Adsorption of Cu(II) by phosphogypsum modified with sodium dodecyl benzene sulfonate

Journal Pre-proof Adsorption of Cu(II) by Phosphogypsum Modified with Sodium Dodecyl Benzene Sulfonate Lina Zhao, Qin Zhang, Xianbo Li, Junjian Ye, Jiuyan Chen

PII:

S0304-3894(19)31762-5

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121808

Reference:

HAZMAT 121808

To appear in:

Journal of Hazardous Materials

Received Date:

1 September 2019

Revised Date:

29 October 2019

Accepted Date:

30 November 2019

Please cite this article as: Zhao L, Zhang Q, Li X, Ye J, Chen J, Adsorption of Cu(II) by Phosphogypsum Modified with Sodium Dodecyl Benzene Sulfonate, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121808

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Adsorption of Cu(II) by Phosphogypsum Modified with Sodium Dodecyl Benzene Sulfonate Lina Zhao a, b, c, Qin Zhang a, b, c*, Xianbo Li a, b, c, Junjian Ye a, b, c, Jiuyan Chen a, b, c a

College of Mining, Guizhou University, Guiyang 550025 National & Local Joint Laboratory of Engineering for Effective Utilization of Regional Mineral Resources from Karst Areas, Guiyang 550025 c Guizhou Key Laboratory of Comprehensive Utilization of Nonmetallic Mineral Resources, Guiyang 550025

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Graphical Abstract

Highlights A new adsorbent was prepared with PG modified by SDBS (SDBS@PG).

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SDBS@PG shows high adsorption capacity and removal rate on Cu(II).

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SDBS@PG can be regenerated and used in several adsorption-desorption cycles.

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New insights into the adsorption mechanism of Cu(II) onto SDBS@PG was proposed.

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AB STRACT

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Phosphogypsum (PG) is a solid waste generated during the wet production of phosphoric acid, and stockpiling PG causes serious pollution to the environment. Therefore, we prepared an adsorption material modified with sodium dodecyl benzene sulfonate (SDBS) based on PG (SDBS@PG). SDBS@PG can be regenerated and used in several adsorption-desorption cycles. The optimum conditions for Cu(II) removal are as follows: the Cu(II) concentration is 10 mg/L, the amount of adsorbent is 1.6 g/L, the pH is 6, and the contact time is 60 min. Under these conditions, the removal rate is 99.23%. The kinetic data of adsorption conform to the pseudo-second-order model. The equilibrium isotherm results are consistent with the Langmuir isotherm equation. Furthermore, plausible mechanisms were proposed: PG was modified with SDBS, which greatly improved the adsorption of Cu(II) onto PG. The main reason is that SDBS is adsorbed on the surface of PG by chemical action in the form of micelles and then Cu(II) is adsorbed on the anionic SDBS micelles of SDBS@PG due to chemical and electrostatic interactions. This work indicates that SDBS@PG can be used for the removal of Cu(II) and is qualified for practical application.

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Keywords: phosphogypsum; sodium dodecyl benzene sulfonate; adsorption; Cu(II)

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1. Introduction

Currently, heavy metal ion pollution is receiving increasing attention because of its bioaccumulation and virulence in nature [1]. With the development of industry, heavy metals have been widely studied as major pollutants in the fields of mineral resource development, ceramics, electroplating, etc. [2]. Cu(II) is a transition metal, but when a large number of Cu(II) ions remain in a body, it is a burden on organs. At present, traditional methods for removing heavy metal ions include biotreatment, chemical treatment, reverse osmosis [3], membrane separation, etc. [4]. However, most of these technologies have a high cost and cause environmental pollution, which limits their widespread use [5].

