Chemosphere 117 (2014) 652–657
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Bioleaching of metals from steel slag by Acidithiobacillus thiooxidans culture supernatant Hong Hocheng ⇑, Cheer Su, Umesh U. Jadhav Department of Power Mechanical Engineering, National Tsing Hua University, No. 101, Sec. 2, Kuang Fu Rd., 30013 Hsinchu, Taiwan, ROC
h i g h l i g h t s Steel industry slag can be used as secondary resource for metals. Bioleaching process can be used to recover metals. The products of microbial metabolism are useful for metal solubilization.
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 26 September 2014 Accepted 29 September 2014
Keywords: Electric arc furnace slag Metals Bioleaching Microorganisms Culture supernatant
a b s t r a c t The generation of 300–500 kg of slag per ton of the steel produced is a formidable amount of solid waste available for treatment. They usually contain considerable quantities of valuable metals. In this sense, they may become either important secondary resource if processed in eco-friendly manner for secured supply of contained metals or potential pollutants, if not treated properly. It is possible to recover metals from steel slag by applying bioleaching process. Electric arc furnace (EAF) slag sample was used for bioleaching of metals. In the present study, before bioleaching experiment water washing of an EAF slag was carried out. This reduced slag pH from 11.2 to 8.3. Culture supernatants of Acidithiobacillus thiooxidans (At. thiooxidans), Acidithiobacillus ferrooxidans (At. ferrooxidans), and Aspergillus niger (A. niger) were used for metal solubilization. At. thiooxidans culture supernatant containing 0.016 M sulfuric acid was found most effective for bioleaching of metals from an EAF slag. Maximum metal extraction was found for Mg (28%), while it was least for Mo (0.1%) in six days. Repeated bioleaching cycles increased metal recovery from 28% to 75%, from 14% to 60% and from 11% to 27%, for Mg, Zn and Cu respectively. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Steel making industries are producing large amount of slag. With the world production of steel at about 900 million tons in 2002; by-products of over 300 million tons was generated (Hiltunen and Hiltunen, 2004). In Europe the total amount of steel slag generated in 2004 was about 15 million tons. Now a days in Europe this steel industry slag is reused for road construction (45%), interim storage (17%), internal recycling (14%), fertilizer (3%), hydraulic engineering (3%), and for cement production (3%) (Gahan et al., 2009). Although these recycling techniques are in use, yet significant amount of slag is being dumped into landfills (11%) or stockpiled for long periods at steel plants (Gomes and Pinto, 2006; Gahan et al., 2009). Because of the presence of heavy metals, the steel industry slag is considered as hazardous waste (Langova and Matysek, 2010). Therefore questions are being raised ⇑ Corresponding author. Tel.: +886 3 5162097; fax: +886 3 5722840. E-mail address: [email protected] (H. Hocheng). http://dx.doi.org/10.1016/j.chemosphere.2014.09.089 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
on landfilling of steel industry slag, as well as on its recycling (Proctor, 2000). For this reason steel slag must be treated in order to reduce its metal content (Vestola et al., 2010). Various aqueous solutions have been used for hydrometallurgical treatment of steel industry slag. However, these methods are favorable only when recoverable metals are present at relatively high levels (Langova and Matysek, 2010; Vestola et al., 2010). The microbial leaching process may offer a suitable alternative for industrial wastes containing low concentrations of metals (Solisio et al., 2002). Limited studies have been carried out on the bioleaching of metals from steel industry slag. Banerjee (2007) compared bioleaching potential of Acidithiobacillus ferrooxidans with a fungus. Bioleaching of 76–78% Zn and Al from EAF sludge was demonstrated by Solisio et al. (2002) using At. ferrooxidans in presence of a sulfur containing substrate. Gahan et al. (2009) utilized the BF slag as a neutralizing agent during pyrite oxidation by a mixed mesophilic culture. In another study Vestola et al. (2010) used mixed culture of sulfur oxidizing bacteria for recovery of Zn and Fe. The present study was carried out to find a suitable
H. Hocheng et al. / Chemosphere 117 (2014) 652–657
lixiviant for metal solubilization from an EAF slag containing low level of metals. The potential of culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger was compared for the solubilization of metals from an EAF slag. Information obtained during this study will help to establish an industrial-scale process for solubilization of metals from an EAF slag. 2. Materials and methods 2.1. Materials, preparation and test of EAF slag An EAF slag sample was collected from local company. The bulk density of an EAF slag was 3.59 (±0.0014) g cm 3. The particle size distribution of an EAF sample was measured by laser particle sizer. The mean particle diameter was calculated from granulometric data. An EAF slag sample of mean particle diameter 324.7 (±234.6) lm (data not shown) was prepared from the main sample by grinding and then it was used for bioleaching experiment. The surface area of an EAF slag particle was determined by Brunauer–Emmett–Teller (BET) method. The surface area found was 2.6832 m2 g 1 (data not shown). Ten milliliter aqua regia was added to 1 g EAF slag, in 100 mL beaker and agitated overnight at 150 rpm. Then the contents were heated at 50 °C for 1 h. After acid leaching, the small amount of residue was removed by filtration. Sample was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES). Table 1 shows chemical composition of this EAF slag sample. Elemental analysis of an EAF slag sample indicated that the major constituent of the sample was Fe. Other elements present in an EAF slag are Aluminum oxide (Al2O3), Magnesium oxide (MgO), Phosphorous pentoxide (P2O5). The presence of metals like Cr, Mo, Ba, and V is also detected. X-ray diffraction analysis was carried out for an EAF slag by X-ray powder diffractometer (TTRAX III, Rigaku, Japan, Co.). The XRD pattern of an EAF slag sample is very complex, with several overlapping peaks resulting from the many minerals present in the sample (Fig. 1). The mineral phases present in an EAF slag sample are magnetite (Fe3O4), calcite (CaCO3), Hematite (Fe2O3), Wustite (FeO), lime (CaO) and portlandite Ca(OH)2. It was not possible to detect presence of sulfide by the XRD analysis. 2.2. Growth of microorganisms and collection of cell free culture supernatant At. thiooxidans 80191, At. ferrooxidans 13823 and A. niger 34770 were obtained from the Food Industry Research and Development Institute (FIRDI), Taiwan. The basal 317 medium contained the Table 1 Chemical composition of EAF slag. Sr. no.
