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Fig.
1.
Particle size distributions of the low grade copper sulfide ore sample.
| Cite this article as: |
Wei Chen, Shenghua Yin, Qing Song, Leiming Wang, and Xun Chen, Enhanced copper recovery from low grade copper sulfide ores through bioleaching using residues produced by fermentation of agricultural wastes, Int. J. Miner. Metall. Mater., 29(2022), No. 12, pp.2136-2143. https://dx.doi.org/10.1007/s12613-021-2392-1
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Demand for various metals has been increased continuously with the development of economy and society over the past decade [1–2]. Because high grade resources have been exploited extremely all over the world, it is imperative to exploit low grade ores to meet the ever-increasing metal demand [3–4]. For instance, copper ores, whose grade is lower than 0.7%, occupy 57% of the total copper reserves in China. In fact, many resources cannot be extracted adequately due to the lack of technology support, which not only cause a lot of wastes, but also lead to a large amount of land occupations and serious environment pollutions (e.g., soil and water deterioration), especially in mining process of metals [5–6]. Application of untraditional approach such as bioleaching to extract metal resources has been applied in industrial production [7–8]. Bioleaching is referred as a suitable and sustainable method to extract metals from low grade ores, which is of low cost and simple operation. Application of appropriate additives including metal cation, Cl− [9], forced aeration [10], appropriate temperature and surfactant [11−12], granulating using fine particles [13–14] can promote the bioleaching efficiency. There are still many problems need to be solved, such as more efficient extraction and lower environmental pollution [15], however.
China owns a huge population and different crops are cultivated to meet the grain consumption [16]. Therefore, a great amount of wastes are produced by agricultural activities [17]. For instance, about 45 million tons of agricultural wastes were produced in China each year according to previous studies [18–19]. In practice, proper disposal has not been applied to agricultural wastes [20]. Most of them are burned in the open air, which cause environmental pollution and waste of bio-resources [7]. Besides, a large amount of agricultural wastes have been left unused or improperly disposed. Agricultural wastes contribute to the degradation of soil [20]. Though some methods for agricultural waste disposal have great prospects (e.g., biogas technology and animal feeds), the potential such as heavy metal pollution caused by such methods still need to be kept a lookout [21].
Many studies have proved that cellulose (e.g., agricultural wastes and waste newspaper) has bright prospects in low grade copper sulfide ores bioleaching. For instance, waste newspaper was studied by Panda et al. [22] during bioleaching, and the results showed that copper recovery increased by 93.43% in comparison with sample without adding waste newspaper. There are a lot of cellulose contained in agricultural wastes, such as rice straw and wheat straw. Passivation layer attached on ores surface, such as jarosite, mainly formed by Fe3+ hydrolysis can lead to obstacle in contact between ore, bacteria, and bioleaching solution, thus reducing leaching recovery [7]. The influence of agricultural wastes after acid hydrolysis on the low grade copper sulfide ores bioleaching was investigated by Yin et al. [7], and the results revealed a strong promoting effect when adding 1g powdered agricultural wastes after hydrolysis. Enhanced copper recovery mentioned above is mainly because that various reducing substances can be produced by cellulose hydrolysis using acid, which can improve conversion of Fe3+ to Fe2+. Thus, passivation layer formed by Fe3+ hydrolysis can be reduced, which lead to sufficient contact between ores and lixiviant. Moreover, conversion between Fe3+ and Fe2+ can nourish the bacteria, which result in improved bacteria activity [23]. Although cellulose after hydrolysis using acid on low grade copper sulfide ores bioleaching was studied extensively, effect of residues produced by fermentation of agricultural wastes on bioleaching was neglected.
In order to investigate the impact of residues produced by fermentation of agricultural wastes, bioleaching experiments of low grade copper sulfide ores were carried out. In addition, some important leachable characteristics, such as Cu2+ concentration, oxidation–reduction potential (Eh), pH value, Fe3+ concentration, and bacteria concentration were monitored. Moreover, the microbial community and dominant species in the bioleaching solution were detected by 16S rDNA analysis. Therefore, the results indicated in this paper should provide promising application of agricultural wastes and copper extraction from low grade copper sulfide ores.
