Facile preparation of a tetraethylenepentamine-functionalized nano magnetic composite material and its adsorption mechanism to anions: competition or cooperation

A tetraethylenepentamine (TEPA)-functionalized nano-Fe3O4 magnetic composite material (nFe3O4@TEPA) was synthesized by a facile one-pot solvothermal method. It was characterized by elementary analysis (EA), powder X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). The results show that the nFe3O4@TEPA has an average size of ∼20 nm, with a saturation magnetization intensity of 48.2 emu g−1. Its adsorption properties were investigated by adsorbing fluorine ions, phosphate, Cr(vi) and their co-existing water system. The adsorption performance was studied as a function of solution pH, initial concentration of ions, contact time and temperature for each ion. The adsorption of the multi-ion co-existing system was studied via batch tests, XPS and FTIR analyses. The effect of co-existing ions was studied through Box-Behnken Design (BBD) and response surface methodology (RSM). It can be deducted that the adsorption mechanism of an individual fluorine ion or phosphate was mainly related to electrostatic attraction, while that of Cr(vi) might be mainly related to electrostatic attraction and coordination interactions. For the fluorine ion and phosphate bi-component system, their adsorption was competitive via ion exchange. For the Cr(vi), fluorine ion and phosphate tri-component co-existing system, Cr(vi) took priority for adsorption and could replace the absorbed fluorine ion or phosphate by competitive reaction, but not vice versa.


Introduction
Elevated levels of oxyanions (e.g. arsenite, arsenate, chromate, phosphate, selenite, selenate, borate, nitrate, etc.) and monoatomic anions (e.g. uoride, chloride, bromide, and iodide) have been found in the environment and they can be harmful to both humans and wildlife. 1-3 Some of these anions have become the most frequently detected in ground water at hazardous waste sites and have been placed on the top of the priority list of toxic pollutants by the U. S. EPA. 4 Treatment of anion-containing wastewater prior to discharge is essential. Conventional techniques, such as reduction, reverse osmosis, electrodialysis, ion exchange, and adsorption, have been used for removing these anions from wastewater. 5,6 However, the reduction followed by precipitation has some disadvantages, i.e., higher waste treatment equipment costs, signicantly higher consumption of reagents, and signicantly higher volume of sludge generated. 7 Although reverse osmosis and electrodialysis are superior in recovering some of the oxyanions, such as Cr(VI), it is difficult to reduce the oxyanions in the effluent to an acceptable level. 6 As far as ion exchange is concerned, it is an attractive approach in treating the wastewater containing anions, but ion exchange system is the complexity in regenerating the resin. 8 Recently, Fe 3 O 4 -based magnetic nanoparticles (MNPs) have found to be simple, convenient, and powerful approaches for the separation and purication of environmental samples, and removal of toxic pollutants, including various ions, in water. [9][10][11][12][13][14][15] However, most reports are focused on the treatment of one-ion component solution. Those for co-existing solutions, especially for adsorption mechanism investigating are quite limited. 5,9,11 The industrial effluents oen contain several kinds of oxyanions and monoatomic anions, the study of which is very complicated because of their synergistic, antagonistic or noninteractive effects on their adsorption. The traditional onefactor-at-a-time approach to run and analyze the experiments cannot successfully predict possible interactions between the oxyanions and monoatomic anions in industrial wastewater. Thus, it is necessary to investigate the simultaneous removal process in multi-component system containing oxyanions and monoatomic anions.
Multivariate analysis allows signicant reduction in the number of experiments in addition to the description of independent variables impact on the process. This can contribute to the development and optimization of the multi-component system while it signicantly decreases the cost of experiments. Response surface methodology (RSM) is of the most popular methods applied in researches on adsorption processes. 16,17 The RSM is a useful statistical tool used to design experiments in which factors and their levels are determined. Aer handling the response of the experiments, the results are obtained by analyzing the response according to the RSM. A mathematical model is set via RSM by considering both linear and nonlinear relationships between independent variables, namely factors and response. If interactions affect the response, it can be mathematically modeled which allows for the optimization of the response. Based on such model, response surface graph and contours are provided, which help to visualize the shape of response surface. 18,19 Thus, simultaneous modeling and optimization of variables are required to achieve an economic and popular wastewater treatment.
