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Vol. 9. Issue 1.
Pages 948-959 (January - February 2020)
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Vol. 9. Issue 1.
Pages 948-959 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.11.034
Open Access
Carboxyl-rich carbon nanocomposite based on natural diatomite as adsorbent for efficient removal of Cr (VI)
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Zhiming Sun, Bixuan Liu, Mingzhe Li, Chunquan Li
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chunquanli@cumtb.edu.cn

Corresponding author.
, Shuilin Zheng
School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, PR China
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Tables (5)
Table 1. BET specific surface area, pore volume and average pore size for DE, DE@S and C-DE@S composite.
Table 2. Fitted parameters of adsorption kinetics models.
Table 3. Parameters of Langmuir, Freundlich and Elovich isotherms for Cr (VI) adsorption.
Table 4. Adsorption capacity for metal cations by C-DE@S nanocomposite in comparison with other reported adsorbents.
Table 5. Thermodynamic parameters of Cr (VI) adsorption on DE@S, C–S and C-DE@S nanocomposite.
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Abstract

Carboxyl-rich carbon nanocomposite based on natural diatomite was successfully synthesized as an effective adsorbent for removing Cr(VI) in aqueous solution in this work. The obtained carboxyl-rich carbon nanocomposites were comprehensively characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), N2 adsorption-desorption apparatus, zeta potential and Fourier transform infrared spectrometer (FTIR). After systematic and detailed study of the adsorption process, it is found that the initial pH of Cr(VI) aqueous solution has an important effect, and pH=1 should be the optimum condition for Cr(VI) adsorption at room temperature. It is proved that the introduction of carboxyl-containing functionalized groups was beneficial to improve the adsorption capacity. The adsorption process of Cr(VI) conforms to the pseudo-second-order kinetic model and Langmuir isotherm, and the adsorption behaviour is endothermic and spontaneous. Additionally, the maximal adsorption capacity towards Cr(VI) can reach to 142.857mg/g at 298.15K, which is significantly improved compared with the sample without carboxyl functionalization (19.4mg/g). Moreover, the prepared adsorbent can be recycled and reused at least four times after being washed by alkali solution. This study indicated that the carboxyl-rich carbon nanocomposite based on diatomite would be promising in removing heavy metal ions (Cr(VI)) from wastewater.

Keywords:
Carbon nanocomposite
Acrylic acid
Adsorption
Diatomite
Cr(VI)
Full Text
1Introduction

Nowadays, with the development of urbanization and industrialization, the problem of environment pollution are gradually disturbing the human beings [1]. As is well known, water pollution is an important issue that affecting people life and health around the world. Among them, Hexavalent chromium (Cr(VI)) is one of the most toxic sources of water pollution, the US government stipulates that the highest acceptable Cr(VI) concentration in drinking water is 50μg/L [2,3]. So far, various technologies have been developed around the world to remove Cr(VI), such as adsorption [4], chemical precipitation [5], photocatalytic reduction [6], membrane systems [7], electrodeposition [8] and ion exchange process [9]. At present, the main treatment method that can be used during the water treatment is the adsorption technology, which has the advantages of easy to operate, lower operating cost and fewer secondary products [10].

Carbon materials are considered to be one of the most suitable adsorbents for removing pollutants from wastewater because of its porous, strong adsorption capacity, chemical inertness and environment friendly [11,12]. But the use of activated carbon is limited because of its high price and serious agglomeration problem [13,14]. In order to solve these problems, many scholars have tried to load carbon nanoparticles onto natural mineral surface, such as attapulgite [15], illite [16], palygorskite [17] and montmorillonite [18]. Through inducing the surface of carbon composite materials to generate a large number of oxygen-containing groups, it will have more active sites than the single carbon materials, which is conducive to improving the adsorption capacity of pollutants. Moreover, the use of non-metallic mineral materials as carriers can not only effectively reduce costs, but also improve the adsorption capacity of pollutants.

