From spent dye-loaded palygorskite to a multifunctional palygorskite/carbon/Ag nanocomposite

Abstract

Palygorskite (PAL) has been widely used for adsorption removal of dyes from wastewater, but the dye-loaded PAL is usually discharged as solid waste because it is hardly regeneratable by conventional elution processes. As the aim was to efficiently utilize the dye-loaded PAL waste, we employed a facile one-pot hydrothermal process to transform the spent methyl violet (MV)-loaded PAL into multifunctional ternary palygorskite/carbon/Ag nanoparticles (PAL/C/AgNPs) nanocomposites in the assistance of AgNO₃. The MV-loaded PAL serves as a carbon precursor, reducer of Ag(I) and supporter of the in situ formed AgNPs. Structure characterizations confirmed that the MV dye was transformed into carbon species, and AgNPs were formed on the PAL with good dispersion, and the addition of Ag(I) ions promoted carbonization of the MV molecules. The as-prepared nanocomposites exhibited excellent adsorption and catalytic performance. Adsorption evaluation showed that the nanocomposite prepared at 5 mass% of AgNO₃ dosage gives the best adsorption properties, and 99.2% of methylene blue (MB), 86.9% of MV, 68.7% of chlortetracycline hydrochloride (CTC) and 46.2% of tetracyclines (TC) were rapidly removed from 100 mg L⁻¹ of the aqueous solution using 0.5 g L⁻¹ of the adsorbent. The removal ratio increased with increasing dosage of adsorbent, and the minimum usage amounts of adsorbent for the thorough removal of MB, MV, CTC and TC molecules were 1.0 g L⁻¹, 1.2 g L⁻¹, 3.5 g L⁻¹ and 4.5 g L⁻¹, respectively. Moreover, the nanocomposites could rapidly catalyze the conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) within 6.5 min with a catalytic rate constant of 0.0120 s⁻¹, and the catalytic activity is still retained after 8 cycles of reuse. In addition, the nanocomposite can be re-generated into new adsorbents by the same hydrothermal process after adsorption of organic matters, which open a new sustainable avenue to efficiently utilize waste dye-loaded PAL and develop new adsorption and catalysis materials.

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1. Introduction

As eco-friendly materials of the “greening 21^st century material world”, natural silicate clay minerals have been frequently used as potential adsorbents for environmental applications.^1–3 Palygorskite (PAL, also known as attapulgite) is a naturally occurring hydrated magnesium aluminum silicate clay mineral with a 2 : 1 ribbon-layer structure, nanorod-like crystal morphology and a rich number of pores and surface groups.^4–6 By virtue of the special crystal structure and its surface properties, PAL has been widely used in many fields such as in nanocomposites,^7,8 carriers of catalysts,^9,10 colloidal agents¹¹ and adsorbents.^12–14 Among them, the application of PAL as an adsorbent is especially of concerned. For one thing, the large specific surface area, good ion-exchange capability and rich surface groups of PAL make it easily capture adsorbates through hydrogen-bonding, ion-exchange and complexation interactions of silanol groups with adsorbates;^15–17 for another, the isomorphous substitution of Al³⁺ for Si⁴⁺ in the tetrahedral layer and divalent cations (i.e., Mg²⁺, Fe²⁺, Zn²⁺ and Ca²⁺) for Al³⁺ in the octahedral layer make PAL have negative surface charges,¹⁸ so PAL more easily generates electrostatic attraction to cationic molecules or ions.¹⁹ As a result, PAL has been widely studied as a potential low-cost, safe and efficient adsorbent in both academic and industrial areas.

So far, great progress about the research on adsorption of PAL for dyes exists. It has been revealed that PAL has strong interactions with cationic dye molecules because the dye molecules can not only be adsorbed on the surface of PAL, but also can enter the nano-pores of PAL to form a stable hybrid structure.^20,21 It has been confirmed that the mysterious “Maya Blue” pigment, with a bright color and a superior stability that can resist acid, alkaline, UV light, thermal and biological attack, is made from dyes and PAL.^22–24 The formation of the stable dye/PAL hybrid structure means that the dye molecules adsorbed PAL are difficult to desorb using conventional eluents such as acidic/basic solutions or organic solvents, and this has been confirmed by our previous work.^25–27 Therefore, after PAL has been used for decontamination of dye wastewaters, the spent dye-loaded PAL is hard to regenerate for reusage by conventional elution process, and it is usually discarded as spent solid waste. For one thing, the solid waste brings the risk of environmental pollution; for another, the PAL resources were wasted. Therefore, it is extremely significant to fully utilize these solid resources to develop useful new materials, but it is still a challenging subject.

