Biosorption–Incineration–Leaching–Smelting Sequential Process for

Article pubs.acs.org/IECR

Biosorption−Incineration−Leaching−Smelting Sequential Process for Ru Recovery from Ru-Bearing Acetic Acid Waste Solution Sung Wook Won,† In Seob Kwak,*,‡ Juan Mao,*,§ and Yeoung-Sang Yun*,∥ †

Department of Marine Environmental Engineering and Institute of Marine Industry, Gyeongsang National University, 38 Cheondaegukchi-gil, Tongyeong, Gyeongnam 650-160, Republic of Korea ‡ RTI Engineering R&D Center, Daejeon 306-220, Republic of Korea § Department of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ∥ School of Chemical Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea ABSTRACT: This study introduces a new method for the recovery of metallic Ru from Ru-bearing acetic acid waste solution through a biosorption−incineration−leaching−smelting sequential process. As a powerful biosorbent for binding anionic Ru complexes, polyethylenimine- (PEI-) modified bacterial biosorbent fiber (PBBF) was prepared by extruding a blended slurry of chitosan and Corynebacterium glutamicum biomass in fiber form, applying a PEI coating, and cross-linking with glutaraldehyde. Biosorption isotherm studies revealed that PBBF showed a much higher Ru uptake than raw biomass and anion-exchange resin (Lewatit MonoPlus M600). In addition, PBBF was stable at water contents less than 10%, and the Ru uptake of PBBF increased with increasing temperature. After sorption, Ru-sorbed PBBF was incinerated, and the metallic form of Ru was recovered. Several oxidizing agents, such as sodium hypochlorite, potassium permanganate, and aqua regia, were sequentially used to leach out the impurities from the ash. Through X-ray photoelectron spectroscopy analysis, it was found that the recovered Ru was present in solid metallic form. According to X-ray fluorescence spectrometry analysis, the metallic Ru achieved from this method had a high weight percentage of 99.75%.

1. INTRODUCTION Platinum-group metals (PGMs), namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum, have been exploited for industrial applications because of their outstanding catalytic properties. About 75% of the acetic acid utilized in the chemical industry is obtained by the carbonylation of methanol.1 Of the PGMs, ruthenium is a versatile catalyst; in particular, organometallic ruthenium carbine and alkylidene complexes have been found to be highly efficient catalysts for use in acetic acid chemistry.2 These catalysts, containing PGMs in complex compounds with organic materials, are discharged in industrial effluents, not only causing environmental problems but also wasting limited resources. Hence, the recovery of these precious metals from acetic acid manufacturing effluents is environmentally and economically attractive. Traditionally, hydrometallurgical and pyrometallurgical processes have been used for the recovery of precious metals or removal of heavy metals from industrial effluents. However, these methods have some disadvantages such as low recovery efficiencies, incomplete metal removal, high capital costs, high chemical and/or energy requirements, and secondary wastes that require disposal.3,4 Adsorption processes have been considered as among the most attractive emerging processes for the removal and/or recovery of precious metals.5 However, the recovered metals in the concentrated solution obtained through adsorption exist in ionic form and should be reduced again to metallic form using reducing agents. Therefore, it is necessary to revise the existing process through a suitable process for the recovery of these precious metals from organic acid effluents. © XXXX American Chemical Society

Biosorption is known as an emerging technology that is capable of recovering or removing ionic pollutants, such as metals and dyes, from aqueous solutions using various biomaterials.6 Inactive Ru species are usually in anionic organic form in acetic acid waste solution. Hence, the main sorption behavior takes place between binding sites on the surface of the biosorbent and anionic metal complexes. Many different biomaterials have been studied as biosorbents, but most show low sorption capacities compared to those of commercial ionexchange resins. As an alternative sorption enhancement technique, a surface modification method has been found to be effective.7,8 As previously reported, coating biomaterials with an ionic polymer, such as polyethylenimine (PEI), which contains plenty of primary and secondary amine groups, is also an efficient way to obtain high-performance biosorbents for ionic pollutants.9 For the disposal of spent biosorbent, incineration has been shown to be an effective pathway for the recovery of precious metals in metallic form.10,11 However, real industrial acetic acid wastewaters contain a variety of metals, which inevitably means the presence of impurities in the incinerated ash. Leaching with several oxidizing agents, such as NaClO, KMnO4, and aqua regia, could be followed by further recovery to remove the impurities, such as Mo, Ni, Fe, and K. A new process for recovering metallic ruthenium from acetic acid effluent is proposed herein, with a combined sequential process of biosorption−incineration−leaching−smelting. PreReceived: March 24, 2015 Revised: June 29, 2015 Accepted: June 30, 2015

