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\dateposted{2025-03-31}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Effectiveness of plant-based biosorbents in textile dye removal
from aqueous solutions: isotherm, kinetic, and thermodynamic studies}

\alttitle{Efficacit\'{e} des biosorbants d'origine v\'{e}g\'{e}tale
dans l'\'{e}limination des colorants textiles des solutions aqueuses :
\'{e}tudes isotherme, cin\'{e}tique et thermodynamique}

\author{\firstname{Abdelaziz} \lastname{Arroussi}\CDRorcid{0000-0002-0394-1928}\IsCorresp}
\address{Laboratory of Saharan Natural Resources, Faculty of Natural
and Life Sciences, University of Ahmed Draia, Adrar, Algeria}
\email[A. Arroussi]{arroussi.abdelaziz@univ-adrar.edu.dz}

\author{\firstname{Hamza} \lastname{Laksaci}\CDRorcid{0000-0001-5971-1606}}
\address{Laboratory of Energies, Environment and Information System,
Faculty of Materials Sciences, Mathematics and Informatics, University
of Ahmed Draia, Adrar, Algeria}

\author{\firstname{Mohammed} \lastname{Djaafri}\CDRorcid{0000-0001-9257-5378}}
\address{Unit\'{e} de Recherche en Energie Renouvelables en Milieu
Saharien (URER.MS), Centre de D\'{e}veloppement des Energies
Renouvelables (CDER), Adrar 01000, Algeria}

\author{\firstname{Ahmed} \lastname{Tahri}\CDRorcid{0000-0002-7684-5523}}
\addressSameAs{3}{Unit\'{e} de Recherche en Energie Renouvelables en Milieu
Saharien (URER.MS), Centre de D\'{e}veloppement des Energies
Renouvelables (CDER), Adrar 01000, Algeria}

\author{\firstname{Mohammed Amine} \lastname{Dahou}\CDRorcid{0000-0002-9495-8193}}
\addressSameAs{2}{Laboratory of Energies, Environment and Information System,
Faculty of Materials Sciences, Mathematics and Informatics, University
of Ahmed Draia, Adrar, Algeria}

\author{\firstname{Slimane} \lastname{Kalloum}\CDRorcid{0000-0001-7960-1156}}
\addressSameAs{2}{Laboratory of Energies, Environment and Information System,
Faculty of Materials Sciences, Mathematics and Informatics, University
of Ahmed Draia, Adrar, Algeria}

\author{\firstname{Salah} \lastname{Jellali}\CDRorcid{0000-0002-4095-4154}}
\address{Centre for Environmental Studies and Research, Sultan Qaboos
University, Muscat, Al-Khoud 123, Oman}

\author{\firstname{Mejdi}\nobreakauthor\lastname{Jeguirim}\CDRorcid{0000-0003-2401-5824}}
\address{The Institute of Materials Science of Mulhouse (IS2M),
University of Haute-Alsace, University of Strasbourg, CNRS, UMR 7361,
F-68100 Mulhouse, France}

\keywords{\kwd{Date palm petiole}\kwd{Biosorbent} \kwd{Textile dye}
\kwd{Adsorption} \kwd{Treatment}}

\altkeywords{\kwd{P\'{e}tiole de palmier dattier} 
\kwd{Biosorbant}
\kwd{Colorant textile} 
\kwd{Adsorption} 
\kwd{Traitement}}

\begin{abstract}
The pollution of water resources by hazardous contaminants is a major
obstacle to the provision of safe and potable water worldwide.
Cost-effective, innovative, renewable, and environmentally friendly
technologies are essential for wastewater treatment. In the current
work, the potential of using raw date palm petiole (DPP) for the
removal of Bezaktiv Marine S-BL (SBL) dye is investigated 
under a wide range of experimental conditions in batch mode.
Experimental results show that the removal of SBL dye  by DPP is highly
dependent on contact time, initial pH, initial dye concentration, and
temperature. The removal of SBL dye  is relatively rapid where the
equilibrium state is reached within a contact time of 120 min.
Moreover, the biosorbent exhibits high stability in SBL adsorption
ability over a wide pH range (4--12). Furthermore, the SBL dye
adsorption increases with increase  in initial concentration. 
The maximum adsorption capacity of the SBL dye on DPP is evaluated to be 
110 mg/g, which is much higher than numerous other materials. In addition, a
thermodynamic study indicates that the adsorption of SBL dye by DPP is
a feasible exothermic and spontaneous process. 

