Monitoring And Mechanism Analysis Of Microbial Reduction Process Of HgS Nanoparticles

Mercury（Hg）is a global environmental pollutant,mainly from natural sources such as volcanic eruptions and anthropogenic emissions from fossil fuel combustion.The biogeochemical cycle of Hg,which determines the transport and transformation of mercury in various ecosystems and human exposure and health risks,is critical for study of Hg.Hg,released into the atmosphere,enters the surface and water through dry and wet deposition.It is generally considered that Hg is eventually immobilized in sediments in the form of an inactive species,mercury sulfide（Hg S）,through a series of chemical and biological transformations.What’s more,numerous studies have demonstrated that Hg S is formed as the by-products in many chemical processes（such as natural gas,oil production）.In addition,Hg removal from industrial wastewater is generally achieved by the precipitation of Hg S.However,it has been proved that Hg S is bioavailable and can be methylated by microbial methylators.Therefore,environmental toxicity of Hg S has been underestimated.With the development of analytical techniques in recent years,mercury sulfide nanoparticles(Hg S_NP),which have been demonstrated to be prevalent in various environmental setting,are regarded as an important Hg species.Understanding the transformation processes of Hg S_NPis critical to better assessing the risk of Hg S.In this paper,the reactivation process of Hg S_NPby microbes mediated reduction under anaerobic conditions was studied.Based on this objective,cold vapor atomic fluorescence spectrometry,isotope tracer,inductively coupled plasma mass spectrometry,microbial fuel cells（MFC）,and other technical methods were used to explore the reactivation process of Hg S_NP.In this study,we mainly focused on monitoring and mechanism elucidation of the Hg S_NPreduction process mediated by iron reducing bacteria G.sulfurreducens PCA.We established a platform to monitor the reduction Hg S_NPof iron reducing bacteria using cold vapor atomic fluorescence spectrometry.The results showed that Hg S_NPcan be rapidly reduced by G.sulfurreducens PCA cells and the reduction ratio reached up to a maximum of 37%within 0.5 h.Compared with Hg²⁺-dissolved organic matter（DOM）and Hg²⁺exposure,it was found that Hg S_NPreduction was the highest,suggesting that reactivation of Hg S_NPby microbe mediated reduction could not be ignored.To determine the origin of Hg⁰,we used isotope tracing to distinguish the reduction process of Hg in nanoparticles,on the surface and in the solution.The results showed that the Hg²⁺on the surface of Hg S_NPwas easier to be reduced compared with other Hg²⁺,indicating that owing to the semiconductor character of Hg S_NP,extracellular electrons were transferred to the surface of particles leading to the reduction of Hg²⁺on the surface.Moreover,it was also found that the reduction of Hg S_NPdecreased with prolonged aging,suggesting the crystal structure played an important role in reduction process.However,3%of the Hg S_NP,which had aged for 4 years,still could be reduced,implying that the Hg S_NPexisting in natural environment also had the reduction potential.The effect of Fe and Mn to the reduction of Hg S_NPwas also explored.The results showed that Fe and Mn promoted the reduction of Hg S_NP.It has been reported that Fe and Mn can act as electron shuttling during the extracellular transfer.By using MFC equipment,we demonstrate that Fe and Mn were the dominant acceptors of electrons in exposing medium,suggesting that the role of electron shuttling in Hg S_NPreduction.By monitoring the microbial reduction process of Hg S_NPand elucidating the reduction mechanism,it is proved that the microbial reduction of Hg S_NPis a non-negligible reactivation process in anaerobic environment.The produced Hg⁰and the subsequent oxidation process could significantly affect the bioavailability of Hg in sediments and the methylation potential.