Gueguen Bleuenn
email: bleuenn.gueguen@univ-brest.fr

Jeffry V Sorensen

Lalonde Stefan

Pena Jasquelin

Brandy M Toner

Rouxel Olivier

Bleuenn Gueguen

Variable Ni isotope fractionation between Fe-oxyhydroxides and implications for the use of Ni isotopes as geochemical tracers

Keywords: Fe-oxyhydroxides, Sorption experiments, Nickel isotopes

Nickel (Ni) isotopes have recently emerged as a new biogeochemical tracer in marine environments, but our understanding of the mechanisms of Ni isotope fractionation in natural systems with regards to its fractionation by mineral surfaces is incomplete. This study aims to provide experimental constraints on Ni isotope fractionation during adsorption to goethite and 2-line ferrihydrite, two Fe minerals that vary in terms of distinct crystalline properties. We conducted two types of adsorption experiments: one with variable pH (5.0 to 8.0) and constant initial Ni concentration, one at a constant pH of 7.7 and variable initial Ni concentrations. Isotopic measurements were made on both the solid phase and the supernatant solutions in order to determine the Ni isotope fractionation factors (Δ60/58Nimin-aq = δ60/58Nimin - δ60/58Niaq) between the mineral and aqueous phases. Our results show preferential adsorption of lighter Ni isotopes during adsorption of Ni to Fe oxyhydroxides presumably under conditions of near equilibrium conditions. Adsorption to goethite generates the greatest fractionation, with Δ60/58Nimin-aq = -0.77 ± 0.23‰ (n = 14, 2sd), whereas adsorption to 2-line ferrihydrite samples yield Δ60/58Nimin-aq = -0.35 ± 0.08‰ (n = 16, 2sd). Using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we found that Ni forms an inner-sphere complex and that its coordination environment does not vary significantly with pH nor with surface loading. In addition, we found no evidence of Ni incorporation into the mineral. We suggest that the more than two-fold increase in Ni isotope fractionation in goethite relative to 2-line ferrihydrite is due to the lower Ni-Fe coordination number in the second shell, which results in the formation of a weaker surface complex and thus favors the adsorption of lighter Ni isotopes. These results show that Ni isotope fractionation during sorption by Fe-oxyhydroxides is dependent on mineralogy, which has important implications for the use of Ni isotopes as environmental tracers and the interpretation of their record in sedimentary rocks.

Introduction

Nickel is a micro-nutrient in modern oceans and displays a nutrient-type profile in seawater, where it is depleted in surface waters due to biological uptake [START_REF] Bruland | Oceanographic distributions of Cadmium, Zinc, Nickel, and Copper in the North Pacific[END_REF][START_REF] Price | Colimitation of phytoplankton growth by Nickel and Nitrogen[END_REF][START_REF] Morel | The Biogeochemical Cycles of Trace Metals in the Oceans[END_REF]. The evolution of marine biogeochemical Ni cycling has become a topic of much interest as ancient metalliferous deposits because Banded Iron Formations (BIF) appear to record a dramatic decrease in marine Ni concentrations through geological time [START_REF] Konhauser | Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event[END_REF][START_REF] Konhauser | The Archean Nickel Famine Revisited[END_REF], attributed to a shift from ultramafic volcanism and eruption of Ni-rich komatiites to basaltic volcanism as the Earth's crust cooled. Iron-rich sedimentary rocks such as Granular Iron Formations (GIF) and BIF were deposited from seawater throughout much of the Precambrian and have been widely used as an archive to reconstruct the geochemical composition of seawater in Precambrian marine environments, in which Fe-oxyhydroxides were the carriers of trace metals (e.g., [START_REF] Bekker | Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes[END_REF]. Hence, Ni isotope systematics of Iron Formations hold great promise for the study of ancient biogeochemical cycles and paleo-environments. But first, the mechanisms of Ni isotope fractionation during Fe-oxyhydroxide precipitation and burial need to be well constrained.

The geochemical cycling of Ni in modern oceans is largely controlled by input from continental weathering and riverine transport and removal from seawater through precipitation in authigenic Mn-rich sediments [START_REF] Sclater | On the marine geochemistry of nickel[END_REF][START_REF] Gall | Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic[END_REF][START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF]. Preliminary investigations have shown that seawater is characterized by an average δ 60/58 Ni value of 1.44 ±0.15‰ (2sd, n=29) relative to the NIST SRM 986 international standard, which is heavier than the average crustal value (estimated at 0.05 ±0.05‰; [START_REF] Gueguen | Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC-ICP-MS[END_REF]. The riverine input flux is estimated at a δ 60/58 Ni value of 0.80‰ with a range of values from 0.29 ‰ to 1.34‰ [START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF]. The oceanic output flux, dominated by authigenic Mn-oxyhydroxides [START_REF] Krishnaswami | Authigenic transition elements in Pacific pelagic clays[END_REF][START_REF] Leinen | The origin of paleochemical signatures in North Pacific pelagic clays: Partitioning experiments[END_REF][START_REF] Koschinsky | Sequential leaching of marine ferromanganese precipitates: Genetic implications[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF]Peacock and Sherman, 2007b), has a δ 60/58 Ni value of ~1.7‰ (i.e., the average Ni isotope composition of hydrogenetic ferromanganese crusts as reported by [START_REF] Gall | Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic[END_REF] and [START_REF] Gueguen | Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers[END_REF]). The isotopically heavy signature of rivers relative to the bulk crust implies that a light Ni isotopic reservoir should be retained in the continental crust, probably during soil formation [START_REF] Ratié | Nickel isotope fractionation during tropical weathering of ultramafic rocks[END_REF]. Based on isotopic mass balance considerations, [START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF] identified a missing source of isotopically heavy Ni in modern oceans.

Alternatively, the heavy Ni isotopic composition of seawater could be balanced by an oceanic sink enriched in light Ni isotopes, perhaps in sulfides [START_REF] Gueguen | Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC-ICP-MS[END_REF][START_REF] Hofmann | Comparing orthomagmatic and hydrothermal mineralization models for komatiite-hosted nickel deposits in Zimbabwe using multiple-sulfur, iron, and nickel isotope data[END_REF] associated with anoxic or suboxic marine sediments enriched in organic matter that accumulate in reducing environments (e.g., continental margins with strong upwelling regimes, restricted anoxic basins, fjords). However, the role of organic-rich sediments in the Ni isotope mass balance is unclear as bulk δ 60/58 Ni of organic-rich sediments display a large range from 0.28‰ to 2.5‰ [START_REF] Porter | Characterising the nickel isotopic composition of organic-rich marine sediments[END_REF].

Large terrestrial Ni reservoirs exist in ultramafic complexes [START_REF] Manceau | Heterogeneous distribution of nickel in hydrous silicates from New Caledonia ore deposits[END_REF][START_REF] Valeton | Genesis of nickel laterites and bauxites in greece during the jurassic and cretaceous, and their relation to ultrabasic parent rocks[END_REF][START_REF] Butt | Nickel Laterite Ore Deposits: Weathered Serpentinites[END_REF], and weathering of these complexes can lead to significant Ni enrichment in laterites. While Ni released during weathering of ultramafic complexes will have a greater affinity for Mn-oxides [START_REF] Manceau | Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia[END_REF], a significant portion of Ni is in fact sequestered in the abundant Fe-oxides (mostly goethite) formed during lateritization (Manceau et al., 2000;[START_REF] Quantin | Sources and extractibility of chromium and nickel in soil profiles developed on Czech serpentinites[END_REF]Dublet et al., 2012;[START_REF] Dublet | XAS evidence for Ni sequestration by siderite in a lateritic Ni-deposit from New Caledonia[END_REF][START_REF] Ratié | Nickel isotope fractionation during tropical weathering of ultramafic rocks[END_REF]. In a recent study, [START_REF] Ratié | Nickel isotope fractionation during tropical weathering of ultramafic rocks[END_REF] demonstrated that weathering of ultramafic complexes leads to the enrichment of light Ni isotopes (by -0.47 ‰) in Ni-rich laterites relative to unweathered bedrock, consistent with the preferential loss of heavy Ni isotopes to the mobile phase and eventually to the riverine flux. Nickel in these laterites is either incorporated into the mineral lattice or sorbed onto the mineral surface.

Nickel sorption on Fe-oxyhydroxides may therefore influence the isotope signature of Ni in weathering environments, and in turn, the riverine flux of Ni to the oceans.

The use of Ni isotopes as biosignatures has also garnered attention in the recent literature and the evolution of Ni biogeochemical cycling in the ancient oceans may be recorded in the Ni isotope composition of coeval authigenic sediments. However, understanding the isotope fractionation of metals such as Ni during adsorption to Fe-oxides and other authigenic minerals is paramount to accurately reconstruct the biogeochemical cycling of nutrients using the sedimentary record. [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] reported the equilibrium Ni isotope fractionation factor during Ni sorption to ferrihydrite (experiments done at room temperature), showing an enrichment of the light Ni isotopes on the solid phase by -0.35 ±0.10 ‰ (1 sd). However, further studies are needed in order to evaluate in more detail the mechanisms and range of parameters controlling Ni isotope fractionation during adsorption onto mineral phases, including the effects of coordination environment or the type of mineral phase (e.g., poorly crystalline versus more crystalline phases). For instance, different Ni isotope fractionation could be expected among different Fe-oxyhydroxides based on coordination chemistry considerations, because this effect has been observed for other metal/metalloid isotope systems such as Cu, Ge, Mo and Zn [START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF][START_REF] Li | First-principles study of Ge isotope fractionation during adsorption onto Fe(III)-oxyhydroxide surfaces[END_REF][START_REF] Wasylenki | The molecular mechanism of Mo isotope fractionation during adsorption to birnessite[END_REF][START_REF] Pokrovsky | Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides[END_REF]. These experimental data are crucial to interpret the trace metal isotopic records in Iron Formations, where the mechanism of formation of Fe-oxyhydroxides-precursor mineral phases and scavenging of trace metals are poorly known, and to evaluate the preservation of the isotopic signal during recrystallization. The role of iron oxides recrystallization on the Ni cycling is also important to consider as it could have impacted the Ni signal preserved in Iron Formations [START_REF] Frierdich | Trace element cycling through iron oxide minerals during redox-driven dynamic recrystallization[END_REF].

