Robust and all-inorganic absorbent based on natural clay nanocrystals with tunable surface wettability for separation and selective absorption

Abstract

We have developed a simple method for fabrication of a robust, three-dimensional all-inorganic porous attapulgite (ATP) monolith based on natural clay nanocrystals using CaCO₃ as a hard template. By a simple surface modification with polydimethylsiloxane (PDMS) by the chemical vapour deposition method, the resulting hydrophilic ATP monolith can be tuned to be superhydrophobic and superolephilic for selective absorption of oils or weak polarity organics from water. Also, the surface wettability of the PDMS-treated ATP monolith can be reversibly tailored from superhydrophobic to hydrophilic by simple thermal treatment, making it a multifunctional absorbent for absorption of oils, non-polar or polar organics, and metal ions from water only by simply tuning its surface wettability.

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Introduction

Severe water pollution arising from oil spillage and chemical leakage has aroused serious concern and worldwide attention with the rapid development of modern industry. As a result, the development of efficient technology/materials for removal of oils or organic contaminates from water is of great importance to address environmental issues. The traditional approaches for cleanup of oil spills or organic contaminates typically involve combustion, the use of oil booms, barriers, and skimmers.¹ However, these approaches have limitations in poor efficiency while at times also causing other types of pollution during cleanup.² Alternatively, the absorption technology has been regarded as one of promising remedies to this point. To date, a number of absorbents including clay,³ active carbon,^4,5 polymers,^6–8 carbon nanotubes,⁹ etc. have been developed for removal of oil spills or organic contaminants from water. However, these absorbent materials have their respective drawbacks such as slow adsorption kinetics, poor selectivity and limited working capacity, which limit their practical use on a large scale. In recent years, the creation and utilization of the surface superhydrophobicity of a solid for direct separation or selective absorption of oils or hydrophobic organic solvents from water have generated an increased interest. There are several kinds of porous absorbents with superhydrophobic and superoleophilic properties, such as superwetting nanowire membrane,¹⁰ superhydrophobic nanoporous polydivinylben-zene,¹¹ conjugated microporous polymers,^12,13 carbon nanotubes sponges,¹⁴ graphene sponges,^15,16 inorganic mesh films^17–19 and sponges.²⁰ have been reported so far. These materials have great advantages over those traditional absorbent materials because of their excellent selective absorption performance, fast absorption kinetics, good working capacity and recyclable use performance. In the most cases, however, the need of complicated or lengthy processes and high production cost still remains a key challenge for realizing them into practical applications. Meanwhile, the environmental and ecological risk of using these absorbents in application remains unclear and the disposal of them after use may also cause new environmental pollution. Therefore, the exploitation of functional materials that fulfill such multivariable performance requirements for efficient removal of oils and organics from water should be of special interest. Attapulgite (ATP) as a kind of hydrated magnesium aluminium silicate usually presents in nature as fibrillate mineral. In this work, we used the natural, low-cost ATP nanocrystals for fabrication of three-dimensional porous ATP monolith employing CaCO₃ as a hard template. By a simple surface modification with polydimethylsiloxane (PDMS, a low surface energy material) by chemical vapour deposition (CVD) method, the resulting hydrophilic ATP monolith can be tuned to be superhydrophobic and superolephilic for selective absorption oils or weak polarity organics from water. Also, the surface wettability of PDMS-treated ATP monolith can be reversibly tailored from superhydrophobic to hydrophilic by simple thermal treatment, which can also be used as a functional absorbent for removal of metal ions or hydrophilic organics from water. To our knowledge, such clay-based absorbent with tunable surface wettability has never been reported and may have great potentials for water treatment and oil spills cleanup.

Experimental section

Materials

The ATP [Mg₅Si₈O₂₀(OH)₂(OH)₄·4H₂O] was purchased from Guangming ATP Co. Ltd, Anhui province, China. For preparation of PDMS (Sylgard184, Dow Corning) substrates, the curing agent and base part were mixed in a 1 : 15 weight ratio and then heated at 80 °C for 3 h.

Purification of ATP

In order to remove the impurities, the natural ATP was treated by aqueous HCl. In a typical procedure, ATP (20 g) and sodium hexametaphosphate (0.4 g) were mixed in distilled water and kept for 24 h at room temperature followed by washing with abundant water. After dying, the ATP and aqueous HCl (1 M, 40 mL) were mixed and kept for 24 h at room temperature. Then, the HCl treated ATP was washed with abundant deionized water until no Cl⁻ was detected and dried in vacuum at 50 °C for overnight.

