Preparation Of Metal Oxides With Supercritical CO₂ Drying And Their Applications On Adsorption

Metal oxide represents a type of important inorganic nanomaterials, and has been widely applied as adsorbent, catalyst, semiconductor, light emitting diode, and ceramics among others. In this work, the commonly used magnesium oxide, zirconium oxide and nickel oxide were studied, precipitation method combined with supercritical CO2 drying was used to prepare metal hydroxides, and then the metal oxides were obtained after calcination.MgO with ultra low bulk density was prepared by precipitation reaction of Mg(NO3)2·6H2O with NaOH and supercritical CO2 drying. The influence of the way of addition of the reactant, reactant concentration, reaction temperature and drying methods on the specific surface area and pore volume of the prepared MgO was investigated. Results showed that adding Mg(NO3)2 solution into NaOH aqueous solution at a constant speed, insuring the precipitation reaction was run under relatively high pH value, led to obtain MgO powder with high specific surface area and pore volume; the specific surface area and pore volume of MgO increase with the increase of the reaction temperature. When the reaction temperature was 90℃ , the minimum bulk density of MgO was only 0.0247 g/mL from supercritical CO2 drying, which is obviously lower than that (0.1 g/mL) of MgO aerogel reported in literature.Using nitrate salts and NaOH as raw materials and reacting under different temperatures, Zr(OH)4 and Ni(OH)2 were obtained with supercritical CO2 drying. Results showed that the pore volumes of Zr(OH)4 and Ni(OH)2 increased with the increase of the reaction temperature, but the pore volumes of ZrO2 and NiO after calcinating were of little difference under different reaction temperatures. The highest pore volume of ZrO2 was 0.98cm3/g which is close to that (1.0 cm3/g) reported in literature from supercritical ethanol drying. Apart from that, the highest pore volume of NiO was 1.14cm3/g which is higher than that (0.9 cm3/g) reported in literature.The adsorption performance of the as-prepared MgO for Pb2+ from its aqueous solution was studied. The effects of the pH, adsorption time, temperature, adsorbent dosage of and the initial Pb2+ concentration on the adsorption process were investigated. The highest adsorption capacity of 1909 mg/g was obtained at pH=6,25 ℃, with adsorption time of 4 h and adsorbent dosage of 0.15 g/L, which is close to the highest value (1980 mg/g) reported in literature. The adsorption data fitted Langmuir isotherm very well, and showed a pseudo-second order kinetics process. Results also showed that the adsorption was actually a solid-liquid interfacial cation exchange process:Pb2+replaced the Mg2+in MgO crystal lattices.The adsorption performance of the as-prepared ZrO2 and NiO was studied referring to the experimental conditons reported in literature. The highest adsorption capacity of NiO for Pb2+is 860 mg/g, which is close to the result in the literature (909 mg/g). The highest adsorption capacity of ZrO2 for C20H12NaNaO7S is 9.54 mg/g, which is close to the result in the literature (9.12 mg/g). However, the adsorption rate is faster and equilibrium adsorption time is shorter than those reported in literature.