbicide formulation; Groundwater
I. I n t r o d u c t i o n
A n e v a l u a t i o n o f rural drinking water sources in the U n i t e d States indicates that m o r e than 97% is derived f r o m u n d e r g r o u n d sources, along with 55% o f all livestock water and 40% o f all irrigation water [ 1 ]. A c c o r d i n g to U.S. C e n s u s data, m o r e than 30 million p e o p l e obtain their drinking water f r o m private wells. In addition to serving the needs o f rural A m e r i c a , g r o u n d w a t e r served 4 0 % o f the population using public w a t e r supplies in 1980, w h i c h includes nearly 74 mil* Corresponding author. Note: This article reports the results of research only. Mention of a pesticide in this article does not constitute a recommendation for use by the U.S. Department of Agriculture nor does it imply registration under FIFRA as amended. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. Elsevier Science B.V. SSDI 0168-3659( 94)00111-1
lion people [ 1 ]. A l t h o u g h g r o u n d w a t e r c o n t a m i n a t i o n has m a n y sources, e v i d e n c e suggests that agriculture's relative contribution m a y be significant [ 2 ] . Large quantities o f pesticides are used e x t e n s i v e l y throughout the U n i t e d States and several h a v e been identified as sources of g r o u n d w a t e r contamination [ 3,4]. Point source contamination f r o m spills, e x c e s s i v e application and i m p r o p e r disposal m a g n i f y the problem. Pesticides such as metribuzin, atrazine, alachlor, aldicarb, and carbofuran, which are k n o w n to have s o m e degree of persistence in the environment, are a m o n g those implicated as g r o u n d w a t e r contaminants [ 5 ]. A l a c h l o r [ 2 - chloro - N - ( 2,6 - diethylphenyl ) - N ( m e t h o x y m e t h y l ) a c e t a m i d e ) is a w i d e l y used herbicide for w e e d control in corn, soybean, and m a n y other crops [ 6 ]. As a result o f its w i d e usage ( a p p r o x i m a t e l y 85 million pounds per year [ 7 ] ), alachlor residues m a y contaminate groundwater, streams and rivers due to
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spraying, spills, leaching, and runoff. The use of controlled release formulations has the potential to reduce the surface run-off and leaching of soil-applied herbicides and decrease the amount of herbicide being applied to the soil. Both of these events would have favorable impacts on the environment [ 8]. In addition, the use of controlled release formulations may reduce the losses to evaporation, co-distillation, and photolysis
[9]. Alginate gels have been used as matrices for preparing controlled release formulations (CRF) of herbicides [10,11]. Incorporation of kaolin to alginate formulations of herbicides has been shown to slow down the release of herbicides [ 12]. The authors have shown that a granular formulation of metribuzin based on linseed oil, kaolin, and alginate considerably reduced the release rate of metribuzin in comparison with conventional formulations and alginate formulations without linseed oil [13]. Once the chemical is released from the CRF, it would be expected to leach at the same rate as the chemical from a conventional formulation. Because CRFs permit only low concentrations of the free pesticide to be available for leaching or runoff during precipitation, these processes are reduced. Surface runoff presents a more complex problem because the CRF must not only contain the chemical, but must also remain in place on the field during heavy rainfall. A recent report indicated that a microcapsule formulation of alachlor (ME) may have an adverse effect on surface runoff because greater losses of alachlor were observed from the ME formulation than from a granular formulation in runoff experiments [14]. Granular formulations should present a less mobile form of the pesticide during run-off events. In an earlier report [ 13 ], linseed oil was incorporated into an emulsion of clay, alginate, emulsifier, herbicide, and water in a one-step process. In this manner the linseed oil was dispersed throughout the matrix of the granules. Dropping of the soluble sodium alginate slurry into calcium chloride forms insoluble calcium alginate granules which are generally spherical in shape. Linseed oil on the surface of the granules polymerizes on drying to form a coating [ 13]. The granular beads prepared in the alginate-kaolin-linseed oil ( A K L O ) process had a coating of linseed oil, which is visible without magnification as a shiny surface, and were yellow in color. This is contrasted to beads prepared without linseed oil, which have a dull, powdery
appearing surface that is off-white. The beads prepared with lignin were dark brown in color. Light photomicrographic cross-sections of the A K L O granules in the metribuzin study [ 13] clearly showed an increased density at the outer edge which was attributed to linseed oil polymer formation on the surface of the alginate bead. Considerably reduced release rates of metribuzin in comparison with a conventional formulation and with alginate formulations without linseed oil were found with the A K L O formulations [ 13]. In the present study, linseed oil and other vegetable oils were incorporated into the alginate-kaolin formulation to determine their effect on the release rate of alachlor from granular formulations. In addition, lignin-alginate formulations were evaluated and shown to be effective for controlling the release of alachlor.
