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Colloids and Surfaces A 604 (2020) 125279 Contents lists available at ScienceDirect Colloids and S...

Colloids and Surfaces A 604 (2020) 125279 Contents lists available at ScienceDirect Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa A magnetic ion exchange resin with high efficiency of removing Cr (VI) T a a a, b a a Yafeng Ren , Youhua Han , Xingfeng Lei *, Chuan Lu , Jin Liu , Guoxian Zhang , Baoliang Zhanga, Qiuyu Zhanga,* a School of Chemistry and Chemical Engineering, Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions of Ministry of Education, Northwestern Polytechnical University, Xi’an, Shaanxi, 710072, PR China b Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610213, China G R A P H I C A L A B S T R A C T A R T I C LE I N FO A B S T R A C T Keywords: The removal of chromium ions is a major concern for the metal-processing plant. In this study, the magnetic ion Magnetic material exchange resin (MR) was prepared given the function of the ion exchange and magnetic separation. The ad- Ion exchange resin sorption characteristics of Cr (VI) by magnetic ion exchange resin were investigated. The ion type of resin, pH of Adsorption and desorption solution, contacting time, initial Cr (VI) concentration and adsorption temperature were studied which showing Cr (VI) a great influence on the removal of Cr (VI). Consequently, the adsorption capacity could reach the maximum 197 mg/g under optimized conditions. Langmuir and Freundlich isotherm models were used to fit the equili- brium data at 298 K. The result demonstrated that the adsorption equilibrium could be well fitted by the Langmuir models. Besides the pseudo-first-order and second-order kinetics models were used to fit the ad- sorption kinetics, the adsorption behavior could be well fitted by the second-order kinetics model which in- dicated that the adsorption process was chemical adsorption. The thermodynamic parameters ΔH, ΔS and ΔG were calculated at 298 K. The negative ΔG and ΔH values indicated that the adsorption process of Cr (VI) removal by MR was spontaneous and endothermic. The results showed that MR could keep a stable adsorption capacity and magnetism. 1. Introduction was increasingly becoming a global problem which has caused serious pollution to soil and water, directly affecting the living conditions of With the development of industry, the pollution of heavy metal ions human beings. Chromium was one of the most toxic metals, in ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Lei), [email protected] (Q. Zhang). https://doi.org/10.1016/j.colsurfa.2020.125279 Received 18 May 2020; Received in revised form 28 June 2020; Accepted 7 July 2020 Available online 13 July 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved. Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 particular, the hexavalent chromium (Cr (VI)) is strongly toxic, carci- 2. Experimental section nogenic and teratogenic, and attacks the liver, kidney and lungs with the form of CrO42− or HCrO4-, therefore it is critical to reduce the 2.1. Materials concentrations of Cr (VI) to meet emission requirements. A wide range of methods has been developed to remove Cr (VI), such as chemical FeSO4·7H2O, FeCl3·6H2O, ammonia solution (25 %), potassium di- reduction and precipitation, adsorption, membrane separation, ion-ex- chromate were purchased from Sinopharm Chemical Reagent Co., Ltd, change method and electrodialysis [2,3]. 4-vinylbenzyl chloride (VBC), divinylbenzene were bought from Meryer Among the available treatment methods, chemical reduction and (Shanghai) Chemical Technology Co., Ltd. All the above chemical re- precipitation had been thought of the most effective technique for the agents were analytical pure. Water used throughout the work was ul- removal Cr (VI) from wastewater. In recent years electrochemical re- trapure produced by an apparatus for pharmaceutical purified water duction had been well developed: Fe3+ converted to Fe2+ under the (Aquapro Co. Ltd.). action of a DC electric field, and the Cr (VI) converted to Cr (III) then precipitated in the form of Cr(OH)3 correspondingly. Some dis- 2.2. Preparation of oleic acid bilayer-grafted Fe3O4 advantages existed in this system such as higher equipment costs, large consumption of chemical reagents; high volume of sludge generation All the magnetic nanoparticles studied were synthesized using. The activated carbon showing excellent adsorption performance chemical co-precipitation method in our laboratory. 