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Mechanochemical Route for Synthesizing KMgPO4 and NH4MgPO4 for Application as Slow-Release Fertilizers Solihin,† Qiwu Zhang,† William Tongamp,*,‡ and Fumio Saito† Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1, Katahira, Aoba-Ku, Sendai 980-8577, Japan, and Faculty of Engineering and Resource Science, Akita UniVersity, 1-1 Tegata-Gakuen machi, Akita 010-8502, Japan
This article introduces a mechanochemical (MC) process for the synthesis of KMgPO4 and NH4MgPO4 by milling starting materials in a planetary ball mill. First, KMgPO4 was prepared by milling KHPO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min at mill rotational speeds of 500-600 rpm. Washing of KMgPO4 in water for 700 h released only up to 20-25% of K+ and PO42- ions into solution. Second, NH4MgPO4 was prepared by milling NH4HPO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min at mill rotational speeds of 300-700 rpm. Washing of NH4MgPO4 in water for 500 h released only up to 10-20% of NH4+ and PO42- ions into solution. The MC process could be developed to synthesize KMgPO4 and NH4MgPO4 for possible application as slow-release fertilizers. 1. Introduction According to the World Bank, the world population will increase to become 9.22 billion in the year 2075,1 and this necessitates the high food productivity of agricultural products. Such a high productivity of agricultural products could be achieved by increasing the crop yield per unit area of land, in addition to the land area used for agriculture. However, the land used for agriculture tends to eventually be reduced, when it is converted to housing, industrial use, and the like.2,3 High crop yields are normally achieved by fixing the soil properties through the addition of fertilizer. Fertilizers assist crop yields, but their efficiency is usually low because of the lack of balance between the speed at which nutrients are released from fertilizers and the speed at which nutrients are adsorbed by plant roots. The nutrients that are not adsorbed by plant roots eventually descend toward underground water and pollute it.4 Currently, these drawbacks can be avoided by applying fertilizers that are able to release nutrients very slowly, known as slow-release fertilizers, such as KMgPO4 and NH4MgPO4. The common methods for synthesizing KMgPO4 and NH4MgPO4 involve physical methods such as dispersing ordinary fertilizer in a matrix or encapsulating ordinary fertilizer so that nutrient release is slowed by diffusion.5-8 Application of a mechanochemical synthesis route to prepare similar compounds, namely, the intercalation of urea (NH2CONH3) into the kaolin structure, and comparison against the aqueous suspension method were reported in 2009 by Mako´ et al.9 It is well-known that kaolin and other hydrated silicate minerals undergo amorphization against grinding operations and that alkali-alumina-phosphate glass does exist.10,11 Recently, application of a mechanochemical route involving the milling of two or more elements or compounds for the synthesis of new compounds was reported by our research group.12-17 Tongamp et al.18 successfully used a mechanochemical route to prepare the nitrate form of layered double hydroxide as a possible slowrelease nitrate fertilizer. The main purpose of this article is to provide information on the mechanochemical synthesis of KMgPO4 and NH4MgPO4 * To whom correspondence should be addressed. E-mail:
[email protected]. † Tohoku University. ‡ Akita University.
through solid-state reactions between mixtures of KH2PO4 + Mg(OH)2 and NH4H2PO4 + Mg(OH)2 for the two compounds, respectively. This is followed by an investigation of the solubility or release behavior of potassium, phosphate, and ammonium ions in water at room temperature. Characterization of the solid samples by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectroscopy and solution analysis by liquid ion chromatography were performed to elucidate the MC process applied in the current work. 2. Experimental Section 2.1. Mechanochemical Synthesis. Two chemical reagents, KH2PO4 and Mg(OH)2, were used as starting materials for the synthesis of KMgPO4, and another two chemicals, NH4H2PO4 and Mg(OH)2, were used for the synthesis of NH4MgPO4. All chemicals were of reagent grade and were supplied by Wako Pure Chemical Industries, Osaka, Japan. A 2-g mixture of starting materials of equimolar ratio (1:1) with a ball/sample weight ratio of about 40 was used. The mill used for synthesizing KMgPO4 and NH4H2PO4 was a planetary ball mill (Pulverisette 7, Fritsch GmbH, Idar-Oberstein, Germany) that had two mill pots (45 cm3 inner volume each) made of stainless steel with seven steel balls of 15-mm diameter. The mill speed in these experiments ranged between 100 and 700 rpm, and the milling time was fixed at 120 min. 2.2. Characterization Techniques. The starting and milled samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry-differential thermal analysis (TG-DTA), and Fourier transform infrared (FT-IR) analyses. The XRD analysis was carried out using a Rigaku RINT-2200/PC system with a Cu KR irradiation source (λ ) 1.5405 Å) at 40 kV and 50 mA for both the milled samples and the starting mixture to determine their phases and observe changes in phase formation. Samples were analyzed in continuous scan mode between 5° and 90° in 2θ. The morphology of the milled powder was observed by scanning electron microscope (Hitachi S-2150). TG-DTA of the as-prepared samples was conducted with a Thermo Plus TG-8120 instrument (Rigaku) at a heating rate of 10 °C min-1 in air to monitor the
10.1021/ie901780v 2010 American Chemical Society Published on Web 02/04/2010
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Figure 2. FT-IR profiles of KH2PO4 + Mg(OH)2 mixtures milled at different mill rotational speeds.
