Adsorption Performances and Mechanisms of Amidoxime Resin

Nov 24, 2015 - Negative values of ΔG indicated that adsorption processes for both metal ... And recovery of gallium from the Bayer process, which is ...
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Research Article pubs.acs.org/journal/ascecg

Adsorption Performances and Mechanisms of Amidoxime Resin toward Gallium(III) and Vanadium(V) from Bayer Liquor Zhuo Zhao,*,† Xiaohang Li,† Yanquan Chai,† Zhongsheng Hua,† Yanping Xiao,‡ and Yongxiang Yang‡ †

School of Metallurgical Engineering, Anhui University of Technology, 243002 Maanshan, Peoples’ Republic of China Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands

‡

ABSTRACT: The adsorption of Ga(III) and V(V) onto an amidoxime resin (LSC700) has been investigated. The parameters studied include the effects of contact time, initial metal ions concentration, and temperature by batch method. The adsorption capacity of the resin for Ga(III) and V(V) was found to be 29.24 and 22.60 mg/g, respectively. The adsorption kinetics of Ga(III) and V(V) onto LSC700 could be well elucidated with Lagergrensecond-order equation. The Langmuir isotherm fitted the experimental data well, indicating a homogeneous adsorption. Thermodynamic parameters, involving ΔG, ΔH, and ΔS were also calculated from graphical interpretation of the experimental data. Negative values of ΔG indicated that adsorption processes for both metal ions onto LSC700 were spontaneous. The standard heats of adsorption ΔH were found to be endothermic, and the entropy change values ΔS were calculated to be positive for the adsorption. The adsorption mechanism of LSC700 toward Ga(III) was exploited using FT-IR spectroscopy indicating the formation of a complex between the amidoxime group of LSC700 and [Ga(OH)4]− through oxygen binding mode. Finally, a novel process for separation and recovery of gallium and vanadium from Bayer liquor was proposed. KEYWORDS: Gallium, Vanadium, Kinetics, Isotherms, Adsorption mechanisms

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INTRODUCTION Gallium is a scattered metallic element, with a content of 10 ppm in the Earth’s crust.1 Vanadium has an abundance in nature of 150 ppm, similar to zinc and more common than copper or nickel.2 Although gallium and vanadium are widely spread, they are considered quite rare,3 because they are generally not found as discrete minerals but instead occur in combination. For example, Ga is mainly concentrated in bauxite and zinc minerals. Trace amounts of Ga are also found in aluminosilicate reserves and clays such as diaspore and kaolin.4 Vanadium is never found in its pure mineral state, but it occurs in combination with various minerals which include carnotite (K 2 (UO 2 ) 2 (V 2 O 8 )·1−3H 2 O), roscoelite (K(V 3 + , Al, Mg)2AlSi3O10(OH)2), vanadinite (Pb5(VO4)3Cl), mottramite (PbCu(VO4) (OH)), and patronite (V4+(S22−)2).5 Due to the widespread distribution of these elements, it is not considered economic to mine mineral deposits merely for the purposes of recovering Ga or V; therefore they are usually obtained as byproducts, from the processing of other minerals. Although recovered as an adjunct to the processing of other metals, there is good reason to maximize the economic recovery of these two elements. Gallium has attracted much research interest, in particular due to the excellent semiconducting properties of materials where Ga is combined with group VA elements. For example, gallium arsenide (GaAs) and gallium nitride (GaN) are suitable for the manufacture of high technological optical devices, such as advanced semiconductors, DVD’s, laser diodes, and other electronic applications.4 © XXXX American Chemical Society

Vanadium is an important product that is used in ferrous and nonferrous alloys due to its high tensile strength, hardness, and fatigue resistance.6 Furthermore, there is significant interest in vanadium redox chemistry,7 new generation batteries, and capacitors.8 Due to increased use of Ga and V in these new applications, present primary resources are considered to be insufficient to satisfy increasing demand.9 A significant amount of gallium and vanadium is present in bauxite;10 the world supply of recoverable gallium hosted in bauxite deposits was reported on the order of 2 million tonnes. And recovery of gallium from the Bayer process, which is generally used to refine bauxite to alumina, is now the only way to get gallium from bauxite. In addition, it is estimated that 10− 50 kilotonnes of V2O5 enters the Bayer process. As such, Bayer liquor presents a significant resource for these increasingly in demand elements and any improvement to extracting Ga and V from these liquors is likely to be welcomed as an alternate revenue source. Many methods have been developed for recovering gallium from Bayer liquor,11 including fractional precipitation (lime method and carbonation method), electrochemical methods (mercury cathode electrolysis,12 cementation13), solvent extraction, and ion exchange. The fractional precipitation method introduces lime or CO2 for coprecipitation of gallium Received: April 18, 2015 Revised: October 10, 2015

