Novel dendrimer-like magnetic bio-sorbent based on modified orange

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9692-9700

pubs.acs.org/journal/ascecg

Novel Dendrimerlike Magnetic Biosorbent Based on Modified Orange Peel Waste: Adsorption−Reduction Behavior of Arsenic Fanqing Meng,† Bowen Yang,† Baodong Wang,‡ Shibo Duan,† Zhen Chen,† and Wei Ma*,† †

Department of Chemistry, Dalian University of Technology, Dalian 116023, PR China National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, PR China

‡

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S Supporting Information *

ABSTRACT: In this work, a novel porous biosorbent (HF-D) based on orange peel (OP) was prepared by an efficient and simple method. The prepared HF-D showed well-ordered dendrimerlike structures and remarkable adsorption performance for the removal of arsenic. Particularly, morphology and structure details proved that the dendrimerlike 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) was 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 not only provides an efficient biosorbent 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

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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 activity discharge.1 There are various As species that have been found in natural water as inorganic oxyanions of trivalent arsenite (AsO33−, As(III)), and pentavalent arsenate (AsO43−, As(V)).2 Research has shown 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 has been 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,9 ion exchange,10 membrane filtration,11 and adsorption methods.12,13 Among those techniques, the adsorption process is considered to © 2017 American Chemical Society

be one of the most promising techniques due to its economics, effectiveness, and social acceptability.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 its low cost, ready availability, environmental friendliness, and high efficiency.15,16 Various raw biomass can be used to produce biosorbents such as apple peels,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 secondary water contamination during the purification process are to be developed.22 Recently, several methods were applied in biomass modification to get potential bioadsorbents by introducing chemicals such as citric acid, acetic acid, lithium chloride,23 and potassium hydroxide.24 Iron compounds showed many advantages for the bioadsorbents production.25,26 (i) Biomass can be modified by iron compounds at lower temperature (about 180 °C) in the atmosphere. (ii) Iron compounds have a low toxic effect on the living body, thus they are environmentally friendly. Received: April 25, 2017 Revised: September 11, 2017 Published: September 18, 2017 9692

DOI: 10.1021/acssuschemeng.7b01273 ACS Sustainable Chem. Eng. 2017, 5, 9692−9700

Research Article

ACS Sustainable Chemistry & Engineering

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 57Fe Mössbauer spectra were collected using a Mössbauer spectrometer operating in constant acceleration mode and equipped with a 50 mCi 57Co(Rh) source. Trace amounts of heavy metals were detected using an inductively coupled plasmaoptical emission spectroscopy (ICP-OES Agilent: 720). The concentration of iron in aqueous solution was detected by an Atomic absorption spectrometry (AAS PerkinElmer instrument co., LTD: 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 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 coexistence 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:

(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 production of mesoporous structures with high specific surface area. Thus, biosorbents modified by iron compounds are beneficial for achieving industrial scale applications. However, conventional magnetic biosorbent modified processes usually require harsh conditions, which not only take time and energy but also lead to the elimination of functional groups.25,26 The hydrothermal carbonization (HTC) process was considered as an ecofriendly route to modify biomass materials in a closed system under mild conditions and low temperatures (150−250 °C).27 The HTC process exhibits more advantages than the conventional process for the reason that it not only retains the rich functional groups but it is also economical and ecofriendly. The resulting HTC biomass tends 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 bioadsorbents which deeply depends on the morphology. As far as the authors know, no studies have been reported for preparing porous OP based bioadsorbent with high specific surface area and richly functional groups by the HTC process via iron solution. In this work, a novel dendrimerlike structure HF-D based on OP through the HTC process in the Fe(II)/Fe(III) ions solution was synthesized. The characterization results demonstrated that the dendrimerlike 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 groundwater solutions were studied. Finally, the adsorption−reduction behavior was explored by Fourier transform infrared (FTIR), X-ray photoelectron (XPS), and Mössbauer spectra. 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.

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qe = (Co − C t)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 groundwater 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, 15.7, 60, and 400 μg/L, respectively. An aliquot of groundwater (50 mL) was treated with 10 wt % HF-D (10 mg). The effect of coexisting 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. First, 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 arseniccontaining HF-D in 0.5 M NaOH solution. Typically, the 1 g arseniccontaining HF-D was suspended in 100 mL NaOH solution, and the 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

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 Dendrimerlike Structure HF-D. Fresh OP was prepared and used within 2 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 dried at 60 °C for 24 h. Approximately 5 g of the dried OP was transposed to a 100 mL stainless steel autoclave (Anhuikemi, China) containing approximately 50 mL of Fe(II)/Fe(III) ion 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 °C for 10 h. The mixture was filtered and washed by the DI water to get the powder samples that were abbreviated as OP-R, 5 wt % HF-D, 10 wt % HF-D, and 15 wt % HF-D. 9693


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ACS Sustainable Chemistry & Engineering literature29 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).

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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 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 loading content of Fe, indicating that Fe enhanced the OP carbonization. Material Characterization and Properties. Details about the morphology of the raw OP, obtained OP-R, and HF-D were examined in Figure 1. Color change from brown to dark

Figure 2. Low-magnification TEM images of (a) raw OP, (b) OP-R, and(c) 10 wt % HF-D. High-magnification TEM images of (d) raw OP, (e) OP-R, and (f) 10 wt % HF-D.

amorphous carbon (Figure S1b and c). The formation of those branches could be attributed to directional growth of the amorphous carbon that drives iron particle directional movement by magnetic action. For the 15 wt % HF-D, the structure morphology was changed due to the coverage of some macropores by the crystal iron nanoparticles (Figure S1e 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 Figure 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 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 obtained HF-D displayed strong intensity at 3250−3550, 1100−1350, 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 groups,17,33,34 respectively. After the HTC treatment process, the νCH signal at 2850 cm−1 of the raw OP began to flatten, suggesting the polymerization and

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

brown was observed after surface modification of the OP. Comparing the raw OP with OP-R, a large number of dendrimerlike macrospores were revealed to exist in the 10 wt % HF-D. The morphology was further investigated by TEM analysis (Figure 2). Notable differences in the morphology of the raw OP, obtained OP-R, and HF-D were observed. As shown in Figure 2, the morphology of the prepared bioadsorbent has been both affected by the introduction of iron and the hydrothermal process. The interfacial structure of the branch was investigated by HR-TEM analysis. As shown in Figure 2f, the spacing lattice fringe of 0.252, 0.245, and 0.492 nm were implied the presence of (110)Fe2O3 planes,30 (111)FeO(OH) planes,31 and (111)magnetite planes,32 respectively. Moreover, it has been found that the dendrimerlike structure was significantly affected by the Fe/biomass ratios. To possess an overall understanding of the dendrimerlike structure HF-D material, the high-magnification TEM images of three HF-D were studied. As shown in Figure S1a 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 dendrimerlike structure. The HRTEM images of the 10 wt % HF-D revealed that the dendrimerlike structure was formed by surrounding the iron nanoparticles inside the branch with the 9694


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Table 1. Summary of As(V) Adsorption Capacity of Various Adsorbents adsorbent organic biochar magnetite nanoparticles magnetic biochar Fe−Mn chitosan bead γ-Fe2O3 biochar OP-R 10 wt % HF-D

surface area (m2 g−1)

capacity (mg/g)

ref

190 39.0 193 248

16.2 27.8 2.53 54.2 30.1 37.1 81.3

20 26 33 34 17 this work this work

164 396

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 Figure 5b. It can be seen that the adsorption efficiency increased sharply with iron loading content up to 10 wt %, where it began to decrease. This could be attributed to the decrease of surface area when the Fe/biomass ratio increased from 10 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 Figure S2. The 10 wt % HF-D shows a type of IV isotherm with H3 hysteresis loop, which indicates the mesoporous nature.35 Effect of Solution pH on Adsorption Capacity of As(V). The effects of pH on the adsorption of As(V) are shown in Figure 6a. 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) appeared at pH 6.0 and decreased monotonically with increasing pH from 8 to 12. Those results were consistent with the pH dependent on As(V) speciation and surface charge of the HF-D. As shown in the ATR-FTIR spectrum (Figure S3), bonds at 906, 873−871, and 763−755 cm−1 which originate from H2AsO4− and HAsO42− appeared at pH 3 and 6. At pH 9, only one band around 854 cm−1 appeared which stems from the As−O stretching vibration of the HAsO4 2− entity.36 The adsorption of As(V) would be promoted by electrostatic attraction between negatively charged As(V) species and the positively charged HF-D surface when the pH is 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 Figure S4, the IEP of the HF-D decreased from 7.3 to 4.5 when As(V) was absorbed. This may be because 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 Figure 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 As(V) was adsorbed, and two peaks at 883 and 794 cm−1 were observed in the ATR-FTIR spectrum which were assigned to the symmetric stretch vibration (νs) of As−O−Fe and the weakly bound 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 coexisting cations was shown in Figure S5. As the adsorption results illustrated, there was no significant

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

Figure 4. 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.

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, which indicated that the formation of the Fe−OH groups in the HF-D. This result was also uncovered 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 dendrimerlike biomass prepared in this work showed relatively large adsorption capacity of 81.3 mg/g as compared to the literature. Figure 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. 9695


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Figure 5. (a) Adsorption capacity analysis of the OP-R and 10 wt % HF-D. (b) Effect of Fe loading content on As(V) removal. The measurements were performed 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, and (d) initial concentration and temperature for As(V) adsorption on the 10 wt % HF-D.

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 increasing Ca2+ and Mg2+. This may be because the Ca2+ and Mg2+ ions may compress the double layer and make the internal pores more accessible, enhancing the adsorption of arsenate accordingly.37,38 Figure 6b shows the effect of coexisting inorganic ions on arsenic adsorption. 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 fact that 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. Figure 6c shows the adsorption capacity of 10 wt % HF-D on As(V) in the presence of different concentration of humic acid (humic acid concentrations 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 and 1 mg/L humic acid. When the concentrations 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 Figure 6d. It can be observed that As(V) adsorption capacity was increased with the initial concentration of As(V) and temperature increase, 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 (DR) isotherms have been investigated. The Langmuir theory was studied to assume a homogeneous type of adsorption within the adsorbent. The DR 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 (Figure 7a and b) are listed in Table 2. These data indicated that the adsorption of 9696


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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.

Table 2. Adsorption Isotherm Constants for 10 wt % HF-D Langmuir temperature (K) 283 298 313 323

−1

−1

qmax (mg g )

KL (L mol )

90.35 93.13 105.6 109.8

× × × ×

7.4 5.6 1.9 1.6

3

10 103 103 103

Dubinin−Radushkevich (DR) R

2

−1

RL

0.998 0.998 0.997 0.998

qmax (mg g )

0.288 0.348 0.612 0.652

KD (mol2 kJ−2) −4

4.6 × 10 5.3 × 10−4 7.8× 10−4 1.2 × 10−3

112.5 101.4 94.48 90.31

R2

E (kJ mol−1)

