Adsorption Properties Of LDHs And LDHs Nanocomposite For Aqueous Thiocyanate Ions And Acid Scarlet GR

Layered double hydroxides (LDHs) are new kind inorganic materials, and have potential value in different industries, such as catalysts, anti-flocculation, medicament, and wastewater treatment, owing to their unique electro kinetic property, layered structure and anion exchange ability. Recently, there is currently a huge interest in the research of LDHs in preparation and application. In our studies, Cl"-containing Magnesium aluminium layered double hydroxides (MgAl-Cl-LDHs) were prepared by co-precipitation method and characterized by modern analyze technique. The adsorption properties of MgAl-Cl-LDHs for the removal of thiocyanate hazardous anions from aqueous solution were studied, and the adsorption mechanisms were discussed. The LDHs nanocomposites, layered double hydroxides which alkyl sulfonates as interlayer anions, were prepared by ion-exchanging method. The structure of the LDHs nanocomposites was characterized, and the adsorption properties of the nanocomposites for Acid Scarlet GR (AS-GR) in water were investigated.1. Preparation and Characterisation of MgAl Layer double hydroxidesMagnesium aluminium layered double hydroxides in which the interlayer anions are chloride anions (MgAl-Cl-LDHs) with differrent Mg/Al mol rations were prepared by co-precipitation method using AlCl3·6H2O, MgCl2·6H2O and NH3-H2O being raw materials. The MgAl-Cl-LDHs was studied by all kinds of chemical analysis methods and the modern techniques. The structure and morphology of MgAl-Cl-LDHs were characterized by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR);1the electrokinetic property of MgAl-Cl-LDHs was measured by a micro-electrophoretic mobility detector and average particle diameter measured with a Zetasizer. The results showed that:The Mg/Al molar ratio in synthetic samples are always more than that before mixing. When the molar ratio of Mg/Al between 2:1 and 4:1, the MgAl-Cl-LDHs hexagonal multilayer in structure, which is similar with the structure of hydrotalcite crystal. The diameter of particles is about 100-115 nm. The MgAl-Cl-LDHs particles had perpetual positive charges and the isoelectric pointes of MgAl-Cl-LDHs for the molar ratio 1:1,2:1,3:1 and 4:1 are at pH values of 10.8,11.5,12.0 and 12.3, respectively.2. Thiocyanate removal from aqueous solution by MgAl-CI-LDHsThe use of a chloride-containing synthetic hydrotalcite sol (MgAl-Cl-LDHs) as adsorbent to remove thiocyanate from aqueous solution was investigated. MgAl-Cl-LDHs was prepared by co-precipitation and was characterized by HRTEM, particle size, XRD and FT-IR. The experiments showed that MgAl-Cl-LDHs was particularly effective in removing thiocyanate due to its small particle size and the high zeta potential. The adsorption of thiocyanate on MgAl-Cl-LDHs was favored when the initial solution pH was in the range 3-10, though the most effective pH range was between 4.5 and7.5. The adsorption reached equilibrium within 150 min. The interaction between the surface sites of MgAl-Cl-LDHs and thiocyanate ions may be a combination of both anion exchange and surface complexation. The pseudo-second-order model best described the adsorption kinetics of thiocyanate onto MgAl-Cl-LDHs. The equilibrium isotherm showed that the adsorption of thiocyanate on MgAl-Cl-LDHs was consistent with the Langmuir equation and the saturated adsorption capacity of MgAl-Cl-LDHs for thiocyanate was 98.3 mg/g at 20℃. The regenerated MgAl-Cl-LDHs in FeCl3 solution can be used repeatedly in adsorption-regeneration cycles. The results showed that LDHs can be used as a new adsorbent for thiocyanate removal from aqueous solution because of its high adsorption capacity and rapid adsorption rate.3. Preparation and Characterisation of the LDHs nanocompositesAn important property of LDHs is their ability to exchange anions located in the interlayer space for others. Through the incorporation of guest inorganic or organic anions into a layered host by ion exchange interaction, during which the layered structure and composition of LDHs may change accordingly, novel functional materials engineered with special properties can be synthesized. In this study, a LDH precursor with an Mg/Al of 2:1 was prepared. LDHs nanocomposites were obtained by intercalation of sodium alkyl sulfonates with different alkyl chain length (guest) into the LDH precursor (host). The LDHs nanocomposites were characterized by XRD,FT-IR etc. Experimental results show that the concentrations of MgAl-Cl-LDHs and sodium alkyl sulfonates were essential to the preparation of LDHs nanocomposites. The intercalation cannot be achieved either when the concentration of sodium alkyl sulfonates was too low (<20 mmol/L) or when the concentration of LDHs precursor was too high (≥1.00 wt%). Alkyl sulfonate anions can exchange the Cl- in the interlamination of LDH precursor, through which nanocomposites with well-organized crystalline structure can be obtained. We found that, when the concentrations of all sodium alkyl sulfonates were 20 mmol/L, with the increase in the alkyl chain length, the interlayer spacing of the samples were increased from 0.773 nm to 2.090 nm,2.287 nm and 2.497 nm, respectively. The gallery heights were simultaneously increased from 0.293 nm of the precursor to 1.610 nm,1.807 nm and 2.017 nm after intercalation. Also with the increase in the alkyl chain length, the surfaces of the LDHs nanocomposites were modified to hydrophobic.4. Adsorption properties of the LDHs nanocomposite for the model anionic dye Acid Scarlet GRThe LDHs nanocomposites (MgAl-CH3(CH2)5SO3-LDHs), layered double hydroxides which six alkyl sulfonates as interlayer anions, were prepared. The use of MgAl-CH3(CH2)5SO3-LDHs adsorbent to remove Acid Scarlet GR (AS-GR), as a model anionic dye, from aqueous solution was investigated. Adsorption experiments were carried out as a function of temperature, pH, contact time and adsorbent dosage. The results showed that MgAl-CH3(CH2)5SO3-LDHs was particularly effective to remove AS-GR. the effective pH was about 9.0. The adsorption of AS-GR on MgAl-CH3(CH2)5SO3-LDHs reached equilibrium within 4 h. The appropriate adsorbent dosage was 1.00 g/L.The adsorption process was endothermic in nature. The equilibrium isotherm showed that the adsorption of AS-GR onto MgAl-CH3(CH2)5SO3-LDHs was consistent with the Freundlich equation. The interaction between the surface sites of LDHs nanocomposite and the dye ions may be a combination of both sueface adsorption and partition.