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Clay-based formulations of metolachlor with reduced leaching

Clay-based formulations of metolachlor with reduced leaching
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  Ž . Applied Clay Science 18 2001 265– r locate r clay Clay-based formulations of metolachlor with reduced leaching Arno Nennemann a , Yael Mishael b , Shlomo Nir  b , Baruch Rubin b ,Tamara Polubesova b , Faıza Bergaya c , Henri van Damme  c , Gerhard Lagaly  a, ) ¨ a  Institute of Inorganic Chemistry, Christian-Albrechts Uni Õ ersity, D-24098 Kiel, Germany b  Department of Soil and Water Sciences, Hebrew Uni Õ ersity of Jerusalem, POB 12, Reho Õ ot 76100, Israel c Centre de Recherche sur la Matiere Di Õ isee, Centre National de la Recherche Scientifique, 1 b rue de la Ferollerie, ` ´ F-45071 Orleans cedex 2, France ´ Received 12 November 1999; received in revised form 31 January 2000; accepted 5 July 2000 Abstract The current research in herbicide application aims to develop formulations that reduce leaching of the herbicide to deeplayers of the soil and to concentrate its biological activity at the top layers. Adsorption of metolachlor on clay minerals, theirorganic derivatives or pillared forms provides the best possibility to develop slow-release formulations. Metolachlor is aselective pre-emergence herbicide widely used in irrigated crops to control annual weeds. It is adsorbed by bentonites andmontmorillonites, but the amount adsorbed strongly depends on the type of bentonite and possible pretreatment reactions.Wyoming bentonites adsorbed considerable amounts of metolachlor but other bentonites did not bind this herbicide. Anacid-activated pillared montmorillonite was also an effective adsorbent of metolachlor. Modification of this sample bypreadsorbing different amounts of benzyl trimethylammonium ions did not influence the level of herbicide adsorption. Thebiological efficiency of the formulations was tested with bioassay soil columns. Slow-release formulations could be preparedwith raw bentonites and the acid-activated pillared montmorillonite. A formulation, prepared by adsorbing metolachlor fromaqueous solution on the acid-activated pillared montmorillonite, showed high herbicide activity at the top 10 cm, and did notdiffuse significantly to greater depths. This formulation should allow a better weed control than the commercialformulations. q 2001 Elsevier Science B.V. All rights reserved. Keywords:  Bentonites; Bioassays; Herbicides; Leaching; Metolachlor; Pesticides; Pillared montmorillonite 1. Introduction The increased use of herbicides poses environ-mental and economic problems. Volatilization andphotodegradation reduce the active amounts, leach-ing and surface migration contaminate surface andground water. The loss of efficiency must be com- ) Corresponding author. Ž .  E-mail address: G. Lagaly . pensated by increasing frequency and dose of herbi-cide application and, therefore, raises contaminationand costs.Metolachlor is a selective pre-emergence herbi-cide widely used in irrigated crops such as potato,peanuts, corn and sunflower to control annual weeds.Due to the relatively high solubility of metolachlor Ž . in water 530 mg r l , it has a high potential to leachand migrate through the soil profile and contaminate Ž ground water Cohen et al., 1984, 1986; Chesters et 0169-1317 r 01 r $ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-1317 01 00032-1  ( ) A. Nennemann et al. r  Applied Clay Science 18 2001 265–275 266 al., 1989; Gallagher et al., 1996; Pasquarell and . Boyer, 1996 . Metolachlor was also detected in Ž streams and rivers Albanis et al., 1994; Fischer et . Ž al., 1995 , in lakes Schottler and Eisenreich, 