Chitosan-coated-magnetite with Covalently Grafted Polystyrene Based Carbon Nanocomposites for Hexavalent Chromium Adsorption

Hongbo Gu ^1,*, Xiaojiang Xu ¹, Hongyuan Zhang ¹, Chaobo Liang ³, Han Lou ¹, Chao Ma ¹, Yujie Li ¹, Zhanhu Guo ² and Junwei Gu^3,*

¹ Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, China

² Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee. 37966, USA

³ MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’ an, Shaanxi, 710072, China

* Corresponding Author(E-mail) : hongbogu2014@tongji.edu.cn (H. Gu); gjw@nwpu.edu.cn (J. Gu)

Abstract 

A chitosan-coated-magnetite with covalently grafted polystyrene (g-PS) based magnetic carbon nanocomposites composed of carbon coated magnetite nanoparticles with carbonized g-PS microsphere are prepared for effective hexavalent chromium (Cr(VI)) adsorption. The adsorption measurements have indicated that the fabricated magnetic nanocomposites FC515 exhibit an effective Cr(VI) adsorption performance and the optimal pH for Cr(VI) adsorption on FC515 is 3.0 at 298 K. The Cr(VI) adsorption kinetics on FC515 is found to follow the pseudo-second-order behavior with a room temperature initial adsorption rate of 7.106 g mg^-1 min^-1 for the solution with an initial Cr(VI) concentration of 6.5 mg L^‑1 and pH value of 3.0. The adsorption isotherm test illustrates that the Langmuir isotherm model with a monolayer adsorption is well fitted for Cr(VI) adsorption on FC515 than the Freundlich isotherm model. The thermodynamic parameters including negative standard Gibb’s free energy change and standard enthalpy change suggest that the Cr(VI) adsorption on FC515 is spontaneous and exothermic. After 5 cycles, FC515 still maintain 86% of Cr(VI) adsorption capacity, exhibiting a good reusability in practice. The zeta-potential and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the Cr(VI) adsorption on FC515 is mainly from the electrostatic interaction between FC515 and Cr(VI). This work provides a new method to design the novel structure of magnetic carbon nanocomposites with natural polymer chitosan and thermoplastic PS for Cr(VI) wastewater treatment.

Table of Content

A chitosan-coated-magnetite with covalently grafted polystyrene (g-PS) based magnetic carbon nanocomposites composed of carbon coated magnetite nanoparticles with carbonized g-PS microsphere exhibit effective Cr(VI) adsorption performance.

Keywords

Chitosan          Magnetite          Grafted polystyrene          Carbon nanocomposites          Cr(VI) adsorption

1. Introduction

Hexavalent chromium (Cr(VI)) released from electroplating, leather tanning, printing, and pigments is considered as a hazardous and toxic contaminant in water system.¹ In order to achieve the US Environmental Protection Agency (US-EPA) limitation (total Cr lower than 100 mg L^-1 in drinking water),^2, 3 it’s urgently required to remove Cr(VI) from waste water system. Currently, among various kinds of technologies for Cr(VI) removal from wastewater including chemical precipitation, ion exchange, membrane filtration, and biomass, adsorption is a competitive method due to its simplicity, low cost, and high efficiency.⁴ Hence, it’s demanded to design and develop the novel adsorbent materials towards Cr(VI) removal with high efficiency and low cost.

