| Rare earth elements,known as the"seasoning"of contemporary industry,are important strategic and non-renewable resources In the process of production and development,a large amount of wastewater contained rare earth ions is generated,resulting in the waste of a large number of rare earth resources,and also seriously threatening the ecological environment.Recycling rare earth elements from wastewater can not only promote the sustainable development of rare earth resources,but also reduce environmental pollution.Adsorption method has become a popular technology for separating and recycling of rare earth resources because of its advantages of high efficiency,simple operation and broad application prospect.However,this technology suffers from the low adsorption capacity and weak selectivity,thus limiting the development.In addition,rare earth wastewater usually contains highly concentrated ammonium（NH4+）,which brings adverse effects on the separation and recovery of rare earths.Therefore,the development of a material with high adsorption capacity and strong selectivity is the key to achieve the separation and recovery of rare earth resources from wastewater.In this study,the selective adsorption and recycler of rare earth elements was carried out by constructing functional MOFs composite materials.（1）The functional MOFs material（MIL-101（Cr）-NH-DTPA）was compositedby grafting diethylenetriamine pentaacetic acid（DTPA）onto MIL-101（Cr）-NH2 surface through amidation reaction under hydrothermal condition using MIL-101（Cr）-NH2 as carrier and DTPA as functional modifier.Based on the analysis of field emission scanning electron microscopy（SEM）,N2 adsorption-desorption isotherms and thermogravimetry（TG）,the DTPA can be embedded into the porous structure of MIL-101（Cr）-NH2,and the corresponding specific surface area decreased from 2044 m2/g to 980 m2/g,but did not change the overall morphology and structure.In addition,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FT-IR）and X-ray diffraction（XRD）analysis showed that-COOH on DTPA reacted with primary amine on the surface of MIL-101（Cr）-NH2 to form-CONH directly connected with C-N bond,but did not change the main element composition and valence state of the carrier.Meanwhile,the content of-COOH on the surface of MIL-101（Cr）-NH2 increased from 0.008 to 0.357mmol/g after DTPA was introduced.（2）The effects of preparation temperature and DTPA capacity on the adsorption properties were investigated.The low temperature（40℃）could not meet the kinetic requirements of amidation reaction,and the equilibrium reaction was shifted to the unfavorable direction for the formation of amide groups.However,when the temperature was too high（80℃）,a part of-NH2 was oxidized to-NO2,which results in the decrease of reactive sites and the degree of amidation.Although excessive DTPA can increase the-COOH content,it can diminish the specific surface area and pore volume of the material,thereby reducing the active sites for the adsorption of rare earth elements.When the ratio of DTPA to MIL-101（Cr）-NH2 was set to 1:1,MIL-101（Cr）-NH-DTPA compositedat 60℃had the highest adsorption capacity for La（Ⅲ）,Eu（Ⅲ）and Er（Ⅲ）,which were 69.78,103.01 and 83.41 mg/g,respectively.（3）The separation and recycle mechanism for rare earth elements by MIL-101（Cr）-NH-DTPA composite was elucidated.The adsorption of rare earth elements was in accordance with Langmuir monolayer adsorption and can be considered as a reaction process of spontaneous heat absorption and entropy increment.Under acidic conditions,rare earth ions can chelate and exchange with-COOH and-NH2,respectively,where chelation is the main mechanism of action for the separation and recycleof rare earth elements using MIL-101（Cr）-NH-DTPA.（4）The performance and mechanism of the functionalized composite MIL-101（Cr）-NH-DTPA for the separation and recycling of rare earth ions from wastewater with rare earth ammonia nitrogen were explored The results showed that the removal rates of La（Ⅲ）,Eu（Ⅲ）and Er（Ⅲ）achieved by MIL-101（Cr）-NH-DTPA were 73.5%,88.9%and 82.9%,respectively,under the condition without NH4+and the optimal dosage of1 g/L.And the corresponding adsorption capacities were 8.01,7.27 and9.17 mg/g,respectively.In addition,the removal rates of element removal were studied.The results showed that the removal rates of La（Ⅲ）,Eu（Ⅲ）and Er（Ⅲ）by MIL-101（Cr）-NH-DTPA were still above 70%when the coexistence of NH4+in rare earth solution was ranged from100 to1500 mg N/L.Ft-IR and XRD analysis showed that the adsorption of La（Ⅲ）,Eu（Ⅲ）and Er（Ⅲ）by MIL-101（Cr）-NH-DTPA was still achieved by the formation of complexation ligands with rare earth elements in the presence of highly concentrated NH4+,while NH4+did not occupy the corresponding complexation sites.In this study,DTPA-modified MIL-101（Cr）-NH2 material was composited,which addesses the problems of low adsorption capacity and poor selectivity of traditional MOFs.It could be used for the selective separation and recycle of rare earth elements in rare earth wastewater with NH4+,providing methodological guidance and theoretical reference for the recycling of rare earth resources by adsorption method.38 figures,16 tables,106 references... |
