The recent report entitled Water for People – Water for Life of the World Water Assessment Programme of the UNESCO says that more than 6000 people die every day due to water-related diseases, including diarrhoea, worm infections, and infectious diseases.
In addition, organic pollutants from industrial wastewater from pulp and paper mills, textiles and leather factories, steel foundries, and petrochemicals refineries, are a major cause of illness in parts of the world where regulations do not necessarily protect people from such industrial outflows.
So the availability of drinking quality water is fast becoming a major socio-economic issue across the globe, especially in the developing world. However, water purification technology is often complicated, requires sophisticated equipment and is expensive to run and maintain. Moreover, it usually requires a final costly disinfection stage.
Now a team of scientists at the Ian Wark Research Institute at the University of South Australia are tackling this by taking a nanotechnology approach to water purification – a move that has the potential to prevent disease and poisoning from affection millions of people.
Research professor Peter Majewski and biomolecular chemist Chiu Ping Chan have investigated how silica particles can be coated easily with a nanometre-thin layer of surface active material (SAM) based on a hydrocarbon with a silicon-containing anchor. The coating is formed through a chemical self-assembly process so involves nothing more than stirring the ingredients to make the active particles.
These active particles were then tested to demonstrate that they could remove biological molecules, pathogens such as the polio virus, bacteria such as Escherichia coli, and Cryptosporidium parvum – a waterborne parasite.
Writing in the International Journal of Nanotechnology (Vol5Nos2/3 2008), the researchers point out that novel materials have attracted considerable interest in recent times for the de-centralised treatment of water in order to remove organic contaminants. These include iron/iron oxide nanoparticles and both nano- and micron-sized photocatalysts.
“However, these methods always require additional treatment processes, such as sophisticated filtering to remove the nanoparticles from water or UV illumination, which is not always feasible in de-centralised water treatment scenarios and require additional infrastructure for water purification as well as electric energy,” they write.
They go on to explain that while the feasibility of surface modification in terms of controlling hydrophobicity, introducing and controlling the surface chemistry, and synthesis of crystalline oxide thin films via functionalised SAMs has been elaborated, their application on silica particles for water treatment has not been studied widely.
A SAM is a closely packed, highly ordered array of chained hydrocarbon molecules containing various numbers of CH2-units.
The SAM is simply described as a hydrocarbon with the general formula A-(CH2)n-B. B represents the bonding group, such as trichlorosilyl (-SiCl3) and trimethoxysilane (Si(OCH3)3) forming tightly covalent Si-O-Si-bonds to the surface atoms of silicon and silica. However, bonding with titanium and titania via Si-O-Ti bonds have also been observed.
‘A’ denotes the head group, chosen from among a number of possible species, such as sulphonate (-SO3H) and amine (-NH2), respectively. The length of the hydrocarbon molecules and the related thickness of the SAM are calculated based on the numbers of CH2-units were calculated to vary between about 0.6nm (three CH2-units) and about 2.5nm (17 CH2-units).
Immersed in aqueous solutions, some of the head groups like sulphonate head groups tend to deprotonate and to form negatively charged surfaces. Head groups like NH2 are known to deprotonate at high pH-values and to capture protons at low pH-values forming positively charged surfaces. Therefore, by carefully choosing of the SAM and pH-value of the solution, in which the SAM is immersed, negatively as well as positively charged surfaces can be obtained.
Majewski and Chan then went on to study the removal of bio-molecules and pathogens of different natures from water by silica particles coated with the functionalised SAM monolayers. The results clearly show that organic species can efficiently be removed at pH ranges of drinking water by stirring the coated particles in the contaminated water for up to 60minutes and finally filtering the powder.
