Selective adsorption of phenanthrene dissolved in Tween 80 solution using activated carbon derived from walnut shells

Accepted Manuscript Selective adsorption of phenanthrene dissolved in Tween 80 solution using activated carbon derived from walnut shells Xin Zheng, Heng Lin, Yufang Tao, Hui Zhang PII:

S0045-6535(18)31099-3

DOI:

10.1016/j.chemosphere.2018.06.025

Reference:

CHEM 21557

To appear in:

ECSN

Received Date: 9 January 2018 Revised Date:

23 April 2018

Accepted Date: 3 June 2018

Please cite this article as: Zheng, X., Lin, H., Tao, Y., Zhang, H., Selective adsorption of phenanthrene dissolved in Tween 80 solution using activated carbon derived from walnut shells, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.06.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Selective adsorption of phenanthrene dissolved in Tween 80 solution using activated

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carbon derived from walnut shells

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Xin Zhenga,b, Heng Lina,b, Yufang Taoa,b, Hui Zhanga,b,*

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a

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Remediation Material Engineering Technology Research Center, Wuhan University,

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Wuhan 430079, China

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b

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Department of Environmental Science and Engineering, Hubei Environmental

Shenzhen Research Institute of Wuhan University, Shenzhen 518057, China

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* Corresponding author: Tel: + 86-27-68775837; Fax: +86 27 68778893. E-mail: [email protected]

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Abstract

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In order to remove phenanthrene (PHE) from surfactant solution, activated carbon

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(AC)

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Brunauer-Emmett-Teller (BET), field-emission scanning electron microscopy

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(FESEM), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron

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spectroscopy (XPS).

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effectively removed and the latter could be economically recovered after adsorption

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by the prepared AC.

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of AC play important roles in the PHE adsorption process.

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process could best be described using the pseudo-second-order model and adsorption

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isotherm results indicated that the Langmuir model best fitted the data.

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thermodynamic parameters, including enthalpy change, Gibbs free energy change and

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entropy change were calculated.

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80 recovery reached 95% and 90%, respectively.

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provided an efficient alternative for selective adsorption of PHE and recovery of

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Tween 80 after the soil washing processes.

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regenerated with ethanol and even if AC were regenerated twice PHE removal

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reached 80%.

from

waste

walnut

shells

and

characterized

by

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prepared

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For solutions containing PHE and Tween 80, the former was

The π-π interactions and oxygen containing functional groups

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was

The adsorption kinetics

Adsorption

The results suggest that AC

After adsorption AC could be

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Under optimal conditions, PHE removal and Tween

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Keywords: Activated carbon; Selective adsorption; Polycyclic aromatic hydrocarbons;

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Tween 80; Recovery

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ACCEPTED MANUSCRIPT 1. Introduction

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Polycyclic aromatic hydrocarbons (PAHs) which are organic pollutants composed of

35

two or more fused aromatic rings (Gharibzadeh et al., 2016) are always found in

36

groundwater and soils at sites involved in coal processing, coal storage, coke oven

37

plants, and as a result of coal tar spillage (Paria and Yuet, 2006).

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their hydrophobicity, have a very low solubility in water and are easily adsorbed by

39

clay minerals and organic matter in contaminated soils (Gómez et al., 2010a).

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Moreover, PAHs are known to be mutagenic and carcinogenic (Chen and Liao, 2006;

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Liu et al., 2016a) and can be harmful to human health (Gan et al., 2009; Gharibzadeh

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et al., 2016) which makes remediation of PAH-contaminated soils highly desirable.

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PAHs, because of

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A variety of methods have been used for the remediation of PAH-contaminated soils,

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including incineration (Acharya and Ives, 1994), thermal desorption (Kuppusamy et

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al., 2017), chemical oxidation (Usman et al., 2012; Lemaire et al., 2013),

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bioremediation (Xiong et al., 2017; Yu et al., 2017) and soil washing (Ahn et al.,

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2008a; Gong et al., 2010; Vizcaíno et al., 2012; Kuppusamy et al., 2017).

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the processes used thus far, surfactant-enhanced soil washing has proved to be an

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effective technology for the remediation of soils contaminated by PAHs (Lau et al.,

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2014; Trellu et al., 2016; Cheng et al., 2017).

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of PAHs in aqueous solution to a great degree by trapping the targeted hydrophobic

53

molecules into the hydrophobic cores of surfactant micelles (Ahn et al., 2008a;

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3

Among

Surfactants can increase the solubility

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Gharibzadeh et al., 2016).

