Preparation Of Metal-organic Frameworks And Their Adsorption Study For Precious Metal Ions

The precious metals Au（Ⅲ）and Ag（I）have excellent performance,wide use and important industrial value,but there is a large loss of precious metal resources in industrial production activities and precious metal natural resources are limited.Therefore,it is very important to recover Au（Ⅲ）and Ag（I）from wastewater effectively.Adsorption technology has been widely used for its advantages such as simple operation,mature method,high efficiency and environmental friendliness.Various biomass feedstocks have been used as biological adsorbents to recover Au（Ⅲ）and Ag（I）,but they have some limitations such as weak adsorption capacity and poor selectivity.Therefore,it is of great significance to develop adsorption materials with fast reaction speed and strong adsorption capacity.MOFs are hybrid inorganic-organic crystalline material with central metal coordinating to organic ligands,and are featured with ordered pore structure and abundant functional groups,which make them have important application prospects in metal ion adsorption.However,studies regarding recycling precious metals by MOFs are limited.According to the HASB principle,N,O and S can interact strongly with Au（Ⅲ）and Ag（I）.Based on the above background,three organic ligands（2,6-naphthalene dicarboxylic acid,benzotriazole and 3-amino-5-mercapto-1,2,4-triazole）were selected to react with Zn（NO₃）₂·6H₂O respectively to prepare three MOFs（Zn-NDC,Zn-BTA and Zn-AMT）for the recovery of Ag（I）and Au（Ⅲ）.The kinetics,isotherms,thermodynamics and underlying mechanisms of Ag（I）and Au（Ⅲ）adsorption were studied systematically via batch adsorption tests and surface analysis.The main conclusions are as follows:1.An oxygen-containing MOF（Zn-NDC）was synthesized by solvothermal reaction of Zn（NO₃）₂·6H₂O with 2,6-naphthalene dicarboxylic acid,which was used for the efficient recovery of Ag（I）from aqueous solution.The adsorption performance of Zn-NDC for Ag（I）was systematically investigated.The results show that the overall adsorption rate was determined by liquid film diffusion.The maximum adsorption capacity of Zn-NDC for Ag（I）is up to 844 mg/g,which is better than many existing Ag（I）adsorption materials.The nonlinear Sips model is in good agreement with the experimental results,indicating that the adsorption of Ag（I）by Zn-NDC is not uniform.In addition,the adsorption process is spontaneous and exothermic.Zn-NDC shows excellent selectivity and good recycling performance towards Ag（I）.After two cycles,the adsorption property only decreases by 6%.The adsorption mechanism of Zn-NDC for Ag（I）is mainly electrostatic attraction and coordination.2.A new MOF material（Zn-BTA）was synthesized by solvothermal reaction of Zn（NO₃）₂·6H₂O with benzotriazole,which was used to recover Ag（I）from aqueous solution.Different from the adsorption of Ag（I）by Zn-NDC,the p H value of Ag（I）solution has no effect on the adsorption capacity.The adsorption ratio of Zn-BTA reached 100%at low Ag（I）concentration,and the maximum adsorption capacity of Ag（I）reached 588 mg/g.The adsorption of Ag（I）by Zn-BTA is an endothermic process and the adsorption isotherm conforms to Langmuir model.The adsorption mechanism of Zn-BTA for Ag（I）is mainly O coordination.3.Zn-BTA was synthesized by solvothermal reaction of Zn（NO₃）₂·6H₂O with3-amino-5-mercapto-1,2,4-triazolium for the recovery of Au（Ⅲ）and Ag（I）from aqueous solution.The one-step method introduces"S",which makes up for the deficiency of long synthesis cycle and complex steps in the post-modification synthesis method.The experimental results show that Zn-AMT has excellent adsorption effect on Au（Ⅲ）and Ag（I）,with the maximum adsorption capacity up to892 mg/g and 968 mg/g,respectively.The optimum p H value of Zn-AMT for Au（Ⅲ）adsorption is 3,while p H value has no effect on the adsorption of Ag（I）by Zn-AMT.At low concentration,100%adsorption of Au（Ⅲ）and Ag（I）can be achieved by Zn-AMT.The adsorption of Au（Ⅲ）and Ag（I）by Zn-AMT is endothermic and the disorder degree increases spontaneously.Zn-AMT has excellent affinity for Au（Ⅲ）and Ag（I）,and has good reusability.The adsorption mechanism of Zn-AMT for Au（Ⅲ）is mainly electrostatic attraction,coordination and partial reduction,while for Ag（I）it is mainly through coordination.