Template-Free Synthesis of Alkaline Earth Vanadate Nanomaterials

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Template-Free Synthesis of Alkaline Earth Vanadates Nanomaterials from Leaching Solutions of Oil Refinery Waste Guowu Zhan, Wei Cheng Ng, Shin Nuo Koh, and Chi-Hwa Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03748 • Publication Date (Web): 30 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018

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ACS Sustainable Chemistry & Engineering

Template-Free Synthesis of Alkaline Earth Vanadates Nanomaterials from Leaching Solutions of Oil Refinery Waste Guowu Zhan,† Wei Cheng Ng,‡ Shin Nuo Koh,§ and Chi-Hwa Wang*†

†

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, 117585, Singapore ‡

NUS Environmental Research Institute, National University of Singapore, 1 Create Way,

Create Tower #15-02, 138602, Singapore §

Sembcorp Industries Ltd., 30 Hill Street #05-04, 179360, Singapore

KEYWORDS. Nanomaterials, Carbon black waste, Leaching, Vanadium recovery, Oil refinery

*Corresponding

author,

Tel.:

+65-65165079,

Fax:

+65-67791936,

[email protected]

ACS Paragon Plus Environment

1

E-mail

address:

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. Currently, industrial wastes are typically combusted at incineration plants and the resulting ashes sent to landfill. This practice raises issues such as potential toxicity of the leachate to the environment, as well as the loss of valuable metals left in the waste. Therefore, from both economic and environmental standpoints, it is becoming increasingly important for the recovery of valuable metals from industrial waste. In this work, a transformative recovery of vanadium (conversion nearly 100%) from leaching solutions of carbon black waste (metal leaching efficiency > 97%) which originates from oil refineries in Singapore, has been systematically studied. Interestingly, three distinctive alkaline earth vanadate nanomaterials were successfully synthesized from the leaching solutions by a facile hydrothermal process, i.e., single-crystal calcium vanadate nanorods (Ca10V6O25) oriented along the [001] direction, strontium vanadate nanorods with ellipsoid-like assembly (Sr10V6O25), and barium vanadate polyhedral nanoparticles (Ba3V2O8). Importantly, we have demonstrated that this facile metal recovery route enables the morphology control of vanadate nanomaterials without templates in the system. Moreover, the vanadate nanomaterials obtained are potential wide band-gap semiconductors for electronic devices and semiconducting glasses.

■ Introduction

Nowadays, there is an increasing interest in the research of creating opportunities from wastes such as food waste, biomass waste, e-waste, oil fly ash, and the like,1-5 they could be potentially valuable resources for the preparation and synthesis of fuels and high-value chemicals (materials). According to the 2016 BP Statistical Review of World Energy, Singapore has an oil refining capacity of 1.514 million barrels per day (1.6% share of the world total).6 In Singapore, the petrochemical industry generates about 30 tons of carbon black waste (a petroleum coke)


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daily as an unavoidable by-product from the processing of crude oil. Since vanadium is the most abundant heavy metal in crude oil,7, 8 it was reported that the derived carbon black waste contains a noteworthy quantity of vanadium (>14000 ppm).9,

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The high market value of vanadium

motivates the recovery of vanadium from such waste. To simplify the metal recovery, firstly, complete dissolution of valuable metals from carbon black waste could be achieved by advanced leaching processes. Subsequently, for the separation and recovery of metal ions in leaching solutions or wastewater in general, various technologies have been developed, such as chemical precipitation, reactive crystallization, adsorption, ion exchange, electrochemical removal, biotechnological processes, and membrane separations.11-15 The chemical methods by precipitation or reactive crystallization are considered the simplest and the most efficient pathway due to their low cost and ease of handling in industry. Additionally, the reactive crystallization method allows ultimate conversion of metal ions to value-added products in a one-pot process. Our previous study found that the carbon black waste is hazardous to human health due to the observation that vanadium could impair the antioxidant enzymatic activities of human cell lines.10 In addition to environmental concerns, we recognize that the vanadium in the carbon black waste could be a valuable resource for vanadate nanomaterials. Recently, vanadate nanomaterials have attracted tremendous interest due to their abundant applications in fields such as chemical sensor,16 transparent conductor,17 photocatalyst,18,

