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\begin{document}

\title{A Novel Preparation and Vapour Phase Modification of 2D-open Channel
Bio-adsorbent Via ATRP for Uranium Separation}



\author[1]{Mudasir Ahmad}%
\author[1]{Jianquan  Ren}%
\author[1]{Tao  Xiu}%
\author[2]{Mehraj-ud-din  Naik}%
\author[1]{Qiuyu Zhang}%
\author[1]{Baoliang Zhang}%
\affil[1]{Northwestern Polytechnical University}%
\affil[2]{Jazan University}%


\vspace{-1em}



  
  \date{May 15, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
An economical and highly uranium extraction from seawater remains a
crucial task for energy sources and environmental safety. Aiming for
improving mass transfer rate of uranium adsorption from seawater, a new
synthetic strategy was adopted for the fabrication of 2D-open channel
microporous bio-adsorbent for uranium extraction from seawater. Herein,
the in-situ ATRP grafting approach was adopted to graft divinylbenzene,
polyacrylonitrile onto the surfaces of microporous frameworks via the
vapour phase method. The post-synthetic functionalization was carried
out by hydrothermal method, where amidoxime groups are
structure-directing agents to trap uranium. Further, amidoxime groups
not only enhanced hydrophilicity but also adjusts adsorbents pKa. AO-Fc
faces minimum interference of competing ions and achieves a high uranium
adsorption capacity of 8.57\selectlanguage{ngerman}±0.02 and 409±1 mg/g in seawater and
simulated solution. Despite its stable structure, AO-Fc exhibits a long
life span and negligible weight loss revealed AO-Fc could be applied as
a potential adsorbent for radionuclide.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{A Novel Preparation and Vapour Phase Modification of 2D-open
Channel Bio-adsorbent Via ATRP for Uranium Separation}

Mudasir Ahmad \textsuperscript{a, b}, Jianquan Ren \textsuperscript{a},
Tao Xiu \textsuperscript{a}, Mehraj-ud-din Naik \textsuperscript{c},
Qiuyu Zhang \textsuperscript{a, b}, Baoliang Zhang \textsuperscript{a,
d,} 11Corresponding author, Northwestern Polytechnical University, Youyi
Road 127\#, Xi'an (710072), China. Email: blzhang@nwpu.edu.cn

\textsuperscript{a} School of Chemistry and Chemical Engineering,
Northwestern Polytechnical University, Xian, China, 710072.

\textsuperscript{b} Xian Key laboratory of Functional Organic porous
materials, Northwestern Polytechnical University, China, 710129.

\textsuperscript{c} Department of Chemical Engineering, College of
Engineering, Jazan University, Jazan 45142, Kingdom of Saudi Arabia.

\textsuperscript{d} Shaanxi Engineering and Research Center for
Functional Polymers on Adsorption and Separation, Sunresins New
Materials Co. Ltd., Xi'an, 710072, China.

\textbf{Abstract:} An economical and highly uranium extraction from
seawater remains a crucial task for energy sources and environmental
safety. Aiming for improving mass transfer rate of uranium adsorption
from seawater, a new synthetic strategy was adopted for the fabrication
of 2D-open channel microporous bio-adsorbent for uranium extraction from
seawater. Herein, the in-situ ATRP grafting approach was adopted to
graft divinylbenzene, polyacrylonitrile onto the surfaces of microporous
frameworks via the vapour phase method. The post-synthetic
functionalization was carried out by hydrothermal method, where
amidoxime groups are structure-directing agents to trap uranium.
Further, amidoxime groups not only enhanced hydrophilicity but also
adjusts adsorbents pKa. AO-Fc faces minimum interference of competing
ions and achieves a high uranium adsorption capacity of 8.57\selectlanguage{ngerman}±0.02 and
409±1 mg/g in seawater and simulated solution. Despite its stable
structure, AO-Fc exhibits a long life span and negligible weight loss
revealed AO-Fc could be applied as a potential adsorbent for
radionuclide.

\textbf{Keywords:} Microporous bioadsorbent; 2D-open channel;
vapor-phase modification; ATRP; uranium

\textbf{Introduction}

A rapid increase in energy demands has given rise to energy shortage,
nuclear energy with the advantage of low carbon emission play an
important role in sustainable energy source\textsuperscript{1,2}.
Uranium plays a key role in nuclear energy sources, therefore its
extraction is very important for continuous energy
supply\textsuperscript{3-5}. However, it is estimated uranium in the
terrestrial ores is limited and at the current usage, the terrestrial
resources might be ended at the end of the current
century\textsuperscript{6}. Comparatively, it is estimated 4.5 billion
tons of uranium in seawater, which is a thousand times higher than
terrestrial ores\textsuperscript{7-9}. Therefore, a new strategy needs
to develop to extract efficient uranium from seawater to fill the energy
gap, environmental protection and sustainable development. However, the
presence of competing ions, low uranium concentration (3.3 ppb) and
high-cost synthetic preparation hinder the development of materials for
the extraction of uranium from natural seawater\textsuperscript{10-14}.
Various materials such as porous organic polymers
(POPs)\textsuperscript{15,16}, covalent organic frameworks
(COFs)\textsuperscript{17-19}, porous aromatic frameworks
(PAFs)\textsuperscript{3,20}, metal-organic frameworks
(MOFs)\textsuperscript{21,22} and biopolymers\textsuperscript{23-25}have
been used for the extraction of uranium from seawater. Among all of
them, amidoxime-modified adsorbents are promising with the advantage of
high selectivity towards uranium in seawater\textsuperscript{26-29}.
However, the development of most of the material is either expensive,
difficult to synthesize, poor stability and low uranium adsorption
efficiency in seawater. Recently, efforts have been devoted to
developing advanced materials via simple and facile methods from
inexpensive raw materials with ample uranium adsorption capacity.

