Use of Biogenic Silica in Porous Alginate Matrices for Sustainable

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Use of biogenic silica in porous alginate matrices for sustainable fertilization with tailored nutrient delivery Mailson de Matos, Bruno Dufau Mattos, Blaise L. Tardy, Orlando J. Rojas, and Washington Luiz Esteves Magalhaes ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04331 • Publication Date (Web): 30 Dec 2017 Downloaded from http://pubs.acs.org on January 2, 2018

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ACS Sustainable Chemistry & Engineering

Use of biogenic silica in porous alginate matrices for sustainable fertilization with tailored nutrient delivery. Mailson de Matos§, Bruno D. Mattos§,†, Blaise L. Tardy†, Orlando J. Rojas†,‡,*, Washington L. E. Magalhães§,&,* §

Integrated Program in Engineering & Materials Science, Federal University of Paraná, Polytechnic Center, Curitiba 81531‑990, Brazil. †

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, FI-00076 Aalto, Finland.

‡

Department of Applied Physics, School of Science, Aalto University, FI-00076 Aalto, Finland. &

Embrapa Florestas, Estrada da Ribeira, Km 111, Colombo 83411‑000, Brazil.

* Corresponding authors: (O.J.R.) [email protected] and (W.L.M.M.) [email protected]

KEYWORDS. Slow-release; nitrogen; sodium alginate; cellulose nanofibrils; biogenic silica.

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ABSTRACT Population growth coupled with significant pressure for clean agricultural practices puts a heavy burden on conventional crop treatments that target high yields with minimal cropland expansion. Optimization of fertilization systems is required as part of the solutions to current megatrends. Herein, we present a sustainable strategy to achieve controlled release formulations for nitrogen fertilization. Specifically, we used interfacial engineering to design alginate-based matrices that incorporated biogenic silica particles to achieve increased interfacial area for dynamic entrapment and release of ammonium nitrate. The incorporation of biogenic silica in the alginate matrix provided a porous architecture spanning length scales from the micro- (within particles) to the macro- (within the polymeric matrix) levels, leading to tunable patterns of nitrogen release. Alginate-biogenic silica granules approached the European requirements of “slowrelease” compositions. At optimized silica content, 15% of the nitrogen was released within 24h and 56% over 28 days. The complete nitrogen dissolution was achieved after 60 days. The experimental results and kinetic models provided insights on the mechanisms driving the nitrogen release from the alginate-silica matrix as a function of the pore-polymer hybrid architectures.


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INTRODUCTION Human nutritional needs will soon exacerbate the capacity needs of cultivable croplands. A population of 9 billion is estimated by 2050, which puts a heavy pressure toward an increase in cereal production of at least 40%.1 Obviously, there is a limit in croplands available. Thus, agricultural practices need to sustainably obtain higher cereal yields instead of creating new croplands. Under this current scenario, large amounts of fertilizers (ca. 190 million tons/year) are applied to increase the cereal production.2 Nevertheless, the traditional fertilization processes and the nutritional needs of plants often do not match,3 leading to a situation where up to 70% of the conventional fertilizers are leached into soils or volatilized.4 Besides the reduction in efficiency, the excessive leaching of fertilizers causes hazardous gaseous emissions and water eutrophication.3 Thus, a well-controlled strategy for fertilization is needed to avoid pollution, as well as, burst delivery of nutrients.5,6 As will be shown in this work, this is an important area where material interfaces can play a pivotal role. Nitrogen is the most demanded nutrient by plants since it plays a fundamental role in the biosynthesis of amino acids, proteins, enzymes, RNA, DNA, ATP, and chlorophyll. Nitrogen deficiency leads to an arrested growth of the crops, typically tracked by the development of yellow and pale leaves.7 Nitrogen fertilization is demanded because soils are unable to meet the nitrogen needs in crops. Urea-coated systems are state-of-the-art controlled release formulations for fertilization.3,4 The release is “controlled” through coating of the urea granule with materials of given physical and chemical nature as well as by controlling the thickness of the coating. The coating strategy does not necessarily promote slow, continuous release of the nutrient, but it promotes a delayed dissolution of the nutrient.3,4 In this sense, a burst delivery of nutrient occurs after the (bio)degradation of the coating material. In parallel, urea undergoes several biological,