However, soil washing generates large amounts of

55

effluent containing surfactant and PAHs, which can cause secondary contamination if

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the effluent is not treated appropriately and the high cost of the surfactant restricts the

57

widespread application of soil washing (Zhou et al., 2013; Li et al., 2014).

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be more advantageous, environmentally friendly and economical if the surfactant

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could be recovered and reused after the soil washing process.

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treatment (Gharibzadeh et al., 2016), solvent extraction (Lee et al., 2002),

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electrochemical treatment (Gómez et al., 2010b) and adsorption (Ahn et al., 2007;

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Rosas et al., 2013; Zhou et al., 2013; Li et al., 2014) have all been employed to

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recover the surfactant from the soil washing effluent.

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methods, adsorption may have the advantages of lower cost and decreased pollution,

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higher efficiency and ease of operation (Ahn et al., 2010; Zhou et al., 2013; Liu et al.,

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2014a).

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organic wastewater (Yang et al., 2009), which has been used in the recovery of

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surfactants from a number of PAH-contaminated soil washing effluents.

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(2007, 2008a, 2008b, 2010) used commercial AC to adsorb PAHs from soil washing

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effluents and obtained about 91% of phenanthrene (PHE) removal.

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(2015a) employed a fixed-bed with commercial AC to recover surfactant solutions

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from soil washing effluents and 80% surfactant retention was attained.

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(2016b) separated PAHs from rhamnolipid solution using commercial AC and the

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maximum adsorption capacity for PHE was around 43.9 mg g−1.

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It would

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Therefore, biological

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Compared with other recovery

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Additionally, activated carbon (AC) is a common type of adsorbent to treat

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Ahn et al.

Zhou et al.

Liu et al.

As mentioned

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above, all the AC used in the recovery of surfactant was commercial and the

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adsorption capacity was limited (up to 58.84 mg g−1).

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spent AC was not investigated extensively.

Moreover, the regeneration of

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Walnut shells are a major agricultural waste with a production of 100,000 ton/year in

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China alone (Yang and Qiu, 2010).

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preparation of AC and has been utilized, for example, in the adsorption of dyes (Yang

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and Qiu, 2010; Alimohammadi et al., 2016; Ashrafi et al., 2017), heavy metals (Yi et

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al., 2015) and antibiotics (Nazari et al., 2016).

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the use of walnut shells AC for the adsorption of PAHs from soil washing effluents,

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and consequently, in this study PHE was selected as a model PAH and Tween 80 as

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the nonionic surfactant (Zhao et al., 2016).

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prepare AC, with a high adsorption capacity, from waste walnut shells; (2) to explore

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the feasibility of selective adsorption of PHE from simulated soil washing effluents;

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(3) to investigate the factors which influence PHE adsorption; (4) to measure the

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adsorption kinetics, determine the adsorption isotherms and calculate thermodynamic

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parameters; (5) to regenerate the spent AC; and (6) to explore the PHE adsorption

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mechanism.

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process to remove PHE from the surfactant solution.

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recovered and could potentially be reused in soil washing, and at the same time

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greatly reduce the cost of soil washing.

However, there has been no report on

The objectives of this work were: (1) to

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It is an excellent and cheap precursor for the

In summary, the significance of this study was to provide a feasible

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The surfactant solution is

ACCEPTED MANUSCRIPT 96 2. Materials and methods

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2.1 Materials

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Phenanthrene was supplied by Sun Chemical Technology (Shanghai) Co. Ltd and

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Tween 80, phosphoric acid (H3PO4), hydrochloric acid (HCl), potassium bromide

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(KBr), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium bicarbonate

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(NaHCO3), ammonium thiocyanate (NH4SCN), chloroform (CHCl3), cobalt nitrate

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hexahydrate (Co(NO3)2·6H2O), ethanol (C2H5OH) and acetonitrile (CH3CN) were

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supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).

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chemicals were used without further purification.

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All

2.2 Preparation of AC

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Walnut shells were washed with deionized water, dried at 60 ºC, crushed using a

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grinder and sieved to thirty mesh.

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was then impregnated with 30 wt% phosphoric acid at the ratio of 2 g H3PO4

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solution/g crushed shells for 24 h at room temperature.

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was then thermally activated under a nitrogen atmosphere in a tube furnace at 700 ºC

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for 2 h.