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lithium battery,20 biological

imaging and therapy.21 Vanadate materials also exhibit electrical and magnetic properties because of the presence of magnetic V4+ ions.22-24 It is well accepted that downsizing the materials to the nanoscale can significantly benefit their applications due to many size-dependent properties. For instance, rare earth vanadate nanoparticles are very promising candidates for


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applications as in vivo multifunctional probe, as they can perform a target-oriented delivery of the active compound.21 One-dimensional vanadate nanomaterials (nanorods, nanobelts, and nanotubes) with high surface area to volume ratio exhibit better electrochemical and photocatalytic properties, thereby they are more promising than their bulk counterparts.25 Based on these considerations, in this paper, we report our recent research work on the facile preparation of alkaline earth vanadate nanomaterials with controlled morphology from leaching solutions of carbon black waste by wet chemical methods. Firstly, chemical species of both acid and alkaline leaching solutions were determined, and their concentrations were quantified. Then, the recovery of vanadium from the leaching solutions was conducted by using a reactive crystallization method. Three different alkaline earth vanadate nanomaterials were obtained, including calcium vanadate, strontium vanadate, and barium vanadate. The optimizations of the recovery process and product characterizations were systematically studied, with the aim of enhancing both the vanadium recovery yield and product purity. ■ MATERIALS AND METHODS

Materials. Carbon black waste was collected from an oil refinery in Singapore. The following chemicals were used as received without further purification: sodium hydroxide (99%, VWR Chemicals), nitric acid (69%, VWR Chemicals), calcium nitrate tetrahydrate (99%, SigmaAldrich), calcium chloride (98%, Merck), calcium acetate monohydrate (>99.9%, Fisher Chemicals), calcium hydroxide (>96%, Fluka), calcium carbonate (Sigma-Aldrich), barium hydroxide monohydrate (>98%, Sigma-Aldrich), barium chloride dihydrate (99%, Merck), barium acetate (99%, Sigma-Aldrich), barium carbonate (Fisher Chemicals), strontium chloride


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hexahydrate (>99%, Merck), strontium acetate (Strem Chemicals), and magnesium acetate tetrahydrate (>99%, Fluka). Deionized water was used for the experiments. Leaching Process. The as-received wet carbon black waste sample was dried in an oven at 150oC for 12 h. Then, 2.5 g of the dry solid was dispersed in 50 mL of nitric acid (0.5 M) or NaOH (1 M) in a sonicator (150 W) for 1 h. Afterward, the leaching solution was collected by a membrane filter (Nylon, pore size: 0.45 µm) under vacuum filtration condition. The pH values were 0.47 and 13.8, for the acid and alkaline leaching solutions, respectively. Metal Recovery Process. For alkaline leaching solution, 5 mL of the leaching solution was mixed with a certain amount of alkaline earth metal salts. Then, the mixture was stirred at room temperature for 10 min before hydrothermal treatment at 200oC for 12 h. Subsequently, the white color solid was collected by centrifugation and washing with water for three times. Vanadium recovery from acid leaching solution was also studied. Unless specifically mentioned, the recovery processes for acid leaching solution were similar to those for alkaline leaching solution except that 5 mL of NaOH solution (1 M) was added before the addition of alkaline earth metal salts. To investigate and compare the effect of temperature, reactions were also conducted at room temperature for 12 h while keeping other conditions similar. Characterization Techniques. Morphologies of the nanomaterials were characterized by transmission electron microscopy (TEM, JEM-2010). The crystallographic information was analyzed by selected-area electron diffraction (SAED, JEM-2100F) and X-ray diffraction (XRD, Bruker D8 Advance). The elemental maps were recorded by energy-dispersive X-ray (EDX, Oxford Instruments; Model 7426). The oxidation state of the elements was analyzed by X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical). Metal concentrations in


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solutions were measured by inductively coupled plasma-atomic emission spectrometry (ICPOES, Optima 7300DV, Perkin Elmer). The liquid leaching solutions were characterized by UV−vis spectrometry (UV-2450, Shimadzu) and band gap energies of the solid samples were examined by diffused reflectance spectra in a UV-VIS-NIR spectrophotometer (Shimadzu 3600). 51