1.99 g was the most commonly used in the field of wastewater treatment FeSO4·7H2O (0.007 mmol) and 3.87 g FeCl3·6H2O (0.014 mmol) were which have proved to have good adsorption properties as a result of its successively dissolved in 100 mL deionized water at 70 °C under con- large specific surface area from 500−1500 m2/g, complete internal tinuous nitrogen purging. Then NH3·H2O was slowly added into the pore structure and large number of carboxyl functional groups [6,7]. medium until pH 9 with vigorous stirring of 1000 rpm when heated up Generally, the adsorption process was simple with a variety of mate- to 80 °C and the medium was kept for 5 min. After that, 1.2 mL oleic rials, but most of them had problems such as low adsorption rate, low acid was added into the flask, and the medium was kept for another regeneration efficiency and short serve life in the performance of Cr 30 min. The reaction flask naturally cooled to room temperature. The (VI) removal. obtained Fe3O4 nanoparticles were washed several times to remove The essence of the ion-exchange method was the displacement re- impurities with ethanol and deionized water. The black product was action between exchangeable ions on ion exchange agents and other transferred into a 250 mL flask with 100 mL deionized water and then isoelectric ions in solution, which was a special adsorption process, 0.1 M HCl was added dropwise into the above solution till pH 6 under usually reversible chemical adsorption [9–11]. Therefore, the ion ex- vigorous stirring. The black substance was separated by a magnet and change seemed to be the most practical method to remove Cr (VI) in then washed several times with ethanol and deionized water. Finally, drinking water with its simplicity, efficiency and chemical stability. In the oleic acid bilayer-grafted Fe3O4 (OA-Fe3O4) was dried under va- recent years, the magnetic separation had been widely developed in cuum at 40 °C for 4 h. The schematic diagram of the preparation of pure biochemistry, environmental purification, cell biology, etc., which had Fe3O4 to OA-Fe3O4 was shown in Fig. 1 (a). many advantages such as faster separation, shorter processing time, larger capacity, easier operation and convenient recovery. The 2.3. Preparation of magnetic ion exchange resin combination of adsorption method and magnetic separation method had been widely used in wastewater treatment with heavy metals and 40 g 4-vinylbenzyl chloride (VBC), 8 g crosslinker divinylbenzene mineral processing, which could effectively remove metal ions and (DVB), 0.5 g initiator azobisisobutyronitrile (AIBN) and OA-Fe3O4 were organics. MIEX produced by Australia was an acrylic anionic resin mixed to form a homogeneous black oil phase. The oil phase was added with quaternary ammonium functional group and exchangeable to the water phase with 7 g gelatin and 700 mL deionized water. The chloride ions. It could be exchanged with other negatively charged suspension polymerization reacted for 8 h at 75 °C and the magnetic ions in water, then undergo solid-liquid separation by magnetic field, particles ripened for 2 h after heated up to 80 °C. which could remove the pollutants in the water mainly due to the 5 g magnetic particles prepared above were mixed with trimethy- smaller particle size (150−180 μm), high specific surface area. How- lamine aqueous solution of 4 times the resin mass after swelling by the ever there were still some disadvantages during current usage, such as solvent. The quaternization reaction was carried out for 24 h. The low exchange capacity (2.55 mmol/g), poor strength, easy