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).