Principal Research Results

Development of Simplified Determination Methodfor Selenium in Aqueous Solution
Background
The Japanese national effluent standard for selenium (Se) has been set at 0.1 mg/L since June 1994. In coal-fired power
plants, Se is detected in flue gas desulfurization (FGD) wastewater because a trace amount of Se is contained in coal. The concentration of Se in wastewater largely varies depending on the coals employed. Thus, there is a high need for process monitoring forcontrolling Se concentration in wastewater.

Objectives
This study aims to propose a simplified determination method for Se that can be applied to the process monitoring, and tooptimize various operating parameters for accurate Se determination. In addition, it aims to demonstrate its applicability to FGDwastewater.

Principal Results
1. Development of simplified Se determination method using a selenium hydride gas sensor
A simple and rapid determination method for Se (IV)＊ 1 in aqueous solutions was proposed. The method employs anamperometric gas sensor, which is used as a gas leak detector for selenium hydride (H2Se) in the working environment. The analytical procedure is as follows: an HCl solution is added to a sample containing Se (IV), and the subsequent addition of a NaBH4 solution
generates H2Se, which is measured by the sensor. With a simple experimental apparatus (Fig. 1), we confirmed that in principle, theproposed method can determination Se (IV) in aqueous solution.
2. Optimization of operating parameters
To improve the analytical accuracy of this method, we built up a new experimental system that offers precise control onvarious operating parameters. A highly linear calibration curve was obtained for Se (IV) standard solution by optimizing the operating
parameters (Fig. 2). The optimized parameters were carrier gas flow rate, NaBH4 flow rate, concentration of NaBH4, and concentration of HCl. The stability of the signal at low concentrations region was improved in this system (Table 1). This is because resolutionof the signal profile was improved by optimizing the operating parameters and using high sensitivity H2Se gas sensor (Fig. 3).
3. Application to FGD wastewater
The proposed method was applied to FGD wastewater in coal-fired power plants. The Se (IV) concentrations were notdetermined due to two interferences, one of which was inhibition of H2Se generation caused by organic compounds in the samples.
The other is a large negative interference caused by I- contained in the wastewaters. These two interferences were removed by thesample pretreatment, which was thermal decomposition in a mixed solution of H2SO4 and HNO3 (Fig. 4). The measured values with
our method were in good agreement with those obtained with ICP-AES when the FGD wastewater samples are taken after pretreatment. This pretreatment is effective for the removal of interferences, but takes 4-5 hours for completion. Therefore, the pretreatment
method should be largely shortened and simplified.

Future Developments
To build up a Se process monitor, a rapid pretreatment and a simple reduction method of Se (VI) to Se (IV) will be established.
Main Researcher: Hiroyuki Masaki, Ph.D.,
Research Scientist, Environmental Chemistry Sector, Environmental Science Research Laboratory
Reference
H. Masaki, et al, 2007, “Development of simplified monitor for selenium in aqueous solution (Part 1) - Review and applicabilityevaluation of simple determination method -”, CRIEPI Report V06014 (in Japanese).
H. Masaki, et al, 2008, “Development of simplified monitor for selenium in aqueous solution (Part 2) - Optimization of the analysis
parameters and applicability to desulfurization wastewaters -”, CRIEPI Report V07006 (in Japanese).

＊1：Se mainly exists in wastewater as oxyanion of Se (IV) and Se (VI). Se (VI) is generally reducted to Se (IV) for the determination.

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2. Environment

Reductant(NaBH4)

H2Se sensor

Gas-cleaning withCuSO4 solution
Se (IV) solution
HCl (6 mol/L)H2Se wasgenerated

Fig.1 The simple apparatus forSemeasurement.

Fig.2 A calibration curve for Se (IV) standardsolution.

A NaBH 4 solution is added to a mixed solution of Se (IV)and HCl solution, which generates H 2Se. The generated
H2Se gas is measured with an amperometric gas sensor.

A highly linear calibration curve was obtained in the proposed methodby optimizing the operating parameters; the carrier gas flow rateof 500mL/min, the NaBH4 flow rate of 30 mL/min, the NaBH4 concentrationof 3.0 g/L, and the HCl concentration of 6.0 mol/L. A simple andrapid determination method from aqueous Se (IV) with a detection
limit of 0.002 mg/L and a quantitative limit of 0.005 mg/L

Table 1 Stability of H2Se signal
Concentraton of Se[mg/L]

Relative standard deviation [%]Simple apparatus(no parameter control)

Fundamental experiment set up(parameters controlled)

0.01

14.0

4.6

0.05

5.6

4.6

0.1

3.9

3.2

0.5

3.9

1.5

A relative standard deviation of the signal intensity was used as an index forthe stability of the signal. At low concentrations region,the signal
intensity was stabilized by optimizing operating parameters on a H2Se gas
sensor with a high resolution.

Fig.3 Comparison of signal profiles of H2Se
for 0.1 mg/L Se solution.
The resolution of the signal profile was improvedby
optimizing operating parameters and using high sensitivity H2Se
gas sensor

Fig.4 Se (IV) concentration in FGD wastewater after pretreatment of thermal decomposition.
The measured values with our method were in good agreement with those obtained with ICP-AES when the samples were thermally
pretreated. Since the thermal pretreatment takes 4-5 hours, it should be largely shortened and simplified.

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