In the Laboratory
Reducing the Use of Agrochemicals: A Simple Experiment
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M. M. Vidal,* Olga M. S. Filipe, and M. C. Cruz Costa Departamento de Ciências Exactas e do Ambiente – CERNAS, Escola Superior Agrária de Coimbra, Bencanta, 3040-316 Coimbra, Portugal; *
[email protected] Owing to great agricultural needs and technological progress, the use of chemicals such as pesticides and fertilizers is an accepted practice today. However, the effect of pesticides and fertilizers on surface water and groundwater quality is becoming a concern (1, 2). Agrochemicals may be carried along with the water that moves downward from the surface until they eventually reach the groundwater (leaching). Slowrelease formulations are excellent alternatives to the conventional soluble formulations, because active substances are released at a slow and constant rate over an extended period of time. Consequently, less frequent applications are required and plants are able to take up the agrochemicals without waste by leaching (3). The goal of this experiment is to introduce students to polymeric-based controlled-release agrochemicals that minimize leaching into groundwater while maintaining an adequate quantity for the desired agrochemical benefits. A gelatin hydrogel containing inorganic phosphorus was prepared. Hydrogels are three-dimensional polymer networks in which individual hydrophilic polymer chains are connected, in the case of the gelatin, by hydrogen bonds. Recently, various polymer-supported or microencapsulated agrochemicals have been introduced in agriculture to limit the undesirable side effects associated with conventional formulations. Biodegradable matrices are preferred to prevent further environmental pollution created by nondegradable carriers (3). Gelatin is a naturally occurring polymer, relatively inexpensive, and a good film or particle-forming substance. Students prepare a controlled-release fertilizer and characterize the release kinetics. They then compare the application of the fertilizer as a controlled-release system to the water-soluble form. Experimental
Preparation of Gelatin Gels Containing Inorganic Phosphorus Sustained release of inorganic P was achieved by including KH2PO4 into a gelatin polymeric support. The gelatin gels were not submitted to any further physical or chemical crosslinking. Gelatin and KH2PO4 solutions were prepared from Merck (Germany, Darmstad) pro-analysis reagents. Gelatin gels containing phosphorus (GelP) were prepared from a 20% (w兾v) aqueous solution of gelatin. Gels were obtained on the bottom of 50-mL glass beakers (3.8-cm diameter) by dissolving different quantities of KH2PO4 in 10 mL of the gelatin solution at 40 ⬚C. Gels were partially dried and stored at 4 ⬚C, for 7 days. They were ground with a mortar and pestle immediately before use. Gelatin gels contained either 100 mg (GelP100) or 400 mg (GelP400) of P. The water weight lost ratio ∆W was calculated according to the
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formula ∆W =
W0 − W7 × 100% W0
(1)
where W0 is the initial gel weight and W7 is the gel weight after storage time. For both GelP100 and GelP400 the water weight lost was 25–28%.
Determination of Inorganic P Release from Gelatin Gels Ground GelP400 and GelP100 formulations were wrapped in a cheesecloth and immersed in fresh distilled water (100 mL) with magnetic stirring for 120 min at room temperature. Released inorganic P was monitored by the vanadomolybdophosphoric acid colorimetric method (4, 5). Fractions, 100 µL, of the released solution were collected at regular time intervals and added to 1 mL of the vanadate– molybdate reagent and 4 mL of distilled water in a test tube. After 10 min, the absorbance of each solution was measured at 470 nm using a Hitachi U2000 spectrophotometer. The absorbance measurements were converted to P concentrations using a P calibration curve and the total quantity of P released was calculated. Soil–Water Release Tests The soil–water release studies of P from gels and unencapsulated P were performed in a glass column (5.0-cm i.d., 15.5-cm height ) containing soil. Both ground GelP and unencapsulated P were mixed separately with 50 g of soil. A measured quantity of distilled water (50 mL) was added to the top of a column. The water passing through the column was collected, filtered by a 0.45 µm GF兾C membrane filter, and analyzed for released P by spectrophotometry as described above. Four irrigations were simulated. All the water passing through the column was collected before the next irrigation, and volume of each irrigation was measured. Hazards The gelatin matrix formulations of inorganic P are not toxic; they can be safely handled and disposed. Results and Discussion
Determination of Inorganic P Release from Gelatin Gels Gelatin-matrix formulations of inorganic P, GelP100 and GelP400, were tested and release of inorganic P from gels was evaluated against distilled water. A simple analysis of the inorganic P release was provided by Sinclair and Peppas (6),
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In the Laboratory 120.0
100.0
Mt/M0 (%)
80.0
60.0
40.0
20.0
0.0 0
2
4
6
8
10
12
t / min
80.0
y ⴝ 21.748x ⴚ 0.118 R 2 ⴝ 0.988
Mt/M0 (%)
60.0
40.0
y ⴝ 19.377x ⴙ 1.124 R 2 ⴝ 0.989
20.0
0.0 0
1
2
4
3
t / min Figure 1. Fractional release (Mt/M0) of P versus t1/2 from GelP into 100 mL of distilled water for GelP100 (䊊) and GelP400 (䊉): (top) results for 120 minutes (t1/2 = 11) and (bottom) results for the first 12 minutes.
Fraction P Released (%)
50
40
30
20
10
0 0
1
2
3
4
5
Irrigation Water Figure 2. Percentage of P released over four irrigation waters: GelP400 (䊊) formulation and 400 mg of unencapsulated P (䊉).
