MECHANISMS OF NI SORPTION TO MN OXYHYDROXIDE FROM EXAFS AND IMPLICATIONS FOR NI ISOTOPE MASS BALANCE IN THE OCEANS

Many marine organisms are crucially dependent on nickel as a trace-metal co-factor for ureases, (de)hydrogenases, and methyl co-enzyme M reductase. Despite the importance of these enzymes to the C and N cycles, little is known about controls on the Ni concentration in the global oceans now or in the geologic past. Sorption of Ni to Mn oxyhydroxides in ferromanganese crusts has been proposed as a primary control on seawater Ni concentration [1], and such crusts have been found to be enriched in heavier isotopes of Ni compared to seawater [2,3]. The isotopic result is rather puzzling, as an isotopically light sink was expected, given that seawater is considerably heavier than known sources of Ni to the oceans.

To examine the molecular mechanisms by which Ni is incorporated into Mn oxyhydroxides, we conducted sorption experiments at low and high ionic strength with synthetic birnessite. We measured Ni stable isotope ratios and collected Ni EXAFS spectra from our experimental products. At low ionic strength, in short-duration experiments, we found that sorbed Ni is 1.4‰ lighter than dissolved Ni. This contrasts with what is observed between seawater and natural crusts. We can fit the EXAFS spectra for these samples well with a linear combination of Ni in triple corner-sharing complexes on {001} (85%) and Ni occupying Mn vacancy sites (15%). In synthetic seawater (I=0.72m) at short durations, we observed an even larger fractionation (up to 4.0‰), although the fractionation was significantly smaller in longer duration experiments. EXAFS spectra for Ni in high ionic strength experiments are fit well with 65% of Ni in triple corner-sharing complexes and 35% in Mn vacancy sites. We hypothesize that competitive sorption on {001} by other cations drives Ni into Mn vacancy sites at a faster rate than observed at low ionic strength. We further speculate that the isotopic fractionation we observe is dominated by a kinetic effect that will continue to decrease over time and may even reverse once equilibrium is approached and nearly all sorbed Ni is occupying Mn sites within the birnessite structure.

[1] Peacock &. Sherman, 2007, Chem. Geol.; [2] Cameron & Vance 2014, GCA; [3] Gall et al., 2013, EPSL.