Materials and methods

We used synthetic mineral phases to investigate surface driven Ni isotope fractionation. The use of synthetic mineral phases allowed for Ni K-edge EXAFS spectroscopy on samples with low Ni concentration without interference from significant amounts of incorporated Ni. Goethite (α-FeOOH) and 2-line ferrihydrite were synthesized by raising the pH of a Fe(NO3)3 solution with KOH at 25°C. Goethite was formed by heating the solid precipitate at 70°C for 60 hours following the protocol of [START_REF] Cornell | The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses[END_REF]. The precipitates were washed three times with ultrapure water and centrifuged at 7,000 rpm for 30 minutes. The goethite was stored freeze-dried while the 2-line ferrihydrite was synthesized within 48 hours of each experiment and stored at -30°C. The synthetic minerals were characterized by BET (Brunauer-Emmett-Teller) surface area, powder X-ray diffraction, and electrophoretic mobility. For BET, samples were dried and degassed at 150°C before specific surface area was measured by N2 adsorption (Micromeritics Tristar II 3020).

The specific surface areas for goethite and 2-line ferrihydrite were 25.5 and 119 ± 3 m 2 /g, respectively. Bulk powder X-ray diffraction was performed using a Pananalytical X'Pert Pro diffractometer (Characterization Facility, University of Minnesota). Mineral surface charge, which is reflected in surface electrokinetic potential, was determined using a ZetaPALS analyzer (Brookhaven Instruments Corp.) for 0.98 -1.44 g/L of mineral suspended in ultrapure water. Instrumental settings consisted of an applied voltage of 7.54V and electric field of 9.29 V/cm. Data were acquired over 3 cycles of 5 measurements per mineral (Electronic Annex, Table A1).

Nickel sorption experiments

For the Ni sorption experiments, a stock solution of 17 mmol/L Ni (1000 mg/L Ni) was made with a Ni(NO3)2•H2O(aq) salt (Fischer Scientific). Aqueous suspensions were prepared with 0.15 g of goethite or 2-line ferrihydrite in 150 mL of 0.05 mol/L NaNO3 in 250 mL Teflon bottles. The Fe-oxyhydroxides were allowed to equilibrate for 24 hours at the target experimental pH before the addition of an aliquot of Ni from the stock solution. Nickel was added slowly using a micropipette over a 15 minutes' period, with continuous mixing with a Teflon-coated stir bar to reduce changes to the pH of the suspension. A control consisting of the mineral in 0.05 mol/L NaNO3 solution with no Ni added was included with each set of experiments and Ni in the blank did not exceed 0.1% of the total amount of Ni measured during Ni sorption experiments. Samples were named according to the adsorbent (i.e., "Fh" for 2-line ferrihydrite and "Goe" for goethite) and the type of experiment (i.e., "pH" for pH-dependent experiments, "C" for varying initial Ni concentration experiments and "T" for time-dependent experiments).

Nickel sorption experiments (Table 1) were performed as a function of pH (pH 5.0, 6.0, 7.0, 7.7 and 8.0) with an initial Ni concentration of 170 µmol/L Ni (10 mg/L Ni). A separate set of Ni sorption experiments were also performed at pH 7.7 as a function of initial Ni concentration (17,85,170,341,and 426 µmol/L Ni). The suspension pH was maintained constant within ± 0.2 pH units with a custom-built pH control system consisting of 6 Hanna Instruments HI 504 controllers, each equipped with a Thermo Scientific Orion 9110 pH electrode. Adjustments of the pH were made with additions of 0.05 mol/L HCl and 0.05 mol/L NaOH using two 3-way General Valve TM solenoid valves and a Manostat Carter peristaltic cassette pump. The pH electrodes were calibrated using pH 4 and pH 7 commercial buffers before each experiment. After the addition of Ni, the suspension was allowed to react for 24 hours before the solution was separated from the solids by vacuum filtration (0.22 µm polycarbonate filter, Millipore). The solids were not rinsed prior to storage or analysis. After filtration, the supernatant solution was refrigerated at 2°C until analysis by ICP-MS and the solid material on filter was stored at -20° C. The solid-phase samples on the filter membranes were cut in half and reserved for Ni K-edge EXAFS spectroscopy and isotopic analysis. Time-dependent experiments were done to check whether kinetic isotopic effects are affecting our experiments (Table 2). They were performed at a fixed pH of 7. A maximum equilibration time of 24 hours was chosen based on our initial test timedependent experiments with goethite and ferrihydrite in which the aqueous Ni concentration stabilized after 18 to 23 hours of reaction, consistent with experimental design implemented by [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF]. These findings are consistent with bulk chemical equilibrium and were used to choose the reaction time for the adsorption experiments. This also prevents the potential effect of diffusion-controlled penetration of Ni into Fe (oxy)hydroxides on Ni isotope fractionation, since this process has been shown to occur over long (week to month) timescales, while surface adsorption reaches completion within a few hours [START_REF] Bruemmer | Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals[END_REF]. As discussed below, at bulk chemical equilibrium the mineral continues to exchange Ni with the aqueous phase, so isotopic changes are still possible. By contrast, no net change in loading to the mineral occurs, implying that the Ni concentration in the mineral is constant.

Nickel K-edge EXAFS spectroscopy

Nickel K-edge EXAFS spectroscopy measurements were conducted at the Advanced Photon Source (Argonne National Laboratory) at beamline 20-BM in fluorescence mode using a 12-element Canberra germanium detector. The Ni-sorbed minerals were prepared for Ni K-edge EXAFS analysis by thawing to room temperature and transferring from the filter to a mylar adhesive with a microspatula. Multiple mylar layers were stacked (6-8 layers) and the edges sealed to form an envelope around the sample. Measurements were conducted at room temperature and individual scans were examined for systematic changes in peak-position or shape to detect photon-induced sample alteration; none was observed. Multiple scans were averaged to improve the signal to noise ratio. The Athena program was used for averaging replicate scans and energy calibration (Ni foil with inflection point set to 8334.5 eV), and the Artemis program was used to generate phase and amplitude paths for Ni K-edge EXAFS fitting [START_REF] Ravel | ATHENA, ARTEMIS, HEPHAESTUS: data analysis for Xray absorption spectroscopy using IFEFFIT[END_REF]. Data reduction, including background subtraction and conversion to k-space, and shell-by-shell fitting were performed using the SixPack software package [START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF] with the following settings: E0=8347 eV, Rbkg =1.0 Å [START_REF] Kelly | Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy[END_REF], a Gaussian pre-edge function, quadratic post-edge function, no clamping, default spline knots value, a Kaiser-Bessel window function, and a k-weight =3. The amplitude reduction factor, S0 2 = 0.96, was determined by fitting the first coordination shell of aqueous published standards (NiCl2 and Ni-EDTA) [START_REF] Peña | Mechanisms of nickel sorption by a bacteriogenic birnessite[END_REF]. Fits were evaluated using the calculated R-factor with a value of 0.05 or less representing a reasonable fit [START_REF] Kelly | Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy[END_REF].

X-ray absorption spectra were collected from 8130 to 9300 eV. Models for the adsorption geometry of Ni were based on the assumption that Ni coordinates to Fe octahedra via edge-sharing or corner-sharing linkages. First, the Ni-O shell was fit with all parameters varied. The optimized parameters for the first shell were constrained in subsequent fits, which included second-shell Ni-Fe paths.

The Ni-Fe second shell was modeled assuming to the occurrence of short and long bond distances, following the model used for Ni/Zn sorption on ferrihydrite (Arai, 2008;Cismasu et al., 2013). Specifically, two types of bond orientations between Fe and Ni octahedral were modeled. Type 1 is comprised of a Ni octahedron that shares an edge, two bond via two shared oxygen molecules, with an Fe octahedra (mineral surface). Type 2 is comprised of a Ni octahedron that shares a corner, or one bond via a shared oxygen, with an Fe octahedra (mineral surface). Type 1 bond distances are shorter and share more bonds with the mineral surface than Type 2. Distortion of the Ni-O coordination sphere can be influenced by either the number of bonds with the mineral surface, or geometric strain introduced due to the type of surface site (e.g., number/type of Fe octahedra) to which the Ni binds.

