Few-layered metal-organic framework nanosheets as a highly selective and efficient scavenger for heavy metal pollution treatment

Journal Pre-proofs Few-layered metal-organic framework nanosheets as a highly selective and efficient scavenger for heavy metal pollution treatment Jie Li, Qingyun Duan, Zheng Wu, Xuede Li, Ke Chen, Gang Song, Ahmed Alsaedi, Tasawar Hayat, Changlun Chen PII: DOI: Reference:

S1385-8947(19)32601-4 https://doi.org/10.1016/j.cej.2019.123189 CEJ 123189

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 July 2019 1 October 2019 14 October 2019

Please cite this article as: J. Li, Q. Duan, Z. Wu, X. Li, K. Chen, G. Song, A. Alsaedi, T. Hayat, C. Chen, Fewlayered metal-organic framework nanosheets as a highly selective and efficient scavenger for heavy metal pollution treatment, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123189

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Few-layered metal-organic framework nanosheets as a highly selective and efficient scavenger for heavy metal pollution treatment Jie Lia*, Qingyun Duana, Zheng Wua, Xuede Lia, Ke Chenb, Gang Songc, Ahmed Alsaedid, Tasawar Hayatd, Changlun Chenb*,d a School

of Resources and Environment, Anhui Agricultural University, Hefei 230036,

P. R. China bKey

Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, P. R. China cGuangdong

Provincial Key Laboratory of Radionuclides Pollution Control and

Resources, School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China dNAAM

Research Group, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia *Corresponding

author. E-mail: [email protected] (J. Li), [email protected] (C.

Chen). Phone: 86-551-65592788, Fax: +86-551-65591310.

1

ABSTRACT: Current techniques trialed for the sequestration of heavy metals (HMs) are typically metal ion specific. The deployment of adsorbents as amenable choices for the simultaneous elimination of several HMs remained a big challenge. Herein, few-layered CoCNSP nanosheets with periodically aligned sulfur species were exfoliated from layered metal-organic frameworks (MOFs) through a facile-operating method. Laboratory scale experiments revealed that CoCNSP nanosheets showed selective uptake in the order of Co(II), Zn(II), Cd(II), Ni(II) << Cu(II) < Pb(II) < U(VI) < Hg(II) in both single-component and multi-component systems. Attractively, CoCNSP nanosheets showed high distribution coefficients (Kd~106–107 mL/g), fast capture dynamics (< 40 min), and ultrahigh uptake capacities (716, 661, 534, and 325 mg/g for Hg(II), U(VI), Pb(II), and Cu(II), respectively). Further, CoCNSP nanosheets could tolerate the influence of ionic strength (up to 100 mM), maintained high adsorption efficiency over a wide pH range, and showed good recycle performance for three times. X-ray photoelectron spectroscopy and theoretical density functional theory calculations demonstrated that the superior adsorption ability dominantly originated from the inner-sphere coordination between sulfur species and HMs. Finally, the traits of simple synthetic method, low-cost and non-toxic raw ingredients, as well as remarkable capture performance, make CoCNSP nanosheets superior to other scavengers. These findings reflect the important synthetic achievement in the deployment of MOFs for alleviating the environmental problems caused by HMs. Keywords: CoCNSP nanosheets; Heavy metal ions; Sulfur species; Scavenger 2

3

1. Introduction Water polluted by toxic heavy metals (HMs) has been an environmental concern throughout the world because of their harmful effects on human beings and aquatic lives [1, 2]. HMs, especially cadmium, lead, and mercury have attracted the greatest concern due to their high toxicity and prevalence [3]. The urgency and challenge of effective removal of HMs from wastewater have been stressed previously [4]. Collectively, effective capture and removal of HMs are mandatory. As a facile, low-cost, efficient, and scalable separation method, the adsorption method is optimal among all the techniques trialed to alleviate metal pollution, especially the excellent uptake efficiency from dilute solutions [5-9]. Classical adsorbents, such as clays [10], zeolites [11], metals oxides [12], biochar [13], polymers [14] and biomaterials [15] generally suffer from slow kinetics, low capacities and low selectivity. Additionally, their selectivity would be disturbed seriously under the interference of coexisting background electrolytes (Ca2+, Mg2+, K+, and Na+). Furthermore, their stability and/or adsorption performance may also be affected by the alternation of water chemistry, including solution pH, ionic strength, competitive organic matters, etc. These bottlenecks can be traced to the chemo-physical interaction between adsorbents and HMs: (i) the physical adsorption triggered by electrostatic interactions was susceptible to solution pH and ionic strength; and (ii) the chemical adsorption of transitional metals was widely caused by coordination actions via surface oxygen-containing functionalities. Based on the principle of hard and soft acids and bases (HSAB), oxygen-containing functionalities 4

