Field Scale Mobility and Transport Manipulation of Carbon

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Remediation and Control Technologies

Field scale mobility and transport manipulation of carbonsupported nanoscale zero-valent iron (nZVI) in fractured media Meirav Cohen, and Noam Weisbrod Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01226 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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Environmental Science & Technology

Manuscript TOC:

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1 ACS Paragon Plus Environment


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Field scale mobility and transport manipulation of carbon-supported

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nanoscale zerovalent iron (nZVI) in fractured media

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Meirav Cohen1 and Noam Weisbrod*1

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Affiliation 1: The Zuckerberg Institute for Water Research, Blaustein

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Institutes for Desert research, Ben Gurion University of the Negev,

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Israel

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*Corresponding author (Tel: +972-8-6596979 , Mobile: +972-52-8795756, Fax: +972-8-6596909, Secretary: +972-

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6596714)

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E-mail addresses: [email protected] (M. Cohen); [email protected] (N. Weisbrod)


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Abstract

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In field applications, mostly in porous media, transport of stabilized nano zerovalent iron

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particles (nZVI) has never exceeded a few meters in range. In the present study, the

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transport of Carbo-Iron Colloids (CIC), a composite material of activated carbon as a

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carrier for nZVI stabilized by carboxymethyl cellulose (CMC), was tested under field

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conditions. The field site lies within a fractured chalk aquitard characterized by moderately

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saline (~13 mS) groundwater. A forced gradient tracer test was conducted where one

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borehole was pumped at a rate of 8 L/min and CMC-stabilized CIC was introduced at an

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injection borehole 47 meters up-gradient. Two CIC-CMC field applications were

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conducted: one used high 100% wt. CMC (40 g/l) and a second used lower 9% wt. loading

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(~2.7 g/l). Iodide was injected as a conservative tracer with the CIC-CMC in both cases.

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The ratio between the CIC-CMC and iodide recovery was 76 and 45% in the high and low

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CMC loading experiments, respectively. During the low CMC loading experiment, the

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pumping rate was increased, leading to an additional CIC recovery of 2.5%. The results

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demonstrate the potentially high mobility of nZVI in fractured environments and the

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possibility for transport manipulation through the adjustment of stabilizer concentration and

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transport velocity.



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Keywords

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nZVI, Colloid transport, fractured media, groundwater remediation, in situ, CMC.


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1. Introduction

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Over the past 20 years, the use of nano zerovalent iron (nZVI) for groundwater

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remediation has gained much attention. Due to its high reactivity and mobility potential,

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the application of nZVI is considered a promising technique for contaminated aquifer

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remediation.1–4 Reactivity of nZVI is reflected in the effective transformation of many

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contaminants, most notably chlorinated organic compounds.1,4,5 However, nZVI mobility

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in groundwater is typically very limited and remains a significant constraint for efficient

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nZVI application in remediation processes.5–7

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High surface area per mass and magnetic forces between nZVI particles result in

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agglomeration and deposition, thereby limiting nZVI mobility.8,9 The main approaches

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for improving nZVI mobility are through the application of stabilizers such as coatings

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and surface functionalizations like carboxymethyl cellulose (CMC). Such negatively

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charged polymers can provide electrostatic repulsion between the particles and

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between the usually charged soil surface.10 Additionally, polymer shear-thinning

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behavior, where the polymers exhibit decreasing viscosity with increasing shear rate,

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also can be used for stabilization.11 Another method that has been proven to increase

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nZVI mobility is through the addition of carrier colloids such as activated carbon (AC).5–

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7,12–14

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stability in suspension can be manipulated and consequently their mobility.15,16 CMC is

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one of the most prevalent and efficient stabilizers used, which functions by inducing

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electrosteric repulsion between particles.5 One example of an efficient nZVI carrier

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colloid is “Carbo-Iron Colloids” ((CIC).7,12 CIC are colloidal-sized AC particles,

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containing nZVI structures within the porous carbon grains. The AC carrier colloids act

In fact, by varying stabilizer concentration and type, nZVIs particle size and



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as a spacer preventing nZVI interactions. Additionally, CIC lower effective density and

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decrease zeta potential in comparison to bare nZVI, resulting in the nZVI’s increased

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stability.7,12,17 When supplemented with CMC, CIC were shown to be highly mobile in

