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. 2020 Sep 2;5(36):23372–23377. doi: 10.1021/acsomega.0c03329

A Tandem Adsorption-Catalysis Strategy for the Removal of Copper Ions and Catalytic Reduction of 4-Nitrophenol

Muhua Chen †, Yingyun Gao †, Bo Fu †,*, Fan Yang ‡,*
PMCID: PMC7496003  PMID: 32954189

Abstract

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In this work, a consecutive adsorption-catalysis approach to remove Cu2+ ions and catalytic reduction of 4-nitrophenol (4-NP) is proposed. Attapulgite (ATP) nanorods are utilized as adsorbents to enrich Cu2+ ions from contaminated water. Subsequently, the adsorbed ions were in situ reduced to construct Cu-loaded ATP catalysts. The catalytic activities of the composite ATP-Cu catalysts are evaluated by 4-NP reduction in the presence of NaBH4. The optimal ATP-Cu50 sample prepared by putting ATP into a 50 mg L–1 CuSO4 solution could complete the catalytic reaction within 4 min. Moreover, the Cu-deposited ATP nanorods can be integrated into a continuous flow catalytic system, and the 4-NP can be rapidly reduced. This method sheds lights on the fabrication of ATP-based hybrid catalysts and the removal of multiple water pollutants.

Introduction

Attapulgite (ATP, also called as palygorskite) is a natural inorganic mineral clay with a layered ribbon shape.1,2 The structure of ATP consists of two bands of tetrahedral silica, which are connected by aluminum ions in octahedral coordination. The theoretical formula of ATP can be defined as (Al,Mg,Fe)5Si8O20(OH)2(OH2)4•4H2O, and the actual composition of ATP relies on the producing area.3 Due to its sustainability, abundance in nature, low toxicity, high surface area, and good mechanical strength, ATP has found many applications in de-coloring agents, adsorptive materials, catalyst supports, and inorganic membrane fillers.4−7

Featured by negative charges and exchangeable cations, ATP is expected to adsorb various metals from contaminated water.2,8 Several prior works demonstrated that ATP-based materials have promising abilities in heavy metal ion (Cr, Cu, and Ni) removal.9−12 For example, Wang and co-workers utilized the hydrothermal approach to modify ATP and the adsorption capacity for Cu2+ ions could reach 210.6 mg g–1.13 The recovery of nanoscaled ATP limits its wider utilization. In this context, tremendous efforts have been dedicated to improve the recyclability of ATP: (1) nanoconstructing magnetic ATP composites, (2) fabricating filtration membranes/aerogels, and (3) preparing millimeter-sized polymeric beads.14−19 Despite the abovementioned methods facilitating the ATP recovery, other problems still remained. For instance, the desorption of metal ions and regeneration of adsorbents often involve harsh operating conditions, such as the use of large amounts of acid and high energy cost.6,20,21

ATP nanorods have a zeolite-like pore structure, high aspect ratio, and high surface area, which make them a potential catalyst support.22,23 The needle-shaped ATP could substantially improve the metal particle dispersion, and thus is regarded as a promising alternative for the conventional carrier of metal catalysts.24 Previously, Chen et al. doped Cu elements on ATP through a wet impregnation method.25 The prepared catalysts presented superior NH3 oxidation activity. Wang et al. encapsulated magnetic ATP into a polymer matrix and utilized them for Ag+ adsorption.26 The Ag+ adsorbed materials were subsequently treated with NaBH4 to acquire Ag particles deposited catalysts.

Inspired by these works, herein, we design a consecutive adsorption-catalysis approach to remove metal ions and catalytic reduction of nitroaromatic compounds. Combined with the good adsorption capacity of ATP and catalytic ability of metal elements, the adsorbed metal ions can be in situ reduced to obtain hybrid ATP-metal catalysts. At the first step, the adsorption performance of pristine ATP to Cu2+ ions was explored in detail. Following that, the adsorbed Cu2+ ions were in situ reduced to prepare Cu-loaded ATP catalysts in the presence of NaBH4. After depositing Cu particles, 4-nitrophenol (4-NP) reduction was conducted to explore the catalytic properties of ATP-Cu composites. Because of the high surface area and abundant adsorption sites of ATP, the high distribution and narrow size of Cu particles are achieved. In particular, for the purpose of flow catalysis, the ATP-Cu composite catalysts were fabricated into a filter membrane and presented a good stability and efficiency. The ATP-Cu composite has potential application in the adsorption of Cu2+ ions, and further can be used as recyclable and low-cost catalytic materials. This tandem adsorption-catalysis method provides a new route for the construction of metal–ATP composites and the removal of water contaminants. This process could simplify the desorption treatments and avoid the complicated regeneration procedures.