As is well known, the dye-loaded PAL waste is mainly composed of dyes and PAL nanorods. It has been found that the organic species loaded on PAL can be transformed into carbon species under hydrothermal conditions. For example, Chen et al. synthesized an attapulgite caly@carbon nanocomposite (APT@C) adsorbent by a hydrothermal carbonization process using glucose as the carbon precursor,²⁸ which shows good adsorption capacities of 177.74 mg g⁻¹ for Cr(VI) and 263.83 mg g⁻¹ for Pb(II). Wu et al. prepared a PAL/carbon adsorbent through a hydrothermal process using glucose as the carbon precursor,²⁹ which enhanced the adsorption capacity of PAL for phenol by 74%. These works demonstrate that the formation of carbon species on PAL by a hydrothermal process contributes a lot to improve the adsorption properties. But, most of the research is focused on the usage of expensive glucose, or other organic molecules, as a carbon precursor. Comparatively, solid waste with the loading of organic species is a potential low-cost and easily available raw material to fabricate new carbon-containing composite adsorbents, which is a eco-friendly approach to fully utilize waste resources and develop new functional materials.

Clay/AgNPs nanocomposites are important functional materials that have been intensively applied in catalytic,^30,31 antibacterial,³² sensing,³³ and electronic³⁴ fields. A variety of methods, e.g., solid grinding-heating,³⁰ UV irradiation,³⁵ photo-reduction,³⁴ chemical reduction,^36,37 chemical plating³⁸ and electrochemical methods,³⁹ have been employed to prepare clay/AgNPs nanocomposites. It has been confirmed from the above research that the reduction reaction from Ag(I) to Ag⁰, as the main formation mechanism of AgNPs, is the main approach to fabricate AgNPs-decorated silicate clay nanocomposites. Yang et al., found that Ag(I) can be in situ reduced on the surface of cellulose under hydrothermal conditions to form AgNPs/cellulose nanocomposites with no need of additional reductants.⁴⁰ In this process, organic cellulose serves as both reductant and carrier of AgNPs, and the organic matter has a stronger reduction capability to Ag(I) ions. This implies that the dye molecules in the dye-loaded PAL is a potential reductant to reduce Ag(I) as Ag⁰, and then form AgNPs. Moreover, previous research has confirmed that the existence of oxidants can promote the decomposition and transformation of MV or MB molecules.^41–43 Therefore, fabrication of AgNPs- and carbon-loaded nanocomposite by the hydrothermal reaction using the dye as reductant, Ag(I), as the oxidant and PAL as the supporter is expected to be feasible. However, research concerning the preparation of AgNPs-loaded nanocomposite from dye-loaded PAL waste is rare.

With the aim to effectively utilizing dye-loaded PAL solid waste and designing new multifunctional nanocomposites, a simple one-pot hydrothermal process was employed by us to treat dye-loaded PAL in the presence of AgNO₃. It is encouraging that the a ternary PAL/C/AgNPs nanocomposites were formed with good dispersion of AgNPs on PAL. In this process, the dye-loaded PAL waste as the reductant, carbon precursor and supporter, and AgNO₃ was the oxidant and precursor of AgNPs. Ag(I) promoted the carbonization of MV molecules and simultaneously, was reduced to AgNPs by MV molecules. The carbon species were formed on PAL rods, and the PAL rods effectively prevented the aggregation of AgNPs. The resultant nanocomposites ingeniously integrated the advantages of PAL rods, carbon species and AgNPs, and exhibited excellent adsorptive and catalytic performances.

2. Experimental

2.1. Materials

The spent MV-loaded PAL used in this work is the solid waste of natural PAL (produced from the Huangnishan Mine, Xuyi, Jiangsu, China) after adsorption of MV from aqueous solution. The adsorption capacity of the PAL for MV was measured to be 156.05 mg g⁻¹, and the content of MV in the dye-loaded PAL waste was about 15 mass%. Silver nitrate (AgNO₃, A.R. grade) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd (Tianjin, China). MB (Indicator grade), MV (A.R. grade), 4-nitrophenol (4-NP, A.R. grade) and sodium borohydride (NaBH₄, A.R. grade) were all purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Chlortetracycline hydrochloride (CTC, USP grade), and tetracyclines (TC, USP grade) were purchased from Aladdin Reagent Inc. (Shanghai, China). The molecular structure of MB, MV, CTC and TC are illustrated in Fig. S1 (ESI).† All the other reagents were of analytical grade, and deionized water was used to prepare all the solutions.

2.2. Preparation of PAL/C/AgNPs nanocomposites

The MV-loaded PAL powder (2.0 g) was dispersed in 80 mL of deionized water under ultrasonication and stirring to form a uniform dispersion. Then, 0.02, 0.05, 0.10 and 0.20 g of AgNO₃ (1, 2.5, 5 and 10% of the mass of MV-loaded PAL, respectively) was dissolved in the above dispersion, respectively. Afterwards, the aqueous dispersions were placed in a 100 mL Teflon-lined stainless steel autoclave, sealed, and reacted at 180 °C for 24 h. After the reactor was naturally cooled to room temperature, the solid product was separated by centrifugation, fully washed with deionized water until no free ions were detected in it. The solid product was dried at 65 °C under vacuum, and then gently ground and passed through a 200-mesh sieve. The as-prepared nanocomposites were coded as PAL/C/AgNPs-1, PAL/C/AgNPs-2.5, PAL/C/AgNPs-5 and PAL/C/AgNPs-10, according to the dosage of AgNO₃.

