Reactivity of Adsorbed Trace Metals at Metal (Oxyhydr)oxide Surfaces as Probed with Isotope Exchange
ORCID
0000-0001-8937-4209
Date of Award
5-8-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Metal (oxyhydr)oxide surfaces regulate the accessibility of trace metals in soils and aquatic systems. Up to half of the trace metals in urban environments, river systems, and contaminated soils are associated with mineral surfaces yet this pool often remains bioaccessible. Past studies of adsorption-desorption suggest that macroscopic rates of adsorption at mineral surfaces are likely mass transfer-limited and show biphasic rates, with a fast-adsorbing fraction equilibrating in seconds to minutes and a slower-adsorbing pool binding over a timescale of hours to days. In addition, adsorption-desorption hysteresis has been observed for some metals after aging at metal (oxyhydr)oxide surfaces, indicating that trace metals can become entrapped or incorporated. Traditional approaches to investigate the dynamics of solid-associated metals, such as acid desorption and selective chemical extractions, perturb chemical equilibrium in ways that prevent the accurate quantification of labile fractions and exchange rates at mineral surfaces. Decreasing the pH may alter the aggregation properties of mineral particles and coordination environment of adsorbed metals. Isotope exchange avoids these complicating factors and allows for the accurate quantification of labile fractions and exchange rates without perturbing chemical equilibrium. This technique relies on the Principle of Detailed Balancing, which states that at equilibrium, forward and reverse rates of reaction are equal but not zero. By isotopically labeling the dissolved pool after metal of natural isotopic abundance has adsorbed to the surface, rates of exchange between the surface-adsorbed pool and dissolved pool can be measured. The goal of this thesis is to assess the reactivity of adsorbed metals at metal (oxyhydr)oxide surfaces by accurately quantifying labile fractions and exchange rates in conditions realistic to aquatic systems. Lead is a potent neurotoxin to organisms even at low concentrations and its dynamics at mineral surfaces affect its mobility in the environment. To probe the reactivity of the adsorbed Pb(II) pool at metal oxide surfaces, isotope exchange experiments were conducted between dissolved and adsorbed pools at the rutile surface at pH values of 5, 6, and 7. Rates of exchange were pH-dependent with half-times of exchange decreased from ~6 hours at pH 5 to ~5 minutes at pH 7. More than 95% of the surface-adsorbed Pb(II) fraction was exchangeable, with a fast-exchanging pool accessible on a timescale of minutes and a slower-exchanging pool accessible on a timescale of hours to days. Pb LIII-edge EXAFS spectra reveal that binding mechanisms do not vary substantially with pH or Pb(II) loading, indicating that pH-dependent exchange rates are not attributable to the coordination environment. Particle aggregation increases with decreasing pH, suggesting that the slower exchange at pH 5 may be associated with intraparticle and interparticle diffusion. These observations reveal that the adsorbed Pb(II) pool can desorb rapidly to replace the dissolved pool in response to incremental changes in solution chemistry such as a decrease in pH, increase in ionic strength, or increase in the complexing ligand concentration. Goethite is a widely abundant and reactive mineral in soils that is a known scavenger of trace metals. Adsorption-desorption hysteresis has been observed for some metal cations after aging with goethite, suggesting that these ions may become entrapped by or incorporated in the mineral. To investigate the systematics of irreversible trace metal adsorption after aging with goethite, three metals (Ni, Zn, and Cd) were selected that represent a range of ionic radii varying by 35% but have hydrated radii that only differ by 4%. Each metal was aged with goethite and at timepoints of 2 days, 30 days, 60 days, and 120 days isotope exchange experiments were performed. The results from these experiments reveal that the non-labile fraction scales with the ionic radius. After 120 days aging, 8% of the Cd was non-exchangeable and 70% of the Ni was irreversibly bound. Spectroscopy and total digestions suggest that the irreversibly bound pool is incorporated into the goethite solids. This work provides a pathway for the incorporation of trace metals into goethite in oxic conditions. Incorporation may limit trace metal availability for biogeochemical cycling and preserve isotope ratios of fluids in contact with iron (oxyhydr)oxides early in the aging process. Aquatic environments possess a wide array of co-occurring trace metals that may interact at mineral surface binding sites. The presence of multiple metals may have synergistic or competitive effects on irreversible binding. To study how mixed-metal systems may influence labile trace metal fractions and exchange rates between dissolved and adsorbed pools at iron (oxyhydr)oxide surfaces, two metal pairs (Ni-Zn and Ni-Cd) were selected for comparison with single metal systems from our previous chapter. Isotope exchange experiments were performed at aging timepoints of 2 days, 30 days, and 60 days. These experiments suggest that competition for surface sites increases lability for all three cations on short timescales. After 60 days, all three cations were less labile at the goethite surface than in single metal systems. Competition for higher-affinity sites on the {021} surface may drive a larger proportion of each trace metal to lower affinity sites on the {110} and {101} faces, which have more porous Fe vacancy defect sites and may be more favorable for incorporation. Initial incorporation may also have a synergistic effect on further incorporation by locally opening the goethite lattice with substitution of larger cations. This study suggests that real aquatic systems may display complex synergistic and competitive effects at mineral surfaces, which affect their availability. Through these effects, a larger proportion of key trace metal nutrients and toxic metals with larger ionic radii may become irreversibly bound. This work present some of the first trace metal isotope exchange data between adsorbed and dissolved pools at mineral surfaces. These isotope exchange studies suggest that intraparticle and interparticle diffusion rates play an important role in exchange rates between bulk solution and the surface-adsorbed pool. At metal (oxyhydr)oxide surfaces, the surface-adsorbed pool can exchange rapidly, indicating that the labile fraction can respond rapidly to replace the dissolved pool due to incremental shifts in chemical conditions. This research also shows that a non-exchangeable fraction may exist at mineral surfaces and this pool shows a dependence on ionic radius and mineral surface properties. The presence of multiple ions may introduce complex competitive and synergistic effects that influence labile fractions. Lastly, this study provides evidence that face specific reactivity and defects in minerals may play an important role in the lability of trace metals.
Language
English (en)
Chair and Committee
Jeffrey Catalano
Recommended Citation
Ledingham, Greg, "Reactivity of Adsorbed Trace Metals at Metal (Oxyhydr)oxide Surfaces as Probed with Isotope Exchange" (2024). Arts & Sciences Electronic Theses and Dissertations. 3036.
https://openscholarship.wustl.edu/art_sci_etds/3036