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DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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spectra for the resin before and after adsorption were recorded on a Bruker TENSOR 27. After being dried, about 25 mg of resin was ground and mixed with 3600 mg of KBr powder, using an agate mortar and pestle. Aliquots of 200 mg of the powder mixture were compressed to form discs (13 mm diameter) in an evacuated stainless steel die for the analysis. FTIR spectra were acquired in the 4000−400 cm−1 wavelength region with 4 cm−1 resolution. Adsorption Methodology. Adsorption Capacity. Adsorption experiments were carried out at 298 K in a 250 mL glass bottle with a stopper. A total of 1 g of LSC700 was introduced to a 200 mL Bayer liquor. The bottle was then shaken for 24 h under 200 rpm. After the separation of resin from the solution, the concentration of gallium and vanadium was analyzed by ICP. The adsorption capacity (Q) of LSC700 for gallium/vanadium and distribution coefficient (D) were calculated according to eqs 1 and 2.

oxide and alumina; subsequently the precipitates are treated by several purification steps and continued with a sodium hydroxide dissolution.14 One major problem of this method involves the destruction of the aluminate solution, which is changed to contain 60 g/L of NaHCO3; it therefore cannot be reintroduced into the Bayer process without further treatment.11 The electrochemical method utilizes aluminate solution itself as an electrolyte and mercury as a cathode; gallium can be deposited at the cathode by electrolysis to form amalgam. This method makes possible the recovery of gallium without destroying the circulating liquor. However, it always involves the problems of the coelectrodeposition of other impurities such as vanadium due to the high electrolysis potential. In addition, the solubility of gallium in mercury is only 1.3%, resulting in an excessive use of a large amount of mercury and special safety precautions.12 During the past two decades, recovering gallium from Bayer liquor by solvent extraction has attracted numerous investigations, and usually they adopted Kelex 100 as a solvent due to its good extraction ability for gallium in aluminate solution.15 However, the main disadvantages of this method include low extraction rate, high cost, and sensitivity to process parameters. Ion exchange has been accepted as the most efficient method, especially in China, and is now widely applied in industrial practice due to its compact process circuit, fast reaction kinetics, high recovery, and good selectivity for gallium over aluminum.11However, the coextraction of vanadium remains as the major unsolved problem.16 It has been found that during the extraction of gallium by ion-exchange resin, a considerable amount of vanadium was also extracted and was difficult to elute, resulting in vanadium buildup in the resin upon recirculation, thus reducing the resin loading capacity for gallium extraction. Simultaneously, a serious operational problem such as frequent change of the ion exchange column would be caused by the buildup of vanadium. Possible solutions, such as shortening the contact time between Bayer liquor and the resin and scrubbing with concentrated sodium hydroxide, were proposed. However, as a result, the gallium recovery would decrease and some side effects such as the generation of some wastewater would happen. In this study, a newly developed resin was employed to systematically investigate the adsorption of gallium and vanadium from Bayer liquor. Based on the clarification of adsorption mechanisms, a novel pathway for separation and recovery of gallium and vanadium from Bayer liquor is proposed.