The objective of this study is to investigate its adsorption properties of uorine ion, phosphate, Cr(VI) and their coexisting water system. On the basis of the adsorption performance investigating of single-component for each ion, the adsorption of multi-component of the co-existing system was statistically studied. Presumed mechanisms were deeply investigated based on batch tests, thermodynamic and kinetic studies, XPS and FTIR characterization and RSM analyses. The overall procedure of the present work was shown in Scheme 1.

Preparation of nFe 3 O 4 @TEPA
The overall preparation of nFe 3 O 4 @TEPA was produced using a polyol-media one-pot solvothermal method. 4.0 g of FeCl 3 -$6H 2 O, and 12.0 g of NaAc were dissolved in 120 mL ethylene glycol. This solution was stirred vigorously at room temperature for 10 min to form a stable orange solution. 40 mL of TEPA was then added with constant stirring for 30 min until completely dissolved. The mixture solution was then transferred to Scheme 1 Overall procedure of the nFe 3 O 4 @TEPA facile preparation and its adsorption to anions. a Teon-lined autoclave and heated at 180 C for 8 h. Aer the autoclave cooled to room temperature, the resulting nFe 3 O 4 @-TEPA was isolated under magnetic eld and washed with water and ethanol to remove redundant reagents and impurities. The as-prepared nFe 3 O 4 @TEPA was dried in a vacuum oven at 60 C for 12 h and stored in a sealed bottle for further use.

Characterization
Transmission electron microscopy (TEM) images were obtained on a JEM-2100F Lorentz-Transmission Electron Microscopy (TEM) at an accelerating voltage of 200 kV. The magnetic properties of magnetic particles were measured using a vibrating sample magnetometer (VSM, Lake Shore 7410). Powder X-ray diffraction (XRD) patterns were collected on an Xray diffractometer (Bruker D8 Advance) with CuKa radiation at l ¼ 0.154 nm operating at 40 kV and 40 mA. The elementary analysis results of the nitrogen contents in nFe 3 O 4 @TEPA were measured using an elementary analysis (EA, Thermo Fisher Flash-1112). Fourier Transform Infrared spectrometer (FTIR, Thermo Nicolet, USA) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRADLD) were used to investigate the adsorption mechanism. Dynamic light scattering (DLS, Nano ZS-90) was used to determine the mean particle size.
The content of Fe 3 O 4 in nFe 3 O 4 @TEPA was calculated from the amount of leached Fe, which was measured by using a spectrophotometer (722, Shanghai, China) according to the standard colorimetric method 11 aer digesting nFe 3 O 4 @TEPA in 12 mol L À1 HCl solution. The concentration of uoride ion (F À ), phosphate or Cr(VI) in the aqueous solution was analyzed following the standard methods for examination of water and wastewater. 20 Briey, the concentration of uoride ion was carried out using combined uoride-specic ion-selective electrode carried out using combined uoride-specic ionselective electrode on a SevenMulti™ instrument (Mettler Toledo). The concentration of phosphate was analyzed spectrophotometrically by the molybdenum blue method at 690 nm by adding (NH 4 ) 6 Mo 7 O 24 and SnCl 2 -HCl solutions followed by being kept in the dark for 10 min at room temperature (722, Shanghai, China). The concentration of Cr(VI) ions in the aqueous solution was analyzed with a spectrophotometer at a wavelength of 540 nm aer acidication of samples with 1 N H 2 SO 4 and reaction with 1,5-diphenyl carbazide to produce a purple color complex for colorimetric measurement (722, Shanghai, China).

