Fabrication of manganese dioxide/carbon/attapulgite composites derived from spent bleaching earth for adsorption of Pb(II) and Brilliant green

Abstract

Manganese dioxide/carbon/attapulgite ternary composites were fabricated via a facile hydrothermal method based on spent bleaching earth. It is worth noting that the residual organic matter of the spent bleaching earth not only served as a low-cost available carbon precursor, but also as a reductant for the formation of manganese dioxide based on the redox with KMnO₄. Using the organic dye of Brilliant green and the heavy metal ion of Pb(II) as model pollutants, the effect of the critical factors on the adsorption properties have been systematically investigated, including the sample preparation conditions, contact time and initial concentration of pollutants. The results reveal that the adsorption properties of the as-prepared composites are well dependent on the concentration of KMnO₄, and the maximum adsorption capacity toward Brilliant green and Pb(II) can reach 199.99 mg g⁻¹ and 166.64 mg g⁻¹ while the concentration of KMnO₄ is 12% and 16%, respectively.

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1. Introduction

Clay mineral/carbon composites, as a novel class of adsorbents, have recently drawn considerable attention due to their special structure and physicochemical properties.^1,2 The features such as high removal efficiency, low operational cost, simple manipulation process and easy functional modifications endow them with potential applications in the removal of pollutants from wastewater. Considering the benefits of clay mineral/carbon composites, the clay mineral/carbon composites have been prepared in the last decades. Chen et al.³ fabricated the attapulgite@carbon nanocomposite, displaying a high adsorption ability for Cr(VI) and Pb(II) ions in aqueous solutions with maximum adsorption capacities of 177.74 and 263.83 mg g⁻¹. Ai et al.⁴ reported that montmorillonite@carbon were designed by a facile hydrothermal route using the glucose biomass as a carbonaceous source and evaluated their performance as adsorbents for MB from water (194.2 mg g⁻¹). Zhou et al.⁵ found that paper-like cellulose acetate/organo-montmorillonite composites could used as a adsorbent and the adsorption capacity of the composites for Acid Scarlet G dye reached 85.7 mg g⁻¹. It is not difficult to find that the pure organic compounds were selected as carbon source in the reported clay mineral/carbon composites, accordingly, most methods mentioned above fail to meet the low-cost, easy access and convenient application requirements, which need to be taken into consideration. From the viewpoint of economic, environmental and sustainable issues, a prospective carbon precursor emanating from the spent bleaching earth (SBE) for preparation of clay mineral/carbon composites would be considered to be even more worthwhile due to low cost and abundance. SBE, a waste material, is derived from the process of refining of vegetable oil. The main composition of SBE contains nonhydratable phospholipids, natural pigment, fatty acid, vitamin and 10% normal grease, etc. It has been reported that palm oil mills in Malaysia produce a considerable amount of spent bleaching earth annually as a by-product of industrial production.^6,7 The valorization of SBE, by converting them into valuable adsorption materials by the deposition of carbon onto clay mineral layers via heat treatment, also plays an environmental objective to make the ecosystem more sustainable.⁸

In addition, metal oxides such as Al₂O₃, TiO₂, MnO₂ and Fe₃O₄ are considered to be the most outstanding candidate of adsorption materials to remove organic pollutants and heavy metal ions from wastewater.^9–11 MnO₂ is of considerable importance in technological applications, including ion-exchange, molecular adsorption, catalysis, and electrochemical supercapacitors owing to their outstanding structural flexibility combined with novel chemical and physical properties.^12–15 In particular, the MnO₂-based adsorbents have attracted intensive interest owing to their high natural abundance, high specific surface area, environmental compatibility, abundant surface functional groups and strong affinities for some organic dyes and heavy metal ions such as methylene blue, Congo red, Cu(II), Pb(II), Cd(II), Zn(II), As(V) and Cr(VI).^16–19 In general, based on redox reactions between Mn²⁺ and MnO₄⁻, MnO₂ have been successfully fabricated by a variety of chemical routes, including sol–gel, thermal decomposition, hydrothermal method and solid state reaction.^20–24 However, these methods mentioned above are either complicated or require strict conditions limiting the wide application in the field of adsorption. For example, sol–gel is relatively time-consuming, which requires a long cycle to prepare materials and easily affects by the environment to a great extent. In the case of the solid state reaction, incomplete fulfillment of the reaction constrains its wide application. Recent investigation shows that various structured MnO₂ can be prepared through a simple route by the reduction of KMnO₄ by organic acids or alcohols.^25–27 The methodologies adopted in the preparation of MnO₂ are meaningful because they are fast, simple, low-cost and eco-friendly. The process involves one manganese precursor of KMnO₄, simplifying the post-synthesis treatment and thereby increasing the viability in commercial applications.²⁸ In order to enhance the adsorption efficiency and flexible operations of MnO₂-based composites, it is a feasible strategy to combine MnO₂ with carbon materials for the removal of the pollutants. It seems to be a promising attempt to combine the merits of the individual components and achieve a possible synergic effect to improve the performances of composites. Wang et al.²⁹ have synthesized KMnO₄ modified bamboo charcoal through microwave irradiation for the adsorption of Pb(II). Wang et al.³⁰ report a redox strategy to prepare MnO₂ coated carbon nanotubes, and the as-prepared composites exhibit a significant improvement of Pb(II) adsorption. At present, it does not attracted any attention to use SBE as a promising precursor for the preparation of MnO₂-based composites for water treatment.

