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BY 4.0 license Open Access Published by De Gruyter Open Access December 3, 2022

Zirconium-modified attapulgite was used for removing of Cr(vi) in aqueous solution

  • Yani Liu , Lei Xu , Qingyun Wang , Tong Zou , Cheng Cao EMAIL logo , Qiqi Fang , Nan Zhang and Yongcheng Wang EMAIL logo
From the journal Open Chemistry


This work fabricated the zirconium-modified attapulgite (Zr@ATP) for removing Cr(vi) ions in aqueous solutions. According to X-ray diffraction, scanning electron microscopy, TEM, Fourier transform infrared, and X-ray photoelectron spectroscopy analyses, Zr was successfully grafted onto the attapulgite rod surface. Cr(vi) adsorption onto Zr@ATP surface fitted well with the Langmuir isotherm and pseudo-second-order kinetic models, which suggested that the adsorption is primarily chemisorption. When the pH of the aqueous solution is 3, Zr@ATP achieved the highest Cr(vi) absorption, of about 32.84 mg/g. Density functional theory studies revealed that the hydroxyl functional group introduced through the modification process supplies more active sites to form the hydrogen bond with CrO 4 2 and HCrO 4 .

Graphical abstract

1 Introduction

In recent years, many pollutants generated by human intervention and activities have entered the environment, which has destroyed the natural ecological balance leading to severe water pollution [1,2]. The exposure of our natural aquatic ecosystem to heavy metal (HM) ions, the most common water pollutants, have aroused wide attention owing to its high mobility and toxicity. Besides, HM ions are non-biodegradable and detrimental [35]. HM ions generated during domestic waste contamination, transportation, and manufacturing may seriously impair human health even at low concentrations. More seriously, the discharged HMs have a cumulative effect on the ecosystem [6,7]. As one of the common HM pollutants, Cr(vi) has attracted widespread attention due to its high toxicity risks [8,9]. Cr(vi) invades into the human body via the skin, respiratory system, mucous membrane, and digestive tract, and gets accumulated in the human body. Considering these hazardous effects, effective removal of Cr(vi) from water bodies has become the focus of the current research [912].

Fortunately, extensive treatment technologies such as photocatalytic degradation, membrane filtration, adsorption, and ion exchange methods have been explored for eliminating Cr(vi) from the water bodies [9,13,14]. As an economic and well-established technology, adsorption is extensively utilized for sewage treatment. The current research on adsorbing agents primarily concentrates on metal-organic frameworks, natural or modified clay materials/biochar, and activated carbon [1518]. As a natural one-dimensional nanomaterial, attapulgite (ATP) has been widely used in many fields since its discovery. Such a wide acceptance of the material can be attributed to its excellent physical and chemical properties [12,1921]. Due to the presence of impurities, high surface energy, self-aggregation, and other issues, the adsorption capacity of natural raw attapulgite is low [22]. Therefore, ways to improve the adsorption performance of natural attapulgite through modification, compounding, and other methods are also the current research focus in this field. Moreover, the mechanism of Cr(vi) adsorption have not revealed from energetic and atomic perspectives. Density functional theory (DFT) calculation is utilized as an effective approach to study the adsorption process of HM ions [2325].

Considering these facts, the Zr-modified attapulgite (Zr@ATP) composite materials were prepared by using the chemical coprecipitation method. Thereafter, the preparation conditions of the Zr@ATP were also optimized. In this article, we have conducted various assays for investigating Zr@ATP’s performance on Cr(vi) adsorption at different conditions. The Zr@ATP was characterized using powder X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and Fourier transform infrared (FT-IR) spectroscopy. DFT calculation has also been conducted for investigating the plausible mechanism determining adsorption. These results would shed valuable light on the fabrication of the best Zr@ATP under optimized conditions and can unveil its adsorption mechanism of Cr(vi).

