Influence Of Modification Conditions On Adsorption And Stabilization Of Heavy Metals By Zeolites

Heavy metals in water and sediment will play a toxic role in aquatic animals and plants,and can also endanger human health through food chain enrichment and bioaccumulation.With the development of industry and the exploitation of metal resources,the treatment of heavy metal pollution in sediment has become one of the important subjects of environmental protection in China.At present,most of the river or lake sediment with heavy metal pollution are treated with ectopic treatment,and the contaminated sediment is stabilized and stabilized after dredging and dewatering.Zeolite is an ideal curing agent because it has the characteristics of the large specific surface area,high porosity and strong ion exchange capacity.China has abundant zeolite resources,large reserves and low cost,but there are a few shortcomings and limitations in the adsorption and removal of heavy metals by natural zeolite.In this experiment,natural zeolite was modified to improve its solidification and stabilization effect on heavy metals in sediment.The influence of different modification methods on the adsorption of heavy metals by zeolite was studied.The modification mechanism was analyzed,and the best scheme of zeolite modification was explored.In this experiment,natural zeolite was chosen as the experimental raw material and modified by various methods.The effects of modifier,zeolite particle size,calcination temperature and modification order on the adsorption of heavy metal ions Cd²⁺,Zn²⁺and Cu²⁺on the modified zeolite were studied by adsorption test.The best zeolite modified method was selected.The adsorption process was fitted by kinetic and thermodynamic models.Characteristics of the modified zeolite were characterized by BET specific surface area,scanning electron,Fourier infrared spectrum and X ray diffraction.The adsorption mechanism of modified zeolite was analyzed.The modified zeolite was used for the solidification test of heavy metal contaminated sediment.TCLP toxicity leaching test and the BCR extraction test were used to evaluate the curing effect of the modified zeolite on the heavy metals in the sediment and determine the optimum dosage of the modified zeolite.The main conclusions are as follows:?1?The best modification scheme for Cd²⁺and Zn²⁺removal is that the natural zeolite with particle size 2-4mm is roasted at 600 C in temperature,and then modified with modifier NaOH.The maximum removal rate of Cd²⁺and Zn²⁺in the solution can reach 95%and 79%respectively.The best modification scheme for Cu²⁺removal is to modify lanthanum chloride as a modifier for the natural zeolite with particle size of 0.5-1mm,and the removal rate of Cu²⁺in the solution can reach 65%.?2?The results of thermodynamic adsorption isothermal model show that the adsorption of heavy metal ions Cd²⁺,Zn²⁺and Cu²⁺is in agreement with the Langmuir adsorption isotherm model.Depending on the equation of Langmuir adsorption isotherm,the maximum adsorption capacity of modified zeolite for Cd²⁺,Zn²⁺and Cu²⁺is 25.256mg/g,22.157mg/g and 21.097mg/g respectively.The adsorption kinetics analysis showed that the adsorption process of Cd²⁺,Zn²⁺and Cu²⁺in the modified zeolite was in line with the quasi two order kinetic model.The equilibrium adsorption capacity of the modified zeolite was 10.193mg/g,8.851mg/g and 8.667mg/g respectively in the concentration of 100mg/L.?3?The results of TCLP toxicity leaching test and BCR extraction test showed that the optimum application amount of the modified zeolite cured by Cd and Zn in the sediment was10%,and the optimum application amount of the modified zeolite cured by Cu was 15%.Under the optimal application ratio,the leaching amount of Cd is reduced from 0.122mg/kg to0.055mg/kg,and about 50%is converted into residue state in the form of acid extractable state and reducible state in the form of Cd.The leaching amount of Zn is reduced from 24.9mg/kg to 10.23mg/kg,and 40%are converted to residue state in the presence of acid extractable state,and the leaching amount of Cu is reduced by 5.21mg/kg.Up to 0.62mg/kg,about 55%of the acid extractable and reducible were transformed into oxidizable and residual state.?4?The mechanism of adsorption of heavy metal ions was analyzed by X ray diffraction,BET specific surface area and Fourier transform infrared spectroscopy.It was found that the modification process did not destroy the basic skeleton structure of the zeolite,but it would cause some O-H or Si-O to disconnect and combine with the modifier to produce new chemical bond.This is conducive to the substitution of Na⁺or La³⁺for zeolites.High temperature modification will remove some impurities in the zeolite structure and make some zeolite bound water evaporate.These reasons make the specific surface area,pore volume and pore size of the zeolite improved in varying degrees after modification,which is more conducive to the entry of heavy metal ions into the structure of zeolite,so the adsorption effect is better.