2. Materials and methods 2.1. Chemicals
Alachlor (technical grade, 94% pure) and a commercial formulation of alachlor ( L A S S O II) were provided by Monsanto Company, St. Louis, MO. Technical grade alachlor was recrystallized several times from ethyl acetate-hexane mixtures at 4°C until no significant impurities were detected by HPLC (method described in HPLC analysis). The recrystallized alachlor was clear rock-like crystals and with a mp of 40.5-41.5°C. The crystals were hand ground to 20-40 mesh in a porcelain mortar with a glass pestle. Sodium alginate, Kelgin MV, was obtained from Kelco, Division of Merck and Company, San Diego, CA, and kaolin clay from Thiele Kaolin Company, Wren, GA. Organosolv lignin was supplied by Repap Technologies, Valley Forge, PA. A suspension of 20% lignin in 0.1 N NaOH was usually prepared prior to use in the formulations. Raw linseed oil was purchased at a local hardware store (Harry's Ace Hardware, New Orleans, LA), soybean oil and corn oil (Hunt-Wesson, Inc., Fullerton, CA) from a local supermarket. Tween 20 (polyoxyethylenesorbitan monolaurate) was purchased from Sigma Chemical Co., St. Louis, MO. Deionized water purified with a NANOpure Ultrapure Water System through a 0.2/~m final filter from Barnstead (Barnstead/Thermolyne Corporation, Dubuque,
A.B. Pepperman, J.-C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23
Iowa) was used throughout the study. All other chemicals were either HPLC grade or reagent grade. 2.2. Formulations and extraction procedures
Formulations containing alachlor were typically prepared by first dissolving alachlor (0.3-1.0%) in methanol ( 5 - 1 1 % ) . The oil (0--10%) and Tween 20 ( 0 . 5 1.0%) were then added and stirring maintained with an overhead stirrer at 200 rpm for 10 min. The water (amount to the balance of 100%) was added very slowly into the stirring mixture (started at 200 rpm, gradually increased to 300 rpm). Caution was taken not to add water too fast at the beginning to prevent alachlor from precipitation and aggregation. The clay ( 4 - 1 0 % ) or lignin (5%) was added and the mixture was stirred at 350 rpm for 10 min. The stirring speed was increased to 400--450 rpm and the alginate (1%) was added to the mixture. The formulation mixture was stirred for 1 h. or until a homogeneous slurry was obtained. It then was dropped through disposable pipets into 0.25 M calcium chloride (about twice the weight of the mixture) to form gelatinous beads. The beads added to the CaC12 were weighed and allowed to harden for 2-5 min. before removing excess water by vacuum filtration through a coarse-frit Buchner funnel. The beads were rinsed with purified water and excess water was removed again. The beads were spread on aluminum foil to air-dry for two weeks at room temperature. The formulation filtrate and its rinsate were combined and saved for the HPLC determination of % active ingredient in formulation. Calculation of percent active ingredient in the formulation was achieved by multiplying the weight of alachlor used in the preparation of the formulation (W) by the quotient from division of the wet weight of the beads (X) divided by the weight of the total ingredients (Y), subtracting the weight of alachlor found in the filtrate (Z, determined by HPLC) and dividing that result by the weight of the dried beads ( D B ) . An indirect method was used because there was no direct method available at the time this work was done. We are currently developing a direct method which involves disintegration of the granule by solubilizing the alginate polymer, utilizing sodium EDTA and will report on this in the near future. % active ingredient = [ ( W. X~ Y) - Z ] / D B
19
A typical controlled release study of an alachlor formulation in static water was conducted by adding 110 g of an accurately weighed sample of formulation to 1 1 of purified water in a 1-1 glass Erlenmeyer flask with stopper. The static water method was chosen to determine the Fick's law relationships [15] under steady state conditions and was used for all the experiments because conditions are readily duplicated. The amount of sample used in the study was based on % alachlor in the formulation so that the theoretical maximum release of alachlor from the formulation would not exceed 200 ppm (solubility of alachlor in water at 25°C is about 240 ppm). A 1.05-ml aliquot of the extract was taken at prescribed intervals. Immediately prior to taking the aliquot the sample flask was gently shaken by inversion of the flask several times to homogenize the extract. When the release of alachlor reached equilibrium, a second extraction was conducted to determine whether additional alachlor would be released from the formulation. For the experiment, after equilibrium was reached, the formulation was separated from its extract and was allowed to dry at room temperature for 2-3 days. The dried formulation was then put in 1 1 of fresh water for further release. 2.3. H P L C analysis
Confirmation and quantitation of alachlor were performed on a high performance liquid chromatographic system from Waters (Waters Chromatographic Division, Millipore Corporation, Milford, MA.). The system consisted of a model 712 WISP auto-sampler, a model 600E Powerline multisolvent delivery system controller, a data system with model 991 Photodiode Array Detector V.6.22A Powerline software, a model 5200 printer plotter, and two detectors: a model 991 photodiode array detector (PDA) and a model 486 IEEE tunable absorbance detector with analytical flow cell. Two detectors were installed parallel to each other and an automated switching valve was used to switch the direction of the flow to the detector. A Waters NovaPak C 18 stainless steel column, 300 mm long X 3.9 mm i.d., 4 / z m particle size was used. The mobile phase was acetonitrile:water (65:35, v / v ) , isocratic at a flow rate of 0.7 ml/min, and the solvents were sparged with helium at the flow rate of 30 ml/min. For quantitative determination of alachlor concentrations, the model 486 detector was selected and was set at 215 nm, 1 AU
20
A.B. Pepperman, J. C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23 -
120
full scale. Injection v o l u m e was 20/xl, each sample was run twice, and the run time was 15 min. P D A spectrosc o p y was used to acquire U V / V i s i b l e spectra and for confirmation. A stock standard o f alachlor, 1.00 m g / m l , was prepared by dissolving recrystallized alachlor in H P L C grade acetonitrile. W o r k i n g standards, 1 - 2 0 0 ppm, w e r e prepared f r o m the dilution of the stock standard with m o b i l e phase (65:35 = a c e t o n i t r i l e / w a t e r ) . For H P L C analysis, samples o f 1.05 ml were taken (in either the controlled release study or c o m b i n e d formulation filtrate-rinsate) and to this was added 1.95 ml acetonitrile. This was m i x e d with a vortex m i x e r for 30 s, then filtered through a 0.22-/.zm M i l l e x - G V filter ( M i l l i p o r e Corp., Bedford, M A . ) before injection into the H P L C . Standards o f alachlor were run routinely as a control. There was no interfering peaks f r o m any of the additives under the H P L C conditions used.
3. R e s u l t s a n d d i s c u s s i o n
3.1. Effect o f aging and use o f lignin Lignin was used in two formulations, one with ( B ) and one without ( A ) alachlor. T h r e e alachlor-clay formulations, one with linseed oil ( E l , one with soybean oil ( F ) , and one without oil ( G ) were also prepared ( T a b l e 1 ). Results f r o m controlled release studies on Table 1 Alachlor formulations using various fillers and oils Formulation
Age of formulation (days)
Type of oil
Type of filler
Percent active ingredient
A B C D E F G
20 20 228 214 19 18 17
linseed soybean linseed soybean -
lignin lignin kaolin kaolin kaolin kaolin kaolin
0 9.35 6.33 6.29 6.17 6.30 7.32
Formulation A = 0% alachlor, 5.0% lignin, 1.0% alginate. Formulation B = 1.0% alachlor, 5.0% lignin, 1.0% alginate. Formulations C,, D, E, F - 1.0% alachlor, 6.0% oil, 10.0% kaolin, 1.0% alginate. Formulation G = 1.0% alachlor, 0% oil, 10.0% kaolin, 1.0% alginate.
