Enhanced Adsorption of 2,4-Dichlorophenol by Nanoscale Zero

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Enhanced adsorption of 2, 4–dichlorophenol by nanoscale zerovalent iron loaded on bentonite and modified with a cationic surfactant Hong Liu, XIA RUAN, Dongye Zhao, Xianyuan Fan, and Tao Feng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03864 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Enhanced adsorption of 2, 4–dichlorophenol by nanoscale zero-valent iron loaded on bentonite and modified with a cationic surfactant Hong Liu, * a Xia Ruan, a Dongye Zhao, b Xianyuan Fan a and Tao Feng a a.

College of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan, 430081, China. Environmental Engineering Program, Department of Civil Engineering, Auburn University, Auburn, AL 36849, USA Corresponding author's E-mail address：[email protected] b.

ABSTRACT: Bentonite-supported nanoscale zero-valent iron (Bent-nZVI) was modified by cetyltrimethylammonium bromide (CTMAB), a cationic surfactant, to form a composite material (Bent-nZVI/CTMAB) for adsorption and reduction of 2, 4-dichlorophenol (2, 4-DCP). The results show that the removal rate of 2, 4-DCP increased from 7.6% by Bent-nZVI to 66.6% by Bent-nZVI/CTMAB, and this improvement increased with increasing concentration of the modifier. The surfactant modification altered the Zeta-potential of bentonite from negative to positive and increased the specific surface area of Bent-nZVI from 1.18 m2 g-1 to 2.01 m2 g-1, which was beneficial to the adsorption of neutral or anionic 2, 4-DCP onto Bent-nZVI/CTMAB. SEM image, FTIR spectra and HPLC chromatograms showed that 2, 4-DCP existed in Bent-nZVI/CTMAB in the form of needle-like crystals and was not reduced by the nZVI particles after the removal. Yet, the possible reduction intermediates, 2-chlorophenol, 4-chlorophenol and phenol, were not detected in the solution or solid phase after the removal of 2, 4-DCP. Hence, the enhanced removal of 2, 4-DCP was mainly attributed to the adsorption rather than the reduction. contaminants, cationic surfactants are often used as a 1. INTRODUCTION modifier of bentonite.12-13 Zhou et al.14 utilized bentonite modified with octadecyl dimethylbenzyl ammonium Nanoscale zero-valent iron (nZVI) has been a focus of chloride to remove 2, 4-dichlorophenol (2, 4-DCP) and the wastewater treatment and groundwater remediation due to removal efficiency increased from 14.0% by bentonite to its lower redox potentials and larger specific surface area 99.3% by modified material. The improvement of compared with commercial iron powder.1-2 Choe et al.3 adsorption capability of modified bentonite toward reported the reduction of trichloroethylene (TCE) by nZVI hydrophobic organics was attributed to the change of surface and the main dechlorination products were ethane and property of bentonite from hydrophilic to hydrophobic methane. Shih et al.4 investigated to degrade owing to the substitutions of inorganic cations (i.e., Na+ and hexachlorobenzene using nZVI and found that 40% of Ca2+) in bentonite interlayers by organic cations.15 In general, hexachlorobenzene was dechlorinated in 96 hours at 25 oC. bentonite modified with organic surfactants is often referred However, nZVI is easy to agglomerate into larger to as organobentonite. particles, resulting in a significant decrease of reductive Since organobentonite has better adsorption performance reactivity. Some improvements have been developed to to organic contaminants than raw bentonite, researchers overcome this disadvantage. Frost et al.5 applied natural have loaded nZVI on the organobentonite to facilitate palygorskite as the carrier of nZVI to enhance its reactivity reductive removal of pentachlorophenol (PCP) from for degradation of methylene blue. Zhang et al.6 utilized aqueous solution, and reported that the material could kaolin-supported nZVI to remove heavy metal ions. rapidly and completely dechlorinate PCP to phenol with an Bentonite is a kind of natural clay mineral as efficiency of 96.2% within 2 h.16 Nevertheless, it was palygorskite and kaolin and has been used extensively due generally difficult to obtain a complete dechlorination of to its low-cost, higher adsorption capacity and special PCP by iron alone. For instances, Kim et al.17 found that layered structure.7-8 It was reported that the removal 2,3,4,5-tetrachlorphenol and 2,3,5,6-tetrachlorphenol were efficiency of Cr (VI) was increased from 66% to 99% by the main reduction products of PCP in ZVI system. Cheng et using bentonite as the support of nZVI.9 al.18 also reported that there was only a trace amount of Bentonite is hydrophilic and negatively charged under phenol detected and the concentration of 2-chlorophenol typical environmental conditions, while organic (2-CP) cumulated was lower than 1 mg L-1 in the contaminants are generally hydrophobic. Consequently, the dechlorination of 50 mg L-1 PCP by nZVI. On the other adsorption of organic contaminants, particularly nonionic or hand, as mentioned above, 2, 4-DCP could be adsorbed with anionic organics, onto raw bentonite is less effective.10-11 To an efficiency of more than 90% by bentonite modified with enhance its adsorption performance towards organic