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hydrochloric acid solution for 30 min and then washed with deionized water to neutral

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pH.

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sealed in a hermetic vessel for further use.

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This material was dried at 80 ºC overnight and

The impregnated material

After being cooled to room temperature, the material was soaked in 0.1 M

Finally, the AC was dried at 105 ºC for 24 h and sieved (200 mesh) and then

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2.3 Characterization of AC

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The morphology of the AC was observed with a Zeiss SIGMA field-emission

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scanning electron microscopy (FESEM).

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by nitrogen adsorption at 77 K via the Brunauer-Emmett-Teller (BET) equation on an

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ASAP 2020 analyzer.

The pHPZC was measured by a batch equilibration technique

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(Zhou et al., 2015b).

The Fourier transform infrared (FTIR) spectroscopy was

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performed on a Nicolet 5700 FTIR Spectrometer using KBr pellets.

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was employed to quantify the acidic groups present on the AC surface (Ge et al.,

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2015).

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were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi,

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Thermo Fisher).

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Boehm titration

Detailed information regarding surface functionalities present on the AC

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The specific surface area was calculated

2.4 Batch sorption experiments

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A stock of solution was prepared by dissolving PHE in Tween 80 solution.

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influence factor experiments, 100 mL solution containing PHE (20 mg L−1) and

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Tween 80 (1-20 g L−1) was applied into the 250 mL conical flask.

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required amount of AC (0.05-0.6 g L−1), the conical flasks were placed on orbital

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shakers with a speed of 200 rpm at 25 ± 1 ºC for 24 h, which was the equilibrium time

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as determined from the sorption kinetics experiments.

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withdrawn with a syringe and filtered through a 0.45 µm nylon filter for the analysis

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In the

After adding the

Then the samples were

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of PHE and Tween 80.

The PHE removal and Tween 80 loss were calculated

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according to the difference between the initial and equilibrium concentrations in

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aqueous solutions as expressed in Eqs. (1) and (2). PHE removal % =

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Tween 80 loss % =

[PHE]0

× 100%

[Tween 80]0 ‒ [Tween 80]e [Tween 80]0

(1)

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[PHE]0 ‒ [PHE]e

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× 100%

(2)

where [PHE]0 (mg L−1) and [Tween 80]0 (g L−1) are the initial concentration of PHE

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and Tween 80, respectively; and [PHE]e (mg L−1) and [Tween 80]e (g L−1) are the

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equilibrium concentration of PHE and Tween 80, respectively.

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In the sorption kinetics experiments, 100 mL solution containing PHE (10, 20 and 30

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mg L−1) and Tween 80 (5 g L−1) was dosed into the 250 mL conical flask.

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adding 0.3 g L−1 AC, the conical flasks were placed on orbital shakers with a speed of

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200 rpm at 25 ± 1 ºC.

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withdrawn and filtered through a 0.45 µm nylon filter before analysis.

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of PHE adsorbed qt (mg g−1) at a certain time t (h) was calculated from Eq. (3) (Li et

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al., 2017):

After

At selected time intervals of 0-24 h, the samples were The amount

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qt =

[PHE]0 ‒ [PHE]t V

(3)

m

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where [PHE]t (mg L−1) is the concentration of PHE at time t (h) in solution; V (L) is

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the total volume of the solution; and m (g) is the mass of AC added.

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The procedure of adsorption isotherms experiments was similar to that sorption 8

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kinetics experiments, and the PHE concentrations were ranged from 2 to 100 mg L−1

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while the Tween 80 concentration and AC dosage were fixed at 5 and 0.3 g L−1,

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respectively.

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calculated from Eq. (4): qe =

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The amount of PHE adsorbed at the equilibrium qe (mg g−1) was

[PHE]0 ‒ [PHE]e V

(4)

m

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The concentrations of PHE in aqueous solution were determined with a Shimadzu

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LC-20AB high-performance liquid chromatography (HPLC) fitted with a diode array

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detector (SPD-M20A) and a Shimadzu Shim-pack VP-ODS column (4.6 mm × 150

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mm, 5 µm) using acetonitrile/water (v/v, 70/30) as the mobile phase at a flow rate of 1

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mL min−1 at a wavelength of 254 nm.

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determined by the cobalt ammonium thiocyanate color-developing method (Smullin

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et al., 1971).

