2. Experimental section
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.
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. 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 oC 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.
Preparation of amidoxime modified charcoal (AO-Fc)
The amidoximation process was carried out by adopting a previous method
with small modifications36. 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 oC for 24 h. After
cooling, the product was washed with distilled water several times,
dried in a vacuum at 60 oC for 12 h and the obtained
product was 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 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
(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 initial concentration of metal ions (mg/L) and
Va volume of seawater used for adsorption (L).
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.