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

octadecyl dimethylbenzyl ammonium chloride.14 Therefore, it is necessary to determine the key mechanism underlying the enahcned removal of hydrophobic chlorinated hydrocarbons by nZVI/bentonite/surfactant composites, i.e., to what extents the enhanced removal was due to adsorption and reductive dechlorination. This knowledge is closely associated with the subsequent treatment of the target organic contaminants. In this study, bentonite-supported nZVI was prepared firstly and then bentonite-nZVI (denoted as Bent-nZVI) was modified with a common cationic surfactant, known as cetyltrimethylammonium bromide (CTMAB), to form a bentonite-nZVI/CTMAB composite (denoted as Bent-nZVI/CTMAB). 2, 4-DCP was employed as a model contaminant to evaluate the reactivity and adsorption capacity of the material. Bentonite, Bent-nZVI, and Bent-nZVI/CTMAB before and after the removal of 2, 4-DCP were characterized by a scanning electron microscopy (SEM). The efficiencies of various materials for the removal of 2, 4-DCP were tested. High performance liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FTIR) and ion chromatograph (IC) were employed to investigate the mechanisms of 2, 4-DCP removal by Bent-nZVI/CTMAB. Zeta potential, median diameter and specific surface area of bentonite, Bent-nZVI and Bent-nZVI/CTMAB modified at different CTMAB concentrations were measured to understand the effects of surface properties on the removal mechanism. 2. MATERIALS AND METHODS 2.1. Chemicals and Materials. Cetyltrimethylammonium bromide was purchased from Yuanju Bioscience Technology Company (China). Bentonite used in this study was obtained from Youxing Company (China). Nanoscale zero-valent iron was purchased from Nanjing Emperor Nano Material Company (China). Other chemicals were of analytical grade, and were purchased from Tianli Chemical Reagent Company (China), including NaBH4, FeSO4·7H2O, NaOH, 2, 4-dichlorophenol and iron powder. All chemicals were used as received. All solutions were prepared with distilled water. 2.2. Preparation and Characterization of Materials. To prepare Bent-nZVI/CTMAB, 5.6 g bentonite was placed into 0.2 M FeSO4 solution (250 mL), the mixture was stirred for 30 min at 250 rpm under a nitrogen atmosphere. Subsequently, the NaBH4 solution with the same

Page 2 of 10

concentration and volume as FeSO4 solution was added dropwise. After the addition of NaBH4 solution, the mixture was stirred for 15 min. Then, 100 mL CTMAB solution (prepared at 33, 82.5, 165, 330 and 660 mM by dissolving the surfactant with 50~80 ℃ distilled water) was added to form a mixed liquor, with the CTMAB concentration being 5.5, 13.7, 55, 82.5 and 110 mM, respectively. The mixture was stirred for another 20 min and then the particles formed were settled and separated from the mixture by vacuum filtration. The particles were washed three times with ethanol and then vacuum-dried. The dried materials were sealed in plastic sample bags and kept in kerosene. The reduction of FeSO4 by NaBH4 could be described by Equation (1), (1) The preparation of Bent-nZVI followed a similar procedure as described above except that no CTMAB was added in this case. For both Bent-nZVI/CTMAB and Bent-nZVI, the mass ratio of bentonite to Fe0 was set at 2:1. For the preparation of Bent/CTMAB, 250 mL FeSO4 solution and 250 mL NaBH4 solution were replaced by 500 mL distilled water. Then, CTMAB was loaded on bentonite following similar procedure as described above except only 33 mM CTMAB solution (100 mL) was added to the mixture. The concentrations of 2, 4-DCP, 2-chlorophenol (2-CP), 4-chlorophenol (4-CP) and phenol were determined by a high performance liquid chromatography (UltiMate 3000, DIONEX) equipped with a C-18 column. A kind of methanol-water mixture (75:25, V/V) was used as the mobile phase. The corresponding analytical wavelengths were 285 nm, 280 nm and 275 nm for 2, 4-DCP, 2-CP/4-CP and phenol, respectively. The concentration of Cl- was measured by an ion chromatograph (ICS-90, DIONEX). The morphology of the materials was characterized using a scanning electron microscope (Nava 400 Nano, FEI). FTIR spectra were recorded using a FT-IR spectrometer (Vertex 70, Bruker). The surface potential was analyzed by a Zeta-potential analyzer (Zeta Probe, Colloidal Dynamics). The median diameter and specific surface area of the materials in aqueous solution were measured by a laser diffraction particle size analyzer (Mastersizer 2000, Malvern). 2.3. Batch Experiments. Batch experiments were carried out in flasks with 3-necks under a nitrogen atmosphere. Different materials were added respectively into 300 mL 2,


Page 3 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4-DCP solutions at the dosage of 1 g L-1. At given time intervals, the samples were taken and filtered through a 0.45 µm membrane, and then filtrates were analyzed for 2, 4-DCP, 2-CP, 4-CP and phenol. The removal rate of 2, 4-DCP can be expressed as: (2) where C0 and Ct are the concentrations of 2, 4-DCP at time 0 and t, respectively. For the analyses of 2, 4-DCP and the possible reduction products (2-CP, 4-CP and phenol) in the solid phase, selected solid samples were separated from the liquid phase by vacuum filtration, and then extracted with 10 mL methanol before analysis. The extraction was assisted with ultrasonication (20 kHz) for 120 min. 3. RESULTS AND DISCUSSION 3.1. SEM Images of Materials. Figure 1 shows the SEM images of bentonite, Bent-nZVI, Bent-nZVI/CTMAB and Bent-nZVI/CTMAB after the 2, 4-DCP removal. As shown (a)

