44721-GB10
Aggregation-Based Growth and Reaction Mechanisms of Iron Oxyhydroxide Nanoparticles
In Year 2 of this grant, we conducted macroscopic batch experiments on the following systems:
1) Uptake of Cu(II) and Zn(II) onto nanoparticle suspensions both before and after aggregation was induced by independent variations in pH and ionic strength
2) Desorption of Cu(II) and Zn(II) from nanoparticle suspensions following aggregation induced by independent variations in pH and ionic strength
3) Real-time adsorption and desorption of Cu(II) on iron oxyhydroxide nanoparticles using an ion selective electrode after the particles have undergone aggregation by increased ionic strength (1.0 M), pH (pH 10), and temperature (75°C, 4 days).
X-ray absorption spectroscopic analyses of selected samples from the above macroscopic batch experiments have been conducted in order to assess changes in sorbed metal(loid) speciation as a function of aggregation mechanism, desorption, time, and/or temperature.
We have also conducted triplicate surface area measurements of dried nanoparticle aggregate samples to track changes in surface area with aggregation.
We have generated from these studies the following initial conclusions:
1) Metal(loid) species which sorb quickly to the iron oxyhydroxide particles (As(V), Cu(II)) appear to passivate the particle surface, impeding the growth of the nanoparticles with progressive aging; in contrast, species that sorb more slowly (Hg(II), Zn(II)) have considerably less impact on particle growth.
2) Changes in the speciation of metal(loid)s with increasing aggregation state suggest shifts in the mode of metal uptake as a function of aggregation, possibly indicating structural incorporation of the metal(loid) into the nanoparticle aggregates.
3) Desorption of metals from nanoparticle aggregates appears to remove the more weakly-associated sorbed complexes, leaving behind species which are more strongly bound, perhaps again as structurally-incorporated or surface precipitated phases.
4) Different mechanisms of aggregation result in dramatically different degrees of metal uptake and retention, indicating that “aggregation” is a term more representative of a continuum of aggregation states, from loosely-aggregated to tightly-aggregated.
5) Metal ion adsorption rates are very rapid and thus roughly comparable regardless of aggregation mechanism; however, desorption rates and behaviors are more variable, with temperature-aggregated particles initially desorbing larger proportions of sorbed metals but resorbing them over the course of several hours.
This work has involved the training of 6 undergraduate students in basic lab skills, adsorption/desorption batch experimentation, and sample analysis using BET surface area measurements, atomic adsorption spectrometry, and EXAFS spectroscopy. All students have also had the experience to present their results in professional national and/or international conferences.
Broadly speaking, this research explores the interactions between the dissolved forms of potentially toxic heavy metals and nanosized particles of iron oxyhydroxides which may serve as an effective strategy by which to remove such metals from the aqueous phase and concentrate them in the solid phase where they can be more easily controlled and retained. By investigating at the molecular level the specific processes by which metals may be adsorbed, incorporated, and (co)-precipitated with these nanoparticles, we can obtain better insight into the fundamental mechanisms of metal uptake at the solid-water interface. These findings represent direct contributions to the field of low-temperature aqueous geochemistry.
