Facile and green one-pot hydrothermal formation of hierarchical

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Materials and Interfaces

Facile and green one-pot hydrothermal formation of hierarchical porous magnesium silicate microspheres as excellent adsorbents for anionic organic dye removal Panpan Sun, Lin Xu, Xuezhen Jiang, Heng Zhang, and Wancheng Zhu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04841 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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Industrial & Engineering Chemistry Research

Facile and green one-pot hydrothermal formation of hierarchical porous magnesium silicate microspheres as excellent adsorbents for anionic organic dye removal Panpan Sun, Lin Xu, Xuezhen Jiang, Heng Zhang, Wancheng Zhu* Department of Chemical Engineering, Qufu Normal University, Shandong 273165, China Abstract Uniform hierarchical porous magnesium silicate (Mg3Si4O10(OH)2) microspheres (diameter: 200-350 nm) assembled by ultrathin nanosheets are successfully fabricated through a facile and green one-pot hydrothermal method (140 ºC, 12.0 h) without the aid of any organic additives or templates, using abundant MgCl2·6H2O, NH4Cl, NH3·H2O and Na2SiO3·9H2O as the raw materials. The as-formed porous microspheres possess a large specific surface area of 551.08 m2 g-1, a pore volume of 0.84 cm g-1, and a mean pore diameter of 6.12 nm. Based on the effects of the hydrothermal parameters, a probable self-sacrificing template assisted formation mechanism of the Mg3Si4O10(OH)2 microspheres is proposed. When utilized as adsorbents, the adsorption isotherms for methyl blue (MB) and Congo red (CR) are both well fitted by the Langmuir adsorption model, with the maximum capacities for MB and CR as 1890 mg g-1 and 576 mg g-1, respectively, superior to most of the referenced adsorbents. Meanwhile, the pseudo-second-order kinetic model and intra-particle diffusion model are also favorable to interpret the kinetic data and adsorption mechanism. Moreover, the recycling performance also indicates the as-formed porous microspheres as excellent adsorbents for removal of MB and CR with satisfactory reusability.

*

Corresponding author. Tel.: +86-537-4453130. Email: [email protected] (W.C.

Zhu).

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Keywords: hierarchical porous microspheres; magnesium silicate; dye removal; adsorption; methyl blue; Congo red 1. Introduction Water contamination with hazardous organic dyes has become a serious issue and drawn much concern throughout the world in the past few decades, because the organic dyestuffs are stable to light and oxidation as well as resistant to aerobic digestion, and also some synthetic dyes are quite toxic or even mutagenic, carcinogenic and teratogenic, which imposes a severe threat on human beings.1-5 Anionic reactive dyes are commonly employed in paper mills, distilleries, tannery, textile industries, and food companies, etc.. Congo red (CR) is representative benzidine-based anionic dye (with negative charges in aqueous solution6), and is recognized to metabolize to benzidine, a known human carcinogen.7 Therefore, it is imperative to eliminate the anionic containing effluents (such as MB8 and CR) in the waste-water to alleviate the water issues confronted by society. As reported, some conventional methods have been applied to remove toxic organic

dyes

including

adsorption,9-12

photocatalytic

degradation,13,14

biodegradation,15

flocculation-coagulation,16,17 internal electrolysis,18 ion-exchange,19 and membrane filtration.20,21 Among the above-mentioned approaches, the adsorption technique has been widely considered as superior to others due to its environmental benignity and cost-effective operation as well as high removal efficiency. Recently, hierarchical porous nanostructures have opened up new opportunities for wastewater treatment, owing to their extraordinary textural properties (e.g. large specific surface area, high pore volume and uniform pore size), well-defined porous networks and thermal stability. In particular, the three-dimensional (3D) porous nanostructures with substantial specificity can

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satisfy the requirements as promising adsorbents for environmental remediation22 and energy storage,23 not only rendering huge interface and large space for guest species, but also enhancing the mass transfer efficiency. As a matter of fact, tremendous achievements have been obtained to fabricate novel porous adsorbents to tackle with hazards organic dyes. For example, silica@mesoporous magnesium silicate nanotubes,24 and hierarchical SnO2,25 exhibited a high adsorption capacity for rodamine B,24 and a high adsorption rate toward methylene blue,25 respectively. Hierarchical porous NiO spheres,26 and urchin-like α-FeOOH,27 hollow spheres demonstrated high adsorption capacities for the removal of CR. And the short-range-ordered Cu2O mesoporous spheres displayed an excellent adsorption ability for methyl orange.28 Among them, magnesium silicates has shown particularly promising adsorption capacity of organic dyes or heavy metal cations from wastewater, due to its high specific surface area, abundant porous structure and good stability.24,29-33 However, pre-preparation of the SiO2 templates (nanotubes or nanospheres) needs carbon nanotubes (CNTs) in the presence of CTAB or TREG (triethylene glycol),24,31,32 or absolute ethanol and EDTA,33 leading to relatively complicated synthetic procedure with intrinsic energy-consuming and environmentally malign characteristic.24,30-33 On the other hand, the adsorption capacities of the organic dyes on the reported magnesium silicates were still relatively low, rarely higher than 800 mg g-1.24,30 As a consequence, it still remains a great challenge to exploit facile, environmentally friendly and versatile routes to 3D porous magnesium silicates with superb adsorption performance. Herein, we report a facile and green one-pot hydrothermal synthetic route to the hierarchical porous Mg3Si4O10(OH)2 microspheres as the excellent adsorbents for MB and CR without the aid of any organic additives or templates, by using inorganic Na2SiO3·9H2O as the template precursor

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and silicon source. Meanwhile, the effects of process parameters on the hydrothermal products are investigated in detail, based on which a precursor SiO2 templated self-sacrificing formation mechanism is proposed. Distinctly, the Mg3Si4O10(OH)2 microspheres with hierarchical porous structure deliver excellent adsorption capacities for MB (1890 mg g-1) and CR (576 mg g-1) with good recyclability. Moreover, the adsorption kinetics and adsorption mechanism are also deliberately and comprehensively investigated. 2. Experimental 2.1. Materials Magnesium chloride hexahydrate (MgCl2·6H2O) was supplied by Sinopharm Chemical Reagent Co. Ltd (China). Ammonium chloride (NH4Cl) and sodium silicate (Na2SiO3·9H2O) were received from Damao Chemical Reagent Factory (Tianjin, China). Ammonia solution (NH3·H2O W/%=25-28%) was purchased from Laiyang Economic and Technological Development Zone (Yantai, China). Methyl blue (C37H27N3Na2O9S3) was obtained Basf Chemical Reagent Factory (Tianjin, China). Congo red (C32H22N6Na2O6S2) was received from Yuanhang Reagent Factory (Shanghai, China). All chemicals were A.R. grade and used as received without further treatment. 2.2. Synthesis of the hierarchical porous Mg3Si4O10(OH)2 microspheres In a typical procedure, 0.1525 g of MgCl2·6H2O (0.75 mmol), 0.5348 g of NH4Cl (10 mmol) and 0.3149 g Na2SiO3·9H2O (1.10 mmol) were individually dissolved into 10.0 mL, 20.0 mL and 20.0 mL of deionized (DI) water. Then, the as-obtained NH4Cl solution, 1.0 mL NH3·H2O (25~28 W/%) and Na2SiO3 solution were successively dropwise added into the above MgCl2 solution under vigorous magnetic stirring. After the mixture was stirred at room temperature (R.T.) for 10 min, the resultant slurry was transferred into a Teflon-lined stainless steel autoclave with a