The Ni K-edge EXAFS model was based on published coordination number (CN) ratios of a short and long Ni-Fe and Zn-Fe paths in the second shell of Ni K-edge EXAFS data from Ni/Zn-adsorbed 2-line ferrihydrite samples (Table 3) [START_REF] Manceau | Surface structural model for ferrihydrite[END_REF][START_REF] Carvalho-E-Silva | Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy[END_REF]Arai, 2008;Dublet et al., 2012;Cismasu et al., 2013). Reported CN ratios for Ni-sorbed 2-line ferrihydrite ranged from 0.4 :0.7 (CNshort :CNlong) with a maximum total second-shell CN of 5 (Arai, 2008). Thus, CNshort :CNlong ratios of 1:1 to 4:5 were tested in the model. A ratio of 2:3 best described the Ni K-edge EXAFS data in both fit statistics and uncertainty of varied parameters. In the final model, two type 1 Ni-Fe paths were modeled for every three type 2 Ni-Fe paths. The uncertainties on the CN values are between 0.144 and 0.931 (estimated to be ~25% of the CN value) and the uncertainties on the interatomic distances are between 0.0078 and 0.0618 Å.

Aqueous speciation of Ni

The aqueous speciation of Ni was calculated for the initial experimental conditions using the MINEQL+ v.4.6 software program (Environmental Research Softaware) with default stability constants. The calculations used the initial Ni concentrations (17, 85, 170, 341, and 426 µmol Ni/L), pH range (5.0 to 8.0 in 20 steps), ionic strength (0.05 mol/L NaNO3), and partial pressure of CO2 g (0.03943 vol%) for a system in contact with the atmosphere and showed that no Ni precipitation is predicted for our experimental conditions.

We used Ni surface complexation values for bidentate adsorption on goethite [ 2 =SOH + Ni 2+ =2(SOH)Ni + 2H + , log K = -2.45 ] from [START_REF] Xu | Bidentate Complexation Modeling of Heavy Metal Adsorption and Competition on Goethite[END_REF], and for ferrihydrite, monodentate adsorption per [START_REF] Dzombak | Surface complexation modeling : hydrous ferric oxide[END_REF] (their classic two-site ferrihydrite model, strong and weak sites, as already included in vMINTEQ). For aqueous speciation calculations during adsorption modeling, the default vMINTEQ thermo.vdb database was used, which is an expanded version of the MINTEQA2 database. Between pH 5.0 and 7.0, the dominant Ni species in aqueous solution, under the initial experimental conditions considered here, is the Ni 2+ (hexaquo) (> 88 %). In addition to Ni 2+ (hexaquo), NiNO3 + is present and accounts for > 11% of the Ni in solution. For the experiments with a pH value greater than 7.0, the aqueous Ni species include minor contributions from NiHCO3 + and NiCO3 0 . Slow addition of Ni during the experiments over a 15-minutes period reduced the risk of precipitation of Ni above pH 7.8. For the experiments at pH 7.7, Ni occurs as: (1) 83.7% Ni 2+ (hexaquo); (2) 10.7 % NiNO3 + ; (3) 4.1 % NiHCO3 + ; and (4) 1.5 % NiCO3 0 for initial Ni concentrations ranging from 17 to 426 µmol/L.

Concentration determinations and column chromatographic purification of Ni

All manipulations were performed in a class 1000 clean room and exclusively used acid-cleaned plasticware and teflonware. Solid samples were digested in 6 mol/L HCl at room temperature for 72 hours. The total concentrations of Ni and Fe in the solids were determined by quadrupole ICP-MS (X-series2, Thermo Scientific) at the Pôle Spectrométrie Océan (PSO, Brest, France). Nickel concentrations in the filtrates were first estimated by mass balance using the initial concentrations in solutions used at the start of each experiment and the Ni concentrations in the solids determined by quadrupole ICP-MS at the end of the experiment.

Nickel concentrations were then recalculated by isotope dilution after isotope measurements.

For the separation of Ni onto chromatographic columns, an aliquot of each filtrate was evaporated to dryness at 80°C prior to redissolution in 1 ml of 6 mol/L HCl. A full description of the purification method can be found in [START_REF] Gueguen | Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC-ICP-MS[END_REF]. Briefly, our two-step separation procedure involved: (1) separation of Fe onto an anion exchange chromatographic resin (BioRad AG1-X8, 100-200 mesh); (2) separation of Ni from the remaining matrix using a Ni-specific resin (Ni-Spec by Eichrom). This Ni-resin contains dimethylglyoxime (DMG) functional groups, which complexes Ni in ammonium citrate solutions at pH 8-9. Nickel is then eluted with 3 mol/L HNO3 after breakdown of the Ni-DMG complex.

Ni isotope mass spectrometry and double-spike correction scheme

Nickel isotope compositions were measured by MC-ICP-MS (Neptune, Thermo Scientific) at the PSO (Brest, France) using a double spike for mass bias correction following the method described in [START_REF] Gueguen | Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC-ICP-MS[END_REF]. The double spike is a mixture of 61 Ni and 62 Ni that was added to samples at spike/natural ratio of 1 prior to the chromatographic separation on Ni-specific resin. Corrected ratios were calculated using an iterative scheme following the method described by [START_REF] Siebert | Determination of molybdenum isotope fractionation by double-spike multicollector inductively coupled plasma mass spectrometry[END_REF]. An ESI (Elemental Scientific Inc., USA) desolvating introduction system (ApexQ) was employed for all measurements. The nine Faraday cups of the instrument allowed simultaneous measurement of Ni isotopes 58 Ni, 60 Ni, 61 Ni, 62 Ni, 64 Ni, and potential isobaric interferences from Zn (measured on 66 Zn) and Fe (measured on 57 Fe). Argon oxide interferences (e.g., 40 Ar 18 O + on 58 Ni + and 40 Ar 17 O on 57 Fe) were resolved using the medium resolution mode of the MC-ICP-MS. Nickel isotope values were reported according to the conventional delta notation (equation 1) relative to the Ni international isotopic standard NIST SRM 986 : δ 60/58 Ni (‰) = ( 60/58 Nisample/ 60/58 NiNIST986 -1) x 1000

(1)

For each sample, 50 measurement cycles were performed, resulting in an internal standard error (2se) value typically between 0.02 and 0.04 ‰. The external precision was evaluated through the duplicated measurements of geostandards (USGS Nod-P-1, δ 60/58 Ni=0.33‰; and USGS Nod-A-1, δ 60/58 Ni=1.04‰) and was found to be ~0.06 ‰ (2sd).

Results

Nickel sorption by Fe oxyhydroxides

The Zeta potential measurements at pH 7 indicated that 2-line ferrihydrite exhibits a net negative surface charge with a value of -3.44 mV (Electronic Annex, Table A1). Zeta potential measurements at pH 7 indicated that goethite exhibits a net positive surface charge with a value of +3.46 mV. 2-line ferrihydrite has more Ni-sorbed at pH 7 than goethite, which is consistent with the more negative surface charge of 2-line ferrihydrite relative to goethite.

Surface complexation modeling

Constant capacitance surface complexation modeling (CCM) for Ni adsorption to 2line ferrihydrite and goethite (Figure 2) was implemented using adsorption parameters provided by [START_REF] Dzombak | Surface complexation modeling : hydrous ferric oxide[END_REF], [START_REF] Xu | Bidentate Complexation Modeling of Heavy Metal Adsorption and Competition on Goethite[END_REF] and [START_REF] Davis | Surface complexation modeling in aqueous geochemistry[END_REF], in order to verify that sorption processes control Ni behavior in our experiments. Our experimental data for 2-line ferrihydrite were in good agreement with the surface complexation model calculated with parameters from [START_REF] Dzombak | Surface complexation modeling : hydrous ferric oxide[END_REF] (Figure 2). The best fit of our data for Ni adsorption to goethite was obtained with a constant capacitance model (Figure 2) generated using adsorption constants from [START_REF] Xu | Bidentate Complexation Modeling of Heavy Metal Adsorption and Competition on Goethite[END_REF] for bidentate adsorption in the Nigoethite system, our BET-measured surface area of 25.5 m 2 /g, solid concentrations close to 1 g/L, and a goethite site density of 1 site per nm 2 . Adopting higher literature values for the latter (e.g., 2.3 sites per nm 2 [START_REF] Davis | Surface complexation modeling in aqueous geochemistry[END_REF] or 6 sites per nm 2 [START_REF] Xu | Bidentate Complexation Modeling of Heavy Metal Adsorption and Competition on Goethite[END_REF])

results in model adsorption values significantly higher than what was observed. Finally, it is important to point out that the CCM is a simple surface complexation model that integrates a smaller number of parameters in comparison to the diffuse-layer model and the triple-layer models [START_REF] Hayes | Surface complexation models: An evaluation of model parameter estimation using FITEQL and oxide mineral titration data[END_REF] and thus may not approximate natural conditions as closely.

Nickel K-edge EXAFS Spectroscopy

Nickel K-edge EXAFS spectra were collected for six Ni-sorbed 2-line ferrihydrite samples (Fh-pH-05, Fh-C-07, Fh-C-08, Fh-C-09, and Fh-C-10) and one Ni-sorbed goethite sample (Goe-C-06) as indicated in Table 1. The Ni K-edge EXAFS spectra collected for the 2-line ferrihydrite samples at pH 7.7 and 8.0 are similar in phase and amplitude despite different Ni-loadings and pH values (displayed in k-space with best fits; Figure 3). All Ni Kedge EXAFS spectra were fit with ~ 6 Ni-O atom pairs in the first-shell at a distance of 2.05 ± 0.01 Å (Table 3). For the Ni-Fe second-shell atom pairs, two sub-shells were required to describe the data: ~ 1.5 Ni-Fe (type 1) at 3.10 ± 0.02 Å and ~ 0.6 Ni-Fe (type 2) at 3.56-3.60 ± 0.06 Å. Errors in the second shell coordination numbers were estimated using the fractional parameter error as an indicator. As these error values were low we estimated CN error to be ~25% of the value.

