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\DOI{10.5802/crchim.366}
\datereceived{2024-06-29}
\daterevised{2024-10-26}
\dateaccepted{2024-11-22}
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\dateposted{2025-02-21}
\begin{document}

%\copyeditor{RB}

\begin{DefTralics}
{\catcode`\:=11\xdef\deuxpoints{:}}
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\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
%\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
%\makeatother

\CDRsetmeta{articletype}{research-article}


\title{Hydrothermal carbonization of date palm pits and pine nut
shells: fuel characteristics of hydrochar, and surface characteristics and
adsorption application of activated hydrochar}

\alttitle{Carbonisation hydrothermale des noyaux de dattes et des
coques de pin: caract\'{e}ristiques combustibles de l'hydrochar,
caract\'{e}ristiques de surface et application d'adsorption de
l'hydrochar activ\'{e}}

\author{\firstname{Amina} \lastname{Bedoui}\CDRorcid{0000-0001-5861-1750}\IsCorresp}
\address{Laboratory of Process Engineering \&amp; Industrial Systems
(LR11ES54), National School of Engineers of Gabes, University of Gabes,
St Omar Ibn El Khattab, 6029, Gabes, Tunisia}
\email[A. Bedoui]{amina.bedoui@gmail.com}

\author{\firstname{Hanen} \lastname{Nouri}\CDRorcid{0000-0003-2229-9157}}
\addressSameAs{1}{Laboratory of Process Engineering \&amp; Industrial
Systems (LR11ES54), National School of Engineers of Gabes, University
of Gabes, St Omar Ibn El Khattab, 6029, Gabes, Tunisia}

\author{\firstname{Mouzaina} \lastname{Boutaieb}\CDRorcid{0000-0002-9408-828X}}
\addressSameAs{1}{Laboratory of Process Engineering \&amp; Industrial
Systems (LR11ES54), National School of Engineers of Gabes, University
of Gabes, St Omar Ibn El Khattab, 6029, Gabes, Tunisia}


\author{\firstname{Riheb} \lastname{Ghouil}}
\addressSameAs{1}{Laboratory of Process Engineering \&amp; Industrial
Systems (LR11ES54), National School of Engineers of Gabes, University
of Gabes, St Omar Ibn El Khattab, 6029, Gabes, Tunisia}

\author{\firstname{Beatriz}\nobreakauthor\lastname{Ledesma}\CDRorcid{0000-0001-8070-1154}}
\address{Applied Physics Department, Industrial Engineering School,
University of Extremadura, Avda. de Elvas, s/n, 06006 Badajoz, Spain}

\author{\firstname{Maria} \lastname{Alonso}\CDRorcid{0009-0004-8767-2156}}
\addressSameAs{2}{Applied Physics Department, Industrial Engineering
School, University of Extremadura, Avda. de Elvas, s/n, 06006 Badajoz,
Spain}

\author{\firstname{Silvia} \lastname{Roman}\CDRorcid{0000-0001-8851-2710}}
\addressSameAs{2}{Applied Physics Department, Industrial Engineering
School, University of Extremadura, Avda. de Elvas, s/n, 06006 Badajoz,
Spain}

\author{\firstname{Monia} \lastname{Guiza}\CDRorcid{0000-0002-6213-9580}}
\addressSameAs{1}{Laboratory of Process Engineering \&amp; Industrial
Systems (LR11ES54), National School of Engineers of Gabes, University
of Gabes, St Omar Ibn El Khattab, 6029, Gabes, Tunisia}

\thanks{Agencia Espa\~{n}ola de Investigaci\'{o}n 
(PID2020-116144RB-I00/AEI/10.13039/501100011033)}

\shortrunauthors

\keywords{\kwd{Carbonization}
\kwd{Hydrothermal}
\kwd{HC}
\kwd{Solid/water ratio}
\kwd{Activated HC}}

\altkeywords{\kwd{Carbonisation}
\kwd{Hydrothermale}
\kwd{HC}
\kwd{Rapport solide/eau}
\kwd{HC activ\'e}}

\begin{abstract}
This work 
aimed at producing hydrochars (HCs) from agricultural waste to
investigate their potential as solid fuels and, following physical
activation, as precursors of activated carbon. Date palm pits (DPPs) and
pine nut shells (PNSs) were hydrothermally carbonized at 250 \textdegree
C for 1 h in various solid/water ratios (1:8; 1:12, and 1:48). The
derived HCs were then activated at 850 \textdegree C using steam 
\mbox{(1 h;}
70 vol\% H\textsubscript{2}O in N\textsubscript{2}). 
The impact of the solid/liquid ratio
and the type of biomass on HC properties was highlighted. Fuel
characteristics of HC were examined via ultimate analysis, higher heating
value (HHV), and solid yield. Surface characterization was
determined by several analytical techniques, including N\textsubscript{2}
adsorption (N\textsubscript{2}), X-ray diffraction, scanning electron
microscopy, and Fourier transform infrared spectroscopy. The
solid/water ratio had no significant effect on HC characteristics. As
fuel, all HCs had high carbon content (\%C) and HHV. Their calculated
H/C and O/C ratios determined on the HCs showed improved stability as
fuels. The HC from DPP with solid/water ratio 1:48 had the highest HHV
(29.7 MJ/kg). The PNS HC had the highest mass yield of 48\%.
Textural properties showed a significant number of oxygen groups and a
low specific surface area. Activated HCs demonstrated high Brunauer--Emmett--Teller surface
areas (857 and 827 m\textsuperscript{2}/g for DPP and PNS, respectively)
and high total pore volumes (0.582 and 0.541 cm\textsuperscript{3}/g
for DPP and PNS, respectively). Thus, the energetic properties of the HC
show its potential to be used as solid fuel while the textural
qualities of the activated HC demonstrate its high adsorbent ability.
\end{abstract}