The low grade copper sulfide ores in this study were provided by a copper mine in Fujian Province, China. The main chemical composition of the ores including Cu (0.75wt%), Fe (1.62wt%), S (1.10wt%), CaO (0.30wt%), MgO (0.04wt%), Al2O3 (5.19wt%), and SiO2 (91.00wt%). The result of mineral phase analysis showed that Cu grade was at 0.75wt%, including 0.04wt% free copper oxide, 0.06wt% primary copper sulfide, 0.64wt% secondary copper sulfide, and 0.01wt% combined copper oxide. Most copper occurs in form of sulfides including chalcopyrite, enargite, and chalcocite. Other metal-bearing minerals were CaO, MgO, and Al2O3. The detailed gradation of mineral sample of the low grade copper sulfide ore was analyzed using a laser particle sizer, which indicated a strong inhomogeneity characteristic according to Fig. 1.
Bacteria were obtained from acidic waste water of copper mines, China. In order to improve bacteria activity, laboratory-scale culture experiments were carried out repeatedly. In order to provide energy for the bacterial growth and reproduction, culture medium named 9K [23] including (NH4)2SO4 (3.00 g·L−1), MgSO4·7H2O (0.50 g·L−1), KCl (0.10 g·L−1), K2HPO4 (0.50 g·L−1), Ca(NO3)2 (0.01 g·L−1), and FeSO4·7H2O (44.20 g·L−1) was selected, and temperature 30°C, pH value 2.0, and shaking speed 120 r·min−1 were chosen as the culture environment.
Agricultural wastes including rice straw, wheat straw, grass, leaf, and waste paper were produced by agricultural activities. Agricultural wastes were cut into small fragments of area measuring less than 0.5 cm2 before the experiments. Then 4500 mL water was mixed with 500 g agricultural wastes after cutting uniformly. Mixture including water and agricultural wastes were placed in an airtight container for fermentation after adjusting the pH value to 6.8. The preprocessing was performed at a constant temperature condition at 30°C for a month. The agricultural wastes obtained after fermentation were used during the bioleaching experiments and subsequent analytical studies. Agricultural wastes after fermentation herein were called as AWF throughout the text.
Bioleaching experiments were conducted in a laboratory scale to investigate the effect of AWF on low grade copper sulfide ores bioleaching. 0K medium including (NH4)2SO4 (3.00 g·L−1), MgSO4·7H2O (0.50 g·L−1), KCl (0.10 g·L−1), K2HPO4 (0.50 g·L−1), and Ca(NO3)2 (0.01 g·L−1) was applied as lixiviant. All experiments were taken out in 250 mL flasks. Firstly, 180 mL 0K medium and 20 mL bacteria solution (4.5 × 108 cells·mL−1) were inoculated to flask. Then 20 g low grade copper sulfide ores were added into flask. The pH value in flask was adjusted to 2.0 in the beginning. Finally, flasks containing bioleaching solution and low grade copper sulfide ores were placed in a shaker. The temperature of 30°C and shaking speed of 120 r·min−1 were applied during bioleaching. All experiments were carried out in triplicate, and detailed bioleaching experiment schemes were given in Table 1.
| Experimental samples | AWF / (g·L−1) | Copper recovery / % | Maximum Cu2+ concentration / (mg·L−1) |
| TA-1 | 0.00 | 67.82 | 508.60 |
| TA-2 | 2.50 | 71.51 | 536.20 |
| TA-3 | 5.00 | 78.35 | 587.60 |
| TA-4 | 7.50 | 64.25 | 481.80 |
| TA-5 | 10.00 | 60.74 | 455.50 |
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Main chemical composition of ores was detected by Atomic Absorption Spectroscopy. The mineral phase analysis was tested through selective dissolved experiments by Chinese Academy of Sciences. The pH value variation during bioleaching was detected by a pH meter (PHS-3E, Inesa, China). Concentrations, such as Cu2+ and Fe3+ were tested by Inductive Coupled Plasma Emission Spectrometer (ICP Optical Spectrometer, OPTIMA 7000DV, America). Eh of lixiviant was detected by Smart Sensor AR8010+ (Range 0.00–20.00 mg·L−1, accuracy ±0.4 mg·L−1). Counts of the inoculum and bacteria concentration were measured by optical microscope (Carl Zeiss Axio Lab A1, Germany). The microbial community was detected by 16S rDNA analysis.