Results and Discussion

Adsorption of Cu2+ Ions

The XRD patterns of purified attapulgite (ATP) is shown in Figure 1a. The characteristic peaks located at 8.4, 13.7, 19.8, 21.5, and 27.5o are ascribed to the reflection of (110), (200), (040), (310), and (400) crystallographic planes of ATP nanorods, respectively (JCPDS no. 21-0958). The peak at 8.4o indicates the orthorhombic phase of ATP. Other small peaks might be associated with the oxides. For example, the significant peak is centered at 26.5o corresponding to the quartz.27 For the FT-IR spectroscopy of ATP (Figure 1b), the band around 3430 cm–1 is attributed to the stretching vibrations of O–H groups of bar-shaped ATP. The bands at 1036 cm–1 is assigned to the stretching vibration of Si-O–Si groups.28 The N2 sorption curve of ATP is displayed in Figure 1c and presents a type IV isotherm curve according to IUPAC classification.7 The adsorption–desorption isotherms are overlapped at P/P0 ≤ 0.4, indicating the presence of micropores. The surface area of ATP is calculated to be 112 m2 g–1, and the average pore size is estimated to be 3.8 nm. The high surface area and abundant surface groups endow ATP with the potential for metal ions adsorption and enrichment. Figure 1d shows the SEM image of pristine ATP, the rod-like ATP has a length of 1–2 μm and a diameter of 20–30 nm.27

Figure 1.

Figure 1

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(a) XRD pattern, (b) FT-IR spectroscopy, (c) N2 sorption curve, and (d) SEM morphology of pristine ATP nanorods.

The adsorption isotherm of Cu2+ ions over ATP is recorded in Figure 2a. With the increase of initial Cu2+ concentrations, the adsorption capacities present a growing trend. The sorption amount increased from 38.8 to 160.7 mg g–1 as the C0 was increased from 100 to 600 mg L–1. The calculated regression coefficient values (R2) for the Freundlich model is 0.999, higher than that of the R2 of the Langmuir model (0.993) (Table S2, Supporting Information). This indicates that the Freundlich model fits better for ATP in the experimental conditions.29 The qmax calculated by the Freundlich model is 171.2 mg g–1, which is comparable with ion-exchanged zeolites and carbon materials. When the original Cu2+ ions were below 200 mg L–1, the ions could be removed in a high ratio. Hence, the following adsorption experiments were performed with Cu2+ ions from 25 to 150 mg L–1. After Cu2+ ions adsorption, Cu particles loaded catalysts are obtained by NaBH4 reduction. The color change depicted in Figure 2b reveals the successful deposition of Cu elements on ATP. Figure 2c illustrates XRD patterns of ATP-Cu composite catalysts. There are no obvious diffraction peaks of Cu particles, which might be ascribed to the low loadings and high dispersion of Cu particles. ATP-Cu composites are further characterized by FT-IR (Figure 2d), the intensity of the Si–O stretching vibration peak (1036 cm–1) displayed a declined tendency with the increase of Cu contents. This is correlated with the presence of Cu elements, which alter the surface interactions of Si–O bonds.4,27 In addition, no Cu–O interactions were detected form FT-IR curves. The peaks below 800 cm–1 are assigned to Fe–O stretching vibrations. In addition, ICP-OES results show that the Cu content (wt %) in ATP-Cu25, ATP-Cu50, ATP-Cu75, ATP-Cu100, ATP-Cu125, and ATP-Cu150 were 0.52, 1.22, 1.45, 1.62, 2.11, and 2.36 wt %, respectively.

Figure 2.

Figure 2

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(a) Adsorption isotherm for Cu2+ ions over pristine ATP nanorods, (b) photographs illustrated the color change of ATP with different Cu loadings, (c) XRD patterns of ATP-Cu catalysts, and (d) FT-IR spectra of ATP-Cu catalysts.

To further explore the changes of porosity and morphology over Cu-loaded ATP nanorods, ATP-Cu50 was selected as the model sample. As illustrated in Figure 3a, ATP-Cu50 displays a surface area of 63 m2 g–1. The decreased surface area compared with original ATP (112 m2 g–1) reveals the introduction of Cu particles occupied some pore channels of ATP. The SEM image in Figure 3b demonstrates that Cu-deposited ATP still remains a rod-like shape. No significant morphology changes can be observed from ATP-Cu50 and pristine ATP. As displayed in Figure 3c, ATP nanorods have a length of 1–2 μm and a width of 20–30 nm. Metal particles are successfully deposited on the ATP surface. No significant aggregation of Cu nanoparticles can be observed in the TEM image.30,31 In addition, the successful deposition of Cu elements can be further proved by the EDS diagram (Figure 3d).

Figure 3.

Figure 3

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(a) N2 sorption curve, (b) SEM morphology, (c) TEM image, and (d) EDS diagram of ATP-Cu50 catalyst.