Diatomite is widely used for filter agents, catalyst carriers and building materials due to the high porosity, strong permeability and good chemical stability [19,20]. Because of its strong ion exchange ability and abundant adsorption sites, diatomite can be used to adsorb pollutant molecules such as dyes, heavy metals and radionuclides in aqueous solution [21]. In addition, the unique discoid morphology of diatomite makes carbon nanoparticles can be loaded effectively. Therefore, constructing composite material derived from diatomite and carbon nanoparticles not only can solve the agglomeration problem of carbon nanoparticles, but also improve the adsorption capacity and reduce the cost of adsorption materials, which indicates that the carbon nanocomposite based on diatomite is a promising material in environment remediation [22].

In our preliminary work, we fabricated the diatomite@carbon nanocomposite via a mild hydrothermal carbonization process by using sucrose as carbonaceous source and diatomite as the carrier. Although diatomite was introduced to solve the agglomeration problem of single carbon material, nevertheless, the obtained composites have a few active sites which restrains the adsorption capacities of the products due to the high reaction temperature [23,24].

Combining with previous studies, we found that the presence of carboxyl, hydroxyl, carbonyl, ketone and other oxygen-containing functional groups can further improve the adsorption capacity of hydrothermal products [25]. Therefore, we further introduced oxygen-containing functional groups into diatomite@carbon nanocomposite through chemical modification. It was reported that carboxyl-containing functional groups could be created by adding a certain amount of acrylic acid, which could effectively improve the adsorption capacity [26]. Furthermore, to the best of our knowledge, there are no reports of carboxyl-rich carbon nanocomposite based on diatomite at present.

Hence, in this work, a kind of carboxyl-rich carbon nanocomposite based on natural diatomite were fabricated through a typical hydrothermal treatment with the aid of acrylic acid. The obtained adsorbent were fully characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), N2 adsorption-desorption apparatus, zeta potential and Fourier transform infrared spectrometer (FTIR). Finally, the main adsorption mechanism of adsorbent for Cr(VI) was also discussed.

2Materials and methods2.1Materials

The purified diatomite was provided from Linjiang City, Jilin Province, China. Deionized water was used throughout the whole experiment. Sucrose (C12H22O11), acrylic acid (C3H4O2), potassium dichromate (K2Cr2O7), hydrochloric acid (HCl), sodium hydroxide (NaOH) and other chemicals used in the experiment were purchased from Beijing Reagent Co (Beijing, China). All reagents are analytical grade and used without further purification.

2.2Preparation of the carboxyl functionalized diatomite@carbon nanocomposite

The diatomite@carbon nanocomposite (denoted as DE@S) mix solution was prepared by using sucrose and diatomite as carbon sources and carriers, respectively [18]. 4.0g of diatomite was dispersed in 110ml of sucrose solution (containing 12.0g sucrose). The same solution of carboxyl-rich carbon nanocomposite based on diatomite (denoted as C-DE@S) was prepared by adding 4.0g of diatomite, which was dispersed in 110ml of sucrose and acrylic acid mixed solution (containing 12.0g sucrose and 16.0ml acrylic acid). The two mixed solutions were dispersed by ultrasound for 15min at room temperature to form a stable suspension, then put into the Teflon-lined stainless-steel autoclave, respectively. And the autoclave was closed in an oven at 180°C for 24h.

Finally, after the hydrothermal reaction, we took out the autoclave and cooled to room temperature. The as-synthesized black product was centrifuged at 1000r/min for 8min, then washed alternately with deionized water and ethanol by filtration, and dried the filter cake in oven at 60°C for 24h.

For comparison, bare carbon material (denoted as S), bare diatomite (denoted as DE) and bare carboxyl-rich carbon modified by acrylic acid (denoted as C–S) were obtained by a similar procedure as DE@S.