There were three phases during fermentation processes of agricultural wastes including hydrolysis, acid generation, and gas production [24]. The reaction mechanisms were showed in Eqs. (1) to (3).
| (C6H10O5)n+nH2O→nC6H12O6 | (1) |
| (C6H10O5)n+nH2O→3nCH3COOH | (2) |
| CH3COONH4+H2O→CH4+NH4HCO3 | (3) |
In order to investigate the changes and products during fermentation of agricultural wastes, gas produced by fermentation process was collected and the temperature was monitored at regular intervals. The result indicated that around 0.15 m3 gas was produced in total during fermentation process. The temperature monitoring result showed that the highest temperature in the airtight container was 32.3°C, which was higher than constant temperature of 30°C mentioned in Section 2.3. It is reported that heat was released during fermentation process according to previous study [25], which showed accordance to the result above.
The effect of AWF on copper recovery from low grade copper sulfide ores was shown in Table 1 and Fig. 2(a). Compared to maximum Cu2+ concentration of 508.60 mg·L−1 in Sample TA-1, that in Samples TA-4 and TA-5 decreased, only 481.80 and 455.50 mg·L−1. Adding relatively high bulk of AWF did not result in higher copper recovery. Higher Cu2+ concentration of 536.20 and 587.60 mg·L−1 were obtained in Samples TA-2 and TA-3, however. The results illustrated that adding appropriate-bulk AWF can enhance bioleaching efficiency.
In order to investigate the effect of AWF on copper recovery further, low grade copper sulfide ore residues of Samples TA-1 to TA-5 after bioleaching were taken into X-ray diffraction (XRD) analysis (Fig. 3). The result showed that great difference was found among the five samples. Less jarosite was detected in Samples TA-2 and TA-3 compared to Sample TA-1, Samples TA-4 and TA-5. Furthermore, XRD analysis was carried out to investigate the difference between agricultural wastes and AWF. The result showed that large molecules were the main components of agricultural wastes. However, the compounds of AWF belonged to small molecules mainly. Thus, newly-made Cu(OH)2 solution was applied to identify whether reducing substances were produced or not. Brick-red precipitation was found when AWF was mixed fully with newly-made Cu(OH)2 solution under alkaline and heating condition, which illustrated that reducing substances were produced in AWF. Using acid-processed cellulose (e.g. rice straw and waste newspaper) to extract copper from low grade copper sulfide ores in earlier studies have been studied [23], which was realized mainly through forming less jarosite covered on ore surface. The reducing substances produced by acid-processed cellulose could act as the electron donor with Fe3+ to facilitate microbial reduction reactions under low O2 levels [23]. Therefore, the possible reason for promoting effect of AWF on low grade copper sulfide ores bioleaching can be explained as follows. Reducing substance as a by-product produced during fermentation of agricultural wastes can react with Fe3+, which promoted the conversion of Fe3+ into Fe2+. The development of passivation layer formed by Fe3+ hydrolysis was restrained, and jarosite mainly formed by Fe3+ hydrolysis thus can be decreased just as that found in Sample TA-2 and Sample TA-3. The reaction mechanism was shown in Eq. (4).
| C6H12O6+24Fe3++12H2O→24Fe2++30H++6HCO−3 | (4) |
Variation of pH value during low grade copper sulfide ores bioleaching was similar no matter whether AWF were applied or not. The pH value rose at first, then dropped sharply and finally fell in fluctuation according to Fig. 2(b). Increase of pH value may be resulted from the consumption of H+. For example, abundant oxides (CaO, MgO, and Al2O3) in low grade copper sulfide ores can react with H+. Oxidation processes of Fe2+ to Fe3+ consumed lots of H+ and the reaction was shown in Eq. (5).