2 Materials and methods

2.1 Chemicals and instruments

The attapulgite clay (MgO 2.84%, Al2O3 16.04%, SiO2 69.94%, Fe2O3 4.74%, K2O 3.62%, and other composition 2.82%) was obtained from Fen Jun Mining Co. Ltd (Linze, Gansu, China). Zirconium chloride octahydrate, sodium hydroxide, and nitric acid are the analytical reagents used. This work adopted the SHZ-B constant-temperature water-bath vibrator (Shanghai Boxun Medical Biological Instrument Corp., China) for adsorption assay studies. Meanwhile, the PB-10 pH meter (Sartorius, Germany) was utilized to adjust Cr(vi) solution pH. The Atomic Absorption Spectroscopy technique (Shimfusa, Japan) was employed for measuring Cr(vi) concentration. The Tecnai G2 F20 Field emission transmission electron microscope (FEI, American) and ULTRA Plus 55 scanning electron microscope that installed the energy-dispersive X-ray (Zeiss, Germany) were employed to observe sample element distributions and morphological structures. Moreover, specific surface area (SSA) was measured by using the Autosorb-iQ automatic surface area and pore size analyzer (Quantachrome, American) based on nitrogen (N2) sorption–desorption at 77 K. The BET theory was used as the test approach for the measurement. Barrett–Joyner–Halenda formula was utilized to determine sample pore volume and size. In addition, the D8 Advance X-ray Diffractometer (Bruker Corporation, USA) was utilized to obtain powder XRD patterns under CuKα radiation (λ = 1.542 Å), at 40 kV, 100 mA, and 5°/min scanning rate, within the 2θ = 5–50° range. The Escalab 250Xi electron spectrometer (Thermo Fisher Scientific) was adopted for measuring XPS under 300 W AlKα radiation. FTIR spectra were recorded with an FTS-3000 Fourier transform infrared spectrometer (Digilab, USA) using KBr pellets at 4,000–400 cm−1.

2.2 Preparation of the Zr@ATP

The Zr@ATP was prepared under varying conditions by using the chemical coprecipitation method. To investigate the effect of pH, the Zr@ATP was prepared by the following procedure: 2.5 g of attapulgite clay and 3.22 g of ZrOCl2·8H2O were accurately weighed and mixed with 120.0 mL of ultrapure water. The mixture was then subjected to a 30 min ultrasonic treatment and 4 h stirring under ambient temperature. Later, the mixed sample was divided into five equal parts, and then the pH was adjusted to the range of 1.0–9.0. The mixed sample was stirred for a period of 12 h, followed by centrifugation, washing thrice, and drying under 105°C. The dried samples were finely ground and screened. Similarly, the different mass proportions of ATP and ZrOCl2·8H2O (1:9, 2:8, 3:7, 4:6, 5:5, and 6:4) were also prepared by adopting the aforementioned method. Moreover, the samples with different reaction times (2–12 h) were also prepared to optimize the condition for the fabrication of the best Zr@ATP.

2.3 Adsorption experiment

The present work investigated the adsorption dynamics at 298 K for diverse periods (range, 3–60 min). Later, 10 mg of the adsorbing agent was mixed with 20 mL of Cr(vi) solution (50 mg/L). The study also analyzed the impact of varying the Cr(vi) content. This was done by mixing 10 mg of the adsorbing agent with Cr(vi) solution (20 mL) at different concentrations, varying from 10 to –45 mg/L. HNO3 or NaOH was utilized to adjust the pH of Cr(vi) solution in the range of 1–7 to investigate the effect of different pH levels on the adsorption capacity. Equations (1) and (2) were utilized to determine adsorbing agents’ equilibrium Cr(vi) adsorption capacity (Q e, mg/g) and removal efficiency (η):

(1) Q e = ( C 0 C e ) V m ,

(2) η = ( 1 C e / C 0 ) × 100 % .

In the above equations, C 0 and C e stand for initial and final contaminant concentrations (mg/L), respectively, V indicates the volume of contaminant solution (L), and m represents the adsorbent weight (g). Further, supernatants were collected and subjected to centrifugation at 8,000 rpm, followed by Cr(vi) level measurement with Atomic Absorption Spectroscopy.

