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\DOI{10.5802/crchim.381}
\datereceived{2024-07-26}
\daterevised{2024-11-17}
\dateaccepted{2025-01-27}
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\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Synthesis of a novel biocomposite from orange peels and its
application for the removal of diclofenac from aqueous solutions:
assessment of adsorption characteristics}

\alttitle{Synth\`{e}se d'un nouveau biocomposite \`{a} partir
d'\'{e}corces d'orange et leur application pour l'\'{e}limination du
diclof\'{e}nac en solution aqueuses : evaluation des
caract\'{e}ristiques d'adsorbant}

\author{\firstname{Lynda} \lastname{Jmai}}
\address{Chemical Engineering Department, National 
Engineering School of Gabes, Tunisia}
\email[L. Jmai]{lynda.jemai@gmail.com}

\author{\firstname{Sami} \lastname{Guiza}\CDRorcid{0000-0003-1967-9677}}
\addressSameAs{1}{Chemical Engineering Department, National
Engineering School of Gabes, Tunisia}
\email[S. Guiza]{sami\_guiza@yahoo.fr}

\author{\firstname{Salah} \lastname{Jellali}\CDRorcid{0000-0002-4095-4154}\IsCorresp}
\address{Center for Environmental Studies and Research, Sultan Qaboos
University, Al-khod 123, Oman}
\email[S. Jellali]{s.jelali@squ.edu.om}

\author{\firstname{Mohamed} \lastname{Bagane}\CDRorcid{0000-0002-8526-7614}}
\addressSameAs{1}{Chemical Engineering Department, National 
Engineering School of Gabes, Tunisia}
\email[M. Bagane]{drmbag1420@yahoo.fr}

\author{\firstname{Mejdi}\nobreakauthor\lastname{Jeguirim}\CDRorcid{0000-0003-2401-5824}}
\address{Institut de Sciences des
Mat\'{e}riaux de Mulhouse, Universit\'{e} de Haute-Alsace, 3 Rue Alfred Werner, 68093 Mulhouse, France}
\email[M. Jeguirim]{mejdi.jeguirim@uha.fr}

\shortrunauthors

\keywords{\kwd{Pollution}\kwd{Drugs}\kwd{Diclofenac}\kwd{Orange peel}\kwd{Magnetic composite}\kwd{Adsorption}}

\altkeywords{\kwd{Pollution}\kwd{M\'{e}dicaments}\kwd{Diclof\'{e}nac}\kwd{Ecorces d'orange}\kwd{Composite magn\'{e}tique}\kwd{Adsorption}}

\begin{abstract}
This work aims to synthesize and to characterize an easily recoverable
magnetic composite (orange peel composite [OPC]) from orange peel (OP) 
and to apply it as an
effective adsorbent for the removal of an industrial drug (diclofenac
(DFC)) from aqueous solutions in batch mode. The OP and OPC
characterization through various techniques including Fourier transform
infrared, scanning electron microscopy coupled with 
energy-dispersive spectroscopy, X-ray diffraction,
Brunauer--Emmett--Teller, and thermogravimetric analysis
shows that the OPC has interesting physicochemical properties in
comparison with numerous other adsorbents. The DFC removal by the OPC
is found to be time-dependent, and the equilibrium state is obtained
after 90 min. Moreover, at a temperature of 30 \textdegree C, the DFC
adsorption capacity by this magnetic material is estimated to be
37.0 mg\tralicstex{\textperiodcentered\text{g}\textsuperscript{--1}}{${\cdot}$g$^{-1}$}, which is higher than various adsorbents. In
addition, the thermodynamic study results indicate that DFC removal
is a feasible, exothermic, and spontaneous process. All these results
prove that magnetized OP wastes can be considered  a
promising material for the efficient removal of DFC from aqueous
solutions under a wide range of experimental conditions. 
{\vspace*{-2pt}}
\end{abstract}

\begin{altabstract}
Ce travail a pour objectif de synth\'{e}tiser et de caract\'{e}riser un
composite magn\'{e}tique (OPC) facilement r\'{e}cup\'{e}rable \`{a}
partir d'\'{e}corce d'orange (OP) et de l'appliquer comme adsorbant
efficace pour l'\'{e}limination d'un m\'{e}dicament industriel
(Diclof\'{e}nac (DFC)) \`{a} partir de solutions aqueuses en mode
batch. La caract\'{e}risation de l'OP et de l'OPC par diverses
techniques, notamment l'infrarouge \`{a} transform\'{e}e de Fourier
(FTIR), la microscopie \'{e}lectronique \`{a} balayage coupl\'{e}e
\`{a} la spectroscopie X \`{a} dispersion d'\'{e}nergie (SEM/EDS), la
diffraction des rayons X (DRX), l'analyse de Brunauer--Emmett--Teller
(BET) et l'analyse thermogravim\'{e}trique (ATG/DTA), montre que l'OPC
poss\`{e}de des propri\'{e}t\'{e}s physico-chimiques int\'{e}ressantes
en comparaison avec de nombreux autres adsorbants. L'\'{e}limination du
DFC par l'OPC s'est av\'{e}r\'{e}e d\'{e}pendante du temps du contact
et l'\'{e}tat d'\'{e}quilibre est obtenu apr\`{e}s 90 min. De plus,
\`{a} une temp\'{e}rature de 30 \textdegree C, la capacit\'{e}
d'adsorption du DFC par ce mat\'{e}riau magn\'{e}tique a \'{e}t\'{e}
\'{e}valu\'{e}e \`{a} 37,0 mg\tralicstex{\textperiodcentered\text{g}\textsuperscript{--1}}{${\cdot}$g$^{-1}$}, ce qui est bien
sup\'{e}rieur \`{a} celui de divers autres adsorbants. De plus, les
r\'{e}sultats de l'\'{e}tude thermodynamique indiquent que
l'\'{e}limination du DFC est un processus favorable, exothermique et
spontan\'{e}. Tous ces r\'{e}sultats prouvent que les d\'{e}chets
d'\'{e}corces d'orange magn\'{e}tis\'{e}s peuvent \^{e}tre
consid\'{e}r\'{e}s comme mat\'{e}riaux prometteurs pour une
\'{e}limination efficace du DFC \`{a} partir de solutions aqueuses pour
un large intervalle de conditions exp\'{e}rimentales.
\end{altabstract}

\maketitle

{\vspace*{-2pt}}

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\end{noXML}

\section{Introduction} \label{sec1}

Various industries such as paper mills, textiles, coatings, and medical
treatment have grown significantly to meet the population's daily
living needs.  This  has led to rapid economic development but at the
same time has increased environmental pollution risks~\cite{1}. Indeed,
high industrialization and intensive use of chemicals and medicines in
everyday life have been pointed out as being responsible for the
release of a variety of substances into the environment including
emerging pollutants~\cite{2}. Typical wastewater treatment systems are
not capable of sufficiently removing these types of
pollutants~\cite{3}. Therefore, these emerging pollutants have been
found to be present in the receiving environment in several
{countries~\cite{3}}. As a consequence, industrial wastewater abound with
these pollutants needs to be treated adequately \mbox{before} its discharge
into the environment~\cite{4}. One of the major sources of
pharmaceutical compound discharge into the environment is hospitals due
to their related healthcare services. The treatment of water
contaminated by emerging pharmaceutical pollutants has been the subject
of several studies. They include the use of membrane
filtration~\cite{5}, biodegradation~\cite{6}, catalytic oxidation or
reduction degradation~\cite{7}, and adsorption~\cite{8}. During the
last few decades, adsorption technology has become an effective
process, recognized for its simplicity, efficiency, and ability to
eliminate these pollutants even when present in low
concentrations~\cite{9}. Many products have been tested as adsorbents,
including hydrogels~\cite{10}, activated carbon~\cite{11},
zeolites~\cite{12}, polymers and carbon nanotubes~\cite{13}, and
composites~\cite{14}. Iron biocomposites seem to be one of the most
interesting adsorbents because of their reuse ability, non-toxicity,
high adsorption performance, cost-effectiveness, and especially their
ease of separation from aqueous solutions through magnets~\cite{4}.

