Biochar Derived from Sawdust Embedded with Molybdenum Disulfide

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Biochar Derived from Sawdust Embedded with Molybdenum Disulfide for Highly Selective Removal of Pb2+ Hongshan Zhu, Xiaoli Tan, Liqiang Tan, Changlun Chen, Njud S. Alharbi, Tasawar Hayat, Ming Fang, and Xiangke Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00388 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Nano Materials

Biochar Derived from Sawdust Embedded with Molybdenum Disulfide for Highly Selective Removal of Pb2+ Hongshan Zhu,a,b Xiaoli Tan*,a,b,c Liqiang Tan,a,b Changlun Chen,b,d,e Njud S. Alharbi,d Tasawar Hayat,e Ming Fang*,a Xiangke Wanga,c

a

School of Environment and Chemical Engineering, North China Electric Power

University, Beijing 102206, P.R. China b

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, P.R.

China c

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

Institutions, Soochow University, Suzhou, Jiangsu 215123, P.R. China d

Department of Biological Sciences, Faculty of Science, King Abdulaziz University,

Jeddah 21589, Saudi Arabia e

NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia

* Corresponding author. Email: [email protected]. or [email protected] (Xiaoli Tan); [email protected] (Ming Fang).

Abstract Surface interactions between the adsorbents and heavy metal ions play an important role in the adsorption process, and appropriately decorating the material’s surface can significantly improve the removal capacity of the adsorbents. So, the objective of this study is to modify biochar by coating with molybdenum disulfide 1

ACS Paragon Plus Environment

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(MoS2) for enhancing the adsorption of Pb2+. The biochar pyrolyzed at 600 oC was chosen as the base to combine the flower-like MoS2 (MoS2@biochar) by solvothermal reaction, in which the abundant S-containing functional groups may promote the elimination of Pb2+. The prepared MoS2@biochar exhibits excellent adsorption capacity (189 mg/g) to Pb2+ in water solution. The adsorption of Pb2+ maintains a high level under the circumstance of coexisting ions (Mg2+, Ca2+, Co2+ and Cd2+), suggesting the high selectivity for Pb2+. The adsorption mechanism of Pb2+ on MoS2@biochar is mainly ascribed to the inner-sphere surface complexation, in particular, metal-sulfur chemical complexation. The easily recycled MoS2@biochar still has high adsorption capacity for Pb2+. This work demonstrates that the MoS2@biochar is an excellent candidate of adsorbent for Pb2+ removal. Keywords: Molybdenum disulfides, Biochar, Sawdust, Pb2+, Adsorption Introduction Due to the increasing discharge of industrial effluent with heavy metal ions, it is an imperative and vital assignment to search a facile way to treat heavy metal ions from contaminated water.1-3 Heavy metal ions (such as: Cr, Pb, Ni, Cd and Hg) can be accumulated in living organisms via the food chain and cause great damage to the liver, lungs, and kidneys.2 Lead ion (Pb2+) is believed as the notable toxic ion in wastewater.4,5 Even a low concentration (5 ppb) may significantly harm the human body. For these reasons, the concentration limit for Pb2+ is set by legislative regulations of the World Health Organization in potable and surface waters to be less than 0.01 mg/L.6 Recently, numerous methods, including co-precipitation, membrane filtration, adsorption, permeation, electrolysis and coagulation have been used to 2


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dispose Pb2+.7-9 However, some of the technologies are inefficient and high cost at very low Pb2+ concentration. Adsorption is considered as the most preferred technique because of the special merits of highly cost effective, easy to operate, high efficiency and availability.1,3 Adsorbents such as activated carbons, natural bio sorbent, polymer particles, nanocomposites and mesoporous materials, have been employed to eliminate Pb2+ from water solutions.9-15 However, these materials do not meet the demand of Pb2+ treating for the lack of high adsorption performance, rapid adsorption kinetics, or good recyclability. Therefore, developing environmental friendly and high performance adsorbents for Pb2+ removal from contaminated water has been an urgent concern. Biochar is a carbon-rich product by pyrolyzing of biomass resources under oxygen deficient environment.5,16 Owing to its stability and special physicochemical properties (economically favorable, high stability, large surface area and environment-friendly), biochar has been widely used in pollution treatment including heavy metal ions, organic compounds and greenhouse gases, etc.17,18 The adsorption mechanism usually relies on the combined action of physical attraction and chemical reaction (electrostatic attraction, π-interaction, ion exchange, and complexation) on the active sites of biochar surface.16,19,20 Nevertheless, the adsorption performance of raw biochar does not satisfy the need of heavy metal ions removal for the relatively less adsorption sites on the surface.19 It is known that the elimination capacity of biochar depends on the amount of surface negatively charged functional groups, which are beneficial to adsorb the positive Pb2+ ion by complexing actions or 3