Adsorption experiments
A stock solution of uoride ion (F À ), phosphate or Cr(VI) at concentration of 1000 mg L À1 was prepared by dissolving a known quantity of potassium uoride (KF), potassium dihydrogen phosphate (KH 2 PO 4 ) or potassium dichromate (K 2 Cr 2 O 7 ) in ultrapure water. Batch adsorption experiments were carried out in 150 mL stoppered asks, each of which contained 25.00 mL of uoride ion (F À ), phosphate or Cr(VI) individual solutions or co-existing solutions of varying concentration, from 10 to 1000 mg L À1 . A 0.02 g amount of nFe 3 O 4 @TEPA was added into each ask and shaken at 150 rpm in a thermostatic shaker. 0.5 mol L À1 HNO 3 and 0.5 mol L À1 NaOH solutions were used for pH adjustment, ranging from 2.0 to 10.0. Adsorption kinetic and thermodynamic studies at different temperatures (25-45 C), with contacting time ranging from 1 to 180 min, were also carried out. Effect of co-existing ions was studied through Box-Behnken Design (BBD) and the response surface methodology (RSM). The post-adsorption solutions were separated magnetically under a NdFeB magnet.
According to the concentrations before and aer adsorption, the equilibrium adsorption capacity (q, mg g À1 ) of the studied anions absorbed to the nFe 3 O 4 @TEPA is calculated using eqn (1): 21 where C 0 and C e represent the initial solution concentration and the equilibrium concentration of uoride ion (F À ), phosphate or Cr(VI) (mg L À1 ), V is the volume of the solution (mL), m is the adsorbent dosage (mg), the same hereinaer.
where, q e and q t are the adsorption capacities at equilibrium and at time t (mg g À1 ), respectively. k 1 (min À1 ), k 2 (g (mg À1 min À1 )) are the adsorption rate constants, k id is the intraparticle diffusion rate constant (mg (g À1 min À1/2 )), C is the intercept (mg g À1 ).
For the adsorption isotherm studies, two adsorption isotherms, Langmuir model (eqn (5)) 21,23 and Freundlich model (eqn (6)) were applied to analyze the adsorption data. 21,24 where q m and K L are the Langmuir constants related to the maximum adsorption capacity and apparent heat change, respectively, while K F is a Freundlich constant related to adsorption capacity and 1/n is a Freundlich constant related to the adsorption intensity.

Characterization of nFe 3 O 4 @TEPA
The TEM images of nFe 3 O 4 @TEPA were shown in Fig. 1(a). All the size data reect the averages of about 100 particles and are calculated according to eqn (7): 25 where U is the polydispersity index, D n is the number-average diameter, D w is the weight-average diameter, and D i is the diameter of the determined microspheres. It revealed that the nFe 3 -O 4 @TEPA particles were multidispersed with some aggregation at an average diameter of around 20 nm ( Fig. 1(a)), with D n at 21.5, D w at 22.8, and U at 1.06. In order to check the aggregation behavior, we carried out the DLS experiments of nFe 3 O 4 @TEPA from 0-10 min. As shown in Fig. 1(b), the particles aggregated gradually and the intensity average diameter measured by DLS increased from 25 nm to 40 nm aer 1 minute, and 75 nm aer 10 minutes. This might be due to the dipolar magnetic interaction between the magnetic cores and hydrogen bonds between the amino groups on the surface of the magnetic cores.