Component
Weight (lg g
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Iron oxide (Fe2O3) Calcium oxide (CaO) Aluminum oxide (Al2O3) Magnesium oxide (MgO) Phosphorous pentoxide (P2O5) Manganese oxide (MnO) Sodium oxide (Na2O) Potassium oxide (K2O) Silicon dioxide (SiO2) Cr Mo Ba V Pb Zn Cu Ni
3645.76 1231.24 352.95 313.0 308.33 185.42 18.36 4.24 3.93 8.80 8.18 3.03 2.46 1.08 0.741 0.719 0.14
1
)
653
followings per litre of glass-distilled water: 0.3 g (NH4)2SO4, 3.5 g K2HPO4, 0.5 g MgSO47H2O, and 0.25 g CaCl2. The pH was adjusted to 4.5 with sulfuric acid. 1.0% (w/v) elemental sulfur was pre-sterilized and added to 317 medium for growth and maintenance of At. thiooxidans. The flask culture was used as an inoculum (10% v/v) after ten days incubation at 30 °C temperature and 150 rpm shaking speed. The basal 9 K medium was used for growth of At. ferrooxidans. FeSO4 was used as energy source for growth of At. ferrooxidans (Jadhav et al., 2013). Sucrose medium was used for the growth of fungi (Xu and Ting, 2009; Jadhav and Hocheng, 2014; Hocheng et al., 2014). Cell free culture supernatant was collected as described by Jadhav and Hocheng (2013). 2.3. Water washing and bioleaching of EAF slag A portion of the EAF slag was prewashed with distilled water. A 10 g L 1 sample of dried EAF slag was mixed together with distilled water in a conical flask and stirred at 30 °C for 24 h. Aliquots of the water sample were taken and sent to metal analysis. An EAF slag was allowed to settle and the water was decanted to collect washed slag. The prewashed sample was dried at 60 °C overnight and subjected to bioleaching tests. Typically, 1 g L 1 of the prewashed EAF slag sample was mixed separately with culture supernatant of At. thiooxidans, At. ferrooxidans and A. niger. After incubating for sufficient time in a shaking incubator the suspension was filtered and the clear solution was sent for metal content analysis by means of ICP-OES. Effect of incubation time on metal recovery was studied. For this 1 g L 1 of the prewashed sample of EAF slag was mixed with At. thiooxidans culture supernatant. The flask was then incubated for 12 d in a shaking incubator and aliquots of the samples were taken at 3, 6, 9 and 12 d to analyze metal content by means of ICP-OES. Effect of repeated bioleaching cycles on metal recovery was studied. 1 g L 1 of the prewashed EAF slag was mixed with At. thiooxidans culture supernatant. The flask was then incubated for 6 d in a shaking incubator. After 6 d, slag and culture supernatant were separated. The fresh At. thiooxidans culture supernatant was again mixed with a slag sample and incubated for 6 d. This was repeated four times. After each bioleaching cycle aliquots of the samples were taken to analyze metal content by means of ICP-OES. The concentration of metals in the leach liquors was analyzed by Kontron S-35, ICP-OES. 3. Results and discussion 3.1. Effect of water washing The steel slags are alkaline in nature (Ziemkiewicz, 1998). The calcium-alumina-silicate complexes present in steel slag cause the pH to rise to high level (Gahan et al., 2009). This alkaline nature of slag can affect the bioleaching process. The water washing treatment was used to overcome this problem. An initial pH of an EAF slag sample was 11.2. After first water washing it remained the same. The second water washing reduced the pH significantly and it became 8.5, while after third water washing a slight decrease in pH observed (8.3). No change in the pH of an EAF slag sample was observed after the fourth water washing experiment (data not shown). Therefore in further experiments, an EAF slag sample was water washed for three times, dried and then used for bioleaching experiment. The metal solubilization during water washing experiment was studied. A negligible amount of metals dissolved in water. Also a decrease in metal solubilization was observed in successive water wash treatments except for Ni (Fig. 2). The Ni solubilization increased in the second water wash
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and again it decreased in the third water wash. The reason behind this is unclear. The water washing experiment helped in reducing the pH of the slag but did not contribute significantly in metal extraction. 3.2. Bioleaching of metals from EAF slag An initial bioleaching experiment was carried out to extract metals from an EAF slag sample. The culture supernatants of three microorganisms were tested for their potential of metal extraction, viz. At. thiooxidans, At. ferrooxidans, and A. niger. In a previous study it was found that A. niger produced 0.104 M citric acid (Jadhav and Hocheng, 2014; Hocheng et al., 2014). The pH of this A. niger culture supernatant was 1.1. The pH of At. ferrooxidans culture supernatant was 2.0. In the present study, the production of sulfuric acid by At. thiooxidans was studied for 14 d. Also the change in pH of growth medium during growth of At. thiooxidans was studied. Before inoculation of At. thiooxidans cells, the pH of the medium was 4.5. The pH of growth medium reduced to 3.3 after incubating