In our previous study, the attapulgite/carbon composites prepared by a one-step carbonization process, using one dimensional APT-based SBE as a low-cost available raw material, performed as high efficient adsorbent for removal of heavy metal ions and dyes from aqueous solutions.^31,32 Herein, a facile and economic hydrothermal method is developed to fabricate manganese dioxide/carbon/attapulgite (MnO₂/C/APT) ternary composites based on SBE in the presence of KMnO₄. The as-prepared composites can be served as adsorbents for the removal of organic dye of Brilliant green (BG) and the heavy metal ion of Pb(II). The effect of the concentration of KMnO₄, initial pH, hydrothermal time, hydrothermal temperature, contact time and initial concentration of pollutants are also systematically investigated. In addition, the feasible adsorption mechanism is also proposed.

2. Experimental

2.1. Materials

SBE, which is mainly composed of attapulgite clay and about 22.3% of organic matters (such as grease, natural pigment, fatty acid, etc.), was provided by The W clay Industries Sdn Bhd (Malaysia). BG (sulfate of di-(p-diethylamino)triphenyl carbonyl anhydride; molecular formula: C₂₇H₃₄N₂O₄S; color index: 42 040; molecular weight: 482.63; nature: basic green 1) was received from Shanghai Sinopharm Chemical Reagent Co., Ltd., China, and used without further purification. The molecular structure and ball-stick model are as illustrated in Fig. S1 (ESI†). All the other agents used were analytical grade and used without further purification. Ultrapure water used in experiments had a resistivity of 18.25 MΩ cm.

2.2. Fabrication of MnO₂/C/APT ternary composites

The MnO₂/C/APT ternary composites were synthesized through a facile hydrothermal route. The detailed procedure is as follows: 1.0 g of SBE was dispersed into 60 mL deionized water, and KMnO₄ was added with mass ratio of 1%, 2%, 4%, 6%, 8%, 10%, 12%, 16% and 20% under vigorous magnetic stirring for 2 h to form a homogeneous precursor. And then the above mixture was sealed in a Teflon-lined stainless steel autoclave (100 mL capacity) and the temperature was maintained at 140 °C for 1 h. The obtained solid product was washed with ethanol and deionized water in sequence after being cooled to room temperature, and dried in a vacuum at 60 °C for 12 h. The hydrothermally treated SBE samples in the presence of KMnO₄ were denoted as MCA₁, MCA₂, MCA₃, MCA₄, MCA₅, MCA₆, MCA₇, MCA₈ and MCA₉ according to the above mass ratio of KMnO₄ to SBE, respectively. As a control, carbon/APT (CA) composites without the addition of KMnO₄ were also synthesized derived from the spent bleaching earth under same conditions.

2.3. Characterization

Fourier transform infrared spectroscopy (FTIR) spectra of the samples were recorded on a Fourier transform infrared spectrometry (Thermo Nicolet NEXUS TM, USA) in the range of 4000–400 cm⁻¹ using KBr pellets. The morphologies of the samples were observed using a JSM-6701F Field Emission Scanning Electron Microscope (FESEM) (JEOL, Tokyo, Japan) at an acceleration voltage of 10.0 kV and a working distance of 10 mm at high vacuum mode after coating the sample with gold film. The weight percent of carbonaceous species was determined by thermogravimetric analysis (TGA) (STA 6000, Perkin Elmer, USA) at a heating rate of 10 °C min⁻¹ from 30 °C to 800 °C in an oxygen atmosphere. Power X-ray diffraction (XRD) (Pana XPERT PRO, Netherlands) patterns were performed using an X-ray diffractometer with a Cu Kα (1.540598 Å) radiation at a scan rate of 0.05° s⁻¹, running at 40 kV and 30 mA. The zeta potentials of suspensions were measured on a Malvern Zetasizer Nanosystem with irradiation from a 633 nm He–Ne laser (Malvern Zeta voltmeter (ZEN3600), Britain). The solid/liquid ratio of the suspension for test is 0.5/100 (w/w), dispersed with deionized water by high-speed stirring. The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method and the pore volume was estimated by the Barrett–Joyner–Halenda (BJH) method at 77 K (ASAP 2020 M, Micromeritics Instrument Corporation, USA). The samples were dried and outgassed at 105 °C for 4 h before N₂ adsorption. UV-vis spectra were recorded on a UV-vis spectrophotometer (SPECORD 200, Analytik Jera AG).

2.4. Adsorption experiments

Analytical grade acetate or dye was used to prepare stock solutions of 1000 mg L⁻¹ of heavy metal ion or dye, which were further diluted to the required concentrations before use.

