Self-template Etching Synthesis of Urchin-like Fe3O4 Microspheres for

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Self-Template Etching Synthesis of Urchin-Like Fe3O4 Microspheres for Enhanced Heavy Metal Ions Removal Yong Yu,† Yang Li,‡ Yongqiang Wang,*,† and Bingfang Zou*,†,§ †

Key Laboratory for Special Functional Materials of the Ministry of Education, ‡College of Life Sciences, and §School of Physics and Electronics, Henan University, Kaifeng 475004, People’s Republic of China

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ABSTRACT: Hierachical Fe3O4 microspheres with superparamagnetic properties are attractive for their superior structural, water-dispersible, and magnetic separation merits. Here self-template etching route was developed to create optimal porous structure in superparamagnetic Fe3O4 microspheres by using the oxalic acid (H2C2O4) as etching agent. A plausible formation mechanism of the urchin-like Fe3O4 microspheres was proposed based on systematic investigation of the etching process, which involved two stages including pore-forming step based on size-selective etching and poreexpanding step based on further etching. The as-synthesized Fe3O4 microspheres exhibited urchin-like structure with specific surface area and pore-size tunable, water-dispersible, and superparamagnetic properties. The optimal urchin-like Fe3O4 microspheres demonstrated superior performance including fast magnetic separation and high removal capabilities for the heavy metals ions like Pb2+ (112.8 mg g−1) and Cr(VI) (68.7 mg g−1). This work will shed new light on the synthesis of urchin-like microspheres for superior performance. chrysanthemum or urchin-like α-FeOOH and α-Fe2O3 microspheres were synthesized by controlling the growth process in different solvents and showed high adsorption capability toward heavy metal ions or organic pollutants in water treatment.9−12 However, these nonmagnetic iron oxide adsorbents made solid/liquid separation and recovery difficult, and magnetic versions of iron oxide sorbent nanomaterials offer a viable solution to simplify manipulation process. Thus, hierarchical or porous magnetic Fe3O4 microspheres are more attractive as an effective absorbent.16 For example, porous Fe3O4 microspheres was reported by direct calcining them or precursors in protecting atmosphere, but ferromagnetism and poor water-soluble samples were realized after calcinations which often caused aggregation during the adsorption process in solution.17,18 Porous and superparamagnetic Fe3O4 particles were reported by using surfactant-modulated solvothermal method, however the specific surface area was still very low which restrained their adsorption capability.7,19 Unlike αFeOOH or α-Fe2O3 with hexagonal lattice structure, the intrinsic Fe3O4 cubic lattice crystalline structure made it difficult to grow into urchin-like structure, especially with the required superparamagnetic size.20,21 Here we report a facile and robust route to construct Fe3O4 microspheres with superparamagnetic and urchin-like structure

1. INTRODUCTION Heavy metals in both natural water supplies and industrial wastewater streams are a critical health and environmental issue due to their high toxicity and bioaccumulation in the human body through the food chain.1 Today, there is an urgent and worldwide demand in developing advanced technologies to mitigate the toxicity and to provide a safe living environment for humans. Compared with chemical precipitation, reverse osmosis, ion exchange, and ultrafiltration, the adsorption techniques can be considered as one of the best available techniques to remove metal ions from wastewater, owing to the convenient operation process, wide applicability, high efficiency, cost effectiveness, and low energy requirement.2 With the rapid advancement of nanotechnology, nanomaterials as absorbent have been widely investigated and exhibited great advantages in environmental treatment nowadays.3,4 Among the nanomaterials, those with porous nanostructures have aroused researcher’s great interests due to high specific surface areas and large pore volumes, and these features are beneficial for mass diffusion and transport, which enables them to have enhanced adsorption performances in wastewater remediation.5,6 Recently, there is a growing interest in using iron oxide nanomaterials for heavy metal cleanup due to their resourceful, cheap, and nontoxic advantages, and various hierarchical or porous iron oxide nanostructures are developed and studied as adsorbent of heavy metal ions in environmental remediation.7−15 For example, hierarchical nanostructures like © XXXX American Chemical Society