2.3Characterizations

S-4800 field emission scanning electron microscope (SEM) (the energy of accelerator beam is 3.0kV) was used to observe the surface micro-morphology of the samples. Energy dispersive spectrometer (EDS) was employed to detect the surface composition of different samples (Hitachi, Japan). The infrared absorption characteristic peaks of the samples were analyzed by Nicolet IS10 Fourier transform infrared spectrometer (FTIR) with the KBr pellet technique, and the scanning range was 400–4000 cm−1[27]. The N2 isotherms adsorption-desorption curves were determined by Micromeritics ASAP2020 instrument. All the data about the specific surface area, pore volume and pore diameter of the samples could be obtained. The zeta potential of the samples were tested by a Malvern ZEN 3600 Zetasizer, and the pH values of the suspensions were adjusted by NaOH or HCl solution.

2.4Batch adsorption experiments

In order to study the adsorption process of adsorbent materials, a batch adsorption experiments have been carried out in this study. A certain amount of potassium dichromate (K2Cr2O7) was dissolved into deionized water to obtain different concentrations of Cr(VI) solution. Before starting the adsorption reaction, 50mg of DE, S, C–S, DE@S and C-DE@S nanocomposite were added into 160ml of Cr(VI) aqueous solution firstly, and then maintained stirring by magnetic force at room temperature. After a certain time, the 3.0ml of suspension was taken and filtered with 0.22μm filter membrane to separate the adsorbent from the solution. The chromogenic solution was prepared according to the method of determination of Cr(VI) in aqueous solution similar to previous report [28]. Finally, the absorbance of Cr(VI) was determined by a UV–vis spectrophotometer (UV-9000S, Shanghai Metash, China) at the wavelength of 540nm. The adsorption amounts of Cr(VI) per unit mass of the sample, Qe (mg/g), were calculated according to the following Eq. (1):

where C0 and Ct are initial and equilibrium concentration of the Cr(VI) solution (mg/L), respectively. V is the volume of aqueous solution containing Cr(VI) (mL), M is the mass of the sample (g).

To study the effect of initial pH on the removal of Cr(VI), 0.1mol/L NaOH or 0.1mol/L HCl solutions was selected to regulate the pH of Cr(VI) solution in the range of 1–10.

In order to study the adsorption properties of the composite materials, 50mg samples were dispersed in 160ml Cr(VI) solution (25mg/L, pH=1.0) and stirred to test the removal efficiency of Cr(VI) at different adsorption time points. To further describe the adsorption equilibrium of the composite materials for Cr (VI) in solution, 50mg samples were dispersed in 160ml Cr(VI) solution with different concentrations (50–300mg/L) and stirred at 298.15K, 308.15K and 318.15K, respectively. All the adsorption experiments were carried out in triplicate and the average values were calculated.

3Results and discussion3.1Characterization of the synthesized composites3.1.1SEM and EDS analysis

The morphology and elemental compositions of DE, DE@S, C-DE@S and S nanocomposites were obtained by SEM and EDS analysis as shown in Fig. 1. The original DE carrier (Fig. 1a and b) clearly revealed the disc-like porous morphology and smooth surface [29,30]. Compared with DE, the surface of DE@S and C-DE@S became rough and evenly loaded with 50∼100nm microspheres after the hydrothermal carbonization (Fig. 1c, d, e and f). According to EDS spectrum (h and i), the C content of DE@S increased from 0% to 6.38%, and the characteristic peaks of Si element decreased obviously, which indicated the loaded species were amorphous carbon microspheres derived from S, which made the surface become rough. EDS spectrum (i, j) suggested that the content of C in C-DE@S composites increased from 6.38% to 37.97% and the content of Si element decreased continuously. The result showed that the addition of acrylic acid during hydrothermal carbonization could lead to generate a large number of oxygen-containing functional groups on the surface of C-DE@S composites [18].

Fig. 1.

SEM images of diatomite (a and b), DE@S (c and d), C-DE@S (e and f), S material (g), and EDS result of diatomite (h), DE@S (i), C-DE@S (j).

(1.11MB).

On the contrary, without the carriers, obvious agglomeration of carbon nanoparticles occurred as shown in Fig. 1g, which was not favorable for a high-efficiency adsorption process. The results demonstrated that the presence of carrier in the hydrothermal process can effectively prevent the usual homogeneous nucleation of carbon species from the bulk solution, and promote heterogeneous deposition of carbonaceous matter on the external carriers for the formation of well-defined nanostructured composites [31].

