\makeatletter
\@ifundefined{HCode}
{\documentclass[screen,CRCHIM,Unicode,published]{cedram}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\def\jobid{crchim20230776}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\let\Lbreak\break
\def\hyphen{\text{-}}
\def\mn{\phantom{$-$}}
\def\bcaption#1#2#3{\caption{Caption continued on next page.}\end{figure*}\setcounter{figure}{#2}\begin{figure*}\vspace*{-1pc}\caption{(\textbf{cont}.)\space#1\space #3}}
\def\refinput#1{}
\def\back#1{}
\def\rmDelta{\Delta}
\def\botline{\\\hline}
\newenvironment{inftab}{}{}
\def\0{\phantom{0}}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\DOI{10.5802/crchim.367}
\datereceived{2023-09-04}
\daterevised{2024-01-11}
\datererevised{2024-08-16}
\dateaccepted{2024-11-22}
\ItHasTeXPublished
}
{\documentclass[crchim]{article}
\def\CDRdoi{10.5802/crchim.367}
\let\newline\break
\makeatletter
\def\CDRsupplementaryTwotypes#1#2{}
\def\bcaption#1#2#3{\caption{#1\space#3}}
}
\makeatother

\usepackage{upgreek}

\foreignlanguage{english}{\gdef\deuxpoints{:}}

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

% \copyeditor{RB}

\dateposted{2025-02-13}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Synthesis and characterization of ZSM-12 zeolite for uranium(VI)
adsorption: isotherm, kinetic, and thermodynamic investigations}

\alttitle{Synth\`{e}se et caract\'{e}risation de la z\'{e}olite ZSM-12
pour l'adsorption de l'uranium(VI) : \'{e}tude cin\'{e}tique,
thermodynamique et isothermes d'adsorption}

\author{\firstname{Sihem} \lastname{Khemaissia}}
\address{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Rachid} \lastname{Abaidia}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Fatima} \lastname{Houhoune}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Messaoud} \lastname{Bennemla}\CDRorcid{0009-0001-4714-8817}\IsCorresp}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}
\curraddr[M. Bennemla]{CRND, Nuclear Research Center of Draria, BP 43 Sebala-Algiers, Algeria}
\email[M. Bennemla]{m-bennemla@crnd.dz}

\author{\firstname{Naima} \lastname{Bayou}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Djamila} \lastname{Haddad}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Asma} \lastname{Benturki}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Fatiha} \lastname{Bendjeriou}}
\addressSameAs{1}{Nuclear Research Center of Draria, BP 43 16050, Sebala, Draria, Algiers, Algeria}

\author{\firstname{Faiza}\nobreakauthor\lastname{Zermane}}
\address{Saad Dahlab University, Blida 1, Algeria}

\thanks{Nuclear Research Center of Draria (Algiers)}

\begin{abstract}
In this study, a porous adsorbent was synthesized hydrothermally from a
molar composition gel of 10Na\tsub{2}O:Al\tsub{2}O\tsub{3}:100SiO\tsub{2}:2000H\tsub{2}O:20TPABr. 
X-ray diffraction confirmed
the presence of a ZSM-12 zeolite phase, and nitrogen adsorption
analysis revealed a Brunauer--Emmett--Teller specific surface area of 343.7 m\tsup{2}/g for
the synthesized material. The ZSM-12 zeolite was then used for the
recovery of uranium(VI) from aqueous solutions with initial uranium
concentrations of 10 and 150 mg/L. Operating parameters such as pH,
contact time, solid/liquid ratio, and temperature were systematically
investigated. The optimum conditions were found to be consistent for
both concentrations, with a pH value of 3, a contact time of 60 min,
and a temperature of 20 \textdegree C. However, the solid/liquid ratio
varied from 5 g/L for 10 mg/L uranium concentration to 8 g/L for
150 mg/L concentration. The Langmuir isotherm was well matched by
ZSM-12, and its kinetics were consistent with the pseudo-second-order
model. The adsorption of uranyl ions by ZSM-12 material is spontaneous,
and both chemisorption and physisorption processes occur simultaneously
during the adsorption. The thermodynamic parameters indicate that the
adsorption of U(VI) on ZSM-12 is exothermic. The negative value of \tralicstex{Δ}{$\rmDelta$}S\text{\textdegree} indicates that the system disorder and randomness
decrease as the adsorption proceeds and the liquid phase adsorbate
components are immobilized on the surface of the adsorbent material.
\end{abstract}

\begin{altabstract}
Dans cette \'{e}tude, un adsorbant poreux est synth\'{e}tis\'{e} par
voie hydrothermale \`{a} partir d'un gel de composition molaire
suivante 10Na\tsub{2}O\deuxpoints\text{Al}\tsub{2}O\tsub{3}\deuxpoints 100SiO\tsub{2}\deuxpoints 2000H\tsub{2}O\deuxpoints 20TPABr. La DRX a confirm\'{e} la pr\'{e}sence de la
phase de ZSM-12 et l'analyse par BET a r\'{e}v\'{e}l\'{e} une surface
sp\'{e}cifique de 343.7 m\tsup{2}/g. La z\'{e}olithe ZSM-12 est
utilis\'{e}e dans la r\'{e}cup\'{e}ration de l'uranium(VI) \`{a}
partir de solutions aqueuses ayant des concentrations initiales de 10
et 150 mg/L. Les param\`{e}tres op\'{e}ratoires tels que le pH, le
temps de contact, le rapport solide/liquide et la temp\'{e}rature
\'{e}taient syst\'{e}matiquement \'{e}tudi\'{e}s. Les conditions
optimales se sont r\'{e}v\'{e}l\'{e}es coh\'{e}rentes pour les deux
concentrations initiales : un pH de 3, un temps de 60 min et une
temp\'{e}rature de 20 \textdegree C. Cependant, l'\'{e}tude du
rapport solide/liquide a r\'{e}v\'{e}l\'{e} un optimum de 5 g/L pour la
concentration initiale de 10 mg/L et 8 g/L pour la concentration
initiale de 150 mg/L. L'\'{e}tude cin\'{e}tique a montr\'{e} une
excellente corr\'{e}lation avec le mod\`{e}le du pseudo-deuxi\`{e}me
ordre. L'adsorption des ions uranyle par le mat\'{e}riau ZSM-12 est
spontan\'{e}e, et les processus de chimisorption et de physisorption
ont lieu en m\^{e}me temps. L'\'{e}tude thermodynamique a montr\'{e}
que l'adsorption de l'U(VI) sur le ZSM-12 est exothermique. La valeur n\'{e}gative de \tralicstex{Δ}{$\rmDelta$}S\text{\textdegree} indique que le d\'{e}sordre et le caract\`{e}re aléatoire du système diminuent au fur et à mesure que l'adsorption progresse et que les composants de l'adsorbat en phase liquide sont immobilis\'{e}s à la surface du mat\'{e}riau adsorbant.
\end{altabstract}


\keywords{\kwd{ZSM-12 synthesis}
\kwd{Adsorption}
\kwd{Uranium(VI)}
\kwd{Equilibrium}
\kwd{Isotherm}}

\altkeywords{\kwd{Synth\`{e}se de ZSM-12}
\kwd{Adsorption}
\kwd{Uranium(VI)}
\kwd{\'{E}quilibre}
\kwd{Isotherme}}


