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Reductive leaching of low-grade manganese ore with bamboo sawdust: Study of bamboo sawdust and glucose degradation
⁎Corresponding author at: Strategic Mineral Niche Area, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, Pulau Pinang, Malaysia. suhaina@usm.my (Suhaina Ismail),
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
This study was conducted to identify the pathways by which bamboo sawdust (BSD) degrades into sugar (predominantly glucose) and the resultant glucose degrades into an organic acid during Mn leaching. Synthetic Mn ore (SMO) and low-grade Mn ore (LGMO) were leached with BSD as a reductant in sulfuric acid. Mn was determined by complexometric titration with ethylenediaminetetraacetic acid. Sugar derivatives and organic acids were determined by high-performance liquid chromatography. The samples were leach liquor treated in 4.0 M H2SO4 at 100 °C and sampled every 15 min until 360 min. BSD, which contained cellulose (56.11%) and hemicellulose (28.6%), and cellulose was degraded into glucose as the major sugar. The highest glucose concentration produced in the SMO and LGMO leaching system were 28.54 ppm and 19.45 ppm, respectively. The produced glucose reduced the Mn ores and was also degraded into organic acids, predominantly formic acid. The highest concentration of extracted Mn was 0.15 ppm for SMO and 0.06 ppm for LGMO. The highest formic acid concentration was less than 0.005 ppm for both ores. The results indicate that the formed glucose preferentially participated in Mn reduction rather than undergoing degradation. Thus, complete extraction of Mn could not be achieved because O2 competed with Mn to degrade glucose into an organic acid.
Keywords
Low grade manganese ore
Bamboo sawdust
Reductive leaching
Glucose degradation
1 Introduction
Mn is the fourth-most consumed metal in the world (Alaoui et al., 2016). The largest producer and consumer is China, which consumes 14 million tons per year (Wang et al., 2017; Dan et al., 2016). In response to increasing demand for Mn, Malaysia is becoming increasingly competitive in Mn production. Mn resources in Malaysia are located in several states: Johor, Kelantan, Pahang, and Terengganu. Some Mn mines in Malaysia include Pertama Ferroalloys Sdn. Bhd., OUS Mining in Pahang, OM Holding Ltd., which is currently building a smelter in Sarawak, and Sakura Ferroalloys Sdn. Bhd. in Sarawak. The diversity of Mn investors stems from the prediction that worldwide consumption of Mn will continue to increase (Dan et al., 2016).
The extraction of Mn ore is a complex process because of its associated impurities. In the present work, Mn extraction from Malaysian low-grade manganese ore (LGMO) is conducted using bamboo sawdust (BSD) as a reducing agent in sulfuric acid for commercial use. A complete hydrometallurgy process is used to extract the LGMO using BSD as a reducing agent in H2SO4. This alternative method is considered a low-cost and low-pollution process because it requires lower temperatures than pyrometallurgy. The use of biomass is considered to ameliorate global warming (Li Feng et al., 2016). The attractiveness of this approach is that bamboo is readily available, particularly in Asia. More than one-half of aggregate bamboo resources are located in Asia (Mohan et al., 2015). In Malaysia itself, bamboo is abundant in almost all states. Malaysia widely harvests bamboo for construction and clothing. Hence, the use of green BSD biomass in Malaysia as a reducing agent is an attractive approach to extracting Mn from LGMO without harming the environment.
Almost all previous reports characterizing bamboo have been focused on bamboo composites, cellulose fibers, or pulp production (Cao et al., 2014; Zakikhani et al., 2014; Zhang, 2014) and much research in recent years has focused on bamboo as a precursor to produce bio-fuel and bio-polymer (Dong, 2020; Lin et al., 2019). The literature contains little research involving the characterization of BSD for leaching purposes (Ismail et al., 2013). The hydrolysis of BSD is a complex process because cellulose is crystalline and hemicellulose is amorphous, and the crystalline material is more difficult to hydrolyze than the amorphous material. The hydrolysis process degrades BSD and converts it into reducing sugars. In principle, cellulose degrade into glucose, galactose and mannose (Van der Weijden et al., 2002), whereas hemicellulose easily degraded into xylose (Van der Weijden et al., 2002; Lin et al., 2019). This reducing sugar is the main reductant contributing to Mn2+ reduction study. However, because the BSD structure is complex, the mechanism of its conversion reaction to glucose and xylose are unknown.