Charcoal-based materials are highly interested due to their low cost,
porous structure, and environmentally friendly
nature\textsuperscript{1,30-32}. Recently, amidoxime modified hollow
fibers were used for uranium adsorption\textsuperscript{33}. Zhou et al.
reported 1D porous adsorbent from wood waste for adsorption of
polycyclic aromatic hydrocarbons\textsuperscript{34}. Liao et al.
reported pig manure-based biochar for uranium
extraction\textsuperscript{35}. Wang et al. reported 1D amidoxime
modified bamboo charcoal for uranium extraction from
seawater\textsuperscript{36}. The drawbacks of already available
amidoxime modified charcoal-based adsorbents for uranium extraction are
as follows; (i) Regarding the analysis and reporting the adsorption
results, the experimental adsorption uncertainty is currently missing,
(ii) the preparation method is difficult or low functionalization yield
and or perform well under inert atmospheres, (iii) use of high cost
solvents for the preparation and purification process and (iv)
requirement of typical experimental procedure with critical conditions.
In this wok we could sort out these problems as well by carrying a
simple method by carbonization process in vacuum and subsequent vapor
phase modification via ATRP to obtain highly functionalized ultra-porous
material. Then after the post-synthetic modification was carried to
convert nitrile groups into amidoxime functional groups onto the surface
of 2D channels by hydrothermal method. The uncertainty in uranium
adsorption experiment was measurement by using classic concepts of error
propagation.

\textbf{2. Experimental section}

\textbf{Materials}

Wood (Chinese fir) was obtained from Heilongjiang province, China.
Acrylonitrile and azobisisobutyronitrile, uranyl nitrate, sodium
chloride, and sodium bicarbonate were purchased from Changzhou Qidi
Chemical Co., Ltd. Divinylbenzene, hydroxylamine hydrochloride. NaOH and
ethanol were obtained from Hubei Jusheng Technology Co., Ltd. The
double-distilled water was obtained from a laboratory filtration plant.

\textbf{Preparation of nitrile functionalized charcoal (CN-Fc)}

Wood (Chinese fir, 5 g) was used as a starting material for the
preparation of charcoal in this study. A simple carbonization process
was carried out at 600 \textsuperscript{o}C with a heating rate of
10\textsuperscript{o}C/min in a vacuum for 3 h. Then, acrylonitrile (5
g) and azobisisobutyronitrile (0.1 g) and charcoal (0.5 g) were added to
a beaker for 30 min. The samples were taken and kept in a glass tube
which was 100 mm in length and 30 mm in diameter and the tube was kept
in an oven at 180 \textsuperscript{o}C for 5 h. To achieve higher
nitrile content, 0.8 g of divinylbenzene was added in a solution of
acrylonitrile (5 g), azobisisobutyronitrile (0.1 g) under the same set
of reaction conditions mentioned above. The obtained products were
denoted as CN-Fc and CN-Fc1, where 1 represents an absence of
divinylbenzene in the reaction system.

\textbf{Preparation of amidoxime modified charcoal (AO-Fc)}

The amidoximation process was carried out by adopting a previous method
with small modifications\textsuperscript{36}. Hydroxylamine
hydrochloride (0.48 g) and NaOH (0.25 g) were dissolved in ethanol (15
mL) using an ultrasonicator for 15 min. In the meantime, CN-Fc (0.48 g)
and the prepared solution were transferred in 50 mL Teflon lined
autoclave, which was kept at 70 \textsuperscript{o}C for 24 h. After
cooling, the product was washed with distilled water several times,
dried in a vacuum at 60 \textsuperscript{o}C for 12 h and the obtained
product was AO-Fc.

\textbf{Characterization}

The morphology was analyzed by using FEI Verios G4 scanning electron
microscopy (SEM) and the energy dispersive spectrum (EDS) was carried
out by using a Bruker Everhart-Thornley Detector. For chemical structure
determination, FTIR (Bruker TENSOR 27 spectrophotometer) and Powder
X-ray diffraction (Thermo Scientific 7000 diffractometer) were used. The
hydrophilicity was determined using a contact angle analyzer. The
thermal stability was analyzed using a Mettler Toledo Thermogravimetric
analyzer with a heating rate of 10 \textsuperscript{o}C/min under a
nitrogen atmosphere in a temperature range of
40-800\textsuperscript{o}C. The X-ray photoelectron spectrum was
obtained by using Kratos Axis Ultra DLD analyzer and the peak fitting of
performed using XPS peak fitting program version 4.1. Nitrogen
adsorption-desorption was carried out using the Tristar3020
Micromeritics analyzer.

