Soil science
Z. Movahedi Rad; M. Hamidpour; A. Tajabadipour
Abstract
Introduction
Recently, layered double hydroxides (LDHs) with a unique structure and unbeatable characteristics have been widely studied and investigated in various fields. One of these fields is the investigating the potential of these compounds to supply essential nutrients for plants. Several studies ...
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Introduction
Recently, layered double hydroxides (LDHs) with a unique structure and unbeatable characteristics have been widely studied and investigated in various fields. One of these fields is the investigating the potential of these compounds to supply essential nutrients for plants. Several studies have reported the application of LDHs as fertilizers for macronutrients and micronutrients. These compounds have a very high potential as fertilizers and can increase agricultural productivity. Micronutrients such as Zn, Cu and Mn can be structurally incorporated in the metal hydroxide layer. According to recent research, LDHs have shown a suitable potential to release micronutrients. However, more studies are needed to enhance our understanding of the mechanism and reaction of layered double hydroxides in different conditions. Although various studies have explored the potential of LDHs as slow-release fertilizers, our research focuses on the role of citric acid and tartaric acid and as well as the ratio of divalent to trivalent cations on the kinetics of Zn, Mn and Mg release from Mg-Zn-Mn-Al-LDH intercalated with nitrate.
Materials and Methods
All chemicals used in this study including citric acid (C6H8O7.H2O), tartaric acid (C4H6O6) KCl, Zn (NO3)2.6H2O, Mn(NO3)2.4H2O Mg(NO3)2.6H2O and Al(NO3).9H2O were of analytical grades, purchased from Chem-Lab or Merck Chemical Corporations. Solutions were prepared using decarbonated ultrapure water (electrical resistivity = 18 MΩcm). The LDHs were synthesized by co-precipitation method at constant pH = 9.2-9.6. Two types of LDHs were synthesized by varying the M+2(Zn+Mn+Mg)/M+3(Al) ratios of 3:1 and 4:1 in the precursor solution while stirring vigorously in a nitrogen atmosphere. The pH was kept at 9.2-9.6 by adding volumes of 3 M NaOH. The LDH crystals were allowed to ripen in the mixture for 2 hours, after which the precipitates were centrifuged at 3000 rpm for 20 min and washed several times with distilled water and placed in an oven at 70°C for 8 hours to dry.
The chemical composition of the synthesized layered double hydroxides (LDHs) was analyzed using furnace atomic absorption spectrophotometry (SavantAA, GBC) following acid digestion. The physical, chemical, and morphological characteristics of the LDHs were assessed through several techniques, including X-ray diffraction (Panalytical X Pert Pro X-ray diffractometer), field emission scanning electron microscopy (FE-SEM, Sigma VP), Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS10 spectrometer), and Brunauer-Emmett-Teller (BET, BELSORP Mini II) analysis.
A batch study was conducted to evaluate the effects of varying M²⁺/M³⁺ ratios in LDHs and the influence of citric acid and tartaric acid on the release of Zn, Mn, and Mg from LDH (3:1) and LDH (4:1). In brief, 0.01 g of synthesized LDH was placed in a centrifuge tube and mixed with 10 ml of background electrolyte (0.01 M KCl) and 1.25 mM of citric acid or tartaric acid, maintaining an initial pH of 6–7 at a constant temperature of 25 ± 0.5 °C. Blank samples (without ligands) were also included for comparison. The suspensions were shaken for time periods ranging from 5 to 720 minutes at an agitation speed of 180 rpm. After shaking, the supernatant was separated by centrifugation at 4000 rpm for 20 minutes. The concentrations of Zn, Mn, and Mg in the supernatant solutions were determined using graphite furnace atomic absorption spectrophotometry.
To describe the time-dependent release of Zn, Mn, and Mg, several kinetic models were tested. Among the five models evaluated, the pseudo-second-order and power function models provided the best fit for the kinetic data. Additionally, the chemical species present in the initial solution and the saturation index (SI) of various minerals were predicted using the Visual MINTEQ 3.1 computer code.