The concentrations of Tween 80 were

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3

Results and discussion

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3.1 Characterization of the prepared AC

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The characterization results are given in Table S1, Figure 1, Figure S1 and Figure S2,

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respectively.

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Figure 1(a) show a Type IV isotherm which indicates the presence of a mesoporous

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surface on the AC.

N2 adsorption-desorption isotherms and pore diameter distribution in

Moreover, an obvious hysteresis loop (H1 Type), due to 9

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capillary condensation, further proved the existence of a mesoporous structure and the

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auxiliary chart in Figure 1(a) shows that the pore size is uniform with an average pore

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diameter of 5.92 nm.

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can be seen that it is irregular with a clear pore structure.

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surface analysis results, it can be concluded that AC consists of a mesoporous rather

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than microporous structure.

Figure 1(b) exhibits the surface morphology of the AC and it

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Thus, along with the BET

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Figure S1 shows that the value of pHPZC was obtained when pHinitial was equal to

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pHfinal and corresponds to about pH 3.

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surface of the AC will be negatively charged when the pH of the solution is greater

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than 3.

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This low value of pHPZC indicates that the

Figure S2 shows the XPS survey spectrum of fresh AC and the elements C, O and P

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could be detected; the presence of small amounts of P resulted from the H3PO4

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impregnation process.

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Figure 1(d) includes four signals which are attributed to graphitic type of C atoms (C1,

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284.0 eV) (Li et al., 2017); carbon species of alcohol and ether groups (C2, 285.5 eV)

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(Puziy et al., 2008); carboxylic/ester/lactone groups (C3, O=C-O, 288.5 eV) (Fang et

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al., 2014); and π-π* transitions in aromatic rings (C4, 290.4 eV) (Li et al., 2017).

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AC C

Also, the C1s photoelectron spectrum of the fresh AC in

199 200

It can be seen from Figure 1(e) that in the fresh AC FTIR spectrum there are seven 10

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main peaks at 3426 cm−1, 2919 cm−1, 2845 cm−1, 2357 cm−1, 1640 cm−1, 1385 cm−1

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and 1092 cm−1.

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O-H stretching vibration of adsorbed water on the surface of AC ) (Salehi et al.,

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2017); 2919 cm−1 and 2845 cm−1 (C-H symmetric and asymmetric stretching

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vibration) (Salehi et al., 2017); 2357 cm−1 (the C-O vibrations of adsorbed carbon

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dioxide); 1640 cm−1 (C=C stretching of aromatic rings) (Bernard et al., 2018); 1385

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cm−1 (carboxyl O=C−O stretching vibration) (Wang et al., 2014); and 1092 cm−1

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(alkoxy C−O stretching vibration) (Wang et al., 2014).

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The absorption bands are assigned as: 3426 cm−1 (hydroxyl groups

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In the Boehm titration process, NaOH solution was used to titrate carboxyl, lactone

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and phenolic groups; Na2CO3 titrated carboxyl and lactone groups; and NaHCO3 only

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titrated carboxyl groups.

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the different consumptions of the three basic solutions and the results are shown in

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Table 1.

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groups account for the main surface modification, lactone groups are much less

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prevalent and there are very few phenolic groups present.

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The result is consistent with the work of Ge et al. (2015) that carboxyl

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The amount of acidic groups was calculated according to

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3.2 The factors influencing selective adsorption

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In order to evaluate the efficiency and practicability of removing PHE and recovering

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Tween 80 by selective adsorption, parameter selectivity (S) was introduced which was

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expressed as the ratio of PHE removal percentage to Tween 80 loss percentage: 11

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S=

PHE removal percentage

(5)

Tween 80 loss percentage

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An S value larger than 1 indicates that more PHE relative to Tween 80 was removed

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which suggested a selective adsorption process and the higher the S value the better.

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225 Solution pH is usually an important factor in any adsorption process because pH

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could influence the electrostatic and dispersive interactions between the adsorbate and

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the adsorbent (Shi et al., 2013).

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values used there was over 95% PHE removal and Tween 80 loss percentage was low

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(less than 10%) under most situations.

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little influence on the PHE removal and Tween 80 recovery so, for cost and ease of

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operations, all further experiments were carried out at the natural solution pH of about

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6.4.

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Figure 2(a) shows clearly in that at each of the pH

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This result indicates that solution pH had

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Figure 2(b) shows that as the AC dose was increased from 0.05 to 0.3 g L−1, the

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percentage of PHE removed increased from 49.2% to 95.1% and reached 98.8% when

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the AC dose was 0.6 g L−1.

