In Situ Synthesis of Î³-AlOOH and Synchronous Adsorption Separation

Research Article pubs.acs.org/journal/ascecg

In Situ Synthesis of γ‑AlOOH and Synchronous Adsorption Separation of V(V) from Highly Concentrated Cr(VI) Multiplex Complex solutions Hailin Zhang,†,‡ Ping Li,*,† Zheming Wang,§ Xin Zhang,§ Shili Zheng,† and Yi Zhang† †

National Engineering Laboratory for Hydrometallurgical Cleaner Production, Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 1 North Second Street, Zhongguancun, Haidian District, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, No. 19A Yuquan Road, Shijingshan District, Beijing 100049, People’s Republic of China § Physcial and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: Boehmite (γ-AlOOH) was synthesized to selectively adsorb V(V) from K2CrO4−KVO3−H2O solutions with highly concentrated Cr(VI) and low concentration V(V). The synthesized γ-AlOOH has a BET surface area of 433.2 m2/g and an average pore size of 3.5 nm. It possesses a maximum adsorption capacity of V(V) of 1.53 mmol/g from K2CrO4−KVO3−H2O solutions. The adsorption of V(V) onto γ-AlOOH follows the Langmuir isotherm model and pseudo-second-order kinetics equation by forming innersphere complexes while the Cr(VI) adsorption forms both inner-sphere and outer-sphere chromate complexes depending on solution pH. The γ-AlOOH was further synthesized in situ by adding HNO3 into the K2CrO4−KAlO2− KVO3−H2O solutions and then used for synchronous adsorption of V(V) and Cr(VI), resulting in increased adsorption capacity of V(V) of 2.88 mmol/g and decreased adsorption capacity of Cr(VI) to 0.073 mmol/g, respectively. In the latter process, adsorption pH values were adjustable, and adsorption reached equilibrium instantaneously, supporting a novel in situ synthesis and adsorption integration strategy with adjustable surface charge of adsorbent and disappearance of diffusion effect. KEYWORDS: Green separation, Boehmite, Chromate solutions, In situ synthesis and synchronous adsorption, Hazardous residues reduction

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INTRODUCTION

An alternative method to prepare chromium compounds is the liquid phase oxidation process.10 In this process, the chromite ore was oxidized in the submolten media (KOH + O2) at 300 °C, and the K2CrO4−KAlO2−KVO3−H2O multiple complex solutions were obtained by a solid−liquid separation step.11,12 Calcium compounds such as CaO and Ca(OH)2 were added to separate V(V) (∼1g/L) and Al(III) (∼10 g/L) from the highly concentrated K2CrO4 solutions (∼100 g/L of Cr(VI)), taking advantage of the low solubilities of Ca(VO3)2 and Ca(AlO2)2. However, Ca(VO3)2 and Ca(AlO2)2 and the highly carcinogenic CaCrO4, called Cr(VI)-containing residues, need further treatment due to strong environmental concerns. There is an urgent demand for green separation to eliminate the residues, where the key is to effectively remove low concentration V(V) from highly concentrated Cr(VI)-containing solutions without the calcium compounds addition.

The recent interest in chromium and vanadium research focuses on their cleaner production application and elimination of highly toxic Cr(VI) and V(V) pollutants, such as Cr(VI)- or V(V)-containing residues and wastewaters which are mainly derived from the their production and use.1−3 The selective separation of Cr(VI) and V(V) is the most difficult step for the cleaner production of chromium and vanadium due to the similar chemical properties of Cr(VI) and V(V). Solvent extraction has been a successful method to effectively separate Cr(VI) from highly concentrated V(V)-containing solutions produced from the vanadium compounds production since the development of the liquid−liquid extraction process in the 1960s,4 and a number of improved extraction processes have been reported recently.5−9 However, a drawback of the method was that it was almost impossible to extract V(V) from highly concentrated Cr(VI)-containing solutions produced from the chromium compounds production owing to the low selectivity of extraction and strong oxidizing property of Cr(VI). © 2017 American Chemical Society

