Adsorption and surface reaction properties of synthesized magnetite nano-particles

Abstract: The surface chemistry of inorganic materials is of great significance in a number of industrially important processes such as separation of ore by flotation, catalysis, water purification, paper coating and pharmaceutical industry. The purpose of this study has been firsly to develop a method for optimal synthesis of magnetite nanoparticles and secondly to utilize these particles as adsorbent in order to investigate their adsorption/desorption properties. The magnetite nanoparticles were synthesized by coprecipitation of FeCl2 and FeCl3 in alkaline media. The precipitated magnetite was analysed with XRD, TEM and FTIR spectroscopy. For evaluation of the mechanism of magnetite formation via coprecipitation method, iron isotopic measurement was applied and compared with magnetite produced from oxidation of ferrous hydroxide (paper 1). No fractionation of iron isotopes was observed for the magnetite synthesized by coprecipitation, whilst the magnetite formed from ferrous hydroxide showed higher abundance of 54Fecompared to 56Fe in the beginning of the reaction. The synthesized magnetite was coated with a primary layer of oleate and subjected to high temperature in air and argon atmosphere (paper 2).Oleate was selected as a model for Atrac which is a collector used for separation of apatite from magnetite. A combination of thermal analysis and infrared spectroscopy method were used in this study. It was found that calcination of the magnetite-oleate system in air involves oxidation of the double bond of oleate and formation of intermediate oxygen-rich molecules. Thermal decomposition of magnetite coated with a primary layer of oleate under argon atmosphere exhibits two steps weight loss. The first step at ~330oC is associated with oleate desorption/decomposition and an enthalpy change of ÄH = 49.86 J/g. Another weight loss occurs at elevated temperature (740oC) leading to partial reduction of magnetite to wustite and iron. In another work, the synthesized magnetite was deposited over an ATR internal reflection element to study adsorption of carbonate and sulphate anions in-situ. It was concluded from the IR spectra that there are two carbonate species on the surface at pH=8, one tightly bond carbonate as inner sphere complex with monodentate binuclear geometry and the other one is a loosely bond outer-sphere hydrogen bonded carbonate. Adsorption of sulphate was also studied using in-situ ATR spectroscopy (paper 3). Three maxima at 1115, 1044 and 979 cm-1 were observed, based on second derivative spectral method analysis. From the adsorption isotherm, the Langmuir affinity constant, K, was estimated to be 1.2344 x 104 M-1 at pH=4, implying a ∆G0ads= - 33.3 KJ/mol at T= 298oK. The kinetic of adsorption showed an initial fast adsorption especially at higher concentrations, eventually reaching an equilibrium plateau value. The calculated pseudo first order rate constant was (0.09± 0.01) min-1.

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