Facile synthesis of flower-like CoFe2O4 particles for efficient sorption of aromatic organoarsenicals from aqueous solution

Journal Pre-proofs Facile synthesis of flower-like CoFe2O4 particles for efficient sorption of aromatic organoarsenicals from aqueous solution Jue Liu, Bing Li, Guowei Wang, Lifan Qin, Xue Ma, Yuanan Hu, Hefa Cheng PII: DOI: Reference:

S0021-9797(20)30149-1 https://doi.org/10.1016/j.jcis.2020.02.004 YJCIS 25999

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

3 December 2019 2 February 2020 3 February 2020

Please cite this article as: J. Liu, B. Li, G. Wang, L. Qin, X. Ma, Y. Hu, H. Cheng, Facile synthesis of flower-like CoFe2O4 particles for efficient sorption of aromatic organoarsenicals from aqueous solution, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.02.004

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Facile synthesis of flower-like CoFe2O4 particles for efficient sorption of aromatic organoarsenicals from aqueous solution Jue Liua,1, Bing Lib,2, Guowei Wanga,3, Lifan Qina,4, Xue Maa,5, Yuanan Hub,6, Hefa Chenga,*,7 aMOE Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China bMOE Laboratory of Groundwater Circulation and Evolution, School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China 1Email: [email protected] 2Email: [email protected] 3Email: [email protected] 4Email: [email protected] 5Email: [email protected] 6Email: [email protected] 7*Corresponding author Email: [email protected]; Telphone: +86-10-6276-1070; Fax: +86-10-6276-7921

Abbreviates: Roxarsone (ROX), p-arsanilic acid (p-ASA), 4-hydroxyphenylarsonic acid (4HPAA), 2-aminophenylarsonic acid (2-APAA), Phenylarsonic acid (PAA), 2nitrophenylarsonic acid (2-NPAA), Scanning electron microscopy (SEM),Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX), Powder X-ray diffraction (XRD), Vibrating sample magnetometer (VSM),Accelerated surface area and porosimetry system (ASAP), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS)

Abstract Hypothesis Aromatic organoarsenicals are heavily used as poultry feed additives, and the application of manure containing these compounds could release toxic inorganic arsenic into the environment. Bimetal ferrites are recognized as promising sorbents in organoarsenicals removal for Fe-O-As formation and easy magnetic separation.

Experiments Herein, a flower-like CoFe2O4 sorbent was synthesized through an environmentalfriendly process.

Findings The flower-like CoFe2O4 particles have abundant mesopores and a large surface area of 48.4 m2/g. At an equilibrium concentration of 80 mol/L, the sorption capacities towards p-arsanilic acid (p-ASA), roxarsone (ROX), 4-hydroxyphenylarsonic acid (4HPAA), 2-aminophenylarsonic acid (2-APAA), phenylarsonic acid (PAA), and 2nitrophenylarsonic acid (2-NPAA) were 38.1, 45.7, 38.7, 39.3, 33.0, and 32.8 mg/g, respectively. Langmuir model and pseudo-second-order kinetics could well fit the sorption isotherms and rates. The sorption performance was better under acidic conditions due to electrostatic attractions. Humic acid (HA) and PO43- inhibited the sorption through competing for sorption sites, while Fe3+ promoted sorption due to additional Fe-O-As formation. CO32- suppressed the sorption. The experimental observations and DFT calculations suggest that the sorption happens mainly through Fe-O-As formation. Meanwhile, flower-like CoFe2O4 could be regenerated. The high sorption capacities, together with its magnetic property, make it an attractive sorbent for removing aromatic organoarsenicals from wastewater. Keywords: Flower-like CoFe2O4 sorbent; sorption; aromatic organoarsenicals; Fe-OAs formation; DFT calculations; magnetic separation; regeneration.

Introduction Aromatic organoarsenicals are heavily used as poultry feed additives because they can control intestinal parasites and enhance feed efficiency [1]. With little absorption and metabolic conversion in animal bodies, most of the administrated compounds remain unchanged when excreted in the manure [2], and subsequently enter the environment. Although aromatic organoarsenicals are of low-toxicity, they can release toxic inorganic arsenic (As(III) and As(V)) through abiotic and biotransformation [3, 4], thus causing contamination on soil and water. Therefore, there is a significant need to remove these organoarsenicals before spreading the animal manure on farmlands to prevent their releases into the environment. 2

