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Facile synthesis of tea waste/Fe3O4 nanoparticle composite for hexavalent chromium removal from aqueous solution

The modification of biomass waste, as a multifunctional composite, has received tremendous attention for resource utilization and recycling. In this study, tea waste, which is a high level generator of biomass waste, was loaded with nano-Fe3O4 particles to prepare a magnetic tea waste/Fe3O4 (TW/Fe3O4) composite through a facile chemical co-precipitation approach. BET, SEM, TEM, XRD, magnetic properties, FTIR, XPS were used to characterize the TW/Fe3O4 composite. A superparamagnetic TW/Fe3O4 composite (Fe3O4: about 20 nm) was successfully synthesized and possessed the advantages of tea waste and nano-Fe3O4 particles. A chromium(VI) adsorption experiment showed that this material has a strong adsorption capacity for aqueous chromium ions, which reached 75.76 mg g−1 based on the Langmuir model. The adsorption process could be well fitted by a pseudo-second-order kinetic model and Langmuir, Temkin and Dubinin–Radushkevich (D–R) isotherm models, and was spontaneous and endothermic according to the thermodynamic analysis. The TW/Fe3O4 composite revealed good reusability and the removal rate was more than 70% after five recycling cycles. The mechanism of Cr(VI) removal involved electrostatic attraction, reduction process, ion exchange, surface complexation, etc. 70% of Cr(VI) was reduced to Cr(III) in this investigation. This study indicated that a TW/Fe3O4 composite could be an attractive option for heavy metal treatment.


Introduction
Chromium is one of the most worrying pollutants in the environment. The primary sources of chromium include the electroplating, cement, fertilizer, textile and steel industries. 1 The speciation of chromium determines its environmental behavior. The main valences of chromium are Cr(VI) and Cr(III). The mobility and solubility of Cr(VI) are higher in aqueous solution. Cr(VI) may exert a toxic effect on biological systems. 2 Hence, removal of Cr(VI) from the water system is crucial.
Many approaches, including chemical precipitation, ionexchange, electrochemical, reduction, membrane ltration, and adsorption, 3,4 have been proposed for the removal of Cr(VI) from aqueous solution. Due to the advantages of convenient operation, lower cost, and higher efficiency, the adsorption method has been widely applied. Currently, adsorbents derived from biomass or modied biomass have drawn great attention. [5][6][7][8] With the development of the tea industry, the amount of tea waste has also increased. Tea waste could act as a good absorbent for heavy metal removal from wastewater because of its abundant organic functional groups and cellulose-based pore structure. The removal of metals by tea waste has been reported in previous literature, including Cu 2+ , [9][10][11][12] Cd 2+ , [10][11][12] Zn 2+ , 11,13 Ni 2+ , [13][14][15] and Pb 2+ . 9,[11][12][13] Meanwhile, the removal of Cr(VI) by tea waste has also been investigated in previous studies. 16,17 However, it is difficult to separate powdered adsorbent from an aqueous solution for recycling and the adsorption capacity of biomass for heavy metals is limited. The introduction of magnetic nano-particles into the adsorbent could solve this problem. Chen et al. 18 found that loading of Fe 3 O 4 nanoparticles into the composite could improve the magnetic and adsorption properties of raw composite. Nano-Fe 3 O 4 material had been widely used in wastewater treatment based on its superparamagnetic properties, high surface area, easy separation, lower toxicity, etc. 19,20 Meanwhile, the loading of nano-Fe 3 O 4 onto a biomass adsorbent could also enhance the dispersibility and stability of nano-Fe 3 O 4 particles. 21 A preliminary investigation into a tea waste/nano particle composite was also carried out. The preparation of a tea waste/ nano Fe 3 O 4 composite has been used to remove Ni(II) from aqueous solution. 22 Babaev et al. 23 investigated Cr(VI) removal by spent tea-supported magnetite nano-particles from aqueous solutions. The preparation method was co-precipitation (FeCl 2 -$4H 2 O + NaOH) and the maximum adsorption capacity was 30 mg g À1 . However, most studies focused on the adsorption thermodynamics or kinetics, and investigation of the adsorption capacity of a tea waste/nano particle composite was limited. The removal of Cr(VI) by tea waste/nano Fe 3 O 4 has rarely been reported, especially the adsorption mechanism and potential recycling.
Therefore, tea waste was chosen as the matrix for the synthesis of a composite. Nano-Fe 3 O 4 particles were loaded into the tea waste by a chemical co-precipitation method to prepare a magnetic tea waste/Fe 3 O 4 composite. The composite was applied to remove Cr(VI) from aqueous solution. The purposes of this study are (1) to investigate the effect of proper parameters, kinetics, and equilibrium on Cr(VI) adsorption on a tea waste/Fe 3 O 4 composite; (2) to explore the interaction mechanism between Cr(VI) and the tea waste/Fe 3 O 4 composite; (3) to assess the potential recycling of the tea waste/Fe 3 O 4 composite acting as Cr(VI) absorbent. Our results showed that a tea waste/ Fe 3 O 4 composite can be facilely prepared and could be used to remove Cr(VI) from aqueous solution.

