Analysis of Thorium (Th) and Uranium (U) in radioactive wastewater sample in accordance with American Public Health Association (APHA) 3125 method

The main issue with thorium (Th) and uranium (U) in radioactive wastewater is the potential of these elements to contaminate the water bodies. The unregulated radioactive wastewater release can affect the ecosystem and potentially harm aquatic life. Therefore, there is a need to establish the analysis of Th and U using standard method to obtain better sensitivity and good confidence in the final report. In this study, the determination of uranium and thorium was employed using the American Public Health Association (APHA) method 3125 standard method. The method performance and method detection level (MDL) samples for APHA 3125 were prepared in the deionized water and digested using hot block (APHA 3030E) and microwave digester (APHA 3030K). The quantification of 238U, 235U and 232Th in the method performance and MDL samples was performed using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). From the results, the method performance samples for both digestion methods were in accordance with the APHA requirement. The MDL obtained from APHA 3030E for 232Th, 238U, and 235U were 5, 7, and 8 μg L−1 respectively. Meanwhile, the MDL obtained from the microwave digester for 232Th, 238U, and 235U, and were 2, 3, and 3 μg L−1 respectively. The APHA 3030K digestion technique gives lower and better detection limit compared to the APHA 3030E technique. Based on APHA 3030K MDL, 232Th was detected in the radioactive wastewater sample while 238U and 235U were less than MDL. Meanwhile, for APHA 3030E, all elements were below MDL. This study will enhance the detection of uranium and thorium in the radioactive wastewater sample towards better environmental monitoring programs at the national and global levels.


Introduction
The radioactive Thorium (Th) and Uranium (U) elements are ubiquitously present in the earth's crust with established important applications [1].Concerns over release of these elements and their possible ecological effect on the environment are increasing due to rapid urbanization and industrialization.Various agencies or committees such as the World Health Organization (WHO) have introduced generic guideline values relating to the maximum levels of these elements that can be released into or be contained in the environment [2].Nowadays, Th and U are fundamental in critical technologies, thus the uses and extraction of these elements are increasing leading to the discharge into the environment with potential risk to humans and biota [3,4].1308 (2024) 012015 IOP Publishing doi:10.1088/1757-899X/1308/1/012015 2 Determination of Th and U in water samples such as wastewater, river water, groundwater and tap water can be challenging due to the low-level detection in the majority of the water samples and potential chemical and physical interferences in the samples.In recent years, due to advanced technology of inductively coupled plasma-mass spectrometer (ICP-MS), Th and U quantification was able to be done in low-level samples with fast, reliable and good precision analysis [5,6].Furthermore, in conjunction with the established standard method such as the American Public Health Association (APHA) method, the Th and U elements quantification can be performed and produce reliable results by following strict method performance criteria and quality control procedures.For Th and U quantification, the APHA 3125 method was the most suitable method for the simultaneous measurement of Th and U in water samples.
Principally in the APHA 3125 method, the water sample is introduced into an argon-based, hightemperature radio frequency plasma by using pneumatic nebulization.The energy from the plasma transfers to the water sample and causes desolvation, atomization, and ionization of elements.Ions generated by these energy-transfer processes are extracted from the plasma by the vacuum interface, and separated based on mass-to-charge ratio by the mass spectrometer [7].The ions passing through the mass spectrometer are counted by an electron multiplier and the resulting information is processed by a computer-based data-handling system.In APHA 3125, water samples are recommended to be digested using the APHA 3030E method (hot-block digestion).However, this method also allows other digestion techniques such as APHA 3030K by using the microwave digester technique.Furthermore, the two digestion methods based on APHA 3030E and APHA 3030K were required to perform the method performance to evaluate the method capability and verification of the obtained result.
Among the method performance criteria recommended in the APHA method, every batch of analysis requires the determination of method blank (MB), laboratory fortified blank (LFB), laboratory fortified matrix (LFM), laboratory fortified matrix duplicate (LFMD) and relative percentage difference (RPD) between LFM and LFMD in every batch sample in order produce a reliable result [8].Furthermore, APHA also requires the determination of method detection level (MDL) in every method to set acceptable levels for reporting limit for each Th and U in real environmental samples.This study aims to determine and compare the method performance and MDL for Th and U in water samples using APHA 3030E and APHA 3030K digestion techniques and quantification by using APHA 3125 method.The findings of this study can be applied in the reporting of Th and U in water samples and can be used to implement further action in environmental management pollution protection improvement.

Materials
In this study, the standard calibration was purchased from Perkin Elmer, USA, and consisted of 10 µg ml -1 of Th and U elements.A single standard of rhodium (Rh) was obtained from the Scharlau brand for internal standard purposes during analysis.Concentrated nitric acid with trace metal grade was purchased from Fisher Chemical.Deionized water (DIW) for washing, soaking and sample preparation was based on the high-quality reagent water specification based on the APHA 1080 method [9].

Preparation of quality control samples
In this study, two metal digestion methods were used to compare the quality control (QC) samples consisting of method performance and MDL samples for Th and U determination in water samples.The first method is the APHA 3030E method (hot block digestion) [10].For method performance of APHA 3030E, two blank, laboratory fortified blank (LFB), unspike, laboratory fortified matrix (LFM) and laboratory fortified matrix duplicate (LFMD) samples were prepared during the analysis.The seven replicates of MDL samples for the APHA 3030E method were also prepared and analyzed together with method performance samples.Briefly, for the APHA 3030E method, 10 ml of quality QC samples were added to the 50 ml tube.The samples were added with 0.5 ml of concentrated nitric acid and placed in the block heater.The temperature of the block heater was adjusted to 105ºC.The tube caps for all the IOP Publishing doi:10.1088/1757-899X/1308/1/0120153 samples were placed on top of the tube without screwing.The samples were digested for a minimum of 2 hours and more concentrated nitric acid was added until observation of clear solution.Upon complete digestion, the sample volume was adjusted and recorded.The details on the spiking regime for QC samples for method performance and MDL determination based on APHA 3030E were tabulated in Table 1.The second method for MDL determination is APHA 3030K (microwave digester) [11].For method performance of APHA 3030K, two blank, laboratory fortified blank (LFB), unspike, laboratory fortified matrix (LFM) and laboratory fortified matrix duplicate (LFMD) samples were prepared during the analysis.The seven replicates of MDL samples for the APHA 3030E method were also prepared and analyzed together with method performance samples.Briefly 45 ml of quality control samples were added into microwave digester teflon vessels.Then the samples were added with 5 ml of concentrated nitric acid.The vessels were capped, tightened with vessel holders and placed in microwave digester turntable.The digestion procedure was programmed in the microwave digester system.The samples were heated to 160 ± 4 for 25 minutes for the first stage of digestion.For the second stage, the samples were digested with a slow rise to 170 ± 4 in 10 minutes.The vessels were allowed to cool and the solution was transferred into a clean polypropylene tube.The final sample volume was adjusted and recorded.The details on the spiking regime for QC samples for method performance and MDL determination based on APHA 3030K were tabulated in Table 2.
The determination of Th and U in the QC samples was carried out using Inductively Coupled Plasma Mass Spectrometer (ICP-MS) Nexion 350X, Perkin Elmer (USA) at mass 232 for Th ( 232 Th), 238 and 235 for U ( 238 U and 235 U) [7].Briefly, the water samples are introduced to argon-based, high-temperature radio-frequency plasma via pneumatic nebulization.As energy is transferred from the plasma to the sample stream, the target elements are dissolved, atomized, and ionized.The resulting ions are extracted from the plasma through a differential vacuum interface and separated based on their mass-to-charge (m/z) ratio via mass spectrometry.The collision cell technology with helium gas is used to improve sensitivity.For 232 Th, 238 U and 235 U quantification, the standard gas mode was set in the ICP-MS software configuration.In the ICP-MS, an electron multiplier detector is used to count the separated ions, and a computer-based data-management system is used to process the resulting information.The internal standard calibration was used for the quantification of 232 Th, 238 U and 235 U elements.Two calibration points of calibration blank and 200 µg l -1 were established with correlation of coefficient more than 0.995 [7].The calibration verification standard with concentration at 100 µg l -1 was analyzed for every ten samples and the values ranged between 90 and 110%.

Determination of method performance criteria
In the method performance criteria, blank, LFB, unspike, LFM and LFMD samples were analyzed to check the accuracy of the digestion procedure and ICP-MS measurement.The RPD was determined based on LFM and LFMD concentration to determine the difference between the two duplicate sample results.The calculation of LFB, LFM, LFMD and RPD recovery were as follows: (3)

Determination of method detection level (MDL)
The MDL was prepared according to Table 1 and Table 2 for APHA 3030E and APHA 3030K respectively.The MDL was prepared and analyzed at least seven replicates of this solution over 3 days to ensure that the MDL determination is more representative of routine measurement.The standard deviation (SD) of the seven replicates was calculated and MDL was determined by using the SD and student t value by using the formula below:

𝑁𝑁
(5) IOP Publishing doi:10.1088/1757-899X/1308/1/0120155 where x is the value of data distribution, μ is the population mean, and N is the total number of observations.

Determination of Th and U in radioactive wastewater sample
The radioactive wastewater sample was collected from the Low-Level Effluent Treatment Plant (LLETP) facility in the Waste Technology Centre (WASTEC), Malaysian Nuclear Agency, Malaysia.The radioactive wastewater sample was collected in the pre-cleaned polypropylene tube and preserved with nitric acid: DIW (1:1) solution until pH fell below 2. The samples were stored in the refrigerator at 4 ± 2ºC in the laboratory prior to analysis.For the determination of 232 Th, 238 U and 235 U, the radioactive wastewater sample was tested by spiking mixture of the standard solution with the final concentration of 25 µg l -1 .The radioactive wastewater sample was digested using APHA 3030E and APHA 3030K methods as described previously in the determination of the MDL part.The level of 232 Th, 238 U and 235 U in the digested radioactive wastewater sample was analyzed using ICP-MS.The digestion procedure for both methods was evaluated by spike recovery of the standard Th and U solution into radioactive wastewater sample with the formula as below.

Method performance of Th and U in water sample
In this study, the method performance was studied to verify the established APHA method in our laboratory.This project involved the determination of 232 Th, 238 U and 235 U in water using two digestion techniques (APHA 3030E and APHA 3030K) and quantitation was done using ICP-MS (APHA 3125).

Method detection level of Th and U in water sample
Figure 1 shows the MDL for 232 Th, 238 U, and 235 U in water samples digested according to APHA 3030E and APHA 3030K methods.The MDL obtained from APHA 3030E for 232 Th, 238 U, and 235 U were 5, 7, and 8 µg L -1 respectively.Meanwhile, the MDL obtained from the microwave digester for 232 Th, 238 U, and 235 U, and were 2, 3, and 3 µg L -1 respectively.Based on the MDL result, the MDL of APHA 3030K for Th and U elements were lower and better than the MDL of APHA 3030E method.The microwave digestion technique was able to achieve better stability and sensitivity at lower concentrations than the hot block digestion technique.

Level of Th and U in radioactive wastewater sample
The capability of the APHA 3030E and APHA 3030K digestion methods was tested using real radioactive wastewater sample.Based on Figure 2, the percentage recovery of the spike 232 Th, 238 U, and 235 U in the radioactive wastewater sample was in the range of 70% and 130% for both APHA 3030E and APHA 3030K methods [8].Both methods show good performance criteria within the APHA requirement for spiking percentage recovery.Furthermore, the percentage recovery value of 232 Th, 238 U, and 235 U in APHA 3030K was closer to a good level compared to the APHA 3030E method.Therefore, the digestion of 232 Th, 238 U, and 235 U in radioactive wastewater sample was better using APHA 3030K than the APHA 3030E method.
Figure2.Percentage recovery of spike radioactive wastewater sample based on APHA 3030E and APHA 3030K methods.
The determination of the reporting concentration of 232 Th, 238 U, and 235 U in the radioactive wastewater sample must be compared using the MDL of 232 Th, 238 U, and 235 U in water samples with respective digestion methods.From Figure 3(a), the concentration of 232 Th, 238 U, and 235 U in the radioactive wastewater sample based on the APHA 3030E method was lower than the MDL of the APHA 3030E method.Therefore, based on the APHA 3030E method, 232 Th, 238 U, and 235 U were reported less than their respective MDL.In the meantime.In Figure 3(b), 232 Th was detected above the MDL value for the APHA 3030K method.The concentration of 232 Th in the sample was reported at 3.6 µg L -1 .Meanwhile, for 238 U and 235 U based on the APHA 3030K method, both elements were reported lower than their respective MDL.This finding shows the good capability and sensitivity of the APHA 3030K method compared to the APHA 3030E method.The lower MDL values for 232 Th, 238 U, and 235 U by using the APHA 3030K method than the APHA 3030E method were due to the closed system digestion of the microwave digester compared to open digestion by using hot block digester.The closed system of microwave provides minimum volatilization and stability of elements of interest during the digestion process [12]

Conclusions
In this study, two methods of digestion (APHA 3030E and APHA 3030K) for 232 Th, 238 U, and 235 U determination using the APHA 3125 method were performed to determine method performance and MDL for each element in water samples.Based on method performance criteria, both methods produce good value and percentage recovery for LFB, LFM, LFMD and RPD.For 232 Th, 238 U, and 235 U MDL determination, APHA offers lower value of MDL compared to APHA 3030E.Due to the difference in digestion technique, APHA 3030K offers better sensitivity in the detection of 232 Th compared to the APHA 3030E method.Therefore, it is recommended to evaluate method performance criteria and MDL in every digestion method prior to elements quantification to provide desirable reporting levels in various types of water samples.

Figure 1 .
Figure 1.Comparison of MDL of 232 Th, 238 U, and 235 U in water sample.

Figure 3 .
Figure 3.Comparison of MDL and obtained radioactive wastewater concentration value (a) APHA 3030E; (b) APHA 3030K [1] Shahin M S A, Saad E A, Shazly A, Ezzat A and El-mongy S A 2020 Improved treatment approaches for trace analysis of Uranium and Thorium using Ultra-sensitive ICP-MS technique J. Nucl.Radiat.Phys.15 23-33.[2] Kasar S, Murugan R, Arae H, Aono T and Sahoo S K 2020 A microwave digestion technique for the analysis of rare earth elements, Thorium and Uranium in geochemical certified reference materials and soils by inductively coupled plasma mass spectrometry molecules 25. [3] Ma L, Dang D H, Wang W, Evans R D and Wang W X 2019 Rare earth elements in the Pearl River Delta of China: Potential impacts of the REE industry on water, suspended particles and oysters Environ.Pollut.244 190-201.[4] Soromotin A, Moskovchenko D, Khoroshavin V, Prikhodko N, Puzanov A, Kirillov V, Koveshnikov M, Krylova E, Krasnenko A and Pechkin A 2022 Major, trace and rare earth element distribution in water, suspended particulate matter and stream sediments of the Ob River Mouth Water (Switzerland) 14. [5] Verplanck P L, Antweiler R C, Nordstrom D K and Taylor H E 2001 Standard reference water samples for rare earth element determinations Appl.Geochem.16 231-44.[6] Wysocka I 2021 Determination of rare earth elements concentrations in natural waters -A review

Table 1 .
The spiking regime of QC samples for APHA 3030E method.

Table 2 .
The spiking regime of QC samples for APHA 3030K method.

Table 3 .
Method performance result for APHA 3030E digestion technique.

Table 4 .
Method performance result for APHA 3030K digestion technique. .