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\DOI{10.5802/crchim.385}
\datereceived{2024-05-06}
\daterevised{2024-08-22}
\datererevised{2025-01-20}
\dateaccepted{2025-02-12}
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%\copyeditor{RB}

\dateposted{2025-03-24}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Hydrophobic deep eutectic solvents for the sono-degradation of
organic pollutants in wastewater}

\alttitle{Solvants eutectiques profondes hydrophobes pour la
sono-d\'{e}gradation des polluants organiques dans les eaux us\'{e}es}

\author{\firstname{Deborah Oluwatomilola} \lastname{Adeoye}\CDRorcid{0000-0002-9201-7891}\IsCorresp}
\address{Directorate of Research and Development, Nigerian Institute of
Leather and Science Technology, Zaria, Kaduna State, Nigeria}
\address{Department of Chemical Engineering, Ahmadu Bello University,
Zaria, Kaduna State, Nigeria}
\email[D. O. Adeoye]{yawehst2006@gmail.com}

\author{\firstname{Zaharaddeen Sani} \lastname{Gano}\CDRorcid{0000-0002-7334-5531}}
\address{Department of Petrochemical and Allied Products, National
Research Institute for Chemical Technology, Zaria, Kaduna State,
Nigeria}

\author{\firstname{Omar\nobreakauthor Umar}\nobreakauthor\lastname{Ahmed}}
\address{Department of Chemical and Petroleum Engineering, Bayero
University, Kano, Kano State, Nigeria}

\author{\firstname{Suleiman Mohammed} \lastname{Shuwa}}
\addressSameAs{2}{Department of Chemical Engineering, Ahmadu Bello
University, Zaria, Kaduna State, Nigeria}

\author{\firstname{Abdulazeez Yusuf} \lastname{Atta}\CDRorcid{0000-0001-8515-6300}}
\addressSameAs{2}{Department of Chemical Engineering, Ahmadu Bello
University, Zaria, Kaduna State, Nigeria}

\author{\firstname{Baba Yakubu} \lastname{Jibril}}
\addressSameAs{2}{Department of Chemical Engineering, Ahmadu Bello
University, Zaria, Kaduna State, Nigeria}

\keywords{\kwd{4-Nitrophenol}\kwd{Decanoic
acid}\kwd{Degradation}\kwd{Dodecanoic acid}\kwd{Hydrophobic deep
eutectic solvent}\kwd{Menthol}\kwd{Phenol}}

\altkeywords{\kwd{4-nitroph\'{e}nol}\kwd{Acide
d\'{e}cano\"{\i}que}\kwd{D\'{e}gradation}\kwd{Acide
dod\'{e}cano\"{\i}que}\kwd{Solvant eutectique profond
hydrophobe}\kwd{Menthol}\kwd{Ph\'{e}nol}}

\thanks{National Research Institute for Chemical Technology
(FGN/AB2021/ERGP30122610).}

\begin{abstract}
Phenol and 4-nitrophenol (4-NP) are hazardous, recalcitrant, and
biorefractory contaminants found in wastewater generated from process
industries.
Even in low concentrations, they are highly poisonous. The
degradation of these pollutants in wastewater is imperative due to
their solubility in water and their toxicity to the biosphere. In this
study, two long-chain, organic acid based hydrophobic deep eutectic
solvents (HDESs) were prepared, characterized, and used in synergy with
hydrogen peroxide for the ultrasonic degradation of these pollutants in
wastewater. These HDESs are menthol--decanoic acid (MC\textsubscript{10}) and
menthol--dodecanoic acid (MC\textsubscript{12}). The HDESs were used in synergy
with hydrogen peroxide for the degradation of phenol and 4-NP. The
effects of different process variables such as pH, oxidant dose,
temperature, and time on phenol and 4-NP degradation efficiency in
simulated wastewater were studied. For phenol degradation, pH 4
provided the best result while for 4-NP, pH 5 provided the best result.
For the MC\textsubscript{10}-HDES/H\textsubscript{2}O\textsubscript{2} system, an oxidant dose of 15\%
\tralicstex{\textit{v/v}}{$v/v$} and temperature 60 \textdegree C yielded the best degradation
efficiency of 76\% for phenol and 94.46\% for 4-NP while for the
MC\textsubscript{12}-HDES/H\textsubscript{2}O\textsubscript{2} system, an oxidant dose of 17\% \tralicstex{\textit{v/v}}{$v/v$} and
temperature 60 \textdegree C yielded the best degradation efficiency of
58.98\% for phenol and 61.08\% for 4-NP.
The process parameters with the best results were then used
to treat refinery wastewater. The percentage reductions in total
organic compounds (TOC) were 88.75 and 67.08\% for MC\textsubscript{10}- and
MC\textsubscript{12}-HDES/H\textsubscript{2}O\textsubscript{2} systems respectively. The recyclability
of the HDESs was also investigated for three runs. The percentage of
TOC reduced from 88.75 to 33.33\% for MC\textsubscript{10} and from 67.08 to
11.25\% for MC\textsubscript{12}. These results  prove this process to be
effective for the treatment of wastewater contaminated with phenol and
4-NP.
\end{abstract}