1oo
~ 8o o
~ 60 c
~ 4o
0
0
500
1,000
1,500
2,000
2,500
3,000
Time(hrs)
Fig. 1. Effect of formulation type and age on alachlor release rate. Formulation B, alginate-lignin = Ill; Formulation C, alginate-kaolinlinseed oil (aged) • ; Formulation D, alginate-kaolin-soybean oil (aged) = O; Formulation E, alginate-kaolin-linseed oil (fresh } rT: Formulation F, alginate-kaolin-soybean oil (fresh) = ~>: Formulation G, alginate-kaolin - (3. Vertical lines at 720, 1290, and 2292 h represent the points at which the beads were collected, air-dried and placed in fresh water. formulations B, E, F, G and two aged alachlor formulations with linseed oil ( C ) or soybean oil ( D ) indicated that both oils and the lignin retarded the release of alachlor from the formulations as c o m p a r e d to the alginate-kaolin formulation G (Fig. 1 ). A l a c h l o r was c o m p l e t e l y released f r o m G after 30 days whereas only one-third to half o f the alachlor was released f r o m the formulations containing either alginate-lignin or alginate-kaolin-oil. The aged formulations s h o w e d somewhat slower release rates than the corresponding freshly prepared formulations. In our previous work, the release rate of metribuzin was profoundly affected by aging [ 13] but the effect for alachlor was less dramatic. This difference was attributed to the solubilities o f the metribuzin ( 1200 p p m ) and alachlor ( 242 p p m ) since the metribuzin w o u l d be partitioned more readily into the water as it enters the bead, the resistance by the p o l y m e r i c film to water flow both into and out of the bead w o u l d be more important for metribuzin. There were only small differences b e t w e e n the soybean oil and linseed oil formulations in release rates of alachlor. Similar release profiles for the lignin and the two oil formulations, indicates that of the steps i n v o l v e d in the release ( 1 ) diffusion o f water into the bead, (2) partitioning o f alachlor into the water, and (3) diffusion to the surface o f the bead for release: step 2 is the rate determining step. In the static water test, eventually an equilibrium is reached b e t w e e n the alachlor in the bead
A.B. Pepperman, J.-C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23 Table 2 Effect o f variation o f oil content on release rate o f alginate-kaolinlinseed oil formulations Formulation
H I J K L M N
Percent linseed oil
0 2 4 6 8 10 -
Percent active ingredient
7.21 6.09 5.34 4.72 4.27 3.90 15 a
Percent release o f alachlor 24 h
600 h
19.03 15.32 10.71 7.98 7.45 6.48 46.14
89.02 50.97 43.61 37.50 31.50 28.29 69.81
Formulations H, I, J, K, L, M all contained 1.0% alachlor, 1.0% alginate, 0.5% T w e e n 20, 10.0% kaolin, a n d v a r y i n g a m o u n t s o f oil as specified. a L A S S O II, g r a n u l a r (contains 1 0 - 1 2 % n a p h t h a l e n e ) .
and the water surrounding the bead and no more release is observed until the granules are placed in fresh water. When the extractions had reached equilibrium, the separation-drying-refreshing process (SDR) was repeated three times over a total period of about 120 days. Further release of alachlor was apparent after each drying and fresh water cycle but the increases after each SDR were smaller than the previous cycle (Fig. 1 ). The data for formulations C, D, E, and F, reinforce the earlier observation with metribuzin [ 13] that in the closed static water release system there is an initial kinetic release of the herbicide followed by a thermodynamic equilibrium wherein the partitioning of the available herbicide between the oil in the bead and the surrounding water occurs. This type of behavior and release profile is consistent with Kydonieus's description of a 'reservoir system without controlling membrane' [ 15 ]. The lignin formulation (B) also demonstrated an equilibrium process which was subject to further release on refreshing the water indicating an absorption-partitioning phenomenon is occurring. Adsorption of herbicides by lignin has been previously reported [ 16]. 3.2. Variation of linseed oil Six formulations containing the same amounts of alachlor (1%), alginate (1%), clay (10%), Tween 20 (0.5%), and methanol (5%), but with varying amounts
21
of linseed oil (0, 2, 4, 6, 8, 10%; Table 2; H-M) were prepared. The percent active ingredient in the air-dried alginate beads ranged from 7.2% for the 0% linseed oil formulation to 3.9% for the 10% oil formulation. The controlled release study was conducted for the six alginate formulations (H-M) and one commercial formulation, LASSO II (N), revealed that release rates of alachlor from the alginate formulations were inversely proportional to the amount of the linseed oil present in the formulations (Fig. 2). Five of the formulations show a stepwise increase in release rate with decreasing oil content (the 8% oil formulation was not graphed for clarity of presentation but the values fell between the 6% and 10% oil formulations). The differences were further reflected in the total release of 100% for H (no oil) vs. 55% recovery for M ( 10% oil) at the conclusion of the study (68 days total time). After the first 600 h the release rate had leveled-off and a refresh cycle, as described above, was conducted. More alachlor release was observed. Formulation N (LASSO II) had a higher initial release rate as observed in the steeper slope of the release curve in the first 24 h. Release rate differences are further demonstrated by examining the release at two distinct times (Table 2). The alginate formulations containing linseed oil (I-M) had, after 24 h, released about one-fourth of what they would release at 600 h. whereas the LASSO II had, after 24 h. released about two-thirds of its 600-h release total. 120
i
100
_
80
-
o o
--
o
--
•
60
0
40 20
0
500
1,000 Time (hre)
1,500
2,000
Fig. 2. Effect o f linseed oil variation on alachlor release rate. Formulations H, I, J, K and M all contained 1.0% alachlor, 1.0% alginate, 0.5% T w e e n 20, 10.0% kaolin, and v a r y i n g a m o u n t s o f linseed oil as specified: H, 0 % oil = II; I, 2% oil = 0 ; J, 4 % oil = O ; K, 6 % oil = C]; M, 10% oil = O ; N, L A S S O II = O . The vertical line at 6 0 0 h represents the point at w h i c h the beads were collected, air-dried and placed in fresh water.
