Adsorption-desorption characteristics of nitrate, phosphate and sulfate on Mg-Al layered double hydroxide

The adsorption-desorption behavior of nitrate on Mg-Al layered double hydroxide (4:1) was compared with that of phosphate and sulfate as two common anions in soil solution. Based on the results, the kinetics of anion adsorption on the LDH followed a pseudo-second order model. The adsorption process was found to be exothermic for nitrate and endothermic for phosphate and sulfate. The adsorption data were best described by the Freundlich model for nitrate and by the Langmuir model for phosphate and sulfate. In this study, the synthesized LDH exhibited higher adsorption rate and adsorption capacity for nitrate compared to sulfate and phosphate. The maximum adsorption capacity values were 1.90, 0.28 and 0.13 mmol g(-1) for nitrate, phosphate and sulfate, respectively. However, the Langmuir equation constants related to the adsorption energy were found to be 0.210, 10.731 and 3.021 L mmol(-1), respectively. The distribution coefficient (K-d) was higher for nitrate than phosphate and sulfate at high initial concentrations. Moreover, the values of hysteresis index based on the Freundlich exponent were 97.5, 22.5 and 79.0% for nitrate, phosphate and sulfate, respectively. The FTIR spectroscopy indicated a strong band related to the nitrate intercalated into the interlayer space of the LDH. (C) 2013 Elsevier B.V. All rights reserved.