\begin{altabstract}
Le ph\'{e}nol et le 4-nitroph\'{e}nol (4-NP) sont des contaminants
dangereux, r\'{e}calcitrants et bior\'{e}fractaires que l'on trouve
dans les eaux us\'{e}es g\'{e}n\'{e}r\'{e}es par les industries de
transformation. M\^{e}me \`{a} faible concentration, ils sont
extr\^{e}mement toxiques. La d\'{e}gradation de ces polluants dans les
eaux us\'{e}es est imp\'{e}rative en raison de leur solubilit\'{e} dans
l'eau et de leur toxicit\'{e} pour la biosph\`{e}re. Dans cette
\'{e}tude, deux solvants eutectiques profonds hydrophobes (HDES) \`{a}
base d'acide organique \`{a} longue cha\^{i}ne ont \'{e}t\'{e}
pr\'{e}par\'{e}s, caract\'{e}ris\'{e}s et utilis\'{e}s en synergie avec
le peroxyde d'hydrog\`{e}ne pour la d\'{e}gradation par ultrasons de
ces polluants dans les eaux us\'{e}es. Ces HDES sont l'acide
menthol-d\'{e}cano\"{i}que (MC\textsubscript{10}{) et l'acide
menthol-dod\'{e}cano\"{i}que (MC}\textsubscript{12}{). Les HDES ont \'{e}t\'{e}
utilis\'{e}s en synergie avec le peroxyde d'hydrog\`{e}ne pour
g\'{e}n\'{e}rer des HDES/H}\textsubscript{2}O\textsubscript{2} {systemes destin\'{e}s \`{a}
la d\'{e}gradation du ph\'{e}nol et du 4-NP. Les effets de
diff\'{e}rentes variables de proc\'{e}d\'{e} telles que le pH, la dose
d'oxydant, la temp\'{e}rature et le temps sur l'efficacit\'{e} de
d\'{e}gradation du ph\'{e}nol et du 4-NP dans des eaux us\'{e}es
simul\'{e}es ont \'{e}t\'{e} \'{e}tudi\'{e}s. Pour la d\'{e}gradation
du ph\'{e}nol, le pH 4 a donn\'{e} le meilleur r\'{e}sultat tandis que
pour le 4-NP, le pH 5 a donn\'{e} le meilleur r\'{e}sultat. Pour le
HDES/H}\textsubscript{2}{O}\textsubscript{2} {Systeme MC}\textsubscript{10}{, une dose d'oxydant de 15 \%
\tralicstex{\textit{v/v}}{$v/v$} et une temp\'{e}rature de 60 \textdegree C ont donn\'{e} la meilleure
efficacit\'{e} de d\'{e}gradation, tandis que pour le
HDES/H}\textsubscript{2}{O}\textsubscript{2} {systeme MC}\textsubscript{12}{, une dose d'oxydant de 17 \%
\tralicstex{\textit{v/v}}{$v/v$} et une temp\'{e}rature de 60 \textdegree C ont donn\'{e} la meilleure
efficacit\'{e} de d\'{e}gradation. Ces r\'{e}sultats ont ensuite
\'{e}t\'{e} utilis\'{e}s pour traiter les eaux us\'{e}es des
raffineries. Le pourcentage de r\'{e}duction des compos\'{e}s
organiques totaux (COT) \'{e}tait respectivement de 88,75 et 67,08 \%
pour le MC}\textsubscript{10} {et le MC}\textsubscript{12}{. La recyclabilit\'{e} des HDES a
\'{e}galement \'{e}t\'{e} \'{e}tudi\'{e}e lors de trois essais. Le
pourcentage de COT est pass\'{e} de 88,75 \`{a} 33,33 \% pour le
MC}\textsubscript{10} {et de 67,08 \`{a} 11,25 \% pour le MC}\textsubscript{12}. Ces
r\'{e}sultats obtenus prouvent que ce proc\'{e}d\'{e} est efficace pour
le traitement des eaux us\'{e}es contamin\'{e}es au ph\'{e}nol et au
4-NP.
\end{altabstract}

\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{mark-1}

Phenol is a precursor for numerous products such as herbicides, drugs,
paints, cosmetics, and lubricants with its primary use as an
intermediate in the production of phenolic resins (e.g.,
phenol--formaldehyde resins). It is also converted to a precursor of
plastics via condensation with acetone, which results in bisphenol A
for the manufacture of polycarbonates and epoxide resins, which forms
its other major use. This in addition to phenolic resins constitutes
about two-thirds of its total use. When phenol and phenol-containing
compounds (alkylphenols or diphenols) are condensed with formaldehyde,
the end product is Bakelite, a synthetic thermosetting phenolic resin.
Other important compounds obtained from phenol include cyclohexanone (a
material for the production of nylon), alkylphenols 
(nonylphenol or its ethoxylated product, which is used for
the production of non-ionic detergents). Secondary uses of phenol and its
compounds include  a multifaceted raw material for  manufacturing 
drugs (aspirin and pharyngitis medicines), carbolic soap (a component
of industrial paint strippers in the aviation industry), and cosmetics
(such as sunscreens, hair dyes, and skin lightening
products)~\cite{1,2,3}.\looseness=-1 

Phenol is considered one of the most hazardous organic contaminants in
wastewater since it is highly poisonous even  at low
concentrations. Phenols, total phenols, and phenolics in remediation
studies represent either phenol or its compounds in wastewater.
Phenolics are swiftly ingested via skin and eye contact, and
inhalation, and they cause burning sensations on the skin and in the
eyes. Aside from their corrosive effects, they cause skin
sensitivity. As stated by the Occupational Safety and Health
Administration (USA), the limit of phenol contact with the skin should
be less than 5 mg/l, and it is considered that the ingestion of 1~g of
phenols is fatal to humans~\mbox{\cite{4,5,6}.} Phenol and its derivatives are
well-known recalcitrant organic contaminants that are generated from
numerous industrial processes. They normally exist in the effluents of
numerous relevant industries~\mbox{\cite{7,6,8,9,10}} such as refineries
(6--500~mg/l), coking operations (28--3900~mg/l), coal processing
(9--6800~mg/l), petrochemical plants (2.8--1220~mg/l), and other
sources such as pharmaceuticals, plastics, wood products, paint, pulp,
and paper industries containing about 0.1--1600~mg/l
phenols~\cite{11,4,12,5,6}. For safety and the prevention of environmental
issues, it is imperative to eliminate them from the wastes of different
production units of these industries before they are discharged into
the biosphere.