The magnitude and imaginary portion of the Fourier transformed Ni K-edge EXAFS spectra and fits are displayed in Figure 4. The only notable difference in the Ni K-edge EXAFS derived fit parameters obtained for 2-line ferrihydrite and goethite was a lower Ni-Fe coordination number in the type 1 second shell for Ni-sorbed goethite. Nickel sorbed to goethite had on average 1.0 ± 0.4 Fe in its second shell, while Ni-sorbed to 2-line ferrihydrite had an average of 2.2 ± 0.56 Fe atoms in its second shell (Table 3). This difference can be viewed by comparing peak amplitudes of the two minerals whereby 2-line ferrihydrite samples have greater peak amplitudes than goethite. The Ni K-edge EXAFS data are also consistent with the formation of inner-sphere complexes with no evidence for a separate Ni phase.

The Ni isotope composition of sorbed Ni and supernatant solutions

The Ni isotope composition of the initial Ni stock solution was determined at δ 60/58 Ni = -0.33 ± 0.05 ‰ (2sd, n=4, Table 1). The Ni isotope compositions of mineral-associated Ni (i.e, sorbed onto 2-line ferrihydrite and goethite) and solution are reported in Table 1. 2). The Ni isotope fractionation factor ∆ 60/58 Nimin-aq is between -0.77 and -0.72 ‰ (average ∆ 60/58 Nimin-aq = -0.75 ± 0.05 ‰; 2sd, n=4) and ∆ 60/58 Nimin-aq values are between -0.73 to -0.62 ‰ (average ∆ 60/58 Nimin-aq = -0.67 ± 0.09 ‰; 2sd, n=4) for the two experiments respectively.

Discussion

Previous work on non-traditional isotope systems, such as Mo [START_REF] Barling | Molybdenum isotope fractionation during adsorption by manganese oxides[END_REF][START_REF] Goldberg | Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides[END_REF][START_REF] Kashiwabara | A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides[END_REF][START_REF] Wasylenki | The molecular mechanism of Mo isotope fractionation during adsorption to birnessite[END_REF], Zn [START_REF] Pokrovsky | Zinc stable isotope fractionation during its adsorption on oxides and hydroxides[END_REF]Juillot et al., 2008), Cu [START_REF] Balistrieri | Fractionation of Cu and Zn isotopes during adsorption onto amorphous Fe(III) oxyhydroxide: Experimental mixing of acid rock drainage and ambient river water[END_REF][START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF], Tl [START_REF] Rehkämper | Thallium isotope variations in seawater and hydrogenetic, diagenetic, and hydrothermal ferromanganese deposits[END_REF], Cd [START_REF] Wasylenki | Cadmium isotope fractionation during adsorption to Mn oxyhydroxide at low and high ionic strength[END_REF], Hg [START_REF] Jiskra | Solution Speciation Controls Mercury Isotope Fractionation of Hg(II) Sorption to Goethite[END_REF], Fe [START_REF] Icopini | Iron isotope fractionation during microbial reduction of iron: The importance of adsorption[END_REF][START_REF] Mikutta | Iron isotope fractionation and atom exchange during sorption of ferrous iron to mineral surfaces[END_REF][START_REF] Beard | Iron isotope fractionation between aqueous ferrous iron and goethite[END_REF][START_REF] Wu | Stable Iron Isotope Fractionation Between Aqueous Fe(II) and Hydrous Ferric Oxide[END_REF][START_REF] Wu | Stable iron isotope fractionation between aqueous Fe(II) and model Archean ocean Fe-Si coprecipitates and implications for iron isotope variations in the ancient rock record[END_REF][START_REF] Frierdich | Iron isotope fractionation between aqueous Fe(II) and goethite revisited: New insights based on a multidirection approach to equilibrium and isotopic exchange rate modification[END_REF], Ge [START_REF] Li | First-principles study of Ge isotope fractionation during adsorption onto Fe(III)-oxyhydroxide surfaces[END_REF][START_REF] Pokrovsky | Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides[END_REF], and Se [START_REF] Mitchell | Selenium sorption and isotope fractionation: Iron(III) oxides versus iron(II) sulfides[END_REF] have shown that adsorption reactions are often accompanied by significant isotope fractionation processes that may vary according to sorbent composition and sorbate structure.

By contrast, sorption of Cr(VI) to Fe-oxyhydroxides is not accompanied by significant isotope fractionation [START_REF] Ellis | Using Chromium Stable Isotope Ratios To Quantify Cr(VI) Reduction: Lack of Sorption Effects[END_REF]. Hence, the combination of spectroscopic and isotopic measurements is essential for understanding isotopic fractionation processes. Here, we discuss the result of both approaches to address the mechanisms and significance of Ni isotope fractionation during Ni sorption to Fe-oxyhydroxides.

Surface coordination of Ni at the mineral surface

Coordination chemistry (i.e., coordination number, bond length) correlates with the extent of isotope fractionation because shorter bond lengths (i.e., stiffer bonds) or lower coordination numbers favor enrichment of heavier isotopes (e.g., [START_REF] Schauble | Applying Stable Isotope Fractionation Theory to New Systems[END_REF][START_REF] Pokrovsky | Zinc stable isotope fractionation during its adsorption on oxides and hydroxides[END_REF]. The first coordination shell of a metal is comprised of oxygen atoms. Variations of the coordination number of the metal (e.g., between the aqueous phase and the solid phase)

during adsorption of a transition metal to the surface of any given Fe-oxyhydroxide mineral is a common cause for isotope fractionation. For instance, heavy isotope enrichment during Zn sorption on ferrihydrite was accounted for by a decrease in the coordination number of the aqueous species relative to the adsorbed species. According to EXAFS data, the adsorption of Zn onto ferrihydrite produces a significant shift in Zn coordination number, from 6 in the solution to 4 in the solid, while Zn sorbed on goethite occurs in octahedral coordination, i.e., similar to aqueous Zn. The larger isotope fractionation factor measured for ferrihydrite relative to goethite can thus be explained by a different coordination environment of the sorbed Zn (i.e., 4 for ferrihydrite and 6 for goethite) and differences in Zn-O bond stiffness (i.e., 1.96 Å for ferrihydrite and 2.11 Å for goethite; Juillot et al., 2008). For Ni, isotopic fractionation cannot be related to changes in the coordination number between aqueous Ni and sorbed Ni because Ni exclusively occurs in coordination 6 in the natural environment.

The length of Ni -O bonds in water is 2.1 Å compared to the 2.05 Å observed when Ni is sorbed to the mineral surface [START_REF] Soper | A neutron diffraction study of hydration effects in aqueous solutions[END_REF], therefore, heavier Ni isotopes enrichment should be expected during sorption to the mineral surface. The systematic enrichment in lighter Ni isotopes on the mineral phase found in this study and in [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] precludes this option. [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] suggest that distortion of the coordination environment after

Ni sorption to ferrihydrite could explain light Ni isotopes enrichment of the mineral surface.

Given our Ni K-edge EXAFS data quality it was not possible to support or refute this hypothesis because we could not achieve fitting multiple Ni-O paths to the Ni-O shell (first coordination shell). However, the number of bonds between Ni and surface oxygens, which is reflected in the second shell parameters (i.e., surface complex geometry), may cause slight distortion or distribution of the electron density in the first coordination shell, even if they cannot be captured by the Ni K-edge EXAFS data. Thus, we argue that the number of bonds between Ni and surface oxygens could distort the first coordination shell.

Consistent with previous studies (Manceau et al., 2000;[START_REF] Carvalho-E-Silva | Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy[END_REF]Arai, 2008;Dublet et al., 2012) adsorbed Ni displayed octahedral coordination (Electronic Annex, Table A2). For an inner-sphere surface complex of Ni on Fe oxyhydroxides, the second major coordination shell is composed of Fe atoms. The interatomic distances between Ni and Fe, as measured by Ni K-edge EXAFS spectroscopy, define the local coordination environment of the Ni surface species and Ni sorbed on ferrihydrite and goethite have been studied in detail (Waychunas et al., 2002;Arai, 2008;Juillot et al., 2008;Dublet et al., 2012;Cismasu et al., 2013). It was demonstrated that Ni sorbed on ferrihydrite occurs in octahedral coordination between Ni-O6 and Fe-O6 polyhedra with interatomic distances in the range of 3.05-3.08 Å for edge-sharing type 1 surface complexes, 3.19-3.12 Å for edge-sharing type 2, and 4.03-4.07 Å for corner-sharing (Arai, 2008). Type 1 edge sharing is between Ni and Fe octahedra in the chains and type 2 surface complexes is between Ni and Fe octahedra in the rows (Arai, 2008). The interatomic distances modeled here for goethite samples reflect the three Fe-Fe second-shell interatomic distances for edge-sharing coordination between Ni and a chain of Fe-O6 octahedra (i.e., type 1), edge-sharing between chains of Fe-O6 octahedra (i.e., type 2), and corner-sharing Fe-O6 octahedra between double chains [START_REF] Manceau | Local structure of ferrihydrite and feroxyhite by EXAFS spectroscopy[END_REF].

