ORIGINAL PAPER

High efficacy on diclofenac removal by activated carbon producedfrom potato peel waste

M. Bernardo1 • S. Rodrigues1 • N. Lapa2 • I. Matos1 • F. Lemos3 • M. K. S. Batista1,4 •

A. P. Carvalho4 • I. Fonseca1

Received: 21 January 2016 / Revised: 12 April 2016 / Accepted: 21 May 2016 / Published online: 6 June 2016

� Islamic Azad University (IAU) 2016

Abstract In the present study, a novel porous carbon

obtained by K2CO3 activation of potato peel waste under

optimized conditions was applied for the first time as liq-

uid-phase adsorbent of sodium diclofenac in parallel with a

commercial activated carbon. The biomass-activated car-

bon presented an apparent surface area of 866 m2 g-1 and

well-developed microporous structure with a large amount

of ultramicropores. The obtained carbon presented leaching

and ecotoxicological properties compatible with its safe

application to aqueous medium. Kinetic data of laboratory-

made and commercial sample were best fitted by the

pseudo-second-order model. The commercial carbon pre-

sented higher uptake of diclofenac, but the biomass carbon

presented the higher adsorption rate which was associated

with its higher hydrophilic nature which favoured external

mass transfer. Both adsorbents presented adsorption iso-

therms that were best fitted by Langmuir model. The

biomass carbon and the commercial carbon presented

adsorption monolayer capacities of 69 and 146 mg g-1,

and Langmuir constants of 0.38 and 1.02 L mg-1,

respectively. The better performance of the commercial

sample was related to its slightly higher micropore volume,

but the most remarkable effect was the competition of

water molecules in the biomass carbon.

Keywords Activated carbon � Adsorption � Diclofenac �Potato peel wastes

Introduction

Pharmaceutical compounds are considered emerging con-

taminants of aquatic environments with potential haz-

ardous effects on biota and human health (Rivera-Utrilla

et al. 2013; Vasquez et al. 2014; Burkina et al. 2015).

Nevertheless, they remain unregulated despite their

increasing consumption and continuously input to the

environment. The presence of these compounds as well as

of other micropollutants has been reported worldwide in

different water bodies, including effluents of wastewater

treatment plants (WWTP) and even in drinking water (Luo

et al. 2014; Evgenidou et al. 2015; Huerta-Fontela et al.

2011; Zekker et al. 2012a, b). This situation poses signif-

icant direct risks to the aquatic biota and indirectly to

humans through the water catchments to produce drinking

water.

European Directive 2013/39/EU (EU 2013) regarding

priority substances in the field of water policy addresses for

the first time the risks posed by pharmaceutical products in

the environment. The Directive established that a ‘watch

list’ of emerging aquatic pollutants must be created and

three pharmaceutical derived compounds, among which is

Electronic supplementary material The online version of thisarticle (doi:10.1007/s13762-016-1030-3) contains supplementarymaterial, which is available to authorized users.

& M. Bernardo

maria.b@fct.unl.pt

1 LAQV, REQUIMTE, Departamento de Quımica, Faculdade

de Ciencias e Tecnologia, Universidade Nova de Lisboa,

2829-516 Caparica, Portugal

2 LAQV, REQUIMTE, Departamento de Ciencias e

Tecnologia da Biomassa, Faculdade de Ciencias e

Tecnologia, Universidade Nova de Lisboa, 2829-516

Caparica, Portugal

3 IBB - Centro de Engenharia Biologica e Quımica, Instituto

Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais,

1049-001 Lisbon, Portugal

4 Centro de Quımica e Bioquımica, Faculdade de Ciencias,

Universidade de Lisboa, 1749-016 Lisbon, Portugal

123

Int. J. Environ. Sci. Technol. (2016) 13:1989–2000

DOI 10.1007/s13762-016-1030-3

diclofenac, shall be included in the list. Therefore, it is

expected that in the near future a legal framework will be

set up, which will promote the development of efficient and

sustainable methods for the removal of these contaminants

from wastewater and drinking water.

Diclofenac (DCF), or 2-[(2,6-dichlorophenyl) amino]

phenyl] acetic acid (Figure S.1), is a widely used nons-

teroidal anti-inflammatory agent, frequently supplied as

monosodium salt, for oral or topical application. Topic

application is the main source of diclofenac release into

water, since it was found that only 6–7 % is absorbed

(Vieno and Sillanpaa 2014), being the remaining washed

off from the skin or clothes.

Concerning the oral application, DCF is extensively

metabolized by human organism: Around 65–70 % of the

administered dose is excreted in urine and 20–30 % in

feces (Vieno and Sillanpaa 2014; Zhang et al. 2008).

Although the majority of DCF is excreted as metabolites, a

small fraction is excreted as unmetabolized DCF. Six

metabolites of DCF have been detected in human plasma,

urine and/or feces: 40-hydroxy-DCF, 5-hydroxy-DCF, 30-hydroxy-DCF, 4,50-dihydroxy-DCF, 30-hydroxy-40-meth-

oxy-DCF and 40-hydroxy-30-methoxy-DCF (Vieno and

Sillanpaa 2014).

Although there has been an effort to improve biological

treatments in WWTPs for nitrogen and organic carbon

compounds (Zekker et al. 2012c, 2014, 2015, 2016), the

removal percentages of DCF in WWTPs are usually low,

21–40 %, (Zhang et al. 2008) due to its poor biodegrada-

tion during biological treatment.

