Novel Carbon Paper@Magnesium Silicate Composite Porous Films

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Novel Carbon Paper@Magnesium Silicate Composite Porous Films: Design, Fabrication, and Adsorption Behavior for Heavy Metal Ions in Aqueous solution Renyao Huang, Li He, Tao Zhang, Dianqing Li, Pinggui Tang, and Yongjun Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01557 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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ACS Applied Materials & Interfaces

Novel Carbon Paper@Magnesium Silicate Composite Porous Films: Design, Fabrication, and Adsorption Behavior for Heavy Metal Ions in Aqueous solution Renyao Huang, a Li He, a Tao Zhang, b Dianqing Li, a Pinggui Tang, a and Yongjun Fenga,* a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing, 100029, China. b Beijing Center for Physical & Chemical Analysis, No. 27 Xisanhuan North Road, Beijing, 100089, China. ABSTRACT: It is of great and increasing interest to explore porous adsorption films to reduce heavy metal ions in aqueous solution. Here, we for the first time fabricated carbon paper@magnesium silicate (CP@MS) composite films for the high-efficiency removal of Zn2+ and Cu2+ by a solid-phase transformation from hydromagnesite coated carbon paper (CP@MCH) precursor film in a hydrothermal route, and detailedly examined adsorption process for Zn2+ and Cu2+ as well as the adsorption mechanism. The suitable initial pH range is beyond 4.0 for the adsorption of the CP@MS to remove Zn2+ under the investigated conditions and the adsorption capacity is mainly up to the pore size of the porous film. The composite film exhibits excellent adsorption capacity for both of Zn2+ and Cu2+ with the corresponding maximum adsorption quantity of 198.0 mg g-1 for Zn2+ and 113.5 mg g-1 for Cu2+, which are advantage over most of those reported in the literature. Furthermore, the adsorption behavior of the CP@MS film follows the pseudo-secondorder kinetic model and the Langmuir adsorption equation for Zn2+ with the cation-exchange mechanism. Particularly, the CP@MS film shows promisingly practical applications to remove heavy metal ions in water by an adsorptionfiltration system. KEYWORDS: Porous composite film, Magnesium silicate, Adsorption behavior, Heavy metal ions, Adsorption-filtration

1. INTRODUCTION So far, great amount of industrial waste water containing excessive heavy metal ions such as Pb2+, Hg2+, Zn2+, Cu2+ has been discharged into environment,1-3 which are seriously threating the ecosystem’s sustainable development and human being’s health beyond the permissible limits.4 For example, Zn2+ and Cu2+ are two of the typical heavy metal ions based on the World Health Organization (WHO),5 in drink water, the WHO suggested limit is 2 mg L-1 for Cu2+ and 5 mg L-1 for Zn2+,6 and the excessive intake of Zn2+ for human can cause irritability, muscular stiffness, anorexia and nausea.7 Some experimental technologies have been explored to eliminate these ions from the waste water, e.g., chemical treatment,8 biological treatment,9 membrane separation,10 and adsorption.11 Yet, the high migration in water and the non-biodegradability significantly hinder the development of practical technologies. Therefore, it is of great urgency and high requirement to explore new technologies and novel-structure materials to remove excessive heavy metal ions from our environment. Recently, magnesium silicate (MS) has shown excellent and promising adsorption capacity for heavy metal ions in wastewater, and furthermore various kinds of different morphological MS materials have been fabricated such as nanorods,11-12 nanosphere,13-14 nanotube,15 ordered mesoporous,16-17 and flower-like MS18 so on. However, the currently investigated MS materials in the literature were most in the form of powder,

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which are unpractical to a certain extent because of highly difficult separation of this adsorbent from the massive treated solution,19 and possible secondary pollution. More recently, some advanced adsorption films/fiber have been developed and exhibited excellent adsorption properties related to the corresponding powders.20-23 To our best knowledge, yet, porous MS film has not been reported. Therefore, it is of great interest and remains a big challenge to design and fabricate hierarchical MS films for the high-efficiency and easy-operation removal of heavy metal ions from water. Here, we designed and developed novel magnesium silicate coated carbon paper composite porous films (CP@MS) by a solid-phase transformation reaction from hydromagnesite coated carbon paper (CP@MCH) precursor films in a hydrothermal route, which were synthesized in a hexamethylene tetramine (HMT)assisted hydrothermal method, and investigated the adsorption behavior for Zn2+ and Cu2+ in water. Furthermore, we further examined the practical performance in a home-made adsorption-filtration system. Scheme 1 shows the preparation process of the CP@MS porous film from the CP via the CP@MCH precursor in the hydrothermal route under relative mild conditions.