to cause magnetic ion exchange resin with exchangeable Cl− (MR-Cl) was ob- secondary pollution and loss of resin mass. The NDMP was another tained after dried under vacuum at 60 °C for 24 h. MR-SO4 and MR−OH type of magnetic ion exchange resin with glycidyl methacrylate ske- could be obtained by treating MR-Cl with Na2SO4 (aq) and NaOH (aq) leton created showed excellent adsorption capacity about organics respectively.. Compared with acrylic and glycidyl methacrylate, the good ri- gidity of styrene could give the resin more stability and regeneration, 2.4. Adsorption studies while there were few reports about the magnetic ion-exchange resin of styrene skeleton at present. The adsorption experiments were mainly performed by the ad- In this study, a new type of magnetic ion-exchange resin with high sorption capacity of chromium ions referred to China's national stan- strength and adsorption capacity was prepared by the following pro- dard Water Quality-Determination of total chromium (GB 7466−87). cess: (1) magnetic nanoparticles were prepared by co-precipitation For equilibrium experiments, the adsorbent mass was fixed at 0.01 g method, then modified by double-layer oleic acid to improve Fe3O4 unless otherwise stated. The experiments were carried out by con- nanoparticles lipophilicity; (2) oleic acid-grafted Fe3O4 was added in tacting MR adsorbent with a 25 mL Cr (VI) sample solution in 50 mL the suspension polymerization system to prepare magnetic polymer conical flask in a thermostatic shaker agitated at 120 rpm for 4 h. At the particles; (3) magnetic ion exchange resin with exchangeable chloride end of each experiment, samples were filtered through a 0.45 μm mi- ions was prepared by quaternary ammonium reaction using trimethy- croporous membrane filter, then 2.5 mL filtrate was taken by pipette to lamine after solvent swelling; (4) The morphology, structure and ad- dilute to 50 mL, and a complexing reagent 1,5-diphenyl carbazide, sorption performance of Cr (VI) were characterized. 0.5 mL 1:1 H2SO4 and 0.5 mL 1:1 H3PO4 were subjected to add into the above solution. UV–vis spectrophotometer (BlueStar, LabTech) oper- ated at a wavelength of 540 nm was employed to determine the left concentration of Cr (VI) ions in the samples. The effects of pH (pH 1–9), initial concentration of Cr (VI) (c0 20, 40, 60, 80, 100, 120 mg/L) and 2 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 1. Schematic illustration of (a) the formation process for OA-Fe3O4 from pure Fe3O4. (b) adsorption–desorption-regeneration cycles. temperature (T 25, 35, 45, 55 °C) on Cr (VI) adsorption capacity were solution to reach adsorption saturation state. The initial adsorption studied. And the adsorption kinetics and isothermal adsorption curve capacity was measured and saturated brine was used to treat the MR-Cl were explored. Standard acid 0.1 M HCl and base 0.1 M NaOH solutions for another 6 h to desorption absolutely after that. The resin was wa- were used for pH adjustment. shed with distilled water until no residual chlorine ion existed in the The experiments of the kinetic study of Cr (VI) adsorption were solution detected by silver nitrate solution and dried in dynamic va- performed with initial Cr (VI) concentrations of 150 mg/L at different cuum at 60 °C. In this study six consecutive adsorption-desorption-re- intervals at 25 °C. The Cr (VI) amount adsorbed by a unit mass of ad- generation experiments were carried out at room temperature. sorbent at the various time was calculated by Eq. (1) : (c0 − ct ) V 2.6. Characterization methods of magnetic nanoparticles and magnetic qt = resin (MR) m (1) where qt was the time-dependent amount of Cr (VI) adsorbed per unit Powder X-ray diffraction (XRD) patterns were acquired on a mass of adsorbent (mg/g), ct was the bulk-phase Cr (VI) concentration Shimadzu XRD-7000 s diffraction instrument with Cu Kα radiation (λ (mg/L) at any time t, V was the volume of sample (L) and m was the 1.542 Å) over the scan range 10°-80°. The hysteresis loops of samples adsorbent mass (g). In this study, c0 