Novel Dendrimerlike Magnetic Biosorbent Based on Modified Orange

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Novel dendrimer-like magnetic bio-sorbent based on modified orange peel waste: adsorption-reduction behavior of arsenic Fanqing Meng, Bowen Yang, Baodong Wang, Shibo Duan, Zhen Chen, and Wei Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01273 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Sustainable Chemistry & Engineering

Novel dendrimer-like magnetic bio-sorbent based on modified orange peel waste: adsorption-reduction behavior of arsenic Fanqing Meng1, Bowen Yang1, Baodong Wang 2, Shibo Duan1, Zhen Chen1, Wei Ma1* 1

Department of Chemistry, Dalian University of Technology, Dalian 116023, PR China. 2

National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, PR, China.

Corresponding author *Address: Department of Chemistry, Dalian University of Technology, Dalian 116023, PR China. * Email address: [email protected] (Wei Ma); *Tel: +86(411) 8470 6303; Fax: +86(411) 8470 7416;

Abstract: In this work, a novel porous bio-sorbent (HF-D) based on orange peel (OP) was prepared by an efficient and simple method. The prepared HF-D showed well ordered dendrimer-like structures and remarkable adsorption performance for the removal of arsenic. Particularly, morphology and structure details proved that the dendrimer-like structure was formed by surrounding the crystal iron nanoparticles with amorphous carbon. Furthermore, the influence of various environmental factors (initial pH, coexisting inorganic ions, humic substances, etc.) on the adsorption of As (V) were investigated. An extraordinary As (V) adsorption capacity (81.3 mg/g) has been observed when the Fe/biomass ratio was 10 wt%. Adsorption and characterization data proved that the behavior of As (V) on HF-D was an adsorption-reduction process, including complex adsorption and inner reduction. In addition, the adsorption kinetics and isotherms were analyzed. The regeneration and application in groundwater solution of the prepared HF-D were discussed. This study

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not only provides an efficient bio-sorbent for removing arsenic but also a low cost and environmentally friendly method for synthesizing porous biomass based materials. Keywords: Orange peel; Dendrimer-like structure; Adsorption; Reduction; Arsenic

Introduction

Water pollution caused by arsenic (As) has dire consequences to public health worldwide because of its potential toxic and carcinogenic effects on human. Arsenic can be released from both natural and anthropogenic sources such as natural weathering reactions, biological activities and anthropogenic activities discharge

(1)

. There are

various of As species have been found in natural water as inorganic oxyanions of trivalent arsenite (AsO33−, As (III)) and pentavalent arsenate (AsO43−, As (V))

(2)

.

Researches showed that short-term exposure to high levels of As can cause skin, bladder, and lung cancers because arsenic can concentrate in the human body by ingestion of arsenic in drinking water (3-4). In order to control the effects of arsenic on people's health, the World Health Organization (WHO) adopted a new regulation to lower the drinking water standard of As from 50 to 10 µg/L

(5)

. However, arsenic

poisoning was a common occurrence in certain regions of some developing countries due to the lack of water quality surveillance and public awareness

(6-7)

. Therefore,

there is a great demand of finding a low cost and high efficiency method for the removal of As from aqueous solutions to protect the environment and human health. In recent years, many techniques were used for arsenic removal such as chemical coagulation

(8)

, reverse osmosis

adsorption methods

(9)

, ion exchange

(10)

, membrane filtration

(12-13)

(11)

and

. Among those techniques, the adsorption process is


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considered as one of the most promising technique due to its economical, effective and socially acceptable

(14)

. Various types of adsorbents have been synthesized and

applied for the removal of As in past years. The biomass based adsorbent was considered to be one of the most efficient adsorbents to remove arsenic from aqueous solutions owing to the low cost, ready availability, environmental friendliness and high efficiency

(15-16)

as the apple peels

. Various raw biomass can be used to produce bio-sorbents such

(15)

, cottonwood (17), sawdust (18), waste tea (19), wheat straw

(20)

and

pteris vittata (21). Notable disadvantages of such biomass based adsorbent are the solid residues into water and low adsorption performance on arsenic. Efficient and universal low cost adsorbents that will not produce a secondary water contamination during the purification process are to be developed (22). Recently, several methods were applied in the biomass modification to get potential bio-adsorbents by introducing the chemicals such as citric acid, acetic acid, lithium chloride

(23)

and potassium hydroxide

advantages for the bio-adsorbents production

(24)

. Iron compounds showed many

(25-26)