Various techniques have been adopted for the elimination of organic
pollutants in industrial wastewater. Some of the conventional
techniques for the elimination of phenolics in industrial wastewater
are membrane filtration, coagulation/flocculation, ion exchange,
electrolysis, adsorption on activated carbon, chemical reduction, and
advanced oxidation processes (AOPs)~\cite{13,14,15}. Among others, AOPs
have gained more popularity due to their ability to extensively
eliminate these pollutants~\cite{8,10,16}. The AOPs refer to the procedures
that largely use non-selective hydroxyl radicals for eliminating
organic pollutants in wastewater. They can work in synergy with other
substances and techniques to provide a robust remediation of
wastewater. When in synergy with other substances and techniques, AOPs
are referred to as integrated advanced oxidation processes
(I-AOPs)~\cite{17}. The I-AOPs have been established to provide efficient
synergistic and holistic treatment of wastewater (in terms of better
degradation efficiency, low energy consumption, less use of reagent
and chemicals, lower cost of operation, etc.), which their
non-integrated counterparts have not been able to achieve
\cite{13,14,16,21}. The Fenton reaction is one of
the widely used AOPs for the degradation of pollutants in wastewater.
It refers to~{{the}} catalyzed
{{decomposition of hydrogen peroxide by ferrous ions to
produce hydroxyl radicals}}. The primary Fenton reaction involves the
reaction between ferrous iron (Fe$^{2+}$) and hydrogen peroxide
(H$_{2}$O$_{2}$) to produce hydroxyl radicals ({\textbullet}OH) and
ferric iron (Fe$^{3+}$)~\cite{13,14}.

Numerous factors are capable of interfering with and limiting the
efficiency of I-AOPs~\cite{18,19,20,21,22}. One such factor is the
non-selective nature of the hydroxyl radical generated from hydrogen
peroxide~\cite{23} and the presence of organic and inorganic
ions~\cite{19}, which cumulatively reduce the efficiency of AOPs.
Alternative oxidizing agents have been researched that can generate
selective/target-oriented radicals that can improve the efficiency of
wastewater treatment; among these are peroxyacids. These are peroxides
produced by the reaction of carboxylic acids with hydrogen peroxide. They are
strong oxidants and are more reactive than hydrogen
peroxide~\cite{24,25}. The oxidation capability of peroxides is
connected to the electronegativity of the substituent of peroxyacids. 
Since the electronegativity of the substituent of peroxyacid is higher than that
of hydrogen, it tends to be a more aggressive oxidizing agent. Aside
from the fact that peroxyacids are strong oxidizing agents, they are
an excellent source for the generation of OH radicals because the
peroxy--oxygen atom bond can be easily dissociated. This in turn makes it easy
to undergo disintegration by homolytic fission~\cite{26}. 

The most used peroxyacid in wastewater treatment technology is
peracetic acid (PAA)~\cite{17}. Among organic peroxyacids, 
PAA solutions are the least stable. At room temperature, 40\% of PAA solutions
lose 1 to 2\% of PAA monthly~\cite{27}. For organic peroxyacids, the
shorter the alkyl chain, the less stable they are. This is because of
the higher percentage share of active oxygen in their molar
mass~\cite{28}. The longer the alkyl chains of peroxyacids, the more
stable they are. Given the potential of replacing the conventional
oxidant in AOP techniques for organic pollutant degradation with
peroxyacids, this study sought to investigate the use of carboxylic
acid based hydrophobic deep eutectic solvents (HDESs)/hydrogen
peroxide based system to treat phenol-contaminated wastewater. The HDESs
are water immiscible deep eutectic solvents (DESs) 
that are capable of distinct phase separation
when mixed with water. These HDES/H$_{2}$O$_{2}$ systems were produced
in situ from the interaction of HDESs with hydrogen peroxide in the
presence of ultrasound, and they were then used for the enhanced
degradation of phenolic micro-pollutants in wastewater. The HDESs were
synthesized combining menthol with decanoic and menthol with dodecanoic
acids. Their precursors are natural and widely used in human life.
Menthol is extracted from pure natural peppermint or peppermint oil.
Decanoic acid is often used to make esters for perfumes and fruit
flavors and as an intermediate for food-grade additives. Dodecanoic
(lauric) acid is naturally present in various animal and vegetable oils
and is the main component of coconut oil and palm kernel oil, and it
can also be used as a food additive. 