1993; . Spalding et al., 1994 and in surface water at the Ž highest concentration out of 17 herbicides Sense- . man et al., 1997 .A positive correlation was reported between the Ž organic matter, the clay mineral content in weight . percent , the surface area of the soil and its capacityto adsorb metolachlor. There was a negative correla-tion between these properties and the migration rate Ž of metolachlor in the soil Peter and Weber, 1985; . Bosetto et al., 1994; Zheng and Cooper, 1996 .Several attempts were made to prepare formula-tions of metolachlor that pose less environmentalrisk and improve agricultural economy. These at-tempts involved the use of starch encapsulation Ž . Gorski et al., 1989; Wienhold et al., 1993 and Ž formulations based on alginate Vollner, 1990; Gerstl . et al., 1998a,b . Clays and clay minerals representanother group of additives for slow-release formula-tions. 2:1 clay minerals provide different adsorption Ž sites for neutral molecules external and internalcations, surface oxygen atoms, silanol and aluminol . groups at the edges, etc. . By modifying the clayminerals with organic cations, the surface is madehydrophobic and the adsorption of pesticidemolecules is enhanced. Decades ago, pyridiniummontmorillonite was used to stabilize the herbicide Ž  N  ,  N  -di- n -propyl thiocarbamate Mortland and Meg- . gitt, 1966; Mortland, 1968 . Later, this idea wastaken up to protect herbicides against photodegrada- Ž tion Margulies et al., 1992, 1993; El-Nahhal et al., . 1999a and to reduce leaching and volativity of alachlor and metolachlor. Adsorption of alachlor and Ž metolachlor on a bentonite SWy-1, Source Clays . Repository was very efficient when the clay mineralwas modified by cation exchange with benzyl Ž . trimethyl ammonium ions BTMA . The raw ben-tonite only adsorbed 3% of the amount of meto-lachlor added, whereas bentonite preadsorbed with0.5 mmol BTMA r g and 0.8 mmol BTMA r g ad-sorbed, respectively, 25% and 20% of the amount Ž . added El-Nahhal et al., 1998, 1999b . A further possibility is opening the interlayerspaces by pillaring reactions to promote penetrationof the pesticide molecules between the layers. Inaddition, modification of the gallery by organic Ž cations or molecules can increase adsorption Zielkeand Pinnavaia, 1988; Srinivasan and Fogler, 1990; . Michot and Pinnavaia, 1991; Mishael et al., 1999 .Studies on the release of alachlor from Al-pillaredmontmorillonite and its transport in soil columnsrevealed the potential use of pillared montmoril-lonites for slow-release formulations of herbicides Ž . Gerstl et al., 1998a .We describe the application of unmodified ben-tonites, montmorillonites, and an acid-activated pil-lared montmorillonite in slow-release formulations.The reduced leaching by montmorillonite modifiedwith benzyl trimethylammonium cations was re- Ž ported in the two preceding papers El-Nahhal et al., . 1998, 1999a,b . 2. Materials and methods 2.1. Metolachlor  w  Ž Metolachlor 2-chloro-  N  - 2-ethyl-6-methylphe- . Ž .  x nyl -  N  - 2-methoxy-1-methylethyl acetamide , C - 15 H ClNO , molecular mass 283.8, was received from 22 2 Ž . Chem Service West Chester, PA, USA, purity 99% . Ž . Agan Chemical Israel provided the commercialformulation of this herbicide. 