The industry of phosphoric acid produces a calcium-rich by-product known as phosphogypsum, which is usually stored in large stacks of millions of tons. Up to now, no commercial application has been widely implemented for its reuse because of the significant presence of potentially toxic contaminants. This work confirmed that up to 96% of the calcium of phosphogypsum could be recycled for CO2 mineral sequestration by a simple two-step process: alkaline dissolution and aqueous carbonation, under ambient pressure and temperature. This CO2 sequestration process based on recycling phosphogypsum wastes would help to mitigate greenhouse gasses emissions. Yet this work goes beyond the validation of the sequestration procedure; it tracks the contaminants, such as trace metals or radionuclides, during the recycling process in the phosphogypsum. Thus, most of the contaminants were transferred from raw phosphogypsum to portlandite, obtained by dissolution of the phosphogypsum in soda, and from portlandite to calcite during aqueous carbonation. These findings provide valuable information for managing phosphogypsum wastes and designing potential technological applications of the by-products of this environmentally-friendly proposal.

The weathering reactions of hydration and carbonation of nanostructured calcium oxide with atmospheric moisture and carbon dioxide have been characterized. This work is the first-to-date combined kinetic and nanostructural research on CaO oriented to two key processes for different systems, i.e. hardening of construction materials and carbon mineral sequestration. The evolution of the precipitated crystalline phases was monitored by X-ray diffraction and thermogravimetry, along with structural characterization by nitrogen physisorption, electron microscopy and small-angle scattering. Complete hydration of the samples was always found prior to the onset of carbon sequestration, which depended on the nanostructure of the samples. 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 combined research was completed by developing an empirical description of the weathering reactions in terms of a two-process Random Pore Model. Finally, this work aimed to determine the role of the nanostructure of samples based on industrial wastes as one of the most important factors for developing efficient carbon sequestration technologies.Peer reviewe

A new carbon mineralization method was designed based on a sequestration agent synthesised exclusively from industrial wastes. Phosphogypsum waste from the fertiliser industry was dissolved into caustic aqueous waste from the aluminium anodising industry. The resulting precipitate consisted of katoite (Ca3Al2(OH)12, a Si-free hydrogrossular solid solution end-member of the Al-containing hydrogarnet) and thenardite (Na2SO4); the latter easily removed by rinsing with water. The carbonation performance of this katoite-rich sequestration agent was evaluated using two different methods, by bubbling in aqueous media and by weathering. Both procedures yielded high carbonation efficiencies (80% and 100%, respectively), and resulted in a solid precipitate composed primarily of calcite (CaCO3) and aluminium hydroxide (Al(OH)3). Priority attention was given to the transfer of trace elements and radionuclides of the uranium series typically present in the phosphogypsum. Results confirmed that the traces were transferred to resulting final solid carbonate at concentrations similar to those present in the raw phosphogypsum. In conclusion, these carbonated minerals would trap substantial amounts of CO2 and produce final materials with similar civil engineering uses to those proposed for current phosphogypsum wastes. This work offers new methods for jointly managing specific industrial wastes oriented to more sustainable industrial processes and controlling CO2 emissionsPeer reviewe