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Environmentally Friendly in situ Regeneration of Graphene Aerogel as a Model Conductive Adsorbent Meilan Pan, Chao Shan, Xiaolin Zhang, Yanyang Zhang, Chanyuan Zhu, Guandao Gao, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02795 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Environmental Science & Technology

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Environmentally Friendly in situ Regeneration of

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Graphene Aerogel as a Model Conductive

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Adsorbent

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Meilan Pan†, Chao Shan‡ §, Xiaolin Zhang‡ §, Yanyang Zhang‡ §,

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Chanyuan Zhu‡ and Guandao Gao‡ §*, Bingcai Pan‡ §

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Submitted to Environmental Science & Technology

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† Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of

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Education), Tianjin Key Laboratory of Environmental Remediation and Pollution

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Control, College of Environmental Science and Engineering, Nankai University,

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Tianjin 300071, China

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‡ State Key Laboratory of Pollution Control and Resource Reuse, School of

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Environment, Nanjing University, Nanjing 210023, China

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§

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Research Center for Environmental Nanotechnology (ReCENT), Nanjing University, Nanjing 210023, China

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* Corresponding author, Guandao Gao Tel./Fax: +86-25-89681675

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E-mail: [email protected]

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Address: School of the Environment, Nanjing University, Nanjing 210023, China

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ACS Paragon Plus Environment


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Graphical Abstract:

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ABSTRACT

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Adsorption is a classical process widely used in industry and environmental

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protection, and the regeneration of exhausted adsorbents, as the reverse process of

35

adsorption, is vital to achieve a sustainable adsorption process. Chemical and thermal

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regeneration, which feature high costs and environmental side effects, are classical

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but not environmentally friendly methods. Herein, a new regeneration method based

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on an electrochemical process using graphene aerogel (GA) as a model conductive

39

adsorbent was proposed. First, 3D GA was prepared to adsorb organic and inorganic

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pollutants, avoiding the inconvenience of using powdered graphene. Then, the

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exhausted GA was cleaned by the electrochemical desorption/degradation of adsorbed

42

organic pollutants if undesired and the electro-repulsion of adsorbed metal ions in the

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absence of any additional chemicals, showing a high processing capability of 1.21 L

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g-1 GA h-1 and low energy consumption (~0.2 kWh m-3 solution). The mechanisms

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involved in the electrochemistry-induced desorption process cover a decline in the

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GA adsorption performance depended on the electrochemically adjustable surface

47

charge conditions, and the further repulsion and migration of adsorbates subject to the

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strong in situ electric field. This work has important implications for the development

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of environmentally friendly regeneration processes and qualified adsorbents, as well

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as the application of a green and efficient regeneration concept for traditional

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adsorption processes.

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INTRODUCTION

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Adsorption is a traditional and effective process applied widely in the chemical,

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pharmaceutical, food and water/wastewater industries. For a long time, much effort

55

has been made to improve the adsorption properties, including the adsorption

56

capacity, adsorption rate and preferential adsorption. However, the moderate

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regeneration of saturated adsorbents is vital for sustainable operation of the adsorption

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processes. Conventional regeneration strategies for adsorbents include chemical

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desorption by the addition of a high concentration of acid or base or organic solvents

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for various requirements

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which are effective but have numerous inherent drawbacks, such as significant

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chemical consumption and the generation of secondary waste streams

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high operational costs.

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susceptible to chemicals and operational temperatures. Currently, a novel,

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environmentally friendly and efficient regeneration strategy is still desired, and

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desorption based on physical factors, such as light, sound, and electric and magnetic

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fields in the absence of any additional chemicals, is a promising alternative.

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Electrochemistry is a potential regeneration technology because the adsorption

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capacity of the adsorbents depends on the surface charge conditions, which can be

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adjusted electrochemically. To realize electrochemical regeneration, adsorbents are

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required to be adsorbable, highly conductive, electrochemically active and chemically

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stable.

