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\DOI{10.5802/crchim.265}
\datereceived{2023-08-04}
\daterevised{2023-10-01}
\dateaccepted{2023-10-05}
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\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Clean Processes for Sustainable Energy and Environmental Applications}
%\def\cdr@specialtitle@french{Mat\'eriaux et proc\'ed\'es propres pour des applications \'energ\'etiques et environnementales}
%\makeatother

\title{Mechanosynthesis, characterization, and adsorptive properties of
Mg--Al-LDH and Zn--Al-LDH for olive mill wastewater treatment}

\alttitle{Synth\`{e}se m\'{e}cano-chimique, caract\'{e}risation et
propri\'{e}t\'{e}s d'adsorption de Mg--Al-LDH et Zn--Al-LDH pour le
traitement de margines}

\author{\firstname{Khaled} \lastname{Hosni}\CDRorcid{0000-0002-3928-2434}\IsCorresp}
\address{University of Carthage, National Center for Research in
Materials Sciences (CNRSM), Laboratory of Composite Materials and Clay
Minerals, Soliman, Tunisia}
\email[K. Hosni]{hosnikhaled@gmail.com}

\author{\firstname{Khaled} \lastname{Mahmoudi}\CDRorcid{0000-0001-6210-1673}}
\addressSameAs{1}{University of Carthage, National Center for Research
in Materials Sciences (CNRSM), Laboratory of Composite Materials and
Clay Minerals, Soliman, Tunisia}
\email[K. Mahmoudi]{mahmoudikhaled1984@gmail.com}

\author{\firstname{Manel} \lastname{Haraketi}\CDRorcid{0009-0006-0677-5108}}
\addressSameAs{1}{University of Carthage, National Center for Research
in Materials Sciences (CNRSM), Laboratory of Composite Materials and
Clay Minerals, Soliman, Tunisia}
\email[M. Haraketi]{mharaketi@gmail.com}

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

\author{\firstname{Ezzeddine} \lastname{Srasra}}
\addressSameAs{1}{University of Carthage, National Center for Research
in Materials Sciences (CNRSM), Laboratory of Composite Materials and
Clay Minerals, Soliman, Tunisia}
\email[E. Srasra]{srasra.ezzedine@gmail.com}

\keywords{\kwd{Layered double
hydroxides}\kwd{Mechanochemistry}\kwd{Peptization}\kwd{Olive mill
wastewater treatment}}

\altkeywords{\kwd{Hydroxydes doubles
lamellaires}\kwd{M\'{e}canochimie}\kwd{Peptisation}\kwd{Margines}}

\shortrunauthors

\begin{abstract}
Industrial olive oil production is of fundamental economic importance
for many Mediterranean countries. However, this industry generates huge
amounts of toxic olive mill wastewater (OMW), which could represent a
serious threat to human health and environmental biodiversity. In the
current study, calcined layered double hydroxides (LDHs) were
synthesized through a mechanochemical process involving the manual
grinding of magnesium or zinc- and aluminum-nitrate salts in an agate
mortar, followed or not by a peptization process. The experimental
results showed that non-peptized LDHs have a layered structure with
relatively low crystallinity. However, the peptization process resulted
in LDHs with regular particles exhibiting~high crystallinity and
thermal stability. These LDHs achieved a significant improvement in the
quality of OMW. Indeed, after 44~h of contact time, the removed amounts
of chemical oxygen demand (COD) and biological oxygen demand (BOD) were
assessed at approximately 300 and 100~mg${\cdot}$g$^{-1}$, respectively.
Moreover, the discoloration rate of this effluent was more than 90\%.
Overall, the results demonstrate the convenience of the
mechanosynthesis of hydrotalcite and the high efficiency of OMW
treatment, which is promising for the potential applications of
calcined LDH in environmental clean-up and remediation of contaminated
water.
\end{abstract}

\begin{altabstract}
La production industrielle d'huile d'olive rev\^{e}t une importance
\'{e}conomique fondamentale pour de nombreux pays
m\'{e}diterran\'{e}ens. Cette industrie g\'{e}n\`{e}re d'\'{e}normes
quantit\'{e}s de margines (OMW) qui repr\'{e}sentent une menace
s\'{e}rieuse pour la sant\'{e} humaine et l'environnement. Dans la
pr\'{e}sente \'{e}tude, des hydroxydes doubles lamellaires (LDH)
calcin\'{e}s ont \'{e}t\'{e} synth\'{e}tis\'{e}s par un processus
m\'{e}cano-chimique impliquant le broyage manuel de sels de
magn\'{e}sium ou de zinc et de nitrate d'aluminium dans un mortier
d'agate, suivi ou non d'un processus de peptisation. Les r\'{e}sultats
exp\'{e}rimentaux ont montr\'{e} que les LDH non peptis\'{e}s
pr\'{e}sentent des caract\'{e}ristiques d'une structure en couches mais
avec une faible cristallinit\'{e}. En revanche, le processus de
peptisation a permis d'obtenir des LDH avec des particules
homog\`{e}nes pr\'{e}sentant une cristallinit\'{e} et une stabilit\'{e}
thermique \'{e}lev\'{e}es. Ces LDH ont permis d'am\'{e}liorer
sensiblement la qualit\'{e} des OMW. En effet, apr\`{e}s 44~h de
contact, les quantit\'{e}s \'{e}limin\'{e}es de demande chimique en
oxyg\`{e}ne (COD) et de demande biologique en oxyg\`{e}ne (BOD) ont
\'{e}t\'{e} respectivement~\'{e}valu\'{e}es \`{a} environ 300 et 
100~mg${\cdot}$g$^{-1}$. De plus, le taux de d\'{e}coloration de cet
effluent a \'{e}t\'{e} \'{e}valu\'{e}e \`{a} plus de 90~\%. Dans
l'ensemble, les r\'{e}sultats exp\'{e}rimentaux d\'{e}montrent la
commodit\'{e} de la synth\`{e}se m\'{e}cano-chimique pour la
synth\`{e}se de LDH efficaces pour le traitement de margines.
\end{altabstract}