. (i) Biomass can be modified by

iron compounds at lower temperature (about 180 ℃) in the atmosphere. (ii) Iron compounds have a low toxic effect on the living body, thus environmentally friendly. (iii) Biomass modified by iron compounds has good magnetic performance and physical stability, which is very suitable for water treatment and separation. (iv) Iron modified biomass is beneficial to the producing of the mesoporous structure with high specific surface area. Thus, bio-sorbents modified by iron compounds are beneficial for achieving industrial scale applications. However, conventional magnetic



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bio-sorbents modified process usually requires harsh condition, which not only takes time and energy, but also leads to the elimination of functional groups (25-26). Hydrothermal carbonization (HTC) process was considered as an eco-friendly route to modify biomass materials in a closed system under mild conditions and low temperatures (150-250 ℃)

(27)

. The HTC process exhibits more advantages than the

conventional process for the reason that it not only retains the rich functional groups but also economical and eco-friendly. The resulting HTC biomass tend to have low specific surface area, even though it contains a large amount of oxygenated functionalities

(27-28)

. It is another important factor to limit the application of the

bio-adsorbents which is deeply depends on the morphology. As far as the authors know, no studies have been reported for preparing porous OP based bio-adsorbent with high specific surface area and richly functional groups by HTC process via iron solution. In this work, a novel dendrimer-like structure HF-D based on OP through HTC process in the Fe(II)/Fe(III) ions solution was synthesized. The characterization results demonstrated that the dendrimer-like structure was formed by surrounding the iron nanoparticles inside the branch with the amorphous carbon. The effects of the solution pH, temperature, initial concentration of As (V) ions, coexisting inorganic ions and humic substances on the adsorption capacity of As (V) were investigated. In addition, adsorption isotherms and kinetics were discussed. The application performance and stability of the HF-D in real ground water solution were studied. Finally, the adsorption-reduction behavior was explored by FTIR, XPS and


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Mössbauer spectrum. Studies described here help us to characterize Fe(II)/Fe(III) modified biomass and assess its suitability for the removal of arsenic from water solution.

Experimental section Chemicals and Reagents. Ferrous nitrate, ferric nitrate, sodium arsenate and

sodium arsenite were purchased from Aladdin Chemical (Dalian, China). Mercury (II) chloride, cadmium chloride and sodium antimonate were purchased from Sigma-Aldrich (Shanghai, China). Disodium chromate, sodium molybdate, sodium nitrate and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. Navel oranges were purchased from a local grocery store. Humic acid was purchased from Oubote (Shandong, China). All the reagents used in the study were analytical grade and all solutions were prepared with deionized (DI) water. Synthesis of OP-R and dendrimer-like structure HF-D. Fresh OP was prepared and used within two weeks of creation for all syntheses. Fruits were first rinsed thoroughly using DI water. Colored peels were carefully cut away from the fruit and then further cut into small pieces and dry at 60 ℃ for 24 hours. Approximately 5g of the dried OP was transposed to a 100 mL stainless steel autoclave (Anhuikemi, China) containing approximately 50 mL of Fe(II)/Fe(III) ions solution (the Fe(II)/Fe(III) ratio was 2/3, the iron/biomass ratio is 0, 5, 10 and 15 wt%) and allowed to agitate for 24 h. The mixture was then heated in a commercial oven at 180 ℃ for 10 h. The mixture was filtered and washed by the DI water to get the powder samples that abbreviated as OP-R, 5 wt% HF-D, 10 wt% HF-D and 15 wt%



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HF-D. Characterization of OP-R and HF-D. FTIR spectra of three HF-D before and after adsorption of arsenic were recorded within the range of 4000-400 cm-1 with KBr as the matrix by using a Bruker MPA FT-IR spectrophotometer. X-ray diffraction (XRD) patterns of the OP-R and three HF-D were recorded using Rigaku: D/Max 2400 with a GADDS powder X-ray diffractometer with a Cu Kα (λ = 1.54 Å) source at 40 kV and 40 mA over a range of 2θ angles from 5° to 80°. For electron microscopy measurements, the high resolution transmission electron microscopy (HRTEM, 200 keV, JEOL, JEM-2100f) and the scanning electron microscopy (SEM, America, FEI: NOVA Nano SEM 450) were used. The As, Fe and O components of As (V) free and As (V) loaded 10 wt% HF-D were further identified using X-ray photoelectron spectroscopy (XPS, ThermoFisher: ESCALAB 250Xi using mono Al Kα as the photoexcitation source). C 1s (C-C bond) was calibrated at 284.5 eV. The magnetic property of HF-D was evaluated using the Vibrating Sample Magnetometer (Changchun Kejiao: JDM-13).