Many technologies have been developed for aromatic organoarsenicals removal, such as photocatalytic degradation, photo-oxidation, and sorption [5-8]. In general, sorption has the advantages of low cost, high efficiency, and simple operation [9, 10]. Many types of sorbents, including polymers [11], carbon materials [12], metal oxides [13, 14], natural minerals [15, 16], and metal-organic frameworks (MOFs) [17-19] have been developed for removal of organoarsenicals in recent years. Among these sorbents, Fe-based ones have attracted much attention with the distinct advantages of high sorption efficiency induced by Fe-O-As coordination [20], low cost, low toxicity, and easy regeneration. Sorption of organoarsenicals on a range of naturally existing Fe-containing materials, including goethite [21], hematite [22], and iron humate [23] has been evaluated. To improve the overall performance, MnFe2O4 [24], Fe3O4@graphene nanocomposites [20, 25], goethite-based composites [26], and carbon nanotubes/C@Fe /chitosan [27] were also synthesized and used to sorb organoarsenicals. Table 1 summarizes the sorption capacities of various Fe-based sorbents towards aromatic organoarsenicals reported recently in the literature. Although Fe-based sorbents have been researched extensively for the sorption of organoarsenicals, previous studies mostly focused on the sorption of one or two organoarsenicals (i.e., p-arsanilic acid (p-ASA) and/or roxarsone (ROX)) and rarely paid attention to the desorption and regeneration of the sorbents, which are important for the cost of sorption treatment. Since organoarsenicals have an arsenic acid group that is similar with arsenate [28], CoFe2O4, which has strong sorption capacity towards inorganic arsenic [29], is likely to work well at organoarsenicals removal. Moreover, it has been reported that bimetal ferrites (e.g., CoFe2O4) have enhanced surface hydroxyl (M-OH) species, which promote the formation of Fe-O-As complexes, compared to single ferrites [30]. Moreover, the high saturation magnetization of CoFe2O4 makes it a useful sorbate for easy regeneration [31, 32]. Although CoFe2O4 could be a promising sorbent for organoarsenicals, the particles of micro/nano sizes agglomerate easily, consequently reducing the available surface sorption sites and decreasing the sorption capacity. In this work, the flower-like CoFe2O4 particles were prepared through a facile and environment-friendly two-step method, including the hydrothermal reaction and calcination. During the calcination process, the organic components escape as gaseous molecules [33] from the precursor, leaving the resulting particles porous structures and low tendency of aggregation [34], which resolves the agglomeration problems for most micro/nano-sized particles easily. Then the sorption capacities were evaluated with six typical organoarsenicals, including p-ASA, ROX, 4-hydroxyphenylarsonic acid (4-HPAA), 2-aminophenylarsonic acid (2-APAA), phenylarsonic acid (PAA), and 2-nitrophenylarsonic acid (2-NPAA) as model sorbates. The crystalline structure, morphology, elemental composition, and magnetic property of the as-prepared sorbent were systematically characterized. The sorption isotherms of the flower-like CoFe2O4 particles towards the model organoarsenicals and the sorption kinetics were studied, while the effects of solution pH, ionic strength, as well as the coexisting humic acid (HA) and common ions, were investigated. The reusability of flower-like CoFe2O4 particles was also tested. The sorption mechanism of organoarsenicals on flower-like 3

CoFe2O4 particles was elucidated based on the experimental observations and spectroscopy results.

Materials and methods 2.1 Materials Six aromatic organoarsenicals, including p-ASA (C6H8AsNO3, >98%), ROX (C6H6AsNO6, >98%), 4-HPAA (C6H7AsO4, >98%), 2-APAA (C6H8AsNO3, >98%), PAA (C6H7AsO3, >98%), and 2-NPAA (C6H6AsNO5, >98%), were obtained from TCI Chemicals (Shanghai, China). Analytical grade FeCl3·6H2O, CoCl2·6H2O, ascorbic acid, and urea were supplied by Sinopharm Chemicals (Shanghai, China), Heowns Biochem (Tianjin, China), Xilong Scientific (Shantou, Guangdong, China), and Tongguang Fine Chemicals (Beijing, China), respectively, and used as received. HA was purchased from Sigma-Aldrich (St. Louis, MO, USA). Stock solutions of aromatic organoarsenicals (2.67 mmol/L) were prepared by dissolving aromatic organoarsenicals in ultrapure water produced by a Milli-Q Reference system (Merck KGaA, Darmstadt, Germany).

Preparation and characterization of flower-like CoFe2O4 particles The flower-like CoFe2O4 particles were prepared through a modified two-step method according to a literature [35]. Specifically, 1.441 g of FeCl3·6H2O, 0.634 g of CoCl2·6H2O, and 1.5 g of ascorbic acid were dissolved in 80 mL ultrapure water, followed by 1.2 g of urea addition. The solution was then transferred into a Teflon lined hydrothermal autoclave reactor, and subsequently heated at 160 °C for 6 h in an electric oven. After cooling, the obtained precursor was washed sequentially with ultrapure water and ethanol to remove residual chemicals, then freeze-dried. The final product (flower-like CoFe2O4 particles) was acquired by calcining the precursor at 500 °C for 2 h. The morphologies and microstructures of as-prepared flower-like CoFe2O4 particles were characterized by scanning electron microscopy (SEM, S-4800, Hitachi) and transmission electron microscopy (TEM, F30, FEI). Elemental distribution of the particles was characterized with elemental mapping obtained with energy dispersive X-ray spectroscopy (EDX) attached to the TEM instrument. The crystallinity of the flower-like CoFe2O4 particles was characterized with powder X-ray diffraction (XRD, X-Pert3 Powder, PANalytical) using Cu Kα radiation. The magnetic property of the particles was characterized with magnetic hysteresis loop obtained on a vibrating sample magnetometer (VSM, BHV-50HTI, Rigaku). The specific surface area and pore distribution of the particles were quantified with nitrogen adsorption-desorption on an accelerated surface area and porosimetry system (ASAP 2020, Micromeritics). The zeta potentials of the particles before and after sorption of aromatic organoarsenicals were measured on a Zetasizer (Nano-ZS90, Malvern) in the pH range of 3 – 10. Infrared (IR) spectra measurement was conducted on a Fourier transform infrared spectrometer (FTIR, Nicolet iS50, ThermoFisher) in the 4

wavenumber range of 400 – 4,000 cm-1. The surface chemical compositions of the particles before and after sorption of aromatic organoarsenicals were analyzed on an X-ray photoelectron spectrometer (XPS, AXIS Supra/Ultra, Kratos) with an Al K source.

