Periclase surface hydroxylation during dissolution

Abstract

Periclase (001) surfaces were etched in dilute acid at pH 2 and 4. X-ray reflectivity measurements on a reference crystal constrained the initial roughness of these surfaces to be approximately 30 Å. The reference crystal and the crystal reacted at pH 2 were analyzed by Elastic Recoil Detection Analysis (ERDA) for proton penetration. After reaction the etched sample showed proton penetration to a depth of at least 5000 Å while the reference crystal showed no significant proton inventory. Down to 900 Å, the H Mg ratio in the etched sample was approximately 2, consistent with near-surface protonation of the MgO to form hydroxylated brucite-like layers. Protonation is a far more likely mechanism to explain the proton profile than precipitation because this reaction was completed over 16 orders of magnitude below saturation with brucite. Formation of a hydroxylated near-surface layer on periclase during dissolution explains why the dissolution rates of periclase and brucite are identical in the pH range 2–5; the detachment rates are the same because the surface structures are the same. This suggests that even for this ionic solid in acid, the dissolution reaction involves a two-step mechanism with a rapid single protonation step of near-surface oxygen atoms and a slower, rate determining second protonation step. In general, product phases such as brucite are likely to be better developed under natural weathering conditions of near-neutral pH because the second step of protonation (and thus full hydration of the detaching cation, e.g., Mg +2) is much slower than in acid. Our proposed protonation mechanism relates field observations of the periclase weathering reaction to laboratory dissolution, hydration, and dehydration experiments.