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Sugarcane-converted Graphene-Like Material for the Super-High Adsorption of Organic Pollutants from Water via Co-assembly Mechanisms Xin Xiao, Baoliang Chen, Lizhong Zhu, and Jerald L. Schnoor Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03639 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Environmental Science & Technology

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Sugarcane-converted Graphene-Like Material for the Super-High Adsorption of

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Organic Pollutants from Water via Co-assembly Mechanisms

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Xin Xiao1,2, Baoliang Chen1,2*, Lizhong Zhu1,2, Jerald L. Schnoor3

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1. Department of Environmental Science, Zhejiang University, Hangzhou 310058, China;

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2. Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China

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3. Department of Civil and Environmental Engineering, University of Iowa, Iowa City, IA, USA

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*

To whom correspondence may be addressed. Phone & Fax: 86-571-88982587 E-mail: [email protected]

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1

ACS Paragon Plus Environment


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Abstract

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A sugarcane-converted graphene-like material (FZS900) was fabricated by carbonization and activation.

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The material exhibited abundant micropores, water-stable turbostratic single-layer graphene nanosheets,

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and a high BET-N2 surface area (2280 m2 g-1). The adsorption capacities of FZS900 toward naphthalene,

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phenanthrene, and 1-naphthol were 615.8, 431.2, and 2040 mg g-1, respectively, which are much higher

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than those of previously reported materials. The nonpolar aromatic molecules induced the turbostratic

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graphene nanosheets to agglomerate in an orderly manner, forming 2-11 graphene layer nanoloops,

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while polar aromatic compounds induced high dispersion or aggregation of the graphene nanosheets.

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This phase conversion of the nanosized materials after sorption occurred through co-assembly of the

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aromatic molecules and the single-layer graphene nanosheets via large-area π-π interactions. An

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adsorption-induced partition mechanism was further proposed to explain the nanosize effect and

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nanoscale sorption sites observed. This study indicates that commonly available biomass can be

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converted to graphene-like material with super-high sorption ability in order to remove pollutants from

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the environment via nanosize effects and a co-assembly mechanism.

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Table of Contents

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Morphology variation before and after sorption

Before sorption

After NAPH sorption

After PHE sorption

graphene nanosheets

Co-assembly sorption mechanism for ultra-high sorption

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After 1-NAPH sorption

After MB sorption

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Introduction

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Thousands of industrial and natural chemical compounds discharge into receiving waters every day.

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Toxic and persistent organic micro-pollutants threats human health,1 making cost-effective and

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appropriate remediation materials and technologies necessary. Sorption is relatively mature, low-cost

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and highly efficient technology for pollutant remediation, and researchers continue to seek effective

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sorbents with higher surface areas, partition coefficient and sorption capacities.2 For traditional

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carbon-rich sorbents (such as charcoal, soot, and biochar),3, 4 the condensed structures consume their

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large sorption potential via limiting internal sites.5 Therefore, novel sorbents with extended structures

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are gaining more and more attentions.

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Graphene is a novel two-dimensional material made of single-atom-thick carbon with a honeycomb

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crystal lattice,6, 7 and it shows great potential for environmental applications.8-10 With its high theoretical

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surface area (2630 m2 g-1) and strong π-π interactions,9, 11 graphene seems to be an ideal material to sorb

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toxic aromatic pollutants. However, graphene is not economically affordable to manufacture, on large

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scale even when prepared via commonly known Hummers method, which also requires large chemical

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inputs. Chemical vapor deposition (CVD) can produce high-quality graphene but the process requires

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large resource and energy input. Biomass sources, such as soybean oil, cookies, chocolate, grass, plastics,

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roaches, and dog feces, have been studied as precursors for CVD in order to reduce resource

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consumption.12, 13 However, large-scale production is still difficult to achieve economically with the

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CVD method. Moreover, due to strong hydrophobic interactions and large-area π-π interactions,

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graphene nanosheets are unstable in water and tend to self-aggregate before associating with pollutant

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molecules,14 which inhibits the adsorption of single-layer graphene nanosheets and then limits their

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potential applications. Besides graphene, for other novel carbon-rich sorbents (such as fullerene, and

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carbon nanotubes), the self-aggregate interaction consumes lots of sorption sites. Therefore, the 4


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development of sorbent with stable expanded structure in aqueous phase is challenging and necessary

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for environmental application.

