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

In-situ kinetics and mechanisms of Ni2+ 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 (Mn0.053+Na0.23+)-Mn-Tc(H2O)(0.69)H+ (0.06) [(Mn4+ (0.86)Mn(0.04)(3+)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 Ni2+ adopted mainly a TC configuration, and had a Debye-Waller factor about four times higher than layer Mn. Steady-state was reached after similar to 2.2 h of contact time, and the final structural formula of the solid was (Ni0.122+Mn3+)-Ni-Tc-Mn-Tc Na-0.05(+) 0.12H2O0.36H0.01+(Mn(0.87)(4+)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.