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Detection tube method for trace level arsenic
Journal of Environmental Chemical Engineering 3 (2015) 40–45
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Detection tube method for trace level arsenic Yoshiaki Kiso a , Satoshi Asaoka b, * , Yuki Kamimoto c, Seiya Tanimoto a , Kuriko Yokota a a
Department of Environmental and Life Sciences, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8586, Japan Research Center for Inland Seas, Kobe University, 5-1-1 Fukaeminamimachi, Higashinada, Kobe 658-0022, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2014 Accepted 18 November 2014
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 1 by visual determination. A small column packed with the poly(vinylchloride) particles coated with a quaternary ammonium salt was used as a detection tube. Molybdoarsenic heteropoly acid (molybdenum blue) was derived from arsenate under modified 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 sodium dichloroisocyanuric acid. The color band length in the detection tube was correlated linearly with the arsenic (As(III) + As(V)) concentration in the range of 0.01–0.1 mg As L 1, and the relative standard deviations in the concentration range were around 1%. Arsenic was successfully detected at 0.01 mg As L 1 using this detection tube. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Arsenic Spot test Detection tube Molybdenum blue WHO guide line
Introduction Arsenic pollution in potable drinking water has been observed around the world, and many people have been afflicted 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 1. 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 1, 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
* Corresponding author. Tel.: +81 78 431 6357; fax: +81 78 431 6357. E-mail address: [email protected] (S. Asaoka). http://dx.doi.org/10.1016/j.jece.2014.11.017 2213-3437/ ã 2014 Elsevier Ltd. All rights reserved.
(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 effluents 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 formation between 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 development 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 modification 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 1 using a detection tube method in order to enable visual determination with an inexpensive analytical kit. The color
Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40–45
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 modified. The reaction conditions controlling the disturbance with co-existing components and the procedure of As(III) detection were also examined. Experimental
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coated 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 C 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.
Reagents and solutions Procedure of detection tube method Standard solutions: arsenate and arsenite standard solutions were prepared with Na2HAsO47H2O and with As2O3, respectively, and the concentration range was 0.002–1.0 mg As L 1. Reagent A: 3.6 g of ammoniummolybdeate ((NH4)6Mo7O244H2O), 0.0822 g of potassium antimonyl tartrate (K(SbO)C4H4O61/2H2O), and 22 mL of H2SO4 were dissolved in 100 mL of distilled-deionized water, where the purchased reagents (reagent grade) were used without further purification. 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 1 was also prepared. Reagent D: the dichloroisocyanuric acid (DCI) solution of 0.75 g L 1 was prepared with C3Cl2N3NaO3. 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 (NH4)6Mo7O244H2O, K(SbO)C4H4O61/ 2H2O and H2SO4 contained 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 1), and absorbance at 840 nm was monitored for 60 min with a UV–vis spectrophotometer (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 C 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 1) and oxidation of arsenite was conducted for 10 min after shaking. The reaction temperature was controlled in the range from room temperature to 80 C. 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 C 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 1). 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 was
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 1). The mixture was allowed to react at 60 C 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 1) 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 using the same procedure as described above. Results and discussion Modification 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 1 arsenate 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 influence 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 (NH4)6Mo7O244H2O, 0.0274 g of K(SbO) C4H4O61/2H2O, and 22 mL of H2SO4 in 100 mL of distilleddeionized water. When this reagent was used, the final concentration of each chemical was equal to that described in US Standard method [14]. The concentrations of (NH4)6Mo7O244H2O and K (SbO)C4H4O61/2H2O 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 1 solution, the absorbance reached stable value rapidly by using the Reagents X2 and X3. In the case of 0.1 mg As L 1 solution, the reaction rate decreased obviously.
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Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40–45
Table 1 Modification on the color development reagent. Reagent
Table 2 Modification of content of H2SO4.