Results and Discussion
The results indicated that the calculated molar ratio of divalent cations to trivalent cations closely matched the molar ratios used in the synthesis of the layered double hydroxide (LDH) samples. The X-ray diffraction (XRD) patterns for both LDH (3:1) and LDH (4:1) samples exhibited strong and sharp peaks corresponding to the 003 and 006 reflections, confirming the layered structure of the synthesized materials. Fourier-transform infrared (FT-IR) spectroscopy revealed two significant absorption bands around 3480 cm⁻¹ and 1620 cm⁻¹ in all synthesized LDH materials, which are indicative of stretching vibrations associated with the O-H groups in the hydroxide layers and the interlayer water molecules. Additionally, a sharp band at approximately 1382 cm⁻¹ in LDH (3:1) and a band at around 1354 cm⁻¹ in LDH (4:1) were attributed to the antisymmetric stretching mode of the nitrate anion present in the LDH structure. The specific surface areas of LDH (3:1) and LDH (4:1) were measured at 5.50 m²/g and 16.54 m²/g, respectively. Correspondingly, the average pore diameters were found to be 1.92 nm for LDH (3:1) and 2.55 nm for LDH (4:1), indicating differences in porosity between the two samples. The time-dependent cumulative release of Zn, Mn, and Mg from LDH (3:1) and LDH (4:1) in the presence and absence of citric acid and tartaric acid was investigated. The release of these micronutrients was accelerated in the presence of both organic acids. The release process appeared to occur in two stages: during the initial stage (0 to 50 minutes), the release rate of Zn, Mn, and Mg was rapid, followed by a period from 50 to 720 minutes where the release rate either fixed or slightly increased.
In this research, among the non-linear models which were used to determine the release kinetics of Zn, Mn, and Mg, the one with the highest R2 values was chosen. The R² values ranged from 0.81 to 0.99 for the pseudo-first-order model, 0.89 to 0.93 for the pseudo-second-order model, 0.97 to 0.99 for the Elovich model, 0.89 to 0.99 for the power function model, and 0.55 to 0.86 for the parabolic diffusion model. Ultimately, the pseudo-second-order and power function models were chosen to analyze the kinetic data. The amount of Zn, Mn and Mg released at equilibrium (qe) were higher in the presence of citric acid (42%) compared to tartaric acid. Additionally, the release of these elements was greater from LDH (4:1) than from LDH (3:1). This suggests that increasing the ratio of divalent cations to trivalent cations reduces the stability of LDH, enhancing the release of micronutrients.
Conclusion
The results of this study demonstrated that the release of Zn, Mn, and Mg from layered double hydroxides (LDHs) was influenced by time, the type of low molecular weight organic acid, and the ratio of divalent to trivalent cations in the LDH structure. Kinetic modeling revealed that the release rates of Zn, Mn, and Mg from LDH with a 4:1 ratio were higher than those from LDH with a 3:1 ratio. Additionally, the dissolution rates of LDHs were faster in the presence of citric acid compared to tartaric acid. To further assess the potential of LDHs as slow-release micronutrient fertilizers in calcareous soils, additional greenhouse and soil experiments are recommended.
Soil science
Z. Movahedi Rad; M. Hamidpour
Abstract
Introduction Recently, layered double hydroxides (LDHs) have attracted significant attention due to their various applications, particularly as slow-release fertilizers for essential plant nutrients. Several studies have reported the release of nitrate and phosphorus from LDHs. Additionally, micronutrients ...
Read More
Introduction Recently, layered double hydroxides (LDHs) have attracted significant attention due to their various applications, particularly as slow-release fertilizers for essential plant nutrients. Several studies have reported the release of nitrate and phosphorus from LDHs. Additionally, micronutrients such as zinc (Zn), copper (Cu), and manganese (Mn) can be structurally incorporated into the metal hydroxide layers. Recent research indicates that LDHs have considerable potential for releasing these micronutrients. However, further studies are needed to enhance our understanding of the mechanisms and reactions of LDHs under different conditions. Currently, there is a lack of information regarding the divalent (M2+) to trivalent cation (M3+) ratios in LDHs and the influence of malic acid on the release of Zn, Mn, and magnesium (Mg) from these compounds. This study aimed to investigate the effects of malic acid and the ratio of M2+/M3+ on the kinetics release of Zn, Mn and Mg from Mg-Zn-Mn-Al-LDH intercalated with nitrate. Materials and MethodsAll chemicals used in this study including malic acid (C4H6O5), KCl, Zn(NO3)2.6H2O, Mn(NO3)2.4H2O, Mg(NO3)2.6H2O and Al(NO3).9H2O were of analytical grades, purchased from Chem-Lab or Merck Chemical Corporations. The