Introduction
Cellulose has been considered in the development of eco-friendly materials. Particularly, cellulose nanofibers (CNF) have raised interest for their suitability for water purification and heavy metal removal (Voisin et al.
2017). Chemical pretreatments have been used to reduce the energy consumed in CNF production and to introduce functional groups and charges on its surface. Such pretreatments include TEMPO oxidation, sulfonation, cationization, and phosphorylation (Klemm et al.
2018). The nanoscopic dimensions of CNF result in materials with a high surface area, which enhances interaction with metal ions (Bethke et al.
2018). Consequently, anionic CNF has been studied for removal of several heavy metals (Ma et al.
2012; Liu et al.
2015a). In this application, uranium is highly relevant given its occurrence in natural water as a result of leaching from mineral deposits and industrial processes, e.g., mining waters and nuclear fuel cycle facilities (WHO
2011; Kapnisti et al.
2018). In Finland, for example, concentrations as high as ~ 15 mg/L have been determined for uranium in water obtained from drilled wells (Asikainen and Kahlos
1979). Uranium removal from these waters is important since it is both hazardous to the environment and toxic to humans. It can cause a kidney failure due to its chemical toxicity, which is typically of greater concern compared to its radioactivity (Kapnisti et al.
2018).
In aqueous media, two stable oxidation states are common for uranium, U(IV) and U(VI) (Aly and Hamza
2013; Xie et al.
2019). Under aerobic conditions, uranium is present in aqueous solutions in its hexavalent form as uranyl ions (UO
22+) (Sylwester et al.
2000; Cai et al.
2017; Sarafraz et al.
2017), which predominate in acidic environments (Riegel and Schlitt
2017). As pH increases, hydrolysis of uranyl leads to the formation of
\(\left( {{\text{UO}}_{2} } \right)_{p} \left( {\text{OH}} \right)_{q}^{{\left( {2p - q} \right)}}\) species (Berto et al.
2012). The uranyl ion can also form various complexes with carbonates present in groundwater (Xie et al.
2019). Due to uranyl complexation, in this manuscript U(VI) is discussed instead of UO
22+ since we consider conditions where various UO
22+ complexes are present.
The presented information points to the urgent need of technologies that are effective for uranium removal or to prevent the release of toxic concentrations of uranium into the environment (Ghasemi Torkabad et al.
2017). Common techniques used for heavy metal extraction include membrane processes, ion exchange, adsorption, precipitation, and solvent extraction (Chen et al.
2017; Xue et al.
2017; Sarafraz et al.
2017). As a low-cost, easily applicable alternative, adsorption has been considered promising for the removal of U(VI) from aqueous solutions (Su et al.
2018). In such application, bioadsorbents are sought after given a number of advantages including costs, geographical availability and the possibility of metal recovery after incineration.
Due to the high affinity of phosphoryl groups to uranium (Zhou et al.
2015), phosphorylated materials such as lignin (Bykov and Ershov
2009), pine wood sawdust (Zhou et al.
2015), graphene oxide (Liu et al.
2015b; Chen et al.
2017), carbon spheres (Yu et al.
2014), chitosan (Sakaguchi et al.
1981; Morsy
2015), cactus fibers (Prodromou and Pashalidis
2013), polyethylene (Shao et al.
2017), zirconium (Um et al.
2007), and mesoporous silica (Sarafraz et al.
2017; Xue et al.
2017), have been demonstrated for uranium removal. Phosphorous-based functional groups act as chelating agents and thus favor binding of uranyl species (Xie et al.
2019). Phosphorylated cellulose nanomaterials have been studied for adsorption of Fe
3+ (Božič et al.
2014), Cu
2+, and Ag
+ (Liu et al.
2015a). In addition, CNF bearing bisphosphonate has been shown to be efficient for vanadium removal (Sirviö et al.
2016) while phosphorylated CNF nanopapers have been prepared for the removal of copper (Mautner et al.
2016). The removal of U(VI) has been studied with TEMPO oxidized CNF (TO-CNF), which achieved an adsorption capacity of 167 mg/g (Ma et al.
2012), and with carboxycellulose nanofibers prepared by nitro-oxidation, presenting a maximum adsorption capacity of 1470 mg/g (Sharma et al.
2017). However, to the best of our knowledge, there are no reports on the removal of U(VI) using phosphorylated CNF (PHO-CNF), which is surprising given the prospects indicated for other metals in the earlier studies.
In addition to chemical chelation, surface sorption mechanisms occur at the solid–liquid interface, such as physisorption, complexation, ion exchange, and precipitation, all of which play a crucial role for uranium removal (Xie et al.
