Rosin-Coated Fertilizers

The effect of coating the fertilizers with paraffin wax or a mixture of paraffin wax and rosin and the effect of temperature (25, 30, and 35 °C) were...
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Ind. Eng. Chem. Res. 2005, 44, 2288-2291

Controlled Release of Paraffin Wax/Rosin-Coated Fertilizers Ahmed A. Ibrahim and Baba Y. Jibril* Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

For effective deliveries of nutrients to plant species, we have studied release profiles of KCl, NH4NO3, KNO3, (NH4)2SO4, and K2SO4 as fertilizer substrates. The effect of coating the fertilizers with paraffin wax or a mixture of paraffin wax and rosin and the effect of temperature (25, 30, and 35 °C) were explored. The dissolution rate and ultimate amounts dissolved were in the order of KCl > NH4NO3 > KNO3 > (NH4)2SO4 > K2SO4. The order was unchanged whether the fertilizers were coated or not. Coating with paraffin wax exhibited ultimate amounts dissolved lower than those of uncoated samples. Wax/rosin-coated samples attained dissolutions similar to those of uncoated samples but at slower rate. The results showed wax/rosin coating to be more effective than wax-only coating, but the difference diminished for low dissolving fertilizer such as K2SO4. When wax-coated samples were tested at 25, 30, and 35 °C, the rate of dissolution increased but the order of the ultimate amount dissolved was unchanged. As the temperature was increased, the range of the ultimate amounts dissolved between KCl and K2SO4 narrowed. The ranges are 0.09, 0.07, and 0.06 mol/dm3 for 25, 30, and 35 °C, respectively. 1. Introduction Controlled delivery of fertilizers, pesticides, herbicides, and other materials offers a prospect of efficient material utilization and decreases in the detrimental effects of overdosage. The degree of utilization of nitrogen fertilizers, for instance, is in the range of 3050%.1 This leads to nitrogen losses with resulting increases in the production cost, environmental degradations, and food contaminations. Public concerns on such challenges have increased the drive toward developing controlled-release (CR) technology that supplies fertilizer nutrients at an optimum rate to plant species. There are different degrees of success by different proposed technologies with respect to the optimum release of the fertilizer.2 These technologies are categorized as (i) nitrogen-reaction products, (ii) coated fertilizers, and (iii) matrix-type formulation. In the second category, the release is controlled by a protective water-insoluble coating of a water-soluble fertilizer core (substrate). The effects of the coating thickness,3,4 types of coating material,4-6 and release profile7,8 are among the important subjects that are still being investigated. The rate of release of the fertilizer is determined by diffusion through the coating. Sulfur-coated urea,9,10 polyethylene-coated urea,11,12 and superphosphate13 were studied to elucidate the significance of diffusion in the release of the substrate. In a previous report from our laboratory, the dissolution profiles of different commercial fertilizers (monoammonium phosphate, diammonium phosphate, NP, NPK-4, and NPK-14 grades and granular triple superphosphate) were shown.7 Attempts were made to propose a model for dissolution and diffusion of the substrate for both coated and uncoated samples.8,14 The types of coating material and additive are also important. It was found that biodegradable polyurethane foam (PUF) material effectively * To whom correspondence should be addressed. Tel.: +99614676897. Fax: +96614678770. E-mail: [email protected].

controlled the release of ammonium sulfate as a fertilizer. When a biomass was added to PUF, the release ratio was increased with an increase in the amount of the biomass.5 In addition, the type of fertilizer was found to affect the porosity of the coating material. This, in turn, enhanced the transport of macromolecules through the porous structure of the coating.4 In another report, materials of different thermal-oxidative stability were tested. The variation in their properties was useful in designing a CR formulation suitable for particular plant physiological needs.6 Garden and potted plant studies with CR systems have shown beneficial effects in promoting the development of the plant roots system, vegetative growth, and reproductive development.15 Furthermore, a nutrient-deficient stream was improved by slow release of a fertilizer. The study found the rate of fertilizer pellet dissolution to be independent of the pellet size (2-9 g) and water temperature (8.0-14.5 °C).16 The material for the controlled release of a fertilizer is important because it will determine the production cost and, hence, the purchase prices. A recent review on the controlled release of a nitrogen fertilizer through polymeric membrane devices has emphasized the need for exploration of new inexpensive materials and further investigations on parameters important for developing new systems.17 There are few reports in the literature that address these issues and, in particular, the effects of the temperature on the fertilizer dissolution. This paper reports the dissolution profile of KCl, NH4NO3, KNO3, (NH4)2SO4, and K2SO4 as fertilizer substrates. The effects of the temperature on the rate of dissolution of both coated and uncoated samples were also explored. The aim of this report was to shed light on the importance of the types of fertilizer substrates and coating materials toward designing new CR systems and improving the existing ones. Paraffin wax and rosin are particularly suitable as coating materials because of their low cost and lower likelihood of contaminating the soil.