\textbf{Uranium extraction from natural seawater}

A homemade experimental setup (Figure S1) was developed to investigate
the adsorption capacity of the AO-Fc in simulated seawater and natural
seawater. The simulated seawater was obtained by dissolving uranyl
nitrate (0.017 g), sodium chloride (25 g), and sodium bicarbonate
(0.193) in distilled water and further, diluted to obtain different
uranium initial concentrations of 0.003 to 1 mg/L. The influence of pH
on uranium adsorption, contact time, initial uranium concentration, and
the presence of various competing ions was investigated. The pH of the
solution was set by adding the required quantity of 0.1 M HNO3 and NaOH
solution. For each experiment, the adsorbent was packed in a syringe to
hold it and let the penetration of the adsorption solution. The solution
was pumped using a lincolin pump with a flow speed of 150 mL/min at room
temperature.

For real seawater, we have packed adsorbent (7 mg) in a syringe to hold
the adsorbent. A real seawater 20 L was obtained from the South China
Sea near Shandong province. The adsorption capacities of metal ions were
estimated by collecting a 5 mL sample every day for 39 days. The metal
ions concentration was determined by using an inductively coupled plasma
emission spectrometer (ICPS-MS, 6300, ThermoFisher Scientific). The
uranium adsorption capacity (q\textsubscript{e} mg/g) and adsorption
efficiency (Ads \%) can be written as shown in equation 1 and 2.

\(q_{e}=\frac{\left(C_{o}-C_{e}\right)}{m}\times V\) (1)

\(Ads\ \%=\frac{\left(C_{o}-C_{e}\right)}{C_{o}}\times 100\) (2)

Where q\textsubscript{e} is the equilibrium adsorption capacity weight
per unit mass (mg/g) C\textsubscript{o} and C\textsubscript{e} represent
initial concentration and equilibrium concentration weight per unit
volume (mg/L), V is a volume of solution in liters (L) and mass of
adsorbent in grams (g).

To determine the adsorbent regeneration, the adsorbent (7 mg) was eluted
by using a 21 mL elution solution
(Na\textsubscript{2}CO\textsubscript{3}, 1 M and
H\textsubscript{2}O\textsubscript{2}, 0.1 M) at room temperature for 1
hour to remove the adsorbed uranium from the adsorbent surface. The
elution efficiency can be expressed as shown in equation 3.

\(\frac{C_{\text{el}}\times V_{\text{el}}}{\left(C_{a}-C_{o}\right)\times V_{a}}\times 100\)(3)

Where C\textsubscript{el} denotes elution concentration (mg/L),
V\textsubscript{el} is the elution volume (L),
C\textsubscript{a}represents uranium concentration in seawater after
adsorption (mg/L), C\textsubscript{o} initial concentration of metal
ions (mg/L) and V\textsubscript{a} volume of seawater used for
adsorption (L).

\textbf{Results and discussion3.1 Adsorbent synthesis and
characterization}

To achieve efficient uranium extraction from seawater, it is critical to
develop highly porous adsorbent material with efficient stability and
selectivity toward uranium in natural seawater. As shown in Figure 1,
two different approaches have been adopted to functionalize the vergin
fir-charcoal (Fc) and subsequent post-synthetic functionalization was
followed to enhance uranium adsorption of targeted adsorbent in
seawater. Firstly, Fc undergoes vapor phase modification with
acrylonitrile and azobisisobutyronitrile as an initiator to obtain
nitrile functionalized material (CN-Fc) via ATRP. Secondly, the
functionalization was enhanced by adding divinylbenzene as an additional
component under the same set of experimental conditions to yield CN-Fc.
In post-synthetic functionalization under alkaline conditions, nitrile
groups were converted into amidoxime groups via a hydrothermal process.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1 Step by step preparation of AO-Fc adsorbent material

