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

Abstract

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.

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1. 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, ion-exchange, electrochemical, reduction, membrane filtration, 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 modified biomass have drawn great attention.^5–8

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 Cu²⁺,^9–12 Cd²⁺,^10–12 Zn²⁺,^11,13 Ni²⁺,^13–15 and Pb²⁺.^9,11–13 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 Fe₃O₄ nanoparticles into the composite could improve the magnetic and adsorption properties of raw composite. Nano-Fe₃O₄ 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-Fe₃O₄ onto a biomass adsorbent could also enhance the dispersibility and stability of nano-Fe₃O₄ particles.²¹

A preliminary investigation into a tea waste/nano particle composite was also carried out. The preparation of a tea waste/nano Fe₃O₄ 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 (FeCl₂·4H₂O + NaOH) and the maximum adsorption capacity was 30 mg g⁻¹. 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 Fe₃O₄ 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-Fe₃O₄ particles were loaded into the tea waste by a chemical co-precipitation method to prepare a magnetic tea waste/Fe₃O₄ 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/Fe₃O₄ composite; (2) to explore the interaction mechanism between Cr(VI) and the tea waste/Fe₃O₄ composite; (3) to assess the potential recycling of the tea waste/Fe₃O₄ composite acting as Cr(VI) absorbent. Our results showed that a tea waste/Fe₃O₄ composite can be facilely prepared and could be used to remove Cr(VI) from aqueous solution.

2. Materials and methods

2.1. Synthesis of TW/Fe₃O₄ 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/Fe₃O₄ composite (TW: tea waste) and the reactions are as shown in eqn (1) and (2). To obtain nano-Fe₃O₄ particles, 5.2 g of FeSO₄·7H₂O and 7.4 g of FeCl₃·6H₂O were put in a three-mouth flask 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 N₂ 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.

Fe²⁺ + 2Fe³⁺ + 8OH⁻ → Fe₃O₄ + 4H₂O (1)

Fe₃O₄ + TW + 4H₂O → TW − Fe₃O₄ (2)

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.

2.2. Batch adsorption experiments

The Cr(VI) stock solution was prepared by dissolving 0.2829 g of K₂Cr₂O₇ 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/Fe₃O₄ composite (0.04 g) and 40 mL of Cr(VI) solution of 40 mg L⁻¹ 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/Fe₃O₄ composite and 40 mL of Cr(VI) solution of 25 mg L⁻¹ were added to a 50 mL centrifugal tube to give a solid/liquid ratio in the range of 0.1–3 g L⁻¹ (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 μm filter 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, C₀ and C_e are the initial and equilibrium concentrations of Cr(VI) (mg L⁻¹), respectively. Herein, q_e is the adsorption amount at equilibrium (mg g⁻¹) and V is the volume of Cr(VI) solution (L); m is the mass of the TW/Fe₃O₄ composite (g).

Adsorption kinetics. The Cr(VI) solution and TW/Fe₃O₄ composite were mixed in a 100 mL Erlenmeyer flask. 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⁻¹, 30 mg L⁻¹, and 50 mg L⁻¹.where, q_t is the adsorption amount (mg g⁻¹) at time t (min). C_t is the concentration of Cr(VI) at time t (mg L⁻¹).

Adsorption isotherm. An amount of TW/Fe₃O₄ 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⁻¹. 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.

2.3. Characterization

The textural properties of tea waste and TW/Fe₃O₄ composite were analyzed with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) at 77 K under N₂ 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/Fe₃O₄ 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/Fe₃O₄ composite was also analyzed by X-ray diffraction (XRD, Bruker, D8 Advance, Germany). The magnetic properties of the TW/Fe₃O₄ composite and the adsorbed Cr(VI) composite were determined by a SQUID instrument (MPMS XL-7, Quantum Design, USA). The functional groups of TW/Fe₃O₄ composite and absorbed Cr(VI) composite were measured by Fourier Transform Infrared Spectroscopy (Nicolette is50, Thermo Fourier, USA).

The concentration of released metals (Ca²⁺, Na⁺, K⁺, Mg²⁺) from the TW/Fe₃O₄ 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 Ca²⁺, Na⁺, K⁺, Mg²⁺ from the TW/Fe₃O₄ 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/Fe₃O₄ composite was determined by an X-ray Photoelectron Spectrometer (Thermo-VG Scientific, ESCALAB 250, USA). The Cr-laden TW/Fe₃O₄ composite was obtained by shaking 1 g L⁻¹ composite with 50 mg L⁻¹ at pH 2 for 10 h.

