Research Article pubs.acs.org/journal/ascecg
Layered Double Hydroxides: New Technology in Phosphate Fertilizers Based on Nanostructured Materials Luíz Paulo Figueredo Benício,*,† Vera Regina Leopoldo Constantino,‡ Frederico Garcia Pinto,§ Leonardus Vergütz,† Jairo Tronto,*,§ and Liovando Marciano da Costa† †
Departamento de Solos, Universidade Federal de Viçosa, Viçosa, MG Brazil CEP 36570-900 Fundamental Chemistry Department, Institute of Chemistry, University of São Paulo, Avenue Professor Lineu Prestes, 748, CEP 05508-000 São Paulo, SP, Brazil § Universidade Federal de Viçosa, Campus Rio Paranaíba, Rodovia MG-230, km 08, CEP 38810-000 Rio Paranaíba, MG, Brazil ‡
S Supporting Information *
ABSTRACT: Layered Double Hydroxide (LDH) intercalated with phosphate ions (LDH-phosphate) was synthesized by ionexchange method from a precursor containing nitrate ions between the layers. The materials were characterized by X-ray diffraction (XRD), attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis coupled to differential scanning calorimetry and mass spectrometry (TGA-DSC-MS), specific surface area (BET), scanning electron microscopy (SEM), and elemental analysis. We hypothesized that LDH-phosphate can be used as a slow release fertilizer and help increase phosphate fertilization efficiency in tropical weathered soils. This new fertilizer technology was tested in a kinetic study of P release and a bioassay with controlled conditions of light, humidity, and temperature using maize (Zea mays) as our indicator plant. The bioassay was performed using an adaptation of the Neubauer method, wherein the LDHphosphate was compared to the commercial triple superphosphate (TSP) fertilizer in two different weathered soils: a sandy soil and a clayey soil. Under the bioassay experimental conditions, LDH-phosphate increased plant productivity (mass of dry matter), height, and the content of phosphorus (P) in the dry matter. In addition, LDH-phosphate promoted an increase in the soil pH value, contributing to decrease the P adsorption capacity of the soil, making it more available to the plants. KEYWORDS: Agronomic efficiency, Fertilization, Nanostructured materials, New fertilizer, Slow release, Phosphorus
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INTRODUCTION
tion, an alternative is to enhance productivity, which can be reached by different ways, including genetic improvement and agronomic management of crops. Specially in tropical weathered soils, the current greatest agricultural frontier, one of the most important agricultural practices is fertilizer management. The appropriate fertilizer management is both economically and environmentally crucial. The use of fertilizers allows for maximizing productivity and profitability, optimizing the use of the soil and decreasing the necessity of new areas. Additionally,
The continuing increase of the world’s population requires a rise in agricultural production. Forecasts indicate that the world population will reach 9.2 billion people in 2050, an increase of 2.3 billion people in the next 35 years.1 Despite being a lower growth rate in relation to the years 1970 to 2010, when the world population increased by 3.2 billion people, the food production must increase in order to reach the food demand of the global population.1 Studies of the Food and Agricultural Organization of the United Nations (FAO) reported in 2009 pointed out that food production would need to increase up to 70% until 2050 to meet this demand.2 To avoid the necessity of incorporating new productive areas, and consequently increasing deforesta© 2016 American Chemical Society
Received: July 28, 2016 Revised: November 25, 2016 Published: December 1, 2016 399
DOI: 10.1021/acssuschemeng.6b01784 ACS Sustainable Chem. Eng. 2017, 5, 399−409
Research Article
ACS Sustainable Chemistry & Engineering
in an acidic solution (pH 5.2) in a more sustained way. The authors also reported that the reaction rate is directly related to intercalated phosphate species, with the monovalent H2PO4− form being released more slowly than the others are. Tests carried out by Koilraj et al.33 using a LDH of composition NiAlphosphate as a source of P for Ulva lactuca cultivated in hydroponics have showed that a suspension of LDH provided a higher plant growth than KH2PO4 or P enriched seawater. These results corroborate with works performed by Everaert et ́ et al.,35 and Bernardo et al.36 which ensure that al.,34 Benicio LDHs are promising matrices to engender new slow release fertilizers. However, to the best of our knowledge, the works reported so far do not show a good understanding of the agronomic aspects of LDH application as a source of P in the soil−plant system. Thus, the present work aims to evaluate a new fertilizer technology based on LDH nanostructured systems intercalated with phosphate anions and compare the results with a commercial source of P fertilizer, assessing the dry matter production, height, and P uptake by the plant.