At. thiooxidans for 7 d due to production of 0.0006 M sulfuric acid. The pH further decreased to 1.8 after 14 d due to production of 0.016 M sulfuric acid (Fig. 3). These results are in accordance to Blais et al. (1992) who showed a drop in pH from 7.1 to 1.7, due to acidification. Fig. 4 shows metal extraction efficiency of culture supernatants of At. thiooxidans, At. ferrooxidans, and A. niger. In case of Mg, Mn, Cr, Mo and Pb, At. thiooxidans culture supernatant found more efficient for metal extraction. For Ni 14.8 (±0.94), 100 (±2.68) and 16.7 (±0.88)% recovery was observed using culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger respectively (data not shown). 100% recovery of Si was observed for all the three supernatants used while for Ca 13.5 (±0.30), 10.6 (±0.14) and 11.8 (±0.12)% recovery was obtained in three days using culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger respectively (data not shown). These results suggest that metal extraction by culture supernatants of At. thiooxidans and A. niger depend on presence of sulfuric acid and citric acid respectively. Another experiment was carried out to determine the leaching agent in case of At. ferrooxidans culture supernatant. The 9 K med-
Fig. 1. X-ray diffraction pattern for an EAF slag.
Fig. 2. Metal extraction during water washing of an EAF slag.
H. Hocheng et al. / Chemosphere 117 (2014) 652–657
Fig. 3. Time course of the production of sulfuric acid by At. thiooxidans and change in pH during the course.
Fig. 4. Bioleaching of metals from an EAF slag by culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger.
Fig. 5. Effect of incubation time on bioleaching of metals from an EAF slag by At. thiooxidans culture supernatant.
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ium without bacteria and Fe2+ ions was used for metal extraction from an EAF slag. The metal extraction for Mg (14.1%), Ni (11.3%), Al (8.3%), Cu (7.3%), Fe (6.4%), Mn (6.3%), Zn (6.2%), Cr (1.0%), Pb (0.01%), Mo (0.03%) was observed during this experiment (data not shown). This metal extraction occurred due to acidic nature of 9 K medium. The metal extraction increased slightly when At. ferrooxidans culture supernatant was used (Fig. 4). The possibility of precipitation of Fe increases at higher pH (Changqiu et al., 2006; Daoud and Karamanev, 2006). Therefore while using At. ferrooxidans culture supernatant for bioleaching experiment the pH was maintained to 2.0 by external addition of sulfuric acid. An increase in metal extraction by At. ferrooxidans culture supernatant as compared to 9 K medium might be due to this external addition of sulfuric acid. These results suggest that major metal extraction by At. ferrooxidans culture supernatant occurred due to low pH rather than Fe3+. An external addition of sulfuric acid may also be responsible for increased Ni (100%) extraction by At. ferrooxidans culture supernatant. Although an efficiency of At. thiooxidans culture supernatant for Zn and Cu extraction is less as compare to At. ferrooxidans culture supernatant, the difference is not significantly large. Also for Mg, Mn, Cr, Mo and Pb, At. thiooxidans culture supernatant showed better bioleaching performance. According to Vestola et al. (2010), in industrial solid waste materials the metals are present in the form of oxides, carbonates and silicates rather than sulfides. It is easier to leach metals via sulfuric acid generated by acidophiles rather than conventional bioleaching with ferric iron (Vestola et al., 2010). Therefore, At. thiooxidans culture supernatant has been used in further bioleaching experiments. 3.3. Effect of incubation time on metal bioleaching At. thiooxidans culture supernatant having pH 1.8 was collected and used to study effect of incubation time on metal bioleaching from an EAF slag sample. The metal extraction increased with an increase in incubation time for all the metals tested. Major metal extraction was achieved in first 3 d and then the metal extraction increased slightly for 6 and 9 d. Further incubation for 12 d has not increased metal extraction (Fig. 5). From the results it is clear that there is a negligible difference in metal extraction at 6 and 9 d. Therefore 6 d incubation is considered optimum incubation time for metal extraction. These results are comparable with Vestola et al. (2010) who studied effect of various conditions on metal extraction from steel industry slag. They showed 36% and 32% metal extraction for Zn and Fe respectively in 42 d. In the