The chemical structure was confirmed by FTIR and XPS analysis. The FTIR
spectra of Fc, CN-Fc, CN-Fc1 and AO-Fc were shown in Figure 2a. All
spectra have similar shapes in the vibration band of carbonaceous
material and a peak at 3500-3700 cm\textsuperscript{-1} was observed due
to the presence of OH groups\textsuperscript{36,37}. Afterward, Fc was
modified with the aid of azobisisobutyronitrile and divinylbenzene to
yield CN-Fc. On comparing the FTIR spectrum of CN-Fc and CN-Fc1, the
peaks in the spectra at 2242 cm\textsuperscript{-1} were assigned to C[?]N
and high peak intensity witnessed the higher payload of nitrile groups
were seen on CN-Fc than CN-Fc1\textsuperscript{38}. The post-synthetic
functionalization further converted C[?]N groups into C=N, C-N and N-O
confirmed by respective peaks at 1638 cm\textsuperscript{-1}, 1380
cm\textsuperscript{-1} and 928 cm\textsuperscript{-1} respectively,
indicating the presence of amidoxime groups on the surface of
Fc\textsuperscript{39}. The presence of amidoxime groups were further
investigated by XPS analysis (Figure 2b-d); in C 1s high-resolution
spectra of AO-Fc (Figure 2b) showed two peaks at 284.45 eV and 285.5 eV
assigned to C-N and C=N; in N 1s spectrum of AO-Fc (Figure 2c) showed
two peaks at 400.05 eV and 398.40 eV assigned to N-H and C=N
respectively\textsuperscript{36}, and the O 1s spectra of AO-Fc (Figure
2d) showed two peaks at 532.5 eV and 531.9 eV assigned to
C(NH\textsubscript{2})-N-OH and O-H groups
respectively\textsuperscript{40}. The powder XRD patterns of Fc, CN-Fc
and AO-Fc were shown in Figure 2e. The XRD patterns showed peaks around
16.2\textsuperscript{o} which can be indexed (101) plane and confirmed
the amorphous nature of the material\textsuperscript{41}. Furthermore, a
broad peak was seen at around 25\textsuperscript{o} and a small peak
nearly 43\textsuperscript{o}, which can be indexed (0002), (1010/1011)
plane of graphite and are consistent with the presence of amorphous
carbon\textsuperscript{42}. The contribution of amidoxime groups on the
surface of AO-Fc was calculated by carrying thermogravimetric analysis.
Compared with Fc, a significant weight loss (80 \%) was observed in
AO-Fc in the temperature range of 300 \textsuperscript{o}C to
550\textsuperscript{o}C (Figure 2f). The higher weight loss in AO-Fc was
due to the large amount organic groups were thermally decayed, which
confirmed the weight contribution of amidoxime groups taken to 80 \% of
the weight loss of AO-Fc.

Figure 2 FTIR spectra (a), high-resolution XPS spectra of C 1s (b) N 1s
(c) and O 1s (d), powder XRD pattern (e), and TGA curves (f)

SEM morphology of the vertical and horizontal cross-section of Fc,
CN-Fc, CN-Fc1, and AO-Fc were shown in Figure 3. The structure of Fc was
seen with two-dimensional rectangular two open-end vessels arranged in a
proper array with a breadth of 28.6 \selectlanguage{greek}μ\selectlanguage{english}m (Figure S2a) and height of 18.3
\selectlanguage{greek}μ\selectlanguage{english}m (Figure 3abc) and (Figure S2b). SEM images of CN-Fc and CN-Fc1 were
not changed obviously after chemical modification (Figure 3de). However,
high magnified SEM images showed the rectangular walls were covered with
small structures that confirmed the presence of chemical groups.
Comparison to Fc, AO-Fc SEM images showed thick wall rectangular vessels
(high magnified image) (Figure 3f) without any collapse or damage with a
height and breadth of approximately 27.02 \selectlanguage{greek}μ\selectlanguage{english}m (Figure S2c) and 14.75 \selectlanguage{greek}μ\selectlanguage{english}m
(Figure S2d). In other words, the AO-Fc was successfully prepared by
amidoximation reaction of nitrile groups.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Figure 3 Vertical cross-section SEM images of Fc (a,b), and horizental
cross section SEM images of Fc (c), CN-Fc (c), CN-Fc1 (c) and AO-Fc(c)

Figure 4 showed dark-field images of CN-Fc, CN-Fc1 and AO-Fc with the
corresponding elemental mapping. The images clearly showed the high
number of nano precipitates as illustrated in corresponding Figure 4a-i
and their elemental percentage was reported in Table 1. These
precipitates were mostly C, N, and O which are arranged in a rectangular
homogenous pattern. The C and N elements of CN-Fc and CN-Fc1 were
distributed in a rectangular fashion (Figure 4dgeh) confirming the
presence of C and N elements in plenty and arranged in a homogenous
pattern. Figure 4fi showed the presence of O in rectangular patterns
confirmed the successful conversion of nitrile groups into amidoxime
groups.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Figure 4 STEM+EDS maps of CN-Fc, CN-Fc1and AO-Fc

The bet surface area, pore volume and pore size of Fc, CN-Fc, CN-Fc1 and
AO-Fc were analyzed and the results were reported in Table 2. After
functionalization of Fc, both surface area and pore volume were
decreased from 155 m\textsuperscript{2}/mg to 46 m\textsuperscript{2}/mg
and 0.07 cm\textsuperscript{3}/g to 0.007 cm\textsuperscript{3}/g
(Figure 5), because of the large number of amidoxime groups occupying
the pores space of Fc. Compared with amidoxime modified bamboo
charcoal\textsuperscript{36}, AO-HNTs\textsuperscript{39} and
fiber\textsuperscript{43} used for uranium extraction, the specific
surface area of AO-Fc was relatively higher which might increase uranium
adsorption capacity in seawater. The super-hydrophilicity of the
material was determined by analyzing water contact angle measurements.
The water contact angle of Fc and CN-Fc showed hydrophilicity and
reached the water contact angle of 149\textsuperscript{o} and
82.3\textsuperscript{o} at the time of water contact with the adsorbent
surface for 13 seconds (see video S1 and S2). On the other side, the
water angle of AO-Fc reached 0\textsuperscript{o} angle at a time gap of
0.02 seconds indicating AO-Fc possessed super-hydrophilicity due to the
presence of enough hetero groups onto the surface of AO-Fc (see video
S3).

