Wood technology removes dye pollutants
More than 80% of dye pollutants in wastewater can be removed by a wood-based technology, says the team at Chalmers University of Technology, Sweden.
By using cellulose nanocrystals (CNCs) with a high adsorption capacity, they report their purifying cellulose powder is tunable for different pollutants.
The research has been conducted with the Malaviya National Institute of Technology Jaipur in India, where dye pollutants in textile industry wastewater are said to be a widespread problem.
The paper, published in the journal Industrial & Engineering Chemistry Research by ACS Publications, reveals how the CNCs are synthesised at mild temperatures using acid hydrolysis of microcrystalline cellulose by sulphuric acid.
'CNCs by a simple acid treatment become enriched with negatively charged surface functionalities and shows good adsorption capacity,' says the paper on Cellulose Nanocrystals Derived from Microcrystalline Cellulose Selective Removal of Janus Green Azo Dye.
As the contaminated water passes through the cellulose powder, pollutants are adsorbed, with sunlight catalysing their breakdown. Lead researcher Gunnar Westman says it is a cost-effective and simple system.
Although they have excess cellulose to the amount of dye adsorbed and want to further optimise the ratios, they have found that roughly 5g of cellulose adsorbs 0.3g of dye.
The Chalmers team is now supporting further testing by their Indian colleagues. So far, laboratory tests with four samples of water from nearby textile industries show that more than 80% of the dye pollutants are removed.
For real-world testing, they plan to make a kilo-batch of the material and create a small side-stream from the main stream of industrial wastewater and measure dye concentrations.
The Chalmers researchers say the cellulose powder can be prepared from almost any cellulose source by sulphuric acid hydrolysis. The sulphate groups coordinate to the dyes. They have not currently recycled the cellulose.
The team has investigated the effects of pH, temperature, concentration and loading capacity on adsorption capacity. After a certain CNC dose of 0.5g/l, the paper reports that the 'adsorbent achieved the maximum removal efficiency'. The researchers infer that the 'reduced adsorption at higher dosage might be due to unavailability of the adsorbent’s active sites and low surface area because of aggregation of the adsorbent molecules'.
The effects of temperatures ranging from 10-50°C and a pH from 1-13 have been explored. Westman thinks the pH and treatment time can be optimised.
The cellulose nanocrystals may be able to also filter other water pollutants. A previous study from the same group showed that pollutants of toxic hexavalent chromium – common in wastewater from mining, leather and metal industries – are removable with a similar cellulose-based material. They are also exploring the removal of antibiotic residues.
'We have the possibility to make the cellulose surface cationic or anionic as well as hydrophobic. The sulphated nanocellulose can be seen as anionic and by chemical treatment of the sulphate group we can convert them into cationic or hydrophobic. The hydrophobic may be used as sorbents for oils,' Westman explains.
They say a significant challenge for scaling up the process is the cubic metres of wastewater that the cellulose material system can handle in its lifetime.