\section{Materials and methods}\label{mark-2}

\subsection{HDES synthesis}

Menthol (99\% assay) was purchased from Molychem while decanoic acid
(98\% assay) and dodecanoic acid (99\% assay) were purchased from Loba
Chemie. \mbox{Binary} mixtures of monoterpene (menthol) and carboxylic acids
(decanoic acid and dodecanoic acid) were prepared by adding the
components into glass vessels in the mole ratio 1:1. The synthesis was
conducted by stirring on a hotplate magnetic stirrer at 80~\textdegree C
until a homogeneous liquid mixture was obtained with no visible
precipitate. The solvent was then cooled to room temperature and stored
in airtight sample bottles for further studies~\cite{29}.
Table~\ref{tab1} shows the mole ratio of menthol and organic acids
combined for the synthesis of the HDESs and their physical appearance.

\begin{table*}
\caption{\label{tab1} Composition of HDESs}
\begin{tabular}{lcccc}
\thead
{HBA} &amp; {HBD} &amp; {Abbreviation} &amp; {Mole ratio} &amp; {Physical appearance} \\
\endthead
\xmorerows{1}{Menthol} &amp; Decanoic acid   &amp; MC$_{10}$ &amp; 1:1  &amp; Colorless liquid  \\
        &amp; Dodecanoic acid &amp; MC$_{12}$ &amp; 1:1  &amp; Colorless liquid
\botline
\end{tabular}
\end{table*}


\subsection{Degradation process}

Simulated wastewater was prepared by dissolving 30~mg of
analytical-grade phenol and 4-NP crystals in 1~l of distilled
water. In addition, refinery wastewater was obtained from a refinery in the
south-south region of Nigeria. Ferrous sulfate anhydrous weighing 3~g (98\%
assay) was dissolved in the contaminated (refinery and simulated)
wastewater to catalyze the homolysis of the HDES--HDES/H$_{2}$O$_{2}$
system. The pH of the wastewater was adjusted using 0.1~M NaOH or 0.1~M
H$_{2}$SO$_{4}$ to 3--6. Sample bottles were filled with 20~ml of contaminated wastewater, 
and varying
volumes of the stoichiometric mixture of HDES ${+}$ H$_{2}$O$_{2}$ were
added. These samples were placed inside an ultrasonicator at a sonication speed 
of 40 kHz.
During the sonication process, the
HDES/H$_{2}$O$_{2}$ systems were generated in situ. The time and
temperature of the degradation process were varied. After appropriate
sonication-enhanced reaction, the samples were removed and left for
phase separation between the HDES phase and the treated water. The water
phase was taken for analysis to determine the concentrations of phenol, 4-NP, and total
organic compounds (TOC).

\subsection{Analysis}

\subsubsection{Fourier transform infrared (FTIR) for HDES}\label{mark-i.}

The spectrum of each sample is determined by a Cary 630 FTIR
Spectrometer (Agilent Technologies). A small quantity of each sample
is scooped with a spatula and placed on the spectrometer sampling
surface. With the aid of an in-built interferometer, the spectrometer
measures the energy transmitted to the sample. Emitted infrared
radiation from the black body (furnace) hits the interferometer, where
spectral encoding occurs. The resultant signal is then transmitted
through the sample surface where specific energy wavelengths are
absorbed. The beam finally proceeds through the detector and is passed
to the processing computer for the Fourier transform of energy signals.

\subsubsection{Moisture content (hydrophobicity)}\label{mark-ii.}

Six sample bottles were filled with 10~ml of the prepared HDES, and
10~ml of distilled water was added to each of them. There was proper
agitation of the mixture at the speed of {120~rpm for 30~min}, and
the samples were left for 24 h for phase separation. The HDES
phases were then removed from the sample mixture using a syringe and
set aside for moisture content analysis using a Karl Fischer Moisture
Analyzer, which was operated at room temperature. The moisture content
of each sample was determined by a Metrohm 756 Karl Fischer Coulometer
Analyzer.

\subsubsection{Phenol and 4-NP concentration} \label{mark-iii.}

An ultraviolet--visible spectrophotometer was used to determine the
initial phenol and 4-NP concentrations (before treatment) and the final
concentrations (after treatment) of both the refinery and simulated
effluents at 270~nm and 320~nm respectively after scanning and plotting
their calibration curves. {The calibration curves for phenol and 4-NP
are established in the concentration range of 0--60~ppm}, with an
$R^{2}$ value of 0.9909 for the phenol curve and 0.9964 for the
4-NP curve. The efficiency of degradation was calculated using
Equation~(\ref{eq1}):
{\begin{equation}
\mbox{Degradation efficiency }(\%)
=\frac{C_{\mathrm{initial}}-C_{\mathrm{final}}}{C_{\mathrm{initial}}}\times 100  
\label{eq1}
\end{equation}}\unskip
where $C_{\mathrm{initial}}$ is the  initial pollutant concentration and
$C_{\mathrm{final}}$ is the final pollutant concentration.

\subsubsection{Analysis of TOC}\label{mark-iv.}

In the determination of the TOC for both untreated and treated
wastewater, the titration method of analysis was used. A sample of 4~ml
was transferred into a digestion tube, and 1~ml of 0.0667~M potassium
dichromate (99\% assay) was added; 5~ml of concentrated sulfuric acid
was added while mixing constantly. Two blank tubes were prepared (i.e.,
with reagents but without sample). The sample tubes and one blank tube were
placed in a pre-heated block at 150~\textdegree C for 30~min and
later removed for cooling down. The other blank tube was left unheated. The contents
of the tube were quantitatively transferred to labeled 100~ml conical
flasks, and 0.3~ml (3--4 drops) indicator solution (1.485~g
o-phenanthroline monohydrate + 0.695~g ferrous ammonium sulfate
hexahydrate in 100~ml of distilled water) was added. While using a
magnetic stirrer to ensure good mixing, all samples and blanks were
titrated with acidified ferrous ammonium sulfate solution (99\%
assay). The endpoint was a color change from green/violet to red. The
titer values were recorded for each sample, the heated blank, and the
unheated blank. The TOC in the raw and the treated
refinery effluent was monitored to determine the efficiency of the
degradation process. 
{\begin{equation}\label{eq2}
\%\ \mbox{TOC reduction}=\frac{\mathrm{TOC}_{\mathrm{raw}}-
\mathrm{TOC}_{\mathrm{treated}}}{\mathrm{TOC}_{\mathrm{raw}}}\times 100
\end{equation}}\unskip
where TOC$_{\mathrm{raw}}$ is the TOC of raw refinery effluent and
TOC$_{\mathrm{treated}}$ is the  TOC of the HDES/H$_{2}$O$_{2}$ system
treated  refinery effluent.