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Biomass is derived from numerous sources and comparatively cheaper; it could be the agricultural

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waste and commonly found in large quantity around the world every year, and also requires management

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and disposal.15 For example, in 2005, the worldwide production of sugarcane waste exceeded 1018

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million tons.16 The conversion of biomass to graphene-like material composed of single- or a few-layer

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nanosheets has the potential to solve the challenges mentioned above. In this study, an inexpensive and

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stable graphene-like material was successfully synthesized via the carbonization and activation of

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sugarcane as a representative biomass source. The preparation of the graphene-like material and possible

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reaction route is illustrated in Fig. 1. Briefly, FeCl3 and ZnCl2 were mixed with sugarcane biomass in a

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water bath, and then the mixture was carbonized at 900 °C under N2 atmosphere, during which α-Fe and

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Zn nanoparticles were generated in situ via reaction with the reduction gas released from the thermal

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degradation of the sugarcane.17 The ZnCl2 and Zn nanoparticles evaporated due to the low boiling point

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of 723 °C and 906 °C, respectively18 (Fig. 1). The metal components in the carbonized solid included

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α-Fe, Fe3C, and FeO(OH), based on the characterization of the X-ray diffraction pattern (Fig. S1), which

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functioned as templates for the graphene nanosheets. After the metal residues were removed by HCl, the

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final carbonized sugarcane materials were obtained and are denoted FeCl3-ZnCl2-sugarcane-900°C (abbr.

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FZS900) (Fig. 1). The adsorption capabilities of naphthalene, phenanthrene, 1-naphthol, methylene blue,

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and toluene by FZS900 were evaluated. The nano-structure of FZS900 before and after sorption were

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characterized by high-resolution transmission electron microscope (HR-TEM), and employed to prove

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the novel co-assembly mechanism.

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Materials and Methods

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Materials. Sugarcane was purchased from fruit market nearby Zijingang Campus, Zhejiang University,

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Hangzhou, China. The sugarcane was peeled off properly and thoroughly rinsed three time with mill-Q

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water before use. Analytical grade FeCl3 and ZnCl2 were purchased from Shanghai Chemicals, Shanghai,

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China. Naphthalene (NAPH > 98%) and methylene blue (MB > 98%) were from Acros Organics.

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Phenanthrene (PHE > 98%) was supplied by J&K Chemical. 1-Naphthol (1-NAPH > 99%) was obtained

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from Aladdin. Toluene was obtained from Hangzhou Chemical Reagent Co., Ltd., China with the

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analytical grade. All other solvents such as methanol, and acetonitrile that were used for high

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performance liquid chromatography (HPLC) were purchased from Honeywell/Burdick & Jackson, with

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HPLC grade. The eluent water was mill-Q water.

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Fig. 1. Preparation of the graphene-like FZS900 material derived from sugarcane biomass and the possible reaction route.

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FZS900 Preparation. The schematic preparation procedures of FZS900 is shown in Fig. 1. In

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general, FZS900 was produced by simultaneous graphitization and activation and with FeCl3 and ZnCl2

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from agricultural biomass waste (sugarcane) which is abundantly available in China.20 Briefly, 3.0 g

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sugarcane bulk material was placed into a 100 mL beaker with 50 mL of 3.0 mol L-1 FeCl3 aqueous

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solution. After adding 9.0 g ZnCl2, the mixture was placed into a stirred water-bath at 80 ºC for 2 days.