electrostatic attractions. Thus, fabrication and modification of biochar are indispensable to obtain new composites with optimal performance. Nowadays, nanomaterials (such as Fe3O4, MgO, MnO2, SiO2 and MoS2) have been applied as adsorbents for heavy metal ions removal due to their high activity.5,12,21,22 Among them, the MoS2 exhibits remarkably adsorption performance and has been known as a potential material for the separation or elimination of heavy metal ions from wastewater.22-25 The mechanism responsible for Pb2+ adsorption by MoS2 is attributed to the abundant intrinsic sulfur atoms (potential binding sites) on the surface. The sulfur atom is a soft base, which is greatly beneficial to complexation with the soft acid (Pb2+) according to the theory of the Pearson Hard Soft Acid Base.23,24 However, a considerable amount of sulfur atoms inside the layer-stacked MoS2 could not bind Pb2+.24,26 If large specific surface nanostructures can be obtained, most of the S atoms may be exposed and thus improve the adsorption ability to the highest extent. Therefore, a strategy was implemented to improve the adsorption ability by combining MoS2 nanosheets on the surface of biochar (MoS2@biochar), to expose more S-containing groups by a one-pot solvothermal method. The biochar was obtained by a slow pyrolysis reaction. The as-prepared MoS2@biochar can serve as an excellent material to remove Pb2+ from contaminated water (Scheme 1). The influence of environmental condition, reusability and the interaction mechanisms between MoS2@biochar and Pb2+ were investigated. This work presents an alternative and satisfactory strategy towards highly selective removal of Pb2+. 4


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Scheme 1. Schematic diagram of the synthesized and adsorption process.

Experimental section Materials and reagents The sawdust was purchased from Sunshine Pet Products Company (Zaozhuang city of Shandong province, PR China). The ammonium tetrathiomolybdate ((NH4)2Mo2S4), hydrochloric acid (HCl), and lead chloride (PbCl2) were supplied by Sinopharm Chemical Reagent Co. Ltd. Chemicals were analytical reagent grade, and the Milli-Q water (>18.2 MΩ) was prepared by Millipore (Milli-Q Academic) water purification system in this experiment. Preparation of biochar and MoS2@biochar The biochars were produced via a facile pyrolysis of sawdust at temperature 400, 500, 600, 700 and 800 oC for 8 h in a tube furnace under argon atmosphere (0.5 L/min). Then the obtained pristine biochars were naturally cooled down to ambient temperature.

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The (NH4)2Mo2S4 (1.0 g) and biochar (1.0 g, pyrolytic temperatures at 600 oC) were added into 100 mL Milli-Q water in a 200 mL beaker. The pH was adjusted to 5.5. Then the mixture was introduced into a Teflon-lined stainless stell autoclave and kept at 200 oC for 8 h. After that, the obtained sample was centrifuged, and rinsed with Milli-Q water and ethyl alcohol repeatedly, then freeze-dried under vacuum before collection. To optimize the experimental results, samples with different mass ratios of (NH4)2Mo2S4 to biochar (1:4, 1:2, 1:1, 2:1, 4:1) were fabricated and named as

