Microscopic Mechanism Of Pb,Cu And Organic Carbon Sequestration During The Transformation Of Iron Oxides Under The Influence Of Natural Organic Matter

Iron oxides are ubiquitous in natural soils and sediments.In the natural environment,the transformation of iron oxides often associates with natural organic matter(NOM)and heavy metals.NOM and heavy metals will therefore affect the transformation rate and mineral composition of iron oxides.At the same time,the process of iron oxide transformation will affect the environmental behaviors of NOM and heavy metals.Although many studies have been done concerning the behavior of heavy metals in "heavy metal-NOM","heavy metal-iron oxide" and "iron oxide-heavy metal-NOM" systems.There is still a lack of knowledge on the dynamic interactions among NOM,iron oxides,and heavy metals especially the nano scale mechanisms during the iron oxide transformation process,which has significant implications on the fate of metals and carbon(C)in the environment.In this study,iron oxide transformation process in the presence of NOM and heavy metal was explored.Cs-STEM together with XAS were used to unveil the underlying nanoscale mechanisms within the interactions of “iron oxides,NOM and heavy metals(Pb/Cu)”.Results in this study will benefit a better understanding of the behaviors of heavy metals in the natural environment and also the geochemical cycling of NOM.We expect that our results will help to elucidate the mechanism of NOM-mineral interactions and to develop models for predicting the dynamic interactions among NOM,minerals and heavy metals.The major findings are as follows:1.The distribution mechanisms of Pb and hematite at sub-nano scales during the transformation of ferrihydrite were unveiled.Batch extraction experiments,STEM-EDS elemental mapping and line scan results showed that Pb can be incorporated into hematite nanoparticles during the transformation process.Sub-nano scale STEM analysis,XRD fine scan and XAS analysis provided direct evidences for Pb incorporation into the crystal structures of hematite nanoparticles,in which Pb distributed along the zone axis and enlarged the lattice distance of hematite.Additionally,Pb was mainly associated with some specific hematite lattice planes of(012),(104),(110)and(113).2.Iron oxide transformation under the influence of Pb and HA was thoroughly studied and the mechanisms of Pb-HA-iron oxide interactions were elucidated.Results indicated that HA and Pb hindered the ferrihydrite transformation to hematite.STEM analysis showed that,compared with the compact pure hematite,hematite nano particles in the presence of HA and Pb always had a porous edge structure.In addition,EDS mapping and EDS/EELS line scan results clearly indicated that HA and Pb can penetrate into the nano pores of the porous hematite particles.3.The mechanisms of HA complexation with iron oxides in the presence of Pb ions were elucidated with Cs-STEM.Results indicated that Pb ions made the hematite aggregates become more disperse in the “Hematite-Pb-HA” adsorption system.With the presence of Pb,HA was not only adsorbed on the edges of hematite nanoparticles,but also localized in the extended areas away from the hematite particles/aggregates,resulting in a heterogeneous distribution of organic carbon(OC)in hematite-HA systems.Our imaging methods provided direct visualization of the mineral-SOM-metal interactions at nano scales,which shed lights on the underlying mechanisms for OC stabilization in natural environments and can enable more robust constraints in the theoretical models describing organic matter-mineral interactions.4.Two types of NOM,fulvic acid and humic acid(FA and HA)were used to investigate the impact of FA and HA on the kinetics of Cu release from iron oxides during the iron oxide transformation.The kinetic experiments showed that Cu release decreased with the reaction time,indicating that iron oxide transformation inhibited Cu release.This is due to the incorporation of Cu into the structure of hematite during the iron oxide transformation.Both FA and HA enhanced Cu release,which resulted in more labile Cu on iron oxides.5.The temporal changes of Cu speciation and nano scale distribution of FA and HA on iron oxides during the transformation of iron oxide in the presence of Cu and FA/HA were determined with Cs-STEM and EXAFS.Results indicated that both HA and FA resulted in the formation of ellipsoidal hematite particles with a loose structure,and the spatial distribution of HA and FA on iron oxides was significantly different.Results indicated that both FA and HA slowed down the iron oxide transformation process.EXAFS and Cs-STEM results unveiled that Cu can be incorporated into the hematite nano particles during the iron oxide transformation process,while FA and HA hindered this process,leading to the increase of Cu release in both the stirred-flow and batch extraction experiments with the presence of FA and HA.