\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}

Nuclear facility operations generate significant quantities of
aqueous effluents containing transuranic elements. These elements
originate from various stages of the nuclear fuel cycle, including ore
processing and nuclear fuel production/development. These long-lived
radionuclides are of significant international concern because of their
dual chemical and radiological toxicity, which can have serious effects
on human health and wildlife \cite{1,2}. 
To reduce the risk to human
health, the United States Environmental Protection Agency has
established a maximum contaminant level of 
$30~\upmu\mathrm{gU}{\cdot}\mathrm{L}^{-1}$ for radionuclides in drinking
water, a limit that has been adopted by many countries \cite{3}. The
World Health \mbox{Organization} limits the concentration of uranium in
drinking water to $15~\upmu\mathrm{gU}{\cdot}\mathrm{L}^{-1}$. In this
context, the effective remediation of wastewater contaminated with
radioactive uranium has become a critical issue from both social and
environmental perspectives \cite{4}. Due to the complexity of 
chemical and radiochemical composition of radioactive waste, its
treatment poses a significant challenge that must be taken very
seriously. The removal of radioactive elements in liquid effluents must
meet strict requirements regarding the limits of radioactive substances
and other contaminants. To meet the standards set by national and
international regulations, the waste must be treated to reduce both the
volume and concentration of radioactive compounds and other toxic
solutes in the effluent \cite{5}.


Several processes are available for the removal of uranium from aqueous
solutions, including precipitation, membrane separation,
electrodeposition, coagulation, reverse osmosis, ultrafiltration,
phytoremediation, solvent extraction, and ion
exchange~\cite{6,7,8,9,10}. However, most of these techniques have
certain drawbacks, such as high operating costs, inadequate metal
removal, and the generation of large volumes of sludge~\cite{11}.
Adsorption using low-cost adsorbents and biosorbents is recognized as
an effective and economical method for removing heavy metals at low
concentrations. Various materials, such as activated
carbon~\cite{12,13}, synthetic polymers~\cite{14}, biomass \cite{15},
clay minerals~\cite{16}, metal oxides~\cite{17}, and a wide range of
inorganic materials, including zeolites, have been developed and used
for radionuclide adsorption~\cite{18,19}. Zeolites, as low-cost
adsorbents, have attracted considerable interest for the removal of
uranium and other trace elements from effluents due to their excellent
adsorption capacity, which is attributed to their large specific
surface area and developed porosity. Their long lifetime and radiation
resistance also provide significant advantages for the treatment of
radioactive waste~\cite{5}. They have been investigated as
cation-exchange adsorbents for heavy metals present in low-activity
wastes and, more recently, for the treatment of medium- and
high-activity wastes~\cite{20}. Many zeolites have been used in the
recovery of uranium from liquid radioactive waste, including
clinoptilolite, montmorillonite, heulandite, analcime, phillipsite,
chabazite, and other minerals, as well as synthetic zeolites such as
NaA, NaY, X, 4A, P1~\cite{5}, and NKF-6~\cite{7}. Zeolites are
a family of aluminosilicates with an open structure with
well-distributed micropores. Their general formula can be expressed as
follows: ${\text{M}}_{{x/n}}((\mathrm{SiO}_{2})
_{{x}}(\mathrm{AlO}_{2})_{{y}})(\mathrm{H}_{2} \mathrm{O})_{{z}}$,
where \text{M} is an extra-framework cation with a valence of $n$, which
compensates for the negative charges of the framework, enabling
zeolites to function as cation exchangers.

The primary objective of this study is twofold: first, to synthesize a
large pore ZSM-12 nanomaterial and second, to evaluate its performance
as an adsorbent for uranium(VI) from aqueous effluents. In addition,
the study investigates sorption isotherm models, kinetics, and various
thermodynamic parameters associated with the adsorption process.


\section{Material and methods}

\subsection{Reagents and chemicals}

All the chemicals and reagents used in this study were of analytical
grade. Uranium nitrate (UO\tsub{2}
(NO\tsub{3})\tsub{2}${\cdot}$6H\tsub{2}O, 99\%), nitric acid
(HNO\tsub{3}, 63\%), aluminum oxide (Al\tsub{2}O\tsub{3}, 98\%),
arsenazo (III), and tetrapropylammonium bromide (TPABr, 98\%) were
purchased from Merck. Sodium hydroxide (NaOH, 98\%) was sourced from
Sigma-Aldrich and Aerosil 200 (SiO\tsub{2}, 100\%) was purchased from
Degussa.  Ethylenediaminetetraacetic acid
(C\tsub{10}H\tsub{16}N\tsub{2}O\tsub{8}, 99\%) was provided by
PanReac, 4-nitrophenol (C\tsub{6}H\tsub{5}NO\tsub{3}) was obtained from
Biochem, and chloroacetic acid (ClCH\tsub{2}COOH, 99\%) was purchased
from Fluka.

The wastewater used was generated during the processing of uranium ore
at the Draria Nuclear Research Center (CRND). Atomic absorption
spectroscopy, flame photometry, and ultraviolet--visible (UV--vis) spectroscopy were used to
determine the chemical composition of the effluent, with concentrations
of U(VI) 151.69~mg${\cdot}$L$^{-1}$; Na, K, and Ca(II) 9.679, 0.752, and
2.708, respectively;  Fe(III) 4.502~mg${\cdot}$L$^{-1}$.


\subsection{Synthesis of the adsorbent} 

The hydrothermal synthesis of ZSM-12 zeolite was carried out in a
stainless steel autoclave coated with Teflon. The molar composition of
the gel is expressed as follows: 
10Na\tsub{2}O:Al\tsub{2}O\tsub{3}:100SiO\tsub{2}:2000H\tsub{2}O:20 
TPABr \cite{22}. The gel obtained was stirred for 1~h until
homogenization occurred and then placed in an oven and heated to a
temperature  of 140~\textdegree C for 6~days. The autoclave was then
cooled under running water. The crystallized product collected after
filtration was washed several times with distilled water until it had a
pH of 7 and then dried in an oven at 80~\textdegree C for 24~h.


\subsection{Characterization} 

The crystallographic structure of the prepared ZSM-12 was analyzed by
powder X-ray diffraction (XRD, Bruker AXS D8 Advance). The surface
functional groups were identified using Fourier transform infrared
(FTIR, UATR, PerkinElmer) spectroscopy. The specific surface area was
determined by a N\tsub{2} adsorption--desorption analyzer (Micromeritics,
ASAP 2010). The specific surface area was calculated by the
Brunauer--Emmett--Teller (BET) method while the pore size distribution
and pore volume were calculated by the BJH model.


\subsection{Adsorption experiments}

A uranium solution with a concentration of 1~g${\cdot}$L$^{-1}$ was
prepared through the dissolution of 2.11~g of uranyl nitrate salt
UO\tsub{2} (NO\tsub{3})\tsub{2}6H\tsub{2}O in distilled water. A small
amount of concentrated nitric acid (HNO\tsub{3}) was added to prevent
hydrolysis. Subsequently, uranium solutions of different concentrations
were obtained by diluting the original solution with distilled water. 