| 4Fe2++O2+4H+→4Fe3++2H2O | (5) |
Severe hydrolysis of Fe3+ and oxidation of S0 generated a great deal of H+, which resulted in the following decrease of pH value. As shown in Fig. 2(b), decrease of pH value in Sample TA-1 was distinct up to 0.37, which was much more notable than that in Samples TA-2 to TA-5. Reactions of Fe3+ hydrolysis were shown in Eqs. (6) to (8). Jarosite (e.g. KFe3(SO4)2(OH)6, (NH)4Fe3(SO4)2(OH)6, (H3O)4Fe3(SO4)2(OH)6, and (KH3O)4Fe3(SO4)2(OH)6) produced by Fe3+ hydrolysis covered on ores surface, which was an potential cause to low copper recovery during low grade copper sulfide ores bioleaching [7], also, to other many bioleaching techniques [11]. The distinct decrease of pH value resulted from Fe3+ hydrolysis in Sample TA-1 showed great accordance with the XRD analysis results mentioned in Section 3.2.
| [Fe(H2O)6]3++H2O→[FeOH(H2O)5]2++H3O+ | (6) |
| [FeOH(H2O)5]2++H2O→[Fe(OH)2(H2O)4]++H3O+ | (7) |
| [Fe(H2O)6]3++2[Fe(OH)2(H2O)4]++2SO2−4+P+→PFe3(SO4)2(OH)6↓+2H3O++10H2O | (8) |
Variation of Eh value in Samples TA-1 to TA-5 was shown in Fig. 2(c), which indicated clearly that Eh value rose strikingly at first and then became stable. The maximum Eh value of Samples TA-1 to TA-5 differed from 589 to 688 mV. Based on Fig. 2(c), Sample TA-3 owned the highest Eh value in comparison with other samples, up to 688 mV. Especially, Eh value in Sample TA-3 was much higher than that in Sample TA-5. Oxidation of Fe2+ to Fe3+ during low grade copper sulfide ores bioleaching assisted by dissolved oxygen promoted Eh value significantly and leaded to a high Fe3+ concentration in the lixiviant according to previous study [26]. Thus it can be concluded that conversion from Fe2+ to Fe3+ may explain high Eh value in Sample TA-3. Moreover, bacteria can be nourished by oxidation from Fe2+ to Fe3+ [10], thus bacterial activity may be related to the promoted Eh value resulted from oxidation of Fe2+ to Fe3+.
Fe3+ concentration increased rapidly firstly, then decreased slowly according to Fig. 2(d). The reactions took place in lixiviant to realize copper release may explain the continuous increase of Fe3+ concentration according to Eqs. (9) to (11). Fe3+ concentration in Samples TA-1 to TA-5 declined slowly around day 14. Fe3+ concentration in Samples TA-1 to TA-5 at day 19 declined 2.74, 1.43, 0.70, 2.06, and 2.41 mg·L−1, compared to the maximum value. Severe Fe3+ hydrolysis was believed to be the main reason for decline of Fe3+ concentration, which leaded to decrease of pH value and formation of precipitation further [23].
| 4CuFeS2+17O2+2H2SO4→4CuSO4+2Fe2(SO4)3+2H2O | (9) |
| CuS+Fe2(SO4)3→CuSO4+S+2FeSO4 | (10) |
| Cu2S+Fe2(SO4)3→CuSO4+CuS+2FeSO4 | (11) |
Maximum Fe3+ concentration in Samples TA-1 to TA-5 was 68.60, 76.14, 85.26, 62.79, and 57.31 mg·L−1. Maximum Fe3+ concentration in Samples TA-4 and TA-5 decreased by 5.81 and 11.29 mg·L−1 compared to that in Sample TA-1. While that in Sample TA-3 increased by 16.66 mg·L−1 compared to that in Sample TA-1. High Fe3+ concentration may explain high Eh value in Sample TA-3 mentioned in Section 3.3.2. It can be inferred that adding appropriate-bulk AWF could improve Fe3+ concentration. Although Sample TA-3 owned the highest Fe3+ concentration, little decrease of pH value and few jarosite were found. It seemed that adding appropriate-bulk AWF decreased Fe3+ hydrolysis and maintained high Fe3+ concentration, thus reducing the formation of jarosite.
