IMPACT OF SALT ON METAL LEACHING FROM COAL FLY ASH

Transcription

1 2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN - May 5-7, IMPACT OF SALT ON METAL LEACHING FROM COAL FLY ASH Jay E. Renew, P.E. 1, Kirk M. Ellison 2, Keith Hendershot 1, Jacob Rajterowski 1, and Ching-Hua Huang, Ph.D. 3 1 Southern Research, 317 Covered Bridge Road, Cartersville, Georgia; 2 Southern Company, 600N18th Street / 14N-8195, Birmingham, Alabama; 3 Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia CONFERENCE: 2015 World of Coal Ash ( KEYWORDS: salt, metal, coal fly ash, magnesium chloride, calcium chloride, sodium chloride, zero liquid discharge, and flue gas desulfurization ABSTRACT The coal-fired power industry is expected to encounter wastes that are characteristically unlike traditional coal-combustion residuals (CCRs). These CCRs could include coal fly ashes (CFAs) impacted by new air emission controls and zero liquid discharge (ZLD) residuals from flue gas desulfurization (FGD) wastewater processing. Halides in coal accumulate in FGD wastewater and are left as a residual in the wastewater. Hence, these CCRs could contain higher salt concentrations than traditional CCRs. One question facing the coal-fired industry is the impact of the introduction of high quantities of salts and brines to existing industry landfills. Specifically, what is the impact of salt on metal leaching from CFA and other CCRs? CFA can contain metals including arsenic, cadmium, chromium, mercury, and selenium. The objective of the current study is to evaluate the impact of salt on metal leaching from CFA. This objective was accomplished by adding salts (sodium chloride, magnesium chloride, and calcium chloride) to the leachant for United States Environmental Protection Agency (USEPA) Method 1313 for an evaluation of CFA. Results from the experiment showed increased leaching of several metals from CFA in the presence of salt.

2 1. Introduction The coal-fired power industry is expected to encounter materials that are characteristically unlike traditional coal-combustion residuals (CCRs). These CCRs could include coal fly ashes (CFAs) impacted by new air emission controls and zero liquid discharge (ZLD) residuals from flue gas desulfurization (FGD) wastewater processing. These CCRs could contain higher salt concentrations than traditional CCRs. New Air Emissions Controls New air emissions controls such as calcium bromide addition, brominated powdered activated carbon injection, and wet FGD additives added to coal could change the characteristics of CCRs that the industry has historically managed. In particular, sodium-based dry sorbent injection (DSI) may produce a highly soluble, high salt by-product that must be disposed in a landfill. ZLD FGD Residuals In a typical wet FGD system, lime slurry is sprayed into the flue gas in order to remove sulfur dioxide. Along with sulfur dioxide, halides from the coal are also removed from the flue gas [2]. These halides accumulate in the wet FGD systems in the form of calcium, magnesium, sodium, chloride ions (with some sulfate). Along with sulfur dioxide and salt removal from flue gas, significant concentrations of arsenic, cadmium, chromium, mercury, and selenium are also scrubbed [1]. This situation has caused the United States Environmental Protection Agency (USEPA) to propose wastewater treatment regulations through the Steam Electric Power Generating Effluent Guidelines. Among various treatment options for FGD brines, ZLD approaches are gaining significant interest from the coal-fired power industry. Although the coming regulations may not require ZLD, future regulations could be more stringent. If a total dissolved solids (TDS) limit is added to the regulations in the future, ZLD treatment may be required for FGD wastewater. The impact of a TDS limit has already been seen in Pennsylvania for the natural gas industry. In 2008, TDS in the Monongahela River (PA) reached levels up to 925 mg/l (mainly chloride and sulfate) [3-5]. As a result of these high TDS levels, 13 public drinking water systems exceeded their secondary maximum contaminant limit (MCL) for chloride under the Safe Drinking Water Act [3, 4] and the State of Pennsylvania subsequently implemented a strict TDS discharge limit for all industrial outfalls. This TDS limit drove the disposal of brines from the natural gas industry from local wastewater treatment plants (which are usually not equipped to remove dissolved solids) to deep well injection or evaporation coupled with solidification/stabilization (S/S) with cement prior to landfill disposal. Potential ZLD methods for FGD wastewater include evaporators & crystallizers, wastewater spray dryers, and brine concentrators. The final solid products produced by these technologies are as follows: 1. Evaporators & Crystallizers - crystallized salt. 2. Wastewater Spray Dryers CFA / salt mixture.