All the adsorption measurements were carried out in a thermostatic orbital shaker (THZ-98A) at a constant temperature of 25 °C and constant speed of 160 rpm using a batch technique. Both heavy metal ion and dye pollutant were employed as pollutants in wastewater for adsorption measurements. For the adsorption of heavy metal ion, 25 mg of the as-prepared adsorbents were added into 25 mL of Pb(II) aqueous solution with a initial concentration of 200 mg L⁻¹. After a certain period of adsorption time, the supernatant solution was separated and collected from the adsorbent by centrifugation at 4500 rpm for 10 min. The remaining concentrations of Pb(II) solution was measured spectrophotometrically using 1,10-phenanthroline monohydrate as the complexing agent by monitoring the absorbance changes at the wavelength corresponding to maximum absorbance of 533 nm.³³

For the adsorption of dye, BG was employed as the probe for measurements. Typically, the adsorption measurement was the same as those for Pb(II). A UV-vis spectrophotometer was used to determine the remaining concentration of the BG solution by standard spectrophotometric methods with maximum absorbance of 624 nm. As for the pH value-dependent effect investigation, BG aqueous solution was adjusted by 1.0 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH solutions to pH values from 2 to 10. Typically, after adding a certain amount of acid or base solution, a small volume of solution was transferred and measured by a pH meter (Fisher Science Education). This procedure was continued until the solution was adjusted to the required pH value, and then the adsorption procedures were carried out.

Each experiment was triplicated under identical conditions. The adsorption capacity of the adsorbent was calculated according to the following equation:

where Q_e is the adsorption capacity of the adsorbent (mg g⁻¹), C₀ and C_e are the initial and equilibrium concentrations (mg L⁻¹), V and m are the total volume of solution (mL) and the equivalent mass value of adsorbents used (mg), respectively.

2.5. Reusability of the MnO₂/C/APT ternary composites

In order to evaluate the regeneration ability of the MnO₂/C/APT ternary composites, the consecutive adsorption–desorption process was performed for six times, and the adsorption capacity of the adsorbent regenerated for different times was obtained. In each cycle, the desorption of BG-loaded MCA₇ composite or Pb(II)-loaded MCA₉ composite was done using 0.1 mol L⁻¹ HCl solution as the desorbing agent. 25 mg of BG-loaded MCA₇ composite or Pb(II)-loaded MCA₉ composite was contacted separately with 25 mL 0.1 mol L⁻¹ HCl solution and placed in a thermostatic shaker and shaken for 240 min at 160 rpm. At the end of the experiment, the supernatant was discarded by centrifugation and the solid was washed with distilled water for several times, dried in a vacuum at 60 °C. After each cycle of desorption and regeneration experiments, the desorbed MCA composites can be employed directly for the next reuse cycle.

3. Results and discussion

3.1. Characterization of MnO₂/C/APT ternary composites

The XRD analyses are very effective to confirm the change of crystal structure. The XRD patterns of CA and MCA ternary composites are depicted in Fig. 1a. It can be seen that diffraction peaks of 8.4°, 16.4°, 19.8° and 21.5° are obviously observed, which can be assigned to the (110), (130), (040) and (300) planes of the APT.^34,35 In contrast to the XRD pattern of CA, there are only two peaks observed for the MCA samples at 2θ = 12.7° and 24.8° corresponding to that of the standard MnO₂ with diffraction peaks of (001) and (002), respectively, which are indexed to the α-MnO₂ structure (JCPDS no. 44-0141).^36,37 The XRD patterns show that the MnO₂ samples are poorly crystalline. The peaks indicate the small size of the generated MnO₂ or the presence of stacking defects in the samples.^38,39 In addition, it is worth mentioning that the relative intensity of the main diffraction peaks of APT decreases with the increasing KMnO₄ concentration. It suggests that the content of MnO₂ in the composites increases with the increase in the KMnO₄ concentration.

Fig. 1 (a) XRD patterns of CA, MCA₂, MCA₄, MCA₇ and MCA₉, (b) FTIR spectra of CA, MCA₂, MCA₄, MCA₇ and MCA₉, (c) EDS curve and element mapping images of MCA₄, the inset shows the atomic ratio of C, O and Mn and (d) TGA curves of CA and MCA₇.

