Treatment Of Dyeing Wastewater By Layered Double Hydroxides

To solve the problems of wide band gap, low adsorption efficient and unavailable of traditional, Layered double hydroxides with Zn, Cu, Ni in the layer were synthesized as metal-doped photo catalysts. The enhancement of photocatalytic activity was explained on the basis of electronic structure of LDHs which played an important role for absorption of visible light, surface hydroxyl groups, pore size distribution and crystallite size. Then based on the adjustment of the species and ratios of anions and the rule of band gap, guide the synthesis of LDHs photocatalyst with various band gaps. We use the UV diffuse reflectance, XRD, FTIR to characterize the structure and photocatalytic properties of the hydrotalcite materials. Based on the synergistic mechanism of adsorption and photocatalytic properties, the effect of different factors are investigated and the Kinetics of LDHs are also studied. Meanwhile, Layered double hydroxides was calcined at high temperature to synthesize Layered double oxide(LDO). The effects of various parameters such as temperature, initial pH values, dosage of adsorbent and competitive anions were studied. The adsorption isotherms, kinetic models and the regeneration of MgAl-LDO were also investigated.(1) CuMgAl-LDHs with stable structural property can be synthesized by addition of Cu element with high catalytic activity into MgAl-LDH layers. With the illumination of visible light, the Cu would result in a state of defect energy between valence band and conduction band with the contribution of d orbital. Accompanied with the projection of the electron, photo-generated carriers were seized to creat HO-which could degrade RhB due to its strong oxidizing property. The degradation rate (η%) with a fixed CuMgAl-LDHs dose of3g/L was found to be85.2%among which the ratio of photocatalytic degradation to total degradation is over50%, when temperature and pH were286.15K,7.84, respectively.(2) Ni3-nMgnAl layered double hydroxides with different Ni/Mg/Al molar ratios have been successfully synthesized by the co-precipitation method. The structure and properties of the photocatalysts were characterized by XRD and UV-Vis. The band gap of the samples were near to2.50eV according to Kubelka-Munk formula. The results show that the photocatalytic activity of LDHs can be improved with the metal cations of appropriate proportion and species incorporated in octahedral layers of LDHs. The degradation rate (η%) with a fixed Ni1Mg2Al-LDHs dose of1g/L was found to be99.7%among which the ratio of photocatalytic degradation to total degradation is over70%, when temperature and pH were303.15K. Meanwhile, it is discovered that the stability of the sample gradually decreased with increasing of Ni in the layer.(3) This paper presents a novel photochemical method for the photocatalytic property of ZnMgAl layered double hydroxides (ZnMgAl-LDHs). Three molar ratios of samples were synthesised by the co-precipitation method at a constant pH of9.5. The effect of pH, the dosage of photocatalyst, contact time and temperature were investigated. The result showed that the appropriate amount and ratio of metal ions replacing Mg-ion could conduce photocatalytic activity of LDHs. Meanwhile, the kinetics of the process was investigated.(4) The adsorption thermodynamics and mechanism of MgAl-LDO for acid red88(AR88), acid orange3(AO3), acid violet90(AV90) from aqueous solution were investigated. The adsorption isotherms were measured at different temperatures,and the functions of thermodynamics were also calculated. The results indicate that the equilibrium isotherms of uptake of acid dye by LDO were well fitted by the Langmuir equation, and thermodynamic parameters such as△rG,△rH,△rS were calculated from Langmuir constants. The negative values confirm the spontaneous exothermic nature of adsorption. Three kinetics models were us ed to fit the kinetics experimental data, and it was found that the pseudo-second order kinetics model could be used to describe the uptake process appropriately. The value of Ea was calculated to be higher than20.0kJ·mol-1, which suggests that the process of uptake bromide is controlled by the reaction rate of bromide with the CLDH rather than diffusion. The Gibbs free energy was calculated to be10to15kJ·mol-1because of Hydrogen bonds between the layers and dye molecular. According to the calculation of Materials Studio5.5, the mechanism of adsorption process was that most dye anions were adsorbed on the surface of MgAl-LDO accompanied with intercalated into the layer.