Application of ultrafine bubbles for enhanced carbonation of municipal solid waste incineration ash during direct aqueous carbonation

Municipal solid waste incineration fly ash was selected as the alkaline Ca-bearing solid waste, and a series of direct aqueous carbonation experiments using 10% CO₂ gas were conducted to showcase the capability of ultrafine bubbles (UFBs) in enhancing carbonation efficiency. Results from the experiments, conducted using a one-pass water flow system, revealed that carbonation without UFBs increased the CO₂ content of the ash from 59 to 200 mgCO₂/g (an increase of 141 mgCO₂/g), while the presence of UFBs elevated it to 237 mgCO₂/g (an increase of 178 mgCO₂/g). Consequently, the introduction of UFBs led to a 26% increase in CO₂ content in ash [(178−141) / 141]. This improvement was primarily attributed to the enhanced carbonation efficiency for particles ≥ 46 µm. The positive impact of UFBs was more evident (62% increase in CO₂ content in ash) in experiments using a water circulation system, where carbonation proceeded at a faster rate compared to the one-pass water flow system. In terms of the mechanism, X-ray diffraction and scanning electron microscopy analyses indicated that UFBs facilitated the removal of CaCO₃ deposition, which inhibited Ca(OH)₂ dissolution. To the best of our knowledge, this study is the first to demonstrate the favorable influence of UFBs on fly ash carbonation efficiency.