Detection of Hexavalent Chromium by Copper Sulfide

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Detection of Hexavalent Chromium by Copper Sulfide Nanocomposites Meihong Zhan, Hongmei Yu, Lihua Li, David T. Nguyen, and Wei Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04501 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Analytical Chemistry

Detection

of

Hexavalent

Chromium

by

Copper

Sulfide

Nanocomposites Meihong Zhan†, Hongmei Yu*,†, Lihua Li*,†, David T. Nguyen‡, and Wei Chen*,†, ‡ † School

of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051,

People’s Republic of China ‡Department

of Physics, The University of Texas at Arlington, Arlington, Texas 76019-0059, United

States

AUTHOR INFORMATION Corresponding authors: *E-mail: [email protected] (H.M. Yu); [email protected] (L.H. Li). Phone: 86-412-592-8590; Fax: 86-412-592-9627; E-mail: [email protected]. Phone: 817-272-1064; Fax: 817-272-3637.

Notes The authors declare no competing financial interest.

ABSTRACT Chromium (Cr) is a vital environmental contaminant. In environmental matrices, Cr presents dominantly in hexavalent chromium [Cr(VI)] and trivalent chromium [Cr(III)], which are two different inorganic states. Cr(VI) is a well-known human carcinogen, while Cr(III) is a naturally occuring micronutrient for the human body. Hence, speciation of Cr is crucial for ensuring environmental water and food safety. The distinction between each chromium species is almost impossible with commonly used methods like atomic spectrometric techniques, due to the low level of Cr and the high complexity of the matrix. Typically, a preconcentration step is required prior to determining and improving the selectivity and ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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detection limit of the inspection instrument. For this process, nanocomposites, which are emerging high-quality adsorbents are used. However, preparation of previous nanocomposites suffered from trivial synthesis process; had high reagent consumption and was time-consuming. Therefore, we succinctly designed and fabricated the novel triadic silica gel-supported copper sulfide (CuS) nanocomposites, for ultrasensitive detection of Cr(VI). CuS nanocomposites in a solid phase system were successfully applied to extract Cr(VI) in reference materials, such as food and water samples. Cr(VI) were detected by use of flame atomic absorption spectrometry (FAAS). By loading 30 mL of sample solution, a linear range of 0.5-300 μg L-1 with R=0.998, a detection limit of 0.15 μg L-1 and an accuracy of 1.7% (20 μg L-1, n=11) were obtained by this method. The detection limit and the precision of this procedure are superior to those reported in the literature with same detection technique, indicating that CuS nanocomposites have a good potential for Cr(VI) detection which is essential for food safety and human health.

Keywords: CuS nanocomposites; Speciation; Solid phase extraction; Cr(VI)

INTRODUCTION Chromium, a commonly used engineering metal, has become an alarming environmental pollutant due to its extensive application in industrial and chemical processing.1,2 Unfortunately, due to the widespread use of Cr, the soil and water has become severely contaminated in recent years.3,4 Chromium can be divided into numerous chemical forms systematically, Cr(III) and Cr(VI) are the two major species present in the environment due to higher stability.5 Cr(III) is required for the metabolism in the body, while Cr(VI) is a highly toxicmetal-oxyanion which is carcinogenic and mutagenic to the human body. So, it is important to accurately monitor and determine the content of chromium in environment and food. However, due to the low concentration of chromium and the complexity of the matrix, it is almost impossible to sensitively determine the trace chromium in actual samples by current universal determination technologies.6-8 Therefore, it is imperative to develop more efficient separation/ preconcentration methods for chromium detection and speciation. Some commonly used methods are ACS Paragon Plus Environment

