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\DOI{10.5802/crchim.329}
\datereceived{2024-03-20}
\daterevised{2024-06-15}
\dateaccepted{2024-07-24}
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\dateposted{2024-11-13}
\begin{document}

\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{Synthesis of Mg-, Al- and Mg/Al-date palm fronds modified
biochars: characterization and investigations on phosphorus adsorption
characteristics}

\alttitle{Synth\`{e}se de biochars modifi\'{e}s de d\'{e}chets de
palmier dattiers par du magn\'{e}sium et/ou aluminium :
caract\'{e}risation et investigations des caract\'{e}ristiques
d'adsorption du phosphore}

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

\author{\firstname{Samar} \lastname{Hadroug}\CDRorcid{0000-0002-5254-0725}}
\address{Water Research and Technologies Centre, Echo-park of Borj
Cedria, University of Carthage, Tunisia}
%\email{samarhadroug@gmail.com}

\author{\firstname{Malik} \lastname{Al-Wardy}\CDRorcid{0000-0002-9105-2804}}
\addressSameAs{1}{Centre for Environmental Studies and Research, Sultan
Qaboos University, Muscat, Al-Khoud 123, Oman}
%\email{mwardy@squ.edu.om}

\author{\firstname{Helmi} \lastname{Hamdi}\CDRorcid{0000-0002-4152-9192}}
\address{Food, Water and Waste Sustainability Program, Center for
Sustainable Development, College of Arts and Sciences, Qatar
University, P.O. Box 2713, Doha, Qatar}
%\email{hhamdi@qu.edu.qa}

\author{\firstname{Jamal} \lastname{~Al-Sabahi}\CDRorcid{0000-0001-7788-8229}}
\address{College of Agricultural and Marine Sciences, Sultan Qaboos
University, Muscat, Al-Khoud 123, Oman}
%\email{jamal@squ.edu.om}

\author{\firstname{Imene} \lastname{Bekri}\CDRorcid{0000-0003-2914-1635}}
\address{National Center of Materials Research, Echo-park of Borj
Cedria, University of Carthage, Tunisia}
%\email{bekrimene@gmail.com}

\author{\firstname{Ahmed} \lastname{Al-Raeesi}}
\addressSameAs{1}{Centre for Environmental Studies and Research, Sultan
Qaboos University, Muscat, Al-Khoud 123, Oman}
%\email{aalraeesi@squ.edu.om}

\author{\firstname{Wissem} \lastname{Hamdi}\CDRorcid{0000-0002-4753-0901}}
\address{Higher Institute of the Sciences and Techniques of Waters,
Gabes University, Tunisia}
%\email{wissemhemdi@yahoo.fr}

\author{\firstname{Mejdi} \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}
%\email{mejdi.jeguirim@uha.fr}

\shortrunauthors

\keywords{\kwd{Lignocellulosic
biomass}\kwd{Pre-modification}\kwd{Pyrolysis}\kwd{Metal-biochar
composites}\kwd{Phosphorous recovery}}

\altkeywords{\kwd{Biomasse
lignocellulosique}\kwd{Pr\'{e}-modification}\kwd{Pyrolyse}\kwd{Composites
m\'{e}tal-biochar}\kwd{R\'{e}cup\'{e}ration de phosphore}}

\thanks{Sultan Qaboos University (CL/SQU-QU/CESR/23/01), Qatar
University (IRCC-2023-004)}

\begin{abstract}
Phosphorus (P) recovery by engineered biochars for a further subsequent
use in agriculture promotes circular economy and sustainability
concepts. In this study, three biochars were synthesized at
500 {\textdegree}C from pre-impregnated date palm fronds (DPFs) with
1 M solutions containing either MgCl\textsubscript{2} (DPF-Mg-B), AlCl\textsubscript{3} (DPF-Al-B) or a combination of MgCl\textsubscript{2}/AlCl\textsubscript{3} (DPF-Mg/Al-B).
Subsequently, the structural, textural, and surface chemistry
properties of these biochars were assessed. Then, the P recovery of
these biochars from aqueous solutions was evaluated under various
experimental conditions. Characterization results revealed that the
synthesized biochars exhibited high loads deposition of Mg- and/or
Al-based nanoparticles. The DPF-Mg/Al-B showed the most interesting
physicochemical properties. Notably, the corresponding Mg and Al
contents; BET surface area and total pore volume were evaluated to 99.6
and 136.0 mg\textperiodcentered\text{g}\textsuperscript{--1};
180.0 m\textsuperscript{2}\textperiodcentered\text{g}\textsuperscript{--1} and
0.09 cm\textsuperscript{3}\textperiodcentered\text{g}\textsuperscript{--1}, respectively. Batch P
recovery assays showed that P recovery efficiency by the three studied
biochars was influenced by the contact time, the initial aqueous pH, P
concentration and also the presence of foreign anions, especially
nitrates and chlorides. Moreover, owing to its attractive
properties,{\unskip\break} the DPF-Mg/Al-B exhibited the highest P
recovery capacity (14.1 mg\textperiodcentered\text{g}\textsuperscript{--1}) which is 1.3
and 1.7 higher than that of DPF-Mg-B and DPF-Al-B, respectively. The
kinetic and isotherm experimental data modelling suggest that P
recovery process was governed by both physical and chemical mechanisms.
These P-loaded biochars have the potential to serve as promising
fertilizers, offering a sustainable alternative to synthetic
counterparts.
\vspace*{-2pt}
\end{abstract}

\begin{altabstract}
La r\'{e}cup\'{e}ration du phosphore (P) par des biochars modifi\'{e}s
pour une utilisation ult\'{e}rieure en agriculture favorise les
concepts d'\'{e}conomie circulaire et de d\'{e}veloppement durable.
Dans cette \'{e}tude, trois biochars ont \'{e}t\'{e}
synth\'{e}tis\'{e}s \`{a} 500 {\textdegree}C \`{a} partir de feuilles
de palmier dattier (DPF) pr\'{e}-impr\'{e}gn\'{e}es avec des solutions
contenant soit du MgCl\textsubscript{2} (DPF-Mg-B), du AlCl\textsubscript{3} (DPF-Al-B) ou un
m\'{e}lange de MgCl\textsubscript{2}/AlCl\textsubscript{3} (DPF-Mg/Al-B). Par la suite, les
propri\'{e}t\'{e}s chimiques structurelles, texturales et de surface de
ces biochars ont \'{e}t\'{e} \'{e}valu\'{e}es. Ensuite, la
r\'{e}cup\'{e}ration du P par ces biochars \`{a} partir de solutions
aqueuses a \'{e}t\'{e} \'{e}valu\'{e}e sous diverses conditions
exp\'{e}rimentales. Les r\'{e}sultats de la caract\'{e}risation ont
r\'{e}v\'{e}l\'{e} que des nanoparticules de Mg et/ou d'Al ont
\'{e}t\'{e} d\'{e}pos\'{e}es sur les biochars synth\'{e}tis\'{e}s. Le
DPF-Mg/Al-B a montr\'{e} les propri\'{e}t\'{e}s physico-chimiques les
plus int\'{e}ressantes. Notamment, les teneurs correspondantes en Mg et
Al ; la surface BET et le volume total des pores ont \'{e}t\'{e}
\'{e}valu\'{e}s \`{a} 99,6 et 136,0 mg\textperiodcentered\text{g}\textsuperscript{--1} ;
180,0 m\textsuperscript{2}\textperiodcentered\text{g}\textsuperscript{--1} et
0,09 cm\textsuperscript{3}\textperiodcentered\text{g}\textsuperscript{--1}, respectivement. Les essais de
r\'{e}cup\'{e}ration du P ont montr\'{e} que cette efficacit\'{e} est
influenc\'{e}e par le temps de contact, le pH aqueux initial, la
concentration en P ainsi que la pr\'{e}sence d'autres anions, notamment
les nitrates et les chlorures. Le biochar DPF-Mg/Al-B a
pr\'{e}sent\'{e} la capacit\'{e} de r\'{e}cup\'{e}ration de P la plus
\'{e}lev\'{e}e (14,1 mg\textperiodcentered\text{g}\textsuperscript{--1}), soit 1,3 et 1,7
sup\'{e}rieure \`{a} celle du DPF-Mg-B et du DPF-Al-B, respectivement.
La mod\'{e}lisation des donn\'{e}es exp\'{e}rimentales cin\'{e}tiques
et isothermes sugg\`{e}re que le processus de r\'{e}cup\'{e}ration du P
est r\'{e}gi par des m\'{e}canismes \`{a} la fois physiques et
chimiques. Ces biochars charg\'{e}s en P peuvent \^{e}tre utilis\'{e}s en
agriculture \`{a} la place des fertilisants synth\'{e}tiques.
\end{altabstract}

