Detection tube method for trace level arsenic [623657]

Detection tube method for trace level arsenic
Yoshiaki Kisoa, Satoshi Asaokab,*, Yuki Kamimotoc, Seiya Tanimotoa, Kuriko Yokotaa
aDepartment of Environmental and Life Sciences, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8586, Japan
bResearch Center for Inland Seas, Kobe University, 5-1-1 Fukaeminamimachi, Higashinada, Kobe 658-0022, Japan
cEcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
ARTICLE INFO
Article history:
Received 1 September 2014
Accepted 18 November 2014Available online 3 December 2014
Keywords:
ArsenicSpot testDetection tubeMolybdenum blueWHO guide lineABSTRACT
Arsenic pollution of surface and ground waters has been reported in many developing countries, and it is
therefore an important task to detect arsenic rapidly using a simple and inexpensive tool. This work
focused on the detection of arsenic at 0.01 mg As L/C01by visual determination. A small column packed
with the poly(vinylchloride) particles coated with a quaternary ammonium salt was used as a detectiontube. Molybdoarsenic heteropoly acid (molybdenum blue) was derived from arsenate under modi fied
reaction conditions. The molybdenum blue solution (20 mL) was introduced into the detection tube by
suction with a syringe to form color band. As(III) was measured after oxidation with sodiumdichloroisocyanuric acid. The color band length in the detection tube was correlated linearly with thearsenic (As(III) + As(V)) concentration in the range of 0.01 –0.1 mg As L
/C01, and the relative standard
deviations in the concentration range were around 1%. Arsenic was successfully detected at
0.01 mg As L/C01using this detection tube.
ã2014 Elsevier Ltd. All rights reserved.
Introduction
Arsenic pollution in potable drinking water has been observed
around the world, and many people have been af flicted with
serious arseniasis [1–6]. It is therefore an urgent subject for the
people in these regions to employ arsenic removal technologies to
decontaminate drinking water supplies and to monitor arsenic
pollution levels of the surface and groundwater. Common arsenic
removal technologies include precipitation and adsorption with a
metal oxide such as iron oxide [7–11]. Hydrotalcite compounds are
also useful adsorbents for As(V) [12,13] . In these methods, As(III) is
removed after oxidation to As(V), and hence, it is an important
consideration in the monitoring of the As(V) concentration in
treated water.
The World Health Organization (WHO) has recommended an
acceptable arsenic level for drinking water at 0.01 mg As L/C01. As(V)
concentration can be detected by the molybdenum blue (MB)
method, which is similar to the phosphate detection method.
However, the MB method cannot directly detect As(V) at 0.01 mg
As L/C01, and preconcentration of arsenic is necessary because of
extremely low absorbance of MB at this concentration level
[14–17]. Therefore, atomic absorption spectrometry (AA) or an
inductively coupled plasma atomic emission spectrometry(ICP-AES) is commonly used for detecting low level arsenic. These
analytical methods are not convenient for rapid monitoring. A
rapid and simple detection method is required for the following
cases: on-site checking of risk level of source water for drinking use
and of ef fluents from small-scale arsenic removal plants. Even for
field research, a rapid monitoring tool may be useful.
Some kinds of spot test kits for detecting arsenic are available
including a test strip type and a gas-detection tube type [18,19]
based on the generation of arsine. These spot tests enable visual
determination, but the accuracy is usually low because the
detected concentration increases exponentially with the increase
of color intensity or color band length.
In our previous works [20 –24], detection tube methods were
developed for the monitoring of phosphate, nitrite/nitrate or
ammonium. The color band was formed by ion-pair formationbetween anionic colored compound developed with analyte and
quaternary ammonium salt coated on polyvinyl chloride (PVC)
particles. The methods needed two steps, i.e., the color develop-
ment and the color band formation, but higher accuracy was
obtained because of the linear relationship between the analyte
concentration and the color band length in the detection tube. In
addition, it was indicated that the detection range can be easily
controlled by modi fication of the preparation condition of the
adsorbent and/or the measurement conditions [24].
In this study, we focused on the visual detection of As(V) at
0.01 mg As L
/C01using a detection tube method in order to enable
visual determination with an inexpensive analytical kit. The color* Corresponding author. Tel.: +81 78 431 6357; fax: +81 78 431 6357.
