The dynamics of trace metals at mineral surfaces influence their fate and bioavailability in the environment. For some trace metals, desorption hysteresis has been observed after aging with iron (oxyhydr)oxides1, suggesting that metal cations may become incorporated or sequestered. In this study, we selected three metals to represent a range of cation sizes (Cd, Ni, or Zn) and performed isotope exchange experiments after aging with goethite for 2 days, 1 month, 2 months, and 4 months. In these experiments, natural isotopic abundance Cd, Ni, or Zn was equilibrated with goethite and at each aging timepoint a 100 mL subsample of the suspension was collected. 50 mL of the solution was removed from this subsample to measure the dissolved metal concentration and was replaced with 50 mL of a compositionally identical solution but with the dissolved metal of interest isotopically enriched in 116Cd, 62Ni, or 68Zn. The evolution of the isotope ratio of the dissolved metal was then measured over a period of five days. Dynamic light scattering and transmission electron microscopy measurements were collected to assess potential changes in particle morphology and aggregation. We find that the exchangeability of the adsorbed metal pool followed the order Cd &gt; Zn &gt; Ni, mirroring the trend in cation size. For Ni(II), the smallest cation with the radius closest to that of Fe(III) in goethite, there is a notable decrease in the exchangeable fraction with time, from 80 percent at 2 days to 52 percent after 2 months. This finding suggests that nickel may become progressively incorporated after aging with goethite even in the absence of Fe(II) or organic ligands that promote recrystallization. For Zn(II), the exchangeable fraction does not change with time, however, rates of exchange slow with aging. Dynamic light scattering measurements indicate that this decrease in exchange rate may be attributable to particle aggregation over time. Cd(II), the largest cation studied, shows nearly identical exchange rates after aging. This study also demonstrates that larger cations adsorbed to iron oxyhydroxides may be largely bioavailable at circumneutral pH because they are capable of rapid and near-complete desorption.

Reactivity and availability of adsorbed trace metals after aging with goethite

ACS Spring 2022

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