Pharmaceuticals are able to be readily removed by

adsorption processes (Rivera-Utrilla et al. 2013; Huerta-

Fontela et al. 2011; Delgado et al. 2012). The major

advantages of adsorption against the conventional physic-

ochemical treatments (chlorination, coagulation, floccula-

tion, membranes) and advanced oxidation processes

(ozonation, ultraviolet and gamma radiation, electro-oxi-

dation) are that (1) no toxic or pharmacologically active

intermediate products are generated, and (2) adsorbents are

likely to be reused. Moreover, adsorption offers the pos-

sibility of using low-cost and easily available precursors,

such as biomass wastes, to synthesize activated carbon

adsorbents (Jodeh et al. 2015; Fernandez et al. 2015;

Saucier et al. 2015; Torrellas et al. 2015).

Biomass wastes, such as those generated in the agrofood

industries, have been used as precursors of activated car-

bons and successfully applied to the removal of several

pharmaceutical compounds (Baccar et al. 2012; Cabrita

et al. 2010; Rovani et al. 2014; Mestre et al. 2014; El-

Shafey et al. 2014). This constitutes a potential low-cost

alternative in obtaining feedstock materials for adsorption-

based technologies, allowing at the same time the

management of wastes, often without market value,

through a high added-value valorization pathway.

The removal of diclofenac through activated carbons

obtained from biomass waste such as cocoa shell, orange

peel wastes, olive waste cake and peach stones, among

others, has been recently studied (Baccar et al. 2012; Jodeh

et al. 2015; Jung et al. 2013; Fernandez et al. 2015; Saucier

et al. 2015; Torrellas et al. 2015). The data reported in

these works were very promising, opening the possibility to

test other different types of biomass wastes with world

significance.

Potato is one of the most produced and consumed car-

bohydrate worldwide. In 2013, the world production of this

crop was around 376 million tons, being Asia the major

producer (49.7 %) followed by Europe (30.4 %) (FAO-

STAT 2015). Around 3.4 % of the total potato production

is directed to processing industry (FAOSTAT 2015).

During potato processing, about 15–40 % (w/w) of potato

peel residues can be generated, being the major by-product

from this industry followed by starch (Ahokas et al. 2014;

Arapoglou et al. 2010). Potato peel waste (PPW) is either

discarded, composted or used as low value animal feed.

The treatment of PPW through composting has a high cost

for industrial producers, and the added value to the final

product is relatively low when compared, for example,

with activated carbon. Additionally, the potential contam-

ination of PPW with side streams, such as inert materials

(sand, small rocks), pose difficulties to its use in animal

feeding.

PPW is mainly composed by starch and other polysac-

charides such as cellulose, hemicellulose (Arapoglou et al.

2010; Onal et al. 2011; Liang and McDonald 2012), which

makes it a good feedstock to bioethanol production (Ara-

poglou et al. 2010; Khawla et al. 2014). However, the

biological conversion of this waste cannot valorize all the

components of PPW. The production of activated carbons

from PPW then is a potential alternative pathway for its

valorization.

The conversion of PPW into activated carbon was

already studied using zinc chloride (ZnCl2) (Moreno-Pi-

rajan and Giraldo 2011, 2012) and potassium hydroxide

(KOH) (Kyzas and Deliyanni 2015) as activating agents.

In the present work, PPW from a potato processing

industry was used as precursor of activated carbon obtained

by chemical activation with K2CO3. The material obtained

was further applied to the removal of sodium diclofenac

from synthetic effluents, in parallel with a commercial

activated carbon. Diclofenac adsorption experiments were

carried out, and the optimization of several parameters was

performed in order to obtain equilibrium and kinetic data.

To authors’ knowledge, this is the first study dealing with

diclofenac adsorption using PPW-based activated carbon.

1990 Int. J. Environ. Sci. Technol. (2016) 13:1989–2000

123

The PPW-based activated carbon was submitted to

aqueous leaching, and the acute ecotoxic level of the eluate

was determined. The environmental risk assessment of

waste-based adsorbents through leaching and ecotoxico-

logical tests is essential in order to elucidate about the

potential environmental impact associated with their use

when exposed to water, in order to avoid problems of

secondary environmental pollution.

The present work was carried out during 6 months

(September 2014–March 2015) at Faculdade de Ciencias e

Tecnologia - Universidade Nova de Lisboa, Faculdade de

Ciencias - Universidade de Lisboa and Instituto Superior

Tecnico - Universidade de Lisboa.

Materials and methods

Potato peel waste (PPW)

PPW was collected from a potato processing industry and

presented a moisture content of about 80 %. It was sub-

sequently oven-dried at 60 �C, during 48 h, until the

moisture content has been reduced to 8 %. The dried PPW

was milled using a laboratory blade mill and sieved

through 25 meshes.

Elemental analysis to determine carbon, hydrogen,

nitrogen and sulfur contents in PPW was performed on an

Elemental Analyzer Thermo Finnigan—CE Instruments,

model Flash EA 1112 CHNS series, on the basis of sample

combustion dynamics.

The proximate analysis of PPW was based on European

standards for solid biofuels: moisture content at 105 �C(EN 14774-1:2009), volatile matter at 900 �C (EN

15148:2009) and ashes at 550 �C (EN 14775:2009).

Thermogravimetric analysis of PPW was performed in a

Perkin-Elmer instrument, Pyris model, between room

temperature and 850 �C, with a heating rate of 10 �Cmin-1, under N2 flow rate of 20 cm3 min-1.