\begin{altabstract}
L'objectif de ce travail \'{e}tait de produire des hydrochars (HC)
\`{a} partir de d\'{e}chets agricoles afin d'examiner leur potentiel en
tant que combustibles solides et suite \`{a} une activation physique,
comme pr\'{e}curseurs du charbon actif. Des noyaux de dattes (DPP) et
des coques de pin (PNS) ont subi une carbonisation hydrothermale \`{a}
250 \textdegree C pendant 1 h pour diff\'{e}rents rapports solide/eau
(1\deuxpoints8 ; 1\deuxpoints12 et 1\deuxpoints48). Les HCs obtenus ont \'{e}t\'{e} ensuite
activ\'{e}s \`{a} 850 \textdegree C en utilisant la vapeur d'eau (1 h ;
70 vol\% H\textsubscript{2}O dans N\textsubscript{2}). L'impact du
rapport solide/liquide et de la nature de biomasse sur les
propri\'{e}t\'{e}s des HC a \'{e}t\'{e} mis en \'{e}vidence. Les
caract\'{e}ristiques combustibles des HC ont \'{e}t\'{e} examin\'{e}es
via l'analyse ultime, le pouvoir calorifique sup\'{e}rieur (PCS) et le
rendement solide (RS). La caract\'{e}risation de surface a \'{e}t\'{e}
d\'{e}termin\'{e}e par plusieurs techniques analytiques y compris
l'adsorption de N\textsubscript{2} (N\textsubscript{2}), la diffraction des rayons X,
la microscopie \'{e}lectronique \`{a} balayage (MEB) et la
spectroscopie infrarouge \`{a} transform\'{e}e de Fourier (FTIR). Le
rapport solide/eau n'a pas d'effet significatif sur les
caract\'{e}ristiques des HC. En tant que carburant, tous les HC ont une
teneur \'{e}lev\'{e}e en carbone (\%C) et un PCS important. Les
rapports H/C et O/C calcul\'{e}s pour les HC ont d\'{e}montr\'{e} une
stabilit\'{e} accrue en tant que combustibles. Le HC issu de DPP avec
un rapport solide/eau (1\deuxpoints48) pr\'{e}sente le PCS le plus important
(29,7 MJ/kg). Le HC issu de PNS avec un rapport solide/eau (1\deuxpoints8) montre
le rendement massif le plus \'{e}lev\'{e} atteignant 48 \%. Les
caract\'{e}ristiques texturales ont relev\'{e} des concentrations
significatives de groupes oxyg\'{e}n\'{e}s et une surface
sp\'{e}cifique limit\'{e}e. Les HC activ\'{e}s ont d\'{e}montr\'{e} une
surface BET importante (857 et 827 m\textsuperscript{2}/g pour le DPP
et le PNS, respectivement) et un volume total de pores important (0,582
et 0,541 cm\textsuperscript{3}/g pour le DPP et le PNS, respectivement). Par
cons\'{e}quent, les caract\'{e}ristiques \'{e}nerg\'{e}tiques des HC
r\'{e}v\`{e}lent leur potentiel d'utilisation en tant que combustible
solide, tandis que les caract\'{e}ristiques texturales des HC
activ\'{e}s illustrent leur forte capacit\'{e} d'adsorption.
\end{altabstract}

\maketitle

\vspace*{10pt}
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%\let\rmmu\upmu

\section{Introduction}\label{sec1}

Solid waste management is an enormous challenge for contemporary human
civilization as the increasing accumulation of solid waste harms the
environment and human health. In this context, recovery is a step toward
a circular and sustainable economy. Among these waste products,
agricultural residues are considered particularly
significant~\cite{1,2,3}. Their global volume has witnessed a steady
rise, propelled by rapid population growth and heightened by
agricultural demands. Despite their natural biodegradability, these
wastes are frequently considered environmentally harmful, and their
degradation rates are generally insufficient~\cite{4,5}. Therefore,
the reutilization of agricultural waste appears to be a possible
solution to this problem. Due to its lignocellulosic nature,
agricultural waste can be used to produce energy~\cite{5}, sequester
carbon~\cite{6}, and significantly contribute to the creation of
carbonaceous materials and biofuels through various thermochemical
techniques, including pyrolysis, gasification, combustion,
torrefaction, and hydrothermal carbonization (HTC)~\cite{7}. The HTC stands
out as a thermochemical process that modifies the original structure of
biomass through a series of chemical reactions that occur in the
presence of water under subcritical conditions (180--250~\textdegree C)
and under autogenous pressure (10--40~bar)~\cite{1}. This process can be
considered a form of moderate-temperature carbonization, making it more
energy-efficient compared to other thermochemical techniques like
pyrolysis~\cite{8,9,10}. In particular, this process converts wet
precursors (biomass and agricultural wastes) into gas, liquid, and a
carbon-rich solid called hydrochar (HC)~\cite{11,12}. The HC presents
several benefits over its initial biomass, including higher carbon
content, specific surface morphology, increased oxygenated
functional groups, such as hydroxyl and carboxyl groups, and higher
calorific value~\cite{5,13}. Kabakci and Baran~\cite{14} investigated
the fuel and surface characteristics of HCs derived from various
lignocellulosic biomasses. They showed that HTC significantly increases
the higher heating value (HHV) and carbon content of agricultural residues,
although the biomass type also had a significant impact on HC
properties~\cite{14}. Si et~al.~\cite{6} evaluated the liquid--solid
ratio impact during HTC and demonstrated its high effect on HC
properties. Bourafa et~al.~\cite{3} studied the HTC of olive and date
stones at different times and temperatures. They found that all
generated HCs present high mass yield, many acidic and basic functional
groups, and relatively low porosities, although the effect of time and
temperature parameters was also very important. 


In recent years, bio-based HCs have been further processed to tune
them up, mainly through physical or chemical activation, to produce
activated carbons (ACs) with very interesting 
results~\cite{4,15,16,17}. This process significantly increases the
pore volume of HCs by selectively removing the condensed matter
that blocks microporosity~\cite{18,19,20}. Activated HCs have been
investigated for different end uses, including energy 
storage~\cite{16,21} and adsorbents for wastewater
treatments~\cite{17,19}. 

This work focused on two main objectives. The first is the preparation and
fuel characterization of HCs made from two local wastes: date palm
pines (DPPs) and pine nut shells (PNSs). Particular attention was paid to
the effects of biomass type and solid/water ratio (SWR) on the final properties.
Second, the produced HCs were steam-activated to generate ACs with a
well-developed surface and porosity to be used in further applications.


