Phosphorus Release Kinetics of Layer Double Hydroxides: Effect of pH and Divalent to Trivalent Cation Ratios in the Mineral Structure

آب و خاک

Article 7, Volume 38, Issue 3 - Serial Number 95, July and August 2024, Pages 409-399   

Document Type: Research Article

DOI: 10.22067/jsw.2024.87711.1404

Authors

A. Hassanzadeh; M. Hamidpour^*

Soil Science Department, Faculty of Agriculture, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran, respectively.

Abstract

Introduction
Layered double hydroxides (LDH) have gained considerable attention for their potential application in agriculture, serving as a slow release sources of essential nutrients for plants. The appraising of LDH as a favorable fertilizer is in the early development, and more studies on the nutrient release mechanism of LDH are needed to answer the question of how LDH could replace commercial fertilizers for providing the stable nutrients for plants. Although several studies on the release of P from LDH exist in the literature, no information regarding ratios of divalent cation (M²⁺) to trivalent cation (M³⁺) in LDHs on phosphate release from LDHs is available. So, it is important to raise our knowledge about various parameters like pH and time on the solubility of LDHs. This study aimed to investigate the effects of pH and the ratios of M²⁺/M³⁺on the kinetics release of P from Mg-Al-LDH.
 
Materials and Methods
All the chemicals in this research, such as magnesium nitrate hexahydrate (Mg (NO₃)₂.6H₂O) and aluminum nitrate nonahydrate Al(NO₃)₃.9H₂O were of analytical grade and obtained from Merk (USA). The solutions were made with decarbonated pure water without impurities (electrical resistivity = 18 MΩcm). Two nitrate forms of Mg-Al-LDH were synthesized using the co-precipitation method at constant pH by varying the Mg/Al ratios (2:1 and 3:1) in the precursor solution. Briefly, 50 mL of 1M solution containing nitrate salt of divalent cations (Mg(NO₃)₂.6H₂O) and trivalent cations (Al(NO₃)_3.9H₂O) in the appropriate ratios (2:1 and 3:1) were added simultaneously for 2h to 400 mL of 0.01M solution of sodium hydroxide while being stirred vigorously in a nitrogen atmosphere. The pH was kept at 9.5 by adding volumes of 3 M NaOH. Afterward, the material was ripened in the synthesis mixture for 2 h and centrifuged at 3000 rpm for 20 min. The precipitates were washed by three washing-centrifugation cycles with Milli-Q water and subsequently dried at 70 °C. In this study, LDH-P was made by ion exchange. The LDH-N were treated with 0.05 M KH₂PO₄ solutions at pH 7.2. The suspensions were shaken end-over-end for 24h, followed by centrifugation, washing, and drying as described above. After digesting the dried LDHs in aqua regia (3:1 HCl/HNO₃), the total P concentration of the LDHs was determined. The chemical composition of the synthesized LDHs was determined by graphite furnace atomic absorption spectrophotometry (SavantAA, GBC) after acid digestion (3:1 HCl/HNO₃). Crystallization and morphology of the LDHs were characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD patterns were prepared using an x-ray diffractometer (Panalytical x Pert Pro, Netherlands), at scan step time of 1s from 2θ=5° to 2θ=70° (40KV and 30 mA), and with a step size of 0.0260, which were used to identify the mineral phases. The phase purity was surveyed by comparing these XRD diagrams with those found in the literature. The SEM photographs were gained on a scanning electron microscope (Sigma VP, Germany). Fourier Transform Infrared (FTIR) spectrum was done on a Nicolet iS10 FT-IR spectrometer by utilizing KBr pressed disk technique.
A batch study was done to determine the effect of different ratios of M²⁺/M³⁺ in LDHs at different pH 6.0 and 8.0 on the release of P from LDHs. Briefly, 0.01 g of synthesized LDH were put in a centrifuge tube mixed with 10 ml of 0.03M KNO₃ at initial pH=6 and 8. Suspensions were shaken at a constant temperature (25±0.5 °C) and agitation (180 rpm) by using an incubator shaker for 8h. Phosphorus concentration in supernatant solutions was measured by vanadate yellow method at 470 nm wavelength.
In order to investigate the kinetics of phosphorus release, LDH-P1 (2:1) and LDH-P2 (3:1) were used at two initial pHs of 6 and 8. First, 0.012 g of LDH sample was placed in 120 ml of KNO₃ electrolyte solution (with ionic strength of 0.03 M) in an Erlenmeyer flask. The flasks were shaken for 5 to 1175 min by an incubator shaker at 100 rpm. Then the suspensions were centrifuged at a speed of 4000 rpm for 20 minutes and the phosphorus concentration was determined by the method described previously. All experiments were performed with three repetitions. Two equations (pseudo-second-order and parabolic diffusion) were used to fit the kinetics data.
 
Results and Discussion
According to the XRD patterns, the sharpness and reflection of diffraction planes (003) and (006) pertained to layer structures. The basal spacing as calculated by Bragg’s law (nλ = 2d sin θ) were 7.94 and 8.0 Å for Mg-Al-NO₃ with M⁺²/M⁺³ 2:1, 3:1 respectively. The XRD patterns of the LDHs exhibited a distinct characteristic reflection (003), which indicated that the basal spacing decreased as the Mg/Al ratio decreased (higher AEC). In addition, the decreased basal spacing is linked with a decrease in the interlayer spacing. The different basal spacing of LDH were related to the layer charge density, the content of water, and the reorientation of anions in the interlayer of LDH. The intercalation of phosphate anions into Mg/Al LDH is in adaptation with the change toward lower 2θ angles of the (001) reflections corresponding to the expansion of the basal distance d003 compared to the host Mg/Al-NO₃^-.
Two bands of FT-IR spectrums around 3470 and 1655 cm^-1 for all synthesized LDH materials designate stretching vibrations of the O-H group of hydroxide layers and the interlayer water molecules. The band vibration of phosphate was perceived at 1051 cm⁻¹ and 1064 cm^-1, reflecting the formation of inner-sphere surface complex (M-O-P) between dihydrogen phosphate ions and MgAl-LDH materials. It indicated that the phosphate exchange process may be resulted in the formation of bidentate and monodentate surface complexes. According to the SEM images, the well-crystallized and plate-like morphology were typical for layer double hydroxides. The results of the X-ray energy dispersive spectroscopy (EDS) analysis showed, the only elements that existed in the LDH-N were Mg, Al, N, and O, whereas Mg, Al, P, and O were detected in the LDH-P. The results showed that increasing the pH from 6 to 8 in the presence of 0.03 M potassium nitrate background electrolyte led to an increase in phosphorus released from both types of LDH. For example, by increasing the initial pH of suspensions from 6 to 8, the amount of cumulative phosphorus released from LDH-P1 increased from 38.59 mg kg^-1 to 41.91 mg kg^-1 at equilibrium. In all studied pHs, phosphorus release from LDH-P1 in background electrolyte was lower than LDH-P2. For example, at pH 6 and 8, the amount of cumulative phosphorus released from LDH-P2 was 1.46 and 1.33 times higher than LDH-P1 at equilibrium, respectively. The cumulative phosphorus release kinetics from the studied LDHs showed that the amount of phosphorus release accelerated with increasing time. Phosphorus release from LDH continued at a higher rate from 0 to 400 minutes in the first stage and at a slower rate during 400-1175 minutes. Also, based on the results, among the studied kinetic equations, pseudo-second-order and parabolic diffusion equations had the best fit on phosphorus release data.
 
Conclusion
The results of this research showed that the release of phosphorus from LDH is dependent on time, pH and the type of LDH. Based on the results of fitting the kinetics models to the experimental data, the release rate of phosphorus from LDH-P2 (3:1) was higher than that of LDH-P1 (2:1). Cumulative phosphorus release from LDH-P2 compared to LDH-P1 was 46.54, 33.61% higher at pH 6 and 8, respectively.

Keywords

Available phosphorus; Kinetics models; Layered double hydroxide; Slow release fertilizer

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