The Study Of The Effects Of Particle Size And Morphology On Adsorption Thermodynamics And Kinetics Of Nano-MgO

Nanoparticles are widely used in catalysis,environmental protection,biology,and other fields due to their excellent adsorption properties.The adsorption properties of nanoparticles mainly depend on their size and morphology.Because the particle size and morphology of nanoparticles determine their adsorption kinetic parameters and thermodynamic properties which determine their adsorption properties.However,the influences of nano-MgO particle size and morphology on the adsorption properties of malachite green and Pb²⁺are still unclear,and it need to be explained by the influence mechanisms of nano-MgO particle size and morphology on its adsorption thermodynamic properties and kinetic parameters.（1）Theoretical analysis of nano-adsorption:The nano-adsorption theory is based on the assumption that the nano-adsorption process is similar to a chemical reaction.The adsorption kinetics theory accords with the transition state theory of chemical reaction,and the transition state is not affected by the particle size,and the relations between particle size and adsorption kinetic parameters were obtained,and discussed the effect of particle size on the kinetic parameters.In the adsorption thermodynamics,the chemical potentials of nano-adsorbent and the adsorption product were defined to derive the relations between the adsorption thermodynamic properties and the particle size of different morphologies,and the influence regularities and the mechanism of particle size on thermodynamic properties of adsorption were discusse.（2）Experimental study of nano-adsorption:The particle size of nano-MgO has a significant effect on the kinetic parameters of the adsorption of malachite green and Pb²⁺,and the effect of these two adsorption systems are consistent.With the decrease of particle size,the adsorption rate constant k increases,while both the activation energy E_a and the pre-adsorption factor A decrease.Furthermore,lnk,E_a,and lnA all show a good linear relationship with the reciprocal of particle size.These experimental results are consistent with the above theoretical analysis of nano-adsorption kinetics,the impact mechanism is:the molar surface of the nano-particles affects the activation energy E_a of the adsorption process,the molar surface entropy affects the pre-exponential factor A of the adsorption process,the molar surface mole and the molar surface The entropy affects the adsorption rate constant k of the adsorption process.The results are as follows:（1）The preparation of nano-MgO:by sol-gel method,using Mg（CH₃COO）₂·4H₂O as magnesium source,H₂C₂O₄·2H₂O as complexing agent and acidic reagent,and dodecyltrimethylammonium bromide（CTAB）as dispersing agents,changing different experimental conditions to prepare nano-MgO with different particle sizes and morphologies,the average particle size of spherical nano-MgO are 40.4 nm⁹0.2 nm,and the average equivalent size of cubic nano-Mg O is 59.1 nm¹14.7 nm.The concentration of Mg（CH₃COO）₂·4H₂O,reaction temperature,calcining temperature and calcining time have significant effects on the particle size.The influence regularities are that the particle size decreases with the increase of the concentration of Mg（CH₃COO）₂·4H₂O,and increases with increasing reaction temperature,calcination temperature,and the calcination time.Among them,the reaction temperature has the greatest influence on the particle size.（2）The influence regularities of particle size and morphology on the kinetic parameters of adsorption:the particle size of nano-MgO has significant effect on the kinetic parameters of malachite green and Pb²⁺adsorption.With the decrease of particle size,the adsorption rate constant k increases,while the adsorption activation energy E_a and the logarithm of pre-adsorption factor lnA decrease,and linearly vary with the reciprocal of the particle size,respectively.These experimental results are consistent with the above theoretical analysis of nano-adsorption kinetics.The influence mechanisms are that:the molar surface of the nano-particles affects the adsorption activation energy E_a,the molar surface entropy affects the adsorption pre-exponential factor A,the molar surface enthalpy and the molar surface entropy both affects the adsorption rate constant k.（3）The influence regularities of particle size and morphology on the thermodynamic properties of adsorption:the particle size and morphology of nano-MgO have significant effects on the thermodynamics of malachite green and Pb²⁺adsorption.As the nano-MgO particle size decreases,K^o-increases,（35）_aG_m^o-,（35）_aH_m^o-and（35）_aS_m^o-both decrease;and them are each a good linear relationship with the reciprocal of the particle size.For nano-MgO with different morphologies but the same size,the adsorption thermodynamic properties are determined by both the specific surface area and the size of the adsorptive force of the adsorbate.The mechanism of influence of particle size on thermodynamic properties of adsorption are that（35）_aG_m^o-andln K^o-is influenced by the molar surface area and the difference between the surface tension before and after the adsorption,（35）_aS_m^o-by the molar surface area and the difference in temperature coefficient of surface tension,（35）_aH_m^o-by the molar surface area,the difference between the surface tension before and after adsorption and the difference between the temperature coefficient of surface tension before and after adsorption.