Orange fruits are produced in large quantities in the world. In 2023, the
overall produced quantity was estimated to be around 70 million
tons~\cite{15}. For this reason, orange peels (OPs) are considered 
important kitchen wastes~\cite{15}. They represent between 40\% and
50\% of the total fresh mass of oranges~\cite{15}. The global 
OP-produced waste is estimated to be around 35 million 
tones/year~\cite{16}. The management of these wastes is a key priority
for the promotion of circular economy and sustainability concepts.
Orange peel wastes along with other kitchen wastes have been pointed
out as effective adsorbents of various pollutants due to their
specific composition (mixture of lignin, cellulose, and hemicellulose),
and also their richness of various functional groups (hydroxyl,
carboxyl, carboxyl, etc.)~\cite{16}. Orange peel wastes have been used
as raw products for the removal of both organic and inorganic
pollutants from wastewater including heavy metals~\cite{4} and
arsenate~\cite{17}. To obtain better removal efficiencies, OPs
have been turned into activated carbons and used for various organic
pollutants involving pesticides~\cite{18}, dyes~\cite{19}, and
pharmaceuticals~\cite{20}.

Magnetic adsorbents can be synthesized through different methods
including catalysis, hydrothermal, and co-precipitation
methods~\cite{21}. Depending on the experimental conditions, different
iron-based \mbox{oxides} may be formed on the surface of adsorbents. They
may include magnetite, hematite~\cite{22}, 
and iron (III) oxide-hydroxide~\cite{4}.
Magnetic biocomposites have become popular due to their effectiveness
and also their ease of separation from wastewater by the application of
an external magnetic field~\cite{24,25}. Magnetic biocomposites have
been applied for nutrient recovery~\cite{24} and the removal of heavy
metals~\cite{24,25}, dyes~\cite{23,24,25,26,27}, pesticides~\cite{16},
and pharmaceuticals~\cite{28,29}. Different pharmaceuticals have been
treated with magnetic adsorbents such as amoxicillin~\cite{26},
tetracycline~\cite{27}, and ciprofloxacin~\cite{28}. Only a few studies
have investigated the use of magnetic biocomposites for diclofenac
(DFC) removal from aqueous solutions. 

Diclofenac, a widely used non-steroidal anti-inflammatory drug, is
commonly administered for managing pain and inflammation in both humans
and certain animals. Although it is therapeutically effective,
DFC presents a significant environmental hazard. Once introduced
into the environment, primarily through wastewater, it resists
degradation and can accumulate in aquatic ecosystems. Research
indicates that DFC is highly toxic to aquatic life, particularly
fish, where it may cause severe liver and kidney damage~\cite{29}.
Furthermore, DFC has been associated with vulture die-offs in
South Asia, as the drug can build up in the tissues of treated animals
whose carcasses vultures consume~\cite{30}. These potential ecological
impacts underscore the urgent need for strategies to control and limit
the drug's environmental release.

The efficiency of pharmaceutical removal  by magnetic composites depends
on not only the feedstock nature and the contents of iron oxides on the
adsorbents but also the experimental adsorption conditions. For
instance, Rocha et~al.~\cite{31} showed that the adsorption of DFC and
venlafaxine by magnetic activated carbon showed high removal capacity values
of 23.7 and 47.7~mg${\cdot}$g$^{-1}$, respectively. Moreover, Frohlich
et~al.~\cite{32} showed that magnetic NiFe\tsub{2}O\tsub{4}/activated
carbon exhibited a high capability of 261.4 and 97.8~mg${\cdot}$g$^{-1}$
in removing ibuprofen and ketoprofen, respectively. The
physicochemical properties of magnetic biocomposites (Brunauer--Emmett--Teller
[BET] surface area and porosity, nature and richness of functional groups, pH
of \mbox{zero-point} charge, etc.) seem to highly \mbox{influence} the removal
efficiencies of pharmaceuticals from aqueous solutions~\cite{31,32}.
For instance, Thi Minh Tam~\cite{33} demonstrated that at a pH of 6.5
and a surface area of 70.15~m\tsup{2}${\cdot}$g$^{-1}$, 
magnetic-gelatin-modified peanut shell biochar had good affinity for the
removal of DFC (348.9~mg${\cdot}$g$^{-1}$). 

In the current work, we intend to synthesize a magnetic composite from
orange peels (orange peel waste [OPC]) and apply it for DFC removal 
from aqueous solutions
under various experimental conditions. The magnetic properties of this
composite enable its easy separation from aqueous solutions
(after the adsorption process) by the application of an external
magnetic field. This is an important advantage in full-scale
situations. The novelty of our work lies in the development of a
sustainable adsorbent that not only controls a stream of abundant worldwide
agricultural waste (OPs) but also maintains its natural
functional groups that are critical for binding contaminants. Moreover,
the OPC is cheaper and eco-friendlier than typical adsorbents 
(activated carbons, biochars, etc.). As far as we know, this is the
first study dealing with DFC removal from aqueous solutions by using a
magnetic composite synthesized from OPs. Therefore, the main
objectives of this work are as follows: (i) synthesize a magnetic biocomposite
from OPs through the co-precipitation method, (ii) 
characterize it in detail by using various types of analytical apparatus, (iii) and 
assess its effectiveness in removing DFC from aqueous solutions under
different experimental conditions (contact time, pH, initial
concentration, temperature).

\section{Materials and methods}\label{sec2}
\subsection{Chemical reagents}\label{ssec21}

Ferrous sulfate (FeSO\tsub{4}${\cdot}$7H\tsub{2}O), ferric chloride
hexahydrate (FeCl\tsub{3}${\cdot}$6H\tsub{2}O, 97\%), and sodium
hydroxide (NaOH) were purchased from Sigma-Aldrich (Sigma-Aldrich, St.
Louis, MO, USA). The anti-inflammatory drug DFC  manufactured by the
local Tunisian industry is used as an industrial product.

