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\datereceived{2022-03-19}
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\dateposted{2022-12-02}
\begin{document}

\begin{noXML}

%\TopicEF{Materials and Clean Processes for Sustainable Energy and
%Environmental Applications}{Mat\'eriaux et proc\'ed\'es propres pour
%des applications \'energ\'etiques et environnementales}

\title{Valorisation of plastic waste from the beverage industry through
its transformation into adsorbent and solid fuel materials}

\alttitle{Valorisation des d\'{e}chets plastiques de l'industrie des
boissons par leur transformation en mat\'{e}riaux adsorbants et
combustibles solides}

\author{\firstname{Asma} \lastname{Nouira}\CDRorcid{0000-0001-5982-060X}}
\address{University of Tunis El Manar, Faculty of Mathematical,
Physical and Natural Sciences of Tunis, Campus Universitaire 2092 - El
Manar-Tunis, Tunisia}
\address{National Center for Research in Materials Science, Borj
Cedria, Technopole of Borj Cedria, Slimane 8027, Tunisia}
\email[A. Nouira]{asma.nouira@fst.utm.tn}

\author{\firstname{Imene} \lastname{Bekri-Abbes}\IsCorresp}
\addressSameAs{2}{National Center for Research in Materials Science,
Borj Cedria, Technopole of Borj Cedria, Slimane 8027, Tunisia}
\email[I. Bekri-Abbes]{bekrimene@gmail.com}

\author{\firstname{Ezzeddine} \lastname{Srasra}\CDRorcid{0000-0002-3606-559X}}
\addressSameAs{2}{National Center for Research in Materials Science,
Borj Cedria, Technopole of Borj Cedria, Slimane 8027, Tunisia}
\email[E. Srasra]{srasraezzedine@gmail.com}

\author{\firstname{Isabel P.~P.} \lastname{Cansado}\CDRorcid{0000-0003-2085-9157}}
\address{LAQV-Requimte, Portugal}
\address{Mediterranean Institute for Agriculture, Environment and
Development (MED), Instituto de Investiga\c{c}\~{a}o e Forma\c{c}\~{a}o
Avan\c{c}ada, Departamento de Qu\'{i}mica, Escola de Ci\^{e}ncias e
Tecnologia, Universidade de \'{E}vora, Rua Rom\~{a}o Ramalho
$\text{n}^{\mathrm{o}}$59, 7005-671 \'{E}vora, Portugal}
\email[I. P. P. Cansado]{ippc@uevora.pt}

\author{\firstname{Paulo A.~M.} \lastname{Mour\~{a}o}\CDRorcid{0000-0002-3634-2390}}
\addressSameAs{4}{Mediterranean Institute for Agriculture, Environment
and Development (MED), Instituto de Investiga\c{c}\~{a}o e
Forma\c{c}\~{a}o Avan\c{c}ada, Departamento de Qu\'{i}mica, Escola de
Ci\^{e}ncias e Tecnologia, Universidade de \'{E}vora, Rua Rom\~{a}o
Ramalho $\text{n}^{\mathrm{o}}$59, 7005-671 \'{E}vora, Portugal}
\email[P. A. M. Mour\~{a}o]{pamm@uevora.pt}

\shortrunauthors

\keywords{\kwd{Waste PET}\kwd{Activated carbon} \kwd{Chemical and
physical activation}\kwd{Adsorption}\kwd{Solid fuel
materials}\kwd{Herbicides removal}}

\altkeywords{\kwd{D\'{e}chets de PET}\kwd{Charbon actif}\kwd{Activation
chimique et physique}\kwd{Adsorption}\kwd{Mat\'{e}riaux combustibles
solides}\kwd{\'{E}limination des herbicides}}

\begin{abstract}
In the present study, char and activated carbon (AC) materials were
prepared from water bottles, recuperated from the waste collection
point in Mourouj, Ben Arous, Tunisia, by using a rotative horizontal
tubular furnace, on a lab/pilot scale and through chemical and physical
activation. Different samples were characterized by $\text{N}_2$
adsorption isotherms, SEM-EDX, XRD, EA as well as by the determination
of the $\text{pH}_{\mathrm{zpc}}$. The efficiency of the ACs to remove
(2-methyl-4-chlorophenoxyacetic acid (MCPA) and
2,4-dichlorophenoxyacetic acid (2,4-D)) herbicides from aqueous
solutions was also investigated. The results demonstrated that
a maximum adsorption capacity of $192~\text{mg}{\cdot}\text{g}^{-1}$,
for MCPA, and $290~\text{mg}{\cdot}\text{g}^{-1}$ for 2,4-D had been
attained. The adsorption kinetics was fitted to pseudo first, second
order and Elovich model, while the adsorption isotherms were fitted
applying the Langmuir and Freundlich isotherm models. The potential use
as solid fuel materials was evaluated using
elemental analysis and numerical high calorific value (HCV). The
experimental results were compared to the conventional solid fuels
classified in the Van Krevelen graph. The essays performed showed that
the HCV was in the range of 10.2--11.9~MJ/kg.
\end{abstract}

\begin{altabstract}
Dans la pr\'{e}sente \'{e}tude, des produits de carbonisation et du
charbon actif (CA) ont \'{e}t\'{e} pr\'{e}par\'{e}s \`{a} partir de
bouteilles d'eau, r\'{e}cup\'{e}r\'{e}es du point de collecte des
d\'{e}chets \`{a} Mourouj, Ben Arous, Tunisie, en utilisant un four
tubulaire horizontal rotatif, \`{a} l'\'{e}chelle laboratoire/pilote et
par activation chimique et physique. Diff\'{e}rents \'{e}chantillons
ont \'{e}t\'{e} caract\'{e}ris\'{e}s par des isothermes d'adsorption de
$\text{N}_2$, SEM-EDX, XRD, EA ainsi que par la d\'{e}termination du
$\text{pH}_{\mathrm{zpc}}$. L'efficacit\'{e} des CA pour \'{e}liminer
les herbicides (acide 2-m\'{e}thyl-4-chloroph\'{e}noxyac\'{e}tique
(MCPA) et acide 2,4-dichloroph\'{e}noxyac\'{e}tique (2,4-D)) des
solutions aqueuses a \'{e}galement \'{e}t\'{e} \'{e}tudi\'{e}e. Les
r\'{e}sultats obtenus ont d\'{e}montr\'{e} qu'une capacit\'{e}
d'adsorption maximale de $192~\text{mg}{\cdot}\text{g}^{-1}$, pour le
MCPA, et de $290~\text{mg}{\cdot}\text{g}^{-1}$ pour le 2,4-D avait
\'{e}t\'{e} atteinte. Des mod\`{e}les de pseudo-premier ordre, de
second ordre et d'Elovich ont \'{e}t\'{e} appliqu\'{e}s \`{a} la
cin\'{e}tique d'adsorption, tandis que les mod\`{e}les des isothermes
de Langmuir et de Freundlich ont \'{e}t\'{e} appliqu\'{e}es aux
isothermes d'adsorption. La possibilit\'{e} d'utiliser les
\'{e}chantillons \'{e}tudi\'{e}s comme mat\'{e}riaux combustibles
solides a \'{e}t\'{e} \'{e}valu\'{e}e en utilisant l'analyse
\'{e}l\'{e}mentaire et le pouvoir calorifique sup\'{e}rieur (PCS)
num\'{e}rique. Les r\'{e}sultats exp\'{e}rimentaux ont \'{e}t\'{e}
compar\'{e}s aux combustibles solides conventionnels class\'{e}s dans
le graphique de Van Krevelen. Les essais r\'{e}alis\'{e}s ont
montr\'{e} que le PCS \'{e}tait de l'ordre de 10,2--11,9~MJ/kg.
\end{altabstract}

\maketitle
%\vspace*{15pt}
\break
\twocolumngrid

\end{noXML}

\section{Introduction}

Plastic materials are widely used in many domains such as bottling,
packaging, agriculture and industry. However, their excessive
employment and
their disposal in green places can cause serious environmental
problems. To reduce or solve these ecological issues, plastics should
be transformed into value-added products. In the Site ``Invest in
Tunisia'', it was stated that the incentives in the plastics-processing
industry are quite evident and a list containing the strengths and
relevant opportunities to investors in plastics-manufacturing
industries was also included. However, it is worth noting that in
2015 foreign investment in plastics-manufacturing industries
contributed to the creation of 11,000 jobs across the country~\cite{1}.