2.2. Bentonites and montmorillonites Bentonites from Otay, Wyoming, Greece and Ž . Turkey Table 1 were used in raw and purifiedform. The sodium montmorillonites were obtainedby two methods: as decribed by Stul and van Leem- Ž . Ž . put 1982 and Tributh and Lagaly 1986 , carbon-ates were removed by sodium acetate r acetic acidbuffer, iron oxides by dithionite r sodium citrate, and  ( ) A. Nennemann et al. r  Applied Clay Science 18 2001 265–275  267Table 1Layer charge and interlayer cation exchange capacity of montmo-rillonitesSample Layer charge, Interlayer Ž . eq r Si,Al O CEC, meq r g 4 10 Ž . Otay bentonite API a 24 0.33 0.89 Ž . Wyoming Bentonite M40A 0.29 0.78 Ž . Wyoming Bentonite M40A 0.29 0.79 Ž . dithionite r citrate method Ž . Wyoming Bentonite M40B 0.28 0.75 Ž . Milos Bentonite M48 0.31 0.84 Ž . Milos Bentonite M48 0.33 0.89 Ž . dithionite r citrate method Ž . Ordu Bentonite M50 0.33 0.90 organic materials by hydrogen peroxide. The ben-tonite was then fractionated by sedimentation toobtain the montmorillonite-rich fraction  - 2  m m. Inother experiments, only carbonates and iron oxideswere decomposed by reaction with hydrochloric acid Ž . and sodium oxalate Janek et al., 1997 . To ensurecomplete transformation into the sodium form, allsamples were washed several times with 1 M NaCl,dialyzed and freeze-dried. 2.3. Pillared montmorillonite The acid-activated pillared montmorillonite wasprepared in kg batches at the National Technical Ž University of Athens, as described elsewhere Jones . et al., 1997 . The precursor montmorillonite was Ž . Fulcat F22B Laporte . The pillared montmorillonite Ž . Al-Fulcat batch w1 had a specific surface area of 256 m 2 r g. The micropore walls contributed by about50% to the total surface area. The low micropore Ž  3 . volume 0.04 cm r g compared to the total pore Ž  3 . volume 0.26 cm r g reflected the mesoporous char-acter of this sample. The cation exchange capacity, Ž  2 q Ž . CEC, was 0.53 meq r g Cu ethylenediamine ex- . change, Bergaya and Vayer, 1997 . 2.4. Organo deri Õ ati Õ es The raw Wyoming bentonite M40B was ex-changed with benzyl trimethylammonium chloride Ž . Ž . BTMA, purum grade, Fluka . A 5% w r w aqueousdispersion of the bentonite was mixed with the samevolume of an aqueous solution of benzyl trimethyl-ammonium chloride. The amount of BTMA was 7.5 Ž mmol corresponding to twice the interlayer ex- . change capacity, Table 1 . The dispersion was heldat 60–70 8 C for 24 h. The exchange reaction wasrepeated after centrifugation. The sample was washed Ž . with water–ethanol 1:1 per volume in a flow- Ž . through cell Lagaly, 1994 and freeze-dried. CHNanalysis revealed a BTMA content of 0.70 meq r g. Ž BTMA-pillared montmorillonite 0.14 and 0.2 . mmol BTMA r g pillared montmorillonite was pre-pared by dropwise addition of aliquots of 0.01 M Ž . Ž . BTMA bromide Aldrich to 200 ml of 0.5% w r v Ž clay suspensions under continuous stirring Mishael . et al., 1999 . After 24 h of stirring, the suspensionswere centrifuged at 16 000 = g  for 30 min; thesupernatants were discarded and the samples werefreeze-dried. The BTMA concentration in the super- Ž natants was measured by UV–Vis absorption spec-trophotometer Uvikon 810, Hewlett-Packard, Ger- . many . The amounts of BTMA cations adsorbedwere determined from the depletion of the solutions. 2.5. Adsorption isotherms The adsorption of metolachlor on the bentonitesand montmorillonites was measured by batch experi-ments at solid contents of 2.5 g r l. Amounts of 100mg of the adsorbent were dispersed in aqueous solu- Ž tions of increasing metolachlor concentration 0– . 1400  m mol r l . Water was added to reach a finalvolume of 40 ml. The dispersions were equilibrated Ž . in an overhead shaker at room temperature 20 " 3 8 Covernight. Amounts of 2 ml of the dispersions werecentrifuged at 10000 = g  for 1 h at 20 8 C. The Ž supernatants were analyzed by HPLC Waters, re-versed phase system; NovaPak RP18 column, 15 cm, Ž particle size 4  m m; eluent: methanol r water 3:1 per . volume , flow rate 1 ml r min; UV detection at 220 . nm .The metolachlor isotherms on the pillared mont-morillonite were measured in the concentration rangeof 0–1056  m mol r g