Biochar-Mediated Reduction of Nitro Herbicides and Explosives

1. 2012 US Biochar Conference Sonoma State Univ., July 31, 2012 Biochar-Mediated Reduction of Nitro Herbicides and Explosives Seok-Young Oh1, Jong-Gil Son1, Pei C. Chiu2 1Department of Civil and Environmental Engineering University of Ulsan, South Korea 2Department of Civil and Environmental Engineering University of Delaware, U.S.A.

2. Black Carbon (BC) • Soot, charcoal, char, coke, coal, and kerogen • Composed of polycyclic aromatic carbon (so-called graphene) sheets • Important geosorbent for small molecules, such as PAHs and PCBs • Sorption mechanism: π-π electron donor-acceptor (EDA) interaction (Pignatello et al., 2004) • The basis of a current remediation approach: addition of BC materials as sorbents to sediments to reduce the bioavailability and ecotoxicity of PCBs or PAHs (Cho et al., 2009)

3. Are organic compounds sorbed to BC chemically inert? Not always. Graphene moiety of BC may play a role of electron conductor to mediate reductive transformation of the sorbed redox-sensitive compounds.

4. BC-mediated reduction of 2,4-dinitrotoluene by dithiothreitol Graphite (99.9% C) Soot (~90% C, 10% O) (Oh and Chiu, 2009, ES&T)

5. Conceptual schematic diagram for BC-mediated reduction of NACs by reductants (Oh et al., 2012, EG&H )

6. Possible Hypothesis on Mechanisms of BC-Mediated Reduction Graphitic structure (graphene) of BC may be responsible for BC-mediated reduction: BC is a sorption site as well as electron conductor (Oh and Chiu, 2009). Surface functional groups (i.e., quinone) of BC may account for BC-mediated reduction (Kemper et al., 2008).

7. Biochar • Char derived from pyrolysis of biomass • Carbon sequestration: biochar remains in soils for hundreds or thousands of years • → long-term sink of CO2 • Improve the fertility of soils and biomass production • However, still costly compared to trading CO2 value ($4/ton)

8. Other indirect benefits from biochar? How about sorbent and electron transfer catalyst for redox-sensitive contaminants in soils and sediments?

9. Objective • Examine the roles of biochar both as a sorbent and as a redox mediator, in the transformation of nitro herbicides and explosives

10. Hypothesis • Similar to other BC, biochar can also act as a redox mediator to promote the reductive transformation of organic compounds and impact the fate of redox-sensitive chemicals in the environment.

11. Materials and Chemicals • Contaminants: • Nitro herbicides: pedimethalin, trifluralin • Nitro explosives: 2,4-dinitrotoluene (DNT), • hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) • Black carbon • Poultry litter biochar (37.1% C, S.A.: 2.3 m2/g. at 400oC for 4 h) • Biosolid biochar (31.8% C, S.A.: 2.0 m2/g, at 400oC for 8 h) • Graphite powder (99.9% C, S.A.: 13.6 m2/g) • GAC (86.1% C, S.A.: 738 m2/g) • Reductant: dithiothreitol (Eh= -0.33V at pH 7)

12. - At 25oC • 150 rpm shaking • In a glove box Batch Experiments Mininert™ valve 250-mL amber vial 250 mL of solution (no head space) - N2 purging (30 min) - pH 7.0 (20 mM phosphate) - 120 mg of dithiothreitol - 1% of methanol (due to low solubility) - Pendimethalin = 0.0355 mM - Trifluralin = 0.0686 mM 1 g of biochar 0.1 g of graphite powder 0.05 g of GAC • Two control experiments • sorption control: without reductant • reduction control: without black carbon

13. Properties of Biochar

14. SEM image and XRD pattern of biochar PL biochar 2theta = 26.6 (d=0.335 nm) graphite BS biochar

15. Reduction of DNT by dithiothreitol in the presence of PL biochar

16. Reduction of RDX by dithiothreitol in the presence of PL biochar

17. Reduction of pendimethalin by dithiothreitol in the presence of BC Graphite GAC PL biochar BS biochar

18. Reduction of trifluralin by dithiothreitol in the presence of BC Graphite GAC PL biochar BS biochar

19. Product Identification Trifluralin A cyclization product: 2-ethyl-7-nitro-1-propyl-5-(trifluoromethyl)-1H-benzimidazole Gas chromatogram of of hexane extract of trifluralin solution with dithiothreitol and PL biochar after reaction for 120 min Mass spectrum of a cyclization product (93.9% match)

20. Conclusions • Biochar can catalyze the reduction of DNT and RDX by dithiothreitol. • The reduction of pendimethalin and trifluralin by dithiothreitol can also be enhanced in the presence of biochar.

21. Environmental Implication • BC may be involved in the abiotic natural attenuation of nitro herbicides and explosives in subsurface environments • A potential novel site remediation and/or waste treatment approach: BC and other graphitic materials may be applied to soils or sediments contaminated with nitro herbicides and explosives. • Biochar may play a role of catalyst for nitro herbicides and explosives during biochar sequestration.

22. Acknowledgments • Korea Research Foundation Grant 2009-0064688 • Research Grant from Ulsan Green Environment Center • 2nd stage Brain Korea 21 • Professor Mingxin Guo of Delaware State University for preparing the PL biochar used in this study.