Water & Wastewater Asia November/December 2020

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30 FOCUS Fruitful endeavours From tasty treat to wastewater treatment essential, is there anything the pineapple cannot do? Associate Professor Duong Hai-Minh (centre) and his team of researchers at the National University of Singapore (NUS) By Natalie Chew Ever since they were first discovered by the Europeans in 1493, pineapples have fascinated humankind to no end. The spiky fruit is surprisingly versatile, with applications ranging from: - The gourmet (and how the pro-vs-anti pineapple on pizza debate rages on), - The practical (when mixed with sand, pineapple juice makes for quite the decent cleaning solution), and - The eco-friendly (we hear Piñatex is doing quite well these days). Yes, the humble Ananas comosus is capable of a great many things - and as of this year has been found to be a potential boon for the water and wastewater treatment industry. This comes as researchers from the National University of Singapore (NUS) have discovered a more novel use for the tropical fruit by converting its leaves into ultra-light, biodegradable aerogels. “Every year, 27.4m tonnes of pineapples are harvested, and 76.4m tonnes of pineapple leaf waste is generated. This in turn could release harmful chemicals and greenhouse gases such as carbon oxide and nitrogen oxide, that can cause serious environmental problems,” explained Associate Professor Duong Hai-Minh, who leads the nine-person team from the NUS Faculty of Engineering’s Department of Mechanical Engineering. “By converting pineapple leaf waste into high-value materials such as aerogels, we can help to solve environmental problems.” “EIGHTEEN TIMES FASTER THAN CONVENTIONAL METHODS” The process of manufacturing these biodegradable aerogels (referred to by Duong as eco-aerogels) is 18 times faster compared to conventional methods of producing commercial aerogels, needing only 10 to 12 hours to be ready. Duong describes the process. “Firstly, the pineapple fibers are extracted from the leaves by the decortication machine. This is a low-cost but highly effective method to obtain the fibers. Then, the fibers are cut into small pieces and mixed with crosslinker polyvinyl alcohol (PVA) in a desired ratio. The mixture is sonicated for 10 minutes for homogenization and cured at 80°C to promote the cross-linking reaction between the fibers and PVA. After that, the suspension is freeze-dried to produce eco-aerogels. The fabrication process varies according to the type of raw material used. The key steps are: shredding/blending, mixing with water and a small amount of non-toxic chemicals, stirring, ageing, freezing, and freeze-drying.” He further explained that no expensive or hazardous chemicals are used in manufacturing the eco-aerogels, and a 1sqm sheet of eco-aerogel with a 1cm thickness will cost less than $10 to produce, and can be sold for between $30-$50. At present, commercial aerogels on the market are costly, and comparatively speaking a commercially-manufactured blanket thermal insulation sheet made from aerogels of the same size can be sold for over $300. “These eco-aerogels made from pineapple leaf fibres are very versatile,” said Duong. November / December 2020 • waterwastewaterasia.com
FOCUS 31 The pineapple fibres only need 10 to 12 hours to be processed into the eco-aerogel “They are effective as oil absorbents, and we have also demonstrated their potential applications in food preservation and wastewater treatment. This is a big step towards sustainable agriculture and waste management.” Previously, the NUS team had experimented with producing eco-aerogels using other kinds of agricultural and food waste such as sugarcane bagasse, coffee grounds, and okara. PINEAPPLES AND WASTEWATER So what can the eco-aerogel do, exactly? According to Duong, utilisation of the ecoaerogels will be targeted at the removal of heavy metal pollutants in wastewater. “As cities move towards industrialisation, common heavy metal pollutions in wastewater are expected - for example, nickel (II) ion: A non-biodegradable toxic metal ion and known carcinogen. Ingesting water with nickel (II) ion concentration exceeding the permissible limit of 0.02mg/L would affect human organs and the central nervous system. treatment. We would first functionalise or modify our eco-aerogels with different amine groups, which can be very useful when it comes to removing heavy metal ions from wastewater. The porous structure of our aerogels helps with the adsorption process, because it increases the surface area exposed to the wastewater. This application can help with wastewater treatment in areas where high concentrations of heavy metal ions are found in the wastewater.” For example, Duong elaborates, ecoaerogels are able to remove up to four times more nickel ions in industrial wastewater when coated with diethylenetriamine (DETA), as opposed to conventional methods that use naturally-derived clay and syntheticallydoped graphene. What’s even better, he explains, is that this process does not generate secondary waste, and the eco-aerogels can be reused several times - which can go a long way in helping treatment plants achieve higher efficiency at lower costs. Duong and his team are currently working with DP Aerogels and Mapletree to pilot the large-scale production of these eco-aerogels for high-value applications. The eco-aerogels are expected to be in the market within the next three years. All images credited to the National University of Singapore. Eco-aerogels made from pineapple leaf fibres have stronger mechanical properties and they are suitable for use in wastewater This eco-aerogel will prove useful in the removal of heavy metal ions in industrial wastewater November / December 2020 • waterwastewaterasia.com
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