The growth and reproduction of bacteria need not only the supply of carbon, but also the participation of oxygen. Moreover, reactions during low grade copper sulfide ores bioleaching need consume a great volume of oxygen as well. As shown in Table 2, the final D.O. in Samples TA-1 to TA-5 decreased remarkably in comparison with that at first. Sample TA-1 owned the highest D.O. at day 16, which may be due to the relatively low copper recovery with less consumption of oxygen. However, D.O. in samples declined in different degrees in the presence of AWF. The consumption processes of O2 were shown in Eqs. (12) to (14).
| Experimental samples | Dissolved oxygen / (mg·L−1) | ||
| Initial | Day 8 | Day 16 | |
| TA-1 | 7.46 | 5.73 | 4.29 |
| TA-2 | 7.43 | 5.65 | 4.17 |
| TA-3 | 7.42 | 5.62 | 4.18 |
| TA-4 | 7.37 | 5.12 | 3.24 |
| TA-5 | 7.31 | 4.95 | 2.07 |
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| CuFeS2+4Fe3++2H2O+3O2→Cu2++5Fe2++4H++2SO2−4 | (12) |
| CuS+4Fe3++2H2O+O2→Cu2++4Fe2++4H++SO2−4 | (13) |
| Cu2S+6Fe3++2H2O+O2→2Cu2++6Fe2++4H++SO2−4 | (14) |
A significant decline of D.O. was showed in Samples TA-4 (4.13 mg·L−1) and TA-5 (5.24 mg·L−1), which was resulted from floating AWF on the lixiviant surface. Low D.O. in Samples TA-4 and TA-5 might explain low bacteria concentration and copper recovery in Samples TA-4 and TA-5. D.O. in Sample TA-3 dropped 3.24 mg·L−1 at day 16 compared to the initial D.O. Great difference of D.O. was showed between Samples TA-3 and TA-5. Based on the results showed in Table 2 and Fig. 2(a), a conclusion could be drawn that the higher D.O. was detected at day 16, the higher copper recovery was obtained in samples in the presence of AWF. Besides, low D.O. might explain low Fe3+ concentration, bacteria concentration, and copper recovery in Samples TA-4 and TA-5 in view of the importance of oxygen to bioleaching reactions and bacteria.
According to Fig. 4(a), bacteria concentration in Samples TA-1 to TA-5 during low grade copper sulfide ores bioleaching increased slightly at first, then a sharp rise followed and became stable finally. The slight increase of bacteria concentration at first was resulted from bacterial adaption to new environment [7]. The following rapid increase of bacteria concentration was mainly due to Fe2+ produced by low grade copper sulfide ores, which can nourish bacteria. The stable tendency eventually was mainly due to shortage of nutrition, energy, and deterioration of bioleaching environment. Maximum bacteria concentration in Samples TA-2 (7.14 × 107 cells·mL−1) and TA-3 (9.56 × 107 cells·mL−1) was much higher than that in Sample TA-1 (5.93 × 107 cells·mL−1) according to Fig. 4(a). Excess Ca(OH)2 solution was poured into AWF before bioleaching experiment, and little white precipitate was found after five minutes, which illustrated the detection of CO2 in AWF. CO2 and reducing substances produced by fermentation of agricultural wastes might explain this phenomenon. CO2 as a nourishment could improve the growth of bacteria. Moreover, reactions between Fe3+ and reducing substances accelerated the conversion of Fe3+ into Fe2+, which provided energy source for bacteria. In general, adding appropriate-bulk AWF indicated promoting influence on bacteria concentration. However, bacteria concentration in Samples TA-4 (5.42 × 107 cells·mL−1) and TA-5 (4.55 × 107 cells·mL−1) was lower than that in Sample TA-1 (5.93 × 107 cells·mL−1). This might be resulted from the floating AWF, which reduced contact between lixiviant and air. Furthermore, excessive CO2 contained in Samples TA-4 and TA-5 hindered bioleaching reactions and bacteria activities.
