Study On Biogenic Palladium Coupled With Membrane Biofilm Reactor(Pd-MBfR)for Removing Typical Oxidizing Contaminants In Water

Oxidizing contaminants widely exist in natural waters,which seriously threaten human health,so the removal of oxidizing contaminants in water has become a hot research topic in recent years.Common oxidizing contaminants in water include three types:halogenated organics,oxidative heavy metals,and oxyanions.Recently,the representative contaminants with high research interest are 2,4-dichlorophenol（2,4-DCP）,hexavalent chromium（Cr（Ⅵ））and bromate（Br O₃^-）.2,4-dichlorophenol（2,4-DCP）,as a common chlorophenol organic matter,is widely used in agricultural and industrial production processes,but it has been detected to varying degrees in many areas of China.Benzene ring structure and chlorine atoms make it toxic,and 2,4-DCP can accumulate in organisms,causing harm to organisms and the whole ecosystem.Cr（Ⅵ）is a heavy metal largely used in industry,which is commonly used as tanning agent,metal additive,and pigment.Because of its strong oxidation and carcinogenicity,it has been listed as a priority pollutant in the water environment of China and the United States.Br O₃^-is a disinfection byproduct with genotoxicity and is classified as a potential 2B carcinogen by the World Health Organization.The detoxification of oxidizing contaminants through reduction approach has the advantages of cleanliness and high efficiency.The common reduction approaches for water treatment include catalytic reduction and biological reduction.Hydrogen,as a available,clean,and non-toxic electron donor,has been widely used for reducing oxidizing contaminants,but the safe and quantitative supply of hydrogen is facing challenges.In this paper,a biogenic Palladium coupled with Hydrogen-based Membrane Biofilm Reactor（Pd-MBfR）was created.This reactor uses hollow fiber membrane to transfer H₂ safely and efficiently in the form of bubble-free way.The biofilm attached to the fibers is used to disperse and immobile the palladium nanoparticles（Pd NPs,catalyst）.The Pd-MBfR technology integrates hydrogen transport,biological palladium preparation,and contaminants removal in a single reactor system,with the advantages of clean,no secondary pollution,safety,and high efficiency.The removal of 2,4-DCP,Cr（Ⅵ）and Br O₃^-from water by using Pd-MBfR was studied,including treatment efficiency and persistence,degradation pathway of contaminants,microorganism community structure,reaction kinetics,and influencing factors.In addition,the simultaneous experiments in Hydrogen-based Membrane Biofilm Reactor（MBfR）and Hydrogen-based Membrane Palladium-film Membrane Reactor（MPfR）were executed,and the experimental results of three reactors were compared and analyzed to reveal the mechanisms and interactions between palladium catalyst and microorganisms.In the study on 2,4-DCP removal,a MPfR was created by depositing Pd NPs on hollow-fiber membranes via autocatalytic hydrogenation to form a Pd-film.The MPfR was used for hydrodechlorination（HDC）of 2,4-DCP.HDC performances and mechanisms were systematically evaluated,and a continuous-flow dechlorination model was established.Approximately 87%of the input 2,4-DCP was reduced to the end-product phenol,while 2-chlorophenol（2-CP）was an intermediate,but only at 2%.Selective adsorption of the 2,4-DCP onto the Pd-film and fast desorption of phenol facilitated efficient dechlorination.Modeling results represented well the concentrations of 2,4-DCP and its intermediates.It demonstrated three dechlorination pathways:The majority of 2,4-DCP was completely dechlorinated to phenol in an adsorbed state without release of monochlorphenol,some 2,4-DCP was degraded to 2-CP that was released and subsequently adsorbed and reduced to phenol,and a small amount was reduced to 4-CP that was released and subsequently adsorbed and reduced to phenol.Analysis based on Density Functional Theory suggests that the pathway of full dechlorination was dominant due to its thermodynamically favorable adsorption configuration,with both Cl atoms bonded to Pd.MPfR technology enables efficient and full dechlorination.However,the dechlorination product phenol still needs to be further removed.In order to thoroughly remove 2,4-DCP and its