Managing Geochemistry of Multiple Coal. Ash Metals of Potential Concern: World of Coal Ash Practical Geochemical Considerations

Transcription

1 2013 World of Coal Ash (WOCA) Conference - April 22-25, 2013 in Lexington, KY Managing Geochemistry of Multiple Coal Ash Metals of Potential Concern: Practical Geochemical Considerations Jeff Gillow Geochemist ARCADIS U.S., Inc. World of Coal Ash 2013 Imagine 1 13 May the 2013 result 2012 ARCADIS

2 2 Outline Uncertain regulatory context; review of new leach protocols and interpretation of resultant data Understanding geochemical controls on leaching (focus on oxyanions) Valuable insight may be gained from an evaluation of the geochemistry of leachable metals and metalloids Optimize management for new facilities, strategies for existing facilities

3 3 Regulatory Context for CCP and New Leach Protocols

4 4 Co-proposals in 40 CFR 257 Two options: Regulate CCP under RCRA as Subtitle C waste (hazardous waste) when destined for disposal in landfills or surface impoundments. Leave the Bevill Regulatory Determinations in place but set Federal standards for regulation under RCRA subtitle D (issuance of national minimum criteria for landfills and surface impoundments). Under both options, new dam safety requirements will be imposed to address structural integrity. New groundwater monitoring requirements and requirements for groundwater corrective measures No major change to beneficial use cases, but EPA leaves open the possibility of refining certain beneficial uses.

5 5 EPA CCP Characterization Research Initiated in 2005: Evaluate leachability of metals in CCRs Develop (purportedly) more reliable framework vs. TCLP Develop holistic approach to lifecycle management Four reports Report 1: Feb 2006 Impact of emission controls on Hg, As, Se Report 2: July 2008 Expanded leachability assessment Report 3: 2009 Multi-site evaluation for more representative profiling (coal type, emission controls, etc.) Report 4: 2010 Decision Support Tool for CCR assessment & management

6 6 CCP and Trace Element Geochemistry

7 Leached concentration (mg/l) 7 ~alk, low-ca ~alk, ~Ca As B V Cr Mo Ca B V As Cr Mo Ca alk, Ca-rich B V Cr Ca As Mo Leaching behavior of arsenic, boron and other trace elements varies depending upon chemical characteristics of the fly ash. from Izquierdo and Querol, 2012

8 mg/l Trace Element Leaching (ppm levels) Ash impoundments As, B, Cr, Mo, Se ph General trends in leaching behavior from CCP (mg/l) [from Izquierdo and Querol, 2012] 2011 ARCADIS 8 Regulatory Standards (ppb levels) Constituent Federal MCL (ug/l) Cr 50 As 10 Pb 15 Se 50 F 4000 Mo 40 (HA) Cu 1.3 Sulfate 250

9 9 Cation vs. Oxyanion Sorption Behavior Oxyanions: negatively charged mobile at neutral to alkaline ph Chromate (CrO 2-4 ) molybdate (MoO4 2- ), selenate (SeO 2-4 ) Mo sorption ligand-like Stollenwerk, 1995 Cations: positively charged sorb readily at neutral ph cadmium, cobalt, strontium, copper, nickel, zinc Cd sorption cation Dzombak & Morel, ARCADIS

10 ph - Point of Zero Charge (below this ph, surfaces are +, above they are - ) 10

11 11 Common Thread: Electro-negativity enhances mobility of oxyanions SbO 3 - H 2 AsO 4 - HAsO 4 2- B(OH) 3 0 B(OH) 4 - HCrO 4 - CrO 4 2- MoO 4 2- HSeO 3 - SeO 4 2-

12 Arsenic Chemistry Arsenic is soluble unless controlled by an incorporating mineral phase. Sensitive to: ph (surface charge & stability of host minerals) Ionic competition (competing ligands like HCO - 3 and HPO 2-4 ) Soluble sulfide (HS - ) abundance (complexation) As(V) As(III) 12 MCL = 10 µg/l

13 Dissolved Arsenic (ug/l) Zone of highest concentrations Arsenic Concentrations as a Function of Biogeochemical Environment Sulfate reduction: lower concentrations Aerobic environment: solubility controlled Platinum Electrode (ORP/mV)