Preparation of ATP monolith

The purified ATP (10 g) and CaCO₃ (10 g) were mixed in solid state. Then, the mixture was heated to 600 °C with a heating rate of 5 °C min⁻¹ under consecutive nitrogen flow and kept for 1 h. The resulting material was immersed in aqueous HCl (1 M, 20 mL) for 24 h to remove CaCO₃ and washed with abundant deionized water until no Cl⁻ was detected. Then, the sample was dried at 100 °C for 24 h.

Modification of ATP monolith

A certain amount of ATP monolith and a piece of PDMS film were put into a sealed vessel and maintained at 240 °C for 1 h.

Absorption capacity measurement

The absorption capacity of ATP and PDMS treated ATP monolith for organic solvents can be evaluated by volume gain, which is calculated as V_absorbed/V_initial. V_absorbed is the volume of absorbed organic liquid. V_initial is the volume of PDMS treated ATP monolith. V_initial is calculated based on the weigh and density of PDMS treated ATP monolith (0.924 g cm⁻³).

Characterization

The UV-visible absorption spectra were recorded using an UV spectrometer (U-2010, Hitachi High-Tech. Corp.). The morphology of the materials was examined by scanning electron microscopy (SEM, JSM-7601F) instrument after coating the sample with Au film. Water and oil contact angle measurement were performed on a contact angle meter (DSA100, Kruss). The porous structure of the materials was characterized with nitrogen isotherm measured at 77 K using a pore and surface analyzer (Quantachrome, Autosorb-6B). Solid state infrared spectra were recorded in the range of 4000–400 cm⁻¹ using KBr pellet technique on a FT-Raman Module (Nicolet, America) instrument. X-ray photoelectron spectroscopy (XPS) analysis was performed on a ESCALAB250xi spectrometer (Thermon Scientific).

Results and discussion

The ATP is generally characterized by its unique layer chainlike structure with exchangeable cations in its framework channels.²¹ The ideal chemical structure of the used natural ATP is shown in Fig. 1a (inset), which consists of two double chains of the pyroxene-type (SiO₃)⁶⁻ like amphibple (Si₄O₁₁)⁶⁻ running parallel to the fibre axis. From the SEM image of ATP (Fig. 1a), it can be seen that the ATP consists of rod-like nanocrystals with an average diameter of 15–50 nm and a rod length of ca. 1 μm. The ATP nanocrystals are partly bundled together and irregularly aggregated. By employing the CaCO₃ as a hard template, the ATP nanocrystals can be engineered into a porous monolith. The resulting ATP monolith shows a porous network in which ATP nanocrystals connect with each other to form the backbone of the framework (Fig. 1b inset) and the hierarchical micrometer sized pores can be seen clearly from the SEM image (Fig. 1b).

Fig. 1 SEM images of (a) ATP and (b) ATP monolith. Scale bar: (a): 1 μm. (b): 10 μm. Inset: 100 nm.

The porosity of these two materials was evaluated by the nitrogen adsorption and desorption isotherm at 77 K. As shown in Fig. 2, both the ATP and ATP monolith show IV-type adsorption and desorption isotherms, indicating mesoporous feature. The porous properties of ATP and ATP monolith were shown in Table 1. The BET surface area of ATP monolith was calculated to be 71.6 m² g⁻¹. Clearly, the BET surface area and the pore volume of ATP monolith decrease after treatment by CaCO₃. It should be noted that the pore size of ATP monolith reached 8.430 nm, much greater than that of ATP. Such increase in pore size would result in an improvement of its absorption capacity for organics or oils, as reported previously.^22,23

Fig. 2 Nitrogen adsorption–desorption isotherms of ATP and ATP monolith.