Mineral content in PPW sample was determined through

microwave-assisted acidic digestion (3 cm3 H2O2

30 % ? 8 cm3 HNO3 65 % ? 2 cm3 HF 40 %) and neu-

tralization with H3BO3 (4 % w/v), according to the Euro-

pean standard EN 15290:2011, followed by quantification

of several metals and metalloids by atomic absorption

spectrometry equipment (Thermo Elemental Solaar). This

determination was performed in duplicate, and the results

will be presented as the average.

Activated carbon sample

PPW-based activated carbon sample was obtained by

chemical activation with potassium carbonate, K2CO3

(Riedel de-Haen,[99 %), using the solution impregnation

method. The biomass was soaked with a saturated aqueous

solution of K2CO3 at a 1:1 weight ratio, at room temper-

ature and mixed for 2 h, and then dried at 100 �C for 24 h.

The impregnated sample was then carbonized in a quartz

reactor placed in an electric vertical tube furnace controlled

by a PID programmable temperature controller (RKC,

REX-P96), and the temperature inside the furnace was

measured by a thermocouple connected to the PID con-

troller. The sample was heated to 700 �C, at 10 �C min-1,

and kept for 1 h under a N2 flow of 150 cm3 min-1. After

cooling under N2 flow, the sample was washed with dis-

tilled water up to pH 7 and dried at 100 �C overnight. The

activated carbon sample was named as CPPW.

Elemental analysis on CPPW sample was performed as

described above for PPW. The determination of ash content

at 750 �C followed ASTM D 1762 standard protocol for

wood charcoal.

The surface chemistry of the activated carbon sample

was characterized by the determination of the pH at the

point of zero charge (pHPZC), based on the methodology

described by Carabineiro et al. (2012). Briefly, 0.1 g of

carbon sample was mixed with 20 cm3 of 0.1 M NaCl

solution in different flasks. The pH of NaCl solution was

adjusted with the addition of 0.1 M solutions of NaOH or

HCl to the desired initial pH value (between 2 and 12) in

different flasks; the carbon sample was then added to each

flask. The mixtures were shaken for 24 h to reach equi-

librium, and the final pH measured with a Crison pH

2001 m. The pHPZC value corresponds to the plateau of the

curve of pHfinal vs pHinitial.

The CPPW ashes were characterized by X-ray powder

diffraction (XRD) using a Denchtop X-Ray diffractometer

RIGAKU, model MiniFlex II, with software for automatic

data acquisition, operating at 30 kV and 15 mA, and using

Cu Ka radiation. The diffractograms were obtained by

continuous scanning from 15� to 80� (2h) with a step size

of 0.01 (2h) and scan speed of 2� min-1.

The textural properties of the biomass-derived activated

carbon such as apparent surface area, pore volume and pore

size distribution were evaluated by the adsorption isotherm

of N2 at-196 �C obtained in an ASAP 2010 Micromeritics

apparatus. Adsorption measurements were taken after

outgassing the sample overnight under vacuum at 150 �C.Scanning Electron Microscopy (SEM) images were

obtained in a Scanning ElectronMicroscopewith thermionic

emission (Hitachi, model S2400) using an accelerating

voltage of 20 kV, equipped with secondary and backscat-

tered electron detectors and with digital image acquisition.

Leaching test and ecotoxicity analysis

The CPPW activated carbon was leached according to the

European standard EN 12457-2:2002 that has been

Int. J. Environ. Sci. Technol. (2016) 13:1989–2000 1991

123

developed to measure the release of soluble constituents

upon contact with water. The sample was mixed with

deionized water in a single-stage batch test performed at a

L/S ratio of 10 dm3 kg-1, at a constant temperature of

20 �C. The containers (borosilicate glass bottles) were

shaken in a roller-rotating device at 10 rpm, for a period of

24 h. At the end of the leaching test, the mixtures were

allowed to settle for 15 min and the eluates were filtrated

over fiberglass filters GF/C Whatman to minimize the

sorption of organics. The eluates were immediately ana-

lyzed for pH and for total organic carbon (TOC) and

inorganic carbon (IC) using a TOC analyzer, Shimadzu

5000A, operating with the combustion-infrared method.

The ecotoxicity characterization followed the determi-

nation of the inhibitory effect of the eluates on the light

emission of the marine bacterium Vibrio fischeri (Azur

Environmental Microtox� system) according to the ISO

11348-3:1998 standard. The luminescence inhibition of V.

fischeri was evaluated for an exposure period of 5, 15, and

30 min. The results of the ecotoxicity test were expressed

as EC50 (% v/v) values, which represent the effective

concentration of the eluate analyzed that causes a reduction

of 50 % on the V. fischeri bioluminescence.

Diclofenac (DCF) adsorption experiments

Kinetic and equilibrium adsorption studies of diclofenac

were made onto laboratory-made carbon and a commercial

activated carbon, CP, for comparison purposes.

The preparation of the DCF solutions (synthetic

wastewaters) used in these studies was performed by

appropriate dissolution in deionized water of sodium

diclofenac powder (Cayman, purity C99 %). All the pre-

pared solutions presented a pH around 5.

Batch experiments were conducted in amber glass

flasks, and the mixtures of adsorbent–adsorbate were stir-

ring at a speed of 150 rpm in an orbital shaker at room

temperature. After stirring, the liquid and solid phases were

separated by filtration.