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electrochemical separation,5,9 ion exchange,10 cloud point extraction,11 solvent floatation method,12 liquid-liquid extraction13 and solid phase extraction (SPE).6-8,14,15 Among these methods, the mini-column SPE technique has been widely investigated for the separation/preconcentration of chromium ions. This is due to its excellent properties, such as good selectivity, low consumption of organic solvents, simplicity, rapid phase separation and many selectable adsorbents.16 It is worth mentioning that sorbent materials play a vital role in SPE procedure. In this sense, it is our goal to find a predominant adsorbent material or modify existing adsorbents to achieve efficient separation/ preconcentration of chromium.17 In recent years, nanomaterials have attracted great attention as a potential solid phase adsorbents as opposed to traditional adsorbents for the preconcentration and speciation of chromium in environmental samples owing to their surface unsaturation, high specific surface area, excellent mechanical strength, unique electrical properties and high chemical stability.18-20 However, when nanomaterials were directly packed into a mini-column SPE system, they tended to create a flow resistance and deteriorated the separation/preconcentration performance due to their small sizes. Moreover, nanomaterials are easily agglomerated and inactivated in aqueous solutions. Thus, it was important to improve the usability of nanomaterials in SPE such as adsorption capacity, sensitivity, stability and the retention of chromium species.1 For this purpose, it is often implemented that nanomaterials are fixed on a carrier or their surfaces are modified.Silica gel (SG) is one of the most commonly used carriers due to its outstanding properties, such as high adsorption performance, good thermal stability and chemical stability.21 Conventionally, nanomaterials are introduced onto the surface of SG modified by amino groups. Many nanocomposites have been reported for SPE of trace chromium, such as bismuthiol-II-immobilized silica-coated magnetic nanoparticles,21 ruthenium nanoparticles loaded on activated carbon,22 modified multiwalled carbon,13 nano TiO2 immobilized on silica gel,23 nano-Au/TiO2,24 Fe3O4@ZrO2 nanoparticles,25 cross linked chitosan-bound FeC nanoparticles,26 magnetite-graphene oxide27 and SiO2/Al2O3/TiO2.28 However, these nanocomposites for the preconcentration of chromium have a narrow linear range or high detection limit. Also, the preparation processes of these nanocomposites were cumbersome and time-consuming. The present work is focused on simple and rapid preparation the novel nanocomposites as the ACS Paragon Plus Environment


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adsorbent for chromium preconcentration. Systematically, CuS nanoparticles are loaded on the surface of SG coated with polyhexamethylene guanidine hydrochloride (PHMG). PHMG is a cationic polymer-oligomer and is used as an intermediate to realize the assembly of nano-CuS and SG to prepare novel CuS nanocomposites, namely, nano-CuS@PHMG@SG. First, nano-CuS is one of the most important nanomaterials in transition metal sulfide and Cr(VI) can be reduced to Cr(III) by it.29,30 Secondly, the silanol groups on the surface of SG behave like a weak acid, hence they can be wrapped by polar compounds through strong hydrogen bonds or dipole-dipole interactions.31,32 Lastly, PHMG is a thermally stable cationic polymer with guanidyl and amidogen groups. It tends to strongly bind to negatively charged particles.33 Herein, we prepared a novel solid phase adsorbent CuS nanocomposites. The strategy is built on the formation of hydrogen bonds between PHMG and SG, then self-assembly with nano-CuS, which forms a sandwich ternary composite by electrostatic attraction. Then the CuS nanocomposites were successfully applied to preconcentration of Cr(VI) in food and water samples. The accuracy of this system was verified by Cr(VI) standard substance. In addition, the dynamic adsorption model and kinetic model were established. The adsorption mechanisms have been studied in depth. The preliminary results show that this method has a wider linear range, a lower detection limit and a better accuracy. To the best of our knowledge, CuS nanoparticles have never been reported for extraction and determination of Cr(VI) even though they have been extensively investigated for cancer photothermal therapy34,35 and photodynamic therapy.36 The current discovery reported here enriches the applications of CuS nanoparticles with multifunction.

