Cloud Point Extraction of Iron and Its Detection Using Flame Atomic

Chapter 9

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 29, 2016 | http://pubs.acs.org Publication Date (Web): December 1, 2015 | doi: 10.1021/bk-2015-1210.ch009

Cloud Point Extraction of Iron and Its Detection Using Flame Atomic Absorption Spectrometry Alexa Rihana-Abdallah,*,1 Zhe Li,1 and Katherine C. Lanigan2 1Department

of Civil and Environmental Engineering, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Michigan 48221-3038 2Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Michigan 48221-3038 *E-mail: [email protected].

The concentration of iron in drinking water is a significant standard for water quality evaluation and water pipeline corrosion detection. This research aims to develop a simple, low cost, and accurate method based on cloud point extraction (CPE) for iron separation, preconcentration, and determination by flame atomic absorption spectrometry (FAAS) when the metal concentration in the sample is below the limit of detection (LOD). The LOD value, determined based on significant sensitivity change and break in the slope of the standard curve, was 0.5 ppm. By utilizing both 2,4-diamino-4-phenyl-1,3,5-triazine (DPT) and 3-amino-7-dimethylamino-2-methylphenazine (Neutral Red, NR) as chelating agents, and using Triton X-114 as surfactant, iron can be effectively preconcentrated in water samples. The preconcentration procedure was optimized by varying the experimental factors temperature, equilibrium time, pH, and the concentration of the chelating agents and surfactant. Using the optimized experimental conditions, the procedure allows the determination of iron concentration with a detection limit of 0.1 ppm and a 98% recovery. The high reproducibility of the results demonstrates that this method can be successfully applied to the analysis of iron in water samples using only FAAS.

© 2015 American Chemical Society Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Introduction While drinking water must meet United States Environmental Protection Agency (USEPA) safety standards upon leaving water treatment plants, these treatment facilities often face difficulties in maintaining and delivering quality water to consumers. The challenge mostly stems from problems with the distribution system. Aging pipelines corrode with time, causing iron levels in water to increase, and microbial biofilms can build up on corroded, exposed pipes and contaminate the water. These problems compromise the quality of water as it travels through distribution systems. The majority of water distribution system pipes are made of concrete material or of iron derivatives such as cast iron, ductile iron, and steel (1). Of these pipe materials, cast iron and ductile iron are the most prone to corrosion and breakage (1). Pipe corrosion leads to pipe breakage, water loss, and potential water contamination (2). Not only does this corrosion damage pipelines, but it also promotes the buildup of biofilm on pipes and leads to the release of soluble or particulate iron corrosion byproducts to the water. The release of these byproducts increases the iron concentration of the water and may slightly discolor the water making it aesthetically unappealing (1, 2). Unlike lead and copper, iron in drinking water will rarely cause a direct health threat to humans; however, because of the impurities and bacteria accumulation at the inner surface of corroded water pipes, the water becomes unsafe to drink. Additionally, corrosion increases the thickness of deposit sediments in the pipes, thus reducing its cross-sectional area and warranting the use of high pressure water pumps which results in a higher cost of electric energy (3, 4). For all these reasons, the detection and prevention of water-pipe corrosion is extremely important in drinking water transportation. Generally, iron concentration in drinking water is the most typical index of water-pipe corrosion level, and USEPA has established secondary drinking water standards of 0.3 ppm for iron concentration in drinking water (5). Flame atomic absorption spectrometry (FAAS) has a limited detection range compared to inductively coupled plasma-mass spectrometry (ICP-MS) or graphite furnace atomic absorption spectrometry (GFAAS) (6, 7). In fact, the detection limit for iron on the FAAS instrument that was utilized in this research was determined based on a break in the slope of the standard curve to be 0.5 ppm, which is much higher than USEPA required standards. For iron concentrations lower than 0.5 ppm, the absorption was very low and unstable. Since accurate determination of iron concentration in water is critical, and direct measurements are hindered by the poor sensitivity of the FAAS instrument, preconcentration procedures become necessary as the first step in order to enrich the trace amounts of the analytes to a level where instrumental analysis can provide reliable measurements (8). Many research studies on preconcentration of iron in environmental samples can be found in the literature (9–12). Citak and Tuzen (12) introduced a new method of cloud point extraction (CPE) of many heavy metal ions including copper, lead, cadmium, and iron using the chelating agent, 2,4-diamino-4-phenyl-1,3,5-triazine (DPT) (Figure 1A), and a nonionic surfactant, Triton X-114. The solubility and activity of nonionic surfactants depend on the hydrophilic ether linkages in the polyoxyethylene chain (13). Triton 184 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


X-114 is soluble at room temperature; however, as the temperature increases, its solubility decreases and the surfactant becomes less soluble. The “cloud point” is the temperature above which the solution becomes turbid and two phases are formed (14, 15). Dissolved inorganic salts exhibit similar effect on the surfactant solubility in water as does the increase in temperature. Because dissolved salts have greater affinity for water than the ether linkages, they dehydrate the nonionic compound, resulting in the replacement of hydrogen ions with iron ions in the amino groups (14).