Amount of reagent in 100 mL
Reagent
Amount of reagent in 100 mL
Reagent Y1 Reagent Y2 Reagent Y3 Reagent Y4
3.6 3.6 3.6 3.6
(NH4)6Mo7O244H2O (g) K(SbO)C4H4O61/2H2O (g) H2SO4 (mL) Reagent Reagent Reagent Reagent
X1 X2 X3 X4
1.2 2.4 3.6 4.8
0.0274 0.0548 0.0822 0.1096
22 22 22 22
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 H2SO4 concentration in the reaction mixture, reagents containing different H2SO4 concentration were used. As shown in Table 2, the reagents were applied for the solution containing both 0.1 mg As L 1 of As(V) and 10 mg Si L 1 of silica. The effect of H2SO4 concentration 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 influenced 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.
Fig. 1. Time profile of absorbance of molybdenum blue at 840 nm. (A) As(V) concentration: 1.0 mg As L 1; (B) As(V) concentration: 0.1 mg As L 1.
(NH4)6Mo7O244H2O (g) K(SbO)C4H4O61/2H2O (g) H2SO4 (mL) 0.0822 0.0822 0.0822 0.0822
22 30 35 40
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 1, although the As level cannot directly be detected by UV–vis spectrophotometer. The reaction temperature increased in the range of 50–80 C 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 C 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 C, 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. 2. Effect of H2SO4 concentration on absorbance of molybdenum blue interfered with silica. (A) As(V) concentration: 0.1 mg As L 1; (B) As(V) concentration: 0.1 mg As L 1 + 10 mg Si L 1.
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Fig. 3. Effect of reaction temperature on the color development of molybdenum blue.
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 1 of 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 C), and higher temperature and longer reaction time did not influence it at all. Furthermore, Reagent C was used as a reducing agent for the residual oxidant in the oxidation process but may be unnecessary for 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 influenced 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 1 of As(V) standard solution: the color development reagent: Reagent X1, the reaction time:
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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 quantification 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 influenced 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 1 of As(V) can be successfully detected, the average values for 0.01 mg As L 1 and for 0.005 mg As L 1 were not recognized as significantly different from each other on the basis of Student’s t-test (5% of confidence level) due to a large standard deviation. On the other hand, it is concluded that the average CBL for 0.01 mg As L 1 was significantly different from that for 0.03 mg As L 1. From the results, it was difficult to evaluate accurately at 0.01 mg As L 1 of arsenate using this method, but it was possible to evaluate levels lower than 0.03 mg As L 1.
Fig. 4. Relationship between sample volume and color band length formed in the columns. Conditions: standard solution of 0.1 mg As L 1; color development with Reagent X1; BCDMA content of the adsorbent: 0.5%.
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Fig. 6. Recommended procedure for the arsenic detection tube method.
Fig. 5. Calibration curves of arsenic obtained using the proposed detection tube method.
In the case of As(III) + As(V) standard solutions, the average CBL for 0.01 mg As L 1 was significantly different from that for 0.005 mg As L 1, although the difference between the average of CBL for 0.01 mg As L 1 and that for 0.008 mg As L 1 was not significant. The results suggested that the total arsenic concentration of the sample showing lower than 21 mm of CBL is lower than 0.01 mg As L 1. Finally, the procedure of recommended method is shown in Fig. 6. The maximum detection level for arsenic can be tuned by
Table 3 Color band lengths (mm) for 5 times repeated measurement. Concentration (mg As L
0.10 0.08 0.05 0.03 0.01 0.008 0.005 SD: standard deviation.