3.1.2FTIR analysis

Fig. 2 displayed the FTIR spectra of DE, S, DE@S and C-DE@S samples. Seen from Fig. 2a, the sharp peaks at 1028cm−1 and 799cm−1 could be attributed to the SiOSi asymmetric vibration and bending vibration bands, respectively. The band at about 3401cm−1 was ascribed to the stretching vibrations of physically adsorbed water. As can be seen from Fig. 2d, for single carbon nanoparticles, the band at about 3401cm−1 can be ascribed to the HOH stretching vibrations. There are obvious vibration absorption peaks at around 1718cm−1 and 1490cm−1, which are ascribed to CO stretching vibration derived from absorption peaks of carboxyl groups. The vibration absorption peak at 1640 cm−1 is ascribed to CC absorption peak. According to Fig. 2b and 2c, after hydrothermal carbonization, the characteristic peaks strength of diatomite at 1028cm−1 and 799cm−1 were significantly weakened, and several new bands appeared, including the 1490cm−1 and 1718cm−1 bands of CO and 1640cm−1 band of CC [32], indicating that carbon nanoparticles were successfully loaded on the surface of diatomite within the composite materials. In addition, the bands at 2850cm−1 which are characteristic peaks of the anti-symmetric C–H stretching modes also appeared, implying the hydrothermal process was a semi-carbonization process [16].

Fig. 2.

FT-IR spectra of DE (a), DE@S nanocomposite (b), C-DE@S nanocomposite (c) and S material (d).

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3.1.3BET analysis

The porosities of DE, DE@S and C-DE@S adsorbents were analyzed by employing a N2 isotherms adsorption-desorption apparatus. Fig. 3 exhibited the N2 adsorption-desorption isotherms of adsorbents, and the parameters obtained from the N2 adsorption-desorption isotherms (including specific surface area, pore volume and average pore size) were calculated and summarized as displayed in Table 1. As shown in Table 1, the specific surface area and average pore size of DE@S composites prepared by hydrothermal carbonization decreased compared with DE, indicating that the surface of diatomite was covered with carbon particles, and the pore might be blocked partially. However, the specific surface area, pore volume and average pore size of C-DE@S nanocomposite increased significantly. The specific surface area of C-DE@S increased from 12.768m2/g of C-DE to 20.397m2/g after hydrothermal treatment with acrylic acid, the pore volume increased from 0.046 to 0.057cm3/g. The result indicated that the morphology of C-DE@S was optimized in hydrothermal process after adding acrylic acid, and resultant amsorphous carbon microspheres with higher specific surface area were formed. The increase of specific surface area of C-DE@S nanocomposite is beneficial to increase the contact probability with pollutants in water. According to the N2 adsorption-desorption curves (Fig. 3), the adsorption-desorption ability of C-DE@S nanocomposite is better than that of DE and DE@S, which might contribute to the improvement of the adsorption capacity.

Fig. 3.

Nitrogen adsorption–desorption isotherms of DE, DE@S and C-DE@S samples.

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Table 1.

BET specific surface area, pore volume and average pore size for DE, DE@S and C-DE@S composite.

  BET specific surface area (m2/g)  Pore volume (cm3/g)  Average pore size (nm) 
DE  16.922  0.042  7.454 
DE@S  12.768  0.046  6.656 
C-DE@S  20.397  0.057  7.925 
3.2Adsorption studies3.2.1Adsorption kinetics

To determine the adsorption rate and equilibration time in the adsorption process, kinetics studies of DE, S, C–S, DE@S nanocomposite and C-DE@S nanocomposite were carried out and the results were presented in Fig. 4. It can be found that the adsorption ability of DE to Cr(VI) is very low, this is because of the negatively charged surfaces of DE and the negatively charged CrO42− or Cr2O72− in solution [33]. At the same time, the adsorption capacity of DE@S for Cr(VI) was better than DE. The C-DE@S had the highest adsorption capacity for Cr(VI) compared to DE, S, C–S and DE@S nanocomposite, indicating that the material with acrylic acid modification could effectively promote the ability to remove Cr(VI) from wastewater (Fig. 5).

