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\begin{document}

\title{Heavy metal sorption using thiolated oils of Elaeis guineensis and
Glycine max}



\author[1]{Onyeka Okwundu}%
\author[1]{Chinedu Chiama}%
\author[1]{Chimezie Chiama}%
\author[1]{Obiora Muojama}%
\author[1]{Chibuzor Enyekwe}%
\author[2]{Augustine Okaro}%
\author[1]{Cyril Ugwuoke}%
\affil[1]{Nigerian Young Researchers Academy }%
\affil[2]{University of Benin}%


\vspace{-1em}



  
  \date{May 07, 2024}


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\selectlanguage{english}
\begin{abstract}
=Sulphur-modification of matter confers improved heavy metal affinity
and could be exploited in the treatment of heavy metal contaminated
water. This paper is aimed at comparing the selective liquid-liquid Ag+
sorption capacities of normal and thiolated: palm oil (PO), palm kernel
oil (PKO) and soybean oil (SBO), respectively. The vegetable oils were
modified with 1-heptanethiol and the thioether-functionalized (TF) oils
were utilized for the removal of Ag+ present as contaminant in water,
while the unmodified oils acted as controls. Liquid-liquid equilibrium
contact time was determined to be 6 hours. The result achieved after
equilibration, revealed the effectiveness of TF oils in the removal of
Ag+ from a 600 ppm AgNO3 (aq). While TF-SBO reduced the cation
concentration to a level less than the detection limit, TF-PKO only
showed appreciable sorption capacity (below 30 \%) compared to the
sorption-inactive normal oils. The order of Ag+ sorption capacity
(TF-SBO \textgreater{} TF-PO \textgreater{} TF-PKO) was ascribed to the
various levels of unsaturation of fatty acid chains encountered in the
lipids, since higher number of sorption-active TF sites is achievable
with greater degree of fatty acid unsaturation. On that note, highly
unsaturated vegetable oils (such as that of Glycine max) were
recommended.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Man's activities such as industrialization, have led to increase
of~heavy metals~in the environment and this poses great threat to
humans, as well as the land and sea animals. Complete removal or
reasonable reduction in concentration of heavy metals like mercury,
silver and lead in industrial wastes (both liquids and solids) before
disposal is an eco-friendly practice. Several techniques have been
utilized effectively for remediation of heavy metal contamination; such
techniques include: immobilization, extraction, adsorption, chemical
precipitation, ion exchange, reverse osmosis, electrodialysis,
ultrafiltration, nanofiltration, coagulation, flocculation, floatation,
etc.{[}1,2{]}.

Enhanced removal of heavy metals can be achieved by functionalization of
the extracting solvent, adsorbent or membrane with a sulfur-containing
group like the thiol moiety. While the exact interactions between
various sulfur functionalities and heavy metals are not clearly
understood, it is generally accepted that sulfur functionality has a
strong affinity for heavy metals{[}3--7{]}. Murray et al. {[}4{]}
discovered that solid, liquid or gaseous materials contaminated with
heavy metals can be treated by contacting such materials with
thiol-functionalized fatty acid or thiol-functionalized ester of fatty
acids (sulfur-modified bio-oil) for a while, under certain conditions
effective for the sequestration of the metal species by the modified
bio-oil. Before then, several researches {[}8--11{]} on heavy metal
remediation have been carried out by treatment with sulfur-modified
agents. Bantchev et al. {[}12{]} have effectively synthesized (in
presence of ultraviolet radiation at wavelength, \selectlanguage{greek}λ \selectlanguage{english}\textless{} 325 nm)
and characterized thioether-functionalized vegetable oil (TF-VO) and the
molecular composition as well as reaction scheme is as shown in Equation
1 {[}12{]}.

Vegetable oil + Alkanethiol Thiolated oil (1)

This work is aimed at comparing the selective liquid-liquid sorption
capacities of normal and thiolated: palm oil (PO), palm kernel oil (PKO)
and soybean oil (SBO), respectively, in the extraction of silver ions
from synthetic (self-induced) wastewater. This work is justified by the
fact that it supports a healthy ecosystem. Meanwhile, bio-oil is an
environmentally sustainable and renewable material.

\textbf{Materials and Methods}

Two kinds of vegetable oil derived from the fleshy fruits and seeds
of\emph{Elaeis guineensis} (PO and PKO, respectively) were donated by
local farmers in Abatete, Anambra State, while SBO was purchased from
Ose Market, Anambra State, Nigeria.

