Improving Technological Viability of Carbon Sequestration Technologies with Structured Agents and Industrial Wastes

The aim of this research is to contribute to resolve some of the major drawbacks of the carbon sequestration technologies under ambient conditions, such as kinetics and costs. We considered two different strategies to address these problems, namely, researching the role of the micro- or nanostructure of the wastes in the carbonation kinetics under ambient conditions to characterize the mechanisms that govern carbonation of calcium-rich samples by weathering, and synthesizing carbon sequestration agents from industrial wastes which are not able to capture CO₂ by themselves.

Firstly, the enhancement of the kinetics of the sequestration processes by weathering calcium oxide-rich samples was achieved by using micro- and nanostructured waste-based sequesters. This work is the first-to-date combined kinetic and nanostructural research on CaO oriented to carbon mineral sequestration. Complete hydration of the samples was always found prior to the onset of carbonation reaction. The strong dependence of both processes on the nanostructure of the samples was characterized. Hence, carbonation started after 300 h of weathering for samples with a specific surface area of 40 m²/g, whereas carbonation of the samples with 20 m²/g occurred after 550 h. Full carbonation from atmospheric CO₂ (100% efficiency) was obtained in all cases. This experimental research was completed by a empirical description of the weathering reactions in terms of a two-process Random Pore Model. Finally, this work aimed to highlight the role of the structure of the sequesters based on industrial wastes as one of the most important factors for developing efficient carbon sequestration technologies.

On the other hand, simple chemical and physical procedures were considered in order to synthesize affordable carbon dioxide sequesters using industrial by-products, which are being currently discarded. The considered wastes are phosphogypsum by-product from the phosphoric acid production and an aqueous alkaline Na-rich transparent solution from the aluminum industry. The phosphogypsum was considered as calcium source, and the alkaline solution was used as the medium for the dissolution of the phosphogypsum and the subsequent precipitation of calcium-rich hydroxides. These hydroxides were revealed as very efficient carbon dioxide sequestration agents under room conditions. The synthesis of the sequestration agents and the carbon sequestration efficiency were tested by X-ray diffraction, scanning electron microscopy and thermogravimetric analyses. This procedure is proposed not only as a solution for reducing CO₂ atmospheric concentrations, but also for reducing stockpiles of phosphogypsum wastes which caused social controversy in some of its emplacement areas and for recycling the aluminum industry wastes.