The Effects of Oxidant (Nitrate) on Trichloroethylene Degradation by Granular Iron

Transcription

1 The Effects of Oxidant (Nitrate) on Trichloroethylene Degradation by Granular Iron WaterTech 2017 June Lu, SNC-Lavalin Sung-Wook Jeen, Chonbuk National University Lai Gui, Health Canada Robert W. Gillham, University of Waterloo

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3 Background

4 Background The effects of co-existing oxidants (nitrate, O 2, Cr 6+, etc.) on long-term performance of PRBs (permeable reactive barriers) include: Interference with TCE degradation Competition for electrons Changes in corrosion potential (OCP) and ph Formation of iron oxide film Nitrate is often found in groundwater as a co-contaminant of TCE 4

5 Background Iron surface film Double layer of oxide inner layer Fe 3 O Iron 4 outer layer Fe 2 O 3 Outer layer can be removed by auto-reduction Fe 2 O 3 + Fe 0 + 6H + 3Fe H 2 O reductive dissolution 3 Fe 2 O H e- 2 Fe 3 O 4 + H 2 O reductive conversion 5

6 Background In the presence of nitrate A decrease in TCE degradation rates was observed Two mechanisms were proposed by previous studies Formation of passive oxide film as a consequence of an increase in corrosion potential Competition between TCE and nitrate Ritter et al. 2003, Farrell et al. 2000, Schlicker et al

7 Column setup H Reference electrodes L Influent G F E D C B A Iron Effluent Electrical connector Peristaltic pump Solution Reservoir 7

8 Results and Discussion Reaction Kinetics 1.TCE reduction Effective half-life t 1 2 ln(2) Co ln( ) Ce 1. Profile shifted up 2. No longer 1 st order Increased from 2.9 hr to 116 hr after 160 PV 8

9 TCE reduction intermediates, products, and mass balance 9

10 Nitrate reduction Similar trends as TCE reduction Effective half-life increased from 8.0 PV to PV 10

11 Nitrate reduction products, and mass balance 11

12 Results and Discussion Corrosion potential and changes in surface film Corrosion potential changes 12

13 Corrosion potential changes (cont.) NO 3 - conc. (mg/l) Before NO 3 - addition Corrosion potential * (mv) After NO 3 - addition Changed * Measurements were taken from port L 13

14 Intensity / arb. unit Iron surface film changes Dry material and after running Millipore water Green rusts Magnetite Carbon Graphite Millipore Water Hematite Dry Iron Raman Shift / cm -1 14

15 Intensity / arb. unit Iron surface film changes after receiving 150 PV of TCE and nitrate Hematite Magnetite Green rust disappeared Fe(OH) 3 formed Raman Shift / cm -1 15

16 Experimental Results Influent NO. Fe 2+, Effective Fe 3+ Fe 2+ / Fe 3+ Half-life (hr) OCP (mv) at port L Millipore water mg/l TCE + Millipore mg/l TCE + Millipore mg/l NO (80 PV)

17 Results and Discussion Recovery of iron reactivity 17

18 Recovery of Corrosion Potential 18

19 Intensity / arb. unit Iron surface film changes after receiving ca. 223 PV TCE after nitrate removal Magnetite Effluent Green rusts Influent Raman Shift / cm -1 19

20 Experimental Results Influent NO. Fe 2+, Fe 2+ / Fe 3+ Fe 3+ Effective Half-life (hr) OCP (mv) at port L Millipore water mg/l TCE + Millipore 10 mg/l TCE + Millipore mg/l NO 3-10 mg/l TCE + Millipore After NO 3 - removal (80 PV) (90 PV)

21 Conclusions The presence of nitrate causes Decrease in TCE reduction rate (same as Nitrate) Formation of passive iron oxide film (confirmed by Raman measurements) Higher nitrate concentration greater potential change Changes in iron surface condition and loss of reactivity due to nitrate is a reversible process Concurrent adverse effect of both TCE and nitrate degradation suggests that competition between nitrate and TCE for available electrons on the iron surface may not be the main reason for the decline in iron reactivity towards TCE. 21

22 Implications The findings of this study suggest that the use of an iron PRB for remediation of groundwater containing high levels of nitrate, is not suitable due to its adverse effects on iron corrosion potentials and surface film formation. 22

23 Implications At low nitrate levels or for seasonal nitrate plumes, the effect of nitrate should be taken into account for PRB design. A thicker wall may be required to ensure satisfactory TCE removal. Iron Permeable Reactive barrier Nitrate Plume Clean Groundwater Modified environmetal technologies inc. 23

24 Questions

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