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At this time, the sugarcane bulk turned to powder form and FeCl3 and ZnCl2 penetrated inside the

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sugarcane. Then the mixture was moving to a drying oven for ~ 5 days at 80 ºC to evaporate water. The

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solid product was then transferred to a quartz boat and placed into a pipe furnace (GSL-1600X, Hefei

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Kejing Material Technology Co., Ltd, China) under N2 flow (0.6 mL min-1). Before starting the heating

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program, the air inside the pipe furnace was exhausted by using a vacuum pump prior to heating and

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refilled it with 99.999% N2 gas three times. The mixture was heated up to 900 ºC with controlled heating

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rate of 5 ºC min-1 and holding for 1 hour. After naturally cooling down to room temperature, the solid

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was transferred into 100 mL of 2 mol L-1 HCl aqueous solution and stirred for 8 hours three times.

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Copious bubbles were generated during the stirring. After rinsing with milli-Q water 5 times, the residue

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was vacuum freeze-dried (LGJ-18A, Beijing Sihuan Instrument Company, China) and passed through a

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100-mesh sieve (< 0.154mm size). Finally, the graphene-like material was obtained yielding around

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14.12% and referred as “FeCl3-ZnCl2-Sugar900” (abbr. FZS900). The FZS900 was further characterized

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and utilized in batch sorption experiments.

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Batch Sorption Experiments. The methods for the sorption experiments of naphthalene and

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phenanthrene were cited from previous literatures9, 21. Detailed methods regarding the sorption kinetics

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of naphthalene and phenanthrene, and the sorption isotherm of naphthalene, phenanthrene, methylene

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blue, and 1-Naphthol, and the data analysis of sorption isotherm were given in Supporting Information.

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Sorption kinetics results indicated that the required sorption equilibrium time of NAPH and PHE onto

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FZS900 was approximately 5 min and 24 h, respectively (Fig. S2). A stabilized pH (around pH=7.0) for

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both 1-NAPH and MB isotherm sorption experiments was confirmed (Fig. S3) to achieve a stable UV

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spectra. The unchanged UV spectra of 1-NAPH before and after sorption results indicate that there were

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no transformation of 1-NAPH during the sorption (Fig. S4). And the detailed operation conditions for

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anlyzing NAPH and PHE via HPLC were provided in Table S1 and S2, respectively.

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Sorption Test for Toluene Gas. The sorption of toluene, an important volatile organic pollutant,

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onto FZS900 was also tested. 14.99 mg FZS900 were weighed on an empty small glass culture petri dish

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which was placed on a larger glass culture petri dish. Then, a 1 mL toluene droplet was placed on the

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larger dish as shown in Fig. S5. The whole dish was covered with another glass dish for 15 min, and the

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sorbed quantity of FZS900 was determined by calculating the mass difference before and after sorption.

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The sorbed quantity of toluene was 8.09 mg, yielding a sorption quantity of 539.7 mg/g of FZS900.

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Theoretical Calculation of Monolayer Sorption Quantity. Based on the Van der Waals radius,

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the molecular area of NAPH, PHE, 1-NAPH, and MB are calculated to be 0.732 nm2, 1.00 nm2, 0.789

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nm2, and 1.30 nm2, respectively22-24. Properties of those sorbates are given in Table S3. Then the

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theoretical estimated sorption values from maximal monolayer arrangement can be calculated by the

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following equation:

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Where QT refers to the theoretical estimated sorption quantity, mg g-1; SA is the surface area of FZS900,

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m2 g-1, which is 2280 m2 g-1 (Fig. 2); Sm is the molecular area of NAPH (or PHE, or 1-NAPH, or MB),

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nm2; NA is the Avogadro constant,

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g mol-1. The theoretical calculated monolayer sorption quantities of NAPH, PHE, 1-NAPH, and MB

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onto FZS900 are 663.3 mg g-1, 674.9 mg g-1, 692.2 mg g-1, and 931.9 mg g-1, respectively. Considering

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the highest sorption quantities of FZS900 to NAPH, PHE, 1-NAPH, and MB are 615.9 mg g-1, 431.2 mg

mol-1; Mr is the molecular weight of NAPH (or PHE),

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g-1, 2040 mg g-1, and 474.7 mg g-1, then 1-NAPH seems to be the only compound that shows multilayer

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sorption behavior onto FZS900 in this work, which is consistent with a previous report.23 A comparison

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of the adsorption data of FZS900 in the current study with that of traditional and novel materials from

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the literatures was provided in Fig. 3.