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Q=

(C0 − Ct )V W

(1)

D=

C0 − Ct V × Ct W

(2)

where Q is the adsorption capacity (mg/g), C0 and Ct are the initial concentration and the concentration at time t, respectively, of gallium/ vanadium in solution (mg/L), V is the solution volume (L), and W is the dry weight of resin (g). Each determination in the adsorption procedures was repeated three times, and the results were given as average values. Adsorption Kinetics. The adsorption kinetics tests were performed as follows: 1 g of LSC700 was weighed and added into a flask, in which 200 mL of Bayer liquor was added. The flask was then shaken in a shaker at a predetermined temperature. A total of 1 mL of the upper layer of clear solution was taken for analysis at desirable intervals to determine the concentration of gallium/vanadium by ICP. Isothermal Adsorption. Adsorption isotherms were studied by mixing 1 g of LSC700 with 200 mL of metal ion solution of various initial concentrations and temperatures. After 24 h of shaking, the adsorption capacities were calculated also by eq 1.

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RESULTS AND DISCUSSION Characterization of the Resins. FTIR spectra of the resin (Figure 1) were consistent with the presence of an amidoxime

EXPERIMENTAL SECTION

Materials. A strongly basic resin containing an amidoxime functional moiety (LSC700), newly developed by Xi’an electric power resin factory (Xi’an, China), was adopted for the extraction of gallium from Bayer liquor. The Bayer liquor was provided by the pilot plant of Zhengzhou Research Institute of Chalco. Table 1 shows the composition of the Bayer liquor, in which αk means the molar ratio between Na2Ok and Al2O3 in the liquor. Other reagents and solvents were of analytical grade (Tianjin Dern Chemical Regent Limited Company) and used without further purification. Instrument. The concentrations of gallium and vanadium ions in Bayer liquor were measured on an ICPE-9000 (Shimadzu). FTIR

Figure 1. FT-IR spectra of the resin used in experiments.

group. A band at 3399 cm−1 is characteristic of NH2 and OH stretching vibrations. The bands for CN and N−O are seen at 1647 and 935 cm−1, respectively. Other peaks were observed at 2928 and 1385 cm−1, which was ascribed to the stretching vibrations of −CH2− and −CH3. According to the FT-IR spectra, the amidoxime group (−C(NOH)NH2) in the resin was confirmed.

Table 1. Chemical Composition of Bayer Liquor (g/L) Na2Ok

Al2O3

αk

Na2OT

SiO2

Ga(III)

V(V)

149.02

80.71

3.04

175.73

0.78

0.19

0.12 B

DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Adsorption Capacity. According to adsorption experiments data, the adsorption capacities of LSC700 toward gallium and vanadium were calculated in Table 2. From Table 2, it could be note that the LSC700 had high adsorption capacity for Ga(III) and V(V).17,26 Table 2. Adsorption Capacities of Resin for Metal Ions ions

adsorption capacity (mg/g)

adsorption capacity (mg/g) in other literature

Ga(III) V(V)

29.24 22.60

5.75,26 8.6017 1−426

Adsorption Kinetics. The effects of contact time on the adsorption of Ga(III) and V(V) on LSC700 (Figure 2a) show that absorption was rapid for the first 60 min, when the amount absorbed reached 28.01 and 18.59 mg/g, respectively, and then continued with a slower rate. Experimental results show that the amount of adsorbed Ga(III) and V(V) increased with time and approached equilibrium at 120 min for Ga(III) and about 180 min for V(V). Many kinetic models have been developed in order to find intrinsic kinetic adsorption constants. Usually, pseudo-firstorder,18 pseudo-second-order,19 and intraparticle diffusion20 models were used to test the experimental data and thus elucidate the adsorption kinetic process. From the linear form of these three models, equations can be written as follows: First‐order kinetic equation: log(Q 0 − Q e) = log Q 0 −

k1 t 2.303

Second‐order kinetic equation:

(3)

t 1 1 = + t 2 Qe Q0 k 2Q 0

Intraparticle diffusion equation: Q e = k intt 1/2

(4) (5)