Brunauer-Emmett-Teller (BET) surface area

measurements were carried out using an Automatic chemical adsorption instrument (Quantochrome: CHEMBET-3000). Transmission

57

Fe Mössbauer spectra were

collected using a Mössbauer spectrometer operating in constant acceleration mode and equipped with a 50 mCi

57

Co(Rh) source. Trace amounts of heavy metals were

detected using an inductively coupled plasma-optical emission spectroscopy (ICP-OES Agilent: 720). The concentration of iron in aqueous solution was detected by an Atomic absorption spectrometry (AAS Perkin Elmer instrument co., LTD:


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AAnalyst700). Adsorption of toxic ions by HF-D. Stock solutions (1000 mg/L) of As (V and III), Cr(VI), Hg(II), Cd(II), Sb(V) and Mo(VI) were prepared in DI water. Batch removal studies were carried out at the desired adsorption temperatures and the initial concentrations ranging from 0.5 mg/L to 45 mg/L. An aliquot of HF-D (20 mg) was directly added into 25 mg/L heavy metal solution (100 mL), subjected to orbital shaking for 5 h at 300 rpm. At the end of adsorption, the suspension was separated by using an external magnet. The heavy metal concentrations prior to and after adsorption were determined by ICP and AAS. Experimental conditions such as initial concentration, pH and co-existence ions were optimized to evaluate the adsorption efficacy. Solution pH controlled with HCl (1 mol L-1) or NaOH (1 mol L-1) has an important influence on the adsorption process. The amount of heavy metals adsorbed per unit mass of the adsorbent was evaluated by using the following equation:

qe = (Co-Ct)V/M

(1)

Where qe (mg/g) is the amount of heavy metals adsorbed per gram of adsorbent, Co and Ct represent the initial and final heavy metal concentration after adsorption, V is the solution volume (mL), and M is the adsorbent weight (g). The groundwater samples were obtained from the real water solution (Huludao, China). This ground water revealed a high conductivity (243 S/cm) and a high total hardness (119 mg/L as CaCO3, including Ca and Mg). The initial concentration of the As, Sb, Cr and Mo were 11.6 µg/L, 15.7 µg/L, 60 µg/L and 400 µg/L, respectively. An aliquot of groundwater (50 mL) was treated with 10 wt% HF-D (10 mg). The effect of



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co-existing ions on As (V) adsorption were studied by adding an appropriate amount of NaCl, KCl, CaCl2 and MgCl2. In order to evaluate the reusability of the HF-D, three cycles of adsorption-desorption experiments were investigated. Firstly, 1 g HF-D was suspended in 2 L As (V) solution with 50 mg/L at pH 6.0 ± 0.1. After stirring for 24 h, the HF-D was separated from the solution by using an external magnet. Then, the HF-D was regenerated from arsenic-containing HF-D in 0.5 M NaOH solution. Typically, the 1 g arsenic-containing HF-D was suspended in 100 mL NaOH solution, and mixture was stirred for 10 h. Then the regenerated HF-D was separated from the NaOH solution. After washing and drying, it was used in the next adsorption-desorption cycle. Analytical methods. In the arsenic analysis process, the supernatant was analyzed by ICP-OES using an external calibration method. The whole analytical procedure has been commonly suggested in the literature

(29)

which can be found in

the supporting information. To detect As (III) selectively, a mixture of 2% KBH4 and 0.5 wt% KOH was used as reducing solution, and 5 wt% citric acid was used as the carrier solution. Under this condition, only As (III) was converted to AsH3 and detected on the AFS instrument. The concentration of Cd (II), Cr (VI) and Mo (VI) ions of the supernatant was determined by AAS. The concentration of Sb (V) and Hg (II) was determined by Atomic Fluorescence Spectrometry (AFS-830d, China).

Results and discussion Chemical composition analysis. The effect of Fe impregnation ratio on the

chemical composition of the HF-D was investigated by the literature method


(12)

.