2.4 Theoretical calculation method

This work utilized the Vienna ab initio Simulation Package for calculating the energy of adsorption using the DFT framework. Later, exchange–correlation energy was evaluated using Perdew–Burke–Ernzerhof and generalized gradient approximation functionals [26,27]. As for the plane-wave basis set, a 500 eV energy threshold was set for providing precise self-consistent charge density. Then, optimal geometries were identified using 0.001 eV/Å as the convergence criterion of atomic force. Besides, this work has built a surface slab model using 1 × 3 × 1 supercell, and sufficient vacuum space (20 Å) was left. One individual gamma grid was utilized to sample the Brillouin zone to optimize the geometries. The DFT-D3 dispersion correction method was utilized for describing the weak interaction existing between the adsorbate and adsorbent. Equation (3) was utilized to determine the adsorption energy of the system (E ads):

(3) E ads = E sub / metal E sub E metal .

In the above equation, E sub/metal represents the total Zr@ATP or ATP energy post-HM adsorption, E sub indicates ATP or Zr@ATP energy, and E metal is the energy of the Cr(vi) ion solution CrO 4 2 or HCrO 4 [28]. Diverse Cr(vi) existing forms in aqueous solutions at varying pH levels were simulated using Visual MINTEQ ver. 3.0 [11,24].

3 Results and discussion

3.1 Optimal conditions for preparing the Zr@ATP

The preparation conditions are an important factor determining the adsorption performance of the adsorbent. Therefore, selecting the suitable fabrication conditions facilitated the production of Zr-modified ATP adsorbent with desirable architecture and characteristics. Based on the above reasons, the optimal preparation conditions for Zr@ATP were analyzed. According to Figure 1a, the prepared Zr@ATP composite adsorbent material exhibits the best Cr(vi) adsorption capacity at pH 5.0. Since zirconium exists as Zr(OH)4 on the attapulgite clay surface, when the pH is too low, the amount of Zr(OH)4 deposited on the surface will decrease, resulting in a decrease in the active hydroxyl groups and consequently the adsorption capacity. The deposition rate of Zr(OH)4 is too fast when the pH is greater than 5.0, which causes the particle size of Zr(OH)4 to increase and reduce the adsorption sites.

Figure 1 
                  The optimal preparation condition of Zr@ATP, the effect of (a) reaction pH, (b) reaction time, and (c) mass proportion.
Figure 1

The optimal preparation condition of Zr@ATP, the effect of (a) reaction pH, (b) reaction time, and (c) mass proportion.

Figure 1b illustrates that during the preparation process of zirconium-modified attapulgite clay composites, the equilibrium adsorption capacity corresponding to different stirring times fluctuates between 23.6 and 27.5 mg/g. Although different stirring times affect the adsorption capacity, it does not lead to an obvious change. Considering the cost of preparing the adsorbent, the subsequent preparation of the adsorbent was executed in 2 h.

As presented in Figure 1c, when the mass ratio of ATP to Zr(OH)4 is 4:6, the prepared Zr@ATP attains its maximum adsorption capacity (27.09 mg/L) toward Cr(vi). This could be due to the too high content of attapulgite and relatively low proportion of Zr(OH)4, which reduces the hydroxyl content on the composite-adsorbing agent surface and thereby affects the Cr(vi) ion adsorption. However, when the content of Zr(OH)4 is high, multilayer stacking deposits may occur on the surface of attapulgite. Although the proportion of Zr(OH)4 increases, the number of effective adsorbed hydroxyl groups exposed on the composite-adsorbing agent surface shows no obvious increase. In effect, these have no significant contribution in enhancing Cr(vi) adsorption. Moreover, considering the adsorption capacity and cost of Zr(OH)4, all subsequent experiments were carried out at the 4:6 proportion. Thus, the Zr@ATP adsorbent was prepared and characterized under the optimal conditions with a 2 h reaction time at pH = 5, and the mass ratio of ATP to Zr(OH)4 equals 4:6. All subsequent adsorption studies were performed using the Zr@ATP prepared under the aforementioned optimized conditions.

