Advanced Oxidation Processes

Advanced Oxidation Processes (AOPs) refer to a set of high-effective oxidative water treatments that can be used to treat toxic effluents at industrial level, hospitals, and wastewater treatment plants, but not often found at household level. AOPs are successful to transform toxic organic compounds (e.g., drugs, pesticides, endocrine disruptors, etc.) into biodegradable substances. AOPs in general are cheap to install but involve high operating costs due to the input of chemicals and energy required. They are often used as a final step to remove micro-pollutants from the effluents of municipal wastewater treatment plants and for the disinfection of water. The combination of several AOPs is an efficient way to increase pollutant removal and reduce costs.

 

UV disinfection step at Bens sewage treatment plant (A Coruña, Spain)

Many methods are classified under the broad definition of AOPs. The table shows some of the most studied processes. Advanced oxidation generally uses strong oxidising agents like hydrogen peroxide (H2O2) or ozone (O3), catalysts (iron cations, electrodes, metal oxides) and irradiation (with UV light, sunlight, ultrasounds) separately or in combination under mild conditions (low temperature and pressure). Among different available AOPs, those driven by sunlight are particularly attractive due to the abundance of solar radiation in the regions where water scarcity is high and due to their relatively low costs and high efficiencies. In particular, solar heterogeneous photocatalysis is the most promising, as it does not use chemicals and barely needs energy, so it is highly sustainable.

Dark AOPsLight-driven AOPs
Ozonation (O3)Photolysis (UV o UV + H2O2)
Fenton (Fe2+ + H2O2)Photocatalysis (light + photocatalyst)
Electrolysis (electrodes + electricity)Photo-Fenton (light + Fenton)
Sonolysis (ultrasounds)

 

Advanced oxidation involves several steps explained as follows:

  • Formation of strong oxidants, particularly hydroxyl radicals (HO).
Hydroxyl_radical
  • Reaction of these oxidants with organic pollutants in the water producing biodegradable intermediates.
  • Reaction of biodegradable intermediates with more oxidants until complete degradation (i.e., production of water, carbon dioxide and inorganic salts), referred to as mineralisation.
Photocatalysts

Photocatalysis is the acceleration of a photoreaction by the presence of a catalyst. When a semiconductor is irradiated with above band gap (BG) illumination, the radiation energy is absorbed and electrons are promoted from the valence band (VB) to the conduction band (CB) giving rise to the formation of electron-hole (e/h+) pairs. If these charges reach the semiconductor-water interface they may participate in redox reactions.

CienciaASDIS3Photocat

When an electron acceptor (normally dissolved oxygen, O2) and electron donor (usually hydroxyl anion, HO) are adsorbed close to the surface of the semiconductor particle, a series of electron transfer reactions may occur so that different reactive oxygen species (ROS) are produced. The ROS are very active, indiscriminate oxidants, especially the hydroxyl radical (HO). The ROS can not only destroy a large variety of chemical contaminants in water but they may also cause fatal damage to microorganisms by disruption of the cell membrane or by attacking DNA and RNA.

 

The semiconductor most employed in photocatalysis is TiO2. Thus, most published papers relate to this compound, pure or combined:

Taken from L. Zhao et al. Science of the Total Environment 627 (2018) 1253-1263

Most studies have reported that suspended photocatalysts are more efficient due to large surface area available for the reaction. The main drawback of using nano- or micro-particles in suspension is the requirement for post-treatment retrieval or recycling of the catalyst, potentially making the treatment more complex and expensive. Therefore, photocatalysis employing immobilized TiO2 have gained more attention. Unfortunately, immobilizing TiO2 on a solid substrate reduces the surface area available for reaction and limits the mass transfer of reactants to the photocatalyst surface in addition to making it more expensive. Besides, the question of the mechanical stability of the photocatalytic layer arises.

 

The capability of TiO2 to inactivate a wide range of pathogens has been a successful field of research since 1985. E. coli has been the most frequently studied microorganism for water disinfection studies, as it is an indicator of faecal contamination and reference for water quality. Other bacteria (including spore-forming species) and fungi such as Bacillus pumilus, Serratia marcescens, Staphylococcus aureus, total coliforms, Pseudomonas aeruginosa, Salmonella typhimurium, Enterobacter cloacae, Saccharomyces cerevisiae, Candida albicans, and Fusarium solani were also inactivated using TiO2 photocatalysis. In addition, plant pathogens were found to be inactivated in water using TiO2-based catalysts like spores of Fusarium solani, Fusarium oxysporum, Fusarium verticillioides, Fusarium equiseti and Fusarium anthophilum. Other infectious agents such as viruses and prion proteins were also investigated, and some authors demonstrated the inactivation capacity of irradiated TiO2 to degrade and inactivate them in water. Moreover, the capability of immobilized TiO2 to inactivate oocysts of Cryptosporidium parvum was demonstrated under natural sunlight.

 

So far, the broad application of this technique has been mostly hampered by the lack of appropriate photocatalysts that combine efficiency and low cost with mechanical stability and easiness of recovery and reuse. But these problems are solved with ASDIS photocatalyst (patent pending at the Spanish Patent and Trademark Office in Madrid, Application No. P202031314 with priority date 29 December 2020, and at the Ecuador’s National Service of Intellectual Rights in Quito, Application No. SENADI-2021-16937 with priority date 10 March 2021), where the semiconductor is not just immobilized at the surface of the support material, but homogeneously mixed to form a cheap, bulky composite easy to filter out.

ASDIS - Scientific Publications

SHORTLY

ASDIS - Contributions to scientific conferences

ASDIS. An innovative solar water disinfection method that makes use of a recently developed novel photocatalyst

Daniel R. Ramos, Silvio Aguilar, J. Arturo Santaballa, Moisés Canle

18th European Symposium on Organic Reactivity (ESOR2021), Online, September 2021

 

ASDIS: Improved photocatalytic disinfection method

Silvio Aguilar, Daniel R. Ramos, Daniel Rosado, Javier Moreno-Andrés, Briggitte Guerrero, Jimmy Fernández, J. Arturo Santaballa, Moisés Canle

18th European Symposium on Organic Reactivity (ESOR2021), Online, September 2021

 

Photocatalytic transformation of the antibiotic sulfamethoxazole using ASDIS method

Zenydia Marín R., Paula Martinez L., Silvio Aguilar, Daniel R. Ramos, J. Arturo Santaballa, Moisés Canle

18th European Symposium on Organic Reactivity (ESOR2021), Online, September 2021

 

Fotocatalizador simple, económico y efectivo para la degradación de contaminantes orgánicos en medio acuoso

Silvio Aguilar, Daniel R. Ramos, J. Arturo Santaballa, Moisés Canle

1er Workshop de Fotocatálisis LATIN-AOPs, Online, July 2021

 

Evaluación de un nuevo fotocatalizador para la eliminación de herbicidas en efluentes de aguas residuales

Mario Pedreira-Díaz, Daniel R. Ramos, Silvio Aguilar, Federico Pomar, Moisés Canle

1er Workshop de Fotocatálisis LATIN-AOPs, Online, July 2021

 

Inactivación fotocatalítica de cianobacterias: estudio preliminar

Daniel R. Ramos, Silvio Aguilar, Javier Moreno-Andrés, Enrique Nebot

1er Workshop de Fotocatálisis LATIN-AOPs, Online, July 2021

 

Elemento fotocatalizador para descontaminación de fluidos

Silvio Aguilar, Daniel R. Ramos, J. Arturo Santaballa, Moisés Canle

II Feria Nacional de Invenciones Académicas, Ecuador, April 2021