Figure 5 Nitrogen adsorption and desorption isotherms (a) and
distribution of pore size (b) of Fc, CN-Fc, CN-Fc1, and AO-Fc

Table 1 Weight percentage (\%) by elemental analysis\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Sample & Elements & Elements & Elements\tabularnewline
\midrule
\endhead
& C \% & N \% & O \%\tabularnewline
Fc & 100\selectlanguage{ngerman}±0.80 & - & -\tabularnewline
CN-Fc & 91 \selectlanguage{ngerman}±1.2 & 8.5\selectlanguage{ngerman}±0.6 & -\tabularnewline
CN-Fc1 & 95.2\selectlanguage{ngerman}±0.36 & 4.9\selectlanguage{ngerman}±0.12 & -\tabularnewline
AO-Fc & 57.3\selectlanguage{ngerman}±0.36 & 28\selectlanguage{ngerman}±1.54 & 14.7\selectlanguage{ngerman}±0.86\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{ngerman}

Table 2 Nitrogen-adsorption desorption analysis of Fc, CN-Fc1, CN-Fc2
and AO-Fc\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Sample\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Pore size nm\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Pore volume cm\textsuperscript{3}/g\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Surface area m\textsuperscript{2}/mg\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Fc\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
1.3\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.07\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
155\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
CN-Fc\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.5\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.04\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
53\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
CN-Fc1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
1.1\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.05\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
48\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
AO-Fc\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.4\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
0.005\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
46\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{3.2 Uranium recovery tests in simulate seawater}

\emph{3.2.1 Analysis of optimal pH for uranium adsorption}

It is well known the pH value of the adsorption solution markedly
affects the uranium adsorption performance of the adsorbent due to the
presence of different ionic charges present on the adsorbent surface and
the uranium ions. Herein we study the effect of pH on uranium adsorption
in the pH range of 3 to 10. From Figure 6a, as we have seen the
adsorption capacity increased rapidly with increasing pH 3 to 8, at pH 8
the maximum adsorption capacity of AO-Fc was 411\selectlanguage{ngerman}±2 mg reported. With a
further increase in pH, the adsorption capacity started to decrease. The
optimum pH with maximum adsorption capacity of Fc, CN-Fc, and AO-Fc was
5, 6, and 8 confirmed the grafting of amidoxime groups adjusted the pKa
value, resulting in shifting of pH to alkaline which is favorable for
uranium under seawater conditions. The AO-Fc showed a positive shift
towards an optimal pH of 8 towards the alkaline phase whereas the CN-Fc
showed a small shift with an optimal pH of 6, confirming the role of
amidoxime groups to shift the pKa of the base material. Further, NaCl
showed no influence on uranium adsorption of AO-FC, CN-Fc and Fc (Figure
6b) indicated a high concentration of NaCl in seawater would not affect
uranium adsorption capacity of AO-Fc in seawater conditions.

Figure 6 Effect of solution pH (a) and various concentration of NaCl (b)
on uranium adsorption capacity (mg/g)

\emph{3.2.2 Effect of contact time for uranium recovery capacity}

To investigate the adsorption kinetics, the influence of contact on
uranium adsorption onto adsorbent was analyzed for the contact time
ranges of 5 to 450 min. As shown from Figure 7a, AO-Fc showed adsorption
amount increasing rapidly from 5 to 80 min, indicating strong uranium
adsorption from the initial uranium concentration solution. However,
after 80 min the adsorption rate was decreased due to a decrease in
diffusive resistance and active sites on the surface of AO-Fc during the
adsorption process of uranium by amidoxime groups. After 350 min, there
was no significant change in adsorption amount of AO-Fc, confirming the
absorption process achieved the equilibrium of adsorption at 350 min.
CN-Fc and Fc showed lower adsorption capacity in comparison to AO-Fc due
to the presence of low chelating sites and no significant adsorption
capacity was increased after 150 min equilibrium of adsorption
indicating equilibrium of adsorption was achieved. It is worth to report
the longer equilibrium time and slower diffusion were attributed due to
the low surface area and internal microporous structure of AO-Fc. To
analyze the uranium adsorption mechanism, the adsorption kinetics of the
adsorbent was studied by applying linear pseudo first order (LPFO) and
pseudo second order kinetics(LPSO) and non-linear pseudo first order
(NLPFO), and pseudo second order kinetics(NLPSO) (supporting
information).

