Enhanced removal of roxarsone by Fe3O4@3D graphene nanocomposites: synergistic adsorption and mechanism

Chen Tian ab, Jian Zhao c, Jing Zhang d, Shengqi Chu d, Zhi Dang ab, Zhang Lin *ab and Baoshan Xing *e
aSchool of Environment and Energy, The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), South China University of Technology, Guangzhou, Guangdong 510006, China
bGuangdong Engineering and Technology Research Center for Environmental Nanomaterials, South China University of Technology, Guangzhou, Guangdong 510006, China
cCollege of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, P.R. China
dBeijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P.R. China
eStockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA. E-mail: bx@umass.edu; Tel: +1 413 545 5212

Received 17th August 2017 , Accepted 8th September 2017

First published on 8th September 2017


Roxarsone (ROX) is an emerging arsenic pollutant due to its potential degradation into highly toxic inorganic arsenic species in the environment. Adsorbents which can capture ROX with both high capacity and affinity are urgently needed. Herein, a nanocomposite of nano-Fe3O4 and three-dimensional graphene (Fe3O4@RGO) was designed, aiming to simultaneously attract arsenate and benzene groups in ROX. Characterization of the nanocomposite revealed that Fe3O4 nanoparticles (20–50 nm) with exposed (400) planes were highly dispersed on the graphene support. Adsorption experiments showed that Fe3O4@RGO had higher adsorption capacity, affinity, and adsorption rate towards ROX than pristine materials and efficiently removed ROX from both simulated natural and waste waters. The adsorption mechanism was confirmed as a synergetic interaction of As–Fe coordination, hydrogen bonding and π–π interaction. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) results suggested that the As–Fe complexes and hydrogen bonds between Fe3O4@RGO and ROX were stronger than those in pristine nano-Fe3O4, due to the greater number of surface hydroxyls and shorter As–Fe atomic distance in Fe3O4@RGO. The π–π interaction between ROX and the graphene part in Fe3O4@RGO was also enhanced. This study provided a novel idea for designing materials to remove pollutants with both inorganic and organic moieties, such as phenylarsonic acid compounds, from water.



Environmental significance

Roxarsone (ROX) is considered to be an emerging pollutant as it can degrade into highly toxic inorganic arsenic species in natural environments. Reported in this work is the design of a Fe3O4@RGO nanocomposite to simultaneously adsorb the arsenate and benzene ring groups in ROX molecules. The excellent performance of Fe3O4@RGO achieved the expected high adsorption capacity and affinity for ROX, as well as the superior ROX removal in both simulated natural and waste waters, with no ROX degradation. To our knowledge, this is the first molecular-level study to investigate the synergistic adsorption between the designed adsorbent and the two functional groups in organoarsenic compounds. The enhanced removal of ROX was ascribed to the synergetic interactions of As–Fe coordination, hydrogen bonding and π–π stacking between Fe3O4@RGO and ROX. The combination of 3D graphene and Fe3O4 nanoparticles increased the surface hydroxyl amounts in the nanocomposite, and thus further enhanced the As–Fe coordination and hydrogen bonding for superior adsorption. On this basis, this work is expected to stimulate further investigations into the functional design of adsorbents for the removal of emerging pollutants with both inorganic and organic moieties.

1 Introduction

Roxarsone (ROX) is a typical phenylarsonic acid compound which is widely used as an animal feed additive due to its broad antimicrobial properties.1,2 After excretion in manure, water-soluble ROX can be degraded into more toxic inorganic arsenic species (such as arsenate) in 30 days via biotic and abiotic degradation, leading to severe arsenic pollution in soil and groundwater.3–5 It was reported in China that the arsenic content in chicken litter and pig manure was 21.6 and 89.3 mg kg−1, respectively,6 which came from ROX and other arsenic-containing feed additives. Therefore, ROX is considered as a type of emerging contaminant, and its removal is crucial for maintaining water and soil quality and safety.

Since the degradation product of ROX is highly toxic, adsorption is preferable over degradation. In particular, adsorbents with both high capacity and affinity for ROX are urgently needed to restrain the release of ROX during and after adsorption. Investigations so far have mainly focused on materials with high adsorption capacity for ROX.7–9 For example, metal–organic frameworks (MOFs) were found with high adsorption capacity for ROX (up to 730 mg g−1).10 However, the adsorption affinity of MOFs for ROX was relatively low, although the adsorption mechanism included the coordination between the open metal sites and arsenic groups, electrostatic interaction and pore filling effects.11 Moreover, very few studies assessed whether ROX degraded into inorganic arsenic species during adsorption. Therefore, new materials with both high adsorption capacity and strong affinity for ROX need to be designed, synthesized and tested.