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

\subsection{Raw feedstock collection}\label{sec21}

In this study, DPPs and PNSs were used as raw precursors for HC
production. 
The former are the residues of date obtained from date palm
(\textit{Phoenix dactylifera} L.) produced in the southwest region of
Tunisia, and the latter are collected from Pinus pinea trees from Ain
Draham in the north of Tunisia. The collected biomass was washed with
distilled water to eliminate undesirable substances and naturally dried
at ambient temperature for 2--3 days until a constant humidity value
was achieved. Then, the biomass was crushed using a high-speed rotary
cutting mill, sieved to a desired particle size between 1.6 and 
3.15~mm, and then dried in an oven at 105~\textdegree C for 24~h. The dry
biomass was stored in closed flasks and placed in a desiccator for
further use. 

\begin{table*}
\caption{\label{tab1}Chemical properties of DCF}
\begin{tabular}{ll}
\tbody
Chemical name &amp;  2-[2-(2,6-dichlorophenyl) aminophenyl] ethanoic acid \\
{Molecular formula} &amp; C$_{14}$H$_{10}$Cl$_2$NNaO$_2$ \\
$\mathrm{p}K_a$ &amp;  4.15 \\
{Molar mass (g/mol)} &amp; 296.149 ${\pm}$ 0.017\\
{Fusion temperature (\textdegree C)} &amp; 283--285 \\
{Water solubility (g/L)} &amp;  2.37~mg$\cdot$L$^{-1}$ 
(water,~25~\textdegree C) \\
{Maximum absorbance ($\uplambda$, nm)} &amp;  275 
\botline
\end{tabular}
\end{table*}

\subsection{HC preparation}\label{sec22}

The HC  was prepared in a 1.8 L autoclave (Parr 4848 model)
equipped with a controller, which allows the programming of the
experiments, autonomous heating, and sensors for measuring pressure ($P$)
and temperature ($T$).  The HTC runs were carried out at a
temperature of 250~\textdegree C for 90~min with different
SWRs. Weights of 12.5, 50, and 75 g of precursors
were mixed separately with 600 g of water to obtain SWRs of 1/48,
1/12, and 1/8, respectively. The operational conditions were chosen
based on prior methods of HC production from agricultural
residues~\cite{22}. At the end of the HTC process, the autoclave was
immediately placed in a cold-water bath and allowed to cool down to
room temperature. The solid phase was then separated from the liquid
phase by simple filtration, washed many times with distilled water until
neutral pH was attained, and dried at 105~\textdegree C overnight to remove residual
moisture. The obtained HCs were referred to as ``HC-P-R'', where P
represents the precursors (DPP and PNS) and R the solid/water ratio of
HTC. For example, ``HC-DPP-1:8'' denotes ``HC based on DPP with an SWR
of 1/8.

The solid yield (SY) values of the prepared HC (SY (\%)) were
calculated according to the following expression (dry basic):
{\begin{equation}\label{eq1}
\mbox{SY (\%)}=\frac{\text{mass of hydrochar}}{\text{mass of biomass}}\times 100
\end{equation}}\unskip


\subsection{Activation process}\label{sec23}

The obtained HCs were then used to prepare ACs by physical steam
activation. To this end, the precursor was introduced into a
stainless-steel reactor placed in a vertical automated furnace equipped
with a temperature controller, with an initial mass equal to 70 g.
Activation was performed in the presence of a continuous flow of
purified nitrogen gas with a flow rate of 70~mL/min until a temperature
of 700~\textdegree C with a heating rate of 10~\textdegree C/min was attained. At
this temperature, steam water (70~vol\% H$_2$O in N$_2$) was
introduced and the activation continued for 1~h until the temperature increased to 850~\textdegree C.
These optimal values were deduced from previous research~\cite{23}. The
reactor was then slowly cooled down while maintaining the nitrogen flow.
The obtained ACs were denoted as HCA-DPP and HCA-PNS for activated HCs
from DPP and PNS, respectively.

The burn-off values of the synthesized ACs (BO (\%)) were calculated
according to the following expression:
{\begin{equation}\label{eq2}
\mbox{BO (\%)} =\frac{(w_{0}-w_{\mathrm{t}})}{w_{0}}\times 100
\end{equation}}\unskip
where $w_{0}$ and $w_{\mathrm{t}}$ represent, respectively, the initial weight
of the prepared HC and the final weight of the corresponding AC.

\subsection{Adsorption process}\label{sec24}
\subsubsection{Solution preparation}\label{sec241}

Diclofenac sodium salt (DCF) is a common anti-inflammatory nonsteroidal
medicament detected in many waste water effluents from the pharmaceutical
industry and hospital and municipal sewage~\cite{24}, which was  used as an
adsorbate in our work. The DCF was of analytical grade with purity higher
than 99\% and obtained from Prolabo. Its principal characteristics are
listed in Table~\ref{tab1}. The DCF solution was prepared by accurately
weighing 1~g of salt and dissolving it in 1~L of distilled water. Then
the solution was diluted to obtain the target concentrations.

\subsubsection{Batch adsorption test}\label{sec242}

The DCF adsorption experiments by HC-PNS-1:8 and HC-DPP-1:8 were carried
out in a series of 100~mL glass Erlenmeyer flasks. A stock solution of 1~g/L 
of DCF was used for the preparation of 10--200~mg/L DCF solution by
diluting with double-distilled water. 
An amount of 0.1~g of AC was added to each flask at a fixed temperature
of 25~\textdegree C and the pH of solution. 
The solutions were stirred for 24~h to achieve 
thermodynamic equilibrium. The initial and residual DCF concentrations
were measured using UV spectroscopy at a wavelength of 275 nm,
corresponding to the maximum absorbance. The amount of DCF adsorbed
onto activated HC, $q_{\mathrm{e}}$ (mg/g), was calculated using the
following equation:
{\begin{equation}\label{eq3}
q_{\mathrm{e}}=\frac{(C_{0}-C_{\mathrm{e}})}{m}\times V
\end{equation}}\unskip
where $q_{\mathrm{e}}$ is the equilibrium adsorbed amount of DCF
(mg/g), $C_{0}$ and $C_{\mathrm{e}}$ are the initial and 
equilibrium concentrations, respectively, of adsorbate in solution (mg/L), $m$ is the
mass of the adsorbent (g), and $V$ is the volume of solution. 

\subsubsection{Isotherm models}\label{sec243}
The experimental data were fitted to widely used isotherm models
such as Langmuir, Freundlich, and Temkin. 


The Langmuir isotherm model is given as~\cite{15}
{\begin{equation}\label{eq4}
q_{\mathrm{e}}=\frac{q_{\mathrm{m}}\times K_{\mathrm{L}}\times C_{\mathrm{e}}}{1+K_{\mathrm{L}}\times C_{\mathrm{e}}}
\end{equation}}\unskip
where $q_{\mathrm{m}}$ represents the theoretical maximum adsorption capacity (mg/g)
and $K_{\mathrm{L}}$ is the Langmuir constant (L/mg).

The Freundlich equation is given as~\cite{15}
{\begin{equation} \label{eq5}
q_{\mathrm{e}}=K_{\mathrm{f}}\times {C}_{\mathrm{e}}^{1/n_{\mathrm{f}}}
\end{equation}}\unskip
where $K_{\mathrm{f}}$ (L/mg) is the Freundlich adsorption constant and n is the
constant indicative of adsorption intensity.

