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Although macroscopic approaches are important tools to understanding the geochemical behaviours of Ni(II) in the environment, spectroscopic approaches such as X-ray photoelectron spectroscopy (XPS) and especially extended X-ray absorption fine structure (EXAFS) are powerful tools whose use is necessary to elucidate the sorption processes of Ni(II) at the molecular scale 2, 10, 11, 12, 13, 14.Īs is well known, the sorption mechanism of Ni(II) at the solid/water subsurface is strongly dependent on the contact time, pH, ionic strength, organic matter, Ni(II) concentration in the solid and aqueous phases, and temperature as well as the type of adsorbent 1, 2, 5, 6, 7, 8, 9, 10, 15, 16, 17. The main mechanisms that have been postulated for Ni(II) sorption at the solid/water subsurface include ion exchange, surface complexation including inner-sphere complexes (ISCs) and outer-sphere complexes (OSCs), surface-induced precipitation, co-precipitation, and diffusion in particle micropores 3, 4, 5, 6, 7, 8, 9. Recently, the sorption mechanisms of Ni(II) on clay minerals, oxides, and nanomaterials have been extensively studied using macroscopic and spectroscopic techniques. To understand how these processes are affected by environmental factors and to make sound decisions about environmental remediation and protection, it is imperative to understand the sorption mechanism of heavy metals at solid/liquid interfaces 1, 2. The sorption reaction at the solid/liquid interface greatly affects the mobility, speciation, fate and bioavailability of heavy metals in aquatic and soil environments.

A high temperature is beneficial to the fixation of Ni(II) on the CA soil and the formation of a Ni-Al LDH. Under weakly acidic conditions, the sorption mechanism of Ni(II) on the CA soil possibly included at least two processes: (i) a fast accumulation dominated by ion exchange and surface complexation and (ii) the formation of a Ni-Al LDH phase over the long term. The R Ni-Ni (~3.07 Å) suggested that the structure of the Ni(II) adsorbed on the CA soil was basically the same as that of Ni(OH) 2(s), while the Ni-Al shell ( R Ni-Al ~3.16 Å) gradually formed and grew with the increasing contact time. Under alkaline conditions, EXAFS analysis showed that the interatomic distances between Ni and O atoms ( R Ni-O) were approximately 2.04 Å with a typical coordination number ( CN) of ~6.0 O atoms in the contact time range from 2 to 15 days. The sorption of Ni(II) on a calcareous aridisol (CA) soil, one of the major soil types in northwestern China, was investigated using batch and extended X-ray absorption fine structure (EXAFS) approaches in a 0.01 mol/L NaClO 4 solution at different pH values (6.0–10.0), temperatures (25–60 ☌) and contact times (2–15 days).