Preferential adsorption of 2,4-dichlorophenoxyacetate from associated binary-solute aqueous systems by Mg/Al-NO 3 layered double hydroxides with different nitrate orientations

Abstract

Layered double hydroxides (LDHs) are potential scavengers for anionic contaminants. To apply these materials to water treatment, it is essential to understand how the structural properties of LDHs control their preferential binding affinity toward a specific target contaminant in the presence of other anions. In this study, Mg/Al-NO 3 LDHs with different nitrate orientations were investigated for their preferential adsorption of 2,4-dichlorophenoxyacetate (2,4-D). LDH samples containing nitrate ions with parallel (LDH5) and perpendicular (LDH3) orientations exhibited different 2,4-D adsorption characteristics under competition with co-existing anions for surface binding sites. Because 2,4-D is inaccessible to the interlayer region of LDH5, the 2,4-D adsorption occurred mainly on the external surface of LDH5 and was easily subjected to negative effects from increasing concentrations of competing anions, including SO 4 2−, HCO 3 −, Cl −, NO 3 − and Br −. On the contrary, the interlayer nitrate in LDH3 is readily exchanged by 2,4-D. Thus, in conjunction with adsorption on the external surface, 2,4-D can also be adsorbed on the internal surface in the interlayer of LDH3. LDH3 exhibited a much higher 2,4-D adsorption capacity than LDH5, and the 2,4-D adsorption on LDH3 was less affected by the increasing concentration of competing anions. In summary, the selective removal of 2,4-D in solutions with a complex chemical composition may be maximized using Mg/Al-NO 3 LDHs containing perpendicularly oriented interlayer nitrate.