Figure 1. (A) DPT structure (B) Neutral Red structure. Sahin and his coworkers developed a new method of preconcentration and determination of iron in spice samples using Triton X-114 and 3-amino-7dimethylamino-2-methylphenazine (Neutral Red, NR) (Figure 1B) by CPE and flow injection FAAS (16). NR is generally utilized in medical applications as a stain and counterstain, and it has a high dependence on pH (17). The recovery of the method described by Sahin was generally above 98%. However, that experimental method uses a flow injection system, which is utilized for sample loading, separation, and elution. Unfortunately, flow injection accessories are not readily available in most chemistry laboratories. Without a flow injection system, centrifugation methods are used; however, recoveries using centrifugation are typically only around 80%. Comparing Citak and Sahin work (12, 16), the similarities of the experimental factors and the features of the DPT and NR methods can be summarized as follow: 1 2 3

4

Both DPT and NR have similar optimal pH ranges for iron preconcentration, which is 7-8; As chelating agents, both DPT and NR are selective for iron; NR has an obvious red color and that makes the distinction between the aqueous phase and the surfactant phase easier. However, without any addition of NR, the surfactant phase is totally transparent and this makes the separation of both phases, surfactant and aqueous, much harder; As mentioned above, the experimental method using DPT as the chelating agent does not require any extra accessory for iron preconcentration; however, the recovery and reproducibility are not sufficiently high. On the contrary, NR is a chelating agent that can effectively bond with iron ions and generate a high recovery, but sample treatment needed flow injection accessories to achieve this high recovery. 185 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

It was hypothesized that both DPT and NR can be used as chelating agents with centrifugation separation in iron preconcentration to remove the requirement of flow injection.

Experimental Methods


Chemicals The chemicals used in this research were purchased from Sigma-Aldrich Chemical Company at the highest purity available and used as received. DPT stock solution was prepared by dissolving 0.2 g of DPT in 30.0 mL methanol (99%) and was diluted to 100 mL by double distilled water at 70 °C. A NR stock solution was prepared by dissolving 0.1 g of NR (≥ 90%) in 100 mL of double distilled water. An iron(III) chloride (1000 ppm, ≥ 99.99%) stock solution was utilized to make the calibration solution and the spike solution. A buffer solution of pH equal to 8.0 was prepared by diluting an appropriate amount of ammonium hydroxide solution (~28-30%) and NH4Cl (99.99%).

Apparatus A PerkinElmer AAnalyst 400 flame atomic absorption spectrophotometer with an air-acetylene flame and auto-sampler was used for the determination of iron concentration. Flow rates for gases were used at optimized levels. The analytical wavelength was 248.3 nm. All other experimental parameters were set at the system-recommended values. An Oakton waterproof pH meter with ±0.1 accuracy was utilized to determine the pH of each solution. A Grant JBA5 basic unstirred water bath was used for incubation. A VWR mini vortex (120 V) was employed for sample mixing.

Experimental Standard and spike solutions of iron(III) were prepared from dilutions of a 1000 ppm FeCl3 stock solution. The final concentrations of calibration standards were 1.0, 2.0, 3.0, 4.0, and 5.0 ppm, and the correlation coefficient was higher than 99.99% as tested by FAAS. The concentrations of spike solutions ranged from 0.0 to 1.0 ppm with an increase of 0.1 ppm. The absorption signals for each spike solution as well as the blank were detected by FAAS. The result showed that 0.5 ppm is the lowest concentration level that can be determined to be statistically different from a blank with a 99% confidence level. A 100-mL sample solution contained 0.01 mL of iron stock solution, 2.0 mL of NR stock solution, 0.2 mL of 99% Triton X-114, and 2.5 mL of DPT. This solution was buffered to 8.0 and incubated at the temperature of 70 °C for 25 min. After a 20 min centrifugation (VWR clinical 200 centrifuge at 3800 × g), the sample solution was exposed to an ice bath for 15 min, and then the aqueous phase was separated from surfactant phase by careful pipetting (see Figures 2 and 3). The 186 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


surfactant phase left at the bottom of the centrifuge tube was dissolved in 10 mL of 1 M nitric acid in methanol. After mixing for 5 min, the sample solution was analyzed by FAAS along with the standard solutions.

Figure 2. Iron preconcentration without the addition of NR. Photo by Dr. Alexa Rihana-Abdallah.

Figure 3. Iron preconcentration with the addition of NR. Photo by Dr. Alexa Rihana-Abdallah. 187 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The preconcentration process yielded a 10-mL sample volume which is adequate for FAAS detection. Moreover, the reduction of sample volume from 100 mL to 10 mL increased the sample concentration from 0.1 ppm to 1 ppm, a concentration that can be accurately determined by FAAS. Since our purpose was the determination by FAAS of USEPA drinking water standards for iron, which is 0.3 ppm, selecting an initial concentration of 0.1 ppm for preconcentration by CPE was deemed logical to prove the efficacy of the method.

