Hydrotalcite-like solid solutions with variable SO ²⁻₄ and CO ²⁻₃ contents at 50° C

Abstract

Hydrotalcite-like solid solutions have been synthesized by coprecipitation in basic solutions with variable SO ²⁻₄ /CO ²⁻₃ ratios. Chemical determination of CO ²⁻₃ in the interlayer was impossible because of the presence of minor hydromagnesite. SO ²⁻₄ was determined both by chemical analysis and X-ray photoelectron spectroscopy (XPS), the two methods giving similar results. A Raman spectrometry gave additional data on the SO ²⁻₄ /CO ²⁻₃ ratio. Then, the stoichiometry of the anionic interlayers, X _s , X _c , and X _OH were determined, and the influence of X _s on the c′ parameter (increasing from c′=7.97 Å to c′=8.63 Å between X _s =0 and X _s =1) was characterized. In addition, a partitioning curve of SO ²⁻₄ and CO ²⁻₃ between aqueous solutions and hydrotalcite-like compounds was established. Its general shape strongly suggests a miscibility gap between a sulfate-rich end and a carbonate-rich solid solution (maximum SO ²⁻₄ /CO ²⁻₃ about 0.2). This result explains why most of the hydrotalcites synthesized during experimental alteration of basaltic glasses by sea-water (a sulfate-rich solution) are CO ²⁻₃ -rich solid solutions.