\maketitle

{\pagebreak}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Mediterranean countries produce approximately 97\% of the world's
olives~\cite{1}. In the European Union, Spain, Italy, and Greece are
the most important producers, whereas Tunisia is one of the largest
producers in the North African region. Olive oil production is
accompanied by the generation of huge amounts of solid waste (OSW) and
liquid effluents. In Mediterranean countries, the olive mill wastewater
(OMW) quantity was evaluated to be approximately
30~M${\cdot}$m$^{3}$~\cite{1}. Because of their high organic loads and
toxic substance contents, such as polyphenols, the disposal of OMW into
surface waters generally results in significant contamination and
reduction in biodiversity~\cite{2}. Uncontrolled discharge of OMW can
also be responsible for soil and groundwater pollution~\cite{3}. The
phytotoxic activity of OMW is mainly attributable to their richness in
monomeric phenols~\cite{4}, which can seriously inhibit seed
germination and plant development~\cite{5}. Therefore, it is necessary
to identify the adapted technologies for the treatment of OMW prior to
their discharge into the environment.

Different approaches have been developed and tested to valorize olive
mill liquid and solid wastes. They include the production of energy and
valuable materials using thermochemical processes such as pyrolysis and
hydrothermal carbonization~\cite{6,7}. Other treatment technologies
have been developed to reduce the levels of harmful substances in OMW.
They include the use of chemical oxidation processes,
electrocoagulation, precipitation and coagulation, sedimentation,
filtration, osmosis, ion exchange, etc.~\cite{8,9}. Adsorption onto
natural and synthetic materials is a promising method for the treatment
of both urban and industrial effluents because it is low in cost,
practical, and highly effective against a large range of
pollutants~\cite{10}. For the treatment of OMW, numerous adsorbents
have been employed. They primarily consist of resin~\cite{10},
activated clay~\cite{11}, activated carbon~\cite{12}, and
zeolite~\cite{13}. However, these adsorbents typically have significant
prices because of both the price of the substance itself and the cost
of the entire process.\looseness=1

The use of layered double hydroxides (LDHs) offers significant
advantages over many other conventional methods. Indeed, these
materials can exhibit high adsorption capacities, higher selectivity,
easier desorption, better mechanical resistance, and
reusability~\cite{11}. Layered double hydroxides, also known as
hydrotalcite-like compounds (HTlc) or anionic clays, are
two-dimensional lamellar compounds that are formed using divalent and
trivalent metal ions~\cite{12}. They have positively charged layers and
charge-balancing anions located in the interlayer region~\cite{12}. The
application of LDHs for wastewater treatment has attracted special
attention because they are prepared from low-cost precursors and can be
easily regenerated~\cite{13}. 

Various chemical methods have been developed for LDH synthesis. The
most common method is the co-precipitation of metal salts from a mixed
solution at a constant pH in the presence of anionic species that are
intercalated between layers. The co-precipitation method is
time-consuming and produces large amounts of waste~\cite{14,15}.
Various other techniques have been proposed, including precipitation at
variable pH~\cite{16}, sol--gel and hydrothermal synthesis~\cite{17},
structure reconstruction~\cite{18}, and hydrolysis~\cite{19}. Each of
these methods has its own advantages and disadvantages~\cite{20}. The
search for new methods of synthesis that allow the rapid preparation of
LDH has directed attention to solid-state reactions. Among these
methods, mechanochemical activation appears to be a promising
alternative method because of its simplicity and versatility~\cite{21}.
During the mechanochemical reactions, the transformation of the solid
reactants can be induced by milling or manual grinding~\cite{20}. Such
reactions between solid reactants in the absence of a solvent are
significant from both environmental and topochemical
standpoints~\cite{20}. These solvent-free and less conventional
procedures are considered viable routes for the preparation of
LDHs~\cite{22}. Grinding is known to mechanically activate these
reactions. The mechanical energy produced by grinding leads to the
regular arrangement of the particles after proper grinding, whereas
structural imperfections in the particles appear after longer
grinding~\cite{23}. As a result, suitable grinding is necessary to
ensure sufficient contacts and effective collisions between the
reactants. The particle size and purity of hydrotalcite are important
in industrial applications~\cite{24}.

It is important to note that LDHs manufactured by liquid-phase
approaches have been intensively used for the removal of anionic
pollutants (anions and oxyanions of the halogen elements)~\cite{25},
boron species~\cite{26}, oxyanions~\cite{27}, etc. However, only a few
studies have reviewed the application of Mg/Zn--Al-LDHs, which are
synthesized by mechanochemical processes involving the manual grinding
of magnesium or zinc and aluminum nitrate salts in an agate mortar
followed by a peptization process~\cite{28,29}. It is well known that
the specific surface area of materials and surface defects increase as
a result of mechanochemical activation processes~\cite{30}. Such
changes contribute to the significant enhancement of adsorption
performance~\cite{28}. Guo and Reardon~\cite{31} synthesized meixnerite
by grinding MgO and Al(OH)\tsub{3} in two steps to remove fluoride
ions. The results illustrated that the obtained samples could adsorb
anions by an ion-exchange mechanism and also form a new phase with the
anions on the surface of the adsorbent due to its outstanding surface
activity.

