Norfloxacin and Bisphenol-A Removal Using Temperature-Switchable

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Norfloxacin and Bisphenol-A Removal Using Temperature-Switchable Graphene Oxide Na Yao, Xuntong Zhang, Zhen Yang, Weiben Yang, Ziqi Tian, and Limin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07233 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

Norfloxacin and Bisphenol-A Removal Using Temperature-Switchable Graphene Oxide Na Yao †,#, Xuntong Zhang †,#, Zhen Yang †,‡,*, Weiben Yang †,*, Ziqi Tian§, Limin Zhang † †

School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of

Material Cycling and Pollution Control, Nanjing Normal University, Nanjing 210023, China ‡

Changzhou Institute of Innovation & Development, Nanjing Normal University,

Changzhou 213022, China §

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of

Sciences, Ningbo 315201, China #

Both authors contributed equally.

*

Corresponding authors.

E-mails: [email protected] (Zhen Yang), [email protected] (Weiben Yang)

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Graphene oxide (GO) is a competitive candidate used for adsorption of emerging organic contaminants (EOCs) from water. To overcome GO’s spontaneous aggregation tendency in adsorption and to ease contaminant desorption from the adsorbent

for

adsorbent

temperature-switchable

regeneration,

a

modified

hydrophilicity/hydrophobicity,

GO

(P-GO),

obtained

by

with

grafting

temperature-responsive poly(N-n-propylacrylamide) was proposed. Two model EOCs, norfloxacin

(NOR)

and

hydrophilicity/hydrophobicity

bisphenol were

A

employed.

(BPA), P-GO

with showed

distinct significant

temperature-responsive adsorption behaviors: P-GO was more hydrophilic at a lower temperature and was beneficial for the adsorption of hydrophilic NOR; whereas it turned more hydrophobic at a higher temperature and was preferred for the adsorption of hydrophobic BPA. Compared with GO, P-GO under corresponding optimal conditions had comparable large adsorption amounts for NOR due to an “adsorption sites replacement” strategy, and notably enhanced adsorption for BPA because of strengthened hydrophobic association. Main interfacial binding interactions were π-π electron donor-acceptor effect and H-bonding for NOR adsorption, and hydrophobic association and H-bonding for BPA uptake. Based on the temperature-responsive adsorption behaviors and studied interfacial interactions, regeneration of the adsorbent at designed temperatures using water (without additional chemicals) as eluent is realized. This achievement is important for reducing risks of secondary environmental pollution during regeneration and easing further recovery of organic contaminants if needed. 2


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KEYWORDS: graphene oxide, temperature-responsive polymers, emerging organic contaminants, adsorption, hydrophobic association,

1. INTRODUCTION Water contamination caused by emerging organic contaminants (EOCs) has received extensive attention,1 among which antibiotics and endocrine-disrupting chemicals are

two major varieties.2

Owing

to

toxicity,

persistence and

non-biodegradability, EOCs have adverse effects on human health and aquatic ecosystems even at low exposure levels.3 There is an urgent requirement for developing effective technologies to remove EOCs from aquatic environment. Among different methods,4,5 adsorption gains its popularity in elimination of EOCs: It’s advantages contain simple operation, wide adaptability, low cost and high efficiency.6 Numerous adsorbents have been explored.7 Graphene oxide (GO), the famous two-dimensional nanomaterial, is a competitive candidate.8 GO can be facilely synthesized by modified Hummers method, and is abundant with oxygen-containing functional groups and sp2-hybridized regions.9 Such structural features make GO bind organic molecules tightly through hydrogen bonding, electrostatic attraction, hydrophobic association, π-π electron donor-acceptor (EDA) effect, etc.,10,11 ensuring its efficient performance in elimination of organic contaminants. However, two shortcomings, i.e. (i) tendency of spontaneous aggregation and (ii) poor regenerablilty after adsorption, of GO exist.12 From the viewpoint of thermodynamics, GO layers tend to aggregate with each other spontaneously via strong π-π interaction and hydrophobic effect. This tendency inhibits the exposure of active adsorption sites towards contaminants.13 On the other hand, the strong binding interactions between GO and organic contaminants engender difficulty in desorption of contaminants for 3



