in-situ immobilization of mercury in sediment and soil by a new class of stabilized iron sulfide...
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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?