product,the effects of nitrate on2,4-DCP dechlorination and biodegradation in a Pd-MBfR,as well as interactions between the two reductions,were studied.The Pd-MBfR was created by synthesizing Pd NPs that spontaneously associated with the biofilm to form a Pd-biofilm.Without input of nitrate,the Pd-MBfR had rapid and stable catalytic hydrodechlorination:93%of the 100-μM influent 2,4-DCP was continuously converted to phenol,part of which was then fermented via acetogenesis and methanogenesis.Introduction of nitrate enabled phenol mineralization via denitrification with only a minor decrease in catalytic hydrodechlorination.Phenol-degrading bacteria capable of nitrate respiration were enriched in the Pd-biofilm,which was dominated by the heterotrophic genera Thauera and Azospira.Because the heterotrophic denitrifiers had greater yields than autotrophic denitrifiers,phenol was a more favorable electron donor than H₂ for denitrification.This feature facilitated phenol mineralization and ameliorated denitrification inhibition of catalytic dechlorination through competition for H₂.Increased nitrite loading eventually led to deterioration of the dechlorination flux and selectivity toward phenol.The 2,4-DCP-adsorption capacity of Pd-biofilm was 2-to 5-fold greater than that of abiotic Pd NPs-film,and the adsorption accelerated dechlorination by Pd-biofilm,including selectivity to phenol instead of mono-chlorophenols.A mechanistic kinetic model was developed to represent the sequential adsorption and reduction processes.Modeling results represented well the removal of 2,4-DCP and quantified that Pd-biofilm had a strong affinity for adsorbing 2,4-DCP.The strong adsorption increased the volume-averaged concentration of 2,4-DCP concentration inside the Pd-biofilm,compared to the concentration in the bulk liquid.This increase in the local concentration of 2,4-DCP led to a 2-to 4-fold increase in the rate of reduction of 2,4-DCP in Pd-biofilm,compared to the abiotic Pd-film.Thus,coupling Pd NPs with the biofilm promoted 2,4-DCP removal and full dechlorination despite its low concentration in water.Pd-MBfR technology enables simultaneous removal of 2,4-DCP and nitrate in a efficient and cheap way.As for the removal of oxidative Cr（Ⅵ）,Both Pd-MBfR and MBfR can respir water contained 1 mg/L Cr（Ⅵ）,but Pd-MBfR has a better removal efficiency of total chromium,and the Cr（III）concentrations in effluent water were less than 40 mg/L,which were much lower than the Cr（III）concentrations in effluent water of MBfR（300mg/L）.The precipitate of Cr（III）derived from Cr（Ⅵ）reduction was trapped in Pd-MBfR.The soluble organic matter concentrations in MBfR’s effluent were significantly higher than that in Pd-MBfR,which led to more Cr（III）complexing with organic ligands,and thus increasing the solubility of Cr（III）.Microbiological analysis shows that Pd NPs in Pd-MBfR attenuated the inhibition of Cr（Ⅵ）toxicity on biofilms,reduced the cellular secretions for resisting Cr（Ⅵ）toxicity,and lowered the energy metabolism of autotrophic bacteria.The Cr（Ⅵ）concentrations in natural water is up to1-10 mg/L,and within this concentration range,the removal percentages of Cr（Ⅵ）by Pd-MBfR reach 99%;while over 2 mg/L Cr（Ⅵ）inhibited the reduction of Cr（Ⅵ）in MBfR.In addition,nitrate had no effect on the reduction of Cr（Ⅵ）in Pd-MBfR,but significantly inhibited the reduction of Cr（Ⅵ）in MBfR.The hydrogen utilization efficiency of Pd-MBfR is also greater than MBfR.As for the removal of oxyanion Br O₃^-,generally,the Br O₃^-removal in Pd-MBfR and MBfR were similar,which were significantly higher than that in MPfR,indicating that the Br O₃^-reduction by microorganism was much faster than palladium catalyst.The experiments on influencing factors further confirmed that compared with MBfR,Pd-MBfR did not improve Br O₃^-removal rate in most conditions.Thus,MPfR and Pd-MBfR are not the preferred technologies for Br O₃^-removal,as MBfR has lower cost.This paper documents that Pd-MBfR technology has broad spectrum,high efficiency,and environmental friendliness for the removal of oxidizing contaminants,and the synergistic mechanism of palladium catalytic reduction and microbial degradation has been explored,which provides a new method for the remediation of polluted water.