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DOI: 10.1021/acs.iecr.5b01111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

agitated, neutralized with sulfuric acid, and then washed with distilled water. The washed fiber was placed into 1 L of PEI solution (3%, w/v) adjusted to pH 10 and stirred for 6 h at 25 °C. Then, the PEI-coated fiber was cross-linked with glutaraldehyde (0.6%) for 2 h to form imine groups between the amine groups in PEI and the biosorbent fiber. After filtration, the resultant PBBF was washed with distilled water to remove unreacted PEI and glutaraldehyde residues. Finally, the final product was lyophilized and sieved to obtain fibers with a uniform size. 2.3. Stability of PBBF at Different Water Contents. The biosorbent fiber employed in this study was examined to determine whether it is stable in acetic acid waste solutions with different water contents. Each 1 g/L of PBBF was added to 50 mL polypropylene tubes containing 30 mL of acetic acid waste solutions with different water contents, namely, 0%, 5%, 10%, 15%, 20%, and 30%. The tubes were shaken in a shaking incubator at 160 rpm and 25 °C for 24 h. After sorption, the biosorbents were separated from the acetic acid waste solutions by using a high-speed microcentrifuge (Micro 17TR, Hanil, Incheon, Korea) at 8000 rpm for 5 min. The shape of the separated biosorbents was observed, and the Ru concentration remaining in the supernatant was analyzed by ICP-AES after proper dilution. The amounts of Ru sorbed (q, mg/g) by the biosorbents were calculated based on the mass balance equation as follows

viously, various methods have been reported for the separation and purification of Ru such as chemical methods, solvent extraction, precipitation, chromatography, and electrochemical methods.12 In practice, solvent extraction has been widely used for the separation of precious metals from multicomponent mixtures. However, this method requires plenty of expensive organic solvents and consumes a great deal of time because of multiple extraction steps. Precipitation is rarely applied for the separation of Ru from multicomponent wastewaters because of the complexity of the operation and the low degree of separation. On the other hand, our suggested method is mainly composed of two parts, namely, separation and purification. The separation of Ru using biosorption is a simple method compared with other current methods. Also, the attainment of high-purity Ru by sequential incineration, leaching, and smelting can be operated at the laboratory scale and is easy to control. In this study, the Gram-positive bacterium Corynebacterium glutamicum was immobilized as a fiber form of biosorbent using chitosan. PEI-modified bacterial biosorbent fiber (PBBF) was investigated as an appropriate biosorbent for Ru recovery from acetic acid waste solution. The valence state and purity of the Ru recovered by the suggested recovery process were also examined by X-ray photoelectron spectroscopy (XPS) and Xray fluorescence (XRF) spectrometry.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Chitosan (84% deacetylated, MW 3.0 × 105 Da) was supplied by Texanmedtech (Seoul, Korea), and PEI with a molecular weight of 70000 Da was purchased from Habjung Moolsan (Seoul, Korea). Aqua regia solution was prepared by mixing concentrated HCl (37%) and HNO3 (65%) at a volume ratio of 3:1.13 Other reagents such as glutaraldehyde, NaClO, KMnO4, HCl, and HNO3 were of analytical grade. A fermentation bacterial biomass slurry containing 12.6% (w/v) C. glutamicum was obtained from Daesang Co. (Gunsan, Korea) after L-arginine fermentation. Ru-bearing acetic acid wastewater was collected from Samsung BP Chemicals Co., Ltd. (Ulsan, Korea) in precleaned 20-L plastic containers. Anionic Ru iodocarbonyl complexes such as [Ru(CO)3I3]− are contained in this wastewater.14 The main elements contained in the acetic acid waste solution were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES; ICPS-7510, Shimadzu, Kyoto, Japan) and are listed in Table 1.

