13.3 Photodegradation of pollutants and water treatment

Photocatalysis and Advanced Oxidation Processes

Principles of photocatalysis

Photocatalysis uses light energy to drive chemical reactions on the surface of a semiconductor, generating highly reactive species that break down pollutants in water. The core idea is straightforward: a semiconductor absorbs photons and produces reactive radicals that oxidize contaminants into harmless (or less harmful) products.

Here's how the process works, step by step:

1. Light absorption — A photon with energy equal to or greater than the semiconductor's band gap strikes the catalyst. For $TiO_2$ (anatase), that band gap is about $3.2 \text{ eV}$, corresponding to UV light ($\lambda < 387 \text{ nm}$).
2. Electron-hole pair generation — The absorbed photon excites an electron from the valence band to the conduction band, leaving behind a positively charged "hole" ($h^+$) in the valence band.
3. Charge carrier migration — The electron ($e^-$) and hole migrate to the catalyst surface. If they recombine before reaching the surface, no useful chemistry happens, which is why minimizing recombination is a major design goal.
4. Surface redox reactions — At the surface, holes oxidize water or hydroxide ions to form hydroxyl radicals ($OH•$), while electrons reduce dissolved oxygen to form superoxide radicals ($O_2^{•-}$). These reactive oxygen species (ROS) then attack pollutant molecules.

Two degradation pathways operate simultaneously:

• Direct oxidation — Holes at the catalyst surface directly oxidize adsorbed pollutant molecules.
• Indirect oxidation — Hydroxyl radicals generated in solution diffuse and react with pollutants that aren't directly adsorbed on the surface. This pathway often dominates because $OH•$ is an extremely powerful, non-selective oxidant.

Key applications in water treatment:

• Organic dye removal — Decolorizes textile wastewater by breaking down chromophores. Methylene blue is a common model pollutant in research because its degradation is easy to track spectrophotometrically.
• Pharmaceutical waste — Breaks down drug residues like ibuprofen and diclofenac that pass through conventional treatment plants largely intact.
• Pesticide detoxification — Degrades persistent compounds like atrazine in agricultural runoff.
• Disinfection — Inactivates waterborne pathogens such as E. coli through oxidative damage to cell membranes.

Mechanisms of advanced oxidation processes

Advanced oxidation processes (AOPs) are defined by their ability to generate hydroxyl radicals ($OH•$) in sufficient quantities to mineralize organic pollutants, ideally converting them all the way to $CO_2$ and $H_2O$. Two of the most studied AOPs in photochemistry are the $UV/H_2O_2$ and $UV/TiO_2$ systems.

UV/$H_2O_2$ system

This system relies on the photolysis of hydrogen peroxide by UV light:

$H_2O_2 + h\nu \rightarrow 2OH•$

Each photon that cleaves an $H_2O_2$ molecule produces two hydroxyl radicals. These radicals then initiate chain reactions, abstracting hydrogen atoms from organic pollutants and generating carbon-centered radicals that react further with dissolved oxygen. The process continues until the pollutant is mineralized or converted to smaller, biodegradable fragments.

A practical advantage of this system is its simplicity: no solid catalyst to recover. However, it requires continuous dosing of $H_2O_2$, and excess peroxide can actually scavenge $OH•$ radicals, reducing efficiency.

UV/$TiO_2$ system

Here, the semiconductor itself generates the reactive species upon UV irradiation:

$TiO_2 + h\nu \rightarrow e^- + h^+$

The photogenerated holes and electrons then produce multiple types of ROS:

• $h^+ + H_2O \rightarrow OH• + H^+$
• $e^- + O_2 \rightarrow O_2^{•-}$
• $O_2^{•-} + H^+ \rightarrow HO_2•$

The combination of $OH•$, $O_2^{•-}$, and $HO_2•$ gives this system versatility against a wide range of pollutants. The main challenge is electron-hole recombination, which wastes absorbed photon energy and lowers quantum efficiency.

Comparing the two systems:

Photocatalytic Materials and Reactor Design

Comparison of photocatalytic materials

Not all photocatalysts are created equal. The choice of material determines what wavelengths of light you can use, how efficiently charge carriers are generated, and how stable the system is over time.

