Dechlorination Mechanisms Of Iron-based Nanoparticles In Reductive Degradation Of Carbon Tetrachloride

Iron-based nanoparticles,including nanoscale zero-valent iron(nZVI)and iron minerals,have drawn intensive research interest recently due to their potential applications in environmental remediation and biogeochemical element cycling.nZVI has been extensively applied as a low-cost,environmentally-benign reductant for degradation of various recalcitrant organic contaminants in water environment,but its reactivity usually declines over time due to formation of passive iron oxides and is usually complicated by the co-presence of other contaminants,which limits its practical application.On the other hand,the dissolution of iron oxides can led to the formation of secondary iron minerals.Such processes can be significantly affected by biological processes,but the underlying mechanisms remain poorly understood so far.In this thesis,we aim to address the above issues by investigating into the dechlorination behaviors and mechanisms of two typical iron-based nanoparticles(i.e,nZVI and biogenic ferrous sulfide(FeS)nanoparticles using carbon tetrachloride(CT)as a model chlorinated organic compound.CT is an environmentally-ubiquitous recalcitrant contaminant in environment with high potential risks to ecosystem and human health.Firstly,we investigated the performances of nZVI in dechlorination of CT in un-buffered solution.We observed a sustained reactivity of nZVI for dechlorination of CT in water during several consecutive reaction cycles.The dechlorination rate constants increased substantially in Cycle 2,then remained at a high level over several consecutive cycles,and ultimately declined in the last cycle.In the entire process,the solution pH increased only slightly from 7.0 to 7.8,which was different from other unbuffered nZVI reduction systems reported before.Characterization of the particle surface morphology and composition revealed an important role of Fe oxyhydroxides formation in self-buffering the solution pH and sustaining a high nZVI reactivity.This part provides new knowledge on the nZVI dechlorination process and may offer implications for extending the lifetime of nZVI in wastewater treatment and environmental remediation applications.Subsequently.the influence of phosphate on the reductive dechloriantion of CT by nZVI was investigated.In light that phosphate can play a dual role of pH buffer and ferrous-binder in this reaction system,we conducted the dechlorination experiment in both unbuffered and HEPES-buffered systems to distinguish the specific influences and elucidate the underlying mechanisms.Significantly different dechlorination rates were observed in the two system.The HEPES-buffered solution showed considerable physical adsorption of phosphate on nZVI surface and formation of abundant iron phosphate compounds(vivianite)as well as high H2 evolution,which together resulted in fast consumption of nZVI.While in the unbuffered system,the presence of phosphate led to raised pH and decreased nZVI corrosion,which renders longer lifetime and sustained reactivity of nZVI.Lastly,we investigated how the biosynthesis of FeS nanoparticles by Shewanella species would affect their reductive dechlorination.Sulfide is a common contaminants formed in anaerobic processes and can significantly affect the decontamination behaviors of microbes.On the other hand,dissimilatory metal reducing bacteria(DMRB)widely exist in the subsurface environment and are involved in various contaminant degradation and element geochemical cycling processes.Recent studies suggest that DMRB can biosynthesize metal nanoparticles during metal reduction,but it is unclear yet how such biogenic nanomaterials would affect their decontamination behaviors.Here,we found that the dechlorination rates of carbon tetrachloride(CT)by Shewanella putrefaciens CN32 was significantly increased by 8 times with the formation of biogenic ferrous sulfide(FeS)nanoparticles.The pasteurized biogenic FeS enabled 5 times faster dechlorination than abiotic FeS that had larger sizes and irregular structure,confirming a significant contribution of the biogenic FeS to CT bioreduction resulting from its good dispersion and relatively high dechlorination activity.This work highlights a potentially important role of biosynthesized nanoparticles in environmental remediation.