1
)
As(V)
As(III) + As(V) [1:1]
Average
SD
Average
SD
36.9 36.3 33.5 28.9 24.1 23.8 22.6
1.2 0.6 0.7 1.0 1.8 1.1 1.0
34.4 33.2 27.7 25.7 23.3 23.0 20.7
1.1 0.8 0.9 0.6 0.7 0.8 2.3
controlling 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]. In the 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 C for 30 min were prepared in advance, the effect of phosphate on CBL may be reduced, and arsenic concentration can be quantified. 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 in a 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 1, 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 1, 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 1 [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 development 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.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Detection tube method for trace level arsenic Yoshiaki Kiso a , Satoshi Asaoka b, * , Yuki Kamimoto c, Seiya Tanimoto a , Kuriko Yokota a a
Department of Environmental and Life Sciences, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8586, Japan Research Center for Inland Seas, Kobe University, 5-1-1 Fukaeminamimachi, Higashinada, Kobe 658-0022, Japan c EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2014 Accepted 18 November 2014
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 1 by visual determination. A small column packed with the poly(vinylchloride) particles coated with a quaternary ammonium salt was used as a detection tube. Molybdoarsenic heteropoly acid (molybdenum blue) was derived from arsenate under modified 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 sodium dichloroisocyanuric acid. The color band length in the detection tube was correlated linearly with the arsenic (As(III) + As(V)) concentration in the range of 0.01–0.1 mg As L 1, and the relative standard deviations in the concentration range were around 1%. Arsenic was successfully detected at 0.01 mg As L 1 using this detection tube. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Arsenic Spot test Detection tube Molybdenum blue WHO guide line
Introduction Arsenic pollution in potable drinking water has been observed around the world, and many people have been afflicted 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 1. 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 1, 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
* Corresponding author. Tel.: +81 78 431 6357; fax: +81 78 431 6357. E-mail address: [email protected] (S. Asaoka). http://dx.doi.org/10.1016/j.jece.2014.11.017 2213-3437/ ã 2014 Elsevier Ltd. All rights reserved.
(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 effluents 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 formation between 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 development 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 modification 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 1 using a detection tube method in order to enable visual determination with an inexpensive analytical kit. The color
Y. Kiso et al. / Journal of Environmental Chemical Engineering 3 (2015) 40–45
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 modified. The reaction conditions controlling the disturbance with co-existing components and the procedure of As(III) detection were also examined. Experimental
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coated 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 C 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.
Reagents and solutions Procedure of detection tube method Standard solutions: arsenate and arsenite standard solutions were prepared with Na2HAsO47H2O and with As2O3, respectively, and the concentration range was 0.002–1.0 mg As L 1. Reagent A: 3.6 g of ammoniummolybdeate ((NH4)6Mo7O244H2O), 0.0822 g of potassium antimonyl tartrate (K(SbO)C4H4O61/2H2O), and 22 mL of H2SO4 were dissolved in 100 mL of distilled-deionized water, where the purchased reagents (reagent grade) were used without further purification. 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 1 was also prepared. Reagent D: the dichloroisocyanuric acid (DCI) solution of 0.75 g L 1 was prepared with C3Cl2N3NaO3. 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 (NH4)6Mo7O244H2O, K(SbO)C4H4O61/ 2H2O and H2SO4 contained 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 1), and absorbance at 840 nm was monitored for 60 min with a UV–vis spectrophotometer (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 C 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 1) and oxidation of arsenite was conducted for 10 min after shaking. The reaction temperature was controlled in the range from room temperature to 80 C. 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 C 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 1). 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 was
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 1). The mixture was allowed to react at 60 C 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 1) 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 using the same procedure as described above. Results and discussion Modification 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 1 arsenate 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 influence 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 (NH4)6Mo7O244H2O, 0.0274 g of K(SbO) C4H4O61/2H2O, and 22 mL of H2SO4 in 100 mL of distilleddeionized water. When this reagent was used, the final concentration of each chemical was equal to that described in US Standard method [14]. The concentrations of (NH4)6Mo7O244H2O and K (SbO)C4H4O61/2H2O 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 1 solution, the absorbance reached stable value rapidly by using the Reagents X2 and X3. In the case of 0.1 mg As L 1 solution, the reaction rate decreased obviously.
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Table 1 Modification on the color development reagent. Reagent
Table 2 Modification of content of H2SO4.
