Sorptive removal of selected emerging contaminants using biochar in aqueous solution

Sorptive removal of selected emerging contaminants using biochar in aqueous solution

Accepted Manuscript Title: Sorptive removal of selected emerging contaminants using biochar in aqueous solution Author: Eunseon Kim Chanil Jung Jonghu...

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Accepted Manuscript Title: Sorptive removal of selected emerging contaminants using biochar in aqueous solution Author: Eunseon Kim Chanil Jung Jonghun Han Namguk Her Chang Min Park Min Jang Ahjeong Son Yeomin Yoon PII: DOI: Reference:

S1226-086X(16)30007-7 http://dx.doi.org/doi:10.1016/j.jiec.2016.03.004 JIEC 2851

To appear in: Received date: Accepted date:

2-2-2016 1-3-2016

Please cite this article as: E. Kim, C. Jung, J. Han, N. Her, C.M. Park, M. Jang, A. Son, Y. Yoon, Sorptive removal of selected emerging contaminants using biochar in aqueous solution, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.03.004 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.

Sorptive removal of selected emerging contaminants using biochar

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in aqueous solution

a

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Ahjeong Sona,*, Yeomin Yoond,**

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Eunseon Kima, Chanil Jungb, Jonghun Hanc, Namguk Herc, Chang Min Parkd, Min Jange,

Department of Environmental Science and Engineering, Ewha Womans University,

Department of Earth and Environmental Studies, Montclair State University,

M

b

an

Seodaemun-gu, Seoul 120-750, Republic of Korea

Montclair, NJ 07043, USA Department of Civil and Environmental Engineering, Korea Army Academy at Young-Cheon,

d

c

Department of Civil and Environmental Engineering, University of South Carolina, Columbia,

e

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d

te

Kokyungmeon, Young-cheon, Gyeongbuk 770-849, Republic of Korea

SC 29208, USA

Department of Environmental Engineering, Kwangwoon University, 447-1 Wolgye-Dong Nowon-Gu, Seoul, Republic of Korea

*Corresponding author: phone: +82-2-3277-3339; fax: +82-2-3277-3275; e-mail: [email protected] (A. Son)

**Corresponding author: phone: +1-803-777-8952; fax: +1-803-777-0670; e-mail: [email protected] (Y. Yoon)

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Abstract The adsorption of sunscreen compounds (benzophene [BZP] and benzotriazole [BZT]) and

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widely known endocrine-disrupting compounds (bisphenol A [BPA] and 17 β-estradiol [E2]) was investigated using commercially available powdered activated carbon (PAC) and activated

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biochar produced in the laboratory. The removal efficiency by biochar was approximately 5-30%

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higher than that by PAC depending on experimental conditions, presumably due to the higher surface area and pore volume of biochar The removal of compounds followed the order E2 >

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BZP > BPA > BZT: Kf (µg/g)/(mg/L)1/n, Freundlich affinity coefficients, were as follows - 19.7, 19.7, 6.57, and 4.56 for PAC, and 30.2, 28.4, 9.22, and 6.79 for biochar. An increase in pH from

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3.5 to 10.5 decreased the adsorption of BZP, BZT, BPZ, and E2 by 11.5, 11.4, 10.7, and 4.7% by

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compared with PAC.

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biochar, respectively. Overall, biochar had a higher adsorption capacity for all chemicals tested

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Keywords: micropollutants; adsorption; biochar; nuclear magnetic resonance; water treatment

1. Introduction Numerous

emerging

micropollutants,

such

as

endocrine-disrupting

compounds,

pharmaceuticals, and personal care products, have been detected at trace concentrations (< 1 µg/L) in various surface and ground waters and wastewaters globally, some of which have been connected with ecological influences, even at these very low concentrations [1-8]. Reports have raised significant concerns on micropollutants in public health assessments and environmental risks with both regulatory agencies and the public [9-12]. Many emerging organic

1 Page 2 of 31

micropollutants are more polar than ‘conventional’ contaminants (e.g., polycyclic aromatic hydrocarbons and pesticides) and may have numerous acidic and/or basic functional groups [13]. These emerging micropollutants cannot be removed completely during wastewater or water

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treatment [1, 14-23]. In particular, during water treatment, previous studies have reported that coagulation processes typically remove only insignificant percentages of micropollutants in

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aqueous solutions [24, 25]. However, numerous pesticides, pharmaceuticals, and estrogenic

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compounds can be removed significantly using activated carbon [24-29]. The removal degree of activated carbon is governed by the physicochemical properties (shape, size, charge, and

an

hydrophobicity) of the solute and the sorbent (surface area, pore size distribution, surface charge, oxygen content) [30]. The main removal mechanisms for most organic compounds in activated

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carbon adsorption systems are hydrophobic interactions. Due to hydrophobic attractions,

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compounds with log KOW > 2) [13].

d

activated carbon significantly removes most non-polar organic compounds (i.e., those

It is anticipated that biochar will be available for value-added products due to advances in