Chitosan is an abundant natural aminopolysaccharide containing both of glucosamine and acetylglucosamine moieties, which is derived from the N-deacetylation of chitin.⁵ The amino and hydroxyl groups on the polymer backbone of chitosan are able to serve as the chelating sites for the removal of heavy metals from polluted water.⁶ However, the adsorption performance of chitosan is normally controlled by the molecular weight and deacetylation degree of the raw chitin.⁷ In addition, chitosan is sensitive to the solution pH. On one hand, it’s easily dissolved in the acidic solution (especially as solution pH is less than 5); on the other hand, the protonation of amino group on chitosan in the acidic solution could severely decrease its adsorption capacity.⁸ Therefore, it’s commonly needed to be crosslinked to enhance the stability of chitosan. Laus et al.⁹ used epichorohydrin (ECH) and triphosphate (TPP) to covalently crosslink with chitosan as the adsorbent, which possessed the maximum adsorption capacities for Cu(II), Cd(II), and Pb(II) of 130.72, 83.75, and 166.94 mg g^-1, respectively. Wu et al.¹⁰ utilized ECH and triethylenetetramine (TETA) to graft and crosslink chitosan and the obtained modified chitosan exhibited  adsorption capacity for Cu(II) and Ag(I) of 117.6 and 151.20 mg L^-1, respectively. Also, chitosan often serves as the shell material to prepare the magnetic chitosan nanocomposites for heavy metal removal in order to easy recycle and reuse the absorbents. Chen et al.¹¹ prepared the magnetic nanosized chitosan composites, which exhibited the maximum Cu(II) adsorption capacity of 35.5 mg g^-1. Zhu et al.¹² studied the adsorption properties of Pb(II), Cu(II), Zn(II) onto xanthate-modified magnetic chitosan and the maximum adsorption capacities for Pb(II), Cu(II), Zn(II) were 76.9, 34.5, and 20.8 mg g^-1, respectively. Yet, even the magnetic chitosan could be manipulated by an external magnetic field, the coating material chitosan still has the poor acid resistant property. The crosslinking process may also reduce the adsorption capability of chitosan, especially if this process involves in the reaction of amino groups.¹³   

In recent years, magnetic carbon nanocomposites have gained considerable attentions in the heavy metal adsorption due to the high specific surface area and good environmental stability of carbon materials, and easy recycle process of magnetic materials. The carbon shell could protect the magnetic metal oxides from the acid etching and the magnetic core could help recycle of nanocomposites and reuse it for the heavy metal removal. Therefore, many efforts have been dedicated to the preparation of magnetic carbon nanocomposites with novel structures. However, there is still no report on the chitosan based magnetic carbon nanocomposites yet. Currently,  in our previous work, the ECH grafted polystyrene (PS) has served as the carbon precursor to form the magnetic carbon nanocomposites for organic pollutant tetrabromobisphenol A (TBBPA)¹⁴ and toxic heavy metal Cr(VI) removal.² Nevertheless, both PS and chitosan behaved as the carbon precursor to prepare the magnetic carbon nanocomposites for heavy metal removal are rarely reported.

In this paper, magnetite nanoparticles were prepared by a co-precipitation method with the n-octanoic acid as a surfactant, and the chitosan-coated-magnetite with covalently grafted polystyrene based magnetic carbon nanocomposites were synthesized by the chemical reaction combined with high temperature annealing. The effects of parameters during high temperature annealing process on the specific surface area, microstructure and adsorption effect of magnetic carbon nanocomposites were studied. The microstructure, crystal structure, thermal oxidation degradation performance, and magnetic properties of magnetic carbon nanocomposites were determined by a series of characterizations including scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The recovery and reuse performance of magnetic carbon nanocomposites were evaluated by the cyclic adsorption experiments. The Cr(VI) adsorption characteristics was analyzed by adsorption kinetics, isothermal adsorption and adsorption thermodynamics. Finally, zeta potential and XPS characterization were used to explore the Cr(VI) adsorption mechanism. This work is aiming to design a novel magnetic carbon nanocomposite structures with chitosan and PS for Cr(VI) removal from water system.¹⁵

2. Experiment

2.1 Materials

Aluminum chloride anhydrous (AlCl₃), ferric chloride (hexahydrate) (FeCl₃×6H₂O), ethyl acetate (C₄H₈O₂), ferrous sulfate (heptahydrate) (FeSO₄×7H₂O), chitosan((C₆H₁₁NO₄)_n), ethanol (95%, v/v), anhydrous ethanol, acetone (C₃H₆O), epichlorohydrin (C₃H₅ClO), cyclohexane (C₆H₁₂) and ammonium hydroxide (NH₃∙H₂O, 25～28 wt%) were supplied by Sinopharm Chemical Reagent Co., Ltd. Polystyrene (PS) was purchased from Taizhou Suosi education equipment Co., Ltd. 1-butyl-3-methylimidazole (C₈H₁₅ClN₂) and n-octanoic acid (C₈H₁₂O₂) was provided from TCI Company, Shanghai. All the chemicals were used as-received without any further treatment.