The Temkin equation is written as follows~\cite{26}:
{\begin{equation}\label{eq6}
q_{\mathrm{e}}=BLn(A_{\mathrm{T}}C_{\mathrm{e}})
\end{equation}}\unskip
where $B$ and $A_{\mathrm{T}}$ (L/g) are Temkin constants. The isotherm
models' nonlinear equations were solved by Microsoft Excel Solver v.
2016.

\subsection{Material characterization}\label{sec25}

\subsubsection{Elemental analysis}\label{sec251}

Elemental analyses (C, H, and N) were carried out using an automatic
elemental analyzer EA-1108 CHNS (LECO Instruments, Germany) to quantify
the carbon, hydrogen, and nitrogen contents using the standard
specification CEN/TS 15104 (2005). The specification CEN/TS 15289 (2006)
method was used to determine the sulfur content, which was in all cases
very low (${&lt;}$0.01\% by mass). The oxygen content was determined from the
difference.

\subsubsection{HC higher heating value}\label{sec252}

The HHVs of both the raw material and HCs were
measured in a Parr 1351 Adiabatic Oxygen Bomb Calorimeter (norm ISO
1928) according to the technical specification CEN/TS/14918, fitted
with continuous temperature recording. Before the HHV analysis, 
samples of 0.4--0.5 g were dried at 105~\textdegree C for 24~h.

\begin{table*}
\caption{\label{tab2}Ultimate analysis and HHV of raw materials and
their corresponding HCs}
\begin{tabular}{lcccccccccc}
\thead
Precursor &amp; \%C &amp; \%H &amp; \%O$^{*}$ &amp; \%N &amp; O/C &amp; H/C &amp; Moisture  (\%) &amp; Ash  (\%) &amp; HHV  (MJ/kg) &amp; SY  (\%) \\
\endthead
DPP &amp; 47.2 &amp; 7.22 &amp; 44.55 &amp; 1.02 &amp; 0.71 &amp; 1.83 &amp; 13.88 &amp; 1.15 &amp; 19.91 &amp; - \\
HC-DPP-1:8 &amp; 71.2 &amp; 5.61 &amp; 21.34 &amp; 1.84 &amp; 0.22 &amp; 0.95 &amp; \04.40 &amp; 0.42 &amp; 28.21 &amp; 39.46 \\
HC-DPP-1:12 &amp; 69.2 &amp; 6.07 &amp; 22.86 &amp; 1.87 &amp; 0.24 &amp; 1.05 &amp; \03.60 &amp; 0.55 &amp; 28.73 &amp; 38.20 \\
HC-DPP-1:48 &amp; 70.4 &amp; 6.46 &amp; 21.26 &amp; 1.88 &amp; 0.22 &amp; 1.10 &amp; \03.00 &amp; 0.63 &amp; 29.77 &amp; 37.20 \\
PNS &amp; 50.9 &amp; 6.14 &amp; 42.71 &amp; 0.23 &amp; 0.63 &amp; 1.45 &amp; \09.95 &amp; 1.68 &amp; 20.24 &amp; - \\
HC-PNS-1:8 &amp; 70.3 &amp; 5.00 &amp; 24.28 &amp; 0.41 &amp; 0.26 &amp; 0.85 &amp; \03.00 &amp; 1.33 &amp; 26.54 &amp; 48.8\0 \\
HC-PNS-1:12 &amp; 67.7 &amp; 5.40 &amp; 26.48 &amp; 0.42 &amp; 0.29 &amp; 0.95 &amp; \02.30 &amp; 1.29 &amp; 26.84 &amp; 46.34 \\
HC-PNS-1:48 &amp; 68.4 &amp; 5.25 &amp; 25.94 &amp; 0.41 &amp; 0.28 &amp; 0.92 &amp; \03.00 &amp; 1.21 &amp; 27.32 &amp; 42.4\0
\botline
\end{tabular}
\vspace*{3pt}
\end{table*}

\subsubsection{Textural characterization}\label{sec253}

Nitrogen adsorption isotherms at 77 K were generated by Autosorb
equipment (Quantachrome). Before analyses, the selected samples were
outgassed at 250~\textdegree C for 12~h. The apparent
Brunauer--Emmett--Teller (BET) surface area $S_{\mathrm{BET}}$
($\mathrm{m}^{2}{\cdot}\mathrm{g}^{-1}$) was estimated using the BET
theory. The t-plot method was used for measuring the micropore volume
$V_{\mathrm{mic}}$ ($\mathrm{cm}^{3}{\cdot}\mathrm{g}^{-1}$) and the
external surface $S_{\mathrm{EXT}}$ 
($\mathrm{m}^{2}{\cdot}\mathrm{g}^{-1}$).  The BJH method was used for
calculating the mesopore and total pore volume ($V_{\mathrm{me}}$ and
$V_{\mathrm{t}}$,  $\mathrm{cm}^{3}{\cdot}\mathrm{g}^{-1}$, respectively).

\subsubsection{Surface morphology}\label{sec254}

The surface morphology of HCs was determined via scanning electron
microscopy (SEM, Quanta 3D FEG, FEI). The samples were prepared by
depositing about 50 mg of the sample on an aluminum (Al) stud covered
with conductive adhesive carbon tape and then coated with Rh--Pd for 1
min to prevent charging during observations. Imaging was performed
under high-vacuum conditions (${&lt;}6\times10^{-4}$~Pa) at an
accelerating voltage of 30 kV using secondary electrons for high
vacuum~\cite{22}.

\subsubsection{X-ray diffraction}\label{sec255}
An X-ray diffraction (XRD) apparatus was used to study the structure of
the obtained materials and identify the crystalline phases present in
the prepared materials. Analyses were carried out using a Cu K
radiation source operating at a voltage of 40~kV. The diffraction angle
($2\Theta$) was changed from 5 to 90\textdegree~for HC and activated HC.

\subsubsection{Surface chemistry}\label{sec256}

Functional group spectra of selected samples were investigated using a
Fourier transform infrared (FTIR) spectrometer. The FTIR absorption spectra
of the samples were recorded with a Perkin Elmer apparatus (Nicolet
iS5) operating in the wavenumber range between 400 and 4000~cm$^{-1}$.
The bands were assigned using suitable bibliography. 

\vspace*{4pt}

\section{Results and discussion}\label{sec3}
\vspace*{2pt}

\subsection{Fuel characteristics of HCs}\label{sec31}

Ultimate analysis, moisture and ash contents, 
HHV, and SY were determined for the two raw materials
(DPP and PNS) and their corresponding HCs prepared at different SWRs. The
results are presented in Table~\ref{tab2}. 