Synthesis of TW/Fe 3 O 4 composite
Tea waste was collected from a tea factory in Anhui province, China. The pretreatment procedure of tea waste was described in a previous ref. 22. The chemical co-precipitation method had been used to prepare nano-particles with a homogeneous composition and narrow size distribution. 24 This method was facile and widely used to synthesize nano-particles. The product was called a TW/ Fe 3 O 4 composite (TW: tea waste) and the reactions are as shown in eqn (1) and (2). To obtain nano-Fe 3 O 4 particles, 5.2 g of FeSO 4 $7H 2 O and 7.4 g of FeCl 3 $6H 2 O were put in a three-mouth ask and 80 mL of distilled water was added to dissolve the iron compound with vigorous stirring. When the solution was heated to 80 C, 20 mL of ammonium hydroxide (25%) was added. Then, 10 g of tea waste was added into the solution and the reaction proceeded for 30 min at 80 C under constant stirring. The whole process was conducted under the protection of N 2 gas. The reaction product was cooled down to room temperature and then repeatedly washed with distilled water and absolute ethyl alcohol to remove the unreacted chemicals. An external magnet was used to separate the product from the solution. The prepared product was dried in a vacuum oven at 60 C for 10 h.
All the chemicals used were of analytical grade and were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. China. The experiments were determined in triplicate and the average value is shown.

Batch adsorption experiments
The Cr(VI) stock solution was prepared by dissolving 0.2829 g of K 2 Cr 2 O 7 in 1000 mL of distilled water. The effect of pH, adsorbent dosage, contact time, and temperature on Cr(VI) removal was examined through batch adsorption studies.
An amount of TW/Fe 3 O 4 composite (0.04 g) and 40 mL of Cr(VI) solution of 40 mg L À1 were mixed in a 50 mL centrifugal tube. The solution pH was adjusted to 2-8 with 0.1 M HCl or 0.1 M NaOH solutions (pH effect). A varying mass of TW/Fe 3 O 4 composite and 40 mL of Cr(VI) solution of 25 mg L À1 were added to a 50 mL centrifugal tube to give a solid/liquid ratio in the range of 0.1-3 g L À1 (dosage effect). The experiment was conducted in an oscillator with the conditions of temperature 298 K, contact time 12 h and an agitation rate of 180 rpm. When the adsorption reached equilibrium, the suspended solution was separated using an external magnet and was passed through a 0.45 mm lter member. The concentration of Cr(VI) in the supernatant was determined by measuring the absorbance of the purple complex of Cr(VI) with 1,5-diphenylcarbazide at 540 nm by a spectrophotometer (722S, Shanghai Precision, Shanghai, China). The removal rate and adsorption amount were calculated according to formulas (3) and (4).
where, C 0 and C e are the initial and equilibrium concentrations of Cr(VI) (mg L À1 ), respectively. Herein, q e is the adsorption amount at equilibrium (mg g À1 ) and V is the volume of Cr(VI) solution (L); m is the mass of the TW/Fe 3 O 4 composite (g). Adsorption kinetics. The Cr(VI) solution and TW/Fe 3 O 4 composite were mixed in a 100 mL Erlenmeyer ask. The pH of the solution was adjusted to 2 using 0.1 M HCl or 0.1 M NaOH solutions. The agitation speed of the oscillator was 180 rpm. The samples were collected at interval times from 0 to 600 min. The adsorption amount was calculated according to formula (5). The kinetic temperature was set at 25 C, 35 C and 45 C. The initial concentrations of Cr(VI) were 10 mg L À1 , 30 mg L À1 , and 50 mg L À1 .
where, q t is the adsorption amount (mg g À1 ) at time t (min). C t is the concentration of Cr(VI) at time t (mg L À1 ). Adsorption isotherm. An amount of TW/Fe 3 O 4 composite (0.04 g) and 40 mL of Cr(VI) solution were mixed in a 50 mL centrifugal tube. The concentration of Cr(VI) solution was set at 10-60 mg L À1 . The pH of the solution was adjusted to 2 and the agitation speed of the oscillator was 180 rpm. The contact time was 10 h. The temperature of the isotherm was set at 25 C, 35 C and 45 C.