E-mail address: s-asaoka@maritime.kobe-u.ac.jp (S. Asaoka).
http://dx.doi.org/10.1016/j.jece.2014.11.017
2213-3437/ ã2014 Elsevier Ltd. All rights reserved.Journal of Environmental Chemical Engineering 3 (2015) 40 –45
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering
journal homepage: www.elsevier.com/locate/jece

development was caused by molybdenum blue (MB) method
because of the anionic character of molybdoarsenic heteropoly
acid. However, the reaction rate of molybdoarsenic heteropoly acid
formation is slower that that of phosphate, and therefore the
conditions of the color development reaction were modi fied. The
reaction conditions controlling the disturbance with co-existing
components and the procedure of As(III) detection were also
examined.
ExperimentalReagents and solutions
Standard solutions: arsenate and arsenite standard solutions
were prepared with Na
2HAsO 4/C17H2O and with As 2O3, respectively,
and the concentration range was 0.002 –1.0 mg As L/C01.
Reagent A: 3.6 g of ammoniummolybdeate
((NH 4)6Mo 7O24/C14H2O), 0.0822 g of potassium antimonyl tartrate
(K(SbO)C 4H4O6/C11/2H 2O), and 22 mL of H 2SO4were dissolved in
100 mL of distilled-deionized water, where the purchased reagents
(reagent grade) were used without further puri fication.
Reagent B: the ascorbic reagent was prepared with the
following method: ascorbic acid (2 g) and NaCl (8 g) were mixed
and ground.
Reagent C: the ascorbic solution of 0.7832 g L/C01was also
prepared.
Reagent D: the dichloroisocyanuric acid (DCI) solution of
0.75 g L/C01was prepared with C 3Cl2N3NaO 3.
All reagents except DCI were obtained from NACALAI TESQUE
(Kyoto, Japan), while DCI was from Tokyo Chemical Industry
(Tokyo, Japan).
Color development conditions
The concentrations of (NH 4)6Mo 7O24/C14H2O, K(SbO)C 4H4O6/C11/
2H2O and H 2SO4contained in the Reagent A varied in the range
of 1.2 –4.8 g/100 mL, 0.0274 –0.1096 g/100 mL and 22 –40 mL/
100 mL, respectively. One milliliter of the above reagent
solution and 0.1 g of the Reagent B were added into the As
(V) standard solutions (1.0 and 0.1 mg As L/C01), and absorbance at
840 nm was monitored for 60 min with a UV –vis spectropho-
tometer (V-530, Jasco Co., Tokyo, Japan) equipped with a 5 cm
cell, where the temperature of the optical chamber was
controlled by a water circulation unit. The reaction temperature
was controlled at 50, 60, 70, and 80/C14C by circulation of water
from a water bath.
Reaction conditions for arsenite oxidations
The Reagent D of 0.4 mL was added into 20 mL of the arsenite
standard solution (1 mg As L/C01) and oxidation of arsenite was
conducted for 10 min after shaking. The reaction temperature was
controlled in the range from room temperature to 80/C14C. Then,
0.4 mL of Reagent C was added into the reaction mixture to degrade
the residual oxidant. One milliliter of the Reagent A and 0.1 g of the
Reagent B were added into the reaction mixture and heated at 60/C14C
for 30 min to develop molybdenum blue. The absorbance at
880 nm of the final solution was measured by a UV –vis
spectrophotometer (V530, JASCO, Tokyo, Japan). This procedure
was also applied to the arsenate standard solution (1 mg As L/C01).
Preparation of packing materials and detection tubes
The adsorbent packed in the detection tube was prepared
following the method described in our previous works [21,24] . PVC
(particle size: 0.1 mm) was used as a support material and wascoated with a mixture of benzylcetyldimethylammonium chloride
(BCDMA) and biphenyl by the following procedure: (1) same
weight of BCDMA and biphenyl were dissolved in methanol, (2)
PVC was added to the solution, and (3) the methanol was
evaporated with a rotary evaporator and dried in an oven at 60/C14C
for 5 h. The contents of BCDMA and biphenyl were 3.0% each. All
chemicals were obtained from NACALAI TESQUE (Kyoto, Japan).
The detection tube was made by packing the adsorbent (0.130 g)
into poly(propylene) columns (i.d.: 3 mm; length: 60 mm; bed
height: 54 mm; a plastic drinking straw), and two pieces of
melamine foam were used as stopper of the column.
Procedure of detection tube method
As(V) solution: both the Reagent A (1 mL) and the Reagent B
(0.1 g) were added into 20 mL of the As(V) standard solution
(0.005 –0.1 mg As L/C01). The mixture was allowed to react at 60/C14C for
30 min followed by cooling in the water bath (10 min). Two
milliliters of the colored solution were introduced into the
detection tube by suction, where a disposable syringe was
connected to the detection tube with a silicon tube. A syringe
stopper made of stainless steel was used to stabilize the solution
volume introduced into the column. The length of the color band
formed in the column was measured with a ruler. Because the front
of the color band fluctuated a little, maximum and minimum
lengths of the color band were measured, and the average value of
both lengths was used as the color band length (CBL).