Received: March 27, 2017 Revised: May 31, 2017 Published: July 10, 2017 6674

DOI: 10.1021/acssuschemeng.7b00918 ACS Sustainable Chem. Eng. 2017, 5, 6674−6681

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) XRD pattern and TEM image of the synthesized sample. (b) Nitrogen adsorption−desorption isotherms of the synthesized γ-AlOOH and its pore size distribution profile derived by the BJH method.

adsorption time was shortened from 5 h to nearly instantaneous. The process mechanism was preliminarily discussed.

It has been reported that the V(V) and Cr(VI) can be selectively adsorbed by different adsorbents, such as Zr(IV)impregnated collagen fiber, crab shells, and orange juice residue from low concentration Cr(VI)- and V(V)-containing aqueous solutions and wastewaters.13−15 For example, the Zr(IV)impregnated collagen fiber could adsorb V(V) and Cr(VI) with the maximum adsorption capacities of 1.92 and 0.53 mmol/g, respectively, from aqueous solutions with 2−4 mmol/L of Cr(VI) and 12−20 mmol/L of V(V). Dried orange juice residue could be used as a promising sorbent for the recovery of a small quantity of V(V) from Cr(VI)-containing effluent containing 40 mmol/L V(V) and 30 mmol/L Cr(VI), with adsorption capacities of 1.003 and 0.288 mmol/g, respectively, for V(V) and Cr(VI). However, adsorption separation V(V) from the highly concentrated Cr(VI) solutions was seldom reported. As an important common adsorbent, γ-AlOOH displays excellent adsorption performance for many heavy metal ions due to its coordinatively unsaturated AlVI centers and H bonds on the surface. 16,17 Recently, γ-AlOOH with various morphologies and surface properties such as nanorods, nanobelts, flowers, and microspheres have been prepared to exhibit further improved adsorption capacities.18−20 For instance, the γ-AlOOH microspheres exhibited enhanced capacity to adsorb Cd2+.21 The γ-AlOOH can adsorb Cr(VI) from 0.1 mmol/L K2Cr2O7 solutions with Cr(VI) adsorption capacity reaching 1.88 mmol/g. Competitive adsorption of V(V) and Cr(VI) onto γ-AlOOH still remains a challenge despite that V(V) could be adsorbed by mesoporous γAlOOH.22 The present work is aimed at highly selective adsorption separation of V(V) from K2CrO4−KAlO2−KVO3−H2O multiple complex solutions to realize green separation. Competitive adsorption separation was conducted by adding the as-prepared γ-AlOOH into K2CrO4−KVO3−H2O solutions. The γ-AlOOH synthesis procedure and competitive adsorption behavior and mechanisms of V(V) and Cr(VI) ions were studied. To further improve adsorption separation efficiency, a novel synthesis and adsorption integration process was introduced. In this process, the γ-AlOOH was in situ synthesized and V(V) was synchronously adsorbed from the K2CrO4−KAlO2−KVO3− H2O solutions. The results showed that the adsorption separation ratio of V(V) to Cr(VI) reached 46.25, and