EXPERIMENTAL Reagents. 1g L-1 Cr(VI) and Cr(III) stock solutions were prepared with K2Cr2O7 and CrCl3·6H2O (Beijing Chemicals Reagents Co., Beijing, China), respectively. The working solutions with the pH of 1-12 was adjusted by 0.1 mol L-1 HNO3 or NaOH (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were obtained daily by stepwise dilution of standard solution. The PHMG was purchased from Taizhou Suning Chemical Co., Ltd (Taizhou, China). The SG of 80-100 mesh was the product of Qingdao Haiyang chemical Co., Ltd (Qingdao, China). The nano CuS were obtained from our previous work.37 The reference materials GSB07-3174-2014 water sample and GBW10011 wheat provided by Institute for Reference Materials of SEPA (Beijing,China) and Geophysical and ACS Paragon Plus Environment

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Geochemical Exploration Institute (Langfang, China), respectively, were used to verify the accuracy of this method. The double deionized water (DDW; ≥18.2 MΩ cm-1) was used throughout the whole experimental process, and all the chemical reagents used in this experiment are of analytical reagent grade or better. Preparation of nano-CuS.37 A ring-shaped nano-CuS was synthesized via a simple hydrothermal reaction, using Cu(AC)2·H2O and Na2S2O3·5H2O as reactants. 80 mL of Cu(Ac)2·H2O (0.01 mol L-1) water-alcohol solution and was added dropwise to 80 ml of Na2S2O3·5H2O (0.01 mol L-1) water-alcohol solution (All water-alcohol ratio is 3:1) under continuous magnetic stirring at the room temperature. After the dropwise addition was completed, the reaction solution was transferred to a 200 mL reaction kettle. Then the reaction kettle was placed into an oven (Shanghai Jinping Instrument Co., Ltd., Shanghai, China) and maintained at 140 ℃ for approximately 12 hours. When the reaction was finished, the kettle was cooled to the room temperature. The resulting precipitate was centrifuged, washed thoroughly with DDW and alcohol for several times and dried in a DZF-6021vacuum oven (Shanghai Yuezhong Instrument Equipment Co., Ltd., Shanghai, China) at 40 ℃ for 6 hours. Preparation and characterization of CuS nanocomposites. The CuS nanocomposites were fabricated by a three-step process. First, the SG was used as a support material and was activated to remove impurities and increase the content of silanol groups on the SG surface. Specifically, 6 g of SG was dispersed in 60 mL of 1.5 mol L-1 HNO3 solutions, stirred for 30 minutes and then sealed for 16 hours. The SG was filtered, simultaneously flushed with DDW to remove residual acid, and was dried in an oven at 120 ℃ for 6 hours. Secondly, the activated SG was coated with PHMG to prepare PHMG@SG binary material. In detail, 100 mg activated SG was added in 8 mL 1% (m/m) PHMG solution, then thoroughly mixed in a KQ250-DBCNC ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China) for 30 minutes. During this step due to hydrogen bond forces, the SG was wrapped by PHMG. Finally, nano-CuS was loaded on PHMG@SG by electrostatic attraction. 10 mg nano-CuS was then dispersed in 2 mL aqueous solution, then added into the PHMG@SG suspension. The mixture system was shaken and sonicated for 30 minutes. The targets were gathered by centrifugation with a SF-TGL-16M high-speed centrifuge (Shanghai Feiqiaer Analysis Instrument Co., Ltd., Shanghai, China) for 15 minutes at the rate of 10000 revolutions per minute. The precipitates ACS Paragon Plus Environment


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obtained by centrifugation were dried in a vacuum oven at 40 ℃ for 10 hours. CuS nanocomposites were successfully prepared by self-assembly method. The specific process was shown in Figure S-1. The structure and properties of material were characterized by an EQUINOX55 Fourier transform infrared spectroscopy (FT-IR) (Bruker, Germany) and a Zeiss-∑IGMA HD model field emission scanning electron microscopy (FESEM) (Carl Zeiss, Germany) assembly an X-max50 energy dispersive spectrometer (EDS) (Oxford Corporation, UK). SPE-FAAS analysis of Cr. The solid phase extraction experiment was carried out on a BT100SV2-CE peristaltic pump (Baoding Lead Fluid Technology Co., Ltd., Baoding, China) with a silicone rubber tube (4.0 mm o.d.×2.0 mm i.d.) and a 3 cm PTFE mini-column (3.0 mm o.d.×2.0 mm i.d.). The column was packed with 20 mg CuS nanocompositesand was linked to the flow path of the peristaltic pump system, for the separation/preconcentration of Cr(VI). Nanocomposites were activated by driving a 1.2 mol L-1 HNO3 solution for 30 seconds. Also, DDW was used for 1 minute at a flow rate of 2.5 mL min-1 to pass through. The column was then evacuated by air. The 30 mL Cr(VI) sample solution was regulated to the optimal pH of 4.0. Then the solution was inhaled into the pipeline at a rate of 2.5 mL min-1. The Cr(VI) retained on CuS nanocomposites was eluted with 1.0 mL of 1.2 mol L-1 HNO3 solution (flow rate: 2.0 mL min-1) and was determined by an AAnalyst-200 atomic absorption spectrometer (PerkinElmer, Waltham, USA) with a Perkin Elmer Lumina lamp (Cr). The operating conditions are as follows: wavelength: 357.87 nm; slit width: 2.7 mm; slit height: 0.8 mm; air flow: 10.0 L min-1; acetylene flow: 4.1 L min -1; burner height: 10 mm; lamp current: 15 mA. However, Cr(III) needed to be oxidized to Cr(VI) to detect total chromium. 10 mL of 3% (w/v) hydrogen peroxide solution was added into the sample solution (pH 11.75) and heated in a water bath at 80 ℃ for 1 hour. Then the sample solution was boiled for 8 minutes to remove excess H2O2.38 The total content of chromium was determined by the test procedure given above. Then the amount of Cr(III) was calculated by subtraction. Samples pretreatment. Two actual flours were produced in Liaocheng (China) and Jilin (China), respectively. Flours were digested in an MDS-6G multiflux microwave digestion/extraction system (Sineo Microwave Chemistry Technology Co., Ltd., Shanghai, China) with a mixture of 8 mL of HNO3 with 2 mL of H2O2. Specific dissolution procedures were recorded in Table S-1. Four environmental ACS Paragon Plus Environment

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water samples were collected from Anshan (China), including rainwater, lake water, waste water and tap water. Before testing, water samples were filtered by 0.22 µm filter membrane. The pH values of analytical samples were adjusted to 4.0 with HNO3. The contents of chromium in samples were determined by the procedure given above. Batch experiments. The batch experiments were performed on the SPE system at room temperature. In the adsorption isotherm experiments, different concentrations (0.2-10.0 mg L-1) of Cr(VI) at pH 4.0 were driven through the mini-column at the rate of 2.5 mL min-1 for 18 minutes. After preconcentration, the concentrations of Cr(VI) ions in the effluent were determined by FAAS. The adsorption capacity was calculated by Eq. (1): qe 

(C0  Ce )V m

(1)

Where C0 and Ce (mg L-1) represent the initial and equilibrium concentration. qe (mg g-1) is the equilibrium adsorption capacity. V (mL) and m (g) are the sample volume and the mass of CuS nanocomposites, respectively.6, 14 Breakthrough experiments. The breakthrough (Ct/C0-time profile) experiments of the mini-column which described the dynamic performance of the mini-column were carried out on the peristaltic pump mentioned above. In the dynamic adsorption experiments, Cr(VI) ions solutions were pumped into the mini-column under the optimal conditions. Simultaneously, the effluent was taken at regular time intervals of 30 seconds until the Cr(VI) concentration in the effluent was found to be equal to the injection concentration.

RESULTS AND DISCUSSION Preparation conditions of CuS nanocomposites. Cr(VI) ions could be absorbed by the amino group under electrostatic attraction and coordination interaction.6,33,39 Therefore, the effects of the type and concentration of cationic polymer with amino groups were first studied. Three polymers, polydimethyl diallyl ammonium chloride (PDDA), polyhexamethylene biguanide hydrochloride (PHMB) and PHMG, were introduced onto the SG surface at the fixing mass ratio of nano-CuS and SG. The results are illustrated in Figure S-2. The PHMG showed better performance for Cr(VI) adsorption than PHMB and ACS Paragon Plus Environment


