The Study on the Reduction and Toxicity of Pd(Ⅱ) by Microorganisms and Catalytic Degradation of Triclosan by Pd(0)-NPs

Author:Chen Yuan

Supervisor:chen yuan cai


Degree Year:2019





With highly catalytic activity and selectivity,palladium is widely used in catalytic,electronics and chemistry industries.However,palladium is expensive and very rare in the earth,and wastewater containing palladium can also damage our ecosystem.Therefore,it is necessary to recover and repair palladium from the environment.The microbial recovery of palladium has the advantages of low cost and environmental friendliness,and can decrease the toxicity of Pd(Ⅱ).However,the microbial species of reducing Pd(Ⅱ)were still very limited,mainly concentrated in Shewanella oneidensis,Desulfovibrio desulfuricans and Geobacter sulfurreducens,most of which were belonged to anaerobic bacteria and grow slowly.Moreover,Pd(Ⅱ)were reduced by bacteria usually in presence of external electron donors and under strictly anaerobic condition,which are complex and costly.The reduction amount of Pd(Ⅱ)by most of bacteria were also not very high.In this study,two aerobic bacteria for reducing Pd(Ⅱ)were screened to solve these problems.Firstly,a strain identified as Bacillus wiedmannii MSM(B.wiedmannii MSM)was isolated.Experiment indicated that B.wiedmannii MSM can reduce Pd(Ⅱ)without additional electron donors and under aerobic condition,and Pd(Ⅱ)reduction can be enhanced with suitable electron donors and under anaerobic condition.Different electron donors and the cells at different growth period have different influences on Pd(Ⅱ)reduction by B.wiedmannii MSM.XRD and XPS showed that B.wiedmannii MSM can reduce Pd(Ⅱ)at 20-40°C and pH 3.0-7.0in the presence of sodium formate,and the reduction of Pd(Ⅱ)by logarithmic growth cells with sodium formate was best.TEM indicated that Pd(0)-NPs were mainly uniformly distributed in the periplasmic space.However,the autoclaved cells could not synthesize Pd(0)-NPs,proving the role of enzyme in the reduction of Pd(Ⅱ).A few of Pd(0)-NPs was only formed on the surface of Cu(Ⅱ)-treated cells,which proved the main role of periplasmic hydrogenase in the reduction of Pd(Ⅱ).FT-IR results showed that amino,hydroxyl and carboxyl groups of the bacteria were the main functional groups binding to Pd(Ⅱ).The toxicity of Pd(Ⅱ)and Pd(0)-NPs to B.wiedmannii MSM were further discussed.A certain concentration of Pd(Ⅱ)can prolong the growth adaptation period and inhibit the growth of B.wiedmannii MSM,the minimum inhibitory concentration(MIC24h)of Pd(Ⅱ)for 24 h was150 mg L-1.The toxicity of Pd(Ⅱ)to B.wiedmannii MSM would decrease after reduction of Pd(Ⅱ)by B.wiedmannii MSM,which is several times lower than that of adsorption.Under aerobic condition,the bactericidal effect of Pd(0)-NPs on bacteria is mainly through residual Pd(Ⅱ)and oxidative stress,followed by physical puncture.Under anaerobic condition,the bactericidal effect of Pd(0)-NPs on B.wiedmannii MSM is mainly through residual Pd(Ⅱ),followed by physical puncture.Flow cytometry analysis showed that Pd(Ⅱ)could increase cell membrane permeability,decrease cell membrane potential and increase ROS level in cells,thereby inactivating cells.SEM results showed that high concentration of Pd(Ⅱ)would destroy the cell.XPS results showed that ROS(ROS produced by Pd(Ⅱ)and Pd(0)-NPs reacting with O2)would react with hydrocarbons on the cell surface,resulting in decreasing hydrocarbon content,increasing polysaccharide and polypeptide content,and inactivating cells due to oxidative stress and structural damage.Another strain was identified as Bacillus megaterium Y-4(B.megaterium Y-4).With the optimal electron donor(sodium formate),the removal amount of Pd(Ⅱ)by B.megaterium Y-4was as high as 1658.26 mg g-1.XRD and XPS analysis revealed that the bacteria could reduce Pd(Ⅱ)in a wide range of temperature(20-60°C)and pH(2.0-7.0),but could not reduce Pd(Ⅱ)under aerobic condition.The best reduction of Pd(Ⅱ)was by logarithmic phase cells.The cell wall,periplasmic space and intracellular contents of B.megaterium Y-4 all contain different kinds of enzymes for reducing Pd(Ⅱ).In addition,the activity of extracellular and periplasmic enzymes was more sensitive to temperature than intracellular enzymes.Triclosan(TCS)is widely used in human life as a broad-spectrum antibacterial agent.However,TCS has potential hazards to ecosystem and human health.The TCS degradation and removal methods mainly include physical,chemical and biological technologies.The physical method is only a matter transfer from environment.Anaerobic biodegradation of TCS is almost non-degradable.Aerobic biodegradation of TCS is long,and high concentration of TCS would produce inhibitory effects.Because the outer electrons of Pd are arranged at 4d105s0,under certain external conditions,d-orbital electrons can transit to s-orbital,forming d-band holes,which is conducive to activation of H2 and O2.Hydrogen supplying substances and oxygen(even air)can produce hydrogen peroxide in situ under the action of Pd and decompose hydrogen peroxide to produce highly oxidizing·OH.The advantage of this method is that H2O2is produced slowly and gradually,which can ensure that H2O2 is fully utilized and avoid invalid decomposition.At the same time,there is no excess H2O2 to inhibit the catalytic reaction,which greatly improves the utilization rate of H2O2.In this study,the catalytic degradation of TCS by Pd(0)-NPs(Pd(0)-NPs were prepared by B.megaterium Y-4)and formic acid were studied.Gas chromatography-mass spectrometry,electron spin resonance,total organic carbon and ion chromatography analysis indicated that Pd(0)-NPs not only can effectively dechlorination of TCS but also oxidize the dechlorination products of TCS and TCS with low concentration of formic acid.Pd(Ⅱ)was not detected in the solution after catalytic degradation of TCS by Pd(0)-NPs,indicating that Pd(0)-NPs was stable and would not cause secondary pollution.This study provides an environmentally method and some technical support for the remediation and reduction of Pd(Ⅱ)from wastes,the bactericidal mechanism of Pd(Ⅱ)and Pd(0)-NPs on B.wiedmannii MSM was revealed.This study has important guidance and reference value for the preparation of highly efficient and stable Pd(0)-NPs catalysts,a new green environmental degradation method of TCS was established.