10.1021/ie048853d CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005

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Figure 1. CR profiles of different fertilizer substrates at 25 °C.

Figure 2. CR profiles of uncoated, wax-coated, and wax/rosincoated KCl at 25 °C.

2. Experimental Section The materials used as fertilizer substrates were KCl, NH4NO3, KNO3, (NH4)2SO4, and K2SO4, obtained from WINLAB. They were of high purity (assay 99%), except K2SO4, which was a general purpose reagent. Each of the substrates was tested based on a simple procedure. A given mass (18 g) of the fertilizer was pulverized in a hammer mill. Then the amount was dried in an oven at 100 °C for 1 day and then stored in a desiccator. A measured amount of paraffin wax (2 g) or paraffin wax (1.8 g) mixed with rosin (0.2 g) as a coating material was continuously stirred in a beaker maintained at 7080 °C until the wax was completely melted. A measured amount of the respective fertilizer substrate was gradually added to the molten wax to obtain a 90 wt % fertilizer concentration in the matrix. The stirring continued until a uniform distribution of the fertilizer in the wax was observed. The contents of the beaker was then poured into custom-designed Plexiglas molds and left to solidify. The fertilizer formulation was then removed from the molds in the form of small disks (0.75 cm diameter and 0.35 cm height). In the text, the fertilizers are referred to as uncoated, wax-coated, and wax/rosin-coated. The dissolution tests were carried out in deionized water in glass bottles without agitation. The temperature was maintained at 25 ( 0.2 °C. In one set of experiments, the effect of the temperature (25, 30, and 35 ( 0.2 °C) was explored. The concentration of the fertilizer released was monitored with time using a conductivity meter (Omega CDH-420) and an atomic absorption spectrophotometer (model 200A; Buck Scientific Inc.). For the entire concentration range, the two meters gave reproducible and similar results. 3. Results and Discussion The rates of release of different coated and uncoated fertilizer substrates were monitored until no more significant amount of each substrate was dissolved. Figure 1 shows the profile of the dissolution of the substrates at 25 °C. The figure shows KCl to have the highest rate of dissolution, while K2SO4 exhibited the least. At steady state, the concentrations of the two were about 0.15 and 0.07 mol/dm3, respectively. Others showed concentrations between these two values: NH4NO3 (0.14) > KNO3 (0.12) > (NH4)2SO4 (0.09 mol/dm3). All of the fertilizers showed rapid rates of release and

Figure 3. CR profiles of uncoated, wax-coated, and wax/rosincoated NH4NO3 at 25 °C.

almost attain their respective steady-state values after about 75 h. Longer dissolution times showed insignificant effects because no agitation or change in variables such as temperature was made. Figures 2-6 show the dissolution profiles of the individual fertilizer substrates when coated with wax or a mixture of wax and rosin (wax/rosin). Without coating, KCl reached a steady-state (equilibrium) concentration of about 0.15 M in about 75 h, as shown in Figure 2. When the substrate was coated with wax, the rate decreased, with an equilibrium concentration of about 0.13 M. Substrates coated with wax/rosin exhibited higher equilibrium concentrations than those of both uncoated and wax-coated samples, but the rates of release were different. Also, the release trend indicated higher ultimate releasable amounts. The rate showed a slower increase with the wax/rosin, exhibiting a concentration of 0.11 M at 50 h. Similar profiles and effects of wax and wax/rosin coatings were observed for NH4NO3 (Figure 3) and (NH4)2SO4 (Figure 4). For KNO3 (Figure 5) and K2SO4 (Figure 6), there appeared to be no differences in the initial rates between wax and wax/ rosin coatings. Still, at equilibrium, the wax/rosin coating maintained a higher ultimate amount dissolved. The differences in the release profiles of the substrates may be due to their different tendencies to affect the coating material properties such as the pore size. The changes in the pore size have been shown to depend on the type of fertilizer.4