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Adsorption technology plays an important role in environmental protection. [6]. At present, many adsorbents have been used diffusely to deal with heavy metal ions, such as molecular sieves [7], zeolites [8], activated carbon [9], clay [10], and carbon nanotubes [11]. However, these adsorbents still have problems of low removal rate and high cost [12]. Consequently, it is very significant to prepare an adsorbent with a high removal rate, high adsorption capacity and low cost. Phosphogypsum (PG) is solid waste generated during the wet production of phosphoric acid [13]. The main component of PG is CaSO4 ·2H2O [14]. PG contains some impurities, including P2O5 and F- [15]. Approximately 5 tons of PG are obtained for every ton of phosphoric acid produced [16]. Stockpiling PG occupies a large amount of land and results in serious environmental pollution. Hence, the comprehensive utilization of PG has great significance for environmental treatment and resource recycling. Research results have shown that PG has good adsorption performance. L. Yang et al. [17] prepared hydroxyapatite with flue gas desulfurization gypsum and immobilized Cu(II). L. Altas et al. [18] compared the removal of Pb(II) by raw PG with that by lime-conditioned PG and found that both adsorbents could be used as economic substitutes for industrial activated carbon. B. Mesci et al. [19] evaluated the removal of Cu(II) by calcined PG, and the removal rate was 78.34%. Although the replacement of other adsorbents with PG greatly reduces the production cost, there are problems of a low removal rate and complicated operation. Therefore, it is necessary to optimize the modification process to obtain a better PG adsorbent. Physical and chemical modification [20-21] and surface modification [22-24] have been proposed to improve the ability of adsorbents to remove heavy metal ions. Among these methods, surface modification can greatly improve the adsorption properties of adsorbents [25-27]. H.N. Tran et al. [28] prepared a modified zeolite with hexadecyl trimethyl ammonium as a surfactant. The results showed that modified zeolite is a good amphiphilic adsorbent for removing organics and heavy metal ions. Further investigations of the adsorption of heavy metal ions with zeolites modified by hexadecyl trimethyl ammonium bromide have been published by Ren et al. [29], who concluded that hexadecyl trimethyl ammonium bromide adsorbed on zeolites was beneficial to absorbing heavy metal ions. H. Jin et al. [30] used anionic surfactant sodium dodecyl benzene sulfonate (SDBS) to prepare modified fly ash to remove Cu(II). When the Cu(II) concentration was less than 20 ppm, the removal rate was 99.75%. However, the removal of heavy metal ions with PG modified by surface modification has not been reported in the literature. In this study, an adsorbent with low cost was prepared with PG modified by the anionic surfactant SDBS (SDBS@PG). The optimum adsorption conditions for Cu(II) removal by SDBS@PG were studied. The kinetics and equilibrium isotherms of adsorption were also studied. Finally, new insights into the mechanism of Cu(II) on SDBS@PG were proposed to provide theoretical guidance for the utilization of PG resources.

2. Materials and methods

2.1. Materials

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The PG sample was obtained from Guizhou Province, China, and it was a grayish powdery solid. X-ray fluorescence (XRF) spectroscopy (Axios 4kW, Palanco, Netherlands) analysis revealed that CaSO4·2H2O is the main component of PG, with a content of 91.32%. PG contains some impurities, including P2O5 and F-. X-ray diffraction (XRD) (X’Pert PRO, PANalytical, Netherlands) analysis revealed that PG is mainly composed of dihydrate gypsum and contains a small amount of eutectic phosphorus and quartz. Furthermore, the characteristic peak of dihydrate gypsum is located at 11.648°, 20.758°, 23.418°, 29.148° [31]. The test reagents are SDBS (content, ≥90.0%; molecular weight, 348.48; analytical purity; Tianjin Beichen Founder Reagent Factory) and CuSO4·5H2O (content, ≥ 99.0%; molecular weight, 249.69; analytical purity; Tianjin Zhiyuan Chemical Reagent Co., Ltd.). 2.2. Adsorbent preparation

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Pretreatment of PG: The soluble impurities in PG were removed by washing with water, and then the PG was filtered, dried and ground to -75 μm to obtain pretreated PG. SDBS@PG: The pretreated PG was added to the 3.0 g/L SDBS-modified solution with a solid-liquid ratio of 3 g:100 mL, and then this mixture was stirred at room temperature for 60 min, filtered, and dried to obtain modified PG. 2.3. Characterization