Received: April 12, 2018 Revised: June 21, 2018 Published: July 19, 2018 A

DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX

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dissolving Pb(NO3)2 and K2Cr2O7 in deionized water, and Pb2+ and Cr(VI) working solutions were freshly prepared by diluting the above stock solutions. The pH values of the solutions were adjusted using NaOH or HCl. In a typical removal procedure, samples (20 mg) were added to 20 mL of Pb2+ and Cr(VI) solutions with different concentrations, and the containers were sealed and shaken at 300 rpm for 10 h at room temperature. After shaking, the mixtures were placed under an external magnetic field and the samples were separated from the solutions. The Pb2+ and Cr(VI) concentrations in the supernatant solution was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo-Fisher Scientific, U.S.A.). Every experiment was performed in triplicate. The adsorption capacity was roughly estimated using the following equation.

simultaneously, where the solid superparamagnetic Fe3O4 microspheres as template were etched selectively into porous structure. Our idea was based on the solid Fe3O4 microspheres synthesized in self-limiting growth system were composed of broad polydisperse nanoparticles,22 thus, the small nanoparticles in the solid Fe3O4 microspheres could be selectively removed, which artfully circumvented the barrier in direct assembly of Fe3O4 nanoparticles into porous structure with high specific area. The environmentally benign oxalic acid (H2C2O4) was selected as etching agent, and the urchin-like and superparamagnetic Fe3O4 microspheres were successfully synthesized through fast chemical reactions at room temperature, which exhibited larger accessible specific surface area than their counterpart without treatment. Here the continual etching process provided an excellent opportunity to optimize the urchin-like Fe3O4 microspheres with controlled parameters, and the optimal product exhibited a superior adsorption capability for heavy metal ions like Pb2+ and Cr(VI) when used as absorbent in water treatment.

Q e = (C0 − C) × V /W where Qe (mg g−1) is the adsorption capacity, C0 (mg L−1) is the initial concentration of the Pb2+ or Cr(VI) solution, C (mg L−1) is the equilibrium concentration of Pb2+ or Cr(VI) ions, V (L) is the initial volume of the Pb2+ or Cr(VI) solution, and W (g) is the weight of the adsorbent.

3. RESULTS AND DISCUSSION Highly monodisperse and solid superparamagnetic Fe3O4 microspheres stabilized by polyacrylate group were synthesized through a modified solvothermal method, and used as precursor in the following etching process in Figure 1a−d.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Urchin-Like Fe3O4 Microspheres. The uniform Fe3O4 microspheres were synthesized through a modified solvothermal method of our group.23 In a typical synthesis procedure, FeCl3·6H2O (2.7 g) and CH3COONa (7.2 g) were mixed in ethylene glycol (80 mL) under magnetic stirring until they were completely dissolved, and then poly(acrylic acid) (MW 1800, 0.25 g) was added. The above solution was mixed under magnetic stirring for 2 h and then transferred into self-made Teflon-lined stainless-steel autoclaves (120 mL) with mechanical stirrer and nitrogen inlet/outlet. The reaction temperature was set at 120 °C under nitrogen airflow to eliminate oxygen and water for 2 h, and then the reaction system was sealed and heated at 200 °C for 10 h under mechanical stirring. Finally, the solid Fe3O4 microspheres were collected after repeated magnetic separation and washing with distilled water. The above solid superparamagnetic Fe3O4 microspheres (30 mg) were first dispersed well into deionized water (40 mL) under sonication for 30 min in a flask, and then oxalic acid (H2C2O4·2H2O, 126 mg) were added into the above solution under mechanical stirring. The reaction proceeded at room temperature. After 1.5 h, the flask was taken out to separate product under external magnet. After black precipitates were washed with distilled water for several times, they were stored in deionized water for further use. 2.2. Characterization. The size and morphology of the assynthesized materials were measured using a Hitachi S-5500 fieldemission scanning electron microscope (FE-SEM, Tokyo, Japan), JEOL JEM-2010 high-resolution transmission electron microscope (HRTEM, Kyoto, Japan). The composition of products were analyzed by X-ray diffraction (XRD, Philips X’pert diffractometer) and Infrared Fourier Transform Spectrometer (FT-IR, AVATAR360, Nicolet, U.S.). Magnetic measurements were performed with a superconducting quantum interference device (SQUID, Quantum Design MPMS) magnetometer at room temperature (300 K). Thermogravimetry (TG) was carried out on a TAQ600 thermoanalyzer in an air atmosphere. Nitrogen sorption isotherms were obtained at 273 K with a Quadrasorb TM SI Four Station Surface Area Analyzer and Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL). The chemical composition and chemical states of the final products were further investigated by X-ray photoelectron spectroscopy (XPS, Axis Ultra). Ultrathin TEM sample was prepared by embedding urchin-like Fe3O4 microspheres into epoxy and then cut into slices with a thickness of 30 nm by an ultramicrotome (Leica EM UC7). 2.3. Measurement of Adsorption of Heavy Metal Ions. To evaluate the adsorption abilities of our samples, heavy metal adsorption tests were carried out as follows. Pb(NO3)2 and K2Cr2O7 were used as the source of Pb2+ and Cr(VI) ions, respectively. Stock solutions of Pb2+ and Cr(VI) were prepared by

