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A high-capacity and photo-regenerable composite material for efficient adsorption and degradation of phenanthrene in water Wen Liu, Zhengqing Cai, Xiao Zhao, Ting Wang, Fan Li, and Dongye Zhao Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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Environmental Science & Technology

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A high-capacity and photo-regenerable composite material for

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efficient adsorption and degradation of phenanthrene in water

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Wen Liu,† Zhengqing Cai,† Xiao Zhao,† Ting Wang,‡ Fan Li,† Dongye Zhao†,*

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†

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University, Auburn, AL 36849, USA

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‡

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Department of Environmental Engineering, Peking University, Beijing 100871, China

Environmental Engineering Program, Department of Civil Engineering, Auburn

The Key Laboratory of Water and Sediment Sciences, Ministry of Education,

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

* Corresponding author

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D. Zhao, Tel: +01-334-844-6277, Fax: +01-334 844 6290,

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E-mail: [email protected] 1

ACS Paragon Plus Environment


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Abstract

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We report a novel composite material, referred to as activated charcoal supported

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titanate nanotubes (TNTs@AC), for highly efficient adsorption and photodegradation

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of a representative polycyclic aromatic hydrocarbon (PAH), phenanthrene.

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TNTs@AC was prepared through a one-step hydrothermal method, and is composed

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of an activated charcoal core and a shell of carbon-coated titanate nanotubes.

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TNTs@AC offered a maximum Langmuir adsorption capacity of 12.1 mg/g for

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phenanthrene (a model PAH), which is ~11.5 times higher than the parent activated

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charcoal. Phenanthrene was rapidly concentrated onto TNTs@AC, and subsequently

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completely photodegraded under UV light within two hours. The photo-regenerated

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TNTs@AC can then be reused for another adsorption-photodegradation cycle without

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significant capacity/activity loss. TNTs@AC performed well over a wide range of pH,

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ionic strength, and dissolved organic matter. Mechanistically, the enhanced adsorption

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capacity is attributed to the formation of carbon-coated ink-bottle pores of the titanate

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nanotubes, which are conducive to capillary condensation; in addition, the modified

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micro-carbon facilitates transfer of excited electrons, thereby inhibiting recombination

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of the electron-hole pairs, resulting in high photocatalytic activity. The combined high

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adsorption capacity, photocatalytic activity and regenerability/reusability merit

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TNTs@AC a very attractive material for concentrating and degrading a host of

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micro-pollutants in the environment.

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1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are well-known water pollutants for

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their detrimental biological effects, toxicity, mutagenecity and carcinogenicity.1,

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Phenanthrene is one of the most widely detected PAHs in the environment, especially

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at oil or coal tar contaminated sites and printing or dyeing wastewaters.3,

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Phenanthrene has been used as a model PAH in many environmental studies due to its

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universality, persistency and toxicity.1, 5-7

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2

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The adsorption/desorption behaviors for both conventional (e.g., activated carbon)

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and emerging (e.g., carbon nanotubes (CNTs)) materials have been widely studied

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and documented.8-18 In recent years, many researchers have reported that carbon

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nanomaterials (CNMs) can effectively adsorb PAHs.8-17 For instance, single-walled

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and multi-walled CNTs were found to offer 2−4 orders of magnitudes higher

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adsorption capacity for phenanthrene than fullerene.8 In terms of adsorption

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mechanisms, Chen et al. proposed that the strong adsorption of nitroaromatics by

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CNTs was due to π−π interactions between nitroaromatic molecules (electron

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acceptors) and the highly polarizable graphene sheets (electron donors).16 While these

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emerging materials may offer improved performances than conventional ACs, their

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much higher material cost prohibits their practical applications. Moreover, the

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potential nano-toxicity, and occupational and environmental health risks of CNMs

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remain under active investigations.19-21

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Both conventional and emerging carbonaceous adsorbents have been designed to

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adsorb/concentrate hydrophobic contaminants without chemical transformation. Often

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times, such adsorption-based technologies are puzzled by some key technical

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obstacles, such as regeneration/reusability of spent adsorbents and treatment of spent

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regenerant.22, 23 To facilitate “green” and cost-effective regeneration and reuse of the 3



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spent adsorbents, it has been desirable to develop composite materials that combine

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high adsorption capacity and reactivity. For instance, catalysts-modified CNMs have

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been shown to be able to adsorb and catalytically degrade organic compounds,23-25

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and the chemical transformation also regenerates the material.