montmorillonite. Aliquots of anaqueous stock solution of 10 y 3 M metolachlor werediluted with distilled water to 25 ml and added to 5 Ž . ml of a 0.5% w r v clay suspension under continu-ous stirring. After shaking the samples for 24 h at20 " 1 8 C, the suspensions were centrifuged at 10000 = g  for 30 min. Metolachlor was extracted from the  ( ) A. Nennemann et al. r  Applied Clay Science 18 2001 265–275 268 Ž supernatant by ethyl acetate r isooctane 1:9 v r v, . HPLC grade, Sigma, Aldrich . Its concentration wasmeasured by gas chromatography, and the amount Ž adsorbed was determined from the difference El- . Nahhal et al., 1999b . Hewlett-Packard gas chro-matograph 6890, electron-capture detector; Rtx w -5MS Capillary Column, 30 = 0.25 mm, film thick-ness 0.25  m m, from Restek, Bellefonte, PA, USA;carrier gas: nitrogen at a flow-rate of 2 ml r min. Thenitrogen set-up flow rate was 30 ml r min. The injec-tor and detector temperatures were 250 8 C and 280 8 C,respectively. The column was held at 170 8 C for 1min; the temperature was then increased to 250 8 C ata rate of 5 8 C r min and held at this temperature for 5min. Blank recovery was 102 " 3%. 2.6. Preparation of formulations2.6.1. Bentonite Formulations were prepared with bentonite M40B Ž . and its BTMA derivative 100% exchange . Anamount of 1 g of metolachlor was dissolved in 50-mlmethanol. This solution was added dropwise to 600ml of a 1.9% dispersion of bentonite in methanol.Stirring was continued for 12 h. Methanol was evap-orated in a rotational evaporator at 40 8 C; the solidmaterial was then dried at 60 8 C and ground in a ball Ž . mill. The metolachlor content was 9.93% w r w . Aformulation with BTMA-bentonite was prepared in Ž . the same manner metolachlor content 11.0% w r w .In addition, formulations containing mixtures of metolachlor-loaded BTMA-bentonite and unloodedbentonite were prepared. The idea was that a diffu-sion of metolachlor between BTMA bentonite andbentonite may delay the release of metolachlor.Therefore, 7.5 g BTMA bentonite was dispersed in Ž . 500 ml methanol overhead shaker, overnight , and1.7 g metolachlor dissolved in 200 ml methanol wasadded under continuous stirring. The dispersion wasstirred for 24 h before the methanol was evaporated.The powder was dried at 65–75 8 C and mixed with7.2 g bentonite and ground in a ball mill. The Ž . metolachlor content was 9.6% w r w . 2.6.2. Acid-acti Õ ated pillared montmorillonite Appropriate amounts of technical grade meto- Ž . lachlor dissolved in acetone denoted a were added Ž to the dispersed pillared montmorillonite Al-Fulcat . w1 or the BTMA derivative in acetone with a totalvolume of 100 ml. The solvent was evaporated underreduced pressure. Some of the formulations from Ž acetone were washed with distilled water 3000 . ml r 2.5 g clay, denoted a–w , centrifuged at 16000 = g  for 30 min.; the supernatant was discarded andthe solid material was freeze-dried. The metolachlor Ž . content was 6.5% w r w . In addition, formulations Ž . denoted w were prepared from aqueous solutionsby adding technical grade metolachlor to the mont-morillonite dispersions as described for the adsorp- Ž . tion measurements metolachlor content: 7% w r w . 