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capacity of 65.0 mL, which was then heated to 140 ºC and kept in an isothermal state for 12.0 h (heating rate: 5 ºC min-1). Finally, the obtained solid product was collected, filtrated and washed with DI water for three times, and then dried at 60 ºC for 12.0 h for further characterization. 2.3. Adsorption performance evaluation The as-formed porous Mg3Si4O10(OH)2 microspheres were evaluated as adsorbents for the removal of anionic dyes such as MB and CR, from the mimic waste water. Batch adsorption experiments were carried to examine the adsorption isotherm of MB and CR. Typically, 20.0 mg of the Mg3Si4O10(OH)2 microspheres was dispersed into a conical flask (capacity: 50.0 mL) containing 40.0 mL of the pre-prepared MB (or CR) solution, keeping the initial concentration within the range of 100.0-1500.0 mg L-1 (or 50.0-400.0 mg L-1 for CR), and then the flasks were kept under a constant magnetic stirring (rate: 200 rpm) for specific time at R.T.. After the attainment of the equilibrium, the adsorbents were separated by a disposable syringe needle filter (diameter: 13.00 mm, pore size: 0.22 µm). Then the concentrations of MB (or CR) in the filtrate were determined by a UV-vis absorption spectroscopy at a wavelength of 598 nm (or 496 nm for CR). The adsorption capacities of MB (or CR) onto the Mg3Si4O10(OH)2 microspheres were evaluated using the following equation (1):

qe 

(c0  ce )V m

(1)

where qe (mg g-1) is the equilibrium adsorption capacity, c0 (mg L-1) and ce (mg L-1) are the initial and equilibrium concentrations of MB (or CR), respectively. V (L) and m (g) are the volume of the dye solution and the weight of the employed adsorbents, respectively. For accuracy and objectivity, all the adsorption tests were carried out for three times. In order to understand the kinetics of the adsorption, the initial concentration of MB (or CR) 5



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solution was set as 50.0 mg L-1, by adding 20.0 mg of the Mg3Si4O10(OH)2 microspheres into 40.0 mL of the pre-prepared MB (or CR) solution. At the pre-set time intervals, 4.0 mL of the suspension was withdrawn from the system for filtration. Each filtrate obtained at various intervals and also that from the finally leftover, was monitored by the UV-vis absorption spectroscopy to examine the effect of the adsorbents on the adsorption of MB (or CR). The adsorption capacities of the adsorbents for MB (or CR) at time (t) were calculated by the following equation (2):

qt 

(c0  ct )V m

(2)

where qt (mg g-1) is the amount of MB (or CR) adsorbed onto a unit mass of the adsorbent at time t (min); c0 (mg L-1) and ct (mg L-1) represent the initial and real-time concentrations, respectively; V (L) is the volume of the employed MB (or CR) solution, and m (g) is the mass of the employed adsorbents Mg3Si4O10(OH)2 microspheres. 2.4. Recycling performance of the Mg3Si4O10(OH)2 microspheres as adsorbents After adsorption experiment, the MB-adsorbed (or CR-adsorbed) Mg3Si4O10(OH)2 samples were both dried in 60 ºC for 12.0 h. Then the samples were heated to 300 ºC (or 400 ºC for CR-adsorbed) with a heating rate of 2.5 ºC min-1 to remove the adsorbed MB (or CR), leading to the regenerated Mg3Si4O10(OH)2 samples. The adsorbents were regenerated for five times and reused for five adsorptions to evaluate the recyclability and reusability of the porous Mg3Si4O10(OH)2 microspheres as the adsorbents for MB or CR removal. 2.5. Characterization Crystal structure and phase of the as-formed samples were measured by X-ray powder diffractometer (XRD, PANalytical Model X pert3, Netherlands) with a Cu-Kα radiation and a fixed power source (60.0 kV, 55.0 mA). Morphologies and microstructures of the samples were 6


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investigated by a field emission scanning electron microscopy (SEM, JSM 6700F, JEOL, Japan) at 5.0 kV and a high resolution transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) at 120.0 kV. Size distribution of the samples was estimated by directly and randomly measuring ca. 50 particles from the typical SEM images. Energy-dispersive X-ray (EDX) spectrum and elemental mappings of the samples were recorded by corresponding accessories equipped on the TEM. Porous structures of the samples were examined by the N2 adsorption-desorption isotherms measured at 77 K using a pore size and specific surface area analyzer (SSA-4200, Beijing Builder Company, China) after the sample had been degassed at 200 ºC for 1.0 h. Specific surface area was calculated from the adsorption branch isotherm within the relative pressure range of 0.05-0.27 using multipoint Brunauer-Emmett-Teller (BET) method, and the pore size distribution (PSD) was evaluated from the N2 desorption isotherm using the Barrett-Joyner-Halenda (BJH) method. Elemental compositions of the samples were confirmed by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Optima 4300DV Spectrometer, PerkinElmer instruments, USA). Zeta-potential was measured using a zeta potential analyzer (Zetasizer Nano, ZEN3700, Malvern, UK). Thermal decomposition behavior was determined using a thermal-gravimetric analyzer (TGA, Netzsch STA 409C, Germany) at 30-900ºC in N2 atmosphere with a heating rate of 10.0 ºC min-1. Absorption spectra of the initial MB or CR solutions were monitored using a UV-vis spectrophotometer (UV-756 CRT, Shanghai Yoke Instrument and Meter Co., LTD, China) with a characteristic absorption at ca. 598 or 496 nm to analyze the concentration of the MB or CR, respectively. 3. Results and Discussion 3.1. Structure and morphology of the Mg3Si4O10(OH)2 microspheres

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The composition, morphology as well as pore size distribution of the hydrothermal product synthesized at 140 ºC for 12.0 h are presented in Fig. 1, with a molar ratio of SiO32-:Mg2+ = 1.10:0.75, concentration of NH4Cl as 0.20 mol L-1 and the volume of NH3·H2O as 1.0 mL. As shown (Fig. 1(a)), all the diffraction peaks of the product are in good agreement with those of the Talc-2M like structure magnesium silicate (Mg3Si4O10(OH)2, JCPDS No. 13-0558, a = 5.287 Å, b = 9.158 Å and c = 18.950 Å) whereas with a relatively low crystallinity. In addition, to further ascertain the chemical composition, the hydrothermal product was also tested using ICP. As shown in Table S1, the atomic ratio of Si to Mg is somehow higher than the stoichiometric ratio of Si:Mg in the molecule of Mg3Si4O10(OH)2 determined by XRD. This indicates probable co-existence of residual amorphous silica with the as-obtained Mg3Si4O10(OH)2 microspheres. Relatively abundant silica might also be derived from the originally employed excessive silicon source, in view of the original molar ratio of SiO32-:Mg2+as 1.10: 0.75 within the reactant. As a matter of fact, the actual yield of current Mg3Si4O10(OH)2 microspheres is approximately 40.0-57.0%. Thus, the Mg3Si4O10(OH)2 microspheres with relatively low crystallinity have been obtained. The typical SEM image (Fig. 1(b)) illustrates that the as-obtained Mg3Si4O10(OH)2 product particles exhibited a uniform microsphere-like profile with a narrow size distribution (Fig. 1(b1)), and 94% of which had a diameter within the range of 200-350 nm. In addition, the high magnification SEM image (Fig. 1(b2)) clearly demonstrates that the as-prepared Mg3Si4O10(OH)2 microspheres were assembled by multitudes of nanosheets (thickness: 10-30 nm). Apparently, the composition and the size of the present Mg3Si4O10(OH)2 microspheres are quite different with our previously obtained mesoporous Mg3Si2O5(OH)4 microspheres,12 implying the key role of the

