Amino-modified Membrane-like And Gelatinous Zirconium Metal-Organic Framework For Removal Of Pb(Ⅱ) From Apple Juice

China is the main apple producing country,the planting area and output of the world ranking first.With the development of national economy,fresh food has been unable to meet the needs of consumers for apples,and people’s demand for processed products is also increasing,among which the most important processed product of apple is concentrated apple juice.But from the actual production situation,there are a series of quality problems in concentrated apple juice,one of the most serious problems is the heavy metal residue in apple juice.Heavy metal ions not only influence the sense and quality of fruit juice,but also have an adverse effect on human health.At present,adsorption is considered to be one of the most effective methods to remove heavy metal ions from food.However,the traditional adsorption materials have some disadvantages such as poor selectivity,small adsorption capacity and difficulty in recycle.Therefore,it is urgent to develop new adsorbents to remove heavy metal ions from apple juice.In this study,membrane-like and gelatinous nano adsorbents with efficient heavy metal ion removal ability are successfully synthesized based on the amino-modified Zirconium MOF,and a series of characterization methods are used to explore the structure and properties of these two nanomaterials.Finally,the adsorption capacities of Pb（Ⅱ）in apple juice are evaluated respectively.The main research results of this experiment are as follows.1.Herein,a UiO-66-NH₂ functionalized nonwoven fabric composite for highly efficient Pb（Ⅱ）removal is developed by in-situ growth strategy.The polyethylene terephthalate（PET）nonwoven fabric support is activated by 4 mol L^-1 Na OH for 3h to alleviate its inertia and is further modified by 1%polyacrylamide（PAM）for 12h to enhance its surface chemical activity to acquire PAM-PET.Then UIO-66-NH₂-PAM PET membrane nanocomposites were obtained by chemical grafting UIO-66-NH₂-PAM PET crystal on the surface of PAM-PET by Zr-N bond.FT-IR,XRD and XPS characterization methods are used to investigate the morphology and structural properties of the membrane-like nanocomposite.And the results demonstrate the successful modification of PET surface and the facile growth of UiO-66-NH₂ crystals on PAM-PET.The N₂ adsorption desorption isotherm shows that the isotherm of the composite belongs to type I,and the pore size is 0.5-2.0nm,indicating that it mainly contains microporous structure.2.The adsorption capacities of UiO-66-NH₂-PAM-PET membrane-like nanocomposite to heavy metal ions in aqueous solution and apple juice are measured.The experiment shows that the adsorption performance of UiO-66-NH₂-PAM-PET is the best when p H=5.In the batch adsorption experiments,the Pb（Ⅱ）adsorption behavior of the UiO-66-NH₂-PAM-PET composite is systematically investigated by kinetics and isotherms experiments,indicating that the adsorption processes of this MOF composite toward Pb（Ⅱ）are chemisorption and monolayer adsorption.The UiO-66-NH₂-PAM-PET composite exhibits an outstanding adsorption capacity of 711.99 mg g^-1 toward Pb（Ⅱ）and excellent selectivity as well as reusability.The selective order of heavy metal ions was as follows:Pb（Ⅱ）>Cu（Ⅱ）>Cd（Ⅱ）>Zn（Ⅱ）>Mn（Ⅱ）,and even after the sixth reuse,the removal efficiency was still more than 88%.In practical application,UiO-66-NH₂-PAM-PET can effectively remove Pb（Ⅱ）from apple juice.The maximum removal efficiency was 88.0%when the additive amount was 90mg,and has slight effect on p H,soluble solids,color value,light transmittance and so on（p>0.05）.The adsorption mechanism was analyzed by FT-IR and XPS characterization.The results showed that the FT-IR peaks of O-H and N-H in UiO-66-NH₂-PAM-PET were changed to 3340 cm^-1,and the C=O peaks were changed to1656 cm^-1 and 1674 cm^-1 after Pb（Ⅱ）adsorption.Compared with the XPS spectra of UiO-66-NH₂-PAM-PET,the Pb 4f peak was found in the UiO-66-NH₂-PAM-PET,and the binding energy of UiO-66-NH₂-PAM-PET changed from 138.90 e V to 143.75 e V after the adsorption of Pb（Ⅱ）。