Optimal synthesis and mechanistic performance of HCO-doped-(Fe3O4)x adsorbent for Sb(III)/Sb(V) removal from water

Concern over potential antimony mediated toxicity from mining and smelting activities has instigated novel concepts toward removing aqueous antimony ions. The iron based adsorbent Fe3O4/HCO was found to be efficient for treating antimony-containing wastewater However, ineffective methodology for preparation limited its effective adsorption capacity and thus wider application. In this study, a new type of HCO-doped-(Fe3O4)x adsorbent was prepared by co-precipitation method for doping Fe3O4 into HCO sludge (HCO), thereby improving adsorption performance for Sb(III) and Sb(V) ions, with the maximum adsorbing capacity being 44.46 mg/g and 47.91 mg/g, respectively. According to the results of BET, SEM-EDS, XRD and XPS, it were confirmed that the FeOOH and X≡Fe-OH were formed during the preparation process, bring about the increased the surface area, thus resulting in further increase of surface area, hydroxyl groups and the net negative ionic charge. Moreover, the adsorption kinetics followed the pseudo-second-order kinetic model which indicated that adsorption process of Sb(III)/Sb(V) by HCO-doped-(Fe3O4)x adsorbent was controlled by Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 23 October 2020 doi:10.20944/preprints202010.0498.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license. chemical reaction. The main adsorption mechanism is that antimony ion and amorphous iron oxide X≡Fe-OH undergo coordination exchange reaction and complexation reaction with CeO2 or Ce2O3. Furthermore, HCO-doped-(Fe3O4)x could adapt to wide pH and had stable adsorption ability after regeneration. The good adsorption performance of HCO-doped-(Fe3O4)x makes it a potential applications of adsorbent for removal of antimony.


Introduction
Antimony (Sb), if present above a certain concentration within the environmental, is a toxic agent [1,2]. It is a global pollutant and listed as a priority for control by the world health organization (WHO) and United States Environment Agency (USEPA) [3,4]. China has one of the largest antimony deposits in the world and minining is continuing. Thus, antimony contamination of water bodies continues to worsen in China because of anthropogenic and natural source contributions such as weathering of rocks, antimony mining and smelting, as well as the extensive use of antimony products [5][6][7]. For instance, near Xikuangshan mines which is considered as the capital of antimony mining, the Sb concentration in natural water bodies was found to be 37~63 µg/L [6], resulting in one of the most serious eco-environment problems. Sb(III) and Sb(V) are the main oxidation states of Sb found within the natural environment, and the two valence states can inter-transform [4]. At present, there is a lack of understanding of the bio-geochemical cycle, the chemical behavior of antimony, and the lack of efficient treatment technology. Purification of water contaminated by Sb(III)/Sb(V) has thus become an urgent problem in such antimony mining environments of China [8,9].
For the last 20 years, various technologies to purify antimony metals from water such as flocculation [10], chemical precipitation [8], ion exchange [11], bioremediation [12] and adsorption [13]. Among them, adsorption has garnered a great deal of attention [1,8,14], due to its the advantages of low-cost and simplicity. Meanwhile, Fe3O4 as a promising absorbent for antimony-containing wastewater treatment, has the advantage of economy, affective over a wide range of pH and easy recovery. However, the main limitation is low adsorption capacity [15]. Therefore, the preparation of Fe-carrying adsorbents with high antimony removal efficiency and large adsorption capacity is a current research focus [1,9]. Recently, it has been found that Fe3O4 doped with manganese, zirconium, cerium or other metals can be prepared as bimetallic composite adsorbent with porous structure [16][17][18][19], so as to improve its adsorption capacity [4,[20][21][22], and retain the advantages of Fe3O4, such as deposition speed, easy recovery and recycling [23][24][25][26]. One of the most abundant rare earth metals , cerium ions and CeO2 aqueous can carry many high affinity surface hydroxyl groups [27] with cerium ions providing a more positive charge density on the surface of adsorbent, which is conducive to the adsorption and removal of oxyanions of Sb, As and Cr [22,28]. Therefore, cerium and its oxides have received much attention in recent years in purification of water contaminated by metal ion [29,30]. For example, it has been found that pure cerium doped Fe3O4 magnetic adsorbent exhibited a good antimony adsorption performance in aqueous solution [4,27,31,32]. However, Cerium, being a rare strategic metal, thus its high price restricted its extensive use. Therefore, under the guidance of the thought of "resource recycling" and "treat waste with waste", Fe3O4/HCO adsorbent was prepared using the grinding sludge rich in cerium oxide (CeO2. nH2O, referred to as HCO) and Fe3O4 as raw material [14]. As is well known, the preparation process of adsorbent doped metals via modified co-precipitation method would have significant effects; such as increased amount of specific surface area, pore size, hydroxy content, as well as tunable surface charge and electronic properties. These factors can greatly enhance the adsorption capacity and performance. However, the optimum preparation conditions of doped Fe3O4 onto HCO have never reported [14], leading to unclear control of its synthesis. On the other hands, the Ce/Fe molar ratio was also found to be a key factor toward controling the specific surface area and pore size of adsorbent, although the optimum Ce/Fe molar ratio for adsorption of antimony is still unclear. In addition, antimony in natural water mainly exists in the form of Sb(V) [1,2]. The influencing factors and mechanism of Sb(V) removal by Fe3O4/HCO adsorption and the difference with removal of Sb(III) are also in not optimized [14].
In this work, a series of new HCO-doped-(Fe3O4)x adsorbent were synthesized by co-precipitation method using Fe3O4 doped into grinding sludge with HCO rich grinding sludge as the substrate ratio of precursors conditions. Evaluation of their aqueous Sb(III) and Sb(V) sorption capacity, as well as the effect of Ce/Fe molar ratio, pH and reaction time on adsorption were investigated. Moreover, the adsorption isothermal model and adsorption kinetics model of Sb(III)/Sb(V) adsorbed by HCO-doped-(Fe3O4)x were constructed. Also the surface morphology, crystal structure, valence state of Sb(III)/Sb(V) before and after adsorption were investigated, as well as the possible adsorption removal mechanism further evaluated.