2.2 Fabrication of chitosan-coated-magnetite with covalently grafted PS based carbon nanocomposites

First, the PS was grafted with epichlorohydrin (g-PS) as published in the previous work.¹⁶ Second, the FeSO₄∙7H₂O and FeCl₃∙6H₂O with a mole ratio of 1:2 were dissolved in deionized water (25 mL) at room temperature under the magnetic stirring. After that, n-octanoic acid (1.0 mL) was dripped in the above solution and the ammonium hydroxide was added until pH to 10-11. The solution was kept at 80 ^oC in a water bath for 1 h to form the magnetite nanoparticles. Then, the mixture was respectively washed with deionized water and 95% (v/v) ethanol for three times, and dried in an oven at 60 ^oC for 6 h. The sample was calcined in a tube furnace at 350 ^oC for 2 h in N₂ atmosphere in order to increase the crystallinity of magnetite nanoparticles. Third, the magnetite (0.25 g), chitosan (0.25 g) and 1-butyl-3-methylimidazolium (ionic liquid, 7.0 g) were put in a 25 mL of beaker, and the mixture was heated to 50 ^oC in a water bath and mechanically stirred for 1 h. Then, the mixture was washed with deionized water to completely remove the ionic liquid. The mixture was filtered and washed with deionized water and 95% (v/v) ethanol, separately, for three times, and then dried in an oven at 60 ^oC for 12 h. Finally, the g-PS solution was transferred into a 50 mL of three-necked flask, and 0.5 g of chitosan-coated-magnetite nanocomposites was ultrasonically added into the above solution and reflux at 80 ^oC for 1 h. The mixed solution was washed with ethanol for five times, and dried in an oven at 60 ^oC for 12 h. The FTIR and TGA shown in Fig. S1 affirms the formation of covalent bond between chitosan coated magnetite and g-PS. Then the dried samples were placed in a tube furnace and calcined under the N₂ atmosphere to obtain the magnetic carbon nanocomposites. The preparation procedure of chitosan-coated-magnetite with covalently grafted PS based carbon nanocomposites is displayed in Scheme 1. The samples prepared at 450 ^oC with calcination time of 30 min; 500 ^oC with calcination time of 0, 15, and 30 min; 600 ^oC with calcination time of 30 min were respectively indexed as FC430, FC500, FC515, FC530 and FC630. For details, please see the supporting information.

 Scheme 1   Schematic for the preparation process of chitosan-coated-magnetite with covalently grafted PS based magnetic carbon nanocomposites.

2.3 Characterizations

SEM (S-4800 high-resolution field emission scanning electron microscope) and TEM (Tecnai G20 type transmission electron microscope) were used to observe the microstructures of samples. The thermal oxidation degradation characteristics of the samples were investigated by a STA 409 differential scanning calorimeter. The TGA curve was recorded at a heating rate of 20 ^oC min^-1 within a temperature range from 25 to 850 ^oC and an air flow rate of 20 mL min^-1. The crystalline structure of samples was characterized by a D8 Advance X-ray powder diffractometer with Cu-Kα as the diffractive light source (λ = 1.5406 Å) within the diffraction angle ranges from 10 to 70 º at a scanning speed of 10 º min^-1. The element composition and valence state of the prepared samples were analyzed by an AXIS Ultra DLD X-ray photoelectron spectroscopy (XPS). The radiation source was Al-Kα (hν = 1486.6 eV) with an operating voltage of 12 kV and a current of 10 mA, respectively. The resulting element was deconvoluted into peaks on a Shirley background using the Gaussian-Lorentz function (Gaussian = 80%, Lorentzian = 20%). The specific surface area and pore size distribution of the samples were conducted by a Tristar 3020 specific surface area and porosity analyzer. The specific surface area and pore size distribution of samples were calculated based on the BET (Brunauer-Emmett-Teller) equation and the BJH (Barrett-Joyner-Halenda) model. Magnetic measurement was performed on a PPMS-9T (EC-II) physical measurement system at a temperature of 298 K within the magnetic field range of 0-3 T. The surface zeta potentials of the nanoadsorbents were determined by a DLS Particle Size analyzer (Zetasizer Nano-ZS, Malvern, U.K.).

2.4 Cr(VI) adsorption measurements

Cr(VI) adsorption characteristics by the as-prepared magnetic carbon nanoadsorbents was evaluated and all the measurements were conducted in a thermostatic ultrasonic cleaner (model: KH5200DE). The concentration of Cr(VI) was determined by an inductively coupled plasma optical emission spectrometry (ICP-OES, 8300). The initial Cr(VI) concentration was adjusted by diluting 1.0 g L^-1 of stock solution with deionized water. The reported value for each sample was the average of three measurements with a standard deviation of ±5%. The Cr(VI) adsorption amounts q_t and the Cr(VI) removal percentage (R%) were respectively calculated through Equation (S1) and (S2). The details for Cr(VI) adsorption experiments please refer to supporting information.

3. Results  and discussion

3.1 Optimal preparation conditions exploration of magnetic carbon nanocomposites

In order to study the optimal preparation conditions of magnetic carbon nanocomposites, the BET specific surface area of magnetic carbon nanocomposites prepared under different process conditions including FC430, FC500, FC515, FC530, and FC630 is analyzed since the BET results are related to the adsorption performance of materials and the results are shown in Table S1. The FC430, FC500, FC530 and FC630 have a specific surface area of 25, 41, 30, and 24 m² g^-1, correspondingly. And the magnetic nanocomposite FC515 calcined at 500 ^oC for 15 min possesses the largest specific surface area of 58 m² g^-1. This illustrates that the different preparation conditions can cause the different specific surface areas of magnetic carbon nanocomposites.

Meanwhile, the Cr(VI) removal percentage by these magnetic carbon nanocomposites prepared under different conditions has served as a justification to explore the optimal fabrication conditions of chitosan-coated-magnetite with covalently g-PS based magnetic carbon nanocomposites. As mentioned in the experimental section, FC430, FC500, FC515, FC530, and FC630 (10 mg) were added into the 2.0 mg L^-1 of Cr (VI) solution with a pH of 7.0 for ultrasonic treatment of 5 min and the obtained experimental results are displayed in Fig. S2. The removal percentage of Cr(VI) by FC430, FC500, FC515, FC530 and FC630 is 38.6, 59.4, 70.4, 57.0 and 46.4%, respectively. It’s concluded that FC515 exhibits the highest Cr (VI) removal percentage.

Based on the BET results and Cr(VI) removal performance, the optimum conditions for the preparation of magnetic carbon nanocomposites are at a temperature of 500 ^oC for 15 min in the N₂ atmosphere, which is FC515 sample. The subsequent experiments were conducted by using FC515.

3.2 Structure characterization of FC515

In order to study the microstructure characteristics of samples, the SEM images with high and low magnifications for as-prepared Fe₃O₄, chitosan coated Fe₃O₄ and FC515 have been performed and the obtained results are depicted in Fig. S3. The SEM images of Fe₃O₄ with and without n-octanoic acid are shown in Fig. S4. The results manifest that after adding n-octanoic acid, the morphology of Fe₃O₄ nanoparticles becomes uniform. This confirms the role of n-octanoic acid as a surfactant. In Fig. S3a and d, it’s seen that the as-prepared Fe₃O₄ has a spherical structure with a smooth surface and an average diameter about 20-30 nm. After coated with chitosan, Fig. S3b and e, the surface of Fe₃O_4­ become relatively rougher. However, the FC515 has a totally different microstructure, which is composed of big carbon microspheres surrounded by small nanoparticles, Fig. S3c and f. The big carbon microsphere probably from the functionalized PS has a diameter of 300-400 nm. The small nanoparticles may be caused by the carbonized chitosan coated Fe₃O₄.