Making use of bamboo

Bamboo has always been a highly versatile natural material, finding use as a construction material, textile, musical instrument, stake for holding up plants and even food, for both humans and pandas. But it could also potentially make a very effective biofuel feedstock, as it grows quickly, can be harvested easily, and possesses similar concentrations of cellulose and hemicellulose as conventional wood.

Unfortunately, unlike conventional wood, it also possesses comparatively high concentrations of silica, which tend to form a water-insoluble precipitate that hampers the biofuel production process. Now, though, a team of researchers from Canada and China has come up with an efficient method for separating the silica from the rest of the components of bamboo. And this could allow bamboo to be used as a feedstock not only for ethanol but for a whole range of other useful chemicals as well.

In a previous study, the researchers, led by Mark Martinez at the University of British Columbia in Vancouver, had discovered that the silica can be removed from bamboo by treating it with an alkaline solution, and so this formed the basis for their method. They decided to try out four different alkaline solutions – sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH₄OH) and calcium hydroxide (Ca(OH)₂) – to determine which worked best. After treating bamboo chips with each of these solutions to release the silica, Martinez and his team reduced the pH of the solution by bubbling carbon dioxide through it, causing the silica to form a precipitate that could easily be extracted.

As reported in Biotechnology for Biofuels, they found that NaOH and KOH were much more effective at releasing silica from the bamboo than NH₄OH and Ca(OH)₂, with both leading to the extraction of 99% of the silica in the bamboo chips. This is because the cations in NaOH and KOH are particularly effective at binding with the silica to form insoluble deposits. Martinez and his team ultimately decided to employ NaOH for their method, as it is cheaper than KOH and also causes less corrosion.

Once the silica has been removed, the lignin then needs to be taken care of, although some is removed by the alkaline treatment. To extract the rest of the lignin, Martinez and his team employed a pulping technique that involves heating the material left over after the silica has been removed, termed the alkaline pre-extraction liquor (APEL), in the presence of NaOH and sodium sulfide (Na₂S). After the lignin is released, the pH of the solution is lowered further by adding sulfuric acid, causing the released lignin to precipitate out of solution. In total, almost 78% of the lignin can be removed by the alkaline and pulping steps, leaving behind a solution that contains 89% of the cellulose and hemicellulose originally in the bamboo.

Next, this cellulose and hemicellulose is digested by enzymes into its component sugars. Martinez and his team found that these sugars could be fermented into ethanol by yeast at a rate of 0.46g per gram of sugar, such that 93% of the sugars released from the bamboo were converted into ethanol after 96 hours. Alternatively, the researchers showed how the cellulose and hemicellulose could be separated from each other by a combination of xylanase treatment and cold caustic extraction. In this case, the cellulose could be used to produce a range of useful chemicals and materials, including viscose rayon, cellulose acetate, nanocrystalline cellulose, paints and liquid crystal displays.

In addition, both the extracted silica and lignin could find several uses. The silica could be used to produce nanoparticles, composite fillers, pharmaceuticals and catalysts, while the lignin could be burnt as fuel or used as a feedstock for various other chemicals and materials (see Making sense of lignin). Overall, Martinez and his team calculated that their method could turn 1 tonne of bamboo into 92.2kg of ethanol, 10.6kg of silica and 197kg of lignin.

The ethanol yield could even be boosted further, with a little help from pandas. Scientists are currently busy analyzing the enzymes that pandas utilize in their guts to break down bamboo (see Belgian scientists look for biofuel clues in panda poo). Given that breaking down bamboo is exactly what these enzymes are designed to do, they should work better than the bacterial enzymes used in this study. 

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.