Chromium (VI) adsorption and reduction by humic acid coated nitrogen-doped magnetic porous carbon

Journal Pre-proof Chromium (VI) adsorption and reduction by humic acid coated nitrogen-doped magnetic porous carbon Ting Zhang, Shuang Wei, Geoffrey I.N. Waterhouse, Lien Fu, Lei Liu, Weijie Shi, Jianchao Sun, Shiyun Ai PII:

S0032-5910(19)30812-5

DOI:

https://doi.org/10.1016/j.powtec.2019.09.091

Reference:

PTEC 14802

To appear in:

Powder Technology

Received Date: 28 April 2019 Revised Date:

27 September 2019

Accepted Date: 30 September 2019

Please cite this article as: T. Zhang, S. Wei, G.I.N. Waterhouse, L. Fu, L. Liu, W. Shi, J. Sun, S. Ai, Chromium (VI) adsorption and reduction by humic acid coated nitrogen-doped magnetic porous carbon, Powder Technology (2019), doi: https://doi.org/10.1016/j.powtec.2019.09.091. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Chromium (VI) Adsorption and Reduction by Humic Acid Coated Nitrogen-doped Magnetic Porous Carbon Ting Zhanga, Shuang Weia, Geoffrey I.N. Waterhousea,b, Lien Fua, Lei Liua, Weijie Shia,*, Jianchao Sunc, Shiyun Aia,*

a

College of Chemistry and Material Science, Shandong Agricultural University, Taian,

271018, Shandong, PR China

b

School of Chemical Sciences, The University of Auckland, Private Bag 92019,

Auckland, New Zealand

c

School of Environment and Materials Engineering, Yantai University, Yantai,

264005, Shandong, PR China

*

Corresponding author:

Tel: +86 538 8247660 Fax: +86 538 8242251 E-mail address: [email protected] (S.Y. Ai) [email protected] (W. J. Shi)

1

Abstract: A humic acid coated nitrogen-doped magnetic porous carbon (HA-N-MPC) adsorbent was successfully synthesized using lignin isolated from black liquor as the main carbon source. SEM, TEM, FT-IR, XPS and N2 physisorption measurements showed the HA-N-MPC to possess a high specific surface area, a porous three dimensional structure with magnetic Fe3O4 nanoparticles uniformly embedded in the N-doped porous carbon matrix, and an abundance of surface functional groups for metal ion sorption. Cr(VI) adsorption experiments were subsequently carried in aqueous solution at pH 2 using HA-N-MPC as the adsorbent. Results revealed that HA-N-MPC had a high adsorption capacity of 130.5 mg·g-1 for Cr(VI), with partial reduction of toxic Cr(VI) to nontoxic Cr(III) occurring on adsorption. The adsorption kinetics and equilibrium adsorption isotherm determined for Cr(VI) on HA-N-MPC obeyed the pseudo-second-order model and Langmuir isotherm model, respectively. After magnetic separation from the sorption medium and regeneration, HA-N-MPC retained excellent adsorption performance for Cr(VI).

Key words: Hexavalent chromium; Adsorption; Reduction; Humic acid coated nitrogen-doped magnetic porous carbon

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1. Introduction The efficient removal of metal ions from industry wastewater is a global priority, motivating the search for novel sorbents that can reduce metal concentrations in wastewater to below permissible thresholds [1]. Cr (VI), a classic heavy metal pollutants, is found in aquatic ecosystems and drinking water sources due to its widespread use in electroplating, battery manufacture, leather tanning, mining activities, and other application [2]. In general, chromium has two common oxidation states in water: hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Trace concentrations of Cr(VI) can cause serious health problems to human and animals, including DNA damage, mutations, chromosomal aberrations and carcinomas of the respiratory organs. Conversely, Cr(III) is an essential trace element for humans and is generally harmless and immobile in aqueous media [3]. Considering the high solubility, mobility and toxicity of Cr(VI), the World Health Organization (WHO) has set a strict threshold on Cr(VI) levels to limit the maximum permissible concentrations of 0.05 mg · L-1 and 0.1 mg · L-1 in drinking water and industrial wastewater, respectively. Accordingly, it is very essential to develop the economical, green and effective water treatment technologies for removing Cr(VI) from aqueous solutions [4]. It is noteworthy that the reduction of the toxic Cr (VI) to harmless Cr (III) is indispensable in the Cr (VI) removal processes. Various methods have been developed to remove Cr (VI), such as electrocatatalysis chemical precipitation, ion exchange, redox treatments, physical/chemical adsorption and reverse osmosis[5, 6]. 3