Scheme 1. Preparation process of carbon paper@magnesium silicate porous film (CP@MS) via the hydromagnesite coated carbon paper (CP@MCH) precursor in the hydrothermal route.

2. EXPERIMENTAL SECTION 2.1. Materials. The TORAY TGP-H-060 carbon fiber paper (CP) was purchased at Shanghai Hesen Electric Co., Ltd. Magnesium sulfate heptahydrate (MgSO4·7H2O), hexamethylene tetramine (HMT), sodium silicate nonahydrate (Na2SiO3·9H2O), concentrated nitric acid (HNO3, 65-68 wt%), and zinc nitrate (Zn(NO3)2) were all analytical grade and used as received. The water used in this work was deionized water. 2.2. Preparation of CP@MS composite films. Two CP@MS composite films were prepared by a solid-phase transformation reaction from hydromagnesite coated carbon paper (CP@MCH) precursor films in a hydrothermal route. The experimental details were described as following:


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Firstly, the CP@MCH precursor films were synthesized in a HMT-assisted hydrothermal method without any template using MgSO4·7H2O as the Mg resource, HMT as the precipitating agent, and CP as the substrate. Typically, the CP was activated in concentrated nitric acid (65-68 wt%) for 3h, washed with water, and dried at 80℃. Then, 76.5 mg (about 3 X 3 cm2) activated CP was added into 50 mL of the mixed solution containing 1.58 mmol Mg(SO4)2·7H2O and 17.1 mmol HMT under ultrasonication. Subsequently, all the above solution and CP were transferred into a Teflon-lined autoclave (100 mL), kept in an oven at 140℃ for 6 hours, and then lowered to 25℃. Finally, the CP@MCH precursor was obtained after being washed with water and ethanol, and dried at 80℃ overnight. Secondly, two CP@MS composite films were fabricated from a solid-liquid reaction between CP@MCH and different amount of Na2SiO3·9H2O under the hydrothermal conditions. For example, 50 mL of Na2SiO3·9H2O aqueous solution and the prepared CP@MCH precursor film were moved into a Teflon-lined autoclave (100 mL) at 160℃ for 12 hours, and lowered to 25℃. Finally, the CP@MS composite films were obtained after the same washing and drying process with that for the precursor. Here two samples were individually marked as CP@MS-1 and CP@MS-2 for Na2SiO3·9H2O as 0.167 mmol and 0.880 mmol under the same procedure. 2.3. Characterization. Powder X-Ray Diffraction (PXRD) were carried out on a BRUKER D8 ADVANCE X-ray powder diffractometer (Cu Ka radiation, λ = 0.15418 nm) at 10o min-1 for 2θ. X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCALAB 2201 XL spectrometer with a monochromatic Mg Ka X-ray radiation. Morphologies were examined using a Hitachi S-4700 scanning electron microscope (SEM) at 30 kV, and the elemental distribution was examined by energy-dispersive X-ray spectroscopy (EDX) coupled with SEM. Low-temperature nitrogen adsorptiondesorption experiments were performed on an ASAP 2460 analyzer. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method from the adsorption isotherm, and the pore volume and the pore size distribution were evaluated by the Density Functional Theory (DFT) method. The concentrations of metal ions were tested on Shimadzu ICPS-75000 inductively coupled plasma atomic emission spectrometer (ICP-AES). The Mettler Toledo S210 SevenCompact pH Meter was applied to accurately test the pH value. 2.4. Adsorption studies. Generally, the initial pH (pH0) plays the important role in the adsorption behavior. Here, we investigated this effect of CP@MS in 100 mL conical flasks by adding the CP@MS-1 film into 50 mL Zn2+ solution with C0 = 100 mg L-1 in the different pH0 from 1.0 to 7.0. These flasks were shaken in a thermostatic shaker at 25℃ for 12 h, and the corresponding equilibrium concentration of Zn2+ and Mg2+ was tested by the ICP-AES. Also, the corresponding equilibrium pH value was determined by the pH Meter. The adsorption kinetics experiments were performed in the similar process as described in our previous work.18 Typically, prior to the adsorption, the pH0 was changed to 5.0 using dilute HNO3. Later, a certain amount of the CP@MS, CP@MCH and CP films were added into the Zn2+ solution, and then the corresponding flasks were closed and shaken in the shaker for a certain time. After a certain time t, the concentration of Zn2+ after adsorption was immediately tested using the ICP-AES without any centrifugation or filtration. The amount of Zn2+ adsorbed qt was evaluated by the equation:

=

( )×

(1)



where C0 and Ct individually refer to the concentrations of Zn2+ before and after adsorption at time t (mg L-1), , V and m is the volume of Zn2+ solution (L) and the mass of adsorbent (g), respectively, qt is the quantity of Zn2+ adsorbed by the sample (mg/g). The adsorption isotherm experiments were done following the similar process as reported in our previous work.18 The equilibrium concentration of Zn2+ in the solution was determined by ICP-AES to evaluate the equilibrium adsorption capacity after 12h. A home-made equipment consisting of a PMMA pipe (Ф3 cm, 8 cm for height) and a peristaltic pump was developed to examine the practical adsorption performance and recyclability of the CP@MS film (area, 7.1 cm2; thickness, 0.019 cm for each piece of film) for Zn2+ and Cu2+ in aqueous solution. A certain pieces of CP@MS-1 films were fixed in the pipe with two PMMA pore plates, then 100 mL of Zn2+ or Cu2+ solution (C0 = 100 mg L-1, pH0 =5.0) was transported to the top of the CP@MS films in the pipe by pump (the flow rate is 25 mL min-1 for 1 piece of composite film, and 2 mL min-1 for 8 pieces of composite films), and then the concentration of Zn2+ in the solution was tested using ICP-AES. This process was repeated several times to evaluate the recyclability of the prepared composite films. The averaged values after three times’ adsorption experiments were collected for the graphs and tables in this work.

3. RESULTS AND DISCUSSION 3.1. Structure and morphologies. Figure 1a-b presents the XRD patterns of CP, CP@MCH and two CP@MS composite samples in the form of film (a) and powder (b). In Figure 1a, here, the CP sample displays two typical Bragg reflection peaks located at ca. 26.4° and 54.4°/2Ɵ for carbon; as for the CP@MCH sample, besides the diffraction peaks assigned to the CP, others well match the JCPDS card No. 70-1177 for Mg5(CO3)4(OH)2(H2O)4 (MCH) as marked by ‘*’ in the graph; for two CP@MS samples, all the peaks ascribed to the MCH are disappeared and it seems that no new peak occurs. In the inset graph, however, the two samples show typical peaks of MS [Mg3Si2O5(OH)4, JCPDS card No. 86-0404] emerged at 2Ɵ = 19.3°, 33.8°, 36.0°, 42.1°, 58.9° and 60.4° for (100), (110), (-1-11), (-1-12), (-2-12) and (300) as marked in the graph, respectively, which are consistent with those for the MS reported in literatures although the corresponding intensities are slightly low.24-25 The low intensities result from the low mass loading of the MS on the CP, which is ca. 27.3 wt% for CP@MS-1 and 27.8 wt% for CP@MS-2 based on the determination of Si element in the CP@MS by the ICP (The data determined by ICP and the calculated results were shown in Table S1, Supporting Information). The EDX analysis of CP@MCH and CP@MS samples, shown in Figure S1 (Supporting information), also exhibits the change of elements from MCH to MS. In order to further examine the solid-phase transformation, two CP@MS composite samples were ground into powder and then were determined by powder XRD. In Figure 1b, interestingly, a series of new Bragg reflection peaks occur for two CP@MS composite samples as marked in the graph, which are assigned to Mg(OH)2 (JCPDS No. 74-2220) and MgCO3 (JCPDS No. 80-0101) marked with symbols ‘o’ and ‘+’ in the graph, respectively. Furthermore, the related diffraction intensities of both Mg(OH)2 and MgCO3 in CP@MS-2 turns weaker related to CP@MS-1 with increase in the feeding dosage of Na2SiO3. That is to say, both of Mg(OH)2 and MgCO3 further reacted with Na2SiO3 to form MS under the investigated conditions because the solubility product of MS is about 1.3×10-13,26 which is much lower than 1.8×10-11 for Mg(OH)2 and 2.6×10-5 for MgCO3.27-28. Figure 1c-d further