(b)

in Figure 1(a), bentonite exhibited a lamellar structure, however, these lamellas bonded together due to the hygroscopicity of bentonite. Figure 1(b) shows that bentonite-supported nZVI particles were spherical with a diameter of around 123 nm. Some nZVI particles were dispersed owing to the loading on bentonite while others aggregated heavily. In contrast, it is evident from Figure 1(c) that the particle size of nZVI in Bent-nZVI/CTMAB was decreased to about 85 nm and the particles were much better dispersed. Furthermore, most nZVI particles in Bent-nZVI/CTMAB were connected with each other to form many dendritic chains, which was attributed to the magnetic interactions between nZVI particles.19-21 The chain-like, interconnected nZVI particles remained separated and did not aggregated into dense clusters as in Bent-nZVI (Figure 1(b)), which can be attributed to the steric hindrance of CTMAB. CTMAB molecules inserted into the interlayers of bentonite and hindered the aggregations of nZVI, as illustrated in Figure 2(c). (c)

(d)

Figure 1. SEM images of bentonite (a), Bent-nZVI (b), Bent-nZVI/CTMAB (c) and Bent-nZVI/CTMAB after the removal of 2, 4-DCP (d).

This result demonstrates that there were more active sites in Bent-nZVI/CTMAB to adsorb or react with contaminants, and nZVI particles in Bent-nZVI/CTMAB were able to contact more effectively with contaminants than that in Bent-nZVI. Moreover, as shown in Figure 1(d), a number of needle-shaped crystals appeared in Bent-nZVI/CTMAB after the 2, 4-DCP removal. These crystals conform to crystalline morphology of 2, 4-DCP, as the crystal of 2-CP, 4-CP, or phenol is not needle-like. In addition, the corrosion products of nZVI after the reaction are plate-like crystals. Thus, it can be speculated that at least a portion of 2, 4-DCP was removed by the adsorption or surface crystallization on Bent-nZVI/CTMAB. 3.2. Removal Rates of 2, 4-DCP by Various Materials. As shown in Figure 2(a), the removal rate of 2, 4-DCP by Bent-nZVI was only 7.6% in 120 min, even lower than that by raw bentonite (25.5%). The iron powder and nZVI achieved 4.4% and 13.4% removal, respectively. In

contrast, Bent-nZVI/CTMAB removed 66.6%, and Bent/CTMAB 89.9%, increasing the removal by 59.0% and 64.4% while compared to Bent-nZVI and bentonite, respectively. Obviously, the enhanced removal was directly related to the modifier, CTMAB. Figure 2(b) shows the results of 2, 4-DCP removal by Bent-nZVI/CTMAB modified at different CTMAB concentrations. As the concentration of CTMAB increased from 5.5 mM to 55 mM, the removal rate of 2, 4-DCP at 90 min increased rapidly from 54.3% to 80.3%. However, when the CTMAB concentrations were more than 55 mM, the rate enhancement became less significant. For Bent-nZVI/CTMAB, some CTMAB molecules can diffuse into the bentonite interlayers and be adsorbed by cation exchange. Concurrently, CTMAB can also be adsorbed on the surface of bentonite via both cation exchange and hydrophobic bonding,22 which could change the hydrophilic surface of bentonite to hydrophobic surface



100

(a)

rate of 2,4-DCP (%)

90 80 70 60

Bent/CTMAB Bent-nZVI/CTMAB Bentonite nZVI Bent-nZVI Iron powder

50 40 30 20 10 0

Removal rate of 2,4-DCP (%)

100

Removal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 10

80 70 60 50 40

20

40

60

80

100

120

110 mM CTMAB 82.5 mM CTMAB 55 mM CTMAB 13.7 mM CTMAB 5.5 mM CTMAB

30 20 10 0

0

(b)

90

0

20

Time (min)

(c)

40

60

80

100

120

Time (min)

(d)

Figure 2. The removal rates of 2, 4-DCP by different materials (a) and by Bent-nZVI/CTMAB modified with different CTMAB concentrations (b). Experimental conditions: [2, 4-DCP]0 = 0.300 mM, initial pH =5.7, material dosage = 1.0 g L-1. The schematic diagram of Bent-nZVI modified with CTMAB (c). The schematic diagram of solubilization of CTMAB micelles toward 2, 4-DCP and electrostatic attraction between CTMAB micelles and bentontie (d).