present study, 16 (±1.17) and 9.1 (±1.81)% metal extraction for Zn and Fe was achieved in 9 d (Fig. 4). Amount of Ca removed in 3, 6 and 9 d was 13.5 (±0.30), 14.9 (±0.04) and 16.3 (±0.34)% respectively (data not shown). Although the metal extraction efficiency was less as compared to Vestola et al. (2010), it was achieved in less time. Vestola et al. (2010) inoculated the media with a mixed culture, containing Acidithiobacillus spp. and Leptospirillum spp which decreased the pH of the growth medium to 1.5. In the present study, pure culture of At. thiooxidans was used and this microorganism was able to bring the pH of the growth medium to 1.8. These might be the possible reasons for less metal extraction in the present study as compared to Vestola et al. (2010). In spite of initial water washing experiment, the pH of steel slag was 8.3. Therefore some acid produced by At. thiooxidans in culture supernatant may have been consumed by steel slag and it was not available for bioleaching process. Also Vestola et al. (2010) were either able to leach only Zn and Fe or they have not considered other metals. In this respect the present study is found more useful, since more metals have been removed from an EAF slag. Several researchers reported metal recovery processes from steel slag. Langova and Matysek (2010) used acid pressure leaching process for recovery of zinc from steel slag. They obtained high percentage of solubilized metals as compared to bioleaching studies. Also very less time is required for metal extraction by using acid pressure leaching process. But this method has some disadvantages. Use of hydrogen peroxide and acid at high temperature and pressure create safety issues. Also the cost of acid transportation will affect the process economics (Bosecker, 1997). These problems can be avoided using microorganisms. Vestola et al. (2010) used At. thiooxidans while Bayat et al. (2009); and Solisio et al. (2002) used At. ferrooxidans for bioleaching of metals from steel slag. The comparison of bioleaching process explained in the present method with the reported literature show that more number of metals extracted in the present method. The time required for metal extraction was less as compared to reported bioleaching processes. Another experiment was carried out to increase the metal extraction. For this purpose bioleaching cycles were repeated for four times. For each cycle an EAF slag sample was incubated with At. thiooxidans culture supernatant for 6 d. Fig. 6 shows metal extraction after each cycle. It is found that for Mg the metal extraction increased from 28 (±1.38) to 75 (±2.04)%. Similarly for Zn, Cu and Fe the metal extraction increased from 14 (±0.94) to 60 (±0.32)%, 11 (±1.05) to 27 (±0.58)% and 9 (±1.08) to 25 (±0.28)%
Fig. 6. Effect of repeated bioleaching cycles on extraction of metals from an EAF slag by At. thiooxidans culture supernatant.
H. Hocheng et al. / Chemosphere 117 (2014) 652–657
respectively. During repeated bioleaching cycles the metal extraction for Ca increased from 13.5 (±0.30) to 48 (1.06)% (data not shown). A slight increase in metal extraction was also observed for Al, Mn, Cr and Pb. In case of Ni and Mo the metal extraction remained the same throughout the four bioleaching cycles (Fig. 6). The repeated bioleaching cycles increased metal recovery and it required total 24 d for the said amount of metal recovery. These results are advantageous as compared to reported bioleaching processes. Bayat et al. (2009) showed 35% Zn recovery in 24 d, while Vestola et al. (2010) showed 48% Zn recovery in 79 d. 4. Conclusion This study showed that the leaching behavior of metals varied for each microbial culture supernatant. The results suggest that it is easier to leach metals via sulfuric acid generated by At. thiooxidans. An optimum metal extraction was achieved in six days by At. thiooxidans culture supernatant. Considering the time required for metal extraction, the method described in the present study appear to be more advantageous as compared to reported bioleaching processes. The metal extraction efficiency was further increased by applying repeated bioleaching cycles. The two step bioleaching process described in the present study will not only support to overcome pollution problems created by steel industry slags but also to satisfy increasing metal demands. References Banerjee, D., 2007. Metal recovery from blast furnace sludge and flue dust using leaching technologies. Res. J. Chem. Environ. 11, 18–21. Bayat, O., Sever, E., Bayat, B., Arslan, V., Poole, C., 2009. Bioleaching of zinc and iron from steel plant waste using Acidithiobacillus ferrooxidans. Appl. Biochem. Biotechnol. 152, 117–126.