The FTIR spectra of nFe 3 O 4 and nFe 3 O 4 @TEPA were showed in Fig. 2(a). Characteristic band of nFe 3 O 4 occurs at $589 cm À1 . Other typical bands can be assigned as follows, y(-OH): $1429 cm À1 for PEG. Compared with nFe 3 O 4, Aer functionalization by TEPA, typical bands at $1573 cm À1 can be assigned to be the stretching and bending vibrations of the -NH and -NH 2 groups appeared, with a great shi of the bands for d(-CH 2 ). This revealed that the amino groups of TEPA had been successfully graed to the surface of the nFe 3 O 4 . The superparamagnetic properties of the nFe 3 O 4 @TEPA were veried by  Paper the magnetization curve measured by VSM, shown in Fig. 2(b). The saturation moment obtained from the hysteresis loop was found to be 48.2 emu g À1 . The nFe 3 O 4 @TEPA was expected to respond well to magnetic elds without any permanent magnetization, therefore making the solid and liquid phases separate easily. Due to the anti-magnetic property of the TEPA, it was no surprise the saturation moment of nFe 3 O 4 @TEPA lower than that of the naked nano-Fe 3 O 4 (78.6 emu g À1 , as shown in Fig. 2(b)). Interestingly, the saturation moment of the present nFe 3 O 4 @TEPA was much higher than those of our previously reported nano magnetic polymers (NMPs), which was ranged from 12.3 to 5.56 emu g À1 , 26 which might be due to two facts: (1) anti-magnetic polymer anchored onto the Fe 3 O 4 core of the NMPs, which leading a decrease of content percentage of Fe 3 O 4 in the NMPs; (2) by using solvothermal method, amino-groups of TEPA self-assembled gra to the surface of the magnetic cores via hydrogen bonds between the amino groups and active hydroxyl groups of Fe 3 O 4 . The obtained nFe 3 O 4 @TEPA is with good dispersity to avoid the dispersion agglomeration defects in the traditional preparation process. With a large number of amino groups on the surface of the nFe 3 O 4 , it is benecial to form magnetic ordered structure, which leading an increase of the saturation moment. This phenomenon was also found by Yoon, et al. 27 High saturation magnetization (56.1 emu g À1 ) of the Fe 3 O 4 based magnetic polymer composite material-Fe 3 O 4 @DAPF was found when solvothermal method was used for preparation. To further demonstrate the crystal structure of nFe 3 O 4 @TEPA, the XRD patterns of the as-prepared Fe 3 O 4 (without adding TEPA) and nFe 3 O 4 @TEPA were collected (Fig. 2(c)). It indicated that nFe 3 511) and (400) appeared. 28 Elemental analysis results showed that nitrogen percentage of nFe 3 O 4 @TEPA was 18.9%, while the total content of Fe 3 O 4 in the nFe 3 O 4 @TEPA was 58.2%, which was higher than those of our previously reported NMPs, and consisted with the VSM results.
3.2 Adsorption mechanism of the nFe 3 O 4 @TEPA to anions 3.2.1 Effect of pH and adsorption mechanism for the individual ion. The pH effect of uoride ion (F À ), phosphate or Cr(VI) individual solutions at concentration of 50 mg L À1 , respectively, was investigated with the pH values ranging from 2.0 to 10.0, and the results were shown in Fig. 3. For nFe 3 O 4 , the adsorption efficiencies of all the three ions were quite low (at around 5%) and almost not dependent on solution pH, shown in Fig. 3(a). However, the adsorption efficiencies of nFe 3 O 4 @TEPA were much higher than those of nFe 3 O 4 , at 15.5-99.9% depending on different ions and pH values, shown in Fig. 3(b). The high efficiencies might be contributed to the amino-groups of TEPA anchored on the surface of the nFe 3 O 4. The adsorption efficiency of phosphate or Cr(VI) was highly dependent on solution pH. Interestingly, unlike phosphate and Cr(VI), the adsorption capacity of uoride ion (F À ) was almost not dependent on solution pH. These results imply there might be different adsorption mechanism of nFe 3 O 4 @TEPA to these three kinds of anions.
The pH dependency might be related both to the intrinsic structure property of the nFe 3 O 4 @TEPA and the species of anions. Fig. 4(a) showed that the pH pzc of the nFe 3 O 4 @TEPA was identied to be 5.02, implying the outer surface of the nFe 3 O 4 @TEPA is positively charged when pH is below 5.02, and negatively charged when pH is above 5.02. Based on the experimental data of the total concentration, we run Visual MINTEQ 3, which is widely used in recent years to simulate equilibria and speciation of inorganic solutes in aqueous solution. [29][30][31][32] The speciation of Cr(VI), phosphate and uorine under various pH was obtained, as shown in Fig. 4(b)-(d).