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molar ratio of Mg to Si on the composition of the hydrothermally synthesized magnesium silicates. Moreover, the TEM image reconfirms the present Mg3Si4O10(OH)2 particles of spherical-like morphology consisted of distinct nanoflakes subunits (Fig. 1(c)). Furthermore, no legible lattice fringes were detected in the HRTEM image (Fig. 1(c1)) due to the relatively low crystallinity of the Mg3Si4O10(OH)2 microspheres, which is consistent with the XRD result (Fig. 1(a)) and also similar to the previously obtained mesoporous Mg3Si2O5(OH)4 microspheres.12 In order to get deeper insight into the porous structures of the as-synthesized Mg3Si4O10(OH)2 microspheres, the typical N2 adsorption-desorption isotherms and corresponding pore-size distribution were recorded or calculated, as presented in Fig. 1(d, d1). Based on the sorption isotherms (Fig. 1(d)), the as-formed the Mg3Si4O10(OH)2 microspheres illustrated the typical type IV with a H3-type hysteresis loop at a relative pressure P/P0 of 0.40-1.00, suggesting the dominant narrow slit-like mesopores existed between the 2D nanosheets within the microspheres. In addition, the corresponding PSD profile of the Mg3Si4O10(OH)2 microspheres exhibited a sharp peak centered at 4.1 nm (Fig. 1(d1)) and also a broad one between 8-200 nm, confirming the dominant mesopores (diameter: 2-50 nm) as well as macropores (diameter: > 50 nm) formed within the Mg3Si4O10(OH)2 microspheres. The present hierachical porous Mg3Si4O10(OH)2 microspheres exhibited a specific surface area of 551.08 m2 g-1, a pore volume of 0.84 cm g-1, and a mean pore diameter of 6.12 nm. Noticeably, the specific surface area of the present Mg3Si4O10(OH)2 microspheres is almost twice of our previously obtained mesoporous Mg3Si2O5(OH)4 microspheres (281.70 m2 g-1),12 and also much larger than the core-shell Mg3Si2O5(OH)4 spheres (386.39 m2 g-1),29 and Mg3Si4O10(OH)2 hollow spheres (521.00 m2 g-1).30 The large specific surface area, high pore volume and unique hierarchical porous structures of the

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present Mg3Si4O10(OH)2 microspheres suggest their great potential applications as promising adsorbents for the removal of organic dyes from waste water. 3.2. Effects of temperature and time on the composition and morphology of the products To further investigate the intrinsic formation mechanism of the hierarchical porous Mg3Si4O10(OH)2 microspheres, temperature and time dependent experiments were carried out. As depicted (Fig. 2), the resultant white precipitate after the dropwise introduction of Na2SiO3 solution into the system at R.T. was confirmed as amorphous SiO2 nanospheres, ca. 80.0 % of which centering on a diameter of 80-120 nm (Fig. 2(a, b, b1)). Simultaneously, the in situ EDS spectrum reveals that elements including Si, O, Mg and Cu were detected in the amorphous precursor (Fig. 2(c)). The corresponding quantitative molar ratio of Si to O was 26.38:51.04 (quite similar to 1:2), reconfirming the formation of the precursors as SiO2. Meanwhile, the elemental mapping clearly indicates the uniform distribution of the Si and O throughout the nanospheres (Fig. 2(d, e)). In addition, the Mg element was also detected in the EDS spectrum (Fig. 2(c)) and the elemental mapping (Fig. 2(f)), probably due to the multitudes of Mg2+ ions adsorbed on the surfaces of the amorphous SiO2 nanospheres owing to the electrostatic interaction between the Mg2+ ions and hydroxyls of the SiO2 nanospheres.29,34 The signal of Cu was originated from the copper grid during the TEM sample preparation. The temperature played a significant role on the compositions and morphologies of the products, as shown in Fig. 3. When hydrothermally treated at 90 ºC for 12.0 h, the crystallinity of the product got higher than the amorphous SiO2 formed at R.T. (Fig. 2(a)). At the present temperature of 90 ºC, the SiO2 nanospheres gradually dissolve in alkaline solution,12 resulting in silicate-ion groups and subsequent silicate structures. Thus, relatively low crystallinity SiO2

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(JCPDS No. 82-1567) and Mg3Si4O10(OH)2 (JCPDS No. 13-0558) microspheres coexisted with a diameter of ca. 100-250 nm (Fig. 3(a1, b)). With the temperature increasing from 120 ºC to 160 ºC and to 180 ºC, the products are all pure phase of Mg3Si4O10(OH)2 (JCPDS No. 13-0558), with the crystallinity of the products getting higher and higher (Fig. 3(a2-a4)). Specifically, when treated at 120 ºC, low crystallinity Mg3Si4O10(OH)2 microspheres assembled by multitudes of thin nanoflakes were acquired (Fig. 3(a2, c)). As the temperature going up to 140 ºC, the as-confirmed uniform Mg3Si4O10(OH)2 microspheres with narrow size distribution were obtained (Fig. 1(b, b1)). When the temperature was further increased to 160 ºC, higher crystallinity whereas nonuniform and somehow bigger porous Mg3Si4O10(OH)2 microspheres with remarkably thicker nanoflakes emerged (Fig. 3(a3, d)). However, if the temperature got higher to 180 ºC, high crystallinity and more compact and even some attached Mg3Si4O10(OH)2 microspheres appeared with the constitutional lamellar nanosheets severely aggregated with each other owing to the overgrowth at high temperature (Fig. 3(a4, e)). Besides the temperature, the hydrothermal duration time also has played an important role in the hydrothermal formation of the products. Fig. 4 depicts the variation of the compositions and morphologies of the products treated at 140 ºC with the duration time. When hydrothermally treated for 0.5 h, the majority of the amorphous SiO2 nanospheres were gradually transformed to the low crystallinity Mg3Si4O10(OH)2 microspheres (Fig. 4(a1, b)). With the time prolonging to 1.0 h to 6.0 h, the crystallinity of the products got higher and higher (Fig. 4(a2, a3)), and at the same time the morphologies of the products evolved from relatively nonuniform compact microspheres to the relatively large loose ones, which consisted of apparent nanoflakes (Fig. 4(c, d)). When treated for 12.0 h, uniform Mg3Si4O10(OH)2 porous microspheres with narrow size

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distribution were acquired (Fig. 1). Notably however, when treated for a longer time as 18.0 h, the rudimental lamellar nanosheets severely aggregated with each other, giving rise to the occurrence of more compact and even bulky Mg3Si4O10(OH)2 particles (Fig. 4(a4, e)). Additionally, to investigate the variation of the Si/Mg ratio of the samples with increase in the reaction time, samples hydrothermally treated at 140 ºC for 0.5 h, 1.0 h, 6.0 h and 18.0 h were further characterized by ICP analysis (Table S2). As seen, with the increase in the duration time, the atomic ratio of Si:Mg in the product changed from 8:3 (0.5-1.0 h) to 7:3 (6.0-18.0 h), which was also higher than the stoichiometric ratio of that in the molecule of Mg3Si4O10(OH)2 determined by XRD. This suggests the gradual transformation from the original amorphous silica to Mg3Si4O10(OH)2 with prolonging of the hydrothermal time. 3.3. Self-sacrificing template assisted formation of hierarchical porous Mg3Si4O10(OH)2 microspheres Based on the characterization of the precursor (Fig. 2), variation of composition and morphology of the hydrothermal products with temperature (Fig. 3) and time (Fig. 4), as well as ICP results (Table S1 and S2) mentioned above, a probable self-sacrificing template assisted formation mechanism of the hierarchical porous Mg3Si4O10(OH)2 microspheres could thus be figured out, as shown in Fig. 5. Firstly, the dissolution of MgCl2 and NH3·H2O resulted in the corresponding aqueous ions (Fig. 5(a)). Secondly, with the addition of Na2SiO3, the newly dissolved SiO32- ions were mixed with Mg2+ ions, NH4+ as well as OH- ions, and subsequently brought about the room temperature (R.T.) coprecipitation of the amorphous SiO2 nanospheres, with multitudes of Mg2+ ions adsorbed onto the surfaces owing to the electrostatic interaction between the Mg2+ ions and hydroxyls of the SiO2 nanospheres (Fig. 2, and Fig. 5(b)). As the hydrothermal reaction

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proceeding, the OH- ions within the solution gradually etched the SiO2 nanospheres, and then the newly formed SiO32- ions derived from the amorphous SiO2 reacted with the adsorbed Mg2+ ions, rendering the hydrothermal conversion for the original laminar Mg3Si4O10(OH)2 subunits in situ grown on the surfaces of the SiO2 nanospheres (Fig. 4(a1-a4, b-e) and Fig. 5(c)). With the hydrothermal treatment further prolonging, the size of the R.T. intermediate SiO2 nanospheres gradually decreased and eventually served as the self-sacrificing templates, giving rise to the further growth and thus bigger Mg3Si4O10(OH)2 microspheres with relatively thicker and larger subunits. Ultimately, the subsequent cooling, washing and drying enabled the relatively high crystallinity and loose hierarchical porous Mg3Si4O10(OH)2 microspheres (Fig. 1 and Fig. 5(d)). In contrast with our previously reported mesoporous Mg3Si2O5(OH)4 microspheres,12 the present facile one-pot hydrothermal synthesis brought about uniform hierarchical porous Mg3Si4O10(OH)2 microspheres whereas no need of the pre-prepared SiO2 nanospheres as templates, and also in the absence of H3BO3 and ethanol. This reveals the present one-pot process of economical, facile and green characteristic. 3.4. Maximum adsorption capacities of MB and CR on the hierarchical porous Mg3Si4O10(OH)2 microspheres Towards the severe water contamination issue, and from a practical application point of view, satisfactory porous structured adsorbents should exhibit some intrinsic natures such as relatively large specific surface area, high crystallinity, mechanical strength and good recyclability. The porous Mg3Si4O10(OH)2 microspheres those treated at 140 ºC for 6.0-18.0 h (Fig. 4(a3, d)), Fig. 1, and Fig. 4(a4, e)) were preliminarily considered as possible adsorbents for removal of anionic dyes MB and CR from mimic waste water.

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Fig. 6(a) presents the adsorption rate (Fig. 6(a1)) and capacity (Fig. 6(a2)) of MB onto the Mg3Si4O10(OH)2 microspheres (140 ºC, 12.0 h, Fig. 1) as a function of contact time with the initial concentration of 50 mg L-1. Apparently, the adsorption efficiency ((1-ct/c0)×100%) of MB from mimic waste water abruptly approaches to 86.8% within 30 min, further reaches to 94.5% in the coming 150 min, and remains relatively constant thereafter. Ultimately, the removal efficiency of MB via the porous Mg3Si4O10(OH)2 microspheres is delivered as 95.4% within the 240 min (Fig. 6(a1)). Simultaneously, the variation of the adsorption uptake versus the adsorption time is also illustrated in Fig. 6(a2). Obviously, the amount of adsorbed dye increases with the increase in the contact time and then reaches equilibrium. And the ultimate adsorption capacity of MB onto the porous Mg3Si4O10(OH)2 microspheres is confirmed as 92.97 mg g-1 within the 240 min (Fig. 6(a2)). For comparison, the ultimate adsorption capacities of MB onto the Mg3Si4O10(OH)2 microspheres those synthesized at 140 ºC for 6.0 h (Fig. 4(a3, d)) and 18.0 h (Fig. 4(a4, e)) are determined as 88.20 mg g-1 (Fig. 6(a3)) and 90.06 mg g-1 (Fig. 6(a4)), respectively, somehow lower than that for the porous Mg3Si4O10(OH)2 microspheres synthesized at 140 ºC for 12.0 h ((Fig. 6(a2)). Moreover, in order to study the versatility of the porous Mg3Si4O10(OH)2 microspheres (140 ºC, 12.0 h, Fig. 1) as the adsorbents for other anionic dye, removal of CR was also evaluated. Correspondingly, the adsorption behaviors, such as adsorption rate and capacity, were demonstrated in Fig. S1(a1, a2). Clearly, the adsorption rate as well as adsorption capacity of CR are determined as 97.8% (Fig. S1(a1)) and 98.56 mg g-1 (Fig. S1(a2)) respectively, within the contact time of 300 min. Also for comparison, as illustrated, the adsorption capacities of CR onto the microspheres synthesized at 140 ºC for 6.0 h (Fig. 4(a3, d)) and 18.0 h (Fig. 4(a4, e)) are

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comfirmed as 96.10 mg g-1 (Fig. S1(a3)) and 95.94 mg g-1 (Fig. S1(a4)), respectively, both of which are lower than that for the porous Mg3Si4O10(OH)2 microspheres synthesized at 140 ºC for 12.0 h ((Fig. S1(a2)). Taking the adsorption capacities (i.e. qm) for MB or CR into account, the hierarchical porous Mg3Si4O10(OH)2 microspheres hydrothermally synthesized at 140 ºC for 12.0 h (Fig. 1) delivered a prominent adsorption capability superior to the samples prepared at the same temperature for 6.0 h and 18.0 h. Thus, the as-obtained hierarchical porous Mg3Si4O10(OH)2 microspheres synthesized at 140 ºC for 12.0 h (Fig. 1) were selected as the target candidate adsorbents for further systematic evaluation. Adsorption isotherm, describing the adsorption capacity of the adsorbents versus the different initial concentrations of the adsorbate in the solution at equilibrium conditions, indicates the distribution of the adsorbate molecules in the liquid phase and solid phase when the adsorption process reaches an equilibrium state. As known, the Langmuir and Freundlich isotherm models are common mathematical models that have been extensively used to analyze the experimental data for investigation of the adsorption mechanisms. Typically, Langmuir isotherm model refers to an ideal monolayer and homogeneous adsorption, in which the adsorption sites possess constant enthalpy and equal affinity for the adsorbate molecules. The Freundlich isotherm model, in contrast, basically assumes multilayer, reversible and non-ideal adsorption, with non-uniform distribution of adsorption heat and affinities over the heterogeneous surface.35 Mathematically, the expression of Langmuir isotherm model and Freundlich isotherm model are represented by the following eqn (3) and (4), respectively.

qe 

qm k L ce 1  bce

(3)

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qe  k F ce

1/ n

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(4)

where qe (mg g-1) is the amount of adsorbate (MB or CR, here) molecules adsorbed on the adsorption surfaces at equilibrium , qm (mg g-1) is the maximum uptake capacity corresponding to complete monolayer coverage, kL (L mg-1) is the coefficient related to the affinity of the binding site and ce (mg L-1) is the concentration of the adsorbate solution at equilibrium. The slope and intercept of the plots of ce/qe vs ce can be used to calculate qm and kL, respectively. As for the Freundlich model, kF (mg g-1 (L mg-1)1/n) and n are Freundlich constants, which individually represent adsorption capacity and adsorption strength of the adsorbents. The constant of 1/n confirms as the degree of heterogeneity of the adsorbent surface. Eqn (3) can be transformed into eqn (5):

ce c 1   e qe q m k L q m

(5)

Another important parameter, RL, called dimensionless separation factor or equilibrium parameter, which is commonly used to determine the feasibility of the adsorption process, can be obtained from the Langmuir adsorption parameter kL (eqn (6)):

RL 

1 1  k L c0

(6)

where kL (L mg-1) is the Langmuir adsorption constant and c0 (mg L-1) is the initial adsorbate concentration. The value of RL indicates that: (1) RL=0 for irreversible adsorption; (2) 0pHPZC (pH=8.2 for MB, and pH=9.4 for CR), the Mg3Si4O10(OH)2 microspheres possessed negative charges. Although MB (Fig. S3(a)) is a kind of anionic dye, the molecule contains a positively charged group (=N+H-, Fig. S3(a)) which will interact with the negatively charged Mg3Si4O10(OH)2 microspheres.51 Thus, the hierarchical porous structure, relatively high specific surface area, and charge-charge interaction all contribute to the relatively high qm of MB (Table 2). Comparatively, the CR