MoS2@biochar(1:4),

MoS2@biochar(1:2),

MoS2@biochar(1:1),

MoS2@biochar(2:1), and MoS2@biochar(4:1), respectively. In this present work, the biochar without special note means that pyrolyzed at 600 oC, and the MoS2@biochar means the mass ratios to be 1:1. Characterization The morphologies and micro-structures of the MoS2@biochar and biochar were investigated by using scanning electron microscopy (SEM, S-2500) and transmission electron microscopy (TEM, JEM-2010). The phase of the materials was studied through X-ray diffraction (XRD, D/Max-Ra) with Cu-Kα source (λ = 1.54178 Å). The functional groups on the surface of MoS2@biochar were confirmed with a Fourier transformed infrared spectrometer (FT-IR, Nicolet 8700, Thermo Scientific) at a resolution of 4 cm-1 and Raman spectroscopy (RAMANLOG 6, SPEX) at 25 oC. Thermogravimetric analysis (TGA) was performed by using a thermal analyzer (Shimadzu) under N2 atmosphere in the temperature range 40-800 oC. The specific surface area was obtained on a NOVA 4200e instrument under N2 atmosphere and 6


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calculated by Brunauer-Emmett-Teller method. Zeta potentials for biochar and MoS2@biochar at different pH were recorded by a Nanosizer instrument (Malvern Instrument Co.) at 298K. The X-ray photoelectron spectroscopy (XPS) measurement was conducted using ESCALAB 250 with Mg Kα as the source. Batch adsorption experiments The batch experiments of Pb2+ on biochar and MoS2@biochar were conducted in 10 mL polythene tubes. Briefly, the PbCl2 (360 mg/L) solution, and biochars and MoS2@biochar (0.9 g/L) suspensions were prepared via dissolving or dispersing in Milli-Q water, respectively. The influence of ionic strength (0.001-0.1 mol/L NaCl) was also investigated. The pH value was adjusted by using ignorable NaOH or HCl aqueous solution with concentration of 0.1 mol/L, and then the mixture was shaken for 24 h. Finally, the solid substance was separated from the suspensions via centrifugation. The concentration of Pb2+ was measured through spectrophotometry (λ = 616 nm) with chlorophosphonazo complex. The

reusability

of

MoS2@biochar

was

investigated

through

adsorption-elution-regeneration experiments. After adsorption, the MoS2@biochar was regenerated in 0.5 mol/L ethylenediaminetetraacetic acid (EDTA) solutions with shaking for 2 h. The regenerated MoS2@biochar was then applied in the recycle adsorption study. The experiments were carried out for seven times with the similar processes. The adsorption percentage (%), adsorption capacity (Cs, mg/g) and the

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distribution coefficient (Kd) were calculated from the following equations:19,21

Adsorption (%) =

Co－Ce ×100 % Co

(1)

Cs =

Co－Ce ×V m

(2)

Kd =

Co－Ce V × Ce m

(3)

where Ce and C0 (mg/L) are the equilibrium and initial concentrations, respectively. V (mL) is the volume of suspension, and m (g) represents the mass of adsorbents. The data used in the pattern were the average of three same experiments with the error bars less than 5%. Results and discussion

Figure 1. SEM images of biochar (A and B) and MoS2@biochar (C and D), the enlarged view of the red frame of D (E and F); TEM of MoS2@biochar (G), HR-TEM (H); and EDS mapping of MoS2@biochar (I-L).

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The morphologies and micro-structures of biochars and MoS2@biochar were investigated by SEM and TEM. The biochars were in a large scale of approximately several tens of micrometers (Figure 1A, B and Figure S1) with relative smooth with some flakes on the surface, which is suitable for MoS2 loading. The macroscopic carbon skeletons of biochars were not changed obviously with increasing the pyrolysis temperature. After hydrothermal reaction, MoS2 nano structures (the white substance) grew on the surface of biochar (Figure 1C, D). Further enlarged SEM images (Figure 1E, F) demonstrated the MoS2 was flower-like nanostructure constructed by irregular nanosheets. TEM revealed more structure information of the MoS2@biochar (Figure 1G). The interplanar distance of high-resolution transmission electron microscopy (HRTEM) image was measured to be 0.26 nm corresponding to the (100) plane of MoS2 (Figure 1H).27 The images of the energy dispersive X-ray spectroscopy (EDS) mapping showed the elemental distribution of C Ka 1, O Ka 1, S Ka 1 and Mo Ka 1 (Figure 1I-L), which all were similar to the profile of Figure 1G. Furthermore, the outline of S Ka 1 matched with the Mo Ka 1, implying the MoS2 was deposited onto biochar successfully. The phases of the biochar and MoS2@biochar were analyzed by XRD (Figure 2A). For the biochar sample, there were two relatively broad diffraction peaks at approximately 2θ = 23o and 44o, which can be assigned to the disordered graphitic (002) plane and the crystalline graphite structure (101) plane, respectively.23,25 The appearance of (101) diffraction peak implied the relatively high graphitization degree and structure stability of the biochar.23,25 The new peaks of the MoS2@biochar at 2θ = 9



