Preparation Of Amidoxime-functionalized Mesoporous Silica And Its Adsorption Properties Of Uranium

As can be chelated with uranium to form complexes selectively the adsorbentscontaining amidoxime groups are considered the best materials for extraction ofuranium from seawater. However, at present adsorbents of this kind are mainlyexisting in forms of fibers and resins, accompanied with a serises of problems, such asslow adsorption rate, poor hydrophilic, low adsorption capacity, etc. Application oforganic group functionalized mesoporous silica for adsorption of metal ions fromaqueous solution has become a hot research issue, combining excellent properties ofmesoporous silica such as high specific surface and ordered pore structure with theselectivity of organic groups, materials will develop into adsorbents with excellentadsorption properties, for instance, fast adsorption rate, high adsorption capacity, etc.Based on this, mesoporous silica functionalized with amidoxime groups is preparedand its adsorption properties for uranium are investigated in this paper.Firstly, two mesoporous silica SBA-15with different pore channels arecomparatively studied to investigate their adsorption properties for uranium. It isfound adsorption equilibrium reaches within30minutes for both SBA-15, butSBA-15with short channels is superior to SBA-15with long channels in terms ofadsorption capacity, corresponding saturated adsorption capacity is311mg·g^-1、227mg·g^-1，respectively. The optimum pH is6for both, and SBA-15is suitable to dealwith uranium solution with low concentration. Kinetic study indicates pseudo-secondorder rate equation is more fitted to describe kinetic process, and adsorption isothermof SBA-15with short channels is followed with both Langmuir and Freundlichmodels, which reveals a combination of monolayer coverage and multilayeradsorption mode, while adsorption isotherm of SBA-15with long channels isfollowed with Langmuir model, indicating a monolayer coverage adsorption mode. Desorption experiment indicates both adsorption and desorption process are limitedby long channels structure, which further to confirm short channels structure isfacilitated for adsorption. FT-TR shows that the Si-OH and Si-O-Si groups existing inSBA-15are mainly responsible for adsorption of uranium.Secondly, based on SBA-15with short channels, cyano-functionalized SBA-15with short channels is synthetized by co-condensation and post grafting method, thenamidoxime-functionalized SBA-15with short channels is prepared by an oximationreaction. For co-condensation synthesis, cyano-functionalized material prepared withTEOS prehydrolysis for2hours is with the highest cyano content and the highestadsorption capacity for uranium after oximation, and degree of material structureordering decreases with CTES molar ratio increases gradually. While CTES dosagehas no obvious effect on morphology and structure ordering of cyano-functionalizedmaterial prepared by post grafting, which still possess regular thin plateletmorphology and highly ordered two-dimensional hexagonal mesoporous structure.Oximation reaction has no significant effect on morphology and ordering structure ofcyano modified materials, indicating good mechanical stability of cyano materials.Ordering structure together with cyano content coact to affect adsorption properties ofmaterials for uranium. Cyano-functionalized material prepared by co-condensationmethod still owns ordering two-dimensional hexagonal mesoporous structure with acyano content1.34mmol·g^-1when CTES molar ratio is20%, and has the highesturanium uptake516mg·g^-1after oximation, while cyano-functionalized materialsynthetized through post grafting with a higher cyano content4.44mmol·g^-1whenCTES mass molar concentration is20mmol/g, and corresponding adsorption capacityis626mg·g^-1with the best reproducibility after oximation. Compared withco-condensation, material prepared by post grafting has a higher adsorption capacityfor uranium, but also with a higher CTES dosage.Finally, adsorption of uranium on amidoxime-functionalized SBA-15with shortchannels is studied. It is found adsorption is strongly pH-dependent and the maximumadsorption occurs when pH is6, adsorption equilibrium reaches within50minutesand the maximum adsorption capacity is650mg·g^-1. The adsorbent is also suitable to deal with uranium solution with low concentration. Adsorption kinetic is simulated bypseudo-first order and pseudo-second order equations, related regression analysisresults indicate pseudo-second order equation is more fitted to describe adsorptionprocess with a simulated adsorption capacity649mg·g^-1. Besides, isothermal datascorrelate well with both Langmuir and Freundlich models, which indicates theadsorption mode is a combination of monolayer and multilayer coverage, theLangmuir capacity is625mg·g^-1, and Freundlich constant n is3.43, illustratingadsorption force is strong.