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1

4

and thermal treatment

2

in steam or hot water (>60 ℃),

3

as well as

In particular, products in the chemical industry are

Graphene family nanomaterial (GFN) including graphene, graphene oxide and 4


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reduced graphene oxide, a two-dimensional monolayer of carbon atoms, has attracted

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much attention due to its interesting properties, such as high electrical conductivity,

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high thermal stability and superior mechanical flexibility, 5-9 as well as a large specific

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surface area (SSA) of up to 2630 m2/g and the presence of abundant surface

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functional groups,

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treatment.

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difficult to separate and reuse.

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nanotoxicity once discharged into the environment.

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to address the abovementioned challenges, and one effective approach is the

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macroscopization of GFN nanosheets, in which individual GFN sheets are bonded

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together to construct three-dimensional (3D) networks to avoid stacking, while

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facilitating the reuse of GFN sheets. Graphene aerogel (GA) is a typical 3D GFN,

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assembled from GFN nanosheets with high specific surface area (SSA), high

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mechanical strength, and fast mass and electron transport kinetics due to the

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combination of 3D porous structures and the excellent intrinsic properties of GFN.

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22-24

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absorption of petroleum products, fats, and organic solvents, with an absorption

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capacity of 28 L of oil,

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The adsorption capacities for different dyes range from 115 to 1260 mg g-1, which are

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greatly superior to those of polymer adsorbents and most of other porous materials. 14,

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27, 28

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abundant functional groups on the GFN sheets, especially for Cr(Ⅵ) (15.6-139.2 mg

10

11, 12 13-16

making GFN a promising adsorbent for water and wastewater However, GFN sheets tend to restack in water, and they are 17-19

Additionally, nanoscale GFN may also present 20, 21

Much effort has been made

Therefore, GA shows outstanding absorption performance, especially for the

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290 g of petroleum, and 913 g of CCl4 per gram of GA.

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GA also possess significant adsorption capacities for heavy metal ions due to the

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g-1), Pb(Ⅱ) (69-373.8 mg g-1)29 and Cu2+ (46.6-228 mg g-1).

30, 31

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past decade, GA has been proven to be a promising adsorbent. Importantly, GA may

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qualify for electrochemical regeneration due to its high conductivity and

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electrochemical activity.

Generally, over the

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Herein, GA was selected as a model conductive adsorbent for the exploration of

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the electrochemical regeneration mechanism for the design of novel adsorbents

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amendable to electrochemical desorption and the development of environmentally

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friendly regeneration processes. Then, in situ electrochemistry-induced cleaning and

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recycling of GA was explored based on the adsorption of model pollutants (methylene

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blue and Cu2+) and electrochemical regeneration under released and compressed

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conditions,

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electrochemistry-induced-cleaning by the electrochemical desorption/degradation of

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adsorbed organic pollutants as well as the electro-repulsive interaction and enrichment

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of adsorbed metal ions in the absence of any additional chemicals; a strategy for

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further improvement in this electrochemistry-induced regeneration process was also

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proposed.

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EXPERIMENTAL SECTION

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Preparation and characterization of graphene aerogel

respectively.

It

was

found

that

GA

enables

in

situ

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Graphite oxide (GO) was synthesized from natural graphite flakes (average

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particle diameter of 44 μm, 99.95% purity, Qingdao Hengdeli Graphite Co., Ltd.)

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through a modified Hummers’ method. To synthesize graphene aerogel (GA), the GO 6


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sheets were first solvothermally assembled in ethanol to obtain a nearly homogenous

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solution and were then transferred to a sealed Teflon-lined autoclave and heated at

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180 ℃ for 12 hours (Raw GA). Following a slow exchange of ethanol with water, the

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aerogel was freeze-dried and then annealed under argon atmosphere at 600 ℃ for 2

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hours to obtain the final GA (GA-600). Finally, to switch the material to the

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hydrophilic state, the above materials were treated in an ozone system for 15 min;

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for simplicity, this sample is referred to as GA-600-O3 in the figures and as GA in the

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maintext. The schematic representation of the procedure for the preparation of the GA

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samples is shown in Scheme S1.