22
A.B. Pepperrnan, J. -C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23
3.3. O t h e r v e g e t a b l e oils
Six alachlor-oil formulations containing canola, corn, olive, safflower, sunflower, and peanut oil, were evaluated in the laboratory controlled release study for 7 days. Release rates of alachlor were about the same for five of the oils ( 17-19% after 168 h.) but slightly slower for the safflower oil formulation ( 14% after 168 h.). The slower release rate of alachlor-safflower oil formulation may be attributable to the fact that safflower oil has a higher percentage of unsaturated fatty acids (iodine value 142-150) than the other oils [ 17], and is classified as a drying oil (iodine v a l u e > 140). Similar to linseed oil, safflower oil reacts with oxygen in the air and polymerizes to form a film on the surface of safflower oil formulation. However, unlike linseed oil, the safflower oil film is tacky causing agglomeration of the beads. The agglomeration may contribute to the slower release since the surface area of the beads accessible by the water is reduced. Since the non-drying oil formulations such as corn, peanut, and sunflower gave similar release profiles to the linseed oil formulation, the rate-determining step in the release of alachlor appears to be the dissolution of the herbicide in the water that diffuses into the bead. If the polymer coating of the bead was the rate-determining step then the linseed oil would be expected to significantly reduce the rate of release over the non-film forming oils. 3.4. Size v a r i a t i o n
Two formulations containing 1% alachlor, 0.5% Tween 20, 10% kaolin, 1% alginate, 5% dimethyl sulfoxide (all other formulations reported in this paper utilized methanol) and 2% of either linseed oil or soybean oil were prepared. Each formulation was made into three different sizes of beads by dropping through syringe needles of 18, 20, and 23 gauge. The diameters of these dry beads were measured under the microscope, and their average diameters were obtained from the measurements of the twenty representative beads for each size (1.5, 1.3 and 1.2 m m respectively). A controlled-release study was conducted (as described in methods) on these six batches of alachlor beads. The study was carried out for 22 days and no significant differences in release rate was observed in formulations of different sizes and for the two different oils. The amount of alachlor released reflects a percentage
release of 5 5 - 5 7 % for each formulation. The release rate in water was almost linear during the first 24 h and was about 1 p p m / h , but it leveled off rapidly and reached equilibrium in approximately ten days. In the small size range studied, there was no significant effect on the rate of release.
4. Conclusions The use of linseed oil, kaolin, and alginate as the basis of a controlled release formulation of alachlor was successful in that a reduced release rate of the herbicide in comparison with a conventional formulation and with alginate formulations not containing linseed oil was obtained. A m o n g the factors affecting the release rate were the ratio of linseed oil/alachlor in the formulation (the larger the ratio of oil/alachlor in the formulation the slower the release rate) and aging of the dried formulation caused some slowing of the release rate. The controlling factor for reduction of release rates in all formulations appeared to be due to the partitioning of the alachlor between the oil and the water. The polymeric film formed by the linseed oil appeared to be of minor import since non-drying oils such as corn, peanut, and sunflower oil gave similar release rates to the linseed oil formulations. The partitioning phenomenon was most clearly observed as an equilibrium position established in the static water release test which could be displaced to release more alachlor by drying of the beads and addition to fresh water. The use of lignin in combination with alginate produced a formulation with slower release of alachlor than alginate-kaolin and commercial formulations and a similar release profile to the alginate-kaolin-vegetable oil formulations.
References [ 1] W.B. Solley, E.B. Chase, and W.B. Mann, IV, Estimated uses of water in the United States in 1980. Circ. 1001. U.S. Geol. Surv., Washington, DC, 1983. [2] L.K. Lee and E.G. Nelson, The extent and costs of contamination by agriculture. J. Soil Water Conserv. 42, (1987) 243-248. [3] U.S. Environmental Protection Agency. Report to Congress: Nonpoint Source Pollution in the U.S., 1984.
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[4] S.Z. Cohen, S.M. Creeger, R.F. Carsel and C.G. Enfield, Potential pesticide contamination of groundwater for agricultural uses, in: ACS Symposium Series, No. 259, Treatment and Disposal of Pesticide Wastes, American Chemical Society, Washington, DC, 1984, pp. 297-325. [5] A.B. Pepperman, J.W. Kuan and C. McCombs. Alginate controlled release formulations of metribuzin, J. Control. Release 17, ( 1991 ) 105-112. [6] G. Chesters, G.V. Simsiman, J. Levy, B.J. Alhajjar, R.N. Fathulla, and J.M. Harkin, Environmental fate of alachlor and metolachlor. Rev. Environ. Contamin. Toxicol. (1989) pp. 174. [ 7 ] National Research Council. Regulating Pesticides in Food: The Delaney Paradox. National Academy Press, Washington DC, 1987, pp. 52-53. [8] B.D. Riggle and D. Penner, The use of controlled-release technology for herbicides. Rev. Weed Sci. 5 (1990) 1-14. [9] M.M. Schreiber, R.E. Wing, B.S. Shasha, and M.D. White, Bioactivity of controlled release formulations of herbicides in starch encapsulated granules. Proc. Int. Symp. Control. Rel. Bioact. Mater. 15 (1988) 223-224. 10] P.R.F. Barrett, Some studies on the use of alginates for the placement and controlled release of diquat on submerged aquatic plants. Pestic. Sci. 9 (1978) 425--433.
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[ 11 ] W.J. Connick, Jr. Controlled release of the herbicides 2,4-D and dichlobenil from alginate gels, J. Appl. Polymer Sci. 27 (1982) 3341-3348. [ 12] W.J. Connick, Jr., J.M. Bradow, W. Wells, K.K. Steward, and T.K. Van, Preparation and evaluation of controlled-release formulations of 2,6-dichlobenzonitrile, J. Agric. Food Chem. 32 (1984) 1199-1205 and references cited therein. [ 13 ] A.B. Pepperman and J.W. Kuan, Slow release formulations of metribuzin based on alginate-kaolin-linseed oil. J. Control. Release 26 (1993) 21-30. [14] A.L. Kenimer, J.K. Mitchell, and A.S. Felsot, Pesticide formulation and application technique effects on surface pesticide transport. Presentation at 1989 International Winter Meeting of the American Society of Agricultural Engineers, New Orleans, LA. Paper #892506, 12 pp. [ 15 ] A.F. Kydonieus, Controlled Release Technologies: Methods, Theory, and Applications, Vol. 1, pp. 1-19, CRC Press, Inc. Boca Raton, FL. (1980). [16] B.D. Riggle and D. Penner, Kraft lignin adsorption of metribuzin as a controlled-release function evaluation. J. Agric. Food Chem. 40 (1992) 1710-1712. [ 17] J.C. Cowan, Drying Oils, in: Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 8, 3rd edition, Wiley, New York, 1979, pp. 130-150,