Preparation and characterization of HCO-doped-(Fe 3 O 4 ) x
The raw HCO was collected from the LSKJ Co., Ltd sewage treatment plant. The major chemical ingredients of the raw HCO in addition to water (80.5%) are cerium oxide (7.8%), silicon dioxide (4.5%), aluminium oxide (3.8%), calcium oxide (2.8%) and other material (0.6%). Taking a certain amount of HCO, dehydrated and dried it at 100 ℃, ground and then sieved through 100 mesh for standby.
HCO-doped-(Fe3O4)x adsorbents were prepared by modified co-precipitation method [14,33]. Firstly, dry sludge containing 0.1mol, 0.2mol and 0.3mol HCO was added into three 500-ml three-necked flask, respectively. Then 50ml of FeSO4.7H2O (0.020 mol) and 50 ml of FeCl3.6H2O (0.040 mol) solution was added under nitrogen atmosphere. Secondly, 200 ml of 7% ammonia solution was slowly added into the reactant mixture with stirring at 60 ℃ within water bath . Then, the reaction was carried out under nitrogen atmosphere for 2 hours to obtain a solution suspension. It was washed with deionized water and anhydrous ethanol, and then dried it at 80℃. Finally, the materials obtained were ground into a powder and stored for future use. HCO-doped-(Fe3O4)x adsorbents(x=0.5, 1.0 and 1.5,respectivly) were obtained. During the preparation, the possible chemical reactions were described as follows (Eq. (1-7)) [4,14,31].

Adsorption and desorption experiments
The effect of Ce/Fe molar ratio, pH and reaction time, adsorption isotherm and kinetics were carried out in batch adsorption experiments. Desorption experiments were executed to investigate the reusability and regeneration potential of the sorbent [4].