Fe3O4 nanoparticles (NPs), which were confirmed to be excellent adsorbents for arsenic,12 could be a potential candidate for removing ROX because the ROX molecule contains an As-bearing group similar to arsenate (Fig. S1 in the ESI). According to previous studies,13–15 the high affinity of Fe3O4 for arsenate was attributed to the inner-sphere coordination of bidentate binuclear corner-sharing (2C) or tridentate hexanuclear corner-sharing (3C) complexes between Fe and As. Moreover, the {100} facet (i.e., (400) plane) of Fe3O4 was considered to be favorable for the formation of As–Fe 2C complexes.16 It was also suggested that hydroxyls on the surface of Fe3O4 facilitated the inner-sphere coordination between Fe and As,17 thereby greatly enhancing the adsorption affinity. However, the spontaneous agglomeration of nano-Fe3O4 resulted in poor dispersibility in water, causing a significant decrease in surface active sites for binding As, thus severely limiting its application.18

New adsorbents with high affinity for ROX could be designed based on the features of arsenate and benzene ring groups in the ROX molecule (Fig. S1). The nanocomposite of Fe3O4 NPs loaded on three-dimensional (3D) reduced graphene oxide (Fe3O4@RGO) was selected on account of the following hypotheses. Firstly, 3D graphene with an interior connected porous structure has been confirmed to be an excellent support for nano-Fe3O4 to refrain the NPs from aggregation.19–21 By immobilization on 3D graphene, more active sites on Fe3O4 NPs could become accessible. Secondly, the graphene substrate may promote the nucleation and inerratic growth of Fe3O4 NPs, which not only decreases the grain size but also possibly changes the preferential crystallographic facet and increases the amount of hydroxyl groups on the surface. It is expected that the preferential facet and high amount of surface hydroxyls in the nanocomposite would enhance the As–Fe coordination. Thirdly, graphene in the nanocomposite is also expected to adsorb ROX through π–π stacking with the benzene ring groups.22 Once ROX is adsorbed by the As–Fe short-distance interaction and/or other forces, it is very possible that the graphene support in the nanocomposite would undergo π–π interaction with the benzene ring of the same ROX molecule, which would further enhance the interaction between the adsorbent and ROX. Thus, Fe3O4 NP–graphene composites may provide potential synergy for ROX adsorption. Currently, although a few iron-based carbon materials have been used for ROX adsorption,8,23 without a rationally justified design, the merit of these materials could not be well manifested. Then, the roles of iron and carbon in ROX adsorption could be only discussed separately, and no synergetic adsorption mechanisms were considered in these works.

In this work, a Fe3O4@RGO nanocomposite was designed and prepared via in situ crystal growth of Fe3O4 NPs on RGO. The main objectives of this work were therefore to (1) analyze the dispersibility and preferential exposed facets of Fe3O4 NPs on RGO, as well as the chemical bonds between Fe3O4 and RGO, (2) comparatively investigate the adsorption of ROX on Fe3O4@RGO and the original pristine materials to verify the synergetic adsorption, and evaluate the ROX removal potential of Fe3O4@RGO from simulated natural and waste waters, and (3) disclose the factors, such as exposed planes and surface hydroxyls of Fe3O4 in the nanocomposite that are responsible for the formation of As–Fe complexes and hydrogen bonds, and assess the contribution of As–Fe coordination, hydrogen bonding, π–π stacking and electrostatic interaction to the overall adsorption. This study will provide an important theoretical and experimental basis for designing adsorbents with high adsorption capacity and affinity towards ROX.

2 Materials and methods

2.1 Synthesis and characterization of the adsorbents

Fe3O4@RGO was synthesized using an in situ compositing method, via a redox process between ferrous ions and graphene oxide (GO).21 Briefly, a 2 mg mL−1 GO suspension was mixed with 1.0 mmol FeSO4·7H2O at pH 10 and then kept at 90 °C for 6 h to produce a self-assembled nanocomposite (details are described in ESI Section S1). For comparison, pure Fe3O4 NPs synthesized using the same method without GO were named p-Fe3O4, while GO treated using the same procedure without the addition of Fe2+ was named GO-90. Preliminary results revealed that the GO could be only slightly reduced at 90 °C, while treatment under hydrothermal conditions at an elevated temperature of 180 °C facilitated the reduction of GO at the same level as that in Fe3O4@RGO, thus it was named RGO (Fig. S2 and Table S1).

The crystal structure, surface morphology, specific surface area and surface charge of the samples were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) N2 adsorption method and zeta potential analysis, respectively. The active crystal planes of Fe3O4 in the nanocomposite and p-Fe3O4 were examined by high-resolution transmission electron microscopy (HRTEM). Surface elemental composition and functional groups were determined using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The mass content of Fe3O4 in Fe3O4@RGO was determined using a modified ferrozine method.24 Details about the characterization methods are presented in ESI Section S1.

