Carbon Foam-Based Hierarchically Porous

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#-FeOOH nanorods/carbon foam based hierarchically porous monolith for highly effective arsenic removal Xiao Ge, Yue Ma, Xiangyang Song, Guozhong Wang, Haimin Zhang, Yunxia Zhang, and Huijun Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01275 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 3, 2017

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ACS Applied Materials & Interfaces

β-FeOOH nanorods/carbon foam based hierarchically porous monolith for highly effective arsenic removal Xiao Ge, †, ‡ Yue Ma, †, ‡ Xiangyang Song, †, ‡ Guozhong Wang, † Haimin Zhang, † Yunxia Zhang∗,† and Huijun Zhao †,§ †

Key Laboratory of Materials Physics, Center for Environmental and Energy

Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China. ‡

§

University of Science and Technology of China, Hefei 230026, P. R. China.

Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia.

Keywords: β-FeOOH NRs/CF, monolith, porous, arsenic, adsorption ABSTRACT: Arsenic pollution in waters has become a worldwide thorny issue, constituting a severe hazard to the whole ecosystem and public health worldwide. Accordingly, it is highly desirable to devise high-performance adsorbents for arsenic decontamination. Herein, a feasible strategy is developed for in-situ growth of β-FeOOH nanorods (NRs) on three dimensional (3D) carbon foam (CF) skeleton via a simple calcination process and subsequent hydrothermal treatment. The as-fabricated 3D β-FeOOH NRs/CF monolith can be innovatively utilized for the arsenic remediation from contaminated aqueous systems, accompanied by remarkably high 1

ACS Paragon Plus Environment


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uptake capacity of 103.4 mg/g for arsenite and 172.9 mg/g for arsenate. The superior arsenic uptake performance can be ascribed to abundant active sites and hydroxyl functional groups available, as well as efficient mass transfer associated with interconnected hierarchical porous networks. In addition, the as-obtained material exhibits exceptional sorption selectivity toward arsenic over other coexisting anions at high levels, which can be ascribed to strong affinity between active sites and arsenic. More importantly, the free-standing 3D porous monolith not only makes it easy for separation and collection after treatment but also efficiently prevents the undesirable potential release of nanoparticles into aquatic environments while maintaining the outstanding properties of nanometer scale building blocks. Furthermore, the monolith absorbent is able to be effectively regenerated and reused for five cycles with negligible decrease in arsenic removal. In view of extremely high adsorption capacities, preferable sorption selectivity and satisfactory recyclability, as well as facile separation nature, the obtained 3D β-FeOOH NRs/CF monolith will hold a great potential for arsenic decontamination in practical application.

1. INTRODUCTION Arsenic species are ubiquitous and mainly originated from the release of arsenic-bearing sediments, mining, pesticide-insufflation, fertilization and coal combustion, as well as industrial wastewater discharge,1-3 which can cause severe damage to the whole ecosystem and human health through the accumulation in the food chain.4 The World Health Organization (WHO) have classified arsenic as a carcinogen and recommended that the permissive arsenic standard in drinking water 2


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cannot be over 10 ppb.5 In order to protect the human health and eliminate the negative effects on the ecological systems, it is highly desirable to decrease the arsenic concentration to below the discharge standard in a highly efficient manner. Inadequate supply of clean and safe water inspires a rapid development of water treatment technology and reuse to meet the needs of both human beings and the environment. During the past decades, a series of purification techniques have been utilized to remove arsenic species from contaminated waters, including chemical precipitation, ion exchange, reverse osmosis, biological treatment and adsorption.6-9 Unfortunately, many of them suffer from more or less severe drawbacks, such as incomplete removal, a large number of reagents and formation of waste sludge. In particular, adsorption is regarded as one of the ideal candidates on account of easy treatment and low cost together with high efficiency. It is demonstrated that developing adsorption-based purification technique is mainly dependent on the absorbent materials, which are thus a durable research hot topic.10-12 Accordingly, extensive explorations have been made to develop various adsorbents for arsenic decontamination, for example active carbon,13 metal oxides,10, 14 zeolites,15 clays and biomass etc.16, 17 However, these widely used adsorbent materials possess inferior uptake capacities and removal efficiency, unable to satisfy the requirements for the real application. By contrast, iron based absorbents have aroused extensive concerns owing to enhanced capturing ability and preferable affinity for arsenic anions together with several additional advantages including low cost, natural abundance and environmental friendliness. Especially, nano-scaled iron oxide/hydroxides display 3



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enhanced arsenic uptake capacities in comparison with micro-sized counterparts.18 Unfortunately, despite great progress in nanomaterials engineering, nanoparticles are inclined to become aggregation during the water treatment processing,19 resulting in the sacrifice of size effect,20 significant reduction of the intrinsic high specific surface area and low durability for long term operation, eventually deteriorating adsorption performance. Besides, there exists great challenges about post-processing of these powder-shaped particles,21-25 which may inevitably increases the release risk of nanoparticles into the treated water bodies. Accordingly, this may result in not only potential damage to both ecosystem and human health, but also increase the water treatment cost,26,