\subsection{Preparation of magnetic biocomposite}\label{ssec22}

Orange peels were collected from the Nabeul region (northeast of
Tunisia). They were thoroughly washed with deionized water and then
dried in an air oven at 60~\textdegree C for 24~h. Afterwards, the
dried material was crushed and sieved through a 500~${\upmu}$m sieve.
The resulting powder is denoted as OP. The OPC was prepared by the
co-precipitation method as presented by Toledo-Jaldin et~al.~\cite{21}.
Briefly, 50~mL of ferric chloride (FeCl\tsub{3}${\cdot}$6H\tsub{2}O)
solution with a concentration of 0.1~M was mixed, by magnetic stirring
under nitrogen atmosphere, with 50~mL of ferrous sulfate
(FeSO\tsub{4}${\cdot}$7H\tsub{2}O) solution with a concentration 0.05 M.
Then, 6~mL of sodium hydroxide (NaOH) solution with a concentration of
3.5 M (11 {${<}$} pH {${<}$} 12) was added dropwise to this solution. After
the addition of NaOH, a mass of 3~g of OP was added to this mixture and
agitated with a magnetic stirrer for 40~min. The resulting biocomposite
was separated by filtration (0.45~${\upmu}$m), dried at 60~\textdegree
C for 24 h, and then thoroughly washed with deionized water until
obtaining a neutral pH. This solid phase was dried at 60~\textdegree C
(24~h), and then crushed and sieved through a 160~${\upmu}$m sieve. The
obtained OP biocomposite was stored in a air-tight plastic bag for later
use.

\subsection{Biocomposite characterization}\label{ssec23}

The biocomposite characterization was carried out by using different
analytical techniques to assess the following: (i) the morphology and
qualitative composition by  using scanning electron microscopy (SEM)
coupled with energy-dispersive spectroscopy (EDS, Nova NanoSEM 230),
(ii) the presence of crystalline phases by using X-ray diffraction
(XRD, Davinci D8 diffractometer), (iii) the BET surface area 
through N\tsub{2} adsorption/desorption tests
(Quantachrome Instruments), (iv) the major functional groups by Fourier
transform infrared (FTIR) spectroscopy (PerkinElmer Spectrum 100
Spectrometer), (v) the pH of zero-point charge (pH$_{\mathrm{PZC}}$)
according to the pH drift methodology~\cite{35} for a range of initial
pH values from 2 to 10, and (vi) the thermal decomposition through
thermogravimetric measurements (thermogravimetric analysis/differential 
thermal analysis [TGA/DTG]) by SDT Q600 (TA Instruments).

\subsection{Preparation of DFC solutions}\label{ssec24}

The chemical formula of DFC is
C\tsub{14}H\tsub{10}Cl\tsub{2}NO\tsub{2}Na (Figure~\ref{fig1}). It has
a molar weight of 318.13 g${\cdot}$mol$^{-1}$. A stock solution with a
concentration of 100~mg${\cdot}$L$^{-1}$ was prepared and used throughout 
this study to make solutions with desired concentrations.
The pH adjustment of aqueous solutions was carried out through 0.1
M of HCl or NaOH solutions. Their measurements were performed by 
pH-meter Basic 20 (Crison Instruments). The DFC concentrations in the aqueous
solutions before and after adsorption were assessed by using a UV--vis
spectrophotometer (Thermo Scientific Genesys 10S) at a wavelength of
276~nm. \looseness=-1

\begin{figure}
{\vspace*{3pt}}
\includegraphics{fig01}
{\vspace*{3pt}}
\caption{\label{fig1}Structural formula of diclofenac.}
\end{figure}

\subsection{Adsorption experiments}\label{ssec25}

The OPC efficiency in removing DFC from aqueous solutions was carried
out in batch mode by using a 2 L jacketed reactor connected to a 
Julabo~12 \mbox{(BICASA)}
thermostatic bath, allowing one to preset a fixed temperature.
These assays consist in agitating, for a given contact time and
temperature, 0.1~g of the adsorbent in 100~mL of DFC solutions at
specific concentration and pH values. If not specified, these
parameters were used during the following experiments: a contact time of
180~min, a DFC concentration of 100~mg${\cdot}$L$^{-1}$, and a
temperature of 20~\textdegree C. During this work, the effect of
various parameters (contact time, initial pH, initial DFC 
concentrations, and temperature) on the efficacy of DFC removal  by the OPC
was assessed. The variation ranges of these parameters and especially
the initial DFC concentration were carefully chosen on the basis of
previous studies~\cite{36,37}.\looseness=-1

\subsubsection{Effect of contact time: adsorption kinetics}\label{sssec251}

The kinetics of DFC removal by the OPC were studied for contact times
ranging from 5 to 90 min and initial DFC concentrations of 60, 80, and
100~mg${\cdot}$L$^{-1}$. These experiments were carried out for a
temperature of 20~\textdegree C and at a non-adjusted pH value (around
5.5). The adsorbed quantity of DFC at a given time $t$, denoted by  $q_{t}$
(mg${\cdot}$g$^{-1}$), was calculated by the following formula: 
{\begin{equation}\label{eq1}
q_{t} = \dfrac{(C_{0}-C_{t})}{D}
\end{equation}}\unskip
where $C_{0}$ and $C_{t}$ (mg${\cdot}$L$^{-1}$) are the initial and time $t$ 
DFC concentrations, respectively. The parameter $D$ is the dose of
adsorbent (g${\cdot}$L$^{-1}$). The kinetic experimental data were
fitted with four common models: pseudo-first-order (PFO),
pseudo-second-order (PSO), Elovich, and intraparticle diffusion models.
Table~\ref{tab1} presents the equations of these models and the definitions
of their parameters~\cite{4}.

%tab1
\begin{table*}
\caption{\label{tab1}Kinetic models applied in 
the elimination of diclofenac by OPC}
\begin{tabular}{cccc}
\thead
Model & Equation & Linearized equation & Plot \\
\endthead  
Pseudo-first-order & 
$q_{t} = q_{\mathrm{e}} (1-\mathrm{e}^{-{k_{1}}t})$ & 
$\ln (q_{\mathrm{e}}-q_{t}) = \ln q_{\mathrm{e}}-k_{1}t$ & 
$q_{t}$ vs.\  $t$ {\vspace*{5pt}} \\

Pseudo-second-order & 
$q_{t} = \dfrac{k_{2}{q}_{\mathrm{e}}^{2}t}{1+k_{2}q_{\mathrm{e}}t}$ & 
$\dfrac{t}{q_{t}} = \dfrac{1}{k_{2}{q}_{\mathrm{e}}^{2}} + \dfrac{t}{q_{\mathrm{e}}}$ & 
$q_{t}$ vs.\  $t$ {\vspace*{5pt}} \\

Elovich & 
$q_{t} = \dfrac{1}{\beta}\ln (\alpha \beta) + \dfrac{1}{\beta }\ln  (t)$ & 
$q_{t} = \dfrac{1}{\beta \ln (\alpha \beta )} + \dfrac{1}{\beta \ln (t)}$ & 
$q_{t}$ vs.\  $\ln(t)$ {\vspace*{5pt}} \\

Intraparticle diffusion & 
$q_{t}=k_{i}\sqrt{t}$ &
$q_{t}=k_{i}\sqrt{t}$ & 
$q_{t}$ vs.\  $t$ 
\botline
\end{tabular}
\tabnote{$q_{\mathrm{e}}$ is the DFC adsorbed at equilibrium (mg${\cdot}$g$^{-1}$), 
$q_{t}$ is the DFC adsorbed at a given time $t$ (mg${\cdot}$g$^{-1}$), 
$k_{1}$ is the pseudo-first-order rate constant (min$^{-1}$), 
$k_{2}$ is the pseudo-second-order rate constant (g${\cdot}$mg$^{-1}{\cdot}$min$^{-1}$),
$\alpha$~is the initial adsorption rate (mg${\cdot}$g$^{-1}{\cdot}$min$^{-1}$),  
${\beta}$~is the extent of surface coverage (mg${\cdot}$g$^{-1}$), and
$k_{i}$~is the intraparticle diffusion rate constant
(mg${\cdot}$g$^{-1}{\cdot}$min$^{-1}$).\vspace*{-4pt}} 
\end{table*}

The agreement between the measured ($q_{t,\exp}$) and calculated
theoretical ($q_{t,\mathrm{theo}}$) adsorbed quantities was determined
on the basis of the determination coefficient and the mean average
percentage error (MAPE$_{\mathrm{kinetic}}$)~\cite{38}:
{\begin{equation}\label{eq10}
\mathrm{MAPE}_{\mathrm{kinetic}} (\%) = 
\dfrac{\sum _{1}^{N}|(q_{t,\exp}-q_{t,\mathrm{theo}})/q_{t,\exp}|}
{N}100
\end{equation}}\unskip
where $N$ is the number of experimental runs.