controlled release ELSEVIER
Journal of Controlled Release 34 (1995) 17-23
Controlled release formulations of alachlor based on calcium alginate 1 Armand B. Pepperman *, Jui-Chang W. Kuan Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, LA70179, USA
Received 31 March 1994; accepted 7 October 1994
Abstract
Granular controlled release formulations of alachlor were prepared from sodium alginate-kaolin-linseed oil-herbicide slurries dropped onto calcium chloride to form spherical beads which were air-dried and evaluated for release rates in a static water extraction. These formulations gave considerably slower release of alachlor than a commercial formulation and/or the alginate formulation without oil. Factors affecting the rate of release included aging of the dried formulation and the ratio of oil/herbicide in the formulation. The size of the beads had no appreciable effect on the rate of release and several oils, including canola, corn, olive, safflower, peanut, soybean, and sunflower gave similar release rates to linseed oil formulations for alachlor. The use of lignin in combination with alginate also produced a granular formulation with reduced release rates. Keywords: Alachlor; Alginate; Kaolin; Linseed oil; Vegetable oil; Controlled release; Herbicide formulation; Groundwater
I. I n t r o d u c t i o n
A n e v a l u a t i o n o f rural drinking water sources in the U n i t e d States indicates that m o r e than 97% is derived f r o m u n d e r g r o u n d sources, along with 55% o f all livestock water and 40% o f all irrigation water [ 1 ]. A c c o r d i n g to U.S. C e n s u s data, m o r e than 30 million p e o p l e obtain their drinking water f r o m private wells. In addition to serving the needs o f rural A m e r i c a , g r o u n d w a t e r served 4 0 % o f the population using public w a t e r supplies in 1980, w h i c h includes nearly 74 mil* Corresponding author. Note: This article reports the results of research only. Mention of a pesticide in this article does not constitute a recommendation for use by the U.S. Department of Agriculture nor does it imply registration under FIFRA as amended. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. Elsevier Science B.V. SSDI 0168-3659( 94)00111-1
lion people [ 1 ]. A l t h o u g h g r o u n d w a t e r c o n t a m i n a t i o n has m a n y sources, e v i d e n c e suggests that agriculture's relative contribution m a y be significant [ 2 ] . Large quantities o f pesticides are used e x t e n s i v e l y throughout the U n i t e d States and several h a v e been identified as sources of g r o u n d w a t e r contamination [ 3,4]. Point source contamination f r o m spills, e x c e s s i v e application and i m p r o p e r disposal m a g n i f y the problem. Pesticides such as metribuzin, atrazine, alachlor, aldicarb, and carbofuran, which are k n o w n to have s o m e degree of persistence in the environment, are a m o n g those implicated as g r o u n d w a t e r contaminants [ 5 ]. A l a c h l o r [ 2 - chloro - N - ( 2,6 - diethylphenyl ) - N ( m e t h o x y m e t h y l ) a c e t a m i d e ) is a w i d e l y used herbicide for w e e d control in corn, soybean, and m a n y other crops [ 6 ]. As a result o f its w i d e usage ( a p p r o x i m a t e l y 85 million pounds per year [ 7 ] ), alachlor residues m a y contaminate groundwater, streams and rivers due to
18
A.B. Pepperman, J.-C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23
spraying, spills, leaching, and runoff. The use of controlled release formulations has the potential to reduce the surface run-off and leaching of soil-applied herbicides and decrease the amount of herbicide being applied to the soil. Both of these events would have favorable impacts on the environment [ 8]. In addition, the use of controlled release formulations may reduce the losses to evaporation, co-distillation, and photolysis
[9]. Alginate gels have been used as matrices for preparing controlled release formulations (CRF) of herbicides [10,11]. Incorporation of kaolin to alginate formulations of herbicides has been shown to slow down the release of herbicides [ 12]. The authors have shown that a granular formulation of metribuzin based on linseed oil, kaolin, and alginate considerably reduced the release rate of metribuzin in comparison with conventional formulations and alginate formulations without linseed oil [13]. Once the chemical is released from the CRF, it would be expected to leach at the same rate as the chemical from a conventional formulation. Because CRFs permit only low concentrations of the free pesticide to be available for leaching or runoff during precipitation, these processes are reduced. Surface runoff presents a more complex problem because the CRF must not only contain the chemical, but must also remain in place on the field during heavy rainfall. A recent report indicated that a microcapsule formulation of alachlor (ME) may have an adverse effect on surface runoff because greater losses of alachlor were observed from the ME formulation than from a granular formulation in runoff experiments [14]. Granular formulations should present a less mobile form of the pesticide during run-off events. In an earlier report [ 13 ], linseed oil was incorporated into an emulsion of clay, alginate, emulsifier, herbicide, and water in a one-step process. In this manner the linseed oil was dispersed throughout the matrix of the granules. Dropping of the soluble sodium alginate slurry into calcium chloride forms insoluble calcium alginate granules which are generally spherical in shape. Linseed oil on the surface of the granules polymerizes on drying to form a coating [ 13]. The granular beads prepared in the alginate-kaolin-linseed oil ( A K L O ) process had a coating of linseed oil, which is visible without magnification as a shiny surface, and were yellow in color. This is contrasted to beads prepared without linseed oil, which have a dull, powdery
appearing surface that is off-white. The beads prepared with lignin were dark brown in color. Light photomicrographic cross-sections of the A K L O granules in the metribuzin study [ 13] clearly showed an increased density at the outer edge which was attributed to linseed oil polymer formation on the surface of the alginate bead. Considerably reduced release rates of metribuzin in comparison with a conventional formulation and with alginate formulations without linseed oil were found with the A K L O formulations [ 13]. In the present study, linseed oil and other vegetable oils were incorporated into the alginate-kaolin formulation to determine their effect on the release rate of alachlor from granular formulations. In addition, lignin-alginate formulations were evaluated and shown to be effective for controlling the release of alachlor.