The findings of the present study indicate that untreated DPP wastes
can be considered  effective materials for the treatment of textile
dyes in wastewater in general and SBL in particular.
\end{abstract}

\begin{altabstract}
La pollution des ressources en eau constitue un  obstacle majeur et
mondial \`{a} l'approvisionnement en eau potable. La mise  en place de
technologies innovantes, rentables, renouvelables et  respectueuses de
l'environnement est devenue une t\^{a}che essentielle. Dans  ce
travail, le potentiel d'utilisation du p\'{e}tiole de palmier dattier 
brut (DPP) pour l'\'{e}limination du colorant Bleu Marine Bezaktiv S-BL
(SBL) a \'{e}t\'{e} \'{e}tudi\'{e} sous des conditions
exp\'{e}rimentales  vari\'{e}es en mode discontinu. Les r\'{e}sultats
exp\'{e}rimentaux montrent  que l'\'{e}limination du SBL par le DPP
d\'{e}pend essentiellement du temps  de contact, du pH initial, de la
concentration initiale du colorant et de la  temp\'{e}rature. En effet,
l'\'{e}limination du SBL est relativement rapide,  l'\'{e}tat
d'\'{e}quilibre \'{e}tant atteint en 120 min. De plus, le DPP  a
montr\'{e} une grande stabilit\'{e} de capacit\'{e} d'adsorption du SBL
dans une large gamme de pH (4--12). En outre, l'adsorption du SBL
augmente  avec l'augmentation de la concentration initiale, sa
capacit\'{e}  d'adsorption maximale par le DPP a \'{e}t\'{e}
\'{e}valu\'{e}e \`{a}  110 mg/g, ce qui est nettement sup\'{e}rieur
\`{a} de nombreux autres  mat\'{e}riaux.

Par ailleurs, l'\'{e}tude thermodynamique indique que l'adsorption du
SBL  par le DPP est un processus faisable, exothermique et
spontan\'{e}. Les  r\'{e}sultats de cette \'{e}tude sugg\`{e}rent que
les d\'{e}chets de DPP  non trait\'{e}s peuvent \^{e}tre
consid\'{e}r\'{e}s comme des mat\'{e}riaux  efficaces pour le
traitement des colorants textiles dans les eaux us\'{e}es  en
g\'{e}n\'{e}ral, et du SBL en particulier.
\end{altabstract}

\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}


\section{Introduction}\label{sec1}

The rapid eutrophication of aquatic ecosystems driven by the
collective impacts of population growth, urbanization, and contemporary
agricultural methods has become a major concern  worldwide~\cite{1}.  
The frequent release of large amounts of colored
wastewater into natural streams exacerbates the situation, causing
negative impacts on both the environment and human health~\cite{2}. 
This type of wastewater is typically generated by industries that use
dyes to achieve the desired coloration of their products. They include
several sectors such as food, paper, rubber, textiles, plastics, and
so on~\cite{3,4}. Currently, the utilization of dyes is witnessing a
significant surge, with approximately 10,000 different dyes in
existence and an annual production exceeding 0.7 million metric
tons~\cite{5}. The release of synthetic dyes into natural streams
elevates water toxicity, raises chemical oxygen demand levels, and
diminishes light penetration, thereby obstructing photosynthetic
processes~\cite{6}. As a result, the presence of dyes in natural
streams can have severe ecological consequences~\cite{7,8}. At the same
time, access to high-quality water is limited, and it is considered a valuable
resource that has to be absolutely protected~\cite{9}.  Therefore, it
has become increasingly essential to treat wastewater and remove
pollutants, ensuring their safe disposal, and prevent the contamination of
natural water bodies~\cite{10}.