3 3. Brine Concentrator - high salinity sludge. The residuals described may be S/S by mixing the residuals with CFA and lime or Portland cement (PC). However, regardless of the ZLD technology utilized, the residuals will greatly increase the quantity of salt landfilled by the industry. Impact of Increased Salt Concentration on Coal Fly Ash Leaching The question facing the industry is what is the impact of the introduction of high quantities of salts to industry landfills. Specifically, what is the impact on metal leaching from CFAs and other CCRs? It is known that the metals contained in the salts can readily leach, but the potential for the metals incorporated in the CFA itself to be mobilized needs investigation. Depending on the source of coal, CFA can contain significant concentrations of metals including arsenic, cadmium, chromium, mercury, and selenium. Little information exists in literature on the impact of salt on metal leaching from CFAs. Effect of Road Salts on Metal Mobilization in Soils Some studies have been conducted on the impact of salt on metal leaching from soils due to the use of road salt [6, 7]. It was found that salt increases the mobility of metals in soils through the following mechanisms [6]: 1. Cation Exchange. 2. Colloid Dispersion. 3. Chloride Complex Formation. Some of the factors that affect the mobilization of metals in the presence of salt include ph and the available charge sites [6]. Current Study The objective current study was to evaluate the impact of salt on metal leaching from CFA. This was accomplished by adding salts (sodium chloride, calcium chloride, and magnesium chloride) to the extraction fluid specified for United States Environmental Protection Agency (USEPA) Method 1313 for a CFA leaching evaluation [8]. The concentration of metals in the leachate was then analyzed to determine if salts increased metal leaching from CFA. 2. Materials and Methods 2.1 Materials CFA was obtained from a power plant in the southeastern United States that burns subbituminous coal. Calcium chloride dihydrate (CaCl2*2H20), magnesium chloride hexahydrate (MgCl2*6H20), and sodium chloride (NaCl) were obtained from Fisher (Pittsburgh, PA). All salts were American Chemical Society (ACS) grade. Trace metal grade nitric acid (HNO3) was

4 obtained from Fisher. Deionized reagent water was prepared by a Barnstead Nanopure Water Purification System (Dubuque, IA). 2.2 Leaching Procedure The leaching procedure utilized in the project was USEPA Method 1313, Liquid-Solid Partitioning as a Function of Extract ph using a Parallel Batch Extraction Procedure. The first step process was to develop a schedule of nitric acid additions for the leachant. This task was accomplished by mixing 20 g of CFA with 200 ml of deionized water with variable amounts of nitric acid added in order develop a pre-test titration curve. These samples were rotated for 24 hours and the ph was then measured. From this data, a schedule of acid-base additions was developed to reach the target phs. It was not necessary to utilize base in the experiments because the final ph of the CFA without any acid addition was higher than the target phs of, 7.0, and 1 for the project. Without the addition of nitric acid, the CFA has a final ph of 11.7 after agitation with deionized water. Once the schedule of nitric acid addition to produce a final ph of, 7.0, and 1 was developed, the leaching procedure was initiated. The 200 ml leachant (with the correct amount of nitric acid added) was prepared in a 250 ml high density polyethylene (HDPE) bottle. Salt was then added to the leachant to reach the concentrations shown in Table 1. For each chloride level, a sample with a final target ph of, 7.0, and 1 was prepared. The amount of water added with the hydrated salts, CaCl2*2H20 and MgCl2*6H20, was taken into account when adding salt to the leachant. The CFA was then added to the leachant. The mixed samples were then rotated for 24 hours in a TCLP tumbler (Environmental Express, Charleston, SC). Upon completion of rotation, the ph of the samples were immediately measured. Next, the solids and the liquids were separated utilizing 0.7 µm pre-acid washed TCLP filters from Environmental Express. The filtrate was collected in a 250 ml HDPE bottle and sent for analysis. 2.3 Sample Analysis The metal content of the sodium chloride, calcium chloride, and magnesium chloride was analyzed by dissolving the salt followed by dilution and analysis per USEPA Method 6020a [9]. The CFA was analyzed by completely digesting the solid and analyzing the resulting liquid per the requirements of USEPA Method 6020a. The leachate samples were also digested and analyzed per the requirements of USEPA Method 6020a. Because salt increases detection limits of the inductively coupled plasma-mass spectrometer (ICP-MS), concentrations below the reporting limit (RL) down to the method detection limit (MDL) were included in the results of the project.