2.2 Adsorption experiments

Adsorption isotherm studies were conducted with ROX (98%, Aladdin Bio-Chem Technology, China) concentrations from 1 to 100 mg L−1, while the adsorbent concentrations of Fe3O4@RGO, p-Fe3O4, GO-90 and RGO were kept at 0.15 g L−1 in 20 mL solution. The mixtures were shaken at pH 5 ± 0.1 and 25 °C with a speed of 200 rpm for 72 h. After magnetic separation,25 the residual ROX concentrations were measured using a UV-vis spectrometer (UV-2600, Shimadzu) at 223 nm with a solution pH of 4.0 ± 0.1. The possible degradation of ROX during adsorption was investigated by determining the concentrations of inorganic As(V) and As(III) in the aqueous solution using HPLC-ICP-MS (ESI Section S1).26 The adsorption isotherms were fitted with Dubinin–Astakhov (DA), Langmuir and Freundlich models.27,28 For kinetic experiments, 50 mg L−1 ROX solution was mixed with 0.15 g L−1 adsorbents in 300 mL solution and shaken at pH 5 ± 0.1 and 25 °C with a speed of 200 rpm for 1 min to 4 days. The adsorption kinetics was fitted with a pseudo-second-order non-linear kinetic model. The effects of pH on adsorption were investigated by conducting adsorption experiments within the pH range of 3.0–11.0, while the pH values were adjusted with NaOH and HCl solutions. To further investigate the potential applications of Fe3O4@RGO in natural and waste waters, ROX adsorption in the presence of dissolved organic matter (DOM, 0–12 mg C L−1) and in swine manure lixivium (with a DOM concentration of 9.0 mg C L−1 and an added ROX concentration of 1 mg L−1) was studied. Experimental and analytical details are displayed in Section S1. The desorption and reusability of Fe3O4@RGO were determined after washing the used adsorbent with 0.05 mol L−1 NaOH for 1 h at 25 °C with shaking at a speed of 200 rpm.29

2.3 Adsorption mechanism analysis

ROX adsorption as a function of solution pH (3–12) was conducted to estimate the contribution of electrostatic interaction between ROX and the nanocomposite. An ROX solution with a concentration of 50 mg L−1 was mixed with 0.15 g L−1 adsorbents and shaken at 25 °C with a speed of 200 rpm for 72 h. The As–Fe surface complex was analyzed by extended X-ray absorption fine structure spectroscopy (EXAFS). Briefly, the As K-edge X-ray absorption fine structure (XAFS) spectra of As-loaded samples were acquired at beamline 4W1B of the Beijing Synchrotron Radiation Facility (BSRF), China. The hydrogen bonding between the adsorbents and ROX was analyzed using Fourier transform infrared spectroscopy (FTIR). The hydroxyl content in Fe3O4@RGO and p-Fe3O4 was calculated from the O 1s XPS spectra, while the π–π stacking of Fe3O4@RGO and RGO after ROX adsorption was determined from the C 1s spectra. Detailed analysis procedures are provided in Section S1.

3 Results and discussion

3.1 Characterization of the Fe3O4@RGO nanocomposite

The XRD patterns of Fe3O4@RGO exhibited the characteristic peak of graphite (2θ = 13.5°) and the characteristic peaks of Fe3O4 (ICSD No. 84611, Fig. S3). XPS analysis of the Fe 3p spectra also confirmed the presence of Fe3O4 in the nanocomposite (Fig. S4). These results suggested the excellent compositing of Fe3O4 NPs with graphene. The mean crystal size of Fe3O4 NPs in the nanocomposite calculated from the Scherrer equation was around 25 nm, which was obviously smaller than that of p-Fe3O4 (around 34 nm, Table S2), indicating that the in situ growth on the graphene substrate could effectively decrease the crystal size of Fe3O4 NPs. The weight ratio of Fe3O4 and graphene in the nanocomposite was around 4.05[thin space (1/6-em)]:[thin space (1/6-em)]1. The specific surface area of Fe3O4@RGO, p-Fe3O4 and GO-90 was 61.9, 103.1 and 28.1 m2 g−1, respectively (Table S2).