Furthermore, the swift advancement of society has led to the production
of large amounts of low-value materials considered worthless. These
materials are often regarded as waste, and their disposal has become a
significant challenge~\cite{11}. Additionally, some materials found in
nature have little to no practical use. In this context, various
industrial and agricultural wastes have been tested as biosorbents for
dye removal from effluents, such as coffee grounds~\cite{8}, date palm
petiole (DPP)~\cite{12}, palm fronds~\cite{13}, date stones~\cite{14,15},
sugar-extracted spent biomass~\cite{16}, apricot stones~\cite{17}, rice
husk~\cite{18}, olive stones~\cite{19}, banana peel~\cite{20}, hazelnut
shell~\cite{21}{\ldots}, and so on.

{Dyes persist in water for extended periods and are challenging to be
treated because of their low biodegradability}~\cite{22}. Conventional
wastewater treatment technologies, including chemical
precipitation~\cite{23}, electrochemical  oxidation~\cite{24},
coagulation~\cite{25}, membrane filtration~\cite{26}, ion
exchange~\cite{27}, photocatalysis process~\cite{28} reverse
osmosis~\cite{29}, and liquid extraction~\cite{30}, are often
impractical for large-scale industrial applications~\cite{31}.
Moreover, these technologies have numerous drawbacks, including the
generation of toxic byproducts, low effectiveness, high energy
consumption, and the use of expensive reagents~\cite{32,33}.

The adsorption process is considered one of the most cost-effective
and eco-friendly physicochemical techniques for dye removal from
wastewater~\cite{34,35}. It entails transferring dyes from the water
phase to the surface of a solid adsorbent. Compared to other methods,
adsorption offers several advantages. For instance, it is a
straightforward process that does not necessitate specialized equipment
or substantial energy input~\cite{36}. Moreover, it can achieve high
removal efficiency without generating toxic byproducts. Additionally,
it can utilize a range of low-cost and easily accessible adsorbents,
including activated carbon, clay minerals, agricultural waste, and
biosorbents, making it a sustainable and environmentally friendly
option~\cite{2}. 

Several researchers have investigated the use of various agricultural
products and byproducts for removing dyes from
solutions~\cite{37,38,39}. Residues, often consisting of
lignocellulosic materials, possess a natural ability to adsorb waste
chemicals such as dyes and cations from water. This capability arises
from Coulombic interactions between the substrates, facilitating
physical absorption~\cite{40}. These materials are renewable
agricultural wastes that are abundantly available at little to no
cost~\cite{40}. However, most of the recent studies have tried to
physicochemically modify these materials in order to improve their
structural, textural, and surface chemistry properties and therefore
their ability in removing dyes from aqueous~\cite{41,42,43} solutions. 

In the open literature, there are few studies on the use of DPP
material for the preparation of activated carbons~\cite{44,45,46}.
These reports have focused on using such materials as adsorbents with
minimal processing (washing, drying, grinding), thereby reducing
production costs not only by utilizing inexpensive raw materials but
also by eliminating energy costs associated with thermal treatments. As
far as the authors are aware, there is no work that has employed DPP
without further thermal treatment as an adsorbent.

The authors believe that the current work will provide new insights into
the utilization of raw waste materials for treating wastewater. The
clear advantage of this method, compared to others, is its low cost and
eco-friendly aspect. 

This study represents another contribution to our ongoing research on
the removal of water contaminants using untreated agricultural
byproducts without any chemical treatment of the adsorbent. Thus, it
aims to assess the efficiency of DPP in \mbox{removing} Bezaktiv Marine S-BL (SBL) dye 
from textile
effluent. The interaction between the adsorbent and the adsorbate was
assessed through an optimization process, considering factors affecting
adsorption such as adsorbent dose, pH, contact time, and initial
concentration. Furthermore, adsorption kinetics and isotherms were
comprehensively investigated through the use of common models.

\section{Materials and methods}\label{sec2}

\subsection{Biosorbent preparation} 

Date palm petioles  were collected from local farms in Adrar
(Algeria). The biomaterial was subjected to several wash cycles with
hot distilled water to remove dust and subsequently air-dried for two
weeks. Then, the dried sample underwent grinding and sieving to produce
particles within the diameter range of 250--500~${\upmu}$m. This
fraction of DPP was utilized in subsequent experiments and processing. 

