Role of the crystal chemistry on the dissolution kinetics of Fe(III)-rich smectites

• Published in: Geochimica et cosmochimica acta 2024-04, Vol.371, p.162-172
• Main Authors: Masson, D., Robin, V., Joussein, E., Tertre, E., Baron, F.
• Format: Article
• Language: English
• Subjects: Acidic conditions; Crystal chemistry; Dissolution kinetics; Environmental Sciences; Sciences of the Universe; Smectite; Structural Fe(III)
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2024.01.028

Smectites are reactive minerals that are present in surface environments. Most of the smectite dissolution kinetics reported in the literature are related to Al-rich smectites, whereas Fe(III)-rich smectites, which are commonly found in soils, have not been well studied. In this study, the impact of structural Fe(III) in smectites on the dissolution kinetics is determined under acidic conditions. The dissolution kinetics of three pure Fe(III)-rich smectites with different crystal chemistries (Si7.16Fe3+0.84)(Fe3+3.46Mg0.54)O20(OH)4Na1.38, (Si6.86Fe3+1.14)(Fe3+4)O20(OH)4Na1.14, and (Si7.34Fe3+0.66)(Fe3+4)O20(OH)4Na0.66 are investigated at 25 °C using flow-through experiments at pH values between 1.8 and 5.5. The measured dissolution rates are compared with published rates for Al-rich smectites. The concentrations of dissolved Si, Fe and Mg are measured as a function of time, and the retrieved residual solids are analyzed via infrared spectroscopy and transmission electron microscopy. The steady-state dissolution rates of the synthetic smectites, calculated from the amount of Si release (RSi), decrease with increasing pH. A comparison of the dissolution rates as a function of pH does not allow us to observe major effects of the crystal chemistry among the synthetic smectites, demonstrating the need to develop another approach. However, the dissolution rates calculated for synthetic Fe(III)-rich smectites are systematically greater than those of Al-rich smectites. The measured dissolution rates decrease as the aqueous system approaches equilibrium. Different behaviors are observed for Fe(III)-rich (i.e., > 4 Fe(III)/O20(OH)4) and Al-rich smectites as the RSi = f(ΔG) functions differ, with Fe(III)-rich smectites exhibiting greater instability than Al-rich smectites. Under the most acidic conditions investigated (pH 1.8 and 2), the crystal chemistry and mineralogy of the smectite changed during dissolution, with a relatively greater loss of tetrahedral Fe than of octahedral Fe and the formation of amorphous silica phases. These results show that the amount and location of structural Fe(III) in smectites impact the stability of the minerals. This suggests that the dissolution rate of smectites as a function of the ionic activity product classically used to describe reactive transport should consider the effects of the crystal chemistry, in particular, the amount of structural Fe(III), which is rather high, especially in soil smectites.