The dissolution of olivine added to soil: Implications for enhanced weathering

Abstract

Editorial handling by M. Kersten a b s t r a c tChemical weathering of silicate minerals consumes atmospheric CO 2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (''enhanced weathering'') has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg 2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10 À16.4 and 10 À15.5 moles(Mg) cm À2 s À1 . Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg 2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.

Potential and costs of carbon dioxide removal by enhanced weathering of rocks

“…For a grain size of 20 m, the weathering rates from section 2 yield a total global CDR potential of 4.9 Gt CO 2 a −1 and 95 Gt CO 2 a −1 for basalt and dunite, respectively ( figure 3(a)). The weathering rates measured in [16] without plant and root activity would lead to a much smaller potential of 1.9 Gt CO 2 a −1 for dunite. The large spread of rates again demonstrates the need for further research that constrains the uncertainties of field weathering rates.…”

“…Equation (2) represents an idealized dissolution process, neglecting grain surface changes during dissolution. Past studies have referred to an idealized general shrinking core model to derive relative dissolution [15,16]. However, the model is based on perfect rather than highly irregular grains and is not able to convincingly reflect the observed specific surface area of natural mineral material, which can be up to 40 times higher (cf.…”

“…Biotic soil processes and high soil CO 2 concentration exert a strong influence on the weathering rate [24] but need to be better constrained and can only be discussed qualitatively. Taylor et al [5] have shown in an exemplary catchment where their model, which includes biotic processes, is able to reproduce rather fast lab-derived weathering rates [15,[22][23][24][25], whereas Brantley [16] calculated rates from an abiotic soil column experiment, that are at least two orders of magnitude lower. A larger experiment supports this finding for abiotic field weathering rates [26], which points towards a general underestimation if biotic processes such as soil respiration are disregarded.…”

The kinetics of siderophore‐mediated olivine dissolution

“…These factors make research on the dissolution rate of olivine important for understanding the natural carbon cycle as well as the feasibility of CO 2 sequestration techniques based on accelerated silicate mineral dissolution (e.g., Hartmann et al., ; Köhler, Hartmann, & Wolf‐Gladrow, ; Montserrat et al., ; Power et al., ; Renforth, Pogge von Strandmann, & Henderson, ; Shirokova et al., ; Weber & Martinez, ). Here, we explore the kinetics and mechanism of biotic Fe‐bearing silicate mineral dissolution using olivine and deferoxamine B (N′‐{5‐[Acetyl(hydroxy)amino]pentyl}‐N‐[5‐({4‐[(5‐aminopentyl)(hydroxy)amino]‐4‐oxobutanoyl}amino)pentyl]‐N‐hydroxysuccinamide) as a model mineral and ligand, respectively.…”

“…These factors make research on the dissolution rate of olivine important for understanding the natural carbon cycle as well as the feasibility of CO 2 sequestration techniques based on accelerated silicate mineral dissolution (e.g., Hartmann et al, 2013;Köhler, Hartmann, & Wolf-Gladrow, 2010;Montserrat et al, 2017;Power et al, 2011;Renforth, Pogge von Strandmann, & Henderson, 2015;Shirokova et al, 2012;Weber & Martinez, 2017). Here, we explore the kinetics and mechanism of biotic Fe-bearing silicate mineral dis- While laboratory mineral dissolution experiments with purified microbial ligands provide insights into the potential effects of microbial activity on mineral dissolution rates, extrapolating experimental results to natural systems is not straightforward.…”