Figure 1. Low- and high-magnified SEM images, TEM and highmagnified TEM images of (a−d) the solid Fe3O4 microspheres and (e−h) the urchin-like Fe3O4 microspheres.

When the solid superparamagnetic Fe3O4 microspheres were added into the water solution containing oxalic acid, the reaction process started quickly at room temperature. After reacting for 90 min, the product kept spherical morphology, but their structure changed greatly in Figure 1e. The surface of the as-obtained microspheres became obviously rough from the enlarged SEM image in Figure 1f when compared with that in Figure 1b, and the inner structure was not solid anymore but full of pores with funnelling shape in Figure 1g, which is similar to the previously reported hierarchical silica nanospheres.24,25 By comparing single microsphere before and after reaction in Figure 1d,h, it can be seen that a large amount of pores were generated even in the interior of Fe3O4 microsphere. After reaction, the as-obtained product still kept similar crystalline structure with the original solid Fe3O4 microspheres in XRD patterns of Figure S1. Therefore, urchin-like Fe3O4 microspheres were successfully obtained through the designed selective etching route. In the reaction system, oxalic acid (H2C2O4) was selected as etching reagent for its wide usage in removal of iron oxide contaminant in minerals.26 Here chemical reactions involved during the etching process were speculated as follows: B

DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX

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Langmuir H+ + Fe3O4 + C2O24 − → Fe(C2O4 )22 − + H 2O + CO2

area from the TEM image of ultramicrotomed slice. This phenomena indicated that the etching process proceeded throughout the whole microsphere, thus the H+ and C2O42− ions were proposed to permeate into the interior of Fe3O4 microspheres. At the reaction time of 60 min, the size of the asobtained microspheres (S2) are still similar as before, but their surface became rough clearly in Figure 2e, and the microspheres seemed to be a urchin in Figure 2f with center-radial oriented pores in Figure 2g. The ultramicrotomed slice demonstrated the initial formation of hierarchical structure in this reaction stage. With reaction proceeding to 90 min, the size of the as-obtained microspheres (S3) reduced slightly to 470 nm in Figure 2i, but the rough surface still existed in Figure 2j. When compared with the product Figure 2g, the pores became larger even in the center area of the microspheres in Figure 2k. From the intersection image in Figure 2l, the as-obtained microsphere exhibited open threedimensional (3D) porous structure, and the original solid Fe3O4 microsphere were completely transformed into urchinlike Fe3O4 microspheres. When the reaction was prolonged to 3 h, the particle size decreased down to 400 nm in Figure 2m,n, which could be ascribed to the dissolution of outer layer of the microspheres (S4) during the etching process. The leftover skeleton became more porous and loose, showing a tendency to collapse in Figure 2o,p. When the reaction last 6 h, most of the product collapsed, and no spherical particles could be distinguished after 9 h in Figure S3. From the above serial characterizations, the whole Fe3O4 microsphere was etched almost homogeneously in acidic solution, rather than the familiar inward etching like SiO2, Fe2O3 or Cu2O microspheres reported previously, where a smaller cores were left after etching.27−30 Therefore, a different etching mechanism was involved in the present experiment. As mentioned above, it can be inferred that the pore formation mechanism is independent of the etching reagent, and then the inner structure of Fe3O4 microsphere was speculated to be the decisive factor. When the solid Fe3O4 microspheres were ultramicrotomed into slice in Figure S4, the black dots was found to be ascribed as superparamagnetic Fe3O4 nanoparticles. However, many white dots were also observed among these black dots, indicating intrinsic sparse structure of the microspheres. It is these tiny pores interconnected into “channels” that allowed some small ions including the H+ and C2O42− to enter into the interior of Fe3O4 microspheres. It is reported that microspheres were composed of broad polydispersity of nanoparticles in self-limiting reaction system, those big ones have relatively strong etch-resistant capability, while the small nanoparticles along the channels would be preferentially etched.22,31 This process could be monitored by XRD through comparing the half widths of the main peak (311) of Fe3O4, which was determined by mean size of nanoparticles in Fe3O4 microspheres according to Scherer equation. In Figure 3a, it was found that half width (S1) became narrower than that of sample S0, indicating the ratio of small nanoparticles in Fe3O4 microspheres really decreased after etching. This stage was named as pore-forming step based on size-selective etching. It can be imagined that these left big nanoparticles near the “channels”would be etched into smaller ones after the outer small nanoparticles were dissolved, which then decreased the mean size of the nanoparticles and widened the half width (S2) again. In the following procedure, the newgenerated small nanoparticles would be dissolved with the further etching, and then the half width of the product at this