3. Results and discussion

3.1. Characterization of tea waste/Fe₃O₄ composite

The SEM and TEM spectra of the TW/Fe₃O₄ composite are shown in Fig. 1. The raw tea waste had a fibrous structure due to its cellulose and hemicellulose components (Fig. 1(a)). Fig. 1(b) shows the image of the Fe₃O₄ nano-particles. As shown in Fig. 1(c), the nano-Fe₃O₄ particles had been successfully loaded into the tea waste. The surface and interiors of the tea waste were covered with nano-Fe₃O₄ particles. The introduction of nano-Fe₃O₄ particles increases the pore diameter and provides more adsorption sites to remove Cr(VI). As Fig. 1(d) shows, the TW/Fe₃O₄ composite could be obviously seen and appeared to have a cubic spinel structure. The size of the Fe₃O₄ nanoparticles is approximately 20 nm. Thus, the TW/Fe₃O₄ composite had been successfully prepared and was used for the subsequent adsorption experiments.

Fig. 1 SEM photo (a) tea waste; (b) Fe₃O₄; (c) TW/Fe₃O₄ composite and (d) TEM of TW/Fe₃O₄ composite.

The XRD spectrum of the TW/Fe₃O₄ composite is shown in Fig. 2. As can be seen in Fig. 2, a Fe₃O₄ crystal compound could be detected and the values of 2θ were 30.20°(220), 35.54°(311), 43.30°(400), 53.50°(422), 57.30°(511), 62.90°(440).^18,25 Thus, nano-Fe₃O₄ particles had been produced and loaded into the tea waste. The peaks (2θ) of the composite which were located nearby at 17° and 22° may relate to the amorphous carbon based on tea waste.

Fig. 2 XRD spectrum of the TW/Fe₃O₄ composite.

The specific surface area, pore volume and average pore diameter of the TW/Fe₃O₄ composite were 6.906 m² g⁻¹, 0.042 cm³ g⁻¹ and 65.471 nm, respectively. Meanwhile, the specific surface area, pore volume and average pore diameter of the raw tea waste were 0.913 m² g⁻¹, 0.007 cm³ g⁻¹ and 2.611 nm, respectively. Compared with tea waste, the surface area and pore volume of the TW/Fe₃O₄ composite increased significantly. This indicated that more adsorption sites could be provided, benefiting the binding of Cr(VI). However, the average pore diameter of the TW/Fe₃O₄ composite also obviously increased. This means that loading of nano-Fe₃O₄ particles could increase the pore average, consistent with the previous study.²⁶

The introduction of nano-Fe₃O₄ particles could cause the composite to be magnetic.

Magnetic hysteresis loops of the TW/Fe₃O₄ composite and the Cr-adsorbed composite are shown in Fig. 3. As shown in the figure, with the increase in magnetic field, the magnetization of the TW/Fe₃O₄ composite also increased and tended to saturation. The saturation magnetization of the TW/Fe₃O₄ composite and adsorbed-Cr composite were 15.8 emu g⁻¹ and 16.8 emu g⁻¹, respectively. The TW/Fe₃O₄ composite also showed a good superparamagnetic character though the composite had adsorbed the Cr(VI) aqueous solution. The TW/Fe₃O₄ composite could be easily separated from aqueous solution by an external magnet. Thus, the magnetization curves indicated that the TW/Fe₃O₄ composite possesses a good magnetic characteristic. The magnetic property of the TW/Fe₃O₄ composite also remained after Cr(VI) adsorption, suggesting that the TW/Fe₃O₄ composite has the potential for recycling.

Fig. 3 Magnetic hysteresis loops of the TW/Fe₃O₄ composite and Cr(VI) laden on the composite.