The U(VI) adsorption experiments were conducted in closed polyethylene
bottles of 100~mL in duplicate and the mean values were considered as
the final experimental data. In order to determine the optimum
parameters of uranium adsorption, the effect of contact time, pH value,
initial concentration, temperature, and solid/liquid ratio was
investigated. Experiments were conducted with known amounts of ZSM-12
in 20~mL of uranium solutions at a speed of 250~rpm. Once the
adsorption equilibrium was reached, the two phases were separated by
centrifugation and the filtrates obtained were analyzed by a UV--vis
spectrophotometer (Cintra 40 with GBC software) at 652~nm using
arsenazo (III) as the complexing agent~\cite{21}. The adsorption capacity
($Q_{\mathrm{e}}$, mg${\cdot}$g$^{-1}$) and the adsorption
efficiency (\%) of uranium(VI) at equilibrium were calculated
using the following equations:\pagebreak
{\begin{eqnarray}
&\displaystyle
Q_{\mathrm{e}}=\frac{(C_{0}-C_{\mathrm{e}})\times V}{m} 
\label{eqn1}\Seqnsplit
&\displaystyle
R\ (\%)=\frac{(C_{0}-C_{\mathrm{f}})\times 100}{C_{0}} 
\label{eqn2}
\end{eqnarray}}\unskip
where $R$ is the adsorption efficiency; $C_{0}$, $C_{\mathrm{f}}$, and
$C_{\mathrm{e}}$ are the initial, final, and equilibrium uranium(VI)
concentrations in the solution (mg${\cdot}$L$^{-1}$), respectively;
$V$ is the volume of the solution (L); and $m$ is the mass of the
adsorbent (g).

The agreement between the experimental and theoretical adsorbed amounts
by the isothermal models was determined by calculating the average
percentage error (APE \%) according to Equation~(\ref{eqn3}): 
{\begin{eqnarray}\label{eqn3}
\mathrm{APE}_{\mathrm{kinetic}} (\%)=
\frac{\sum | (Q_{t,\exp}-Q_{t,\mathrm{calc}})/Q_{t,\exp}|}{N}\times 100
\end{eqnarray}}\unskip
where $Q_{t,\exp}$ and $Q_{t,\mathrm{calc}}$ (mg${\cdot}$g$^{-1}$)
are the experimental and theoretical adsorbed amounts, respectively, at
time $t$ and $N$ is the number of experimental data.


\subsection{Adsorption isotherms}

The sorption isotherm reveals the nature of the adsorption, which is
directly related to the surface properties of the adsorbent material
and its affinity for the adsorbate. It also provides insight into the
distribution of adsorbate ions at the solid/liquid interface at
equilibrium. Langmuir, Freundlich, and Dubinin--Radushkevich (D--R) models
have been tested for the simulation of uranium adsorption isotherm
data.


\subsubsection{Langmuir isotherm}

The Langmuir model~\cite{22} is based on the assumption that the
maximum adsorption capacity corresponds to the complete coverage of a
monolayer of molecules on the surface of the adsorbent without any
interaction between the adsorbed molecules. The Langmuir equation for a
homogeneous surface is expressed as
{\begin{eqnarray}\label{eqn4}
\frac{C_{\mathrm{e}}}{Q_{\mathrm{e}}}=\frac{1}{Q_{\max}K_{\mathrm{l}}}
+\frac{C_{\mathrm{e}}}{Q_{\max}}
\end{eqnarray}}\unskip
where $Q_{\mathrm{e}}$ is the quantity of solute adsorbed per unit mass
of the adsorbent at equilibrium (mg${\cdot}$g$^{-1}$), $Q_{\max}$ is
the maximum amount adsorbed per unit mass (mg${\cdot}$g$^{-1}$),
$C_{\mathrm{e}}$ is the equilibrium concentration of the solute
(g${\cdot}$L$^{-1}$), and $K_{\mathrm{l}}$ is the Langmuir constant
(L${\cdot}$mg$^{-1}$), which gives an indication of the
adsorbate/adsorbent affinity. The Langmuir constants $K_{\mathrm{l}}$ and
$Q_{\max}$ can be obtained from the slope and intercept of the linear
plot.


\subsubsection{Freundlich isotherm}

The Freundlich isotherm model~\cite{23} describes adsorption on
heterogeneous surfaces in multilayers. The linear form of the
Freundlich equation is given by the following equation: 
{\begin{eqnarray}\label{eqn5}
\mathrm{Log}\, Q_{\mathrm{e}}=\mathrm{Log}\, K_{\mathrm{F}}
+\left(\frac{1}{n}\right)\mathrm{Log}\, C_{\mathrm{e}}
\end{eqnarray}}\unskip
where $C_{\mathrm{e}}$ is the equilibrium concentration of U(VI) in the
liquid phase (mg${\cdot}$L$^{-1}$), and $K_{\mathrm{F}}$ and $n$ are
the Freundlich constants of the system, which denote the adsorption
capacity and adsorption intensity, respectively.


\subsubsection{Dubinin--Radushkevich isotherm}

The D--R model~\cite{24} assumes that the surface
of the material is heterogeneous. This model is more general than the
Langmuir and Freundlich models and is used to describe the
adsorption on both homogeneous and heterogeneous
surfaces~\cite{25}. 


Equation (\ref{eqn6}) gives the linear form of the D--R equation:
{\begin{eqnarray}\label{eqn6}
\mathrm{Ln}\, Q_{\mathrm{e}}=\mathrm{Ln}\, Q_{m}-K\varepsilon ^{2}
\end{eqnarray}}\unskip
where $K$ (mol/J)\tsup{2} is a constant of adsorption energy,
$Q_{m}$ is the adsorption capacity, and ${\varepsilon}$ is the
Polanyi potential given as follows:
{\begin{eqnarray}\label{eqn7}
\varepsilon =RT \mathrm{Ln} (1+1/C_{\mathrm{e}})
\end{eqnarray}}\unskip
where $R$ is the perfect gas constant
(8.314~J${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$) and $T$ is the
temperature in kelvin (K). The values of $Q_{m}$ and $K$ are deduced
from the graph $\mathrm{Ln}\,Q_{\mathrm{e}} = f(\varepsilon^{2})$; the
adsorption energy $E_{\mathrm{a}}$ (kJ/mol) is one of the most
important parameters determining the type of adsorption (chemical or
physical), and it is calculated from Equation~(\ref{eqn8}). When the
adsorption energy ($E_{\mathrm{a}}$) is between 1 and 8, the dominant
mechanism of adsorption is physisorption. In the case where
$E_{\mathrm{a}}$ is between 9 and 16, chemisorption
predominates~\cite{26}.
{\begin{eqnarray}\label{eqn8}
E_{\mathrm{a}}=1/\sqrt{2K}
\end{eqnarray}}\unskip

\subsection{Adsorption kinetics}

In order to study the uptake mechanism of the uranyl ions by the
synthesized ZSM-12 material, two kinetic models were applied to the
experimental kinetic data: the pseudo-first-order model and the
pseudo-second-order model. The equations associated with these kinetic
models are as follows. 

Pseudo-first-order model~\cite{27}:
{\begin{eqnarray}
\mathrm{Log}\, (Q_{\mathrm{e}}-Q_{t})=\mathrm{Log}\,
Q_{\mathrm{e}}-K_{1}\frac{t}{2.303}
\label{eqn9}
\end{eqnarray}}\unskip
Pseudo-second-order~model~\cite{28}:
{\begin{eqnarray}\label{eqn10}
\frac{t}{Q_{t}}=\frac{1}{K_{2}{Q_{\mathrm{e}}}^{2}}+\frac{t}{Q_{\mathrm{e}}}
\end{eqnarray}}\unskip
where $K_{1}$ (min$^{-1}$) and $K_{2}$
(g${\cdot}$mg$^{-1}{\cdot}$min$^{-1}$) are the kinetic constants
of the first- and second-order models; $Q_{\mathrm{e}}$ and $Q_{t}$ are
the quantities of uranium(VI) adsorbed (mg${\cdot}$g$^{-1}$) at
equilibrium and at time $t$ respectively.



