Ionic substitution of Mg2+ for Al3+ and Fe3+ with octahedral coordination in hydroxides facilitate precipitation of layered double hydroxides

Abstract Precipitation of hydrotalcite-like layered double hydroxides (HT-LDHs) from CO 3 2− -SO 4 2− -rich acidic and alkaline aqueous media through ionic substitution of Mg 2+ for Al 3+  + Fe 3+ and vice versa was investigated under ambient conditions. Diffractogram, spectroscopic, microprobe, microscopic, and synchrotron techniques were used to examine the mechanisms involved. The cations facilitated rapid precipitation of HT-LDH in alkaline conditions (pH ≥ 8.2) with SO 4 2− and CO 3 2− as the counter charge balancing interlayer anions, while initial formation of Fe 3+ - and Al 3+ -hydroxides in acidic conditions (pH ≥ 2.4) with subsequent transformation to MgAlFe-type HT-LDH (pH ≥ 8.2) occurred through substitution of Mg 2+ for Al 3+ and Fe 3+ . Substitution of Al 3+ and Fe 3+ in Mg 2+ -hydroxides did not yield HT-LDH, while the reverse, i.e., Mg 2+ substitution in Al 3+ and Fe 3+ -hydroxides, produced initial poorly ordered amorphous HT-LDH that gained better crystallinity and crystallite size upon neutralization. Linear combination fit analyses of XANES data suggest schwertmannite constituted the predominant Fe-phase until pH ∼3.7 followed by ferrihydrite and eventually HT-LDH after pH ≥ 10; basaluminite and epsomite constituted the predominant Al and Mg phases until pH ∼4.5, after which HT-LDH with minor Al(OH) 3 and HT-LDH with brucite, respectively, predominated. The study highlights that Mg 2+ substitution in Al- and Fe-precipitates is the governing mechanism for HT-LDH precipitation in oxic environments through neutralization of acidic cationic aqueous residues.