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Examining the pH dependence of Fe behavior in hydrotalcite-group structures

Jiaxing Ban,Barbara Lothenbach,John L. Provis,George Dan Miron,Zeyu Zhou,Dengquan Wang,Sergey V. Churakov,Bin Ma

Hydrotalcite-group layered double hydroxide (LDH) phases are important in many technical and geological contexts, and in applications ranging from environmental processes to catalysts to cements. This study systematically investigates the roles of Fe in LDH structures across varying pH conditions relevant to concrete environments, combining laboratory-based characterization, synchrotron-based techniques, and thermodynamic modeling. Elevated pH enhances Fe incorporation into the LDH phase, while suppressing ferrihydrite formation. At pH > 11, partial LDH dissolution is observed. Thermodynamic modeling and diffractometry reveal the transformation mechanism: control of the initial pH increases promotes Al(III) and Fe(III) uptake into LDH structures, while further alkalinization (pH > 11) triggers selective Al(III) dissolution, thereby increasing the M 2+ /M 3+ ratio and altering unit cell parameters. These findings elucidate the dynamics between Fe(III) incorporation in LDH and ferrihydrite precipitation, governed by pH-dependent solubility and charge-balance constraints.