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Master’s Thesis Defense “EFFECTS OF TEMPERATURE AND pH ON VOLATILIZATION OF MERCURY AFTER CHEMICAL REDUCTION”

Master’s Thesis Defense “EFFECTS OF TEMPERATURE AND pH ON VOLATILIZATION OF MERCURY AFTER CHEMICAL REDUCTION”. Presented by Jose Vasquez Master in Environmental Engineering Florida International University July 7, 2009. Overview.

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Master’s Thesis Defense “EFFECTS OF TEMPERATURE AND pH ON VOLATILIZATION OF MERCURY AFTER CHEMICAL REDUCTION”

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  1. Master’s Thesis Defense“EFFECTS OF TEMPERATURE AND pH ON VOLATILIZATION OF MERCURY AFTER CHEMICAL REDUCTION” Presented by Jose Vasquez Master in Environmental Engineering Florida International University July 7, 2009

  2. Overview • In the 1950’s around 11,400 metric tons of mercury (Hg) was processed in the Y-12 plant (Oak Ridge, TN) which led to mercury contamination of ground and surface waters. • East Fork Poplar Creek, which runs adjacent to the Y-12 plant, is the main focus of this project because of its high Hg concentration.

  3. Types of mercury in this research • Mercury could be divided into three types: Metal, inorganic and organic mercury • Mercury occurs in three oxidation states: mercuric Hg(II), mercurous Hg(I) and metal mercury Hg(0). • Methyl-mercury is a type of organic mercury and the most toxic form. (Lin et al, 1999)

  4. Chemical cycle of Hg

  5. Hg Health Effects • One form of mercury could have different health effects and exposure than another • Human exposure could be by dermal absorption, inhalation or ingestion • The health effects from elemental mercury include tremor of lips, eyelids, tongue, fingers and sometimes of the whole body • The human body absorbs only 0.01% of elemental mercury while methyl-mercury is nearly absorbed 100% from the intestinal tract • The main problem with the methyl-mercury is its bio-magnification and bio-accumulation. Long term exposure to methyl-mercury can damage the brain, kidneys, and affect the development of the fetus

  6. Current Situation • Remedial efforts started in the 1980s in Poplar Creek. However, Hg remains elevated above regulatory standards. • Reduce the high concentration of inorganic mercury in Poplar Creek to avoid the formation of methylmercury

  7. Some efforts to reduce Hg(II) from Poplar Creek Hg volatilization after chemical reduction and air stripping Flow Management: adding an extra flow of water to the creek to dilute inorganic mercury or relocate the creek channel. Removal of contaminated soil and sediments Stabilize floodplain and stream bank soils using native vegetation Reduction of rainwater infiltration and cleaning of contaminated storm drains

  8. Objectives • Find out if temperature and pH affects the rate of Hg volatilization in water after air stripping and the addition of a chemical reductor. • Evaluate the effectiveness of air stripping as a tool to enhance the mercury volatilization to be collected later in a trap. • Determine the efficacy of stannous chloride (SnCl2)as a reducing agent. • Characterize toxic effects on the environment of SnCl2 addition.

  9. Converting Hg (II) to Hg(0) using chemical reduction • Adding small quantities of Stannous Chloride (SnCl2) could help to convert Hg (II) to Hg (0) which could volatilize easily. (Looney et al, 2003) • Sn: Hg stoichiometric ratios between 5 to 25, could convert more than 94% to Hg(0) (Looney et al, 2003) • Reaction of Hg with tin occurs as a chloride or hydroxide complex: HgCl2+Sn+2↔Hg0+2Cl-+Sn+4

  10. Stannous chloride facts • SnCl2 is a white crystalline solid that is highly corrosive and water soluble. • It is unknown to cause cancer in humans or animals but could produce DNA damage if it is used in large doses. • Large doses of tin have negatively impacted bone structure, reproductive system, the liver and the central nervous system of some mammals like rabbits, mice and rats (LD50 toxic dose of 44.4 mg/kg). • Very low concentration of SnCl2 added to the creek (< 0.01 mg/kg), represents little concern for chronic toxicity.