Compared to raw materials, all HCs showed higher carbon and nitrogen
contents and lower oxygen (\%O) and hydrogen (\%H) contents. In
the precursors, H and O were removed in the form of H$_{2}$O and CO$_{2}$ due to
reactions of dehydration and decarboxylation, respectively, during HTC~\cite{14}. The
carbon enrichment was related to the decrease in the other elements (especially
O) and to the aromatization during HTC~\cite{25}. The change in carbon
contents from the biomass to HC products was more pronounced for DPP
than PNS. The elemental analysis showed that the increase in carbon
contents was greater for HC produced from DPP  with a percentage
increase equal to 50\% while that of PNS HC was equal to 30\%. The
oxygen and hydrogen contents decreased by 52\% and 22\%, respectively,
for DPP, whereas for PNS, the \mbox{decrease} was less significant with
percentages that did not \mbox{exceed} 43\% and 18\%, respectively. This
behavior can be explained by the distinct original composition of the
biomass samples. Previous studies proved that the type of biomass
significantly affects the chemical composition of the obtained
HC~\cite{3,26}. This finding is supported by our investigation, and
could be explained by the specific reactivity in dehydration and
decarboxylation reactions of each biomass~\cite{14}. Moreover, the
differences in the type and concentration of degradation fragments may
affect the extent of recombination processes and the formation of any
HC that also accounts for SY and C capture~\cite{27}.

Decreasing the HTC SWR of both DPP and
PNS at the same time and temperature conditions had little effect on
the atomic content of the produced HC. For example, C, O, H, and N contents
in HC-DPP or -PNS-1:8 and HC-DPP or PNS-1:48 were very similar despite
the large difference between the two ratio values, indicating that a
sixfold reduction in water consumption generates an equivalent
characteristic of HC, with environmental and economic benefits in
reducing wastewater discharge and improving carbon yield per unit
volume. Si et~al.~\cite{6} studied the effect of SWR on corn straw HC
production and showed that reducing 80\% of water consumption produced
HCs with the same performance.

Atomic ratios H/C and O/C are significant indicators of aromaticity stability and
maturity of HCs~\cite{1, 28}. The change in  H/C and O/C
was evaluated using the Van Krevelen diagram, which crossplots the
hydrogen/carbon atomic ratio as a function of  oxygen/carbon atomic
ratio~\cite{1,12} (Figure~\ref{fig1}). The evolution of  O/C and H/C
ratios reveals the degree of deoxygenation of the precursors obtained
through \mbox{dehydration} and decarboxylation~\cite{3}. These processes
\mbox{enable} the removal of hydrogen and oxygen in the form of H$_2$O and
CO$_2$~\cite{3,29}. As can be seen in Figure~\ref{fig1}, the HTC
decreases the O/C and H/C atomic ratios of DPP and PNS, and all HCs
show comparable values. The Van Krevelen diagram confirms that the
hydrothermal process significantly affects the elemental composition of
the products. The evolution of the HC O/C and H/C atomic ratios
suggests that the removal of oxygen in the form of water through
the dehydration process was the primary mechanism for the HTC process.
Our results are consistent with previous research indicating that the
biomass loses groups, such as carboxyl, hydroxyl, and carbonyl, leading
to the breakage of aliphatic side chains and an increase in 
aromaticity of carbon materials~\cite{9}.\looseness=1 

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Van Krevelen diagram of  raw materials and
their corresponding HCs.}
\vspace*{-6pt}
\end{figure}

The H/C and O/C ratios of HC-DPP and HC-PNS obtained under different
SWRs ranged from 0.95 to 1.1 and 0.22 to 0.24 for HC-DPP and from 0.85 to
0.95 and 0.26 to 0.29 for HC-PNS, respectively. The maximum difference
between H/C values was 0.07, and the maximum difference between O/C
values was 0.03. The small difference indicates that reducing the SWR
will neither  significantly affect Van Krevelen ratios nor the HHV. Along these
lines, it would be interesting to investigate the thermal stability of
these materials as secondary HC contribution could be modified with the
SWR.\looseness=-1 

Compared to the four typical materials (anthracite, bituminous,
sub-bituminous, and lignite coal), the H/C and O/C atomic ratios of the
obtained HCs were found to be near those of the lignite and
sub-bituminous regions~\cite{30,31}. 

Another obvious effect is the absence of SWR influence on ash
proportion in HC as this could have been expected as a result of enhanced
demineralization.


On the other hand, the HC HHVs determined (Table~\ref{tab2}) were
higher than their corresponding raw precursors~\cite{14}. Similar
results were found by Park et~al.~\cite{31} and Boutaeib et~al.~\cite{32}
who carbonized algal and pine cone biomass, respectively, and showed
that greater heating values were associated with higher fixed carbon
content. It is worth noting that all the HCs had heating values as high
as bituminous coal~\cite{6}. Such results indicate that the prepared HCs
have the potential to be used as solid fuels for combustion
applications~\cite{14, 22}.

Additionally, the SY of the HC (Table~\ref{tab2}) was unaffected by the
SWR. For example, it ranged between 37.2\% (SWR 1:48) and 39.46\% (SWR
1:8) for HC-DPP while HC-PNS ranged between 42.4\% (mass ratio 1:48)
and 48.8\% (mass ratio 1:8). A more intense effect was observed for the 
biomass type as related to their distinct composition that influences
hydrothermal reactions differently~\cite{5,33}. 
\vspace*{-6pt}

