Study On Biorecovery Methods Of Palladium Nanoparticles,Mechanisms And Its Catalytic Activities Products

Precious metal Palladium?Pd?is a scarce natural resource with increasing industrial demand.The use of microorganisms for biorecovery processes of Pd is a promising approach and recognizes as a potential alternative method to conventional treatment techniques because it mimics natural biogeochemical cycles.In this research,the biorecovery of palladium nanoparticles(Pd⁰-NPs)by Enterococcus faecalis?E.faecalis?and the catalyst activities of the obtained Pd⁰-NPs products are investigated.The biorecovery of Pd⁰-NPs was carried out through studying the bioreduction and biosorption processes.The biorecovery efficiencies and characteristics of Pd⁰-NPs were analyzed by UV-vis spectroscopy,atomic adsorption spectrophotometry?AAS?,X-ray diffraction?XRD?,Fourier transform infrared?FT-IR?spectroscopy,transmission electron microscopy?TEM?and X-ray photoelectron?XPS?.The main works and the obtained results are summarized as follows:?1?Study on the biorecovery of Pd^?through bioreduction process by using E.faecalis cells.The electron donor selection for reduction Pd^?were performed on four kinds of chemicals including sodium acetate,acid acetate,pyruvate and sodium formate,in which,only the sodium formate was identified to be fitting for bioreduction of Pd^?into Pd⁰by E.faecalis.UV-vis and X-Ray diffraction analysis exhibited that the process could successfully reduce Pd^?and generate Pd⁰-NPs.The observation of TEM images showed that the Pd⁰-NPs were associated at the membrane and inside of the cell with predominantly shapes including triangles,hexagons,spherical,and undefined shapes.The effect of basic factors such as biomass dosage,electron donor concentration,the initial Pd^?concentration,temperature and the solution pH was evaluation.Thereby,under the optimum conditions of temperature?40 oC?,sodium formate concentration?25 mM?,biomass?1.2 g/L?,and 210 mg/L Pd^??pH 3.0 – 3.5?,the bioreduction of Pd^?completed within 48 h.The percentage of the Pd⁰-NPs with an average size lower than 10 nm increased from 91.5 to 99.6% when the biomass increased from 1.2 to 3.6 g/L,respectively.?2?The biorecovery of Pd^?through biosorption process was investigated for evaluation of the biosorption potential of E.faecalis in the removal of Pd^?from aqueous solution.The results demonstrated that E.faecalis could be sorbed Pd^?from the solution.In order to finding the optimum biosorption efficiencies,the effect of various parameters such as biomass dosage,the initial Pd^?concentration,incubation temperature,the initial pH solution,and biosorption equilibrium were evaluated.The results of UV-Vis,AAS,TEM,and XPS confirmed that the Pd^?was sorbed on the cell wall and inside the cell,and the biosorption efficiency reached the saturation of 99.4% after 6 h.The optimum conditions for Pd^?biosorption were found to be 1.2 g/L biomass dosage,40 oC incubation temperature,and pH of 3.0 – 3.5 at the initial Pd^?concentration of 210 mg/L.The Pd^?biosorption capacity of the E.faecalis cells was calculated by the Langmuir model(qmax = 186.1 mg Pd^?/g dry cells weight,R2 = 0.995)fitted the equilibrium data better than the Freundlich model?R2 = 0.937?at 30 oC and pH 3.0.The biosorption thermodynamic model indicated that the positive of ?H occurred according to the endothermic nature of the biosorption.Besides,the negative standard Gibb‘s free energy??G?indicated the feasibility and spontaneous nature of the biosorption process.?3?The mechanisms of the biosorption and bioreduction of Pd^?by E.faecalis was elucidated via kinetic models?such as pseudo-first order,pseudo-second order and WeberMorris intraparticle diffusion model?,the changed solution pH during reaction process,FT-IR and XPS analysis.The kinetic models determined diffusion controlled at the early stage of biosorption process and then,chemical interaction between Pd^?and biomass of the cell become the limiting mechanism for diffusion.Observation at the initial solution pH at 3.0 and 3.5 showed that the solution pH was decreased from 3.9 to 2.7 and 2.5 after 6 h in biosorption process,respectively,while increasing above 9.0 in bioreduction process.The presence of function groups on the cell wall were explored by FT-IR and XPS methods.The results proved that the Pd^?biosorption occurred via electrostatic interaction,the complexation of Pd^?with function groups on the cell and intracellular uptake.Whereas,the Pd^?bioreduction mechanisms with sodium formate as electron donor have been elucidated that major occurring by the mechanisms such as ion-exchange and chemical reduction.?4?The biorecovery of Pd^?from the simulated wastewater resources was evaluated.The qualitative and quantitative tests of the influence of single other metal ions such as Pt^?,Au^?,Ag^?,Cu^?and Fe^?on biorecovery process were performed.The effect levels on the decreased biorecovery efficiencies of Pd^?were found as Au???> Pt^?> Ag???> Cu???> Fe^?.In the presence of single metal as Fe^?,E.faecalis cells could be reduced Pd^?into Pd⁰without adding any electron donor through XRD analysis.In the presence of Pd^?,E.faecalis cells could also be reduced Au???into Au⁰without adding any electron donor.Through XPS analysis in biosorption systems of simulated wastewaters,a part of Pd^?could be reduced into Pd⁰with the presence of high concentration of Fe^?without occurring in the presence of Au^?and Ag???.Whereas,in the bioreduction systems,a part of Pd^?was not reduced in the presence of Au^?and Ag^?.?5?The recovery of Pd⁰-NPs products from Bio-Pd⁰-NPs was tested by using autoclave and ultrasonic methods.The higher catalyst activities of Pd⁰-NPs by ultrasonic treatment for reduction toxic Cr^?into non-toxic Cr^?and removal of toxic PCBs from solution were demonstrated.It showed that the optimum efficiency of 630 mg/L Pd⁰-NPs could catalyzed the complete reduction of below 0.5 mM Cr^?after 12 h.The Pd⁰-NPs sizes decreased when increasing of cell dry weight?CDW?:Pd^?ratio from 6:1 to 18:1 as well as the increasing solution pH from 2.0 to 3.5 improved the reduction efficiencies of Cr^?.GC-MS analysis revealed that 5 mg/L toxic PCBs in solution was completely removed by using 210 mg/L Pd⁰-NPs products and 2 mM sodium formate as electron donor.Chloride ions was released in the degradation process of PCBs that demonstrated the catalytic activities of Pd⁰-NPs products for dechlorination from PCBs.In brief,this research proved that E.faecalis was greatly potential for the removal of Pd^?and recovery of Pd⁰-NPs.Simultaneously,the recovered Pd⁰-NPs products showed highly catalytic efficiency in reduction of toxic Cr^?as well as the degradation of toxic PCBs from solution.