Preparation Of Porous Magnesium Oxide And Zinc-aluminum Layered Double Hydroxides In Phosphate Adsorption

Phosphorus is one of critical nutrients for biological and chemical processes in natural water bodies. Its release in a large amount to water body, occurring mainly as phosphate in municipal and industrial wastewaters and agricultural runoff has destroyed the ecological balance of water environmental system, and caused eutrophication. Now, various techniques have been used for phosphate removal, including physicochemical (chemical precipitation with lime, alum, and iron salts; crystallization; electrodialysis; reverse osmosis; ion exchange) and biological (phostrip;A/O; A2/O; Bardenpho; SBR technology) methods. While adsorption has the advantages of little area occupied, simple technique, non-second-pollution and could be used for the collecting and recycling of phosphate, so, its important for phosphate removal and resource utilization.This paper was guided by inorganic porous materials, magnesium oxide and hydrotalcite-like compounds were prepared by simple and convenient methods, and discussed the adsorption capacity for phosphate in water by these materials respectively. Firstly, mesoporous magnesium oxide microspheres were prepared by a simple precipitation and calcination method using sodium poly (4-styrenesulfonate) (PSS) as structure-directing agent. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherms and fourier transform infrared (FT-IR) spectroscopy. PSS plays a key role in the formation of MgO microspheres, the morphology of the MgO changed from irregular aggregated nanoparticles to microspheres in the presence of PSS. The microspheres are composed of at least three levels of hierarchical porous organization:small mesopores (ca.2-5 nm), large mesopores (ca. 10-50 nm) and macropores (50-250 nm). Adsorption of phosphate onto the as-prepared samples from aqueous solutions was investigated and discussed. The results reveal that adsorption process is clearly time dependent such that the majority of phosphate from aqueous solutions was completed in approximately 24 h and pseudo-second-order kinetic equation and intra-particle diffusion model can better describe the adsorption kinetics. Langmuir model provides the better correlation of the experimental data, the adsorption capacities for removal of phosphate were determined using the Langmuir equation and found to be 3.17 and 75.13 mg-g"1 for MgO samples prepared in pure water and in the presence of 1.0 g·L-1 PSS, respectively. The as-prepared mesopores MgO microspheres are found to be effective adsorbent for the removal of phosphate from aqueous solutions as a result of their unique porous structures and high specific surface areas.Then, Zn-Al LDHs were synthesized by a homogeneous precipitation method utilizing urea hydrolysis. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherms and fourier transform infrared (FT-IR) spectroscopy. Adsorption of phosphate by various Zn-Al LDHs and calcined Zn-Al LDHs (LDOs) were investigated. The concentrations of urea and crystallization time have strong influence on morphology in the preparation of LDHs. The formation of Zn-Al LDHs with variable morphologies from irregular nano-plates to loose nanosheet aggregates, solid microspheres, hollow microspheres, is highly dependent on the concentration of urea. The results indicate that pseudo-second-order kinetic equation and intra-particle diffusion model can better describe the adsorption kinetics. Langmuir model provides the better correlation of the experimental data. Calcined Zn-Al LDHs with urea concentration of 0.4 M and crystallization time 36 h showed higher adsorption capacity compared to other calcined or uncalcined LDHs. The maximum loading capacities for removal of phosphate were determined using the Langmuir equation and found to be 232.02 mg·g-1. Its found to be effective adsorbent for the removal of phosphate from aqueous solutions as a result of their unique porous structures and high specific surface areas.