q=

where Ci and Cf are the initial and final Ru concentrations (mg/ L), respectively; V is the working volume of the solution (L); and m is the weight of the biosorbent (g). 2.4. Effect of Temperature on the Ru Biosorption Capacity of PBBF. To evaluate the effect of temperature on Ru biosorption, experiments were carried out with 0.03 g of PBBF and 30 mL of acetic acid waste solution in the range of 20−100 °C. The samples were agitated at 160 rpm for 24 h in a shaking incubator. After reaching the equilibrium state, the samples were centrifuged for liquid−solid separation. The supernatant was used to analyze the residual Ru concentration after appropriate dilution as mentioned above. 2.5. Recovery of Ru from Ru-Sorbed PBBF. First, a combined method of biosorption and incineration was used to recover Ru from spent biosorbent. After biosorption at 25 °C, Ru-loaded PBBF was separated by centrifugation at 8000 rpm for 5 min and washed with acetic acid several times to remove residual Ru complex ions from the surface of the PBBF. The Ru-loaded PBBF was dried at 60 °C for 24 h in an oven. Thereafter, the dried biosorbent was incinerated in an electric furnace at 800 °C for 4 h. The incinerated ash was used for analysis by XPS (Theta Probe AR-XPS System, Thermo Fisher Scientific, Glasgow, U.K.) and XRF (M1MISTRAL, Bruker, Karlsruhe, Germany). These analyses were performed at Korea Basic Science Institute (KBSI) in Busan, Korea. Second, the impurities in the ash were leached out using oxidizing agents such as aqua regia and sodium hypochlorite (NaClO) with hydrochloric acid to obtain high-purity Ru. The incinerated ash was added to a solution of oxidizing agent in a 500 mL round-bottom flask. The mixture was heated to 150 °C for 16 h. The oxidizing-agent-treated ash was washed with distilled water three times and then centrifuged for solid and liquid separation. The final product was analyzed by XRF.

Table 1. Main Elements in Acetic Acid Waste Solution element

concentration (mg/L)

element

concentration (mg/L)

Ru Ir Cr Mn Fe

66.63 16.76 199.87 7.01 378.08

Co Ni Zr Mo

3.89 486.67 5.44 231.01

(C i − Cf )V m

2.2. Preparation of PEI-Modified Bacterial Biosorbent Fiber. The PBBF was prepared according to our previously reported method.15 Briefly, 5 g of chitosan and 100 mL of C. glutamicum slurry were mixed, and then 5 mL of acetic acid was added to the suspension to dissolve the chitosan. The mixture was stirred sufficiently for 24 h. The well-suspended mixture was extruded into 1 M NaOH solution using an injection syringe. The chitosan−C. glutamicum biomass blended fiber was B

DOI: 10.1021/acs.iecr.5b01111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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undesirable characteristic of PBBF as a biosorbent. However, this is no problem when the PBBF is applied for the recovery of Ru from acetic acid waste solutions, because the possible highest content of water in the real waste solutions is 0.25%. The biosorption of Ru by PBBF was examined at different temperatures (20, 40, 60, 80, and 100 °C). PBBF stably existed without any change at all temperatures examined in this work, and the effect of temperature on the biosorption of Ru by PBBF is shown in Figure 1. The results showed that the uptake

Third, to recover much purer Ru from ash, the incinerated ash was sequentially leached using NaClO, KMnO4, and aqua regia at 150 °C for 8 h. The ash was then washed with distilled water and separated by centrifugation. The settled material was smelted using an induction furnace (K-WI-1060, DIK, Korea) at 10 kW and 15−45 kHz. The recovered Ru alloy was analyzed by XRF.

3. RESULTS AND DISCUSSION 3.1. Evaluation of PBBF as a Biosorbent for Ru Biosorption. As reported in a previous work, the biosorption isotherm study was conducted to evaluate the maximum Ru uptakes of raw biomass, PBBF, and anion-exchange resin (Lewatit MonoPlus M600).15 According to the Langmuir model, the maximum Ru uptakes of raw biomass, PBBF, and Lewatit MonoPlus M600 were estimated to be 16.0, 110.5, and 6.7 mg/g, respectively. The maximum Ru uptake of PBBF was enhanced by 6.9- and 16.5-fold compared to those of raw biomass and Lewatit MonoPlus M600, respectively. These results indicate that the Ru biosorption capacity of PBBF was significantly enhanced by the introduction of PEI onto the surface of the biosorbent fiber. The positively charged amine groups in PEI molecules might be responsible for binding anionic Ru complexes through electrostatic attraction.15,16 Therefore, the high Ru biosorption capacity of PBBF is one of the most important advantages of the biosorbent for treating Ru-bearing acetic acid waste solution. To evaluate the stability of PBBF under conditions with different water contents, sorption experiments were carried out in acetic acid waste solutions with water contents of 0%, 5%, 10%, 15%, 20%, and 30%. The results are summarized in Table 2. PBBF was stable up to a water content of 10% , whereas PBBF in acetic acid waste solutions with water contents greater than 10% were unstable and even dissolved. Thus, PBBF is appropriate for use at water contents in the range of 0−10%. On the other hand, the Ru uptake of PBBF decreased from 32.42 to 22.40 mg/g as the water content was increased from 0% to 10%. Actually, the decrease in Ru uptake is an

Figure 1. Effect of temperature on the Ru uptake (sorbent amount, 30 mg; waste solution volume, 30 mL; temperature range, 20−100 °C).

of Ru increased from 28.70 to 33.47 mg/g as the temperature was increased from 20 to 80 °C. Meanwhile, the Ru uptake decreased slightly at 100 °C, likely because of thermal damage to the biosorbent fiber. However, utilization of PBBF for Ru sorption could be favorable for practical applications because the temperature of real acetic acid effluent is close to 80 °C. 3.2. Combined Method of Biosorption and Incineration for Ru Recovery. After Ru biosorption, the Ru-sorbed PBBF was incinerated to recover the Ru in metallic form from the spent biosorbent. XPS analysis was performed to identify the Ru valence existing in the ash. Figure 2 shows the Ru 3d XPS spectrum for the Ru contained in the ash and indicates the

Table 2. Stability of PBBF at Different Water Contents

Figure 2. Ru 3d XPS spectrum of the incinerated ash (incineration conditions: 800 °C, 4 h). C

DOI: 10.1021/acs.iecr.5b01111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research oxidation state of the Ru in the ash. The peak for Ru 3d5/2 around 280.2 eV indicates the presence of metallic Ru.17 From the XPS results, it is likely that Ru in the fibers might be partially reduced to metallic form through incineration with oxidation of organic substances in PBBF. Also, in the same manner as reported in the literature, other PGMs such as palladium and platinum have been successfully recovered in metallic form by the combined process of biosorption and incineration.11,16,18 The Ru-sorbed biosorbent fiber and incinerated ash were analyzed by XRF (see Figure 3). Ru and many other impurities

Figure 4. Comparison by XRF of incinerated and aqua regia-leached ash (incineration conditions: 800 °C, 4 h ; aqua regia-leaching conditions: 200 mL of aqua regia, 150 °C, 16 h).

frequently used as a disinfectant or bleaching agent. In this study, sodium hypochlorite was used to leach out the impurities from the ash. The leaching effect of sodium hypochlorite on the impurities was investigated as presented in Figure 5. These

Figure 3. Comparison by XRF of PBBFs after sorption and after incineration (sorption conditions: 30 mg of sorbent, 30 mL of waste solution, 160 rpm, 24 h; incineration conditions: 800 °C, 4 h).

were present in the biosorbent fiber. After incineration, the ash also contained Ru and many other impurities, except the element of iodine, which was included in the original waste solution. Accordingly, the content of Ru increased from 5.40 to 24.84 wt % through incineration. Although the combined method of biosorption and incineration can recover Ru in metallic form from spent fiber, the recovered Ru still contains many impurities. Thus, an appropriate method that can enhance the purity of the recovered Ru should be explored. Several oxidizing agents were thus used to leach impurities from the ash, as described in the next section. 3.3. Leaching Effect of Aqua Regia. Because Ru is insoluble in aqua regia,19 the ash generated after incineration of Ru-sorbed PBBF was treated with aqua regia twice to leach out impurities such as acid-soluble metals. The XRF results for the aqua regia-treated ash were compared with those for the incinerated ash as shown in Figure 4. In the first treatment of aqua regia, Mo, W, and K were removed from the ash. The contents of Ni and Fe in the ash also decreased from 29.19 and 18.71 wt % to 10.59 and 11.65 wt %, respectively, upon aqua regia leaching. However, the weigh percentage of Cr barely changed after aqua regia leaching. Compared to the content of Ru in the untreated ash, the content of Ru in the aqua regiatreated ash increased to 65.07 wt %. To further remove the impurities from the ash, the aqua regia-treated ash was leached with aqua regia again. In this case, the content of Ru in the ash increased from 24.84 to 72.53 wt % as a result of the sequential treatment with aqua regia. 3.4. Leaching Effect of Sodium Hypochlorite. Sodium hypochlorite (NaClO) is a strong oxidizing agent that is

Figure 5. Comparison by XRF of incinerated and sodium hypochlorite-leached ash (incineration conditions: 800 °C, 4 h ; sodium hypochlorite-leaching conditions: 200 mL of sodium hypochlorite solution, 150 °C, 16 h).