13.20, 32.80, 35.26, 41.66 and 57.70o can be indexed to the MoS2 planes of (002), (100), (103), (015) and (110), respectively.28,29 Additionally, the intensities of biochar at approximately 23o and 44o became weak for MoS2@biochar pattern, which was due to the surface of biochar was covered up by the MoS2.

Figure 2. XRD of biochar and MoS2@biochar (A), FT-IR spectra of biochar and MoS2@biochar (B), Raman spectra of biochar and MoS2@biochar (C), TGA of sawdust (D), BET of biochar (E); and zeta potential of biochar and MoS2@biochar (F).

The biochar and MoS2@biochar were examined by FT-IR to identify the functional groups attached to the surface of the samples (Figure 2B). The wide band 10


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at about 3413 cm-1 was attributed to the hydrogen-bonded O-H stretching vibration.16 The bands at ~ 2916 and 2850 cm-1 were assigned to asymmetric and symmetric C-H stretching vibrations of -CH2-. These bands decreased after the pyrolysis processing, indicated the fatty alkyl chain in biochar decreased.30,31 The bands at 1701 and 1438 cm-1 belonged to the C=O stretching vibration from the various oxygen functional groups.32,33 The bands at 1606, 1336, 875, 797, and 692 cm-1 were attributed to the C=C stretching, C-H stretching (polysaccharides and cellulose), aromatic hydrogen C-H stretching, =C-H stretching, and deformation vibrations of the benzene rings, respectively.32,34-37 For the MoS2@biochar, the weak band appeared at ~480 cm-1, corresponded to the Mo-S stretching vibration, suggested that MoS2 was successfully coated on the surface of biochar.38,39 Raman spectroscopy was also employed to characterize biochar and MoS2@biochar (Figure 2C). Two strong peaks at ~1352 and ~1600 cm−1 originated from the disordered D band (disordered structure) and graphitic G band (graphitic structure) of carbon, respectively.19 The ratio of ID/IG can be used to describe the graphitization degree of biochar, the ratio of about 1.31 meant the biochar has high graphitization degree. In addition, the values of ID/IG gradually increased as the pyrolysis temperature increasing from 400 to 800 oC (Figure S2). Compared with biochar, new characteristic peaks at ~382 and ~407 cm−1 can be observed in MoS2@biochar, which were ascribed to the E2g1 (an in-plane motion of Mo and S atoms) and A1g (an out-of-plane vibration of Mo and S) modes of MoS2.29 Thermogravimetric analysis was implemented to analyze the content of sawdust 11



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in nitrogen atmosphere (Figure 2D). The weight loss below 200 oC was owing to the loss of water. Then the weight loss in the range from 200 to 300 oC, should be ascribed to the loss of organic ligands on the surface of the sawdust. A sharp decrease appeared from 300 to 400 oC, corresponding to the combustion of carbon matrix. For the temperature from 500 to 600 oC, there was no obvious weight loss. The N2 adsorption-desorption isotherms of biochar were shown in Figure 2E. The pore diameters were found to be 2-50 nm. With the pyrolysis temperature increasing from 400 to 600 oC, the N2 adsorption capacity increased due to the increase of pore volume (Figure S3 and Table S1). Further increasing the temperature to 800 oC, the N2 adsorption capacity decreased owing to the collapse of the carbon skeleton and the decrease of the total pore volume.31,40,41 The large difference between the adsorption and desorption isotherms at low pressure could be attributed to the presence of irregular mesoporosity (such as bottle shaped pores and slit pores), which hindered the N2 desorption from the pores. The zeta potentials of biochar and MoS2@biochar were shown in Figure 2F. The zeta potentials of biochar and MoS2@biochar varied from positive to negative with pH

increasing.