\maketitle

\vspace*{-1pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

Date palm trees cultivation (\textit{Phoenix dactylifera}~L.) is an important
agricultural activity in arid and semi-arid regions worldwide,
including the Middle East and North Africa, India, and the
USA~\cite{1,2}. Nowadays, the global cultivated area dedicated to date
palms exceeds 1.381 million hectares and the number of date palm trees
is around 120 million~\cite{3}. Throughout the growth and fruit
harvesting phases of these trees, various residues are generated,
consisting primarily of fronds, trunks, and seeds~\cite{4,5,6}.
Research by Fang {et~al.}~\cite{5} indicates that each productive date
palm tree generates approximately 50~kg of waste annually, resulting in
an annual worldwide production of about 6 million tons of
lignocellulosic residues. Presently, the majority of these wastes are
either incinerated or disposed of in landfills~\cite{6}. 

Recent studies have explored the use of these abundant wastes in
various applications, including energy recovery, animal feeding,
extraction of bio-based materials, soil enrichment, and treatment of
wastewater and gaseous effluents~\cite{1}. One recommended approach for
valorizing these wastes involves their thermochemical treatment through
the pyrolysis process. This method permits their complete conversion
into: (i)~biofuels, serving as a renewable energy source~\cite{7}, and
(ii)~biochars, which can be utilized for agro-environmental
applications such as soil amendments~\cite{1} or efficient adsorbents
for various mineral and organic pollutants including heavy metals,
ammonium, nitrates, dyes, and pharmaceuticals~\cite{8,9,10,11,12}.
\looseness=-1

Simultaneously, huge amounts of phosphorus (P) are lost in urban and
industrial wastewaters, with projections estimating this amount to
reach 2.4 million tons (MT) by 2050~\cite{13}. Together with nitrogen,
P can contribute significantly to the degradation of water quality
through the eutrophication process~\cite{14}. Hence, the recovery of P
from discharged wastewaters and its subsequent reuse within a circular
economy framework can help preserve the environment and mitigate the
excessive depletion of phosphate reserves~\cite{15}. Various
technologies have been explored for P recovery from effluents,
including P precipitation as struvite, P storage through polyphosphates
accumulation, and P adsorption onto tailored materials like
biochars~\cite{16}. The latter method is particularly promising due to
its simplicity, environmental friendliness, and high effectiveness
under specific conditions~\cite{17}.\looseness=-1

The transformation of date palm wastes into biochars for P recovery
from effluents, followed by their use in agriculture as an organic
amendment, represents an outstanding approach that aligns with circular
economy and sustainability principles~\cite{1}. However, pristine
biochars typically exhibit limited adsorption capacity for oxyanions in
general, and P in particular, mainly due to their negatively charged
surface, low specific surface area, and microporosity~\cite{18}.
Consequently, the synthesis of engineered biochars has been identified
as a promising research avenue to enhance their physico-chemical
properties and, subsequently, their P recovery efficiency. In this
regard, the impregnation of raw biomasses with metal-rich
wastes~\cite{17} or solutions rich in magnesium~\cite{19},
calcium~\cite{20}, aluminum~\cite{21}, and iron~\cite{22} has been
experimentally tested at the laboratory scale for the synthesis of
efficient metal-biochar composites. Review papers on the topic have
indicated that there has been no clear relationship between P recovery
efficiency and the type of metal used for biochar
modification~\cite{18,23,24}. Indeed, this process is simultaneously
influenced by various factors, including the metal concentration,
nature of the feedstock, pyrolysis conditions, and adsorption
conditions, particularly pH and initial P
\mbox{concentration}~\cite{23}. \looseness=-1

Additionally, impregnating biomasses with solutions containing salt
mixtures, such as Mg/Al, Mg/Fe, Zn/Al, and Ca/Fe, enables the
production of layered double hydroxides (LDH)-modified biochars,
potentially enhancing P recovery efficiency~\cite{25}. For example, a
research study carried out by Hadroug {et~al.}~\cite{26} demonstrated
that post-modifying poultry manure-derived biochar with Mg/Al instead
of Al alone increased P recovery by a factor of~3.3. A comparable
result was observed by Zheng {et~al.}~\cite{27}, where the P recovery
efficiency of Mg/Al-modified biochar from wheat straw was 15.9 and 1.9
times higher than the same biochar when modified with Mg or Al,
respectively. However, the efficiency of LDH-loaded biochars in P
recovery depends not only on the nature of LDHs loaded onto the biochar
surface~\cite{28} but also on the type of the feedstock and also the
pyrolysis and adsorption conditions~\cite{23}. For instance, the P
recovery capacity of Mg/Al-modified biochars was found to range from
around $7~\text{mg}{\cdot}\text{g}^{-1}$ for coffee ground waste at
500~{\textdegree}C~\cite{29} to over
$153~\text{mg}{\cdot}\text{g}^{-1}$ for wheat straw at
600~{\textdegree}C~\cite{27}. However, very few studies have explored
the use of biochars derived from raw and/or modified date palm wastes
for P recovery from aqueous solutions~\cite{11,12}. Besides, to the
best of our knowledge, the modification of biochars derived from date
palm fronds with both single and mixed salts (i.e., Mg and Al) and
their application for P recovery from aqueous solutions have not been
previously investigated.

Therefore, the primary objectives of this research study are to:
(i)~synthesize and thoroughly characterize the three biochars generated
from the pyrolysis of date palm fronds separately pre-impregnated with
1~M solutions of MgCl$_{2}$, AlCl$_{3}$, or MgCl$_{2}$/AlCl$_{3}$;
(ii)~investigate the efficiency of these three modified biochars in
recovering P from aqueous solutions under different conditions such as
contact time, initial pH, initial P concentration, and the presence of
foreign anions; and (iii)~deduce the benefits of using such P-loaded
materials in agriculture.

\section{Materials and methods} \label{sec2}
\subsection{Biochar synthesis} \label{sec2.1}

Dried date palm fronds (DPFs) were obtained from the agricultural
experiment station of Sultan Qaboos University in Oman. Initially, they
underwent a grinding step using a mechanical grinder, and only the
fraction with particle sizes less than 1~mm was selected. Subsequently,
the ground DPFs were mixed with solutions containing 1~M MgCl$_{2}$, or
1~M AlCl$_{3}$, or a mixture of 1~M MgCl$_{2}$ and 1~M AlCl$_{3}$. This
impregnation process was performed using a magnetic stirrer
(Gallenkamp, Leicestershire, UK) for a contact time of 16
hours and a mass-to-liquid ratio of 1:$25~\text{g}{\cdot}
\text{mL}^{-1}$. This ratio was chosen in order to get a smooth
agitation and consequently a good contact between the feedstock and the
solution components. Following the impregnation step, the solid phases
were separated through centrifugation at 3000~rpm (Beckman,
Indianapolis, USA) and then dried overnight at 85~{\textdegree}C.