The FTIR spectra of tea waste, Fe₃O₄, TW/Fe₃O₄ and Cr(VI)-adsorbed onto TW/Fe₃O₄ are shown in Fig. 4. The FTIR spectra of tea waste, Fe₃O₄, TW/Fe₃O₄ and Cr(VI) adsorbed onto TW/Fe₃O₄ composite were different and the typical functional groups are presented in Table 1. The peaks of the main functional groups in tea waste include 3416 cm⁻¹ (bonded –OH groups), 2924 cm⁻¹ and 2852 cm⁻¹ (aliphatic C–H group), 1647 cm⁻¹ (C=O stretching or aromatic C=C and C=O/C=C stretching), 1536 cm⁻¹ (secondary amine group), 1452 cm⁻¹ (C–H alkanes in aromatic rings), 1370 cm⁻¹ (C–H bending –CH₃ symmetric bending of CH₃), 1234 cm⁻¹ (–SO₃ stretching/P=O), 1149 cm⁻¹ (C–O–C of polysaccharides), 1036 cm⁻¹ (C–O–H stretching), 613 cm⁻¹ (–C–C–).^27,28 Thus, tea waste contains protein-like, aliphatic-like, cellulose-like substances which have hydroxyl, carboxyl, and amine groups, and S- and P-containing functional groups. The obvious peak at 583 cm⁻¹ in the spectrum of Fe₃O₄ indicates Fe–O, suggesting the assignment of Fe₃O₄. Other peaks were at 892 cm⁻¹ (Fe–O–H), 1630 cm⁻¹ and 1400 cm⁻¹ (hydroxyl groups from water adsorbed during the preparation of the TW/Fe₃O₄ composite).^29,30 The peaks of the main functional groups of the TW/Fe₃O₄ composite include 3423 cm⁻¹ (bonded –OH groups), 2923 cm⁻¹ and 2852 cm⁻¹ (aliphatic C–H group), 1630 cm⁻¹ (assigned to C=O and C=C aromatic vibrations), 1518 cm⁻¹ (secondary amine group), 1367 cm⁻¹ (C–O stretching vibration), 1215 cm⁻¹ (C–N stretching), 1151 cm⁻¹ (C–O stretching of ether groups), 1059 cm⁻¹ (C–O stretching of COOH), 579 cm⁻¹ (Fe–O). Thus, the TW/Fe₃O₄ composite retained the functional groups of tea waste and also loaded the functional groups of nano-Fe₃O₄. Meanwhile, the introduction of Fe₃O₄ also changed some functional groups of tea waste and led to a shift in some peaks or the intensity reduction of some peaks, such as 1647 cm⁻¹, 1536 cm⁻¹, 1452 cm⁻¹, 1234 cm⁻¹, and 1036 cm⁻¹.

Fig. 4 The FTIR spectrum of tea waste, Fe₃O₄, TW/Fe₃O₄ and Cr laden on the TW/Fe₃O₄ composite.

Table 1 FTIR spectral characteristics of the TW/Fe₃O₄ composite before and after Cr(VI) adsorption

Tea waste cm^−1	Assignment	Fe3O4 cm^−1	Assignment	TW/Fe3O4			Assignment
				Before adsorption	After adsorption	Difference
3416	Bonded –OH groups	3423	Hydroxyl groups	3423	3406	+17	Bonded –OH groups
2924	Aliphatic C–H group	1630	Hydroxyl groups	2923	2923	0	Aliphatic C–H group
2852	Aliphatic C–H group	1400	Hydroxyl groups	2852	2852	0	Aliphatic C–H group
1647	C=O stretching aromatic C=C C=O/C=C stretching	892	Fe–O–H	1630	1640	−10	Assigned to C=O and C=C aromatic vibrations
1536	Secondary amine group	583	Fe–O	1518	1541	−23	Secondary amine group
1452	C–H alkanes in aromatic rings			1367	1385	−18	C–O stretching vibration
1370	C–H bending –CH3 symmetric bending of CH3			1215	1246	−31	C–N stretching
1234	–SO3 stretching/P=O			1151	1158	0	C–O stretching of ether groups
1149	C–O–C of polysaccharides			1059	1059	0	C–O stretching of COOH
1036	C–O–H stretching			579	579	0	Fe–O
613	–C–C–

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When the Cr(VI) was adsorbed on the TW/Fe₃O₄ composite, some peaks had changed, including 3423 → 3406 cm⁻¹, 1630 → 1640 cm⁻¹, 1518 → 1541 cm⁻¹, 1367 → 1385 cm⁻¹, 1215 → 1246 cm⁻¹. Hence, –OH, C=O and C=C, C–O, the secondary amine groups, and C–N functional group may participate in the adsorption of Cr(VI), referring to surface complexation, ion exchange, etc.