3.2 Characterization of the Zr@ATP

3.2.1 SEM and TEM analysis

The SEM images of ATP and Zr@ATP are as presented in Figure 2a and b. Before the modification process, the morphology of raw ATP presented a smooth rod-crystal structure [29]. After the modification process, the surface of ATP was covered with granular species, indicating that Zr(OH)4 was successfully grafted onto the ATP surface. Figure 2c and d depicts the TEM images for ATP as well as Zr@ATP. Before the modification process, the morphology of ATP resembles a fibrous rod-like structure. And post-modification, the surface of the Zr@ATP sample was covered with Zr(OH)4, which indicates that Zr(OH)4 has been successfully loaded on ATP. It is worth noting that the morphology of ATP has not changed before and after the modification. Moreover, the EDS element mapping of the Zr@ATP sample (Figure S1) was performed to confirm the successful loading of Zr(OH)4 onto the ATP surface.

Figure 2 
                     The SEM images of (a) ATP and (b) Zr@ATP; and the TEM images of (c) ATP and (d) Zr@ATP.
Figure 2

The SEM images of (a) ATP and (b) Zr@ATP; and the TEM images of (c) ATP and (d) Zr@ATP.

3.2.2 BET SSA

Figure 3 demonstrates the N2 adsorption–desorption isotherm of ATP, Zr@ATP, and Zr@ATP after adsorbing Cr(vi) ions, which showed the reversible type-IV isotherm and an H3 hysteresis loop [30]. Type-IV isotherm can be frequently seen within porous adsorbing agents, where capillary condensation takes place. In addition, those adsorbing agents which have an unchanged H3 hysteresis loop always have wedge and crack architectures. There is ample nitrogen molecule condensation within the hysteresis loop at low barometric pressure because of the capillary condensation action [31]. The SSAs of ATP, Zr@ATP, and Zr@ATP after adsorbing Cr(vi) are 235.6, 224.5, and 256.7 m2/g, respectively (Table S1). The SSA of Zr@ATP is smaller than that of ATP, which would be caused by the loading of Zr(OH)4. After the adsorption of Cr(vi), the SSA of Zr@ATP increases, which could be caused by the sludge removed during the process of recycling heat treatment. The presence of water crystals in the attapulgite clay pores is also consistent with the disappearance of the bending vibrational peaks in the infrared spectrum. In addition, the pore volume (Table S1) of ATP, Zr@ATP, and Zr@ATP after adsorption of Cr(vi) are 0.370, 0.531, and 0.357 cm3/g, respectively. Based on the above results, it can be suggested that both physical and chemical reactions coexisted during the whole process, and the Zr@ATP possesses high pore volume and SSAs compared with ATP, indicating its favorable adsorption performance.

Figure 3 
                     N2 adsorption–desorption isotherms for ATP, Zr@ATP, as well as Zr@ATP after adsorbing Cr(vi).
Figure 3

N2 adsorption–desorption isotherms for ATP, Zr@ATP, as well as Zr@ATP after adsorbing Cr(vi).

3.2.3 FT-IR analysis

Figure 4a displays FT-IR spectra of ATP, Zr@ATP, and Zr@ATP after adsorbing Cr(vi). The peak that appeared at 3,555 cm−1 is associated with the representative stretching vibration of the O−H bond of hydroxyl groups in the attapulgite clay, and that measured at 1,647 cm−1 is associated with the bending vibration of the attapulgite adsorbing water and interlayer water [22,32]. The peaks that appeared at 1,188 cm−1 along with 974 cm−1 are associated with the stretching and bending vibrations of the Si−O−Si bond in the SiO2 framework of the attapulgite clay [33,34].

Figure 4 
                     The (a) FT-IR spectra and (b) XRD patterns for ATP, Zr@ATP, and Zr@ATP after adsorbing Cr(vi).
Figure 4

The (a) FT-IR spectra and (b) XRD patterns for ATP, Zr@ATP, and Zr@ATP after adsorbing Cr(vi).

The peak at 1,451 cm−1 (the bending vibration peak of the coordinated water molecule in attapulgite) gets converted to 1,366 cm−1, which is attributed to the Zr@ATP formation. After the zirconization process, the characteristic peaks of attapulgite were found at 3,555, 1,647, and 974 cm−1, and their corresponding strengths were weakened, indicating that the process of zirconization does not affect the structure of attapulgite. The peak at 1,366 cm−1 in Zr@ATP disappeared after the adsorption of Cr(vi), which could be due to the removal of coordinated water during the sludge drying process.

