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

Abstract

Enhanced silicate weathering (ESW) by spreading finely ground silicate rock along the coastal zone to remove atmospheric carbon dioxide (CO2) is a proposed climate change mitigation technique. The abundant and fast-dissolving mineral olivine has received the most attention for this application. However, olivine contains nickel (Ni) and chromium (Cr), which may pose a risk to marine biota during a gigaton-scale ESW application. Herein we derive a first guideline for coastal olivine dispersal based on existing marine environmental quality standards (EQS) for Ni and Cr. Results show that benthic biota are at the highest risk when olivine and its associated trace metals are mixed in the surface sediment. Specifically, depending on local sedimentary Ni concentrations, 0.059–1.4 kg of olivine m–2 of seabed could be supplied without posing risks for benthic biota. Accordingly, globally coastal ESW could safely sequester only 0.51–37 Gt of CO2 in the 21st century. On the basis of current EQS, we conclude that adverse environmental impacts from Ni and Cr release could reduce the applicability of olivine in coastal ESW. Our findings call for more in-depth studies on the potential toxicity of olivine toward benthic marine biota, especially in regard to bioavailability and metal mixture toxicity.

Life Cycle Assessment of Coastal Enhanced Weathering for Carbon Dioxide Removal from Air

“…Finally, the large contribution of the coastal spreading stage to the MET and HTc is mainly traced back to Ni emissions to seawater, whereas TET is mainly affected by olivine overland transportation. Specifically, the Ni that is released during olivine dissolution to seawater can pose a risk to marine biota, with toxicological effects occurring when its concentration in an organism exceeds a certain threshold value . As such, the probable harm in marine environments from increased Ni releases due to olivine spreading and weathering in coastal environments is reflected in the MET score.…”

“…Regarding the olivine impurities, here, a mean concentration of 0.263% was considered for Ni, while Cr and Fe concentrations were not included in the analysis. It has been estimated that 0.059 to 1.4 kg of olivine can be safely spread per m 2 of seafloor without exceeding the European Ni environmental quality standard (0.147 μM) and posing risks to benthic biota . Here, following Flipkens et al, this number was estimated at 1.3 kg m –2 when assuming a mean depth of 10 m, 24 h residence time, 15 °C water temperature, and 10 μm particle size.…”

“…Therefore, their possible indirect addition through CEW 10 could stimulate the biological carbon pump, but more research is required . Alternatively, Ni and chromium (Cr) can be present in olivine-rich rocks, which, if found in elevated concentrations, can cause harm to coastal ecosystems and thus hamper CEW’s eligibility as a CDR . Ni is typically well distributed within olivine’s mineral matrix; , however, this is not the case for Cr, which is concentrated in insoluble Fe-rich particles, implying low bioavailability and lower ecotoxicological concerns compared to Ni .…”

“… 21 Alternatively, Ni and chromium (Cr) can be present in olivine-rich rocks, 33 which, if found in elevated concentrations, can cause harm to coastal ecosystems and thus hamper CEW’s eligibility as a CDR. 34 Ni is typically well distributed within olivine’s mineral matrix; 28 , 35 however, this is not the case for Cr, which is concentrated in insoluble Fe-rich particles, 33 implying low bioavailability and lower ecotoxicological concerns compared to Ni. 34 For this reason, focus was placed here on the effect of Ni concentration as a representative impurity of olivine that can be released in coastal environments.…”

Sustainable scale-up of negative emissions technologies and practices: where to focus

“…Project Vesta (2022), a non-profit organisation aiming to advance the deployment of coastal EW, estimates that the cost of this NETP could range between 34 and 50 $ t −1 net CO 2 removed if implemented at scales above 100 Mt, with costs falling below 100 $ t −1 at the 1-10 Mt scale (Green 2022). Nonetheless, the CDR potential of coastal EW deploying olivine grains is limited by the toxicological effects associated with the nickel and chromium contained in the olivine; 0.51-37 Gt CO 2 could be sequestered until 2100 without putting benthic organisms at risk (Flipkens et al 2021).…”