10th International Conference on
Particle Induced X-ray Emission and its Analytical Applications
PIXE 2004, Portorož, Slovenia, June 4-8, 2004
http://pixe2004.ijs.si/
Quantitative Analysis of Arsenic for
Environmental Water Samples by using PIXE
H. Yamazaki, K. Ishii, Y. Takahashi, S. Matsuyama, Ts. Amartaivan, A. Suzuki, T. Yamaguchi,
G. Momose, K. Hotta, T. Izukawa, K. Mizama, and S. Abe
Dept. of Quantum Science and Energy Engineering, Tohoku University, Sendai 980-8579, Japan
ABSTRACT
In this study, an enhanced sample preparation method for PIXE analysis was developed where dissolved arsenic is oxidized to
the pentavalent state with permanganate ions, and then is adsorbed by ferric hydroxide colloids generated in the solution. The
standard method for collecting the colloids adsorbing arsenic ions on Nuclepore filter of 0.2 µm pores is based on an
investigation of the pH-dependence of the recovery of dissolved arsenic ions and on the calibration curve. The prepared targets
were examined for 5 to 10 minutes by 3 MeV protons (0.7-2 nA beam currents). A lower detection limit of 0.3 ppb arsenic in a
25 ml aquatic sample was obtained based on the 3σ error of the background counts, integrated over the FWHM of arsenic peak
in the PIXE spectrum. The minimum limit of quantification was evaluated to be <1 ppb based on the calibration curve method.
The applicability of PIXE using this sample-preparation technique to the surveillance of the arsenic concentration in river basin
was confirmed where three hot springs were located upstreams as a possible source emitting arsenic to the environment.
Keywords: Quantitative PIXE analysis, Arsenic, River water, Target preparation, Preconcentration
Correspondence: Hiromichi Yamazaki, Tohoku University, [email protected]
1.
INTRODUCTION
The toxic effects of some chemicals in the biosphere have been pointed out previously.[1,2] Arsenic is a
hazardous element, but it is rather difficult to monitor the concentration in drinking water < 10 ppb
(ng/ml) by using spectrophotometric methods.[3-6] PIXE has been proven to be a very useful technique
for the study of the trace element distribution in ecosystems, due to its multielement analysis capability
and to the high speed of analysis of a wide variety of samples. We developed a simple and rapid threestep method for preparing thin uniform targets of inorganic components in both soluble and insoluble
fractions of aqueous samples in combination with preconcentration of trace heavy metals.[7] However,
the PIXE analysis using target preparations developed so far lacks high sensitivity for arsenic.
In this study, an enhanced sample preparation method for PIXE analysis was developed, where
dissolved arsenic is oxidized to the pentavalent state with permanganate ions, and then is adsorbed by
ferric hydroxide colloids generated in the solution. This sample-preparation of dissolved arsenic is then
applied to samples of river water where three hot spring spas are located upstreams.
2.
EXPERIMENTAL
The standard method for collecting ferric hydroxide colloids adsorbing arsenic on a thin filter was tested
by investigating the pH-dependence of the recovery of dissolved arsenic and iron ions, and the obtained
calibration curve covers the concentration range from 1 to 40 ppb (ng/ml) for arsenic. The sample
preparation procedure is as follows. Aliquot of permanganate is added to a 25 ml solution containing a
given amount of arsenic, in which the pH is adjusted to ∼1 by adding conc. HNO3. A chosen amount of
ferric ions is added to the solution on a hot plate at around 80 oC, and then the pH is readjusted to a
selected value by adding 2% NH3 aq. After stirring 2 min, the solution is filtered under suction (ca.250
mmHg) through a 0.2µm-pore Nuclepore filter of 10µm thickness. In order to confirm the applicability
of the sample preparation technique to monitoring arsenic pollution in the environment, we collected 0.5
dm3-volume samples at 6 places in the river valley of 35 km, where three different spas are located
upstreams. Samples were also collected at two spas in the uppermost location and at one place in the
branch of this river, where no spas exist, respectively. Four kinds of PIXE targets were prepared from
each water sample using the ferric hydroxide coprecipitation for arsenic and the three-step method
911.1
911.2
developed previously, that is, the Nuclepore filtration target for insoluble constituents, the
preconcentration target for trace amount of heavy metal ions using a combination of chelation by
dibenzyldithiocarbamate (DBDTC) ions with subsequent condensation into dibenzylidene-D-sorbitol
(DBS) gels, and the deposit target of soluble major constituents.