3.2.4 XRD analysis

This work performed an XRD analysis of the attapulgite clay under different states by an X-ray powder diffraction meter. Figure 4b suggests that the attapulgite clay, after purification, is better detached from the impurities, which makes the adsorption performance of the clay better. The diffraction peaks that appeared at 2θ = 8.36, 19.78, 26.59, and 35.20 are attributed to the attapulgite rod surface [3537]. After the modification process, the position of diffraction peaks of attapulgite clay suffered no obvious change, but the intensity of the diffraction peaks decreased, indicating the successful loading of Zr(OH)4 onto the attapulgite clay surface. According to the spectrum of Zr@ATP after adsorbing Cr(vi), the structure of attapulgite does not change during the recycling process after the adsorption of Cr(vi). From the above results, we can conclude that the Zr@ATP is very stable for adsorbing Cr(vi) in aqueous solutions.

3.2.5 XPS analysis

For confirming Cr(vi)’s valence state after adsorption on the Zr@ATP, the adsorbents were investigated utilizing XPS characterization. According to the full-scan XPS spectra (Figure 5a), the peak of Zr 3d appeared in all samples, and the Cr 2p peak appeared after the adsorption process. Figure 5b presents Zr@ATP’s O 1s spectra before and after Cr(vi) adsorption. Those peaks appearing at 529.88 and 531.18 eV are associated with O element combined with Zr–O and O–H of Zr–OH groups, respectively [38]. These spectral data also indicated that the attapulgite surface is loaded with Zr(OH)4. After adsorbing Cr(vi), the binding energy of the above two peaks and their peak areas decreased and the peaks appearing at 531.98 eV (Si–O–Si) and 532.78 eV (Si−OH) showed no obvious changes, indicating that the Cr(vi) was mainly adsorbed on the –OH groups of Zr@ATP [35,39]. Additionally, the Zr 3d spectra of Zr@ATP before and after Cr(vi) adsorption can be observed from Figure 5c. After adsorbing Cr(vi), the peaks that appeared at 182.48 and 184.78 eV were shifted to the low binding energy, indicating a change in the chemical environment of Zr, which is associated with the elimination of coordinated water during the adsorption process. Figure 5d presents the Cr 2p spectrum of Zr@ATP after adsorbing Cr(vi), indicating the successful Cr(vi) adsorption by Zr@ATP [40]. The peaks of Cr 2p can be divided into three peaks (577.08, 579.28, and 587.38 eV), attributed to Cr(iii) (577.08 eV), CrO3(vi) (579.28 eV), and Cr(vi) (587.38 eV), respectively. This indicates that after being adsorbed on the Zr@ATP, the valence state of Cr underwent no obvious change [13,41].

Figure 5 
                     The (a) full-scan XPS spectra of Zr@ATP before and after Cr(vi) adsorption, high-resolution XPS spectra for (b) O 1s, (c) Zr 3d, along with (d) Cr 2p.
Figure 5

The (a) full-scan XPS spectra of Zr@ATP before and after Cr(vi) adsorption, high-resolution XPS spectra for (b) O 1s, (c) Zr 3d, along with (d) Cr 2p.

3.3 Adsorption properties

3.3.1 Effects of pH

Solution pH represents a critical factor that affects the Cr(vi) adsorption efficiency by adsorbents. According to Figure 6a, at the solution pH = 1–3, the adsorption capacity gradually increases and reaches a maximum at around a pH of 3.0. After increasing the pH value to 4, the adsorption capacity begins to decrease again, which is due to the negatively-charged attapulgite surface as well as the electrostatic repulsion with HCrO 4 , which are unfavorable to the adsorption of Cr(vi). At the same time, there is competitive adsorption of OH and HCrO 4 or CrO 4 2 , and the electrostatic force on the anions containing hexavalent chromium is weakened, which reduces the volume of Cr(vi) adsorbed. Based on the above results, it can be concluded that the adsorption capacity reaches its maximum at pH = 3.0. Therefore, it is more conducive to Cr(vi) adsorption under the condition of pH = 3.0.