Characterization
The textural properties of tea waste and TW/Fe 3 O 4 composite were analyzed with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) at 77 K under N 2 atmosphere. The surface area, pore volume and average pore size were calculated using the nitrogen adsorption isotherms. The multipoint Brunauer-Emmett-Teller method was employed to calculate the surface area. The pore volume and average pore size were obtained from desorption isotherms using the Barrett-Joyner-Halenda method. The surface morphology of the TW/Fe 3 O 4 composite was characterized by a Scanning Electron Microscope (SEM, Sirion 200, FEI company, USA) and a Transmission Electron Microscope (TEM, Tecnai G2 spirit Biotwin, FEI, USA). The crystal composition of the TW/Fe 3 O 4 composite was also analyzed by X-ray diffraction (XRD, Bruker, D8 Advance, Germany). The magnetic properties of the TW/Fe 3 O 4 composite and the adsorbed Cr(VI) composite were determined by a SQUID instrument (MPMS XL-7, Quantum Design, USA). The functional groups of TW/Fe 3 O 4 composite and absorbed Cr(VI) composite were measured by Fourier Transform Infrared Spectroscopy (Nicolette is50, Thermo Fourier, USA).
The concentration of released metals (Ca 2+ , Na + , K + , Mg 2+ ) from the TW/Fe 3 O 4 composite in the supernatant of the equilibrium solution was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer, 2100DV, USA). The corresponding release of Ca 2+ , Na + , K + , Mg 2+ from the TW/Fe 3 O 4 composite with deionized water served as a control.
The concentration of total Cr in the supernatant was determined by atomic absorption spectroscopy (AAS, ZEEnit 700P, Analytik Jena AG, Germany). The concentration of Cr(VI) was analyzed by spectrophotometer. The concentration of Cr(III) in the supernatant was calculated as the difference between the total Cr and Cr(VI) concentrations.
The valence of the Cr bound on the TW/Fe 3 O 4 composite was determined by an X-ray Photoelectron Spectrometer (Thermo-VG Scientic, ESCALAB 250, USA). The Cr-laden TW/Fe 3 O 4 composite was obtained by shaking 1 g L À1 composite with 50 mg L À1 at pH 2 for 10 h.

Characterization of tea waste/Fe 3 O 4 composite
The SEM and TEM spectra of the TW/Fe 3 O 4 composite are shown in Fig. 1. The raw tea waste had a brous structure due to its cellulose and hemicellulose components ( Fig. 1(a)). Fig. 1(b) shows the image of the Fe 3 O 4 nano-particles. As shown in Fig. 1(c), the nano-Fe 3 O 4 particles had been successfully loaded into the tea waste. The surface and interiors of the tea waste were covered with nano-Fe 3 O 4 particles. The introduction of nano-Fe 3 O 4 particles increases the pore diameter and provides more adsorption sites to remove Cr(VI). As Fig. 1  The specic surface area, pore volume and average pore diameter of the TW/Fe 3 O 4 composite were 6.906 m 2 g À1 , 0.042 cm 3 g À1 and 65.471 nm, respectively. Meanwhile, the specic surface area, pore volume and average pore diameter of the raw tea waste were 0.913 m 2 g À1 , 0.007 cm 3 g À1 and 2.611 nm, respectively. Compared with tea waste, the surface area and pore volume of the TW/Fe 3 O 4 composite increased signicantly. This indicated that more adsorption sites could be provided, beneting the binding of Cr(VI). However, the average pore diameter of the TW/Fe 3 O 4 composite also obviously increased. This means that loading of nano-Fe 3 O 4 particles could increase the pore average, consistent with the previous study. 26 The introduction of nano-Fe 3 O 4 particles could cause the composite to be magnetic.
Magnetic hysteresis loops of the TW/Fe 3 O 4 composite and the Cr-adsorbed composite are shown in Fig. 3. As shown in the gure, with the increase in magnetic eld, the magnetization of the TW/Fe 3 O 4 composite also increased and tended to saturation. The saturation magnetization of the TW/Fe 3 O 4 composite and adsorbed-Cr composite were 15.8 emu g À1 and 16.8 emu g À1 , respectively. The TW/Fe 3 O 4 composite also showed a good superparamagnetic character though the composite had adsorbed the Cr(VI) aqueous solution. The TW/Fe 3 O 4 composite could be easily separated from aqueous solution by an external magnet. Thus, the magnetization curves indicated that the TW/ Fe 3 O 4 composite possesses a good magnetic characteristic. The magnetic property of the TW/Fe 3 O 4 composite also remained aer Cr(VI) adsorption, suggesting that the TW/Fe 3 O 4 composite has the potential for recycling.
The    The binding energies at 587.2 and 576.9 eV could be assigned to Cr(III) and binding energies at 578.2 and 580.9 eV could be attributed to Cr(VI). [31][32][33][34][35][36][37] The ratio of Cr(III)/Cr(VI) was 1.68, based on the peak area analysis. The results conrm that both Cr(VI) and Cr(III) coexist on the surface of the Cr(VI)adsorbed TW/Fe 3 O 4 composite. Thus, most of the Cr bound to the adsorbent was in the trivalent state during Cr(VI) adsorption. The results of XPS conrm that the reduction of Cr(VI) to Cr(III) by the TW/Fe 3 O 4 composite is the major process, while more Cr(VI) was bound in a reduced state of Cr(III) during the process.
When the Cr(VI) was reduced to Cr(III), organic compounds in tea waste were responsible for reducing Cr(VI), including carboxyl, methoxy, carbonyl, and hydroxyl groups, and amino groups which could act as electron-donors. Meanwhile, under the acidic atmosphere, few Fe 2+ ions may be oxidized by Cr(VI), although Fe 3 O 4 nanoparticles are relatively stable and further conrmation is needed in future research.