During this work, all tests were repeated thrice and the average
values were given. The standard deviation of all tests was less than
5\%.

\subsubsection{Effect of initial pH}\label{sssec252}

The effect of initial pH values on the removal of DFC by the OPC
was assessed for values varying between 2 and 10. This range
involves both acidic and alkaline conditions. These assays were
performed for fixed contact time, initial DFC concentration, adsorbent
dose, and temperature of 3~h, 10~mg${\cdot}$L$^{-1}$,
1${\cdot}$g${\cdot}$L$^{-1}$, and 20~\textdegree C, respectively. The DFC
adsorbed amount ($q_{\mathrm{e}}$ (mg${\cdot}$g$^{-1}$)) and the corresponding
removal yield ($R$ (\%)) were calculated by the following formulas:
{\begin{eqnarray}
q_{\mathrm{e}} &=& \dfrac{(C_{0}-C_{\mathrm{e}})}{D} \label{eq11} \Seqnsplit
R(\%) &=& \dfrac{(C_{0}-C_{\mathrm{e}})}{C_{0}}100 \label{eq12}
\end{eqnarray}}\unskip
where $C_{0}$ is the initial concentration of DFC, $C_{\mathrm{e}}$
(mg${\cdot}$L$^{-1}$) is the concentration at equilibrium, and $D$ is
the adsorbent dose (g${\cdot}$L$^{-1}$).

\subsubsection{Isotherm modeling and adsorption thermodynamic study}
\label{sssec253}

The multiple point method was applied to study the adsorption isotherms
of DFC on OPC at three different temperatures: 30, 40, and 50~\textdegree
C. This work was carried out for initial DFC concentrations varying
between 10 and 100~mg${\cdot}$L$^{-1}$ and a contact time of 3~h.
Moreover, the initial pH was not adjusted (5.5). The experimental data
of the DFC isotherms were fitted with four well-known models:
Langmuir, Freundlich, Temkin, and 
BET, which is commonly
used to analyze gas adsorption on solid surfaces. The equations of
these models and the definitions of their parameters are given in
Table~\ref{tab2}~\cite{39}. 

As for the kinetic modeling, the suitability of the isotherm
experimental data fitting with these models was judged based on the
determination coefficients between the experimental and predicted
curves as well as the mean average percentage error
(MAPE$_{\mathrm{isotherm}}$) as follows:
{\begin{equation}\label{eq13}
\mathrm{MAPE}_{\mathrm{isotherm}}(\%) = 
\dfrac{\sum _{1}^{N}|(q_{\mathrm{e},\exp}-q_{\mathrm{e},\mathrm{theo}})/q_{\mathrm{e},\exp}|}
{N}100
\end{equation}}\unskip
where $q_{\mathrm{e},\exp}$ and $q_{\mathrm{e},\mathrm{theo}}$ (mg${\cdot}$g$^{-1}$) are
the observed and calculated amounts of adsorbed DFC at equilibrium, respectively, and
$N$ is the number of experimental runs.

To gain a better understanding of the DFC removal process, three
thermodynamic parameters (Gibbs free energy [${\Delta}G$]), enthalpy
[${\Delta}H$], and entropy [${\Delta}S$]) were calculated according
to Equations (\ref{eq10})--(\ref{eq12}) and the Van't Hoff plot
giving the variation of $\ln(K_{\mathrm{d}})$ as a function of
$1/T$~\cite{35,36,37}:
{\begin{eqnarray}
\Delta G &=& \Delta H - T\Delta S \label{eq22} \Seqnsplit
K_{\mathrm{d}} &=& \dfrac{q_{\mathrm{e}}}{C_{\mathrm{e}}} \label{eq23} \Seqnsplit
\ln (K_{\mathrm{d}}) &=& \dfrac{\Delta S}{R} - \dfrac{\Delta H}{RT} \label{eq24}
\end{eqnarray}}\unskip
where $T$ is the absolute temperature (K), $K_{\mathrm{d}}$ is the
distribution coefficient, and $R$ is the  gas  constant
(8.314~J${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$).

\section{Results and discussion}\label{sec3}
{\vspace*{-1pt}}
\subsection{Material characterization}\label{ssec31} 
{\vspace*{-1pt}}
\subsubsection{Structural properties}\label{sssec311}
{\vspace*{-1pt}}
\paragraph{SEM/EDS analysis}\label{ssssec3111}

The SEM analysis shows a complicated surface microstructure with much
roughness for OP (Figure~\ref{fig2}a). In contrast, the OPC exhibits a
distinct surface morphology with more wrinkles (Figure~\ref{fig2}b).
This may be attributed to the deposition of Fe\tsub{3}O\tsub{4} on its
surface~\cite{14}. Moreover, the EDS analyses show that the chemical
composition of the OP and OPC mainly contain carbon (C), oxygen (O), and
calcium (Ca) as reported in other studies~\cite{31,39} while iron
(Fe), sodium (Na), and chloride (Cl) were only found in the OPC.
The presence of these
elements is attributed to both the formation of iron-based materials and the
deposition of elements from the chemical reagents used during the
magnetization process (FeCl\tsub{3}, 6H\tsub{2}O, FeSO\tsub{4},
7H\tsub{2}O, and NaOH). Similar results were found by Praipipat 
et~al.~\cite{4} in the removal of lead by OP.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}SEM/EDS analysis of orange peels (a) and orange
peel composite (b).}
\end{figure*}

%tab2
\begin{table*}
\caption{\label{tab2}Isotherm models used to study the removal of
diclofenac by OPC}
\begin{tabular}{cccc}
\thead
Model & Original equation & Linearized equation & Plot \\
\endthead
Langmuir & 
$q_{\mathrm{e}}=\dfrac{q_{\mathrm{m}}K_{\mathrm{L}}C_{\mathrm{e}}}{1+K_{\mathrm{L}}C_{\mathrm{e}}}$ & 
$\dfrac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\dfrac{1}{K_{\mathrm{L}}q_{\mathrm{m}}}+\dfrac{C_{\mathrm{e}}}{Qq_{\mathrm{m}}}$ & 
$\dfrac{C_{\mathrm{e}}}{q_{\mathrm{e}}}$ vs.\  $C_{\mathrm{e}}$ {\vspace*{5pt}} \\