Sorption experiment Equilibrium sorption of the aromatic organoarsenicals was conducted with 10 mg of sorbent added into 30 mL solution (concentration: 20 – 120 mol/L, pH: 4, ionic strength: 2 mmol/L, temperature: 25 °C) under constant stirring (at 120 rpm) for 24 h (which is sufficient for sorption to reach equilibrium). To evaluate the sorption kinetics, 80 mg of sorbent was added to 150 mL solution containing the aromatic organoarsenicals (concentration: 80 mol/L, pH: 4, ionic strength: 2 mmol/L, temperature: 25 °C) and stirred constantly (at 120 rpm). Samples of the suspension (1.5 mL) were withdrawn at pre-determined time intervals and filtered with a 0.22 m filter membrane. The influence of solution pH on sorption was investigated by adjusting the initial solution pH (within the range of 3 – 10) using 0.1 mol/L HCl or 0.1 mol/L NaOH. The effect of ionic strength on sorption was investigated in the presence of NaCl at different concentrations (0 – 6 mmol/L). The effects of dissolved organic matter (DOM) and co-existing ions were studied with the addition of HA (0 – 24 mg/L), as well as CaCl2, MgCl2, CoCl2, ZnCl2, KCl, CuCl2, AlCl3, MnCl2, NiCl2, FeCl3, NaNO3, Na2SO4, NaHCO3, and NaH2PO4 (all at 1 mmol/L) at pH 4. The desorption and regeneration of the sorbent were conducted with equilibrating 20 mg of sorbent in 40 mL aromatic organoarsenicals solution (concentration: 80 mol/L, pH: 4, ionic strength: 2 mmol/L, temperature: 25 °C) for 24 h. After that, the sorbent was separated by applying an external magnetic field, followed by regenerating the sorbent through immersing in 0.01 mol/L NaOH and 0.01 mol/L HCl for 24 h separately, then washed with ultrapure water. The sorption-regeneration cycles were repeated for five times. The concentrations of aromatic organoarsenicals in aqueous solution were measured on a high performance liquid chromatograph (HPLC, LC-20A, Shimadzu) following the method used previously.[36] In brief, the analytes were eluted on a C18 column (WondaSil, 4.6 × 250 mm, 5 μm) with 30:70 (v:v) methanol and an aqueous solution of 10 mmol/L CH3COONH4 and 0.2% H3PO4 at 1.0 mL/min. The sorption capacities of the flower-like CoFe2O4 particles and removal efficiencies of aromatic organoarsenicals were calculated with the following equations: 𝑞𝑒 = 𝑅=

(𝐶0 ― 𝐶𝑡)𝑉

𝐶0 ― 𝐶𝑡 𝐶0

𝑚

(1)

× 100%

(2)

where qe (mg/g) represents the sorption capacity at the equilibrium, C0 (mg/L) and Ct (mg/L) correspond to the initial and equilibrium concentrations of the aromatic organoarsenicals, V (L) and m (g) are the solution volume and sorbent mass, respectively. 5

Langmuir model was used to fit the sorption isotherms of the flower-like CoFe2O4 particles: 𝐶𝑒

𝐶𝑒

1

(3)

𝑞𝑒 = 𝑞𝑚 + 𝐾𝐿𝑞𝑚

where Ce (mg/L) and qe (mg/g) are the residual concentration and sorption capacity at equilibrium, respectively. qm (mg/g) is the maximum sorption capacity. KL (L/mg) is Langmuir constant. The kinetics of aromatic organoarsenicals sorption on the particles were fitted with pseudo-second-order models: 𝑡 𝑞𝑡

1

𝑡

(4)

= 𝑘 𝑞2 + 𝑞𝑒 2 𝑒

where k2 (gmin/mg) is the rate constant for pseudo-second-order rate equation. qe (mg/g) and qt (mg/g) are the sorption capacities at equilibrium and time t [37], respectively.

DFT calculation methods To further study the effects of functional groups in aromatic organoarsenicals on sorption, the density functional theory (DFT) calculation was performed using the Cambridge serial total energy package (CASTEP) code. The exchange and correlation interactions were treated with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional. For detailed parameters, a cutoff energy of 340 eV was set for the Vanderbilt ultrasoft pseudopotential, geometric convergence tolerances included maximum force of 0.03 eV/A˚, maximum energy change of 10-5 eV/atom, maximum displacement of 0.001 A˚ and maximum stress of 0.5 GPa. Density mixing electronic minimisation was adopted with the self-consistent field (SCF) tolerance set to ‘‘fine’’, in which mode high accuracy of 10-6 eV/atom for energy convergence was guaranteed. The (111) plane of flower-like CoFe2O4 was selected, treated with a 22 supercell, and then an 11.8711.8728 Å unit cell was built to calculate the total energy of pristine flower-like CoFe2O4, denoted as Esurf, and flower-like CoFe2O4 sorbed with each of six aromatic organoarsenicals, denoted as ESurf+sorbate. Besides, Esorbate, the total energy of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA, respectively, was also calculated. Based on the above results, the sorption energy (Eads) could be obtained with the equation: Eads=ESurf+sorbate – Esorbate -ESurf.