Incorporation of Ni into Fe-oxyhydroxides lattices is possible, but our Ni K-edge EXAFS data shows the absence of Ni incorporation in the minerals. [START_REF] Cornell | The effect of Nickel on the conversion of amorphous iron(III) hydroxide into more crystalline iron-oxides in alkaline media[END_REF] reported Ni incorporation into synthetic goethite at ~5% of the total molar fraction and several studies showed that incorporation of Ni in goethite occurs via diffusion into the mineral lattice

where Ni can occupy vacant Fe positions in synthetic goethite [START_REF] Barrow | Rate of desorption of eight heavy metals from goethite and its implications for understanding the pathways for penetration[END_REF][START_REF] Brümmer | Effect of porosity of goethite on the sorption of six heavy metal ions[END_REF]. [START_REF] Fischer | Observations and modelling of the reactions of 10 metals with goethite: adsorption and diffusion processes[END_REF] showed that Ni reaction with the goethite surface is

Nickel isotope fractionation factors during Ni adsorption to Fe-oxyhydroxides

In the case of equilibrium isotopic fractionation, the fractionation factor between mineral and solution, the αmin-aq value (related to Δ 60/58 Nimin-aq values as ≈ 10 3 .ln αmin-aq), ultimately depends on the difference in bond stiffness between two species involved in the reaction. In a closed system, the Ni isotope composition of the solution and mineral-bound Ni are related to the proportion of Ni sorption to the mineral (Figure 5). If equilibrium isotope fractionation is maintained during the experiment, the fractionation factor, Δ 60/58 Nimin-aq, should remain constant regardless of the percentage of Ni adsorption and duration of the experiments. We recognize that equilibrium isotope fractionation is best demonstrated using the 3-isotopes method [START_REF] Wu | Stable Iron Isotope Fractionation Between Aqueous Fe(II) and Hydrous Ferric Oxide[END_REF], which allows the determination of the rate of isotopic

Differences in Ni isotope fractionation between goethite and 2-line ferrihydrite

Our experiments showed that Ni sorbed to goethite is twice more fractionated towards lighter Δ 60/58 Nimin-aq values (~ -0.77 ‰) than Ni sorbed to 2-line ferrihydrite (~ -0.35 ‰).

Nickel species in solution that are likely to be adsorbed onto minerals are the Ni 2+

(hexaquo) species (Peacock and Sherman, 2007a). The surface complexation model reported in Figure 2 predicts that under the experimental conditions used in our study Ni should sorb to goethite to a greater extent than we observed in our experiments. At pH values less than ~ 8.0, the expected point of zero net charge for goethite is such that it should be positively charged.

In contrast, the zeta potential measurements for 2-line ferrihydrite indicate a net negatively charged surface. The net negative surface charge of 2-line ferrihydrite tends to attract dissolved cations to its surface while the net positive surface charge of goethite tends to repel dissolved cations and ferrihydrite has a higher surface area than goethite. This is in agreement with the strong uptake of Ni from solution by 2-line ferrihydrite observed in our experiments and predicted by our surface complexation model (Figure 2).

Two different modes of isotope fractionation are then possible during Ni adsorption to Fe-oxyhydroxides. One involves isotopic exchange among aqueous species in solution followed by preferential adsorption of one of these species onto the mineral phase. This mechanism may lead to contrasting isotope composition between the fluid and mineral, as previously observed for the Mo isotope system [START_REF] Siebert | Molybdenum isotope records as a potential new proxy for paleoceanography[END_REF][START_REF] Barling | Molybdenum isotope fractionation during adsorption by manganese oxides[END_REF]). An alternative mechanism of fractionation may take place directly during metal adsorption and involves the preferential adsorption of one isotope from the aqueous phase onto the solid phase because of, for instance, differences in coordination chemistry or kinetic effects. These two mechanisms are not mutually exclusive and combination of the two is likely to occur in natural systems. [START_REF] Pokrovsky | Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides[END_REF] Mechanism (2) : Mineral surface coverage and distribution of weak and strong sites. [START_REF] Villalobos | Variability in goethite surface site density: evidence from proton and carbonate sorption[END_REF] demonstrated that goethite synthesized with variable NaOH addition rates yield variable site densities that are in turn reflected in adsorption behavior. The goethite minerals synthesized in our study appear to fall on the low end of surface site concentrations reported in literature for goethite. Our Ni K-edge EXAFS data and fits revealed no detectable differences between low and high Ni loadings onto 2-line ferrihydrite. Therefore, we have no evidence to support a fractionation mechanism based on discrimination between weak and strong sorption sites. The Ni loading data when normalized to mineral surface area show no distinct differences between 2-line ferrihydrite and goethite in terms of range or magnitude of Ni surface coverage (Figure 1 and Table 1). This suggests that mineral surface loading does not explain the Ni isotope fractionation observed in our experiments. 3). This difference in surface complex geometry, which may originate from the structure of goethite as discussed below, suggest that the number of bonds formed between Ni and surface oxygen atoms, as indicated by the number of Ni-Fe near neighbors, may influence the distribution of electron density in the first coordination shell of Ni and thus influence its isotopic fractionation.

Of these possible mechanisms, only mechanism (5) 'binding mode of the metal' provides a good explanation for our data. Specifically, we observe a subtle difference between the Ni-Fe second-shell coordination environment when comparing two Ni-sorbed mineral samples having similar Ni loading when expressed in terms of surface area rather than total mass. Nickel sorbed to goethite has fewer Fe atoms in the second-shell than Ni sorbed to 2line ferrihydrite. The goethite structure is known to generate three different surface functional groups based on the number of Fe atoms bonded to a surface oxygen [START_REF] Essington | Surface chemistry and adsorption reactions, Soil and water chemistry[END_REF] : type

(1) ≡Fe3-O 1/2-, type (2) ≡Fe2-O 1-, and type (3) ≡Fe-O 3/2-. In our experiments, sorption of Ni(H2O)6 2+ by site type (3) is most consistent with the results. In addition, fewer Fe atoms in the second coordination shell is consistent with weaker surface complexes and the observed enrichment of light Ni isotopes in the Ni-sorbed to goethite sample leading to larger isotopic fractionation (i.e., larger fractionation factor ∆ 60/58 Nimin-aq) than for 2-line ferrihydrite.

Implications for Ni isotope biogeochemical cycling in modern and ancient surface environments 4.4.1. Nickel isotope systematics during continental weathering

In modern Earth surface environments, Fe-oxyhydroxides are likely an important carrier of Ni in riverine suspended sediments [START_REF] Turekian | The fate of metals in oceans[END_REF][START_REF] Snodgrass | Distribution and behavior of nickel in the aquatic environment[END_REF][START_REF] Callender | Heavy Metals in the Environment -Historical Trends A2 -Holland[END_REF] and in soils (Dublet et al., 2012;[START_REF] Ratié | Nickel isotope fractionation during tropical weathering of ultramafic rocks[END_REF]. [START_REF] Snodgrass | Distribution and behavior of nickel in the aquatic environment[END_REF] showed that Ni occurs in solution (0.5% only), as adsorbed species (3.1%), as precipitated inorganic phases or particle coatings (47%), as organic matter-bound complexes (14.9%) and as crystalline material (34.4%). [START_REF] Martin | Elemental mass-balance of material carried by major world rivers[END_REF] showed that Ni occurs mainly in the truly dissolved phase. However, a proportion of the total dissolved pool consists of Ni sorbed to Fe and Mn-oxides colloids and particles [START_REF] Mouvet | Speciation (including adsorbed species) of copper, lead, nickel and zinc in the Meuse River: Observed results compared to values calculated with a chemical equilibrium computer program[END_REF][START_REF] Gaillardet | Trace Elements in River Waters[END_REF] while a significant proportion of Ni is associated with colloidal organic matter and organic compounds [START_REF] Vasyukova | Trace elements in organic-and iron-rich surficial fluids of the boreal zone: Assessing colloidal forms via dialysis and ultrafiltration[END_REF]. Considering that the proportion of each of these Ni pools in rivers may vary depending on environmental and geological settings, Ni speciation may exert a strong influence on the Ni isotope composition of rivers. Bedrock lithology, rock alteration processes and soil formation may be also important. For example, the Ni isotope composition of Ni-rich laterites forming through weathering of ultramafic complexes indicates an enrichment in light Ni isotopes by -0.47 ‰ compared to the bedrock composition [START_REF] Ratié | Nickel isotope fractionation during tropical weathering of ultramafic rocks[END_REF]. Nickel isotope fractionation during adsorption to Fe-oxyhydroxides might explain the observed light isotopes enrichment in weathering environment, and by isotopic mass balance, the overall enrichment in heavy isotopes in rivers [START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF].

This would indicate a significant role for Fe in Ni isotopic cycling on continental surfaces and more specifically in the delivery of Ni to the oceans both in terms of elemental flux and isotopic composition.