Figure 6 
                     (a) Role of pH in Cr(vi) adsorption ability on Zr@ATP, (b) adsorption kinetics of Cr(vi) on Zr@ATP, (c) Langmuir model, (d) Freundlich model, (e) fitting plots of ln k
                        c to 1/T, and (f) recycling performance of Zr@ATP.
Figure 6

(a) Role of pH in Cr(vi) adsorption ability on Zr@ATP, (b) adsorption kinetics of Cr(vi) on Zr@ATP, (c) Langmuir model, (d) Freundlich model, (e) fitting plots of ln k c to 1/T, and (f) recycling performance of Zr@ATP.

3.3.2 Adsorption kinetics

Surface adsorption to a solid absorbent falls into physisorption and chemisorption. Physisorption is a non-specific loose binding of the adsorbate to the solid via van der Waals type interactions, while chemisorption involves a more specific binding of the absorbate to the absorbent. The Langmuir isotherm best describes the chemisorption processes. To get a deeper insight into the adsorption process, the present work adopted pseudo-first-order and pseudo-second-order kinetic models (equations (4) and (5)) to study the removal mechanism of Cr(vi) by Zr@ATP composite [42,43]:

(4) Q t = Q e [ 1 exp ( k 1 t ) ] ,

(5) Q t = ( Q e 2 k 2 t ) / ( 1 + k 2 Q e t ) .

In the above equations, Q e (mg/g) and Q t (mg/g) represent equilibrium adsorption dosage and equilibrium adsorption volume at t-time (min), respectively; whereas k 1 (min−1) and k 2 (g/mg/min) stand for model adsorption rate constants, respectively.

According to Figure 6b, the Cr(vi) adsorption performance of Zr@ATP is a rapid process. After 20 min of reaction, the adsorption capacity tends to be stable. After 50 min, the adsorption attains equilibrium. The above results could be explained as, in the initial stage, there are many –OH binding sites available on the adsorbing agent surface, and hence faster will be the adsorption rate. As the reaction proceeds, the reduction of binding sites will gradually reduce the adsorption rate and finally reaches adsorption equilibrium.

In line with equations (4) and (5) curves (Figure 6b), the adsorption behavior of Zr@ATP composite material for Cr(vi) is consistent with equation (4). Table 1 displays the correlation coefficients as well as the associated fitting curve parameters for both equations (4) and (5). Equation (5) has a significantly high correlation coefficient as compared to equation (4), and the maximum theoretical adsorption volume measured using equation (5) approaches experimental results more closely, suggesting that equation (5) performs well in describing Cr(vi) adsorption by Zr@ATP. The above results are obtained by assuming that the rate-limiting process is possibly chemisorption or chemisorption related to valence forces by exchanging or sharing electrons between Cr(vi) and Zr@ATP.

Table 1

Pseudo-first-order together with pseudo-second-order kinetic model parameters within the HM ion system

Ion Model
Pseudo-first-order kinetic model
Q e (mg/g) k 1 (L/mg) R 2
Cr(vi) 31.41 0.31007 0.8535
Pseudo-second-order kinetic model
Q e (mg/g) k 2 (L/mg) R 2
Cr(vi) 33.45 0.01564 0.9748

3.3.3 Effect of initial concentrations

This work also performed isothermal experiments at varying Cr(vi) solution concentrations. Langmuir (equation (6)) along with Freundlich isothermal (equation (7)) formula is listed below [44,45]:

(6) Q e = ( K L C e Q m ) / ( 1 + K L C e ) ,

(7) Q e = K F C e 1 / n .

In the above formula, C e (mg/L) stands for Cr(vi) content within the aqueous solution when reaching equilibrium adsorption, Q m (mg/g) and Q e (mg/g) represent adsorption volumes at maximum and equilibrium adsorption determined using Langmuir isothermal model, respectively. Langmuir constant K L (L/mg) can be determined by adsorbing agent’s binding energy, 1/n and K F ((mg/g) (L/mg)1/n ), respectively, stand for constants representing adsorption intensity and density.