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Photocatalytic degradation of persistent organic pollutants including PAHs has

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picked strong momentum.26, 27 While innovative photocatalysts have been consistently

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sought, TiO2 remains the most widely used photocatalyst for its low cost and sound

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photo-activity.2, 7, 28, 29 Derived from TiO2, titanate nanotubes (TNTs) were prepared

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through hydrothermal treatment,30 and have drawn immediate interests for their large

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specific surface area, nano-scale structures, ion-exchange property and good

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photoelectric response.31, 32 TNTs are not only excellent adsorbents for heavy metals,

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but also promising photocatalysts.33-39 However, due to the hydrophilic nature, TNTs

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can hardly adsorb hydrophobic hydrocarbons such as PAHs, which also severely

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inhibits photocatalytic degradation of PAHs.28, 40

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While carbon nanofiber or graphene have been used to enhance the

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photocatalytic activity of TNTs,41,

42

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AC-supported TNTs to our knowledge. Taking advantage of the high adsorption

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capacity of AC and photocatalytic activity of TNTs, we conceived a novel composite

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material, referred to as TNTs@AC, by depositing TNTs onto a common activated

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charcoal. the newly synthesized TNTs@AC is expected to show the following

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synergistic effects: 1) the high adsorption capacity of AC will concentrate the target

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organic pollutants onto the surface of TNTs@AC, facilitating the subsequent

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photocatalytic degradation; 2) the high photocatalytic activity of TNTs will facilitate

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effective degradation of the adsorbed pollutants, which also regenerates the spent

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TNTs@AC; 3) the hydrothermal treatment during the material synthesis may

there has been no information available on

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facilitate micro-AC coating on TNTs, and the AC-amended TNTs will enhance both

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the adsorption capacity and kinetics; and 4) AC on TNTs may serve as electron

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shuttles and prevent recombination of the excited holes and electrons, and thus

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enhance the photodegradation efficacy.

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As such, the overall goal of this study was to develop a novel bi-functional

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material

that

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photodegradation/regeneration for rapid and complete removal of PAHs (and possibly

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other trace organic contaminants) in water. Specifically, using phenanthrene as a

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model PAH, this works aimed to: (1) synthesize and characterize the desired

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TNTs@AC; (2) test the adsorption kinetics and capacity of TNTs@AC; (3) examine

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the effects of various water chemistry conditions on adsorption, including pH, ionic

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strength

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photo-degradation/regeneration efficiency and material reusability; (5) elucidate the

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underlying mechanisms.

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2. Materials and Methods

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2.1. Chemicals

and

offers

natural

both

high

organic

adsorption

matters

capacity

(NOMs);

(4)

and

efficient

evaluate

the

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All chemicals were of analytical grade or higher. Nano-TiO2 (P25, 80% anatase

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and 20% rutile) was purchased from Degussa (Evonik) of Germany. Sodium

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hydroxide (GR) and absolute ethanol were obtained from Acros Organics (Fair Lawn,

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NJ, USA). A DARCO granular activated charcoal (20−40 mesh) was acquired from

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Sigma-Aldrich (St. Louis, MO, USA) and used as received. The key considerations

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for the choice of AC included: 1) moderate adsorption affinity toward the target

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contaminants so that adsorbed contaminants are available for subsequent

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photodegradation, 2) some carbon can be released to modify the TNTs during the

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hydrothermal treatment to facilitate photodegradation, and 3) relatively larger pore

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size to avoid pore clogging. Phenanthrene (Table S1 in Supporting Information (SI))

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was purchased from Alfa Aesar (Ward Hill, MA, USA), and a stock solution of 2 g/L

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was prepared by dissolving phenanthrene in methanol. A standard leonardite humic

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acid (LHA, IHSS 1S104H) containing 64% of total organic carbon (TOC) was chosen

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as the model NOM.41

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2.2. Synthesis and Characterization of TNTs@AC

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TNTs@AC was synthesized through a one-step hydrothermal method based on

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our previous studies on TNTs preparation.37, 38 Typically, 2.4 g AC and 1.2 g TiO2

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were mixed with 66.7 mL of a 10 mol/L NaOH solution. After stirred for 12 h, the

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mixture was transferred into a Teflon reactor with a stainless steel cover, and then

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heated at 130 °C for 72 h. The black precipitate (TNTs@AC) was then separated and

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washed with deionized (DI) water till pH ~7.5−8.5, and then dried at 80 °C for 4 h.