SEM observation revealed that GO-90 had a complete sheet-like structure with a wrinkled surface (Fig. 1a), while RGO had more pores in the sheets (Fig. 1b), indicating that the increased temperature for achieving a high reduction degree could be at the expense of losing morphology features. p-Fe3O4 exhibited irregular and stacked nanoplates with lateral sizes of 200–400 nm (Fig. 1c). Fe3O4@RGO showed a similar sheet-like structure to GO-90, while Fe3O4 NPs with diameters of 30–50 nm were uniformly immobilized on the graphene sheets (Fig. 1d). These results indicated that the graphene support was beneficial for decreasing the particle size of Fe3O4 NPs and restricting their aggregation. In addition, HRTEM images showed that the d-spacing of lattice fringes for p-Fe3O4 and Fe3O4@RGO was 0.48 and 0.21 nm, respectively (Fig. 1e and f), suggesting that the (111) plane is mainly exposed in p-Fe3O4 while the (400) plane is mainly exposed in the Fe3O4 NPs of the nanocomposite. Further studies via the O 1s XPS spectra and Raman spectra confirmed that the immobilization of Fe3O4 NPs on graphene had a significant feature of Fe–O–C bonding (Fig. S5), which agreed with previous studies,30,31 indicating the strong interaction between Fe3O4 and the graphene support.


image file: c7en00758b-f1.tif
Fig. 1 SEM images of GO-90 (a), RGO (b), p-Fe3O4 (c), and Fe3O4@RGO (d) and HRTEM images of p-Fe3O4 (e) and Fe3O4@RGO (f).

3.2 Adsorption behavior of Fe3O4@RGO towards ROX

3.2.1 Adsorption isotherms. Adsorption of ROX on Fe3O4@RGO was investigated, along with p-Fe3O4, GO-90 and RGO for comparison. The adsorption isotherms of ROX on the tested materials are presented in Fig. 2a. Obviously, Fe3O4@RGO displayed higher adsorption capacity (qe) than the three pristine materials. After normalizing with surface area, the adsorption capacity of Fe3O4@RGO was further elevated (Fig. 2b), indicating that the enhanced ROX adsorption on Fe3O4@RGO was independent of the surface area. Better fitting was observed using DA and Langmuir models than that using the Freundlich model (Table 1). As indicated by DA fitting, the maximum adsorption capacity (Qm) followed the order of Fe3O4@RGO > p-Fe3O4 > p-Fe3O4 + GO-90 (calculated from the physical mixture of p-Fe3O4 and GO-90 with a mass ratio of 4.05[thin space (1/6-em)]:[thin space (1/6-em)]1) > RGO > GO-90, while the adsorption affinity (as indicated by E values) followed the order of Fe3O4@RGO ≈ p-Fe3O4 > p-Fe3O4 + GO-90 > RGO > GO-90. Therefore, the combination of Fe3O4 NPs and graphene in Fe3O4@RGO displayed a synergetic enhancement on both adsorption capacity and affinity for ROX.
image file: c7en00758b-f2.tif
Fig. 2 Adsorption isotherms (a), surface area-normalized adsorption isotherms (b), and adsorption kinetics (fitted by the pseudo-second-order non-linear kinetic model) (c) of ROX on Fe3O4@RGO, p-Fe3O4, GO-90 and RGO. In panel (a), the magenta symbols (image file: c7en00758b-u1.tif) stand for the calculated adsorption capacity value of the physical mixture of p-Fe3O4 and GO-90.
Table 1 Parameters of Langmuir, Freundlich, and DA fits for ROX adsorption on Fe3O4@RGO, p-Fe3O4 GO-90, and RGO
Sample Langmuir model Freundlich model DA model
Q m (mg g−1) K L R 2 n K f (mg g−1) R 2 Q m (mg g−1) E b R 2
a The Langmuir, Freundlich, and DA fitting of p-Fe3O4 + GO-90 was conducted from the additive results of residual concentration and adsorption capacity obtained from 80.2% p-Fe3O4 and 19.8% GO-90.
Fe3O4@RGO 454.48 0.78 0.9975 2.70 134.80 0.9562 534.43 4.72 2.44 0.9939
p-Fe3O4 163.93 0.75 0.9930 8.56 69.96 0.9212 172.16 4.81 2.49 0.9999
GO-90 43.10 0.025 0.9948 1.51 6.73 0.9915 60.02 2.42 1.69 0.9980
p-Fe3O4 + GO-90a 111.11 0.044 0.9936 2.05 24.05 0.8911 136.48 3.87 3.27 0.9941
RGO 104.17 0.065 0.9879 1.70 9.71 0.9738 92.78 3.42 2.36 0.9986


The comparison of the Langmuir-fitted maximum adsorption capacity (Qm) and affinity (KL) of Fe3O4@RGO with other adsorbents in the literature is shown in Table S3. It can be seen that although some of the adsorbents had larger Qm, most of the KL values were smaller than those for Fe3O4@RGO, and thus Fe3O4@RGO had a higher adsorption amount at equilibrium in both low and high concentration systems. Moreover, except the ferric and manganese binary oxide,32 none of the reported adsorbents considered the degradation of ROX during adsorption. In comparison, for the as-prepared Fe3O4@RGO, no inorganic As(V) or As(III) was detected in the ROX adsorption supernatant even after 15 days or in the desorption solution in water and NaOH (Fig. S6). Therefore, compared with the reported adsorbents, Fe3O4@RGO was a more effective and environmentally friendly adsorbent towards ROX with both high adsorption capacity and affinity, as well as no risk of releasing inorganic arsenic into the environment.

