Nitrate easily moves through the soil profile and eventually enters the groundwater as a common contaminant. Recently the possibility of using layered double hydroxides (LDHs) to improve nitrate retention capacity (NRC) of soils has been proposed [2]. For successful application of LDHs to remove nitrate, these compounds should have high selectivity for nitrate adsorption in the presence of other anions, particularly phosphate, in complex solutions such as soil solution. In the present study Mg- Al LDHs with M2+/M3+ ratios of 3:1 and 4:1 were synthesized by the coprecipitation method [1]. The synthesized LDHs were characterized by XRD, TGA and SEM techniques. The LDHs were compared in terms of ion adsorption isotherm and selectively in the presence of common anions in the soil solution. Results showed that the M2+/M3+ ratio had a significant effect on structural and adsorption characteristics of Mg-Al-LDHs. The anion exchange capacity of Mg-Al-LDH (3:1) and Mg-Al-LDH (4:1) were determined as 192.7 and 160.6 cmol kg-1, respectively. Nevertheless, Mg-Al-LDH (4:1) showed higher BET specific surface area (64.42 m2g-1) compared to Mg-Al-LDH (3:1) (21.03 m2g-1). The percentage of phosphate adsorbed from the solution contains 1 mM nitrate, phosphate, sulfate and bicarbonate decreased with decreasing M2+/M3+ ratio. However, the reverse was true for nitrate (Fig1, left). The experimental data for phosphate adsorption by Mg-Al-LDH (3:1) and Mg-Al-LDH (4:1) fitted well into Freundlich and Langmuir isotherms, respectively. For nitrate a good agreement between the experimental data and Langmuir isotherm was observed. The highest adsorption capacity from Langmuir isotherm was obtained for Mg- Al-LDH (4:1) probably due to the nitrate ion-sieve property of these LDH. The Mg-Al- LDH (3:1) showed a higher distribution coefficient for phosphate compared to nitrare in all equilibrium concentrations. However, the Mg-Al-LDH (4:1) showed a higher distribution coefficient for phosphate at lower equilibrium concentrations and for nitrate at higher equilibrium concentrations (Fig 1, right).

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