\subsection{HC and activated HC surface properties}\label{sec32}
\vspace*{-6pt}

\subsubsection{Textural properties}\label{sec321}

\begin{table*}
\caption{\label{tab3}BET surface areas and micro- and mesopore volumes of
activated HCs\vspace*{-4pt}}
\begin{tabular}{clccccc}
\thead
Precursor &amp;  Sample &amp;  BO(\%) &amp; 
$S_{\mathrm{BET}}$ (m$^{2}$/g) &amp; 
$V_{\mathrm{T}}$ ($\mathrm{cm}^{3}{\cdot}\mathrm{g}^{-1}$) &amp; 
$V_{\mathrm{micro}}$ ($\mathrm{cm}^{3}{\cdot}\mathrm{g}^{-1}$) &amp; 
$V_{\mathrm{meso}}$ ($\mathrm{cm}^{3}{\cdot}\mathrm{g}^{-1}$) \\
\endthead
\morerows{2}{HC-DPP}  &amp;  HC-DPP-1:8 &amp;  - &amp;  \014.92 &amp;  - &amp;  - &amp;  - \\
&amp;  HC-DPP-1:12 &amp;  - &amp;  \016.59 &amp;  - &amp;  - &amp;  - \\
&amp;  HC-DPP-1:48 &amp;  - &amp;  \020.60 &amp;  - &amp;  - &amp;  - \\
\morerows{2}{HC-PNS}  &amp;  HC-PNS-1:8 &amp;  - &amp;  \041.83 &amp;  - &amp;  &amp;  \\
&amp;  HC-PNS-1:12 &amp;  - &amp;  \043.12 &amp;  - &amp;  - &amp;  - \\
&amp;  HC-PNS-1:48 &amp;  - &amp;  \047.05 &amp;  - &amp;  - &amp;  - \\
\morerows{1}{AC-DPP}  &amp;  HCA-DPP-1:8 &amp;  20.32 &amp;  857.07 &amp;  0.582 &amp;  0.372 &amp;  0.210\\
&amp;  BCA-DPP &amp;  52.9\0 &amp;  524.02 &amp;  0.420 &amp;  0.182 &amp;  0.238 \\
AC- PNS &amp;  HCA-PNS-1:8 &amp;  22.01 &amp;  827.82 &amp;  0.541 &amp;  0.377 &amp;  0.164
\botline
\end{tabular}
\vspace*{-6pt}
\end{table*}

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Nitrogen adsorption--desorption isotherms for
activated HC.}
\end{figure}

The N$_2$ adsorption--desorption isotherms of the activated HC derived
from DPP and PNS are shown in Figure~\ref{fig2}. The associated
textural parameters of HCs and their corresponding activated HCs are
\mbox{illustrated} in Table~\ref{tab3}. Furthermore, the properties of ACs
obtained via steam activation of DPP biochars, as \mbox{obtained} from
previous studies, are also reported in Table~\ref{tab3} for comparison
purposes. 


All HC samples had low BET surface areas (Table~\ref{tab3}). Previous
studies agree about the low BET surface area of HC and attribute it to pore
blockage caused by volatile matter, which fails to reach the liquid
phase during the HTC treatment, as well as condensed or repolymerized
matter~\cite{3,20,34}. 

The BET surface area very slightly increases with decreasing SWR, with
values ranging between 14.92 and 
20.60~$\mathrm{m}^{2}{\cdot}\mathrm{g}^{-1}$ for the HCs derived from
DPP and between 41.83 and 47.05~$\mathrm{m}^{2}{\cdot}\mathrm{g}^{-1}$
for those from PNS. The oxygen and hydrogen contents decrease for DPP by
52\% and 22\%, respectively, whereas the decrease for PNS is less
significant not exceeding 43\% and 18\%, respectively.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}SEM images of HC-DPP-1:8, HC-DPP-1:12,
HC-DPP-1:48, HC-PNS-1:8, HC-PNS-1:12, and HC-PNS-1:48.}
\end{figure*}

In contrast, steam-activated HCs exhibit high N$_2$  adsorption
(Figure~\ref{fig2}). The resulting isotherms have the same shape (type
I according to Langmuir classification), which is typical of
microporous materials, and have mesopores as indicated by the presence
of a hysteresis loop. As can be seen in Table~\ref{tab3}, the surface
area of the activated HC significantly increases compared to raw HC.
The BET surface areas reach quite high values (857~m$^{2}$/g for DPP and
827~m$^{2}$/g for PNS). This finding reveals the significant role of
the activating agent in porous structure development. These results are
consistent with those reported in previous studies. Using potassium
oxalate and melamine as activating agents, Sevilla et~al.~\cite{35}
enhanced the pore volume (from 0.7~g/cm$^{3}$ to 1.6~g/cm$^{3}$) and
$S_{\mathrm{BET}}$ (from 1300--1600~m$^{2}$/g to 1800--3000~m$^{2}$/g)
for the eucalyptus HC. Hsini et~al.~\cite{36} increased the BET surface
area of HC made from prickly pear seed waste from 0.19 to 389~m$^{2}$/g
using CO$_2$ as the activating agent. Furthermore, Liu et~al.~\cite{34} found
activating HCs brought about an improvement in their apparent surface as
compared to those of raw HC, with  $S_{\mathrm{BET}}$ values reaching
367.3~m$^{2}$/g for corn straw and 142.6~m$^{2}$/g for corncob. To
highlight the significance of HTC before activation, the textural
properties of activated biochars were also evaluated (Table~\ref{tab3}).
As mentioned above, the activated biochar (BCA-DPP) was obtained via
steam activation of conventional chars by simple pyrolysis~\cite{23}.
According to Table~\ref{tab3}, the $S_{\mathrm{BET}}$ of HCA-DPP-1:8
increased by approximately 63\%, and  $V_{\mathrm{mic}}$ exhibited
an increase of 38\% as compared to BCA-DPP under the same activation
conditions. The $V_{\mathrm{mes}}$ value remained relatively constant while
the $V_{\mathrm{tot}}$ value increased. It could be concluded that HTC
before activation highly enhances the porous structure of ACs. These
findings are supported by previous works that compared hydrothermal and
conventional processes for activated carbon production~\cite{3,14}.
Additionally, under the same activation conditions, BCA can reach 50\% of
burn-off while activated hydrochar only reaches 20\% for the two
biomasses. The biomass type effect is more pronounced for HC than
activated HC. This outcome is consistent with the elemental analysis
and HHV results, showing that the type of biomass affects the atomic
and energetic properties of HCs. The BET surface area of HC-PNS is larger
than that of HC-DPP. However, the $S_{\mathrm{BET}}$ values of HCA-DPP and HCA-PNS
are similar. This finding is inconsistent with those of previous
studies, proving the significant impact of biomass type on textural
characteristics of activated HCs~\cite{14,31}. \looseness=-1

\subsubsection{Surface morphology}\label{sec322}
\vspace*{-4pt}

The SEM images of the produced HC-DPP-1:8, HC-DPP-1:12, HC-DPP 1:48,
HC-PNS-1:8, HC-PNS-1:12, and HC-PNS-1:48 are depicted in
Figure~\ref{fig3}. The SEM images show that the surface
of HC presents a large number of carbon microsphere structures. 

For both precursors, it is observed that most of the spheres of HC-DPP
and HC-PNS are grouped confirming that the generated HC spheres have
not fully developed into complete individual spheres, and they have a
wide range of sizes. Si et~al.~\cite{6} showed that the type of biomass
presents a determining factor in the surface morphology of HC.

Generally, mechanisms of nucleation and growth are often involved
in the production of carbon microspheres. When HTC intermediate
fragments approach critical saturation concentration, carbon nuclei are
produced. The resulting carbon nuclei grow isotopically to eventually
produce homogeneous carbon spheres, which is beneficial for producing
precursors with high specific surface areas~\cite{37}, leading us to
conclude that our conditions of the hydrothermal process did not reach
their optimum~\cite{38}. 