14 Boron: Chemistry The typical concentration of boron in soil is about 30 mg/kg while the typical concentration in coal fly ash is about 600 mg/kg. Typical concentrations of boron in CCP leachate are less than 10 mg/l, although concentrations higher than 100 mg/l have been observed. Boron is mobile, it does not readily precipitate, and has a relatively low affinity for sorption, particularly in sand environments. Media Range (mg/kg) Typical (mg/kg) Earth s lithosphere 5 to Soil 10 to Coal 1 to Fly Ash 25 to 6, Bottom Ash 5 to FGD Solids 25 to 2, DWEL = 7mg/L Other standards: mg/l

15 Boron: ph of Maximum Sorption on Soil (Sposito, 1989) 15

16 Eh (volts) Selenium: Chemistry HSeO 4-1 H 2 SeO 3.5 HSeO 3 - SeO Se(black) SeO 3 -- FeSe 2.5 FeSe 25 C ph JGillow Wed Jul MCL = 50 µg/l Diagram SeO3 --, T = 25 C, P = bars, a [main] = 10 1, a [H2 O] = 1, a [Fe++ ] = 10 1 Concentration of selenium in CCP leachate observed up to 400 ug/l.

17 Insight from New Leach Tests? 17

18 18 Proposed New Leach Test Protocols EPA Motivation TCLP includes an organic acid (acetic acid) to simulate acidic conditions created by microbial degradation of putrescible waste. SPLP is relevant for the leaching of trace metals by acidic rainwater (ph 4.2 east of the Mississippi; ph 5 west of the Mississippi). Conditions in a CCR landfill can be acidic or alkaline, depending upon the properties of the source coal, and pollution control equipment used in the plant. n samples S 1 A S 2 S n B n n chemical analyses L A L B L n

19 19 EPA Motivation CCR landfill ph can vary (and trace metals are not always more soluble under acidic ph (arsenic, boron, selenium and thallium can be more soluble under alkaline ph conditions) Liquid content of the impoundment/landfill may vary New extraction procedures examine multiple phs (2-13) and liquidsolid ratios. EPA Methods and Pre-Methods Method 1313 (9/12) - Liquid-Solid Partitioning as a Function of Extract ph using a Parallel Batch Extraction Test Method 1316 (10/12) - Liquid-Solid Partitioning as a Function of Liquid-Solid Ratio Using a Parallel Batch Procedure Draft Methods 1314 and 1315 Monolith or Compacted Granular 1 Sample A 1 L 1 n Leaching Intervals Δt 1 Δt 2 Δt n A 2 A n n analytica l samples L 2 L n Monolithic

20 20 Interpretation of Test Results At this point the use of test results has not been clearly stated by EPA clarification is expected in the final CCP rule. The expectation is that leach testing using new procedures will be required for new and existing ash fills. Recent published work by EPA provides insight into how the test results may be used (Thorneloe, 2010): Through the use of the LEAF test methods, leaching behavior can be more accurately predicted resulting in the calculation of a more reliable source term for use in modeling potential impacts to human health and ecosystems.

21 Boron (mg/l) Boron (mg/l) Arsenic (mg/l) Arsenic (mg/l) Example Results from Method th % 95 th % 10 AaFA - Median 100x HBN 10x HBN HBN 1 100x HBN x HBN 0.01 HBN own ph BFA - Median own ph All Fly Ash - Median All Fly Ash - 95th % 5th and 95th % of ph Health Based Number (HBN) ph L/S (L/kg) 10x HBN 100x HBN x HBN 5 th % 95 th % 100x HBN 10x HBN HBN x HBN HBN ph L/S (L/kg) From Kosson et al., 2011

22 Selenium (mg/l) Selenium (mg/l) x HBN 5 th % 95 th % 10 10x HBN 1 HBN ph From Kosson et al., AaFA - Median own ph BFA - Median own ph All Fly Ash - Median All Fly Ash - 95th % 5th and 95th % of ph Health Based Number (HBN) 10x HBN 100x HBN 100x HBN 10x HBN HBN L/S (L/kg)

23 23 Optimized CCP Impoundment and Landfill Management

24 24 Monitored Natural Attenuation Application to Inorganics (ITRC, 2010)

25 25 Monitored Natural Attenuation Application to Inorganics (ITRC, 2010)

26 26 Considerations for Arsenic Sorption Sufficient iron:arsenic molar ratio Stability factors ph Development of negative surface charge at high ph Loss of ferric iron stability at low ph Competing oxyanions (phosphate, carbonate) Displacement of arsenic Stability under changing redox conditions Reduction of arsenic and dissolution of ferric iron minerals OH As OH OH Oxidizing Environments Sorption Arsenic (V) Arsenic (III) Iron (hydr)oxide [Ferrihydrite]

27 27 Treatability Testing Enhanced Mitigation of CCP Leachate Ferric iron addition to fly ash promotes the formation of freshly precipitated iron oxyhydroxide. The Fe(OH) 3 will sorb and retain boron and other oxyanions. ph, iron:trace element target molar ratio Advective-flow column experiments were used to simulate long-term groundwater leach behavior of fly ash.