Wang \etal~\cite{32} milled Mg(OH)\tsub{2} to an activated state before
co-grinding it with Al(OH)\tsub{3} to create the precursor of
Mg--Al-LDH.  The obtained sample showed excellent adsorption capacity
(1110.2~mg${\cdot}$g$^{-1}$) toward methyl orange (MO). Through surface
adsorption, He \etal~\cite{32} successfully removed phenols from water
using an Mg--Al-LDH precursor. They reported outstanding adsorption
capacities of 82.6~mg${\cdot}$g$^{-1}$ for phenol and
356.4~mg${\cdot}$g$^{-1}$ for p-nitrophenol. The data demonstrated that
the disorderly precursor could provide more active sites than other
adsorbents, with a stable crystalline structure for the adsorption of
anionic pollutants~\cite{32}. Therefore, testing this type of LDH for
the treatment of real and chemically complex effluents such as OMW is
of great interest. To date, no study has been conducted on the use of
LDHs prepared using the solid--solid method for OMW treatment. 

In this respect, the main goals of this work are as follows: (i) to
synthesize Mg/Zn--Al-LDHs using a simple solid--solid method; (ii) to
determine how the grinding time and the molar ratio of Mg/Al or Zn/Al
affect the crystallinity of Mg/Zn--Al-LDHs; (iii) to deeply
characterize these synthesized LDHs using infrared spectroscopy,
thermogravimetry/differential thermal analysis (TG-DTA), BET surface
area, and X-ray diffraction (XRD) analyses; and (iv) to study the
adsorptive properties of Mg--Al-LDH and Zn--Al-LDH for olive mill
wastewater treatment.

\section{Experimental}\label{sec2} 
\subsection{Synthesis of Mg/Zn--Al-LDH}\label{ssec21}

A series of Mg--Al-LDH and Zn--Al-LDH were synthesized by a
mechanochemical approach that involves manual grinding in a mortar
followed by peptization. A mixture of magnesium or zinc and aluminum
nitrates was combined with NaOH pellets and manually crushed to create
a paste. To remove unwanted electrolytes, the resulting paste was
washed five times with distilled water. Consequently, the entire
mixture was crystallized in a Teflon bottle for 24~h. To investigate
the effect of the operating conditions on the formation of LDH samples,
various Mg/Al and Zn/Al molar ratios, grinding times, and peptization
temperatures were tested. 

For comparative purposes, two samples of Mg/Zn--Al-LDH were produced
using the traditional co-precipitation approach. This method involves
adding, at room temperature, a solution containing Mg/Zn and Al
dropwise to a solution containing Na\tsub{2}CO\tsub{3} while constantly
stirring~\cite{34}. A solution of NaOH (2N) was added when needed to
maintain the pH of the slurry at~10.

\subsection{Characterization of the synthesized materials}
\label{ssec22} 

Various apparatuses were used to thoroughly characterize the
synthesized LDHs. They include the use of an atomic absorption
spectrophotometer (AAS Vario 6) for metal concentration assessment and
X-ray diffraction (XRD) to determine the main present species and their
degrees of crystallinity. The diffractograms were obtained using
monochromated CuK radiation and a PANalytical X'Pert HighScore Plus'
diffractometer. Moreover, nitrogen adsorption measurements were
performed at ${-}$196~\textdegree C\ with an Autosorb-1 unit
(Quantachrome, USA) to determine the textural properties of the samples
using the multipoint Brunaner--Emmet--Teller (BET) method. Furthermore,
the Fourier-transformed infrared (FT-IR) spectra of the samples were
recorded by using the KBr pellet technique with a Perkin-Elmer FT-IR
(model 783) instrument. Finally, the thermal decomposition of the solid
samples was studied by thermogravimetric analysis (TGA) using a
Shimatzu TG-50 thermobalance. In this study, 10~mg of these samples
were heated from 50 to 700~\textdegree C\ at a gradient of 
10~\textdegree C/min under an argon flow rate of 50~mL/min.\looseness=1

\subsection{Olive mill wastewater sampling and preparation}
\label{ssec23} 

Fresh samples of olive mill wastewater were collected from a
three-phase local automatic mill located in northwest Tunisia during
the oil extraction season. The raw OMW was first centrifuged for 60~min
at 6000~rpm, filtered to remove the solid contents, and stored in a
refrigerator at 4~\textdegree C\ to reduce the effect of
biodegradation. The collected OMW was physicochemically characterized
by determining various parameters such as pH (HENNA instruments P211R),
electrical conductivity (HENNA instruments HI2300), chemical oxygen
demand (COD) by the open reflux method for a digestion time of 
2~h~\cite{33}, total suspended solids (TSS) by filtration through 
0.45~$\upmu$m filters and biological oxygen demand (BOD) by the
respirometric method. Total organic carbon (TOC) was determined by
using the Multi N/C 3100 system 141 (Analytik Jena GmbH, Germany).\looseness=1

\subsection{OMW treatment and kinetic modeling}
\label{ssec24}

The OMW treatment experiments were carried out in 50-ml polyethylene
tubes for the materials with the best properties. All experiments were
performed at the same pH as that of the raw OMW sample (pH 4.7) and at
room temperature (around 25~\textdegree C).\looseness=1

Time-dependent OMW treatment was performed using 250~mg of LDH and 
20~ml of raw OMW. The mixtures were stirred at low speed
(${\sim}$100~rpm) for different time intervals and centrifuged. The total
suspended solid (TSS), chemical oxygen demand (COD), biochemical oxygen
demand (BOD), and total organic carbon (TOC) in the supernatant were
then determined.