A decrease in SWR from 1:8 to 1:48 leads to the continued
development of carbon microspheres. This finding could be attributed to
the nucleation and generation of extended concentrations. Si
et~al.~\cite{6} demonstrated that the lower the SWR, the better the
microsphere development. They also proved that the surface is
relatively smooth for an SWR of 1:1~\cite{6}. In our case, it seems that a
lower SWR also corresponds to a better isolation of these spheres,
which is seen for both feedstocks in the case of the 1:8 ratio. This might
suggest that because the concentration of degradation fragments is
lower, the spheres do not fuse with the other spheres. 
In a previous work, it was
postulated that spheres are placed in layers; in this work, it seems
that for larger concentrations they fuse~\cite{27}. This in turn
corresponds to images where spheres allow a better identification of
the cellular structure of the parent material that is especially
unique to PNS.

A bursting of specific spheres is detected for HC-DPP-1:48 (Figure~\ref{fig3}). 
However, the microspheres of HC-PNS-1:48 retain their form. This
finding could be linked to biomass structure given that the particles
that are most likely produced from the transformation of cellulosic
fractions maintain their cellular structure. Generally, the development
of spherical particles is commonly related to the lignocellulosic
nature of precursors that contain lignin, cellulose, and
hemicellulose~\cite{1,12}. These carbohydrate polymers are composed of
monosaccharides, hexose, and acidic polysaccharides. Therefore, during
the HTC process, these basic units form microspherical particles via
dehydration, decarboxylation, condensation, and polymerization
reactions~\cite{33}. 

\subsubsection{XRD analysis}\label{sec323}
The XRD patterns are essential for evaluating the
carbon accumulation state. Figure~\ref{fig4} illustrates the XRD
spectra of the HCs and their corresponding activated DPP and PNS HCs.
The spectra of HCs show a diffraction peak at
$2\Theta\sim22$\textdegree, indicating  stable cellulose and lignin
phase structures, which suggests that the degradation of organic
components during the HTC process was incomplete~\cite{3}. 

\begin{figure}
\includegraphics{fig04}
\vspace*{-6pt}
\caption{\label{fig4}XRD spectra of HC and the corresponding activated
HCs of (a) DPP  and  (b) PNS.}
\vspace*{-6pt}
\end{figure}

The spectra of HCA-DPP and HCA-PNS exhibit two asymmetric peaks
corresponding to $2\Theta \sim24$\textdegree~and 
$2\Theta \sim43$\textdegree. Following HC activation, the amorphous carbon
peak changed from $2\Theta \sim22$\textdegree~to 24\textdegree~as the
aromatization degree increased, confirming the rearrangement reaction 
of carbon subsequent to cellulose and lignin
degradation~\cite{4}. The
diffraction peak at $2\Theta \sim43$ reveals the amorphous
structure of the precursor and indicates  the aromatization
degree~\cite{39}. The results illustrate the evolution of disordered
carbon from hydrothermal carbonization to the activation process.

\subsubsection{FTIR analysis}\label{sec324}
The FTIR spectra of raw materials, HC, and activated HC are pictured in
Figure~\ref{fig5}a, b, and c. 
As can be seen in Figure~\ref{fig5}a,
the raw DPP and their corresponding HCs present the same absorption
bands with a higher intensity compared to HC. The FTIR analysis of all
samples reveals a distinctive absorption band observed between 3393 and
3438~cm$^{-1}$, associated with the vibration of --OH groups present in
carboxylic and hydroxyl groups~\cite{3, 15}. The two peaks detected
between 2849 and 2927~cm$^{-1}$ mainly result from the C--H stretching
vibrations of aliphatic molecules from the \mbox{cellulose-hydrolyzed}
oligomers~\cite{3, 33}. A peak is discernible between 1710 and
1749~cm$^{-1}$, corresponding to the vibrations of the C${=}$O bonds
due to the carboxyl group of the cellulose~\cite{40}. Additionally, the
spectrum exhibits almost a peak at 1606~cm$^{-1}$ primarily due to 
C${=}$C bond stretching vibrations. Several peaks between 1445 and
1470~cm$^{-1}$ are mostly attributed to the stretching  C${=}$N 
bond~\cite{40}. 

\begin{figure}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig5}FTIR analysis of (a) HC-DPP,  (b) HC-PNS, and 
(c) activated HC (HCA-DPP-1:8 and HCA-PNS-1:8).}
{\vspace*{-2pt}}
\end{figure}


The peak at 1030~cm$^{-1}$ attributed to C--O bond stretching vibration
in cellulose and hemicellulose~\cite{15} appeared only for the raw
precursors. The disappearance of the C--O group from the HC surface
demonstrated that the hemicellulose was degraded. This result was
already verified by  elemental analysis (Table~\ref{tab2}) and by 
fast hemicellulose hydrolysis~\cite{14}.

\begin{table*}
\caption{\label{tab4}Langmuir, Freundlich, and Temkin isotherm model
constants for DCF adsorption on activated HC\vspace*{-2pt}}
\tabcolsep 8pt\begin{tabular}{lllll}
\tbody
Langmuir &amp;  &amp; ${q}_{\mathrm{max}}$ (mg/g) &amp;  $K_{\mathrm{L}}$ &amp; $R^{2}$ \vspace*{3pt}\\
&amp; HCA-PNS-1:8 &amp; 106.383 &amp; 0.777 &amp; 0.991 \\
&amp; HCA-DPP-1:8 &amp; 112.359 &amp; 1.769 &amp; 0.971\vspace*{3pt} \\
Freundlich &amp;  &amp; $K_{\mathrm{f}}$ &amp; $1/n$ &amp; $R^2$ \vspace*{3pt}\\
&amp; HCA-PNS-1:8 &amp; 53.801 &amp; 0.178 &amp; 0.864 \\
&amp; HCA-DPP-1:8 &amp; 64.955 &amp; 0.145 &amp; 0.922\vspace*{3pt} \\
Temkin &amp;  &amp; $B$ &amp; $A_{\mathrm{T}}$ &amp; $R^2$\vspace*{3pt} \\
&amp; HCA-PNS-1:8 &amp; 13.309 &amp; 59.013 &amp; 0.944 \\
&amp; HCA-DPP-1:8 &amp; 12.010 &amp; 250.276 &amp; 0.967
\botline
\end{tabular}
\end{table*}

By investigating the impact of solid/water variation, the spectra of
HC-DPP-1:12 and HC-DPP-1:48 show the disappearance of peaks between
1000 and 1100~cm$^{-1}$ observed in HC-DPP-1:8 and assigned to C--O bond
stretching vibration in cellulose and hemicellulose. Regarding PNS HCs
(Figure~\ref{fig5}b), the spectrum of HC-PNS-1:48 shows the absence of
the peak at 830~cm$^{-1}$ already observed for HC-PNS-1:8 and
HC-PNS-1:12 and ascribed to the aromatic ring's out-of-plane C--H bond
bending. The intensity of peaks at 1602 and 1705~cm$^{-1}$ attributed to
the vibrations of the C${=}$O bonds and C${=}$C bond stretching
vibrations, respectively, significantly decreases for HC-PNS-1:48.

