Environmental Remediation of Chlorinated Hydrocarbons Using

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10976-10985

pubs.acs.org/journal/ascecg

Environmental Remediation of Chlorinated Hydrocarbons Using Biopolymer Stabilized Iron Loaded Halloysite Nanotubes Yang Su,† Yueheng Zhang,† Hang Ke,‡ Gary McPherson,§ Jibao He,∥ Xu Zhang,‡ and Vijay T. John*,† †

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Department of Chemical & Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States ‡ School of Environment, Tsinghua University, 30 Shuangqing Rd, Beijing 100084, China § Department of Chemistry, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States ∥ Coordinated Instrumentation Facility, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States S Supporting Information *

ABSTRACT: Dense nonaqueous phase chlorinated compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE) are widespread groundwater and soil contaminants which cause long-term environmental pollution. Extensive efforts have been carried out to develop materials for in situ remediation particularly using nanoscale zerovalent iron (NZVI) to reduce TCE to relatively innocuous products such as ethane and ethylene. A novel technology is described here that uses earthabundant natural tubular aluminosilicate clays known as halloysite (HNT) to support NZVI. These systems are efficient at the reductive dechlorination of such chlorinated hydrocarbons indicating a pseudo-first order rate constant of 0.1 L g−1 h−1 with NZVI particle size between 5 and 10 nm. The adsorption of the naturally derived polyelectrolytes chitosan and carboxymethyl cellulose on the surface of HNT provides easy dispersibility in aqueous solutions and colloidal stability to the NZVI supported on HNT, with chitosan adsorption leading to stability over a period of 60 h. Observations of transport through packed capillaries using optical microscopy indicate that these biopolymerstabilized composites transport efficiently through porous media at flow rates representative of groundwater flow. Such efficient transport is attributed to the tubular morphology with the particles aligning along flow streamlines. Calculations of the sticking coefficient indicate values as low as 0.1 indicating low attachment to sediment. Such composite materials using sustainable biopolymers and earth abundant clay minerals have potential in the groundwater remediation of chlorinated ethenes. KEYWORDS: Halloysite nanotubes, NZVI, DNAPL, Chlorinated ethene

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INTRODUCTION Trichloroethylene (TCE) is a widespread soil and groundwater contaminant that causes long-term environmental pollution.1−3 TCE is sparingly soluble in water (solubility of 1100 ppm) and has a density of 1.46 g/mL. Due to these properties, TCE spills travel deep into the subsurface of aquifers, and the contaminant accumulates in bedrock fractures and in aquifer pores, from which it slowly dissolves in groundwater forming a plume of contaminated groundwater that is difficult to remediate.1 Extensive efforts have been carried out to develop remediation methods for TCE pollution such as soil vapor extraction, bioremediation, and in situ injection of reactive agents.4−8 Among these approaches, the in situ injection of nanoscale zerovalent iron (NZVI) particles into contaminant plumes is a simple, low cost, and environmentally benign technology and has become a preferred method.5,6,9 The decontamination reaction follows the overall redox reaction: 0

+

C2HCl3 + 4Fe + 5H → C2H6 + 4Fe

2+

−

+ 3Cl

© 2017 American Chemical Society

The objective of in situ injection is to have the dechlorination agent (NZVI) stay colloidally suspended in groundwater and follow groundwater migration, thus continually breaking down the dissolved TCE in a contaminant plume. Compared to granular iron particles, NZVI particles exhibit superior reactivity toward TCE dechlorination because of the large specific surface area of nanoparticles.10 However, the high surface energy and the intrinsic ferromagnetism of NZVI particles larger than the superparamagnetic limit lead to a rapid aggregation of NZVI particles. These agglomerates decrease the mobility of NZVI particles, thus limiting the application of NZVI particles in in situ injection processes. Various hydrophilic or amphiphilic species such as starches, vegetable oils, surfactants, and some polyelectrolytes such as sodium carboxymethyl cellulose (CMC) and poly(acrylic acid) (PAA) have been used to stabilize the NZVI Received: August 18, 2017 Revised: September 21, 2017 Published: October 10, 2017

(1) 10976

DOI: 10.1021/acssuschemeng.7b02872 ACS Sustainable Chem. Eng. 2017, 5, 10976−10985

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) SEM and TEM images of halloysite nanotubes. (b) Halloysite structure showing the Si rich outer layer and the Al rich inner structure of a scroll leaflet.