where k1 is the rate constant of pseudo-first-order adsorption (h−1), k2 is the rate constant of pseudo-second-order adsorption (g/mg), kint is the rate constant of intraparticle diffusion, and Q0 and Qe are the adsorption uptake at equilibrium and at time t, respectively (mg/g). All three of the models were used to fit the kinetics curves; the results are shown in Figure 2b−d. The parameters of the three equations are tabulated in Table 3. As seen from Figure 2b, the Lagergren first-order model fits the data well for initial time, and thereafter the data did not obey the theory. The calculated Q0 values were not in agreement with the experimental Q0 values, suggesting that the adsorption of Ga(III) and V(V) does not follow pseudofirst-order kinetics. Figure 2c shows that the obtained R2 values are above 0.9973. Moreover, the calculated Q0 values are in good agreement with experimental Q0 values. Hence, the pseudo-second-order model appears to be more suitable to describe the adsorption kinetics of Ga(III) and V(V) onto LSC700. Figure 2d indicates a multilinearity with three distinct phases. The initial curved portion relates to the external surface adsorption. The second portion describes the gradual adsorption stage, where intraparticle diffusion control is rate limiting and the final portion (plateau) is attributed to the equilibrium stage. As seen from Figure 2d, the straight line did not pass through the origin, suggesting that intraparticle diffusion may not be the only rate controlling step.

Figure 2. (a) Effect of contact time on the adsorption of Ga(III) and V(V) (under the temperature of 298 K, the stirring rate of 200 rpm). (b) Pseudo-first-order kinetic plots for the adsorption of Ga(III) and V(V) onto LSC700 at 298 K. (c) Pseudo-second-order kinetic plots for the adsorption of Ga(III) and V(V) onto LSC700 at 298 K. (d) Intraparticle diffusion model plots for the adsorption of Ga(III) and V(V) onto LSC700 at 298 K.

Adsorption Isotherm. The isotherm curves for the adsorption of Ga(III) and V(V) onto LSC700 is shown in Figure 3. The isotherm results reveal that the adsorption capacity for both metals increased with the increase of initial C

DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 3. Kinetic Parameters of Ga(III) Adsorption on LSC-600 at Different Temperatures pseudo-first-order

pseudo-second-order

intraparticle diffusion

metal ion

Q0(exp) (mg/g)

k1 (min−1)

Q0(cal) (mg/g)

R2

k2 (g/mg min)

Q0(cal) (mg/g)

R2

kint (mg/g min1/2)

R2

Ga(III) V(V)

29.24 22.60

0.0129 0.0094

9.25 12.65

0.8398 0.8911

0.0041 0.0020

29.65 22.83

0.9998 0.9973

1.04 1.13

0.6288 0.7533

The two most commonly used isotherms, Langmuir and Freundlich, were adopted for further investigation of the adsorption capacity. The linear equations of these two isotherms are represented as follows: Langmuir isotherm:

Ce C 1 = + e Qe bQ 0 Q0

Freundlich isotherm: ln Q e = ln KF +

(6)

1 ln Ce n

(7)

where Qe is the adsorption capacity (mg/g), Ce is the equilibrium concentration of Ga(III)/V(V) (mg/L), Q0 is the saturated adsorption capacity (mg/g), b is an empirical parameter, n is the Freundlich constant related to adsorption intensity, and KF is the binding energy constant reflecting the affinity of the resin for the metal ion. As shown in Table 4 and Figure 4, the regression coefficients R2 indicate that the experimental data are suitable described by the Langmuir isotherm (R2 > 0.99). High temperature is favorable for the adsorption processes for both metals, indicating an endothermic adsorption. As is well-known, the Langmuir equation is applicable to homogeneous adsorption, where the adsorption of each sorbate molecule on to the surface has an equal adsorption activation energy. Conversely, the Freundlich equation is employed to describe heterogeneous systems and reversible adsorption and is not restricted to the formation of a monolayer. Therefore, the adsorption of Ga(III) and V(V) on LSC700 at the temperature range of 298−318 K is homogeneous. In addition, the predicted Q0 for Ga(III) and V(V) is greater than those observed, which may be due to the metal ions having difficulty accessing all of the potential binding sites on the matrix. Adsorption Thermodynamics. The effect of temperature on the adsorption of Ga(III) and V(V) onto LSC700 was investigated by determining the adsorption isotherms at 298, 308, and 318 K. Bayer liquor with initial concentrations of Ga(III) at 190 mg/L and V(V) at 120 mg/L was used in these experiments. Based on experimental data, the distribution coefficients D at different temperature were calculated. It can be found that D increased with the increase of temperature, confirming the endothermic adsorption nature. Plotting log D against 1/T, two straight lines could be obtained (Figure 5). There are three important thermodynamic parameters, i.e. enthalpy ΔH, entropy ΔS, and Gibb’s free energy ΔG,

