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Highly Efficient Interception and Precipitation of Uranium (VI) from Aqueous Solution by Iron-Electrocoagulation Combined with Cooperative Chelation by Organic Ligands Peng Li, Bao Zhun, Xuegang Wang, Ping Ping Liao, Guanghui Wang, Lizhang Wang, Yadan Guo, and Weimin Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05288 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017
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Highly Efficient Interception and Precipitation of Uranium (VI) from
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Aqueous Solution by Iron-Electrocoagulation Combined with
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Cooperative Chelation by Organic Ligands
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Peng Lia,b* , Bao Zhunb, Xuegang Wanga,b, PingPing Liaob, Guanghui Wanga,b, Lizhang Wangc* Yadan Guoa,b and Weimin Zhanga,b a
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China University of Technology, Nanchang City, Jiangxi 330013, China, PR China
b
School of Water Resource & Environmental Engineering, East China University of Technology, Nanchang City, Jiangxi 330013, PR China c
School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou City, Jiangsu 221008, PR China
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ABSTRACT: A new strategy combining iron-electrocoagulation and organic ligands (OGLs)
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cooperative chelation was proposed to screen and precipitate low concentrations (0–18.52 µmol/L)
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of uranium contaminant in aqueous solution. We hypothesized that OGLs with amino, hydroxyl,
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and carboxyl groups, hydrophobically/hydrophiliclly would realize precuring of uranyl ion at
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pH IDA >
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EDTA > PANI, which is in line with their uranium removal performance in the bulk experiments.
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Interestingly, Figure 1 shows that the optimum pH for uranium iron-electrocoagulation after the
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cooperative chelation by OGLs remained at 6.0. This was corroborated by the Zeta potential
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measurements of the obtained precipitated flocs at different pH conditions in Figure 2b. The zeta
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potential at pH 6.0 was much lower than at other pH conditions, while the reactions in the
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iron-electrocoagulation system reached the highest level. The generated iron oxides and
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hydroxides would be important for screening the U-OGLs and the accumulative crystallization.
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The identical optimum solution pH for uranium removal by iron-electrocoagulation in the
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presence and in the absence of cooperative chelation by OGLs also indicates that analogous
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precipitate evolution processes took place before and after the intercalation of OGLs, and that the
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added OGLs did not affect the intrinsic electrochemical reactions of the iron-electrocoagulation
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system.
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Uranium concentration (mg/L)
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Solution pH
Solution pH
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Solution pH
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Initial After Treatment
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d)
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0
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c)
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4.5
6.0
7.5
9.0
Solution pH
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Figure 1. Precipitation of uranium from water with iron-electrocoagulation in the absence and in presence of
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organic ligands: a) in the absence organic ligands; b) IDA; c) Alizarin S; d) PANI; e) EDTA. Initial uranium
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concentrations were in the range of 8.35 to 15.70 µmol/L (2.255 to 4.238 mg/L) as indicated in the figure. The
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current density 0.6 mA/cm2, OGLs-to-U (VI) mole ratio 3:1. The solid blue circles represent the initial
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concentrations of uranium in aqueous solution, while the empty red ones symbol those after 24 mins reactions
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-5.0
a) Zeta potential (mV)
Zeta potential (mV)
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-6.0 -6.5 -7.0 -7.5
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b)
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U with no OGLs
EDAT
Alizarin S
PANI
IDA
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Different ion-electrocoagulation condition
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6
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Solution pH
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Figure 2. Zeta potential of the precipitated flocs obtained at (a) different ion-electrocoagulation condition and (b)
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Alizarin S as organic ligands at different solution pH values. The experimental condition was initial uranium
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concentrations 8.35 to 15.70 µmol/L (2.255 to 4.238 mg/L), current density 0.6 mA/cm2, and OGLs-to-U (VI)
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mole ratio 3:1
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Alizarin S, the best organic ligand for cooperative chelation in the efficient interception and
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precipitation of uranium (VI) from water by iron-electrocoagulation, was selected for the next
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steps of testing. Critical parameters that affect the uranium precipitation, including solution pH,
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applied current density, OGLs-to-U(VI) mole ratio, and initial uranium concentration were
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comprehensively investigated, and the results are presented in Figure 3. As depicted in Figure 3a,
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the optimum solution pH was 6.0, which was consistent with the experimental results in Figures 1
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and 2. A higher applied current density would increase the uranium removal efficiency (Figure 3b),
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which was 60.29%, 73.22%, 90.96%, and 98.50% at the current density of 0.6, 0.8, 1.0, and 1.4
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mA/cm2, respectively. In the presence of Alizarin S, the iron-electrocoagulation method could
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effectively remove uranium to meet the discharge standard, by using a low cell voltage (2.2 V for i
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= 1.0 mA/cm2 and 2.6 V for 1.4 mA/cm2) and a short treatment time. Therefore, both high
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uranium removal efficiency and low treatment cost could be achieved with the proposed
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iron-electrocoagulation method when combined with the OGLs cooperative chelation.