0.987 0.988 0.984 0.991

32.9 30.7 25.3 20.4

Table 3. Fitted Parameters of Pseudo-First-Order and Pseudo-Second-Order Kinetic Models to the Adsorption Kinetics of As(V) on HF-D and OP-R pseudo-first-order absent species HF-D OP-R

qexp (mg g−1) 81.3 37.1

qcal (mg g−1) 85.9 43.4

K1 (h−1) 0.132 0.416

pseudo-second-order R2 0.982 0.981

qcal (mg g−1) 90.2 43.3

K1 (g mg−1 h−1) −3

3.12 × 10 6.67 × 10−3

R2 0.999 0.998

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, shown in Figure S6, yield an apparent activation energy of 11.4 kJ mol−1 for arsenic removal by HF-D. The arsenic adsorption on HF-D turned out to be a chemical adsorption process for the reason that the activation energy is much larger than 4.2 kJ mol−1.44 Application Performance of HF-D. Adsorption of As(III). After the As(V) adsorption process, the used HF-D was separated by an external magnet (Figure S7) and the concentration of As(III) in the supernatant was measured. The analysis data showed 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. Figure 8a 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 cycle results are shown in Figure S8. The value of cycle 0 corresponds to the adsorption capacity of the fresh prepared HF-D. Successive three cycle 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

As(V) on 10 wt % HF-D was well described by the Langmuir and DR models. The essential feature of the Langmuir isotherm can be expressed as RL which referred to the 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 that the arsenic adsorption process is favorable.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 that the adsorption mechanism might be a chemical adsorption process. The calculated thermodynamic parameters of As(V) adsorption are shown in Table S3, including ΔGθ, ΔHθ, and ΔSθ. The calculated function of qe for ΔHθ was provided in the Supporting Information. The negative ΔGθ suggest that the As(V) adsorption process is a spontaneous adsorption reaction.42 The positive ΔSθ 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 ΔHθ also suggest that the adsorption of As(V) on HF-D was an endothermic process. Overall, the large positive enthalpy change and the entropy increase all strongly suggested that As(V) adsorption by HF-D was a spontaneous process. 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 9697


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

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.

flatten. The arsenic loaded HF-D exhibits a new band at 818 cm−1 as compared to HF-D, which can be attributed to the −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 Figure 9a, the XPS spectra of O 1s was deconvoluted into O2− and −OH component peaks by analyzing each component peak area. 45,46 The O 2− 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 highresolution XPS spectra of Fe 3p and As 3d are shown in Figure 9b 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 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

in arsenic and DI water solution for 500 min. As shown in Figure 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 that the HF-D can be used repeatedly and easily regenerated via NaOH treatment. Adsorption of Toxic Ions from Real Groundwater Solution. The real groundwater is more complicated relative to the synthetic solution due to that fact that it contains various kinds of salts, organic matters, and even microbes. Adsorption of arsenic on HF-D from the real groundwater solutions was studied, and the results are shown in Figure 8b. The 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 groundwater 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 Figure 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 9698


Research Article


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

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, 57Fe 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 Mössbauer spectra are listed in Table S4. The 57Fe Mössbauer spectra for As(V) free and As(V) loaded 10 wt % HF-D confirm 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 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +86(411) 8470 6303. Fax: +86(411) 8470 7416 (W.M.). ORCID

Wei Ma: 0000-0002-5359-0273 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the NSFC-Xinjiang Unite Funding (U1403194) and Key projects of Liaoning province (China), 851116 are gratefully acknowledged.

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CONCLUSION In summary, the porous biosorbent based on OP was synthesized by an easy and effective approach. The dendrimerlike 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 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 spectra analysis revealed that the behavior of As(V) on HF-D can be ascribed to an adsorption− reduction process, including −OH group site adsorption and an inner reduction process. Moreover, stability and regeneration tests demonstrated that the HF-D could be used repeatedly and easily regenerated via NaOH treatment. The findings of this paper offer an effective strategy to fabricate porous biosorbents for water treatment.

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hyperfine parameters, TEM images, BET analysis, ATR-FTIR spectra, zeta potential, magnetic hysteresis loops, and effects of different cations on the adsorption and regeneration data (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01273. Arsenic analysis method, thermodynamic calculation method and parameters, adsorption kinetics, Mössbauer 9699


Research Article


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