\subsection{Adsorption thermodynamics}

In order to study the thermodynamic properties of uranyl ion adsorption
by synthesized ZSM-12, thermodynamic analysis was carried out at
different temperatures. The thermodynamic parameters of the adsorption
process such as enthalpy $(\rmDelta H\text{\textdegree})$, standard
entropy $(\rmDelta S\text{\textdegree})$, and standard free enthalpy
$(\rmDelta G\text{\textdegree})$ were determined using 
Equations (\ref{eqn11})--(\ref{eqn13})~\cite{29}: 
{\begin{eqnarray}
&\displaystyle
\mathrm{Ln}\, K_{\mathrm{d}}=\frac{\rmDelta S\text{\textdegree}}{R}
-\frac{\rmDelta H\text{\textdegree}}{RT} 
\label{eqn11}\Seqnsplit
&\displaystyle
\rmDelta G\text{\textdegree}=\rmDelta H\text{\textdegree}
-T\rmDelta S\text{\textdegree} 
\label{eqn12}\Seqnsplit
&\displaystyle
K_{\mathrm{d}}=\frac{C_{\mathrm{i}}-C_{\mathrm{f}}}{C_{\mathrm{f}}}\times \frac{V}{m} 
\label{eqn13}
\end{eqnarray}}\unskip
where $K_{\mathrm{d}}$ is the distribution coefficient
(L${\cdot}$g$^{-1}$), $\rmDelta S\text{\textdegree}$ is the standard
entropy (kJ${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$), $\rmDelta
H\text{\textdegree}$ is the standard enthalpy
(kJ${\cdot}$mol$^{-1}$), $\rmDelta G\text{\textdegree}$ is the free
enthalpy (kJ${\cdot}$mol$^{-1}$), $T$ is the absolute temperature in
Kelvin, $R$ is the perfect gas constant ($R =
8.314~\mathrm{J}{\cdot}\mathrm{mol}^{-1}{\cdot}\mathrm{K}^{-1}$),
$C_{\mathrm{i}}$ and $C_{\mathrm{f}}$ are the initial and final
concentrations of the solute in the solution, respectively
(g${\cdot}$L$^{-1}$), $V$ is the volume of the solution (L), and $m$
is the mass of the adsorbent (g).

\section{Results and discussion}

\subsection{Characterization}

\subsubsection{X-ray diffraction}

The XRD patterns of the synthesized material are illustrated in
Figure~\ref{fig1}. The observed patterns show excellent crystallinity
as evidenced by the prominent peaks, confirming the efficacy of the
hydrothermal synthesis process. 


\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}XRD patterns of synthesized material and ZSM-12 zeolite
indexed by PDF2 \# 00-043-0439.}
\end{figure}



Moreover, the obtained spectrum is consistent with the reference code
number 00-043-0439, confirming the ZSM-12 type phase in accordance with
the existing literature~\cite{30,31}. The peaks at 7.9\textdegree,
8.8\textdegree, 20.8\textdegree, and 23\textdegree\ were successfully
indexed as characteristic diffraction peaks of ZSM-12
zeolite~\cite{32}. Furthermore, the peaks at 11.89\textdegree,
13.24\textdegree, 13.89\textdegree, 14.60\textdegree, and
15.90\textdegree\ provide evidence for the presence of
TPA$^{+}$ cations~\cite{33} with an organized arrangement of the
organic template used to synthesize ZSM-12 zeolite within the crystal
framework~\cite{34}.



\subsubsection{Fourier transform infrared spectrum}

In order to identify the functional groups on the surface of the
prepared material (ZSM-12),  
FTIR spectroscopy analysis was conducted in the wavenumber range of
400--4000~cm$^{-1}$, and the spectrum obtained is shown in
Figure~\ref{fig2}. The absorption band at 3297~cm$^{-1}$ could be
attributed to the elongation of the O--H bond of hydroxyl groups
(alcohols or phenols)~\cite{35}. The small band at 1627~cm$^{-1}$
indicates the C${=}$O stretching vibration in carbonyl groups (aldehydes
or ketones)~\cite{36}. The bands located at 1227 and 1100 cm$^{-1}$
can be assigned to the axial deformation of C--O bonds in carboxylic
acids, alcohols, esters, and ethers~\cite{37}. The bands observed at
798~cm$^{-1}$ and 548 cm$^{-1}$ could be attributed to the external
symmetric Si--O stretching vibration and the internal vibration mode of
tetrahedra (SiO\tsub{4}, AlO\tsub{4}), respectively~\cite{38}. Finally,
the absorption peak observed at 445~cm$^{-1}$ corresponds to Si--O
bending vibration~\cite{38}.

 
\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}FTIR spectrum of the ZSM-12 synthesis material.}
\end{figure}


\subsubsection{Point of zero charge (PZC)} 

The acid--base properties of ZSM-12 zeolites were investigated by means
of the zero charge point pH ($\mathrm{pH}_{\mathrm{PZC}}$) at which the net
charge of the particle is zero; the amounts of positive and negative
charges at this point are equal~\cite{39}. Figure~\ref{fig3} shows the
point where the initial and final pH values intersect. The $\mathrm{pH}_{\mathrm{PZC}}$ value of
the sorbent was observed to be 2.19. This value reflects the acidity of
this zeolite due to the predominance of acidic groups on its
surface~\cite{28}. At the point of PZC, the net charge of the
sorbent is zero and below this point, the sorption of U(VI) is
low. The organic functional groups present in the cells of the sorbent
material are charged in a positive or negative way depending on the pH
of the solution. Due to the negative charge of the sorbent surface, the
sorption process is favored when the pH is above 2.19~\cite{40}.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}PZC determination ($\mathrm{pH}_{\mathrm{PZC}}$) for the
ZSM-12 zeolite.}
\end{figure}


\subsubsection{Nitrogen adsorption--desorption isotherm}

Figure~\ref{fig4} shows the nitrogen adsorption--desorption isotherm of
the synthesized ZSM-12 zeolite, which is type IV
according to the IUPAC classification~\cite{41}. A hysteresis loop is
observed during the desorption process in the range of 0.4 ${<}$ P/P0 ${<}$
0.95, indicating the coexistence of micro- and mesopores~\cite{31,42}.
The large branch of hysteresis indicates that the synthesized ZSM-12
is mainly composed of mesopores accompanied by a few micropores. 
The mesoporosity might be the result of inter/intracrystalline mesopores
corresponding to the zeolitic material as well as the intrinsic
mesoporosity of the amorphous material~\cite{43}. Furthermore, the
synthesized ZSM-12 exhibits a high BET surface area
(343.69~m\tsup{2}${\cdot}$g$^{-1}$), low micropore volume
(0.1067~cm\tsup{3}${\cdot}$g$^{-1}$), and a small mesopore size (3.0~nm)
calculated by BJH models. The narrow mesopore size distribution may be
attributed to the nanocrystals that constitute the
agglomerates~\cite{44}. The high BET specific surface area of ZSM-12
offers abundant adsorption sites, which favors the adsorption of
uranium from aqueous solution. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Nitrogen adsorption--desorption isotherm (a) and pore size
distribution (b) of the ZSM-12 zeolite.}
\end{figure}