Among these techniques, adsorption is considered as the most suitable method for Cr(VI) removal because of its high efficiency, simple operation and easy sorbent regeneration [7]. Adsorbents including activated carbons [8], fiber materials [9], chitosan [10] and zeolites [11] have been successfully applied for the removal of Cr(VI) in wastewater. Unfortunately, some intrinsic drawbacks of these adsorbents such as small surface area, low adsorption capacity, poor mechanical strength and slow kinetics process seriously restrict their widespread application. Furthermore, the second-pollution caused by Cr (VI) ions with high toxicity is still the unnegligible obstacle in adsorption. The simultaneous adsorption and reduction of Cr(VI) in the treatment process is thus the preferable water purification method. Recently, magnetic porous carbon (MPC) materials have attracted a lot of attention as adsorbents of organic and metal ions owing to their high surface area and porosity, as well as easy magnetic separation and recovery. For example, Chen et al. successfully fabricated a porous carbon-encapsulated iron (Fe@PC) composite through carbothermal reduction of Fe(NO3)3 and starch, with the composite displaying excellent adsorption properties for Cr(VI)in water [12]. However, many carbon-based magnetic adsorption materials show weak affinity for Cr(VI) in solution and thus relatively low Cr(VI) removal efficiencies [13]. Recent researches indicated that doping of heteroatoms into carbon materials is a new strategy for improving the adsorption amounts of environmental pollutants. Nitrogen has an extra electron compared to carbon. Accordingly, N-doped carbons have improved the electron

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donating properties, increased hydrophilicity, and more negative surface charge densities compared to conventional pyrolysis derived carbons, all of which can enhance the adsorption of molecules and ions from water [14]. In addition, N-doped magnetic porous carbon materials (N-MPC) typically possess an abundance of cationic imine and amine functional groups, which not only allow the adsorption of negatively charged ions by electrostatic interaction, but also are capable of reducing toxic Cr(VI) to harmless Cr(III) by acting as reducing agents [15]. Therefore, N-MPC has received considerable attention on their applications for the Cr (VI) removal. Improving the adsorption capacity of N-MPC for Cr(VI) ions, enhancing the reduction of Cr(VI) to Cr(III) ions, and incorporating magnetic nanoparticles to achieve facile recycling, are all key focus areas of current research in the field. Raw materials commonly used to prepare N-MPC materials include melamine [16], polypyrrole [17] and dopamine [18]. It is regretful that some of these materials are expensive or non-renewable, so it is still a challenge to develop the low cost, environment friendly and sustainable raw materials for N-MPC fabrication. Efficient biomass utilization is an area of increasing research focus. Lignin is an amorphous three-dimensional heterogeneous polymer with a high molecular weight and the low reactivity, and it is the second-most-abundant natural and renewable raw material after cellulose on earth. Since its 3-dimensional structure is based on aromatic units, lignin is an ideal precursor for making functional porous carbon materials. Black liquor is the main by-product in the pulp and paper industries which contains around 15%

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solids by weight, including lignin, hemicelluloses and various inorganic compounds. Transforming this waste product into useful products is makes good sense from an environmental and economic perspective. Isolating the lignin from black liquor, followed by the manufacture of MPC sorption materials from that lignin, represents a smart and economical strategy for utilizing black liquor wastes. Humic acid (HA) is a natural macromolecular compound produced by the biological and chemical decomposition of plant and animal residues. HA has a framework of large polycyclic aromatic hydrocarbons, with various chemical functional groups including carbonyl, carboxyl, methoxyl, alcoholic hydroxyl, phenolic hydroxyl, ketones, quinones and amino groups distributed over the framework [19]. Due to this abundance of active functional groups, HA can effectively complex with various heavy metal ions. Some research reports show that HA has a high affinity towards Fe3O4 nanoparticles, allowing high dispersion and stabilization of Fe3O4 nanoparticles whilst also reducing the inherent toxicity of nanomaterials [20, 21]. Considering its excellent sorption ability for metal ions and affinity for Fe3O4, coating HA substrates on N-MPC surfaces should offer an efficient strategy for removing Cr(VI) ions from aqueous media. To the best of our knowledge, the application of such a multifunctional adsorbent for Cr(VI) removal has yet to be reported. Herein, we prepared a humic acid coated nitrogen-doped magnetic porous carbon (HA-N-MPC) material using lignin obtained from black liquor as the main carbon

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precursor. The overarching aim of this work was to develop an economical and effective adsorbent for removal and detoxification of Cr(VI) in aqueous solutions (Scheme 1). The design of the HA-N-MPC adsorbent leveraged four aspects: (1) The diverse array of functional groups on HA afforded a high complexation capacity for Cr(VI) ions; (2) HA had a high affinity for both N-MPC and magnetic Fe3O4 nanoparticles, thereby circumventing a traditional difficulty of HA separation from water (by allowing magnetic separation); (3) The presence of abundant amino and hydroxyl groups on

HA-N-MPC stronger enabled chemical reduction of adsorbed

Cr(VI) to Cr(III); and (4) As an abundant, inexpensive and non-toxic carbon source, utilizing of lignin from black liquor conformed green chemistry principles and environmental sustainable resource utilization. Findings of this study were expected to add new knowledge to the low cost sorption technologies for removing Cr(VI) from wastewater.

2. Experimental section 2.1. Materials

Black liquor was obtained from Shandong Tralin Paper Co., Ltd. (Shandong, China). Humic acid (90% fulvicacid) was purchased from Aladin Ltd. (Shanghai, China). K2CO3, Fe(NO3)3·9H2O, urea, N, N-dimethylformamide (DMF) and Diphenyl Carba Zide (DPC) were obtained from Kay Tong Chemical Reagents Co., Ltd. (Tianjin, China). K2Cr2O7, NH3 (25-28%) and H3PO4 (85%) were purchased from

7

Kang De Chemical Reagents Co., Ltd. (Yantai, China). All chemicals were analytical grade and used as received without further purification. Double-distilled water was used in all experiments.