150 mg/L was used to be a uniform were characterized by a vibrating sample magnetometer (VSM, initial Cr (VI) concentration. LakeShore 7307) at room temperature. The surface morphologies of The effects of c0 on Cr (VI) adsorption capacity were studied with m samples were observed by a field emission scanning electron micro- 0.01 g, t 4 h, V 25 mL and T 25 °C. The Cr (VI) percentage removal scope (FESEM, ZEISS EVO 18 Research) with an accelerating voltage of (adsorption efficiency) was determined using Eq. (2) : 15 kV. The transmission electron microscope (TEM) was obtained by c 0 − ce using a JEOL JEM-2010 transmission electron microscope with an ac- R (%) = × 100% celerating voltage of 300 kV. The Cr (VI) concentration was analyzed c0 (2) using a UV–vis spectrophotometer (BlueStar, LabTech). The scheme of where c0 and ce was the initial Cr (VI) concentration (mg/L) and the adsorption Cr (VI) was test by a X-ray Photoelectron Spectroscopy (XPS, equilibrium Cr (VI) concentration (mg/L), respectively. Kratos AXIS Ultra DLD). The schematic diagram of the ad- The adsorption isotherm curves were generated by performing ex- sorption–desorption-regeneration cycles of MR was shown in Fig. 1 (b). periments with different c0 at T 25 °C, t 4 h. The amount of Cr (VI) adsorbed by a unit mass of adsorbent at equilibrium was calculated by 3. Results and discussion Eq. (3) : (c0 − ce ) V 3.1. OA-Fe3O4 characterization qe = m (3) The surface morphology of pure Fe3O4 and OA-Fe3O4 was observed where qe was the amount of Cr (VI) adsorbed per unit mass of adsorbent by TEM in Fig. 2 (a, b). The comparation showed that oleic acid was the at equilibrium (mg/g), c0 and ce was the initial Cr (VI) concentration coated on the surface of Fe3O4 nanoparticles from the microscopic (mg/L) and the equilibrium Cr (VI) concentration (mg/L), respectively, structure like a core-shell. The digital photograph Fig. 2 (c) was OA- V was the volume of sample (L) and m was the adsorbent mass (g). Fe3O4 dispersed in 1,2-dichloroethane solvent and water. Fig. 2 (d) was OA-Fe3O4 dispersed in a hexane solvent and water. The oil-water in- 2.5. Regeneration studies terface was clear and no settlement was found in the two oil/water system after 100 h, which directly indicated that OA-Fe3O4 had lipo- The 0.01 g MR-Cl was treated for 6 h soaked at 150 mg/L Cr (VI) philicity performance. 3 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 2. (a) (b) TEM image of pure Fe3O4 and OA-Fe3O4. (c) digital photos of water and 1,2-dichloroethane dispersed with OA-Fe3O4. (d) digital photos of water and hexane dispersed with OA-Fe3O4. Fig. 3. (a) XRD pattern of OA-Fe3O4, (b) TGA curve of OA-Fe3O4. Fig. 3 (a) showed the XRD pattern of OA-Fe3O4. These characteristic The mass profile exhibited three well defined decreasing steps. The first peaks of OA-Fe3O4 were observed at Bragg angles of 18.3°, 30.1°, 35.4°, mass loss was about 4.8 %, and correspondingly, the inflection tem- 43.1°, 53.4°, 56.9°, 62.5°, 70.9° and 75.0°. These diffraction peaks were perature was 279 °C. The second mass loss was 8.8 %, and a higher assigned to the reflection of (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 inflection temperature was 486 °C. Apparently, for bilayer surfactant- 1), (4 4 0), (6 2 0) and (5 3 3) planes of the face-centered cubic (FCC) coated particles, the weight loss for the first and second steps should be lattice of Fe3O4 respectively, which was consistent with the database in attributed to quantitative mass losses of the outer and inner layers of JCPDS file (PDF No. 19-0629). The XRD pattern further indicated that the coating oleic acid, respectively. Different degradation temperatures the crystalline structure OA-Fe3O4 was not affected by oleic acid, as a could be explained by the different forces of the outer and inner layers. result of no crystalline structure in oleic acid. Fig. 3 (b) presented ty- The third weight loss between 490 and 750 °C may result from the re- pical TGA curves for magnetic particles coated with bilayer oleic acid. ducing gases produced by