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RESULTS AND DISCUSSION Characterization. Figure 1 shows the physicochemical characteristics of synthesized γ-AlOOH. The XRD patterns matched with those of γ-AlOOH (JCPDS No. 49-0133, Figure 1a).23,24 Broad XRD reflection peaks and relatively low diffraction intensities suggested the existence of an amorphous phase of the as-synthesized γ-AlOOH. TEM results (Figure 1a, inset, and Figure S1, Supporting Information) showed that the surface morphology of the γ-AlOOH clearly exhibited irregular nanosheet-like structures, which further formed the agglomerates. The BET gas-sorption measurements were performed to confirm the surface and average pore size of the synthesized γAlOOH (Figure 1b). The mesoporous characteristics of γAlOOH could be confirmed by pore size distribution curves which exhibited a mean pore diameter of 3.5 nm (Figure 1b). The N2 adsorption isotherms in the insets of Figure 1b also revealed a type IV isotherm with a type H4 hysteresis loop, indicating the slit-shaped mesopores and microporosity that were associated with capillary pressure of condensation.25 The specific surface area was 433.2 m2/g, and the pore volume determined at P/P0 = 0.993 reached 0.43 cm3/g. The XRD patterns of the synthesized γ-AlOOH at different temperatures (Figure S2, Supporting Information) indicated poor crystallinity of synthesized γ-AlOOH at low temperature, depending on the decreased peaks intensity and shrinking grain of γAlOOH (Table S1, Supporting Information) in the temperature range from 90 to 25 °C. A preliminary adsorption investigation also showed that the synthesized γ-AlOOH with poor crystallinity possessed better adsorption capacity for V(V) (Figure S3, Supporting Information). Therefore, the poor crystallinity, large specific surface area, and ideal pore size distribution make the mesoporous γ-AlOOH a promising candidate for application in the adsorption of V(V).22 Competitive Adsorption Behavior. Effect of pH. The solution pH determines the adsorption capacity due to its influence on the nature of the ionic species of V(V) and Cr(VI) and the surface properties of γ-AlOOH. The adsorption capacities of V(V) and Cr(VI) onto the γ-AlOOH as a 6675


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Figure 2. Competitive adsorption of V(V) and Cr(VI) ions onto γ-AlOOH as a function of the (a) pH and (b) temperature.

capacity of V(V) to the increased rate of intraparticle diffusion of V(V) ions into the pores of γ-AlOOH at higher temperatures.13 The opposite trend of the adsorption capacity of Cr(VI) could be explained by competition of V(V) and Cr(VI) ions onto the adsorption sites of γ-AlOOH.32 Adsorption Isotherms and Kinetics. To understand the characteristics of adsorption isotherms of V(V) and Cr(VI) onto γ-AlOOH, the effect of the initial concentrations on the V(V) and Cr(VI) ions were studied by using both Langmuir and Freundlich isotherm models:33

function of pH are shown in Figure 2a. The V(V) adsorption capacity decreased from 1.53 to 0.53 mmol/g as the pH increased from 3 to 6.5, while the adsorption capacity of Cr(VI) increased from 0.12 to 0.31 mmol/g. As the pH increased from 6.5 to 12, the Cr(VI) adsorption capacity decreased gradually from 0.31 to 0.0002 mmol/g; the V(V) adsorption capacity increased and reached the highest value of 0.87 mmol/g at pH 9 and then decreased to 0.22 mmol/g at pH 12.26,27 We noticed that the competitive adsorption capacities of V(V) and Cr(VI) onto γ-AlOOH in this manuscript were slightly different from the reported results which was performed by adsorption of V(V) or Cr(VI) individually.13 The competitive adsorption behavior of V(V) and Cr(VI) with varying pH could be explained by the presence of different ionic species of V(V) and Cr(VI). Thermodynamically, the main species of V(V) and Cr(VI) are H3V2O7− and HCrO4− at pH < 7, respectively; HVO42− and CrO42− become dominant at pH ≥ 8 in the Cr(VI)−V(V)-containing solutions.28,29 Electrostatic adsorption of V(V) and Cr(VI) occurred between γ-AlOOH and H3V2O7− and HCrO4− in an acidic environment (pH < 7); a decrease in the V(V) adsorption capacity and an increase in the Cr(VI) adsorption capacity resulted from high ionic strength of HCrO4− in the experimental conditions. In weakly basic solutions (7 < pH < 9), an ion exchange could be used to explain the difference in adsorption capacities between V(V) and Cr(VI). HVO42− with a larger ionic radius possessed a stronger ion exchange capacity with the OH− ions of γAlOOH in comparison with CrO42−, resulting in an increase in the V(V) adsorption capacity and decrease in the Cr(VI) adsorption capacity, respectively. At pH > 9, decreases in the adsorption capacities of V(V) and Cr(VI) ions can be ascribed to the competition between excess OH− ions and HVO42− and CrO42−, which is similar to the literature.30,31 Effect of Temperature. The effect of temperature on the competitive adsorption of V(V) and Cr(VI) ions onto γAlOOH was then investigated with an equilibrium time of 5 h (Figure 2b). For the same K2CrO4−KVO3−H2O solutions (25 g/L of Cr(VI), 1 g/L of V(V)), and γ-AlOOH (0.5 g), the adsorption capacity of V(V) ions at various temperatures increased from 0.53 to 0.94 mmol/g with temperature varing from 25 to 75 °C at pH 9. However, the adsorption capacity of Cr(VI) decreased from 0.175 to 0.02 mmol/g at the same temperature. We attributed the enhancement in the adsorption