In the case of Cr(VI), as shown in Fig. 3, the percentage of uptake Cr(VI) for nFe 3 O 4 @TEPA decreased from 99.9% to 16.8% gradually with an increase of pH value from 2.0 to 10.0. This phenomenon is contributed to the pH-dependent adsorption mechanism. As shown in Fig. 4  Under acidic conditions (pH at 2.0-3.5), amino groups were easier to be protonated (-NH 3 + ), as described by eqn (12).
Electrostatic attraction happened as in eqn (13), 33 leading a decrease of the residue concentration of Cr(VI). With increasing of the pH value, the concentration of H + was decreased, and at the same time the concentration of OH À , which competed with HCrO 4 À , was increased. So the ability of -NH 2 to be protonated was weakened, resulting in the decline of removal efficiency.
With the increasing of pH value, an interesting phenomenon was observed that there was a at, as we found before in aminofunctionalized nano magnetic polymers (NMPs). 26 This implied that besides the electrostatic attraction and ion exchange interactions, coordination interactions might occur in the adsorption process. as in eqn (14). (14) In the case of phosphate, as shown in Fig. 3(c), the percentage of uptake phosphate for nFe 3 O 4 @TEPA increased from 31.9% to 81.4% sharply to a maximum at pH 3.0, then decreased sharply to 26.0% at pH 6.0. As shown in Fig. 4 , depending on the solution pH (pK 1 ¼ 2.15, pK 2 ¼ 7.20, and pK 3 ¼ 12.33). 34,35 With an increase of pH, the nFe 3 O 4 @TEPA surface carried positive charge, and thus would more signicantly attract the negatively charged monovalent H 2 PO 4 À ions in solution, which indicated that the physicochemical adsorption due to electrostatic attraction was the predominant process of phosphate removal, as described by eqn (15). When the pH of the solution increased, the surface became negatively charged, consequently, unfavorable to the phosphate for electrostatic repulsion. In the case of uoride, as shown in Fig. 3(d), unlike Cr(VI) and phosphate, the adsorption capacity was almost not dependent on solution pH. To our knowledge, this is one of the longest pH ranges of the materials for F À adsorption in literature. The percentage of uptake uoride for nFe 3 O 4 @TEPA kept constant at around 95.6% under pH value from 2.0 to 11.0, and gradually decreased to 70.2% at pH 14. Such high uoride removal efficiencies are much better than those in prior reports. For instance, Kong et al. reported the uoride removal efficiency of MHS-MgO/MgCO 3 adsorbent was 86.2%, 83.2% and 76.5% at pH ¼ 11 for initial uoride concentrations of 10, 20 and 30 mg L À1 , respectively. 36 Mohapatra et al. studied the uoride removal efficiency of Mg-doped nano Fe 2 O 3 adsorbent was almost 30% at pH ¼ 11 initial uoride concentration 10 mg L À1 . 37 Swain et al. demonstrated that the uoride removal efficiency of Fe(III)-Zr(IV) binary mixed oxide was about 38% at pH ¼ 11. 38 As shown in Fig. 4(c), uoride mainly existed in the forms of HF, F À , depending on the solution pH (pK ¼ 3.18). Although, some literatures 38,39 showed that in the acidic pH range (pH < 5), weak hydrouoric acid (HF) is present in the experiments may affect deuoridation, no obvious decrease of the uptake uoride was observed in this work. Thus, we assume that besides electrostatic attraction, hydrogen bonds might form in the process of uoride removal, as described by eqn (16)- (18). In the strongly alkaline range (pH > 11), there is a drop at around 25% in adsorption percentage, which may be due to the competition of hydroxyl ions with the uoride and the electrostatic repulsion from the surface of nFe 3 O 4 @TEPA.

Kinetic studies.