V nuclear magnetic resonance (NMR) solution spectra were done on a Bruker AV500

(500MHz) spectrometer. ■ RESULTS AND DISCUSSION

Characterizations of Carbon Black Waste. TEM images of the carbon black waste are displayed in Figure 1a,b. As shown, the carbon black waste particles are spherical with both hollow and non-hollow structures. From the EDX elemental maps (Figure 1c), it was found that the distributions of V, Ni, and Fe, appear to be in the same regions, indicating that the heavy metals may have fused together along with the formation of carbonaceous residues at high temperature (ca. 400oC). The concentrations of V, Ni, and Fe in carbon black waste were determined as 14530 ppm, 3287 ppm, and 1530 ppm, respectively (see Table S1). The vanadium element is predicted to be the fully-oxidized (VV) as suggested by the XPS spectrum in V 2p region (Figure S1). It shows that the V 2p3/2 centered with a binding energy of 517.1 eV, which is consistent with the VV oxide (different from V0 (512.4 eV), VI and VII (513.7 eV), VIII (515.3 eV), and VIV (515.8 eV)).26 Due to the low amounts of Ni and Fe in solid samples, their oxidation states were unable to be analyzed by XPS. Chemical leaching of carbon black waste by using HNO3 (0.5 M) and NaOH (1 M) as leaching agents was performed in this work. In the experiments, 5 wt% of carbon black waste solid was used in the leaching process. Interestingly, it was found that the acid leaching solution contained


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715 ppm of V, 158 ppm of Ni, and 79 ppm of Fe, while only V was present in alkaline leaching solution with a concentration of 705 ppm (Table S1). Therefore, a two-stage leaching process was suggested to separate different metals in the carbon black waste, that is the first stage using alkaline leaching to recover V followed by the second stage using acid leaching to recover Ni and Fe.27 In addition, we found that the leaching efficiency was enhanced when the leaching processes were conducted under sonication (data are shown in Table S2). It was believed that ultrasonic energy has a beneficial effect on metal leaching from carbon black waste, which is consistent with the finding from another report.28 Significantly, the leaching efficiencies of vanadium in both leaching processes were higher than 97%. Characterizations of Leaching Solutions. As illustrated in Figure 1d, the acid leaching solution is blue in color while alkaline leaching solution is colorless. In alkaline leaching solution, vanadium mainly exhibits as monomeric anion VO43−, which could be confirmed from 51

V NMR spectrum showing single resonance at chemical shift of -537 ppm (Figure 1f).29 But,

after adding acid to the alkaline leaching solution, the solution contains cationic hydroxo or oxo species, such as [V(OH)h(OH2)6−h](5−h)+ and pervanadyl ion [VO2]+, with characteristic yellow color by the UV-vis absorption bands at ca. 405 nm (Figure 1e).30, 31 Diverse polyoxovanadates would also form by condensation reactions (e.g., oxolation or olation) between the monomeric species. For instance, the pH of 7, the resonances in 51V NMR spectra (Figure 1f) could be assigned to the decavanadate anion, [V10O28]6− (-424, -506, -524 ppm) and the cation, [VO2]+ (543 ppm). At an extremely acidic condition (pH of 0), the solution turns to colorless again (inset in Figure 1e). Although the complex protonation equilibria and oligomerization equilibria are reversible, it is still difficult to determine the specific species as vanadium has rich coordination chemistry and the chemical species are strongly influenced by the concentration, pH,