Fig. 4.

Adsorption rate curves of Cr(Ⅵ) on DE, S, C–S, DE@S and C-DE@S nanocomposite.

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Fig. 5.

Fitted curves of pseudo-second-order models of the adsorption by the S, C–S, DE@S and C-DE@S nanocomposite.

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Therefore, the deposition of carbon nanospheres on diatomite by hydrothermal reaction is conducive to improving the removal efficiency of diatomite. The removal rate of Cr(VI) is very slow based on previous works [18]. In our experiment, the adsorption equilibrium spent nearly 30h for the DE@S nanocomposite and C-DE@S nanocomposite, which is aligned with the previous literature [34].

To further examine the adsorption mechanism, pseudo-first-order equation, pseudo-second-order equation, intra-particle diffusion model and elorich model were applied to analyze the experimental data. The models can be expressed in the following form [35]:

ln(qe−qt)=lnqe−k1t
qt=kit0.5+C
where k1, k2 and ki are the rate constant of pseudo-first order (min−1), pseudo-second order (min−1) and intra-particle diffusion model (mg/g·min1/2). qe is the saturated adsorption capacity of Cr(VI) (mg/g). qt is the removal amount of Cr(VI) at time t (mg/g). C (mg/g) is a constant revealing the thickness of the boundary layer. α (mg/g·min) is the adsorption rate, and β (g/min) were obtained from the slope and intercept of the linear relationship.

The parameters of four models are calculated and summarized in Table 2. All the values of R2 for DE, S, C–S, DE@S and C-DE@S based on the pseudo-second-order model are higher than 0.99, which gives better fitting in comparison with pseudo first-order model. This indicated that the adsorption process fits the pseudo-second-order model well. Meanwhile, the parameter k2 for DE is much higher than that of the composite, further indicating that the adsorption of Cr(VI) by the composite is a slow process (Table 3).

Table 2.

Fitted parameters of adsorption kinetics models.

  Pseudo-first-orderPseudo-second-order
  k1 (1/min)  qe,cal (mg/g)  R2  k2 (g/(mgmin))  qe (mg/g)  R2 
DE  0.688  0.338  0.862  5.429  0.226  0.999 
0.081  14.585  0.958  0.021  25.056  0.995 
C–S  0.153  25.886  0.99  0.014  34.247  0.999 
DE@S  0.092  24.398  0.989  0.014  39.308  0.996 
C-DE@S  0.073  17.717  0.95  0.018  31.153  0.997 
  Intraparticle diffusionElorich modle
  ki (mg/gh1/2R2  α (mgg.min)  β (g/min)  R2 
DE  0.006  0.204  41945  84.745  0.504 
2.633  0.941  61.688  0.237  0.965 
C–S  4.136  0.826  32.11  0.184  0.92 
DE@S  9.705  0.888  44.54  0.194  0.948 
C-DE@S  3.201  0.993  98.858  0.175  0.988 
Table 3.

Parameters of Langmuir, Freundlich and Elovich isotherms for Cr (VI) adsorption.

  LangmuirFreundlichElovich
  KL(L/mg)Q max (mg/g)  R2  KF (mg1−1/nL1/n/g)  1/n  R2  KE (L/mg)  Qm (mg/g)  R2 
DE  0.0080.601  0.994  0.028  0.479  0.974  10.398  0.093  0.549 
0.02171.429  0.997  24.218  0.2  0.8298  1.44  0.06  0.763 
DE@S  0.038123.46  0.999  23.28  0.299  0.976  0.4588  33.556  0.989 
C–S  0.166136.986  0.999  50.074  0.1978  0.904  6.437  25.840  0.893 
C-DE@S0.057  142.857  0.998  41.018  0.228  0.985  2.389  29.155  0.984 
3.2.2Adsorption isotherms

The adsorption isotherms of Cr(VI) in the solution for DE@S and C-DE@S composites at different temperatures are shown in Fig. 6. The figure shows that higher Cr(VI) removal efficiency could be obtained with increasing the adsorption temperature. This demonstrated that higher temperature is helpful to improve the adsorption capacity of Cr(VI) during the adsorption process.