Kinetic adsorption modeling allows not only the estimation of the
sorption rates but also the deduction of the main possible mechanisms
involved. In the current study, the pseudo-first-order~\cite{34} and
pseudo-second-order equation~\cite{35} were tested.\looseness=1

\section{Results and discussion}\label{sec3}
\subsection{Structure of synthesized LDHs}\label{ssec31}

The XRD patterns of the Mg/Zn--Al-LDH prepared by the mechanochemical
method at a grinding time of 15~min and without peptization for  Mg/Al
$=$\ 3:1 and Zn/Al $=$\ 3:1 are shown in Figures~\ref{fig1}a 
and~\ref{fig1}b, respectively.

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig01}
{\vspace*{2pt}}
\caption{\label{fig1}Powder XRD patterns: (a) Mg--Al-LDH and (b)
Zn--Al-LDH prepared by  (1) co-precipitation (for comparison) (2)
mechanochemical method. Experimental conditions: M/Al: 3:1, grinding
time $=$\ 15~min, and no peptization.}
{\vspace*{2pt}}
\end{figure*}

The X-ray diffraction (XRD) patterns for the Mg--Al-LDH samples
prepared by co-precipitation (for comparison) and by the
mechanochemical method, respectively, shown in Figures~\ref{fig1}a1
and~\ref{fig1}a2, are similar. The difference between these
diffractograms lies in the intensity of the reflection (00l). Both
plots show sharp, symmetrical peaks at lower 2$\uptheta$ values, which
are characteristic of lamellar hydrotalcite-type compounds, and also
indicate a high degree of crystallinity in the samples. Reflections 003
and 006 are attributed to basal reflections, which correspond to
successive stacking of brucite-like sheets. The diffraction peaks
correspond to a hexagonal lattice with R-3m rhombohedral symmetry,
which is commonly used to describe LDH structures. The basal
reflections of (003) indicate the formation of Mg--Al--CO\tsub{3}-LDH
with an interlayer spacing of  0.78~nm~\cite{36}. Furthermore, the XRD
pattern of LDH prepared by the mechanochemical method is consistent
with that of JCPDS No. 41-1428, suggesting that this sample has a
hydrotalcite sheet structure~\cite{37}. 

Concerning the Zn--Al-LDH sample, the XRD diffractogram
(Figure~\ref{fig1}b2) shows the presence of additional reflections
that are different from those of LDH prepared by the conventional
method (Figure~\ref{fig1}b1), indicating a mixture of phases. The
Zn--Al-LDH sample showed a layered structure, as observed from the
peaks detected at 7.82, 3.89, and 2.61~\AA{}, which correspond to
planes (003), (006), and (009), respectively, for a layered
hydrotalcite-like material~\cite{36}. This sample displays a weak and
broad peak at a 2$\uptheta$ value of 11\textdegree\ compared with the
sample prepared by co-precipitation under the same conditions (Zn/Al
$=$\  3). On the other hand, Zn--Al-LDH shows a structure different
from that of the Mg/Al sample. The Zn--Al-LDH phase appears to be
contaminated with Al(OH)\tsub{3} (JCPDS 82-2256).

The infrared spectra of the Mg--Al-LDH and Zn--Al-LDH samples are
shown in Figure~\ref{fig2}. Because of the stretching mode of the --OH
structural groups in the metal hydroxide, they exhibit a broad band at
approximately 3470~cm$^{-1}$~\cite{38}. Both spectra 
(Figures~\ref{fig2}a2 and~\ref{fig2}b2) show a shoulder at 
1638~cm$^{-1}$ which can be attributed to the bending mode of
interlayer water molecules~\cite{39}. Three characteristic bands of
carbonate in hydrotalcite at  ${\sim}$1380~cm$^{-1}$
($\upnu$3)~\cite{40}, 877~cm$^{-1}$ ($\upnu$2) and 
${\sim}$1020~cm$^{-1}$ ($\upnu$1)~\cite{41} and bands around 420 and
668~cm$^{-1}$, which can be attributed to the Al--O and Mg--O bending
modes, respectively.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}FTIR spectra of (a) Mg--Al-LDH and (b)
Zn--Al-LDH prepared by (1) co-precipitation (for comparison) and (2)
the mechanochemical method.}
\end{figure*}