Leaching experiment To detect the leaching of cobalt and iron ions, 10 mg of flower-like CoFe2O4 particles were added into 30 mL of aqueous solutions containing each of six aromatic organoarsenicals (concentration: 80 mol/L, pH: 4, ionic strength: 2 mmol/L, temperature: 25 °C) and stirred for 24 h. Then the samples were filtered with a 0.22 m filter membrane and collected. The metal ions concentrations of the samples were detected using optical emission spectrometer (OES, Optima 5300 DV, PerkinElmer).

6

Results and Discussion Characterization of the flower-like CoFe2O4 particles The precursor of the flower-like CoFe2O4 particles has flower-like structures with flocky surface (Figure S1). Figure 1 shows the morphology and microstructure of the CoFe2O4 particles (after calcination). The spheres inherit the flower-like morphology of the precursor, with diameters in the range of 0.5 – 1.0 m (Figure 1a). The fluffy and porous surface (Figure 1b) suggests they have large surface area and porous structure. And the magnified image (Figure 1c) shows that the flower-like CoFe2O4 particles are composed of many tiny nanoparticles, which overlap with each other to assemble hierarchical flower-like spheres. The TEM images (Figure 1d-1e) confirm that the flower-like CoFe2O4 particles are agglomerations of nanocrystals (ca. 5 nm). High resolution TEM image (Figure 1f) acquired on the edge region of a CoFe2O4 sphere shows a lattice fringe spacing of 0.25 nm, which corresponds to the (311) plane of CoFe2O4. EDX mapping (Figure 1g) shows that the distributions of cobalt, iron, and oxygen in a flower-like CoFe2O4 sphere are homogenous. The XRD pattern of the precursor is shown in Figure S2, which indicates that the precursor is composed of rhombohedral CoCO3 (JCPDs 11-0692) and rhombohedral FeCO3 (JCPDs 83-1764). These crystalline phases are also reported in the literature, in which similar synthesis strategy was adopted [35]. The resulting flower-like CoFe2O4 particles were obtained during calcination accompanied by CO2 escape. The XRD pattern of the particles (Figure 2a) shows the peaks at 30.1, 35.4, 43.1, 53.4, 57.0, and 62.6 are assigned to the (220), (311), (400), (422), (511), and (440) planes of CoFe2O4, respectively, indicating a cubic structure. N2 adsorption-desorption on the flower-like CoFe2O4 particles follows a type IV isotherm of the BDDT classification (Figure 2b), which is indicative of mesoporous structure.[38] The BET surface area (48.4 m2/g) and total pore volume (0.29 cm3/g) calculated from N2 adsorptiondesorption isotherm are relatively high, which could be attributed to the escaping of CO2 from the precursor in the annealing process, leaving significant vacant space inside the particles. And the average pore size is 24.0 nm. The mesoporous structure and large surface area of the sorbent help to enhance the sorption performance. Figures 2c-2e present the Co 2p, Fe 2p, and O 1s XPS spectra of the flower-like CoFe2O4 particles, respectively, while the full-scan spectrum is shown on Figure 7. Five peaks, at 779.9 eV, 781.5 eV, 786.3 eV, 795.8 eV, and 802.5 eV, are observed on the Co 2p spectrum. The peaks at 779.9 eV and 781.5 eV with two satellite peaks at 786.3 eV and 802.5 eV are ascribed to Co2+ in octahedral and tetrahedral sites, respectively. And the peak at 795.8 eV is attributed to the existence of Co3+ [39]. On the Fe 2p spectrum, peaks at 710.4 eV and 724.3 eV are ascribed to Fe2+, while those at 712.5 eV, 717.7 eV, and 727.1 eV are assigned to Fe3+. The O 1s spectrum contains three peaks located at 529.9 eV, 531.4 eV and 533.3 eV, corresponding to the lattice oxygen, hydroxyl groups, and sorbed water on surface of flower-like CoFe2O4 particles, respectively [40]. The magnetic hysteresis loop of flower-like CoFe2O4 particles shows that the saturation magnetization is about 62.0 emu/g, which is 7

sufficient to recover them quickly (within several seconds) from solution with an external magnetic field. (Figure 2f)