Effect of pH
pH is a key factor in the adsorption process and affects the surface charge of the adsorbent, the degree of ionization, and the speciation of metals in the solution. 38 The adsorption of Cr(VI) on the TW/Fe 3 O 4 composite is highly pH dependent. The effect of pH on the adsorption of Cr(VI) onto the TW/Fe 3 O 4 composite was investigated in the pH range from 2 to 8. As shown in Fig. 6(a), pH signicantly affects Cr(VI) removal. With an increase in pH, the removal rate of Cr(VI) signicantly decreased. When the pH was 2, the removal of Cr(VI) was more than 90%. When the pH was 3, the removal of Cr(VI) was lower than 50%. The removal of Cr(VI) decreased when the pH was larger than 3. Thus, pH 2 was a suitable parameter in this work. All subsequent adsorption experiments herein were conducted at pH 2, the optimum pH for adsorption of Cr(VI) onto the TW/Fe 3

Effect of TW/Fe 3 O 4 composite dosage
Adsorbent dosage is also an important parameter to determine its adsorption ability. The effect of dosage on removal rate and adsorption amount is shown in Fig. 6(b). As shown in Fig. 6(b), as the dosage was increased from to 0.1 to 1, the removal rate of Cr(VI) increased from 20% to 90%. When the adsorbent dosage -C-C-was larger than 1, the removal rate increased slightly, suggesting that the adsorption reached a saturated state. The TW/Fe 3 O 4 composite provided enough adsorption sites to bind the Cr(VI). However, adsorption density (adsorption capacity for unit mass of the adsorbent) decreased with an increase in adsorbent concentration due to formula (3). This was consistent with the previous study. 44 Aer comprehensive consideration of removal rate, adsorption capacity, and cost, the optimum absorbent dosage was found to be 1 g L À1 .
3.4. Effect of contact time, initial concentration and temperature on the removal of Cr(VI) The contact time, initial concentration and temperature also had pronounced effects on the removal of Cr(VI) from aqueous solution. The effects of these parameters on Cr(VI) removal rate are shown in Fig. 7. The rate of Cr(VI) uptake was quite fast for the rst 30 min. The adsorption reaction reached equilibrium when the contact time was 10 h. When the initial concentration of Cr(VI) was higher, the rate of Cr(VI) removal was relatively slow at the beginning. With a prolonged contact time, the adsorption ultimately reached equilibrium. The removal rate was higher when the initial concentration of Cr(VI) was lower under the same conditions, as the TW/Fe 3 O 4 composite could provide more adsorption sites. When the initial concentration of Cr(VI) was the same, a higher temperature led to a higher removal rate in the temperature range of 25-45 C. A higher temperature was benecial for the adsorption of Cr(VI) on the TW/Fe 3 O 4 composite.
Pseudo-rst-order logðq e À q t Þ ¼ log q e À k 1 2:303 t (6)  Pseudo-second-order Intra-particle diffusion Evolich where, q t and q e (mg g À1 ) are the adsorption capacities of Cr(VI) ions at time t and equilibrium time, respectively. k 1 (min À1 ) and k 2 (g mg À1 min À1 ) are the pseudo-rst-order and pseudosecond-order rate constants, respectively. k id (mg g À1 min À0.5 ) is the intra-particle diffusion rate constant and C is the intercept (mg g À1 ) and its value stands for the thickness of the boundary layer. a is the initial adsorption coefficient (mg g À1 min À1 ) and b is the desorption coefficient (g mg À1 ). The kinetic tting of Cr(VI) on the TW/Fe 3 O 4 composite at 298 K is shown in Fig. 8.
The tting parameters of pseudo-rst-order and pseudosecond-order kinetics are presented in Table 2. The  correlation coefficient (R 2 ) was larger than 0.99 and indicates that pseudo-rst-order and pseudo-second-order kinetic models could t the adsorption process well. However, the calculated q e from the pseudo-rst-order kinetic model shows a large difference from the q e from experiment. The calculated q e from the pseudo-second-order kinetic model was close to the q e from experiment. Thus, the pseudo-second-order model could better t the adsorption process of Cr(VI) onto the TW/ Fe 3 O 4 composite. Generally, the pseudo-second-order kinetic model was used to describe chemisorption, referring to valency force through the sharing or exchange of electrons between the adsorbent and adsorbate as covalent forces, and ion exchange. 46,49,50 Therefore, the removal rate of Cr(VI) by the TW/Fe 3 O 4 composite was controlled by a chemisorption mechanism.
The tting parameters of the intra-particle diffusion and Evolich models are shown in Table 3. Intra-particle diffusion is a transport process which describes the movement of species from the bulk of the solution to the solid phase. 45 The correlation coefficient (R 2 ) was larger than 0.90, indicating that intraparticle kinetic models could also t the adsorption process. The line does not pass through the origin, which means that the adsorption of Cr(VI) onto the TW/Fe 3 O 4 composite was controlled by chemisorption coupled with intra-particle diffusion, involving electron sharing or electron transfer. 47 The Elovich kinetic equation is used to interpret the kinetics of chemisorption on highly heterogeneous sorbents. 51 The correlation coefficient (R 2 ) was larger than 0.95, indicating that the Evolich model could also t the adsorption process well. Chemisorption plays an important role during the adsorption of Cr(VI) onto the TW/Fe 3 O 4 composite.
Herein, the focus of our research is the adsorption process of the Cr(VI) on the TW/Fe 3 O 4 composite. The reduction of Cr(VI) is the subsequent reaction aer the Cr(VI) reaches the surface of the TW/Fe 3 O 4 composite. The reduction process may inuence the isotherm/kinetics models and belongs to the whole adsorption process. Recently, advanced reduction kinetics have been developed to t the kinetics process and conrmed the "reduction mechanism". 52,53 Further research is needed in the future.

