The adsorbed pool of trace metals at mineral surfaces often represents a large portion of the bioaccessible fraction of these elements in soils and river systems. Desorption hysteresis has been observed for Ni with goethite, suggesting that Ni may become incorporated or entrapped after aging. In this study, we selected two pairs of metals (Ni-Cd and Ni-Zn) to elucidate the potential effects of competitive adsorption on Ni exchangeability. Isotope exchange experiments were performed after aging these metal pairs with 4 g/L goethite at pH 7 for 2 days, 1 month, and 2 months. At each aging timepoint, a subsample of the suspension was collected. From this subsample, half of the solution was removed to measure the dissolved metal concentration and replaced with a compositionally-identical solution but with the dissolved metals of interest isotopically enriched in 62Ni and 116Cd or 68Zn. The evolution of the metal isotope ratios was then measured with time. Reactive transport modeling was used to simulate how mass transfer processes may affect exchange rates. XAS spectra evaluating the Ni coordination after aging will be presented. For both metal pairs, Ni exchangeability decreases substantially after 1 month (relative to the single metal system where only Ni is present), indicating that Zn and Cd may block surface sites less favorable for Ni incorporation or may enhance structural trapping. In Ni-Zn experiments, up to 10% of the Zn becomes inaccessible after aging, which was not observed with only Zn present. If Ni is incorporating into goethite, local distortion of the mineral structure may promote Zn incorporation. Cd exchangeability in Ni-Cd experiments remained unchanged, suggesting that larger cations do not incorporate or become entrapped during competitive adsorption. For all metals, exchange is biphasic with a fast-exchanging fraction accessible to solution on a timescale of minutes and a slow-exchanging pool accessible on a timescale of hours to days. Our reactive transport model demonstrates that particle aggregation may slow exchange rates and increase the size of the slow-exchanging pool. These results suggest that competition influences trace metal accessibility at the goethite surface and that the exchangeable trace metal pool is capable of desorbing in response to an incremental change in chemical conditions on a timescale of minutes to days.

Competitive effects on adsorbed trace metal accessibility after aging with goethite

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