In this study, we prepared Fe-Mg-type hydrotalcites (Fe-HT3.0 and Fe-HT5.0) with different molar ratios and evaluated their adsorption capability for nitrite and nitrate ions from aqueous solution. Fe-HT is a typical hydrotalcite-like layered double hydroxide. Adsorption isotherms, as well as the effects of contact time and pH were investigated, and it was found that Fe-HT can adsorb larger amounts of nitrite and nitrate ions than Al-HT (normal-type hydrotalcite). Adsorption isotherm data were fitted to both Freundlich (correlation coefficient: 0.970-1.000) and Langmuir (correlation coefficient: 0.974-0.999) equations. Elemental analysis and binding energy of Fe-HT surface before and after adsorption indicated that the adsorption mechanism was related to the interaction between the adsorbent surface and anions. In addition, the ion exchange process is related to the adsorption mechanism. The adsorption amount increased with increasing temperature (7-25°C). The experimental data fit the pseudo-second-order model better than the pseudo-first-order model. The effect of pH on adsorption was not significant, which suggested that Fe-HT could be used over a wide pH range (4-12). These results indicate that Fe-HT is a good adsorbent for the removal of nitrite and nitrate ions from aqueous solution.

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In this study, metal complex hydroxide materials containing magnesium to iron in molar ratios of 3:1 and 5:1, referred to in this study as Mg-Fe-CH3.0 and Mg-Fe-CH5.0, respectively, were prepared, and their adsorption capability with respect to fluoride ions was investigated. The physicochemical characteristics of adsorbents were determined using scanning electron microscopy and X-ray diffraction, and specific surface area and the number of hydroxyl groups were calculated. The adsorption behaviors and mechanism of fluoride ions were assessed. The adsorption capability of fluoride ions using Mg-Fe-CH3.0 was greater than that using Mg-Fe-CH5.0. In addition, the amount adsorbed depended on the adsorption temperatures; the adsorption was comparatively less at 5 °C than at 25 °C. Adsorption mechanism of fluoride ions was evaluated by elemental distribution analysis and binding energy. The binding energy of fluorine onto Mg-Fe-CH3.0 and Mg-Fe-CH5.0 could be detected after adsorption. Additionally, it was clear that one of the adsorption mechanisms was related to the ion exchange between fluoride ions and chloride ions in the interlayer space of the Mg-Fe-CH3.0 and Mg-Fe-CH5.0 (correlation coefficient 0.923–0.965). This study illustrates that both Mg-Fe-CH3.0 and Mg-Fe-CH5.0 have a high potential for fluoride ion adsorption from the aqueous phase.