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biorefineries in the near future [31]. Biochar is obtained as a byproduct of the pyrolytic processing of biomass when biofuel is produced during controlled thermal processes and gasification [32]. Additionally, biochar shows potential as a promising adsorbent for the removal of micropollutants due to its better properties, including its surface density of functional groups and highly condensed structure, while the activated product provides a smaller surface area and volume than commercially activated carbon [33]. These properties vary depending on the type of feedstock, pyrolysis conditions (residence time and temperature), and activation. In particular, while higher proportions of aliphatic carbons and functional groups are typical of biochars pyrolyzed at low temperatures, biochar pyrolyzed at higher temperatures contains mainly

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polyaromatic carbons and has a higher microporosity, which enhances organic compound adsorption [32, 34]. In one study, chemically activated biochar resulted in a relatively larger surface area, porous structure, and lower ash content than commercially available activated

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carbon [35]. Many organic forms, including plants, sewage sludge, domestic and industrial wastes, and animal manures, are used as material sources for pyrolysis. The composition of

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elements and the ratio of inorganic components in biomass varies and affects both the product

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yield and quality of bio-oil and biochar [36].

An efficient treatment strategy for micropollutants has been considered using cost-effective

an

adsorption, particularly with biochar in an aqueous environment [31, 37-39]. A recent study reported the effect of temperature on sulfamethoxazole removal using a biochar pyrolyzed at

M

600°C [40]. A separate study discovered distinct adsorption abilities of demineralized pine wood biochar for sulfamethoxazole and sulfapyridine [38]. In that study, their adsorption was reduced

te

d

in the presence of humic acid or a cation (Cu2+). However, there has been little research effort devoted to biochar prepared under different pyrolysis conditions and the inversely proportional

Ac ce p

relationship between biochar and bio-oil production. Also, most previous studies on biochar have not compared its performance with commercially available activated carbon and cover only a few compounds under limited water quality conditions. Thus, the objective of this study was to determine the removal of four micropollutants: sunscreen compounds (benzophene [BZP] and benzotriazole [BZT]) and widely known endocrine-disrupting compounds (bisphenol A [BPA] and 17 β-estradiol [E2]), each having different physicochemical properties, by biochar under various water quality conditions (pH, background ions, ionic strength, and natural organic matter [NOM]). Activated biochar produced in the laboratory was also characterized using conventional analytical methods as well as

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advanced solid-state nuclear magnetic resonance (NMR) techniques to address how these properties determine the mechanism(s) and micropollutant adsorption characteristics.

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2. Material and methods 2.1. Reagents and selected micropollutants

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All standards and chemicals were at least reagent grade and/or of the highest purity available

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commercially. For adsorption experiments, four micropollutants were selected as target compounds. Table 1 summarizes the target EDC/PPCP compounds that were studied by spiking

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the compounds into various synthetic waters. In selecting the target compounds, two issues were considered: (i) their occurrence in source waters and (ii) the physicochemical properties of the

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particular compound. All target compounds were obtained from Sigma-Aldrich (St. Louis, MO,

d

USA). Concentrated spiking solutions of the target compounds were prepared at high

te

concentrations (approximately 500-1,000 mg/L) in methanol to minimize the volume of solvents introduced into the experiments. BZP, BPA, and E2 had low water solubilities and thus could not

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be spiked as neat standards. A small volume of each spiking solution (<5 mL) was injected into a 5-L flask containing a target synthetic water.

2.2. Adsorbents

Biochar samples were produced with torrefied loblolly pine chips (15 × 6 mm) containing bark through thermal treatment at 300°C for 15 min in a laboratory-scale batch tube-furnace (OTF-1200X, MTI Corp., Richmond, CA, USA) under pure nitrogen. A thorough description of the production method has been published previously [31]. The yield of biochar was 42.3%. Biochar samples of 3 g were soaked with 40 mL of a 4 moles/L NaOH solution and incubated

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with shaking (15-min intervals) for 2 h at room temperature. NaOH-impregnated samples were filtered (Buchner filter funnel) to remove excess NaOH solution and dried overnight (105°C oven). The NaOH-impregnated biochar samples were then heated at 800°C for 2 h under a

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nitrogen gas flow (2 L/min) and cooled (10°C/min). The dried samples were rinsed with 0.1 mole/L HCl followed by deionized water until they reached neutral pH, dried at 105°C, milled,

cr

and passed through a 74-μm sieve. Additionally, a commercially available powdered activated

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carbon (PAC, PAC form of F400, Calgon Carbon Corp., Pittsburgh, PA, USA) was used to

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compare its adsorption ability with that of the biochar manufactured in the laboratory.