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Figure 4. CR profiles of uncoated, wax-coated, and wax/rosincoated (NH4)2SO4 at 25 °C.

Figure 7. CR profiles of different wax-coated fertilizer substrates at 25 °C.

Figure 5. CR profiles of uncoated, wax-coated, and wax/rosincoated KNO3 at 25 °C.

Figure 8. CR profiles of different wax-coated fertilizer substrates at 30 °C.

Figure 6. CR profiles of uncoated, wax-coated, and wax/rosincoated K2SO4 at 25 °C.

The result suggests that the tendency of wax to retard the rate of dissolution and decrease the amount of fertilizer to be released depends on the type of fertilizer employed. When the fertilizer substrates with the least dissolution rate (K2SO4 and KNO3) were coated with wax/rosin or wax, initially (until about 150 h) no significant difference in the concentrations was observed. Later, the profiles exhibited trends similar to those of other fertilizers. The wax/rosin-coated sample exhibited a steady-state concentration similar to that

of an uncoated sample, both of which were higher than that of the wax-coated sample. This shows that the mixture of rosin and wax is more effective than wax alone in slowing the release. It is also efficient in ultimately releasing all of the fertilizer material, thereby limiting wastage. On the other hand, wax coating has the disadvantage of retaining residual amounts of substrates. This may pose both economic and environmental challenges. The difference between wax/rosin and wax appeared to be insignificant when a lowdissolution fertilizer was employed. The fertilizer dissolution rate exhibited an increase with an increase in the temperature (25-35 °C), as shown in Figures 7-9. The rates and the order of concentrations at equilibrium [KCl > NH4NO3 > KNO3 > (NH4)2SO4 > K2SO4] remain the same between waxcoated and uncoated samples. The increases in the ultimate amounts dissolved when the temperature was increased (from 25 to 35 °C) were 0.002, 0.007, 0.012, 0.002, and 0.029 mol/dm3 for NH4NO3, KCl, (NH4)2SO4, KNO3, and K2SO4, respectively. It may also be observed that the range of concentrations of the amounts dissolved between KCl and K2SO4 has decreased in the temperature range (Figure 9). At high temperature, the rates of dissolution and diffusion through the wax increase, perhaps and especially because the substrates may have higher effects on the wax material. The rate of dissolution also increases with the temperature. This shows the strong effect of the temperature on the rate.

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wax/rosin-coated samples, the amount released at equilibrium was similar to the case of the uncoated sample. Thus, wax/rosin coating is more effective in controlling the release and more efficient in eventually releasing all of the substrate. Such a difference between wax/ rosin- and wax-coated samples diminished when the fertilizer with the slowest dissolution was tested. The temperature showed a strong effect on the rate of dissolution of the wax-coated sample. The equilibrium concentration range for the substrates narrowed at higher temperature, thus indicating the decrease in the efficacy of the coating material with an increase in the temperature. Literature Cited Figure 9. CR profiles of different wax-coated fertilizer substrates at 35 °C.

Figure 10. CR profiles of different wax/rosin-coated fertilizer substrates at 25 °C.