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Generally, the total concentration of Fe on the three HF-D was determined by the AAS after microwave mineralization. Total organic carbon and total carbon were determined after dry combustion. As the elemental analysis revealed (Table S1), the content of the inorganic carbon increased with increasing the loading content of Fe, indicating the Fe illustrated an enhancement of the OP carbonization. Material Characterization and Properties. Details about the morphology of the raw OP, obtained OP-R and HF-D were examined in Fig. 1. Colors change from brown to dark brown was observed after surface modification of the OP. Comparing the raw OP with OP-R, a large number of dendrimer-like macrospores were revealed to exist in the 10 wt% HF-D. The morphology was further investigated by the TEM analysis (Fig. 2). Notable differences in the morphology of the raw OP, obtained OP-R and HF-D were observed. As shown in Fig. 2, the morphology of the prepared bio-adsorbent has been both affected by the introducing of iron and hydrothermal process. The interfacial structure of the branch was investigated by HR-TEM analysis. As shown in Fig. 2 (f), the spacing lattice fringe of 0.252 nm, 0.245 nm and 0.492 nm were implied the presence of (110)Fe2O3 planes (111)magnetite planes

(30)

, (111)FeO(OH) planes

(31)

and the

(32)

, respectively. Moreover, it has been found that the

dendrimer-like structure was significantly affected by the Fe/biomass ratios. To possess an overall understanding of the dendrimer-like structure HF-D material, the high-magnification TEM images of three HF-D were studied. As shown in Fig. S1 a and b, there was no obvious dendrimer-like structure and crystal structure on the 5 wt% HF-D, suggesting that the iron salt loading content is too small to build the



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dendrimer-like structure. The HRTEM images of the 10 wt% HF-D revealed that the dendrimer-like structure was formed by surrounding the iron nanoparticles inside the branch with the amorphous carbon (Fig. S1 b and c). The formation of those branches could be attributed to directional growth of the amorphous carbon that drive iron particles directional movement by magneticaction. For the 15 wt% HF-D, the structure morphology was changed due to the coverage of some macropores by the crystal iron nanoparticles (Fig. S1 e and f). X-ray Diffraction Spectroscopy (XRD) Analysis. The structure and phase purity of the OP-R and three HF-D were investigated by XRD. As shown in Fig. 3, there were no characteristic peaks in the OP-R material, suggesting the OP-R was amorphous carbon. The diffraction pattern also revealed that the synthesized HF-D was a poorly crystalline material. The small observed signals of crystalline forms were enhanced with the Fe loading content increasing. When the Fe loading content up to 10 wt% and 15 wt%, a small diffraction pattern appeared at 22.36° which corresponded to the graphite (002) reflection. This indicated that the Fe illustrated an enhancement of the OP carbonization, which consists with the chemical composition data. In addition, it can be observed that the main constituents of the three HF-D were measured to be magnetite, FeO(OH), Fe2O3 and FeO. The formation of those crystal constituents would be helpful to build the porous structure by growing the amorphous carbon on the crystal iron nanoparticle, which consists with the HRTEM images. Fourier Transform Infrared Spectroscopy (FT-IR) Analysis. The typical FTIR spectra of the raw OP and HF-D were shown in Figure 4. The spectrum of the


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obtained HF-D displayed strong intensity at 3250-3550 cm-1, 1100-1350 cm-1 and 1712 cm-1 which corresponded to the overlapping signals of νOH, the overlapping signals of C-C/C-O groups and the vibration of -C=O group

(17, 33-34)

, respectively.

After the HTC treated process, the νCH signal at 2850 cm-1 of the raw OP began to flatten, suggesting the polymerization and carbonization of the OP. Furthermore, the FTIR spectrum also showed that the overlapping signals of νOH at 1080 cm-1 became stronger after introducing of the iron salt, indicated that the formation of the Fe-OH groups in the HF-D. This result was also turned out by the XRD and HRTEM data. Adsorption capacity analysis of HF-D. Table 1 lists the comparison of maximum arsenic adsorption capacity of various adsorbents derived from different precursors. It could be concluded that the porous dendrimer-like biomass prepared in this work showed relatively large adsorption capacity of 81.3 mg/g as compared to the literature. Fig. 5a shows the adsorption performances of 10 wt% HF-D and OP-R for the removal of different ions. It can be observed that 10 wt% HF-D shows higher adsorption capacity on negative ions than OP-R. This could be explained that in pH of 6.0, the positively charged HF-D made it more sensitive to negative ions. When the pH is 6, both anions and cations could be adsorbed by the uncharged OP-R for the reason that richly functional groups of amorphous carbon can become efficient adsorption site for pollutants. Surface area (m2 g-1)

Capacity (mg/g)

Reference

Organic biochar

190

16.2

(20)

Magnetite Nanoparticles

39.0

27.8

(26)