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shows the XPS spectra of CP, CP@MCH and CP@MS-1 samples. In Figure 1c, the CP demonstrates C 1s and O 1s signals, the CP@MCH exhibits Mg 2s and 2p signals besides C 1s and O 1s, and the CP@MS displays another Si 2s and 2p signals. In comparison with the CP, the intensity of C 1s signal significantly reduces in both of CP@MCH and CP@MS, probably due to coverage of MCH and MS on CP. In Figure 1d, the XPS spectra for Mg 2p were fitted into two components for CP@MCH with MgCO3 (50.8 eV) and Mg(OH)2 (49.8 eV),29-30 and into three components for CP@MS-1 with MgCO3, Mg(OH)2 and MgSiO3 (50.2 eV).31 Furthermore, one observes the obvious reduction of signal intensities assigned to MgCO3 and Mg(OH)2 from CP@MCH to CP@MS, suggesting the transformation of MCH to MS. Therefore, the so-called solid-phase transformation method is available to prepare the CP@MS composite films from the CP@MCH solid precursor and Na2SiO3 aqueous solution.

Figure 1. (a) and (b), XRD patterns of CP, CP@MCH, CP@MS samples (a) in the form of film, and (b) in the form of powder, (c) XPS survey scans spectra of CP, MCH@CP and CP@MS-1 samples, and (d) fitted peaks corresponding to Mg 2p spectra of CP@MCH and CP@MS-1.

Figure 2 demonstrates the SEM images of CP, CP@MCH and two CP@MS samples. One observes that the CP consists of lots of carbon fibre with smooth surface and an averaged diameter of 7.88 μm. After the hydrothermal reaction between MgSO4.7H2O and HMT at 140 °C for 6h to produce the CP@MCH sample, the surface of carbon fibre is completely covered with abundant MCH micro plates with the size ca. 7∼9 μm and the average thickness ca. 0.78 μm. These MCH micro plates vertically grow on the carbon fibre with uniform size distribution and smooth outside surface. After the following hydrothermal reaction between the CP@MCH precursor and different amount of Na2SiO3 to produce two CP@MS samples, one interestedly observes a lot of porous nanosheets on the surface of MS micro plates, which are big difference from the smooth surface of the CP@MCH precursor. Undoubtedly, the porous structure



favours to improve the possible removal capacity and speed up the corresponding removal rate.32 Additionally, the averaged thickness of the MS plates increases from 1.30 μm for CP@MS-1 to 1.37 μm for CP@MS-2 resulting from more formation of MS on the carbon paper, which are in consistence with the XRD results in Figure 1.

Figure 2. SEM images of CP, CP@MCH, CP@MS-1, and CP@MS-2 samples with different magnification times.

3.2. Pore structure. Figure 3 shows the low-temperature nitrogen adsorption/desorption isotherm and the pore-size distribution of CP, CP@MCH, CP@MS-1, and CP@MS-2 samples. In Figure 3a, the CP@MCH, CP@MS-1, and CP@MS-2 samples show a typical Type H3 hysteresis loop, often resulting from loosely coherent aggregates of plate-like particles,18 and this result is in good agreement with those seen in Figure 2. Figure 3 exhibits the pore-size distribution of four samples in the range of 0.8∼200 nm. The pore-size distribution can be discussed in three sections: 0.8∼5 nm, 5∼30 nm, and 30∼200 nm. Compared with CP, the CP@MCH and the CP@MS samples have more pore in the corresponding range, particularly from 5 nm to 200 nm, which may favour to improve the adsorption capacity and the corresponding rate based on our previous results.18 Figure 3c-d further demonstrates the pore size distribution in the integrated mode. In comparison, obviously, the CP@MS samples have much more big-size pore numbers in the range of 5-200 nm related to the CP@MCH, and the CP@MS-1 does more than the CP@MS-2 from 30 nm to 200 nm, which is in consistence with those seen in Figure 2. Besides, Table 1 lists the calculated BET data, for example, the surface area and the pore volume are increased from the CP to the CP@MS-2, and further the CP@MS-1 has the largest average pore size among all of four samples.