Sorption isotherms and kinetics The sorption capacities of the flower-like CoFe2O4 particles towards the six aromatic organoarsenicals were evaluated in batch sorption experiments. As shown in Figures 3a, more and more aromatic organoarsenicals were sorbed on flower-like CoFe2O4 particles with increasing initial concentrations of aromatic organoarsenicals (20 – 120 mol/L). The sorption capacity is positively relative to initial concentration until a platform is reached. This is because the increasing driving force, produced by the higher concentration gradient between the sorbent surface and aromatic organoarsenicals solution, contributes to mass transfer of aromatic organoarsenicals, resulting in higher sorption capacity However, the number of active sites is limited, which cannot sorb over-the-top aromatic organoarsenicals molecules [41], which explains the appearance of sorption platform. Considering the single constitution and homogenous surface of flower-like CoFe2O4 sorbents, Langmuir isotherm model was used to describe the sorption isotherms. The Langmuir fittings shown in Figure 3b have high correlation coefficients (R2 > 98%). And the calculated maximum sorption capacities towards the aromatic organoarsenicals decrease in the order of ROX (49.8 mg/g), 2-NPAA (44.5 mg/g), pASA (38.9 mg/g), 4-HPAA (36.8 mg/g), 2-APAA (35.6 mg/g), and PAA (32.0 mg/g) (Table 2), whose values and trends are very close to the experimental ones, indicating the Langmuir model is consistent with the sorption isotherms. The obtained results illustrate that the sorption mainly occurs through formation of surface monolayer on the mesoporous sorbent, which can be as ascribed to the following aspects: The sorption active sites are relatively uniform distributed due to single crystal phase, component and homogenous surface of as-prepared flower-like CoFe2O4 particles. Besides, the mesoporous structure with average pore sizes of 24.0 nm makes the sorption happens through surface interaction instead of inside the pore interior like microporous materials [42]. Figures 3c shows that the sorption of aromatic organoarsenicals on flower-like CoFe2O4 particles proceeded quickly within the first 200 min, and then slowed down until equilibria reached within 24 h. The rapid sorption at the initial stage suggests that abundant active sites are accessible to the aromatic organoarsenicals on the external interface of flower-like CoFe2O4 particles. As sorption goes on, the reduced active sites, together with existent sorption reaction, retard the sorption equilibria [43]. With an initial concentration of 80 mol/L, the sorption capacities of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA (after reaching equilibria) are 38.1, 45.7, 38.7, 39.3, 33.0, and 32.8 mg/g, respectively. Considering that the same sorbent is used and sorption condition is set as constant, the varying sorption capacities are attributed to the different structures of sorbates, i.e., different types (i.e., amino, nitro, and hydroxyl groups) and positions of substituted function groups on the aromatic ring in our study, which will be further discussed in Part 3.6. Furthermore, as two stages are included in the sorption process, the sorption kinetics data were fitted with 8

pseudo-second-order model, which is expressed based on the assumption of ratelimiting step existence [43]. It turns out that the pseudo-second-order model fits sorption kinetics well (Figure 3d, Table 2), with R2 for all six aromatic organoarsenicals above 99%, suggesting that the sorption capacity is positively related to the number of active sites and the sorption process is dominated by chemisorption through sharing electrons or formation bonds between sorbent and sorbate.

The effect of solution pH and matrix species Figure 4a shows the solution pH effect on the sorption of aromatic organoarsenicals. The sorption of all six aromatic organoarsenicals on the flower-like CoFe2O4 particles exhibits similar trends. The sorption decreases as the initial solution pH increases from 3 to 5, and changes little with further increasing up to 9. Reduction in sorption occurs again as solution pH increases from 9 to 10. This trend is similar to sorption tendency of p-ASA and dimethylarsonic acid with goethite impregnated graphene oxide-carbon nanotube aerogel [26]. Generally, the effect of solution pH on sorption is attributed to the pH dependence on sorbent surface charge and organoarsenical speciation [24]. Figure 4b indicates that the point of zero charge (pHpzc) of the assynthesized flower-like CoFe2O4 particles is 6.4. Table S1 summarizes the pKa values of the organoarsenicals, which determine their speciation in aqueous solution. As the pKa1 values of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA are 3.4, 1.9, 3.9, 2.0, 3.8, and 3.1, respectively, these aromatic organoarsenicals are mainly negatively charged in experimental pH range. As the solution changes from acidic to circumneutral, the positive surface charges of the flower-like CoFe2O4 particles become less, thus reducing the electrostatic interaction between flower-like CoFe2O4 surface and the anions of aromatic organoarsenicals. At solution pH above 6.4, the flower-like CoFe2O4 particles are negatively charged. The electrostatic repulsion dominates the interaction with the negatively charged organoarsenical species. Apparently, the sorption of organoarsenicals on the flower-like CoFe2O4 particles does not decrease all the way with solution pH increase, which suggests other interaction may contribute to the sorption. Considering the reported interaction between arsenic groups and Febased compounds [44], surface complexation is likely to occur in the sorption process. Similar mechanisms have also been reported in arsenite or arsenate sorption process [45, 46].

The effect of ionic strength and humic acid Figure 4c indicates that the sorption of aromatic organoarsenicals was barely affected by the presence of sodium chloride with the concentration up to 6 mmol/L. In general, ionic strength has significant impact on the sorption mainly induced by outer-sphere complexes, but much less impact on the sorption which the inner-sphere complexes play a vital role [47]. Thus, the organoarsenicals probably sorb on the flower-like CoFe2O4 particles through inner-sphere complexation. Figure 4b also shows that the pHzpc values of flower-like CoFe2O4 particles decreased significantly from 6.4 to 4.8, 4.4, 4.2, 5.0, 4.6, and 4.7 in the presence of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA, respectively. Such significant change in the pHzpc values can be 9

explained by the formation of inner-sphere surface complexes generated by strong bonding [1, 48] instead of weakly-bonded outer-sphere surface complexes [21]. HA is often used as a surrogate for DOM in natural water, whose constituent is complex with many functional groups. Figure 4d shows the impact of HA on organoarsenicals for the flower-like CoFe2O4 particles. The increases in HA concentration led to appreciable reduction in the aromatic organoarsenicals sorption, which was caused by the sorption sites competition between organoarsenicals and HA.