2.2. Characterizations High-resolution transmission electron microscopy (HRTEM) images were recorded on a high resolution transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI Company, USA) operating at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were recorded on a field emission scanning electron microscope (FESEM, SU8010, Hitachi, Japan). X-ray diffraction (XRD) patterns were recorded on a Bruker AXSD 8 advanced powder X-ray diffraction system with Cu Kα-radiation source (Bruker Co., Germany). Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 380 FT-IR spectrometer (Thermo, USA). Samples were dispersed in KBr pellets for the analyses. X-ray photoelectron spectroscopy (XPS) data was obtained on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo, USA) using Mg Kα X-rays (hν = 1253.7 eV) as the excitation source. An Energy-dispersive X-ray spectroscope (EDX) (Oxford, England) with an operating voltage of 5 kV was used to analyze the elemental composition of the products. Thermogravimetric analysis (TGA) and differential thermal gravimetry (DTG) measurements were performed in air on a DTG-60AH instrument (Shimadzu, Japan), using a heating rate of 20 °C min-1. N2 adsorption–desorption isotherms were collected at 77 K using a TriStar II adsorption instrument (Micromeritics, USA). 8

Specific surface areas were calculated from the adsorption isotherms via the Brunauer–Emmett–Teller (BET) method, and pore volumes and cumulative pore volumes were calculated from the adsorption branch via the Barrett-Joyner-Halenda (BJH) method. Magnetic properties were analyzed by a vibrating sample magnetometer (VSM) (LakeShore 7307, USA) at room temperature. UV-Vis absorption spectra were obtained on a Shimadzu UV-2450PC spectrometer (Shimadzu, Japan). Atomic absorption spectrometry data were acquired on a Shimadzu AA6800 spectrometer (Shimadzu, Japan). 2.3. Lignin precipitation from black liquor

The precipitation of lignin from the black liquor waste was achieved as follows. A volume of black liquor was poured into screw capped conical flasks, and the pH adjusted to 2 by the dropwise addition of HCl. Subsequently, the conical flasks were left undisturbed for 24 h at the room temperature to allow settling of the flocs formed by the addition of acid. After 24 h, the contents of the flask were centrifuged at 1207×g for 10 min, and the supernatant discarded. The lignin precipitate was washed several times with double-distilled water followed by oven drying at 60 °C overnight to remove residual water and obtain a constant weight. The obtained lignin was then used to prepare N-doped magnetic porous carbons (N-MPCs).

2.4. Preparation of N-doped magnetic porous carbon (N-MPC) In a typical synthesis, lignin (2.16 g), urea (0.54 g), K2CO3 (2.16 g) and

9

Fe(NO3)3·9H2O (0.34 g) were added to 16 mL of N, N-dimethylformamide (DMF), after which the resulting mixture was placed in an oil bath at 110 °C for 30 min. Subsequently, the reaction mixture was cooled to room temperature, and then washed with DMF and methanol via centrifugation-redispersion cycles. The precipitate obtained was dried at 60 °C in a vacuum oven, then transferred to a tube furnace and heated at 5 °C · min-1 to 700 °C under N2, and held at this temperature for 2 h. The N-MPC product was then cooled to room temperature under N2, rinsed with double-distilled water in three cycles and finally dried at 60 °C overnight. In addition, a control magnetic porous carbon (MPC) sample was prepared via a similar route, except that urea was not included in the synthesis. 2.5. Preparation of humicacid coated N-doped magnetic porous carbon (HA-N-MPC) HA-N-MPC was prepared according to a published method [22]. Briefly, N-MPC (0.5 g) was dispersed in 50 mL of water, and then 4 mL of 25% ammonium hydroxide and HA (0.2 g) were added to the solution sequentially under rapid stirring. The mixture was then heated to 90 °C for 30 min and then cooled to room temperature. The resulting solution was centrifuged at 1207×g for 10 min to isolate the solid, and then washed with double-distilled water three times. The HA-N-MPC product was then dried in a vacuum oven at 60 °C to constant weight and stored in a vacuum desiccator for later use. 2.6. Adsorption experiments