2.3. Evaluation of adsorption properties

Certain amounts of nanocomposites were mixed with 10 mL of aqueous solution containing 100 mg L⁻¹ of MB, MV, CTC and TC, respectively, and then the mixtures were shaken in a thermostatic shaker at 30 °C for 120 min. After shaking for 2 h, the nanocomposites were separated from the solution by centrifugation, and the absorbance of the MB, MV, CTC and TC solutions before and after adsorption were determined using a UV-vis spectrophotometer at their respective maximum absorbance wavelengths (664 nm for MB, 583 nm for MV, 400 nm for CTC and 390 nm for TC) and used to calculate the adsorption capability according to standard calibration curves. The removal ratio of the nanocomposites for MB, MV, CTC and TC in the solution can be calculated by eqn (1):

Removal ratio (%) = (C₀ − C)/C₀ × 100% (1)

where, C₀ (mg L⁻¹) is the initial concentration of the MB, MV, CTC or TC solutions, C (mg L⁻¹) is the concentration of the MB, MV, CTC or TC solution after adsorption.

2.4. Evaluation of catalytic activity

The catalytic activities of the nanocomposites were evaluated through the catalytic reduction reaction of 4-NP in the presence of NaBH₄. Typically, 0.03 g of the PAL/C/AgNPs nanocomposites were added to 100 mL of the 4-NP solution (initial concentration, 20 mg L⁻¹) containing 20 mmol L⁻¹ of NaBH₄. The progress of the catalytic reduction reaction was monitored by UV-vis spectroscopy. At set time intervals, the nanocomposites were instantly separated from the solution by filtering through a filter (0.45 μm), fully washed with deionized water, recycled and used for the next cycle. Eight cycles were conducted to evaluate the reusability of the nanocomposites as a catalyst.

2.5. Characterization

FTIR spectra were measured on a Fourier transform infrared spectrometer (Thermo Nicolet 6700, Thermo Fisher, USA) in the range of 4000–400 cm⁻¹ using KBr pellets. The X-ray diffraction (XRD) patterns were collected using an X'Pert PRO diffractometer equipped with a Cu-Kα radiation source (40 kV, 40 mA) from 3 to 70° (2θ) with a step interval of about 0.017° (X'Pert PRO, PANnalytical, Netherlands). Scanning electronic microscopic (SEM) images were taken using a scanning electronic microscope (JSM-6701F, JEOL, Ltd., Japan) after the samples were coated with a gold film. Transmission electron microscopic (TEM) images were observed using a JEM-1200 EX/S transmission electron microscope (JEOL, Tokyo, Japan). The specific surface area was measured on an ASAP 2010 analyzer (Micromeritics, USA) at 77 K by determining the N₂ adsorption–desorption isotherms. The values of the specific surface area (S_BET) were calculated by the BET equation. The total pore volumes (V_total) were obtained from the volume of liquid N₂ held at the relative pressure P/P₀ = 0.95. The micropore volume (V_micro) was estimated by the t-plot method. XPS data were collected using an ESCALAB 250Xi Instrument (ESCALAB 250Xi, ThermoFisher Scientific, US). Zeta potential was measured on a Nano-ZS model zetasizer instrument (ZEN3600, Malvern, England). UV-vis spectra were measured using a UV 765 spectrophotometer (UV 765, Precision & Scientific Instrument Co., Ltd, Shanghai, China). The elemental composition of the samples was determined using a Kevex energy dispersive spectrometer (JSM-5600LV, Japanese electronic optical co., LTD). The chemical composition of the metal elements was measured on a MiniPal 4 X-ray fluorescence spectrometer (PANalytical Co., Netherland).

3. Results and discussion

3.1. Morphology analysis

Fig. 1 shows the digital photographs, SEM and TEM images, of the MV-loaded PAL and the as-prepared nanocomposites. It is obvious that the MV-loaded PAL is bright purple in color (Fig. 1a), which transforms to a purplish brown color after the hydrothermal process in the presence of 1 mass% AgNO₃ (Fig. 1b). Further increasing the addition amount of AgNO₃, the color of the product becomes darker, and a dark-brown product was obtained when the addition amount of AgNO₃ was 10 mass% (Fig. 1d). The obvious change in color after the hydrothermal reaction intuitively indicates that the MV molecules were carbonized, and the Ag(I) ion was beneficial to the carbonation process.

Fig. 1 Digital images of (a) MV-loaded PAL, (b) PAL/C/AgNPs-1, (c) PAL/C/AgNPs-5 and (d) PAL/C/AgNPs-10; SEM images of (e) MV-loaded PAL, (f) PAL/C/AgNPs-1, (g) PAL/C/AgNPs-5 and (h) PAL/C/AgNPs-10; TEM images of (i) MV-loaded PAL, (j) PAL/C/AgNPs-10.