5 Table 1. Leachant Salt Concentrations. Sample Total Salt Concentration in Leachant (M) Chloride Concentration in Leachant (M) CFA, No Salt 0 0 NaCl Level NaCl Level NaCl Level CaCl2 Level CaCl2 Level CaCl2 Level MgCl2 Level MgCl2 Level Results and Discussion 3.1 Metal Analysis of Solids Table 2 shows the metal analysis for the CFA, sodium chloride, calcium chloride, and magnesium chloride in the project. The results show that the sodium chloride is contaminated with some iron and potassium. The calcium chloride is contaminated with some arsenic, barium, lead, magnesium, potassium, sodium, and tin. The magnesium chloride is contaminated with barium, calcium, iron, nickel, sodium, and tin. These trace concentrations were taken into account when evaluating metal leaching from the CFA.

6 Table 2. Metal analysis of solids in project. CFA NaCl CaCl2 MgCl2 Metal (mg/kg dry) (mg/kg dry) (mg/kg dry) (mg/kg dry) Aluminum 29,20 < MDL < MDL < MDL Antimony 1.2 < MDL < MDL < MDL Arsenic 25.6 < MDL 0.1 < MDL Barium 32 < MDL 10.8 Beryllium 6.3 < MDL < MDL < MDL Cadmium 0.8 < MDL < MDL < MDL Calcium 15,60 < MDL 352, Chromium 70.9 < MDL < MDL < MDL Cobalt 24.6 < MDL < MDL < MDL Copper 36.7 < MDL < MDL < MDL Iron 28, < MDL Lead 30.8 < MDL 0.1 < MDL Magnesium 1,71 < MDL ,00 Manganese 7 < MDL < MDL < MDL Molybdenum 24.3 < MDL < MDL < MDL Nickel 29.0 < MDL < MDL 0.3 Potassium 2, < MDL Selenium 5.5 < MDL < MDL < MDL Silver 0.1 < MDL < MDL < MDL Sodium 1,40 354, Thallium 0.8 < MDL < MDL < MDL Tin 3.2 < MDL 0.9 Titanium 91 < MDL < MDL < MDL Zinc 52.8 < MDL < MDL < MDL