solutions were made with the decarbonated ultrapure water (electrical resistivity = 18 MΩcm). The LDHs were synthesized by co-precipitation method at constant pH=9.2-9.6. Two types of LDHs were synthesized with varying the M+2(Zn+Mn+Mg)/M+3(Al) 3:1 and 4:1 in the precursor solution while being stirred vigorously in a nitrogen atmosphere. The pH was kept at 9.2-9.6 by adding volumes of 3 M NaOH. The crystals of LDH were ripened in the mixture for 2 h and after that, the precipitates were centrifuged at 3000 rpm for 20 min and washed several times with distilled water and placed in an oven at 70 °C for 8 h to dry. The chemical composition of the synthesized LDHs was determined by furnace atomic absorption spectrophotometry (SavantAA, GBC) after acid digestion. The physical, chemical, and morphological characteristics of the LDHs were determined using X-ray diffraction analysis (Panalytical x Pert ProX-ray diffractometer), Fe-SEM (Sigma VP), FT-IR (Nicolet iS10 spectrometer), and BET (BELSORP Mini II) techniques. A batch study was done to determine the effect of different ratios of M2+/M3+ in LDHs and the effect of malic acid on release of Zn, Mn, and Mg from LDH (3:1) and LDH (4:1). Briefly, 0.01 g of synthesized LDH were put in a centrifuge tube mixed with 10 ml background electrolyte (KCl 0.01 M) and 1.25 mM malic acid in initial pH=6-7 and constant temperature (25±0.5 °C). Blank samples (without ligand) were also considered. Suspensions were shaken at periods ranging from 5 to 720 min agitation (180 rpm). Then, the supernatant solution was separated using a centrifuge at a speed of 4000 rpm for 20 min. Zn, Mn, and Mg concentrations in supernatants solutions were determined by graphite furnace atomic absorption spectrophotometry. The effect of pH in the range of 5 to 10 on the release of Zn, Mn, and Mg from LDH was also studied. Two equations (pseudo-second-order and Elovich) were used to fit the kinetics data. Results and DiscussionThe results showed that the calculated molar ratio of divalent cation to trivalent cation was similar to their molar ratio in the solution prepared for the synthesis of LDH samples. The X-ray diffraction patterns of LDH (3:1) and LDH (4:1) samples showed the existence of strong and sharp peaks for 003 and 006 plates. Accordingly, the reflections of the 003 and 006 plates revealed the layered structure of the synthesized LDH materials. Two bands of FT-IR spectrums around 3480 and 1620 cm-1 for all synthesized LDH materials designated stretching vibrations of the O-H group of hydroxide layers and the interlayer water molecules. The sharp characteristic band around 1382 cm−1 in LDH (3:1) and band around 1354 cm-1 in LDH (4:1) was attributed to the antisymmetric stretching mode of nitrate anion in LDH. The specific surface area of LDH (3:1) and LDH (4:1) were 5.50 m2g-1 and 16.54 m2g-1 respectively. The average pore diameters in LDH (3:1) and LDH (4:1) were 1.92 nm and 2.55 nm, respectively. Time-dependent cumulative release of Zn, Mn, and Mg from LDH (3:1) and LDH (4:1) in the presence and absence of malic acid was investigated. Time-dependent Zn, Mn, and Mg release from LDH (3:1) and LDH (4:1) was accelerated in the presence of malic acid. The Zn, Mn, and Mg release from the LDHs was likely to be separated into two stages. In the initial stage from 0 to 60 min, the release rate of Zn, Mn, and Mg was rapid, then either remained constant or slightly enhanced during 60–720 min. In this research, among the non-linear models used to determine the release kinetics of Zn, Mn, and Mg, the result with the highest R2 values was chosen. The R2 values were 0.91–0.99, 0.93–0.99, 0.93–0.99, 0.89-0.99, and 0.55–0.86 for pseudo-first-order, pseudo-second-order, Elovich, power function, and parabolic diffusion, respectively. So, pseudo-second-order and Elovich models were used to analyze kinetic data. The amounts of release of Zn, Mn and Mg were higher from LDH (4:1) than from LDH (3:1) because of greater specific surface area, volume, and pore diameter in LDH (4:1). A comparison of metal release versus time profiles exhibited that dissolution was greatly dependent on the pH. ConclusionThe results of this research indicated that the release of Zn, Mn, and Mg from layered double hydroxides (LDHs) was influenced by factors such as time, ligand, solution pH, and the type of LDH. According to the kinetics models fitted to the experimental data, the release rate of Zn, Mn, and Mg from LDH (4:1) was higher than that from LDH (3:1). In both types of LDHs, the presence of malic acid significantly increased both the rate and amount of Zn, Mn, and Mg release compared to the absence of malic acid. While this study demonstrated that varying the ratios of divalent to trivalent cations can influence the amount and rate of Zn and Mn release, further greenhouse studies are required to confirm the effectiveness of LDH as a slow-release fertilizer in calcareous soils.