27

which dramatically restricts the practical application of

nanostructured adsorbents in the wastewater remediation. Hence, it is of paramount importance to construct effective adsorbents that can maintain the superior performance and simultaneously ensure facile separation from the solution.28, 29 For this purpose, the design of macroscopic monoliths by integrating active nanomaterials onto 3D substrates with hierarchically porous structure is recognized to be one of the most feasible strategies to the aforementioned issues. The 3D hierarchical materials can take full advantage of the synergistic effects of different components, in which interconnected macropores of the hybrid monolith can guarantee fast mass transfer kinetics, reduce mass transfer resistance;30,

31

while

abundant active adsorption sites associated with mesopores and micropores largely increase the effective contact area with contaminants, consequently promoting water treatment performance.32, 33 In view of the macroporous form of 3D monoliths, they 4


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can effectively circumvent the separation issue of nano-adsorbents from solution without the help of auxiliary magnetic or centrifugation techniques.22, 34-36 In this regard, free-standing CF is considered as a prospective support in terms of the intrinsic features, such as low cost,37 abundant availability, 3D interconnected macropores network,38 environmental friendliness,39 highly accessible surface area and excellent mass transfer ability. Based on the above considerations, herein, 3D β-FeOOH NRs/CF monolith with hierarchical porous structure are innovatively designed and fabricated via the pyrolysis of melamine foam and subsequent hydrothermal treatment in iron chloride solution. The structural characteristics of the resulting 3D monolith are evaluated via various techniques. And the adsorption behaviors of arsenite and arsenate on β-FeOOH NRs/CF monolith, including sorption kinetics, sorption isotherms, effects of competing ions and initial pH on removal efficiency, as well as regeneration and recycling performance are investigated in detail to demonstrate their application for wastewater purification. Furthermore, the possible arsenic removal mechanism is elucidated via X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy analyses.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Melamine foam (MF) was provided by Puyang Green Foam Co. Ltd. Potassium dihydrogen phosphate (KH2PO4, 99.5%), sodium sulfate decahydrate (Na2SO4·10H2O, > 99%), sodium chloride (NaCl, 99.5%), hydrogen chloride (HCl, 36%~38%), sodium nitrate (NaNO3, > 99%), nitric acid 5



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(HNO3, 65%~68%), acetonitrile (C2H3N, > 99%) and iron(Ⅲ) chloride (FeCl3·6H2O, 99%) were obtained from Alfa Aesar. 2.2. Preparation of CF. CF was fabricated via a simple pyrolysis process. Briefly, MF was firstly cut into pieces with the size of 4.5 × 3 × 3 cm. And then the carbonization process was carried out in N2 atmosphere with a temperature ramp of 5 °C/min to 700 °C and kept for 120 min. Next, the as-prepared CF was cooled down to ambient temperature. To endow CF with hydrophilicity and ensure more oxygen-containing functional groups, the obtained CF was further hydrothermally treated with 3 M nitric acid at 120 °C for 1 h. Finally, the treated CF was thoroughly rinsed using deionized (DI) water and dried at 60 °C for the following use. 2.3. Fabrication of 3D β-FeOOH NRs/CF Monolith. β-FeOOH NRs were directly grown on the surface of CF by means of a simple hydrothermal process.40 Briefly, 0.0135 mol of FeCl3 and 0.04 mol of NaNO3 were dissolved in a 40 mL of mixed solution containing 120 µL HCl (37.5%), 11.88 mL of DI water and 28 mL of acetontrile with constantly magnetic stirring for 30 min. Subsequently, a piece of acid-treated CF (3 × 1.5 × 0.6 cm) was immersed to the mixture mentioned above and further stirred for 60 min. Next, the reaction system was carried out for hydrothermal treatment at 100 °C for 4 h. Finally, the resultant β-FeOOH NRs/CF monolith was taken out from the suspension and washed thoroughly to remove the products physically absorbed on CF surface, followed by vacuum drying at 60 °C. 2.4. Characterization. The phase structure of the obtained products was identified by powder X-ray diffraction (XRD, X’Pert-Pro MPD) with Cu Kα radiation (λ = 6