Ce C 1 = + e qe qmb qm log qe =

1 log Ce + log k n

(1)

(2)

where qe and qm represent the amount adsorbed at equilibrium and the maximum adsorption capacity (mmol/g), respectively; Ce is the equilibrium concentration in solution (mol/L), and b is the Langmuir adsorption constant related to the free energy of adsorption; k and 1/n are Freundlich constants related to the adsorption capacity and the adsorption intensity, respectively. The isotherms parameters were obtained by nonlinear regression of the experimental data (Table S2 and Figure S4, Supporting Information). For the K2CrO4−KVO3−H2O solutions, the V(V) adsorption data could be best fitted by the Langmuir model (0.999), while that of Cr(VI) was very poor (0.904), but on the other hand, the Cr(VI) had a higher correlation coefficient value for Freundlich (0.993) as compared with that of V(V) (0.949). The values of qm and b of the Langmuir equation were 1.156 mmol/g and 0.108, respectively, demonstrating that the adsorption of V(V) on γAlOOH took place only on a portion of the available sites, and there were no interactions between the adsorption molecules. The values of the Freundlich adsorption isotherm of the adsorption of Cr(VI) were determined from the slope and intercept of the linear plot of log qe versus log Ce, resulting in 1/ n and log k values of 0.5831 and −4.1438, respectively. The fitted Freundlich parameters can be explained with different adsorption energies on uniform adsorption sites which results in preferential adsorption of V(V) ions onto the γ-AlOOH surface despite the highly concentrated Cr(VI) characteristics and antagonistic effect of Cr(VI) in the K2CrO4−KVO3−H2O 6676


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Figure 3. ATR-IR spectra simulation by a combination of Gaussian function profiles of (a) γ-AlOOH as synthesized, (b) γ-AlOOH after V(V)/ Cr(VI) adsorption at pH 4, and (c) γ-AlOOH after V(V)/Cr(VI) adsorption at pH 9.

other researchers.30 It is plausible to assume that the diffusion of Cr(VI) onto the surface of γ-AlOOH was probably the ratecontrolling step in the adsorption of Cr(VI) from the K2CrO4− KVO3−H2O solutions. Adsorption Mechanism. ATR-FTIR spectra offer information about surface molecular signature of samples. Figure 3 shows the ATR-FTIR spectra of γ-AlOOH after V(V) and Cr(VI) ions adsorption in the K2CrO4−KVO3−H2O solutions at pH 4 and 9, respectively, for 5 h. Consistent with the results of Fondeur et al., the four absorption peaks at 844, 880, 994, and 1065 cm−1 were assigned to O−H bending, (HO)−AlO asymmetric stretching, (OH)−AlO angle bending, and O Al−(OH) angle deformation vibrations, respectively.35 To elucidate the coordination environment of V(V) and Cr(VI) on γ-AlOOH, the peaks at 800−1100 cm−1 were fitted using Guassian functions (Figure 3b, c). After γ-AlOOH adsorbed V(V)/Cr(VI) at pH 9, peaks at 1026 cm−1 could be assigned to the mono-oxo, V=O terminal double bonds within VO4 units of slightly different asymmetries, which corresponded to innersphere complexes adsorption.36−38 The peak at 942 cm−1 could be assigned to the Cr−O stretching vibrations (ν3 band) of coordinated chromate.39 In comparison to overlapped peaks of pristine γ-AlOOH at 907 and 880 cm−1, the increased portion of the peak area of adsorbed γ-AlOOH could be ascribed to the splits of the ν3 band, thus indicating formation of inner-sphere surface complexes.40,41 A remarkable difference between the fitted spectra at pH 4 and 9 was the appearance of the peak at