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Batch adsorption experiments were conducted to investigate Cr(VI) adsorption by HA-N-MPC. All sorption experiments were carried out in duplicate. Briefly, HA-N-MPC (0.1 g) was added into the conical flask containing 100 mL of Cr(VI) solution (10 mg · L-1) and shaken thoroughly using a thermostated shaker rotating at a speed of 200 rpm. At specified times, samples were collected, filtered and the residual Cr(VI) ion and total chromium concentrations measured. Similar sorption experiments were performed for the N-MPC and MPC materials. The experiments were optimized for maximum sorption capacity (SC) by varying various parameters, including pH, adsorbent amount and sorption time. The influence of pH on the sorption of Cr(VI) was studied by varying the pH of the solution using 0.1 M HCl or NaOH solutions. Concentrations of Cr(VI) were measured by 1,5-diphenylcarbazide: the collected sample (1.0 mL) was taken into 1.5 mL centrifuge tube, H2SO4 (10 µL, 9.2 M), H3PO4 (10 µL, 8.1 M) and DPC (40 µL, 2 g · L-1) were then added. After incubation of 10 min at room temperature for color development, the collected samples absorbance was determined at 540 nm wavelength by a UV-Vis spectrophotometer. Total chromium in solution was determined by atomic absorption spectrometry. The collected sample (5.0 mL) was added into colorimetric tube, and measured by atomic absorption spectrometry. The determinations were conducted under following conditions: wavelength 357.9 nm, lamp current 12 mA, slit width 0.5 nm, burner height 8 mm, air flux 5 L · min-1, acetylene flow 1.4 L · min-1. In addition, to assess the stability of Fe on the adsorbent at pH=2, HA-N-MPC (0.1 g) was added into the

11

conical flask containing 100 mL of Cr(VI) solution (10 mg · L-1), followed by the same procedure as the above. Total concentrations of Fe in the solutions at different adsorption times were measured using atomic absorption spectrometer. Kinetic adsorption experiments were carried out at pH 2.0. The HA-N-MPC (2 mg) was dispersed in 100 mL of Cr(VI) solution (10 mg · L-1) and shaken thoroughly. The supernatant was withdrawn at appropriate time intervals, and the concentrations of Cr(VI) ion were determined by UV-Vis absorption spectroscopy. To construct the adsorption isotherm, HA-N-MPC (20 mg) was added to chromium solutions with various concentrations at room temperature, and then processed according to same procedure described above. The amounts of Cr(VI) adsorbed on each adsorbent were calculated using the following equation: qe =

(c0 − ce )V m

Where c0 represents the initial Cr(VI) concentration (mg · L-1); ce is the equilibrium Cr(VI) concentration (mg · L-1) in solution after adsorption; V represents the volume of solution (L) and m is the mass of the adsorbent (g).

3. Results and discussion 3.1. Characterization of HA-N-MPC

The morphology of the HA-N-MPC composite was examined by SEM and TEM. SEM image in Fig. 1A indicated that the surface of HA-N-MPC was not smooth, and

12

porous structure could be clearly observed on the surface. The magnified image in Fig. 1B further showed that the porous structure with three dimensional network. Therefore, such special rough and porous structure of HA-N-MPC revealed the potential ability for adsorption of Cr (VI). The TEM images in Fig. 1C and 1D further verified the porous network structure of HA-N-MPC, revealing thin carbon nanosheets interspersed by Fe3O4 nanoparticles. As shown in Fig. 1D, the mean size of the Fe3O4 nanoparticles was ~25-30 nm, with the carbon nanosheets less than 2 nm in thickness. Further, the magnetic Fe3O4 nanoparticles were reasonably uniformly distributed within the carbon matrix. The addition of HA was expected to prevent Fe3O4 agglomeration due to surface complexation-ligand exchange reactions between HA functional groups and the Fe3O4 magnetic nanoparticles [23]. These TEM results demonstrated the successful fabrication of HA-N-MPC. FT-IR spectroscopy was used to identify the different surface functional groups present on N-MPC and HA-N-MPC. As shown in Fig. 2A, the two materials (curve a and curve b) exhibited very similar FT-IR spectra, suggesting they possessed common functional groups. The broad absorption band centered at 3438 cm-1 could readily be assigned to the O–H and N–H stretching vibrations, indicating the presence of surface hydroxyl and amino groups [24]. The peak at 1628 cm-1 was assigned to a C

O

stretching mode of a carboxylic acid or possibly a combination band involving N–H bending coupled with C–N stretching, whilst the peak at 1354 cm-1 was either a CH2 wagging mode or a C–N stretching vibration. The peaks located at 1103 cm-1 and 768

13

cm−1 were assigned to the C–O stretching vibrations and the out-of-plane bending vibration of aromatic C–H bonds, respectively [25]. The band at 543 cm-1 was assigned to Fe–O stretching vibrations of Fe3O4, strong evidence that magnetic nanoparticles were successfully introduced into the carbon matrix [26]. From inset in Fig. 2A, the FT-IR spectrum of the HA-N-MPC subtracted N-MPC (curve i) still had the main characteristic peaks at about 3438 cm-1, 1628 cm-1 and 1354 cm-1, and it was almost coincide with the FT-IR spectrum of HA (curve ii), suggesting that HA-N-MPC material was prepared successfully. In addition, HA had plenty of IR active functional groups such as carboxyl, carbonyl and hydroxyl groups connected with the aliphatic or aromatic carbons in complex macromolecular structures. Most of these functional groups would give IR bands at similar frequencies to the functional groups of N-MPC, which probably explained why the FT-IR spectra for HA-N-MPC and N-MPC were almost indistinguishable. EDX analysis was used to determine the elemental composition of HA-N-MPC (Fig. 2B). The weight percentage of C, N, O and Fe in HA-N-MPC were estimated to be 78.01, 4.41, 7.61 and 1.88 %, respectively, with the atomic ratio of N/C and Fe/C around 0.047 and 0.005, respectively (see inset in Fig. 2B), suggesting the successful preparation of HA-N-MPC with different components. The other elements identified by EDX were Na, Al and Si and represented residues in the lignin derived from black liquor, whilst K derived from the K2CO3 used in the synthesis of N-MPC as a porogen.

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Nitrogen adsorption-desorption measurements were carried out in order to examine the surface area and porosity of HA-N-MPC. Fig. 3 showed the nitrogen adsorptiondesorption isotherms and the pore-size distribution plot of HA-N-MPC, respectively. HA-N-MPC exhibited a type IV adsorption isotherm with an H3-type hysteresis loop in the relative pressure range 0.45-1.0 (Fig. 3A), consistent with a mesoporous structure [27]. The H3-type hysteresis loop without any limiting adsorption at high relative pressures was commonly observed for aggregates of plate-like particles giving rise to slit-like pores [28]. The SEM and TEM images for HA-N-MPC (Fig. 1) revealed such a layered structure composed of 2 nm thick carbon sheets. The BJH pore-size distribution of HA-N-MPC (Fig. 3B) showed that the average diameter of the mesopores to be in the range of 4-5 nm, consistent with the result of the H3 hysteresis loop analysis. The BET surface area of HA-N-MPC was calculated to be 747.84 m2 · g-1, much larger than values previously reported for N-MPC materials. Such a high surface was expected to provide an abundance of adsorption sites for metal ions (e.g., Cr, Cd, Pb, etc.), whilst the mesopores would facilitate ion transport to the adsorption sites [13]. All the results above suggested that the as-prepared HA-N-MPC material should offer good performance for the removal of Cr(VI) ions from aqueous solution, which was confirmed in the experiments below. The paramagnetic properties of prepared HA-N-MPC, regenerated HA-N-MPC and reused HA-N-MPC were analyzed by VSM as shown in Fig. 4. The saturated magnetization values obtained from the hysteresis loop were found to be 7.21, 7.19,

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and 6.95 emu · g-1 for prepared HA-N-MPC, regenerated HA-N-MPC and reused HA-N-MPC, respectively, which showed that the magnetic properties of the absorbents almost unchanged after regeneration and reuse. With such higher saturated magnetization, the magnetic adsorbents were expected to respond well to magnetic fields, making the liquid and solid phases separate easily. 3.2. Cr(VI) removal efficiency of different adsorbents

Fig. 5 showed the Cr(VI) removal performance of HA-N-MPC, N-MPC and MPC. All three materials showed complete Cr(VI) removal of 4 h (at pH 2 and an initial Cr(VI) concentration of 10 mg · L-1). However, HA-N-MPC demonstrated the best Cr(VI) removal performance, with 96% removal achieved in 30 min and complete removal achieved in only 45 min. Compared with HA-N-MPC, the Cr(VI) removal performance of N-MPC was similar (marginally inferior), with the removal percentage around 94% after 30 min and 100% after 1 h. MPC showed only 76% Cr(VI) removal with complete adsorption taking ~4 h. These results indicated that nitrogen-doped carbon materials offered superior Cr(VI) adsorption properties than un-doped carbon materials. This could be rationalized in terms of the large number of active sites on the N-MPC surface, especially amine groups and the pyrrolic and pyridinic sites, which could modify the surface properties and enhance the electrostatic interaction between N-MPC and Cr(VI) [13]. Coating HA on N-MPC further provided large numbers of functional groups such as carboxyl, hydroxyl groups and so on, which could effectively complex Cr (VI) through surface 16

complexation reactions and presented superior performance for Cr (VI) removal. 3.3. Effect of pH and adsorbent amount on Cr (VI) removal Wastewater treatment plants typically processed water with pH values in the range 2-10, thus the Cr(VI) removal efficiency of HA-N-MPC was investigated in this pH range. The results presented in Fig. S1 revealed that solution pH had a major influence on Cr(VI) adsorption by HA-N-MPC. The highest Cr(VI) removal percentage after the 2 h adsorption period was achieved at pH 2, with the Cr(VI) removal efficiency decreasing sharply with increasing the pH in the range 3-6. Almost no Cr(VI) adsorption occurred in the pH range 6-10. The pH dependence of Cr(VI) adsorption on HA-N-MPC related not only the surface properties of HA-MPC, but also the chromium species in solution [29]. Depending on the pH, Cr(VI) may exist in various species in solution, including H2CrO4, HCrO4−, CrO42−, Cr2O72− or HCr2O7− [30]. Generally in acidic pH, chromium ions exist in two forms: as chromic acid (H2CrO4) at pH 1 and as hydrogen chromate ions (HCrO4−) at pH 2-6 [31]. In acidic media, the hydroxyl and amine functional groups presented on HA-N-MPC would be protonated and carried a positive charge, leading to the strong electrostatic attraction of HCrO4− ions. The adsorbed Cr(VI) was then fractionally reduced to Cr() via electron transfer from some electron-donor functional groups [32]. As the pH values increased from 2-6, the adsorption capacity of HA-N-MPC decreased because of the lesser degree of protonation of the active sites on HA-N-MPC. Under alkaline conditions (pH > 7), both the surface of HA-N-MPC and the chromate species (CrO42−)