to the amorphous phase at 400 rpm. When the mill speed was raised to 500 and 600 rpm, characteristic and dominant peaks corresponding to KMgPO4 clearly appeared in the patterns. For the system NH4H2PO4 + Mg(OH)2 (Figure 1b), no new peaks were observed for the mixture milled at 100 rpm, but peaks of the starting materials remained dominant. When the mill speed was increased to 200 rpm, new peaks of NH4MgPO4 · 2H2O appeared; however, peaks of the starting materials continued to appear in the product. When the mill speed was raised above 300 rpm (up to 700 rpm), characteristic peaks of the starting materials completely disappeared, and clear dominant peaks of NH4MgPO4 · 2H2O were seen throughout. The 2 mol of water offered in Mg(OH)2 and NH4H2PO4 appear together as a compound in the final product that could be removed by heat treatment to obtain pure NH4MgPO4 product. The mechanochemically induced solid-state reactions in the two milling systems can be represented by reactions 1 and 2 Figure 1. XRD patterns of (a) KH2PO4 + Mg(OH)2 and (b) NH4H2PO4 + Mg(OH)2 sample mixtures milled at different mill rotational speeds.
decomposition profiles of the ground samples. FT-IR spectra were measured by using a Digilab Excalibur Series FTS-3000 spectrometer with KBr as a diluent. Subsequent release of nutrients into water from the KMgPO4 and NH4MgPO4 materials synthesized mechanochemically was evaluated by dissolving 0.4 g of the product in 100 mL of water with moderate stirring (300 rpm). A liquid ion chromatograph instrument (Shimadzu L10 Series) was used to determine the concentration of nutrient ions in the filtrate. 3. Result and Discussion 3.1. Mechanochemical (MC) Synthesis of KMgPO4 and NH4MgPO4. MC synthesis of KMgPO4 was performed by milling KH2PO4 (1.4 g) and Mg(OH)2 (0.6 g), and that of NH4MgPO4 was performed by milling NH4H2PO4 (1.33 g) and Mg(OH)2 (0.67 g), each at a 1:1 mole ratio. Figure 1 shows X-ray diffraction (XRD) patterns of (a) KH2PO4 + Mg(OH)2 and (b) NH4H2PO4 + Mg(OH)2 sample mixtures milled for 120 min at mill speeds ranging from 100 to 600 rpm. For the system KH2PO4 + Mg(OH)2 (Figure 1a), characteristic peaks of the starting materials continued to remain in the milled products at low mill speeds (100-300 rpm), but reduced
KH2PO4 + Mg(OH)2 f KMgPO4 + 2H2O
(1)
NH4H2PO4 + Mg(OH)2 f NH4MgPO4 · 2H2O
(2)
FT-IR spectral measurements recorded for the KH2PO4 + Mg(OH)2 sample system at various mill rotational speeds are shown in Figure 2. The stretching vibration of the OH group corresponding to Mg(OH)2 at around 3700 cm-1 decreased as the mill rotational speed was increased and disappeared from the samples prepared at 500-600 rpm. This result relates to the XRD results indicating decomposition of the starting materials during milling and subsequent formation of the new product. Similar behavior was also observed for the sample system corresponding to reaction 2. Differential thermal analysis of the solid samples obtained at varying mill speeds was performed for the two sample systems. For the KH2PO4 + Mg(OH)2 system (data not shown), two endothermic peaks at 240 and 400 °C for the sample mixture before milling correspond to the decomposition of KH2PO4 (240 °C) and Mg(OH)2 (400 °C), both releasing moles of water. It is known that KH2PO4 undergoes dissociation at around 240 °C,19,20 and analysis of the solids by XRD after KH2PO4 had been heated at 240 °C showed that KH2PO4 decomposed to KPO3. It has also been reported in the literature that Mg(OH)2 decomposes at around 400 °C.21-24 For the sample mixture milled at 100 rpm, the intensity of the endothermic peak
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Figure 3. DTA profiles of NH4H2PO4 + Mg(OH)2 mixtures prepared by milling at different mill rotational speeds.
Figure 4. SEM images of the KH2PO4 + Mg(OH)2 mixtures milled at different mill rotational speeds.