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An appropriate amount of added OGLs is necessary to reach a satisfactory uranium removal
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efficiency, by improving the uranium interception and reducing the Zeta potential of the growing
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flocculation precursor. As shown in Figure 3c, when the OGLs-to-U(VI) mole ratio was small
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(3:1) means that the
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uranyl ion would be encapsulated by excess organic ligands, producing a higher net negative
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charge. As a result, a higher input cell voltage is required to compress the newly formed double
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layer structure. Meanwhile, the rubber and bonded organic ligands would affect the settlement
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characteristics of the generated flocs. The OGLs-to-U(VI) mole ratio of 3:1 was found to be the
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optimal for uranium iron-electrocoagulation precipitation. At this ligand mole ratio, current
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density of 0.6 mA/cm2, initial uranium concentration of 11.36 µmol/L (3.067 mg/L), and solution
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pH of 6.0, the uranium removal efficiency reached 99.65% (Figure 1c). The change of Fe3+
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concentration with Alizarin S as OGL under various pH conditions in the iron-electrocoagulation
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system is provided in SI, Table S2. At low pH values, a high Fe3+ concentration was detected, and
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it decreased dramatically with increasing of the solution pH value. At pH = 6.0, a very low Fe3+
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concentration was detected in the effluent, indicating that Fe3+ pollution can be avoided.
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The initial uranium concentration affects its removal efficiency and determines the bulk
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electrolysis time needed to achieve an acceptable effluent concentration. Figure 3d shows that the
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uranium removal efficiency decreases with increasing initial uranium concentration, being 60.29%,
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49.95%, 32.13%, and 24.95% for the respective initial uranium concentrations of 12.57, 27.31,
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38.80, and 53.87 µmol/L (3.395, 7.375, 10.475, and 14.544 mg/L), under the operating conditions
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of solution pH 3.0, current density 0.6 mA/cm2, and OGLs-to-U(VI) mole ratio of 3:1. The results
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revealed that uranium precipitation with the iron-electrocoagulation method was an energy reigned
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electrochemical reaction, and the amount of the formed flocculation precursor (iron oxides/
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hydroxides) determined the uranium removal efficiency. A high initial uranium concentration
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should be accompanied by a high current density. After the intercalation of organic ligands, the
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uranium precipitation process was found to also follow the second-order kinetic model (SI, Figure
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S3-6 and Table S1).