At present, Tunisia does not impose strict regulations to minimize
environmental pollution, as the Government has not yet introduced a
serious policy on waste reduction. In 2020, Tunisia produced about
2.6~million tons of solid waste and about 245 thousand tons of
plastics~\cite{2}. It was the 4th consumer of plastic products in the
Mediterranean region and 60\% of this amount of solid waste were sent to
landfills, 16\% were openly thrown away into Nature and only 4\% were
recycled~\cite{3}. Waste management should give priority to recycling
and recovery, which makes it possible to complete the circular economy
cycle~\cite{4}.

\vspace{4pt}
The municipal solid waste can be transformed into adsorbent materials,
such as
activated carbons (ACs).
The production of ACs from a diversity of
carbonaceous raw materials, of natural or synthetic origins, (e.g.
charcoal, peat, lignite~\cite{5}, bituminous coal, cork, olive
pits~\cite{6}, Prosopis juliflora~\cite{7}, polyacrylonitrile~\cite{8}
and pure polyethylene terephthalate) was widely dealt with in previous
research works~\cite{9,10}. It was proven that ACs can be produced
by chemical or physical activation of the precursor. Moreover, a
review showing the results obtained with different activating
agents was presented by Heidarinejad and co-authors in 2020~\cite{11}.
Physical activation happens at a temperature varying between 650
and 900~{\textdegree}C, in an oxidizing atmosphere~\cite{5}. The
textural properties of the ACs change depending on the
precursor and the activating agent as well as the activation
temperature. Indeed, carbon dioxide favours the development of
microporosity while water vapour leads to a more developed
porosity~\cite{12,13,14}. On the other hand, chemical activation
takes place under an inert gas flow at temperatures ranging from 400 to
800~{\textdegree}C. During this process, the precursor is mixed dry
with the activation agent or impregnated in an aqueous solution, before
heating. The most referenced activating agents are phosphoric
acid~\cite{7,15} sodium hydroxide, potassium hydroxide~\cite{10,16,17}
and potassium carbonate~\cite{10}. Indeed, chemical activation
prevents the excessive loss of AC mass observed during physical
activation and leads to ACs with a better-developed
\mbox{porosity}~\cite{10,16,17,18}.

ACs have been successfully utilized in several fields such as
emission control, filtration and purification of gases,
decolourization, adsorption of dyes~\cite{19}, gaseous
pollutants~\cite{20}, heavy metals, pharmaceuticals~\cite{21} and
pesticides~\cite{10,22}.

Nowadays, the main concern of the research community is how to
treat the liquid effluents containing pesticides and herbicides and
reuse in the agriculture without damaging Nature. These sources of pollution,
eutrophication and unaesthetic disturbance to aquatic life present a
potential danger of bioaccumulation. They can be removed from liquid
effluents, using a thermal, physical, biological or chemical process.
Among a diversity of processes available, the adsorption on ACs scores
well in a review paper made by Hattab and Ghaly in 2012  where 18
methods used to treat pesticides were evaluated in terms of
operational cost, implementation area, detoxification ability and ease
of use~\cite{23}.

Most AC materials utilized as solid fuel, for energetic purpose, or as
adsorbent, for pollutants removal from the liquid phase, are derived
from biomass resources which are also used in the feeding of animals.
For that reason, the present work focuses on precursor derived from
plastic waste instead of biomass. Two modes of valorization were
studied. The first one is the conversion of PET waste into AC and its
potential use as an adsorbent for herbicides removal and the second
mode is the potential conversion of PET waste into a solid fuel for
energetic purposes.
\vspace*{70pt}


\section{Materials and methods}\label{sec2}
\subsection{Production of char and activated carbons} \label{sec2.1}
\subsubsection{Char production} \label{sec2.1.1}

The precursor used in this study is PET recuperated from a plastic
waste point collection, in Mourouj, Ben Arous, Tunisia. Waste bottles
were cut into small pieces with dimensions varying between of
0.5--1~cm. Char was produced in a furnace by PET carbonisation under a
nitrogen flow of $70~\text{mL}{\cdot}\text{min}^{-1}$. The furnace is
equipped with a temperature control programmer, allowing the control of
the temperature rate ($10~\text{K}{\cdot}\text{min}^{-1}$) and final
temperature (between 973 and 1073~K) (Figure~\ref{fig1}).

\begin{figure*}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{\label{fig1}Small pieces of waste PET and the rotative furnace
used for char and activated carbon production.}
\end{figure*}

\subsubsection{Activated carbon production by physical{\newline}
activation} \label{sec2.1.2}

After applying a step similar to that of the char production, the
recuperated char was physically activated using carbon dioxide, at
973~K and 1073~K, or air at 1073~K under a fixed flow of
$110~\text{mL}{\cdot}\text{min}^{-1}$ during time intervals ranging
from 15 to 240~min.

\subsubsection{Activated carbon production by chemical{\newline}
activation} \label{sec2.1.3}

To carry out chemical activation, 200~g of PET waste were placed in a
beaker and mixed with previously-crushed 200~g of KOH (Sigma). The
mixture was put in a stainless-steel container, brought to the
furnace and submitted to a heating rate of $10~\text{K}{\cdot}
\text{min}^{-1}$ from room temperature to a final temperature of 753,
873 or 1073~K. The samples were activated and cooled in the presence of
nitrogen flow of $70~\text{mL}{\cdot}\text{min}^{-1}$. The carbonaceous
materials were taken out of the furnace and washed extensively with
distilled water, until the pH of the washing water reached almost~7.

\subsection{Activated carbons characterization} \label{sec2.2}

All ACs were structurally and chemically characterized, at 77~K by
nitrogen adsorption, Scanning Electron Microscopy with
energy-dispersive X-ray spectroscopy
(SEM-EDX), X-ray diffraction (XRD), Fourier Transform
Infrared spectroscopy (FT-IR), and elemental analysis as well as by
determining the point of zero charge ($\text{pH}_{\mathrm{zpc}}$).

\subsubsection{Textural characterisation of the carbon samples}
\label{sec2.2.1}

A full textural characterization of an AC material requires specifying
its apparent surface area, external area, total porous volume (macro,
meso and micropores), mean pore size and pore size distribution. These
characteristics were obtained from the nitrogen isotherm, carried out
at 77~K. The ACs, degassed under a vacuum system for 12~h at 493~K,
were placed in an appropriate cell. The latter was, then, put into the
Quantachrome Autosorb iQ2-C Series equipment where the data obtained by
the adsorption isotherm were recorded.

\subsubsection{XRD} \label{sec2.2.2}

The X-ray diffraction spectra were provided using an X'Pert Pro Pan
analytical X-ray diffractometer, D8~ADVANCE, from Bruker, USA.
Subsequently, different diffractograms were processed by X'pertHight
Score Plus software.

\subsubsection{SEM} \label{2.2.3}

Scanning Electron Micrographs of the selected samples were obtained
employing a Quanta 3D FEG electron microscope purchased from FEI
Company, with a resolution of 30x--200,000~Kx, and an accelerating
voltage varying between 0.2 and 30~kV. These analyses were conducted by
the Servicio de Apoyo \`{a} l'Investigaci\'{o}n, at the Servicio de
An\'{a}lisis Elemental y Molecular of the University of Extremadura,
Spain.