The pyrolysis procedure was identical to the methodology described in
Jellali {et~al.}~\cite{30}. In summary, the dried metal-loaded DPFs were
heated under a nitrogen atmosphere using a tubular furnace (Carbolite,
TF1-1200, Neuhausen, Germany). The pyrolysis temperature and heating
gradient were fixed to 500~{\textdegree}C and
5~{\textdegree}C${\cdot}$min$^{-1}$, respectively. The samples were
maintained at this maximum temperature for 2~h before cooling to
ambient temperature. The resulting solids were thoroughly washed with
deionized water until getting stable electrical conductivity values
corresponding to the elimination of excess metals and chloride ions.
This step was followed by an overnight drying at 85~{\textdegree}C.
Subsequently, to ensure experimental uniformity, these dried biochars
were gently ground and sieved to obtain a particle size range of
0.08--0.5~mm. The resulting pre-modified biochars were designated as:
DPF-Mg-B, DPF-Al-B, and DPF-Mg/Al-B, corresponding to treatments
involving MgCl$_{2}$, AlCl$_{3}$, and a mixture of both salts,
respectively.

\subsection{Biochar characterization} \label{sec2.2}

The surface morphology and qualitative composition of the three
metal-loaded biochars were analyzed using scanning electron microscopy
(SEM) coupled with an energy-dispersive X-ray spectrometer (EDS) (Jeol,
Jsm-7800F, Tokyo, Japan). X-ray diffraction (XRD) patterns were
obtained with a D8-advance diffractometer (Rigaku, Miniflex 600, Tokyo,
Japan), covering diffraction angles ($2{\theta}$) ranging from
10{\textdegree} to 90{\textdegree}. Identification of crystalline peaks
on the biochars was carried out using the ICDD \mbox{database}
(International Centre for Diffraction Data). The contents of major
minerals (Mg, Al, Ca, K, P, etc.) were determined using an X-ray
fluorescence (XRF) unit equipped with a rhodium target X-ray tube
(Rigaku, Nexqc, Tokyo, Japan). Textural properties, including the
Brunauer--Emett--Teller (BET) surface area, total pore volume, and
average pore diameter, were derived from corresponding N$_{2}$
adsorption--desorption isotherms at 77~K using a Micromeritics ASAP
2020 gas adsorption apparatus. Finally, the surface chemistry
properties of the biochars involved the assessment of: (i)~the pH of
zero-point charge through the pH drift method, and (ii)~the main
functional groups through the exploitation of Fourier
transform-infrared (FTIR) analyses spectra by a Miniflex 600
spectrometer (Rigaku, Tokyo, Japan). FTIR spectra were recorded from
KBr pellets of the samples within the wavenumber range of 500 to
4000~cm$^{-1}$. 

\subsection{Phosphorus recovery experiments} \label{sec2.3}
\subsubsection{Preparation and analysis of P solutions} \label{sec2.3.1}

A phosphorus stock solution ($3000~\text{mg}{\cdot}\text{L}^{-1}$) was
prepared by mixing the corresponding mass of Na$_{2}$HPO$_{4}$
(obtained from Sigma Aldrich) into deionized water. This stock solution
was consistently utilized in this study for the preparation of the
desired batch solutions at fixed P concentrations. Following the
completion of the batch adsorption assays, the mixture underwent
filtration through 0.45~${\upmu}$m filters (Whatman, Buckinghamshire,
UK). Phosphorus concentrations in the filtered solutions were
determined using the Fleury method and a UV--visible spectrometer
(UV-1900i, Shimadzu; Kyoto, Japan) at a wavelength of 430~nm. The pH
values of the aqueous solutions were adjusted using 0.1~M HNO$_{3}$ and
NaOH solutions. The pH measurements were conducted with a
pre-calibrated pH meter (Mettler Toledo; Ohio, USA).

\subsubsection{Batch adsorption procedure} \label{sec2.3.2}

Static adsorption experiments were conducted to assess the efficacy of
the synthesized biochars in removing P from aqueous solutions under
diverse experimental conditions. The procedure involved agitating a
specific quantity of biochars in 50~mL of P solution at room
temperature (20 ${\pm}$ 2~{\textdegree}C). The agitation was achieved
using glass Erlenmeyer flasks and a multi-position magnetic stirrer
(Gallenkamp, UK) set at a speed of 600~rpm. Default parameters, unless
otherwise specified, included a biochar dose of
$2~\text{g}{\cdot}\text{L}^{-1}$, a natural pH (unadjusted), an initial
P concentration of $100~\text{mg}{\cdot}\text{L}^{-1}$, and a contact
time of 180~min. These parameters were chosen on the basis of
preliminary assays. The current study investigated the effect of the
following parameters on P recovery by the synthesized biochars:
(i)~contact time varying from 1~min to 180~min, (ii)~initial pH values
between 3 and 11, (iii)~initial P concentrations ranging from 10 to
$100~\text{mg}{\cdot}\text{L}^{-1}$, and (iv)~the presence of competing
anions such as chlorides, sulfates, and nitrates at concentrations of
1000, 400, and $100~\text{mg}{\cdot}\text{L}^{-1}$, respectively. These
concentrations are in the range of urban wastewater
composition~\cite{31}.

At a given time ``$t$'', the amount of recovered P ($q_{t}$) and the
corresponding yield ($\eta_{t}$) were determined using the mass balance
conservation principle as \mbox{follows}:
{\begin{eqnarray}
&\displaystyle q_{t}=\frac{(C_{0}-C_{t}) * V}{m}\label{eq1}\Seqnsplit
&\displaystyle \eta_{t}=\frac{(C_{0}-C_{t})}{C_{0}} * 100 \label{eq2}
\end{eqnarray}}\unskip
where, $C_{0}$ and $C_{t}$ represent the P initial and time ``$t$''
concentrations in the aqueous solution
($\text{mg}{\cdot}\text{L}^{-1}$), respectively, $V$ stands for the
volume of the solution (L), and $m$ is the mass of the adsorbent (g).

The experimental data on P recovery kinetics were fitted to three
standard models: pseudo-first order (PFO), pseudo-second order (PSO),
and intraparticle and film diffusion models. Additionally, the measured
experimental isothermal data were fitted to the Langmuir, Freundlich,
and Dubinin--Radushkevich (D--R) isotherm models. These kinetic models
equations and assumptions were previously presented in the
literature~\cite{30,32}. To assess the agreement between the measured
kinetic or isothermal data ($q_{\mathrm{meas}}$) and the calculated
values ($q_{\mathrm{calc}}$), the related correlation coefficients and
mean average percentage errors (MAPE) were employed, as outlined below:
{\begin{eqnarray}
\text{MAPE}=\frac{\sum_{i=1}^{i=N}\left|\frac{q_{\mathrm{meas}}-
q_{\mathrm{calc}}}{q_{\mathrm{meas}}}\right|}{N} \label{eq3}
\end{eqnarray}}\unskip
where $N$ represents the number of experimental runs.

During this work, batch assays were realized in triplicate and the
average values and standard deviation are presented in the
corresponding figures.

\subsection{Statistical analyses} \label{sec2.4}

The experimental data reported in this work represent the average
values of triplicate measurements. Excel 2016 was used to perform the
regression analysis and to plot the experimental data. The error bars
represent the standard deviation of the measured data.