Adsorption kinetics of monocomponent uoride ion (F À ), phosphate or Cr(VI) onto nFe 3 O 4 @TEPA showed that all the three kinds of anions, the adsorption capacity increased rapidly and reached equilibrium in 10 min and intra-particle process might not be involved in the rate-limiting steps. The adsorption kinetic experimental data t the pseudo-second-order model well for all the studied anions. The activation energies of the adsorption process, E a , are found to 19.56 kJ mol À1 , 23.71 kJ mol À1 , 25.35 kJ mol À1 for uoride ion (F À ), phosphate or Cr(VI), respectively. All are less than 42 kJ mol À1 , indicating that diffusion process was the ratecontrolled step. 40 Detailed discussions were presented in ESI, S1 Kinetic studies and Fig. S1, Table S1 and S2. † 3.2.3 Adsorption capacity of nFe 3 O 4 @TEPA to anions. The adsorption capacities of nFe 3 O 4 @TEPA to uoride ion (F À ), phosphate or Cr(VI) mono-component were investigated. Detailed discussions were presented in ESI, S2 Adsorption capacity and Fig. S2, Table S3 and S4. † The results showed that the Langmuir models t the data well, suggesting a monolayer adsorption. The maximum adsorption capacities (q m ) for uoride ion (F À ), phosphate or Cr(VI) are 163.9, 149.3, and 400 mg L À1 , respectively. Interestingly, although the adsorption to all the studied anions the nFe 3 O 4 @TEPA was spontaneous in nature (DG q < 0), the enthalpy changes (DH q ) for the uoride ion (F À ), phosphate or Cr(VI) were found to be at 38.54, 13.89, 88.03 kJ mol À1 (Table S4 †), respectively, which indicated that the adsorption was endothermic. For physical adsorption, the process of adsorption is usually exothermic, that is, the increase of temperature is not favorable to the adsorption. However, chemisorption is some of endothermic, and some of exothermic. In general, it is thought that the increase in temperature is benecial to chemisorption. 41 Similar results were found in our previous work 13, 17 and in literature. 41 3.2.4 Effect of co-existing ions and presumed mechanism. The effect of co-existing ions experimental studies were investigated with a standard response surface methodology (RSM) design called Box-Behnken Design (BBD). RSM is a useful mathematical and statistical technique for the development of empirical relation between the experimental outputs (responses) and process parameters (factors). A well designed RSM approach leads to optimize the process parameters for improving the responses. The experimental parameters (X 1 (initial concentration of Cr(VI), C(Cr(VI))), X 2 (initial concentration of uoride ion (F À ), C(F)), and X 3 (initial concentration of phosphate, C(P)), for design of experiment strategy are considered at three levels and coded as À1, 0, and +1 for low, middle and high level respectively. The coded and actual values of the independent variables and predicted response of the model were shown in Table S5 and S6. † In the BBD modeling of three factors and three levels, the center point was repeated for ve times in order to improve the accuracy in estimation of errors. The response of the model was analyzed by analysis of variance (ANOVA) and a second-order polynomial model (as shown in eqn (19)) was tted to correlate between the independent variables (X 1 , X 2 and X 3 ) and the response for anions removal in order to predict the of co-existing ions effect.
where Y represents the predicted response variables i.e. the amount of anions adsorbed by nFe 3 O 4 @TEPA, K 0 is the constant coefficient, K i is the linear coefficient of the input factor X i , K ii is the ith quadratic coefficient of the input factors X i , K ij is the different interaction coefficients between input factors X i and X j , and 3 is the error of the model. The soware Design Expert (Version 8.0.6.1) was used for model statistic, like experimental design, determination of the coefficients, data analysis and the graph plotting. The adsorption capacities of nFe 3 O 4 @TEPA to uoride ion (F À ), phosphate or Cr(VI) multi-component solution was carried out by means of BBD of RSM. Quadratic model were used to know the adsorption capacity of the uoride ion (F À ), phosphate or Cr(VI), respectively, shown in ESI eqn (S6)-(S8). † The positive sign and the negative sign of the term indicates the synergetic and antagonistic effect respectively. The ANOVA data shown in Table S7-S9 of ESI † for Response 1 (q(Cr)), Response 2 (q(F)), and Response 2 (q(P)), respectively. The coefficient of determination (R 2 ), which measure the degree of t in the model was found to be 0.9956, 0.9923, 0.9920 and an Adj-R 2 of 0.9900, 0.9824, 0.9817, respectively. In addition, the model is very signicant as evident from its F-value and very low probability p-value. If the p-value is less than 0.05, it indicates that the model is statistically signicant whereas a value higher than 0.05 suggests that the model is not signicant. 42,43 Here, the Fvalues were found to be 177.03, 100.01, 96.19, respectively, and p-value were all < 0.0001 for the model.