The XPS spectrum of the TW/Fe₃O₄ composite and Cr(VI) adsorbed onto TW/Fe₃O₄ are displayed in Fig. 5. As shown in Fig. 5(a), elements in the TW/Fe₃O₄ composite involve C, O, Fe, etc. When the Cr(VI) was adsorbed on the TW/Fe₃O₄ composite, the Cr was detected (as shown in Fig. 5(b)). The fitting of the Cr valence is shown in Fig. 5(c).

Fig. 5 XPS analysis of the TW/Fe₃O₄ composite and absorbed Cr on the TW/Fe₃O₄ composite (a) TW/Fe₃O₄ composite (b) Cr laden on the TW/Fe₃O₄ composite (c) speciation analysis of Cr.

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).^31–37 The ratio of Cr(III)/Cr(VI) was 1.68, based on the peak area analysis. The results confirm that both Cr(VI) and Cr(III) coexist on the surface of the Cr(VI)-adsorbed TW/Fe₃O₄ composite. Thus, most of the Cr bound to the adsorbent was in the trivalent state during Cr(VI) adsorption. The results of XPS confirm that the reduction of Cr(VI) to Cr(III) by the TW/Fe₃O₄ 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 Fe²⁺ ions may be oxidized by Cr(VI), although Fe₃O₄ nanoparticles are relatively stable and further confirmation is needed in future research.

3.2. 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/Fe₃O₄ composite is highly pH dependent. The effect of pH on the adsorption of Cr(VI) onto the TW/Fe₃O₄ composite was investigated in the pH range from 2 to 8. As shown in Fig. 6(a), pH significantly affects Cr(VI) removal. With an increase in pH, the removal rate of Cr(VI) significantly 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/Fe₃O₄ composite.

Fig. 6 Effect of (a) pH and (b) adsorbent mass on Cr(VI) removal.

pH influenced the surface charge and the protonation degree of the TW/Fe₃O₄ composite. The main Cr(VI) species are HCrO₄⁻, CrO₄²⁻, Cr₂O₇²⁻, etc.^39,40 When the pH was lower, the surface charge of the TW/Fe₃O₄ composite caused a protonation reaction and more positive charge formed. The binding site of the TW/Fe₃O₄ composite could interact with Cr(VI) through electrostatic attraction or an ion exchange process. With the increase in pH, OH⁻ increased and competed with HCrO₄⁻, leading to a decrease in Cr(VI) removal efficiency.⁴¹ Meanwhile, the positive surface charge of Fe₃O₄ could attract HCrO₄⁻ through electrostatic interaction under the acidic conditions, consistent with previous studies.^42,43

3.3. Effect of TW/Fe₃O₄ 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 was larger than 1, the removal rate increased slightly, suggesting that the adsorption reached a saturated state. The TW/Fe₃O₄ 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 After comprehensive consideration of removal rate, adsorption capacity, and cost, the optimum absorbent dosage was found to be 1 g L⁻¹.

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 first 30 min. The adsorption reaction reached equilibrium when the contact time was 10 h.

Fig. 7 Effect of contact time, initial concentration and temperature on Cr(VI) adsorption.

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/Fe₃O₄ 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 beneficial for the adsorption of Cr(VI) on the TW/Fe₃O₄ composite.

3.5. Adsorption kinetics

Based on the adsorption kinetics of Cr(VI) onto the TW/Fe₃O₄ composite, experimental data was fitted using pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich models.^45–48 The kinetic equations are expressed as the following eqn (6)–(9).

Pseudo-first-order

Pseudo-second-order

Intra-particle diffusion

q_t = k_idt^1/2 + C (8)

Evolich

q_t = (1/β)ln(αβ) + 1/β ln(t) (9)

where, q_t and q_e (mg g⁻¹) are the adsorption capacities of Cr(VI) ions at time t and equilibrium time, respectively. k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the pseudo-first-order and pseudo-second-order rate constants, respectively. k_id (mg g⁻¹ min^−0.5) is the intra-particle diffusion rate constant and C is the intercept (mg g⁻¹) and its value stands for the thickness of the boundary layer. α is the initial adsorption coefficient (mg g⁻¹ min⁻¹) and β is the desorption coefficient (g mg⁻¹).