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adsorbent regeneration, even using elution with extensive solvent (acidic, alkaline, and/or organic ones).14 Since it is widely acknowledged that reuse of adsorbents after regeneration is one of the most notable advantages in adsorption technique, the latter shortcoming limits GO’s practical applicability in water remediation.15 To overcome the weak points, grafting temperature-responsive polymer chains onto GO’s sheets is proposed to provide potential solution due to following reasons: (i) For one thing, oxygen-containing functional groups of GO are beneficial for chemically bonding polymers chains, and steric hindrance produced by polymer chains on GO sheets could effectively prevent the aggregation of different sheets,16 resulting in more exposed adsorption sites. Meanwhile, the anchored polymer could possibly possess additional adsorption sites.17 (ii) For another, temperature-responsive polymer bonded on GO sheets could make it easier for the release of adsorbed contaminants from the adsorbent.18,19 Temperature-responsive polymers, such as the well-known poly(N-alkylacrylamide), will switch their feature between hydrophilicity and hydrophobicity when temperature rises over or drops below their unique temperature-switches (called the Lower Critical Solution Temperature, LCST).20 Such temperature-switches provide an easy-operated means to either enhance (for uptake of contaminants)

or

weaken

(for

elution

of

contaminants)

hydrophobicity-/hydrophilicity-related binding interactions between adsorbents and contaminants by adjusting water temperature without additional chemicals. More importantly, using water without additional chemicals as eluent could further reduce the risk of secondary environmental pollution during regeneration process,21 solving such a “pain point” in adsorption technique. Despite the above proposal, related work based on this idea is rare. Consequently, a

temperature-switchable

GO

(noted

as

4


P-GO)

modified

by

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poly(N-n-propylacrylamide) (PNNPAM) was proposed as an adsorbent for removal of two EOCs (norfloxacin (NOR) and bisphenol A (BPA), a widely detected antibiotic and an endocrine-disrupting chemical, respectively) from water. Effects of temperature, pH, ionic strength and coexisting natural organic matter (NOM, using humic acid (HA) as the model in this paper) on adsorption performance were studied. Simultaneously, by employing both instrumental and computational chemistry tools, interfacial interactions in adsorption were explored. Based on the adsorption performance and interfacial interactions, regeneration of the adsorbent by using water without additional chemicals was achieved.

2. MATERIALS AND METHODS 2.1. Materials. GO was prepared according to a modified Hummers method.9 PNNPAM with carboxyl end groups (PNNPAM-COOH) was prepared according to reported method.22

Preliminary experiments and literatures demonstrated that different

average repeated unit numbers of a PNNPAM-COOH chain resulted in different LCSTs (Supporting Information (SI) Table S1).23,24 Given the temperature range (0-40 o

C) of commonly available contaminated water, the PNNPAM-COOH with a LCST of

21 oC (close to the middle of the above range) was selected as a model in further P-GO synthesis for better clarifying the temperature effect. P-GO was fabricated by a “graft-to” method as illustrated in Fig. 1a. Detailed synthetic routes, characterization methods and results confirming the chemical structure of P-GO were provided in Supporting Information (SI) Texts S1 to S3 and Figs. S1 to S5. -Fig. 1NOR (pure drugs, logKOW of -1.70) and BPA (> 99.80% purity, logKOW of 3.32), 5



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the two model EOCs employed for comparison, were purchased from Dalian Meilun Biotechnology Co., Ltd. and Tianjin Guangfu Fine Chemical Research Institute, respectively. Their physicochemical properties were listed in SI Table S2. All other chemicals were bought from Sinopharm Chemical Reagent Co. Ltd. Water was ultrapure.

2.2. EOCs Adsorption Adsorption tests were conducted in 10-mL tubes at designed temperatures. All tubes were placed in air bath in an incubator shaker to control the temperature. A shaking speed of 140 rpm was kept to make sure the well dispersibility of adsorbents. 9 mL of contaminant aqueous solution with designed initial concentration and pH in each tube. Then, 10 mg of adsorbent were added. After adsorption, EOC concentrations were detected by Ultra High-performance Liquid Chromatograph coupled with mass spectrometer (Thermo Finnigan TSQ Quantum Series). Controlled examples without adsorbents were tested to exclude the influence of EOCs onto the surface of tubes. Adsorption capacity, Qe (mg/g), was obtained according SI Text S4. Unless specified, 12 h was the time length for adsorption. 12 h is long enough to reach equilibrium. Kinetic and isothermal experiments were conducted at the corresponding optimal pHs. Each data was the average from triplicated tests.