\section{Results}\label{mark-3}

\subsection{HDES characterization}

The FTIR spectra of monoterpene, decanoic
acid, and dodecanoic acid as well as the HDESs synthesized from their
combinations were obtained to investigate and confirm the interaction
of the combining components. This can be found in an earlier publication
by Adeoye et~al.~\cite{29}. 

The resulting spectra when compared with related works reflected the
same kind of shifts at the OH band of the menthol and the carbonyl
bands of decanoic acid and dodecanoic acid. This represents the
interaction of the two components for DES
formation~\cite{30,31,32,33}{.} The hydrogen bond has been
identified to be the major intermolecular interaction between terpenes
and organic acids in the formation of HDES~\cite{30,6,34}.

It is strongly advised to determine the stability of DESs that are to
be applied in aqueous environments to ensure that they will not leach into
the water phase and contaminate it and that no change occurs in the
DES structure due to water absorption. 

Therefore, the HDESs were mixed with water and agitated. 
Figure~\ref{fig1}(a,b) represents images of the HDESs  immediately
after agitation and after 24~h. An oil-in-water emulsion-like mixture
was observed immediately after agitation. A distinct phase separation
was observed between the HDESs and water. The water phase was at the
bottom while the HDES phase was at the top.


\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}HDES in water (a) immediately after agitation and
(b) 24~h after agitation.}
\end{figure}

The moisture content in each HDES was thereafter determined after its
interaction with water for 24~h. The HDESs: MC$_{10}$,
and MC$_{12}$, had moisture content of 3.54\% and 2.64\%
respectively (Figure~\ref{fig2}). The decrease in the moisture absorption
capacity of MC$_{10}$, and MC$_{12}$ was a result
of the increase in the alkyl chain of the organic acids from
C$_{10}$ to C$_{12}$ which resulted in increased
hydrophobicity~\cite{35}.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Test for hydrophobicity of HDESs.}
\end{figure}


After the synthesis and characterization of the HDESs, simulated
wastewater contaminated with phenol and 4-NP was prepared and used to
\mbox{investigate} the effects of pH, oxidant dose, temperature, and time on
the ultrasonic degradation process while using the HDES/H$_{2}$O$_{2}$
systems. 

\subsection{Ultrasonic degradation of phenol and 4-NP}

\subsubsection{Effect of pH}

The effect of pH on the ultrasonic degradation of phenol and 4-NP was
investigated using MC$_{10}$- and MC$_{12}$-HDES/H$_{2}$O$_{2}$ systems
as shown in Figures~\ref{fig3} and~\ref{fig4}. The process was varied 
between pH values of 3 and 6
while keeping other conditions constant. It was observed that the best
degradation efficiency for phenol was achieved at pH 4 while that for
4-NP was at pH 5. As the pH became less acidic, the efficiency of
degradation kept decreasing. This is because as the pH becomes high,
iron precipitates as Fe((OH)$_{3}$), which in turn
reduces the concentration of ferrous ions that can catalyze the
homolysis of HDES/H$_{2}$O$_{2}$ systems to generate radicals. For
Fenton processes, effective pH for the degradation of organic
pollutants was found to be between 3 and 6~\cite{36,37,38}.


\begin{figure}
\includegraphics{fig03}
{\vspace*{-2pt}}
\caption{\label{fig3}Effect of pH on phenol degradation using MC$_{10}$
and MC$_{12}$ HDES.}
\end{figure}


\begin{figure}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}Effect of pH on 4-NP degradation using MC$_{10}$
and MC$_{12}$ HDES.}
\end{figure}


\subsubsection{Effect of oxidant dose}

The effect of oxidant dose on the degradation of phenol and 4NP was
equally investigated using MC$_{10}$- and MC$_{12}$-HDES/H$_{2}$O$_{2}$
systems. The oxidant dose was varied between 5 and 20\% $v/v$ while
keeping the aqueous solution at pH 4 for phenol degradation and pH 5
for 4-NP degradation. The reaction time was  75~min and the temperature
was  60~\textdegree C. In Figure~\ref{fig5}, the degradation efficiency
increased with increasing oxidant dose until it peaked at 15\% $v/v$ for the
MC$_{10}$-HDES/H$_{2}$O$_{2}$ system and at 17\% $v/v$ for the
MC$_{12}$-HDES/H$_{2}$O$_{2}$ system for both phenol and 4-NP and
started declining. According to Kim et~al.~\cite{24}, degradation
increases with increase in oxidant dose. This is because an increased
oxidant dose gives rise to an increase in the concentration of radicals
generated for the degradation process, which results in better
degradation efficiency. At a certain point, an increase in oxidant dose
results in a decrease in degradation efficiency. This is attributed to
the scavenging effect of the excess oxidant on the generated
radicals~\cite{24}. 


\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5} (a) Effect of oxidant on phenol degradation. 
(b) Effect of oxidant on 4-NP degradation.}
\end{figure}


\subsubsection{Effect of temperature and time}

{An increase in temperature favors the decomposition of H}$_{2}${O}$_{2}$,
{leading to a very rapid generation of the hydroxyl radical; it also}
favors {the decomposition of H}$_{2}${O}$_{2}$ into O$_{2}$, a less
reactive species. The effect of temperature was monitored between 
30 and 60~\textdegree C while operating at the best pH and oxidant dose. {This
temperature range was chosen because the degradation of phenolic
pollutants in the literature using integrated methods was operated
within this range. This is to provide ample comparison of the efficacy of
the process.} An increase in \mbox{temperature} was observed to favor the
increase in pollutant degradation for both HDES/H$_{2}$O$_{2}$ systems
as shown in Figures~\ref{fig6} and~\ref{fig7} for phenol and 4-NP
degradation. Ribeiro and Nunes in their review of Fenton
processes for wastewater treatment stated that an increase in
temperature enhances the kinetics of the process because the Fenton
reaction is endothermic in nature~\cite{39}. Moreover, the effect of time
was monitored between 30 and 120~min as seen in Figures~\ref{fig6}
and~\ref{fig7} for both phenol and 4-NP degradation respectively. The
efficiency of degradation was observed to increase throughout the
duration.


\begin{figure}
{\vspace*{3pt}}
\includegraphics{fig06}
{\vspace*{3pt}}
\caption{\label{fig6}(a) Temperature--time profile for phenol degradation using 
MC$_{10}$. (b) Temperature--time profile for phenol degradation using 
MC$_{12}$.}
{\vspace*{2pt}}
\end{figure}


\begin{figure}
{\vspace*{3pt}}
\includegraphics{fig07}
{\vspace*{4pt}}
\caption{\label{fig7}(a) Temperature--time profile for 4-NP degradation 
using MC$_{10}$. (b) Temperature--time profile for 4-NP degradation using
MC$_{12}$.}
{\vspace*{2pt}}
\end{figure}


\begin{table*}
\caption{\label{tab2} Kinetic parameters for first- and second-order
models for the degradation reaction}
\begin{tabular}{cccc}
\thead
First-order kinetics &amp; Temperature (\textdegree C) &amp; $k_{1}$ (min$^{-1}$)  &amp; $R^{2}$ \\
\endthead
\xmorerows{2}{MC$_{10}$-phenol}  &amp; 50 &amp; 0.00571 &amp; 0.9324 \\
 &amp; 55 &amp; 0.00912 &amp; 0.9493 \\
 &amp; 60 &amp; 0.01443 &amp; 0.9788 \\
\xmorerows{2}{MC$_{12}$-phenol}  &amp; 50 &amp; 0.00362 &amp; 0.9679 \\
 &amp; 55 &amp; 0.00507 &amp; 0.9771 \\
 &amp; 60 &amp; 0.006\0\0 &amp; 0.9791 \\
\xmorerows{2}{MC$_{10}$-4-NP}  &amp; 50 &amp; 0.00585 &amp; 0.6197 \\
 &amp; 55 &amp; 0.0079\0 &amp; 0.6829 \\
 &amp; 60 &amp; 0.00964 &amp; 0.6993 \\
\xmorerows{2}{MC$_{12}$-4-NP}  &amp; 50 &amp; 0.00422 &amp; 0.6304 \\
 &amp; 55 &amp; 0.00533 &amp; 0.6585 \\
 &amp; 60 &amp; 0.00715 &amp; 0.6961 
\xbotline
\end{tabular}
\begin{tabular}{cccc}
\thead
Second-order kinetics &amp; Temperature (\textdegree C) &amp; $k_{2}$ (L/mg$^{-1}{\cdot}$min$^{-1}$)  &amp; $R^{2}$ \\
\endthead
\xmorerows{2}{MC$_{10}$-phenol}  &amp; 50 &amp; $3.37\times 10^{-4}$ &amp; 0.9484 \\
 &amp; 55 &amp; $1.88\times 10^{-4}$ &amp; 0.9192 \\
 &amp; 60 &amp; $1.32\times 10^{-4}$ &amp; 0.9219 \\
\xmorerows{2}{MC$_{12}$-phenol}  &amp; 50 &amp; $2.29\times 10^{-5}$ &amp; 0.9488 \\
 &amp; 55 &amp; $4.56\times 10^{-5}$ &amp; 0.9637 \\
 &amp; 60 &amp; $5.97\times 10^{-5}$ &amp; 0.9757 \\
\xmorerows{2}{MC$_{10}$-4-NP}  &amp; 50 &amp; $3.05\times 10^{-4}$ &amp; 0.7020 \\
 &amp; 55 &amp; $4.81\times 10^{-4}$ &amp; 0.8017 \\
 &amp; 60 &amp; $6.96\times 10^{-4}$ &amp; 0.8448 \\
\xmorerows{2}{MC$_{12}$-4-NP}  &amp; 50 &amp; $1.86\times 10^{-4}$ &amp; 0.6860 \\
 &amp; 55 &amp; $2.56\times 10^{-4}$ &amp; 0.7347 \\
 &amp; 60 &amp; $4.06\times 10^{-4}$ &amp; 0.8047
\botline
\end{tabular}
{\vspace*{-3pt}}
\end{table*}


\subsubsection{Kinetics of degradation reaction}

The kinetics of the degradation of phenol and 4-NP using
HDES/H$_{2}$O$_{2}$ systems was modeled using first- and second-order
kinetic equations while varying temperatures between 
50 and 60~\textdegree C. All the reactions were conducted with the best
\mbox{experimentally} obtained parameters. The rate constant and the
regression coefficient were calculated (Table~\ref{tab2}) by following
the model equations (Equations~(\ref{eqn2}) and~(\ref{eq3}))~\cite{40}. Kinetic
parameters were calculated individually for 50, 55, and 60~\textdegree C.
{\begin{eqnarray}
&amp;\displaystyle 
1\mbox{st order: } \ln C_{\mathrm{p}}  = \ln C_{\mathrm{po}}  - k_{1}t \label{eqn2}
\Seqnsplit
&amp;\displaystyle 
2\mbox{nd order: }  \left(\frac{1}{C_{\mathrm{p}}}\right)-\left(\frac{1}{C_{\mathrm{po}}}\right)  
= k_{2}t \label{eq3}
\end{eqnarray}}\unskip
where $C_{\mathrm{p}}$ is the final pollutant concentration,
$C_{\mathrm{po}}$ is the initial  pollutant concentration, $t$ is the
reaction time, and $k_{1}$ and $k_{2}$ are  rate constants for first-
and second-order kinetics.

Two kinetic models were used to fit the experimental data for the
degradation reactions at \mbox{temperatures} 50, 55, and 60~\textdegree C. It
was observed that with every increase in temperature, the reaction rate
constant also increased for the degradation of both \mbox{phenol} and 4-NP.
This shows that an increase in temperature favors the degradation
reaction, which is endothermic. For phenol degradation, the first-order
model gave the best fit with the highest regression coefficient
($R^{2}$) values of 0.9788 and 0.9791 for MC$_{10}$- and
MC$_{12}$-HDES/H$_{2}$O$_{2}$ systems respectively, which is similar to
that obtained in the literature for organic pollutant
degradation~\cite{41,42,43}. While still \mbox{considering} the first-order
kinetics, the rate constants for phenol degradation using the 
MC$_{10}$-HDES/H$_{2}$O$_{2}$ system at higher temperatures (55 and
60~\textdegree C) were found to be about 1.5 times in magnitude
when compared with reactions at lower temperatures (55 and
50~\textdegree C). Similarly, the rate constants at higher temperatures
(55 and 60~\textdegree C) for phenol degradation using the
MC$_{12}$-HDES/H$_{2}$O$_{2}$ system were found to be a magnitude
greater than what was obtained at lower temperatures (55 and
50~\textdegree C). For 4-NP degradation, the second-order model gave the
best fit with the highest regression coefficient ($R^{2}$) values of
0.8448 and 0.8047 for MC$_{10}$- and MC$_{12}$-HDES/H$_{2}$O$_{2}$
systems respectively. Sudha et~al.~in their work on Fenton
degradation of 4-NP concluded that the  second-order kinetic model gave the
best fit for their experimental results~\cite{44}. For the second-order
kinetics, the rate constants for 4-NP degradation using the
MC$_{10}$-HDES/H$_{2}$O$_{2}$ system at higher temperatures (55 and
60~\textdegree C) were found to be about 1.5 times higher in magnitude
than at lower temperatures (55 and 50~\textdegree C). In a similar trend,
the rate constants at higher temperatures (55 and 60~\textdegree C) for
4-NP degradation using the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system were found
to be about 1.5 times higher in magnitude than at lower temperatures
(55 and 50~\textdegree C). Moreover, the reaction rate constants for
pollutant degradation using the MC$_{10}$-HDES/H$_{2}$O$_{2}$ system were
greater than when the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system was used. This
could be attributed to the higher hydrophobicity of MC$_{12}$. The
order of a reaction reflects the sensitivity of the rate of that
reaction to the change in concentration of the reactants per time. For
zero-order reactions, the rate of reaction is constant; this means it
is independent of the concentration of the reacting components. In
first-order reactions, the rate is dependent on the concentration of
only one reactant while for second-order reactions, the rate is
dependent on the square of the concentration of only one reactant or
the product of the concentrations of two reactants.\looseness=-1

\subsection{Refinery effluent degradation}

The best process parameters obtained for the degradation of phenol and
4-NP in simulated wastewater were then used to treat refinery
wastewater that was obtained from the south-south region of Nigeria. The
TOC for raw refinery wastewater was analytically determined to be 
0.024~g/l while that of the treated wastewater when the
MC$_{10}$-HDES/H$_{2}$O$_{2}$ system was used was 0.0027~g/l; when
the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system was used, the TOC value was 
0.0079~g/l. The efficiency of pollutant degradation was monitored using a
percentage reduction in the TOC present in the raw
and treated refinery effluent as calculated by Equation~(\ref{eq1}). The TOC
reduction when the MC$_{10}$-HDES/H$_{2}$O$_{2}$ system was used was
88.75\% while when the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system was used, the
TOC reduction was 67.08\%. The MC$_{10}$-HDES/H$_{2}$O$_{2}$ system was
observed to be more efficient in pollutant degradation than
the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system. As  stated earlier,
this could be a result of the higher hydrophobicity of MC$_{12}$, which limited its
interaction with pollutants in the water phase. The results for each
HDES/H$_{2}$O$_{2}$ system are presented in Table~\ref{tab3}.


{\vspace*{-3pt}}

\subsection{Reusability}

{\vspace*{-3pt}}

The recovery and reusability of materials in engineering processes are
of the utmost importance because they impact the economics of such
processes. The recovered HDESs were used for the generation of
HDES/H$_{2}$O$_{2}$ systems and were recontacted with raw refinery
wastewater for three runs. The percentage of TOC load was observed to
reduce from 88.75 to 33.33\% for the MC$_{10}$-HDES/H$_{2}$O$_{2}$ system
while for the MC$_{12}$-HDES/H$_{2}$O$_{2}$ system, it reduced from 67.08
to 11.25\% for the three runs. This decrease could be a result of the
contamination of the HDES phase with pollutants during the repeated
degradation process. Yet again, the MC$_{10}$-HDES/H$_{2}$O$_{2}$ system
was observed to be more efficient in pollutant degradation than the 
MC$_{12}$-HDES/H$_{2}$O$_{2}$ system all through the recycle runs, and
this could be attributed to the higher hydrophobicity of MC$_{12}$.
Figure~\ref{fig8} shows the graph for the reusability of the
HDES/H$_{2}$O$_{2}$ systems, which reflected a consistent decline in
TOC reduction when it was reused.

\begin{figure}
\includegraphics{fig08}
{\vspace*{-3pt}}
\caption{\label{fig8}Reusability of HDES for the degradation of
pollutants in refinery effluent.}
{\vspace*{-3pt}}
\end{figure}

\begin{table}
\caption{\label{tab3} TOC analysis for raw and treated refinery effluents}
\begin{tabular}{lcc}
\thead
Wastewater &amp; TOC (g/l) &amp; \% TOC reduction \\
\endthead
Raw refinery      &amp; 0.0240   &amp; - \\ 
MC$_{10}$ treated &amp; 0.0027   &amp; 88.75 \\
MC$_{12}$ treated &amp; 0.0079   &amp; 67.08 
\botline
\end{tabular}
\end{table}

{\vspace*{-3pt}}

\section{Conclusion}\label{mark-4.}

{\vspace*{-3pt}}

In this study,
two menthol-based HDESs
from long-chain organic acids were successfully prepared at a molar
ratio of 1:1, and both were liquids at room \mbox{temperature.} The FTIR spectra
of the HDESs revealed remarkable shifts in the O--H stretching bands and
C${=}$O stretching bands of the HDESs when compared with their precursors.
The changes in the O--H stretching bands and C${=}$O stretching bands
resulted from the intermolecular hydrogen bond formed between the
starting materials to produce the HDESs. 

Both HDESs were then used in synergy with H$_{2}$O$_{2}$ to generate
HDES/H$_{2}$O$_{2}$ systems, which were used to degrade phenol and 4-NP
from simulated wastewater. Process variables were monitored, and the
best degradation efficiency for the simulated effluent was obtained. A pH value of 4
gave the best result for phenol degradation while pH 5 gave the best
result for 4-NP. An oxidant dose of 15\% $v/v$ and temperature 60~\textdegree C
yielded 76\% and 94.46\% for phenol and 4-NP respectively when the
the MC$_{10}$-HDES/H$_{2}$O$_{2}$ system was used. Furthermore, an oxidant dose of
17\% $v/v$ and temperature 60~\textdegree C gave the best degradation
efficiency of 58.98\% and 61.08\% for phenol and 4-NP when the
MC$_{12}$-HDES/H$_{2}$O$_{2}$ system was used. The kinetics of the
degradation reaction were studied, and the experimental data obtained were
fitted into first- and second-order kinetic models. The first-order kinetic
model gave the best fit for phenol degradation when using both
MC$_{10}$-HDES/H$_{2}$O$_{2}$ and 
MC$_{12}$-HDES/H$_{2}$O$_{2}$ systems with regression
coefficients of 0.9788 and 0.9791 respectively while the second-order
kinetic model gave the best fit for 4-NP degradation when using both
HDES/H$_{2}$O$_{2}$ systems with regression coefficients of 0.8448 and
0.8047 respectively, all at a temperature of 60~\textdegree C. These
best process parameters were then used to treat refinery wastewater in
which the efficiency of TOC removal was 88.75\% for
MC$_{10}$ and 67.08\% for MC$_{12}$. The recyclability of the spent
HDES/H$_{2}$O$_{2}$ systems was also investigated. For MC$_{10}$, the
percentage reduction of TOC from the wastewater was
from 88.75 to 33.33\% while for MC$_{12}$, it reduced from 67.08 to
11.25\% for three runs.\looseness=-1


\section*{Nomenclature}
\begin{inftab}
\begin{tabular}{ll}
\xtbody
DES &amp; deep eutectic solvent\\
HDES &amp; hydrophobic deep eutectic solvent\\
{MC}$_{10}$ &amp;{menthol--decanoic acid}\\
{MC}$_{12}$ &amp;{menthol--dodecanoic acid}\\
4-NP &amp; para-nitrophenol\\
TOC &amp; total organic compounds\\
FTIR &amp; Fourier transform infrared\\
AOPs &amp; advanced oxidation processes\\
I-AOPs &amp; integrated advanced oxidation processes\\
PAA &amp; peracetic acid\\
HBD &amp; hydrogen bond donor\\
HBA &amp; hydrogen bond acceptor
\end{tabular}
\end{inftab}

{\vspace*{-2pt}}

\section*{Declaration of interests}

{\vspace*{-2pt}}

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

{\vspace*{-2pt}}

\section*{Funding}

{\vspace*{-2pt}}

This project was partly funded by the National Research Institute for
Chemical Technology under the project number FGN/AB2021/ERGP30122610. 

{\vspace*{-2pt}}

\section*{Acknowledgments}

{\vspace*{-2pt}}

The authors express their gratitude to the Department of Chemical
Engineering (Ahmadu Bello University), National Research Institute for
Chemical Technology (NARICT), and Nigerian Institute of Leather and
Science Technology (NILEST) for the support rendered in carrying out
this research.

\back{}

{\vspace*{-2pt}}

\section*{Supplementary data}

{\vspace*{-2pt}}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-385-suppl.pdf}}

{\vspace*{-2pt}}

\printbibliography
\refinput{crchim20240365-reference.tex}

\end{document}
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