The kinetic fitting of Cr(VI) on the TW/Fe₃O₄ composite at 298 K is shown in Fig. 8.

Fig. 8 Pseudo-first-order, pseudo-second-order, intra-particle diffusion, and Elovich kinetics models of Cr(VI) adsorption onto the TW/Fe₃O₄ composite at 298 K.

The fitting parameters of pseudo-first-order and pseudo-second-order kinetics are presented in Table 2. The correlation coefficient (R²) was larger than 0.99 and indicates that pseudo-first-order and pseudo-second-order kinetic models could fit the adsorption process well. However, the calculated q_e from the pseudo-first-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 fit the adsorption process of Cr(VI) onto the TW/Fe₃O₄ 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/Fe₃O₄ composite was controlled by a chemisorption mechanism.

Table 2 Parameters of kinetic models for adsorption of Cr(VI) onto the TW/Fe₃O₄ composite (pseudo-first-order and pseudo-second-order kinetics models)

Initial concentration (mg L^−1)	T (K)	Pseudo-first-order			Pseudo-second-order
		qe	k1	R^2	qe	k2	R^2
10	298	1.7557	0.0070	0.9945	9.1827	0.0175	0.9997
	308	1.3006	0.0065	0.9863	9.3897	0.0243	0.9998
	318	0.9838	0.0057	0.9758	9.5923	0.0302	0.9999
30	298	16.8601	0.0069	0.9915	29.7530	0.0012	0.9964
	308	17.1712	0.0083	0.9940	30.3674	0.0014	0.9982
	318	11.9989	0.0120	0.9897	30.1023	0.0033	0.9998
50	298	20.3470	0.0070	0.9934	42.9738	0.0012	0.9976
	308	21.9149	0.0067	0.9921	45.7875	0.0010	0.9974
	318	22.4647	0.0075	0.9944	48.7329	0.0011	0.9983

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The fitting 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 (R²) was larger than 0.90, indicating that intra-particle kinetic models could also fit the adsorption process. The line does not pass through the origin, which means that the adsorption of Cr(VI) onto the TW/Fe₃O₄ composite was controlled by chemisorption coupled with intra-particle diffusion, involving electron sharing or electron transfer.⁴⁷

Table 3 Parameters of kinetic models for the adsorption of Cr(VI) onto a TW/Fe₃O₄ composite (intra-particle and Evolich kinetics models)

Initial concentration (mg L^−1)	T (K)	Intra-particle model			Evolich model
		kd	C	R^2	α	β	R^2
10	298	0.0840	7.2950	0.9430	2.4696 × 10^6	2.4155	0.9910
	308	0.8310	10.7200	0.9320	7.1403 × 10^0	0.2410	0.9930
	318	0.9810	20.6700	0.9360	4.3934 × 10^1	0.2047	0.9910
30	298	0.0650	7.9780	0.9220	1.5283 × 10^9	3.0488	0.9960
	308	0.8360	11.7100	0.9000	8.3477 × 10^0	0.2355	0.9940
	318	1.0720	21.4100	0.9320	3.7918 × 10^1	0.1868	0.9930
50	298	0.0490	8.4870	0.9280	3.4964 × 10^13	4.0816	0.9820
	308	0.5990	7.5900	0.7900	8.5280 × 10^1	0.3114	0.9710
	318	1.0950	24.1700	0.9190	5.6834 × 10^1	0.1817	0.9930

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The Elovich kinetic equation is used to interpret the kinetics of chemisorption on highly heterogeneous sorbents.⁵¹ The correlation coefficient (R²) was larger than 0.95, indicating that the Evolich model could also fit the adsorption process well. Chemisorption plays an important role during the adsorption of Cr(VI) onto the TW/Fe₃O₄ composite.

Herein, the focus of our research is the adsorption process of the Cr(VI) on the TW/Fe₃O₄ composite. The reduction of Cr(VI) is the subsequent reaction after the Cr(VI) reaches the surface of the TW/Fe₃O₄ composite. The reduction process may influence the isotherm/kinetics models and belongs to the whole adsorption process. Recently, advanced reduction kinetics have been developed to fit the kinetics process and confirmed the “reduction mechanism”.^52,53 Further research is needed in the future.

