In-situ Immobilization of Mercury in Sediment and Soil by A New Class of Stabilized Iron Sulfide Nanoparticles

Zhong Xiong, Feng He, Don Zhao, Mark O. Barnett, and Willie F. Harper Jr,

Department of Civil Engineering Auburn University, Auburn AL

Mercury (Hg)

The U.S. EPA has identified Hg as one of its twelve priority persistent bio-accumulative toxins (PBTs).

Sources: Fossil fuel, Natural degassing of the Earth, Industrial discharges.

The annual anthropogenic Hg emitted in the U.S. totals 158 metric tons.

Bacteria transform Hg to Methylmercury (CH3Hg+).

Concept of Proposed Technology

Soil /

Sediment

Hg pool

Injection and Controlled Dispersion of FeS Nanoparticles

Hg2+

nHg2+

HgS(s) + Fe2+ Ion Exchange

+ FeS(s)

FeS(s)-Hgn

Sorption

+ FeS(s)

CH3Hg+

Bacteria

In a few days/weeks the nanoparticles will agglomerate and grow to larger flocs (up to sub-mm) or be sorbed to soils/sediment surfaces, losing their mobility but continuing to offer prolonged Hg immobilization capacity.

Why FeS?

Highly stable, i.e. extremely insoluble in water and unavailable to biota; Ksp (FeS) = 8x10-19; innocuous to the environment.

Extremely attractive to Hg ions: FeS(s) + Hg2+ HgS(s) + Fe2+ or FeS(s) + nHg2+ FeS-nHg2+ Ksp (HgS) = 2x10-53 (black) or 2x10-54 (red).

Why Nanoparticles?

Can be easily delivered (e.g. sprayed, injected) to near-surface or subsurface of contaminated soils/sediments.

Can be applied in-situ to cap a site, to build a sorption barrier, to trap or extract Hg in soil or sediment pores.

High surface area, highly reactive, and able to diffuse in soil/sediment pores.

Why Stabilizer?

Control the size (agglomeration) and soil/sediment mobility (viscosity) of the nanoparticles.

Enhance Hg immobilization.

Stabilizers: Polysaccharides (Water-soluble Starch or Cellulose).

Low cost, Environmentally friendly, Effective to stabilize metal nanoparticles, and controlling mobility.

Objectives Develop a new class of stabilized, controllable FeS nanoparticles using lo

w-cost and environmentally friendly polysaccharides such as carboxylmethyl cellulose (CMC) as a stabilizer or size-controller.

Test the feasibility of applying the nanoparticles for in-situ immobilization of Hg in soils and sediments.

Preparation of FeS Nanoparticles

Step 1. Prepare CMC and Fe2+ stock solutions containing 0~0.5% (w/w) of CMC and 0.1-1 M Fe2+.

Step 2. Vary the stabilizer-to-Fe molar ratio and mix CMC-Fe2+ solution under purified N2 gas.

Step 3. Add stoichiometric amount of Na2S solution into the above mixture

and allow for reaction under vacuum and at room temperature.

System under vacuum and mixing

System under vacuum and mixing

Transmission Electron Microscope (TEM) Images of FeS Nanoparticles

(a) Fresh 0.5 g/L FeS without a stabilizer

(b) Fresh 0.5 g/L FeS with 0.2% (w/w) CMC

D =38.5 ± 5.4 nm

Mercury Leaching from a Hg-laden Sediment with or without Treatment of FeS Nanoparticles

FeS/Hg molar ratio

0 5 10 15 20 25

Hg

, g

/L

0

50

100

150

200

250

300

Sediment: Clay LoamHg Content: 177.2 mg/kg

Liquid-to-solid = 10:1FeS Treatment pH: 7Mixing: 45 rpmEquilibrium Time: 1 week

At a FeS/Hg molar ratio of 26.5 in batch tests, only 8.5 µg/L of Hg was leached out in the aqueous phase, a 97% drop compared to that without the treatment.

Zhao, D.; Xiong, Z.; Liu, R.; He, F.; Barnett, M.O.; Harper, W.F. Patent pending, PCT/US07/62985 .

Toxicity Characteristic Leaching Procedure (TCLP) Tests

FeS/Hg molar ratio

0 5 10 15 20 25

Hg

, g

/L

0

50

100

150

200

250Sediment: Clay LoamHg Content: 177.2 mg/kg

TCLP Fluid: #1Reaction Time: 18 hrsMethod 1311

At a FeS/Hg molar ratio of 26.5, only 2 µg/L of Hg was extracted in TCLP tests, a 99% drop compared to that without FeS nanoparticles treatment.

Xiong, Z.; Zhao, D.; He, F.; Barnett, M.O.; Harper, W.F. Environmental Science & Technology. In review.

Soil Permeability by Gravity

0.5 g/L FeS stabilized with 0.2% CMC

Non-stabilized 0.5 g/L FeS 30 min

Soil type: sandy soil

1 min 15 min 20 min

Soil Permeability by Pressure

The stabilized FeS nanoparticles are highly mobile in the sediment and breakthrough of the nanoparticles through a sediment column bed occurred at 18 pore volumes.

Pore Volume

0 5 10 15 20 25 30

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0

Sediment: Clay Loam

EBCT = 30 minSLV = 0.13 cm/min

0.5 g/L KBr

0.5 g/L FeS with 0.2% CMC

Column Tests – Sediment Treated with FeS Nanoparticles

When 0.5 g/L of stabilized FeS nanoparticle suspension was passed through a Hg-laden sediment bed, the total Hg leached from the sediment was ~67% less than that in the control test.

Pore Volume

0 10 20 30 40 50 60 70

Hg

, mg

/L

0

20

40

60

80

100

120

0.2% CMC, 0.1 M NaNO3

0.2% CMC, 0.5 g/L FeS

4.9% leachated

14.7% leachated

Sediment: Clay LoamHg Content: 3119.9 mg/kg

EBCT = 30 minSLV = 0.13 cm/min

TCLP Tests on Sediments from Column Tests

The Hg concentration in TCLP extractant for FeS-treated sediment was 77% less than that for the sediment in the control test.

FeS-treated Control

Ext

ract

able

Hg

,, g

/L

0

1000

2000

3000

4000Sediment: Clay LoamHg Content: 3119.9 mg/kg

TCLP Fluid: #1Reaction Time: 18 hrs

0.2% CMC 0.5 g/L FeS

0.2% CMC 0.1 M NaNO3

Summary The stabilized FeS nanoparticles are highly di

spersive and can be injected into Hg-contaminated sediment. 100% breakthrough occurred at 18 pore volumes.

Mercury in soil or sediment can be immobilized effectively by FeS nanoparticles and at a FeS/Hg molar ratio of 26.5, the Hg concentration leached out in the aqueous phase was reduced by 96.8%.

Acknowledgement

Thanks for EPA-STAR and USGS-AWRRI (Alabama Water Resource Research Institute) for funding.

Questions?