2.7. Leaching studies Tin columns, 10 = 10 cm and 25 cm long, were Ž filled with a sandy soil collected from the top 30 cmof a sandy loam soil at the Faculty’s Experimental . Farm in Rehovot, air-dried and sieved  - 2 mm Ž . El-Nahhal et al., 1998 . The column surface was Ž . sprayed with the metolachlor formulations Table 1at a rate of 2.0 kg active ingredient r ha. The experi-ment was performed in triplicates for each formula-tion. The columns were carefully irrigated with 500m 3 water r ha applied in portions during 3–5 h with20-min intervals. This irrigation level was selected toensure water movement up to 30 cm depth. Thecolumns were left for 72 h for equilibration and thensliced along their length to obtain two pots 10 = 5 = Ž 25 cm. Two test plant species, green foxtail  Setaria . Ž Õ iridis  L.  beau Õ ios  and wheat  Triticum aesti Õ um , . cv.  ariel  , were sown in two rows in each pot. Toensure germination and growth, the pots were sprin-kle-irrigated when needed. Sixteen days after sow-ing, the shoot height of the test plants was measuredto estimate the herbicide activity in the column atdifferent soil depths. 3. Results and discussion 3.1. Adsorption on bentonites and montmorillonites Adsorption of metolachlor from aqueous solutionsbasically represents an adsorption process from bi- Ž nary liquid mixtures Dekany et al., 1986a,b; Szanto ´ ´ ´ ´ . et al., 1986, Berger et al., 1997 . Because of the low  ( ) A. Nennemann et al. r  Applied Clay Science 18 2001 265–275  269 solubility of metolachlor in water, the adsorptionprocess may be similar to the adsorption of butanol Ž or pentanol from water on modified silicates Reg- . don et al., 1994, 1998; Dekany et al., 1996 . In fact, ´ ´ metolachlor was preferentially adsorbed from aque-ous solutions. Surprisingly, several bentonites ad-sorbed metolachlor in amounts comparable to theamounts adsorbed by the organo derivatives and the Ž . pillared forms Fig. 1 . The adsorption strongly de-pended on the type of montmorillonite and pretreat- Ž . ment reactions Figs. 2 and 3 . Wyoming bentoniteM40A adsorbed the highest amounts of metolachlor,about 50% of the amount of metolachlor added. Atequilibrium concentrations of 175 ppm metolachlor, ; 180  m mol r g were adsorbed. The reduced adsorp-tion by Wyoming bentonite M40B, 25–35% of theamount of metolachlor added, may be a surprise butvariations are not unexpected when we consider thelarge Wyoming bentonite deposit consisting of dif- Ž . ferent types of bentonites Odom, 1984 . Calciumbentonite of Milos adsorbed less than 50  m mol r g Ž . metolachlor about 16% of the amount added , andthe turkish and Otay calcium bentonite were inactive.Both Wyoming bentonites are sodium-rich andthe superiority to the other bentonites may be causedby the delamination in water, which increases thesurface area accessible to the adsorptive. However,transformation of the Milos and Ordu bentonites to Fig. 1. Adsorption of metolachor on raw bentonites and on the Ž . BTMA derivative of Wyoming bentonite.  v  Wyoming ben- Ž . Ž . Ž . Ž . tonite M40A ;  '  Wyoming bentonite M40B ;  B  Milos Ž . Ž . Ž . Ž . bentonite M48 ;  %  Ordu bentonite M50 ;  ^  BTMA deriva- Ž . tive of Wyoming bentonite M40B .Fig. 2. Adsorption of metolachlor on the sodium montmorillonite Ž . Ž . prepared from Wyoming bentonite M40A .  v  Raw Wyoming Ž . Ž bentonite;  '  sodium form dithionite r citrate extraction and . Ž . Ž . H O treatment ;  ^  sodium form oxalate method . 2 2 the sodium form further reduced metolachlor adsorp-tion. Also, the sodium form of Otay bentonite didnot adsorb metolachlor.The method of bentonite purification is also im- Ž . portant Figs. 2 and 3 . The classical procedure Ž . dithionite r citrate method reduced the amounts ad-sorbed and changed the shape of the adsorptionisotherm. The S-shape indicated a cooperative char-acter of the adsorption on the pure sodium montmo-rillonite, i.e. adsorbed molecules favor the adsorption Fig. 3. Adsorption of metolachlor on sodium montmorillonites Ž . Ž . Ž . prepared from Wyoming M40A  ' ,  ^  and Milos M48 Ž . Ž . bentonite  B , I .  ' , B  dithionite r citrate extraction and H O 2 2 Ž . treatment  ^ , I  oxalate method.
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