The point of zero charge ($\mathrm{pH}_{\mathrm{PZC}}$) of the DPP was
determined using the pH drift method~\cite{35}. A series of flasks
containing 25~mL of a 0.1~mol${\cdot}$L$^{-1}$ KNO3 aqueous solution at
pH values of 2, 4, 6, 8, 10, and 12 were prepared. The initial pH (pHi)
was adjusted using 0.1~mol${\cdot}$L$^{-1}$ KOH or HNO$_3$ aqueous
solutions. Next, 10~mg of DPP was added to each sample. The dispersions
were stirred for 24~h at room temperature, and the final pH (pHf) of
the solution was measured and plotted against pHi. The
$\mathrm{pH}_{\mathrm{PZC}}$ was determined from the plot of pHf ${=}$
f(pHi), where the pH at which the curve intersects the bisector of the
axes is taken as the $\mathrm{pH}_{\mathrm{PZC}}$ of the carbon.

\subsection{Adsorption studies}

A stock solution of SBL at a concentration of 500~mg/L was prepared by
dissolving 500~mg of SBL in 1~L of distilled water and diluted as
required to obtain the desired concentrations.

Batch adsorption experiments were carried out in 250~mL Erlenmeyer
flasks, which were agitated on a shaker at 350~rpm for a specified
contact time. The suspensions were subsequently filtered, and the
remaining concentrations of SBL in the supernatant solutions were
determined using a Cary 60 Series spectrophotometer. Absorbance
calibration curves were recorded against the concentration of the SBL
solution at a maximum wavelength of 599~nm. The selected parameter
ranges, including initial SBL concentration, adsorbent concentration,
and pH, were based on literature reports on SBL adsorption using other
agricultural residues, facilitating a comparison of adsorption
performance.

\subsubsection{Effect of time, pH, and adsorbent dose on adsorption of
SBL onto DPP}

Adsorption kinetic experiments were performed to assess the effect of
contact time on SBL removal. For this purpose, 50~mg of DPP was
agitated in 25~mL of SBL solution (50~mg${\cdot}$L$^{-1}$) at
25~\textdegree C for contact times ranging from 10 to 180~min. Aqueous
solutions were separated from the adsorbent by centrifugation. The
adsorbed amount at a given time $t$, $Q_{t}$ (mg/g), was calculated
using the following equation~\cite{36}:
{\begin{eqnarray}\label{eq1}
Q_{t}=V (C_{0} - C_{t}) /m 
\end{eqnarray}}\unskip
where $C_{0}$ is the initial concentration, $C_{t}$ is the
concentration at time $t$, $V$ is the volume of the solution, and $m$
is the mass of DPP.

The effect of DPP dose was assessed for biosorbent doses varying
between 0.4 and 2~g${\cdot}$L$^{-1}$. Moreover, the effect of pH on SBL removal
was examined within the range of 2 to 12. These effects were evaluated
for an initial dye concentration of 50~mg/L and at equilibrium. All
tests were conducted in triplicate.

\subsubsection{Equilibrium adsorption isotherms}

The adsorption performance of DPP was assessed through isotherm
experiments, where 10~mg of DPP was suspended in 25~mL of aqueous
solutions containing various pollutant concentrations
($C_{\mathrm{SBL}}= 20$, 40, 60, 80, 100, 120, 140, 160, 180,
200~mg/L). The suspensions were stirred for 24~h at 150~rpm to ensure
equilibrium. Once equilibrium was achieved, 5~mL aliquots were
withdrawn from the suspension and centrifuged at 8000~rpm to remove any
suspended particles. The concentrations of residual dye in the
supernatant solutions were measured using a spectrophotometer after
appropriate dilution.

\subsubsection{Thermodynamic studies}

The influence of temperature on the sorption of SBL onto DPP was
examined to determine the nature of the adsorption process. The
adsorption tests were conducted at four temperatures: 15~\textdegree C,
25~\textdegree C, 35~\textdegree C, and 45~\textdegree C. The
experimental results were utilized to determine thermodynamic
functions, specifically standard enthalpy (${\Delta}H$), entropy
(${\Delta}S$), and free energy (${\Delta}G$), calculated using
Equations (\ref{eq2}) and (\ref{eq3})~\cite{37,38}:
{\begin{eqnarray}
\label{eq2}
&amp; \displaystyle
{\Delta}G= -RT \ln\!K_{\mathrm{d}} \Seqnsplit
&amp; \displaystyle
{\Delta}G={\Delta}H-T{\Delta}S 
\label{eq3}
\end{eqnarray}}\unskip
where $K_{\mathrm{d}}$ is the distribution coefficient. The Van't Hoff plot $\ln(K_{\mathrm{d}})$
versus $1/T$ was used to calculate the values of ${\Delta}H$ and
${\Delta}S$.

\subsubsection{Adsorption models}

In order to select an appropriate model for fitting the experimental
data of SBL adsorption onto DPP, the investigation focused on analyzing
the kinetic mechanism by applying the pseudo-first-order~\cite{47},
pseudo-second-order~\cite{48}, and intraparticle diffusion models~\cite{49}
to the experimental data. Moreover, the experimental isothermal data
were analyzed to ascertain the distribution of the adsorbate between
the solid and liquid phases at various equilibrium concentrations,
aiming to better understand the uptake process. Thus, the models of
Freundlich~\cite{50} and Langmuir~\cite{51} were employed to assess the
dye adsorption parameters onto DPP. With further analysis of the
Langmuir equation, the Langmuir isotherm is characterized by a
dimensionless separation factor $R_{\mathrm{L}}$ given  by~\cite{52}:
{\begin{eqnarray}\label{eq4}
R_{\mathrm{L}} = 1/(1+K_{\mathrm{L}} C_{0})
\end{eqnarray}}\unskip
The $R_{\mathrm{L}}$ value indicates whether the process is spontaneous ($0 &lt;
R_{\mathrm{L}}&lt; 1$), unfavorable ($R_{\mathrm{L}}&gt; 1$), linear ($R_{\mathrm{L}}= 1$), or
irreversible ($R_{\mathrm{L}}= 0$).

\section{Results and discussion} 

\subsection{Adsorption kinetics}

To determine the equilibrium time, the impact of contact time on dye
biosorption was investigated. Figure~\ref{fig1} illustrates the
adsorption of SBL on DPP over time. The results show that the SBL
biosorption increases with time and reaches equilibrium after 120~min.
The efficiency of the biosorbent's removal increases quickly in the
initial stage (0--100~min)  due to the abundance of active binding
sites on the biomass~\cite{53}. During the second stage (100--120~min),
biosorption becomes less efficient and reaches saturation. This could
be attributed to the interaction of SBL involving the functional groups
located on the biosorbent surface and intercellular
accumulation~\cite{54}. Based on the findings, a shaking duration of
120~min was opted for subsequent biosorption experiments.

\begin{figure}
\includegraphics{fig01}
\caption{Effect of contact time on removal capacity
($T=25~\text{\textdegree}\mathrm{C}$, $C=50$~mg/L, $V=25$~mL, $m =
50$~mg).\label{fig1}}
\end{figure}

To elucidate the kinetics of the adsorption reactions, focusing on
order and rate constants, a kinetic study was undertaken. The mechanism
controlling SBL biosorption by DPP was explored by analyzing the
experimental data fitting to the pseudo-first-order,
pseudo-second-order, and intraparticle diffusion kinetic models. The
results of the kinetic parameters are presented in Table~\ref{tab1} and
Figure~\ref{fig1}. Based on the assessment of both the correlation
coefficients ($R^{2}$) and the estimated adsorbed amounts at
equilibrium ($Q_{\mathrm{e}}$) values, it is evident that SBL adsorption onto DPP
is more adequately described by the pseudo-first-order model. This
suggests that surface adsorption interactions may be the
rate-determining step, and the adsorption capacity likely correlates
with the number of active sites available on the biosorbent~\cite{55}.