Magnetic biochar

193

2.53

(33)

Fe-Mn chitosan bead

248

54.2

(34)

-

30.1

(17)

OP-R

164

37.1

This work

10 wt% HF-D

396

81.3

This work

Adsorbent

γ-Fe2O3 biochar



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Table 1 Summary of As (V) adsorption capacity of various adsorbents. The effect of Fe/biomass ratio on adsorption capacity of As (V). The effect of Fe/biomass ratio on the adsorption of As (V) is shown in Fig. 5b. It can be seen that the adsorption efficiency increased sharply with iron loading contents up to 10 wt%, where it began to decrease. This could be attributed to the decreasing of surface area when the Fe/biomass ratio increased from 10 wt% to 15 wt %. As the BET data show, the SBET of the HF-D with Fe/biomass ratio of 0, 5, 10, and 15 wt% were 164, 313, 396 and 312 m2 g−1, respectively (Table S2). A rather different SBET was obtained for OP-R and HF-D likely attributed to their morphology morphological feature. The nitrogen adsorption-desorption isotherm of 10 wt% HF-D is shown in Fig. S2. The 10 wt% HF-D shows a type of IV isotherm with H3 hysteresis loop, which indicating a mesoporous nature (35). Effect of solution pH on adsorption capacity of As (V). The effects of pH on the adsorption of As (V) were shown in Fig. 6 (a). It could be seen that the adsorption capacity of HF-D on As (V) was significantly affected by the solution pH. The maximum adsorption capacity of 10 wt% HF-D on As (V) was appeared at pH 6.0 and decreased monotonically with increasing pH from 8 to 12. Those results were consistent with the pH dependent of As (V) speciation and surface charge of the HF-D. As shown in ATR-FTIR spectrum (Fig. S3), bonds at 906, 873-871 and 763-755 cm-1 which originate from H2AsO4- and HAsO42- were appeared at pH 3 and pH 6. At pH 9, only one band around 854 cm-1 was appeared which stems from the As-O stretching vibration of the HAsO4

2-

entity (36). The adsorption of As (V) would be promoted by


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electrostatic attraction between negatively charged As (V) species and positively charged HF-D surface when pH below the isoelectric point (IEP) of HF-D. When the pH was above pHZPC of HF-D, the adsorption of As (V) would be inhibited by electrostatic attraction. As shown in Fig. S4, the IEP of the HF-D decreased from 7.3 to 4.5 when As (V) was absorbed. This may be because that the positively charged site of the HF-D could be covered by the adsorbed As (V) and some complexes were formed at the lower pH. As shown in Fig. S3, after bonding onto HF-D surfaces, the position and intensity of the peaks were altered. In addition, the peak’s intensity was also affected by the solution pH which consisted with the adsorption results. At pH 6, most of As (V) was adsorbed, two peaks at 883 cm-1 and 794 cm-1 were observed in the ATR-FTIR spectrum which assigned to the symmetric stretch vibration (νs) of As-O-Fe and the weakly bound of the physical adsorbed As (V) species, respectively (36)

. Effect of the coexisting inorganic ions on arsenic adsorption. The adsorption

capacity of As (V) on 10 wt% HF-D with increasing co-existing cations was shown in Fig. S5. As the adsorption results illustrated that there was no significant effect of Na+ and K+ on As (V) adsorption with the different cation concentrations. However, the presence of Ca2+ and Mg2+ has a strengthening effect on the adsorption of As (V). Moreover, the adsorption capacity was increased with the Ca2+ and Mg2+ increasing. This may because that the Ca2+ and Mg2+ ions may compress the double layer and made the internal pores more accessible, enhancing the adsorption of arsenate accordingly (37-38)

. Fig. 6 (b) shows the effect of coexisting inorganic ions on arsenic adsorption.



