Comparison of different synthesis routes for Mg–Al layered double hydroxides (LDH): Characterization of the structural phases and anion exchange properties

Nitrate forms of layered double hydroxides (LDHs) were synthesized based on the co-precipitation method under different synthesis conditions (aqueous ammonia solution or potassium hydroxide to control the pH of the solution; magnesium to aluminium ratios of 2:1 and 5:1). In a second step the lab procedure was up-scaled to pilot plant scale. The effects of the synthesis conditions on the structural and textural properties as well as on the anion exchange of the LDH products were investigated using different methods to evaluate the suitability as a soil conditioner. Using a Mg:Al ratio of 2:1 resulted in higher basal spacings compared to a Mg:Al ratio of 5:1, while the type of pH-controlling solution had no effect. Based on nitrogen adsorption isotherms the specific surface area of pores was calculated. The highest values for specific surface areas were found for LDHs synthesized using KOH at a Mg:Al ratio of 5:1. These products had a lower total nitrate adsorption capacity compared to LDHs synthesized at a lower Mg:Al ratio. However, nitrate exchangeability by counter anions for LDHs synthesized using KOH at a Mg:Al ratio 5:1 differed significantly (HCO3− > Cl− > SO42−) indicating that these LDHs are preferable under a multi-anionic environment like the soil solution.