The effect of coexisting ions To evaluate the potential application of the flower-like CoFe2O4 particles for removal of organoarsenicals in natural water, the impact of common ions was also studied. Figure 5 shows that the presence of cations, such as Ca2+, Mg2+, Co2+, Zn2+, K+, Cu2+, Al3+, Mn2+, and Ni2+, had little impact on organoarsenicals sorption, while the presence of Fe3+ significantly increased their sorption (by 1.2 to 1.8 times), which is probably caused by the formation of Fe-O-As complexes. It is also observed that NO3and SO42- barely affected the sorption of organoarsenicals, while CO32- and PO43- had obvious inhibitory effect. (Figure 5) The presence of CO32- (1 mmol/L) reduced the sorption of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA on the flower-like CoFe2O4 particles by 61.8, 83.3, 78.0, 60.8, 79.6, and 75.4 %, respectively. This is possibly attributed to the formation of additional surface complexes, competing for the available organoarsenicals [24]. Similarly, the co-existence of PO43- at 1 mmol/L also caused significant reduction in the sorption of organoarsenicals on the flower-like CoFe2O4 particles. PO43- has structurally similarity with arsenate and is well known to have strong competitive effect on arsenate sorption with metal (hydro)oxides [49]. Thus, it is not surprising that it can significantly reduce the sorption of organoarsenicals on the flower-like CoFe2O4 particles through competing for the same sorption sites [50, 51].

Sorption mechanism It appears that the existence of different substituted functional groups (i.e., amino, nitro and hydroxyl groups) on the aromatic organoarsenicals molecules, and the accordingly their size and shape, made differences on their sorption over the flowerlike CoFe2O4 particles. The effect of solution pH indicates that electrostatic interaction can help in some degree. The influence of ionic strength and competing ions suggests that the formation of inner-sphere Fe-O-As complexes plays a key role in sorption, which is consistent with the finding of organoarsenicals on metal-organic frameworks in a previous study [17]. Figure 6a depicts the FTIR spectra of flowerlike CoFe2O4 particles before and after the sorption of aromatic organoarsenicals. For pristine flower-like CoFe2O4 particles, four major peaks appear, among which two peaks at 3368 and 1627 cm-1 are attributed to the stretching and bending vibration of O-H groups, respectively[46], while peaks at 547 cm-1 and 420 cm-1 can be explained by the stretching vibration of Fe-O and Co-O [52, 53]. 10

After sorption of the organoarsenicals, the peak at 3368 cm-1 disappears owing to the arsenic groups coordinating with the O-H chelated with iron [54], while a number of peaks appear in the range of 700−1600 cm-1 (Figure 6b). The bonds observed in 768 cm-1, which are induced by the formation of inner-sphere complexes, are assigned to vibrations of Fe-O-As [16, 22, 55]. And peaks at around 838 cm-1 are ascribed to AsO stretching vibrations [22], generated by free As-O bonds. Additionally, the features at about 1097 cm-1 in the spectra correspond to in-plane C-H groups bending of aromatic compounds with a contribution from As-C vibration [22]. Otherwise, the bonds in the range of 1420 – 1620 cm-1 are generated by the variable C=C stretching vibrations of benzene ring. And the different positions of bonds are due to the different substituted functional groups on the organoarsenical molecules sorbed on the surface of the flower-like CoFe2O4 particles [56]. This result confirms the formation of inner-sphere complexation on the flower-like CoFe2O4 surface during the sorption of the aromatic organoarsenicals. Figure 7 shows the full XPS spectra of the flower-like CoFe2O4 particles before and after sorption of the organoarsenicals. In full spectrum of pristine flower-like CoFe2O4, peaks can be ascribed to Co 2p, Fe 2p, and O 1s, respectively. After sorption of organoarsenicals, a new peak with binding energy around 45.1 eV emerges, assigned to As 3d. As mentioned in Figure 2e, the peaks centered at 529.9, 531.2, and 533.6 eV, respectively, in O 1s spectrum of pristine flower-like CoFe2O4 correspond to lattice oxygen in metal oxides (M-O), oxygen in hydroxyl groups (MOH), and oxygen in adsorbed water (M-OH2) [57]. Their respective area percentages are 72.2, 19.4, and 8.4%, respectively (Table S2), among which M-OH has been reported to be helpful in coordinating with As-O, thus forming bidentate binuclear, bidentate mononuclear or monodentate mononuclear complexes [43]. After the sorption of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, or 2-NPAA, a new peak at around 532.4 eV appears, which is attributed to the oxygen in As-O groups (Figure 8). Meanwhile, the ratios of oxygen in M-O decrease to 68.7, 63.8, 62.6, 70.1, 68.0, and 69.0%, respectively, suggesting the partial disruption of M-O bonds in the sorption process. The area percentages of M-OH peaks also decline slightly to 17.1, 17.1, 17.7, 17.8, 17.9, and 17.6% after the sorption of p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2-NPAA, respectively, due to the interaction between the organoarsenicals and the hydroxyl groups on sorbent surface [26]. The result is in consistent with the analysis of FTIR spectra. To further prove the contribution of Fe-O-As complexes formation to sorption, the DFT calculations were performed with the resulting sorption configurations illustrated in Figure 9. And it is observed that the sorption energies towards p-ASA, ROX, 4HPAA, 2-APAA, PAA, and 2-NPAA are -3.608, -4.091, -3.692, -3.886, -3.547, and 3.355 eV, respectively. The negative sorption energies confirm the formation of Fe-OAs complexes and indicate that the sorption happens spontaneously. Besides, smaller value of sorption energy means that the sorption affinity between sorbent and sorbate is stronger. And the sorption affinity decreases in the order: ROX > 2-APAA > 4HPAA > p-ASA > PAA > 2-APAA, which matched the experimental kinetics data perfectly. 11