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The competitive adsorption data revealed that the competitive ability seemed to be related to the ion species and ordered as follows: SO42- >CO32- > Cl- >NO3-. This could be attributed to the negatively charged ions could compete with H2AsO4− and HAsO42− to be adsorbed by positively charged HF-D. The highly charged and large size properties of the SO42- make it more competitive than others. Effect of the humic substances on arsenic adsorption. Fig. 6c shows the adsorption capacity of 10 wt% HF-D on As (V) in the presence of different concentration of humic acid (Humic acid concentration of 0.1, 1, 5, 10 and 20 mg/L were assessed). It can be observed that there is no significant decrease of the arsenic adsorption capacity in the presence of 0.1 mg/L and 1 mg/L humic acid. When the concentration of the humic acid were 5, 10 and 20 mg/L, As (V) adsorption capacity decreased with the concentration increasing. Moreover, humic acid increasingly reduced the arsenic adsorption on 10 wt% HF-D by decreasing the solution pH from 9 to 2. Similar results were obtained by Grafe et al. who assumed arsenic adsorption on α-FeOOH in the presence of humic acid (39). Isotherm study. The effects of initial concentration of As (V) and temperature on the adsorption of As (V) onto HF-D are shown in Fig. 6 (d). It can be observed that As (V) adsorption capacity was increased with the initial concentration of As (V) and temperature increasing, which could be mainly attributed to the inner adsorption mechanism between the arsenic species and adsorbents. In order to give deep insights into the understanding of As (V) adsorption behavior, the Langmuir and Dubinin-Radushkevich (D-R) isotherms have been investigated. The Langmuir theory


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was studied to assume homogeneous type of the adsorption within the adsorbent. The D-R isotherm was applied to distinguish between the physical and chemical adsorption of arsenic. The fitting results from the adsorption of As (V) on 10 wt% HF-D (Fig. 7 a and b) are listed in Table 2. These data indicated that the adsorption of As (V) on 10 wt% HF-D was well described by the Langmuir and D-R model. The essential feature of the Langmuir isotherm can be expressed as RL which referred to As (V) equilibrium parameter or separation factor. The RL values in the fitting result were obtained at the range of 0.652 - 0.288, suggesting the arsenic adsorption process is favourable

(40)

. The free energy (E) above 16 kJ mol-1 corresponds to a chemical

process while the value of E < 8 kJ mol-1 represents a physical process (41). As shown in Table 2, the numerical value of E was found to be higher than 16 kJ mol-1 for all studied temperatures, suggesting the adsorption mechanism might be a chemical adsorption process. The calculated thermodynamic parameters of As (V) adsorption were showed in Table S3, including ∆Gθ, ∆Hθ and ∆Sθ. The calculated function of qe for ∆Hθ was provided in support materials. The negative ∆Gθ suggest that As (V) adsorption process is a spontaneous adsorption reaction

(42)

. The positive ∆Sθ can be attributed to the

release of orderly structured hydration water and the increasing of the randomness when As (V) is adsorbed on the HF-D surface (42-43). The positive ∆Hθ also suggest that the adsorption of As (V) on HF-D was an endothermic process. Overall, large positive enthalpy change and the entropy increase all strongly suggested that As (V) adsorption by HF-D was a spontaneous process.



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Langmuir parameters Temperature (K)

qmax

283 298

Dubinin-Radushkevich (D-R)

KL

qmax

-1

(L mol )

90.35

3

(mg g )

93.13

Page 16 of 34

-1

7.4 × 10

3

5.6 × 10

3

R

2

0.998 0.998

RL

0.348

(mol kJ )

R

112.5

-4

0.987

32.9

-4

0.988

30.7

-4

101.4

2

E -2

(mg g )

0.288

KD

-1

4.6 × 10 5.3 × 10

2

(kJ mol-1)

313

105.6

1.9 × 10

0.997

0.612

94.48

7.8× 10

0.984

25.3

323

109.8

1.6 × 103

0.998

0.652

90.31

1.2 × 10-3

0.991

20.4

Table 2. Adsorption isotherm constants for 10 wt% HF-D. Kinetics study. The fitted adsorption kinetics of As (V) with the solution pH 6.0 were studied and the fitted parameters were depicted in Table 3. As expected, both of the arsenic adsorption on OP-R and HF-D were following the pseudo-second model, indicating the behavior of As (V) on HF-D is a combined process which could be assumed by the surface chemical adsorption and the inner complex reduction. Therefore, it would take more time for all of the adsorption site on the HF-D to be filled with As (V). The Arrhenius plots of the rate constants were shown in Fig. S6, yields an apparent activation energy of 11.4 kJ mol-1 for arsenic removal by HF-D. The arsenic adsorption on HF-D was turned out to be a chemical adsorption process for the reason that the activation energy is much larger than 4.2 kJ mol-1 (42). Pseudo-first-order

Pseudo-second-order 2

Absent species

qexp (mg/g)

qcal (mg/g)

K1 (h-1)

R

qcal (mg/g)

K1 (g mg-1 h-1)