BSD degradation into reducing sugars such as glucose occurs through a complex network reaction. However, the produced glucose is also degraded into organic acids such as formic acid. This reaction will affect Mn(IV) reduction. As the produced glucose is degraded into organic acids, its role as a reducing sugar is interrupted. Hence, the chemical pathway of glucose degradation is important. However, to our knowledge, no research on the degradation of glucose from BSD into organic acids has been reported. Furlani et al. (2006) studied the degradation of glucose into organic acids but used glucose directly to conduct the analysis (Furlani et al., 2006). Therefore, the present work was designed to study the pathway of BSD degradation into sugar and subsequent glucose degradation into organic acids while leaching Mn ore. Thus, we characterized the sugars obtained from BSD degradation and subsequent degradation of glucose using high-performance liquid chromatography (HPLC). The BSD degradation during reductive leaching produces glucose as the major product, and the glucose degradation produces formic acid as the major organic acid. This chemical pathway demonstrates why complete extraction of Mn cannot be achieved with this approach: oxidation of glucose into an organic acid competes with the glucose reduction of Mn(IV).
2 Methods
2.1 Low-grade manganese ore
LGMO was obtained from Pahang, Malaysia. The ore was dried, ground, and sieved to −75 μm. The elemental composition of the ore was 15.26% Mn, 4.05% Fe, 2.74% Si, and 2.71% Al. The phases present in the ore were pyrolusite (MnO2; ICDD card no. 98–006-2906), goethite (FeO(OH); ICDD card no. 98–003–4797), and quartz (SiO2; ICDD card no. 98–010-7202).
2.2 Bamboo sawdust
The BSD was used as a potential reducing agent in the reductive leaching of synthetic Mn ore (SMO) and LGMO. The type of bamboo was Gigantochloa scortechinii, which was supplied by the Forest Research Institute Malaysia (FRIM). The culm part of the bamboo was used. The bamboo was washed, dried, cut, and ground. Sampling and sieving were then carried out to obtain samples with particle sizes of −300 μm and −75 μm. The BSD contained 28.60% hemicelluloses, 84.71% holocellulose, and 56.11% cellulose (Muthalib et al., 2018).
2.3 Reductive leaching setup
Reductive leaching was carried out in a 500 mL three-neck flask immersed in a silicone oil bath. The flask was equipped with a condenser and a mechanical stirrer. Thermometers were placed inside the glass reactor flask and in the silicone oil bath to measure the temperature. An electric hotplate was used.
2.4 Reductive leaching process
Twenty-five grams of Mn ore (LGMO or SMO) was added to a sulfuric acid solution at a certain temperature. Then, 7.5 g of BSD was added to the reaction flask. The solution was mechanically stirred, and leaching was continued for 360 min. A 2 mL aliquot was withdrawn at 15 min, 60 min, 120 min, 240 min, and 360 min during the leaching process. These samples were filtered using a 0.45 μm filter paper and then diluted to 50 mL in a volumetric flask with 10% HNO3 to avoid sample precipitation. The reaction was completed at 360 min, and the leached liquor was then filtered. The Mn concentration of each sample was determined by complexometric titration using ethylenediaminetetraacetic acid (EDTA) as a titrant.
2.5 Identification of sugars and organic acid derivatives by HPLC
Sugar derivatives were determined using an HPX-87P (Bio-Rad Aminex) carbohydrate analysis column (300 × 7.8 mm2) and deionized water as the mobile phase. The column temperature was 85 °C. Organic acids were determined using an HPX-87H column (300 × 7.8 mm2) with 5 mM sulfuric acid solution as the mobile phase. The column temperature was 50 °C. The flow rate for both derivatives was 0.6 mL/min. All standards and samples were tested using a refractive index detector. The samples used in sugar identification of BSD and in analysis of the chemical pathway of sugar formed during reductive leaching were the leach liquor of SMO and LGMO leached in 4.0 M H2SO4 at 100 °C for 360 min. This leaching series was chosen because the optimum conditions were achieved using these parameters.
3 Results and discussion
3.1 Sugar identification of bamboo sawdust and glucose degradation by HPLC
Numerous works have discussed the analysis of sugar contents, especially glucose, and organic acids by HPLC (Furlani et al., 2006; Parpinello and Versari, 2000; Gawron et al., 2014). Identified degradation products of glucose include aldonic acids (sugar acids, —C(⚌O)OH), lower aldoses (monomers), formic acid (HCOOH), and CO2 (Furlani et al., 2006). However, the identification of sugar from biomass, particularly BSD, has not yet been reported. The reaction of BSD is thought to be more complex because BSD contains both cellulose, which is crystalline, and hemicellulose, which is amorphous. Both cellulose and hemicellulose need to degrade into sugars (monomers). The monomers formed, such as glucose, can be used to reduce Mn(IV). Hence, the objective of the present work was to characterize the pathway of BSD degradation into reducing sugars, especially glucose. However, monomers such as glucose are themselves degraded into organic acids simultaneously during the leaching reaction. Thus, we also investigated the chemical pathways of sugar derivatives produced during the reductive leaching process. The reducing sugars and organic acids in the leach liquor were analyzed.
3.2 Elucidation of chemical pathway of bamboo sawdust degradation
The sugars produced are important because they function as a reducing agent to reduce Mn(IV). The leaching reaction of SMO and LGMO using BSD in sulphuric acid is represented as below (Hariprasad et al., 2007) and Adel A Ismail et al., 2016).
Generally, cellulose in BSD expressed as [C6H10O5]n will composed to α-D-glucose units. The [C6H10O5]n hydrolyzed into glucose, galactose and mannose based on the sequence of essential. During the hydrolysis process, this BSD also contains hemicellulose which becomes xylose, glucose, arabinose and mannose during hydrolysis proses. These sugars are assumed to react as reductant during leaching process (Van der Weijden et al., 2002; Hariprasad et al., 2007). Oxidation of these sugars by Mn(IV), containing the reducing aldehyde end-group, but possibility also the thermal alcohol and other hydroxyl group. It is noted the hydrolysis process is takes place simultaneously during leaching process. Cellulose → glucose, galactose, mannose Hemicellulose → xylose, arabinose, mannose
Fig. 1 shows the relationship between the concentration of glucose formed (ppm) by hydrolysis and the concentration of extracted Mn. The hydrolysis or degradation of BSD occurred simultaneously during the reductive leaching process. BSD which contains cellulose and hemicellulose, were successfully hydrolyzed or degraded into a reducing sugar. However, the degradation product of BSD was only glucose; no peaks of xylose, arabinose or mannose were observed. These results suggest that glucose plays a major role as a reducing sugar without transforming into other carbohydrates (Furlani et al., Mar. 2006).
Relationship between amount of glucose formed and the amount of Mn extracted: (a) overall result, (b) SMO samples, and (c) LGMO samples.
The degradation trend of BSD was nearly constant (15–25 ppm) for both ores because the hydrolysis occurred constantly during reductive leaching. The concentration of glucose produced using SMO and LGMO was same because the hydrolysis was conducted with the same amount of BSD (7.5 g in 4.0 M H2SO4 solution). Thus, the ores used in this work did not affect the degradation of BSD. A trend of increasing Mn extraction was observed until 120 min; the BSD degradation still occurred constantly throughout the 360 min reductive leaching experiment. This behavior is attributed to the product (glucose) formed by degradation of BSD diffusing to the mineral boundary layer. At a certain time, this boundary layer was compacted with the product layer. Hence, the rate of Mn(II) diffusion back into solution decreased. However, the degradation still occurred, and glucose diffused to the active sites (mineral grains).
One method to enhance the degradation of BSD that contains cellulose is thermal modification by nitrogen. However, this method requires high temperatures (Gawron et al., 2014) because the rate of hydrolysis is directly proportional to temperature (Ismail et al., 2008). In the present work, the temperature at which BSD hydrolysis was conducted was constant at 100 °C. Hence, the hydrolysis rate was constant. In addition, a previous study indicated that cellulose, which is crystalline, is difficult to degrade (Gawron et al., 2014). Fig. 2 shows the cellulose C-bonds broken during degradation into glucose.
Overall degradation of cellulose to glucose.
Cellulose is a chain of glucose molecules linked together by β-glycosidic linkages (covalent bonds that join sugars or monomers to another group). The degradation of cellulose produces glucose, which is a hexose sugar that contains a hydroxymethyl (–CH2OH) group. The hydroxymethyl is linked at carbon 5 (C5). Meanwhile, carbon 1 (C1) in the glucose structure is the reducing end and participates in the reduction of Mn(IV) ions (Olsson and Westman, 2013).
The arrangement of the cellulose structure can be altered to generate intramolecular H-bonding between the primary H of the CHO groups and the oxygen of β-glycosidic bonds (Enslow and Bell, 2012; Olsson and Westm, 2013). The protonation–deprotonation of cellulose under acidic conditions will break the bond between the ring oxygen and anomeric carbon. Fig. 3 shows the proposed reaction pathway of cellulose degradation into glucose (Enslow and Bell, 2012).
Proposed reaction pathway of cellulose degradation during the reductive leaching of Mn (Enslow and Bell, 2012).
The subunits of hemicellulose are described in Fig. 4. Hemicellulose is an amorphous material that is easily hydrolyzed into glucose, xylose, arabinose, and mannose (Van der Weijden et al., 2002). As previously mentioned, arabinose and mannose peaks were not observed in the chromatogram, which suggests that glucose was the major product of degradation, without transforming into other sugars. In addition, no xylose peaks were observed, suggesting that xylose degraded and formed furfural. Fig. 5 shows the reaction pathway of hemicellulose.
Subunits of hemicellulose (Pierson et al., 2013).
Reaction pathway of hemicellulose (Enslow and Bell, 2012).
An intermediate reaction of the formed glucose occurred simultaneously in the leaching system. The formed glucose degraded into formic acid. Specifically, the glucose was shortened by detachment of one carbon atom as formic acid by monocarboxylic polyhydroxyacids.
3.3 Identification chemical pathways of glucose degradation
Typically, the Mn reductive leaching reaction proceeds as shown in Equation (1) (Hariprasad et al., 2007):
However, complete dissolution of Mn(IV) was difficult even under the most favorable conditions because aldose (monomers, normally glucose) reacts with oxidants other than Mn(IV), including O2. In addition to its function of reducing Mn(IV), the glucose formed was also degraded into organic acids, limiting the reduction of Mn(IV) (Furlani et al., Mar. 2006). Hence, some the glucose formed reduced Mn(IV), but some also degraded into organic acids. Equation (2) shows the glucose degradation process via an intermediate reaction during the leaching process:
Fig. 6 shows the relationship between the concentration of formic acid formed as a product of glucose degradation and the concentration of extracted Mn. Formic acid was formed as a product of glucose degradation via an intermediate reaction during the leaching process. In Fig. 6(b) and (c), the trend of formic acid formation is similar to the trend of Mn extraction. The formic acid concentration increased gradually until 120 min and then decreased at the end of leaching. Similar trends were observed for both ores, indicating that the type of ore did not affect the degradation of glucose into organic acids.
Relationship between formic acid and Mn extraction: (a) Overall result, (b) SMO samples, and (c) LGMO samples.