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were negatively charged, with electrostatic repulsions resulting in a very low Cr(VI) removal percentage [33]. Based on the data presented in Fig. S1, the optimal pH was 2 because of the highest Cr(VI) removal percentage. The adsorbent amount was another important parameter that needed to be considered when developing an adsorption technology. The influence of the HA-N-MPC amount on the adsorption of Cr(VI) from aqueous solution was shown in Fig. S2. The Cr(VI) removal efficiency increased from 12% to 90% in 6 h by increasing the amount of adsorbent from 2 mg to 50 mg. The removal percentage increased sharply with increasing HA-N-MPC amount up to 20 mg over the initial 6 h adsorption period and then increased only slightly when the amount was increased to 50 mg. Generally, the higher adsorbent amount in the solution, the more adsorption sites and the greater surface area are available to combine with Cr (VI). However, many phenomena may happen at too large amount of adsorbent, including the overlap of the excessive adsorbent, agglomeration of adsorption sites and the relatively small surface area, leading to a slightly increase for the Cr(VI) removal efficiency. Accordingly, an adsorbent amount of 20 mg was selected for subsequent adsorption experiments. 3.4. Adsorption and reduction of Cr(VI) by HA-N-MPC

To explore the processes of Cr(VI) adsorption and reduction in detail, we studied the removal efficiency of Cr(VI) and the total Cr in solution using HA-N-MPC as adsorbent. Fig. 6 (curve a) showed that Cr(VI) removal efficiency increased 18

rapidly with time, with a removal percentage of 80% achieved after 15 min and 100% removal realized after 45 min. Fig. 6 (curve b) showed the total Cr removal efficiency. A total Cr removal percentage of ~81% was achieved after 45 min due to the excellent adsorption properties of HA-N-MPC, after which the removal percentage slightly increased until the removal percentage remained constant (~85%) after 120 min. The difference between the total Cr and Cr(VI) concentrations demonstrated that a certain amount of Cr( ) existed in solution, which came from the reduction of Cr(VI). These results suggested that the removal of Cr(VI) occurred mainly via adsorption by HA-N-MPC with the reduction of Cr(VI) to Cr(III) by electron donors (amino, hydroxyl, Fe3O4, etc.) of HA-N-MPC also being important. The latter is an advantage of the HA-N-MPC adsorption system because Cr(III) is substantially less toxic than Cr(VI). 3.5. Adsorption kinetics of Cr(VI) on HA-N-MPC To investigate the adsorption process of Cr(VI) on HA-N-MPC, pseudo-first-order (Eq. 1) and pseudo-second order (Eq. 2) kinetic models were applied to the adsorption data: log (qe- qt) = logqe -

K1 t 2.303

t 1 1 = + t 2 q t K 2 qe qe

(1) (2)

Where qe (mg · g-1) is the adsorption capacity at equilibrium, qt (mg · g-1) is the solid-phase loading of Cr(VI) on HA-N-MPC at time (h), k1 (min-1) and k2 (g ·

19

mg-1 · min-1) are the pseudo-first-order rate constant and pseudo-second-order adsorption rate constant, respectively. Fig. 7 (A, B and C) showed the adsorption data of Cr (VI) over HA-N-MPC at different time intervals, and the relevant parameters of the kinetic models calculated by the adsorption data were summarized in Table 1. It was obvious that the calculated correlation coefficient of the pseudo-second-order model (R2 = 0.9623) was much higher than that of the pseudo-first-order model (R2= 0.8168), indicating that the pseudo-second-order kinetic model is the best fit to the adsorption process. In addition, the calculated equilibrium sorption capacity qe,2 (134.2 mg · g-1) was very close to the experimental value of qe,exp (130.5 mg · g-1), further confirmed that the Cr(VI) removal on HA-N-MPC followed the pseudo-second-order model. Therefore it could be concluded that the adsorption of Cr(VI) by HA-N-MPC mainly involved chemical adsorption [34], which was consistent with the results of previous data reported for Cr on porous carbon adsorbents [35, 36]. Here, the Cr(VI) adsorption could mainly be attributed to complexation of Cr(VI) by chemical functional groups such as amines, carboxyl, and hydroxyl groups on M-MPC and HA. Furthermore, HA-N-MPC showed a higher adsorption capacity compared with other reported carbon materials [37, 38].

Table 1 Pseudo-first-order and Pseudo-second-order model parameters for Cr (VI) adsorption on HA-N-MPC.

Pseudo-first-order model

Pseudo-second-order model

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qe,exp (mg · g-1)

K1 (min-1)

qe,1 (mg · g-1)

R2

K2 (g · mg-1 · min-1)

qe,2 (mg · g-1)

R2

130.5

1.807×10-3

125.9

0.8167

2.114×10-5

134.2

0.9623

3.6. Adsorption isotherms for Cr(VI) on HA-N-MPC Adsorption isotherms were constructed to further describe the interaction between Cr(VI) and HA-N-MPC. Two classic isotherm models were used to fit the equilibrium data for Cr (VI) on HA-N-MPC: Langmuir (Eq. 3) and Freundlich (Eq. 4): ce c 1 = + e qe qm K L q m

(3)

1 lnqe= lnKF + lnce n

(4)

Where ce (mg · L-1) is the equilibrium concentration of Cr(VI) in aqueous solution, qm and qe are the maximum adsorption capacity and equilibrium adsorption capacity of HA-N-MPC (mg · g-1), KL (L · mg-1) is the Langmuir constant which is related to the affinity for the binding sites, KF (L · mg-1) and n (dimensionless) are Freundlich constants that indicate the adsorption capacity and adsorption intensity, respectively. The Langmuir isotherm dimensionless separation factor (RL) which describes the favourability of an adsorption process was given by the Eq. (5): RL =

1 1 + K L c0

(5)

The fitted curves for each isotherm model were shown in Fig. 8 (A, B). The calculated Langmuir and Freundlich parameters determined from the slopes and intercepts of Eq. 3 and Eq. 4 were listed in Table 2. The analyses showed that the Langmuir isotherm model provided a better fit with the experimental data since the

21

correlation coefficient R2 value was higher than that obtained by application of the Freundlich isotherm model [39]. The Langmuir model assumed that Cr(VI) adsorption ceased at monolayer coverages and that specific homogenous adsorption sites existed on HA-N-MPC for Cr(VI) [40]. Generally, the separation factor (RL) has one of three different values: RL = 1, 0 < RL < 1 and RL = 0, suggesting linear, favorable and irreversible adsorption, respectively. The calculated RL values at different initial concentrations of Cr (VI) were in the range 0.00314 to 0.0275 (0 < RL < 1), and close to zero, indicating that Cr(VI) adsorption on HA-N-MPC was favorable and a nearly irreversible process (Table 2). The low RL values were evidence for a relatively strong interaction between the HA-N-MPC and the Cr (VI) ions. Therefore, HA-N-MPC due to its high specific surface area and multiple adsorption sites, can be considered a very effective adsorbent for Cr(VI) ions at low pH in aqueous solution. Table 2 Constants and correlation coefficients of Langmuir and Freundlich models for Cr(VI) adsorption onto HA-N-MPC. Temperature (K)

Langmuir equation

Freundlich equation

KL (L · mg-1)

R2

KF (L · mg-1)

n

R2

17.68

0.9435

13.82

2.10

0.7667

293.15

3.7. Magnetic separation and regeneration of the adsorbent The rapid separation and reusability of the HA-N-MPC was subsequently explored to assess its real application potential for Cr(VI) removal from aqueous solution. The 22

magnetic property of HA-N-MPC which allowed rapid removal of the adsorbent from solution was highlighted in Fig. S3 (A, B and C). The figure showed digital photographs for a Cr(VI) solution before and after HA-N-MPC addition. After HA-N-MPC addition, the characteristic color of the Cr(VI) solution disappeared, indicating the high adsorption efficiency of the adsorbent. On applying an external magnetic field, the Cr-HA-N-MPC could be rapidly by separated from solution, leaving a colorless solution. Following Cr(VI) adsorption experiments, the used adsorbent (Cr-HA-N-MPC) was separated magnetically from solution, and then washed thoroughly with double-distilled water to neutrality for the adsorbent regeneration. After magnetic separation and drying at 60 °C for 24 h in vacuum oven, the regenerative adsorbent was reused for Cr (VI) removal. Table S1 showed the adsorption capacity of HA-N-MPC for Cr(VI) before and after the adsorption-desorption cycle. The recovered adsorbent retained 96.0-98.5% of the adsorption capacity of the fresh adsorbent for Cr(VI) removal, indicating that HA-N-MPC still had excellent reusability as an adsorbent for Cr(VI) removal. Importantly, the regeneration process was both green and simple. The remarkable recyclability of HA-N-MPC may in part be due to its inherent mesoporosity and open porous structure, allowing facile diffusion of Cr ions to and from adsorption sites [33]. To assess the structural stability of HA-N-MPC, the Fe leaching concentrations in solution after adsorption of HA-N-MPC were measured at pH 2. As shown in Fig. S4,

23

the concentration of total Fe in solution increased with the prolonging of the adsorption time, and total dissolved Fe reached 1.9 mg · L-1 when the adsorption time was 4 h. This value was comparable to that of the magnetic adsorption materials reported in the literature [41, 42], confirming the synthesized HA-N-MPC in this study exhibited superior stability in acidic solution. On the basis of its high adsorption performance, low cost, good magnetic properties and excellent regeneration capacity, HA-N-MPC represented a very promising candidate for the large scale treatment of Cr(VI) polluted wastewater. 3.8. Cr(VI) removal mechanism analysis

In order to better investigate the removal mechanism of Cr(VI), XPS spectra of the Cr-HA-N-MPC (i.e. HA-N-MPC following Cr(VI) adsorption) were studied and the results were shown in Fig. 9. The XPS survey spectrum of Cr-HA-N-MPC (Fig. 9A) in the binding energy range 800-200 eV revealed the presence of iron, chromium, oxygen, nitrogen and carbon, with carbon and oxygen being the most abundant elements. The high resolution C 1s scan (Fig. 9B) was deconvoluted into three component peaks located at 284.8 eV (C 288.9 eV (O C

C/C C), 286.4 eV (C

O or C

N) and

O) [43]. The N 1s spectrum (Fig. 9C) was deconvoluted into two

peaks, pyridinic-N at 398.4 eV and pyrrolic-N at 400.3 eV [44]. The Cr 2p spectrum for Cr-HA-N-MPC (Fig. 9D) showed two sets of signals. Peaks at 579.5 and 588.8 eV were assigned to the Cr 2p3/2 and Cr 2p1/2 signals, respectively, of a Cr(VI) species, whereas peaks at 577.8 and 587.4 eV corresponded to a Cr (III) species. Comparing 24

the relative intensities (peak areas) of the Cr(VI) and Cr(III) signals, it could be concluded that Cr-HA-N-MPC contained significantly more Cr(III) than Cr(VI) [45]. The XPS results conclusively demonstrated that Cr (VI) was reduced to nontoxic Cr (III) by electron donors (amino, hydroxyl, Fe3O4, etc.) on HA-N-MPC. According to the discussion mentioned above, the removal mechanism of Cr(VI) mainly involved three processes: (1) Due to the large surface area and the developed mesoporous structure, HA-N-MPC easily and quickly captured Cr (VI) species; (2) The negatively charged Cr(VI) species were adsorbed via the electrostatic interaction and chemical interactions onto the surface of HA-N-MPC at low pH in aqueous solution; (3) The adsorbed Cr(VI) were reduced to Cr(III) via electron transfer from some electron-donor on HA-N-MPC adsorbent [46, 47]. Therefore, the HA-N-MPC adsorbent prepared in this work is simple and low-cost, and capable of promoting Cr(VI) removal by concurrent adsorption and reduction.

4. Conclusions A low cost synthetic strategy was developed for transforming lignin in black liquor into a novel porous and magnetically recyclable adsorbent material (HA-N-MPC) for Cr(VI) removal from aqueous media. At pH 2, HA-N-MPC displayed a very high adsorption capacity for Cr(VI) of 130.5 mg · g-1, which was much higher than the sorption capacity reported in the literature for other N-doped carbon adsorbents. Furthermore, the adsorption process obeyed a pseudo-second-order model, wherein Cr(VI) adsorption on HA-N-MPC was mainly controlled by chemical adsorption. The 25

adsorption data agreed well with the Langmuir isotherm model, indicating that Cr(VI) adsorbed on HA-N-MPC up to monolayer coverage. Further, HA-N-MPC showed outstanding reusability, magnetic separation properties and superior stability. All data suggested that HA-N-MPC was a very promising adsorbent materials for the removal of toxic Cr(VI) from aqueous media.

Acknowledgments This work was supported by the National Key R&D Program of China (2017YFD0801504), National Natural Science Foundation of China (Nos. 41701350 and 41771342), and the Natural Science Foundation of Shandong Province, China (No. ZR2017MD002).

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Figure Captions

Scheme 1. Schematic illustration of the fabrication of HA-N-MPC for Cr(VI) removal from aqueous solution.

Fig. 1. SEM images of HA-N-MPC (A, B) and TEM images of HA-N-MPC (C, D).

Fig. 2. (A) FT-IR spectra of N-MPC (curve a) and HA-N-MPC (curve b) (inset: the FT-IR spectra of HA-N-MPC subtracted N-MPC (curve i) and HA (curve ii)). (B) The EDX spectrum of HA-N-MPC (inset: the relative weight percentages and atomic percentages of different elements in HA-N-MPC). Fig. 3. (A) N2 adsorption and desorption isotherms and (B) pore size distribution of HA-N-MPC. Fig. 4. Magnetization curves of prepared HA-N-MPC, regenerated HA-N-MPC and reused HA-N-MPC.

Fig. 5. Cr(VI) removal percentage of different adsorbent materials with time.

Fig. 6. Cr(VI) removal efficiency (a) and the total Cr removal efficiency (b).

Fig. 7. Kinetics of Cr (VI) adsorption on HA-N-MPC (A) effect of contact time; (B) plot for pseudo-first order kinetics and (C) plot for pseudo-second order kinetics.

Fig. 8. (A) Langmuir and (B) Freundlich isotherms for Cr (VI) adsorption on HA-N-MPC (Cr (VI) concentration: 2, 4, 8, 10, 13, 18 mg · L-1).

Fig. 9. XPS spectra for Cr-HA-N-MPC: (A) Survey spectrum, (B) C 1s, (C) N 1s, (D) Cr 2p.

34




A good adsorbent for removal and detoxification of Cr(VI) has been synthesized




The addition of HA can prevent Fe3O4 agglomeration




The removal of Cr(VI) occurs mainly via adsorption and reduction




The prepared adsorbent shows excellent magnetic separation performance