Study On The Functionalized MOF(Metal-Organic Framework)Based Adsorbents

Uranium(U(?))is a highly toxic species released from mining and industrial activities.The closure of several nuclear facilities has led to a lot of uranium contaminated sites in many countries.Exposure to uranium can lead to increased cancer risk,and/or liver damage.Chronic ingestion can lead to internal irradiation and chemical toxicity.Among the many types of the heavy metals,mercury(Hg(?))is considered to be exceedingly hazardous in nature even at very low concentration due to the accumuLative properties of methyl mercury in the body and its toxic effects around the natural environment.Efficient removal of hazardous materials from the environment has become an important issue from a biological and environmental standpoint.Recently,porous metal-organic framework(MOF)materials have been very promising in the adsorption/separation of various liquids and gases due to their unique characteristics.Various targets MOF were prepared by hydrothermal synthesis and solvothermal synthesis method.Different functional groups were grafted onto different MOF by post-synthesis modification or functional adsorbents were prepared for the adsorption of uranium and mercury in water.Objective:By comparing the adsorptive properties of the adsorbents before and after modification toward uranium or mercury ions,it has been determined whether the adsorptive capacity of the functional materials for target ions is better than that of the raw materials.In the static adsorption system,single experimental variable is controlled to determine the optimum adsorption pH of the adsorbents.The adsorption kinetics of functional materials for target ions has been studied in detail.The selective adsorption capacity of the materials before and after modification has been compared and a more convenient method for purifying heavy metal ions in wastewater has been further explored.Methods:Various targets MOF has been prepared byhydrothermal synthesis and solvothermal synthesis method.Different functional groups are grafted onto different MOF by post-synthesis modification or functional adsorbents are prepared for the adsorption of uranium and mercury in water.The concentration of uranium and mercury in solution is determined by trace uranium analyzer and ICP-OES.The change of material structure and morphology has been observed by transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The change of lattice structure of adsorbent is detected by XRD,the types of functional groups are determined by FTIR,the coupling of elements and functional groups in adsorbent is detected by XPS.Results:1.The metal-organic framework material functionalized by grafting amino group of tetraethylenepentamine(TEPA)on the coordinative unsaturated Cr(?)centers is described.Significantly,MIL-101-TEPA 60%exhibits high adsorption capacity(350mg/g)for removal of uranium(?)from water at pH 4.5.At uranium concentration<20mg/L,the isothermal plot is best represented by Freundlich model.At U(VI)concentration approximately>30mg/L,the isotherm is best described by Langmuir model.2.A novel amino-functionalized carbon material derived from MOF-5 has been synthesized successfully via a facile and fast method of short-term high temperature treatment.The resulting material of the amino-functionalized MOF-5(TEPA-C-MOF-5)exhibits high adsorption capacity(550mg/g)for uranium and the adsorbent can reach the adsorption equilibrium at a relatively low pH(3.5).Moreover,the formation of surface carbonized layer under short-term high temperature can improve the stability in water or humidity.Compared with UiO-66(Zr-based MOF),it is founded that only MOF-5(Zn-based MOF)can be introduced into amino group by this method.3.Thiol-functionalized metal-organic frameworks(MIL-101-SH and UiO-66-SH)has been synthesized by a post-synthetic modification method as the proper adsorbents for Hg(?)removal from water.This facile method for the synthesis of UiO-66-SH is developed via condensation reaction between cysteamine and carboxyl groups present in the framework of UiO-66-COOH.MIL-101 is functionalized by grafting amine group of cysteamine on coordinatively unsaturated chromium ions centers,yielding toMIL-101-SH adsorbents.The maximum adsorption capacities of mercury ions for UiO-66-SH and MIL-101-SH adsorbents are 110mg/g and 250mg/g at pH of 5,respectively.Moreover,acidic medium could effectively elute Hg(?)and the adsorbents can be well reusable.4.A novel monolith ZnS-ZIF-8 adsorbent has been designed and developed for Hg(?)removal.The ZnS adsorbent is prepared by the sulfurization of ZIF-8 nanoparticles in situ grown on the surface of ZIF-8-filter paper.The resulting of novel monolith ZnS-ZIF-8 adsorbent shows outstanding capture efficiency toward Hg(?)under the condition of pH?5 within very short time and it can be properly reclaimed after a simple washing by aqueous sodium sulfide.Conclusions:1.TEPA functionalized MIL-101 contains a large number of amino functional groups which can coordinate with radioactive uranyl ions.When the mass ratio of TEPA to MIL-101 is 60%,the adsorption capacity of functionalized metal-organic framework can reach to 350mg/g,which is significantly higher than that of unmodifiedMIL-101.Because of the acid resistance of the material,the material did not dissolve after the low pH analytic experiment.2.A novel amino-functionalized carbon material has been synthesized via a high eff-icient method of short-term high temperature treatment and used as adsorbents for removal of U(?)from aqueous solutions.It could be summed up as follows:(1)The absorption capacity of U(?)is enhanced by the introduction of amino groups in MOF-5.(2)The adsorption equilibrium point is reached at a relatively low pH of 3.5 from wastewater.(3)20%TEPA-C-MOF-5 adsorbent exhibits highly efficient removal U(?)with an adsorption time of less than 20min.(4)After surface carbonization by a short-term high temperature treatment,TEPA-C-MOF-5 adsorbent derived from MOF-5 becomes significantly less sensitive to water.(5)This method of introducing amino is only suitable carboxylic acid based MOF.(6)By comparing with the two kinds of carboxylic acid based on MOF,it is found that TEPA can evaporate during high temperature carbonization process if themetal ions in material possess relatively weak coordination ability with amino groups.3.Cysteamine has been selected to modify the two types of acid-resistant MOF through a post-synthetic modification method and uses as adsorbents for Hg(?)removal from water.The absorption capacity of Hg(?)is enhanced by the introduction of thiol groups in the frameworks.Thiol-functionalized UiO-66-SH andMIL-101-SH adsorbents exhibit highly adsorption capacity(110mg/g and 250mg/g,respectively)for enhancing Hg(?)removal at pH of 5.Possessing good stability and reusability,UiO-66-SH and MIL-101-SH are promising materials as adsorbent in the industrial waste water treatment without secondary pollution.The adsorption efficiency of these modified materials is not declined significantly after regeneration.4.A novel monolith ZnS-ZIF-8 adsorbent grown in situ on a filter paper for Hg(?)removal based on cation exchange.The monolith ZnS-ZIF-8 shows outstanding capture efficiency toward Hg(?)under the condition of pH?5 in extremely short time and it can be reclaimed after a simple washing by aqueous sodium sulfide.In the short time of filtration process,wastewater containing Hg(?)has been separated by the monolith ZnS-ZIF-8 adsorbent without causing secondary pollution.Therefore,this novel monolith ZnS-ZIF-8 is a type of promising adsorbent for Hg(?)capture from industrial contaminated water.Further work will provide a new perspective to design eff-icient mercury scavenger by constructing hybrid structure of filter paper and adsorbent.