7 3.2 Final Leachate ph Table 3 shows the final leachate ph results for the samples in the project. All but five samples were within the desired range of ±0.5 of the target final ph per USEPA Method All but one of the samples outside of the desired range are for target ph 10 samples with high magnesium and calcium chloride concentrations. The leaching procedure for these samples was repeated three times, but the final ph of the leachant was always lower than the target range. The reason that these four samples were outside of the desired range could be as simple as difficulty in measuring the ph at very high salt concentration. In addition, some precipitates may have formed due to the high calcium and magnesium concentrations. The formation of these precipitates may have affected the final ph of the leachant. It was decided to include the samples in the results since the ph values were close to the desired range. Sample Table 3. Measured Final Leachate ph. Target ph, Measured Final ph Target ph 7.0, Measured Final ph Target ph 1, Measured Final ph CFA, No Salt NaCl Level NaCl Level NaCl Level CaCl2 Level CaCl2 Level CaCl2 Level MgCl2 Level MgCl2 Level Metal Leaching Results The portion of a metal leached from CFA was calculated using Equation No. 1: % 100 MCFA is mass of the total metal present in the CFA (20 grams CFA x Metal Concentration in CFA = MCFA). MD is the mass of metal detected in the leachant (Concentration Detected

8 *0.2L = MD). MS is the metal mass contributed to the leachant from the salt due to contamination (Salt Metal Concentration * Mass of Salt Added to Leachant = MS). The following metals were monitored for the project: aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, potassium, selenium, silver, thallium, tin, titanium, and zinc. The results for antimony, arsenic, barium, cadmium, chromium, lead, selenium, and silver are presented in this paper. Antimony Figures 1-4 show the antimony leaching results. A clear trend of increased leaching with increasing salt content is observed for target ph and 7.0. The increase in leaching for the target ph samples is particularly high. The target ph 1 samples also show increasing leaching for the calcium and magnesium chloride samples, but not for sodium chloride. The mechanism for the increased leaching could be chloride complexation and/or cation exchange. Certain oxidation states of antimony have been shown to form complexes with chloride [10]. The formation of these complexes would increase the mobility and leaching of antimony from the CFA. However, differences in leaching appear to exist based on the cation (sodium, calcium, or magnesium) Figure 1. Antimony Leaching Results versus Final Extractant ph (No Salt Addition). ph

9 Figure 2. Antimony Leaching Results for Target ph = Magnesium Chloride Figure 3. Antimony Leaching Results for Target ph = Magnesium Chloride Magnesium Chloride Figure 4. Antimony Leaching Results for Target ph = 1.

10 Arsenic Figures 5-8 show the arsenic leaching results. The results show increasing leaching with the addition of all salts for target ph and 1. For target ph 7.0, an increase in leaching is shown for sodium and magnesium chloride, but not for calcium chloride. The increased leaching of arsenic is likely explained by arsenic - chloride complexation increasing the mobility of arsenic. The results show that calcium chloride decreases arsenic leaching at ph 7.0. This situation is likely the result of the precipitation of calcium arsenate compounds in this ph range Figure 5. Arsenic Leaching Results versus Final Extractant ph (No Salt Addition). ph

11 Magnesium Chloride Magnesium Chloride Figure 6. Arsenic Leaching Results for Target ph = Magnesium Chloride Figure 7. Arsenic Leaching Results for Target ph = Figure 8. Arsenic Leaching Results for Target ph = 1.

12 Barium Figures 9-12 show the barium leaching results. The results show a trend of increasing barium leaching in the presence of all salts for all target ph values except for magnesium chloride at target ph 7.0. The likely mechanism for barium leaching is cation exchange. Calcium obviously has the strongest affinity to exchange with barium. Chloride complexation may also play some role barium leaching Figure 9. Barium Leaching Results versus Final Extractant ph (No Salt Addition). ph

13 Figure 10. Barium Leaching Results for Target ph = Figure 11. Barium Leaching Results for Target ph = Figure 12. Barium Leaching Results for Target ph = 1.

