Zero-valent iron’s effectiveness at dehalogenating chlorobenzenes and its feasibility as a reactive cap Shawn Moderow and Danny Reible University of Texas at Austin Department of Civil, Architectural and Environmental Engineering
Chlorobenzenes, CBs Cl Cl Cl Hexachlorobenzene, HCB Cl Cl Cl Cl Cl Used in the � Pentachlorobenzene, PeCB Cl Cl manufacturing of Cl pesticides, Cl Cl Cl Cl Cl Cl herbicides, dyestuff and rubbers Cl Cl Cl Cl Cl Cl 1,2,3,4-Tetrachlorobenzene 1,2,3,5-Tetrachlorobenzene 1,2,4,5-Tetrachlorobenzene Cl Cl Cl Range of chemical � Cl Cl and physical Cl Cl Cl properties. Cl 1,2,3-Trichlorobenzene 1,2,4-Trichlorobenzene 1,3,5-Trichlorobenzene Cl Cl Cl High MW CBs Cl � strongly sorbing. Cl 1,2-Dichlorobenzene 1,3-Dichlorobenzene Cl Cl 1,4-Dichlorobenzene Low MW CBs are Monochlorobenzene � very volatile. Benzene
Bayou d’Inde (Bayou Den) Tributary of the Calcasieu � River (Calc-a-shoe) outside Lake Charles, Louisiana that has received discharge of metals, PAH, PCBs, CBs, and other chlorinated organics. � Primarily contaminated with HCB. Natural attenuation has been � ineffective at reducing HCB contamination (Yeh and Pavlostathis, 2004). response.restoration.noaa.gov
Sediment Capping � Improve quality of aquatic habitat � Stabilize sediments � Physically isolate sediment contaminants from benthic organisms � Reduce contaminant flux to benthos and water column � Improve surficial substrate
Sand caps � Majority of existing caps � Effective for contaminants strongly sorbed to solid phase of underlying sediment � Easy to place with minimal intermixing � Generally erosion resistant compared to existing bottom but, if necessary, can be supplemented with armoring layer � Often provides much-needed diversity to bottom substrate � Drives sediment layer anaerobic
Active Capping Provides an opportunity for treatment in addition to passive containment • Sorption and sequestration • Chemical and biological treatment From a variety of materials, Zero-Valent Iron (ZVI) was chosen for investigation as an active capping material for the use in the Bayou d’Inde. Why? •ZVI has been shown to be effective at reducing chlorinated aliphatics and PCBs • Can HCB be reduced to less chlorinated benzenes? • Results of literature review inconclusive •Iron is an relatively inexpensive and nontoxic material.
Goals � Assess ZVI potential for reducing chlorobenzenes. - Published reports have shown mixed results on the reactivity of ZVI and CBs.
Zero-valent Iron Funnel and Gate design � MicroScale ZVI - 0.14 for ZVI active capping m 2 /g, 70% < 44 μ m Reactive Zone, ZVI diameter. � Reactive Nanoscale Sand Layer Sand Layer Low Permeability Cap Low Permeability Cap Iron Particles (RNIP) – 33.1 m 2 /g, ~ 70 nm Ground water flow Contaminated Sediment Layer
Zero-Valent Iron Chemistry Fe 0 = Fe 2+ + 2e - Fe 2+ RCl + 2e - + H + = RH + Cl - A. RCl + H + e - Fe 0 + RCl + H + = RH + Cl - + Fe 2+ RH + Cl - Fe 3+ RCl + H + (A) Reduction by zero-valent RH + Cl - B Fe 2+ iron. H 2 O e - (B) Reduction by Fe 2+ . OH - + H 2 (C) Catalyzed hydrogenolysis. Fe 2+ C H 2 O e - RCl OH - + H 2 e - Catalysis RH + Cl -