chemical and physical conversions to finally produce the major nitrogen forms absorbed by plants (ammonium NH4+ and nitrate NO3-).7 The burst release associated with formulations that use urea-coated systems, as well as the metabolic delays for generation of ammonium or nitrate, suggest that alternative formulations are needed for controlled release of nitrogen-based fertilizers. Herein we propose a sustainable strategy to prepare engineered and controlled release formulations (CRF) for nitrogen-based fertilization. We used alginate as a biodegradable polymer to prepare granules and, because of the recent successes for the controlled release of pesticides using nanostructured biogenic silica (BSiO2),8,9 we applied BSiO2 as a filler to control the pore access and thus the release rate from the granules (Figure 1a). We chose sodium alginate as it can be considered as the best non-toxic, natural and biodegradable polymer for bead (granule) formation. In addition, the use of a biodegradable, natural polymer is key to avoid bioaccumulation of synthetic materials. Alginate can also readily cross-link in the presence of a divalent cations (for example, Ca2+) allowing the formation of a rigid shell that entrap substances or small molecules (including electrolytes) in the core.10,11 We incorporated nanostructured biogenic silica particles, which were isolated in high yields from plants,12,13 to increase the porosity of the granules, by up to 70-fold their original value. We show how the incorporation of biogenic silica changes the nitrogen distribution and morphological features of the granules, leading to different combinations of mechanisms for nutrient delivery (e.g. polymer relaxation and diffusion through porous networks) (Figure 1b). The high surface area (ca. 350 m2·g-1) and self-similar architecture of biogenic silica12 makes it an ideal material to introduce porosity into green matrices, different from other inorganic particles of much lower surface area, i.e. dozens of m2·g-1, that were also combined with alginate to prepare CRFs.14,15 We also demonstrate that


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cellulose nanofibrils can be used for reinforcement of the network without significantly affecting the nitrogen release rates. Cellulose abound in nature, is not expensive, and its nanofibrillated form is often applied as material in delivery science.8,16,17 This is an important consideration that shows that the sustainability of the CRF could be improved by avoiding the overuse of a single raw material. Overall, our work thoroughly evaluated the complex interfacial interactions between nitrogen salts, silica, sodium alginate and calcium chloride for the formation of various CRF with engineerable release rate.



Figure 1. Schematics of the preparation of alginate-based controlled release fertilizer using NH4NO3 as nitrogen precursor and biogenic silica as modifiers, as well as, Ca2+ ions as alginate cross-linking for granule formation. The egg-box structure of sodium alginate cross-links is shown (top right) (a).

After the incorporation of biogenic silica particles, the diffusion

mechanisms gradually changed from polymer relaxation/swelling to pore permeation due to increased pore access of the granules (b).

EXPERIMENTAL SECTION Chemicals and Materials. Ammonium nitrate (NH4NO3), sodium alginate (NaAlg) and calcium chloride (CaCl2) were purchase from Sigma Aldrich. Biogenic silica particles (BSiO2) and cellulose nanofibrils (CNF) were produced as previously reported.8,12 Briefly, the silica particles were obtained after acid treatment of horsetail biomass (100 °C, 2 h, 10:1 acid:liquid, H2SO4 2% w·v) followed by calcination at 650 °C for 4 h. The cellulose nanofibrils were obtained by mechanical fibrillation in a Super Masscoloider Masuko Sangyo mill as follow: bleached pine cellulose pulp was suspended in water at 1wt% (w·v-1); then, the water slurry was passed twenty times through the disk grinder at 1500 rpm until a homogeneous and stable suspension is obtained. Transmission electron microscopy images of the isolated silica particles and prepared cellulose nanofibrils are shown in Figure S1. Preparation of the controlled release formulation. An aqueous suspension (100 mL) containing 2% w·v-1 of sodium alginate (NaAlg) and 10% w·v-1 of ammonium nitrate (NH4NO3) was placed in a glass container and mechanically stirred until homogenization. Then, different concentrations of biogenic silica particles were added (1, 5 or 10 w·v-1). Also, the same formulations were prepared adding cellulose nanofibrils (CNF) at 1% w·v-1 concentration. Each