Adsorption isotherms
The equilibrium adsorption isotherm is fundamental in describing the interactive behavior between adsorbates and adsorbent, and is important in the design of adsorption systems. [54][55][56][57] Research into adsorption isotherms is crucially important in the modeling, design, and commercial application of adsorbents. [58][59][60] The Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherms were expressed as the following eqn (10)- (15). Langmuir: Temkin: Dubinin-Radushkevich (D-R) equation: where C e is the equilibrium concentration, mg L À1 ; q e is the adsorption capacity at equilibrium time, mg g À1 ; b is the Langmuir constant, L g À1 ; q max is the maximum adsorption capacity, mg g À1 ; K f is the Freundlich constant, L mg À1 ; 1/n is the heterogeneity of the sorption sites and is an indicator of isotherm nonlinearity; K T is the equilibrium binding constant, L mg À1 ; b T is the Temkin isotherm constant; RT/b T is related to adsorption heat, J mol À1 . q m is the theoretical isotherm saturation capacity (mol g À1 ); b D-R is the Dubinin-Radushkevich isotherm constant (mol 2 kJ À2 ), 3 is the Dubinin-Radushkevich isotherm constant.
The tting curves of the different isotherm equations are displayed in Fig. 9.
The adsorption isotherms and tting results are shown in Table 4. According to the correlation coefficient, Langmuir, Temkin and D-R isotherms could t the adsorption process well. The maximum adsorption capacity of Cr(VI) by the TW/Fe 3 O 4 composite was 75.76 mg g À1 . The maximum adsorption capacity of the TW/Fe 3 O 4 composite is much higher than that of most adsorbents reported in the literature, 5 and thus it can be applied as a highly efficient adsorbent to remove trace Cr(VI) from aqueous solution. The Langmuir constant (b) was in the range of 0-1, indicating that the TW/Fe 3 O 4 composite had a good potential to remove Cr(VI). Thus, the adsorption of Cr(VI) onto the TW/Fe 3 O 4 composite related to monomolecular adsorption. With the increase in temperature, the maximum adsorption capacity gradually increased, suggesting that higher temperatures benet the adsorption process.
A dimensionless separation factor R L was dened as R L ¼ 1/(1 + C 0 b). 61 The value of R L indicates the shape of the isotherm to be either unfavourable (R L > 1), linear (R L ¼ 1), favourable (0 < R L < 1) or irreversible (R L ¼ 0). 62,63 As shown in Fig. 10, R L was lower than 1 in this study and the adsorption was favorable. A higher initial concentration and temperature beneted the adsorption of Cr(VI) onto the TW/Fe 3 O 4 composite. The Temkin isotherm model assumes that the heat of adsorption decreases linearly rather than logarithmically with coverage and describes the chemisorption interaction with primarily electrostatic interactions. In this study, the correlation coefficient of the Temkin model was larger than 0.95. Thus, electrostatic interaction was an important mechanism between Cr(VI) and the TW/Fe 3 O 4 composite, consistent with the result of the pH effect. The D-R isotherm could also well describe the adsorption process of Cr(VI) onto the TW/Fe 3 O 4 composite. When E a is in the range of 1-8 kJ mol À1 , physical interaction plays an important role. When E a is between 8 and 16 kJ mol À1 , the main interaction is an ion exchange process. Chemisorption interaction was the main mechanism when E a ranged from 16 to 40 kJ mol À1 . 64 In this research, E a ranged from 8 to 10 kJ mol À1 when the temperature was between 25 and 45 C. The tting results showed that the main adsorption mechanism was ion exchange between Cr(VI) and the TW/Fe 3 O 4 composite, coexisting with physical interaction. With an increase in temperature, the increased E a suggests that the chemisorption interaction became more pronounced.

Adsorption thermodynamics
In order to better understand the effect of temperature on the adsorption of Cr(VI) on the TW/Fe 3 O 4 composite, the thermodynamic parameters of the adsorption process such as change in standard free energy (DG), enthalpy (DH), and entropy (DS) can be calculated using the eqn (16)- (18). 65,66 where K 0 is the apparent equilibrium constant. R is the ideal gas constant (4.187 J mol À1 K À1 ) and T is Kelvin temperature (K). According to the method suggested by Khan and Singh, 66 the sorption distribution coefficient K 0 for the sorption reaction was determined from the slope of the plot ln(q e /C e ) against C e at various temperatures and extrapolating to zero C e . The values of DH and DS can be obtained from the slope and intercept of a plot of ln K 0 against 1/T and are shown in Fig. 11.
The thermodynamic parameters of Cr(VI) adsorption on the TW/Fe 3 O 4 composite are given in Table 5. The DG value was negative for all three considered temperatures, and this   suggests that the adsorption was a spontaneous process. As the temperature increases, the DG values decrease, indicating that elevated temperatures benet the adsorption process. The DH value was positive and the adsorption process was endothermic. The positive value of DS conrms that the randomness of Cr(VI) increased at the solid-solution interface during the adsorption process.