positions characteristic of KH2PO4 and Mg(OH)2 decreased, and these peaks disappeared from the sample mixtures milled at mill rotational speeds of 300-600 rpm; however, new endothermic peaks appeared at around 100 °C. This new endothermic peak position could be due to the loss of water obtained as a reaction product according to reaction 1. Figure 3 shows DTA patterns of NH4H2PO4 + Mg(OH)2 sample mixtures milled at different mill rotational speeds. The endothermic peak at around 70-100 °C is due to evaporation of ammonia and water from the starting materials due to milling. The endothermic peak at 200 °C is
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due to the decomposition of NH4H2PO4, and that at 400 °C is from the decomposition of Mg(OH)2. As the mill rotational speed was increased, the endothermic peak at 400 °C resulting from the decomposition of Mg(OH)2 disappeared, indicating progress of the MC reaction to form a new product. Figure 4 shows scanning electron microscopy (SEM) images of the KH2PO4 + Mg(OH)2 sample mixture of starting materials milled at varying mill speeds from 300 to 600 rpm. Generally, the morphology of the milled samples exhibits agglomeration of fine particles of several micrometers, although the size of the starting particles was on the nanoscale. The formation of water molecules at higher mill rotational speeds is attributed to the agglomeration of fine particles. Very similar SEM images were observed for both the KH2PO4 + Mg(OH)2 and NH4H2PO4 + Mg(OH)2 sample systems as investigated in this work. 3.2. Solubility of KMgPO4 and NH4MgPO4 · H2O in Water. The solubility of the solid products obtained from the milling operation is shown in Figure 5. For the system KH2PO4 + Mg(OH)2 milled to obtain KMgPO4, the solubilities of potassium (K+), phosphorus (PO42-), and magnesium (Mg2+) ions in water over 700 h at different mill speeds are represented in Figure 5a. KH2PO4 used as a starting material is highly soluble in water and dissolves within several minutes, but Mg(OH)2 is scarcely soluble in water at ambient temperature. For both the sample mixture before milling and the products obtained after milling, Mg dissolution remained consistently below 2%. The solubilities of K+ and PO42- ions followed similar dissolution profiles. For the mixture before milling, complete release of the two nutrients was observed. However, as the mixture was subjected to milling, the solubilities of both K+ and PO42- into solution decreased as the mill rotational speed was increased. At 100 rpm, up to 70% of the PO42- and 50% of the K+ were released into solution, and as the mill rotational speed was increased to 200 rpm, the concentrations of both ions significantly decreased, and only 30% of each was released into solution. The release of both nutrients was reduced only slightly to 25% at 300 rpm, but it remains at between 20% and 25% even at higher mill rotational speeds of 400-600 rpm. For the system NH4H2PO4 + Mg(OH)2 milled to obtain NH4MgPO4, the solubilities of ammonium (NH4+), phosphorus (PO42-), and magnesium (Mg2+) ions in water over 500 h at different mill speeds are represented in Figure 5b. NH4H2PO4 used as a starting material is highly soluble in water and dissolved within several minutes, but Mg(OH)2, as indicated earlier, is not very soluble in water at ambient temperature, so Mg2+ ions in solution remained consistently below 10% at all mill speeds. The solubilities of both NH4+ and PO42- into solution decreased as the mill rotational speed was increased. At 100 rpm, only up to 40% of PO42- and 40% of NH4+ were
Figure 5. Solubilities of KMgPO4 and NH4MgPO4 · 2H2O products prepared by MC reaction at different mill rotational speeds and dissolved in water for 700 and 500 h, respectively.
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released into solution, and as the mill rotational speed was increased to 200-300 rpm, the concentrations of both ions significantly decreased to less than 30%. The release of both nutrients was further reduced to below 20% and remained consistent from 400 to 700 rpm. The solubility profiles for both sample systems show that, as milling progressed to above 200 rpm, MC-induced reactions to form KMgPO4 and NH4MgPO4 · H2O can be achieved and that the ions or nutrients contained in the solid products can be released slowly, which can allow for the control of nutrient release. 4. Conclusions The results in this work clearly show that KMgPO4 and NH4MgPO4 can be synthesized mechanochemically by dry milling in a planetary ball mill and can be summarized as follows: (1) KMgPO4 can be prepared by milling KHPO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min and at mill rotational speeds of 500-600 rpm. (2) Washing of KMgPO4 in water for 700 h releases only up to around 20-25% of K+ and PO42- ions or nutrients into solution. (3) NH4MgPO4 can be prepared by milling NH4HPO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min and at mill rotational speeds of 300-700 rpm. (4) Washing of NH4MgPO4 in water for 500 h releases only up to between 10% and 20% of NH4+ (ammonium) and PO42(phosphate) ions or nutrients into solution. More work should be performed to determine other optimum conditions such as milling time and determine conditions under which complete release of nutrients into solution, such as maximum time of release, occurs. This process offers an alternative synthesis route avoiding other processes involving solution treatment as it is a single dry process. Literature Cited (1) World Population to 2300; Department of Economic and Social Affairs, United Nations: New York, 2004; p 1. (2) Adachi, M.; Kanak, P. Agricultural land conversion and inheritance tax in Japan. ReV. Urban Reg. DeV. Stud. 1999, 11 (2), 127. (3) Wasilewski, A.; Krukowski, K. Land conversion for suburban housing. EnViron. Manage. 2004, 34 (2), 291. (4) Adetunji, M. T. Nitrogen application and underground water contamination in some agricultural soils of South Western Nigeria. Fert. Res. 1994, 37, 159. (5) Riu, L.; Mingzhu, L.; Lan, W. Controlled release NPK compound fertilizer with the function of water retention. React. Funct. Polym. 2007, 67, 769.
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ReceiVed for reView November 11, 2009 ReVised manuscript receiVed January 24, 2010 Accepted January 24, 2010 IE901780V