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molecule (Fig. S3(b)) does not contain positively charged group, which however presents anionic form at present alkaline pH condition.52 This implies that the adsorption capacity of the CR is merely dependent on the textural hierarchical porous structure and high specific surface area of the porous Mg3Si4O10(OH)2 microspheres. Thus, although MB and CR are both anionic dyes, the MB molecules are more prone to be adsorbed on the porous Mg3Si4O10(OH)2 microspheres with a distinct higher qm value, superior to that of CR. Consequently, in consideration of the cost effective factor, eco-friendliness, mild hydrothermal process and high qm, the as-obtained hierarchical porous Mg3Si4O10(OH)2 microspheres can be convincingly and extensively employed as extremely competitive candidates as adsorbents for the removal of anionic dyes such as MB and CR, from mimic waste water. 3.5. Adsorption kinetics for MB and CR on the adsorbents To get deeper understanding about the characteristics of the adsorption process of MB onto the porous Mg3Si4O10(OH)2 microspheres, two typical kinetic models including pseudo-first-order (PFO) and pseudo-second-order (PSO) are used to interpret the adsorption data originated from batch experiments, and the corresponding linear regression equations are expressed as eqn (7) and (8), respectively.

log( qe  qt )  log qe 

k1t 2.303

(7)

t 1 t   2 qt k 2 qe qe

(8)

where qe (mg g-1) and qt (mg g-1) are the amount of MB adsorbed at equilibrium and any time t (min), respectively; k1 (min-1) and k2 (g mg-1 min-1) are the PFO and PSO rate constants, respectively; t (min) is the contact time between the adsorbents and adsorbates. Since the adsorption process of MB on the hierarchical porous Mg3Si4O10(OH)2 microspheres 20


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had reached equilibrium within 150 min (Fig. 6(a)), the adsorption data before equilibrium were fitted through linear regression via the PFO and the PSO models, as shown in (Fig. 7(a)) and (Fig. 7(b)), respectively. The determined corresponding kinetic parameters and determination coefficients (R2) are given in Table 3. As shown, for the adsorption MB, the value of R2 derived from the PFO model is 0.8644. Meanwhile, the calculated qe,cal1 value from the PFO model (50.65 mg g-1) is significantly deviated from the corresponding experimental data qe,exp (92.27 mg g-1), suggesting that the MB adsorption behavior on the porous Mg3Si4O10(OH)2 microspheres can not be well interpreted with the PFO model. In contrast, when the PSO model is adopted, a linear plot of t/qt vs t is obtained (Fig. 7(b)), and the related parameters are also listed in Table 3. As depicted, the determination coefficient R2 delivered remarkably high value (R2=0.9970), implying that the present adsorption process of MB on the porous Mg3Si4O10(OH)2 microspheres can be more appropriate in describing by the PSO model rather than the PFO model. Additionally, the value of qe,cal2 stemmed from the PSO model, is determined to be 92.94 mg g-1, and this appears to be much closer to the experimentally obtained value of qe,exp (92.97 mg g-1) than the value of qe,cal1 (50.65 mg g-1) originated from PFO model does. Taking the correlation coefficient (R2) and calculated qe,cal into consideration, the PSO model can be more suitable to explain the adsorption behavior of MB on the hierarchical porous Mg3Si4O10(OH)2 microspheres. The current adsorption behavior of MB on the hierarchical porous Mg3Si4O10(OH)2 microspheres is analogous to our previous work.37 This indicates that, the adsorption of MB onto the present Mg3Si4O10(OH)2 microspheres might be chemisorption, which is associated with the valence forces by sharing or exchanging electrons between the Mg3Si4O10(OH)2 and MB molecules.37,53 Similar kinetic results have also been reported for the adsorption of MB onto β-cyclodextrin-chitosan,42 and

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hydroxyapatite.54 To determine the actual rate-controlling step involved in the MB sorption process, the intra-particle diffusion model was further employed to confirm the possible transportation of MB molecules into the pores of the Mg3Si4O10(OH)2 microspheres. Weber-Morris intra-particle diffusion model is generally represented as follows:55

qt  K id t 1 / 2  I

(9)

where Kid (mg g-1 min-1/2) is the intra-particle diffusion rate constant, and I (mg g-1) is the intercept that is proportional to the thickness of boundary layer, i.e. the larger the intercept, the greater boundary layer effect is. The plot of qt against t1/2 for the sorption of MB is delivered in Fig. 8(a), and the relevant kinetic parameters such as the slope (Kid) and intercept (I) are tabulated in Table 4. As can be seen (Fig. 8(a)), the regression of qt vs t1/2 are distinctly separated into two linear regions by employing the intra-particle model, and additionally the plot does not pass through the origin, implying that there might be more than one process involved in the adsorption process.56,57 And this is analogous to that drawn in our previous work.12,37 The first stage, i.e. the steeper region with a higher slope of Kid1 in the diffusion model, represents that the external mass transfer of MB molecules from solution and their further binding by those active sites distributed onto the outer surface of Mg3Si4O10(OH)2 microspheres. Comparatively, the second stage, i.e. the relatively flat region with a lower slope of Kid2 is in accordance with the intra-particle diffusion of the adsorbates MB from the exterior surface to the internal pores of the Mg3Si4O10(OH)2 microspheres. And correspondingly, the intercept I1 of the first stage was lower than I2 of the second stage, suggesting that the rate-limiting step lies in the second stage, i.e. the intra-particle diffusion of MB.

22


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It is also notable that however, neither the intercept I1 nor the intercept I2 is equal to 0 in the present system, implying that the intra-particle diffusion is not the entirely sole rate-limiting step and the external mass transfer may also have a significant role in controlling the adsorption rate in view of the relatively large intercept (I1) of the linear portion of the plot. Thus, it is still needed to judge which one exerted a greater influence on the rate of dyes sorption. Subsequently, the adsorption kinetic data were further analyzed by Boyd model.58 Mathematically, the Boyd model can be expressed as eqn (10):

B t   ln(1 

qt )  0.4977 qe

(10)

where qt (mg g-1) and qe (mg g-1) are the amounts of dyes adsorbed on the adsorbent at time t and equilibrium time, respectively. The calculated value Bt vs t is plotted, as demonstrated in Fig. 8(b). The linearity of the plot presents valuable information to distinguish between external mass transfer and intra-particle diffusion controlled mechanism of adsorption.58 Obviously, the plot in Fig. 8(b) does not pass through the origin, confirming that both the external mass transfer and intra-particle diffusion existed in the whole process of MB adsorption on the present Mg3Si4O10(OH)2 microspheres.58 Nevertheless, the fitted plot is not straight line, suggesting that external mass transfer delivered a relatively weak rate control for MB molecules adsorption onto Mg3Si4O10(OH)2 microspheres. In other words, it reconfirms that the entire adsorption of MB on the present hierarchical Mg3Si4O10(OH)2 microspheres can be well interpreted by the Weber-Morris intra-particle kinetic model, and it is the intra-particle diffusion rather than the external mass transfer dominates the rate-controlling step. As for the characteristic adsorption process of the CR onto the hierarchical porous Mg3Si4O10(OH)2 microspheres, the PFO and PSO kinetic models were also used to interpret the 23