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suspension was transferred into a burette, and dropped into a saturated calcium chloride (CaCl2) cross-linking solution, using the distance for droplet formation of 15 cm. After the cross-linking reaction, the obtained granules were collected and dried at 60 °C until a constant mass was reached. The obtained fertilizers are thereafter referred to as CRF0, CRF1, CRF5 and CRF10 for the systems containing 0, 1, 5 and 10 w·v-1 of biogenic silica particles in the precursor mixture. Characterization of fertilizer granules for controlled release. The total nitrogen percentage was quantified using an elemental analyzer (Ementaray Vario Macro Cube). Chlorine, calcium and silicon were quantified through wet chemistry procedures, which are briefly described elsewhere.18 Chlorine was measured using the Mohr’s method, which consists in the solubilization of chlorine in water followed by titration with standard silver nitrate solution. Calcium was first extracted from the granule through acid digestion for further quantification in an atomic absorption spectrophotometer. Silicon was extracted from the granule with hydrochloric and hydrofluoric acid, the resultant silicic and fluorosilicic acid were reacted with molybdate for the complexation of silicon and molybdenum. The colored solution was then measured in a UV-visible spectrophotometer.18 Dried fertilizer granules were ground as preparation for X-ray and infrared analysis. The crystalline states of the granule compounds were observed in an X-ray diffractometer (Cu target), using configurations of 0.018° per step, scan rate of 1.5°·min-1, 2θ angle range from 5 to 65°, room temperature, voltage of 40 kV and current of 20 mA. The XRD patterns were analyzed using the database of the X’Pert HighScore Plus software. Fourier transformed infrared spectra (FT-IR) of precursors and fertilizer granules were taken in a Bruker Tensor 37 spectrometer in transmittance mode. The spectra were collected from KBr discs in the range from 4000 to 400 cm−1, resolution of 2 cm−1 and 32 scans.



The size distribution of the granules was obtained by using sequential sieving with different mesh sizes. This experiment was carried out by sieving 30 g of the granules, and then calculating the mass percent retained in each mesh. Each experiment was conducted in triplicate, for a total of 90 g of granules for CRF. The morphological features and the elemental mapping of the obtained fertilizers were investigated using a scanning electron microscope Tescan VEGA3 equipped with an energy-dispersive X-ray spectroscopy device. The cross-section of the granules was prepared using cryofracture. The specific surface area of the granules was calculated using the Brunauer–Emmett–Teller (B.E.T.) multipoint model in the linear relative pressure range (P·P0-1) from 0.05 to 0.35. The equipment used for these measurements was a Quantachrome pore analyzer, model NOVA 1200e. Nitrogen release profiles and kinetics considerations. The nitrogen release profiles were obtained following the procedures described by the European standard CEN EN 13266. In a typical experiment, 1 g of granule was immersed into 200 mL distilled water, and then NIR spectra were taken at regular intervals from the water for 60 days. Nitrogen was quantified through a calibration curve built in a near-infrared spectrometer, using NH4NO3 aqueous solutions ranging from 0.0078 to 4.0 g·L-1. The calibration curve was validated by the Kjeldahl method for N quantification. Several phenomena can cause the nitrogen release from the granule, such as desorption, diffusion, dissolution, erosion and/or relaxation of the matrix.19 Therefore, representative kinetic models were applied to identify the mechanisms behind nitrogen release from the alginatecellulose-silica granules. Korsmeyer-Peppas,20 zero-order,21 Higuchi,22 and Peppas-Sahlin23 were fitted in the experimental release data. The modelling allowed evaluation of the release rate as a function of a single kinetic parameter (k).


