AB ST R ACT : This work describes the development of potential multi-element slow-release fertilizers obtained by mechanochemical activation of mixtures of kaolinite and ammonium or potassium monohydrogen phosphates. Preliminary results of talc amorphization have also been included. The methodology consists of milling the materials in a high-energy ball mill, where the influence of rotation and time of milling were investigated. The samples were characterized by XRD, FTIR, TGA/DTA, SEM and MAS-NMR. The experimental results explain the slow-release behaviour of the amorphous nanostructured materials in aqueous suspensions, especially the MAS-NMR spectra, which showed the changes in the chemical environment of the elements analysed. The materials displayed slow-release behaviour for phosphates probably because the aluminium ions in the mineral structure interact more thoroughly with phosphate than potassium or ammonium. Nevertheless, in general, all of the nutrients were released slowly.

Sustainable slow-release fertilizers have been reported as environmentally friendly alternatives to highly soluble commercial products. Their main advantages are that they dissolve and release nutrients into soils in a way that assures bioavailability of nutrients to plants over a long period of growth. In addition, novel formulations can reduce or eliminate environmental problems caused by excess use of conventional fertilizers, such as eutrophication and atmospheric pollution. In this study, the solid-state mechanochemical activation method was used to prepare potential fertilizers by milling montmorillonite (MMT) or talc with K2HPO4. Characterizations by several instrumental techniques evidenced phase transformations, while kinetic studies and modeling indicated promising release performance. Even though the potassium release behavior was similar for both systems, the kinetic studies showed that phosphorus release profiles were different. Since potassium struvite (K-struviteMgKPO4·6H2O) was formed during the release experiments, talc based potential slow-release fertilizers displayed slower release behavior compared to MMT.

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The milling process in the solid-state 2:1 clay minerals, montmorillonite and talc, which have different cation exchange capacities, is reported here. Several instrumental techniques were used to monitor systematically the products formed. The dehydroxylation/amorphization of the montmorillonite and talc structures occurs within 3 and 6 h of milling, respectively. Electron paramagnetic resonance spectra indicated that structural Mn2+ was oxidized more quickly in the montmorillonite structure than in talc, while the paramagnetic defects increased during milling. Nuclear magnetic resonance was also used to monitor the environmental changes for Si and Al during milling.

This study describes the behavior of potential slow-release fertilizers (SRF), prepared by the mechanochemical activation of calcined Mg2Al-CO3 or Mg2Fe-CO3 layered double hydroxides (LDH) mixed with dipotassium hydrogen phosphate (K2HPO4). The effects of LDH thermal treatment on P/K release behavior were investigated. Characterizations of the inorganic composites before and after release experiments combined X-Ray diffraction (XRD), Fourier-transform infra-red spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The best release profile (<75% in 28 days and at least 75% release) was obtained for MgAl/K2HPO4 (9 h milling, 2:1 molar ratio, MR). Compared to readily used K2HPO4, milling orthophosphate into LDH matrices decreases its solubility and slows down its release, with 60% and 5.4% release after 168 h for MgAl/K2HPO4 and MgFe/K2HPO4 composites, respectively. Mechanochemical addition of carboxymethylcellulose to the LDH/K2HPO4 composites leads to a noticeable improvement of P release properties.

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Rice husk ash (RHA) is a waste material produced in large quantities in many regions worldwide, and its disposal can be problematic. This work describes a method for using RHA to synthesize silicon and potassium slow-release fertilizer. The extraction of silica from RHA was accomplished by alkaline leaching with KOH. Different KOH concentrations and reaction times were evaluated and the best production of K 2 SiO 3 solution was achieved using 6 mol L -1 and 6 h, respectively. The fertilizer was synthesized by the reaction of K 2 SiO 3 with KAlO 2 in aqueous medium, followed by calcination at 500 °C. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses indicated that the fertilizer composition was similar to mineral kalsilite. Solubility essays indicated lower K + and Si 4+ release percentage in neutral medium. Kinetic mechanisms of release tests can be well explained by the pseudo-second order model. The proposed synthesis seems to be a viable process offering economic and environmental benefits.Keywords: rice husk ash, silicon, potassium, slow-release fertilizer IntroductionNowadays, one of the most important goal of agronomic management is the development of environmentalfriendly fertilizers, which promote a sustainable nutrient management to ensure growth crop yield.1 One method of reducing fertilizer nutrient losses involves the use of slowrelease fertilizers, which have been designed to gradually release nutrients to plants at a rate to coincide with the requirement of crops. The advantages of using slow-release fertilizers instead of the conventional type are various, such as the increased efficiency of the fertilizer, the continuous supply of nutrients for a prolonged period and a decrease of nutrient losses by volatilization and leaching out to surface and ground water. 2,3 Potassium (K) is known as one of the most required nutrient during plant growth. K plays an important role in the energy state of the plant, translocation and storage of assimilates and maintenance of water in plant tissues. 4 For many crops, silicon (Si) is also an important nutrient and its availability to plants is often associated to increase of crop yield. 5Plants under intensive cultivation that require high absorption of Si, such as rice, sugar cane, and grasses in general, can quickly deplete the soluble Si content of soil. This element, therefore, needs to be replaced by fertilization. The application of Si fertilizer can influence plants in two ways: (i) by improvement of the fertility and chemical properties of the soil, and (ii) by direct effects on plant growth and development.6 Positive effects that have been reported include improved plant structure, such as upright leaves and stems, 7 and increased resistance to fungi and insects due to deposition of Si under the plant cuticle. 8 The high Si content of rice husk ash (RHA) has led to interest in its use as a source of Si for plants and for the production of numerous Si-based materials.9 RHA is generated after burning the rice husk, which is a waste from the ric...

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