28 V1 V3 V5 V7 V9 V11 V13 V15 V17 V19 V21 V23 V25 V27 V29 V31 V33 V35 V37 V39 Arsenic Concentrations (mg/l) V1 V3 V5 V7 V9 V11 V13 V15 V17 V19 V21 V23 V25 V27 V29 V31 V33 V35 V37 V39 ph Influent Water Control Column Medium Dose High Dose Enhanced Sorption by Iron: GW affected by leaching from slag 2 Cumulative Pore Volumes Influent ph was 3 6, with arsenic at 5 ppm. Iron-amended backfill reduced arsenic concentrations to <0.01 mg/l in effluent DL=0.010 by ICP DL =0.002 by ICP-MS DL= by ICP-MS Cumulative Pore Volumes ph settled at 7 to 8 for duration of test (37 pore volumes) 28

29 Boron: Treatment Precipitation Most naturally-occurring boron minerals are alkaline o Requires maintenance of ph >11 Can incorporate in some minerals o Hydrotalcite Sorption and co-precipitation Selective resins (2g/L of resin) Aluminum hydroxide, iron hydroxide, magnesium oxides 29 Resin Iron hydroxide

30 Boron Extracted by TCLP (mg/l) Boron Speciation (Fraction of Species) Test Results Boron Control B(OH) 3 B(OH) 4 - Unit 1 Unit 3 Unit 4 - B(OH) 3 + H 2 O = B(OH) 4 + H + pk a = ph

31 Selenium: Treatment Harnessing iron mineral transformations Treatment to <0.1 µg/l Develop lasting treatment capacity and protection of reduced selenium through the buildup of iron mineral phases Se(VI) Se(0) 31 reactive mineral phases in the aquifer Fe2+ sulfides mackinawite greigite pyrite Iron oxides & oxy/hydroxides Fe3+ silicates, clays IRB re-oxidation Amorphous iron oxy/hydroxide goethite ferryhydrite Fe2+ aq Fe2+ carbonate siderite Fe2+ sorbed to Fe3+ minerals Mixed Fe2+/Fe3+ hydroxide green rust carbonate sulfate conversion Mixed Fe2+/Fe3+ oxide magnetite x y y z x From Fendorf, z z y x Selenium (ug/l) Sulfate (mg/l) Se Sulfate Time (days) relative to first injection Active anaerobic biostimulation with ferrous sulfate addition Demonstration of residual treatment capacity

32 Conclusions New regulatory context may require additional GW monitoring and corrective action Understanding geochemical drivers on trace element leaching from CCP will provide for the implementation of optimized approaches to meet new requirements 32

33 Acknowledgements Co-authors: Chris Lutes, Peter Kariher, and Dave Liles (ARCADIS, Durham, NC) Michael Hay (ARCADIS, Boulder, CO) 33

34 Imagine the result 34

35 ASTM C1308 Overview 35 Approved in 1995 to assess release rates from solid nuclear waste Demonstrate no public exposure to leached radionuclides Department of Energy (DOE) and Nuclear Regulatory Agency (NRC) Hanford Site in Richland, WA (DOE Pacific Northwest National Laboratory) Solid cylinders immersed in leaching solution Synthetic or site groundwater Solution volumes replaced sequentially over 11 days

36 ASTM C1308 Overview 36 Recent use for mining applications Adopted by Nevada for backfill geochemical characterization Successfully used to support permitting for the Barrick Goldstrike Mine Being used for the Nevada Copper Pumpkin Hollow Project NDEP is requiring mining laboratories obtain state certification August 1, 2013 WETLAB (Sparks, Nevada)

37 37 Recent Method Application Permit conditions established Strength requirement Compressive strength of 30 psi Minimum composition requirements At least 3 wt. % suitable binder Over 2 wt. % of the binder must be ordinary Portland cement At least 50 % of the binder must be Portland cement Examples 3% Portland cement- no fly ash required 2% Portland cement- 2% fly ash required Minimum composition is project specific