Fig. 1b shows the FTIR spectra of representative samples including CA, MCA₂, MCA₄, MCA₇, and MCA₉. The FTIR technique was employed to investigate the compositions of MnO₂/C/APT ternary composites. For the FTIR spectrum of SBE (Fig. S2†), the absorption bands at 2923 and 2854 cm⁻¹ are related to the asymmetric and symmetric stretching vibrations of the C–H bonds in –CH₃ and –CH₂– groups, and at 1467 cm⁻¹ is attributed to the C–H bending vibration of –CH₂– group.^40,41 After hydrothermal treatment, the intensity of these peaks decreases obviously, which is a evidence that the SBE have been transformed into carbon species during hydrothermal treatment process. The broad band around 3425 cm⁻¹ represents the O–H stretching of the interlayer water molecules and framework hydroxyl groups, while the weak band at 1630 cm⁻¹ is probably due to the bending vibrations of –OH groups of the adsorbed water molecules.⁴² The absorption peak at 1033 cm⁻¹ and 797 cm⁻¹ are both attributed to Si–O stretching vibration.⁴³ The characteristic of the absorption band of the C–C bond at 468 cm⁻¹ might be overlapped with the bending vibration of Si–O.³¹ An obvious change of the spectra for above MCA composites lies in the appearance of a new band at the low-frequency region around 513 cm⁻¹, which is considered as the main characteristic absorption band of MnO₂ corresponding to Mn–O and Mn–O–Mn vibrations.⁴⁴

In order to further confirm the composition of the prepared composites, three MnO₂/C/APT composites of MCA₄, MCA₈ and MCA₉ were selected for EDS analysis. The presence of Mg, Al, Si, O and Fe in composites is ascribed to the existence of attapulgite. Furthermore, the EDS analysis of MCA₄ demonstrates the distribution of carbon, oxygen and manganese elements of MCA₄, suggesting the as-prepared composites are composed of APT, carbon and MnO₂ (Fig. 1c). Besides, the EDS curve and element mapping images of MCA₈ and MCA₉ is shown in Fig. S3 (ESI†). By contrast, it can be found that the manganese content of the three as-synthesized composites gradually increases from 9.81% to 13.46% as the initial KMnO₄ concentration increases from 6% to 20%. In addition, the similar components of the three composites are observed. Therefore, it can be safely concluded that the MnO₂/C/APT ternary composites have been successfully prepared during the hydrothermal process.

TGA analysis is conducted to detect the components as well as the thermal stability of MnO₂/C/APT ternary composites. The TGA curves of CA and MCA₇ under an O₂ atmosphere are shown in Fig. 1d. The TGA curves reveal the dehydration/dehydroxylation and the decomposition of organic groups of the ternary composites. The mass loss at around 100 °C is attributed to the release of the physically adsorbed water on the external surface of samples. A sharp mass loss from around 200 °C to 600 °C can be observed from TGA curve of CA, which can be assigned to the degradation of carbon species including the pyrolysis of the labile oxygen containing functional groups to transform into steam and oxygenated carbon such as CO and CO₂ under oxygen atmosphere.⁴⁵ It also shows that the obtained CA sample via the hydrothermal method possesses a large amount of oxygen-containing functional groups. In the case of MCA₇, there are about 4.56% weight loss is from 200 °C to 300 °C, which is assigned to the decomposition of labile oxygen-containing functional groups present in the material.⁴⁶ In addition, the weight loss in the temperature range from 300 °C to 450 °C could be caused by the loss of more stable oxygen-containing functional groups and the combustion of carbon species.⁴⁷ Furthermore, a weight loss of about 2.23% in the range of 400–500 °C was observed in the TGA curve of MCA₇, which is related to the high valence MnO_x decomposed to a lower valence state along with the removal of the residual carbon species on composites.^27,48

The detailed pore textural characteristics of the samples are analyzed by Brunauer–Emmett–Teller (BET) and nitrogen adsorption–desorption technique. As shown in Fig. 2a, the isotherms are typical type IV isotherm with H3 type hysteresis loop features at high partial pressures according to IUPAC classification.^49,50 At a range of comparatively low relative pressure (P/P₀ < 0.4), the amount of the adsorbed nitrogen increases gradually with the increasing relative pressure. The adsorption and desorption lines completely coincide with each other, implying a monolayer adsorption. This phenomenon infers that the adsorption of nitrogen mainly takes place in the micropores.⁵¹ The sharp increase in the N₂ adsorbed quantity near the high relative pressure range (P/P₀ > 0.4), which indicates that mesopores are dominated in the all samples.^52,53 Fig. 2b shows the pore size distribution of samples. It can be seen that the porosity of the samples is essentially consisted of mesopores, in which the pore size is around range from 2 nm to 40 nm. There are a large number of adsorption sites on the surface of composites due to its high surface area, resulting in ultrahigh adsorption capacity. This will be evidenced in the following discussion of dye or heavy metal ion sorption on MnO₂/C/APT ternary composites.

Fig. 2 (a) N₂ adsorption/desorption isotherms and (b) BJH pore size distribution of CA, MCA₂, MCA₄, MCA₇ and MCA₉.