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For comparison, plain TNTs were also prepared separately via the same procedure but

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without AC,37, 38 and a sample of amended AC was also prepared by subjecting the

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parent AC to the same hydrothermal treatment without TiO2.

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Methods on material characterization are provided in Section S1 in SI. 2.3. Adsorption Kinetic and Isotherm Experiments

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Phenanthrene adsorption kinetic and equilibrium experiments were carried out to

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gauge the adsorption rate and capacity of TNTs@AC. Details on the experimental

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methods are provided in Section S2 in SI.

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2.4. Photo-Regeneration and Reuse of TNTs@AC

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Regeneration of spent TNTs@AC was performed through photodegradation of

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phenanthrene adsorbed on TNTs@AC. Upon adsorption equilibrium, the mixture was

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left still for 1 h to allow the spent TNTs@AC to settle by gravity (>99% of

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TNTs@AC settled). Then, ~90% of the supernatant was removed, and the residual

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solid-liquid mixture was transferred into a glass photo-reactor with a quartz cover.

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The reactor was then placed under UV light (365 nm, 1.42 mW/cm2). After 60 minʼs

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UV irradiation, the liquid was decanted and the solid was extracted for remaining

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phenanthrene using 20 mL methanol at 80 °C for 4 h.43 The regenerated TNTs@AC

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was then reused in another cycle to adsorb and degrade phenanthrene, and the

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adsorption-regeneration cycles were repeated 5 times to probe reusability of the

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material.

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2.5. Chemical Analysis

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Methods for chemical analysis are provided in Section S3 in SI.

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3. Results and Discussion

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3.1. Characterizations of TNTs@AC

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Figure 1 presents scanning electron microscope (SEM) images of the parent AC

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and TNTs@AC. While the surface of AC appeared bulky, flat and smooth (Figure 1a),

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the surface of TNTs@AC appeared rather rough and full of clusters of aggregates

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(Figure 1b). A close-up of the surface revealed that the tubular TNTs formed an

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interweaved network spreading throughout the surface (Figure 1c). The length of the

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nanotubes stretched up to hundreds of nanometers. The energy-dispersive X-ray

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spectra (EDS) (Figures 1d and 1e) reveal four major elements C, O, Na and Ti on the

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surface of TNTs@AC, indicating that TNTs were not just simply coated on AC,

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rather the nanotubes are intermingled with AC, i.e., some AC is also coated on TNTs

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(further confirmed below).

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[Figure 1]

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Figure S1 displays the X-ray diffractometer (XRD) patterns of neat TNTs, AC

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and TNTs@AC. For neat TNTs, the peak at 9.4°, 24.4°, 28.1°, 48.2° and 61.5° are all

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assigned to sodium tri-titanate,34, 37, 44 with a basic structure of NaxH1-xTi3O7 (x =

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0−0.75, depending on the remaining sodium).33, 34, 37 The tri-titanate is composed of

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corrugated ribbons of triple edge-sharing [TiO6] as a skeletal structure and

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H+/Na+ located in interlayers.33,

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interlayer distance (crystal plan (020)) of TNTs.33, 34, 37 For AC, the two peaks at 26°

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and 43° are attributed to the diffractions of crystal planes of graphite (002) and (100),

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respectively.45-47 For TNTs@AC, all the peaks observed for TNTs remained, and in

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addition, the graphite (002) peak was observed, confirming the SEM finding that AC

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is covered by TNTs with some AC coated on the surface TNTs. The Si impurities

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(quartz/cristoballite-SiO2) in the raw AC were removed in TNTs@AC upon the

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hydrothermal-alkaline treatment and the subsequent washing process (Section 2.2).

34, 37

In addition, the peak at 9.4° represents the

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Figure S2 displays X-ray photoelectron spectroscopy (XPS) spectra of AC and

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TNTs@AC, and Table S2 lists the corresponding atomic compositions. 7.1% of Ti

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and 1.7% of Na are detected for TNTs@AC. Based on the Na/Ti ratio and the general

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molecular formula of NaxH1-xTi3O7 for TNTs,34, 37 the compositions of the synthetic

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TNTs can be identified as Na0.7H1.3Ti3O7. Based on carbon content in AC (82.1%) and

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Ti mass added, the overall mass ratio of AC to TNTs in the composite material is

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~1.7:1. The high resolution spectra of C 1s appeared similar before and after the

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hydrothermal treatment (Figure S2b), while the C atomic percent associated with the

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π−π bond increased from 9.9% for AC to 13.0% for TNTs@AC (Table S3), indicating

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that TNTs@AC may offer stronger adsorption of aromatic organic compounds

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through π−π interactions.48 The high resolution spectra of O 1s (Figure S2c) reveal

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that the lattice O increased from 22.6% for AC (due to inorganic oxide impurities 8


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such as SiO2) to 72.8% for TNTs@AC (due to [Ti−O6]) (Table S3),46, 48, 49 confirming

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accumulation of TNTs on AC. The O peak at 532.3 eV in TNTs@AC is assigned to

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Ti−O/C−O, which suggests formation of a linkage of C−O−Ti between TNTs and AC.

