Periodic application of agrochemicals has led to high cost of production and serious environmental pollution. In this study, the ability of montmorillonite (MMT) clay to act as a controlled release carrier for model agrochemical molecules has been investigated. Urea was loaded into MMT by a simple immersion technique while loading of metalaxyl was achieved by a rotary evaporation method. The successful incorporation of the agrochemicals into the interlayer space of MMT was confirmed by several techniques, such as, significant expansion of the interlayer space, reduction of Barrett-Joyner-Halenda (BJH) pore volumes and Brunauer-Emmett-Teller (BET) surface areas, and appearance of urea and metalaxyl characteristic bands on the Fourier-transform infrared spectra of the urea loaded montmorillonite (UMMT) and metalaxyl loaded montmorillonite (RMMT) complexes. Controlled release of the trapped molecules from the matrix was done in water and in the soil. The results reveal slow and sustained release behaviour for UMMT for a period of 10 days in soil. For a period of 30 days, MMT delayed the release of metalaxyl in soil by more than 6 times. It is evident that MMT could be used to improve the efficiency of urea and metalaxyl delivery in the soil.
Agrochemicals are important ingredients for achieving global food security. However, the need for periodic application stemming from the huge losses witnessed due to volatilization, photodegradation, leaching, and surface migration may render agrochemical usage economically unsustainable [
Agrochemical formulations which combine minimum amount of active ingredients (ai) with prolonged efficacy would effectively reduce postapplication losses. Important ways to achieve this are by photostabilisation and slowing the release of the active materials [
Clay minerals are natural and relatively cheap components of soils. They are suitable materials for controlled release (CR) and photostabilized agrochemicals due to their high adsorption power, easily modified surfaces, and their colloidal nature [
Montmorillonite [(Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2
MMT have been used as a drug delivery system (DDS) previously. Recent research indicates that, with proper control of intercalation conditions, MMT can carry nicotine, timolol, ibuprofen, and donepezil among other therapeutic drugs and effectively modulate the drug release behavior. The drug releasing behaviour of MMT can also be modified by intercalation with other nanoparticle materials to form composites such as use of polymers including hydrogels, soluble polymers, and biodegradable and nonbiodegradable polymers [
Intercalation of agrochemicals into montmorillonite may offer multiple benefits. The gallery space would provide room for agrochemical molecules to occupy. The inorganic sheets would offer protection against environmentally induced rapid disintegration while intermolecular forces would ensure sustained release, consequently leading to long treatment periods for the same quantity of active ingredient [
Urea [carbonyl diamine] and metalaxyl [methyl N-(2,6-dimethylphenyl)-N-(2-methoxyacetyl) alaninate] were used as model fertilizer and pesticide molecules, respectively, to study the ability of MMT to store and controllably release agrochemical molecules. The choice of urea which constitutes about 40% of the total global nitrogen fertilizers application was made based on its high aqueous solubility, rapid hydrolysis by soil urease, and photo lability which has led to its considerable and rapid loss in soil [
In this work, commercial MMT was purified using sedimentation and centrifugation techniques. Urea and metalaxyl molecules were successfully incorporated into the interstitial space of MMT by employing simple emersion and rotary evaporation techniques. The clay/agrochemical composites were characterized by electron microscopy (SEM and TEM), Fourier-transform infrared (FT-IR) spectroscopy, powder wide angle X-ray diffraction (WAXRD), N2 sorption studies, and thermal gravimetric analysis/differential thermal analysis (TGA/DTA). Controlled release properties of the loaded complexes were studied in water and in the soil.