The microscopic morphologies of MV-loaded PAL and the nanocomposites were observed by SEM and TEM techniques. As shown in Fig. 1e, the rod crystals of PAL can be clearly observed in the MV-loaded PAL clay, which exist as bulk bundles or aggregates. After the hydrothermal reaction, the structure of the rod crystals do not change, and aggregated rods tend to be dispersed well, and the stacking of rods becomes loose. Compared with PAL rods in MV-loaded PAL, the surface of rods in the nanocomposites are relatively coarser, which implies the formation of carbon species and AgNPs on the PAL. However, the crystal lengths do not obviously change, indicating the crystal structure of PAL has not been broken. The formation of AgNPs can be further confirmed by TEM analysis. It can be seen from Fig. 1i that MV is difficult to observe in the TEM image of MV-loaded PAL because dye molecules are distributed on the adsorbent in the form of molecules, as confirmed by the XRD analysis.²⁷ Although MV fails to be clearly observed on MV-loaded PAL by TEM, the existence of MV can be intuitively confirmed by the purple color of the MV-loaded PAL. In addition, the FTIR spectra also confirms the existence of MV on PAL (Fig. 2). After the hydrothermal reaction, well dispersed nanoparticles on the PAL nanorods were observed (Fig. 1j), and the Ag element was uniformly distributed in the nanocomposite as shown by elemental mapping (Fig. S2, ESI†), which confirms that the AgNPs were in situ formed by the reduction of Ag(I) using the MV molecules as the reductant. In addition, carbon species were also observed around the PAL nanorods, which confirms the formation of carbon during the hydrothermal process. The formation of carbon was also confirmed by C element mapping in the nanocomposite (Fig. S2, ESI†). These observation indicate that the hydrothermal process converts the spent PAL as a ternary nanocomposite with well-dispersed AgNPs, carbon species and a one-dimensional PAL supporter.

Fig. 2 (a) FTIR spectra of MV-loaded PAL and the nanocomposites. (b) Digital images of the supernatant before and after the hydrothermal reaction with different amounts of AgNO₃ present.

3.2. FTIR analysis

The FTIR spectra of MV-loaded PAL and the PAL/C/AgNPs nanocomposites (Fig. 2a) demonstrate the change of surface groups. The characteristic fingerprints of MV at 2849 cm⁻¹ and 2922 cm⁻¹ (asymmetric and symmetric stretching of C–H), 1581 cm⁻¹ (the C=C stretching of an aromatic ring), 1379 cm⁻¹ (the C–N stretching of an aromatic ring), 1483 cm⁻¹ and 1306 cm⁻¹ (the antisymmetric and symmetric deformation bands of CH₃, respectively) can be observed in the spectrum of MV-loaded PAL.⁴⁴ However, these bands sharply weaken and even disappeared after the hydrothermal process in the presence of AgNO₃. This result indicates that the MV molecules were gradually transformed into carbon species. Interestingly, the characteristic bands of MV do not obviously change even after hydrothermally treated at 180 °C in the absence of AgNO₃ (Fig. S3, ESI†). Also, the supernatant of MV-loaded PAL is still purple in color after hydrothermal treatment at 180 °C in the absence of AgNO₃ (Fig. 2b), but it obviously becomes colorless (no dye molecules exist in the solution) after adding AgNO₃. After the hydrothermal reaction at 80 °C in the presence of AgNO₃, the characteristic bands of MV also disappeared in the spectrum of MV-loaded PAL (Fig. S3, ESI†). Simultaneously, the supernatant changes from dark purple to colorless (Fig. S4, ESI†). These observations confirmed the addition of Ag(I) ions may promote the transformation of MV molecules into carbon species.

As described previously, the MV molecules are easily degraded as small molecules under the action of an oxidant.⁴¹ Ag(I) ions, as an oxidant, may also promote the breakage of chemical bonds in MV molecules, which accelerates the carbonization reaction of the MV under hydrothermal conditions.

After the hydrothermal reaction, the characteristic bands of PAL at ∼1200 to 600 cm⁻¹ (characteristic bands of the silicate backbone) do not obviously change,^45,46 which indicates that the crystal structure of PAL does not obviously change after the MV molecules were carbonized during the hydrothermal reaction.

3.3. XRD analysis

The structural change of PAL during the hydrothermal reaction and the formation of Ag⁰ can be further confirmed by XRD analysis (Fig. 3). The XRD diffraction peaks of PAL were observed at 2θ = 8.41° (110 plane), 2θ = 13.76° (200 plane), 2θ = 16.45° (130 plane) and 2θ = 19.84° (040 plane).⁴⁷ Also, the characteristic reflections of some associated minerals, e.g., quartz, and feldspar, were observed, indicating that PAL, quartz and feldspar co-existed in the PAL clay with PAL as the main component. After the hydrothermal reaction, the characteristic reflections of the associated minerals do not change, indicating that the associated minerals do not affect the formation of the nanocomposite. The (200), (130), (040) characteristic reflections of PAL have no obvious change, and only the (110) reflection of PAL slightly shifted from 2θ = 8.41° (for MV-loaded PAL) to 2θ = 8.39° (for PAL/C/AgNPs-1), 2θ = 8.38° (for PAL/C/AgNPs-5), and 2θ = 8.34° (for PAL/C/AgNPs-10), respectively, after the hydrothermal reaction. The shift of the diffraction peaks to a smaller angle indicates that the formation of carbon and AgNPs increased the interplanar spacing of the 110 planes. This indicates the hydrothermal process has a slight influence on the PAL crystal, but it does not affect the crystal backbone structure of PAL. As is shown in the SEM and TEM images, the PAL rod crystal was well retained after the hydrothermal process, which directly confirms that the PAL crystal does not break during the reaction. In the nanocomposite, PAL acts as a supporter of carbon species and AgNPs, so retaining of the rod crystal after the hydrothermal reaction is important for nanocomposite formation. New reflection peaks appeared at 2θ = 38.1°, 44.2° and 64.4° after the hydrothermal reaction that can be ascribed to the characteristic diffractions of (111), (200) and (220) crystal planes of the face-centered cubic metallic silver (JCPDS 87-0720). This proves that Ag(I) ions were reduced to Ag⁰ during the one-pot hydrothermal process. With increasing dosage of AgNO₃, the relative intensity of the diffraction peaks of cubic metallic silver increased, indicating more Ag⁰ could be formed when the dosage of Ag(I) increased.