applications in catalysis.24−27 Halloysite nanotubes (HNTs) constitute a 1:1 aluminosilicate clay mineral, chemically similar to kaolin with the empirical formula Al2Si2O5(OH)4. As shown in Figure 1, HNTs naturally occur as small scroll-like clay sheets which are caused by a lattice mismatch between the two different layers comprising the clay sheet. The external surface consists of siloxane (Si−O−Si) groups and the internal surface consists of aluminol (Al−OH) groups. The structural isomorphic substitution of Si for Al leads to the unequal charge distribution between the cationic Al-rich inner surface and the anionic Si-rich outer surface.28 Naturally occurring HNTs have lengths varying from 0.5 to 10 μm although a typical length is around 0.5−2 μm as shown in Figure 1. The outer diameter of halloysite is around 100 nm with the inner lumen approximately 20 nm. Halloysites thus have aspect ratios ranging from 5 to 20.29 The spacing between the scrolled layers is 0.7 nm in the dehydrated state leading to 10−15 sheets per nanotube. Our objective in this work is to demonstrate the use of halloysite as a support for NZVI to conduct the dechlorination reaction in groundwater saturated sediments. Of relevance is the development of methods to prevent aggregation of the particles and to retain colloidal suspension stability in groundwater. Additionally, the tubular morphology of halloysite allows it to align axially with groundwater streamlines and may enhance transport through porous media. Figure 2 illustrates such transport through porous media indicating that the relatively narrow diameter of halloysite allows penetration through pores not accessible to the isometric spherical morphology. These concepts are elaborated upon in subsequent sections.

Figure 2. Schematic of tubular particles that align along groundwater flow streamlines. Such alignment allows transport through pores that are impenetrable by isometric spherical particles.

particles in order to prevent the particle aggregation.11−18 These organic species can be adsorbed to the surfaces of NZVI particles and provide steric or electrostatic repulsions between particles. A second approach is to stabilize NZVI particles by immobilizing the NZVI particles on solid supports, such as porous silica particles, carbon particles,19−22 and activated carbon.23 Such immobilization follows principles of supported catalyst systems used in heterogeneous catalytic reactions, with the objective of achieving high surface areas for reaction and preventing metal particle aggregation. In contrast to supported metal catalysis in a stationary reactor, however, there is the added aspect that the particles move with groundwater flow, leading to the requirement of a colloidally stable particulate system. In situ injection requires large quantities of material, and there is therefore the need to use earth abundant materials both to mitigate manufacturing costs and to minimize the environmental footprint of manufactured materials. In this paper, we report our findings on the use of halloysite, a natural aluminosilicate clay with a nanotubular morphology, as a support for NZVI. The use of halloysite nanotubes as a support material for metal particles is of much interest due to their high porosities, inertness, and abundance leading to potential

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EXPERIMENTAL SECTION

Materials. Ferric chloride hexahydrate (FeCl3·6H2O), sodium borohydrate (NaBH4, 99%), tetrachloroethylene (PCE, C2Cl4, 99%), trichloroethylene (TCE, C2HCl3, 99%), 1,2 dichloroethylene (mixture of cis- and trans-1,2-DCE, C2H2Cl2, 99%), 1,1 dichloroethylene (1,1-DCE, C2H2Cl2, 99%), sodium carboxymethyl cellulose (mean MW = 90 000), chitosan (low molecular weight, 60K−120 K), and Rose Bengal (C20H2Cl4I4Na2O5, dye content 95%) were purchased from Sigma-Aldrich and used as received. Halloysite nanotubes (HNTs) were obtained from Sigma-Aldrich. Ottawa sand particles (EMD, CAS 10977