3.6. 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.^54–57 Research into adsorption isotherms is crucially important in the modeling, design, and commercial application of adsorbents.^58–60

The Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R) isotherms were expressed as the following eqn (10)–(15).

Langmuir:

Freundlich:

Temkin:

Dubinin–Radushkevich (D–R) equation:

ln(q_e) = ln(q_m) − β_D–Rε² (13)

where C_e is the equilibrium concentration, mg L⁻¹; q_e is the adsorption capacity at equilibrium time, mg g⁻¹; b is the Langmuir constant, L g⁻¹; q_max is the maximum adsorption capacity, mg g⁻¹; K_f is the Freundlich constant, L mg⁻¹; 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⁻¹; b_T is the Temkin isotherm constant; RT/b_T is related to adsorption heat, J mol⁻¹. q_m is the theoretical isotherm saturation capacity (mol g⁻¹); β_D–R is the Dubinin–Radushkevich isotherm constant (mol² kJ⁻²), ε is the Dubinin–Radushkevich isotherm constant.

The fitting curves of the different isotherm equations are displayed in Fig. 9.

Fig. 9 The fitting of Langmuir, Freundlich, Temkin, D–R isotherms for Cr(VI) adsorption onto the TW/Fe₃O₄ composite.

The adsorption isotherms and fitting results are shown in Table 4. According to the correlation coefficient, Langmuir, Temkin and D–R isotherms could fit the adsorption process well. The maximum adsorption capacity of Cr(VI) by the TW/Fe₃O₄ composite was 75.76 mg g⁻¹. The maximum adsorption capacity of the TW/Fe₃O₄ 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/Fe₃O₄ composite had a good potential to remove Cr(VI). Thus, the adsorption of Cr(VI) onto the TW/Fe₃O₄ composite related to monomolecular adsorption. With the increase in temperature, the maximum adsorption capacity gradually increased, suggesting that higher temperatures benefit the adsorption process.

Table 4 Isotherm parameters for the adsorption of Cr(VI) onto a TW/Fe₃O₄ composite at various temperatures

Temperature (°C)	Langmuir			Freundlich
	qmax	b	R^2	Kf	n	R^2
25	62.11	0.3214	0.9609	15.3375	1.8854	0.8876
35	67.57	0.4554	0.9786	19.6957	1.8096	0.9150
45	75.76	0.7174	0.9720	28.6141	1.7010	0.9132

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Temperature (°C)	Temkin			D–R
	A	B	R^2	qm	β	E	R^2
25	3.4305	13.4120	0.9647	3.14541 × 10^−5	0.0150	8.1650	0.9330
35	4.7987	14.6180	0.9795	3.6362 × 10^−5	0.0130	8.7706	0.9580
45	7.6963	16.2970	0.9776	5.2102 × 10^−5	0.0100	10.000	0.9500

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A dimensionless separation factor R_L was defined as R_L = 1/(1 + C₀b).⁶¹ The value of R_L indicates the shape of the isotherm to be either unfavourable (R_L > 1), linear (R_L = 1), favourable (0 < R_L < 1) or irreversible (R_L = 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 benefited the adsorption of Cr(VI) onto the TW/Fe₃O₄ composite.

Fig. 10 The variation of R_L under different initial concentrations and temperatures.

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/Fe₃O₄ 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/Fe₃O₄ composite. When E_a is in the range of 1–8 kJ mol⁻¹, physical interaction plays an important role. When E_a is between 8 and 16 kJ mol⁻¹, the main interaction is an ion exchange process. Chemisorption interaction was the main mechanism when E_a ranged from 16 to 40 kJ mol⁻¹.⁶⁴ In this research, E_a ranged from 8 to 10 kJ mol⁻¹ when the temperature was between 25 and 45 °C. The fitting results showed that the main adsorption mechanism was ion exchange between Cr(VI) and the TW/Fe₃O₄ composite, coexisting with physical interaction. With an increase in temperature, the increased E_a suggests that the chemisorption interaction became more pronounced.

3.7. Adsorption thermodynamics

In order to better understand the effect of temperature on the adsorption of Cr(VI) on the TW/Fe₃O₄ composite, the thermodynamic parameters of the adsorption process such as change in standard free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) can be calculated using the eqn (16)–(18).^65,66

ΔG = −RT ln(K₀) (17)

where K₀ is the apparent equilibrium constant. R is the ideal gas constant (4.187 J mol⁻¹ K⁻¹) and T is Kelvin temperature (K).