Freundlich & 
$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{\frac{1}{n}}$ & 
$\ln Qq_{\mathrm{e}}=\dfrac{1}{n}\ln  C_{\mathrm{e}}+\ln  K_{\mathrm{F}}$ & 
$\mathrm{Ln}(q_{\mathrm{e}})$ vs.\  $\ln (C_{\mathrm{e}})$ {\vspace*{5pt}} \\

Temkin & 
$q_{\mathrm{e}}=B_{1}\ln (K_{\mathrm{T}}C_{\mathrm{e}})$ &
$q_{\mathrm{e}}=B_{1}\ln (K_{t})+B_{1}\ln (C_{\mathrm{e}})$ & 
$q_{\mathrm{e}}$ vs.\  $\ln (C_{\mathrm{e}})$ {\vspace*{5pt}} \\

BET & 
$q_{\mathrm{e}} = \dfrac{q_{\mathrm{m}}K_{1}C_{\mathrm{e}}}{(1-K_{2}C_{\mathrm{e}})[1+(K_{1}-K_{2})C_{\mathrm{e}}]}$ &
$\dfrac{C_{\mathrm{e}}}{q_{\mathrm{e}}(1-K_{2}C_{\mathrm{e}})} = \dfrac{1}{Qq_{\mathrm{m}}K_{1}} + \dfrac{K_{1}-K_{2}}{Q_{\mathrm{m}}K_{1}}C_{\mathrm{e}}$ &
$\dfrac{C_{\mathrm{e}}}{q_{\mathrm{e}}}$ vs.\  $C_{\mathrm{e}}$
\botline
\end{tabular}
\tabnote{$C_{\mathrm{e}}$ is the DFC concentration at equilibrium
(mg${\cdot}$L$^{-1}$), $C_s$ is the DFC concentration at the multilayer
(mg${\cdot}$L$^{-1}$),  $q_{\mathrm{e}}$\ is the adsorbed amount of OPC
at equilibrium (mg${\cdot}$g$^{-1}$), $q_{\mathrm{m}}$ is the maximum amount of
DFC adsorbed on OPC (mg${\cdot}$g$^{-1}$), $K_{\mathrm{L}}$, $K_{\mathrm{F}}$, and
$K_{\mathrm{T}}$ are the adsorption constants of Langmuir,  Freundlich, and
Temkin models, respectively, $K_{1}$ (L${\cdot}$mg$^{-1}$) is the BET
model equilibrium adsorption constant of the first layer, $K_{2}$ is
the BET model equilibrium adsorption constant of the subsequent layers,
$1/n$ is the Freundlich constant depicting the adsorption intensity,
$B_{l}$ is $R/b_{T}$ with $R$ being the gas constant
(8.314~J${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$),  $T$ is the temperature
(K), and $b_{T}$ is the constant related to the heat of adsorption
(J${\cdot}$mol$^{-1}$).}
\end{table*}

\paragraph{XRD results}\label{ssssec3112}

The XRD analysis shows that the OP does not have crystalline phases
(Figure~\ref{fig3}a)~\cite{44}. The OPC shows the presence of magnetite
(Fe\tsub{3}O\tsub{4}) at $2\theta$ values of 30.5\textdegree,
35.7\textdegree, 43.5\textdegree, and 57.7\textdegree{} 
(Figure~\ref{fig3}b). They correspond to crystal plane index values of (220),
(331), (400), and (511) of Fe\tsub{3}O\tsub{4}, respectively, according to the JCPDS
No. 75-0449 standard XRD pattern of Fe\tsub{3}O\tsub{4}~\cite{14}. 
These results show that the OPC contains Fe\tsub{3}O\tsub{4} with an
inverse spinel structure~\cite{45}, indicating that the
co-precipitation preparation method does not have any effect on
changing the Fe\tsub{3}O\tsub{4} phase in the OPC. A similar result was
reported for a biocomposite prepared from OP by the 
co-precipitation method~\cite{14}. The peak existing at $2\theta$ of
24.6\textdegree{} corresponds to amorphous carbon~\cite{41}. 

\begin{figure*}
\includegraphics{fig03}
{\vspace*{-1pt}}
\caption{\label{fig3}XRD pattern of orange peels (a) and orange peel
composite (b).}
{\vspace*{-1pt}}
\end{figure*}

\subsubsection{Textural properties}\label{sssec312}
\paragraph{BET analysis}\label{ssssec3121}

The specific surface areas of OP and OPC were estimated to be 
64.4~m\tsup{2}${\cdot}$g$^{-1}$  and
25.8~m\tsup{2}${\cdot}$g$^{-1}$, respectively. The net reduction of
this parameter after the magnetization process can be attributed to
pore blockage caused by magnetite deposition. An equivalent finding
was reported in previous studies~\cite{42,46}. 

\subsubsection{Surface chemistry properties}\label{sssec313}
\paragraph{FTIR analysis}\label{ssssec3131}

The FTIR spectra of OP and OPC showed the presence of several peaks
(Figure~\ref{fig4}). It can be clearly seen that most of the peaks exist in
both OP and its magnetized form.
Indeed, the highest and largest peak at
3418~cm$^{-1}$ for OP and at 3416~cm$^{-1}$ for OPC is related to the O--H
stretching vibrations of cellulose, hemicellulose, pectin, lignin, and
absorbed water~\cite{14}. The bands measured at 2939~cm$^{-1}$ and
1384~cm$^{-1}$ for OP and 2927~cm$^{-1}$ and 1324~cm$^{-1}$  for
OPC correspond to the C--H stretching and bending
vibrations of methyl, methylene, and methoxy groups~\cite{14}. The peak
at 1743~cm$^{-1}$ for OP can be assigned either to the elastic
oscillation of the C--O bond due to non-ionic carboxyl groups 
\mbox{(--COOH,}
--COOCH\tsub{3}) or to the carboxylic acids or esters in
pectin~\cite{23}. The peak at 1636~cm$^{-1}$ for OP and
1626~cm$^{-1}$ for OPC may be related to the bond of the benzene ring
structure in lignin~\cite{47}. In addition, the band at
1051~cm$^{-1}$ for OP and 1.074~cm$^{-1}$ for OPC is related to the
glycosidic (C--O--C) bond existing mainly in cellulose, hemicellulose,
and pectin~\cite{47}. The conservation of all these bands is due to the
fact that co-precipitation is a smooth method and does not affect
the original functional groups observed in the OP~\cite{48}. The
magnetization process has induced the appearance of a new peak at
590~cm$^{-1}$, which is attributed to the Fe--O group~\cite{49}. This
finding indicates that iron oxides were deposited on the surface of the
OPC. Similar results were reported by Zhao~et~al.~\cite{14} and
Praipipat et~al.~\cite{4} when they produced iron-doped materials from
OPs. 

\begin{figure}
{\vspace*{-2pt}}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}FTIR spectra of orange peels and orange peel
composite.}
{\vspace*{-2pt}}
\end{figure}

\paragraph{pH of zero-point charge}\label{ssssec3132}

The pH$_{\mathrm{PZC}}$ value of the OPC was estimated by the pH
drift method to be 5.8. This value is close to those found for MAC-CP
(magnetic activated carbon from pulp and paper mill)~\cite{31} and
MNP/OPP (magnetic nanoparticles from OP powder)~\cite{45}.
This implies that at pH values lower than 5.8, the adsorbent surface
would be mainly positively charged and consequently would favor DFC
removal through electrostatic interactions.