  11. 0.0018 0.0016 0.0014 0.0012 0.001 Hg(0) uM/L 0.0008 0.0006 0.0004 0.0002 0 0 0.0005 0.001 0.0015 0.002 SnCl2(uM/L) Volatilization of Hg(0) with different doses of SnCl2

  12. Air Stripping • Mercury volatilization could be enhanced by air stripping • Southworth (2006) showed that it is possible to have a complete removal of Hg(0) if air-water ratio is favorable (greater than 20) • Scientists at Savannah River National Lab have been testing this technology, which is in the process to be tested on a larger scale.

  13. Air Water Phase Exchange • Mercury is a highly volatile substance with a large Henry’s law constant (range 0.2 to 0.8) • Hg(0) occurs at the air-water interface while methylmercury is mostly present at the creek sediments. • The rate of volatilization is determined by both the release rate of the dissolved gas and the diffusion rate of mercury in water (Southworth, 2006)

  14. Possible Factors Affecting Hg Volatilization in Water (Literature review) Temperature pH Sunlight Rainfall and Wind Speed Organic Matter Turbulence and depth of the Aquatic System

  15. Temperature and Hg Volatilization • There is no correlation between Hg volatilization and temperature in water (Southworth et al, 2006). • A positive relation of temperature with mercury flux exists in a river or creek (Gardfelt et al,2002). • Loux (2000) affirms that 44% of diurnal mercury flux variations in aquatic systems are because of temperature effects over some chemical properties like Henry’s law constant.

  16. Hg volatilization as a function of Henry’s law constant and air: water ratio C(n)/C(o) =e-(Hn) (Looney et al, 2003) • Where H is Henry’s law constant which differs with temperature. The ratio n is the air volume purged to water volume. • Henry’s law constant determines the gas-liquid equilibrium Source: Sanemasa (1975)

  17. Hg Volatilization and pH • Munthe et al (1992) experiments showed no correlation of aqueous phase oxidation of Hg(0) with pH (5.2-6.2) and temperature. • Xun et al (1987) demonstrated that low pH increased methylmercury production which decreased Hg(0) from water (Earle et al, 2000).

  18. Sunlight Effects on Hg Volatilization • Solar radiation is the main force controlling diurnal variations in dissolved gaseous mercury (DGM) and mercury flux. These variations are the result of reduction and oxidation processes occurring simultaneously. • During my summer internship experiment at ORNL, I concluded that Hg(0) is rapidly oxidized to Hg (II) by sunlight. Volatilization of Hg(0) was affected by photo-oxidation (sunlight) and chemical reduction (SnCl2).

  19. METHODS Experiment Set up • Find the percent of Hg volatilization at pH 5, 7 and 9 for three different temperatures (5, 15 and 25°C) KMnO4 trap Exp I: DMA-80 analyzer Exp II: PSA 10.035 analyzer

  20. Experimental work procedure • Hg Solutions Preparation (pH 5, 7 and 9) • Preparation of SnCl2 • Potassium permanganate(KMnO4) trap • Volatilization • Measure final Hg concentration • Data assessment and analysis

  21. 1. Hg solutions preparation with pH 5, 7 and 9 • 0.01 mol/L of Nitric Acid (NO3) and 0.1 mol/L of base (KOH) were used to adjust pH to required level. • 10 ml of 0.3 ppm Hg solution was made • 1 ml of 0.3 ppm Hg was added to 199 ml of each of the three pH solutions. Then measure Hg concentration and refrigerate. Bench Top Refrigerator Incubator Shaker

  22. 2. Preparation of SnCl2 • 10 ml of SnCl2 solution (10 g/L) was prepared under the hood. • 22.5 ul of this SnCl2 solution was added to each Hg solution before air stripping (final concentration of SnCl2 was 11.3ug/l). • The Sn:Hg ratio was approximately 7

  23. 3. Potassium Permanganate (KMnO4) as a Hg Trap • 0.6 g of KMnO4 was mixed with 95 ml solution of DI water and 5 ml H2SO4. • 8 ml of this solution was used as a mercury trap for each set. • The cap has two metal tubes, one inlet for receiving volatilized Hg and another for outlet acting as a vent.