the degradation of oleic acid. 4 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 4. (a) M−H curve at room temperature for pure Fe3O4, OA-Fe3O4 and MR. 3.2. Characterization of magnetic ion exchange resin (MR) maximum at pH 3. As pH decreasing further forming a strong acid environment, more HCrO4− and CrO42− shifted to Cr2O72−. The de- We performed the magnetic property of pure Fe3O4, OA-Fe3O4 and crease of adsorption capacity was observed as the increase of con- MR. As Fig. 4 (a) shows, the M−H curves of materials were super- centration of Cr2O72− because Cl− exchanged with Cr2O72− with 2:1 M paramagnetic because of zero remanences and coercivity. A hysteresis ratio and the strong competition for adsorption sites between Cr2O72− loop with a coercivity field (Hc) and a saturation magnetization (Ms) of and protons. The same mechanism could be applied to alkaline solution pure Fe3O4 and OA-Fe3O4 were observed. The OA-Fe3O4 could be that Cl− exchanged with CrO42− with 2:1 M ratio and the strong quickly separated in 30 s by an external magnetic field when dispersed competition for adsorption sites between Cr2O72− and OH−. in n-hexane, which could be proved by Fig. 4 (b, c). Fig. 4 (d) was an optical microscope and TEM figures of the MR, where the particles were 3.4. Adsorption kinetic studies full and well-formed. There was no impurity and breakage, which were caused by excellent polymer strength. The saturation magnetization To investigate the adsorption kinetics of Cr (VI), three kinds of resin 4 emu/g of MR was the lowest of three samples. This was caused by the MR-Cl, MR-SO4 and MR−OH were studied for 360 min and the ad- collective effect of other substances OA and polymer framework which sorption capacity of Cr (VI) was analyzed at different contact intervals occupied a certain mass. While the saturation magnetization of MR was as shown in Fig. 5 (b). The adsorption capacity significantly increased lower, they could be quickly separated and collected for recycling by an in the first 60 min then grew slowly. Three types of resins achieved external magnetic field in 30 s which were shown from Fig. 4 (e, f). adsorption equilibrium at 250 min. The adsorption capacities of MR-Cl, (b), (c) photo of OA-Fe3O4 dispersed in n-hexane with and without MR-SO4 and MR−OH were in order from high to low. As for fixed an external magnetic field adsorption sites, the exchangeable amount of SO42− for Cr (VI) was half (d) optical microscope picture of MR of the Cl- because of their different number of charges. In the previous (e), (f) photo of MR soaked deionized water with and without an section, the acidic solution was optimized to the maximum adsorption external magnetic field but the acidic environment would inevitably consume the OH- in the resin causing the number of exchangeable decreased. 3.3. Effect of solution pH on adsorption Cr (VI) The mechanism of Cr (VI) adsorption onto the adsorbent could be studied by using the pseudo-first-order and pseudo-second-order The effects of pH within the range of pH 1–9 on adsorption capacity models to fit the kinetics data. The pseudo-first-order and pseudo- of MR were shown in Fig. 5 (a). With the pH increasing, the adsorption second-order kinetics equations were represented Eq. (5), Eq. (6) capacity of the MR-Cl for Cr (VI) reached up to 197 mg/g at pH 3. There as follows: were three main forms of Cr (VI) in aqueous solution: Cr2O72−, HCrO4- k1 log(qe − qt ) = logqe − t and CrO42− which could convert to each other at different pH, and the 2.303 (5) equation Eq. (4) was as follows : t 1 t = + Cr2 O72 − + H2 O ⇋ 2HCrO4− ⇋ 2H+ + 2CrO42 − (4) qt k2 qe2 qe (6) The protons caused reverse reaction increasing the concentration of where qe (mg/g) and qt (mg/g) were the adsorption capacities of Cr (VI) HCrO4− when the solution was weak acidic. As for MR-Cl, the Cl− in ions at equilibrium and time t, respectively. And k1 and k2 were rate the resin was exchanged with HCrO4− with equal molar ratio as a result constants of pseudo-first-order (min−1) and pseudo-second-order (g/ of the same