Concentrations of DCF in the filtrate were determined

by an UV–Vis spectrophotometer (GBC UV/VIS, model

916) at 276 nm, from calibration curve obtained between 0

and 100 mg dm-3. All experiments were performed in

duplicate, and the results will be presented as the average.

The DCF retained in the adsorbent, qt (mg g-1), was

determined according to Eq. 1:

qt ¼C0 � Ctð Þ

W� V ð1Þ

where C0 and Ct are the initial and final DCF concentra-

tions (mg dm-3), respectively, V is the volume of the

solution (dm3) and W is the adsorbent mass (g).

The effect of pH on DCF uptake was investigated with

an initial pH range between 5 (natural pH of DCF solution)

and 12. In these experiments, 10 mg of activated carbon

was mixed with 25 cm3 of DCF solution, with initial

concentrations of 50 mg dm-3, for a stirring period of

24 h. The initial pH of DCF solutions were adjusted to the

required values by using NaOH 0.1 M or 1 M, or HCl

0.1 M. The pH was measured by using a Crison pH

2001 m.

The effect of contact time in the uptake of DCF was

assessed for stirring periods that ranged from 0 to 48 h

using 10 mg of adsorbent mixed with 25 cm3 of DCF

solution with initial concentration of 50 mg dm-3.

Adsorption isotherms were obtained for a concentration

range of 10–100 mg dm-3, using 10 mg of adsorbent

mixed with 25 cm3 of DCF solution. According to the

results obtained in the kinetic study, stirring times of 17 h

and 24 h were used for CPPW and CP, respectively, as

equilibrium times.

Results and discussion

PPW characterization

Table 1 presents the results from the proximate and ele-

mental analysis of PPW. This material presents a high

content in volatiles which points out the potentialities of

this biomass to be used as activated carbon precursor as the

reactivity of the carbon matrix is linked with the amount of

Table 1 Proximate analysis, elemental analysis and pHPZC of PPW,

CPPW and CP samples

PPW CPPW CPc

Proximate analysis

Moisture (% w/w) 8.4 – –

Volatiles (% w/w) 83.5 – –

Ash (% w/w) 5.7 35.3 3.5

Fixed Carbona (% w/w) 2.4 – –

Elemental analysisb

C (% w/w) 46.4 55.3 93.5

H (% w/w) 7.6 1.3 0.5

N (% w/w) 1.5 0.6 0.4

S (% w/w) n.d. n.d. n.d.

O (% w/w)a 38.8 7.5 5.6

O/C ratio – 0.10 0.05

pHPZC – 7 10

n.d. not detected, PPW potato peel waste, CPPW potato peel waste

carbon, CP commercial carbona By difference, b As-received, c Data from Mestre et al. (2014)

1992 Int. J. Environ. Sci. Technol. (2016) 13:1989–2000

123

volatiles. In fact, it has been reported that high volatile

content is associated with the creation of highly porous

structures within the activated carbon matrix (Yahya et al.

2015). Carbon and oxygen are the main elements present in

PPW, as expected from a biomass mainly composed by

starch and other polysaccharides. This composition is

similar to the one obtained by Liang and Macdonald (2012)

for potato peel waste.

Concerning the inorganic phase composition of PPW,

the values presented in Table 2 show that silicon and

potassium are the main elements. The high potassium

content was already reported in previous works (Hosein-

zadeh et al. 2013; Bozym et al. 2015), which was expected

since potatoes are the highest source of dietary potassium

(White et al. 2009). The high content of silicon was not

expected, but it may be attributed to inert contaminants

from soils (coarse and fine sand) which were visually

observed in PPW.

As expected from a biomass, other alkaline and alkaline

earth elements are also present in significant concentra-

tions. Among the heavy metals, only zinc was detected, but

in low concentrations.

Thermal analysis provides useful information on the

pyrolysis behavior of biomasses. In the case of PPW, the

thermogravimetric (TGA) and differential thermogravi-

metric (DTG) curves (Figure S.2) show that the thermal

degradation of this material under inert conditions can be

divided in three main steps: Step 1 from 50 to 115 �C is

mainly due to water loss; step 2 is characterized by a

marked weight loss (about 50 % w/w) and develops from

240 to 360 �C; step 3, above 360 �C, is characterized by a

slow weight loss of about 35 % (w/w) until 850 �C. TheseTGA/DTG profiles are similar to the ones obtained by Onal

et al. (2011) and by Liang and McDonald (2012) being

possible to observe a DTG intense peak at 300 �C, a

broader peak at 70 �C and also some decomposition cen-

tered at 440 �C.Thermal weight loss of PPW is the combined decom-

position of its main components: Potato starch thermal

degradation occurs mainly at 280–350 �C and undergoes

gradual decomposition over a wide temperature range

(Guinesi et al. 2006); pectin substances decompose

between 180 and 580 �C, being the major weight loss

registered around 240 �C (Aburto et al. 2015); cellulose

decomposes mainly between 315 and 400 �C, hemicellu-

loses in the range of 220–315 �C and lignin for a wide

temperature range of 160–900 �C (Yang et al. 2007).

Primary pyrolysis reactions take place in the tempera-

ture range of 200–400 �C, which results in the formation of

a char residue. At temperatures above 400 �C, the char

slowly undergoes volatilization. At 850 �C, no plateau was

visible in the thermogram, which indicates that the total

decomposition of sample was not achieved.

Characterization of activated carbon samples

The choice of activation conditions was based on previous

studies (Cabrita et al. 2010; Mestre et al. 2007), where it

was demonstrated that biomass-based activated carbons

obtained under conditions used in the present study possess

developed porosity and high adsorption capacity for sev-

eral pharmaceutical compounds.

