Wood-based water filter traps bacteria

Materials World magazine
30 Apr 2017

Cellulose filters with a positively charged polymer surface have been found to trap bacteria in water, as Simon Frost reports.

Wood science is a key area of research for the KTH Royal Institute of Technology, Sweden, which has recently developed wood-based transparent materials, batteries, foams and polystyrene alternatives. On 22 March, World Water Day 2017, the university launched WaterCentre@KTH, a cross-disciplinary unit aimed at uniting scientists, industry and public agencies to collaboratively develop sustainable approaches to water systems and tackle scarcity. A team at KTH has now married these themes, creating a wood cellulose-based water filter that uses layers of charged polymers to extract bacteria.  

Anna Ottenhall, a PhD student at KTH, carried out the research under the supervision of Professor Monica Ek and Dr Josefin Illergård. ‘We have worked on creating a sustainable and non-leaching antibacterial cellulose material for about ten years at KTH,’ Ottenhall told Materials World. ‘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.’

The researchers are using two kinds of readily available filters – cellulosic laboratory filters and commercial coffee filters – as the base material, although others can be used. ‘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,’ Illergård said. 

Using household salt and two oppositely charged polyelectrolytes in solution, these standard filters can be imbued with antibacterial properties, as Illergård explained to Materials World. ‘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,’ she said. 

With this gain in entropy, the surface can be overcharged, turning an initially negative fibre surface into a net-positive surface that can attract 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,’ Illergård explained.  

Rather than kill bacteria by chemical means, the system works by physically removing negatively charged bacteria through electrostatic adsorption. The trapped bacteria are unable to reproduce and die. Foregoing the addition of chemicals to the water prevents leaching potentially harmful chemicals, and also means that the bacteria will not create resistant strains. ‘So far, we have shown that it is possible to remove 99.9% of the bacteria in water using our filter, and this could probably be improved even further with the right filter design,’ Ottenhall said. For the laboratory tests, the team is using standard filter holders, and are yet to experiment with original designs. 

Illergård noted that the positively charged filters have a long shelf life – she has stored treated materials for several years on an open bench in the laboratory without loss of antibacterial properties. The materials also benefit from low cost and high availability, as Ottenhall explained, ‘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. 

‘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.’ 

While the focus at KTH is on using the materials to filter water where a reliable, clean supply is unavailable – whether long-term, through a lack of infrastructure, or in short-term disaster relief – the same principle could be used for application in medical dressings, packaging and sportswear.

The next steps are to develop an efficient filter design and test the materials with larger quantities of water. ‘Our role, however, is that of the scientist – we develop the technology and not the final applications, despite our applied research,’ Illergård said. That’s where the engineers will step in. 

Don’t pass the salt

Graphene oxide boasts an expanding range of potential applications, and can now add water desalination to that list, as a team at the University of Manchester, UK, has developed membranes that can sieve common salts from salt water. 

Until now, graphene-oxide membranes have only demonstrated the ability to filter out nanoparticles, organic molecules and large salts, because when the membranes are exposed to water they swell, allowing water and smaller salts to pass through but catching larger ions or molecules. The Manchester team discovered that the pore size in a membrane could be precisely controlled down to an atomic scale, which can filter common salts out of water to make it drinkable. When salt is dissolved in water, a ‘shell’ of water molecules surrounds the salt molecules. As the water and salt hit the graphene oxide membrane, its capillaries block the salt but let the water pass. The membranes can reject 97% of salt with limited swelling.

Claiming that the method has been demonstrated as suitable for mass production, the team said scaling membranes opens the door for the fabrication of membranes with on-demand filtration capabilities that can filter out ions according to size.

The three main desalination methods currently used – thermal, electrical and pressure – all require a significant amount of energy, requiring between 7–30kWh of energy per 1,000 gallons of water. Membrane systems, such as electrical and pressure, use more energy when filtering water with a high salt content, like seawater. The use of graphene membranes could drastically reduce this cost if commercially scaled.  

With the UN predicting that 14% of the world’s population will experience water scarcity by 2025, the membranes could prove to be a solution to water desalination in areas with a limited water supply. The membranes also show potential to be made on smaller scales to be accessible to countries lacking the funding to commit to large manufacturing plants.

Tunable sieving of ions using graphene oxide membranes go.nature.com/2o8SZk1

Adsorbent materials clean up wastewater

A new adsorbent material made from grapefruit peel is being used to rid waters of heavy metals and organic pollutants. Researchers at the University of Granada, Spain, the Centre for Electrochemical Research and Technological Development and the Centre of Engineering and Industrial Development, Mexico, modified the structure of fruit peel, giving it a greater surface area and porosity to increase its absorbency. 

Luis Alberto Romero Cano, from the Carbon Materials Research Team at the University of Granada’s Faculty of Science, commented, ‘[we] have managed to add functional groups to the material, thus making it selective in order to remove metals and organic pollutants present in the water.’ This also offers a new commercial use for some of the estimated 38.2 million tonnes of fruit peel produced globally each year by the food industry. 

It’s not only fruit peel that is being used to absorb contaminants in water – researchers from the University of Minnesota College of Food, Agriculture and Natural Sciences, USA, have created a nanocomposite sponge that can take up mercury in water. The researchers grew selenium nanomaterials on a polyurethane sponge to produce antimicrobial properties. 

The sponge kills bacteria and converts the contaminants into a non-toxic form to be disposed of via landfill. It removes mercury from tap and lake wastewater in less than five seconds, and from industrial wastewater in less than five minutes, the researchers claim.

Grapefruit peels as biosorbent: characterization and use in batch and fixed bed column for Cu(II) uptake from wastewater bit.ly/2ouF33A