(1) +

H + Fe3O4 → Fe

3+

+ Fe

2+

+ H 2O

(2)

From eq 1, the H2C2O4 could accelerate the dissolution process of Fe3O4 where oxalate formed complex with Fe3+ ions. Experimental results exhibited that highly porous Fe3O4 microspheres could be realized by using oxalic acid solution with low concentration of 25 mM at room temperature (25 °C) for 1.5 h. When inorganic acids like HCl and HNO3 were used to etch the solid Fe3O4 microspheres, the H+ ionized from HCl and HNO3 etch the Fe3O4 microspheres through dissolution process according to eq 2, but ultrahigh concentration of H+ ions (1 M) and high temperature (80 °C) are both required to generate distinguishable pores in solid Fe3O4 microspheres in Figure S2. The control experiments indicate that oxalic acid are better choice as etching reagent, which simplified the manipulation process with less energy. Besides the oxalic acid is more environmentally benign than inorganic acids. Thus, the oxalic acid was considered to be effective etching agent in the present reaction system. To investigate the etching mechanism, the pore formation process was studied through time-depended experiments. Meanwhile, to thorough understanding the interior etching situation, the samples were embedded into epoxy and cut into ultrathin films (30 nm) with an ultramicrotome. As shown in Figure 2, all the products sampled at different reaction intervals

Figure 2. SEM images, TEM images, high-magnified TEM images, and ultramicrotomed slice of urchin-like Fe3O4 microspheres at different reaction intervals: (a−d) 30 min (S1), (e−h) 60 min (S2), (i−l) 90 min (S3), and (m−p) 180 min (S4), respectively. All the scale bar are 200 nm.

were characterized by SEM and TEM microscopy and labeled for convenience as S0 (0 min), S1 (30 min), S2 (60 min), S3 (90 min), and S4 (180 min), respectively. From the Figure 2a, the sample S1 kept spherical morphology and size with original solid Fe3O4 microspheres in Figure 1a, but it can be seen that lots of tiny pores appeared but mainly in the peripheral region of Fe3O4 microspheres in Figure 2b,c. However, the pores are found to bestrewed the whole microspheres even in the center C

DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX

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stoichiometry of Fe3O4 within the uncertainty of calculation, indicating the unchanged magnetite structure. The magnetic hysteresis of the as-obtained urchin-like Fe3O4 microspheres showed the similar magnetic saturation (72 emu g−1) with that of the solid one (77 emu g−1) in Figure 3d and still kept superparamagnetic property as before. Even if they were etched into porous structure, the as-synthesized urchin-like Fe3O4 microspheres dispersed very well in water and could be separated from solution in five seconds, as seen from the inset of Figure 3d. The well-dispersibility could be ascribed to the residual carboxyl groups after etching in Figure 4a.34 And the