Table 1 presents the results of elemental analysis, ash

content and pHPZC of activated carbon samples.

From Table 1, it must be highlighted the significant

content of ashes in CPPW activated carbon, which may be

associated with a concentration effect due to the organic

mass loss during the activation process. The higher con-

tents of nitrogen and oxygen in CPPW sample comparing

with CP may indicate the presence of more polar functional

groups at its surface. Moreover, the higher O/C ratio for

CPPW carbon is indicative of this sample being more

hydrophilic than CP. CPPW carbon presents a neutral

pHPZC, and the commercial activated carbon has basic

nature.

Figure S.3 presents the X-ray diffractogram of CPPW

ashes. A tentative identification of XRD peaks by match

with ICDD database of XRD software (Windows Qualita-

tive Analysis version 6.0, Rigaku Corporation Database:

ICDD PDF-2 Release 2007) was performed.

Table 2 Inorganic composition of PPW

Metals (mg kg-1 d.b.) PPW

Si 6756 ± 1149

K 4920 ± 115

Ca 3309 ± 228

Mg 1184 ± 29

Na 1153 ± 12

Al 579 ± 48

Fe 296 ± 6

Zn 9.5 ± 6.9

Ti \204

Ba \57.3

Pb \35.8

Mo \35.1

Ni \22.6

Cr \17.9

Cu \14.7

Cd \11.5

Sb \0.11

Se \0.33

As \0.12

Hg \0.43

d.b. dry basis, standard deviation of duplicates is presented

Int. J. Environ. Sci. Technol. (2016) 13:1989–2000 1993

123

The following minerals were identified in XRD pattern:

quartz (SiO2, 46-1045); potassium oxide (K2O, 23-0493);

orthoclase (KAlSi3O8, 31-0966); sylvite (KCl, 41-1476);

and sodium aluminum silicate (Na2OAl2O3SiO2, 55-0071).

The presence of these mineral phases is in agreement with

the inorganic composition of the precursor (Table 2), in

which the main components are Si, K and Ca.

The nitrogen adsorption–desorption isotherm of CPPW

activated carbon sample is presented in Figure S.4 and is

typical of materials with well-developed microporous

structure [Type I according IUPAC (Thommes et al. 2015)]

associated with some mesoporosity denoted by the slight

slope in themultilayer region. On the other hand, the analysis

of the isotherm in the region of low relative pressure shows a

somewhat rounder knee which is indicative of the presence

of relatively broad micropore size distribution.

The adsorption data were used to calculate the apparent

surface area (ABET), through the BET equation, and the

total pore volume. Microporosity analysis was evaluated

through the a-plot method taking as reference the isotherm

reported by Rodriguez-Reinoso et al. (1987). The textural

parameters obtained are presented in Table 3 along with

those reported for sample CP. It can be observed that there

are no significant differences regarding apparent surface

area and total pore volume for both carbons; however, the

mesopore volume is higher in the case of CPPW carbon.

The samples also differ in the microporosity characteristics

since the commercial carbon has the higher micropore

volume, with a large amount of supermicropores, while

CPPW sample presents a somewhat smaller micropore

volume constituted mainly by narrower pores.

Figure S.5 presents the results from SEM analysis.

CPPW has a homogeneous surface with sponge-like mor-

phology identical to those observed for biomass-derived

carbons activated with K2CO3 (Gurten et al. 2012).

Leaching test and ecotoxicity analysis of CPPW

The application of standard leaching tests allows evaluat-

ing the potential contaminants mobility from a given

material to water. Therefore, it is important to know the

leaching behavior and ecotoxic properties of adsorbents

originated from waste materials which will be applied in

aqueous media. Assessing the ecotoxicity on the aqueous

soluble fraction of the material provides a more accurate

risk to the environment instead of considering all sample

composition which would provide an overestimated pre-

diction of the ecotoxicity risk.

Table 4 presents the results on the characterization of

CPPW eluate, revealing that the aqueous eluate of CPPW

activated carbon has a neutral pH value, which is consistent

with its pHPZC.

The Council Decision 2003/33/EC defines criteria and

procedures for the acceptance of waste at landfills; these

criteria are based on the leaching limit values for several

parameters in eluate obtained through the application of the

leaching standard EN 12457-2 for the classification of

wastes. According to this European Decision, inert materials

must follow a leaching limit value of 30000 mg kg-1 for

TotalOrganicCarbon (TOC) content. TOCcontent inCPPW

eluate is quite below the limit value defined in the European

Decision; therefore, according to this guideline CPPW

activated carbon can be considered as an inert material.

Despite the release of compounds represented by TOC

and IC from carbon sample, its eluate does not present

ecotoxicity to V. fischeri, being a good indicator that this

adsorbent can be safely applied to aqueous matrices in

what concerns the acute ecotoxicity risk. Further acute and

chronic ecotoxicity tests may be used in future work, as the

production conditions are optimized and higher amounts of

activated carbon will be obtained.

Diclofenac (DCF) adsorption experiments

Effect of pH

Figure 1 presents the effect of solution pH in the uptake of

DCF by the two activated carbons.