\subsubsection{Thermal degradation behavior}\label{sssec314}

The TGA and DTA curves for OP and OPC are shown in Figure~\ref{fig5}a,b. The
TGA curves of the OP material show the presence of three endothermic
decomposition peaks in the tested temperature range of  30~\textdegree
C--800~\textdegree C, corresponding to three weight losses. The first
stage (10.75\%) was observed for a temperature range of  30~\textdegree
C--200~\textdegree C, indicating the removal of water, organic solvent
or residual solvent, and physisorbed and chemisorbed H\tsub{2}O molecules
in the sample~\cite{50}. In the second stage, the decomposition of capping
biomolecules occurred at 220~\textdegree C--400~\textdegree C. The
protein of the OP extract decomposed completely at temperatures
higher than 600~\textdegree C~\cite{50}. 

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig05}
\caption{\label{fig5}Thermogravimetric study of orange peels (a) and
orange peel composite (b).}
{\vspace*{-1pt}}
\end{figure*}

Figure~\ref{fig5}b indicates that the initial mass loss of the OPC
occurred below 100~\textdegree C (4.9\%) and was due to the release of
weakly bonded physisorbed water molecules. The second range between
135 and 250~\textdegree C is related to the degradation of
hemicellulose~\cite{44,45}. The third range 250--375~\textdegree C is
due to cellulose decomposition~\cite{52}. Finally, the degradation
occurring between 375 and 500~\textdegree C corresponds to lignin,
which is more thermally stable and difficult to be degraded~\cite{53}.
Overall, the thermal stability and degradation characteristics of OP
and OPC are in line with those reported for oil-palm wastes~\cite{54}.
This result highlights their high potential to be used as promising
materials for various environmental applications such as wastewater
treatment~\cite{55}.

\subsection{Study of DFC removal by the biocomposite} 
\label{ssec32}

Preliminary assays aiming to assess the ability of the raw material
(OP) in removing DFC were conducted in batch mode for an initial pH of
5.5, a DFC concentration of 100~mg${\cdot}$L$^{-1}$, and a contact time
of 180 min. The results show that despite its relatively higher BET surface
area (64.4~m\tsup{2}${\cdot}$g$^{-1}$), this material was inefficient
in removing DFC from aqueous solutions. This finding may be due to the
fact that the DFC adsorption may be mainly governed by other parameters
such as  structural and surface chemistry properties. For this
reason, only the OPC material was selected for the study of DFC
adsorption. 

\subsubsection{Effect of contact time--kinetic study}
\label{sssec321}

The kinetic removal of DFC by the OPC was conducted under the experimental
conditions given in Section~\ref{sssec251}. The experimental results
are depicted in Figure~\ref{fig6}.

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Kinetic removal of DFC by the orange peel composite
at different initial concentrations ($C_{0}$: initial DFC
concentration; PFO: pseudo-first order).}
\end{figure}

Figure~\ref{fig6} reveals that DFC adsorption on OPC is a very rapid
process. Indeed, after only 5 min of contact time, the amount of removed DFC 
\mbox{represents} around 4.5\%, 7.3\%, and 12.4\% of the totally
adsorbed amount for initial concentrations of 60, 80, and
100~mg${\cdot}$L$^{-1}$, respectively. After that, the DFC adsorption
continues to increase but at a slower rate. The equilibrium is
observed after about 20--25 min for all the studied initial
concentrations \mbox{(Figure~\ref{fig6})}. At this stage, the related
experimental adsorbed amounts ($q_{\mathrm{e},\exp}$) were evaluated as 37.6,
15.2, and 9.8~mg${\cdot}$g$^{-1}$ for initial DFC concentrations of
100, 80, and 60~mg${\cdot}$L$^{-1}$, respectively. This is an important
advantage when scaling up this process. This contact time is much lower
than those observed for DFC removal by a three-dimensional graphene-based
adsorbent~\cite{56}, and activated carbon prepared from olive
stones~\cite{57}.

The experimental data fitting with the four kinetic models shows that
the PFO model is the most appropriate model. Indeed, it exhibits the
highest $R^{2}$ and the lowest MAPE values (Table~\ref{tab3}).
Moreover, the calculated DFC adsorbed amounts at equilibrium by this
model for the three studied initial concentrations were the closest to
the experimental values (Table~\ref{tab3}). This finding suggests that
the DFC adsorption process is mainly controlled by an external
diffusion step~\cite{58}. The PFO model was also found to best fit the
experimental data for DFC adsorption by alginate-/carbon-based
films~\cite{59} and three-dimensional graphene-based
adsorbents~\cite{56}. However, better fitness of PSO  was reported for DFC
adsorption by magnetic carbon nanofibers derived from bacterial
cellulose~\cite{25} and activated carbon derived from olive
stones~\cite{57}. The Elovich model explains pollutant adsorption onto
heterogeneous \mbox{surfaces,} where the rate slows down over time making it
suitable for varying affinities. In contrast, the intraparticle
diffusion model mainly occurs via diffusion from an aqueous solution
though the boundary layer in the first step and then inside the adsorbent's
pores.

%tab3
\begin{table*}
\caption{\label{tab3}Parameters of used kinetic models for DFC
removal by OPC{\vspace*{-2pt}}}
\begin{tabular}{cccccc}
\tbody
\morerows{3}{PFO} & 
$C_{0}$ (mg${\cdot}$L$^{-1}$) &
$k_{1}$ (min\tsup{{-1}}) & 
$q_{\mathrm{e}}$ (mg${\cdot}$g$^{-1}$) & $R^{2}$ & MAPE{\vspace*{2pt}} \\
& 100 & 0.232 & 37.0 & 0.989 & 2.2 \\
& \080 & 0.459 & 14.8 & 0.972 & 5.0 \\
& \060 & 0.474 & \09.7 & 0.969 & 6.7{\vspace*{5pt}} \\

\morerows{3}{PSO} & 
$C_{0}$ (mg${\cdot}$L$^{-1}$) & 
$k_{2}$ (g${\cdot}$mg$^{-1}{\cdot}$min$^{-1}$) & 
$q_{\mathrm{e}}$ (mg${\cdot}$g$^{-1}$) & $R^{2}$ & MAPE{\vspace*{5pt}} \\
& 100 & 0.008 & 39.6 & 0.975 & 4.6 \\
& \080 & 0.044 & 15.6 & 0.954 & 6.6 \\
& \060 & \00.0830 & 10.1 & 0.963 & 6.6{\vspace*{5pt}}\\

\morerows{3}{Intraparticle diffusion} &
$C_{0}$ (mg${\cdot}$L$^{-1}$) & 
\multicolumn{2}{c}{$k_{i}$ (mg${\cdot}$g$^{-1}{\cdot}$min\tsup{0.5})} &
$R^{2}$ & MAPE {\vspace*{2pt}} \\
& 100 & \multicolumn{2}{c}{5.255} & 0.413 & 30.0 \\
& \080  & \multicolumn{2}{c}{2.443} & 0.352 & 20.1 \\
& \060  & \multicolumn{2}{c}{1.655} & 0.094 & 37.2{\vspace*{5pt}}\\