Common photocatalytic materials:

• $TiO_2$ — The most widely studied photocatalyst. Anatase phase is generally more active than rutile due to its slightly higher band gap ($3.2 \text{ eV}$ vs. $3.0 \text{ eV}$) and lower recombination rate. Mixed-phase materials like Degussa P25 (roughly 80% anatase, 20% rutile) often outperform either pure phase because the phase junction helps separate charge carriers.
• $ZnO$ — Similar band gap to $TiO_2$ and sometimes shows higher activity for certain pollutants. Offers good surface area as nanoparticles. The drawback is photocorrosion: $ZnO$ can dissolve under prolonged UV irradiation in acidic conditions.
• $WO_3$ and other metal oxides — Narrower band gaps ($\sim 2.7 \text{ eV}$ for $WO_3$) allow absorption of visible light, but the conduction band position is often too positive to reduce $O_2$ to $O_2^{•-}$, limiting ROS generation.
• Doped and composite materials — Strategies like nitrogen doping of $TiO_2$ or coupling $TiO_2$ with $g\text{-}C_3N_4$ extend light absorption into the visible range and improve charge separation. This is one of the most active areas of current research.

Four factors that control photocatalyst efficiency:

1. Band gap energy — Determines the minimum photon energy (and maximum wavelength) that can activate the catalyst. A smaller band gap captures more of the solar spectrum but may reduce the oxidizing power of the holes.
2. Surface area and porosity — Higher surface area means more active sites for pollutant adsorption and radical generation. Mesoporous and nanostructured catalysts excel here.
3. Crystallinity and defects — Well-crystallized materials have fewer bulk defects that trap and recombine charge carriers. However, controlled surface defects can sometimes improve adsorption.
4. Charge carrier recombination rate — The single biggest efficiency killer. If electrons and holes recombine before reaching the surface, the absorbed photon energy is wasted as heat. Heterojunctions, co-catalysts, and morphology engineering all aim to suppress recombination.

Reactor designs:

Reactor performance ultimately depends on how well you manage light distribution, mass transfer, catalyst loading, and residence time. A reactor with poor light penetration will have a "dead zone" where no photocatalysis occurs, no matter how good the catalyst is.

Challenges in large-scale implementation

Moving photocatalytic water treatment from the lab bench to a real treatment plant is one of the biggest hurdles in this field. The challenges fall into several categories.

Technical challenges:

• Lab-scale results often use idealized conditions (pure water spiked with a single pollutant, high catalyst loading, intense UV lamps). Real wastewater contains mixtures of pollutants, dissolved ions, and natural organic matter that compete for reactive species and foul catalyst surfaces.
• Catalyst deactivation over time requires regeneration strategies (thermal treatment, UV cleaning) that add complexity.
• Uniform light distribution becomes much harder in large reactors. Light intensity drops exponentially with distance from the source (Beer-Lambert law), so thick water layers receive almost no photons at their center.

Economic considerations:

• Capital costs include reactor construction, UV lamp arrays or solar concentrators, and catalyst synthesis.
• Operating costs are driven by energy consumption (UV lamps are energy-intensive), catalyst replacement, and $H_2O_2$ dosing for AOP systems.
• Photocatalysis must compete with established methods like activated carbon adsorption, ozonation, and chlorination, which are cheaper per volume treated for many common pollutants.

Environmental concerns:

• Incomplete mineralization can produce transformation byproducts that are sometimes more toxic than the parent compound. Monitoring byproduct formation is essential.
• Spent catalyst nanoparticles (especially those containing heavy metals from doping) require proper disposal to avoid secondary contamination.
• Life cycle assessment should account for the energy and materials used to manufacture catalysts and UV systems, not just the treatment step itself.

Where the field is heading:

• Developing visible-light-active and solar-driven photocatalysts to reduce dependence on UV lamps and lower energy costs.
• Improving catalyst durability so materials can be reused through hundreds of treatment cycles without significant activity loss.
• Hybrid systems that combine photocatalysis with membrane filtration, biological treatment, or electrochemistry to handle complex real-world wastewater more effectively.
• Pilot-scale demonstrations that bridge the gap between promising lab results and practical deployment.