\section{Results and discussion} \label{sec3}
\subsection{Biochars characterization} \label{sec3.1}

The process of modification had a significant impact on both the
production yields and physico-chemical properties of the derived
biochars as given in Table~\ref{tab1}. Specifically, the biochar yields were
35.7\%, 37.5\%, 38.1\%, and 37.7\% for the raw DPFs, and DPFs
pretreated with Mg, Al, and Mg/Al, respectively. This yield variation
is attributed to the presence of metals on the surface of DPFs, which
diminishes the efficiency of the carbonization process~\cite{33}. Some
minerals, exhibiting a catalytic effect on the decomposition of
cellulose into lower molecules that subsequently condense, contribute
to the increased production yield of biochar~\cite{34}. A comparable
pattern was observed for biochars derived from modified wheat
straw~\cite{27} and a mixture of poultry manure and DPFs~\cite{35}. In
this latter study, due to the relatively high Mg/Al contents deposited
on the feedstock, the production yield of the corresponding biochar was
52.4\% higher than the untreated one.

\begin{table*} %tab1
\tabcolsep=10pt
\caption{\label{tab1}Main properties of the synthesized biochars (ND:
not detected){\vspace*{-2pt}}}
\begin{tabular}{cccccccc}
\thead
\multicolumn{8}{c}{Mineral composition ($\text{mg}{\cdot}\text{g}^{-1}$)}\\
& Ca & P & K & Mg & Al & Fe & S\\
\endthead
DPF-Mg-B & 5.2 & 0.9 & ND & 164.0 & ND & 0.5 & 1.2\\
DPF-Al-B & 5.7 & 0.8 & 0.2 & ND & 160.0 & 0.6 & 0.8\\
DPF-Mg/Al-B & 3.7 & 0.4 & ND & \099.6 & 136.0 & 0.5 & 1.0\botline
{\vspace*{-.5pc}}
\end{tabular}

\begin{tabular}{cccc}
\thead
\multicolumn{4}{c}{Textural properties}\\
& BET surface area ($\text{m}^{2}{\cdot}\text{g}^{-1}$) &
Total pore volume ($\text{cm}^3{\cdot}\text{g}^{-1}$) &
Pore size (nm)\\
\endthead
DPF-Mg-B &225.4 & 0.100 & 14.1\\
DPF-Al-B &130.8 & 0.124 & \07.4\\
DPF-Mg/Al-B &180.0 & 0.090 & \06.9\botline
{\vspace*{-.5pc}}
\end{tabular}

\begin{tabular}{cc}
\thead
\multicolumn{2}{c}{Surface chemistry: pHzpc}\\
\endthead
DPF-Mg-B & 11.00\\
DPF-Al-B & \09.41\\
DPF-Mg-Al-B & \05.58
\botline
\end{tabular}
\end{table*}

As anticipated, XRF analyses revealed elevated Mg, Al, and Mg/Al
contents in the DPF-Mg-B, DPF-Al-B, and DPF-Mg/Al-B samples,
respectively (Table~\ref{tab1}). This increase can be imputed to the
deposition of Mg- and/or Al-based nanoparticles on the surface of the
biochars. A similar trend was reported for previous studies dealing
with biochar modification with Mg and/or Al modified
biochars~\cite{27,36}. This result was supported by the XRD analysis
(Figure~\ref{fig1}). Indeed, the pre-modification of DPF with
MgCl$_{2}$ resulted in the appearance of nanoparticles of magnesium
hydroxide (Mg(OH)$_{2}$) and magnesium oxide (MgO) on the DFP-Mg-B
(Figure~\ref{fig1}a). These peaks were observed at diffraction angles
($2\theta$) of 18.7{\textdegree}, 32.9{\textdegree}, 38.1{\textdegree},
43.0{\textdegree}, 51.0{\textdegree}, 58.8{\textdegree},
62.3{\textdegree}, 68.4{\textdegree}, 72.1{\textdegree},
78.7{\textdegree}, and 81.3{\textdegree}. Similarly, on the DPF-Al-B
surface, distinctive peaks corresponding to boehmite (AlOOH) were
identified at $2\theta$ values of 38.5{\textdegree}, 48.9{\textdegree},
64.9{\textdegree}, and 82.5{\textdegree} (Figure~\ref{fig1}b). Lastly,
various aluminum and magnesium nanoparticles were detected on the
DPF-Mg/Al-B surface, including aluminum oxide (Al$_{2}$O$_{3}$), AlOOH,
MgO, and Mg(OH)$_{2}$ (Figure~\ref{fig1}c).

\begin{figure}
{\vspace*{.25pc}}
\includegraphics{fig01}
\caption{\label{fig1}X-ray diffractograms of DPF-Mg-B~(a),
DPF-Al-B~(b), and DPF-Mg/Al-B~(c) (${\clubsuit}$: Magnesium hydroxide,
${\CIRCLE}$: Magnesium oxide; ${\blacklozenge}$: Boehmite;
${\blacktriangle}$: Aluminum oxide).}
{\vspace*{.15pc}}
\end{figure}

Previous studies by Lee {et~al.}~\cite{37}, Nardis {et~al.}~\cite{38},
and Hadroug {et~al.}~\cite{26} have reported the deposition of Mg- or
Al- or Mg/Al-nanoparticles on modified biochars when using MgCl$_{2}$,
AlCl$_{3}$, and MgCl$_{2}$/AlCl$_{3}$ as modifier agents, respectively.
It is noteworthy that the nature and loads of these nanoparticles
depend largely on the conditions of the impregnation process. For
instance, Zheng {et~al.}~\cite{27} demonstrated the formation of
magnesium aluminate (MgAl$_{2}$O$_{4}$) instead of Mg(OH)$_{2}$. The
pronounced abundance of Mg- and/or Al-nanoparticles on the surface of
the current modified biochars may enhance the potential for P recovery
through the precipitation \mbox{process}~\cite{39}.

The presence of relatively high contents of Mg, Al, and Mg/Al peaks in
the DPF-Mg-B, DPF-Al-B, and DPF-Mg/Al-B, respectively, was further
corroborated through EDS analysis (Figure~\ref{fig2}a--c). Moreover,
SEM images revealed that the modified biochars showcased expanded
micropores (Figure~\ref{fig2}), a result of volatile matter release
during the pyrolysis \mbox{process}~\cite{34}.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig02}
{\vspace*{-2pt}}
\caption{\label{fig2}SEM/EDS analyses of DPF-Mg-B~(a), DPF-Al-B~(b) and
DPF-Mg/Al-B~(c).}
\end{figure*}

This outcome was validated with the N$_{2}$ adsorption/desorption
isotherm analyses, wherein the total pore volume (TPV) for DPF-Mg-B,
DFP-Al-B, and DPF-Mg/Al-B materials was determined to be 0.100, 0.124,
and $0.090~\text{cm}^{3}{\cdot}\text{g}^{-1}$, respectively
(Table~\ref{tab1}). It is noteworthy that the DPF-Mg-B exhibited the
highest BET surface area value
($225.4~\text{m}^{2}{\cdot}\text{g}^{-1}$), \mbox{surpassing} that of
DPF-Mg/Al-B and DPF-Al-B by 1.3 and 1.7 times, respectively
(Table~\ref{tab1}). The comparatively lower surface area of DPF-Al-B
could be attributed to pores blockage caused by the formation of
Al-based nanoparticles. Specifically, the small size of AlOOH and
Al$_{2}$O$_{3}$ facilitates their deposition into the pores, leading to
blockage and consequently a reduction in the related BET surface area.
This outcome is in line with previous research results~\cite{27,38,40}.
For instance, Zheng {et~al.}~\cite{27} showed that the pre-modification
with AlCl$_{3}$ reduced the BET surface area of a biochar generated
from wheat straw pyrolysis by approximately 25.3\%. Across all
synthesized biochars, the average pore diameters remained relatively
small, ranging from 6.9 to 14.1~nm (Table~\ref{tab1}). The
well-developed porosity and relatively high specific surface areas of
the synthesized biochars may enhance the efficiency of P recovery from
aqueous solutions. Concerning surface chemistry properties, the FTIR
spectra of the raw DPF confirm the characteristic features of
lignocellulosic materials, displaying a hydroxyl band (--OH) at
3391~cm$^{-1}$, carbonyl (C${=}$O) stretches associated with
hemicellulose at 1589~cm$^{-1}$, alkene group (C--H) bending vibrations
at 1384~cm$^{-1}$, and polar carboxylic (C--O) stretches linked to
cellulose and lignin vibrations around 1109~cm$^{-1}$~\cite{41}
(Figure~\ref{fig3}). Additionally, a~\mbox{minor} peak corresponding to
the P--O group was identified at 567~cm$^{-1}$~\cite{17}. 
\looseness=-1

\begin{figure*}
\includegraphics{fig03}
{\vspace*{-.6pc}}
\caption{\label{fig3}FTIR spectra of the raw DPF and its pre-modified
biochars with Mg, Al and Mg/Al.}
{\vspace*{-.4pc}}
\end{figure*}