\morerows{3}{Elovich} & 
$C_0$ (mg${\cdot}$L$^{-1}$) & 
$A$ (mg${\cdot}$g$^{-1}{\cdot}$min$^{-1}$) & 
${\beta}$ (g${\cdot}$mg$^{-1}$) & 
$R^{2}$ & MAPE{\vspace*{2pt}} \\
& 100 & 62.760 & 0.168 & 0.922 & 10.100 \\
& \080 & 95.950 & 0.496 & 0.895 & 10.300 \\
& \060 & 67.830 & 0.754 & 0.884 & 17.300
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

\subsubsection{Effect of pH}\label{sssec322}

The effect of initial pH values on DFC removal by the OPC
was studied under the experimental conditions described in 
Section~\ref{ssec25}. The results (Figure~\ref{fig7}) show that the highest
removal yield (85.9\%) was {observed} at a pH of 3. Then, the removal
yield gradually decreases with increase in initial pH value.
The lowest value (7.5\%) was measured at an alkaline pH of 10.4. This
decrease in efficiency is due to the fact that for pH values higher than
the $\mathrm{p}K_{\mathrm{a}}$
of DFC (4.0) and the pH$_{\mathrm{zpc}}$ of the OPC material (5.8),
both the DFC and the OPC particles are negatively charged. In contrast,
for pH values lower than 4, electrostatic interactions are favored
between the positively charged OPC particles and negatively charged\break
DFC molecules.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Effect of pH on diclofenac adsorption by the
orange peel composite ($q_{\mathrm{e}}$: amount of DFC removed  at
equilibrium).}
\end{figure}

Similar results were reported by Yoong et~al.~\cite{60} and Mizzaee 
et~al.~\cite{61} when investigating DFC removal by a chitosan-based
magnetic composite and wood-derived mesoporous activated carbon
respectively. For instance, in the latter study, increasing the pH from
3 to 11 has resulted in a decrease in DFC removal yield from 92.3 to
23.9, respectively~\cite{61}. In our study, we choose to use an initial
pH of 5.5 (without adjustment) even it has lower adsorption capacity.
Indeed, avoiding the use of chemicals for pH adjustment to a value
of 3 has both environmental and economic benefits. 

\subsubsection{Effect of temperature combined with the initial DFC 
concentration and isotherm study} \label{sssec323}

The effect of temperature on the DFC adsorption isotherm by OPC is illustrated in
Figure~\ref{fig8}. It can be clearly seen that the  adsorbed DFC
quantity by the OPC decreases with increase in initial DFC
concentration. Indeed, under fixed experimental conditions, the
highest adsorbed DFC  amounts were estimated as 23.7, 19.0, and
13.6~mg${\cdot}$g$^{-1}$ at temperatures of 30, 40, and 50~\textdegree
C, respectively (Figure~\ref{fig8}). This finding implies that 
temperature increase has a negative impact on the removal of DFC. This
\mbox{behavior} may be explained by the weakness of the physical bond between
DFC molecules and OPC active sites, which decrease with increase
in aqueous temperature~\cite{33}. This result is in line with the
study by Thi Minh Tam~\cite{33}, who reported that an increase in
temperature from 15~\textdegree C to 45~\textdegree C had decreased the DFC
removal efficiency of magnetic-gelatin-modified peanut shell biochar by
39\%. In contrast, the study by Leone et~al.~\cite{62} showed an increase in DFC
removal rate by 14.4\% when the temperature was increased from
10~\textdegree C to 45~\textdegree C. 

\begin{figure}
{\vspace*{-4pt}}
\includegraphics{fig08}
{\vspace*{-4pt}}
\caption{\label{fig8}Effect of temperature on DFC removal by the orange
peel composite ($T$: temperature; BET: BET isotherm model).}
{\vspace*{-4pt}}
\end{figure}

The experimental isotherm data were fitted using the four selected
models---Langmuir, Freundlich, Temkin, and BET---by using Matlab
R2016a software. Table~\ref{tab4} summarizes the obtained results.

%tab4
\begin{table*}
\caption{\label{tab4}Estimated parameters of Langmuir, Freundlich,
Temkin, and BET isotherm models of diclofenac elimination by the orange
peel composite{\vspace*{1pt}}}
\begin{tabular}{ccccccc}
\thead
Isotherm model & Temperature & \multicolumn{5}{c}{Parameters} \\
\endthead
\morerows{3}{Langmuir} 
& $T$ (\textdegree C) & \multicolumn{2}{c}{$K_{\mathrm{L}}$ (L${\cdot}$mg$^{-1}$)} & $q_{\mathrm{m}}$ (mg${\cdot}$g$^{-1}$) & $R^{2}$ & MAPE {\vspace*{2pt}}\\
& 30 & \multicolumn{2}{c}{0.102} & 28.1 & 0.998 & 3.5 \\
& 40 & \multicolumn{2}{c}{0.073} & 22.8 & 0.984 & 5.8 \\
& 50 & \multicolumn{2}{c}{0.109} & 15.4 & 0.988 & 4.3 {\vspace*{5pt}} \\

\morerows{3}{Freundlich} 
& $T$ (\textdegree C) & \multicolumn{2}{c}{$K_{\mathrm{F}}$} & $N$ & $R^{2}$ & MAPE {\vspace*{2pt}}\\
& 30 & \multicolumn{2}{c}{5.500} & 2.6 & 0.973 & 11.0 \\
& 40 & \multicolumn{2}{c}{3.500} & 2.3 & 0.991 & \04.4 \\
& 50 & \multicolumn{2}{c}{4.100} & 3.3 & 0.996 & \02.0 {\vspace*{5pt}} \\

\morerows{3}{Temkin} 
& $T$ (\textdegree C) & \multicolumn{2}{c}{$B$} & $K_{\mathrm{T}}$ (min$^{-1}$) & $R^{2}$ & MAPE {\vspace*{2pt}}\\
& 30 & \multicolumn{2}{c}{6.1} & 1.0 & 0.995 & 3.0 \\
& 40 & \multicolumn{2}{c}{5.0} & 0.7 & 0.979 & 4.1 \\
& 50 & \multicolumn{2}{c}{3.1} & 1.4 & 0.985 & 2.2 {\vspace*{5pt}} \\

\morerows{3}{BET} 
& $T$ (\textdegree C) & $K_{1}$ & $K_{2}$ & $q_{\mathrm{m}}$ (mg${\cdot}$g$^{-1}$) & $R^{2}$ & MAPE {\vspace*{2pt}}\\
& 30 & 0.090 & 0.000 & 29.9 & 0.986 & 4.6 \\
& 40 & 0.166 & 0.006 & 14.1 & 0.995 & 3.2 \\
& 50 & 0.219 & 0.004 & 11.4 & 0.998 & 1.5
\botline
\end{tabular}
{\vspace*{1pt}}
\end{table*}

Table~\ref{tab4} shows that both the BET and Freundlich models fit the
experimental data. Indeed, they \mbox{exhibit} correlation coefficients
close to unity and the lowest MAPE values. This suggests that DFC
removal by OPC occurs heterogeneously and on multilayers. This result
is confirmed by the finding of Jaldin et~al.~\cite{22} when studying
pesticide removal by sugarcane bagasse and peanut shell magnetic
composites. Moreover, the calculated Langmuir parameter values
($R_{\mathrm{L}} = 1 / (1 + K_{\mathrm{L}} \times C_{0})$) are less
than 1, indicating that DFC removal by the OPC is a favorable process.
In addition, under the studied conditions, the calculated Freundlich
parameter ($n$) values are higher than 1 (Table~\ref{tab4}), suggesting
that the adsorption process is favorable. Finally, the estimated Temkin
parameter ($B$) values are lower than 8~kJ${\cdot}$mol$^{-1}$
(Table~\ref{tab4}), implying that this biosorption might involve
physical mechanisms.