The thermogravimetric analyses of the synthesized Mg--Al-LDHs and
Zn--Al-LDHs are shown in Figures~\ref{fig3}a and~\ref{fig3}b,
respectively. The shapes of the curves reflect a good degree of
crystallinity for the two samples. The TGA-DTA patterns are
characterized by a weight loss between 10\% and 14\% due to the
evaporation of the interlayer-water in the temperature range of
50--250~\textdegree C. For Mg--Al--CO\tsub{3}, dehydration occurs in
two steps at 138~\textdegree C\ and 216~\textdegree C\ 
(Figure~\ref{fig3}a). These steps result from the loss of adsorbed
and interlayer water, followed by water coordinated to the interlayer
carbonate. The interlayer carbonate was released as CO\tsub{2} at
approximately 402~\textdegree C \mbox{(Figure~\ref{fig3}a)}. The total mass
loss was estimated to be approximately 37\%. For Zn--Al--CO\tsub{3},
significant mass losses at 174, 239, and 540~\textdegree C\ were
observed (Figure~\ref{fig3}b). The mass losses at 174 and
240~\textdegree C\ are accompanied by a change in the heat flow, which
is the result of adsorbed surface water and interlayer water
evaporation~\cite{42}. The combination of two processes, namely the
dehydroxylation of the Zn--Al-LDH layers and the decomposition of the
interlayer $\mathrm{CO}_{3}^{2-}$ anions, could result in the second
distinct mass loss region (350--500~\textdegree C)~\cite{42}.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}TG and DTA curves for (a) Mg--Al-LDH and (b)
Zn--Al-LDH prepared by the mechanochemical method.}
\end{figure*}

The N\tsub{2} adsorption--desorption isotherms were of type~II for all
samples, which is typical of mesoporous materials (Figure~\ref{fig4}).
All materials possessed zero micropore volume. Adsorption isotherms of
this type are represented by mesoporous materials with no micropores
and strong interactions between the adsorbent and adsorbate molecules.
This type of hysteresis loop is formed when the adsorption and
desorption curves do not coincide and is physically caused by capillary
condensation in the mesopores. According to the IUPAC
classification~\cite{43}, both \mbox{materials} display a type II isotherm
with an H\tsub{3} hysteresis loop, which corresponds to solids with
aggregates of plate-like particles that give rise to slit-shaped
pores~\cite{44}.\looseness=1

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}N\tsub{2} adsorption--desorption isotherms
recorded for Mg--Al-LDH and Zn--Al-LDH: (a) before and (b) after
calcination at 500~\textdegree C.}
{\vspace*{2pt}}
\end{figure*}

Specific surface areas of the Mg--Al-LDH--500 and Zn--Al-LDH--500
were determined by the single point BET method (Table~\ref{tab1}) and
were found to be 121 and 98~m$^{2}{\cdot}$g$^{-1}$ respectively,
greater than the 77 and  65~m$^{2}{\cdot}$g$^{-1}$ values obtained
for their precursors. It has been suggested that a porous system
developed in the calcined samples during the transformation of the
interlayer CO$_{3}^{2-}$ to CO\tsub{2}~\cite{45}.

%tab1
\begin{table*}
\caption{\label{tab1}Textural properties of Mg--Al-LDH and Zn--Al-LDH
before and after calcination at 500~\textdegree C}
\begin{tabular}{ccccc}
\thead
\xmorerows{1}{Sample} & \multicolumn{2}{c}{Mg--Al-LDH} &
\multicolumn{2}{c}{Zn--Al-LDH} \\\cline{2-3}\cline{4-5}
& $S_{\mathrm{BET}}$ (m$^{2}{\cdot}$g$^{-1}$) & $V_{\mathrm{total}}$ (cm$^{3}{\cdot}$g$^{-1}$) &
  $S_{\mathrm{BET}}$ (m$^{2}{\cdot}$g$^{-1}$) & $V_{\mathrm{total}}$ (cm$^{3}{\cdot}$g$^{-1}$) \\
\endthead
Before calcination & \077 & 0.43 & 66 & 0.79 \\
After calcination  & 121  & 0.83 & 98 & 1.20 
\botline
\end{tabular}
\end{table*}

\subsection{Effect of M$^{2+}$/Al$^{3+}$ molar ratio on the properties
of LDHs}\label{ssec32}

The effect of the M\tsup{2+}/Al\tsup{3+} ratio ($R$ values) on the
structure of Mg/Zn--Al-LDHs is shown in Figure~\ref{fig5}. Samples
prepared within the Mg/Al and Zn/Al ranges of 0.5--3 show similar
patterns of natural hydrotalcite. The difference between these samples
is in the intensity of the (00$l$) reflection. As the Mg/Al molar ratio
increased from 0.5 to~3, the intensity of the reflections increased,
corresponding to an increase in crystallinity (Figure~\ref{fig5}). This
figure shows the highest crystallinity for  $R=3$. According to
Bukhtiyarova \etal~\cite{46}, a ratio of 3 produces an energetically
stable LDH phase. This behavior was imputed to the fact that the more
Mg\tsup{2+} or Zn\tsup{2+} were replaced by Al\tsup{3+}, the stronger
the bond between the layers and the intercalary anions, and therefore
the LDH phase became more stable. In contrast, additional energy is
introduced by the distortion of the network created by the difference
in size between the two cations. In addition, a crystalline
hydrotalcite phase was observed in the precipitate for Mg/Al molar
ratios as low as $R=0.5$. This can also be seen in Figure~\ref{fig5}
that well-crystalline hydrotalcite Mg/Al compounds are formed at an
Mg/Al ratio of 1:1. These samples were mixed with NaNO\tsub{3}. The LDH
phase was mixed with Al(OH)\tsub{3} in samples prepared with a
Mg/Al\tsup{3+} ratio of~0.5.

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig05}
{\vspace*{-4pt}}
\caption{\label{fig5}X-ray diffraction (XRD) patterns of LDHs prepared
at a grinding time of 15~min and various Mg/Al and Zn/Al molar ratios
without peptization.}
{\vspace*{-4pt}}
\end{figure*}

For Zn--Al-LDH (Figure~\ref{fig5}), an LDH phase with acceptable
crystallinity was obtained using only a Zn/Al molar ratio of~2. Below
this ratio, the formed LDH phases were very poorly crystallized. These
results are not in good agreement with those found for the synthesis of
LDH by the co-precipitation method, which showed a significant increase
in the crystallinity of LDH when the M\tsup{2+}/Al\tsup{3+} molar ratio
decreased~\cite{47}. Dutta \etal~\cite{48} showed that increasing the
Al\tsup{3+} concentration in Zn--Al-LDH solutions causes an increase
in the crystallinity of the LDH phase.

The FT-IR spectra of the synthesized samples at various Mg/Al and Zn/Al
molar ratios are shown in Figure~\ref{fig6}. These samples show spectra
similar to those of natural hydrotalcite. These IR results, coupled
with the above XRD results, clearly confirm that a ratio of $R=3$ is
the best molar ratio for the preparation of well-crystalline
Mg--Al-LDH and Zn--Al-LDH hydrotalcite-like compounds by the
mechanochemical method.

\begin{figure*}
\includegraphics{fig06}
{\vspace*{4pt}}
\caption{\label{fig6}Infrared spectra of the dry precipitates prepared
at a grinding time of 15~min, various Mg/Al and Zn/Al molar ratios, and
without peptization.}
\end{figure*}

\subsection{Effect of grinding time on the properties of LDHs}
\label{ssec33}

Grinding time and peptization temperature could also highly influence
crystal growth, kinetically or thermodynamically~\cite{49}. These two
key parameters control the crystallinity and particle size of the
synthesized LDHs~\cite{50}. In the current work, the synthesis of
Mg--Al--CO\tsub{3} and Zn--Al--CO\tsub{3} was realized by altering the
grinding times while keeping the other parameters constant. Powder
mixtures were prepared with Mg/Al or Zn/Al ratios of 3:1. The
peptization process was performed at room temperature. The XRD patterns
of the dry precipitates prepared by mechanosynthesis for grinding times
of 15, 30, 60, and 90~min showed patterns similar to those of natural
hydrotalcite (Figure~\ref{fig7}). The main difference between these
samples is in the intensity of the (001) reflection
(Figure~\ref{fig7}).  For Mg--Al-LDH, as the grinding time increases,
the intensity of the reflection increases, corresponding to an increase
in crystallinity.

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}XRD patterns of Mg--Al-LDH and Zn--Al-LDH
prepared for Mg/Al $=$\ 3 and Zn/Al $=$\ 3 at various grinding times
and without peptization.}
\end{figure*}

Similarly, Figure~\ref{fig7} shows the XRD patterns of Zn--Al-LDH
synthesized under different grinding times. When the grinding time was
increased from 15 to 60~min, the crystallinity improved, but peak
heights and peak broadening were observed when the grinding time was
increased to 90~min. Under shorter grinding (i.e., 30~min), the
mechanical energy from shorter grinding is too weak to give rise to
fully grown particles. Under longer grinding (i.e., 90~min),
distortions in the plate stacking and delamination processes may
account for the reduction in peak intensity as well as its
widening~\cite{51}. 

\subsection{Effect of peptization temperature on the properties of LDHs}
\label{ssec34}

The peptization temperature is an important factor in the formation of
all hydrotalcite-type materials via the mechanochemical route. Samples
prepared within the temperature range of ambient room temperature to
150~\textdegree C\ show patterns similar to those of natural
hydrotalcite (Figure~\ref{fig8}). The difference between these samples
is in the intensity of the (00\textit{l}) reflection. The sample of
Mg--Al-LDH prepared at 120~\textdegree C\ shows fine and intense peaks
compared with those prepared at 90~\textdegree C\ and 150~\textdegree
C\ (Figure~\ref{fig8}). The XRD patterns of the Zn--Al-LDH sample
prepared at 90~\textdegree C\ show broad peaks, indicating poor
crystallinity (Figure~\ref{fig8}). As shown in this figure, a layered
structure with high crystallinity was obtained at a peptization
temperature of 120~\textdegree C, regardless of the nature of the
cation. These results are in agreement with those of Zhang and Li
(2013)~\cite{14}, who prepared Mg--Al-LDHs, Zn--Al-LDHs, Ni-Al-LDHs,
and Mg-Fe-LDH samples with high crystallinity at a peptization
temperature of 100~\textdegree C. The results of the above analysis
clearly indicate that Mg/Zn--Al-LDHs materials with high crystallinity
and stability could be obtained after \mbox{peptization}. Furthermore,
the peptization process probably plays a critical role in the formation
of Mg/Zn--Al-LDHs with high crystallinity, and the optimum peptization
temperature mainly depends on the cations used.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}XRD patterns of Mg--Al-LDH and Zn--Al-LDH for
Mg/Al $=$\ 3 and Zn--Al $=$\ 3 at a grinding time of 15~min and at
various peptization temperatures.}
\end{figure*}

\subsection{Textural properties}\label{ssec35} 

The textural properties of the LDHs prepared under different conditions
are presented in Figure~\ref{fig9}. It can be clearly concluded that
the grinding time and molar ratio affect the textural properties the
most. Indeed, the specific surface area of the Mg--Al-LDH samples
increased by approximately 108\% when the Mg--Al molar ratio increased
from 0.5 to 3 and by 32\% when the grinding time increased from 15 to
90~min. Moreover, the Mg--Al-LDH obtained by the mechanochemical
method with an $R$ value of~3, a peptization temperature of
25~\textdegree C, and a grinding time of 90~min has the greatest
surface area (103~m\tsup{2}${\cdot}$g$^{-1}$), making it a potential
material for anionic exchange applications. The specific surface area
of the Zn--Al system increases primarily as a function of the Zn/Al
molar ratio and peptization temperature (Figure~\ref{fig9}). Indeed,
$S_{\mathrm{BET}}$ increased by 225\% when the molar ratio increased
from 0.5 to 3. On the other hand, the specific surface area reached its
maximum for a grinding time of 30~min. The highest surface area was
recorded for the LDH synthesized under the following conditions: Zn/Al
$=$\ 3, $T=150$~\textdegree C, and a grinding time of 15~min. This
behavior is explained by the fact that the grinding procedure ensures
homogeneity of the mixed raw materials. In addition, this process leads
to the formation of primary LDH nuclei. During the heating process, the
nuclei grow to form LDH of high crystallinity. The
Brunauer--Emmett--Teller (BET) surface area is correlated with the
presence of intercrystalline pores and the better crystallinity of the
sample. On the other hand, because of the rapid nucleation in the
mechanochemical process, some Al\tsup{3+} ions might have become
overloaded on the surface sites, resulting in a higher surface charge
and hence a positive surface charge density value. The correlation of
surface charge density and the~Mg:Al ratio with the microporous
structure of LDHs was analyzed by Weir and Kydd~\cite{52}. It is likely
that materials with a high surface charge density contain smaller
pores, and slight changes in the layer composition may yield different
BET surface areas.

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}$S_{\mathrm{BET}}$ for various Mg--Al-LDH and
Zn--Al-LDH samples prepared using the mechanochemical method in terms
of the M/Al molar ratio, grinding time, and peptization temperature.}
\end{figure*}

\subsection{Treatment of olive mill effluent}\label{ssec36}

The OMW sample was characterized according to standard methods for the
examination of water and wastewater~\cite{53}. Table~\ref{tab2} lists
the main physicochemical characteristics of OMW. The pH of the OMW was
acidic (4.7) because of the presence of fatty acids. This low pH value
indicates that biological activities in OMW may be inhibited or very
limited. The OMW electrical conductivity was determined to be 
18.25~mS${\cdot}$cm$^{-1}$, which is relatively significant and can be
attributed to the presence of a high dissolved salt content.
BOD\tsub{5} was equal to 25.0~g${\cdot}$L$^{-1}$, which is comparable to
the value reported by Haddad \etal~\cite{54}. The COD content was
estimated to be 212.0~g${\cdot}$L$^{-1}$, which is exceptionally high and
is mainly due to the presence of organic matter in both suspended and
decanting materials~\cite{9}. On the other hand, OMW seems to be a real
environmental problem. Some preliminary adsorption assays showed that
the adsorbents were not efficient in treating the raw OMW. Therefore,
the OMW was diluted twice with deionized water.

%tab2
\begin{table}
\caption{\label{tab2}Main physicochemical properties of the raw OMW
sample{\vspace*{-3pt}}}
\tabcolsep=3pt
\begin{tabular}{ccc}
\thead
Parameters & Unit & Value \\
\endthead
pH & - & 4.7 \\ 
Conductivity & mS/cm & 18.25 \\ 
Turbidity & NTU & 11,000 \\ 
Total suspended solids (TSS) & mg/L & 1530 \\ 
Chemical oxygen demand (COD) & g/L & 212.0 \\ 
Biological oxygen demand (BOD) & g/L & 25.0 \\ 
Total nitrogen & mg/L & 0.26 \\ 
Total organic carbon (TOC) & g/L & 33.8
\botline
\end{tabular}
{\vspace*{-6pt}}
\end{table}

The COD, BOD, TOC, and TSS reduction kinetics were determined under the
experimental conditions described in Section~\ref{ssec24}. The results
(Figure~\ref{fig10}a) show that the removal of COD by Zn--Al-LDH
increases within the first 3~h. No significant variation was observed
after this period. For Mg--Al-LDH, the efficiency of COD removal
increased with increasing contact time, reaching a state of near
equilibrium after a contact time of 44~h. When Al Bsoul \etal~\cite{55}
treated OMW with titanium oxide nanoparticles, they observed the same
behavior. These authors found that an equilibrium state could be
reached after 4~h of treatment.

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig10}
{\vspace*{2pt}}
\caption{\label{fig10}Effect of contact time on (a) COD, (b) BOD, (c)
TOC, and (d) TSS uptake by Mg--Al-LDH and Zn--Al-LDH.}
{\vspace*{2pt}}
\end{figure*}

The maximum adsorption capacities were found to be
292~mg${\cdot}$g$^{-1}$ and 294~mg${\cdot}$g$^{-1}$ for Mg--Al-LDH and
Zn--Al-LDH, respectively. These adsorbed amounts are much higher than
those reported by Azzam \mbox{\etal}~\cite{56}, where only 10\% of the
contained COD was removed. This removal process may be mainly
attributed to surface complexation in the LDH structure and
intercalation of the OMW organic matter into the LDH interlayer. The
kinetic curves (Figures~\ref{fig10}b, \ref{fig10}c, and \ref{fig10}d)
of the removal of BOD, TOC, and TSS show behavior similar to that
observed for COD. For instance, for Zn--Al-LDH, only a duration of
20~h was necessary to reach an equilibrium state, and the maximum TOC
removed quantity was approximately 92~mg${\cdot}$g$^{-1}$; however, a
duration of 48~h was required for Mg--Al-LDHs. These results indicate
that both Mg--Al-LDH and Zn--Al-LDH have relatively high efficiencies
in removing COD, TOC, BOD, and TSS from the OMW.

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}Visible-UV absorbance for different time periods:
(a) Mg--Al-LDH and (b) Zn--Al-LDH.}
{\vspace*{2pt}}
\end{figure*}

%tab3
\begin{table*}
\caption{\label{tab3}Kinetic parameters for OMW treatment with
Mg--Al-LDH and Zn--Al-LDH}
\begin{tabular}{ccccccccc}
\thead
& & \multicolumn{3}{c}{Pseudo-first order} 
  & \multicolumn{3}{c}{Pseudo-second order} & 
\xmorerows{1}{$Q_{\exp}$} \\\cline{3-5}\cline{6-8}
& & 
$Q_{1}$, mg${\cdot}$g$^{-1}$ & $k_{1}$, $h^{-1}$ & $R_{1}^{2}$ &
$Q_{2}$, mg${\cdot}$g$^{-1}$ & $k_{2}$, $h^{-1}$ & $R_{2}^{2}$ &  \\
\endthead
\morerows{3}{Mg--Al--500~\textdegree C} & TS & 61.1 & 0.117 & 0.693 & 65.6 & 0.004 & 0.984 & 65.5 \\
&  COD &  162.3\0 &  0.100 &  0.699 &  285.3\0 &  0.002 &  0.990 &  292.2\0 \\
&  BOD &  49.2 &  0.087 &  0.737 &  73.5 &  0.013 &  0.990 &  94.1 \\
&  TOC &  57.0 &  0.128 &  0.801 &  81.8 &  0.007 &  0.991 &{\vspace*{5pt}} 81.7 \\ 
\morerows{3}{Zn--Al--500~\textdegree C}  &  TS &  29.8 &  0.115 & 0.765 &  84.2 &  0.022 &  0.999 &  84.6 \\
&  COD &  50.5 &  0.135 &  0.907 &  294.0\0 &  0.014 &  1.000 &  294.2\0 \\
&  BOD &  16.3 &  0.077 &  0.703 &  98.3 &  0.041 &  0.999 &  98.1 \\
&  TOC &  25.6 &  0.116 &  0.785 &  92.1 &  0.029 &  0.999 &  92.0
\botline
\end{tabular}
\end{table*}

In batch-type adsorption systems, an adsorbate monolayer typically
forms on the surface of the adsorbent~\cite{57}. The rate at which an
adsorbate species is transported from the exterior/outer sites to the
interior sites of the adsorbent particles determines how quickly an
adsorbate species is removed from an aqueous solution~\cite{58}. In
addition to allowing the calculation of sorption rates, kinetic
modeling produces appropriate rate expressions representative of
potential reaction mechanisms.

Although the correlation coefficient values are higher for Mg--Al-500
and Zn--Al-500, the experimental $Q_{\exp}$ values do not agree with
the calculated values obtained from the linear plots
(Table~\ref{tab3}). This demonstrates that COD, BOD, TOC, and TS
removal from the body and onto LDH are not first-order kinetics. When
using second-order kinetics, the $t/q$ versus $t$ plot should be
linear. There is no need to know any parameter beforehand, and $Q_{e}$
and $k_{2}$ can be determined from the slope and intercept of the plot.
Furthermore, this procedure is more likely to predict behavior across
the entire range of uptake. The correlation coefficients for the
second-order kinetic model are greater than 0.98, indicating the
\mbox{applicability} of this kinetic equation and the 
\mbox{second-order} \mbox{nature} of
the COD, BOD, TOC, and TS uptake processes on calcined LDH. It is worth
mentioning that our materials can be considered promising materials for
OMW treatment in comparison with eucalyptus sawdust~\cite{59},
activated clay~\cite{60}, and chitosan~\cite{61}.\looseness=1 

Coloring is considered to be an indicator of pollution from dissolved
organic matter~\cite{62}. The studied OMW sample is dark in color
(brown). This coloration is largely due to the presence of phenolic
compounds~\cite{63}, and its variation is related to the types of
distribution of these compounds~\cite{64}. In addition, UV visible
spectroscopy analyses (Figure~\ref{fig11}) showed the existence of UV
absorbance bands around 278 nm, which correspond to the absorption
domain of polyphenols.

From Figure~\ref{fig11}, it can be clearly seen that the treatment of
OMW with Mg- and Zn-LDH significantly decreased the absorption
intensity at 278 nm as the contact time increased. This peak
corresponds to organic matter, particularly polyphenols. After 44~h,
the absorbance measurement, or index of rejection discoloration,
measured in the visible range (at 395~nm), showed that the percentage
of \mbox{discoloration} reached 90\% for Mg--Al-LDH and 97\% for Zn--Al-LDH.

\section{Conclusions}\label{sec4}

This work shows that the properties of calcined Mg--Al-LDH and
Zn--Al-LDH using a mechanochemical approach are dependent on various
parameters, including grinding time, peptization temperature, and metal
ratios. Under some experimental conditions, this rapid, simple, and
inexpensive method permits the production of well-defined and
homogenous LDH particles. These materials achieve high removal
efficiencies for various mineral and organic pollutants in the OMW. In
the future, we intend to complete the conversion of the precursors to
produce almost phase-pure LDH. This stage will be achieved through a
two-step milling operation (dry and wet milling) in a mixer mill while
adjusting the ball/sample ratio or the imparted mechanochemical energy.
Moreover, an ecotoxicological assessment of the treated OMW will be
performed. 

\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.

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