Catalytic Tests

The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) is generally used as the model reaction to test the catalytic activity of metal particles. 4-NP is one of the most toxic mononitrophenols and the Environmental Protection Agency of United States limits its concentration to 10 ppb in natural water.32,33 The catalytic reduction of 4-NP to 4-AP by non-noble Cu particles is an effective and green route and there is no side reactions.34,35 4-NP exhibits a peak at 400 nm due to the formation of BH4– ions in alkaline NaBH4 solution. Without the addition of catalysts, the peak at 400 nm kept unchanged for 24 h. As depicted in Figure 4a, pristine ATP nanorods present a very low catalytic activity and the reaction proceed slowly, indicating that the metal oxides in ATP have negligible catalytic effects. When ATP-Cu50 composite was added into the solution, the signal at 400 nm decreases quickly, and a new signal peak appears at 298 nm. This emerged peak is indexed to 4-AP, and the reduction could be completed within 4 min (Figure 4b). Moreover, the full conversion of 4-NP to 4-AP also can be visualized with the faded color of the solution (inset, Figure 4b).

Figure 4.

Figure 4

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(a) Time-dependent UV–visible absorption spectra for the reduction of 4-NP in the presence of NaBH4 without catalysts, (b) time-dependent UV–visible absorption spectra for the reduction of 4-NP with ATP-Cu50, (c) plots of ln(Ct/C0) against time for the catalytic reduction of 4-NP over different catalysts, and (d) stability tests over ATP-Cu50 for continuous flow catalytic reduction.

The catalysts with different Cu loadings were also tested, and the results are summarized in Figure 4c. The linear relation of ln(Ct/C0) against reaction time indicated that the reactions followed first-order kinetics. The Cu content has an influence on reaction speeds, and ATP-Cu50 is the optimized sample. The differences in apparent rate constants (k) are due to the Cu loading and their distribution. The k value of ATP-Cu50 (36.7 × 10–3 s–1) is one order higher than ATP-Cu25 (3.8 × 10–3 s–1). Further improving Cu contents, the k value showed a decreasing tendency (e.g., 13.8 × 10–3 s–1 for ATP-Cu75, 12.0 × 10–3 s–1 for ATP-Cu100, 4.2 × 10–3 s–1 for ATP-Cu125, and 4.1 × 10–3 s–1 for ATP-Cu150). Generally, the Cu content in the composite catalysts is proportional to the concentrations of precursor CuSO4 solutions. The results suggest that both the quantity of Cu particles and their distribution have effects on catalytic activity.36,37 The transformation of 4-NP to 4-AP follows a typical electron transfer mechanism. Also, the Cu particles existed in the pore channels of ATP nanorods improve the available active sites.38 First, the BH4– ions in the solution could interact with Cu particles to form metal hydrides. Then, the electrons transferred from the BH4– ions to 4-NP molecules through the metal surface and the protons rearranged to produce 4-AP. The abundant porosity and surface groups of ATP substrate promote the 4-NP adsorption and 4-AP migration.39 In addition, ATP-Cu50 is utilized for the next reusability tests. The catalysts are separated from the solution by centrifugation. The catalyst after eight cycles still has a conversion efficiency of 94% (Figure 4d), indicating the good stability. ICP-OES experiments demonstrate that there is no detachment or leaching of Cu particles. The catalytic properties of ATP-Cu50 is comparable with other related catalysts reported in the literature (Table S3, Supporting Information).35,40

Considering the remarkable elimination rate under the batch catalysis, it is expected that the Cu-loaded ATP to perform well under flow conditions.41 In the continuous experiments, ATP-Cu50 was encapsulated into a filter membrane, and a mixed solution of 4-NP and NaBH4 passed through the filter at a certain rate. With a flow speed of 1.0 mL min–1, the residence time of the 4-NP solution (0.01 M) penetrating the membrane is estimated to be 3.6 min. After passing a volume of 1000 mL, the effluent remained colorless, and the 4-NP concentration was close to zero. The maximum volume for the filter was estimated to be 2000 mL as the 4-NP concentration in the effluent was increased to be 12–20 ppb. This indicates that the 4-NP could be quickly and fully reduced, which is due to the high activity and accessibility of Cu particles. Such high performance endows the composite materials with huge potential for practical applications.

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Conclusions

A tandem adsorption-catalysis system was proposed for removing metal ions and catalytic reduction of 4-nitrophenol (4-NP). Attapulgite (ATP) was applied as adsorbents to remove Cu2+ ions from contaminated water. The results showed that ATP nanorods had an excellent metal removal ability. Then, the adsorbed Cu2+ ions were in situ reduced to element Cu particles to prepare Cu-loaded ATP catalysts. The results of EDS and N2 sorption characterizations demonstrated the successful loading of Cu elements on ATP. The catalytic activity of metal-deposited ATP nanorods toward the reduction of 4-nitrophenol was investigated. In addition, by integrating the ATP-Cu50 into a filter, the catalyst showed a remarkable performance for flow catalytic reduction. This consecutive strategy offers new insights for the removal of multiple water contaminants and applications of natural clay materials.