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Figure 3. (a) Nitrogen adsorption/desorption isotherm, (b) pore-size distribution, (c) pore volume distribution of different pore size of CP, CP@MCH, CP@MS-1 and CP@MS-2, and (d) pore volume percentage in different pore size of CP@MS-1 and CP@MS-2. Table 1 BET analysis of CP, CP@MCH and CP@MS samples Sample

Surface area 2 -1 (m g )

Pore volume 3 -1 (cm g )

Average pore diameter (nm)

CP

2.5

0.0061

9.64

CP@MCH

5.0

0.0169

5.18

CP@MS-1

9.6

0.0312

13.0

CP@MS-2

10.6

0.0246

9.30

3.3. Adsorption behavior of CP@MS composite film for heavy metal ions. Figure 4 depicts the adsorption behavior of porous CP@MS films on Zn2+. As for the adsorption materials, the applicable pH rang is one of crucial parameters.3334

For example, when the pH is too high, the heavy metal ions are precipitated;35 when the pH is too low, the

adsorption materials may be dissolved and then loss the adsorption capacity.16 Figure 4a exhibits the influence of pH0 on the adsorption capacity of CP@MS-1 on Zn2+ with the initial concentration of 100 mg L-1 for 12 h. When the initial pH value was below 2.0, almost no adsorption occurred between the CP@MS-1 and Zn2+, and lots of Mg2+ ions were determined in the adsorption system, suggesting that the MS in the composite film was dissolved in this pH range; when the initial pH was increased from 2.0 to 4.0, the equilibrium concentration of Zn2+ significantly decreased from 89.3 mg L-1 to 4.3 mg L-1, the equilibrium pH obviously did from 2.4 to 6.6, and the equilibrium concentration of Mg2+



decreased from 137.0 mg L-1 to 35.5 mg L-1, suggesting that the MS in the composite film was partially dissolved and the rest of MS in the film exhibited the certain adsorption capacity; while the initial pH was in the range of 4.0∼7.0, all the three parameters was close to the constant value indicating that the MS was stable in this initial pH range. In the last pH range, interestingly, the equilibrium concentration of Mg2+ still remains 36.1∼36.8 mg L-1 in the solution probably because of the cation exchange between Zn2+ and Mg2+,36-37 and the slow decrease in the equilibrium concentration of Zn2+ was observed from 2.0 mg L-1 at pH = 6.0 to 0.9 mg L-1 at pH = 7.0 probably due to precipitation of Zn2+.38 On the other hand, the morphologies of CP@MS-1 sample were tested at three initial pH values such as 1.0, 3.0, and 4.0 in Figure S2 (Supporting Information). For example, at pH =1.0, the MS was completely dissolved; at pH = 3.0, the MS was partially dissolved and no MS nanosheet was observed; at pH = 4.0, the porous morphologies remain without destruction. That is to say, the stable adsorption is in the pH0 range of 4.0∼7.0 for MS, which is in agreement with other report.16 Therefore, all of the later adsorption tests were performed under the initial pH=5.0. Figure 4b shows the adsorption capacity of CP, CP@MCH, CP@MS-1 and CP@MS-2 for Zn2+ in the aqueous solution at pH0=5.0. As for all four samples, the adsorption of Zn2+ arrives at the equilibrium after 7 hours. Therefore, all the thermodynamics data in adsorption isotherm experiments were collected after the adsorption for 12 hours to ensure the equilibrium state later. Among the investigated samples, obviously, CP@MS-1 shows the highest adsorption capacity with the equilibrium quantity of 187.3 mg g-1, which is more than three times related to CP@MS-2. Noteworthy, the CP@MCH precursor and the CP substrate show very low and even negligible adsorption for Zn2+ under the investigated conditions. The results suggest: (1) magnesium silicate in the composite film is the active compound for the adsorption of Zn2+ since the precursor and the substrate display insignificant adsorption capacity; (2) the pore structure of the film plays the key role in the adsorption of Zn2+, especially, the big pore size favours to improve the adsorption capacity based on the comparison between CP@MS-1 and CP@MS-2. Generally, two kinetic models and one diffusion model have been well developed to investigate the adsorption mechanism. For example, the pseudo-first-order kinetic model describes the adsorption of liquid/solid system and assumes the rate of occupation of adsorption sites to be proportional to the number of unoccupied sites (eqn. (2));39 the pseudo-second-order kinetic model considers the chemical adsorption as the limiting step involving electron transfer, exchange, or sharing (eqn. (3));40 the intra-particle diffusion model involves three diffusion steps such as the external surface adsorption or the boundary layer diffusion, the intra-particle diffusion, and the final equilibrium stage (eqn. (4)).41-42 Here, Figure 4c-d and Figure S3 (Supporting Information) further demonstrate the adsorption behavior of two CP@MS samples for Zn2+ using the above three models. Also, Table 2 summarizes the calculated results on the basis of the following equations. $