A greatly promoting effect with improved bacteria concentration and copper recovery from low grade copper sulfide ores in the presence of appropriate-bulk AWF were discussed in the previous Section 3.2. Herein three samples labeled Samples TB-1 to TB-3 were detected by 16S rDNA analysis, which represented Samples TA-1, TA-3, and TA-5, respectively. Results of 16S rDNA analysis identified difference of microbial community structure in the three samples. Fig. 4(b) showed that top 35 bacteria of the three samples. Fusobacterium, Serratia, Delftia, and other five bacteria were the dominant bacteria in Sample TB-1. While Staphylococcus and Methanosaeta owned high proportion in Sample TB-3. The dominant bacteria in Sample TB-2 were classified into nine different kinds mainly. An operational taxonomic unit (OTU) represented a bacterial species here. Total OTU among the three samples was 74 OTUs. The amount of OTU in Samples TB-1 to TB-3 was 55, 58, and 54. Among all OTUs, 38 OTUs were shared by the three samples. The unique OUT of Samples TB-1 toTB-3 was 8, 7, and 7. Thus, microbial community structure was successfully differentiated after adding AWF.
It could be seen from Table 3 that Acidithiobacillus ferrooxidans, Lactobacillus acetotolerans, Bacteroides plebeius, Alicyclobacillus disulfidooxidans, and Moraxella osloensis were main species in samples according to the result of 16S rDNA analysis at day 8 and 16. Acidithiobacillus ferrooxidans referred as one of the most significant bioleaching bacteria held the highest proportion at both day 8 and 16 in Samples TB-1 to TB-3. The proportion of Acidithiobacillus ferrooxidans in Samples TB-1 to TB-3 at day 8 was 8.69%, 13.84%, and 7.31%. While that in Samples TB-1 to TB-3 at day 16 was 14.75%, 28.63%, and 6.91%. The Acidithiobacillus ferrooxidans in Sample TB-3 was much lower than that in Samples TB-1 and TB-2 distinctly, which illustrated that adding large bulk of AWF caused impediment to growth and reproduction of Acidithiobacillus ferrooxidans. Acidithiobacillus ferrooxidans was an aerobic bacteria, whose growth and reproduction need the participation of air. Adding large bulk of AWF led to the floating of AWF in lixiviant surface, which blocked the contact between air (especially O2) and bacteria, thus resulting in low proportion of Acidithiobacillus ferrooxidans in Sample TB-3. However, Acidithiobacillus ferrooxidans in Sample TB-2 was much higher than that in Sample TB-1. This may be because CO2 (contained in AWF) and Fe2+ (produced by reactions between reducing substances and Fe3+) could provide energy sources to promote the growth and production of Acidithiobacillus ferrooxidans in Sample TB-2.
| Species | Proportion / % | |||||||
| Sample TB-1 | Sample TB-2 | Sample TB-3 | ||||||
| Day 8 | Day 16 | Day 8 | Day 16 | Day 8 | Day 16 | |||
| Acidithiobacillus ferrooxidans | 8.69 | 14.75 | 13.84 | 28.63 | 7.31 | 6.91 | ||
| Alicyclobacillus disulfidooxidans | 0.00 | 0.00 | 1.01 | 2.53 | 0.92 | 1.86 | ||
| Lactobacillus acetotolerans | 1.27 | 1.22 | 1.19 | 1.21 | 1.08 | 3.63 | ||
| Bacteroides plebeius | 0.00 | 0.00 | 0.37 | 1.47 | 0.41 | 1.64 | ||
| Moraxella osloensis | 2.17 | 3.26 | 1.97 | 2.18 | 0.00 | 0.00 | ||
| Others | 87.87 | 80.77 | 81.62 | 63.98 | 90.28 | 85.96 | ||
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Proportion of Lactobacillus acetotolerans in Samples TB-1 to TB-3 at day 8 was 1.27%, 1.19%, and 1.08%. That in Samples TB-1 to TB-3 at day 16 was 1.22%, 1.21%, and 3.63 %. The proportion of Lactobacillus acetotolerans in Samples TB-1 and TB-2 between day 8 and 16 indicated little difference, which illustrated adding appropriate-bulk AWF had no effect on Lactobacillus acetotolerans. The proportion of Lactobacillus acetotolerans in Sample TB-3 increased distinctly along with low copper recovery, however. Lactobacillus acetotolerans need a small amount of air to grow and adding relatively large bulk of AWF reduced D.O. in Sample TA-5, which may explain high proportion of Lactobacillus acetotolerans in Sample TB-3. Further, Lactobacillus acetotolerans in Sample TB-3 at day 16 increased a lot compared to that at day 8. Therefore, it could be concluded that Lactobacillus acetotolerans showed little influence on this low grade copper sulfide ores bioleaching.