number of charges, so adsorption capacity reached a (mg min). The linear fitting of experimental adsorption data in the 5 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 5. (a) the influence of initial pH value on the adsorption on MR-Cl. Temp 25 °C, c0 100 mg/L; (b) effect of adsorption time on the adsorption for different MR. Temp 25 °C, pH 3. Fig. 6. (a) (b) (c) linear fitting of Pseudo-first-order model of MR-Cl, MR-SO4 and MR−OH. (d) (e) (f) linear fitting of Pseudo-second-order model of MR-Cl, MR-SO4 and MR−OH. Table 1 3.5. Effect of initial solution concentration of Cr (VI) on adsorption Kinetic parameters for Cr (VI) adsorption to three kinds of MR. behavior Magnetic Pseudo-first-order model Pseudo-second-order model Resin The efficiency of Cr (VI) adsorption was affected by the initial metal −1 2 qe (mg k1 (min ) R qe (mg k2 (g R2 ion concentration. The adsorption efficiency and adsorption capacity g−1) g−1) mg−1 min−1) were tested at different initial concentration of Cr (VI) (c0 20, 40, 60, MR-Cl 196.31 0.051 0.867 204.31 5.02 × 10−5 0.999 80, 100, 120 mg/L). The test results were shown in Fig. 7 (a, b) below. MR-SO4 175.08 0.018 0.919 204.02 1.03 × 10−4 0.999 At low concentration, as for a fixed resin dosage, the total number of MR-OH 156.39 0.024 0.973 177.62 1.81 × 10−5 0.999 adsorption sites were limited, which resulted in adsorption efficiency decreasing as the concentration increasing. The adsorption capacity was increasing continuously with the c0 getting bigger before reaching pseudo-first-order kinetic (log(qe−qt) vs t) and pseudo-second-order the saturation adsorption capacity. In this study, the equilibrium ad- kinetic (t/qt vs t) equations were shown in Fig. 6 respectively. The data sorption capacity obtained was 197, 175, 162 mg/g corresponding to obtained by fitting was shown in the following Table 1. The high cor- the MR-Cl, MR-SO4 and MR−OH, respectively (Fig. 7). relation coefficient values (R2 values of 0.999, 0.999, 0.999) was cal- culated from pseudo-second-order kinetic model, which indicated that the adsorption kinetics of resin was controlled by the model. And the 3.6. Adsorption isotherms adsorption capacity at equilibrium qe calculated from pseudo-second- order kinetic model while the calculated theoretical values were higher To describe adsorption processes and mechanisms involved in this than the experimental value. The MR adsorption kinetics of Cr (Ⅵ) Cr (VI) removal studies, different adsorption isotherm models have could be described better by pseudo-second-order kinetic model. been used. The Equilibrium adsorption capacity was obtained at dif- ferent initial concentration of Cr (VI) from 20 mg/L to 120 mg/L at 6 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 7. (a) (b) Effect of initial concentration on adsorption efficiency and equilibrium capacity. 25 °C. Table 2 Langmuir was a monolayer adsorption model which was assumed Isotherm parameters for Cr (VI) adsorption to three kinds of MR. that the solid surface was uniform and there was no interaction between Magnetic Langmuir model Freundlich model the adsorbed molecules , while the Freundlich model was an em- Resin 1 2 pirical adsorption model whose surface was non-uniform. The qm (mg b (L/mg ) R k n R2 equations were represented as Eq. (7), Eq. (8) follows: g−1) ce c 1 MR-Cl 191.82 21.37 0.999 2.06 × 10−22 0.0968 0.742 = e + MR-SO4 173.85 0.6539 0.988 0.01 0.6746 0.589 qe qm bqm (7) MR-OH 160.85 0.3552 0.964 2.73 × 10−5 0.3479 0.884 qe = kce1/ n (8) model, the adsorption process was monolayer adsorption. In addition, where qe (mg/g) and ce (mg/L) were adsorption capacity and residual the maximum adsorption capacity qm (191.82, 173.85 and 160.85 mg/ Cr (VI) concentration at equilibrium. b (L/mg) was the Langmuir con- g) calculated by Langmuir model was closed to the real experiment stant related to the adsorption energy, and the qm was the maximum value (196.31, 178.70 and 158.90 mg/g). This indicated that MR has adsorption capacity. n and k were Freundlich constant corresponding to great potential as an adsorbent for the removal of Cr (VI) because of its the intensity and capacity of adsorption. The related equilibrium high adsorption capacity. parameters for Langmuir and Freundlich isotherms models were de- termined from the corresponding linear fitting of ce/qe vs. ce and ln qe vs. ln ce shown in Fig. 8 respectively. The results are listed in Table 2. 