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The Fourier transform infrared spectroscopy (FTIR) spectra (Figure S3 and Section

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S4 in SI) also confirm the new peak at 1081 cm-1 that is assigned to C−O−Ti bond.47

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In addition, the C−OH peak of AC at 1091 cm-1 was not only much lowered, but also

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shifted to 1097 cm-1 in TNTs@AC,50, 51 indicating decreased carbon-oxygen groups.

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The hydrothermal treatment and loading of TNTs lowered the measured BET

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surface area from 566.1 m2/g for AC to 471.6 m2/g for TNTs@AC, and the pore

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volume from 0.61 to 0.52 cm3/g (Table S4). Considering the compositions of

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TNTs@AC (AC:TNTs mass ratio of 1.7: 1) and the specific surface area of TNTs

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(272.3 m2/g),38 the measured BET surface area of TNTs@AC is very close to the

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calculated value 470.6 m2/g, suggesting that the hydrothermal treatment did not

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significantly alter the AC surface area. However, the measured pore volume is much

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lower than the theoretical value of 0.85 cm3/g of TNTs@AC (calculated as the

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weighted average of the mean pore volumes for neat AC and TNTs), which supports

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the postulate that some micro-AC may have intruded into the pores of TNTs during

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the hydrothermal treatment. TNTs@AC exhibits a bimodal pore size distribution

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profile with a primary peaking at ~4 nm and a secondary peaking at 2−2.5 nm (Figure

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S4b), which are attributed to the pores of AC and conversion of larger pores (>10 nm)

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of TNTs into more micropores in TNTs@AC, respectively.52 The pore size

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distribution (Figure S4b) also indicates that most of the larger pores (>10 nm) in AC

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disappeared in TNTs@AC, suggesting that these macro-pores may also be blocked

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due to the leached AC particles and/or growth of TNTs at the mouth of the

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macro-pores. It is less likely for TNTs to intrude into the internal finer pores due to

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their interwoven tubular structure and size exclusion. The AC coating on the

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nanotubes reduced the mean pore diameter of TNTs from 18.6 to 3.7 nm of

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TNTs@AC. Figure S5 shows that zeta potential of TNTs@AC is much less negative

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than neat TNTs, indicating that AC coating on TNTs shielded part of the functional

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groups (−OH/−ONa) on TNTs.35, 37 The pHPZC values were measured to be 3.1 for

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TNTs@AC, 2.6 for TNTs and 6.8 for AC (Table S4).38

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3.2. Adsorption and Desorption of Phenanthrene

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Figure 2a shows adsorption kinetics of phenanthrene by TNTs@AC. TNTs@AC

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displayed rapid uptake rate. The adsorption equilibrium was reached in 180 min, with

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a high removal efficiency of 96.8% at equilibrium, and most (>92%) of the adsorption

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capacity was filled in the first 60 min. In contrast, the parent AC showed much slower

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kinetics and lower phenanthrene capacity (74.9% removal at 600 min). Furthermore,

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the hydrothermally treated AC showed only slightly enhanced kinetics and

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equilibrium uptake compared to the original AC, and much lower capacity than

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TNTs@AC. These observations indicate that the TNTs play an important role in

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phenanthrene adsorption by providing more accessible sites and added adsorption

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capacity. Table S5 shows that the pseudo-second-order model best-fits the

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experimental kinetic data (R2 =1) for TNTs@AC, whereas the intraparticle diffusion

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model performs worst (see Section S6 for the models), which differs from standard

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AC where film or intraparticle diffusion often controls the adsorption rate,53

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suggesting that the rate-controlling step for TNTs@AC is due to chemical

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interactions.54

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[Figure 2]

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Figure 2b compares the adsorption isotherms of phenanthrene by TNTs@AC,

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parent AC, treated AC, and neat TNTs in the low concentration range of

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