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porous and fractured environments.7,16,17

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Laboratory scale experiments, mostly conducted at low ionic strength (IS) (mostly below

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10 mM though higher IS of up to ~200 mM have been tested) in pure pre-treated silica

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sand, confirmed the increased mobility and high recoveries of stabilized nZVI in porous

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media.7,18–20 Nevertheless, stabilized nZVI mobility in natural media was shown to be

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notably lower due heterogeneities inducing increased surface charge, different type and

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content of natural organic matter and sorptive clay material encouraging interactions

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with the media.18,21,22 Additionally, in high IS environments, enhanced particle

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aggregation and interaction with the aquifer matrix were shown to decrease stabilized

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nZVI mobility.16,21,23–26

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The literature is still scarce in field scale studies of stabilized nZVI. In most field studies,

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nZVI migration is on the order of 1 to 5 m, with an estimated influence radius of 3-30

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m.3,4,17,27–31 These field studies which explored nZVI transport have mostly been

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conducted in porous media. Though their applicability to aquifer remediation has been

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tested,4,27,28

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Europe, for example, had been conducted in fractured media,32 little attention has been

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given to nZVI transport potential in naturally fractured systems. It has been suggested

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that at higher water flow velocities in fractured media, nZVI travel distance will be

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enhanced.24,33 In many instances, fracturing of porous media is performed in order to

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increase its permeability and subsequently enhance nZVI mobility.5,29,34 It is well

and despite the fact that by 2012 about 25% of nZVI applications in


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recognized that fractured systems and preferential flow paths can enable fast migration

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of solutes and colloids due to higher shear forces and larger conduits.35,36,37 Cohen and

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Weisbrod (2018)16 in a set of laboratory experiments in a naturally fractured chalk core

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showed that transport of iron nanoparticles in fractures depended mostly on the particle

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stability.

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This study’s objective was to test nZVI mobility potential on a field scale in naturally

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fractured rock. The study site – a fractured aquitard characterized by moderately saline

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groundwater (~13 mS) – enabled examination of nZVI transport under saline conditions.

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Transport potential and manipulation of nZVI were tested through the alteration of

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stabilizer concentrations and flow velocities, where the distance between the injection

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and pumping wells was about 50 m. This also enabled examination of the stabilizer

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loading effect on nZVI transport in-situ. A preliminary conservative tracer test was

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conducted in the field with uranine and naphthionate, both found to be stable under

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saline conditions.38 Next, two CIC injections are reported on where scenarios of highly

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versus slightly stabilized CIC were tested. Together with CIC, Iodide (that in contrast to

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uranine and naphthionate does not sorb to CIC) was injected as a conservative tracer.

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2. Materials and Methods 2. 1

Field site

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The site is located in the Negev desert of Israel within an industrial area (Figure 1). It

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lies over the Eocene chalk formation ranging from 150 to 285 m thick.

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formation has in the past been assumed to provide a natural hydrogeological barrier to

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groundwater flow due to the matrix low permeability.

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chalk formation is highly fractured, enabling fast migration of water and contaminants

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from land surface to groundwater and within the saturated system. In the field site,

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groundwater underlying the industrial complex was found to be contaminated by various

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volatile and nonvolatile halogenated organic compounds, heavy metals, and halogenide

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anions.40 As part of later remediation efforts, the site’s fracture system was mapped and

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studied. It was found that apart from fractures oriented along bedding planes, most

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fractures are vertical and comprise two main fracture systems. The first dominant

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fracture system is orientated NE–SW (azimuth 50–600), and the second is oriented

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NW–SE (310–3400).41 Groundwater flow occurs mostly within the two main fracture

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systems, the bedding planes and the intersection between the three,41 probably through

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a network of channels.39

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This chalk

Yet, as later became evident the


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Figure 1 – Schematic illustration of the field site: (a) The industrial municipality where the field site lies;

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The red circle indicates the injection area; (b) Top view of the study site boreholes; blue dashed arrows

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represent general flow direction; (c) Side view of boreholes' orientation at the injection site. Note that all

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boreholes are slanted in 68° or 45° normal to the orientation of one of the two prevailing fracture systems

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and crossing the fractures below the water table.

