COMPETITIVE EFFECTS ON ADSORBED NICKEL ACCESSIBILITY AT THE GOETHITE SURFACE

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. Adsorption-desorption hysteresis has been observed for Ni with goethite, suggesting that Ni may become incorporated or entrapped after aging. However, desorption experiments may perturb the coordination environment of adsorbed metals, the distribution of labile Fe(III), and mineral aggregation properties, influencing the interpretation of labile metal fractions. Isotope exchange enables quantification of rates of exchange between adsorbed and dissolved metal pools without perturbing chemical equilibrium. In this study, we aged dissolved Ni as well as Ni-Zn and Ni-Cd mixtures with goethite at pH 7. At aging timepoints of 2 days, 1 month, and 2 months, isotope exchange experiments were performed to measure labile fractions of each metal and quantify rates of exchange between adsorbed and dissolved pools. In samples with a recalcitrant Ni pool, X-ray absorption near edge structure (XANES) was used to identify whether the residual fraction was incorporated, entrapped as an adsorbed species, or present as a surface precipitate.

We find that in the single metal system, 58% of the adsorbed Ni was labile after 2 months aging. With Zn and Cd present, Ni accessibility decreases to 42% and 44%, respectively. XANES spectra of the residual Ni pool in all three systems are consistent with incorporated nickel. These results indicate that Zn and Cd may block strong adsorption sites that are less favorable for Ni incorporation. Exchange rates are biphasic with a fast-exchanging fraction labile to solution on a timescale of minutes and a slow-exchanging pool labile on a timescale of hours to days. This study demonstrates a pathway by which Ni incorporates into goethite independent of coprecipitation or recrystallization and indicates that competitive adsorption increases the size of the recalcitrant Ni pool.