Preparation Of Chitosan/Metal-Organic Framework Composites And Their Adsorption Properties Of Chromium(Ⅵ) In Water

As a strategic metal element widely used in industry,chromium（Cr）cannot be ignored for its rapid migration and accumulation,strong toxicity,and carcinogenicity.Adsorption is considered a green and effective technological measure to solve heavy metal pollution in water.Therefore,searching for efficient,environmentally acceptable,and cost-effective adsorbents is crucial for human health and environmental safety.As organic-inorganic porous materials with tunable pore structures and abundant active sites,metal-organic framework materials（MOFs）are regarded as excellent potential adsorbents.However,the majority of MOFs are powdered and need to be modified to optimize their properties due to issues with easy aggregation,poor acid stability,and hard separation in an aqueous solution.Chitosan is a naturally occurring,inexpensive biomolecular material containing a large number of reactive groups.It can also be easily combined with other materials to obtain composite materials with specific functions.In this paper,MIL-125-NH₂ was used as raw material and modified with chitosan and ferric tetroxide to construct two chitosan-based organic framework composites,which were applied to the adsorption of Cr（Ⅵ）in water.The physical and chemical properties such as morphological structure and functional groups were analyzed by microscopic characterization.Batch adsorption tests were used to investigate the composites’ability to adsorb Cr（Ⅵ）,and the optimum adsorption conditions were identified.The adsorption mechanism was revealed by combining the model-fitting results and characterization.The following are the key conclusions:（1）The composite was prepared by modifying MIL-125-NH₂ using chitosan.By comparing the effect of different chitosan to MIL-125-NH₂ mass ratios on the adsorption effect,it was found that the composite MCS-1 showed the most effective adsorption performance on Cr（Ⅵ）when chitosan:MIL-125-NH₂=3:1.SEM,FTIR,and XRD were used to characterize MCS-1,and the results confirmed that the composite MCS-1 was successfully prepared and was in the form of a round sphere with a relatively smooth surface.The magnetic properties were investigated and the composites were found to have good magnetic properties both before and after adsorption and could be quickly separated and recovered from the solution under the applied magnetic field.The composite material MCS-1 demonstrated the best adsorption impact on Cr（Ⅵ）at 303 K,p H=2,and dosage of 0.010 g,according to batch adsorption experiments.The highest amount of absorption was 105.93 mg/g.The PSO model and the Langmuir model are both supported by the entire adsorption process,demonstrating that it is a chemisorption taking place on the surface of a homogenous monomolecular layer.The materials’acid stability tests verified that the composites were stable in a range of acid concentrations,and the desorption tests demonstrated the composites’excellent reusability.The coexisting anions have an influence on the adsorption process,with the degree of influence being PO₄^3-（12.55 mg/g）>SO₄^2-（8.3 mg/g）>Cl^-（1.68 mg/g）.In addition,the adsorption of Cr（Ⅵ）by MCS-1 mainly involves a significant number of hydroxyl and amino groups on the material surface,achieved by hydrogen bonding interactions,electrostatic attraction,and reduction;（2）Given MCS-1’s effective ability to remove heavy metals and its stability in acids,we further modified MIL-125-NH₂ using chitosan and magnetic trioxide to obtain magnetic chitosan-based metal-organic framework composites in order to make the composites to be rapidly separated and recovered from the water.Compared with other materials,the composite FMCS-1（chitosan:MIL-125-NH₂=3:1）showed the best adsorption effect under the same conditions.EDS,SEM,BET,FTIR,VSM,and XRD were used to examine the structural as well as physical and chemical features of FMCS-1.The findings demonstrated that the composite FMCS-1 is a round,spherical substance made up of MIL-125-NH₂,chitosan,and Fe₃O₄,which can be quickly separated and recovered in the presence of an applied magnetic field before and after adsorption.The maximum adsorption capacity of 109.46 mg/g for Cr（Ⅵ）was reached at a temperature of 303 K,p H=2,and a dose of 0.010 g of FMCS-1.The adsorption of Cr（Ⅵ）by FMCS-1 occurs on the surface of a homogenous monomolecular layer of the substance,and it can be analyzed using both the PSO kinetic model and the Langmuir model.This process is shown to be dominated by chemisorption.The magnetic composites are stable in a range of acidic solution concentrations,according to tests on acid stability.The removal rate of the composite material can still reach more than 50%after conducting 5 cycles of regeneration experiments.Furthermore,electrostatic attraction,hydrogen bonding,and reduction are the major components of the Cr（Ⅵ）adsorption mechanism by FMCS-1.This research confirmed that the composite materials obtained by modifying MOFs with chitosan and magnetic ferric tetroxide not only effectively solve the problem of easy aggregation of materials,but also have excellent adsorption potential.It can stably exist in acidic solutions and is easy to separate and recover from the solution.This research offered two efficient and convenient adsorbents,expanding the application range of MOFs in water treatment and providing scientific basis for the preparation of adsorbents for treating heavy metal polluted water bodies and the resource management of Cr（Ⅵ）.