3.7. Effect of temperature on adsorption Cr (VI) It could be seen that the adsorption isotherm data fitted the Langmuir model better than the other model as a result of the higher The effect of temperature on adsorption Cr (VI) was studied at dif- correlation coefficient values (R2 values of 0.999, 0.988, 0.964) than ferent adsorption temperature (25, 35, 45, 55 °C). As Fig. 9 (a) shows, the Freundlich model. Based on the assumptions of the Langmuir the equilibrium adsorption capacity decreased gradually with the Fig. 8. (a) (b) (c) linear fitting of Langmuir isotherm model of MR-Cl, MR-SO4 and MR−OH. (d) (e) (f) linear fitting of Freundlich isotherm model of MR-Cl, MR-SO4 and MR−OH. 7 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 9. (a) effect of temperature on equilibrium adsorption capacity. (b) the thermodynamic plot of log (qe/ce) and 1000/T. temperature increasing, which indicated that low temperature was 3.9. Cr (VI) adsorption mechanism of MR conducive to adsorption. The experimental data obtained at different temperatures were used in calculating the thermodynamic parameters To further analyze the Cr (VI) removal mechanism by MR, XPS was such as Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) ac- used to examine the elemental composition for the removal of Cr (VI). cording to the following Eq. (9), Eq. (10): As shown in Fig. 11 (a), six major peaks at binding energies of 983.69, 527.25, 399, 282.25, 268 and 195 eV, representing O KLL, O 1s, N 1s, qe ΔH ΔS C1s, Cl 2s and Cl 2p respectively were observed for the virgin sorbent logK = log =− + ce 2.303RT 2.303R (9) MR. Significant changes can be seen in this figure after 4 h for Cr (VI) adsorption, confirming that Cr (VI) was adsorbed successfully onto the ΔG = ΔH − T ΔS (10) material. When completing regeneration after 6 cycles, XPS peaks were almost same with virgin sorbent, further proved that the MR has good where K was the equilibrium constant and R was the universal gas recycling performance. The Cr high resolution spectra were fitted in constant. The qe (mg/g) and ce (mg/L) were adsorption capacity and Fig. 11 (b); the peaks at 586.4 and 577.0 eV represent Cr2p3/2 and Cr2p1/ residual Cr (VI) concentration at equilibrium ΔH and ΔS could be ob- 2 orbitals respectively which were assigned to Cr (VI). All the tained from the slope and intercept of the line obtained by the linear above XPS data showed good adsorption efficiency of the sorbent MR. fitting of log (qe/ce) and 1000/T. The change of ions within a cycle (taking HCrO4− as a target ion) was The linear fitting of the data of the three resins (Fig. 9 b) illustrates shown in Fig. 11 (c). It can be seen that the adsorption and desorption the thermodynamic parameters of the three resins fit well from the high were attributed to anion exchange process which were determined by correlation coefficient. According to the thermodynamic parameters the solution environment outside the MR. Table 4 depicted the values of calculated at 298 K at Table 3, the ΔG and ΔH of all MR were negative some other adsorbent capacities for Cr(VI) reported in the literature. values illustrating the adsorption process of Cr (VI) was spontaneous We can clearly see that the adsorption capacity of 197 mg/g at ambient and an exothermic process and the adsorption of Cr (VI) was more ef- temperature is among the highest recorded values. fective at lower temperatures. 