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Figure 3. (a) TEM images of HNTs loaded with iron nanoparticles with the inset being a high resolution image of iron nanoparticles. Loading of iron within the lumen of halloysite is due to capillary influx of the precursor solution into the lumen. (b) EDS analysis of HNT-NZVI and pristine HNT. 14808-60-7, Fisher) were sieved, and sand grains >300 μm were used in capillary and column experiments. Deionized (DI) water, produced from an Elga water purification system (Medica DV25) to a resistance of 18.2 MΩ was used in all experiments. Halloysite-NZVI Composite Preparation. The wetness impregnation method was used to prepare halloysite-FeCl3 (HNT-FeCl3) composites. In detail, 0.3 g of HNTs were loosely packed in a container with dropwise addition of FeCl3 solution (184 g/L) up to 500 μL. The dense slurry was then dried into a powder. Nanoscale zerovalent iron loaded halloysite (HNT-NZVI) was prepared by using liquid phase NaBH4 reduction as previously reported,30 leading to an iron loading amount of 6.4 wt %. Specifically, 0.8 g of HNT-FeCl3 particles was placed in a vial followed by the dropwise addition of 5 mL of 0.5 M NaBH4 solution. After cessation of visible hydrogen evolution, the particles were centrifuged and washed with N2 purged D.I. water before use. The HNTs were labeled with Rose Bengal according to the procedures reported by Dorota.31 Thus, 2.5 g of HNTs was added into 100 mL of 10 × 10−5 M Rose Bengal aqueous solution, and the suspension was stirred continuously for 96 h at ambient temperature while being kept in the dark. The excess Rose Bengal was washed away by centrifugation until the supernatant showed no adsorption at 548 nm (the maximum absorption band for Rose Bengal) using UV/vis adsorption spectroscopy. Characterization and Analysis. Scanning electron microscopy (SEM, Hitachi S-4800, operated at 3 kV) and transmission electron microscopy (TEM, FEI Tecnai G2 F30, operated at 200 kV) along with energy-dispersive spectroscopy (EDS) were used to characterize the pristine HNT and the HNT-NZVI composite. The dechlorination reactivity was tested through batch experiments using TCE dechlorination as the specific example. Thus, 0.8 g of HNT-NZVI was dispersed in 20 mL of water and placed in a 40 mL reaction vial equipped with a Mininert valve. 0.1% Pd (w/w of NZVI) was added using K2PdCl6 aqueous solution.32 15 μL of a TCE stock solution (20 g/L TCE in methanol) was spiked into this vial, resulting in an initial TCE concentration of 15 ppm. The reactions were monitored through headspace analysis33 using a HP 6890 gas chromatograph (GC) equipped with a J&W Scientific capillary column (30 m × 0.32 mm) and a flame ionization detector (FID). The sample was injected splitless at 220 °C. The oven temperature was held at 75 °C for 2 min and then ramped to 150 °C at a rate of 25 °C/min and maintained at 150 °C for 8 min to ensure adequate peak separation. Optical microscopy (Leica DMI REZ) was used to characterize the transport properties of the HNT-NZVI composite in packed capillary systems. We also used confocal microscopy (Leica SP2 AOBS 2004) to observe the transport of Rose Bengal-labeled halloysites (HNT-RB)

through the packed capillaries. To facilitate discussion of the results, further aspects of the capillary transport experiments are described in conjunction with the observations.

Figure 4. (a) TCE removal from solution and gas product evolution rates for HNT-NZVI composite. M/Mo is the fraction of the original TCE remaining and P/Pf is the ratio of the gas product peak to the gas product peak at the end. (b) Representative GC trace of headspace chromatographic analyses showing TCE degradation and reaction product evolution at various reaction times. 10978


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Figure 5. (a) Structures of carboxymethyl cellulose (CMC) and chitosan. (b) Sedimentation curves of 0.25 g/L HNT-NZVI in 0.5% (w/w) CMC solution, 0.5% (w/w) chitosan solution and water. One mM NaCl was added as the electrolyte. The normalized turbidity is defined as the ratio of the measured time-dependent turbidity to the initial turbidity of the colloidal suspensions.