2.3. Adsorption experiments

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Table 1 describes the characteristics of the micropollutants that were studied by spiking

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them together into different synthetic waters. A stock suspension of 1,000 mg/L of each

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adsorbent was prepared for the batch adsorption experiments. Four target micropollutants (initial concentrations of approximately 500-1,000 g/L each) were placed together in contact with PAC

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and biochar as mixed components in binary mixtures with salts (NaCl, Na2SO4, or CaCl2) or humic acid at varying pHs and ionic strengths. Prior to the experiments, each solution was buffered with 2 mmoles/L phosphate buffer solution, and the pH was adjusted using 1 mole/L HCl or 1 mole/L NaOH solution. Applied adsorbent doses ranged from 0 to 50 mg/L for kinetic and adsorbent dose-response experiments, which were conducted with 1, 4, and 24 h contact times. PAC and biochar adsorption experiments were conducted at an adsorbent dose of 50 mg/L with a 4 h contact time. While adsorption experiments are generally conducted for 7 days to reach equilibrium, kinetic experiments confirmed that 4 h was sufficient time for the solution to reach a pseudo-equilibrium state. Screw-cap amber vials (40 mL) were used for the kinetic and

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isotherm experiments and were shaken in a shaker (WiseShake SHO-2D, Seoul, Korea) using a separate container for each duplicate at 100 rpm.Most adsorption experiments (adsorption test; isotherm and kinetic experiment) were conducted in duplicate and triplicate to achieve higher

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coincidence and repeatability. Control treatments contained target micropollutants, but no PAC or biochar. Adsorbents were removed from the samples by filtering with a 0.7-m (25 mm GF/F)

cr

glass-fiber filter. All adsorbents were hydrated for 24 h in distilled water prior to use and added

us

as a slurry to the samples. The adsorption data obtained in the experiments were fitted to a Freundlich isotherm model in Eq. (1): 1/ n

an

qe  K f Ce

(1),

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where qe is the solid-phase concentration (mg/g), Ce is the equilibrium solution phase concentration (mg/L), Kf is the Freundlich affinity coefficient [(mg/g)/(mg/L)(1/n)], and n is a

Characterization of adsorbents

Ac ce p

2.4.

te

d

dimensionless number related to surface heterogeneity.

A PerkinElmer 2400 Series II Elemental Analyzer (PerkinElmer, Waltham, MA, USA) was used for elemental analyses. While ash content was determined by heating the biochars to 750°C, oxygen content was also calculated by subtracting the ash and carbon, hydrogen, and nitrogen contents from the sample total mass. The Brunauer-Emmett-Teller (BET) surface area was determined with a Gemini VII 2390p surface area analyzer (Micromeritics, Norcross, GA, USA),and the total pore volume was determined from the adsorbed quantity of N2 at P/P0 = 0.95. Solid-state 13C direct polarization/magic angle spinning (DP/MAS) NMR spectra were acquired with a 3.2 mm MAS probe on a Varian Inova 500 spectrometer (Palo Alto, CA, USA). The 13C NMR spectra, combined with dipolar-recoupled NMR methods, were used for quantitative 6 Page 7 of 31

structural analyses of the PAC and biochar. Detailed experimental conditions for the NMR experiments are described elsewhere [41]. The adsorption surface charge was estimated as a function of pH for each adsorbent at a concentration of 50 mg/L by zeta potential measurements

Analytical methods

cr

2.5.

ip t

(Brookhaven ZetaPals, Holtsville, NY, USA).

us

To carry out sensitive and simple analysis of the target compounds, high-performance liquid chromatography coupled to mass spectrometry (HPLC-MS/MS) and HPLC-fluorescence were

an

carried out on Agilent Technologies 6410 and 1200 Series instruments (Santa Clara, CA, USA), respectively. HPLC-MS/MS was equipped with electrospray ionization (ESI) apparatus and a

M

C18 reverse column (150 mm × 4.6 mm, 5 m) (Agilent Technologies, Santa Clara, CA, USA)

d

for BZP and BZT determination. For HPLC-fluorescence, detection was carried out using a

te

fluorescence detector at an excitation wavelength of 280 nm and an emission wavelength of 310 nm to identify BPA and E2. The mobile-phase solvent profile was 30% DI water and 70%

Ac ce p

methanol for 10 min at a constant flow rate of 1 mL/min and with a sample injection volume of 10 L. Chromatographically separated samples for HPLC-MS/MS were analyzed under the following conditions: ESI negative ionization mode; drying gas flow, 10 mL/min; nebulizer pressure, 345 kPa; drying gas temperature, 350°C; fragmentor, 110 V; collision energy, 25 V; precursor ion, 183 m/z; and product ion, 105 m/z for BZP and 65 m/z for BZT. Each compound concentration in the samples was determined against an external calibration curve with five different concentrations of 1, 10, 50, 100, and 500 g/L prepared in DI water. Calibration standards of 50 M were run between approximately every 10 samples. The method detection limits were approximately 0.5 g/L for BZP and BZT by HPLC-MS/MS and approximately 1 7 Page 8 of 31

g/L for BPA and E2 by HPLC-fluorescence.

3. Results and discussion

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3.1. Characterization of PAC and biochar

The characterization of adsorbents is significant because adsorption is greatly influenced by

cr

the physicochemical properties of the adsorbent. Both elemental analyses and solid-state NMR

composition and

13

us

were used to characterize the commercially available PAC and activated biochar. The elemental C NMR spectra of PAC and biochar differed significantly (Fig. 1a). The

an

precursor biochar for the activated biochar was pyrolyzed in the absence of oxygen, allowing the material to be fully charred. The oxygen content (20.2%) of PAC was slightly lower than N-

M

biochar (21.3%), resulting in carbon contents of 59.3% and 72.6%, respectively (Table 2). The

d

H/C ratio of 0.032 for PAC and 0.127 for biochar indicated that PAC was slightly more

te

carbonized than biochar.

