AC composite – As alternative to nano-iron for groundwater treatment

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 8 1 7 e3 8 2 6

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Carbo-Iron e An Fe/AC composite e As alternative to nano-iron for groundwater treatment Katrin Mackenzie*, Steffen Bleyl, Anett Georgi, Frank-Dieter Kopinke Helmholtz Centre for Environmental Research e UFZ, Department of Environmental Engineering, Permoser Str. 15, D-04318 Leipzig, Germany

article info

abstract

Article history:

Carbo-Iron
1 is a novel colloidal composite consisting of activated carbon colloids (ACC)

Received 1 November 2011

with a d50 particle size of 0.8 mm and anchored deposits of zero-valent iron clusters. This

Received in revised form

study discusses the principal material properties of Carbo-Iron colloids (CIC) relevant for

5 April 2012

groundwater treatment in comparison to commercially available nano-sized zero-valent

Accepted 8 April 2012

iron (nZVI). CIC with 10e25 wt% Fe0 have been developed and tested in laboratory studies

Available online 26 April 2012

for their suitability as dehalogenation reagent and are especially designed to overcome some limitations known from the utilization of nZVI: CIC combine the sorption properties

Keywords:

of ACC and the chemical reactivity of nZVI. In column tests, flushed-in CIC showed an

In-situ groundwater remediation

enhanced mobility in sediment material compared to nZVI, without the need for colloid

Injectable colloids

stabilizers. However, adding 1e3 wt-% of carboxymethyl cellulose (CMC) related to CIC as

nZVI

colloid stabilizer was found to assure long-lived stable suspensions under laboratory

Fe/AC

conditions which may additionally support the already improved mobility of the CIC and

Composite

the homogeneity of particle deposition on the sediment matrix. The hydrophobic character

Dechlorination

of the ACC carrier provides a high affinity of CIC to non-aqueous phase liquids (NAPL). In

Carbo-Iron

undisturbed flow, the reactive particles are collected at the water-NAPL interface. The reagent accumulation at the organic phase is necessary for a successful source attack. ª 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

The main focus for research on the utilization of iron in groundwater treatment has been shifted in the last years to nano-sized zero-valent iron (nZVI) because of its advantages over larger iron particles (e.g. Nurmi et al., 2005; Zhang and Elliot, 2006) for an extended application profile. As with granular iron, the dehalogenation reaction (of the halogenated pollutant RX) follows the equation: Fe0 þ RX þ H2O/Fe2þ þ OH þ RH þ X. The heterogeneous dehalogenation reaction profits from the higher specific surface area when shifting from micro- and granular iron to

nZVI. The reactivity increases and the material offers the possibility of the in-situ generation of reactive subsurface zones by injection of particle suspensions. Nevertheless, remediation of larger source zones can still be seen skeptical as to whether enough iron mass can be transported in the vicinity of the source to have a significant effect on the source mass and emission rates from the source (e.g. Fagerlund et al., 2012). Source remediation remains one of the great challenges for in-situ applications. Although nZVI shows the desirable properties such as high dehalogenation activity and injectability into deep and overbuilt aquifers, it also shows undesirable properties such as a pronounced agglomeration

* Corresponding author. Tel.: þ49 341 235 1760; fax: þ49 341 235 451760. E-mail address: [email protected] (K. Mackenzie). 1 Carbo-Iron is registered as a trademark in Germany. 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.04.013

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tendency, untimely deposition, blocking of flow paths and hence a very limited mobility under aquifer conditions (Zhang, 2003; Johnson et al., 2007; Yang et al., 2007). Hydrophilized carriers, polyelectrolytes as colloid stabilizer or other surface modifiers such as triblock copolymers have been utilized in attempts to mobilize nZVI better (Schrick et al., 2004; Saleh et al., 2007). However, in the present work, the utilization of an iron-containing colloidal composite with inherent improved subsurface behaviour of the iron reagent is preferred rather than adding surface modifiers which would have additional impact on the aquifer chemistry. Results from mobility studies using quasi-soluble activated carbon colloids (ACC) showed that the properties of the carbon particles could provide the ideal refinement to nZVI concerning mobility in porous media, agglomeration and controllable deposition behaviour (Georgi et al., submitted for publication). The combination of the reactivity of iron and the transport properties of the ACC would provide both better transport properties than nZVI and a higher affinity of the iron reagent to non-aqueous phase liquids (NAPL). In addition, the sorptive enrichment of organic pollutants at the hydrophobic ACC carrier could be advantageous for their efficient destruction at the neighbouring reactive centres. The combination of reactive metals and various carbon materials has recently attracted interest in many ways (e.g. Plagentz et al., 2006; Hoch et al., 2008; Choi et al., 2009; Pereira et al., 2010; Sunkara et al., 2010; Zhan et al., 2011). However, a synergistic effect towards a more efficient reaction in water treatment can only be achieved when both components play complementary roles. Plagentz et al. (2006) showed that the utilization of iron and activated carbon in sequential treatment zones is a useful tool for some applications, e.g. for mixtures of halogenated and hydrophobic non-halogenated pollutants, or metal ions in combination with halogenated substances. A true synergistic effect for the dehalogenation is not found in such cases. Dechlorination rates remain unchanged in comparison to cases where iron is applied alone. For iron on the surface of carbon black (80 m2/g) prepared by carbothermal reduction, the combination of the materials showed improved properties for the reduction of Cr(VI) (Hoch et al., 2008). Also uniform carbon microspheres 500 nm from carbothermal and aerosol-based processes, modified at the outer surface with Fe0 or Fe0/Pd, were successfully studied as dehalogenation agents (Zhan et al., 2011; Sunkara et al., 2010). For another composite, an ironeAC material with larger particles (100 mesh), which were carbothermally generated and used for treatment of waste water, two environmentally relevant reactions were described: oxidative dye destruction and reduction of Cr(VI) (Pereira et al., 2010). The metal/carbon approaches all try to benefit from the combination of the carbon material with iron. The differences to the aforementioned examples is that the CIC material is not only small enough for subsurface transport, it also carries the iron mainly in the inner carbon framework which ensures that the majority of the carbon-surface properties are maintained. In the present paper, ACC was utilized in CIC generation because of its cost efficiency and known environmental compatibility. In order to tailor the aspired improved in-situ subsurface reagent for groundwater treatment based on the combination

of AC and iron, the requirements for the target applications have to be defined. We did this using model assumptions originating from filtration theory. For a high subsurface mobility, particles should be optimized in size, density, agglomeration tendency and surface charge. Particle deposition during transport through subsurface sediments depends on the collision frequency of the particles with sediment surfaces and on the attachment probability in case of collision (Elimelech et al., 1995). Since silica, the predominant subsurface mineral, is negatively charged under aquifer conditions, particle transport is favoured by a negative colloid surface and a low ionic strength of the suspension medium. For CIC and process design, the question arises: what parameters can be changed to improve the suitability of iron as a colloidal reagent for plume treatment? (i) The composite particles need to provide long-term reactivity and should have a more negative surface charge than pure iron. For reactive nano-iron particles (RNIP) produced by Toda/Kogyo a zeta potential of z z þ20 mV at pH ¼ 7 and 1 mM KNO3 as electrolyte was measured. (ii) A polyelectrolyte can be added as colloid stabilizer and its concentration can be optimized. Any addition of chemicals in form of modifiers into the groundwater should be kept to the absolute minimum. Therefore, particles which from the start have a more negative surface charge are to be preferred. (iii) A further important parameter determining particle mobility in porous media is the optimal particle size which allows highest mobility. For colloids with low density (2.5 g cm3), the highest mobility in tests using sand beds was found for the particle size range of 0.5e2 mm (Elimelech et al., 1995; Tufenkji and Elimelech, 2004). Therefore, in addition to the particle size, (iv) the density difference between water and the reactive material is a parameter which can be adjusted when tailoring particles. What can be done to design particles with a better suitability for source attack than nZVI? Here, a high affinity or compatibility to NAPL is necessary such that the reaction can occur at or within the organic phase. Emulsified zero-valent iron (EZVI) has been successfully tested in a field-scale demonstration, where it proved miscible with the organic phase (Quinn et al., 2005). Nevertheless, the addition of considerable amounts of chemicals (oil and surfactants) to the reactive agent is necessary in order to achieve the close contact between the reactants. In addition, EZVI shows less subsurface mobility. The better solution, in our opinion, would be an a priori modified material in order to avoid the additional chemicals. To achieve this, (v) the surface of the colloid particles should be more hydrophobic than that of pure iron. Nonetheless, for larger and difficult to access pollution sources, in-situ remediation with reactive particles which are consumed has to be questioned because insufficient delivery of reactive material mass may occur. The present paper introduces the composite material Carbo-Iron, which is tailored according to the criteria described. Nano-iron clusters are placed within the pore structure of colloidal, “quasi water soluble” ACC particles with a d50 ¼ 0.8 mm. This sorption-active and reactive material is evaluated regarding its suitability for the in-situ generation of subsurface reactive treatment zones and its behaviour towards NAPL. In this paper we introduce the material, show its main properties such as general colloid behaviour and reactivity, and

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 8 1 7 e3 8 2 6

exemplarily compare the particle transport and compatibility with residual organic phases to those of pure nZVI particles.

2.

Experimental section

2.1.

Materials

The AC SA Super was purchased from Norit (Germany). The point of zero charge (PZC) was found at pH ¼ 8.2 to 9.0 (potentiometric titration and immersion technique). The AC was finely ground in the presence of deionized water (horizontal mill 200 AHM, Alpine Hosokawa, Germany) to form ACC. By means of laser diffraction measurements, a particle size distribution of d50 ¼ 0.8 mm and d90 ¼ 1.6 mm (Metasizer, Malvern Instruments Ltd., UK) was determined. The iron salts (e.g. Fe(NO3)3$9H2O, FeCl3, FeC6H5O7$nH2O) were purchased from Merck and chlorinated hydrocarbons from SigmaeAldrich. All chemicals were used without further purification. Carboxymethyl cellulose (CMC) with a substitution degree of 0.6e0.95 and a molecular weight of about 50,000 g mol1 was obtained from Fluka. RNIP was purchased from Toda Kogyo Corp., Japan (Fe0 content found: 44 wt%) with a surface area of 24 m2 g1 (BET). The RNIP will be used as a representative of the class of nZVI. Ethylene (99.95%) was purchased from Fluka. Details about the tailored syntheses of CIC and discussion of preparation pathways are provided in Mackenzie et al. (2007), Bleyl et al. (2012). CIC were synthesized by doping ACC with iron salts by wet impregnation in aqueous suspension and transformation of the iron precursor to iron oxide/ hydroxide during particle separation by pH increase. After drying, the iron oxide/ACC system was reduced to form CIC either (i) by using hydrogen as reducing agent (H2/N2 with 30 vol% H2) in the temperature range of 450e520  C (midtemperature mode) to form mid-temperature CIC (MT-CIC), or (ii) carbothermal at 700  C (high-temperature mode) under inert atmosphere (N2) to form high-temperature CIC (HT-CIC). The synthesis procedure for studied CIC samples are summarized in Table 1 and SI 3-1 (Supp. Inform.).

2.2.

Carbo-Iron characterization

The specific surface areas of the CIC samples and ACC were determined by means of BET measurements (Micromeritics ASAP 2010, nitrogen adsorption). For analysis of the Fe0 contents in composites and RNIP, the reaction of ZVI to hydrogen in acidic suspension was utilized according to Fe0 þ 2Hþ/Fe2þ þ H2. Only few milligrams of the samples were given into a 4-mL-vial, carefully excluding air. After addition of 1 mL of Ar-purged half-concentrated HCl to the sample and shaking the vial for >10 min, the H2 concentration in the headspace was measured by means of GC/TCD (HP6850 equipped with an HP PLOT column, Agilent Technologies Inc.). Since the preparation procedure of CIC varied, Table SI 3-1, (Supp. Inform.) gives a summary of the products which were mainly used within this study and which represent the particles for each type. Deviation from those materials listed will be stated where discussed.

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Transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) pattern were used to assess iron cluster sizes. Details about these methods are summarized in the Supporting Information section.

2.3.

Dechlorination studies

The dechlorination batch experiments were carried out in 250-mL amber screw-cap bottles equipped with Mininert
valves. Between 20 mg and 7 g Fe0 as RNIP or CIC was added to 200 mL reaction solution (typically deionized water, tap water, 0.2 M NH3/NH4Cl or NaHCO3 buffer solution with/without polyanionic stabilizers), which was extensively purged with N2 prior to adding the reagents. After washing with degassed buffer solution, the particles were given into the reactor and re-dispersed for a few minutes in an ultrasonic bath. The reaction was started by adding the desired amount of trichloroethylene (TCE) as methanolic solution. From this point on, the bottle was continuously shaken (60 rpm). In order to avoid a pH rise in unbuffered reaction solutions, the pH values were adjusted by means of pH-stat titration (Titristat TL alpha, Schott Germany) using 0.1 N HNO3. Most experiments were carried out at pH values between 8 and 9 where pressure increase due to hydrogen formation was low. The reaction kinetics was monitored by headspace analyses of dechlorinated products, using a GCeMS device (QP 5000, Shimadzu Corp., equipped with a 60 m DB1 thick-film (5 mm) capillary column). The analysis of the reaction products (mainly ethylene and ethane) has the advantage that their sorption to AC can be neglected (experimentally confirmed, see Supp. Inform. section) and provides an immediate analytical answer. In addition, the chloride concentration of the reaction medium was analysed using an ion chromatograph (IC25, Dionex, equipped with an IonPac
AS15/AG15). For reasons of direct comparison of different products, in most of the experiments standard conditions were chosen (buffer pH ¼ 8.5, cFe(0) ¼ 1 g L1, cCOC ¼ 30 mg L1).

2.4.

Mobility tests

For exemplary mobility studies, column experiments were conducted using glass-fibre supported PVC columns (di ¼ 1.2 cm, l ¼ 23 cm) packed with a sieve fraction (0.25e0.5 mm) of washed sand (Weferlingen, Germany) and placed in a vertical orientation. Porosity of the bed was ε z 0.4. The packed bed was rinsed with oxygen-free He-flushed eluent for >5 h prior to contact with the colloid suspensions. The freshly prepared re-dispersed suspensions containing CIC or RNIP were pumped through the column from bottom to top. The storage vessel for the suspension was also flushed with inert gas throughout the experiment. The mean pore velocity n for the quartz bed was set to 2 cm min1. During and after the injection of the suspensions, the effluent was cut into samples of 3 mL at the column outlet. The carbon and Fe0 contents were determined by total carbon analysis (TOC analyzer 5000, Shimadzu) and H2 evolution after acidification, respectively. After the particles elution ceased, the bed’s loading with particles and their distribution were measured by slicing the column into 1-cm segments within a glove box. The contents of the segments were suspended in water, re-dispersed in an

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ultrasonic bath and analyzed for their carbon and iron contents as described above.

2.5.

NAPL affinity

Dehalogenation in pure organic phase was carried out in a manner similar to that described above. The reaction media were exhaustively dried TCE or dried TCE with a defined amount of water added. The generation of ethane and ethylene was followed by headspace sampling above the solvent phase. The solubility of the two gases in TCE was taken into consideration. For simulation of particle interaction with organic phases, a TCE droplet (d ¼ 4 mm) was placed into a horizontally placed glass reactor and exposed to a slow-flowing CIC suspension. Using a digital microscope (Keyance Inc.), the interaction of CIC with the droplet was studied (Fig. SI 7-1, Supp. Inform.).

3.

Results and discussion

3.1.

Carbo-Iron colloids

The combination of the porous activated carbon grain with metallic iron in a true composite material is realized in CarboIron
. The density of the water-filled composite material is about 1.7 g cm3. Accordingly, the particle size should be chosen in the range of 0.5e2 mm in order to achieve optimized suspension stability and particle mobility in the subsurface (Elimelech et al., 1995; Tufenkji and Elimelech, 2004). With a size distribution of d50 ¼ 0.8 mm and d90 ¼ 1.6 mm, CIC fit perfectly in this range and form colloidal suspensions in a broad concentration range, thus justifying the term “CarboIron colloids”. Transmission electron microscopy (TEM) images of CarboIron particles demonstrated the close contact between the ACC and the iron structures (Fig. SI 2-1, Supp. Inform.). The majority of the iron occurs as small clusters on the inner carbon surface, most of them about 50 nm in diameter. XRD analysis revealed iron species with mean cluster sizes also in the range of 50 nm which supports the data extracted from TEM images (Fig. SI 1-1, Supp. Inform.). However, further efforts are necessary to apply other surface analyses such as CO or H2 chemisorption in a reproducible manner in order to measure the true surface area accessible for the pollutants. In this study these methods did not lead to success. The freshly produced CIC are pyrophoric. Therefore, several possibilities were evaluated for mildly deactivating the iron surface during synthesis while maintaining the iron content. The optimization of the synthesis is described in detail in Bleyl et al. (2012). Two strategies for reaching this goal were evaluated. (i) Iron was permitted to react with a deactivator to form a surface layer by adding small amounts of steam, oxygen, hydrogen sulphide or hydrogen chloride. (ii) The deposition of carbon or silicon coatings on the iron surface from precursors such as acetylene or silanes aimed at the formation of a protecting layer attached to the iron surface. Most of these protection approaches took place at the expense of the iron content and reduced it drastically. In the end, adding a sufficient amount of HCl (1.5 wt% related to

mCIC) to the pyrophoric material produced the best results. The air-stable CIC maintained nearly all Fe0 and preserved its reactivity in the aqueous phase Bleyl et al. (2012). Table SI 3-1 (Supp. Inform.) provides a summary of synthesis and characterization data of typical CIC.

3.2.

Dechlorination studies

CIC show in principle the same dehalogenation ability and react with the same spectrum of halogenated pollutants as known from nZVI, since the same reaction mechanisms apply. The same products and similar product selectivities are found for the dechlorination of TCE with RNIP, micro-iron and CIC. Unfortunately, with CIC, TCE degradation cannot be monitored by means of headspace analysis because of superposition with adsorption processes. However, both ethene and ethane are only marginally adsorbed at the AC body, which makes them good probes for monitoring the reduction kinetics. Almost no partially dechlorinated intermediates are found from TCE. Ethane, ethylene and traces of acetylene are readily released from the CIC particles. Partially chlorinated intermediates seem to stay adsorbed at the surface, which increases their chance to be fully dechlorinated before detachment and release into the aqueous phase (as chlorinefree C2 hydrocarbons). Whereas chloride is found in the reaction suspension in stoichiometric concentrations, the yield of detected organic products is markedly lower than 100%, as can be seen in Fig. 1. The yield of chlorine-free C2-hydrocarbons in the presented experiment came up to almost 60% of the converted TCE. Kinetic evaluations of the dechlorination reactions therefore were carried out using the product formation kinetics. The TCE degradation was monitored on the basis of the C2 formation as ð1  cC2 ;t =cC2 max;exp Þ, where cC2 ;t is the concentration of the sum of C2-hydrocarbons formed at the reaction time t and cC2 max;exp the maximal occurring C2 concentration within this experiment. A closed C-balance was not reached. As we already know from bare iron (Liu et al., 2007), also CIC are able to initiate coupling reactions of the intermediately formed chlorine-free hydrocarbon species (e.g. acetylene and radicals). By means of extraction of CIC particles after reaction, small fractions of hydrocarbons >C2 up to traces of C8, were found. The portion of the higher-molecularweight products varies depending on the preparation procedure of the CIC. The metal surface-area-normalized dechlorination rate coefficient, kSA (L m2 h1), is used as general descriptor of reactivity when comparing Fe0 reagents rather than the pseudo-first-order rate constant kobs (h1) (Johnson et al., 1996). For calculation of kSA for CIC samples, the specific surface area of the Fe0 clusters in CIC has to be determined. Since CO chemisorption measurements for iron are disputed, and in our case may be additionally affected by various adlayers (Benziger and Madix, 1980), other means of surface area estimation were used. Both TEM and XRD measurements (Supp. Inform.) revealed mean cluster sizes of approximately 50 nm for a variety of CIC samples. Assuming spherical iron structures and a tight cluster size distribution, the specific surface area of the Fe0 comes to as z 15.5 m2/g. For

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A

0 -1

-1

ln(1-c/cmax, exp)

0

5.8 g L Fe 0.58 g L-1 Fe0 0.092 g L-1 Fe0

-2 y0.092 = -0.0089x -3

y5.8 = -0.0101x

-4

y0.58 = -0.0096x

-5 0

100

200

300

400

500

Time [h]

B

8

2.4

k SA CICs

Fig. 1 e Kinetics of TCE dechlorination using air-stabilized MT-Carbo-Iron with 18 wt % Fe0 (upper: product yield, lower: first-order kinetics plot, where (1Lct/c100% conversion) for chloride and (1Lct/cmax, exp) for C2 hydrocarbons represent TCE degradation from chloride- and C2hydrocarbon-formation data, respectively; c0, TCE [ 30 mg LL1, cFe(0) [ 4 g LL1, pH0 [ 9 buffered with 0.25 M NaHCO3 and adjusted with NaOH).

simplification, this value is taken for the calculation of kSA for all CIC samples based on the relationship kSA ¼ kobs/ (as$cFe) ¼ kobs/(as$cCIC$xFe) ¼ kobs/ra, where cFe and cCIC are the Fe0 and CIC suspension concentrations, respectively, xFe is the Fe0 content of the CIC particles and ra (m2 L1) is the surface area concentration of Fe0 in the reaction mixture. As expected from literature data, the observed dechlorination rate of RNIP strictly changes with the iron concentration in the suspension (Johnson et al., 1996), whereas Fig. 2 shows that the kSA for CIC roughly remains constant. Its seen on the one hand that the dechlorination of TCE follows a pseudo-first-order kinetics, and on the other hand that the suspension concentration of Fe0 could be reduced markedly without causing a decrease in the observed dechlorination rate for CIC. kobs for the dechlorination of TCE is almost independent of the total iron concentration in suspension cFe for a wide range of CIC concentrations, as is illustrated by the grey area in Fig. 2 showing the data set for typical MT-CIC. The kSA values, however, differ by more than 1.5 orders of magnitude between kSA ¼ 1$104 L m2 h1 and 6.7$103 L m2 h1 for the highest and the lowest CIC concentrations tested, respectively. In all cases, TCE was found to be predominantly in the sorbed state (Kd, TCE, CIC z 21,500 L kg1 for standard conditions, please see Supp. Inform. for sorption data.). Obviously, the interplay of sorption and reaction can contribute to the higher reactivity,

1.8

k obs CICs

4

1.2

2

0.6

0

k obs . 102 [h-1]

k SA . 103 [L (m2 h)-1]

k SA RNIP 6

0.0 0

1

2

3

4

5

6

7

-1

c Fe [g L ]

Fig. 2 e A) Evaluation of the pseudo-first-order kinetics of the TCE dechlorination with various Carbo-Iron concentrations (MT-CIC-02, sulfide-stabilized Carbo-Iron with 13 wt% Fe0, c0, TCE [ 30 mg LL1, pH [ 8.5 buffered with 0.2 M NH4Cl/NH3-buffer). (B) TCE dechlorination kinetics data for the CIC sample MT-CIC-02 and RNIP applying various Fe0 concentrations (c0, TCE [ 30 mg LL1, pH [ 8.5).

which means a more efficient utilization of the iron reagent. A detailed study of CIC’s interplay between the sorptive accumulation of the pollutants and their efficient chemical destruction will be subject of a forthcoming paper. In the standard reactivity tests (1 g L1 Fe0 and 30 mg L1 TCE at pH ¼ 8.5) higher specific reactivities were observed for the various CIC samples than for all pure iron samples under study. Nevertheless, the CIC samples showed varying reactivity in dechlorination experiments, depending on the iron content and the preparation procedure (Table 1). Those CIC generated by a similar preparation procedure and having comparable iron contents showed a similar behaviour in the test reactions. Therefore, in this study, typical examples for CIC types are presented. Despite many similarities to pure iron, the main difference of CIC is the neighbourhood of CarboIron’s iron to the sorption-active centres of the ACC. The adsorber collects the pollutants such that the Fe centres in CIC are always confronted with a higher local pollutant concentration than its counterpart e the nZVI particle e which only has to deal with the bulk phase concentration of the pollutant in water (calculation of sorptive enrichment in Supp. Inform.). A marked concentration enhancement in the vicinity of the iron centres could in principle be provided by any carbon surface which shows considerable sorption affinity for the

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Table 1 e Selected CIC types and nZVI. Preparation characteristics, iron content and reaction rate coefficients (cFe(0) z 1 g LL1, c0,TCE [ 30 mg LL1, pH [ 8.5, data marked with * at pH [ 9).