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The surface morphology of PG before and after modification was observed by scanning electron microscopy (SEM) (ΣIGMA+X-Max20, Zeiss, Germany). Surface functional groups were characterized by Fourier transform infrared (FTIR) spectroscopy (iS50, Nicolet, America). The scanning wavenumber range was 400-4000 cm-1. The thermal stability and specific surface area of PG and SDBS@PG were investigated by thermogravimetric analysis (TGA) (TGA 2, Mettler, Switzerland) and the Brunauer-Emmett-Teller (BET) method (ASAP 2460, micromeritics, America), respectively. The change in the binding energy of PG surface characteristic elements before and after modification was analyzed by X-ray photoelectron spectroscopy (XPS) (K-Alpha+, Thermo Scientific, America) to determine whether there was chemical adsorption on the PG surface. The zeta potential of PG before and after modification was measured by a Zeta potentiometer (DelsaNano C, Beckman coulter, America ). The Cu(II) concentrations were determined by atomic absorption spectrophotometry (AAS) ( 240FS AA, Agilent, Australia). 2.4. Adsorption experiment All adsorption experiments were conducted in conical flasks that contained 50 mL Cu(II) solution, which was stirred at a speed of 180 r/min in a thermostatic vibrator (SHA-B, Li Chen, China). To obtain optimal conditions for Cu(II) removal, the

effects of pH (1-7), amount of adsorbent (0.4-2.4 g/L) and contact time (5-120 min) were investigated. The formulas of the adsorption capacity (qt) and removal rate (R) are as follows:

qt  R

(C0  Ct )V m

(1)

(C0  Ct ) 100% C0

(2)

where C0 (mg/L) and Ct (mg/L) represent the Cu(II) concentrations at time 0 and t, respectively; V (L) is the volume of Cu(II) solution; and m (g) is adsorbent quality.

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2.5. Desorption and recycling experiment

3.1.

Characterization

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3. Results and discussion

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To improve the use efficiency of the adsorbent, the effects of eluents on the desorption of Cu(II) from Cu(II)-loaded SDBS@PG were studied at room temperature. Three kinds of eluents (H2SO4, NaCl and NaOH) at concentrations of 0.1 mol/L were used for the desorption experiment. To further determine whether the adsorbent can be reused, adsorption-desorption experiments were conducted at room temperature for three cycles. The Cu(II)-loaded SDBS@PG was recovered from a 0.1 mol/L NaOH solution by filtration, washed, and then reused in the next cycle of adsorption experiments.

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Fig. 1 presents the surface morphology of PG before (a) and after (b) modification by SDBS. Fig. 1 (a) shows that the original PG was mainly dominated by oblique crystal columns, and the crystal morphology was mainly rhomboid [18]. As shown in Fig. 1 (b), a large number of small, white, spherical objects are attached to the surface of PG.

Fig. 1. SEM images of PG before (a) and after (b) SDBS modification

Nitrogen adsorption-desorption isotherms (a) and pore size distributions of PG and SDBS@PG (b) are shown in Fig. 2 The BET and Barret-Joyner-Halenda (BJH)

methods were used to determine the specific surface area and pore diameter distribution, respectively. The results showed that the specific surface area of PG is 20.41 m2/g, whereas the BET surface area of SDBS@PG is decreased to 16.58 m2/g. This can be explained by the fact that as SDBS is grafted onto the PG surface, the PG particles become larger. In addition, the single point total pore volume of SDBS@PG (0.0486 cm3/g) is less than that of PG (0.0569 cm3/g). The main reason for this difference is that SDBS was adsorbed on the surface of PG and blocked the pore of PG, leading to a decrease in the pore volume of SDBS@PG. 40

PG SDBS@PG

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Fig. 2. Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of PG and SDBS@PG

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Fig. 3 shows the FTIR spectra of PG, SDBS@PG and Cu(II)-loaded SDBS@PG. The peaks located at approximately 3549 cm-1, 3493 cm-1 and 3400 cm-1 are attributed to O-H stretching vibrations of the crystallized water of PG. The peaks at approximately 1686 cm-1 and 1622 cm-1 are attributed to O-H bending vibrations of the crystallized water. The peak at approximately 1124 cm-1 is attributed to the γ3 stretching vibration of SO42-. The peak at approximately 669 cm-1 and 600 cm-1 are attributed to the γ4 stretching vibration of SO42-. Compared with the spectrum of PG, the spectrum of SDBS@PG contains two new peaks. The peak located at approximately 1477 cm-1 is attributed to the bending vibration of C-H bonds (-CH2, -CH3), and that at 877 cm-1 is attributed to the bending vibration of aromatic hydrocarbons. This result indicated that SDBS was successfully adsorbed onto PG, while in other regions of the spectrum, there are characteristic peaks of PG [18]. In addition, in the spectrum of Cu(II)-loaded SDBS@PG, a C–H bending vibration peak attributed to monosubstituted benzene (BZMA unit) was observed at approximately 786 cm-1 [32]. The main reason for the new peak is the chemical interaction between SDBS@PG and Cu(II) to form a complex compound [33].