\begin{table}
\caption{\label{tab1}Adsorption kinetic model parameters}
\tabcolsep=3pt
\begin{tabular}{ccc} 
\thead 
\multicolumn{2}{c}{Parameters} &amp; Values \\ 
\endthead 
\morerows{2}{Pseudo-first-order}  &amp; $k_1$ (min$^{-1}$)  &amp; \00.0248 \\
 &amp; $Q_{\mathrm{e}}$ (mg${\cdot}$g$^{-1}$) &amp; 13.96\0\0\\
 &amp; $R^2$ \vspace*{5pt}&amp; 0.987\\
\morerows{2}{Pseudo-second-order}  &amp; $k_2$ (g${\cdot}$mg$^{-1}{\cdot}$min$^{-1}$) &amp; \00.0013\\
 &amp; $Q_{\mathrm{e}}$ (mg${\cdot}$g$^{-1}$) &amp; 17.66\0\0\\
 &amp; $R^2$ \vspace*{5pt}&amp; 0.978\\
\morerows{2}{Intraparticle diffusion}  &amp; $K_{\mathrm{diff}}$
(min$^{-1}$) &amp; \01.1198\\
&amp; $C$ &amp; \00.5873\\
&amp; $R^2$ &amp; 0.913
\botline
\end{tabular}
\end{table}


\subsection{Effect of biosorbent dose}

The adsorbent dose is a vital parameter that dictates the capacity of
an adsorbent for a given {initial} concentration of the adsorbate under
{particular} {operating} conditions. To identify the optimal
\mbox{adsorbent} mass for maximum removal efficiency, the effect of varying
the mass of DPP on the adsorption of SBL at a constant adsorbate
concentration was investigated. The results indicated that SBL uptake
increased as the dose increased (Figure~\ref{fig2}). Specifically,
values of the removal rate $Y$ (\%) ($Y=\stfrac{c_1-c_f}{c_t}$)
increased from 26\% to 96\% as the adsorbent dose increased from 10 to
50~mg/25~mL. This behavior is due to the presence of more active
adsorption sites for increased doses that can react with SBL molecules.
A similar pattern was noted in a study on the removal of methylene blue
from an aqueous solution using fruit kernels~\cite{56} and with
calcium-rich biochars for removing amoxicillin~\cite{57}.

\subsection{Solution pH}

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Adsorbent dose effect on biosorption capacity
($Q_{\mathrm{e}}$; $T=25~\text{\textdegree}\mathrm{C}$, $C=50$~mg/L, $V=25$~mL,
$t=120$~min).}
\end{figure}

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Effect of solution pH on removal capacity
($Q_{\mathrm{e}}$; $T=25~\text{\textdegree}\mathrm{C}$, $C=50$~mg/L, $V=25$~mL,
$t=120$~min, $m=50$~mg).}
\end{figure}

Solution pH plays a crucial role in adsorption processes as it directly
influences the surface charge and the characteristics of ionic species
in the adsorbates~\cite{35}. The pH effect was investigated across a
range of 4 to 12, with the evolution of $Q_{\mathrm{e}}$ values shown in
Figure~\ref{fig3}. The SBL adsorption capacity decreased as the pH
increased from 4 to 6. This decrease in adsorption capacity can be
attributed to the surface charge of the adsorbent, which leads to
electrostatic repulsion between the adsorbent surface and negatively
charged adsorbate molecules. At higher pH levels, the surface of the
adsorbent becomes negatively charged through the deprotonation of
functional groups, thereby enhancing the {attraction} between the
adsorbent and SBL molecules and increasing the adsorption capacity. 
The \mbox{observed} trend in SBL
removal with changing pH can be elucidated by
considering the point of zero charge ($\mathrm{pH}_{\mathrm{PZC}}$) of
the adsorbent. There is a correlation between 
$\mathrm{pH}_{\mathrm{PZC}}$ (Figure~\ref{fig4}) and biosorption
capacity, which varies with pH and aids in understanding the behavior
of suspended biomaterials in \mbox{solution}~\cite{58,59}. As depicted in
Figure~\ref{fig3}, SBL removal is favorable at pH values greater than
$\mathrm{pH}_{\mathrm{PZC}}$ (5.5), where the surface charge of the
biosorbent becomes negative ($\mathrm{pH} &gt;
\mathrm{pH}_{\mathrm{PZC}}$). In this pH range, the positively charged
SBL molecules will be removed through electrostatic interactions.
Relatively similar behavior was observed for Bezaktiv Orange S-RL 150
removal by a biochar derived from palm flower stalk waste~\cite{39}.
\looseness=-1

