On the nature of calcium magnesium acetate road deicer

Abstract

CMA (calcium magnesium acetate) road deicers have gained popularity in recent years as an environmentally friendly alternative to traditional rock salt, and as an industrial adsorbent for removing H2S and other odorous acid gases from gas streams. Despite its increasing commercial use, its exact composition and crystalline structure remained unknown, with subsequent problems in evaluating properties of commercial CMA. Various synthetic routes towards formation of crystalline calcium magnesium acetates were investigated. From aqueous solutions preferential formation of calcium monohydrates or calcium monohydrate acetic acid solvates is observed. Crystals of genuine mixed metal calcium-magnesium acetate were obtained from hot glacial acetic acid. Material suitable for analysis by single crystal X-ray diffraction, SC-XRD, was obtained by slight reduction of solvent volume at 80 °C for several hours. CMA crystallizes in the orthorhombic space group Pnma with a formula of Mg2Ca(OAc)6 (OAc = acetate anion), with a magnesium to calcium ratio of two to one. Under the same conditions, but in the absence of magnesium, the acetic acid solvate of calcium acetate, Ca(OAc)2(HOAc), is obtained, which is also described. Multicrystalline XRD and EDS analysis data of ground CMA samples match those of commercial CMA. Single crystal structural analysis finds an unusually large unoccupied void space of 9.4% of the unit cell volume. Thermal gravimetric analysis, TGA, gives an upper limit of one water molecule per formula unit of CMA, leaving the void space at least partially unoccupied. This helps to better understand CMAs unusually low density, which had been an issue when used as a commercial road deicer, having been described as being easily blown off road surfaces when applied in crystalline or powder form.