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1.5478 Å). The morphologies and microstructural observations were carried out on field emission scanning electron microscope (FSEM, SU8020, Hitachi) and transmission electron microscope (TEM, Philips TecnaiG2 F20). The element compositions and chemical mappings were examined using energy-dispersive spectroscopy (EDS) equipped in FESEM. FT-IR and Raman spectra of the samples were recorded on Nicolet 6700 spectrophotometer and Renishaw invia Raman microspectrometer, respectively. Zeta-potentials were recorded on Zetasizer (Nano ZS ZEN3600, Malvern Instruments Ltd., UK). N2 adsorption-desorption isotherms were carried out on Autosorb-iQ-Cx (Quantachrome, USA). XPS spectra were obtained on an ESCALAB 250 (Thermo, USA) with the monochromatized X-ray source at 1486.6 eV. The arsenic content in the solution was determined using inductively coupled plasma atomic emission spectrometer (ICP-AES, ICP-6300, Thermo Fisher Scientific). 2.5. Batch Sorption Experiments. A series of batch adsorption experiments were carried out at room temperature and sorption performance of the as-prepared monolith was investigated by altering contact time, initial solution pH and arsenic concentration. 2.5.1. Sorption Kinetics. The uptake kinetic experiments were performed by adding 10 mg of the adsorbent to 20 mL of arsenic solution (10 mg/L). pH of the initial solution was adjusted to the desired value with diluted HCl/NaOH solution (0.1 M). After different duration, part of the supernatant was extracted and the residual arsenic concentration was determined by ICP-AES analyses. 7



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2.5.2. Adsorption Isotherm. Equilibrium studies of arsenic species were evaluated by varying the initial arsenic concentration (1~1000 mg/L) under neutral pH. 10 mg of the adsorbent was added to 20.0 mL of arsenic solution at various initial concentrations and mixed for 24 h on a shaker to reach the adsorption equilibrium. The final arsenic concentration was determined by ICP-AES measurements. 2.5.3. Effect of Initial pH. In order to investigate the influence of initial pH on arsenic uptake, 10 mg of the absorbent was added to 20 mL of arsenic solution (10 mg/L), in which the initial solution pH was adjusted to the desired value (2~10) using diluted HCl or NaOH solution (0.1 M). After adsorption equilibrium, the final arsenic concentration was analyzed using the same method described above. 2.5.4. Effect of Competing Ions. The influence of competing ions on arsenic removal efficiency was investigated in the presence of coexisting ions (SO42-, Cl-, HCO3-, NO3-, PO43-) with two different concentrations (0.1 or 1 mM). The arsenic concentration was fixed at 20 ppm and the dosage of adsorbent was kept at 0.5 g/L. 2.5.5.

Desorption-regeneration

Cycles.

Several

adsorption-desorption

-regeneration cycles were conducted to evaluate the sustainability of β-FeOOH NRs/CF monolith. After the monolith absorbent was saturated with the arsenic solution, it was withdrawn from the treated solution and regenerated by 0.5 M of NaOH solution to remove arsenic species from the sorbent surface, followed by washing with distilled water, 0.05 M HCl and again distilled water. After desorption, the recovered monolith were subjected to the next four cycles of adsorption– desorption with the same procedure as described above. 8


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3. RESULTS AND DISCUSSION 3.1. Structural Analyses. The fabrication of 3D β-FeOOH NRs/CF monolith is achieved by a simple route, as illustrated in Scheme 1. In this design, the commercially available MF is first selected as the raw material for the CF framework. With a simple pyrolysis treatment of MF at 700 ℃ for 2 h in an inert atmosphere, the original white sponge develops into the black CF monolith, accompanied by the obvious volume shrinkage and mass reduction of 83.54%, which can be ascribe to less carbon-containing amount in the original MF.37 Despite the great changes, the 3D framework architecture is perfectly retained in the resulting CF sponge. It should be noted that the obtained CF sponge is highly graphitic and hydrophobic. In order to ensure the following efficient loading of β-FeOOH NRs, the bare CF sponge is subjected to further strong acid treatment to provide abundant oxygen-containing groups, which is indispensible for the following wetting of the iron precursors on its surface. Afterwards, the acid-activated CF monolith is impregnated into an aqueous FeCl3 solution containing NaNO3, HCl and acetontrile, followed by the hydrothermal reaction at 100 °C for 4 h. Based on the corresponding optical image, the resultant sponge is converted from black into khaki, suggesting the successful grafting of FeOOH on CF skeleton. Importantly, carboxyl radicals may play the part of nucleation sites for the heterogeneous crystallization and in-situ construction of β-FeOOH on the surface of the acid-activated CF substrate via the possible complexation effect between Fe3+ and carboxyl groups, which can contribute to strong adhesion between CF and β-FeOOH NRs. The abundant hydroxyl and carboxyl 9



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functional groups endow the resulting β-FeOOH NRs/CF composites with good hydrophilicity. Additionally, the as-fabricated CF and β-FeOOH NRs/CF are in a free-standing monolithic form and retain the foam-like structures similar to the original MF. After loading β-FeOOH NRs, the mass of CF increases from 261 mg to 589 mg. That is, the weight ratio of β-FeOOH NRs to CF in the resultant composite is 1.26: 1. It should be noted that the weak physically absorbed nanoparticles on the outside of β-FeOOH NRs/CF monolith can be removed by simple washing. When exposed to the repeated bending or mild ultrasound treatment, no visible peeling-off occurs, confirming good flexibility and strong interfacial interactions between CF and β-FeOOH NRs, which is favorable for the subsequent recycling use.