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where Mt is the mass of inorganic P released at time t, M0 is the total mass of encapsulated P, k is the diffusion kinetic constant, and n is the diffusion exponent. The value of the exponent is a good indication of the release mechanism. When fractional release, Mt兾M0, is linear with the square root of time, the release-curve profiles match Fick’s law. The profiles of P release obtained with both GelP400 and GelP100 are shown in Figure 1. As shown in the bottom graph, both experimental release curves can be considered linear up to 10–15 min, 3.2 < t1兾2 < 3.9, corresponding to a fractional release of 70%. After this time interval, there is a release rate decrease for both gels. The total release of P for GelP400 was obtained at 120 min while that for GelP100 was obtained at 40 min. We conclude that the release mechanism is controlled by diffusion, as shown by the Fickian behavior during the first 15 minutes. After this time the P release is controlled by gelatin dissolution. In fact, dissolution in aqueous solution is typical of gelatin gels prepared without further crosslinking and a relationship between inorganic P quantities and gel mechanical properties can be observed.
Soil–Water Release of Unencapsulated P A soil–water system is proposed to follow the movement of P downward in a soil column prepared with a mixture of soil and unencapsulated P. Two different quantities of unencapsulated P, 100 mg and 400 mg, were studied. A measured quantity of water was added to both columns to observe the movement of P along the column. Quantities of P, in the water collected at the bottom of the columns, were plotted as a function of the irrigation water. Similar profiles were obtained for both 100 mg and 400 mg quantities of unencapsulated P, and total leaching P loss is under 100%, which is consistent with P-fixation capacity of soil (2, 7). Results of 400-mg unencapsulated P are shown in Figure 2. We observe that the levels of leached unencapsulated P are significant only for the first two irrigation waters. Thus, to maintain P levels in the soil longer, a new P application is necessary following the second irrigation water. The data suggest that an application of P more often and in smaller doses would be more beneficial. Soil–Water Release of GelP GelP400 and GelP100 formulations were used as controlled-release systems and the same soil–water system was used to indicate the leachability of these formulations for comparison with unencapsulated P (see discussion above). For the first irrigation the quantity of P leached is less for GelP formulations when compared with corresponding quantities of unencapsulated P (Figure 2). According to the results obtained in kinetic studies, and since similar free water in the soil column had not drained out, the difference on P leaching between unencapsulated P and GelP can be assigned to a controlled-release ruled by a diffusion mechanism. Over the next three irrigation waters controlled-P release is ruled by the gelatin dissolution, showing quantities of leached P always above 10%. Production of a more long-term delivery system (over a period of 1 to 3 moths) would require gelatin stabilization by using crosslinking procedures delaying the gelatin dissolution process. Chemical crosslinking of gelatin is accomplished traditionally with dilute solutions of glutaraldehyde or formaldehyde, but these crosslinking agents have been shown to be toxic. Therefore other crosslinking
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methods have been studied using natural molecules, such as glyceraldehyde, genipin, dextran, and alginate (8, 9).
We thank Victor M. S. Gil for the revision of the manuscript.
Summary Gelatin gel containing inorganic P provides controlledrelease of an agrochemical. Release curves show a linear relationship between release and square root of time, implying a diffusion-controlled release mechanism. The relatively low cost, the ease of preparation, and the use of biodegradable gels make these systems an ideal choice to give students their first contact with this kind of device. The availability of inorganic P to the leaching medium was more uniform, lasting over a larger number of irrigation waters, than when applied by conventional methods. Students realize that applications of fewer doses with large concentrations of agrochemicals are inefficient because they provide concentrations beyond a crop’s needs. These high concentrations can escape from soil into water sources causing serious health risks and environmental pollution. On the other hand, application of an active ingredient immobilized in a polymeric matrix results in a continuous release to the soil at a controlled rate that is advantageous because it improves nutrient uptake and absorption and reduces pollution effects. W
Acknowledgment
Supplemental Material
Additional background information and a detailed procedure, including Excel spreadsheets to aid in the calculations, are available in this issue of JCE Online.
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Literature Cited 1. Pretty, J. N. Regenerating Agriculture: Policies and Practice for Sustainability and self-Reliance; Joseph Henry Press: Washington, DC, 1995. 2. Campbell, K. L.; Edwards, D. R. Agricultural Nonpoint Source Pollution: Watershed Management and Hydrology; Ritter, W. F., Shirmohammadi, A., Eds.; Lewis Publishers: Boca Raton, FL, 2001; pp 91–109. 3. Bajpai, A. K.; Giri, A. React. Funct. Polym. 2002, 53, 125– 141. 4. Standard Methods for the Examination of Water and Wastewater, 20th ed.; Clesceri, L. S., Greenberg, A. E., Eaton, A. D., Eds.; American Public Health Association: Washington, DC, 1998. 5. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; W. H. Freeman Company: New York, 1998. 6. Sinclair, G. W.; Peppas, N. A. J. Membrane Sci. 1984, 17, 329– 331. 7. Xia, K.; Pierzynski, G. J. Chem. Educ. 2003, 80, 71–74. 8. Kosmala, J. D.; Henthorn, D. B.; Peppas L. B. Biomaterials 2000, 21, 2019–2023. 9. Yao, C. H.; Liu, B. S.; Chang, C. J.; Hsu, S. H.; Chen, Y. S. Mater. Chem. Phys. 2004, 83, 204–208.
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