Figure 3. (a) Half widths of the main peak (311), (b) pore size distribution of the as-obtained Fe3O4 microspheres (S0−S4), (c) XPS spectrum of the fitted Fe 2p peak from the as-obtained microspheres, and (d) the magnetization curves of solid and urchin-like Fe3O4 microspheres.

stage became narrow correspondingly (S3). With the newgenerated small nanoparticles disappearing completely, the relative number of big nanoparticles (S4) increased and caused a narrow half width. With the etching process proceeding, the size and volume of the evacuated spaces are increasing. This stage was then named as the pore-expanding step based on further etching, which was still size-selective etching fundamentally. In the following procedure, continual dissolution of nanoparticles caused the residual skeleton loosely and finally collapsed, as seen in Figure S3. The advantage of such continual etching process was that the parameters of the resultant microspheres could be tailored in a controlled manner from the above experiment results. The parameters including specific surface areas and pore sizes distribution were characterized by N2 adsorption and desorption isotherm measurements, which could quantitatively reveal the structural changes of the porous Fe3O4 microspheres during the reaction. The BET surface area of the as-obtained Fe3O4 microspheres at different reaction intervals are 45.4 (S0), 75.9 (S1), 84.2 (S2), 94 (S3), and 78.5 m2 g−1 (S4), respectively. Therefore, the specific surface area of the resultant Fe3O4 microspheres after etching treatment could reach as high as 94 m2 g−1, more than 2× their solid counterpart before etching, which is comparable with urchin-like α-FeOOH hollow spheres reported previously.9 And the average pore diameters derived from desorption data exhibited an increasing trend from 3.7 nm (S0), 10.5 nm (S1), 12.9 nm (S2), and 20 nm (S3) to 23 nm (S4) with the etching time in Figure 3b. The regular changes of specific surface area and pore size of these as-obtained products indicated unique advantages of the designed etching strategy. In the XPS spectrum of the asobtained microspheres (S3) in Figure 3c, the relative area ratio of the deconvoluted Fe 2p3/2 peak assigned to Fe2+ and Fe3+ was calculated to be 0.31:0.69, which is consistent with the

Figure 4. (a) IR spectra of the Fe3O4 microspheres before (I) and after (II) etching, (b) the TG curve of the Fe3O4 microspheres before (I) and after (II) etching, (c) the effect of pH on the Pb2+ adsorption performance of the urchin-like Fe3O4 microspheres (S3), and (d) the zeta potential of the urchin-like Fe3O4 microspheres.

content of PAA resided in Fe3O4 microspheres after etching was close to 5%, which is lower than that of the original solid Fe3O4 microspheres according to the TG curve in Figure 4b. Additionally, it was found that the as-synthesized urchin-like Fe3O4 microspheres demonstrated excellent stability in harsh conditions. For example, the as-synthesized urchin-like Fe3O4 microspheres sill kept the same hierarchical structure after strong sonication for several hours in Figure S5. And they could still preserve the urchin-like structure after immersing in HCl solution (pH 1) for 48 h, and they just seemed to be further etching slightly, as seen in Figure S6. From the above characterizations, the resultant urchin-like and superparamagnetic Fe3O4 microspheres are suitable as absorbent in wastewater treatment for their physical and chemical stable, water-dispersible and magnetic separable merits. Here, highly toxic Pb2+ ions were selected as cationic model heavy metal ions. Since the aqueous pH determined the adsorbent surface charge and the degree of ionization, and speciation of the adsorbed species, the optimal pH for Pb2+ adsorption on urchin-like Fe3O4 microspheres was first explored through changing pH values of the model wastewater from 2 to 7, where 20 mg urchin-like Fe3O4 microspheres were dispersed in 20 mL of Pb2+ ions with concentration of 100 mg/ L. As illustrated in Figure 4c, the Pb2+ adsorption on urchinlike Fe3O4 microspheres is obviously pH dependent as expected due to the different kinds of lead speciation in the solutions with different pH values.32 It is well-known that the electrostatic attractions between Fe−O− and Pb2+ become D