This

was

ascribed

to

the

introduction

of

deprotonated

oxygen-containing functional groups, as well as the deprotonated sulfur. The point of zero charge (pHPZC) values of biochar and MoS2@biochar were 3.3 and 3.2, respectively. XPS was used to study the surface properties including the chemical states and the surface oxygen-containing functional groups. The peaks of Mo and S elements 12


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confirmed the successful combination of MoS2 on the surface of biochar (Figure S4A). The peaks of Mo3d3/2, Mo3d5/2, and S2p1/2, S2p3/2 can be further investigated in the high-resolution Mo3d and S2p curves (Figure S4B, C), respectively.27,42-45 The high-resolution C1s curve for the MoS2@biochar was divided into four individual component peaks (Figure S4D), which is corresponded to C-O, C-C/C=C, O=C-O, and C=O, respectively.17,31,33,43 The high-resolution O1s curve for the MoS2@biochar was also investigated (Figure S4L), with three peaks corresponding to the C-O, O=C-O, and C=O, respectively.31,44 The high-resolution C1s and O1s curves of the biochars was similar to the MoS2@biochar (Figure S5). Interestingly, the C/O atomic ratio decreased from 400 to 800 oC (Figure S4A), indicating the high temperature carbonization was unfavorable for enhancing the oxygen-related surface functional groups. All results confirmed that the S functional groups from MoS2 were exposed on the surface of the MoS2@biochar, while amount of oxygen-related functional groups (-COOH, -OH) still coexisted. All of them contributed to the Pb2+ adsorption.

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Figure 3. Adsorption isotherms of Pb2+ on biochar (150 mg/L) and MoS2@biochar (150 mg/L) (A and B) (the solid line was simulated by Langmuir model, and the dash line by Freundlich model), linear plots of lnKd versus Ce (C), and ln Ko versus 1/T of Pb2+ adsorption on MoS2@biochar (D). pH = 5.0, [NaCl] = 0.001 mol/L.

To investigate the adsorption performance and possible removal mechanism of biochars and MoS2@biochar, the adsorption isotherms of Pb2+ onto biochars and MoS2@biochars were displayed in the Figure 3A. The amount of Pb2+ adsorbed on the adsorbents increased with increasing Pb2+ concentration, because an increase in collisions between high concentration of Pb2+ and the active sites of the adsorbents. The MoS2@biochar enhanced Pb2+ adsorption capability compared with biochar due to the excellent adsorption of MoS2. Langmuir and Freundlich models were utilized to analyze the data,21 and Table S2 showed the thermodynamic parameters. The Langmuir and Freundlich models are shown as Eqs. (4) and (5), respectively.21 14


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Ce =

bCmax Ce 1 + bCe

(4)

Cs = K FCen

(5)

Where b is the constant of the equation. Cmax is the maximal amount of Pb2+ attached onto adsorbents. KF and 1/n are the adsorption performance and the degree of dependence of adsorption. The adsorption data for Pb2+ on biochar were fitted with Langmuir model well over the Freundlich model (Figure S6). The removal performance of the biochar may coordinate with Pb2+ through carboxyl (-COOH), hydroxyl (-OH) and/or Pb-π interaction.16 Although the oxygen-containing functional groups decreased with increasing temperature, the adsorption of biochar is mainly due to its large surface area and abundant aromatic groups.16 As the pyrolysis temperature increased to 700 oC, the structure began to collapse, and the surface area and benzene ring structure decreased. The adsorption process of various MoS2@biochar (4:1, 2:1, 1:1, 1:2, 1:4) was also simulated by Langmuir model (Figure S7). The maximum adsorption

capacities

of

MoS2@biochar(1:4),

MoS2@biochar(1:2),

MoS2@biochar(1:1), MoS2@biochar(2:1), and MoS2@biochar(4:1) were 112, 157, 189, 205, and 209 mg/g, respectively. The high adsorption capacity was mainly attributed to the interaction between the S atom and the Pb2+. It is worth to note that with increasing the content of MoS2, the adsorption capacity increased rapidly, and then became slowly due to the aggregation of the MoS2 on biochar. Therefore, MoS2@biochar(1:1) was employed to investigate the adsorption performance for Pb2+ at the following experiments. 15



