2. Experimental section
Materials
Wood (Chinese fir) was obtained from Heilongjiang province, China. AN
and AIBN, uranyl nitrate, sodium chloride, and sodium bicarbonate were
purchased from Changzhou Qidi Chemical Co., Ltd. DVB, hydroxylamine
hydrochloride (HAHC). NaOH and ethanol were obtained from Hubei Jusheng
Technology Co., Ltd. The double-distilled water was obtained from a
laboratory filtration plant.
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 oC with a heating rate of 10oC/min in a vacuum for 3 h. Thereafter, fir charcoal
(0.5 g) was added to a beaker containing AN (5 g) and AIBN (0.1 g) for
30 min. The sample was taken out and put in a glass tube (100 mm in
length and 30 mm in diameter). The glass tube was kept in an oven at 180oC for 5 h. To achieve highly polymer grafted
materials, 0.8 g of DVB was added to a solution of AN (5 g), and AIBN
(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 DVB in the reaction system.
Preparation of amidoxime modified charcoal (AO-Fc)
The nitrile group conversion was carried out by adopting a previous
method with small modifications36. HAHC (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 a 50 mL Teflon lined autoclave, which was kept at 70oC for 24 h. After cooling, the product was washed
with distilled water several times, dried in a vacuum at 60oC for 12 h and the obtained product was denoted as
AO-Fc.
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 oC/min under a
nitrogen atmosphere in a temperature range of 40-800oC. 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.
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 (qe 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 qe is the equilibrium adsorption capacity weight
per unit mass (mg/g) Co and Ce represent
initial concentration and equilibrium concentration weight per unit
volume (mg/L), V is the volume of solution in liters (L), and m
represents mass of adsorbent in grams (g).
To determine the adsorbent regeneration, the adsorbent (7 mg) was eluted
by using a 21 mL elution solution
(Na2CO3, 1 M and
H2O2, 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 Cel denotes elution concentration (mg/L),
Vel is the elution volume (L), Carepresents uranium concentration in seawater after adsorption (mg/L),
Co is the initial concentration of metal ions (mg/L) and
Va is the volume of seawater used for adsorption (L).
Results and discussion3.1 Adsorbent synthesis and characterization
To achieve efficient uranium extraction from seawater, it is very
difficult to prepare a highly porous adsorbent material with higher
selectivity and adsorption capacity for uranium in natural seawater.
Recently DVB has been used for the preparation of porous polymers for
separation purposes37. In this work, polymerization of
AN with AIBN as an initiator in presence of DVB as a crosslinking agent
has led to active porous polymer chains grafted onto the FC. A well as
well developed nanoporous polymer chains with large accessible nitrile
groups, which were then converted into amidoxime groups under alkaline
conditions for selective adsorption of uranium from seawater with
tunable adsorption capacity and surface
functionality38. Two polymers grafted carbonous
materials were synthesized by free radical polymerization, that is the
comonomer AN, and initiator AIBN was polymerized and grafted on FC which
was denoted as CN-FC1. By adding the DVB under the same set of reaction
conditions mentioned above, it was possible to adjust the pore size and
surface structure of the polymer chains grafted onto FC hereafter
denoted as CN-FC (Figure 1).