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temperature, and ionic strength. In the acid leaching solution, the blue color was ascribed to the presence of Ni2+ and Fe2+ ions, as both ions showed intensive absorption of visible light (see UVvis spectra in Figure S2a). Interestingly, by simply adding NaOH to the acid leaching solution (pH value raised to 13.4), the blue color receded while precipitates with pale-brown color formed (Figure S2b). The ICP test confirmed that both Ni and Fe were almost removed in the supernatant liquid, and only a negligible amount of V was precipitated. This result further affirms that Ni and Fe ions are responsible for the blue color of acid leaching solution. Metal Recovery from Alkaline Leaching Solution. As discussed above, vanadium ions contribute the highest value in the carbon black waste. Thereby, one feasible and economical way of recovering these valuable vanadium ions is to prepare alkaline earth vanadate nanomaterials by treating the leaching solutions with alkaline earth salts. The use of alkaline earth metal compounds (such as calcium (Ca), strontium (Sr), and barium (Ba)) offers advantages such as cost-effectiveness and easy scalability. Since only VO43− ion predominates in the alkaline leaching solution (Fe and Ni ions were nearly non-existent, see Table S1), it shall be a suitable precursor solution for fabricating vanadate nanomaterials. Herein, five different calcium salts and four different barium salts were employed for vanadium recovery from alkaline leaching solution, and vanadium conversions (calculated from the concentration of residual vanadium in solution) at both room temperature and hydrothermal conditions were compared and summarized in Figure 2a,b. Different trends were found for the two processes. For instance, in hydrothermal synthesis, there was no pronounced effect of the anions on vanadium recovery, as almost complete recovery of vanadium could be achieved by using any of the alkaline earth salts. The carbonate gave a significantly lower recovery efficiency, suggesting that having solvable raw materials is crucial for the formation of vanadate product. However, for reactions at room


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temperature, different calcium salts show very different efficiency for vanadium recovery. In particular, no vanadium was recovered when calcium carbonate or barium carbonate was used at room temperature condition. Generally, higher vanadium recovery could be achieved at the hydrothermal condition. Besides the difference in recovery efficiency, the morphology of the products was strongly affected by the treatment temperature (room temperature vs. hydrothermal) due to the adjusting of crystal nucleation and growth process, which will be addressed shortly (vide infra). As illustrated in Figure 2c, by changing the alkaline earth salts input, we found that unlike other alkaline earth metals, Mg hardly recovers any vanadium ions from the leaching solution probably indicating a large formation energy of Mg vanadate.32 In this case, only Mg(OH)2 with plate-like shape (edge length < 200 nm) was produced (refer to the XRD and TEM characterizations in Figure S3). Moreover, at room temperature, Sr salts and Ba salts showed higher activity over Ca salts in terms of vanadium recovery (i.e., full recovery). Furthermore, the effect of calcium nitrate amount was evaluated to minimize the cost. As shown in Figure 2d, apparently, the vanadium recovery was highly dependent on the concentration of calcium salt. Incomplete vanadate recovery was observed when the calcium salt concentration was lower than 26 mM. The product purity will be studied shortly by XRD and XPS analysis (vide infra). Characterizations of Calcium Vanadate. The crystal composition, morphology, and size of the obtained alkaline earth vanadates were analyzed in detail by XRD, XPS, TEM, SAED, and EDX techniques. As shown in Figure 3a-c, the calcium vanadate materials are one-dimensional nanorods with the bush-like assembly. The length and diameter of the nanorods are about 720 nm and 90 nm, respectively, so the aspect ratio of the nanostructure is around 8:1. The XRD characterization (Figure 4a) shows that the calcium vanadate product can be ascribed to the


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hexagonal Ca10V6O25 phase (JCPDS card no. 52-0649).33, 34 Three different interplanar spacing of the crystals (0.350 nm, 0.319 nm, and 0.291 nm) are measured in a high-resolution TEM (HRTEM) image (Figure 3d), which can be assigned to (002), (210), and (211) crystal planes, respectively. The angle between the (002) and (211) planes is 65°, matching well with the hexagonal crystal structure. Likewise, the single crystallinity of Ca10V6O25 was also confirmed by selected area electron diffraction (SAED) pattern. As shown in Figure 3e, the electron beam was incident along the [11ത0] of a single-crystal nanorod. Moreover, the main growth orientation of the nanorod was determined as direction (c-axis). EDX spectra and elemental maps show the even distribution of Ca, V and O elements over the whole nanostructure. The XPS spectra (see Figure S4) of Ca10V6O25 indicate that Ca is in +2 oxidation state as the 2p3/2 and 2p1/2 binding energies are at 346.7 eV and 350.3 eV, respectively. V is in +5 oxidation state, but V 2p3/2 spectrum could be decomposed into two peaks at 516.6 eV (75%) and 517.8 eV (25%), indicating two different electron densities of vanadium. It should be noted that the product was pure Ca vanadate (Ca10V6O25 phase) when the molar concentration of calcium nitrate salt was 8 mM or 26 mM. However, Ca(OH)2 phase (JCPDS card no. 04-0733, marked by purple triangles in Figure 4a) was detected as the molar concentration of calcium salt increased to 42 mM, and CaCO3 phase (JCPDS card no. 05-0586, marked by pink spheres) appeared at the highest amount of calcium salt (84 mM). It was also observed that the length of the nanorods increased as the amount of salts increased (Figure S5). As such, it is concluded that both a high recovery rate (97.6%) of vanadium and a high purity (~100%) of calcium vanadate product could be simultaneously achieved by treating alkaline leaching solution with 26 mM of calcium nitrate, which corresponds to 11% Ca2+ in excess of the stoichiometric ratio (Ca2+/V5+ = 10:6).