(Figure 2(c) presents a schematic diagram of the modification), and thus increase the affinity of Bent-nZVI/CTMAB towards hydrophobic 2, 4-DCP. Furthermore, CTMAB is a large organic cation with 12 carbon atoms and can create an organic partition medium for 2, 4-DCP uptake through the conglomeration of their flexible alkyl chains. However, when the equilibrium concentration of CTMAB was greater than the critical micelle concentration (CMC, the CMC of CTMAB is 0.9 mM at 30 oC), CTMAB micelles (as shown in Figure 2(d)) will be formed. On the one hand, the partition of 2, 4-DCP into the hydrophobic cores of the micelles could enhance the removal rate of 2, 4-DCP;23 moreover, as the shell of CTMAB micelles is positively charged while bentontie is negatively charged, some micelles will be immobilized on bentonite due to the electrostatic attraction between the CTMAB micelles and bentontie (shown in Figure 2(d)). The immobilized micelles may increase the uptake of 2, 4-DCP through a solubilization.24 On the other hand, the free micelles in the solution can compete for 2, 4-DCP, which inhibits the removal of the contaminant. As a result, at lower concentrations of CTMAB, more micelles or hemimicelles are formed on the bentonite surface, and the

removal rate of 2, 4-DCP increased with increasing CTMAB concentration. At elevated surfactant concentrations, the bentonite surface became more occupied and more free micelles formed. Moreover, as illustrated in Figure 2(b), the removal rate was much faster in the first 10 min, where 40-80% of 2, 4-DCP was removed. The rapid initial rate indicates that the adsorption played a major role rather than the dechlorination reaction as the latter would take much longer time. Similar results were reported by others. Kim et al.17 tested the PCP removal by ZVI and found that sorption may account for more than 50% removal of the initial mass of PCP. The subsequent more gradual removal can be due to both adsorption and reduction reaction.17 Recycling tests were conducted to analyze the stability of Bent-nZVI/CTMBA. 67.5% of 2, 4-DCP (50 mg/L) was removed by Bent-nZVI/CTMBA modified with 13.7 mM CTMAB after 120 min. Subsequently, the spent material was filtered and dried. Its regeneration was performed at 120 oC for 2 h under an air atmosphere to decompose 2, 4-DCP and avoid CTMAB being oxidized. The first recycling exhibited 58.1% removal towards 2, 4-DCP, the second 49.6%, and the third 45.2%. Thus, the loss rate of adsorption capability of Bent-nZVI/CTMBA was less than


10% while it was used repeatedly. 3.3. Characterizations of Surface Properties of Materials. As discussed in 3.2, the adsorption of cationic CTMAB could result in the development of positive charge on the surface of Bent-nZVI, and the formation of surface CTMAB micelles and hemimicelles may shield the negative potential and surface reactive sites of bentonite in Bent-nZVI/CTMAB. To verify it experimentally, Zeta-potential, median diameter and specific surface area (measured in aqueous solution by a Malvern Mastersizer 2000) of raw bentontie, Bent-nZVI and Bent-nZVI/CTMAB were determined (Figure 3). As shown in Figure 3(a), Zeta potential of raw bentontie was -2.52 mV while that of Bent-nZVI was 0.55 mV, which may be ascribed to that some surfaces of bentontie were occupied by zero-valent iron particles. The CTMAB modified Bent-nZVI exhibited a more positive Zeta-potential and this trend increased with increasing concentration of cationic CTMAB, indicating that CTMAB altered the surface charge property of Bent-nZVI. Apparently, the modified materials would favorably take up both neutral and anionic forms of 2, 4-DCP (i.e., it can work well over a broad pH range). Hence, the removal rate of 2, 4-DCP by Bent-nZVI/CTMAB was enhanced owing to the added CTMAB sinks as well as the more favorable surface potential. However, as the CTMAB concentration was greater than 55 mM, Zeta potential of Bent-nZVI/CTMAB decreased from 1.93 mV to 1.57 mV. The decline of Zeta potential can be due to partial (a)

12

1.93mV

Median diameter Specific surface area

2.01 2 -1 mg

11

2.0

0.55mV

0.5

0.65mV

Bent Bent-nZVI

-0.5

Bent-nZVI/ Bent-nZVI/ Bent-nZVI/ Bent-nZVI/ CTMAB-5.5 CTMAB-13.7 CTMAB-55 CTMAB-110

-1.0 -1.5

9.50 um

10 9 8

8.30 1.53 um 2 -1 mg

7.60 um

7.98 um 1.41 2 -1 mg

1.6

1.4

7 1.18 2 -1

6

-2.0

1.8

1.61 2 -1 mg

mg

5.21 um

1.14 2 -1 mg

1.2

2

-1

1.34mV

1.0

-2.5

11.65 um

(b)

1.57mV

1.5

0.0

coverage of the cationic heads due to accumulation of more surfactant molecules on the bentonite surface and enhanced stabilization of nZVI (i.e., some nZVI particles are released from the surface by CTMAB). Figure 2(b) shows variations of median diameter and specific surface area. The median diameter (denoted as D50) is used to represent the equivalent diameter of the largest particle when the cumulative distribution percentage of particles reached 50% at the curve of particle size distribution. D50 of Bent-nZVI (11.65 µm) was larger than that of bentonite (8.30 µm) owing to the loading of nZVI, while that of Bent-nZVI/CTMAB-5.5 (5.21 µm) decreased markedly. The drop of D50 should be attributed to the disaggregation of bentonite caused by the insertion of CTMAB, and the dispersing of nZVI resulting from the steric hinerance of C-C chain of CTMAB and the stabilization effects of the surfactants on the nZVI particles. Nevertheless, as the concentration of CTMAB increased from 5.5 mM to 110 mM, D50 increased gradually from 5.21 µm to 9.50 µm, due to accumulation of more CTAMB on the surface, and the specific surface area decreased from 2.01 m2 g-1 to 1.14 m2 g-1 accordingly. The drop of specific surface area can be explained as a result of the generation of CTMAB micelles. The generation of CTMAB micelles reduced the amount of CTMAB molecules inserting into bentontie interlayers and caused the charge neutralization with bentontie, which would promote aggregation and decrease the surface area of Bent-nZVI/CTMAB.