Scheme 1. Illustration of the fabrication process of 3D β-FeOOH NRs/CF composite. The morphologies and microstructures of the as-obtained products are firstly investigated by the SEM observation. As displayed in Figure 1a, the pristine CF possesses very clean and smooth surface as well as an interconnected porous framework inherited from the MF sponge, which can serve as the support for the following growth of β-FeOOH NRs. As expected, the CF substrate becomes roughened after the hydrothermal reaction, indicating the anchoring of β-FeOOH NRs 10


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on the surface of CF. Figure 1b–d presents images of β-FeOOH NRs grown on the CF substrate with different magnifications. From the three axes joint of 3D skeleton, the whole CF substrate is uniformly and densely covered by β-FeOOH NRs (Figure 1c). Furthermore, these β-FeOOH NRs are well interwoven and grown nearly vertical to the substrate surface, with the diameter of 100∼180 nm and the length of 450∼700 nm along the growth direction. The element compositions of the resulting composite are clearly identified by EDS spectrum (Figure S1), in which carbon and nitrogen are derived from CF, whereas iron and oxygen are assigned to β-FeOOH. Further EDS mapping analyses (Figure 1e and f) reveal the uniform spatial distribution of Fe, O, N and C elements throughout the examined backbone region of 3D frameworks, providing clear support for the formation of β-FeOOH NRs/CF composite. In addition, TEM observation is employed to gain more information about β-FeOOH NRs and the corresponding image is shown in Figure 1g. Evidently, the single β-FeOOH NR is spindle-shaped, in accordance with SEM observations. And the corresponding high-resolution TEM analysis demonstrates that the interplanar spacing is roughly 0.26 nm, assigned to (400) lattice plane of β-FeOOH (Figure 1h). Simultaneously, the selected-area electron diffraction pattern displays discontiguous spots of orthorhombic FeOOH (inset in Figure 1h), indicates that β-FeOOH is single-crystal. All the aforementioned results provide solid evidence for the successful fabrication of β-FeOOH NRs/CF monolith.

11



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Figure 1. (a) SEM image of bare CF; (b-d) SEM images of β-FeOOH/CF at different magnifications; (e) and (f) EDS mappings of the three axes joint of 3D β-FeOOH/CF skeleton; (g) and (h) TEM and HRTEM images of a single β-FeOOH NR, respectively.

12


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Figure 2. (a) XRD and (b) Raman spectra of CF and β-FeOOH NRs/CF. The crystal structure of the resulting β-FeOOH NRs/CF is studied via XRD analysis, as displayed in Figure 2a. Several typical peaks at 11.8◦, 16.8◦, 26.7◦, 34.0◦, 35.2◦, 39.2◦, 46.4◦, 55.9◦ can be indexed to the (110), (200), (310), (400), (211), (301), (411) and (521) planes of the β-FeOOH (JCPDS 34-1266). In addition, the wide diffraction peak around 25◦ can be ascribed to (002) plane of graphite carbon. The findings further reveal the successful fabrication of β-FeOOH NRs/CF composites, consistent with EDS analyses. Raman spectra are further employed to characterize the lattice vibrational behaviors of pristine CF and β-FeOOH NRs/CF composites. As illustrated in Figure 2b, two vibration peaks at 1361 and 1592 cm-1 are ascribed to D and G bands of CF, respectively. As for β-FeOOH NRs/CF, three new peaks appears at 284, 393 and 685 cm-1 (inset in Figure 2b), which can be attributed to Fe−O and O– H vibrations of β-FeOOH, indicating the successful decoration of β-FeOOH on the CF framework.41 In general, the intensity ratio of D- and G-band (ID/IG) is utilized to assess the carbon hybridization state and the disorder degree of materials. Obviously, ID/IG value of β-FeOOH NRs/CF composite is very close to that of bare CF (1.08 vs.

