| Previous studies reported that microbes and natural organic matter(NOM)could mediate the reduction of metal ions and graphene oxide(GO)to metal nanoparticles(MNPs)and reduced GO(rGO),respectively,which provided a new strategy for the green synthesis of nanomaterials.MNPs-rGO nanocomposites exhibited unique and excellent performances in fields of optics,catalysis,electronics,photoelectricity,and pollutant removal,etc.However,up to now,there were few reports about the preparation of MNPs-rGO composites using microbes and NOM.On the other hand,microbes and NOM are ubiquitous in natural environments.The study on microbes-and NOM-mediated formation of MNPs-rGO composites is also beneficial for the better understanding of the natural source/fate of nanomaterials in environments and has great environmental significance.In the present study,the formation of MNPs-rGO composites through the reduction of Au(Ⅲ)or Ag(Ⅰ)mediated by a model dissimilatory metal reducing bacterium—Shewanella oneidensis MR-1,and a typical NOM—Suwannee River humic acid(SRHA)was reported.The formed nanomaterials were characterized by a series of techniques and the mechanisms of microbes-and NOM-mediated reduction of metal ions and formation of MNPs-rGO were also analyzed.The main research contents and results are as follows:The Au nanoparticles(AuNPs)-rGO nanocomposite was prepared through the one-pot bioreduction of Au(Ⅲ)and GO by S.oneidensis MR-1 under growing conditions.AuNPs with an average size of 7.7 um were uniformly anchored on rGO nanosheets,and the N atoms might be doped into rGO during the microbial synthesis process.The effects of the formed AuNPs-rGO on the chemical,electrochemical,and biological reduction of nitro-aromatic compounds(NACs)were investigated.It was found that the biogenic AuNPs-rGO could promote the reduction of NACs,and its promoting performance was higher than those of AgNPs,rGO,AuNPs+rGO mixture,and chemically synthesized AuNPs-rGO.The formed AuNPs-rGO could also facilitate the bioreduction of NACs by MR-1 mutant strains lacking Mtr pathway components.Both MR-1 cells and their metabolites were vital for the formation of AuNPs/rGO,and the presence of GO could enhance the reduction of Au(Ⅲ).The extracellular polymeric substance(EPS)of S.oneidensis MR-1 was extracted and used to prepare AuNPs-rGO by in situ reduction of Au(Ⅲ)in the presence of rGO.The AuNPs-rGO formation process was analyzed.EPS molecules could be adsorbed on rGO surface through chemical complexation,and the coexistence of EPS and Au(Ⅲ)could promote their adsorption on rGO.The results of 3D excitation-emission matrix fluorescence spectroscopy showed that Au(Ⅲ)could bind with the protein and humic acid(HA)components of EPS,and the binding rate and ligand stability of Au(Ⅲ)-protein binding were higher than those of Au(Ⅲ)-HA binding.The roles of different molecular weight-fractionated EPS components in AuNPs formation were also investigated.It was found that the small molecular weight EPS was the main reducing agent of EPS,and the large molecular weight EPS could act as coating reagent to increase the stability of AuNPs.The carboxyl/carboxylate-containing substances in EPS may play crucial roles in stabilizing AuNPs.HA is one of the most important NOMs in the environment.SRHA was also used as a typical HA to reduce Ag(Ⅰ)for the formation of Ag nanoparticles(AgNPs).SRHA was able to reduce Ag(Ⅰ)to AgNPs in darkness at ambient temperature,no matter at high(40 mg/L)or low(1 μg/L)Ag(Ⅰ)concentrations.The reduction of Ag(Ⅰ)could be fitted with the pseudo-second-order reaction kinetics.The growth of AgNPs followed the Ostwald ripening and oriented attachment mechanisms.The formed AgNPs were mainly spherical and were stable in the solution without distinct aggregation.It was found that the increase of pH(6-9)and the coexistence of HCOO-,CH3COO-,CO32-,and SO42-could promote the formation of AgNPs.The formed AgNPs would coalesce to large aggregates under acidic conditions or in the presence of SO42-.HA in real environmental water sample could also convert Ag(Ⅰ)to AgNPs.Synchrotron radiation X-ray absorption fine structure(XAFS)spectroscopy analysis showed that Ag(I)was first bound to the carboxylic groups of SRHA to form Ag-OOC-ligands,which were then reduced to AgNPs.Besides AgNPs,Ag(Ⅰ)could also be partly transformed to Ag-S-.The SRHA was also used to in situ reduce Ag(Ⅰ)in the presence of rGO for the preparation of AgNPs-rGO nanocomposites.It was found that the formation of free AgNPs in the solution was suppressed by coexisting rGO,and most Ag(Ⅰ)was reduced on rGO surface,resulting in the formation of AgNPs-rGO.The average size(4.4 nm)and the size range(0-12 nm)of rGO-supported AgNPs were smaller than the average size(7.0 nm)and the size range(0-50 nm)of free AgNPs formed in the absence of rGO,respectively,which might be because the reduction rate of Ag(Ⅰ)on rGO surface was much higher than that in the aqueous phase.The toxicity of Ag(Ⅰ)was weakened after being reduced to AgNPs and AgNPs-rGO,and the toxicity of AgNPs-rGO was weaker than those of AgNPs and rGO.The XAFS study showed that Ag(Ⅰ)was first bound with SRHA,forming Ag-OOC-complexes,which could be adsorbed on rGO surface and rapidly reduced to AgNPs.The adsorbed Ag-OOC-could also be partially converted to Ag-S-.The rGO alone could also adsorb Ag(Ⅰ),reduce Ag(Ⅰ)to AgNPs or transform Ag(Ⅰ)to Ag-S-.The phenolic hydroxyl on rGO surface played essential roles in the Ag(Ⅰ)adsorption and reduction processes.Collectively,all the above results in this dissertation showed that microbes and NOM could in situ reduce metal ions to metal-graphene nanocomposites in the presence of graphene,and revealed important roles of carboxyl groups in NOM-mediated reduction of metal ions.The results of this study could provide an environmentally friendly alternative for the preparation of metal-graphene nanocomposites,and improve our understanding of the natural source,the environmental fates and risks,and the interaction of/between different nanomaterials in aqueous environments. |
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