Study on determination of mercury species in sediment using selective extraction technique

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ***. TRINH THI THUY STUDY ON DETERMINATION OF MERCURY SPECIES IN SEDIMENT USING SELECTIVE EXTRACTION TECHNIQUE Major: Analytical Chemistry Code: 9.44.01.18 SUMARY OF DOCTORAL THESIS IN CHEMISTRY Hanoi - 2018 The thesis has been completed at: Institute of Chemistry - Graduate university science and technology – Vietnam Academy of Science and Technology Science supervisor: 1.Assoc.Prof.Dr Vu Duc Loi 2.Assoc.Prof.Dr Le Thi Trinh Reviewer 1: Reviewer 2: Reviewer 3: The thesis was defended at National level Council of Thesis Assessment held at Graduate University of Science and Technology – Vietnam Academy of Science and Technology at , on , 201 Thesis can be further referred at: -The Library of Graduate University of Science and Technology -National Library of Vietnam 1 INTRODUCTION 1. Background Mercury and its compounds are chemicals which are great bio accumulative potential causing serious effects on human health and the environment. Mercury is used in many industries such as chemicals, fertilizers, plastics, electrical engineering, electronics, cement, paint, silver and gold in mineral ores, fluorescent lamps, barometer, thermometer, blood pressure monitor, cosmetics... According to the United Nations Environment Program (UNEP), Asia's rapid economic growth has accelerated the growth of industries that use mercury in production, making it the largest source of mercury, accounting for nearly 50% of the world's waste. According to the Ministry of Industry and Trade's 2016 national mercury inventory report, Vietnam has four major production sectors related to the use and emission of mercury: the manufacture and use of lighting equipment: fuel use coal in industrial activities; Use of mercury and compounds in the health sector and small- scale manual gold mining. Total amount of mercury imported into Vietnam in 2014 is about 14000 kg. However, there is no investigation to clarify the path and purpose of the use of mercury and mercury compounds sold in the domestic market. In October 2013, Vietnam signed the Minamata Convention on Mercury, which shows the concern and attention of state management agencies on mercury pollution, including monitoring, pollution control, minimize mercury using and emissions. The toxicity of mercury depends on its chemical form. In general, inorganic mercury is more toxic than organic mercury, mercury element and sulfide mercury are less toxic. The only form of mercury is methyl mercury, which can accumulate in fatty tissues, as well as in fish and other animals. Therefore, the determination of the content of different chemical forms of mercury in environmental samples, biological samples is very important, especially the sediment samples which accumulate many pollutants from the waste sources and are the habitats for many aquatic plants. Currently, there are a number of scientific studies on the method of determining mercury forms in different samples in the world, but there are not many comprehensive studies on sample processing to extract existing forms of mercury in sediment samples. The International Organizations and countries haven’t also issued standards and guidelines for the determining mercury species in sediment samples except for the US Environmental Protection Agency (EPA) standard. In Vietnam, there is no standard procedure for the analysis of total mercury content and mercury species in sediment samples as there are very few studies evaluating the presence of mercury and its forms in the environment. Therefore, the study on “Study on the determination of mercury species in sediment using selective extraction technique” was conducted. 2. Objectives of this dissertation This study will be achieved by aiming to: - Build the analytical procedure of determination of mercury species in sediment using selective extraction technique - Assess of the reliability of the method of determination of mercury speciation in sediment - Apply the result of study to determine forms of mercury in sediment at a certain area. 3. The composition of the thesis - Investigate, select optimal conditions and validate of the analytical method of total mercury content in sediment; - Investigate and develop the procedure of determination of methyl mercury content in sediment by gas chromatography using Gas Chromatography - Electron Capture Detector (GC-ECD) with a capillary column instead of packed columns used previously; - Investigate and develop the procedure of determination of concentration of methyl mercury in sediment using selective extraction techniques and atomic absorption spectrometry. - Develop the procedure of selective extraction to determination of mercury species in sediment samples. 