Higher Inorganic CO ₂ Removal Despite Slower Weathering in an Enhanced Weathering Experiment With Steel Slags and Basalt

The natural process of silicate weathering has inspired two CO₂ removal technologies: enhanced weathering and mineral carbonation. Here, in a 15-month mesocosm experiment, both approaches were combined, with the aim of maximising CO₂ removal. To do so, pre-carbonated steel slags (basic oxygen furnace (BOF) and argon oxygen decarburisation (AOD) slag) with mineral carbonation rates ranging from 0.04 to 0.13 t CO₂/t rock were applied to soil planted with maize. Other treatments included uncarbonated slags and basalt for comparison. Aside from the commonly assessed leachate and soil solid carbonate pool, which contribute to inorganic CO₂ removal, also other soil pools (plant, exchangeable, (hydr)oxide, SOM) not directly linked to CO₂ removal were assessed. These alternative sinks are key to better understand the low inorganic CO₂ removal efficiencies reported in many experiments. Indeed, also in this experiment, the realised inorganic CO₂ removal of enhanced weathering remained low in all treatments (< 0.014 t CO₂/t rock), except for the highest carbonated slag treatment, which removed slightly more CO₂ (0.04 t CO₂/t rock). Thus, with a high degree of prior mineral carbonation, the inorganic CO₂ removal during enhanced weathering was increased. This was the case even though carbonated slags weathered slower. These results demonstrate that active weathering does not necessarily imply high inorganic CO₂ removal. While slags weathered almost entirely after only 4 months, less than 5.3% of the theoretical possible CO₂ removal was realised. If weathering occurred too rapidly, the formation of secondary minerals such as (hydr)oxides and/or aluminosilicate clays likely immobilised base cations, thereby constraining inorganic CO₂ removal. For uncarbonated slags, ~58% (BOF) and 70% (AOD) of the added base cations were likely locked in (hydr)oxides/aluminosilicate clays. These findings underline that understanding the fate of weathering products (beyond the leachate and carbonate pool) is key to assess the 'true' inorganic CO₂ removal potential of enhanced weathering.