Development Of Functional Bio-nanocapsules For Hexavalent Chromium Reduction

As an important metal raw material,chromium is widely used in metal smelting,electroplating,leather tanning and other industries.The heavy use of chromium causes it to be largely discharged into the environment,especially into water bodies.As a typical heavy metal pollutant,hexavalent chromium seriously threatens the ecological environment and harms human health.Therefore,how to efficiently treat chromium-containing wastewater is an important issue for the wastewater treatment.Among various treatment methods for chromium-containing wastewater,biological treatment methods have the advantages of simple process and low cost,but also have problems such as low treatment rate and poor stability.In order to solve the above problems,in this thesis,the typical dissimilatory metal-reducing bacteria,Shewanella oneidensis MR-1,was used as the model bacterium.Shewanella was encapsulated by in-situ modification and two functionalized bio-nanocapsules were constructed.The synthesized Shewanella encapsulated bio-nanocapsules showed higher stress resistance and stronger hexavalent chromium treatment performance than unmodified Shewanella cells.Further,the mechanism of bio-nanocapsules to improve the removal of hexavalent chromium was explored.The main research results of the paper are as follows:(1)Based on the in-situ polymerization technology of polydopamine(PDA),a method for constructing biological nanocapsules was developed.First,the free-radical polymerization of dopamine under aerobic alkaline conditions was used to modify a polydopamine layer on the surface of bacterial cell,which resulted in the formation of cell@PDA bio-nanocapsule.Further optimization determined the optimal reaction conditions:the concentration of dopamine was 4 mg/mL,the concentration of bacteria was OD₆₀₀=4,and the reaction time was 3 h.Under these conditions,the cell viability of the synthesized bio-nanocapsules reached 91.6%,and bio-nanocapsules showed higher electron transfer efficiency and resistance than unmodified cells.(2)The removal performance of Cr(VI)by S.oneidensis MR-1@PDA biological nanocapsules was improved,and the mechanism for performance improvement was explored.Based on the construction method of biological nanocapsules,S.oneidensis MR-1@PDA bio-nanocapsules were prepared.Studies on the removal performance of Cr(VI)showed that bio-nanocapsules had higher electron utilization rate(24.0%)and Cr(VI)removal kinetic constant(0.075 h^-1),which were 11.7 times and 6.2 times that of unmodified Shewanella,respectively.Further,with genetic engineering and transcriptomic analyses,it was found that the nanoencapsulation could enhance the extracellular electron transfer efficiency of Shewanella,inhibit the transport of chromium ions into the cell,and reduce the oxidative damage to cells.Thus,the biocapsules showed improved Cr(VI)tolerance,enhanced metabolic activity,reduced electrons loss,and in turn improved the removal performance of Cr(VI).(3)S.oneidensis MR-1@PDA/FeS bio-nanocapsules were constructed to further enhance the removal performance of Cr(VI).Using the ferrous sulfide(FeS)synthesis ability of Shewanella,the FeS nanoparticle layer was formed on the surface of S.oneidensis MR-1@PDA biological nanocapsule by self-assembly modification.Finally,S.oneidensis MR-1@PDA/FeS bio-nanocapsule was constructed.Studies on the removal performance of Cr(VI)showed that S.oneidensis MR-1@PDA/FeS bio-nanocapsules had higher stability and higher chromium removal efficiency(The kinetic constant of hexavalent chromium removal reached 0.196 h^-1,which was 10.3 times that of unmodified Shewanella).