Abstract
Coupled diffusion equations are used to describe the dissolution of porous ionic solids in acidic solutions. The analysis shows that subsurface dissolution can occur when calcium hydroxide is attacked by dilute aqueous solutions of hydrochloric acid; because highly mobile OH− diffuses out of the pore solutions more rapidly than Ca2+ diffuses in, some solid dissolves to keep the subsurface pore solution saturated with calcium hydroxide. With concentrated acid solutions, precipitation can occur just below the solid's surface due to the large influx of Ca2+ leached from the surface. The predictions are verified qualitatively by leaching gel-stabilized suspensions of calcium hydroxide. It is shown that coupled ionic diffusion in saturated pore solutions can produce subsurface dissolution or precipitation without chemical reaction when, for example, calcium hydroxide dissolves in aqueous calcium chloride solutions.
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