CO2 geological sequestration can remove emerging contaminants in groundwater: The important role of secondary mineral carbonates.

Carbon dioxide (CO2) injection has been proposed as a strategy for carbon sequestration, while uncertainties persist regarding its effects on groundwater. Concerns have been raised that CO2 mineralization and sequestration could potentially lead to groundwater contamination. However, our study demonstrates its capability to mitigate pollution. The injection of CO2 facilitates the rapid dissolution of minerals, releasing Ca(II), Mg(II), and Fe(II) and forming secondary carbonate minerals, such as CaCO3, MgCO3, and FeCO3. The in-situ generated FeCO3 can activate oxygen to produce hydroxyl radicals (•OH) under oxic condition, thereby enhancing the degradation of emerging organic contaminants in groundwater, such as 2,4,6-tribromophenol, flurbiprofen, diclofenac, carbamazepine, phenol, and sulfamethoxazole. Mechanism studies suggest that this process is enhanced by the conversion of in-situ formed FeCO3 into a two-dimensional goethite nanosheet structure, which provides a larger specific surface area and enables more Fe(II) to be adsorbed on the mineral surface. The formation of Fe-O coordination bonds effectively reduces the loss of •OH at the interfacial reaction layer. The study further distinguishes and quantifies the contributions of different Fe(II) forms to •OH generation. The transformation pathways of the six contaminants and the toxicity of their intermediates are also analyzed. CaCO3 and MgCO3 do not exhibit the ability to degrade pollutants, but play a role in carbon mineralization. This work reveals that secondary minerals generated through the CO2 mineralization and sequestration process display simultaneous capabilities of contaminant degradation and carbon fixation. Such activities are pivotal not only for the environmental fate and transformation of emerging contaminants in groundwater but also for regulating the carbon cycle.