Adsorption of nitrate and nitrite from aqueous solution onto calcined (Mg–Al) hydrotalcite of different Mg/Al ratio

Hydrotalcites (HTx) of different Mg/Al ratio (x was Mg/Al ratio; 2, 3, 4) were synthesized by co-precipitation method. Calcined hydrotalcites (CHTx) at 500 °C have been shown to recover their original layered structure by adsorbing nitrate or nitrite from aqueous solution. The adsorption kinetics study showed that the Pseudo-second-order kinetic model could be used to describe both nitrate and nitrite removal satisfactorily. The equilibrium isotherm showed that the Langmuir model gave a better fit to the experimental data than the Freundlich model. As the Mg/Al ratio increases from 2 to 4; the electric charge density between the layers becomes weaker; the interlayer spacing increases and the sample shows greater adsorption capacity. The CHT4 exhibits maximum adsorptive capacity, and the maximum adsorption capacity obtained by Langmuir model for nitrate adsorption was 34.36 mg N/g while for nitrite was 37.17 mg N/g. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray analysis (EDX) and Fourier transform infrared spectroscopy (FTIR).

► Calcined hydrotalcites of different Mg/Al ratio (2, 3, 4) was tested for NO3- and NO2- removal. ► Pseudo-second order and Langmuir model fitted kinetics and isotherm satisfactorily. ► Higher Mg/Al ratio exhibited larger interlayer spacing and showed greater adsorption capacity. ► The Qmax of calcined sample (Mg/Al = 4) was 34.36 mg N/g for NO3- and 37.17 mg N/g for NO2-. ► The detrimental effect of co-existing anion decreased as: CO32- > SO42- > F− > Cl−.