results are very similar to the leaching effects of aqua regia. After the treatment with NaClO, the content of Ru in the ash was enhanced to 69.27 wt %. Other impurities such as Mo, W, and K were largely removed from the ash. Portions of the Ni and Fe were leached out as well. However, a phenomenon similar to that observed in leaching with aqua regia occurred: Cr remained in the ash after the treatment with sodium hypochlorite. Although sodium hypochlorite leads to the leaching of impurities from the incinerated ash, it requires an improved method for recovering Ru with high purity. 3.5. Suggestion of a Biosorption−Leaching−Smelting Process for Recovery of Ru and Its Proof of Concept. According to the results of section 3.3 and section 3.4, aqua regia and sodium hypochlorite can be used as oxidizing agents for leaching out metal impurities present in the ash. On the D

DOI: 10.1021/acs.iecr.5b01111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research other hand, the Cr was seldom leached from the ash using these agents. Thus, potassium permanganate (KMnO4 ) was employed to remove Cr from the ash, and a combined process of biosorption, incineration, leaching, and smelting was suggested for the recovery of high-purity metallic Ru from Ru-containing acetic acid waste solution (Figure 6). The series

Figure 7. Comparison by XRF of incinerated and NaClO/KMnO4/ aqua regia-leached ash (incineration conditions: 800 °C, 4 h; NaClO/ KMnO4/aqua regia-leaching conditions: 200 mL of NaClO, 200 mL of KMnO4, 200 mL of aqua regia, 150 °C, 8 h).

A combined method of biosorption and incineration was used to recover Ru from Ru-loaded biosorbent, but the resultant ash contained many impurities as well as Ru. The incinerated ash was subjected to leaching using NaClO and aqua regia. The content of Ru in the oxidizing-agent-treated ash was raised to 69.27 and 72.53 wt %. To achieve higher-purity Ru, the impurities in ash were removed by sequential leaching and smelting. As a result of this process, a lump of metallic Ru was obtained, and the Ru content was evaluated to be 99.75 wt % by XRF. Therefore, the proposed sequential process of biosorption, incineration, leaching, and smelting can be considered as a promising and effective method for the recovery of Ru from acetic acid waste solution.

Figure 6. Suggested process for the recovery of high-purity metallic Ru from Ru-bearing acetic acid waste solution.

of leaching systems were mainly composed of sodium hypochlorite, potassium permanganate, and aqua regia as oxidizing agents. To confirm the possibility and effectiveness of this proposed process, the NaClO−KMnO4−aqua regia consecutive leaching system was applied to recover high-purity Ru, and then the treated ash was smelted using an induction furnace at 10 kW and 15−45 kHz. As a result, a lump of metallic Ru was obtained and analyzed by XRF. As shown in Figure 7, most of the metal impurities such as Mo, Ni, Cr, Fe, Mn, W, and K were removed, and high-purity Ru was obtained by the NaClO−KMnO4−aqua regia sequential leaching system. The content of Ru in the ash was recorded as 99.75 wt %, although a small amount of Ir (less than 0.25 wt %) existed in the ash. This indicates that potassium permanganate is an appropriate oxidant for removing Cr from the ash. Because KMnO4 is a very strong oxidant, it can likely convert Cr to an oxidized form such as Cr(III) at 150 °C.20 Therefore, the NaClO−KMnO4−aqua regia sequential leaching system can be also considered as an effective method for the recovery of highpurity metallic Ru.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-42-671-2133. Fax: +82-42-671-2134. E-mail: kwak@ rtieng.com (I.S.K.). *Tel.: +86-27-8779-2155. Fax: +86-27-8779-2101. E-mail: [email protected] (J.M.). *Tel.: +82-63-270-2308. Fax: +82-63-270-2306. E-mail: [email protected] (Y.-S.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1002177). This research was also supported by an NRF grant funded by the Korean government (MEST) (NRF-2014R1A2A1A09007378).

4. CONCLUSIONS PBBF was used as a potential biosorbent for Ru recovery from Ru-containing acetic acid waste solution. The characteristics of PBBF, such as its stability at different water contents and temperatures, were evaluated. As a result, PBBF was found to be stable in the range of 0−10% water contents, and Ru uptake was found to decrease with increasing water content. In addition, an increase in temperature led to an enhancement of the Ru sorption capacity of PBBF.

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REFERENCES

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DOI: 10.1021/acs.iecr.5b01111 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX