Adsorption Mechanism Of Ethidium Bromide On Non-expanded Illite And Expanded Sodium Montmorillonite

The problem of water environment pollution is becoming increasingly prominent,among those many pollutants,there is a type of pollutants that can interact with DNA.It has strong mutation and teratogenicity,and can accumulate in the organism with the food chain and cause irreversible life,so it is especially important to remove such pollutants.As a representative of such pollutants,ethidium bromide is extensively used as a DNA dye in biological and chemical laboratories,which is often discharged directly into the sewer without treatment,but because of its biological resistance,which cannot be effectively removed by microbial degradation in water treatment plants and the environment.In order to remove ethidium bromide quickly,conveniently,and economically,two clay minerals with different properties,non-expanded illite and expanded sodium montmorillonite,were used in this study.Through the performances of illite and sodium montmorillonite on removing ethidium bromide in aqueous solution,the influence of environmental factors on the adsorption of ethidium bromide on two clay minerals,and through the characterization experiments of clay minerals before and after adsorption,the adsorption behavior and mechanism were explored.Experimental results show that when the illite adsorbs ethidium bromide with a concentration of 0.25 mmol/L,it can reach a removal rate of nearly 100%.As the concentration of ethidium bromide increases,the removal rate gradually decreases.When its concentration reached 5.0 mmol/L,the removal rate decreased to 10%.In the process of ethidium bromide adsorbing by illite,the reaction rate was faster in the first hour.With the increase of the adsorption time,the removal rate increased and the reaction rate gradually decreased.After 8 hours,the adsorption basically reached equilibrium.By increasing the ambient temperature of the illite adsorption from 25℃to 55℃,the removal rate of ethidium bromide increased from 19%to 21%.When the p H of the reaction solution increased from 1 to 11,the removal rate of ethidium bromide at a higher level of hydrogen ion concentration（p H=1）is about 19%.After the p H of the solution increases,that is the hydrogen ion concentration decreases,the removal rate of ethidium bromide gradually increases,and after the p H increases to 7,the removal rate stabilizes at about 22%;In the case of adding disparate orders Na⁺of ionic strength to the ethidium bromide solution,when the solution has 0.001 mmol/L ionic strength Na⁺,the adsorption removal rate of ethidium bromide is 22%.When the Na⁺ionic strength in the solution increases to 1.0 mmol/L,the removal rate of ethidium bromide decreased to 19%.The removal rate of sodium montmorillonite reached nearly 100%when the ethidium bromide solution with an initial concentration of 2.0 mmol/L.When the concentration of the ethidium bromide solution increased to 12.0 mmol/L,the removal rate decreased to 45%.Sodium montmorillonite adsorption of ethidium bromide occurred mainly within the first half of hour,and the adsorption rate remained basically unchanged after half an hour.After increasing the ambient temperature of the sodium montmorillonite adsorption from 25℃to 55℃,the removal rate of ethidium bromide increased by 7%.When the p H increased from 1 to 11,whether in the acidic conditions where the concentration of coexisting cations（H⁺）in the solution is at a higher level,or in the alkaline conditions where the coexistence of cations is at a lower level,the ethidium bromide adsorption rates remained at 83%.In the case of adding different orders Ca²⁺of ionic strength to ethidium bromide solution,when the solution has 0.001mmol/L ionic strength,the ethidium bromide adsorption removal rate is 83%,and when the ionic strength in the solution increase to 1.0 mmol/L,the ethidium bromide removal rate was reduced to 65%.Based on the adsorption data of illite and sodium montmorillonite for ethidium bromide,the adsorption kinetics,isotherm,and thermodynamic fitting studies were conducted.The adsorption of illite and sodium montmorillonite on ethidium bromide both fit well to Pseudo-second-order kinetics and Langmuir isothermal adsorption models,respectively.The adsorption of ethidium bromide by illite is an exothermic reaction with spontaneous increase in degrees of freedom.The adsorption capacity is0.10 mmol/g,the initial adsorption rate is 0.136 mmol/（g.h）,and the adsorption rate constant is 15.042 g/（mmol.h）;The adsorption of ethidium bromide by sodium montmorillonite is an endothermic reaction with spontaneous increase of freedom.The adsorption capacity is 1.09 mmol/g,the initial adsorption rate is 28.986 mmol/（g.h）,and the adsorption rate constant is 28.618 g/（mmol.h）.By detecting the four main desorbed cations of potassium,calcium,sodium,and magnesium in the reaction solution after adsorption,it was found that the amount of desorbed cations in the adsorption of illite and sodium montmorillonite is proportional to the amount of adsorbed ethidium bromide,which is directly proportional to the existence of a cationic adsorption mechanism for both of the ethidium bromide adsorption.The main types of desorbed cations are different due to the different materials.In the results of fourier transform infrared spectroscopy,the characteristic peaks of ethidium bromide were polarized on the illite and sodium montmorillonite after the adsorption,indicating that a strong interaction occurred during the adsorption process.Scanning the illite and sodium montmorillonite before and after the adsorption in the range of 2°～26°of the X-ray diffractometer,and judging the change of the interlayer distance by the change in the d₀₀₁ peak.It was confirmed that the interlayer spacing of illite before and after adsorption did not change,and the sodium montmorillonite layer spacing increased from 13.44?when the concentration of the ethidium bromide was 0mmol/L to 16.73?at 6 mmol/L,reaching the maximum interlayer expansion,and thereafter it does not change with increasing concentration.In the thermogravimetric analysis,the adsorption amount of ethidium bromide is small,so that the overall mass of illite after adsorption is reduced less after temperature rise,and the thermal cracking temperature does not change compared to the original ethidium bromide,indicating that the adsorption occurs on the outer surface.The sodium montmorillonite exhibited a mass loss peak at 590℃when adsorbing high concentration ethidium bromide,which was 58℃higher than the original ethidium bromide,this is because part of the ethidium bromide is adsorbed on the inner surface of sodium montmorillonite and enters the interlayer to increase the thermal cracking temperature.However,this position peak did not appear when ethidium bromide concentration was low,indicating that the adsorption mainly occurred on the outer surface at this time.