According to the method suggested by Khan and Singh,⁶⁶ the sorption distribution coefficient K₀ 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 ΔH and ΔS can be obtained from the slope and intercept of a plot of ln K₀ against 1/T and are shown in Fig. 11.

Fig. 11 Plotting of ln(K_d) against 1/T.

The thermodynamic parameters of Cr(VI) adsorption on the TW/Fe₃O₄ composite are given in Table 5. The ΔG value was negative for all three considered temperatures, and this suggests that the adsorption was a spontaneous process. As the temperature increases, the ΔG values decrease, indicating that elevated temperatures benefit the adsorption process. The ΔH value was positive and the adsorption process was endothermic. The positive value of ΔS confirms that the randomness of Cr(VI) increased at the solid–solution interface during the adsorption process.

Table 5 Parameters of thermodynamic models for the adsorption of Cr(VI) onto a TW/Fe₃O₄ composite

Temperature (K)	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)
298	−2.5817	11.964	48.770
308	−3.0214
318	−3.5593

---

3.8. 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 after the contact between the TW/Fe₃O₄ composite and Cr(VI) solutions, the Cr(III) concentration rose in solution. This result also confirmed that a chemical redox reaction occurs during the adsorption process.

Fig. 12 Concentration of total Cr, Cr(VI), Cr(III) under different parameters (a) pH (b) adsorbent mass (c) contact time.

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/Fe₃O₄ 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⁻¹). Cr(III) could be detected in the solution of the adsorption experiment after 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/Fe₃O₄ 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). 30% of the Cr(VI) was adsorbed onto the surface of the TW/Fe₃O₄ and existed in solution.

where, reduction (%) is the reduction ratio of Cr(VI). Cr(III)_solution, Cr(VI)_solution are the concentration of Cr(III), Cr(VI) in solution when the adsorption reached equilibrium (mg L⁻¹). C₀ is the initial concentration of Cr(VI). V is the volume of Cr(VI) solution (L) 1.68/2.68 is the ratio of Cr(III)/[Cr(VI) + Cr(III)] based on the peak area analysis of XPS.

3.9. Desorption and regeneration experiment

Desorption and regeneration batch experiments were designed to study the recoverable potential of the TW/Fe₃O₄ composite. The conditions for the adsorption experiments were 50 mg L⁻¹ 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⁻¹) was used to desorb the adsorbent for 2 h. The Cr(VI) was determined by a spectrophotometric method. Then, HCl (0.5 mol L⁻¹) was used to regenerate the absorbent.

The Cr(VI) removal rate of the TW/Fe₃O₄ composite after five 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/Fe₃O₄ composite, consistent with the measurement of magnetic hysteresis loops (Fig. 4). Desorption of the Cr(VI) from the TW/Fe₃O₄ 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/Fe₃O₄ composite has the potential for regeneration and reusability.

Fig. 13 Desorption–regeneration experiment of the TW/Fe₃O₄ composite for Cr(VI) removal.

3.10. Proposed mechanisms

The mechanism of Cr(VI) removal is not fully understood and involves many models or steps.^69–76 The adsorption mechanism of Cr(VI) onto the TW/Fe₃O₄ 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/Fe₃O₄ composite involved adsorption-coupled 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 confirmed 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/Fe₃O₄ composite played an important role during the adsorption–reduction process.

The ion exchange process is also an important mechanism. The concentrations of Ca²⁺, Mg²⁺, Na⁺, K⁺ in the supernatant were determined and deionized water served as a control. The ion exchange capacities of Ca²⁺, Mg²⁺, 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/Fe₃O₄ composite. The ion exchange capacity was higher when the initial concentration of Cr(VI) was higher. Ca²⁺ and K⁺ ions participate more in the ion exchange process, consistent with previous studies.⁷⁸ The results also confirm the conclusions of the kinetic and isotherm models.

Table 6 The release of Ca²⁺, Mg²⁺, Na⁺, K⁺ during adsorption by a TW/Fe₃O₄ composite at 25 °C^a

Samples	The net amount of released cations (mequiv. g^−1)				Sum
	Ca^2+	Mg^2+	Na^+	K^+
a Note: 10 mg L^−1 to 25 °C means the concentration of Cr(VI) was 10 mg L^−1 and the operating temperature was 25 °C during the adsorption kinetics experiment.
10 mg L^−1 to 25 °C	0.0243	−0.0015	−0.0568	0.0448	0.0109
30 mg L^−1 to 25 °C	0.0264	−0.0003	−0.0505	0.1080	0.0837
50 mg L^−1 to 25 °C	0.0259	0.0003	−0.0431	0.1714	0.1545

---

In brief, the adsorption mechanism of Cr(VI) onto the TW/Fe₃O₄ 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 Fe²⁺ ions may be oxidized by Cr(VI) although Fe₃O₄ nanoparticles are relatively stable. The reaction process follows eqn (21).⁴³ The converted Cr(III) was retained by the TW/Fe₃O₄ 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/Fe₃O₄ composite by carbon–oxygen groups or weak acid surface groups.^79–83

HCrO₄⁻ + 7H⁺ + 3e⁻ → Cr³⁺ + 4H₂O (20)

3Fe²⁺ + CrO₄²⁻ + 4H₂O → Cr³⁺ + 3Fe³⁺ + 8OH⁻ (21)

In conclusion, the proposed mechanism of Cr(VI) removal by the TW/Fe₃O₄ composite is as displayed in Fig. 14. The main steps follow: first step, Cr(VI) is adsorbed quickly onto the surface of the TW/Fe₃O₄ composite due to electrostatic attraction (protonated functional groups or positive Fe₃O₄ nanoparticles) or an ion exchange process or surface complexation. 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/Fe₃O₄ 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.

Fig. 14 Proposed mechanism of Cr(VI) removal by the TW/Fe₃O₄ composite.

4. Conclusions

In this study, a tea waste/nano-Fe₃O₄ composite was facilely prepared and used to remove Cr(VI) from aqueous solution. The maximum removal of Cr(VI) by the TW/Fe₃O₄ composite was observed at pH 2. The adsorption process data was found to best fitted by a pseudo-second-order kinetic model. The maximum adsorption capacity was higher and reached 75.76 mg g⁻¹ based on the Langmuir model. Adsorption of hexavalent chromium ions by the TW/Fe₃O₄ 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/Fe₃O₄ 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/Fe₃O₄ composite could be applied as a novel adsorbent for Cr(VI) removal from industrial wastewater.

List of symbols

bT	Temkin isotherm constant (J mol^−1)
C0	Initial Cr(VI) concentration (mg L^−1)
Ce	Cr(VI) concentration in solution at equilibrium (mg L^−1)
Ct	Concentration of Cr(VI) at t time (mg L^−1)
Kf	Freundlich isotherm constant (mg g^−1) (mg L^−1)^−1
k1	Pseudo-first-order rate constant (L min^−1)
k2	Pseudo-second-order rate constant (g mg^−1 min^−1)
kid	Intraparticle diffusion rate constant (mg g^−1 min^−0.5)
K0	Apparent equilibrium constant
m	Amount of adsorbent (TW/Fe3O4) composite (g)
qt	Adsorption capacities of Cr(VI) ions at time t (mg g^−1)
qe	Adsorption capacities of Cr(VI) ions at equilibrium time (mg g^−1)
qmax	The maximum adsorption capacity based on Langmuir model (mg g^−1)
qm	Maximum adsorption capacity (mg g^−1) in D–R model
R	Ideal gas constant, 8.314 J mol^−1 K^−1
R^2	Coefficient of determination
RL	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
ΔG	Gibbs free energy (kJ mol^−1)
ΔH	Enthalpy (kJ mol^−1)
ΔS	Entropy (J mol^−1 K^−1)
ε	Dubinin–Radushkevich isotherm constant
α	The initial adsorption coefficient in Evolich model (mg g^−1 min^−1)
β	The desorption coefficient in Evolich model (g mg^−1)
βD–R	Dubinin–Radushkevich isotherm constant (mol^2 kJ^−2)

Acknowledgements

This study was financially supported by Natural Science Foundation of China (No. 51609001), National Major Water Project (No. 2015ZX07103007) and Anhui Agricultural University Youth Fund Project (No. 2014zr004, jz2015-23).

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