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less than 10% in all instances indicating that larger AC doses promoted the PHE

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removal.

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that there are more effective adsorption sites.

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removal and the full use of AC, the AC dose of 0.3 g L−1 was selected in all further

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experiments.

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It was noteworthy that Tween 80 loss percentages were

This result is easy to understand since the higher adsorbent dose means

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In order to make sure of high PHE

ACCEPTED MANUSCRIPT 243 Figure 2(c) shows the effect of changing the concentration of Tween 80 and as the

245

concentration of Tween 80 is increased the percentage of PHE removed decreased

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gradually from 99.6% to 80.7% though the Tween 80 loss percentages were less than

247

10% in all situations.

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could obstruct the adsorption process of PHE.

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higher surfactant concentration, more surfactant micelles would remain in the aqueous

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state, which can solubilize more PAH and decrease PAH sorption.

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the entrance of the micropores of AC would be blocked by more surfactant molecules

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(Rosas et al., 2013; Liu et al., 2016b), and consequently, fewer pores are available for

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PHE adsorption.

Thus it can be concluded that higher Tween 80 concentrations

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According to Liu et al. (2016b), at

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In the meanwhile,

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Overall, it was found that all the S values were much greater than 1 which meant that

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the selective adsorption was a good method for removal of PHE and recovery of

257

Tween 80 from soil washing effluent.

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3.3 Adsorption mechanism

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To investigate adsorption mechanism, the AC surface before and after adsorption was

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first characterized by FESEM.

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covered by a thin film from the surface-adsorbed Tween 80 after adsorption.

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would block the micropores of AC, and consequently decrease the surface area of AC

As can be seen from Figure 1(c), the AC surface was

13

This

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(Ahn et al., 2007; Yang et al., 2009).

Table S1 indicates that the specific surface area

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and pore volume of AC reduced from 410.84 to 324.52 m2 g−1 and 0.61 to 0.50 cm3

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g−1, respectively, while the average pore diameter of AC rose from 5.92 to 6.21 nm.

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The effects of surface chemistry of AC on PHE adsorptive removal can be studied by

269

XPS, FTIR and Boehm titration.

270

confirm the oxygen containing functional groups on the surface of AC, which is

271

consistent with the results obtained by Boehm titration.

272

in aromatic rings (peak C4 in Figure 1d) indicates the existence of π electron on the

273

surface of AC which may be related to PHE adsorption mechanism.

274

spectrum of spent AC (Figure 1e) shows the characteristic peaks of PHE at 811 cm−1

275

and 726 cm−1 (C-H out of plane bend) (Gupta and Gupta, 2016) indicating the

276

adsorption of PHE on the AC.

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intensity of C=C stretching of aromatic rings at 1640 cm−1 was shifted to 1667 cm−1

278

and was strengthened. This proves the important role of π-π interactions in the

279

adsorption process between PHE and AC (Pei et al., 2013; Wang et al., 2014) and it

280

was obvious that π-π interactions strengthened the double bands and resulted in

281

blue-shift of C=C stretching vibration bands.

282

1385 cm−1 was slightly enhanced; and the alkoxy C−O band shifted from 1092 cm−1

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to 1073 cm−1 after adsorption.

284

confirm the interactions between the oxygen containing functional groups of the AC

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The XPS peaks of C2 and C3 in Figure 1(c)

The FTIR

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In addition, π-π* transitions

AC C

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Additionally, the spent AC spectrum shows that the

The O=C−O stretching vibration at

These clear changes in the spent AC FTIR spectrum

14

ACCEPTED MANUSCRIPT 285

and PHE (Wang et al., 2014).

286 287

Based on the result and the literature, the selective adsorption of PHE from the Tween

288

80 solution can be considered to proceed via the following steps.

289

would be formed when the Tween 80 concentration is higher than critical micelle

290

concentration (CMC) (Haigh, 1996; Mousset et al., 2014).

291

solubility can be increased by being partitioned into the hydrophobic cores of Tween

292

80 (Chun et al., 2002; Zhou and Zhu, 2005).

293

80 solution containing PHE, the micelles with PHE partitioned into their hydrophobic

294

core (PHE-containing micelles) would be sorbed onto the solid/liquid interface (Liu et

295

al., 2016b), and form hemi-micelles on the surface of AC (Lanzon and Brown, 2013).