Table 3 Textural characteristics of activated carbon samples

Textural parameter CPPW CPa

ABET (m2g-1) 866 907

Vtotal (cm3 g-1) 0.40 0.43

Vmeso (cm3 g-1) 0.08 0.03

Vmicro (cm3 g-1) 0.32 0.40

Vultra (cm3 g-1) 0.21 0.16

Vsuper (cm3 g-1) 0.11 0.24

ABET—apparent surface area evaluated through BET equation, Vmi-

cro—micropore volume evaluated through a-plot method, Vtotal—total

pore volume determined by the amount of nitrogen adsorbed at

P/Po = 0.98, Vmeso—mesopore volume determined by the difference

between Vtotal and Vmicro. Vultra—volume of ultramicropores

(Ø\ 0.7 nm), Vsuper—volume of supermicropores (0.7\Ø\ 2 nm)

CPPW potato peel waste carbon, CP commercial carbona Data from Mestre et al. (2014)

Table 4 Characterization of

aqueous eluate of CPPW acti-

vated carbon

CPPW eluate

pH 7.4

TOC (mg kg-1) 1071

IC (mg kg-1) 595

EC50 (% v/v)

5 min [99

15 min [99

30 min [99

1994 Int. J. Environ. Sci. Technol. (2016) 13:1989–2000

123

In the pH range studied, sodium DCF is dissociated and

so the species that will interact with the carbon samples

presents negative charge. No studies were made at lower

pHs because the solubility of DCF remarkably decreases

due to the progressive protonation of the specie (pKa = 4)

(Meloun et al. 2007; Kincl et al. 2004).

Considering the pHPZC of the carbons (Table 1), it is

expected that positive electrostatic interaction occurs

between carbons and DCF anion in the different pH ranges.

For CPPW activated carbon, it is observed a decrease in

DCF uptake at pH above its pHPZC (7), which can be

attributed to electrostatic repulsion since carbon surface

starts acquiring negative charge. The same reason explains

the decrease in DCF uptake above pH of 9 in the case of

carbon CP, since this pH value is in the vicinity of the

pHPZC of CP. Regardless the solution pH, carbon CP pre-

sented higher uptake of DCF compared to CPPW sample.

However, it must be noted that at high pH values, where

electrostatic repulsion plays a role, the uptakes of DCF by

the two carbons are still significant, which means that other

types of interactions must be considered to explain the

adsorption mechanism. Attending to the structure of DCF

(Figure S.1), we cannot disregard the possibility that p–pelectron donor–acceptor interactions between the aromatic

rings of DCF and polarizable graphene sheets of carbons

occur (Baccar et al. 2012; Jung et al. 2015), or hydrogen

bonding between the carboxylic or N–H group of DCF and

activated carbon functional groups, or van der Waals forces

(Moreno-Castilla 2004).

Given the results, it was chosen for both carbons a pH of

5 as optimum for subsequent adsorption assays, i.e., DCF

solution was used without pH adjustment.

Effect of contact time–kinetic data

Figure 2 presents the kinetic curves of DCF adsorption for

both activated carbons.

A marked uptake in the first hours of contact time can be

observed, after what the adsorption processes proceed

slowly until the equilibrium was attained at around 17 h for

CPPW and 24 h to CP carbon. It must be noted that in any

case there was still some DCF species remaining in the

solution after the contact time, so the plateau reflects the

equilibrium of the adsorption process and not the exhaus-

tion of DCF species in solution. At equilibrium, CPPW

sample removed around 70 % of DCF and CP sample

presented removal efficiency around 90 %.

The kinetic data were fitted to pseudo-first- and pseudo-

second-order nonlinear kinetic models (Ho 2006) using

nonlinear regression adjusted with the minimization of sum

square error function, (SSE,Pn

i¼1 ðqe;calc � qe;expÞ2i ),between the calculated and experimental data. The best-

fitting model was chosen according to the correlation

between theoretical models with experimental data asses-

sed through the determination coefficient, R2.

The kinetic data of both carbons were best fitted (higher

R2) by the pseudo-second-order model (Fig. 2), which

implies that the rate limiting step is chemical adsorption

involving electronic forces through the sharing or exchange

of electrons between the adsorbent and ionized DCF.

The results reported in Table 5 show that the uptake of

DCF in the equilibrium for CP is higher and CPWW pre-

sents a higher overall adsorption rate.

CP sample presented a higher micropore volume,

namely the supermicroporous volume which favoured the

higher initial adsorption uptake presented by this sample

(Mestre et al. 2007).

The overall adsorption process can be described by the

following consecutive steps (1) transport of solute in the

bulk of the solution; (2) diffusion of solute across the so-

Fig. 1 Effect of solution pH on DCF uptake. Conditions: adsorbent

mass = 10 mg; initial DCF concentration = 50 mg dm-3; contact

time = 24 h. Error bars of duplicates are presented. CPPW potato

peel waste carbon, CP commercial carbonFig. 2 Kinetic data of DCF adsorption adjusted to pseudo-second-

order kinetic model. Conditions: adsorbent mass = 10 mg; initial

DCF concentration = 50 mg dm-3; pH 5. Exp experimental data,

calc calculated data. Error bars of duplicates are presented. CPPW

potato peel waste carbon, CP commercial carbon

Int. J. Environ. Sci. Technol. (2016) 13:1989–2000 1995

123

called liquid film surrounding adsorbent particles (external

mass transfer) (3) diffusion of solute in the liquid contained

in the pores of adsorbate particle and along the pore walls

(intraparticle diffusion); (4) adsorption and desorption of

solute molecules on/from the adsorbent surface (surface

reaction) (Gupta and Bhattacharyya 2011). One of these

steps is the slowest and controls the overall rate of

adsorption, but a combined effect of steps is also possible.