\subsubsection{FTIR} \label{2.2.4}

The ACs' chemical functional groups were identified based on the FTIR
spectra, obtained using a Perkin Elmer Two FTIR Spectrophotometer, from
Perkin Elmer, USA, by applying the KBr disc
method with a resolution of $4~\text{cm}^{-1}$ and 20 scans between
4000 and $450~\text{cm}^{-1}$.

\subsubsection{Determination of the point of zero charge} \label{2.2.5}

The $\text{pH}_{\mathrm{zpc}}$ was evaluated by first suspending
a mass of 7\% (w/v) of AC in solutions of
$\text{HNO}_3$, NaOH or $\text{NaNO}_3$ (Riedel). The suspensions were
maintained for 2 days in a thermostatic stirring bath at room
temperature. Then, they were filtered, and the pH of the solution was
measured, to identify the $\text{pH}_{\mathrm{zpc}}$ of ACs.

\subsection{Herbicides removal from the aqueous phase} \label{sec2.3}

Two herbicides were used. The first is
2-Methyl-4-chlorophenoxyacetic acid
(MCPA---$\text{C}_9\text{H}_9\text{ClO}_3$) (Sigma), which is a
powerful and widely used hormone-selective systemic herbicide, with a
water solubility of $825~\text{mg}{\cdot} \text{L}^{-1}$ at
298~K~\cite{24}. The second herbicide is 2,4-Dichlorophenoxyacetic acid
(2,4-D---$\text{C}_8\text{H}_6\text{Cl}_2\text{O}_3$) (Sigma), which is
an herbicide belonging to the phenoxy acetic acid family, with a water
solubility of $660~\text{mg}{\cdot}\text{L}^{-1}$ at 298~K~\cite{25}.

In an Erlenmeyer, 10~mg of AC were added to 25~mL of an aqueous
solution of MCPA or 2,4-D, with concentrations varying between 5 and
$250~\text{mg}{\cdot}\text{L}^{-1}$. To study the kinetics of MCPA and
2,4-D removal from the liquid phase, the suspensions were kept under
agitation in a thermostatic shaker bath, for a period varying between
15~min and 48~h and at 22~{\textdegree}C, 30~{\textdegree}C and
40~{\textdegree}C. The effect of pH of the solution on maximum
adsorption capacity of the ACs was also evaluated, at pH solutions
ranging from 1.7 to~11.2.

To quantify the herbicide present in the solution, after different
equilibrium times, small aliquots were collected, filtered with a nylon
membrane filter of $0.45~\upmu$m (Filtres Fioroni, France) and analyzed
by UV/Vis spectroscopy, using a UV--Vis spectrophotometer, Nicolet
Evolution 300. The quantification was done at a wavelength of 228 and
280~nm, for MCPA, as well as 230 and 285~nm for 2,4-D, respectively.
Both herbicides were quantified employing an external pattern and the
amount ${Q}_{\mathrm{ads}}$ ($\text{mg}{\cdot}\text{g}^{-1}$) of
the adsorbed herbicide was determined applying the following equation:
{\begin{equation}\label{eq1}
{Q}_{\mathrm{ads}}=V(C_{0}-C_{\mathrm{eq}})/m
\end{equation}}\unskip
$C_0$ and $C_{\mathrm{eq}}$ are respectively the initial and
equilibrium herbicide concentrations ($\text{mg}{\cdot}
\text{L}^{-1}$), $V$ is the \mbox{volume} of the solution (L) and $m$
denotes the mass of the dry AC~(g).

\subsection{Potential use of the activated carbons as a solid fuel}
\label{sec2.4}
\subsubsection{Elemental analysis} \label{sec2.4.1}

The elemental analysis, which determines the percentages of
carbon, hydrogen, sulphur, nitrogen and oxygen present in each AC, was
carried out by the Servicio de Apoyo {a} l'Investigaci\'{o}n, at the
Servicio de An\'{a}lisis Elemental y Molecular of the University of
Extremadura. Other elements, present on the ACs surfaces, such as
potassium, iron and calcium, were identified by SEM-EDX.

\subsubsection{Higher calorific value (HCV) and energy{\newline}
content} \label{sec2.4.2}

The HCV of the ACs was determined by using elemental analysis and
applying the following equation~\cite{29}:
{\begin{eqnarray}
\text{HCV}\ ({\text{MJ}}/{\text{kg}}) &=& -1.3675+0.1317
\times \%\text{C}\nonumber\\
&& +\, 0.7009\times \% \text{H}+0.0318\times \%\text{O} \label{eq2}
\end{eqnarray}}\unskip
where \%C, \%H, \%O are the percentage (wt) of C, H and O present in
the sample and determined by EA.

\section{Results and discussion} \label{sec3}
\subsection{Characterization of the ACs} \label{sec3.1}
The char and the
ACs were produced on a semi-industrial pilot horizontal furnace, with a
rotative system, at the University of \'{E}vora, Portugal. Such furnace
allows producing a large amount of AC in each batch. Although the pilot
furnace is still in the early stages of operation, it plays an
important role in improving the industrialization of the ACs
\mbox{production}.{\looseness=-1}

During the physical activation with $\text{CO}_2$ or airflow, the
burn-off of the produced ACs increased while the yield decreased with
the rise of the activation time. Concerning the ACs prepared by
chemical activation, with KOH, the yield decreased with the
augmentation of the activation temperature. However, the yield was
always higher than that obtained with physical activation, as presented
in Table~\ref{tab1}.

\subsubsection{Textural characterization of the produced{\newline}
activated carbons} \label{sec3.1.1}

Figure~\ref{fig2} shows the nitrogen adsorption isotherms, obtained at
77~K for the carbonized sample (char) and ACs prepared from PET waste
by physical activation with airflow and $\text{CO}_2$ at 1073~K, or by
chemical activation with KOH at different temperatures. The textural
parameters obtained from the analysis of the~$\text{N}_2$ isotherms,
provided by applying the Brunauer--Emmelt--Teller (BET) and
Dubinin-Radushkevich (DR) equation and alfa-s method, are included in
Table~\ref{tab1}.

\begin{figure}
\includegraphics{fig02}
{\vspace*{-.2pc}}
\caption{\label{fig2}Nitrogen adsorption isotherms obtained, at 77~K,
of different ACs prepared, from waste PET, by chemical activation with
KOH, and by physical activation with $\text{CO}_2$ and airflow. Open
symbols---adsorption, filled symbols---desorption.}
{\vspace*{-.4pc}}
\end{figure}