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RESULTS AND DISCUSSION

Chemical characterization and morphological features of CRF granules. The complexation between the divalent calcium cations and the negatively charged carboxylate groups from alginate resulted in a water insoluble, cross-linked polymeric network, arranged in the so called “egg-box” structure (Figure 1a).24,25 As a result, spheroidal rigid structures formed readily (Figure 2) with nitrogen entrapped in the polymeric matrix (Figure 3a). Most of the granules, ~92%, were sized between 2.0 – 4.8 mm, and the remaining ~8% displayed diameters > 4.8 mm. Negligible amounts had diameters between 1.0 – 2.0 mm (0.6%) or < 1.0 mm (0.3%). The incorporation of biogenic silica particles, up to 10% w·v-1 (with respect to the precursor mixture), did not alter the granule size significantly (Figure 2a to 2d).



Figure 2. Visual appearance of the obtained alginate-based granules without (CRF0) (a) and with biogenic silica particles at 1 (CRF1, b), 5 (CRF5, c) and 10% (CRF10, d) of the formulation. Included are also scanning electron microscopy images taken from the surface (top row) and cross-section (bottom row) of the granules at different magnifications (column e = CRF0, f = CRF1, g = CRF5 and h = CRF10).

The addition of biogenic silica clearly changed the morphological features of the granules, yielding a 70-fold increase in the surface area. The CRF0 granule presented a specific surface area of 0.1 m2·g-1 compared to 7.0 m2·g-1 observed for CRF10. The nitrogen content in the


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granule slightly decreased after the incorporation of the biogenic silica particles (Figure 3a). We hypothesize that the increased porosity may have facilitated the permeation of the nitrogen precursor out of the granule during and after the ionic complexation. As a matter of fact, micrometric pores are clearly observed in the CRF5 granules, as well as a deformed/crackedshaped granules for samples with the highest silica content (CRF10) (Figure 2h). Thus, the addition of 5% of biogenic silica in the precursor mixture is a threshold value for high enough porosity without compromising granule integrity as far as its deformation. Nitrogen is incorporated mainly in three forms: ammonium nitrate, ammonium chloride or calcium nitrate. From the chemical quantification results, we hypothesize that nitrogen was mostly entrapped as ammonium nitrate, since the calcium/chloride ratio follows the original 1/2 source ratio (Figure 3a). The calcium content fixed in the granule did not vary among the formulations (Figure 3a). Chlorine was incorporated in the granules in the range of ca. 12-15 w·w-1. The chloride counter ion accumulation derived from the cross-linking solution (CaCl). From the overall chemical quantification, the obtained fertilizers presented other important nutrients and beneficial substances, such as calcium (Ca), chlorine (Cl), and silicon (Si). The chlorine incorporation in the granule likely occurred as ammonium chloride (Figure 3b). The interaction of NH4+ with Cl- ions induced the crystallization of ammonium chloride upon drying, which corroborate the characteristic XRD peaks at 47.0° and 58.3°. The formation of NH4Cl mostly occurred for the CRF5 and CRF10 samples, as their increased porosity allowed a better diffusion of Cl counter ions, from the cross-linking solution to the inner core of the granule. The diffraction peak at 32.9° is related to the crystalline organization of NH4Cl, while the peak at 32.6° corresponds to NH4NO3 (Figure 3b). The elemental mapping of chlorine in the granules confirmed this hypothesis (Figure 4). As expected, a broad peak at 21.8° appeared in