Generally, step (1) is ignored; thus, solute transfer is usu-

ally controlled by either external mass transfer or intra-

particle diffusion or both.

The intraparticle diffusion model allows identifying the

controlling step, or the combination of steps (Gupta and

Bhattacharyya 2011):

qt ¼ kid � t1=2 þ h ð2Þ

where kid is the intraparticle diffusion constant in mg g-1

h-0.5 and h is a constant related to the thickness of the

boundary layer (mg g-1). If the plot of qt versus t1/2 gives a

straight line, then the adsorption process is controlled by

intraparticle diffusion only. However, if the data exhibit

multi-linear plots, then two or more steps influence the

sorption process.

Figure S.6 presents the intraparticle diffusion plots for

the adsorption of DCF in the studied activated carbons.

It is possible to identify differentiated steps for both

carbons. The first linear step represents the adsorbate dif-

fusion in the boundary layer (external mass transfer), the

second one represents the intraparticle diffusion and the

last one represents the equilibrium plateau.

For both carbons, it is observed the presence of the first

adsorption step which means that the effect of external

mass resistance cannot be neglected.

The parameters obtained with the intraparticle diffusion

model are presented in Table 5, being possible to observe

that the intraparticle diffusion constant for CPPW is lower

than for CP carbon as well as the boundary layer thickness.

The lower value of boundary layer thickness to CPWW

sample indicates a smaller film resistance to mass transfer

surrounding adsorbent particles. As previously referred,

CPPW carbon presents a more hydrophilic nature due its

higher O/C ratio (Table 1); therefore, a higher wettability

of this carbon is expected which reduces the boundary

layer thickness. Consequently, DCF molecules dissolved in

the water medium should be more rapidly transferred to

porous structure, which probably determines the higher

overall adsorption rate obtained from the fitting of the

pseudo-kinetic model.

Given the size of DCF molecule: molecular length,

1.01 nm; molecular width, 0.719 nm; and molecular

height, 0.484 nm (Jung et al. 2013), it was not expected

any restrictions to pore diffusion. However, given the more

hydrophilic nature of CPPW carbon, the competitive

retention of water molecules may lead to the formation of

water clusters that may result in, at least partial, pore

blockage hindering the access of DCF species to the

micropore network (Mestre et al. 2007; Moreno-Castilla

2004; Ruiz et al. 2010; Franz et al. 2000). Therefore, the

lower intraparticle diffusion constant for CPPW carbon

may be associated with the deceleration of pore diffusion

process due to the water competitive effect in these sam-

ples which presents the larger fraction of narrow

micropores.

Effect of initial concentration of DCF–adsorption

isotherms

DCF adsorption isotherms of DCF onto the two activated

carbon samples are presented in Fig. 3. Both carbons pre-

sented L-type isotherms (Limousin et al. 2007) associated

with progressive saturation of the adsorbents. The data

presented show that higher equilibrium uptake (qe) is

obtained with the commercial carbon.

The isotherms data were fitted to Freundlich and

Langmuir models (Limousin et al. 2007) using nonlinear

regression adjusted with the minimization of sum square

error function, (SSE,Pn

i¼1 ðqe;calc � qe;expÞ2i ), between the

calculated and experimental data (Table 6). The best-fitting

model was chosen according to the correlation between

theoretical models with experimental data assessed through

the determination coefficient, R2.

According to the results of Table 6, both curves were

best fitted by the Langmuir model that presented the higher

determination coefficients. CP sample presented the higher

adsorption monolayer capacity, qm, as well as the higher

Table 5 Kinetic parameters obtained with pseudo-second-order

nonlinear kinetic model and with the intraparticle diffusion model

CPPW CP

Pseudo-second-order nonlinear kinetic model

R2 0.852 0.857

qe (mg g-1) 74 115

k2 (g mg-1 h-1) 0.075 0.045

h (mg g-1 h-1) 412 588

Intraparticle diffusion model

kid (mg g-1 h-0.5) 6.0 10.5

h (mg g-1) 50.4 73.8

R2 0.998 0.971

R2, determination coefficient; qe, DCF uptake at equilibrium; k2,

pseudo-second-order rate constant; h, initial adsorption rate; kid,

intraparticle diffusion constant; h, constant related to the thickness of

the boundary layer

CPPW potato peel waste carbon, CP commercial carbon

1996 Int. J. Environ. Sci. Technol. (2016) 13:1989–2000

123

Langmuir constant, KL, which shows that this carbon has

higher affinity to DCF molecule.

These results are related to both the pore structure and

surface chemistry of carbons.

Given the slightly higher micropore volume of CP

(Table 3), it was expected a slightly higher DCF uptake

capacity for this carbon. The marked difference between the

monolayer capacities of carbon samplesmay be related to the

pore blockage caused bywatermolecules’ competition in the

more hydrophilic CPPW carbon, as previously referred.

Moreno-Castilla (2004) also highlighted the role of

carbon mineral matter content in the adsorption process.

Usually, high mineral matter content has a deleterious

effect on the adsorption because it can block the porosity of

the carbon matrix and can preferentially adsorb water due

to its hydrophilic character. That is the case of CPPW.

The lower affinity of sample CPPW to DCF molecule

can also be attributed to the higher amount of hydrophilic

groups in the surface of activated carbon. Aromatic com-

pounds such as DCF are usually adsorbed in flat position on

the graphene layers, and in this situation the adsorption

driving forces would be due to p–p dispersion interactions

between the aromatic rings of adsorbate and the aromatic

structure of the graphene layers (Moreno-Castilla 2004).