These targets were irradiated for 5 to 10 minutes in a vacuum chamber by 3 MeV protons (0.7-10 nA
beam currents, 4 mm beam diameter). X-rays from targets were measured with two Si(Li) detectors;
No.1 detector (0.012 mm thick Be window) with a low geometric efficiency is well suited for the
detection of elements of low atomic number Z ≤ 20, and No.2 detector (0.025 mm thick Be window) with
a 0.5-2 mm Mylar absorber and high geometric efficiency allows the detection of X-rays > 5 keV.[8] A
target containing Fe3+ and Cu2+ of a known amount (40 ppb in a 25 ml solution) was prepared by a
DBDTC-DBS preconcentration technique, and used as an external standard for normalization of PIXE
spectra for the filtration targets. For PIXE-spectrum analysis, we used a least-squares fitting computer
code based on the pattern analysis method which has been developed in our laboratory. [9,10] The lower
detection limit was obtained based on the 3σ statistical error of the background counts.
3. RESULTS AND DISCUSSION
Ferric hydroxide colloids possess amphoteric ion-exchange property with the isoelectric point in the
vicinity of pH 8.[11,12] Then, the pH and the pore size of the filter were changed, and the collection rate
of iron and arsenic ions was examined. Figure 1 shows the mean value and the standard deviation of the
recoveries of Fe and As by the filter of 0.2 µm pores for three samples prepared under identical
condition. In order to confirm separation of AsO43- from constituents of river water, the test solutions
were prepared to contain 5 ppm (µg/ml) SO42- and 10-ppm K+ as major components, as well as 50 ppb
Pb2+ as an element interfering at the PIXE analysis of arsenic. The recovery of Fe3+ added to 1 ppm is
1.00±0.07 in a wide pH region, resulting in good quantitative collection of AsO43- at 5.5< pH <8.
However, the recovery of arsenic in low concentrations like 10 ppb decreases on the acidic side of pH<5,
due to the adsorption of anions coexisting in 100-500 times higher concentrations. The recovery also
decreases on the alkaline side of pH>8, since the anionic adsorption capacity of ferric hydroxides
appreciably decreases. Lead of low concentration like 50 ppb is appreciably coprecipitated on ferric hydroxides in a wide pH region. However, As and Pb at low concentrations are adsorbed quantitatively by
iron hydroxides in the solution condition of pH 6-7, and the PIXE spectrum analysis program based on
the pattern analysis method can be used to quantify these elements within an error margin of ±10 %.
In order to evaluate the reliability of the quantitative PIXE analysis for dissolved arsenate ions, a
calibration curve was measured using targets, which were prepared at pH 6.0 by adding 1 ppm Fe3+ to a
25 ml of arsenate solution in concentrations ranging from 1 to 40 ppb, as shown in Fig.2. A linear
relationship is observed between the initial concentrations of arsenic added to the solutions and the concentrations converted from the PIXE analysis values of arsenic scavenged on the filter. An error margin
of 6% or less was obtained. The detection limit in the present PIXE measurement setup with 0.7-1.2 µC
irradiation of 3 MeV protons is around 0.3 ppb for a 25 ml sample, where the yields of characteristic X1.2
50
1
40
/ppb
0.4
0.2
20
10
0
-0.2
3
30
Exp
0.6
[As]
Recovery
0.8
4
5
6
pH
7
8
9
10
Fig.1 Elemental recovery in ferric hydroxide precipitation method.
PIXE analysis of 2µC irradiation, Initial Conc./ppm; As ○:0.01,
□:0.05,◇:0.1; ▽: Fe 1.0; ◆:Pb 0.05; △:K 10; ●:SO42- 5.