32

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The microscopic features and morphology of GA were characterized by scanning

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electron microscopy (SEM, Hitachi S-3400N) and transmission electron microscopy

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(TEM, JEOL JEM-200CX). Brunauer-Emmett-Teller (BET) measurements using N2

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absorption were performed using an Autosorb-IQ-MP (Quantachrome) surface area

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analyzer. X-ray diffraction (XRD) analysis of the samples was carried out with an

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X-ray diffractometer (Shimadzu 6000) under Cu Kα radiation at a 2θ scanning rate of

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2°/min. The surface functional groups were characterized by X-ray photoelectron

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spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The XPS

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data were collected by a PHI 5000 VersaProbe (U1VAC, Japan) with a resolution

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below 0.2 eV, and the C1s core-level spectra were analyzed using the XPS Peak 4.1

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software. The FT-IR spectra were recorded on a Thermo Fisher spectroscope (Nicolet

137

iS5) in the 4000-500 cm-1 region with a resolution of 4 cm-1 in transmission mode.

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Electrochemical experiments

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We designed an adsorption column equipped with supply power components

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(Scheme 1) to enable the adsorption, desorption and in situ electrochemistry

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experiments.35 Scheme 1 shows the schematic (A) and images (B-D) of the

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electrochemical device. A polycarbonate column was designed to allow simultaneous

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electrochemistry experiments, and its bottom casing was drilled to provide openings

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for the cathodic and anodic leads. The main components of the electrochemical casing

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were the titanium anodic ring-connector (2) separated by an insulating silicone rubber

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seal (3) with a perforated titanium cathode (4). One columnar GA (diameter = 2.6 cm,

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volume = 15.8 cm3, weight = 49.5 mg) was packed into the cylindrical adsorption

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column, and the GA was in good contact with the perforated titanium plate (2) located

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at the bottom of the column to enable in situ electrochemistry if necessary. This novel

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setup can accelerate mass transfer, compared with a traditional batch reactor, by using

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convection instead of diffusion.

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All filtration experiments were completed using this modified electrochemical

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filtration column. The sample was placed directly into the apparatus. Then, the

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electrode system was installed such that the GA was in close contact with the anode.

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Next, the height of the inlet water was adjusted to ensure that there were no spaces in

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the GA. The filtration casing was then sealed and primed with DI water using a needle

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syringe to remove any air in the internal aerogel that could restrict the flow. Water

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was then peristaltically pumped (Masterflex) through the filter at a rate of 1.0 ± 0.1 8


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mL min-1. Sample aliquots were collected directly from the filter casing outlet and

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analyzed immediately after collection.

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RESULTS AND DISCUSSION

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Structural characterization of GA

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The profiles of the as-prepared GA (cylindrical shape, diameter = 2.6 cm, height

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= 3 cm, volume = 15.8 cm3, weight = 49.5 mg) are shown in Figures 1a, b. One GA

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sample (49.5 mg) has an effective volume of 15.8 cm3 with over 99% porosity, and its

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density of 3.13 g L-1 approached that of air gas (1.29 g L-1). The microscopic features

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and morphology of the as-prepared GA were characterized by scanning electron

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microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure

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1. As shown in Figure 1c, the structure of the three-dimensional (3D) networks

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enables the macroscopization of GO sheets, in which GA exhibited a macroporous

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structure with well-defined interconnected pores with sizes ranging from 30 to 50 μm,

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and the high-magnification SEM image in Figure 1d clearly revealed that the pore

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walls consisted of layered GO nanosheets, as was also verified by the XRD patterns

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shown in Figure S1 (Supporting Information). The TEM images in Figures 1e and 1f

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further confirmed the presence of layered GO nanosheets, and the crystalline structure

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of GA was confirmed by the well-defined diffraction spots in the selected area

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electron diffraction (SAED) pattern via HRTEM, which is in accordance with

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previous reports.

