The role of trivalent cations and interlayer anions on the formation of layered double hydroxides in an oxic-CO2 medium

Abstract

Most common occurrences of magnesium (Mg2+), aluminum (Al3+), iron (Fe3+), carbonate (CO32−), and sulfate (SO42−) in environmental settings can lead to formation of layered double hydroxides. This study investigated the role of different trivalent cations and divalent interlayer anions on the physicochemical properties of hydrotalcite-like layered double hydroxides. Hydrotalcite-like compounds (HTLCs) were synthesized at ambient temperature by co-precipitation while maintaining Mg2+ as the primary divalent cation and varying Al3+ and Fe3+ as the trivalent cations and SO42− and CO32− as the interlayer anions. X-ray diffractograms, Raman and infrared spectra, and scanning electron microscopy images confirmed the precipitation of rounded fibrous HTLCs. BET analyses showed that the MgAlCO3-type HTLC had the greatest surface area (101.7m2/g), followed by MgAlSO4 (93.9m2/g), MgFeCO3 (81.3m2/g), and MgFeSO4 (17.8m2/g). Substitution of Fe3+ increased lattice parameters compared to Al3+ substitution. Incorporation of SO42− as an interlayer anion was favored during Fe3+ substitution but reduced the crystallinity of the HTLC. Carbonate was the preferred interlayer anion, but available SO42− occupied the interlayer spaces together with CO32− if a high charge deficiency was maintained during HTLC precipitation. Two major stages of mass loss occurred (≤250 and 250–450°C) for all the samples with better thermal stability for SO42− vs. CO32− as interlayer anion and Al3+ vs. Fe3+ as trivalent cation.