\subsection{Adsorption studies}

\subsubsection{Initial pH effect}

The effect of initial pH on the adsorption of uranium(VI) by the
synthesized zeolitic material ZSM-12 was studied in the pH range of 1.5
to 9.0 using initial concentrations of 10 and 150~mg/L. As shown in
Figure~\ref{fig5}, the adsorption efficiency of uranyl ions increases
in the pH range of 1.5 to 3 and then decreases with increasing pH. The
highest levels of adsorption efficiency, namely 93\% and 50\% for
initial concentrations of 10 and 150~mg/L, respectively, are observed
at a pH of 3. These results can be explained by considering the
concentration of hydrogen ions (H$^{+}$), the pH of the charge zero
point ($\mathrm{pH}_{\mathrm{PZC}}$), and the speciation of uranium(VI)~\cite{45}. At
low pH values ($\mathrm{pH} < \mathrm{pH}_{\mathrm{PZC}} = 2.19$), the active surface
functional groups of ZSM-12 zeolite, which are mainly the silanol
(Si--OH) and aluminol (Al--OH) groups, and the adsorption of
$H^{+}(aq)$ on the zeolite surface cause an increase in the solution
pH. Under such conditions, the electrostatic repulsion between
$\mathrm{UO}_{2}^{2+}$ and ($\mathrm{SiOH}_{2}^{+}$, $\mathrm{AlOH}_{2}^{+}$)
leads to a decrease in the adsorption rate of uranyl ions~\cite{46,47}.
In the pH range 2.19 ${<}$ pH ${<}$ 3, the surface of ZSM-12 becomes
negatively charged with increasing pH. The predominance of the
$\mathrm{UO}_{2}^{2+}$ species in this pH zone leads to an increased
adsorption efficiency of uranium(VI) via electrostatic attraction
between the positively charged uranium species ($\mathrm{UO}_{2}^{2+}$)
and the negatively charged functional groups on the surface of ZSM-12.
Finally, at pH values higher than 3.0, the surface charge of ZSM-12
remains negative, but complex uranium species such as
UO\tsub{2}(OH)$^{+}$ and $(\mathrm{UO}_{2})_{2}(\mathrm{OH})_{2}^{2+}$
appear~\cite{48}. The low adsorption percentage observed in this zone
can be explained by the lack of affinity between the material and
hydroxylated species.


\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Effect of pH on the adsorption efficiency of uranium(VI).
$T=20 \pm 2~\text{\textdegree}\mathrm{C}$; contact time ${=}$ 60~min;
$R$ (S/L) ${=}$ 10~g${\cdot}$L$^{-1}$.}
\end{figure}


\subsubsection{Effect of contact time} 

The impact of contact time on the adsorption efficiency of uranium(VI)
by the ZSM-12 zeolite adsorbent was explored in a time range of 
5--300~min. As shown in Figure~\ref{fig6}, the adsorption efficiency of
uranyl ions by the synthesized material rises with increasing contact
time until it reaches a plateau~\cite{48}. The uptake of uranium by
ZSM-12 can be delineated into two distinct stages: an initial rapid
process occurring within the first 60~min followed by a subsequent
very slow phase. Approximately 95\% and 50\% of the total uranyl ions
are adsorbed from uranyl nitrate solutions of 10 and
150~mg${\cdot}$L$^{-1}$, respectively, during the first hour. After this
period, most of the active sites on the adsorbent become saturated,
marking the end of sorption. It is noteworthy that in the case of the
150~mg${\cdot}$L$^{-1}$ uranyl nitrate solution, the kinetic curve shows a
slight decrease in the adsorption rate due to material saturation. A
contact time of 1~h is chosen for further investigation.

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Effect of contact time on U(VI) sorption on the
synthesized ZSM-12 material. $T= 20 \pm 2~\text{\textdegree}\mathrm{C}$;
pH ${=}$ 3; $R$ (S/L) ${=}$ 10~g${\cdot}$L$^{-1}$.}
\end{figure}


The mechanism of the surface reaction between uranyl ions
($\mathrm{UO}_{2}^{2+}$) and the active sites of ZSM-12 zeolites was
tested using the two kinetic equations, namely, pseudo-first-order
and pseudo-second-order (Figure~\ref{fig7}). The kinetic parameters
calculated from the two theoretical models and the values of the
regression coefficients are given in Table~\ref{tabI}. 
From Figure~\ref{fig7}, we can see that our experimental data follow very
well the two kinetic models applied to our system ($R^{2} > 0.96$) but
with better regression coefficients ($R^{2}> 0.999$)  if pseudo-second-order
reactions are allowed. Therefore, it can be assumed that the adsorption
process is controlled by  pseudo-second-order kinetics. These results
are in agreement with previous research studies, which mentioned that the
pseudo-second-order kinetic model described data better than the
pseudo-first-order model when investigating U(VI) adsorption   by
different adsorbents such as cross-linked magnetic chitosan
beads~\cite{50}, magnetic Schiff base~\cite{51}, chitosan
tripolyphosphate beads~\cite{52}, and cross-linked chitosan~\cite{53}.


\begin{figure*}
\vspace*{-1pt}
\includegraphics{fig07}
\vspace*{-6pt}
\caption{\label{fig7}First-order plot (a) and second-order plot (b) for U(VI) adsorption
 using ZSM-12 as adsorbent. $T = 20 \pm 2~\text{\textdegree}\mathrm{C}$; 
pH ${=}$ 3; $R$ (S/L) ${=}$ 10~g${\cdot}$L$^{-1}$.}
\vspace*{-4pt}
\end{figure*}

\begin{table*}
\caption{\label{tabI}Kinetic parameters for the sorption of U(VI) onto ZSM-12\vspace*{-2pt}}
\begin{tabular}{lcccccc}
\thead
& \multicolumn{3}{c}{Pseudo-first-order} & \multicolumn{3}{c}{Pseudo-second-order} \\\cline{2-4}\cline{5-7}
 & $Q_{\mathrm{e}}$  (mg${\cdot}$L$^{-1}$) & $K_{1}$ (min$^{-1}$) & $R^{2}$ 
 & $Q_{\mathrm{e}}$  (mg${\cdot}$L$^{-1}$) & $K_{2}$ (g${\cdot}$mg$^{-1}{\cdot}$min$^{-1}$) & $R^{2}$ \\
\endthead 
\parbox[t]{7pc}{\raggedright [U]\textsubscript{0}$= 10~\mathrm{mg}{\cdot}\mathrm{L}^{-1}$;\break  $R$ (S/L) ${=}$ 5~g${\cdot}$L$^{-1}$} 
& 0.731 & 0.225 & 0.967 & 0.952 & 1.796 & 0.9999\vspace*{2pt} \\
\parbox[t]{7pc}{\raggedright [U]\textsubscript{0}$= 150~\mathrm{mg}{\cdot}\mathrm{L}^{-1}$;\break  $R$ (S/L) ${=}$ 8~g${\cdot}$L$^{-1}$} 
& 2.33\0 & 0.045 & 0.777 & 7.194 & 0.100 & 0.998\0 
\botline
\end{tabular}
\vspace*{-2pt}
\end{table*}


\subsubsection{Solid/Liquid ratio effect}


Batch experiments were conducted using different solid/liquid ratios to
examine the impact of this parameter on the adsorption of uranyl ions
by the ZSM-12 zeolite. The results are shown in Figure~\ref{fig8}. It is
evident that the uranium adsorption efficiency increases with rise
in solid/liquid ratio, reaching a maximum at 5~g${\cdot}$L$^{-1}$ and 
8~g${\cdot}$L$^{-1}$ for initial concentrations of 10~mg${\cdot}$L$^{-1}$ and
150~mg${\cdot}$L$^{-1}$, respectively. Correspondingly, the adsorption
efficiency of U(VI) increases from 54.9\% to 89\% for the
5~g${\cdot}$L$^{-1}$ ratio and from 11.32\% to 50.58\% for the 8~g/L ratio,
after which a slight decrease is observed. This increasing trend may be
attributed to the growing number of adsorption sites and the enhanced
contact probability between uranyl ions and the available adsorption
sites~\cite{54}. Conversely, the decreasing trend could be due to the
presence of a large number of active particles, causing overlapping and
aggregation, thereby reducing the total surface area available for
uranium ion adsorption~\cite{55}.


\begin{figure}
\includegraphics{fig08}
\vspace*{-3pt}
\caption{\label{fig8}Effect of the solid/liquid ratio on U(VI) sorption on
the synthesized ZSM-12 material. pH ${=}$ 3; contact time ${=}$ 60~min;
$T = 20 \pm 2~\text{\textdegree}\mathrm{C}$.}
\vspace*{-7pt}
\end{figure}


\subsubsection{Initial uranium concentration effect} 

The effect of the initial concentration of uranium on its adsorption by
the synthesized ZSM-12 was investigated by varying the concentration of
uranium from 5 to 200~mg/L. The experiments were conducted for two
solid/liquid ratios of 5~g${\cdot}$L$^{-1}$ and 8~g${\cdot}$L$^{-1}$. The graphs
depicting the uranium uptake efficiency in relation to the initial
concentration are presented in Figure~\ref{fig9}.


\begin{figure}
\includegraphics{fig09}
\caption{\label{fig9}Effect of the initial concentration on U(VI) sorption 
on the synthesized ZSM-12 zeolite. pH ${=}$ 3; $T
= 20 \pm 2~\text{\textdegree}\mathrm{C}$; contact time ${=}$
60~min.}
\end{figure}

The curves in Figure~\ref{fig9} show that the adsorption efficiency of
the synthesized ZSM-12 is inversely proportional to the increase in
initial uranium concentration. It is known that at low concentration,
the mobility of uranium ions is high, which favors their uptake by the
adsorbent. However, their mobility \mbox{decreases} as their concentration
increases, causing a decrease in adsorption efficiency. In another way,
increasing the initial concentration of uranium ions increases the
charges of the uranium species in the solution, which increases the
Coulomb repulsions, leading to a decrease in the adsorption efficiency.
A similar trend was observed by Monika Jail et~al.\ in their studies on
the removal of Cr(VI), Cu(II), and Cd(II) using iron oxide/activated
carbon nanoparticles~\cite{56}. The experimental uranium adsorption
capacities ($Q_{\mathrm{e}}$) of the synthesized material were determined by
plotting the adsorption capacity as a function of a steady-state
uranium concentration as shown in Figure~\ref{fig9}. The maximum
experimental adsorption capacities of the prepared ZSM-12 zeolite were
found to be equal to 14 and 12~mg${\cdot}$g$^{-1}$ for the two solid--liquid
ratios of 5 and 8~g${\cdot}$L$^{-1}$, respectively. Table~\ref{tabII}
highlights the superior adsorption capacities of ZSM-12 zeolite,
especially at different solid--liquid ratios, in comparison with various
zeolite-based adsorbents reported in the literature.

The Langmuir, Freundlich, and D--R models are used to fit the data of
the adsorption process of uranium(VI) on ZSM-12 material. The
results are shown in Figure~\ref{fig10}, and the obtained parameters
are listed in Table~\ref{tabIII}.

\begin{figure*}
\includegraphics{fig10}
\vspace*{-5pt}
\bcaption{\label{fig10}}{9}{Isothermal experimental and fitted data
with Langmuir, Freundlich, and D--R models for uranium(VI) adsorption
on ZSM-12 zeolite. (a) pH ${=}$ 3; $T$ ${=}$~$20 \pm
2~\text{\textdegree}\mathrm{C}$; contact time ${=}$ 60~min; $R$ (S/L) ${=}$
5~g${\cdot}$L$^{-1}$. (b) pH ${=}$ 3; $T = 20 \pm
2~\text{\textdegree}\mathrm{C}$; contact time ${=}$ 60~min;  $R$ (S/L)
${=}$ 8~g${\cdot}$L$^{-1}$.}
\vspace*{4pt}
\end{figure*}

\begin{table*}
\caption{\label{tabII}Comparison of maximum adsorption capacities of ZSM-12 zeolite
with other adsorbents based on zeolite from the literature}
\begin{tabular}{lcccc}
\thead
Samples & pH & $R$ (S/L) (g${\cdot}$L$^{-1}$) & $Q_{\max}$ (mg${\cdot}$g$^{-1}$) & Ref. \\
\endthead 
ZSM-12  & 3 & 5 & 14 & This work \\ 
ZSM-12  & 3 & 8 & 12 & This work \\ 
Natural zeolite clinoptilolite  & - & - & 1.2 & \cite{57} \\ 
Natural zeolite clinoptilolite  & 6 & 200 & 0.34 & \cite{57} \\ 
Modified clinoptilolite  & 6 & 200 & 4.66 & \cite{57} \\ 
Hematite material  & 7 & 25 & 3.54 & \cite{58} \\ 
{Manganese oxide coated zeolite (MOCZ)}  & 4 & 5 & 15.1 & \cite{59} \\ 
Ammonium modified natural zeolite  & 5 & 20 & 2 & \cite{60}  
\botline
\end{tabular}
\end{table*}

\begin{table}
\caption{\label{tabIII}Adsorption isotherm parameters for U(VI) on ZSM-12 material}
\tabcolsep2pt\fontsize{9.5}{12}\selectfont
\begin{tabular}{lcc}
\thead
& $R$ (S/L)  ${=}$ 5~g${\cdot}$L$^{-1}$
& $R$ (S/L)  ${=}$ 8~g${\cdot}$L$^{-1}$ \\ 
\endthead
\multicolumn{3}{c}{Freundlich model} \vspace*{4pt}\\ 
$K_{\mathrm{F}}$ & \mn\0\02.491 & \mn\0\01.035 \\ 
$n$ & \mn\0\02.398 & \mn\0\01.73\0 \\ 
$R^{2}$ & \mn\0\00.989 & \mn\0\00.897 \\ 
APE (\%) & \mn\082.21\0 \vspace*{6pt} & \mn\091.38\0 \\ 
\multicolumn{3}{c}{Langmuir model} \vspace*{4pt}\\ 
$Q_{\max}$ (mg${\cdot}$g$^{-1}$)   & \mn\016.667 & \mn\012.315 \\ 
$K_{\mathrm{l}}$ (L${\cdot}$g$^{-1}$) & \mn\0\00.116 & \mn\0\00.086 \\ 
$R^{2}$ & \mn\0\00.983 & \mn\0\00.978 \\ 
$\rmDelta G\text{\textdegree}$ (kJ${\cdot}$mol$^{-1}$) & \0${-}$24.93\0 & \0${-}$24.20\0 \\ 
APE (\%) & \mn\019.05\0 \vspace*{6pt} & \mn\0\02.625 \\ 
\multicolumn{3}{c}{Dubinin--Radushkevich model} \vspace*{4pt}\\ 
$Q_{\max}$ (mg${\cdot}$g$^{-1}$)   & \mn\049.708 & \mn\081.546 \\ 
$K \times 10^{-9}$ (mol${\cdot}$J$^{-1}$)\tsup{2}
& \mn\0\03.11\0 & \mn\0\04.73\0 \\ 
$E_{\mathrm{a}}$ (kJ${\cdot}$mol$^{-1}$) & \mn\012.68\0 & \mn\010.28\0 \\ 
$R^{2}$ & \mn\0\00.989 & \mn\0\00.928 \\ 
APE (\%) & \mn255.06\0 & \mn314.23\0 
\botline
\end{tabular}
\end{table}


The results show that for a solid--liquid ratio of
5~g${\cdot}$L$^{-1}$, the three models fit the experimental data well
with high correlation coefficients ($R^{2} > 0.98$). However, the
calculated APEs for the Freundlich and D--R models are very high
(82.21\% and 255.06\%, respectively). Furthermore, the corresponding
adsorption capacity of U(VI) predicted by the Freundlich model is very
low (2.49~mg${\cdot}$g$^{-1}$) and that obtained with the D--R model is
very high (49.708~mg${\cdot}$g$^{-1}$) compared to the experimental
value  (14~mg${\cdot}$g$^{-1}$). For a solid--liquid ratio of
8~g${\cdot}$L$^{-1}$, the correlation coefficient ($R^{2}$) of the
Langmuir model is higher than that of the Freundlich and D--R models.
In addition, the $Q_{\max}$ value (12.315~mg${\cdot}$g$^{-1}$) for the
Langmuir model is more similar to the experimental value
(12~mg${\cdot}$g$^{-1}$). Based on the above results, it can be
concluded that the Langmuir model is the most suitable for representing
the adsorption equilibrium isotherm of uranium(VI) by the ZSM-12
material. This suggests that ZSM-12 provides specific homogeneous sites
and sorption of U(VI) ions on the monolayer generated by the ZSM-12
material~\cite{58,61}. The Langmuir separation parameter
($K_{\mathrm{l}}$) is between 0 and 1, indicating that the adsorption
of U(VI) on ZSM-12 is favorable. The estimation of the thermodynamic
parameters from the Langmuir model requires using the value of the
Langmuir isotherm constant ($K_{\mathrm{l}}$) in the van't Hoff
equation by multiplying it with the molar weight of 
uranium(VI)~\cite{62}. The free energy is evaluated to be ${-}$24.93 and
${-}$24.20~kJ${\cdot}$mol$^{-1}$ for the two studied solid--liquid
ratios. According to the negative values of energy $\rmDelta G\text{\textdegree}$,
the adsorption process is both feasible and spontaneous. The values of
energy between 0 and ${-}$20~kJ${\cdot}$mol$^{-1}$ are generally
indicative of physisorption while the values between ${-}$80 and
${-}$400~kJ${\cdot}$mol$^{-1}$ are indicative of chemisorption~\cite{63}.
The negative $\rmDelta G\text{\textdegree} $ values indicate that both chemisorption
and physisorption occur simultaneously during the adsorption process.


\subsubsection{Temperature effect and thermodynamics}

The temperature effect was studied by varying the temperature of the
uranyl nitrate solution from 20~\textdegree C to 60~\textdegree C for
the two initial uranium concentrations of 10~mg${\cdot}$L$^{-1}$ and 
150~mg${\cdot}$L$^{-1}$. The experiments were carried out with a thermostatic
bath. The curves in Figure~\ref{fig11} clearly show that the increase
in temperature was inversely proportional to the uranium uptake by the
studied material ZSM-12. This indicates that the adsorption process was
exothermic in nature and that the adsorption of uranium(VI) ions on
ZSM-12 was favored at low temperatures.  This result can be attributed
to fact that the interaction between the active groups on the surface
of the adsorbent and the $\mathrm{UO}_{2}^{2+}$ ions was weaker at higher
temperature\unskip\break \cite{48,64}.


\begin{figure}
\vspace*{3pt}
\includegraphics{fig11}
\vspace*{4pt}
\caption{\label{fig11}Effect of temperature on uranium uptake by
zeolite ZSM-12. $R$ (S/L) ${=}$ 8~g${\cdot}$L$^{-1}$; contact time
${=}$ 60~min;  pH ${=}$ 3.}
\end{figure}

Thermodynamic parameters, such as the enthalpy  $\rmDelta H\text{\textdegree}$ 
and the entropy $\rmDelta S\text{\textdegree}$ of the adsorption
reaction, are obtained from the slope and intercept of 
Van't Hoff plots (Figure~\ref{fig12}) and are
shown in Table~\ref{tabIV}.


\begin{figure}
\includegraphics{fig12}
\vspace*{-4pt}
\caption{\label{fig12}Variation of Ln $K_{\mathrm{d}}$ versus $1/T$ for uranium
adsorption on ZSM-12 zeolites.}
\vspace*{-2pt}
\end{figure}

\begin{table*}
\caption{\label{tabIV}The thermodynamic parameters $\rmDelta H\text{\textdegree}$,
$\rmDelta S\text{\textdegree}$, and $\rmDelta G\text{\textdegree}$ of
uranium adsorption by the ZSM-12 material for the two initial
concentrations of 10 and  150~mg${\cdot}$L$^{-1}$\vspace*{-2pt}}
\begin{tabular}{ccccc}
\thead
[U]\textsubscript{0} mg${\cdot}$L$^{-1}$ & Temperature  (K) 
& $\rmDelta H\text{\textdegree}$  (kJ${\cdot}$mol$^{-1}$) & $\rmDelta S\text{\textdegree}$  (kJ${\cdot}$mol$^{-1}{\cdot}$K$^{-1}$)
& $\rmDelta G\text{\textdegree}$  (kJ${\cdot}$mol$^{-1}$) \\ 
\endthead
\morerows{3}{\010}  & 293.15 & \morerows{3}{${-}$33.289}  & \morerows{3}{${-}$0.105}  & \0${-}$2.508\0 \\
 & 313.15 &  &  & \0${-}$0.408\0 \\
 & 323.15 &  &  & \0\mn0.626\0 \\
 & 333.15 &  &  & \0\mn1.691\0\vspace*{4pt} \\ 
\morerows{3}{150}  & 293.15 & \morerows{3}{${-}$67.090}  & \morerows{3}{${-}$0.237}  & \0\mn2.4975 \\
 & 313.15 &  &  & \0\mn7.2475 \\
 & 323.15 &  &  & \0\mn9.658\0 \\
 & 333.15 &  &  & \mn11.997\0  
\botline
\end{tabular}
\vspace*{-2pt}
\end{table*}


The standard enthalpy of the adsorption reaction of uranyl ions by the
ZSM-12 material was found to be equal to
${-}$33.289~kJ${\cdot}$mol$^{-1}$ for the 10~mg${\cdot}$L$^{-1}$
solution and ${-}$67.09~kJ${\cdot}$mol$^{-1}$ for the 
150~mg${\cdot}$L$^{-1}$ solution. These results indicate that the
adsorption process for the two initial concentrations of uranyl nitrate
solutions is exothermic.