2.3. Interfacial Interaction Investigation Interfacial

interactions

were

demonstrated

by

both

instrumental

and

computational chemistry tools. They included FTIR and XPS spectra, and density functional theory (DFT) calculations. Details about DFT calculations were in SI Text S5. 6


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2.4. Assessment of P-GO Reuse After EOCs were adsorbed onto P-GO under optimal conditions, the solid and liquid phases were separated by centrifugation at 8000 rpm for 10 minutes. Then, a known volume of eluent (water without additional chemicals) was used to desorb EOCs from P-GO (NOR was desorbed at 35 oC while BPA was desorbed at 15 oC; Reasons for the selection of temperature were based on the adsorption behaviors of P-GO as discussed below). After desorption equilibrium was reached, solid P-GO was separated by centrifugation for reuse. Eight repeated cycles were conducted to assess the recyclability.

3. RESULTS AND DISCUSSION 3.1. Characterization of P-GO 3.1.1. Improved Dispersibility Grafting polymer chains on GO sheets is above proposed to inhibit aggregation of adsorbents. This point is verified. As illustrated in X-ray diffraction (XRD) patterns of solid samples (Fig. 1b), graphite and GO exhibit peaks located at 2θ = ~26o and ~11o, respectively, which are in agreement with reported results.25 After the insertion of PNNPAM (with specific wide peak at 2θ = ~23o),26 interlayer spacing of sheets is enlarged with a smaller 2θ value (~9o). According to previous literature,27 steric hindrance caused by the grafted polymer chains could improve of the dispersibility of P-GO in water. Atomic force microscope (AFM) observation on fully dispersed primary nanosheets (1-2 layers, very low concentration (0.02 wt.‰) in water) is shown in Fig. 1c. It is evident that primary sheets of P-GO (2-3 nm) are thicker and rougher than 7



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those of GO (~1 nm) because of the anchoring of PNNPAM.16,28 However, when the concentration of adsorbents increases to 1 wt.‰, it is found from particle size distribution curves (Fig. 1d) that P-GO with larger primary particles in Fig. 1c exhibits smaller aggregate sizes than GO at each temperature. This phenomenon directly demonstrates P-GO has better dispersibility (anti-aggregation ability) than GO in water. Various factors could affect the dispersibility, including steric hindrance, surface

charge

property,

and

hydrophilicity/hydrophobicity.13

Although

hydrophobicity of P-GO increases (according to water contact angles (WCA) in Fig. 1e), the better dispersibility of P-GO here is due to both steric hindrance among grafted PNNPAM chains and P-GO’s relatively larger absolute value of zeta potentials (SI Fig. S6). 3.1.2. Temperature-switchable Hydrophilicity/Hydrophobicity WCA

of

adsorbents

at

different

temperatures

demonstrate

temperature-switchable hydrophilicity/hydrophobicity of P-GO (Fig. 1e). It is above expected and will be later confirmed to make the desorption of contaminants easier in adsorbent regeneration process. For GO, like most other hydrophilic materials, increased temperature generates slight decrease of WCA because of the enhanced molecular thermal motion.29 However, WCA of P-GO notably rises from 29o to 54o when temperature increases from 15 to 35 oC, which is ascribed to PNNPAM’s “hydrophilicity-to-hydrophobicity” transition accompanied by temperature crossing LCST. Additionally, molecular dynamics (MD) computational results of a PNNPAM chain in water (Fig. 1f and SI Fig. S7), owning 21 repeated units and grafted onto a GO sheet, give theoretical support. It is apparent the PNNPAM chain becomes more collapsed and more hydrophobic with fewer intermolecular H-bonds (from 51 to 21) to water molecules (SI Fig. S7) but more intramolecular H-bonds (from 1 to 6) (Fig. 8


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1f) as temperature rises from 15 to 35 oC. It could therefore be inferred that P-GO would possess higher Qe than GO towards more hydrophobic forms of contaminations at a high temperature (>LCST).