Median diameter (um)

2.0

Zeta Potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Specific surface area (m g )

Page 5 of 10

5 1.0 Bent

-2.52mV

Bent-nZVI

Bent-nZVI/ Bent-nZVI/ Bent-nZVI/ Bent-nZVI/ CTMAB-5.5 CTMAB-13.7 CTMAB-55 CTMAB-110

Figure 3. The Zeta-potential (a), median diameter and specific surface area (b) of different materials. Bent-nZVI/CTMAB modified with 5.5, 13.7, 55 and 110 mM CTMAB were denoted as Bent-nZVI/CTMAB-5.5, Bent-nZVI/CTMAB-13.7, Bent-nZVI/CTMAB-55 and Bent-nZVI /CTMAB-110, respectively.

Combining the results and discussion in 3.2 and 3.3, it was concluded that the surface properties of materials play

an important role in the removal of 2, 4-DCP. Comparatively, the 2, 4-DCP removal was more dependent on immobilized



500

-CH2 2854

3745 O-H

1645 Ar-C=C

O-H

3745

2923 -CH3

-CH2

(c) 2854

1396 C-O 1000

(b)

1645 Ar-C=C

1398 C-O

675 C-Cl

Ar-C=C 1531-1533

2923 -CH3

(a)

680 C-Cl

surfactant molecules (hemimicelles and micelles) and the associated surface charge property, which was evidenced by the consistent alteration of surfactant concentration and Zeta-potential with that of 2, 4-DCP removal rate. The variation of surface area due to the modification exhibited positive effect on the 2, 4-DCP removal only when the concentration of CTMAB was 5.5 mM. Hence, the mechanism of 2, 4-DCP removal by Bent-nZVI/CTMAB is more likely attributed to the adsorption. 3.4. Analysis of FTIR Spectra. Figure 4 shows the FTIR absorption spectra of CTMAB (a), 2, 4-DCP solution with the concentration of 0.300 mM (b), Bent-nZVI/CTMAB after the removal of 2, 4-DCP (c), and bentonite (d). Compared with the spectra of bentonite, there were some peaks assigned to organic function groups appeared in the spectra of Bent-nZVI/CTMAB after the removal. The peaks at 2923 cm-1 and 2854 cm-1 were attributed to the stretching vibrations of methyl (-CH3) and methylene (-CH2), respectively. These peaks also existed in the spectra of CTMAB, indicating that the hydrocarbon groups of CTMAB entered into bentonite. The bands at 1531-1533 cm-1 and 1645 cm−1 were attributed to the overlap of aromatic C=C skeletal vibration, and the peaks at 680 cm-1, 1398 cm-1 and 3745 cm-1 were assigned to the stretch vibration of C-Cl, C-O and O-H, respectively. Meanwhile, all of these peaks were also observed in the spectra of 2, 4-DCP solution, which demonstrated that 2, 4-DCP was adsorbed on Bent-nZVI/CTMAB after the removal. This result was consistent with needle-like SEM image of 2, 4-DCP observed in Figure 1(d) and verified the adsorption mechanism of 2, 4-DCP removal from another perspective. Additionally, the peaks related to Si-O-Al/Mg (around 500 cm-1) and Si-O-Si (around 1050 cm-1) were not altered, implying that the crystal structure in bentonite was hardly affected by the modification. 3.5. Analysis of Intermediates of 2, 4-DCP Reduction by Bent-nZVI/CTMAB. Figure 5 shows the HPLC chromatogram of 2, 4-DCP solution before and after the 2, 4-DCP removal by Bent-nZVI/CTMAB. The retention time of 2, 4-DCP was 6.437 min. It was found that the area of 2, 4-DCP absorption peak decreased with the increase of removal time. For its possible reduction products, 2-CP, 4-CP and phenol, the positions of absorption peaks located at 4.193 min, 3.942 min and 3.587 min, respectively. There were no peaks appeared at the corresponding retention time of these reduction products after the removal was carried out for 120 min. Moreover, the possible intermediates were also

Al/Mg-O 468 523 Si-O-Al/Mg 792 Si-O 1037 1089 Si-O-Si

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 10

(d)

1531-1533 Ar-C=C

1500

2000

2500

3000

3500

4000

Wavenumber (cm-1)

Figure 4. FTIR spectra of CTMAB (a), 2, 4-DCP solution (b), Bent-nZVI/CTMAB after the removal (c) and bentonite (d).