gaseous product. The corresponding chromatogram in Figure 4b indicates negligible intermediates and an evolving gaseous product (ethane+ ethylene lumped into a single chromatographic peak). A pseudo first order initial rate constant km of 0.0912 L g−1 h−1 is calculated based on the mass of zerovalent iron, with the assumption of first order kinetics serving as a quantitative guideline to compare various systems.34 The fast evolution of the major products and the relative absence of intermediates such as dichloroethylene and vinyl chloride which tend to stay adsorbed until complete conversion to ethane indicate that the HNT-NZVI system is effective in the reductive dechlorination of TCE. The morphology of the HNT-NZVI composite particles after reaction is shown in Figure S2a (Supporting Information) indicating transformation of spherical particles of zerovalent iron to sheet-like structures after oxidation. Enhancement of Colloidal Stability through Polymer Adsorption. The colloidal stability of HNT particles is essential to assess the ability of these materials to follow contaminant plumes in groundwater and thus design appropriate in situ remediation methods. In this experiment, the sedimentation of a suspension containing 250 mg/L of HNT-NZVI was used to monitor colloidal stability, with the addition of electrolyte (1 mM NaCl) to simulate natural groundwater conditions.35 Results of the sedimentation experiment are shown in Figure 5b. We note that even pristine HNT-NZVI particles take approximately 24 h to sediment, an indication of minimal aggregation. Subsequent experiments were done with the addition of the water-soluble biopolymers carboxymethylcellulose (CMC) and chitosan. Structures of the two sustainable polymers are shown in Figure 5a. CMC is a cellulose derivative which can be easily obtained by replacing the CH2OH group in the glucose unit with a carboxymethyl group. As a food-grade additive, CMC has been widely used as a cost-effective stabilizer in industries, food and

Table 1. Zeta Potential Measurement of HNT-NZVI Suspensions solution 0.25 g/L HNT-NZVI in 1 mM NaCl solution 0.25 g/L HNT-NZVI in 1 mM NaCl solution with 0.5% w/w CMC 0.25 g/L HNT-NZVI in 1 mM NaCl solution with 0.5% w/w chitosan

Z potential (mV) −29.12 −39.83 45.42

The colloidal stability of HNT-NZVI suspensions was monitored with a nephelometric turbidimeter (DRT100B, Scientific Inc.) over a period of 60 h. Zeta potential values of the HNT-NZVI suspensions were done by measuring the electrophoretic mobility using the phase analysis light scattering (PALS) technique (Nanobrook ZetaPALS, Brookhaven Instruments). Macroscopic method (column breakthrough tests) combined with capillary microscopy method were used to study the transport characteristics of HNT-NZVI particles.

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RESULTS AND DISCUSSION Morphological Characterizations. Figure 3a shows the TEM image of HNT-NZVI. The inset is a high resolution TEM, indicating NZVI particles in the 5−10 nm range have been deposited on the HNT surface and are relatively evenly distributed. EDS analysis (Figure 3bi) confirms the presence of iron on the HNT. The control sample of pristine HNT (Figure 3bii) only exhibits elemental peaks of Al, Si, and O, with the Cu elemental peak attributed to the TEM sample grid. The X-ray diffraction and X-ray photoelectron spectroscopy data of the HNT-NZVI composite are shown in the Supporting Information, S1. Reactivity Characteristics of Pristine HNT-NZVI Composites. Figure 4a illustrates the removal of TCE from solution indicating both removal of TCE and evolution of the 10979