Hence, the sorption of flower-like CoFe2O4 particles towards aromatic organoarsenicals is dominated by the formation of inner-sphere complexation (Fe-OAs), which is also confirmed by the experimental results and spectroscopic analysis. Moreover, electrostatic interaction contributes to the sorption on the flower-like CoFe2O4 particles, proved by pH influence. The possible mechanism is illustrated in Scheme 1.

Leaching experiment The stability of sorbent and secondary pollution brought by cobalt ions release need to be taken into consideration for practical application, besides the sorption efficiency. Thus, the leaching of cobalt ions after sorption is detected and the data are shown in Figure S3. The cobalt ions release for all six aromatic organoarsenicals is below 1.6 mg/L. This result suggests the material is stable and eco-friendly.

Desorption and regeneration of flower-like CoFe2O4 particles With excellent magnetic property, the flower-like CoFe2O4 particles can be easily separated from aqueous solution with an external magnetic field. Efficient desorption of the sorbed aromatic organoarsenicals would allow them to be reused. Figure 9a shows the desorption of aromatic organoarsenicals from the flower-like CoFe2O4 particles under various conditions. Water and ethanol media only bring rather poor desorption of the aromatic organoarsenicals, demonstrating that the sorption on the flower-like CoFe2O4 particles is not mainly realized by forming simple outer-sphere complexes. Besides, considering intense decreases of sorption capacities in alkaline environment, 0.01 mol/L NaOH aqueous solution was used to desorb aromatic organoarsenicals and showed effective desorption from the flower-like CoFe2O4 particles because the arsenic groups are likely exchanged with water through hydrolysis. And no obvious change was observed in desorption efficiency with higher-concentration NaOH solution (0.1 mol/L) as desorption media. Since the effective desorption is obtained from the NaOH solution instead of conventional solvents (water and ethanol), the desorption test supports the surface complexation mechanism in some degree, owing to the replacement of sorbed arsenic groups by OH. Meanwhile, regeneration is another side need to be considered. As shown in Figure 9b, the flower-like CoFe2O4 particles only desorbed with NaOH possessed poor regeneration efficiency. With further immersed in HCl solution, the sorbent showed effective sorption performance, indicating that the sorption sites may be activated through protonation process. To evaluate the reusability of the flower-like CoFe2O4 particles in the sorptive removal of aromatic organoarsenicals, the sorption efficiencies at sorbing the six orgaoarsenicals were evaluated in five sorption-desorption cycles (Figure 10). Overall, the sorption efficiencies of the flower-like CoFe2O4 particles towards all the aromatic organoarsenicals gradually decreased over the regeneration cycles, but still retained 73.8, 71.5, 53.1, 64.2, 46.0, 74.6 % of the initial ones for p-ASA, ROX, 4HPAA, 2-APAA, PAA, and 2-NPAA, respectively, after 4 cycles of reusability. These reductions are attributed to the gradual loss of active sites during regeneration process. 12

This phenomenon has also been reported for other types of sorbents [45, 58]. However, the performance of the flower-like CoFe2O4 particles is still competitive with other sorbents for removal of organoarsenicals at low initial concentrations. Further study will investigate the mechanism responsible for the reduction in sorption capacity and develop strategies to avoid this.

Conclusions A hierarchical flower-like CoFe2O4 sorbent was synthesized through a facile two-step method. The sorbent has a saturation magnetization of 62.0 emu/g, making it easy to be recovered with an external magnetic field. The as-synthesized sorbent has a large surface area of 48.4 m2/g and pore volume of 0.29 cm3/g (average pore sizes of 24.0 nm). The isotherms of aromatic organoarsenicals sorption on the flower-like CoFe2O4 particles could be described by Langmuir isotherm model, while their kinetics observed the pseudo-second-order model. The maximum sorption capacities of the flower-like CoFe2O4 particles towards p-ASA, ROX, 4-HPAA, 2-APAA, PAA, and 2NPAA reached 38.9, 49.8, 36.8, 35.6, 32.0, and 44.5 mg/g, respectively. The p-ASA and ROX sorption capacities of the flower-like CoFe2O4 particles are comparable or higher than that of many other Fe-based sorbents with a relatively low initial arsenic concentration [24],[26],[59],[60]. For the electrostatic interaction playing an important role, the sorption of aromatic organoarsenicals on the flower-like CoFe2O4 particles exhibited strong pH dependence. Fe3+ facilitated the sorption of aromatic organoarsenicals on the flower-like CoFe2O4 particles through contributing to the formation of Fe-O-As complexes, while HA, CO32-, and PO43- inhibited their sorption through competing for the surface active sites. The above studies, together with spectra analysis and DFT calculations, consistently indicate that the sorption of aromatic organoarsenicals on the flower-like CoFe2O4 particles is mainly due to the formation of inner-sphere surface complexes. Moreover, the flower-like CoFe2O4 sorbent could be easily recollected, desorbed, and regenerated over multiple cycles. Thus, the flower-like CoFe2O4 particles synthesized in this work hold promises in removing aromatic organoarsenicals from aqueous stream. Notes: Declarations of interest: none Acknowledgements The constructive comments of the anonymous reviewers on an earlier version of this manuscript are greatly appreciated. This work was supported in parts by the Natural Science Foundation of China (Grant Nos. 41725015 and 41673089)