\begin{table*} %tab1
\fontsize{9}{9.9}\selectfont
\tabcolsep=3pt
\caption{\label{tab1}Textural parameters of carbonaceous samples
obtained from the analysis of the nitrogen adsorption isotherms\vspace*{-4pt}}
\begin{tabular}{ccccccccccc}
\thead
& & \multicolumn{5}{c}{Physical activation} &&
\multicolumn{3}{c}{Chemical activation}\\\cline{3-7}\cline{9-11}
& Carbonisation&
\multicolumn{2}{c}{$\text{CO}_2$} && \multicolumn{2}{c}{Air} &&
\multicolumn{3}{c}{PET/KOH 1:1}\\
\cline{3-4}\cline{6-7}\cline{9-11} && \multicolumn{5}{c}{Activation
time (min)} && \multicolumn{3}{c}{Temperature (K)}\\
\cline{3-7}\cline{9-11} && 60 & 120 && 15 & 240 && 723 & 873 & 1073\\
\endthead
Sample name & Char &
\parbox[t]{3.3pc}{PET-860-$\text{CO}_2$} &
\parbox[t]{3.5pc}{PET-8120-$\text{CO}_2$} &&
\parbox[t]{3.3pc}{PET-815-Air} &
\parbox[t]{3.3pc}{PET-8240-Air} &&
\parbox[t]{3.3pc}{PET-KOH-450} &
\parbox[t]{3.3pc}{PET-KOH-600} &
\parbox[t]{3.3pc}{PET-KOH-800}\\
Yield (\%) & 19.1\0\0 & 18.8\0\0 & 18.5\0\0 && 15.9\0\0 & 14.4 && 27.0\0\0 & 22.5\0\0 & 20.2\0\\
$A_{\mathrm{BET}}~(\text{m}^2{\cdot}\text{g}^{-1})$ & 4.3\0 & 20.2\0\0 & 77.8\0\0 && 25.2\0\0 & 621.2\0 && 16.6\0\0 & 50.2\0\0 & 732.7\0\0\\
$A_s~(\text{m}^2{\cdot}\text{g}^{-1})$ & 27.3\0\0 & 42.2\0\0 & 38.9\0\0 && 25.9\0\0 & 22.7 && 26.0\0\0 & 98.6\0\0 & 93.2\0\\
$V_s~(\text{cm}^3{\cdot}\text{g}^{-1})$ & \00.001 & \00.001 & \00.039 && \00.022 & \0\00.26 && \00.004 & \00.019 & \00.31\\
$V_0~(\text{cm}^3{\cdot}\text{g}^{-1})$ & --- & \00.004 & \00.002 && \00.007 & \0\00.17 && \00.005 & \00.007 & \00.21\\
$\text{L}_0~(\text{nm})$ & --- & --- & --- && 4.6\0 & \0\01.18 && --- & --- & \02.86\\
$\text{pH}_{\mathrm{zpc}}$ && 9.13 & 8.81 && 8.36 & \0\09.28 && 7.77 & 8.17 & \08.34
\botline
\end{tabular}
\xxtabnote{${A}_{\mathrm{BET}}$ ($\text{m}^2{\cdot} \text{g}^{-1}$):
BET surface area, ${A}_s$ ($\text{m}^2{\cdot}\text{g}^{-1}$):
Apparent surface area, $\text{L}_0$ (nm): mean pore size, $V_s$
($\text{cm}^3{\cdot}\text{g}^{-1}$): porous volume, $V_0$
($\text{cm}^3{\cdot}\text{g}^{-1}$): micropore volume,
$\text{pH}_{\mathrm{ZPC}}$: pH at the point of zero charge.
{\vspace*{-.6pc}}}
\end{table*}

The char presents low affinity and adsorption capacity for $\text{N}_2$
molecules which does not exceed $2~\text{mmol}{\cdot}\text{g}^{-1}$ and
a low specific surface area of $4.3~\text{m}^2{\cdot}\text{g}^{-1}$.
The isotherms of ACs prepared by physical activation shows a
hysteresis cycle that extends to a low relative pressure. This
hysteresis cycle was more evident in samples having lower volume and
narrow mean pore size (PET-860-$\text{CO}_2$, PET-8120-$\text{CO}_2$,
PET-815-Air, PET-KOH-450-1:1 and PET-KOH-600-1:1). However, a similar
hysteresis cycle was presented for PET-8240-Air, which confirms the
presence of narrow pores, whose average dimension was 1.18~nm.

The physical activation with airflow seems more effective than that
with $\text{CO}_2$: at the same temperature, the burn-off
obtained with airflow after 15~min was superior to the burn-off
with $\text{CO}_2$ after 60~min. Obviously, the increase in
the activation time favoured the development of textural parameters,
producing AC with a surface area of
$621.2~\text{m}^2{\cdot}\text{g}^{-1}$ and a pore volume of
$0.26~\text{cm}^3{\cdot}\text{g}^{-1}$ (PET-8240-Air), after an
activation time equal to four hours.

As far chemical activation is concerned, and with the same ratio of PET
and KOH (1:1), the apparent surface area and porous volume increased
significantly with the rise in the activation temperature. The AC
produced at 1073~K (PET-KOH-800-1:1) exhibited an apparent surface area
of $732.7~\text{m}^2{\cdot}\text{g}^{-1}$ and a porous volume of
$0.31~\text{cm}^3{\cdot}\text{g}^{-1}$. In some previous
works~\cite{9,10}, authors published data about the production of ACs
from recycled PET. However, the carbon materials produced were
obtained in a small furnace, where all parameters were judiciously
controlled. The use of this pilot furnace allowed obtaining a higher
amount of AC than that provided on a laboratory scale. Whereas
the time spent for
each batch was similar in both situations, the amount of the
monthly-produced ACs was about 100 times higher than that produced with
the laboratory furnace, which reduced the AC final price production.

\subsubsection{SEM-EDX spectroscopy} \label{sec3.1.2}

The ACs were analysed based on SEM-EDX spectroscopy. Figure~\ref{fig3}
shows some representative images of the SEM and EDX micrographs
obtained for char and ACs. It was difficult to identify a porous
structure on the surface of the char sample, which is in accordance
with the low micro-pore volume obtained from the $\text{N}_2$ isotherm.
Two different areas were identified on the PET-8240-Air. The area in
the foreground corresponds to a porous structure, while in the background
the AC seems completely compact without any identified porous
structure. For PET-KOH-800-1:1, the activation allowed larger porous
structure.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}SEM and EDX micrographs of char and ACs (each mark
corresponds to: (a)~$40~\upmu$m, (b) and (c)~$50~\upmu$m.}
\end{figure*}

It is clear that potassium was present in the ACs produced by chemical
activation with KOH. The ACs activated with $\text{CO}_2$ contains
also a small amount of potassium. In the ACs activated with airflow,
some heteroatoms (e.g. iron, silicon, aluminium, chromium and calcium)
were present. The presence of these impurities was attributed to the
low quality of the airflow used in the activation process.


\subsubsection{Characterization of the activated carbon by FTIR}
\label{sec3.1.3}

The FTIR spectra of the ACs prepared by chemical activation with KOH
and physical activation with airflow at 1073~K are presented in
Figure~\ref{fig4}. The stretching vibrations of OH groups (alcohols,
phenols and carboxylic acids) present on aromatic rings were observed
between 3000 and $3500~\text{cm}^{-1}$~\cite{27}. The stretching and
deformation of $\text{NH}_2$ and N--C--N bonds were identified in the ranges
3200--$3400~\text{cm}^{-1}$ and 570--620 and $1140~\text{cm}^{-1}$--$1190~\text{cm}^{-1}$,
respectively. The C--H
stretching bands at about 2925 and $2852~\text{cm}^{-1}$ were present
with different intensities for both samples~\cite{27}. The absorption
band noticed at 1600 and $1623~\text{cm}^{-1}$ corresponds,
respectively, to the C${=}$C and C${=}$O stretching vibrations of the carbonyl
groups near the $\text{NH}_2$ group. However, the bands at 1034 and
$1106~\text{cm}^{-1}$ are associated with C--O stretching vibrations.
The bands appearing at $1600~\text{cm}^{-1}$ refer to the axial
deformation of the carbonyl groups~\cite{17}, while those at
$1400~\text{cm}^{-1}$ correspond to
C${=}$N
stretching and C--O stretching of the carboxyl groups and
$\text{CH}_2$
bending~\cite{17,27}. The absorption bands at 656--$568~\text{cm}^{-1}$
and $685~\text{cm}^{-1}$ show
N--H
bending out of the plan and the ring deformation in the plane,
respectively.

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}FTIR spectra of PET-KOH-800-1:1 and PET-8240-Air.}
{\vspace*{-.3pc}}
\end{figure}

Some thermally-stable aromatic groups, such as pyrrole and
pyridinic-like functionalities, were identified, in overlap with the
bands corresponding to the stretching mode associated to C--N, C${=}$N, on
the profile of IR spectra around 1034--$1106~\text{cm}^{-1}$.

