3.1 Fundamentals of advanced oxidation

Advanced Oxidation Processes (AOPs) are powerful chemical treatments that zap tough pollutants in wastewater. They work by creating super-reactive hydroxyl radicals that break down stubborn contaminants conventional methods can't handle.

AOPs use various oxidants like hydrogen peroxide and ozone to generate these radicals. Factors like pH, oxidant dose, and UV light intensity affect how well they work. Understanding these processes helps engineers design more effective wastewater treatment systems.

Advanced Oxidation Processes (AOPs)

Advanced oxidation processes in wastewater treatment

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• AOPs are chemical treatment methods that remove organic and inorganic contaminants from wastewater by generating highly reactive hydroxyl radicals ($\cdot$OH) to oxidize and degrade pollutants (pesticides, pharmaceuticals)
• Effective in treating recalcitrant, non-biodegradable, and toxic compounds that are difficult to remove using conventional wastewater treatment methods (chlorination, activated sludge)
• Often used as a tertiary or polishing step in wastewater treatment after primary (screening, sedimentation) and secondary (biological) treatment to further improve water quality
• Can be used as a pretreatment step to enhance the biodegradability of wastewater before biological treatment by breaking down complex molecules into simpler, more easily degradable compounds

Generation of hydroxyl radicals

• Hydroxyl radicals ($\cdot$OH) are the primary oxidizing species in AOPs with a high oxidation potential (2.8 V), making them highly reactive, non-selective, and short-lived
• Can oxidize a wide range of organic (pesticides, dyes) and inorganic (cyanides, sulfides) contaminants
• Generation of hydroxyl radicals in AOPs can be achieved through various methods:
  1. UV/H2O2: Photolysis of hydrogen peroxide (H2O2) by ultraviolet (UV) light
     • $H2O2 + hν → 2 \cdot OH$
  2. Ozone-based processes: Decomposition of ozone (O3) in water or in combination with UV light or H2O2
     • $O3 + H2O → 2 \cdot OH + O2$
     • $O3 + hν → O2 + O(^1D); O(^1D) + H2O → 2 \cdot OH$
     • $O3 + H2O2 → \cdot OH + \cdot HO2 + O2$
  3. Fenton and photo-Fenton processes: Reaction of ferrous iron (Fe2+) with H2O2, enhanced by UV light in photo-Fenton
     • $Fe2+ + H2O2 → Fe3+ + \cdot OH + OH-$
     • $Fe3+ + H2O + hν → Fe2+ + \cdot OH + H+$
• Hydroxyl radicals react with contaminants through various pathways:
  • Hydrogen abstraction: $\cdot OH + RH → R \cdot + H2O$
  • Electrophilic addition: $\cdot OH + C=C → \cdot C-C-OH$
  • Electron transfer: $\cdot OH + RX → RX \cdot + + OH-$

Oxidants and Factors Influencing AOPs

Comparison of AOP oxidants

• Hydrogen peroxide (H2O2)
  • Relatively stable and easy to handle
  • Requires activation by UV light or catalysts to generate hydroxyl radicals
  • Effective in UV/H2O2 or Fenton processes
• Ozone (O3)
  • Strong oxidant with a high oxidation potential (2.07 V)
  • Can directly oxidize some contaminants (iron, manganese) or decompose in water to generate hydroxyl radicals
  • Effective in ozonation, O3/UV, and O3/H2O2 processes
• Persulfate (S2O82-)
  • Stable at room temperature with a high redox potential (2.01 V)
  • Can be activated by heat, UV light, or transition metals to generate sulfate radicals (SO4$\cdot$-) which have a longer lifetime and higher selectivity compared to hydroxyl radicals
• Peroxymonosulfate (HSO5-)
  • Forms sulfate radicals and hydroxyl radicals upon activation by transition metals, heat, or UV light
  • Effective in sulfate radical-based advanced oxidation processes (SR-AOPs)

Factors affecting AOP efficiency

• pH
  • Affects the speciation and reactivity of oxidants and target contaminants
  • Optimal pH varies depending on the specific AOP and target contaminants
  • Fenton processes are most effective in acidic conditions (pH 2.5-3.5)
  • Ozonation is more effective in alkaline conditions due to enhanced formation of hydroxyl radicals
• Oxidant dose
  • Higher oxidant doses generally lead to increased contaminant removal
  • Excessive doses may lead to scavenging effects and reduced efficiency
  • Optimal dose depends on the specific AOP, target contaminants, and water matrix (dissolved organic matter, alkalinity)
• UV light intensity and wavelength
  • Higher UV intensity enhances the generation of reactive species and contaminant removal
  • UV wavelength affects the absorption and activation of oxidants
    • H2O2 absorbs UV light at wavelengths below 280 nm
    • Ozone absorbs UV light at wavelengths below 300 nm
• Presence of scavengers
  • Inorganic ions (bicarbonate, carbonate, chloride) and natural organic matter can scavenge reactive species
  • Scavengers compete with target contaminants for reactive species, reducing the efficiency of AOPs
  • Pretreatment or removal of scavengers (ion exchange, activated carbon) may be necessary to improve AOP performance
• Catalyst type and dose (for catalytic AOPs)
  • Catalysts enhance the generation of reactive species and contaminant removal
  • Common catalysts include transition metal ions (Fe2+, Cu2+), metal oxides (TiO2, ZnO), and activated carbon
  • Optimal catalyst dose depends on the specific AOP and water matrix
  • Excessive catalyst doses may lead to scavenging effects or the formation of unwanted byproducts (bromate, chlorate)