Table 1 The porous properties of the ATP monolith and ATP

Item	ATP monolith	ATP
BET surface area (m^2 g^−1)	71.6	163.9
Total pore volume (cm^3 g^−1)	0.151	0.411
Average pore diameter (nm)	8.430	2.202

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The porous ATP monolith has poor selectivity for separation and absorption of oils or hydrophobic organics from water on account of its hydrophilic property. Surface-coating with materials of low-surface free energy, such as fluoroalkylsilane and trimethylchlorosilane, was reported to result in a significant improvement on the hydrophobicity of materials and absorption selectivity for organics.^24–26 Thus, to tune its surface wettability, we treated the ATP monolith with PDMS by CVD method. To investigate the effect of surface chemical compositions on the surface wettability of PDMS treated ATP monolith, XPS and FT-IR were performed. As shown in Fig. 3a, for both ATP monolith and PDMS treated ATP monolith, the peaks at 284.73 eV, 532.73 eV, 154.33 eV and 103.13 eV are observed and attributed to C1s, O1s, Si2s and Si2p, respectively.²⁷ Compared with ATP monolith, however, the intensity of Si2p peak in the XPS spectrum increased after PDMS treatment, which could be contributed to PDMS deposition on the surface of ATP monolith. In the FT-IR spectra (Fig. 3b), for ATP monolith, the stretching bands and bending vibration bands of –OH present at 3430 and 1640 cm⁻¹, respectively. The peaks at 1030 and 793 cm⁻¹ are attributed to Si–O stretching of ATP. These characteristic peaks are good agreement with previous work.^21,28 For ATP monolith after PDMS treatment, the peak at 793 cm⁻¹ shifted to 799 cm⁻¹. Referring to previous work²⁹ and others,¹⁰ during the CVD, the thermal pyrolysis of PDMS would happen and lead to the cleaving of Si–O bond to some extent, and then generate short PDMS chains to form a polymer layer on the ATP monolith and subsequently to crosslink, thus resulting in a silicon coating on the surface of ATP monolith.

Fig. 3 (a) XPS spectra of ATP monolith before and after PDMS treatment. (b) FT-IR spectra of ATP monolith before and after PDMS treatment.

As expected, the surface wettability of ATP monolith changes from hydrophilicity to superhydrophobicity and superoelophilicity with a water contact angle (CA) of 151.4° and octane CA of ca. 0° after treatment with PDMS by CVD, as seen in Fig. 4b. As shown in Fig. 4b (down), the water droplets kept spherical and rolling when placed on the PDMS treated ATP monolith, indicating a superhydrophobic behaviour. Such surface superhydrophobicity should be attributed to the nanometer-sized surface roughness of the ATP monolith and hydrophobic chemistry of PDMS coating, in accordance with XPS and FT-IR, which are two key factors for fabrication of surface superhydrophobicity.²⁴ Taking great advantages of such surface superhydrophobicity and superoleophilicity, oils or non-polar organic solvents can be easily absorbed and separated from water by the PDMS treated ATP monolith.

Fig. 4 (a) Schematic of surface wetting switchability of ATP monolith. (b) Snapshots of octane (top) and water droplets (down) on the PDMS treated ATP monolith surface. Inset is the CA measurement of octane (top) and water (down). (c) Snapshots of octane and water droplets on the ATP monolith surface.

As seen in Fig. 5a(1–3), when a small piece of the PDMS treated ATP monolith (about 1.0 × 0.77 × 0.4 cm³) touched the octane–water mixture, the octane (dyed with Red oil O) was absorbed into the PDMS treated ATP monolith in a several seconds without absorbing any water, giving an excellent absorption selectivity. Such fast absorption kinetics should be attributed to superoleophilic nature of the PDMS treated ATP monolith combining with its mesoporous features where absorbing of octane by capillary action of its surface nano- or micrometre-sized pores may occur as well. Compared with those superwetting absorbents especially prepared from organic polymers which have limitations in their poor thermal or chemical stability, the PDMS treated ATP monolith shows excellent thermal stability (coating of PDMS by CVD at 240 °C) and chemical stability (insoluble in acid or base) that can tolerate to harsh conditions owing to its inorganic silicate in nature. As shown in Fig. 5a(4), the octane absorbed in the PDMS treated ATP monolith can be burned directly after absorption process, but no obvious change in the surface morphology of the ATP monolith was observed, making the ATP monolith the robust absorbent material.

Fig. 5 (a) Removal of octane from water using PDMS treated ATP monolith (1–3) and burning of absorbed octane (4). UV-visible adsorption spectra of methylene blue (b) and Cu²⁺ (c) by ATP monolith.