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It is well known that hydrothermal treatments facilitate the preparation of nanoscale vanadium materials with well-defined size, shape, and structures.33,

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Thus, the effect of reaction

temperatures (room temperature vs. hydrothermal temperature) on the product morphology evolution was also studied herein. As expected, the product obtained from room temperature transformation has irregular shapes as displayed in Figure S6, even though XRD patterns of the calcium vanadate products are identical (Figure S7). Also, we found that the morphologies of the vanadate products were quite different by using different calcium source for the reactive crystallization process, as indicated by Figure S8. This is due to the fact that nucleation and growth behavior of vanadate were affected by the anions of the precursors. In nanoscience, the electronic structures and properties of many nanomaterials are highly dependent on their size.36 From this standpoint, calcium nitrate and calcium acetate would be recommended as precursors for vanadium recovery due to the relatively well-defined morphology of the products. Characterizations of Strontium Vanadate. As illustrated in Figure 5, the morphology of Sr vanadate is quite different from the Ca vanadate. The ellipsoid-like structures of Sr vanadate are formed via self-assembly of many individual 1D short nanorods (length of ca. 110 nm, refer to the structural model, inset Figure 5b). The strong and sharp peaks in XRD patterns (Figure 4b) show that Sr vanadate products are highly crystalline, which can be indexed to the hexagonal phase of Sr10V6O25. The clear lattice fringe with an interplanar spacing of 0.290 nm matches well with the distance between two (300) planes of hexagonal Sr10V6O25. Likewise, the SAED patterns of several assembled nanorods shown in Figure 5e indicate the single crystalline nature of each individual nanorod. Figure S9 gives the XPS spectra of Sr10V6O25. The binding energies of 2p3/2 level of vanadium and 3d5/2 level of strontium are 517.2 eV and 133.2 eV, respectively, in accordance with their chemical composition. Similar to the previous result, as shown in Figure


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4b, no impurity phase was detected if the amount of strontium salt was low (e.g., 8 mM or 22 mM), and the respective vanadium recovery was 49.2% and 95.7%. However, as the molar concentration of strontium salt increases to 84 mM, strontium carbonate was found in addition to the predominant vanadate product. Therefore, high-purity and uniform strontium vanadate nanomaterials can be synthesized at an optimal strontium chloride salt amount of 22 mM, in which the vanadium recovery was 95.7%. As clearly seen from Figure 5f,g, the Sr, V, and O elements are uniformly distributed over the structure. Moreover, we found that the usage of strontium acetate resulted in a similar product to the one by using strontium chloride (see Figure S10). Again, strontium vanadate products obtained from room temperature have irregular morphology (see Figure S11). Characterizations of Barium Vanadate. Another interesting material derived from the leaching solution is barium vanadate. As shown in Figure 6, barium vanadate is in the form of isolated polyhedral nanoparticles with 56±16 nm in size. According to XRD results (Figure 4c), the product can be identified as barium orthovanadate Ba3V2O8. The interplanar spacing of the crystal shown in Figure 6d is 0.288 nm which can be assigned to (110) plane in Ba3V2O8. SAED pattern in Figure 6e confirms the crystalline nature of the particles. In addition, EDX spectra (Figure 6f) and elemental maps (Figure 6g) further confirm the elemental compositions of the product. The different morphology of barium vanadate with the other two vanadates (Ca, Sr) is caused by their intrinsically different crystal structures (trigonal phase vs. hexagonal phase). Importantly, the morphology and size control of our barium orthovanadate product are even better than the one reported in literature which uses pure Na3VO4 chemical as a precursor under microwave solvothermal treatment followed by further heat-treatment (600oC, 3 h).37 The XPS characterizations suggest that Ba and V are in their expected +2 and +5 oxidation states,


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respectively (Figure S12), consistent with the literature.24 As the molar concentration of barium acetate was 8 mM and 24 mM, the recovery of vanadium was 54.7% and 99.2%, respectively. Pure product was formed under these conditions (Figure 4c). Carbonate was found as a byproduct when a higher amount of salt (84 mM) was used. As a result, barium acetate with a concentration of 24 mM was used to achieve both high recovery efficiency and high purity of the product. Furthermore, we found that the barium vanadate product has a similar shape when using barium chloride as a precursor, as illustrated in Figure S13. Similarly, the barium vanadate prepared at room temperature exhibits poor morphology uniformity (Figure S14). Metal Recovery from Acid Leaching Solution. Apart from alkaline leaching solution, the three metals (Fe, Ni, and V) in acid leaching solution can also be utilized. The following integrated processes were used to recover all the three metals: (a) adding NaOH to the acid leaching solution to precipitate the Ni and Fe ions, and (b) adding alkaline earth salts to recover V ions. It was observed that recovery of Fe and Ni was highly dependent on the concentration of NaOH. As shown in Figure S15a, the recovery efficiencies of Fe and Ni were 5.2% and 87.4 %, respectively, by adding NaOH (0.2 M) to the acid leaching solution, while complete removal of the two metals was achieved when NaOH concentration was raised to 0.5 M. The precipitation was based on reactions: Mn+ + n(OH)− ⇆ M(OH)n↓. Based on XRD (Figure 4d) and EDX (Figure S16) characterizations, the Ni and Fe ions were finally precipitated in the forms of Ni(OH)2 nanoplates and Fe2O3 polyhedral particles, respectively. Fe2O3 was likely obtained from the decomposition of α-FeOOH during the hydrothermal treatment.38 In terms of vanadium recovery in the form of calcium vanadate, apparently, as shown in Figure S15b, vanadium recovery was highly dependent on the amount of calcium salts. Moreover, it should be noted that without the addition of NaOH to the acid leaching solution, vanadium was hardly recovered even


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by adding a lot of calcium salts. This indicates that acidic environment favors the formation of condensed vanadate species (e.g., polyoxovanadates) and the crystallization of vanadate product is therefore not feasible. Similarly, the effects of anions of the precursors and different alkaline earth cations (such as Mg2+, Ca2+, Sr2+, and Ba2+) on vanadium recovery were also investigated (refer to Figure S15c,d). The trends are similar to our aforementioned observations in Figure 2. TEM images of the recovered solids from acid leaching solution are displayed in Figure S17. Overall, nearly 100% V, Ni, and Fe could be leached then recovered via the developed reactive crystallization methods, though the products would be mixtures of Ca10V6O25 nanorods, Ni(OH)2 nanoplates and Fe2O3 nanopolyhedrons. Optical Property Study. Furthermore, optical properties of the as-prepared alkaline earth vanadates were studied by UV–vis–NIR diffuse reflectance spectrum. As shown in Figure S18, the measured band gap energies for Ca vanadate, Sr vanadate, and Ba vanadate are 4.1 eV, 4.3 eV, and 3.9 eV, respectively. It was found that the band gaps are slightly larger than that of micron-sized alkaline earth vanadates.39 The semiconducting properties of alkaline earth vanadates have been reported, which is associated to hoping process.40, 41 In fact, the alkaline earth vanadates obtained belong to the wide bandgap semiconductor materials, unlike the transition metal vanadates (e.g., BiVO4, MnV2O6) with narrow band gap known as photocatalysts.42, 43 Therefore, these semiconductor materials might find promising application in electronic devices to be operated at higher temperature or larger voltages,44, 45 or semiconducting glasses.41 ■ Conclusions


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In summary, we have developed a new synthetic protocol to fabricate vanadate nanomaterials from the leaching solutions of oil refinery waste. Accordingly, three different alkaline earth vanadate nanomaterials have been fabricated, including calcium vanadate nanorods (Ca10V6O25), strontium vanadate nanorods (Sr10V6O25), and barium vanadate polyhedral nanoparticles (Ba3V2O8). Moreover, the resultant products with high purity and uniform morphology/size are ideal candidates for electronic devices and semiconducting glasses. By estimation, one ton of dry carbon black waste will yield approximately 52.5 kg, 75.1 kg, and 91.5 kg of such Ca, Sr, and Ba vanadate nanomaterials, respectively. Overall, the proposed synthetic protocol offers not only a novel pathway for the low-cost preparation of uniform vanadate nanomaterials in the absence of an auxiliary surfactant, but also a potential solution to mitigate hazardous vanadium pollution. ■ ASSOCIATED CONTENT

Supporting Information. More results for carbon black waste leaching experiments, materials characterizations and applications, supplementary figures (Figures S1 to S18) and tables (Tables S1, S2) are available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: C. H. Wang: Tel.: +65-65165079; Fax: +65-67791936; E-mail address: [email protected] ORCID Guowu Zhan: 0000-0002-6337-3758 Chi-Hwa Wang: 0000-0002-1040-2121


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■ ACKNOWLEDGMENT This research programme is funded by the National Research Foundation Singapore, Sembcorp Industries Ltd and National University of Singapore under the Sembcorp-NUS Corporate Laboratory (R261-513-008-281 & R261-513-008-282). We thank Prof. Hua Chun Zeng for providing laboratory facilities for the execution of this project. ■ REFERENCES (1) Lin, C. S. K.; Pfaltzgraff, L. A.; Herrero-Davila, L.; Mubofu, E. B.; Abderrahim, S.; Clark, J. H.; Koutinas, A. A.; Kopsahelis, N.; Stamatelatou, K.; Dickson, F.; Thankappan, S.; Mohamed, Z.; Brocklesby, R.; Luque, R. Food Waste as a Valuable Resource for the Production of Chemicals, Materials and Fuels. Current Situation and Global Perspective. Energy Environ. Sci. 2013, 6 (2), 426-464. doi: 10.1039/C2EE23440H. (2) Ashokkumar, M.; Narayanan, N. T.; Gupta, B. K.; Reddy, A. L. M.; Singh, A. P.; Dhawan, S. K.; Chandrasekaran, B.; Rawat, D.; Talapatra, S.; Ajayan, P. M.; Thanikaivelan, P. Conversion of Industrial Bio-Waste into Useful Nanomaterials. ACS Sustainable Chem. Eng. 2013, 1 (6), 619-626. doi: 10.1021/sc3001564. (3) Maroufi, S.; Mayyas, M.; Sahajwalla, V. Novel Synthesis of Silicon Carbide Nanowires from E-Waste. ACS Sustainable Chem. Eng. 2017, 5 (5), 4171-4178. doi: 10.1021/acssuschemeng.7b00171. (4) Yang, C.; Tan, Q.; Liu, L.; Dong, Q.; Li, J. Recycling Tin from Electronic Waste: A Problem That Needs More Attention. ACS Sustainable Chem. Eng. 2017, 5 (11), 9586–9598. doi: 10.1021/acssuschemeng.7b02903. (5) Navarro, R.; Guzman, J.; Saucedo, I.; Revilla, J.; Guibal, E. Vanadium Recovery from Oil Fly Ash by Leaching, Precipitation and Solvent Extraction Processes. Waste Manage. 2007, 27 (3), 425-438. doi: 10.1016/j.wasman.2006.02.002. (6) www.bp.com/statisticalreview. (7) Sasaki, T.; Maki, H.; Ishihara, M.; Harayama, S. Vanadium as an Internal Marker to Evaluate Microbial Degradation of Crude Oil. Environ. Sci. Technol. 1998, 32 (22), 3618-3621. doi: 10.1021/es980287o. (8) Breslin, V. T.; Duedall, I. W. Vanadium Release from Stabilized Oil Ash Waste in Seawater. Environ. Sci. Technol. 1988, 22 (10), 1166-1170. doi: 10.1021/es00175a006. (9) Dong, P.; Maneerung, T.; Ng, W. C.; Zhen, X.; Dai, Y.; Tong, Y. W.; Ting, Y.-P.; Koh, S. N.; Wang, C.-H.; Neoh, K. G. Chemically Treated Carbon Black Waste and Its Potential Applications. J. Hazard. Mater. 2017, 321 62-72. doi: 10.1016/j.jhazmat.2016.08.065. (10) Zhen, X.; Ng, W. C.; Fendy; Tong, Y. W.; Dai, Y.; Neoh, K. G.; Wang, C.-H. Toxicity Assessment of Carbon Black Waste: A by-Product from Oil Refineries. J. Hazard. Mater. 2017, 321 600–610. doi: 10.1016/j.jhazmat.2016.09.043. (11) Fu, F.; Wang, Q. Removal of Heavy Metal Ions from Wastewaters: A Review. J. Environ. Manage. 2011, 92 (3), 407-418. doi: 10.1016/j.jenvman.2010.11.011.


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List of Figures

Figure 1. Characterizations of carbon black waste particles. (a-b) TEM images, inset in (b): photograph of dry carbon black waste in a glass bottle, (c) EDX elemental mapping, (d) UV-vis spectra of the leaching solutions, insets: photographs of the leaching solutions, (e) UV-vis spectra of the alkaline leaching solution after adjusting pH, insets illustrate the color change, and (f) 51V NMR spectra recorded for alkaline leaching solutions at different pH values.


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Figure 2. Recovery of vanadium from alkaline leaching solution at both room temperature and hydrothermal condition: (a) the effect of different calcium salts, keeping the constant calcium ion molar concentration as 84 mM, (b) the effect of different barium salts, keeping the constant barium ion molar concentration as 84 mM, (c) the effect of alkaline earth metals, keeping the constant metal salt amount as 0.42 mmol (molar concentration: 84 mM), and (d) the effect of the amount of calcium nitrate. Note: the initial vanadium concentration in the leaching solution was 705 ppm and the volume of the leaching solution was 5 mL.


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Figure 3. Characterizations of calcium vanadate nanorods prepared by using alkaline leaching solution (V: 705 ppm, volume: 5 mL) with calcium nitrate under hydrothermal treatment. (a-c) TEM images, inset in (b): structural model, (d) high-resolution TEM image, inset is the corresponding fast Fourier transform (FFT) pattern, (e) SAED pattern; the inset shows the illuminated area, (f) EDX spectra, and (g) EDX elemental mapping.


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Figure 4. (a) XRD patterns of calcium vanadate products by using different molar concentration of calcium nitrate, (b) XRD patterns of strontium vanadate products by using different molar concentration of strontium chloride, (c) XRD patterns of barium vanadate products by using different molar concentration of barium acetate, and (d) XRD pattern of the precipitate solids from the acid leaching solution (V: 715 ppm, volume: 5 mL) with the addition of NaOH aqueous solution (1 M).


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Figure 5. Characterizations of strontium vanadate nanorod with ellipsoid-like structure prepared by treating alkaline leaching solution (V: 705 ppm, volume: 5 mL) with strontium chloride. (a-c) TEM images, inset in (b): structural model, (d) HRTEM image, inset the corresponding FFT pattern, (e) SAED, inset shows the illuminated area, (f) EDX spectra, and (g) EDX elemental maps.


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Figure 6. Characterizations of barium vanadate polyhedral nanoparticles prepared by treating alkaline leaching solution (V: 705 ppm, volume: 5 mL) with barium acetate. (a-c) TEM images, inset in (b) shows the particle size distribution, inset in (c) shows a structural model, (d) HRTEM image, inset the corresponding FFT pattern, (e) SAED, inset shows the illuminated area, (f) EDX spectra, and (g) EDX elemental maps.


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FOR TABLE OF CONTENTS USE ONLY Synopsis: We have developed a transformative recovery of vanadium from leaching solutions of oil refinery waste into alkaline-earth vanadate nanomaterials.


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Template-Free Synthesis of Alkaline Earth Vanadate Nanomaterials

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