The Modification Of Clayey Dolomite By Calcination And Carbonation And Its Performances To Remove Cadmium

Highly active porous materials composed of periclase and calcite nanocomposites(PCN)could be prepared after the calcination/carbonation treatment of clayey dolomite,which has great potential for heavy metal pollution control in water and soil.In this paper,the effect of coexisting steam and hydration pretreatment on the carbonation of calcined clayey dolomite was studied by fixed bed reactor and mass spectrum on-line monitoring,the structural transformation of clayey dolomite after calcination and carbonation was analyzed by XRD,FTIR,TG,SEM,TEM,BET.The effect of soild-liquid rate,initial pH,contact time,initial concentration on cadmium ions removal by PCN were investigated through batch experiments,meanwhile,the removal mechanism was discussed by XRD,FTIR,TEM characterization of reaction product.The ability of PCN continuously remove cadmium ions from water was investigated through dynamic tests.The results showed that:(1)The main component of clayey dolomite is dolomite,with a mass fraction of 86.42%,and showing a sub-micron rhombohedral structure,the secondary component is palygorskite,showing a nano-rod structure.(2)The initial and complete carbonation temperatures of calcined clayey dolomite under anhydrous conditions were 400~oC and 600~oC,respectively.The presence of 10%steam decreased the initial and complete temperatures by 100~oC,respectively.The complete carbonation temperature was reduced to 400~oC when the calcined clayey dolomite was hydrated and dried before carbonation.However,the calcined/hydrated clayey dolomite which not dried can be directly carbonated at room temperature,and the carbonation rate(newborn calcite mass fraction)firstly decreased,then increased with the increase of temperature in the range of 50-500~oC.(3)The calcination and carbonation treatment of clayey dolomite were accompanied by the increase and decrease of specific surface area,respectively,and the crystallite sizes of calcite and periclase produced after calcination/carbonation were about 35 nm and 30 nm,respectively,the newborn calcite displayed a higher thermochemical activity.(4)In the batch experiments of cadmium removal by PCN,the concentration of cadmium decreases and then becomes stable with the increase of soild-liquid ratio.The initial pH value of solution has a slight effect on the removal efficiency of cadmium ions by PCN,for different initial pH of cadmium solution,it all has good removal efficiency and the removal rate is all above 99.8%.The equilibrium time is longer when the initial concentration of cadmium is higher.The removal kinetic of cadmium ions follows the pseudo-second-order model equation(R~2?0.996).The Langmuir isothermal model can well describe the removal of cadmium ions by PCN.The thermodynamic results reveal that the adsorption belongs to chemical adsorption.The main mechanism of removing cadmium ions in aqueous solution by PCN is since magnesium oxide slowly reacts with water to produce magnesium hydroxide,and the reaction results in the increase of the solution pH value,inducing the hydrolysis of cadmium ions to produce the surface precipitation.(5)The breakthrough bed volume(BV)was 3744 and the saturation removal capacity was 247.3 mg/g when using PCN as dynamic column packing to continually treat cadmium wastewater.