Figure 3B showed the equilibrium adsorption isotherms of MoS2@biochar at different temperature (15, 25, and 35 oC) and the thermodynamic parameters were presented in Table S2. The correlation coefficient (R2), residual sum of squares (RSS), and chi-squared distribution (χ2) calculated by Langmuir model were better than that by Freundlich, implying the Langmuir model was available to describe the adsorption of MoS2@biochar. Furthermore, the Pb2+ may interact with the uniform binding sites on the MoS2@biochar surface.46 The maximum adsorption capabilities of Pb2+ on MoS2@biochar were 177, 189, and 204 mg/g at 15, 25, and 35 oC, respectively. High temperature can increase the adsorbate’s diffusion rate and the equilibrium adsorption capacity of the MoS2@biochar. By comparison to the previous studies (Table S3), the MoS2@biochar had a superior adsorption capacity toward Pb2+, which was due to the enhancement of the Pb-sulfur complexation.22 The results clearly indicated the application potential of the MoS2@biochar as an effective adsorbent for the uptake of Pb2+ from the polluted water. To well study the adsorption mechanism of Pb2+ onto MoS2@biochar, the changes of enthalpy (∆H0), Gibbs free energy (∆G0) and entropy (∆S0) (see supporting information) were shown in Figure 3C, D and Table S4 by thermodynamic equilibrium calculation.21,46 The positive ∆H0 value suggested an endothermic reaction for the adsorption, indicating the dehydration energy was higher than the heat release of the Pb2+ adhering to the [email protected],48 The negative ∆G0 values revealed a spontaneous process, and the positive ∆S0 value implying the great disorderness of the solid-solution systems.47,48

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Figure 4. Adsorption kinetics of Pb2+ on MoS2@biochar (150 mg/L). Pseudo-second-order kinetic model (A), and intraparticle diffusion model (B). pH = 5.0, [Pb2+] = 60 mg/L, [NaCl] = 0.001 mol/L, T = 25 oC.

The adsorption kinetics for Pb2+ onto the MoS2@biochar was shown in Figure 4A. The Pb2+ adsorption increased quickly during the first 35 min due to the abundant active sites on the surface of MoS2@biochar, then increased slowly until reaching equilibrium owing to the diffusion into pores (interior surface interaction) and the relatively low residual concentration of Pb2+. For interpreting the mechanism of the adsorption, the experimental data were fitted by the pseudo-second-order and intraparticle diffusion models (see Supporting Information) (Figure 4A, B), and the calculated relevant parameters were summarized in Table S5.21 The R2, RSS, and χ2 suggested the pseudo-second-order model described the kinetics adsorption well, and a chemisorption process (strong chemical forces) was the rate limiting step for Pb2+ removal.21,49 To further describe the diffusion mechanism, the intraparticle diffusion model was utilized to simulate the adsorption data. Especially, the values of intercept (A) can reflect the extent of the boundary layer thickness (the large A values indicate a great 17



boundary layer effect).50 The fitting results of Ct versus t1/2 according to the intraparticle diffusion equation was shown in Figure 4B. It should also be noted that the curve did not across the zero. Thus, the reaction rate was affected not only by intraparticle diffusion.50 According to the fitting parameters (R2id, RSS, and χ2), four steps of the adsorption process can be obtained. In the first step, the Pb2+ can be rapidly attached (higher kid) on the surface of MoS2@biochar due to the small A value. Then the removal process was subject to intraparticle, pore diffusion for the second step and the relatively low residual concentration of Pb2+ for the third step due to the uptake rates kid decelerated. Furthermore, the values of A at the second and third steps were 27.4 and 44.6 mg/g, respectively. So, the adsorption can also be influenced by the transfer of Pb2+ from the boundary layer from liquid phase to MoS2@biochar. A plateau at the fourth step indicated the adsorption equilibrium due to the binding sites were fully occupied by Pb2+ ions. The above analysis suggested that the multiple rate limited steps of chemisorption, physisorption and intraparticle diffusion may control the adsorption of Pb2+ on the MoS2@biochar. The functional groups of MoS2@biochar and the stratified structure of MoS2 may have important positive and efficient effect on the adsorption of Pb2+.