Following chemical impregnation, important alterations were evident in
the FTIR spectra of the biochars. Notably, a new narrow peak emerged at
3695~cm$^{-1}$, attributed to the --OH stretching vibration resulting
from the formation of Mg(OH)$_{2}$ nanoparticles~\cite{17}.
Furthermore, the intensities of various peaks significantly increased
(Figure~\ref{fig3}). For instance, the --OH stretching vibration
intensity in the spectra of the three modified biochars rose notably
due to the formation of magnesium and aluminum nanoparticles.
Additionally, the intensities of oxygen-based functional groups
associated with aluminum oxides (i.e., C${=}$O, C--O) have also increased
as it can be seen in Figure~\ref{fig3}. Inversely, the C--H peak nearly
disappeared in the spectra of the three pre-modified biochars,
confirming the successful doping of Mg and/or Al into the synthesized
biochars. A similar result was found by Deng {et~al.}~\cite{42} in their
study on P recovery using Al-modified biochar generated from corn cob
wastes.

Finally, the pre-modification of date palm fronds with Mg, Al, and
Mg/Al led to the detection of new peaks that are associated with Mg--O
at 475~cm$^{-1}$~\cite{43} and Al--O bands at 877~cm$^{-1}$~\cite{40},
respectively.

The determination of the pHzpc for the modified biochars was conducted
using the pH drift method. As expected, the highest value was observed
for DPF-Mg-B (11.00), followed by DPF-Mg/Al (9.41) (Table~\ref{tab1}).
This can be attributed to the alkaline nature of Mg(OH)$_{2}$ formed
during the modification process. Similarly, elevated pH values were
observed for a Mg-modified food waste biochar (11.03)~\cite{41},
Mg/Al-modified date palm waste-derived biochar (10.02)~\cite{12}, and
Mg-modified pig manure-derived biochar (10.40)~\cite{38}. This outcome
suggests that P recovery through electrostatic interactions will be
advantageous across an expanded pH range (for aqueous pH values
${<}$pHzpc). Conversely, DPF-Al-B exhibited a relatively low pHzpc value
(5.58), indicating that for common wastewater (typically with a pH
higher than 6.5), P recovery through electrostatic interactions is
likely to be minimal. Acidic pHzpc values were also reported for
Al-modified pig manure-derived biochars~\cite{38}.

\subsection{Phosphorus recovery assays} \label{sec3.2}

The influence of key experimental parameters on P recovery using the
three DPF-modified biochars was evaluated based on the conditions
outlined in \mbox{Section}~\ref{sec2.3}. The principal findings are
elaborated upon in the following sections.

\subsubsection{Effect of contact time: kinetic study} \label{sec3.2.1}

Figure~\ref{fig4} illustrates the correlation between the recovered P
amounts and the contact time. It is evident that P recovery is
contingent on time. Specifically, the rate of P adsorption experiences
a substantial increase in the initial phase of the experiment (up to
10~min) before gradually diminishing and stabilizing at equilibrium
after 3~h. At a contact time of 10~min, the recovered P rates by
DPF-Mg-B, DPF-Al-B, and DPF-Mg/Al-B constituted 54\%, 47\%, and 81\% of
the total recovered amounts, respectively. The slower kinetic observed
for DPF-Mg-B even if it has more interesting textural properties (see
Table~\ref{tab1}) may be imputed to the fact this biochar recovers P
through a combination of physical and chemical mechanisms (i.e.,
complexation and precipitation). The relatively rapid P retention can
be attributed to the facile transfer of this macroelement from aqueous
solutions to the surface of biochar particles through electrostatic
interactions~\cite{41}. Subsequently, the slower secondary kinetic
phase can be ascribed to P ions transfer into the pores of the modified
biochars through intraparticle diffusion~\cite{44}. Ultimately, an
\mbox{equilibrium} state, characterized by quasi-constant adsorbed
amounts, is achieved after around 2--3~h, signifying the net decrease
of available adsorption sites~\cite{30}. A similar trend has already
been observed for P recovery using various engineered
biochars~\cite{12,29,41}. The required time to reach the equilibrium
state is dependent on the biochar properties (i.e., number of active
sites and accessibility easiness) and also the adsorption experimental
conditions (i.e., initial concentration and agitation rate)~\cite{45}.

\begin{figure}
\includegraphics{fig04}
{\vspace*{-.25pc}}
\caption{\label{fig4}Measured and predicted P recovery kinetic data by
the Mg-, Al-, and Mg/Al-modified DPF-derived biochars.}
{\vspace*{-.15pc}}
\end{figure}

It is noteworthy that the equilibrium time for the three synthesized
biochars is shorter than those reported for Mg/Al-modified coffee
ground wastes (12~h)~\cite{29}, 20~h for calcium-rich
biochar~\cite{35}, and 24~h for Mg-modified corn straw-derived
biochar~\cite{43}.\ Biochars with reduced equilibrium contact times are
more appealing as they imply lower energy costs when scaling up the
process to real-case applications.

The fitting of experimental data with kinetic models (Figure~\ref{fig4}
and Table~\ref{tab2}) reveals that the PSO model stands out as the most
suitable. Across all three biochars, this model demonstrates the lowest
MAPE and the highest correlation coefficients ($R^{2}$). This outcome
suggests that the primary \mbox{driving} force \mbox{behind} P recovery
is likely chemical mechanisms such as electron exchange/sharing with
functional groups on the biochar~\cite{43}.\ Additionally, the
calculated rate constants ($k_{2}$) were found to be 0.0156, 0.068, and
0.0198 for Mg-DPF-B, Al-DPF-B, and Mg/Al-DPF-B, respectively
(Table~\ref{tab2}). This implies that, due to a higher loading of Mg-
and Al-based nanoparticles, Mg/Al-DPF-B exhibits the highest removal
rate. Furthermore, for DPF-Mg-B and Al-DPF-B, the intraparticle
diffusion coefficients are higher than the film diffusion coefficients.
This indicates that the critical step for these two biochars is the
diffusion of P through the boundary layer~\cite{26}. In contrast,
intraparticle diffusion for Mg/Al-DPF-B appears to be the limiting step
in P recovery. This is supported by the relatively high P amount
adsorbed at the initial stages of the batch experiment
(Figure~\ref{fig4}). The DPF-Mg/Al-B exhibited the highest P removal
capacity ($12.7~\text{mg}{\cdot}\text{g}^{-1}$) (Table~\ref{tab2}).
This aspect will be discussed later in the isotherm section.

