Effects of humic acid and fulvic acid on the sequestration of copper and carbon during the iron oxide transformation

In natural environments, the iron oxide transformation is associated with natural organic matter (NOM) and heavy metals. However, there is still a lack of knowledge on the dynamic interactions among NOM, iron oxides, and heavy metals during the iron oxide transformation process, which has significant implications on the fate of metals and carbon (C) in the environment. In this study, two types of NOM, fulvic acid and humic acid (FA and HA) were used to elucidate the influence of NOM on the iron oxide transformation and the dynamic distribution and speciation of Cu and C during the iron oxide transformation. The X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that both FA and HA decreased the rates of ferrihydrite transformation to hematite. The EXAFS analysis of Cu speciation and spherical aberration corrected scanning transmission electron microscope (Cs-STEM) showed that Cu could be incorporated into the hematite particles while HA and FA hindered the incorporation of Cu, consistent with the chemical extraction results. Compared with FA, HA resulted in more extractable Cu by diluted acid and less Cu incorporation into iron oxides. The Cs-STEM results showed that both HA and FA resulted in the formation of ellipsoidal hematite particles with a loose structure. Batch extraction experiments and the Cs-STEM imaging, collectively, indicated that HA might penetrate into the loose and porous structure of hematite nanoparticles, which was less evident for FA. Results in this study help to understand the environmental behaviors of C and Cu during the iron oxide transformation process and also shed light on the nanoscale mechanisms of C sequestration within iron oxides.