Adsorption experiments
For the effect of Ce/Fe molar ratio, five 100 ml solutions were prepared with an initial

Adsorption isothermal experiment
For adsorption isothermal experiment, 0.2g HCO-doped-(Fe3O4)x adsorbents were added into triangular bottle and then made up to 100 ml with concentration gradient of (10-200 mg/L for Sb(III) ) and (10-100 mg/L for Sb(V)) [34], respectively. The pH of solution was maintained at pH 5.0 using 0.1 mol/L HCl or 0.1 mol/L NaOH with shaking at 150 r/min for 6 h at the temperature of 25±1℃. After filtration with 0.45 µm membrane, the residual concentration of Sb(III)/Sb(V) was measured. The adsorption isotherm models, such as Langmuir and Freundlich [8,9,14], were used to fit the experimental data. The model formulas of Langmuir and Freundlich models are shown in Eq. (8) and Eq. (9), respectively.
Where, Ce (mg/L) is the concentration of Sb(III)/Sb(V) in the solution at adsorption equilibrium, qe (mg/g) and qmax (mg/g) are equilibrium adsorption capacity and maximum adsorption capacity respectively, b (L/mg ) is the adsorption constant of Langmuir, Kf and 1/n are the constants of Freundlich.

Adsorption kinetics experiment
According to the experimental results in section 2. concentration of Sb, the removal rate and the adsorption capacity of Sb was calculated. The data were fitted by the Pseudo-first-order and Pseudo-second -order kinetic models [8,9,14].
Where, t (min) is adsorption time, qt (mg/g) and qe (mg/g) are the adsorption quantity at time t and the adsorption equilibrium, respectively. k1 (min -1 ) is the adsorption rate constant of Pseudo-first-order kinetic models, k2 (g/(mg·min)) is the adsorption rate constant of the Pseudo-second-order kinetic models.

Desorption experiment
In order to evaluate regeneration performance of this adsorbents, Sb(III)/Sb(V) of HCO-doped-(Fe3O4)x adsorbents were desorbed and adsorbed with 0.1mol/L NaOH solution according to the reports in the literature [4,14]. After filtration and drying, the regenerated adsorbents were reused to evaluate recycle ability.

Analysis method
The standard stock solution of Sb(III)/Sb(V) of 1.0 g/L was prepared with antimony potassium tartrate and potassium pyroantimonate [14,18]. Appropriate amount of stock solution was taken and diluted to obtain the required concentration of Sb(III)/Sb(V) solution.
All reagents used in the experiment were analytical pure or superior pure, and the experimental water was deionized water.
The test analysis was completed within 24h after the experiments, and the blank test was conducted with deionized water. With this method the minimum detection concentration is 1 µg/L, and antimony recovery at more than 90% within 1% error.
3 Results and discussion