Fig. 6.

Adsorption isotherm for Cr(Ⅵ) onto the (a) DE@S and (b) C-DE@S nanocomposite under three different temperatures.

(0.13MB).

The application of adsorption isotherm is of great significance to describe the interaction effect within the composites [18,36]. The Langmuir, Freundlich and Elovich isotherm models were used to fit the isotherm data of the adsorption process for Cr(VI) in solution. The Langmuir, Freundlich and Elovich equations can be expressed as:

where qe is the equilibrium adsorption capacity (mg/g), ce represents the equilibrium concentration in solution (mg/L), KL, KF and KE are the Langmuir, Freundlich and Elovich equilibrium constant, qm represents the maximum adsorption capacity of the adsorbent (mg/g). Experimental values of qm and KL are calculated from the slope and intercept of the linear plot of ce/qe against ce. 1/n represents the degree of heterogeneity of the adsorbent surface and the favorability of adsorption.

As shown in Fig. 7, the data of adsorption experiment that fitted by Langmuir, Freundlich and Elovich isotherm models were displayed. And the parameters of the three isotherm models of Cr(VI) adsorption for different adsorbents were calculated and shown in Fig. 7, the results indicated that the adsorption data were better fitted by the Langmuir model, which means that the uptake of Cr(VI) by the S, C–S, DE@S and C-DE@S adsorbents are dependent on the monolayer adsorption model. The saturated adsorption amounts of DE, S, DE@S, C–S and C-DE@S nanocomposites calculated by Langmuir isothermal adsorption model are 0.601, 71.429, 123.460, 136.986 and 142.857mg/g, respectively. It is clear that the adsorption capacity of the C-DE@S nanocomposites is much higher than those of DE and S, which means that the modification of DE by carbon nanopheres and acrylic acid could effectively enhance the adsorption capacity of natural diatomite towards Cr(VI). Especially, the Cr(VI) adsorption capacity of the C-DE@S nanocomposite dramatically increased with the addition of acrylic acid compared with other references [37–39]. In addition, although the adsorption capacities of C–S and C-DE@S are similar, the application cost of C-DE@S nanocomposite is much lower than that of C–S since the addition of diatomite could significantly reduce the production cost. Moreover, the saturated adsorption amount of C-DE@S nanocomposite was among the highest compared with other adsorbents reported recently in the literatures (Table 4). Therefore, the prepared C-DE@S nanocomposite should be a potential adsorbent for removing Cr(VI) from wastewater cinsidering its relatively low cost and high adsorption capacity.

Fig. 7.

Langmuir (a), Freundlich (b) and Elovich (c) isotherms for Cr (VI) by the S, C–S, DE@S and C-DE@S nanocomposite.

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Table 4.

Adsorption capacity for metal cations by C-DE@S nanocomposite in comparison with other reported adsorbents.

Materials  Contaminants  Adsorption capacity (mg/g)  Reference 
Activated carbon (AC)  Cr(VI)  18.9–53.7  [37] 
Carbon nanotubes (CNTs)  Cr(VI)  9.50  [38] 
Chitosan-crosslinked-poly nanohydrogel (CN-cl-PL(AA)NHG)  Cr(VI)  26.49  [39] 
Activated carbon prepared from peanut shell  Cr(VI)  16.26  [40] 
Activated carbon was prepared by chemical activation of peanut shell with H3PO4  Cr(VI)  43.67  [41] 
Illite@carbon nanocomposite (I@C)  Cr(VI)  149.25  [42] 
DE@S nanocomposite  Cr(VI)  123.46  This work 
C-DE@S nanocomposite  Cr(VI)  142.86  This work 
3.2.3Thermodynamic study

In order to better understand the adsorption process of Cr(VI) by the C-DE@S nanocomposite, the three thermodynamic parameters i.e. Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were calculated, the relationship of them is as follows [43–45]:

where ΔG(kJ/mol) is the standard free energy change, T(K) is the absolute temperature and R(kJ/(mol·K)) is the ideal gas constant, KL is the dimensionless equilibrium constant, ΔH(kJ/mol) is the enthalpy change, ΔS(kJ/(molK)) is the entropy change during adsorption.