The vegetable oil samples were first analyzed for their physicochemical
properties and then part of each was modified by reacting them with
excess 1-heptanethiol in the presence of ultraviolet light (UV at a
wavelength of 300 nm) such that they become thioether functionalized
(TF) {[}12{]}. In a typical modification experiment, 100 g of oil was
reacted with 30 g, 20 g or 65 g of heptanethiol to obtain crude TF-PO,
TF-PKO, or TF-SBO, respectively. Thereafter, the modified oil (TF-VO)
samples were respectively recovered from the crude mixture (with
excess/unreated heptanethiol) by simple distillation. The
sulphur-modification reaction is as given in Figure 1.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1: Structural representation of the sulphur-modification
reaction for TF-VO formation.}

AgNO\textsubscript{3}, a product of Sigma Aldrich, was used to introduce
Ag contaminant in water. A stock 600 ppm aqueous solution of
AgNO\textsubscript{3} was prepared using deionized water. Cation
extraction experiments were carried out using 20 mL of the contaminated
water for each run. First, the equilibrium liquid-liquid contact time
was determined by adding roughly 3.5 g of each of the TF oils to 20 mL
of contaminated water in masked conical flask mounted on a stirrer, all
within an enclosed dark chamber, due to the light sensitivity of silver
salts. In each case, the resulting two-phase mixture was stirred at 200
rpm for up to 10 hours. In the interim of stirring, samples were taken
after: 0.5, 1, 2, 4, 6, 8 and 10 hours, and separated. Treated water
samples were collected from the clear aqueous layer recovered after
centrifugation of the stirred oil-water mixture and Inductively Coupled
Plasma Mass Spectrometer (ICPMS-2030 Shimadzu) was used to analyze the
water samples, at 328 nm {[}5{]}, while a commercial standard Ag
solution (QMX Laboratories Ltd, UK) was used for calibration.

In order to determine the respective sorption capacities, various
amounts of the vegetable oils in the range of 0.1 to 10 g, were
respectively added to 20 mL of contaminated water and brought to
equilibrium (stirred for the determined equilibrium contact
time/period). Separation and analysis of treated water were performed as
earlier stated. In this case however, normal oils were also considered.

\textbf{Results and discussion}

Analyses of the normal/raw vegetable oils resulted in the
physicochemical properties shown in Table 1. Oil is a mixture of organic
compounds (predominantly, fatty acid esters of glycerol) and from the
table, the oil samples had viscosities greater than water (about 1
mm\textsuperscript{2}/s) and densities less than water (about 1000
kg/m\textsuperscript{3}); therefore, when mixed with water, the mixture
can be readily separated by centrifugation. Considering the fatty acid
profile, the PKO was composed of the shortest fatty acid chains with
only 9 \% unsaturation -- fewest thiolation (reaction) sites. On the
other hand, SBO had the longest of chains and highest level of
unsaturation (more than 80 \%).

\textbf{Table 1: Physicochemical properties of the normal vegetable
oils}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& PO & PKO & SBO\tabularnewline
\midrule
\endhead
Acid value (mg KOH/ g) & 1.85 & 0.88 & 1.20\tabularnewline
Saponification value (mg KOH/ g) & 174.3 & 237.0 & 151.6\tabularnewline
Iodine value (g I\textsubscript{2}/100 g) & 53.8 & 18.4 &
135.3\tabularnewline
Moisture content (wt. \%) & 1.01 & 0.12 & 0.07\tabularnewline
Actual density at 28\textsuperscript{o}C (kg/m\textsuperscript{3}) &
920.96 & 913.72 & 916.08\tabularnewline
Kinematic viscosity at 28 \textsuperscript{o}C (mm\textsuperscript{2}/s)
& 30.13 & 29.42 & 28.94\tabularnewline
Appearance & red & Brownish & Yellowish\tabularnewline
Flash point (\textsuperscript{o}C) & 319 & 314 & 319\tabularnewline
Fire point (\textsuperscript{o}C) & 353 & 348 & 350\tabularnewline
\textbf{Fatty acid profile} & \textbf{Fatty acid profile} &
\textbf{Fatty acid profile} & \textbf{Fatty acid profile}\tabularnewline
Caproic acid, C6:0 (\%) & -- & 0.2 & --\tabularnewline
Capryric acid, C8:0 (\%) & -- & 5.2 & --\tabularnewline
Capric acid, C10:0 (\%) & -- & 4.7 & --\tabularnewline
Lauric acid, C12:0 (\%) & -- & 62.4 & --\tabularnewline
Myristic acid, C14:0 (\%) & 1.2 & 12.9 & --\tabularnewline
Palmitic acid, C16:0 (\%) & 41.8 & -- & 7.2\tabularnewline
Palmitoleic acid, C16:1 (\%) & 0.3 & -- & --\tabularnewline
Stearic acid, C18:0 (\%) & 3.9 & -- & 3.3\tabularnewline
Oleic acid, C18:1 (\%) & 38.5 & 6.0 & 20.9\tabularnewline
Linoleic acid, C18:2 (\%) & 10.7 & 2.6 & 56.8\tabularnewline
Linolenic acid, C18:3 (\%) & 1.1 & -- & 8.1\tabularnewline
\bottomrule
\end{longtable}

{[}CHART{]}

\textbf{Figure 2: Influence of contact time on the
Ag\textsuperscript{+}sequestration at TF-VO loading of 3.5 g per 20 mL
of contaminated water}

Furthermore, to ascertain the liquid-liquid equilibrium contact time,
the influence of contact time on Ag\textsuperscript{+} sequestration at
TF-VO loading of 3.5 g per 20 mL of contaminated water, was investigated
and the result is as shown in Figure 2. From the figure, TF-PKO quickly
attained a stable extraction efficiency after 4 hours, while other TF
oils reached 6 hours. A general equilibrium contact time of 6 hours was
chosen and used hereafter.

{[}CHART{]}

\textbf{Figure 3: Comparative extraction capacities of the normal and TF
oils}

Now, considering the cation removal efficiency of both the normal and TF
oils, Figure 3 gives the comparative extraction capacities of the normal
and TF oils. All normal oils were inefficient. TF-SBO was outstanding at
all concentration, attaining 100 \% Ag\textsuperscript{+} removal
efficiency at a loading of 5 g per 20 mL of contaminated water. It
reduced the Ag\textsuperscript{+}concentration to a value less than 0.5
ppm (the detection limit of the analytical device). This extraction
performance was maintained as the oil loading was increased up to 10 g.
TF-PO followed after with a somewhat linear trend up to 8 g oil loading,
but its extraction capacity did not exceed 70 \%. TF-PKO had the least
sorption capacity of all the TF-VOs. Considering the nature of the
vegetable oils, SBO had the highest concentration of double bonds
(unsaturation) which is necessary for thiol-ene addition reaction.
Hence, TF-SBO had the highest active sorption (TF) site concentration
for heavy metal removal. The normal oils lacked such site and TF-PO had
much of it than TF-PKO. With reference to Figure 2, the poor
concentration of sorption sites in TF-PKO, or better said, the presence
of only few sites in TF-PKO could have caused it to attain equilibrium
earlier than TF-PO and TF-SBO, since less Ag ions need to be captured to
saturated the functionalized sites. Economically speaking, the use of
TF-SBO would save more oil due to its relatively high sorption capacity
-- little amount was needed to achieve the highest cation removal
efficiency. Use of raw oils in this regard only served the purpose of
control experiment.

\textbf{Conclusions and recommendations}

This study was aimed at comparing the selective liquid-liquid Ag+
sorption capacities of normal and thiolated: palm oil (PO), palm kernel
oil (PKO) and soybean oil (SBO), respectively.

It was found that TF-SBO had the highest sorption capacity, followed by
TF-PO and then TF-PKO, while the normal oils were ineffective.
Sulfur-modification of oils renders them effective for extraction of
heavy metals like silver in aqueous media. The concentration of heavy
metal sorption active (TF) sites in the sulphur-modified oil, favorably
determines its sorption capacity. The choice of bio-oil for
sulfur-modification should be one with relatively high total unsaturated
bonds.

Future works should focus on scaling up and optimizing the extraction
process. This study relied on bench-scale experiments. It is also
necessary to determine the effects of stirring speed and other process
conditions on the extraction process in order to optimize the use of
TF-VO in extracting heavy metals.

Finally, implementation of the outcome of this study in remediating
heavy metal-contaminated water is recommendable, and is already in
progress. We propose a heavy metal remediation plant based on the
process flow diagram shown in Figure 4. TF-VO would be prepared or
purchased and stored. Contaminated water and TF-VO would be pumped
respectively to the arms of a long passive T-mixer with bends (M-1).
Studies have shown that this kind of mixers are effective for contacting
fluid phases down to molecular level {[}13,14{]}. Further mixing could
be achieved with active stirred tank with residence time of about 1
hour, as against the 6 hours spent in laboratory. The mixed fluid could
be separated using a hydro-cyclone (or series of it, if necessary) at
high feed pump pressure. It is recommended that used TF-VO (with trapped
metals) be treated and recycled for process sustainability.

\textbf{Figure 4. Process flow diagram of the proposed heavy metal
remediation plant.}

\textbf{Acknowledgement}

The authors received no financial support for the research, authorship
and publication of this article. They declare that no interest or
relationship whatsoever has influenced their objectivity in the reported
work.

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