Chemical Compatibility of a Polymer-Modified GCL

Transcription

1 Chemical Compatibility of a Polymer-Modified GCL SWANA NW SYMPOSIUM PRESENTED BY: Melody A. Adams May 1, 2015

2 INTRODUCTION Waste product at a mine site regulated under EPA Subtitle D regulations Waste source was filter cake product from the treatment of process water, surface water and groundwater at the facility Airspace was critical so GCL was selected as a replacement for the compacted soil liner Concern that filter cake contained elements that could negatively impact performance of GCL Worked with manufacturer (CETCO) to find a solution

3 GCL SELECTION GCLs containing sodium bentonite are routinely utilized in a variety of containment applications. When in contact with a compatible solution, bentonite hydrates and swells, producing a uniform layer with extremely low hydraulic conductivity (on the order of 10-9 cm/sec). Hydraulic performance can be impacted by metal salts in leachate such as: Calcium and magnesium High ionic strength solutions

4 GCL SELECTION How is performance impacted? In certain aggressive chemical environments, the interlayer sodium ions in the bentonite can be replaced with cations dissolved in the water resulting in less bentonite swell, higher porosity, and increased GCL hydraulic conductivity. High concentrations of calcium and magnesium, as well as high ionic strength solutions are common sources of compatibility problems. Initial TCLP testing on filter cake indicated presence of these constituents.

5 COMPATIBILITY ANALYSIS A synthetic leachate was prepared from the filter cake and found that concentration of concerning elements were not compatible with standard bentonite Parameter Result Calcium 700 mg/l Magnesium 330 mg/l Potassium 32 mg/l Sodium 64 mg/l Sulfur 1,025 mg/l Ionic Strength 0.13 M RMD 0.02 M 1/2 ph 8

6 COMPATIBILITY ANALYSIS The method devised by Kolstad et al. (2004 and 2006) was used as a first tier analysis. The following formula incorporates the ionic strength (I) and the RMD to estimate a long-term GCL hydraulic conductivity. Log K c /Log K DI = I RMD I 2 RMD where: I = ionic strength (Molarity [M]) of the site-specific leachate = 0.13 RMD = ratio of monovalent cation concentration to the square root of the divalent cation concentration (M 1/2 ) in the site-specific leachate = 0.02 [Na RMD = + + K + ] [Ca 2+ + Mg 2+ ] K DI = GCL hydraulic conductivity with deionized water (cm/sec). K c = GCL hydraulic conductivity when hydrated and permeated with site-specific leachate (cm/sec) = 2.7x10-8 cm/sec.

7 CHEMICAL COMPATIBILITY Blue shaded area represents water chemistry where standard sodium bentonite based GCL performs well. Outside the blue is where performance declines Data on site specific leachate fell outside the blue area Bentonite, K 10-8 cm/s Data points where Bentonite k 10-8 cm/s. Project Data Point

8 COMPATIBILITY ANALYSIS The 1 st tier of testing indicated that bentonite may experience decreased performance with the site leachate. The 2 nd tier of testing included index tests with site leachate with both bentonite and polymer-modified bentonite. Select polymers can help reduce the effects of cation exchange and elevated ionic strength by swelling, filling pore space, and partly offsetting the reduced sodium bentonite swell.

9 MATERIALS Bentonite Polymer

10 BENTONITE+POLYMER

11 INDEX TESTS Fluid loss (left) and free swell (right) 100psi (2 g of clay) FL 18 ml (with clean water) FS 24 ml/2 g (with clean water)

12 INDEX TEST RESULTS Index Test Values With Project Representative Leachate Untreated Polymer-Modified Required Bentonite Index Test Values With DI Water Index Properties Bentonite Bentonite Untreated Bentonite Fluid Loss ml 35.4 ml 18 ml max. Free Swell 12 ml/2 grams 19.5 ml/2 grams 24 ml/2 grams min. While not as good as bentonite in clean water, the polymer-modified bentonite indicated better performance with the site leachate. Fluid loss was 290% lower and free swell was 62% higher.

13 TIER 3 TESTING Index tests provided a good indication that the polymermodified GCL would demonstrate better performance with the site leachate (target 5x10-9 cm/sec) as compared to bentonite. The final tier of testing included a hydraulic conductivity test with the site leachate using a polymer-modified, sodium bentonite GCL. Tests were to run for a minimum of 2 pore volumes.

14 HYDRAULIC CONDUCTIVITY TEST

15 SUMMARY OF TESTING Over 1,200 hours of testing were required to reach two pore volumes of flow through the GCL specimen. Hydraulic Conductivity = 7.5x10-10 cm/sec. Drawback of a low permeability value is that it requires more time to reach chemical equilibrium, or even 2 pore volumes of flow. Recommendation: start compatibility testing as soon as possible.

16 PROJECT SPECIFICATIONS Project specifications were written around a performance value with the site leachate. Required the manufacturer to meet a hydraulic conductivity value, with a minimum number of pore volumes, with the specific site chemistry. Allowed multiple GCL manufactures to bid project.

17 CONCLUSION While a standard sodium bentonite-based GCL is a chemically compatible barrier for many common waste streams, certain chemical environments can negatively impact the hydraulic performance of sodium bentonite. A tiered approach was used to demonstrate GCL chemical compatibility and hydraulic performance. Project specifications were written around the project-specific testing.

18 Questions are welcome. Thank you for your interest.