\subsubsection{Determination of the pH at the point of zero charge}
\label{sec3.1.4}

The pH at the point of zero charge ($\text{pH}_{\mathrm{zpc}}$) of the
ACs was assessed and the values are included in
Table~\ref{tab1}. The ACs produced by physical activation have a more
basic character ($\text{pH}_{\mathrm{zpc}}$ from 8.36 to~9.28),
compared to the ACs prepared with chemical activation
($\text{pH}_{\mathrm{zpc}}$ from 7.77 to~8.34).

\subsubsection{Characterization of the activated carbon by XRD}
\label{sec3.1.5}

PET-8240-Air and PET-KOH-800-1:1 were characterized by X-ray
diffraction and the spectra are presented in Figure~\ref{fig5}. The
spectra have the same global aspect. In the range between 10{\textdegree} and
50{\textdegree}, both profiles exhibit a very broad diffraction peak
and the absence of a sharp peak reveals a predominantly amorphous
structure. The XRD pattern exhibits two broad peaks at
${\sim}$25{\textdegree} (attributed to 002 plane) and 42{\textdegree}
(attributed to 100 plane), resulting from the stacking of some
graphene-like layers in both ACs.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Diffractogram of PET-KOH-1:1-800 and
PET-8240-Air.}
{\vspace*{4pt}}
\end{figure}

\subsection{The use of the activated carbons to remove herbicides from
the liquid phase} \label{sec3.2}

The efficiency of the ACs presenting the highest surface area and
micro-pore volume in removing MCPA and 2,4-D present on the liquid
phase was tested. The influence of pH, contact time and
temperature on the herbicide removal was also studied.

\subsubsection{MCPA and 2,4-D adsorption} \label{sec3.2.1}

Figure~\ref{fig6} shows the performance of the different ACs in
removing MCPA and 2,4-D from the liquid phase. The latter reveals that
2,4-D maximum adsorption capacity varies from
$30~\text{mg}{\cdot}\text{g}^{-1}$ for PET-KOH-600 to
$290~\text{mg}{\cdot}\text{g}^{-1}$ for PET-KOH-800-1:1. However, the
maximum adsorption capacity (${Q}_{\mathrm{max}}$) of MCPA ranges from
$70~\text{mg}{\cdot}\text{g}^{-1}$ for PET-8120-$\text{CO}_2$ to
$192~\text{mg}{\cdot}\text{g}^{-1}$ for PET-KOH-1:1-800. The \mbox{difference}
in the maximum adsorption capacities displayed by various ACs
highlights the importance of their \mbox{textural} properties. In the
presence of a well-developed porous volume with an average pore size
accessible to the herbicides, a favourable pH of the solution can
improve the performance of the ACs.\looseness=-1

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}2,4-D and MCPA isotherms adsorption onto different
prepared ACs.}
\end{figure}

The comparison of the ${Q}_{\mathrm{max}}$ of PET-KOH-800-1:1 with some
adsorbents cited in the literature is presented in Table~\ref{tab2}. It
is interesting to report that the ACs prepared from waste PET provided
the best ${Q}_{\mathrm{max}}$. It is also obvious that waste PET plastic
could be a good precursor for the preparation of adsorbent to treat
agriculture wastewater.

\begin{table*}[p!] %tab2
\fontsize{9}{10}\selectfont
\tabcolsep=3pt
\caption{\label{tab2}Comparison of the maximum adsorption capacity
(${Q}_{\mathrm{max}}$) of 2,4-D and MCPA in ACs prepared during this
work with data found in the literature}
\begin{tabular}{ccccc}
\thead
Adsorbent & Herbicide & Experimental condition & ${Q}_{\mathrm{max}}$
($\text{mg}{\cdot}\text{g}^{-1}$) & Ref.\\
\endthead
\morerows{1}{PET-8120-$\text{CO}_2$} & MCPA &
\morerows{5}{\parbox[t]{5cm}{\centering Physical or chemical
activation of waste PET, done at 1073~K}} & 48.8 & \morerows{5}{Our
study}\\
& 2,4 D && 120 &{\vspace*{2pt}}\\
\morerows{1}{PET-KOH-800-1:1} & MCPA && 192 &\\
& 2,4 D && 290 &{\vspace*{2pt}}\\
\morerows{1}{PET8240 AIR} & MCPA && 41 &\\
& 2,4 D & & 75 &{\vspace*{8pt}}\\

Polyaniline-$\text{Fe}_3\text{O}_4$ & 2,4D & \morerows{1}{Solvothermal
method} & 60.97 & \morerows{1}{\cite{32}}\\
Polypyrrole-$\text{Fe}_3\text{O}_4$ & 2,4D & & 96.15 &{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Graphene oxide coated with porous iron
oxide} & 2,4D & Solvothermal process & 67.26 &
\cite{33}{\vspace*{8pt}}\\

Cotton plant ash & 2,4D & Thermal process & 0.64 &
\morerows{1}{\cite{34}}{\vspace*{5pt}}\\
Cotton plant char & 2,4D && 3.93 &{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Activated charcoal decorated with
$\text{Fe}_2\text{O}_3$ nanoparticles} & 2,4D & Green synthesis &
255.10 & \cite{35}{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Nano-activated carbon derived from palm
coir waste} & 2,4D & \parbox[t]{5cm}{\centering Single-step
carbonization-KOH activation approach} & 50.25 &
\cite{36}{\vspace*{8pt}}\\

Corn cob biochar & 2,4D & 600~{\textdegree}C for 4 h\,+\,HF
modification & 37.40 & \cite{37}{\vspace*{8pt}}\\

Groundnut shell & 2,4D & Combustion & 0.87 &
\morerows{2}{\cite{38}}\\
Groundnut shell biochar & 2,4D & Pyrolysis & 3.02 &\\
Groundnut shell AC & 2,4D & Chemical activation & 250 &{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Carbonaceous materials
C--$\text{C}_2\text{Cl}_6$ and C--$\text{C}_6\text{Cl}_6$} & 2,4D &
\parbox[t]{6cm}{\centering Combustion of the mixtures containing
hexachloroethane or hexa-chlorobenzene with sodium azide} & 895 &
\cite{39}{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Polymer-based activated carbon-polyvinyl
alcohol} & 2,4D && 55.9 & \cite{40}{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Commercial granular activated carbon
Filtersorb 300 (GAC F300)} & 2,4D & Commercial & 181.82 &
\cite{41}{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Iron doped graphitic carbon cotton
nanostructures} & 2,4D & \parbox[t]{5cm}{\centering Impregnation with
a $\text{FeCl}_36\text{H}_2\text{O}$ + microwave irradiation} & 33 &
\morerows{1}{\cite{42}}{\vspace*{2pt}}\\
\parbox[t]{4cm}{\centering Iron doped graphitic carbon filter paper
nanostructures} & 2,4D & \parbox[t]{5cm}{\centering Impregnation with
a $\text{FeCl}_36\text{H}_2\text{O}$ + microwave irradiation} & 77
&{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Activated carbons from wood composites} &
MCPA & Physical activation with carbon dioxide & 413 &
\cite{43}{\vspace*{8pt}}\\

Switchgrass biochar & MCPA & Fast pyrolysis switchgrass biochar & 50 &
\cite{44}{\vspace*{8pt}}\\

\parbox[t]{4cm}{\centering Organic polymer resin Lewatit VP OC
1163}\vspace*{2pt} & MCPA & Commercial & 92.55 & \cite{45}
\botline
\end{tabular}
\end{table*}

\subsubsection{Influence of the temperature on the efficiency of
different ACs to remove MCPA and 2,4-D from the liquid phase}
\label{sec3.2.2}

The influence of the temperature on 2,4-D and MCPA adsorption was
evaluated. The amounts of herbicides adsorbed by each AC at different
equilibrium concentrations are presented in Figure~\ref{fig7}. It is
clear that the increase in temperature had a slight influence on the
efficiency of the ACs in removing 2,4-D. However, the adsorption of
MCPA can be considered as an endothermic process. Jui {\etal} concluded
that the adsorption of Swiss blue by Acacia nilotica was an endothermic
process~\cite{28}.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Temperature influence on the MCPA and 2,4-D
adsorption on PET-KOH-1:1-800.}
{\vspace*{-2pt}}
\end{figure}

\subsubsection{Influence of pH} \label{sec3.2.3}

To study the interaction between ACs and both herbicides when pH of
the solution changes,{\break} it is necessary to compare the
$\text{pH}_{\mathrm{zpc}}$ of different adsorbents with the pKa
of herbicides. The $\mathrm{pH}_{\mathrm{zpc}}$ is defined as the pH
value where, in the liquid, the surface charge of the adsorbent is
about zero. At a pH solution lower than
$\mathrm{pH}_{\mathrm{zpc}}$, the adsorbent
surface is positively charged and at a pH solution greater than
$\text{pH}_{\mathrm{zpc}}$, the AC surface has a net negative charge.
The $\mathrm{pH}_{\mathrm{zpc}}$ value of PET-KOH-1:1-800 is equal to
8.34. The (pKa) of a chemical compound determines the degree of
ionization of the molecule at different pH~solutions.

For pH solution higher than pKa value, the compound exists
predominantly in an anionic form. The pKa values of the MCPA and 2,4~D
are 3.1 and 2.73, respectively. The adsorption trends of both
herbicides on ACs, as a function of the pH solution, are similar, as
presented in Figure~\ref{fig8}. The latter reveals that the adsorption
of MCPA and 2,4~D decreases rapidly in the pH region between 2--4. The
highest adsorption capacity values are attained at pH almost equal
to~2. At this value, at which the neutral form of the two herbicides is
predominant, the surface charge of the AC is positive. At pH solution
near~2, the interaction \mbox{between} the neutral forms of both herbicides
and ACs are more favourable. The decrease in the maximum adsorption
values at pH above 7 is due to electrostatic repulsion between the
negatively-charged adsorbent surface ($\text{pH} >
\text{pH}_{\mathrm{zpc}}$) and the anionic form of the herbicide
molecules ($\text{pH} > \text{pKa}$).

\begin{figure}
\includegraphics{fig08}
{\vspace*{4pt}}
\caption{\label{fig8}Influence of pH on MCPA and 2,4 D adsorption onto
PET-KOH-1:1-800.}
\end{figure}

\subsubsection{Kinetic study} \label{sec3.2.4}

The adsorption kinetics of MCPA and 2,4-D on PET-KOH-1:1-800 is
presented in Figure~\ref{fig9}. It is obvious that, in the first
minutes of contact, the adsorption of both herbicides is rapid and the
equilibrium is reached after approximately 7~h. It is also obvious that
the adsorption equilibrium is achieved for all considered
concentrations after a contact time of 24~h between 2,4-D and the
adsorbent. In the same experimental conditions, the adsorption capacity
of PET-KOH-800-1:1 is lower for 2,4-D compared to MCPA, which
highlights the slower adsorption kinetics for 2,4-D at 298~K. The MCPA
isotherm shape suggests a monolayer coating of the activated carbon
surfaces.

\begin{figure}
\includegraphics{fig09}
{\vspace*{-.3pc}}
\caption{\label{fig9}Kinetic study of MCPA adsorption, with different
initial concentrations, on PET-KOH-1:1-800 at pH~$=$~6.5.}
\end{figure}

Nevertheless, the high removal of both herbicides, mainly from diluted
solutions, in a short period, presented by this AC allows removing
herbicides from the liquid phase more economical. On the other hand,
the optimisation of the nitrogen flow, activation temperature, and
activation time during the ACs production improves the control of the
textural characteristics of the ACs produced. As some of these
\mbox{parameters} can still be adjusted, ACs more efficient in removing
herbicides from the liquid phase may be obtained.

\paragraph*{Kinetic modeling}

Herbicides adsorption onto activated carbons could be modelled by the
first-order, second-order and Elovich models.

\paragraph*{Pseudo first order kinetic model}

The integrated form of the pseudo first order kinetic model is
expressed by the following equation:
{\begin{equation} \label{eq3}
\text{Ln}(q_e-q_t)=\text{Ln}\,q_e-k_1t
\end{equation}}\unskip
where $k_1$ is the pseudo-first-order rate constant
($\text{h}^{-1}$), $q_t$ represents the quantity of herbicide
adsorbed at time $t$ and $q_e$ is the quantity
($\text{mg}{\cdot}\text{g}^{-1}$) adsorbed at equilibrium and
saturation.

\paragraph*{The pseudo second-order model}

The integrated pseudo-second order model is represented by the
following equation:
{\begin{equation}\label{eq4}
\frac{t}{q_t}=\frac{1}{k_2\cdot q_e^2}+t/q_{e}
\end{equation}}\unskip
where $k_{2}$ denotes the pseudo second order rate constant
($\text{h}^{-1}$).

\paragraph*{Elovich model}

The integrated form of this model is expressed using the following
equation:
{\begin{equation}\label{eq5}
q_t=\frac{1}{\beta}\cdot\text{Ln}\,\alpha\beta+\frac{1}{\beta}\ln\ln(t)
\end{equation}}\unskip
where $\alpha$ ($\text{mg}{\cdot}\text{g}^{-1}{\cdot}\text{h}^{-1}$)
is the initial adsorption rate and $\beta$ ($\text{mg}{\cdot}
\text{g}^{-1}$) designates the desorption \mbox{constant}.

Figure~\ref{fig10} presents the fitting of the results provided by the
kinetic models (pseudo first-order and second-order model and Elovich
model) while \mbox{Table}~\ref{tab3} resumes the kinetic parameters for
the different models. For the pseudo first-order model, the
values of $q_e$ obtained experimentally are not in agreement with those
calculated, which shows the inapplicability of the model in herbicides
adsorption ACs. On the other hand, the pseudo second-order model
fits well with high~$R^2$. The resulting $q_e$ values are closer to the
calculated $q_e$ than those obtained by the pseudo-first order model.
It can be also noticed that the Elovich model is efficiently employed
to describe chemisorption on highly heterogeneous adsorbents. $R^{2}$
coefficients of Elovich model are relatively lower, compared to those
of the second order model, but they are better than those provided by
the first-order model. In addition, it can be seen that the values of
$\alpha$ and $\beta$ vary as a function of the initial MPCA and 2,4~D
herbicide concentration. In fact, with the increase in initial
herbicide concentration from 10 to $200~\text{mg}{\cdot}\text{L}^{-1}$,
$\alpha$ rises from 0.08 to 25.95 for MCPA, and from 1.09 to 8.03 for
2,4~D. On the other hand, $\beta$ decreases from 0.38 to 0.029 for
MCPA, and from 0.156 to 0.020 for 2,4~D. Thus, $\beta$ minimizes with
the increase of the herbicide initial concentrations and~$\alpha$. The
same observations were reported in the literature~\cite{43,44}. This
trend in $\alpha$ and $\beta$ reflects the enhancement of the reaction
rate with the increase in initial concentration~\cite{45}.\looseness=1

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig10}
{\vspace*{1pt}}
\caption{\label{fig10}Kinetic modeling of MCPA and 2,4-D onto KOH-800.}
%{\vspace*{-5pt}}
\end{figure*}

\vspace{4pt}
\subsubsection{MCPA and 2,4-D adsorption isotherms}

MCPA and 2,4-D adsorption isotherms are presented in
Figure~\ref{fig11}. The isotherms were analysed by applying the
Langmuir and Freundlich equations used in several research
works~\cite{10,30,32}. The Langmuir equation was essentially applied to
isotherms obtained on homogeneous solids, where the adsorption was
limited to the monolayer. However, the Freundlich equation was mainly
employed to describe the adsorption on heterogeneous adsorbents where
the adsorption is non-linear. By fitting Langmuir and Freundlich
equations to the experimental data, the following parameters were
determined: $n_{mL}$ monolayer capacity, $K_L$ Langmuir constant,
$K_F$ Freundlich constant and $n_F$ Freundlich exponent. As some
produced ACs had a less developed porous {structure,} the adsorption
isotherms of MCPA and 2,4-D were more irregular, due to the constraints
of the herbicides to access the interior of the pores, and the fit
to the Langmuir and Freundlich equations were more difficult to
obtain. In~most of the MCPA-adsorbent systems, the adjustment to the Langmuir
equation was normally better. The MCPA isotherm obtained on
\mbox{PET-KOH-800-1:1} after a contact time of 24~h shows that the parameters
provided by the Langmuir fit were
$n_{mL}=232.6~\text{mg}{\cdot}\text{g}^{-1}$ and $K_{L}=
0.049~\text{dm}^3{\cdot}\text{mol}^{-1}$. However, those given by
Freundlich fit were $n_{F}=1.74$ and $K_{F} =
217~\text{mg}{\cdot}\text{g}^{-1}\
(\text{dm}^3{\cdot}\text{mg}^{-1})^{1/n}_{F}$ and the maximum
adsorption capacity inferred directly from the isotherm was
$n_{\mathrm{max}}=196.9~\text{mg}{\cdot}\text{g}^{-1}$. On the other
hand, the 2,4-D isotherm obtained on PET-KOH-800-1:1 after a contact
time of 24~h demonstrates that the parameters provided by the Langmuir
fit were {$n_{mL}= 192.3~\text{mg}{\cdot}\text{g}^{-1}$} and
$K_{L} = 0.26~\text{dm}^{3} {\cdot}\text{mol}^{-1}$. However,
those given by Freundlich fit were $n_{F} = 3.38$ and $K_F =
314.3~\text{mg}{\cdot}\text{g}^{-1}\ (\text{dm}^3{\cdot}
\text{mg}^{-1})^{1/n}_{F}$ and the {maximum} adsorption capacity obtained
directly from the isotherm was $n_{\mathrm{max}}=
267.1~\text{mg}{\cdot}\text{g}^{-1}$.\looseness=1

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}Adsorption isotherms of MCPA (left) and 2,4-D
(right) were obtained after different equilibrium times.}
{\vspace*{4pt}}
\end{figure*}

\begin{table*}[t!] %tab3
\fontsize{9.5}{12}\selectfont
\tabcolsep=3pt
\caption{\label{tab3}Comparison between the experimental and calculated
$q_e$ values of first, second-order adsorption kinetics and at different
initial concentrations (10--200~ppm) of 2,4-D and MCPA{\vspace*{-2pt}}}
\begin{tabular}{cccccccccccccc}
\thead
\raisebox{3pt}{\xmorerows{1}{Pollutant}} &
\raisebox{3pt}{\morerows{1}{\parbox[t]{2cm}{\centering
Initial \mbox{concentration} (ppm)}}}\vspace*{2pt} &
\morerows{1}{\raisebox{-20pt}{$k_1$ ($\text{h}^{-1}$)}} &
\multicolumn{2}{c}{\parbox{2cm}{\centering\raisebox{.6pc}{}First-order
kinetic model}}\vspace*{2pt} &&
\multicolumn{4}{c}{\raisebox{.3pc}{Second-order kinetic model}} &&
\multicolumn{3}{c}{\raisebox{.3pc}{Elovich model}}\\
\cline{4-5}\cline{7-10}\cline{12-14}
&& & $q_e$ (cal) & $R^2$ && $q_e$ (cal) & $q_e$
(exp) & $k_2$ (h/mg) & $R^2$ && $\alpha$ & $\beta$ & $R^2$\\
\endthead
\morerows{5}{MCPA} & \010 & 0.1\0\0 & 8 & 0.96 && \011.11 & \010 & 0.02\0\0 & 0.990 && \00.08 & 0.38\0\0 & 0.98\\
& \025 & 0.23\0 & \023.6\0 & 0.92 && \047.62 & \045 & 0.018\0 & 0.999 && \01.96 & 0.14\0\0 & 0.93\\
& \050 & 0.18\0 & \065.8\0 & 0.97 && 100 & \086 & 0.004\0 & 0.998 && 10.28 & 0.058\0 & 0.99\\
& \075 & 0.11\0 & \096.5\0 & 0.88 && 125 & \095 & 0.001\0 & 0.996 && 21.76 & 0.034\0 & 0.99\\
& 100 & 0.18\0 & 118 & 0.85 && 166.7\0 & 138 & 0.002\0 & 0.997 && 25.24 & 0.030\0 & 0.95\\
& 200 & 0.1\0\0 & \099.5\0 & 0.87 && 200 & 192 & 0.003\0 & 0.997 && 25.95 & 0.029\0 & 0.95{\vspace*{3pt}}\\
\morerows{5}{2,4-D} & \010 & 0.05\0 & \0\02.9\0 & 0.64 && \021.37 & 21.74 & 0.04\0\0 & 0.999 && \01.09 & 0.156\0 & 0.75\\
& \025 & 0.034 & \0\04.14\0 & 0.76 && \047.39 & 47.62 & 0.015\0 & 0.999 && \03.21 & 0.077\0 & 0.75\\
& \050 & 0.17\0 & \010.3\0 & 0.9\0 && 123.46 & 125 & 0.0013 & 0.97\0 && \03.41 & 0.029\0 & 0.96\\
& \075 & 0.1\0\0 & \0\09.02 & 0.78 && 138.89 & 142 & 0.0017 & 0.98\0 && \04.81 & 0.0224 & 0.94\\
& 100 & 0.045 & \0\08.17 & 0.97 && 172.41 & 200 & 0.0015 & 0.999 && \08.03 & 0.0201 & 0.94\\
& 200 & 0.21\0 & 134.3\0 & 0.97 && 289 & 333 & 0.0055 & 0.999 && 7.7 & 0.02\0\0 & 0.92
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

On the isotherms presented in Figure~\ref{fig10}, the adsorbent does
not attain its maximum \mbox{adsorption} \mbox{capacity.} The $n_{L}$ and $K_{F}$
values are very close, but lower than the maximum adsorbed amount
($n_{\mathrm{max}}$), obtained experimentally. PET-KOH-800-1:1 has an
apparent maximum adsorption capacity similar to the values obtained by
ACs commonly used in wastewater treatments. Thus, the achieved
maximum adsorption capacity is higher than the value indicated in
Table~\ref{tab2} since a well-defined step has not yet been \mbox{obtained.}

\begin{table*}[t!] %tab4
\fontsize{10}{11}\selectfont\tabcolsep=8pt
\caption{\label{tab4}ACs elemental composition{\vspace*{-3pt}}}
\begin{tabular}{cccccccc}
\thead
Samples & C/\% & H/\% & N/\% & O/\%${}^*$ & C/\% (EDX) & O/\% (EDX) &
$\text{pH}_{\mathrm{zpc}}$\\
\endthead
Carbonized & 87.9 ${\pm}$ 2.5 & 2.4 ${\pm}$ 0.1\0 & 0.02 & \09.68 & 87.9 ${\pm}$ 1.8\0 & 12.1 ${\pm}$ 1.8\0 & ---\\
PET-860-CO2 & 79.8 ${\pm}$ 0.9 & 1.3 ${\pm}$ 0.13 & 0.01 & 18.9\0 & 63.1 ${\pm}$ 3.7\0 & 30.6 ${\pm}$ 3.7\0 & 9.13\\
PET-8120-$\text{CO}_2$ & 79.9 ${\pm}$ 3.9 & 1.3 ${\pm}$ 0.4\0 & --- & 18.8\0 & 58.1 ${\pm}$ 5.2\0 & 35.9 ${\pm}$ 2.9\0 & 8.81\\
PET-815-Air & 80.8 ${\pm}$ 1.9 & 1.3 ${\pm}$ 0.03 & --- & 17.9\0 & 64.7 ${\pm}$ 2.3\0 & 25.6 ${\pm}$ 3.1\0 & 8.36\\
PET-8240-Air & 83.4 ${\pm}$ 1.1 & 1.0 ${\pm}$ 0.04 & --- & 15.6\0 & 61.7 ${\pm}$ 12.8 & 30.9 ${\pm}$ 7.8\0 & 9.28\\
PET-KOH-450-1:1 & 80.9 ${\pm}$ 2.5 & 3.0 ${\pm}$ 0.1\0 & --- & 16.1\0 & 69.7 ${\pm}$ 15.5 & 27.1 ${\pm}$ 12.9 & 7.77\\
PET-KOH-600-1:1 & 84.4 ${\pm}$ 1.4 & 2.8 ${\pm}$ 0.3\0 & --- & 12.8\0 & 74.4 ${\pm}$ 5.7\0 & 24.2 ${\pm}$ 5.7\0 & 8.17\\
PET-KOH-800-1:1 & 82.9 ${\pm}$ 1.9 & 2.0 ${\pm}$ 0.4\0 & --- & 15.1\0 & 79.9 ${\pm}$ 3.7\0 & 19.2 ${\pm}$ 3.5\0 & 8.34
\botline
\end{tabular}
\tabnote{${}^*$~The $\text{O}_2$ percentage was determined by
difference to 100\%.
{\vspace*{-10pt}}}
\end{table*}

\subsection{Application of the ACs as a solid fuel}
\subsubsection{Ultimate analysis of the ACs}

Char and different ACs samples were chemically characterised by
elemental analysis and the C, H, S, N and O percentages were
determined. The two major elements identified directly were carbon and
hydrogen. The N was present in trace amounts and the S was not
quantified, as presented in Table~\ref{tab4}. The latter shows that C
percentage ranges from 79.8 to 84.4\% and the hydrogen varies between
1.0 to 3.0\%. Using physical activation, the carbon increment can
be explained by the increase in activation time and the appearance
of C${=}$N and C--O functional groups at the surface of the carbonized
sample. These groups are useful, especially in catalysis and
adsorption \mbox{applications}.\looseness=-1

\break
The carbon and oxygen contents at the char and ACs external surface
were also evaluated by EDX. For the char sample, only a slight
difference was observed between the EDX and EA results. \mbox{However,} EDX
provided significantly lower carbon percentage and higher oxygen
percentage, compared to EA. This difference was clearer for ACs
prepared by \mbox{physical} activation. It is well known that the
analysis performed through elemental analyzers is based on the
combustion of the sample and the analysis of the corresponding evolved
gas products~\cite{46}. For example, carbon, hydrogen, nitrogen, and
sulfur elements release $\text{CO}_2$, $\text{H}_2\text{O}$, NOx and
$\text{SO}_2$,\break respectively.

It is worth noting that the elemental analysis conducted in EXD
technique is limited to the detection of the functional groups at the
surface of the samples. Thus, the results obtained by applying EDX and
EA prove that the functional groups containing O were mainly localised
at the external surfaces and were introduced during the activation
process. More evidence about the nature of these groups can be drawn
from FTIR analysis.

\break
The preparation of the ACs by chemical activation with KOH
demonstrates that, as the activation temperature increased, the
difference between the carbon and oxygen content, on the external
surface area and AC matrix, was less observed. This result demonstrates
the remarkable stability of the ACs prepared by chemical activation at
higher temperatures, which agrees with the findings of Borghei
{\etal}~\cite{17} and Chen {\etal}~\cite{18}.
%\vspace*{30pt}

\subsubsection{The Van Krevelen graph and energy content}

In general, the Van Krevelen graph is used to illustrate the elemental
composition results and
%{\vadjust{\break}}
\mbox{cross-plot} the H:C atomic ratios as a function
of the O:C atomic ratios of fuels~\cite{47}. A lower ratio of H/C and
O/C indicates the improvement in the energy potential of fuels since
low H/C and O/C ratios generate less $\text{CO}_2$, water vapor and
smoke. The molar ratios of H/C and O/C obtained for char and ACs are
presented in Figure~\ref{fig12}. It is well known that high
energy-content fuels, such as anthracite and bituminous coal,
show low H/C and O/C molar ratios due to the high degree of
coalification. On the other hand, low-rank fuels, such as biomass, show
the opposite H/C and O/C ratio trend~\cite{48}. It is obvious that
carbonaceous materials with lower ratio of H/C and O/C are considered
as promising solid fuels~\cite{49}.

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig12}
{\vspace*{-3pt}}
\caption{\label{fig12}Van-Krevelen diagram of char and ACs.}
{\vspace*{-3pt}}
\end{figure*}

\subsubsection{Energy content and higher heating values}

The high heating values (HHV) of the char, produced ACs and
conventional solid fuels are included in Figure~\ref{fig12}
and their comparison is presented in Figure~\ref{fig13}. The HHV
obtained by different ACs are 11.89, 10.05, 10.06, 10.18, 10.32, 11.4
and 10.95~MJ/kg for char, PET-860-$\text{CO}_2$,
PET-8120-$\text{CO}_2$, PET-815-Air, PET-8240-Air,
PET-KOH-450, PET-KOH-600 and PET-KOH-800, respectively. The results
indicate that various prepared carbonaceous materials have comparable
energy content and are in the same range as the conventional solid
fuels, as shown in Figure~\ref{fig13}.

%\break
\section{Conclusion}

The present research is a contribution to the circular economy. It
highlights the importance of common urban waste valorization by
converting PET waste into ACs used as solid fuel or as adsorbents to
remove herbicides.

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig13}
{\vspace*{-1pt}}
\caption{\label{fig13}HHV comparison with different conventional solid
fuels.}
{\vspace*{-2pt}}
\end{figure*}


%\break
The activated carbons, produced in a lab-scale furnace by physical
activation with $\text{CO}_2$ or water vapor or by chemical
activation with KOH, were successfully used to remove herbicides from
liquid \mbox{effluents}. The ACs produced by physical or chemical
activation showed interesting porous development. However, the chemical
activation produced ACs with higher apparent surface area and pore
volume. Due to its important textural and chemical \mbox{characteristics,} the
PET-KOH-1:1-800 was employed as an adsorbent to remove MCPA and 2,4-D
from the liquid phase. For PET-KOH-1:1-800, the herbicides adsorption
was predominantly controlled by the total porous volume and the
apparent surface area. \mbox{PET-KOH-1:1-800} presented at 298~K a
maximum adsorption capacity of $192~\text{mg}{\cdot}\text{g}^{-1}$ and
290 for MCPA and 2,4-D, respectively. It must be noted that the
adsorption of both herbicides fits well the pseudo second order
kinetic.

Van Krevelen diagram revealed that the H/C and O/C molar ratios of char
and ACs were similar to those of sub-bituminous and bituminous
materials. From this study, it can be concluded that the use of PET
plastic as raw material for the ACs production has ecological and
economic benefits. This material can be used in the future for the
treatment of liquid effluents or as solid fuels instead of\break biomass.

\section*{Conflicts of interest}

Authors have no conflict of interest to declare.

\section*{Acknowledgements}

The authors gratefully acknowledge the Tunisian Ministry of Higher
Education and Scientific Research for funding this research under a
scholarship for an internship and to the Funda\c{c}\~{a}o para a
Ci\^{e}ncia e a Tecnologia (FCT, Portugal) national funds --
(PEst-OE/QUI/UI0619/2014).

\CDRGrant[FCT]{PEst-OE/QUI/UI0619/2014}

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