Determination,of,total,sulfur,in,geothermal,water,by,inductively,coupled,plasma-atomic,emission,spectrometry

Bing-bing Liu,Mei Han,Jia Liu,Na Jia,Chen-ling Zhang,Lin Zhang

Institute of Hydrogeology and Environmental Gelogy, Chinese Academy of Geological Science; Key Laboratory of Groundwater Science and Engineering, Ministry of Natural Resources, Zhengding 050803, Hebei, China.

Abstract: Sulfur speciation and concentration in geothermal water are of great significance for the research and utilization of the water resources.In most situations,it is necessary to determine the total sulfur in geothermal water.In this study,the method was established for the determination of determining total sulfur content — the inductively coupled plasma-atomic emission spectrometry (ICP-AES),with the wavelength of 182.034 nm selected in spectral line of sulfur.It was identified that the optimal working conditions of the ICP-AES instrument were 1 200 W for high frequency generator power 9 mm for vertical observation height,0.30 MPa atomizer pressure,and 50 r/min analytical pump speed.The matrix interference of the method was eliminated by the matrix matching method.Using this method,sulfur detection limit and minimum quantitative detection limit were 0.028 mg/L and 0.110 mg/L,respectively,whilst the linear range was 0.0-100.0 mg/L.The recovery rate of sample was between 90.67% and 108.7%,and the relative standard deviation (RSD) was between 0.36% and 2.14%.The method was used to analyze the actual samples and the results were basically consistent with the industry standard method.With high analysis efficiency,the method has low detection limit and minimum quantitative detection limit,wide linear range,good precision and accuracy,and provides an important detection method for the determination of total sulfur in geothermal water.

Keywords: High frequency generator power;Vertical observation height;Atomizer pressure;Matrix matching method;Standard addition method

Sulfur plays a key role on the development and evolution of life and the formation of the earth’s(Mitchell,2021;Hammerli et al.2021).As one of important components in geothermal water,the sulfurous species mainly includes sulfide and sulfate minerals,which comes from seawater,atmosphere,degassing of rocks and magma,etc (Robin et al.2020;Banks et al.2021).The total sulfur in geothermal water is closely related to the control of the emission of hydrogen sulfide in the environment (Robin et al.2020;Banks et al.2021;Liu et al.2017).Therefore,the rapid and accurate determination of total sulfur content in geothermal water is of great significance for the in-depth research of geothermal water.

The determination methods of total sulfur mainly include flame-atomic absorption spectrometry (FAAS) (Ozbek and Akman,2016;Ozbek and Baysal,2015;Kapitány et al.2020),graphite furnace-atomic absorption spectrometry (GF-AAS)(Marrocos et al.2020),inductively coupled plasmaatomic emission spectrometry (ICP-AES) (Li et al.2017;Ahmadi et al.2017;Hu et al.2017;Hu et al.2018;Wang and Shi,2014;Guo et al.2012;Wang and Wang,2020;Zhang et al.2017),inductively coupled plasma-mass spectrometry (ICP-MS) (Schurr et al.2020;Wang et al.2016;Xu et al.2016),ion chromatography (IC) (Wang et al.2021).As the F-AAS,GF-AAS and ICP-MS have narrow linear ranges and many interference factors,they cannot be used for practical work.The time duration of the IC method is long with a low working efficiency.

In comparison with the other methods,the ICPAES has more advantages in wide linear range,high sensitivity,strong anti-interference ability,simple operation,and etc.It is an ideal method for the determination of total sulfur in geothermal water.In this experiment,the total sulfur in geothermal water was hence analysed by using the ICPAES method.First,the analytical spectral lines of elemental sulfur were selected,and then the working conditions of the spectrometer were optimized,including high frequency generator power,vertical observation height,atomizer pressure and analytical pump speed.At the same time,the matrix matching and standard addition methods were comparatively considered,in order to eliminate the interference factors..As a result,the matrix matching method was selected due to its high efficiency and wide application range.Using this method to analyse actual samples,the results were basically consistent with industry standard method-ion chromatography.ICP spectral interference factors are controllable,and the analysis accuracy and precision are high;the spectrometer is easy to operate and the samples do not need pretreatment with high sample analysis efficiency.The linear range and detection limit of the method are better than other methods.The ICP spectrometer has a high compatibility with different samples and is suitable for the determination of geothermal water samples with high salinity and high total sulfur content.This method greatly improves the detection efficiency and detection accuracy of total sulfur in geothermal water,and provides an efficient and suitable analytical method for the determination of total sulfur in geothermal water.