Fig. 6(a) indicates that the organic acid concentrations increased beyond 15 min. At the beginning of leaching (0–15 min), only the hydrolysis of BSD to monomers (glucose) occurred. The glucose formed was not yet degraded because the leaching reaction had just started. The formic acid concentration during this time was 0 ppm during the leaching of both SMO and LGMO. Fig. 6(a) also shows that the glucose formed favored Mn reduction over glucose degradation. Only a small amount of the glucose formed was degraded into formic acid. The highest concentration of formic acid was less than 0.005 ppm for both ores, whereas the highest concentration of extracted Mn was 0.15 ppm for SMO and 0.06 for LGMO. However, the degradation of glucose still occurred and interrupted its function as a reducing for Mn(IV). Competition occurred between the formed glucose functioning as reducing agent and degrading into an organic acid. Thus, the reduction of Mn(IV) occurred but incomplete Mn extraction was achieved.
Glucose has been previously reported to degrade to gluconic acid first, then to glyceric acid, glycolic acid, and formic acid (Furlani et al., 2006). However, the researchers did not identify a gluconic acid peak, whereas a formic acid peak was clearly observed. These results suggest that glucose was completely degraded to formic acid and that the degradation reaction rates were high. This result is in good agreement with previous work (Furlani et al., 2006).
In the present work, we encountered some limitations in that the total reducing sugars formed were unknown because Mn reduction occurred even at the beginning of the reaction, suggesting that the degradation of BSD to monomers was rapid. Reduction occurred with 15 min, indicating that some of the formed glucose had reacted as a reducing sugar. Meanwhile, the progression of the leaching reaction showed that some glucose was functioning as a reducing sugar and some was degraded into formic acid. Consequently, identifying the total formed reducing sugars was difficult. We therefore only identified the reducing sugars throughout the leaching process, without considering the glucose that had reacted to reduce Mn(IV).
Fig. 7 displays the overall reductive leaching process. The leaching process starts with the hydrolysis of BSD with H2SO4. The hydrolysis occurs simultaneously during leaching. This process is ongoing in the leaching system. During the hydrolysis, the BSD, which contains both cellulose and hemicellulose (identified during the analysis), is degraded. Cellulose is crystalline and is degraded into glucose (major), galactose, and mannose. Meanwhile, hemicellulose, which is amorphous, is degraded into xylose, and arabinose. In this work, the sugar formed (glucose) diffuses into the Mn ores, where the reduction of Mn(IV) to Mn(II) occurs. Finally, Mn(II) diffuses back into the solution. However, during hydrolysis, an intermediate reaction occurs. The glucose formed during hydrolysis is also degraded into organic acids such as gluconic, glyceric, glycolic, and formic acids. This intermediate reaction interrupts the function of glucose as a reducing agent. Hence, the chemical pathway of sugar formed during reductive leaching (glucose) was identified.
Proposed pathway of BSD degradation and sugar degradation.
4 Conclusion
The results presented here suggest that the degradation of BSD, which contains cellulose and hemicellulose, during reductive leaching produces glucose as the major product. However, an intermediate reaction of the formed glucose occurs simultaneously in the leaching system. Thus, complete extraction of Mn cannot be achieved because glucose also reacts with oxygen to degrade into an organic acid. In this work, the glucose degradation produces formic acid as the major organic acid. On the basis of HPLC analysis, the formed glucose preferentially participates in Mn reduction rather than glucose degradation. This result is expressed by the highest concentration of extracted Mn (0.15 ppm and 0.06 ppm for SMO and LGMO, respectively) being substantially greater than the highest concentration of formic acid (<0.005 ppm) for both ores.
Acknowledgement
The authors are thankful to School of Materials and Mineral Resources Engineering, USM and Fundamental Research Grant Scheme (FRGS), Ministry of Higher Education Malaysia, Account Number (203.PBAHAN.6071359). The technical support from late Mr. Kemuridan Mat Desa, Ms. Haslina Zulkifli, Mr. Hasnor Husin, Mr. Khairi and Mr. Azrul are also much appreciated.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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