This is contrary to an earlier report that showed no temperature effects, although lower temperature values and lower ranges were used.16 The temperature also showed the effect on the residual amounts that remained in the core of the coating material. At 35 °C, wax-coated substrates showed concentration values close to that of the uncoated sample. This shows that the effectiveness of the coating decreases with an increase in the temperature. The combined effect of rosin and wax is shown in Figure 10. When the two materials were mixed and used for coating, the releases of fertilizers were slower than those in the cases of wax, rosin, or no coating. This indicates that the mixed coatings may offer more effective release regimes and longer times before the equilibrium concentration is reached. 4. Conclusion CR profiles of different fertilizers have been studied. The rates of dissolution of the fertilizers over time and the concentrations at equilibrium were found to be in the following decreasing order: KCl > NH4NO3 > KNO3 > (NH4)2SO4 > K2SO4. When the fertilizers were coated with wax/rosin or wax only, the rate of dissolution showed a higher decrease for wax/rosin samples than for wax-coated samples. Wax-coated samples showed a lower ultimate amount to be released than wax/rosin by retaining some amounts of the substrates. For the

(1) Chatzoudis, G. K.; Rigas, F. P. Polymeric conditioners effects on leaching of nitrogen fertilizers in soil columns. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 1998, 33, 765. (2) Goertz, H. M. Controlled Release Technology. Encyclopedia of Chemical Technology, 4th ed.; John Wiley & Sons: New York, 1993; Vol. 7, p 251. (3) Tomaszewska, M.; Jarosiewicz, A.; Karakulski, K. Physical and chemical characteristics of polymer coatings in CRF formulation. Desalination 2002, 146, 319. (4) Tomaszewska, M. Preparation and properties of flat-sheet membranes from poly(vinylidene fluoride) for membrane distillation. Desalination 1996, 104, 1. (5) Ge, J.; Wu, R.; Shi, X.; Yu, H.; Wang, M.; Li, W. Biodegradable polyurethane materials from bark and starch. II. Coating material for controlled-release fertilizer. J. Appl. Polym. Sci. 2002, 86, 2948. (6) Hanafi, M. M.; Eltaib, S. M.; Ahmad, M. B. Physical and chemical characteristics of controlled release compound fertilizer. Eur. Polym. J. 2000, 36, 2081. (7) Al-Zahrani, S. M. Utilization of polyethylene and paraffin waxes as controlled delivery systems for different fertilizers. Ind. Eng. Chem. Res. 2000, 39, 367. (8) Ko, B.-S.; Cho, Y.-S.; Rhee, H.-K. Controlled Release of Urea from Rosin-Coated Fertilizer Particles. Ind. Eng. Chem. Res. 1996, 35, 250. (9) Rindt, D. W.; Blouin, G. M.; Gestinger, J. G. Sulfur Coating on Nitrogen Fertilizer to Reduce Dissolution Rate. J. Agric. Food Chem. 1968, 16, 773. (10) Blouin, G. M.; Rindt, D. W.; Moore, O. E. Sulfur-Coated Fertilizer for Controlled Release: Pilot Plant Production. J. Agric. Food Chem. 1971, 19, 801. (11) Salman, O. A. Polymer Coating of Urea Prills to Dissolution Rate. J. Agric. Food Chem. 1988, 36, 616. (12) Salman, O. A. Polyethylene-Coated Urea. 1. Improved Storage and Handling Properties. Ind. Eng. Chem. Res. 1989, 28, 630. (13) Subrahmanyan, K.; Dixit, L. A. Effect of different coating materials on the pattern of phosphorus release from superphosphate. J. Indian Soil Sci. 1988, 36, 461. (14) Al-Zahrani, S. M. Controlled-Release of Fertilizers: Modeling and Simulation. Int. J. Eng. Sci. 1999, 37, 1299. (15) Wybraniec, S.; Schwartz, L.; Wiesman, Z.; Markus, A.; Wolf, D. Release Characteristics of Encapsulated Formulations Incorporating Plant Growth Factors. J. Environ. Sci. Health, Part B 2002, 37, 235. (16) Sterling, M. S.; Ashley, K. I.; Bautista, A. B. Slow-Release Fertilizer for Rehabilitating Oligotrophic Streams: A Physical Characterization. Water Qual. Res. J. Can. 2000, 35, 73. (17) Dave, A. M.; Mehta, M. H.; Aminabhavi, T. M.; Kulkarni, A. R.; Soppimath, K. S. Review on Controlled Release of Nitrogen Fertilizers through Polymeric Membrane Devices. Polym.-Plast. Technol. Eng. 1999, 38, 675.

Received for review November 28, 2004 Revised manuscript received January 19, 2005 Accepted January 31, 2005 IE048853D