usually act as hard Lewis acid, which are prone to bind the hard Lewis base (e.g., Ca2+, Mg2+, K+, and Na+) rather than soft Lewis base (e.g., Pb2+, Hg2+ and Cd2+). Different from oxygen-containing groups, thio groups (S2−) are soft Lewis acid according to the principle of HSAB and are beneficial for the selective interaction with late transition HMs because of the soft-soft interaction effect. Layered metal sulfides [16-18], thio-functionalized layered double hydroxide [2, 19], thio-modified covalent organic frameworks [20] and porous organic polymer [21] as well as MoS42- intercalated polypyrrole [1] have therefore been developed and proved their effective capture abilities for HMs based on this concept. Some of these adsorbents, however, still face sorts of challenges, such as improper distribution of functionalities, poor stability in a broad pH range, etc. which necessitates the design of advanced materials for efficient removal of HMs. Metal-organic frameworks (MOFs) are a kind of well-known porous materials with promising applications in the extraction of various toxic HMs [5, 22]. The tunable nature of MOFs favors for alleviating environmental issues caused by toxic HMs. Readily accessible surfaces and densely populated chelating groups via purposeful design are pivotal to MOFs as scavengers. Accordingly, the particle architecture of MOFs and their readily accessible surfaces have a crucial impact on the adsorption dynamics. The total number and kinds of adsorption sites determine their adsorption capacity and selectivity. MOFs now available for the removal of HMs mainly have three-dimensional (3D) bulk structures on a micrometer scale. Their performances are often compromised by the deeply wrapped or buried interior active 5

sites. Conversely, two-dimensional (2D) MOFs showing the traits of both 2D layered nanomaterials [23] and MOFs [24, 25] are anticipated to overcome these abovementioned drawbacks: (i) large surface areas expose much more surface functional groups; (ii) highly open structure prompts more inner species exposed as readily accessible functional groups; (iii) periodic distribution of chelating groups in 2D MOFs facilitates their cooperation in metal binding, and (iv) the periodic structure of MOFs favors rational improvement and investigating the relationship between structure and performance. In this context, we believe that 2D MOFs can provide an alternatively attractive platform that combats the problems of improper distributed or buried functional groups occurred in other materials, thus showing great promise for mitigating environmental pollution. To the best of our knowledge, only one study reported very recently has focused on the potential application of 2D MOFs for HM removal [26]. Herein, few-layered sulfur-functionalized MOF (abbr. CoCNSP) nanosheets were prepared through a facile-operating exfoliation strategy from the precursor of layered bulk Co(CNS)2(pyz)2 (pyz = pyrazine) MOF (Fig. 1). The as-prepared CoCNSP nanosheets were further tested for their ability to adsorb and remove eight kinds of HMs (i.e., Hg(II), U(VI), Pb(II), Cu(II), Cd(II), Ni(II), Zn(II) and Co(II)) from aqueous solutions. We observed high distribution coefficients, rapid dynamics and large adsorption amounts for Hg(II), U(VI), Pb(II) and Cu(II) in both mono-component and multi-component capture experiments. We also observed superior endurance in high ionic strength, excellent adsorption performance in a wide 6

pH range, and good recycle performance for three times. The interaction mechanisms between HMs and CoCNSP nanosheets were explored on the base of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculation, which determines the important features that made CoCNSP nanosheets a kind of superior adsorbent for environmental remediation applications. Our studies therefore put forth a promising design strategy in improving the adsorption efficiency of MOFs for addressing environmental issues.

2. Materials and methods 2.1. Preparation of few-layered CoCNSP nanosheets Layered bulk Co(CNS)2(pyz)2 MOF was prepared based on the reported procedures [27]. For the preparation of few-layered CoCNSP nanosheets, 120 mg of the obtained bulk precursors were weighed into 240 mL of ultrapure water. Then the layered precursors were ultrasonically exfoliated for one hour (Brandson, KQ-600B, 110 W, 40 KHz); the precursors after ultrasound were further stirred vigorously for 8 h and kept standing for one day. After removing the unexfoliated sediments by centrifugation, the upper colloidal suspension was rotary-evaporated at room temperature to achieve the few-layered CoCNSP powders. 2.2. Characterization The crystalline structures of the obtained products were identified by using X-ray powder diffractions (PXRD) on a Rigaku smartlab 9 KW diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphology of the few-layered CoCNSP nanosheets was observed by using a transmission electron microscope (TEM; Tecnai-G2 20 7

E-TWIN 200 kV). Atomic force microscopy (AFM) measurements were performed by using a Bruker multimode-8 scanning probe microscope. The zeta potential values of the few-layered CoCNSP nanosheets were conducted by using a Zeta Potential Analyzer (Brookhaven Instrument Corp., Holtsville, New York). The Fourier transform infrared spectroscopy (FTIR) spectra of the original CoCNSP and postadsorption samples were obtained on a Nicolet Magana-IR750 spectrometer. XPS spectra of the products and postadsorption samples were conducted on a Thermo VG RSCAKAB 250X. 2.3. Metal ion capture experiments Uptake experiments were carried out in a series of solutions spiked with individual metal ion (i.e., Co(II), Cu(II), Zn(II), Pb(II), Cd(II), Hg(II), U(VI) or Ni(II)) from their nitrate salts or their mixture at bench level. Once added the CoCNSP nanosheets into the solutions for a certain time, the supernatant solutions were obtained by centrifugation. The concentrations of HMs were measured by using inductively coupled plasma-optical emission spectrometer (ICP-OES) or ICP-mass spectroscopy (ICP-MS) (≤1 ppb). The mixture pH values were adjusted by using negligible volumes of 0.01 M HCl or NaOH. Experiments containing all eight HMs were conducted to study the selectivity of CoCNSP nanosheets. To be specific, an ~10 mg/L concentration for each ion was mixed with 0.03 g of CoCNSP nanosheets (V/m = 1000 mL/g). The adsorption capacity for Cu(II), Pb(II), Hg(II) or U(VI) was obtained by conducting uptake experiments under gradient concentrations at 25 oC with a contact time of 24 h. Uptake dynamic experiments of Cu(II), Pb(II), Hg(II) and 8

U(VI) were performed within different contact time (10−540 min). Specifically, 0.03 g of CoCNSP nanosheets was weighed into 30 mL of 10 mg/L aqueous metal ion solution (V/m = 1000 mL/g). The HMs in the suspensions were centrifuged and analyzed after scheduled time intervals. For the recycle experiments, an ~10 mg/L aqueous metal ion solution was mixed with 0.03 g of CoCNSP nanosheets (V/m = 1000 mL/g). After the adsorption was finished, the postadsorption sample was stirred in thioglycol solution (60 mL) for 60 min, then separated and purified by ultrapure water for next use. For the breakthrough experiment, about 3.0 g of CoCNSP nanosheets were packed into a column with an inner diameter of ~10 mm. A Hg(II) aqueous solution with an initial concentration of 5 mg/L was passed through the column with a flow of 2 mL/min. The Hg(II) concentrations in the effluent were measured by using ICP-MS. The distribution coefficient (Kd, mL/g) was calculated according to the formula of Kd = ([(C0 − Cf)/Cf] × V)/m, where C0 and Cf (mg/L), respectively, represent the initial HM concentration and the final one. V (mL) represents the volume of solution, and m (g) represents the adsorbent amount. The adsorption rate (%) is obtained according to the formula of (C0 − Cf)/C0 × 100. The adsorption amount (qm) is measured according to the formula of qm = (C0 − Cf) × V/m. 2.4. Theoretical calculations DFT calculations were applied to probe the coordination modes of HMs by CoCNSP nanosheets at the atomic level. A single cluster model of CoCNSP unit was chosen to save computational efficiency (Fig. 1). All studied structures were 9

optimized by the M06-2X function with the Gaussian 09 software package. The 6-31G (d, p) basis set was used for light atoms (C, H, N, O) and the Stuttgart−Dresden 1-electron and 19-electron ECPs (SDD) basis set was used for Co(II), Pb(II), Hg(II) and U(VI) atoms. The Solvation Model based on Density model (SMD) was employed to perform the Self Consistent Reaction Field-effect (SCRF) in a solution environment. The adsorption energy (Ead, kcal/mol) was calculated according to the equation of Ead = ECoCNSP + ECo(II)/Pb(II)/Hg(II)/U(VI) – ECoCNSP+Co(II)/Pb(II)/Hg(II)/U(VI). The atomic dipole moment corrected Hirshfeld (ADCH) charge analysis was applied to investigate the changes of electron density after CoCNSP contacting with Co(II), Pb(II), Hg(II) and U(VI). The electrostatic potential (ESP) and reduced density gradient (RDG) methods were applied to further elucidate the interaction between CoCNSP and HMs [28]. By using VMD version 1.9.1 [29], we achieved the ESP-mapped van der Waals (vdW) surface and the RDG isosurfaces.

3. Results and discussion 3.1. Characterization of the CoCNSP nanosheets The schematic diagrams for the synthetic procedures and molecular structures of few-layered CoCNSP nanosheets are graphed in Fig. 1. Firstly, pink-colored Co(CNS)2(pyz)2 precursor was prepared according to the previous report [27]. In the layered structure of bulk precursor, each Co atom is coordinated with two CNS− groups arrayed at the transaxial position and with four pyz linkers located along the equatorial direction. Employing top-down ultrasonic method in an ethanol solution, green-colored powders of few-layered CoCNSP nanosheets were obtained by rotary 10

evaporation. In each CoCNSP layer, the CNS− groups were aligned symmetrically along its two sides with an S···S distance of 9.476 Å. The successful preparation of few-layered CoCNSP nanosheets was first proved by the PXRD technique. Fig. 2A reveals that the crystallographic (110) peak disappeared in the PXRD pattern of CoCNSP nanosheets with respect to that of the bulk precursor. This observation suggests that the bulk precursors are exfoliated layered-by-layered to nanosheets. The TEM image of the obtained products (Fig. 2B) reveal the ultrathin CoCNSP nanosheets are randomly stacked with curling edges. The selected area electron diffraction (SAED) pattern of CoCNSP nanosheets further testifies their crystalline characteristics (Fig. 2C). The AFM topological image (Fig. 2D) observes a large ultrathin nanosheet with a lateral area of 0.5 μm. The height profile shows a sub-4.0 nm thickness (Fig. 2E), related to few-layered CoCNSP nanosheets as verified by the S···S distance of 9.476 Å (Fig. 1). The observations mentioned above demonstrate the successful exfoliation of layered bulk precursor to few-layered CoCNSP nanosheets via a facile-operating ultrasonic exfoliation method. 3.2. Heavy metal uptake by using CoCNSP nanosheets For comparison, both layered bulk precursor and few-layered CoCNSP nanosheets were tested their abilities to capture HMs (i.e., Co(II), Cu(II), Zn(II), Pb(II), Cd(II), Hg(II), U(VI) or Ni(II)) from their individual solutions. The affinity of an adsorbent toward HMs can be evaluated by the distribution coefficient Kd. Generally, adsorbents with Kd values in the range of ~104−105 are regarded as exceptional materials for contaminant removal [30]. The capture amounts and Kd 11

values for different HMs are displayed in Fig. 3. At the initial solution pH during the preparation of their individual metal ion solutions, bulk precursor shows poor uptake abilities for all eight HMs with very low uptake rates (<35 %), Kd values (~102 mL/g) as well as low uptake amounts. After exfoliation, their uptake amounts and Kd values all improve. It is noteworthy that CoCNSP nanosheets display superior uptake performance for Cu(II), Pb(II), U(VI) and Hg(II). Specifically, > 99.4 % of removal, >105 mL/g of Kd values as well as >8 mg/g of uptake amounts are obtained for Pb(II) and Cu(II). Moreover, > 99.9 % of uptake rates, >106 mL/g of Kd values and > 9 mg/g of uptake amounts are obtained for U(VI) and Hg(II). These observations disclose the superior capture ability of few-layered CoCNSP nanosheets to that of layered bulk precursor, which proves the advantage of exfoliation for improving uptake performance. This can be explained by the reason that exfoliation can expose the inaccessible interior sulfur species, which are the main chelating sites for HMs. Fig. 3 also shows the competitive uptake of HMs by bulk precursor and CoCNSP nanosheets in a mixed ion state. CoCNSP nanosheets still show superior removals and Kd values (>104 mL/g) as compared to the bulk precursor, further confirming the significant role of the material’s architecture. Compared to individual metal adsorption

experiments,

about

52-fold

(2.70×106/5.18×104),

24-fold

(1.85×105/7.69×103), 146-fold (1.29×107/8.85×104) and 8-fold (1.90×105/2.43×104) decrease in Kd values are observed for U(VI), Pb(II), Hg(II) and Cu(II), respectively. Competition among coexisting HMs during mixed treatment might be the reason for 12

the decrease in Kd values. A selectivity order of Co(II), Zn(II), Cd(II), Ni(II) << Cu(II) < Pb(II) < U(VI) < Hg(II) is observed, which is consistent with the individual metal adsorption experiment. The HSAB theory can be the reason for this selectivity [1, 2, 19, 30]. Basically, the dangling thio groups (soft base) in CoCNSP provide powerful bonding affinity toward soft HMs (i.e., Hg(II) and Pb(II)). Consequently, CoCNSP offered high adsorption performance for Hg(II) and Pb(II) due to the favorable soft-soft

interactions

(Hg/Pb⋯S)

over

unfavorable

bonding

interactions

(Co/Ni/Zn/Cd⋯S). In terms of U(VI), although UO22+ ions are widely deemed as hard Lewis acid cation, previous studies prove that sulfur species are efficient for UO22+ uptake through the powerful interactions between the soft Lewis basic sulfur species and relatively softer Lewis acidic UO22+ ions [16, 31-34]. Gu et al. even observed that U(VI) immobilization onto modified MoS2 was through covalent bonds between UO22+ and S2− in virtue of XPS results and DFT calculations [35]. Thus, UO22+ ions are softer Lewis acid centers than previously thought. The HSAB theory can therefore rationalize the observed selectivity order. 3.3. Relative selectivity of CoCNSP nanosheets Inspired by the aforementioned high adsorption performance, we wanted to see if CoCNSP nanosheets had any prospects to separate HMs from each other, which is a challenging issue encountered in mining operations [2]. An adsorption experiment was conducted in a mixed solution containing only Cu(II), Pb(II), U(VI) and Hg(II) (~10 mg/L for each ion) to further identify the relative selectivity order. Decreased amounts (0.02 and 0.005 g) of CoCNSP nanosheets enough to uptake only one of 13

HMs were employed, and the results are tabulated in Table 1. The separation factor (SF) for A and B (SFA/B) calculated from KdA/KdB can evaluate the separation degree of one ion from the other. Generally, good separation factor is regarded as >100 according to the definition and applications of SF [2]. When 0.02 g of CoCNSP nanosheets is used, ~99.95 % removal of Hg(II) discloses the high affinity of CoCNSP nanosheets toward Hg(II). The SFHg(II)/Cu(II) and SFHg(II)/Pb(II) values are ~153.8 and 113.8, respectively, proving a higher affinity for Hg(II) than for Cu(II) and Pb(II). Meanwhile, the SFU(VI)/Cu(II), SFU(VI)/Pb(II) and SFPb(II)/Cu(II) values are ~2.1, 1.6 and 1.4, respectively, suggesting a slightly higher selectivity toward U(VI) than Cu(II)/Pb(II) as well as slightly higher selectivity toward Pb(II) than Cu(II). When only 0.005 g of CoCNSP nanosheets were used, the adsorption rates of Cu(II), Pb(II), U(VI) and Hg(II) are 22.16 %, 40.62 %, 51.54 % and 96.54 %, respectively. The adsorption rate follows an order of Cu(II) < Pb(II) < U(VI) << Hg(II), which is consistent with the individual and mixed metal adsorption experiments. 3.4. Metal adsorption studies Time-course uptake experiments disclose that metal uptake by the CoCNSP nanosheets is kinetically efficient, displaying high removal rates and Kd values (Fig. 4A). After equilibrium, heavily metal-polluted solution (~10 mg/L) decreases to 3, 8, 10 and 3 ug/L for Hg(II), U(VI), Pb(II) and Cu(II), respectively, well below the drinking water standards [36]. As depicted in Fig. 4A, the adsorption for all four HMs achieves equilibrium within ~60 min. The fast adsorption kinetics are in stark contrast to the long contact time (several hours or days) required for other porous materials [5, 14