In natural environments, the reaction rate for the transformation of ferrihydrite to goethite can be relatively rapid (< 500 days) and does not depend on pH [START_REF] Schwertmann | Long-term in vitro transformation of 2line ferrihydrite to goethite/hematite at 4, 10, 15 and 25°C[END_REF], but ferrihydrite can be metastable in some environments [START_REF] Navrotsky | Size-Driven Structural and Thermodynamic Complexity in Iron Oxides[END_REF][START_REF] Toner | Mineralogy of iron microbial mats from loihi seamount[END_REF]. The presence of large amounts of Ni incorporated in Fe-oxyhydroxides has been shown to slow the transformation of poorly crystalline Fe-oxyhydroxides to more crystalline phases [START_REF] Cornell | The effect of Nickel on the conversion of amorphous iron(III) hydroxide into more crystalline iron-oxides in alkaline media[END_REF][START_REF] Cornell | The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses[END_REF]. During dissolution of Fe(III)oxyhydroxides, Ni is released more slowly than Fe, which leads to enrichment in Ni in the residual solid phase [START_REF] Cornell | The effect of Nickel on the conversion of amorphous iron(III) hydroxide into more crystalline iron-oxides in alkaline media[END_REF]. The presence of organic ligands in seawater may also affect the crystallization of ferrihydrite and its transformation to goethite [START_REF] Cornell | Influence of organic-anions on the crystallization of ferrihydrite[END_REF]. Therefore, in the case of differential Ni isotope fractionation between the two mineral phases as observed in this study, the overall Ni isotope composition of Feoxyhydroxides may be controlled by the nature of the initial mineral phase precipitated.

Based on our results, we propose that Ni adsorption on Fe-oxyhydroxides is likely to be a major mechanism for explaining the enrichment in heavy Ni isotopes in river water compared to the Bulk Silicate Earth by ~ 0.6-0.7 ‰ [START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF]. However, one should also consider the possibility that the present-day main river discharges (e.g., Amazon river) may contain some anthropogenic Ni as a by-product of fossil fuel burning, which may alter both the natural concentration of Ni in rivers and its isotopic composition.

For instance, [START_REF] Gueguen | Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC-ICP-MS[END_REF] report a  60/58 Ni value of 0.5 ‰ in coal, which is heavier than the Bulk Silicate Earth isotopic composition. Such anthropogenic input could also explain the apparent Ni isotopic imbalance in the ocean [START_REF] Cameron | Heavy nickel isotope compositions in rivers and the oceans[END_REF], thereby implying that Ni delivery to the ocean occurs under non steady-state conditions.

Application to Precambrian seawater chemistry and ancient Ni biogeochemical cycling

The marine biogeochemical cycle of Ni experienced major perturbations during the Precambrian [START_REF] Konhauser | Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event[END_REF][START_REF] Konhauser | The Archean Nickel Famine Revisited[END_REF], including an apparent sharp decrease in the Ni delivery to the ocean at the onset of the Great Oxidation Event, at ca. 2.5

Ga. Under the overall anoxic conditions and high Ni bioavailability of the Archean oceans, microorganisms such as methanogens may have been important primary producers. Given that methanogens fractionate light Ni isotopes by up to ~ -1.5 ‰ [START_REF] Cameron | A biomarker based on the stable isotopes of nickel[END_REF], such a shift in Ni utilization in the oceans should have left characteristic imprints in the Ni isotopic composition of Precambrian seawater. Because Fe-oxyhydroxides were probably an important host phase for Ni in ancient marine environments [START_REF] Konhauser | Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event[END_REF], an understanding of Ni isotope fractionation during sorption to ferrihydrite or goethite is critical for inferring the Ni isotopic composition of Precambrian seawater from the Precambrian sediment record [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF]. Tracing the evolution of Ni isotopic composition in Precambrian seawater by investigating BIFs could provide meaningful insights into the coevolution of the chemical composition of seawater with biological evolution and diversification [START_REF] Bekker | Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes[END_REF]. However, Fe-oxides such as hematite and magnetite are the predominant carrier of Ni in these deposits, although accessory minerals such as silicates (e.g., stilpnomelane) and carbonates may be also important. Because the nature and origin of the Fe-oxyhydroxide precursor in BIF remain poorly known, interpretations based on the Ni isotope composition in BIF should be made with significant caution given the range of possible fractionation factors for different mineral types (e.g., ferrihydrite vs. goethite) and adsorption mechanisms.

Our results show that Ni isotope fractionation during abiotic reactions involving adsorption on Fe-oxides are of the same order of magnitude as for biotic reactions (i.e., [START_REF] Cameron | A biomarker based on the stable isotopes of nickel[END_REF] and vary between different mineral surfaces i.e., from ~ -0.35 ‰ (2-line ferrihydrite) to ~ -0.77 ‰ (goethite). Hence, the use of Ni isotopes as biosignatures should be approached with caution, as Ni isotope fractionation produced by abiotic processes may mask any isotope signals related to biotic processes. This issue is critical for BIF studies because the large extent of Fe precipitation in seawater, leading to the accumulation of freshly formed

Fe-oxyhydroxides at the seafloor as likely BIF precursors, may also shift the isotopic composition Ni remaining in seawater.

In a previous study, [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] suggested that Ni isotopes in BIF should be offset by ~ -0.35 ‰ relatively to seawater. We further propose that such an offset is likely a minimum value, considering that fractionation up to ~ -0.77 ‰ is observed during sorption to goethite, regardless of potential kinetic isotope effects. Clearly, further study investigating the preservation of Ni isotope signatures during Fe-oxyhydroxide recrystallization to hematite and magnetite is warranted in order to directly apply Ni isotope systematics to the BIF record.

Although [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] showed that there was apparently no isotope fractionation during aging of ferrihydrite and recrystallization to goethite and hematite, they did not determine the proportion of ferrihydrite that has recrystallized, and longer experimental durations as well as heating of the system should be explored to better evaluate the effect of ferrihydrite aging on Ni isotopes. The effect of silica [START_REF] Eickhoff | Nickel partitioning in biogenic and abiogenic ferrihydrite: The influence of silica and implications for ancient environments[END_REF] and the possibility that it could affect the Ni isotope fractionation factor during BIF formation, has also to be considered when investigating the BIF record.

Summary

Consistent with previous studies [START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF], we found that Ni isotopes (Manceau et al., 2000;Waychunas et al., 2002;Arai, 2008;Juillot et al., 2008;Dublet et al., 2012;Cismasu et al., 2013).

Sample type

  To address this problem, we conducted Ni adsorption experiments on two synthetic Fe-oxyhydroxides, 2-line ferrihydrite and goethite, as a function of pH, variable Ni loading to pre-formed minerals, (i.e., no co-precipitation) and duration of experiments. Our goal is to mechanistically understand the isotope fractionation during Ni adsorption to Fe-phases with different crystallinity, with special consideration to Ni speciation, coordination, and structure of adsorbent. The local coordination environment of Ni at the mineral surface was evaluated by Ni K-edge EXAFS spectroscopy, while the isotopic fractionation of Ni between solid and aqueous phases were measured by Multi Collector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS) using double-spike ( 61 Ni-62 Ni) correction of instrumental mass bias. Our results build on the results of Wasylenki et al. (2015) regarding the causes for Ni isotope fractionation. We expand on the role of Fe-oxides in the transfer of continental Ni to the oceans and in the biogeochemical cycling of Ni isotopes, furthering the development of Ni isotopes as a geochemical tracer in surface environments.

  7 and at two different initial Ni concentrations of 170 µmol/L and 426 µmol/L, in order to assess the potential effect of dissolved Ni concentration (i.e., Ni loading) on Ni isotope fractionation factor. Four different batches were prepared for each initial Ni concentration and were allowed to react for 18 (or 19 hours for 2-line ferrihydrite), 20, 22, and 23 hours, respectively.

  loadings of Ni onto both 2-line ferrihydrite and goethite expressed in terms of the percent of Ni sorbed for pH-dependent and varying initial [Ni] experiments are displayed in Table 1. The sorption of Ni onto 2-line ferrihydrite for pH ranging from 5.0 to 8.0 (i.e., pHdependent experiments, with initial Ni concentration of 170 µmol/L) resulted in Ni uptake between < 1% and 45% translating into 0.02 to 0.62 µmol Ni/m 2 mineral (Figure 1a). At pH 7.7 (varying initial Ni concentrations experiments, 17 to 426 µmol/L Ni), final Ni loadings on 2-line ferrihydrite ranged from 0.11 to 1.71 µmol Ni/m 2 mineral (Figure 1), which correspond to a percentage of sorbed Ni between 25% and 78%. Nickel sorption to goethite at pH 5.0, 6.0, and 8.0 resulted in Ni uptake between < 1% and 4.3% of the initial 170 µmol/L aqueous Ni (Figure 1), corresponding to Ni loadings ranging from 0.04 and 0.28 µmol Ni/m 2 . At pH 7.7, Ni uptake from solutions with initial concentrations of 17, 170, and 426 µmol/L Ni yielded final Ni loadings onto the mineral between 0.63 to 1.33 µmol Ni/m 2 , corresponding to percentage of sorbed Ni between 8% and 19%.