13



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1.11), indicating that the resulting β-FeOOH NRs/CF monolith retains the structural integrity of the pristine CF framework. Furthermore, the D band of β-FeOOH NRs/CF shows an obvious blue-shift by 22 cm-1 compared with the pristine CF, indicating the strong interaction between β-FeOOH NRs/CF and CF as well as well incorporation of β-FeOOH NRs/CF on the surface of CF skeletons, consistent with the observation in the literature.42 N2 adsorption/desorption isotherm is employed to investigate the specific area and pore characteristic of the resultant β-FeOOH NRs/CF monolith. From Figure S2, it can be seen that the adsorption-desorption curves CF and β-FeOOH NRs/CF exhibit type-IV isotherm with an obvious hysteresis loop, in which the corresponding BET surface areas for CF and β-FeOOH NRs/CF are 10 and 107 m2/g, respectively. The pore size distribution of the resulting β-FeOOH NRs/CF is obtained based on Barret-Joyner-Halenda (BJH) method (inset in Figure S2) and the corresponding pore volume for CF and β-FeOOH NRs/CF are 0.095 and 0.023 cm3/g, respectively. Clearly, the resulting β-FeOOH NRs/CF monolith has a broad pore size distribution from micro/mesopores to macropores, in which the macropores are beneficial for liquid transport and micro/mesopores can largely increase the effective contact area with the contaminant ions. It is noteworthy that the adsorption-desorption branches of the resultant β-FeOOH NRs/CF sample are not in a close loop. The possible reason is the existence of micropores, originated from the inner intervals (ca. 1~2 nm) of the thin β-FeOOH NRs, as shown in Figure S3. In addition, some microporous channels are not open, preventing the efficient escape of the absorbed gas and resulting in the 14


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formation of the unclose loop. Similar unclose N2 adsorption-desorption loops were also found in the previous reports.43, 44 Therefore, it seems reasonable to surmise that the β-FeOOH NRs/CF monolith will ensure abundant sorption sites and feasible mass-transfer paths for enhanced arsenic uptake performance. 3.2. Arsenic Sorption Kinetics. In an effort to investigate the sorption behavior of β-FeOOH NRs/CF monolith towards arsenic species, the time-dependent arsenic adsorption kinetics are presented in Figure 3a, in which the concentration of arsenic solution is 10 ppm at a fixed adsorbent dosage of 0.5 g/L. Obviously, the initial sorption rates for both As(III) and As(V) species are very fast and then reach gradually to an equilibrium within 6 h. For As(V), the removal percentage is as high as 89.3% within just 60 min; after the following 280 min treatment, the residual As(V) is completely removed. By contrast, merely about 65.5% of As(III) is removed within the first 60 min, indicating faster removal kinetics of As(v) than As(III) by β-FeOOH NRs/CF monolith. More importantly, the final As(III) or As(V) concentration is lower than 0.01 ppm after treatment, which satisfies the sanitary requirement for arsenic in drinking water recommended by WHO.5 Therefore, the obtained β-FeOOH NRs/CF monolith can remove effectively As(III) and As (V) at trace levels in a highly efficient manner to meet the stringent arsenic discharge standard. In addition, the obtained kinetics data (plots of t/Qt versus t) can be well evaluated using the pseudo-second-order kinetic model with high correlation coefficients (R2 > 0.99, Figure 3b), revealing that arsenic adsorption onto β-FeOOH NRs/CF monolith are likely governed by chemisorption. Furthermore, the adsorption rate constants (k2 15



values calculated from the plot slope) of both As(III) and As(V) are determined to be 0.00194, 0.00297 g/mg/min, respectively. Obviously, k2 value of arsenate is found to be about 1.5 times higher than that of arsenite. Similar phenomena were also found for the uptake of arsenite and arsenate onto ferrihydrite.45 1.2

20

(a)

(b)

1.0

As(V)

0.6 WHO standard of 0.01 ppm

0.4

t/Qt (min/g/mg)

As(III)

0.8

Ct/C0

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15 2

R

10

68 .99 =0

2

R

=0

90 .99

5

0.2 0.0

0 0

100

200

300

400

500

600

700

800

0

50

100

150

200

250

300

350

Time (min)

Time (min)

Figure 3. Arsenic adsorption kinetics data (a) and the corresponding fitting curves on β-FeOOH NRs/CF via pseudo-second-order kinetic model (b). (10 ppm of arsenic species, 0.5 g/L of sorbent dosage, pH 6.0 and temperature 25◦C) 3.3. Arsenic Sorption Isotherm. According to the sorption kinetic behavior, the sorption isotherm are investigated with different initial arsenic concentration (1~1000 mg/L) under pH 6.0 with the contact time of 24 h. As illustrated in Figure 4a, the uptake amount of As(V)/As(III) on β-FeOOH NRs/CF monolith increases with increasing

arsenic

concentration.