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Specifically, the field injection site is located at the convergence of two washes, the

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Naim and the Hovav ephemeral washes (Figure 1a, b). This location is down-gradient

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from the industrial center (Figure 1a) and is characterized by brackish water (TDS

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~8000 mg/L - Table S7) and low organic contaminant concentrations (BTEX ~ 3 µg/L



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and organic halides ~ 8 µg/L). Seven slanted boreholes were drilled in the test area,

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crossing the fracture network below the water table (Figure 1b-c). Most boreholes were

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drilled at an angle of 68° normal to the orientation of the prevailing fracture system.39,41

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The boreholes are 25-40 m deep, and they are open to the chalk matrix with the

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exception of casings installed in the upper 2-3 meters of loose, granular media.

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Previous forced gradient tracer tests conducted in the boreholes at this site showed that

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transport mostly depends on hydrodynamic dispersion and the concentration decay rate

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at the injection borehole.39 Tests yielded high recoveries; up to 80% of uranine injected

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at RH11c and pumped at RH11a39 was recovered.

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2. 2

Field site characterization

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A pumping test for groundwater characterization (see Supporting information (SI)) and

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three sets of independent transport experiments carried out in the field site between

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December 2014 and May 2016 are described.

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In experiment I, two different conservative tracers, uranine and naphthionate, were

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injected into two different boreholes, RH11c and RH11f, respectively (see SI for details),

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in order to understand the feasibility of injecting CIC into the fracture system.

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Experiment II, in which CIC were injected with high stabilizer loadings, aimed at

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exploring the maximum potential migration of CIC in the fractures. Due to the very high

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CIC recoveries in experiment II, CIC particles with lower stabilizer loading were injected

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in experiment III. This last experiment aimed at exploring the possibility of manipulating

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the CIC migration. CMC:CIC ratios explored are in the range of higher and lower

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stabilizer loadings considered relevant for field applications.17,28,30,42–44


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2. 3

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Experiment II: 1st CIC injection – CMC 100% wt. 2.3.1 The particles

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CIC was manufactured by Scientific Instruments (Dresden GmbH, Germany). The CICs

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were produced with a mixture of nZVI and iron oxide nanoparticle load of 7 and 13%

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wt., respectively. Particle size distribution according to the manufacturer was d10= 0.56

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µm, d50= 1.16 µm, d90= 2.13 µm (nZVI mean cluster size ~0.05 µm). Particle size of

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selected samples from the field experiment was also tested (section 2.3.3). 2.3.2 Exp. II: Preparation of injection solution on site

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According to the manufacturer7 and sorption tests conducted (SI section S7), the

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maximum CMC loading on CIC is about 6–9 wt%. Excess stabilizer loadings can

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contribute to particle stability and subsequent mobility due to increased viscosity and

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increased CMC layer coverage.43,45 In this experiment, the main goal was to explore the

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maximum potential CIC mobility. Therefore, in light of the saline environment and long

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travel distance, a high CMC (90 kg/mol) loading of about 80 wt% on CIC mass (initial

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injection borehole viscosity of 1.9 cP and density of ~1g/cm3) was used to assure high

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recovery.



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Figure 2 – Schematic illustration of the experimental setup: (a) The pumping and injection borehole

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RH11a and RH11c, respectively; (b) Scheme of equipment outside of the RH11c injection borehole; (c)

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Cross-section of RH11c borehole; (d) Zoom-in of RH11c at around 25 m depth where the packer was

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installed.

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CMC 90 kg/mol (Sigma Aldrich) was used as the stabilizer. In preliminary laboratory

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tests, uranine and naphthionate underwent fast sorption to CIC, while iodide did not.

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Also, iodide is not found in the local groundwater at high concentrations (unlike Cl and

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Br – Table S7). Therefore, iodide was used as a soluble conservative tracer, which was

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added as KI (Sigma Aldrich). The CIC solution was prepared on-site in a 160 L barrel

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(Figure 2). 120 L groundwater was pumped from RH11a into the barrel situated at


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RH11c (Figure 2a-b). To maintain nZVI anaerobic and active, the barrel was sealed

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and nitrogen purged, as has been conducted in field applications where degradation

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should take place17,30. While purging nitrogen, mixing started with a large customized

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field mixer. Next, 4.8 kg CMC and 500 g KI were added to the barrel. The solution was

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mixed for one hour to allow for CMC dissolution, then 6 kg of CIC were added and the

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solution, and mixing continued for an additional 1.5 hours. Due to the slow solubility, the

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mixing was continued the next day for 5 additional hours until a homogenous solution

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was achieved. Eventually, 4.9 Kg were injected, resulting in a CMC loading of ~97 wt%

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and initial CIC and CMC concentrations of ~41 and 40 g/l, respectively. Next, the 120 L

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solution was injected, similarly to experiment I (section 2.2.1), by pouring the solution

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into three pipes positioned inside the borehole (Figure 2c-d). This took a few minutes.