1.1 Instrument and reagent

The instrument used in this experiment was the Thermo Fisher Scientific ICAP 6300,a inductively coupled plasma-atomic emission spectrometer model with its working conditions shown in Table 1.Relevant solutions and reagents,including standard reserve solution of sulfur (1 000 μg/mL),standard reserve solution of potassium,sodium,calcium and magnesium (1 000 μg/mL),standard reserve solution of iron,strontium and barium (100 μg/mL),were all from China Institute of Metrology.The water used for this experiment was ultrapure water and conformed to water for analytical laboratory use specification and test methods(GB/T 6682-2008).

Table 1 Working conditions of instrument

1.2 Experimental method

Due to the high salinity of geothermal water,it is easy to block the atomizer and affect the accuracy of analytical results when the total sulfur is determined by ICP spectrometry.Therefore,the samples with high salinity need to be diluted before testing.

2.1 Determination of analytical spectral lines of sulfur

There are three analytical spectral lines for sulfur in the spectral line library of ICP spectrometer,of which the wavelengths are 182.624 nm,180.731 nm,182.034 nm,respectively.The relative intensity and average background intensity of the three spectral lines are shown in Fig.1.Which shows that the sensitivity is in the wavelength order of 180.731 nm＞182.034 nm＞182.624 nm.Calcium with a wavelength of 183.801 nm have a strong spectral interference to sulfur with a wavelength of 180.731 nm.The concentration of calcium in geothermal water is high,and the interference spectrum peak is very strong.Therefore,the wavelength of 182.034 nm with a slightly low sensitivity was selected in this experiment as analytical spectral line of sulfur with complete waveform,small background and good linear correlation.

Fig.1 Relative intensity and average background intensity of 3 spectral lines of sulfur

2.2 Optimization of instrument working conditions

2.2.1 High frequency generator power

A 10.0 mg/L sulfur standard solution and a blanksolution were respectively detected in the condition of the high frequency generator power ranging from 1 000 W to 1 300 W,with the other working conditions remaining unchanged.The results shown in Fig.2 indicate that with the increase of the high frequency generator power,the intensity of both the standard and blank solutions increase.The higher the intensity,the higher the sensitivity of the method is.However,considering the high frequency generator power may affect service life of the instrument,the power value of 1 200 W was used in this experiment.

Fig.2 Influence of high frequency generator power on the intensity of sulfur standard solution and blank solution

2.2.2 Vertical observation height

The minimum vertical observation height of this ICP spectrometer is 8 mm.The intensities of both the 10.0 mg/L sulfur standard solution and blank solution were analysed under the condition wth the vertical observation height ranging from 8 mm to 14 mm,whilst the other working factors remain unchanged.The results shown in Fig.3 indicate that the analytical intensities become stable when the vertical observation height falls between 8 mm and 9 mm.After that,as the increase of the vertical observation height causes the gradual decrease in the intensity and sensitivity values of the instrument.Since the stability of the spectrometer for the vertical observation height at 8 mm was low,its height of 9 mm was hence selected in this experiment.

Fig.3 Influence of vertical observation height on the intensity of sulfur standard solution and blank solution

2.2.3 Atomizer pressure and analytical pump speed

The atomizer pressure and the analytical pump speed are two related factors.A two-factor fourlevel orthogonal experiment was designed for selecting the appropriate atomizer pressure and analytical pump speed in this experiment,as shown in Table 2.The spectral intensity ratio (I1/I0) of sulfur standard solution and blank solution under different combinations of atomizer pressure and pump speed was investigated.The result is shown in Fig.4,where the intensity ratios of A10 and A16 are higher than the others.The greater the intensity ratio,the better the atomization effect is.However,larger atomizer pressure and analytical pump speed could affect the service life of the atomizer and pump tube.In order to achieve the best working condition of the atomizer,the atomizer pressure and pump speed were 0.30 MPa and 50 r/min,respectively,in this experiment.

Fig.4 Influence of atomizer pressure and analytical pump speed on the spectral intensity ratio (I1/I0) of sulfur standard solution and blank solution

Table 2 Selection of different atomizer pressure and analytical pump speed

2.3 Elimination of interference factors

In the ICP analysis,interference factors mainly include spectral interference and matrix interference.Spectral interference refers to spectral line overlap interference and background interference.Most of the spectral interference could be eliminated by selecting the appropriate analytical spectral line and the synchronous background correction function of the spectrometer.There is serious matrix interference when using ICP spectrometry to determine total sulfur in geothermal water with high salinity and complex matrix composition.In this experiment,the standard addition method and the matrix matching method were used to eliminate matrix interference.