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ACS Sustainable Chemistry & Engineering medical fields.36−38 Chitosan is a cationic linear polysaccharide obtained from the deacetylation of chitin. Derived from species of fungi as well as seafood processing wastes such as shrimp and crab shells, chitin is the second most abundant natural biopolymer, after cellulose.39,40 Chitosan is biodegradable and biocompatible and has been extensively studied in a variety of applications including water treatment, biomaterials, and food additives. Both CMC and chitosan are widely available and inexpensive. We observe that the colloidal stability of the HNT-NZVI composite is greatly enhanced with the addition of these polymers, with 60% of HNT-NZVI particles remaining suspended after 60 h in 0.5% (w/w) CMC solution and 76% of HNT-NZVI particles remaining suspended in 0.5% (w/w) chitosan solution. Zeta potential measurements shown in Table 1 indicate the reversal of charge from a negative zeta potential upon CMC adsorption, to a strongly positive zeta potential upon chitosan adsorption. We propose that the enhancement of stability with chitosan is the result of adsorption of the cationic biopolymer to the Si rich anionic exterior of halloysite. With CMC, adsorption may primarily occur within the lumen on the cationic inner surface of halloysite, and the greater enhancement in colloidal stability with chitosan may reflect the attachment of chitosan to the external surface. The choice of biopolymer to be used is a function of the porous media characteristics, as it is important that the polyelectrolyte coated particles do not become adsorbed onto the sediment grains. Nevertheless, the use of widely available biopolymers such as CMC and chitosan to enhance HNT-NZVI colloidal stability points to the development of sustainable technology. The Supporting Information, S3, lists viscosity versus shear rate for CMC and chitosan stabilized HNT suspensions indicating that viscosity levels remain close to those of water (1 mPa s) at polymer concentrations up to 0.25 g/L. The low viscosities imply applicability of such material suspensions for direct injection into TCE contaminated groundwater saturated sediments. The question arises as to whether the adsorbed polymer layer on the HNT nanotubes inhibits reactivity. Accordingly, we conducted dechlorination reactivity tests with the adsorbed polymer. The reactivity of HNT-NZVI particles with 0.5% (w/w) CMC toward TCE is shown in Figure 6a, with the corresponding test of HNT-NZVI particles with 0.5% (w/w) chitosan shown in Figure 6b. In all experiments, 0.8 g of HNT-NZVI particle was used and the initial TCE concentration was 15 ppm. Both systems indicate efficient reaction with the pseudo first order rate constant km determined as 0.104 and 0.121 L g−1 h−1 for the CMC and chitosan stabilized HNT-NZVI composite, respectively. The values are fully comparable to the values obtained for pristine HNT-NZVI, and the slight enhancement may be a consequence of TCE adsorption to the polymer thus enhancing the local concentration of TCE in the vicinity of the NZVI. Dechlorination reactivity tests of HNT-NZVI particles toward PCE, 1,2-DCE and 1,1-DCE are shown in the Supporting Information (S4). In all cases, high reactivities to the chlorinated ethenes are observed indicating the efficiency of HNT-NZVI composite particles. Transport Characteristics of HNT-NZVI Composite Particles. Transport characteristics of the HNT-NZVI particles were examined through capillary transport experiments that we had developed earlier.41 The advantage of capillary experiments is the minimal use of materials and the ability to visualize bands of the dark colored HNT-NZVI particles moving through the capillary. Additionally, optical microscopy imaging of the capillary

Figure 6. TCE removal from solution and gas product evolution rates for HNT-NZVI composite with (a) 0.5% w/w CMC and (b) 0.5% w/w chitosan. M/Mo is the fraction of the original TCE remaining and P/Pf is the ratio of the gas product peak to the gas product peak at the end.

enables visualization of particle aggregates deposited on sediment grains. Figure 7a shows the schematic experimental setup. In detail, a glass capillary open at both ends (1.5−1.8 mm i.d × 100 mm length, Corning, NY) was packed with wet Ottawa sands to a 3 cm length. Then 30 μL of particle suspension was injected into the capillary, and the capillary was placed horizontally to simulate horizontal groundwater flow. A continuous water flow was provided by a syringe pump at a flow rate of 0.1 mL/min for 5 min. The exit point of the capillary was plugged with glass wool in order to maintain the capillary packing during water flush and also to collect the eluted particles. Optical microscopy was used to observe the transport of the particles through the packed sand grains. Figure 7b shows the photograph of the capillary with the HNT-NZVI suspension, and Figure 7c shows the same capillary after the water flush. As shown in Figure 7c, the particles were collected by the glass wool, and there is a small evidence of particle aggregates on the sand grains. In contrast, RNIP particles (10DS, Toda Kogyo) quickly aggregate even prior to the water flush, as shown in Figure 7d. Figure 7e shows that most of the RNIP particles are trapped between the sand grains after the water flush. Capillary transport experiments for CMC and chitosan stabilized HNT-NZVI suspensions are shown in the Figure 8. The sand grains were notably clean after water flush. These images directly demonstrate that the CMC and chitosan stabilized HNT-NZVI particles can readily transport through the porous media of the capillary and become captured by the glass wool. The added polyelectrolytes, adsorbed onto the surface of HNT-NZVI particles, prevent particle aggregation and increase 10980


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Figure 7. Characterization of the capillary transport experiments. (a) Experimental setup: flow rate, 0.1 mL/min; sand length, 3 cm; the injected suspension volume, 0.03 mL. Photographs of capillary (b) containing HNT-NZVI particles and (c) after water flush, (d) containing RNIP suspension, and (e) after water flush. Panels i−iii show optical micrographs of particles at different locations. All scale bars are 100 μm.