14 Cadmium Figures show the cadmium leaching results. A large increase in leaching is seen with increasing concentrations of all salts at target ph 7.0. No increase in leaching is seen at target ph. At target ph 1, no increase in leaching is seen except at very high chloride concentrations. Cadmium is very soluble at low ph [11, 12]; hence, all of the available cadmium will readily leach at lower phs anyway so the addition of salt would not have an impact as shown in Figure 10. At high ph values, cadmium forms very insoluble cadmium hydroxide. Hence, there is very little leaching of cadmium at ph 10 except at very high concentrations of chloride concentrations. At target ph 7.0, cadmium is not strongly soluble or insoluble; hence its solubility can be influenced by salt addition. The increase in leaching is likely due to either chloride complexation or cation exchange [6]. Figure 11 provides support for chloride complextation since the cadmium leaching closely tracks chloride concentration regardless of cation (sodium, calcium, or magnesium) Figure 13. Cadmium Leaching Results versus Final Extractant ph (No Salt Addition). ph

15 Figure 14. Cadmium Leaching Results for Target ph = Figure 15. Cadmium Leaching Results for Target ph = Figure 16. Cadmium Leaching Results for Target ph = 1.

16 Chromium Figures show the chromium leaching results. The results show a slight increase in leaching with increasing salt content for all cations at target ph. At target ph 7.0, an increase in leaching is seen with increasing calcium and magnesium chloride content. The increase is much higher for calcium chloride versus magnesium chloride. At target ph 1, a large increase in leaching is seen with increasing calcium chloride content. At the same target ph, a slight increase in leaching is seen with sodium chloride addition and a decrease in leaching is seen with magnesium chloride addition. The highest chromium leaching occurs at low and high ph values. It appears that the chromium leaching increases tracks chloride concentration. Hence, chloride complexation may play a role in chromium mobilization as seen in a previous study [7] Figure 17. Chromium Leaching Results versus Final Extractant ph (No Salt Addition). ph

17 Figure 18. Chromium Leaching Results for Target ph = Figure 19. Chromium Leaching Results for Target ph = Figure 20. Chromium Leaching Results for Target ph = 1.

18 Lead Figures show the lead leaching results. The results show a large increase in leaching with increasing concentration of all salts at target ph. At target ph 7.0, an increase in leaching is only seen at the highest chloride concentrations. A decrease in leaching is seen with increasing salt concentrations at target ph 1. The likely mechanism for increased lead leaching at target ph and 7.0 is the formation of soluble lead chloride complexes (PbCl +, PbCl3 -, PbCl4 2- ). These soluble complexes have been shown to form in concentrated magnesium and calcium solutions [13]. Figure 16 shows that the increase in lead leaching closely tracks the chloride concentration regardless of cation. The leaching impact is obviously not as strong for target ph 7.0. The mechanism for the decrease in leaching at target 1 ph is the formation of insoluble lead chloride (PbCl2) at that ph. All of the salts enhance the formation of this precipitate Figure 21. Lead Leaching Results versus Final Extractant ph (No Salt Addition). ph

19 Figure 22. Lead Leaching Results for Target ph = Figure 23. Lead Leaching Results for Target ph = Figure 24. Lead Leaching Results for Target ph = 1.

20 Selenium Figures show the selenium leaching results. The results show that increasing salt concentrations do not have a large impact on selenium leaching ph Figure 25. Selenium Leaching Results versus Final Extractant ph (No Salt Addition).

21 Figure 26. Selenium Leaching Results for Target ph = Figure 27. Selenium Leaching Results for Target ph = Figure 28. Selenium Leaching Results for Target ph = 1.

22 Silver Figures show the silver leaching results. The silver results mirror the lead results. Increased silver leaching is seen with increasing chloride concentration at target ph and 7.0. The increase in silver leaching is due to the formation of soluble silver chloride complexes. In fact, the use of concentrated magnesium and calcium chloride solutions has been proposed for mining silver and lead [13]. As with lead, silver leaching is not increased at the high target ph. This situation is likely due to the formation of insoluble silver chloride Figure 29. Silver Leaching Results versus Final Extractant ph (No Salt Addition). ph

23 Figure 30. Silver Leaching Results for Target ph = Figure 31. Silver Leaching Results for Target ph = Figure 32. Silver Leaching Results for Target ph =