  Figure 5 presents δ 60/58 Ni values of the mineral and the associated filtrate versus

  continuous with time and that Ni is incorporated via diffusion in the lattice structure of the mineral.[START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] reported nearly identical isotopic signatures for surface sorbed Ni and incorporated Ni, suggesting that the influence of incorporated Ni is negligible in the isotopic budget and that potential Ni isotope fractionation during diffusion can be precluded.The best Ni K-edge EXAFS model for Ni sorbed to 2-line ferrihydrite data, with two Ni-Fe distances in the second shell, is consistent with a short Ni-Fe distance produced by edge-sharing polyhedra (3.10 ± 0.02 Å) in addition to a longer Ni-Fe distance produced by a combination of edge-sharing and corner-sharing polyhedra not resolved by our Ni K-edge EXAFS data (3.57 ± 0.06 Å). The data quality for the Ni-sorbed goethite prevented a robust second-shell fit to the Ni K-edge EXAFS. However, the fit results were consistent with a Ni-Fe atom pair at 3.07 ± 0.03 Å and revealed a difference relative between goethite and 2-line ferrihydrite in the Ni-Fe coordination number. The Ni K-edge EXAFS measurements on goethite and 2-line ferrihydrite from this study also indicate that the Ni coordination environment was similar among all samples. Therefore, the difference in Ni isotope fractionation between the two minerals may be due to minor differences in surface site density and distribution. Since no detectable difference in surface speciation was observed as a function of Ni loading to the 2-line ferrihydrite surface at pH 7.7 or as a function of pH (7.7 versus 8.0), alternative variable parameters such as protonation state of the mineral, surface complex geometry (e.g., number of bonds between Ni and mineral surface), surface coverage or bond lengths between the two minerals might have influenced Ni isotope fractionation.

  exchange and extrapolate the isotope fractionation factors during incomplete isotope exchange. Here, we mainly relied on the constant Ni isotope fractionation factor between solution and mineral during the reaction progress in order to derive equilibrium Δ60/58 Nimin-aq values. Results obtained for 2-line ferrihydrite and goethite (i.e., pH dependent, concentration dependent, and time dependent) are for the most part consistent with isotopic equilibrium in a closed system (i.e., constant Δ 60/58 Nimin-aq values), although significant departure from this relationship is observed at low percentage of Ni sorption (Figure7) and in one goethite sample (i.e., the variable initial Ni concentrations experiments with 19.3% Ni-sorbed (Figure5)).In order to derive meaningful Ni isotope fractionation factors from the experimental data, the % of Ni adsorption should be generally comprised between 5% to 95%, i.e., too low or too high values may pose problem in the determination of  60/58 Ni of the mineral and solution, respectively. Using this more restricted dataset, we determine an average ∆ 60/58 Niminaq = -0.32 ±0.20 ‰ for the 2-line ferrihydrite and ∆ 60/58 Nimin-aq = -0.77 ±0.23 ‰ for goethite.For time-dependent experiments, we determine a range of ∆ 60/58 Nimin-aq values for the 2-line ferrihydrite between -0.32 ±0.04 and -0.38 ±0.06‰, and for goethite between -0.65 ±0.07 and -0.77 ±0.07‰. Therefore, ∆ 60/58 Nimin-aq values do not vary within uncertainties over the course of the experiments. This is in agreement with previous results of[START_REF] Wasylenki | Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations[END_REF] showing no variations in ∆ 60/58 Nimin-aq values for 2-line ferrihydrite for experiment duration between a few hours to 30 days. There is also no appreciable difference in the magnitude of Ni isotope fractionation factor depending on the initial Ni concentration and sorption %, i.e., ∆ 60/58 Nimin-aq = -0.35 ±0.01 ‰ (2sd, n=4) for an initial Ni concentration of 170 µmol/L and ∆ 60/58 Nimin-aq = -0.36 ±0.05 ‰ (2sd, n=4) for an initial Ni concentration of 426 µmol/L (Table2). Average ∆ 60/58 Nimin-aq values for 2-line ferrihydrite and goethite timedependent experiments are also similar to the average value calculated for the pH-dependent and varying initial Ni concentrations experiments suggesting that our experiments are at isotopic equilibrium. The duration of experiments is the same for low Ni-sorbed samples than for high Ni-sorbed samples. Therefore, if there were any kinetic effects we would expect them to be similar whatever the amount of sorbed Ni or that larger isotope fractionation would occur for low Ni-sorbed samples compared to high Ni-sorbed samples which is not the case in our experiments. Although these experiments did not allow the determination of isotopic exchange rates between the solution and the mineral, which would have required shorter experiment duration, we suggest that isotopic equilibrium was likely achieved in our experiments. This assumption is also consistent with previous goethite adsorption experiments for Cu and Zn isotopes systematics[START_REF] Pokrovsky | Zinc stable isotope fractionation during its adsorption on oxides and hydroxides[END_REF][START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF] for which isotope fractionation factors were similar for short duration experiments (~20 hours) than for long duration experiments (~ 100 days). The smaller Ni isotope fractionation factors for goethite and 2-line ferrihydrite at low % (Figure 7) can be explained in several ways: (1) non-equilibrium fractionation (i.e., kinetic fractionation processes) due to partially irreversible reaction between sorbed Ni on the mineral and Ni in solution; (2) a change in coordination chemistry in the mineral with pH and/or Ni loading; and (3) retention of aqueous Ni on the mineral during filtration, leading to incomplete separation of Ni in mineral and Ni in solution. As explained above, time-dependent experiments suggest that isotopic equilibrium is achieved which, thus, discounts hypothesis (1). Hypothesis (2) can also be ruled out since no change in coordination chemistry as a function of pH was observed in the Ni K-edge EXAFS measurements. We propose that the low ∆ 60/58 Nimin-aq values measured in samples prepared at low pH may be due to the presence of a small fraction of the supernatant solution that remained trapped in the mineral even after filtering, which would mute Ni isotope fractionation during adsorption.Using the final Ni concentration measured in supernatant solutions and the estimated volume of water present in the mineral, we calculated that the supernatant solution should contribute between 0.0002 and 0.0070 µmol Ni/m 2 for 2-line ferrihydrite. Hence, for low Ni-loading in the 2-line ferrihydrite experiments, Ni in solution may account for between 13 % and 44 % of the total Ni in the sorbed fraction. This mixing effect is the most plausible explanation (hypothesis 3) for the variation in Δ 60/58 Nimin-aq values (i.e., decrease) at low Ni-loading.

  identified five mechanisms that could be responsible for equilibrium isotope fractionation during sorption reactions: (1) protonation of metal species in solution or on the mineral surface; (2) mineral surface coverage and distribution of weak sites versus strong sites; (3) bond distances between the metal and oxygen atoms and the presence of other atoms in the mineral structure; (4) change in coordination between the fluid and the solid phase; and (5) binding mode of the metal. Below we evaluate each of these mechanisms in light of the results from our study. Mechanism (1) : Protonation of aqueous or sorbed Ni. As a divalent cation, Ni has a relatively low ionic potential, e.g., in contrast to Fe 3+ and Al 3+ . Hence, Ni(H2O)6 2+ does not readily undergo hydrolysis and Ni tends to form relatively weak aqueous complexes with ligands when compared to trivalent cations. In addition, because we expect the presence of a single aqueous Ni species, Ni(H2O)6 2+ , in our experiments there should be no isotopic fractionation occurring in solution.

  Mechanism (3) : Bond distances between the metal and oxygen atoms and the presence of other atoms in the mineral structure. The minerals studied here were synthesized in the laboratory and are free of impurities. In addition, as discussed above, the Ni-O firstshell interatomic distances are indistinguishable among our 2-line ferrihydrite and goethite sorbents. Mechanism (4): Change in coordination between the fluid and the solid phase. Based on thermodynamic modeling, we expect the aqueous Ni speciation is > 80% Ni 2+ (hexaquo), Ni(H2O)6 2+ , and in octahedral coordination with O. Based on our Ni K-edge EXAFS data, the Ni sorbed to 2-line ferrihydrite and goethite is also in octahedral coordination with O. Mechanism (5): Metal bonding environment. The Ni K-edge EXAFS data and fit results indicate that the local coordination environment for Ni at the mineral surface is similar for all Ni loadings and pH values for 2-line ferrihydrite. To determine whether we can extend this finding to goethite, we compared two samples for which Ni K-edge EXAFS data were collected : (1) Goe-C-06 (1.33 µmol Ni/m 2 ) and Fh-C-10 (1.71 µmol Ni/m 2 ). These two samples have similar Ni loadings when the amount of adsorbed Ni is normalized by specific surface area (Table 1) and the Ni K-edge EXAFS-derived fit parameters for the Ni-O shell are identical within errors. In other words, the Ni species have the same coordination number and the same bond lengths for Ni-O (recall some of the coordinating oxygens are contributed by the mineral and waters of hydration). The only difference in Ni speciation between the two samples is a lower Ni-Fe coordination number in the second shell for Ni-sorbed goethite. On average, the Ni sorbed to goethite has only 1.0 Fe (range 0.6-1.5) in its second shell, while Nisorbed to 2-line ferrihydrite has 2.2 Fe (range 1.8-2.7) (Table

  Photon Source, Argonne National Laboratory); Lindsey Briscoe for construction of the pH

Figure 1 :

 1 Figure captions:

Figure 2 :

 2 Figure 2: Surface complexation modeling (SCM) using a Constant Capacitance Model

Figure 3 :

 3 Figure 3: Summary of Ni K-edge EXAFS spectra in k-space for Ni-sorbed 2-line ferrihydrite

Figure 4 :

 4 Figure 4: Summary of Fourier transformed data (magnitude and imaginary part) and fits for

Figure 5 :

 5 Figure 5: Nickel isotope composition (‰) of minerals (circles) and supernatant solutions

Figure 6 :

 6 Figure 6: Ni isotope fractionation factors (Δ 60/58 Nimin-aq = δ 60/58 Nimineral -δ 60/58 Niaqueous) versus

Figure 7 :

 7 Figure 7: Ni isotope fractionation factors (Δ 60/58 Nimin-aq = δ 60/58 Nimineral -δ 60/58 Niaqueous) versus

  Figure1

Table captions :Table 1 :

 captions1 Sample information, summary of experimental conditions, Ni loading to the mineral

	and Ni isotope composition (‰) of mineral phases and supernatant solutions for pH-
	dependent experiments, variable Ni concentration experiments. Non-filled symbols

correspond to the very low sorbed Ni samples (see text for discussion).