also determined by setting the measurement wavelength at 275 nm and 280 nm, which were the maximum absorption wavelength of phenol and 2-CP (4-CP), respectively. The results were illustrated in Figure 5(a) and (b). Similarly, no peaks were identified at 4.193 min, 3.942 min or 3.587 min. On the other hand, after 2, 4-DCP was removed by various materials, the concentrations of chloride ions in the solution were measured and the corresponding results were listed in Table 1. [Cl-] varied in the range of 0.02~0.71 mg L-1, indicating that the C–Cl bond on benzene ring were hardly broken in the removal process. 0 min

(b)

(a)

3.942 min

3.942 min

3.587 min

3.587 min

4.193 min

4.193 min

60 min 120 min 3.5

0

3.5

4.0 Time (min)

1

2

3

4.0 Time (min)

4

5

6

7

Time (min)

Figure 5. HPLC chromatograms of 2, 4-DCP solution at different removal time, λmax= 285 nm. (a): HPLC chromatogram of 2, 4-DCP solution after 120 min removal, λmax= 280 nm. (b): HPLC chromatogram of 2, 4-DCP solution after 120 min removal, λmax= 275 nm. Experimental conditions: [2, 4-DCP]0 = 0.300 mM, initial pH = 5.7, material dosage = 1.0 g L-1.

Meanwhile, the contents of 2, 4-DCP and its possible reduction products in the solid phase after the removal were determined by extracting the materials using methanol as an extractant, which was assisted with ultrasonication for 120 min before analysis. In order to compare with [2, 4-DCP] before the removal, the determined contents were divided by 300 mL (it was the volume of 2, 4-DCP solution before the removal) and presented as [2, 4-DCP]solid, [2-CP]solid,


Page 7 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[4-CP]solid and [phenol]solid, respectively (i.e., the content of 2, 4-DCP in the solid phase using nZVI as a removal material was 0.018 mmol, and 0.018 mmol divided by 300 mL equaled to 0.043 mM, then [2, 4-DCP]solid in nZVI after the removal was 0.043 mM). Table 1 shows the concentrations of 2, 4-DCP and its possible reduction products in solution and solid after 120 min removal by various materials. Again, the potential reduction products, 2-CP, 4-CP and phenol, were not detected in the solid phase, which was consistent with the investigation reported by others.25-26 However, the contents of 2, 4-DCP in Bent-nZVI/CTMAB after the removal and Bent/CTMAB after the removal reached 0.178 mM and 0.242 mM, respectively, indicating that 59.3% and 80.7% of 2, 4-DCP was adsorbed by Bent-nZVI/CTMAB and Bent/CTMAB. Therefore, it was verified again that the enhanced removal of 2, 4-DCP was mainly attributed to the adsorption rather than the reduction. Based on the mass balance calculations, the loss of 2, 4-DCP after the removal varied from 4.3% for Bent-nZVI to 9.3% for bentonite, which probably arised from the errors in the measurement, and the mass loss in the operation process as well as the residue of 2, 4-DCP in the materials. The reactions between Fe0 and chlorinated organic were reported as follows:27-29

(3) (4) (5) Hence, a higher reactivity of ZVI could only be obtained when the initial pH of solution is below 5.0 (acidic conditions).30 Furthermore, there was no effective catalyst, such as Pd or Ni, in our system. As a result, very little H2 produced and thus the reaction (5) was limited. Moreover, the C–Cl covalent bond on the benzene ring is stable and difficult to be broken. On the other hand, the CTMAB micelles could retard the contacting of nZVI with 2, 4-DCP, which was unfavorable to the 2, 4-DCP reduction by nZVI.31 However, a great amount of literatures reported that ZVI or nZVI could effectively degrade chlorinated organic solvents.32-34 The reasons might be manifold. If Fe0 was pretreated or washed using H2SO4 or HCl, the reaction (3)-(5) could occur more easily.35 In addition, the reductions may be carried out at a greater dosage of ZVI or a higher temperature.36-37 Another reason for rapid and complete dechlorination of chlorinated organic may be the coating iron with palladium38 or nickel39 as a catalyst.

Table 1. The concentrations of 2, 4-DCP and its possible reduction products in solution and solid after removal by various materials Bent-nZVI

bentonite

iron powder

nZVI

Bent-nZVI/ CTMAB

[Cl-] (mg L-1) [2-CP] ([2-CP] solid) (mM) [4-CP] ( [4-CP] solid) (mM) [phenol] ( [phenol] solid ) (mM) [2, 4-DCP] (mM) [2, 4-DCP]solid (mM) [2, 4-DCP]+[2, 4-DCP]solid (mM) Loss rate of 2, 4-DCP (%)

0.71 0.54 0.03 0.02 0.38 0.284 0.229 0.227 0.266 0.102 0.003 0.043 0.051 0.018 0.178 0.287 0.272 0.278 0.284 0.280 4.3 9.3 9.1 5.3 6.7 Experimental conditions: [2, 4-DCP]0= 0.300 mM,initial pH = 5.7, material dosage = 1.0 g L-1, removal time = 120 min.