Table~\ref{tab5} presents an effectiveness comparison of DFC removal  by
OPC with other engineered adsorbents. It clearly shows that the OPC can
be considered a promising adsorbent. However, it is obvious that the OPC
exhibits much lower efficiency than activated carbons and biochars
(Table~\ref{tab5}). This is attributed to the fact that these materials
usually have very improved physicochemical properties. For instance, a
magnetic-gelatin-modified peanut shell biochar~\cite{33} has a DFC
removal capacity 10 times higher than our material. This can be
partially explained by the fact that it has a BET surface area of
222.75~m\tsup{2}${\cdot}$g$^{-1}$, which is much higher than that of our
OPC (25.8~m\tsup{2}${\cdot}$g$^{-1}$), in addition to the enhancement of its
surface chemistry properties after modification. However, our
synthesized OPC is cheaper and eco-friendlier than typical biochars.

%tab5
\begin{table*}
\caption{\label{tab5}Comparison of diclofenac removal by the orange
peel composite with other engineered adsorbents}
\begin{tabular}{ccc}
\thead
Adsorbent & \parbox[t]{8pc}{\centering Diclofenac adsorption capacity
(mg${\cdot}$g$^{-1}$)} & Reference \\
\endthead
Magnetite rice husk activated carbon & 270.0 & \cite{60} \\
Carbon nanotube tanned leather waste & \020.6 & \cite{63} \\
Magnetic carbon from nanofibers & \048.8 & \cite{25} \\
\parbox[t]{20pc}{\centering Alginate-/carbon-based films from sodium alginate solution and commercial activated carbon} & \029.9 & \cite{59}\vspace*{2pt} \\
Magnetic-gelatin-modified peanut shell biochar & 348.9 & \cite{33} \\
\parbox[t]{20pc}{\centering Lignin-based magnetic nanoparticle adsorbent from alkaline bamboo} & 106.4 & \cite{64}\vspace*{2pt} \\
Orange peel composite & \037.0 & This work 
\botline
\end{tabular}
\end{table*}

Thermodynamic parameters were calculated from the Van't Hoff
diagram according to the methodology given in Section~\ref{ssec32}.
The results (Table~\ref{tab6}) showed that all the calculated ${\Delta}G$
values at {different} temperatures were negative. This indicates that the
adsorption phenomenon is feasible and spontaneous~\mbox{\cite{65,66}.}
Moreover, the enthalpy (${\Delta}H$) is negative
(${-}$22.3~kJ${\cdot}$mol$^{-1}$), indicating that the DFC adsorption
process is exothermic. In addition, this value is less than
40~kJ${\cdot}$mol$^{-1}$, suggesting that DFC adsorption by OPC
involves mainly physical {mechanisms.} The ${\Delta}S$ value is
negative (${-}$0.071~J${\cdot}$mol$^{-1}{\cdot}$k$^{-1}$), suggesting
that the adsorption is accompanied by an orientation of  molecules
leading to a reduction in the state of molecular disorder~\cite{67}.
Similar results were reported for DFC adsorption by
zeolites/resins~\cite{58}.

%tab6
\begin{table}
\caption{\label{tab6}Thermodynamic parameters of diclofenac adsorption
by the orange peel composite}
\begin{tabular}{cccc}
\thead
$T$ (\textdegree C) & 
\parbox[t]{4pc}{\centering ${\Delta}H$ (kJ${\cdot}$mol$^{-1}$)} &
\parbox[t]{5pc}{\centering ${\Delta}S$ (kJ${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$)} &
\parbox[t]{3.5pc}{\centering ${\Delta}G$ (kJ${\cdot}$mol$^{-1}$)}\vspace*{2pt} \\
\endthead
30 & \morerows{2}{${-}$22.273}&  \morerows{2}{${-}$0.071} & ${-}$1.626 \\
40 &  & & ${-}$0.921 \\
50 &  & & ${-}$0.216
\botline
\end{tabular}
\end{table}

It can be noted that textural properties do not highly affect DFC
removal by our synthesized OPC. Indeed, even if this material has a lower
surface area than the raw OP, it efficiently adsorbs DFC from aqueous
solutions under a wide range of experimental conditions. In this context, Thi Minh
Tam~\cite{33} reported that even if magnetized biochar has a lower
specific area (70.2~m\tsup{2}${\cdot}$g$^{-1}$) than raw biochar
(222.7~m\tsup{2}${\cdot}$g$^{-1}$), its DFC removal capacity was around
3.3 times higher. Other properties may influence this process to a greater extent. For
instance, on the basis of the pH effect and isotherm modeling
studies, it appears that the DFC removal is highly influenced by the
electrostatic interaction mechanism. However, chemical interactions
may be also involved in this process. Indeed, it has been previously
proven that ${\uppi}$--${\uppi}$ interactions established between
the aromatic ring in DFC and the OPC may play a crucial role in this
process~\cite{68}. In addition, DFC adsorption through complexation
with functional groups (C--H stretching vibration) may be
important under specific conditions~\cite{68}. Further investigations
are required to better elucidate the contribution of each one
of them. Moreover, sustainable management of the DFC-loaded OPC is
an important research work to be carried out in the future. An
effective desorption step followed by the use of an advanced oxidation
process may be applied~\cite{69}. Furthermore, laboratory column
experiments are needed in order to assess the scale-up design
parameters of large-scale columns. As detailed in the recent work of
Bian et~al.~\cite{70}, this operation will be based on the combination
of these laboratory assay outputs and theoretical concepts. 

\section{Conclusion}

The current work shows that a magnetic OPC could be
considered  a promising material for removing DFC from aqueous
solutions under a wide range of  experimental conditions. The in-depth
\mbox{characterization} by using numerous techniques and the experimental and
modeling adsorption analysis indicate that DFC removal by
adsorption is favorable, spontaneous, exothermic, and involves mainly
physical mechanisms. The highest removal efficiency was achieved for
low acidic pH values due to a favorable electrostatic interaction
mechanism. Additional research will be undertaken to assess this
OP-derived adsorbent in removing/oxidizing DFC and also
other typical pharmaceuticals from actual wastewater. Moreover, dynamic
assays involving laboratory columns and reactors are necessary
for the following processes: (i) evaluate the effect of key adsorption parameters (flow
rate, adsorbent bed heights) and (ii)  guarantee a successful and
smooth scaling up of laboratory devices to real-world situations. 
\looseness=-1

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\section*{Declaration of interests}

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The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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\section*{Acknowledgments}

{\vspace*{-2pt}}

The authors were supported by the National Engineering School of Gabes
(ENIG), University of Gabes, Tunisia and the Instituto de Ciencia de
Materiales de Madrid, Spain. They express special thanks to Professors
Pilar Aranda and Marga Darder for their valuable help and work
co-supervision. 

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