Concentration of different Cr species under different parameters
The concentrations of different Cr species under various parameters are shown in Fig. 12. It was found that the concentrations of total and hexavalent chromium were not equal, which meant that aer the contact between the TW/ Fe 3 O 4 composite and Cr(VI) solutions, the Cr(III) concentration rose in solution. This result also conrmed that a chemical redox reaction occurs during the adsorption process. As shown in Fig. 8(a), when the pH was lower than 4, Cr(III) could be detected in the solution. This indicated that Cr(VI) was easily reduced to Cr(III) under acidic conditions. The experimental result was in line with the effect of pH on Cr(VI) removal.
The effect of adsorbent dosage is presented in Fig. 8(b). The experimental conditions were consistent with the effect of dosage on Cr(VI) removal. When the solid-liquid ratio was larger than 0.25, the reduction reaction occurred under acidic conditions. When the solid-liquid ratio was larger than 1, the concentration remained stable because the Cr(VI) was adsorbed onto the TW/Fe 3 O 4 composite and was reduced to Cr(III). Cr(VI) could not be detected in the solution.
The effect of contact time is described in Fig. 8(c). The experimental conditions were equal to those of the adsorption kinetics (initial concentration of Cr(VI) was 50 mg L À1 ). Cr(III) could be detected in the solution of the adsorption experiment aer 5 min. The concentration of Cr(III) increased as the contact time increased. When Cr(VI) was reduced to Cr(III) on the surface of the TW/Fe 3 O 4 composite, part of the Cr(III) was released into solution due to electrostatic repulsion. 67 The proportion of reduced Cr was calculated by means of XPS analysis and the results of Fig. 8(c) using formula (19). The results showed that 70% of Cr(VI) was reduced to Cr(III).

Desorption and regeneration experiment
Desorption and regeneration batch experiments were designed to study the recoverable potential of the TW/Fe 3 O 4 composite.
The conditions for the adsorption experiments were 50 mg L À1 initial concentration of Cr(VI), 40 mL of solution, adsorbent mass 0.1 g, pH 2, temperature 25 C, 180 rpm for 10 h. When the adsorption process reached equilibrium, 50 mL of NaOH (0.5 mol L À1 ) was used to desorb the adsorbent for 2 h. The Cr(VI) was determined by a spectrophotometric method. Then, HCl (0.5 mol L À1 ) was used to regenerate the absorbent. The Cr(VI) removal rate of the TW/Fe 3 O 4 composite aer ve recycles of desorption-regeneration is shown in Fig. 13 and the removal rate was still higher at more than 70%, indicating the potential recycling of the TW/Fe 3 O 4 composite, consistent with the measurement of magnetic hysteresis loops (Fig. 4). Desorption of the Cr(VI) from the TW/Fe 3 O 4 composite was favored by weak alkaline conditions 68,69 and HCl could be a choice for the regeneration of adsorbent. 25,67 Therefore, the TW/Fe 3 O 4 composite has the potential for regeneration and reusability.

Proposed mechanisms