Detailed information on the structure of BET specific surface areas and pore structure parameters of CA and the as-prepared MnO₂/C/APT ternary composites are summarized in Table S1 (ESI†). By comparison, it can be seen that SBE exhibits the smallest surface area and pore volume of 2.3 m² g⁻¹ and 0.002 cm³ g⁻¹, respectively.³¹ The above result is attributed to the pores in SBE might be filled with the residual organic matters. The as-synthesized CA (140 °C, 1 h) exhibits a relatively large specific surface area of 46.9 m² g⁻¹, which is about 20 times as much as that of SBE. When MnO₂ is anchored on the surface of CA, the specific surface areas and pore volumes of all MnO₂/C/APT ternary composites clearly change. It is worthwhile to note that the surface areas and pore volumes of the MnO₂/C/APT ternary composites increase significantly with the increase in the KMnO₄ concentration from 1% to 20%, especially MCA₉ composite with the maximum surface areas of 94.6 m² g⁻¹. Compared with that of SBE, it increases by around 40 times after being decorated with MnO₂ nanoparticles. Therefore, we have reason to believe that the SBE treated with KMnO₄ can provide high specific surface area and a large number of exposed surface active sites, indicating that such MnO₂/C/APT ternary composites have excellent potential in the application of wastewater treatment.^54,55

3.2. Mechanisms of MnO₂/C/APT ternary composites formation

As shown in Fig. 3, MnO₂/C/APT ternary composites were fabricated via a facile hydrothermal method based on the low-cost available SBE. The three reactions may be involved in the whole preparation process as follows: (1) hydrothermal carbonization of organic matter, (2) the redox reaction between organic matter and KMnO₄, and (3) the redox reaction between the generated carbon species and KMnO₄. The above reactions simultaneously occur, but the dominant reaction in the hydrothermal process depends on the concentration of KMnO₄. At lower KMnO₄ concentration, the overall preparation process was controlled by hydrothermal carbonization of organic matter and the redox between organic matter and KMnO₄, and the content of obtained carbon species is higher than that of MnO₂ nanoparticles. With the increase in the concentration of KMnO₄, the residual organic matter and the generated carbon species can simultaneously interact with KMnO₄. At this point, the residual organic matter and a much greater amount of carbon species are consumed accompanied with the deposition of more and more MnO₂ nanoparticles. Therefore, this stage is controlled by the redox reaction between KMnO₄ and organic matter or carbon species, but the redox reaction between KMnO₄ and carbon species is dominant over that of organic matter. However, the redox between carbon species and KMnO₄ might be stopped due to the generated MnO₂, which might prevent from the contact of carbon species and MnO₄⁻. Consequently, a small amount of carbon species may react with excessive KMnO₄, meaning the incomplete reaction of KMnO₄ with the further increase in the KMnO₄ concentration. Therefore, the residual organic matter of SBE acts as both a low-cost available carbon precursor and a reductant to react with MnO₄⁻.

Fig. 3 Schematic illustration of the synthetic route of the MnO₂/C/APT ternary composites for the adsorption of BG and Pb(II).

To achieve a better understanding of the role of the residual organic matter of spent bleaching earth, the digital photographs of supernatant fluid and products before and after the hydrothermal reaction for SBE and the bleaching earth are provided. As shown in Fig. S4 (ESI†), the supernatant fluid of bleaching earth after hydrothermal reaction has no distinct change, and the colour of supernatant fluid and products remain deep red and pale yellow, respectively. It suggests that KMnO₄ is not involved in reaction due to the absence of reductant. As SBE is hydrothermally treated in the presence of KMnO₄, the colour of supernatant fluid changes from deep red to colorless while the colour of products is brown. The above result demonstrates that the residual organic matter of spent bleaching earth is crucial to prepare the ternary composites.

3.3. Optimization of reaction conditions

In order to investigate the adsorption performance of the as-prepared MnO₂/C/APT ternary composites, it is important to investigate the equilibrium properties of the adsorption process. It is well-known that the initial KMnO₄ concentration in the hydrothermal process is a indirect factor to construct composites composed of carbon and MnO₂, influencing their components, structures and relevant properties, while the content of MnO₂ is a direct factor to influence the adsorption behavior of MCA composites. The content of MnO₂ is determined by the concentration of KMnO₄. The digital photographs of all as-synthesized composites are shown in Fig. 4a. For CA, the color of the sample appears light yellow. After being modified with KMnO₄ solution, the digital photographs clearly show the colour change of the samples with the increase in the initial concentration of KMnO₄. By comparison with the CA, the colours of the MnO₂/C/APT ternary composites change from light yellow to crimson when the initial KMnO₄ concentration increases from 1% to 20%. Therefore, it can be inferred that not only the MnO₂/C/APT ternary composites have been successfully prepared, but also the content of MnO₂ in the ternary composites gradually increases, which is also consistent with EDS analyses.

Fig. 4 (a) Digital photographs of the MnO₂/C/APT ternary composites obtained after hydrothermal treatment at 140 °C for 1 h at different initial KMnO₄ concentration and (b and c) effect of initial KMnO₄ concentration on adsorption for BG and Pb(II).