33

GA had a specific surface area of 2178 m2 g-1 due to the large

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number of mesopores and micropores, with a wide size distribution from 1 nm to 30

180

nm, as indicated in Table S1 and Figure S2.

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Figure 1. Profile and Microscopic Features of Graphene Aerogel (GA). Profile and cross-sectional images (a, b), SEM images (c, d) and TEM images (e, f) of GA. The inset shows the selected area electron diffraction (SAED) pattern.

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184 185

Furthermore, Fourier transform infrared spectroscopy (FT-IR), shown in Figure

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2a, revealed numerous GA functional groups with peaks at ~3,233 cm-1 (O-H

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stretching vibrations), ~1,710 cm-1 (C=O stretching vibrations) and ~1,298 cm-1 (C-O

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stretching vibrations). X-ray photoelectron spectroscopy (XPS) was also conducted,

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and the spectra are presented in Figures 2b and S3, in which the deconvolution of the

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C 1s peak showed the presence of C-C (~284.5 eV), C-O (~286.1 eV) and O=C-O

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(~288.6 eV) groups. Previous work reported that the specific functional groups have a

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significant effect on the adsorption capacity and electrical conductivity of a material.

193

34, 35

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30~50 μm, the numerous micropores and large SSA of 2178 m2 g-1 (see Table S1)

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together with the adjustable functional groups qualify GA as a competitive adsorbent.

Generally, the as-prepared 3D GA is equipped with macropores with sizes of

10


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Figure 2. FT-IR (a) and XPS (b) Spectra of the Samples..

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198 199 200

Equipment and design for the in situ electrochemical regeneration of GA

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All filtration experiments were completed using the modified electrochemical

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filtration casing method (Scheme 1), as described in the experimental section. The

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upper limit of the liquid residence time at 1 mL min-1 in the column is τ ≤ 15.8 min.

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The average macropore diameter of GA is 30.5 ± 20.0 μm; therefore, if a molecule

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is located at the center of the largest pore, the maximum distance to a graphene

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surface is (30.5+20.0)/2 = 25.25 μm. The maximum molecular diffusion time, td =

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ld2/(2D), to the graphene surface can be estimated using this distance and the diffusion

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coefficient, D = 10-5 cm2 s-1 = 103 μm2 s-1. Thus, an influent molecule will collide

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with the graphene nanosheet surface with a maximal characteristic time of 0.32 s, and

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during the residence time, τ ≤ 15.8 min, a single molecule could undergo 1000

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collisions with a graphene interface. Generally, GA has plentiful adsorption and

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reaction sites, and the residence time is long enough for adsorption and reaction on

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GA (49.5 mg) in the novel adsorption devices with the assistance of electrochemistry.

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Scheme 1. Depiction and Images of the Electrochemical Regeneration Apparatus. (A) Design of the modified polycarbonate column consisting of GA (1), a titanium plate (2) pressed into GA, an insulating silicone rubber separator and seal (3), and a perforated titanium cathode (4). (B) Device for the electrochemical desorption/degradation process. (C) Profile of GA. (D) SEM image of GA.

220 221

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Organic pollutants: adsorption and electrochemistry cleaning via in situ

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electrochemical regeneration

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GA is expected to not only possess a high adsorption capacity but also exhibit

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high electrical conductivity and electrochemical activity, which may allow GA to

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desorb/degrade adsorbed organic pollutants. Figure 3a shows the adsorption and

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subsequent desorption/oxidation of MB, as a model organic pollutant, on GA as a

228

function of applied voltage. From the adsorption breakthrough curve, [MB]eff/[MB]in

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versus t, in the absence of electrochemistry, for GA, we found that the effluent MB

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concentration was below the limit of detection prior to breakthrough, with a sorption

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capability of 18.3 ± 1.1 mg g-1. At t = 0, an attempt was made to oxidize adsorbed 12


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dyes by the application of a potential of 1-3 V after a dye monolayer was formed on

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the GA surface, which was expected to enable the in situ electrochemical regeneration

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of GA. An immediate decrease in [MB]eff/[MB]in was observed, which continued for