As(III) solution: the standard solutions (0.005 –0.1 mg As L/C01)
containing both As(III) and As(V) (50/50) were prepared. The 20 mL
solution was oxidized by adding 0.4 mL of the Reagent D for 10 min
at room temperature. The residual DCI was decomposed by adding
0.4 mL of the Reagent C. The Reagent A (1 mL) and the Reagent B
(0.1 g) were added into the mixed solution, and the color
development and color band formation were documented usingthe same procedure as described above.
Results and discussionModi fication of the color development reagent
When molybdenum blue (MB) method was employed for the
detection of arsenate, the following two concerns were pointed
out: the color development reaction was very slow especially in the
case of low arsenate concentration. When the reagent solution
mentioned in the US Standard method [14] was employed for
0.1 mg As L
/C01arsenate solution, stable absorbance of molybdenum
blue was obtained after 120 min. Another concern was the
instability of the ascorbic acid solution used commonly in the
MB method. The solution cannot be stored beyond 1 week.
In order to address the latter, Reagent B was used in this work
because solid ascorbic acid is stable and NaCl used as a builder did
not in fluence color intensity of molybdenum blue. Acceleration of
the color development was examined by increasing the reagent
concentration and raising the reaction temperature.
The basic reagent solution (Reagent X1) was prepared by
dissolving 1.2 g of (NH 4)6Mo 7O24/C14H2O, 0.0274 g of K(SbO)
C4H4O6/C11/2H 2O, and 22 mL of H 2SO4in 100 mL of distilled-
deionized water. When this reagent was used, the final concentra-
tion of each chemical was equal to that described in US Standard
method [14]. The concentrations of (NH 4)6Mo 7O24/C14H2O and K
(SbO)C 4H4O6/C11/2H 2O increased as shown in Table 1 .
The absorbance of the reaction mixture at 840 nm was
monitored for 60 min and the results are shown in Fig. 1 . In the
case of 1.0 mg As L/C01solution, the absorbance reached stable value
rapidly by using the Reagents X2 and X3. In the case of
0.1 mg As L/C01solution, the reaction rate decreased obviously.Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40 –45 41

However, it is imperative to use the Reagent X2 or X3 in order to
accelerate the reaction. Considering the results shown in Fig. 1 (B),
Reagent X3 gave a stable absorbance after 20 min, and therefore,
was the reagent of choice in the following study.
The detection of low level phosphate or arsenate was also
disrupted with silica. Because the effect of silica on the detection is
influenced by H 2SO4concentration in the reaction mixture,
reagents containing different H 2SO4concentration were used. As
shown in Table 2 , the reagents were applied for the solution
containing both 0.1 mg As L/C01of As(V) and 10 mg Si L/C01of silica. The
effect of H 2SO4concentration on the absorbance is expressed in
Fig. 2 . In the case of Y2, the maximum absorbance of the solution
containing arsenate alone increased, but silica exerted a positive
effect on the absorbance. When Reagent Y1 was used, the
maximum absorbance was not in fluenced by silica and was
achieved in ca. 30 min. Considering the results shown in Figs. 1 and
2, Reagent Y1 was the most suitable reagent and therefore used in
the following experiments as Reagent A.Reaction temperature
Reagent A accelerated the color development reaction, but a
more rapid reaction may be necessary for the detection of
0.01 mg As L/C01, although the As level cannot directly be detected
by UV –vis spectrophotometer. The reaction temperature in-
creased in the range of 50 –80/C14C for acceleration of the reaction.
The temperature of the cell unit of the spectrophotometer was
controlled at a certain level, and the absorbance was monitored
for 60 min. The results are shown in Fig. 3 . Stable absorbance was
obtained in the range of 50 –60/C14C of the reaction temperature.
Higher temperature gave rather unstable absorbance, and this
may be caused by evaporation of the water from the reaction
mixture. The recommended reaction temperature was at
50–60/C14C, but higher temperature may be acceptable when an
airtight bottle is used. In the case of field monitoring, a portable
burner can be used to provide a water bath.
[(Fig._1)TD$FIG]
Fig. 1. Time pro file of absorbance of molybdenum blue at 840 nm. (A) As(V)
concentration: 1.0 mg As L/C01; (B) As(V) concentration: 0.1 mg As L/C01.[(Fig._2)TD$FIG]
Fig. 2. Effect of H 2SO4concentration on absorbance of molybdenum blue interfered
with silica. (A) As(V) concentration: 0.1 mg As L/C01; (B) As(V) concentration:
0.1 mg As L/C01+1 0m gS iL/C01.Table 2
Modi fication of content of H 2SO4.