the appropriate fertilizer management avoids losses of nutrients to the environment, reducing the contamination of soils, water, and atmosphere. In this context, fertilizer dose should be applied correctly, but also new fertilizer technologies should be developed. These technologies should ensure that the fertilizer release of nutrients would synchronize with plant demand, enhancing the efficiency of fertilizers and the recovery of nutrients by the plants. Phosphorus (P) is one of the more limiting nutrients in agriculture globally, mainly in tropical soils.3 It is due to the acidic nature of these soils, with low P availability and the strong adsorption of orthophosphate ions by the clay mineral fraction of these soils (especially in Fe and Al (hydroxides).4−8 The agriculture industry is the main consumer of phosphorus in the world, consuming around 80−90% of all P globally extracted.9 A great part of this demand is due to the low effectiveness of phosphate fertilizers, with just 5−30% of the total phosphorus applied to the soils being used by crops. The remaining P is lost or restrained into fractions not available for the plants, especially by the adsorption/fixation processes of P in the soils.10,11 To minimize this problem, we need to improve the recycling of P and develop new technologies to reduce P losses and increase its effectiveness.12 The high demand and low efficiency of phosphate used in the agriculture, associated with the fact that the main source of P (phosphate rocks) is not a renewable resource, make the search for new technologies mandatory to optimize P usage in agriculture. In this context, new fertilizer technologies and better management of phosphate fertilization are frequently evaluated, including the use of less soluble natural rock phosphates,13−15 thermophosphates,16,17 alternative sources such as tailings of rock industry for phosphate fertilizers,18−20 and phosphate fertilizers coated with polymers.21−23 Currently, with the emergence and advance of nanotechnology, nanofertilizers have been recognized as new materials with high potential for agriculture.24,25 Layered double hydroxides (LDHs) are nanostructured materials with promising potential for usage as fertilizers. The LDH structure comprises the stacking of positively charged layers intercalated with hydrated anions. Each positive layer is formed by octahedral units containing metal cations in the center and hydroxyl anions positioned at their vertices. These octahedral units share the edges, forming planar layers, where positive charges are neutralized by the presence of anions between the layers. Together with water molecules, the anions promote the stacking of layers, which culminates in the LDH structure. LDH can be represented by the following general formula: [MII(1−x)MIIIx(OH)2]Ax/m·nH2O, wherein M2+ is a divalent cation, M3+ is a trivalent cation, and Am− represents the intercalated anion with the m− charge. Phosphate ions can be interleaved between the LDH layers. Because of its specific structure, the LDHs provide physical protection to the intercalated phosphate ions, decreasing the direct contact of these ions with the soil. In addition, the layered material can release P in a more controlled way, making it possible to try to meet plant demand (gradual release in time). In the last years, many works about the use of LDH in agriculture have been found, most of them reporting the sustained release of agrochemicals.26,27 Other studies have reported the synthesis and characterization of LDH intercalated with phosphate as well its sustained release in aqueous or buffered medium.28−31 Studies performed by Woo et al.32 showed that phosphate ions interleaved in LDH were released
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EXPERIMENTAL SECTION
Synthesis of Layered Double Hydroxides. The following ́ reagents with a degree of analytical purity from Vetec Quimica or Sigma-Aldrich were used for the synthesis of LDH: Mg(NO3)2 6H2O, Al(NO3)3 9H2O, KNO3, K2HPO4, and NaOH. Water used in the synthetic procedures was distilled, deionized, and decarbonated. Initially, a LDH precursor intercalated with NO3− anions was synthesized by the coprecipitation method at a constant pH value.37 The use of a LDH precursor is necessary since the synthesis of LDH intercalated with phosphate anions by the coprecipitation method is challenging: metal cations as magnesium and aluminum precipitate in the presence of phosphate anions, and phosphate anions in solution present buffer action.38 For the precursor isolation, 500 mL of a solution containing 0.500 mol/L of Mg2+ and 0.250 mol/L of Al3+ were slowly added to 290 mL of a solution containing 1.476 mol/L of NO3− ions (using KNO3 as source), under strong agitation and N2 atmosphere. To keep the pH value constant at 10 (±0.5) during the synthesis, a potentiometric titrator containing a solution 2.0 mol/L of NaOH was used. After the LDH precipitation, the suspension was treated hydrothermally at 65 °C for 24 h. After hydrothermal treatment, the solid was filtered under reduced pressure and washed with water. The LDH sample was freeze-dried for 120 h. After being dried, the material was crushed and sieved through 80 mesh. The LDH precursor was nominated as LDH-NO3. As mentioned above, the anion exchange method was used for the phosphate anion intercalation into LDH-NO3. In this process, 2.0 g of the precursor was suspended in 200 mL of a solution of 1.0 mol/L of K2HPO4. The suspension was kept for 24 h at 65 °C under N2 atmosphere. Afterward, the material was filtered under reduced pressure, washed with water, and dried under reduced pressure in the presence of silica gel at room temperature. The LDH sample produced by the anionic exchange method was nominated as LDH-P. Characterization. The following techniques were used for physicochemical characterization of LDH: X-ray diffraction (XRD), Fourier transform infrared spectroscopy with accessory of attenuated total reflectance (ATR-FTIR), Fourier transform Raman (FT-Raman) spectroscopy, specific surface area, medium volume of pores and medium diameter of pores (the three measurements were done by BET method), thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry (TGA-DSC-MS), and scanning electron microscopy (SEM). To quantify the metals Mg and Al and the content of P, 0.25 g of the samples was mixed with 8 mL of a 12.0 mol/L HCl solution and heated to 180 °C until total dissolution of the samples. In the extracts obtained, the amount of Mg and Al were determined by inductively 400
DOI: 10.1021/acssuschemeng.6b01784 ACS Sustainable Chem. Eng. 2017, 5, 399−409
Research Article
ACS Sustainable Chemistry & Engineering coupled plasma optical emission spectrometry (ICP-OES). The amount of P in LDH-P was determined by molecular absorption spectrophotometry.39 The contents of C, N, and H in LDH samples were determined in an elemental analyzer PerkinElmer 2400 Series II CHNS/O. In Brazil, according to the law 6.934/81, a material should have a minimum guarantee of P2O5 to be considered a phosphate fertilizer. The amount of phosphate soluble in H2O, 2% citric acid, and neutral ammonium citrate (NAC) was measured according to the Association of Official Agricultural Chemist (AOAC)40 procedure to compared to the commercial sources. Kinetics of P Release. Experiments of P release were performed on a continuous stirred-flow system using deionized water. The system consists of a high performance liquid chromatography (HPLC) piston pump and a Teflon reaction chamber of 17 mL of internal volume. The solution (deionized water) is pumped into the chamber in a 1 mL/min flow by a bottom hole and constantly stirred (100 rpm) by a magnet stirrer. The sample is kept in the chamber by a 0.45 μm cellulose membrane placed in the upper part of the chamber. The solution outlet is located on the top part of the reaction chamber, and a fraction collector collects the solution passing through the chamber. The LDH and triple superphosphate (TSP) samples were placed into the chamber, and the outlet solution was collected every 2 min up to 150 min. The P extracted in the fractions was determined by molecular absorption spectrophotometry.39 Bioassays. The bioassays were performed in a growth chamber under controlled conditions of temperature and light (Figure S1, Supporting Information). The randomized block design (RBD) was used in a factorial scheme 2 × 2 × 5 with four replications. The following factors were evaluated: two types of soil (sandy and clayey), two sources of P (LDH-P and TSP), and five doses of P (0, 15, 30, 45, and 60 mg/kg) which have been corrected for the 100 g of soil used (Figure S2, Supporting Information). The characteristics of the soils used in this work are presented in Table 1.
mg/kg of Zn; 1.23 mg/kg of Cu; and 2.86 mg/kg of Mn, divided into three applications: the first one 6 days after sowing (DAS), the second one 11 DAS, and the third one 17 DAS. Twenty-five DAS, the plants were evaluated for their height and harvested for the shoot dry matter (SDM) productivity. After being cut, the plants were stored in paper bags and placed in an oven under air circulation at 65 °C for 96 h for drying. After this step, the samples were weighed, ground in a Willey mill, and sieved through a 20-mesh sieve. After that, nitropercloric digestion was carried out to quantify shoot nutrient contents.42 P was quantified by a more sensitive colorimetric method38 and the other nutrients by ICP-OES. The evaluated variables SDM, height, shoot P content, and soil pH value were submitted to analysis of variance and regression following the applied P doses. Also, in order to compare sources of P in the two different soils, all P doses were averaged and compared by Tukey’s test (p < 0.05). The following agronomic indexes of nutrient utilization were also calculated: partial factor productivity (PFP), agronomic efficiency (AE), and recovery rate (RR). PFP shows the production of SDM per unit of P applied and is calculated according to eq 1:
Table 1. Characteristics of the Soils Used in This Work: Sandy and Clayey
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pH (H2O)
P content (mg/dm3)f
Prem (mg/L)d
PFP = (PP /DAP)
The agronomic efficiency (AE), which shows the increase in production due to each unit of P applied, is calculated by eq 2:
AE P = (PP − P0)/DAP
4.36 5.27 sanda (g/kg)
1.0 0.9 siltb (g/kg)
34.4 13.1 clayc (g/kg)
3.99 4.79 organic matter (%)
sandy soil clayey soil
799.0 350.0
20.0 30.0
190.0 620.0
1.01 1.77
(2)
The recovery rate of P (RR) is calculated by eq 3:
RR P = [(CF − CF0)/DAP] × 100
(3)
where PP is a measurement of productivity (SDM) for each P rate applied, DAP is the dose of P applied as fertilizer, P0 is the productivity (SDM) of the treatment without fertilizer, CF is the shoot P content for the treatment with P application, and CF0 is the shoot P content of the control. The equations and indexes were obtained from Doberman.43
RESULTS AND DISCUSSION Characterization. The X-ray diffraction patterns of the prepared LDH samples are presented in Figure 1. The XRD
CEC (cmolc/dm3)e
sandy soil clayey soil
(1)
a
0.05 to 2 mm. b0.002 to 0.05 mm. c