The FTIR results showed that the HC still contained most of the ether
functional groups under these hydrothermal conditions and the variation in
SWR is not very noticeable.

Most functional groups have disappeared for activated HCs
(Figure~\ref{fig5}c). It may produce carbon-containing gas, which would
increase the organic matter's condensation and show that the lignin
structure was degraded under the activation process~\cite{4}. This
behavior may be attributed to the high activation temperature
(850~\textdegree C); only the most stable groups are conserved at this
temperature~\cite{41,42}. The FTIR spectrum shows only an absorption band
at 3490~cm$^{-1}$ related to O--H vibration and a weak band around
1126~cm$^{-1}$ for HCA-DPP with a lower intensity than the HC, which is
attributed to the vibration of C--O bonds in lactones, epoxides, and
ether structures~\cite{43}. The HCA-PNS spectrum exhibits additional
peaks at 1500, 1089, and 890~cm$^{-1}$, which are absent
for HCA-DPP. 
Tu et~al.~\cite{4} attributed the band at 1500~cm$^{-1}$  to a C${=}$C
polyaromatic bond, which further improved aromaticity for activated HC.
Saputra et~al.~\cite{43} related the peak
between 1109--1114~cm$^{-1}$ with a high intensity to the C--O group and
the peak between 871--873~cm$^{-1}$ to C--H alkene groups.

\section{Batch adsorption test}\label{sec4}

Adsorption isotherms describe the equilibrium and interaction of the
adsorbate and adsorbent. Equilibrium occurs when the adsorbate
concentration in the solution is in dynamic equilibrium with the
adsorbent concentration. The isotherm adsorption constants indicate the
affinity of the activated HC adsorbent surface characteristics for DCF.

Figure~\ref{fig6} presents experimental adsorption isotherms of DCF
onto HCA-PNS-1:8 and HCA-DPP-1:8 and a comparison between isotherm
models Langmuir, Freundlich, and Temkin described by  
Equations~(\ref{eq4})--(\ref{eq6}). 

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Adsorption isotherms of DCF on activated HC
adsorbents: HCA-PNS-1:8 and HCA-DPP-1:8 at temperature 
25~\textdegree C.}
\end{figure}

The plots of experimental data show that the two isotherms have a
similar shape. According to the Giles classification, the obtained
adsorption isotherms are of type H (high affinity), indicating that the
quantity adsorbed increases with  equilibrium concentration. The
amount of DCF adsorbed increases from 9.57 to 110.84 mg/g for
HCA-PNS-1:8 and from 9.64 to 110.58 mg/g for HCA-DPP-1:8. The DCF
amounts adsorbed by HCA-DPP-1:8 are slightly more than that adsorbed by
HCA-PNS-1:8, which is in agreement with the textural properties already
found for these produced activated hydrochars.

By comparing the regression coefficients $R^{2}$ from
Table~\ref{tab4} and from the plotted isotherm model curves shown in
Figure~\ref{fig6}, the DCF adsorption on two activated HCs in this study
is best fitted to the Langmuir model. 
The regression coefficient is
$R^{2}=0.99$ and the maximal adsorption capacity of the Langmuir model is
112.359~mg/g for HCA-DPP-1:8. The Freundlich model is the least fitted model
to experimental data as it diverges from the experimental data
at higher DCF concentrations.


\section{Conclusions}\label{sec5}
Date palm pits and pine nut shells were hydrothermally carbonized at
250~\textdegree C for 90 min for different SWRs of 1:8,
1:12, and 1:48. All HCs have high carbon contents, high SYs
(near 40\%) and HHV, and low ash contents. HC-DPP-1:48 presents the
\mbox{highest} HHV with a value of 29.7 MJ/kg and a low ash content of 
0.63\%. Surface properties were evaluated through SEM images, BET analysis,
and FTIR spectra. 
HCs show a large number of carbon microsphere structures, a
significant number of oxygen groups, and a low BET surface area. The
effect of biomass type on HC properties was more pronounced than that
of SWR evolution, which was very slight for the
properties evaluated. Steam activation of HCs for 1~h at 850~\textdegree
C led to high BET surface areas, reaching 857~m$^2$/g and 827~m$^2$/g
for HCA-DPP-1:8 and HCA-PNS-1:8, respectively. Activated biochars
prepared under the same activation conditions had a BET surface area
of 524~m$^2$/g. These outcomes demonstrate the role of HTC prior to
activation. The fuel qualities of HCs and the textural characteristics
of activated HCs produced from DPP are better than those derived from
PNS.

\section*{Nomenclature}
\mbox{}
\begin{inftab}
\begin{tabular}{ll}
\xtbody
{HC} &amp;  {hydrochar} \\
{HTC} &amp; {hydrothermal carbonization} \\
{DPP} &amp; date palm pit \\
{HC-DPP} &amp; hydrochar from date palm pit \\
{HCA-DPP} &amp; activated hydrochar of date palm pit \\
{BCA-DPP} &amp; activated biochar of date palm pit \\
{PNS}  &amp; pin nut shell \\
{HC-PNS} &amp; hydrochar from pine nut shell \\
{HCA-PNS} &amp; activated hydrochar of pine nut shell \\
{SY} &amp; solid yield \\
{SWR} &amp;  solid/water ratio \\
{DCF} &amp; diclofenac sodium salt 
\end{tabular}
\end{inftab}

\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*{Acknowledgment}

The authors are grateful to Agencia Espa\~{n}ola de Investigaci\'{o}n
for financial help through project
(PID2020-116144RB-I00/AEI/10.13039/501100011033) to the  Servicios de
Apoyo a la Investigaci\'{o}n de la UEX (SAIUEX) for their support on
characterization analysis. 

\CDRGrant[AEI]{PID2020-116144RB-I00/AEI/10.13039/501100011033}

\back{}

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\refinput{crchim20240527-reference.tex}

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