2. Materials and methods 2.1. Chemicals
Alachlor (technical grade, 94% pure) and a commercial formulation of alachlor ( L A S S O II) were provided by Monsanto Company, St. Louis, MO. Technical grade alachlor was recrystallized several times from ethyl acetate-hexane mixtures at 4°C until no significant impurities were detected by HPLC (method described in HPLC analysis). The recrystallized alachlor was clear rock-like crystals and with a mp of 40.5-41.5°C. The crystals were hand ground to 20-40 mesh in a porcelain mortar with a glass pestle. Sodium alginate, Kelgin MV, was obtained from Kelco, Division of Merck and Company, San Diego, CA, and kaolin clay from Thiele Kaolin Company, Wren, GA. Organosolv lignin was supplied by Repap Technologies, Valley Forge, PA. A suspension of 20% lignin in 0.1 N NaOH was usually prepared prior to use in the formulations. Raw linseed oil was purchased at a local hardware store (Harry's Ace Hardware, New Orleans, LA), soybean oil and corn oil (Hunt-Wesson, Inc., Fullerton, CA) from a local supermarket. Tween 20 (polyoxyethylenesorbitan monolaurate) was purchased from Sigma Chemical Co., St. Louis, MO. Deionized water purified with a NANOpure Ultrapure Water System through a 0.2/~m final filter from Barnstead (Barnstead/Thermolyne Corporation, Dubuque,
A.B. Pepperman, J.-C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23
Iowa) was used throughout the study. All other chemicals were either HPLC grade or reagent grade. 2.2. Formulations and extraction procedures
Formulations containing alachlor were typically prepared by first dissolving alachlor (0.3-1.0%) in methanol ( 5 - 1 1 % ) . The oil (0--10%) and Tween 20 ( 0 . 5 1.0%) were then added and stirring maintained with an overhead stirrer at 200 rpm for 10 min. The water (amount to the balance of 100%) was added very slowly into the stirring mixture (started at 200 rpm, gradually increased to 300 rpm). Caution was taken not to add water too fast at the beginning to prevent alachlor from precipitation and aggregation. The clay ( 4 - 1 0 % ) or lignin (5%) was added and the mixture was stirred at 350 rpm for 10 min. The stirring speed was increased to 400--450 rpm and the alginate (1%) was added to the mixture. The formulation mixture was stirred for 1 h. or until a homogeneous slurry was obtained. It then was dropped through disposable pipets into 0.25 M calcium chloride (about twice the weight of the mixture) to form gelatinous beads. The beads added to the CaC12 were weighed and allowed to harden for 2-5 min. before removing excess water by vacuum filtration through a coarse-frit Buchner funnel. The beads were rinsed with purified water and excess water was removed again. The beads were spread on aluminum foil to air-dry for two weeks at room temperature. The formulation filtrate and its rinsate were combined and saved for the HPLC determination of % active ingredient in formulation. Calculation of percent active ingredient in the formulation was achieved by multiplying the weight of alachlor used in the preparation of the formulation (W) by the quotient from division of the wet weight of the beads (X) divided by the weight of the total ingredients (Y), subtracting the weight of alachlor found in the filtrate (Z, determined by HPLC) and dividing that result by the weight of the dried beads ( D B ) . An indirect method was used because there was no direct method available at the time this work was done. We are currently developing a direct method which involves disintegration of the granule by solubilizing the alginate polymer, utilizing sodium EDTA and will report on this in the near future. % active ingredient = [ ( W. X~ Y) - Z ] / D B
19
A typical controlled release study of an alachlor formulation in static water was conducted by adding 110 g of an accurately weighed sample of formulation to 1 1 of purified water in a 1-1 glass Erlenmeyer flask with stopper. The static water method was chosen to determine the Fick's law relationships [15] under steady state conditions and was used for all the experiments because conditions are readily duplicated. The amount of sample used in the study was based on % alachlor in the formulation so that the theoretical maximum release of alachlor from the formulation would not exceed 200 ppm (solubility of alachlor in water at 25°C is about 240 ppm). A 1.05-ml aliquot of the extract was taken at prescribed intervals. Immediately prior to taking the aliquot the sample flask was gently shaken by inversion of the flask several times to homogenize the extract. When the release of alachlor reached equilibrium, a second extraction was conducted to determine whether additional alachlor would be released from the formulation. For the experiment, after equilibrium was reached, the formulation was separated from its extract and was allowed to dry at room temperature for 2-3 days. The dried formulation was then put in 1 1 of fresh water for further release. 2.3. H P L C analysis
Confirmation and quantitation of alachlor were performed on a high performance liquid chromatographic system from Waters (Waters Chromatographic Division, Millipore Corporation, Milford, MA.). The system consisted of a model 712 WISP auto-sampler, a model 600E Powerline multisolvent delivery system controller, a data system with model 991 Photodiode Array Detector V.6.22A Powerline software, a model 5200 printer plotter, and two detectors: a model 991 photodiode array detector (PDA) and a model 486 IEEE tunable absorbance detector with analytical flow cell. Two detectors were installed parallel to each other and an automated switching valve was used to switch the direction of the flow to the detector. A Waters NovaPak C 18 stainless steel column, 300 mm long X 3.9 mm i.d., 4 / z m particle size was used. The mobile phase was acetonitrile:water (65:35, v / v ) , isocratic at a flow rate of 0.7 ml/min, and the solvents were sparged with helium at the flow rate of 30 ml/min. For quantitative determination of alachlor concentrations, the model 486 detector was selected and was set at 215 nm, 1 AU
20
A.B. Pepperman, J. C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23 -
120
full scale. Injection v o l u m e was 20/xl, each sample was run twice, and the run time was 15 min. P D A spectrosc o p y was used to acquire U V / V i s i b l e spectra and for confirmation. A stock standard o f alachlor, 1.00 m g / m l , was prepared by dissolving recrystallized alachlor in H P L C grade acetonitrile. W o r k i n g standards, 1 - 2 0 0 ppm, w e r e prepared f r o m the dilution of the stock standard with m o b i l e phase (65:35 = a c e t o n i t r i l e / w a t e r ) . For H P L C analysis, samples o f 1.05 ml were taken (in either the controlled release study or c o m b i n e d formulation filtrate-rinsate) and to this was added 1.95 ml acetonitrile. This was m i x e d with a vortex m i x e r for 30 s, then filtered through a 0.22-/.zm M i l l e x - G V filter ( M i l l i p o r e Corp., Bedford, M A . ) before injection into the H P L C . Standards o f alachlor were run routinely as a control. There was no interfering peaks f r o m any of the additives under the H P L C conditions used.
3. R e s u l t s a n d d i s c u s s i o n
3.1. Effect o f aging and use o f lignin Lignin was used in two formulations, one with ( B ) and one without ( A ) alachlor. T h r e e alachlor-clay formulations, one with linseed oil ( E l , one with soybean oil ( F ) , and one without oil ( G ) were also prepared ( T a b l e 1 ). Results f r o m controlled release studies on Table 1 Alachlor formulations using various fillers and oils Formulation
Age of formulation (days)
Type of oil
Type of filler
Percent active ingredient
A B C D E F G
20 20 228 214 19 18 17
linseed soybean linseed soybean -
lignin lignin kaolin kaolin kaolin kaolin kaolin
0 9.35 6.33 6.29 6.17 6.30 7.32
Formulation A = 0% alachlor, 5.0% lignin, 1.0% alginate. Formulation B = 1.0% alachlor, 5.0% lignin, 1.0% alginate. Formulations C,, D, E, F - 1.0% alachlor, 6.0% oil, 10.0% kaolin, 1.0% alginate. Formulation G = 1.0% alachlor, 0% oil, 10.0% kaolin, 1.0% alginate.
1oo
~ 8o o
~ 60 c
~ 4o
0
0
500
1,000
1,500
2,000
2,500
3,000
Time(hrs)
Fig. 1. Effect of formulation type and age on alachlor release rate. Formulation B, alginate-lignin = Ill; Formulation C, alginate-kaolinlinseed oil (aged) • ; Formulation D, alginate-kaolin-soybean oil (aged) = O; Formulation E, alginate-kaolin-linseed oil (fresh } rT: Formulation F, alginate-kaolin-soybean oil (fresh) = ~>: Formulation G, alginate-kaolin - (3. Vertical lines at 720, 1290, and 2292 h represent the points at which the beads were collected, air-dried and placed in fresh water. formulations B, E, F, G and two aged alachlor formulations with linseed oil ( C ) or soybean oil ( D ) indicated that both oils and the lignin retarded the release of alachlor from the formulations as c o m p a r e d to the alginate-kaolin formulation G (Fig. 1 ). A l a c h l o r was c o m p l e t e l y released f r o m G after 30 days whereas only one-third to half o f the alachlor was released f r o m the formulations containing either alginate-lignin or alginate-kaolin-oil. The aged formulations s h o w e d somewhat slower release rates than the corresponding freshly prepared formulations. In our previous work, the release rate of metribuzin was profoundly affected by aging [ 13] but the effect for alachlor was less dramatic. This difference was attributed to the solubilities o f the metribuzin ( 1200 p p m ) and alachlor ( 242 p p m ) since the metribuzin w o u l d be partitioned more readily into the water as it enters the bead, the resistance by the p o l y m e r i c film to water flow both into and out of the bead w o u l d be more important for metribuzin. There were only small differences b e t w e e n the soybean oil and linseed oil formulations in release rates of alachlor. Similar release profiles for the lignin and the two oil formulations, indicates that of the steps i n v o l v e d in the release ( 1 ) diffusion o f water into the bead, (2) partitioning o f alachlor into the water, and (3) diffusion to the surface o f the bead for release: step 2 is the rate determining step. In the static water test, eventually an equilibrium is reached b e t w e e n the alachlor in the bead
A.B. Pepperman, J.-C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23 Table 2 Effect o f variation o f oil content on release rate o f alginate-kaolinlinseed oil formulations Formulation
H I J K L M N
Percent linseed oil
0 2 4 6 8 10 -
Percent active ingredient
7.21 6.09 5.34 4.72 4.27 3.90 15 a
Percent release o f alachlor 24 h
600 h
19.03 15.32 10.71 7.98 7.45 6.48 46.14
89.02 50.97 43.61 37.50 31.50 28.29 69.81
Formulations H, I, J, K, L, M all contained 1.0% alachlor, 1.0% alginate, 0.5% T w e e n 20, 10.0% kaolin, a n d v a r y i n g a m o u n t s o f oil as specified. a L A S S O II, g r a n u l a r (contains 1 0 - 1 2 % n a p h t h a l e n e ) .
and the water surrounding the bead and no more release is observed until the granules are placed in fresh water. When the extractions had reached equilibrium, the separation-drying-refreshing process (SDR) was repeated three times over a total period of about 120 days. Further release of alachlor was apparent after each drying and fresh water cycle but the increases after each SDR were smaller than the previous cycle (Fig. 1 ). The data for formulations C, D, E, and F, reinforce the earlier observation with metribuzin [ 13] that in the closed static water release system there is an initial kinetic release of the herbicide followed by a thermodynamic equilibrium wherein the partitioning of the available herbicide between the oil in the bead and the surrounding water occurs. This type of behavior and release profile is consistent with Kydonieus's description of a 'reservoir system without controlling membrane' [ 15 ]. The lignin formulation (B) also demonstrated an equilibrium process which was subject to further release on refreshing the water indicating an absorption-partitioning phenomenon is occurring. Adsorption of herbicides by lignin has been previously reported [ 16]. 3.2. Variation of linseed oil Six formulations containing the same amounts of alachlor (1%), alginate (1%), clay (10%), Tween 20 (0.5%), and methanol (5%), but with varying amounts
21
of linseed oil (0, 2, 4, 6, 8, 10%; Table 2; H-M) were prepared. The percent active ingredient in the air-dried alginate beads ranged from 7.2% for the 0% linseed oil formulation to 3.9% for the 10% oil formulation. The controlled release study was conducted for the six alginate formulations (H-M) and one commercial formulation, LASSO II (N), revealed that release rates of alachlor from the alginate formulations were inversely proportional to the amount of the linseed oil present in the formulations (Fig. 2). Five of the formulations show a stepwise increase in release rate with decreasing oil content (the 8% oil formulation was not graphed for clarity of presentation but the values fell between the 6% and 10% oil formulations). The differences were further reflected in the total release of 100% for H (no oil) vs. 55% recovery for M ( 10% oil) at the conclusion of the study (68 days total time). After the first 600 h the release rate had leveled-off and a refresh cycle, as described above, was conducted. More alachlor release was observed. Formulation N (LASSO II) had a higher initial release rate as observed in the steeper slope of the release curve in the first 24 h. Release rate differences are further demonstrated by examining the release at two distinct times (Table 2). The alginate formulations containing linseed oil (I-M) had, after 24 h, released about one-fourth of what they would release at 600 h. whereas the LASSO II had, after 24 h. released about two-thirds of its 600-h release total. 120
i
100
_
80
-
o o
--
o
--
•
60
0
40 20
0
500
1,000 Time (hre)
1,500
2,000
Fig. 2. Effect o f linseed oil variation on alachlor release rate. Formulations H, I, J, K and M all contained 1.0% alachlor, 1.0% alginate, 0.5% T w e e n 20, 10.0% kaolin, and v a r y i n g a m o u n t s o f linseed oil as specified: H, 0 % oil = II; I, 2% oil = 0 ; J, 4 % oil = O ; K, 6 % oil = C]; M, 10% oil = O ; N, L A S S O II = O . The vertical line at 6 0 0 h represents the point at w h i c h the beads were collected, air-dried and placed in fresh water.