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Figure 5. Effect of solution pH on Pb2+ adsorbed on MoS2@biochar (150 mg/L) at 0.1, 0.01 and 0.001 M NaCl (A); the effect of adsorbent content on Pb2+ adsorbed on MoS2@biochar (B). [Pb2+] = 60 mg/L, T = 25 oC.

The adsorption of adsorbents for heavy metal ions is usually sensitive to the pH of aqueous solutions owing to the effect of pH on the existence forms of metal ions and the surface chemical behaviors of adsorbents. The adsorption of MoS2@biochar versus pH were studied in the range of 2.0-6.0 to avoid the hydroxide precipitation of Pb2+ (Figure 5A).51 It revealed that the Pb2+ adsorption on MoS2@biochar increased dramatically with the pH increasing, and then increased slowly as pH was higher than 6.0. At pH Cd2+ > Co2+ > Ca2+ > Mg2+ > Mg2+ and Ca2+. The high selectivity can be attributed to: (1) Compared with Ca2+ (-1505 kJ/mol), Mg2+ (-1830 kJ/mol), Co2+ (-1915 kJ/mol) and Cd2+ (-1755 kJ/mol), Pb2+ had a relatively low Gibbs free energy of hydration (-1425 kJ/mol). The low hydration energies of divalent cations were adsorbed preferably on the surface of the adsorbents in contrast with high hydration energies;23 (2) The adsorption process mainly relied on metal-sulfur complexation. The order of selectivity towards the Pb2+, Cd2+, Co2+, Ca2+, and Mg2+ was in accordance with the trend of softness.22 Thus, the specific selective adsorption of MoS2@biochar can be ascribed to the strong soft-soft interactions of Lewis acid and base. It is well known that the contribution of electrostatic attraction for adsorbents is small compared to inner-sphere complexation. In consideration of practical application, the recycle and desorption performance of MoS2@biochar were studied. The chemical bond between Pb2+ and MoS2@biochar was strong, thereby EDTA (as a strong ligand and non-destructive regeneration chelating agent to Pb2+) may be a suitable agent used for the desorption of MoS2@biochar. After treating by 0.5 M EDTA (Figure 8B), the adsorption performance of MoS2@biochar still maintained high level for Pb2+, only a slight decrease was observed during seven times cycling. Thus, the MoS2@biochar can be repeatedly used for the removal of Pb2+ ions from contaminated water. Conclusions In conclusion, the MoS2@biochar has been successfully fabricated by a one-pot facile solvothermal method, and the MoS2@biochar exhibited outstandingly capturing 24


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capacity (189 mg/g), extraordinary affinity and excellent selectivity to Pb2+. The enhancement of the adsorption can be ascribed to the Pb-S complexation (the strong soft-soft interactions of Lewis acid and base). Intraparticle diffusion model and pseudo-second-order kinetic can be used to well describe the adsorption process, and the adsorption of Pb2+ on the MoS2@biochar was controlled by multiple rate limited steps of physisorption, chemisorption and intraparticle diffusion. The adsorption performance has been kept a high level in various environments including coexisting ions (Mg2+, Ca2+, Co2+, and Cd2+), and the MoS2@biochar had a good recycle property by using EDTA as an eluent. These results confirmed the MoS2@biochar could be a promising candidate for the treatment of Pb2+ in practical applications. Acknowledgments Financial supports from National Natural Science Foundation of China (U1607102, U1504107, 11675210), Science Challenge Project (TZ2016004), the Fundamental Research Funds for the Central Universities (2018ZD11), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions are acknowledged. Supporting Information Supporting text, Figures S1 to S9, Tables S1 to S5, and references.

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TOC

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Biochar Derived from Sawdust Embedded with Molybdenum Disulfide

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