Dissolution kinetics of platinum mine by-products for nature-based and engineered carbon dioxide removal

In addition to deep reductions in greenhouse gas emissions, active removal of atmospheric CO 2 is essential to limit global warming to <2 °C above pre-industrial levels. Dissolution of crushed silicate rocks by carbonic acid converts CO 2 into stable carbon species (alkalinity, carbonate minerals), but its effectiveness for CO 2 removal (CDR) depends on identifying suitably abundant and reactive feedstocks. Here, we assess dissolution rates of crushed by-products (overburden, processed tailings, smelter slags) from platinum group metal (PGM) mining operations within the Bushveld Complex, South Africa, during reaction with water and CO 2 under far from equilibrium conditions. Mine tailings had fastest bulk dissolution rates (∼10 -10 to 10 -8 mol m -2 s -1 ), with fresh material sourced directly from concentrator conveyor belts reacting faster than tailings collected from settling ponds. Glassy slag material dissolved at rates of 10 -11 to 10 -9 mol m -2 s -1 , whereas plagioclase-rich overburden rocks reacted more slowly, with dissolution rates of 10 -12 to 10 -10 mol m -2 s -1 . Finer-grained overburden material generally dissolved more slowly than coarser fractions, likely due to particle clustering, surface passivation and/or enrichment of slower-reacting clays in finer material. Under CO 2 -saturated conditions, relative bulk dissolution rates of PGM by-products were generally faster (by ∼1-3 orders of magnitude) than for other potential CDR feedstocks, such as basalt and gabbro. At the global scale, weathering of PGM by-products has the potential to sequester up to one gigatonne of CO 2 per year, of which the Bushveld Complex may account for as much as 75%. However, we caution that while our experiments show that PGM by-products have high intrinsic reactivity, they are unlikely to deliver durable and substantial CDR under field conditions where factors such as availability of CO 2 and soil water flow rate may inhibit dissolution. The CDR potential of PGM by-products is only likely to be fully realised with process intensification, such as using reactors or through microbial augmentation.