Bacterial enzymes can replace chemicals in recycling
Millions of tonnes of waste paper is generated every year. Just a small fraction of that is recycled through deinking—detachment of ink particles from the paper and removal of this ink by washing. This is done using hazardous chemicals like sodium hydroxide and hydrogen peroxide. This produces toxic effluents, drastically increasing the cost of wastewater treatment.
Indian researchers have now devised an ecological deinking method using bacterial enzymes. The new method, developed by researchers from the Kurukshetra University and Ballarpur Industries Limited in Haryana, is environment friendly and cost-effective.
Paper is made of fibres derived from plants. These fibres contain sugar molecules, like xylan and pectin, and a chemical compound known as lignin. These fibres need to be broken down to seperate ink from the paper surface. For the experiment, the researchers grew Bacillus pumilus, a bacterium that produces xylanase and pectinase, the enzymes that can digest the plant sugar molecules in paper. They then shredded school waste paper, written over with blue ballpoint pen ink, into small pieces. These shreds were soaked overnight in tap water at room temperature and then washed, ground, dried, mixed with water and made into pulp. The pulp was subjected to bacterial enzymes for 1-4 hours at 40-65°C.
The researchers found that enzymatic treatment dissolved the sugars in the paper fibres, leaving only lignin molecules behind. This loosened the bond between paper fibres and ink particles, seperating ink from the paper pulp. Ink floated to the top of the container and was removed by skimming. The ink-free pulp was washed and pressed into sheets.
A dose of 15 International Units (IU) xylanase and 3 IU pectinase is sufficient to deink 1 gram of paper pulp, say the researchers. The enzymes removed maximum ink at a pH of 8.5, leading to an increase in brightness and whiteness of the pulp. “This shows that the enzymes can work in alkaline conditions and, unlike chemical deinking, the process does not involve any acids which can end up in the effluents. This makes them suitable for paper industries. Maximum enzymatic deinking was observed at 50°C, while chemical deinking peaks at 70°C,” says lead researcher Ritu Mahajan, of the Department of Biotechnology in Kurukshetra University.
The study has been published in the September issue of Bioresource Technology. The researchers say if enzymatic and chemical deinking processes are used in combination for deinking, it can bring down the use of toxic chemicals by 50 per cent.
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