Characterization analysis
The apparent morphology characteristics of HCO sludge and HCO-doped-(Fe3O4)x adsorbents (x=0.5,1 and 1.5) are shown in Fig. 1. The surface of HCO sludge exhibited a smoother surface typical of less-pores structure ( Fig. 1(a)). After doping Fe3O4, more crystaline particles, typical of more-porous structure appeared on the surface of the adsorbent ( Fig. 1 (b, c, d)). Moreover, the surface roughness porosity increased with the increase of Ce/Fe molar ratio ( Table 1). The EDS energy spectrum (Fig. S1, and and O(17.28-26.51%). The actual ratio Ce/Fe molar ratios were 0.49, 1.14 and 1.72, respectively, which were very close to the design values. The results indicated that Fe was successfully doped into HCO sludge by the improved co-precipitation method [4], and the adsorbents with porous and multi-active sites had been formed.
The porous characteristics of the adsorbents were confirmed by the SBET parameters which were measured by nitrogen adsorption-desorption measurements ( Table 1). As shown in Table 1, the SBET was found to be 98.64 m 2 /g for the sample of HCO-doped-(Fe3O4)0.5, and it was 2.4 times higher than the value of HCO sludge sample (40.60 m 2 /g). Moreover, when the Ce/Fe molar ratio increased from 0.5 to 1.5, both of the SBET value (from 98.64 m 2 /g to 108.57 m 2 /g) and he total pore volume (Vtot) (0.159 cm 3 /g to 0.224 m 3 /g) increased, but the average pore diameter (6.77nm to 4.82 nm ) decreased a similar result also can be found from cerium doped Fe3O4 by Qi et al [4]. It can be inferred that increase of Ce/Fe molar ratio could provide more active sites for diffusion and adsorption [4,35], which was consistent with the conclusion of Qi et al. [4]. We attempted to improve the Ce/Fe molar ratio to more than 1.5, but adsorbents with more-porous structure weren't formed. The reason for this outburst was still uncertain, and expected to be further researched.
The HCO sludge and as-prepared HCO-doped-(Fe3O4)x were characterized using XRD to identify the phases present ( Fig.2(a)). From the diffraction pattern, it was suggested that all the space group could be indexed in the cubic Fd3m comparing data with PDF-4+. As shown in Fig.2 420) and (422) Cerianite crystal faces [36], respectively. However, in HCO-doped-(Fe3O4)x adsorbents, the intensity of the Cerianite character diffraction peak decreased and the position of character diffraction peak changed, suggesting that Fe doping changed the crystal structure of Cerianite [4,28], and formed a lot of multi-pore adsorbents. Combined with the analysis of XPS peak software, the results implied that the compound reaction between Fe3O4 and HCO occurred in the preparation process (Eq. (6)), and formed FeCe2O4 which was favorable for antimony adsorption [4,14].  The full scan XPS spectrum for all kinds of HCO adsorbent exhibited in Fig.2 (b). The results showed that the main components of HCO sludge and HCO-doped-(Fe3O4)x adsorbent were O and Ce, which was consistent with EDS analysis results (Fig.S1). Moreover, new peaks corresponding to Fe were detected in Fe containing HCO-doped-(Fe3O4)x adsorbents, indicating that Fe was successfully loaded in HCO sludge.
There were two strong diffraction peaks at the binding energies of 711ev and 724ev (Fig.   2(c)), and previous studies have shown that Fe at 711ev was mainly in the form of FeOOH and Fe3O4, and at 724ev was mainly in the form of Fe2O3 [4]. Therefore, it was inferred that Fe mainly existed in forms of three states of matter: FeOOH, Fe3O4 and Fe2O3 [4,27,31,32].
Fortunately, the three amorphous iron oxides showed excellent antimony removal properties [16,37,38], implying doping Fe in HCO sludge achieves the expected purpose. XPS scanning could be applied to verify the sample on account of its sensitivity to Sb(III)/Sb(V) and Fe 2+ /Fe 3+ ions. Fig. 2 [4,14]. So, we inferred that Ce 3+ and Ce 4+ were two valence states in HCO-doped-(Fe3O4)x adsorbent. The hydrolysis of cerium dioxide and dicerium trioxide ions can carry hydroxyl group [27], which could promote the removal and adsorption of antimony ions. 10    The results showed that the Sb(III) removal rate increasing from 86.69% to 95.46% with Ce/Fe molar ratio increasing from 0.5 to 1.5 (Fig.3). In addition, a positive correlation was found between Sb(III) removal rate and Ce/Fe molar ratio, which was absolutely consistent with other obtained results by Qi et al. [4,31]. However, with the increase of Ce/Fe molar ratio from 0.5 to 1.5, the Sb(V) removal rate first increased from 54.36% to 78.57%, then decreased to 67.75%, this also means there was not linear relationship between them. The results show optimum efficiency for removal of Sb(III) is when Ce/Fe molar ratio is 1.5, but optimum for removal of Sb(v) is when Ce/Fe molar ratio is 1. Therefore, the removal characteristics of Sb(III) onto HCO-doped-(Fe3O4)x are much different from those of Sb(V).

Effect of Ce/Fe molar ratio
The reason might be due to the different reaction process between Sb(III) and Sb(V) with HCO-doped-(Fe3O4)x. It is generally known that the hydroxyl group with high affinity carried by Ce2O3 play a major role in the process of adsorption and removal of Sb(III) [27]. Therefore, more hydroxyl group would be formed with the increase of Ce/Fe molar ratio, and corresponding Sb(III) removal rate increased. However, Sb(V) ions were mainly removed by the complexation of FeOOH and X≡Fe-OH [4,28]. The higher concentration of HCO doped elements hampered the nucleation process of amorphous iron particles, and corresponding the structure and valence of amorphous iron (Fe3O4) were affected [4], thereby reducing the removal performance of Sb(V). In addition, the removal performance of Sb(III) by HCO-doped-(Fe3O4)x was significantly higher than those of Sb(V) under the same conditions.
In conclusion, HCO-doped-(Fe3O4)1.5 and HCO-doped-(Fe3O4)1 had the best adsorption and removal effect on Sb(III) and Sb(V), respectively. Consequently, they were used for subsequent Sb(III) and Sb(V) adsorption and removal studies.

Effect of pH and adsorption time
The pH of the solution to be treated is an important factor affecting the adsorption capacity of the adsorbent [1,2]. The effect of different initial pH on Sb(III) and Sb(V) removal by HCO-doped-(Fe3O4)x adsorbent are shown in Fig.4(a). As can be seen, the removal rate of  [4,8]. It was generally known that pH would determine the existence of Sb(III) and Sb(V) in aqueous solution [39], and also affect the degree of hydrolysis of amorphous iron such as FeOOH in HCO-doped-(Fe3O4)1.5 [4]. Sb(III) mainly exists in the form of Sb(OH)3 or H3SbO3 in the aqueous solution with the pH value ranging from 2.0 to 7.0 [4,39]. Both of Sb(OH)3 and H3SbO3 can easy interact with Fe3O4 and Ce2O3 to form X≡Fe-Sb(OH)2 precipitates [40] and CeSbO3 [41] when the pH value of solution is between 2.0 to 7.0. Therefore, the removal rate of Sb(III) has been maintained in the higher level. But if pH continues to rise, FeOOH formation and iron ion dissolution would be inhibited [9], which would lead to the decrease of Sb(III) removal rate, alsoconsistent with the research conclusion of Fan et al. [35]. As shown in Fig.3, at pH values higher than 2.0, Sb(V) adsorption decreased with further increase in the pH of the solution. The adsorption behavior can be explained in terms of the distribution of Sb(V) species. When the pH was 2-2.7, Sb(V) mainly exists in the form of H3SbO4 in the aqueous solution [4,39], which benefits the adsorption of antimony through substitution of hydroxyl ions coordinated with the metal ions loaded on the HCO-doped-(Fe3O4)1.0. However, at pH values higher than 2.7, the concentration of Sb(OH)6increased with an increase in pH [4,39], and resulting in a decrease in Sb(V) adsorption through growing competition between OHand Sb(OH)6 - [35]. Overall, HCO-doped-(Fe3O4)x show better absorption capability at relatively wide range of pH in comparison with other iron containing or iron loaded composite adsorbents (Table 2) [37,[42][43][44][45]. In conclusion, the optimal pH for HCO-doped-(Fe3O4)x adsorbing Sb(III) and Sb(V) should be controlled at 2-7 and 2-4, respectively. Adsorption equilibrium time is an important parameter to evaluate the performance of adsorbent. The results showed that the equilibrium time was about 4h and 8h ( Fig.4 (b)), respectively, and indicated that adsorptive velocity of Sb(III) on HCO-doped-(Fe3O4)1.5 was higher than that of Sb(V). The main reason for this was that the adsorption of Sb(III) on HCO-doped-(Fe3O4)1.5 adsorbent belongs to monolayer adsorption [4,14] and the adsorption process of adjacent adsorption sites will not affect each other. HCO-doped-(Fe3O4)1.0 adsorbent adsorption Sb(V) belongs to multilayer adsorption, and the adsorption speed is determined by the adsorption rate related to solution concentration and temperature [8], which is consistent with the conclusions of the subsequent adsorption isothermal model. It is noteworthy to mention that the time to achieve equilibrium was shorter for this composite than that of other iron associated composite adsorbents (Table.3). For example Fe/Mn bimetallic oxide (Sb(III)(5h)) [16], Fe/Zr loaded orange waste(Sb(III) and Sb(V)(24h)) [19], Fe3O4 loaded zeolite(Sb(V)(13h)) [46], and so on. Consequently, HCO-doped-(Fe3O4)x may have wide application potential in the future.

Adsorption isothermal
The isotherm adsorption model, for which the definition and fitting accuracy are related to the type of adsorption materials, antimony ion valence, initial concentration, pH value and other factors [8,9,35], can reflect the relationship between adsorption capacity and solution concentration of adsorption materials at a specific temperature. The Sb(III)/Sb(V) adsorption capacities of the HCO-doped-(Fe3O4)x at pH 5.0 (25℃) were shown in Table 2. As shown in Table 2, the higher regression coefficient (R 2 =0.98) implied that Langmuir model was suitable for representing the adsorption process of Sb(III) onto HCO-doped-(Fe3O4)1.5. As is well known, the Langmuir model assume that all adsorption sites on the surface of adsorption matrix had the same solute affinity, and the adsorption process of adjacent adsorption sites would not affect each other [8,14,35]. We concluded that the adsorption of Sb(III) took place on the surface of HCO-doped-(Fe3O4)1.5, which was defined as monolayer adsorption and chemical adsorption [8]. At the same time, the higher regression coefficient (R 2 =0.945) implied that Freundlich model was suitable for describing the adsorption process of Sb(V) onto HCO-doped-(Fe3O4)1.0, which wasn't consistent with the conclusions of Qi et al [4,8]. In addition, we could make further inference that Sb(V) was most easily adsorbed by HCO-doped-(Fe3O4)1.0 because of the parameter 1/n (0.486) less than 0.5. Also the adsorption process included single-layer (chemical) and multi-layer (physical) adsorption [8], belonging to synergistic adsorption [8,35]. Moreover, the value 1/n of Sb(III) from HCO-doped-(Fe3O4)1.5 was smaller than that of Sb(V) with HCO-doped-(Fe3O4)1.0, suggesting that Sb(III) was more easily adsorbed by HCO-doped-(Fe3O4)x composite adsorbent than Sb(V) [47]. The calculated maximum adsorption capacity for Sb(III) and Sb(V) was 44.46 mg/g and 47.91 mg/g, respectively, which demonstrated that it has good adsorption properties in comparison with other iron containing composite adsorbents (Table 3), such as MnFe2O4 adsorption Sb(III) (10.66 mg/g) [48], Fe3O4/HCO adsorption Sb(III) (22.85 mg/g) [14],Ce-doped magnetic biochar adsorption Sb(V) (25.0 mg/g) [28], Fe(III) modified fungal aerobic sludge adsorption Sb(V) (19 mg/g)) [49] and other ferric adsorbents. However, the maximum adsorption capacity in this study is smaller than those of other iron containing composite adsorbents (Table 3), Fe-Zr bimetallic oxide (60.4 mg/g) [44], Fe/Mn bimetallic oxide (214.28 mg/g) [16], Fe/Zr loaded orange waste (170.45-227.67 mg/g) [19], still provided some obvious advantages for practical aqueous removal of Sb(III)/Sb(V) because of achieving the concept of disposal waste with waste and recycling.

Adsorption kinetics
The adsorption kinetic model can describe the potential rate control and adsorption mechanism of material transport as well as chemical reaction in the adsorption process [14,35]. Calculated parameters on adsorption kinetics of Sb(III)/Sb(V) on HCO-doped-(Fe3O4)x are shown in Table 4. As seen in Table 4, the regression coefficient (R 2 ) showed suggested that the Sb(III)/Sb(V) adsorption kinetics could be better fitted by pseudo-second-order kinetic model. We inferred that electron sharing or electron transfer was the main source of adsorption power [14,28], and the adsorption process was controlled by the chemical reaction [14,28]. Moreover, the calculated qe values (37.845 mg/g for Sb(III) and 44.029 mg/g for Sb(V)) were close to the experimental values (44.46 mg/g and 47.91 mg/g), which further implied that the whole adsorption process of Sb(III) and Sb(V) was controlled by the solid-liquid interface chemical reaction [35]. Furthermore, Elovich model also fitted well for the adsorption of Sb(III) and Sb(V) onto HCO-doped-(Fe3O4)x (date not shown), implying that heterosphere diffusion reaction may be some role in the adsorption process [8,9].