In our investigation, Pb(II) was employed as heavy metal ion for adsorption, while BG was used as an organic pollutant. All of the samples are evaluated and the results are displayed in Fig. 4b and c. For BG, the adsorption capacity of MnO₂/C/APT ternary composites for BG first gradually increases with the increase in the initial KMnO₄ concentration in the range of 1–12%, and reaches the maximum at 12%, and then keeps constant. As for Pb(II), the adsorption capacity of MnO₂/C/APT ternary composites significantly increases with the increase in the KMnO₄ concentration from 1% to 4%, and then gradually increases until the maximum value is reached at 20%. When the initial KMnO₄ concentration is above 20%, the supernatant after hydrothermal method still present light red due to the incomplete reaction of the exceedingly high KMnO₄ concentration in the solution. This significant phenomenon can be confirmed by the digital photographs of supernatant before and after hydrothermal reaction with the initial concentration of 16% and 20% (Fig. S5†). Moreover, it is clearly observed that the maximum adsorption capacities of 199.99 mg g⁻¹ for BG and 166.64 mg g⁻¹ for Pb(II) were obtained at the initial KMnO₄ concentration of 12% and 20%, respectively. The adsorption capacity of the ternary composites to BG and Pb(II) enhances 1.5 and 3 times compared with that of the as-prepared CA (133.88 mg g⁻¹ for BG and 56.01 mg g⁻¹ for Pb(II)) after incorporating of MnO₂, respectively. That is to say, adsorption properties of the MnO₂/C/APT ternary composites are highly dependent on the content of MnO₂ and the difference in the adsorption mechanism, and further studies were then carried out with the initial KMnO₄ concentration of 12% (MCA₇) for BG and 20% (MCA₉) for Pb(II) with satisfactory adsorption efficiency, respectively.

The adsorption properties of the resultant MnO₂/C/APT ternary composites as adsorbents were further studied at different hydrothermal temperature (120 °C, 140 °C and 160 °C) and hydrothermal time (0.5 h, 1 h, 2 h, 4 h and 8 h), respectively. As shown in Fig. 5a and b, the adsorption capacity for BG and Pb(II) significantly increases with the increasing hydrothermal temperature from 120 to 140 °C, and then the adsorption capacity decreases as the temperature is above 140 °C, and the maximum value is obtained at 140 °C for BG (199.99 mg g⁻¹) and Pb(II) (166.64 mg g⁻¹). In addition, the hydrothermal time also affect the adsorption capacity of the samples and the maximum adsorption capacity can be observed at 1 h, as shown in Fig. 6a, and it can be found that the morphology of MnO₂/C/APT composites is similar with the increase in the reaction time (Fig. 6b). In addition, it can be found that the color of the supernate is light red when the hydrothermal reaction is proceeded at 120 °C for 1 h and at 140 °C for 0.5 h, respectively. Then it turns into colorless with the increase in the reaction temperature and time, as depicted in Fig. 5c and 6c, respectively. Therefore, it can be conclude that the optimum reaction conditions are 140 °C and 1 h.

Fig. 5 Effect of hydrothermal temperature on adsorption capacity (a) for MCA₇ of BG, (b) for MCA₉ of Pb(II) and (c) digital photographs of supernatant before and after hydrothermal reaction in the different hydrothermal temperature for MCA₇ and MCA₉ (hydrothermal time: 1 h).

Fig. 6 Effect of hydrothermal time on adsorption capacity (a) for MCA₇ of BG and for MCA₉ of Pb(II), (b) FESEM images of MCA₇ prepared at different hydrothermal time and (c) digital photographs of supernatant before and after hydrothermal reaction in the different hydrothermal time for MCA₇ and MCA₉ (hydrothermal temperature: 140 °C).

3.4. Adsorption properties towards BG

It is well-known that the adsorption behaviors of pollutants by adsorbents are highly dependent on the initial pH of solution. On the basis of the study mentioned above, MCA₇ was selected to study the effects of solution pH on the adsorption capacity for BG (Fig. 7a). To investigate the pH-dependence, the pH values were adjusted from 2 to 10. It is observed that the adsorption capacity of the MCA₇ for BG has no obvious fluctuation within the pH range studied. Moreover, the high adsorption capacity has been achieved for any one of pH. Based on zeta potential analysis (Fig. S6†), MCA₇ is an ionic ternary composite in the whole pH range, and even become increasingly negative as the pH values of the suspension increases, meaning that its surface charge is essentially affected by changing the pH values and some of the oxygen-containing functional groups are located at the surface of the MCA₇, which will be provide the number of surface active sites available for the cationic contaminants. In addition, the introduction of carbon species and MnO₂ has been proved to be a simple and effective protocol to increase the surface negative charge, which can provide an advantage for the removal of pollutants under wider environmental pH conditions.

Fig. 7 Effect of (a) pH, (b) initial concentration, (c) contact time on the adsorption of MCA₇ for BG and (d) the reusability of the as-prepared MCA₇ for BG adsorption.

At the lower pH values, a large quantity of hydrogen ions existed in the solutions can inhibit the ion-exchange interaction between BG cations and exchangeable cations on the surface of MCA₇. On the contrary, the content of hydrogen ions in the system decreases when pH of the initial BG solution increases, which lead to weakening of competitive adsorption onto the binding sites between hydrogen ions and BG.⁵⁶ Consequently, adsorption capacity of the adsorbent increases gradually, but no obvious change occurs. On the whole, it should be noted that the pH of the solution can influence the adsorption capacity for targeted pollutant to a certain extent, but this influence is not obvious due to a minor difference between various pH, which means that the resultant ternary composite can be potentially applied in a wide pH range as promising adsorbent for the treatment of polluted water in environment pollution cleanup.