13

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List of Figure Captions Figure 1. SEM (a, b), TEM (c-e) and HRTEM (f) images and the corresponding EDX elemental mapping (g) of flower-like CoFe2O4 particles. Figure 2. XRD pattern (a), N2 adsorption-desorption isotherms (with the pore size distribution shown on the insert) (b), XPS spectra of Co 2p, Fe 2p, O 1s (c-e), and M-H curve (f) of flower-like CoFe2O4 particles. Figure 3. Sorption isotherms (a), Langmuir fittings (b), sorption kinetics (c), and pseudo-second-order fittings (d) of the six aromatic organoarsenicals on flower-like CoFe2O4 particles. Figure 4. Sorption of the six aromatic orgaoarsenicals on flower-like CoFe2O4 particles: effect of initial solution pH on sorption efficiency (a), zeta potentials of flower-like CoFe2O4 particles with/without aromatic orgaoarsenicals (b), effect of solution ionic strength on sorption efficiency (c), and effect of HA on sorption efficiency (d). Figure 5. Effect of co-existing ions (at 1 mmol/L each) on sorption of the six aromatic orgaoarsenicals on flower-like CoFe2O4 particles. Figure 6. FTIR spectra of flower-like CoFe2O4 particles before and after sorption of the six aromatic organoarsenicals in 400-4,000 cm-1 (a), and in 700-1,600 cm-1 (b). Figure 7. Full XPS spectra of flower-like CoFe2O4 particles before and after sorption of the six aromatic organoarsenicals. Figure 8. O1s XPS spectra of the flower-like CoFe2O4 particles with aromatic organoarsenical sorbed. Figure 9. Equilibrium configurations of the sorption of six aromatic organoarsenicals on flower-like CoFe2O4 ((a): p-ASA; (b): ROX; (c): 4-HPAA; (d): 2-APAA; (e): PAA; and (f): 2-NPAA). Scheme 1. Proposed sorption mechanisms of aromatic organoarsenical (taking pASA as an example) on flower-like CoFe2O4 particles. Figure 10. Desorption and Regeneration conditions of flower-like CoFe2O4 particles: desorption of the six aromatic organoarsenicals by various media (a), and regeneration of the six aromatic organoarsenicals with/without HCl (b). Figure 11. Reuse performance over five cycles after regeneration using 0.01 mol/L NaOH and 0.01 mol/L HCl.

18

Table 1. Comparison of the sorption capacities of Fe-based sorbents towards aromatic organoarsenicals reported in the literatures. Sorbent

Initial concentration

pH

Sorption capacity (mg/g)

(mg/L)

ROX

p-ASA

References

MnFe2O4

400

natural

51.5

59.5

[24]

MIL-101(Fe)

400

natural

508.0

379.7

[56]

Fe-Mn framework

130

7.5

/

201.9

[59]

Fe(OH)3

130

7.5

/

76.0

[59]

-FeOOH /GO-CNT α-FeOOH

200

/

/

102.0

[26]

200

/

/

14.5

[26]

Fe-doped sludge

0.5

3

/

5.5

[60]

Iron humate

300

5

/

169.5

[23]

Fe3O4@RGO

100

5

454.5

/

[20]

Fe3O4

100

5

163.9

/

[20]

biochar

Table 2. Summary of the Langmuir and pseudo-second-order fitting parameters for sorption of the six aromatic organoarsenicals on flower-like CoFe2O4 particles. Sorbate

Langmuir qm (mg·g-1)

Pseudo-second-order KL (L·mg-1)

R2

k2

R2

(g·mg·min-1) p-ASA

38.941

1.6386

0.9854

0.0015972

0.99962

ROX

49.751

1.6275

0.9990

0.0014504

0.99998

4-HPAA

36.805

0.8475

0.9885

0.0020687

0.99999

2-APAA

35.625

0.5924

0.9964

0.0017925

0.99996

PAA

32.020

0.3072

0.9822

0.0065031

0.99991

2-NPAA

44.464

0.0940

0.9993

0.0032783

1.00000

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*Declaration of Interest Statement

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Dec. 3, 2019

Credit Author Statement Jue Liu: Conceptualization; Data curation; Methodology; Software; Writing - original draft; Writing - review & editing. Bing Li: Data curation; Visualization; Writing - original draft. Guowei Wang: Data curation; Visualization; Writing - original draft. Lifan Qin: Data curation; Visualization. Xue Ma: Data curation; Methodology. Yuanan Hu: Conceptualization; Project administration; Resources; Supervision. Hefa Cheng: Conceptualization; Funding acquisition; Supervision; Writing - review & editing.

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