[ASAP] Deriving Nickel (Ni(II)) and Chromium (Cr(III)) Based Environmentally Safe Olivine Guidelines for Coastal Enhanced Silicate Weathering

Achieving the goals set in the 2015 Paris agreement to limit global warming to well below 2 °C will require drastic reductions in anthropogenic greenhouse gas emissions during the coming decades.(1) Additionally, carbon dioxide (CO₂) will also need to be captured from the atmosphere by so-called negative emission technologies (NETs).(2) These NETs are needed to be able to achieve net zero CO₂ emissions by the year 2050 and net negative CO₂ emissions during the second half of the 21st century.(3,4) One of the proposed NETs is coastal enhanced silicate weathering (ESW). This technique aims to artificially speed up the natural CO₂-consuming chemical weathering of a silicate mineral by supplying gigatons of finely ground source rock to the dynamic coastal environment.(5)

The mineral olivine (Mg_2xFe_2(1–x)SiO₄) is a prime candidate for coastal ESW because of its relatively fast dissolution rate and widespread abundance.(5−7) Billions of tonnes of olivine are globally present in mafic and ultramafic igneous rocks, and each year approximately 8 Mt is mined from dunite (rock containing >90% olivine) or serpentinite (metamorphosed olivine-rich ultramafic rock) deposits for metallurgical use.(8−10) Relatively low energy costs and low CO₂ emissions associated with grinding olivine source rock to 100 μm particles make this a desirable grain size for use in coastal ESW.(6) However, several studies indicate that grain sizes of 10 μm or smaller might be needed to ensure significant olivine dissolution and CO₂ uptake in the coming decades.(6,7,11)

During olivine dissolution, protons are consumed (eq 1), which leads to a shift in the equilibrium of the acid–base reactions of the seawater–carbonate system (eq 2) to the right. Consequently, seawater total alkalinity (TA) will increase, and additional CO₂ can be taken up from the atmosphere (eq 3).(5) This artificial seawater TA increase to drive atmospheric CO₂ removal is called ocean alkalinization (OA) or ocean alkalinity enhancement (OAE).(12,13)(1)(2)(3)

Combining these reactions gives the overall dissolution reaction of olivine (eq 4), from which we can derive that theoretically 4 mol of CO₂ is sequestered in the form of bicarbonate (HCO₃^–) per mole of dissolved olivine.(14)(4)

In addition to CO₂ sequestration, the proton consumption during dissolution could counteract ocean acidification. Furthermore, the released iron (Fe) and silicon (Si) are necessary nutrients for phytoplankton growth and could therefore result in additional atmospheric CO₂ uptake and increased abundance of silicifiers (e.g., diatoms), but these aspects remain to be investigated.(5,13) In contrast to these associated benefits, olivine also contains chromium (Cr) and nickel (Ni), which are toxic to marine organisms above certain threshold concentrations.(5,15)

Nickel is a transition metal that is mainly used in the production of stainless steel and other Ni alloys.(16) In the crystal structure of olivine, Ni²⁺ is homogeneously distributed in the M1 octahedral binding sites, substituting for other divalent cations such as Mg²⁺ or Fe²⁺, thereby forming a nickel silicate (Ni₂SiO₄).(17,18) The Ni content in olivine ranges from 2.4 to 12 mmol of Ni mol^–1 of olivine depending on the site of origin.(14,18,19) Nickel is an essential component of nine enzymes associated with carbon, nitrogen, and oxygen cycling in marine microorganisms.(20,21) However, for other marine biota Ni essentiality has not been recognized and the uptake mechanisms are not well-known.(22) Exposure to elevated concentrations of Ni can lead to toxicological effects as a consequence of three main toxicity mechanisms: (1) ionoregulatory disruption (mainly Ca²⁺, Mg²⁺, and Fe^2+/3+), (2) respiratory toxicity as a result of an allergic type reaction of respiratory epithelia, and (3) reactive oxygen species induced oxidative stress.(16,22) However, the relative importance of the different toxicity mechanisms is not well-known for marine organisms.(22)

In contrast to nickel, chromium is not homogeneously distributed in olivine, but rather is present in iron (Fe)-rich areas, likely as Cr³⁺ in the form of water-insoluble chromite (FeCr₂O₄).(18) Reported Cr concentrations in olivine from the Norwegian Åheim mine (the largest exploited olivine source) vary considerably, ranging from 0.19 mmol mol^–1 of olivine to 6.6 mmol mol^–1 of olivine.(14,18,19,23,24) In the marine environment Cr occurs in two stable oxidation states, Cr³⁺ and Cr⁶⁺, which differ significantly in their environmental and biological behaviors.(25) Hexavalent Cr occurs as chromate (CrO₄^2–) or dichromate (Cr₂O₇^2–) anions in aquatic systems, which can easily cross biological membranes via nonspecific anion channels and are therefore considered highly bioavailable and potentially very toxic.(25−27) After entering the cell, Cr(VI) is reduced to Cr(III). This reduction is accompanied by the production of reactive oxygen species (ROS) which can damage cell membranes, proteins, and DNA when in excess of antioxidant molecules.(27,28) By contrast, trivalent chromium is predominantly present as Cr(OH)₃ or Cr(OH)₂⁺ in the marine environment, which are chemical species of less ecotoxicological concern compared to Cr(VI) species due to their low aqueous solubilities.(25,29)

A body of literature is available on the acute and chronic toxicities of Ni²⁺ and Cr³⁺/Cr⁶⁺ toward marine biota.(30−37) Most of the data have been derived from laboratory studies that exposed single biological species to single metal ions (e.g., Ni²⁺ or Cr³⁺) under optimal conditions (e.g., constant pH and temperature). With the use of various approaches, e.g., species sensitivity distributions (SSDs), these data are used to derive marine metal environmental quality standards (EQS), which are threshold metal concentrations in seawater or sediment that are considered to be sufficiently protective for the aquatic environment.(38) These EQS are used by industries, the government, and environmental agencies as a guidance tool in the setting of regulations.(39)