MT-CIC-01 MT-CIC-02 MT-CIC-03 MT-CIC-04 MT-CIC-05 MT-CIC-06 (HT)-MT-CIC HT-CIC-01 HT-CIC-02 RNIP

Precursor, stabilization (reductant; Tred)

Fe0 content xFe [wt%]

kobs$102 [h1]

kSA$103 [L m2 h1]

Fe(III)-nitrate, pyrophoric (H2; 500  C) Fe(III)-nitrate, H2S-stabilized (H2; 500  C) Fe(III)-nitrate, H2S-stabilized (H2; 500  C) Fe(III)-nitrate þ NaOH, air-stabilized (H2; 500  C) Fe(III)-chloride þ NH4OH, HCl-stabilized, (H2; 500  C) Fe(III)-nitrate þ NaOH, HCl-stabilized (H2; 500  C) Fe(III)-nitrate þ NaOH, (H2; 500  C), shortly tempered at 700  C, stabilized with TMCSd Fe(III)-citrate, pyrophoric (C, CO; 700  C) Fe(III)-nitrate þ NaOH, HCl-stabilized (C, CO; 700  C) iron oxide nanoparticles (reduction in H2 atmosphere)

19 13 25 19.5 20 19 19

1.95* 1.1 4.2 0.9c* 2.1* 1.25* 2.75

1.25a 0.7 2.7 0.6 1.3 0.8 1.8

11.2 20 44 (core)

1.2 1.4 0.95

0.8 0.9 0.4b

a Estimation of specific iron surface area for CIC from mean size of Fe0 structures of 50 nm, derived from TEM images and XRD (see Figs. SI 1-1 and SI 2-1, Supp. Inform.). b SSA measured, meaning an average particle size of d ¼ 40 nm for an average density of r ¼ 6.4 g cm3 (44 wt% Fe0 and 56 wt% magnetite). c Typically, air-stabilized samples have a lag time before the dechlorination starts with its normal reaction rate Bleyl et al. (2012). In the case of MT-CIC-04, the lag time was 100 h. d Trimethylchlorosilane.

target contaminants. Activated carbon is in our opinion the best choice as carrier material because of the enrichment of hydrophobic pollutants by several orders of magnitude and its porous structure. Other carbon materials such as graphite and carbon black as carrier material should also be able to markedly increase the local contaminant concentration, but in contrast to the porous ACC, the iron would be placed at the outer surface of the nonporous carbon grain. The aspired synergistic effect of the combination of carbon and iron towards a more efficient dehalogenation reaction can only be achieved when neither component is an encumbrance to the other. In the case of the mechanical combination of nZVI and graphite by intensive ball milling under inert atmosphere (data not shown), we found that the nZVI particles produced were fully coated by the soft carbon material. The dehalogenation efficiency was therefore drastically diminished. Only after ultrasonic treatment of the reaction suspension, the graphite coat could be broken up and the iron restarted the dehalogenation process with its original reaction rate as known from the uncoated particles.

3.3.

Surface properties

Compared to pure iron, the surface properties of the composite material particles are strongly influenced by the carbon carrier, which is beneficial for transport and hydrophobicity. In some of the recent reports about the combination of iron and carbon materials, carriers with a low porosity (e.g. carbon black) are used, where the iron is mostly placed on the outer surface of the particle grain (Zhan et al., 2011; Sunkara et al., 2010). The overall particle properties in such cases are therefore dominated by the iron itself. Depending on origin and production conditions, AC samples possess various proportions of basic and acidic surface sites. Commercially available AC samples typically have zeta potentials of z ¼ 35 to 20 mV at pH ¼ 7 and moderate ionic strength (Julien et al., 1998). The ACC used in this study have a zeta potential of z ¼ 27 mV at neutral conditions before iron impregnation (Georgi et al., submitted for publication). For the various CIC