mobility of multiwalled carbon nanotubes in porous media, was applied. The collector efficiency is the contribution of effects from particle diffusion (ηD), particle interception by collector grains (ηI), and particle sedimentation due to gravity (ηG)47 with the resulting expression derived for the “side-contact” (the sidewall of the tubular particle contacting the collector) mechanism of collection:

colloidal stability of the suspension through steric and electrostatic repulsion. To increase sensitivity of the capillary setup to microscopic observation, we used Rose Bengal-labeled HNTs (HNT-RB) for the transport experiments that would allow the use of confocal microcopy for fluorescence imaging of the capillary. As shown in Figure 9, after the water flush, there is no fluorescence observed in the sand grain section of the capillary indicating negligible adsorption of HNT-RB to sand grains. The glass wool at the end of the column captures the HNT-RB particles, and fluorescence is clearly observed in this section of the column. While qualitative, the capillary transport experiments are instructive and illustrate the ability of HNTs to move readily through groundwater saturated sediments. As stated earlier, the facile transport characteristics of HNT are ascribed to their tubular structure. Such tubular particles can readily mobilize through porous media in comparison to isometric spherical particles due to their tendency to align with the groundwater flow when their aspect ratio is sufficiently large. Penetration of sediment pores is thus a function of their circular cross sections.42−44 To quantify such an effect, we calculated the single collector efficiency of tubular particles. The collector efficiency (η0) represents the frequency of a particle colliding with the surface of a collector (sediment grain)45 with low collector efficiencies implying enhanced mobility through porous media. To calculate the collector efficiency of the HNTs, the mathematical model developed by Carroll and co-workers46 for the

2/3 ⎡ ⎛ 1 + (1 − ( dP )2 )1/2 ⎞⎤ ⎢ ⎥ l ⎜ ⎟ η0 = ηD + ηI + ηG = 4.03⎢kT ln dP ⎜ ⎟⎥ ⎝ ⎠⎦ ⎣ l 2/3 − 1/3 ⎞ ⎛ d ⎛3 ⎞ l ⎜⎜3πμdCυ0⎜ dP2l⎟ ( )2/3 (1 − ( P )2 )1/2 ⎟⎟ ⎝2 ⎠ dP l ⎠ ⎝ ⎛ ⎞ ⎤ ⎡ ⎛ ⎞2 ⎛ ⎞ ρP − ρ)g (dP2l)2/3 ⎟ 0.146( d d d 1 P + ⎢ ⎜ P ⎟ ⎜3 − ⎟⎥ + ( P )2/3 ⎜ ⎜ μυ [1 − ( dP )2 ]1/2 ⎟ ⎢⎣ 2 ⎝ dC ⎠ ⎝ dP + dC ⎠⎥⎦ l 0 ⎝ ⎠ l ⎛ 1 + [1 − ( dP )2 ]1/2 ⎞ l ⎟ ln⎜ dP ⎜ ⎟ ⎝ ⎠ (2) l

where l is the length of the tubular particle (m), dC is the diameter of the collector (m), dP is the particle diameter (m), ρP is the particle density (g cm−3), ρ is the fluid density (g L−1), μ is the fluid viscosity (kg m−1 s−1), ν0 is the undisturbed uniform flow velocity (m s−1), T is the absolute temperature (K), and k is the Boltzmann constant. For a typical 1 μm long HNT with a diameter of 100 nm, the collector efficiency is calculated to be 0.00087. The single-collector 10981


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Figure 8. Characterization of the capillary transport experiments with polymer stabilized HNT-HZVI. Photographs of capillary (a) containing HNT-NZVI particles with 0.5% w/w CMC and (b) after water flush, (c) containing HNT-NZVI particles with 0.5% w/w chitosan, and (d) after water flush. Panels i and ii show optical micrographs of sediments and particles at different locations. Panel iii illustrates accumulation on glass wool at the end of the capillary. All scale bars are 100 μm.

Figure 9. Characterization of HNT-RB particles in the capillary transport experiment. (a) HNT-RB suspension with 0.5% w/w CMC and (b) HNT-RB suspension with 0.5% w/w chitosan. The particle suspension was injected in the capillary followed by a continuous water flush. Confocal images of (i) sand grains and (iii) glass wool taken in the bright-field mode and (ii) the same sand grains and (iv) glass wool taken in the fluorescence mode (λexc = 561 nm). All scale bars shown in the images represent 10 μm.