the diffractograms of the samples with higher silica content (CRF5 and CRF10), confirming the presence of silicon dioxide.26 Nonetheless, as indicated by the X'pert Highscore Plus software, the peaks at 16.5, 28.7 and 30.3° are ascribed to sodium silicate. This crystallization occurred as silica underwent deprotonation at pH above 426 (precursor solution has pH 5.1) thus promoting interaction with Na+ ions. Figures 3c and 3d show the FT-IR spectra of precursor materials and CRF samples, respectively. For the CRF samples, the mid-IR region ca. 3345 cm-1 refers to several vibration modes of OH groups (from alginate and silica); however, the high intensity peaks at 3140 and 3030 cm-1 correspond to the vibrations of the ammonium ion.27 Nitrate ion’s peak appeared at 820 cm-1.27 The peak at 1390 cm-1 was observed in the ammonium nitrate spectrum (Figure 3c) and refers to the ionic nitrate group, which has D3h symmetry;28,29 however, for the CRFs spectra this peak was divided into two, one at 1400 and the other at 1385 cm-1 (Figure 3d). The abovementioned peaks are attributed to the asymmetric stretching of -NO2 from nitrate moiety with C2v symmetry, which occur when nitrate interacts with a bivalent metallic ion,28,29 in this case Ca2+. Considering that calcium nitrate formation was not indicated by the XRD analyses, we hypothesize that nitrate is interacting with the calcium ions from the “egg-box” calcium-alginate complexation. The peak at 1610 cm-1 (Figure 3c), related to the asymmetric deformation of the carboxyl groups of the alginate, shifted to 1630 cm-1 after the cross-linking complexation between negatively charged alginate carboxylates and Ca2+ ions.30 As expected, the peaks at 1090 and 470 cm-1 were only observed in the samples containing silica particles (CRF1, CRF5 and CRF10) and they refer to the covalent Si-O-Si bonds.26


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Figure 3. Overall chemical composition of the controlled release fertilizers (a). XRD patterns showing the presence of ammonium nitrate in the granules and the possible transformation of ammonium nitrate into ammonium chloride in the systems CRF5 and CRF10 (b). FT-IR spectra of the precursor materials (c) and the obtained controlled release formulations (d).

As discussed prior, the instantaneous cross-linking reaction of alginate and calcium ions promptly formed a rigid structure, resulting in a high content of nitrogen entrapped into the granule. Figure 4 shows that nitrogen is homogeneously distributed in the polymeric matrix. A well-defined “shell” structure was observed for the samples CRF0 (Figure 4a) and CRF1



(Figure 4b), which concentrated chloride ions from the cross-linking solution. Precipitation of ammonium chloride may have contributed to the shell formation. The increased porosity of the formulations CRF5 and CRF10 was responsible for the diffusion of the Cl ions to the inner parts of the granule. The absence of calcium in the “shell” layer confirms its dissociation from chlorine after the cross-linking.

Figure 4. Electron image and chemical mapping of nitrogen, chlorine, calcium and silicon obtained from the cross-section of the CRF0 (a), CRF1 (b), CRF5 (c) and CRF10 (d).


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Nitrogen release profiles obtained using the CEN EN 13266 method. Both ammonium and nitrate salts are highly soluble in water; however, a significant decrease in their solubility rates was observed after their entrapment in the alginate-based matrices (Figure 5). The complete nitrogen release occurred after 60 days in those granules containing the lowest silica contents (CRF0 and CRF1). On the other hand, the nitrogen release profiles of the granules with increased porosity (CRF5 and CRF10) reached the equilibrium stage (“complete release”) approximately after 28 days. Considering the associated error, all granules have released the same amount of nutrient after the first 24h. The nitrogen released within 24h was probably related to those ions adsorbed at the outer layers of the granule. Nevertheless, the incorporation of silica particles in the granules resulted in significant changes in the release profiles after the first day. The increased porosity of the granules explains the faster nitrogen release for the CRF5 and CRF10 samples. The porous architecture of the granule spanning across different length scales (micro within biogenic particles and macro within alginate matrix) facilitated the flow of solvent (water) in and out of the matrix, thereby resulting in faster nitrogen dissolution. In general, the mechanism of flowing-diffusion-release changes with increase of the porosity of the material. In general, when considering release rates, an increased pore access is responsible to an increase in the release rate of the loaded/entrapped molecule/ion.4,31,32 However, at the same time that the pore access is increased, the higher surface area increases the density of binding sites between cargo and carrier. For the CRF10 sample, the high content of biogenic silica particles increased the nanoscale features of the granule, which, as a consequence, intensified the interactions between surfaces and nitrogen, resulting in a slower release rate when compared to the CRF5 sample. The increase in binding sites between carrier and cargo was previously discussed as one