samples, zeta potentials were found in the range of z ¼ 7 mV to 29 mV at pH ¼ 7 in 1 mM KNO3 as electrolyte (Fig. SI 6-1, Supp. Inform.). The differences may be due to various Fe contents as well as modified syntheses. Compared to z ¼ þ20 mV measured for pure iron particles, the shift in surface charge to negative zeta potentials is a significant improvement towards higher subsurface mobility. CIC forms stable suspensions for shorter time periods also in concentrations 100 mg L1. Over longer periods of time they are not sufficiently stable; small concentrations of CMC added as stabilizer enhance the suspension stability significantly. Studies on sorption of CMC on CIC revealed that the maximal reachable surface loading is about 7 wt% (Fig. SI 6-2, Supp. Inform.), but CIC suspension with concentrations of about 1e3 wt% CMC (rel. to cCIC) were found to be already sufficiently long-term stable even in tap water. As for pure ACC, it is in accordance with the hypothesis that macromolecules such as CMC are excluded from the inner pore volume of the porous CIC carbon structure and are adsorbed mainly at the outer surface. With low-molecular weight stabilizers, such as surfactants, the maximum loading is much higher (20e35 wt%), which indicates adsorption within the micropores of the ACC (Georgi et al., submitted for publication). This stabilizing effect of CMC on CIC suspensions was also observed in sedimentation experiments which are described in the Supporting Information section.

3.4.

CIC mobility

Zhang (2003) stated, in his review about the application of nZVI in environmental remediation, that the mobility of ironbased NPs is to be regarded as a challenge rather than as a risk. In order to evaluate the transport properties of the particles in any column, the mobility can be defined as an operational parameter (Eq. (1)): particles which are transported through the column are regarded as mobile. Particles which do not reach the exit are immobile.

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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 8 1 7 e3 8 2 6

xmobile ¼

mCIC;out $100% mCIC;in

(1)

In this equation (Eq. (1)), mCIC, out is the effluent mass of CIC particles and mCIC, in is the influent mass of CIC particles. In initial experiments, the mobilities of CIC and RNIP were compared in simplified sand beds. Packed columns of various lengths (l ¼ 20e100 cm) were used. In this work, only an example from a standard test with washed quartz over a bed length of 22 cm will be discussed in order to show the principle transport behaviour of CIC compared to RNIP in the model bed volume (for RNIP transport: e.g. Phenrat et al., 2010 and further work of this group). Further results of mobility studies varying e.g. the bed material (sieved sediment up to undisturbed liners), the injection medium (pH, ionic strength, colloid stabilizer) and the particle concentrations will be subject of a forthcoming paper. With the help of the simplified column experiments, the difference in material transport between CIC and pure iron particles becomes apparent. We compared the transport behaviours of pure ACC and RNIP in a mixed suspension with that of CIC suspensions. Fig. 3A reflects the mobility characteristics of the pure single materials through the column whereas Fig. 3B shows the mobility of the composite particles. As expected, ACC (D in Fig. 3A) is much more mobile than iron ( in Fig. 3A). When CIC suspensions are pumped through the column, the simultaneous transport of both components (iron and carbon) is observed as would be expected for particles of a true composite material (simultaneous appearance of the elements C and Fe at the column outlet in Fig. 3B). This can be observed even under the more unfavourable pH conditions (pH ¼ 7 instead of pH ¼ 8.1 for the mixture). Compared to pure iron, CIC show a markedly improved mobility in the test experiments. After no particles

were eluted anymore, the bed loading and the particle distribution were measured by slicing the glass-fibre supported PVC column into 1-cm segments within a glove box. The content of each segment was analyzed for its carbon and iron contents. The deposited particles formed a thin but homogeneous layer on the silica grains (Fig. 3C). The loading under more environmental-near conditions (real sediment, longer contact, lower flow velocity.) is expected to be considerably higher. However, the experiment under simplified laboratory conditions clearly shows that the composite material achieves the desired effect of improved mobility in comparison to pure iron.

3.5.

The affinity of the subsurface reagent to NAPL is regarded as a precondition for a successful source attack. Unfortunately, nZVI particles are repelled by the hydrophobic solvent phase. In practical applications, the necessary hydrophobicity is provided e.g. by emulsifiers (Quinn et al., 2005), forcing the particles into the NAPL phase, but further diminish the mobility which would also be necessary to reach the target zones. The hydrophobicity of the ACC carrier a priori gives the CIC a higher affinity to the residual organic phase. Often, the affinity of particles to the solvents is illustrated by their partitioning within a two-phase system in shaken vials (see Supp. Inform.). Different affinities can be shown in this way. However, this procedure cannot be used as a model for real groundwater conditions. In this paper, undisturbed groundwater flow is simulated by a horizontal flow of highly diluted CIC suspension over an immobile NAPL droplet. The solvent collects and enriches the particles at its surface, as can be seen from the microscope images in Fig. 4. How much material can

A

B 90

60

mobility [%]

90 mobility [%]

Carbo-Iron affinity to NAPL

ACC RNIP

30

60

carbon

30

iron 0

0 5

10

15

0

sediment loading with CIC [mg kg-1]

exchanged pore volumes

5

10

15

exchanged pore volumes

C 100

20

CIC iron 50

10

0

0 5

7

9

11

13

15

17

19

travel distance in sediment bed [cm]; (column inlet: l=0 cm)

21

sediment loading with Fe [mg kg -1]

0

Fig. 3 e Results of column experiments using washed quartz sand (sieve fraction 0.25e0.5 mm) as fixed bed (lcolumn [ 22 cm; V_ susp [ 1 mL minL1, cCMC [ 0.20 cparticle). (A) Mobility of a mixed suspension of RNIP and ACC particles (cACC [ 145 mg LL1, cFe(0) [ 57 mg LL1, pH [ 8.1) in comparison to the (B) Mobility of CIC; analysis of the single components travelling through the column (cCIC [ 219 mg LL1, xFe(0) [ 14.6 wt%, pH [ 7). (C) Loading of the sediment matrix in experiment (B) after CIC passage and flushing with water until particle elution finished.