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Through comparing of the adsorption capacities and specific surface areas of S1, S2, and S3, the pore size should be above 12.9 nm for the free diffusion of Pb2+ into the interior of Fe3O4 microspheres, much larger than the size of lead species in our experiment. The reason was proposed that the carboxylic acid groups of the PAA anionic hydrogel are ionized above pH 4, and then the hydrogel themselves will swell due to the electrostatic repulsion and large osmotic swelling force by the presence of ions.35 As a result, the PAA hydrogel swelled in the pores will block the diffusion of lead species into the interior of microspheres. Thus, in addition to high specific surface area, the pores with large enough size are requisite for high adsorption capacity, especially for those absorbents modified with functional organic groups. The detailed relationship between the removal ability of optimal urchin-like Fe3O4 microspheres (S3) and the concentration of the Pb2+ solution was investigated and illustrated by an adsorption isotherm in Figure 5. Through model simulation, the adsorption data are better fitted by the Langmuir model, and the maximum adsorption capacity of urchin-like Fe3O4 microspheres (S3) was calculated to be 112.8 mg g−1, much higher than that of the original Fe3O4 microspheres (S0, 31.8 mg g−1), which directly exhibited the advantages of hierarchical structure. When compared with previously reported iron oxide nanoabsorbents, as listed in Table 1, the adsorption capacity of the as-obtained product is

more numerous as increasing pH improved the deprotonation of Fe−OH surface groups.33 It should be noted that the poly(acrylic acid) (PAA) that distributed in the interior of solid Fe3O4 microspheres also became available during the pore formation process, which could enhanced the adsorption capacity through binding COO− with Pb2+ electrostatically.34 The zeta potential of the as-obtained urchin-like PAA-modified Fe3O4 microspheres as a function of pH was shown in Figure 4d, and the surface potential became more negative with increasing pH value of the solution, which implied that more Fe−O− and −COO− groups are produced. Thus, a quick increase in Pb2+ removal was observed as solution pH rose from 2.0 to 5.0. As the solution pH was above 5.0, the Pb2+ removal still increased in Figure 4a; however, the lead removal may also occur due to the onset of metal hydrolysis precipitation of lead hydroxide, and this will introduce uncertainty with respect to adsorption versus precipitation.32 Therefore, here the solution with pH 5 was selected for Pb2+ adsorption on urchin-like Fe3O4 microspheres. The structural effect on adsorption performance was systematically studied by using the as-obtained urchin-like Fe3O4 microspheres with different specific surface areas and pore sizes (S0−S4). From the results in Figure 5a, the

Table 1. BET Surface Areas and the Maximal Pb2+ Adsorption Capacities (Qm) of the as-Obtained Urchin-Like Fe3O4 Microspheres (S3) and Other Adsorbents adsorbents

S (m2 g−1)

Qm (mg g−1)

Fe3O4 nanospheres Fe3O4 nanoparticles Fe3O4 nanoparticles urchin-like α-FeOOH hollow spheres chrysanthemum-like α-FeOOH microspheres urchin-like Fe3O4 microspheres

11.3 12.7 43 96.9 120.8

19 53.1 36 80 103

38 32 33 9 12

94

112.8

this work

ref

obviously much higher than that of pure Fe3O4 nanoparticles (36 mg g−1), because here the Fe3O4 nanoparticles remained after selective etching formed urchin-like structures rather than pure Fe3O4 nanoparticles, which severely aggregated randomly due to the high active surfaces.33 Meanwhile, the specific surface area of the as-obtained product was greatly increased due to the formation of open 3D hierarchical superstructures, which is comparable with that of urchin-like α-FeOOH hollow spheres.9 More importantly, the adsorption capacity of the asobtained product was higher than that of hierarchical αFeOOH nanomaterials reported previously,9,12 which could be ascribed that more carboxyl groups attached on the Fe3O4 microspheres are available to bind more Pb2+ ions with increasing specific surface area.36,37 Therefore, the surfacemodified PAA molecules not only improve the water dispersibility, but also contribute to the adsorption performance. And the superparamagnetic characteristic of the asobtained product simplified during the adsorption process greatly compared with those hierarchical α-FeOOH or αFe2O3 nanomaterials. The recycling ability of the urchin-like Fe3O4 microspheres (S3) were also investigated which was often considered as an important factor to evaluate the cost effectiveness. As shown in Figure 5c, the removal efficiency still