\begin{table*} %tab2
\caption{\label{tab2}Recovery kinetic parameters of P by the Mg-, Al-,
and Mg/Al-modified DPF-derived biochars ($T = 20 \pm
2$~{\textdegree}C, pH ${=}$ 6.8, Dose ${=}$ 2~$\text{g}{\cdot}\text{L}^{-1}$,
$C_{0} = 100~\text{mg}{\cdot}\text{L}^{-1}$)}
\begin{tabular}{lcccc}
\thead
& Parameter & DPF-Mg-B & DPF-Al-B & DFP-Mg/Al-B\\
\endthead
& $q_{e,\mathrm{exp}}$ ($\text{mg}{\cdot}\text{g}^{-1}$) & 6.2 & 7.9 & 12.7{\vspace*{8pt}}\\
\morerows{2}{Pseudo first order model} & $k_{1}$ (min$^{-1}$) & 0.0093 & 0.0176 & 0.0372\\
& $R^{2}$ & 0.973 & 0.928 & 0.825\\
& MAPE (\%) & 52.5 & 29.4 & 30.7{\vspace*{8pt}}\\
\morerows{3}{Pseudo second order model} & $k_{2}$ ($\text{g}{\cdot}\text{mg}^{-1}{\cdot}\text{min}^{-1}$) & 0.0156 & 0.0068 & 0.0198\\
& $q_{e,\mathrm{calc}}$ ($\text{mg}{\cdot}\text{g}^{-1}$) & 6.4 & 8.5 & 12.9\\
& $R^{2}$ & 0.869 & 0.978 & 0.980\\
& MAPE (\%) & 14.2 & 9.3 & 5.6{\vspace*{8pt}}\\
\morerows{1}{Film diffusion model} & $D_{\mathrm{f}}$ (${\times}10^{-12}~\text{m}^{2}{\cdot}\text{s}^{-1}$) & 0.19 & 1.24 & 3.4\\
& $R^{2}$ & 0.981 & 0.975 & 0.990{\vspace*{8pt}}\\
\morerows{1}{Intraparticle diffusion model} & $D_{\mathrm{ip}}$ (${\times}10^{-12}~\text{m}^{2}{\cdot}\text{s}^{-1}$) & 1.72 & 1.83 & 1.3\\
& $R^{2}$ & 0.871 & 0.930 & 0.990
\botline
\end{tabular}
{\vspace*{.15pc}}
\end{table*}

\subsubsection{Impact of initial pH} \label{sec3.2.2}

The initial aqueous pH significantly influences the efficiency of P
recovery by the three modified biochars (Figure~\ref{fig5}). For
Al-DPF-B and Mg/Al-DPF-B, the recovered P amounts decrease as the
initial pH values increase. The highest values, at an initial pH of
3.0, were determined to be 23.5 and
$15.3~\text{mg}{\cdot}\text{g}^{-1}$ for Al-DPF-B and Mg/Al-DPF-B,
respectively. This behavior was anticipated and is primarily attributed
to the positive charge carried by these biochar particles at aqueous pH
levels lower than their respective pHzpc values of 5.58, and 9.41,
respectively. Within this pH range, aluminum oxides and magnesium
oxides undergo protonation, forming ${\equiv}$AlOH$^{2+}$ and
${\equiv}$MgOH$^{+}$ species, consequently attracting P
anions~\cite{26,41}. On the contrary, for pH values higher than their
pHzpc, the surfaces of \mbox{Al-DPF-B} and Mg/Al-DPF-B predominantly carry
negative charges and negatively charged P compounds (mainly
$\text{HPO}_{4}^{2-}$) will compete with hydroxyl ions, resulting in P
recovery as a result, diminishing the electrostatic attraction of P
anions. Moreover, under these conditions, hydroxyl ions may compete
with P anions for adsorption onto biochars particles. Hence, for both
DPF-Al-B and DPF-Mg/Al-B, the electrostatic interaction mechanism is
partially responsible for P recovery. Similar trends have previously
been reported in studies investigating P recovery by various engineered
biochars~\cite{40,41}.

\begin{figure}
\includegraphics{fig05}
{\vspace*{-.25pc}}
\caption{\label{fig5}Impact of initial pH values on P recovery by Mg-,
Al-, and Mg/Al-modified DPF-derived biochars.}
{\vspace*{-.15pc}}
\end{figure}

In contrast, DPF-Mg-B exhibited an opposing behavior. Despite having a
high pHzpc (11.02), the recovered amounts of P increase with the rise
of initial pH values (Figure~\ref{fig5}). This result suggests the
involvement of P complexation and probably precipitation mechanisms. In
this context, different precipitates such as magnesium phosphates
(Mg$_{3}$(PO$_{4}$)$_{2}$) and/or di-magnesium phosphates (MgHPO$_{4}$)
may form and contribute to P recovery process~\cite{43,46}. 

\subsubsection{Effect of initial concentration: isotherm study}
\label{sec3.2.3}

The recovered P amount increases with the rise in the initial aqueous
concentration (Figure~\ref{fig6} and \mbox{Table}~\ref{tab3}). Indeed,
when the P aqueous concentration rises from 10 to
$100~\text{mg}{\cdot}\text{L}^{-1}$, the recovered amounts for
DPF-Mg-B, DPF-Al-B, and DPF-Mg/Al-B increase by 4.8, 1.8, and 3.3
times, respectively (Figure~\ref{fig6}). The prediction of the measured
data with the Freundlich, Langmuir, D--R isotherm models showed that the
former model was the most \mbox{suitable.} This model \mbox{exhibits} relatively high
correlation coefficients and low MAPE (Table~\ref{tab3}). Consequently,
phosphorus recovery by these DPF-pre-modified biochars occurs
heterogeneously and on multilayers on the modified biochars~\cite{47}.
Furthermore, the P recovery process is favorable, given that the
Freundlich constant ``$n$'' for all biochars falls within the\break
range of 1--10.

\begin{figure}
{\vspace*{.3pc}}
\includegraphics{fig06}
{\vspace*{-.25pc}}
\caption{\label{fig6}Impact of the initial P concentration on its
recover by the three DPF-modified biochars: observed and predicted data
with Freundlich, Langmuir and D--R models.}
{\vspace*{-.25pc}}
\end{figure}

\begin{table*} %tab3
\caption{\label{tab3}Isotherm adsorption parameters for P recovery by
the DPF pre-modified biochars ($T = 20 \pm 2$~{\textdegree}C, $t =
3$~h, Dose ${=}$ 2~$\text{g}{\cdot}\text{L}^{-1}$, pH ${=}$ 6.8){\vspace*{-3pt}}}
\fontsize{10}{11.5}\selectfont
\begin{tabular}{lcccc}
\thead
Isotherm & Parameter & DPF-Mg-B & DPF-Al-B & DFP-Mg/Al-B\\
\endthead
\morerows{3}{Langmuir} & $K_{\mathrm{L}}$ ($\text{L}{\cdot}\text{mg}^{-1}$) & 0.016 & 0.182 & 0.096\\
& $q_{m,\mathrm{L}}$ ($\text{mg}{\cdot}\text{g}^{-1}$) & 10.8 & 8.3 & 14.1\\
& $R^{2}$ & 0.982 & 0.864 & 0.970\\
& MAPE (\%) & 8.3 & 11.2 & 6.7{\vspace*{4pt}}\\
\morerows{3}{Freundlich} & $n$ & 1.5 & 5.8 & 2.8\\
& $K_{\mathrm{F}}$ & 0.341 & 3.636 & 2.843\\
& $R^{2}$ & 0.970 & 0.981 & 0.971\\
& MAPE (\%) & 6.3 & 3.0 & 6.8{\vspace*{4pt}}\\
\morerows{3}{D--R} & $q_{m,\mathrm{D}\text{-}\mathrm{R}}$ ($\text{mg}{\cdot}\text{g}^{-1}$) & 6.6 & 7.7 & 11.8\\
& $E$ ($\text{kJ}{\cdot}\text{mol}^{-1}$) & 1.6 & 3.7 & 3.7\\
& $R^{2}$ & 0.956 & 0.839 & 0.937\\
& MAPE (\%) & 18.3 & 14.5 & 6.0
\botline
\end{tabular}
{\vspace*{-3pt}}
\end{table*}