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a)
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Uranium concentration (mg/L)
Uranium concentration (mg/L)
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d) c0=14.544 mg/L
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Figure 3 Uranium concentration on dependent of reaction time with iron-electrocoagulation in presence of Alizarin
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S at different pH (a) (i=0.6 mA/cm2, OGLs-to-U(VI)=3:1, c0=11.11-18.52 µmol/L (3.0-5.0 mg/L)), current density
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(b) (pH 3.0, OGLs-to-U(VI)=3:1, c0=11.11-18.52 µmol/L (3.0-5.0 mg/L)), different OGLs-to-U(VI) molar ratio (c)
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(pH 3.0, i=0.6 mA/cm2, c0=11.11-18.52 µmol/L (3.0-5.0 mg/L)), and initial uranium concentration 0 to 55.56
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µmol/L (0 to 15.0 mg/L) (d) (pH 3.0, i=0.6 mA/cm2, OGLs-to-U(VI)=3:1)
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Elimination of Organic Ligand Pollution. Figure 4 shows that adding the OGLs could raise the
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organic pollution index, with the chemical oxygen demand (CODCr) value ranging from 40 to 80
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mg/L. Fortunately, the introduced organics would also be removed during the subsequent
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iron-electrocoagulation process. Hydrophilic macromolecule such as Alizarin S and EDTA (with
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hydrophilic hydroxyl and carboxyl groups, respectively) present higher precipitation efficiency
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than the hydrophilic small molecule IDA and the hydrophobic macromolecule of PANI. The COD
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removal efficiency with IDA, PANI, EDTA, and Alizarin S as organic ligands were 82.97%,
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71.61%, 88.97%, and 86.89%, respectively. The different CODCr removal for different organic
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ligands would be ascribed to their affinity and nucleation-promotion capacity with the flocculation
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precursors generated in the iron-electrocoagulation system. The hydrophilic substances would be
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easily captured and encapsulated in the crystallized ferrite species. In other words, the hydrophilic
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groups have an intrinsic affinity for the iron hydroxides. The larger molecular weight and fine
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hydrophilicity of Alizarin S that endows adequate flocculation reactions in iron-electrocoagulation
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system also facilitate its uranium precipitation in U-Alizarin S chelates.
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a)
b) COD removal efficiency (%)
120 100 80 60 40 20 0 IDA
PANI
EDTA
Alizarin S
OGLs species
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Figurer 4 The COD value (a) and removal efficiency (b) during uranium iron-electrocoagulation with different
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organic ligands species. Experimental condition: pH 6.0, current density 0.6 mA/cm2, OGLs-to-U(VI) mole ratio
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3:1, initial uranium concentration 11.11 to 18.52 µmol/L (3.0-5.0 mg/L)
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Mechanism of Uranium Precipitation in Iron-electrocoagulation System. The precipitated
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flocs in iron-electrocoagulation system were collected for the following three cases under the
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initial uranium concentration of 11.11–18.52 µmol/L (3.0–5.0 mg/L): (1) with no uranium and
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OGLs, (2) with uranium but no OGLs, and (3) with uranium and Alizarin S as OGL. The flocs
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were characterized with SEM, elemental mapping, EDS, and TEM methods for their
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microstructure observation and elemental composition detection. XRD, FT-IR, and XPS
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techniques were employed to explore the corresponding precipitation mechanisms. As shown in
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Figure 5a, d, and g, the colloidal aggregates of pure iron oxides/hydroxides were flake-shaped,
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fully crystallized nanoparticles with size of appropriately 26.2 nm. The fine crystals on the
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surfaces were sodium sulfate granules separated out during the bulk electrolysis. Once the
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uranium participated in the iron-electrocoagulation reactions, crystalline uranium composites were
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observed on the surface and edge of the particles of the iron oxides and hydroxides (Figure 5b),
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and the morphology of the precipitated flocs became disorganized with a defective lattice structure
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and marginal fault (shown in Figure 5b, e, and h). The size of the formed nanoparticles decreased
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to 8.6 nm. After the intercalation of Alizarin S, the SEM image (Figure 5c) showed that fine
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nanoparticles of uranium composites were gathered on the particle surface and edge of pure iron
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oxides and hydroxides, the lattice structure of the flocs became increasingly disordered, and the
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size of the formed nanoparticles further decreased to 4.7 nm. Uranium composites were also found
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as fine nanoparticles on the surface and edge of iron oxides and hydroxides when IDA, PANI, or
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EDTA was used as organic ligands in the iron-electrocoagulation system, with the respective
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nanoparticle size of 4.7, 2.6, and