22
A.B. Pepperrnan, J. -C. W. Kuan / Journal of Controlled Release 34 (1995) 17-23
3.3. O t h e r v e g e t a b l e oils
Six alachlor-oil formulations containing canola, corn, olive, safflower, sunflower, and peanut oil, were evaluated in the laboratory controlled release study for 7 days. Release rates of alachlor were about the same for five of the oils ( 17-19% after 168 h.) but slightly slower for the safflower oil formulation ( 14% after 168 h.). The slower release rate of alachlor-safflower oil formulation may be attributable to the fact that safflower oil has a higher percentage of unsaturated fatty acids (iodine value 142-150) than the other oils [ 17], and is classified as a drying oil (iodine v a l u e > 140). Similar to linseed oil, safflower oil reacts with oxygen in the air and polymerizes to form a film on the surface of safflower oil formulation. However, unlike linseed oil, the safflower oil film is tacky causing agglomeration of the beads. The agglomeration may contribute to the slower release since the surface area of the beads accessible by the water is reduced. Since the non-drying oil formulations such as corn, peanut, and sunflower gave similar release profiles to the linseed oil formulation, the rate-determining step in the release of alachlor appears to be the dissolution of the herbicide in the water that diffuses into the bead. If the polymer coating of the bead was the rate-determining step then the linseed oil would be expected to significantly reduce the rate of release over the non-film forming oils. 3.4. Size v a r i a t i o n
Two formulations containing 1% alachlor, 0.5% Tween 20, 10% kaolin, 1% alginate, 5% dimethyl sulfoxide (all other formulations reported in this paper utilized methanol) and 2% of either linseed oil or soybean oil were prepared. Each formulation was made into three different sizes of beads by dropping through syringe needles of 18, 20, and 23 gauge. The diameters of these dry beads were measured under the microscope, and their average diameters were obtained from the measurements of the twenty representative beads for each size (1.5, 1.3 and 1.2 m m respectively). A controlled-release study was conducted (as described in methods) on these six batches of alachlor beads. The study was carried out for 22 days and no significant differences in release rate was observed in formulations of different sizes and for the two different oils. The amount of alachlor released reflects a percentage
release of 5 5 - 5 7 % for each formulation. The release rate in water was almost linear during the first 24 h and was about 1 p p m / h , but it leveled off rapidly and reached equilibrium in approximately ten days. In the small size range studied, there was no significant effect on the rate of release.
4. Conclusions The use of linseed oil, kaolin, and alginate as the basis of a controlled release formulation of alachlor was successful in that a reduced release rate of the herbicide in comparison with a conventional formulation and with alginate formulations not containing linseed oil was obtained. A m o n g the factors affecting the release rate were the ratio of linseed oil/alachlor in the formulation (the larger the ratio of oil/alachlor in the formulation the slower the release rate) and aging of the dried formulation caused some slowing of the release rate. The controlling factor for reduction of release rates in all formulations appeared to be due to the partitioning of the alachlor between the oil and the water. The polymeric film formed by the linseed oil appeared to be of minor import since non-drying oils such as corn, peanut, and sunflower oil gave similar release rates to the linseed oil formulations. The partitioning phenomenon was most clearly observed as an equilibrium position established in the static water release test which could be displaced to release more alachlor by drying of the beads and addition to fresh water. The use of lignin in combination with alginate produced a formulation with slower release of alachlor than alginate-kaolin and commercial formulations and a similar release profile to the alginate-kaolin-vegetable oil formulations.
References [ 1] W.B. Solley, E.B. Chase, and W.B. Mann, IV, Estimated uses of water in the United States in 1980. Circ. 1001. U.S. Geol. Surv., Washington, DC, 1983. [2] L.K. Lee and E.G. Nelson, The extent and costs of contamination by agriculture. J. Soil Water Conserv. 42, (1987) 243-248. [3] U.S. Environmental Protection Agency. Report to Congress: Nonpoint Source Pollution in the U.S., 1984.
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[4] S.Z. Cohen, S.M. Creeger, R.F. Carsel and C.G. Enfield, Potential pesticide contamination of groundwater for agricultural uses, in: ACS Symposium Series, No. 259, Treatment and Disposal of Pesticide Wastes, American Chemical Society, Washington, DC, 1984, pp. 297-325. [5] A.B. Pepperman, J.W. Kuan and C. McCombs. Alginate controlled release formulations of metribuzin, J. Control. Release 17, ( 1991 ) 105-112. [6] G. Chesters, G.V. Simsiman, J. Levy, B.J. Alhajjar, R.N. Fathulla, and J.M. Harkin, Environmental fate of alachlor and metolachlor. Rev. Environ. Contamin. Toxicol. (1989) pp. 174. [ 7 ] National Research Council. Regulating Pesticides in Food: The Delaney Paradox. National Academy Press, Washington DC, 1987, pp. 52-53. [8] B.D. Riggle and D. Penner, The use of controlled-release technology for herbicides. Rev. Weed Sci. 5 (1990) 1-14. [9] M.M. Schreiber, R.E. Wing, B.S. Shasha, and M.D. White, Bioactivity of controlled release formulations of herbicides in starch encapsulated granules. Proc. Int. Symp. Control. Rel. Bioact. Mater. 15 (1988) 223-224. 10] P.R.F. Barrett, Some studies on the use of alginates for the placement and controlled release of diquat on submerged aquatic plants. Pestic. Sci. 9 (1978) 425--433.
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[ 11 ] W.J. Connick, Jr. Controlled release of the herbicides 2,4-D and dichlobenil from alginate gels, J. Appl. Polymer Sci. 27 (1982) 3341-3348. [ 12] W.J. Connick, Jr., J.M. Bradow, W. Wells, K.K. Steward, and T.K. Van, Preparation and evaluation of controlled-release formulations of 2,6-dichlobenzonitrile, J. Agric. Food Chem. 32 (1984) 1199-1205 and references cited therein. [ 13 ] A.B. Pepperman and J.W. Kuan, Slow release formulations of metribuzin based on alginate-kaolin-linseed oil. J. Control. Release 26 (1993) 21-30. [14] A.L. Kenimer, J.K. Mitchell, and A.S. Felsot, Pesticide formulation and application technique effects on surface pesticide transport. Presentation at 1989 International Winter Meeting of the American Society of Agricultural Engineers, New Orleans, LA. Paper #892506, 12 pp. [ 15 ] A.F. Kydonieus, Controlled Release Technologies: Methods, Theory, and Applications, Vol. 1, pp. 1-19, CRC Press, Inc. Boca Raton, FL. (1980). [16] B.D. Riggle and D. Penner, Kraft lignin adsorption of metribuzin as a controlled-release function evaluation. J. Agric. Food Chem. 40 (1992) 1710-1712. [ 17] J.C. Cowan, Drying Oils, in: Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 8, 3rd edition, Wiley, New York, 1979, pp. 130-150,