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PDDA at any same level. In practice, the PHMG was selected as an excellent intermediate for the preparation of the CuS nanocomposites. Furthermore, the effects of the PHMG concentration in the range of 0.25%-2.0% (m/m) have been investigated systematically and the results were also listed in Figure S-2. It showed that when the concentration PHMG is increased from 0.25% (m/m) to 1% (m/m), the absorbance value of Cr(VI) increases rapidly and reaches the maximum. Almost no variation of the absorbance is observed when further increasing the concentration up to 2.0% (m/m). Thus 1% (m/m) of PHMG is selected as the intermediate for the synthesis process. Nano-CuS has exhibited a strong reducibility to Cr(VI).29,30 The influences of mass ratio of nano-CuS and SG in the range of 1:5-1:50 were explored. In addition, the adsorptions of Cr(VI) by single SG, single nano-CuS and PHMG@SG were researched under the consistent experimental conditions, respectively. The results were recorded in Figure S-3. It could be found that SG has almost no the retention ability for Cr(VI) ions due to the negative electricity of its surface, but nano-CuS can contribute partial retention. And for the ternary CuS nanocomposites, the recovery of Cr(VI) were stable around the maximum, which was within the range of 1:5-1:30 of mass ratio of nano-CuS and SG. With the continuous decrease of the ratio to 1:50, the amount of Cr(VI) ions adsorbed on CuS nanocomposites was reduced but still better than that retained on PHMG@SG and single nano-CuS. Comprehensive consideration, the mass ratio of nano-CuS to SG is 1:10 for the subsequent experiments. Materials characterization. The FESEM and EDS of CuS nanocomposites and transmission electron microscope (TEM) and X-ray diffraction (XRD) of nano-CuS data obtained were investigated in Figure 1 (a, b, c, d and e). It was observed from the FESEM micrograph (Figure 1a) that the morphology of CuS nanocomposites is irregular and its surface has many small particles. Furthermore, the surface of the composites was magnified 20,000 times (Figure 1b), and was observed that the surface had a certain roughness. In addition, C, N and Cl elements of PHMG and Cu and S elements of nano-CuS were observed on the EDS spectrum of CuS nanocomposites (Figure 1c) besides two characteristic elements Si and O of SG. Furthermore, the size, crystal form and purity of the small particles on the surface of CuS nanocomposites were also studied. Figure (1d) showed the particle size and morphology of the nano-CuS by the use of TEM. Figure (1d) showed that nano-CuS was a ring-shaped particle which was similar to the texture of the surface of the composites. The ring-shaped nano-CuS was self-assembled by ACS Paragon Plus Environment

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irregular rods with a length of 30-70 nm and a width of 20-40 nm. The crystal structure and crystal phase composition of nano-CuS were analyzed by XRD pattern as shown in Figure 1e. The diffraction pattern of the nano-CuS we used is consistent with the standard diffraction spectrum of the hexagonal crystal structure of CuS in the JCPDS card (NO.06-0464, a=3.804Å, c=16.378Å). When the values of 2θ are 31.82 º , 47.94 º and 29.28 º , the diffraction peaks of CuS in the nanoparticles are stronger which correspond to diffraction crystal plates 103, 110 and 102, respectively. This data demonstrates that the nanoparticles on the surface of composites are pure and highly crystalline nano-CuS. The above demonstrated the successful compounding of nano-CuS@PHMG@SG composites.

c

110

8000

116

101

4000

100

c

006

6000

108

ee

d

105

b

102 103

a

Intensity Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

0 20

30

40

50

60

70

2θ(degree)

Figure 1. FESEM (a: mag=100 X; b: mag=20,000 X) and EDS (c) profiles of CuS nanocomposites and TEM (d) and XRD (e) of nano-CuS. The investigation of FT-IR was presented to prove the combination of the nanocomposites. The FT-IR spectra of PHMG (a), SG (b), before absorption (c) were shown in Figure S-4. The bands approximately 3000-4000 cm-1 were attributed to the stretching vibrations of O-H and N-H. And the stretching vibration of N-H can be masked by the broad and strong absorption peak of O-H. In spectral line c, not only the characteristic absorption peaks of SG (curve b) are detected, but also the absorption peaks of the functional groups carried by PHMG (curve a) are also found. Specifically, the peaks at 1121 cm-1 and 817 cm-1 are attributed to the stretch of Si-O and Si-O-Si in SG (curve b). The absorption peaks at 1686 cm-1 and 1473 cm-1 are assigned to the stretching vibration of C=N and C-N.40