adsorption data, and the adsorption mechanism was quite similar to that of MB. Specifically, the the adsorption behavior of CR may also be chemisorption (Fig. S4), and also belongs to the intra-particle diffusion controlled adsorption (Fig. S5). 3.6. Recyclability of the adsorbents The regeneration performance or recyclability of the adsorbents is extremely important for their practical applications. For the current case, the MB or CR-adsorbed Mg3Si4O10(OH)2 samples were separated from the dye-containing solution after adsorption and then dried at 60 ºC for 12.0 h. The pristine white Mg3Si4O10(OH)2 powders turned into deep blue/red ascribed to the adsorption of MB/CR. To evaluate the reusability of the hierarchical porous Mg3Si4O10(OH)2 microspheres as the adsorbents, the MB or the CR-adsorbed samples are calcined through a facile thermal conversion at 300 ºC or 400 ºC (lower than that required for the phase transformation of Mg3Si4O10(OH)2, Fig. S6) for 2.0 h (heating rate: 2.5 ºC min-1), respectively, so as to eliminate the adsorbed MB or CR molecules and bring about the regenerated samples. The regenerated samples were employed as the adsorbents for the next cycle of the adsorption of MB or CR. Typically, as in the current test, five cycles of regeneration and adsorption were performed, and the results were illustrated in Fig. 9. As shown, The removal efficiency of the original hierarchical porous Mg3Si4O10(OH)2 microspheres for MB and CR reached 95.4% (Fig. 6(a1)) and 97.8% (Fig. S1(a1)), respectively. With the successive adsorption-regeneration processes going on, the removal efficiency of the samples slightly decreased and individually remained at 91.5% and 96.6% after five cycles (Fig. 9), implying the as-prepared hierarchical porous Mg3Si4O10(OH)2 microspheres as excellent candidate adsorbents for removal of anionic dyes from mimic waste water with satisfactory recyclability.

24


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In order to investigate the stability of the adsorbents, compositions of the Mg3Si4O10(OH)2 microspheres before the first and after the fifth cycles of adsorption were monitored, as depicted in Fig. S7. Apparently, all the diffraction peaks of the samples are well in accordance with the standard Mg3Si4O10(OH)2 (JCPDS No. 13-0558) except for a slight decrease in the crystalinity of the recycled samples, indicating the satisfactory chemical stability of the current adsorbents even after five cycles of the adsorption for MB (Fig. S7(a2)) and CR (Fig. S7(a3)), respectively. This suggests that, the present hierarchical porous Mg3Si4O10(OH)2 microspheres are highly efficient stable adsorbents for MB or CR removal from simulate waste water with satisfactory recyclability and reusability. 4. Conclusion In summary, uniform hierarchical porous Mg3Si4O10(OH)2 microspheres (diameter: 200-350 nm) consisted of ultrathin nanosheets are successfully formed through a facile and green one-pot hydrothermal method in the absence of any organic additives or templates. The as-synthesized hierarchical porous microspheres possess a large specific surface area of 551.08 m2 g-1, a pore volume of 0.84 cm3 g-1, and a mean pore diameter of 6.12 nm. According to the influences of the hydrothermal parameters on the products, a probable self-sacrificing template assisted formation mechanism is put forward. When employed as the adsorbents for the removal of the anionic dyes MB and CR, the as-prepared porous microspheres exhibit the satisfactory maximum adsorption capacities qm of 1890 mg g-1 and 576 mg g-1, respectively, superior to most of the referenced adsorbents. Taking the facile and green preparation, environmental friendliness as well as satisfactory recyclability into consideration, the as-obtained porous Mg3Si4O10(OH)2 microspheres are undoubtedly revealed as novel competitive adsorbents candidate for removal of anionic

25



organic dyes such as MB and CR from mimic waste water, which may also serve as promising catalyst supports in diverse fields, e.g. heterogeneous catalysis. Acknowledgements This work is supported by the National Natural Science Foundation of China (no. 21276141), the State Key Laboratory of Chemical Engineering, China (no. SKL-ChE-17A03). The authors also want to thank the referees for the constructive suggestions on the improvement of the work. Supporting Information Adsorption isotherm models and kinetic models for adsorption of CR, zeta potential, TG-DSC curves, and XRD pattern of the hierarchical porous Mg3Si4O10(OH)2 microspheres, molecular structures of MB and CR, XRD patterns of the regenerated adsorbents, as well as the ICP analysis of the products hydrothermally synthesized at 140 ºC for different time. Supplementary information is available free of charge on the ACS publication website. http://pubs.acs.org. References (1) Barylak, M.; Cendrowski, K.; Mijowska, E. Application of Carbonized Metal-Organic Framework as Efficient Adsorbent of Cationic Dye. Ind. Eng. Chem. Res. 2018, 57, 4867-4879. (2) Ji, H. F.; Song, X.; Shi, Z. Q.; Tang, C. Q.; Xiong, L.; Zhao,W. F.; Zhao, C. S. Reinforced-Concrete Structured Hydrogel Microspheres with Ultrahigh Mechanical Strength, Restricted Water Uptake, and Superior Adsorption Capacity. ACS Sustainable Chem. Eng. 2018, 6, 5950-5958. (3) Chang, H. H.; Chao, Y. H.; Pang, J. Y.; Li, H. P.; Lu, L. J.; He, M. Q.; Chen, G. Y.; Zhu, W. S.; Li, H. M. Advanced Overlap Adsorption Model of Few-Layer Boron Nitride for Aromatic Organic Pollutants. Ind. Eng. Chem. Res. 2018, 57, 4045-4051. (4) Hu, J.; Deng, W. J.; Chen, D. H. Ceria Hollow Spheres as an Adsorbent for Efficient Removal of Acid Dye. ACS Sustainable Chem. Eng. 2017, 5, 3570-3582. (5) Yang, L. Y.; Zhang, Y. Y.; Liu, X. Y.; Jiang, X. Q.; Zhang, Z. Z.; Zhang, T. T.; Zhang, L. The investigation of synergistic and competitive interaction between dye Congo red and methyl blue on magnetic MnFe2O4. Chem. Eng. J. 2014, 246, 88-96. (6) Purkait, M. K.; DasGupta, S.; De, S. Adsorption of eosin dye on activated carbon and its surfactant based desorption. J. Environ. Manage. 2005, 76, 135-142. (7) Mall, I. D.; Srivastava, V. C.; Agarwal, N. K.; Mishra, I. M. Removal of congo red from aqueous solution by bagasse fly ash and activated carbon: kinetic study and equilibrium isotherm analyses. Chemosphere 2005, 61, 492-501. 26