Particularly,

β-FeOOH

NRs/CF

monolith

demonstrates enhanced uptake capacity towards As(V) over As(III), similar to the previous report about arsenic sorption.46 Besides, the sorption isotherms for both arsenate and arsenite species can be well fitted using Langmuir model (R2 > 0.995, Figure 4b), revealing the monolayer sorption behavior of As(V) and As(III), which can be ascribed to homogeneous distribution of the sorption sites on the resulting 16


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β-FeOOH NRs/CF monolith. The corresponding saturated sorption capacities for As(V) and As(III) are 172.9 and 103.4 mg/g, respectively, superior to other adsorbents except for FeOOH microboxes summarized in Table 1, revealing that the obtained β-FeOOH NRs/CF monolith in this study is an effective arsenic decontaminant. The excellent arsenic removal performance may be attributed to abundant active sites and 3D hierarchical porous network structure of the obtained β-FeOOH NRs/CF monolith. 180 160

12

(a) 10

140 120

(b)

8

Ce/Qe (g/L)

Qe (mg/g)

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


100 80 60 40

As(V) As(III)

20

6

2

R

=0

59 .9 9

4 2 .999 R =0

2

5

0

0 0

200

400

600

800

1000

0

200

Ce (mg/L)

400

600

800

1000

Ce (mg/L)

Figure 4. (a) Sorption isotherms of β-FeOOH NRs/CF towards As(III) and As(V); (b) the corresponding plots of Ce/Qe vs. Ce. Experimental conditions: 0.5 g/L of sorbent dosage; pH 6.0 and temperature 25 ◦C. Table 1. Comparison of arsenic adsorption capacity on various absorbents. Sorption capacity Adsorbents

BET pH

(mg/g)

(m2/g)

References

As(III)

As(V)

13.6

18.2

7.0

196

47

6.7

11.7

7.0

—

48

Mag-Fe-Mn

46.76

—

7.0

—

49

Fe3O4

46.06

16.65

7.0

179

50

Fe2O3@C

29.4

17.9

—

858

18

TiO2-coated carbon nanotube Fe/Mn oxy-hydroxide

17



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zero-valent Iron

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3.5

—

7

—

51

Fe-Cu oxide

122.3

82.7

7

—

52

Fe-Ce oxide

—

68.96

5.5

135.9

53

Fe-Mn oxide

118.5

62.31

5.0

265

54

Fe-Ti oxide

85.0

14.0

7.0±0.1

77.8

55

γ-Fe2O3

109.5

39.1

neutral

56.1

15

FeOOH microboxes

192.19

250.0

4.0~7.0

147.28

56

β-FeOOH NRs/CF

103.4

172.9

6.0

107.13

Present study 3.4. Effect of Initial Solution pH on Arsenic Sorption. Among different sorption parameters, initial pH value is one of the most critical parameters affecting arsenic uptake. With the purpose of assessing the influence of initial pH value on sorption capacity, the adsorption experiments are carried out using 20 ppm of arsenic solution with varying initial pH values (2~10). From Figure 5a, arsenic uptake capacities on β-FeOOH NRs/CF monolith slightly decrease with the increase of pH, indicating a pH-dependent process. This adsorption behavior can be explained by different surface charges of adsorbents and variation in arsenic species under various pH values. To better understand the interaction of different arsenic species available at different pH, the relationship between zeta potential of β-FeOOH NRs/CF and pH is displayed in Figure 5b, in which the isoelectric point (IEP) of β-FeOOH NRs/CF monolith is approximately 8.2. At lower pH values (below IEP), the surface of adsorbent is positively charged on account of ample protonated hydroxyl radicals, facilitating the uptake of anions via electrostatic attraction. Since the predominate species of As(V) are H2AsO4− and HAsO42−, while As(III) exists mainly in the form of a nonionic 18


Page 19 of 36

H3AsO3 below IEP, the electrostatic interaction between negatively charged arsenate and protonated groups leads to enhanced sorption capacity for As(V) over As(III). With increasing pH values (above IEP), the synergistic effects based on deprotonation of hydroxyl radicals and negative surface charges result in the decrease of arsenic sorption capacity owing to the electrostatic repulsion effect. It should be noted that electrostatic attraction is not the sole reason for arsenic uptake owing to the chemical adsorption behavior of arsenic species on β-FeOOH NRs/CF monolith. Additionally, complexation effect between arsenic species and absorbent may be also responsible for effective arsenic removal, which will be discussed later. 50

80

(a)

As(V) As(III)

(b)

60

Zeta potential (mV)

45 40

Qe (mg/g)

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


35 30 25 20

40 20 0 -20 -40

15

-60 0

2

4

6

8

10

12

0

2

4

pH

6

8

10

12

pH

Figure 5. (a) Effect of initial solution pH on sorption of As(III)/As(V) by β-FeOOH NRs/CF; (b) zeta potentials of β-FeOOH NRs/CF under various pH. Experimental conditions: 20 ppm of initial arsenic concentration, 0.5 g/L of adsorbent dose. 3.5. Effects of Coexisting Anion on Arsenic Sorption. Interfering anions including nitrate, sulfate, chloride and phosphate exist widely in natural waters and industrial effluents, which can interfere with the uptake of arsenic by competing for surface binding sites available or altering the surface charge of adsorbent. To investigate the selective binding characteristics of β-FeOOH NRs/CF monolith 19