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Additionally, 2993 cm-1 and 721 cm-1 are contributed by the stretching and bending vibration of C-H in PHMG (curve a), respectively.14, 41 It is confirmed from the above results that the CuS nanocomposites are successfully prepared. Adsorption mechanisms of CuS nanocomposites for Cr(VI). The FT-IR spectrum of after absorption was shown in Figure S-4d. Compared with the curve c, there were two new peaks at 1460 cm-1 and 1408 cm-1 which were corresponded to the stretching vibrations of S=O.30 It confirmed the existence of redox between the nano-CuS and Cr(VI). The Cr(VI) ions adsorbed on nano-CuS are reduced to Cr(III), simultaneously, the nano-CuS is partly converted to CuSO4. The adsorption process was depicted in Figure 2.The contribution rate of each material for adsorption Cr(VI) obtained from Figure S-3 was given in Figure 2a. It could be seen that 18.51% of the contribution was provided by nano-CuS and 81.38% were contributed by the PHMG. As expected, SG was basically non-responsive to Cr(VI) ions. Based on the results of analysis and calculation in Figure 2a, we can assume that the surface of SG is covered with many exposed PHMG, and only a small number of PHMG surface self-assembled with nano CuS. Therefore, we designed the structure diagram of the CuS nanocomposites, as shown in Figure 2b. The mechanisms of Cr(VI) adsorption on the CuS nanocomposites are also shown in Figure 2b. As shown in the schematic diagram, the PHMG surface has a large amount of positive charge, which facilitates the adsorption of Cr(VI) in the form of anions by the electrostatic attraction. Meanwhile, part of the Cr(VI) ions is captured by the nano-CuS particles, due to their strong reducibility. Thus, the adsorption ability of the CuS nanocomposites is significantly improved by the synergism of nano-CuS and PHMG. In combination with Figure S-4 and Figure 2, the adsorption mechanisms of CuS nanocomposites for Cr(VI) are confirmed as mainly electrostatic attraction simultaneously accompanied with the reduction of Cr(VI) to Cr(III).


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Figure 2. Schematic diagram of the adsorption process of Cr(VI) on CuS nanocomposites. (a) Contribution rate of each material; (b) Mechanisms of Cr(VI) adsorption on the CuS nanocomposites. Equilibrium study. Adsorption isotherm is the relationship between the equilibrium adsorption capacity (qe) of CuS nanocomposites and the concentration (Ce) of Cr(VI) ions in the residual liquid under a definite temperature. Langmuir and Freundlich models are commonly used to simulate the relationship between qe and Ce.14, 25 The adsorption isotherms of Cr(VI) were shown in Figure 3. It was found that the mass transfer resistance of Cr(VI) ions between the adsorbent and the liquid phase was overcome, as the equilibrium concentration of Cr(VI) ions increased, the adsorption capacity also increased. The relationship between the equilibrium adsorption capacity and the residual Cr(VI) concentration at room temperature was observed. The experimental data was then analyzed by using the Langmuir model (Eq.2) and Freundlich model (Eq.3). qe 

qmKLCe 1  KLCe 1

qe  K F Ce n

(2) (3)

Where qm (mg g-1) was the maximum adsorption capacity, KL (L mg-1) was the adsorption strength constant of Langmuir, and KF (mg1-1/n L1/n g-1) and n were Freundlich constants, and Ce (mg L-1) was the concentration in the liquid phase. All the parameters of the adsorption model were shown in Table S-2.In addition, the adsorption performance could be judged by the separation factor or equilibrium parameter RL (Eq.4).42-44



RL 

1 1  K LC0

(4)

The RL value indicated whether the adsorption reaction was irreversible (RL=0), advantageous (0

Detection of Hexavalent Chromium by Copper Sulfide

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