2.3.1 Matrix matching method

The matrix matching method is to maintain the same salt content of standard solution as that of sample solution.Through researching a large number of geothermal water quality test reports,it was found that most of the geothermal water contained potassium,sodium,calcium,magnesium,iron,strontium and barium.According to their average concentration levels,solutions of potassium (100 mg/L),sodium (500 mg/L),calcium (500 mg/L),magnesium (500 mg/L),iron (10 mg/L),strontium(10 mg/L),barium (10 mg/L) were added in sulfur standard series.The concentrations of sulfur standard series were 0.0 mg/L,1.0 mg/L,10.0 mg/L,20.0 mg/L,50.0 mg/L and 100.0 mg/L,respectively.The standard series solutions were determined under the best working conditions of the spectrometer,and the standard curve was drawn with the intensity on ordinate and the concentration on abscissa.The linear regression equation,correlation coefficient,linear range and the concentration of sample were listed in Table 3.

2.3.2 Standard addition method

The standard addition method is to add a certain quantity of standard solution of known concentration to the samples tested,and to determine the concentrations of the sample before and after the addition.The geothermal water sample was firstly divided into 6 portions,with a volume of 50 mL for each part.Then the 200 mg/ L sulfur standard solutions of 0 mL,0.5 mL,5.0 mL,10.0 mL,25.0 mL,50.0 mL were respectively added in the partitioned samples,which were diluted to 100 mL volumetric flask.The sulfur standard solution was detected in the ICP spectrometer.The resulting concentrations of the standard solutions were used as the abscissa and the intensity used as the ordinate to draw the linear regression equation.The total sulfur in the tested samples was analyzed and the relative deviations of the results from the two methods were listed in Table 3.

Table 3 Determination results of total sulfur in geothermal water samples by matrix matching method and standard addition method (n=6)

It can be seen from Table 3 that the results of the methods are basically the same,which further suggests that both quantitative methods can be used for the determination of total sulfur in geothermal water.However,the standard addition method requires adding standard solution to each sample,which requires a lot of work and is suitable for a small number of samples.The matrix matching method is suitable for continuous batch sample detection.The matrix matching method was therefore chosen in this experiment.

2.4 Method technical indicators

2.4.1 Detection limit and minimum quantitative detection limit

The method detection limit and the minimum quantitative detection limit were calculated according to Technical guideline for the development of environmental monitoring analytical method standards (HJ 168-2020).In this case,detection limit of the selected method is defines as 3.143 times of the standard deviation,and the minimum quantitative detection limit is defined as 4 times of the detection limit,which are 0.028 mg/L and 0.110 mg/L,respectively.In addition,the comparison of the limits between the methods of ICP Spectroscopy and ion chromatography are listed in Table 4.

2.4.2 Precision and accuracy of the method

The concentrations of total sulfur in five samples of geothermal water were determined under the optimum conditions of the instrument and the recovery rate.The results are shown in Table 5,where the recovery rate=(measured value of the added standard sample -measured value of the sample)/the amount of added standard × 100%.The recovery is between 90.67% and 108.7% and the relative standard deviation (RSD) is between 0.36% and 2.14%,which indicates that the method had a good precision,and can achieve the demand of the analysis of total sulfur in geothermal water.

2.5 Geothermal water sample analysis

Geothermal water samples from 6 different regions were selected,and the total sulfur in the samples was detected by using the ICP Spectroscopy method.The relative standard deviation (RSD) was calculated by 6 parallel determinations.At the same time,the samples were pretreated to convert all sulfur into sulfate,and then the industry standard method (DZ/T0064.51-2021),i.e.the ion chromatography,was used to analysed the sulfate concentration.The detected results were then converted back to total sulfur contents.The comparison results were shown in Table 6.The results of the two methods for the determination of total sulfur in geothermal water are basically consistent.The relative standard deviation is between 0.30% and 3.73%,and the relative deviation is ≤ 3.73%,which further verified the reliability and stability of the method.

Table 4 Method detection limit,lower limit of determination and linear range compared with DZ/T 0064.51-2021

Table 5 Method precision and accuracy test (n=6)

Table 6 Determination of total sulfur in geothermal water samples from different regions (n=6)

After determining analytical spectral lines of sulfur,optimizing the working conditions of the instrument and eliminating the interference factors,the detection limit,minimum quantitative detection limit and linear range of the method are all better than the recommended values in the method for analysis of groundwater quality in Chinese geological and mineral industry standards (DZ/T 0064.51-2021).The precision of the method can fulfill the requirements of analysis and testing with low detection limit,wide linear range and high sensitivity.It realizes the continuous determination of total sulfur for large quantities of geothermal water samples and provide theoretical basis and data support for further research and development of geothermal water.

Acknowledgements

This study was supported by the Fundamental Research Fund Project (SK201908) of the Institute of Hydrogeology and Environmental Geology,Chinese Academy of Geological Sciences.