Preparation And Adsorption Properties Of Porous Magnesium Silicate Crystals

Magnesium silicate as a green pollution-free adsorption material,high adsorption efficiency,is widely used in waste water,waste gas treatment,biodiesel refining,radioactive waste encapsulation and other fields.During the preparation of magnesium silicate,there are many factors affecting its structure and adsorption properties.This paper adopts precipitation-hydrothermal method,using heavy magnesium water from the hydration and carbonization of magnesite as the magnesium source,and sodium silicate as the silicon source to study the effects of raw material concentration,molar ratio of Si and Mg,reaction temperature,p H value of precursor solution,hydrothermal time,hydrothermal temperature and calcination temperature on the sample phase composition,microscopic morphology,specific surface area,etc.Three-dimensional flower-like porous magnesium silicate crystals with high specific surface area were prepared and represented by two typical heavy metal ions.The adsorption properties of the obtained porous magnesium silicate crystals to wastewater containing heavy metal ions such as Cu²⁺and Pb²⁺and oleic acid were investigated.The results show that（1）the optimum preparation conditions for porous magnesium silicate crystals are as follows:The concentration of Mg²⁺was 0.2mol/L,the concentration of Si O₃^2-was 0.3mol/L,the molar ratio of silicon to magnesium was 3:2,the reaction temperature was 70℃,the p H of precursor solution was 9±0.01,the hydrothermal temperature was 150℃,and the hydrothermal time was 12h.Under these conditions,three-dimensional flower-spherical micromesoporous porous magnesium silicate crystals with specific surface area of 539.9m²/g were prepared.The hydrothermal time and temperature have obvious influence on the morphology of magnesium silicate.When the hydrothermal time is extended to 48h,the magnesium silicate crystal changes from a granular packing structure to a three-dimensional network porous structure.When the hydrothermal temperature is increased to 150℃,the porous magnesium silicate crystal morphology is three-dimensional flower ball and the structure is complete.（2）The sample of magnesium silicate was calcined at 200℃～600℃,and the activated magnesium silicate crystal with porous structure was prepared.With the increase of calcination temperature,the crystal morphology changed from three-dimensional network structure to three-dimensional flower ball structure.With the increase of calcination temperature,the specific surface area of the sample first increases and then decreases.When the calcination temperature is 200℃,the specific surface area reaches the maximum of 564.38m²/g,which is 6.13%higher than that of the sample without calcination.When the calcination temperature is higher than 300℃,the porous crystal structure dissolves,the morphology tends to be granular accumulation structure,and the specific surface area decreases sharply.The calcination temperature has a certain influence on the pore structure.With the increase of the calcination temperature,the pore volume of the crystal increases first and then decreases.When the calcination temperature is 300℃,the pore volume of porous magnesium silicate crystal reaches the maximum value,0.365cm³/g.（3）The calcined porous magnesium silicate crystals have strong adsorption properties for heavy metal ions(Cu²⁺,Pb²⁺)and oleic acid.When the calcination temperature is 200℃,the adsorption capacity of Cu²⁺is the highest,which is 232.25mg/g.Compared with the inactive sample,the adsorption capacity was increased by 37.7%.The crystal activated at 400℃has strong adsorption capacity for Pb²⁺ion,and the saturation adsorption capacity is 194.22mg/g,which is 7.0%higher than that of the inactive sample.The fitting result（R²）of the quasi second-order kinetic model is closer to 1 than that of the quasi first-order kinetic model,and the theoretical equilibrium adsorption capacity(q_{e exp})calculated by the quasi second-order adsorption kinetics is closer to the actual measured equilibrium adsorption capacity(q_{e cal}).Therefore,the adsorption of heavy metal ions(Cu²⁺,Pb²⁺)by porous magnesium silicate crystal conforms to the quasi second-order adsorption kinetic fitting results.When simulating the adsorption of free fatty acids in vegetable oil,the crystal obtained by calcination at 300℃has strong adsorption activity,and the removal rate of oleic acid reaches 19.3%.