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In situ Synthesis of #-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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00918 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017
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In situ Synthesis of γ-AlOOH and synchronous adsorption separation of V(V) from highly concentrated Cr(VI) multiplex complex solutions Hailin Zhang,a,bPing Li,*,aZheming Wang, c Xin Zhang,c Shili Zheng,a and Yi Zhanga a
National Engineering Laboratory for Hydrometallurgical Cleaner Production ,Techno
-logy, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 1 North 2nd Street, Zhongguancun, Haidian District, Beijing, 100190, People’s Republic of China b
University of Chinese Academy of Sciences, No.19A Yuquan Road, Shijingshan
District, Beijing, 100049, People’s Republic of China c
Physcial and Computational Science Directorate, Pacific Northwest National
Laboratory, Richland, WA, 99354, USA *
Corresponding author, E-mail:
[email protected] Tel: +86-10-82544856; fax: +86-10-82544856 Hailin Zhang, Ping Li, Zheming Wang, Xin Zhang, Shili Zheng and Yi Zhang ABSTRACT Boehmite (γ-AlOOH) was synthesized to selectively adsorb V(V) from K2CrO4-KVO3-H2O solutions with highly concentrated Cr(VI) and low concentrated 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 inner-sphere 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 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. 1
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KEYWORDS：Green separation, Boehmite, Chromate solutions, In situ synthesis and synchronous adsorption, Hazardous residues reduction
INTRODUCTION 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 high concentrated V(V)-containing solutions produced from the vanadium compounds production since the development of 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). An alternative method to prepare chromium compounds is the liquid phase oxidation process.10 In this process, the chromite ore was oxidized in the sub-molten media (KOH +O2) at 300 oC, 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 advantages 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 2
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without the calcium compounds addition. 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 mmol/g 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 centres and H bonds on the surface.16,17 Recently, γ-AlOOH with various morphologies and surface properties such as nanorods, nanobelts, flowers, 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 the V(V) could be adsorbed by mesoporous γ-AlOOH.22 The present work is aimed at highly selective adsorption separation of V(V) from the 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 3
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situ
synthesized
and
V(V)
was
synchronously
adsorbed
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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 adsorption time was shortened from 5 h to nearly instantaneousness. The process mechanism was preliminarily discussed. 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, Fig. 1a).23,24 Broad XRD reflection peaks and relatively low diffraction intensities suggested the existence of 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 nano-sheet-like structures, which further formed the agglomerates.
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.
The BET gas-sorption measurements were performed to confirm the surface and average pore size of the synthesized γ-AlOOH (Figure 1b). The mesoporous characteristics of γ-AlOOH 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 Fig. 1b also revealed a type IV isotherm with a type H4 hysteresis loop, indicating the slit-shaped mesopores and microporosity that were associated with 4
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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 γ-AlOOH (Table S1, Supporting Information) in the temperature range from 90 to 25 oC. 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 as 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 function of pH are shown in Figure 2a. The V(V) adsorption capacity decreased from 1.53 mmol/g 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 mmol/g to 0.31 mmol/g. As the pH increased from 6.5 to 12, the Cr(VI) adsorption capacity decreased gradually from 0.31 mmol/g 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 the 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