solutions. Comparison of the adsorption capacities of V(V) and Cr(VI) onto γ-AlOOH with those of other adsorbents reported in the literature (Table S3, Supporting Information) demonstrated that γ-AlOOH is a superior adsorbent for V(V).13−15 Pseudo-first-order and pseudo-second-order models are adopted to determine the adsorption kinetics of V(V) and Cr(VI) on the adsorption capacities of V(V) and Cr(VI) as a function of time, and the corresponding equations are expressed as eqs 4 and 5, respectively. ln(qm − q) = ln qm − k1t

(3)

t 1 1 = + t 2 q q k 2qm m

(4)

where t (h) is the time, and k1 and k2 (h−1) are the pseudo-firstorder and pseudo-second-order models constants, respectively. Here, qm (mmol/g) is the equilibrium adsorption capacity, and q (mmol/g) is the amount of adsorption at time t (h). As shown in Table S2 and Figure S5 (Supporting Information), the adsorption of V(V) followed the pseudosecond-order model (R2 > 0.9997), indicating that adsorption of V(V) ions onto γ-AlOOH is a chemisorption process.34 Table S2 (Supporting Information) also shows that ln (qm − q) is linearly proportional to adsorption time. As observed, the adsorption of Cr(VI) onto γ-AlOOH was more suitable to the pseudo-first-order model, different from the pseudo-secondorder model of Cr(VI) adsorption onto γ-AlOOH reported by 6677


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ACS Sustainable Chemistry & Engineering Table 1. Adsorption Capacity on γ-AlOOH at Different Concentrations of V(V) and Cr(VI) V adsorption capacity mmol/g

Cr adsorption capacity mmol/g

K2CrO4 g/L

KVO3 g/L

KAlO2 g/L

0 min

30 min

60 min

0 min

30 min

60 min

94 188 94

2.70 2.70 1.35

8.20 8.20 8.20

2.83 2.61 1.74

2.79 2.65 1.69

2.84 2.59 1.72

0.082 0.070 0.083

0.075 0.077 0.081

0.085 0.056 0.079

Figure 4. (a) Plots of internal diffusion model. (b) Plots of external diffusion model for competitive adsorption of V(V) and Cr(VI) on γ-AlOOH.

814 cm−1, which was closely related to the Cr−OH stretching vibrations, indicating the presence of hydrogen chromate (HCrO4−).42,43 Combined with the possibly overlapped bands at 939 cm−1, 878 cm−1 resulted from nonprotonated monodentate complexes; Cr(VI) adsorbed onto γ-AlOOH at pH 4 suggests formation of outer-sphere surface complexes. Thus, V(V) adsorption onto the γ-AlOOH surface formed inner-sphere complexes, while Cr(VI) adsorption formed innersphere complexes and outer-sphere chromate complexes at the acidic and alkaline circumstances, respectively. XPS measurement was conducted for γ-AlOOH pastes before and after adsorption of V(V) and Cr(VI) to further characterize the surface states of the samples.44 Figure S6 (Supporting Information) presented the XPS spectra with clear Al 2p, V 2p, Cr 2p, and O 1s peaks, where V 2p (Figure S6c, Supporting Information) and Cr 2p (Figure S6d, Supporting Information) XPS spectra indicated that the valence state of vanadium and chromium after adsorption were +5 and + 6, respectively,45−47 and V(V) and Cr(VI) ions adsorption occurred on the surface of γ-AlOOH, consistent with the result of the ATR-FTIR spectra (Table S5, Supporting Information). Additionally, the binding energy of Al 2p at around 74.4 eV and the peak shape also indicated that the dominating chemical state of Al atoms in γ-AlOOH was Al(III) (Figure S6a, Supporting Information).44 The O 1s peaks are designated as surface O2− (530.1−530.3 eV), OH− groups (531.5−532.0 eV), and free H2O (532.8− 533.4 eV), respectively.48 As shown in Figure S6b, Figure S7, and Table S4 (Supporting Information), OH− groups were dominant, and the percentage of OH− groups slightly decreased after adsorption of V(V) and Cr(VI). The decrease in the number of OH− groups could be attributed to the fact that the more OH− bonds were exposed to adsorb V(V) and Cr(VI) ions by liberating surface hydroxyls in the adsorption process.46