No Moraxella osloensis was found in Sample TB-3 in the presence of AWF according to Table 3. While low proportion of Moraxella osloensis was found in Samples TB-1 and TB-2. Low D.O. was yielded in Sample TB-3, which was mainly resulted from floating of large-bulk AWF. Moraxella osloensisis referred as aerobic bacteria and air is required in its growth and reproduction. Thus, the low D.O. may explain why Moraxella osloensis was not detected in Sample TB-3. Bacteroides plebeius and Alicyclobacillus disulfidooxidans were not detected in Sample TB-1 in the original inoculums at day 8 and were not found by 16S rDNA analysis at day 16 as well. However, low proportion of Bacteroides plebeius and Alicyclobacillus disulfidooxidans were found in Samples TB-2 and TB-3. It could be drawn that Bacteroides plebeius and Alicyclobacillus disulfidooxidans may be brought into lixiviant by AWF. Meanwhile, inhibiting and promoting effect on low grade copper sulfide ores bioleaching was found in Samples TB-2 and TB-3. Thus it could be inferred that Bacteroides plebeius and Alicyclobacillus disulfidooxidans owned little effect on this low grade copper sulfide ores bioleaching.
The heatmap of beta diversity index was indicated in Fig. 4(c), which illustrated the difference of microbial community structure among Samples TB-1 to TB-3. The small coefficient value represented a small difference of species diversity between two samples according 16S rDNA analysis. The difference coefficient value between Sample TB-1 and Samples TB-2 and TB-3 was 0.375 and 0.362. However, the coefficient value between Samples TB-2 and TB-3 was only 0.218. Smaller difference showed between samples in the presence of AWF. Thus, it could be concluded that AWF affected microbial community structure greatly. Adding AWF not only affected the growth and reproduction of bacteria through D.O., but also influenced bioleaching reactions to realize energy supply for bacteria. The results indicated good accordance with bacteria species among the three samples mentioned in Section 3.4.2.
Improved bioleaching performances with enhanced copper recovery and bacteria concentration, using low grade copper sulfide ores and agricultural waste, were obtained in this study. The effect of agricultural wastes on environment was investigated further. Cellulose, as a widespread substance was the main compound of agricultural wastes, which corresponded to three different types including C30H34O13, C23H26O4N2, and C13H8O. As for component of residues produced by fermentation of agricultural wastes, many short chain molecules substances, such as C9H10O2 was found. Thus, agricultural wastes showed little impact on the environment according to XRD analysis and toxicity analysis.
Bioleaching residues in samples using and without using AWF were taken into toxicity analysis, respectively. For one thing, bioleached low grade copper sulfide ore residues were detected in SiO2 significantly. For another thing, in bioleaching solution residue, heavy metal ions, such as Zn2+, As3+, and Hg2+ were found in an extreme low concentration in all samples, which illustrated that the use of agricultural wastes during bioleaching caused little impact on the environment. However, the pH value of bioleaching solution residue in all samples was low, which may result in water pollution and soil degradation. Therefore, more attempts to deal with low pH value of bioleaching solution residue when using agricultural wastes to recover copper from low grade copper sulfide ores should be conducted.
It is important to keep a low cost in bioleaching, which is of great significance to the sustainable development of mine. When 2.50 and 5.00 g·L−1 AWF were respectively applied during bioleaching, the key characteristics of Samples TA-2 and TA-3 are better than that of Sample TA-1 (no AWF was added). The price of agricultural wastes, acid, and water used for fermentation is cheap. Compared to benefit from increased copper recovery of 3.69% and 10.53% in Samples TA-2 and TA-3, cost of materials for fermentation can be negligible. Therefore, better economic benefits can be obtained when such technique is used.
Potential benefits using residues produced by fermentation of agricultural wastes during bioleaching of low grade copper sulfide ores were obtained, which improved copper recovery, concentration, and performance of bacteria. The use of fermentation of agricultural wastes was very attractive and offered several advantages as compared to the conventional bioleaching method with considerable potential for further scale-up bioleaching studies on low grade copper sulfide ores in an eco-friendly and economic manner. External addition of biodegradable or fermentable organic material in bio-heapleaching operations may promote heterotrophic microbes, which may result in biofouling problems over long term, however. Thus, further industrial experiments using fermentation of agricultural wastes to recovery copper, environment assessment, and bacteria performance should be carried out.
