The Adsorptive Characteristics Of Persimmon Tannin-Based Metal Adsorbents For Recovery Of Au(Ⅲ),Pd(Ⅱ) And Pb(Ⅱ) From E-Wastes

In this research,persimmon powder,persimmon fallen leaves,barks and purified persimmon tannin were utilized to prepare four different persimmon tannin based bio-sorbents using various methods and named as PPFR,NPFL,Kaki-TRF and Fe3O4@Si O2@PT,respectively.Their adsorption characterics towards Au(Ⅲ),Pd(Ⅱ)and Pb(Ⅱ)were investigated.Different parameters affecting the adsorption process were examined using batch adsorption tests.Various fitting models were applied for experimental data such as pseudo-first rate model and Langmuir equation.The adsorption mechanisms were proposed based on the characterization by the methods of FTIR,SEM,XRD and XPS.Finally,a mimic column was used to detect the application potential.The main results are as following:1.Precious metal ion of Au(Ⅲ)could be selectively adsorbed by PPFR from the industrial leached liquior.The solid-liquid ratio and diluted factor were optimized after the comparison of adsorption capacities.As revealed by characterization,the appearance of gold particles after adsorption indicated occurrence of redox reaction during the adsorption process.The dynamic adsorption capacity of PPFR was 100 B.V.indicated from the test of industrial leach liquors.Compared with other two commercial adsorbents,the selective adsorption capacity of PPFR was superior rendering promising application in practice.2.The treatment of persimmon fallen leaves by Na OH could significantly promote the adsorption amount towards Pb(Ⅱ).The p H exerted great influence on adsorption capacity of NPFL towards Pb(Ⅱ)and the optimal p H was about 5.0.The highest adsorption capacity of NPFL towards Pb(Ⅱ)is 256.41 mg g-1 as fitted by Langmuir.Compared with other leaf-based adsorbents,NPFL exhibited good potential for Pb(Ⅱ)removal from aqueous solutions.The proposed mechanism could be summarized as(1)the electrostatic attraction between negatively charged active sites on NPFL and Pb(Ⅱ);(2)the ionic exchange between H+ of phenol hydroxyl group and Pb(Ⅱ);(3)the chelation between hydroxyl group and carboxylate ligand on NPFL and Pb(Ⅱ)with the formation of Pb(Ⅱ)complex.The facile preparation,low-cost of raw material and high removal efficiency make NPFL a good candidate as metal ions adsorbent.3.Kaki-TRF is porous material containing numerous phenol hydroxyl groups.The optimal p H for adsorption of Au(Ⅲ)is 3.0.The maximum adsorption capacity of Kaki-TRF towards Au(Ⅲ)is 258.07 mg g-1 after calculated by Langmuir modelling,surpassing other porous bio-based adsorbents.The proposed adsorption mechanism might be electrostatic attraction,in situ reduction and aggregation of nano-gold.Formaldehyde-free,reusable for at least 5 times and based on waste branches of persimmon trees render it a novel and green tannin rigid foam.4.A core-shell magnetic composite was prepared by immobilizing persimmon tannin(PT)through glutaraldehyde on aminated Fe3O4@Si O2 microspheres,and the synthetic Fe3O4@Si O2@PT was utilized for adsorptive recovery of Au(Ⅲ)and Pd(Ⅱ)from aqueous solutions.The adsorption capacities of Fe3O4@Si O2@PT for Au(Ⅲ)and Pd(Ⅱ)were as high as 917.43 and 191.47 mg g-1,respectively.Their magnetization saturation values are all higher than 20 emu g-1 and coercivity lower than 50 Oe(superparamagnetic characteristics at room temperature)which declared that a high magnetic separation efficiency could be obtained.As a consequence,Fe3O4@Si O2@PT is a composite material with magnetic response and superior adsorption amounts.In summary,persimmon tannin-based adsorbents have superior adsorption capacities for heavy/precious metal ions from e-wastes and the adsorption amounts are positively correlated with the content and purity of persimmon tannin.The main proposed adsorption mechanism towards Au(Ⅲ)is oxidation-reduction,and the dominating adsorption mechanism for Pb(Ⅱ)is chelation.As for Pd(Ⅱ),both oxidation-reduction and chelation are involved in the adsorption process.