0
0
10
20
[As]
Ini
30
/ppb
40
50
Fig.2 Experimental results([As]Exp) vs. nominal concentration
([As]Ini) for calibration measurements of dissolved arsenic at
pH 6. PIXE analysis of 0.7-1.2 µC irradiation
Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical
Applications , Portorož, Slovenia, June 4-8, 2004
911.3
ray peak is close to the 3σ statistical error of the background counts. On the other hand, the detection
limit of PIXE for a deposit target was found to be 4 ppb As in a 25 ml sample by irradiation of 6 µC
protons. Since the detection limit of flame-less atomic absorption spectrometry generating AsH3, which
is used for the routine analysis of arsenic in environmental water samples, is around 0.8 ppb, a sensitive
monitoring of arsenic in aqueous environment can be carried out by PIXE analysis for the ferric
hydroxide coprecipitation target of arsenic. Also, the PIXE target preparation that uses the oxidation
reaction of As with permanganate ions is suitable for all As species in the environment, including As
taken up into organic materials, and hence a low valence state can be targeted.
Figure 3 shows the elemental concentrations in both insoluble and soluble fractions of river water and
hot-spring water. The As concentration was determined by PIXE using ferric hydroxide coprecipitation
and the other elemental concentrations were analyzed with the three-step method of preparing PIXE
targets. The As concentration in the river rises gradually by influx from three spas, and the concentration
is almost constant downstream of the spas. Though As is contained in the soluble fractions of the spa
samples, the concentration falls below 100 ppb, legally restricted as the discharge limit of arsenic to the
aqueous environment. It is found that the As concentration is quite low at the branch where no hot
springs exist upstream. The highly sensitive PIXE analysis of As scavenged by ferric hydroxide
precipitation can clarify the source of arsenic at ultra low concentrations. In addition, a chemical background concerning the distribution of arsenic in river water was clarified by the PIXE analysis for the
three kinds of targets. That is, concentrations of Al, Si and Fe in the insoluble fraction of this river water
are three to ten times lower compared with other rivers in our city. [8] Because the redox potential of
river water exposed sufficiently to the atmosphere is considered not to be different from the potential
around 280 mV of fresh water, arsenic is thought to be oxidized to arsenate, the pentavalent state, in the
soluble fraction of the river water containing small amounts of soil minerals adsorbing arsenate ions.
Minor Components
Mg
Al
Si
P
S
Cl
K
Ca
Fe
Sr
1
0.1
0.01
P1 S1 P2
Upstream
S2 P3 P4
P5 P6 P7 35 km
Downstream
Concentration / ppm
Concentration / ppm
Major Components
10
0.1
0.01
Ti
Mn
Cu
Zn
As
Pb
0.001
35 km
P1 S1 P2 S2 P3 P4 P5 P6 P7
Upstream
Downstream
Fig.3 Elemental concentrations in river water and hot spring water samples. PIXE analysis of 1-3 µC irradiation,
Open marks: Soluble components, Solid marks: Insoluble components, P: Sampling points along the Natori river
(P4 is the sampling point in the branch in the river), S: Hot spring spas are located upstreams.
REFERENCES
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8. Yamazaki, H., Ishii, K., Takahashi, Y., et al., Int. J. PIXE,, 11, 79-92 (2001).
9. Murozono, K., Ishii, K., Yamazaki, H., et al., Nucl. Instrum. Meth. B150, 76-82 (1999).
10. Ishii, K., and Morita, S., Nucl. Instrum. Meth. B3, 57-61 (1984).
11. Clearfield, A., ed., “Inorganic Ion Exchange Materials,” Florida, CRC Press, Inc., 1982, pp.161-196.
12. Anderson, M. A., and Rubin, A. J., “Adsorption of Inorganics at Solid-Liquid Interface,” Michigan, Ann Arbor
Science, 1981, pp.183-218.
Proceedings of the 10th International Conference on Particle Induced X-ray Emission and its Analytical
Applications , Portorož, Slovenia, June 4-8, 2004