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Experimental determination of point of zero charge.}
\end{figure}

\subsection{Isotherm adsorption studies}

The adsorption isotherms describe the relationship between the amount
of adsorbate adsorbed per unit mass of the adsorbent and the
concentration of the adsorbate in an aqueous solution at
equilibrium~\cite{60,61}. The SBL adsorption isotherm onto DPP is
presented in Figure~\ref{fig5}. The isotherm reveals that as the
initial concentration of SBL increases, the biosorbed quantity rises
until reaching a saturation point, indicating that all active sites on
the adsorbent surface are occupied. This suggests homogeneous
adsorption of SBL through ionic interactions \mbox{facilitated} by the
negatively charged surface of the adsorbent~\cite{44}. The isotherm
belongs to type L according to the Giles classification, indicating
strong interaction between the adsorbed molecules and the surfaces of
the  adsorbent~\cite{62}.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Adsorption isotherms of SBL onto DPP
($T=25~\text{\textdegree}\mathrm{C}$, $m = 50$~mg, $V=25$~mL,
$t=120$~min, pH ${=}$ 10).}
\end{figure}

The adsorption data were fit to two different isotherm models,
namely Freundlich and Langmuir, to characterize the adsorption of dye
solutions onto DPP biochar. These models offer a deeper understanding
of the adsorption process, which is crucial to evaluating or designing
an adsorption system~\cite{63}. The modeling outcomes are shown in 
Figure~\ref{fig5} and Table~\ref{tab2}. By comparing the $R^{2}$
values (Table~\ref{tab2}), the Langmuir model more accurately describes
the \mbox{adsorption} of SBL. The dimensionless equilibrium constant
($R_{\mathrm{L}}$) values for the Langmuir isotherm were lower than 1
(0.064--0.552), indicating that the adsorption process is spontaneous.
Furthermore, the parameter $1/n$ in the Freundlich isotherm, which
indicates the adsorption intensity of SBL dye on the applied adsorbent,
had values below unity (0.314), suggesting that the adsorption process
is favorable and implies that significant adsorption can occur even at
high dye concentrations. The Langmuir isotherm model was opted for 
assessing the maximum adsorption capacity, indicating the adsorbent's
complete monolayer coverage. The adsorption capacity, denoted as
$Q_{\mathrm{max}}$ and measuring the maximum sorption capacity at full
monolayer coverage, had a value of 106.17~mg/g.\looseness=-1

\begin{table}
\caption{\label{tab2}Adsorption isotherm model parameters{\vspace*{-2pt}}}
\begin{tabular}{ccc} 
\thead 
\multicolumn{2}{c}{Parameters} &amp; Values \\
\endthead 
\morerows{2}{Langmuir}  &amp; $k_{\mathrm{L}}$ &amp; \0\00.081 \\
 &amp; $Q_{\mathrm{max}}$ (mg${\cdot}$g$^{-1}$) &amp; 106.17\0 \\
 &amp; $R^2$ \vspace*{5pt}&amp; \0\00.992 \\
\morerows{2}{Freundlich}  &amp; $K_{\mathrm{F}}$ &amp; 19.25 \\
 &amp; $1/n$ &amp; \0\00.314 \\
 &amp; $R^2$ &amp; \0\00.945 
\botline
\end{tabular}
\end{table}

\begin{table}
\caption{\label{tab3}Adsorption thermodynamic parameters{\vspace*{-2pt}}}
\begin{tabular}{cccc} 
\thead 
\parbox[t]{1.9cm}{\centering Temperature (K)} &amp; 
\parbox[t]{1.5cm}{\centering $\Delta G$ (kJ${\cdot}$mol$^{-1}$)} &amp; 
\parbox[t]{1.5cm}{\centering $\Delta H$ (kJ${\cdot}$mol$^{-1}$)} &amp; 
\parbox[t]{1.2cm}{\centering $\Delta S$ (J${\cdot}$mol$^{-1}$)}\vspace*{2pt} \\
\endthead 
288 &amp; ${-}$12.451 &amp; \morerows{3}{${-}$80.760} &amp; \morerows{3}{${-}$237.38}\\
298 &amp; \0${-}$9.910 &amp; &amp; \\
308 &amp; \0${-}$7.682 &amp; &amp; \\
318 &amp; \0${-}$5.287 &amp; &amp; 
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table}

