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

IntroductionLayered 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 MethodsAll 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).