% Pseudo-first-order kinetic： !(" − ) = !(" ) − &.''

Pseudo-second-order kinetic： = $

%

& & "

+

"

Intra-particle diffusion： = $)*+ √ + -

(2) (3) (4)


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where qe (mg g-1) and qt (mg g-1) are the adsorbed quantity for Zn2+ at the equilibrium state at a certain time t (h); k1 (h−1), k2 (g mg-1 h-1), and kdif (mg g-1 h-1/2) are individually the rate constant for the corresponding model in the eqns. (2)(4), and C is the intercept. Furthermore, the parameters such as k1, k2, kdif, C and qe, can be extracted from the slope and intercept.

2+

2+

Figure 4. (a) The equilibrium concentration of Zn and Mg , and the equilibrium pH value as a function of the pH0 value from 1.0 to 7.0 in 2+ -1 the adsorption of Zn by CP@MS-1 when C0 = 100 mg L and t = 12 h; (b) Variation of adsorption capacity of CP, CP@MCH, CP@MS-1, and 2+ -1 2+ CP@MS-2 with time for Zn with C0 = 100 mg L ; (c) Pseudo-second-order kinetic, and (d) intra-particle diffusion kinetics for Zn by two CP@MS film samples.

For both of two CP@MS samples, the adsorption kinetic curves match much better the Pseudo-second-order model with the better linear relationship in Figure 4c, and the relatively higher R2 value, in comparison with the Pseudo-firstorder model in Figure S3 and Table 2. In both cases, the R2 values are not well close to 1. Figure 4d further demonstrates that the adsorption behavior of two CP@MS samples matches much well the intra-particle diffusion model. Obviously, CP@MS-1 exhibits much faster diffusion in the first two steps with bigger slope compared with CP@MS-2, suggesting that the CP@MS-1 has more highly efficient surface adsorption process than the CP@MS-2 because of the higher surface utilization efficiency generated by larger pore size. The large pore size favours to accelerate the diffusion rate of Zn2+ into the inner side and react with MS.



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2+

Table 2 Pseudo-first-order adsorption kinetic and Pseudo-second-order adsorption kinetic constants for Zn adsorption

Pseudo-first-order model

qe,exp

Sample

-1

-1

-1

Pseudo-second-order model 2

-1

-1

-1

2

(mg g )

qe,cal (mg g )

k1(h )

CP@MS-1

187.3

281.3

1.657

0.967

207.0

0.467

0.995

CP@MS-2

59.82

89.51

0.708

0.867

75.48

0.497

0.947

R

qe,cal (mg g )

k2(mg g h )

R

Figure 5a shows the adsorption isotherm data of two CP@MS samples for Zn2+ after the adsorption for 12 hours, which were fitted using the Langmuir model (eqn. (5))43 in Figure 5b and the Freundlich model (eqn. (6))44 in Figure S4 (Supporting Information) in the linear form. The fitted results are listed in Tables 3 and S3 (Supporting Information), which are in better agreement with the Langmuir model. Based on the Langmuir equation and the fitted line in Figure 5b, the maximum adsorption capacity was evaluated and listed in Table 3. Langmuir:

" "

=

% ./

+

"

(5)

%/2

Freundlich： " = .0 "