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Fig. 3. FTIR spectra of PG, SDBS@PG and Cu(II)-loaded SDBS@PG

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Mass fraction(%)

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Fig. 4 shows the TGA curves of PG (a) and SDBS@PG (b). Both PG and SDBS begin to dehydrate at approximately 110°C. This is mainly due to the loss of crystallized water in PG. At 250°C, the weight loss degrees of PG and SDBS@PG are 19.68% and 19.11%, respectively. Overall, the results showed that SDBS grafted onto PG had little effect on the thermal stability of PG.

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Fig. 4. TGA curves of PG (a) and SDBS@PG (b) under an atmosphere at a heating rate of 10℃/min

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Fig. 5 presents the XPS spectra of PG and SDBS@PG. Table 1 shows the content and binding energy of surface characteristic elements before and after PG modification. Compared with the original PG, it is obvious that the contents of O and Ca decrease and the percentage content of C increases significantly, indicating that the CaSO4·2H2O of the PG surface was wrapped by SDBS. Furthermore, compared with that of the original PG, the binding energy of Ca2p on the surface SDBS@PG decreased by 2.47 eV, which shows that the Ca on the surface of PG has undergone chemical changes [34]. Thus, SDBS was grafted onto the PG surface by chemical adsorption.

O1s SDBS@PG PG

Counts(s)

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Fig. 5. XPS spectra of PG and SDBS@PG

Adsorption experiment

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To determine the optimal conditions for Cu(II) adsorption, the pH, amount of adsorbent, and contact time were studied. Furthermore, the kinetics of the adsorption and equilibrium isotherms were also investigated.

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3.2.1. Adsorption test of Cu(II) on SDBS@PG Fig. 6 shows the adsorption capacity and removal rate of PG before and after modification. The adsorption conditions were as follows: Cu(II) concentration: 10 mg/L; amount of adsorbent: 1.6 g/L; temperature: 298 K; contact time: 60 min; initial pH: 6.0. Compared with that on PG, the adsorption of Cu(II) on SDBS@PG was greatly improved, and the removal rate of Cu(II) by SDBS@PG is approximately 60% higher than that by PG.

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Fig. 6. Adsorption test for Cu(II) removal by PG and SDBS@PG (298 K)

3.2.2. Effect of pH on Cu(II) removal Since Cu(II) easily precipitates in alkaline solution, the pH is controlled at 1-7. The adsorption of Cu(II) was not good, as shown in Fig. 7 (a), at pH<3 but then increased

gradually as the pH increased in the range of pH 3-6. The reason is that H+ and Cu(II) compete to occupy the adsorption sites on the SDBS@PG surface in a strongly acidic solution. The larger the H+ concentration is, the stronger the competition, so the adsorption capacity and removal rate are low. When the pH values increase gradually, the competition of H+ decreases, so the adsorption capacity and removal rate increase gradually. Therefore, pH = 6 is the best pH for the test. Fig. 7 (b) shows that the zeta potentials of PG before and after modification decrease as the pH increases, and the surfaces are all negatively charged. However, compared with the trend for PG, the trend for SDBS@PG decreases dramatically, and the negative charge is stronger than that of PG, which indicates that SDBS significantly reduces the surface potential of PG. Therefore, SDBS successfully modified PG. 8

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Fig. 7. The effect of pH on the Cu(II) removal (a) (Cu(II) concentration: 10 mg/L; amount of adsorbent: 1.6 g/L; temperature: 298 K; contact time: 60 min) and zeta potential of PG before and after modification at different pH values (b)

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3.2.3. Effect of the amount of adsorbent on Cu(II) removal The effect of the amount of adsorbent on Cu(II) removal by SDBS@PG is shown in Fig. 8. The removal rate radically increases as the amount of adsorbent increases, which is caused by the increase in the number of adsorption active sites. However, the adsorption capacity decreases when the amount of adsorbent increases, which is caused by the initial constant Cu(II) concentration. After comprehensive consideration, the optimal amount of adsorbent is 1.6 g/L. When the optimal amount of adsorbent is obtained, the removal rate is 98.39%, and the adsorption capacity is 6.15 mg/g.

