層狀雙氫氧化物的電荷密度及層間硝酸根離子的排列方式對其吸附除草劑2,4-D的影響

Abstract

Chlorinated phenoxyacetic acid herbicides, such as 2,4-dichlorophenoxyacetic acid（2,4-D）, are widely used for controling weeds. Due to its low pKa value of 2.73, 2,4-D exists as an anionic species in the environment and has high mobility in soils and can easily migrate to non-target area by leaching. Layered double hydroxides (LDHs) have high specific surface area and anionic exchange capacity (AEC), so they have high potential in immobilizing inorganic and organic anion contaminants in water. However the adsorption is not specific to 2,4-D, and the existences of other anions may lower the adsorption efficiency of LDHs for 2,4-D. A better understanding of the adsorption mechanisms of LDHs will be essential for selective removal of 2,4-D by LDHs from water. In this study, the effects of the positive charge density and nitrate orientation of LDHs on the adsorption of 2,4-D were investigated. Mg/Al-NO3 LDH with Al3+/(Mg2++Al3+) molar ratios of 1/3 (LDH3), 1/4 (LDH4), 1/5(LDH5) were synthesized and used as the adsorbents. The kinetic study showed that the adsorption could reach equilibrium in 10 minutes. Among the samples, LDH3 has the highest layer charge density and contains nitrate with an orientation perpendicular to the hydroxide sheets, so the maximum adsorption of 2,4-D on LDH3 was as high as 89% of its anionic exchange capacity. The 2,4-D adsorption of LDH3 occurred mainly through ion exchange for interlayer nitrate. On the contrary, LDH5 with a low 2,4-D adsorption capacity due to the low accessibility of 2,4-D to the interlayer space. The accessibility was restricted by the small basal spacing of LDH5 as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheet. Thus, the 2,4-D adsorption occurred mainly on the external surface of the material, and the maximum adsorption of 2,4-D on LDH5 was 16 ％ of its anionic exchange capacity. For LDH4 that contains interlayer nitrate with both parallel and perpendicular orientations, the adsorption characteristics was between those of LDH3 and LDH5, and the maximum amount of 2,4-D adsorbed on LDH4 was 66% of the anionic exchange capacity. The results of competitive adsorption revealed that the adsorptions of 2,4-D on LDH3 and LDH4 are less affected by the coexistences of other anions, such as Cl-、Br 、NO3-、HCO3-、SO42-, in solution. The kinetic adsorption curves of LDH3 and LDH4 in different temperatures had a good fit with pseudo-second order kinetic model. The rate of the adsorption of 2,4-D on LDHs increasing with temperature, and LDH3 required more activation energy to adsorb 2,4-D then LDH4. The results of this study suggested that LDH3 is a better adsorbent for removing 2,4-D from water.