Introduction

Water is typically purified by passing it through layers of porous materials, each specially selected for its ability to remove specific forms of contamination. All porous materials can filter particles by size, many have extra capabilities thanks to their chemical properties. For example, charcoal - or activated carbon - is a porous component of household and industrial water filters that can also bind large or electron-rich molecules via the van der Waals forces, and catalyse the breakdown of other chemicals such as molecular chlorine. While there are many different types of filters, we can categorise and compare them using their measurable physical and chemical properties. The key physical properties are:

• Pore size - average or maximum size of pores in the material
• Porosity - volume of the filter not occupied by solid material
• Tortuosity - length of paths through the filter compared with a straight line
• Adhesion - the strength of hydrogen bond interactions between the fluid and filter
• Kinetic rate constants - parameters defining how the filter material affects chemical reactions in the fluid

These properties contribute to the filter’s flow rate, measured in units of fluid filtered per unit surface area of the filter per unit time (e.g. Ga m^-2 h^-1). Altering the filter’s flow rate generally affects the filter’s efficiency at removing its targets, as increasing the flow rate typically involves increasing the pore size or reducing the surface area of the filter in contact with the fluid. This trade-off is the main hurdle in industrial water filtration, where demands for high-quality filtration require very low flow rates. Industrial processes are further complicated by blockage of the pores, fouling of the filter by organisms and the lack of a complete set of materials that will exhaustively filter all contaminants - heavy metal ions, small organic molecules and non-polar compounds remain difficult to filter without prior chemical treatments that themselves need to be filtered out.

Turbidity

Phytochelatin-dCBD metal binding assay

Nickel filtration assay

The nickel ions are an example of heavy metals, poisonous even in relatively small concentrations in water and notoriously difficult to filter with current filtration methods. Therefore our filtration concept was tested against a concentration of nickel in water that far exceeds the safe limits. We have attempted to filter high amount of nickel (250 μM) through the cellulose filters grown by the G. Xylinus ATCC53582 strain (K1321305). The phytochelatin-dCBD fusion (K1321110) was coated on the surface of the cellulose to make a nickel specific functionalised ultra filtration membrane. To test the two membranes we have used coffee press. As a control measure we have also attempted to the filter the nickel solution through the cellulose that was not further functionalised.

Results

References