Values of "Prob > F"less than 0.0500 indicate model terms are signicant. As shown in the ANOVA data in Table S7-S9, † for the adsorption of Cr(VI), the linear terms, C(Cr(VI)) and C(P) are signicant; while for adsorption of uoride ion (F À ), besides the linear terms, C(F), and C(P), quadratic terms of C(F), 2 and one cross-product coefficients C(Cr(VI))C(F) and C(F)C(P) are significant; for adsorption of phosphate, except the one cross-product coefficients C(F)C(P) is not signicant, all the other model terms are signicant.
In response surface plots, the adsorption of anions can be better explained by the interaction of all the three factors. The three dimensional plots and contour plots were used to know the effect of two parameters in their experimental range for the removal of anions while the third parameter remains at zero level. From the shape of contour plot, one could be able to explain the nature and extents of interaction between the experimental factors, i.e., the effects of the co-existing ions in the present work. Circular and elliptical shape of contour plots shows the signicant interaction between the experimental factors in the model. Therefore maximum adsorption capacity can be explain on the basics of these experimental factors, i.e., the effects of the co-existing ions, here. The effect of the coexisting uoride ion (F À ) and phosphate to the adsorption of Cr(VI) was shown in Fig. 5(a), while the effect of the co-existing Cr(VI) and phosphate to the adsorption of uoride ion was shown in Fig. 5(b) and the effect of the co-existing Cr(VI) and uoride ion to the adsorption of phosphate was shown in Fig. 5(c), respectively. Those of the bi-component systems were shown in the Fig. S3 in the ESI. † As shown in Fig. 5, for the tricomponent co-existing system, either uorine ion or phosphate, had little effect on the adsorption competition to Cr(VI). Cr(VI) took priority for adsorption and could replace the absorbed uorine ion or phosphate by competitive reaction. As shown in Fig. S3, † for the uorine ion and phosphate bi-component system, the adsorption of them was competitive via ion exchange.
The adsorption mechanism could be conrmed by XPS and FTIR analyses of nFe 3 O 4 @TEPA before and aer adsorption of the studied anions. The survey scan of XPS spectra of nFe 3 O 4 @TEPA before (a) and aer adsorption of phosphate (b) Cr(VI) (c), uoride ion (d) and the co-existing the three anions (e) were shown in Fig. 6. From the survey scan of XPS spectra (Fig. 6), new peaks owing to P2p, Cr2p and F1s appeared aer adsorption of phosphate (b), aer adsorption of Cr(VI) (c), aer adsorption of uoride ion (d) and aer adsorption of the coexisting the three anions (e), suggesting that the phosphate, Cr(VI) and uoride ion were successfully adsorbed on the surface of nFe 3 O 4 @TEPA. High-resolution XPS spectra of nFe 3 O 4 @TEPA aer adsorption of the co-existing the three anions were shown in Fig. S4. † As shown in Fig. S4(a), † the characteristic peaks for Cr(VI) (Cr2p 1/2, 587.5 eV; Cr2p 3/2, 579.4 eV) appeared, no obvious peaks assigned to Cr(III), normally appeared at Cr2p 1/2, 586.3 eV; Cr2p 3/2, 577.1 eV, were observed, 44,45 which implied that the main species existed in the surface of the nFe 3 O 4 @TEPA was Cr(VI), reduction to Cr(III) hardly occurred, differing from our previous founding. 12 The characteristic peaks for phosphate (P2p, 113.0 eV) and uoride (F1s, 685.2 eV) can be found in Fig. S4(b) and (c), † respectively, which clearly conrmed the successful adsorption of phosphate 46 and uoride. 47 As shown in Fig. S4(d), † aer adsorption, the peaks of N1s appeared at 398.8 eV with a broader band range, which could be attributed to protonated amine groups (-NH 3 + ) and the further formation of -NH 3 + /anions. 