| Montmorillonite has been widely used because of its high cation exchange capacity,large surface area,chemical stability and mild effect on the environment.Perivous researches mostly focused on monovalent,divalent cations,for example:the adsorption of Li+?Cd2+?Cue+ions and their migration on montmorillonite,few about trivalent cations like Cr3+,which have been regarded as one of the most seriously polluted metal ions and extensively employed in industrial manufacture include leather tannery,electroplating,dyes,etc.Excess Cr3+ can do directly damage on metabolism of bodies,induce stimulation and erosion on skin,even lead to cancer.The mechanism of its adsorption and migration means great to both production and environment.In this paper,on the one hand,the changes of Ca-Montmorillonite(Ca-RM)before and after the adsorption of Cr3+,then thermal treatment under different temperatures(200??300??500??600??700?)and further rehydration have been studied;on the other hand,investigated the changes of the modified Ca-M by hexadecyl trimethylammonium bromide(CTAB)at three different cation exchange capacity(CEC)coverages before and after the adsorption of Cr3+,and migration under at 250?.At last,the desorption of Ca-M,the modified Ca-M after the adsorption of Cr3+at pH=3 and pH=7 were evaluated.The results about Ca-M showed that Cr3+ was in the form of[Cr(H2O)6]3+existing in the interlayer of TOT sheet by replacing Ca2+.When heated under 500?,the d001 decreased,the OH stretching and the mid-wavenumber regions shifted to higher wavenumbers and ESR signals enhanced.After rehydration,d001 increased,suggested that the structure could still be partly swollen as FTIR spectra shifted to lower wavenumbers and ESR signals did change compared to the heated samples but without rehydration.Those facts indicated some Cr3+ might enter into the hexagonal cavities.When the temperature were above 500?,a new phase,i.e.eskolaite(Cr2O3),occurred in the product according to XRD patterns.The decreasing of the 001 diffraction to 0.97nm was similar to the height of TOT sheet and 060 diffraction turning from sharp and narrow into weak and broad suggested that the montmorillonite broke down to finer particles after heating and might form a phase similar to pyrophyllite.The fact that Cr3+ migrated into the deep the hexagonal cavities was suggested by some OH stretching vibrations shifted slightly to lower wavenumbers and intensity changes of signal at g= 1.97 at their ESR spectra after rehydration.The results about the modified Ca-M presented that the value of d0011 increased greatly and the adsorption capacity declined with the CTAB arranged in various ways.According to the ICP-AES test and FTIR results,3564 cm-1 was assigned to Cr3+Cr3+OH and did not transform when heated at 2500C,indicating that Cr3+ replaced octahedral Al3+ partly.Simultaneously,together with the change of g-factor value in the ESR spectra,all those demonstated that part of Cr3+ ions removed to the hexagonal cavities and partial octahedron site.As to desorption,the content of Cr3+ in acidic the solution were significantly more than alkaline solution in the results of desorption,which is related to the exchange between H+ and Cr3+ in the surface,the collapse of M layers due to H+ bonded with structural hydroxyl or surface O2-on M.While,Cr3+ reacted with OH-in alkaline solutions,end up with polymeride.lt concluded that alkaline conditions was preferred to acidic conditions to store M after the adsorption of metal ions Cr3+,for CTAB-M is better. |
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