Chemicals now touch practically every aspect of our daily lives. Complex organic pollutants such as herbicides, pesticides, hormones and other active pharmaceutical ingredients, as well as waste products from the oil, gas and chemical industries are now being detected in natural water courses and even in drinking water.
Only in the past decade have analytical methods become sufficiently advanced to detect them. Public awareness of these compounds is becoming much more high profile as information of their persistence in the environment and their impact on human health becomes available.
Evaluating exactly how much of a risk these micropollutants pose to humans is a more complex issue. Even parts per trillion levels are considered potentially problematic due to their cumulative effects in the body and through the food chain. The problem is exacerbated by the fact that 10’s or 100’s of different compounds may be present in a water sample and the overall toxicological effect of these cocktails is completely unknown.
The responsibility for removal lies with the municipal water treatment facility. However, pharmaceuticals are usually non-volatile, water-soluble, often charged molecules and many of them pass through treatment plants designed to deal with conventional pollutants. In addition, agricultural use of herbicides and pesticides such as aminopyralid and the active component in slug pellets, metaldehyde, is also now under scrutiny amid concerns about their entry into river systems.
The developing situation poses a significant challenge for a wide range of sectors wishing to remove organic contaminants, not just the municipal drinking water and wastewater treatment plants. Government legislation on the use and removal of environmentally persistent pollutants from industrial waste streams before discharge is becoming more demanding and, in many cases, waste is being stockpiled whilst effective treatments are sought.
The group of technologies termed Advanced Oxidation Processes (AOPs), such as ozonation – oxidation using ozone – is extremely effective, removing most of the compounds even at relatively low doses. High energy UV combined with hydrogen peroxide is similarly effective in reducing the levels of micopollutants to below the current detection limits.
Another AOP is heterogeneous photocatalysis. It can be defined as the acceleration of a photoreaction in the presence of a catalyst. This method has the ability to oxidise organic and inorganic substrates using a light source and a semiconductor catalyst. The catalyst can carry out substrate oxidation and reduction reactions simultaneously using UV light of long wavelengths, including the 5% or so present in sunlight.
This approach offers a number of advantages over those mentioned above for removing organic species from the environment, because the process gradually breaks down and mineralizes the contaminant molecule, no residue of the original material remains and therefore no sludge requiring disposal to landfill is produced. The catalyst itself is unchanged during the process and little or no consumable chemicals are required. These characteristics result in considerable savings and a simpler operation of the equipment involved. The process will continue to work at very low concentrations allowing sub part-per-million consents to be achieved. The most common semiconductor used for photocatalysis in this setting is titanium dioxide (TiO2).
TiO2 is a non-toxic, white pigment, used in everyday paints, plastics, cosmetics and foods. In addition, it has the ability to act as a photocatalyst. In the presence of near-UV light (<388nm), TiO2 generates reactive oxygen species in water which, in turn, can completely mineralise organic pollutants. Hydroxyl radicals are usually the most important radicals formed in TiO2 photocatalysis, rapidly attacking pollutants at the surface and possibly in solution as well. The oxidation potential of the hydroxyl radical is greater than ozone, hydrogen peroxide, hypochlorous acid and chlorine.
TiO2 has been used in the form of a nanoparticulate suspension, or as a thin film coating on sheets and pipe work in water treatment settings. Degussa P25 is a commercially available nanopowder form of TiO2 and has been used in many studies of photocatalytic degradation. It is widely regarded as the benchmark material for such studies.
The huge number of academic articles published on its use demonstrate that TiO2 photocatalysis is a versatile technology, capable of degrading many of the environmental pollutants of concern today. However, two major obstacles exist to the wider acceptance of this technology in an industrial-scale setting.
Firstly, because the mechanism of target breakdown is a chemical reaction, rather than a passive adsorption, the speed at which this reaction occurs will be governed by many factors. One of the most important of these is the available surface area of the catalyst. Nanoparticulates offer extremely high surface areas for both adsorption of target pollutants and their subsequent destruction through reactive oxygen species generation. The format in which the catalyst is presented, therefore, has a major role to play in maximising the rate at which the depollution takes place.
Secondly, has been the difficulty in subsequent recovery of the catalyst from the treated water. Whilst P25 TiO2 nanoparticles are used as the benchmark material in terms of activity, the expense and practical difficulties involved in removing it from treated water has been a major discouragement to potential users. Other approaches such as pelletised TiO2 and coated pipe work/screens clearly reduce these recovery problems, but have a significantly lower surface area and therefore reaction rate. To compensate for this reduced surface area, large curtain screens or very long networks of pipes are used, so facilities based around these approaches tend to be extremely large and expensive to put in place.
A study was carried out in 2011 by Robert Gordon University, Aberdeen, UK, where the performance of MTL Photospheres was compared with P25 nanoparticulate catalyst. The results generated indicated that the Photosphere coating has a comparable activity to that of P25, the industry benchmark.
Until a material is developed which addresses the rate and handling problems simultaneously, TiO2 photocatalysis will continue to struggle to be accepted as a mainstream depollution technology.
Using its proprietary technologies in particle coating, MTL has developed an alternative solution to the above problem, through the synthesis of a highly active TiO2 photocatalyst on the surface of a buoyant, filterable microparticulate core. This material has been termed Photospheres.
TiO2 coated hollow glass microspheres offer significant advantages over conventional pelletised and nanoparticulate TiO2 materials currently used in some specialist water treatment processes:
- It is filterable and reusable.
- It provides the option of being used either in enclosed reactors or in tanks/lagoons depending on climate and nature of polluant.
- The inherent buoyancy keeps TiO2 at the surface, close to the illumination source (particularly in an open tank setting).
- The continuous, covalently bound shell minimises shedding of TiO2 into target solvent stream.
- The material offers a high surface area when compared to pellets or coated plates & pipes.