Fig. 3 XRD patterns of MV-loaded PAL and the nanocomposites.

3.4. XPS analysis

XPS is an effective approach to reveal the oxidation state of the Ag in the nanocomposites. As shown in Fig. 4a–c, the basic elements of PAL (Mg, Al, Si, O and Fe) can be observed in the survey scans of the PAL/C/AgNPs nanocomposites, and the Ag⁰ 3d characteristic peaks were observed in the XPS survey curves of the nanocomposite, indicating Ag was loaded on the nanocomposites. In the high resolution scans of the Ag 3d peaks (Fig. 4b), two peaks with binding energy (BE) = 368.50 and 374.50 eV can be clearly observed, which are assigned to the characteristic peaks of Ag⁰ 3d_5/2 and Ag⁰ 3d_3/2, respectively.^48,49 This proves that Ag(I) ions were reduced to Ag⁰ during the one-pot hydrothermal process. In addition, it was found that the relative intensity of the two peaks increases with increasing dosage of AgNO₃, indicating that more Ag⁰ was formed at higher dosages of AgNO₃, which is consistent with the XRD results. It can be determined by the XRF technique that the Ag contents in PAL/C/AgNPs-1, PAL/C/AgNPs-5 and PAL/C/AgNPs-10 are 0.60, 2.84 and 3.52%, respectively. Correspondingly, the carbon contents in PAL/C/AgNPs-1, PAL/C/AgNPs-5 and PAL/C/AgNPs-10 are about 0.61, 0.55 and 0.45%, respectively, which confirm the formation of Ag⁰ consumes some MV and carbon species.

Fig. 4 (a) XPS survey scanning spectra of the nanocomposites; (b) high-resolution scanning of Ag 3d.

3.5. Formation mechanism of PAL/C/AgNPs nanocomposites

It has been confirmed by the above discussion that a series of PAL/C/AgNPs nanocomposites were synthesized via a facile one-pot hydrothermal process using the spent dye-loaded PAL waste as the raw materials in the presence of AgNO₃. The dye-loaded PAL may serve as carbon precursor, reductant for Ag(I) and the supporter for carbon species and AgNPs. The redox reaction in the hydrothermal process the between MV molecules and AgNO₃, and the carbonization of organic matters simultaneously occurs, but the former was the controlling step, which was inferred from the following phenomenon. In the absence of AgNO₃, the carbonization of organic matters hardly occurs even at 180 °C, whereas it happened easily at a very low hydrothermal temperature of 80 °C in the presence of 10 mass% of AgNO₃. In other words, the transformation of organic matters was highly dependent on the concentration of AgNO₃ rather than the hydrothermal temperature.

However, the generation and crystallization of AgNPs exhibit a rather different trend. It not only depends on the concentration of AgNO₃, but also depends on the hydrothermal temperature to a large degree. To achieve a better understanding of this deduction, the PAL/C/AgNPs-10 nanocomposites prepared at different hydrothermal temperatures were characterized by XRD instruments. As shown in Fig. 5, the characteristic diffraction peaks of the face-centered cubic silver strengthened with rising reaction temperature. According to previous reports,⁴¹ and our experimental results, the redox reactions between Ag(I) and MV molecules could occur under hydrothermal conditions. The MV induces the reduction of Ag(I) to Ag⁰, and itself was transformed into carbon species. Thus, increasing the hydrothermal temperature may improve the reaction activity of MV molecules and promote conversion of Ag(I) to AgNPs. In this study, to achieve a high conversion rate of Ag(I), the reaction temperature of 180 °C was used to fabricate the nanocomposites.

Fig. 5 XRD patterns of the nanocomposites prepared at different reaction temperature.