The van’t Hoff equation is used to evaluate the variation of an equilibrium constant with temperature. The enthalpy (ΔH) and entropy (ΔS) can be determined by slope and intercept of the plot of lnKL vs 1/T, and the results are given in Table 5. G<0 proved that the adsorption behavior was spontaneous. The value of G increases with the increase of temperature, which indicates that higher reaction temperature is beneficial to the adsorption. S>0 indicates that the adsorption process is endothermic and that the randomness of the solid solute interface increases during the adsorption process.

Table 5.

Thermodynamic parameters of Cr (VI) adsorption on DE@S, C–S and C-DE@S nanocomposite.

△H0 (kJ/mol)△S0 (kJ/mol)△G0 (KJ/mol)
298.15308.15318.15
DE@S  10.930  0.067  −8.999  −9.667  −10.336 
C–S  31.776  0.149  −12.719  −14.211  −15.256 
C-E@S  42.325  0.176  −10.265  −12.030  −13.264 
3.2.4Effect of initial pH

As we know, the intial pH value has a significant impact on the adsorption process, because the intial pH determines the presence form of chromium ions in the solution and the surface charge of the adsorbent. The initial pH effect towards zeta potential values of the adsorbent is presented in Fig. 8. The zeta potential values of C-DE@S nanocomposite decreased and changed from positive to negative when the pH varied from acidic to alkaline conditions, this is because the hydroxyl and carboxyl groups on the surface of C-DE@S nanocomposite are protonated. The isoelectric point of the C-DE@S nanocomposite is pH=2, and the surface charge was positive when pH<2, which is beneficial for the retention of anionic contaminants.

Fig. 8.

Zeta potential values of C-DE@S nanocomposite at various pH.

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The relationship between initial pH and the adsorption capacity of the C-DE@S nanocomposites is shown in Fig. 9. When pH=1.0, the adsorption capacity of C-DE@S towards Cr(VI) reached to the maximum. Afterwards, the adsorption efficiency decreased as pH value increased.

Fig. 9.

Effect of initial pH on the adsorption capacity of C-DE@S nanocomposite.

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At an initial pH=1.0, the Cr(VI) mainly exists in anionic form (HCrO4), which can be efficiently captured by the positively charged surface of the C-DE@S nanocomposite by electrostatic attraction. With the increase of pH, the zeta potential value of the C-DE@S nanocomposites changed from positive to negative, which will inhibit the Cr(VI) adsorption by the C-DE@S nanocomposite. Meanwhile, the HCrO4 gradually converted to divalent CrO42−[46,47]. Thus, the removal efficiency gradually decreased when the pH increased. According to the pH, zeta potential and adsorption capacity analysis results in Figs. 8 and 9, it is indicated that the electrostatic attraction between anionic HCrO4 and the positively charged surface of the C-DE@S nanocomposite should be mainly responsible for the Cr(VI) adsorption process [3,48]. In other words, due to the protonation process of surface active sites (i.e. COOH, CO, SiOH, etc.) at low pH values, the electrostatic attraction between the positive charge on the surface of C-DE@S nanocomposite and the anion HCrO4 of Cr(VI) charged for the enhancement mechanism of adsorption [49,50], which generally occurred at the surface of carbon materials in the process of heavy metals adsorption.

3.2.5Changes of material

For the adsorption study, the characterization is very important to see the changes in the functional group and surface morphology of the adsorbent after adsorption. Therefore, the morphology, element composition and surface functional groups of C-DE@S nanocomposites adsorbed Cr (VI) were compared by SEM, EDS (shown in Fig. 10) and FT-IR analysis (shown in Fig. 11).

Fig. 10.

SEM images of unadsorbed (a) and adsorbed (c) of C-DE@S nanocomposites, EDS images of unadsorbed (b) and adsorbed (d) of C-DE@S nanocomposites.