Interestingly, this hydrophobic PDMS coating can be removed by heating the PDMS treated ATP monolith above 400 °C (Fig. 4a). As seen in Fig. 4c, the PDMS treated ATP monolith loses its superhydrophobic property after heating at 400 °C and exhibits a hydrophilic property that can absorb both oils and water, just as commercial available sponges. The ATP has been reported to have excellent adsorption performance for metal ions and hydrophilic organic dyes due to its high ion-exchange capacity and special surface area.^3,30–32 In this way, the ATP monolith can also be used for adsorption of metal ions and organic dyes from water after removal of the PDMS coating. In this case, methylene blue (initial concentration C₀ = 10 mg L⁻¹) and Cu²⁺ (initial concentration C₀ = 200 mg L⁻¹) were employed as targets. The adsorption of the dye and Cu²⁺ was determined using a UV-visible spectrophotometer at wavelengths of 663 nm and 440 nm for methylene blue and CuSO₄ solutions, respectively. The absorbance decreases from 1.4 to 0.1 for methylene blue (Fig. 5b) and 0.7 to 0.1 for Cu²⁺ (Fig. 5c) after 48 h treatment, showing a good adsorption performance of the ATP monolith. Quite different from the natural ATP whose nanocrystals can be readily dispersed in water individually and difficult to be recovered from water for removal of metal ions and hydrophilic organic dyes, our material formed as bulk material thus facilitate the recovery of the ATP monolith during adsorption procedure for practical operation.

More importantly, the surface superhydrophobicity can be regained by treating the ATP monolith (which lose its superhydrophobicity over 400 °C) using PDMS by CVD method, suggesting a switchable surface wetting behavior between superhydrophobicity and hydrophilicity (Fig. 4a). In this way, the ATP monolith can be used as multifunctional absorbent for absorption of oils, non-polar or polar organics and metal ions from water only by simply tuning its surface wettability. To our knowledge, only a few of reports involves such multifunctional absorbent with tunable surface wettabilities that can absorb organic contaminants from water.^10,17

The absorption capacity of the PDMS treated ATP monolith was evaluated and the results are shown in Fig. 6. Compared with the natural ATP that shows a low absorption capacity ranging from 0.24 cm³ cm⁻³ to 1.20 cm³ cm⁻³, the absorbency of PDMS treated ATP monolith increase twice from 1.21 cm³ cm⁻³ to 2.82 cm³ cm⁻³ for a variety of oils and organic solvents. As reported in our previous studies,^22,23,29 increase of the mesopore volume or pore size of porous absorbent would lead to an increase in its absorption capacities for organic solvents. In this case, engineering of ATP nanocrystals into three-dimensional network obviously increases its pore size (see Table 1) which generates additional space to allow more organic molecules to be held, leading to an increase in organics uptake. Although the organics absorption capacity of the PDMS treated ATP monolith was lower compared to the recently reported superwetting absorbents such as carbon nanotube sponges,¹⁴ conjugated microporous polymers,^12,13 MnO₂ nanowire membrane¹⁰ and spongy graphene,¹⁵ it still has the advantages of low-cost production, easy to be scaled up, robustness, environmental friendly, high separation efficiency and selectivity. We anticipate that a further design on the micro- and nano-scale structures of the ATP monolith might further improve its organics absorption capacity.

Fig. 6 The absorption capacity of PDMS treated ATP and ATP monolith for various organic solvents and oils.

Conclusions

In summary, robust, three-dimensional porous ATP monolith based on natural clay nanocrystals using CaCO₃ as a hard template was fabricated. By treatment with PDMS using CVD method, the ATP monolith exhibit superhydrophobicity with a water CA of 151.4° and excellent oleophilicity, which makes the ATP monolith efficient absorbent for selective absorption of organics and oils from water. Besides, removing the hydrophobic PDMS coating just by heating the material above 400 °C, the ATP monolith regained a hydrophilic property and can remove dyes and metal ions from water. This switchable surface wettabilities between superhydrophobicity and hydrophilicity make the porous ATP monolith be multifunctional absorbent. Taking advantages of low cost production and easy to scale up, the all-inorganic porous ATP monolith might be a promising substitute for the conventional absorbent materials and may have great potentials as environmentally friendly absorbent for a wide range of applications such as water treatment and oil spills cleanup.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51263012, 51262019) and Gansu Provincial Science Fund for Distinguished Young Scholars (Grant no. 1308RJDA012).

Notes and references

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