37, 38]. For Hg(II) and U(VI), >99.9 % of removal rates and >106 mL/g of Kd values are obtained within only 20 min. Under the same experimental conditions, >105 and >106 mL/g of Kd values are reached for Pb(II) and Cu(II) within 40 min. The slightly lower uptake dynamics of Pb(II) and Cu(II) can be traceable to their intermediate soft acid traits, which offer weaker affinity. Pseudo-first-order and pseudo-second-order (PSO) models were applied to determine the uptake dynamics [5]. The adsorption amounts (qe,cal) calculated from the PSO model closely approximate to the experimental ones (qe,exp) (Fig. 4C and Table S1). The correlation coefficients (R2) disclose that the uptake process can be well described by the PSO model, reflecting a chemisorption process [39]. The k2 values follow an order of Hg(II) > U(VI) > Pb(II) > Cu(II), which reflects their different uptake rates. In addition, a breakthrough experiment conducted under the initial Hg(II) concentration of 5 mg/L reveals that the column removal amount at the breakthrough point (the drinking water standards of < 2.0 ug/L) is about 15.0 mg/g (Fig. S1), which is very low due to the low Hg(II) initial concentration and short contact time of column operation. An adsorption equilibrium study was conducted to evaluate CoCNSP nanosheets as a potential scavenger. Fig. 4D shows that the metal ion uptake capability increases with the rise of C0. In terms of Hg(II) uptake, high removal rates (>99 %) with large Kd values (3.76×104−1.26×107) are obtained over a C0 range of 12−412 mg/L. The adsorption capacities of Cu(II), Pb(II), U(VI) and Hg(II) are 325, 534, 661 and 716 mg/g, respectively. For comparison, Table 2 tabulates the performance of various 15

thiol/sulfur-based capturers for HMs. It is apparent that CoCNSP nanosheets go far beyond that of many other reported materials, which can be traced to the abundant readily accessible sulfur species that are orderly aligned in the surface of CoCNSP nanosheets. Herein, the Langmuir and Freundlich equations were employed to elucidate the uptake behaviors [5], and the fit parameters are listed in Table S2. The data points perfectly conform to the Langmuir equation, indicating that a monolayer uptake process. The constant b value can represent an indicator of affinity toward a specific ion. The b values (L/mg) are 0.242, 0.185, 0.108 and 0.091 for Hg(II), U(VI), Pb(II) and Cu(II), respectively, disclosing that CoCNSP nanosheets have stronger affinity toward Hg(II) and U(VI) ions than for Pb(II) and Cu(II) ions. 3.5. Possible removal mechanism The effects of water chemistry, including solution pH and ionic strength on the adsorption performance, are crucial for the evaluation of the broad application of an adsorbent, which can also indirectly reflect the underlying adsorption mechanism. Fig. 5A reveals that CoCNSP nanosheets are stable in neutral and basic conditions with no Co(II) leaking. In contrast, the CoCNSP nanosheets start to dissolve under relatively high acidic pH conditions with a portion of Co(II) leaking into the solution, which suggests the dissolution of CoCNSP nanosheets. Fig. 5A shows that Hg(II) maintains >91.9 % of adsorption rates and >104 mL/g of Kd values in the pH range of 3.15–8.87, reflecting superior Hg(II) uptake at a broad pH range. The adsorption for Cu(II), Pb(II) and U(VI) are relatively poor at pH < 4.02. However, >92.2 % of removals and >104 mL/g of Kd values are obtained in the pH range of 5.02–8.87, 16

reflecting different removal toward these HMs under different conditions. Fig. 5B shows the zeta potentials of CoCNSP nanosheets at different solution pH and the pH of zero point charge (pHZPC) is ~3.20. Fig. S2 shows the Hg(II), U(VI), Pb(II) and Cu(II) species under different pH values. CoCNSP nanosheets show consistent high adsorption rates at both pH values larger or less than the pHZPC, which reflect that electrostatic interactions are not controlling factor dominating the uptake behaviors. Compared with the weak physical adsorption triggered by the surface charges, powerful chemisorption based on coordination via thio groups can be responsible for the superior uptake performance of CoCNSP nanosheets, which is consistent with the above-mentioned uptake study. To further verify this speculation, the uptake efficiency affected by different ionic strength (1–100 mM of NaNO3) was studied. Fig. 5C shows that the increase in ionic strength do not induce any discernible effect on the removal of these HMs, confirming that surface charges related to the alternation in solution pH or ionic strength are not the dominant driving force for metal uptake. The consistency efficiency under different ionic strength discloses that the chemical adsorption is through inner-sphere complexation. FTIR and XPS spectroscopy further traced the possible mechanisms of metal ion removal by the CoCNSP nanosheets. The FTIR spectra of original and used CoCNSP nanosheets are showed in Fig. 5D. After the removal of HMs, the characteristic peaks at 2060 cm-1 assigned to the typical stretch mode of CNS- groups [22] either diminish or disappear, indicating the strong binding interactions between HMs and CNSgroups. The XPS spectra of metal-laden CoCNSP are showed in Fig. 6. Original and 17