4. CONCLUSIONS Compared with Bent-nZVI, the removal of 2, 4-DCP by Bent-nZVI/CTMAB was enhanced owing to the modification with CTMAB. The variations of 2, 4-DCP removal rates were dependent on the changes of surfactant loading and surface properties of Bent-nZVI/CTMAB, especially on Zeta potential. Furthermore, it was found from SEM image, FTIR spectra and HPLC chromatogram that adsorbed 2, 4-DCP in Bent-nZVI/CTMAB was present as needle-like crystals and was not reduced by the nZVI particles. In addition, the typical reduction intermediates (2-CP, 4-CP and phenol) were not detected

Bent/ CTMAB 0.43 0.037 0.242 0.279 7.0

in both aqueous and solid phases after the 2, 4-DCP removal. Hence, we can conclude that the enhanced removal of 2, 4-DCP was attributed to the added sink effect of Bent-nZVI/CTMAB due to the modification of Bent-nZVI with CTMAB, rather than the reduction reaction by nZVI. The results may help prepare new composite materials for more effective removal of 2,4-DCP and other persistent chlorinate organic compounds. ACKNOWLEDGEMENTS This work was supported by National Science



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Foundation of China (Grant 41230638), the Research Funds from Department of Enviromental Protection in Hubei Province (Grant 2015HB05), and Excellent Doctorial Dissertation Cultivation Grant from Wuhan University of Science and Technology (2016WKD06).

Page 8 of 10

(10) Zhu, L. Z.; Chen, B. L. Sorption behavior of p-nitrophenol on the interface between anion-cation organobentonite and water. Environ. Sci. Technol. 2000, 34, 2997-3002. (11) Kang, Q.; Zhou, W. Z.; Li, Q.; Gao, B. Y.; Fan, J. X.; Shen, D. Z. Adsorption of anionic dyes on poly (epicholorhydrin dimetylamine) modified bentonite in REFERENCES single and mixed dye solutions. Appl. Clay. Sci. 2009, (1) Hwang, Y.; Lee, Y. C.; Mines, P. D.; Yun, S. H.; 45, 280-287. Andersen, H. R. Nanoscale zero-valent iron (nZVI) synthesis in a Mg-aminoclay solution exhibits (12) Tian, S. L.; Zhu, L. Z.; Shi, Y. Characterization of sorption mechanisms of VOCs with organobentonites increased stability and reactivity for reductive using a LSER approach. Environ. Sci. Technol. 2004, decontamination. Appl. Catal. B-Environ. 2014, 147, 38, 489-495. 748-755. (2) Sun, Z. M.; Zheng, S. L.; Ayoko, G. A.; Frost, R. L.; Xi, (13) Bartelt-Hunt, S. L.; Burns, S. E.; Smith, J. A. Nonionic organic solute sorption onto two Y. F. Degradation of simazine from solutions by organobentonites as a function of organic-carbon diatomite-supported nanosized zero-valent iron content. J. Colloid, Interf. Sci. 2003, 266, 251-258. composite materials. J. Hazard. Mater. 2013, 263, (14) Zhou, Y.; Jin, X. Y.; Lin, H. F.; Chen, Z. L. Synthesis, 768-777. characterization and potential application of (3) Choe, S.; Lee, S. H.; Chang, Y. Y.; Hwang, K. Y.; Khim, organobentonite in removing 2, 4-DCP dichlorophenol J. Rapid reductive destruction of hazardous organic 0 from industrial wastewater. Chem. Eng. J. 2011, 1, compounds by nanoscale Fe . Chemosphere, 2001, 42, 176-183. 367-372. (4) Shih, Y. H.; Hsu, C. Y.; Su, Y. F. Reduction of (15) Shen, Y. H. Preparation of organobentonite using nonionic surfactants, Chemosphere, 2001, 44, 989-995. hexachlorobenzene by nanoscale zero-valent iron: (16) Li, Y. M.; Zhang, Y.; Li, J. F.; Zheng, X. M. Enhanced Kinetics, pH effect, and degradation mechanism. Sep. removal of pentachlorophenol by a novel composite: Purif. Technol. 2011, 76, 268-274. nanoscale zerovalent iron immobilized on (5) Frost, R. L.; Xi, Y. F.; He, H. P. Synthesis, organobentonite. Environ. Pollut. 2011, 159, characterization of playgorskite supported zero-valent 3744-3749. iron and its application for methylene blue adsorption. (17) Kim, Y. H.; Carraway, E. R. Dechlorination of J. Colloid. Interf. Sci. 2010, 341, 153-161. pentachlorphenol by zero-valent iron and modified (6) Zhang, X.; Lin, S.; Lu, X. Q.; Chen, Z. L. Removal of zero-valent iron. Environ. Sci. Technol. 2000, 34, Pb(Ⅱ) from water using synthesized kaolin supported 2014-2017. nanoscale zero-valent iron. Chem. Eng. J. 2010, 163, (18) Cheng, R.; Zhou, W.; Wang, J. L.; Qi, D. D.; Guo, L.; 243-248. Zhang, W. X.; Qian, Y. Dechlorination of (7) Tahir, S. S.; Rauf, N. Removal of Fe(Ⅱ) from the pentachlorophenol using nanoscale Fe/Ni particles: wastewater of a galvanized pipe manufacturing role of nano-Ni and its size effect. J. Hazard. Mater. industry by adsorption onto bentonite clay. J. Environ. 2010, 180, 79-85. Manage. 2004, 73, 285-292. (19) Zhang, L.; Manthiram, A. Chains composed of (8) Zhu, L. Z.; Li, Y. M.; Zhang, J. Y. Sorption of nanosize metal particles and identifying the factors organobentonite to some organic pollutants in water. driving their formation. Appl. Phys. Lett. 1997, 70, Environ. Sci. Technol. 1997, 31, 1407-1410. 2469-2471. (9) Shi, L. N.; Lin, Y. M.; Zhang, X.; Chen, Z. L. Synthesis, (20) Shih, Y. H.; Tai, Y. T. Reaction of decabrominated characterization and kinetics of bentonite supported diphenyl ether by zerovalent iron nanoparticles. NZVI for the remaoal of Cr(VI) from aqueous solution. Chemosphere, 2010, 78, 1200-1206. Chem. Eng. J. 2011, 171, 612-617.


Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Phenrat, T.; Saleh, N.; Sirk, K.; Tilton, R. D.; Lowry, G. V. Aggregation and sedimenttation of aqueous nanoscale zerovalent iron dispersions. Environ. Sci. Technol. 2007, 41, 284-290. (22) David, R. B.; Christopher, P. A. In situ modification of an aquifer material by a cationic surfactant to enhance retardation of organic contaminants. J. Contam. Hydrol. 1992, 10, 325-337. (23) Wagner, J.; Chen, H.; Brownawell, B. J.; Westal, J. C. Use of cationic surfactants to modify soil surfaces to promote sorption and retard migration of hydrophobic organic compounds. Environ. Sci. Technol. 1994, 28, 231-237. (24) Zhao, X.; Gong, Y. Y.; O’Reilly, S. E.; Zhao, D. Y. Effects of oil dispersant on solubilization, sorption and desorption of polycyclic aromatic hydrocarbons in sediment–seawater systems. Mar. Pollut. Bull. 2015, 92, 160-169. (25) Alessi, D. S.; Li, Z. H. Synergistic effect of cationic surfactants on perchloroethylene degradation by zero-valent iron. Environ. Sci. Technol. 2001, 18, 3713-3717. (26) Campbell, T. J.; Burris, D. R.; Roberts, A. L. Trichlorethylene and tetrachloroethylene reduction in a metallic iron-water-vapor batch system. Environ. Toxicol. Chem. 1997, 16, 530-625. (27) Wei, J. J.; Xu, X. H.; Wang, D. H. Catalytic dechlorination of o-chlorophenol by nanoscale Pd/Fe. J. Environ. Sci. 2004, 16, 621-623. (28) Hu, J. Z.; Chen, S. J.; Xie, N. Z. Zero-valent iron dechlorination of 4-chlorophenol in soils. Ecol. Environ. 2005, 14, 349-352. (29) Cheng, R.; Wang, J. L.; Zhang, W. X. Comparison of reductive dechlorination of p-chlorophenol using Fe0 and nanosized Fe0. J. Hazard. Mater. 2007, 144, 334-339. (30) Xiong, Z. K.; Yuan, Y.; Lai, B.; Yang, P.; Zhou, Y. X. Comparative study on the reactivity of Fe/Cu

bimetallic particles and zero valent iron (ZVI) under different conditions of N2, air or without aeration. J. Hazard. Mater. 2015, 297, 261–268. (31) Saleh, N.; Kim, H. J.; Phenrat, T.; Matyjaszewski, K.; Tilton, R. D.; Lowry, G. V. Ionic strength and composition affect the mobility of surface-modified FeO nanoparticles in water-saturated sand columns. Environ. Sci. Technol. 2008, 42, 3349-3355. (32) Lien, H. L.; Zhang, W. X. Nanoscale iron particles for complete reduction of chlorinated ethenes. Colloid Surface A. 2001, 191, 97-105. (33) Lowry, G. V.; Johnson, K. M. Congener-specific dechlorination of dissolved PCBs by microscale and nanoscale zerovalent iron in a water/methanol solution. Environ. Sci. Technol. 2004, 38, 5208-5216. (34) Song, H.; Carraway, E. R. Reductive of chlorinated ethanes by nanosized zero-valent iron: kinetics, pathways, and effects of reaction conditions. Environ. Sci. Technol. 2005, 39, 6237 -6245. (35) Ravary C.; Lipczynska-Kochany, E. E. 209th Natl. Meet., Am. Chem. Soc., Div. Environ. Chem. 1995, 35, 738-740. (36) Alessi, D. S.; Li, Z. H. Synergistic effect of cationic surfactants on perchloroethylene degradation by zero-valent iron. Environ. Sci. Technol. 2001, 18, 3713-3717. (37) Chuang, F.; Larson, R. A.; Wessman, M. S. Zero-valent iron-promoted dechlorination of polychlorinated biphenyls. Environ. Sci. Technol. 1995, 29, 2460 -2463. (38) Wang, C. B.; Zhang, W. X. Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs. Environ. Sci. Technol. 1997, 31, 2154-2156. (39) Han, Y.; Li W.; Zhang, M. H. Catalytic dechlorination of monochlorobenzene with a new type of nanoscale Ni(B)/Fe(B) bimetallic catalytic reductant. Chemosphere, 2008, 72, 53-58.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Only


Page 10 of 10

Enhanced Adsorption of 2,4-Dichlorophenol by Nanoscale Zero

Recommend Documents