Construction And Application For The Recovery Of Low-concentration Precious Metal Ions Of Hexagonal WO₃-based Reductive Ion Exchange Membrane

Precious metals have special physical and chemical properties,which are widely used in industrial fields.With the rapid development of global industry,the demand for precious metals is increasing.However,the content of precious metals in the earth’s crust is low,so they have become a very important strategic resource.With the increase in the amount of precious metals in the industrial field,the wastewater containing precious metal ions discharged from the industry is also increasing year by year.So how to treat wastewater and recover precious metals has attracted widespread attention.but in terms of rapid and effective treatment of low-concentration precious metal wastewater is still not well resolved.In this paper,we use two materials with capacitive properties to construct a two-dimensional membrane,and use the material’s appropriate redox potential to treat low-concentration precious metal ion wastewater by reducing ion exchange.1.Preparation of hexagonal WO3 reductive ion exchange membrane and its reduction properties for noble metal ionsWe use Na2WO4·2H2O,H2C2O4,(NH4)2SO4 as raw materials,deionized water as solvent,HCl as pH regulator,and prepared hexagonal phase tungsten trioxide(h-WO3)by hydrothermal synthesis.We grinded polyvinylidene fluoride(PVDF),Ketjen black(KB),and h-WO3 with the solvent N-methylpyrrolidone(NMP)together,then constructed a hexagonal phase tungsten trioxide reductive ion exchange membrane(h-WO3 RIM)on the titanium mesh.The reduction potential of h-WO3 RIM in the acid electrolyte is 0.303 eV,the specific capacitance is 423 F/g at a current density of 0.5 A/g,and the capacitance does not decrease significantly after 500 cycles.Putting the reduced h-WO3 RIM in a solution containing noble metal ions(AgNO3,HAuCl4,H2PtCl6 and K2PdCl4 solutions with a concentration of 0.1 mol/L),the noble metal ions will migrate to the h-WO3 RIM surface and spontaneously undergo a reduction reaction.At the same time,the H+in the membrane will be released,which is equivalent to a reductive ion exchange reaction between precious metal ions and h-WO3 RIM.The reducing ion exchange membrane can directly reduce precious metal ions to high-purity elemental substances which can be recovered through simple centrifugation and vacuum suction filtration,and has excellent recycling performance.The reduced h-WO3 RIM coil is placed in a wastewater treatment device for low-concentration precious metal ion wastewater reduction/recovery properties test.The maximum removal efficiency of 100 μg/L AgNO3 solution is 99.98%,and 10 mg/L low-concentration HAuCl4,K2PdCl4 and H2PtCl6 solutions can reach the maximum removal efficiency of 98.63%,97,81%and 98.37%,respectively.When the solution contains non-precious metal ions and organics,h-WO3 RIM can still maintain a high recovery effect at low-concentration precious metal ions condition.Compared with the traditional capacitive deionization method that also uses capacitive materials,this decoupling step-by-step wastewater treatment method can prevent impurity ions in the water from occupying positions in the h-WO3 crystals,so the regeneration process of h-WO3 RIM is simple and thorough,and the usage life of the material is longer.2.Preparation of activated carbon AC@h-WO3 reductive ion exchange membrane and research on its precious metal reduction/recovery propertiesActivated carbon(AC)is a commonly used electric double layer capacitor material,and has good adsorption properties.Based on the preparation of h-WO3 RIM,we constructed AC@h-WO3 RIM composite material with reducing ion exchange performance on the basis of h-WO3 RIM.The electrochemical performance test of the prepared AC@h-WO3 RIM composite material in acid electrolyte shows that the AC@h-WO3 RIM composite material has the properties of electric double layer capacitance.AC has little effect on oxidation-reduction potential of h-WO3 RIM and can increase the specific capacitance of h-WO3 RIM.The specific capacitance of the composite material is 470 F/g at a current density of 0.5 A/g.When the reduced AC@h-WO3 RIM is placed in a 1000 mg/L AgNO3 solution,the surface of the AC can adsorb the silver element reduced by the AC@h-WO3 RIM through ion exchange.The dynamic recovery test results show that AC@h-WO3 RIM still has excellent reduction/recovery capabilities for precious metal ions in the solution when the low-concentration precious metal ion solution is passed at a large flow rate of 10 mL/min and 20 mL/min.