Reducing Marble-SO₂ Reaction Rate by the Application of Certain Surfactants

Abstract

Sulfur dioxide (SO₂), prevalent in the modern urban environment of industrial countries, attacks calcite (CaCO₃)in marble. As a result, a gypsum (CaSO₄·2H₂O) crust is produced at rain-sheltered surfaces while areas exposed to rain experience accelerated erosion. We have investigated theeffect of certain surfactants as protective agents against SO₂ attack. We report that the anions oxalate (C₂O₄ ^-2) and oleate (C₁₇H₃₃COO^-) from solutions of their highly soluble alkali salt species areable to replace carbonate (CO₃ ^-2) in calcite producingless reactive substrate of oxalate and oleate of calcium. Experiments to measure the protection obtained by these treatments were carried out in the laboratory and field conditions at nearly 1ppm and 10 ppb SO₂ concentrations, respectively. We found that these treatments provided significantprotection to marble exposed in sheltered areas, up to 30% reduction of reaction rate by treatment with 2 × 10^-4M sodium oleate and up to 14% by a 2 × 10^-3 M with potassium oxalate solutions, but become ineffective over long term exposure when applied to surfaces exposed to rain.Carrara marble was used in the reported study. Ion chromatographywas the analytical tool, which allowed precise measurements of ionic concentrations of these salts, the amount of their uptakeby marble, and the thickness of the gypsum crust. X-ray diffraction allowed determination of the new minerals formed at the marble surface by the treatment with surfactants.