R2

HF-D

81.3

85.9

0.132

0.982

90.2

3.12 × 10-3

0.999

OP-R

37.1

43.4

0.416

0.981

43.3

6.67 × 10-3

0.998

Table 3. Fitted Parameters of Pseudo-first-order and Pseudo-second-order Kinetic Model to the Adsorption Kinetics of As (V) on HF-D and OP-R. Application Performance of HF-D. Adsorption of As (III). After As (V) adsorption process, the used HF-D was separated by an external magnet ( Fig. S7) and the concentration of As (III) in supernatant was measured. The analysis data showed


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that no As (III) ions were found in the supernatant, indicating that the produced As (III) might be also adsorbed by the HF-D. In order to prove this, the adsorption performance of HF-D on As (III) was studied. Fig. 8 (a) shows that the HF-D also reveals a high adsorption capacity for 45.3 mg/g on As (III). The analysis above demonstrated that the HF-D can be used as a potential absorbent to remove both As (III) and As (V) in aqueous solution. Desorption and Regeneration. The adsorption and regeneration analysis method are provided in the supporting information and cycles results are shown in Fig. S8. The value of cycle 0 corresponds to the adsorption capacity of the fresh prepared HF-D. Successive three cycles adsorption and regeneration experiments were repeated and the adsorption performance of the HF-D slightly decreased with the regeneration cycle number. The adsorption efficiency for arsenic adsorption in cycle 3 was found to be 89%. In addition, the stability of the HF-D was investigated by soaking HF-D particles in arsenic and DI water solution in 500 min. As shown in Fig. S9, the concentration of the total released iron in two solutions were all lower than 0.3 mg/L, indicating the good mechanical stability of the HF-D. Desorption and regeneration details of As (V) proved out that the HF-D can be used repeatedly and easily regenerated via NaOH treatment. Adsorption of toxic ions from real ground water solution. The real ground water is more complicated relative to the synthetic solution due to it contains various kinds of salts, organic matters and even microbes. Adsorption of arsenic on HF-D from the real ground water solutions was studied, and the results are shown in Fig. 8 (b). The



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removal rate of As, Sb, Cr and Mo on the HF-D were 99.2%, 98.7%, 90.3% and 86.1, which are much higher as compared to OP-R. The analysis above turn out that the synthesized HF-D can be used as a potential adsorbent to remove toxic ions from real ground water solution. Adsorption Mechanism of As (V) on HF-D. The FT-IR spectrum of HF-D before and after loading As (V) are presented in Fig. 4. The distinct band at 3418 and 1023 cm-1 are attributed to the stretching vibration of the -OH and Fe-OH groups of the HF-D, respectively. After loading of As (V), the Fe-OH peak and -OH peak of the HF-D began to flatten. The arsenic loaded HF-D exhibits a new band at 818 cm-1 as compared to HF-D, which can be attributed to -O-As bond. It could be concluded that some of As (V) species were adsorbed by the -OH groups of the HF-D. The XPS analysis for evidence of the inner adsorption mechanism was also performed. As shown in Fig. 9 (a), the XPS spectra of O 1s was deconvoluted into O2- and -OH component peaks by analyzing each component peak area

(45-46)

. The O2- component

peaks increased from 10.6% to 40.8% and -OH component peaks decreased from 89.4% to 59.2% when the arsenic was loaded, suggesting that the -OH groups are changed into -O-R groups (i.e. C-O-As or Fe-O-As). Reduction Mechanism of As (V) on HF-D. The High-resolution XPS spectra of Fe 3p and As 3d are shown in Fig. 9 b and c. In this result, the Fe 3p peak and the As 3d peak were deconvoluted into Fe (III), Fe (II) and As (V), As (III) component peaks by analyzing each component peak area, respectively (46). The deconvoluted data show that there is a great change in the oxidation state of As (V) and Fe atoms after arsenic


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was loaded. The As-free HF-D sample had 60.6% Fe (III) and 39.4% Fe (II), which agreed well with the oxidant dosage. For As (V) loaded HF-D sample, Fe (III) component peaks increased to 69.4% and Fe (II) component peaks decreased to 30.6%, indicating that there is a significant oxidation of Fe (II) to Fe (III) in the adsorption process. To identify this inner reduction process,

57

Fe Mössbauer

spectroscopy was employed. The acquired Fe Mössbauer spectra of the As(V) free HF-D and As(V) loaded HF-D are shown in Figure 10, and values of the Mössbauer hyper-fine parameters obtained from fitting the respective the respective Mössbauer spectra are listed in Table S4. The 57Fe Mössbauer spectra for As (V) free and As (V) loaded 10 wt% HF-D confirm that the oxidation of Fe(II) to Fe(III) on the HF-D surface

(47,48)

. The As loaded 10 wt% HF-D sample had 69.2% Fe(III) and 30.8%

Fe(II), which agrees well with the XPS data. On the basis of the XPS and Mössbauer spectrum results, the behavior of As (V) on the HF-D surface was turned out to be an adsorption-reduction process. The presence of the reduction process may facilitate the arsenic adsorption through electron transfer process between As and Fe.

