 |
|
|
|
|
|
|
|
|
|
|
|
 |
|
||||||||||
 |
 |
 |
|
||||||||
 |
 |
|
|||||||||
 |
|
||||||||||
 |
 |
 |
 |
 |
 |
 |
 |
|
|||
 |
|
||||||||||
 |
|
||||||||||
| EXAFS | ||||||||||||||||||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||
|
Experiment
Chemicals. The reagents used were at least of analytical grade. Millipore (Milford, MA, USA) Milli-Q Plus (MQ) water (18.2 M
cm) was used for all solution preparations. Synthetic As
O
and AsO
were purchased from Merck (Darmstadt, Germany) and monomethylarsonic acid (CHAsO(OH)) and dimethylarsinic acid ((CH)AsO(OH)) were gifts from the late Professor K.J. Irgolic (Karl-Franzens University, Graz, Austria). For wet chemical analyses stock solutions of these As compounds containing 1000 mg l
were prepared in water and kept at a temperature of 4 °C; appropria te dilution of the stock solutions with water yielded working solutions (prepared fresh daily) with As concentrations of 1-100 ng ml. The following As-containing minerals (partly gifts from the geology museum, University of Exeter in Cornwall, UK) were used as standards in the X-ray absorption spectroscopy analyses: pharmacosiderite (KFe
(AsO)(OH).6-7HO), scorodite (FeAsO.2HO), arsenolite (natural AsO), realgar (AsS) and orpiment (AsS).
Soil samples. Samples were taken near a calciner in Cornwall, located approximately 4 km west of the Camborne-Redruth-St. Day orefield, which ceased operation in the 1920s. Samples to establish the depth profile were collected from the calciner's residue dump, indicated by the black spot in Figure 1. Samples R076, R077 and R078 were extracted from distinct layers visible on the exposed side of the residue dump pit, where the recently-added cover of clean soil had been eroded. Sample R076 was from the top layer (15-20 cm), R077 from the middle layer (20-25 cm) and R078 from the bottom layer (25-30 cm). Sample R075 is a mixed soil sample representing the soil in a depth from 0-30 cm, which was taken approximately 3 m from the edge where the depth profile samples were collected. Samples were dried and sieved at 2 mm, followed by grinding in a zirconium planetary mill to a fine powder.
Total elemental concentrations were determined by a Hewlett-Packard 4500 PLUS ICP-MS spectrometer [19] after open acid digestion with HClO and HF of the soil sample in a Teflon beaker on a hot plate [19].
X-ray absorption spectroscopy. As K-edge absorption spectra (EXAFS [Extended X-ray Absorption Fine Structure] and XANES [X-ray absorption near-edge structure]) of the soil samples, and reference As compounds and minerals were recorded at the BM29 beamline of ESRF in Grenoble and at the X1 station of HASYLAB at DESY (Hamburg, Germany). A Si(311) double-crystal monochromator was used at both beamlines. The resolution at the As K-edge (11866.7 eV) was 1 eV at BM29 and 1.5 eV at X1. The high-order harmonics were effectively eliminated by detuning of the monochromator crystals to the 60% of the rocking curve maximum, using the beam-stabilization feedback control.
The absorption spectra of the soil samples with high As level (R077 and R078) and the reference As samples were obtained in transmission mode. The intensity of the X-ray beam was measured by three consecutive ionization chambers. At the BM29 station the chambers were filled with an Ar/He mixture at 2 bar, with 14%, 47 % and 47% of Ar. At the X1 station the first ionization chamber was filled with an Ar/N mixture (50/50), and the second and third with an Ar/Kr mixture (90/10) at ambient pressure.
Powder samples were prepared on multiple layers of adhesive tape (30 in case of soil samples and 10 - 20 in case of reference samples) and inserted between the first and second ionization chamber. Stacking of layers to the attenuation of ~1 above the As K-edge improved the homogeneity of the samples. The absorption spectra were measured within the interval [-250 eV ..900 eV] relative to the edge. In the XANES region equidistant energy steps of 0.5 eV were used, while for the EXAFS region equidistant k-steps (
k > 0.03 
) were adopted with an integration time of 2s/step.
For the low-arsenic content soils (R075 and R076), the fluorescence detection mode at the X1 station of HASYLAB was used, exploiting a seven pixel Ge-fluorescence detector. The absorption spectra were obtained as the ratio of the fluorescence detector signal and the signal of the incident photon beam from the first ionization chamber. An integration time of 12s/step was adopted.
In all experiments, the exact energy calibration was established with simultaneous absorption measurements on metallic arsenic placed between the second and the third ionization chamber.
|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
E-mail:iztok.arcon@p-ng.si Last change: 28-Jun-2006 |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