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~15 min until it reached the equilibrium value of 0.97 for 1 V, 0.64 for 2 V and 0.06

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for 3 V. The absence of dye breakthrough at 3 V indicates that the primary MB loss

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mechanism is oxidation. The >94% oxidation of influent and adsorbed MB in a single

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pass through the GA network (49.5 mg and 15.8 cm3) is an impressive result based on

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the processing capability of 3800 L m-3 GA h-1 or 1.21 L g-1 GA h-1 and low energy

240

consumption (~0.2 kWh m-3 solution). Generally, as an adsorbent, GA can adsorb and

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enrich organic pollutants with high BET surface area, and then, functioning as an

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anode, GA can simultaneously and efficiently oxidize the adsorbed pollutants in the

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absence of additional chemicals, qualifying GA as an electrochemically cleaning

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adsorbent for the removal of organic contaminants.

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Figure 3. Adsorption and Electrochemical Degradation/Desorption of MB on GA. [MB]in = 10 ± 0.1 mg L-1, J = 1.0 ± 0.1 mL min-1, and adsorbed methylene blue adsorption and oxidation at potentials of 1 V (blue triangles), 2 V (red circles), and 3 V (black squares) on GA-600-O3 (a); Adsorption isotherms and electrochemical degradation of MB on raw GA at 3 V (b), in which raw GA did not undergo high-temperature annealing and ozone treatment, showing the original graphene aerogel compared to the GA, [MB]in = 20 ± 0.1 mg L-1, [Na2SO4]in = 50 ± 0. 1 mg L-1, J = 1.0 ± 0.1 mL min-1. Adsorption isotherms and electrochemical degradation of MB on GA-600 at 3 V (c).

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255 256

In the control experiment shown in Figure 3b-c, raw GA without annealing can

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adsorb a much higher amount of MB (1005 mg g-1) than in previous reports (Table S2)

258

due to the plentiful surface functional groups (see Table S1 and S3) and its

259

hydrophilic state; however, adsorbed MB cannot be efficiently removed or enriched,

260

with a high [MB]eff/[MB]in = 0.85. The annealed GA without O3 treatment (GA-600)

261

loses nearly all adsorption capacity (1.2 ± 0.1 mg g-1). Clearly, annealing makes GA

262

conductive (see Table

263

desorption/degradation of adsorbates, and the ozonation process switches the material

264

to the hydrophilic state,32 allowing the aqueous solution to access the GA pores and

265

facilitating the subsequent adsorption interaction. Regulation of the GA surface

266

conditions (Figure 2a, 2b) to balance the adsorption and regeneration capability is

267

key for the development of effective adsorption and electrochemical regeneration

S1),

which

is

beneficial for the electrochemical

14


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technologies based on GA, which will be further optimized in our future works.

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Briefly, 3D configurations of GA are desired, with macropores of 30~50 μm as diffuse

270

channels to facilitate reactant/product transfer, and the numerous micropores, large

271

SSA up to 2178 m2 g-1 for adsorption and reaction sites qualify GA as a promising

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adsorbent and enable efficient adsorption-electrochemical regeneration.

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Inorganic metal ions: adsorption and enrichment by electrochemistry-induced

274

desorption

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Adsorption and enrichment: Metal ions in wastewater treatment fields are

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another major kind of pollutant, and here, we also tested the GA adsorption and

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electrochemical regeneration system using the typical copper ion as a model ion.

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Figure 4a shows the Cu2+ adsorption breakthrough curve, [Cu2+]eff/[Cu2+]in versus t,

279

in the absence of electrochemical treatment for GA ([Cu2+]in = 6.0 ± 0.1 mg L-1). The

280

effluent Cu2+ concentration was below the limit of detection prior to breakthrough.

