Photo-Catalysis for Water Treatment

In current scientific world, semiconductor photocatalytic process is showing a great potential as an environmental friendly, sustainable and low-cost treatment technology in water and waste-water industry. The remarkable potential of this technology has already been demonstrated in removing the persistent organic compounds and micro-organisms from water. The hurdle that is stopping this advanced oxidation technology from being commercialized is the lack of technique to recover the catalyst particles after the water treatment process.

 Rapid development, industrialization, population growth and long-term droughts have increased the demand for clean water. Shortage of water sources is a global issue. Researchers are constantly putting effort in discovering a practical solutions to yield more viable water resources. Some strategies that can resolve problem for short time are rainwater harvesting and catchment storage for stormwater. It is estimated  that roughly 4 Billion people in the world have experienced water scarcity in their life. Millions and millions of people have also died because of severe waterborne diseases. Due to increasing discharge of micropollutants and contaminants into the water resources the death toll is going to increase and more and more people are going to experience water scarcity. Looking at this scenario, the need for advanced, low-cost and high efficient water treatment technologies for water treatment is increasing everyday. 

Municipal waste-water is also considered as a major water resources because it constitute one of the largest accumulation of water. Reuse of those water can provide abundant amount of clean water supply. Those water contains suspended solids, health threatening coliforms and soluble refractory organic compounds, which are all tedious and expensive to remove. Currently available technology like adsorption and coagulation only changes the contaminants from one form to another but doesn't destroy it completely. Other technology like sedimentation and filtration can generate toxic bi-products that can act as a secondary pollutants. These contaminants are highly redundant and has increased concerns around the world. Chlorination is one of the most effective process in destroying the pollutants but it creates bi-products that are mutagenic and carcinogenic for human.

This has increased the research in the field of Advanced Oxidation Processes. This process works in the concept of generating highly reactive species like H2O2, OH-, O2-, O3, etc. These helps in mineralization of refractory organic compounds and water pathogens. Photocatalysis involving TiO2, ZnO, Fe2O3, CdS, GaP and ZnS has proved high efficiency in degrading a wide range of ambigous refractory organics into readily biodegradable compounds. Finally, those compounds change or mineralize to innocuous CO2 and water. Researchers has focused more in TiO2 semiconductor catalysis because of its promising high potential. It is the most chemically active photocatalyst under the photon energy of 300 nm to 390 nm. It also remains intact after many more repeated catalytical cycles. It also does not form any toxic bi-products. TiO2 also has high chemical and thermal stability. It also has strong mechanical properties. 

There are many important features of heterogenous photocatalysis that has made it feasible in water treatment industries. Some of them are: 

1. Ambient operating temperature and pressure

2. Complete mineralization of parents and their intermediate compounds without secondary pollution

3. Low operating costs

It is already well established that highly reactive species generated using photo-induced charge separation is effective in microbial inactivation and organic mineralization. It also does not create any toxic secondary pollutants. Still there is a challenge in separating the catalyst after the water treatment process. This is the main obstacle that is preventing it from being industrialized. The fine powders of these catalysts create a strong tendency for catalyst agglomeration during the operation. Such particles have large surface area to volume ratio and high surface energy.  Catalysts particle agglomeration is highly detrimental in views of particles size preservation, surface area reduction and its reusable lifespan.  Other technical challenges include lower photoactivity range  and integration with photo-catalytic reactor system. Understanding of the theory behind common reactor operational parameters and their interactions is also inadequate and presents a difficult task for process optimization. A number of commonly made errors in studying kinetic modelling on either the photo-mineralization or photo-disinfection have also been seen over the past decades. 

References:

Chong, M. N., Jin, B., Chow, C. W., & Saint, C. (2010). Recent developments in photocatalytic water treatment technology: a review. Water research, 44(10), 2997-3027.