Results and Discussion The average iron preconcentration recovery was as high as 98.9% and was calculated as the ratio of the measured sample concentration over the theoretical sample concentration. The experimental factors, including pH, temperature, concentration of chelating agents and surfactants were optimized. The data points in Figures 4 through 8 represent the average of five replicates as well as their standard deviations.

Figure 4. Effect of Temperature on Recovery (bars represent standard deviation of the mean). 188 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Effect of Temperature on Recovery Temperature is a significant experimental factor for iron preconcentration using CPE because the chelating efficiency relies heavily on temperature. High temperature not only increases the solubility of DPT and NR in water, but it also increases the probability of collision between iron ions and these chelating agents. Additionally, high temperature allows surfactant agents to mix more thoroughly with the insoluble conjugates formed by the iron ions and the chelating agents, resulting in a more efficient removal from water sample. As can be illustrated by the results in Figure 4, percent recovery was increased by an increase of temperature. According to the experimental data, 70 °C is the optimal temperature, and thus it was chosen for all additional studies.

Effect of pH on Recovery The pH value is another important factor to iron preconcentration that can affect the activity of chelating agents. In this research, the optimum pH range is narrow because of the high sensitivity of the chelating reaction. As the experimental data show in Figure 5, at a pH value of 8.0, extraction efficiency reached its maximum with a 98% recovery. For all further experiments, a pH of 8.0 was selected.

Figure 5. Effect of pH on Recovery (bars represent standard deviation of the mean). 189 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 6. Effect of Triton X-114 concentration on Recovery (bars represent standard deviation of the mean).

Effect of Concentration of Surfactant Agent and Chelating Agents on Recovery Triton X-114 is a homogeneous solution at 0 °C but exhibits a cloud point at around 20 °C (14, 15). It can mix with the hydrophobic complex of chelating agents and metal ions and can be removed from aqueous solutions based on this characteristic. The effect of Triton X-114 concentration on the CPE recovery was examined. As exhibited by Figure 6, iron recovery increased tremendously with the increase of the surfactant concentration up to 0.2% (v/v), then slightly decreased and plateaued. The optimal concentration of 0.2% (v/v) for Triton X-114 was thus chosen for the remaining experiments. Maximizing the extraction efficiency depends also on optimizing the concentrations of the chelating agents. In this work, DPT is the predominant chelating agent, as it conjugates with iron ions by replacing the hydrogen in amino group and, consequently, forms a hydrophobic substance. Neutral Red, on the other hand, acts a secondary chelating agent. Although added in small concentrations, NR was found to further enhance the extraction procedure. As illustrated in Figure 7, with the increase of NR concentration from 0.5 mL/L to 2 mL/L, the recovery percentage improves from 80 to almost 100 %. The optimal value of NR concentration was 2 mL/L and this value was used subsequently when evaluating the effect of DPT concentration on CPE extraction efficiency. 190 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 7. Effect of Concentration of NR on Recovery (bars represent standard deviation of the mean).

Figure 8. Effect of Concentration of DPT with 2 mL/100 mL (v/v) of NR on Recovery (bars represent standard deviation of the mean). 191 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The effect of DPT concentration of percentage recovery can be seen in Figure 8. At zero concentration of the chelating agent, iron recovery was only 20%. The recovery increased sharply as DPT concentration increases up to 2.5 mL/100 mL, then remained constant. The recovery efficiency reached its maximum of 98.9% when the DPT and NR amounts in the solution were 25 mL/L and 2 mL/L, respectively. In this research, the concentrations of surfactant agents and chelating agents were optimized to reach maximum recovery of iron by CPE. As demonstrated in Figures 6 through 8, the optimal concentrations of Triton X-114, NR, and DPT are 0.2 mL/100 mL, 2 mL/100 mL, and 2.5 mL/100 mL, respectively. Equilibrium time was also investigated and the value of 25 min was found to be optimal for iron extraction efficiency. Table 1 summarizes the optimal values of the experimental factors with concentrations given as w/v or v/v percentage.

Table 1. Optimal Experiment Factors and Their Corresponding Value Optimized Factors

Optimized Value

pH

8.0

Concentration of DPT

0.005% (w/v)

Concentration of NR

0.002% (w/v)

Concentration of Triton X-114

0.2% (v/v)

Equilibrium Temperature

70 °C

Equilibrium Time

25 min

Conclusion With the optimization of the experimental factors, pH, concentrations of chelating agents and surfactant, equilibrium temperature, equilibrium time, iron(III) samples of 0.1 ppm were separated, preconcentrated, and detected by FAAS. The recoveries of this CPE method were maximized at 98.9%. Additionally, the original FAAS detection limit of 0.5 ppm for iron was reduced by 80% to 0.1 ppm when using the preconcentration step. The experimental results indicate that this new method is a simple, rapid, and inexpensive method for iron separation, preconcentration, and determination in aqueous phase samples. Moreover, this method not only can be applied to FAAS detection, but also to other instrumental methods, such as ICP-MS or GFAAS. Furthermore, based on the results of recent research completed by the authors, this method can potentially be employed for the separation, preconcentration, and detection of other trace elements at low concentration such as lead. 192 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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193 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Cloud Point Extraction of Iron and Its Detection Using Flame Atomic

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