towards As(III) and As(V), batch sorption experiments are performed in the presence of Cl-, NO3-, SO42-, PO43- with two different concentrations and the corresponding results are displayed in Figure 6. Apparently, there is no obvious decrease about arsenic removal efficiency with coexisting chloride, nitrate and sulfate ions, although their concentrations (1 mM) are far higher than that of arsenic species. By contrast, the presence of phosphate (0.1 M) results in an obvious reduce of As(III) sorption capacity from 22.2 to 14.7 mg/g (Figure 6a). Furthermore, As(III) sorption capacity declines greatly to 9.9 mg/g when phosphate concentration increases to 1 mM. Likewise, the coexistences of chloride, nitrate, and sulfate have almost no adverse effect on As(V) uptake, whereas the coexistence of 1 mM phosphate gives rise to remarkable decrease of arsenate sorption capacity from 40 to 23.2 mg/g (Figure 6b). The decrease can be assigned to structural coincidence between As(III)/As(V) oxoanions and PO43-, whereas PO43- may compete for the sorption sites with As(III)/As(V). The identical interference for arsenic uptake caused by phosphate anions was also demonstrated in the previous reports.10, 11 30

50

(a)

None

0.1 mM

(b)

1 mM

25

None

0.1 mM

1 mM

40

20

Qe (mg/g)

Qe (mg/g)

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 20 of 36

30

15

20

10 10

5 0

0 None

Chloride

Nitrate

Sulfate

Phosphate

None

Chloride

Nitrate

Sulfate

Phosphate

Figure 6. Effect of coexisting ions on arsenic uptake by β-FeOOH NRs/CF: (a) As(III); (b) As(V). (20 ppm of arsenic species, 0.5 g/L of adsorbent dose). 20


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


3.6. Sorption–regeneration Cycles. Apart from fast adsorption kinetics, high adsorption capacity and preferable affinity for the target contaminant ions, an optimal adsorbent should maintain superior stability and reusability. Sorption and regeneration experiments for arsenic species are performed to investigate the reusability of the as-synthesized β-FeOOH NRs/CF monolith for arsenic removal (Figure 7), in which NaOH (0.5 M) is employed as the regenerant and then the recovered β-FeOOH NRs/CF monolith is reutilized for the uptake of arsenic. Notably, the sorption capacities for both As(III) and As(V) display a slight decrease after five sorption-recycle, resulted from the fact that some of active sites within β-FeOOH NRs/CF cannot be effectively regenerated by NaOH treatment. In spite of that, the regenerated β-FeOOH NRs/CF monolith still maintains over 76% of arsenic removal efficiency after the fifth cycle, paving the way for sustainable arsenic remediation. 50 As(V) As(III) 40

Qe (mg/g)

Page 21 of 36

30

20

10

0 1

2

3

4

5

Cycles

Figure 7. Arsencic sorption capacity on the as-synthesized β-FeOOH NRs/CF monolith after different adsorption-regeneration cycles. Initial arsenic concentration: 20 ppm; adsorbent dose: 0.5 g/L. 3.7. Mechanism of Arsenic Sorption. To identify the interaction between β-FeOOH NRs/CF monolith and As(III)/As(V) species, FT-IR spectra of β-FeOOH 21



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Page 22 of 36

NRs/CF before and after As(III)/As(V) uptake are compared, as shown in Figure 8a. For the pristine β-FeOOH NRs/CF, two absorption bands at 3400 and 1600 cm-1 are attributed to coordinated water molecules and C=O stretching vibration, respectively. After As(III) or As(V) uptake, the aforementioned two peaks have almost no any alteration, which indicates the physically adsorbed H2O molecules and C=O bonds are not responsible for the uptake of arsenic species. By contrast, the band at 1400 cm-1 assigned to –OH in-plane deformation vibration of Fe–OH bond15,

57

obviously

weakens, suggesting that the surface hydroxyl groups might be substituted by adsorbed arsenic species. Additionally, several new peaks at 870~810 cm-1 appear after arsenic uptake, which can be ascribed to synergistic effects based on symmetric and asymmetric stretching vibrations of As–O bond in the As–O–Fe linkage.58 The above findings provide solid evidence for the surface complexation of arsenic species on the designed adsorbent. Particularly, compared with As(III)-loaded β-FeOOH NRs/CF, the peak assigned to As-O bonds is more sharp and strong after adsorption of As(V), indicative of stronger affinity of hydroxyl groups for As(V) and consistent with higher adsorption capacity towards As(V). Therefore, excellent removal efficiency of β-FeOOH NRs/CF monolith towards As(III)/As(V) species may be reasonably attributed to not only the formation of As-O-Fe chemical bonds but also the replacement of M–OH radicals by arsenic species. As a result, all the collective effects ensure excellent arsenic removal efficiency from the contaminated water. In order to reveal further the adsorption mechanism of arsenic species, the surface compositions and the variation of binding energies of β-FeOOH NRs/CF before and 22