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Fig. 8. The effect of the amount of adsorbent on Cu(II) removal (Cu(II) concentration: 10 mg/L; pH: 6.0; temperature: 298 K; contact time: 60 min)

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3.2.4. Effect of contact time for Cu(II) removal Fig. 9 shows the effect of contact time on Cu(II) removal by SDBS@PG. As the time increases, the adsorption capacity increases rapidly, and adsorption equilibrium is almost reached 60 min. At this time, the adsorption capacity of Cu(II) is 6.202 mg/g, 11.663 mg/g and 15.768 mg/g when the concentration is 10, 20 and 30 mg/L, respectively. 16

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Fig. 9. The effect of contact time on Cu(II) removal (pH: 6.0; temperature: 298 K; amount of adsorbent: 1.6 g/L; Cu(II) concentration: 10, 20, 30 mg/L)

3.2.5. Adsorption kinetics The equations of the pseudo-first-order kinetics model (3), pseudo-second-order kinetics model (4), Elovich adsorption equation (5), and intraparticle diffusion model (6) are as follows [35]. They were adopted to study the adsorption kinetics of Cu(II) on SDBS@PG, and Fig. 10 shows the analysis results.

ln (qe  qt )  ln qe  k1t

(3)

t 1 t   2 qt k 2 qe qe

(4)

qt  a  b ln t

(5)

qt  kintt 0.5  C

(6)

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where qe (mg/g) is the equilibrium adsorption capacity; qt (mg/g) is the capacity at time t; and k1, k2, a, b, k and C are all kinetic equation constants. The pseudo-first-order (Fig. 10 (a)) and the pseudo-second-order (Fig. 10 (b)) kinetic models were fitted. The simulated parameters are shown in Table 2. For the pseudo-second-order kinetic model, the values of theoretical qe are closer to the experimental qe, value, and the correlation coefficients (R2) are all greater than those of the pseudo-first-order model. Furthermore, the Elovich adsorption equation (Fig. 10 (c)) and the intraparticle diffusion model (Fig. 10 (d)) were used for analysis. The kinetic parameters are also shown in Table 2. The curve of the Elovich adsorption equation and the intraparticle diffusion model both have a certain offset when the time increases. The R2 values of the three different concentrations are all low. The simulation of the intraparticle diffusion model found no straight lines through the origin, so the adsorption process is not controlled by only intraparticle diffusion [36]. In summary, the experimental kinetic data conform to the pseudo-second-order equation. 2

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Fig. 10. Fitting curves of dynamic modes

3.2.6. Adsorption isotherms Fig. 11 shows the adsorption isotherms for Cu(II) removal by SDBS@PG. The equilibrium adsorption capacities increase when the concentrations increase. Furthermore, the Langmuir and Freundlich models were used to assess the effect of temperature on the adsorbent equilibrium capacity. The equations of the Langmuir (7) and Freundlich (8) models are given as follows [37]: Langmuir:

Ce C 1   e qe bqm qm

(7) (8)

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1 Freundlich: log qe  log K F  log Ce n

where qe (mg/g) and qm (mg/g) are the adsorption capacity at equilibrium and the maximum adsorption capacity, Ce (mg/L) is the concentration at adsorption equilibrium, b (L/mg) is the adsorption strength, KF (mg/g) is the adsorption coefficient, and 1/n is the adsorption index.

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Fig. 11. Adsorption isotherms for Cu(II) removal (pH: 6.0; amount of adsorbent: 1.6 g/L; contact time: 90 min)

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Fig. 12 and Table 3 show the simulation results of the Langmuir and Freundlich models at different temperatures and the isotherm simulation parameters, respectively. The R2 values of the Langmuir isotherm adsorption model are all higher than the values of the Freundlich model at the three test temperatures. The Cu(II) removal by SDBS@PG is consistent with the Langmuir isotherm adsorption model. In addition, the adsorption belongs to the single molecular layer adsorption [38], and 23.33 mg/g is the maximum adsorption capacity.