of the main factors affecting the decrease/control of the release rate in controlled release formulations.9 The incorporation of cellulose nanofibrils in the alginate-silica granules did not significantly change the nitrogen release profiles (Figure S2). From this result, we suggest the possibility to use other biodegradable polymers as filler or reinforcement agents in the preparation of fertilizer granules with tuned porosity. For instance, it could be possible to improve the mechanical integrity of the granules,33 while keeping the morphological features responsible for tuning the release rate. Also, the incorporation of a filler compound is an elegant strategy to increase the sustainability of the material, which is achieved by avoiding the overuse of one specific precursor.

Figure 5. Nitrogen release profiles as a function of time for the alginate-based beads containing different amounts of biogenic silica particles.


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Noteworthy, the nitrogen release profiles observed for the samples CRF0 and CRF1 reached the requirement of a “slow-release fertilizer” according to the standardized criteria of the CEN EN 13266 regulation. This norm stipulates that a slow release formulation must present up to 15% of nutrient release within 24 hours and a maximum of 75% in the first 28 days.34 The mechanisms of nitrogen release out from the granule changed after the incorporation of biogenic silica (Table 1). For instance, the Korsmeyer-Peppas kinetic model fitted very-well the experimental data for the nitrogen release profiles of the CRF0 and CRF1 granules. The diffusion coefficient for these samples were between 0.8 and 0.95, which indicates an anomalous release behavior driven by a combination of diffusion and relaxation of the polymeric matrix.20 The lower coefficient of determination for the Higuchi model infers that it does not strictly follow Fickian-diffusion, mainly because swelling occurs in the alginate-silica matrix.35 However, the coefficients (K1 and K2) obtained from the Peppas-Sahlin kinetic model showed that the nitrogen release has higher dependence on diffusion mechanisms than matrix relaxation (K1 > K2). This model is a combination of a parabolic diffusion component (K1 · t0.5, which gives the K1 coefficient) and zero order (K2 · t, which gives the K2 coefficient), in which a predominant kinetic mechanism can be determined.23

Table 1. Coefficient of determination, and kinetic coefficients of the kinetic models tentatively applied to describe the release rates of nitrogen out from alginate-biogenic silica granules. Model* Korsmeyer-Peppas n

