Waste Green Sands as Reactive Media for PRBs

Transcription

1 Waste Green Sands as Reactive Media for PRBs Craig H. Benson Geo Engineering, University of Wisconsin-Madison Taeyoon Lee Korean Research Institute and Industrial Science & Technology Gerald Eykholt Eykholt Consulting, Madison, WI RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 23

2 What is waste green sand? Granular material used for molds for metal casting A blend of sand, binder, organic additive, water Generated by addition of components creating excess volume (not truly a waste product) Residual iron provides reactivity, organic carbon provides sorptive capacity

3 Typical Composition Bentonite 7% Water 5% Organic 3% Base Sand 85% Waste sands also contain 2-12% iron particles derived from casting process

4 Percent Finer (%) Particle Size Distribution Uniform fine sand with 2-15% fines Foundry Sands from WI, IL, MI, & IN 1.1 Particle Diameter (mm).1

5 Green Sand is Really Black Sand Usually dry (< 5% water content) and easy to handle, but can contain debris

6 Why consider green sand? Can be obtained at no cost. In some cases, transportation cost is provided by foundry Beneficial reuse of industrial byproduct Fosters sustainable development Appears effective (see presentation)

7 Objectives Assess hydraulic conductivity, reactivity, and sorptive capacity of green sands Evaluate long-term reactivity Evaluate potential field scenarios Assess leaching of metals and PAHs (not in this presentation)

8 Materials 12 greens sands from foundries in Wisconsin, Illinois, and Indiana TCE along with vinyl chloride, 1,1-dichloroethylene, trans-1,2-dichloroethylene, and cis-1,2- dichloroethylene Alachlor and acetyl alachlor (both from Monsanto Corporation); metolachlor and MBP (both from Novartis Crop Protection) ZVI particles from Peerless Metal Powders and Abrasives Co. (mean particle size =.7 mm, specific surface area of.87 m 2 /g)

9 Hydraulic Conductivity Hydraulic Conductivity SP SM-SC SP-SM SP Chemical SP SC-SM SP-SM SC-SM.81.5 SC-SM SW-SM SM SP-SM Saturated Hydraulic Conductivity (m/d) Total Organic Carbon (%) USCS Classification Binder Type Green Sand

10 TCE Sorption Isotherms Sorbed Concentration (mg/kg) Sand 1 Sand 2 Sand 3 Sand 4 Sand 5 Sand Equilbrium Concentration (mg/l) Linear within range of concentrations used Intercepts indicate nonlinearity at low concentrations

11 TCE Partition Coefficients vs. TOC 5 2 Partition Coefficient, K (L/kg) p Low Intermediate High Partition Coefficient, K (L/kg) p K p (L/kg) = 4.76 TOC(%) R 2 = Total Organic Carbon, TOC (%) Total Organic Carbon, TOC (%) 4 Linear K p -TOC relationship in intermediate TOC range. K p is approx. 2x higher than expected based on K oc and f oc

12 Batch Reactivity Tests 1. Green Sand Iron Relative Concentration, C/C Test A Test B.2 Test C Test D Test E Test F Time (hr) Conducted on iron extracted from sand Approximately first order Similar rate coefficients as Peerless iron

13 Test Iron Source Initial Conc. (mg/l) Iron Surface Area/Volume (m 2 /L) Dissolved Oxygen (mg/l) NaCl (M) Rate Constant (L/m 2 -hr) Partition Coefficient (L/kg) A Green Sand B Green Sand < C Green Sand D Green Sand < E Green Sand F Green Sand G Peerless Iron H Peerless Iron I Peerless Iron J Peerless Iron

14 Comparison of Normalized Rate Coefficients Apparent First-Order Rate Constant, K obs (1/hr) Test A (Green Sand Iron) Test B (Green Sand Iron) Test C (Green Sand Iron) Test D (Green Sand Iron) Test E (Green Sand Iron) Test F (Green Sand Iron) Test G (Peerless Iron) Test H (Peerless Iron) Test I (Peerless Iron) Test J (Peerless Iron) Slope =.153 (Green Sand Iron) Slope =.172 (Peerless Iron) K obs vs. SSA approximately linear for both green sand iron and Peerless iron Comparable normalized rate coefficients Specific Surface Area of Iron, SSA (m 2 /L)

15 Pump Column Set-Up Effluent Reservoir Sampling port Ceramic disk Reactive Media Glass fiber filter Glass column Influent Reservoir Sampling port

16 1. Typical Breakthrough Curve Normalized TCE Concentration (C/C ) van Genuchten solution Green Sand 12 Dry density = 1.6 Mg/m Seepage Velocity =.86 m/d Dispersivity =.52 m Total Porosity =.39 Effective Porosity =.37 K = 1.9 L/kg p Pore Volumes of Effluent (PVE) 3 Breakthrough data analyzed using van Genuchten solution to ADRE with instantaneous sorption and firstorder reactions Fit by least-squares minimization

17 Results of Column Tests Tests conducted on two sands with very little (<.1%) iron and two sands with moderate to high amounts of iron K SA and K p are comparable to those obtained from batch tests Green Sand Total Porosity (n) Effective Porosity (n e ) n e /n K p (L/kg) K obs (1/hr) SSA (m 2 /L) K SA (L/m 2 -hr)

18 Long-Term Column Tests K SA (L/m 2 -hr) 1-3 Sand 12 Peerless Iron 1-4 Alachlor Modest reduction in K SA (2-3x) over 15 pore volumes Comparable effect on Peerless iron PVE

19 Normalized TCE Concentration (C e /C o ) Seepage Velocity (m/d) Field Scenarios or 1.5 Barrier Thickness (m) 1..5 C = 2 mg/l C = 4 mg/l Based on van Genuchten s steady- state solution PRBs 1 m wide are practical for lower seepage velocities (<.1 m/d) and modest iron contents (>2%) Higher seepage velocities (1 m/d) required a thicker barrier or higher iron content Iron Content (%)

20 Summary Green sands have high sorptive capacity for TCE and chlorinated herbicides (4. L/kg to 5 L/kg). Isotherms are approximately linear and partition coefficient is linearly related to TOC (1 < TOC < 4%). Reactivity of green sand iron determined from batch tests (iron alone) or column tests (in green sand) is comparable to that of Peerless iron. Comparable partition coefficients and rate coefficients obtained using column and laboratory tests. Long-term reactivity appears to be comparable to that of Peerless iron.

21 Acknowledgement Financial support provided by the Wisconsin Department of Natural Resources and the Wisconsin Groundwater Research Advisory Council. Green sands were provided by the participating foundries.