Reagent Amount of reagent in 100 mL
(NH 4)6Mo 7O24/C14H2O (g) K(SbO)C 4H4O6/C11/2H 2O (g) H 2SO4(mL)
Reagent Y1 3.6 0.0822 22
Reagent Y2 3.6 0.0822 30Reagent Y3 3.6 0.0822 35Reagent Y4 3.6 0.0822 40Table 1Modi fication on the color development reagent.
Reagent Amount of reagent in 100 mL
(NH
4)6Mo 7O24/C14H2O (g) K(SbO)C 4H4O6/C11/2H 2O (g) H 2SO4(mL)
Reagent X1 1.2 0.0274 22
Reagent X2 2.4 0.0548 22Reagent X3 3.6 0.0822 22Reagent X4 4.8 0.1096 2242 Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40 –45

Reaction conditions for arsenite oxidations
The additive amount of DCI was ten times of the equivalent
amount in order to oxidize 1 mg As L/C01of As(III), and 0.4 mL of
Reagent D was added into 20 mL of the arsenite standard solution.
The residual DIC after the oxidation was decomposed by Reagent C,
where the applied amount of Reagent C was equivalent to that of
added DCI. The effects of the reaction time and the reaction
temperature were examined. The reaction was conducted
completely for 10 min at room temperature (22/C14C), and higher
temperature and longer reaction time did not in fluence it at all.
Furthermore, Reagent C was used as a reducing agent for the
residual oxidant in the oxidation process but may be unnecessaryfor the following reason: the amount of ascorbic acid in 0.4 mL
Reagent C is only 1.5% of 0.1 g of Reagent B.
Color band formation in the detection tube
The detection tube method developed in our previous work is
based on ion-pair formation between anionic colored compound
and cationic BCDMA coated on the PVC particle [20 –22]. When the
colored compound is completely entrapped with the packing
material, the color band length increased proportionally with
amount of the colored compound, i.e., with concentration of the
analyte. The color band length may be in fluenced by several
parameters as follows: the content of BCDMA in the packing
material, diameter of the detection tube, ionic valence of the
colored compound and the amount of the colored compound
(concentration of the analyte and volume of the solution
introduced into the tube).
While very low arsenic level detection is a goal of this method, it
is necessary that a large volume of the reaction mixture
(molybdoarsenic heteropoly acid solution) is required to be
introduced into the slim detection tube. In addition, higher
content of BCDMA may be suitable for elucidating a clear front of
the color band, although it may give a shorter color band.
Considering that a few milliliters of the reaction mixture is applied
into the detection tube, a short column is necessary based on the
result of a preliminary examination: when a long detection tube
(100 mm long) was used, the application of 2 mL reaction mixture
into the column required more than 10 min due to high hydraulic
resistance.
The relationship between the introduced volume and CBL
was examined in preliminary experiments conducted under the
following conditions: 0.1 mg As L
/C01o fA s ( V )s t a n d a r ds o l u t i o n :
the color development reagent: Reagent X1, the reaction time:120 min, column: 3 mm diameter and 40 mm long, and BCDMA
content of the adsorbent: 0.5%. The amount of the packing
material was very stable; the average amount was 0.129 g and
the relative standard deviation was 1.70%. The experiments
were repeated 5 times for each sample volume. The color band
was formed clearly in the column, although the color developed
with Reagent X1 was light. The CBL was correlated linearly with
the sample volume as shown in Fig. 4 . It can be pointed out that
a very stable color band length was obtained for each sample
volume, and this may be caused by using a stopper for a
syringe. The results also indicated that the detection range can
be tuned by the sample volume.
The optimum conditions proposed in this work were employed
for the two types of standard solutions to form color bands in the
detection tube: As(V) standard solutions and As(III) + As(V)
standard solutions. In the case of the latter standard solutions,
color development was conducted after the oxidation process. The
overall procedure was repeated 5 times for each standard solution.
The calibration curves are shown in Fig. 5 . In the case of As(V)
standard solutions, the calibration curve was not linear, and similar
results were obtained for the color band using the phosphate
detection tube method [20]. This may be caused by the following
properties of molybdoarsenic heteropoly acid: a large molecular
weight (low diffusivity), tri-valence of the ion, multilayer
adsorption and self-coagulation property at high concentration.
Even when a calibration curve is not linear, a stable calibration
curve with the low standard deviation is useful for accurate
quanti fication of As(V). In the case of the As(III) + As(V) standard
solutions, a linear calibration curve was obtained, where it was
pointed out that the results might be in fluenced by the oxidation
process.