3.6. Adsorption properties

The adsorption performance of the as-prepared PAL/C/AgNPs nanocomposites was evaluated using MV, MB, CTC and TC as the target molecules. As shown in Fig. 6, the adsorption removal ratio of the as-prepared PAL/C/AgNPs nanocomposites for each organic molecule increased with increasing dosage of AgNO₃ reaching the optimum value and then decreased. The PAL/C/AgNPs-5 nanocomposite shows the best adsorption capability for each organic matter. In addition, the maximum removal ratio of the four organic molecules using PAL/C/AgNPs-5 as the adsorbent follows the sequence of MB (99.2%) > MV (86.9%) > CTC (68.7%) > TC (46.2%). The minimum usage dosage of PAL/C/AgNPs-5 for the thorough removal of each pollutant from 100 mg L⁻¹ of initial solution was also evaluated. As shown in Fig. 7, the removal efficiency of each pollutant gradually increases with the dosage of PAL/C/AgNPs-5. The minimum dosage for complete removal of MB, MV, CTC and TC were 1.0 g L⁻¹, 1.2 g L⁻¹, 3.5 g L⁻¹ and 4.5 g L⁻¹, respectively. The change of solution color before and after adsorption was monitored with digital photographs for intuitively showing the progress of adsorption (Fig. S5, ESI†). A gradual decolorization of the MB, MV, CTC and TC solutions was observed with increasing dosage of adsorbent, which confirms that the nanocomposite derived from low-cost spent PAL adsorbent is an efficient adsorbent for the removal of dyes and antibiotics.

Fig. 6 The adsorption removal ratio of the nanocomposites for organic pollutants MB, MV, CTC and TC.

Fig. 7 UV-vis spectra of MB, MV, CTC and TC solutions (initial concentration, 200 mg L⁻¹) after absorption using different dosages of PAL/C/AgNPs-5.

As a good adsorbent suitable for practical application, both high adsorption capacity and good reusability is usually required.^50,51 In this study, it has been confirmed that the PAL/C/AgNPs nanocomposite adsorbed organic molecules can be regenerated by a desorption process using 1.0 mol L⁻¹ CH₃COOH solution as the eluent. Fig. 8a shows the removal efficiencies of the PAL/C/AgNPs-5 nanocomposite for MB (100 mg L⁻¹ in the initial solution) after five cycles of reuse. It can be seen that the removal ratio of PAL/C/AgNPs-5 for MB from 100 mg L⁻¹ of MB solution slightly decreased, but it still reaches about 76.2% after 5 cycles, which confirmed the excellent adsorption capacity and stability of the nanocomposite.

Fig. 8 (a) Reusability of the PAL/C/AgNPs-5 nanocomposite for MB adsorption; (b) removal efficiency of MB by the regenerated PAL/C/AgNPs-5 nanocomposite through calcination for different times for MB.

As discussed above, the regeneration of the nanocomposite by a desorption process using CH₃COOH solution as the eluent can retain the adsorption capability to a certain degree, but the adsorption ratio of the nanocomposite after being regenerated for 5 times is still limited. In order to improve the regeneration efficiency of the MB-loaded PAL/C/AgNPs-5 after adsorption–desorption for 5 cycles, the nanocomposite was regenerated by carbonized at 180 °C for 4 h via the facile hydrothermal method to form new clay/carbon adsorbents. The adsorption-carbonization process was conducted for 8 times, and the adsorption properties of the resultant adsorbent were evaluated. As shown in Fig. 8b, the removal efficiency of the regenerated PAL/C/AgNPs-5 nanocomposite for MB does not obviously decrease after regeneration for five times. Although a decrease of the adsorption ratio is observed after 5 regenerations, the removal ratio is still higher than 70.0% after 8 regenerations. This adsorption-carbonization regeneration process provides a sustainable approach to recycle the spent dye-loaded PAL adsorbent and can be used for the reutilization of other spent organic-matter-rich clay waste.

3.7. Adsorption mechanism

As discussed above, the nanocomposite shows excellent adsorption capacities to dyes and antibiotics. An exploration of the adsorption mechanism may be helpful to the design of new types of adsorbents.⁵² According to previous reports,^53,54 the adsorption of cationic pollutants was associated with electrostatic attraction and surface complexation. On the one hand, the adsorption of cationic pollutants onto the PAL/C/AgNPs nanocomposite is mainly attributed to electrostatic attraction. It is obviously observed from Fig. 9a that all the nanocomposites are negatively charged, which would be beneficial to capturing cationic dyes via electrostatic interactions.¹³ However, the adsorption ratio of the nanocomposites is not only dependent on surface charges. This indicates the adsorption of PAL/C/AgNPs nanocomposite for cationic matter is associated with other actions (i.e., surface complexation, hydrogen bonds).

Fig. 9 (a) Zeta potential of the nanocomposites; (b) FTIR spectra of the PAL/C/AgNPs-5 nanocomposite before and after adsorption of MB.