2 - Apply the analytical procedure to determine the content of total mercury and mercury speciation in surface sediment samples (ponds, lakes) in Minh Khai trade village, Van Lam, Hung Yen; sediment core at Han River estuary, Da Nang city and assess their pollution level in the environment. 3 CHAPTER 1: LITERATURE REVIEW 1.1 Mercury and mercury compounds: Introduce mercury and mercury compounds in terms of physical and toxicological properties, the metabolic pathways of mercury in the environment. 1.2 Source of Mercury, Mercury Compounds Emissions: Summarize sources, current status of mercury emissions into the environment in the world and in Vietnam. 1.3 Classification of the existence of mercury: Describes the classification of mercury forms in the environment, classify the forms of mercury in soil and sediment. 1.4 Methods of Determining Mercury Content: Overview the methods for quantifying mercury from post- processing of Hg 2+ and methylmercury. 1.5 Research in and outside the country related to thesis: Summarize studies on methods of determining the total content of mercury in sediment, studies on sequential extraction methods, selection of mercury forms in sediment, some guidelines on the quantification of mercury and mercury forms in environmental samples. 1.6 Overview of sampling sites: Provide information on sampling sites for the study: The study was conducted on two matrix samples: surface sediment samples collected in the Minh Khai plastic recycling village, Nhu Quynh district, Hung Yen province and marine sediment collected from Han River estuary in Da Nang city. Base on references related to the method of determination of mercury species in sediment, show that: - The determination of total mercury content in sediments is not sufficiently scientific for evaluation the mobility, bioaccumulation potential, the impacts of mercury and its compounds the environment and ecosystem. In addition to the analysis of total mercury content, the concentration of mercury species in the sample should be determined in order to obtain a complete assessment. - For the procedures of determination of the total content of mercury, there have been many studies on methods of sample handling and quantitative techniques of total mercury content in sediment samples and other matrix samples. Several analytical method guidelines for determination of total mercury content in sediment were issued by US EPA, Japan. However, when applying these guidelines, laboratories have more or less changed or developed the methods. Therefore, the methods should be validated to ensure the reliability and accuracy of the results of the analysis under the actual conditions of existing laboratories. - Researches on sample preparation and quantification of methyl mercury in sediment is not very much, especially in Vietnam. According to international studies, methyl mercury in extract solution could be quantified by using GC instrument coupled with high sensitivity detector such as ECD, AAS, AFS, MS; there are small number of studies using CV-AAS or DMA to quantify MeHg. Therefore, it is very necessary to select, survey and evaluate the reliability of the analytical method in real experimental conditions. - There have been many studies on procedures of determination of some mercury species in soil and sediment samples, but there have been little agreement among authors on the classification and sequential extraction of species, also very few guideline standards for systematically validation of method. Hence, it is necessary to research on classification of mercury species, sequential extracting procedures, optimistically condition and evaluation of reliability as well as understanding of structural phase change of the sample after each extraction step to evaluate selectivity of extraction. Based on the above-mentioned research issues, we chose the research thesis: “Study on the determination of mercury species in sediment using selective extraction technique” 4 CHAPTER 2: MATERIALS AND METHODS 2.1 Research subjects - Process analysis Mercury Species in sediment: total mercury, methyl mercury, organic mercury total, water- soluble and mercuric oxides, mercuric sulfide. - Sediment samples: + Quality control samples: method blank, matrix spike, duplicate and laboratory control sample + Environmental Samples: The sediment core samples from Han River estuary, Da Nang city; surface sediment samples were taken at a pond, lake and river in Minh Khai Plastic Recycling Village, Van Lam District, Hung Yen Province. 2.2 Research methods: The research methods used in the thesis: 2.2.1. Literature review of publications 2.2.2. Quantification method and equipment + Using the Cold Vapor Atomic Absorption Spectrometry (CV - AAS) method to measure and determine mercury Species after the sample digestion specializing Hg 2+ . + Gas Chromatography – Electron Capture Detector: determine methyl mercury 2.2.3. Data processing methods The experimental results were processed using Microsoft Excel 2010, Origin 8.5, SPSS - 20 software. 