The initial concentration is often used for adsorption studies, because they can describe the adsorption uptake rate at which a pollutant is removed from aqueous solutions and provide valuable data for understanding the interaction mechanism of the adsorbent with the adsorbate of sorption reactions. The effect of initial concentration on the adsorption capacities of MnO₂/C/APT ternary composites is obtained by batch tests for the various initial BG concentrations, from 10 mg L⁻¹ to 700 mg L⁻¹ at room temperature, are shown in Fig. 7b. It can be seen that the adsorption capacity increases from 9.45 to 385.65 mg g⁻¹ for MCA₇ with an increase in the initial BG concentration from 10 to 500 mg L⁻¹, and then achieve equilibrium at the concentration higher than 500 mg L⁻¹. A reasonable interpretation about this phenomenon is availability of sufficient vacant active sites at lower concentration which had limited adsorbate to occupy on adsorbent. The superfluous unsaturated vacant sites present in the adsorption process at low concentration transformed into lower adsorption uptake of pollutant from solution. Similar scenario observed has been reported.⁵⁷ On the other hand, this may be due to the fact that the increasing BG concentration generates the maximum driving force to overcome all the mass transfer resistances of BG from the aqueous phase to solid phase resulting in higher probability of collision between BG and the active adsorption sites. That is, all the active adsorption sites have been utilized at higher BG concentrations, and then the adsorption capacity reached the maximum and almost kept the equilibrium state.

To further understand the characteristics adsorption process, another parameter seems also indispensable, that is, the contact time until the adsorption system reaches its equilibrium. As can be seen from Fig. 7c, the adsorption efficiency of MCA₇ increases rapidly in the initial period for BG, and then remains almost constant as the contact time is above 4 h. The adsorption rate of MCA₇ composite was found to be rapid and more than 99% of BG was removed at equilibrium in 4 h with an initial concentration of 200 mg L⁻¹, indicating an excellent adsorption rate of the prepared MnO₂/C/APT ternary composites. This can be explained based on the fact that the high availability of the adsorption sites can be provided in the initial period to allow these adsorbate molecules to disperse freely and quickly, thus they would be easily encountered and captured by MCA₇. Upon further increasing the contact time, the adsorption sites have occupied by adsorbate molecules, and then adsorbate molecules are aggregated densely to larger particles, which are not beneficial for other adsorbate molecules to enter and be adsorbed. That is to say, the adsorption capacity hardly increased with prolonged time, and then remained constant until the adsorption equilibrium. In summary, the fast adsorption rate makes the MnO₂/C/APT ternary composites convenient to utilize in practical applications.

In addition, the reusability of the synthesized adsorbent is quite a crucial factor, since better repeated availability can effectively reduce the overall cost of the adsorbent. In our measurements, BG-loaded MCA₇ was selected as an example for reusability studies. The BG-adsorbed MCA₇ was treated with 0.1 mol L⁻¹ HCl solution, as the desorbing agent, to regenerate the adsorption sites, which was proven to be suitable for desorption of BG due to the similar consistency and electrostatic repulsions. Then the regenerated adsorbent was utilized again to adsorb the BG solution to study its adsorption stability, and six successive cycles were performed (Fig. 7d). It can be seen that the MCA₇ still possesses 98.4% adsorption capacity for BG after six cycles of reuse, indicating that this ternary composite has a good reusability for BG adsorption. The slight decrease of the adsorption capacity may be attributed to the incomplete desorption of the BG adsorbed on MCA₇. Accordingly, the results from the regeneration experiments show that the as-prepared MnO₂/C/APT ternary composites can be used as an efficient recyclable adsorbent for the treatment of wastewater.