2019). In this work, we study the removal of uranium with PHO-CNF using several batch adsorption approaches. Furthermore, we compare the uranium removal efficiency of PHO-CNF with different phosphorylation degrees with that determined for native CNF and TO-CNF. The nanocelluloses are characterized by Fourier transform infrared spectroscopy (FT-IR), zeta potential measurements, and transmission electron microscopy (TEM) imaging. Results from the scanning electron microscopy (SEM), TEM, and X-ray photoelectron spectroscopy (XPS) after adsorption are found to give insights into the adsorption mechanism of U(VI). The effect of pH on the adsorption of U(VI) and selectivity against other metals are also studied. Adsorption data at pH 6 are fitted to the Langmuir, Freundlich, and Sips isotherm models. The data was found to fit best with the Sips isotherm, demonstrating a maximum adsorption capacity of 1550 mg/g, the highest among the values reported so far for organic adsorbents.
Experimental
Materials
Native CNF was produced at Aalto University from bleached birch pulp by a method reported previously (Rajala et al.
2016). PHO-CNF and TO-CNF were obtained from Betulium Oy, Finland. The concentrations of phosphoryl groups provided by the manufacturer were 0.66 and 1 mmol/g for the PHO-CNF samples referred to as PHO-CNF
0.66 and PHO-CNF
1.00, respectively. The concentration of carboxylate groups in TO-CNF was 1 mmol/g. Uranyl acetate, Arsenazo III, ascorbic acid, perchloric acid, and nitric acid were all obtained from Sigma Aldrich. Milli-Q water (Millipore) was used for the preparation of all solutions.
Characterization of CNF
The electrostatic charge of CNF was determined with a Zetasizer Nano-ZS90 (Malvern), reported as zeta potential, and measured at pH 3, 5, 7, and 9 (adjusted by using 1 M HCl and NaOH) from 0.05 wt% CNF suspensions. The different types of CNF were imaged with a FEI Tecnai 12 TEM operating at 120 kV. For sample preparation, 3 µL of 0.01 wt% CNF suspension was drop casted on a copper grid with an ultrathin carbon support film and the excess solution was blotted with filter paper after 1 min of contact time, followed by drying under ambient conditions. Thereafter, 3 µL of 2% uranyl acetate was drop casted onto the dried CNF sample in order to stain the sample. The excess solution was blotted with filter paper after 1 min of contact time, followed by drying under ambient conditions. FT-IR spectra of freeze-dried CNF samples were recorded with Nicolet 380 FT-IR Spectrometer using an ATR accessory. The spectra were recorded in the region of 400–4000 cm−1 with 0.5 cm−1 intervals.
Adsorption experiments
A dry mass of 5 mg of the respective nanocellulose and a total volume of 15 mL of solution were used in the adsorption experiments, unless otherwise mentioned. Experiments were conducted at room temperature (21–22 °C), which remained stable throughout the experiments. A stock solution of 2000 mg/L uranyl acetate was used to prepare the solutions. The adsorption isotherm studies were conducted with initial uranium concentrations of 10, 25, 50, 100, 200, 300, 400, and 500 mg/L. Experiments at different pH and with different CNF types were performed using an initial concentration of 100 mg/L uranium. For the isotherm study, for the comparison between CNF types, and for the selectivity study, the pH was adjusted to 6 using 2 M HCl and NaOH. In all experiments, glass vials were filled with 15 mL of the suspension containing uranium and CNF and sonicated to disperse the fibrils in an ultrasonic bath at 37 kHz for 5 min. The samples were then left in a shaker at 200 rpm for 55 min to reach the equilibrium. After the adsorption process, samples were taken from the solutions and filtered with 0.1 µm filters (Whatman). Samples without CNF having similar initial U(VI) concentrations were used as controls to analyze any possible adsorption of U(VI) onto the filters. Based on this analysis, the adsorption of uranium onto the filters was found to be negligible. Uranium concentrations were determined spectrophotometrically with Arsenazo III method (Khan et al.
2006) using a microplate reader (Synergy H1) to determine the absorbance of the solutions at 651 nm. Briefly, 25 μL of ascorbic acid (100 g/L), 175 μL of Arsenazo III (0.07 w/v% in 3 M perchloric acid) and 50 μL of sample were added to the wells of a microwell plate. If necessary, the samples were diluted to reach a maximum U(VI) concentration of 10 mg/L before mixing with ascorbic acid and Arsenazo III. For each sample, two parallel measurements were conducted with the plate reader and the average of these values reported.
Selectivity experiments were conducted using a concentration of 10 mg/L for all the metals tested (U, Zn, Mn, and Cu). Other typical ions present in natural waters were also added to the solution according to Table S1 (adapted from Sankar et al.
2013). In the selectivity tests, two different amounts of PHO-CNF
1.00 were used, 5 mg and 0.25 mg (values given as dry mass in 15 mL solution). Metal concentrations in the samples used in the selectivity tests were analyzed with inductively coupled plasma mass spectrometry (ICP-MS) (PerkinElmer, NexION 300X). For ICP-MS analysis, the samples were diluted to a maximum concentration of 1 mg/L and digested with 5% (vol.) concentrated HNO
3 (68–70%) before analysis.
The percentage of U(VI) removed in adsorption studies was calculated based on Eq. (
1):
$$U\;removal\;\left( \% \right) = \frac{{C_{0} - C_{f} }}{{C_{0} }} \times 100\%$$
(1)
and the equilibrium adsorption capacities (
qe) were calculated using Eq. (
2):
$$q_{e} = \frac{{\left( {C_{0} - C_{e} } \right)V}}{m}$$
(2)
where
C0,
Cf, and
Ce are the initial, final and equilibrium concentrations (mg/L) of U(VI), respectively,
V is the volume (L) and
m is the mass of adsorbent (g) used.
Langmuir, Freundlich, and Sips isotherm models were used for fitting the experimental adsorption data. The Langmuir and Freundlich isotherm models are shown in Eqs. (
3) and (
4), respectively:
$$\frac{{C_{e} }}{{q_{e} }} = \frac{{C_{e} }}{{q_{max} }} + \frac{1}{{q_{max} K_{L} }}$$
(3)
$${ \ln }q_{e} = { \ln }K_{F} + \frac{1}{n}{ \ln }C_{e}$$
(4)
where
qmax is the maximum adsorption capacity (mg/g) and
KL is the Langmuir adsorption constant (L/mg).
KF is the Freundlich isotherm constant and
n is the dimensionless heterogeneity factor.
The Sips isotherm model, a combination of the Langmuir and Freundlich isotherms, is expressed as Eq. (
5):
$$\ln \left( {\frac{{q_{e} }}{{q_{m} - q_{e} }}} \right) = \frac{1}{n}\ln C_{e} + \ln K_{s}$$
(5)
where
qm is the maximum adsorption capacity (mg/g) and
Ks is the median association constant.
Characterization of PHO-CNF1.00 after adsorption
The uranium stock solution was diluted to reach final concentrations of 50, 100, 250, and 500 mg/L. After mixing with PHO-CNF1.00, sonication and shaking as described earlier, a 10 μL drop of the PHO-CNF1.00 uranium suspension was drop casted onto the carbon tape on aluminum stubs and the stubs were placed in a -80 °C freezer overnight and freeze dried at − 50 °C. For SEM imaging, the samples were sputter-coated with 3 nm of platinum-palladium and observed using an acceleration voltage of 1.6 kV with a scanning electron microscope (Zeiss Sigma VP). TEM samples for imaging after U(VI) adsorption were prepared by drop casting from the suspension with initial U(VI) concentration of 100 mg/L similarly as described in the characterization section without additional uranium staining.
X-ray photoelectron spectroscopy
Samples for XPS were prepared by vacuum filtration of PHO-CNF
1.00 onto 0.1 μm filters after adsorption of U(VI) from the initial concentrations of 0, 100 mg/L, and 500 mg/L. After vacuum filtration, the filter cakes were frozen at − 80 °C and freeze-dried to obtain dry films. An electron spectrometer (AXIS Ultra, Kratos Analytical, UK) with monochromatic Al Kα irradiation at 100 W under neutralization was used for the measurements. Three different spots from each film were scanned and elemental surface compositions of the films were determined from low resolution survey scans. High resolution measurements of uranium U 4f, carbon C1
s and oxygen O1
s were also conducted. Pure cellulose filter paper (Whatman) was used as an in situ reference in all measurements. Data analysis was performed with CasaXPS software, using fitting parameters customized for celluloses and the C–O component of the high resolution C1
s at 286.7 eV as the energy reference for all the spectra (Beamson and Briggs
1992; Johansson and Campbell
2004).
Uranium speciation
An ion speciation software (PHREEQC) was used to determine the uranyl speciation at pH range 3–7, and in the simulated drinking water used for selectivity experiments (Tables S2 and S3). The U(VI) concentration used for the calculations was 100 mg/L.
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