Release of Zinc, Manganese and Magnesium from Mg-Zn-Mn-Al-LDH: Effect of Malic Acid and Divalent to Trivalent Cation Ratios in Mineral Structure

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.

All 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.

The 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.

The 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.