Controllable Preparation Of Microbial Nano-palladium And Its Catalytic Properties In The Reduction Of Recalcitrant Contaminants

Due to its high-performance in hydrogenation reactions,palladium nanoparticles?Pd NPs?is extremely advantageous in catalyzing the reduction of some refractory pollutants?eg.nitroaromatic,halogenated compounds and azo dyes,etc.?,and is also potentially applied to treatment of various water environmental pollutions?such as groundwater,chemical,pharmaceutical,printing and dyeing wastewater,etc.?.As showing plenty of advantages compared to traditional chemical methods,such as low-cost,greenness and sustainable,Pd NPs produced by microbial systems have become a quite hot and application prospects.However,the disadvantages of current synthetic techniques are that the Pd NPs synthesized by microorganisms is limited in low catalytic activity,poor conductivity and difficult to apply directly.Thus,this study focused on the controllable preparation of suspended Pd-cells and their catalytic activities in different systems;constructing a reinforced system of conductive nanoparticles?carbon nanotubes and graphene?composite d Pd-cells,clarifying its role in mediating the long-range electron transport and clarified the mechanism of the degradation of refractory pollutants;further,a new method for the in-situ controllable construction of three-dimensional nano-Pd catalytic layer by electrode active biofilm?EAB?was proposed,which allows the Pd synthesis,immobilization and electrochemical application at single electrode.The palladized cells with different Pd NPs sizes and distribution properties were prepared by changing the ratio of cell dry weight?CDW?to palladium ions.A specific expression of exposed area-NPs sizes-extracellular distribution relationship was established,which has a direct correlation with the catalytic performance.The Pd-cell with higher exposed area?9.3±0.1×105 nm²/mg Pd?,exhibited the highest catalytic performance towards reduction of nitrobenzene and p-chlorophenol aromatic compounds in suspension scenario(the k are 0.082min^-11 and 7.8×10^-33 min^-1,respectively).In the electrocatalytic system,the coverage of deposited Pd NPs on the cell surface played a crucial role in boosting the conductivity of the biocatalyst.Pd-cells prepared with CDW:Pd ratios at 1:6were characterized as high coverage?98%?,a conductive path can be established,thus exhibiting good electrocatalytic activity.In order to make full use of Pd NPs with small particle size,which prepared under high CDW to Pd ratio,the electrochemical catalytic activities of Pd-cells is expected to be enhanced by compositing the Pd-cells with conductive nano material?CNTs or GO?.According to the cyclic voltammetry?CV?and chronoamperometric?CA?analysis,it was shown that the el ectrochemical active surface area of Pd in Pd-cells-CNTs10?the ratio of Pd/CNTs was 1/10 w/w?was dramatically increased by 68 times and the steady current density for electrocatalytic reduction of NB was increased by 5 times under a constant potential of-0.55 V.The removal of nitrobenzene reaches 95%within 12 h,and promotes the conversion to the final ptodut aniline.On the other hand,Pd-cells-r GO composites can be assembled by microbial through one-step reduction of Pd???and GO.Due to the good electrical conductivity of r GO and its unique two-dimensional network structure,Pd-cells-r GO with less than 25 times quantity of carbon adding was enough to enhance the electrocatalytic activity toward NB reduction compared to Pd-cells-CNTs under same Pd loading.EAB formed by G.Sulfurreducens serving as the green synthesizer and stabilizer for in situ fabricating EAB-Pd electrocatalyst that can be applied directly.As a result,the EAB-Pd showed conductive network and good stability,which delivered 7.1,4.7 and 5.5-fold increase of current compared to the suspension bio-Pd?Sus-Pd?in the reductive degradation of nitro-,azo-and chloroaromatics,respectively.Superior performance of EAB-Pd was also observed in comparison with the commerci al Pd catalyst.This article put forward the key parameters affecting the formation and catalytic performance of EAB-Pd,and proposed a controllable construction method for EAB-Pd.The high Pd loading per unit cell?Pd/cells>0.18 pg/cell?indicated high coverage and densely distributed nanoparticles on bacteria surface,which could form an effective electron transfer pathway to accelerate electrons transport through conductive structure and promote the reaction efficiency.The operation parameters such as the ratio of EAB-Pd biomass to initial Pd???concentration?cells/Pd ??and the exposure time to Pd precursor solution played decisive roles in the growth and final morphology of Pd NPs.Therefore,the controllable construction scheme of EAB-Pd was clarified.The controllable synthesis strategy of microbial nano-Pd proposed in this study provided technical basis for promoting the application of biological nanotechnology in the environmental field.It also makes microbial nanomaterials more versatile in the electrochemical application.These principles outlined here are expected to be also helpful for other biogenetic metal synthesis and application.