Amount of reagent in 100 mL
Reagent
Amount of reagent in 100 mL
Reagent Y1 Reagent Y2 Reagent Y3 Reagent Y4
3.6 3.6 3.6 3.6
(NH4)6Mo7O244H2O (g) K(SbO)C4H4O61/2H2O (g) H2SO4 (mL) Reagent Reagent Reagent Reagent
X1 X2 X3 X4
1.2 2.4 3.6 4.8
0.0274 0.0548 0.0822 0.1096
22 22 22 22
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 H2SO4 concentration in the reaction mixture, reagents containing different H2SO4 concentration were used. As shown in Table 2, the reagents were applied for the solution containing both 0.1 mg As L 1 of As(V) and 10 mg Si L 1 of silica. The effect of H2SO4 concentration 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 influenced 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.
Fig. 1. Time profile of absorbance of molybdenum blue at 840 nm. (A) As(V) concentration: 1.0 mg As L 1; (B) As(V) concentration: 0.1 mg As L 1.
(NH4)6Mo7O244H2O (g) K(SbO)C4H4O61/2H2O (g) H2SO4 (mL) 0.0822 0.0822 0.0822 0.0822
22 30 35 40
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 1, although the As level cannot directly be detected by UV–vis spectrophotometer. The reaction temperature increased in the range of 50–80 C 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 C 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 C, 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. 2. Effect of H2SO4 concentration on absorbance of molybdenum blue interfered with silica. (A) As(V) concentration: 0.1 mg As L 1; (B) As(V) concentration: 0.1 mg As L 1 + 10 mg Si L 1.
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Fig. 3. Effect of reaction temperature on the color development of molybdenum blue.
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 1 of 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 C), and higher temperature and longer reaction time did not influence it at all. Furthermore, Reagent C was used as a reducing agent for the residual oxidant in the oxidation process but may be unnecessary for 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 influenced 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 1 of As(V) standard solution: the color development reagent: Reagent X1, the reaction time:
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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 quantification 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 influenced 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 1 of As(V) can be successfully detected, the average values for 0.01 mg As L 1 and for 0.005 mg As L 1 were not recognized as significantly different from each other on the basis of Student’s t-test (5% of confidence level) due to a large standard deviation. On the other hand, it is concluded that the average CBL for 0.01 mg As L 1 was significantly different from that for 0.03 mg As L 1. From the results, it was difficult to evaluate accurately at 0.01 mg As L 1 of arsenate using this method, but it was possible to evaluate levels lower than 0.03 mg As L 1.
Fig. 4. Relationship between sample volume and color band length formed in the columns. Conditions: standard solution of 0.1 mg As L 1; color development with Reagent X1; BCDMA content of the adsorbent: 0.5%.
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Fig. 6. Recommended procedure for the arsenic detection tube method.
Fig. 5. Calibration curves of arsenic obtained using the proposed detection tube method.
In the case of As(III) + As(V) standard solutions, the average CBL for 0.01 mg As L 1 was significantly different from that for 0.005 mg As L 1, although the difference between the average of CBL for 0.01 mg As L 1 and that for 0.008 mg As L 1 was not significant. The results suggested that the total arsenic concentration of the sample showing lower than 21 mm of CBL is lower than 0.01 mg As L 1. Finally, the procedure of recommended method is shown in Fig. 6. The maximum detection level for arsenic can be tuned by
Table 3 Color band lengths (mm) for 5 times repeated measurement. Concentration (mg As L
0.10 0.08 0.05 0.03 0.01 0.008 0.005 SD: standard deviation.
1
)
As(V)
As(III) + As(V) [1:1]
Average
SD
Average
SD
36.9 36.3 33.5 28.9 24.1 23.8 22.6
1.2 0.6 0.7 1.0 1.8 1.1 1.0
34.4 33.2 27.7 25.7 23.3 23.0 20.7
1.1 0.8 0.9 0.6 0.7 0.8 2.3
controlling 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]. In the 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 C for 30 min were prepared in advance, the effect of phosphate on CBL may be reduced, and arsenic concentration can be quantified. 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 in a 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 1, 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 1, 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 1 [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 development 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.
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