Study On Adsorption Performance And Mechanism Of Arsenic In Water By Cerium/zirconium Based Metal Organic Frameworks

Arsenic（As）is a metal-like element widely found in water,atmosphere and land,and it is highly toxic.With the continuous development of social industrialization,the burning of arsenic-containing fuels and the discharge of arsenic-containing wastes will cause arsenic pollution,threatening people’s lives and health.As arsenic pollution incidents occur frequently at home and abroad,how to improve the current situation of arsenic pollution has become very urgent.There are various water arsenic treatment technologies,such as coagulation precipitation method,biological method,membrane technology and adsorption method.Among them,the adsorption method is a simple,effective,safe,green,and easy-to-manage method to remove heavy metals in water,and is widely used in water treatment processes.However,shortcomings such as low adsorption capacity and poor selectivity restrict the development of adsorbents.In recent years,metal organic framework（MOF）has become an emerging adsorption material due to its large specific surface area,adjustable metal center and organic ligand structure.It has shown strong application potential in the fields of energy and environment.In this paper,three MOFs,Zr-UIO-66,Ce-UIO-66 and Ce-MOF-808,were synthesized by hydrothermal synthesis of zirconium-based metal organic frameworks,and studied their effects on the two valence states of arsenic（As（III）and As（V））adsorption performance and mechanism research,the main research content is as follows:（1）Ce-UIO-66 and Zr-UIO-66 adsorb As（V）.The experimental results of p H influence are as follows:when p H<4,the adsorption rate decreases as the p H rises,and when p H>4,the p H value increases.The rising removal rate increases,indicating that the strong acid or weak alkaline conditions are beneficial to the adsorption of As（V）.The experimental results of adsorption kinetics show that the adsorption process of As（V）is better fitted by the pseudo-second-order kinetic equation,indicating that chemical adsorption is the main adsorption method.The fitting results of the As（V）intra-particle diffusion equation and Boyd equation show that the adsorption is controlled by the two processes of intra-particle diffusion and particle outer surface diffusion.The results of adsorption isotherms show that the Langmuir model of Zr-UIO-66 has a higher degree of fit,with a maximum adsorption capacity of 325.26 mg/g,while Ce-UIO-66 fits better with Freundlich model,with a maximum adsorption capacity of 355.67 mg/g.PO₄^3-will have a strong inhibitory effect on the adsorption of As（V）,and CO₃^2-ions will slightly inhibit the adsorption effect.Toxicity leaching test（TCLP）results show that As,Ce and Zr are almost undetectable in the leaching solution.Fourier infrared spectroscopy and X-ray photoelectron spectroscopy techniques were used to characterize the adsorbent before and after adsorption.It was found that:Zr-UIO-66 and Ce-UIO-66 mainly use organic ligands for terephthalic acid Formic acid exchange and hydroxyl unsaturated coordination sites for As（V）adsorption.（2）Ce-UIO-66 and Zr-UIO-66 adsorb As（III）.The removal rate always increases with the rise of p H,and the removal rate rises faster under alkaline conditions.The results of kinetics show that the results of the adsorption process of As（III）are better fitted with pseudo-second-order kinetics,indicating that chemical adsorption is the main adsorption method.The results of adsorption thermodynamics study showed that for As（III）,the adsorption capacity of Zr-UIO-66 reached 161.66 mg/g,while that of Ce-UIO-66 was only 5.52 mg/g,which was 30 times difference.The coexisting ions are mainly PO₄^3-,and CO₃^2-has a significant effect on the removal of As（III）.FTIR and XPS were used to characterize the adsorbent.Possible adsorption mechanism:Zr-UIO-66 mainly adsorbs As（III）through the BDC exchange of the ligand and the hydroxyl unsaturated coordination site,while Ce-UIO-66 only has ligand BDC exchange,and the single adsorption mechanism results in the low adsorption capacity of Ce-UIO-66.（3）Ce-MOF-808 adsorbs As（III）and As（V）.The results of the p H experiment shows that whether for As（III）or As（V）,Ce-MOF-808 can maintain a high adsorption rate to them under wide p H conditions.PO₄^3-will strongly inhibit the adsorption of As（V）by the adsorbent,but only slightly inhibit As（III）.The fitting result of Langmuir isotherm shows that the adsorption capacity of Ce-MOF-808 on As（V）reached 217.80mg/g,and the adsorption capacity of As（III）reached 402.10 mg/g.Kinetic experiments show that the adsorption can reach equilibrium within 1 h.The MOF maintains good adsorption performance in different environmental water.Based on the results of XPS and FTIR analysis,it is proposed that the adsorption mechanism of Ce-MOF-808 on As in water is organic ligand trimellitic acid exchange and hydroxyl unsaturated coordination site adsorption.The massive leakage of BTC in the solution after adsorption indicates that Ce-MOF-808 is prone to ligand exchange when As（III）is adsorbed,and As（V）is more likely to bind to unsaturated sites.In summary,Zr-UIO-66 has a general affinity for As（V）.By changing the central metal of the MOF,the synthesized Ce-UIO-66 has a greatly improved affinity for As（V）,but Ce-UIO-66 has a greater affinity for As（V）.III)The low removal rate may be due to the difference in the coordination mode between As（III）and As（V）by Ce-UIO-66.Ce-MOF-808 synthesized by changing the strategy of organic ligands has good affinity for As（V）and As（III）,which may be due to the physical and chemical properties of Ce-based MOF,such as the morphology and pore structure of the Ce-based MOF.The body change has changed,so it is beneficial to the coordination of As（III）and As（V）.In this paper,the removal performance of three MOFs of Ce-UIO-66,Zr-UIO-66 and Ce-MOF-808 on As（V）and As（III）in water are studied,and the reaction mechanism is discussed,providing theoretical and technical basis for the research removal of arsenic.