As expected, DPP biochar exhibited a higher $Q_{\mathrm{max}}$ value 
than numerous other materials reported in the scientific literature. 
Palm frond biochar was used for the removal of methylene blue 
from aqueous solutions, presenting an adsorption capacity of
70.87~mg/g~\cite{13}, 
which is lower than the value \mbox{obtained} in
this work. The DPP-derived activated carbon presented a
sorption capacity of 53.76~mg/g for indigo carmine~\cite{46}. Date palm
fiber had shown a maximum removal capacity of 18.45~mg/g for 
Ni ion and 16.15~mg/g for Cd ion elimination~\cite{64}. Therefore, the
DPP biochar can serve as a promising adsorbent for wastewater treatment.

\subsection{Thermodynamic studies}

Temperature exerts a notable influence on the sorption process, which
can be explained through thermodynamic parameters~\cite{16}. The
thermodynamic functions evaluated from the variation of distribution
coefficient ($K_{\mathrm{d}}$) in Figure~\ref{fig7} are listed in
Table~\ref{tab3}. The overall ${\Delta}G$ values are negative, varying
between ${-}$12.451 and ${-}$5.287~kJ/mol 
for temperature levels of 15--45~\textdegree C, demonstrating that the
biosorption process occurred spontaneously. The negative value of
${\Delta}H$ (${-}$80.76~kJ/mol) indicates the exothermic nature of the
biosorption process. This fact was confirmed by the higher amount of dye removal
observed at lower temperatures (Figure~\ref{fig6}). Furthermore, the
negative value of ${\Delta}S$
(${-}$237.38~J${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$) indicates decreased
randomness at the solid/solution interface, with no significant changes
occurring in the internal structure of the adsorbent during the
adsorption process~\cite{65}. The ${\Delta}H$ and ${\Delta}S$ values of
DPP--SBL are comparable to those reported by Aichour
et~al.~\cite{44} when investigating methyl orange removal by 
DPP-derived biochar.

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Temperature effect on SBL removal by DPP
($C=200$~mg/L, $m = 50$~mg, $V=25$~mL, $t=120$~min, pH ${=}$ 10).}
\end{figure}

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Plot of $\ln K_{\mathrm{d}}$ versus $1/T$.}
\end{figure}

\section{Conclusion}

This study primarily aims to determine the feasibility of using 
DPP biomass as a cost-effective \mbox{adsorbent} for
dye-contaminated wastewater. The sorption of SBL dye by DPP waste
reveals significant influences of sorbent dose, initial
concentration, pH, and contact time on SBL removal. The dye
concentration emerged as the most crucial factor {affecting} both
{adsorption} capacity and color removal efficiency. The evaluation of
adsorption isotherms revealed that the Langmuir isotherm, with a
correlation coefficient ($R^{2}$) of 0.993, best fit the experimental
data. Additionally, the monolayer adsorption capacity of the dye on DPP was
determined to be 106.17~mg/g. The pseudo-first-order kinetic model,
with an $R^{2}$ value of 0.987, was also identified as the most
suitable model for describing the adsorption process. The results of kinetic,
equilibrium, and thermodynamic studies supported the feasibility of
utilizing DPP biomass as an efficient adsorbent for dye removal.
Utilizing this locally sourced biomass could help alleviate the
financial burden associated with valorizing agricultural byproducts for
water treatment applications.

\section*{Declaration of interests}

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.

\section*{Data availability statement}

No datasets were generated or analyzed during the current study.

\back{}

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\refinput{crchim20240673-reference.tex}

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