Experimental Section

Materials and Reagents

The natural clay attapulgite (ATP) was purchased from Xuyi, China. To remove the impurities, the raw ATP (10 g) was immersed into a 20% NaCl solution (100 mL) under stirring for 12 h at 50 °C, washed with deionized water, and dried at 80 °C for 12 h. Copper sulfate pentahydrate (CuSO4•5H2O) and sodium borohydride (NaBH4, >98%) were supplied by Sinopharm Chemical Reagent Co. In addition, 4-nitrophenol (4-NP, >99%) was bought from Shanghai Macklin Biochemical Co.

Fabrication of Cu-Loaded ATP Nanaorods

First, CuSO4 solutions (60 mL) with different concentrations (25, 50, 75, 100, 125, and 150 mg L–1) were prepared and used as ion-containing pollutants. ATP nanorods (200 mg) were added into the solution under stirring and kept at room temperature for 24 h. Subsequently, NaBH4 solution (0.1 M, 40 mL) was added dropwise, and the solution color turn from green into black gray. The resulting mixed solution was then centrifuged, washed with de-ionized water, dried at 80 °C overnight, and ground to fine powder. The metal adsorbed ATP nanaorods were denoted as ATP-Cux, where x refers to the precursor Cu2+ ions concentration (mg L–1).

Characterization

The chemical compositions of purified ATP (Table S1, Supporting Information) were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermal Fisher). The microstructure and morphology of ATP-Cu composites were explored using a scanning electron microscope (SEM, JSM-7600, JEOL Led., Japan) operated at an accelerating voltage of 10 kV. Elemental composition of the metal-loaded samples was recorded by using energy-dispersive X-ray spectroscopy (EDX) analysis. The crystalline structures of all samples were measured by X-ray diffraction (XRD, Rigaku Ultima IV, Japan) with Cu Kα radiation (k = 0.1542 nm) at 40 kV. Fourier transform infrared spectra (FT-IR, Thermo Electron Nicolet-360, USA) were conducted to reveal the group interactions between metal particles and ATP nanorods. Nitrogen adsorption–desorption isotherms were measured at liquid nitrogen temperature (77 K) using a volumetric adsorption analyzer (Micromeritics ASAP 2020, Micrometritics, USA). The specific surface areas and the pore diameter distributions were determined by the BJH (Barret–Joyner–Halenda) method. Inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted on a Varian ICP-OES Vista Pro.

Adsorption of Cu2+ Ions

The adsorption capacity of ATP adsorbents for Cu2+ ions was calculated as follows:

graphic file with name ao0c03329_m001.jpg

where V indicates the metal salt solution volume (L), C0 is the initial metal ion concentration (mg L–1), Ct stands for the Cu2+ ion concentration in the solution after adsorptions, and m is the adsorbent mass (mg). The concentration of Cu2+ ions was tested by using an atomic absorption spectrometer (PerkinElmer, USA).

Catalytic Reduction of 4-NP

To test the catalytic activity of ATP-Cu samples, the reduction of 4-NP was employed as the model reaction. In brief, NaBH4 (0.4 mg mL–1, 50 mL) and 4-NP (0.01 M, 500 μL) were mixed in a 100 mL glass beaker under magnetic stirring. Afterward, 4 mg of ATP-Cu composite catalysts was added. At a given time, the remaining concentration of 4-NP (λmax = 400 nm) in the solution was determined by UV–vis absorption spectrophotometer (Shimadzu, Japan). Finally, the used catalysts were centrifuged from the solution, rinsed with de-ionized water, dried overnight and then reused for the next cycle.

Moreover, long-time stability of the catalysts was explored by a continuous flow system. During the tests, 4 mg of ATP-Cu were fixed on a filter membrane (0.2 μm) by passing the suspended Cu-adsorbed ATP solution through the membrane. Following that, the mixed solution of 4-NP (0.01 M, 500 μL) and NaBH4 (0.4 mg mL–1) was injected through the filter at a rate of 1.0 mL min–1 with the assistance of an external pressure. The UV spectrophotometer was used to record the conversion efficiency.

Acknowledgments

The authors thank the financial support of the National Natural Science Foundation of China (21606133) and the Support Program for Young Scholars of Nanjing University of Finance and Economics.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03329.

  • Main chemical compositions of purified ATP, the parameters of the Langmuir model and the Freundlich model, and the comparison of different catalysts for reduction of 4-NP (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03329_si_001.pdf (76.2KB, pdf)

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