The mechanism of Cr(VI) removal is not fully understood and involves many models or steps. [69][70][71][72][73][74][75][76] The adsorption mechanism of Cr(VI) onto the TW/Fe 3 O 4 composite is a complicated process and multiple interactions coexist. According to the research of Saha and Orvig, 70 there are four potential mechanisms explaining Cr(VI) sorption by biosorbents: (a) anionic adsorption, (b) adsorption-coupled reduction, (c) anionic and cationic adsorption and (d) reduction and cationic adsorption. According to an analysis of the above results, the mechanism of Cr(VI) sorption by the TW/Fe 3 O 4 composite involved adsorptioncoupled reduction.
According to the analysis of the FTIR, functional groups (-OH, C]O and C]C, C-O, secondary amine groups, C-N) could participate in the adsorption-reduction process. Herein, the oxygen-containing or nitrogen-containing groups played three functions. First, functional groups could adsorb Cr(VI) through surface complexation, an ion exchange process, etc. Second, a functional group could be protonated by the H + in the solution, especially the amine functional groups. A protonated functional group could reduce Cr(VI) to Cr(III). 73,74,77 The Cr(III) was conrmed by the analysis of XPS (Fig. 5) and the difference between total Cr and Cr(VI) (Fig. 13). Third, the functional groups could also bind with Cr(III) through surface complexation, an ion-exchange process, etc. Thus, functional groups of the TW/Fe 3 O 4 composite played an important role during the adsorption-reduction process.
The ion exchange process is also an important mechanism. The concentrations of Ca 2+ , Mg 2+ , Na + , K + in the supernatant were determined and deionized water served as a control. The ion exchange capacities of Ca 2+ , Mg 2+ , Na + , K + were calculated and are presented in Table 6. The sum of ion exchange capacity increased when the Cr(VI) was removed by the TW/Fe 3 O 4 composite. The ion exchange capacity was higher when the initial concentration of Cr(VI) was higher. Ca 2+ and K + ions participate more in the ion exchange process, consistent with previous studies. 78 The results also conrm the conclusions of the kinetic and isotherm models.
In brief, the adsorption mechanism of Cr(VI) onto the TW/ Fe 3 O 4 composite referred to physical adsorption, electrostatic interaction, reduction, ion exchange, surface complexation, etc.
Meanwhile, Cr(VI) in solution had been adsorbed onto the surface of the adsorbent and then reduced to Cr(III) by a heterogeneous redox process under an acid environment (as shown in eqn (20)). A few Fe 2+ ions may be oxidized by Cr(VI) although Fe 3 O 4 nanoparticles are relatively stable. The reaction process follows eqn (21). 43 The converted Cr(III) was retained by the TW/Fe 3 O 4 composite and part of the Cr(III) was released into solution due to electrostatic repulsion. Some portion of the Cr(III) could be adsorbed onto the TW/Fe 3 O 4 composite by carbon-oxygen groups or weak acid surface groups. 79 In conclusion, the proposed mechanism of Cr(VI) removal by the TW/Fe 3 O 4 composite is as displayed in Fig. 14. The main steps follow: rst step, Cr(VI) is adsorbed quickly onto the surface of the TW/Fe 3 O 4 composite due to electrostatic attraction (protonated functional groups or positive Fe 3 O 4 nanoparticles) or an ion exchange process or surface complexation.  Table 6 The release of Ca 2+ , Mg 2+ , Na + , K + during adsorption by a TW/Fe 3 O 4 composite at 25 C a