used CoCNSP nanosheets show Co 2p binding energies center at ~781, 986, 796 and 803 eV. The Co 2p orbital energies in the original sample and the postadsorption samples are very similar, indicating no discernible changes in the oxidation of Co occurred after the adsorption of HMs. S 2p binding energy observed in the range of 161.4–166.3 eV is assigned to different S2− groups (Fig. 6B′, C′, D′, E′). The loading of HMs causes a slight change in the S 2p binding energy, indicating an alteration of the local bonding environment occurrence. Compared with the original sample exposed in the open air, the peaks centered at ~168 eV in the used samples are from SO42− impurity, indicating some sulfur groups are oxidized. Specifically, about 20 %, 15 %, 6 %, and 4 % sulfur groups are oxidized into SO42− after the contact with Hg(II), U(VI), Pb(II), and Cu(II) for 24 h. These results reflect the stability of CoCNSP nanosheets in the open air and

Pb(II) and Cu(II) solutions, while other

sulfur-functionalized adsorbents such as Sx–LDH [40] must be stored in vacuum to preserve their activities. This stability facilitates the excellent performance of CoCNSP nanosheets in the recycle experiments (Fig. S3A). CoCNSP nanosheets show superior adsorption rate (>90 %) until the third cycle. The restacking of nanosheets evidenced by the presence of the (110) planes in the postadsorption sample (Fig. S3B) and the partial oxidation of sulfur groups (Fig. 6B′, C′, D′, E′) during the adsorption process are responsible for the dropped adsorption efficiency in the following cycles. For the Hg(II) postadsorption sample (Fig. 6B″), the Hg 4f levels centered at ~106.1 (Hg 4f5/2) and ~101.9 (Hg 4f7/2) eV confirm the Hg2+ immobilization [41]. As shown in Fig. 6C″, the U 4f5/2 and U 4f7/2 energies at 393.1 18

and 382.2 eV, respectively, are associated with UO22+ in the adsorbed sample [31]. In Fig. 6D″, the peaks occurred at ~144.5 (Pb 4f5/2) and 139.6 eV (Pb 4f7/2) are assigned to Pb 4f signals [1]. For Cu(II) (Fig. 6E″), four characteristic peaks observed at ~962.7 and 955.1 eV (Cu 2p1/2) as well as ~943.1 and 934.6 eV (Cu 2p3/2), correspond to the Cu 2p energy of Cu2+ [42]. The single state of the adsorbed HMs suggests the absence of redox activities during their contact with sulfur groups. To further confirm the aforementioned possible uptake mechanisms, the DFT calculations were conducted to probe the coordination modes of HMs and CoCNSP nanosheets. The possible binding mode of Co(II) and CoCNSP nanosheets was also probed to give a reasonable explanation for the selectivity mentioned above. After DFT calculations, the most stable configurations and representative structural parameters of metal ion-laden CoCNSP are displayed in Fig. 7A. In the optimized structures, Hg(II), U(VI) and Pb(II) are mainly coordinated with sulfur atoms, which confirm that S2− groups exert an enormous function on their removal. The distances between HMs and sulfur atom are 2.802, 2.921, 2.359 and 3.962 Å for Pb(II), U(VI), Hg(II) and Co(II), respectively. The Ead values of Hg(II)-, U(VI)-, Pb(II)- and Co(II)-laden CoCNSP are 9.43, 10.77, 8.75 and 0.97 kcal/mol, respectively. These observations disclose stronger interaction occurred in Hg(II), U(VI) and Pb(II) uptake than Co(II) capture by CoCNSP nanosheets. Thus, the selectivity order in the individual or mixed adsorption experiments can be rationalized. The Ead values follow an order of U(VI) > Hg(II) > Pb(II), inconsistent with the batch experimental results of Hg(II) > U(VI) > Pb(II), probably due to the structural differences between one 19

simple structural unit for DFT modeling and actual CoCNSP nanosheets. The ADCH charge analysis of CoCNSP nanosheets in the presence of HMs are graphed Fig. 7B. It reveals that positively charged U(VI), Hg(II) and Pb(II) prefer to bind with the negatively charged sulfur species, reducing the negative surface potential of CoCNSP. Compared with Co(II), more positively charged U(VI), Hg(II) and Pb(II) atoms indicate stronger electrostatic attraction interactions, which could be regarded as coordinate covalent bonds. The results of ADCH charge analysis are in a good agreement with that of ESP analysis (Fig. 7C). We also conducted RDG analysis to illustrate the strength of the interaction between HMs and CoCNSP nanosheets. The front views for the RDG isosurfaces with a color bar are graphed in Fig. 8. The surface color is distinguished by blue, green and red based on the values of sin(λ2)ρ. Blue reflects powerfully attractive interaction and red reflects a steric effect. The color of the RDG isosurface between Hg(II)/U(VI)/Pb(II) and the sulfur atom in the CNS- group is mostly blue, which revealed that their binding was strong. On the contrary, the dominant color of the RDG isosurface between Co(II) and sulfur atom is green (Fig. 8), suggesting the weak binding interaction between Co(II) and CoCNSP nanosheets, which supports the above experimental data.