The Langmuir and Freundlich fitting curves obtained are shown in Figure 6c and d. Compared with the Freundlich isotherm model, the Langmuir isotherm model is well fitted. From Table 2, it is evident that the Langmuir isotherm equation can better describe Cr(vi) adsorption via the adsorbent than the Freundlich isotherm equation, suggesting a monolayer adsorption mechanism.

Table 2

Langmuir, together with Freundlich isotherm model parameter within the HM ion system at varying temperature conditions

Temperature (K) Model
Langmuir isotherm model
Q m (mg/g) k L (L/mg) R 2
298 37.37 0.2879 0.9575
308 34.95 0.3294 0.9670
318 34.19 0.2554 0.9502
Freundlich isotherm model
k F ((mg/g) (L/mg)1/n ) 1/n R 2
298 14.44 0.2605 0.8092
308 13.65 0.2592 0.8734
318 11.81 0.2863 0.8136

3.3.4 Thermodynamic study

According to Figure 6c and d, as the temperature increases to 318 K, the equilibrium Cr(vi) adsorption volume via Zr@ATP declines. This is because the increase in temperature will deactivate the active sites on the attapulgite surface, leading to decreased adsorption volume. And the adsorption experiment results at varying temperatures point out that Cr(vi) adsorption onto Zr@ATP follows the characteristics of physical adsorption, i.e., equilibrium adsorption volume declines with a rise in temperature.

Thermodynamic parameters, such as enthalpy change (ΔH Θ), entropy change (ΔS Θ), and Gibbs free energy (ΔG Θ) helped to predict the temperature dependence of adsorption. They were determined according to equations (8)–(10) in the present work [22,46,47]:

(8) k c = q e / C e ,

(9) G Θ = RT ln k c ,

(10) ln k c = S Θ / R H Θ RT .

In the above equations, k c (dimensionless) represents equilibrium constant, K L stands for adsorption experiment-derived Langmuir constant, T and R indicate absolute temperature (in K) and universal gas constant (8.314 J/(mol K)), respectively. ΔS Θ and ΔH Θ were predicted based on the ln k c vs 1/T plot (Figure 6e).

According to Table 3, the adsorption process has a negative ΔH Θ, indicating an exothermic nature of the Cr(vi) adsorption process onto Zr@ATP. Low temperature benefits adsorption progression. Calculated ∆G Θ < 0 indicates that the adsorption is spontaneous. The ∆G Θ calculated at different temperatures were −1.514, −1.287, and −1.119 kJ/mol, suggesting that the process is primarily physisorption. The negative ∆S Θ indicates that a displacement reaction occurs after the adsorption of Cr(vi), along with the replacement of the –OH from the adsorbent surface. In other words, the adsorption system involves a reaction of decreasing disorder.

Table 3

Thermodynamic parameters of Cr(vi) adsorption onto Zr@ATP within the HM ion system

T (K) ΔH Θ (kJ/mol) S Θ [J/(mol K)] G Θ (kJ/mol)
298 –7.416 –19.84 –1.514
308 –1.287
318 –1.119

According to Figure 6f, in comparison with the original adsorption, the regeneration performance of Zr@ATP dropped from 87.82 to 71.65% after three consecutive cycles. This figures out that the adsorbent possesses high adsorption capacity and recycling performance and can be efficiently utilized to remove Cr(vi) from an aqueous solution.