48 Similar phenomena were observed in the XPS spectra of O1s (Fig. 4(e)), peaks of O1s appeared at $531.1 eV and $529.5 eV, assigned to C-O-C and C-OH groups, broadening with a slight shi of binding energies. In the XPS spectra of C1s (Fig. 4(f)), the carbon atoms can be found in two chemically different positions, leading to two differing C1s binding energies: C-O-C ($282.6 eV) and C-O-C ($284.0 eV) or C-OH ($286.5 eV). Changes in atomic concentration of the key elements aer the adsorption were summarized in Table S10. † The main elements of the nFe 3 O 4 @TEPA were Fe, O, N and C. Compared with the initial pre-adsorbed material, the chromium, phosphate, and uoride atomic percent of the sample was 1.88%, 1.01% and 0.91% aer adsorption experiment. It conrmed that the studied anions were undoubtedly adsorbed onto the surface of nFe 3 O 4 @TEPA. The FTIR spectra of nFe 3 O 4 @TEPA before (a) and aer adsorption of phosphate (b), Cr(VI) (c), uoride ion (d) and the co-existing the three anions (e) were showed in Fig. 7. In Fig. 7(a), the broad peak appeared at $3360 cm À1 and $1573 cm À1 can be assigned to be the stretching and bending vibrations of the -NH and -NH 2 groups. While aer adsorption, in Fig. 7(b)-(e) the characteristic bands at $1573 cm À1 disappeared along with the appearance of the bands at $1630 cm À1 , which may be attributed to the interaction between amino groups and the phosphate, Cr(VI) and uoride groups, subsequently weakened the N-H bonding and resulted in a large shi ($80 cm À1 ). The characteristic peaks of Cr(VI) at $940 cm À1 and $760 cm À1 can be observed in the absorption of HCrO 4 À and the typical peak at $540 cm À1 for the "Cr-N" also appeared as shown in Fig. 7(b) and (e). The characteristic peaks of the phosphate groups at 543 cm À1 were also observed in Fig. 7(c) and (e), corresponding to the -P-O and -O-P-O groups, respectively. 3.2.5 Reusability investigation. The reusable of the nFe 3 -O 4 @TEPA was evaluated by comparing the average adsorption efficiency of a mixture solution of uoride ion (F À ), phosphate and Cr(VI) at each concentration at 20 mg L À1 . The postabsorbed nFe 3 O 4 @TEPA was extracted with 1% NaOH methanol solution for 30 min, and for another adsorption to get the next adsorption efficiency. The results were shown in Fig. 8, indicating that nFe 3 O 4 @TEPA could be used for at least 10 cycles with a loss of less than 2.8% upon recovery on average. No obvious decrease in the adsorption capacity efficiency was found. The VSM experiments of the recycled nFe 3 O 4 @TEPA  were tested. The saturation moments obtained from the hysteresis loops were found to be 48.0-47.6 emu g À1 from 1 cycle to 10 cycles, (as shown in Fig. 8 (insert)). Compared with the saturation moment of the fresh-prepared nFe 3 O 4 @TEPA (48.2 emu g À1 ), which implied that no obvious decrease reduction of the magnetic strength was found.

Conclusion
A tetraethylenepentamine (TEPA)-functionalized nano-Fe 3 O 4 magnetic composite materials (nFe 3 O 4 @TEPA) was synthesized by a facile one-pot solvothermal method. The as-prepared nFe 3 O 4 @TEPA exhibited a homogeneous morphology, strong affinity ability, and high magnetic responsiveness for the adsorption of ions. The adsorption of the multi-ion co-existing system was studied via batch tests, XPS and FTIR analyses, and analyzed via response surface methodology (RSM). The adsorption mechanism of multi-ion component system was intensively studied.

Conflicts of interest
There are no conicts to declare. This journal is © The Royal Society of Chemistry 2018