Adsorption Efficiency And Associated Mechanisms Of Microplastics For Manganese In The Coexistence Of Cations With Different Valences

Both microplastics and heavy metal pollutants have the characteristics of refractory degradation,and microplastics are easy to adsorb heavy metal pollutants i n the water environment and have potential impacts.There are not only typical pollutants in natural water,but also some common inorganic ions,which will participate in the interaction between microplastics and heavy metals and affect their adsorption behavior.This study focused on the adsorption kinetics,adsorption isotherms and thermodynamics of Mn（II）adsorption by polypropylene（PP）microplastic particles under the condition of coexisting cations of different valence states and concentrations.The microscopic characteristics of plastics were analyzed,and the mechanism of the effect of coexisting cations on the adsorption of Mn（II）by PP particles was systematically explored.The results showed that the presence of Na⁺,Ca²⁺and Al（III）all inhibited the adsorption of Mn（II）on PP microplastics,and the adsorption effect of Mn（II）was enhanced with the increase of ion valence.An increase in the concentration of coexisting cations led to a decrease in the equilibrium adsorption capacity.According to the model analysis,under the influence of coexisting cation species,concentration and temperature,the adsorption kinetic model was more in line with the description of the pseudo-first-order kinetic model,that was,the adsorption of ions on microplastics was controlled by the diffusion step.The internal diffusion model showed that the adsorption process basically conform ed to the three stages of liquid film diffusion,intraparticle diffusion and dynamic adsorption equilibrium,and the control steps of the adsorption process were not unique.The Elovich model indicated that the diffusion process of adsorption was heterogeneous.Combining the Langmuir model and the Dubinin-Radushkevich adsorption equation（D-R model）with a high degree of fitting in the isotherm adsorption model,it was considered that the adsorption of Mn（II）on the PP surface conform ed to the monolayer adsorption theory of the Langmuir model and the of heterogeneous surface adsorption and micropore filling theory of the D-R model.The calculation results of thermodynamic parameters（ΔG,ΔS andΔH）indicated that the adsorption of Mn（II）on the PP surface was spontaneously endothermic and the disorder of the solid-liquid interface increased.TheΔG value between-20 k J/mol and 0 k J/mol meant that physical and weak chemical adsorption coexisted during the adsorption process.Combined with the fitting results of the internal diffusion model,it could be seen that the higher the concentration of coexisting cations,the greater the impact on the boundary layer effect,and the effect of Ca²⁺on the boundary layer is greater than that of Na⁺and Al（III）under the condition of low concentration of coexisting cations.Langmuir model parameters showed that the affinity between Mn（II）and PP particles decreased with the increase of ion concentration and valence,but the adsorption effect was always in a good state.According to the analysis of the isotherm adsorption model,the effect of 0.05 mmol/L Ca²⁺and Al（III）on the adsorption strength was higher than that of the temperature gradient in this experiment.However,the change of temperature,coexisting cation concentration and valence would not cause the adsorption of Mn（II）on PP microplastics to change from a physical effect to a chemical effect.The value of thermodynamic parameterΔG decreased with the increase of temperature,indicating that the increase of temperature was favorable for the occurrence of Mn（II）adsorption.It is speculated that the increase of temperature would make the molecular chains in the amorphous region of PP particles more disordered,and the increase of free volume would provide more adsorption sites for Mn（II）.At the same time,the increase of temperature also increase d the collision probability between Mn（II）and adsorption sites,which increased the equilibrium adsorption capacity.Combined with the adsorption efficiency and microscopic characterization analysis of Mn（II）under the condition of different kinds of coexisting cations,it was found that the effects of Na⁺,Ca²⁺and Al（III）on the adsorption of Mn（II）on PP microplastics were mainly physical,and Al（III）may also have some degree of chemical influence.The main principles of physical inhibition were:the competitive adsorption of physical forms;the weakening of the activity of Mn（II）led to the weakening of the electrostatic interaction between Mn（II）and PP;the compression of the extra-granular electric double layer of PP reduce d the attractive force between PP and Mn（II）;the aggregation of PP microplastics induced by high valence or high concentration of coexisting cations led to a decrease in the number of adsorption sites.In addition,since Al（III）had a great influence on the p H of the solution,the neutralization effect of H⁺on the negative charge on the surface of PP could not be ignored in the presence of Al（III）.The chemical inhibition principle was that Al（III）might preferentially combine with—COO^-in the form of complexation,thereby hindering the complexation of Mn（II）.In addition,when the Al（III）concentration reached 0.05 mmol/L,the liquid film diffusion and intraparticle diffusion stages could not be clearly distinguished,especially when the concentration was above 0.10 mmol/L,the adsorption of Mn（II）on PP microplastics could hardly be detected.This was because H⁺and Al（III）preferentially occupy the PP surface and surround ed the PP particles,hindering the chance of Mn（II）interacting with PP.When the Al（III）concentration was large enough and there was enough H⁺in the aqueous solution,the surface electrical properties of the microplastics would reverse to a positive value,resulting in the original electrostatic attraction between Mn（II）and PP particles becoming electrostatic repulsion and no adsorption could occur.In addition,the hydrolysis of Al（III）may formed a colloidal particles,so that Mn（II）adsorb ed with the colloid first and could not reach the PP surface.