Q = Kkp · t + b Zero order

Parameter

CRF0

CRF1

CRF5

CRF10

R²

0.98

0.98

0.89

0.91

Kkp

2.3e-3 ± 1.6e-3

9.0e-4 ± 6.0e-4

0.05 ± 0.10

1.6e-5 ± 5.5e-5

n

0.81 ± 0.09

0.93 ± 0.10

0.44 ± 0.31

1.68 ± 0.52

R²

0.97

0.98

0.87

0.91



Q = K0 · t + b

K0

6.0e-4 ± 3.2e-5

Higuchi

R²

0.96

6.0e-4 ± 2.5e-5 0.93

1.2e-3 ± 2.0e-4

0.91 -3

-2

0.85

0.02 ± 1.4 e

1.8e ± 1.7 e

3.4e ± 4.3e

2.8e ± 4.7e-3

Peppas-Sahlin

R²

0.98

0.98

0.89

0.89

Q = K1 · t0.5 + K2 · t

K1

7.3e-3 ± 4.0e-3

2.4e-3 ± 3.4e-3

0.03 ± 0.02

0.08 ± 0.02

+b

K2

4.0e-4 ± 1.0e-4

5.0e-4 ± 1.0e-4

8.4e-5 ± 1.1e-3

1.5e-3 ± 9.0e-4

+b

-2

1.4e-3 ± 2.0e-4

KH

Q = Kh · t

0.5

-3

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-3

-3

* the integrated equations consider the initial concentration of nitrogen in the release media equal to zero, and that only desorption occurred. Q is the fraction of released fertilizer at time t. n is the diffusivity coefficient. R2 is the coefficient of determination. Kkp, K0, Kh, K1 and K2 are the release rate coefficient for the Korsmeyer-Peppas, zero order, Higuchi and Peppas-Sahlin models. On the other hand, no model could properly fit the nitrogen release profiles obtained for the CRF5 and CRF10 granules (Table 2). The release profiles presented three well-demarked stages, which is likely to be derived from the combination of several kinetic mechanisms (e.g. pore permeation and diffusion, relaxation, swelling, and others). Also, this particular release profile was previously discussed as being a result of degradation and erosion of the matrix.36 Cost considerations are some of the next steps toward implementation and commercialization of our CRFs. Based on the costs of the raw materials and processing, the bio-based CRFs appear to be competitive compared to other CRFs in current use. We provide a back-of-the-envelope calculations together with Table S1 in the Supporting Information document; a forthcoming report will consider these aspects in detail.

CONCLUSIONS This study highlights the potential of biogenic silica particles, isolated in high yields from plants, to incorporate porosity in bio-based fertilizer granules. Hus, controlled release fertilizer systems, containing up to 18% of entrapped nitrogen, were successfully synthesized. Silica


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content in alginate matrices played an important role in the nutrient entrapment and release profiles because of different morphological features and chemical distribution. CRF0 and CRF1 granules approached the standards of “slow-release” composition according to the CEN EN 13266 normative. These formulations presented 15-18% of the nitrogen released within 24 h and 56% after 28 days. Korsmeyer-Peppas, zero-order, Higuchi and Peppas-Sahlin kinetic models were used to identify the mechanisms driving the nitrogen dissolution, with diffusion and matrix relaxation being the dominant ones. The granules with zero or lowest silica content fitted the release rate of a zero-order model, meaning a linear relationship between nitrogen release and time. A zeroorder fitting is especially attractive for technical applications, once the nitrogen delivery over time can be easily designed and estimated. The incorporation of cellulose nanofibrils did not alter the nitrogen delivery performance, but greatly expands the utilization of different raw materials for a more sustainable preparation of controlled release formulations. In sum, by using bio-based materials and ammonium nitrate as only nitrogen source, we were able to prepare an affordable slow release fertilizer that compete with controlled release formulations (CRF) based on synthetic materials. The technical application of our proposed CRFs could bring benefits to both the environment and the crops, representing a unique solution compared to systems in current use. In fact, it may be possible to avoid or strongly limit nitrogen accumulation in the environment (up to ~70%) while meeting the demands of plants for nutrients in a more efficient manner. These are positive sustainability gains that are demonstrated here by using surface science together with incorporation of safe biopolymers and nano/microparticle carriers.



ASSOCIATED CONTENT Supporting Information. Transmission electron microscopy images of the obtained biogenic silica particles and cellulose fibrils, nitrogen release profiles of the CRF with incorporated cellulose nanofibrils, and cost evaluation.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel: +358-(0)50 512 4227 * E-mail: [email protected] Tel: +55 (41) 3675 5712

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors are grateful for the support of the Brazilian CNPq, CAPES, and Fundação Araucária. We are thankful to the Academy of Finland through Centres of Excellence Programme (2014 –2019) under Project 264677 “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER) as well as NordForsk Project “High-Value Products from Lignin”.

REFERENCES (1)

Maxmen, A. Crop pests: Under attack. Nature 2013, 501 (7468), S15–S17.

(2)

FAO. World fertilizer trends and outlook to 2018; 2015. 20 ACS Paragon Plus Environment

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For Table of Contents Use Only TOC and Synopsis Incorporation of nanostructured biogenic silica particles into alginate-matrices provides different pore-polymer hybrid architectures responsible for tuned nutrient delivery rates.


Use of Biogenic Silica in Porous Alginate Matrices for Sustainable

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