Figure 3. Adsorption isotherms of (a) Ga(III) and (b) V(V) at 298 K, 308 K, 318 K (under the initial concentration of Ga(III): 100 mg/L, 189 mg/L, 300 mg/L, 400 mg/L, 500 mg/L, 600 mg/L and V(V): 120 mg/L, 200 mg/L, 300 mg/L, 400 mg/L, 500 mg/L, 600 mg/L respectively).

concentration. The adsorption capacity of V(V) increased quickly in the low vanadium concentration ranging from 0 to 120 mg/L and was kept unchanged at higher initial concentrations. This sensitivity of adsorption under the lower concentration of metal ions indicates that the adsorption is dominated by the electrostatic attraction (physical adsorption).21 Consequently, the electrostatic attraction does play a primary role in the complex formation between V(V) ions and the functional group in the resin. In addition, the adsorption of Ga(III) is relatively insensitive with the initial concentration, which indicates that the adsorption may be dominated by the chelating interaction (chemical adsorption). Figure 3 also indicates that LSC700 has good adsorption capacity for Ga(III) and V(V). Table 4. Coefficients of Langmuir and Freundlich isotherm Langmuir model

Freundlich model

adsorbates

T (K)

Q0 (mg/g)

b(L/mg)

R2

KF (mg/g)

n

R2

Ga(III)

298 308 318 298 308 318

55.28 62.50 64.18 48.08 50.10 52.19

0.0337 0.0524 0.0649 0.0534 0.1216 0.1392

0.9978 0.9993 0.9992 0.9987 0.9993 0.9998

7.84 11.31 13.41 11.62 17.00 18.81

2.92 3.21 3.47 4.02 5.03 5.30

0.8887 0.8954 0.9360 0.8096 0.7277 0.8157

V(V)

D

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Table 5. Thermodynamic Parameters for the Adsorption of Ga(III) and V(V) onto LSC700 metals

ΔH0(kJ/mol)

ΔS0(J/mol)

T (K)

ΔG0(kJ/mol)

R2

Ga(III)

47.74

218.90

16.69

114.88

−17.49 −19.68 −21.87 −17.49 −18.69 −19.84

0.9791

V(V)

298 308 318 298 308 318

0.9875

addition, the reaction is favored and gets easier at higher temperatures. The positive values of ΔH indicated that the adsorption process was an endothermic process. The heat of adsorption values above 20 kJ/mol are generally taken to indicate chemical sorption processes.22 From Table 5, ΔH values are respectively 47.74 and 16.69 kJ/mol for Ga(III) and V(V), which reveals that the adsorption processes for Ga(III) may be chemisorption. The positive values of ΔS referred to the increased randomness at the solid−solution interface. Adsorption Mechanism. Generally, when amidoxime chelating resins adsorb metal ions, the coordination between the amidoxime group and metal ion can occur in three different modes23 (Figure 6). In the first mode, the metal ion forms Figure 4. Linear fitting curves with Langmuir model for (a) Ga(III) and (b) V(V).

Figure 6. Three possible bonding motifs between amidoxime and metal ions: I, oxygen; II, chelate; III, η2.

Figure 5. Plots of log D versus 1/T for the adsorption of Ga(III) and V(V).

bonds with the oxygen atom. In the second mode, the metal ion is bound to both oxygen and nitrogen donor atoms, forming a five-membered chelate ring. And the third mode exhibits η2 binding with the N−O bond of oximido. From the FI-IR spectra before and after adsorption (Figure 7), it was found that the characteristic stretching bands of CN, −CH3, and −CH2− were unchanged. However, the stretching vibration bands for N−O and O−H shifted from 935 to 950 cm−1 and 3399 cm−1 to 3421 cm−1, respectively. Simultaneously, a new band was found at 592 cm−1, which is ascribed to the stretching vibrations of Ga−O.24 These phenomena indicate that the coordination between Ga and N−O−H happened through the dissociation of the H+ in the hydroxyl and subsequent formation of a bond between bonding electrons coming from the O atom and Ga ion, respectively. It is worth noting that, from the FT-IR spectrum after adsorption, the characteristic stretching band of Ga−N24 at 800−850 cm−1 was not found, suggesting the coordination between Ga and N might not

respectively. The slope and intercept of log D vs 1/T was used for the ΔH and ΔS according to eqs 8−10. The Gibb’s free energy change of the adsorption is associated with the distribution coefficient (D) in the linear form of eq 8. ΔG 0 = −RT ln D