Desorption
The repeated adsorption/desorption test of HCO-doped-(Fe3O4)x was conducted with 0.2 mol/L NaOH solution as desorption agent. As shown in Table 5, HCO-doped-(Fe3O4)x maintained a good removal performance for Sb(III) and Sb(V) (86.55% and 71.33%) after 3 cycle, reflecting its high stability and good regeneration performance. The main reason is that there is competition adsorption between OHions and Sb(III) and Sb(V) ion on the surface of HCO-doped-(Fe3O4)x [8]. This performance lays a good foundation for the reuse of HCO-doped-(Fe3O4)x adsorbent and the recycling of Sb, and has a good economic value.

Adsorption mechanism
Based on the foregoing analysis, some possible adsorption mechanisms of Sb(III)/Sb(V) by HCO-doped-(Fe3O4)x could be summarized as follows: (1) The Pseudo-second-order kinetic model fitted better for the adsorption of Sb(III) and Sb(V) onto HCO-doped-(Fe3O4)x, and the adsorption process was controlled by the chemical reaction at the solid-liquid interface [4,14,28]. Furthermore (2)The type of ligand exchange reaction played a major role in adsorption process of Sb(III)/Sb(V) ions. As shown in Fig. 5, with doping Fe3O4, the compound FeCe2O4 was produced (Eq. (6)), which hinted that more negatively charged groups, hydroxyl groups existed on the surface of the HCO-doped-(Fe3O4)x [8]. Afterwards, electron transfer, ion transfer and sharing may occur in the hydrolyzation process of of FeCe2O4, and forming amorphous hydrated iron oxide X≡Fe-OH with a double electron layer structure [4,50], which could provide higher specific surface area and more hydroxyl groups. Furthermore, coordination body exchange reactions of the X≡Fe-OH with the Sb(III)/Sb(V) ions, may result in complex compounds containing antimony, such as FeOH2SbO3 (Eq. (14)), X≡Fe-Sb(OH)2 (Eq. (15)), FeOSb(OH)6 (Eq. (18)) and Fe-O-Sb(OH)5 -(Eq. (19)) [4,14], and had been identified by the XRD results. Therefore, the adsorption capacity and Sb(III)/Sb(V) adsorption performance was improved remarkably.
(3) Complexation and oxidation reactions also were favorable for adsorption of antimony.
As shown in Fig. 2(a) and Fig. 2(c, e), it could be concluded that Ce element in HCO-doped-(Fe3O4)x was mainly in the form of CeO2 and Ce2O3 [4,14]. The peak binding energy of Ce(III) shifted after adsorption of Sb(III)/Sb(V), and indicated that Ce2O3 reacted with Sb(III) or Sb(V) to generate Ce-O-Sb compounds in the form of CeSbO3(Eq. (12)) or CeSbO4(Eq. (17)) [4]. The results of Fig.2(f) showed that a prominent peak appeared at the binding energy of 531.68ev, indicating the existence of Sb(V) valence state [28], meanwhile, both of the peak binding energy of Ce and Fe shifted (Fig. 2(c) and Fig. 2(e)). Considering the oxidizing ability of CeO2 [51,52], it was inferred that Sb(III) could be oxidized to Sb(V) by CeO2 and Fe2O3 during the adsorption process, to form Ce-O-Sb and Fe-O-Sb complexes [4,28], and then Sb(V) was removed by the reaction of formulas (Eq. [18][19]. In addition, there no Sb(III) absorption peak appeared after Sb(V) adsorption, stating clearly no reduction reaction occurred in this process.
Moreover, the HCO-doped-(Fe3O4)x adsorbent could be readily regenerated using 0.1mol/L NaOH solution and be repeatedly used. HCO-doped-(Fe3O4)x adsorbent can adsorb Sb(III)/Sb(V) with high adsorption capacity and adsorption speed can be used as a promising adsorbent to remove antimony from aqueous solution.