All the chemicals used in this study were of high purity. Montmorillonite K10 was purchased from Alfa Aesar, USA; metalaxyl Pestanal (99.9%), P-dimethylaminobenzaldehyde, urea (>99%), hydrochloric acid, and trichloroacetic acid (99.0%) were purchased from Sigma-Aldrich, UK. All the reagents apart from montmorillonite were used as purchased without further purification. Water used in the experiments was purified by Millipore Milli-Q system to a resistivity of 18.2 MΩ
Montmorillonite nanoparticles (MMT NP) were obtained from commercial MMT. Particles having diameter greater than 2
Preliminary tests were conducted on the as-purchased MMT denoted by MMT MP and MMT NP using varied concentrations of urea. MMT (20 mg) samples were suspended in 4 mL of 0.17, 1.67, and 8.33 moles/litre aqueous urea solutions for 24 hours at RT using a magnetic stirrer at 300 rpm. The suspensions were centrifuged at 10 000 rpm for 10 min and urea concentration of the supernatant was determined by UV/Vis spectrophotometry method described by Wanyika et al. [
Based on initial preliminary results, MMT NP and 8.33 moles/litre aqueous urea solutions were chosen for all the subsequent experiments. First, equilibration time was determined by suspending 20 mg of MMT NP into 4 mL of urea solution for 15 min, 30 min, 60 min, 100 min, 200 min, and 300 minutes, followed by determination of the urea content in the supernatant. Secondly, using an excess of established equilibration time, an experiment to study the reaction kinetics was conducted with 20 mg of MMT NP and 4 mL of urea solution for 1 min, 5 min, 10 min, 15 min, 20 min, 30 min, 45 min and 60 minutes. Finally, MMT NP (500 mg) was dispersed in 20 mL of 8.33 moles/litre aqueous urea solutions for 24 hours at room temperature using a magnetic stirrer at 300 rpm. The suspensions were centrifuged at 10 000 rpm for 10 min and then oven-dried at 80°C for 4 hours. The urea-MMT NP composite denoted by UMMT was used for characterization and release studies.
MMT NP (230 mg) was suspended in 20 mL of water containing 9.8 mg of metalaxyl. Dissolution of metalaxyl in water was facilitated by sonication for 10 minutes. The suspension was stirred for 2 hours, and then the solvent was evaporated to ~5 mL using a rotary evaporator at 45°C and reduced pressure. The MMT NP intercalate was isolated by centrifugation and the product, denoted by RMMT, dried at RT for 24 hours.
Electron microscope images were collected on scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, Tecnai G2 20 S-TWIN). Infrared (IR) spectra were recorded on a FT-IR spectrophotometer (SPECTMM ONE B) using KBr discs in the range of 400–4000 cm−1. X-ray powder diffraction patterns (XRD) were obtained on a diffractometer (LR 39487C XRD) using Ni-filtered Cu K
The static release profiles were studied for a period of 10 days with samples collected and analyzed daily. All the 10 measurements were started simultaneously. UMMT (10.0 mg) was accurately weighed. It was suspended in 3 mL of pure water in conical bottles. The suspensions were stirred at room temperature and centrifuged after every time lapse. Urea content of the supernatant was determined using the UV-Vis method previously described.
UMMT (26 mg) and a physical mixture comprising 11.7 mg urea and 14.3 mg MMT were well mixed with 18 g of dry soil (<2 mm in diameter) and put in 25 mL separating funnels at room temperature and water (12 mL) was added. Control experiment was done with 11.7 mg of pure urea mixed with 18 g of the soil while blank experiment was done with 18 g of soil alone. A fraction of water (5 mL) was eluted after every 24 hours; 5 mL of pure water was added back in order to maintain a constant volume of water slightly above the soil surface throughout the experiment. The elute was filtered through 0.22
The release of metalaxyl from RMMT was carried out by suspension of 8.0 mg of RMMT in 4.0 mL of water contained in glass bottles and sealed with screw caps for different time periods. Different experiments for the different periods of time were set up simultaneously. In all cases, the release kinetics was obtained in triplicate. After each time lapse, the bottles were hand-shaken and centrifuged at 10 000 rpm for 10 minutes. The supernatant was filtered and analyzed by high performance liquid chromatography (HPLC). The following HPLC conditions were used: detector, UV,
The controlled release characteristics of the RMMT were determined with soil filled glass separating funnels with a volume of 25 cm3. Pure metalaxyl (1.6 mg) corresponding to the amount entrapped in the RMMT was used as control. The soil columns were settled by the addition of 20 mL of water, 50 mg of RMMT was placed on top of soil columns, and 10 mL of pure water was applied on a three-day interval for one month to the tubes. The elutes were filtered through 0.22
Electron microscopy images and corresponding particle size distributions of as-purchased MMT (MMT MP) and purified MMT (MMT NP) are displayed in Figure
SEM and corresponding TEM images of (a)-(b) as-purchased MMT and (c)-(d) MMT NP. Inset: particle size distributions.