Preliminary Experiments � Batch tests � 20 ml aqueous solution in 40 ml vials � pH ~7 � ~8 grams of acid pretreated MicroScale Fe 0 � Anaerobic Environment � ~3.2 μ M Chlorobenzene isomer � Run over 24- 48 hrs � CB recovery with injection of Hexane, 10 - 20 min � Continuously shaken using Shaker table
Preliminary Results (Trichlorobenzene) 1 0.9 0.8 0.7 0.6 C/Co 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 20 25 30 Time, Hours 1,2,4-Trichlorobenzene 1,2,4-Trichlorobenzene control
Preliminary Results (Hexchlorobenzene) 1.0 0.10 0.9 0.09 0.8 0.08 0.7 0.07 PeCB C/Co 0.6 0.06 HCB C/Co 0.5 0.05 0.4 0.04 0.3 0.03 0.2 0.02 0.1 0.01 0.0 0.00 0 10 20 30 40 50 60 Time, Hours HCB HCB Control PeCB
Experimental Adjustments � Ensure iron activity � Longer extraction times to recover reactant sorbed to iron � Enhance mixing throughout with tumbler (relative to shaker table) � Lower pH (2.7) to maximize potential for reductive dechlorination
Hexachloroethane G C /E C D A re a o f D e c h lo rin a tio n b y p ro d u c t 2.00 25000 1.80 1.60 20000 Control 1.40 1.20 15000 m ic r o M 1.00 Expected at pH 7 0.80 10000 Observed 0.60 pH 2.7 0.40 5000 0.20 0.00 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Time, Hours
Results - pH 2.7 0.070 0.007 0.040 0.0040 0.035 0.0035 0.060 0.006 0.030 0.0030 0.050 0.005 H C B M icro M o les m icro m o le PeC B PeC B M icro M o les m icro m o le T eC B 0.025 0.0025 0.040 0.004 0.020 0.0020 0.030 0.003 0.015 0.0015 0.020 0.002 0.010 0.0010 0.010 0.001 0.005 0.0005 0.000 0.000 0.000 0.0000 0 10 20 30 40 50 0 10 20 30 40 50 60 Tim e, hours Tim e, Hours HCB Total Moles Control, no ZVI Pentachlorobenzene Control HCB Total Moles w / ZVI Pentachlorobenzene Theoretical Mass injected 1,2,3,4-Tetrachlorobenzene PeCB Total Moles w / ZVI 1,2,3,5-Tetrachlorobenzene and 1,2,4,5-Tetrachlorobenzene •Observed reductions in HCB and PeCB were 0.8% and 0.7 %. •Observed reductions in 1,2,3,4-TeCB, 1,2,3,5-TeCB and 1,2,4,5-TeCB were negligible (0.04, 0.05 and 0 %, respectively)
Micro vs Nano Iron � Nanoscale iron requires much lower iron loading rate compared with microscale to achieve equivalent surface areas per gram. Microscale Nanoscale Iron loading g/L 400 6.3 Surface Area m2/g 0.14 33.1 SA conc m2/L 56 209
Nanoscale Iron Reduction of PCB Cl Cl 3.12 % , 59 % 3.0 0.30 Cl C 2.5 0.25 l PeCB micro M TeCB micro M Cl Cl 2.0 0.20 Cl Cl 0.73 % , 15 % 1.5 0.15 Cl Cl Cl Cl C 1.0 0.10 l Cl 0.5 0.05 Cl 1.26 % , 27 % 0.0 0.00 Cl 0 10 20 30 40 50 60 C l Time, Hours PeCB 1,2,3,4-TeCB 1,2,3,5-TeCB 1,2,4,5-TeCB
Nanoscale Iron Reduction of HCB 4.0 0.40 3.5 0.35 3.0 0.30 2.5 0.25 PeCB uM HCB uM 2.0 0.20 1.5 0.15 1.0 0.10 0.5 0.05 0.0 0.00 0 10 20 30 40 50 60 Tim e, Hours HCB PeCB
Summary Percent Reduction Microscale Fe NanoScale Fe pH 7 pH 2.7 pH 7 pH 2.7 24 / 48 hrs 24 / 48 hrs 24 / 48 hrs 24 / 48 hrs HCB 0.61 / 0.78 0.50 / 0.80 0.30 / 0.36 2.22 / 3.33 PeCB -- 0.34 / 0.69 -- 2.70 / 5.11 1,2,3,5-TeCB -- 0.05 -- -- 1,2,4,5-TeCB -- 0 -- -- 1,2,3,4-TeCB -- 0.04 -- -- 1,2,4-TCB 0 -- -- --
Conclusions � ZVI has shown limited reactivity with CBs � ZVI cannot be recommended as a reactive capping material for the purpose of reducing CBs. � Larger MW CBs have greater reactivity with ZVI (HCB>PeCB>TeCBs, although all show small reactivity).
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