(1) Effects of AWF on copper recovery and microbial community during low grade copper sulfide ores bioleaching were investigated. Results illustrated that agricultural wastes can not only produce environmental-friendly biogas through fermentation, but also promote copper extraction and dominant bacteria proportion (Acidithiobacillus ferrooxidans) when 5 g·L−1AWF was used.
(2) Mechanism of AWF to improve copper recovery was proposed. Analysis showed that reducing substances produced during fermentation of agricultural wastes can inhibit Fe3+ hydrolysis, which reduced formation of precipitation and accelerated copper extraction. Such research should be useful for application of low grade copper sulfide ores and agricultural wastes.
(3) Environmental and economic analysis showed that application of AWF during bioleaching caused little impact on environment, attention on handling low pH should be paid, however. Additionally, although economic benefits can be obtained using AWF during bioleaching, further industrial experiments should be carried out to find effect of AWF on copper recovery, environment assessment, and bacteria performance.
This work was financially supported by the Key Program of National Natural Science Foundation of China (Nos. 52034001 and 51734001), the Innovation Team in Key Fields of Ministry of Science and Technology of the People’s Republic of China (No. 2018RA400), the 111 Project (No. B20041), the Fundamental Research Funds for the Central Universities (No. FRF-TP-18-003C1), and China Scholarship Council (No. 202006460037).
The authors declare no conflict of interest.
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| 3. | Wei Chen, Huiyang Tang, Shenghua Yin, et al. Copper recovery from low-grade copper sulfides using bioleaching and its community structure succession in the presence of Sargassum. Journal of Environmental Management, 2024, 349: 119549. DOI:10.1016/j.jenvman.2023.119549 |
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| 6. | Emmanuel Konadu Sarkodie, Luhua Jiang, Kewei Li, et al. A review on the bioleaching of toxic metal(loid)s from contaminated soil: Insight into the mechanism of action and the role of influencing factors. Frontiers in Microbiology, 2022, 13 DOI:10.3389/fmicb.2022.1049277 |
Figures(4) / Tables(3)
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| Experimental samples | AWF / (g·L−1) | Copper recovery / % | Maximum Cu2+ concentration / (mg·L−1) |
| TA-1 | 0.00 | 67.82 | 508.60 |
| TA-2 | 2.50 | 71.51 | 536.20 |
| TA-3 | 5.00 | 78.35 | 587.60 |
| TA-4 | 7.50 | 64.25 | 481.80 |
| TA-5 | 10.00 | 60.74 | 455.50 |
DownLoad:
CSV
| Experimental samples | Dissolved oxygen / (mg·L−1) | ||
| Initial | Day 8 | Day 16 | |
| TA-1 | 7.46 | 5.73 | 4.29 |
| TA-2 | 7.43 | 5.65 | 4.17 |
| TA-3 | 7.42 | 5.62 | 4.18 |
| TA-4 | 7.37 | 5.12 | 3.24 |
| TA-5 | 7.31 | 4.95 | 2.07 |
DownLoad:
CSV
| Species | Proportion / % | |||||||
| Sample TB-1 | Sample TB-2 | Sample TB-3 | ||||||
| Day 8 | Day 16 | Day 8 | Day 16 | Day 8 | Day 16 | |||
| Acidithiobacillus ferrooxidans | 8.69 | 14.75 | 13.84 | 28.63 | 7.31 | 6.91 | ||
| Alicyclobacillus disulfidooxidans | 0.00 | 0.00 | 1.01 | 2.53 | 0.92 | 1.86 | ||
| Lactobacillus acetotolerans | 1.27 | 1.22 | 1.19 | 1.21 | 1.08 | 3.63 | ||
| Bacteroides plebeius | 0.00 | 0.00 | 0.37 | 1.47 | 0.41 | 1.64 | ||
| Moraxella osloensis | 2.17 | 3.26 | 1.97 | 2.18 | 0.00 | 0.00 | ||
| Others | 87.87 | 80.77 | 81.62 | 63.98 | 90.28 | 85.96 | ||
DownLoad:
CSV