281

The adsorption capacity of GA for Cu2+ was 68.2 mg g-1, which is nearly equal to the

282

maximum sorption capacity of Cu2+ to GA 36 and higher than in the previous reports

283

of the maximum Cu2+ adsorption on other adsorption materials, such as 15.5 mg g-1

284

for trihydroxamic acid-functionalized carbon materials,

285

oxide/Fe3O4 composites, 38 and 37.8 mg g-1 for a traditional ion exchange resin. 39 The

286

large GA sorption capacity observed in this study is attributed to the 3D network

287

structure, the abundance of surface functional groups and the high specific surface

288

area of 2178 m2 g-1 of GA compared to < 1000 m2 g-1 for other adsorption materials.

37

18.3 mg g-1 for graphene

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Figure 4. Adsorption and Different Desorption Strategies of Cu2+ on GA. Adsorption (a) in the conditions of [Cu2+]in = 6.0 ± 1.0 mg L-1, J = 1.0 ± 0.1 mL min-1. Desorption of adsorbed Cu2+ (b): 100 mM HCl treatment (pink upside-down triangles), 0.62 mM HCl treatment (black squares), electrochemical treatment (blue triangles) and electrochemical/0.62 mM HCl treatment (red circles). The other conditions are J = 1.0 ± 0.1 mL min-1 and V = 3 V. The inset shows the same plot with a magnified y-axis to show the low-concentration data. Comparison of the recovery efficiency for the different treatments (c), in which the black line (black squares) represents the ratios of the sum of influent H+ to the sum of effluent desorbed Cu2+. Effluent pH when Cu2+ is desorbed by the different treatments (d).

299

300 301

Traditional desorption：We then explored the traditional regeneration efficiency

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of saturated GA-Cu2+, as shown in Figures 4b-4d. The conventional regeneration

303

method for metal ion adsorbents generally requires a high concentration of

304

hydrochloric acid of > 100 mM.

305

mM HCl, and the results in Figure 4b indicated that the initial collected effluent

306

concentration of Cu2+ was as high as 260.5 mg L-1 with [Cu2+]eff/[Cu2+]in = 40.4,

307

suggesting that adsorbed Cu2+ was exchanged with excess H+ and that the recovery

40, 41

For comparison, GA was regenerated by 100

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ratio was nearly 100%. However, the ratio of influent H+ vs the desorbed Cu2+,

309

H+inf/Cu2+desorb, was close to 200, indicating that H+ must be in large excess for

310

thorough Cu2+ desorption (Figure 4c), and the pH value, ~1.0, of the effluent solution

311

is too low (Figure 4d) to discharge into the environment. Neutralization of the

312

concentrated acidic solution is required, according to the discharge standard, and here,

313

3.8 kg of Ca(OH)2 per cubic meter of solution is required to neutralize H+, leading to

314

a large amount of secondary waste discharging into the environment. Obviously, this

315

is an effective but undesirable strategy.

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Electrochemistry-induced desorption and optimization: Electrochemical

317

desorption is an environmentally friendly strategy that does not require the use of

318

additional chemicals. An obvious initial desorption is observed in Figure 4b, and the

319

effluent [Cu2+] was 20.3 mg L-1. Unfortunately, the recovery ratio of Cu2+ was 34.7%

320

at 3 V. Increasing the applied voltage contributes to enhance desorption efficiency, but

321

it may also result in greater water electrolysis and consumes additional energy. A

322

combined process of electrochemistry together with the equivalent H+ for the

323

desorption of Cu2+ (H+inf/Cu2+desorb = 2 in theory) was carried out. Here, additional H+

324

is desired only to couple with the functional groups on GA once Cu2+ is desorbed, and

325

thus, no redundant H+ remains in the effluent. The specific mechanism will be

326

explained in detail in the next section. As a result, it can be clearly seen that the

327

aliquot of the collected effluent contained a high concentration of Cu2+ (99.9 mg L-1)

328

at 3 V, the recovery ratio of Cu2+ was more than 90% based on the combined process

329

induced by electrochemistry, and the effluent pH was 4.7 after 30 min, enabling facile 17



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330

neutralization and avoiding the requirement of high alkali concentration compared to

331

traditional acidic desorption (pH ~1.0). As a control, an equivalent amount of H+ was

332

utilized to only desorb Cu2+. As a result, the desorption ratio was only 38.2%, and the

333

effluent Cu2+ was 3.0 mg/L with an effluent pH of 3.0 (Figure 4b-4d). Obviously,

334

electrochemistry induces the desorption process with highly concentrated effluent

335

Cu2+, enabling a high recovery ratio and the negligible residue of H+ with equivalent

336

H+ addition to Cu2+. This is an environmentally friendly in situ regeneration strategy

337

with the enrichment of Cu2+.