The elemental analyses were consistent with the 13C solid-state NMR results (Table 2). The C DP/MAS NMR spectra showed that PAC has a more aromatic character on the basis of a

Ac ce p

13

stronger peak at 108-148 ppm, corresponding to aryl carbons, than biochar. However, biochar has a slightly higher aliphatic carbon fraction: paraffinic or alkyl (0-45 ppm), methoxyl (45-63 ppm), carbohydrate (63-108 ppm), and carboxyl carbons (165-187 ppm). Quantitative analyses of the

13

C DP/MAS experiments showed that PAC had a more condensed aromatic

structure with higher aromaticity based on the larger non-protonated carbon fraction, which agreed well with the lower H/C ratio than that of biochar. These results indicate that PAC has higher hydrophobicity than biochar.

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N2 adsorption experiments were conducted to determine the porous structures (BET surface area and pore volume) of PAC and biochar (Table 2). The chemically activated biochar exhibited a higher surface area (1360 m2/g) and pore volume (0.95 cm3/g) than those of commercial PAC

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(972 m2/g and 0.53 cm3/g). The biochar, having a high surface area and pore volume, may be a promising sorbent from renewable biomass that can potentially replace coal-based activated

cr

carbons, such as PAC. The amount of ash (4.7%) in the activated biochar was far lower than the

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commercial PAC (20.1%). The lower ash content of biochar was due to its property as a feedstock, following in the order of livestock manure > corn or wheat stover > hard wood > soft

an

wood [36]. The higher ash content of coal-based PAC barely contributes to binding hydrophobic organic compounds, except for the interaction with ash-preferring species, such as some dyes

M

and metal ions [42, 43]. Thus, not only the effective surface area and pore volume of the

d

adsorbent, but also the total aromaticity may be diminished, resulting in reduced adsorption

te

capacity.

pHpzc (i.e., the pH at which the electrical charge density on the surface of an adsorbent is

Ac ce p

zero) is a significant parameter for explaining the efficiency of the adsorption process. Under conditions of pH > pHpzc, the adsorbent surface becomes negatively charged, thereby attracting or repelling micropollutants, depending on their anionic or cationic functional groups. As shown in Fig. 1b, the pHpzc was approximately 9 for both PAC and biochar, while PAC’s pHpzc was slightly lower than that of biochar.

3.2. Kinetic and adsorbent dose-response experiments Kinetic experiments were monitored by collecting samples after 1, 4, and 24 h of contact with PAC and biochar. Representative adsorbent dose-response data for the target

9 Page 10 of 31

micropollutants are shown in Fig. 2. Removal varied depending on the target compound, dose, and contact time. Approximately 45, 50, and 25 mg/L of PAC was required to achieve 50% BZP, BPA, and E2 removal, respectively, from the model water with a contact time of 1 h, whereas

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significantly lower doses were required for contact times of 4 and 24 h. Significantly lower doses were required for biochar to achieve the same degree of adsorption. The removal of BZT (< 25%

cr

at 50 mg/L and 24 h) was significantly lower than those of the other compounds.

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Fig. 3 shows the adsorption of target compounds to PAC and biochar with a contact time of 24 h. A linearized Freundlich isotherm model was used to fit the sorption data. Table 3 lists the

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fitting parameters of isotherms for the various compounds investigated in this study. Currently, the most frequently applied models to describe the adsorption of organic compounds onto

M

activated carbons are the Freundlich and Langmuir isotherms [24, 39, 44]. The Freundlich

d

isotherm model describes multi-layer adsorption; the compounds initially interact with the PAC

te

and biochar surfaces and then with each other. Langmuir isotherms assume a single-layer adsorption process, where the chemicals only interact with the PAC and biochar surface

Ac ce p

throughout the adsorption process. While both Freundlich and Langmuir isotherms were used in this study, Langmuir isotherm data are not shown due the low correlation (R2) values. The higher correlation values for the Freundlich method (Table 3) suggest that the adsorption in this study can be better explained using the Freundlich model. A clear trend of adsorption onto both PAC and biochar was observed for the target compounds: the removal of compounds follows the order E2 > BZP > BPA > BZT (Kf, (g/g)/(mg/L)1/n; 19.7, 19.7, 6.57, and 4.56 for PAC, and 30.2, 28.4, 9.22, and 6.79 for biochar, respectively. These adsorption phenomena are related to the physicochemical properties of adsorbates/adsorbents and the water quality conditions (e.g., pH,

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ionic strength/background ion, and humic acid). The influence of these factors on the adsorption of target compounds is discussed in the following sections.