On account of the above ATR-FTIR and XPS spectra results, it could be confirmed that the V(V) and Cr(VI) ions were selectively adsorbed onto γ-AlOOH, and the surface of γAlOOH was covered with monodentate vanadate complexes in addition to a small amount of chromate complexes. In Situ Synthesis of γ-AlOOH and Synchronous Adsorption of V(V) from K2CrO4−KVO3−KAlO2−H2O Solutions. We conducted in situ synthesis of γ-AlOOH and synchronous adsorption of V(V) from the K2CrO4−KVO3−KAlO2−H2O solutions to realize green separation. Table 1 shows the results of in situ γ-AlOOH synthesis and synchronous adsorption results of V(V) and Cr(VI). As observed, with the extension of reaction time from 0 to 60 min, the adsorption capacities of V(V) basically remained unchanged, indicating that the γAlOOH adsorption equilibrium was reached with the completion of γ-AlOOH synthesis. Compared to the competitive adsorption of V(V) and Cr(VI) onto γ-AlOOH (see Competitive Adsorption Behavior section), the observed adsorption time was significantly reduced with the in situ synthesis of γ-AlOOH and synchronous adsorption of V(V), which could be ascribed to the effective diffusion between the γ-AlOOH interface and V(V) and Cr(VI). However, it is difficult to determine the relative diffusion coefficients because of the very short adsorption time. The Weber−Morris diffusion model was adopted to identify the rate-limiting step in the competitive adsorption process between γ-AlOOH and V(V) and Cr(VI) since it is well documented that the film and intraparticle diffusion dictates the metal ion adsorption time.49 The corresponding equations can be described as follows: qt = kpt 1/2 + c 6678

(5) DOI: 10.1021/acssuschemeng.7b00918 ACS Sustainable Chem. Eng. 2017, 5, 6674−6681

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Figure 5. (a) Transient pH values and environmental pH values in the process of in situ γ-AlOOH synthesis. (b) Zeta potential of γ-AlOOH at different pH values (conditions: temperature, 25 °C; time, 6 h; V(V), 1g/L; Cr(VI), 25g/L).

⎛ q ⎞ ln⎜⎜1 − t ⎟⎟ = −k f t qm ⎠ ⎝

transient pH values changed from 8.5 to 4.5 during the HNO3 addition and then rocketed upward to 8.0 within 10 s, while the solution pH decreased from 12.5 to 8.0 and kept constant at different interval times. Zeta potential of γ-AlOOH at different pH was measured by adding synthesized γ-AlOOH into K2CrO4−KVO3−H2O solutions, taking into consideration the affinity between surface charge and pH value. As shown in Figure 5b, the pHzpc (point of zero charge) of γ-AlOOH was found to be 8.2. Below the pHzpc, the γ-AlOOH surface will carry a net positive charge. Hence, the low transient pH values in the in situ γ-AlOOH synthesis and synchronous adsorption of V(V) and Cr(VI) will lead to a change in the net charge of the γ-AlOOH surface from quasi-neutral to positive and further increase the surface potential of γ-AlOOH, effectively reducing the adsorption energy barrier of V(V), which will promote the V(V) adsorption. The in situ synthesis and synchronous adsorption results show that the adsorption depends critically on the formation of γ-AlOOH, realizing the synthesis and adsorption integration. In this process, the adsorption pH is adjustable, and the adsorption happens during the formation of γ-AlOOH, improving the adsorption capacity and shortening the adsorption time, respectively.