3.2.2 Adsorption kinetics. The adsorption kinetics of ROX on the tested materials are shown in Fig. 2c. Adsorption equilibrium was reached around 720 min for Fe3O4@RGO, whereas longer times were required for the other samples. All the adsorption kinetics followed the pseudo-second-order model (Table S4), indicating that chemical sorption controlled the sorption rate and the number of active sites on the sorbent determined the adsorption capacity.33 The rate constant (k2) of Fe3O4@RGO was 2.38 × 10−4 g mg−1 min−1, which was higher than those of the three pristine materials (Table S4). The enhanced adsorption rate of Fe3O4@RGO, together with its higher adsorption capacity and affinity, was possible due to the stronger chemical interaction between Fe3O4@RGO and ROX, which is discussed in detail in the Adsorption mechanism of Fe3O4@RGO towards ROX section.
3.2.3 Reusability of Fe3O4@RGO. In this work, NaOH solution was chosen as the desorption solution to test the reusability of Fe3O4@RGO. The result indicated that the adsorption capacity of the used Fe3O4@RGO did not diminish significantly after four cycles (Fig. S7). FTIR analysis clearly showed the same characteristic peaks of Fe3O4@RGO after the fourth adsorption cycle with the pristine sample, while XRD patterns also showed the characteristic peaks of Fe3O4 after the fourth cycle (Fig. S8). This result suggested the stable structure properties of Fe3O4@RGO during the repeated regeneration process.
3.2.4 Performance of Fe3O4@RGO in simulated natural and waste waters. DOM (dissolved organic matter) is the major inhibitor in natural water for the adsorption of organic pollutants.34 Therefore, the effect of DOM on ROX adsorption was investigated and the results are shown in Fig. 3a. The ROX removal rate slightly decreased from 98.6% to 93.5% when the DOM concentration increased from 0 to 12 mg C L−1. Since DOM is present at levels of milligrams per liter in natural water, the slight decrease in ROX removal rates suggested that Fe3O4@RGO could be potentially applied to ROX removal in natural water.
image file: c7en00758b-f3.tif
Fig. 3 (a) Effect of DOM on the removal rate of ROX adsorbed on Fe3O4@RGO, with an initial ROX concentration of 1 mg L−1. (b) The removal rate of ROX and the residual arsenic concentration in a natural swine manure lixivium with Fe3O4@RGO dosages of 0.15 and 0.30 g L−1.

Since swine manure is a significant source of ROX in wastewater and the composition of organic matter in swine manure could represent certain real wastewater,35 the removal of ROX in swine manure lixivium was investigated to evaluate the potential application of Fe3O4@RGO in real wastewater. As shown in Fig. 3b, the 90 min removal rate of ROX by Fe3O4@RGO at dosages of 0.15 and 0.30 g L−1 reached 92% and 99%, respectively. Moreover, the dosage of 0.30 g L−1 Fe3O4@RGO decreased the arsenic concentration in ROX from 285 to 3.1 μg L−1, which satisfied the arsenic standard in surface water of China (50 μg L−1, GB3838-2002), suggesting the excellent removal of ROX by Fe3O4@RGO in waste water. Therefore, Fe3O4@RGO is potentially applied to ROX removal in both natural and waste waters.

3.3 Adsorption mechanism of Fe3O4@RGO towards ROX

3.3.1 Identification of involved adsorption mechanisms.
Electrostatic interaction. To reveal the role of electrostatic interaction during adsorption, the correlation between ROX adsorption capability and pH values for all the four adsorbents was conducted. As shown in Fig. S9, Fe3O4@RGO presented the highest adsorption capacity at pH 5.0, which then sharply decreased with increasing pH. p-Fe3O4 displayed a similar trend to Fe3O4@RGO, although the maximum adsorption occurred at pH 7. Zeta potential measurements showed that the point of zero charge (pHPZC) for Fe3O4@RGO, p-Fe3O4, RGO and GO-90 was at pH 5.8, 7.3, 2.9 and <2, respectively (insert of Fig. S9). Thus, the maximum adsorption of both Fe3O4@RGO and p-Fe3O4 occurred almost at the pHPZC, indicating that electrostatic attraction was not the primary mechanism for ROX adsorption on Fe3O4@RGO at pH 5. In addition, ROX was mainly negatively charged in the tested pH range (3–11) since the aqueous dissociation constants of ROX molecules were 3.49, 5.74 and 9.13, respectively (Fig. S1). Due to similar values between the pKa (5.74) of ROX and the pHPZC of Fe3O4@RGO, negative charge-assisted hydrogen bonds may exist between Fe3O4@RGO and ROX, which was reported between carboxylated and hydroxylated carbon nanotubes and weak acids (e.g., phthalic acid).36 Meanwhile, negatively charged GO-90 and RGO displayed decreasing adsorption capacity in the tested pH range, but still showed a certain level of ROX adsorption. Therefore, other interactions in addition to electrostatic interaction must exist between the graphene support and ROX molecules.
π–π interaction. The C 1s XPS spectra were analyzed to reveal the interaction between the graphene support and ROX in Fe3O4@RGO, setting RGO as a comparison (Fig. 4). The C–C/C[double bond, length as m-dash]C band proportion of Fe3O4@RGO was close to that of RGO, indicating a similar reduction level of GO in the two materials. After adsorbing ROX, the band proportion increased from 74.61% to 90.52% and 76.41% to 82.77% for Fe3O4@RGO and RGO, respectively. The increase in this band proportion was attributed to the π–π interaction between the graphene support and ROX molecules. Moreover, the increase range of the band proportion in Fe3O4@RGO was higher than that in RGO, suggesting a stronger π–π interaction between Fe3O4@RGO and ROX, which might result from the synergistic effect of electrostatic attraction. At pH 5, Fe3O4@RGO was still positively charged while RGO was negatively charged. Such a discrepancy resulted in the difference of the electrostatic interactions between ROX and the two adsorbents. In other words, the weak electrostatic attraction would narrow the molecule distance between Fe3O4@RGO and ROX, thus promoting the formation of π–π stacking between the graphene part and ROX.
image file: c7en00758b-f4.tif
Fig. 4 C 1s spectra of Fe3O4@RGO (a) and RGO (b) before and after ROX adsorption.

As–Fe complex. In view of the higher adsorption capacity of p-Fe3O4 (163.93 mg g−1) than that of GO-90 (43.10 mg g−1) and the dominant content of Fe3O4 (80.2%, w/w) in the nanocomposite, the adsorption of ROX on Fe3O4@RGO was considered mainly from the Fe3O4 NPs in the nanocomposite. The As–Fe interaction between Fe3O4@RGO and ROX was studied via XAFS analysis. The X-ray absorption near-edge structure (XANES) data together with the As 3d spectra from the XPS results confirmed that no ROX was degraded into inorganic As(V) or As(III) during adsorption by Fe3O4@RGO and p-Fe3O4 (Fig. S10), which was consistent with the HPLC-ICP-MS results. The unfiltered k3-weighted arsenic K-edge EXAFS data and their Fourier transform (FT) spectra can be found in Fig. 5. As shown in Fig. 5b, the R-space spectra consisted of a predominant signal derived from As–O first-neighbor contributions and a weaker signal assigned to As–Fe second-neighbor contributions. The As–O first-neighbor contributions were fitted with 3.2–3.4 oxygen atoms at 1.69 ± 0.02 Å for both samples (Table 2), which agreed with the calculation result of the tetrahedral structure around As in phenylarsonic acid compounds.37 The second-neighbor fitting results indicated that the As atom was surrounded by Fe atoms at 3.30 and 3.35 Å for Fe3O4@RGO–ROX and p-Fe3O4–ROX (Table 2), respectively, suggesting the formation of 2C complexes in both samples.13
image file: c7en00758b-f5.tif
Fig. 5 Arsenic K-edge XAFS data of Fe3O4@RGO–ROX and p-Fe3O4–ROX samples: unfiltered k3-weighted K-space EXAFS data (a). Magnitude part of the FT R-space (b). Real part of the FT R-space (c). Experimental and calculated curves are displayed as red open circles and black solid lines, respectively. Results of the k3-weighted fit are reported with their corresponding FT (Table 2).
Table 2 Results of shell-by-shell fitting of EXAFS data for samples with adsorbed ROX
Sample Atomic path CNa R (Å) σ 2 (Å) ΔEd (eV) R-Factore
a CN: number of neighbors. b R (Å): interatomic distances. c σ 2 (Å): Debye–Waller factor. d ΔE (eV): difference between the user-defined threshold energy and the experimentally determined threshold energy, in electron volts. e R-Factor: goodness of fitting. f As–Fe: As–Fe second-neighbor pairs. g MSAs–O–O: multiple scattering. The second-neighbor contributions of both samples were fitted using As–Fe pairs and an additional multiple-scattering (MS) contribution of As–O–O–As corresponding to Fe(AsO4)·2H2O. The amplitude reduction factor (SO2) was fixed at 0.95. Values in the parentheses are standard errors of the fitted parameters.
Fe3O4@RGO–ROX As–O 3.2 (0.1) 1.69 (0.02) 0.002 (0.001) 6.4 (1.5) 0.023
As–Fef 1.7 (0.3) 3.30 (0.05) 0.01
MSAs–O–Og 12 3.08 0.001
p-Fe3O4–ROX As–O 3.4 (0.1) 1.69 (0.02) 0.003 (0.001) 5.6 (1.2) 0.021
As–Fef 1.3 (0.2) 3.35 (0.05) 0.008
MSAs–O–Og 12 3.10 (0.05) 0.001


However, the coordination numbers (CN) of Fe atoms around the As atom in the second-neighbor contribution of Fe3O4@RGO–ROX and p-Fe3O4–ROX were different (Table 2). Fe3O4@RGO–ROX had a higher As–Fe coordination number of 1.7 than p-Fe3O4–ROX (1.3). It was reported that the lower number of neighbors could be attributed to the occurrence of outer-sphere complexes.15 Thus, the higher number of neighbors in Fe3O4@RGO–ROX suggested the lower impact of outer-sphere complexes. Meanwhile, the As–Fe distance of Fe3O4@RGO–ROX (3.30 Å) was a little shorter than that of p-Fe3O4–ROX (3.35 Å), indicating that compositing with graphene narrowed the As–Fe atomic distance during adsorption. Thus, EXAFS analysis confirmed that the compositing of Fe3O4 and 3D graphene was beneficial to the formation of an As–Fe inner-sphere complex, and enhanced the bond strength between Fe and As atoms in the complex.


Hydrogen bonds. Hydrogen bonds between the surface hydroxyl groups on the Fe3O4 surface (Fe–OH) and ROX molecules also played an important role during the adsorption, since the surface functional groups in Fe3O4@RGO were almost electroneutral and the hydroxyl groups in ROX were unionized at pH 5 (Fig. S1).38 The FTIR spectra of Fe3O4@RGO and p-Fe3O4 before and after adsorbing ROX are shown in Fig. S11. The characteristic peaks around 3500 and 1167 cm−1 were assigned to the stretching vibration and bending vibration of Fe–OH, respectively.39,40 Compared with p-Fe3O4, the obvious increase in intensity of the two bands in Fe3O4@RGO suggested that more Fe–OH groups existed in the nanocomposite, which was consistent with the XPS results discussed in the following section. It was reported that the formation of hydrogen bonds would cause a red-shift of the –OH vibration peaks.41 Thus, the red-shift of the Fe–OH characteristic peaks in Fe3O4@RGO from 3507 and 1167 cm−1 to 3520 and 1178 cm−1 after ROX adsorption indicated that Fe–OH groups participated in the adsorption as hydrogen bonding sites.
3.3.2 Synergetic adsorption mechanism of Fe3O4@RGO. The higher adsorption capacity and affinity of Fe3O4@RGO relative to the pristine materials could probably be attributed to the enhanced As–Fe coordination and hydrogen bonding between Fe3O4@RGO and ROX molecules. The enhancement of As–Fe coordination may be partly promoted by the exposed facets of Fe3O4 NPs in the nanocomposite. It was reported that the dominant 2C complexes were formed between the adsorbed As(V) and the {100} facet of magnetite.16 As mentioned above, the HRTEM results showed the (400) plane as the major exposed facet of Fe3O4 NPs in Fe3O4@RGO, which belonged to the {100} facet of magnetite. Therefore, the enhancement of As–Fe coordination might partly result from the (400) planes exposed on the Fe3O4@RGO surface. Besides the exposed planes, Fe–OH groups also played a very important role in As–Fe coordination, since previous work revealed that the formation of the As–Fe inner-sphere complex was realized through the replacement of hydroxyl groups in Fe–OH with arsenate.42,43 As shown in Fig. 6a, the O 1s XPS spectra of Fe3O4@RGO before and after adsorption could be deconvoluted into four peaks at 530.5, 531.6, 532.2, and 533.3 eV, which referred to Fe–O/As–O from Fe3O4 and ROX, –OH/Fe–O–C, H2O, and C–O–C, respectively.44,45 The proportion of –OH peaks in the O 1s spectra was 46.06% and 20.16% for Fe3O4@RGO and p-Fe3O4, respectively (Fig. 6). Considering the formation of the 2C complex between As and Fe, it was calculated that the Fe–OH species participating in the formation of the As–Fe complex constituted 26.30% of the total oxygen in Fe3O4@RGO, which was much higher than that in p-Fe3O4 (15.96%, Fig. S12 and Table S5). This result suggested that more As–Fe complexes could be formed in Fe3O4@RGO. Since the O 1s spectra revealed that the proportion of –OH/Fe–O–C groups in the nanocomposite was 46.06%, it could be concluded that the Fe–OH species replaced by arsenate accounted for more than one half of the –OH/Fe–O–C groups in the nanocomposite.
image file: c7en00758b-f6.tif
Fig. 6 O 1s XPS spectra of Fe3O4@RGO (a) and p-Fe3O4 (b) before and after ROX adsorption.