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Next, the pipes were removed, and circulation and sampling pumps (Figure 2c-d) were

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initiated. More details can be found in the SI (sections S3 and S2).

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2.3.3 Experiment. II: Chemical analysis

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All samples were analyzed for CIC concentration with a UV-VIS spectrophotometer at a

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wavelength 508 nm.15,20 RH11c samples were analyzed for iodide by EPA Method 30046

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using a Dionex 4500i ion chromatograph (Sunnyvale, CA USA). RH11a lower

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concentration (maximum concentration ~3.5 mg/L) iodide samples were analyzed by

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ICP-MS (Agilent ICP-MS 8800, CA USA) (details in SI section S5).

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Selected groundwater samples from the two boreholes were tested for particle size,

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viscosity and density (SI, Tables S2-S5). The solution density was measured using a

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portable density meter (model DA-130N Kyoto Electronics Manufacturing (Kem) Co.,



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Shanghai, China). Viscosity was measured using rolling-ball viscometer (Lovis 2000

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M/ME, Anton Paar GmbH, Gratz, Austria). Particle size was analyzed by dynamic light

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scattering (DLS) using a particle size analyzer (90plus, Brookhaven Instruments Co.,

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Holtsville NY, USA).

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2. 4

Experiment III: 2nd CIC injection – CMC 9% wt. 2.4.1 Experiment III: Particle suspension

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All specifications for the particles were the same as in the previous experiment with the

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exception that all of the 20% wt. iron loading consisted of nZVI (no iron oxides) due to

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manufacturer supply issues. According to the manufacturer and to independent tests

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conducted at the laboratory, this does not affect CIC stability and should subsequently

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not affect its' mobility.14

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2.4.2 Experiment III: Particle preparation on site

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In order to test nZVI mobility manipulation, this experiment was conducted using a lower

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stabilizer concentration. For that purpose, a low CMC loading of about 5 wt% was

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applied. 110 L of RH11a solution was pumped into a 120 L barrel situated at RH11c.

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The barrel was sealed, nitrogen was injected, and mixing was initiated. Following one

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hour of nitrogen injection, 500 g KI and 300 g CMC were added to the solution.

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Following 2 more hours of mixing, 6 kg of CIC were added, and the mixing continued for

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an additional 2.5 hours. As CIC aggregated, thereby clogging the barrel outlet, manual

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mixing was applied while purging in nitrogen for another 40 minutes. The lower part of

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the barrel tubing that was blocked by aggregated particles was opened manually, and

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the removed solution was retuned back into the barrel after removing the aggregated 14 ACS Paragon Plus Environment

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CIC particles. When the solution was homogenous, the barrel was sealed again and

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mixed for 20 more minutes. Eventually, 3.2 Kg were injected, which resulted in a CMC

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loading of ~9 wt% and initial CIC and CMC concentrations of ~29 and 2.7 g/l,

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respectively. Next, the 110 L solution was injected similarly to experiment II (section

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2.3.2). The experiment was conducted in the same manner as in experiment II (SI –

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section S3) with the exception of some equipment changes (SI – section S4). To

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examine the role of the hydraulic gradient and subsequent flow velocity change, the

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pumping rate at RH11a was elevated to 17.1 L/min (from 8 L/min) towards the end of

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the experiment. This was done 61 hours after injection, when the CIC were close to the

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detection limit at the extraction borehole (RH11a). Samples were analyzed as described

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in section 2.3.3.

