Biochar-Mediated Reduction of Nitro Herbicides and Explosives

Seok-Young Oh1, Jong-Gil Son1, Pei C. Chiu2

1Department of Civil and Environmental EngineeringUniversity of Ulsan, South Korea

2Department of Civil and Environmental EngineeringUniversity of Delaware, U.S.A.

2012 US Biochar ConferenceSonoma State Univ., July 31, 2012

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)

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.

BC-mediated reduction of 2,4-dinitrotoluene by dithiothreitol

Time (day)

0 5 10 15 20C

on

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(m

M)

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DNT4A2NT2A4NTDATDNT (control w/ soot only)

Time (day)

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)

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DNT4A2NT2A4NTDATDNT (control w/ graphite only)DNT (control w/ DTT only)

(Oh and Chiu, 2009, ES&T)

Graphite (99.9% C) Soot (~90% C, 10% O)

Conceptual schematic diagram for BC-mediated reduction of NACs by

reductants

(Oh et al., 2012, EG&H )

Possible Hypothesis on Mechanisms of BC-Mediated Reduction

1. 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).

2. Surface functional groups (i.e., quinone) of BC may account for BC-mediated reduction (Kemper et al., 2008).

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)

Other indirect benefits from biochar?

How about sorbent and electron transfer catalyst for redox-sensitive contaminants in soils and sediments?

Objective

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

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.

Materials and Chemicals

• Contaminants: Nitro herbicides: pedimethalin, trifluralinNitro explosives: 2,4-dinitrotoluene (DNT),

hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

• Black carbon1.Poultry litter biochar (37.1% C, S.A.: 2.3 m2/g. at 400oC for 4 h)2.Biosolid biochar (31.8% C, S.A.: 2.0 m2/g, at 400oC for 8 h)3.Graphite powder (99.9% C, S.A.: 13.6 m2/g)4.GAC (86.1% C, S.A.: 738 m2/g)

• Reductant: dithiothreitol (Eh= -0.33V at pH 7)

Batch Experiments

- At 25oC-150 rpm shaking- In a glove box

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

Mininert™ valve

Two control experiments- sorption control: without reductant- reduction control: without black carbon

Properties of Biochar

Types of biochar

Surface area (m2/g)

Elemental contents* (%)

C H O N

Poultry litter (PL)

2.3 37.1 2.3 13.9 5.2

Biosolids (BS) 19.5 31.8 3.4 19.4 4.4

Types of biochar

Al B Ca Cu Fe K Mg Mn P S Zn

Poultry litter (PL)

0.48 0.01 4.91 0.08 0.24 5.32 1.35 0.09 2.71 1.10 0.10

Biosolids (BS) 0.65 0.01 8.14 0.27 0.43 1.62 1.60 0.40 1.02 0.59 0.65

SEM image and XRD pattern of biochar

PL biochar

BS biochar

2

15 20 25 30 35 40 45 50 55 60

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nsity

(C

PS

)

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2

15 20 25 30 35 40 45 50 55 60

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(C

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0

10000

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2theta = 26.6 (d=0.335 nm) graphite

Reduction of DNT by dithiothreitolin the presence of PL biochar

Time (hr)

0 50 100 150 200 250

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cent

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DNT4A2NT2A4NTDAT DNT, reductant-free control (biochar only)DNT, biochar-free control (dithiothreitol only)

Reduction of RDX by dithiothreitolin the presence of PL biochar

Time (hr)

0 100 200 300 400 500

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cent

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n (m

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RDXFormaldehydeRDX, reductant-free control (biochar only)RDX, biochar-free control (dithiothreitol only)

Reduction of pendimethalin by dithiothreitol in the presence of BC

Time (min)

0 20 40 60 80 100 120

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of P

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(C

/C0)

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1.0

Graphite 0.1g DTT 0.12g (=0.778 mol)Graphite 0.1g + DTT 0.12g

Time (min)

0 20 40 60 80 100 120

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GAC 0.05gDTT 0.12g (=0.778 mol)GAC 0.05g + DTT 0.12g

GACGraphite

Time (min)

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lativ

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PL biochar 1gDTT 0.12g (=0.778 mol)PL biochar 1g + DTT 0.12g

Time (min)

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(C

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BS biochar 1gDTT 0.12g (=0.778 mol)BS biochar 1g + DTT 0.12g

BS biocharPL biochar

Reduction of trifluralin by dithiothreitol in the presence of BC

Time (min)

0 20 40 60 80 100 120

Re

lativ

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on

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of T

riflu

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(C

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Graphite 0.1gDTT 0.12g (=0.778 mol)Graphite 0.1g + DTT 0.12g

Time (min)

0 20 40 60 80 100 120

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ativ

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ncen

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ion

of T

riflu

ralin

(C

/C0)

0.0

0.2

0.4

0.6

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1.0

GAC 0.05gDTT 0.12g (=0.778 mol)GAC 0.05g + DTT 0.12g

GACGraphite

Time (min)

0 20 40 60 80 100 120

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lativ

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of T

riflu

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(C

/C0)

0.0

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1.0

PL biochar 1gDTT 0.12g (=0.778 mol)PL biochar 1g + DTT 0.12g

Time (min)

0 20 40 60 80 100 120

Rel

ativ

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of T

riflu

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(C

/C0)

0.0

0.2

0.4

0.6

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1.0

BS biochar 1gDTT 0.12g (=0.778 mol)BS biochar 1g + DTT 0.12g

BS biocharPL biochar

Product Identification

Trifluralin

A cyclization product:2-ethyl-7-nitro-1-propyl-5-(trifluoromethyl)-1H-benzimidazole

Mass spectrum of a cyclization product (93.9% match)

Gas chromatogram of of hexane extract of trifluralin solution with dithiothreitol and PL biochar after reaction for 120 min

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.

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.

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.