Table 2 :

 2 Sample information, summary of experimental conditions, Ni loading to the mineral and Ni isotope composition (‰) of mineral phases and supernatant solutions for time-

	dependent experiments.

Table 3 :

 3 Summary of Ni K-edge EXAFS fit parameters.

Table 1 :

 1 Sample information, summary of experimental conditions, Ni loading to the mineral and Ni isotope composition (‰) of mineral phases and supernatant solutions for pH-dependent and variable Ni experiments.

	Mineral	Sample name	Type of experiment	pH	[Ni] aq initial (μmol/L)	Final loading to mineral (µmol Ni/g) c	Final loading to mineral (µmol Ni/m 2 ) c	% of Ni sorption d	δ 60/58 Ni mineral	2se e	δ 60/58 Ni aqueous	2se	Δ 60/58 Ni min-aq	f	2se g	Mass balance (‰)
	2-line Ferrihydrite a	Fh-pH-01	pH dependent	5.0	170	1.99	0.02	1.2	-0.36	0.02	-0.32	0.03	-0.03		0.03	-0.32
	2-line Ferrihydrite	Fh-pH-02	pH dependent	6.0	170	4.77	0.04	2.9	-0.47	0.02	-0.35	0.03	-0.12		0.04	-0.35
	2-line Ferrihydrite	Fh-pH-03	pH dependent	7.0	170	38.21	0.32	23.4	-0.57	0.02	-0.30	0.03	-0.27		0.04	-0.36
	2-line Ferrihydrite	Fh-pH-04	pH dependent	7.7	170	66.61	0.56	41.7	-0.52	0.02	-0.15	0.03	-0.37		0.04	-0.31
	2-line Ferrihydrite	Fh-pH-05*	pH dependent	8.0	170	73.81	0.62	45.3	-0.55	0.02	-0.17	0.04	-0.38		0.05	-0.34
	2-line Ferrihydrite	Fh-C-06	Varying initial [Ni]	7.7	17	12.97	0.11	78.1	-0.41	0.02	-0.02	0.04	-0.39		0.04	-0.33
	2-line Ferrihydrite	Fh-C-07*	Varying initial [Ni]	7.7	85	38.64	0.32	46.8	-0.51	0.02	-0.18	0.04	-0.32		0.04	-0.34
	2-line Ferrihydrite	Fh-C-08*	Varying initial [Ni]	7.7	170	41.31	0.35	24.8	-0.56	0.02	-0.22	0.04	-0.34		0.04	-0.30
	2-line Ferrihydrite	Fh-C-09*	Varying initial [Ni]	7.7	341	131.24	1.10	39.2	-0.54	0.03	-0.13	0.04	-0.40		0.05	-0.29
	2-line Ferrihydrite	Fh-C-10*	Varying initial [Ni]	7.7	426	203.14	1.71	49.8	-0.53	0.03	-0.28	0.04	-0.24		0.05	-0.40
	Goethite b	Goe-pH-01	pH dependent	5.0	170	1.00	0.04	0.6	-1.00	0.03	-0.30	0.03	-0.70		0.04	-0.31
	Goethite	Goe-pH-02	pH dependent	6.0	170	1.33	0.05	0.8	-1.13	0.03	-0.25	0.04	-0.88		0.05	-0.25
	Goethite	Goe-pH-03	pH dependent	8.0	170	7.17	0.28	4.3	-1.19	0.03	-0.23	0.03	-0.96		0.05	-0.27
	Goethite	Goe-C-04	Varying initial [Ni]	7.7	170	15.96	0.63	9.5	-1.08	0.02	-0.27	0.04	-0.80		0.04	-0.35
	Goethite	Goe-C-05	Varying initial [Ni]	7.7	341	17.56	0.69	19.3	-1.15	0.02	-0.12	0.04	-1.03		0.04	-0.35
	Goethite	Goe-C-06*	Varying initial [Ni]	7.7	426	33.86	1.33	7.9	-1.07	0.02	-0.29	0.03	-0.78		0.04	-0.35
	Ni standard solution	-

Table 2 :

 2 Sample information, summary of experimental conditions, Ni loading to the mineral and Ni isotope composition (‰) of mineral phases and supernatant solutions for the time-dependent experiments.

						Final	Final
	Mineral*	Sample name	Duration of experiment (hours)	pH	[Ni] aq initial (μmol/L)	loading to mineral (µmol	loading to mineral (µmol	% of Ni sorption	δ 60/58 Ni mineral	2se	δ 60/58 Ni aqueous	2se	Δ 60/58 Ni min-aq	2se	Mass balance (‰)
						Ni/g)	Ni/m 2 )
	Goethite	Goe-T-07	18	7.7	170	41.0	1.6	22.3	-0.98	0.05	-0.21	0.06	-0.77	0.07	-0.38
	Goethite	Goe-T-08	20	7.7	170	41.7	1.6	22.0	-0.96	0.05	-0.19	0.05	-0.77	0.07	-0.36
	Goethite	Goe-T-09	22	7.7	170	42.2	1.7	22.6	-0.98	0.08	-0.26	0.03	-0.72	0.08	-0.42
	Goethite	Goe-T-10	23	7.7	170	42.5	1.7	22.0	-0.98	0.04	-0.24	0.04	-0.74	0.05	-0.41
	Goethite	Goe-T-11	18	7.7	426	65.3	2.6	17.2	-0.88	0.06	-0.25	0.04	-0.62	0.07	-0.36
	Goethite	Goe-T-12	20	7.7	426	50.6	2.0	14.4	-0.99	0.06	-0.27	0.03	-0.73	0.07	-0.37
	Goethite	Goe-T-13	22	7.7	426	56.7	2.2	15.6	-0.94	0.03	-0.29	0.06	-0.65	0.07	-0.39
	Goethite	Goe-T-14	23	7.7	426	51.6	2.0	12.9	-0.97	0.05	-0.29	0.07	-0.68	0.08	-0.38
	2-line Ferrihydrite	Fh-T-11	19	7.7	170	225.2	1.9	92.4	-0.36	0.04	0.00	0.05	-0.36	0.06	-0.33
	2-line Ferrihydrite	Fh-T-12	20	7.7	170	224.5	1.9	92.9	-0.37	0.04	-0.02	0.03	-0.35	0.05	-0.35
	2-line Ferrihydrite	Fh-T-13	22	7.7	170	223.5	1.9	93.2	-0.36	0.02	-0.01	0.04	-0.35	0.04	-0.34
	2-line Ferrihydrite	Fh-T-14	23	7.7	170	322.7	2.7	95.0	-0.39	0.05	-0.05	0.03	-0.34	0.05	-0.38
	2-line Ferrihydrite	Fh-T-15	19	7.7	426	444.4	3.7	91.5	-0.41	0.04	-0.03	0.05	-0.37	0.06	-0.37
	2-line Ferrihydrite	Fh-T-16	20	7.7	426	443.1	3.7	91.5	-0.43	0.04	-0.05	0.05	-0.38	0.06	-0.40
	2-line Ferrihydrite	Fh-T-17	22	7.7	426	392.9	3.3	85.7	-0.37	0.02	-0.05	0.04	-0.32	0.04	-0.33
	2-line Ferrihydrite	Fh-T-18	23	7.7	426	443.1	3.7	91.3	-0.42	0.06	-0.06	0.01	-0.36	0.06	-0.39
	*See Table 1 for footnotes.

Table S2 :

 S2 Summary of EXAFS fit parameters for previous Ni-sorption and -incorporation experiments. Data for Zn are shown for comparison. Literature data is from

1,2,3 See Table2and main text.

* Samples for which Ni K-edge EXAFS data were collected. a 2-line ferrihydrite (0.014 mol Fe/g dry mineral) b Goethite (0.015 mol Fe/g dry mineral)

c Final loading corresponds to the amount of Ni per gram or m 2 of mineral in the reactor. The amount of mineral added per reactor measured was ~ 0.15 g.

d Ni sorbed fractions (%) were determined by mass balance using measured amount of Ni in solution and adsorbed onto minerals. e 2se corresponds to the two standard error of the mean determined on the 50 measurement cycles on the MC-ICP-MS for each sample (see text for explanation).

g '2se' corresponds to error propagation of the ∆ 60/58 Ni min/aq values (2se = √((2se mineral ) 2 + (2se aqueous ) 2 )).

h The initial Ni standard solution used for experiments was processed several times through chromatography columns. The two standard deviation (2sd, n=4) was calculated on these duplicates. i Nod-A-1 and Nod-P-1 are duplicates of USGS geostandards measured for monitoring the reproducibility of the data.

f The difference in isotopic composition between the mineral and the supernatant solution is expressed as ∆ 60/58 Ni min-aq = δ 60/58 Ni mineral -δ 60/58 Ni aqueous . This value corresponds to the fractionation factor during Ni adsorption on the mineral surface. A two-standard error was determined for this value by error propagation. Bleuenn Gueguen 1,2* , Jeffry V. Sorensen 3 , Stefan V. Lalonde 1 , Jasquelin Peña 4 , Brandy M.