(6)

where qe (mg g-1) and qm (mg g-1) is the equilibrium and the maximum adsorption quantity, respectively; Ce (mg L-1) is the equilibrium concentration; KL (L mg-1) is the Langmuir adsorption constant, KF is the Freundlich isotherm constant and n presents the adsorption intensity. One observes that the qm value remarkably increases from 67.98 mg g-1 for CP@MS-2 to 198.0 mg g-1 for CP@MS-1, probably resulting from larger pore size and more utilization surface area. In Figure 5c, on the other hand, the CP@MS-1 has also exhibited excellent adsorption properties for Cu2+, which is another typical toxic heavy metal in our life. The adsorption for Cu2+ also follows the Langmuir equation and the qm was calculated to be 113.5 mg g-1. That is to say, the developed CP@MS composite film in this work has shown promising adsorption performance to remove both of Zn2+ and Cu2+ in aqueous solution. In comparison, Table 4 summarizes the maximum adsorption quantities of some reported adsorbents for Zn2+ and Cu2+ in the literatures. The CP@MS-1 composite film exhibits the highest qm value of 198.0 mg g-1 for Zn2+ among the literatures and the higher value of 113.5 mg g-1 for Cu2+ than most of them except for PI−PS−PAA membrane58 and PDCMAA film59.In both cases, yet, the authors just investigated the adsorption capacity towards Cu2+, not Zn2+. 2+

2+

Table 3 Langmuir isotherms parameters for Zn and Cu adsorption Metal Sample KL (L/mg) qm (mg/g) R2 2+

CP@MS-1

0.291

198.0

0.998

2+

CP@MS-2

0.227

67.98

0.994

2+

CP@MS-1

0.599

113.5

0.999

Zn Zn

Cu

Besides, Figure 5d further demonstrates the nice linear relationship of the molar ratios between the released Mg2+ from CP@MS-1 and the adsorbed Zn2+ as well as the adsorbed Cu2+ during the adsorption process , which were extracted from the data in Table S3 (Supporting Information). The results consist with those described by Song and his co-authors,45 suggesting that the adsorption mechanism of CP@MS-1 for Zn2+ and Cu2+ is the cation-exchange mechanism between the interlayer Mg2+ with the guest Zn2+ and Cu2+ in a typical two dimension layered structure. 16 Furthermore, the cation-exchange mechanism further supports the intraparticle diffusion kinetic model, which is consistent with particle diffusion being the slow rate controlling step. It is worthy to noting that the permission of Mg2+


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in the drink water is 450 mg L-1,45 which is much higher than those such as 5 mg L-1 for Zn2+ and 2 mg L-1 for Cu2+. Therefore, the CP@MS composite film is one of acceptable and effective absorbents to remove Zn2+ and Cu2+ in water.

Figure 5. (a) Langmuir isotherm and (b) linear fitting of Langmuir isotherm of two CP@MS samples for Zn2+, (c) Langmuir isotherm of 2+ 2+ 2+ CP@MS-1 for Cu , inset graph presents the linear fitting result, (d) the molar ratio between Mg released from CP@MS-1 and Zn (or Cu2+) adsorbed by the CP@MS-1. 2+

2+

Table 4 Comparison of various adsorbent and the adsorption ability for Zn and Cu in the literature Adsorbents

BET area 2 (m /g)

Pore size (nm)

qm for Zn2+ (mg/g)

qm for Cu2+ (mg/g)

Ni@Mg(OH)2

124.2

5.5

36.11

-

46

Microporous AMH-3

-

-

27.97

-

47

MoS4-LDH

-

-

9.10

-

48

Magnetite nanorods

-

-

107.3

-

49

Biochar

115.5

5.88

23.26

-

50

polymer-modified Fe3O4

-

-

43.4

-

51

Fe3O4/MnO2

118

3.3

8.15

-

52

Biomass based hydrogel*

3.12

2.96

121.2

75.4

53

PVA/silica membrane

164.7

2.11

-

70.1

54

PAO/SiO2

-

-

-

88.9

55

PVP/CeO2 nanofiber

37.6

1.95

-

84.8

36

PAN-oxime nanofiber

-

-

-

52.7

56

78.03

57

260.5

58

176.0

59

Phosphorylated PAN fibers PI−PS−PAA membrane

38

PDCMAA film CP@MS-1

9.6

13.0

198.0


113.5

Ref.