296

The formation of hemi-micelles also provides an additional PHE partitioning site

297

(Guha and Jaffé, 1996; Liu et al., 2016b).

298

will diffuse into the pores of AC, and the “empty micelles” will then be exchanged

299

with new PHE-containing micelles (Guha and Jaffé, 1996).

300

can be recovered through activated carbon adsorption.

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As a result, the PHE

The PHE molecules in the hemi-micelle

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When the AC was dispersed in Tween

As a result, Tween 80

AC C

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Briefly, micelles

302

3.4 Adsorption kinetics of PHE

303

Firstly, pseudo-first-order and pseudo-second-order models (Zheng et al., 2016) were

304

applied to the adsorption kinetic process of the AC.

305

adsorbed quickly during the initial 5 h and the adsorption is virtually completed after 15

As seen in Figure 3(a), PHE is

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24 h under three different initial concentrations of PHE.

307 The non-linear and linear forms of pseudo-first-order model are represented by:

309

qt = qe 1 ‒ e‒k1 t

310

lnqe ‒ qt = lnqe ‒ k1 t

(7)

where k1 (h−1) is the adsorption rate constant of pseudo-first-order model.

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(6)

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312

314

The non-linear and linear forms of pseudo-second-order model are represented by:

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k2 q2 t

qt = 1 + k eq t 2 e

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t qt

= q + k t

1

e

2 2 qe

(9)

where k2 (g mg−1 h−1) is the adsorption rate constant of pseudo-second-order model.

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(8)

Figures 3(b) and (c) show the data plotted using the two kinetic models and it is

319

evident that the pseudo-second-order model (Figure 3c) best describes the adsorption

320

process with the kinetic parameters shown in Table S2.

321

pseudo-second-order adsorption rate constant k2 ranged from 0.26 to 0.06 g mg‒1 h‒1

322

with 10–30 mg L‒1 PHE.

323

when 120 mg L‒1 PHE was removed from TX100 solution using commercial AC (Liu

324

et al., 2014a), and the k2 value changed to 0.12 g mg‒1 h‒1 when 60 mg L‒1 PHE was

325

removed from rhamnolipid solution (Liu et al., 2016b).

AC C

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It can be seen that the

The corresponding k2 value was only 5.04×10‒3 g mg‒1 h‒1

326 16

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To further investigate the diffusion mechanism between solutes and particles, the

328

intra-particle diffusion model proposed by Weber and Morris is introduced and the

329

equation is shown below (Liu et al., 2015):

330

qt = kdi t0.5 + Cdi

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(10)

where kdi (mg g−1 h−0.5) is the adsorption rate constants of Weber and Morris

332

intra-particle diffusion model; Cdi (mg g−1) represents the effect of the boundary layer

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on diffusion.

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Generally speaking, an adsorption process on a porous adsorbent can be divided into

336

three stages: (1) external diffusion: the adsorbates go through the liquid film to the

337

external surface of the adsorbent and this stage is also known as boundary layer

338

diffusion or film diffusion; (2) intra-particle diffusion: the adsorbates move from the

339

exterior surface of the adsorbent to the internal structure of the adsorbent; (3) the last

340

stage: the adsorbates are quickly adsorbed to the active sites on the adsorbent and this

341

step is too fast to be a rate-limiting step (Yu et al., 2012 and 2016).

342

above description, it can be seen that the adsorption rate may be controlled by the first

343

stage and/or the second stage.

344

parameters of intra-particle diffusion model are shown in Table S2.

345

the intra-particle diffusion is the rate-limiting step in the first phase since the linear

346

fitting line passes through the origin (Yu et al., 2016).

347

fitting line deviates from the origin, indicating the adsorption process is controlled by

AC C

EP

TE D

335

According to the

The fitting plot is demonstrated in Figure 3(d) and the

17

As can be seen,

In the second phase, the

ACCEPTED MANUSCRIPT 348

both external and intra-particle diffusion.

The adsorption equilibrium reached in the

349

third phase (Li et al., 2016), and less time is required to reach equilibrium at lower

350

initial PHE concentration.

RI PT

351 3.5 The Adsorption isotherm

353

The classical Freundlich and Langmuir isotherm models were used to describe the

354

PHE adsorption isotherm (Foo and Hameed, 2010).

355

Freundlich fitting curve and the Langmuir fitting curve are given in Figure 4.