Figure 5. (a) Adsorption capacities of Pb2+ of the as-obtained urchinlike Fe3O4 microspheres (S0−S4), (b) the comparison of Pb2+ adsorption isotherms between the urchin-like (S3) and solid Fe3O4 (S0) microspheres, (c) the cycling performance of the as-obtained urchin-like Fe3O4 microspheres (S3), and (d) the adsorption capacities of several heavy metal ions.

adsorption capacities of Pb2+ ions of urchin-like Fe3O4 microspheres (S0−S4) were 31.8, 77.1, 91.4, 107.4, and 93.7 mg g−1, respectively, and the adsorption capacities of samples increased with their increasing specific surfaces in general. According to the electrostatic adsorption mechanism between the iron oxide/PAA surface groups and the Pb2+ ions, the adsorption capacities should be proportional with the surface area.9 However, when the relationship between specific surface areas and adsorption capacities were further studied, the adsorption capacities was not strictly in proportion to specific surface areas. For example, the as-obtained urchin-like Fe3O4 microspheres (S3) demonstrated Pb2+ ions adsorption capacity 3.55× but with its specific surface area only 2.1× of the solid counterpart. The different proportions indicated that partial surface area of the Fe3O4 microspheres are not accessible. E

DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX

Langmuir retain above 80% after five adsorption−desorption cycles, indicating its good recyclability. Besides, other cations including Ni2+, Co2+, Ca2+, and Cd2+ were also investigated by using the as-synthesized urchin-like Fe3O4 microspheres (S3), where the adsorption concentrations of these cations were 500 mg/L, and these cations could also be absorbed like Pb2+, as shown in Figure 5d. Additionally, the Cr(VI) adsorption performance was also studied by using the as-obtained urchin-like Fe3O4 microspheres considering that Fe3O4 nanomaterials were good nanoabsorbents toward Cr(VI). According to the experimental results, the adsorption capacity of the optimal urchin-like Fe3O4 microspheres (S3) reached 68.7 mg g−1 in Figure S7, which is much higher than the previously reported iron oxide nanomaterials summarized in the Table S1.9−12,17,39 It was reported that the adsorbed partial Cr(VI) ions could be reduced to Cr3+ by Fe2+, thus here the porous structure will provide more active sites which could incorporated Cr3+ into the porous structure, and then enhance the Cr(VI) removal capability.18 From the above serial tests, the adsorption capability of Fe3O4 microspheres was greatly enhanced after controlled etching treatment, which should be ascribed to the unique open 3D hierarchical superstructures with large pore channels and highly accessible surface areas.

ACKNOWLEDGMENTS

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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/acs.langmuir.8b01219. Supporting figures and table (PDF).

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This work was supported by the Natural Science Foundation of China (Nos. 51102077 and 51372070), the Program for Science and Technology Innovation Talents in Universities of Henan Province (No. 16HASTIT009), the Natural Science Foundation of Henan University (No. 0000A40409), Changjiang Scholars and Innovative Research Team in University (PCS IRT15R18), Henan Province Natural Science Foundation Research Project Funding (162300410040), and Key Project of Education Department of Henan Province (No. 14B430010).

4. CONCLUSION A selective etching strategy was successfully developed to create adjustable hierarchical structure in superparamagnetic Fe3O4 microspheres by using the environmentally benign oxalic acid. The excellent urchin-like structure, waterdispersible and superparamagnetic properties made the assynthesized urchin-like Fe3O4 microspheres very suitable as nanoabsorbents in water treatment, and the optimal urchin-like Fe3O4 microspheres demonstrated superior capability with high adsorption capacities to Pb2+ (112.8 mg g−1) and Cr(VI) (68.7 mg g−1), and the superparamagnetic property also made the dispersion and separation process easily. Additionally, the adjustable specific surface areas and pore sizes of the asobtained urchin-like Fe3O4 microspheres provided an opportunity to studied the effect of the urchin-like structures on adsorption performance. This work provided a novel synthesis strategy for designing hierarchical structure materials in future.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Fax: +86 3712 3881358. Tel.: +86 3712 3881358. ORCID

Yongqiang Wang: 0000-0002-1877-0308 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

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DOI: 10.1021/acs.langmuir.8b01219 Langmuir XXXX, XXX, XXX−XXX