The variation of hydroxyl groups in the nanocomposite also indicated that the hydrogen bonds between Fe3O4@RGO and ROX were stronger than those of the pristine p-Fe3O4. Deducting the Fe–OH groups forming the As–Fe complex, the proportion of residual hydroxyl groups in p-Fe3O4 was only 4.20%, suggesting that the hydrogen bonding between p-Fe3O4 and ROX was extremely weak. Meanwhile, the hydroxyl amount in Fe3O4@RGO that could form hydrogen bonds with ROX was calculated to be 19.76%. Hence, the higher content of hydroxyl groups in Fe3O4@RGO could enhance both the As–Fe coordination and hydrogen bonding between the adsorbent and ROX molecules.

Since the hydroxyl proportion of hydrogen bonds (19.76%) in the total oxygen amount of Fe3O4@RGO was lower than that of the As–Fe complex (26.30%, Table S5), it could be inferred that As–Fe coordination had a higher contribution to ROX adsorption than hydrogen bonding. Moreover, it was reported that the binding energy of the As–Fe complex was around 20 kcal mol−1, which was higher than those of hydrogen bonding (∼8 kcal mol−1) and π–π interaction (∼10 kcal mol−1).37,46,47 Therefore, considering the different binding energies of the three interactions, as well as the high content of Fe3O4 in Fe3O4@RGO and high adsorption capacity of Fe3O4 for ROX, it could be concluded that the contribution of the three forces to ROX adsorption was in the order: As–Fe coordination > hydrogen bonding > π–π interaction.

A synergistic mechanism for adsorption of ROX on the Fe3O4@RGO nanocomposite is illustrated in Fig. 7. As–Fe coordination, hydrogen bonding and π–π stacking were considered to be the dominant forces for ROX adsorption. The co-existence of the graphene support and Fe3O4 NPs helped Fe3O4 expose (400) planes and shortened the As–Fe atomic distance, which likely promoted the formation of the As–Fe complex, while the graphene support also contributed to the adsorption through π–π stacking. Moreover, the abundant hydroxyl groups present on the surface of Fe3O4@RGO enhanced both the As–Fe coordination and hydrogen bonding. The weak electrostatic attraction force brought more ROX molecules close to the substrate and enhanced the π–π stacking. Hence, the synergy between Fe3O4 NPs and the graphene support in Fe3O4@RGO greatly strengthened the adsorption capability and affinity of Fe3O4@RGO for ROX.


image file: c7en00758b-f7.tif
Fig. 7 Schematic diagram for the proposed synergetic mechanism of ROX adsorption on Fe3O4@RGO. The maximum adsorption occurred at pH 5, at which the surface of Fe3O4@RGO was almost electroneutral. The dominant adsorption forces were considered to be As–Fe coordination, hydrogen bonding and π–π stacking.

4 Conclusions

In this work, a Fe3O4@RGO nanocomposite was designed for effective removal of ROX which is an emerging arsenic pollutant. To our knowledge, this is the first molecular-level study to investigate the synergistic adsorption between the designed adsorbent and the two functional groups of phenylarsonic acid compounds. The excellent performance of the Fe3O4@RGO nanocomposite achieved higher adsorption capacity and affinity for ROX, compared with pristine p-Fe3O4, RGO and GO-90, and no ROX was degraded into inorganic arsenic after adsorption and desorption. Moreover, the ROX adsorption rates of Fe3O4@RGO in DOM-containing solution and swine manure lixivium were both over 90%, suggesting Fe3O4@RGO as a potential adsorbent to remove ROX in both natural and waste waters. The enhanced removal of ROX was ascribed to the synergistic As–Fe coordination, hydrogen bonding and π–π stacking between the adsorbent and adsorbate. In addition, graphene in the nanocomposite not only promoted the dispersibility of Fe3O4 NPs, but also increased the surface hydroxyl amounts, thus enhancing the As–Fe coordination and hydrogen bonding for superior adsorption. On this basis, this work is expected to stimulate further investigations into the functional design of adsorbents for the removal of emerging pollutants with both inorganic and organic moieties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the China Postdoctoral Science Foundation (no. 2016M600654), the National Natural Science Foundation of China (grant no. 21477129), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), and the Fundamental Research Funds for the Central Universities (no. 2017PY009 and 2017BQ054). The authors thank the beamline 4W1B (Beijing Synchrotron Radiation Facility) for providing the beam time.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7en00758b

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