Both linear and non-linear plots of pseudo first order model and pseudo
second order model and their kinetic parameters were shown in Figure 7b,
Figure S3, Figure S4, Table 3 and Table S1. The kinetic parameters of
linear plots of pseudo first order model and pseudo second order model
were obtained by easily plotting a graph between ln
(q\textsubscript{e}-q\textsubscript{t}) versus t and t/qt versus t and
the relevant parameters for studying the uranium adsorption were listed
in Table 3 and Table S1. The kinetic parameters of non-linear plots of
pseudo first order model and pseudo second order model were calculated
by plotting a graph between q\textsubscript{t} versus t and the relevant
parameters were shown in Table S2. From the results of both linear and
non-linear model fit (Table 3, Table S1 and Table S2), the correlation
coefficient (R\textsuperscript{2}) of the pseudo second order model was
close to unity than pseudo first order model, confirming the pseudo
second order was better to discuss the adsorption behavior over entire
adsorption process. The pseudo second kinetic model is based on the
assumption that the rate-limiting step follows the chemisorption process
through sharing valance electrons between uranium and the chelating
site. Also, the validity of pseudo second order model fit was
investigated by calculating p-value, q\textsubscript{ecal} value (Figure
S5) and statistical errors(supporting information), indicating the
experimental data were best fitted with the pseudo second order.
According to the theoretical calculation of Azizian et al. the
adsorption experiment was carried out at a high initial concentration of
solute \textgreater{}250 mg/L, the adsorption data were best fitted with
pseudo first order rather than pseudo second order\textsuperscript{44}.

Figure 7 Adsorption kinetics of uranium onto AO-Fc, CN-Fc and Fc, the
effect of contact time (a) and linear fitting by the pseudo second order
(b) (T = 303 K and adsorbent dosage = 0.01 mg/L)

Table 3 Kinetic parameters for uranium obtained by linear regression.

Pseudo second order model\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
Adorbent & K\textsubscript{2} & q\textsubscript{eexp} &
q\textsubscript{ecal} & R2 & p-value & SSE & SAE & X\textsuperscript{2}
& \selectlanguage{greek}Δ\selectlanguage{english}q(\%)\tabularnewline
AO-Fc & 0.00012\selectlanguage{ngerman}±2E-05 & 410.000\selectlanguage{ngerman}±0.004 & 400\selectlanguage{ngerman}±10 & 0.996 & 9.6E-08 & 15.9
& 9.2 & 12.2 & 3.2\tabularnewline
CN-Fc & 0.00094\selectlanguage{ngerman}±7E-05 & 30.00\selectlanguage{ngerman}±0.01 & 29\selectlanguage{ngerman}±1 & 0.996 & 1.1E-08 & 1.6 & 4.7
& 1.0 & 3.3\tabularnewline
Fc & 0.00124\selectlanguage{ngerman}±0.0001 & 19.000\selectlanguage{ngerman}±004 & 19\selectlanguage{ngerman}±1 & 0.989 & 1.1E-07 & 1.2 & 10.0 &
1.8 & 3.5\tabularnewline
\bottomrule
\end{longtable}

\emph{3.2.3 Effect of equilibrium concentration on uranium recovery}

To determine the adsorption capacity of AO-Fc, equilibrium adsorption
studies were performed in a simulated uranium solution at different
initial uranium concentrations. As shown in Figure 8a, the uranium
adsorption amount increased rapidly with increasing the initial uranium
concentration, but in due course, the absorption capacity reached a
plateau, which meant the adsorption capacity had reached the maximum
adsorption capacity of the adsorbent. It was attributed, to more
opportunities to adsorbent to intact with the higher uranium initial
concentration until the maximum adsorption capacity was achieved.
Further, the adsorption capacity of AO-Fc was higher than CN-Fc and Fc
due to the presence of amidoxime groups. To measure the adsorption
amount of adsorbent for uranium, various linear and non-linear isotherm
models such as Langmuir isotherm model, Freundlich isotherm model, and
Temkin isotherm model were applied to treat the experimental adsorption
data at various uranium concentrations (Supporting information).

The fitting results of AO-Fc, CN-Fc and Fc were shown in Figure 8b,
Figure S6 and Figure S7 and the values of various parameters are listed
in Table 4 and Table S3 and Table S4. From results listed in Table 4 and
Table S3 and Table S4, for AO-Fc the Langmuir isotherm model showed a
higher regression coefficient (R\textsuperscript{2}=0.998) than
Freundlich isotherm(R\textsuperscript{2}=0.944) and Temkin isotherm
(R\textsuperscript{2}=0.985), confirming the uranium adsorption onto the
surface of AO-Fc followed a homogenous and monolayer adsorption
coverage. Moreover, the applicability of the isotherm model was checked
by calculating various statistical errors. For AO-Fc the Langmuir
isotherm model showed lower error values as compared to Freundlich
isotherm and Temkin isotherm, indicating the experimental data were best
fitted with the Langmuir isotherm. Also, for AO-Fc the
q\textsubscript{m} (mg/g) calculated from the Langmuir isotherm model
was 420\selectlanguage{ngerman}±12 mg/g at an equilibrium concentration of (0.5 mg/L), which was
compared with the previously reported amidoxime adsorbents till now
(Table 5). The results from Table 5 showed higher adsorption capacity
for AO-Fc, indicating that it had greater potential to extract uranium
from an aqueous solution.