collector surface. In other words, when the longitudinal axis of the cylinder and the center of the isometric sphere both lie along a streamline, the sphere surface is closer to the collector thus increasing van der Waals forces and enhancing the probability of capture by the collector. By collecting the particles eluted out of the capillary and the particles adhering to the glass wool, it is possible to determine the attachment efficiency (α) defined as the ratio of particles that

efficiency of the isometric spherical particle (same volume as a cylinder with length 1 μm and diameter 100 nm) with particle diameter dP = 246.6 nm, calculated using the well-established Tufenkji−Elimelech model47 (details in the Supporting Information, section S5) is 0.0027, clearly showing the increased probability of spherical particles being trapped onto sand grains. We note that the greater diameter of the isometric spherical particle implies a greater proximity of the spherical particle to the 10982


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important factors with storage in an inert atmosphere or as suspensions in NaBH4 serving to prevent oxidation.9 All of these are aspects of continuing research essential to translating the concepts described here to technological application.

attach to the sediment to the particles that collide with the grains.45,47 Thus α=−

dC 2 C ln( ) 3 (1 − f )Lη0 C0

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(3)

where dC is the average diameter of sand grains (dC = 500 μm), C is the ratio of particle concentration in the effluent to that in

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02872. X-ray diffraction characterization of HNT-NZVI composite particles (S1); characterizations of HNT-NZVI composite particles after dechlorination reaction (S2); viscosity of CMC and chitosan stabilized HNT suspensions (S3); dechlorination reactivity tests of HNT-NZVI particles toward PCE, 1,2-DCE, and 1,1-DCE (S4); calculations of the single-collector efficiency for spherical particles (S5); and calculations of the attachment coefficient (S6). (PDF)

C0

the influent, f is the porosity of the packed sand grains (f = 0.32), L is the length of the column (L = 0.03 m), and η0 is the single collector efficiency. Regardless of whether CMC or chitosan is used as the colloidal stabilizer, we have found that over 95% of the particles are eluted within one elution volume, with α values in the range of 0.1, the low values indicating rapid elution as indicated by eq 3 (C/C0 → 1). The attachment efficiency can also be determined from column elution experiments20,48,49 and the Supporting Information, section S6, describes such elution experiments using a 50 mL glass buret (diameter = 1.3 cm) packed with wet Ottawa sand. Again, we note the removal of almost all particles within one elution volume when stabilized with CMC or chitosan. The control experiment of nonfunctionalized halloysite shows a reduced level of 60% removal over a single elution volume further implying the advantage of stabilization with sustainable biopolymers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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Vijay T. John: 0000-0001-5426-7585

CONCLUSIONS In summary, we have described a technology for the reductive in situ remediation of chlorinated compounds by using earthabundant, environmentally benign, and sustainable materials. Important aspects of the technology are the following: (1) the use of earth abundant halloysite nanotubes as a support for nanoscale zerovalent iron with the 5−10 nm NZVI particles exhibiting high levels of reactivity in the reductive dechlorination of chlorinated ethenes; (2) the use of sustainable biopolymers such as carboxymethylcellulose and chitosan to enhance colloidal stability of the HNT-NZVI composites in aqueous solutions; and (3) the tubular morphology of HNT that facilitates transport through porous media as seen in capillary and column transport experiments. We feel that these characteristics of the biopolymerstabilized HNT nanotubes indicate potential for using these materials in in situ remediation where the particles are injected into plumes of dissolved contaminants in groundwater saturated sediments, travel with the plumes, and break down the contaminants to light gases that gradually volatilize out of the sediment. While we have introduced the first application of halloysites to in situ groundwater remediation, there are significant opportunities for further improvement. Novel methods of introducing metal particles between the sheets and in cracks of halloysite have recently been introduced by Vinokurov and co-workers.50 Such methods may enhance NZVI loading on halloysite and make the composites less susceptible to reactive site deactivation by groundwater foulants such as natural organic matter. Carbonization of halloysite may additionally improve reactivity by creating adsorption sites for chlorinated compounds thus increasing local concentrations near the reactive iron species.20,51 In contaminated groundwater sediments, the TCE can exist as dissolved contaminants and as bulk material sequestered in pores. The advantage of particle supported NZVI is that the particles can adsorb to the bulk TCE-water interface to form particle stabilized emulsions (Pickering emulsions) to degrade bulk TCE.52 Storage of HNT-NZVI and delivery are additional

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the National Science Foundation (Grant 1236089) is gratefully acknowledged.

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REFERENCES

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