Title: Tailored Formation of Mineral Carbonates in the Presence of Various Chemical Additives for In-situ and Ex-situ Carbon Storage
Description: The reduction and stabilization of atmospheric CO2 concentration is currently one of the most challenging problems being investigated. Carbon mineralization has recently received much attention as one of the most promising options for CO2 sequestration. The engineered weathering of silicate minerals as a means of permanent carbon storage has unique advantages such as the abundance of naturally occurring calcium and magnesium-bearing minerals and the formation of environmentally-benign and geologically stable solids via a thermodynamically favored carbonation reaction. However, several challenges need to be overcome to successfully deploy carbon mineralization on a large-scale. The current limitation of the carbon mineralization scheme for permanent storage of anthropogenic CO2 is the slow reaction kinetics, since the natural weathering of silicate minerals occurs on geological time-scales. Another problem of mineral carbonation is that the cost of the carbon mineralization process for sequestration is dominated by up front energy costs during the mineral processing and carbonation.

In this study, chemically enhanced mineral dissolution via various chelating agents was investigated to accelerate the overall reaction rate of ex-situ and in-situ mineral carbonation. To reduce the overall cost of the carbon mineralization process, the utilization of solid products as value-added materials, e.g. precipitated magnesium carbonates (PMC) and precipitated calcium carbonates (PCC), was studied. Wollastonite (CaSiO3) and antigorite, which is a kind of serpentine (Mg3(OH)4(Si3O5)) group minerals, were selected for this work. They are representative of calcium silicate minerals and magnesium silicate minerals, respectively. This work starts with development of an experimental framework for the systematic investigation of mineral dissolution and carbonation behaviors with mineral pre-processing considerations (e.g., the removal of fines (< 5 μm) to standardize the reaction surface of the minerals), experimental set-up (e.g., syringe pump reactor for the investigation of mineral dissolution and high temperature, high pressure batch reactor for the study of direct aqueous mineral carbonation) and post reaction analyses (e.g., the evaluation of various carbon analysis techniques for the accurate estimation of the extent of carbon mineralization).

Accelerated wollastonite weathering is experimentally studied first. For large scale carbon mineralization, generally Mg-bearing silicate minerals such as serpentine or olivine (Mg2SiO4) are the most suitable minerals due to not only their significant abundance in nature but also their high capacity. New York State, however, has one of the largest deposits of wollastonite in the United States and is considered to be a suitable place to adapt CO2 mineralization using Ca-bearing minerals as a CO2 storage option. Moreover, the technologies developed for enhancing carbonation of Ca-bearing minerals can also be applied to the industrial wastes with similar chemistry, such as steel slag and cement kiln dust. The effect of various types of chelating agents on the dissolution rate of wollastonite minerals is explored to accelerate its weathering rate. It is found that chelating agents such as acetic acid and gluconic acid can significantly improve the dissolution kinetics of wollastonite even at a much diluted concentration of 0.006 M by complexing with calcium in the mineral matrix.

Calcium extracted from wollastonite is then reacted with a carbonate solution to form PCC, and the study shows that by controlling the reaction temperature, the morphological structure of the synthesized PCC can be tuned for various applications (i.e., paper fillers, plastic fillers and construction materials). Microbial and chemical enhancement of ex-situ and in-situ antigorite carbonation is investigated as well as synthesis of PMC to mimic commercially available CaCO3-based filler materials. The effect of various chelating agents, including volatile fatty acids produced via anaerobic digestion of food waste, on antigorite dissolution is investigated in a syringe pump reactor. It is found that oxalate performs best among over fifteen kinds of chelating agents on accelerating dissolution rate of antigorite minerals. Among the volatile fatty acids, valerate works best on antigorite dissolution followed by acetate. The concentration of valerate, however, is very low in the produced mixture of volatile fatty acids via anaerobic digestion.

On the other hand, acetate is the dominant component in the mixture, so it is considered as the most valuable product of anaerobic digestion of food waste. Magnesium extracted from antigorite is then reacted with carbonates to form precipitated magnesium carbonates. The effects of various chelating agents, reaction time, reaction temperature and pH on the mean particle size, particle size distribution, composition, and particle morphological structures of precipitated magnesium carbonates are systematically studied. Finally, the effect of volatile fatty acids on direct aqueous mineral carbonation is studied in a high temperature, high pressure batch reactor with antigorite and olivine minerals to predict the effect of volatile fatty acids on in-situ mineral carbonation. Volatile fatty acids can enhance the overall reaction rate via direct aqueous mineral carbonation route slightly. Volatile fatty acids may be not good enough for accelerating ex-situ direct aqueous mineral carbonation. However, they may be suited to in-situ mineral carbonation, which takes years.