356

The non-linear and linear forms of Langmuir isotherm model are represented by:

SC

352

L

1

358

qe

e

= q + K 1

1

L qm []e

m

M AN U

q K []

e m L qe = 1 + K []

357

The original data, the

(11) (12)

where qm (mg g−1) is the maximum adsorption capacity of PHE; KL (L mg−1) is

360

Langmuir constant related to free energy of adsorption.

TE D

359

363 364

The non-linear and linear forms of Freundlich isotherm model are represented by:

AC C

362

EP

361

1

qe =KF [PHE]ne

(13)

1

lnqe = n ln[PHE]e + lnKF 1

365

where KF mg g1  L mg1 n and

366

intensity.

(14) 1 n

are Freundlich constants related to adsorption

367 368

It is evident that Langmuir model fits the data better than Freundlich model and the R2 18

ACCEPTED MANUSCRIPT value of the Langmuir model was 0.994 (shown in Table S3), indicating a very good

370

fit with the data, and this adsorption model represents monolayer adsorption and

371

assumes that adsorption sites have a definite number and location and every adsorbent

372

molecule possesses constant enthalpy and activation energy (Foo and Hameed, 2010).

373

Based on the Langmuir model, the maximum adsorption capacity qm was calculated to

374

be 247.54 mg g‒1, which is much higher than the literature report. The qm value was

375

only 43.9 mg g−1 when PHE was removed from rhamnolipid solution with

376

commercial AC (Liu et al., 2016b) and this value changed to 58.84 mg g−1 when

377

TX100 was used as the surfactant instead (Liu et al., 2014a).

378

M AN U

SC

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369

3.6 Adsorption thermodynamics

380

Thermodynamic parameters including Gibbs free energy change (∆G0 (kJ mol−1)),

381

enthalpy change (∆H0 (kJ mol−1)) and entropy change (∆S0 (J K−1 mol−1)) were

382

calculated using the following equations (Liu et al., 2014b):

383

EP

TE D

379

lnKd =

(15)

384

∆G0 = ∆H0 ‒ T∆S0

(16)

∆S0

∆H0 RT

AC C

R

−

385

where R is the gas constant, and T is the temperature (K).

386

coefficient (L g−1) that was calculated from the following equation:

387

Kd =

qe

Kd is the distribution

(17)

[PHE]e

388

Experiments were conducted at temperatures of 293.15, 303.15 and 313.15 K and the

389

linear regression of lnKd ∼

1 T

and the fitting equation based on Eq. (15) are shown in 19

ACCEPTED MANUSCRIPT The values of ∆H0 and ∆S0 were calculated from the equation and the

390

Figure S3.

391

data is given in Table 2.

392

adsorption process is exothermic and higher temperatures decrease adsorption.

393

positive value of ∆S0 reflected the affinity of the AC for PHE and the system

394

randomness increased after the adsorption process and the negative values of ∆G0

395

indicated that the PHE adsorption process was spontaneous (Murugesan et al., 2014).

The negative value of ∆H0 suggests that the PHE

SC

RI PT

The

396 3.7 Regeneration of the AC

398

It would be wasteful and cause secondary pollution if AC was no re-used, so

399

regeneration of the AC was investigated using the cheap and nontoxic solvent, ethanol.

400

After use, the spent AC was filtered and regenerated by shaking with ethanol in a

401

conical flask on an orbital stirrer, filtered, dried, cooled and retained for future use.

402

As seen in Figure 1(e), the FTIR spectrum of AC after regeneration indicated that the

403

two characteristic peaks at 811 cm−1 and 726 cm−1 of PHE had disappeared showing

404

that complete desorption of PHE had occurred.

405

and Figure 5 illustrates that PHE removal decreased slightly after regeneration but it

406

still reached 80% which was an acceptable result.

The regeneration was repeated twice

AC C

EP

TE D

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397

407 408

4

Conclusions

409

An effective adsorption process to remove PHE and recover Tween 80 surfactant

410

solution from soil washing effluent was investigated using activated carbon 20

ACCEPTED MANUSCRIPT 411

synthesized from waste walnut shells.

412

concentrations were optimized.

413

influence on PHE removal and the optimal conditions for selective adsorption

414

occurred when the solution was at natural pH, AC dose was 0.3 g L−1 and Tween 80

415

concentration was 5 g L−1.

416

and PHE removal and Tween 80 recovery percentage reached 95% and 90%,

417

respectively.

418

functional groups of AC and PHE are both important in the adsorption process.

419

Adsorption of PHE fitted fit well to the pseudo-second-order kinetic model and the

420

Weber and Morris intra-particle diffusion model.

421

best isotherm model which represents monolayer absorption and the PHE adsorption

422

process was exothermic and spontaneous as ∆H0 and ∆G0 were both negative.

423

could be efficiently regenerated with ethanol and subsequent reuse (twice) resulted in

424

80% of PHE removal.

425

effluent and the Tween 80 surfactant solution could be reused.

RI PT

The results showed that solution pH had no

SC

Good selectivity was displayed under optimal conditions

M AN U

The π-π interactions and the interactions between oxygen containing

TE D

Langmuir isotherm model is the

AC

EP

Overall, PHE was effectively removed from soil washing

AC C

426

The solution pH, AC dose and Tween 80

427

Acknowledgements

428

This work was supported by Wuhan Applied Basic Research Project (Grant No.

429

2016060101010074) and Shenzhen Basic Research Plan Project (Grant No.

430

JCYJ20150508152951667).

431

polishing this manuscript is also greatly appreciated.

The generous help of Professor David H. Bremner in

21

ACCEPTED MANUSCRIPT 432 433

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613

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618

modeling of selective phenanthrene removal from surfactant solutions. Colloid

619

Surf. A 470, 100-107.

AC C

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617

29

ACCEPTED MANUSCRIPT Table 1 AC surface functional groups from Boehm titration Amount (mmol g−1)

Carboxyl groups

0.9275

Lactones

0.1000

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Functional groups

0.0025

AC C

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Phenolic groups

ACCEPTED MANUSCRIPT Table 2 Thermodynamic parameters of PHE adsorption

∆H0 (kJ mol−1)

∆G0 (kJ mol−1)

∆S0 (J K−1 mol−1)

20.473

303.15 K

313.15 K

‒7.402

‒7.607

‒7.812

AC C

EP

TE D

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‒1.400

293.15 K

AC C

EP

TE D

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(a)

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ACCEPTED MANUSCRIPT

(b)

AC C

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(c)

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ACCEPTED MANUSCRIPT

(d)

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(e)

Figure 1. Characterization of AC: (a) N2 adsorption-desorption isotherm and pore diameter distribution of fresh and spent AC; (b) FESEM image of fresh AC; (c)

TE D

FESEM image of spent AC; (d) C1s spectrum of fresh AC; (e) FTIR spectra ([PHE] =

AC C

EP

100 mg L−1, [AC] = 0.3 g L−1, [Tween 80] = 5 g L−1)

M AN U

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ACCEPTED MANUSCRIPT

AC C

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(a)

(b)

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ACCEPTED MANUSCRIPT

(c)

Figure 2. Effects of: (a) pH ([PHE] = 20 mg L−1, [AC] = 0.3 g L−1, [Tween 80] = 5 g L−1); (b) AC dose ([PHE] = 20 mg L−1, [Tween 80] = 5 g L−1, pH 6.4); and (c)

AC C

EP

TE D

Tween 80 concentration ([PHE] = 20 mg L−1, [AC] = 0.3 g L−1, pH 6.4)

M AN U

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ACCEPTED MANUSCRIPT

AC C

EP

TE D

(a)

(b)

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AC C

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(c)

(d)

Figure 3. Adsorption kinetics of PHE: (a) adsorption equilibrium time; (b) pseudo-first-order model; (c) pseudo-second-order model; (d) Weber and Morris intra-particle diffusion model

M AN U

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ACCEPTED MANUSCRIPT

AC C

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Figure 4. Adsorption isotherm of PHE: Langmuir model and Freundlich model.

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ACCEPTED MANUSCRIPT

Figure 5. Reuse of AC ([PHE] = 20 mg L−1, [AC] = 0.3 g L−1, [Tween 80] = 5 g

AC C

EP

TE D

L−1, pH 6.4)

ACCEPTED MANUSCRIPT Highlights

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AC was prepared from walnut shell to adsorb phenanthrene from Tween 80

•

RI PT

solution 247.54 mg g‒1 adsorption capacity, 95% phenanthrene removal and 90% Tween 80 recovery

π-π interactions and oxygen containing groups play important roles in adsorption

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The activated carbon could be regenerated and reused at least two times

AC C

EP

TE D

M AN U

SC

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