4. Conclusions 3.8. Regeneration study In this work, we prepared the pure Fe3O4, OA-Fe3O4 and MR, and characterized the morphology, magnetic property, crystal structure. In this study, the MR-Cl was selected to study the regeneration and Adsorption experiment for removal of Cr (VI) was studied showing that circulation because it had a high adsorption capacity. Six consecutive MR could be used as an effective adsorbent for Cr (VI) removal from adsorption-desorption-regeneration experiments were carried out at aqueous solutions. The MR had a high adsorption at pH 3. The ad- room temperature (Fig. 10 (a)) with an adsorption capacity loss of 7.11 sorption process occurred rapidly within the first 60 min then nearly % (197 to 183 mg/L). Fig. 10 (b) was MeH curve of no-adsorption and attained equilibrium at 240 min. The maximum adsorption capacity after 6 adsorption–desorption-regeneration cycles of MR which showed could reach 197 mg/g when the initial concentration was 100 mg/L, that the saturation magnetization (Ms) of MR declined from 4.0 to adsorption temperature was 298 K and the time was 4 h. The Freundlich 3.8 emu/g. This was mainly because the polymer crosslinked structure isotherm was more effective to describe the removal equilibrium. of the magnetic ion exchange resin protected the magnetic nano- Adsorption process could be fitted well by the pseudo-second-order particles and alkaline groups inside the resin, effectively increased the kinetics model. The thermodynamic parameters indicated that the ad- service life of the resin, and gave it a stable adsorption capacity and sorption process was spontaneous and exothermic. After 6 adsorption- magnetic responsiveness. desorption-regeneration cycles, the MR could keep a stable adsorption capacity and magnetic responsiveness. The MR was demonstrated to be a good adsorbent to remove Cr (VI) in water solution. The main work in Table 3 the future is to investigate the removal performance of magnetic resins Thermodynamic data of Cr (VI) adsorption process at 298 K. on other heavy metal ions and organic pollutants and study its serve life Magnetic Resin ΔG (kJ mol−1) ΔH (kJ mol−1) ΔS (J mol−1 K−1) further under different conditions. MR-Cl −5.64 −26.61 −70.37 CRediT authorship contribution statement MR-SO4 −4.41 −24.01 −65.76 MR-OH −1.78 −23.96 −67.87 Yafeng Ren: Conceptualization, Methodology, Formal analysis, 8 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 Fig. 10. (a) capacity of MR-Cl for 6 adsorption–desorption-regeneration cycles. (b) M−H curve of no-adsorption and after 6 adsorption–desorption-regeneration cycles of MR. Fig. 11. (a) wide scan of XPS spectra before and after adsorption and regeneration after 6 cycles. (b) narrows scan of XPS spectra of Cr2p after 4 h adsorption. (c) the change of ions within a cycle (taking HCrO4− as a target ion). Writing - original draft. Youhua Han: Investigation. Xingfeng Lei: Declaration of Competing Interest Resources. Chuan Lu: Resources. Jin Liu: Data curation. Guoxian Zhang: Software. Baoliang Zhang: Resources. Qiuyu Zhang: Funding The authors declared that they have no conflicts of interest to this acquisition, Supervision. work. We declare that we do not have any commercial or associative Table 4 The maximum adsorption capacity of Cr(VI) compared with some reported works. Adsorbent Removal scheme Qmax (mg/g) Ref natural peach gum polysaccharide (PGP) with multiple amine groups electrostatic adsorption & reduction by amino 188.32 corncob biochar decorated Anion exchange & reduction by amine and 19.23 with polypyrrole hydroxyl porous carbon-encapsulated iron electrostatic adsorption & reduction by iron 2.5 magnetic greigite/biochar composites electrostatic adsorption & reduction by iron 23.25 hexametaphosphate intercalated green rust reduction by iron 92.25 PPy/Fe3O4 magnetic nanocomposite electrostatic adsorption 208.77 poly([2-(methacryloxy)ethyl]trimethylammonium chloride) modified magnetic chitosan electrostatic adsorption & anion exchange 153.85 particles Magnetic anion exchange resin electrostatic adsorption & anion exchange 197 This work 9 Y. Ren, et al. Colloids and Surfaces A 604 (2020) 125279 interest that represents a conflict of interest in connection with the Physicochem. Eng. Asp. 579 (2019) 123685. work submitted U.O. Aigbe, R. Das, W.H. 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