The more hydrophilic surface groups, which are located at

the edges of the basal planes, remove electrons from the p-electron system, creating positive holes in the conducting

p-band of the graphitic planes. This would lead to weaker

interactions between the p-electrons of the DCF aromatic

rings and the p-electrons of the basal planes (Torrellas

et al. 2015; Moreno-Castilla 2004; Franz et al. 2000).

On the other hand, Jung et al. (2015) assumed that in its

lowest energy configuration, DCF interacts with some

functional groups at carbon’s surface with one aromatic

ring oriented parallel and the other perpendicular to the

adsorbent surface. In this configuration, it is unlikely that

DCF molecule can diffuse in ultramicropores, and even it

can access to a fraction of this type of porosity, which is the

major fraction of the micropore volume of sample CPPW,

the packing of the species will be less effective.

Table 7 presents a comparison between different bio-

mass-based activated carbons used for DCF adsorption

concerning the adsorption capacity. Although a direct

comparison may be subjective due to different experi-

mental conditions and synthesis methods, it allows having

a perception about the performance of PPW-based acti-

vated carbon. It can be observed that PPW activated carbon

presents a higher adsorption capacity for DCF than the

obtained with the other adsorbents which highlight the

Fig. 3 DCF adsorption isotherms on activated carbons. Conditions:

adsorbent mass = 10 mg; contact time = 17 h to CPPW and 24 h to

CP; pH 5. Exp experimental data, calc calculated data. Error bars of

duplicates are presented. CPPW potato peel waste carbon, CP

commercial carbon

Table 6 Langmuir and

Freundlich isotherm parameters

for the adsorption of DCF with

the activated carbons

Activated carbon Langmuir Freundlich

R2 qm (mg g-1) KL (L mg-1) R2 KF 1/n

CPPW 0.931 68.5 0.38 0.916 33.4 0.18

CP 0.920 146.0 1.02 0.837 66.2 4.52

R2, determination coefficient; qm, DCF adsorption capacity; KL, Langmuir constant; KF, Freundlich con-

stant in mg1-1/n (L)1/n/g; 1/n, related to the adsorption affinity or surface heterogeneity

CPPW potato peel waste carbon, CP commercial carbon

Table 7 DCF adsorption

capacity of different biomass-

derived activated carbons found

in literature

Biomass precursor Adsorption capacity qm (mg g-1) References

Potato peel waste 68.5 Present work

Cyclamen persicum (herbaceous plant) 22.0 Jodeh et al. (2015)

Orange peels 5.7–52.2 Fernandez et al. (2015)

Cocoa shell 56.3 Saucier et al. (2015)

Olive waste cake 56.2 Baccar et al. (2012)

Int. J. Environ. Sci. Technol. (2016) 13:1989–2000 1997

123

potential of this type of biomass-derived carbons for DCF

removal from wastewaters.

Conclusion

Potato peel waste-based activated carbon was obtained

through chemical activation with K2CO3. The biomass-

derived activated carbon presented a well-developed

microporous structure with a high fraction of ultramicro-

pores (53 % of the total pores). The potato peel-based

carbon was applied to the removal of sodium diclofenac

from water samples, and its performance compared with

that of a commercial activated carbon. The removal effi-

ciencies of DCF were around 70 % with experimental

carbon and 90 % with commercial carbon.

Adsorption kinetics of both carbons were best fitted by

the pseudo-second-order model; the commercial carbon

presented a higher uptake of diclofenac in the equilibrium

(115 mg g-1), but laboratory-made sample presented the

higher adsorption rate (0.075 g mg-1 h-1).

It was found that the global adsorption rate of DCF

on activated carbons is controlled simultaneously by

external mass transfer (boundary layer) followed by the

intraparticle diffusion. Both porous features and surface

chemistry of the samples controlled the adsorption rate

and the removal efficiency of DCF from aqueous media.

The more hydrophilic biomass carbon (higher O/C ratio)

presented slower intraparticle diffusion due to the

competitive retention of water molecules; on the other

hand, its higher hydrophilic nature enhanced its wetta-

bility which lowered the boundary layer thickness

(50.4 mg g-1) as well as the kinetic restrictions asso-

ciated with external mass transfer.

In the equilibrium experiments, both adsorbents were

best fitted by the Langmuir model. Although the com-

mercial carbon presented the higher adsorption monolayer

capacity (146 mg g-1), as well as the highest Langmuir

constant, the potato peel waste-based activated carbon

presented higher uptake capacity of DCF than that obtained

with different biomass-derived activated carbons. More-

over, the waste-based carbon presented leaching and eco-

toxicological properties compatible with its safe

application to environmental compartments.

The results obtained in the present study allowed con-

cluding that diclofenac adsorption is highly dependent on

the hydrophilic nature of the adsorbent as well as on the

presence of narrow micropores.

Acknowledgments The authors would like to acknowledge the

Portuguese Foundation for Science and Technology (FCT) for the

financial support with the Post-Doc grants SFRH/BPD/93407/2013

and SFRH/BPD/84542/2012, respectively. The authors also thank

FCT for the financial support to CQB and LAQV/REQUIMTE

through the projects UID/MULTI/00612/2013 and UID/QUI/50006/

2013, respectively. The authors thank Quimitejo for providing car-

bon CP.

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