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Colloid Interf. Sci. 2012, 386, 277-284. (40) Wang, F. Y.; Fan, Y. L.; Ni, J. J.; Xu, T. T.; Song, J. M. Co-based ternary nanocomposites: synthesis and their superior performances for hydrogenation of p-nitrophenol and adsorption for methyl blue. J. Nanopart. Res. 2016, 18, 1-8. (41) Shu, Y. H.; Shao, Y. M.; Wei, X. Y.; Wang, X.; Sun, Q. Q.; Zhang, Q. Y.; Li, L. S. Synthesis and characterization of Ni-MCM-41 for methyl blue adsorption. Micropor. Mesopor. Mater. 2015, 214, 88-94. (42) Fan, L. L.; Zhang, Y.; Luo, C. N.; Lu, F. G.; Qiu, H. M.; Sun, M. Synthesis and characterization of magnetic β-cyclodextrin-chitosan nanoparticles as nano-adsorbents for removal of methyl blue. Int. J. Biol. Macromol. 2012, 50, 444-450. (43) Feng, Y.; Li, Y.; Xu, M. Y.; Liu, S. C.; Yao, J. F. Fast adsorption of methyl blue on zeolitic imidazolate framework-8 and its adsorption mechanism. RSC Adv. 2016, 6, 109608-109612. (44) Bai, Z. Q.; Zheng, Y. J.; Zhang, Z. P. One-pot synthesis of highly efficient MgO for the removal of Congo red in aqueous solution. J. Mater. Chem. A 2017, 5, 6630-6637. (45) Wang, L. X.; Li, J. C.; Wang, Z. T.; Zhao, L. J.; Jiang, Q. Low-temperature hydrothermal synthesis of α-Fe/Fe3O4 nanocomposite for fast Congo red removal. Dalton Trans. 2013, 42, 2572-2579. (46) Ai, L. H.; Yue, H. T.; Jiang, J. Sacrificial template-directed synthesis of mesoporous magnesium oxide architectures with superior performance for organic dye adsorption. Nanoscale 2012, 4, 5401-5408. (47) Chaukura, N.; Mamba, B. B.; Mishra, S. B. Conversion of post consumer waste polystyrene into a high value adsorbent and its sorptive properties for Congo Red removal from aqueous solution. J. Environ. Manage. 2017, 193, 280-289. (48) Lu, X. H.; Zheng, D. Z.; Xu, M.; Huang, Y. Y.; Xie, S. L.; Liu, Z. Q.; Liang, C. Lun.; Liu, P.; Tong, Y. X. General Electrochemical Assembling to Porous Nanowires with High Adaptability to Water Treatment. CrystEngComm, 2011, 13, 2451-2456. (49) Wang, Q.; Wang, X. F.; Shi, C. L. LDH Nanoflower Lantern Derived from ZIF-67 and Its Application for Adsorptive Removal of Organics from Water. Ind. Eng. Chem. Res. 2018. 57, 12478-12484. (50) Maiti, D.; Mukhopadhyay, S.; Devi, P. S. Evaluation of Mechanism on Selective, Rapid, and Superior Adsorption of Congo Red by Reusable Mesoporous α-Fe2O3 Nanorods. ACS Sustainable Chem. Eng. 2017, 5, 11255-11267. (51) Sharma, P.; Hussain, N.; Borah, D. J.; Das M. R. Kinetics and Adsorption Behavior of the Methyl Blue at the Graphene Oxide/Reduced Graphene Oxide Nanosheet-Water Interface: A Comparative Study. J. Chem. Eng. Data 2013, 58, 3477-3488. (52) Maiti, D.; Mukhopadhyay, S.; Devi, P. S. Evaluation of Mechanism on Selective, Rapid, and Superior Adsorption of Congo Red by Reusable Mesoporous α-Fe2O3 Nanorods. ACS Sustainable Chem. Eng. 2017, 5, 11255-11267. (53) Liu, J. Y.; Lai, M. W.; Wong, L. H.; Chiam, S. Y.; Li, S. F. Y.; Ren, Y. Immobilization of dye pollutants on iron hydroxide coated substrates: kinetics, efficiency and the adsorption mechanism. J. Mater. Chem. A 2016, 4, 13280-13288. (54) Zhang, F.; Yin, X.; Zhang, W.; Ji, Y. Optimizing decolorization of methyl blue solution by two magnetic hydroxyapatite nanorods, J. Taiwan Inst. Chem. E. 2016, 65, 269-275. (55) Doğan, M.; Abak, H.; Alkan, M. Adsorption of methylene blue onto hazelnut shell: Kinetics, 29



mechanism and activation parameters. J. Hazard. Mater. 2009, 164, 172-181. (56) Vimonses, V.; Lei, S.; Bo, J.; Chow, C. W. K.; Saint, C. Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials. Chem.Eng. J. 2010, 148, 354-364. (57) Naiya, T. K.; Chowdhury, P.; Bhattacharya, A. K.; Das, S. K. Saw dust and neem bark as low-cost natural biosorbent for adsorptive removal of Zn(II) and Cd(II) ions from aqueous solutions. Chem. Eng. J. 2009, 148, 68-79. (58) Kumar, K. V.; Ramamurthi, V.; Sivanesan, S. Modeling the mechanism involved during the sorption of methylene blue onto fly ash. J. Colloid Interf. Sci. 2005, 284, 14-21.

30


Page 30 of 38

MB

1 µm

qm=538 mg g-1

100

qm=1842 mg g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Removal efficiency (%)

Page 31 of 38

80

94.1

98.5

95.697.8

CR

97.9 96.6 97.5 95.9 91.5 91.5

60 40 20 0

1

2

3

4

Number of cycles

Table of Contents

31


5

(a)

JCPDS No. 13-0558 Mg3Si4O10(OH)2

(b)

Page 32 of 38

(b2) 20 nm 30 nm 10 nm

40

50

60

2

(c)

70

80

1 µm

Diameter (nm)

(d)

500

0.025

(d1)

0.020

-1

Volume (cc g )

3

(c1)

-1

30

dV(logD) [cm g A ]

20

(b )

Frequency (%)

10

42 36 40 1 35 30 24 18 19 12 6 2 4 0 100 150 200 250 300 350 400 450

20 nm

500 nm

400

0.015 0.010 0.005

300

0.000 10

100

Pore Diameter (nm)

200 100 0

100 nm

0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

Fig. 1. XRD pattern (a), SEM (b, b1) and TEM (c, c1) images, N2 adsorption-desorption isotherms (d) of the hierarchical porous Mg3Si4O10(OH)2 microspheres hydrothermally synthesized at 140 ºC for 12.0 h. Insets (b1) and (b2) show the size distribution and a high resolution SEM image, (c1) and (d1) demonstrate a high resolution TEM image and pore diameter distribution of the Mg3Si4O10(OH)2 microspheres, respectively. The red vertical lines in (a) indicate the standard pattern of Mg3Si4O10(OH)2 (JCPDS No. 13-0558).

(a)

10

20

30

40

50

60

2

70

80

(c) O

35 36 1 30 30 25 20 15 10 14 10 5 10 0 60 70 80 90 100 110 120 130 140

(b )

Frequency (%)

Intensity (a.u.)

(b)

100 nm

Diameter (nm)

(d)

(e)

(f)

Si

O

Mg

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a.u.)


Cu Si Mg 0

2

Cu 4

6

Energy (keV)

8

10

Fig. 2. XRD pattern (a), SEM image (b), EDS spectrum (c) and elemental mappings (d-f) of the room temperature precipitate. Inset (b1) shows the size distribution of the SiO2 nanospheres.


Page 33 of 38

(a)

JCPDS No. 13-0558 Mg3Si4O10(OH)2

(a4)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

(a3) (a2) JCPDS No. 82-1567 SiO2 13-0558 Mg3Si4O10(OH)2

(a1)

20

30

40

50

2

60

70

80

(c)

(b)

1 µm

500 nm (d)

(e)

1 µm

1 µm

Fig. 3. XRD patterns (a) and SEM images (b-e) of the hydrothermal products obtained at different temperatures for 12.0 h. Temperature (ºC): (a1, b) 90; (a2, c) 120; (a3, d) 160; (a4, e) 180. The black and red vertical lines in (a1) indicate the standard pattern of SiO2 (JCPDS No. 82-1567) and Mg3Si4O10(OH)2 (JCPDS No. 13-0558), respectively.



(a)

JCPDS No. 13-0558 Mg3Si4O10(OH)2 (a4)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

Page 34 of 38

(a3)

(a2) (a1)

20

30

40

50

2

(b)

60

70

80

(c)

100 nm (d)

100 nm (e)

1 µm

1 µm

Fig. 4. XRD patterns (a) and SEM images (b-e) of the hydrothermal products (a1-a4, b-e) synthesized at 140 ºC for different time. Time (h): (a1, b) 0.5; (a2,c) 1.0; (a3, d) 6.0; (a4, e) 18.0. The red vertical lines in (a) indicate the standard pattern Mg3Si4O10(OH)2 (JCPDS No. 13-0558).