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3.3. Adsorption differences among adsorbates and adsorbents

The target compounds include single or multiple charged groups, as well as polar groups

cr

(hydroxyl, carbonyl, and amine) with aromatic rings. All target compounds are primarily neutral

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species under acidic conditions at pH 3.5. The effect of pH on the adsorption of target compounds is shown in Fig. 4. Generally, the adsorption by PAC and biochar decreased slightly

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with increasing pH for the target compounds, excluding BZP for PAC. BZP (pKa, -7.5) adsorption increased slightly with increasing pH, although it is unclear why this occurs. The

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adsorption affinity towards adsorbents increases significantly with the pH < pKa, while the

d

adsorption affinity decreases sharply when pH > pKa value [45]. This is presumably because the

strong

te

electronic coupling influences the adsorptive interaction with each adsorbent. BPA, having a electron-withdrawing

hydroxyl

group,

had

a

repulsive

interaction

with

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π-electron acceptor-rich aromatic rings on the adsorbents [46], resulting in inhibition of π-π electron donor-acceptor (EDA) interactions. However, less variation in adsorption affinity due to their negative or higher pKa values allowed BZP and E2 to show strong hydrophobic interactions through a wider range of pH values (Fig. 4). In addition, E2 was most likely non-ionizable across the pH range from 3.5 to 10.5, and thus the influence of pH was insignificant on the adsorption, with minimal variation (< 5%). The adsorption contrast among compounds with varying pH may be attributable to the different octanol-water partition coefficient (KOW, indicating hydrophobicity). However, this is true only if the compounds are non-ionizable, independent of pH. Thus, the use of the distribution coefficient (D) is more realistic and preferred, because it

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avoids the overestimation of hydrophobicity [47]. The pH-dependent D values of the target compounds were calculated and reported on a logarithmic scale at pH 3.5, 7.0, and 10.5 (Table 1). The adsorption of BZT and BPA decreased by 2.1-8.2 and 4.5-8.4%, respectively, with an

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increase in pH from 7.5 to 10.5 due to drops in their hydrophobicity (log DOW 1.27 to -1.01 for BZT and 3.44 to 2.64 for BPA), thus lowering the interaction with the adsorbent when pH > pKa.

cr

While several adsorption studies of organic compounds have emphasized hydrophobic

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interactions in various solutions [25, 26, 31], the adsorption of organic compounds cannot be interpreted by only one or two mechanisms. These results showed that biochar had a higher

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adsorption for all target compounds compared with PAC (Figs. 2-4), although the aromaticity of biochar (62.5%) was lower than that of PAC (69.4%). While high aromaticity contributed to

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effective adsorption [48], the smaller surface area and pore volume of PAC relative to biochar

d

significantly limited the adsorption capacity (Table 2). Additionally, elemental composition,

te

structural characteristics, and surface properties of adsorbents greatly affect their adsorption performance. The higher polarity of biochar, based on its polarity index (N/C + O/C), enhanced

Ac ce p

the adsorption affinity towards polar compounds due to delocalization of the aromatic π-cloud. This role of polarizability may lead to induced electrostatic interactions (i.e., π-π interaction, πstacking, and London dispersion forces [49]). Additionally, a previous study showed a positive relationship between the polarities of compounds and the sorption coefficient, log KOC [31], which may be interpreted through other intermolecular interactions such as dipolar and dispersion forces, resulting in higher adsorption affinity [50]. Greater contributions of carbohydrate (63-108 ppm), carboxyl (165-187 ppm), and carbonyl carbons (187-220 ppm), as determined from the 13C NMR spectra in biochar, are attributable to the polar functional groups, suggesting that a polarity provider induces a higher adsorption capacity. In addition, the

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adsorption activity might be attributable to π-H-bonding interactions due to the higher portion of O-containing (polar) functional groups in biochar [51], although this mechanism was largely inhibited by different values of π-electron-dependent polarizable interactions, EDA interactions,

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and a specific non-covalent force remaining between π-electron-rich moieties (π-electron donors) and π-electron-depleted moieties (π-electron acceptors) throughout the entire pH range [52, 53].

cr

Thus, these effects resulted in a strong interaction between compounds (π acceptors) and

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adsorbents having aromatic benzene rings (π donors) as well as hydrophobic interactions [50]. Previous studies have shown that the adsorption of organic compounds was significantly

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influenced by micropore filling and sieving effects [46, 54, 55]. Relatively smaller pore filling and sieving effects occurred with biochar due to its larger micro- and macro-pore volumes (0.307

d

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and 0.643 cm3/g) versus those of PAC (0.216 and 0.314 cm3/g, respectively) (Table 2).

te

3.4. Effect of electrolyte species, ionic strength, and humic acid on adsorption The effects of electrolyte species on the adsorption of target compounds onto PAC and

Ac ce p

biochar were examined by varying the background ions (NaCl, Na2SO4, and CaCl2) and their concentrations (10, 50, and 100 mM). As shown in Fig. 5, the adsorption of E2 by both PAC and biochar was essentially unaffected by the presence of background ions and their concentration ( < 5%). However, an increase in the NaCl and Na2SO4 concentration from 0 to 100 mM increased the adsorption of BZP and BZT by 6-19% and 3-12%, respectively. Previous studies have shown that increased ionic strength can enhance the adsorption of compounds onto carbonaceous materials due to a screening effect of the surface charge, produced by adding a salt [56, 57]. However, other researchers have observed that increasing the ionic strength had an insignificant effect on the adsorption of various organic compounds [58-60]. The observation found in this

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study could possibly be explained by the previously mentioned “salting-out” effect (i.e., the reduced solubility of organic compounds in aqueous salt solutions), which has been found to be strong with Na+ and Ca2+ [61]. This mechanism could presumably increase the accessibility of

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the surfaces of PAC and biochar to the target compounds. However, the adsorption data in Fig. 5 suggest that this mechanism may only apply in the presence of Na+. The experiments showed

cr

that the adsorption of the target compounds decreased with increasing CaCl2 concentration by 3-

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17%. This is presumably because divalent calcium ions can complex those compounds, possibly

tend to reduce their ability to access micropores.

