Mechanisms of Ni(Ⅱ) Sorption at the Palygorskite-Solution Interface
revealed from EXAFS, HRTEM and DRS Investigation
Xinxin Moa, W. Goua and W. Lia,*
a Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, China
xxmo@smail.nju.edu.cn
Sorption reactions at the mineral/water interface control the
speciation, mobility, and bioavailability of trace metals in soil
environments. It is indispensable to precisely understand the mechanisms
of metal sorption reactions in the perspective of both geochemical
processes and environmental remediation. The development of synchrotron
radiation X-ray absorption fine structure spectroscopy (XAFS) technique,
an advanced analytical method, has made it possible to definitively
discriminate among the sorption mechanisms at an atomic/molecular level. Extensive
XAFS studies have been employed to investigate the mechanisms of heavy
metals on traditional layer-structure types (e.g. 1:1 type such as
kaolinite, and 2:1 type such as montmorillonite, illite, pyrophyllite),
however, metal sorption on chain-structure clays, another significant
kind of clays such as sepiolite and palygorskite, have barely been
studied. Hence, a combination of X-ray absorption fine structure
spectroscopy (XAFS), high-resolution transmission electron microscopy
(HTTEM), and diffused reflectance spectroscopy (DRS) is applied to
examine the microscopic mechanisms in the Ni/palygorskite system and
explore the contribution of mineral structures to metal sorption
mechanisms.
The results demonstrated that the mechanisms of Ni sequestration vary as
a function of reaction time, pH, ionic strength, and temperature. At low
pH and low ionic strength (e.g. below pH 7 and I=0.001 M), the sorption
of Ni was dominated by outer-sphere surface complexation. This was
confirmed by the interatomic the coordination numbers determined by
XAFS. As pH and ionic strength increased, inner-sphere sutface
complexation gradually became dominant. At higher pH and higher ionic
strength (e.g. above pH 7.5 and I=0.1 M), the Ni-Ni distances
(RNi-Ni=3.09 Å) determined by EXAFS suggested the formation of
α–Ni(OH)2 type of precipitates, which was different from those of
Ni-Al LDH (RNi-Ni=3.06 Å) and β–Ni(OH)2 (RNi-Ni=3.13 Å). Wavelet
transform analysis of XAFS data and DRS characterization further
confirmed this result. Ni-rich surface precipitates formed with the
initial Ni concentration as low as 0.07mM, with the sorption densities
of Γ=0.09 μmol m-2, which corresponds to a 0.3% monolayer coverage for
palygorskite. This observation is to our surprise because surface
precipitates would not form at montmorillonite and γ-Al2O3 at this
low level of Ni concentration. The comparation of Ni sorption mechanisms
on palygorskite, sepiolite, montmorillonite and γ-Al2O3 revealed
that for both the Ni/palygorskite and Ni/sepiolite systems, the sorption
isotherms exhibited as linear shape and the precipitate phases were
determined to be α–Ni(OH)2 precipitates, which significantly differed
from the Ni/montmorillonite and Ni/γ-Al2O3 systems, indicating the
similarity and uniqueness of the palygorskite-sepiolite group, both of
which are a typical kind of chain-structure phyllosilicates. The
findings presented in this study add substantial novel fundamental
knowledge to improve current understanding of the Ni sorption and
sequestration at the mineral/water interfaces, which influences the fate
and transport of Ni on larger scales in the environment.