| Layered double hydroxides (LDHs) are host-guest materials with permanent positive charged layers and exchangeable interlayer anions. Recently, the studies on the synthesis, properties and applications of LDH have been increased greatly. LDH have shown great promise as sorbents for the removal of anionic contaminants from water. The main mechanism involves in the sorption of sorbates by LDH include Van der Waals interaction, surface complexation and ion exchange, etc. However, due to the preferential attraction of polar water molecules to the hydrophilic surface, they are less effective at binding organic compounds. Through the incorporation of organic anions such as alkyl sulfonates, organo-LDHs can be produced, in which the layered structure and composition of LDHs may change. The resulting organo-LDHs are novel functional materials engineered with hydrophobic surfaces with stable structures; making them powerful sorbents for many organic pollutants.In this study, organo layered double hydroxide with different Mg/Al molar ratios (2:1,3:1and4:1) containing dodecylsulfonate (DSO) as the interlamellar anions were prepared through co-precipitation, structural reconstruction and co-percipitation methods. The synthesized materials were characterized by X-ray diffraction (XRD), infra-red spectroscopy (FT-IR), elemental analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetry-differential thermal analysis (TG-DTA). Both the co-precipitation and the structural reconstruction method can obtain ideal products with good layered structure and well regularity. However, the co-precipitation method has superiority in its convenient and lower energy consumption. The ability of organo-LDHs to remove humic acid (HA) from an aqueous medium was studied as a function of pH, sorbent dosage, initial HA concentration, contact time and ionic strength. The sorption mechanism was also discussed. The results show that:Compared with inorganic LDHs, the surfaces of organo layered double hydroxides (LDHs) are less hydrophilic and more organophilic, thus their ability to remove organic contaminants from water can be significantly enhanced. The HA sorption capacities of all three LDH-DSO sorbents were significantly larger than the HA adsorption capacity of inorganic LDH-CO32-. The sorption was a combination of both partitioning and specific adsorption. The sorption of HA to organo-LDH fits well with a "dual-mode model" equation as well as the Freundlich isotherm equation. Calculations of the specific adsorption and partition contributions showed that (a) at lower HA concentrations, the adsorption effect dominates and the data conforms to the Langmuir isotherm;(b) at higher HA concentrations, the partition effect dominates and the isotherms were approximately linear. In the mechanism of HA sorption on LDH-DSO surfaces, ligand exchange and electrostatic interactions are more important than hydrophobic interactions. The sorption kinetics data fit the pseudo second order model. Because LDH-DSO shows a buffering capacity over a broad pH range (from5to9), the sorbent has a wide range of effective application. An increase in NaCl ionic strength causes a slight decrease in percentage removal of HA.The LDH-DSO showed high adsorption capacities to three different kinds of dyes, i.e. acid dyes, reactive dyes and basic dyes. At pH5-9, the changes in pH have low influence to the adsorption capacity. The adsorption kinetics data fit the pseudo second order model. The adsorption data fitted both the Langmuir model and the Freundlich model. The adsorption is a heterogeneous and reversible process. An increase in the temperature is favorable to the adsorption.The high adsorption capacity of LDH-DSO shows that, due to the hydrophobic surface character, LDH-DSO has high affinity to organic pollutants in water. LDH-DSO could be used as a high efficiency sorbents in the removal of organic pollutants from water. |
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