The FTIR spectra of PAL/C/AgNPs-5 nanocomposites before and after adsorption of MB were analyzed to get insight into the interactions between the PAL/C/AgNPs nanocomposites and the cationic pollutants. As shown in Fig. 9b, the FTIR spectrum of MB-loaded PAL/C/AgNPs-5 shows the characteristic fingerprint of MB in the range of 1600 to 1200 cm⁻¹. The stretching vibration of O–H groups at around 3410 cm⁻¹ shifts to 3396 cm⁻¹ after adsorption of MB. In addition, the absorption bands of Al–Fe³⁺–OH (at 3616 cm⁻¹), Al–Mg–OH (at 3585 cm⁻¹) and Si–O–H (at 1027 and 981 cm⁻¹) become blunt and broad.^45,46 All the above results reveal a strong interaction between O–H groups and –N– groups of MB, where the nitrogen atom of the R–N– group (R: C₁₆H₁₈ClN₂S) serves as the hydrogen-bonding acceptor to form intermolecular hydrogen bonding with the O–H groups on PAL/C/AgNPs-5 nanocomposite (Fig. S6†).⁵⁵ In addition, the generated micro-pores provide enough vacant sites to capture and encapsulate cationic molecules. As shown in Table 1, the t-plot micro-pore surface area of the MV-loaded PAL increased about 30–140 fold after being converted to PAL/C/AgNPs nanocomposites, which is caused by the generation of carbon species during the hydrothermal process. The newly formed micro-pores play a positive role to improve the adsorption. The carbon species provide enough vacant sites to hold cationic molecules, whereas the exposed O–H groups on PAL and the negative surface charges provide strong forces to capture and immobilize them.

Table 1 Pore structure parameters of MV-loaded PAL and the nanocomposites^a

Samples	SBET (m^2 g^−1)	Smicro (m^2 g^−1)	Sext. (m^2 g^−1)	Vmicro (cm^3 g^−1)	Vtotal (cm^3 g^−1)
a Vmicro, micro-pore volume.
MV-loaded PAL	87.7	2.1	85.6	—	0.2500
PAL/C/AgNPs-1	204.8	65.7	139.1	0.0294	0.3348
PAL/C/AgNPs-2.5	99.6	213.1	—	0.0969	0.1054
PAL/C/AgNPs-5	46.8	273.9	—	0.1260	0.1352
PAL/C/AgNPs-10	37.2	232.5	—	0.1051	0.0729

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3.8. Catalytic properties

The catalytic properties of the nanocomposites were evaluated using the catalytic reduction of 4-NP to 4-aminophenol (4-AP) as a model reaction. 4-NP is one of the most common organic pollutants in wastewater, and its reduction product 4-AP is a potent intermediate for the preparation of many analgesic and antipyretic drugs such as paracetamol, acetanilide, phenacetin and so forth.⁵⁶ 4-AP is also used widely as a photographic developer, corrosion inhibitor, anticorrosion-lubricant, and hair-dyeing agent.⁵⁷ In recent years, the reduction reaction of 4-NP to 4-AP using NaBH₄ as the reductant and noble metal nanoparticles as catalysts has been intensively investigated for the production of 4-AP inndustrially.^58–60 Among these noble metal nanoparticles, AgNPs received much attention due to their advantages, e.g., lower cost and high interfacial activity.

As shown in Fig. 10a, the aqueous solution of 4-NP show a maximum UV-vis absorbance peak at 315 nm. After the addition of fresh NaBH₄ solution, the peak shifts to 400 nm due to the formation of 4-nitrophenolate ions under alkaline conditions. Upon the addition of a small amount of PAL/C/AgNPs-10 nanocomposite, the absorbance at 400 nm quickly decreases until it disappear entirely at the reaction time of 390 s, while the absorbance at 293 nm (the maximum absorbance wavelength of 4-AP) simultaneously increases. This confirmed that 4-NP was rapidly transformed to 4-AP during catalysis.^59,60

Fig. 10 (a) UV-vis spectra of the 4-NP solution after catalytic reduction using PAL/C/AgNPs-10 as the catalyst. Catalytic kinetic curves of (b-1) NaBH₄, (b-2) PAL/C/AgNPs-1, (b-3) PAL/C/AgNPs-2.5, (b-4) PAL/C/AgNPs-5 and (b-5) PAL/C/AgNPs-10. (c) Linear fitting curves of −ln(C/C₀) versus time t. (d) Conversion rate of 4-NP to 4-AP within 7 min after 8 reuse cycles for PAL/C/AgNPs-10.

However, the absorption peak of 4-NP at 400 nm does not change after reaction in the absence of PAL/C/AgNPs nanocomposites (Fig. 10b), indicating addition of NaBH₄ only cannot trigger the reduction reaction, and the PAL/C/AgNPs catalyst is essential to trigger the conversion from 4-NP to 4-AP. It was found from Fig. 10b that the conversion ratio from 4-NP to 4-AP increased with increasing the loading amount of AgNPs. This is because the existence of more AgNPs with good distribution and dispersion on the surface of PAL/C is helpful for increasing the contact area of active AgNPs with 4-NP and make the transfer of electrons easier. Thus, the reaction rate would be greatly enhanced. Also, the dependence of catalytic rate on the content of AgNPs confirms the importance of the catalyst to the reaction.

The rate constant k was calculated to investigate the conversion rate of 4-NP to 4-AP. In the reaction system, the amount of NaBH₄ is in excess compared to 4-NP, which remained essentially constant during the reaction. Hence, pseudo-first-order reaction would be a reasonable assumption to evaluate the performance of the catalysts (eqn (2)).

−ln(C/C₀) = k_obst (2)

where, C (mg L⁻¹) is the concentration of 4-NP in the solution at time t, C₀ (mg L⁻¹) is the initial concentration of 4-NP and k_obs (s⁻¹) is the first order rate constant.

