Adsorption Characteristics Of Cadmium By Organically Modified Clay Minerals And Its Immobilization Effect On Cadmium Contaminated Soil

The problem of heavy metal pollution with excessive cadmium content in natural environment is becoming more and more serious,which seriously threaten the ameliorating of environment quality.The removement of heavy metal cadmium hasbeen recently focused on in the research of environment science and engineer.Modified clay minerals are widely used as adsorbent in the remediation of heavy metals.However,the modification effect is different in different types of clay minerals and environment.The reason and mechanism of this difference have not been clearly identified,and should be furtherly investigated.In the present study,montmorillonite,kaolin and illite were selected to be organically modifiedwith three different types of organic substances such as potassium humate,chitosan and glycine.The adsorption performance and mechanism of organically modified clay minerals on cadmium in solution were studied through the analysis of mineral morphological characteristics and batch adsorption experiments,and the organic modified minerals with the best adsorbability on cadmium were selected.The passivation effect of the best modified mineral on Cadmium would be verified in polluted soil with the pot experiment.The main results are as follows:(1)Fourier transform infrared spectroscopy showed that glycine characteristic functional groups such as-CH₂-,C-N and COO-were coated into the surface of glycine modified sodium montmorillonite Na-M-G.The results of X-ray diffraction of sodium montmorillonite modified by chitosan indicated that the interlayer space of sodium montmorillonite with chitosan was successfully increased by 0.2 nm.The maximum cation exchange capacity of Na-M-HA modified by potassium humate is 2.31 mmol·g^-1.Small pores were formed on the surface of sodium montmorillonite modified by potassium humate and glycine with scanning electron microscopy,which made the structure loose,while the surface of chitosan modified sodium montmorillonite was more cohesive.The surface morphology of organically modified kaolin and illite was not changed.Compared with calcium based and sodium based clay minerals,the specific surface area of minerals modified by chitosan was slightly decreased,which was increases by 21.84%and 19.20%on average for the minerals modified by potassium humate and glycine.(2)The fitting results of adsorption kinetic model show that the adsorption process of organically modified clay minerals for Cd²⁺conforms to the quasi second-order kinetic model,and the adsorption equilibrium time is about 120 min.The isothermal adsorption results show that except that potassium humate modified sodium montmorillonite and calcium montmorillonite are more in line with Freundlich model,the adsorption process of other organic modified clay minerals for Cd²⁺is in line with Langmuir model.The theoretical adsorption capacity of sodium montmorillonite Na-M-HA modified by potassium humate for Cd²⁺is the largest,which is 73.72 mg·g^-1,which is 22.53%and 81.77%higher than that of the other two organic modified sodium montmorillonites,respectively.The thermodynamic model results show that the adsorption capacity of Na-M-HA for Cd²⁺increases with the increase of temperature,?H>0??G<0??S>0,indicating that the adsorption is spontaneous endothermic and the degree of chaos increases in the adsorption process.In addition,the increase of pH value of water can promote the adsorption of Cd²⁺by Na-M-HA;The increase of ionic strength and the valence of metal cations will inhibit the adsorption of Cd²⁺by Na-M-HA,indicating that ion exchange and complexation reaction are the adsorption mechanism of Cd²⁺by Na-M-HA.(3)Potassium humate modified sodium montmorillonite with 2%addition of Na-M-HA2had the best reduction effect on soil available cadmium and cadmium accumulation in lettuce,which was 36.02%and 56.81%lower than CK without addition.Compared with CK,Na-M-HA2 treatment increased the proportion of low bioactive oxidative cadmium and residual cadmium by 0.31%and 9.5%,respectively.The fresh weight of lettuce treated with Na-M-HA was 1.80-2.53 times that of CK treated without Na-M-HA.Potassium humate modified Na-M-HA can not only significantly increase the biomass of lettuce,promote the growth and development of root system,but also reduce the cadmium content in lettuce and the available cadmium content in soil.In conclusion,different organically modified sodium and calcium based clay minerals were prepared by loading different organics.Compared with sodium based and calcium based clay minerals,Na-M-HA modified by potassium humate not only increases the cation exchange capacity and specific surface area,but also significantly improves the adsorption capacity of cadmium;Appropriate addition of Na-M-HA can also promote the growth and development of lettuce in cadmium contaminated soil and reduce the bioavailability of cadmium in soil.Therefore,sodium montmorillonite modified by potassium humate has high environmental remediation ability and application potential.