Results on the preliminary adsorption of urea onto MMT MP and MMT NP are shown in Figure
Efficiency of MMT NP and MMT MP in adsorption of urea.
The time dependence of urea adsorption by MMT NP is exhibited in Figure
Time dependence of urea adsorption by MMT NP. Inset: calibration curve for urea determination.
Pseudo-1st and -2nd order sorption kinetic models for intercalation of urea molecules into MMT NP interlayer. Inset: pseudo-2nd order kinetic parameters.
FT-IR spectra of purified MMT, UMMT, and RMMT hybrids are shown in Figure
FT-IR spectra of MMT NP, UMMT, and RMMT.
WAXRD patterns of MMT NP, UMMT, and RMMT are given in Figure
WAXRD patterns of MMT NP, UMMT, and RMMT.
Nitrogen sorption isotherms and their corresponding pore size distributions are displayed in Figures
Physicochemical properties of MMT NP, UMMT and RMMT.
|
|
Pore size (nm) | |
---|---|---|---|
MMT NP | 311 | 0.50 | 5.7 |
UMMT | 85 | 0.21 | 7.9 |
RMMT | 267 | 0.42 | 6.5 |
N2 adsorption-desorption isotherms of MMT, UMMT, and RMMT.
BJH pore size distributions of MMT NP, UMMT, and RMMT.
TGA/DTA thermograms of UMMT and RMMT are exhibited in Figure
TGA/DTA curves of UMMT and RMMT.
TGA/DTA curves for MMT NP used in (a) UMMT and (b) RMMT.
The SR profile of UMMT in water and in soil is exhibited in Figure
Time dependence release of urea from UMMT nanocomposite in water and soil.
IR spectrum of UMMT and corresponding spectra after releasing urea in water for different days.
Urea molecules located within the interlayer space of MMT NP could be protected against decomposition by photochemical, thermal, enzymatic, and other catalytic activities of soils unlike free-urea molecules on the surface of soil particles. When they are in contact with soil water and adsorbed water of soil particles, they are readily transferred into the waters by diffusion. When they are hydrolyzed to ammonia, MMT can quickly adsorb urea-derived ammonia through physical and chemical interactions not only because MMT has both high water holding and cation exchange capacities but also because MMT and the ammonia tend to be located very closely. Therefore, MMT is expected to play an essential role in suppressing emission of ammonia through delivery of urea into inner soils and adsorption of ammonia.
The release profile of RMMT in water and in soil is shown in Figure
Release profiles of RMMT in (a) water and (b) soil.
Intercalation of urea and metalaxyl into purified MMT was achieved. The entrapped/intercalated molecules possess the same physical and chemical properties with those in free state. Sorption kinetic studies demonstrated that the carrier materials could adsorb high concentrations of the agrochemicals by physisorption. High adsorption efficiency would facilitate the use of less delivery carriers and prolong the release period. Physical adsorption ensures that the chemical nature of the guest molecules is not altered.
The release of the intercalated molecules was significantly retarded which is sine qua non for controlled release application.
The author declares that there is no conflict of interests regarding the publication of this paper.
This research was supported by the German Academic Exchange Service (DAAD) and Jomo Kenyatta University of Agriculture and Technology (JKUAT).