Figure 8 Adsorption isotherm of uranium onto AO-Fc, CN-Fc and Fc, the
effect of uranium initial concentration (a) and linear fitting by the
Langmuir isotherm (T=303 K; adsorbent dosage = 0.01 mg/L and t = 350
min)

Table 4 Isothermmal parameters for uranium obtained by linear regression\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
\begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage} & \begin{minipage}[b]{0.07\columnwidth}\raggedright\strut
\textbf{Langmuir isotherm model}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Adsorbent\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
q\textsubscript{eexp} (mg/g)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
K\textsubscript{L} (L/mg)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
q\textsubscript{ecal} (mg/g)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
q\textsubscript{m} (mg/g)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
R\textsuperscript{2}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
SSE\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
SAE\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
X\textsuperscript{2}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\selectlanguage{greek}Δ\selectlanguage{english}q(\%)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
AO-Fc\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
409\selectlanguage{ngerman}±1\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
73.036380100\selectlanguage{ngerman}±2E-09\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
410.65\selectlanguage{ngerman}±0.02\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
420\selectlanguage{ngerman}±12\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
0.998\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
70\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
67\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
237\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
7\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
CN-Fc\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
30\selectlanguage{ngerman}±1\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
10.280936\selectlanguage{ngerman}±3E-06\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
29.560\selectlanguage{ngerman}±0.002\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
32.5\selectlanguage{ngerman}±0.7\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
0.998\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
Fc\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
17\selectlanguage{ngerman}±2\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
6.82781\selectlanguage{ngerman}±4E-05\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
16.330\selectlanguage{ngerman}±0.004\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
19\selectlanguage{ngerman}±2\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
0.999\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
4\strut
\end{minipage} & \begin{minipage}[t]{0.07\columnwidth}\raggedright\strut
3\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{3.3 Uranium recovery in seawater}

To analyze the uranium adsorption capacity of AO-Fc in real seawater,
adsorbent (7 mg) was added in a homemade adsorbent holder (syringe,
Figure S1) and exposed to 20 L seawater and the complete home-made
uranium adsorption set-up was shown in (Figure S1). The amount of
uranium adsorbed from seawater was analyzed by knowing uranium
concentration at different time intervals using (ICP-MS). For industrial
applications, the adsorbent can be directly used to lower surface
seawater for uranium extraction. The uranium adsorption kinetic results
revealed that AO-Fc showed 6.60 mg/g adsorption capacity within one
week, which was 75\% of the total uranium adsorption. The maximum
adsorption capacity reached 8.57 mg/g with 100 \% adsorption efficiency
after 11 days of the adsorption experiment (Figure 9ab). In comparison,
AO-Fc showed efficient uranium adsorption from seawater (Table 5).
Uranium adsorption capacity reflects the direct measurement of the
amount of uranium adsorbed by an adsorbent and the distribution
coefficient (K\textsubscript{d}), determines the adsorbent affinity
towards adsorbate. These results indicated that the adsorbent retaining
high uranium adsorption capacity in presence of other coexisting ions
might have a correspondingly higher K\textsubscript{d} value than other
coexisting ions in Figure 9c. Based on that, the evaluation of
K\textsubscript{d} (L/g)is an important parameter for choosing the most
suitable adsorbent for further analysis.

Figure 9 Kinetics of uranium extraction (a,b) and distribution
coefficient (K\textsubscript{d}) of various ions in natural seawater (b)

Table 5 Comparison of various adsorbents for uranium extraction in
seawater\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
Adorbent & Volume of seawater (L)/location & Adsorbent mass(mg/L) &
Adsorption time(days) & Adsorption capacity(mg/g) &
References\tabularnewline
\midrule
\endhead
UiO-66-AO & 1/Bohai Seawater & 1 & 3 & 2.7 &
\textsuperscript{45}\tabularnewline
Anti-UiO-66 & 10 & 2 & 30 & 4.6 & \textsuperscript{46}\tabularnewline
Anti-UiO-66 & Simulated solution & - & - & 2.84 &\tabularnewline
UiO-66-3C4N & 100/South China Sea & 0.2 & 28 & 6.9 &
\textsuperscript{47}\tabularnewline
UiO-66-3C4N & Simulated solution & - & & 380.3 &\tabularnewline
BP@CNF-MOF & 100/ South China Sea & 1 & 42 & 6.8 &
\textsuperscript{48}\tabularnewline
UiO-66-NH-(AO) & 25/South China Sea & 0.2 & 8 & 5.2 &
\textsuperscript{49}\tabularnewline
AOOBCH & Simulated seawater & - & 30 & 6.7 &
\textsuperscript{36}\tabularnewline
AO-Fc & 20/South China Sea & 0.35 & 11 & 8.6 & Present
work\tabularnewline
AO-Fc & Simulated solution & - & 0.236 & 409\selectlanguage{ngerman}±1 &\tabularnewline
\bottomrule
\end{longtable}