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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 8 1 7 e3 8 2 6

Fig. 4 e Images taken with a digital microscope (Keyence). A: Fresh droplet of TCE, B: Enrichment of CIC at the surface of the TCE droplet in water after 30 min of particle contact and rinsing with fresh water (ddroplet [ 4 mm, cCIC [ 10 mg LL1, nsuspension [ 0.3 cm minL1), C: Particle enrichment at the wateresolvent interface.

phase. It may lead to different results at different reaction conditions (e.g. c0, TCE and pH value). The main advantage of CIC over pure iron in this context is that CIC are able to travel to residual phases without needing to be forced to enter them: the organic phase collects and enriches the CIC at the watereorganics interphase.

n (1-c/cmax,exp) relative to Fe availability

reaction time [h]

formation of ethylene as c/c max, theory Fe content [%]

be transported to source zones is not yet known and has to be studied. This still remains as one of the challenges for application as in-situ method. However, providing the material at the scene of action e at the NAPL-water interface where all reactants are present in maximum concentrations is a precondition of successful reaction. In addition, the ability of CIC to efficiently dechlorinate TCE depends on the presence of water (Berge and Ramsburg, 2010). Fig. 5 shows that CIC suspended within dry TCE show no reaction. In a two-phase system with equal volumes of TCE and water, CIC show reaction kinetics comparable to those in a TCE phase where only a 6-fold stoichiometric surplus of water to iron is introduced. In both cases, CIC are completely suspended inside the TCE phase. Under groundwater conditions, the residual organic phase is always watersaturated. In contrast to experiments in aqueous suspension such as depicted in Fig. 1 (iron-excess conditions), Fig. 5 reflects the iron consumption kinetics rather than the TCE conversion (iron-deficient conditions). Surprisingly, the initial 1st order rate coefficient kobs, Fe z 0.8$102 h1 is in the same order of magnitude as kobs, TCE z 1.1$102 h1 measured in the aqueous suspensions under iron-excess conditions (Table 1). In terms of initial specific reaction rates of ZVI these are higher in the 1 1 wet TCE phase (1$102 gTCE g1 Fe(0) h , c0, TCE ¼ 1464 g L ) than in the aqueous phase where iron is provided in excess 1 at c0, TCE ¼ 30 mg L1). The higher TCE (3.3$104 gTCE g1 Fe(0) h concentration in the TCE phase outperforms the unbeneficial reaction medium. However, the result of this comparison is only valid for the chosen reaction conditions in the aqueous

100

80

0

50

100

150

200

250

0.0 -0.5

y = -0.0081x -1.0 -1.5 -2.0

60

40

2 phases: TCE and H2O TCE phase + 25 µl H2O

20

TCE phase

0 0

100

200

300

400

500

600

700

reaction time [h]

Fig. 5 e Reaction of MT-CIC-02 in the presence of a TCE phase. The ethylene formation is shown relative to the maximal possible ethylene yield. Comparison of dried and water-saturated TCE phase (VTCE [ 20 mL, mCIC [ 100 mg, xFe, CIC [ 13 wt%), insertion: logarithmic presentation of the initial reaction kinetics in the 2-phase system.

w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 8 1 7 e3 8 2 6

4.

Conclusion

In summary, the present paper describes the behaviour of the composite material Carbo-Iron for several major properties in comparison to RNIP, a widely studied in-situ subsurface reagent. It could be shown that CIC can provide tailored surface properties. Particle size and surface charge were favourable for better particle transport in column experiments, which minimised the need for additional colloid stabilizer. The direct comparison in column tests showed that the material is more mobile than nZVI, which was one of the intentions for combining activated carbon colloids with nanostructured iron. The close combination of both materials is also beneficial for efficient pollutant destruction e the sorptive enrichment of the hydrophobic contaminants seems to assist the chemical reaction. Organic solvent droplets which were brought into contact with CIC suspensions were found to collect the particles at the wateresolvent interface. The combination of all the properties and observations described above makes CIC appear suitable for both plume control and source attack. At present, the materials’ ecotoxicological behaviour is being studied and the first field test is in progress.

Acknowledgements The authors thank the BMBF (German Federal Ministry of Education and Research) for financial support within the research projects SAFIRA and Fe-NANOSIT.

Appendix A. Supplementary data Supplementary data related to this article can be found online at doi:10.1016/j.watres.2012.04.013.

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