3.2. Application of P-GO to EOCs Adsorption 3.2.1. For Single-Contaminant Solutions The ability for P-GO to adsorb NOR or BPA is evaluated at two different temperatures (15 and 35 oC) over the pH range from 3 to 11 (Fig. 2). GO is tested for comparison. Species distributions of NOR or BPA as functions of pH are also depicted in Fig. 2 (dotted lines) as references for better discussion. -Fig. 2(i) In the case of uptake of NOR (Fig. 2a), Qe always reaches the largest value (Qe,max) for each adsorbent at pH=7 where neutral species of NOR are dominant. It is easily understood that anionic NOR species under alkaline condition is not favored by negatively surface-charged GO and P-GO (SI Fig. S6) due to electrostatic repulsion; whereas the phenomenon that cationic species at pH LCST but weakened at temperature < LCST due to the unique temperature-responsiveness of PNNPAM chains, providing guidance for desorption process.

3.4. Reuse of P-GO after Regeneration Using Water without Additional Chemicals After EOC adsorption under corresponding optimal conditions, regeneration of P-GO was tested by using water at controlled temperature without additional chemicals, based on the above studied adsorption behaviors and interfacial interactions. To be more specific, NOR was adsorbed at 15 oC and desorbed at 35 oC; 14


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while BPA was adsorbed at 35 oC and desorbed at 15 oC. Under such conditions, GO could not be regenerated, but P-GO could. Qe,max changes of P-GO in eight cycles of reuse are given in Fig. 5. Meantime, theoretical Qe,max values (listed in SI Table S6) have also been calculated according to the method described in SI Text S8, based on Langmuir adsorption equilibrium model. The experimental Qe,max values in each cycle is found reasonable by comparisons with corresponding theoretical ones. -Fig. 5According to Fig. 5, both Qe,max values for NOR and BPA are found to undergo notable decline, remaining ~85% and ~70% of the original values, respectively, in the first two cycles. This is because P-GO, especially GO sheets of P-GO, still owns a certain adsorption capacity for each contaminant at the corresponding temperature selected for desorption. However, after that, Qe,max values maintain stable in the following six cycles. From another point of view, the “effective” adsorption capacities of P-GO, which can be fully re-utilized after the adsorbent is regenerated for several times, could be regarded as ~120 mg/L for NOR and ~30 mg/L for BPA. The different behavior in adsorption/desorption processes for NOR and BPA indicates the difference in mechanisms between adsorption/desorptions of NOR and BPA. (1) In the case of NOR, the contaminant molecules are able to be adsorbed onto GO sheets by π-π EDA effect and H-bonding (Fig. 4d) as aforementioned. This interaction is significantly inhibited if the grafted PNNPAM branches become hydrophobic at 35 oC. As the MD simulation results shows in Figure 1f, when temperature rises from 15 to 35 oC, PNNPAM will cover a larger area of GO surface with the projection length increasing from 16.739 to 22.446 Å. This will engender fewer available adsorption sites for π-π EDA effect and H-bonding on the sheets. Consequently, fewer NOR molecules are able to be attached in desorption process at 15



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35 oC. On the other hand, NOR attached on PNNPAM chains via H-bonding (Fig. 4d) will also be weakened when temperature rises to 35 oC, because PNNPAM branches prefer to form intramolecular, instead of intermolelcular, H-bonds at 35 oC. Reasons from both aspects make it possible for the desorption of NOR from P-GO at 35 oC. (2) In the cases of BPA, as aforementioned, hydrophobic association between PNNPAM and BPA predominates (Fig. 4e). When the temperature changes from 35 to 15 oC, PNNPAM branches become more hydrophilic with an enhanced hydration layer. Such a layer inhibits the approach of hydrophobic BPA molecules, leading to the desorption of the contaminant. To sum up, grafting PNNPAM chains onto GO sheets realizes the original proposal, providing an example to enable the temperature-switchable GO to be recycled using water without additional chemicals. The choice of temperature for either adsorption or desorption in such an approach could be on the basis of the hydrophilicity/hydrophobicity (i.e. logKow) of contaminants. Compared to reported means using inorganic salt or alcohol solution as eluent,38-40 such an achievement is of significance in reducing the risk of secondary environmental pollution during regeneration, and easing further recovery of organic contaminants if needed. In order to increase or decrease eluent temperature for regeneration, energy and capital costs are needed. It should be marked that these costs are in a controllable range because both time length and eluent volume for desorption in practical application

are

controllable.