\textbf{3.4 Adsorbent stability and interaction with uranyl ions}

The morphology of the adsorbent after uranium extraction/elution was
analyzed by using SEM analysis. From Figure 10, we see the SEM images
after uranium extraction (Figure 10a) and uranium elution (Figure 10b)
confirmed that seawater exposed AO-Fc maintained the integrity of
structure after reuse of several cycles for a long time of seawater
exposure. The detailed chemical composition of the uranium-loaded
adsorbent (AO-Fc-U) adsorbent was analyzed by XPS and FTIR analysis and
the results were compared with the adsorbent before uranium adsorption
(AO-Fc) (Figure 11). The uranium-loaded adsorbent (AO-Fc-U) showed two
peaks at 390.9 eV and 380.1 eV assigned to U 4f\textsubscript{5/2} and U
4f\textsubscript{7/2} confirmed the presence of uranium on AO-Fc (Figure
11a). As shown in Figure 11b, two types of Nitrogen peaks were observed
at 400.05 eV and 398.4 eV assigned to N-H and C=N respectively. And also
in accordance with Figure 11c, the binding energy of both N-H and C=N
shifted to higher binding energy at 400.9 eV and 399.1 eV. The positive
shift in N-H and C=N groups confined the coordination of nitrogen and
uranium ions. To further analysis the uranium-loaded adsorbent was
analyzed by FTIR. Compared with the results (Figure 11d), the uranium
loaded spectra showed the shifts at various peaks or increase peak
intensity. What is more, in the FTIR spectrum of AO-Fc, two new peaks
were observed at 937 cm\textsuperscript{-1} and 1161
cm\textsuperscript{-1} confirming the presence of uranium on
AO-Fc\textsuperscript{50,51}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image21/image21}

\end{center}
\end{figure}

Figure 10 Uranium adsorbed AO-Fc exposed to natural seawater for 39 days
(a) and uranium striped AO-Fc with elution solution
Na\textsubscript{2}CO\textsubscript{3} and
H\textsubscript{2}O\textsubscript{2} (b)

Figure 11 High-resolution XPS spectra of U 4f (a), N 1s before (b) and
after uranium adsorption (c), and FTIR spectra (d)

\textbf{3.5 Adsorbent regeneration and elution efficiency}

Reusability is an important parameter for the industrial application of
adsorbents that influences the cost of the adsorbent. For the
reusability test, AO-Fc (7 mg) was used in 20 L seawater and the elution
solution (1 M Na\textsubscript{2}CO\textsubscript{3} and
H\textsubscript{2}O\textsubscript{2} 0.1 M) was fixed 3 mL per milligram
of seawater exposed adsorbent. For uranium elution, the elution
efficiency reached 99.9 \% within 20 min, indicating the desired elution
solution was better for uranium removal from AO-Fc (Figure 12a). The
overall decrease in adsorption capacity and elution efficiency of AO-Fc
were 1.8 \% and 2 \% up to seven cycles (Figure 12b,c), confirming the
adsorbent can be used for several cycles, which would significantly
decrease the uranium extraction cost. The decrease in adsorption
capacity and elution efficiency might be due to the strong binding of
uranyl ions with active functional groups. The adsorbent showed
excellent structural stability and negligible loss of adsorbent (Figure
S8) after seven adsorption-desorption cycles of AO-Fc (Figure 12b,c).

Figure 12 Elution efficiency of uranium in 1 M
Na\textsubscript{2}CO\textsubscript{3} and
H\textsubscript{2}O\textsubscript{2} 0.1 M (a), elution efficiency and
uranium adsorption capacity of AO-Fc from natural seawater (b,c)

\textbf{Conclusion}

In conclusion, the new low-cost and facile prepared amidoxime modified
adsorbent were analyzed qualitatively and quantitively and can be
considered as a potential adsorbent for uranium recovery from seawater.
AO-Fc showed significant adsorption capacity and higher uranium
selectivity, which is attributed due to amidoxime groups being present
onto the surface of the 2D-open channel. In addition, the addition of
amidoxime groups not only enhanced the adsorption capacity and cracks
the hydrophilicity but also adjusted the adsorbent pKa value.

The BET surface area of AO-Fc was lower as compared to Fc, indicating
the surface area was not a grim factor for the adsorption mechanism. The
uranium adsorption on AO-Fc was strongly dependent on pH value confirmed
the adsorption was mainly occurred due to surface complexation.

The experimental adsorption simulation by various isotherm models
exhibited that the R\textsuperscript{2} value of Langmuir isotherm was
close to unity obtained by linear regression. Also, the
q\textsubscript{e} calculated and the statistical analysis confirmed the
adsorption data were best fitted with the Langmuir isotherm than other
isotherm models.

Kinetic studies showed very fast uranium adsorption and the equilibrium
of adsorption was achieved in 11 days, which was a lower contact time as
compared to the adsorbents reported in Table 5 for uranium extraction
from natural seawater.

The desorption and regeneration showed the adsorbent is economically and
environmentally efficient. The higher adsorption capacity of AO-Fc
towards uranium exhibits that the materials can be potential adsorbents
for uranium extraction under seawater conditions.

\textbf{Acknowledgments}

This work was financially supported by the National Natural Science
Foundation of China (No.21975206).

\textbf{Conflict of interest}

The authors declare no conflict of interest.

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\textbf{Hosted file}

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