This work


The best advantage of the adsorption films is able to easily adjust,60-61 continuously adsorb,62 and simply separate from the adsorption solution compared with the corresponding powder materials. We have further extended to examine the simulative continuous adsorption behavior using a home-made adsorption-filtration system as shown in Scheme 2. The results in Figure 6a suggest that the removal efficiency of Zn2+ and Cu2+ increases with the increase of film pieces. When one piece of CP@MS-1 film was used, the maximum removal efficiency was only 15.9% and 12.6% for Zn2+ and Cu2+ respectively, while if 8 pieces of CP@MS-1 were used, the removal efficiency was improved to 99.8% for Zn2+ and 98.1% for Cu2+. That is to say, an aqueous solution containing Zn2+ or Cu2+ (C0 = 100 mg L-1) was treated though 8 pieces of CP@MS-1 films to remain 0.2 mg L-1 for Zn2+ or 1.9 mg L-1 for Cu2+ in the water, which is much lower than the required limit by the WHO. Also, the permeance (F) of the CP@MS-1 film was calculated from the data as summarized in Table S4 (Supporting Information) : the F value is 278.9 L m-2 h-1 bar-1 for 1 piece of CP@MS-1 with the flow rate of 25 mL min-1, and 32.4 L m-2 h-1 bar-1 for 8 pieces of CP@MS-1 at 2 mL min-1.63 The recyclability of practical adsorbent material is another crucial property. Figure 6b further displays the recyclability of CP@MS-1 sample after seven cycles with 100 mL, 100 ppm Zn2+ solution. When one piece of CP@MS-1 film was used, after four cycles, it shows a promising recyclability with removal efficiency of 14.4%, which remains ca. 90% compared with the first recycle adsorption ability. In further experiment, eight pieces were loaded and after five cycles the concentration of Zn2+ is still lower than the limit by WHO, which is promising for practical applications.

Scheme 2. A home-made adsorption-filtration system used to test the adsorption performance of CP@MS-1 films.


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2+

2+

2+

Figure 6. (a) Removal efficiency of Zn or Cu , and (b) recyclability of Zn with different pieces of CP@MS-1 composite films in the 2+ 2+ home-made adsorption-filtration system for 100 mL aqueous solution under the initial pH = 5.0. The initial concentration of Zn or Cu -1 ions is 100 mg L .

4. CONCLUSIONS For the first time, we developed carbon paper@magnesium silicate composite adsorption films (CP@MS) by a solid-phase transformation reaction from hydromagnesite coated carbon paper (CP@MCH) precursor film in a mild hydrothermal route. The CP@MS film as one of low-cost adsorption materials shows the excellent adsorption capacity for two typical heavy metals such as Zn2+ and Cu2+ in aqueous solution with a cationexchange mechanism. For example, the maximum adsorption quantity is 198.0 mg g-1 for Zn2+ and 113.5 mg g1

for Cu2+ under the investigated conditions, respectively. The approximately initial pH range is beyond 4.0 for

the adsorption of the CP@MS for Zn2+. Furthermore, the pore size plays one of the crucial roles on the adsorption capacity of the CP@MS film. Also, the composite films exhibit promisingly practical applications for the removal of heavy metals in water by an adsorption-filtration system.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXX. Additional figures including EDX spectra of the CP@MCH and CP@MS samples, SEM images of CP@MS-1 after immersing in different pH aqueous solution, pseudo-first-order kinetic for Zn2+ by CP@MS-1, linear fitting of Freundlich isotherm for Zn2+ adsorption; tables including mass fraction of MS on CP@MS and transformation percentage from MCH to MS, Freundlich isotherms parameters for adsorption of Zn2+, and ICP data of Zn2+, Cu2+ and Mg2+ after adsorption and the molar ratio between Mg2+ released from the CP@MS-1 and Zn2+ (or Cu2+) adsorbed on CP@MS-1 film

AUTHOR INFORMATION



Corresponding Author *(Y.J. Feng) E-mail: [email protected]

ORCID Dianqing Li: 0000-0001-6761-8946 Pinggui Tang: 0000-0003-1866-7527 Yongjun Feng: 0000-0001-9254-6219

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Key R&D Program of China (No. 2016YFB0301600), Natural Science Foundation of China, and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1205).

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