2.2.4. Optimize and validate analytical procedures: Design experiments for calculation of LOD, LOQ, accuracy, accuracy and uncertainty measurement. 2.3 Chemicals, tools: Presenting chemicals and devices for research in the full report. 2.4 Experiment 2.4.1 Sample preparation for the study: present sample collection techniques, preparation of samples for research: Blank method samples, laboratory control sample (add the standard chemical to the cleaned sediment). 2.4.2 Evaluation of the reliability of result of the total mercury content in sample: Method validation and determination . 2.4.3 Investigation and evaluation of the procedure for determination of methyl mercury content in sediment: To investigate the process of determination of methylmercury content in sediment by CV-AAS and GC- ECD method, then validate the use of two analytical processes. 2.4.4 Investigation and evaluation of selective sequential extraction of some forms of mercury in sediment: Investigation of selective sequential extraction of some forms of mercury in sediment, evaluation of the reliability of survey process. In this study we selected the following classification: Form F1: organic mercury Form F2: Soluble in water, HgO Form F3: mercuric sulfide Form F4: Residual form (fractional residue is a fraction of Hg bound to elements that can not be extracted by the previous reactants). 2.4.5 Apply the procedures to determine the concentration of mercury speciation in the sediment: Determine the total mercury, methyl mercury and other forms in surface sediment and sediment cores taken at the Han estuary, Da Nang city; surface sediment samples were taken at a pond, lake and river in Minh Khai Plastic Recycling Village, Van Lam District, Hung Yen Province. 5 CHAPTER 3: RESULTS AND DISCUSSION 3.1 Result of confirming the use value of the total mercury content analysis The calibration curve was established using the standard solution of methyl mercury - Cysteine and carry out digestion of sample under the same conditions as for the environmental sample (calibration curve is built on blank sample matrix). First, measure the signal strength by repeatedly measuring each standard point at five times. The results show that the measurement signal is stable (the relative standard deviations (RSD) of signal of standard sample were under 15%). The linear range was from 0.05 to 1.4μg Hg/L. Validation result of the process of determining the total mercury content was shown in the following table: Table 3.6: Summarize the result of method validation of the T - Hg analysis process No Parameter Result Requirement of AOAC 1 Limit of detection and Limit of quantitation LOD = 1,04 ng/g LOQ = 3,45 ng/g 4< R = 5,52 < 10 2 Repeatability of the method RSD = 0,933- 4,53 % RSD < 15% 3 A ccuracy of the method (Recovery productivity) R = 89,76 ÷ 103,80%. 80 ≤ R ≤ 110% 4 Estimation of expanded measurement uncertainty U (%) 13,72% 3.2 Investigation and evaluation of the determination of level of methyl mercury 3.2.1 Determination of concentration of methyl mercury using CV-AAS a) Investigation of some extracted conditions The overall recoveries obtained from the experiments investigating the factors that affect the extraction efficiency of methyl mercury were shown in Figure 3.1. Figure 3.2: Summarize the results of investigating factors in the process of sample determination of methyl mercury by CV-AAS method 6 Based on the results of the study, we propose an experimental procedure to determine methyl mercury in sediment by the CV-AAS as follows: Weigh approximately 2.0 gram of sediment sample into 50mL glass centrifuge tube. Add 10.0mL of 6M HCl to the centrifuge tube, shake the mixture in a horizontal shaker, for 5 minutes. Centrifuge for 10 minutes at 2400 rpm, separate the water phase into a new centrifuge tube. Add 20.0 mL Toluene to the centrifuge tube, shake the mixture in a horizontal shaker, for 15 minutes. The mixture is then centrifuged for 20 minutes at 2400 rpm. Separate organic phase, repeat this step 2 times. Transfer the whole extract to a centrifuge tube, add 1 ml of L-Cyanine, shake for 20 minutes, centrifuge for 3 minutes at 2000 rpm, extract for L-Cystine. The concentration of methyl mercury in the final extract solution was determined by CV-AAS. b) Evaluate the reliability of the analysis procedure Determine the Limit of detection and Limit of quantitation of method: Limit of detection (LOD) and Limit of quantitation (LOQ) of established analytical procedure were calculated from result of the analysis of the sample in a given matrix containing small amount of