As shown in Fig. 10c, good linear correlation was observed in the plots of ln(C/C₀) versus time (t) for each nanocomposite, so the kinetic reaction rate constant k can be calculated by the slope of the linear plots and the results are listed in Table 2. The catalytic reaction rate of the nanocomposites for the reduction of 4-NP follows the order: PAL/C/AgNPs-10 > PAL/C/AgNPs-5 > PAL/C/AgNPs-2.5 > PAL/C/AgNPs-1, which further confirms that the catalytic efficiency is closely related to the content of AgNPs. In the reaction process, the AgNPs on the nanocomposite initiate the catalytic reaction by conveying electrons from the donor BH₄⁻ to the acceptor 4-NP when the 4-NP molecule contacts with the nanocomposite through the adsorption action on the interface.⁶⁰ The AgNPs serve as a “transfer station” of electrons, which first accept electrons from BH₄⁻ ions, and then release electrons to 4-NP. After electrons are obtained from AgNPs, the 4-NP was rapidly reduced as 4-AP. Increasing amounts of AgNPs is very beneficial to the transfer of electrons and thus, accelerates the catalytic reduction reaction. In the absence of AgNPs (transfer station of electrons), the electrons cannot transfer from BH₄⁻ to 4-NP, so the reduction reaction does not occur by only adding NaBH₄.

Table 2 Catalytic kinetic parameters for the conversion reaction of 4-NP

Catalysts	kobs (s^−1)	R^2
PAL/C/AgNPs-1	0.0007	0.9879
PAL/C/AgNPs-2.5	0.0013	0.9805
PAL/C/AgNPs-5	0.0021	0.9758
PAL/C/AgNPs-15	0.0120	0.9920

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In addition to the satisfactory catalytic activity, the nanocomposite also shows good reusability. From Fig. 10c, the nanocomposite exhibited excellent catalytic performance without significant reduction in the conversion rate of 4-NP even after running for 8 cycles, which is extremely important to the practical application of the nanocomposite as a catalyst. The superior activity and stability of the nanocomposite make it a potential highly efficient catalyst for environmentally friendly applications.

4. Conclusions

A series of multifunctional PAL/C/AgNPs nanocomposites were successfully fabricated by a simple one-pot hydrothermal process using spent dye-loaded PAL as a raw material in the presence of Ag(I). This study was aimed at the comprehensive utilization of spent PAL and the design of new nanocomposite materials, and covered the following innovation points: (a) a facile transformation of MV-loaded PAL into PAL/C/AgNPs nanocomposite by a simple one-pot hydrothermal process was achieved, as inspired by a “from nature, for nature” sustainable strategy; (b) the spent dye-loaded PAL with a certain amount of organic species was an efficient reductant to in situ conversion of Ag(I) to AgNPs with excellent catalytic activities; (c) the transformation of dyes and Ag(I) can be controlled by altering the concentration of AgNO₃ and the hydrothermal temperature; (d) a nanocomposite that combined the advantages of PAL rods, carbon species and AgNPs was obtained, and is an excellent adsorbent and catalyst.

The adsorption experiment showed that the PAL/C/AgNPs nanocomposite could effectively remove organic pollutants from contaminated water, which could remove almost all of the MB and MV molecules from an aqueous solution at the adsorbent dosage of 1 g L⁻¹ and 1.2 g L⁻¹, respectively. The nanocomposite with 5 mass% of AgNO₃ shows the best adsorption performance. The formed carbon species provided enough vacant sites to hold cationic molecules, whereas the exposed O–H groups of PAL and the negative surface charges provided strong forces to capture and immobilize them. Besides, the nanocomposites could rapidly and efficiently, catalyze the reduction reaction of 4-NP in the presence of NaBH₄, and exhibit excellent stability after running 8 cycles, which can be used as a promising catalyst to produce 4-AP industrially. As a whole, this simple synthesis route opens a new avenue to design multifunctional materials on the condition of using spent PAL adsorbents.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (no. 51403221 and 21377135), “863” Project of the Ministry of Science and Technology, People's Republic of China (no. 2013AA032003), and Jiangsu Provincial Joint Innovation and Research Funding of Enterprises, Colleges and Institutes (no. BY2015056-01) and the National Science Foundation of Gansu province, P.R. China (no. 1308RJZA308) for financial support of this research.

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Footnote

† Electronic supplementary information (ESI) available: The molecular structure of MB, MV, CTC and TC; FTIR spectra of the hydrothermal treated MV-loaded PAL at 180 °C (HSPA-180) in the absence of Ag(I) and the nanocomposites prepared at different hydrothermal temperatures in the presence of 10% AgNO₃; Digital photos of the supernate after being treated at different hydrothermal reaction temperature; Digital photos of the supernate before and after adsorption by different dosage of PAL/C/AgNPs-5 nanocomposite; a proposed mechanism for the adsorption of MB onto the PAL/C/AgNPs nanocomposite and a proposed mechanism for the catalytic conversion of 4-NP as 4-AP using PAL/C/AgNPs nanocomposite as the catalyst. See DOI: 10.1039/c6ra03981b

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