Fig. 7. Computed tomography (CT) scans for quartz column collected from different locations (bottom and top) after 8 and 22 d of reaction. (a) Bottom (20 mm from the inlet) after 8 d of reaction; (b) top (60 mm from the inlet) after 8 d of reaction; (c) bottom (20 mm from the inlet) after 22 d of reaction; (d) top (60 mm from the inlet) after 22 d of reaction.