Abstract
The sorption mechanism of nickel (Ni) at the illite/water interface was investigated using batch, sorption modelling, extended X-ray absorption fine structure (EXAFS), and extraction approaches. The results showed that Ni(II) sorption on illite was strongly dependent on pH, contact time, temperature, and initial Ni(II) concentration. At a low initial Ni(II) concentration, the ion exchange species of ≡X2Ni° and the inner-sphere complexes including ≡SsONi+, ≡SwONi+ and ≡SwONiOH° species are observed on the sorption edges of Ni(II) on illite. As the initial Ni(II) concentration increased to 1.7 × 10−3 mol/L, precipitates including surface-induced precipitation of s-Ni(OH)2 and amorphous Ni(OH)2 became more significant, especially under neutral to alkaline conditions. EXAFS analysis confirmed that Ni-Al layered double hydroxide (LDH) can gradually form with an increase in the contact time. At pH 7.0, α-Ni(OH)2 was produced in the initial stage and then transformed to the more stable form of Ni-Al LDH with increasing contact time because of the increased Al3+ dissolution. With an increase in temperatures, α-Ni(OH)2 phase on illite transformed to Ni-Al LDH phase, indicating a lower thermodynamic stability compared to Ni-Al LDH phase. These results are important to understand the geochemical behaviors to effectively remediate soil contaminated with Ni(II).
Highlights
The migration and retention of Ni(II) in contaminated soil and groundwater is largely controlled by its sorption and desorption behaviors at the solid/water interface
Previous studies have confirmed that Ni-Al layered double hydroxide (LDH) can form very quickly within several minutes, which is questionable given that mineral dissolution is generally slow
To the best of our knowledge, discussions of the relationship between Ni(II) speciation and availability at the illite/water interface is scarce, especially using the Extended X-ray absorption fine structure (EXAFS) technique; this relationship is critical to understand the geochemical behaviors of Ni(II) in the environment
Summary
To explore the sorption species and mechanism of Ni(II) on illite, the surface complexation model and MINTEQ 3.1 code were combined to study the sorption edges at the different initial Ni(II) concentrations, and the fit results are shown in Fig. 3 and Table 1. No changes in the RSFs for the Ni(II)-adsorbed illite at pH 10.0 suggested that the primary sorption mechanism did not change This is consistent with the XPS analysis, showing that IE and ISCs are dominant in a low pH range and that the precipitates, including Ni(OH)[2] and Ni-Al LDH, are contributing to Ni(II) sorption under alkaline conditions (Fig. SI-6). Additional characteristics and reactions under different conditions, such as the initial concentration, particle size and S/L ratio, are important to assess and predict the effects of these precipitate phases in environmental systems
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