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2. 5

Data analysis

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Solutes and particle concentrations at the injection borehole exhibited exponential

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decay, as previously reported,39 in the form:

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= (1)

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where Ct is the concentration at time t (M/L3), C0 is the concentration at the injection

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borehole immediately following the injection, t is the time (T) and A and α are fitting

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parameters, where α (1/T) represents the decay rate at the injection borehole. Initial

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tracer concentration (C0) at the injection borehole was extrapolated from the fitted

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Equation 1 at t=0. Breakthrough curves (BTCs) were normalized to initial injection

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concentration (C/C0), and recoveries were calculated according to the extrapolated C0.



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First arrival time was calculated as C/C0≥0.0001. Uranine BTC (experiment I) modeling

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is provided in SI, section S7.

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3. Results 3.1

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Experiment I: Conservative tracers

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Conservative tracer BTCs can be observed in Figure 3a-b. At the pumping borehole,

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uranine was first observed 23 min from its injection, and a high uranine mass recovery of

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about 83% was recorded (Figure 3a-b, Table 1). A similarly high recovery (~80%) and

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fast arrival were reported for uranine injected at RH11c and pumped at RH11a by

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Kurtzman et al (2005).39

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Table 1: Transport experiment results BTC

Exp. No.

I

Injection borehole

Sampling borehole

First arrival (min)

Main BTC peak (h)

C/C0 max

Uranine

RH11a

23

4

0.014

83.375

RH11c

0.08

2.57

Naphtionate

RH11a

195

17.4

0.145

16.19

RH11f

0.20

1.75

Naphtionate

RH11c

245

26.5

0.006

67.83

RH11f

0.20

0.82

Iodide

RH11a

30

11.7

0.0136

82.66

RH11c

0.05

CIC

RH11a

34.2

6.7

0.0118

62.77

RH11c

0.07

Iodide

RH11a

23

8.2

0.0135

36.1/*38.75

RH11c

0.19

CIC

RH11a

19

8.3

0.0062

16.36/*18.87

RH11c

0.08

Tracer

∆H Exp Mass between Borehole decay recovery (%) boreholes -1 (h ) (m)

II

1.48

III

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2.89

*Overall mass recovered following increased pumping rate


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0.025 0.025

a

Naphthionate NaphthionateatRH11c RH11C

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NaphthionateatRH11a RH11A Naphthionate Concentration (mg/l) Concentration (mg/l)

C/C 0

C/C0

b

6060

0.015 0.015 0.010 0.01

0.01 0.010

0.005 0.005 0.000 0

0.005 0.005

100

8080

0.025 0.025 0.020 0.02

0.015 0.015

Uranine U ranine 1010m m Uranine U ranine 2020m m Naphthionate N aphthionate Uranine Exp. decay U ranine Exp. decay model Naphtionate Exp. decay N aphthionate Exp. decay model

100

Uranineat RH11A Uranine RH11a

0.02 0.020

c

120 120 100 100 80 80 60 60 40 40 20 20 00

4040

0

5

10

15

20

2020

d

0

2

4

6

8

10

00

0 0.000

0

20 20

40 40

60 60

80 80

00

100 100 120 120 140 140

20 20

40 40

60 60

80 80

100 100

120 120

140 140

Time Time(hour) (hour)

TTime ime(ho ur) (hour)

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Figure 3 – Experiment I (Conservative tracers). Note that axes units of the inset figures (b and d) are

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identical to those of the containing figures (a and c). (a) Conservative tracer BTCs - uranine at RH11a

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(squares) and naphthionate (circles) at RH11c and (diamonds) at RH11a. Concentration values are

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normalized to the injection concentration (C/C0) vs. time; (b) Zoom-in on first 20 hours. (c) Concentration

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of uranine (squares and diamonds at 10 and 20 m depth respectively) and naphthionate (circles) in

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injection boreholes, RH11c and RH11f, respectively. Dashed black line indicates uranine decay rate and

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red dashed line indicates naphthionate decay rate; (d) Zoom-in on uranine during the first 10 hours in

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RH11c injection borehole. Note the different concentrations at the two depths due to the insufficient

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mixing.

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During experiment I, the mixing at the injection borehole was insufficient, and thus it

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took about 3 hours until a uniform concentration along the water column was

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established (Figure 3c-d). This resulted in an initially higher concentration in the upper

268

part (10 m) of the borehole compared to the bottom part (20 m) (Figure 3d). The initial

269

(

Field Scale Mobility and Transport Manipulation of Carbon

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