24 4. Conclusions The presence of salt increased leaching of the following metals from CFA: Antimony increased leaching with all salt additions and target ph values except for sodium chloride at target ph 1. Arsenic - increased leaching with all salt additions and target ph values except for calcium chloride at target ph 7.0. Barium increased leaching with all salt additions at target ph, calcium and sodium chloride addition at target ph 7.0, and calcium chloride addition at ph 1. Cadmium increased leaching with all salt additions at target ph 7.0 and for calcium chloride addition at target ph 1. Chromium increased leaching with all salt additions at target ph, calcium and magnesium chloride addition at target ph 7.0, and calcium chloride addition at target ph 1. Lead increased leaching with all salt additions at target ph and calcium chloride addition at target ph 7.0. Silver increased leaching with all salt additions at target ph and sodium and calcium chloride at ph 7.0. Selenium leaching did not show a discernable trend in the presence of increasing salt concentrations. However, more research is needed on selenium leaching in high salt environments. 5. Acknowledgements This project is funded by Southern Company Services. Development of the project is especially noted from Mr. Kirk Ellison of Southern Company. Technical assistance and technical consultation from Dr. Ching-Hua Huang of the Georgia Institute of Technology is also especially noted. 6. References 1. Steam Electric Power Generation Point Source Category: Final Detailed Study Report, USEPA, Editor. 2009: Washington, DC. 2. Huang, Y.H., P. K. Peddi, H. Zeng, C. Tang and X. Teng, Pilot-Scale Demonstration of the Hybrid Zero-Valent Iron Process for Treating Flue-Gas-Desulfurization Wastewater: Part I. Water Science & Technology, : p Entrekin, S., M. Evans-White, B. Johnson, and E. Hagenbuch, Rapid Expansion of Natural Gas Development Poses a Threat to Surface Waters. Frontiers in Ecology and the Environment. 9(9): p Shaping Proposed Changes to Pennsylvania s Total Dissolved Solids Standard. 2009, Penn State University - College of Agricultural Sciences, Agricultural Research and Cooperative Extension. 5. Kargbo, D.M., R. G. Wilhelm, and D. J. Campbell, Natural Gas Plays in the Marcellus Shale: Challenges and Potential Opportunities. Environmental Science & Technology, : p

25 6. Nelson, S.S., D. R. Yongek and M. E. Barber, Effects of Road Salts on Heavy Metal Mobility in Two Eastern Washington Soils. Journal of Environmental Engineering, : p Amrheln, C., J. E. Strong, and P. A. Mosher, Effect of Deicing Salts on Metal and Organic Matter Mobilization in Roadside Soils. Environmental Science & Technology, : p USEPA, Method Liquid-Solid Partitioning as a Function of Extract ph Using a Paralell Batch Extraction Procedure. 2012, USEPA: USEPA. 9. USEPA, Method 6020a - Inductively Coupled Plasma Mass Spectrometry. 2007, USEPA. 10. Johnson, C.A., H. Moench, P. Wersin, P. Kugler, and C. Wenger, Solubility of Antimony and Other Elements in Samples Taken from Shooting Ranges. Journal of Environmental Quality, : p Akhter, H., L.G. Butler, S. Branz, F.K. Cartledge, and M.E. Tittlebaum, Immobilization of As, Cd, Cr and Pb-Containing Soils by Using Cement or Pozzolanic Fixing Agents. Journal of Hazardous Materials, : p Cartiedge, F.K., L. G. Butler, D. Chalasani, H. C. Eaton, F. P. Frey, E. Herrera, M. E. Tittlebaum, and S. Yang, Immobilization Mechanisms in Solidification/Stabilization of Cd and Pb Salts Using Portland Cement Fixing Agents. Environmental Science & Technology, : p Sinadinovic, D., G. Kamberovic, and A. Sutic, Leaching Kinetics of Lead from Lead ( II) Sulphate in Aqueous and Solutions. Hydrometallurgy, : p