Concerning the Dubinin--Radushkevich (D--R) model, the calculated free
energy values for the three investigated biochars were below
$8~\text{kJ}{\cdot}\text{mol}^{-1}$ (Table~\ref{tab3}), suggesting that
the P adsorption may be governed by physical processes. This
observation contradicts the results of the kinetic study, which
indicates that P recovery involves mainly chemical mechanisms (see
Section~\ref{sec3.2.1}). Consequently, as proposed by Zhu
{et~al.}~\cite{43}, P recovery by the modified biochars might entail a
combination of both physical and chemical processes. It is noteworthy
that DPF-Mg/Al-B emerges as a promising material since its efficiency
is much higher than some pristine, and Fe- or La-modified
biochars~\cite{48,49,50}. \mbox{Moreover}, its P recovery capacity
($14.1~\text{mg}{\cdot}\text{g}^{-1}$) is 7.8, and 2.3 times larger
than biochars derived from the pyrolysis of Al-modified pine
residues~\cite{21} and Mg/Al-modified almond shell~\cite{51}
(Table~\ref{tab4}). However, this capacity remains lower than that of
various other biochars modified with Mg- and/or Ca-rich natural
products such as sepiolite~\cite{42}, powder marble wastes~\cite{17},
and dolomite~\cite{52}.

\begin{table*} %tab4
\fontsize{8.7}{10.7}\selectfont
\caption{\label{tab4}Comparison of DPF-derived biochars efficiency in P
recovery with other Mg- and/or Al-modified biochars
($q_{m,\mathrm{L}}$: Langmuir's adsorption capacity){\vspace*{-2pt}}}
\tabcolsep=3pt
\begin{tabular}{lccccc}
\thead
\parbox[t]{2cm}{\centering Feedstock and provenance} &
\parbox[t]{2cm}{\centering Treatment conditions} &
\parbox[t]{2cm}{\centering Pyrolysis conditions} &
\parbox[t]{2cm}{\centering Adsorption conditions}\vspace*{2pt} &
\parbox[t]{1.1cm}{\centering $q_{m,\mathrm{L}}$ ($\text{mg}{\cdot}\text{g}^{-1}$)} &
Reference\\
\endthead
\parbox[t]{2cm}{\centering Wheat straw, China}\vspace*{6pt} &
\parbox[t]{3cm}{\centering Impregnation with 0.5~M MgCl$_{2}$ solution} &
\parbox[t]{2.8cm}{\centering $T = 600$~{\textdegree}C; $G = 10$~{\textdegree}C/min; $t=2$~h} &
\parbox[t]{3.3cm}{\centering $C_0 = 10$--$2000~\text{mg}{\cdot}\text{L}^{-1}$; $D = 7~\text{g}{\cdot}\text{L}^{-1}$; $t=-h$; $T = -$} &
\0\09.64 & \cite{27}\\

\parbox[t]{2cm}{\centering Sugar cane straw, Brazil}\vspace*{6pt} &
\parbox[t]{3cm}{\centering Impregnation with 0.03~M MgCl$_{2}$, 6H$_{2}$O} &
\parbox[t]{3.1cm}{\centering $T = 350$~{\textdegree}C; $G = 10~\text{{\textdegree}C}{\cdot}\text{min}^{-1}$; $t=-h$} &
\parbox[t]{3.3cm}{\centering $C_0 = 0$--1000~mg/L; $D = 2$~g/L; $t=24$~h; $T = -$} &
17.7 & \cite{53}\\

\parbox[t]{2cm}{\centering Rice husk, Vietnam} &
\parbox[t]{3cm}{\centering Impregnation with 3.3~M MgCl$_{2}$ for 2~h} &
\parbox[t]{3.1cm}{\centering $T = 500$~{\textdegree}C; $G = -$~{\textdegree}C/min; $t = 1$~h} &
\parbox[t]{3cm}{\centering $C_0 = 10$--100~mg/L; $D = 1$~g/L; pH ${=}$ 8; $t = 2$~h; $T = 25$~{\textdegree}C}\vspace*{6pt} &
17.1 & \cite{19}\\

\parbox[t]{2cm}{\centering Tea waste, Turkey}\vspace*{6pt} &
\parbox[t]{3cm}{\centering Impregnation with Al (NO$_{3}$)$_{3}$ solution for 2~h} &
\parbox[t]{3.1cm}{\centering $T = 300$~{\textdegree}C; $G = 3$~{\textdegree}C/min; $t = 3$~h} &
\parbox[t]{3.4cm}{\centering $C_0 = 10$--$100~\text{mg}{\cdot}\text{L}^{-1}$; $D = 1~\text{g}{\cdot}\text{L}^{-1}$; $t = 24$~h; $T = -$} &
\05.0 & \cite{54}\\

\parbox[t]{2cm}{\centering Pine residues, South Korea} &
\parbox[t]{3cm}{\centering Impregnation with 0.71~M recovered aluminum solution from sludge for 24~h}\vspace*{5pt} &
\parbox[t]{3.1cm}{\centering $T = 500$~{\textdegree}C; $G = 10$~{\textdegree}C/min; $t = 0.25$~h} &
\parbox[t]{3.1cm}{\centering $C_0 = 12.5$--$400~\text{mg}{\cdot}\text{L}^{-1}$; $D = 2$~g/L; pH ${=}$ 7; $t = 48$~h; $T = 25$~{\textdegree}C} &
11.9 & \cite{21}\\
& \parbox[t]{3cm}{\centering Impregnation with 0.71~M of AlCl$_{3}$ solution for 24~h}\vspace*{5pt} &&& \01.8 &\\
& \parbox[t]{3cm}{\centering Impregnation with 0.71~M of Al$_{2}$(SO4)$_{3}$ solution for 24~h}\vspace*{6pt} &&& 14.4 &\\

\parbox[t]{2cm}{\centering Almond shell derived biochar, China}\vspace*{6pt} &
\parbox[t]{3cm}{\centering Impregnation with 0.3~M MgCl$_{2}$ and 0.1~M AlCl$_{3}$}
& - &
\parbox[t]{3.1cm}{\centering $C_0 = -$; $D = 1$~g/L; pH~${=}$~6.5; $t = 12$~h; $T = 30$~{\textdegree}C} &
\06.1 & \cite{51}\\

\parbox[t]{2cm}{\centering Cattail biomass powder, China}\vspace*{6pt} &
\parbox[t]{3cm}{\centering Impregnation with 0.4~M MgCl$_{2}$ and 0.2~M AlCl$_{3}$} &
\parbox[t]{3.1cm}{\centering $T = 500$~{\textdegree}C; $G = -$; $t = 2$~h} &
\parbox[t]{3.1cm}{\centering $C_0 = 5$--80~mg/L; $D = 0.5$~g/L; pH ${=}$ 7.2; $t = 24$~h; $T = 25$~{\textdegree}C} &
35.2 & \cite{28}\\

\parbox[t]{2cm}{\centering Rice straw, Japan} &
\parbox[t]{3cm}{\centering Impregnation with 0.03~M MgCl$_{2}$ and 0.01~M AlCl$_{3}$}\vspace*{6pt} &
\parbox[t]{3.1cm}{\centering $T = 475$~{\textdegree}C; $G = -$; $t = 2$~h} &
\parbox[t]{3.1cm}{\centering $C_0 = 25$--500~mg/L; $D = 2$~g/L; pH ${=}$ 3.0; $t = 24$~h; $T = -$~{\textdegree}C} &
192.0\0 & \cite{55}\\