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(Ce/qe)/(g/L)

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Fig. 12 Isothermal model fitting curves at different temperatures

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3.2.7. Comparison of Cu(II) adsorption on different modified PGs The relevant literature on Cu(II) adsorption on modified PG was compared in this study, as shown in Table 4. Compared with other modified PGs reported in the literature, SDBS@PG has obvious advantages in Cu(II) adsorption because of its higher adsorption capacity and higher removal rate. Therefore, SDBS@PG is a potentially efficient and low-cost adsorbent. 3.2.8. Reusability of SDBS@PG The experimental results of desorption are shown in Fig. 13. As shown in Fig. 13, when 0.1 mol/L NaOH was used for the desorption experiment, the removal rate of Cu(II) reached a maximum of 98.21%. Therefore, compared with H2SO4 and NaCl, NaOH is a better eluent to desorb adsorbed Cu(II) from the Cu(II)-loaded SDBS@PG and is used in recycling experiments.

R(%)

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NaCl

NaOH

Fig. 13. The effects of SDBS@PG on Cu(II) removal under different regeneration conditions

Fig. 14 shows that the removal rate of Cu(II) in three cycles by SDBS@PG is basically constant. After three cycles, the removal rate remained at approximately 98%. The results showed that SDBS@PG may be regenerated and used in several adsorption-desorption cycles and has qualified for practical application.

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Fig. 14. The effect of regeneration times of SDBS@PG on Cu(II) removal

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3.2.9. Mechanism As shown in Fig 6, the removal rate of Cu(II) by SDBS@PG is approximately 60% higher than that by PG. However, the BET surface area of SDBS@PG is smaller than that of PG. Therefore, it is necessary to study the mechanism. The results are shown in Figs. 15 and 16. Modification mechanism: (1) A new adsorption material was obtained by SDBS modification, i.e., SDBS@PG. Compared with that of PG, the binding energy of Ca2p on the surface of SDBS@PG shows a 2.47 eV displacement (Fig. 5 and Table 1). The main mechanism is that the Ca on the surface of PG has undergone chemical changes. In other words, the SDBS modifier is adsorbed on the surface of PG by chemical adsorption. Z. Duan, et al. also obtained a similar conclusion [34]. (2) Y. Dong et al. [41] and J.L. Anderson et al. [42] noted that when the surfactant was at a lower concentration, a hydrophobic monolayer surface was formed at the liquid-air interface (the hydrophobic tails faced the air, and the hydrophilic head faced the liquid). When the concentration of surfactant exceeded the critical micelle concentration (CMC), micelles were formed (hydrophobic tail faced the center, hydrophilic head outward), and the charged hydrophilic head of the micelles interacted strongly with the ions. In addition, the simulations of F. Palazzesi et al. [43] and C. Yang et al. [44] showed that most of the micelles were bilayer or spherical micelles. In this study, when the SDBS concentration was lower than that of CMC, a hydrophobic monolayer surface was formed, as shown in Fig. 15(a). When the SDBS concentration was higher than that of CMC, an SDBS bilayer or spherical micelles was formed, as shown in Fig. 15(b). Obviously, the negatively charged head of SDBS@PG is prone to electrostatic attraction with positively charged ions [45], while the monolayer surface is not conducive to adsorption. In summary, SDBS was chemically adsorbed onto the surface of PG to form a bilayer or spherical micelles, as shown in Fig. 15(c). Adsorption mechanism: (1) SDBS was grafted onto the surface of PG in the form of bilayer micelles, which enhances the electrostatic attraction between Cu(II) and SDBS@PG [30]. In addition,

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the FTIR spectra of the Cu(II)-loaded SDBS@PG in Fig. 3 show that the adsorption mechanism involves the formation of a complex compound due to the chemical interaction between Cu(II) and SDBS@PG. In conclusion, Cu(II) was adsorbed on the anionic SDBS bilayer micelles of SDBS@PG due to chemical and electrostatic interactions, as shown in Fig. 16 (a). (2) SDBS was grafted onto the surface of PG in the form of spherical micelles. On the one hand, Cu(II) was adsorbed onto the anionic SDBS spherical micelles of SDBS@PG due to chemical and electrostatic interactions. On the other hand, Cu(II) could be dissolved and diffused into the micelles[46]. SDBS was grafted to the surface of PG in the form of spherical micelles, which have more binding sites and stronger electrostatic attraction to Cu(II) than bilayer micelles, as shown in Fig. 16 (b). (3) Since PG is slightly negatively charged, as presented in Fig. 7(b), and Cu(II) is positively charged, it is expected that free Cu(II) will be readily adsorbed on the surface of PG because of electrostatic attraction, as shown in Fig. 16 (c). This is the secondary reason for Cu(II) removal.

