Engineering And Architecture Finely Regulated UiO-66 For The Enhanced Heavy Metal Ions Elimination Performance

Recently,water pollution problems induced by heavy metal ions had been widely concerned due to their huge threat for human and aquatic animals and plants health.Therefore,it was essential to exploit effective techniques for purifying heavy metal ions-containing wastewater.Among them,photocatalysis and adsorption with the merits of low-cost,low energy consumption,rapid reaction rate and free of secondary pollution were regarded as the prospective wastewater treatment techniques.Among metal-organic frameworks(MOFs),UiO66,constructed by Zr-O cluster(Zr6O6)and terephthalic acid ligand,was widely studied for the field of water remediation due to its large specific surface area,high porosity,rich active sites,outstanding water/chemical stability and high-adjustable structure.However,UiO-66 possessed wide band gap and faster photo-induced carrier recombination efficiencies,which inhibited the photocatalytic activities.Moreover,the microcellular structure of UiO-66 also influenced the mass transfer process.To solve the above problems,the structure of UiO-66 would be finely tuned via functional group modification and defect engineering,achieving the photocatalytic Cr(VI)reduction and selective Cr(VI)and Pb(Ⅱ)adsorption.Series of advanced characterizations and experiments were performed to confirm the structural features,removal performances and corresponding structure-function relationships.And then,the corresponding removal mechanism were explored via experiments,characterizations and density functional theory(DFT)calculations.The specific results of this study were described as follows:1.Hydroxyl modified UiO-66((OH)2-UiO-66-X%,X represents the mass content ratio of introduced 2,5-dihydroxyterephthalic acid)was prepared via a solvothermal reaction between zirconium tetrachloride,benzene-1,4-dicarboxylic acid,as well as 2,5dihydroxyterephthalic acid.Series of characterizations like thermogravimetric analyses(TGA),X-ray photoelectron spectra(XPS)and elemental analyses(EA)and so on were performed to confirm the successful modification of hydroxyl group.It was found that hydroxyl groups could not only extend the light absorption region to longer wavelength in visible range to narrow the band gap of photocatalyst,but also act as the intramolecular hole scavenger to boost the photo-induced charge carrier separation to enhance Cr(VI)reduction.The results indicated that the optimal(OH)2-UiO-66-20%photocatalyst could accomplish 100.0%Cr(Ⅵ)(10.0 mg L-1)reduction within 40.0 min under low-power ultraviolet LED light irradiation.Meanwhile,(OH)2-UiO-66-20%still exhibited excellent photocatalytic reduction performance in real water with abundant inorganic co-existing ions or under the real sunlight irradiation.The cyclic experiments demonstrated the good cyclicity and stability of(OH)2-UiO-66-20%Electron paramagnetic resonance(EPR)and electrochemical tests confirmed that the main active species were photo-generated electrons and superoxide(O2·-)radicals during the photocatalytic process.This study provided a high-efficient photocatalyst for the treatment of Cr(Ⅵ)-containing wastewater by photocatalysis.2.In this work,A facile modulating strategy was adopted to fabricate hierarchically porous HP-UOH-X(HP,UOH and X represent the hierarchical pores,UiO-66-(OH)2 and the dosage of benzoic acid,respectively)via introducing benzoic acid with different dosages into the precursor solution of UiO-66-(OH)2.Series of characterizations like TGA-DSC,XPS,high-resolution transmission electron microscopy(HR-TEM)and so on were performed to confirm that the defective sites originated from the missing linker.The formation of hierarchical pores boosted the exposure of-OH groups and the Cr(Ⅵ)mass transfer in HP-UOH-X,which vastly enhanced its sorption capacities and sorption rates.The optimal adsorbent(HP-UOH-80)displayed better sorption capacity(266.74 mg g-1)toward Cr(Ⅵ)(T=308 K,pH=2.0)and faster diffusion rate(k1=14.21 mg g-1 min0.5,k2=6.25 mg g-1 min0.5)than those of the pristine UiO-66-(OH)2 and other HP-UOH-X.The corresponding mechanism was proposed that the-OH groups and the defect sites played dominant contribution on Cr(Ⅵ)adsorption,which could be affirmed by Fourier transform infrared spectra(FTIR)and XPS.In addition,the strategy of photocatalytic Cr(Ⅵ)reduction for desorption was introduced to replace traditional chemical desorption.In all,this work presented one effective adsorbent and sustainable approach for Cr(Ⅵ)elimination.3.To create adsorption sites for cationic heavy metal ions,a green and facile approach was proposed to modulate NH2-UiO-66 for purpose of obtaining SS-NH2-UiO-66-X("X"implied the dosage of used SS)using seignette salt(SS).The generation of abundant vacancies with the formation of hierarchical pores boosted their sorption performance for lead(Pb(Ⅱ)),which strengthened the mass transfer of Pb(Ⅱ)in SS-NH2-UiO-66-X interior.Series of characterizations like TGA-DSC,XPS,HR-TEM,EA,EPR,X-ray absorption spectroscopy(XAS)and so on were performed to confirm that the defective sites originated from the missing linker and Zr-O clusters.Particularly,the optimal SS-NH2-UiO-66-5 exhibited good adsorption capacity toward Pb(Ⅱ)(186.14 mg g-1)and fast diffusion rate(32.1 mg g-1·min0.5)at 25℃ and initial pH=5.46,which were about 34.2 and 66.9 times higher than those of the pristine NH2-UiO-66,respectively.SS-NH2-UiO-66-5 could selectively capture the Pb(Ⅱ)from simulated wastewater containing different co-existing ions.The mechanism was proposed that the defect sites played a significant role in boosting the Pb(Ⅱ)capture performance,which was further affirmed by XAS and XPS.The density functional theory(DFT)calculations illustrated that the hierarchical pores and rich vacancies enhanced the Pb(Ⅱ)mobility towards the adsorption active sites and reduced the adsorption energy between SS-NH2-UiO-66-X and Pb(Ⅱ).This study was the first case to construct MOFs adsorbent with rich metal vacancies,which not only provided an effective adsorbent for the treatment of Pb(Ⅱ)-containing wastewater,but also provided a certain guiding significance for designing and fabricating defective MOFs.