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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


after arsenic adsorption are further investigated by XPS, as illustrated in Figure 8b-f. For the freshly prepared β-FeOOH NRs/CF (Figure 8b), the peaks at 285, 398.4, 531.8 and 710.7 eV correspond to C 1s, N 1s, O 1s and Fe 2p, respectively, indicating the existence of C, N, O and Fe elements in the as-obtained product, in accordance with the EDS spectrum. After the uptake of As(V) or As(III), several new peaks assigned to As 3d and As (A) can be clearly observed, indicating that arsenic species are successfully immobilized on the surface of the adsorbent. As revealed in Figure 8c, high-resolution XPS spectrum of Fe 2p exhibits two prominent bands at 710.7 and 728.3 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, respectively. After As(III) treatment, the binding energy of Fe 2p shifts slightly to high-energy edge, indicating the chemical interactions between iron and arsenic species associated with the potential formation of As–Fe or As–O–Fe bonds. In the case of As(V)-loaded β-FeOOH NRs/CF, a more positive shift (ca. 0.5 eV) is also observed for Fe 2p spectrum, suggesting a better affinity between β-FeOOH NRs/CF and As(V) and thus resulting in higher uptake capacity for arsenate. The detailed O 1s spectrum of the pristine β-FeOOH NRs/CF (Figure 8d) can be deconvoluted into three peaks at 529.7, 531.4 and 533.2 eV, assigned to the binding energies of oxygen bonded to metal (M-O), hydroxyl bonded to metal (M-OH) and adsorbed H2O molecules, respectively.11 Moreover, H–O peak is significantly stronger than that of M–O and H–O–H, confirming the existence of abundant hydroxyl radicals on β-FeOOH NRs/CF monolith. After the uptake of As(III) (Figure 8e), the H–O ratio decreases from 55.18% to 50.75%, owing to the possible replacement of 23



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 24 of 36

hydroxyl groups by H2AsO3- and H2AsO4- species during the adsorption process, consistent with the results of the above FT-IR analyses. By contrast, the percentage of M–O increases slightly from 12.75% to 17.26%, suggesting the formation of new chemical bonding between metal and oxygen (e.g. As–O bond). Additionally, the integral area of the peak assigned to H2O molecules displays a negligible change, suggesting that H2O is not responsible for As(III) uptake. Similarly, the percentage of M–O increases from 12.75% to 17.40% after the adsorption of As(V) (Figure 8f), which can be ascribed to the formation of Fe-O-As bonds.10,

59, 60

On the basis of all

above results, it is possible that As(V) species are firstly adsorbed on surface of β-FeOOH NRs/CF followed by surface complexation and ion exchange with hydroxyl groups.

24


Page 25 of 36

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


Figure 8. FT-IR spectra of β-FeOOH NRs/CF before and after arsenic uptake (a); XPS spectra of β-FeOOH NRs/CF before and after arsenic uptake: (b) survey spectrum; (c) Fe 2p; (d) O 1s before arsenic uptake; (e) O 1s after As(III) uptake; (f) O 1s after As(V) uptake.

4. CONCLUSIONS In summary, 3D hierarchical β-FeOOH NRs/CF monolith has been successfully fabricated via the simple calcination followed by hydrothermal treatment, which can 25



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

be exploited as the efficient arsenic adsorbents. Thanks to abundant active sites from β-FeOOH NRs and efficient mass/ions transport pathways associated with

interconnected hierarchical pore network, the as-fabricated β-FeOOH NRs/CF monolith not only exhibits excellent sorption capacities for both As (V) and As (III), but also removes effectively 10 ppm of arsenic species to below the acceptable standard for drinking water set by WHO. Besides, the as-obtained absorbent possesses preferable sorption affinity for arsenic species in the presence of co-existing ions together with excellent recyclability, indicating its huge potential as a highly effective arsenic capture. More significantly, in view of the macroscopic size, the β-FeOOH NRs/CF monolith can be conveniently extracted from the treated water bodies without the help of auxiliary magnetic or centrifugation techniques, circumventing the intractable separation and potentially secondary pollution risk of the powdered nano-adsorbents. All these features make the fabricated β-FeOOH NRs/CF monolith an ideal candidate for the effective arsenic remediation from the contaminated water.

ASSOCIATED CONTENTS Supporting Information. EDS spectrum of β-FeOOH NRs/CF; N2 sorption/desorption isotherms and the corresponding pore-size distribution of the obtained samples; TEM image of a single β-FeOOH NR.

AUTHOR INFORMATION Corresponding Authors ∗Email: [email protected] 26


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Grant No. 51572263,

51272255,

51472246),

Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09030200) and National Basic Research Program of China (Grant No. 2013CB934302).

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