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(a) SDBS concentration < CMC

(b) SDBS concentration ≥ CMC

Fig. 16. Adsorption mechanism

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4. Conclusions

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Fig. 15. Modification mechanism

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(1) In this work, a new adsorbent with low cost was prepared consisting of PG modified by SDBS, which has a strong ability to remove Cu(II). When the Cu(II) concentration is 10 mg/L, the dosage of adsorbent is 1.6 g/L, the pH value is 6 and the adsorption time is 60 min, the removal rate of Cu(II) is 99.23%. (2) The kinetic data of adsorption conform to the pseudo-second-order equation; the experimental equilibrium isotherm results are more consistent with the Langmuir isotherm model than with the Freundlich model. (3) The adsorbed Cu(II) can be desorbed from the Cu(II)-loaded SDBS@PG using 0.1 mol/L NaOH as the eluent. Regenerated SDBS@PG can be used in several adsorption-desorption cycles, so SDBS@PG was qualified for practical application. (4) Plausible mechanisms were proposed: PG was modified with SDBS, and SDBS@PG has a strong ability to remove Cu(II). The main reason is that SDBS is chemically adsorbed on the surface of PG in the form of bilayer micelles or spherical micelles, and then Cu(II) is adsorbed on the anionic SDBS micelles of SDBS@PG due to chemical and electrostatic interactions; additionally, Cu(II) is adsorbed on the PG, which is slightly negatively charged. (5) The results show that SDBS can be used for surface modification of PG and has a good adsorption effect on Cu(II). This study provides theoretical guidance for the resource utilization of PG.

Acknowledgment This research was funded by the National Key R&D Program of China (Grant No. 2018YFC1903501)

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Table 1 Analysis of surface characteristic elements on PG and SDBS@PG with XPS Elements Binding energy (eV) Chemical shift (eV) Element content (%)

PG

SDBS@PG

Ca2p

348.68

-

12.27

O1s

531.85

-

61.71

C1s

285.07

-

9.8

S2p

169.15

-

16.22

Ca2p

346.21

2.47

10.35

O1s

531.85

-

46.36

C1s

285.07

-

30.23

S2p

169.15

-

13.06

Pseudo-first-order kinetics

qe (mg/g) k1 (min)

R12

0.038

0.335

0.937

20

0.029

2.282

0.747

30

0.050

6.843

0.880

qe (mg/g)

0.440

0.999

11.916

0.061

0.999

16.474

0.021

0.998

Elovich

Ci (mg/L)

b

R2

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a

k2 (g·mg-1·min-1） R12

6.237

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Pseudo-second-order kinetic

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Ci (mg/L)

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Table 2 Kinetic parameters for Cu(II) removal

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Adsorbent

Intraparticle diffusion model

K (mg·g ·min-0.5)

C

Rint2

-1

5.747

0.114

0.966

0.050

5.841

0.893

20

8.033

0.948

0.840

0.395

8.924

0.642

30

9.113

1.487

0.836

0.696

10.141

0.939

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Table 3 Isotherm parameters for Cu(II) removal by SDBS@PG Temperature/K

Parameter

Langmuir

298

308

318

b (L/mg)

2.310

2.252

2.146

qm (mg/g)

17.590

19.463

23.326

0.995

0.998

0.990

KF (mg/g)

9.387

10.298

13.130

1/n

0.187

0.229

0.198

2

0.975

0.977

0.967

R Freundlich

R

2

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Model

Table 4 Comparison of Cu(II) adsorption on different modified PGs qm (mg/g)

R (%)

References

Calcined PG

-

78.34

[19]

Lime-preconditioned PG

2.824

-

[39]

Microwave-preconditioned PG

3.937

-

[40]

SDBS@PG

23.33

99.23

This study

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Adsorbent