Conclusion In summary, the porous bio-sorbent based on OP was synthesized by an easy and

effective approach. The dendrimer-like structure was formed by surrounding the crystal iron nanoparticles with amorphous carbon, which was proved by characteristic data. The obtained HF-D exhibited a remarkable adsorption performance on arsenic (~81.3 mg/g) when the Fe/biomass ratio is 10 wt % and pH 6. The studies of adsorption kinetics and isotherms demonstrated that the adsorption process can be



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well fitted by the pseudo-second order model and Langmuir model. The RL (0.760-0.311), the free energy (17.6-21.3 kJ mol-1) and the activation energy (> 4.2 kJ mol-1) all proved that the arsenic adsorption process was a favorable chemical adsorption process. In addition, FTIR, XPS and Mössbauer spectrum analysis revealed that the behavior of As (V) on HF-D can be ascribed to an adsorption-reduction process, including -OH groups site adsorption and inner reduction process. Moreover, stability and regeneration tests demonstrated that the HF-D could be used repeatedly and easily regenerated via NaOH treatment. The finding of this paper offering an effective strategy to fabricate porous bio-sorbents for water treatment.

Acknowledgements Financial supports from the NSFC-Xinjiang Unite Funding (U1403194) are

gratefully acknowledged.

Supporting Information Arsenic analysis method, thermodynamic calculation method and parameters,

adsorption kinetics, Mössbauer hyperfine parameters, TEM images, BET analysis, ATR-FTIR spectra, zeta potential, magnetic hysteresis loops, effects of different cations on the adsorption and regeneration data are supplied as the supporting information.

References

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Figures and Captions:

Figure 1. Optical micrographs of (a) raw OP, (b) OP-R and (c) 10 wt% HF-D. SEM micrographs of surface of (d) raw OP, (e) OP-R and (f) 10 wt% HF-D.

Figure 2. Low-magnification TEM images of (a) raw OP, (b) OP-R and (c) 10 wt%


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HF-D. High-magnification TEM images of (d) raw OP, (e) OP-R and (f) 10 wt% HF-D.

Figure 3. The XRD patterns of the OP-R, 5 wt% HF-D, 10 wt% HF-D and 15 wt% HF-D.

Figure 4. The FT-IR spectra of raw OP, As (V) free OP-R, 5 wt% HF-D, 10 wt% HF-D, 15 wt% HF-D and As (V) adsorbed 10 wt% HF-D.



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Figure 5. (a) The adsorption capacity analysis of the OP-R and 10 wt% HF-D. (b) The effect of the Fe loading content on As (V) removal. The measurements were did at pH 6 and in 25 mg/L ion solution with the temperature of 298 K.

Figure 6. Effect of (a) pH, (b) coexisting ions, (c) humic acid, (d) initial concentration and temperature for As (V) adsorption on the 10 wt% HF-D.


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Figure 7. (a) Langmuir and (b) Dubinin-radushkevich plots for the adsorption of arsenic onto 10 wt% HF-D at different temperatures.

Figure 8. (a) The removal of As (III) by OP-R and 10 wt% HF-D. (b) The removal of toxic ions by the 10 wt% HF-D in real ground water solution. (Control with pH at 6 and in 298 K, As removal rate is the total As species)



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Figure 9. High-resolution XPS spectra in (a) O 1s regions, (b) Fe 3p and (c) As 3d of As-free HF-D and As adsorbed HF-D. a

b

Figure 10. Low-temperature 57Fe Mössbauer spectra of (a) As(V) free and (b) As(V) loaded HF-D.


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For Table of Contents use only:

This study provides a new method for synthesizing porous adsorbent to remove arsenic, which is low cost and environmental friendliness.



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This study provides a new method for synthesizing porous biosorbent to remove arsenic, which is low cost and environmental friendliness. 378x193mm (96 x 96 DPI)


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Novel Dendrimerlike Magnetic Biosorbent Based on Modified Orange

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