In-situ determination of the kinetics and mechanisms of nickel adsorption by nanocrystalline vernadite

In-situ kinetics and mechanisms of Ni 2+ uptake by synthetic vernadite were determined at pH 5.8 and I = 0.1 M NaCl using wet chemistry, atomic-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and synchrotron high-energy X-ray scattering (HEXS) in both the Bragg-rod and pair distribution function formalisms. The structural formula of the initial solids was TC Mn 3+ 0.05 Na + 0.23 (H 2 O) 0.69 H + 0.06 [(Mn 4+ 0.86 Mn 3+ 0.04 vac 0.1 )O 2 ], where species under brackets form the layer having “vac” layer vacancies, and where other species are present in the interlayer, with TC standing for “triple-corner sharing” configuration. According to HEXS and STEM-EELS, adsorbed Ni 2+ adopted mainly a TC configuration, and had a Debye-Waller factor about four times higher than layer Mn. Steady-state was reached after ~ 2.2 h of contact time, and the final structural formula of the solid was TC Ni 2+ 0.12 TC Mn 3+ 0.05 Na + 0.12 H 2 O 0.36 H + 0.01 [(Mn 4+ 0.87 vac 0.13 )O 2 ]. Atomic-scale imaging of the solids also evinced the presence of minor Ni adsorbed at the crystal edge. The retention coefficient R D = 10 3.76 ± 0.06 L kg −1 , computed from PDF data modelling and solution chemistry results, was in agreement with those available in the literature.