4. Conclusions In conclusion, we have demonstrated the deployment of few-layered CoCNSP nanosheets as an amenable platform for the removal of HMs (i.e., Cu(II), Pb(II), Hg(II), and U(VI)). The key findings are: (i) selective adsorption in the order of 20

Co(II), Zn(II), Cd(II), Ni(II) << Cu(II) < Pb(II) < U(VI) < Hg(II) in both single-component and multi-component adsorption experiments; (ii) high distribution coefficients (Kd~106–107 mL/g) and fast capture dynamics for Cu(II), Pb(II), Hg(II) and U(VI), as well as large saturated uptake capacities (716 mg/g for Hg(II), 661 mg/g for U(VI), 534 mg/g for Pb(II), and 325 mg/g for Cu(II)); (iii) superior endurance under high ionic strength and excellent adsorption performance in a wide pH range; and (iv) good recycle performance for Cu(II) and Pb(II) for three times. Therefore, few-layered CoCNSP nanosheets are a fascinating amenable platform for the decontamination of HMs-polluted water due to the facile preparation strategy, low-cost and non-toxic precursors as well as outstanding removal abilities for HMs. Notes The authors declare no competing financial interests.

Acknowledgments Financial support from the National Natural Science Foundation of China (21806001), the Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences, and Funded by the Research Fund Program of Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources (2017B030314182), and theory calculations supported by the University of Science and Technology of China Supercomputer Centers are acknowledged.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.CEJ.2019. 21

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28

Fig. 1. Schematic diagrams for the synthetic procedures and molecular structures of few-layered CoCNSP nanosheets.

29

Fig. 2. PXRD pattern (A), TEM image (B), selected-area electron diffraction pattern of CoCNSP nanosheets (C), AFM image (D) and height profile (E) of few-layered CoCNSP nanosheets.

30

10M

A CoCNSP

9

CoNCSP

Bulk

100k

6

300.0

3 0

12

Co

U

Zn

Pb Cd Ni

Hg

Cu

Co 100k

C

CoCNSP

Bulk

8 6

D

Zn

Pb Cd

CoCNSP

Ni

Hg Cu

Bulk

1.0k

4

500.0

2 0

U

Kd (mL/g)

10

qe (mg/g)

B

Bulk

Kd (mL/g)

qe (mg/g)

12

Co

U

Zn

Pb Cd

Ni

Hg

Cu

Co

U

Zn

Pb Cd

Ni

Hg Cu

Fig. 3. Comparison of uptake amounts (A) and Kd values (B) by bulk precursor and CoCNSP nanosheets in a single ion state; and comparison of uptake amounts (C) and Kd values (D) by bulk precursor and CoCNSP nanosheets in a mixed ion state.

31

A

3M

10 Hg(II) U(VI) Pb(II) Cu(II)

8 6 4

Kd (mL/g)

Concentration (mg/L)

12

B Hg(II) U(VI) Pb(II) Cu(II)

2M

1M

2 0

0 0

20

40

60

0

20

Time (min) 12

60

80

100

120

140

Time (min) 900

C

10

D

750 Hg(II) U(VI) Pb(II) Cu(II)

8 6 4

600

qe (mg/g)

qt (mg/g)

40

PSO model

2

450 300 Hg(II) Pb(II)

150

0

U(VI) Cu(II)

0 0

20

40

60

80

100

120

140

0

Time (min)

200

400

600

800

1000 1200 1400

Ce (mg/L)

Fig. 4. Adsorption kinetics curves of HMs by CoCNSP nanosheets at m = 0.03 g, V = 30 mL, C0 = 10 mg/L, T = 298 K: concentration change following contact time (A); Kd values at different contact time (B) and adsorption amount as a function of contact time (C). Adsorption isotherm curves of HMs by CoCNSP nanosheets (D) at m = 0.03 g, V = 30 mL, contact time = 24 h, T = 298 K.

32

80

75

Hg(II) U(VI) Pb(II) Cu(II)

60 45

60 40

30

20

15 0 120

1 C

2

3

Hg(II)

4

5

6

pH

U(VI)

7

8

Zeta potential (mV)

Adsorption (%)

90

5

100

A

Co leaking (%)

105

0

B

0 -5 -10 -15

9 10 11

2

3

5

6

7

8

pH Pb(II)

D

Cu(II)

100 80 60 40 20

U(VI) Cu(II)

Intensity (a.u.)

Adsorption (%)

4

Pb(II) Hg(II) Blank -

0

SCN

0

10 50 100 1 5 NaNO3 concentration (mM)

500

1000 1500 2000 2500 3000 3500 4000 -1

Wavenumber (cm )

Fig. 5. The effect of pH on the uptake of HMs by CoCNSP nanosheets and Co leaking under different solution pH (A); the zeta potentials of the CoCNSP nanosheets at different solution pH (B); the effect of ionic strength on the uptake of HMs by the CoCNSP nanosheets (C); and the FTIR spectra of the CoCNSP nanosheets before and after the contact with HMs (D).