3.5. The feasible adsorption mechanism

It is well-known that the content of MnO₂ and the adsorption mechanism are crucial factors in the adsorption process, influencing their adsorption properties. To gain further insight into the interaction between MnO₂/C/APT ternary composites and pollutants during the adsorption process, FTIR results of MnO₂/C/APT ternary composites with the highest adsorption capacity for BG and Pb(II) were evaluated before and after adsorption, as shown in Fig. 8a and b, respectively. As can be seen from Fig. 8a, the broad and strong band at around 3433 cm⁻¹ is related to the stretching vibration of O–H groups, which shifts to 3428 cm⁻¹ after adsorption of BG indicating the interaction between O–H and –N– of BG.⁵⁸ The band located at 1580 cm⁻¹ was assigned to stretch vibration of C=N bond contained in the quinoid structure of BG.⁵⁹ The shift of this band to higher wavenumbers (1592 cm⁻¹) after adsorption of the BG dye reveals the involvement of the quinoid structure in the adsorption process. It also can be observed that the band at 1630 cm⁻¹ corresponded to the overlapping of C=C and O–H shifts to lower wavenumbers (1619 cm⁻¹) after adsorption of the BG dye, and the intensity of this band obviously weakens, indicating that the mechanism of interaction between the BG dye with the MCA₇ should also occur by the π–π interaction and hydrogen-bonding interaction of the BG dye with the MCA₇.^60,61 Additionally, the characteristic bands of BG at 1341, 1275 1183, 698 and 575 cm⁻¹ are recorded accompanied with the shift of these bands in the FTIR spectrum of MCA₇ after adsorption of BG. This change further suggests the strong interaction between the BG dye and MCA₇. In addition, the MCA₇ obtained by hydrothermal process is also negatively charged due to the presence of oxygen-containing groups, which is testified by the zeta potential results. Then it is regarded that the electrostatic interaction occurs between negatively charged MCA₇ and positively charged BG dye. The above results indicate that π–π interaction, hydrogen-bonding and electrostatic interaction are mainly involved in the adsorption process.

Fig. 8 FTIR spectra of (a) MCA₇ before and after BG adsorption and (b) MCA₉ before and after Pb(II) adsorption.

Detailed band variation data on MCA₉ samples before and after uptake of Pb(II) are presented in Fig. 8b. The one of mechanism of Pb(II) removal by the MCA₉ ternary composite can be explained on the bases of interaction between Pb(II) and the active sites on the surface of composites. The presence of MnO₂ nanoparticles and oxygen-containing groups on the surface of the MCA₉ make the surface more negative, which might increase the electrostatic interaction between the positive Pb(II) ions and the composite. As can be observed, the band at 3429 cm⁻¹ is normally assigned to the stretching vibration of O–H groups which shifted 3433 cm⁻¹ after the adsorption of Pb(II). This shift indicated that the adsorption could occur through the interaction between heavy metal ions and the surface hydroxyl groups.⁶² In particular, the band at 1635 cm⁻¹ is attributed to the O–H bending vibration combined with Mn atoms. It is noticeable that the uptake of Pb(II) onto MCA₉ results in a slightly decrease in the relative intensity of the Mn–OH absorption band. This observation may be ascribed to the fact that Pb(II) ions undergo ion-exchange with the protons present on the O atoms of Mn–OH groups, which further demonstrates that the Pb(II) adsorption onto MCA₉ is controlled by ion-exchange mechanism.⁶³ In addition, after adsorption of Pb(II) by MCA₉, the band around 513 cm⁻¹ associated with the Mn–O stretching vibration are significantly shifted to higher wavenumbers of 522 cm⁻¹. Therefore, the obvious shift of O–H groups and Mn–O bands may be attributed to formation of inner-sphere complexes between Pb(II) and MCA₉. It is considered that the inner-sphere complexation reactions may be described by the following reaction (2)–(4). Here –OH or –O⁻ comes from carbon or Mn atom on MCA₉ surface.⁶⁴

MCA₉–OH + Pb²⁺ → (MCA₉–O–Pb)⁺ + H⁺ (2)

MCA₉–O⁻ + Pb²⁺ → (MCA₉–O–Pb)⁺ (3)

MCA₉–OH⁺ + Pb²⁺ + H₂ → MCA₉–OPbOH + 2H⁺ (4)

4. Conclusions

In summary, MnO₂/C/APT ternary composites were successfully fabricated via a facile and scalable hydrothermal method based on a low-cost available waste of the spent bleaching earth. The results confirmed that the initial KMnO₄ concentration in the hydrothermal process is a dominant factor to construct composites, affecting their components, structures and adsorption properties. The synthesized ternary composites afforded a remarkably enhanced adsorption rate and high adsorption capacity toward Pb(II) and Brilliant green, and the adsorption capacity toward BG and Pb(II) can reach 199.99 mg g⁻¹ and 166.64 mg g⁻¹, respectively. Regeneration studies showed the feasibility of the reuse of the adsorbent materials. Taking into account the simple process, low cost, fast adsorption rate, high adsorption capacity and recyclability, the as-prepared ternary composites can be developed as large-scale, low-toxicity, green-process, cost-effective and high-performance adsorbents for the removal and immobilization of dyes and heavy metal ions from wastewater.

Acknowledgements

The authors would like to thank “863” Project of the Ministry of Science and Technology, P. R. China (No. 2013AA032003) and the National Natural Science Foundation of China (21377135 and 51403221) for the financial support of this research.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Molecular structure and ball-stick model of BG, FTIR spectrum of SBE, EDS curves and element mapping images of MCA₈ and MCA₉, digital photographs of SBE and the bleaching earth reaction liquid and products before and after the hydrothermal reaction, digital photographs of supernatant before and after hydrothermal reaction with the initial KMnO₄ concentration of 16% and 20%, zeta potential of MCA₇, N₂ adsorption/desorption analyses of CA, MCA₂, MCA₄, MCA₇ and MCA₉. See DOI: 10.1039/c5ra26362j

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