Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe
journal contributionposted on 18.04.2017, 00:00 by Bin Ma, Alejandro Fernandez-Martinez, Sylvain Grangeon, Christophe Tournassat, Nathaniel Findling, Francis Claret, Ayumi Koishi, Nicolas C. M. Marty, Delphine Tisserand, Sarah Bureau, Eduardo Salas-Colera, Erik Elkaïm, Carlo Marini, Laurent Charlet
Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42–) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanismsedge sorption, interfacial dissolution-reprecipitationare at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.
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edge sites complexessorption isothermCaMoO 4 precipitationMo geometryanion uptake mechanismsCaAl LDHsbatch isotherm experimentscontrol anion uptaketime-resolved synchrotron-based X-ray diffractionMo K-edge X-ray absorption spectraanion exchange mechanismsanion exchange processLayered Double HydroxidesMultiple Sorption ModesStructural Probe Layeredinterlayer anion exchange