Summary'Zero Valent Iron (ZVI) in-situ Reactive Zones (RZ), including iron filing permeable walls and micro- and nano-scale direct injection zones, have been constructed at hundreds of locations across North America to treat groundwater contaminated by chlorinated aliphatic hydrocarbons (CAH) such as Trichoroethylene (TCE). The CAH occur in dissolved phase form and, in some cases, as constituents of dense non-aqueous phase liquids (DNAPL) near the source of subsurface release. Dissolved CAH molecules can be fully or partially degraded via abiotic surface catalyzed redox reactions when the molecules are proximate to ZVI. High mass loading or use of an emulsified form can provide good treatment of relatively high CAH concentrations or mass flux. However, wide spread use of ZVI across both the source and dissolved phase plume is often limited due to challenges associated with subsurface distribution and cost (ITRC, 2011). Furthermore, the ZVI RZ design life is finite and hydraulic and reactivity failure modes exist (Phillips, et. al., 2000). Spatial or temporal coupling of ZVI with other compatible treatment technologies will lead to improved management of the DNAPL source zone and more effective combined remedy strategies for managing the entire plume. Biogeochemical Reductive Dehalogenation (BiRD) is an innovative engineered process for in-situ generation of reactive iron sulfides (for brevity, FeS), is compatible with ZVI, and offers source zone and plume-wide treatment optimization potential. The degree of optimization achievable is site specific and is influenced by the creativity displayed in the spatial and temporal coupling. A ZVI-BiRD combined remedy strategy for a concept DNAPL-dissolved phase plume is presented.

Scenario'–'CAH'DNAPL'Source'

Zone,'GW'Plume,'Aged'ZVI'RZ

ZVI'Modes'of'FuncCon'/'Failure'This in-situ chemical reduction (ISCR) process promotes abiotic surface catalyzed reaction pathways. ZVI is physically emplaced often resulting in abrupt physical/biogeochemical contrasts and accelerated corrosion and precipitation can reduce reactivity. Cementation filling of pores can cause preferential flow through and around the RZ.

β – Elimination Pathway

Cis-DCE will transform to acetylene. β-elimination typically dominates. TCE half-lives hours (fresh) to several months (aged).

Sequential Hydrogenolysis Pathway

Basic ZVI Corrosion Reactions Oxic Setting:

2 Fe0 + O2 + 2 H2O = 2 Fe2+ + 4 OH-

4 Fe2+ + O2 + 2 H+ = 4 Fe3+ + 2 OH-

Anoxic Settings (Microbial Induced Corrosion possible too): Fe0 + 2 H2O = Fe2+ + H2 + 2 OH-

ReacCvity'and'Hydraulic'Failure'

Coupling:'Plume'Containment'' This concept consists of temporally sequenced RZs of iron filings and nanoscale FeS coatings/particles. To re-establish hydraulic and reactive control at the toe of the plume, BiRD reactants are injected directly into the permeable wall to generate, in situ, FeS coatings on the aged ZVI particles. CAH (TCE) half-life returned to < 30 days.

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James Studer, P.E. (jstuder@infrasur-llc.com) (InfraSUR LLC, Albuquerque, New Mexico, USA) '!

Combined'Remedy'Example'

ZVI (with emulsified form option) coupled to BiRD RZ for SZ, intermediate plume BiRD RZ(s), and coupled ZVI-BiRD at toe.

Literature'cited'

D. H. Phillips, B. Gu , D. B. Watson , Y. Roh , L. Liang , and S. Y. Lee. 2000. Performance Evaluation of a Zerovalent Iron Reactive Barrier: Mineralogical Characteristics. Environ. Sci. Technology 34 (19): 4169–4176.

ITRC. 2011. Permeable Reactive Barrier: Technology Update. PRB-5. Washington, D.C.: Interstate Technology & Regulatory Council. Itrcweb.org.

D. Enning, J. Garrelfs. 2014. Corrosion of Iron by Sulfate-Reducing Bacteria: New Views of An Old Problem. Appl. Environ. Microbiol. 80: 1226-1236.

DNAPL Source Zone and Dissolved Phase Plume Treatment: ZVI and BiRD Combined Remedy Strategy

The'BiRD'Process'''

This in-situ biogeochemical reduction (ISBGR) process promotes the same two abiotic pathways and can be installed using direct injection or trenching methods. If sufficient iron is present in RZ then iron supplement is not necessary. Physical/biogeochemical contrasts are less abrupt, and precipitation and cementation are less likely to occur, compared to ZVI. FeS mass loading complete within 30 days.

Step 1 – Dissimilatory Sulfate Reduction with Labile Organic CH2O + ½ SO4

2- HCO3 + ½ HS- (ag) + H2O + H+

Step 2 – Geochemical (and Biological) Iron Conversion

3 HS- + 2 FeOOH(s) 2 FeS(s) + So + H2O + 3 OH-

Step 3 – Dehalogenation (TCE half-life 30 days or less) 4/9 FeS + C2HCl3 + 28/9 H2O 4/9 Fe(OH)3 + 4/9 SO4

2-

+ C2H2 + 3 Cl- + 35/9 H+

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Coupled'ZVIOBiRD'for'DNAPL'SZ'''

This concept consists of spatially sequenced RZs of micro or nanoscale ZVI and nanoscale FeS to address the DNAPL source zone at the head of the plume. ZVI particles are injected into the shallow sand above the clay that held up the DNAPL. The BiRD reactants are injected into the lower sand to develop, in-situ, a RZ consisting of FeS particles and coatings on the native sand grains.

Source: ITRC 2011

ZVI wall is aged ten years. Corrosion, surface passivation, and cementation develops at abrupt biogeochemical interface and progresses inward.

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