338

Mechanism and implication of electrochemistry-induced desorption

339

Further understanding of the electrochemistry-induced desorption mechanism is

340

beneficial for the development of novel desorption processes and related adsorption

341

composites. The results in Figure 5a indicated that GA was fully deprotonated at pH

342

6, and the Cu2+ in aqueous solution started began to disorder even participating at pH

343

> 6.0 based on the simulation by visual MINTEQ shown in Figure S4. Under our

344

experimental conditions (pH ~5.0), Cu2+ is adsorbed on GA by ion exchange based on

345

the deprotonation of GA, and one part of the Cu2+ can interact with GA by other

346

function such as interaction between π electrons and Cu2+, which can be supported by

347

the fact that the zeta potential of Cu2+-adsorbed GA (GA-Cu2+) is greater than that of

348

GA in Figure 5b. Moreover, the -COO- adsorption groups contents on one gram GA

349

is ~ 1.30 mmol based on O/C and –COO- ratio on GA calculated by the XPS data in

350

Tabel S3 in SI, which is not enough to exchange all Cu2+ (1.06 mmol Cu/g GA), so 18


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351

partial adsorbed Cu2+ are interacted with GA by other function. Correspondingly, we

352

speculated that electrochemistry-induced desorption mainly involved the two

353

mechanisms shown in Figure 5c.

354 355 356 357 358 359 360 361 362

Figure 5. Mechanisms of the Electrochemistry-induced Desorption Process. Influence of the solution pH on the zeta potential of GA (a) over a pH range of 0~12. The zeta potential of GA with different regeneration treatments (b). Speculated change in the surface chemical composition of the graphene sheets of GA after different treatments (c), (c1) treated by 100 mM HCl, (c2) treated by 0.62 mM HCl, (c3) treated by electrochemistry, and (c4) treated by electrochemistry together with 0.62 mM HCl, where the red data in (c) represents the desorption ratios, and the red graphene sheets in c3 and c4 represent the increasing polarization conditions and loss of electrons in GA compared with the gray graphene sheets in c1 and c2.

363

19



Page 20 of 27

364 365

First, the adsorption capacity of the adsorbents depends on the surface charge

366

conditions. Moreover, the conditions can be changed by an applied potential. The

367

potential of the anode can reduce the electron density of GA to zero, which is denoted

368

the potential of zero charge (PZC), and even to positive charge conditions if enough

369

positive potential is added. Correspondingly, Cu2+ adsorbed on the inner sphere by

370

specific adsorption will preferentially leave the adsorption sites due to the decrease in

371

the electron density, as shown in Figure 5c3. Second, electrochemistry-induced Cu2+

372

repulsion and migration from the adsorption sites, based on an increase in the

373

electrochemical repulsion potential under up to 1010 V/m of the electrical field of the

374

EDL, accounts for the partial desorption performance. Correspondingly, one Cu2+ is 20


Page 21 of 27


375

electrostatically expelled from the GA functional groups, such as –COO-, as shown in

376

Figure 5c, and two H+ are required based on ion exchange rules. Interestingly, the

377

subsequent increase in the effluent pH from 10 min to 30 min (Figure 4d, blue line)

378

indicates that this kind of desorption requires H+ participation through

379

electrochemistry-assisted ion exchange. However, the low H+ content in solutions due

380

to the limited decomposition of water molecules restricts the exchange process with

381

Cu2+ under electrochemical conditions, even though many Cu2+ are still adsorbed on