However,

concerns

might

arise

about

temperature-induced energy-consuming in adsorption. Artificially increasing or decreasing temperature of large amounts of real contaminated water for obtaining higher Qe would engender considerably extra energy-consuming. For instance, if the initial temperature of NOR-contaminated water is 30 oC, lowering temperature to a 16


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value below 21 oC (LCST of P-GO in this work) needs a lot of energy. Such concerns can be solved by selecting appropriate temperature-responsive polymer chains (both type

and

chain

length)

with

a

suitable

LCST

to

prepare

designed

temperature-switchable GO. This is feasible because LCST values of different types of temperature-responsive polymer with different chain lengths can be adjusted from 0 to 60 oC.23,24 For instance, without changing the temperature of a large amount of NOR-contaminated water originally at 30 oC, a temperature-switchable GO with a LCST higher than 30 oC can work as an effective hydrophilic adsorbent for the elimination of NOR. Additionally, another problem in regeneration of the adsorbent is the difficulty in separation of powder-like P-GO from water after use. In this work, well solid-liquid separation was achieved by centrifugation at 8000 rpm for 10 mins in authors’ lab. However, the operation of centrifugation in real water treatment will consume a lot of energy and capital cost. According to authors’ previous study on magnetic GO-based adsorbents,30 magnetic carrier technology (MCT) could be an efficient approach to save the cost and make the separation process easier. For example, introducing magnetic nanoparticles (such as Fe3O4) onto GO sheets of the adsorbent could facilely make the GO-based adsorbents be separated from water very fast in magnetic field, further improving practical water treatment efficiency.

4. CONCLUSIONS In summary, a unique temperature-switchable hydrophilicity/hydrophobicity adsorbent (P-GO) was designed and synthesized by grafting temperature-responsive PNNPAM chains onto GO sheets, and its adsorption performances for two EOCs (NOR and BPA) extractions were clarified through systematical batch adsorption 17



experiments. Results presented that adsorption amount of NOR reached 147mg/g while BPA reached 43mg/g under respective optimal adsorption conditions. Temperature lower than LCST of PNNPAM, when PNNPAM chains were hydrophilic, was beneficial for the uptake of hydrophilic NOR; whereas temperature higher than LCST, when the polymer chains were hydrophobic, promoted the adsorption of hydrophobic BPA. Such results were explained by instrumental and computational chemistry tools: π-π EDA effect and H-bonding were main interfacial binding interactions for NOR adsorption; hydrophobic association and H-bonding were for BPA uptake. A new desorption method was thereby developed using water without additional chemicals, based on the temperature-responsive adsorption behaviors of P-GO. Such a desorption method was cleaner with lower secondary pollution risk in adsorbents regeneration process, and could facilitate further recovery of organic matters if needed.

Supporting Information Additional characterization methods, characterization results, calculation methods, model descriptions, relevant tables and figures.

ACKNOWLEDGEMENTS The National Natural Science Foundation of China (51608275), the Natural Science Foundation of Jiangsu Province of China (BK20150981); the National Major Project of Science and Technology Ministry of China (2017ZX07202-004); the Foundation of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); the Scientific Computing Center of NNU. 18


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Captions Figure 1. (a) Schematic diagram of grafting PNNPAM onto GO sheets; (b) XRD patterns of GO, P-GO and graphite; (c) AFM images of GO and P-GO (insert: thickness of nanoplatelets); (d) Particle size distribution curves of GO and P-GO at different temperatures; (e) Static water contact angles of GO and P-GO at different temperatures; (f) Simulated conformation change of PNNPAM chains in water at different temperatures. Figure 2. Adsorption of (a) NOR and (b) BPA onto GO and P-GO (dotted lines are species distributions of contaminants); (c) Adsorption behaviors of P-GO in NOR-BPA binary-solutions. Figure 3. Effects of (a and b) ionic strength and (c and d) HA on NOR and BPA adsorption onto P-GO. Figure 4. (a) FTIR spectra of contaminants and P-GO loaded with contaminants; (b and c) XPS spectra of P-GO loaded NOR and BPA; (d and e) The most stable conformations of complexes formed between contaminants and segments of P-GO according to DFT calculation results. Figure 5. Qe changes in eight adsorption-desorption cycles (P-GO loaded with (a) NOR and (b) BPA was regenerated by water without additional chemicals at (a) 35 and (b) 15 oC, respectively).

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Norfloxacin and Bisphenol-A Removal Using Temperature-Switchable

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