Negative values of the adsorption entropy $(\rmDelta S\text{\textdegree})$
indicate that the transition of uranyl ions from the aqueous phase to
the solid phase (adsorption) results in a reduction in the degree of
freedom of the adsorbate, hence the favorable nature of the adsorption
and the affinity of the adsorbent toward uranyl ions~\cite{65,66}.
Negative values of free energy ($\rmDelta G\text{\textdegree}_{\mathrm{ads}}$),
in the case of an initial uranium concentration of 10~mg${\cdot}$L$^{-1}$,
suggest the feasibility and spontaneity of the adsorption reaction,
especially at low temperatures. The increase in $\rmDelta
G\text{\textdegree}_{\mathrm{ads}}$ with increasing temperature signifies
a rise in disorder during adsorption. This phenomenon can be
attributed to the redistribution of energy between the adsorbent and
the adsorbate~\cite{67}. The positive free enthalpy observed at
temperatures of 50~\textdegree C and 60~\textdegree C indicates that an
external energy supply is required for the \mbox{process} to occur. For an
initial uranium concentration of 150~mg${\cdot}$L$^{-1}$, $\rmDelta
G\text{\textdegree}$ values of the sorbent are obtained as positive,
which concludes the reaction to be nonspontaneous, but it shows a range
of spontaneity in the reverse direction~\cite{68}. In addition, the
spontaneous reaction decreases after the initial concentration becomes
more significant, and the spontaneous reactivity is negatively
correlated with the initial concentration of U(VI)~\cite{69}.


\subsection{Reusability of the sorbent}

\begin{figure}
\includegraphics{fig13}
\vspace*{-4pt}
\caption{\label{fig13}Sequential adsorption--desorption cycles of uranium(VI) on
ZSM-12 zeolites. [U]\textsubscript{0}$= 150~\mathrm{mg}{\cdot}\mathrm{L}^{-1}$; $T =20
\pm 2~\text{\textdegree}\mathrm{C}$; contact time ${=}$ 180~min;
$R$ (S/L) ${=}$ 8~g${\cdot}$L$^{-1}$.}
\vspace*{-2pt}
\end{figure}

\begin{scheme*}
{\vspace*{-2pt}}
\includegraphics{sc01}
\vspace*{4pt}
\caption{\label{sch1}The interaction of surface functional groups of ZSM-12 zeolite
with uranyl ions ($\mathrm{UO}_{2}^{2+}$).}
{\vspace*{-6pt}}
\end{scheme*}

Reusability and regeneration of the used sorbent is an important factor
in any removal process. The regeneration process was carried out using a 
solution of HNO\tsub{3} as the desorption agent. The effect of the
concentration of HNO\tsub{3} solution was studied in the range of
0.1--1.5~mol${\cdot}$L$^{-1}$. According to Figure~\ref{fig13}, a
1.0~mol${\cdot}$L$^{-1}$ solution of HNO\tsub{3} is the best eluent for the
desorption of U(VI) from  ZSM-12. After the first adsorption, 84.05\% of the adsorbed uranium
ions were desorbed, indicating that some
adsorption sites with adsorbates cannot be regenerated~\cite{71}. The
fresh adsorbent has 84.05\% sorption sites and its adsorption capacity
remains at 9.26~mg${\cdot}$g$^{-1}$ after five adsorption--desorption cycles,
which is 65.82\% of the fresh adsorbent in the first cycle. In
subsequent cycles, the sorption of uranium may be reduced by sorbent
loss or partial degradation during the elution process~\cite{72} and
by the accumulation of uranium molecules on the surface of the ZSM-12
sorbent~\cite{73}. However, efforts will be made to improve reuse in
the future.\looseness=-1

\subsection{Adsorption mechanism}

The adsorption mechanism of U(VI) on ZSM-12 zeolites is mainly ion
exchange as presented in Scheme~\ref{sch1}. The divalent cation
($\mathrm{UO}_{2}^{2+}$) is exchanged with two Na$^{+}$
cations of the ZSM-12 zeolite as already observed by Caputo and
Pepe~\cite{49} for some toxic and noxious cations, such as
Pb\tsup{2+} and Ba\tsup{2+}, according to 
Equation (\ref{eqn14}):
{\begin{eqnarray}\label{eqn14}
2(\mathrm{Na}\hyphen\mathrm{ZSM12})+\mathrm{UO}_{2}^{2+}\rightarrow 
\mathrm{UO}_{2}(\mbox{ZSM12})_{2}
\end{eqnarray}}\unskip



\subsection{Uranium recovery from a real nuclear effluent by synthesized ZSM-12 material}

In order to assess the practical performance of ZSM-12, this material
was used for the recovery of uranium from wastewater generated during
the processing of uranium ore at the CRND.
The optimal parameters obtained from the adsorption study with
synthetic uranium solutions were applied. The wastewater consisted
mainly of U(VI), Na, K, Ca(II), and Fe(III) with concentrations of
151.69, 9.679, 0.752, 2.708, and 4.502~mg${\cdot}$L$^{-1}$, respectively.
Uranium adsorption efficiencies of 38.86\%, 41.135\%, and 37.57\% were
obtained with an average yield of 39.18\%. As a result,
the presence of various ions such as Na, K, Ca(II), and Fe(III)
significantly influenced the adsorption behavior of ZSM-12 toward
U(VI), resulting in a 21.64\% decrease in adsorption efficiency. The
initial concentrations of Fe(III), Ca(II), and Na decreased by
65.32\%, 76.60\%, and 62.72\%, respectively. Due to their similar
charge and ionic nature, these cations can inhibit the assimilation of
uranium ions by participating in competitive adsorption by either
trapping or occupying  adsorption sites. This competition leads to a
reduction in adsorption efficiency for uranium ions~\cite{70}.


\section{Conclusion}

In the present work, microporous--mesoporous ZSM-12 zeolite was
successfully synthesized by a hydrothermal process, using
TPABr as the structuring agent. The
structure of ZSM-12 was characterized by XRD, FTIR, and nitrogen
adsorption--desorption, which revealed a high specific surface area of
343~m\tsup{2}/g. Adsorption studies demonstrated its efficacy in
adsorbing 93\% and 50\% of the initial uranyl ions
($\mathrm{UO}_{2}^{2+}$) from uranium solutions of 10 and 150~mg/L,
respectively, with relatively rapid kinetics well represented by the
second-order kinetic model. The equilibrium data fit better to the
Langmuir isotherm model. The adsorption energies determined by the Langmuir
model confirmed that the adsorption of uranyl ions by the synthesized
ZSM-12 material was spontaneous and both chemisorption and physisorption
processes occurred simultaneously. The adsorption capacities of the ZSM-12
material were found to  be 14.8~mg${\cdot}$g$^{-1}$
and 12~mg${\cdot}$g$^{-1}$ for
initial uranium concentrations of 10 and 150~mg${\cdot}$L$^{-1}$,
respectively. Thermodynamic parameters indicated that the adsorption of
uranium by ZSM-12 zeolite was favored at low temperatures. It can be
concluded that the synthesized ZSM-12 zeolite is an effective adsorbent
for uranium recovery.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgment}
This research received financial backing from the Draria Nuclear
Research Center (CRND, Algiers). The authors express their
gratitude to the General Directorate of the CRND 
for their financial support.

\back{}

\bibliographystyle{crchim}
\bibliography{crchim20230776}
\refinput{crchim20230776-reference.tex}

\end{document}