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Blais, J., Auclair, J., Tyagi, R., 1992. Cooperation between two Thiobacillus strains for heavy metal removal from municipal sludge. Can. J. Microbiol. 38, 181–187. Bosecker, K., 1997. Bioleaching: metal solubilization by microorganisms. FEMS Microbiol. Rev. 20, 591–604. Changqiu, W., Shengfeng, M., Anhuai, L., Jiangong, Z., 2006. Experimental study on formation conditions of ammoniojarosite and its environmental significance. Acta Geol. Sin.-Engl. 80, 296–301. Daoud, J., Karamanev, D., 2006. Formation of jarosite during Fe2+ oxidation by Acidithiobacillus ferrooxidans. Miner. Eng. 19, 960–967. Gahan, C., Cunha, M., Sandstrom, A., 2009. Comparative study on different steel slags as neutralizing agent in bioleaching. Hydrometallurgy 95, 190–197. Gomes, J., Pinto, C., 2006. Leaching of heavy metals from steelmaking slags. Rev. Metal Madrid 42, 409–416. Hiltunen, R., Hiltunen, A., 2004. Environmental aspects of the utilization of steel industry slags. In: VII International Conference on Molten Slags, Fluxes and Salts. The South African Institute of Mining and Metallurgy, pp. 357–362. Hocheng, H., Hong, T., Jadhav, U., 2014. Microbial leaching of waste solder for recovery of metal. Appl. Biochem. Biotechnol. 173, 193–204. Jadhav, U., Hocheng, H., 2013. Extraction of silver from spent silver oxide–zinc button cells by using Acidithiobacillus ferrooxidans culture supernatant. J. Clean. Prod. 44, 39–44. Jadhav, U., Hocheng, H., 2014. Use of Aspergillus niger 34770 culture supernatant for tin metal removal. Corros. Sci. 85, 248–255. Jadhav, U., Hocheng, H., Weng, W., 2013. Innovative use of biologically produced ferric sulfate for machining of copper metal and study of specific metal removal rate and surface roughness during the process. J. Mater. Process. Technol. 213, 1509–1515. Langova, S., Matysek, D., 2010. Zinc recovery from steel-making wastes by acid pressure leaching and hematite precipitation. Hydrometallurgy 101, 171–173. Proctor, D., 2000. Physical and chemical characteristics of blast furnace, basic oxygen furnace, and electric arc furnace steel industry slags. Environ. Sci. Technol. 34, 1576–1582. Solisio, C., Lodi, A., Veglio, F., 2002. Bioleaching of zinc and aluminium from industrial waste sludges by means of Thiobacillus ferrooxidans. Waste Manage. 22, 667–675. Vestola, E., Kuusenaho, M., Närhi, H., Tuovinen, O., Puhakka, J., Plumb, J., Kaksonen, A., 2010. Acid bioleaching of solid waste materials from copper, steel and recycling industries. Hydrometallurgy 103, 74–79. Xu, T., Ting, Y., 2009. Fungal bioleaching of incineration fly ash: metal extraction and modeling growth kinetics. Enzyme Microb. Technol. 44, 323–328. Ziemkiewicz, P., 1998. Steel slag: applications for AMD control. In: Proceedings of the Conference on Hazardous Waste Research, pp. 44–62.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Bioleaching of metals from steel slag by Acidithiobacillus thiooxidans culture supernatant Hong Hocheng ⇑, Cheer Su, Umesh U. Jadhav Department of Power Mechanical Engineering, National Tsing Hua University, No. 101, Sec. 2, Kuang Fu Rd., 30013 Hsinchu, Taiwan, ROC
h i g h l i g h t s Steel industry slag can be used as secondary resource for metals. Bioleaching process can be used to recover metals. The products of microbial metabolism are useful for metal solubilization.
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 26 September 2014 Accepted 29 September 2014
Keywords: Electric arc furnace slag Metals Bioleaching Microorganisms Culture supernatant
a b s t r a c t The generation of 300–500 kg of slag per ton of the steel produced is a formidable amount of solid waste available for treatment. They usually contain considerable quantities of valuable metals. In this sense, they may become either important secondary resource if processed in eco-friendly manner for secured supply of contained metals or potential pollutants, if not treated properly. It is possible to recover metals from steel slag by applying bioleaching process. Electric arc furnace (EAF) slag sample was used for bioleaching of metals. In the present study, before bioleaching experiment water washing of an EAF slag was carried out. This reduced slag pH from 11.2 to 8.3. Culture supernatants of Acidithiobacillus thiooxidans (At. thiooxidans), Acidithiobacillus ferrooxidans (At. ferrooxidans), and Aspergillus niger (A. niger) were used for metal solubilization. At. thiooxidans culture supernatant containing 0.016 M sulfuric acid was found most effective for bioleaching of metals from an EAF slag. Maximum metal extraction was found for Mg (28%), while it was least for Mo (0.1%) in six days. Repeated bioleaching cycles increased metal recovery from 28% to 75%, from 14% to 60% and from 11% to 27%, for Mg, Zn and Cu respectively. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Steel making industries are producing large amount of slag. With the world production of steel at about 900 million tons in 2002; by-products of over 300 million tons was generated (Hiltunen and Hiltunen, 2004). In Europe the total amount of steel slag generated in 2004 was about 15 million tons. Now a days in Europe this steel industry slag is reused for road construction (45%), interim storage (17%), internal recycling (14%), fertilizer (3%), hydraulic engineering (3%), and for cement production (3%) (Gahan et al., 2009). Although these recycling techniques are in use, yet significant amount of slag is being dumped into landfills (11%) or stockpiled for long periods at steel plants (Gomes and Pinto, 2006; Gahan et al., 2009). Because of the presence of heavy metals, the steel industry slag is considered as hazardous waste (Langova and Matysek, 2010). Therefore questions are being raised ⇑ Corresponding author. Tel.: +886 3 5162097; fax: +886 3 5722840. E-mail address: [email protected] (H. Hocheng). http://dx.doi.org/10.1016/j.chemosphere.2014.09.089 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