The binding energy of O 1s changed from 531.65 e V to 531.65 e V and the binding energy of N 1s changed from 399.87 e V to 399.70 e V.The results show that the adsorption mechanism of Pb（Ⅱ）on UiO-66-NH₂-PAM-PET is as follows:（1）Pb（Ⅱ）is first transferred from the bulk solution to the solid-liquid interface,and then adsorbed on the surface of UiO-66-NH₂-PAM-PET through physical adsorption and electrostatic attraction.（2）UiO-66-NH₂-PAM-PET provides a large number of-NH₂,-COOH and-CO-NH₂groups,which can capture Pb（Ⅱ）rapidly through electrostatic surface complexation and ion exchange.3.A straightforward strategy was developed to synthesize amorphous Zr-MOG-12 by gelatinization of UiO-66-NH₂ via a facile standing time-controlled crystallization method.Zr-MOG-12 was synthesized by solvothermal method after the reactants Zr Cl₄（Zr Cl₄）and 2-amino-terephthalic acid（NH₂-BDC）were left standing at room temperature for 12h.SEM shows that Zr-MOG-12 nanoparticles present uneven spherical morphology and the particle size is～16 nm.The X-ray diffraction（XRD）spectra indicate that Zr-MOG-12 is synthesized successfully.The nitrogen adsorption-desorption isotherm of Zr-MOG-12 is identified as a combination of type I and IV isotherm,and the pore size distribution is0.67-1.61 nm and 4.36-35.61 nm,which reveals coexist of micropore and mesopore in the structure of gels.TGA analyses illustrates that Zr-MOG-12 has great thermal stability.FTIR shows that the precursor Zr Cl₄ obtains oxide and hydroxide bridges by Zr-O and Zr-OH bond after standing at room temperature,which promotes the formation of[Zr₆O₄（OH）₄]¹²⁺clusters and induces rapid and excessive nucleation rate,and further leads to the coordination perturbation of UiO-66-NH₂ nanoparticles to achieve Zr-MOG-12.4.The adsorption properties of Zr-Mog-12 gel nanomaterials for heavy metal ions in aqueous solution and actual apple juice samples were studied.The experiments show that the optimum p H of Zr-MOG-12 for Pb（Ⅱ）adsorption is 5.0.The as-prepared Zr-MOG-12integrates the hierarchical porous structure of gel-based soft materials and the amorphous structure of low-coordinated active sites,which exhibits superior adsorption capacity for Pb（Ⅱ）and excellent selectivity and reusability.The adsorption kinetics and adsorption isotherms study indicate that this adsorption process are a chemisorption and monolayer adsorption process.The maximum Pb（Ⅱ）adsorption capacity can reach to 475.46 mg g^-1.Moreover,Zr-MOG-12 has good selectivity and excellent reusability for Pb（Ⅱ）.Even in the eighth adsorption-desorption cycle,the removal rate of Pb（Ⅱ）by Zr-MOG-12 is still above77%.In the experiment of removing Pb（Ⅱ）from apple juice,Zr-MOG-12 showes excellent adsorption ability to Pb（Ⅱ）from juice.The maximum removal efficiency was 91.9%,and has slight effect on all sensory indexes of juice（p>0.05）,indicating that Zr-MOG-12 is a promising candidate to removal Pb（Ⅱ）in practical sample.A series of characterization methods were used to analyze the adsorption mechanism of Zr-MOG-12 gel-like nanomaterials.After Pb（Ⅱ）adsorption,the N-H peak of Zr-MOG-12 at 3458 cm^-1disappeared,and the O-H and C=O peaks were transferred from 3348 cm^-1 to 3361 cm^-1 and from 1656 cm^-1 to 1651 cm^-1,respectively,indicating that there was a strong affinity between Zr-MOG-12 and Pb（Ⅱ）.In addition,compared with Zr-MOG-12,the characteristic peak of Pb 4f appeared in the XPS spectra of Zr-MOG-12-Pb,the relative content of O-Metal bond increased from 22.79%to 43.74%,the relative content of C=O and C-OH bond decreased from 38.52%to 32.88%and 38.69%to 23.38%,respectively,and the relative content of-NH increased from 31.98%to 36.28%,and the relative content of-NH₂decreased from 68.02%to 63.72%.The results show that the surface complexation and ion exchange between Pb（Ⅱ）and-COOH and-NH₂ on Zr-MOG-12 are the main removal mechanisms of Pb（Ⅱ）.In this experiment,membrane-like and gelatinous nanomaterials were synthesized on the basis of zirconium metal-organic framework.A series of results show that the two nanomaterials synthesized have good removal ability for heavy metal ions in aqueous solution and apple juice.In contrast,the maximum adsorption capacity of UIO-66-NH2-PAM-PET for Pb（Ⅱ）in aqueous solution is greater than that of Zr-Mo G-12,so it is more suitable for the removal of heavy metal ions in aqueous solution.The maximum removal rate of Pb（Ⅱ）in apple juice by Zr-MOG-12 is higher than that of UIO-66-NH2-PAM PET,so it is more suitable for removing heavy metal ions in apple juice.