3.3.5 DFT calculations

To confirm the existing form of Cr(vi) in aqueous solution at diverse pH levels, different Cr(vi) existing forms were simulated using Visual MINTEQ ver. 3.0 [11,24]. According to Figure 7, when the pH of the solution is less than 5, the existing forms of Cr(vi) is mainly HCrO 4 . The CrO 4 2 species appears at pH above 5, and the concentration of CrO 4 2 species decreases. Considering this, the adsorption process of CrO 4 2 and HCrO 4 on the ATP and Zr@ATP was investigated with the aid of the DFT calculations. The optimized structures of ATP and Zr@ATP are as presented in Figure 8a and b. Figure 8c and d corresponds to the structures of ATP and Zr@ATP after HCrO 4 adsorptions. From the optimized structures (Figure 8c and d) of ATP and Zr@ATP after adsorbing HCrO 4 , it becomes clear that the HCrO 4 forms a hydrogen bond with the Zr(OH)4 grafted on the surface of ATP and has no obvious weak interaction with the hydroxyl groups on the ATP surface. This indicates that the hydroxyl functional group provided by the Zr(OH)4 has an excellent adsorption performance toward HCrO 4 . Simultaneously, CrO 4 2 adsorbed on the ATP and Zr@ATP was also investigated. As depicted in Figure S2, due to the high surface energy of ATP, the hydroxyl group on the ATP surface and the neighboring hydroxyl functional group possibly forms a hydrogen bond. This hinders CrO 4 2 and H atom on the ATP surface from hardly forming any hydrogen bonds, thus, inducing the poor adsorption ability. After being modified with Zr(OH)4, many hydroxyl functional groups are introduced, which can efficiently form hydrogen bonds with CrO 4 2 and HCrO 4 , leading to an increased adsorption capacity.

Figure 7 
                     The existing forms of Cr(vi) in aqueous solutions under different pH.
Figure 7

The existing forms of Cr(vi) in aqueous solutions under different pH.

Figure 8 
                     The optimized structures of (a) ATP, (b) Zr@ATP, (c) ATP after adsorbing 
                        , and (d) Zr@ATP after adsorbing 
Figure 8

The optimized structures of (a) ATP, (b) Zr@ATP, (c) ATP after adsorbing HCrO 4 , and (d) Zr@ATP after adsorbing HCrO 4 .

For further understanding of the adsorption process, adsorption energy (E ads) of the system was calculated. As presented in Table S3, before modifying the ATP, the adsorption energy E ads of ATP toward CrO 4 2 and HCrO 4 are −9.13 and −15.46 eV, respectively, indicating that the adsorption of HCrO 4 on ATP is more stable. After the modification process, the adsorption energy (E ads) of Zr@ATP toward CrO 4 2 and HCrO 4 are −28.02 and −28.06 eV, respectively, suggesting that the modification process could reduce the adsorption energies of both the systems and make the adsorption a more facile process [23,48]. The above results can be explained by the modification process introducing more hydroxyl groups onto the ATP surface, thereby supplying more active sites to form hydrogen bonds with CrO 4 2 and HCrO 4 and reducing the adsorption energy [48].

4 Conclusions

In summary, the Zr-modified attapulgite was prepared to eliminate the Cr(vi) ions from aqueous solutions. According to XRD, SEM, TEM, and FT-IR analysis results, Zr showed successful grafting onto the attapulgite surface. Through the optimization of reaction conditions, the ideal parameters for the preparation of Zr@ATP were determined as reaction pH of 5, reaction time of 2 h, and mass proportion of ATP:Zr = 4:6. Cr(vi) adsorption onto Zr@ATP surface was well fitted with Langmuir isotherm and pseudo-first-order kinetic models, which verified that the process is primarily physisorption. As obtained from DFT calculation, the hydroxyl functional group introduced through the modification process reduced the adsorption energy and supplied more active sites to form hydrogen bonds with CrO 4 2 and HCrO 4 . Above experimental results and theoretical calculation analysis suggest that modification markedly enhances the adsorption performance of ATP toward Cr(vi), which provides further guidance for the preparation and application of high-performance attapulgite-based adsorbents.


The present study was funded by National Natural Science Foundation of China (21761011 and 21865007).

  1. Author contributions: Yani Liu: conceptualization, formal analysis, investigation, methodology, validation, visualization, writing – original draft. Lei Xu: conceptualization, data curation, formal analysis. Qingyun Wang: data curation, formal analysis, funding acquisition. Tong Zou: investigation, methodology. Cheng Cao: investigation, methodology, supervision, funding acquisition. Qiqi Fang: software, validation. Nan Zhang: conceptualization and validation. Yongcheng Wang: project administration, resources, supervision, and writing review and editing.

  2. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

  3. Data availability statement: All data generated or analyzed during this study are included in this published article (and its supplementary information files).


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Received: 2022-09-01
Revised: 2022-10-28
Accepted: 2022-11-05
Published Online: 2022-12-03

© 2022 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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