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Structural Characterization.

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BET-N2: The surface area and pore size distribution of FZS900 were analyzed by the

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Brunauer–Emmett–Teller method (BET-N2) on NOVA-2000E surface area analyzer using N2 as the gas

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molecule (0.162 nm2). In brief, 17.47 mg of FZS900 was degassed under vacuum (to less than 0.1 Pa)

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for 12 h at 378 K. After it cooled down to room temperature naturally, the N2 sorption-desorption

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analysis started under liquid N2 temperature 77K. Surface area was determined by the Brunauer -

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Emmett - Teller (BET) method, using seven data sorption points, between relative pressures of 0.05 and

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0.3 based on the monolayer adsorption theory. Pore distribution was calculated by all the desorption data

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points according to the Barrett – Joyner - Halenda (BJH) method. Data are presented in Fig. 2 (A, E).

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SEM: Scanning electron microscope (SEM) (FEI Quanta 3D FEG) was utilized to observe the

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surface morphology of FZS900. A tiny sample was directly adhered to the double-coated

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carbon-conductive tabs on stubs. Then the stub was gilded with Pt for 60 s to enhance the sample

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conductivity prior to SEM observation under 5kV. Data are presented in Fig. 2 (B, F).

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AFM: Atomic force microscope (AFM) images of FZS900 on cleaved mica were taken under a

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tapping mode by atomic force microscopy (AFM, SPI-3800N, Seiko Co.) After sonicating for 30

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minutes, a droplet of FZS900 dispersion ~0.01mg/mL was cast onto the fresh mica surface. The sample

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was then kept under an infrared lamp at room temperature to allow the water to evaporate before the

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AFM investigation. Results are shown in Fig. S6.

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TEM: Morphologies at the nanoscale and atomic scale of FZS900, FZS900+NAPH, and

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FZS900+PHE were observed by applying high-resolution transmission electron microscope (HR-TEM).

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In order to observe the nanosheet structure of FZS900, a low boiling point solvent, chloroform was used

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to exfoliate the nanosheet graphene in FZS900 according to previous reports25, 26 because a low boiling

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point solvent can be easily removed before the sample enters the vacuum system of TEM. To be more

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specific, 4.0 mg FZS900 sample was added into a 40 mL glass bottle with 40mL trichloromethane (AR,

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Sinopharm Chemical Reagent co., Ltd, China). The suspension was further sonicated (SK7200HP,

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Shanghai Kedao Ultrasonic Instrument Co, Shanghai, China) at 40% power (53 kHz, power: 350W) for

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30 min. After centrifuging at 500 rpm for 90 min, the supernatants were dropped into carbon-film-coated

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copper micro-grid. Since CHCl3 is a low boiling point solvent, it dried fast in several minutes before

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observations with HR-TEM (FEI Tecnai G2 F20 S-TWIN) operated at 200kV attached with energy

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X-ray dispersive spectrometry (EDS) to achieve the element distribution of FZS900. The corresponding

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images are shown in Fig. 2(C, D, G, H) and the corresponding EDS results was given in Fig. S7. For the

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morphology observation of FZS900 before sorption, the original FZS900 sample was tested by directly

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putting the sample (dispersed in water) onto the carbon-film-coated copper micro-grid, and for FZS900

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after sorption (including NAPH, PHE, 1-NAPH, and MB sorption), the highest-sorbate-concentration

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mixture in the isotherm experiments was dipped onto carbon-film-coated copper micro-grids in order to

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achieve the morphology of FZS900 with the highest sorption quantity in our experiments. HR-TEM

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(FEI Tecnai G2 F20 S-TWIN) operated at 200kV was applied for FZS900 morphology variation after

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NAPH, PHE, 1-NAPH, and MB sorption, the corresponding images were shown in Fig. 4(A, C, G, I)

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and Fig. S2(A, B, C, D, F, G, H). EDS spectrum of FZS900+PHE was also recorded to obtain the

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element distribution and was provided in Fig. S7. To achieve better results, HR-TEM equipped with

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Cs-corrector (FEI Titan 80-300) was also operated at 80kV for the observation of FZS900 sorbed with

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PHE. Corresponding results are presented in Fig. 4(E) and Fig. S2(E).

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XRD: X-ray powder diffraction (XRD) was applied to monitor the mineral phase of FZS900 before

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HCl washing. The sample directly used the carbonized solid after pyrolysis of FeCl 3, ZnCl2, and

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sugarcane. The spectrum was obtained at a diffraction angle region of 5−70° by an X-ray diffractometer

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(RIGAKU D/MAX 2550/PC, Japan) with Cu−Kα radiation. The resulting spectrum is shown in Fig. S1.

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XPS: X-ray photoelectron spectroscopy (XPS) is a powerful and sensitive technology to measure

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the surface elemental compositions of solid samples. The XPS characterization was performed via a VG

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Escalab Mark II with a scanning step of 0.5 eV from 0 to 1000 eV. The results was shown in Fig. S8.

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The C 1s spectra of FZS900 and FZS900+PHE samples were recorded at 284.4 eV with a scanning step

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of 0.2 eV as shown in Fig. S9.

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Raman: Raman spectra of FZS900, FZS900+PHE, and FZS900+1-NAPH were characterized to

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monitor the transformation of the graphene-like nano-sheet layers by a Lab Ram HRUV Raman

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spectrometer (JDbin-yvon, FR). The laser excitation wavelength was 514 nm, and the Raman spectra

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was measured from 100 cm-1 to 3500cm-1 with a step of 0.444 cm-1. The FZS900+PHE and

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FZS900-1-NAPH

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highest-sorbate-concentration mixture from the isotherm experiments with a porous alumina membrane

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(pore size: 200nm, Whatman). The membrane with FZS900+PHE and FZS900+1-NAPH was further

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oven-dried for Raman characterization. Results are shown in Fig. S9.

samples

for

Raman

test

were

prepared

by

vacuum

filtrating

the

206

FTIR: Fourier transform infrared spectroscopy (FTIR) was utilized to record the functional groups

207

of FZS900 before and after 1-NAPH and MB sorption. Briefly, after freeze drying, the samples was

208

mixed with KBr (at a ratio of 0.5 mg sample/ 100 mg KBr), grinded, and pressed. Then the FTIR spectra

209

was recorded using a Nicolet 6700 spectrometer in the 400~4000cm-1 region with resolution of 1 cm-1.

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For all the samples, the air background spectrum was subtracted, and 64 interferograms were collected.

211

The corresponding spectra are shown in Fig. S10.

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Results and Discussion

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Morphology of FZS900. The surface and structural morphology of FZS900 was observed by different

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characterization techniques. N2 sorption-desorption results at 1 atm and 77 K using the

216

Brunauer-Emmett-Teller method (BET-N2) show that FZS900 has a sorption capacity of 2866 mg g-1 for

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N2 (Fig. 2A), which is much higher than that reported for other metal-organic frameworks.27 The

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calculated surface area of FZS900 (2280 m2 g-1) is close to the theoretical surface area of single-layer

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graphene nanosheets (2630 m2 g-1), implying a less agglomerated graphene nanosheet structure. The

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pore size distribution (Fig. 2E, red line) indicated that abundant mesopores (2-50 nm) were present in

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FZS900, which was also observed in the SEM images (Fig. 2B and 2F). In addition, the AFM images of

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FZS900 show that the graphene nanosheets have a depth of ~1.2 nm (Fig. S6). Although the pure single

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layer graphene has a thickness of 0.34 nm, it is widely believed that a depth around 1 nm could be seen

224

as single-layered structure due to the existence of functional groups, structural defects and adsorbed

225

water molecules.28,

226

demonstrated the microporous (

Sugar Cane-Converted Graphene-like Material for ... - ACS Publications

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