Capacity, selectivity, and reversibility for nitrate exchange of a layered double-hydroxide (LDH) mineral in simulated soil solutions and in soil

The leaching of nitrate is an important way of N losses from agricultural soils in humid regions. Nitrate leaching is difficult to control as most soils under crop production do not have anion-exchange properties, and nitrate remains mobile in the solution. The present work evaluated the potential use of a synthetic layered double-hydroxide (LDH) mineral as a nitrate exchanger in soil. The LDH used was a chloride form of a magnesium-aluminum layered double hydroxide ol with the formula: [Mg0.822+Al0.183+(OH)(2)](0.18)+[(Cl-)(0.18) 0.5(H2O)](0.18-). Experiments were carried out in aqueous solutions as well as in soil with the following objectives: (1) to characterize the nitrate adsorption capacity on the LDH, (2) to study its selectivity for nitrate adsorption in solution, (3) to evaluate the reversibility for nitrate exchange, and (4) to study the nitrate adsorption capacity and nitrate diffusion towards the LDH in soil. In aqueous solutions, the nitrate adsorption capacity was determined to be 1.54 mol(c) kg(-1) LDH at nitrate concentrations typically encountered in soils under crop production. In solution, the LDH showed a high selectivity for nitrate adsorption compared to other anions. The LDH exchanged nitrate reversibly without a measurable reduction of the exchange capacity for at least 15 times. Under soil conditions, the extension of the nitrate depletion zone from the LDH surface reached up to about 8 cm within one week, confirming its high nitrate adsorption capacity also in soil. The LDH mineral evaluated in this work has the potential to be used as a nitrate exchanger in crop production, aiming to control the movement of nitrate in soil and thereby reducing the risks of nitrate leaching.