on landfilling of steel industry slag, as well as on its recycling (Proctor, 2000). For this reason steel slag must be treated in order to reduce its metal content (Vestola et al., 2010). Various aqueous solutions have been used for hydrometallurgical treatment of steel industry slag. However, these methods are favorable only when recoverable metals are present at relatively high levels (Langova and Matysek, 2010; Vestola et al., 2010). The microbial leaching process may offer a suitable alternative for industrial wastes containing low concentrations of metals (Solisio et al., 2002). Limited studies have been carried out on the bioleaching of metals from steel industry slag. Banerjee (2007) compared bioleaching potential of Acidithiobacillus ferrooxidans with a fungus. Bioleaching of 76–78% Zn and Al from EAF sludge was demonstrated by Solisio et al. (2002) using At. ferrooxidans in presence of a sulfur containing substrate. Gahan et al. (2009) utilized the BF slag as a neutralizing agent during pyrite oxidation by a mixed mesophilic culture. In another study Vestola et al. (2010) used mixed culture of sulfur oxidizing bacteria for recovery of Zn and Fe. The present study was carried out to find a suitable
H. Hocheng et al. / Chemosphere 117 (2014) 652–657
lixiviant for metal solubilization from an EAF slag containing low level of metals. The potential of culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger was compared for the solubilization of metals from an EAF slag. Information obtained during this study will help to establish an industrial-scale process for solubilization of metals from an EAF slag. 2. Materials and methods 2.1. Materials, preparation and test of EAF slag An EAF slag sample was collected from local company. The bulk density of an EAF slag was 3.59 (±0.0014) g cm 3. The particle size distribution of an EAF sample was measured by laser particle sizer. The mean particle diameter was calculated from granulometric data. An EAF slag sample of mean particle diameter 324.7 (±234.6) lm (data not shown) was prepared from the main sample by grinding and then it was used for bioleaching experiment. The surface area of an EAF slag particle was determined by Brunauer–Emmett–Teller (BET) method. The surface area found was 2.6832 m2 g 1 (data not shown). Ten milliliter aqua regia was added to 1 g EAF slag, in 100 mL beaker and agitated overnight at 150 rpm. Then the contents were heated at 50 °C for 1 h. After acid leaching, the small amount of residue was removed by filtration. Sample was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES). Table 1 shows chemical composition of this EAF slag sample. Elemental analysis of an EAF slag sample indicated that the major constituent of the sample was Fe. Other elements present in an EAF slag are Aluminum oxide (Al2O3), Magnesium oxide (MgO), Phosphorous pentoxide (P2O5). The presence of metals like Cr, Mo, Ba, and V is also detected. X-ray diffraction analysis was carried out for an EAF slag by X-ray powder diffractometer (TTRAX III, Rigaku, Japan, Co.). The XRD pattern of an EAF slag sample is very complex, with several overlapping peaks resulting from the many minerals present in the sample (Fig. 1). The mineral phases present in an EAF slag sample are magnetite (Fe3O4), calcite (CaCO3), Hematite (Fe2O3), Wustite (FeO), lime (CaO) and portlandite Ca(OH)2. It was not possible to detect presence of sulfide by the XRD analysis. 2.2. Growth of microorganisms and collection of cell free culture supernatant At. thiooxidans 80191, At. ferrooxidans 13823 and A. niger 34770 were obtained from the Food Industry Research and Development Institute (FIRDI), Taiwan. The basal 317 medium contained the Table 1 Chemical composition of EAF slag. Sr. no.
Component
Weight (lg g
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Iron oxide (Fe2O3) Calcium oxide (CaO) Aluminum oxide (Al2O3) Magnesium oxide (MgO) Phosphorous pentoxide (P2O5) Manganese oxide (MnO) Sodium oxide (Na2O) Potassium oxide (K2O) Silicon dioxide (SiO2) Cr Mo Ba V Pb Zn Cu Ni
3645.76 1231.24 352.95 313.0 308.33 185.42 18.36 4.24 3.93 8.80 8.18 3.03 2.46 1.08 0.741 0.719 0.14
1
)
653
followings per litre of glass-distilled water: 0.3 g (NH4)2SO4, 3.5 g K2HPO4, 0.5 g MgSO47H2O, and 0.25 g CaCl2. The pH was adjusted to 4.5 with sulfuric acid. 1.0% (w/v) elemental sulfur was pre-sterilized and added to 317 medium for growth and maintenance of At. thiooxidans. The flask culture was used as an inoculum (10% v/v) after ten days incubation at 30 °C temperature and 150 rpm shaking speed. The basal 9 K medium was used for growth of At. ferrooxidans. FeSO4 was used as energy source for growth of At. ferrooxidans (Jadhav et al., 2013). Sucrose medium was used for the growth of fungi (Xu and Ting, 2009; Jadhav and Hocheng, 2014; Hocheng et al., 2014). Cell free culture supernatant was collected as described by Jadhav and Hocheng (2013). 2.3. Water washing and bioleaching of EAF slag A portion of the EAF slag was prewashed with distilled water. A 10 g L 1 sample of dried EAF slag was mixed together with distilled water in a conical flask and stirred at 30 °C for 24 h. Aliquots of the water sample were taken and sent to metal analysis. An EAF slag was allowed to settle and the water was decanted to collect washed slag. The prewashed sample was dried at 60 °C overnight and subjected to bioleaching tests. Typically, 1 g L 1 of the prewashed EAF slag sample was mixed separately with culture supernatant of At. thiooxidans, At. ferrooxidans and A. niger. After incubating for sufficient time in a shaking incubator the suspension was filtered and the clear solution was sent for metal content analysis by means of ICP-OES. Effect of incubation time on metal recovery was studied. For this 1 g L 1 of the prewashed sample of EAF slag was mixed with At. thiooxidans culture supernatant. The flask was then incubated for 12 d in a shaking incubator and aliquots of the samples were taken at 3, 6, 9 and 12 d to analyze metal content by means of ICP-OES. Effect of repeated bioleaching cycles on metal recovery was studied. 1 g L 1 of the prewashed EAF slag was mixed with At. thiooxidans culture supernatant. The flask was then incubated for 6 d in a shaking incubator. After 6 d, slag and culture supernatant were separated. The fresh At. thiooxidans culture supernatant was again mixed with a slag sample and incubated for 6 d. This was repeated four times. After each bioleaching cycle aliquots of the samples were taken to analyze metal content by means of ICP-OES. The concentration of metals in the leach liquors was analyzed by Kontron S-35, ICP-OES. 3. Results and discussion 3.1. Effect of water washing The steel slags are alkaline in nature (Ziemkiewicz, 1998). The calcium-alumina-silicate complexes present in steel slag cause the pH to rise to high level (Gahan et al., 2009). This alkaline nature of slag can affect the bioleaching process. The water washing treatment was used to overcome this problem. An initial pH of an EAF slag sample was 11.2. After first water washing it remained the same. The second water washing reduced the pH significantly and it became 8.5, while after third water washing a slight decrease in pH observed (8.3). No change in the pH of an EAF slag sample was observed after the fourth water washing experiment (data not shown). Therefore in further experiments, an EAF slag sample was water washed for three times, dried and then used for bioleaching experiment. The metal solubilization during water washing experiment was studied. A negligible amount of metals dissolved in water. Also a decrease in metal solubilization was observed in successive water wash treatments except for Ni (Fig. 2). The Ni solubilization increased in the second water wash
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and again it decreased in the third water wash. The reason behind this is unclear. The water washing experiment helped in reducing the pH of the slag but did not contribute significantly in metal extraction. 3.2. Bioleaching of metals from EAF slag An initial bioleaching experiment was carried out to extract metals from an EAF slag sample. The culture supernatants of three microorganisms were tested for their potential of metal extraction, viz. At. thiooxidans, At. ferrooxidans, and A. niger. In a previous study it was found that A. niger produced 0.104 M citric acid (Jadhav and Hocheng, 2014; Hocheng et al., 2014). The pH of this A. niger culture supernatant was 1.1. The pH of At. ferrooxidans culture supernatant was 2.0. In the present study, the production of sulfuric acid by At. thiooxidans was studied for 14 d. Also the change in pH of growth medium during growth of At. thiooxidans was studied. Before inoculation of At. thiooxidans cells, the pH of the medium was 4.5. The pH of growth medium reduced to 3.3 after incubating
At. thiooxidans for 7 d due to production of 0.0006 M sulfuric acid. The pH further decreased to 1.8 after 14 d due to production of 0.016 M sulfuric acid (Fig. 3). These results are in accordance to Blais et al. (1992) who showed a drop in pH from 7.1 to 1.7, due to acidification. Fig. 4 shows metal extraction efficiency of culture supernatants of At. thiooxidans, At. ferrooxidans, and A. niger. In case of Mg, Mn, Cr, Mo and Pb, At. thiooxidans culture supernatant found more efficient for metal extraction. For Ni 14.8 (±0.94), 100 (±2.68) and 16.7 (±0.88)% recovery was observed using culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger respectively (data not shown). 100% recovery of Si was observed for all the three supernatants used while for Ca 13.5 (±0.30), 10.6 (±0.14) and 11.8 (±0.12)% recovery was obtained in three days using culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger respectively (data not shown). These results suggest that metal extraction by culture supernatants of At. thiooxidans and A. niger depend on presence of sulfuric acid and citric acid respectively. Another experiment was carried out to determine the leaching agent in case of At. ferrooxidans culture supernatant. The 9 K med-
Fig. 1. X-ray diffraction pattern for an EAF slag.
Fig. 2. Metal extraction during water washing of an EAF slag.
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Fig. 3. Time course of the production of sulfuric acid by At. thiooxidans and change in pH during the course.
Fig. 4. Bioleaching of metals from an EAF slag by culture supernatants of At. thiooxidans, At. ferrooxidans and A. niger.
Fig. 5. Effect of incubation time on bioleaching of metals from an EAF slag by At. thiooxidans culture supernatant.
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ium without bacteria and Fe2+ ions was used for metal extraction from an EAF slag. The metal extraction for Mg (14.1%), Ni (11.3%), Al (8.3%), Cu (7.3%), Fe (6.4%), Mn (6.3%), Zn (6.2%), Cr (1.0%), Pb (0.01%), Mo (0.03%) was observed during this experiment (data not shown). This metal extraction occurred due to acidic nature of 9 K medium. The metal extraction increased slightly when At. ferrooxidans culture supernatant was used (Fig. 4). The possibility of precipitation of Fe increases at higher pH (Changqiu et al., 2006; Daoud and Karamanev, 2006). Therefore while using At. ferrooxidans culture supernatant for bioleaching experiment the pH was maintained to 2.0 by external addition of sulfuric acid. An increase in metal extraction by At. ferrooxidans culture supernatant as compared to 9 K medium might be due to this external addition of sulfuric acid. These results suggest that major metal extraction by At. ferrooxidans culture supernatant occurred due to low pH rather than Fe3+. An external addition of sulfuric acid may also be responsible for increased Ni (100%) extraction by At. ferrooxidans culture supernatant. Although an efficiency of At. thiooxidans culture supernatant for Zn and Cu extraction is less as compare to At. ferrooxidans culture supernatant, the difference is not significantly large. Also for Mg, Mn, Cr, Mo and Pb, At. thiooxidans culture supernatant showed better bioleaching performance. According to Vestola et al. (2010), in industrial solid waste materials the metals are present in the form of oxides, carbonates and silicates rather than sulfides. It is easier to leach metals via sulfuric acid generated by acidophiles rather than conventional bioleaching with ferric iron (Vestola et al., 2010). Therefore, At. thiooxidans culture supernatant has been used in further bioleaching experiments. 3.3. Effect of incubation time on metal bioleaching At. thiooxidans culture supernatant having pH 1.8 was collected and used to study effect of incubation time on metal bioleaching from an EAF slag sample. The metal extraction increased with an increase in incubation time for all the metals tested. Major metal extraction was achieved in first 3 d and then the metal extraction increased slightly for 6 and 9 d. Further incubation for 12 d has not increased metal extraction (Fig. 5). From the results it is clear that there is a negligible difference in metal extraction at 6 and 9 d. Therefore 6 d incubation is considered optimum incubation time for metal extraction. These results are comparable with Vestola et al. (2010) who studied effect of various conditions on metal extraction from steel industry slag. They showed 36% and 32% metal extraction for Zn and Fe respectively in 42 d. In the
present study, 16 (±1.17) and 9.1 (±1.81)% metal extraction for Zn and Fe was achieved in 9 d (Fig. 4). Amount of Ca removed in 3, 6 and 9 d was 13.5 (±0.30), 14.9 (±0.04) and 16.3 (±0.34)% respectively (data not shown). Although the metal extraction efficiency was less as compared to Vestola et al. (2010), it was achieved in less time. Vestola et al. (2010) inoculated the media with a mixed culture, containing Acidithiobacillus spp. and Leptospirillum spp which decreased the pH of the growth medium to 1.5. In the present study, pure culture of At. thiooxidans was used and this microorganism was able to bring the pH of the growth medium to 1.8. These might be the possible reasons for less metal extraction in the present study as compared to Vestola et al. (2010). In spite of initial water washing experiment, the pH of steel slag was 8.3. Therefore some acid produced by At. thiooxidans in culture supernatant may have been consumed by steel slag and it was not available for bioleaching process. Also Vestola et al. (2010) were either able to leach only Zn and Fe or they have not considered other metals. In this respect the present study is found more useful, since more metals have been removed from an EAF slag. Several researchers reported metal recovery processes from steel slag. Langova and Matysek (2010) used acid pressure leaching process for recovery of zinc from steel slag. They obtained high percentage of solubilized metals as compared to bioleaching studies. Also very less time is required for metal extraction by using acid pressure leaching process. But this method has some disadvantages. Use of hydrogen peroxide and acid at high temperature and pressure create safety issues. Also the cost of acid transportation will affect the process economics (Bosecker, 1997). These problems can be avoided using microorganisms. Vestola et al. (2010) used At. thiooxidans while Bayat et al. (2009); and Solisio et al. (2002) used At. ferrooxidans for bioleaching of metals from steel slag. The comparison of bioleaching process explained in the present method with the reported literature show that more number of metals extracted in the present method. The time required for metal extraction was less as compared to reported bioleaching processes. Another experiment was carried out to increase the metal extraction. For this purpose bioleaching cycles were repeated for four times. For each cycle an EAF slag sample was incubated with At. thiooxidans culture supernatant for 6 d. Fig. 6 shows metal extraction after each cycle. It is found that for Mg the metal extraction increased from 28 (±1.38) to 75 (±2.04)%. Similarly for Zn, Cu and Fe the metal extraction increased from 14 (±0.94) to 60 (±0.32)%, 11 (±1.05) to 27 (±0.58)% and 9 (±1.08) to 25 (±0.28)%
Fig. 6. Effect of repeated bioleaching cycles on extraction of metals from an EAF slag by At. thiooxidans culture supernatant.
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respectively. During repeated bioleaching cycles the metal extraction for Ca increased from 13.5 (±0.30) to 48 (1.06)% (data not shown). A slight increase in metal extraction was also observed for Al, Mn, Cr and Pb. In case of Ni and Mo the metal extraction remained the same throughout the four bioleaching cycles (Fig. 6). The repeated bioleaching cycles increased metal recovery and it required total 24 d for the said amount of metal recovery. These results are advantageous as compared to reported bioleaching processes. Bayat et al. (2009) showed 35% Zn recovery in 24 d, while Vestola et al. (2010) showed 48% Zn recovery in 79 d. 4. Conclusion This study showed that the leaching behavior of metals varied for each microbial culture supernatant. The results suggest that it is easier to leach metals via sulfuric acid generated by At. thiooxidans. An optimum metal extraction was achieved in six days by At. thiooxidans culture supernatant. Considering the time required for metal extraction, the method described in the present study appear to be more advantageous as compared to reported bioleaching processes. The metal extraction efficiency was further increased by applying repeated bioleaching cycles. The two step bioleaching process described in the present study will not only support to overcome pollution problems created by steel industry slags but also to satisfy increasing metal demands. References Banerjee, D., 2007. Metal recovery from blast furnace sludge and flue dust using leaching technologies. Res. J. Chem. Environ. 11, 18–21. Bayat, O., Sever, E., Bayat, B., Arslan, V., Poole, C., 2009. Bioleaching of zinc and iron from steel plant waste using Acidithiobacillus ferrooxidans. Appl. Biochem. Biotechnol. 152, 117–126.
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