Abstract
Carbonate formation in waste from the steel industry could constitute a nontrivial proportion of the global requirements for removing carbon dioxide from the atmosphere at a potentially low cost. To utilize this potential, we examined atmospheric carbon dioxide sequestration in a >20 million ton legacy slag deposit in northern England, United Kingdom. Carbonates formed from the drainage water of the heap had stable carbon and oxygen isotope values between -12 and -25 ‰ and -5 and -18 ‰ for δ13C and δ18O, respectively, suggesting atmospheric carbon dioxide sequestration in high-pH solutions. From the analyses of solution saturation states, we estimate that between 280 and 2900 tons of CO2 have precipitated from the drainage waters. However, by combining a 37 year long data set of the drainage water chemistry with geospatial analysis, we estimate that <1% of the maximum carbon-capture potential of the deposit may have been realized. This implies that uncontrolled deposition of slag is insufficient to maximize carbon sequestration, and there may be considerable quantities of unreacted legacy deposits available for atmospheric carbon sequestration.
Highlights
The steel industry produces around half a billion tons of an alkaline material globally each year.[1]
This, alongside historic deposits of iron and steel slag, creates environmental issues associated with highly alkaline leachates.[8,9]
The dissolution of these materials releases calcium and magnesium ions from oxide and silicate minerals and glass and amorphous material, which raises pH beyond that encountered in most natural settings: Ca2SiO4 + 4H2O → 2Ca2+ + H4SiO4 + 4OH−
Summary
The steel industry produces around half a billion tons of an alkaline material (known as slag) globally each year.[1]. There was a significant decline in saturation index in waters draining to the Dene Burn (Figure 2A; PMK: −856; P = 0.002) irrespective of flow condition, which showed no significant long-term trend (PMK: 220; P = 0.421) This may be indicative of gradual exhaustion of alkalinity generating minerals (e.g., Ca silicates, free lime, or portlandite) in the slag deposits with weathering (see Supporting Information S7).[12] No significant trend in SIcalcite or flow in the Howden Burn is apparent (Figure 2B; SIcalcite: PMK: 9.3: P = 0.80; flow: PMK: 22; P = 0.95), a clear decline and subsequent recovery in SIcalcite is apparent between the mid-1980s and the early part of the 21st century, respectively.
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