Wastewater Treatment: Advanced Oxidation Processes

Contaminants of emerging concern (CECs) are a direct consequence of anthropogenic activities in today’s society. This term usually refers to compounds from different sources with a range of chemical compositions. Their presence in the environment is not significant, but recent studies highlight their high risk due to toxicological effects from constant and prolonged exposure, which can have chronic and harmful effects.

The CECs detected include substances such as food additives, pharmaceutical compounds, illegal drugs, steroid hormones, pesticides, flame retardants, surfactants and products derived from modern lifestyles such as caffeine and nicotine. In response to the lack of control over CECs, the European Union passed Directive 2013/39/EU and tentatively established the CEC Watch List (2015/495/EU) to register and control these substances, given that their effects on the environment and humans are still largely unknown.

Traditional Wastewater Treatment Systems

Contaminated water is generally treated in conventional wastewater treatment plants (WWTPs). The treatment process is divided into four consecutive subprocesses:

Preliminary treatment: This consists of a series of processes aimed at removing all materials capable of damaging any part of the wastewater treatment system. Through processes such as decantation, filtration and separation, a great majority of the macroscopic material that comes in with the contaminated water is removed.
Primary Treatment: Physical separation systems are then applied to isolate materials such as sediments and macroscopic organic matter.
Secondary Treatment: The goal here is to eliminate organic matter that could not be removed in previous phases. Wastewater undergoes biological treatment, in which bacteria, protozoa, fungi, algae and worms feed on organic matter as nutrients and transform it into CO₂ and H₂O. However, secondary treatment is a very delicate phase within the entire wastewater treatment system because of the conditions required for proper use of specific microorganisms, but also because the presence of non-biodegradable chemical compounds can deteriorate the conditions of the biological treatment and eliminate the microorganisms used, thus incapacitating the plant’s biological treatment.
Tertiary Treatment: This consists of a series of specific processes that are necessary depending on the required quality of the water to be discharged. Treatments such as chlorination, ozonation and UV light radiation are included as key procedures for the elimination of persistent microorganisms, as well as oxidation of organic contaminants that could not be removed in previous phases.

Despite the performance of technologies applied in WWTPs, they are not designed for the elimination of CECs. For example, pharmaceuticals such as carbamazepine and diclofenac, artificial sweeteners such as sucralose, x-ray contrast compounds and halogenated chemicals such as fungicides and herbicides persist throughout the treatment process in WWTPs while removal rates of less than 25% are achieved. As a result, a significant amount of these organic contaminants ends up being released into the aquatic environment with the associated environmental consequences.

Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are an attractive and promising option for the effective removal of organic contaminants. AOPs are defined as processes that involve the generation of reactive oxygen species (ROS) at a concentration high enough to purify wastewater. The hydroxyl radical (OH^–) is one of the most oxidizing ROS due to its high reactivity, which makes it react quickly with any organic molecule in the medium. This, combined with its low selectivity, makes OH^– a key species for mineralization of any organic contaminant, with CO₂ and H₂O as byproducts. However, this also means that the lifespan of this radical species is very short, which makes on-site production necessary during its application.

AOPs are currently based mainly on UV/O₃, UV/O₃/H₂O₂, Fenton, photo-Fenton, non-thermal plasmas, sonolysis, photocatalysis, radiolysis, supercritical water oxidation processes and generation of other reactive oxygen species.

The different systems capable of carrying out advanced oxidation processes highlight the versatility of ROS in reacting with organic contaminants, making them potential alternatives to traditional wastewater treatment systems for the elimination of CECs. Among AOPs, solar photocatalysis systems are of particular interest because they can be considered an eco-friendly method driven by clean energy, which involves milder conditions than other AOP-based systems and conventional technologies. The possibility of avoiding hazardous, unhealthy heavy metal-based catalysts and strong chemical oxidizing/reducing agents, as well as the benefit of using the sun as a free green light source make photocatalysis a promising technology for the generation of radical species and wastewater treatment.

Photocatalysis

In general, photocatalysis can be homogeneous or heterogeneous, depending on the physical state of the photocatalyst and the reacting species. In homogeneous photocatalysis, all reacting species are in similar physical states, i.e. liquid, solid or gaseous. In contrast, heterogeneous photocatalysis involves photocatalysts or reacting species in a different physical state from the other reaction components. The nature of the photocatalyst used is also very important for proper design of AOP-based wastewater treatment systems. Photocatalysts can be organic or inorganic, transition metals in the form of complexes, dissolved or in the form of suspensions or fixed beds. Understanding the photochemical behaviour of catalysts is a key point in the treatment of contaminated water.

Homogeneous photocatalysts exhibit high activity and selectivity due to their uniform distribution in the reaction medium, making them an attractive proposal for the removal of CECs. However, their use is limited to laboratory scale due to their high toxicity, low photostability and the high cost associated with the post-treatments necessary to prevent their release into the environment. These restrictions can be overcome by heterogenizing homogeneous photocatalysts to facilitate their recovery from the reaction medium.

Heterogeneous Photocatalysis Based on Semiconductors

The heterogeneous photocatalysis process can be divided into four independent steps:

Transport of reactants from the fluid to the surface of the photocatalyst.
Adsorption of at least one of the reactants.
Light absorption and reaction on the surface of the photocatalyst.
Desorption of the products.

Heterogeneous photocatalysts can be based on organic molecules attached generally to inorganic and semiconductors supports. For a semiconductor to act as a photocatalyst, two important requirements must be met: first, the semiconductor must have an energetically suitable band gap to absorb the incident radiation. Second, the reduction and oxidation potentials of the semiconductor’s conduction band (CB) and valence band (VB) must be favourable to carry out reduction and oxidation reactions, respectively. Moreover, these redox reactions can generate reactive oxygen species that eliminate organic contaminants in the medium.

It is generally accepted that when a semiconductor is irradiated with energy greater than or equal to the band gap (E_g), it can absorb this radiation by promoting an electron (e^–) to the conduction band and leaving a hole (h⁺) in the valence band. Depending on the energy position of the conduction and valence bands, the photogenerated electron-hole pair can carry out reduction and oxidation reactions. In the presence of oxygen and water, the formation of reactive oxygen species is thermodynamically favoured if the energy of the photogenerated electron in the CB is lower than the reduction potential of O₂ and the energy of the photogenerated hole is higher than the reduction potential of H₂O. Semiconductors with this band arrangement are of great interest for wastewater treatment and CEC removal.

Therefore, semiconductor-based photocatalysis for the removal of CECs and wastewater treatment is a promising strategy. New synthetic methods for photocatalysts, combined with the development of LED light sources, position photocatalysis as a new ally in the removal of organic contaminants.

Author: Oscar Cabezuelo Gandia, PhD · AIMPLAS Decarbonization Group