Assessing the Benefits and Drawbacks of Using Mine Residues for Enhanced Rock Weathering

Enhanced rock weathering (ERW) is a promising carbon dioxide removal (CDR) technology that involves spreading silicate rock powder in agricultural and silvicultural settings to trap CO₂ by increasing soil alkalinity and promoting the formation of secondary carbonate minerals [1]. Many previous ERW trials have used newly mined rock for their amendments, resulting in embedded carbon emissions. To avoid these emissions, alkaline and environmentally safe mine residues could be used for ERW [2]. Here, we assess the carbon drawdown potential of two mine residues (kimberlite, serpentinite) and three newly mined agricultural amendments (basalt, metabasalt, wollastonite). We explore the use of geochemical analyses and remote sensing to monitor CO₂ drawdown and the introduction of potentially hazardous transition metals into soil solids, plants, and water.

 

Pea (Pisum sativum L.) plants were grown in acidic soil (pH = 4.9) amended with each rock type at four spreading rates (1, 5, 10, and 50 t/ha). This growth chamber trial ran for three months, with leachate samples collected throughout, and soil samples collected at completion. Over 3 months, the alkalinity of drainage waters from pots of all amendment types significantly (p < 0.05) increased compared to controls, while soil inorganic carbon increased significantly (p < 0.05) for four of five rock types (all but metabasalt). After 3 months, visible, near infrared (VNIR), and shortwave infrared (SWIR) scans of the soils showed increased abundances of carbonate minerals on the surfaces of soil colloids in the amended pots.

 

Among the transition metals analyzed (e.g., Cd, Co, Cr, Ni) in drainage waters, plants, and soil solids, significant increases in concentration (p < 0.05) were only detected for nickel (10 mg/L) and only in leachates from soils amended with high amounts of serpentinite (50 t/ha), which remains below the Canadian regulatory standard (14 mg/L). Further, significant increases (p < 0.05) in nickel concentration were seen in the soil solids for both kimberlite- and serpentinite-amended pots, resulting in contamination (63 and 140 mg/kg respectively) significantly above (p < 0.05) the Canadian regulatory limit (37 mg/kg). Finally, a significant increase (p < 0.05) in nickel concentration was seen in the edible portion of the pea plants grown in soils amended with serpentinite, but the concentration remained significantly below (p < 0.05) the EU regulatory limit (10 mg/kg).  The remaining drainage waters, plants and soil solids contained transition metal concentrations below regulatory limits.

 

While this study demonstrates the potential for CDR through ERW using mine residues, it also highlights   contamination risks that need to be weighed when determining deployment strategies, locations, and amendment rates if mine residues are to be used.

 

[1] Paulo et al. (2021), Appl Geochem, 129, 104955.

[2] Power et al. (2024), Environ Sci Technol, 58, 43-52.