The average value and the standard deviation of CBL are
summarized in Table 3 . Although the results for As(V) suggested
that 0.01 mg As L/C01of As(V) can be successfully detected, the
average values for 0.01 mg As L/C01and for 0.005 mg As L/C01were not
recognized as signi ficantly different from each other on the basis of
Student ’st-test (5% of con fidence level) due to a large standard
deviation. On the other hand, it is concluded that the average CBL
for 0.01 mg As L/C01was signi ficantly different from that for
0.03 mg As L/C01. From the results, it was dif ficult to evaluate
accurately at 0.01 mg As L/C01of arsenate using this method, but it
was possible to evaluate levels lower than 0.03 mg As L/C01.[(Fig._3)TD$FIG]
Fig. 3. Effect of reaction temperature on the color development of molybdenum blue.
[(Fig._4)TD$FIG]
Fig. 4. Relationship between sample volume and color band length formed in the
columns. Conditions: standard solution of 0.1 mg As L/C01; color development with
Reagent X1; BCDMA content of the adsorbent: 0.5%.Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40 –45 43

In the case of As(III) + As(V) standard solutions, the average CBL
for 0.01 mg As L/C01was signi ficantly different from that for
0.005 mg As L/C01, although the difference between the average of
CBL for 0.01 mg As L/C01and that for 0.008 mg As L/C01was not
significant. The results suggested that the total arsenic concentra-
tion of the sample showing lower than 21 mm of CBL is lower than0.01 mg As L
/C01.
Finally, the procedure of recommended method is shown in
Fig. 6 . The maximum detection level for arsenic can be tuned bycontrolling of the volume of the colored solution applied into
the detection tube: when higher level arsenic is targeted,
smaller volume of the colored solution must be introduced into
a detection tube. In addition, considering that Reagent D is also
unstable during long time storage, it may be better that solid
DCI is added into a sample solution. A similar procedure was
used in the detection tube method for ammonium [23].I nt h e
case of using solid DCI, the mixture of DCI (0.1 g) and NaCl
(30 g) is prepared and 0.1 g of the mixture is added into 20 mL
of a sample solution, where the mixture should be well ground
and mixed.
This method must be interfered by phosphate, because similar
molybdenum blue is developed. The color development from
phosphate can be achieved with Reagent X1 within 10 min at room
temperature, but arsenic is not colorized under the same
conditions. When two calibration curves for phosphate at room
temperature for 10 min and at 60/C14C for 30 min were prepared in
advance, the effect of phosphate on CBL may be reduced, and
arsenic concentration can be quanti fied. In addition, it can be
emphasized that both the detection tube (column and packing
material) and the color development reagents can be prepared
easily and at low cost. When the detection tube kits are prepared ina laboratory, the kits may be a helpful tool for the monitoring of
arsenic pollution in a wide area.
Conclusions
In this work, an alternative detection tube method for arsenic
was developed. The proposed method enabled detection of arsenic,
As(III) and As(V), at 0.01 mg As L
/C01, although the reaction needed
moderate heating and 30 min for stable reaction to occur. The color
band length was correlated almost linearly with the arsenic
concentration in the range of 0.01 –0.1 mg L/C01, and low deviations
of color band length were obtained. It was indicated that low level
arsenic could be detected compared to common spot test kits
(LOQ; 0.2 mg L/C01[25]). In addition, considering that the detection
range is controlled by the sample volume introduced into the
detection tube, a wider range of arsenic concentration may be
detected by tuning the application volume of the colored sample
into the detection tube. The test kits (reagents and column) for this
method can be prepared easily at low cost, because the reagent and
materials may be commonly purchased anywhere. The develop-
ment of a convenient tool for heating the reaction is under
development, but common tools used for recreational outdoor
activities may be useful for this purpose.Table 3
Color band lengths (mm) for 5 times repeated measurement.
Concentration (mg As L/C01) As(V) As(III) + As(V) [1:1]
Average SD Average SD
0.10 36.9 1.2 34.4 1.1
0.08 36.3 0.6 33.2 0.80.05 33.5 0.7 27.7 0.90.03 28.9 1.0 25.7 0.60.01 24.1 1.8 23.3 0.70.008 23.8 1.1 23.0 0.8
0.005 22.6 1.0 20.7 2.3
SD: standard deviation.[(Fig._5)TD$FIG]
Fig. 5. Calibration curves of arsenic obtained using the proposed detection tube
method.[(Fig._6)TD$FIG]
Fig. 6. Recommended procedure for the arsenic detection tube method.44 Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40 –45

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