Page 35 of 38

Mg2+ OH-

SiO32-

Hydrothermal

Hydrothermal

conversion

further growth

R.T. coprecipitation

OH

NH4+

OH

(b)

(a)

Mg2+

(c)

(d)

Fig. 5. Facile self-sacrificing template assisted synthesis of the hierarchical porous Mg3Si4O10(OH)2 microspheres. (a) Dissolution of the reactants led to aqueous Mg2+ ions, and ionization of the alkali NH3·H2O resulted in NH4+ and OH- ions; (b) Dropwise addition of SiO32- ions at room temperature (R.T.) brought about the coprecipitation of the SiO2 microspheres, with multitudes of Mg2+ ions adsorbed onto the surfaces due to the electrostatic interaction between the Mg2+ ions and hydroxyls of the SiO2 microspheres; (c) With the hydrothermal treatment going on, the OH- ions within the solution gradually etched the SiO2 microspheres, and the newly formed SiO32- ions reacted with the adsorbed Mg2+ ions, rendering the hydrothermal conversion for the original laminar magnesium silicate (Mg3Si4O10(OH)2) subunits in situ grown on the surfaces of the SiO2 microspheres; (d) With the hydrothermal time further prolonging, the SiO2 microspheres eventually served as the self-sacrificing templates, giving rise to the further growth and thus final bigger hierarchical porous Mg3Si4O10(OH)2 microspheres with relatively thicker and larger subunits.

100

(a) 1.0 (a2)

0.8

6h 12 h 18 h

(a4) (a3)

40

0.4 0.2 0.0

20

(a1)

0

30

60

0 90 120 150 180 210 240

t (min)

(c) 2100

(b)1800

qe (mg g )

1800

1500

qe (mg g )

0.8

1200

0.6

(b1)

0.4

600

0.2

300

0.0

0

200

400

1500 1200

ce/qe

900

-1

-1

60

-1

ct/c0

0.6

80

qt mg g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


900 600

0

200

600

400

600

-1800

ce (mg L )

-1 800

ce (mg L )

1000 1200

1000 1200

300 0

200

400

600

-1 800

ce (mg L )

1000 1200

Fig. 6. Variation of the adsorption rate (a1) and capacities (a2-a4) of MB onto the Mg3Si4O10(OH)2 microspheres hydrothermally synthesized at 140 ºC for different time, with an initial concentration and dosage of MB as 50.0 mg L-1 and 40 mL, respectively, and the adsorption isotherms fitted with the Langmuir (b) or Freundlich (c) model by non-linear regression, with the concentration of MB changing within the range of 100.0-1500.0 mg L-1. Hydrothermal time (h): (a1, a2) 12.0; (a3) 6.0; (a3) 18.0. The inset (b1) indicates the corresponding plot of ce/qe vs ce by linear regression.



(b)1.0

(a) 2.00 1.75

0.8

log(qe-qt)

1.50

0.6

t/qt

1.25 1.00

0.4

0.75

0.2

0.50 0.25 0

15

30

45

t (min)

60

75

0.0

90

0

15

30

45

t (min)

60

75

90

Fig. 7. Pseudo-first-order (a) and pseudo-second-order (b) kinetic plots derived from linear regression for the adsorption of MB on the hierarchical porous Mg3Si4O10(OH)2 microspheres (20 mg), with an initial concentration and dosage of MB as 50.0 mg L-1 and 40.0 mL, respectively.

(b)5

80

4

-1

)

(a) 100

3

60

Bt

qt (mg g

2

40

1

20

0

0 0

2

4

t

1/2 6

8 1/2 10 (min )

12

14

16

-1

0

30

60

90

t (min)

120

150

Fig. 8. Weber-Morris model (a) and Boyd model (b) for the adsorption of MB on the hierarchical porous Mg3Si4O10(OH)2 microspheres (20 mg), with an initial concentration and dosage of MB as 50.0 mg L-1 and 40.0 mL, respectively.

100

Removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

80

MB

CR

96.6 97.9 98.5 97.8 97.5 95.9 91.5 95.6 91.5 94.1

60 40 20 0

1

2

3

4

Number of cycles

5

Fig. 9. Recycling performance of the as-synthesized hierarchical porous Mg3Si4O10(OH)2 microspheres as adsorbents for the removal of MB and CR from the mimic waste water.


Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Corresponding fitting parameters originated from the non-linear regression by using Langmuir isotherm model and Freundlich isotherm model for the adsorption of MB and CR on the hierarchical porous Mg3Si4O10(OH)2 microspheres. Organic

Langmuir isotherm model

dye

qm (mg g )

kL (L mg )

R

MB

1890

0.0194

CR

576

0.1762

-1

Freundlich isotherm model RL

kf

1/n

R2

0.9749

0.0332-0.3401

205.75

0.3243

0.9285

0.9508

0.0140-0.1019

138.70

0.2752

0.8193

-1

2

Table 2. Comparison of the maximum adsorption capacities (qm, mg g-1) for MB and CR on various adsorbents. Organic

adsorbents

dyes

MB

qm

(m g )

(mg g )

2

-1

References -1

Magnetic β-cyclodextrin-chitosan

/

2780

[42]

ZIF-8 nanoparticles

/

2500

[43]

Hierarchical porous Mg3Si4O10(OH)2 microspheres

551.0

1890

This work

SBP nanorods

17.1

1863

[36]

953/1588

[37]

Hierarchical porous γ-AlOOH/Al2O3 microspheres Graphene

76.8/158.6 353.0

1520

[38]

Barium phosphate nano-flake

2.5

1500

[39]

Co-based ternary nanocomposites

19.8

1100

[40]

MnFe2O4 particles

155.7

498

[5]

843

189

[41]

164.1

3236

[44]

/

1297

[45]

mesoporous MgO architectures

94.0

689

[46]

Hierarchical porous Mg3Si4O10(OH)2 microspheres

551.0

576

This work

hierarchical flower-like NiO microspheres

107.0

535

[26]

500/357

[47]

Ni-MCM-41 powders MgO rods α-Fe/Fe3O4 nanocomposite

CR

*SBET

Conjugated

microporus

polymer/sulphonic-group

752.0/510.0

carrying resin Porous La(OH)3 naowires

67.8

481

[48]

-AlO(OH) nanowires/Mg-Al-LDH/C nanosheets

288.0

447

[10]

LDH nanoflower

214.3

329

[49]

Urchin-like α-FeOOH hollow spheres

53.8

275

[27]

Mesoporous α-Fe2O3 nanorods

39.0

160

[50]

*SBET, specific surface area.



Page 38 of 38

Table 3. Kinetic parameters fitted by the pseudo-first-order model and pseudo-second-order model for the adsorption of MB and CR on the hierarchical porous Mg3Si4O10(OH)2 microspheres, with an initial concentration of the organic dyes as 50.0 mg L-1. Organic

qe,exp

dyes

(mg g )

pseudo-first-order kinetic model

-1

qe,calc1

k1 (min-1)

R2

(mg g )

pseudo-second-order kinetic model qe,calc2

k2 (mg g-1 min-1)

R2

(mg g )

-1

-1

MB

92.97

50.65

0.0174

0.8644

92.94

1.157×10-4

0.9970

CR

98.56

35.99

0.042

0.5785

96.25

0.0054

0.9976

Table 4. Parameters fitted by the Weber-Morris model for the adsorption of MB and CR onto the hierarchical porous Mg3Si4O10(OH)2 microspheres, with an initial concentration of the organic dyes as 50.0 mg L-1. Kid1 (mg g-1 min-1/2)

Kid2 (mg g-1 min-1/2)

I1 (mg g-1)

I2 (mg g-1)

MB

25.34

0.87

2.4077

81.5501

CR

23.94

0.17

0.0273

92.6891

Organic dyes


Facile and green one-pot hydrothermal formation of hierarchical

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