33

790

780

803.5

810

786.1

790

780

Binding energy (eV)

810

785.6

800

790

780

Co 2p

D

770

810

800

790

780

770

781.0

796.6 803.5

810

800

785.5

790

780

Binding energy (eV)

770

163.2

S 2p 164.4 168.9

E'

95

382.2 U 4f

C" 393.1

Satellite

Binding energy (eV)

D"

139.6

144.5

Pb 4f

148 146 144 142 140 138 136 134

Binding energy (eV)

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

Co 2p

D'

100

396 392 388 384 380 376

168.3

174 171 168 165 162 159

Binding energy (eV)

E

163.4

Binding energy (eV)

781.5

796.8 785.5 803.0

162.4

174 171 168 165 162 159

Intensity (a.u.)

Intensity (a.u.)

Binding energy (eV)

105

Binding energy (eV)

162.3

S 2p

Hg 4f

106.1

110

168.5

C'

101.9

B"

163.3

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

796.6 803.2

S 2p

174 171 168 165 162 159

770

781.3

Co 2p

C

Binding energy (eV)

B'

780.9

796.4

800

174 171 168 165 162 159

770

Intensity (a.u.)

Intensity (a.u.)

Co 2p

B

168.9

Intensity (a.u.)

800

Binding energy (eV)

164.5

Intensity (a.u.)

810

163.3

S 2p

Intensity (a.u.)

796.8 802.8 786.4

A'

163.1

S 2p 164.4 169.0

174 171 168 165 162 159

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

781.0

Intensity (a.u.)

Co 2p

A

E"962.7

Cu 2p 934.6

955.1

943.1

960

950

940

930

Binding energy (eV)

Fig. 6. The XPS peak deconvolution of the CoCNSP nanosheets before (A, A′) and after adsorbed Hg(II) (B, B′, B″), U(VI) (C, C′, C″), Pb(II) (D, D′, D″) and Cu(II) (E, E, E″), respectively.

34

Fig. 7. Optimized structures (A1-A4), Hirshfeld charge analyses (B1-B4) and electrostatic potential (C1-C4) of the CoCNSP nanosheets contacting with Pb(II), U(VI), Hg(II) and Co(II), respectively. The isosurface has a value of 0.001 e Bohr−3.

35

Fig. 8. Color-filled RDG mapped isosurface of CoCNSP nanosheets contacting with Pb(II), U(VI), Hg(II) and Co(II). The value of sin(λ2)ρ on the surfaces is represented by filling color according to the color bar in the middle space.

36

Table 1 Selective adsorption results of CoCNSP nanosheets for Cu(II), Pb(II), U(VI) and Hg(II). CoCNSPa)

0.005 g

0.02 g

U(VI)

Cu(II)

Hg(II)

Pb(II)

U(VI)

Cu(II)

Hg(II)

Pb(II)

C0 (ppm)

10.38

10.02

13.02

10.07

10.65

10.53

13.02

10.07

Cf (ppm)

5.03

7.80

0.45

5.98

0.3823

0.7709

0.0067

0.5564

Kd (mL/g) Removal (%) a)

5.32×103 1.42×103 1.40×105 3.42×103 51.54

22.16

96.54

40.62

6.71×104 3.16×104 4.86×106 4.27×104 96.41

92.68

99.95

94.47

30 mL solution of Cu(NO3)2, Pb(NO3)2, UO2(NO3)2 and Hg(NO3)2, 10 ppm

concentration per ion.

37

Table 2 Adsorption capacities of various thiol/sulfur-based capturers for HMs. Ions

Cu(II)

Pb(II)

U(VI)

Hg(II)

Adsorbents

qm (mg/g)

Ref.

CoCNSP

325

This work

MoS4 LDH

181

[2]

MoS4-Ppy

111

[1]

Mg2Al–LS–LDH

64

[43]

Sx-LDH

127

[40]

CoCNSP

534

This work

Mn-MoS4

357

[19]

Fe-MoS4

345

[30]

MoS4 LDH

290

[2]

MoS4-Ppy

78

[1]

Mg2Al–LS–LDH

123

[43]

Sx-LDH

383

[40]

CoCNSP

661

This work

[Me2NH2]2[Ga2Sb2S7]·H2O

196

[32]

[Et2NH2]2-[Ga2Sb2S7]·H2O

144

[32]

FJSM-SnS

338.43

[31]

MoS2

23.7

[35]

GA-SO3H

148.4

[44]

PVP/MoS2

117.9

[35]

NPS-GLCs

294.16

[45]

Sx-LDH

330

[33]

CoCNSP

716

This work

Mn-MoS4

594

[19]

Fe-MoS4

582

[30]

MoS4 LDH

500

[2]

MoS4-Ppy

210

[1]

FJI-H12

439.8

[22]

Sx-LDH

483

[40]

38

Zr-DBMD

197

[46]

UiO-66-SH

785

[41]

ZrSulf

824

[47]

PAF-1-SH

1014

[21]

2D-NCS

1698

[26]

39

Declaration of interests

☒

The authors declare that they have no known competing financial interests or

personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

40

Graphical Abstract

41

Highlights 

CoCNSP nanosheets with periodically aligned sulfur species were exfoliated.

 CoCNSP nanosheets showed fast capture dynamics and ultrahigh uptake capacities.  CoCNSP nanosheets had good recycle performance.  Spectroscopic and DFT analysis demonstrated superior adsorption performance.

42