  24. Air stripping for 10 minutes. Air flow=4 L/hr for each vial. The air-water ratio of 33 (4000mL/60 min)*10 min)/ 20 ml=33) Experiment I: Each set consists of 4 vials (one for control 2 and the rest for triplicates) connected to the air source (inlet) and to the trap (outlet) Experiment II: Only one vial at the time 4. Volatilization

  25. Controls • Control 1: Three samples (pH 5, 7 & 9) which did not receive either air stripping or SnCl2. (Initial Hg Concentration) • Control 2: Nine samples (each combination of pH and temperature) which received air but not SnCl2

  26. 4. Measuring Hg concentrationAnalyzer DMA-80 • The samples were analyzed using EPA method 7473. (Triplicates) • This machine is equipped with a drying decomposition furnace, a catalyst tube, an amalgamator, a Hg source lamp, a detector and a DMA-80 software. • The detection limit for this instrument is 0.5 ppb

  27. Mercury Analyzer PSA 10.035 • The method used in this experiment was Standard Procedure SERCMLAB SOP-001-04, which is a modification of EPA test method 1631 • This method is based on atomic fluorescence techniques and vapor generation • This system is generally used for reading mercury concentration within the range of 0.1 ppt to 10 ppm

  28. Data AnalysisFinal and initial Hg concentrations A higher final Hg concentration in samples with higher pH after applying SnCl2 and air stripping. Hg initial is control 1.

  29. Percentage of Hg volatilization versus pH 95% 85% 78% Higher volatilization with lower pH

  30. Percentage of Hg volatilization versus pH for each temperature

  31. Percentage of Hg volatilization versus temperature 89.5% 81.6% 86.6%

  32. Percentage of Hg volatilization versus temperature for different pH

  33. Results • It was demonstrated that a very low concentration of SnCl2 (Sn:Hg ratio of 7) was able to volatilize approximately an average of 85% of Hg (0). • Because of the very low concentration of Hg involved in this study, it was difficult to predict the rate of volatilization with a very low margin of error • If enough air is stripped into the solution (air: water ratio=30), there would be an effective Hg volatilization.

  34. Results • There is a lower Hg volatilization with higher pH level, given that the concentration of hydroxide ions (OH-) increases with higher pH. These ions are acting as oxidant agents competing against the reductor agent (SnCl2) • Little correlation was found between temperature and Hg volatilization. However, it is possible to observe an increase in Hg volatilization with an increase in temperature if the average volatilization value for each temperature is calculated.

  35. Conclusions • The technique of adding a reductor like SnCl2 and posterior air stripping were able to convert more than 80% of inorganic mercury to elemental mercury. • Air stripping is required to enhance the volatilization of gaseous mercury. Air stripping without any chemical reduction results in an inefficient volatilization (<18%). • The combination of chemical reduction and air stripping could be economically feasible and practical to reduce mercury in aqueous systems. • Higher pH is related to lower volatilization. • The results from the experiments do not show a clear correlation of Hg volatilization with temperature.

  36. Recommendations • Reducing the pH value in any aqueous system could help to increase Hg volatilization; however, a complete analysis of other effects should be studied, like a possible increase in the production of methylmercury. • More research is necessary to find toxic effects on the environment and human health when SnCl2 is used as a chemical reductor in an aqueous system. • Collection of more data is recommended to reduce uncertainties and to be able to confirm the correlation of mercury volatilization with temperature.

  37. Recommendations • The low cost of applying a small amount of SnCl2 and air stripping could make this technique appropriate to use on a bigger scale (in a creek or river). • Brainstorming new ideas with different groups involved with Hg pollution is recommended to find other feasible alternatives to reduce Hg concentration in water and soil. • Removal of Hg(0) from water should be in absence of sunlight, because of the rapid oxidation of Hg(0) to Hg (II).

  38. Acknowledgment • This research work has been sponsored by the DOE/FIU Science & Technology Workforce Initiative, a program developed by the US Department of Energy and Florida International University’s Applied Research Center (FIU-ARC) • Thanks to Dr. Katsenovich and Dr. Lagos from ARC for their support and valuable assistance. • I would also like to express my appreciation to my committee members Dr. Tansel, Dr. Laha and Dr. Tang for their guidance and time dedicated to this thesis.

  39. Questions?? Thank you.

  40. C(n)/C(o) =e-Hn Where air: water ratio is 30 and H vary with temperature

  41. Hg(0) Volatilization after Reduction by SnCl2 and Photo-oxidation by Sunlight SnCl2 was added to the sample SnCl2 was added to the Hg sample before receiving direct sunlight. 10 ml of water were taken from this sample every 5 minutes (experiment at ORNL)

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