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Design and kinetic study of sustainable potential slowrelease fertilizer obtained by mechanochemical activation of clay minerals and potassium monohydrogen phosphate. Roger Borges, Vanessa Prevot, Claude Forano, and Fernando Wypych Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04378 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016
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Industrial & Engineering Chemistry Research
Design and kinetic study of sustainable potential slow-release fertilizer obtained by mechanochemical activation of clay minerals and potassium monohydrogen phosphate.
Roger Borges1,2,3, Vanessa Prevot2,3, Claude Forano2,3 and Fernando Wypych1*
1
Department of Chemistry, Universidade Federal do Paraná, PO Box 19032, 81531-980
Curitiba - PR, Brazil. 2
Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, Université Blaise
Pascal, BP 10448, F-63000 Clermont-Ferrand, France. 3
CNRS, UMR 6296, F-63178 Aubière, France
*
E-mail:
[email protected] Abstract 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 1 ACS Paragon Plus Environment
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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 fertilizer displayed slower release behavior compared to MMT.
Keywords:
slow-release,
potential
fertilizer,
mechanochemical
montmorillonite, potassium struvite.
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activation,
talc,
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Industrial & Engineering Chemistry Research
1. Introduction Currently, most inorganic fertilizers containing N, P and K have high solubility or are volatile. Thus they can be easily lost through the action of wind and water, and as a consequence pollute the atmosphere and water bodies, in the latter case by accumulation of nutritive elements, causing eutrophication.1 These characteristics lead to the use of huge amounts of fertilizers to ensure food production and the negative effects became worse. Moreover, the increasing scarcity of phosphate resources threatens food security and requires changing the approach to fertilizer management. In this context, the study of environmentally friendly methods and products is important. One possibility is based on the development of sustainable slow-release fertilizers (SSRF). According to the literature reports they can be synthesized by many methods.2-7 The main one consists to mix the soluble fertilizer with different substances, mainly polymers to limit the diffusion. A few years ago, the mechanochemical activation method was also reported as an interesting approach to produce SSRF,8-10 being a simple method that does not require solvents. For instance, Zhang et al.8 described the synthesis of KMgPO4 and NH4MgPO4 by milling Mg(OH)2 with KH2PO4 and NH4H2PO4, respectively, and found slowrelease behavior, with only 20% of HPO42- being release after 500 h. Many variables in the formulations and/or production processes can affect the slow-release behavior of the product.11 Interestingly, the use of different clay minerals in the mechanochemical process was also described as producing efficient SSRFs. Clay minerals are of particular interest since they are abundant and environmentally friendly. Kaolinite, for instance, when milled with KH2PO4 or NH4H2PO4 produces amorphous phases that can release K, N and P slowly.9,11 To better understand the behavior of the SSRFs, the study of the release kinetics and mechanisms is of great interest. Even if solubilization and release/delivery of chemical species are common physico-chemical processes involved in many applications most of the
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studies have been reported on the mechanisms and kinetics involved for drug formulation and delivery.12 The release of plant nutrients from natural or formulated compounds involves similar processes; however the mechanisms have been less investigated than for drugs. Controlled release of fertilizers involves similar processes, such as dissolution, erosion, diffusion, adsorption/desorption, ion exchange and swelling, which depend on many physicochemical parameters such as the solubility and surface properties of the solid compounds, the permeation characteristics of the medium (pore size and connectivity) and other physicochemical parameters of the surrounding environment (pH, temperature and ionic strength). However, chemical fertilizer systems are in many respects quite different, since they do not involve encapsulation of organic molecules (namely drugs) in polymeric matrices or hydrogels. Mathematical models that describe the kinetics of the overall release (dissolution and diffusion) have been widely discussed in the literature for drug delivery.13 Some of these models have also been used to evaluate the performance of SSRFs, such as natural phosphate rocks or synthetic compounds loaded with phosphate.14 Briefly, two main competitive phenomena must be considered: i) the release is controlled by the dissolution process (zero-order kinetics), through either the erosion of the whole matrix (homogeneous erosion) or surface erosion;14 or ii) the release is controlled by the diffusion process, so Fick’s second law or the Higuchi equation can be applied, depending on the solubility of the shell or the core active species. The first-order model depends only on the initial concentration (C0). It often better reflects osmotic processes or chemical elimination with no change in the morphology of the solid during dissolution. The pseudo first-order model introduces in the equation a release coefficient that attenuates the change of Ct/C0 over time. The Higuchi model is widely used to fit drug delivery phenomena due to its realistic
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description of the release conditions (C0>>Csat, unidirectional diffusion, low impact swelling, constant diffusivity, Ceq
