Engineering soils to act as carbon sinks

Abstract

Soils containing calcium (Ca) and magnesium (Mg) bearing waste silicate minerals may be intentionally engineered to capture and store atmospheric carbon (C). Within the soil environment these minerals can capture and store atmospheric C through the process of weathering that releases Ca and Mg which then precipitate as carbonate minerals. Like natural silicates, silicate ‘wastes’ and artificial silicates sequester C through carbonation

of calcium (Ca2+) and magnesium (Mg2+). Terrestrial CO2 sequestration may be promoted by the inclusion of these reactive mineral substrates in soils, and many waste sites and urban and anthropogenic soils already contain quantities of these materials.

The UK Government is currently committed to reducing carbon emissions by 80% in 2050 (against a 1990 baseline) and soils have a role to play, acting as sinks for carbon. It is proposed that soil engineering measures could harness the high C turnover of the global pedologic system, ~120Pg C a-1 , to develop an efficient method of enhanced

weathering. Artificial silicates have the potential to capture 192-333 Mt C a-1 , representing 2.0-3.7% of contemporary global C emissions; natural silicates present a carbon capture potential many orders of magnitude greater. Mineral carbonation in an artificial soil setting has the potential to capture inorganic carbon comparable to organic

carbon accumulation. Soils of this type can accumulate 20-30 kg C m2 as carbonates (≥ organic carbon content in natural soils, ~17.5 kg C m2 for rural soils in the UK).

Laboratory investigations were carried out on a number of experimental scales, from meso-scale flow-through reactors to micro-scale batch experiments, to determine the rate at which Ca and Mg could be supplied from suitable materials in engineered soil systems to perform a carbon capture function. Environmental factors were controlled for each in order to constrain their contribution to the overall process. Batch experiments were carried out at standard temperature and pressure (STP) to investigate effects of changes in solute concentration, water chemistry, agitation and particle size. pH controlled experiments were run at STP from pH 3-8, to determine the effects of pH changes on the weathering of wollastonite. Flow-through weathering experiments at STP investigated the effects of time, water chemistry, hydrogeological conditions and addition of CO2 on the weathering of steel slag. Analytical results demonstrate that Ca leaches rapidly from a number of Ca-rich artificial minerals providing great potential for carbon capture to occur on human-relevant timescales. Steel slag was shown to weather at a log rate of -9.39 to -11.88 mol Ca m-2 sec-1 in laboratory settings and -7.11 to - 7.56 mol Ca m-2 sec-1 under ambient environmental conditions in the field over 975 days.

Anthropogenic soils, known to contain substantial quantities of Ca and Mg-rich minerals derived from industrial and demolition activity (including iron and steel slag, cement and concrete), were systematically sampled across two field sites. Analysis illustrated mean soil carbonate values of 21.8 ± 4.7% wt to 41.16 ± 9.89 wt % demonstrating that a large quantity of soil carbonate forms and persists in these

environments, formed at a rate of 18kg CO2 t-1 a-1 . Stable isotope data ( 13C, 18O) confirm that up to 81% of C in these pedogenic carbonates is atmospherically derived. 14 C data also suggest that a significant proportion of the C present in carbonates

analysed is ‘modern’. Applying a current CO2 trading cost of £8-£12 t-1 CO2, the potential value of CO2 sequestration at a study site was calculated to be £51,843 £77,765 ha-1 after 58% of its carbonation potential had been exploited.

The studies contained in this thesis add to a growing body of evidence for the formation of carbonate minerals in soil settings where Ca/Mg-bearing silicate minerals occur. They also support the idea that engineered soils could be effectively utilised for carbon sequestration. Soil engineering for carbon capture provides a comparatively cheap, easy and attractive way of beginning to offset the environmental impact of certain industrial processes. Carbonation of waste silicates is a useful exercise in ‘closing the loop’ on C emissions produced in their manufacture. Carbon capture taking place on sites containing industrial waste materials is of interest to a variety of stakeholders: site owners, third sector bodies and local and national legislative bodies. Effective, low- energy field-scale implementation of mineral carbonation through soil engineering could assuage current constraints on economic performance of enhanced weathering technologies and highlight the importance of soil carbon storage.