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due to bridging organic compounds in solution prior to the adsorption step [62], which would

Competitive adsorption was assessed in the presence of humic acid, representing the

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relatively complex organic carbon present in natural water. The effects of humic acid on the

d

adsorption of target compounds on PAC and biochar were insignificant in this study (Fig. 6).

te

Possible explanations may include competition against occupying active adsorption sites and hydrophobic interactions between micropollutants and humic acid. A previous study showed that

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NOM reduced atrazine adsorption to approximately 35% under non-adsorbent conditions; this was presumably due to precipitation with a hydrophobic interaction between the NOM and atrazine, which contains a heterocyclic aromatic ring, and direct site competition and pore blockage due to the small size of atrazine [63]. However, the major adsorption capacity of hydrophobic chemicals on the adsorbent was predominant, while NOM disperses not only adsorbents [64] but also the compounds [65] in the solution or initiates ionization through interactions with diverse functional groups on NOM.

4. Conclusions

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Overall, this study demonstrated higher removal efficiencies of four target micropollutants (BZP, BZT, BPA, and E2) using biochar produced in the laboratory versus commercially available PAC. Analysis of the adsorbents by solid-state NMR showed that biochar had higher

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polarity moieties, with more alkyl, methoxyl, O-alkyl, and carboxyl carbon content than PAC, while the aromaticity of PAC was higher than that of biochar. Additionally, PAC contained

cr

mostly aromatic moieties, with lower H/C and O/C ratios than the highly polarized biochar,

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which contained diverse polar functional groups, suggesting that a polarity provider can induce a higher adsorption capacity and the higher surface area and pore volume of biochar. The

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adsorptive capacity of both adsorbents consistently followed the order E2 > BZP > BPA > BZT, while in general the degree of adsorption increased with increasing contact time. This is

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presumably due to their hydrophobicity following the same order, which leads to increased

d

hydrophobic interactions. Generally, an increase in pH slightly decreased the adsorption,

te

presumably because electronic coupling influences the adsorptive interaction with each adsorbent. Also, an increase in ionic strength with NaCl and Na2SO4 increased adsorption

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slightly, possibly due to the “salting-out” effect, while the adsorption of the target compounds decreased with increasing CaCl2 concentrations. These findings suggest that an understanding of the adsorption system with adsorbents can contribute significantly to improving the removal efficiency of undesirable chemicals under varying water quality conditions.

Acknowledgements This research was supported by Ministry of Science, ICT, and Future Planning in Korea (2015M3C8A6A06012735). This research was also supported by the Korea Ministry of Environment, ‘GAIA Project, 2015000540003’.

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[11] L.N. Vandenberg, T. Colborn, T.B. Hayes, J.J. Heindel, D.R. Jacobs, Jr., D.H. Lee, J.P. Myers, T. Shioda, A.M. Soto, F.S. vom Saal, W.V. Welshons, R.T. Zoeller, Reprod. Toxicol. 38 (2013) 1.

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[12] R.T. Zoeller, T.R. Brown, L.L. Doan, A.C. Gore, N.E. Skakkebaek, A.M. Soto, T.J. Woodruff, F.S.V. Saal, Endocrinology 153 (2012) 4097. [13] S.A. Snyder, P. Westerhoff, Y. Yoon, D.L. Sedlak, Environ. Sci. Technol. 20 (2003) 449. [14] J. Ryu, J. Oh, S.A. Snyder, Y. Yoon, Environ. Monit. Assess. 186 (2014) 3239. [15] N. Vieno, M. Sillanpaa, Environ. Int. 69 (2014) 28. [16] Y. Luo, W. Guo, H.H. Ngo, N. Long Duc, F.I. Hai, J. Zhang, S. Liang, X.C. Wang, Sci. Total. Environ. 473 (2014) 619. [17] B. Yang, G.G. Ying, J.L. Zhao, S. Liu, L.J. Zhou, F. Chen, Water Res. 46 (2012) 2194. [18] C. Park, Y. Fang, S.N. Murthy, J.T. Novak, Water Res. 44 (2010) 1335. [19] Q. Sui, J. Huang, S. Deng, G. Yu, Q. Fan, Water Res. 44 (2010) 417.

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[22] Z.H. Liu, Y. Kanjo, S. Mizutani, Sci. Total. Environ. 407 (2009) 731.

[23] Y. Yoon, J. Ryu, J. Oh, B.G. Choi, S.A. Snyder, Sci. Total. Environ. 408 (2010) 636.

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[24] L. Joseph, L.K. Boateng, J.R.V. Flora, Y.G. Park, A. Son, M. Badawy, Y. Yoon, Sep. Purif. Technol. 107 (2013) 37.

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[26] Y.M. Yoon, P. Westerhoff, S.A. Snyder, M. Esparza, Water Res. 37 (2003) 3530. [27] M. Chen, P. Xu, G. Zeng, C. Yang, D. Huang, J. Biotechnol. Adv. 33 (2015) 745-755.

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[32] B. Chen, D. Zhou, L. Zhu, Environ. Sci. Technol. 42 (2008) 5137. [33] L. Ji, Y. Shao, Z. Xu, S. Zheng, D. Zhu, Environ. Sci. Technol. 44 (2010) 6429. [34] Y. Chun, G. Sheng, C.T. Chiou, B. Xing, Environ. Sci. Technol. 38 (2004) 4649. [35] R. Azargohar, A. Dalai, Appl. Biochem. Biotech. 131 (2006) 762. [36] D.A. Laird, R.C. Brown, J.E. Amonette, J. Lehmann, Biofuel. Bioprod. Bior. 3 (2009) 547. [37] T. Li, X. Han, C. Liang, M.J.I. Shohag, X. Yang, Environ. Technol. 36 (2015) 245. [38] M. Xie, W. Chen, Z. Xu, S. Zheng, D. Zhu, Environ. Pollut. 186 (2014) 187. [39] C. Jung, L.K. Boateng, J.R.V. Flora, J. Oh, M.C. Braswell, A. Son, Y. Yoon, Chem. Eng. J. 264 (2015) 1. [40] T. Li, X. Han, C. Liang, M. Shohag, X. Yang, Environ. Technol. (2014) 1. [41] J. Park, J. Meng, K.H. Lim, O.J. Rojas, S. Park, J. Anal. Appl. Pyrol. 100 (2013) 199. 17 Page 18 of 31

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[45] K. Yang, W. Wu, Q. Jing, L. Zhu, Environ. Sci. Technol.42 (2008) 7931. [46] L. Ji, F. Liu, Z. Xu, S. Zheng, D. Zhu, Environ. Sci. Technol.44 (2010) 3116.

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Ac ce p

te

d

M

an

us

cr

ip t

[65] L.C. Konradt Moraes, R. Bergamasco, C.G. Tavares, D. Hennig, M. Carvalho Bongiovani, Int. J. Chem. React. Eng. 6 (2008) A87.

19 Page 20 of 31

Figure captions Fig. 1. (a) Solid-state

13

C DP/MAS NMR spectra for PAC and biochar samples. (b) Zeta

ip t

potential curves as a function of pH for each type of adsorbent (dose of adsorbent = 50 mg/L).

cr

Adsorbent: PAC (○); biochar (●).

an

of (1) 1 h, (2) 4 h, and (3) 24 h (pH = 7; NaCl = 10 mM).

us

Fig. 2. Effect of contact time on adsorption by (a) PAC and (b) biochar at various contact times

M

Fig. 3. Freundlich model isotherms for adsorption by PAC and biochar (contact time = 24 h; pH

d

= 7; NaCl = 10 mM).

Ac ce p

mg/L; NaCl = 10 mM).

te

Fig. 4. Effect of pH on adsorption by PAC and biochar (contact time = 4 h; adsorbent dose = 20

Fig. 5. Effect of background ions on adsorption by (a) PAC and (b) biochar (contact time = 4 h; adsorbent dose = 20 mg/L; pH = 7).

Fig. 6. Effect of humic acid on adsorption by PAC and biochar (contact time = 4 h; adsorbent dose = 20 mg/L; pH = 7; NaCl = 10 mM).

20 Page 21 of 31

ip t cr us an M d te Ac ce p Fig. 1

21 Page 22 of 31

BZP

BPA

BZT

1.0

E2

ip t

0.6 0.4

cr

C/C0

0.8

(a.1)

0.0 1.0

(b.1)

an

0.6 0.4

M

C/C0

0.8

0.2

(b.2)

te

d

(a.2)

0.0 1.0 0.8 0.6

Ac ce p

C/C0

us

0.2

0.4 0.2

(a.3)

0.0

0

10

20

30

40

Dose (mg/L)

(b.3) 50

60 0

10

20

30

40

50

60

Dose (mg/L)

Fig. 2

22 Page 23 of 31

BZT

BZP

BPA

E2

-3.0 (b) (a)PAC PAC

ip t

-4.0 -4.5

cr

-5.0 -5.5 -6.0 -6.5 -4

-3

-2

-1

an

-5

us

ln qe (mg/g)

-3.5

0

ln C e (mg/L) BZT

BZP

(a) Biochar (b) Biochar

d

-4.0

-5.5

te

-4.5

Ac ce p

ln qe (mg/g)

-3.5

-5.0

E2

M

-3.0

BPA

-6.0 -6.5

-5

-4

-3

-2

-1

0

ln C e (mg/L)

Fig. 3

23 Page 24 of 31

PAC

1.0

Biochar

pKa -7.5

ip t

0.6 0.4

cr

C/C0

0.8

(a) BZP

(b) BZT

M

0.6 0.4

d

0.2

0.0

3.5

te

pKa 9.6-10.2

(c) BPA

7.0

pH

10.5

(d) E2 14.0 0.0

pKa 10.13 3.5

7.0

10.5

14.0

pH

Ac ce p

C/C0

0.8

0.0

pKa 8.2

an

0.0 1.0

us

0.2

Fig. 4

24 Page 25 of 31

BZP

BZT

BPA

E2

1.0

ip t

0.6 0.4

cr

C/C0

0.8

0.2

us

0.0

NaCl (mM)

NaCl (mM)

an

1.0

0.6

M

C/C0

0.8

0.4

d

0.2 0.0

Ac ce p

0.8

C/C0

Na2SO4 (mM)

te

Na2SO4 (mM) 1.0

0.6 0.4 0.2 0.0

0

20

40

60

80

CaCl2 (mM)

100

120 0

20

40

60

80

100

120

CaCl2 (mM)

Fig. 5

25 Page 26 of 31

BPA

BZT

BZP

E2

1.0

ip t cr

0.6 0.4

us

C/C0

0.8

0.2

(a) PAC

0.4

M

Ac ce p

0.2

d

0.6

te

C/C0

0.8

0.0

an

0.0 1.0

(b) Biochar 0

5

10

15

20

25

Humic acid (mg/L)

Fig. 6

26 Page 27 of 31

Table 1. Properties of target micropollutants used in this study.

Bisphenol A [BPA] (Plasticizer)

pH 10.5

Log Kow

182.2

3.43

3.43

3.43

3.43

119.1

1.30

1.27

-1.01

228.1

M

17-estradiol [E2] (Hormone)

3.44

273.2

3.75

pKaa

ip t

pH 7.0

cr

Benzotriazole [BZT] (Sunscreen)

pH 3.5

-7.5

us

Structure

Benzophenone [BZP] (Sunscreen)

1.30

8.2

3.44

2.64

3.44

9.6 -10.2

3.75

3.55

3.75

10.3

te

d

chemicalize.org by ChemAxon (http://www.chemicalize.org)

Ac ce p

a

Log Dowa

MW (g/mol)

an

Compound [ID] (Use)

27 Page 28 of 31

ip t cr

Table 2. Properties of PAC and biochar (modified from [31]).

C (%)

H (%)

N (%)

O (%)

PAC

59.3

0.16

0.31

20.2

Biochar

72.6

0.77

0.65

21.3

H/C

Polarity index

Ash (%)

SA-N2 (m2/g)

0.255

20.1

0.221

4.7

N/C

O/C

0.032

0.004

0.127

0.001

M an

Samples

us

Elemental composition, aromatic ratio, ash content, BET-N2 surface area (SA-N2), and cumulative pore volume Pore volume (cm3/g) micropore

macro-pore

972.3

0.216

0.314

1360

0.307

0.643

Quantitative spectral analysis for solid-state 13C DP/MAS NMR (calculated based on 100% carbon in each biomass)

Samples

Alkyl

Methoxyl

Aromatic C (%)

ed

Aliphatic C (%)

Carbohydrate

Carbonyls (%)

Aryl

O-aryl

Carboxyl

Carbonyl

108–148 ppm

148–165 ppm

165–187 ppm

187–220 ppm

Aliphatic C (%)

Aromatic C (%)

Aromaticity (%)a

Polar C (%)b

0–45 ppm

45–63 ppm

PAC

2.78

3.70

21.4

53.5

9.67

5.55

3.43

27.9

63.2

69.4

43.8

Biochar

7.14

4.81

21.5

45. 7

9.91

8.22

2.79

33.5

55.6

62.5

47.2

ce pt

63–108 ppm

Aromaticity = aromatic C (108–165 ppm) / [ aliphatic C (0–108 ppm) + aromatic C (108–165 ppm)]

b

Total polar carbon = (45-108 ppm) + (148-220 ppm)

Ac

a

29 Page 29 of 31

Table 3. Freundlich and Langmuir isotherm parameters for adsorption of target micropollutants onto PAC and biochar. PAC

Adsorbate

Biochar

1/n

R2

Kf

1/n

R2

BZP

19.7

0.160

0.985

28.4

0.132

0.990

BZT

4.56

1.625

0.993

6.79

1.321

0.996

BPA

6.57

0.115

0.997

9.22

0.214

0.995

E2

19.7

0.096

0.990

30.2

0.142

0.992

cr

Ac ce p

te

d

M

an

Unit of Kf: (g/g)/(mg/L)

us

1/n

ip t

Kf

31 Page 30 of 31

Graphical abstract

Aliphatic C (0-108)

COOH C=O (165-220)

ip t

Aromatic C (108-165)

us

cr

Paraffinic C (0-45)

200

150

100

50

M

250

an

Biochar

0

PAC

-50

Ac ce p

te

d

Chemical shifts, ppm

32 Page 31 of 31