Samples
The net amount of released cations (mequiv. g À1 )
Second step, some portions of Cr(VI) were reduced to Cr(III). Third step, some Cr(III) was adsorbed onto the surface of the TW/Fe 3 O 4 composite through ion exchange or surface complexation. Last step, part of the Cr(III) was released into solution due to electrostatic repulsion. Then, NaOH and HCl acted as the desorption reagent and regeneration reagent, respectively.

Conclusions
In this study, a tea waste/nano-Fe 3 O 4 composite was facilely prepared and used to remove Cr(VI) from aqueous solution. The maximum removal of Cr(VI) by the TW/Fe 3 O 4 composite was observed at pH 2. The adsorption process data was found to best tted by a pseudo-second-order kinetic model. The maximum adsorption capacity was higher and reached 75.76 mg g À1 based on the Langmuir model. Adsorption of hexavalent chromium ions by the TW/Fe 3 O 4 composite is spontaneous in nature, endothermic, and results in increased randomness at the adsorbent/adsorbate interface, due to an investigation of the thermodynamics. Desorption and regeneration experiments revealed that the TW/Fe 3 O 4 composite had a good potential for recycling. The mechanism of Cr(VI) removal involved an adsorption-reduction process, referring to physical adsorption, electrostatic interaction, reduction, ion exchange, surface complexation, etc. 70% of the Cr(VI) was reduced to Cr(III) in this investigation. Therefore, the TW/Fe 3 O 4 composite could be applied as a novel adsorbent for Cr(VI) removal from industrial wastewater.
List of symbols b T Temkin isotherm constant (J mol À1 ) C 0 Initial Cr(VI) concentration (mg L À1 ) C e Cr(VI) concentration in solution at equilibrium (mg L À1 ) C t Concentration of Cr(VI) at t time (mg L À1 ) K f Freundlich isotherm constant (mg g À1 ) (mg L À1 ) À1 k 1 Pseudo-rst-order rate constant (L min À1 ) k 2 Pseudo-second-order rate constant (g mg À1 min À1 ) k id Intraparticle diffusion rate constant (mg g À1 min À0.5 ) K 0 Apparent equilibrium constant m Amount of adsorbent (TW/Fe 3 O 4 ) composite (g) q t Adsorption capacities of Cr(VI) ions at time t (mg g À1 ) q e Adsorption capacities of Cr(VI) ions at equilibrium time (mg g À1 ) q max The maximum adsorption capacity based on Langmuir model (mg g À1 ) q m Maximum adsorption capacity (mg g À1 ) in D-R model R Ideal gas constant, 8.314 J mol À1 K À1 R 2 Coefficient of determination R L Dimensionless separation factor T Kelvin temperature (K) V Volume of Cr(VI) solution (L) 1/n The heterogeneity of the sorption sites and an indicator of isotherm nonlinearity DG Gibbs free energy (kJ mol À1 ) DH Enthalpy (kJ mol À1 ) DS Entropy (J mol À1 K À1 ) 3 Dubinin-Radushkevich isotherm constant a The initial adsorption coefficient in Evolich model (mg g À1 min À1 ) b The desorption coefficient in Evolich model (g mg À1 ) b D-R Dubinin-Radushkevich isotherm constant (mol 2 kJ À2 )