| Heavy metals are toxic,hard-degradable and easy-accumulated in organisms,thus constitute great harm to ecological environment.As an environmental friendly and easy-operated treatment technology,adsorption has been used in the treatment of heavy metals and other pollutants.Sepiolite,a natural clay mineral,has large reserves,low price,good adsorption and ion exchange performance,is one of the ideal environmental adsorbents.It is of great significance to develop modified sepiolite adsorbents for heavy metals treatment by using sepiolite's unique chain layered structure.Based on the review of the properties and structural characteristics of the original sepiolite and the application of the modified sepiolite in the control of heavy metal pollution,this paper investigated the adsorption performance of the original sepiolite for a heavy metal ion Cd(?) by compared with another chain layered clay mineral palygorskite to analyze the differences of the adsorption performance and mineral structure between them.The alkaline sepiolite was prepared by the molten salt method,and then the adsorption performance for Cd(?) ,adsorption mechanism and recycling effect were analyzed.Furthermore,the adsorption performance and mechanism of alkaline sepiolite for Cd(?) and phosphate P(V)in a binary pollution system was explored.The main results are as follows:(1)The adsorption capacity of primitive sepiolite to Cd(?) (18.63 mg/g)was better than palygorskite(12.99 mg/g).The adsorption of Cd(?) by the original sepiolite conforms to the quasi-second-order kinetic equation.X-ray diffraction(XRD),fourier transform infraid spectroscopy(FTIR),x-ray photoelectron spectroscopy(XPS),specific surface and pore structure analyzer(BET)and solid-state nuclear magnetic characterization(NMR)showed that the number of octahedral sheets in the sepiolite unit was more than palygorskite unit,which may be the reason why sepiolite has a larger specific surface area and more surface hydroxyl groups than palygorskite;the mechanism of primitive sepiolite adsorption of Cd(?) is that Cd(?) forms a complex with the silicon hydroxyl group on the surface of sepiolite(2)Alkali etching sepiolite was prepared by molten salt method,and the best conditions for preparing alkali etching sepiolite were determined as follows: the best powder ratio was sepiolite: odium carbonate: odium nitrate = 1:0.7:5.0;the best calcination time for preparing alkali etching sepiolite was 2 h,and the best calcination temperature was 350 ?.XRD,BET,FTIR and NMR were used to characterize the structure of alkali etched sepiolite.The results showed that the molten salt method did not destroy the crystal structure of sepiolite,formed defects and cross-sections on the surface of sepiolite,and increased the specific surface area of sepiolite.The adsorption experiment showed that the adsorption capacity of alkali etched sepiolite to Cd(?) is 33.30 mg/g,which is 44% higher than that of original sepiolite.After three cycles of adsorption desorption,the removal rate of Cd(?)by alkali etching sepiolite did not decrease obviously,which indicated that the adsorption regeneration performance of alkali etching sepiolite was good.(3)In the binary system of Cd(?) and phosphate P(V),the adsorption capacity of alkali etched sepiolite to Cd(?) and P(V)was higher than that of Cd(?) or P(V)in the single system.The adsorption kinetics experiment showed that the existence of P(V)shortened the equilibrium adsorption time of Cd(?) .The p H edge experiment of binary system showed that a small amount of Cd(?) was adsorbed at low p H,which indicated that the adsorption of P(V)provided the adsorption site for Cd(?) .XPS results showed that in the binary system,Si-OH of sepiolite etched by alkali adsorbed Cd(?)and P(?)through ligand exchange.Compared with the single system,the binding energy of Cd(?)in the binary system changed significantly,which showed that in addition to ligand exchange,Cd(?)could be adsorbed by alkali-etched sepiolite on its surface through forming ternary complex with P(?). |
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