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 (CH 3 Hg + ).

Concept of Proposed Technology Soil / Sediment Hg pool Injection and Controlled Dispersion of FeS Nanoparticles Hg 2+ nHg 2+ HgS(s) + Fe 2+ Ion Exchange + FeS(s) FeS(s)-Hg n Sorption + FeS(s) CH 3 Hg + 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; K sp (FeS) = 8x ; innocuous to the environment.  Extremely attractive to Hg ions: FeS(s) + Hg 2+  HgS(s) + Fe 2+ or FeS(s) + nHg 2+  FeS-nHg 2+ K sp (HgS) = 2x (black) or 2x (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 low- 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 Fe 2+ stock solutions containing 0~0.5% (w/w) of CMC and M Fe 2+. Step 2. Vary the stabilizer-to-Fe molar ratio and mix CMC-Fe 2+ solution under purified N 2 gas. Step 3. Add stoichiometric amount of Na 2 S solution into the above mixture and allow for reaction under vacuum and at room temperature. 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 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 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 min15 min20 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.

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.

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.

Summary The stabilized FeS nanoparticles are highly dispersive 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.

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