(6)

where kp and kf (mmol/g·h−1) are the intraparticle and external diffusion rate constant, respectively. Here, qm (mmol/g) is the equilibrium adsorption capacity, and qt (mmol/g) is the amount of adsorption at time t (h). A plot of qt vs t1/2 (Figure 4) showed that it was a dual-linear relationship. The initial portion of the plot refers to the film diffusion, whereas the second linear portion can be ascribed to the intraparticle diffusion, implying that more than one process affected the adsorption.50,51 Figure 4 showed that ln (1 − qt/ qm) had excellent correlation with time t, and the correlation coefficient value for V(V) reached 0.988 while that of Cr(VI) was 0.98, indicating that the adsorption data could be fitted by the film diffusion very well. Note that the film diffusion was the rate-limiting step for the whole adsorption process. Such diffusion usually indicates the system has poor mixing, dilute concentration of adsorbent, small particle size, and high affinity of adsorbate for adsorbent, some of which is similar to the competitive adsorption system.52 Thus, it can be indirectly inferred that the in situ synthesis and synchronous adsorption eliminated the film diffusion and even the whole diffusion process, consistent with the fact that equilibrium adsorption time of V(V) and Cr(VI) decreased from 5 h to nearly instantaneous. It was also found that the maximum adsorption capacity of V(V) reached 2.88 mmol/g, which is much higher than the value of 1.53 mmol/g that was obtained by adding γ-AlOOH into K2CrO4−KVO3−H2O solutions. The in situ synthesis and synchronous adsorption ratio of V(V) to Cr(VI) reached 46.25, demonstrating that the process further promoted the preferable adsorption of V(V) in the K2CrO4−KVO3−KAlO2−H2O solutions. The online monitoring of pH values were used to explore such behavior. In the online process, one pH probe was placed under the surface of solutions (about 1 cm) close to the HNO3 addition position to determine transient pH value related to the in situ γ-AlOOH synthesis, and the other was located at the bottom of the beaker far from the acid addition position to determine solution pH. Figure 5a shows the measured transient pH and solution pH. As observed, the

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CONCLUSIONS The γ-AlOOH nanosheets were successfully synthesized by reacting KAlO2 with HNO3 and selectively adsorbed V(V) and Cr(VI) from the K2CrO4−KVO3−H2O solutions with highly concentrated Cr(VI) and low concentration V(V). The γAlOOH nanosheets were further synthesized in situ and synchronously adsorbed V(V) and Cr(VI) from the K2CrO4−KAlO2−KVO3−H2O solutions. The adsorption energy barrier of V(V) was reduced, and the film diffusion between the γ-AlOOH interface and V(V) and Cr(VI) was eliminated effectively, resulting in a much improved V(V) adsorption capacity and significant reduced adsorption time, respectively. Such a synthesis and adsorption integration process as an option to the calcium addition method realized the residues source reduction. These results should be very useful for the green adsorption separation of V(V) from chromate solutions in the chromium compounds production 6679


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and enrich the understanding of hydrometallurgical green separation to adsorption extraction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00918. Experimental details, TEM image, XRD patterns at different temperatures, XPS spectra of samples, and detailed information on adsorption kinetics and isotherms, including six figures and four tables. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-82544856. Fax: +86-10-82544856. ORCID

Shili Zheng: 0000-0001-9474-9503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51574212, U1403195) and the Major State Basic Research Development Program of China (2013CB632605).

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REFERENCES

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In Situ Synthesis of Î³-AlOOH and Synchronous Adsorption Separation

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