MeHg. Ten replicates of MK8 sample were performed. The LOD andd LOQ of the built-up process were 0.34ng Hg/g and 1.12 ng Hg/g if using 2 grams of dry sediment samples for analysis, respectively. Evaluate the accuracy of the analytical process: Accuracy were evaluated at three levels as repeatability and precision using sediment samples and matrix sample spiked. Relative standard deviations (RSD) obtained were lower than 8.29%, it was lower than the acceptable RSD for the 10 ppb sample analyzes by AOAC (RSD Criteria <21%). Thus, the analytical procedure has been constructed to ensure the required repeatability. The determination of the trueness was considered through determination of recoveries using matrix spike. The recoveries from spiked samples at three concentration levels were ranged from 88.51% to 114.00%. This result is consistent with AOAC requirements (at the ng/g levels, the required recoveries are between 60 and 115%). Measurement uncertainty of method: expanded measurement uncertainty of method U = 24,34 (%). 3.2.2 Determination of concentration of methyl mercury using GC/ECD a) Optimization of conditions of GC/ECD to determine methyl mercury  Select the column When we used 03 column types DB - 608, DB - 5; DB - 17 to separated the analyst, realized that using the DB - 608 column give the pick signal more stable than the other two columns. Therefore, we choose the DB - 608 column (30m x 0.25mm x 0.25μm) for the next steps of the methyl mercury analysis on GC/ECD.  Determination of operation conditions of equipment Survey methyl mercury determining conditions on GC/ECD equipment, column DB - 608 is carried out in accordance with parameters: temperature of detector, temperature of injector, program of furnace temperature in different conditions. From the experimental results, 3 rd condition was selected for quantitation of Methyl mercury by GC/ECD. The optimization control are: the injector temperature is 220 0 C, the detector temperature is 280 0 C, the gradient of temperature at column oven: initial temperature at 50 0 C (keep 1 minute) and increased to 240 0 C with rate of temperature of 20 0 C/min (kept at the last temperature of 15 minutes). Under this condition, the retention time of methyl mercury is in the range of 6.62 ÷ 6.67 minutes (Figure 3.3) 7 7.27.157.17.0576.956.96.856.86.756.76.656.66.556.56.456.46.356.3 950,000 900,000 850,000 800,000 750,000 700,000 650,000 600,000 550,000 500,000 450,000 400,000 350,000 300,000 250,000 200,000 150,000 100,000 50,000 0 -50,000 -100,000 M e H g RT [min] MeHg200ppb2.DATAµV 6.826.86.786.766.746.726.76.686.666.646.626.66.586.566.546.526.56.486.466.446.426.4 2,200,000 2,100,000 2,000,000 1,900,000 1,800,000 1,700,000 1,600,000 1,500,000 1,400,000 1,300,000 1,200,000 1,100,000 1,000,000 900,000 800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000 0 -100,000 -200,000 M e H g M e H g M e H g M e H g M e H g RT [min] MeHg500ppb1.DATA MeHg200ppb2.DATA MeHg100ppb2.DATA MeHg1000ppb Pha loang2.DATA MeHg50ppb2.DATA µV a) Diagram of standard at 200ppb b) Diagram of standard at different concentration Figure 3.3: Chromatographic of methyl mercury standard samples Next, the limit of detection and the limit of quantitative of the instrument with optimization condition. Five replicates of standard samples at 0.5ppb and signal-to-noise ratio (S/N) was calculated. Results of standard deviation and mean values of S/N, IDL and IQL values are shown in Table 3.2. Table 3.17: Results of IDL and IQL determination First Second Third 4 th time 5 th time Mean Concentration (ppb) 0,5 0,5 0,5 0,5 0,5 Pick area (µV) 117,7 93,4 88,1 76,3 102,1 Noise 69,27 48,90 45,70 51,20 71,30 S/N 1,699 1,910 1,928 1,490 1,432 1,69 IDL 0,883 0,785 0,778 1,007 1,048 0,90 IQL 2,943 2,618 2,594 3,355 3,492 3,00 According to the results, the Instrument detection limit (IDL) for MeHg is 0.90 ppb and the Instrument Quantitation limit (IQL) is 3.00 ppb. Those values allows the quantification of trace levels of methyl mercury in the sample after cleansing and enrichment. b) Optimization of conditions of sample preparation Synthesis results from the GC/ECD methylmercury determination process are shown in Figure 3.4 below. Thus, the research has selected the parameters for the process of determining methyl mercury by GC/ECD method as follows: Carefully weigh about 2 grams of sediment sample into 50mL glass centrifuge tube. Add 5.0 mL of KOH/CH3OH (25%), sonication for 45 minutes. Add 5mL of 4M H2SO4 saturated with CuSO4 solution, 5mL of KBr 4M solution and 3 mL of Toluene solution, shake for 3 minutes, then centrifuge at 2200 rpm for 10 minutes, muscle. Add 3 ml of solvent Toluene to the rest, repeating the extraction process twice. Collect the whole organic extracts to the new centrifuge tube. Add 1 ml of L-Cysteine solution 2%, shake for ...