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Fig. 11.

FT-IR spectra of unadsorbed and adsorbed of C-DE@S nanocomposite.

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Seen from Fig. 10, there was a significant change between unadsorbed C-DE@S and adsorbed C-DE@S nanocomposites in terms of morphology. Though a large number of carbon nanoparticles were still loaded on the surface of diatomite disk, it was almost covered by the abundant anion HCrO4. Which could also be verified by the EDS diagram, the adsorbed surface of C-DE@S nanomaterials contains a large number of Cr elements. The result indicated that the Cr(VI) ions can be effectively adsorbed on the surface of C-DE@S nanocomposites, so as to achieve the purpose of rapid removal of Cr (VI) from the water body.

FT-IR is one kind of techniques that can well display the change of functional groups. As seen in Fig. 11, the asymmetric vibration and bending vibration bands of Si-O-Si at 799cm−1 and 1028cm−1 are enhanced for the C-DE@S nanocomposite after adsorption, and the tensile vibration of physically adsorbed water at 3401cm−1 are also enhanced. A wide CO stretching vibration band appeared at 1591cm−1 which covered the vibration bands of 1718cm−1 and 1640cm−1 after adsorption. Meanwhile, a bending vibration peak of CH appeared at 1400cm−1, which can be attributed to the change of functional groups caused by the adsorption of Cr(VI).

3.2.6Desorption and regeneration experiment

The regeneration performance of adsorbent is of great importance in industry. The desorption experiment of C-DE@S nanocomposites were conducted with 0.1mol/L NaOH solution. As can be seen from Fig. 12, the adsorption rate of adsorbates decreases slightly in each cycle, but the adsorption adsorption performance remains high after four cycles. As for the regeneration mechanism, the chromium desorbed at room temperature might be attributed to the hydrolysis neutralization reaction of the adsorbed HCrO4 on the surface of C-DE@S nanocomposites in an alkaline solution (0.1M NaOH). The most probable mechanism that could explain the chromium desorption with NaOH is illustrated as folllows:

{≡R−COO}−HCrO4 + Na+ + OH → {≡R−COO}–Na+ + CrO42− +H2O

Fig. 12.

Reusability of C-DE@S nanocomposite for Cr (VI) adsorption.

(0.08MB).

Due to the high concentration of OH in alkaline condition, the main specie formed by alkaline regeneration is CrO42−. In general, it is indicated that the prepared C-DE@S nanocomposites have good reusability, which is of great significance to reduce the cost of industrial application.

4Conclusions

In summary, we found that adding a certain amount of acrylic acid to the hydrothermal preparation of DE@S can produce carboxyl-containing functional groups, and the introduction of such oxygen-containing functional groups may change the surface properties and functional group structure of the material, thereby improving the adsorption properties of Cr(VI). Adsorption kinetics studies show that the pseudo-second-order kinetic equation can accurately describe the adsorption process of C-DE@S nanocomposites for Cr(VI). Thermodynamic study showed that the adsorption process was accorded with the Langmuir isothermal adsorption equation. The adsorption capacity of DE@S and C-DE@S nanocomposites were 123.460mg/g and 142.857mg/g, respectively. The adsorption behavior of DE@S and C-DE@S nanocomposites for Cr(VI) is monolayer adsorption, and the adsorption process is an endothermic spontaneous reaction process. According to the surface properties of the nanocomposites, the possible mechanism for removing Cr(VI) is proposed, which should be mainly attributed to the combination of the positive charge surface of the nanocomposites with the anion HCrO4 in Cr(VI) solution at pH=1 due to the protonation and electrostatic attraction. In addtion, the C-DE@S nanocomposite is proven to have good reusability and low application cost. To sum up, our study provided a promising adsorbent (C-DE@S nanocomposites) for the effective removal of toxic Cr(VI) from wastewater.

Conflict of interests

The author declares no conflicts of interest.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001) and the Yueqi Young Scholar Project, China University of Mining & Technology (Beijing) (2017QN12).

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