Elephant dung, as it turns out, is an excellent source of cellulose for paper manufacturing in countries where trees are scarce, scientists reported at the 255th National Meeting & Exposition of the American Chemical Society (ACS). Also, in regions with plenty of farm animals such as cows, upcycling manure into paper products could be a cheap and environmentally sound method of repurposing this agricultural waste.
The idea for the project germinated on Crete, where Dr. Alexander Bismarck noticed goats munching on summer-dry grass in the small village where he was vacationing. "I realized what comes out in the end is partially digested plant matter, so there must be cellulose in there," he said.
"Animals eat low-grade biomass containing cellulose, chew it and expose it to enzymes and acid in their stomach and then produce manure. Depending on the animal, up to 40% of that manure is cellulose, which is then easily accessible," Bismarck said. That means it should require much less energy and fewer chemical treatments to turn this partially digested material into cellulose nanofibers relative to starting with raw wood, he suggested.
After working with goat manure, Bismarck, who is at the University of Vienna, Austria, post-doc Andreas Mautner and graduate students Nurul Ain Kamal and Kathrin Weiland moved on to dung from horses, cows and, eventually, elephants.
The supply of raw material is substantial: Parks in Africa that are home to hundreds of elephants produce tons of dung every day, and cattle farms in the U.S. and Europe yield "mountains" of manure, according to Mautner.
The researchers treat the manure with a sodium hydroxide solution. This partially removes lignin — which can be used later as a fertilizer or fuel — as well as other impurities, including proteins and dead cells. To fully remove lignin and to produce white pulp for making paper, the material has to be bleached with sodium hypochlorite. The purified cellulose requires little, if any, grinding to break it down into nanofibers in preparation for use for paper, in contrast to conventional methods.
"You need a lot of energy to grind wood down to make nanocellulose," Mautner said, but using manure as a starting material, "you can reduce the number of steps you need to perform, simply because the animal already chewed the plant and attacked it with acid and enzymes. You inexpensively produce a nanocellulose that has the same or even better properties than nanocellulose from wood, with lower energy and chemical consumption," he said.
The dung-derived nanopaper could be used in many applications, including as reinforcement for polymer composites or filters that can clean wastewater before it's discharged into the environment, Bismarck said. His team is working with an industrial consortium to further explore these possibilities. The nanopaper could also be used for writing paper, he added.
The researchers are investigating whether the process can be made even more sustainable by first producing biogas from manure and then extracting cellulose fibers from the residue. Biogas, which is mostly methane and carbon dioxide, can then be used as a fuel for generating electricity or heat.
The work was supported by the Austrian Academic Exchange Service, the South Africa Department of Science & Technology, Europe 2020, the Ministry of Education Malaysia, the University of Vienna and the Vienna Zoo.