A water sample before filtration using cellulose filters with a positively charged polymer surface, as compared to before. Photo credit: David Callahan/KTH Royal Institute of Technology
Cellulose filters with polymer surfaces that have been positively charged are capable of trapping bacteria in water, according to KTH Royal Institute of Technology in Sweden, where wood science is one of the main pillars of research.
On World Water Day 2017, KTH launched the WaterCentre@KTH, a unit that span across multiple disciplines with the aim of brining scientists, industry, and public agencies together to create and develop sustainable approaches to water systems while simultaneously addressing water scarcity. One of the teams at the WaterCentre@KTH was able to integrate all the themes into developing a water filter based on cellulose that uses numerous layers of positively charged polymers to separate the bacteria from the water.
Anna Ottenhall, a student reading for her PhD at KTH, carried out the groundbreaking research under Professor Monica Ek and Dr Josefin Illergård’s guidance.
“We have worked on creating a sustainable and non-leaching antibacterial cellulose material for about ten years at KTH,” Ottenhall said to Materials World Magazine in an interview. “Dr Illergård has done a lot of the fundamental research regarding the material. Exploring different applications for the antibacterial cellulose, she had the idea that it might be possible to use it for water treatment. She wanted to use the material for an application that would make a difference in the world.”
Although cellulosic laboratory filters and commercial coffee filters are both currently being used by the research scientists as a base material, others can be used as well. “We have shown in earlier work that it is possible to use our technology on different types of wood-based pulp fibres and other materials as well,” Dr Illergård explained.
According to Dr Illergård, with household salt and two oppositely charged polyelectrolytes, the filters can be suffused with antibacterial properties. “We use physical modification, where long, charged polymers (polyelectrolytes) are adsorbed onto the fibre surface. The surface has negatively charged groups that attract positively charged polymers, but what drives the adsorption is the gain in entropy as the polymer releases its counter ions, which are needed to stabilise it,” Dr Illergård continued.
The gain in entropy means that the surface can be overcharged, thus turning what was originally a negative fibre surface into a net-positive surface capable of attracting bacteria. “By repeating this step with a negatively charged polymer you can add another layer, alternating with oppositely charged polymers to create a polyelectrolyte multilayer system, ending with a positive layer,” Dr Illergård explained to Materials World Magazine.
Thus, rather than killing off the bacteria through chemical means, the system simply physically removes the negatively charged bacteria through electrostatic adsorption. The bacteria, trapped, will be unable to reproduce and eventually die. Additionally, by not adding chemicals to the water, the bacteria in the water are also not given the chance to develop resistant strains.
“So far, we have shown that it is possible to remove 99.9 per cent of the bacteria in water using our filter, and this could probably be improved even further with the right filter design,” Ottenhall said.
Dr Illergård also noted that the filters – positively charged – have an extended shelf life, having stored several treated materials for a number of years on a bench in the laboratory with no loss of antibacterial properties. The materials are also cost-efficient and readily available.
“One of our main targets is to develop a cheap and simple water filter that doesn’t require electricity – just gravity – to run water through it. We have shown that it is possible to remove bacteria using a simple coffee filter that is modified to gain a positive charge. The polymers are also affordable and used in low concentrations,” Ottenhall explained. “Nanoparticle-based filtration materials are generally much more expensive. The process doesn’t rely on chemicals like chlorine or expensive metals like silver to deactivate bacteria, as in other filtration methods.”
Now, the next steps will be to create an efficient filter design as well as test the materials with increasingly large water quantities before turning their applied research over to the engineers for scaling.
“Our role is that of the scientist – we develop the technology and not the final applications, despite our applied research,” Dr Illergård said.
Source: Materials World Magazine