382

GA. Further increasing the anode potential resulted in electrochemical reactions, such

383

as water and/or adsorbate oxidation (refer to MB desorption/oxidation in Figure 3),

384

which are not beneficial for the desorption of metal ions. The addition of an

385

equivalent amount of acid to Cu2+ is an alternative strategy, in which the additional H+

386

is utilized only to couple with –COO- once Cu2+ is desorbed and no redundant H+

387

remains in the effluent. Consequently, a combined process induced by

388

electrochemistry together with the equivalent H+ (H+inff/Cu2+desorp = 2 is required in

389

theory; here, the value is 2.9 experimentally) was proposed. It can be seen that the

390

effluent pH (red line in Figure 4d) was nearly fixed during the first 30 min due to the

391

influent H+inff consumption and then starts to decrease at 30 min due to the end of the

392

desorption process, confirming the desorption process described above.

393

Mechanisms for the electrochemistry-induced desorption of metal ions were

394

proposed as follows: 1) the decrease in the adsorption capacity of metal ions on GA

395

due to a change in the surface charge conditions results in the removal of metal ions

396

from the adsorption sites and 2) the further repulsion and migration of metal ions 21



Page 22 of 27

397

under the assistance of a strong in situ EDL electric field improve the

398

electrochemistry-induced desorption process. In future work, novel strategies will be

399

proposed based on the above mechanisms, and we plan to combine GA with

400

nanoparticles featuring strong electrochemical self-polarization capacity to generate in

401

situ a strong internal electric field around the adsorption sites, which is expected to

402

realize more efficient electrochemical regeneration based on the mechanism explored

403

in this study.

404

Environmental applications

405

A 3D GA network was successfully prepared to achieve high surface area,

406

suitable pore size distribution and high conductivity for the adsorption of organic

407

pollutants and metal ions, avoiding the inconvenience of using powdered

408

nanographene. Importantly, this approach may realize the environmentally friendly in

409

situ regeneration of model pollutants based on GA by regulating its surface conditions

410

to balance the adsorption and regeneration capabilities. Adsorbed organic pollutants if

411

undesired can be desorbed and then degraded electrochemically in the absence of any

412

additional chemicals. For adsorbed metal ions, the mechanisms involved in the

413

electrochemistry-induced

414

electrochemically adjusted adsorption performance and the further repulsion and

415

migration of metal ions subject to the strong in situ EDL electric field. In general,

416

electrochemical regeneration is a potential regeneration technology for conductive

417

adsorbents, and our research may have important implications for the design of

desorption

process

cover

the

decline

in

the

22


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418

graphene-based adsorbents and the development of environmentally friendly

419

regeneration processes, as well as for the understanding of the electrochemical

420

regeneration mechanism; this approach can also be used as a green and efficient in

421

situ regeneration method for traditional adsorption processes.

422

ASSOCIATED CONTENT

423

Supporting Information

424

The schematic representation of the procedure for the preparation of GA samples,

425

XRD, the pore size distribution, BET, XPS O1s spectra of samples, the O/C ratio and

426

functional ratio, reported the adsorption capacity of GA for MB and Cu 2+, and

427

speciation of Cu2+ in aqueous solution under different pH simulated by visual

428

MINTEQ are available in the Supporting Information (SI) on the ACS Publication

429

website at DOI: XXXXXX.

430 431

AUTHOR INFORMATION

432

Corresponding Author

433

* Guandao Gao Tel./Fax: +86-25-89681675. E-mail: [email protected].

434

Notes

435 436

The authors declare no competing financial interest. ACKNOWLEDGMENTS

437

This work was supported by the National Key Research and Development

438

Program of China (2016YFA0203104), the National Natural Science Foundation of

439

China (21577069), and Research and Development Program of Jiangsu Province

23



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440

(BE2017710). M. Pan thanks Prof. Yongsheng Chen, Dong Sui and Huicong Chang

441

from Nankai University for their kind assistance in the preparation of GA.

24


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