(8)

ΔG 0 = ΔH 0 − T ΔS 0

(9)

log D =

ΔS 0 ΔH 0 − 2.303R 2.303RT

(10)

where D is the distribution coefficient (mL/g) and R is the gas constant (J/mol K). Table 5 shows the calculated thermodynamic parameters. As can be seen from Table 5, ΔG values were negative, indicating the spontaneous nature of the adsorption process under the experimental conditions. In E

DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. FT-IR spectra of the resin before and after adsorption.

happen. Accordingly, a conclusion could be made that oxygen binding mode I might be the main binding motif of AO toward Ga(III) in Bayer liquor, since in the strong basic Bayer liquor, Ga(III) is usually present as [Ga(OH)4]−. When the O atom bonds with Ga, the released H+ will take one OH− from [Ga(OH)4]− to form H2O. The adsorption mechanism of Ga(III) onto LSC700 might be described as Figure 8. Figure 9. Scheme for recovery of both gallium and vanadium from Bayer liquor.

(physical adsorption) might play a primary role in the complex formation between V(V) ions and a functional group in the resin, while the adsorption of Ga(III) may be dominated by chelating interaction (chemical adsorption). Both the adsorption of Ga(III) and V(V) can be expressed better with Langmuir adsorption isotherms, which shows the homogeneous characteristics of the sorption sites on resin. Thermodynamic parameters, including ΔG, ΔH, and ΔS were calculated based on the experimental data. The ΔH values are respectively 47.74 and 16.69 kJ/mol for Ga(III) and V(V), which reveals that the adsorption process for both metal ions was an endothermic process and the adsorption processes for Ga(III) may be chemisorption. By comparing the FT-IR spectrum before and after adsorption, it was found that the amidoxime group of LSC700 coordinated with [Ga(OH)4]− through oxygen binding mode with the formation of a complex compound. Finally, based on the experimental results, the coextraction of vanadium when extracting gallium from Bayer liquor by ion exchange could be solved by the proposed process, which can separately recover gallium and vanadium from Bayer liquor.

Figure 8. Adsorption reaction of Ga(III) onto LSC700.

Proposal for Separation and Recovery of Gallium and Vanadium from Bayer Liquor. Based on the above research, a process for separately recovering gallium and vanadium from Bayer liquor is proposed in Figure 9. After adsorption, sodium aluminate solution is reintroduced back to the Bayer process, while gallium and vanadium are loaded onto resin. Afterward, the loaded resin is subjected to a two-stage desorption to elute gallium and vanadium, respectively. Dilute sulfuric acid (0.01M/0.02M), EDTA, and acetylacetone, which have been proven efficient for desorption of gallium,25 could be adopted at stage one. Almost all gallium and only a little vanadium will be eluted. Since the adsorption of vanadium by LSC700 may be dominated by physical adsorption, the second desorption stage in the process could utilize eluents such as sulfuric acid (0.9M) and ammoniacal solutions,26 which were used for desorption of vanadium from loaded carbon, as well as concentrated HCl solutions.27 The efficient desorption of V from LSC700 will be tested in future experiments. After two-stage desorption, gallium-enriched and vanadium-containing solutions are treated by traditional industrial procedures to produce gallium and V2O5 respectively.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 138 5550 5238. E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS The amidoxime resin LSC700 exhibits good extractive properties for both gallium and vanadium from Bayer liquor in terms of loading capacity and kinetics. Adsorption kinetic processes for Ga(III) and V(V) were found to follow a secondorder model. The desorption isotherm studies showed that the adsorption of V(V) was quite sensitive under lower concentration, which indicated the electrostatic attraction

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51104002, 51274005, and 51204002). REFERENCES

(1) Połedniok, J. Speciation of scandium and gallium in soil. Chemosphere 2008, 73 (4), 572−579.

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DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.5b00307 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX