Fe(ii)(aq) uptake of Mg(ii)–Al(iii)/Fe(iii)–SO4/CO3 HTLCs under alkaline conditions: adsorption and solid state transformation mechanisms

Abstract

Abiotic reduction of Mg(II)–Al(III)/Fe(III)–SO₄/CO₃ hydrotalcites (HTLCs) was investigated under three anoxic abiotic reaction conditions: (1) a target pH of 8 and 10 mM Fe(II)_(aq), (2) a target pH of 8 and 0.5 mM Fe(II)_(aq), and (3) a target pH of 10 and 0.5 mM Fe(II)_(aq). Under both Fe(II)_(aq) concentrations and at the targeted pH of 8, the greatest Fe(II)_(aq) uptake, Mg(II)_(aq) release, and least deviation in pH was observed for Fe-based HTLCs (Mg(II)–Fe(III)–SO₄/CO₃) irrespective of the interlayer ion (SO₄ or CO₃). In contrast, the Al-based HTLCs showed the greatest pH drift from the target pH of 8; this phenomenon was attributed to the remaining total Fe(II + III) in solution. At the target pH of 10 and 0.5 mM Fe(II)_(aq), the pH of the Al-based HTLCs was greater than the Fe-based HTLCs despite 99% of the Fe(II) being removed from solution in both cases. The greatest Mg(II)_(aq) release was again observed for the Fe-based HTLCs. Unique to these reaction conditions was the readsorption of the released Mg(II)_(aq) by the Mg–Fe–CO₃ HTLC. The major reaction phases produced from the adsorption of Fe(II)_(aq) onto Mg(II)–Al(III)/Fe(III)–SO₄/CO₃ HTLCs were green rust (GR) and magnetite (MG). Two types of phase transformation mechanisms are suggested to occur during the adsorption of Fe(II)_(aq) onto Mg(II)–Al(III)/Fe(III)–SO₄/CO₃ HTLCs, both dependent upon the metal ion (Al vs. Fe) and independent of the interlayer ions (SO₄ or CO₃). For Fe-based HTLCs (Mg–Fe–SO₄/CO₃), surface adsorption of Fe(II)_(aq) and surface precipitation allows electron transfer (between Fe(II) ↔ Fe(III)) and subsequent atom exchange to occur at the surface as well as within the bulk HTLC structure. For the Al-based HTLCs (Mg–Al–SO₄/CO₃), this surface adsorption of Fe(II)_(aq) and surface precipitation still occurs but electron transfer is limited to the precipitated surface phase due to the redox inactivity of the Al(III) groups in the HTLC structure.