Mineral carbonation in soils : engineering the soil carbon sink

Abstract

Rapid anthropogenic climate change is one of the greatest challenges that human civilisation will face in the 21st century. A 25-180 % increase in atmospheric carbon dioxide content since the early 1800’s and a predicted increase of 2-3% each year will lead to a 2-6°C rise in tropospheric temperatures. The consequences of increased atmospheric temperatures are profound and would put unsustainable strain on human infrastructure, which was conservatively estimated in the Stern Review (2006) to cost approximately 20% of GDP. Given the political, technical, economic and social barriers preventing the transition to a low carbon economy, there is an unequivocal need to research ‘geoengineering’ technologies that can bridge the gap between carbon emission reduction targets and actual emissions. Soil mineral carbonation is one such technology. The atmosphere is one of the smallest carbon pools at the Earth’s Surface (depending on how each pool is demarcated). Soils turn over the quantity of carbon in the atmosphere in under a decade and collectively form one of the largest carbon pools (3-4 times the quantity of carbon in the atmosphere). Land use change since the agricultural revolution has released 256 GtC (40 % of anthropogenic emissions). Research investigating the potential for carbon accumulation in soils is primarily focused on restoring organic carbon concentration to pre-agricultural values through modification of farming practices. The research presented in this thesis is the first that explores the potential of increasing the inorganic carbon pool as an emissions mitigation technology. Inorganic carbon accumulation is promoted by introducing divalent cation rich (predominantly calcium and magnesium) silicate and hydroxide minerals into the soil, which weather and supersaturate the soil solution with respect to carbonate minerals (predominantly calcite, aragonite, magnesite and dolomite). The carbon in the resultant precipitate is derived from the atmosphere. This is analogous to mineral carbonation technologies which induce carbonate precipitation from silicate weathering in industrial scale reactors at elevated temperatures and pressures. However, carbonation in soil exploits natural weathering processes to the same effect with minimal energy and infrastructure input. The research presented in this thesis broadly investigates soil mineral carbonation by contributing work towards the fundamental issues associated with application of soil mineral carbonation technology. Research activity described herein covers a range of laboratory batch weathering experiments, field work, geochemical modelling, plant growth trials, soil microcosm experiments and literature reviews. While eclectic, all work packages contribute to the same goal of describing the efficacy, effectiveness and potential impacts of soil mineral carbonation. The efficacy of mineral carbonation technology is primarily limited by the availability of appropriate silicate bearing material. A literature search suggests that approximately 15-16 Gt a- 1 of silicate rich ‘waste’ materials are produced as a consequence of human activity. This has a carbon capture potential between 190 and 332 MtC a-1, which is equivalent to other emissions mitigation strategies. Quarrying silicate specifically for carbonation is a suggested strategy that may be able to store on the order of 102 GtC a-1 (based on two sites in the US). Therefore, mineral carbonation may form part of global mitigation strategies collectively equivalent to 14 GtC a-1 to stabilise the CO2 concentration of the atmosphere at 500 parts per million by volume. Considering that the potential capacity of soil mineral carbonation is sufficient to act as a substantial emissions mitigation strategy it was appropriate to investigate issues associated with the application of such a technology. In the first instance, sites known to contain silicates were investigated. These include soils developed on natural silicates (on the Whin Sill in Northumberland), construction and demolition waste (at a brownfield site and waste transfer stations) and slag (at a former steelworks). Interpretation of fieldwork results suggests that inorganic carbon accumulation is rapid (up to 38 gC kg-1(soil) a-1), and is orders of magnitude xxv greater than organic carbon accumulation in natural soils. The average concentration of inorganic carbon (20-30 Kg m-3) is equivalent to organic carbon in natural soils. The unusually light carbon and oxygen isotope ratios of the carbonate (-3.1 ‰ and -27.5 ‰ for δ13C and -3.9 ‰ and -20.9 ‰ for δ18O) were used to determine that up to 55% of the carbon was derived from the atmosphere. The rate of carbon capture, which is the same as the precipitation rate of carbonate, is a function of solution chemistry. The more supersaturated a solution is with respect to a carbonate mineral, the more rapid the precipitation rate. Saturation of a solution is a function of divalent cation and carbonate anion concentration. Therefore, the supply of each of these components was investigated in laboratory experiments. Batch weathering experiments were used to investigate the supply of calcium from artificial silicates (hydrated cement gel). Up to 70-80 % of the calcium contained in the mineral was removed, which is consistent with efficiencies reported for conventional mineral carbonation. The log rate of weathering was between -10.66 and -6.86 mol Ca cm-2 sec-1, which is several orders of magnitude greater than that usually reported for natural silicates. Microcosm experiments were conducted to investigate the rate of supply of carbonate from the organic carbon mineralisation in high pH solutions. The research clearly demonstrates that high pH solutions inhibit the breakdown of organic carbon as a function of nutrient supply. Where organic carbon was successfully mineralised the log rates (-3.4 mmol g-1(field moist soil) sec-1) were equivalent to that found in previous studies. While the influx of dissolved carbonate mineral components into the soil solution is the primary controlling step in the rate of carbon accumulation, there is a complex relationship between soil physical properties and geochemistry. This was highlighted in a numerical model that was constructed for this thesis, which suggests that soil pore volume and particle size distribution are important variables. An additional numerical model was constructed to investigate the transportation of silicate material to the application site. This model suggests that an economics of soil mineral carbonation is a function of transport costs, the value of the silicate material and the price of carbon. Field observations, growth trials, microcosm experiments and previous research suggest a complex interaction between biology, weathering and carbonate precipitation. Additional work is required to investigate carbonate precipitation mediated by plant and microorganism activity and the degree to which soil mixed with silicates impact on ecosystem functioning. This research has demonstrated that mineral carbonation in soils could form a substantial emissions mitigation strategy, but additional work is required in a number of areas to which this thesis provides a suitable foundation.