\parbox[t]{2cm}{\centering Date Palm Fronds, Oman} &
\parbox[t]{3cm}{\centering Impregnation with 1~M MgCl$_{2}$} &
\parbox[t]{3.1cm}{\centering $T = 500$~{\textdegree}C; $G = 5$~{\textdegree}C/min; $t = 2$~h} &
\parbox[t]{3.1cm}{\centering $C_0 = 10$--$100~\text{mg}{\cdot}\text{L}^{-1}$; $D = 2~\text{g}{\cdot}\text{L}^{-1}$; $t = 180$~min, $T = 20$~{\textdegree}C}\vspace*{3pt} &
\08.3 & \parbox[t]{.8cm}{\centering This study}\\
& Impregnation with 1~M AlCl$_{3}$ &&& 10.8 &\\
& \parbox[t]{3cm}{\centering Impregnation with 1~M MgCl$_{2}$ and AlCl$_{3}$} &&& 14.1 &\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

\subsubsection{P recovery in presence of other anions} \label{sec3.2.4}

The influence of chloride, nitrate, and sulfate anions on P recovery by
the three modified biochars is shown in Figure~\ref{fig7}. The results
indicate that, for all biochars, there was no discernible competition
between P and sulfate ions. However, the co-presence of chlorides and
nitrates resulted in a reduction of P adsorption for Mg-DPF-B and
Al-DPF-B. The most pronounced effect was observed for Al-DPF-B, where
the P adsorption capacity decreased by 16.6\% and 11.5\% in the
presence of chlorides and nitrates, respectively (Figure~\ref{fig7}).
This outcome may be attributed to the higher electronegativity of
chlorides compared to nitrates and sulfates ions~\cite{56}. It is worth
noting that no significant competition with typical anions (i.e.,
Cl$^{-}$, $\text{SO}_{4}^{2-}$, and $\text{NO}_{3}^{-}$) was reported
for various engineered biochars~\cite{41,57}. For example, the P
recovery efficiency of a calcium-modified biochar was only reduced by
less than 5\% when using tap water or river water instead of deionized
water~\cite{57}. Notably, for the most efficient biochar (DPF-Mg/Al-B),
no significant competition effect was observed, making this biochar a
potential material for selectively adsorbing P from wastewater
(Figure~\ref{fig7}). Consequently, this P-enriched biochar could be
employed in agriculture as an attractive fertilizer, serving as an
alternative to synthetic commercial ones.

\begin{figure}
{\vspace*{.3pc}}
\includegraphics{fig07}
\caption{\label{fig7}Effect of foreign anions presence on P recovery by
DPF-Mg-B, DPF-Al-B and DPF-Mg/Al-B.}
{\vspace*{-.3pc}}
\end{figure}

{\vspace*{-2pt}}

\subsection{Benefits and challenges of using the{\hfill\break} P-loaded
biochars} \label{sec3.3}

{\vspace*{-2pt}}

The synthesized biochars (i.e., DPF-Mg/Al-B) offer a potential solution
for recovering phosphorus (P) and other nutrients present in urban
wastewaters for a further subsequent use in agriculture. For this
reason, the regeneration of the P-loaded modified biochars was not
studied in this work. The recovery process can be facilitated by using
a dedicated continuous stirring tank reactor (CSTR) or laboratory
column~\cite{17,58}.\ These systems can be implemented in real
wastewater treatment plants and serve as tertiary treatment.\ For the
case of CSTR system, the separation of the nutrient-loaded biochars
from treated wastewater can be achieved by using a specialized settling
device at the downstream of this system~\cite{23}. However, it's
important to note that the efficiency of P recovery in CSTR or column
mode is usually lower than that observed in batch conditions due to the
lower hydraulic residence time~\cite{35,59}. This removal efficiency
decrease should be absolutely taken into consideration in real-case
applications since they are typically performed under dynamic
conditions. The up-scaling process can be performed according to the
methodology suggested by Bian {et~al.}~\cite{60}. The resulting P-loaded
biochars (e.g., DPF-Mg/Al-B) can be utilized in agriculture as
slow-release fertilizers, providing an environmentally friendly
alternative to synthetic fertilizers~\cite{28}. This approach holds
significant advantages, including sustainable management of large
quantities of date palm wastes, environmental preservation against
pollution (groundwater, \mbox{surface} water, and air), reduction of
greenhouse gas emissions, promotion of the circular economy concept,
and decreased expenses for farmers~\cite{61}.

However, the synthesis and on-site application of nutrients-loaded
biochars face numerous complex challenges~\cite{62}, including:
\begin{itemize}
\item Technical considerations such as optimizing pyrolysis conditions,
particularly heating temperature and residence time, ensuring efficient
separation between loaded biochar and wastewater, and reducing the
leaching of heavy metals (i.e., Al) from biochars into subsurface
components (soil, groundwater, cultivated plants).
\item Economic viability constraints requiring substantial efforts to
reduce the cost of biochar production.\ Depending on feedstock nature
and pyrolysis conditions, biochar production costs could exceed
US\dollar17 per kg~\cite{63}.\ Utilizing renewable energy sources and
valorizing the generated biofuels (bio-oil and biogas) during the
pyrolysis process for pyrolyzers self-functioning can significantly
reduce these costs.\looseness=-1
\item Social acceptance challenges, that is mainly exacerbated by the
negative perceptions among some farmers and consumers regarding the use
of waste-derived materials in agriculture. Addressing this issue
requires tailored dissemination and communication campaigns across
various media channels to enhance awareness and understanding of
biochar production and safe application.
\item Policy hurdles, necessitating the implementation and fair
enforcement of dedicated laws governing biochar production from organic
wastes within a circular economy framework. Incentives aimed at
encouraging farmers to transition from synthetic fertilizers to
P-loaded biochars can boost these efforts. Additionally, fostering
concrete and efficient coordination among key stakeholders, civil
society actors, and policymakers is an essential step. 
\end{itemize}

\section{Conclusions}

\vspace*{-2pt}

In this investigation, three pre-modified biochars derived from date
palm fronds were produced using impregnation with MgCl$_{2}$,
AlCl$_{3}$, and a mixture of MgCl$_{2}$ and AlCl$_{3}$ solutions. The
results indicate that the biochars modified with Mg and Mg/Al exhibit
noteworthy physico-chemical properties, enabling efficient P recovery
from aqueous solutions under a broad range of experimental conditions.
The respective P recovery capacities for these two biochars were
determined to be 10.8 and $14.1~\text{mg}{\cdot}\text{g}^{-1}$, which
are higher than various raw and modified biochars reported in the
literature. The phosphorus recovery process seems to include both
physical and chemical mechanism. Utilizing these P-enriched biochars as
slow-release fertilizers in agriculture aligns with the sustainability
and circular economy principles. However, further investigations are
essential to assess their impact on the physical, chemical,
microbiological, and hydrodynamic characteristics of agricultural
soils, as well as on plant growth and \mbox{productivity}.
\vspace*{-2pt}

\section*{Declaration of interests}

\vspace*{-2pt}

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.
\vspace*{-2pt}

\section*{Funding}

\vspace*{-2pt}

The authors would like to thank Sultan Qaboos University and Qatar
University for funding this research work in the joint projects
CL/SQU-QU/CESR/23/01 and IRCC-2023-004, respectively.

\CDRGrant[SQU]{CL/SQU-QU/CESR/23/01}
\CDRGrant[QU]{IRCC-2023-004}

\vspace*{-2pt}

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